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Department of Obstetrical and Gynecological Sciences (A.R., M.M.), University of Torino, 10126 Torino, Italy; and Laboratoire d’Eylau (J.T.), 75116 Paris, France
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Abstract |
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 Top Abstract I. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
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I. Introduction |
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TopAbstractI. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
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,25-dihydroxyvitamin D3 receptors. All members of this
superfamily share similar structural and functional characteristics and
have highly conserved structural domains involved in DNA and ligand
binding. The activation of steroid hormone receptors regulates the
transcriptional activity of specific genes, thus mediating the
so-called classic, or genomic, actions of steroids. A detailed description of the structure and function of the classic steroid receptors is beyond the scope of this review and has already been published (see reviews in Refs. 1–5). Briefly, within the intact cell the unoccupied form of the classic steroid receptor resides in the nucleus or in the nucleus and cytoplasm, dependent on the receptor and cell type. Steroid hormones act in the target tissues with a mechanism that may be summarized in the following steps: 1) free steroid enters the target cell by passive diffusion through the plasma membrane, passes through the nuclear membrane (in the case of nuclear receptors), and binds with high affinity to the receptor; 2) the steroid-receptor complex undergoes activation, a process that involves conformational changes (tertiary and quaternary structure) and enables the receptor to bind to selective sites on the chromatin; 3) the activated steroid-receptor complex interacts with specific DNA sequences referred to as steroid response elements, usually located upstream of the steroid-responsive genes; 4) the activated steroid-receptor complex acts as a transcription factor modulating the synthesis of specific mRNAs and proteins, in turn responsible for the final effect of the hormone.
A large body of experimental data obtained in several cell systems suggests the existence of steroid effects that cannot be explained by the classic model of steroid-target cell interaction. These effects may rather be explained by the existence of signal-generating steroid receptors on the cell surface and have been called nonclassic, nongenomic steroid effects. In general, nongenomic steroid effects have the following characteristics: 1) are too rapid (from seconds to a few minutes) to be compatible with the involvement of changes in mRNA and protein synthesis; 2) can be observed even in highly specialized cells that do not accomplish mRNA and protein synthesis (e.g., spermatozoa) or by cell clones lacking the steroid nuclear receptors; 3) can be elicited even by steroids coupled with high-molecular weight substances that do not pass across the plasma membrane and do not enter into the cell; 4) are not blocked by inhibitors of mRNA or protein synthesis; 5) may not be blocked by antagonists of the classic, genomic steroid receptors (unless these antagonists react with structurally related steroid-binding domains that may be common to both the genomic and the nongenomic steroid receptors); 6) are highly specific, as steroids with very similar, but not identical, chemical structure may show various degrees of potency in exerting them.
This review summarizes the current knowledge about the nongenomic
actions of steroid hormones in the reproductive tissues, describes the
intimate mechanisms by which these effects are accomplished, and
analyzes the cross-talk pathways between nongenomic and genomic actions
of steroids in the reproductive system (Table 1
). All the effects that are dealt with
in this review can be induced by physiological concentrations of
steroid hormones (i.e., concentrations with which each
respective cell can be confronted in the physiological context).
Effects of steroids at supraphysiological (pharmacological)
concentrations are not included in this paper.
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II. Nongenomic Actions of Estrogens |
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TopAbstractI. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
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A. Granulosa cells
In chicken and pig granulosa cells taken from preovulatory
follicles, measurements with the Ca2+-reporter dye Fura-2
show an immediate (<5 sec) 4- to 8-fold increase in intracellular
Ca2+ ([Ca2+]i) after the addition of
10-6 to 10-10 M 17ß-estradiol
(17ß-E2) (6). This effect is specific of estrogenic
compounds, as estrone, 17-E2, and estriol are as
effective as 17ß-E2 in increasing [Ca2+]i,
whereas progesterone (P), pregnenolone, testosterone (T),
androstenedione, and 5-dihydrotestosterone (5-DHT) are
uneffective up to a concentration of 10-5 M.
The estrogen-triggered [Ca2+]i increase is not affected
by the classic estrogen receptor (ER) antagonist tamoxifen or by the
RNA and protein synthesis blockers, such as actinomycin-D and
cycloheximide. The rapid [Ca2+]i increase is not reduced
by incubating these cells in Ca2+-free medium containing
the Ca2+ chelator EGTA or by pretreating them with the
Ca2+ channel blockers La3+, Co2+,
methoxyverapamil, and nifedipine. The mechanism of the rapid estrogen
action in this experimental model is therefore different from that
displayed in other cell systems by other steroid hormones, such as
vitamin D3 and P, which stimulate the influx of external
Ca2+ through membrane channels (see Section
III). In granulosa cells, the estrogen-induced rise of
[Ca2+]i is abolished by pretreating the cells with
inhibitors of inositol phospholipid hydrolysis such as neomycin and
U-73,122 (7). The 17ß-E2-induced [Ca2+]i
increase in rat granulosa cells, therefore, seems to be the result of
the release of Ca2+ from intracellular stores triggered by
inositol 1,4,5-triphosphate (InsP3) generated by a steroid
receptor-induced hydrolysis of membrane phosphatidylinositol
4,5-biphosphate. The very brief (5 sec) time elapsed between
17ß-E2 addition and the [Ca2+]i peak
probably represents the time needed for enough InsP3 to
accumulate from the breakdown of membrane phosphoinositides and
bind to Ca2+-releasing receptors on intracellular
storage vesicles.
In mammalian granulosa cells, the estrogen-induced
[Ca2+]i rise could be an important regulating mechanism,
possibly able to modulate the function of Ca2+-binding
signal proteins (e.g., calmodulin), affect the secretion of
Ca2+-sensitive autocrine and/or paracrine factors
(e.g., transforming growth factor-), stimulate cell
proliferation, and modify the cell-to-cell intraovarian communication
(8). Moreover, since the classic ER is a substrate of the
calmodulin-dependent protein kinase II (9), the [Ca2+]i
transient could even mediate a cross-talk interaction between ERs
located on the plasma membrane and the genomic, classic ER.
B. Endometrial cells
In cultured rat endometrial cells, 17ß-E2 elicits
both functional and morphological changes that appear too rapid to be
mediated via the classic genomic pathway. The addition of
10-9 M 17ß-E2 rapidly (<10 min)
stimulates 45Ca2+ uptake from endometrial cells
(10), and this effect appears to be mediated via surface
estrogen-binding sites, whose existence has been reported in rat
endometrial and liver cells (11, 12, 13). Furthermore, the intravenous
injection of 17ß-E2 to ovariectomized rats rapidly
(within 1 min) modifies cell morphology, increasing the number and
clustering of microvilli in the endometrial cells and the luminal
surface ciliation (14). Similar morphological changes may be obtained
after exposure to diethylstilbestrol (14). Ultrastructural analysis
confirms the rapid estrogen-induced increase in microvillar density and
length and shows that the structural modifications induced by estrogens
are biphasic, as they appear further enhanced 7 min after
17ß-E2 injection. 17ß-E2 is even able to
elicit micropinocytotic activity in the endometrial cells within 2 min
of its intravenous injection to ovariectomized rats (15). It is of
interest that morphological effects similar to those induced by
17ß-E2 in endometrial microvilli can be evoked in other
cell types by peptidic growth factors, whose action is mediated by
surface receptors. For instance, such effects are produced by the
epidermal growth factor in cultured sympathetic neurons (16) and by the
nerve growth factor in the human carcinoma cell line A-431 (17).
C. Oocytes
Human oocytes at the germinal vesicle stage develop a rapid
(within a few seconds) Ca2+ influx in response to
17ß-E2 (18). The addition to oocytes of 10-6
M 17ß-E2, either free or linked to a
membrane-impermeant conjugate with BSA, induces Ca2+ influx
followed by a series of oscillations in the free cytosolic
Ca2+ concentration ([Ca2+]i) that are
dependent on the continuous presence of Ca2+ in the
incubation medium (18). The presence of 17ß-E2 in the
medium does not influence the progression of oocytes through meiotic
maturation (to metaphase I and metaphase II stages) but improves the
subsequent fertilization potential of the in vitro matured
oocytes. In fact, oocytes that reach the metaphase II stage and are
inseminated in vitro show better cleavage rate and
development of the resulting embryos. Thus, 17ß-E2 does
not appear to affect the nuclear maturation of human oocytes but exerts
a beneficial effect on their cytoplasmic maturation (18). A
subthreshold concentration of 17ß-E2 (10-7
nM) also promotes Ca2+ oscillations in germinal
vesicle-stage human oocytes, but with a delay of 1–3 h after hormone
addition (19). This slow nongenomic response may be closer to the
in vivo condition, when oocytes are exposed to slow
progressive changes in ovarian follicular steroid concentrations rather
than to an abrupt increase. It should be noted that spontaneous
Ca2+ oscillations can be observed in mouse germinal vesicle
oocytes (20) but never in human oocytes (18, 19). Ca2+
oscillations, however, represent a typical early reaction of oocytes to
the fertilizing spermatozoon in various mammalian species (21)
including humans (22, 23). It remains to be determined whether the
priming of oocytes with 17ß-E2 during their previous
intrafollicular development is important for the process of
fertilization. Both the spontaneous Ca2+ oscillations of
mouse oocytes (20) and the 17ß-E2-induced
Ca2+ oscillations of human oocytes (19) stop after the
germinal vesicle breakdown and do not reappear in metaphase I or
metaphase II oocytes.
