From the Laboratories of Pathology and
§ Biochemistry, NCI, National Institutes of Health,
Bethesda, Maryland 20892-1500
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ABSTRACT |
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The interleukin-2 CD28 response element (CD28RE)
acts as a composite enhancer, in conjunction with a
3-12-O-tetradecanoylphorbol-13-acetate response element
(TRE)-like element, to confer CD28 receptor-dependent inducibility to the interleukin-2 promoter in T-cells. When inserted as
a single copy upstream of a basal promoter, this composite enhancer,
termed the CD28RE-TRE, is both highly active and CD28-inducible in
transactivation assays. A multicomponent nuclear protein complex that
binds the CD28RE-TRE was isolated by DNA affinity chromatography from
nuclear extracts of mitogen- and CD28 receptor-costimulated human
T-cells. Immunological and biochemical analyses of this complex reveal
the presence of c-Rel, ATF-1, and CREB2 as major DNA-binding
components. Coexpression of c-Rel in combination with ATF-1, CREB2, or
ATF-1/CREB2 leads to synergistic transactivation of a CD28RE-TRE
reporter plasmid in quiescent Jurkat T-cells. Furthermore,
CD28-dependent transactivation of the CD28RE-TRE is
specifically inhibited by cAMP response element-binding protein (CREB)
dominant-negative expression vectors. Moreover, mutant promoter
constructs in which the internal 5
-CD28RE and 3
-TRE-like sequences
have been topologically positioned 180° out of phase with one another
show loss of mitogen- and CD28-dependent inducibility. Finally, the addition of the CREB-binding transcriptional coactivator p300 leads to a dramatic CREB-dependent increase in both
mitogen- and CD28-mediated transactivation of the CD28RE-TRE. These
findings demonstrate that full physiological responsiveness to CD28
receptor stimulation in T-cells is dependent on topologically linked
sequences within the CD28RE-TRE composite enhancer and provide strong
support of a direct role for the CREB family of transcription factors and p300/CREB-binding protein coactivator proteins in cytokine gene
induction during T-cell activation.
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INTRODUCTION |
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The extensive characterization of the molecular processes governing immune activation of T-lymphoctyes has produced one of the most instructive and informative model systems in the field of signal transduction biology (1-3). A major area of interest to both signal transduction biologists and immunologists has been the elucidation of the mechanisms through which combined signals, derived from the T-cell surface, are integrated to achieve specific biological responses in the activated T-cell. In the case of T-lymphocyte immune activation, the generation of a fully functional immune response is dependent on the integration of dual signals that originate from cell-cell interactions between the antigen-reactive T-cell and the antigen-presenting B-cell or macrophage. The first of these signals is provided by the T-cell receptor, which makes contact with its target antigen in the context of the major histocompatibility complex type II receptor on the surface of the antigen-presenting cell. In cell culture systems, this first antigen receptor-dependent signal can be replaced pharmacologically by mitogenic combinations such as ionomycin and phorbol 12-myristate 13-acetate (PMA)1 or phytohemagglutinin and PMA (4). The second signal is derived from additional events that are triggered by receptor-mediated interactions with accessory molecules on the T-cell surface. One such accessory molecule is CD28, which interacts with its counter-receptors B7-1 and B7-2 on antigen-presenting cells (5-10). The interaction between the B7 counter-receptors and CD28 can be mimicked to a large extent by activating monoclonal antibodies (e.g. mAb 9.3) specific for the human CD28 receptor (11, 12). Together with mitogenic signals that mimic the occupied T-cell antigen receptor, the CD28 receptor agonists provide a costimulus that leads to full T-lymphocyte activation. Although CD28 stimulation alone is non-mitogenic and does not stimulate T-cells, inhibition or loss of CD28 function in the presence of antigenic stimulation results in blunting of T-cell activation and ultimately can lead to anergy (13).
