| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
B activation by G-protein-coupled receptors
Department of Pharmacology, University of Illinois College of Medicine, Chicago, Illinois
Correspondence: Dr. Richard D. Ye, Department of Pharmacology, MC868 University of Illinois, College of Medicine, 835 South Wolcott Avenue, Chicago, IL 60612-7343. E-mail: yer{at}uic.edu
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONGPCR-MEDIATED EXPRESSION OF...CHEMOKINE-INDUCED PRODUCTION OF...SIGNALING MECHANISMSBIOLOGICAL RELEVANCE AND FUTURE...REFERENCES |
|---|
B is one of the most important transcription
factors responsible for the expression of these proinflammatory genes.
Recent studies have shown that GPCRs utilize several different pathways
to activate NF-B. These pathways differ from the ones induced by
classic cytokines in that they are initiated by heterotrimeric
G-proteins, but they converge to IB phosphorylation and nuclear
translocation/modification of the NF-B proteins. GPCR-induced
NF-B activation provides an effective means for local expression of
cytokine and growth factor genes due to the wide distribution of these
receptors. Chemokine-induced, GPCR-mediated production of chemokines
constitutes an autocrine regulatory mechanism for the growth of certain
malignant tumors and enhances the recruitment of leukocytes to sites of
inflammation.
Key Words: gene expression • transcription factors • NF-B • signal transduction
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONGPCR-MEDIATED EXPRESSION OF...CHEMOKINE-INDUCED PRODUCTION OF...SIGNALING MECHANISMSBIOLOGICAL RELEVANCE AND FUTURE...REFERENCES |
|---|
subunits and closely associated ß
subunits
[1
]. The complexity of the signaling pathways initiated
by GPCRs is illustrated by the presence of numerous G-proteins,
including 18 subunits, which can be classified into four groups, 12
ß subunits, and 5 subunits [2
].
Leukocytes express a large number of receptors for chemokines and
classic chemoattractants [3
, 4
]. These are
GPCRs that primarily couple to the Gi class of G-proteins
for chemotaxis and other cellular functions including degranulation and
generation of superoxide anions [5
]. Many laboratories
have demonstrated that classic chemoattractants and chemokines can
stimulate inflammatory cytokine gene expression in monocytes,
neutrophils, and lymphocytes. In some cases, up-regulation of cytokine
mRNAs is followed by enhanced secretion of cytokines. Thus,
chemoattractants participate in the induction of inflammatory cytokine
expression, a task often accomplished by tumor necrosis factor (TNF)
, interleukin (IL)-1ß, and bacterially derived products such as
lipopolysaccharide and peptidyl glycans. More recent findings on the
expression of chemokine receptors in nonhematopoietic cells suggest
that chemokines can regulate cell growth and differentiation in part
through transcription factor activation. In addition to
chemoattractants, other GPCR agonists, including proteases,
neuropeptides, and lipid mediators, have been found to regulate
transcriptional activities in a variety of cells. These observations
suggest that GPCR regulation of transcription is not confined to
leukocytes but is broadly applicable to other types of cells. This
review provides a brief summary of recent progress in GPCR-mediated
transcription regulation with an emphasis on the activation of nuclear
factor (NF) B, a transcription factor that profoundly influences
leukocyte functions and the inflammation process [6
,
7
].
|
GPCR-MEDIATED EXPRESSION OF CYTOKINES AND OTHER PROINFLAMMATORY FACTORS |
|---|
|
TOPABSTRACTINTRODUCTIONGPCR-MEDIATED EXPRESSION OF...CHEMOKINE-INDUCED PRODUCTION OF...SIGNALING MECHANISMSBIOLOGICAL RELEVANCE AND FUTURE...REFERENCES |
|---|
-interferon
to induce IL-1 expression [9
]. The same group also
reported secretion of TNF- from C5a-stimulated monocytes
[10
]. In addition, C5a has been known to stimulate the
production of IL-6 and IL-8 [11
12
13
]. Induction of
cytokine secretion by C5a is the result of enhanced transcription
[13
14
15
]. However, at least two published reports
suggest that a translational signal must come from sources other than
C5a, such as endotoxin or IL-1 itself [14
,
15
]. These discrepancies might be attributable to
differences in experimental conditions. In addition to C5a, other
chemoattractants including PAF, LTB4, and fMLF have been
shown to stimulate cytokine gene expression and protein secretion in
monocytes and neutrophils [16
17
18
19
20
].
