| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Departments of Physiology (J.R.G.C., S.G.M., W.G., S.J.L.) and of Obstetrics and Gynaecology (J.R.G.C., S.G.M., S.J.L.), University of Toronto, Toronto, Ontario, Canada M55 1A8; Program in Development and Fetal Health (J.R.G.C., S.J.L.), Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario, Canada M5G 1X5; MRC Group in Fetal and Neonatal Health and Development (J.R.G.C., S.J.L.); Department of Obstetrics and Gynaecology, and Cellular and Molecular Medicine (W.G.), University of Ottawa, Ottawa, Ontario, Canada K1H 8L6
 |
Abstract |
|---|
 Top Abstract I. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
|
I. Introduction |
|---|
|
TopAbstractI. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
Preterm birth, where there is asynchrony between the labor process and fetal maturation, occurs in 8–10% of all pregnancies, and its incidence has changed little in the past 40 yr (1). Indeed, factors such as low socioeconomic status of some inner-city populations, the tendency for women to choose to start a family at an older age, and the impact of fertility treatment are contributing to an increase in the incidence of preterm delivery (2, 3). Improved neonatal care, however, continues to reduce the mortality rate due to prematurity, although preterm birth remains the primary cause of neonatal death. In North America, the cost of caring for infants in the neonatal intensive care nursery during the first months of life has been estimated at $5–6 billion annually (3). That figure does not take into account the extraordinary emotional stress to the family of the prematurely delivered infant. Nor does it take into account the long-term costs required for chronic care of these infants, some of whom have major motor and/or mental handicaps and/or long-term neuro-developmental complications. To prevent preterm birth effectively, we need to understand the fundamental processes that switch the myometrium from its relative quiescence during pregnancy to the activated and contractile state at the time of labor. We will develop the thesis that regulation of myometrial function requires both endocrine and mechanical controls. Furthermore, it is now evident that the cause of preterm labor may vary at different times during pregnancy and will not necessarily reflect acceleration of the processes at term gestation. The ability to recognize these various causes of premature delivery, in a clinical setting, and then provide appropriate treatment remains a major clinical challenge. Furthermore, it is evident that prevention of preterm delivery may not always be desirable, particularly if the fetus is allowed to develop in a hostile intrauterine environment.
Causes of preterm birth in general fall into three categories: iatrogenic, where there is demonstrable complication of pregnancy such as preeclampsia or fetal distress that requires obstetrical intervention; premature rupture of the fetal membranes with or without infection; and, idiopathic preterm labor. The relative importance of these causes varies. However, most sources consider that approximately 30–40% of preterm birth is associated with an underlying infective process, and 40–50% of preterm births are idiopathic.
In this review, we will focus attention on experimental studies in the sheep, the species of choice for many investigators concerned with understanding the processes of birth (4). We shall then extrapolate from the sheep to an understanding of parturition in primates, particularly in the human. Our central thesis is that the processes of birth are remarkably similar, at a fundamental level, across species, and in both sheep and human the fetus, through activation of its hypothalamic-pituitary-adrenal (HPA) axis, plays a central and crucial role. We shall examine how the fetal HPA axis may be activated in response to a stress circumstance during pregnancy, e.g., hypoxemia, such as that perhaps associated with reduced uteroplacental perfusion in preeclampsia. It will be apparent that the fetal signal provokes increased outputs of stimulatory PGs and other uterotonins from intrauterine tissues. It is evident now that there is a progression from fetal to maternal control of intrauterine PG production. Furthermore, the regulation of PG synthesis and metabolism in fetal trophoblasts and maternal uterus is effected by different mechanisms.
|
II. Regulation of Myometrial Contractions |
|---|
|
TopAbstractI. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
We have found it useful to divide the uterine phenotype into different stages of the parturition process (10). The uterus is relatively quiescent during 95% of pregnancy, corresponding to phase 0 of parturition. Activation corresponds to phase 1 and is effected predominantly by mechanical input, and through regulation by uterotrophins such as estrogen. Stimulation corresponds to phase 2, when endogenous uterotonins, including PGs and oxytocin (OT), act on the activated myometrium. Postpartum involution corresponds to phase 3. In this sequence of events, the "initiation" of parturition corresponds to the transition from phase 0 to phase 1, although clearly one could argue that initiation started much earlier in gestation (11).
Contraction of the myometrium at term or preterm depends upon
conformational changes in the actin and myosin molecules, which allow
actin and myosin filaments to slide over each other, ultimately leading
to a shortening of the myocyte (Fig. 1
and Refs. 12, 13). The confirmational changes (involving
cross-bridge cycling of the myosin head) require ATP, which is
generated by myosin after phosphorylation of the 20-kDa light chains of
myosin by the enzyme myosin light chain kinase (MLCK). This enzyme is
central to signaling pathways that both stimulate and inhibit
myometrial contractions (14, 15). MLCK is activated through interaction
with the calcium binding protein calmodulin (CAM), which in turn
requires 4 Ca2+ ions for its own activation.
Binding of calcium-activated CAM to MLCK induces a conformational
change in the enzyme, allowing MLCK to phosphorylate the 20-kDa light
chains of myosin. MLCK can also undergo phosphorylation by protein
kinase A (PKA, cAMP-activated protein kinase), which reduces the
affinity of the enzyme for calcium calmodulin (Ca-CAM) and leads to its
inactivation (14, 16). Regulation of MLCK has been reviewed extensively
(17, 18). It is evident that activity of this enzyme is altered by
intracellular pathways that regulate levels of calcium and of cAMP and
is critical for the development of uterine contractility. Uterotonins
generally increase intracellular calcium levels
([Ca2+]i), by increased
influx of Ca2+ through receptor-operated
channels, or release of calcium from intracellular stores including
sarcoplasmic reticulum (see Ref. 19). Agents that inhibit myometrial
activity do so by increasing intracellular levels of cyclic nucleotides
cAMP or cGMP, which in turn inhibit release of calcium from
intracellular stores or reduce MLCK activity. Binding of agents such as
ß-adrenergic agonists, relaxin and prostacyclin, to myometrial
receptors activates adenylate cyclase activity, leading to an increase
in cAMP generation, while uterine inhibitors such as nitric oxide (NO)
activate guanyl cyclase, increasing cGMP. In collaborative studies,
Pato et al. (20) characterized MLCK purified from pregnant
sheep myometrium. The enzyme had an apparent molecular mass of 160 kDa
and high substrate specificity for myosin light chains. Sheep
myometrial MLCK has an absolute requirement for
Ca2+ and CAM for activation; in the absence of
Ca-CAM, MLCK is inactive. On binding Ca-CAM, MLCK undergoes a
conformational change that exposes the catalytic site, which can then
phosphorylate the 20-kDa myosin light chains to initiate contraction.
Relaxation is achieved either by dephosphorylation of MLC-20 by the
catalytic subunits of type 2A phosphatase (21) or by reduction in MLCK
activity. The latter is achieved, as discussed, by reduction in
[Ca2+]i, resulting in
dissociation of Ca-CAM from MLCK. Sheep myometrial MLCK is also a
substrate for PKA, which phosphorylates serine residues on the sheep
myometrial enzyme in the presence or absence of bound Ca-CAM. The
ability of PKA to inhibit myometrial MLCK activity, even in the
presence of agonists that increase
[Ca2+]i, provides a
biochemical rationale for the finding that agents that increase
intracellular cAMP inhibit uterine contractions even in the presence of
calcium-activating agents such as OT and stimulatory PG. Ca-CAM can
also activate phosphodiesterase to increase the breakdown of cAMP.
|
-, and
-subunits. The -subunit activates adenylate cyclase to initiate
cAMP synthesis. cAMP, in turn, activates PKA, which then phosphorylates
a series of regulatory proteins. Activated PKA either phosphorylates
MLCK to reduce its ability to bind Ca-CAM or phosphorylates a
membrane-binding site for Ca2+ that increases
calcium binding and reduces free intracellular calcium concentrations. Regulation of myometrial calcium levels has been reviewed extensively (see Refs. 12, 22, 23, 24). Free resting Ca2+ increases from 150 nM to about 500 nM during contraction through influx of extracellular Ca2+ or by the release of Ca2+ from intracellular binding sites or intracellular organelles (25, 26). Extracellular Ca2+ enters cells through receptor-operated or voltage-gated channels. Release of intracellular Ca2+ from sarcoplasmic reticulum is activated through the phosphoinositol (PI) pathway. Binding of a uterotonin to its plasma membrane receptor activates a G protein transducer, coupled to phospholipase C, which frees inositol trisphosphate (IP3) and diacylglycerol (27, 28). Free IPs, especially IP3, increase cellular calcium from intracellular storage sites. Interestingly, IP3 binding in myometrium was inhibited by calcium, suggesting that this might provide a mechanism for regulating the IP3 response by oscillating [Ca2+]i. Diacylglycerol formed during IP3 turnover may stimulate PKC to phosphorylate cellular proteins such as MLCK or be rapidly phosphorylated by diacylglycerol kinase to phosphatidic acid, a naturally occurring Ca2+ ionophore, or lead to release of arachidonic acid by cellular lipases, resulting in production of eicosanoids (see below).
Function of the myometrium during labor at term or preterm requires highly developed cell-to-cell coupling, effected through formation of intercellular GAP junctions within adjacent cell membranes (14, 29). The proteins forming GAP junctions are termed connexins and are classified according to their apparent molecular weights (30). Connexins are arranged into hexameric hemichannels, which become aligned across adjacent cells to form an interconnecting pore that allows low-resistance electrical or ionic coupling between the cells and provides a pathway for metabolite transfer (31). Hundreds of individual channels arrange themselves into an organized plaque to form a GAP junction. Regulation of connexins occurs at the level of transcription and translation (31, 32); mechanisms also operate to control transport of connexin protein to the cell membrane and to direct assembly into connexons, through apposition, clustering, and formation of functional channels (33, 34). This complex process is poorly understood, although it is influenced by steroids and by mechanical stretch (35). GAP junction formation requires the presence of cell adhesion molecules, and in early studies, Meyer et al. (36) showed that appearance of GAP junctions in transfected S180 cells was blocked by coincubation with antisera to liver cell adhesion molecule.
Garfield (see Refs. 14, 16) established clearly that an absence of GAP junctions in the pregnant myometrium was responsible for high-input resistance of these smooth muscle cells and poor coordination of uterine contractions. There is a massive increase in numbers of GAP junctions with the onset of labor, which significantly enhances electrical coupling and allows the myometrium to develop synchronous high amplitude contractions (37). An increase in GAP junctions with labor onset has been found in all species studied. In the rat, levels of connexin-43 (CX-43) mRNA and protein were low during pregnancy but increased some 48 h before labor (38, 39). Highest levels of mRNA and protein were found during delivery itself. This is critical because the half-life of GAP junctions may be as short as 1–2 h, and hence continued synthesis would be required to maintain labor. Increases in CX-43 mRNA have been reported in sheep and human myometrium with the onset of labor and correlated with increases in CX-43 protein (37, 38). Permeability of GAP junctions may be facilitated through phosphorylation at consensus serine and tyrosine sites within the cytoplasmic domain of CX-43. Garfield (14) demonstrated that cell-to-cell communication in the myometrium is reduced by elevated [Ca2+]i and increased levels of cAMP. Importantly, more recent studies have shown that the pattern of CX-43 in myometrium during pregnancy differs from that of CX-26. Connexin-26 expression is elevated in midgestation in the rat and appears to be associated more with uterine quiescence (7, 8).
|
III. Pregnancy: Phase 0 of Parturition |
|---|
|
TopAbstractI. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
s subunits of G
proteins and increase intracellular levels of cAMP (40, 41, 42). Relaxin also elevates myometrial cAMP and inhibits OT-induced turnover of phosphoinositide (PI) by the action of cAMP-dependent protein kinase. Relaxin exerts a dual role in the inhibition of myometrial contractility and in the regulation of connective tissue changes in the cervix (43, 44). Porter and colleagues (45, 46) were among the first to show that relaxin suppressed spontaneous uterine contractility in the rat and guinea pig, but sensitivity to OT was preserved. Thus, the major action of relaxin is one of frequency modulation (47). Hansell et al. (48) and others have demonstrated that relaxin is expressed in the human fetal membranes, decidua, and placenta, consistent with its exerting paracrine/autocrine effects on intrauterine tissues (49, 50, 51). Relaxin gene expression is dramatically up-regulated in patients with preterm, premature rupture of membranes (PPROM) (49). Relaxin receptors have been localized to decidua and chorionic trophoblast cells, and the protein acts through these to up-regulate expression of matrix metallo-proteinases (MMP), especially MMP1, MMP3, and MMP9. Similarly, relaxin increases MMP expression in cervical tissue at term. Administration of exogenous relaxin stimulates separation of the pubic symphysis in those species in which it is a prerequisite for delivery (52). In addition, in pigs and rats, relaxin appears necessary for maintaining evolution of spontaneous uterine contractility in late pregnancy and for maintaining a high frequency of live births (43). In vitro studies have shown that relaxin blocks the action of stimulants such as OT, carbachol, and norepinephrine on the myometrium, through mechanisms involving PKA-mediated phosphorylation of PLC-linked G proteins. This in turn inhibits IP3 turnover and the increase in [Ca2+]i (22). Although the precise role that relaxin plays during pregnancy remains to be determined, it may be particularly useful in maintaining uterine quiescence during the period when progesterone concentrations are falling and estrogen levels are beginning to increase before the onset of labor (see Ref. 12). In addition, there are reports that relaxin may act centrally to increase circulating plasma OT and vasopressin concentrations by an opioid-independent mechanism (53). It is now known that OT is produced within human intrauterine, choriodecidual tissues. It remains to be established whether a similar relationship exists between relaxin and OT synthesized within the intrauterine compartment in women.
Lye and Challis (54, 55) first showed, some 20 yr ago, that prostacyclin infused into nonpregnant sheep inhibited uterine contractility in vivo. In parallel studies a similar inhibitory effect of prostacyclin was observed on human myometrium (56), and it is clear now that prostacyclin represents the major eicosanoid present within the pregnant myometrium of many species (57), including human. In human term pregnant myometrial strips maintained in vitro, the initial response to PGI2 was contraction, but this was followed by relaxation (58, 59). It is now recognized that PGI2 acts through specific IP receptor species to increase adenylate cyclase activity and elevate intracellular cAMP (60). Other agents such as CRH also stimulate output of cAMP from myometrial cells and act synergistically with PGI2 in a paracrine/autocrine fashion (61). The role of CRH in pregnancy maintenance and parturition will be discussed later in this review.
More recently, interest has arisen over the potential role of NO as an endogenous inhibitor of myometrial contractility (62). Increases in endogenous synthesis of NO by administration of the NO precursor L-arginine, or the NO donor sodium nitroprusside, inhibit myometrial contractions in the rat and human (62). Nitroprusside has been shown to decrease force and 20-kDa myosin light chain phosphorylation in human myometrial strips, although the tissue is not as sensitive as vascular smooth muscle. Nitric oxide synthase (NOS) isoforms have been detected using RT-PCR in human fetal membranes and choriodecidua (62). Levels of mRNA-encoding inducible NOS (iNOS) are highest in human myometrium at preterm, not in labor patients, and decrease with a corresponding fall in iNOS protein in myometrium collected at term (see Ref. 10). Several authors have suggested that NO acts in a paracrine manner, potentially in conjunction with progesterone to effect myometrial quiescence during pregnancy, although this position has been disputed. There is a decrease in NOS activity of decidua and myometrium in species such as rat before parturition in a manner that would presumably diminish its inhibitory influence on the uterus. Furthermore, studies by Chwalisz and Garfield (62) have shown that, at term in the rat, there is a corresponding increase in NO production by inflammatory cells of the cervix, indicating a role for NO in cervical effacement and relaxation as its influence on the myometrium is diminished.
