Cytokine signaling and Epstein-Barr virus-mediated cell transformation

Epstein-Barr virus (EBV) latent infection is tightly associated with the development of lymphoid and epithelial human malignancies. The disruption of cell-growth checkpoints is mediated by a limited number of viral proteins that interfere with signal transduction mechanisms and transcription control in the infected cell. Genetic and biochemical evidence supports the notion that EBV-mediated transformation relies extensively on interference with cytokine signaling networks. This is achieved through direct modulation of cytokine receptor signaling mechanisms as well as alterations in the expression levels of various cytokines. The principal effector of these interventions is the EBV latent membrane protein 1 (LMP1) which plays a central role in the transformation process. This viral protein mimics activated receptors of the tumor necrosis factor receptor superfamily to promote cell growth and antiapoptotic mechanisms. LMP1 and other EBV latent proteins upregulate cytokines and growth factors which participate in autocrine and paracrine loops that are likely to promote cell transformation and modulate immune responses. This report will review the molecular mechanisms that underlie the disruption of cytokine signaling mechanisms in EBV-mediated transformation with a particular emphasis on the LMP1 mechanism of function.

Effects of the NIK aly mutation on NF-kappaB activation by the Epstein-Barr virus latent infection membrane protein, lymphotoxin beta receptor, and CD40

Homozygosity for the aly point mutation in NF-kappaB-inducing kinase (NIK) results in alymphoplasia in mice, a phenotype similar to that of homozygosity for deletion of the lymphotoxin beta receptor (LTbetaR). We now find that NF-kappaB activation by Epstein-Barr virus latent membrane protein 1 (LMP1) or by an LMP1 transmembrane domain chimera with the LTbetaR signaling domain in human embryonic kidney 293 cells is selectively inhibited by a wild type dominant negative NIK comprised of amino acids 624-947 (DN-NIK) and not by aly DN-NIK. In contrast, LMP1/CD40 is inhibited by both wild type (wt) and aly DN-NIK. LMP1, an LMP1 transmembrane domain chimera with the LTbetaR signaling domain, and LMP1/CD40 activate NF-kappaB in wt or aly murine embryo fibroblasts. Although wt and aly NIK do not differ in their in vitro binding to tumor necrosis factor receptor-associated factor 1, 2, 3, or 6 or in their in vivo association with tumor necrosis factor receptor-associated factor 2 and differ marginally in their very poor binding to IkappaB kinase beta (IKKbeta), only wt NIK is able to bind to IKKalpha. These data are compatible with a model in which activation of NF-kappaB by LMP1 and LTbetaR is mediated by an interaction of NIK or a NIK-like kinase with IKKalpha that is abrogated by the aly mutation. On the other hand, CD40 mediates NF-kappaB activation through a kinase that interacts with a different component of the IKK complex.

Activation of the I kappa B alpha kinase (IKK) complex by double-stranded RNA-binding defective and catalytic inactive mutants of the interferon-inducible protein kinase PKR

The interferon (IFN)-inducible double stranded (ds) RNA-activated protein kinase PKR plays an important role in protein synthesis by modulating the phosphorylation of the alpha-subunit of eukaryotic initiation fact 2 (eIF-2 alpha). In addition to translational control, PKR has been implicated in several signaling pathways leading to gene transcription. For example, PKR induces I kappa B alpha kinase (IKK) activity and I kappa B alpha phosphorylation leading to the induction of NF-kappa B-mediated gene transcription. Recent findings suggested that NF-kappa B activation by PKR does not require the catalytic activity of the kinase. Here, we provide novel evidence that induction of IKK and NF-kappa B activities proceeds independently of the dsRNA-binding properties of PKR and also verify the kinase-free role of PKR in this process. We also show that the effects of PKR mutants on IKK and NF-kappa B activation are independent of cell transformation but are dependent on the amount of the mutant PKR proteins expressed in cells. These data strongly support an indirect role of PKR in I kappa B alpha phosphorylation by modulating IKK activity through pathways that do not utilize the enzymatic and dsRNA-binding properties of PKR.

The X-linked lymphoproliferative syndrome gene product SH2D1A associates with p62dok (Dok1) and activates NF-kappa B

The X-linked lymphoproliferative syndrome (XLP) is a genetic disorder in which affected males have a morbid or fatal response to Epstein-Barr virus infection. The XLP deficiency has been mapped to a gene encoding a 128-residue protein, SH2D1A, which is comprised principally of a Src homology 2 (SH2) domain. We now report that SH2D1A associates with Dok1, a protein that interacts with Ras-GAP, Csk, and Nck. An SH2D1A SH2 domain mutant that has been identified in XLP does not associate with Dok1, in accord with the hypothesis that this interaction is linked to XLP. The association of SH2D1A with Dok1 also depends on phosphorylation of Dok1 Y(449) in the sequence ALYSQVQK. Further, overexpression of SH2D1A is found to activate NF-kappaB in 293T cells. NF-kappaB activation by SH2D1A does not depend on the wild-type SH2 domain and is inhibited by a dominant-negative IkappaB kinase beta. Thus, SH2D1A can affect multiple intracellular signaling pathways that are potentially important in the normal effective host response to Epstein-Barr virus infection.

Epstein-barr virus transformation: involvement of latent membrane protein 1-mediated activation of NF-kappaB

Epstein-Barr virus (EBV) transforms resting primary human B lymphocytes into indefinitely proliferating lymphoblastoid cell lines in vitro and is associated with several human malignancies in vivo. Recombinant EBV genetic analyses combined with in vitro B lymphocyte transformation assays demonstrate that latent infection membrane protein 1 (LMP1) is essential for EBV-mediated lymphocyte transformation. LMP1 has no intrinsic enzymatic activity but instead aggregates cellular proteins of the tumor necrosis factor receptor signaling pathway to activate transcription factor NF-kappaB. Mutants rendering LMP1 defective in these protein interactions are impaired in their abilities to activate NF-kappaB in reporter gene assays. Concordantly, EBV recombinants with LMP1 mutations that are compromised for NF-kappaB activation are impaired for growth transformation. Thus, EBV-mediated growth transformation is genetically and biochemically linked to LMP1-mediated activation of NF-kappaB.

