Unique signaling properties of CTAR1 in LMP1-mediated transformation

The Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) gene is considered the EBV oncogene as it is necessary for EBV-mediated transformation of B lymphocytes and itself transforms rodent fibroblasts. LMP1 activates the NF-kappaB, phosphatidylinositol 3-kinase (PI3K)-Akt, mitogen-activated protein kinase, and Jun N-terminal protein kinase signaling pathways through its two signaling domains, carboxyl-terminal activating regions 1 and 2 (CTAR1 and CTAR2). CTAR1 and CTAR2 induce signal transduction pathways through their direct (CTAR1) or indirect (CTAR2) recruitment of tumor necrosis factor receptor-associated factors (TRAFs). CTAR1 is necessary for LMP1-mediated transformation as well as activation of PI3K signaling and induction of cell cycle markers associated with G(1)/S transition. In this study, activation of PI3K-Akt signaling and deregulation of cell cycle markers were mapped to the TRAF-binding domain within CTAR1 and to the residues between CTAR1 and CTAR2. LMP1 CTAR1 also activated the MEK1/2-extracellular signal-regulated kinase 1/2 signaling pathway, and this activation was necessary for LMP1-induced transformation of Rat-1 fibroblasts. Dominant-negative forms of TRAF2 and TRAF3 inhibited but did not fully block LMP1-mediated transformation. These findings identify a new signaling pathway that is uniquely activated by the TRAF-binding domain of LMP1 and is required for transformation.     

TRAF6 is a critical mediator of signal transduction by the viral oncogene latent membrane protein 1

The oncogenic latent membrane protein 1 (LMP1) of the Epstein-Barr virus recruits tumor necrosis factor-receptor (TNFR)-associated factors (TRAFs), the TNFR-associated death domain protein (TRADD) and JAK3 to induce intracellular signaling pathways. LMP1 serves as the prototype of a TRADD-binding receptor that transforms cells but does not induce apoptosis. Here we show that TRAF6 critically mediates LMP1 signaling to p38 mitogen-activated protein kinase (MAPK) via a MAPK kinase 6-dependent pathway. In addition, NF-kappaB but not c-Jun N-terminal kinase 1 (JNK1) induction by LMP1 involves TRAF6. The PxQxT motif of the LMP1 C-terminal activator region 1 (CTAR1) and tyrosine 384 of CTAR2 together are essential for full p38 MAPK activation and for TRAF6 recruitment to the LMP1 signaling complex. Dominant-negative TRADD blocks p38 MAPK activation by LMP1. The data suggest that entry of TRAF6 into the LMP1 complex is mediated by TRADD and TRAF2. In TRAF6-knockout fibroblasts, significant induction of p38 MAPK by LMP1 is dependent on the ectopic expression of TRAF6. We describe a novel role of TRAF6 as an essential signaling mediator of a transforming oncogene, downstream of TRADD and TRAF2.     

Negative regulation of Epstein-Barr virus latent membrane protein 1-mediated functions by the bone morphogenetic protein receptor IA-binding protein,BRAM1

The latent membrane protein 1 (LMP1) of Epstein-Barr virus causes cellular transformation and activates several intracellular signals, including NF-kappaB and c-Jun N-terminal kinase. Using yeast two-hybrid screening with the LMP1 C-terminal sequence as bait, we demonstrate that BRAM1 (bone morphogenetic protein receptor-associated molecule 1) is an LMP1-interacting protein. BRAM1 associates with LMP1, both in vitro and in vivo, as revealed by confocal microscopy, glutathione S-transferase pull-down, and co-immunoprecipitation assays. This association mainly involves the C-terminal half of BRAM1 comprising the MYND domain and the CTAR2 region of LMP1, which is critical in LMP1-mediated signaling pathways. We show that BRAM1 interferes with LMP1-mediated NF-kappaB activation but not the JNK signaling pathway. Because the CTAR2 region interacts with the tumor necrosis factor (TNF-alpha receptor-associated death domain protein, it is interesting to find that BRAM1 also interferes with NF-kappaB activation mediated by TNF-alpha. BRAM1 interferes LMP1-mediated and TNF-alpha-induced NF-kappaB activation by targeting IkappaBalpha molecules. Moreover, BRAM1 inhibits the resistance of LMP1-expressing cells to TNF-alpha-induced cytotoxicity. We therefore propose that the BRAM1 molecule associates with LMP1 and functions as a negative regulator of LMP1-mediated biological functions.     