Other inhibitors of uterine activity include calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), and endogenous ß-adrenergic agonists (63). These compounds act through increasing intracellular cAMP and/or decreasing intracellular calcium availability (64).
|
IV. Myometrial Activation: Phase 1 of Parturition |
|---|
|
TopAbstractI. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
Overall regulation of myometrial activity is genetically
regulated (Fig. 2). Different species
have gestations of varying lengths, and studies involving embryo
transfer suggest that it is the genotype of the fetus that controls the
length of pregnancy. For example, when sheep embryos from short
gestation or long gestation breeds were implanted into random
gestation-age recipients, parturition occurred at the appropriate time
for the fetal rather than maternal genotype (66, 67). There is a
variety of mechanisms by which the fetal genotype can influence
pregnancy length, and we have proposed that it includes both endocrine
and mechanical signals. In initial studies, Ou and Lye (68) found,
using unilaterally pregnant rats, that while expression of CAP genes,
CX-43 and OT receptor (OTR), increased in the gravid uterine horn in
labor, there was no parallel increase in the nongravid horn, even
though both horns were exposed to the same systemic hormonal changes.
Next, these workers showed that when a small 3-mm diameter tube was
placed into one uterine horn of bilaterally ovariectomized nonpregnant
animals, there was a significant increase in mRNA levels encoding CX-43
in that horn, compared with the contralateral horn. Control experiments
showed that this result was not due to the presence of a foreign body
within the uterus. Administration of progesterone to these animals
blocked stretch-induced increases in CX-43 expression.
|
receptor (FP receptor), or OTR were
measured. In animals treated at day 15 and studied at day 20, there was
no effect of the Silastic tube in increasing CX-43 transcripts, but in
animals studied at the time of labor there was a dramatic increase in
the numbers of CX-43 transcripts to values similar to those seen in the
contralateral pregnant horn. There was little change in CX-43
transcripts in the nonpregnant control horn. These data suggest that
stretch of the myometrium appears capable of up-regulating
contraction-associated proteins, but the ability to do so is highly
dependent on the endocrine environment. If the stretch stimulus is
applied during pregnancy, it is inadequate to induce CX-43, and
presumably its activity is inhibited by circulating concentrations of
progesterone. However, at term, when maternal systemic progesterone
levels have decreased, stretch itself is adequate to produce the same
level of CX-43 expression as in the pregnant horn containing the fetus. The molecular mechanisms by which stretch increases CX-43 and OTR expression remain to be determined (69). In other systems, such as cardiac myocytes, stretch activates multiple intracellular signaling pathways through shear stress response elements in the promoter of some stretch-responsive genes (70). The CX-43 gene contains such an element, suggesting that if wall tension contributes to the regulation of CAP genes in the myometrium, regulation of uterine growth through pregnancy will be important in determining the level of shear stress. Lye and colleagues (8) have argued that, during pregnancy, uterine growth follows three distinct phases: an initial phase during the first trimester where uterine growth is due to hyperplasia and controlled by endocrine factors, a second phase during the second and third trimester in which growth is closely matched to increased fetal size, and a final phase in which there is a decline in uterine growth in comparison to fetal growth, and hence an increase in uterine wall stretch and tension. They speculate that progesterone is necessary to support stretch-induced hypertrophy of the uterus during midgestation in concert with increasing fetal size. Near term, the fall in progesterone, observed in most animal species (see below), leads to a decline in uterine growth relative to fetal growth and hence increased tension development, which in turn results in increased CAP gene expression and contributes to myometrial activation. Since the decrease in circulating progesterone appears critical for the altered influence of stretch on myometrial CAP gene expression, we shall consider the endocrine pathways that result in progesterone withdrawal.
A. Activation: role of fetal HPA maturation
More than 35 yr ago, Professors Sir Graham (Mont) Liggins and
Geoffrey Thorburn, working in the sheep and goat, showed
conclusively in those species that the fetus, in utero,
appeared to provide the trigger mechanism for the onset of parturition
and that it did so through activation of the fetal HPA axis. An
endpoint of activation of this axis is progesterone withdrawal. We
shall suggest that the primate fetus similarly affects gestation
lengths through activation of the HPA axis. However, in human gestation
there is no systemic progesterone withdrawal, and we shall argue that,
in women, sustained circulating concentrations of progesterone are
indeed required at term to effect regionalization of myometrial
contractility and promote relaxation of the lower uterine segment.
Early studies in the fetal sheep showed that ablation of the fetal pituitary gland, the fetal adrenal gland, pituitary stalk section, or lesioning of the fetal paraventricular nucleus (PVN) resulted in prolongation of gestation (71, 72, 73), whereas the infusion to the fetal lamb in utero of ACTH or of a glucocorticoid resulted in premature parturition within 3–5 days of beginning the infusion. These studies provided experimental verification of the concept developed from observations of naturally occurring prolonged gestation in sheep attributable to ingestion of the teratogen Veratrum californicum at a specific time of gestation. In those animals, gestation length was prolonged by up to 60 or 70 days, although fetal growth continued. Fetuses exhibited gross malformations, including cyclopean characteristics. At autopsy, the pituitary and adrenal glands were remarkably hypoplastic as a result of impaired pituitary development at an early gestational age (see Ref. 81).
Several groups of workers provided clear evidence for maturation of fetal HPA function in the sheep fetus during late gestation (74, 75, 76). There are progressive increases in fetal plasma ACTH1–39 and cortisol in the plasma of the late-gestation fetal sheep (77, 78, 79, 80); the initial increases in ACTH precede the rise in cortisol (79), but fetal cortisol increases in an exponential fashion over the last 10 days of gestation, with highest concentrations being established immediately before term (80). This is consistent with the fact that ACTH is important in the development of the adrenal cortex in late gestation. Similar maturation of pituitary adrenocortical function has been demonstrated in several other species, including the guinea pig, which represents a species that gives birth to mature young. The prepartum surge of cortisol is important for the maturation of several organ systems, particularly the lungs and kidneys (see Ref. 81). It is also critical for normal development of programming of the brain. However, the simultaneous increase in fetal plasma ACTH and cortisol has remained somewhat of a paradox because, under normal circumstances, one would expect elevations in fetal plasma cortisol concentration to inhibit further ACTH secretion. Mechanisms have developed to override the influence of negative feedback in the fetus in late gestation, a relationship now described in the guinea pig as well as in the sheep (see below).
Recent studies have explored the molecular mechanisms underlying changes in fetal pituitary adrenocortical activation in late gestation sheep. Developmental changes in CRH mRNA in the fetal hypothalamic PVN were examined by in situ hybridization (82). By day 60 of gestation, CRH mRNA was detectable in the fetal PVN. There was an increase in CRH mRNA expression by day 120 of gestation and a further substantial up-regulation of CRH gene expression in the last 20 days of pregnancy. This was followed by a decrease in CRH expression in the PVN of the newborn lamb. Throughout development, expression of CRH mRNA appears to be confined to parvocellular fields of the PVN, with no expression detected in magnocellular neurons (82). Recent studies have confirmed that the changes in CRH mRNA are translated to CRH peptide in the fetal hypothalamus, indicating a close association between transcription and translation of the CRH gene during development.
In the fetal pituitary, expression of the ACTH precursor, POMC, is detectable in the inferior region of the pars distalis by day 60 of gestation. Levels of POMC mRNA in the superior and inferior regions of the pars distalis increased with progression of gestation until around day 120, when there was a further increase in expression, peaking at term (83, 84). The increase in POMC expression is combined with a remarkable reorganization of the corticotrophs toward the inferior aspect of the fetal pituitary gland. This pattern of expression was sustained in the newborn lamb. In the fetal pars intermedia, the developmental profile of POMC mRNA was quite different. Relatively high levels were present by day 60 of gestation; these increased between days 60 and 100 and then remained relatively constant for the remainder of gestation. Early controversy concerning changes in expression of POMC mRNA in fetal pituitary tissue appears to result from differences in methodologies. The use of in situ hybridization clearly allows separation of different zones of the fetal pituitary gland, whereas erroneous results may have been obtained through use of Northern blot analysis (85, 86). In a recent carefully conducted study obtaining pituitary tissue from fetuses at specific times in late gestation and during the labor process itself, the lack of negative feedback on POMC mRNA, and the sustained increase in POMC mRNA levels, was clearly demonstrated (87). The change in regional distribution of POMC mRNA in the pars distalis may indicate the transition of fetal-like to adult-like corticotrophs that has been described at this time (see below). Changes in POMC mRNA in the pars distalis are reflected by increased levels of ir-ACTH and by increased immunostaining for ACTH in pituitary corticotrophs (83, 84); at term ir-ACTH-positive cells represent about 15% of the total cell number in the pars distalis.
Arginine vasopressin (AVP) is also an important regulator of fetal pituitary ACTH secretion and is expressed in the fetal PVN relatively early in gestation (88). AVP mRNA is present in the supraoptic nucleus, PVN, and the accessory magnocellular nuclei by day 60 of gestation (82). Differential expression of magnocellular and parvocellular AVP is evident in the PVN by day 80. In magnocellular neurons, AVP mRNA increases with gestational age, whereas parvocellular expression of AVP remains relatively unchanged. Levels of AVP mRNA increase dramatically in both regions of the PVN in the newborn lamb. It is suggested that magnocellular AVP is involved primarily in fetal fluid homeostasis, while parvocellular AVP is important in stimulation of the pituitary corticotroph (84). There is a close correlation between AVP mRNA levels and ir-AVP in the anterior hypothalamus, as there is for CRH. The increase in parvocellular AVP mRNA in the newborn may be associated with the stress of the novel extrauterine environment. Axons containing AVP and OT have been identified in a zone of the pars distalis adjacent to the pars intermedia in fetal sheep. These axons are probably those of magnocellular neurons and may represent a mechanism by which magnocellular AVP and OT directly affect ACTH release in vivo.
CRH and AVP induced dose-dependent increases in ACTH output from ovine fetal pituitary cells in vitro (89); at equimolar concentrations AVP was more potent than CRH. Simultaneous administration of CRH and AVP showed an additive interaction between the neuropeptides (90). Treatment with CRH significantly increases POMC mRNA levels in sheep pituitary cells harvested at day 120 and day 138 of gestation. However, CRH treatment of cells collected from fetuses at term failed to affect POMC synthesis. AVP increased POMC mRNA levels in cells obtained at day 138 of gestation; in pituitary cells from late-gestation fetuses, AVP and CRH are equally potent in the induction of POMC synthesis. Cortisol has little negative feedback effect on basal output of ACTH in these cells but inhibits CRH-stimulated ACTH output and POMC gene expression.
Studies by Lu et al. (91) showed that ovine fetal pituitary membranes expressed CRH receptor activity as early as day 100 of gestation. CRH binding increased to its highest levels at around day 135 (term, 145–150 days) and then decreased progressively through late gestation (92). Recent studies have extended these measurements to show that levels of mRNA encoding fetal pituitary CRH-receptor type I may follow a similar profile (J. C. Rose, personal communication), and this may account for the altered outputs of ACTH in response to CRH stimulation in vivo (see below). Factors regulating CRH receptor expression have been examined in vivo and in vitro. In vitro studies indicated that CRH down-regulated expression of its own receptor and cortisol produced a similar attenuation of binding activity (92).
In vivo studies demonstrated that CRH was more potent than
AVP in stimulating ACTH output by pituitary tissue from chronically
catheterized fetal sheep in late gestation (93, 94). The response
profiles, however, are quite different. AVP induced a transient rise in
plasma ACTH while CRH stimulated a more sustained increase (95).
Subsequently, it was demonstrated that AVP concentrations are about 5
times those of CRH in the hypophyseal portal circulation of adult sheep
(96), and it remains possible that the relative importance of AVP in
fetal corticotroph activation in utero may be greater than
that of CRH (97). Fetal pituitary responsiveness to CRH increases
between day 110 and 125 and then decreases toward term (79). This
relative insensitivity of the pars distalis to CRH may reflect the
increase in negative feedback influence of rising endogenous cortisol
concentrations, or the decrease in CRH binding sites indicated above
(79). Simultaneous administration of CRH and AVP results in an ACTH
response that is greater than when either neuropeptide is administered
independently, and the interaction is synergistic in nature, at least
at around day 115 of gestation (95). CRH and AVP affect the
corticotrophs through different second messenger systems. CRH exerts
this action through up-regulating a
Gs-adenylate
cyclase-linked membrane receptor and increasing intracellular levels of
cAMP (89). AVP acts through V1b receptors to
stimulate PI turnover, stimulating phospholipase C and activating
protein kinase C.
POMC is processed through different endoproteases, prohormone convertase 1 (PC-1) and prohormone convertase 2 (PC-2), to yield a spectrum of products. Recent studies have demonstrated that both PC-1 and PC-2 are present in fetal sheep pituitary tissue in late gestation. However, expression of these enzymes does not change with labor, and it seems unlikely that the increase in ACTH output is attributable to altered prohormone convertase activity (87, 98). However, the pattern of POMC-derived peptides from the fetal pituitary does change in the plasma of the fetal lamb in late pregnancy (99). Several groups of investigators have reported that large molecular weight POMC-derived ACTH precursor peptides are present in the circulation (100). The concentrations of these larger molecular weight forms decrease prepartum, whereas those of ACTH1–39 increase. Because the larger molecular weight peptides may act to antagonize the action of ACTH1–39 on adrenocortical cells (101, 102, 103), a decrease in their concentration prepartum would presumably facilitate ACTH action and an increase in adrenal glucocorticoid secretion (104). The sources of these peptides may be different (105, 106, 107). Studies in hypothalmo-pituitary-disconnected fetuses have led to the suggestion that the pars intermedia may be a potential source of large molecular weight peptides, whereas the pars distalis is the primary source of ACTH1–39. In addition, the ovine fetal lung and placenta express POMC mRNA and contain ir-ACTH. It is not clear whether these potential sources of ACTH contribute to circulating ACTH1–39 in a meaningful manner or whether the peptides have paracrine/autocrine actions within the tissues of origin.
Thus, the temporal relationship between hypothalamic-CRH and pituitary
POMC expression is consistent with the simultaneous increase in plasma
ACTH and cortisol observed in late gestation (84, 108, 109, 110).
Nevertheless, the mechanism by which CRH mRNA and POMC mRNA increase in
the presence of high plasma glucocorticoid concentrations is not clear.
One possible mechanism is that, in the fetus, glucocorticoid feedback
thresholds within the brain and pituitary become modified. This may
occur at several levels (Fig. 3). We have
reported that glucocorticoids up-regulate expression of corticosteroid
binding globulin (CBG) mRNA in the fetal liver, and of circulating CBG,
which is the opposite of the response in adult sheep (111, 112). In the
fetus, the pattern of CBG glycosylation varies from that in adult
animals, but the glycoprotein increases in concentration in the fetal
circulation and maintains a relatively constant free cortisol
concentration for most of pregnancy (112, 113). Near term, however, the
increase in adrenal cortisol output exceeds the CBG binding capacity,
resulting in a sudden increase in free cortisol concentration over the
final hours before birth (114). It appears that this increase in free
cortisol before parturition is a consistent observation across
different animal species (115). More recently, we have demonstrated
expression of CBG mRNA and the presence of CBG immunoreactive protein
in other fetal tissues including the kidney, pancreas, and pituitary
(115). CBG mRNA has been localized to fetal pituitary cells by in
situ hybridization, and its pattern of distribution appears to
differ from that of POMC, with greater abundance in superior regions of
the gland. As yet, there are no studies demonstrating colocalization of
CBG with ACTH-producing cells in fetal pituitary tissue.
|
A further mechanism by which glucocorticoid feedback could be altered locally is through modification of corticosteroid receptor expression (117). The ovine fetal pituitary expresses type II glucocorticoid receptor (GR) from relatively early in gestation, and the levels of GR mRNA increase toward term (118), consistent with glucocorticoid effects in modulating the switch from fetal to adult corticotroph cell types in the pituitary (106). During the course of labor, there is a dramatic decrease in levels of GR mRNA in the fetal pars distalis, suggesting that the potential for glucocorticoid negative feedback decreases in the pituitary during the course of labor. More important, perhaps, is the demonstration that there are decreases in immunoreactive GR in the hypothalamic PVN near term. These changes were specific to CRH- and AVP-positive parvocellular neurons. More recently, we showed that GR mRNA levels in the PVN of fetal sheep and guinea pigs decrease in late gestation, and in fetal sheep levels of GR mRNA in the hippocampus also fall prepartum. The hippocampus represents a major site of glucocorticoid feedback for HPA function, and there are a number of direct and indirect connections between the limbic system and the PVN. Together these data suggest that a reduction in the potential for glucocorticoid feedback occurs in late gestation in brain structures that are central to glucocorticoid negative feedback action (119).