Role of the TRAF binding site and NF-kappaB activation in Epstein-Barr virus latent membrane protein 1-induced cell gene expression

In this study, we investigated the induction of cellular gene expression by the Epstein-Barr Virus (EBV) latent membrane protein 1 (LMP1). Previously, LMP1 was shown to induce the expression of ICAM-1, LFA-3, CD40, and EBI3 in EBV-negative Burkitt lymphoma (BL) cells and of the epidermal growth factor receptor (EGF-R) in epithelial cells. We now show that LMP1 expression also increased Fas and tumor necrosis factor receptor-associated factor 1 (TRAF1) in BL cells. LMP1 mediates NF-kappaB activation via two independent domains located in its C-terminal cytoplasmic tail, a TRAF-interacting site that associates with TRAF1, -2, -3, and -5 through a PXQXT/S core motif and a TRADD-interacting site. In EBV-transformed B cells or transiently transfected BL cells, significant amounts of TRAF1, -2, -3, and -5 are associated with LMP1. In epithelial cells, very little TRAF1 is expressed, and only TRAF2, -3, and -5, are significantly complexed with LMP1. The importance of TRAF binding to the PXQXT/S motif in LMP1-mediated gene induction was studied by using an LMP1 mutant that contains alanine point mutations in this motif and fails to associate with TRAFs. This mutant, LMP1(P204A/Q206A), induced 60% of wild-type LMP1 NF-kappaB activation and had approximately 60% of wild-type LMP1 effect on Fas, ICAM-1, CD40, and LFA-3 induction. In contrast, LMP1(P204A/Q206A) was substantially more impaired in TRAF1, EBI3, and EGF-R induction. Thus, TRAF binding to the PXQXT/S motif has a nonessential role in up-regulating Fas, ICAM-1, CD40, and LFA-3 expression and a critical role in up-regulating TRAF1, EBI3, and EGF-R expression. Further, D1 LMP1, an LMP1 mutant that does not aggregate failed to induce TRAF1, EBI3, Fas, ICAM-1, CD40, and LFA-3 expression confirming the essential role for aggregation in LMP1 signaling. Overexpression of a dominant form of IkappaBalpha blocked LMP1-mediated TRAF1, EBI3, Fas, ICAM-1, CD40, and LFA-3 up-regulation, indicating that NF-kappaB is an important component of LMP1-mediated gene induction from both the TRAF- and TRADD-interacting sites.

Epstein-Barr virus-transforming protein latent infection membrane protein 1 activates transcription factor NF-kappaB through a pathway that includes the NF-kappaB-inducing kinase and the IkappaB kinases IKKalpha and IKKbeta

The Epstein-Barr virus oncoprotein latent infection membrane protein 1 (LMP1) is a constitutively aggregated pseudo-tumor necrosis factor receptor (TNFR) that activates transcription factor NF-kappaB through two sites in its C-terminal cytoplasmic domain. One site is similar to activated TNFRII in associating with TNFR-associated factors TRAF1 and TRAF2, and the second site is similar to TNFRI in associating with the TNFRI death domain interacting protein TRADD. TNFRI has been recently shown to activate NF-kappaB through association with TRADD, RIP, and TRAF2; activation of the NF-kappaB-inducing kinase (NIK); activation of the IkappaB alpha kinases (IKKalpha and IKKbeta); and phosphorylation of IkappaB alpha. IkappaB alpha phosphorylation on Ser-32 and Ser-36 is followed by its degradation and NF-kappaB activation. In this report, we show that NF-kappaB activation by LMP1 or by each of its effector sites is mediated by a pathway that includes NIK, IKKalpha, and IKKbeta. Dominant negative mutants of NIK, IKKalpha, or IKKbeta substantially inhibited NF-kappaB activation by LMP1 or by each of its effector sites.

A fusion of the EBV latent membrane protein-1 (LMP1) transmembrane domains to the CD40 cytoplasmic domain is similar to LMP1 in constitutive activation of epidermal growth factor receptor expression, nuclear factor-kappa B, and stress-activated protein kinase

The EBV latent infection transforming protein, LMP1, has six hydrophobic transmembrane domains that enable it to aggregate in the plasma membrane and a 200-amino acid carboxyl-terminal cytoplasmic domain (CT) that activates nuclear factor-kappaB and induces many of the phenotypic changes in B lymphocytes that accompany CD40 activation. Since the phenotypic effects of LMP1 are similar to those of activated CD40, we now compare signaling from the LMP1 CT with that from the CD40 CT fused to the LMP1 transmembrane domains. The LMPCD40 chimera was similar to LMP1 in nuclear factor-kappaB activation and in up-regulation of epidermal growth factor receptor expression. CD40 ligation was known to activate the stress-activated protein kinase, and both LMPCD40 and LMP1 are now shown to induce stress-activated protein kinase activity in the absence of ligand. Deletion of the first four transmembrane domains of LMP1 abrogated LMP1 aggregation in the plasma membrane and nearly abolished signaling from LMP1 or the LMPCD40 chimera. These results highlight the role of LMP1 as a constitutively active receptor similar to CD40 and provide a novel approach for the generation of ligand-independent receptors.

The Epstein-Barr virus-induced Ca2+/calmodulin-dependent kinase type IV/Gr promotes a Ca(2+)-dependent switch from latency to viral replication

The switch from latency to viral replication in Epstein-Barr virus (EBV)-transformed human B cells is mediated by Zta, the protein product of immediate-early EBV gene BZLF1. BZLF1 transcription is normally suppressed in EBV-transformed B cells but can be induced in some cell lines upon ligation of surface immunoglobulin by mechanisms that include the activation of Ca(2+)-dependent signaling pathways. The multifunctional Ca2+/calmodulin-dependent kinase type IV/Gr (CaMKIV/Gr) is normally absent in primary human B cells, but its expression is induced by the EBV oncoprotein LMP1 in the course of B-cell growth transformation by EBV. In this study, we demonstrate that activated CaMKIV/Gr induces transcription from the BZLF1 promoter and upregulates the expression of Zta in permissive cells. Transcriptional activation of the BZLF1 promoter by CaMKIV/Gr is dependent on the CREB/AP1 binding element ZII and is greatly augmented by the Ca2+/calmodulin-dependent phosphatase calcineurin. These results outline a virus-regulated mechanism involving CaMKIV/Gr which promotes transition from latency to productive viral replication in response to Ca(2+)-mobilizing extracellular signals.