TAK1 is a component of the Epstein-Barr virus LMP1 complex and is essential for activation of JNK but not of NF-kappaB

Epstein-Barr virus latent membrane protein 1 (LMP1) activates NF-kappaB and c-Jun N-terminal kinase (JNK), which is essential for LMP1 oncogenic activity. Genetic analysis has revealed that tumor necrosis factor receptor-associated factor 6 (TRAF6) is an indispensable intermediate of LMP1 signaling leading to activation of both NF-kappaB and JNK. However, the mechanism by which LMP1 engages TRAF6 for activation of NF-kappaB and JNK is not well understood. Here we demonstrate that TAK1 mitogen-activated protein kinase kinase kinase and TAK1-binding protein 2 (TAB2), together with TRAF6, are recruited to LMP1 through its N-terminal transmembrane region. The C-terminal cytoplasmic region of LMP1 facilitates the assembly of this complex and enhances activation of JNK. In contrast, IkappaB kinase gamma is recruited through the C-terminal cytoplasmic region and this is essential for activation of NF-kappaB. Furthermore, we found that ablation of TAK1 resulted in the loss of LMP1-induced activation of JNK but not of NF-kappaB. These results suggest that an LMP1-associated complex containing TRAF6, TAB2, and TAK1 plays an essential role in the activation of JNK. However, TAK1 is not an exclusive intermediate for NF-kappaB activation in LMP1 signaling.     

Roles of TNF receptor-associated factor 3 in signaling to B lymphocytes by carboxyl-terminal activating regions 1 and 2 of the EBV-encoded oncoprotein latent membrane protein 1

TNFR-associated factor (TRAF)3, an adaptor protein that binds the cytoplasmic domains of both CD40 and the EBV-encoded oncoprotein latent membrane protein (LMP)1, is required for positive signaling by LMP1 but not CD40 in B lymphocytes. The present study further investigated how TRAF3 participates in LMP1 signaling. We found that TRAF3 mediates signaling both through direct interactions with the C-terminal activating region (CTAR)1 of LMP1 and through indirect interactions with the CTAR2 region of LMP1 in mouse B cells. Notably, our results demonstrated that the CTAR2 region appears to inhibit the recruitment of TRAF1 and TRAF2 to membrane rafts by the CTAR1 region. Additionally, the absence of TRAF2 in B cells resulted in only a modest reduction in CTAR1-mediated signals and no detectable effect on CTAR2-mediated signals. CTAR1 and CTAR2 cooperated to achieve the robust signaling activity of LMP1 when recruited to the same membrane microdomains in B cells. Interestingly, TRAF3 deficiency completely abrogated the cooperation between CTAR1 and CTAR2, supporting the hypothesis that TRAF3 participates in the physical interaction between CTAR1 and CTAR2 of LMP1. Together, our findings highlight the central importance of TRAF3 in LMP1-mediated signaling, which is critical for EBV persistent infection and EBV-associated pathogenesis.     

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.     

Epstein-Barr Virus-Encoded Latent Membrane Protein 1 Activates the JNK Pathway through Its Extreme C Terminus via a Mechanism Involving TRADD and TRAF2

The transforming Epstein-Barr virus-encoded latent membrane protein 1 (LMP1) activates signalling on the NF-κB axis through two distinct domains in its cytoplasmic C terminus, namely, CTAR1 (amino acids [aa] 187 to 231) and CTAR2 (aa 351 to 386). The ability of CTAR1 to activate NF-κB appears to be attributable to the direct interaction of tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2), while recent work indicates that CTAR2-induced NF-κB is mediated through its association with TNF receptor-associated death domain (TRADD). LMP1 expression also results in activation of the c-Jun N-terminal kinase (JNK) (also known as stress-activated protein kinase) cascade, an effect which is mediated exclusively through CTAR2 and can be dissociated from NF-κB induction. The organization and signalling components involved in LMP1-induced JNK activation are not known. In this study we have dissected the extreme C terminus of LMP1 and have identified the last 8 aa of the protein (aa 378 to 386) as being important for JNK signalling. Using a series of fine mutants in which single amino acids between codons 379 and 386 were changed to glycine, we have found that mutations of Pro³⁷⁹, Glu³⁸¹, Ser³⁸³, or Tyr³⁸⁴ diminish the ability of LMP1 CTAR2 to engage JNK signalling. Interestingly, this region was also found to be essential for CTAR2-mediated NF-κB induction and coincides with the LMP1 amino acid sequences shown to bind TRADD. Furthermore, we have found that LMP1-mediated JNK activation is synergistically augmented by low levels of TRADD expression, suggesting that this adapter protein is critical for LMP1 signalling. TRAF2 is known to associate with TRADD, and expression of a dominant-negative N-terminal deletion TRAF2 mutant was found to partially inhibit LMP1-induced JNK activation in 293 cells. In addition, the TRAF2-interacting protein A20 blocked both LMP1-induced JNK and NF-κB activation, further implicating TRAF2 in these phenomena. While expression of a kinase-inactive mutated NF-κB-inducing kinase (NIK), a mitogen-activated protein kinase kinase kinase which also associates with TRAF2, impaired LMP1 signalling on the NF-κB axis, it did not inhibit LMP1-induced JNK activation, suggesting that these two pathways may bifurcate at the level of TRAF2. These data further define a role for TRADD and TRAF2 in JNK activation and confirm that LMP1 utilizes signalling mechanisms used by the TNF receptor/CD40 family to elicit its pleiotropic activities.     