In addition to classic feedback processes, there are several other mechanisms by which fetal HPA axis activation may occur. Expression of pro-enkephalin mRNA rises to a maximum in the parvocellular PVN of fetal sheep at day 135 of gestation and then decreases in older animals (120). A fall in hypothalamic pro-enkephalin mRNA occurs with intrafetal infusion of cortisol at day 135, suggesting that the prepartum rise in endogenous cortisol may inhibit parvocellular pro-enkephalin synthesis. CRH and met-enkephalin are present in the same secretory granules in rodents, and met-enkephalin inhibits CRH-stimulated ACTH secretion from fetal pituitary cells in vitro. Thus, a decrease in met-enkephalin production may facilitate corticotroph function near term (120). OT has been implicated in the control of ACTH secretion in adult sheep, and OT stimulates ACTH output from the fetal pituitary cells in vitro. OT mRNA is present in both magnocellular and parvocellular fields of the PVN and SON and follows a similar developmental profile to AVP mRNA, raising the possibility that it too may influence fetal pituitary function.
In fetal sheep, the kinetically determined production of cortisol from the adrenal gland increases during the last 20–25 days of gestation (77, 121). In part, this results from the increase in drive to the adrenal from rising levels of ACTH, but, in part, it is attributable to maturation of fetal adrenal function (122). Indeed, in hypophysectomized fetuses treated with a continuous low-level infusion of ACTH, plasma cortisol concentrations increased and parturition occurred at around the normal time, consistent with fetal adrenal maturation as the overriding influence (123).
Ovine fetal adrenal responsiveness changes dramatically during the course of pregnancy (124, 124, 125, 126). Adrenal cells collected from animals at days 50–70 of gestation secrete cortisol in response to ACTH stimulation in amounts similar to or greater than adrenal tissue from term fetuses (127). However, between approximately days 90–110 of pregnancy the adrenal is relatively insensitive to ACTH stimulation (124). It is now clear that this pattern of response is due, in large part, to decreased gene expression of P450C17 and P450SCC steroidogenic enzymes in fetal adrenal cortical cells at midgestation (128, 129). The abundance of mRNAs for these enzymes is increased by ACTH administration to the fetus (130, 131). Although 3ß-HSD may be rate limiting to cortisol production in the first half of pregnancy (132), immunoreactive (ir)-3ß-HSD-positive cells are present throughout the zona fasiculata of the fetal adrenal cortex from day 50 until term (133). The midgestational decrease in P450C17 may result by TGFß inhibiting ACTH-induced stimulation to P450C17, as demonstrated in vitro in ovine fetal and adult adrenal cells (134). Recent studies have demonstrated that ACTH receptor mRNA is detectable from around day 60 of gestation (135). There is a modest increase through pregnancy and then a substantial increase between days 126–128 and days 140–141 (135). Thus, the low level of basal adrenal responsiveness to ACTH around day 100 of gestation is not due to lack of ACTH receptor expression, but may be attributable, in part, to very low concentrations of ACTH in the fetal circulation at that time (136). The increase in ACTH receptor expression in late gestation would appear to contribute to increased adrenal responsiveness near term. The factors responsible for up-regulating ACTH receptor mRNA abundance are unclear (137). These may include ACTH itself, cortisol, or local intraadrenal interaction with IGF-II and/or decreased influence of TGFß (138, 139, 140).
Both in vivo and in vitro studies have shown that fetal adrenal maturation can be advanced by ACTH1–24 administration (110, 141, 142, 143). Exogenous ACTH in vivo enhances coupling between ACTH receptor and adenylate cyclase and enhanced capacity for cAMP generation (144, 145, 146). ACTH treatment in vivo also increased expression and activity of P450C17, P450C11, P450C21, and 3ß-HSD (130, 147). The adrenal responds to ACTH early in gestation, although continued trophic input is required to maintain increased levels of gene expression. Interestingly, when ACTH was administered to fetuses in vivo as pulses, rather than as a continuous infusion, it led to a pattern of fetal adrenal steroidogenesis that favored cortisol over corticosterone output (i.e., directed P450C17 activity). Thus, the pulse pattern of endogenous ACTH secretion in vivo may affect the pattern of adrenal activation (148, 149).
These studies suggest that ACTH-induced increases in adrenal steroidogenic enzymes, particularly P450C17, is essential to allow C21 steroids to proceed through the 17-hydroxy pathway leading to cortisol biosynthesis (130, 150). An obligatory role for an increase in ACTH drive to the fetal adrenal as a prerequisite for increased responsiveness, however, has been challenged recently. When hypophysectomized fetal sheep were infused at a constant, but low level of ACTH, there was a normal rise in fetal cortisol concentration; later, maternal progesterone levels decreased and birth occurred at about the expected time (123). The molecular mechanisms underlying this fascinating result clearly require elucidation.
We have hypothesized that fetal stress, perhaps reflected in diminished fetal arterial P02, constitutes a stimulus for preterm birth. Experimental hypoxemia has been used extensively to investigate fetal HPA activation (151, 152). Many studies have shown that even modest reductions in fetal arterial P02 induce robust increases in fetal plasma ACTH and cortisol concentrations (153, 154). Release of CRH and AVP into the hypophysial portal system is abolished in the hypothalamo-pituitary-disconnected (HPD) fetus (152), and these animals are incapable of mounting an ACTH response to stress, implying that increased ACTH output requires hypothalamic input. Studies by Akagi and colleagues (155) demonstrated that changes in fetal P02 of only 4–5 mm Hg were adequate to elicit increased ACTH concentrations in the circulation of the fetal lamb. This level of oxygen change is similar to that seen during spontaneous contractures in late gestation sheep, raising the possibility that uterine activity itself may contribute part of the stimulus to increased fetal HPA maturation. Whether chronic stress is a stimulus to birth at term (156) or contributes only to some cases of preterm labor is unclear at the present time.
At 135 days’ gestation, hypoxia (P02 reduction by 8 mm Hg) significantly increased CRH mRNA in parvocellular PVN and POMC mRNA in the pars distalis within 6 h. This response, however, was attenuated by concurrent infusion of cortisol, indicating effective glucocorticoid feedback mechanisms in vivo at this time (157). After 48 h of sustained hypoxemia, levels of POMC in the pars distalis were elevated, but expression in the pars intermedia was decreased (158). This suggests differential regulation of these two zones of the fetal pituitary, consistent with observations that dopamine, likely from the fetal arcuate nucleus, tonically inhibits pituitary POMC synthesis, and this inhibition is exacerbated in the presence of hypoxemia. Infusion of bromocriptine, a dopamine D2 receptor agonist at day 130 of gestation, produced a 50% decrease in pars intermedia POMC mRNA levels, without affecting POMC mRNA in the pars distalis (159). Thus, the fetal D2 receptor system is functional in late pregnancy, but the fetal pars intermedia does not appear to secrete ACTH1–39 in amounts that alter fetal adrenal function.
Activation of fetal HPA function in response to hypoxemia, however, is a critical aspect of the story leading to preterm birth (160, 161). A sustained pulsatile hypoxemic stimulus is adequate to up-regulate HPA gene expression, plasma ACTH, and cortisol concentrations. It is reasonable to predict that sustained hypoxemia in conditions of fetal compromise predisposes to fetal HPA activation and would result in premature birth (162, 163).
B. Activation mechanism by which cortisol changes placental steroid
and PG synthesis (Fig. 4)
Fetal cortisol acts on the sheep placenta to alter the pattern of
steroidogenesis; as a result, progesterone output falls and estrogen
concentrations increase (164, 165, 166, 167). These changes in placental steroid
output are associated with increased expression and activity of
placental P450C17 (168, 169). This is a critical
difference between the sheep and the human, where this enzyme is not
induced in the placenta at term. Ovine placental tissue contains
P450arom activity, and up-regulation of this gene
also occurs in late gestation. For many years the general thesis has
been that placental estrogen production is limited in ovine pregnancy
and occurs in abundance only at term with the induction of placental
P450C17 as a result of glucocorticoid action
(170, 171, 172). The fall in progesterone and later increase in maternal and
fetal estrogen concentrations have been considered as providing the
stimulus to increased PG output by intrauterine tissues, with
consequent increase in myometrial contractility (173, 174, 175, 176, 177, 178, 179, 180).
|
There are other troubling features of the currently accepted model (185). Several groups of investigators have used either immunohistochemical techniques for localization of PGHS-1/-2, or PGHS-2, or in situ hybridization for PGHS-2 mRNA, or measurements of PGHS and/or PGHS-2 activity in ovine placental cells and microsomal preparations (186, 187, 188, 189), to show that PG production by the sheep placenta increases progressively through the last 20–25 days of gestation (190, 191, 192, 193, 194, 195). Placental output of PGs is not confined to the immediate 24–48 h before spontaneous parturition (196, 197). The increase in PGHS expression and activity in the placenta correlates closely with the progressive increase in plasma PGE2 concentrations in the circulation of the chronically catheterized fetal lamb (191, 198). The increase in circulating PGE2 in the fetus bears a striking temporal relationship to the increase in plasma cortisol concentration (198, 199). Louis et al. (200) first reported, more than 25 yr ago, that infusion of PGE2 into the ovine fetus in late gestation stimulated an increase in the plasma cortisol concentration at a time when the fetal adrenal gland was relatively unresponsive to ACTH stimulation. Later studies have shown that the effect of PGE2 infused into the fetus on fetal HPA function could be exerted at any one or all of the hypothalamic, pituitary, or adrenal levels (201, 202). Thus, the progressive increase in output of PGE2 appears to contribute to the drive to fetal HPA function and augments the stimulus supplied by ACTH to the fetal adrenal (201, 203). Indeed, fetal PGE2 infusion will provoke premature delivery of the ovine fetus (204). Placental PGE2 output would not be subjected to negative feedback regulation by cortisol and may contribute to the apparent lack of negative feedback relationship between ACTH and cortisol in the late gestation ovine fetus.
Recent studies have suggested that in addition to PGE2 stimulating output of cortisol by the fetal adrenal gland (205, 206), fetal cortisol, and not estrogen, may affect placental PGHS-2 activity and contribute to the rise in fetal plasma PGE2 concentrations. Evidence in support of this suggestion included the observation that infusion of estrogen into fetal lambs in late pregnancy was without stimulatory effect on levels of placental PGHS-2 mRNA (207), although estrogen infusion into nonpregnant adult sheep did increase PGHS-2 expression in the endometrium (see also below). Studies with human amnion cell cultures and chorion trophoblast cells have suggested that glucocorticoids may up-regulate PGHS-2 gene expression in these tissues. Infusion of cortisol to fetal sheep in late gestation also increased levels of PGHS-2 mRNA and immunoreactive PGHS-2 protein (by Western blotting) in placental trophoblast cells. This effect was independent of changes in estrogen, since a similar stimulation of placental PGHS-2 mRNA levels was observed when cortisol was infused in the absence or presence of the aromatase inhibitor, 4-hydroxyandrostenedione.
Using immunohistochemistry we showed that the
P450C17 enzyme and PGHS-2 both localized to
trophoblast epithelial cells, but not binucleate cells in ovine
placentomes (208). Moreover, the appearance of ir-PGHS-2 clearly
preceded that of P450C17. Collectively,
therefore, these data offer strong reasons to refute the current model
of endocrine events occurring in the placenta of the sheep in late
gestation and suggest that a different sequence likely pertains. This
is summarized in Fig. 4
. We have argued elsewhere that during late
gestation in the fetal sheep, increased output of cortisol from the
fetal adrenal gland progressively up-regulates PGHS-2 gene expression
in placental trophoblast cells (208). The mechanism of this action
remains unresolved. It may depend on trophoblast-specific transcription
factors generated in response to elevations of cortisol, or it could be
a direct action of cortisol since early studies reported a full GRE
consensus sequence at approximately 760 bp upstream from the PGHS-2
transcription start site. We suggest that increased PGHS-2 expression
in the sheep placenta contributes to increased
PGE2 output into the fetal circulation. Fetal
PGE2 drives the fetal HPA axis in a positive
feed-forward fashion (Fig. 4). PGE2, and not
cortisol, is responsible for up-regulation of
P450C17 in placental trophoblast cells. This
occurs in a manner analogous to the effect of
PGE2 on P450C17 induction
in ovine and bovine adrenal tissue. Ovine placental tissue expresses
PGE receptor subtypes (EP1-EP4), but any changes in their expression
during the course of late gestation remain to be determined (see Ref.
208). We have suggested further that increased
P450C17 in the placenta allows the conversion of
C21 
5 steroids directly through to 5 C19 steroids, precursors for
estrogen biosynthesis, as demonstrated by Flint et al. (209)
and Mason and colleagues (210) some years ago. A crucial difference of
the current hypothesis is that this change is superimposed on an
already substantial basal output of estrogen by the sheep placenta
(measured as conjugated estrogens in maternal plasma and urine), and
contributes principally to the terminal increase in maternal estradiol
concentrations. This increase in estrogen is required for expression of
CAP genes in the ovine myometrium and for expression of PGHS-2 in
maternal endometrial tissue, predominantly endometrial epithelium. We
have found that whereas the increase in placental (fetal trophoblast)
expression of PGHS-2 after intrafetal cortisol administration was
unaffected by concurrent infusion of 4-hydroxyandrostenedione, maternal
endometrial up-regulation of PGHS-2 and output of 13–14
dihydro-15-keto PGF2 (PGFM) into the maternal
circulation occurred with cortisol infusion but was blocked by
concurrent administration of the aromatase inhibitor (211). Thus, in
sheep it appears that the fetal placenta and maternal endometrium exist
as two separate sites of PG synthesis in late gestation and that these
are differentially regulated. In fetal placenta, PGHS-2 is increased by
cortisol, independent of changes in estrogen output, whereas in
maternal uterine tissue, up-regulation of PGHS-2 and maternal plasma
PGFM is dependent upon increased estrogen production (Fig. 4).
Current studies are directed at examining this hypothesis further. Using immunohistochemistry and Western blot analysis, it is evident that GR is expressed in ovine placental tissue, predominantly in uninucleate trophoblast cells. Estrogen receptor (ER) mRNA and activity have been demonstrated in maternal endometrium but is apparently lacking in placental trophoblast (212). Hence, it is difficult to envisage how estrogen could provide a stimulus to placental PG production as previously hypothesized. It remains to be shown whether glucocorticoids affect placental PGHS activity directly or indirectly. However, in early studies we have demonstrated that glucocorticoids increase output of PGE2 by ovine placental trophoblast cells maintained in culture, and this effect is abolished by addition of meloxicam, a specific inhibitor of PGHS-2 activity.
