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B and Steroid Receptor-Signaling Pathways
Laboratory of Signal Transduction, Molecular Endocrinology Group, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709
| Abstract |
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B (NF-
B)
B is a dimeric transcription factor
B is an inhibitor of NF-
B
B
B interacts with multiple transcription
factors and transcriptional co-factors
B family members in
immunity and development
B interactions
B and GR Antagonism: Physiological Significance?
B and Other Steroid Hormone Receptors | I. Introduction |
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B and glucocorticoid-mediated signal transduction cascades.
Glucocorticoid receptor-
(GR) and NF-
B are inducible
transcription factors with diametrically opposed functions in the
regulation of immune and inflammatory responses. NF-
B is known to
mediate the transcriptional activation of a variety of cytokines and
cytokine-induced genes involved in immunity. GR, long known to function
as a suppressor of immunity and inflammation, inhibits the expression
of many of the same cytokines and cytokine-induced genes that are
activated by NF-
B. However, because these genes have no identifiable
glucocorticoid responsive elements (GRE) either within their promoters
or their intragenic regions, the mechanism by which glucocorticoids
repress these genes was unclear.
Recent research indicates that GR and NF-
B physically interact
(1, 2, 3) and function as mutual transcriptional antagonists (1, 4). These
findings have rekindled interest in, and encouraged re-examination of,
these interactions as the basis for mutual functional antagonism of GR
and NF-
B. While physiological processes in a variety of tissues and
organ systems are regulated by glucocorticoids and are likely to be
modulated by GR interactions with the ubiquitous NF-
B, the
immune/inflammatory response has been the primary focus of current
research on NF-
B/GR antagonism. Since both these transcription
factors are known to be potent regulators of the immune system,
elucidation of the mechanisms by which GR and NF-
B negatively
interact not only provides the basis for understanding their role in
the precise control of normal immune response, but also opens up the
possibility of novel therapeutic intervention in immune pathology.
The purpose of this review is to describe our current knowledge of
NF-
B and GR-signaling pathways, including overviews of the NF-
B
and steroid hormone receptor families of transcription factors and
their regulatory proteins, their general mechanisms of action, the cell
types in which they function, and their regulated genes. Special
consideration will then be given to the newest findings concerning the
interactions between these two transcription factors, and the
physiological significance of these findings in terms of immunity and
inflammation. Finally, emerging data concerning interactions between
NF-
B and other steroid hormone receptors will be considered.
II. Nuclear Factor- B (NF- B)
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B was identified more than a decade ago by Sen and Baltimore
(5) as an enhancer-binding protein controlling Ig
-light chain gene
expression in B cells. This seminal paper identified the NF-
B
protein as an activity in electrophoretic mobility shift assays that
specifically retarded the migration of
-light chain enhancer DNA
containing the 10-mer sequence 5'-GGACTTCC-3'. Research that followed
this initial finding indicated that this
-enhancer binding activity
was only found in nuclear extracts from B cells that were at an
appropriate stage to express Ig
-light chain, and showed that
NF-
B was an essential factor for the function of the
-light chain
enhancer (6, 7). Because of this apparent B cell specificity, NF-
B
was considered to be a tissue-restricted transcription factor. However
NF-
B was later found to be expressed not only in B cells, but in
other immune cells as well. In non-B cell lymphocytes, NF-
B was
found to be an inactive protein sequestered in the cytoplasm, which
could be activated with phorbol ester and lipopolysaccharide, rather
than as the constitutively active nuclear protein seen in B cells
(8, 9, 10). Of particular interest was the finding that, in T cells with
latent human immunodeficiency virus (HIV) infection, NF-
B could be
activated by a variety of compounds that cause lymphokine secretion.
Once activated, NF-
B enhanced the expression of HIV in these cells
(10). This exciting finding led to a proliferation of research in the
fledgling field of NF-
B research.
Today, NF-
B is recognized as a ubiquitously expressed transcription
factor that can be activated in a wide variety of cell types. Despite
the discovery that NF-
B enjoys a wide pattern of cellular
expression, the particular importance of NF-
B to cells of the immune
system remains. In immune cells, NF-
B has now been shown to
positively regulate the expression of a wide variety of genes involved
in mammalian immune and inflammatory responses, including cytokines,
cell adhesion molecules, complement factors, antiapoptotic factors, and
immunoreceptors (for a detailed list, refer to Table 1
).
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B is a dimeric transcription factor
B functions as a dimeric DNA-binding protein that comprises
subunits from a family of related proteins called the Rel family of
transcriptional activators. The general mechanism of NF-
B signaling
by the prototypical p65/p50 heterodimer, with phosphorylation by the
catalytic subunit of protein kinase A (PKAc), is depicted in Fig. 1
B1), p100/p52 (NF-
B2), and the
Drosophila melanogaster proteins Dorsal (a dorsoventral
pattern formation gene) and Dif (which mediates immune response in
Drosophila larvae) (11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22). As shown in the accompanying
diagram, p50 and p52 are proteolytic cleavage products of precursor
proteins that possess both a Rel region and an I
B-like region
(described below). There is also evidence of a Rel-like protein in
Saccharomyces cerevisiae that is involved in cell growth
(23). Recently, a more distantly related and less well characterized
Drosophila transcriptional activator protein was identified.
Termed Relish, this protein possesses both Rel-like and I
B-like
(ankyrin repeats; see below) domains and is believed to regulate
embryogenesis and some immune responses (24). These Rel proteins
possess a highly conserved region of approximately 300 amino acids
known as the Rel homology domain. Contained within this Rel homology
domain are the DNA recognition/binding, dimerization, and nuclear
localization functions of NF-
B (25, 26, 27, 28). A more detailed description
of the domain structure of p65 is found in a subsequent section of the
text.
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B factors with different sequence specificities (22, 28, 29, 30, 31, 32).
While not all combinations of NF-
B dimers have been shown to form,
and not all dimers that can form have been shown to have physiological
relevance, several transcriptionally active NF-
B dimers have been
identified, including relB/p50, Rel B/p52, p65/p50, p65/p65, p65/c-Rel,
and possibly p50/p50. The relevance of p50 homodimers ability to
induce transcription in yeast and cell-free systems compared with its
function in mammalian cells remains a matter of dispute (see Refs.
33, 34, 35, 36, 37 for varying viewpoints).
Since p50 and p52 lack the transactivation domain found in the other
Rel family members, it is not clear whether homodimers of these
proteins have any transcriptional activity in native environments. One
study (38) indicates that p52 homodimers can directly transactivate
when associated with the I
B-like protein bcl-3. In
general, however, it is believed that an NF-
B dimer must possess at
least one Rel transactivation domain to activate transcription in
vivo. The most widely accepted function for p50 and p52 homodimers
is as transcriptional repressors that block NF-
B sites in the DNA
from transcriptionally active NF-
B dimers (39). Evidence for a
repressive function of p50 homodimers was found in human T cells, where
p50 homodimers constitutively expressed in the nucleus bind DNA and
repress NF-
B-dependent transcription. This repression was overcome
by overexpression of bcl-3, which has been shown to prevent
binding of p50 homodimers to NF-
B sites in vitro and to
activate NF-
B-mediated transcription in cultured cells (35, 40).
Whether this observed bcl-3-mediated increase in
transcription is entirely due to its repressive effect on p50
homodimers is unclear, however, because bcl-3 can function
as a transactivator at NF-
B sites when it trimerizes with otherwise
transcriptionally inactive p52 homodimers (38).
The composition of NF-
B dimers determines sequence specificity by
combining different DNA recognition loops in the Rel homology domains
of different subunits. For example, p65 homodimers preferentially bind
DNA enhancer motifs that are not well recognized by p65/p50
heterodimers or p50/p50 homodimers (31). Such variety in dimerization
may contribute to the cell type specificity of NF-
B response. For
example, cell type differences in NF-
B activity might depend on
which subunits of NF-
B are expressed (and in what ratios) and on the
NF-
B activating signals to which these different cells respond.
Dimer composition may also affect interactions with
inhibitory/regulatory proteins such as the various members of
the I
B family of proteins (22, 28, 32, 35, 41, 42, 43). The ability to
modulate gene expression in a cell type-specific or temporally
regulated manner by altering the expression patterns of various Rel
subunits affords NF-
B-mediated signal transduction the potential for
both great flexibility and tight control.
Several gene knockout mice that are deficient in one or more specific
NF-
B subunits have been generated. The phenotypes of these
transgenic animals vary widely and will be discussed in detail in a
later section of this review. When taken as a whole, the information
gleaned from NF-
B knockouts solidifies the importance of NF-
B to
immune function, inflammatory response, and development.
p65/p50: the "Classical" NF-
B. The prototypical and
most thoroughly studied NF-
B dimer is the p65/p50 heterodimer. When
these two subunits are coexpressed at comparable levels in the cell,
their affinity for each other is higher than the affinity of either
homodimer, and therefore the heterodimer is preferentially formed (22).
This heterodimer has a high affinity for the consensus NF-
B DNA
sequence 5'-GGGRNNYYCC-3' (22, 28) and is generally considered to be
the predominant, inducible (i.e., activatable by
extracellular signals, nonconstitutive) form of NF-
B in most cells.
For this reason, throughout the remainder of this text, the term
NF-
B will refer to the "classical" NF-
B p65/p50 heterodimer
unless otherwise specified.
As the transcriptionally active subunit of the prototypical NF-
B
heterodimer, the structure of the p65 (Rel A) polypeptide has been
extensively studied. The transactivation function of p65 has been
mapped to a 120-amino acid region of the C terminus, which contains two
distinct and independent transactivation domains (27). The first domain
comprises 30 amino acids at the extreme C terminus and is known as
TA1, whereas TA2 is located within the adjacent
90 amino acids of the C-terminal region of p65. These transactivation
domains share a common sequence motif that is induced to form an
-helical conformation when bound to target molecules (27, 44).
Phorbol ester-induced phosphorylation of the TA2 domain
between amino acids 442 and 470 has been shown to enhance the
transcriptional activity of p65 (44). Recent crystallographic data (45)
have also revealed the structure of DNA-bound p65/p50. These data
demonstrate that the subunits dimerize via ß-sheet sandwich
structures in the C-terminal dimerization domains, and contacts with
the DNA consensus sequence are made using loops from the ends of the N-
and C-termini. Beg et al. (46) have shown that the nuclear
localization sequence located in the middle of the p65 polypeptide is
the target for interaction with the inhibitory subunit I
B.
C. The regulatory subunit I
B is an inhibitor of NF-
B
Not long after the initial identification of NF-
B, Baeuerle and
Baltimore (47) identified an inhibitory protein of 6070 kDa that
specifically associates with NF-
B dimers, forming a trimer that
cannot bind DNA and is retained in the cell cytoplasm. They termed this
protein I
B, for inhibitor of NF-
B. More recently, there have been
multiple I
B proteins identified with different specificities for
NF-
B dimers and different cell type distributions. This family of
I
Bs now includes I
B
, I
Bß, I
B
, I
B-R, Bcl-3,
p50/p105(C-terminus), p100/p52(C-terminus), and the Drosophila
melanogaster proteins Cactus and (probably) Relish (14, 15, 16, 17, 21, 24, 28, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57). The I
B family of proteins is outlined schematically
in Fig. 3
. There is also evidence that
the yeast Saccharomyces cerevisiae expresses an I
B-like
protein (23). I
B
and I
Bß both preferentially interact with
dimers containing p65, but they are responsive to different signaling
pathways in the cell and may be differentially expressed and employed
in different cell types (28, 58). The p100/p52 and p50/p105 molecules
contain both an NF-
B-like and an I
B-like region. The I
B region
can be cleaved from the NF-
B region to yield two functional and
distinct molecules or can act intramolecularly to inhibit the function
of the NF-
B portion of the intact molecule.