A large part of these receptor-mediated responses are governed at the
level of transcription and, consequently, are mediated by multiple
transcription factors that regulate the expression of genes encoding
various cytokines responsible for propagating the immune activated
state. One such cytokine is the T-cell growth factor interleukin-2
(IL-2) (14). An enhancer within the interleukin-2 promoter that confers
inducibility to CD28 costimulation was first identified as a
5-AGAAATTCCA-3
sequence located between positions
164 and
154,
relative to the start of transcription (10, 15). Subsequent analysis of
factors that can bind to and transactivate this sequence has identified
a prominent role for the Rel family of transcription factors, in
particular c-Rel and RelA, in CD28-mediated transcriptional regulation
(16-18). More recently, there have been reports suggesting a role for
other transcription factors in governing CD28 responsiveness, including
NFAT (nuclear factor of activated T-cells), the high mobility group I/Y proteins, and an
undescribed 35-kDa protein (19-21). These newer findings clearly imply
that the precise mechanisms through which CD28 responsiveness is
mediated at the interleukin-2 promoter have yet to be elucidated.
This investigation was initiated with the hypothesis that the CD28RE
may act physiologically as a composite element functionally linked to a
non-consensus 3-TPA response element (TRE; 5
-AGAGTCA-3
) located at
positions
155 to
146. This non-consensus TPA response element
(IL-2-TRE or NF-IL-2B) has been previously recognized as a site that is
important for induction of AP-1-dependent transcriptional activity at the IL-2 promoter (22-24). This site, like other TREs, can
bind in vitro to members of the basic leucine zipper (bZIP) class of transcriptional regulators, including the dimeric Fos/Jun AP-1
family (25, 26) and the related ATF/CREB family of proteins (27-29).
The close juxtaposition of these CD28RE and TRE sites led to the
speculation that, together, they may mediate CD28 responsiveness through their ability to coordinate a simultaneous occupancy by Rel-related and bZIP transcription factor complexes. Henceforth, this
composite element will be referred to as the CD28RE-TRE enhancer.
We propose that the CD28RE-TRE sequence, which spans positions 174 to
146 in the interleukin-2 promoter, can act individually as a single
enhancer element to promote the transcriptional activation of the IL-2
gene. Evidence provided shows that a single copy of the CD28RE-TRE,
when linked to a minimal basal promoter, can mediate a dramatically
increased mitogen- and CD28-dependent transcriptional activation, which parallels the physiological regulation of
interleukin-2 production. A DNA affinity-purified multicomponent
complex containing c-Rel, ATF-1, and CREB2 was isolated from nuclear
extracts of mitogen/anti-CD28 mAb-activated Jurkat T-cells and binds
cooperatively to the CD28RE-TRE. Exogenous expression of c-Rel in
combination with ATF-1, CREB2, or ATF-1/CREB2 leads to synergistic
transactivation of the CD28RE-TRE in unstimulated quiescent T-cells.
Furthermore, anti-CD28 mAb inducibility of the CD28RE-TRE requires
phase-dependent alignment of the CD28RE and TRE sequences
within the enhancer. CREB dominant-negative expression vectors inhibit
endogenous mitogen/anti-CD28 mAb inducibility. Finally, we show that a
near 100-fold superinduction of the CD28RE-TRE by enforced expression
of the CREB-binding coactivator p300 is CREB-dependent.
These findings establish that the CD28RE-TRE enhancer functions as a
composite element in response to CD28 costimulation and provide
compelling evidence for a prominent and direct role for the CREB family
of proteins and p300/CREB-binding protein in the transcriptional
activation of interleukin-2.
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EXPERIMENTAL PROCEDURES |
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Materials-- Diisopropyl fluorophosphate, leupeptin, pepstatin A, phenylmethylsulfonyl fluoride, Hepes, Tween 20, bovine serum albumin, PMA, forskolin, DNA-cellulose, and poly[d(I-C)] were purchased from Sigma. Triton X-100 and Bio-Rex 70 resin were from Bio-Rad. Phytohemagglutinin was from Murex Diagnostics Ltd. Rapamycin and ionomycin were from Calbiochem. Cyclosporine was from Sandoz. CNBr-activated Sepharose 4B and acetyl-CoA were from Pharmacia Biotech Inc. [14C]Chloramphenicol was from Amersham Life Science, Inc.
