Kappa B site-dependent activation of the interleukin-2 receptor alpha-chain gene promoter by human c-Rel

Sam68 modulates the promoter specificity of NF-κB and mediates expression of CD25 in activated T cells

CD25, the alpha chain of the interleukin-2 receptor, is expressed in activated T cells and has a significant role in autoimmune disease and tumorigenesis; however, the mechanisms regulating transcription of CD25 remain elusive. Here we identify the Src-associated substrate during mitosis of 68 kDa (Sam68) as a novel non-Rel component in the nuclear factor-kappaB (NF-κB) complex that confers CD25 transcription. Our results demonstrate that Sam68 has an essential role in the induction and maintenance of CD25 in T cells. T-cell receptor engagement triggers translocation of the inhibitor of NF-κB kinase alpha (IKKα) from the cytoplasm to the nucleus, where it phosphorylates Sam68, causing complex formation with NF-κB in the nucleus. These findings reveal the important roles of KH domain-containing components and their spatial interactions with IKKs in determining the binding targets of NF-κB complexes, thus shedding novel insights into the regulatory specificity of NF-κB.

Diacylglycerol kinase ζ limits the generation of natural regulatory T cells

Natural regulatory T (nT(reg)) cells are important for maintaining tolerance to self- and foreign antigens, and they are thought to develop from thymocytes that receive strong T cell receptor (TCR)-mediated signals in the thymus. TCR engagement leads to the activation of phospholipase C-γ1, which generates the lipid second messenger diacylglycerol (DAG) from phosphatidylinositol 4,5-bisphosphate. We used mice that lack the ζ isoform of DAG kinase (DGKζ), which metabolizes DAG to terminate its signaling, to enhance TCR-mediated signaling and identify critical signaling events in nT(reg) cell development. Loss of DGKζ resulted in increased numbers of thymic CD25(+)Foxp3(-)CD4(+) nT(reg) cell precursors and Foxp3(+)CD4(+) nT(reg) cells in a cell-autonomous manner. DGKζ-deficient T cells exhibited increased nuclear translocation of the nuclear factor κB subunit c-Rel, as well as enhanced extracellular signal-regulated kinase (ERK) phosphorylation in response to TCR stimulation, suggesting that these downstream pathways may contribute to nT(reg) cell development. Indeed, reducing c-Rel abundance or blocking ERK phosphorylation abrogated the increased generation of nTreg cells by DGKζ-deficient thymocytes. The extent of ERK phosphorylation correlated with TCR-mediated acquisition of Foxp3 in immature thymocytes in vitro. Furthermore, the development of nT(reg) cells was augmented in mice in which ERK activation was selectively enhanced in T cells. Together, these data suggest that DGKζ regulates the development of nT(reg) cells by limiting the extent of activation of the ERK and c-Rel signaling pathways.

Regulation of Transcriptional Networks by PKC Isozymes: Identification of c-Rel as a Key Transcription Factor for PKC-Regulated Genes

BACKGROUND

Activation of protein kinase C (PKC), a family of serine-threonine kinases widely implicated in cancer progression, has major impact on gene expression. In a recent genome-wide analysis of prostate cancer cells we identified distinctive gene expression profiles controlled by individual PKC isozymes and highlighted a prominent role for PKCδ in transcriptional activation.

PRINCIPAL FINDINGS

Here we carried out a thorough bioinformatics analysis to dissect transcriptional networks controlled by PKCα, PKCδ, and PKCε, the main diacylglycerol/phorbol ester PKCs expressed in prostate cancer cells. Despite the remarkable differences in the patterns of transcriptional responsive elements (REs) regulated by each PKC, we found that c-Rel represents the most frequent RE in promoters regulated by all three PKCs. In addition, promoters of PKCδ-regulated genes were particularly enriched with REs for CREB, NF-E2, RREB, SRF, Oct-1, Evi-1, and NF-κB. Most notably, by using transcription factor-specific RNAi we were able to identify subsets of PKCδ-regulated genes modulated by c-Rel and CREB. Furthermore, PKCδ-regulated genes condensed under the c-Rel transcriptional regulation display significant functional interconnections with biological processes such as angiogenesis, inflammatory response, and cell motility.

CONCLUSION/SIGNIFICANCE

Our study identified candidate transcription factors in the promoters of PKC regulated genes, in particular c-Rel was found as a key transcription factor in the control of PKCδ-regulated genes. The deconvolution of PKC-regulated transcriptional networks and their nodes may greatly help in the identification of PKC effectors and have significant therapeutics implications.

Defects in the Bcl-2-regulated apoptotic pathway lead to preferential increase of CD25 low Foxp3+ anergic CD4+ T cells

Defects in the Bcl-2-regulated apoptotic pathway inhibit the deletion of self-reactive T cells. What is unresolved, however, is the nature and fate of such self-reactive T cells escaping deletion. In this study, we report that mice with such defects contained increased numbers of CD25(low)Foxp3(+) cells in the thymus and peripheral lymph tissues. The increased CD25(low)Foxp3(+) population contained a large fraction of cells bearing self-reactive TCRs, evident from a prominent increase in self-superantigen-specific Foxp3(+)Vβ5(+)CD4(+) T cells in BALB/c Bim(-/-) mice compared with control animals. The survival rate of the expanded CD25(low)Foxp3(+) cells was similar to that of CD25(high)Foxp3(+) CD4 T cells in vitro and in vivo. IL-2R stimulation, but not TCR ligation, upregulated CD25 on CD25(low)Foxp3(+)CD4(+) T cells in vitro and in vivo. The expanded CD25(low)Foxp3(+)CD4(+) T cells from Bim(-/-) mice were anergic but also had weaker regulatory function than CD25(high)Foxp3(+) CD4(+) T cells from the same mice. Analysis of Bim(-/-) mice that also lacked Fas showed that the peripheral homeostasis of this expanded population was in part regulated by this death receptor. In conclusion, these results show that self-reactive T cell escapes from thymic deletion in mice defective in the Bcl-2-regulated apoptotic pathway upregulate Foxp3 and become unresponsive upon encountering self-Ag without necessarily gaining potent regulatory function. This clonal functional diversion may help to curtail autoaggressiveness of escaped self-reactive CD4(+) T cells and thereby safeguard immunological tolerance.