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Bs identified thus far are the presence
of multiple ankyrin repeat motifs, involved in the protein-protein
interaction between NF-
B and I
B, an acidic region in the C
terminus of the proteins, and a C-terminal PEST (pro-glu/asp-ser-thr)
sequence (21, 51). Whereas the acidic region of the I
Bs is believed
to be involved in interaction with the NF-
B DNA-binding/nuclear
localization region, and the PEST sequence may be involved in
regulating I
B degradation, the functions of these two regions have
not been conclusively demonstrated and remain a matter of debate.
(Although beyond the scope of this review, evidence supporting both
sides of this question is discussed in Ref. 21).
D. Activation and function of NF-
B
Constitutive NF-
B activity in B cells is largely attributable
to the p50-cREL heterodimer (28, 59, 60). However, in most other cell
types, NF-
B activity is largely inducible, and the prototype p65/p50
NF-
B is retained in the cytoplasm of an unstimulated cell by its
association with I
B
. When an extracellular stimulus, such as a
cytokine (e.g., tumor necrosis factor-
), viral protein,
lipopolysaccharide, or an oxidative stressor activates the
NF-
B-signaling pathway, I
B
is phosphorylated at serines 32 and
36 (61). Phosphorylated I
B
is a target for ubiquitination at
lysines 21 and 22 (21, 62, 63), which leads to rapid removal of
I
B
via the proteosomal degradation pathway and, consequently,
unmasking of the p65 nuclear localization sequence and movement of
NF-
B to the nucleus of the cell (64, 65). [There are two different
but related ser/thr kinases, identified by several groups and now
termed IKK-1 and IKK-2, that are generally accepted to be responsible
for the inducible phosphorylation of I
B (66, 67, 68, 69). A very recent
study indicates that these IKKs have a greater phosphorylation activity
on I
B when it is bound to NF-
B, thus explaining the ability of
active IKK and free I
B to coexist within a cell and regulate NF-
B
(70). Regulation of the activity of these kinases provides an indirect
mechanism for the regulation of NF-
B activation.]
Although not yet completely understood, the activation of NF-
B
involves not only the dissociation of I
B and translocation to the
nucleus, but also phosphorylation of the p65 subunit by PKAc. This PKAc
is associated with the NF-
B/I
B heterotrimer in the cytoplasm and
is believed to be held inactive by the association with I
B. Once
I
B dissociates, the PKAc becomes active, phosphorylates serine 276
of p65 (at a consensus PKA site in the Rel domain of the protein), and
dramatically increases the transcriptional activity of NF-
B (71).
Recent studies show that this PKA-mediated phosphorylation of p65 is
required for recruitment of CREB-binding protein and the closely
related p300 (CBP/p300), a transcriptional coactivator discussed
below, by NF-
B p65 (72). There is evidence that other protein
kinases may also be important regulatory proteins for p65. For example,
casein kinase II has been shown to mediate the interleukin-1
(IL-1)-stimulated phosphorylation of p65 in fibroblasts and hepatoma
cells (73). It is possible that the phosphorylation required for p65
activation is mediated by various kinases in a cell type-specific
manner.
The classical p65/p50 NF-
B heterodimer is a major inducer of
inflammatory genes, and there are a large number of proinflammatory,
extracellular signals that can activate NF-
B. These stimuli include
viruses [such as herpes simplex virus and adenovirus] and
viral proteins; bacterial products like lipopolysaccharide;
inflammatory cytokines like TNF
, IL-1, and IL-2; and a variety of
DNA-damaging agents and oxidative stressors (reviewed in Ref. 22). Once
activated, the nuclear NF-
B binds to cognate NF-
B sites in the
chromatin and modulates gene expression. A variety of
NF-
B-responsive genes involved in immune response and inflammation
due to these extracellular proinflammatory signals are induced by
activated NF-
B. New synthesis of I
B causes retention of NF-
B
in the cytoplasm and attenuation of NF-
B-mediated transcriptional
activation and provides a feedback mechanism for modulating the extent
and duration of inflammatory responses by the cell.
Genes that are known to be regulated by NF-
B are listed in Table 1
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In addition to the well studied role of NF-
B in the immune system
(including lymphocyte development, inflammatory response, and host
defense mechanisms), more recent work has also implicated NF-
B as an
important regulator of apoptosis and embryonic development (74).
E. The transcription factor NF-
B interacts with multiple
transcription factors and transcriptional cofactors
With the advent of yeast two-hybrid screening techniques, a
variety of enhancer binding transcription factors have been found to
interact with a variety of nuclear proteins. Many of these interacting
nuclear proteins have been identified as transcriptional cofactors,
which have the ability to either enhance or repress the transactivation
function of transcription factors (for example, see Refs. 75, 76, 77). As
knowledge in the field of transcriptional regulation expands, a
consensus is forming that transcription factors do not usually function
alone as independent activators of transcription. Rather than the
classical view that these proteins bind DNA and then directly contact
the basal transcriptional machinery, the current understanding is that
DNA-binding transcription factors often interact simultaneously with
multiple nuclear proteins that are either transcription factors
themselves or accessory proteins. The resulting large
transcription-regulatory complexes interact with the basal
transcription apparatus to modulate gene expression. The combination of
transcription-regulatory proteins "piled up" on the promoter of a
gene allows for cell type, developmental, promoter, and stimulus-driven
specificities in gene expression regulated by a transcription factor.
Examples of such promoter complexes are depicted in reports by Merika
et al. (75), which deals with the "enhanceosome"
complex, and Shibata et al. (78), which examines complexes
containing members of the nuclear receptor superfamily.
As the list of proteins that interact with NF-
B and alter its
transactivation function grows, it is becoming clear that NF-
B is no
exception to this general rule of transcription-regulatory complex
formation. The complexity and potential flexibility of NF-
B effects
in the cell are suggested both by the broad range of
NF-
B-interacting proteins that have been identified and by the fact
that some of these proteins appear to have context-dependent effects,
acting antagonistically with NF-
B in some cases and synergistically
in others. The p65 subunit of NF-
B has been shown to directly
interact in vitro with the general transcription factors
TFIIB and TATA-binding protein (TBP) (79). A functional interaction
between TBP and p65 (as well as c-Rel), which activates
NF-
B-dependent transcription, has been shown in cultured insect and
mammalian cells (79, 80, 81, 82). These data reveal a potentially
important mechanism for NF-
B-mediated regulation of the basal
transcriptional apparatus.
As previously mentioned, Bcl-3 was originally identified to
be an inhibitory subunit of NF-
B and is now known to function as a
specific transcriptional coactivator of the p52 and p50 members of the
Rel family (38). HMG-1 (high mobility group-1) coactivates
NF-
B-mediated transcription, and SRC-1 specifically coactivates
NF-
B p50 (83, 84). Members of the CCAAT-enhancer binding protein
(C/EBP) family of bZIP proteins (C/EBP
, C/EBPß, and
C/EBP
) bind multiple members of the Rel family (p65, p50, and Rel).
While C/EBP
and C/EBP
function as corepressors (85), C/EBPß can
either repress or synergize with NF-
B. On the acute-phase response
element of the rat angiotensinogen gene, NF-
B and C/EBPß
antagonize by competing for overlapping binding sites in the DNA (86),
while on the promoters for IL-6 and IL-8, NF-
B and C/EBPß activate
transcription synergistically apparently by cooperative DNA binding
(87, 88). Members of the signal transducers and activators of
transcription (STAT) family of transcriptional regulators have also
been shown to interact with NF-
B. STAT6, normally a coactivator
protein, has also been shown to function as a corepressor for NF-
B,
presumably by competing with NF-
B for overlapping binding sites in
the promoter region of the E-selectin gene (89). Conversely, STAT1 and
NF-
B have been shown to synergistically activate transcription of
proinflammatory genes in mouse fibroblasts, although the mechanism of
this interaction is not well defined (90). The transcriptional
coactivator protein CBP/p300 has been shown to interact with many
transcription factors, and its interactions with p65 are a focus of
much current research. Not only has CBP/p300 been shown to be a
coactivator of p65-mediated transactivation (72, 91), but the specific
interaction between p65 and CBP/p300 has been shown to be required for
assembly of the "enhanceosome," an assembly of transcriptional
cofactors that synergize and mediate transcription of interferon-ß
(75).
In addition to interaction with a variety of transcriptional cofactors,
NF-
B is also capable of interaction with other nuclear proteins that
are themselves activatable transcription factors. An example of such an
interaction is found between NF-
B and AP-1 (Fos-Jun). When these two
dimeric transcription factors interact via the Rel homology domain of
p65 and the bZIP region of Fos and Jun, the transactivation function of
each factor is potentiated (92). NF-
B and the GR provide another
example of NF-
B/transcription factor interaction. For years, the
mechanism by which glucocorticoids exerted their potent
antiinflammatory effects was not understood. It is now clear that
NF-
B and the ligand-dependent GR can directly interact. In contrast
to the potentiation of function with NF-
B and AP1, the result of the
GR/NF-
B interaction is usually mutual transcriptional antagonism.
(For angiotensinogen and probably several other hepatic acute-phase
reactant genes, however, it appears that NF-
B and GR positively
interact at the acute phase response element to activate
transcription (93, 94, 95).) NF-
B/GR antagonism provides a fundamental
mechanism for the regulation of many immune and inflammatory responses
and therefore will be discussed in detail in a separate section of the
text.
Other members of the steroid receptor family have also been found to
interact with NF-
B. Androgen receptor (AR) and NF-
B have been
shown to be mutual antagonists in cultured cell systems, and evidence
for such antagonism in rat liver in vivo has also been
demonstrated (4, 96). For progesterone receptor (PR) and estrogen
receptor (ER), there is also evidence of negative interaction with
NF-
B, although the reciprocal antagonism of NF-
B p65 by PR and ER
appears to be a cell type-specific phenomenon (4, 97, 98).
F. Transgenic animals suggest a complex role for NF-
B family
members in immunity and development
As mentioned earlier, the generation of transgenic mice with
deficiencies in specific NF-
B subunits has underscored the
importance of this family of transcription factors in development and
the immune system. The phenotypic severity of the different knockout
animals ranges from deficiencies in specific immune responses to
embryonic lethality. The most dramatic Rel family knockout is p65/RelA,
whose phenotype is embryonic lethal due to massive apoptosis in the
liver. In addition, cultured embryonic fibroblasts from these animals
demonstrate profound deficiencies in inducible NF-
B activity, but
unaffected basal NF-
B activity. These findings point to p65 as being
critical for activatable, but not constitutive, NF-
B activity and
suggests a significant role for p65 in both embryonic development and
regulation of apoptosis (99).