C. HPA function in the primate fetus and activation of
parturition
The role of the human and subhuman primate fetus in controlling
gestation length has been, until recently, less clearly defined than
that of the sheep fetus. However, over the past few years it has become
apparent that mechanisms leading to activation of fetal HPA function in
primates bear considerable similarity to processes in sheep, and
that fetal cortisol and fetal adrenal C19 steroids appear to play an
important role. In 1933, Malpas (213) in a study of gestation length in
human pregnancies complicated with anencephaly concluded that
"... . the fetal pituitary and adrenal glands was responsible for
the trigger to the neuromuscular expulsive mechanism that led to the
onset of labor. " Early observations indicated that the
mean length of gestation in anencephaly, after exclusion of cases with
polyhydramnios, was similar to controls, but the proportions of preterm
and postmature births were both higher (see Ref. 12). Similar results
have been obtained after experimental anencephaly in rhesus monkeys
(214). In monkeys, fetal hypophysectomy predisposed to prolongation of
gestation (215), but fetal adrenalectomy was without effect on
gestational length, although five of eight fetuses died in that study
(see Ref. 12). Initial studies indicated that removal of the fetus, but
leaving the placenta in utero (fetectomy) had little effect
on gestation length. However, more recent studies have indicated
clearly that placental retention after fetectomy was significantly
longer (195 days) compared with 164 days in controls (216). Fetectomy
in baboon pregnancy did not affect gestation length, although maternal
estradiol concentrations fell to basal values and progesterone
concentrations were reduced by 20–45% (217, 218, 219). Overall, these
experiments are difficult to interpret. The numbers and observations
are invariably small, no attempt is generally made to sustain uterine
volume and the stretch stimulus to the myometrium, and it is
technically very difficult to operate on the primate fetus without
stimulating uterine contractility.
In intact rhesus monkeys, as in the baboon and human, there is an increase in maternal estrogen concentrations in late gestation that parallels an increase in the concentrations of fetal adrenal C19 steroids, particularly DHEA and DHEA-sulfate (DHAS) (220, 221). Maternal estrogen concentrations increase progressively and then more rapidly in the later phases of human gestation; estriol, derived in substantial part from precursors of fetal adrenal origin, rises rapidly in maternal plasma and urine in late pregnancy at term, and in preterm labor (221). When androstenedione was infused into pregnant rhesus monkeys at about three-quarters of the way through gestation, there was an increase in maternal plasma estrogen concentrations and premature birth (222). This effect was blocked by the coinfusion of the aromatase inhibitor 4-hydroxyandrostenedione, which prevented maternal endocrine changes and changes in fibronectin in the fetal membranes and inhibited the nocturnal increases in uterine myometrial contractility (223). Elevations of maternal systemic estrogen concentrations by infusion increased myometrial activity, but did not produce premature delivery or fetal membrane changes. It was suggested that in the primate, as in the sheep, estrogen is important for the normal processes of parturition. The failure of exogenous estrogen to stimulate sustained uterine contractility, even though locally produced estrogen formed after C19 steroid infusion was effective, led the authors to suggest that the estrogen had to be generated near to its site of paracrine/autocrine action (223).
D. HPA maturation in the primate fetus
There is emerging strong evidence that maturation of HPA function
occurs in the primate fetus in a manner generally analogous to that
discussed above in the sheep fetus. Excellent reviews by Pepe and
Albrecht (221, 224) and by Mesiano and Jaffe (225) have provided
detailed analyses of pituitary-adrenal function in the primate fetus.
In the human, baboon, and monkey fetus the pituitary is necessary for
adrenal maturation and steroidogenesis, at least during the second half
of gestation. Adrenal development is impaired in anencephalic human
fetuses. In the baboon fetus treated in late gestation with
betamethasone, there was suppression of fetal pituitary POMC mRNA and
reductions in fetal adrenal weight, and 3ß-HSD fetal adrenal ACTH
receptor mRNA levels (221). The authors concluded that increased
expression of fetal adrenal ACTH receptor and mRNA species encoding
steroidogenic enzymes depended upon fetal pituitary ACTH stimulation.
In the human fetus, ACTH activity is present in the pituitary by 5 weeks’ gestational age, and CRH- and AVP-like activity is present in the fetal hypothalamus by approximately 12 weeks gestation (226). CRH1–41, in addition to a large molecular weight form of CRH, are contained within the human fetal hypothalamic tissue. CRH and AVP synergize in promoting ACTH release from the human fetal pituitary tissue in early gestation, and the stimulatory effect of CRH and ACTH output was reproduced by 8-bromo-cAMP (see Ref. 12).
Levels of POMC mRNA in anterior pituitary tissue from fetal baboons
increased significantly from mid (day 100) and late (day 165) gestation
(term = day 184) in nontreated animals, and there was a
corresponding increase in pituitary cells expressing ACTH peptide (227, 228). In the baboon it has been suggested that this increase in fetal
pituitary POMC mRNA levels might be associated with increased pituitary
CRH receptor activity, rather than increased expression of CRH peptide
in hypothalamic nuclei. However, administration of estrogen to
midgestation baboons resulted in an increase in levels of POMC mRNA-
and ACTH-positive corticotrophs in pituitary tissue to values that
approached, but remained significantly different from, those at term
(228). Pepe et al. (229) have argued that this increase in
POMC is secondary to an effect of estrogen on placental 11ß-HSD
activity, particularly 11ß-HSD-2. In previous studies, these
investigators have shown increased expression of placenta 11ß-HSD-2
in the baboon during pregnancy and have shown that activity of this
enzyme is increased by treatments that increase estrogen and decreased
with inhibition of estrogen production or action (221, 229). In
midgestation, the relatively lower levels of placenta 11ß-HSD-2 allow
passage of maternal cortisol into the fetal compartment and relative
suppression of fetal HPA activity (221). With increased 11ß-HSD-2
activity at day 160, there would be diminished maternal cortisol
reaching the fetus (230), allowing the fetal HPA axis to escape from
the presumed negative feedback of maternal cortisol. This would allow
increases in POMC gene expression, ACTH output, and fetal adrenal
maturation. These results are compatible with observations that
production of cortisol by the primate fetal adrenal gland is relatively
low for much of gestation (231, 232). The bulk of the gland is occupied
by the fetal zone with relative deficiency of 3ß-HSD, and predominant
formation of C19 5 steroids, particularly DHAS (233, 234, 235). In late
gestation, there is an increase in ACTH receptor mRNA and 3ß-HSD
activity in the definitive zone of the fetal adrenal, and a decrease in
ACTH receptor mRNA and formation of DHAS in the fetal zone (236, 237, 238).
The expression of fetal adrenal enzymes P450C17
and P450SCC remained relatively unchanged during
gestation. Thus, there are subtle differences between fetal adrenal
development in the primate and sheep. In the former, expression of
3ß-HSD appears rate limiting toward adrenal cortisol output whereas
in the ovine species, expression of P450C17
appears to regulate fetal adrenal steroidogenesis.
In primate pregnancy, estrogen production in the placenta depends
extensively on the provision of C19 precursor steroids, predominantly
from the fetal adrenal gland (239, 240). Fetal adrenal DHAS can be
converted to estrone and estradiol in the placenta, and approximately
50% of circulating maternal estrone and estradiol are derived from
placental aromatization of fetal DHAS; the remainder is formed from
maternal adrenal C19 steroids (239, 241). Activation of the
pituitary-adrenal axis of the fetus occurs in late gestation. There is
a progressive increase in the concentration of DHAS in the fetal
circulation, which mirrors an increase in maternal plasma estriol
concentration (maternal estriol is formed in the placenta from the
precursor 16-hydroxy-DHAS that is 90% of fetal origin and formed in
the fetal liver from adrenal DHAS). This pattern of fetal adrenal
activation, reflected in plasma DHAS concentrations, resembles the time
course of increase for plasma cortisol in the fetal sheep. Recent
studies have shown that the fetal adrenal in primates is divided into
the outer adult zone that produces predominantly aldosterone, the fetal
zone that produces DHAS, and the transitional zone, interposed between
the adult and fetal cortex, which produces predominantly cortisol
(225). Thus, the elegant studies of Mesiano and Jaffe (225) and Coulter
and colleagues (242), have shown that P450scc is
expressed throughout the primate fetal adrenal gland.
P450C17 is not expressed in the definitive zone
but is expressed in the transitional and fetal zones.
P450C21 is expressed throughout the gland.
3ß-HSD is not expressed in the fetal adrenal at midgestation but is
expressed in the definitive and transitional zone in late gestation
fetuses. P450C11 is expressed in the transitional
zone in midgestation and throughout the fetal adrenal cortex in late
gestation. ACTH stimulates steroidogenesis in the transitional and
fetal zone; the major products in late pregnancy are cortisol from the
former and DHAS from the latter. Both in vitro and in
vivo studies show dependence on ACTH for fetal adrenal
steroidogenesis. More recent studies, however, have indicated that CRH,
potentially of placental origin (see below), can also stimulate the
fetal zone to produce DHAS (243). In addition, this zone of the fetal
adrenal appears to respond to trophic inputs from the fetal pituitary
other than ACTH. ER-/ß mRNA is also expressed in fetal and
definitive-transitional zones of the baboon fetal adrenal cortex at
mid- and at late gestation (244). The presence of ER in the adrenal
cortical cells provides an additional mechanism by which estrogen
mediates ACTH-dependent functional maturation of the primate fetal
adrenal gland. In addition, previous studies had shown that
estrogens increase availability of LDL-cholesterol as precursor for
adrenal steroidogenesis (245, 246).
The difference in fetal adrenal architecture between the sheep and
primate fetus has been regarded by many as a clear obstacle to
extrapolating from the sheep model of parturition to the primate.
However, it is now apparent that similarities between these species are
greater than the perceived differences (247). In both the sheep and
primate fetus the fetal adrenal produces increased amounts of cortisol
in late gestation (247). It is relatively unprofitable to make detailed
comparison of the minutiae of temporal changes in plasma cortisol
because of differences in binding to circulating CBG, transplacental
transfer from the mother, and tissue levels of 11ß-HSD isozymes in
the fetus that could locally regulate cortisone-to-cortisol
interconversion. In both sheep and primate, the feto-placental unit
also produces increased amounts of estrogen. In the primate, that
estrogen results primarily from placental aromatization of precursors
generated within the fetal (and to a certain extent maternal) adrenal.
There is no induction of placental P450C17 at
term, and the primate placenta does not metabolize C21 steroids through
to estrogen. In the sheep, a similar fetal-placental unit of
estrogen production exists in pregnancy. The major fetal adrenal
precursors are both 5 and 4 C19 steroids produced from the
developing zona fasiculata reticularis. At term, the prepartum rise in
fetal cortisol results directly or indirectly in increased expression
of P450C17 in the ovine placenta, which at that
time becomes capable of metabolizing 5 C21 steroids to estrogen.
Thus, the apparent difference in the pattern of estrogen biosynthesis
between sheep and primate at term, in its simplest term,
reflects the source of C19 precursor steroid. The mechanisms of HPA
activation may vary. However, in the primate, the C19 precursor comes
from the fetal zone of the fetal adrenal gland. In the sheep, that
precursor comes in part from the fetal adrenal, but there are
additional estrogen precursors produced in the placenta under the
influence of cortisol from the fetal adrenal gland. We suggest that
these differences are ones of degree rather than of absolute
distinction.
The role of estriol in the processes leading to the onset of human parturition has remained unresolved over many years. Maternal estriol concentrations reflect fetal hepatic 16-hydroxylation of DHAS produced from the fetal adrenal gland. It might be anticipated that estriol concentrations in the maternal circulation would increase in response to fetal stress and might be predictive of impending preterm delivery. Maternal estriol levels increase exponentially toward normal term. Lachelin and colleagues (248, 249) have shown that maternal plasma and salivary estriol concentrations are elevated further in a subset of patients with diagnosis of preterm labor. Since estriol may affect uterine CAP gene expression (249), it could contribute to the progressive increase in uterine responsiveness in primate pregnancy during the third trimester of gestation, and its measurements may be of predictive value in delineating patients at risk of premature delivery (249, 250).
E. Placental progesterone and human pregnancy: the enigma of the
progesterone block
A fall in the plasma progesterone concentration is the single most
common endocrine event associated with parturition across species (12, 250, 251). Administration of exogenous progesterone at term not only
blocks the expression of CAP genes, but blocks the onset of labor
(252). Even in the human, where there is no evidence of a fall in
maternal plasma or uterine tissue progesterone, administration of the
progesterone receptor (PR) antagonist RU486 leads to increased uterine
activity and induction of labor (253). In human pregnancy, the
luteoplacental shift in progesterone production occurs by 5–6 weeks’
gestation (254). Progesterone is synthesized from pregnenolone by
placental syncytiotrophoblast and by chorionic trophoblasts (Fig. 6 and
Ref. 255). However, the levels of 3ß-HSD mRNA, protein, and activity
do not change in these tissues with labor at term or preterm (256),
although regional changes in 3ß-HSD expression might still occur
(257). For example, expression of 15-hydroxyprostaglandin
dehydrogenase (PGDH; the major PG metabolizing enzyme) in chorion is
regulated by progesterone (see below) and levels correlated with
3ß-HSD in tissue collected adjacent to the placenta, but not in the
cervical region. In this lower segment, it was suggested that the
action of progesterone in maintaining PGDH tonically was overcome near
term by the inhibitory influence of proinflammatory cytokines (see
below). There are reports of an increase in the estrogen-progesterone
(E:P) ratio in amniotic fluid of women during labor; however, these
changes are not impressive (250). We have referred to suggestions that
maternal estriol, which increases during term and preterm labor, might
promote myometrial activation and labor contractions, but this
possibility requires stronger experimental verification (249).
Alternatively, another progesterone-like steroid, possibly a
progesterone metabolite that interacts with the PR, might serve as the
active progestagen in human pregnancy and decline before labor, or
progesterone could be converted to an inactive metabolite that
displaces progesterone from its receptor (258, 259, 260). To date, there are
no clear data to support either of these possibilities. Erb et
al. (261) reported recently that levels of allopregnanolone, the
3,5-reduced metabolite of progesterone that can bind to
-aminobutryic acid-A receptors and inhibits uterine smooth
muscle, did not decrease with labor. The 5ß- metabolite blocks OT
binding to its receptor and inhibits OT-induced contractions in the
human myometrium. However, there is also no evidence that levels of
this metabolite decrease at term.
|
Although recent exciting data have shown that progesterone can bind
directly to the oxytocin receptor (OTR) and inhibit its signaling
(262), the majority of the actions of progesterone are mediated through
a nuclear ligand-inducible transcription factor, the PR. It has been
suggested that a functional withdrawal of progesterone may involve
antagonism of its action at the level of the PR or PR interaction with
transcriptional machinery (8). This might include a decrease in PR
expression, a switch in PR isoforms, a change in expression of receptor
accessory proteins (e.g., heat shock proteins and receptor
coactivators/repressors), or increased expression of endogenous
antagonists of progesterone or PR (such as cortisol, TGFß, or
phospholipids). Three isoforms of the PR have been described: the
full-length PR-B and the truncated isoforms, PR-A and PR-C. In mammals,
PR-B functions predominantly as an activator of progesterone-responsive
genes, while PR-A acts as a modulator or repressor of PR-B function and
of other nuclear receptors including the GR, possibly because it lacks
one of the three activation function domains (AF3) contained within
PR-B (263). Notably, progesterone repression of estrogen-induced gene
expression was effected through PR-B and not through PR-A. The
expression of PR-A and PR-B isoforms is regulated differentially during
development and by hormone treatment. The PR-C isoform (
60 kDa),
which has C-terminal transactivating domains and lacks the first zinc
finger of the DNA binding domain, can dimerize with and modify
(possibly inhibit) transcriptional activity of both PR-A and PR-B.