NOD mice are defective in proteasome production and activation of NF-kappaB

The nonobese diabetic (NOD) mouse is an animal model of human type I diabetes with a strong genetic component that maps to the major histocompatibility complex (MHC) of the genome. We have identified in NOD lymphocytes a specific proteasome defect that results from the lack of the LMP2 subunit. The pronounced proteasome defect results in defective production and activation of the transcription factor NF-kappaB, which plays an important role in immune and inflammatory responses as well as in preventing apoptosis induced by tumor necrosis factor alpha. The defect in proteasome function in NOD mouse splenocytes was evident from impaired NF-kappaB subunit p50 and p52 generation by proteolytic processing and impaired degradation of the NF-kappaB-inhibitory protein IkappaBalpha. An obligatory role of MHC-linked proteasome subunits in transcription factor processing and activation has been established in a spontaneous-disease model and mutant cells similarly lacking the MHC-encoded subunit. These data suggest that NOD proteasome dysfunction is due to a tissue- and developmental-stage-specific defect in expression of the MHC-linked Lmp2 gene, resulting in altered transcription factor NF-kappaB activity, and that this defect contributes to pathogenesis in NOD mice. These observations are consistent with the diverse symptomatology of type I diabetes and demonstrate clear sex-, tissue-, and age-specific differences in the expression of this error which parallel the initiation and disease course of insulin-dependent (type I) diabetes mellitus.

In Vivo Inhibition of NF-κB in T-Lineage Cells Leads to a Dramatic Decrease in Cell Proliferation and Cytokine Production and to Increased Cell Apoptosis in Response to Mitogenic Stimuli, But Not to Abnormal Thymopoiesis

To understand the role of NF-κB complexes in T cell development and activation, we have generated transgenic mice in which RelA and c-Rel complexes were selectively inhibited in the T-lineage cells by specific expression of a trans-dominant form of IκBα. Transgene expression did not affect the thymic development, but led to lowered numbers of splenic T cells and to a dramatic decrease in the ex vivo proliferative response of splenic T lymphocytes. Analysis of IL-2 and IL-2Rα expression demonstrated that the perturbation of the proliferation response was not attributable to an abnormal expression of these genes. In contrast, expression of IL-4, IL-10, and IFN-γ was strongly inhibited in the transgenic T cells. The proliferative deficiency of the transgenic T cells was associated with an increased apoptosis. These results point out the involvement of NF-κB/Rel family proteins in growth signaling pathways by either regulating proteins involved in the IL-2 signaling or by functionally interfering with the cell cycle progression.

Nuclear Factor-κB Mediates TNF-α Inhibitory Effect on α2(I) Collagen (COL1A2) Gene Transcription in Human Dermal Fibroblasts

Among its plethora of activities as an inflammatory mediator, TNF-α has potent regulatory control on extracellular matrix production and degradation. Earlier studies have documented that TNF-α inhibits type I collagen gene (COL1A2) expression at the transcriptional level, but the characterization of the transcription factors involved has been elusive. In the present study, using transient cell transfection of human dermal fibroblasts with a battery of 5′ end deletion/chloramphenicol acetyltransferase (CAT) reporter gene constructs, we have characterized the TNF-α response element of the COL1A2 promoter. The TNF-α response element was attributed to a specific region that comprises noncanonical activator protein-1 (AP-1) (CGAGTCA) and NF-κB (AGAGTTTCCC) binding sites. TNF-α effect was eliminated by a 2-bp substitution mutation in the NF-κB1 binding half site of the NF-κB cis element. Electrophoretic mobility shift assays (EMSA) showed that recombinant human NF-κB heterodimers as well as NF-κB1 and RelA homodimers, but not AP-1, were capable of binding this element. Further, EMSA with human fibroblast nuclear extracts demonstrated enhanced binding of a single, specific complex within 5 min of TNF-α stimulation, which reached a plateau by 1 h and was not affected by preincubation of cells with cycloheximide. Gel supershift assays identified the complex as the NF-κB (p50/p65) heterodimer, whereas Abs to nuclear factor of activated T cells (NF-AT) and Jun family members failed to recognize the complex. These data suggest that in fibroblasts TNF-α activates and initiates the nuclear translocation of NF-κB that binds a divergent NF-κB element and plays a critical role in the observed inhibition of α2(I) collagen gene transcription.

Impaired Fetal Thymocyte Development After Efficient Adenovirus-Mediated Inhibition of NF-κB Activation

We introduce a new experimental system combining adenovirus-mediated gene transfer and fetal thymic organ culture (FTOC). This system allowed us to efficiently express in developing thymocytes a mutant form of the NF-κB inhibitor IκBα (mut-IκB) and to study the maturation defects occurring when NF-κB activation is inhibited during fetal development. Fetal thymocytes infected with adenovirus containing mut-IκB were found to develop normally until the CD44−CD25+, CD4−CD8− double-negative stage, while production of more mature double-positive and single-positive populations was strongly decreased. Proliferation, as measured by the percentage of cells in cycle appeared normal, as did rearrangement and expression of the TCR β-chain. However, apoptosis was much higher in FTOC infected with adenovirus containing mut-IκB than in FTOC infected with a control virus. Taken together, these results suggest that NF-κB plays a crucial role in ensuring the differentiation and survival of thymocytes in the early stages of their development.

Cooperation between SMAD and NF-kappaB in growth factor regulated type VII collagen gene expression

We have previously demonstrated that transforming growth factor-beta (TGF-beta) and pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) or interleukin-1beta, synergistically enhance the expression of type VII collagen gene (COL7A1) in human dermal fibroblasts in culture (Mauviel et al., 1994). Recently, we identified a SMAD-containing complex, rapidly induced by TGF-beta and binding the region [-496/-444] of the COL7A1 promoter, responsible for COL7A1 gene transactivation (Vindevoghel et al., 1998a). In this report, we demonstrate that TGF-beta and TNF-alpha response elements are distinct entities within the COL7A1 promoter. In particular, we demonstrate that the TNF-alpha effect is mediated by NF-kappaB1/RelA (p50/p65) and RelA/RelA (p65/p65) NF-kappaB complexes binding the TNF-alpha response element (TaRE) located in the region [-252/-230], with RelA acting as the transcriptional activator. Finally, we provide definitive evidence for the role of both TGF-beta and TNF-alpha response elements as enhancer sequences, functioning in the context of a heterologous promoter in an additive manner in response to TGF-beta and TNF-alpha. This study provides the first identification of a functional interaction between the two immediate-early transcription factors, SMAD and NF-kappaB, to activate the expression of an extracellular matrix-related gene, COL7A1.