The remainder of Rel knockout animals display normal embryonic development, but all show serious impairment of immune function. Immune deficiencies vary with each Rel protein knocked out, and in different cell types with a single type of Rel knocked out, supporting the idea of cell type specificities in the function and expression of the Rel family members. Knockouts of c-Rel (100) have profoundly impaired T cell function and proliferation as well as lymphoid hyperplasia. Knocking out Rel B results in multiorgan inflammation, T cell deficiencies, an inability to differentiate dendritic cells that cannot be compensated by expression of other Rel subunits, myeloid hyperplasia, and extramedullary hemopoiesis (28, 100). p50/p105 Knockouts exhibit B cells deficient in antibody production and proliferation (74). Interestingly, p105 knockouts (which express functional p50 but no inhibitory portion of the precursor molecule and consequently have enhanced p50 activity) suffer chronic organ inflammation, susceptibility to bacterial infection, and lymphoid hyperplasia. These different and opposing phenotypes in B cells, T cells, and macrophages suggest that the p50 homodimer functions as either a transactivator or transrepressor in a cell type-specific manner (101). Knockouts of p100 (which express functional p52 but no inhibitory portion of the precursor) die in the early postnatal period from gastric hyperplasia and exhibit overactive immune responses such as lymphocyte overproliferation and increased cytokine production (102). Animals that lack bcl-3 expression can neither mount T helper responses nor antigen prime B or T cells (103).
The creation of double knockouts in Rel family members has revealed some unexpected phenotypes and suggests that Rel members may have redundant functions, again speaking to the importance of maintaining Rel function for proper immune response and development. For example, when p50 and p52 are both disrupted, mice are incapable of maturing osteoclasts and B cells. This leads to a phenotype of severe osteopetrosis and abnormal splenic/thymic architecture (104). Double p50/RelB knockouts die in the early postnatal period from massive organ inflammation that is far more severe than seen in a single-knockout animal (105).
| III. Steroid Hormones/Receptors: Glucocorticoids and the Glucocorticoid Receptor (GR) |
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The steroid hormone receptors are a family of structurally similar,
modular proteins that include GR forms
and ß, PR forms A and B,
mineralocorticoid receptors (MR), and ER forms
and ß. Steroid
hormone receptors belong to the nuclear receptor superfamily. All
members of the nuclear receptor superfamily are believed to share a
common ancestry, and all function via a similar mechanism of action, as
ligand-dependent, DNA-binding transcription factors that interact with
the basal transcriptional apparatus. Other classes of nuclear receptors
include the thyroid, retinoid, and orphan receptors (114). Orphans are
receptors with structural/sequence similarity to other nuclear
receptors but for whom a ligand and/or function has not been
identified.
Members of the steroid/thyroid/retinoid receptor superfamily share a
variable amino-terminal transactivation domain, a central and well
conserved DBD, and a moderately conserved carboxy-terminal domain
responsible for binding ligand (110, 114). Within the steroid hormone
receptor family, ER is the least conserved member. It has a highly
homologous DBD but differs significantly from the other steroid
receptors in primary structure outside this region. Because ER differs
from the other steroid receptors and shares homology with thyroid
hormone receptors, it is considered by some to be a separate subfamily
within the steroid hormone receptor family. A schematic diagram
comparing the primary structure of the steroid hormone receptors is
found in Fig. 4
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-form of the GR (GR
) as a specific example.
As described above, GR
is a ubiquitously expressed (115)
phosphoprotein of the steroid/thyroid/retinoid receptor superfamily.
When GR
and other steroid receptors form high-affinity bonds with a
cognate ligand, they interact with specific DNA sequences in the genome
and function as transcription factors. Glucocorticoids are lipophilic steroid hormones and, as such, are found in circulation associated with "carrier" proteins such as corticosteroid-binding globulin (in the case of endogenous steroids) and albumin (for both endogenous and synthetic steroids) (116). The most widely accepted mechanism for glucocorticoid entry into the cell is by free diffusion of these lipophilic molecules across the lipid bilayer of the cell into the cytoplasm. However, there is some evidence (107, 108, 117) suggesting that glucocorticoid entry into cells is a regulated process involving specific membrane-associated receptors distinct from the classical intracellular GRs that are discussed in detail here. These membrane-associated receptors are believed to signal via G proteins and may also mediate the rapid, nongenomic effects of glucocorticoids that have been observed, particularly in neuronal tissues (108, 118).
In the absence of glucocorticoid hormone, classical GR
is retained
in the cytoplasm in an inactive (i.e., DNA
binding-incompetent) state by its association with the regulatory heat
shock proteins hsp 90 and hsp 56 (119, 120). The inactive conformation
of GR exhibits high affinity for ligand. Once inside the cell,
glucocorticoid binds the high-affinity cytoplasmic GR and induces a
poorly understood process known as receptor activation. Activation of
GR involves a change in receptor conformation, dissociation from
regulatory heat shock proteins, and hyperphosphorylation. Activated
receptor rapidly translocates to the cell nucleus and binds to specific
DNA sequences as a homodimer (121, 122, 123, 124). The DBD/dimerization domain of
GR has been well characterized (125) and consists of two zinc ions
coordinated with eight cysteine residues to form two peptide loops
called zinc fingers. Each zinc finger is followed by an amphipathic
-helix. GR DBDs bind cooperatively to specifically spaced target
half-sites in the DNA, and this specific DNA association induces
receptor dimerization. The subunits of GR then interact with adjacent
major grooves of the DNA via their amphipathic
-helices. The
specific palindromic sequences in the promoter regions of
glucocorticoid-responsive genes to which activated GR dimer binds are
known as GREs, or glucocorticoid responsive elements. The consensus GRE
sequence is: 5'- GGTACA nnnTGTTCT-3' (126). Once bound to a GRE,
homodimeric GR induces or increases transcription of the target gene,
presumably by interacting with the basal transcription apparatus (127).
In addition to its function as a ligand-dependent activator of
transcription, GR also functions as a negative regulator of
transcription in a specific subset of glucocorticoid-responsive genes.
Many of these glucocorticoid-repressed genes contain a negative GRE
(nGRE). nGREs are less well defined than positive GREs, and their
gene-repressive function appears to be context dependent. The mechanism
of action for GR on an nGRE likely involves displacement of a positive
regulatory protein from the promoter (126, 128, 129).
Recently, research in our own and other laboratories has focused on a
variant form of human GR, termed GRß, which arises from alternative
splicing of the GR mRNA transcript. Originally cloned along with the
GR
isoform in 1985 (130), GRß was largely ignored because it was
not found to bind ligand or activate transcription of
glucocorticoid-responsive reporter genes (130, 131). GRß is identical
to GR
through the first 727 amino acids but diverges in the carboxy
terminus, where it lacks the last 50 amino acids found in GR
but has
an additional 15 nonhomologous amino acids past the point of
divergence. As with GR
, GRß is widely expressed in adult and fetal
human tissues but differs from GR
in that it is localized to the
cell nucleus independent of the presence of ligand. The most recent
results from transient transfection assays demonstrate that human (h)
GRß functions as a dominant negative regulator of hGR
transactivation (Refs. 132, 133 ; R. H. Oakley and J. A.
Cidlowski, unpublished observations). Although the extent to which
hGRß functions as a dominant negative in vivo is not yet
clear (132, 133, 134), these studies suggest that tissue- or cell
type-specific expression patterns of hGRß and hGR
may function to
modulate the glucocorticoid responsiveness of tissues.
C. Glucocorticoid physiology
Glucocorticoids are synthesized in the zonae
fasciculata/reticularis of the adrenal cortex and released into
circulation in response to a wide range of stressful stimuli
(e.g., starvation, pain, surgery, trauma, emotional stress,
extreme heat or cold, and cellular damage). Their release is
orchestrated by the hypothalamic-pituitary-adrenal (HPA) axis, where
hypothalamic CRH acts on the pituitary to cause release of ACTH, and
ACTH then stimulates the adrenal gland to release glucocorticoid.
Glucocorticoids have a vast array of functions within the body, and an organism cannot survive without them (135). One of the first identified functions of glucocorticoids, and the one from which their name was derived, is as an important regulator of intermediary metabolism. Their main role in intermediary metabolism is to up-regulate the process of gluconeogenesis in the liver, kidney, and skeletal muscle, which results in elevated levels of blood glucose. Glucocorticoids also modulate fat, protein, and glutamine metabolism, as well as bone turnover. The importance of glucocorticoids in regulation of metabolism is underscored by their role in the "metabolic syndrome," a collection of metabolic disorders including hyperlipidemia, hyperinsulinemia, insulin resistance, and hypertension, which is believed to be caused in part by hypersensitivity to cortisol (136, 137). Immune cell apoptosis is also known to be a glucocorticoid-regulated process (138), and there are glucocorticoid effects on the central nervous system that are as yet poorly understood. In addition to broad effects in the adult organism, glucocorticoids are known to have profound effects on fetal development and parturition and are especially important to fetal lung maturation.
Some of the most dramatic and clinically relevant effects of glucocorticoids are those on the immune system. Glucocorticoids are potent suppressors of immune response and inflammation. These characteristics have made synthetic glucocorticoids the drug of choice for therapeutic intervention in a broad range of autoimmune and inflammatory disorders. The benefits of glucocorticoids in treatment of rheumatoid arthritis were first recognized nearly 60 yr ago (139). Currently, a variety of synthetic glucocorticoids are employed in the treatment of systemic lupus erythematosus, inflammatory bowel disease, psoriasis, eczema, and asthma (3, 140, 141, 142). They are also used to suppress the host immune system and prevent rejection during organ transplantation. Interestingly, glucocorticoids enjoyed widespread use in the clinic, based primarily on empirical evidence of their efficacy, even before much information was available concerning their mechanisms of antiinflammatory and immunosuppressive action. Today, multiple mechanisms of antiinflammatory and immunosuppressive glucocorticoid action have been put forth in the literature, but the relative importance of these mechanisms is still unclear, and there is a good possibility that additional mechanisms are yet to be discovered.
The general mechanisms by which glucocorticoids exert their cellular
effects have been elucidated over the past 15 yr, yet specifically
understanding the immunosuppressive mechanism of glucocorticoid action
has proven to be a conundrum. Although glucocorticoids are capable of
reducing the number of lymphocytes both by redistribution of peripheral
lymphocytes to the lymph nodes and by inducing lymphocyte apoptosis and
growth suppression in the thymus (143, 144, 145), these functions are
insufficient to explain the potent immunosuppressive action of
glucocorticoids. Glucocorticoids are also known to suppress the
expression of proinflammatory cytokines, which are key regulators of
the immune response. As outlined above, glucocorticoids are now known
to act via a cytosolic receptor that functions as a ligand-dependent
transcription factor, modulating the expression of genes with GREs in
their promoters. However, the majority of proinflammatory genes (such
as cytokines) that are suppressed by glucocorticoids have no such
responsive elements in their promoters, which might explain the role of
glucocorticoids in their regulation. It was not until the role of
NF-
B in the immune system and interactions between NF-
B and GR
were identified that feasible mechanisms for the powerful
glucocorticoid-mediated immune suppression could be proposed.
D. GR/NF-
B interactions
GR activation results in altered expression of many genes that
affect a variety of cellular processes. A listing of genes that are
regulated by glucocorticoids can be found in Table 2![]()
. Brief
inspection of Table 2
indicates that many genes involved in immune and
inflammatory responses in the cell, particularly proinflammatory
cytokines and cell adhesion molecules, are modulated by
glucocorticoids.