GPCR agonists other than classic chemoattractants and chemokines have
also been shown to induce the expression of proinflammatory cytokines,
growth factors, and adhesion molecules. For example, thrombin, a
component of the blood coagulation system and a mitogen for
fibroblasts, can induce the expression of several cytokines, chemokines
[21
, 22
], and adhesion molecules
[23
]. Substance P has been found to stimulate NF-B
activation and IL-8 gene expression through the neurokinin (NK)-1
receptor [24
]. This function might be responsible for
its neurogenic inflammatory and immunomodulatory activities.
Lysophosphatidic acid (LPA) stimulates the activation of NF-B
[25
] as well as serum response factor
[26
], permitting modulation of multiple gene expression
events. A partial list of GPCR agonists that have been known to
stimulate NF-B activation is given in Table 1 .
|
|
CHEMOKINE-INDUCED PRODUCTION OF CHEMOKINES AND GROWTH FACTORS |
|---|
|
TOPABSTRACTINTRODUCTIONGPCR-MEDIATED EXPRESSION OF...CHEMOKINE-INDUCED PRODUCTION OF...SIGNALING MECHANISMSBIOLOGICAL RELEVANCE AND FUTURE...REFERENCES |
|---|
(GRO)/melanoma growth-stimulating activity (MGSA) and its homologues
GROß and GRO [47
, 48
]. Continuous
expression of GRO, ß, and confers tumor-forming capacity to
melan-a, an immortalized murine melanocyte cell line
[49
]. This effect of the GRO chemokines is abrogated by
antibodies that interfere with binding of the chemokines to their
receptors, suggesting that the GRO chemokines function by activating
the receptors. Norgauer et al. used reverse transcriptase
(RT)-polymerase chain reaction (PCR) and flow cytometry to identify the
expression of CXCR2 in melanoma cell lines and normal melanocytes
[50
]. Blockade of GRO binding to CXCR2 partially
inhibits cell growth, suggesting a correlation between the
GRO-CXCR2 interaction and serum-independent melanoma growth
[50
].
The expression of GRO is regulated in part at the transcription
level. Binding sites for several transcription factors, including
NF-B and Sp1, have been found in the promoter regions of GRO,
ß, and [51
52
53
]. Similarly, the expression of IL-8
is regulated by NF-B, activator protein-1 (AP-1), and NF–IL-6
[54
55
56
]. Several laboratories have investigated the
possibility that the GRO chemokines and IL-8 bind CXC receptors and
stimulate an autocrine amplification loop. Metzner et al. reported that
the A431 epidermoid carcinoma cell line expresses higher levels of
GRO, IL-8, and CXCR2 when compared with expression in normal
keratinocytes [57
]. The two chemokines have been found
to induce proliferation of the A431 cells, and neutralizing antibodies
against either CXCR2 or the chemokines inhibit cell growth
[57
]. A study conducted by Takamori and coworkers
suggests that IL-8 and GRO stimulate the growth of a pancreatic
tumor line, Capan-1, through binding and activation of CXCR2. Their
experimental results indicate that pretreatment of the cells with
pertussis toxin (PTX) or a specific anti-CXCR2 antibody inhibits cell
growth, implying a role of Gi proteins in CXCR2 signaling
[58
]. A study by Wang and Richmond has demonstrated that
GRO activates NF-B through a pathway involving Ras, MAP/ERK
kinase kinase-1 (MEKK1), and p38 [39
]. These findings
suggest that the constitutive high-level expression of GRO and
IL-8 found in certain tumor cells is the result of CXCR2-mediated
transcriptional activation and that this autocrine loop plays a role in
the sustained growth of these tumor cells (Fig. 1
).
|
[61
]. KSHV-GPCR is a
constitutively active receptor that binds several chemokines
[62
], among which IL-8 and GRO further stimulate its
activation [63
] whereas inducible-protein 10 (IP-10) and
stromal cell-derived factor-1 (SDF-1) serve as inverse agonists
[64
, 65
]. Among mammalian GPCRs, KSHV-GPCR
is most homologous to CXCR2. Mutation of an aspartic acid at position
138 in CXCR2 to a valine (as in KSHV-GPCR) leads to constitutive
activation of the receptor similar to KSHV-GPCR [66
].
