Rel-dependent induction of A1 transcription is required to protect B cells from antigen receptor ligation-induced apoptosis

Characterization of a human REL-estrogen receptor fusion protein with a reverse conditional transforming activity in chicken spleen cells

Overexpression of the human REL transcription factor can malignantly transform chicken spleen cells in vitro. In this report, we have created and characterized a cDNA encoding a chimeric protein (RELDelta424-490-ER) in which sequences of a highly transforming REL mutant (RELDelta424-490) are fused to the ligand-binding domain of the human estrogen receptor (ER). Surprisingly, RELDelta424-490-ER is constitutively nuclear in A293 cells, and RELDelta424-490-ER activates transcription in the absence, but not in the presence, of estrogen in kappaB-site reporter gene assays. Furthermore, RELDelta424-490-ER transforms chicken spleen cells in the absence of estrogen, but the addition of estrogen blocks the ability of RELDelta424-490-ER-transformed cells to form colonies in soft agar, even though estrogen induces increased nuclear translocation of RELDelta424-490-ER in these cells. ERalpha can also inhibit REL-dependent transactivation in trans in an estrogen-dependent manner, and ERalpha can interact with REL in vitro. Thus, the RELDelta424-490-ER fusion protein shows an unusual, reverse hormone regulation, in that its most prominent biological activities (transformation and transactivation) are inhibited by estrogen, probably due to an estrogen-induced interaction between the ER sequences and sequences in the Rel homology domain. Nevertheless, these results indicate that the continual activity of REL is required to sustain the transformed state of chicken spleen cells in culture, suggesting that direct and specific inhibitors of REL may have therapeutic efficacy in certain human lymphoid cancers.

Guggulsterone inhibits NF-kappaB and IkappaBalpha kinase activation, suppresses expression of anti-apoptotic gene products, and enhances apoptosis

Guggulsterone, derived from Commiphora mukul and used to treat obesity, diabetes, hyperlipidemia, atherosclerosis, and osteoarthritis, has been recently shown to antagonize the farnesoid X receptor and decrease the expression of bile acid-activated genes. Because activation of NF-kappaB has been closely linked with inflammatory diseases affected by guggulsterone, we postulated that it must modulate NF-kappaB activation. In the present study, we tested this hypothesis by investigating the effect of this steroid on the activation of NF-kappaB induced by inflammatory agents and carcinogens. Guggulsterone suppressed DNA binding of NF-kappaB induced by tumor necrosis factor (TNF), phorbol ester, okadaic acid, cigarette smoke condensate, hydrogen peroxide, and interleukin-1. NF-kappaB activation was not cell type-specific, because both epithelial and leukemia cells were inhibited. Guggulsterone also suppressed constitutive NF-kappaB activation expressed in most tumor cells. Through inhibition of IkappaB kinase activation, this steroid blocked IkappaBalpha phosphorylation and degradation, thus suppressing p65 phosphorylation and nuclear translocation. NF-kappaB-dependent reporter gene transcription induced by TNF, TNFR1, TRADD, TRAF2, NIK, and IKK was also blocked by guggulsterone but without affecting p65-mediated gene transcription. In addition, guggulsterone decreased the expression of gene products involved in anti-apoptosis (IAP1, xIAP, Bfl-1/A1, Bcl-2, cFLIP, and survivin), proliferation (cyclin D1 and c-Myc), and metastasis (MMP-9, COX-2, and VEGF); this correlated with enhancement of apoptosis induced by TNF and chemotherapeutic agents. Overall, our results indicate that guggulsterone suppresses NF-kappaB and NF-kappaB-regulated gene products, which may explain its anti-inflammatory activities.

The transcription factors c-rel and RelA control epidermal development and homeostasis in embryonic and adult skin via distinct mechanisms

Determining the roles of Rel/NF-kappaB transcription factors in mouse skin development with loss-of-function mutants has been limited by redundancy among these proteins and by embryonic lethality associated with the absence of RelA. Using mice lacking RelA and c-rel, which survive throughout embryogenesis on a tumor necrosis factor alpha (TNF-alpha)-deficient background (rela(-/-) c-rel(-/-) tnfalpha(-/-)), we show that c-rel and RelA are required for normal epidermal development. Although mutant fetuses fail to form tylotrich hair and have a thinner epidermis, mutant keratinocyte progenitors undergo terminal differentiation to form an outer cornified layer. Mutant basal keratinocytes are abnormally small, exhibit a delay in G(1) progression, and fail to form keratinocyte colonies in culture. In contrast to the reduced proliferation of mutant keratinocytes during embryogenesis, skin grafting experiments revealed that the mutant epidermis develops a TNF-alpha-dependent hyperproliferative condition. Collectively, our findings indicate that RelA and c-rel control the development of the epidermis and associated appendages during embryogenesis and regulate epidermal homeostasis in a postnatal environment through the suppression of innate immune-mediated inflammation.

Up-regulation of Bfl-1/A1 via NF-κB activation in cisplatin-resistant human bladder cancer cell line

The potent anti-cancer agent cis-diamminedichloroplatinum (II) (cisplatin) is currently used for treating bladder cancer. However, clinical use of this drug for long periods is often limited because of the appearance of cisplatin-resistant bladder tumor cells. We employed the method of a differential display reverse transcriptase polymerase chain reaction to identify the differentially expressed genes in the parental human bladder cancer cell line, T24 and three cisplatin-resistant cell lines. We report here that cisplatin-resistant cell lines overexpress Bcl-2 family protein Bcl-2-related gene expressed in fetal liver (Bfl-1)/A1 as compared with their parental cell. Cisplatin and gamma-irradiation induced expression of Bfl-1/A1 in T24R2 cells but not in T24 cells. Among Bcl-2 family members, Bfl-1/A1 showed the most significant alteration of the expression level in resistant cells. The nuclear translocation of nuclear factor-kappaB (NF-kappaB) by cisplatin and gamma-irradiation selectively occurred in T24R2 cells. Mitochondrial depolarization and cell death by cisplatin were also prevented in T24R2 cells. Moreover, Bfl-1/A1 inhibited cisplatin- and TNF-alpha-induced apoptosis in BOSC23 cells. Our findings suggest that the induction of Bfl-1/A1 by NF-kappaB may be important in controlling resistance to cisplatin responses in bladder tumor cells.

Genetic deletion of glycogen synthase kinase-3beta abrogates activation of IkappaBalpha kinase, JNK, Akt, and p44/p42 MAPK but potentiates apoptosis induced by tumor necrosis factor

Glycogen synthase kinase (GSK)-3beta is a constitutively active, proline-directed serine/threonine kinase that controls growth modulation and tumorigenesis through multiple intracellular signaling pathways. How GSK-3beta regulates signaling pathways induced by cytokines such as tumor necrosis factor (TNF) is poorly understood. In this study, we used fibroblasts derived from GSK-3beta gene-deleted mice to understand the role of this kinase in TNF signaling. TNF induced NF-kappaB activation as measured by DNA binding in wild-type mouse embryonic fibroblasts, but deletion of GSK-3beta abolished this activation. This inhibition was due to suppression of IkappaBalpha kinase activation and IkappaBalpha phosphorylation, ubiquitination, and degradation. TNF-induced NF-kappaB reporter gene transcription was also suppressed in GSK-3beta gene-deleted cells. NF-kappaB activation induced by lipopolysaccharide, interleukin-1beta, or cigarette smoke condensate was completely suppressed in GSK-3beta(-/-) cells. Deletion of GSK-3beta also abolished TNF-induced c-Jun N-terminal kinase and p44/p42 mitogen-activated kinase activation. Most surprisingly, TNF-induced Akt activation also required the presence of GSK-3beta. TNF induced expression of the NF-kappaB-regulated gene products cyclin D1, COX-2, MMP-9, survivin, IAP 1, IAP 2, Bcl-x(L), Bfl-1/A1, TRAF1, and FLIP in wild-type mouse embryonic fibroblasts but not in GSK-3beta(-/-) cells, and this correlated with potentiation of TNF-induced apoptosis as indicated by cell viability, annexin V staining, and caspase activation. Overall, our results indicate that GSK-3beta plays a critical role in TNF signaling and in the signaling of other inflammatory stimuli and that its suppression can be exploited as a potential target to inhibit angiogenesis, proliferation, and survival of tumor cells.