D. Spermatozoa
The incubation of human spermatozoa in the presence of
17ß-E2 stimulates sperm motility (24, 25),
[3H]tetracycline binding capacity (26), oxygen uptake,
and lactate production (27, 28), as well as the metabolization of
several substrates (27). Furthermore, the in vitro
sperm-agglutinating property of selected sera is significantly
decreased by estrogens (29). The addition of 17ß-E2 (30
ng/ml) to the sperm incubation medium is even claimed to improve the
results of the zona-free hamster ova penetration test by human
spermatozoa of both fertile and infertile men (30). Since spermatozoa
are known to have a densely packaged DNA, and protein synthesis in
spermatozoa is very scarce and limited to mitochondria, a genomic
action of steroids is not possible in this cell type. The above
mentioned effects of estrogens are thus necessarily mediated by a
nongenomic mechanism.
A study by Hyne and Boettcher (31) reported the existence of specific, low-affinity binding sites for 17ß-E2 on human spermatozoa for which other steroids could compete. A subsequent autoradiographic study observed that the binding of [3H]17ß-E2 to spermatozoa can be reduced by coincubation with other steroids, such as progestagens, which can strongly compete for the 17ß-E2 receptor (32). The presence of high-affinity (KD 6.6 x 10-10 M) binding sites for 17ß-E2 on human sperm has been further confirmed: after fractionation of 17ß-E2-bound spermatozoa, they are detectable mainly in the membrane fraction (75–85%), whereas the nuclear fractions shows only about 10% of the total bound radioactivity, and no radioactivity is detected in the cytosolic fraction (33). Other autoradiographic data confirm that the plasma membrane is the site of receptors with specificity for 17ß-E2, which are mostly concentrated in the central part of the sperm tail (34). In competition experiments, the binding sites for [3H]17ß-E2 appear to be specific, the labeled steroid being displaced by cold 17ß-E2. Specific binding sites for [3H]17ß-E2 are not detectable in the sperm cytosol and in sperm nuclei isolated after homogenization, detergent treatment, and centrifugation (35).
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III. Nongenomic Actions of P |
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B. Oocytes
Several data suggest that the physiological stimulus triggering
the resumption of meiotic division in amphibian oocytes is P. In
Xenopus laevis, P acts at the oocyte plasma membrane to
reinitiate the first meiotic division (37). The photoaffinity labeling
technique identifies a monomeric membrane binding site for P with an
approximate mol wt of 110,000, whose activation by P or the synthetic
progestin R5020 induces germinal vesicle breakdown (38, 39). In
Xenopus oocyte, P is effective in increasing
[Ca2+]i, leads to a sudden rise in intracellular pH,
decreases membrane conductance, and finally regulates the activity of
plasma membrane enzymes (40). Some of them (e.g., serine
proteases) are activated by P addition; others (e.g.,
adenylate cyclase) undergo down-regulation. In amphibian oocytes, the
binding of P to plasma membrane receptors activates several systems
associated with intracellular signal transduction:
1. One system is linked to the activation of a phosphatidylinositol diphosphate-specific phospholipase C that generates InsP3 and a smaller quantity of diacylglycerol (DAG): both metabolites are subsequently released into the cytosol and reach a maximum after approximately 15 min of oocyte exposure to P (37).
2. Both in Rana pipiens (41) and Xenopus laevis (42, 43) oocytes, P inhibits adenylate cyclase, thus reducing cAMP production and stimulating the Na+/H+ exchange as well as changes in Ca2+ and K+ conductance. In vitro addition of P elicits an almost immediate decrease in cAMP concentration inside the oocyte, even if the enzyme has previously been stimulated by pretreating the oocytes with cholera toxin (42). Furthermore, the addition of physiological concentrations (30 x 10-9 M) of P to intact oocytes selectively inhibits membrane-bound adenylate cyclase activity, which represents about 30% of the total enzyme (44). The inhibition of X. laevis oocyte adenylate cyclase involves a G protein; in fact, the level of inhibition of plasma membrane adenylate cyclase by both P and 2',5'-dideoxyadenosine, which acts as a potent progesterone receptor (PR) agonist, correlates with slowing of guanine nucleotide exchange in photoaffinity labeling assays (43). The inhibitory action of P is abolished by the ion Mn2+ (43).
3. Another consequence of P action in the amphibian oocyte is the stimulation of massive phospholipid turnover in the oocyte plasma membrane (45) and the activation of plasma membrane-bound, Ca2+-dependent protein kinase C (PKC) (46). In fact, the addition of GTP to the isolated plasma membrane of R. pipiens oocytes increases both the P-induced protein phosphorylation (46) and the activation of a trypsin-like serine protease, which is released from the plasma membrane within 1 min after P binding to its receptor (45).
4. In R. pipiens oocytes, P induces N methylation of phosphatidylethanolamine within seconds, and this reaction is followed by an increased synthesis of phosphatidylcholine and further conversion into phosphatidylmonomethylethanolamine within the first minute of the steroid addition. The latter conversion is associated with a sharp, transient rise of DAG, an increase in the phosphorylation of plasma membrane proteins, and the first peak of a biphasic Ca2+ release from the oocyte surface (47). Since inhibitors of phospholipid N methylation are able to block P-induced meiosis, and exogenous phosphatidylmonomethylethanolamine alone is able to induce meiosis, N-methyltransferase is likely to be a downstream element of the signal transduction cascade mediating the P effect on oocyte maturation (47).
C. Spermatozoa
Follicular fluid (FF) mixed with the oviductal fluid or trapped in
the extracellular matrix of the cumulus oophorus represents
a source of P acting on spermatozoa at the fertilization site. P is
able to stimulate the motility and acrosomal exocytosis of human sperm
preserving sperm viability in vitro (48) and also enhances
the acrosomal exocytotic response of mouse sperm to the subsequent
exposition to the zona pellucida glycoprotein ZP3 (49). P has also been
observed to increase the percentage of spermatozoa exhibiting
hyperactivated motility at very low concentrations (3.1 ng/ml) and
within 10 min of addition to the incubation medium (50), and to
increase the velocity of spermatozoa when added to peritoneal fluid
(51). A significant correlation between P levels in FF and the ability
of FF to induce the acrosomal exocytosis of human sperm has been
noticed after charcoal delipidation of FF and its subsequent
supplementation with exogenous P (52). A final concentration of 10 µg
P/ml almost completely restores (88%) the stimulating effect of FF on
the acrosomal reactivity (52). Data obtained combining organic
precipitation and HPLC suggest that P and 17-hydroxy-P are the major
substances responsible for the acrosomal reaction-inducing activity of
FF (53). Moreover, charcoal treatment, as well as preincubation of FF
with P-specific antibodies, prevents the FF-induced Ca2+
influx in human spermatozoa, a phenomenon that is strictly related to
acrosomal exocytosis (54).