These findings raise the possibility that KSHV-GPCR stimulates the
expression of chemokines that can bind to and further activate this
receptor as seen with CXCR2. Two recent studies have confirmed that
KSHV-GPCRs indeed activate transcription factors [67
,
68
] that in turn stimulate the secretion of IL-8, IL-6,
monocyte chemoattractant protein-1 (MCP-1), TNF-, and IL-1ß
[67
]. The induced IL-8 expression is further enhanced by
GRO [67
] and reduced by IP-10 [68
]
consistent with the properties of these chemokines as the respective
agonist and inverse antagonist for KSHV-GPCR [64
,
65
]. Another virus-derived GPCR, US28, recently has been
shown to be a constitutively active receptor that induces NF-B
activation [69
]. Thus, virally derived GPCRs, like their
mammalian equivalents, might utilize autocrine (e.g., production of
GRO) and paracrine (e.g., secretion of VEGF) mechanisms to regulate
the growth of infected cells and the nearby host cells (Fig. 1)
.
In leukocytes, autocrine regulation can help to maintain a basal level
of chemokines, similar to that observed with IL-1ß and TNF-. IL-8
has been shown to induce the expression of IL-8 mRNA in isolated blood
monocytes [70
]. Browning et al. demonstrated that
Sepharose-immobilized IL-8 could stimulate IL-8 secretion in isolated
blood mononuclear cells [71
]. It is also known that
incubation of isolated peripheral blood monocytes results in
time-dependent expression of MCP-1 [72
]. Expression of
MCP-1 is regulated in part by NF-B and Sp1 [73
].
Other leukocytes also respond to chemokine stimulation with expression
of cytokine and chemokine genes. With cDNA array technology,
investigators can now obtain a gene expression profile with relative
ease and match specific transcriptional events with the expression of a
given chemokine. The induced expression of proinflammatory factors,
especially those with chemotactic activities, promotes recruitment of
leukocytes to the sites of inflammation.
|
SIGNALING MECHANISMS |
|---|
|
TOPABSTRACTINTRODUCTIONGPCR-MEDIATED EXPRESSION OF...CHEMOKINE-INDUCED PRODUCTION OF...SIGNALING MECHANISMSBIOLOGICAL RELEVANCE AND FUTURE...REFERENCES |
|---|
B activation. GPCRs can either stimulate or inhibit
NF-B activation depending on the G-proteins that are involved. Based
on structure, the subunit of G-proteins can be divided into four
classes: Gs, Gi, Gq, and
G12 [2
]. At least one member in each G
class and several Gß proteins have been shown to regulate NF-B
activation (Fig. 2
).
|
B activation
[78
79
80
]. ß-Adrenergic agonists may also increase the
production of IB, thereby inhibiting NF-B activation
[81
].
Gq and phospholipase C (PLC) ß
The Gq family of subunits consists of
q, 11, 14, and
16 [2
]. The mouse equivalent of
16 is 15 [82
].
Collectively, these G-proteins couple a large number of GPCRs for
activation of PLC-ß. Among the GPCRs that have been shown to activate
NF-B, many couple to Gq. These include receptors for
thrombin, PAF, endothelin, 5-hydroxytryptamine, LPA, and bradykinin
(Table 1) . In several experiments, it has been shown that phosphokinase
C (PKC) inhibitors could block activation of NF-B by some of these
agonists, suggesting the potential involvement of PKC in
G-protein-mediated NF-B activation [25
,
83
].