Plasmids--
The wild-type and mutant CD28RE-TRE-CAT reporter
plasmids were constructed using the following synthetic
oligonucleotides: 5-AGCTTGGGGGTTTAAAGAAATTCCAAAGAGTCATCAGAAGAGG-3
and
5
-TCGACCTCTTCTGATGACTCTTTGGAATTTCTTTAAACCCCCA-3
for the wild-type
CD28RE-TRE, 5
-AGCTTGGGGGTTTAAAGAAATTCCAATCGATAGAGTCATCAGAAGAGG-3
and
5
-TCGACCTCTTCTGATGACTCTATCGATTGGAATTTCTTTAAACCCCCA-3
for the
phase-mutated CD28RE-TRE (WB3/4),
5
-AGCTTGGGGGTTTAACTCCCGGAAAAAGAGTCATCAGAAGAGG-3
and
5
-TCGACCTCTTCTGATGACTCTTTTTCCGGGAGTTAAACCCCCA-3
for the mCD28RE-TRE mutant,
5
-AGCTTGGGGGTTTAAAGAAATTCCCCCTCTAACGCAGAAGAGG-3
and
5
TCGACCTCTTCTGCGTTAGAGGGGGAATTTCTTTAAACCCCCA-3
for the
CD28RE-mTRE mutant, 5
-AGCTGGGTTTAAAGAAATTCCA-3
and
5
-TCGATGGAATTTCTTTAAACCC-3
for the CD28RE half-site
(WB5/6), and 5
-AGCTAAGAGTCAT-3
and 5
-TCGAATGACTCTT-3
for the TRE
half-site (WB7/8). Each duplex oligonucleotide contains a
HindIII site at the 5
-end and a SalI site at the
3
-end. The oligonucleotides were annealed, phosphorylated, and ligated
as a single copy immediately upstream of the c-fos minimal
promoter sequences in the
56-CAT vectors (30). The CREB1, CREB2,
ATF-1, c-Fos, and p300 expression vectors (RSV-CREB, CDM7-CREB2,
RSV-ATF-1, pBK28, and CMV-p300-E2, respectively) have been previously
described (30-34). The IL-2-CAT (pIL-2-CAT) and TRE-CAT (p10) reporter
plasmids have been previously described (35, 36). The c-Rel expression
vector and 4XCD28RE reporter plasmids (17) were a generous gift from
Dr. Tse-Hua Tan (Baylor College of Medicine). CREB dominant-negative
expression vectors encoding truncated (bZIP) and acidic
extension-modified (aZIP) CREB constructs have been described
(37-39).
Cell Culture and Transfection Assays-- Jurkat T-cells were cultured in RPMI 1640 medium (Life Technologies, Inc.), 10% fetal calf serum, and 100 units/ml penicillin/streptomycin at 37 °C in 5% CO2. Cells (1-2 × 107) were resuspended in 250 µl of ice-cold phosphate buffer with 2.5-8 µg of reporter plasmid and 1-8 µg of the indicated expression vectors, followed by electroporation (200 V, 1180 microfarads). The cells were then resuspended in 5 ml of complete RPMI 1640 medium at 37 °C. Following transfection and resuspension, the cells were either unstimulated or stimulated with either 1 µM ionomycin and 50 ng/ml PMA or 1 µM ionomycin and 50 ng/ml PMA plus a 1:1000 dilution of mAb 9.3 ascites. All transfections studies were performed in duplicate (or more) in all cases and are representative of at least two independent experiments. CAT activity in cell extracts was determined as described (40). Quantitation of CAT activity was performed using a Molecular Dynamics PhosphorImager. Results are expressed as the mean ± S.E.
