
* Department of Medical Biochemistry, Arabian Gulf University, Manama, Bahrain; and
Department of Biology, American University of Beirut, Lebanon
Correspondence: Dr. Wassim Y. Almawi, Department of Medical Biochemistry, College of Medicine & Medical Sciences, Arabian Gulf University, P.O. Box 22979, Manama, Bahrain. E-mail: wassim{at}agu.edu.bh
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B, NF-AT, and AP-1,
through direct and indirect mechanisms. GCs induced the gene
transcription and protein synthesis of the NF-
B inhibitor, I
B.
Activated GR antagonized transcription factor activity through
protein:protein interaction. This involved complexing with and
inhibition of transcription factor binding to DNA (simple model),
association with factor bound at its DNA site (composite
model), and/or through interaction of GRE-bound GR with
DNA-bound transcription factor (transmodulation model). Finally, GR
competed with transcription factors for nuclear coactivators
(competition model), including CBP and p300. Remarkably, GR did not
affect the assembly of the preinitiation complex but acted proximally
in inhibiting transcription factor activity and thus transcriptional
initiation.
Key Words: GRE NF-
B NF-AT AP-1 transactivation transrepression
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|
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B
(NF-
B). This review focuses on the effect of GCs on
transcription-factor activity. For discussion about other aspects of
GC-antiproliferative effects, the reader is referred to reviews
published elsewhere [1
, 5
]. |
|
|---|
B (stimulated by PKC and
other signaling pathways), where they bind the interleukin (IL)-2
enhancer and stimulate IL-2 transcription [7
] (Fig. 1)
.
An orderly expression of cytokine genes and their high-affinity
receptors ensues, followed by induction of T-cell proliferation
[8
].
![]() View larger version (36K): [in a new window] |
Figure 1. T-cell activation cascade. T-cell activation through the multimeric
T-cell receptor-CD3 complex results in the activation of several
intracellular signaling pathways leading to phosphorylation or
dephosphorylation of cytoplasmic target molecules. These converge at
provision and nuclear translocation of transcription factors NF- B,
NF-AT, and the AP-1 complex, which subsequently bind their specific
upstream DNA sites, hence facilitating RNA polymerase II-directed
activity.
|
(IFN-
) [19
],
tumor necrosis factor-
(TNF-
) [20
], and the
colony-stimulating factors (CSF), macrophage (M)-CSF
[21
], granulocyte (G)-CSF [22
], CSF-1
[23
], and granulocyte-macrophage (GM)-CSF
[14
]. Inhibition of cytokine expression was specific for
the GCs because non-GC steroids failed to affect cytokine synthesis
[24
] or T-cell proliferation [25
] and as
the GR antagonist, RU-486 abrogated GC effects [4
,
11
, 16
]. Inhibition of cytokine expression
by GCs involved transcriptional and posttranscriptional events (see
below) and resulted in a significant reduction in cytokine secretion
and a profound inhibition of T-cell-effector function. Furthermore,
recombinant cytokines abrogated GC effects, including apoptosis
[26
], inhibition of cytokine expression
[15
], and suppression of T-cell proliferation
[10
]. Kinetic differences exist between different GCs in
inhibiting cytokine expression and/or T-cell activation, depending on
the GC itself, stimulus used, and activation status
[25
]. |
|
|---|
B inhibitor, I
B
[27
, 28
], and the latter is
exemplified by the demonstrated effect of GCs in suppressing the
expression of IL-2 and other cytokine genes [1
]. In the cytoplasm, GRs exist as an inactive complex with two molecules of heat-shock protein (HSP-90) and other cytosolic proteins [29 ]. Upon binding its cognate ligand (GCs), GR undergoes conformational changes, dissociates from HSP-90 binding, and subsequently translocates to the nucleus where it transiently associates with HSP-56. GR, in turn, dissociates from HSP-56 and binds as a dimer to conserved palindromic DNA sequences, the GREs, which comprise two boxes spaced by three variable nucleotides (GGTACAnnnTGTTCT), each box binding one of the two GR zinc fingers [30 ]. GREs are located in variable copy numbers and are found at variable distances from the TATA box in the promoter region of GC responsive genes, including cytokine genes.
The GR, a member of the steroid superfamily (which includes steroid,
thyroid hormone, vitamin D, and retinoic acid receptors
[31
]), consists of three domains: a hormone-binding
domain, a highly conserved DNA-binding domain, and an N-terminal region
[31
]. An intact GRE site [32
] and DNA
binding domain within the GR [33
] are required for
efficient transcriptional modulation by GCs. Binding of activated GRs
complex to GRE-DNA elements results in downstream inhibition of
cytokine gene expression in a cis-acting (steric hindrance)
[34
] or, alternatively, trans-acting fashion, the latter
involving induction of the synthesis of specific inhibitor(s).