To determine the mechanism by which q regulates
transcription, constitutively active mutants of q are
used in transfected cell lines. It has been demonstrated that a Q209L
mutation of q, when expressed in transiently transfected
cells, produces the same effect as bradykinin in NF-B activation
[31
]. Furthermore, q-mediated NF-B
activation involves the IB kinases (IKK), including IKK1 (IKK)
and IKK2 (IKKß), and can be blocked by a repressor of IB
[31
]. Inhibitors for phosphatidylinositol (PI) 3-kinase
(PI3K) as well as dominant negative constructs of PI3K and its
downstream effector Akt (PKB) partially block the
q-mediated NF-B activation [31
]. These
results suggest that q uses the PI3K pathway, in
addition to the PLC-ß pathway, to activate NF-B.
Other members of the Gq family have also been shown to
regulate transcription. It is reported that the virus-derived chemokine
receptor US28 uses 11 to activate NF-B and
constitutively active 11 enhances this response
[69
]. Another member of the Gq family,
16, is primarily expressed in hematopoietic cells and
has been known to stimulate PLC-ß activation [84
].
Whereas the biological function of this G-protein remains unknown, it
has been well recognized that 16 couples to a large
number of GPCRs, including most chemokine and chemoattractant receptors
[85
86
87
]. Targeted deletion of mouse 15
leads to reduced PLC-ß activation in response to C5a stimulation,
supporting the notion that 16 is responsible for some of
the chemoattractant-induced cellular responses [88
]. A
recently published paper demonstrates that 16 indeed can couple
chemokine and chemoattractant receptors, such as the receptors for
fMLF, C5a, C3a, and IL-8, to NF-B activation in transfected cells
[37
]. Whether these GPCRs use 16 for
transcriptional regulation in leukocytes remains unknown, but one study
has shown that reduction of 16 protein levels in an
erythroleukemia cell line can impair the expression of the ß-globin
gene [89
].
The Gq proteins directly activate PLC-ß, resulting in the
generation of second messengers IP3 and diacyl
glycerol. These molecules promote the activation of conventional PKC
(cPKC) and the release of Ca2+ from intracellular stores.
Elevation of intracellular Ca2+ further activates cPKC
(Fig. 2)
. The functions of these second messengers in GPCR-mediated
gene transcription have been confirmed. Shahrestanifar et al. have
reported that LPA-induced NF-B activation can be blocked by
buffering the rise of intracellular Ca+ with
1,2-bis(O-aminophenyl-ethane-ethane)-N1N1N'1N'-tetraacetic
acid and by PKC inhibitors [25
]. Yang et al. have
demonstrated dependence of 16-mediated NF-B
activation on intracellular Ca2+ and cPKC
[37
]. Several PKC isoforms, including cPKC, are known
activators of NF-B based on early studies of the effect of phorbol
esters on NF-B activation. Thus, Gq-mediated NF-B
activation is the result of signaling through PLC-ß that most likely
converges to an established pathway involving cPKC and IKKs.
A recent publication suggests a novel mechanism by which
q might regulate transcription [90
].
Tubby, a protein containing a positively charged groove implicated in
DNA binding, has been shown to bind to q and is
associated with phosphatidylinositol 4,5-bisphosphate. Activation of
q leads to release of Tubby from plasma membrane and
translocation to nuclei, presumably through hydrolysis of
phosphatidylinositol 4,5-bisphosphate by PLC-ß. This mode of action
is similar to the release and translocation of the p50/p65 subunits of
NF-B to the nucleus as a result of IB degradation
[91
]. It is interesting that Tubby release from the
plasma membrane is not affected by the ß subunits, which
dissociate from the subunits on G-protein activation and can
activate PLC-ß. Experimental data indicate that s and
o do not stimulate the release of Tubby
[90
], and it is not clear whether nuclear translocation
of Tubby affects NF-B activation.