Affinity Purification of the CD28RC--
A CD28RE-TRE DNA
affinity column was generated by coupling 4 mg of 5-amino-modified
duplex oligonucleotide encoding the CD28RE-TRE sequence (see above) to
4 ml of CNBr-activated Sepharose 4B resin using a previously described
procedure (41). Jurkat T-cells were grown to a final density of 8 × 105/ml in 8 liters of RPMI 1640 medium. The cells were
then harvested and resuspended in a final volume of 2 liters of RPMI
1640 medium. The resuspended cells were stimulated, for 8 h, by
the addition of 1 µM ionomycin, 50 ng/ml PMA, and a
1:1000 dilution of affinity-purified anti-CD28 mAb 9.3 (courtesy of
Carl June, Naval Medical Research Institute). Following stimulation,
nuclei were isolated, and nuclear extracts were prepared as described
previously (42). All subsequent manipulations were performed at
4 °C. The nuclear extract (final volume of 40 ml) was dialyzed
against 4 liters of buffer containing 10 mM Hepes, 175 mM NaCl, 1 mM sodium azide, 0.05% Tween 20, 4 mM
-mercaptoethanol, 0.5 mM
phenylmethylsulfonyl fluoride, and 1 µg/ml leupeptin, pH 7.5 (buffer
A). Following dialysis, the extract was passed over an 8-ml
DNA-cellulose column at 10 ml/h. The eluate was collected and then
loaded onto a 4-ml CD28RE-TRE oligonucleotide DNA affinity column at 3 ml/h. The column was then washed successively with 20 ml of buffer A
containing 175 mM NaCl, 250 mM NaCl, and 600 mM NaCl. A protein peak was eluted from the column with
buffer A containing 1.2 M NaCl. This peak was assayed for
CD28RE-TRE DNA binding activity and then stored at
80 °C. Under
these conditions, the affinity-purified CD28RC was stable for at least
24 months with no loss in the quantity or quality of the DNA binding
activity.
Antibodies-- Antibodies against c-Rel, CREB2, ATF-4 (CREB2), and ATF-1 were obtained from Santa Cruz. The anti-CD28 mAb 9.3 ascites was obtained from Bristol-Myers Squibb Pharmaceutical Research Institute (Seattle, WA).
Electrophoretic Mobility Shift Assays-- EMSA analysis and antibody supershift analysis were performed on 4 and 8% polyacrylamide gels at 24 °C as described previously (42). Competition analysis with duplex DNA oligonucleotides was carried out with oligonucleotides that had been normalized for concentration by absorbance at 260 nm, followed by a final adjustment by densitometric scanning of the duplex oligonucleotides analyzed on ethidium bromide-stained gels.
Determination of Interleukin-2 Concentration-- Interleukin-2 concentrations were determined by enzyme-linked immunoadsorbent analysis using the Quantikine immunoassay kit (R&D Systems) according to the manufacturer's instructions.
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RESULTS |
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Mitogen- and Anti-CD28 mAb-costimulated Induction of the CD28RE-TRE Parallels the Transcriptional Activation of the Interleukin-2 Promoter and the Induction of IL-2 Secretion in Jurkat T-cells-- As has been shown previously, ionomycin/PMA-induced activation of interleukin-2 production and IL-2 promoter activity in Jurkat T-cells is further up-regulated by costimulation with anti-CD28 mAb (Fig. 1A) (10). This inducibility is similar to the transcriptional activation of the intact IL-2 promoter-chloramphenicol acetyltransferase gene reporter plasmid (IL-2-CAT) in transiently transfected Jurkat T-cells (Fig. 1B, left panel). The activity of the transfected interleukin-2 promoter is closely paralleled by the ionomycin/PMA and anti-CD28 mAb inducibility of a reporter plasmid containing a single copy of the CD28RE-TRE linked to a basal promoter (Fig. 1B). Interleukin-2 production, IL-2 promoter activity, and activation of the CD28RE-TRE enhancer were not induced by pretreatment of quiescent cells with anti-CD28 mAb alone (data not shown; see Refs. 5-10).