The effect of P on acrosomal reaction is due to its capacity of inducing a rapid increase in free cytosolic [Ca2+]i, which is evident after a few seconds of exposure to the steroid (54). The increase in [Ca2+]i is entirely due to the influx of extracellular Ca2+, as it may be blocked by the Ca2+ channel-blocker La3+ or by the addition of the Ca2+-chelator EGTA to the incubation medium (54). The effect of P on Ca2+ influx is dose-dependent, with small effects being observed with 10-8 to 10-9 M and maximal effects with 10-5 to 10-6 M P (54). It has been calculated that P is able to initiate the acrosomal reaction in vitro at concentrations ranging from 1 µg/ml (approximately 3 x 10-6 M) to 100 ng/ml (55). P stimulates the rapid influx of Ca2+ in parallel with a rapid hydrolysis of phosphatidylinositol 4,5-biphosphate (56); both phenomena are blocked by preventing the entry of extracellular Ca2+ by La3+ addition, showing that the latter is a sequela, and not the cause, of the former. Thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ ATPase Ca2+-pump, and thus a mobilizer of intracellular Ca2+, can initiate the acrosomal reaction in human sperm (57). Since there is no apparent endoplasmic reticulum in the cytoplasm of mature sperm, the Ca2+ contained in the acrosome, and possibly the mitochondria, is likely to be involved in these phenomena. Although some authors reported that the Ca2+ influx after P addition is even higher in the absence of Na+ (58), Na+ seems to play a role in these phenomena since the absence of Na+ in the medium causes inhibition of the P-mediated increase in human sperm [Ca2+]i and of the acrosomal exocytosis (59).
Trypsin-like activity in sperm might also play a role in the P-mediated [Ca2+]i increase. In fact, preincubation of capacitated sperm with either benzidamine hydrochloride or 4'-acetamidophenyl,4-guanidinobenzoate, two inhibitors of trypsin-like enzymes, is able to inhibit the P-induced acrosomal reaction by 68–85% by blocking the membrane fusion events of the acrosome reaction (60). Moreover, the action of P on human spermatozoa is enhanced in the presence of an acrosin activator from the human FF (61). The action of proteases, however, needs to be regulated by a yet unknown mechanism because the addition of trypsin to the incubation medium removes the P-binding activity from the sperm surface without affecting sperm motility, viability, and acrosomal reactivity (62). Sperm surface trypsin-like activity, represented by the acrosomal enzyme acrosin (63) or by an acrosin-like protease from the seminal vesicles (64), is claimed to be important in cleaving a P-binding protein (corticosteroid-binding globulin, CBG), finally producing a high local P concentration (65, 66).
The existence of specific receptors for P on the plasma membrane of spermatozoa is suggested by the observation that P conjugated with BSA [P-3-(O-carboxymethyl)oxime: BSA], a large molecule incapable of crossing the plasma membrane, is able to increase [Ca2+]i and to stimulate the acrosomal reaction (67, 68). Moreover, the [Ca2+]i-increasing effect of P is extremely rapid, dose-dependent, and not counteracted by RU486, a potent antagonist of the nuclear PR (69). RU486 is ineffective even in inhibiting the P-induced stimulation of hyperactivated motility (50). Further, neither RU486 nor ZK98299, another blocker of the classic genomic receptor, are able to avoid the P-mediated Ca2+ entry into spermatozoa, and this process may not be mimicked by synthetic progestins such as megestrol, norgestrel, medroxyprogesterone acetate, norethynodrel, norethindrone, and R5020, which interact with the nuclear PR (70). Direct evidence of the presence of specific binding sites for P on the sperm plasma membrane has been provided by a study with P coupled to fluorescein isothiocyanate-labeled BSA: the P-BSA complex binds tightly to the sperm plasma membrane (71). This study, however, does not demonstrate that the binding is specific, since competition tests showing the ability of unlabeled P to displace the bound labeled P have not been performed. Fluorescence microscopy and flow cytometry indicate that only about 10% of spermatozoa possess PRs on their surface, and thus are able to bind the steroid and undergo the acrosome reaction (71). The binding of P appears to be limited to the sperm acrosomal region of the plasma membrane, and a marked variability among individual spermatozoa may be observed (71). These observations are consistent with three other studies: the first reporting that only 30% of spermatozoa have detectable PRs on their heads (67), the second localizing PRs at the equatorial region of the sperm head by means of immunofluorescence with the monoclonal antibody MAb C26s, directed against the C-terminal P-binding domain of the intracellular PR (72), and the third showing that the P-mediated increase in [Ca2+]i begins in the midhead region and spreads rapidly (within 1 sec) anteriorly over the rest of the head (73). Some authors, however, hypothesize that a greater proportion of sperm in an ejaculate can possess PRs in a functionally inactive form; their activation could be asynchronous and stimulated by the increase of extracellular Ca2+ (74). More recent findings indicate that about 90% of spermatozoa may respond to P with an abrupt increase in [Ca2+]i (75, 76), and thus the size of the sperm population having PRs seems to be greater than originally believed.
The apparent contradiction among these studies probably originates from
the presence of different types of PRs in the sperm plasma membrane.
Many features of the sperm response to P, in fact, suggest the
implication of at least three types of surface receptors, by means of
which P targets, respectively, 1) a plasma membrane Ca2+
channel (PR1), 2) a membrane-associated protein tyrosine kinase (PTK;
PR2), and 3) a plasma membrane chloride channel (PR3) (77) (Fig. 1). The tyrosine kinase-associated PR
(PR2) seems to be the one visualized by the hormone-binding assay
because those spermatozoa that bind the BSA conjugate also increase
their phosphotyrosine content and undergo the acrosome reaction (78).
This PTK-associated PR is probably responsible both for the effect of P
on the acrosome reaction and on hyperactivated motility (79). It
appears to be sensitized during the capacitation process, during which
an initial phosphorylation event takes place under the control of
redox- (80) and cAMP-regulated (81) mechanisms. When tyrosine
phosphorylation is stimulated, human spermatozoa become particularly
sensitive to physiological concentrations of P, responding to the
P-generated Ca2+ transient with high rates of sperm-oocyte
fusion occurring after 5 min of exposure to the steroid (80). On the
other side, when P is administered to uncapacitated spermatozoa, they
rapidly develop an insensitivity to further stimulation that is due
both to the down-regulation of the cellular mechanisms necessary to
generate a Ca2+ transient, and to interferences with the
downstream events coupling Ca2+ influx with a biological
response (80). Tyrosine phosphorylation may be the site at which
premature exposure to P exerts its suppressive effect; as in other cell
types, cell-specific phosphorylation events may influence steroid
receptor interaction with the transcription apparatus, altering
receptor-mediated induction mechanisms (82, 83). The PR responsible for
the rapid opening of the Ca2+ channel (PR1) is active in a
higher percentage (more than 90%) of spermatozoa (75, 76), but has
different ligand-binding properties than the receptor responsible for
PTK activation. Moreover, it is not capable of initiating the P-induced
acrosome reaction. A pharmacological dissection of the P effects on
Ca2+ influx and on tyrosine phosphorylation may be obtained
in peculiar experimental conditions (77). The third PR (PR3) is likely
to be a 
-aminobutyric acid (GABA)A-receptor/chloride
channel complex, and probably mediates the Cl- fluxes
occurring during acrosomal exocytosis (84). It does not appear to be
involved in the P-induced Ca2+ influx (85). Thus, P effects
on spermatozoa, and perhaps the nongenomic effects of steroids on cells
in general, appear to be mediated by multireceptor systems rather than
by a single type of receptor. When one member of this system is not
operational, the biological response may fail even if the other types
of receptor function normally. P receptors in spermatozoa are probably
relevant to their fertilizing ability: in fact, an impaired P-binding
capacity with subsequent failure in generating the typical
Ca2+ influx wave and the acrosomal reaction has been
observed in some cases of unexplained infertility (86), and spermatozoa
from oligospermic men seem to be less prompt to respond to P with an
increase in [Ca2+]i, probably for a lack of PRs on the
sperm surface (87).
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-face (88). This observation opens an interesting perspective of
synthesizing steroids capable of interacting specifically with the
sperm surface receptor without interacting with the genomic PR;
selective agonists and antagonists of the membrane PR could be used as
fertility-enhancing drugs, or as contraceptives, respectively. A close
contact between the steroid and the surface receptor across the C,
D-ring upper edge (C-11, C-12, C-17) appears to be intimately involved
in P membrane-PR interaction (88). After binding to the PTK-associated surface receptor, P produces receptor aggregation on the sperm surface (62), which may be instrumental in at least some of the known P effects. Artificial aggregation of PR complexes, as generated by adding anti-P antibodies to spermatozoa previously incubated with subthreshold concentrations of P, is able to trigger a Ca2+ influx into the sperm and is followed by the acrosomal reaction within a few minutes (89). Under appropriate conditions (inhibition of immediate exocytosis), a spontaneous aggregation of ligand-occupied P-binding sites on the sperm surface can be demonstrated (62). P is also able to increase the fluidity of human sperm plasma membrane, to aggregate membrane vesicles, to induce the fusion of these vesicles, and to render the membrane permeable to hydrophilic molecules such as carboxyfluorescein (90). The increase in membrane fluidity, which facilitates receptor aggregation, is a common finding when spermatozoa are incubated in capacitating conditions: the decrease in cholesterol/phospholipid ratio inside sperm membranes occurs during capacitation, and is a prerequisite for acrosomal reaction (91).