The Gi proteins
The Gi family of subunits has been implicated in
coupling chemokine and chemoattractant receptors to leukocyte functions
such as chemotaxis, superoxide generation, and degranulation [reviewed
in ref. 5
and 92]. A number of these receptors, including those for
fMLF, C5a, C3a, PAF, LTB4, SDF-1, IL-8, and GRO, have
been reported to mediate activation of transcription factors such as
NF-B (Table 1)
. The i (and o) proteins
are substrates for PTX-catalyzed ADP-ribosylation. This covalent
modification of the G-protein carboxyl terminus interrupts receptor and
G-protein interaction. Indeed, NF-B activation and gene expression
induced by some of these GPCRs is sensitive to PTX [17
,
18
, 32
]. However, there are reported cases
in which PTX does not affect or only partially blocks the activation of
NF-B [13
, 41
]. One explanation for this
discrepancy is that receptors for chemokines and chemoattractants can
also utilize PTX-insensitive G-proteins, such as 16 and
q, for activation of NF-B and other transcription
factors.
The i-subunits are known for their ability to inhibit
adenylyl cyclase, but these G-proteins do not directly activate
downstream effectors such as PLC-ß [1
]. It is unlikely
that i-coupled GPCRs activate NF-B through reduction
of intracellular cAMP levels, because i proteins are
more effective in opposing the rise of cAMP levels (e.g. by forskolin),
and stimulation of leukocytes by these chemoattractants is not
associated with drastic changes in cAMP levels. It is generally
believed that the release of Gß proteins on Gi
activation contributes to many of the GPCR functions, including
chemotaxis [93
, 94
], mitogen-activated
protein (MAP) kinase activation [reviewed in ref. 95
], and
transcriptional regulation [96
]. The role of Gß in
the activation of NF-B is discussed below.
G12 and RhoGEF
The G12 family of subunits consists of
12 and 13. These subunits are widely
distributed and are known for their abilities to regulate sodium-proton
exchange, cell proliferation, transformation, and apoptosis
[97
98
99
100
]. Hill et al. reported that G12 and
G13 mediate LPA-induced activation of the transcription
factor serum response factor (SRF) [26
]. G12
and G13 do not directly activate PLC-ß. Instead, they use
the Rho family of small GTPases as effectors [26
].
Kozasa and coworkers have reported that activation of the small
GTPase RhoA by 13 is mediated through a guanine
nucleotide exchange factor, p115RhoGEF [101
,
102
]. This exchange factor contains Dbl-homology
(DH) and pleckstrin homology domains for interacting with RhoA
and an N-terminal RGS domain that negatively regulates the activation
of the subunits in part through its property as a GTPase-activating
protein [101
]. In another study, Mao and coworkers have
demonstrated inhibition of LPA- and thrombin-induced SRF activation by
a DH-domain deletion mutant of p115RhoGEF and by RGS12
[103
]. Their results also indicate direct activation of
SRF by p115RhoGEF. These studies demonstrate relay of signals from the
G12 class of G-proteins to small GTPases.
Receptors for thrombin, LPA, endothelin, and thromboxane A2
can activate SRF in cells lacking q and
11 in an RGS12-sensitive manner [38
]. In
comparison, SRF activation by the type-1 muscarinic receptor and
1-adrenergic receptor depend exclusively on
q and/or 11, indicating lack of
12 and 13 coupling. The nonreceptor
tyrosine kinases Tec and Bmx might also play a role in the activation
of SRF [104
]. Tec and Bmx most likely act downstream of
GPCRs that couple to 12/13 and induce SRF in a
RhoA-dependent manner. Another tyrosine kinase, Pyk2, has been found to
mediate type-1 muscarinic receptor signaling to serum response element
downstream of 12 and 13
[105
].
The function of 12 and 13 in leukocyte
development and activation has not been identified. GPCRs that perform
specific leukocyte functions, including receptors for chemokines and
classic chemoattractants, are known for their coupling to the
Gi proteins. Two recent studies demonstrate that G2A, a
GPCR identified as an inducible receptor predominantly expressed in T
and B progenitors [106
], couples to 13
[107
, 108
]. G2A appears to slow cell cycle
progression and cause accumulation of cells at growth phase G2/M
[106
]. Activation of 13 by G2A results in
transformation of fibroblasts, through a pathway that involves RhoA and
can be inhibited by the RGS domain of p115RhoGEF [108
].