According to the former, GR-GRE binding is viewed as that of a masking
effect, where GR binds to sites overlapping the binding sites of basal
[35
] and induced [36
] transcription
factors, resulting in steric interference with the binding of
transcription/enhancing factors (NF-
B, NF-AT) to their putative DNA
sites. The latter involves expression of a specific GC-induced
inhibitor of T-cell activation [37
], including the
NF-
B antagonist I
B [27
]. It should be noted here
that although evidence supporting both possibilities is documented,
conclusions drawn must be viewed in the context of the cell type and
gene studied.
|
|
|---|
B [34
,
39
, 40
], and NF-AT [13
,
33
, 41
]. Reduction in transcription factor
availability and function subsequently translated into reduction and
eventually cessation of transcription in target genes. It is noteworthy
that GR did not interfere with the assembly of the preinitiation
complex (including association of RNA polymerase II with TFII complex
of nuclear proteins), thereby localizing its effect at DNA sites
upstream of the TATA box [42
]. Several mechanisms were
postulated for GC effects, including induction of production of NF-
B
inhibitor, I
B [27
, 28
, 43
];
protein:protein interaction, where GR prevented transcription-factor
binding to DNA sites [44
, 45
], repressed
the activity of the transcription factor without affecting its binding
onto DNA [34
, 42
], or associated with the
factor, thereby becoming functionally (repressive) active; and
competition with transcription factor for nuclear coactivators
[46
, 47
]. Elucidation of the exact
mechanism of action of the GCs and the scope of interaction of the
activated GR with AP-1, NF-
B, NF-AT, and other transcription factors
are of paramount importance toward development of a more
efficacious anti-inflammatory and immunosuppressive regimen.
B SYNTHESIS
|
|
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B, a member of the mammalian rel gene family, which
comprised p105/p50, p100/p52, p65 (RelA), RelB, and others
[48
], is a heterodimer of p65 (RelA) and p50 and in the
inactivated state is sequestered in the cytoplasm through the ankyrin
repeats of its specific inhibitor, I
B (Fig. 2
). I
B, a member of a large family of inhibitory molecules that
comprised I
B
, I
Bß, I
B
, and I
B
[49
], masked the nuclear localization signal of NF-
B,
resulting in its retention in the cytoplasm. Activation by
extracellular signals supposedly induced the phosphorylation and
ubiquitinylation of I
B by specific I
B kinases (IKK
and
IKKß), leading to its rapid proteolytic degradation and thus the
release of NF-
B [49
] (Fig. 2)
. NF-
B then undergoes
nuclear translocation and binds its decameric DNA response element as a
homo- or heterodimer comprising p50 and p65 (RelA) subunits, thus
stimulating the transcription of NF-
B-regulated genes
[48
, 49
], including cytokine and I
B
genes [28
, 50
]. Persistent NF-
B
activation, in turn, leads to increased I
B synthesis and sequesters
cytosolic NF-
B, thereby attenuating its activity [48
,
50
].
![]() View larger version (58K): [in a new window] |
Figure 2. Induction of I B synthesis by GCs. Cellular activation results in
phosphorylation of I B (mediated by the I B kinases IKK and ß)
and its dissociation from NF- B binding and degradation by
proteasomes. NF- B (p50 and p65) then translocates to the nucleus
where it binds B sites. By inducing I B synthesis, GC provides for
increasing I B availability, which subsequently binds to and
sequesters NF- B in the cytosol, hence preventing it from
translocating to the nucleus.
|
B
synthesis [27
, 28
]. GC treatment of phorbol
ester 12-O-tetradecanoylphorbol 13-acetate (TPA)-stimulated
Jurkat cells [27
], TNF-stimulated HeLa cells
[28
], vascular epithelial cells of Crohns disease
patients [43
], lipopolysaccharide (LPS)-stimulated
macrophages [20
], and brain cells [51
]
resulted in a concentration-dependent induction of I
B synthesis.
According to these studies, GC treatment induced I
B synthesis
(assessed by gel shift and nuclear run-on transcription assays) without
affecting its phosphorylation and subsequent degradation
[28
]. This constituted a negative feedback loop, whereby
I
B binds to and sequesters NF-
B in the cytosol and prevents it
from translocating to the nucleus [27
, 28
].
It was even proposed that I
B acted at the level of nucleus, binding
and removing NF-
B from binding
B sites [27
,
28
], although the universality of this finding remains to
be determined.