The role of Rel/NF-kappa B proteins in viral oncogenesis and the regulation of viral transcription

Rel/NF-kappa B is a ubiquitous transcription factor that consists of multiple polypeptide subunits, and is subject to complex regulatory mechanisms that involve protein-protein interactions, phosphorylation, ubiquitination, proteolytic degradation, and nucleocytoplasmic translocation. The sophisticated control of Rel/NF-kappa B activity is not surprising since this transcription factor is involved in a wide array of cellular responses to extracellular cues, associated with growth, development, apoptosis, and pathogen invasion. Thus, it is not unexpected that this versatile cellular homeostatic switch would be affected by a variety of viral pathogens, which have evolved mechanisms to utilize various aspects of Rel/NF-kappa B activity to facilitate their replication, cell survival and possibly evasion of immune responses. This review will cover the molecular mechanisms that are utilized by mammalian oncogenic viruses to affect the activity of Rel/NF-kappa B transcription factors and the role of Rel/NF-kappa B in the regulation of viral gene expression and replication.

Lymphotoxin-beta receptor signaling complex: role of tumor necrosis factor receptor-associated factor 3 recruitment in cell death and activation of nuclear factor kappaB

The binding of heterotrimeric lymphotoxin, LT alpha1 beta2, to the LTbeta receptor (LTbeta R), a member of the tumor necrosis factor receptor (TNFR) superfamily, induces nuclear factor kappaB (NF-kappaB) activation and cell death in HT29 adenocarcinoma cells. We now show that treatment with LT alpha1 beta2 or agonistic LTbeta R antibodies causes rapid recruitment of TNFR-associated factor 3 (TRAF3) to the LTbeta R cytoplasmic domain. Further, stable overexpression of a TRAF3 mutant that lacks the RING and zinc finger domains inhibits LTbeta R-mediated cell death. The inhibition is specific for LTbeta R cell death signaling, since NF-kappaB activation by LT alpha1 beta2 and Fas-mediated apoptosis are not inhibited in the same cells. The mutant and endogenous TRAF3s are both recruited at equimolar amounts to the LTbeta R, suggesting that the mutant disrupts the function of the signaling complex. These results implicate TRAF3 as a critical component of the LTbeta R death signaling complex and indicate that at least two independent signaling pathways are initiated by LTbeta R ligation.

Fascin, a sensitive new marker for Reed-Sternberg cells of hodgkin's disease. Evidence for a dendritic or B cell derivation?

Immunohistochemical localization of human fascin, a distinct 55-kd actin-bundling protein, was determined for a wide variety of lymphoid tissues (364 specimens total). In non-neoplastic tissues, reactivity was highly selective and localized predominantly in dendritic cells. In the thymus, this protein was distinctly localized to medullary dendritic cells. In reactive nodes, interdigitating reticulum cells of T zones, cells in subcapsular areas, and cells of the reticular network were reactive, with variable reactivity observed for follicular dendritic cells. Splenic dendritic cells of the white pulp and sinus-lining cells of the red pulp were reactive. Endothelial cells of all tissues exhibited variable reactivity. Lymphoid cells, myeloid cells, and plasma cells were uniformly nonreactive. In the peripheral blood, only dendritic (veiled) cells were reactive for fascin. A striking finding was observed for cases of Hodgkin's disease (total 187 cases). In all cases of nodular sclerosis (132), mixed cellularity (34), lymphocyte depletion (2), and unclassified types (5), all or nearly all Reed-Sternberg cells and variants were immunoreactive for fascin. Neoplastic cells exhibited strong diffuse cytoplasmic staining and frequently assumed dendritic shapes, particularly in the nodular sclerosis type, producing an interdigitating meshwork or syncytial network of cells. In cases of mixed cellularity type, neoplastic cells generally appeared more discrete. In all 14 cases of nodular lymphocyte predominance type, L&H variants were nonreactive. By contrast, neoplastic lymphoid cells of only 24 of 156 (15%) other lymphoid neoplasms (127 B cell, 27 T cell, and two null cell evaluated) were reactive for fascin. Fascin represents a highly effective marker for detection of certain dendritic cells in normal and neoplastic tissues, is an extremely consistent marker for Reed-Sternberg cells and variants of Hodgkin's disease (except L&H types), and may be helpful to distinguish between Hodgkin's disease and non-Hodgkin's lymphoma in difficult cases. The staining profile for fascin raises the possibility of a dendritic cell derivation, particularly an interdigitating reticulum cell, for the neoplastic cells of Hodgkin's disease, notably in nodular sclerosis type. However, as fascin expression may be induced by Epstein-Barr virus infection of B cells, the possibility that viral induction of fascin in lymphoid or other cell types must also be considered in Epstein-Barr virus-positive cases.

Epstein-Barr virus LMP1 induction of the epidermal growth factor receptor is mediated through a TRAF signaling pathway distinct from NF-kappaB activation

The Epstein-Barr virus (EBV)-encoded LMP1 protein induces several cellular changes including induction of epidermal growth factor receptor (EGFR) expression and activation of the NF-kappaB transcription factor. Two domains within the carboxy terminus have been identified that activate NF-kappaB. In this study, mutational analysis of the LMP1 protein indicated that the proximal NF-kappaB activation domain, which is identical to the TRAF interaction domain (amino acids 187 to 231), is essential for induction of the EGFR. The distal NF-kappaB activation domain (amino acids 352 to 386) did not induce expression of the EGFR. In contrast, the two domains both independently activated a kappaB-CAT reporter gene and induced expression of the NF-kappaB-regulated A20 gene in C33A epithelial cells. These results indicate that induction of the EGFR by LMP1 involves the TRAF interaction domain and that activation of NF-kappaB alone is not sufficient. Northern blot analysis revealed that induction of EGFR and A20 expression is likely to be at the transcriptional level. Interestingly expression of CD40 in the C33A cells also induced expression of the EGFR. Overexpression of either TRAF3 or an amino-terminal-truncated form of TRAF3 (TRAF3-C) inhibited signaling from the LMP1 TRAF interaction domain but did not affect signaling from the distal NF-kappaB activation domain. These data further define the mechanism by which LMP1 induces expression of the EGFR and indicate that TRAF signaling from LMP1 and CD40 activates a downstream transcription pathway distinct from NF-kappaB that induces expression of the EGFR.