Analysis of PR expression is complicated by the multiple mRNA and protein species of the receptor. A decrease in PR immunostaining in myometrium at term has been reported but, given the multiple isoforms of PR, these data are difficult to interpret. There is no change in PR mRNA in myometrium or membranes with labor, and no evidence of change in PR-B or A + B mRNA nor in any immunoreactive PR isoforms in samples of lower segment myometrium during labor that might indicate a decrease in progesterone signaling (G. Erb, N. McLusky, and S. J. Lye, unpublished results). There was increased expression of heat shock proteins (HSP)-90 and HSP-56 as well as the steroid receptor coactivators SRC-1 and TIF-2 (G. Erb and S. J. Lye, unpublished results). These coactivators may interact with several steroid receptors, but any interaction with PR should increase rather than decrease its transcriptional capability. There are limited data on ER expression in myometrium with labor. However, in the lower uterine segment at term, ER mRNA, protein, and high-affinity binding all appear to be very low.
There are several candidates for potential endogenous antagonists of
progesterone action. TGFß has been proposed as an endogenous
antiprogestin that reduces progesterone stimulation of genes such as
enkephalinase (264). Others have reported that a phospholipid extract
of human fetal membranes was capable of inhibiting progesterone
binding, but not estrogen binding. Cortisol itself may compete with
progesterone in the placenta or membranes to regulate the gene for CRH
(263). We have found (see below) that while progestagens such as
medroxyprogesterone acetate (MPA) increase PGDH activity in human
placental and chorion trophoblasts, this effect is reversed by
cortisol. At the present time, it is not clear whether these are
separate actions through GR and PR, or whether cortisol and MPA compete
for PR-GR binding. Although four upstream GREs have been identified
within the PGDH promoter, no putative PRE has been identified.
Cytokines [interleukin-1ß (IL-1ß), tumor necrosis factor-
(TNF)] also decrease PGDH activity, but their interaction with
progesterone as putative antiprogestins remains unexplored. In recent
studies, Stevens et al. (265) reported that CRH receptor
type 1 (CRH-R1) was expressed preferentially in myometrium and fetal
membranes of human gestation. Levels of CRH-R1 increased in myometrium
collected from patients in term and preterm labor but, importantly,
levels of CRH-R1 in lower segment myometrium were consistently much
higher than levels of CRH-R1 in the fundal region (265). CRH acts
through CRH-R1 to increase levels of cAMP and promote uterine
relaxation (61). We therefore proposed that the role of CRH-R1 in the
lower uterine segment was to promote relaxation of this region during
labor and to facilitate descent of the fetus (61, 265). These data
indicated that there might be mechanisms by which CRH-R1 expression was
regulated differentially in the fundus and the lower segment during
labor. In independent studies, Sparey et al. (266) reported
that levels of PGHS-1 and PGHS-2 proteins were also expressed at
greater levels in the lower than upper uterine segment. Connexin-43
protein, in contrast, was expressed at much greater levels in the upper
uterine segment. Myometrial GS protein was
uniformly expressed in both lower and upper segments and down-regulated
at the time of parturition. These authors also concluded that
differential expression of these genes might be important to allow
cervical ripening before and dilatation during labor, with orderly
propagation of uterine contractions (266).
Our own data suggest considerable differences in the expression of CAP genes in the human myometrium during labor compared with other species. In contrast to observations in myometrium of rats, sheep, and cows, Teoh et al. (267, 268) did not observe any increase in the expression of CAP genes, including CX-43, OTR, and the PG receptors that are linked to stimulation of contractile pathways (FP, EP1, and EP3 receptor subtypes, including four splice variants of the EP3 receptor) in lower segment myometrium at labor. However, Teoh et al. (267) did observe increased expression of connexin-26, the EP4 receptor and CRH-R1 receptor that might be expected to promote myometrial relaxation after an increased generation of cAMP. It is known that connexin-26 is positively regulated by progesterone.
What is the relevance of these observations to the effect of
progesterone on the myometrium and the apparent lack of withdrawal of
the progesterone block to the myometrium in human pregnancy? We propose
that the biological basis for the onset of labor in animals and in
humans is essentially similar. Both require activation of the
myometrium and the generation of uterotonins to generate labor
contractions. In human fetal membranes and myometrium, however,
regional differences in gene expression allow functional autonomy
during labor. We suggest that this functional autonomy may be critical
for the efficient and effective delivery of the fetus and speculate
that this is a mechanism associated with evolution to bipedal life. We
have suggested that this regionalization is established through the
action of progesterone. Early studies, e.g., those of
Wiqvist and colleagues (269), support this hypothesis. These authors
found that PGF2 had little effect on the
fundal myometrium, but was stimulatory in lower segment specimens taken
before labor. PGE2 induced a biphasic
dose-dependent response. However, PGF2 and
PGE2 always stimulated fundal myometrium
collected during spontaneous labor. PGE2 induced
inhibition in lower segment samples collected at that time while
PGF2 had no effect.
We speculate that during pregnancy, progesterone limits the generation of stimulatory PG in chorion by inducing high expression of PGDH (see below), and it also inhibits the expression of CAP genes within the myometrium, thereby maintaining the muscle in a quiescent state (8). Functional regionalization of both chorion and myometrium at term is engineered by progesterone. In the cervical, but not fundal, region of chorion, there is a local decrease in PGDH (10), increased production of PGE, and later matrix remodeling. In the myometrium, functional withdrawal of progesterone in the fundus induces CAP gene expression and myometrial activation. Enhanced progesterone signaling in the lower uterine segment, however, promotes the expression of genes that induce relaxation, facilitating descent of the fetus (8). The mechanisms inducing functional withdrawal of progesterone in fundal myometrium and cervical chorion need not necessarily be the same (270). Cortisol and/or cytokines may antagonize progesterone- induced PGDH activity in chorion (see below). In myometrium, potential mechanisms include changes in PR isoforms, steroid receptor co-activator/repressors, or other putative antagonists of progesterone action. We speculate that this concept of human labor provides an explanation as to why progesterone levels remain high in this species. Rather than being an impediment to labor onset, we suggest that progesterone is required to induce lower segment relaxation and the safe and efficient delivery of the primate fetus.
Recent exciting studies have pointed to a role for progesterone in
maintaining cervical function during pregnancy, and to metabolism of
progesterone within the cervix as being a critical step in cervical
dilatation and parturition. Mahendroo and colleagues (271, 272) showed
that parturition was delayed in mice lacking steroid 5-reductase
type 1 enzyme. They showed subsequently that basal and stimulated
levels of uterine contractility were similar in these animals and in
wild-type controls. However, cervical distention did not occur in
5-reductase-deficient animals, and cervical compliance was less on
day 20 of gestation than earlier in pregnancy. As expected, relaxin,
which is known to promote cervical ripening, induced delivery in both
wild-type and 5-reductase knockout animals. Subsequent studies
demonstrated that while serum progesterone concentrations declined in
knockout animals in a manner generally similar to that of controls, the
concentration of progesterone in cervical tissue and in whole uterus
remained elevated. As expected, cervical ripening and parturition
occurred after ovariectomy. Thus, these studies point to the role of
progesterone metabolism in facilitating normal cervical dilatation that
must accompany uterine contractility to allow birth (273). In the
uterus of pregnant mice, progesterone can be metabolized at term
through either 5-reductase or 20 -HSD pathways. In the cervix,
however, there is limited 20-HSD activity, and normally
5-reductase provides the pathway for progesterone metabolism,
progesterone withdrawal, and cervical ripening and dilatation (272).
Further studies of other genes associated with cervical ripening are
clearly warranted in this fascinating model, as are measurements of
5-reductase activity in human cervix from patients at term and
preterm labor.
|
V. Myometrial Stimulation: Phase 2 of Parturition |
|---|
|
TopAbstractI. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
A. Stimulation: role of OT
OT is a nonapeptide synthesized by hypothalamic magnocellular
neurons located in the supraoptic and paraventricular nuclei
(275, 276, 277). Hypothalamic OT is released into the circulation from the
posterior pituitary. Its classical effects include promoting myometrial
contractility during late pregnancy and parturition and stimulating
milk release from the mammary gland in lactation (275, 278, 279). The
dilemma surrounding the role of OT in the process of labor arose when
it was unclear whether levels of OT in the maternal circulation
actually increased before the onset of labor (279, 280). The recent
report that mice bearing a null mutation in the OT gene have normal
pregnancies and labors may reflect a compensatory effect of AVP (281, 282). Studies showing the relative ineffectiveness of OTR antagonists
in preventing preterm labor, however, suggest that while this hormone
contributes to labor, it may not be an essential element (283).
One aspect of the solution to the apparent discrepancy between circulating OT levels and parturition was the dramatic increase in myometrial sensitivity to OT before and during labor, associated with a several-fold increase in myometrial OTR gene expression, which coincides with peak uterine responsiveness (276, 284, 285, 286). Thus, changes in circulating OT levels would not be necessary for the peptide to have a physiological role in labor (280). A parallel conclusion is drawn from the 24-h pattern of OT secretion, and myometrial sensitivity (287). Recent studies also suggest that OT may act as a local mediator of parturition. OT gene expression has been demonstrated in the human and rat uterus and fetal membranes (288, 289, 290). In the rat, fetal membranes, placenta, and uterus synthesize OT mRNA transcripts with extended poly-A tails (289). Levels of OT mRNA in rat fetal membranes declined from gestational day 14 to term, but uterine OT transcripts increased during gestation 150-fold and exceeded levels of OT mRNA in the hypothalamus at term (289). Human fetal membranes, amnion, chorion, and decidua synthesize OT mRNA, and levels of OT mRNA transcripts increased in these tissues at the time of parturition (290). In vitro studies with rat and human chorio-decidual tissue have indicated that estrogen, generated locally, could up-regulate OT gene expression (291, 292, 293), consistent also with the presence of an ERE in the OT promoter region (293). Other, fascinating studies have indicated that OT may promote uterine activity by antagonizing the relaxant effect of CRH through receptors coupled to adenylate cyclase (see below). The general consensus is that OT appears to have a role to play in the stimulus to uterine contractility at term and in uterine involution (294). Whether that role is indispensable remains in dispute.
B. Stimulation: role of PGs
There is a substantial body of evidence to support a role for PGs
in the labor process, at term and preterm (207, 295). PGs contribute to
the transition from phase 1 to phase 2 rather than initiating the labor
process. Mice carrying null mutations for genes encoding the
PGF2 receptor (296), cytosolic phospholipase
A2, and prostaglandin synthase type 1 (PGHS-1) (297) have delayed labor
onset although neonatal viability is diminished. Mice lacking the
PGHS-2 gene (298) have not been studied in relation to gestation length
and pregnancy outcome because fertility is impaired, and ovulation and
implantation are blocked. Lack of PGF2 (FP)
receptor prevents effective luteolysis at the end of gestation, so
plasma progesterone concentrations are maintained. In these animals OTR
expression in the uterus is suppressed, presumably in response to the
elevation in progesterone, since ovariectomy allowed OTR up-regulation
and delivery. The extent to which information from these murine models
is applicable to human gestation may be questioned, since the primary
site of PG action is at the level of the corpus luteum, which is not
required for pregnancy maintenance in women after the first 5–6 weeks
of pregnancy. Perhaps the best indicator for a role of PG in
parturition in primates as well as sheep and other species is the
measurement of increased PG output before the appearance of labor-like
myometrial contractions (299, 300, 301) and the effectiveness with which
drugs that block PG synthesis suppress myometrial contractility and
prolong gestation length.
PGs are formed from membrane phospholipids through the initial activity of phospholipase A2 or C isozymes forming unesterified arachidonic acid (302, 303, 304). PLA2 isozymes, localized by immunostaining to fetal membranes and myometrium (305), may include the larger molecular mass (85–110 kDa) cytosolic form (cPLA2), as well as secretory types I, II, and III, extracellular 14-kDa forms. Activation of secretory PLA2 (sPLA2) requires millimolar concentrations of calcium, whereas cPLA2 is activated at micromolar calcium concentrations (see Ref. 8).
Cytosolic PLA2 translocates to the cell membrane in response to agonist stimulation and liberates arachidonic acid from the sn-2 position of phospholipid (306). Activity of cPLA2 is reportedly greater in amnion from patients not in labor at term or preterm than from patients in labor, explained as depletion of cPLA2 at this time (304). Previous studies had shown that cPLA2 expression was up-regulated in WISH cells, a transformed amnion epithelial cell line, in response to cytokine stimulation, and that this occurs in parallel with increased expression of PGHS-2 by these cells (307, 308). The general consensus, however, is that in human pregnancy, expression of PLA2 increases gradually in fetal membranes during gestation but does not increase appreciably at the time of labor (309).
Arachidonic acid is further metabolized to the intermediate PGH2 by PGHS enzymes, which have both cyclooxygenase and peroxidase activities (310, 311). There are two forms of PGHS; both are heme proteins composed of two approximately 70-kDa subunits. The constitutive form (PGHS-1) and the inducible form (PGHS-2) are distinct gene products although they have considerable sequence homology, and their cDNAs are 60–65% homologous (312). PGHS-1 has similar properties to other housekeeping genes. PGHS-2 is characteristically up-regulated by growth factors and cytokines. The activity of PGHS-1 and PGHS-2 is inhibited by a wide spectrum of nonsteroidal antiinflammatory drugs. These differ in their Ki values for the two PGHS isoforms, suggesting the potential to develop specific inhibitors of either isoform for therapeutic management (313, 314, 315).
Arachidonic acid may also be metabolized through different lipoxygenase pathways including 5-lipoxygenase, platelet-type-12-lipoxygenase, leukocyte-type-12-lipoxygenase, and 15-lipoxygenase (316). Arachidonic acid metabolism through 5-lipoxygenase forms 5 H(P)ETE, which can be converted to leukotriene A4 (LTA4), which is subsequently hydrolyzed to LTB4 or LTC4. 12-Lipoxygenase or 15-lipoxygenase activity results in the formation of 12-H(P)ETE and 15H(P)ETE. There are some suggestions that these products can weakly stimulate contractility of smooth muscle. It has also been suggested that arachidonic acid metabolism in human fetal membranes during pregnancy is directed preferentially toward lipoxygenase products, but there is a progressive switch toward the more potent PGHS (also cyclooxygenase, COX) activity at term (317). Primary PGs are formed from PGH2 through the activity of specific isomerases and synthases. There is unfortunately very little information concerning the expression, localization, and change in activity of these enzymes in intrauterine tissues at term or preterm labor, and this will be an obvious area of further investigation.
The major pathway in the metabolism of PGE2 and
PGF2 involves the action of a type 1
NAD+- dependent PGDH that catalyzes oxidation of
15-hydroxy groups resulting in formation of 15-keto and 13,14
dihydro-15-keto metabolites with reduced biological activity (318, 319). We have reported that PGDH expression and activity are decreased
in choriodecidual tissue of women at spontaneous and preterm labor (see
below), raising the possibility that failure to inactivate PGs produced
within intrauterine tissues during pregnancy may be one cause of
preterm labor (320).
The action of PGs is exerted through specific receptors including the
four main subtypes for PGE2, EP1, EP2, EP3, and
EP4, and FP for PGF2 (60, 321). EP1 and EP3
receptors mediate contractions of smooth muscle through intracellular
signaling pathways that elevate free calcium and decrease intracellular
cAMP (27). EP2 and EP4 receptors are coupled through adenylate cyclase
and increase cAMP formation, leading to relaxation of smooth muscle.
Consistent with this, various groups have reported that EP2 expression
in myometrium is higher preterm than at term. In the rat, parturition
is associated with down-regulation of EP receptor subtypes and with
up-regulation of myometrial FP receptors, effecting a switch from
inhibition to stimulation.