Overexpression of p65 and c-Jun Substitutes for B7-1 Costimulation by Targeting the CD28RE Within the IL-2 Promoter

The role of Rel and activation protein-1 (AP-1) in IL-2 promoter activity in B7-1- and leukocyte function-associated Ag-3 (LFA-3)-costimulated T cells has been evaluated. We demonstrate that overexpression of c-Jun but not c-Fos increases IL-2 promoter activity in both B7-1- and LFA-3-costimulated Jurkat T cells. Cotransfection of both c-Jun and c-Fos substitutes for B7-1 costimulation in driving an activation protein-1 response element but not for the IL-2 promoter. Overexpression of Rel proteins demonstrated that p65-expressing Jurkat cells transcribed equally well a nuclear factor κβ reporter construct when costimulated with B7-1 or LFA-3, but transcription of IL-2 promoter or CD28 response element (CD28RE)-driven reporters was superior in B7-1-costimulated cells. Combined expression of c-Jun and p65 induced vigorous transcription of IL-2 promoter- and CD28RE-driven reporter constructs in both LFA-3- and B7-1-costimulated Jurkat cells. Mutating the CD28RE but not the upstream nuclear factor κβ-binding site in the IL-2 promoter reduced B7-1-driven transcription >90%. The results implicates a major role of the CD28RE in the integration of p65/c-Jun-mediated transcription within the IL-2 promoter. We suggest that the transition from an autocrine LFA-3-driven immune response to a B7--induced paracrine immune response involves the activation of c-Jun and p65, which target the CD28RE region of the IL-2 promoter.

The N-terminal domain of IkappaB alpha masks the nuclear localization signal(s) of p50 and c-Rel homodimers

Members of the Rel/NF-kappaB family of transcription factors are related to each other over a region of about 300 amino acids called the Rel Homology Domain (RHD), which governs DNA binding, dimerization, and binding to inhibitor. At the C-terminal end of the RHD, each protein has a nuclear localization signal (NLS). The crystal structures of the p50 and RelA family members show that the RHD consists of two regions: an N-terminal section which contains some of the DNA contacts and a C-terminal section which contains the remaining DNA contacts and controls dimerization. In unstimulated cells, the homo- or heterodimeric Rel/NF-kappaB proteins are cytoplasmic by virtue of binding to an inhibitor protein (IkappaB) which somehow masks the NLS of each member of the dimer. The IkappaB proteins consist of an ankyrin-repeat-containing domain that is required for binding to dimers and N- and C-terminal domains that are dispensable for binding to most dimers. In this study, we examined the interaction between IkappaB alpha and Rel family homodimers by mutational analysis. We show that (i) the dimerization regions of p50, RelA, and c-Rel are sufficient for binding to IkappaB alpha, (ii) the NLSs of RelA and c-Rel are not required for binding to IkappaB alpha but do stabilize the interaction, (iii) the NLS of p50 is required for binding to IkappaB alpha, (iv) only certain residues within the p50 NLS are required for binding, and (v) in a p50-IkappaB alpha complex or a c-Rel-IkappaB alpha complex, the N terminus of IkappaB alpha either directly or indirectly masks one or both of the dimer NLSs.

Human cytomegalovirus induces interleukin-8 production by a human monocytic cell line, THP-1, through acting concurrently on AP-1- and NF-kappaB-binding sites of the interleukin-8 gene

Cytomegalovirus (CMV) infection induced interleukin-8 (IL-8) gene transcription in a human monocytic cell line, THP-1 cells, leading to IL-8 secretion. The functional analysis of the IL-8 gene revealed that both AP-1- and NF-kappaB factor-binding elements were involved in conferring the responsiveness to CMV. Moreover, electrophoretic mobility shift assays demonstrated that CMV induced the formation of NF-kappaB and AP-1 complexes. These results suggest that CMV activates these transcriptional factors, resulting in IL-8 gene expression.

Perturbation of the T lymphocyte lineage in transgenic mice expressing a constitutive repressor of nuclear factor (NF)-kappaB

Members of the nuclear factor (NF)-kappaB/Rel family transcription factors are induced during thymic selection and in mature T lymphocytes after ligation of the T cell antigen receptor (TCR). Despite these findings, disruption of individual NF-kappaB/Rel genes has revealed no intrinsic defect in the development of mature T cells, perhaps reflecting functional redundancy. To circumvent this possibility, the T cell lineage was targeted to express a trans-dominant form of IkappaBalpha that constitutively represses the activity of multiple NF-kappaB/Rel proteins. Transgenic cells expressing this inhibitor exhibit a significant proliferative defect, which is not reversed by the addition of exogenous interleukin-2. Moreover, mitogenic stimulation of splenocytes leads to increased apoptosis of transgenic T cells as compared with controls. In addition to deregulated T cell growth and survival, transgene expression impairs the development of normal T cell populations as evidenced by diminished numbers of TCRhi CD8 single-positive thymocytes. This defect was significantly amplified in the periphery and was accompanied by a decrease in CD4(+) T cells. Taken together, these in vivo findings indicate that the NF-kappaB/Rel signaling pathway contains compensatory components that are essential for the establishment of normal T cell subsets.

Physical interactions between Ets and NF-kappaB/NFAT proteins play an important role in their cooperative activation of the human immunodeficiency virus enhancer in T cells

The transcriptional regulatory elements of many inducible T-cell genes contain adjacent or overlapping binding sites for the Ets and NF-kappaB/NFAT families of transcription factors. Similar arrays of functionally important NF-kappaB/NFAT and Ets binding sites are present in the transcriptional enhancers of human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2), suggesting that this pattern of nuclear protein binding sites reflects an evolutionarily conserved mechanism for regulating inducible T-cell gene expression that has been co-opted during HIV evolution. Despite these findings, the molecular mechanisms by which Ets and NF-kappaB/NFAT proteins cooperatively regulate inducible T-cell gene expression remained unknown. In the studies described in this report, we demonstrated a physical interaction between multiple Ets and NF-kappaB/NFAT proteins both in vitro and in activated normal human T cells. This interaction is mediated by the Ets domain of Ets proteins and the C-terminal region of the Rel homology domains of NF-kappaB/NFAT proteins. In addition, the Ets-NF-kappaB/NFAT interaction requires the presence of DNA binding sites for both proteins, as it is abolished by the DNA intercalating agents propidium iodide and ethidium bromide and enhanced by the presence of synthetic oligonucleotides containing binding sites for Ets and NF-kappaB proteins. A dominant-negative mutant of NF-kappaB p50 that binds DNA but fails to interact with Ets proteins inhibits the synergistic activation of the HIV-1 and HIV-2 enhancers by NF-kappaB (p50 + p65) and Ets-1, suggesting that physical interaction between Ets and NF-kappaB proteins is required for the transcriptional activity of the HIV-1 and HIV-2 enhancers. Taken together, these findings suggest that evolutionarily conserved physical interactions between Ets and NF-kappaB/NFAT proteins are important in regulating the inducible expression of T-cell genes and viruses. These interactions represent a potential target for the development of novel immunosuppressive and antiviral therapies.