The domains of apoptosis: a genomics perspective

Apoptosis plays important roles in many facets of normal physiology in animal species, including programmed cell death associated with fetal development or metamorphosis, tissue homeostasis, immune cell education, and some aspects of aging. Defects in the regulation of apoptosis contribute to multiple diseases associated with either inappropriate cell loss or pathological cell accumulation. Host-pathogen interactions have additionally provided evolutionary pressure for apoptosis as a defense mechanism against viruses and microbes, sometimes linking apoptosis mechanisms with inflammatory responses. To a large extent, the apoptosis machinery can be viewed as a network, with different nodes connected by physical interactions of evolutionarily conserved domains. These domains can serve as signatures for identification of proteins involved in the network. In particular, the caspase recruitment domains (CARDs); death effector domains (DEDs); death domains (DDs); BIR (baculovirus IAP repeat) domains of inhibitor of apoptosis proteins (IAPs); Bcl-2 family proteins; caspase protease domains; and endonuclease-associated CIDE (cell death-inducing DFF45-like effector) domains are found in common in proteins involved in apoptosis. In the genomes of mammals, genes encoding proteins that carry one or more of these signature domains are often present in multiple copies, making up diverse gene families that permit tissue-specific and highly regulated control of cell life and death decisions through combinations of stimulus-specific gene expression and complex protein interaction networks. In this Review, we organize the repertoire of apoptosis proteins of humans into domain families, drawing comparisons with homologs in other vertebrate and invertebrate animal species, and discuss some of the functional implications of these findings.

The Bcl-2 family member Bfl-1/A1 is strongly repressed in normal and malignant plasma cells but is a potent anti-apoptotic factor for myeloma cells

Terminal B-cell differentiation is a multi-step process, from short-lived plasmablasts to mature long-lived plasma cells (PC). The anti-apoptotic Bcl-2 family member Bfl-1/A1 plays a critical role in the survival of mature B cells. However, its potential involvement at the later stages of B-cell development remains highly controversial. Our aim was thus to clarify the place of Bfl-1/A1 in the biology of normal PC and in the pathogenesis of multiple myeloma (MM), the major PC dyscrasia. Using gene expression profiling and quantifiable reverse transcription polymerase chain reaction experiments, we found a similar down-regulation of Bfl-1/A1 in both normal immature plasmablasts and mature PC when compared with B cells. In myeloma cells, the level of Bfl-1/A1 was low and Bfl-1/A1 was not a nuclear factor kappaB-inducible gene. Collectively, these data demonstrate that Bfl-1/A1 is not involved in the prolonged survival of normal mature PC, and that Bfl-1/A1 deregulation is not a common oncogenic event in MM. However, overexpression of Bfl-1/A1 by retroviral transduction promoted autonomous survival of an interleukin-6-dependent myeloma cell line and rendered it less sensitive to dexamethasone. Thus, Bfl-1/A1 transduction could be an interesting tool to obtain myeloma cell lines from primary samples and to favour the in vitro generation of antibody-secreting, long-lived normal PC.

Protein Farnesyltransferase Inhibitor (SCH 66336) Abolishes NF-κB Activation Induced by Various Carcinogens and Inflammatory Stimuli Leading to Suppression of NF-κB-regulated Gene Expression and Up-regulation of Apoptosis

Ras farnesyltransferase inhibitor (FTI) exhibit antiproliferative and antiangiogenic effects through a mechanism that is poorly understood. Because of the known role of Ras in the activation of transcription factor NF-kappaB and because NF-kappaB-regulated genes can control cell survival and angiogenesis, we postulated that FTI mediates its effects in part by modulating NF-kappaB activation. Therefore, in the present study we investigated the effect of FTI, SCH 66336, on NF-kappaB and NF-kappaB-regulated gene expression activated by a variety of inflammatory and carcinogenic agents. We demonstrate by DNA-binding assay that NF-kappaB activation induced by tumor necrosis factor (TNF), phorbol 12-myristate 13-acetate, cigarette smoke, okadaic acid, and H(2)O(2) was completely suppressed by SCH 66336; the suppression was not cell type-specific. This FTI suppressed the activation of IkappaBalpha kinase (IKK), thus abrogating the phosphorylation and degradation of IkappaBalpha. Additionally, TNF-activated Ras and SCH 66336 inhibited the activation. Also, overexpression of Ras (V12) enhanced TNF-induced NF-kappaB activation, and adenoviral dominant-negative Ras (N17) suppressed the activation, thus suggesting the critical role of Ras in TNF signaling. SCH 66336 also inhibited the NF-kappaB-dependent reporter gene expression activated by TNF, TNFR1, TRADD, TRAF2, NIK, and IKK but not that activated by the p65 subunit of NF-kappaB. The TNF-induced NF-kappaB-regulated gene products cyclin D1, COX-2, MMP-9, survivin, IAP1, IAP2, XIAP, Bcl-2, Bfl-1/A1, TRAF1, and FLIP were all down-regulated by SCH 66336, which potentiated apoptosis induced by TNF and doxorubicin. Overall, our results indicate that SCH 66336 inhibited activation of NF-kappaB and NF-kappaB-regulated gene expressions induced by carcinogens and inflammatory stimuli, which may provide a molecular basis for the ability of SCH 66336 to suppress proliferation and angiogenesis.

Maximal apoptosis of renal cell carcinoma by the proteasome inhibitor bortezomib is nuclear factor-κB dependent

Advanced renal cell carcinoma (RCC) is resistant to cytotoxic chemotherapy, and immunotherapy has modest activity. Proteasome inhibitors represent a novel class of anticancer agents that have activity across a wide spectrum of tumor types. We investigated the efficacy of the proteasome inhibitor bortezomib (VELCADE, formerly known as PS-341) in RCC and found that bortezomib potently induces apoptosis of RCC cell lines. Blockade of the nuclear factor-κB (NF-κB) pathway is considered a crucial effect in bortezomib-induced apoptosis, but the dependence on NF-κB inhibition for bortezomib-mediated death has not been formally demonstrated. Thus, we also studied the contribution of NF-κB inhibition as a mechanism of bortezomib-induced apoptosis in RCC cells, which display constitutive NF-κB activation. Ectopic expression of the NF-κB family members, p65 (Rel A) and p50 (NF-κB1), markedly reduced bortezomib-induced apoptosis. However, when we used selective genetic and chemical inhibitors of NF-κB, we found that NF-κB blockade was not sufficient to induce apoptosis of RCC cells. Thus, we conclude that maximal bortezomib-induced apoptosis is dependent on its NF-κB inhibitory effect, but NF-κB-independent effects also play a critical role in the induction of apoptosis by bortezomib. This represents the first report to formally demonstrate that bortezomib-induced NF-κB blockade is required to achieve the maximum degree of apoptosis by this drug.

Involvement of Rel/NF-kappa B transcription factors in senescence

Senescence is now established as a genetically controlled phenomenon that alters different cell functions, including proliferation, apoptosis, resistance to stress, and energetic metabolism. Underlying changes in gene expression are governed by some transcription factors, whose expression or activity must change with senescence as well. Transcription factors of the Rel/NF-kappa B family are good candidates to participate in the establishment of senescence. Arguments range from correlation between cell functions controlled by these factors and cell functions altered during senescence, to phenotypes resulting from in vitro manipulations of Rel/NF-kappa B activity.