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B-induced genes
in Table 1
B and GR are physiological antagonists. A blockage of
NF-
B-mediated transcriptional activation by GR, rather than direct
repression of proinflammatory genes by GR, could reconcile the
GR-mediated repression of these genes with their lack of nGREs. For
this reason, the interaction between NF-
B and GR in immunity and
inflammation has become an active and rapidly expanding area of
research. Possible mechanisms of NF-
B/GR antagonism are presented
below.
1. Evidence for direct NF-
B/GR interactions. It has been
determined that GR and the p65 subunit of NF-
B physically interact,
and the consensus is that this (presumably direct) physical interaction
involves the Rel homology domain of p65 and the DBD of GR (Refs. 1, 2, 3, 4, 146 and our unpublished observations). It has also been
demonstrated that the physiological antagonism of GR and NF-
B is
based on a mutual transcriptional antagonism, rather than an alteration
in the expression levels of these two transcription factors in the cell
(4). Initial hypotheses concerning NF-
B/GR antagonism focused on the
possibility that the physical interaction between these two factors
impaired the DNA-binding functions of each and therefore blocked the
ability of each to activate transcription. However, subsequent studies
on this question have provided contradictory results, and little
conclusive evidence exists suggesting that the DNA-binding function of
either transcription factor is impaired under conditions in which a
mutual transcriptional antagonism occurs (2, 3, 147, 148). In fact, we
have observed that NF-
B p65 is capable of blocking GR
transactivation of a reporter gene (4), but cannot interfere with
homologous down-regulation of GR (a process that requires GR to bind
intragenic GREs; J. C. Webster and J. A. Cidlowski,
unpublished observations). This argues against a mechanism of
repression that involves a simple blockage of DNA binding.
To further address the question of mutual transcriptional repression,
other possible mechanisms that might drive the NF-
B/GR antagonism
were considered by several groups. One approach taken by our group was
to functionally dissect both the GR and NF-
B to determine which
subunits/domains of these transcription factors are required for mutual
transcriptional repression. A summary of these findings is presented in
Fig. 5
. First, it was determined that the
p65 subunit, but not the p50 subunit, of the NF-
B heterodimer could
repress GR-mediated transactivation of a reporter gene. The ability of
a series of GR deletion mutants (missing portions of the ligand-, DNA
binding-, or transactivation domain) to negatively interact with p65
was then assessed. These experiments showed that multiple domains of GR
are required to repress p65 transactivation. While the importance of
the DBD of GR to the p65-GR interaction was already known (1, 3), the
data demonstrated that deletion of either one of the two zinc fingers
that are found in this domain, as well as deletion of the
steroid-binding domain or large portions of the transactivation domain
of GR, also abolishes the repressive effect of GR on NF-
B.
Interestingly, the reciprocal repression of GR by p65 was less
selective, since all transcriptionally active mutants of GR were
repressed by p65. These findings suggested that the reciprocal
physiological antagonism between NF-
B and GR might not be based on
entirely reciprocal mechanisms of transcriptional antagonism. One
mutant GR that was examined is both constitutively nuclear and
transcriptionally active (since it has no ligand-binding domain and is
ligand independent), yet it is still repressed by p65, suggesting that
the mechanism of p65-GR antagonism does not involve cytoplasmic
sequestration of the factors, but rather occurs within the nuclear
compartment (4).
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B requires multiple domains
of both proteins (1, 4, 146, 149), while physical interaction requires
only a subset of those domains (the GR DNA-binding region and the p65
Rel region), argues against a simple mechanism of antagonism in which
direct physical contact of GR and p65 is sufficient to prevent DNA
binding and subsequent transactivation. More recent data suggest that
the mechanism of mutual antagonism between NF-
B and
glucocorticoid-signaling pathways involves transcriptional cofactors
that mediate the interactions of NF-
B and the GR. The emerging
evidence, which supports the idea of GR-cofactor interactions
modulating GR signaling, is presented in detail in a separate section
below.
One possible mechanism by which cofactors might mediate GR-p65
antagonism is as a target for competition. A limiting cellular pool of
a common transcriptional cofactor, required by both GR and NF-
B for
full transactivation, might result in repression of one transcription
factors basal or induced activity when the other transcription factor
was activated and had bound up all available cofactor. While no one
cofactor has been identified that would occupy this role, one candidate
currently being examined in both our own and other laboratories (146)
is CBP/p300. The recent proliferation of yeast two-hybrid assays has
demonstrated that many transcription factors (such as steroid hormone
receptors and nuclear receptors), cofactors (such as SRC-1), and
proteins of the basal transcriptional machinery (RNA polymerase II,
TATA-binding protein, and TFII-B, for example) can interact with CBP,
and a view of CBP as a central "adapter/integrator" protein, which
brings a complex set of transcriptional proteins together on the DNA,
is forming (78, 79, 150, 151). Both GR (see below) and NF-
B interact
with CBP/p300, making CBP an attractive candidate for a role as
mediator of NF-
B/GR antagonism, either by functioning as an
adapter/cointegrator or as the limiting pool of a mutually required
transcriptional cofactor. This idea of CBP functioning as a mediator of
transcription factor antagonism was first proposed by Kamei and
co-workers (150), who provided evidence that nuclear receptor-mediated
repression of AP-1 activation is a result of competition for limiting
amounts of CBP.
While it seems logical to consider the role of cofactor (in particular,
CBP) competition in NF-
B/GR antagonism, in vitro studies
by Caldenhoven et al. (1) demonstrate that GR-mediated
repression of an NF-
B reporter is overcome by increased expression
of p65 and therefore argue against such a mechanism. Another potential
problem with the hypothesis that CBP might be limiting is that
activation of NF-
B or GR should globally inhibit all receptors that
require CBP for their activity. This simple model of transcription
factor competition for CBP would apparently not allow for the specific
nature of the NF-
B/GR antagonism. However, the more complex proposal
that CBP might function as a cointegrator and mediate the antagonism of
NF-
B and GR by bringing the two factors together in a specific
inactive conformation is promising in that it allows for a specific
repressive effect even when faced with both the relatively promiscuous
interactions of CBP/p300 and the in vitro data that show
that GR repression of NF-
B can be titrated out by overexpression of
p65.
3. Other mechanisms of NF-
B/GR interaction. Another
potential mechanism of NF-
B/GR antagonism which does not involve
direct physical interaction of GR and NF-
B requires further
examination. Recent work from our laboratory (J. C. Webster and
J. A. Cidlowski, unpublished observations) indicates that NF-
B
increases the expression of hGRß, the endogenous dominant negative
inhibitor of hGR
transactivation. Therefore, a cell in which NF-
B
is activated may have a higher ratio of GRß:GR
and, consequently,
impaired GR transactivation. This would provide a dual mechanism of GR
repression by NF-
B: directly by mutual transcriptional antagonism
and indirectly by increasing the relative amounts of a dominant
negative regulator of GR. Such a mechanism would be reminiscent of a
dual mechanism of NF-
B repression by GR (directly by transcriptional
antagonism and indirectly by inducing I
B
), which has been
identified and is discussed in Section IV of this
manuscript.
4. Evidence for a functional antagonism of GR by NF-
B. That
two independent mechanisms of GR repression by NF-
B likely exist
within the same cell suggests that maintaining negative control on
GR-signaling pathways is of physiological importance. Although most
interest in NF-
B/GR antagonism is focused on the role of GR in
dampening NF-
B actions, the antagonism of GR function by NF-
B is
probably equally important to the physiology of an organism. While not
yet supported by direct evidence, it may be that
glucocorticoid-responsive tissues with an activated inflammatory
response (mediated by an activated NF-
B) become somewhat resistant
to glucocorticoid signaling because GR function is blocked.
The repression of GR by NF-
B may also be relevant in patients with
steroid-resistant asthma. The pathology of asthma is caused by chronic
inflammation of the airway epithelium, and the treatment of choice for
controlling this inflammation is administration of glucocorticoid (152, 153). However, a subpopulation of asthmatics do not respond to
antiinflammatory treatment with glucocorticoids. Some of these
steroid-resistant asthmatics have an abnormally low number of GRs, and
others demonstrate reduced ligand binding affinity (presumably a
reversible effect due to the actions of IL-1 and IL-4), which can
explain the lack of glucocorticoid responsiveness (154, 155). Other
steroid-resistant asthmatics show no defects in their GRs or in steroid
absorption or clearance (152). While there are data that suggest that
abnormal AP-1 (activator protein 1)-GR interactions reduce GR DNA
binding in these patients (152, 156), it will also be important to
determine whether these individuals have any abnormality in NF-
B
regulation that leads to excessive NF-
B activation. It is
conceivable that a chronically high level of NF-
B activity would
lead not only to chronic inflammation, but also to glucocorticoid
resistance by blocking of the GR-signaling pathway. Should abnormal
NF-
B regulation be found in these patients, targeting both NF-
B-
and GR-signaling pathways simultaneously could provide a promising
approach to effective antiinflammatory therapy in this otherwise
refractory population.
E. GR interacts with other transcription factors and
transcriptional cofactors
As discussed, GR binds to specific regions of the chromatin and
alters the basal level of transcription for responsive genes. However,
as with NF-
B, GR is now known to interact with a variety of nuclear
regulatory proteins and other transcription factors that can modulate
its function and alter its interaction with DNA and/or the basal
transcription machinery. Interactions with different
cofactors/transcription factors may contribute to cell type
specificities in glucocorticoid responsiveness and GR-mediated gene
expression.
1. Transcriptional cofactors and GR. Some proteins function as transcriptional coactivators for the GR, enhancing, or in some instances, enabling the transcriptional activity of ligand-activated GR. Examples of proteins recently identified as GR coactivators are CBP/p300 (cAMP-response element binding protein), GRIP1/TIF2 (GR-interacting protein-1 in mouse, transcriptional intermediary factor 2 in humans), HMG-1 proteins, 143-3 eta, STAT 5 (signal transducer and activator of transcription-5), GRIP 170, hRPF1, RAP 46, and SRC-1 (steroid receptor coactivator-1) (76, 78, 127, 157, 158, 159, 160, 161, 162, 163, 164, 165). Proteins that interact with steroid and other nuclear receptors and serve as transcriptional corepressors, including N-CoR (nuclear receptor corepressor) and SMRT (silencing mediator for retinoid and thyroid hormone receptors), have also been identified (78, 164, 166, 167). While it has not yet been conclusively determined whether any of these recently identified corepressors specifically interact with GR, it is likely that as more corepressors are identified and their mechanisms of action are clarified, corepressors that specifically affect GR-mediated transcription will be identified.
2. Transcription factors and GR. In addition to interacting with transcriptional cofactors, GR has been shown to interact with other proteins that are themselves independent transcription factors. This "cross-talk" has been shown to alter the transcriptional properties of both GR and the other transcription factors. For example, GR cross-talk is observed with members of the ubiquitous OTF (octamer transcription factor) family of constitutive transcriptional activators (168). Oct-1 and Oct-2 interactions with GR in some cell types and promoter contexts enhance the transcriptional activity of both factors via a cooperative binding mechanism, while in other contexts Oct-1/GR physical interactions result in functional interference (169). Interaction with GR represses Oct-2A transactivation, possibly due to competition for transcriptional cofactors (170), and transcriptional repression of the bovine PRL (PRL3) gene by GR has been shown to be enhanced by interactions with Oct-1 and another homeodomain protein, Pbx (171). GR has also been shown to repress the activity of the transcription factor GATA-1 via direct physical interaction in mouse erythroleukemia cells (172) and to synergize with the developmentally regulated transcription factor hepatocyte nuclear factor-1 in liver and liver-derived cell lines (173).