Several reports disputed induction of I
B synthesis as the mechanism
by which GCs repressed gene activation, which was based along several
lines of reasoning. First, it was based on the finding that in many
cell types, GC treatment did not stimulate [52
] or
actually decreased I
B synthesis, as was shown for TNF-
-activated
endothelial cells [34
] and IL-1ß-stimulated epithelial
cells [45
]. GR mutants, which did not enhance I
B
synthesis, repressed NF-
B activity [40
]. Conversely,
induction of I
B synthesis by synthetic GC analogues failed to
repress NF-
B activity [40
]. Furthermore, GC-mediated
down-regulation of NF-
B was described to be independent of I
B
induction [39
, 40
, 53
],
because despite clear induction of I
B synthesis by GCs, increased
I
B synthesis did not [34
, 39
], or only
partially affected [20
], GC anti-inflammatory and
immunosuppressive effects and because GC effects were resistant to
cycloheximide treatment [34
]. This effectively argued
against induction of de novo I
B synthesis or a GC-mediated
stabilization of cytosolic NF-
B association with I
B
[28
] as potential mechanisms by which GCs repress
transcriptional of cytokine genes.
This indicated that GC induction of I
B synthesis and thus antagonism
of NF-
B activity is an independent event [53
,
54
] or is cell-type specific [39
,
55
]. However, the latter is questioned, because
contradicting effects of GCs on I
B synthesis were observed in the
same tissue and cell types. This was exemplified by the demonstrated
capacity of GCs according to some studies to induce I
B synthesis in
brain cells [51
] and in L929 cells [55
]
but in sharp disagreement with other studies, which showed that GCs did
not affect I
B levels in brain cells [54
] or in L929
cells [34
]. This prompted the speculation that GC
effects on I
B synthesis and subsequently on NF-
B synthesis may be
highly cell-specific [39
] or a result of specific
activation signals, and this questioned whether stimulation of I
B
synthesis by GCs is even required or sufficient to repress NF-
B
activity [40
].
|
|
|---|
B, and AP-1.
According to this model, proposed and increasingly being adopted by
several investigators, the GC antiproliferative effect is viewed as the
result of binding GR to a critical site within the transcription factor
prior to DNA binding (simple model) or following association with the
DNA-binding site of the transcription factor (composite model).
Although direct association of the GR with DNA, for example, through
binding negative GREs, was found not to be obligatory, it was not ruled
out completely. Protein:protein interaction between hormone-activated
GR and transcription factor resulted in interference with the
functional capacity of the transcription factor to stimulate
transcription. |
|
|---|
B [34
,
52
, 56
], and AP-1 [56
,
57
], thereby abolishing their capacity to bind DNA. In
exerting its effect, GR did not alter the nuclear translocation
[13
, 56
] or inhibit the synthesis of the
transcription factor [45
, 52
] but acted by
interfering with DNA binding through reciprocal masking of a specific
domain within the GR and the transcription factor (Fig. 3)
[38
]. This did not result in a competitive displacement
of transcription factor bound to DNA but was associated with antagonism
of binding [56
, 58
], evidenced by the
coimmunoprecipitation of GR and nuclear transcription factor
[13
, 44
] and by the reversal of the GC
effect by overexpression of transcription factor [44
],
although the universality of the latter conclusion could not be
confirmed. From suppression of a key signaling pathway involved in
transcription-factor activation, the capacity of the GR to inhibit
transcription-factor binding may have, in principle, exemplified a key
mediator of AP-1 activation [59
] or possibly as a
consequence of earlier antagonism of other transcription factors
[13
], evidenced by the demonstrated capacity of the GCs
to antagonize AP-1 binding through inhibition of c-Jun
NH2-terminal kinase (JNK).
![]() View larger version (55K): [in a new window] |
Figure 3. The "simple model" of protein:protein interaction. According to the
simple model, the hormone-activated GR did not affect the availability
or translocation of transcription factors (exemplified by NF-AT and
NF- B) but rather acted more distally in binding to the transcription
factor in the nucleus and/or cytosol, thereby forming a complex, which
failed to bind to DNA.
|
|
|
|---|
B [39
, 42
], which, in turn,
destabilized the interaction of basal transcription factors (TFIID)
with the TATA-binding domain (TBD). In addition to NF-
B, GR also
inhibited AP-1 activity without altering the binding of AP-1 (Fos-Jun)
or itself binding to DNA [61
], as evidenced by the
capacity of GR mutants lacking the DNA-binding domain to bind to and
inhibit AP-1 activity [57
, 62
].