Fatal Acute Lymphoblastic Leukemia in Mice Transgenic for B Cell-Restricted bcl-xLand c-myc

Expression of the c-myc gene is frequently dysregulated in malignant tumors and translocations of c-myc into the Ig H chain locus are associated with Burkitt's-type lymphoma. There is indirect evidence that bcl-x, an anti-apoptotic member of the bcl-2 gene family, may also contribute to a variety of B lymphoid tumors. In this study, we show that mice transgenic for both B cell-restricted c-myc and bcl-x(L) developed aggressive, acute leukemias expressing early B lineage and stem cell surface markers. Of interest, the tumor cells proliferated and differentiated down the B cell developmental pathway following in vitro treatment with IL-7. Analysis of sorted leukemic cells from spleen indicated constitutive expression of sterile micro and kappa transcripts in combination with evidence for D-J(H) DNA rearrangements. Several B cell-specific genes were either not expressed or were expressed at low levels in primary tumor cells and were induced following culture with IL-7. IL-7 also increased V-Jkappa and V-DJ(H) rearrangements. These data demonstrate oncogenic synergy between c-myc and bcl-x(L) in a new mouse model for acute lymphoblastic leukemia. Tumors in these animals target an early stage in B cell development characterized by the expression of both B lineage and stem cell genes.

Role of Bcl-2 family members in immunity and disease

The different members of the Bcl-2 family are essential regulators of programmed cell death. These different members share one or more Bcl-2 homology domains, required for their ability to regulate each other. In this review, we describe current knowledge of the functions of different Bcl-2 members and their potential roles in disease and immunity.

Caspase-8-Dependent HER-2 Cleavage in Response to Tumor Necrosis Factor α Stimulation Is Counteracted by Nuclear Factor κB through c-FLIP-L Expression

The oncoprotein HER-2/neu is a prosurvival factor, and its overexpression has been correlated with poor prognosis in patients with breast cancer. We report that HER-2 is a new substrate for caspase-8 and that tumor necrosis factor alpha (TNF-alpha) stimulation leads to an early caspase-8-dependent HER-2 cleavage in MCF7 A/Z breast adenocarcinoma cells defective for nuclear factor kappaB (NFkappaB) activation. We show that the antiapoptotic transcription factor NFkappaB counteracts this cleavage through induction of the caspase-8 inhibitor c-FLIP. Our results also demonstrate that this HER-2 cleavage contributes to the TNF-alpha-induced apoptosis pathway because ectopic expression of an uncleavable HER-2 protects NFkappaB-defective cells against TNF-alpha-mediated cell death. Therefore, we propose an original model in which NFkappaB exerts a new antiapoptotic function by counteracting TNF-alpha-triggered cleavage of the HER-2 survival factor.

The c-Rel transcription factor and B-cell proliferation: a deal with the devil

Activation of the Rel/NF-kappaB signal transduction pathway has been associated with a variety of animal and human malignancies. However, among the Rel/NF-kappaB family members, only c-Rel has been consistently shown to be able to malignantly transform cells in culture. In addition, c-rel has been activated by a retroviral promoter insertion in an avian B-cell lymphoma, and amplifications of REL (human c-rel) are frequently seen in Hodgkin's lymphomas and diffuse large B-cell lymphomas, and in some follicular and mediastinal B-cell lymphomas. Phenotypic analysis of c-rel knockout mice demonstrates that c-Rel has a normal role in B-cell proliferation and survival; moreover, c-Rel nuclear activity is required for B-cell development. Few mammalian model systems are available to study the role of c-Rel in oncogenesis, and it is still not clear what features of c-Rel endow it with its unique oncogenic activity among the Rel/NF-kappaB family. In any event, REL may provide an appropriate therapeutic target for certain human lymphoid cell malignancies.

Genes and genomics of autoimmune inflammation: from Rel to TRAIL

Development of autoimmune diseases requires coordinated expression of a myriad of genes. To explore the spectrum and global patterns of gene expression during autoimmune inflammation, we have recently performed functional genomic studies of autoimmune inflammation in the central nervous system (CNS). Inflammation in the CNS not only induced the expression of many immune-related genes, but also significantly altered the gene expression profile of neural cells. A number of unique clusters of genes were identified, which represent putative immune and nervous responses in autoimmune inflammation. This review will focus on two clusters of genes that we have been studying during the past few years: the Rel/nuclear factor (NF)-kappaB family and the tumor necrosis factor (TNF) family.

NF-kB inhibitor blocks B cell development at two checkpoints

Members of the NF-kB transcription factor family are differentially expressed in the B cell lineage. Disruption of individual or two NF-kB subunits exhibits distinct defects in B lymphocyte development, activation, and survival. However, the role each NF-kB plays during B cell development has been obscured by molecular compensation. To address this issue, a trans-dominant form of IkBα was transduced into bone marrow cells to act as a pan-inhibitor of NF-kB using a retroviral system. While the development of T-lymphocytes and myeloid cell lineages was not grossly affected by the transduced IkBα gene, a significant reduction in the number and percentage of B lineage cells was apparent in IkBα transduced chimeric mice. IkBα expression decreased the percentage of pre-B and immature B cell subsets in the bone marrow and further impaired the development of follicular mature B cells and marginal zone B cells in the periphery. Introduction of the Bcl-X transgene completely restored the pre-B and immature B cell pool in the bone marrow. However, despite a significant improvement of overall viability of the B cell lineage, Bcl-X expression was insufficient to overcome the maturation block resulting from NF-kB inhibition. Together, our study suggests that NF-kB activity is required for two distinct checkpoints during B cell development: one is for pre-B/immature B cell viability, the other is to provide both survival and maturation signals to ensure the proper development of follicular mature B cells.

NF-kappaB in pancreatic cancer

Although the genetic profile of pancreatic cancer is emerging as a result of much research, the role of specific genetic alterations that initiate tumorigenesis and produce its cardinal clinical features of locally aggressive growth, metastasis, and chemotherapy resistance remains unresolved. Recently, a number of studies have shown that the inhibition of constitutive NF-kappaB activation, one of the frequent molecular alterations in pancreatic cancer, inhibits tumorigenesis and metastasis. It also sensitizes pancreatic cancer cell lines to anticancer agent-induced apoptosis. Therefore because of the crucial role of NF-kappaB in pancreatic cancer, it is a potential target for developing novel therapeutic strategies for the disease. In vivo and in vitro models that mimic the tumorigenic phenotypes in the appropriate histological and molecular concert would be very useful for confirming the suspected role of the pancreatic cancer signature genetic lesions and better understanding the molecular basis of this disease.

Estrogen stimulates microglia and brain recovery from hypoxia-ischemia in normoglycemic but not diabetic female mice

Diabetic hyperglycemia increases ischemic brain damage in experimental animals and humans. The mechanisms are unclear but may involve enhanced apoptosis in penumbral regions. Estrogen is an established neuroprotectant in experimental stroke. Our previous study demonstrated that female diabetic db/db mice suffered less damage following cerebral hypoxia-ischemia (H/I) than male db/db mice. Here we investigated the effects of diabetes and estrogen apoptotic gene expression following H/I. Female db/db and nondiabetic (+/?) mice were ovariectomized (OVX) and treated with estrogen or vehicle prior to H/I; brains were analyzed for damage and bcl-2 family gene expression. OVX increased ischemic damage in +/? mice; estrogen reduced tissue injury and enhanced antiapoptotic gene expression (bcl-2 and bfl-1). db/db mice demonstrated more damage, without increased bcl-2 mRNA; bfl-1 expression appeared at 48 hours of recovery associated with infarction. To our knowledge, this is the first description of bfl-1 in the brain with localization to microglia and macrophages. Early induction of bfl-1 expression in +/? mouse brain was associated with microglia; delayed bfl-1 expression in diabetic brain was in macrophages bordering the infarct. Furthermore, estrogen replacement stimulated early postischemic expression of bcl-2 and bfl-1 and reduced damage in normoglycemic animals but failed to protect the diabetic brain.