Association of TRAF1, TRAF2, and TRAF3 with an Epstein-Barr virus LMP1 domain important for B-lymphocyte transformation: role in NF-kappaB activation

The Epstein-Barr virus (EBV) transforming protein LMP1 appears to be a constitutively activated tumor necrosis factor receptor (TNFR) on the basis of an intrinsic ability to aggregate in the plasma membrane and an association of its cytoplasmic carboxyl terminus (CT) with TNFR-associated factors (TRAFs). We now show that in EBV-transformed B lymphocytes most of TRAF1 or TRAF3 and 5% of TRAF2 are associated with LMP1 and that most of LMP1 is associated with TRAF1 or TRAF3. TRAF1, TRAF2, and TRAF3 bind to a single site in the LMP1 CT corresponding to amino acids (aa) 199 to 214, within a domain which is important for B-lymphocyte growth transformation (aa 187 to 231). Further deletional and alanine mutagenesis analyses and comparison with TRAF binding sequences in CD40, in CD30, and in the LMP1 of other lymphycryptoviruses provide the first evidence that PXQXT/S is a core TRAF binding motif. The negative effects of point mutations in the LMP1(1-231) core TRAF binding motif on TRAF binding and NF-kappaB activation genetically link the TRAFs to LMP1(1-231)-mediated NF-kappaB activation. NF-kappaB activation by LMP1(1-231) is likely to be mediated by TRAF1/TRAF2 heteroaggregates since TRAF1 is unique among the TRAFs in coactivating NF-kappaB with LMP1(1-231), a TRAF2 dominant-negative mutant can block LMP1(1-231)-mediated NF-kappaB activation as well as TRAF1 coactivation, and 30% of TRAF2 is associated with TRAF1 in EBV-transformed B cells. TRAF3 is a negative modulator of LMP1(1-231)-mediated NF-kappaB activation. Surprisingly, TRAF1, -2, or -3 does not interact with the terminal LMP1 CT aa 333 to 386 which can independently mediate NF-kappaB activation. The constitutive association of TRAFs with LMP1 through the aa 187 to 231 domain which is important in NF-kappaB activation and primary B-lymphocyte growth transformation implicates TRAF aggregation in LMP1 signaling.

Tumor necrosis factor receptor-associated factor (TRAF)-1, TRAF-2, and TRAF-3 interact in vivo with the CD30 cytoplasmic domain; TRAF-2 mediates CD30-induced nuclear factor kappa B activation

CD30 is a member of the tumor necrosis factor receptor superfamily, which can transduce signals for proliferation, death, or nuclear factor kappa B (NF-kappa B) activation. Investigation of CD30 signaling pathways using a yeast two-hybrid interaction system trapped a cDNA encoding the tumor necrosis factor receptor-associated factor (TRAF)-2 TRAF homology domain. TRAF-1 and TRAF-3 also interacted with CD30, and > 90% of in vitro-translated TRAF-1 or -2, or 50% of TRAF-3, bound to the CD30 cytoplasmic domain. TRAF-1, -2, and -3 bound mostly, but not exclusively, to the carboxyl-terminal 36 residues of CD30. The binding was strongly inhibited by a CD30 oligopeptide centered around a PXQXT (where X is any amino acid) motif shared with CD40 and the Epstein-Barr virus transforming protein LMP1, indicating that this motif in CD30 is an important determinant of TRAF-1, -2 or -3 interaction. At least 15% of TRAF-1, -2, or -3 associated with CD30 when coexpressed in 293 cells. The association was not affected by CD30 cross-linking. However, cross-linking of CD30 activated NF-kappa B. NF-kappa B activation was dependent on the carboxyl-terminal 36 amino acids of CD30 that mediate TRAF association. TRAF-2 has been previously shown to have a unique role in TRAF-mediated NF-kappa B activation, and NF-kappa B activation following CD30 cross-linking was blocked by a dominant negative TRAF-2 mutant. These data indicate that CD30 cross-linking-induced NF-kappa B activation is predominantly TRAF-2-mediated.

CD40-induced growth inhibition in epithelial cells is mimicked by Epstein-Barr Virus-encoded LMP1: involvement of TRAF3 as a common mediator

CD40, a member of the tumour necrosis factor receptor family, is expressed on the surface of B lymphocytes where its ligation provides a potent survival signal. CD40 is also expressed in basal epithelial cells and in a number of different carcinomas where its function remains unknown. We observed that contrary to the studies in normal B cells, CD40 ligation in carcinoma cell lines and in normal primary epithelial cells resulted in growth inhibition and enhanced susceptibility to apoptosis induced by anti-neoplastic drugs, TNF-alpha, Fas and ceramide. This effect was also observed in CD40-transfected Rat-1 fibroblasts. The expression of Bcl-2 did not affect growth inhibition induced by CD40 ligation in epithelial cells but the Epstein - Barr Virus-encoded latent membrane protein 1 (LMP1) blocked the effect. Whilst transient expression of LMP-1 resulted in the inhibition of epithelial cell growth, this effect was not observed with a LMP1 mutant lacking the binding domain for TRAF3, a protein which may mediate signal transduction by interacting with the cytoplasmic domains of both CD40 and LMP1. Transient expression of TRAF3 also inhibited epithelial cell growth, whilst expression of a dominant-negative TRAF3 partially blocked the inhibitory effect of CD40 ligation and of transient LMP1 expression. These results suggest that CD40 regulates epithelial cell growth in a manner mimicked by LMP1 and implicate TRAF3 as a common mediator in the transduction of the growth inhibitory signals generated via the CD40 and LMP1 pathways.