Several mechanisms mediating the transduction of the P signal inside the sperm cell have been studied (see also Section V). The P-induced onset of hyperactivated motility seems to be linked to an increased 3',5'-cAMP concentration inside the spermatozoon (79), which in turn is responsible for cAMP-dependent kinase (protein kinase A; PKA) activation. The stimulating effect of human FF on acrosomal reactivity, which has been thoroughly studied (92, 93, 94, 95, 96), is inhibited by KT5720, a PKA inhibitor (97). Human spermatozoa incubated for 2 h with increasing P concentrations show a significant, dose-dependent increase in intracellular cAMP that occurs biphasically, with one peak after 30 min and another peak after 120 min incubation. These effects of P on cAMP levels are paralleled by a significant rise in the percentage of hyperactivated spermatozoa, occurring at the same times. The effect of P on intracellular cAMP concentration is Ca2+-dependent, is mimicked by the Ca2+-ionophore A23187, and is inhibited by the PTK inhibitor genistein, but not by the genomic PR antagonist RU486. Since phosphodiesterase inhibitors, such as pentoxyfilline, have been shown to increase hyperactivation in human spermatozoa (98, 99), the P-induced increase in cAMP could also depend on the inhibition of phosphodiesterase.
The intracellular mechanisms by which P elicits the acrosome exocytosis
and prepares the subsequent sperm-oocyte fusion are complex. They are
schematically represented in Fig. 1
.
The aggregation of the PTK-associated type of P receptors stimulates the tyrosine phosphorylation of a 94-kDa phosphoprotein, which, in turn, is able to activate intracellular effectors leading to acrosomal exocytosis (78). This protein is a unique, germ cell-specific receptor PTK localized at the sperm plasma membrane in the acrosomal region; it may enhance its kinase activity in response to both P and solubilized zona pellucida, behaving as a receptor PTK with regard to the zona glycoprotein ZP3 and as a receptor-coupled PTK with regard to P (100). The PR responsible for tyrosine phosphorylation (PR2) is expressed in only 10% of spermatozoa, aggregates within minutes, and stimulates the PTK which, in turn, leads to the opening of voltage-dependent, PTK-dependent Ca2+ channels. The P-induced increase in tyrosine phosphorylation is detectable after at least 5–10 min after P addition to spermatozoa, contrasting with the rapidity of the P-induced Ca2+ influx occurring within a few seconds after P treatment (100). This rapid Ca2+ influx depends on the direct effect of P on voltage-independent Ca2+ channels, probably coupled with a different type of PR (PR1). The result of the direct P-induced, rapid activation of the voltage-independent Ca2+ channels first, and of the second messenger-operated activation of voltage-dependent Ca2+ channels in a second time, is a biphasic Ca2+ influx into sperm. The P effect on tyrosine phosphorylation, in any case, is not a simple consequence of the P-induced Ca2+ influx because a similar Ca2+ influx can be obtained with the use of a Ca2+ ionophore without producing the PTK-mediated phenomena leading to the acrosomal exocytosis (77, 100).
A unique steroid receptor/Cl- channel complex (resembling but not identical to a GABAA receptor/Cl- channel complex) is possibly involved in the P-initiated acrosomal reaction of sperm. Both GABA and the GABAA-receptor agonist muscimol induce acrosomal exocytosis in murine spermatozoa (101), but this effect cannot be observed with human sperm (80). In the mouse, P can also potentiate the ability of GABA agonists to stimulate the acrosome reaction (101). The GABAA receptor/Cl- channel complex blockers picrotoxin or pregnenolone sulfate and the GABAA receptor antagonist bicuculline are effective in reducing the P-initiated increase in [Ca2+]i (69) and the acrosomal reaction of human and porcine sperm (84, 102). The absence of extracellular Cl- in the culture medium does not prevent P from stimulating the Ca2+ entry and the rise in [Ca2+]i, but the acrosomal reaction occurs only in the presence of Cl- (103). Such observations may be explained by the existence of a third type of plasma membrane PR (PR3), different from that initiating the rapid increase in [Ca2+]i and from the one associated with PTK activation. PR3 (similar to the GABAA/Cl- channel complex) is thought to mediate an increased Cl- influx leading to a subsequent reversal of HCO3-/Cl- exchange (85, 103). Another possible model is that a chloride influx induces an initial hyperpolarization of the cell, as happens when GABA activates neuronal GABAA-receptor/Cl- channel complexes (84), and this hyperpolarization is then followed by a proton efflux, depolarization, and activation of voltage-sensitive Ca2+ channels. The hypothesis of the existence of PR3 is supported by the observation that steroids that are very active on the GABAA/Cl- channel are poor stimulators of Ca2+ influx (85). Moreover, known modulators of GABAA/Cl- channels, such as picrotoxin, diazepam, GABA, muscimol, and pentobarbital, are ineffective in influencing basal [Ca2+]i or the Ca2+ flux induced by P (80, 85).
Several other substances appear to play a role in coupling P binding to
its receptors on the sperm surface and Ca2+ fluxes through
the sperm membrane (Fig. 1, in square brackets). The P-PR
complex is thought to exert the following effects:
1. Activation of phospholipase D, which in turn elevates the intracellular concentration of phosphatidic acid, a substance able to promote Ca2+ influx (104).
2. Modulation of polyamine biosynthesis increasing spermidine
production, in turn promoting Ca2+ influx (105). Inhibitors
of polyamine synthesis, such as -difluoromethylornithine, an
inhibitor of putrescine synthesis, or MDL73811, an inhibitor of
S-adenosylmethionine decarboxylase (required for spermine
and spermidine synthesis), are effective in blocking the P-induced
Ca2+ influx and the acrosomal reaction, but preincubation
with putrescine or spermidine reverses such a block (105).
3. Stimulation of the production and release of platelet-activating factor (PAF), a phospholipid able to induce acrosomal exocytosis (106, 107, 108, 109, 110). Treatment of human spermatozoa with P results in the activation of phospholipase A2 (PLA2) and an increase of [3H]acetate incorporation into PAF is observed (111, 112); such newly synthesized PAF could act autocrinally on spermatozoa and play a role in mediating P-induced Ca2+ influx and acrosomal reaction.
4. Stimulation of protease-driven mechanisms, which have been hypothesized in the P-induced acrosomal reaction: the acrosomal reaction of human spermatozoa, in fact, is impaired by protease inhibitors (113).
5. Activation of phospholipase C, thus increasing the intracellular content in InsP3 and DAG, in turn mediating the stimulation of PKC (56, 114). However, while in most hormonally responsive cells the phospholipase C-mediated generation of InsP3 is responsible for the release of Ca2+ from intracellular stores and consequently for the increase in [Ca2+]i, in spermatozoa the synthesis of InsP3 appears to be a consequence rather than a cause of the Ca2+ influx induced by P. In fact, the phospholipase C inhibitor U73122 is uneffective in disrupting the P-induced Ca2+ transient in human spermatozoa (80).
The involvement of the G protein system in the P-induced stimulation of sperm motility and acrosomal reactivity has never been demonstrated. Pertussis toxin, which catalyzes the ADP ribosylation of G proteins and thereby inhibits their function, does not influence the P-mediated Ca2+ influx and the acrosome reaction (80, 115). Moreover, although G proteins insensitive to this toxin have been identified in mouse spermatozoa (116), they have not been detected in spermatozoa from mammalian species including the human (117).
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IV. Nongenomic Actions of Androgens |
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TopAbstractI. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
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-DHT rapidly
(20–40 sec) increase the intracellular Ca2+ concentration
(118). This effect is inhibited by preincubation with either the
nonsteroidal antiandrogen hydroxyflutamide or the 5-reductase
inhibitor finasteride (118), indicating that T acts via a
receptor-mediated mechanism and, at least partially, after conversion
to 5-DHT. Aromatization of T to 17ß-E2 consistently
reduces the T-induced Ca2+ entry into the cell (118),
suggesting that T effect is mediated by specific androgen binding
sites, but not by ERs. Other steroids, like P, are completely inactive
in Sertoli cells (118). The abrupt increase of [Ca2+]i
can be prevented by the removal of extracellular Ca2+ or by
the pharmacological blockade of voltage-dependent or
voltage-independent membrane Ca2+ channels (118),
indicating that the androgen effect on Sertoli cells is accomplished by
transplasmalemmal influx of extracellular Ca2+.