Basal phosphorylation of the PEST domain in the I(kappa)B(beta) regulates its functional interaction with the c-rel proto-oncogene product

The product of the c-rel proto-oncogene (c-Rel) belongs to the NF-kappaB/Rel family of polypeptides and has been implicated in the transcriptional control of cell proliferation and immune function. In human T lymphocytes, c-Rel is sequestered in the cytoplasmic compartment by constitutively phosphorylated inhibitors, including I(kappa)B(alpha) and I(kappa)B(beta). Studies with bacterially expressed forms of these inhibitory proteins revealed that unphosphorylated I(kappa)B(alpha) but not I(kappa)B(beta) assembles with c-Rel and inhibits its DNA binding activity. Furthermore, latent I(kappa)B(beta)-c-Rel complexes derived from mammalian cells were sensitive to phosphatase treatment, whereas I(kappa)B(alpha)-c-Rel complexes were resistant. We have identified a constitutive protein kinase in unstimulated T cells that associates with and phosphorylates I(kappa)B(beta) in vitro. The substrate specificity, electrophoretic mobility, and antigenic properties of this I(kappa)B(beta)-associated kinase (BAK) suggest identity with casein kinase II (CKII), an enzyme known to mediate basal phosphorylation of I(kappa)B(alpha). Phosphorylation of recombinant I(kappa)B(beta) by either BAK or CKII restored the capacity of this inhibitor to antagonize the DNA binding activity of c-Rel. Peptide mapping and mutational analyses localized the bulk of the basal phosphorylation sites in I(kappa)B(beta) to the C-terminal PEST domain, which contains two potential acceptors for CKII-mediated phosphoryl group transfer (Ser-313 and Ser-315). Point mutations introduced into the full-length inhibitor at Ser-313 and Ser-315 led to a significant reduction in the phosphorylation of I(kappa)B(beta) and severely impaired its c-Rel inhibitory function in vivo. Taken together, these findings strongly suggest that basal phosphorylation of the PEST domain of I(kappa)B(beta) at consensus CKII sites is required for the efficient formation of latent I(kappa)B(beta)-c-Rel complexes.

Sp1 binding plays a critical role in Erb-B2- and v-ras-mediated downregulation of alpha2-integrin expression in human mammary epithelial cells

The human alpha2-integrin gene is transcriptionally downregulated in a nontumorigenic human mammary epithelial cell line, MTSV1-7, and its clonal variant HB2, overexpressing the Erb-B2 oncogene. In this study, we have used deletion mutations within the alpha2-integrin promoter inserted 5' of the chloramphenicol acetyltransferase or luciferase reporter genes to identify the element that is responsible for the Erb-B2-mediated downregulation. The results of the transient-transfection assay showed that the Sp1 binding element located in the core region (positions --64 to +1) of the alpha2-integrin promoter plays an essential role in the alpha2-integrin promoter activity and its downregulation by Erb-B2. By gel shift assay, we have demonstrated that this element binds with a high degree of affinity not only to Sp1, but also to Sp3. The downregulation of the alpha2-integrin promoter activity could also be achieved by overexpression of v-Hras (v-ras), suggesting that the signals generated by Erb-B2, which lead to downregulation of the alpha2-integrin gene expression, may proceed through the ras pathway. Both the Erb-B2- and the v-ras-overexpressing cells exhibited a Sp1 DNA binding activity lower than that of the parental line, while the relative levels of Sp1 protein in these cells were not altered. The Erb-B2- and v-ras-mediated downregulation could be reversed by the overexpression of Sp1 and by a dominant negative variant of ras (rasN17), confirming the importance of Sp1 and the ras pathway. The inhibitory effects of Erb-B2 on transcriptional activity of the alpha2-integrin promoter were observed in transient-cotransfection assays using alpha2-integrin reporter plasmids and plasmids expressing the Erb-B2 or v-ras oncogene. The same effects were seen when an alpha2-integrin reporter gene construct was transfected into MTSV1-7 or HB2 cells permanently overexpressing Erb-B2 or v-ras. The effects of Erb-B2 or v-ras on the transcriptional activity of the alpha2-integrin promoter were observed in nontumorigenic luminal epithelial cell lines (MTSV1-7 and HB2) as well as in the breast cancer cell line T47D. These data suggest that in luminal epithelial cells and the breast cancers which develop from them, the Erb-B2 proto-oncogene signaling leads to inhibition of (alpha)2(beta)1-integrin gene expression and could contribute to the disruption of tissue architecture seen in breast cancers.

The nuclear factor YY1 suppresses the human gamma interferon promoter through two mechanisms: inhibition of AP1 binding and activation of a silencer element

Our group has previously reported that the nuclear factor Yin-Yang 1 (YY1), a ubiquitous DNA-binding protein, is able to interact with a silencer element (BE) in the gamma interferon (IFN-gamma) promoter region. In this study, we demonstrated that YY1 can directly inhibit the activity of the IFN-gamma promoter by interacting with multiple sites in the promoter. In cotransfection assays, a YY1 expression vector significantly inhibited IFN-gamma promoter activity. Mutation of the YY1 binding site in the native IFN-gamma promoter was associated with an increase in the IFN-gamma promoter activity. Analysis of the DNA sequences of the IFN-gamma promoter revealed a second functional YY1 binding site (BED) that overlaps with an AP1 binding site. In this element, AP1 enhancer activity was suppressed by YY1. Since the nuclear level of YY1 does not change upon cell activation, our data support a model that the nuclear factor YY1 acts to suppress basal IFN-gamma transcription by interacting with the promoter at multiple DNA binding sites. This repression can occur through two mechanisms: (i) cooperation with an as-yet-unidentified AP2-like repressor protein and (ii) competition for DNA binding with the transactivating factor AP1.