Estrogen stimulates microglia and brain recovery from hypoxia-ischemia in normoglycemic but not diabetic female mice

Diabetic hyperglycemia increases ischemic brain damage in experimental animals and humans. The mechanisms are unclear but may involve enhanced apoptosis in penumbral regions. Estrogen is an established neuroprotectant in experimental stroke. Our previous study demonstrated that female diabetic db/db mice suffered less damage following cerebral hypoxia-ischemia (H/I) than male db/db mice. Here we investigated the effects of diabetes and estrogen apoptotic gene expression following H/I. Female db/db and nondiabetic (+/?) mice were ovariectomized (OVX) and treated with estrogen or vehicle prior to H/I; brains were analyzed for damage and bcl-2 family gene expression. OVX increased ischemic damage in +/? mice; estrogen reduced tissue injury and enhanced antiapoptotic gene expression (bcl-2 and bfl-1). db/db mice demonstrated more damage, without increased bcl-2 mRNA; bfl-1 expression appeared at 48 hours of recovery associated with infarction. To our knowledge, this is the first description of bfl-1 in the brain with localization to microglia and macrophages. Early induction of bfl-1 expression in +/? mouse brain was associated with microglia; delayed bfl-1 expression in diabetic brain was in macrophages bordering the infarct. Furthermore, estrogen replacement stimulated early postischemic expression of bcl-2 and bfl-1 and reduced damage in normoglycemic animals but failed to protect the diabetic brain.

Nuclear factor kappa B-dependent activation of the antiapoptotic bfl-1 gene by the Epstein-Barr virus latent membrane protein 1 and activated CD40 receptor

Suppression of the cellular apoptotic program by the oncogenic herpesvirus Epstein-Barr virus (EBV) is central to both the establishment of latent infection and the development of EBV-associated malignancies. We have previously shown that expression of the EBV latent membrane protein 1 (LMP1) in Burkitt's lymphoma cell lines leads to increased mRNA levels from the cellular antiapoptotic bfl-1 gene (also known as A1). Furthermore, ectopic expression of Bfl-1 in an EBV-positive cell line exhibiting a latency type 1 infection protects against apoptosis induced by growth factor deprivation (B. N. D'Souza, M. Rowe, and D. Walls, J. Virol. 74:6652-6658, 2000). We now report that LMP1 drives bfl-1 promoter activity through interactions with components of the tumor necrosis factor receptor (TNFR)/CD40 signaling pathway. We present evidence that this process is NF-kappa B dependent, involves the recruitment of TNFR-associated factor 2, and is mediated to a greater extent by the carboxyl-terminal activating region 2 (CTAR2) relative to the CTAR1 domain of LMP1. Activation of CD40 receptor also led to increased bfl-1 mRNA levels and an NF-kappa B-dependent increase in bfl-1 promoter activity in Burkitt's lymphoma-derived cell lines. We have delineated a 95-bp region of the promoter that functions as an LMP1-dependent transcriptional enhancer in this cellular context. This sequence contains a novel NF-kappa B-like binding motif that is essential for transactivation of bfl-1 by LMP1, CD40, and the NF-kappa B subunit protein p65. These findings highlight the role of LMP1 as a mediator of EBV-host cell interactions and may indicate an important route by which it exerts its cellular growth transforming properties.

To be, or not to be: NF-kappaB is the answer--role of Rel/NF-kappaB in the regulation of apoptosis

During their lifetime, cells encounter many life or death situations that challenge their very own existence. Their survival depends on the interplay within a complex yet precisely orchestrated network of proteins. The Rel/NF-kappaB signaling pathway and the transcription factors that it activates have emerged as critical regulators of the apoptotic response. These proteins are best known for the key roles that they play in normal immune and inflammatory responses, but they are also implicated in the control of cell proliferation, differentiation, apoptosis and oncogenesis. In recent years, there has been remarkable progress in understanding the pathways that activate the Rel/NF-kappaB factors and their role in the cell's decision to either fight or surrender to apoptotic challenge. Whereas NF-kappaB is most commonly involved in suppressing apoptosis by transactivating the expression of antiapoptotic genes, it can promote programmed cell death in response to certain death-inducing signals and in certain cell types. This review surveys our current understanding of the role of NF-kappaB in the apoptotic response and focuses on many developments since this topic was last reviewed in Oncogene 4 years ago. These recent findings shed new light on the activity of NF-kappaB as a critical regulator of apoptosis in the immune, hepatic, epidermal and nervous systems, on the mechanisms through which it operates and on its role in tissue development, homoeostasis and cancer.

Dying for NF-κB? Control of cell death by transcriptional regulation of the apoptotic machinery

The transcription factor nuclear factor kappaB (NF-kappaB) is a pleiotropic protein complex that is activated from a sequestered, cytoplasmic form by pro-inflammatory extracellular signals and cellular stress. Several hundred cellular genes have been shown to be regulated by NF-kappaB, including cytokines, chemokines and adhesion molecules. Nearly eight years ago, a flurry of publications showed that loss or suppression of NF-kappaB results in an enhanced sensitivity to apoptosis. In the ensuing years, activation of NF-kappaB has become almost synonymous with enhanced cell survival, although more recent data suggests that this transcription factor plays a more complex role in the regulation of cell death.

The Bcl-2 family: roles in cell survival and oncogenesis

Apoptosis, the cell-suicide programme executed by caspases, is critical for maintaining tissue homeostasis, and impaired apoptosis is now recognized to be a key step in tumorigenesis. Whether a cell should live or die is largely determined by the Bcl-2 family of anti- and proapoptotic regulators. These proteins respond to cues from various forms of intracellular stress, such as DNA damage or cytokine deprivation, and interact with opposing family members to determine whether or not the caspase proteolytic cascade should be unleashed. This review summarizes current views of how these proteins sense stress, interact with their relatives, perturb organelles such as the mitochondrion and endoplasmic reticulum and govern pathways to caspase activation. It briefly explores how family members influence cell-cycle entry and outlines the evidence for their involvement in tumour development, both as oncoproteins and tumour suppressors. Finally, it discusses the promise of novel anticancer therapeutics that target these vital regulators.

Cyclin E and Bcl-xL cooperatively induce cell cycle progression in c-Rel−/− B cells

Aberrant overexpression of the c-rel protooncogene is associated with lymphoid malignancy, while c-rel deletion produces severe lymphoproliferative defects and immunodeficiency. To investigate the mechanism of c-rel-induced proliferation and cell cycle progression in B lymphocytes, we have compared signaling events elicited through the BCR in c-rel-/- and wild-type B cells. BCR stimulation of c-rel-/- B cells fails to induce proper cyclin expression, resulting in G1 phase arrest, but it is unclear whether these defects are in fact secondary events of decreased B-cell survival, since c-rel deletion also affects the expression of antiapoptotic genes such as bcl-xL. Here, we use the bcl-xL transgene to correct the viability of c-rel-deficient B cells, and show that the inhibition of apoptosis does not necessarily confer hyperproliferation of B cells activated through the BCR. c-rel-/- B cells still fail to enter the S phase despite improved survival by bcl-xL overexpression, suggesting that c-Rel-associated cell cycle progression is dependent on more than just enhanced cell viability. Overexpression of cyclin E protein, however, can cooperate with Bcl-xL to restore cell cycle progression to c-rel-/- B cells via induction of the cyclin-CDK/Rb-E2F pathway. Furthermore, we show that c-Rel can directly regulate transcription of the e2f3a promoter/enhancer, which is then likely to lead to transcriptional activation of the cyclin E promoter by E2F3a. Hence, these studies provide clear evidence that control of lymphocyte proliferation via c-Rel is linked to a cyclin-dependent process, and suggest that c-Rel not only activates antiapoptotic signaling but also the induction of cell cycle progression.