3. AP-1 and GR. A particularly important physical interaction
occurs between GR and the ubiquitous dimeric transcription factor
activator protein-1 (AP-1). GR can interact with both Fos and Jun
subunits of the AP-1 heterodimer (although it appears that Fos may be
the preferential target for GR) and alter the interaction of both
transcription factors with DNA, resulting in reciprocal repression of
AP-1 and GR transactivation functions. GR also similarly interacts with
Fos and Jun homodimers to alter transcriptional properties of both
factors (174, 175, 176, 177, 178, 179). AP-1 (Fos-Jun) is a proinflammatory transcription
factor that is induced by a variety of cytokines and by phorbol ester
(180). Like NF-
B, AP-1 has been shown to activate transcription of
genes involved in inflammatory diseases such as rheumatoid arthritis
and asthma (180, 181, 182), and AP-1 and NF-
B have been shown to act
synergistically in the induction of some proinflammatory genes in lung
epithelium (182). In addition, many inflammatory genes that are
repressed by glucocorticoids but do not have nGREs in their promoters
do carry sites for AP-1 as well as for NF-
B (183). These data
suggest that AP-1 repression by GR is another important mechanism of
antiinflammatory and immunosuppressive action by glucocorticoids. It is
important to bear in mind that, since NF-
B and AP-1 can synergize in
the proinflammatory pathway, GR repression of AP-1 may be an important
indirect mechanism for suppressing NF-
B-mediated immune responses.
4. GR and other steroid hormone receptors. It has also been demonstrated that GR can interact with other steroid receptors, resulting in altered transcriptional activity of genes responsive to each reporter. In some cases, the mechanism of interaction is known to involve heterodimerization. For example, GR and MR heterodimerize, and coexpression of GR and MR in a given cell may influence the cell type-specific pattern of gene activation (184). When AR and GR heterodimerize, mutual transcriptional inhibition results (185). GR and progesterone receptor A (PR-A) have also been found to interact. Although the mechanism of interaction is not yet clear, it has been demonstrated that PR-A can function as a dominant negative regulator of GR transactivation (186).
IV. NF- B and GR Antagonism: Physiological Significance?
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B is a key proinflammatory and proimmune transcription
factor, and ligand-activated GR is a potent suppressor of immunity and
inflammation. The negative cross-talk that has been demonstrated
between these two transcription factors with opposing functions has
clear implications for the importance of these two transcriptional
modulators in immune regulation. Given their profound effects on
immunity and inflammation, it is conceivable that NF-
B/GR mutual
antagonism provides the primary control mechanism by which an organism
maintains homeostasis in situations where host defense is activated.
The signals that activate NF-
B, such as bacterial or viral infection
and oxidative stress, are perceived as stressful stimuli by the
organism. These stimuli also activate the HPA axis, with the end result
being the release of high levels of glucocorticoid into circulation.
Therefore, both the pro- and antiinflammatory pathways are activated by
these environmental stressors. The presumption is that GR-mediated
immunosuppression serves to limit the cellular damage that would be
caused by an excessive immune response.
While a strong case can be made for the importance of NF-
B in
glucocorticoid regulation of immunity and inflammation, it is important
to maintain perspective and consider NF-
B/GR effects within the
context of all known GR-mediated effects on immune function. For
example, AP-1 may also be a key mediator of these glucocorticoid
effects. As discussed above, AP-1 regulates expression of
proinflammatory genes both independently and synergistically with
NF-
B, and the well described antagonism of AP-1 by GR suggests that
glucocorticoid effects on immune/inflammatory cascades are
mediated through AP-1 dependent pathways in much the same manner that
they are mediated by NF-
B. AP-1/GR antagonism also suggests that
proinflammatory genes whose transcription is synergistically activated
by AP-1 and NF-
B might be even more sensitive to
glucocorticoid-mediated suppression than those that are activated by
only one of these two transcription factors. There is also evidence
that GR can directly regulate the expression of some proinflammatory
cytokines and related genes via transcriptional and posttranscriptional
mechanisms. For example, the IL-8 gene has been shown to be repressed
in a fibrosarcoma cell line via a GRE in its 5'-flanking sequence
(187), glucocorticoids dose-dependently destabilize IL-6 mRNA via a
receptor-mediated pathway (188), and a novel GRE has been identified in
the murine IL-2 receptor
gene (189). Therefore, it is likely that
multiple mechanisms contribute to the potent immunosuppressive and
antiinflammatory effects of glucocorticoids in vivo.
However, the interaction between NF-
B and GR is multifaceted,
involving mutual transcriptional antagonism, feedback mechanisms
involving cytokine cascades, and regulation of I
B, which is
discussed below. Multiple mechanisms of antagonism afford multiple
sites of regulatory control and flexibility of the interaction. Such
flexibility and capacity for precision regulation make NF-
B an
attractive candidate for the primary mediator of glucocorticoid effects
in the immune system.
As discussed, an additional level of control of NF-
B by GR involves
regulation of I
B
expression. I
B
, the inhibitory subunit of
NF-
B, has been shown to be up-regulated by active GR (190, 191, 192).
This positive regulation of I
B
by GR presumably dampens
proinflammatory effects of NF-
B and therefore is another mechanism
by which glucocorticoids exert their antiinflammatory effects, keeping
immune responses in check. The extent to which this second mechanism of
NF-
B regulation by GR influences immune function is an area that
requires further study. Since NF-
B is a family of transcription
factors with differing responsiveness to activating pathways and
I
B
repression, as well as different cellular distributions,
induction of I
B
by GR cannot account for all of the
antiinflammatory effects of glucocorticoids. Recent studies by Heck
et al. (193) support this idea. They demonstrate that a
dimerization-deficient GR mutant represses NF-
B yet cannot induce
I
B
expression. In addition, they show that some
glucocorticoid analogs can activate I
B
but do not repress
NF-
B. Thus, I
B
induction by GR is a separable process from
GR-mediated repression of NF-
B transactivation. The fact that
separate and concurrent pathways for repression of NF-
B by GR exist
suggests that it is necessary for an organism to keep immune and
inflammatory responses under tight control, and that glucocorticoids
are an important physiological means of maintaining this control.
The idea that NF-
B is an important physiological repressor of
GR-mediated cellular processes is less well developed, but this may
prove to be an equally important result of GR/NF-
B cross-talk. The
question of whether prior activation of NF-
B in a normally
steroid-responsive cell confers a degree of glucocorticoid resistance
requires further study. It may be that the efficacy of pharmacological
doses of glucocorticoid for control of inflammation is modulated by the
level of NF-
B expression in the target cell. Or, a state of
inflammation (and elevated NF-
B activation) might impinge on other
functions of the glucocorticoid-mediated signaling pathway that occur
at physiological levels of the hormone, thus influencing normal
metabolic or developmental processes, for example.
The role of NF-
B/GR interactions in nonimmune processes and cell
types is not yet clear. We know that both NF-
B and GR enjoy broad
ranges of expression within the various tissues of an organism, and
that these two factors are often expressed in the same cell types.
Whether they are mutual antagonists in nonimmune cells is largely a
function of whether both pathways would be activated simultaneously
under any circumstances. No definitive evidence exists that, outside
the immune system, a balance between GR- and NF-
B-mediated signaling
is physiologically relevant in an intact animal. However, since such a
balance is mimicked in cultured COS-1 cells (4), the possibility that
some states of steroid resistance are mediated by NF-
B should be
considered. One might also speculate that NF-
B/GR antagonism might
be important in regulating apoptosis or programmed cell death. GR is
known to be an inducer of thymocyte apoptosis (194), although its role
in apoptosis in other cell types has not been established. NF-
B p65
knockout mice are embryonic lethal due to massive liver apoptosis (99),
suggesting a possible role for NF-
B as a general suppressor of
apoptosis. Whether these roles as apoptotic inducer and suppressor,
respectively, are simultaneously filled in any one cell type is as yet
an open question.
V. Interactions Between NF- B and Other Steroid Hormone Receptors
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B and GR has been identified. That
interactions between NF-
B and GR might also be relevant in other
physiological situations is a logical assumption given their widespread
distribution and broad range of regulated genes. Ongoing research
concerning NF-
B and GR function outside the immune system should
clarify this issue.
The great structural similarity of GR to the other members of the
steroid hormone receptor family, as well as the fact that they share a
common mechanism of action as ligand-dependent transcription factors,
begs the question of whether other steroid hormone receptors (such as
estrogen, progesterone, or androgen) might also interact with NF-
B
via mechanisms like those proposed for GR. While our understanding of
this question is still sparse, evidence that such interactions do occur
is beginning to emerge and is summarized below.
A. Androgen receptor (AR)
AR is closely related to GR in terms of both structure and
sequence specificity (they share 78% sequence identity in their DBDs).
GR and AR have been shown to interact with and repress AP-1 via similar
mechanisms (195). Therefore, it would not be surprising that AR might
also interact with NF-
B in a manner similar to that observed for GR.
Transient cotransfection experiments performed in our laboratory
indicate that AR and NF-
B (p65) are mutual transcriptional
antagonists (4). While the in vivo significance of this
antagonism has yet to be established, it is possible that inflammatory
agents that activate NF-
B in vivo may interfere with
normal androgen signaling, thus producing androgen insensitivity in
tissues where AR is normally expressed and exposed to ligand. Since we
have shown AR/NF-
B antagonism to be mutual, one might also speculate
that androgen-responsive tissues might be relatively insensitive to
NF-
B-induced inflammation.
Interestingly, Supakar et al. (96) have observed physical
interaction of NF-
B p50 and p65 subunits with the promoter region of
the AR gene in vitro (by electrophoretic mobility shift
assay). This interaction of the NF-
B and AR signaling pathways is
different than any identified for GR, and it will be interesting to
determine whether a similar mechanism for GR/NF-
B antagonism can be
identified. They suggest that age-dependent androgen insensitivity of
the liver may be related to increased levels of p50 homodimer expressed
in aging liver, which bind to the AR gene and subsequently block the
androgen-signaling pathway. The extent to which the in vitro
interactions of AR and NF-
B reflect physiological interactions that
occur within an organism remains to be seen.
Another interesting consideration for future research into NF-
B/AR
cross-talk is the ability of AR and GR to heterodimerize (185). Since
both these receptors antagonize NF-
B as homodimers, it is of
interest to consider whether they maintain this function as a
heterodimer, and whether cofactor or cell type specificities change
under these circumstances.
B. Estrogen receptor (ER)
Several groups (4, 84, 92, 196) have shown that NF-
B p65 and ER
interact. Interestingly, while all groups show that p65 represses
ER
-mediated transactivation in transient cotransfection assays,
whether or not this transcriptional antagonism is reciprocal is less
clear. Our studies using COS-1 cells indicate that ER is incapable of
reciprocally repressing NF-
B signaling. Stein and Yang (98) show a
mutual antagonism in some cell lines and one-way antagonism in others.
A more recent report assessing the regulation of NF-
B signaling in
cell lines confirms that ER antagonizes NF-
B in a cell type-specific
manner consistent with a cofactor-modulated process but could not
demonstrate a role for known ER cofactors in this specificity (197).
This suggests that for ER, there may be cell type-specific cofactors
that are important modulators of the ER/NF-
B interaction but have
not yet been identified.