Rel-deficient T cells exhibit defects in production of interleukin 3 and granulocyte-macrophage colony-stimulating factor

The c-rel protooncogene encodes a subunit of the NF-kappa B-like family of transcription factors. Mice lacking Rel are defective in mitogenic activation of B and T lymphocytes and display impaired humoral immunity. In an attempt to identify changes in gene expression that accompany the T-cell stimulation defects associated with the loss of Rel, we have examined the expression of cell surface activation markers and cytokine production in mitogen-stimulated Rel-/- T cells. The expression of cell surface markers including the interleukin 2 receptor alpha (IL-2R alpha) chain (CD25), CD69 and L-selectin (CD62) is normal in mitogen-activated Rel-/- T cells, but cytokine production is impaired. In Rel-/- splenic T cell cultures stimulated with phorbol 12-myristate 13-acetate and ionomycin, the levels of IL-3, IL-5, granulocyte- macrophage colony-stimulating factor (GM-CSF), tumor necrosis factor alpha (TNF-alpha), and gamma interferon (IFN-gamma) were only 2- to 3-fold lower compared with normal T cells. In contrast, anti-CD3 and anti-CD28 stimulated Rel-/- T cells, which fail to proliferate, make little or no detectable cytokines. Exogenous IL-2, which restitutes the proliferative response of the anti-CD3- and anti-CD28-treated Rel-/- T cells, restores production of IL-5, TNF-alpha, and IFN-gamma, but not IL-3 and GM-CSF expression to approximately normal levels. In contrast to mitogen-activated Rel-/- T cells, lipopolysaccharide-stimulated Rel-/- macrophages produce higher than normal levels of GM-CSF. These findings establish that Rel can function as an activator or repressor of gene expression and is required by T lymphocytes for production of IL-3 and GM-CSF.

RelA is a potent transcriptional activator of the CD28 response element within the interleukin 2 promoter

T-cell activation requires two different signals. The T-cell receptor's recognition of a specific antigen on antigen-presenting cells provides one, and the second signal comes from costimulatory molecules such as CD28. In contrast, T cells that are stimulated with antigen in the absence of the CD28 costimulatory signal can become anergic (nonresponsive). The CD28 response element (CD28RE) has been identified as the DNA element mediating interleukin 2 (IL-2) gene activation by CD28 costimulation. Our previous work demonstrates that the Rel/NF-kappa B family proteins c-Rel, RelA (p65), and NFKB1 (p50) are involved in the complex that binds to the CD28RE. We also showed that c-Rel, but not NFKB1 (p50), can bind to the CD28RE and activate CD28RE-driven transcription in cotransfection assays. However, the role of RelA (p65) in CD28 signaling has not yet been addressed. We provide evidence that RelA (p65) itself bound directly to the CD28RE of the IL-2 promoter and other lymphokine promoters. In addition, RelA (p65) was a potent transcriptional activator of the CD28RE in vivo. We show that a RelA (p65)-c-Rel heterodimer bound to the CD28RE and synergistically activated the CD28RE enhancer activity. We also demonstrate that activated Raf-1 kinase synergized with RelA (p65) in activating the CD28RE enhancer activity. Interestingly, a soluble anti-CD28 monoclonal antibody alone, in the absence of other stimuli, also synergized with RelA (p65) in activating the CD28RE. Furthermore, we show that RelA (p65) activated expression of the wild-type IL-2 promoter but not the CD28RE-mutated IL-2 promoter. A combination of RelA (p65) and NFKB1 (p50) also activated the IL-2 promoter through the CD28RE site. These results demonstrate the functional regulation of the CD28RE, within the IL-2 promoter, by Rel/NF-kappa B transcription factors.

FK506 inhibits antigen receptor-mediated induction of c-rel in B and T lymphoid cells

Stimulation of B and T cells via the antigen receptor, by phorbol ester or by phorbol ester and ionomycin, leads to nuclear translocation of the inducible transcription factor NF-kappa B, comprising the p50 and p65 rel-related polypeptides. In this report we show that c-rel is a component of the antigen receptor-induced kappa B binding proteins in both B and T cells. Whereas NF-kappa B can be induced by phorbol ester alone, optimal induction of c-rel requires stimulation by both phorbol ester and ionomycin, the dual signal that is necessary for proliferation of untransformed lymphocytes. Furthermore, c-rel induction is blocked by the immunosuppressive drug FK506 that is known to inhibit B and T cell activation. c-rel-dependent transactivation of the interleukin-2 receptor alpha chain (IL-2R alpha) promoter is augmented by coexpression of calcineurin, suggesting the involvement of a calcineurin-dependent intracellular pathway. Our results identify c-rel as a target of immunosuppressive agents and illustrate the similarity of activation pathways in both B and T cells.

Regulation of cell-type-specific interleukin-2 receptor alpha-chain gene expression: potential role of physical interactions between Elf-1, HMG-I(Y), and NF-kappa B family proteins

The interleukin 2 receptor alpha-chain (IL-2R alpha) gene is rapidly and potently induced in T cells in response to mitogenic stimuli. Previously, an inducible enhancer between nucleotides -299 and -228 that contains NF-kappa B and CArG motifs was identified. We now report the characterization of a second essential positive regulatory element located between nucleotides -137 and -64 that binds Elf-1 and HMG-I(Y). This element had maximal activity in lymphoid cells, paralleling the cell type specificity of Elf-1 expression. Transcription from the IL-2R alpha promoter was inhibited when either the Elf-1 or the HMG-I(Y) binding site was mutated. Coexpression of both proteins activated transcription of the -137 to -64 element in COS-7 cells. Elf-1 physically associated with HMG-I and with NF-kappa B p50 and c-Rel in vitro, suggesting that protein-protein interactions might functionally coordinate the actions of the upstream and downstream positive regulatory elements. This is the first report of a physical interaction between an Ets family member and NF-kappa B family proteins. These findings provide significant new insights into the protein-protein and protein-DNA interactions that regulate cell-type-specific and inducible IL-2R alpha gene expression and also have implications for other genes regulated by Elf-1 and NF-kappa B family proteins.

v-rel Induces ectopic expression of an adhesion molecule, DM-GRASP, during B-lymphoma development

In an effort to identify aberrantly expressed genes in v-rel-induced tumors, monoclonal antibodies were developed that reacted selectively with avian B-cell tumors. One antibody, HY78, immunoprecipitated a 120-kDa glycoprotein (p120) from cells that express v-rel. N-terminal amino acid sequencing of p120 identified a 27-amino-acid sequence that is also present in DM-GRASP, an adhesion molecule belonging to the immunoglobulin superfamily. Evidence from tissue distribution, immunological cross-reaction, PCR amplification, cDNA cloning, and DNA sequence shows that p120 is indeed DM-GRASP. Northern (RNA) analysis using a probe from the DM-GRASP gene identified a 5.3-kb transcript in mRNA from bursa, thymus, and brain as well as from v-rel-induced B-cell lymphomas but not from bursal B cells. The induction of this protein by v-rel during the development of bursal B-cell lymphomas appears, therefore, to be ectopic in nature. Overexpression of v-rel or c-rel in chicken embryonic fibroblasts, B-cell lines, and spleen mononuclear cells induces the expression of DM-GRASP. The ratio of DM-GRASP to v-Rel was fivefold higher than that of DM-GRASP/c-Rel in a B-cell line, DT95. Interestingly, the presence of HY78 antibody inhibits the in vitro proliferation of v-rel-transformed cells but not cells that immortalized by myc. These data suggest that DM-GRASP is one of the genes induced during v-rel-mediated tumor development and that DM-GRASP may be involved in the growth of v-rel tumor cells.