Collectively, this indicated that GR acted by forming a complex with
the transcription factor at the binding site of the latter, which, in
turn, attenuates and eventually terminates transcription (Fig. 4
). However, it remains to be seen whether the association of GR with the
transcription factor is sufficient to repress the transcriptional
activity of the latter or, in addition, requires the participation of a
corepressor as was suggested (Fig. 4)
[42
].
![]() View larger version (55K): [in a new window] |
Figure 4. The "composite model" of protein:protein interaction. In the
composite model, ligand-activated GR did not bind DNA but associated
with transcription factor bound at its respective DNA site, as shown
for NF- B bound to the B site. This association inhibited
downstream transcriptional activities, without affecting the correct
assembly of the preinitiating complex at the TATA box. GR may act
directly (B) or may require the participation of corepressor(s)
(C).;8>
|
|
|
|---|
B,
AP-1, AP-3, Oct-1, and NF-AT [13
, 33
,
67
], and evidence of the formation of a complex of these
factors with GR (assessed by double immunoprecipitation) was shown. Correct spacing between GRE and transcription factor binding sites was necessary for GR to exert its effect. Interaction of ligand-bound GR with AP-1, where GRE sites were positioned within a few bases from the AP-1-binding site, resulted in repression of AP-1-driven activity [64 ]. This did not result from occupancy of the AP-1 site or from inhibition of AP-1 binding, because both factors were shown to be clearly bound to their DNA sites, but rather involved a direct protein:protein association of GR with c-Fos and thus repression of AP-1 activity [64 ]. It was interesting to note that composition of AP-1 influenced GR activity according to this model, because Jun-Fos heterodimers, but not Jun-Jun homodimers, associated with GR [68 ] and because c-Fos (but not c-Jun) induced the expression of GR [69 ] and associated with it [64 , 70 ]. Collectively, this suggested that the association of transcription factors with GR, coupled with the close proximity of their DNA-binding sites to GRE sites, prevented correct assembly of the preinitiation complex and hence initiation of proper transcription.
|
|
|---|
B [71
],
and as GRs were shown to antagonize transcription factors, it was
suggested that GRs acted, at least in part, by competing with nuclear
transcription factors for nuclear coactivators (Fig. 5
) [47
, 72
]. In support of this were the
findings that CBP augmented GR-suppressive effects [73
],
enhanced the association of GR with CBP, and hence suppressed
NF-
B activity [46
, 74
], and as
overexpression of CBP, abrogated GR-mediated repression of NF-
B
activity [46
]. GCs down-regulated mRNA and protein
accumulation of SRC-1, a key "adaptor" coactivator, thereby
providing for an autoregulatory loop of GC action [75
].
Insofar as coactivators, including SRC-1 [46
,
76
] and CBP [72
], were described as an
integral link between basal transcription factors and other
transcription factors, including GR and NF-
B [72
,
76
], competition for a limited amount of nuclear
coactivators between GR and other induced transcription factors, at
least in part, antagonized transcription factors (Fig. 5)
.
![]() View larger version (47K): [in a new window] |
Figure 5. The "competition model". Coactivator proteins, by associating with
transcription factors (TF) bound at their specific DNA sites
(TF-binding sites) and the preinitiation complex of basal TFs and RNA
polymerase II, facilitate TF effects, thereby promoting transcription.
GR, by binding to the nuclear coactivator, competes with TF for the
coactivator, thereby breaking the link between TF and the preinitiation
complex, thus repressing transcription.
|
B [39
], independently of
CBP levels in the cell. Increased AP-1 and NF-
B binding was
associated with overall reduced CREB binding [56
],
thereby questioning whether reduced CREB and other coactivator function
and GR repression of transcription factors were related. Indeed, the
transactivation and transrepression functions of the GR were shown to
be separate entities [40
, 77
], and the
requirement for direct association with specific AP-1 or NF-
B
subunits on overall GR function (activation or repression), without
necessarily involving an adaptor or competing for a nuclear
coactivator, is well-documented [39
, 64
,
78
]. Additional studies are required to confirm or
alternatively rule out competition for nuclear coactivator(s) as a
mechanism by which GRs antagonize transcription factors. |
|
|---|
B translocation and hence its binding to the I
B DNA
site through a protein:protein interaction [83
] and to
interact with c-Fos, thereby inhibiting the translocation of AP-1
(dimer of Fos and Jun) and subsequently its DNA binding
[82
]. Collectively, this suggested that GC antagonism of
NF-
B and AP-1 activity results, at least in part, from induction of
GILZ. |
|
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Received October 2, 2001; accepted October 15, 2001.
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