While the exact mechanisms of ER/NF-
B antagonism are not yet clear,
the physiological importance of these interactions is suggested by
several recent reports. For example, estrogens are known to be key
regulators of bone metabolism, and their withdrawal is related to the
bone mineral loss of osteoporosis. It has been shown by several groups
that this bone-protective effect of estrogen is due, at least in part,
to estrogen-mediated repression of IL-6, a cytokine that is
transcriptionally up-regulated by NF-
B and regulates bone metabolism
(196, 198, 199). While each of these groups attributes this IL-6
inhibition to ER interference with NF-
B binding to the IL-6
promoter, the mechanism by which ER blocks NF-
B binding is not
agreed upon. Galien and Garcia (196) show evidence for ER/NF-
B
complex formation that prevents DNA binding, while Kurebayashi et
al. (198) assert that ER cannot prevent NF-
B DNA binding. Sun
et al. (199) show that estrogen blocks I
B
degradation,
suggesting that the inhibitory effects of estrogen on NF-
B function
are due to the sustained presence of the inhibitory subunit of NF-
B.
ER/NF-
B interactions have also been implicated in the progression of
breast cancer. Nakshatri and co-workers (58) show that progression of a
mammary carcinoma cell line from ER-positive nonmalignant to
ER-negative metastatic correlates with constitutive activation of
NF-
B and altered expression of both NF-
B and I
B. They
hypothesize that, since ER represses both constitutive and inducible
NF-
B activity, overexpression of NF-
B-inducible genes in
ER-negative cells contributes to malignancy and chemotherapeutic
resistance. Multiple mechanisms for the interaction of ER and NF-
B
in this breast cancer model have been proposed. In addition to the
previously described mechanisms of 1) repression of NF-
B DNA binding
by physical association with ER, and 2) the regulation of I
B
expression by estrogen, these authors propose that ER might interfere
with NF-
B signaling by increasing expression of PR. As described in
the subsequent section, PR has also been shown to negatively interact
with NF-
B.
C. Progesterone receptor (PR)
As with ER, both our group and others have demonstrated a
repressive effect of p65 on PR (B-form) transactivation, and the
physical interaction of NF-
B and PR has also been demonstrated
in vitro (4, 97). Again, it is less clear whether PR
reciprocally represses NF-
B transactivation, and as with ER/NF-
B
mutual antagonism, this may be dependent on the cell type in which the
interaction occurs. No clear physiological role for NF-
B/PR
antagonism has been elucidated. However, it has been proposed that,
since most progesterone target tissues (such as breast and endometrium)
express receptors for cytokines that regulate NF-
B, there might be a
physiological relevance to p65/PR antagonism in normal cellular
processes in these cells. Clearly, much more data concerning this
question must be available before a firm role for PR/p65 antagonism can
be postulated.
While it is not clear that the mechanisms of mutual antagonism are the
same for GR and NF-
B as they are for the other steroid hormone
receptors and NF-
B, it is logical that the physiological result of
the antagonism would be similar: a cell type-specific homeostasis
brought about by opposing transcriptional effects on a gene or group of
genes. The cell type specificity of response would depend on the
cofactors present within a cell, as well as the pattern of expression
for NF-
B and steroid hormone receptors in that cell type. Even if
all necessary transcriptional factors are present within the cell, a
physiologically relevant antagonism would depend on the temporally
coordinated activation of both transcription factors by ligand or
extracellular signal. Whether such conditions exist for these steroid
and other nuclear receptors with NF-
B in a single cell in
vivo remains to be shown. If they do, this may present an
important mechanism by which cells may become steroid resistant. At
present, more questions than answers are available concerning NF-
B
interaction with the steroid hormone receptor family. For NF-
B and
GR in the immune system, however, evidence is strong that the
interactions of their signaling pathways provide a fundamental
homeostatic control mechanism.
| VI. Summary/Conclusions |
|---|
|
|
|---|
Both pro- and antiinflammatory mechanisms must be present and
functional for a cell (and by extension, an organism) to survive in the
face of environmental stimuli that elicit an immune response. These
pathways, when activated simultaneously, provide homeostasis by
"pulling" the cell in opposite directions. Two candidate signaling
pathways for this physiological balancing act are the NF-
B and
GR-mediated signaling pathways. These pathways, as discussed in this
review and depicted in Fig. 6
, have
important and opposing roles in immune function. NF-
B is a
transcription factor which induces the expression of many genes that
participate in immune and inflammatory responses, while GR is a
transcription factor that serves as a potent antiinflammatory agent and
immune suppressor. Their interactions within the cell, while not yet
completely understood, appear to be an important, possibly even the
primary, mechanism of immune homeostasis. For this reason, research on
GR-NF-
B interaction continues to expand as many groups strive to
fill in the "black boxes" where the GR and NF-
B-signaling
pathways cross.
|
B or the steroid hormone receptors, with the basal
transcriptional machinery is an important mechanism for tightly
regulating gene-mediated physiological processes such as immunity and
inflammation. However, the mechanism by which GR elicits its
antiinflammatory effects has, until recently, proven to be a conundrum,
since the majority of proimmune/proinflammatory genes repressed by
glucocorticoids have no identifiable GREs in their promoters that would
allow GR to interact with the basal transcription machinery. The
relatively recent discovery that GR and NF-
B are mutual
transcriptional antagonists has given researchers a handle on potential
mechanisms of immune regulation by glucocorticoids.
The functional interaction of NF-
B and GR has thus far been
demonstrated only in in vitro and cell culture systems, so
it is important to consider whether these regulatory factors interact
in precisely the same way within a more "physiological" context.
Clearly, the field of NF-
B/GR research must move toward
understanding how these two factors interact in an intact animal.
However, understanding how such complex and broadly used signaling
pathways interact within an intact organism is a daunting task. As
discussed in this review, both NF-
B and GR have cell/tissue-specific
effects, and both cross-talk with other steroid hormone receptors,
transcription factors, and a variety of other cellular signaling
cascades that are functioning simultaneously within an animal. The
design of tissue-specific transgenic or inducible NF-
B and GR
knockout animals would provide a powerful tool for understanding
functional interactions of NF-
B and GR in immunity and inflammation
(as well as other processes mediated by these two factors) in a more
physiological context. Given our present scientific tools, however, the
most valuable means of understanding NF-
B/GR cross-talk is probably
still to examine the interaction in a relatively artificial context
where a single pathway or response can be evaluated.
Despite the limitations of currently available model systems, our
understanding of the elegant interplay between NF-
B and GRs in terms
of immune regulation is rapidly improving. Gaining insight into
mechanisms of NF-
B/GR antagonism not only opens up the possibility
of novel, transcription-based therapy in immune pathology, but also
raises the exciting possibility that NF-
B may also interact with GR
or another steroid hormone receptor in other cell types to regulate
other important physiological and developmental processes. As the body
of research concerning NF-
B/GR expands, we may find that GR/NF-
B
antagonism in the immune system is simply the prototype of a
fundamental NF-
B/steroid receptor factor cross-talk mechanism that
affords tight homeostatic control of critical cellular processes in a
variety of cell types.
Preliminary studies indicate that NF-
B also interacts with androgen,
estrogen, and PRs. These findings suggest that we may find sex-specific
differences in immunity or other NF-
B-mediated responses in an
organism due to cross-talk with gonadal steroid-mediated signaling
pathways. It also suggests the existence of a general nuclear
receptor/NF-
B cross-talk mechanism that may exert widespread
influence on cell type-specific gene transcription. In light of this
possibility, continued research into the mechanism of GR/NF-
B
antagonism is truly exciting.
Additional, recently published references of interest:
Sheppard K-A, Phelps KM, Williams AJ,
Thanos D, Glass CK, Rosenfeld MG, Gerritsen ME, Collins T 1998
Nuclear integration of glucocorticoid receptor and nuclear factor-
B
signaling by CREB-binding protein and steroid receptor coactivator-1.
J Biol Chem 273:2929129294
Hofmann TG, Hehner SP, Bacher S, Droge W, Schmitz
ML 1998 Various glucocorticoids differ in their ability to induce
gene expression, apoptosis and to repress NF-
B-dependent
transcription. FEBS Lett 441:441446
| Addendum/Erratum |
|---|
|
|
|---|
| Footnotes |
|---|
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L. Sunday, C. Osuna, D. N. Krause, and S. P. Duckles Age alters cerebrovascular inflammation and effects of estrogen Am J Physiol Heart Circ Physiol, May 1, 2007; 292(5): H2333 - H2340. [Abstract] [Full Text] [PDF] |
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H.-Y. Lee, T. J Acosta, M. Tanikawa, R. Sakumoto, J. Komiyama, Y. Tasaki, M. Piskula, D. J Skarzynski, M. Tetsuka, and K. Okuda The role of glucocorticoid in the regulation of prostaglandin biosynthesis in non-pregnant bovine endometrium J. Endocrinol., April 1, 2007; 193(1): 127 - 135. [Abstract] [Full Text] [PDF] |
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C. G. Radu, C. J. Shu, S. M. Shelly, M. E. Phelps, and O. N. Witte Positron emission tomography with computed tomography imaging of neuroinflammation in experimental autoimmune encephalomyelitis PNAS, February 6, 2007; 104(6): 1937 - 1942. [Abstract] [Full Text] [PDF] |
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P. Germain, B. Staels, C. Dacquet, M. Spedding, and V. Laudet Overview of Nomenclature of Nuclear Receptors Pharmacol. Rev., December 1, 2006; 58(4): 685 - 704. [Abstract] [Full Text] [PDF] |
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K Vagnerova, M Kverka, P Klusonova, P Ergang, I Miksik, H Tlaskalova-Hogenova, and J Pacha Intestinal inflammation modulates expression of 11{beta}-hydroxysteroid dehydrogenase in murine gut. J. Endocrinol., November 1, 2006; 191(2): 497 - 503. [Abstract] [Full Text] [PDF] |