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A DNA Affinity-purified Multicomponent Complex from Ionomycin/PMA- and Anti-CD28 mAb-stimulated Jurkat T-cells Binds Cooperatively to the CD28RE-TRE Sequence-- A DNA-binding protein complex was isolated from ionomycin/PMA/anti-CD28 mAb-stimulated Jurkat T-cells by DNA affinity chromatography (Fig. 2). EMSA analysis of the affinity-purified CD28RC reveals the presence of three major complexes of varying mobility, designated A, B, and C (in order of decreasing electrophoretic mobility). A dose-response analysis of increasing the concentration of the CD28RC shows a cooperative increase in the formation of the B and C complexes, whereas the A complex increases linearly (Fig. 2, right panel). Although the appearance of the C complex is variable and could represent aggregation, the differential increase in the formation of these complexes suggests that the B and C complexes may be assembled from more than one component.
Analysis of DNA-Protein Complexes Bound to Wild-type and Mutant CD28RE-TRE Sequences Shows Dependence on Both the CD28RE and TRE Regions for Both Specific DNA Binding and Transactivation-- Competition analysis of the CD28RE-TRE DNA binding activity of the affinity-isolated material was carried out with unlabeled wild-type CD28RE-TRE duplex oligonucleotides or mutant CD28RE-TRE sequences containing 8- and 9-base transversions in the CD28RE (mCD28RE-TRE) and TRE (CD28RE-mTRE) regions of the composite enhancer (Fig. 3A). Sequence mutations in either the CD28RE or TRE region impair specific competition of the A and B complexes (Fig. 3B). Direct comparison of the ability of the A and B complexes to form on wild-type versus mutant CD28RE-TRE sequences shows that the formation of the A complex has a strong dependence on the integrity of both the CD28RE and TRE regions of the enhancer (Fig. 3C). The formation of the B complex is completely abolished by a mutation in the TRE region of the enhancer sequence; however, a complex that comigrates with the wild-type B complex is still capable of forming on the enhancer sequence with a mutation in the CD28RE region. Thus, both the CD28RE and TRE regions of the CD28RE-TRE composite enhancer are important for sequence-specific DNA binding and formation of DNA-protein complexes formed on the CD28RE-TRE. Interestingly, as shown in Fig. 3E, results from a screen of inhibiting/supershifting antibodies showed that antibodies to c-Rel, ATF-1, and CREB2 caused significant inhibition of the B complex and a coincident formation of supershifted complexes when viewed after longer autoradiographic exposures (Fig. 4A). In contrast, the B complex formed on the mutant mCD28RE-TRE enhancer is inhibited by antibodies to c-Rel and ATF-1, but is not inhibited by anti-CREB2 antibodies. Thus, although a complex resembling the B complex does form on the mutant altered in the CD28RE region, it does not appear to be identical to the B complex formed on the wild-type enhancer.
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Affinity-purified CD28RC Is Enriched in c-Rel, ATF-1, and CREB2-- As suggested in Fig. 3E, analysis of the affinity-purified CD28RC by EMSA antibody supershift analysis consistently showed the formation of minor, although consistently reproducible antibody-supershifted complexes specific for c-Rel, ATF-1, and CREB2 (Fig. 4A, left panel). The supershift complexes were seen in multiple experiments and did not appear in control EMSA analysis of mixtures of radiolabeled probe and antibody alone (Fig. 4A, right panel). Western blot analysis of the affinity-purified CD28RC shows the presence of c-Rel, ATF-1, and CREB2 cross-reactive polypeptides at approximate molecular masses of 75, 42-40, and 35 kDa, respectively (Fig. 4B). Moreover, each of these cross-reactive polypeptides is enriched severalfold in comparison to the crude nuclear extract (starting material) (Fig. 4C).
Exogenous Expression of c-Rel and ATF-1 in Quiescent Jurkat T-cells Induces Synergistic Transactivation of the CD28RE-TRE-- Comparison of cotransfection of combinations of c-Rel with the c-Fos, CREB, or ATF-1 expression vector and the CD28RE-TRE-CAT reporter plasmid construct showed a higher activity and significantly higher synergy between c-Rel and ATF-1 (Fig. 5, compare normalized A-C). The c-Rel/ATF-1 combination induced transactivation of the CD28RE-TRE that was nearly 25-fold higher compared with c-Rel alone and more than 5-fold higher compare with ATF-1 alone. Mixtures of c-Fos and c-Rel show measurable synergy, but the total activity above c-Rel alone was much lower compared with c-Rel/ATF-1.