Interestingly, in avian and porcine (but not rat) granulosa cells (the
female analog of Sertoli cells), 17ß-E2 increases the
cytosolic [Ca2+]i independently of extracellular
Ca2+, releasing the ion from the intracellular stores via
the activation of phosphoinositide pathways (6), as accomplished by
peptides (119). BSA-conjugated T, which is unable to cross the
plasmalemma of the Sertoli cell, is still effective in rapidly
increasing cytosolic Ca2+ (118). This observation, together
with the rapidity of the androgen effect and its dependence on
extracellular Ca2+, suggests that the effect of T and its
5-reduced metabolite 5-DHT may be mediated directly at the plasma
membrane. Since T and FSH have synergistic effects in initiating
spermatogenesis (120), and the increase of [Ca2+]i is one
of the signal transduction mechanisms of FSH action in Sertoli cells
(121), it may be postulated that the interaction between T and FSH in
these cells is mediated via common effects on Ca2+ fluxes.
B. Oocytes
Though incapable of inducing a Ca2+ response,
androstenedione (A), the main intrafollicular androgen, influences the
Ca2+ response of human germinal-vesicle oocytes to
17ß-E2 (see Section II). When
10-6 M A is added to oocytes during ongoing
Ca2+ oscillations induced by previous addition of
10-6 M 17ß-E2, the oscillations
stop; the 17ß-E2-induced Ca2+ oscillations of
human germinal-vesicle oocytes are also disabled to a variable degree
by previous exposure of the oocytes to A, even when the oocytes are
washed from the hormone before the treatment with 17ß-E2
(19). Because 17ß-E2 appears to support oocyte
cytoplasmic maturation, acting by a nongenomic,
Ca2+-mediated mechanism (18), these observations may be
interpreted to suggest that the quality of mature human oocytes
recovered from large ovarian follicles is conditioned by the
estrogen-to-androgen ratio to which the oocytes have been temporarily
exposed in the midfollicular phase (19).
|
V. Signal Transduction Pathways Involved in Nongenomic Steroid Effects |
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TopAbstractI. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
|---|
Most nongenomic actions of steroids yet described involve
Ca2+ as a second messenger. Thus, we will first focus on
the fundamental phenomena underlying the steroid-induced
[Ca2+]i increase and subsequently show what parallel
signaling mechanisms steroids use, in addition to the Ca2+
signal, and what downstream elements are involved in the transduction
of all these signals to corresponding effectors of the cellular
biological response (Fig. 2).
|
The initial Ca2+ influx has been shown to stimulate several
bifurcating signal transduction pathways (Fig. 2). One of these
pathways employs cAMP whose intracellular concentration in human
spermatozoa undergoes a Ca2+-dependent increase in response
to P (79), although another study has failed to detect any P-induced
cAMP response in the same system (125). This discrepancy may be
explained by a particular kinetics of the P-induced cAMP increase
reported by Parinaud and Milhet (79) and characterized by two peaks
separated by a period during which no increase is detectable. It is not
known whether the observed P-dependent cAMP increase is due to
stimulation of adenylate cyclase or to inhibition of phosphodiesterase.
Another pathway activated downstream of the Ca2+ influx
(Fig. 2) involves the stimulation of hydrolysis of plasma membrane
phosphoinositides by phosphoinositide-specific phospholipase C (PIC)
leading to the formation of two important second messengers, DAG and
InsP3 (56, 114, 126, 127). The DAG produced by this pathway
amplifies its own production by stimulating the activity of
phosphatidylcholine-specific phospholipase C, another DAG-generating
enzyme in mammalian spermatozoa (126). In many biological systems, DAG
exerts its signaling role by activating PKC. However, the actual
presence of PKC in spermatozoa, claimed by some workers (128, 129), was
questioned by others (130). It now seems clear that PKC plays a role in
transducing steroid-generated signals in mammalian spermatozoa (131, 132) although its activity in these cells is extremely low (133, 134).
When activated by DAG, the two PKC isoforms found in bovine
spermatozoa, PKC and PKCßI, rapidly translocate from the cytosol
to the membrane fraction, presumably with the participation of membrane
PKC-binding proteins, collectively termed RACKs (Receptors for
Activated C-Kinase) that have been identified in the bovine sperm
membrane fraction (132).
The pathways leading, respectively, to the increase in the
intracellular cAMP content and to the stimulation of PKC appear to play
an important role in the amplification of the initial steroid-induced
[Ca2+]i increase (Fig. 2). This amplification is mediated
by opening, through phosphorylation by PKC, of a plasma membrane
Ca2+ channel leading to additional Ca2+ influx,
on the one hand, and by the release of Ca2+ stored in the
sperm acrosome through cAMP-gated channels in the acrosomal membrane
(135) on the other hand. Unlike the plasma membrane Ca2+
channel responsible for the initial steroid-induced Ca2+
influx, both the PKC-dependent plasma membrane channel and the
cAMP-gated acrosomal channel are voltage-dependent (135). This may be
important because the channel involved in the initial Ca2+
influx shows a poor cation selectivity and its opening also leads to
Na+ influx (58). Together with Cl- efflux via
GABAA-like receptors/Cl- channels, which is
also directly activatable by P and related steroids in mammalian
spermatozoa (84, 85, 101, 136, 137), this Na+ influx seems
to be responsible for membrane depolarization events necessary for the
optimal function of these voltage-dependent channels. Sperm plasma
membrane depolarization in response to P has been experimentally
demonstrated (58). A testis-specific L-type voltage-dependent
Ca2+ channel has been cloned and suggested to be the
voltage-dependent plasma membrane Ca2+ channel employed in
the cell signaling by P in mammalian spermatozoa (124).
As to the intracellular Ca2+ release from the sperm acrosome, the involvement of the InsP3 released from membrane phosphoinositides by PIC stimulated through the steroid-induced Ca2+ influx is also possible as InsP3 receptors have been detected in spermatozoa of several mammalian species and localized to the outer acrosomal membrane of mouse spermatozoa (138). The need for these Ca2+ signal amplification events for the generation of biologically efficient response may explain why the P-induced acrosome reaction can be mimicked (57) or potentiated (139) by thapsigargin, a mobilizer of InsP3-sensitive intracellular Ca2+ stores. The observation of the P-induced Ca2+ waves and Ca2+ oscillations in human spermatozoa (137) also argues in favor of the existence of a complex Ca2+ exchange mechanism in these cells because such phenomena would be hardly compatible with the presence of only one mechanism of Ca2+ influx through a single type of plasma membrane channel. The reason why the inhibitors of voltage-dependent Ca2+ channels nifedipine and verapamil do not inhibit the P-induced [Ca2+]i increase in human spermatozoa (58) is not clear. A recent study suggests that a prolonged incubation with nifedipine is required for an efficient inhibition of the P-induced Ca2+ influx and acrosome reaction in human spermatozoa (124). There may also be interspecies differences relating to the relative importance of the Ca2+ influx and the Ca2+ release mechanisms in the amplification of the initial steroid-induced Ca2+ signal.
The above mechanisms of Ca2+ signal generation and amplification were essentially discovered and analyzed using spermatozoa of several mammalian species as a model. However, fragmentary data that are available from studies in other systems reveal the presence of similar events, although the order in which individual mechanisms are employed is sometimes different. Therefore, in human oocytes the initial Ca2+ signal induced by 17ß-E2 is also generated by the action of the steroid at the outer surface of the plasma membrane and relies on Ca2+ influx through a yet unknown type of channel; this influx is only secondarily amplified by Ca2+ release from intracellular stores (18). In contrast, the 17ß-E2-induced [Ca2+]i increase in chicken granulosa cells appears to be primarily due to Ca2+ release from InsP3-sensitive stores after the generation of InsP3 from plasma membrane phosphatidylinositol 4,5-biphosphate by PIC stimulated by the steroid (6). However, measurable [Ca2+]i rises in human granulosa cells in response to androstenedione are precluded both by voltage-dependent Ca2+ channel blockers and thapsigargin (V. Machelon, F. Nomé, and J. Tesarik, unpublished observation), suggesting either that the initial limited Ca2+ influx (below the sensitivity threshold of the technique employed) is subsequently amplified by Ca2+-induced Ca2+ release from internal stores or that an initial limited Ca2+ release is amplified by capacitative Ca2+ entry (store depletion-induced opening of plasma membrane Ca2+ channels) from the extracellular space. Thus, the principle of cross-amplification of the initial steroid-induced Ca2+ signal by a combination of Ca2+ influx and Ca2+ release appears to be a widely spread characteristic of nongenomic steroid effects although different cell types may differ as to the source of the initial [Ca2+]i increase triggering the amplification reaction.