1. PG synthesis.
Regulation of PGHS-2 and PGHS-1 genes are
clearly multifactorial (322, 323, 324). There are two nuclear factor
(NF)-
B binding elements within the proximal promoter region
of PGHS-2 (325, 326). p50 And p65, key members of the NF-B
Rel family of proteins are present in trophoblasts and
likely serve as mediators of cytokine-induced up-regulation of PGHS-2
expression (327). The PGHS-2 promoter also includes response elements
resembling NF-IL6, GRE, CRE, and AP2 sites (323, 325). Levels of PGHS-2
are increased up to 80-fold in response to various cytokines and growth
factors, whereas levels of PGHS-1 are usually increased only 2- to
3-fold in response to these stimulators (328, 329). Studies in several
species, including the human, have indicated that the PGHS-2 isoform is
the principal form of the enzyme involved in the increased PG
production seen at the time of parturition. Effects of CRH in
up-regulating PG output, at least within fetal membranes (see below),
is likely mediated through proximal CRE sequences (326). Although
glucocorticoids inhibit PGHS-2 expression in WISH cells and in most
other cell types, apparently by interference with the NF-B signaling
system (330), they stimulate PGHS expression and activity in
trophoblast-derived cells including amnion, and chorionic trophoblast
(58, 331, 332, 333, 334). Kniss (327) reported a similar effect of dexamethasone
in stimulating PGHS-2 mRNA expression in human breast adenocarcinoma
cells. The stimulatory effect of glucocorticoids on PGHS gene
expression in human fetal membranes is central to our current
hypotheses of human parturition and will be discussed in more detail
below.
In human pregnancy, the PG synthesizing and metabolizing enzymes are
compartmentalized discretely between the amnion and chorion, decidua,
and myometrium (Fig. 5; Refs. 335, 336). PGHS activity predominates in amnion, PGE2
is the principal PG formed (337), and there is an increase in PG
synthesis and levels of PGHS-2, but not PGHS-1 mRNA at preterm and term
labor (338, 339, 340, 341, 342, 343). Immunohistochemical and in situ
hybridization studies have localized the PGHS-2 enzyme and mRNA to the
amnion epithelium (344, 345, 346), the subepithelial cells in the mesenchyme
and in the chorion laeve trophoblasts with lower expression found in
decidua (347, 348, 349). Decidua has been reported to produce increased
amounts of PGs at the time of labor, but this is not a consistent
observation (348). Human decidua is made up of decidualized stromal
cells, bone marrow-derived macrophages, and other cell types including
trophoblasts that interface with chorion (350). Variability in cell
populations used for in vitro studies may contribute to the
variability of responses that have been obtained. In chorion,
interposed between amnion and decidua, PGDH activity predominates,
although PGHS is also expressed (347, 351). Output of PGs and PGHS
activity is greater in chorion from patients at spontaneous labor than
at elective term cesarean section; in preterm labor chorion both PGHS-1
and PGHS-2 mRNA levels are increased (352, 353).
|
It remains crucial to understand regulation of PGHS-1 and PGHS-2
expression in human fetal membranes and to delineate the major site of
PG production at term and preterm labor (Fig. 5). These may not
necessarily be the same. For example, instances of preterm labor may be
associated with elevated PG production in amnion or chorion, whereas
term labor may require increased PGHS-2 expression in decidua and
myometrium (344). Given that PGs act generally as paracrine or
autocrine regulators, it will be exceedingly difficult to obtain
in vivo evidence for altered PG production specifically at
these sites. Amniotic fluid concentrations of PGs increase at labor,
and the initial changes precede the onset of myometrial contractility.
Levels of PGF2 in amniotic fluid presumably
reflect, in part, production from decidua, since
PGE2 and not PGF2 is the
major eicosanoid formed from amnion and chorion (Figs. 5 and 6). However, these measurements probably
provide no more than a crude estimate of the pattern of PG change at a
local cellular level and give no information concerning receptor
subtypes and distribution (358).
Primary cultures of mixed and purified cells from human amnion or chorion have been used extensively as models to study the regulation of PG formation in response to cytokines, growth factors, CRH, and lipopolysaccharides. In addition, the amnion-derived epithelial cell line (WISH cells) has also been used extensively (359, 360, 361). A crucial reservation with all of these in vitro studies is that, in general, single compounds have been studied in isolation of the in vivo environment; the extent to which results can be extrapolated from in vitro to in vivo will remain, unfortunately, a matter of conjecture.
Many cytokines have been shown to act on amnion, chorion leave, and
decidua to increase PG output (360, 362, 363, 364). IL-1ß stimulates PG
output by cultured amnion, chorion leave, and decidua (195, 317, 365)
while IL-6 stimulates PG output by decidua and amnion (366, 367). IL-8
did not alter PG production by chorion or decidua, but augmented the
stimulatory action of other cytokines (368). The effect of IL-1ß is
certainly associated with increased expression of PLA-2 and PGHS-2
(329). The action of IL-1ß can be reduced by the naturally occurring
receptor antagonist, which has been shown to prevent IL-1ß-induced
labor in mice (369). IL-1ß stimulation of PGHS in amnion and chorion
may be mediated through the NF-B system (370, 371, 372). In WISH cells
stimulated with interleukin-1ß, I-B was degraded by more than
90% within 15 min of stimulation, and this was associated temporally
with nuclear translocation and binding of NF-B (373). PGHS-2 mRNA
was increased within 30 min and reached steady state by 4 h.
PGHS-2 protein then increased more than 80-fold, and this was
associated with a corresponding time-dependent increase in PG
production. Inhibition of I-B degradation by calpain-I inhibition
blocked NF-B translocation, and increases in PGHS-2 mRNA and
protein, and PG synthesis (373). Wang and Tai (374) provided
similar information and showed that in WISH cells, dexamethasone
blocked IL-1ß-mediated stimulation of PGE2
output consistent with the general model of mutual transcriptional
antagonism from GR/NF-B interaction (330).
Human amnion cells can be maintained as mixed populations in culture or can be separated into primary epithelial cells and cells of the subepithelial mesenchymal layer (375). We have reported recently that the output of PG by mesenchymal cells exceeds that of epithelial cells in the basal state. Epithelial cell production of PGs was stimulated by glucocorticoids, whereas there was no significant change in the already elevated output of PGs from mesenchymal cells (375). Previously, in mixed cultures, glucocorticoids and IL-1ß were shown to increase PGHS-2 mRNA, protein, and PGE2 output predominantly from the subepithelial mesenchymal cells (331, 376). It remains possible that this apparent difference can be explained by epithelial-mesenchymal cell interaction, and current studies are directed at resolving this issue.
The effect of glucocorticoids on primary cultures of amnion cells and
on chorion trophoblast cells is surprising (377) and striking (323, 378, 379). Although dexamethasone inhibited PGE2
output by freshly dispersed amnion cells, it stimulated
PGE2 output by amnion cells after 4–5 days in
culture (376). The effect was dose dependent and associated with
increased expression of PGHS-2 mRNA and protein. The activity of
glucocorticoids is also receptor mediated and can be inhibited by
addition of GR antagonist (380). In previous studies, we had localized
GR to amnion epithelial cells, subepithelial fibroblasts, and chorion
laeve trophoblasts in human pregnancy (381). GR exists as both -form
and ß-form (330). GR is retained in the cytoplasm in an inactive
state by its association with the regulatory heat shock proteins such
as HSP-56 and HSP-90. GRß, formed from alternate splicing of the same
mRNA transcript as GR, is localized in the cell nucleus independent
of binding to ligand. It appears that GRß functions as a dominant
negative regulator of GR transactivation. Thus, earlier studies of
GR localization to cell types within human fetal membranes require
repeating with specific identification of GR and GRß forms.
Peptides such as CRH could be released from amnion epithelial cells to act in a local paracrine manner and up-regulate PGHS-2 expression in mesenchymal cells (see Ref. 207). Full thickness fetal membranes treated in culture with CRH were stimulated to increase output of PGE2 and increased levels of PGHS-2 mRNA within 4 h in culture. Thus, the stimulatory effect of glucocorticoids on PG production by amnion, known to involve an intermediary protein synthetic step, could be the result of synergistic epithelial-mesenchymal interaction, in addition to, or instead of, any direct effect on amnion cell types. Similar interactions may contribute to the response to cytokines such as IL-1ß in vitro (382). Interestingly, recent studies have shown that in amnion explants, in contrast to chorion and decidua, the antiinflammatory cytokine IL-10 stimulates rather than inhibits PG production, and the normally antiinflammatory cytokine IL-4 stimulates PGE2 output in amnion cultures (329). The authors have suggested that amnion may therefore be refractory to inhibitory cytokines as part of an evolutionary mechanism designed to expedite the parturition processes.
Over the past 10 yr, in vitro studies have generated an impressive list of substances capable of increasing PG output by human fetal membranes in culture (383, 384, 385, 386). Clearly, availability of free calcium is a critical requirement. Epidermal growth factor (EGF), platelet activating factor (PAF), and agents that activate protein kinase C stimulate PG output (387, 388). Importantly, ß-sympathomimetic drugs and agents that increase intracellular cAMP levels also increased PG output by cultured chorion and decidual cells (389). Catecholamines are present in increasing concentrations in human amniotic fluid in late gestation (390), and both amnion and decidua express components of the adenylate cyclase system, which undergoes stimulation with ß-agonists such as isoproteronol (391). Effects of these activators of adenylate cyclase can be mimicked by (Bu)2cAMP or phosphodiesterase inhibitors such as methylxanthine (389). Studies such as these may help explain the disappointing lack of efficacy of ß2-sympathomimetic drugs in sustaining uterine quiescence when used in the treatment of preterm labor (392). Although these compounds are effective in the short term by elevating cAMP and decreasing activity of MLCK, in the longer term elevations of cAMP may up-regulate PGHS-2 through a proximal CRE, resulting in increased output of stimulatory PGs, uterotonins whose action the administration of ß2-mimetic was intended to antagonize.
2. PG metabolism. The major metabolizing enzyme for PGs (393),
PGDH, is exquisitely localized in fetal membranes to trophoblast cells
of chorion (Fig. 5). Thus, it could act as a metabolic barrier to the
passage of unmetabolized PGs, generated in amnion or chorion, and
prevent their reaching the underlying decidua or myometrium in a
biologically active form (354, 394, 395). Some years ago, we identified
a group of patients presenting in idiopathic preterm labor with
deficiency of PGDH in chorion trophoblast cells (396). There was a
further reduction of ir-PGDH, PGDH mRNA, and PGDH activity in chorion
trophoblast cells, but not placental trophoblast, in patients in
preterm labor with an underlying infective process (397). Thus, with
preterm labor in the presence of an inflammatory response, loss of
chorion trophoblast cells leads to loss of PGDH activity. PGs
generated, for example in response to elevations of cytokines, will not
be metabolized and will be available to stimulate underlying
myometrium.
In idiopathic preterm delivery, in the absence of infection, it is
clear that PGDH activity is specifically regulated in chorion
trophoblast (Fig. 7). During in
vitro studies with chorion trophoblast cells maintained in
culture, we found that the glucocorticoids, cortisol and dexamethasone,
inhibited PGDH activity and decreased levels of PGDH mRNA (398).
Cortisone was as effective as cortisol, since chorion trophoblasts
contain 11ß-HSD Type 1 (11ß-HSD-1) capable of reducing cortisone to
biologically active cortisol (399). This activity could be inhibited by
carbenoxolone, an active ingredient of licorice. Chorion trophoblast
cells also expressed 3ß-HSD and converted pregnenolone to
progesterone (400, 401). Inhibition of 3ß-HSD activity with
trilostane led to decreased PGDH activity and reduced levels of PGDH
mRNA in the cells. These could be restored by concurrent addition of
progesterone, or of the synthetic progestagens, MPA or R5020 (398).
Effects of these compounds, in turn, were antagonized by onapristone
and RU486, inhibitors of progesterone action (398, 402). Furthermore,
inhibition of PGDH mRNA and activity by cortisol could be reversed by
addition of progesterone (320).
|
(403). Previously, Karalis and Majzoub
(404) provided evidence that similar interaction between progesterone
and cortisol for binding to GR explains the interactive effect of these
compounds on the output of CRH by placenta trophoblast cells.
In recent studies we found that CRH also decreased PGDH activity in
chorion trophoblast cells in a dose-dependent fashion (F. Patel and
J. R. G. Challis, unpublished observations). We believe this
activity is mediated through cAMP generation, since CRH binds to CRH-R1
species in fetal membranes where it may increase cAMP, and cAMP
decreases PGDH activity (405), presumably acting through a consensus
CRE in its promoter region. Thus a pattern is emerging that several
agents which up-regulate PGHS-2 in human fetal membranes (CRH,
cortisol, IL-1ß, TNF) down-regulate PGDH in chorion (Fig. 8). Effects of cortisol in the membranes
may be enhanced by local conversion of cortisone to cortisol, through
the reductase activity of chorionic 11ß-HSD-1 (406). The activity of
this enzyme is increased by PGE2 and
PGF2 in a dose-dependent fashion that is
associated with, and dependent upon, a transient increase in
intracellular Ca2+ (N. Alfaidy and J.
R. G. Challis, unpublished results). Therefore, a further
feed-forward paracrine/autocrine loop exists in which increased output
of PG should stimulate 11ß-HSD-1, resulting in increased production
of cortisol, which leads to further increases in PGHS-2 and decreases
in PGDH (Fig. 8).
|
increase PG synthesis, these cytokines
decrease PGDH activity and PGDH gene expression (409, 410).
Importantly, IL-10, the antiinflammatory cytokine that attenuates
IL-1ß-induced up-regulation of PGHS, also reverses IL-1ß
down-regulation of PGDH (409). The importance of this observation is
that PGs generated within amnion and chorion in the lower segment may
escape metabolism in chorion specifically in that region at the time of
labor to reach the cervix and effect effacement and dilatation.
3. PGs and infection. Approximately 30–40% of preterm labors
are associated with an underlying infective process. Romero, Mitchell,
and collaborators (411, 412, 413) have demonstrated elegantly the role for
infection in preterm labor. Bacterial organisms themselves secrete
phospholipases, resulting in increased release of arachidonic acid from
intrauterine tissues and increased PG production. Alternatively,
bacterial endotoxin, such as lipopolysaccharide, acts on amniotic or
membrane macrophages, causing either PG release or further release of
cytokines (414, 415, 416, 417). Cytokines in turn elevate PG production within
amnion, chorion, and decidua as discussed previously (418).
Administration of cytokines or bacterial endotoxins to pregnant mice
provokes premature delivery and allows examination of the precise
temporal sequence of events in infection-driven preterm labor (419, 420). A number of cytokines including IL-1ß, TNF, IL-6, and IL-8
(neutrophil-activating protein-1) are increased in amniotic fluid of
patients undergoing preterm labor associated with infection (421, 422, 423, 424).
Cytokines are produced not only by macrophages, but are synthesized and
secreted by human fetal membranes in decidua, and these tissues may be
the sources of the cytokines found in amniotic fluid. IL-1ß, IL-6,
and IL-8 mRNA were expressed in amnion, chorion leave, and decidua,
particularly in tissues obtained after labor. In addition, cultured
decidual and chorion cells produce IL-6 and IL-8 when stimulated with
IL-1ß and TNF, and amnion produces IL-8 in response to IL-1ß
(425). Thus, these studies have led to the suggestion that there is a
complex cytokine network at the chorio-decidual interface, as has been
proposed to exist in other tissues (269). It is also possible that
cytokines cause release of other uterotonins, including OT and CRH in
decidua (426, 427), myometrium, and/or placenta. These compounds may
affect the myometrium directly or indirectly. Lipopolysaccharide also
inhibits replication of amnion cells, and it has been suggested that
this might be a mechanism by which lipopolysaccharide
contributes to premature ruptured membranes.