Similar to the LPA receptors, G2A activates SRF in transfected cells
[108
]. Recent experimental data from our laboratory
indicate that constitutively active mutants of 13 can
induce NF-B in a RhoA-dependent fashion [68
] (Fig. 2)
. Our preliminary results also suggest that a GPCR derived from KHSV,
ORF74, can use 13 to activate NF-B and to stimulate
IL-8 secretion [68
]. Thus, G-proteins of the
G12 family may play a potential role in regulating
proinflammatory gene expression.
Gß, PI3K, and Akt/PKB
Gß dissociates from G on activation and can independently
stimulate PLC-ß. Furthermore, Gß binds to the regulatory
subunits of PI3K and has been shown to activate these lipid kinases
[109
110
111
112
].
Several approaches have been taken to investigate the role of Gß
in NF-B activation. Expression of the Gß scavengers, such as
transducin and a C-terminal peptide of GPCR-specific kinase 2, results
in reduced NF-B activation in bradykinin-stimulated cells
[31
]. Expression of ß12 in
HeLa cells leads to NF-B activation independently of agonist
stimulation [31
]. In COS-7 cells, expression of
ß21 and ß22
but not ß12 and
ß51 was found to enhance NF-B
activation by the virus-derived GPCR US28 [69
]. The
difference in abilities of these ß proteins to stimulate NF-B
activation may be cell specific. It was found that both HeLa and COS-7
cells lack PLC-ß2 [37
], the PLC isoform that can be
readily activated by Gß [113
, 114
].
Furthermore, NF-B activation by ß12 is not
significantly affected by the PLC-ß inhibitor U-73122 (R. D. Ye,
unpublished results). These findings indicate a limited function of
PLC-ß in mediating Gß-induced NF-B activation in the cell
systems studied and suggest the presence of other signaling pathways.
To determine whether NF-B activation by Gß involves PI3K, Xie
et al. [31
] treated B2 bradykinin receptor-transfected
HeLa cells with wortmannin and LY294002, inhibitors for PI3K.
Subsequent stimulation of these cells with bradykinin resulted in
markedly reduced NF-B activation [31
]. Expression of
myristoylated p110 of PI3K also stimulates NF-B activation. That
PI3K is a downstream effector of GPCRs for NF-B activation has also
been reported using cultured cells [30
] and blood
monocytes [32
], although the role of Gß was not
investigated in these studies. In leukocytes, the p110 isoform of
PI3K is activated by Gß on chemoattractant stimulation
[112
, 115
]. The activation is mediated
through the regulatory subunit of this PI3K, p101 [116
],
which contains a ß binding site [117
]. It has also
been shown that fMLF-induced NF-B activation involves PI3K that can
be immunoprecipitated by an antibody against the p85 regulatory
subunit, suggesting that PI3K isoforms other than p110 might be
involved in GPCR-mediated NF-B activation in leukocytes
[32
].
The serine/threonine protein kinase Akt (PKB) is a downstream effector
of PI3K and plays a role in cell survival [118
]. Several
published studies have indicated the involvement of Akt in
cytokine-stimulated and PI3K-mediated NF-B activation
[119
120
121
122
]. Akt has also been implicated in
G-protein-mediated NF-B activation, because bradykinin can stimulate
Akt phosphorylation and a dominant-negative form of Akt can partially
inhibit NF-B activation by bradykinin or by
ß12 [31
]. In these studies
in which epithelial cells are used, Akt activation is most likely
downstream of the endogenous p110 or p110ß isoforms of PI3K
[123
]. However, the p110 isoform of PI3K can also
activate Akt [124
].
The mechanisms by which PI3K and Akt activate NF-B are not fully
understood. Although some studies suggest that Akt is part of the IKK
complex [121
, 122
], others report tyrosine
phosphorylation of IB [119
] and transactivation
through p65 phosphorylation [125
] to be responsible for
the activation of NF-B by PI3K and Akt. Bradykinin-induced NF-B
activation has been shown to involve phosphorylation of IKK
[31
], but it is not clear whether this is the direct
action of Akt (Fig. 2)
.