B. Ca2+ signal transduction
A key molecule realizing the downstream transduction of the
steroid-generated Ca2+ signal (Fig. 2) is DAG resulting
from the Ca2+-dependent activation of PIC and of
phosphatidylcholine-specific phospholipase C (reviewed in Ref.127). In
addition to the activation of PKC (see above), a major role of DAG in
spermatozoa is to activate phospholipase A2 (140) with an
ensuing generation of fatty acids and lysophospholipids (141), which
then act as direct effectors of the biological response (membrane
fusion during the acrosome reaction).
Another mechanism by which the Ca2+ signal is transduced to effectors of membrane fusion is the hydrolysis of phosphatidylinositol-4,5-bisphosphate by the action of Ca2+-dependent PIC: the removal of phosphatidylinositol-4,5-bisphosphate alleviates its inhibitory effect on actin-severing proteins, which thus become active and depolymerize F-actin localized between the plasma membrane and the outer acrosomal membrane and representing a physical barrier to membrane fusion (135, 142). Last, but not least, the Ca2+-dependent, steroid-induced increase in sperm cAMP (79) is likely to activate cAMP-dependent PKA. In fact, the effect of FF (with P as the main active component) on the human sperm acrosome reaction can be inhibited by KT5720, a PKA inhibitor (97). Both of the latter two signal transduction pathways appear to require, in addition to Ca2+, a simultaneous stimulation through other, Ca2+-independent steroid response mechanisms, namely the protein tyrosine kinase system and the GABAA-like receptor/Cl- channel (see below).
C. The PTK system
The implication of protein tyrosine phosphorylation in transducing
the P-generated signal in spermatozoa has been suspected since the
observation that artificial, antibody-mediated cross-linking of
steroid-receptor complexes on the sperm surface leads to an additional
Ca2+ influx and induces the acrosome reaction in those
spermatozoa that fail to respond to the addition of the hormone alone
(89). A subsequent study confirmed the working hypothesis that, as in
other systems in which ligand-receptor binding leads to receptor
aggregation, P induces tyrosine phosphorylation in human spermatozoa;
this study also pointed out a 94-kDa sperm phosphoprotein as a major
Triton-soluble substrate for tyrosine phosphorylation in human
spermatozoa (78). This phosphorylation was later shown not to be
required for the initial P-induced Ca2+ influx (77, 143)
and, inversely, external Ca2+ was not required for the
P-induced stimulation of tyrosine phosphorylation (78).
A deeper analysis of the dynamics of the P effect on protein tyrosine
phosphorylation suggests that this effect is less rapid than the
P-induced Ca2+ response. This conclusion is based on the
following observations. First, ligand-induced aggregation of a human
sperm P receptor, visualized with the use of P conjugated with
fluorescein-labeled BSA, needed several minutes to develop (62) as
opposed to the several seconds that are required for the beginning of
the P-induced Ca2+ response (see above). Second, the
initial, PTK-independent P-induced [Ca2+]i increase,
monitored in single sperm cells, is followed, after 2–10 min, by a
second major [Ca2+]i increase, this time a PTK-dependent
one (100). Interestingly, this delayed, PTK-dependent
[Ca2+]i increase is only seen in about 10% of
spermatozoa, whereas the initial, PTK-independent [Ca2+]i
increase may be observed in 35% of the cells (100). Depending on the
sensitivity setting of the assay system, this initial P-induced
[Ca2+]i increase can be detected in up to 100% of viable
spermatozoa exposed to this hormone (75, 137). Because so many
spermatozoa can generate a Ca2+ response to P but so few
(
10%) actually bind the conjugate of P with fluorescein-labeled BSA
(71), activate protein tyrosine phosphorylation (78, 100), and develop
the biological response (71, 89), it has been suggested that
spermatozoa possess at least two types of plasma membrane steroid
receptor, one responsible for the Ca2+ influx, ubiquitous
and undetectable by fluorescent steroid conjugate binding, and the
other responsible for PTK activation, restricted to a small sperm
subpopulation and detectable by fluorescent steroid conjugate binding
(144). The stimulation of the former receptor may functionally activate
the latter because a gradual increase in sperm Ca2+ load
was related to the recruitment of new cohorts of spermatozoa capable of
steroid conjugate binding followed by the acrosome reaction (74). The
relatively slow response of the PTK-coupled steroid receptor may also
partly explain the reported failure to observe an increase in tyrosine
phosphorylation after a short (5 min) exposure of human spermatozoa to
P (76).
The mechanism by which the steroid-generated signal is transduced
downstream of PTK activation is largely unknown for the time being. The
delayed, PTK-dependent [Ca2+]i increase observed in human
spermatozoa (100) may be involved. If this is the case, this
[Ca2+]i increase is likely to be due to Ca2+
release from internal stores rather than to Ca2+ influx
because P exerts a genistein-sensitive stimulatory effect on the human
sperm acrosome reaction even in the actual absence of extracellular
Ca2+ provided that spermatozoa have been previously loaded
with Ca2+ using a Ca2+ ionophore (77).
Activation of an InsP3-gated Ca2+ channel by
tyrosine phosphorylation was reported in mouse T cells and was
suggested to modulate intracellular Ca2+ release during T
cell activation (145), and a similar mechanism may be involved in the
opening of the InsP3 receptor regulating Ca2+
release from the sperm acrosome. Furthermore, the P-activated PTK may
be required, together with Ca2+, for the activation of
PIC (135, 142), which is responsible for the generation of DAG that
subsequently acts to activate phospholipase A2, resulting
in the generation of fatty acids and lysophopholipids needed for
membrane fusion (see above).
There is also evidence for a cross-talk between the PTK and PKA signaling pathways because A-kinase anchor proteins (AKAPs), responsible for the targeting of PKA to specific subcellular locations, are major substrates of PTK in the Triton-insoluble sperm protein fraction (146). The phosphorylation status of AKAPs is controlled by Ca2+ via calmodulin-dependent protein tyrosine dephosphorylation (146). It remains to be determined whether AKAPs are also substrates of the P-activatable sperm PTK.
D. The GABAA-like receptor/Cl- channel
The involvement of Cl- efflux (136) through the
P-activated GABAA-like receptor/Cl- channel
represents a well documented feature of the P-induced acrosome reaction
(72, 85, 101). However, Cl- fluxes may not explain all the
effects supposedly mediated by the GABAA-like receptor in
spermatozoa, such as the increase in the intracellular concentration of
cAMP (79) or the stimulation of DAG generation (127). In fact the
GABAA-like receptor of mammalian spermatozoa appears to be
linked to the PTK system because the inhibition of PTK inhibits the
GABA-induced generation of DAG in mouse spermatozoa (127) and the
P-induced Cl- efflux in human spermatozoa (147). It is
possible that the ligand-bound GABAA-like receptor
comigrates with PTK, forming functionally active complexes in which PTK
is activated and activates, in its turn, PIC responsible for DAG
production and adenylate cyclase responsible for cAMP synthesis or
inactivates phosphodiesterase, thus slowing down cAMP degradation.
Alternatively, phosphorylation by PTK may be important for the function
of the GABAA-like receptor/Cl- channel. Such a
modulation has been reported for GABAA receptors in neurons
(148). These possible relationships still remain to be experimentally
assessed.
|
VI. Cross-Talk Between the Nongenomic and Genomic Responses of Cells to Steroids |
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TopAbstractI. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
|---|
). In this scenario, the resultant global steroid effect on the
cell will be a superimposition of the two distinct receptor-mediated
events of which one may condition or modulate the other. In fact, the
PKC pathway, known to be involved in some nongenomic steroid effects,
potentiates androgen-mediated gene expression of a mouse vas
deferens-specific protein (160), and the 17ß-E2-binding
mechanism of the human nuclear ER is regulated by tyrosine
phosphorylation (161). It is not known whether, inversely, events set
in motion by the genomic response mechanism can influence the
nongenomic response. It remains to be determined how different cell
types regulate the relative contributions of both kinds of response to
steroids in concrete physiological and pathological situations. |
VII. Conclusions |
|---|
|
TopAbstractI. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
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Acknowledgments |
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Footnotes |
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1 The writing of this review was supported by C.E.G. (Study Group in
Gynecological Chronoendocrinology), Torino, Italy. 