The PEST-like sequence of I kappa B alpha is responsible for inhibition of DNA binding but not for cytoplasmic retention of c-Rel or RelA homodimers

In most cells, proteins belonging to the Rel/NF-kappa B family of transcription factors are held in inactive form in the cytoplasm by an inhibitor protein, I kappa B alpha. Stimulation of the cells leads to degradation of the inhibitor and transit of active DNA-binding Rel/NF-kappa B dimers to the nucleus. I kappa B alpha is also able to inhibit DNA binding by Rel/NF-kappa B dimers in vitro, suggesting that it may perform the same function in cells when the activating signal is no longer present. Structurally, the human I kappa B alpha molecule can be divided into three sections: a 70-amino-acid N terminus with no known function, a 205-residue midsection composed of six ankyrin-like repeats, and a very acidic 42-amino-acid C terminus that resembles a PEST sequence. In this study we examined how the structural elements of the I kappa B alpha protein correlate with its functional capabilities both in vitro and in vivo. Using a battery of I kappa B alpha mutants, we show that (i) a dimer binds a single I kappa B alpha molecule, (ii) the acidic C-terminal region of I kappa B alpha is not required for protein-protein binding and does not mask the nuclear localization signal of the dimer, (iii) the same C-terminal region is required for inhibition of DNA binding, and (iv) this inhibition may be accomplished by direct interaction between the PEST-like region and the DNA-binding region of one of the subunits of the dimer.

Identification of the rel family members required for virus induction of the human beta interferon gene

We have carried out experiments to determine which members of the rel family of transcription factors are involved in virus induction of the beta interferon (IFN-beta) gene. First, we examined the inducibility of artificial DNA binding sites that preferentially interact with different homo- or heterodimeric combinations of rel proteins in vitro. We found that only those sites capable of binding the p50/p65 heterodimer are virus inducible. Second, we analyzed a series of mutant rel DNA-binding sites in the context of the intact IFN-beta promoter. We found a correlation between (i) sites capable of binding both the p50/p65 heterodimer and the high-mobility-group protein HMG I(Y) and (ii) virus inducibility. Third, cotransfection of the IFN-beta gene enhancer/promoter with plasmids capable of expressing several different rel proteins revealed that only the combination of p50 and p65 efficiently activated transcription. Finally, we have used antibodies directed against different rel proteins to show that virus-inducible protein-DNA complexes assembled on the IFN-beta enhancer in vitro contain both p50 and p65. We conclude that the p50/p65 heterodimer is responsible for the NF-kappa B-dependent activation of the IFN-beta gene promoter in response to virus infection.

Effect of CD28 signal transduction on c-Rel in human peripheral blood T cells

Optimal T-cell activation requires both an antigen-specific signal delivered through the T-cell receptor and a costimulatory signal which can be delivered through the CD28 molecule. CD28 costimulation induces the expression of multiple lymphokines, including interleukin 2 (IL-2). Because the c-Rel transcription factor bound to and activated the CD28 response element within the IL-2 promoter, we focused our study on the mechanism of CD28-mediated regulation of c-Rel in human peripheral blood T cells. We showed that CD28 costimulation accelerated the kinetics of nuclear translocation of c-Rel (and its phosphorylated form), p50 (NFKB1), and p65 (RelA). The enhanced nuclear translocation of c-Rel correlated with the stimulation of Il-2 production and T-cell proliferation by several distinct anti-CD28 monoclonal antibodies. This is explained at least in part by the long-term downregulation of I kappa B alpha following CD28 signalling as opposed to phorbol myristate acetate alone. Furthermore, we showed that the c-Rel-containing CD28-responsive complex is enhanced by, but not specific to, CD28 costimulation. Our results indicate that c-Rel is one of the transcription factors targeted by CD28 signalling.

Effect of CD28 signal transduction on c-Rel in human peripheral blood T cells

Optimal T-cell activation requires both an antigen-specific signal delivered through the T-cell receptor and a costimulatory signal which can be delivered through the CD28 molecule. CD28 costimulation induces the expression of multiple lymphokines, including interleukin 2 (IL-2). Because the c-Rel transcription factor bound to and activated the CD28 response element within the IL-2 promoter, we focused our study on the mechanism of CD28-mediated regulation of c-Rel in human peripheral blood T cells. We showed that CD28 costimulation accelerated the kinetics of nuclear translocation of c-Rel (and its phosphorylated form), p50 (NFKB1), and p65 (RelA). The enhanced nuclear translocation of c-Rel correlated with the stimulation of Il-2 production and T-cell proliferation by several distinct anti-CD28 monoclonal antibodies. This is explained at least in part by the long-term downregulation of I kappa B alpha following CD28 signalling as opposed to phorbol myristate acetate alone. Furthermore, we showed that the c-Rel-containing CD28-responsive complex is enhanced by, but not specific to, CD28 costimulation. Our results indicate that c-Rel is one of the transcription factors targeted by CD28 signalling.