Identification of TRAF6, a novel tumor necrosis factor receptor-associated factor protein that mediates signaling from an amino-terminal domain of the CD40 cytoplasmic region

CD40 signalings play crucial roles in B-cell function. To identify molecules which transduce CD40 signalings, we have utilized the yeast two-hybrid system to clone cDNAs encoding proteins that bind the cytoplasmic tail of CD40. A cDNA encoding a putative signal transducer, designated TRAF6, has been molecularly cloned. TRAF6 has a tumor necrosis factor receptor (TNFR)-associated factor (TRAF) domain in its carboxyl terminus and has a RING finger domain, a cluster of zinc fingers and a coiled-coil domain, which are also present in other TRAF family proteins. TRAF6 does not associate with the cytoplasmic tails of TNFR2, CD30, lymphotoxin-beta receptor, and LMP1 of Epstein-Barr virus. Deletion analysis showed that residues 246-269 of CD40 which are required for its association with TRAF2, TRAF3, and TRAF5 are dispensable for its interaction with TRAF6, whereas residues 230-245 were required. Overexpression of TRAF6 activates transcription factor NFkappaB, and its TRAF-C domain suppresses NFkappaB activation triggered by CD40 lacking residues 246-277. These results suggest that TRAF6 could mediate the CD40 signal that is transduced by the amino-terminal domain (230-245) of the CD40 cytoplasmic region and appears to be independent of other known TRAF family proteins.

Tumor necrosis factor receptor associated factor 2 is a mediator of NF-kappa B activation by latent infection membrane protein 1, the Epstein-Barr virus transforming protein

Latent infection membrane protein 1 (LMP1), the Epstein-Barr virus transforming protein, associates with tumor necrosis factor receptor (TNFR) associated factor 1 (TRAF1) and TRAF3. Since TRAF2 has been implicated in TNFR-mediated NF-kappa B activation, we have evaluated the role of TRAF2 in LMP1-mediated NF-kappa B activation. TRAF2 binds in vitro to the LMP1 carboxyl-terminal cytoplasmic domain (CT), coprecipitates with LMP1 in B lymphoblasts, and relocalizes to LMP1 plasma membrane patches. A dominant negative TRAF2 deletion mutant that lacks amino acids 6-86 (TRAF/ delta 6-86) inhibits NF-kappa B activation from the LMP1 CT and competes with TRAF2 for LMP1 binding. TRAF2 delta 6-86 inhibits NF-kappa B activation mediated by the first 45 amino acids of the LMP1 CT by more than 75% but inhibits NF-kappa B activation through the last 55 amino acids of the CT by less than 40%. A TRAF interacting protein, TANK, inhibits NF-kappa B activation by more than 70% from both LMP1 CT domains. These data implicate TRAF2 aggregation in NF-kappa B activation by the first 45 amino acids of the LMP1 CT and suggest that a different TRAF-related pathway may be involved in NF-kappa B activation by the last 55 amino acids of the LMP1 CT.

Circulating human dendritic cells differentially express high levels of a 55-kd actin-bundling protein

This study was initiated to examine the differential expression of an evolutionary conserved human 55-kd actin-bundling (p55) protein that is induced in B lymphocytes by Epstein-Barr virus infection. Our study demonstrates that p55 is specifically expressed at constitutively high levels in human peripheral blood dendritic cells and lymph node (interdigitating) dendritic cells. Blood dendritic cells constitute a minority (< 2%) of all blood leukocytes but are a distinct population of potent antigen-presenting cells. Immunofluorescence microscopy with a monoclonal antibody specific for p55 showed that 87% of peripheral blood dendritic cells stained brightly in the cytoplasm and in the veiled cytoplasmic extensions. In contrast, monocytes, granulocytes, T cells, and B lymphocytes showed no expression of the p55 protein. Western blot analysis confirmed that only the dendritic cell component of peripheral blood expressed high levels of p55. Staining of human lymph node sections demonstrated selective expression of the p55 antigen by dendritic cells in the T-cell-dependent areas but not in the B cell follicles. p55 is likely to be involved in the organization of a specialized microfilament cytoskeleton in the dendritic cells, and the anti-p55 antibody should be useful for further characterization of this important population of antigen-presenting cells in clinical transplantation, HIV-1 pathogenesis, and autoimmune diseases.

The Epstein-Barr virus nuclear protein 2 acidic domain forms a complex with a novel cellular coactivator that can interact with TFIIE

Epstein-Barr virus nuclear antigen 2 (EBNA 2) activates transcription of specific genes and is essential for B-lymphocyte transformation. EBNA 2 has an acidic activation domain which interacts with general transcription factors TFIIB, TFIIH, and TAF40. We now show that EBNA 2 is specifically bound to a novel nuclear protein, p100, and that p100 can coactivate gene expression mediated by the EBNA 2 acidic domain. The EBNA 2 acidic domain was used to affinity purify p100. cDNA clones encoding the p100 open reading frame were identified on the basis of peptide sequences of the purified protein. Antibody against p100 coimmunoprecipitated p100 and EBNA 2 from Epstein-Barr virus-transformed lymphocyte extracts, indicating that EBNA 2 and p100 are complexed in vivo. p100 overexpression in cells specifically augmented EBNA 2 acidic domain-mediated activation. The coactivating effect is probably mediated by p100 interaction with TFIIE. Bacterially expressed p100 specifically adsorbs TFIIE from nuclear extracts, and in vitro-translated p56 or p34 TFIIE subunit can independently bind to p100. p100 also appears to be essential for normal cell growth, since cell viability was reduced by antisense p100 RNA and restored by sense p100 RNA expression.