Phospholipase Cgamma2 provides survival signals via Bcl2 and A1 in different subpopulations of B cells

PLCgamma2 plays a critical role in B cell receptor (BCR) signaling and its targeted deletion results in defective B cell development and function. Here, we show that PLCgamma2 deficiency specifically blocks B cell maturation at the transitional type 2 (T2) to follicular (FO) B cell transition and the PLCgamma2 pathway regulates survival of B cells. BCR-induced apoptosis is dramatically enhanced in all subsets of splenic PLCgamma2-deficient B cells, especially in T2 and FO B cell subpopulations. We also find that all splenic PLCgamma2-deficient B cell subpopulations express abnormally low levels of Bcl-2 protein. In addition, PLCgamma2 deficiency disrupts BCR-mediated induction of A1 expression. Enforced expression of Bcl-2 prevents BCR-induced apoptosis in all splenic PLCgamma2-deficient B cell subpopulations and partially restores the numbers of PLCgamma2-deficient FO B cells. In contrast to Bcl-2, enforced expression of A1 preferentially prevents BCR-induced apoptosis in PLCgamma2-deficient FO B cells and partially restores the numbers of these B cells. Therefore, the PLCgamma2 pathway provides a survival signal via regulation of Bcl-2 in all splenic B cell subpopulations and via additional induction of A1 in mature FO B cells.

Transcriptional Regulation of Type I Diabetes by NF-κB

Development of type I diabetes requires coordinated expression of myriad genes responsible for the initiation and progression of the disease. Expression of these genes are regulated by a small number of transcription factors including the Rel/NF-kappaB family. To determine the roles of the Rel/NF-kappaB family in type I diabetes, we studied multiple low-dose streptozotocin-induced diabetes in mice deficient in either c-Rel or NF-kappaB1. We found that mice deficient in each of these NF-kappaB subunits were resistant to streptozotocin-induced diabetes. However, the mechanisms of the disease resistance may differ in different cases. Deficiency in c-Rel selectively reduced Th1, but not Th2 responses, whereas NF-kappaB1 deficiency had little effect on T cell responses to anti-CD3 stimulation. Death of dendritic cells was accelerated in the absence of NF-kappaB1, whereas death of macrophages and granulocytes was affected primarily by c-Rel deficiency. Furthermore, Stat-1 expression was significantly reduced in macrophages deficient in NF-kappaB1, but not c-Rel. These results indicate that both c-Rel and NF-kappaB1are essential for the development of type I diabetes and that strategies targeting each of these subunits would be effective in preventing the disease.

NF-kappaB activation in premalignant mouse tal-1/scl thymocytes and tumors

TAL-1/SCL activation is a common genetic event in pediatric T-cell acute lymphoblastic leukemia (T-ALL). Expression of tal-1/scl or a DNA binding mutant of tal-1/scl induces arrest of thymocyte development, resulting in decreases in double-positive and single-positive CD4 thymocytes. Moreover, nuclear p65/p50 heterodimers are detected in premalignant tal-1/scl and mut tal-1/scl thymocytes, suggesting that E2A depletion may induce developmental arrest and stimulate NF-kappaB activation. Increased NF-kappaB activity is also observed in tal-1/scl tumors and bcl-2 is overexpressed. To examine the contribution of NF-kappaB to tal-1/scl tumor growth in vivo, we expressed a mutant form of IkappaBalpha in tal-1/scl tumor cells. Although expression of mutant IkappaBalpha inhibited the tumor necrosis factor alpha (TNF-alpha)-induced NF-kappaB response, it had no effect on tumor growth in mice. These data suggest that NF-kappaB activation is an early event in tal-1/scl-induced leukemogenesis, associated with arrest of thymocyte development, and does not appear to contribute to tal-1/scl-induced tumor growth.

A1/Bfl-1 expression is restricted to TCR engagement in T lymphocytes

We analyzed regulation of the prosurvival Bcl-2 homologue A1, following T-cell receptor (TCR) or cytokine receptor engagement. Activation of CD4(+) or CD8(+) T cells by antigenic peptides induced an early but transient IL-2-independent expression of A1 and Bcl-xl mRNA and proteins, whereas expression of Bcl-2 was delayed and required IL-2. Cytokines such as IL-2, IL-4, IL-7 or IL-15 prevented apoptosis of activated T cells that effect being associated with the maintenance of Bcl-2, but not of A1 expression. However, restimulation of activated or posteffector T cells with antigenic peptide strongly upregulated A1 mRNA and maintained A1 protein expression. IL-4, IL-7 or IL-15 also prevented cell death of naive T cells. In those cells, cytokines upregulated Bcl-2, but not A1 expression. Therefore, in naive, activated and posteffector T cells, expression of A1 is dependent on TCR but not on cytokine receptor engagement, indicating that A1 is differently regulated from Bcl-xl and Bcl-2.

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

Contribution of BCAP to maintenance of mature B cells through c-Rel

Mice deficient in the B cell adaptor for phosphoinositide 3-kinase (BCAP) have reduced numbers of mature B lymphocytes, which show defects in cell survival and proliferation. We found here that the NF-kappa B (Rel) pathway was impaired in BCAP-deficient mature B cells and that NF-kappa B target genes, indispensable for cell survival and division, were not induced in response to B cell receptor (BCR) stimulation. Among the NF-kappa B (Rel) family, expression of c-Rel was specifically reduced in BCAP-deficient B cells. Retrovirus-mediated reintroduction of c-Rel restored the pool size of immunoglobulin (Ig)M(lo)IgD(hi) mature B cells in the spleen as well as proliferative responses to BCR stimulation. These results indicate BCAP is essential in the maintenance of mature B cells through functional coupling with c-Rel.

Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 is required for activation of NF-kappaB in response to diverse cellular stresses

Nuclear factor kappaB (NF-kappaB) serves to coordinate the transcription of genes in response to diverse environmental stresses. In this report we show that phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2) is fundamental to the process by which many stress signals activate NF-kappaB. Phosphorylation of this translation factor is carried out by a family of protein kinases that each respond to distinct stress conditions. During impaired protein folding and assembly in the endoplasmic reticulum (ER), phosphorylation of eIF2alpha by PEK (Perk or EIF2AK3) is essential for induction of NF-kappaB transcriptional activity. The mechanism by which NF-kappaB is activated during ER stress entails the release, but not the degradation, of the inhibitory protein IkappaB. During amino acid deprivation, phosphorylation of eIF2alpha by GCN2 (EIF2AK4) signals the activation of NF-kappaB. Furthermore, inhibition of general translation or transcription by cycloheximide and actinomycin D, respectively, elicits the eIF2alpha phosphorylation required for induction of NF-kappaB. Together, these studies suggest that eIF2alpha kinases monitor and are activated by a range of stress conditions that affect transcription and protein synthesis and assembly, and the resulting eIFalpha phosphorylation is central to activation of the NF-kappaB. The absence of NF-kappaB-mediated transcription and its antiapoptotic function provides an explanation for why eIF2alpha kinase deficiency in diseases such as Wolcott-Rallison syndrome leads to cellular apoptosis and disease.