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References |
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TopAbstractI. IntroductionII. Nongenomic Actions of...III. Nongenomic Actions of...IV. Nongenomic Actions of...V. Signal Transduction Pathways...VI. Cross-Talk Between the...VII. ConclusionsReferences |
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-hydroxyprogesterone. Novel stimulators of
calcium influx in human sperm. J Biol Chem 265:1376–1380-aminobutyric acid on human sperm
motility and hyperactivation. Mol Hum Reprod 2:733–738 and ßI in bovine spermatozoa, and
their regulation by calcium and phorbol esters. Biol Reprod 56:454–459[Abstract]
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 T. R. Rankouhi, J. T. Sanderson, I. van Holsteijn, C. van Leeuwen, A. D. Vethaak, and M. van den Berg Effects of Natural and Synthetic Estrogens and Various Environmental Contaminants on Vitellogenesis in Fish Primary Hepatocytes: Comparison of Bream (Abramis brama) and Carp (Cyprinus carpio) Toxicol. Sci., September 1, 2004; 81(1): 90 - 102. [Abstract] [Full Text] [PDF]
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 O. Kretz, L. Fester, U. Wehrenberg, L. Zhou, S. Brauckmann, S. Zhao, J. Prange-Kiel, T. Naumann, H. Jarry, M. Frotscher, et al. Hippocampal Synapses Depend on Hippocampal Estrogen Synthesis J. Neurosci., June 30, 2004; 24(26): 5913 - 5921. [Abstract] [Full Text] [PDF]
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 F. Barletta, C.-W. Wong, C. McNally, B. S. Komm, B. Katzenellenbogen, and B. J. Cheskis Characterization of the Interactions of Estrogen Receptor and MNAR in the Activation of cSrc Mol. Endocrinol., May 1, 2004; 18(5): 1096 - 1108. [Abstract] [Full Text] [PDF]
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 A. R. Beker-van Woudenberg, H. T.A. van Tol, B. A.J. Roelen, B. Colenbrander, and M. M. Bevers Estradiol and Its Membrane-Impermeable Conjugate (Estradiol-Bovine Serum Albumin) During In Vitro Maturation of Bovine Oocytes: Effects on Nuclear and Cytoplasmic Maturation, Cytoskeleton, and Embryo Quality Biol Reprod, May 1, 2004; 70(5): 1465 - 1474. [Abstract] [Full Text] [PDF]
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C. A. Heinlein and C. Chang Androgen Receptor in Prostate Cancer Endocr. Rev., April 1, 2004; 25(2): 276 - 308. [Abstract] [Full Text] [PDF]
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 S. Cabodi, L. Moro, G. Baj, M. Smeriglio, P. Di Stefano, S. Gippone, N. Surico, L. Silengo, E. Turco, G. Tarone, et al. p130Cas interacts with estrogen receptor {alpha} and modulates non-genomic estrogen signaling in breast cancer cells J. Cell Sci., March 15, 2004; 117(8): 1603 - 1611. [Abstract] [Full Text] [PDF]
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 T. Sato, T. Matsumoto, H. Kawano, T. Watanabe, Y. Uematsu, K. Sekine, T. Fukuda, K.-i. Aihara, A. Krust, T. Yamada, et al. Brain masculinization requires androgen receptor function PNAS, February 10, 2004; 101(6): 1673 - 1678. [Abstract] [Full Text] [PDF]
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W. B. Nothnick, X. Zhang, and H.-E Zhou Steroidal Regulation of Uterine Edema and Tissue Inhibitors of Metalloproteinase (TIMP)-3 Messenger RNA Expression Is Altered in TIMP-1-Deficient Mice Biol Reprod, February 1, 2004; 70(2): 500 - 508. [Abstract] [Full Text] [PDF]
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 B. H. Welter, E. L. Hansen, K. J. Saner, Y. Wei, and T. M. Price Membrane-bound Progesterone Receptor Expression in Human Aortic Endothelial Cells J. Histochem. Cytochem., August 1, 2003; 51(8): 1049 - 1055. [Abstract] [Full Text]
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E. O. Pietrobon, M. De Los Angeles Monclus, A. J. Alberdi, and M. W. Fornes Progesterone Receptor Availability in Mouse Spermatozoa During Epididymal Transit and Capacitation: Ligand Blot Detection of Progesterone-Binding Protein J Androl, July 1, 2003; 24(4): 612 - 620. [Abstract] [Full Text] [PDF]
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 R. M. LOSEL, E. FALKENSTEIN, M. FEURING, A. SCHULTZ, H.-C. TILLMANN, K. ROSSOL-HASEROTH, and M. WEHLING Nongenomic Steroid Action: Controversies, Questions, and Answers Physiol Rev, July 1, 2003; 83(3): 965 - 1016. [Abstract] [Full Text] [PDF]
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Y. Zhu, C. D. Rice, Y. Pang, M. Pace, and P. Thomas From the Cover: Cloning, expression, and characterization of a membrane progestin receptor and evidence it is an intermediary in meiotic maturation of fish oocytes PNAS, March 4, 2003; 100(5): 2231 - 2236. [Abstract] [Full Text] [PDF]
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Y. Zhu, J. Bond, and P. Thomas Identification, classification, and partial characterization of genes in humans and other vertebrates homologous to a fish membrane progestin receptor PNAS, March 4, 2003; 100(5): 2237 - 2242. [Abstract] [Full Text] [PDF]
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 A. C. McDonnel, E. A. Van Kirk, D. D. Isaak, and W. J. Murdoch Inhibitory Effects of Progesterone on Plasma Membrane Fluidity and Tumorigenic Potential of Ovarian Epithelial Cancer Cells Experimental Biology and Medicine, March 1, 2003; 228(3): 308 - 314. [Abstract] [Full Text] [PDF]
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H. Schwarzenbach, P. R. Manna, D. M. Stocco, G. Chakrabarti, and A. K. Mukhopadhyay Stimulatory Effect of Progesterone on the Expression of Steroidogenic Acute Regulatory Protein in MA-10 Leydig Cells Biol Reprod, March 1, 2003; 68(3): 1054 - 1063. [Abstract] [Full Text] [PDF]
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S.A. Adeoya-Osiguwa, S. Markoulaki, V. Pocock, S.R. Milligan, and L.R. Fraser 17{beta}-Estradiol and environmental estrogens significantly affect mammalian sperm function Hum. Reprod., January 1, 2003; 18(1): 100 - 107. [Abstract] [Full Text] [PDF]
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C.-W. Wong, C. McNally, E. Nickbarg, B. S. Komm, and B. J. Cheskis Estrogen receptor-interacting protein that modulates its nongenomic activity-crosstalk with Src/Erk phosphorylation cascade PNAS, November 12, 2002; 99(23): 14783 - 14788. [Abstract] [Full Text] [PDF]
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C. A. Heinlein and C. Chang The Roles of Androgen Receptors and Androgen-Binding Proteins in Nongenomic Androgen Actions Mol. Endocrinol., October 1, 2002; 16(10): 2181 - 2187. [Abstract] [Full Text] [PDF]
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Y.-H. Suh and F. Checler Amyloid Precursor Protein, Presenilins, and alpha -Synuclein: Molecular Pathogenesis and Pharmacological Applications in Alzheimer's Disease Pharmacol. Rev., September 1, 2002; 54(3): 469 - 525. [Abstract] [Full Text] [PDF]
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 B. M. Jacobsen, J. K. Richer, S. A. Schittone, and K. B. Horwitz New Human Breast Cancer Cells to Study Progesterone Receptor Isoform Ratio Effects and Ligand-independent Gene Regulation J. Biol. Chem., July 26, 2002; 277(31): 27793 - 27800. [Abstract] [Full Text] [PDF]
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E. Garcia Dos Santos, M. N. Dieudonne, R. Pecquery, V. Le Moal, Y. Giudicelli, and D. Lacasa Rapid Nongenomic E2 Effects on p42/p44 MAPK, Activator Protein-1, and cAMP Response Element Binding Protein in Rat White Adipocytes Endocrinology, March 1, 2002; 143(3): 930 - 940. [Abstract] [Full Text] [PDF]
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J.J. Peluso, G. Fernandez, A. Pappalardo, and B.A. White Characterization of a Putative Membrane Receptor for Progesterone in Rat Granulosa Cells Biol Reprod, July 1, 2001; 65(1): 94 - 101. [Abstract] [Full Text] [PDF]
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L. O'Donnell, K. M. Robertson, M. E. Jones, and E. R. Simpson Estrogen and Spermatogenesis Endocr. Rev., June 1, 2001; 22(3): 289 - 318. [Abstract] [Full Text] [PDF]