Two carboxyl-terminal activation regions of Epstein-Barr virus latent membrane protein 1 activate NF-kappaB through distinct signaling pathways in fibroblast cell lines

Latent membrane protein 1 (LMP1), an Epstein-Barr virus transforming protein, is able to activate NF-kappaB through its carboxyl-terminal activation region 1 (CTAR1) and 2 (CTAR2), but the exact role of each domain is not fully understood. Here we show that LMP1 activates NF-kappaB in different NF-kappaB essential modulator (NEMO)-defective cell lines, but not in cells lacking both IkappaB kinase 1 (IKK1) and 2 (IKK2). Mutational studies reveal that CTAR1, but not CTAR2, mediates NEMO-independent NF-kappaB activation and that this process largely depends on IKK1. Retroviral expression of LMP1 mutants in cells lacking either functional NF-kappaB inducing kinase (NIK), NEMO, IKK1, or IKK2 further illustrates distinct signals from the two activation regions of LMP1 for persistent NF-kappaB activation. One originates in CTAR2, operates through the canonical NEMO-dependent pathway, and induces NFKB2 p100 production; the second signal originates in CTAR1, utilizes NIK and IKK1, and induces the processing of p100. Our results thus help clarify how two functional domains of LMP1 persistently activate NF-kappaB through distinct signaling pathways.     

The C-terminal activating region 2 of the Epstein-Barr virus-encoded latent membrane protein 1 activates NF-kappaB through TRAF6 and TAK1

Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) is oncogenic and indispensable for EBV-mediated B cell transformation. LMP1 is capable of activating several intracellular signaling pathways including the NF-kappaB pathway, which contributes to the EBV-mediated cell transformation. Two regions in the cytoplasmic carboxyl tail of LMP1, namely C-terminal activating regions 1 and 2 (CTAR1 and CTAR2), are responsible for NF-kappaB activation, with CTAR2 being the main NF-kappaB activator. Although the CTAR1-mediated NF-kappaB activation was previously shown to be TRAF3-dependent, we showed here that the CTAR2-mediated NF-kappaB activation is mainly TRAF6-dependent but TRAF2/5-independent. In contrast to the interleukin-1 receptor/toll-like receptor-mediated NF-kappaB pathways, the CTAR2-mediated NF-kappaB pathway does not require MyD88, IRAK1, or IRAK4 for TRAF6 engagement. Furthermore, we showed that TAK1 is required for NF-kappaB activation by LMP1. Thus, LMP1 utilizes two distinct pathways to activate NF-kappaB: a major one through CTAR2/TRAF6/TAK1/IKKbeta (canonical pathway) and a minor one through CTAR1/TRAF3/NIK/IKKalpha (noncanonical pathway).     

CD40 signaling through tumor necrosis factor receptor-associated factors (TRAFs). Binding site specificity and activation of downstream pathways by distinct TRAFs.