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Exogenous Expression of CREB2 Alone or in Combination with c-Rel Also Induces Significant Up-regulation of CD28RE-TRE Transactivation-- CREB2 also shows moderate synergy with c-Rel (Fig. 6A). Interestingly, single component dose-response curves with increasing amounts of CREB2 show significant transactivation of the CD28RE-TRE (Fig. 6B). This finding is quite provocative since prior studies have described that CREB2 acts as a repressor of transcription for cognate cyclic AMP response elements (33). Finally, combinations of c-Rel, ATF-1, and CREB2 act coordinately and synergistically to induce CD28RE-TRE transactivation nearly 30-fold above background levels (Fig. 6C).
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CREB Dominant-negative Expression Vectors Selectively Inhibit CD28RE-TRE Transactivation-- Two different dimer-specific CREB dominant-negative expression vectors were used to demonstrate that the physiological activation of the CD28RE-TRE requires CREB family members. The first dominant-negative CREB was generated by truncation of the transactivation domain to leave only the DNA-binding bZIP domain of CREB (38). These bZIP-CREB constructs specifically dimerize with CREB family members including CREB1, CREB2, and ATF-1 to form dimeric hybrid DNA-binding complexes with reduced transactivation potential. Overexpression of these constructs leads to transcriptional repression through competitive quenching of transactivation. The second construct was a truncated CREB leucine zipper modified by an acidic coiled-coil extension (aZIP) in place of the basic DNA-binding domain (37-39). This acidic extension has DNA-mimetic properties and binds with high affinity to the basic DNA-binding domain of endogenous CREB. Dimers formed between these acidic extension leucine zipper constructs (aZIP-CREB) and endogenous CREB are stable hybrid complexes that are incapable of binding DNA and thus have no transactivation potential. Overexpression of the aZIP constructs produces a more avid and selective inhibition of sequence-specific transactivation (39).
Expression of both the aZIP and bZIP CREB dominant-negative constructs in Jurkat T-cells selectively inhibits mitogen/anti-CD28 mAb-induced transactivation of the CD28RE-TRE (Fig. 7A). As predicted, the aZIP-CREB dominant-negative construct is much more effective than the bZIP form. The inhibitory actions of the CREB dominant-negative expression vectors are specific since, although they repress transactivation from CREB-dependent cAMP response element sites (data not shown), they do not inhibit transactivation from an AP-1-dependent TRE expression vector (Fig. 7B). The slightly enhanced transactivation of the TRE by the bZIP vector reflects the lower specificity of the bZIP-CREB versions compared with the aZIP-CREB versions and is probably due to their ability to generate leucine zipper hybrids with AP-1 that retain intrinsic DNA binding activity.
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CD28 Inducibility of the CD28RE-TRE Depends on Correct Topological Phasing of Its Internal CD28RE and TRE Sequences-- A mutant CD28RE-TRE enhancer (WB3/4) was constructed in which the CD28RE and TRE sequences are separated by an intervening 5 bases that places each sequence 180° out of phase (on opposite faces of the DNA double helix) with each other. Transactivation from reporter plasmid constructs bearing this mutant shows nearly complete abrogation of CD28 responsiveness (Fig. 8A). Although, as previously published, an expression vector containing four multimerized tandem copies of the CD28RE is responsive to both mitogen and mitogen/anti-CD28 mAb stimulation (17), the magnitude of its induction is still less than that produced by a single copy of the CD28RE-TRE. Furthermore, reporter plasmid constructs containing a single copy of either of the CD28RE-TRE half-sites (WB5/6 for CD28RE and WB7/8 for the TRE region) are uninducible. Finally, the phase-disrupted CD28RE-TRE (WB3/4) is also only minimally induced by the coexpression of c-Rel and ATF-1. Likewise, c-Rel and ATF-1 coexpression shows very little induction of the multimerized 4XCD28RE, which lacks the TRE sequences (Fig. 8B).