Activation of NF-kappa B in vivo is regulated by multiple phosphorylations

The activation of nuclear factor kappa B (NF-kappa B) in intact cells is mechanistically not well understood. Therefore we investigated the modifications imposed on NF-kappa B/I kappa B components following stimulation and show that the final step of NF-kappa B induction in vivo involves phosphorylation of several members of the NF-kappa B/I kappa B protein families. In HeLa cells as well as in B cells, TNF-alpha rapidly induced nuclear translocation primarily of p50-p65, but not of c-rel. Both NF-kappa B precursors and I kappa B alpha became strongly phosphorylated with the same kinetics. In addition to the inducible phosphorylation after stimulation, B lymphocytes containing constitutive nuclear NF-kappa B revealed constitutively phosphorylated p65 and I kappa B alpha. Phosphorylation was accompanied by induced processing of the precursors p100 and p105 and by degradation of I kappa B alpha. As an in vitro model we show that phosphorylation of p105 impedes its ability to interact with NF-kappa B, as has been shown before for I kappa B alpha. Surprisingly, even p65, but not c-rel, was phosphorylated after induction in vivo, suggesting that TNF-alpha selectively activates only specific NF-kappa B heteromers and that modifications regulate not only I kappa B molecules but also NF-kappa B molecules. In fact, cellular NF-kappa B activity was phosphorylation-dependent and the DNA binding activity of p65-containing NF-kappa B was enhanced by phosphorylation in vitro. Furthermore, we found that the induction by hydrogen peroxide of NF-kappa B translocation to the nucleus, which is assumed to be triggered by reactive oxygen intermediates, also coincided with incorporation of phosphate into the same subunits that were modified after stimulation by TNF-alpha. Thus, phosphorylation appears to be a general mechanism for activation of NF-kappa B in vivo.

Lipopolysaccharide induction of tissue factor gene expression in monocytic cells is mediated by binding of c-Rel/p65 heterodimers to a kappa B-like site

Exposure of monocytic cells to bacterial lipopolysaccharide (LPS) activates the NF-kappa B/Rel family of proteins and leads to the rapid induction of inflammatory gene products, including tissue factor (TF). TF is the primary cellular initiator of the coagulation protease cascades. Here we report the characterization of a nuclear complex from human monocytic cells that bound to a kappa B-like site, 5'-CGGAGTTTCC-3', in the 5'-flanking region of the human TF gene. This nuclear complex was activated by LPS with kinetics that preceded induction of the TF gene. In vitro binding studies demonstrated that the TF site bound translated c-Rel and p65 homodimers but not p50/p65 heterodimers or p50 homodimers. Base-pair substitutions in the TF site indicated that the presence of a cytosine at position 1 precluded binding of NF-kappa B. In fact, under low-ionic-strength conditions, the TF complex did not migrate with translated p50/p65 dimers but instead comigrated with c-Rel/p65 dimers. Antibodies against the NF-kappa B and Rel proteins and UV cross-linking studies revealed the presence of c-Rel and p65 and the absence of p50 in the TF complex and further showed that c-Rel/p65 heterodimers selectively bound to the TF kappa B-like site. Functional studies indicated that the TF site conferred LPS inducibility on a heterologous promoter and was transactivated by c-Rel or p65. Taken together, our results demonstrated that binding of c-Rel/p65 heterodimers to a novel kappa B-like site mediated LPS induction of TF gene expression in monocytic cells.

Lipopolysaccharide induction of tissue factor gene expression in monocytic cells is mediated by binding of c-Rel/p65 heterodimers to a kappa B-like site

Exposure of monocytic cells to bacterial lipopolysaccharide (LPS) activates the NF-kappa B/Rel family of proteins and leads to the rapid induction of inflammatory gene products, including tissue factor (TF). TF is the primary cellular initiator of the coagulation protease cascades. Here we report the characterization of a nuclear complex from human monocytic cells that bound to a kappa B-like site, 5'-CGGAGTTTCC-3', in the 5'-flanking region of the human TF gene. This nuclear complex was activated by LPS with kinetics that preceded induction of the TF gene. In vitro binding studies demonstrated that the TF site bound translated c-Rel and p65 homodimers but not p50/p65 heterodimers or p50 homodimers. Base-pair substitutions in the TF site indicated that the presence of a cytosine at position 1 precluded binding of NF-kappa B. In fact, under low-ionic-strength conditions, the TF complex did not migrate with translated p50/p65 dimers but instead comigrated with c-Rel/p65 dimers. Antibodies against the NF-kappa B and Rel proteins and UV cross-linking studies revealed the presence of c-Rel and p65 and the absence of p50 in the TF complex and further showed that c-Rel/p65 heterodimers selectively bound to the TF kappa B-like site. Functional studies indicated that the TF site conferred LPS inducibility on a heterologous promoter and was transactivated by c-Rel or p65. Taken together, our results demonstrated that binding of c-Rel/p65 heterodimers to a novel kappa B-like site mediated LPS induction of TF gene expression in monocytic cells.

Differential regulation of the c-myc oncogene promoter by the NF-kappa B rel family of transcription factors

The murine c-myc gene contains two elements responsive to the rel-oncogene-related family of NF-kappa B factors. Previously we have shown that factor binding to these two NF-kappa B elements mediates induction of transcription of the c-myc promoter upon interleukin-1 treatment of human dermal fibroblasts and human T-cell leukemia virus type I tax gene expression in T cells (D. J. Kessler, M. P. Duyao, D. B. Spicer, and G. E. Sonenshein, J. Exp. Med. 176:787-792, 1992; M. P. Duyao, D. J. Kessler, D. B. Spicer, C. Bartholomew, J. L. Cleveland, M. Siekevitz, and G. E. Sonenshein, J. Biol. Chem. 267:16288-16291, 1992). To begin to delineate the specific roles of the individual members of the NF-kappa B family, here we have tested their effects on activation of a c-myc promoter/exon 1-CAT construct in NIH 3T3 cells. Classical NF-kappa B (p65/p50) was a potent transcriptional activator of the c-myc promoter. Cotransfection with either p65 alone or p65 in combination with p50 mediated significant induction. In contrast, expression of either v-rel or chicken c-rel failed to transactivate, while murine c-rel induced c-myc promoter activity only slightly. Furthermore, induction by classical NF-kappa B was inhibited by coexpression of either v-rel or chicken c-rel. Thus, individual members of the rel family have differential effects of the c-myc promoter, which can modulate overall transcriptional activity and allow for precise regulation of this oncogene under diverse physiologic conditions.