Tumor necrosis factor receptor-associated factors (TRAFs) associate with the CD40 cytoplasmic domain and initiate signaling after CD40 receptor multimerization by its ligand. We used saturating peptide-based mutational analyses of the TRAF1/TRAF2/TRAF3 and TRAF6 binding sequences in CD40 to finely map residues involved in CD40-TRAF interactions. The core binding site for TRAF1, TRAF2, and TRAF3 in CD40 could be minimally substituted. The TRAF6 binding site demonstrated more amino acid sequence flexibility and could be optimized. Point mutations that eliminated or enhanced binding of TRAFs to one or both sites were made in CD40 and tested in quantitative CD40-TRAF binding assays. Sequences flanking the core TRAF binding sites were found to modulate TRAF binding, and the two TRAF binding sites were not independent. Cloned stable transfectants of human embryonic kidney 293 cells that expressed wild type CD40 or individual CD40 mutations were used to demonstrate that both TRAF binding sites were required for optimal NF-kappaB and c-Jun N-terminal kinase activation. In contrast, p38 mitogen-activated protein kinase activation was primarily dependent upon TRAF6 binding. These studies suggest a role in CD40 signaling for competitive TRAF binding and imply that CD40 responses reflect an integration of signals from individual TRAFs.     

LMP1 signal transduction differs substantially from TNF receptor 1 signaling in the molecular functions of TRADD and TRAF2

The Epstein-Barr virus latent membrane protein 1 (LMP1) binds tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) and the TNFR-associated death domain protein (TRADD). Moreover, it induces NF-kappaB and the c-Jun N-terminal kinase 1 (JNK1) pathway. Thus, LMP1 appears to mimick the molecular functions of TNFR1. However, TNFR1 elicits a wide range of cellular responses including apoptosis, whereas LMP1 constitutes a transforming protein. Here we mapped the JNK1 activator region (JAR) of the LMP1 molecule. JAR overlaps with the TRADD-binding domain of LMP1. In contrast to TNFR1, LMP1 recruits TRADD via the TRADD N-terminus but not the TRADD death domain. Consequently, the molecular function of TRADD in LMP1 signaling differs from its role in TNFR1 signal transduction. Whereas NF-kappaB activation by LMP1 was blocked by a dominant-negative TRADD mutant, LMP1 induces JNK1 independently of the TRADD death domain and TRAF2, which binds to TRADD. Further downstream, JNK1 activation by TNFR1 involves Cdc42, whereas LMP1 signaling to JNK1 is independent of p21 Rho-like GTPases. Although both LMP1 and TNFR1 interact with TRADD and TRAF2, the different topologies of the signaling complexes correlate with substantial differences between LMP1 and TNFR1 signal transduction to JNK1.     

Role of tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2) in distinct and overlapping CD40 and TNF receptor 2/CD120b-mediated B lymphocyte activation

Members of the tumor necrosis factor receptor (TNFR) family play a variety of roles in the regulation of lymphocyte activation. An important TNFR family member for B cell activation is CD40. CD40 signals stimulate B cell TNF-alpha secretion, which subsequently signals via TNFR2 (CD120b) to enhance B cell activation. Although the function of the pro-apoptotic and pro-inflammatory receptor TNFR1 (CD120a) has been the subject of much research, less is understood about the distinct contributions of CD120b to cell activation and how it stimulates downstream events. Members of the tumor necrosis factor receptor family bind various members of the cytoplasmic adapter protein family, the tumor necrosis factor receptor-associated factors (TRAFs), during signaling. Both CD40 and CD120b bind TNF receptor-associated factor 2 (TRAF2) upon ligand stimulation. Wild type and TRAF2-deficient B cells expressing CD40 or the hybrid molecule (human) CD40 (mouse)-CD120b were examined. CD40- and CD120b-mediated IgM secretion were partly TRAF2-dependent, but only CD40 required TRAF2 for c-Jun N-terminal kinase activation. CD40 and CD120b used primarily divergent mechanisms to activate NF-kappaB, exemplifying how TNFR family members can use diverse mechanisms to mediate similar downstream events.     

LMP1 protein from the Epstein-Barr virus is a structural CD40 decoy in B lymphocytes for binding to TRAF3

Epstein-Barr virus is a human herpesvirus that causes infectious mononucleosis and lymphoproliferative malignancies. LMP1 (latent membrane protein-1), which is encoded by this virus and which is essential for transformation of B lymphocytes, acts as a constitutively active mimic of the tumor necrosis factor receptor (TNFR) CD40. LMP1 is an integral membrane protein containing six transmembrane segments and a cytoplasmic domain at the C terminus that binds to intracellular TNFR-associated factors (TRAFs). TRAFs are intracellular co-inducers of downstream signaling from CD40 and other TNFRs, and TRAF3 is required for activation of B lymphocytes by LMP1. Cytoplasmic C-terminal activation region 1 of LMP1 bears a motif (PQQAT) that conforms to the TRAF recognition motif PVQET in CD40. In this study, we report the crystal structure of this portion of LMP1 C-terminal activation region-1 (204PQQATDD210) bound in complex with TRAF3. The PQQAT motif is bound in the same binding crevice on TRAF3 where CD40 is bound, providing a molecular mechanism for LMP1 to act as a CD40 decoy for TRAF3. The LMP1 motif is presented in the TRAF3 crevice as a close structural mimic of the PVQET motif in CD40, and the intermolecular contacts are similar. However, the viral protein makes a unique contact: a hydrogen bond network formed between Asp210 in LMP1 and Tyr395 and Arg393 in TRAF3. This intermolecular contact is not made in the CD40-TRAF3 complex. The additional hydrogen bonds may stabilize the complex and strengthen the binding to permit LMP1 to compete with CD40 for binding to the TRAF3 crevice, influencing downstream signaling to B lymphocytes and contributing to dysregulated signaling by LMP1.     