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p300 Dramatically Up-regulates Anti-CD28 mAb-stimulated Transactivation of the CD28RE-TRE through CREB-- The p300 protein is a member of the coactivator CREB-binding protein family that augments the transcriptional activation of numerous transcription factors by linking them to the basal transcriptional machinery through direct protein-protein interactions (43). As well as showing tight interaction with ATF/CREB family members, p300 has also been recently demonstrated to have binding sites for interactions with Rel family members (43, 44). The exogenous addition of p300 to Jurkat T-cells leads to a near 100-fold increase in transactivation compared with quiescent cells and increases mitogen-stimulated transactivation nearly 30-fold (Fig. 9A). The effect in CD28-stimulated cells is dose-dependent and is detectable with nanogram amounts of the p300 expression vector (Fig. 9B). p300 induction of the CD28RE-TRE is CREB-dependent since it is effectively inhibited by CREB dominant-negative expression vectors (Fig. 9C).
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DISCUSSION |
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We have shown that CD28 inducibility is maximally conferred to the
interleukin-2 promoter via a composite enhancer element formed by the
linkage of the CD28RE with a 3-TRE (also known as the NF-IL-2B site)
that we refer to as the CD28RE-TRE. This composite enhancer is active
in vivo in Jurkat T-cells when linked as a single copy to a
minimal promoter. Furthermore, the inducibility of this element
parallels the physiological induction of interleukin-2 production in
response to mitogen and mitogen/anti-CD28 mAb stimulation. Characterization of a DNA affinity-purified complex that binds to the
CD28RE-TRE revealed the presence of the transcription factors c-Rel,
ATF-1, and CREB2. Exogenous expression of c-Rel in combination with
ATF-1 or CREB2 shows synergistic transactivation of the CD28RE-TRE in
quiescent T-cells. Moreover, inhibition of the physiological activation
of the CD28RE-TRE can be by achieved through cotransfections studies
using dimer-specific CREB dominant-negative expression vectors. Full
CD28 responsiveness of the CD28RE-TRE to physiological stimulation or
the action of exogenous combinations of c-Rel and ATF-1 requires the
presence and correct phasing of the CD28RE and TRE sequences within the
CD28RE-TRE composite element. Finally, the mitogen/anti-CD28 mAb
induction of the CD28RE-TRE is dramatically up-regulated by the
CREB-binding coactivator protein p300.
Although a role for the Rel-related B proteins as downstream targets
in CD28 signaling has been well established (45-50), a direct role for
the CREB family in CD28 responses and/or regulation of interleukin-2
signaling has not been reported. Recent transgenic mouse studies, in
which CREB activity was functionally abrogated by the expression of a
dominant-negative CREB, report a dramatic loss of interleukin-2
expression (51). This result was interpreted as an indirect effect
secondary to the reduced CREB-dependent production of the
fos/jun AP-1 subunits. The present work argues that the deficit in these mice may be a more direct effect of attenuated CREB action at the interleukin-2 promoter. In support of
this notion, there have been recent reports that
CREB-dependent transcriptional activity can be induced in
T-cells via dual mitogen- and CD28-responsive pathways (52).
The CREB family of proteins were originally characterized as factors that are responsive to increased intracellular levels of cyclic AMP through direct phosphorylation of CREB1 at serine 133, and at analogous positions on other CREB family members, by protein kinase A (53, 54). It is well known that increases in intracellular cAMP lead to inhibition of numerous selective mitogen-dependent kinase cascades in T-cells and blunt their mitogen-dependent activation and proliferation (55-57). In addition, others have reported that p65 is a target of the immunosuppressive action of protein kinase A (58). This apparent paradox has been resolved by the elucidation of other mitogen-induced signal transduction cascades that can lead to the phosphorylation and activation of the CREB family members in the absence of elevated cAMP and protein kinase A (52, 59, 60). A primary target of these pathways includes the recently described p90rsk family of CREB kinases (60). There is significant evidence to suggest that these kinases may play a major role in the induction of CREB during T-cell activation. Thus, signaling through the T-cell receptor is likely to occur through the cooperative induction of two distinct transcription factor families: the Rel family (including c-Rel, p65, p50, and RelB) and the CREB family (including CREB1, ATF-1, and CREB2). Thus activated, the cross-talk between these transcription factors would be expected to exert a cooperative influence within the interleukin-2 promoter at the level of the CD28RE-TRE.