Activation of mast cells by immunoglobulin E-receptor cross-linkage, but not through adenosine receptors, induces A1 expression and promotes survival

BACKGROUND

Mast cells are a potent source of mediators that regulate the inflammatory response in allergy and asthma. Mast cells can be activated through different receptors, for example, via cross-linkage of the high-affinity IgE receptor (Fc epsilon RI) and by adenosine acting on specific receptors. We have recently described mast cell survival of an IgE receptor activation by up-regulation of the anti-apoptotic gene A1.

OBJECTIVE

To compare mast cell survival and expression of A1 after activation through the Fc epsilon RI and by an adenosine agonist.

METHODS

Bone marrow-derived, cultured mouse mast cells (BMCMC) were activated either with IgE+antigen or with the adenosine receptor agonist 5'-N-ethylcarboxamido adenosine (NECA). Release of beta-hexosaminidase, cell viability, phosphorylation of Akt and IkB-alpha, and expression of pro-survival and pro-apoptotic genes were measured after activation.

RESULTS

Activation of BMCMC with NECA caused the release of beta-hexosaminidase, although to a lesser extent than after Fc epsilon RI activation (33% and 98%, respectively). Activation by both NECA and Fc epsilon RI stimulated phosphorylation of Akt (Ser473 and Thr308) and IkB-alpha (Ser32), both of which are implicated in the regulation of cell survival. However, only cells that were activated through Fc epsilon RI, but not by NECA, expressed A1 and exhibited an increased survival rate compared to the control.

CONCLUSION

These results show that adenosine receptor activation of BMCMC does not induce the same survival programme in mast cells as does activation through Fc epsilon RI. These findings may be important for understanding the role that mast cells play in asthma provoked by different stimuli.

Inhibition of B-cell death does not restore T-cell-dependent immune responses in CD40-deficient mice

Signalling through CD40 is essential for the development of immunoglobulin G (IgG) antibody responses, germinal centres and B-cell memory against T-dependent antigens. In addition, engagement of CD40 in B cells promotes cell survival by inducing the expression of anti-apoptotic members of the bcl-2 family of cell-death regulators. In the present study we analysed whether T-dependent immune responses can be developed in mice deficient in CD40 if the anti-apoptotic activity mediated by the engagement of CD40 in B cells is compensated by the constitutive over-expression of anti-apoptotic genes of the bcl-2 family. We showed that the over-expression of either hbcl-2 or hbcl-xL transgenes in B cells is not sufficient to restore IgG antibody responses and germinal centre formation in CD40-deficient mice. These results indicate that CD40 functions, other than those mediated through survival, are required for the establishment of T-dependent B-cell responses.

Nur77 as a survival factor in tumor necrosis factor signaling

The immediate-early gene Nur77, which encodes an orphan nuclear receptor, is rapidly induced by various stress stimuli, including tumor necrosis factor (TNF). Nur77 has been implicated in mediating apoptosis, particularly in T cells and tumor cells. We report here that Nur77 can play a role in antagonizing apoptosis in TNF signaling. Nur77 expression is strongly induced by TNF. Interestingly, unlike most antiapoptotic molecules, this induced expression of Nur77 is largely independent of NF-kappa B. Ectopic expression of Nur77 can protect wild-type, TRAF2-/-, and RelA-/- cells from apoptosis induced by TNF, whereas expression of a dominant-negative form of Nur77 (DN-Nur77) accelerates TNF-mediated cell death in the mutant cells. In mouse embryonic fibroblasts, Nur77 remains in the nucleus in response to TNF and is not translocated to the mitochondria, where it was reported to mediate apoptosis. Our results suggest that Nur77 is a survival effector protein in the context of TNF-mediated signaling.

Overcoming resistance of cancer cells to apoptosis

Discovery of the B cell lymphoma gene 2 (Bcl-2 gene) led to the concept that development of cancers required the simultaneous acquisition, not only of deregulated cell division, but also of resistance to programmed cell death or apoptosis. Apoptosis is arguably the common pathway to cell death resulting from a range of therapeutic initiatives, so that understanding the basis for the resistance of cancer cells to apoptosis may hold the key to development of new treatment initiatives. Much has already been learnt about the apoptotic pathways in cancer cells and proteins regulating these pathways. In most cells, apoptosis is dependent on the mitochondrial dependent pathway. This pathway is regulated by pro- and anti-apoptotic members of the Bcl-2 family, and manipulation of these proteins offers scope for a number of treatment initiatives. Effector caspases activated by the mitochondrial pathway or from death receptor signaling are under the control of the inhibitor of apoptosis protein (IAP) family. Certain proteins from mitochondrial can, however, competitively inhibit their binding to effector caspases. Information about the structure of these proteins has led to initiatives to develop therapeutic agents to block the IAP family. In addition to development of selective agents based on these two (Bcl-2 and IAP) protein families, much has been learnt about signal pathways that may regulate their activity. These in turn might provide additional approaches based on selective regulators of the signal pathways.

Inhibition of constitutive NF-kappa B activation in mantle cell lymphoma B cells leads to induction of cell cycle arrest and apoptosis

Constitutive activation of the NF-kappaB has been documented to be involved in the pathogenesis of many human malignancies, including hemopoietic neoplasms. In this study, we examined the status of NF-kappaB in two non-Hodgkin's lymphoma cell lines derived from mantle cell lymphoma (MCL) samples and in patient MCL biopsy specimens by EMSA and confocal microscopic analysis. We observed that NF-kappaB is constitutively activated in both the MCL cell lines and in the MCL patient biopsy cells. Since NF-kappaB has been shown to play an important role in a variety of cellular processes, including cell cycle regulation and apoptosis, targeting the NF-kappaB pathways for therapy may represent a rational approach in this malignancy. In the MCL cell lines, inhibition of constitutive NF-kappaB by the proteasome inhibitor PS-341 or a specific pIkappaBalpha inhibitor, BAY 11-7082, led to cell cycle arrest in G(1) and rapid induction of apoptosis. Apoptosis was associated with the down-regulation of bcl-2 family members bcl-x(L) and bfl/A1, and the activation of caspase 3, that mediates bcl-2 cleavage, resulting in the release of cytochrome c from the mitochondria. PS-341or BAY 11-induced G(1) cell cycle arrest was associated with the inhibition of cyclin D1 expression, a molecular genetic marker of MCL. These studies suggest that constitutive NF-kappaB expression plays a key role in the growth and survival of MCL cells, and that PS-341 and BAY 11 may be useful therapeutic agents for MCL, a lymphoma that is refractory to most current chemotherapy regimens.

NF- kappa B2/p100 induces Bcl-2 expression

The NF-kappaB2/p100 and bcl-3 genes are involved in chromosomal translocations described in chronic lymphocytic leukemias (CLL) and non-Hodgkin's lymphomas, and nuclear factor kappaB (NF-kappaB) protects cancer cells against apoptosis. Therefore, we investigated whether this transcription factor could modulate the expression of the Bcl-2 antiapoptotic protein. Bcl-2 promoter analysis showed multiple putative NF-kappaB binding sites. Transfection assays of bcl-2 promoter constructs in HCT116 cells showed that NF-kappaB can indeed transactivate bcl-2. We identified a kappaB site located at position -180 that can only be bound and transactivated by p50 or p52 homodimers. As p50 and p52 homodimers are devoid of any transactivating domains, we showed that they can transactivate the bcl-2 promoter through association with Bcl-3. We also observed that stable overexpression of p100 and its processed product p52 can induce endogenous Bcl-2 expression in MCF7AZ breast cancer cells. Finally, we demonstrated that, in breast cancer and leukemic cells (CLL), high NF-kappaB2/p100 expression was associated with high Bcl-2 expression. Our data suggest that Bcl-2 could be an in vivo target gene for NF-kappaB2/p100.