The paradigm of infection-driven preterm labor has been proposed as a means of understanding regulation of PG production in labor at term (348). However, preterm labor in the absence of infection can occur without demonstrable changes in amniotic fluid PGE concentrations and apparently without enhanced PG biosynthetic activity in fetal membranes. It has been argued that changes in PG and cytokine concentrations in the amniotic fluid of women in preterm labor with infection are not reproducible, and that these compounds accumulate there as a result of preterm labor, rather than as a cause (428). It has also been argued that invasion of the amniotic sac by microorganisms occurs when labor has been initiated, when tissues of the forebag are exposed. Furthermore, since parturition is an inflammatory process, the presence of mediators of inflammation in amniotic fluid could be a natural event of parturition without arguing for a role of infection as a cause of preterm labor. The body of evidence currently available has tended to counter this latter view. However, as in all human studies of this type, it is extremely difficult to delineate precisely the cause-and-effect sequence of relationships. Furthermore, a low-grade inflammatory response, where accumulation of cytokines occurs without an infective process, may be present normally at term and contribute to the stimulus of labor or remain as a parallel, but unrelated, event.
C. Stimulation: role of CRH
Over the past 10 yr there has been considerable interest in the
possible role that CRH, produced from intrauterine tissues, plays in
the regulation of human pregnancy and parturition (429, 430). Pro-CRH
mRNA is present in placental tissue (431) and decidua in increasing
amounts during pregnancy. These levels correlate with increased
concentrations of ir-CRH peptides in the placenta and with the
exponential increase in CRH1–41 concentrations in maternal
peripheral plasma (432, 433, 434, 435). CRH also increases in cord plasma,
although the concentrations are generally lower than those in the
maternal compartment (432, 436, 437). Several groups of investigators
have reported that maternal plasma CRH concentrations are elevated
significantly in the plasma of patients presenting in preterm labor
(433, 438, 439, 440) and may be used to discriminate patients presenting in
preterm labor who will deliver within 24–48 h from those patients with
a similar diagnosis, but in whom labor is not imminent (441).
The biological activity of CRH in maternal plasma is attenuated by the
presence of a circulating CRH binding protein (CRH-BP), produced in the
liver and placenta (429, 442). CRH-BP blocks the ability of circulating
CRH to promote ACTH release from pituitary corticotrophs, and it
inhibits the stimulatory effect of CRH on uterine PG production.
Concentrations of CRH-BP decrease during the last 5–6 weeks of normal
pregnancy and before preterm labor, coincident with the increase in
maternal CRH concentrations (443), and apparently in response to
increased CRH secretion. In the placenta, CRH is produced by
syncytiotrophoblast and intermediate trophoblasts (444), and
immunoreactive CRH localizes to these cell layers (429, 445). In
culture, CRH output from placental and chorion trophoblast cells is
inhibited by nitric oxide and progesterone and increased by
catecholamines, OT, cytokines, and glucocorticoids (Fig. 9; Refs. 427, 444). Majzoub and
colleagues (446, 447) demonstrated that dexamethasone increases levels
of CRH mRNA in placental trophoblast cells maintained in culture in a
time- and dose-dependent fashion, although later suggested that this
"apparent" stimulation resulted in fact from reversal of
progesterone-induced inhibition of CRH expression (263, 404).
Glucocorticoids compete with and displace progesterone from GR
binding, and diminished inhibition is measured as an apparent increase
in secretion of CRH.
|
Based upon these results, and the demonstration of activation of fetal
HPA function in response to hypoxemia in animal fetuses, we proposed
that the human fetus would also respond to an adverse intrauterine
environment such as acute hypoxemia with activation of the fetal HPA
axis (10). With time, increased pituitary drive to the adrenal
increases steroidogenic enzyme potential and cortisol output. Fetal
cortisol, then acting through placental and/or membrane GR,
up-regulates placental CRH gene expression, leading to the increased
CRH concentrations in the plasma of patients presenting in preterm
labor. Accordingly, cord CRH concentrations are elevated in the
presence of intrauterine growth restriction (IUGR), or decreased values
of cord PO2 (440, 452). CRH is a vasodilator in
the placental vascular bed and reverses the vasoconstrictor influence
of PGF2 (453). In the placenta, the
vasodilator action of CRH is associated with up-regulation of the
NO-cyclic GMP pathway. Hence, elevations of CRH within the placenta
should signal increased blood flow and correction of a hypoxemic insult
to the fetus. However, if the hypoxemia persists, placental CRH output
presumably remains elevated (Fig. 10).
CRH, secreted into the fetal circulation, drives further pituitary ACTH
secretion and also drives DHAS output from the fetal zone of the fetal
adrenal gland (243); hence, maternal estrogen output should rise as a
secondary response to fetal distress. Increased estrogen leads to
uterine activation. CRH contributes to increased expression of PGHS
(451) by up-regulating adenylate cyclase activity in placental and
membrane cells (61). It will be recalled that the PGHS promoter
contains a CRE. Thus, we speculate that activation of a feed-forward
loop in response to a hostile intrauterine environment is a mechanism
by which a compromised fetus may signal preterm labor and induce
premature delivery (Fig. 10). In addition, maternal stress with
elevations of maternal glucocorticoid concentrations may also
contribute to elevations of placental CRH output and preterm birth.
Hobel and colleagues (454) reported increases in maternal CRH
concentrations in women with elevated scores for perceived stress and
anxiety. These values predicted preterm labor, even as early as 20–24
weeks of gestation.
|
A further reservation is related to CRH receptor specificity. CRH
exerts its effects through activating specific G protein-coupled
receptors, which exist in two subtypes: CRH-R1 and CRH-R2. These arise
from different genes with multiple splice variants (455). The two
receptors share approximately 70% homology at the amino acid level.
CRH-R1 exists in at least three variant forms (R1, R1ß, and R1C).
Recently, an additional form, CRH-R1D, has been isolated, which is
identical to CRH-R1 except that it contains an exon deletion
resulting in loss of 14 amino acids in the seventh transmembrane domain
(456). CRH-R2 exists in at least three splice variant forms (R2,
R2ß, and R2). CRH-R1 predominates in human myometrium (455, 457).
CRH-R2 is expressed in fetal membranes, but at lower levels than
CRH-R1. Parenthetically, this pattern is reversed in rats in which
CRH-R2 predominates in myometrium (Y. Stevens and J. R. G.
Challis, unpublished observations). CRH-R2 has higher specificity for
urocortin than CRH, raising the possibility that in rodent gestation,
placental output of urocortin rather than CRH, may determine activity
of this pathway.
Because CRH-R1 is linked to the adenylate cyclase system through
GS regulatory proteins, it is not surprising
that CRH stimulates cAMP output by human myometrial cells maintained
in vitro (61). Herein lies the paradox. CRH-induced
increases in cAMP should inhibit myometrial activity, through
mechanisms described above, yet elevations in maternal peripheral
plasma CRH concentration are suggested to predict women at risk of
increased uterine activity and preterm labor (61). This may
be resolved if CRH action on myometrium is independent of effects on PG
synthesis in other tissues (458). Affinity of CRH binding in myometrium
increases with pregnancy, and then decreases in late gestation (459).
Hence, we (8) and others (61) have speculated that during gestation CRH
acts as a myometrial relaxant, rather than as a uterotonin. At term, OT
up-regulates protein kinase C, which phosphorylates CRH receptor
protein resulting in its desensitization and loss of inhibitory
influence (61, 460). Stevens et al. (265) showed that levels
of mRNA for CRH-R1 heptohelical glycoprotein increase in lower segment
myometrium from patients in labor whether at term or preterm. Hence,
CRH may contribute to regionalization of uterine activity responses at
this time, producing inhibition of activity, or relaxation in the lower
segment, but stimulation of activity through up-regulation of PG
synthesis in the fundal region of the uterus.
In our view, the putative role of CRH in pregnancy maintenance and
parturition remains unclear. The concept of placental CRH as "a
placental clock controlling the length of human pregnancy" implied a
stimulatory effect on the myometrium (461, 462), which is difficult to
reconcile with the known biochemical effects of CRH (61). Certainly,
CRH augments OT- and PGF2-induced
contractility of myometrial strips in vitro (289, 463).
However, it decreases output of PGI2 by
myometrial cells and has no direct stimulatory action on its own.
Perhaps increased levels of CRH are required to sustain relaxation,
rather than stimulation, of the uterus through late gestation. However,
lowered concentrations of CRH in maternal plasma are associated with
postterm delivery in which, presumably, relative myometrial quiescence
has been maintained. Resolution of this interesting dilemma in which a
single ligand may have different actions depending upon differential
expression of its receptor subtypes and coupling through second
messenger systems is required as a scientific basis to understanding
CRH action in pregnancy (61).
|
VI. Application to Clinical Preterm Labor |
|---|
|
TopAbstractI. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
Both these approaches, however, act on agents of phase 2 parturition, in which uterine activation has already taken place. Inhibition of uterotonin action or secretion does not necessarily affect myometrial activation, although recent studies in sheep treated with nimesulide, a PGHS-2 inhibitor, have shown reversal of some CAP gene expression. Ideally, a future strategy for preterm labor diagnosis and management should address uterine activation. Those studies will require careful animal studies before the introduction of new drugs into clinical practice. A satisfactory outcome may be to delay rather than actually to prevent preterm birth, providing that there is improvement in mortality and morbidity of the newborn.
We remain concerned about the capricious use of glucocorticoids in preterm labor patients (467). There is no question of the beneficial effect of these compounds in promoting pulmonary maturation in infants of women who give birth prematurely within an appropriate time for treatment. However, a central thesis of this review is that glucocorticoids provide a stimulus to the labor process and that evidence is accumulating to suggest that the model derived from animal experiments may have substantial applicability to the human. We recognize from animal studies that repeated administration of glucocorticoids to pregnant animals produces, in a dose-dependent fashion, inhibition of fetal growth (468). Prenatal corticosteroids alter postnatal HPA function and the setting of negative feedback. Prenatal corticosteroids, in animals, may result in the development of hypertension postnatally, and in a pattern of pancreatic response to a glucose load that resembles insulin resistance (469). Prenatal and postnatal administration of corticosteroids affect levels of type 1 and type 2 GRs in critical brain regions, particularly the hippocampus, associated with memory and, in later life, with memory loss and neurodegenerative disease. Future research into the control of preterm labor, and to the tocolytic management of the patient at risk of preterm labor, will need to define the relative risks and benefits of different management paradigms that may be proposed (469).
|
Acknowledgments |
|---|
|
Footnotes |
|---|
1 Work in the authors’ laboratories has been supported by Medical
Research Council (MRC) Group and operating grants from the MRC of
Canada. 
|
References |
|---|
|
TopAbstractI. IntroductionII. Regulation of Myometrial...III. Pregnancy: Phase 0...IV. Myometrial Activation: Phase...V. Myometrial Stimulation: Phase...VI. Application to Clinical...References |
|---|
adrenoceptor concentration. J
Pharmacol Exp Ther 240:44–504C-21 steroids in coexisting, genetically
dissimilar twin lamb fetuses throughout late gestation. Endocrinology 114:703–711 release. Biol Reprod 33:67–78[Abstract]
MSH, MSH, ACTH and
ßendorphin/ßlipotrophin in the fetal sheep pituitary: an
ontogenetic study. J Dev Physiol 8:355–368[Medline]
-hydroxylase and 3ß-hydroxysteroid dehydrogenase in the
integration of gonadal and adrenal steroidogenesis via the 5 and
4 pathways of steroidogenesis in mammals. Biol Reprod 56:789–799[CrossRef][Medline]
5-4 isomerase, tyrosine hydroxylase and
phenylethanolamine N-methyl transferase in the adrenal glands of sheep
fetuses throughout gestation and in neonates. J Reprod Fertil 96:127–134-hydroxylase (P45017) and cytochrome P450
side-chain cleavage (P450scc) messenger ribonucleic acid in
the sheep placenta. Am J Obstet Gynecol 180:1215–1221[CrossRef][Medline]
in parturition in sheep.
Nature 232:629–631[CrossRef][Medline]
-hydroxylase/C-17,20-lyase,
aromatase and sulphatase in dexamethasone-induced and natural
parturition. J Endocrinol 122:351–359. Endocrinology 95:547–553 and ß in the baboon fetal adrenal gland.
Endocrinology 140:5953–59615—4 isomearase in the human placenta
and fetal membranes during pregnancy and labor. Gynecol Obstet Invest 35:199–203[Medline]
5
4 isomerase in
human placenta and fetal membranes. J Clin Endocrinol Metab 75:956–961[Abstract]
-reduction signal the onset of
labor? Steroids, in press
proteins in
the upper and lower segments of the human uterus during pregnancy and
labor. J Clin Endocrinol Metab 84:1705–1710-reduced androgens play a key role in murine parturition. Mol
Endocrinol 10:380–392-reductase type 1 knockout mice
is due to impaired cervical ripening. Mol Endocrinol 13:981–992B
and steroid receptor-signaling pathways. Endocr Rev 20:435–459 and PGE2
binding to rat myometrium during gestation, parturition, and
postpartum. Am J Physiol 258:E740–E747
in human amnionic WISH cells
by various stimuli occurs through distinct intracellular mechanisms.
J Pharmacol Exp Ther 280:1065–1074B
involvement in IL-1ß-induction of GM-CSF and COX-2: inhibition by
glucocorticoids does not require 1-B. Biochem Soc Trans 25:154S
B mediates
interleukin-1ß-induced expression of cyclooxygenase-2 in human
myometrial cells. Am J Obstet Gynecol 181:359–366[CrossRef][Medline]
B pathway in human amnion derived WISH cell.
Prostaglandins Leukot Essent Fatty Acids 59:63–69[CrossRef][Medline]
levels in primary monolayer
cultures of epithelial cells from human proliferative endometrium.
Endocrinology 113:1274–1279 on the activity and expression of prostaglandin H
synthase-2 and the NAD+-dependent
15-hydroxyprostaglandin dehydrogenase in cultured term human villous
trophoblast and chorion trophoblast cells. J Clin Endocrinol Metab 84:4645–4651,
13,14-dihydro-15-keto-prostaglandin F2 (PGFM) and
11-deoxy-13,14-dihydro-15-keto-11, 16-cyclo-prostaglandin E2
(PGEM-LL) in preterm labor. Prostaglandins 37:149–161[CrossRef][Medline]
in the initiation of human parturition.