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 J Dupont and D Le Roith Insulin-like growth factor 1 and oestradiol promote cell proliferation of MCF-7 breast cancer cells: new insights into their synergistic effects Mol. Pathol., June 1, 2001; 54(3): 149 - 154. [Abstract] [Full Text]
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A. Revelli, C. Costamagna, F. Moffa, E. Aldieri, S. Ochetti, A. Bosia, M. Massobrio, B. Lindblom, and D. Ghigo Signaling Pathway of Nitric Oxide-Induced Acrosome Reaction in Human Spermatozoa Biol Reprod, June 1, 2001; 64(6): 1708 - 1712. [Abstract] [Full Text]
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 G. P. H. Leung, S. B. Cheng-Chew, and P. Y. D. Wong Nongenomic effect of testosterone on chloride secretion in cultured rat efferent duct epithelia Am J Physiol Cell Physiol, May 1, 2001; 280(5): C1160 - C1167. [Abstract] [Full Text] [PDF]
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S.-H. Park, M. Taub, and H.-J. Han Regulation of Phosphate Uptake in Primary Cultured Rabbit Renal Proximal Tubule Cells by Glucocorticoids: Evidence for Nongenomic as Well as Genomic Mechanisms Endocrinology, February 1, 2001; 142(2): 710 - 720. [Abstract] [Full Text] [PDF]
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J. L. Maller The elusive progesterone receptor in Xenopus oocytes PNAS, January 2, 2001; 98(1): 8 - 10. [Full Text] [PDF]
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P. Thomas, J. Pinter, and S. Das Upregulation of the Maturation-Inducing Steroid Membrane Receptor in Spotted Seatrout Ovaries by Gonadotropin During Oocyte Maturation and Its Physiological Significance Biol Reprod, January 1, 2001; 64(1): 21 - 29. [Abstract] [Full Text]
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J. Tian, S. Kim, E. Heilig, and J. V. Ruderman Identification of XPR-1, a progesterone receptor required for Xenopus oocyte activation PNAS, December 8, 2000; (2000) 250492197. [Abstract] [Full Text]
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E. Falkenstein, H.-C. Tillmann, M. Christ, M. Feuring, and M. Wehling Multiple Actions of Steroid Hormones---A Focus on Rapid, Nongenomic Effects Pharmacol. Rev., December 1, 2000; 52(4): 513 - 556. [Abstract] [Full Text] [PDF]
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F.M. Lyng, G.R. Jones, and F.F.G. Rommerts Rapid Androgen Actions on Calcium Signaling in Rat Sertoli Cells and Two Human Prostatic Cell Lines: Similar Biphasic Responses Between 1 Picomolar and 100 Nanomolar Concentrations Biol Reprod, September 1, 2000; 63(3): 736 - 747. [Abstract] [Full Text]
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T. Morrison, L. Waggoner, L. Whitworth-Langley, and B. J. Stith Nongenomic Action of Progesterone: Activation of XenopusOocyte Phospholipase C through a Plasma Membrane-Associated Tyrosine Kinase Endocrinology, June 1, 2000; 141(6): 2145 - 2152. [Abstract] [Full Text] [PDF]
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A. K. Loomis and P. Thomas Effects of Estrogens and Xenoestrogens on Androgen Production by Atlantic Croaker Testes In Vitro: Evidence for a Nongenomic Action Mediated by an Estrogen Membrane Receptor Biol Reprod, April 1, 2000; 62(4): 995 - 1004. [Abstract] [Full Text]
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C. Cudeville, F. Mondon, B. Robert, R. Rebourcet, T.-M. Mignot, C. Benassayag, and F. Ferré Evidence for Progesterone Receptors in the Human Fetoplacental Vascular Tree Biol Reprod, March 1, 2000; 62(3): 759 - 765. [Abstract] [Full Text]
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 J. Onishi, M. Meinz, J. Thompson, J. Curotto, S. Dreikorn, M. Rosenbach, C. Douglas, G. Abruzzo, A. Flattery, L. Kong, et al. Discovery of Novel Antifungal (1,3)-beta -D-Glucan Synthase Inhibitors Antimicrob. Agents Chemother., February 1, 2000; 44(2): 368 - 377. [Abstract] [Full Text]
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Estrogen: Consequences and Implications of Human Mutations in Synthesis and Action J. Clin. Endocrinol. Metab., December 1, 1999; 84(12): 4677 - 4694. [Abstract] [Full Text]
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V. Giguère Orphan Nuclear Receptors: From Gene to Function Endocr. Rev., October 1, 1999; 20(5): 689 - 725. [Abstract] [Full Text]
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M. Campbell-Thompson, G. Y. Lauwers, K. K. Reyher, J. Cromwell, and K. T. Shiverick 17{beta}-Estradiol Modulates Gastroduodenal Preneoplastic Alterations in Rats Exposed to the Carcinogen N-Methyl-N'-Nitro-Nitrosoguanidine Endocrinology, October 1, 1999; 140(10): 4886 - 4894. [Abstract] [Full Text]
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B. Moosmann and C. Behl The antioxidant neuroprotective effects of estrogens and phenolic compounds are independent from their estrogenic properties PNAS, August 3, 1999; 96(16): 8867 - 8872. [Abstract] [Full Text] [PDF]
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S. A. Lund, J. Murdoch, E. A. Van Kirk, and W. J. Murdoch Mitogenic and Antioxidant Mechanisms of Estradiol Action in Preovulatory Ovine Follicles: Relevance to Luteal Function Biol Reprod, August 1, 1999; 61(2): 388 - 392. [Abstract] [Full Text]
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P. M. Wise, M. J. Smith, D. B. Dubal, M. E. Wilson, K. M. Krajnak, and K. L. Rosewell Neuroendocrine Influences and Repercussions of the Menopause Endocr. Rev., June 1, 1999; 20(3): 243 - 248. [Abstract] [Full Text]
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 J.L. Kornyei, Z. Vertes, A. Oszter, K.A. Kovacs, Ch.V. Rao, and M. Vertes Opioid peptides inhibit the action of oestradiol on human myometrial cells in culture Mol. Hum. Reprod., June 1, 1999; 5(6): 565 - 572. [Abstract] [Full Text] [PDF]
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B. S. Jaiswal, I. Tur-Kaspa, J. Dor, S. Mashiach, and M. Eisenbach Human Sperm Chemotaxis: Is Progesterone a Chemoattractant? Biol Reprod, June 1, 1999; 60(6): 1314 - 1319. [Abstract] [Full Text]
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M. Luconi, M. Muratori, G. Forti, and E. Baldi Identification and Characterization of a Novel Functional Estrogen Receptor on Human Sperm Membrane That Interferes with Progesterone Effects J. Clin. Endocrinol. Metab., May 1, 1999; 84(5): 1670 - 1678. [Abstract] [Full Text]
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 B. L. Lokeshwar, G. G. Schwartz, M. G. Selzer, K. L. Burnstein, S.-H. Zhuang, N. L. Block, and L. Binderup Inhibition of Prostate Cancer Metastasis in Vivo: A Comparison of 1,25-Dihydroxyvitamin D (Calcitriol) and EB1089 Cancer Epidemiol. Biomarkers Prev., March 1, 1999; 8(3): 241 - 248. [Abstract] [Full Text] [PDF]
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 P. K. Coyle Hormonal Effects on The Immune System Multiple Sclerosis, December 1, 1998; 4(6): 509 - 510. [PDF]
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 G. R. Ehring, H. H. Kerschbaum, C. Eder, A. L. Neben, C. M. Fanger, R. M. Khoury, P. A. Negulescu, and M. D. Cahalan A Nongenomic Mechanism for Progesterone-mediated Immunosuppression: Inhibition of K+ Channels, Ca2+ Signaling, and Gene Expression in T Lymphocytes J. Exp. Med., November 2, 1998; 188(9): 1593 - 1602. [Abstract] [Full Text] [PDF]
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T. Ördög, J. R. Goldsmith, M.-D. Chen, M. A. Connaughton, J. Hotchkiss, and E. Knobil On the Mechanism of the Positive Feedback Action of Estradiol on Luteinizing Hormone Secretion in the Rhesus Monkey J. Clin. Endocrinol. Metab., November 1, 1998; 83(11): 4047 - 4053. [Abstract] [Full Text]
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J. Tian, S. Kim, E. Heilig, and J. V. Ruderman Identification of XPR-1, a progesterone receptor required for Xenopus oocyte activation PNAS, December 19, 2000; 97(26): 14358 - 14363. [Abstract] [Full Text] [PDF]
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