The Epstein-Barr virus transforming protein LMP1 engages signaling proteins for the tumor necrosis factor receptor family

The cytoplasmic C-terminus of Epstein-Barr virus (EBV) latent infection membrane protein 1 (LMP1) is essential for B lymphocyte growth transformation and is now shown to interact with a novel human protein (LMP1-associated protein 1 [LAP1]). LAP1 is homologous to a murine protein, tumor necrosis factor receptor-associated factor 2 (TRAF2), implicated in growth signaling from the p80 TNFR. A second novel protein (EBI6), induced by EBV infection, is the human homolog of a second murine TNFR-associated protein (TRAF1). LMP1 expression causes LAP1 and EBI6 to localize to LMP1 clusters in lymphoblast plasma membranes, and LMP1 coimmunoprecipitates with these proteins. LAP1 binds to the p80 TNFR, CD40, and the lymphotoxin-beta receptor, while EBI6 associates with the p80 TNFR. The interaction of LMP1 with these TNFR family-associated proteins is further evidence for their role in signaling and links LMP1-mediated transformation to signal transduction from the TNFR family.

The Epstein-Barr virus LMP1 cytoplasmic carboxy terminus is essential for B-lymphocyte transformation; fibroblast cocultivation complements a critical function within the terminal 155 residues

Recombinant Epstein-Barr viruses (EBVs) were made with mutated latent membrane protein 1 (LMP1) genes that express only the LMP1 amino-terminal cytoplasmic and six transmembrane domains (MS187) or these domains and the first 44 amino acids of the 200-residue LMP1 carboxy-terminal domain (MS231). After infection of primary B lymphocytes with virus stocks having small numbers of recombinant virus and large numbers of P3HR-1 EBV which is transformation defective but wild type (WT) for LMP1, all lymphoblastoid cell lines (LCLs) that had MS187 or MS231 LMP1 also had WT LMP1 provided by the coinfecting P3HR-1 EBV. Lytic virus infection was induced in these coinfected LCLs, and primary B lymphocytes were infected. In over 200 second-generation LCLs, MS187 LMP1 was never present without WT LMP1. Screening of over 600 LCLs infected with virus from MS231 recombinant virus-infected LCLs identified two LCLs which were infected with an MS231 recombinant without WT LMP1. The MS231 recombinant virus could growth transform primary B lymphocytes when cells were grown on fibroblast feeders. Even after 6 months on fibroblast feeder layers, cells transformed by the MS231 recombinant virus died when transferred to medium without fibroblast feeder cells. These data indicate that the LMP1 carboxy terminus is essential for WT growth-transforming activity. The first 44 amino acids of the carboxy-terminal cytoplasmic domain probably include an essential effector of cell growth transformation, while a deletion of the rest of LMP1 can be complemented by growth on fibroblast feeder layers. LMP1 residues 232 to 386 therefore provide a growth factor-like effect for the transformation of B lymphocytes. This effect may be indicative of the broader role of LMP1 in cell growth transformation.

Epstein-Barr virus nuclear protein 2 transactivation of the latent membrane protein 1 promoter is mediated by J kappa and PU.1

Expression of the Epstein-Barr virus (EBV) latent membrane protein 1 (LMP-1) oncogene is regulated by the EBV nuclear protein 2 (EBNA-2) transactivator. EBNA-2 is known to interact with the cellular DNA-binding protein J kappa and is recruited to promoters containing the GTGGGAA J kappa recognition sequence. The minimal EBNA-2-responsive LMP-1 promoter includes one J kappa-binding site, and we now show that mutation of that site, such that J kappa cannot bind, reduces EBNA-2 responsiveness by 60%. To identify other factors which interact with the LMP-1 EBNA-2 response element (E2RE), a -236/-145 minimal E2RE was used as a probe in an electrophoretic mobility shift assay. The previously characterized factors J kappa, PU.1, and AML1 bind to the LMP-1 E2RE, along with six other unidentified factors (LBF2 to LBF7). Binding sites were mapped for each factor. LBF4 is B- and T-cell specific and recognizes the PU.1 GGAA core sequence as shown by methylation interference. LBF4 has a molecular mass of 105 kDa and is probably unrelated to PU.1. LBF2 was found only in epithelial cell lines, whereas LBF3, LBF5, LBF6, and LBF7 were not cell type specific. Mutations of the AML1- or LBF4-binding sites had no effect on EBNA-2 transactivation, whereas mutation of the PU.1-binding site completely eliminated EBNA-2 responses. A gst-EBNA-2 fusion protein specifically depleted PU.1 from nuclear extracts and bound in vitro translated PU.1, providing biochemical evidence for a direct EBNA-2-PU.1 interaction. Thus, EBNA-2 transactivation of the LMP-1 promoter is dependent on interaction with at least two distinct sequence-specific DNA-binding proteins, J kappa and PU.1. LBF3, LBF5, LBF6, or LBF7 may also be involved, since their binding sites also contribute to EBNA-2 responsiveness.

Genetic and biochemical evidence that EBNA 2 interaction with a 63-kDa cellular GTG-binding protein is essential for B lymphocyte growth transformation by EBV

Epstein-Barr virus (EBV) nuclear protein 2 (EBNA 2) is an acidic transcriptional transactivator of virus and cell gene expression and is essential for growth transformation of primary B lymphocytes. EBNA 2 transactivation of response elements (E2REs) can be mediated by interaction with a GTGGGAA-specific DNA-binding factor(s). We now purify the factor by S-Sepharose and EBNA 2 affinity chromatography and identify it as a single 63-kDa protein. The protein is shown to specifically coimmunoprecipitate with EBNA 2 from lymphoblasts transfected with an EBNA 2 FLAG expression vector. Mutation of GTG to TCT in a GTGGGAA motif common to the Cp, LMP2, and LMP1 promoters results in loss of recognition by p63. EBNA 2 amino acids 310-336 are sufficient for p63 binding. The only motif in this 27 amino acid sequence which is common to the EBNA 2 genes of EBV types 1 and 2 is GPPWWPP (I/V) (C/R) DP, which is therefore likely to mediate p63 interaction. Mutation of WW to SS or FF ablates interaction with p63, indicating that both the hydrophobic and aromatic characteristics of WW are essential for its "key" interaction with p63. EBNA 2 with a WW mutated to SS is also unable to marker rescue primary B lymphocyte transforming virus from cells infected with an EBNA 2-deleted virus, while otherwise isogenic wild-type EBNA 2 readily marker rescues transforming virus in parallel experiments. EBNA 2 transactivation through the Cp E2RE is completely abolished by the WW to SS mutation while transactivation of -234 to +40 LMP1 E2RE is only partially affected. These genetic and biochemical experiments support the hypothesis that EBNA 2 WW interaction with a p63 GTGGGAA-binding protein is essential for EBV-mediated cell growth transformation because it specifically associates EBNA 2 with its response elements. This enables the EBNA 2 acidic domain to transcriptionally transactivate specific genes.