J Clin Endocrinol Metab 76:1332–1339[Abstract]
(CRH-1) and the CRH-C variant receptor. J Clin Endocrinol Metab 83:1376–1379This article has been cited by other articles:
 |
|
 |
|
  A J Tyson-Capper, E A Shiells, and S C Robson Interplay between polypyrimidine tract binding protein-associated splicing factor and human myometrial progesterone receptors J. Mol. Endocrinol., July 1, 2009; 43(1): 29 - 41. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. R. Mendelson Minireview: Fetal-Maternal Hormonal Signaling in Pregnancy and Labor Mol. Endocrinol., July 1, 2009; 23(7): 947 - 954. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. R. Kramer and C. R. Hogue What Causes Racial Disparities in Very Preterm Birth? A Biosocial Perspective Epidemiol. Rev., June 26, 2009; (2009) mxp003v3. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. R. Norwitz A Blood Test to Predict Preterm Birth: Don't Mess with Maternal-Fetal Stress J. Clin. Endocrinol. Metab., June 1, 2009; 94(6): 1886 - 1889. [Full Text] [PDF]
|
|
|
|
 |
|
 K. L. Connor, F. H. Bloomfield, M. H. Oliver, J. E. Harding, and J. R. G. Challis Effect of Periconceptional Undernutrition in Sheep on Late Gestation Expression of mRNA and Protein From Genes Involved in Fetal Adrenal Steroidogenesis and Placental Prostaglandin Production Reproductive Sciences, June 1, 2009; 16(6): 573 - 583. [Abstract] [PDF]
|
|
|
|
|
|
A. Imperatore, Wei Li, F. Petraglia, and J. R. G. Challis Urocortin 2 Stimulates Estradiol Secretion From Cultured Human Placental Cells: An Effect Mediated by the Type 2 Corticotrophin-releasing Hormone (CRH) Receptor Reproductive Sciences, June 1, 2009; 16(6): 551 - 558. [Abstract] [PDF]
|
|
|
|
 |
|
 P. Jeyasuria, J. Wetzel, M. Bradley, K. Subedi, and J. C. Condon Progesterone-Regulated Caspase 3 Action in the Mouse May Play a Role in Uterine Quiescence During Pregnancy Through Fragmentation of Uterine Myocyte Contractile Proteins Biol Reprod, May 1, 2009; 80(5): 928 - 934. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. C. Timmons, A.-M. Fairhurst, and M. S. Mahendroo Temporal Changes in Myeloid Cells in the Cervix during Pregnancy and Parturition J. Immunol., March 1, 2009; 182(5): 2700 - 2707. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. R. Challis, C. J. Lockwood, L. Myatt, J. E. Norman, J. F. Strauss III, and F. Petraglia Inflammation and Pregnancy Reproductive Sciences, February 1, 2009; 16(2): 206 - 215. [Abstract] [PDF]
|
|
|
|
 |
|
 K.K. Ryckman, H.N. Simhan, M.A. Krohn, and S.M. Williams Predicting risk of bacterial vaginosis: the role of race, smoking and corticotropin-releasing hormone-related genes Mol. Hum. Reprod., February 1, 2009; 15(2): 131 - 137. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. Helguera, M. Eghbali, D. Sforza, T. Y. Minosyan, L. Toro, and E. Stefani Changes in global gene expression in rat myometrium in transition from late pregnancy to parturition Physiol Genomics, January 8, 2009; 36(2): 89 - 97. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. A. Carvajal, A. M. Delpiano, M. A. Cuello, J. A. Poblete, P. C. Casanello, L. A. Sobrevia, and C. P. Weiner Brain Natriuretic Peptide (BNP) Produced by the Human Chorioamnion May Mediate Pregnancy Myometrial Quiescence Reproductive Sciences, January 1, 2009; 16(1): 32 - 42. [Abstract] [PDF]
|
|
|
|
 |
|
 P. Arthur, M. J. Taggart, B. Zielnik, S. Wong, and B. F. Mitchell Relationship between gene expression and function of uterotonic systems in the rat during gestation, uterine activation and both term and preterm labour J. Physiol., December 15, 2008; 586(24): 6063 - 6076. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. J Henderson, P. E Hartmann, T. J M Moss, D. A Doherty, and J. P Newnham Disrupted secretory activation of the mammary gland after antenatal glucocorticoid treatment in sheep Reproduction, November 1, 2008; 136(5): 649 - 655. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Dubicke, A. Akerud, M. Sennstrom, R. Rafik Hamad, B. Bystrom, A. Malmstrom, and G. Ekman-Ordeberg Different secretion patterns of matrix metalloproteinases and IL-8 and effect of corticotropin-releasing hormone in preterm and term cervical fibroblasts Mol. Hum. Reprod., November 1, 2008; 14(11): 641 - 647. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Zhang, R. Sundaram, W. Sun, and J. Troendle Fetal Growth and Timing of Parturition in Humans Am. J. Epidemiol., October 15, 2008; 168(8): 946 - 951. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Torricelli, L. Galleri, C. Voltolini, G. Biliotti, P. Florio, M. De Bonis, and F. Petraglia Changes of Placental Kiss-1 mRNA Expression and Maternal/Cord Kisspeptin Levels at Preterm Delivery Reproductive Sciences, October 1, 2008; 15(8): 779 - 784. [Abstract] [PDF]
|
|
|
|
 |
|
 M. Serrano-Sanchez, Z. Tanfin, and D. Leiber Signaling Pathways Involved in Sphingosine Kinase Activation and Sphingosine-1-Phosphate Release in Rat Myometrium in Late Pregnancy: Role in the Induction of Cyclooxygenase 2 Endocrinology, September 1, 2008; 149(9): 4669 - 4679. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. E. Michael and A. T. Papageorghiou Potential significance of physiological and pharmacological glucocorticoids in early pregnancy Hum. Reprod. Update, September 1, 2008; 14(5): 497 - 517. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. Latendresse and R. J. Ruiz Bioassay Research Methodology: Measuring CRH in Pregnancy Biol Res Nurs, July 1, 2008; 10(1): 54 - 62. [Abstract] [PDF]
|
|
|
|
|
|
Z. Zeng, M. C. Velarde, F. A. Simmen, and R. C.M. Simmen Delayed Parturition and Altered Myometrial Progesterone Receptor Isoform A Expression in Mice Null for Kruppel-Like Factor 9 Biol Reprod, June 1, 2008; 78(6): 1029 - 1037. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Ciarmela, E. Wiater, and W. Vale Activin-A in Myometrium: Characterization of the Actions on Myometrial Cells Endocrinology, May 1, 2008; 149(5): 2506 - 2516. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Sato, H. Chisaka, K. Okamura, and J. R.G. Challis Effect of the Interaction Between Lipoxygenase Pathway and Progesterone on the Regulation of Hydroxysteroid 11-Beta Dehydrogenase 2 in Cultured Human Term Placental Trophoblasts Biol Reprod, March 1, 2008; 78(3): 514 - 520. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Guoyang Luo, T. Morgan, M. O. Bahtiyar, V. V. Snegovskikh, F. Schatz, E. Kuczynski, E. F. Funai, A. T. Dulay, S.-T. J. Huang, C. S. Buhimschi, et al. Single Nucleotide Polymorphisms in the Human Progesterone Receptor Gene and Spontaneous Preterm Birth Reproductive Sciences, February 1, 2008; 15(2): 147 - 155. [Abstract] [PDF]
|
|
|
|
|
|
V. Casciani, E. Marinoni, A. D. Bocking, M. Moscarini, R. Di Iorio, and J. R. G. Challis Opposite Effect of Phorbol Ester PMA on PTGS2 and PGDH mRNA Expression in Human Chorion Trophoblast Cells Reproductive Sciences, January 1, 2008; 15(1): 40 - 50. [Abstract] [PDF]
|
|
|
|
|
|
J. D. Roizen, M. Asada, M. Tong, H.-H. Tai, and L. J. Muglia Preterm Birth without Progesterone Withdrawal in 15-Hydroxyprostaglandin Dehydrogenase Hypomorphic Mice Mol. Endocrinol., January 1, 2008; 22(1): 105 - 112. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
V. Sitras, R.H. Paulssen, H. Gronaas, A. Vartun, and G. Acharya Gene expression profile in labouring and non-labouring human placenta near term Mol. Hum. Reprod., January 1, 2008; 14(1): 61 - 65. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. E Ackerman IV, T. L.S Summerfield, D. D Vandre, J. M Robinson, and D. A Kniss Nuclear Factor-Kappa B Regulates Inducible Prostaglandin E Synthase Expression in Human Amnion Mesenchymal Cells Biol Reprod, January 1, 2008; 78(1): 68 - 76. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Guo, Z. Yang, W. Li, P. Zhu, L. Myatt, and K. Sun Paradox of Glucocorticoid-Induced Cytosolic Phospholipase A2 Group IVA Messenger RNA Expression Involves Glucocorticoid Receptor Binding to the Promoter in Human Amnion Fibroblasts Biol Reprod, January 1, 2008; 78(1): 193 - 197. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. T. Ross, I. C. McMillen, F. Lok, A. G. Thiel, J. A. Owens, and C. L. Coulter Intrafetal Insulin-Like Growth Factor-I Infusion Stimulates Adrenal Growth But Not Steroidogenesis in the Sheep Fetus during Late Gestation Endocrinology, November 1, 2007; 148(11): 5424 - 5432. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P Florio, P J Lowry, C Benedetto, L Galleri, M Torricelli, A Giovannelli, R Battista, F M Reis, and F Petraglia Maternal plasma corticotropin-releasing factor (CRF) and CRF-binding protein (CRF-BP) levels in post-term pregnancy: effect of prostaglandin administration Eur. J. Endocrinol., September 1, 2007; 157(3): 279 - 284. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C Chandras, T E Harris, A L. Bernal, D R E Abayasekara, and A E Michael PTGER1 and PTGER2 receptors mediate regulation of progesterone synthesis and type 1 11{beta}-hydroxysteroid dehydrogenase activity by prostaglandin E2 in human granulosa lutein cells J. Endocrinol., September 1, 2007; 194(3): 595 - 602. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. C. Wathes, D. R. E. Abayasekara, and R. J. Aitken Polyunsaturated Fatty Acids in Male and Female Reproduction Biol Reprod, August 1, 2007; 77(2): 190 - 201. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Markovic, M. Vatish, M. Gu, D. Slater, R. Newton, H. Lehnert, and D. K. Grammatopoulos The Onset of Labor Alters Corticotropin-Releasing Hormone Type 1 Receptor Variant Expression in Human Myometrium: Putative Role of Interleukin-1{beta} Endocrinology, July 1, 2007; 148(7): 3205 - 3213. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. M. Blanks, Z.-H. Zhao, A. Shmygol, G. Bru-Mercier, S. Astle, and S. Thornton Characterization of the molecular and electrophysiological properties of the T-type calcium channel in human myometrium J. Physiol., June 15, 2007; 581(3): 915 - 926. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M.-J. Leroy, E. Dallot, I. Czerkiewicz, T. Schmitz, and M. Breuiller-Fouche Inflammation of Choriodecidua Induces Tumor Necrosis Factor Alpha-Mediated Apoptosis of Human Myometrial Cells Biol Reprod, May 1, 2007; 76(5): 769 - 776. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Torricelli, A. Giovannelli, E. Leucci, G. De Falco, F. M. Reis, A. Imperatore, P. Florio, and F. Petraglia Labor (Term and Preterm) Is Associated With Changes in the Placental mRNA Expression of Corticotrophin-Releasing Factor Reproductive Sciences, April 1, 2007; 14(3): 241 - 245. [Abstract] [PDF]
|
|
|
|
|
|
R. M. Rizek, C. S. Watson, S. Keating, H.-H. Tai, J. R. G. Challis, and A. D. Bocking 15-Hydroxyprostaglandin Dehydrogenase Protein Expression in Human Fetal Membranes With and Without Subclinical Inflammation Reproductive Sciences, April 1, 2007; 14(3): 260 - 269. [Abstract] [PDF]
|
|
|
|
 |
|
 M. Kawamata, Y. Tonomura, T. Kimura, Y. Sugimoto, T. Yanagisawa, and K. Nishimori Oxytocin-induced phasic and tonic contractions are modulated by the contractile machinery rather than the quantity of oxytocin receptor Am J Physiol Endocrinol Metab, April 1, 2007; 292(4): E992 - E999. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. Li, E. Unlugedik, A. D. Bocking, and J. R.G. Challis The Role of Prostaglandins in the Mechanism of Lipopolysaccharide-Induced proMMP9 Secretion from Human Placenta and Fetal Membrane Cells Biol Reprod, April 1, 2007; 76(4): 654 - 659. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
I. Garcia-Verdugo, D. Leiber, P. Robin, E. Billon-Denis, R. Chaby, and Z. Tanfin Direct Interaction of Surfactant Protein A with Myometrial Binding Sites: Signaling and Modulation by Bacterial Lipopolysaccharide Biol Reprod, April 1, 2007; 76(4): 681 - 691. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. S. Knight, C. E. Pennell, R. Shah, and S. J. Lye Strain Differences in the Impact of Dietary Restriction on Fetal Growth and Pregnancy in Mice Reproductive Sciences, January 1, 2007; 14(1): 81 - 90. [Abstract] [PDF]
|
|
|
|
|
|
A. C. Buser, E. K. Gass-Handel, S. L. Wyszomierski, W. Doppler, S. A. Leonhardt, J. Schaack, J. M. Rosen, H. Watkin, S. M. Anderson, and D. P. Edwards Progesterone Receptor Repression of Prolactin/Signal Transducer and Activator of Transcription 5-Mediated Transcription of the {beta}-Casein Gene in Mammary Epithelial Cells Mol. Endocrinol., January 1, 2007; 21(1): 106 - 125. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Gidlof, A. Wedell, and A. Nordenstrom Gestational Age Correlates to Genotype in Girls with CYP21 Deficiency J. Clin. Endocrinol. Metab., January 1, 2007; 92(1): 246 - 249. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Astle, R. Newton, S. Thornton, M. Vatish, and D.M. Slater Expression and regulation of prostaglandin E synthase isoforms in human myometrium with labour Mol. Hum. Reprod., January 1, 2007; 13(1): 69 - 75. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. B. Hardy, B. A. Janowski, D. R. Corey, and C. R. Mendelson Progesterone Receptor Plays a Major Antiinflammatory Role in Human Myometrial Cells by Antagonism of Nuclear Factor-{kappa}B Activation of Cyclooxygenase 2 Expression Mol. Endocrinol., November 1, 2006; 20(11): 2724 - 2733. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Kanitz, W. Otten, and M. Tuchscherer Changes in endocrine and neurochemical profiles in neonatal pigs prenatally exposed to increased maternal cortisol. J. Endocrinol., October 1, 2006; 191(1): 207 - 220. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S.R. Sooranna, P.L. Grigsby, N. Engineer, Z. Liang, K. Sun, L. Myatt, and M.R. Johnson Myometrial prostaglandin E2 synthetic enzyme mRNA expression: spatial and temporal variations with pregnancy and labour Mol. Hum. Reprod., October 1, 2006; 12(10): 625 - 631. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Marinoni, C. Zacharopoulou, A. Di Rocco, C. Letizia, M. Moscarini, and R. Di Iorio Effect of Betamethasone In Vivo on Placental Adrenomedullin in Human Pregnancy Reproductive Sciences, September 1, 2006; 13(6): 418 - 424. [Abstract] [PDF]
|
|
|
|
 |
|
 H. Wang, S. K. Dey, and M. Maccarrone Jekyll and Hyde: Two Faces of Cannabinoid Signaling in Male and Female Fertility Endocr. Rev., August 1, 2006; 27(5): 427 - 448. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. A. Carvajal, R. J. Vidal, M. A. Cuello, J. A. Poblete, and C. P. Weiner Mechanisms of Paracrine Regulation by Fetal Membranes of Human Uterine Quiescence Reproductive Sciences, July 1, 2006; 13(5): 343 - 349. [Abstract] [PDF]
|
|
|
|
|
|
J. L. Sarno, F. Schatz, C. J. Lockwood, S.-T. J. Huang, and H. S. Taylor Thrombin and Interleukin-1{beta} Regulate HOXA10 Expression in Human Term Decidual Cells: Implications for Preterm Labor J. Clin. Endocrinol. Metab., June 1, 2006; 91(6): 2366 - 2372. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
X. You, R. Yang, X. Tang, L. Gao, and X. Ni Corticotropin-Releasing Hormone Stimulates Estrogen Biosynthesis in Cultured Human Placental Trophoblasts Biol Reprod, June 1, 2006; 74(6): 1067 - 1072. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. Li, L. Gao, Y. Wang, T. Duan, L. Myatt, and K. Sun Enhancement of Cortisol-Induced 11{beta}-Hydroxysteroid dehydrogenase Type 1 Expression by Interleukin 1{beta} in Cultured Human Chorionic Trophoblast Cells Endocrinology, May 1, 2006; 147(5): 2490 - 2495. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. Schmitz, B. A Levine, and P. W Nathanielsz Localization and steroid regulation of prostaglandin E2 receptor protein expression in ovine cervix. Reproduction, April 1, 2006; 131(4): 743 - 750. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
|