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W. L. Whittle, A. C. Holloway, S. Lye, J. R. G. Challis, and W. Gibb The Pattern of Glucocorticoid and Estrogen Receptors May Explain Differences in Steroid Dependency of Intrauterine Prostaglandin Production at Parturition in Sheep Reproductive Sciences, October 1, 2006; 13(7): 506 - 511. [Abstract] [PDF] |
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L. Sunday, M. M. Tran, D. N. Krause, and S. P. Duckles Estrogen and progestagens differentially modulate vascular proinflammatory factors Am J Physiol Endocrinol Metab, August 1, 2006; 291(2): E261 - E267. [Abstract] [Full Text] [PDF] |
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X. Gu, S. Ke, D. Liu, T. Sheng, P. E. Thomas, A. B. Rabson, M. A. Gallo, W. Xie, and Y. Tian Role of NF-{kappa}B in Regulation of PXR-mediated Gene Expression: A MECHANISM FOR THE SUPPRESSION OF CYTOCHROME P-450 3A4 BY PROINFLAMMATORY AGENTS J. Biol. Chem., June 30, 2006; 281(26): 17882 - 17889. [Abstract] [Full Text] [PDF] |
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A. Cernetich, L. S. Garver, A. E. Jedlicka, P. W. Klein, N. Kumar, A. L. Scott, and S. L. Klein Involvement of gonadal steroids and gamma interferon in sex differences in response to blood-stage malaria infection. Infect. Immun., June 1, 2006; 74(6): 3190 - 3203. [Abstract] [Full Text] [PDF] |
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M. G. Rosenfeld, V. V. Lunyak, and C. K. Glass Sensors and signals: a coactivator/corepressor/epigenetic code for integrating signal-dependent programs of transcriptional response Genes & Dev., June 1, 2006; 20(11): 1405 - 1428. [Abstract] [Full Text] [PDF] |
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M. D. Ashworth, J. W. Ross, J. Hu, F. J. White, D. R. Stein, U. DeSilva, G. A. Johnson, T. E. Spencer, and R. D. Geisert Expression of Porcine Endometrial Prostaglandin Synthase During the Estrous Cycle and Early Pregnancy, and Following Endocrine Disruption of Pregnancy Biol Reprod, June 1, 2006; 74(6): 1007 - 1015. [Abstract] [Full Text] [PDF] |
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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] |
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G. Juhasz-Vedres, E. Rozsa, G. Rakos, M. B. Dobszay, Z. Kis, J. Wolfling, J. Toldi, A. Parducz, and T. Farkas Dehydroepiandrosterone Sulfate Is Neuroprotective when Administered Either before or after Injury in a Focal Cortical Cold Lesion Model Endocrinology, February 1, 2006; 147(2): 683 - 686. [Abstract] [Full Text] [PDF] |
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J. Chen, H. K. Kinyamu, and T. K. Archer Changes in Attitude, Changes in Latitude: Nuclear Receptors Remodeling Chromatin to Regulate Transcription Mol. Endocrinol., January 1, 2006; 20(1): 1 - 13. [Abstract] [Full Text] [PDF] |
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M. Ayaori, S. Sawada, A. Yonemura, N. Iwamoto, M. Ogura, N. Tanaka, K. Nakaya, M. Kusuhara, H. Nakamura, and F. Ohsuzu Glucocorticoid Receptor Regulates ATP-Binding Cassette Transporter-A1 Expression and Apolipoprotein-Mediated Cholesterol Efflux From Macrophages Arterioscler. Thromb. Vasc. Biol., January 1, 2006; 26(1): 163 - 168. [Abstract] [Full Text] [PDF] |
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F. Wieser, J.-L. Vigne, I. Ryan, D. Hornung, S. Djalali, and R. N. Taylor Sulindac Suppresses Nuclear Factor-{kappa}B Activation and RANTES Gene and Protein Expression in Endometrial Stromal Cells from Women with Endometriosis J. Clin. Endocrinol. Metab., December 1, 2005; 90(12): 6441 - 6447. [Abstract] [Full Text] [PDF] |
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C. J. Shu, S. Guo, Y. J. Kim, S. M. Shelly, A. Nijagal, P. Ray, S. S. Gambhir, C. G. Radu, and O. N. Witte Visualization of a primary anti-tumor immune response by positron emission tomography PNAS, November 29, 2005; 102(48): 17412 - 17417. [Abstract] [Full Text] [PDF] |
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M. J. Walters, M. J. Paul-Clark, S. K. McMaster, K. Ito, I. M. Adcock, and J. A. Mitchell Cigarette Smoke Activates Human Monocytes by an Oxidant-AP-1 Signaling Pathway: Implications for Steroid Resistance Mol. Pharmacol., November 1, 2005; 68(5): 1343 - 1353. [Abstract] [Full Text] [PDF] |
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A. Mouihate, S. Ellis, E.-M. Harre, and Q. J. Pittman Fever suppression in near-term pregnant rats is dissociated from LPS-activated signaling pathways Am J Physiol Regulatory Integrative Comp Physiol, November 1, 2005; 289(5): R1265 - R1272. [Abstract] [Full Text] [PDF] |
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A. Razmara, D. N. Krause, and S. P. Duckles Testosterone augments endotoxin-mediated cerebrovascular inflammation in male rats Am J Physiol Heart Circ Physiol, November 1, 2005; 289(5): H1843 - H1850. [Abstract] [Full Text] [PDF] |
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T. Rhen and J. A. Cidlowski Antiinflammatory action of glucocorticoids--new mechanisms for old drugs. N. Engl. J. Med., October 20, 2005; 353(16): 1711 - 1723. [Full Text] [PDF] |
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G. Nalbandian, V. Paharkova-Vatchkova, A. Mao, S. Nale, and S. Kovats The Selective Estrogen Receptor Modulators, Tamoxifen and Raloxifene, Impair Dendritic Cell Differentiation and Activation J. Immunol., August 15, 2005; 175(4): 2666 - 2675. [Abstract] [Full Text] [PDF] |
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M. Moayeri, J. I. Webster, J. F. Wiggins, S. H. Leppla, and E. M. Sternberg Endocrine Perturbation Increases Susceptibility of Mice to Anthrax Lethal Toxin Infect. Immun., July 1, 2005; 73(7): 4238 - 4244. [Abstract] [Full Text] [PDF] |
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A. Bouman, M. J. Heineman, and M. M. Faas Sex hormones and the immune response in humans Hum. Reprod. Update, July 1, 2005; 11(4): 411 - 423. [Abstract] [Full Text] [PDF] |
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G. Soucy, G. Boivin, F. Labrie, and S. Rivest Estradiol Is Required for a Proper Immune Response to Bacterial and Viral Pathogens in the Female Brain J. Immunol., May 15, 2005; 174(10): 6391 - 6398. [Abstract] [Full Text] [PDF] |
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N. Yoshikawa, K. Yamamoto, N. Shimizu, S. Yamada, C. Morimoto, and H. Tanaka The Distinct Agonistic Properties of the Phenylpyrazolosteroid Cortivazol Reveal Interdomain Communication within the Glucocorticoid Receptor Mol. Endocrinol., May 1, 2005; 19(5): 1110 - 1124. [Abstract] [Full Text] [PDF] |
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H. Igarashi, K. L. Medina, T. Yokota, M. I. D. Rossi, N. Sakaguchi, P. C. Comp, and P. W. Kincade Early lymphoid progenitors in mouse and man are highly sensitive to glucocorticoids Int. Immunol., May 1, 2005; 17(5): 501 - 511. [Abstract] [Full Text] [PDF] |
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S. Lee and C. Rivier Role Played by Hypothalamic Nuclear Factor-{kappa}B in Alcohol-Mediated Activation of the Rat Hypothalamic-Pituitary-Adrenal Axis Endocrinology, April 1, 2005; 146(4): 2006 - 2014. [Abstract] [Full Text] [PDF] |
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K. Hsu, R. J. Passey, Y. Endoh, F. Rahimi, P. Youssef, T. Yen, and C. L. Geczy Regulation of S100A8 by Glucocorticoids J. Immunol., February 15, 2005; 174(4): 2318 - 2326. [Abstract] [Full Text] [PDF] |
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J. E. Eickhoff and M. Cotten NF-{kappa}B activation can mediate inhibition of human cytomegalovirus replication J. Gen. Virol., February 1, 2005; 86(2): 285 - 295. [Abstract] [Full Text] [PDF] |
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P. B. Cornet, C. Galant, Y. Eeckhout, P. J. Courtoy, E. Marbaix, and P. Henriet Regulation of Matrix Metalloproteinase-9/Gelatinase B Expression and Activation by Ovarian Steroids and LEFTY-A/Endometrial Bleeding-Associated Factor in the Human Endometrium J. Clin. Endocrinol. Metab., February 1, 2005; 90(2): 1001 - 1011. [Abstract] [Full Text] [PDF] |
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H. Garside, A. Stevens, S. Farrow, C. Normand, B. Houle, A. Berry, B. Maschera, and D. Ray Glucocorticoid Ligands Specify Different Interactions with NF-{kappa}B by Allosteric Effects on the Glucocorticoid Receptor DNA Binding Domain J. Biol. Chem., November 26, 2004; 279(48): 50050 - 50059. [Abstract] [Full Text] [PDF] |
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B. M. Necela and J. A. Cidlowski Mechanisms of Glucocorticoid Receptor Action in Noninflammatory and Inflammatory Cells Proceedings of the ATS, November 1, 2004; 1(3): 239 - 246. [Full Text] [PDF] |
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Z. Szatmary, M. J. Garabedian, and J. Vilcek Inhibition of Glucocorticoid Receptor-mediated Transcriptional Activation by p38 Mitogen-activated Protein (MAP) Kinase J. Biol. Chem., October 15, 2004; 279(42): 43708 - 43715. [Abstract] [Full Text] [PDF] |
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S. Ducreux, F. Zorzato, C. Muller, C. Sewry, F. Muntoni, R. Quinlivan, G. Restagno, T. Girard, and S. Treves Effect of Ryanodine Receptor Mutations on Interleukin-6 Release and Intracellular Calcium Homeostasis in Human Myotubes from Malignant Hyperthermia-susceptible Individuals and Patients Affected by Central Core Disease J. Biol. Chem., October 15, 2004; 279(42): 43838 - 43846. [Abstract] [Full Text] [PDF] |
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H. Harant and I. J. D. Lindley Negative cross-talk between the human orphan nuclear receptor Nur77/NAK-1/TR3 and nuclear factor-{kappa}B Nucleic Acids Res., October 5, 2004; 32(17): 5280 - 5290. [Abstract] [Full Text] [PDF] |
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M. T. Rae, D. Niven, H. O. D. Critchley, C. R. Harlow, and S. G. Hillier Antiinflammatory Steroid Action in Human Ovarian Surface Epithelial Cells J. Clin. Endocrinol. Metab., September 1, 2004; 89(9): 4538 - 4544. [Abstract] [Full Text] [PDF] |
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S. Chu, Y. Nishi, T. Yanase, H. Nawata, and P. J. Fuller Transrepression of Estrogen Receptor {beta} Signaling by Nuclear Factor-{kappa}B in Ovarian Granulosa Cells Mol. Endocrinol., August 1, 2004; 18(8): 1919 - 1928. [Abstract] [Full Text] [PDF] |
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S. Zelivianski, R. Glowacki, and M.-F. Lin Transcriptional activation of the human prostatic acid phosphatase gene by NF-{kappa}B via a novel hexanucleotide-binding site Nucleic Acids Res., July 7, 2004; 32(12): 3566 - 3580. [Abstract] [Full Text] [PDF] |
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Y. Iwasaki, M. Asai, M. Yoshida, T. Nigawara, M. Kambayashi, and N. Nakashima Dehydroepiandrosterone-Sulfate Inhibits Nuclear Factor-{kappa}B-Dependent Transcription in Hepatocytes, Possibly through Antioxidant Effect J. Clin. Endocrinol. Metab., July 1, 2004; 89(7): 3449 - 3454. [Abstract] [Full Text] [PDF] |