Epstein-Barr virus infection induces expression in B lymphocytes of a novel gene encoding an evolutionarily conserved 55-kilodalton actin-bundling protein

A novel human mRNA whose expression is induced over 200-fold in B lymphocytes by latent Epstein-Barr virus (EBV) infection was reverse transcribed, cloned, and sequenced. The mRNA is predicted to encode a protein containing four peptides which precisely match amino acid sequences from a previously identified 55-kDa actin-bundling protein, p55. In vitro translation of the cDNA results in a 55-kDa protein which binds to actin filaments in the presence of purified p55 from HeLa cells. The p55 mRNA is undetectable in non-EBV-infected B- and T-cell lines or in a myelomonocytic cell line (U937). Newly infected primary human B lymphocytes, EBV-transformed B-cell lines, latently infected Burkitt tumor cells expressing EBNA2 and LMP1, a chronic myelogenous leukemia cell line (K562), and an osteosarcoma cell line (TK143) contain high levels of p55 mRNA or protein. In EBV-transformed B cells, p55 localizes to perinuclear cytoplasm and to cell surface processes that resemble filopodia. The p55 mRNA is detected at high levels in spleen and brain tissues, at moderate levels in lung and placenta tissues, and at low levels in skeletal muscle, liver, and tonsil tissues and is undetectable in heart, kidney, pancreas, and bone marrow tissues. Immunohistochemical staining of human brain tissue demonstrates p55 localization to the perinuclear cytoplasm and dendritic processes of many, but not all, types of cortical or cerebellar neurons, to glial cells, and to capillary endothelial cells. In cultured primary rat neurons, p55 is distributed throughout the perinuclear cytoplasm and in subcortical filamentous structures of dendrites and growth cones. p55 is highly evolutionarily conserved since it shows 40% amino acid sequence identity to the Drosophila singed gene product and 37% identity to fascin, an echinoderm actin-bundling protein. The evolutionary conservation of p55 and its lack of extensive homology to other actin-binding proteins suggest that p55 has specific microfilament-associated functions in cells in which it is differentially expressed, including neural cells and EBV-transformed B lymphocytes.

Characterization of a chicken cDNA encoding the retinoblastoma gene product

We have isolated a chicken cDNA that encodes the retinoblastoma susceptibility gene product (RB). The predicted amino acid sequence of the chicken RB protein is highly similar to that of the mouse, human and Xenopus RB proteins in regions of known functions; however, chicken RB has distinct species-specific differences, including a shorter N-terminal region as compared to the mouse and human RB proteins. In vitro-translated chicken RB co-migrates on SDS-polyacrylamide gels with endogenous RB synthesized in transformed chicken spleen cells. Finally, chicken RB is located in the nucleus of chicken embryo fibroblasts when overexpressed from a retroviral vector.

A Ca2+/calmodulin-dependent protein kinase, CaM kinase-Gr, expressed after transformation of primary human B lymphocytes by Epstein-Barr virus (EBV) is induced by the EBV oncogene LMP1

CaM kinase-Gr is a multifunctional Ca2+/calmodulin-dependent protein kinase which is enriched in neurons and T lymphocytes. The kinase is absent from primary human B lymphocytes but is expressed in Epstein-Barr virus (EBV)-transformed B-lymphoblastoid cell lines, suggesting that expression of the kinase can be upregulated by an EBV gene product(s). We investigated the basis of CaM kinase-Gr expression in EBV-transformed cells and the mechanisms that regulate its activity therein by using an EBV-negative Burkitt lymphoma cell line, BJAB, and BJAB cells converted to expression of individual EBV proteins by single-gene transfer. CaM kinase-Gr expression was upregulated in BJAB cells by EBV latent-infection membrane protein 1 (LMP1) but not by LMP2A or by nuclear proteins EBNA1, EBNA2, EBNA3A, and EBNA3C. In LMP1-converted BJAB cells, the kinase was functional and was dramatically activated upon cross-linking of surface immunoglobulin M. Overlapping cDNA clones that encode human CaM kinase-Gr were sequenced, revealing 81% amino acid identity between the rat and human proteins. Transfection of BJAB cells with an expression construct for the human enzyme resulted in a functional kinase which was shown by epitope tagging to localize primarily to cytoplasmic and perinuclear structures. Induction of CaM kinase-Gr expression by LMP1 provides the first example of a Ca2+/calmodulin-dependent protein kinase upregulated by a viral protein. In view of the key role played by LMP1 in B-lymphocyte immortalization by EBV, these findings implicate CaM kinase-Gr as a potential mediator of B-lymphocyte growth transformation.

v-Rel and c-Rel are differentially affected by mutations at a consensus protein kinase recognition sequence