The role of Rel/NF-κB transcription factors in B lymphocyte survival

Among the transcriptional mediators of the various signal transduction pathways that have been shown to regulate lymphocyte apoptosis, Rel/NF-kappaB transcription factors have emerged as key regulators of B cell survival during their differentiation and in their activation by antigens or mitogens. The aim of this review is to bring together recent findings on Rel/NF-kappaB regulation of B cell survival and to present an integrated model of how these transcription factors control apoptosis in a signal and differentiation-stage specific manner. In addition to providing a contemporary view of Rel/NF-kappaB regulated B cell survival, the model described here is aimed to serve as a paradigm for how Rel/NF-kappaB family members control survival in different cell types during differentiation and in response to the plethora of signals that impinge on this master transcriptional regulatory pathway.

IKK beta is required for peripheral B cell survival and proliferation

NF-kappaB activity in mammalian cells is regulated through the IkappaB kinase (IKK) complex, consisting of two catalytic subunits (IKKalpha and IKKbeta) and a regulatory subunit (IKKgamma). Targeted deletion of Ikkbeta results in early embryonic lethality, thus complicating the examination of IKKbeta function in adult tissues. Here we describe the role of IKKbeta in B lymphocytes made possible by generation of a mouse strain that expresses a conditional Ikkbeta allele. We find that the loss of IKKbeta results in a dramatic reduction in all peripheral B cell subsets due to associated defects in cell survival. IKKbeta-deficient B cells are also impaired in mitogenic responses to LPS, anti-CD40, and anti-IgM, indicating a general defect in the ability to activate the canonical NF-kappaB signaling pathway. These findings are consistent with a failure to mount effective Ab responses to T cell-dependent and independent Ags. Thus, IKKbeta provides a requisite role in B cell activation and maintenance and thus is a key determinant of humoral immunity.

Cell adhesion-mediated drug resistance (CAM-DR) is associated with activation of NF-kappa B (RelB/p50) in myeloma cells

The microenvironment has been shown to influence tumor cell phenotype with respect to growth, metastasis, and response to chemotherapy. We have utilized oligonucleotide microarray analysis to identify signal transduction pathways and gene products altered by the interaction of myeloma tumor cells with the extracellular matrix component fibronectin that may contribute to the antiapoptotic phenotype conferred by the microenvironment. Genes with altered expression associated with fibronectin cell adhesion, either induced or repressed, were numerically ranked by fold change. FN adhesion repressed the expression of 469 gene products, while 53 genes with known coding sequences were induced by twofold or more. Of these 53 genes with two fold, or greater increase in expression, 11 have been reported to be regulated by the nuclear factor-kappa B (NF-kappa B) family of transcription factors. EMSA analysis demonstrated NF-kappa B binding activity significantly increased in cells adhered to fibronectin compared to cells in suspension. This DNA binding activity consisted primarily of RelB-p50 heterodimers, which was distinct from the NF-kappa B activation of TNF alpha. These data demonstrate the selectivity of signal transduction from the microenvironment that may contribute to tumor cell resistance to programmed cell death.

Combined deficiency of p50 and cRel in CD4+ T cells reveals an essential requirement for nuclear factor kappaB in regulating mature T cell survival and in vivo function

Signaling pathways involved in regulating T cell proliferation and survival are not well understood. Here we have investigated a possible role of the nuclear factor (NF)-kappaB pathway in regulating mature T cell function by using CD4+ T cells from p50-/- cRel-/- mice, which exhibit virtually no inducible kappaB site binding activity. Studies with these mice indicate an essential role of T cell receptor (TCR)-induced NF-kappaB in regulating interleukin (IL)-2 expression, cell cycle entry, and survival of T cells. Our results further indicate that NF-kappaB regulates TCR-induced expression of antiapoptotic Bcl-2 family members. Strikingly, retroviral transduction of CD4+ T cells with the NF-kappaB-inducing IkappaB kinase beta showed that NF-kappaB activation is not only necessary but also sufficient for T cell survival. In contrast, our results indicate a lack of involvement of NF-kappaB in both IL-2 and Akt-induced survival pathways. In vivo, p50-/- cRel-/- mice showed impaired superantigen-induced T cell responses as well as decreased numbers of effector/memory and regulatory CD4+ T cells. These findings provide the first demonstration of a role for NF-kappaB proteins in regulating T cell function in vivo and establish a critically important function of NF-kappaB in TCR-induced regulation of survival.

NF-kappa B-dependent assembly of an enhanceosome-like complex on the promoter region of apoptosis inhibitor Bfl-1/A1

Expression of the prosurvival Bcl-2 homologue Bfl-1/A1 is induced by NF-kappa B-activating stimuli, while B and T cells from c-rel knockout mice show an absolute defect in bfl-1/a1 gene activation. Here, we demonstrate NF-kappa B-dependent assembly of an enhanceosome-like complex on the promoter region of bfl-1. Binding of NF-kappa B subunit c-Rel to DNA nucleated the concerted binding of transcription factors AP-1 and C/EBP beta to the 5'-regulatory region of bfl-1. Optimal stability of the complex was dependent on proper orientation and phasing of the NF-kappa B site. Chromatin immunoprecipitation analyses demonstrated that T-cell activation triggers in vivo binding of endogenous c-Rel, c-Jun, C/EBP beta, and HMG-IC to the bfl-1 regulatory region, coincident with selective recruitment of coactivators TAFII250 and p300, SWI/SNF chromatin remodeling factor component BRG-1, and basal transcription factors TATA-binding protein (TBP) and TFIIB, as well as hyperacetylation of histones H3 and H4. These results highlight a critical role for NF-kappa B in bfl-1 transcription and point to the need for a complex and precise regulatory network to control bfl-1 expression. To our knowledge, this is the first demonstration of enhanceosome-mediated regulation of a cell death inhibitor.

A1 is a growth-permissive antiapoptotic factor mediating postactivation survival in T cells

The regulation of cell death in activated naive T cells is not well understood. We examined the expression of A1, an antiapoptotic member of the Bcl-2 family, following activation of naive mouse splenocytes. A1 gene expression was strongly but transiently induced during the first day of activation, with a peak at 2 to 6 hours, whereas Bcl-2 mRNA was simultaneously transiently down-regulated. Transgenic (Tg) overexpression of A1-a in T cells via the lck distal promoter resulted in decreased apoptosis following activation either with concanavalin A or with antibodies to CD3 and CD28 and led to a doubling of T-cell yield by 5 days. Tg A1-a also partially protected thymocytes from several proapoptotic stimuli but did not protect T-cell blasts from cell death induced by reactivation via the T-cell receptor. Tg Bcl-2 and Tg A1-a showed a similar ability to reduce apoptosis in both resting and activated T cells. However, in activated splenocyte cultures, the increase in 5-day T-cell yield observed with Tg Bcl-2 was only half that produced by Tg A1-a. This difference could be attributed at least in part to the fact that A1, unlike Bcl-2, did not inhibit S-phase entry of activated cells. The A1 protein may represent an adaptation of the Bcl-2 gene family to the need for survival regulation in the context of a proliferative stimulus.

Mutations in the v-Rel transactivation domain indicate altered phosphorylation and identify a subset of NF-kappaB-regulated cell death inhibitors important for v-Rel transforming activity

Consistent with the constitutive activation of Rel/NF-kappaB in human hematopoietic tumors, the v-Rel oncoprotein induces aggressive leukemia/lymphomas in animal models. v-Rel is thus a valuable tool to characterize the role of Rel/NF-kappaB in cancer and the mechanisms involved. Prior studies by our group identified a serine-rich domain in v-Rel that was required for biological activity. Here, we investigated the molecular basis for the transformation defect of specific serine mutants. We show that the transforming efficiency of these mutants in primary lymphoid cells is correlated with their ability to mediate kappaB site-dependent transactivation and with specific changes in phosphorylation profiles. Interestingly, coexpression of the death antagonists Bcl-xL and Bcl-2 significantly increased their oncogenicity, whereas other NF-kappaB-regulated death inhibitors showed little or no effect. The fact that a subset of apoptosis inhibitors could rescue v-Rel transactivation mutants suggests that their reduced transcriptional activity may critically affect expression of defined death antagonists essential for oncogenesis. Consistent with this hypothesis, we observed selection for high endogenous expression of Bcl-2-related death antagonists in cells transformed by weakly transforming v-Rel mutants. These results emphasize the need for Rel/NF-kappaB to efficiently activate expression of a subset of antiapoptotic genes from the Bcl-2 family to manifest its oncogenic phenotype.