A precise understanding of the molecular nature of the protein complexes that bind the CD28RE-TRE in activated T-cells remains incomplete. Due to the inherent dimeric structure of the Rel and ATF/CREB family members, there is likely to be considerable heterogeneity in the composition of the linked complexes that form on the CD28RE-TRE. The precise stoichiometry of the components of the CD28RC is not known. DNA binding analysis of the CD28RC indicates that both the CD28RE and TRE regions of the enhancer contribute to the specific formation of DNA-binding complexes, and immunological data indicate that the integrity of the DNA-binding B complex is dependent on c-Rel, ATF-1, and CREB2 (Figs. 3E and 4A). Less is known about the biochemical nature of the A complex, although its specific formation correlates well with the transactivation potential of the wild-type enhancer (Fig. 3). Moreover, it cannot be determined for certain at this time whether or not the major complexes seen by EMSA represent the CD28RC holocomplex formed on the CD28RE-TRE enhancer in solution or instead reflect more stable intermediates derived from the assembly or decay of a larger parent complex that does not survive electrophoresis. In view of these points, it would be premature to rule out possible added roles for other Rel or leucine zipper family members as participants in the cross-talk that mediates the transcriptional activation of the CD28RE-TRE.
It has been well established that CD28 costimulation of antigen- or mitogen-activated T-cells results in synergistic increases in interleukin-2 production (5-10). A complete evaluation of this observation requires a careful consideration of the various levels through which the intersection of numerous cellular processes and pathways can result in synergistic increases in interleukin-2 production. CD28 ligation by cellular receptors or activating antibody alone does not induce either IL-2 transcription or CD28 enhancer activation (5-10). Together with mitogen/antigen stimulation, the increased levels of IL-2 RNA are due both to activated transcription and to increased RNA stability (61, 62). This dual effect defines one level of synergy. The synergistic effects of CD28 costimulation on transcriptional activation are likely to occur at numerous levels within the T-cell nucleus. Such synergistic targeting could involve any of the following: 1) the cooperative association of Rel and CREB transcription factor family members with CD28RE-TRE enhancer DNA; 2) a simultaneous "bidentate" recruitment of transcriptional coactivators such as p300 and CREB-binding protein by c-Rel and CREB; 3) CD28-dependent post-translational modification of c-Rel, CREB, or p300; or 4) the action of other, yet undefined, DNA-binding factors. Obviously, any one of these possibilities could account for the transcriptional synergy observed upon CD28 costimulation. None are mutually exclusive, and there is likely to be significant interplay between most. A delineation of the roles and interdependence of these mechanisms awaits further investigation. Regardless, it is clear that cross-talk between the Rel and ATF/CREB family of transcription factors is likely to be play a prominent role in the transcriptional regulation of IL-2 gene expression in both stimulated and costimulated T-cells.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed: NIH, Bldg. 10, Rm. 2N212, Bethesda, MD 20892-1500. Tel.: 301-496-1055; Fax: 301-402-0043; E-mail:kevgar{at}helix.nih.gov.
1 The abbreviations used are: PMA, phorbol 12-myristate 13-acetate; mAb, monoclonal antibody; IL-2, interleukin-2; CD28RE, CD28 response element; CD28RC, CD28RE-TRE-binding (response) complex; TPA, 12-O-tetradecanoylphorbol-13-acetate; TRE, TPA response element; bZIP, basic leucine zipper; aZIP, acidic extension leucine zipper; CREB, cAMP response element-binding protein; CAT, chloramphenicol acetyltransferase; EMSA, electrophoretic mobility shift assay.
2 Data in Table I were compiled from previous studies (17, 20, 21, 25, 64).
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