The avian retroviral oncoprotein v-Rel and its cellular homolog c-Rel are members of a family of related site-specific DNA-binding proteins. Towards the carboxy-terminal end of the highly conserved Rel homology (RH) domain in the majority of Rel proteins, there is a consensus recognition sequence for protein kinase A (PK-A). We have investigated the importance of this sequence (Arg-Arg-Pro-Ser) for several functional properties of v-Rel and c-Rel. Disruption of the PK-A sequence by a two amino acid insertion between the arginine and the proline residues completely abolished the ability of v-Rel and c-Rel to bind a kappa B site in vitro. When the phosphorylatable serine in this sequence (Ser-275 in v-Rel, Ser-266 in c-Rel) was replaced by an alanine, DNA binding by v-Rel was not affected, whereas the ability of c-Rel to bind DNA was reduced approximately fourfold by this mutation. Similarly, a serine to tryptophan change greatly reduced the DNA-binding ability of c-Rel, whereas v-Rel was not appreciably affected by this change. When this serine was replaced by an acidic amino acid, DNA binding by v-Rel was reduced approximately twofold and the DNA-binding activity of c-Rel was nearly abolished. Glutaraldehyde cross-linking experiments indicated that mutations at the PK-A recognition site that reduced DNA binding also negatively affected protein oligomerization, which is likely to be responsible for the reduced ability of mutant v-Rel and c-Rel proteins to bind DNA. Domain-swapping experiments showed that structural differences between v-Rel and c-Rel in the central region of the proteins are primarily responsible for the higher sensitivity of c-Rel to a serine to alanine mutation in the PK-A site. One difference between v-Rel and c-Rel, a glutamine to alanine change in v-Rel located three amino acids carboxy-terminal to the PK-A phosphorylatable serine (Ala-278 in v-Rel; Glu-269 in c-Rel), is mainly responsible for the lack of an effect on DNA binding by v-Rel when Ser-275 is replaced by alanine. That is, a v-Rel double mutant (v-275A/278E) showed reduced DNA-binding and transforming abilities as compared with v-Rel and v-275A. Similarly, the mutations in c-Rel that affected DNA binding showed a corresponding effect on the ability of c-Rel proteins to activate transcription in yeast from a reporter gene containing upstream Rel binding sites.(ABSTRACT TRUNCATED AT 400 WORDS)

p105, the NF-kappa B p50 precursor protein, is one of the cellular proteins complexed with the v-Rel oncoprotein in transformed chicken spleen cells

Active NF-kappa B-like transcription complexes are multimers consisting of one or two members of a family of proteins related to the c-Rel proto-oncoprotein. We have isolated a chicken cDNA encoding p105, the precursor protein for the p50 subunit of NF-kappa B. Sequence analysis shows that chicken p105 is approximately 70% identical to the mouse and human p105 proteins, containing the Rel homology domain in its N-terminal 370 amino acids and several ankyrinlike repeats in the C-terminal portion of the protein. The Rel homology domain is particularly highly conserved between chicken and mammalian p50, and an in vitro-synthesized, truncated chicken p105 protein, containing sequences that correspond to the predicted p50 protein, bound to a consensus kappa B site in an electrophoretic mobility shift assay. In v-Rel-transformed chicken spleen cells, v-Rel is found in high-molecular-weight complexes which include cellular proteins of approximately 124 kDa (p124) and 115 kDa (p115). Here we report that in vitro-produced p105 comigrates with p124 from v-Rel-transformed spleen cells and that p105 and p124 appear to be identical by partial proteolytic mapping with V8 protease. Furthermore, both p105 and p50 can complex directly with v-Rel and chicken c-Rel in vitro. However, in vitro association with p105 by v-Rel does not necessarily correlate with transformation, since one nontransforming v-Rel mutant can associate with p105 in vitro.

A protein kinase-A recognition sequence is structurally linked to transformation by p59v-rel and cytoplasmic retention of p68c-rel

The Rel family of proteins includes a number of proteins involved in transcriptional control, such as the retroviral oncoprotein v-Rel, c-Rel, the Drosophila melanogaster developmental protein Dorsal, and subunits of the transcription factor NF-kappa B. These proteins are related through a highly conserved domain of approximately 300 amino acids, called the Rel homology domain, that contains dimerization, DNA binding, and nuclear targeting functions. Also within the Rel homology domain, there is a conserved consensus sequence (Arg-Arg-Pro-Ser) for phosphorylation by cyclic AMP-dependent protein kinase (PKA). We used linker insertion mutagenesis and site-directed mutagenesis to determine the importance of this sequence for the transformation of avian spleen cells by v-Rel and the subcellular localization of c-Rel in chicken embryo fibroblasts (CEF). The insertion of 2 amino acids (Pro-Trp) within this sequence completely abolished transformation and transcriptional repression by v-Rel and resulted in a shift in the localization of c-Rel from cytoplasmic to nuclear in CEF. When the conserved Ser within the PKA recognition sequence was replaced by Ala, there was no significant effect on transformation and transcriptional repression by v-Rel or on cytoplasmic retention of c-Rel. However, when this Ser was changed to Asp or Glu, transformation and transcriptional repression by v-Rel were significantly inhibited and c-Rel showed a diffuse nuclear and cytoplasmic localization in CEF. Although a peptide containing the recognition sequence from v-Rel can be phosphorylated by PKA in vitro, this site is not constitutively phosphorylated to a high degree in vivo in transformed spleen cells incubated with okadaic acid. Our results indicate that the transforming and transcriptional repressing activities of v-Rel and the cytoplasmic retention of c-Rel are dependent on the structure of the conserved PKA recognition motif. In addition, they suggest that phosphorylation at the conserved PKA site could have a negative effect on transformation and transcriptional repression by v-Rel and induce the nuclear localization of c-Rel.

Oncogenic transformation by vrel requires an amino-terminal activation domain

The mechanism by which the products of the v-rel oncogene, the corresponding c-rel proto-oncogene, and the related dorsal gene of Drosophila melanogaster exert their effects is not clear. Here we show that the v-rel, chicken c-rel, and dorsal proteins activated gene expression when fused to LexA sequences and bound to DNA upstream of target genes in Saccharomyces cerevisiae. We have defined two distinct activation regions in the c-rel protein. Region I, located in the amino-terminal half of rel and dorsal proteins, contains no stretches of glutamines, prolines, or acidic amino acids and therefore may be a novel activation domain. Lesions in the v-rel protein that diminished or abolished oncogenic transformation of avian spleen cells correspondingly affected transcription activation by region I. Region II, located in the carboxy terminus of the c-rel protein, is highly acidic. Region II is not present in the v-rel protein or in a transforming mutant derivative of the c-rel protein. Our results show that the oncogenicity of Rel proteins requires activation region I and suggest that the biological function of rel and dorsal proteins depends on transcription activation by this region.