The survival of antigen-stimulated T cells requires NFkappaB-mediated inhibition of p73 expression

We have explored the interactions between the NFkappaB and Cdk-Rb-E2F pathways in controlling T cell fate following antigen stimulation. The inhibition of NFkappaB in antigen-stimulated T cells results in apoptosis but does not inhibit E2F activation and S phase entry. IkappaB-induced apoptosis coincides with the superinduction of p73 expression and activity. G1 Cdk activity is required for IkappaB-induced apoptosis and the induction of p73. Importantly, p73 deficiency rescues activated T cells from the apoptosis resulting from the inhibition of NFkappaB. Thus, Cdk2 activation sends signals for both cell cycle progression and apoptosis, the latter of which must be blocked by NFkappaB to allow for proliferation.

The role of nuclear factor-kappaB essential modulator (NEMO) in B cell development and survival

The transcription factor nuclear factor-kappaB (NF-kappaB) is essential for immune and inflammatory responses. NF-kappaB essential modulator (NEMO) is a scaffolding component of the IkappaB kinase complex required for NF-kappaB activation in vitro. Because NF-kappaB activation is involved in B cell development and function, we set out to determine whether NEMO is required for these processes. NEMO(-/-) mice die very early during embryogenesis, and fetal livers from NEMO(-/-) embryos can not reconstitute either B or T lymphopoiesis in irradiated host mice. We therefore used NEMO(-/-) embryonic stem cells and the OP9 in vitro differentiation system to demonstrate that NEMO is not required for B cell development but plays an important role in B cell survival.

NF-kappaB activation in cancer: a challenge for ubiquitination- and proteasome-based therapeutic approach

Nuclear factor-kappa B (NF-kappaB) activation relies primarily on ubiquitin-mediated degradation of its inhibitor IkappaB. NF-kappaB plays an important role in many aspects of tumor development, progression, and therapy. Some types of cancer are characterized by constitutive NF-kappaB activity, whereas in others such activity is induced following chemotherapy. NF-kappaB-harboring tumors are generally resistant to chemotherapy and their eradication requires NF-kappaB inhibition. Here we describe the mechanisms of NF-kappaB activation in normal and tumor cells, review prevalent notions regarding the factor's contribution to tumorigenicity and discuss present and future options for NF-kappaB inhibition in cancer. The ubiquitination-mediated activation of NF-kappaB is intersected by another cancer-associated protein, beta-catenin. We, therefore, compare the related activation pathways and discuss the possibility of differential targeting of the involved ubiquitination machinery.

Inhibition of IkappaB kinase by a new class of retinoid-related anticancer agents that induce apoptosis

The transcription factor NF-kappaB is overexpressed or constitutively activated in many cancer cells, where it induces expression of antiapoptotic genes correlating with resistance to anticancer therapies. Small molecules that inhibit the NF-kappaB signaling pathway could therefore be used to induce apoptosis in NF-kappaB-overexpressing tumors and potentially serve as anticancer agents. We found that retinoid antagonist MX781 inhibited the activation of NF-kappaB-dependent transcriptional activity in different tumor cell lines. MX781 was able to completely inhibit tumor necrosis factor alpha-mediated activation of IkappaB kinase (IKK), the upstream regulator of NF-kappaB. Inhibition of IKK activity resulted from direct binding of MX781 to the kinase, as demonstrated by in vitro inhibition studies. Two other molecules, MX3350-1 and CD2325, which are retinoic acid receptor gamma-selective agonists, were capable of inhibiting IKK in vitro, although they exerted variable inhibition of IKK and NF-kappaB activities in intact cells in a cell type-specific manner. However, N-(4-hydroxyphenyl)-retinamide, another apoptosis-inducing retinoid, and retinoic acid as well as other nonapoptotic retinoids did not inhibit IKK. Inhibition of IKK by the retinoid-related compounds and other small molecules correlated with reduced cell proliferation and increased apoptosis. Reduced cell viability was also observed after overexpression of an IKKbeta kinase-dead mutant or the IkappaBalpha superrepressor. The induction of apoptosis by the retinoid-related molecules that inhibited IKK was dependent on caspase activity but independent of the retinoid receptors. Thus, the presence of an excess of retinoic acid or a retinoid antagonist did not prevent the inhibition of IKK activation by MX781 and CD2325, indicating a retinoid receptor-independent mechanism of action.

Apoptosis or growth arrest: modulation of the cellular response to p53 by proliferative signals

Activation of the tumor suppressor p53 after genotoxic insults may result in two different responses: growth arrest or apoptosis. In this study, we analysed how mitogenic stimulation of primary mouse lymphocytes influences p53 signaling upon gamma-irradiation. We found that G(0) lymphocytes rapidly went into p53-dependent apoptosis, whereas stimulated lymphocytes went into a p53-dependent, p21-mediated growth arrest. The switch in p53 response upon stimulation did neither result from a switch in transcriptional activation of major p53 target genes, nor from the high level of p21 expressed in stimulated, irradiated cells. Growth stimulation, however, led to the upregulation of the antiapoptotic factors Bcl-x(L) and Bfl-1. In resting cells, p53 induced apoptosis after gamma-irradiation was accompanied by a breakdown of the mitochondrial membrane potential (psi(m)) that was counteracted by growth stimulation. We propose that growth stimulation intercepted p53 proapoptotic signaling at the level of mitochondrial integrity, most likely by upregulating the antiapoptotic factors Bcl-x(L) and Bfl-1. Upregulation of Bcl-x(L) and of Bfl-1 upon growth stimulation was mediated by the PKC-dependent activation of NF-kappaB. Consequently, blocking PKC activity restored apoptosis in stimulated, irradiated splenocytes. The inherent coupling of growth stimulation with antiapoptotic signaling in primary lymphocytes might provide hints as to how precancerous lymphocytes bypass the need for mutational inactivation of p53. Thus, our findings might explain the relatively low frequency of p53 mutations in lymphomas in comparison to other tumor entities.

Enhanced B cell expansion, survival, and humoral responses by targeting death receptor 6

Targeted disruption of death receptor (DR)6 results in enhanced CD4(+) T cell expansion and T helper cell type 2 differentiation after stimulation. Similar to T cells, DR6 is expressed on resting B cells but is down-regulated upon activation. We examined DR6(-/-) B cell responses both in vitro and in vivo. In vitro, DR6(-/-) B cells undergo increased proliferation in response to anti-immunoglobulin M, anti-CD40, and lipopolysaccharide. This hyperproliferative response was due, at least in part, to both increased cell division and reduced cell apoptosis when compared with wild-type B cells. Consistent with these observations, increased nuclear levels and activity of nuclear factor kappaB transcription factor, c-Rel, and elevated Bcl-x(l) expression were observed in DR6(-/-) B cells upon stimulation. In addition, DR6(-/-) B cells exhibited higher surface levels of CD86 upon activation and were more effective as antigen-presenting cells in an allogeneic T cell proliferation response. DR6(-/-) mice exhibited enhanced germinal center formation and increased titers of immunoglobulins to T-dependent as well as T-independent type I and II antigens. This is the first demonstration of a regulatory role of DR6 in the activation and function of B cells.