The Rho small GTPases
It has been known for several years that the Rho family of small
GTPases (RhoA, Cdc42, and Rac1) mediates the effect of
12 and 13 in activating SRF
[26
]. Subsequent studies have indicated that these small
GTPases also play a role in NF-B activation. Transfection of
constitutively activated forms of RhoA, Cdc42, and Rac1 into COS-7
cells has been shown to stimulate the expression of a NF-B-driven
luciferase reporter [126
]. Furthermore, NF-B
activation by the small GTPases may facilitate SRF activation by
increasing the nuclear contents of cis-regulatory enhancer
binding protein-like factor proteins [127
]. RhoA
has been shown to play a role in GPCR-mediated NF-B activation
[29
], and Rac1 has been implicated in NF-B activation
through Toll-like receptor 2 [128
].
The Rho small GTPases have been known to regulate cytoskeletal
rearrangement [129
]. Therefore, it is possible that the
Rho GTPases regulate transcription factor activation through this
mechanism. Rosette and Karin studied the effects of agents that alter
microtubule polymerization and found an association of microtubule
depolymerization with NF-B activation [130
].
Disruption of actin cytoskeleton by antibody binding of the
5ß1 integrin has been linked to
collagenase-1 gene expression, which is controlled by NF-B and
requires Rac1 activation [131
]. Reactive oxygen species
have also been suggested to play a role in NF-B activation by the
small GTPase Rac1 [131
132
133
]. Jefferies and O’Neill
recently reported that Rac1 regulation of IL-1-induced NF-B
activation is independent of IB degradation but involves NF-B
transactivation by p65 [134
]. Several Rac1 effectors,
such as p21-activated kinase-1 (PAK-1) [135
] and plenty
of SH3s (POSH) [136
], have been implicated
in Rac1-mediated NF-B activation.
|
BIOLOGICAL RELEVANCE AND FUTURE DIRECTIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGPCR-MEDIATED EXPRESSION OF...CHEMOKINE-INDUCED PRODUCTION OF...SIGNALING MECHANISMSBIOLOGICAL RELEVANCE AND FUTURE...REFERENCES |
|---|
B and other transcription
factors. What then is the biological significance for GPCR-mediated
NF-B activation given that potent inducers of NF-B such as
TNF- and IL-1ß already exist? Based on the studies summarized
above, GPCR-mediated transcriptional regulation serves several
important functions: (1) it provides an effective means for local
production of cytokines and growth factors because of the wide
distribution of GPCRs in various tissues and organs; (2) it often
produces a synergistic effect with cytokine-induced transcriptional
activation (there are already several examples demonstrating synergy
between GPCRs and receptors for TNF- and IL-1ß in transcriptional
activation); (3) it plays a unique role in regulating cell growth and
leukocyte recruitment through an autocrine mechanism. The last function
is best exemplified in the sustained growth of certain tumor cells,
which relies on continued production of the GRO family of
chemokines. Some virus-encoded GPCRs can stimulate transcription factor
activation and induce cytokine and growth factor biosynthesis,
resulting in pathological changes of the host and creating a favorable
environment for the survival of the infected cells [60
,
61
, 67
68
69
]. An important feature of GPCR-mediated transcriptional regulation is the ability of these receptors to couple to a diverse array of G-proteins. Identification of the role of each G-protein in transcriptional regulation will be of great importance because this information will allow us to understand how various GPCR-mediated pathways are initiated. Of particular interest are the biological functions of G16, G12, and G13 in leukocyte development because these are less investigated but potentially important areas of research. Published results indicate that activation of G-proteins generates signals that converge with the transcriptional activation pathways used by cytokine receptors. Exactly at which point the signals converge must be determined and this requires more detailed studies on GPCR-mediated transcriptional regulation. A better understanding of the mechanisms of GPCR signaling will help us to appreciate the potential of these receptors in converting environmental stimuli to transcriptional events.
|
ACKNOWLEDGEMENTS |
|---|
Received August 17, 2001; accepted August 20, 2001.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGPCR-MEDIATED EXPRESSION OF...CHEMOKINE-INDUCED PRODUCTION OF...SIGNALING MECHANISMSBIOLOGICAL RELEVANCE AND FUTURE...REFERENCES |
|---|
B and secretion of IL-8 induced by the G protein-coupled receptor of Kaposi’s sarcoma associated herpesvirus involves G13 and RhoA J. Biol. Chem. In press