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C. S. Park, H. Lim, K. J. Han, S. H. Baek, H. O. Sohn, D. W. Lee, Y.-G. Kim, H.-Y. Yun, K. J. Baek, and N. S. Kwon Inhibition of Nitric Oxide Generation by 23,24-Dihydrocucurbitacin D in Mouse Peritoneal Macrophages J. Pharmacol. Exp. Ther., May 1, 2004; 309(2): 705 - 710. [Abstract] [Full Text] [PDF] |
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M. A. Iannone, C. A. Simmons, S. H. Kadwell, D. L. Svoboda, D. E. Vanderwall, S.-J. Deng, T. G. Consler, J. Shearin, J. G. Gray, and K. H. Pearce Correlation between in Vitro Peptide Binding Profiles and Cellular Activities for Estrogen Receptor-Modulating Compounds Mol. Endocrinol., May 1, 2004; 18(5): 1064 - 1081. [Abstract] [Full Text] [PDF] |
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G. Wu, P. Brouckaert, and T. Olivecrona Rapid downregulation of adipose tissue lipoprotein lipase activity on food deprivation: evidence that TNF-{alpha} is involved Am J Physiol Endocrinol Metab, May 1, 2004; 286(5): E711 - E717. [Abstract] [Full Text] [PDF] |
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J. A. Ospina, H. N. Brevig, D. N. Krause, and S. P. Duckles Estrogen suppresses IL-1{beta}-mediated induction of COX-2 pathway in rat cerebral blood vessels Am J Physiol Heart Circ Physiol, May 1, 2004; 286(5): H2010 - H2019. [Abstract] [Full Text] [PDF] |
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C. Y. Hong, J. H. Park, R. S. Ahn, S. Y. Im, H.-S. Choi, J. Soh, S. H. Mellon, and K. Lee Molecular Mechanism of Suppression of Testicular Steroidogenesis by Proinflammatory Cytokine Tumor Necrosis Factor Alpha Mol. Cell. Biol., April 1, 2004; 24(7): 2593 - 2604. [Abstract] [Full Text] [PDF] |
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M. U. De Martino, N. Bhattachryya, S. Alesci, T. Ichijo, G. P. Chrousos, and T. Kino The Glucocorticoid Receptor and the Orphan Nuclear Receptor Chicken Ovalbumin Upstream Promoter-Transcription Factor II Interact with and Mutually Affect Each Other's Transcriptional Activities: Implications for Intermediary Metabolism Mol. Endocrinol., April 1, 2004; 18(4): 820 - 833. [Abstract] [Full Text] [PDF] |
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C.-d. Geng and W. V. Vedeckis Steroid-Responsive Sequences in the Human Glucocorticoid Receptor Gene 1A Promoter Mol. Endocrinol., April 1, 2004; 18(4): 912 - 924. [Abstract] [Full Text] [PDF] |
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H. Leis, A. Page, A. Ramirez, A. Bravo, C. Segrelles, J. Paramio, D. Barettino, J. L. Jorcano, and P. Perez Glucocorticoid Receptor Counteracts Tumorigenic Activity of Akt in Skin through Interference with the Phosphatidylinositol 3-Kinase Signaling Pathway Mol. Endocrinol., February 1, 2004; 18(2): 303 - 311. [Abstract] [Full Text] [PDF] |
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J. Wu, Y. Li, J. Dietz, and D. S. Lala Repression of p65 Transcriptional Activation by the Glucocorticoid Receptor in the Absence of Receptor-Coactivator Interactions Mol. Endocrinol., January 1, 2004; 18(1): 53 - 62. [Abstract] [Full Text] [PDF] |
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K. Sun and L. Myatt Enhancement of Glucocorticoid-Induced 11{beta}-Hydroxysteroid Dehydrogenase Type 1 Expression by Proinflammatory Cytokines in Cultured Human Amnion Fibroblasts Endocrinology, December 1, 2003; 144(12): 5568 - 5577. [Abstract] [Full Text] [PDF] |
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Z. Strakova, M. Szmidt, S. Srisuparp, and A. T. Fazleabas Inhibition of Matrix Metalloproteinases Prevents the Synthesis of Insulin-Like Growth Factor Binding Protein-1 during Decidualization in the Baboon Endocrinology, December 1, 2003; 144(12): 5339 - 5346. [Abstract] [Full Text] [PDF] |
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A. Takuma, T. Kaneda, T. Sato, S. Ninomiya, M. Kumegawa, and Y. Hakeda Dexamethasone Enhances Osteoclast Formation Synergistically with Transforming Growth Factor-{beta} by Stimulating the Priming of Osteoclast Progenitors for Differentiation into Osteoclasts J. Biol. Chem., November 7, 2003; 278(45): 44667 - 44674. [Abstract] [Full Text] [PDF] |
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S. Altuwaijri, H.-K. Lin, K.-H. Chuang, W.-J. Lin, S. Yeh, L. A. Hanchett, M. M. Rahman, H.-Y. Kang, M.-Y. Tsai, Y. Zhang, et al. Interruption of Nuclear Factor {kappa}B Signaling by the Androgen Receptor Facilitates 12-O-Tetradecanoylphorbolacetate-Induced Apoptosis in Androgen-sensitive Prostate Cancer LNCaP Cells Cancer Res., November 1, 2003; 63(21): 7106 - 7112. [Abstract] [Full Text] [PDF] |
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S. W. Chung, B. Y. Kang, and T. S. Kim Inhibition of Interleukin-4 Production in CD4+ T Cells by Peroxisome Proliferator-Activated Receptor-{gamma} (PPAR-{gamma}) Ligands: Involvement of Physical Association between PPAR-{gamma} and the Nuclear Factor of Activated T Cells Transcription Factor Mol. Pharmacol., November 1, 2003; 64(5): 1169 - 1179. [Abstract] [Full Text] [PDF] |
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T. RHEN, S. GRISSOM, C. AFSHARI, and J. A. CIDLOWSKI Dexamethasone blocks the rapid biological effects of 17{beta}-estradiol in the rat uterus without antagonizing its global genomic actions FASEB J, October 1, 2003; 17(13): 1849 - 1870. [Abstract] [Full Text] [PDF] |
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L. Sun, A. C. Nicholson, D. P. Hajjar, A. M. Gotto Jr., and J. Han Adipogenic differentiating agents regulate expression of fatty acid binding protein and CD36 in the J744 macrophage cell line J. Lipid Res., October 1, 2003; 44(10): 1877 - 1886. [Abstract] [Full Text] [PDF] |
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J. Frasor, J. M. Danes, B. Komm, K. C. N. Chang, C. R. Lyttle, and B. S. Katzenellenbogen Profiling of Estrogen Up- and Down-Regulated Gene Expression in Human Breast Cancer Cells: Insights into Gene Networks and Pathways Underlying Estrogenic Control of Proliferation and Cell Phenotype Endocrinology, October 1, 2003; 144(10): 4562 - 4574. [Abstract] [Full Text] [PDF] |
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C. L. Raison and A. H. Miller When Not Enough Is Too Much: The Role of Insufficient Glucocorticoid Signaling in the Pathophysiology of Stress-Related Disorders Am J Psychiatry, September 1, 2003; 160(9): 1554 - 1565. [Abstract] [Full Text] [PDF] |
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K. De Bosscher, W. Vanden Berghe, and G. Haegeman The Interplay between the Glucocorticoid Receptor and Nuclear Factor-{kappa}B or Activator Protein-1: Molecular Mechanisms for Gene Repression Endocr. Rev., August 1, 2003; 24(4): 488 - 522. [Abstract] [Full Text] [PDF] |
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G.U. Meduri, P. Carratu, and A.X. Freire Evidence of biological efficacy for prolonged glucocorticoid treatment in patients with unresolving ARDS Eur. Respir. J., August 1, 2003; 22(42_suppl): 57s - 64s. [Abstract] [Full Text] [PDF] |
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D. Soulet and S. Rivest Polyamines play a critical role in the control of the innate immune response in the mouse central nervous system J. Cell Biol., July 21, 2003; 162(2): 257 - 268. [Abstract] [Full Text] [PDF] |
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K. Wang, D. Scheel-Toellner, S. H. Wong, R. Craddock, J. Caamano, A. N. Akbar, M. Salmon, and J. M. Lord Inhibition of Neutrophil Apoptosis by Type 1 IFN Depends on Cross-Talk Between Phosphoinositol 3-Kinase, Protein Kinase C-{delta}, and NF-{kappa}B Signaling Pathways J. Immunol., July 15, 2003; 171(2): 1035 - 1041. [Abstract] [Full Text] [PDF] |
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S. Nadeau and S. Rivest Glucocorticoids Play a Fundamental Role in Protecting the Brain during Innate Immune Response J. Neurosci., July 2, 2003; 23(13): 5536 - 5544. [Abstract] [Full Text] [PDF] |
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J. F. Kuebler, Y. Yokoyama, D. Jarrar, B. Toth, L. W. Rue III, K. I. Bland, P. Wang, and I. H. Chaudry Administration of Progesterone After Trauma and Hemorrhagic Shock Prevents Hepatocellular Injury Arch Surg, July 1, 2003; 138(7): 727 - 734. [Abstract] [Full Text] [PDF] |
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F. A. Patel, J. W. Funder, and J. R. G. Challis Mechanism of Cortisol/Progesterone Antagonism in the Regulation of 15-Hydroxyprostaglandin Dehydrogenase Activity and Messenger Ribonucleic Acid Levels in Human Chorion and Placental Trophoblast Cells at Term J. Clin. Endocrinol. Metab., June 1, 2003; 88(6): 2922 - 2933. [Abstract] [Full Text] [PDF] |
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A. Mouihate and Q. J. Pittman Neuroimmune Response to Endogenous and Exogenous Pyrogens Is Differently Modulated by Sex Steroids Endocrinology, June 1, 2003; 144(6): 2454 - 2460. [Abstract] [Full Text] [PDF] |
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R. Marienfeld, M. J. May, I. Berberich, E. Serfling, S. Ghosh, and M. Neumann RelB Forms Transcriptionally Inactive Complexes with RelA/p65 J. Biol. Chem., May 23, 2003; 278(22): 19852 - 19860. [Abstract] [Full Text] [PDF] |
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A. J. Casal, J.-S. Silvestre, C. Delcayre, and A. M. Capponi Expression and Modulation of Steroidogenic Acute Regulatory Protein Messenger Ribonucleic Acid in Rat Cardiocytes and after Myocardial Infarction Endocrinology, May 1, 2003; 144(5): 1861 - 1868. [Abstract] [Full Text] [PDF] |
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S. Gupta, S. Gyomorey, S. J. Lye, W. Gibb, and J. R. G. Challis Effect of Labor on Glucocorticoid Receptor (GRTotal, GR{alpha}, and GR{beta}) Proteins in Ovine Intrauterine Tissues Reproductive Sciences, April 1, 2003; 10(3): 136 - 144. [Abstract] [PDF] |
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C. Lukas-Croisier, C. Lasala, J. Nicaud, P. Bedecarras, T. R. Kumar, M. Dutertre, M. M. Matzuk, J.-Y. Picard, N. Josso, and R. Rey Follicle-Stimulating Hormone Increases Testicular Anti-Mullerian Hormone (AMH) Production through Sertoli Cell Proliferation and a Nonclassical Cyclic Adenosine 5'-Monophosphate-Mediated Activation of the AMH Gene Mol. Endocrinol., April 1, 2003; 17(4): 550 - 561. [Abstract] [Full Text] [PDF] |
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S. G. Kallapur, B. W. Kramer, T. J. M. Moss, J. P. Newnham, A. H. Jobe, M. Ikegami, and C. J. Bachurski Maternal glucocorticoids increase endotoxin-induced lung inflammation in preterm lambs Am J Physiol Lung Cell Mol Physiol, April 1, 2003; 284(4): L633 - L642. [Abstract] [Full Text] [PDF] |
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K. Okamoto, H. Hirata, S. Takeshita, and D. A. Bereiter Response Properties of TMJ Units in Superficial Laminae at the Spinomedullary Junction of Female Rats Vary Over the Estrous Cycle J Neurophysiol, March 1, 2003; 89(3): 1467 - 1477. [Abstract] [Full Text] [PDF] |
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