Differential regulation of the c-myc oncogene promoter by the NF-kappa B rel family of transcription factors

The murine c-myc gene contains two elements responsive to the rel-oncogene-related family of NF-kappa B factors. Previously we have shown that factor binding to these two NF-kappa B elements mediates induction of transcription of the c-myc promoter upon interleukin-1 treatment of human dermal fibroblasts and human T-cell leukemia virus type I tax gene expression in T cells (D. J. Kessler, M. P. Duyao, D. B. Spicer, and G. E. Sonenshein, J. Exp. Med. 176:787-792, 1992; M. P. Duyao, D. J. Kessler, D. B. Spicer, C. Bartholomew, J. L. Cleveland, M. Siekevitz, and G. E. Sonenshein, J. Biol. Chem. 267:16288-16291, 1992). To begin to delineate the specific roles of the individual members of the NF-kappa B family, here we have tested their effects on activation of a c-myc promoter/exon 1-CAT construct in NIH 3T3 cells. Classical NF-kappa B (p65/p50) was a potent transcriptional activator of the c-myc promoter. Cotransfection with either p65 alone or p65 in combination with p50 mediated significant induction. In contrast, expression of either v-rel or chicken c-rel failed to transactivate, while murine c-rel induced c-myc promoter activity only slightly. Furthermore, induction by classical NF-kappa B was inhibited by coexpression of either v-rel or chicken c-rel. Thus, individual members of the rel family have differential effects of the c-myc promoter, which can modulate overall transcriptional activity and allow for precise regulation of this oncogene under diverse physiologic conditions.

Tumor necrosis factor and interleukin-1 lead to phosphorylation and loss of I kappa B alpha: a mechanism for NF-kappa B activation

Nuclear factor kappa B (NF-kappa B) is a critical regulator of several genes which are involved in immune and inflammation responses. NF-kappa B, consisting of a 50-kDa protein (p50) and a 65-kDa protein (p65), is bound to a cytoplasmic retention protein called I kappa B. Stimulation of cells with a variety of inducers, including cytokines such as tumor necrosis factor and interleukin-1, leads to the activation and the translocation of p50/65 NF-kappa B into the nucleus. However, the in vivo mechanism of the activation process remains unknown. Here, we provide the first evidence that the in vivo mechanism of NF-kappa B activation is through the phosphorylation and subsequent loss of its inhibitor, I kappa B alpha. We also show that both I kappa B alpha loss and NF-kappa B activation are inhibited in the presence of antioxidants, demonstrating that the loss of I kappa B alpha is a prerequisite for NF-kappa B activation. Finally, we demonstrate that I kappa B alpha is rapidly resynthesized after loss, indicating that an autoregulatory mechanism is involved in the regulation of NF-kappa B function. We propose a mechanism for the activation of NF-kappa B through the modification and loss of I kappa B alpha, thereby establishing its role as a mediator of NF-kappa B activation.

Tumor necrosis factor and interleukin-1 lead to phosphorylation and loss of I kappa B alpha: a mechanism for NF-kappa B activation

Nuclear factor kappa B (NF-kappa B) is a critical regulator of several genes which are involved in immune and inflammation responses. NF-kappa B, consisting of a 50-kDa protein (p50) and a 65-kDa protein (p65), is bound to a cytoplasmic retention protein called I kappa B. Stimulation of cells with a variety of inducers, including cytokines such as tumor necrosis factor and interleukin-1, leads to the activation and the translocation of p50/65 NF-kappa B into the nucleus. However, the in vivo mechanism of the activation process remains unknown. Here, we provide the first evidence that the in vivo mechanism of NF-kappa B activation is through the phosphorylation and subsequent loss of its inhibitor, I kappa B alpha. We also show that both I kappa B alpha loss and NF-kappa B activation are inhibited in the presence of antioxidants, demonstrating that the loss of I kappa B alpha is a prerequisite for NF-kappa B activation. Finally, we demonstrate that I kappa B alpha is rapidly resynthesized after loss, indicating that an autoregulatory mechanism is involved in the regulation of NF-kappa B function. We propose a mechanism for the activation of NF-kappa B through the modification and loss of I kappa B alpha, thereby establishing its role as a mediator of NF-kappa B activation.

Interaction between NF-kappa B- and serum response factor-binding elements activates an interleukin-2 receptor alpha-chain enhancer specifically in T lymphocytes

We find that a short enhancer element containing the NF-kappa B binding site from the interleukin-2 receptor alpha-chain gene (IL-2R alpha) is preferentially activated in T cells. The IL-2R alpha enhancer binds NF-kappa B poorly and is only weakly activated by the NF-kappa B site alone. Serum response factor (SRF) binds to a site adjacent to the NF-kappa B site in the IL-2R enhancer, and both sites together have strong transcriptional activity specifically in T cells. Surprisingly, the levels of SRF constitutively expressed in T cells are consistently higher than in other cell types. Overexpression of SRF in B cells causes the IL-2R enhancer to function as well as it does in T cells, suggesting that the high level of SRF binding in T cells is functionally important.

Interaction between NF-kappa B- and serum response factor-binding elements activates an interleukin-2 receptor alpha-chain enhancer specifically in T lymphocytes

We find that a short enhancer element containing the NF-kappa B binding site from the interleukin-2 receptor alpha-chain gene (IL-2R alpha) is preferentially activated in T cells. The IL-2R alpha enhancer binds NF-kappa B poorly and is only weakly activated by the NF-kappa B site alone. Serum response factor (SRF) binds to a site adjacent to the NF-kappa B site in the IL-2R enhancer, and both sites together have strong transcriptional activity specifically in T cells. Surprisingly, the levels of SRF constitutively expressed in T cells are consistently higher than in other cell types. Overexpression of SRF in B cells causes the IL-2R enhancer to function as well as it does in T cells, suggesting that the high level of SRF binding in T cells is functionally important.

The interleukin 2 CD28-responsive complex contains at least three members of the NF kappa B family: c-Rel, p50, and p65

Optimal activation of T cells requires at least two signals. One signal can be delivered by the antigen-specific T-cell receptor, and the second signal is provided by the costimulatory molecule(s) delivered by the antigen-presenting cell. CD28 is a T-cell surface molecule and stimulation through this protein plays an important role in delivering the second activation signal. In this report, we show that in human peripheral blood T cells, CD28-mediated signal transduction involves the rel family proteins--c-Rel, p50, and p65. Treatment of peripheral blood T cells with phorbol 12-myristate 13-acetate (PMA) and anti-CD28 monoclonal antibody (mAb) results in augmentation of nuclear c-Rel, p50, and p65, and this augmentation can occur in the presence of the immunosuppressant cyclosporin A. It is also shown in this report that, in response to PMA/anti-CD28 mAb or anti-CD3/anti-CD28 mAb, c-Rel, p50, and p65 are associated with CD28-responsive element present in the promoter of the human interleukin 2 gene. The functional significance of c-Rel involvement in the CD28-responsive complex is demonstrated by transient transfection analysis, where cotransfection of c-Rel augments the level of expression of a chloramphenicol acetyltransferase reporter gene linked to the CD28-responsive element.