EB病毒潜伏膜蛋白1通过结合TRAFs调控NF-κB

为了探讨EB病毒潜伏膜蛋白1(LMP1)的致瘤机制,对鼻咽癌中LMP1激活重要的核转录因子NF-κB机制进行了研究.首先,采用免疫共沉淀-蛋白质印迹在稳定表达LMP1的鼻咽癌细胞系HNE2-LMP1中证实LMP1与TRAF1,2,3结合形成免疫共沉淀复合物,进一步以野生型LMP1及其三种突变体的鼻咽癌细胞系LMP1（野生型,wt）、HNE2-LMP1 del187～351(CTAR1缺失型)、HNE2-LMP1(1～231)(CTAR2缺失型)、HNE2-LMP1(1～187)(羧基端胞浆区缺失型)、HNE2-pSG5(空白载体型)为材料,结合NF-κB报道基因质粒（pGL2-NF-κB-luc）的荧光素酶活性表达分析NF-κB的活性,证实:较之母细胞, 野生型LMP1活化NF-κB达13.8倍, LMP1(1～187)几乎不活化NF-κB,LMP1(1～231)活化NF-κB达4.9倍, LMP1(del187～351)活化NF-κB达9.1倍;TRAF1过表达升高LMP1(wt)及LMP1(1～231)介导的NF-κB活性,而对LMP1(del 187～351)活化NF-κB无影响;TRAF3过表达或TRAF3负显性突变体抑制LMP1(wt)及LMP1(1～231)介导的NF-κB活性,而不影响LMP1(del 187～351)活化NF-κB; TRAF2过表达升高LMP1(wt)、LMP1 (1～231)及LMP1(del 187～351)介导的NF-κB活性.这些结果表明：鼻咽癌中LMP1通过TRAF1、TRAF2或TRAF3调控NF-κB,TRAF1和TRAF3主要通过CTAR1发挥作用,TRAF2的作用主要是通过CTAR1和CTAR2介导的.     

Molecular mechanisms of TNFR-associated factor 6 (TRAF6) utilization by the oncogenic viral mimic of CD40, latent membrane protein 1 (LMP1)

Latent membrane protein 1 (LMP1), encoded by Epstein-Barr virus, is required for EBV-mediated B cell transformation and plays a significant role in the development of posttransplant B cell lymphomas. LMP1 has also been implicated in exacerbation of autoimmune diseases such as systemic lupus erythematosus. LMP1 is a constitutively active functional mimic of the tumor necrosis factor receptor superfamily member CD40, utilizing tumor necrosis factor receptor-associated factor (TRAF) adaptor proteins to induce signaling. However, LMP1-mediated B cell activation is amplified and sustained compared with CD40. We have previously shown that LMP1 and CD40 use TRAFs 1, 2, 3, and 5 differently. TRAF6 is important for CD40 signaling, but the role of TRAF6 in LMP1 signaling in B cells is not clear. Although TRAF6 binds directly to CD40, TRAF6 interaction with LMP1 in B cells has not been characterized. Here we tested the hypothesis that TRAF6 is a critical regulator of LMP1 signaling in B cells, either as part of a receptor-associated complex and/or as a cytoplasmic adaptor protein. Using TRAF6-deficient B cells, we determined that TRAF6 was critical for LMP1-mediated B cell activation. Although CD40-mediated TRAF6-dependent signaling does not require the TRAF6 receptor-binding domain, we found that LMP1 signaling required the presence of this domain. Furthermore, TRAF6 was recruited to the LMP1 signaling complex via the TRAF1/2/3/5 binding site within the cytoplasmic domain of LMP1.     

Epstein-Barr virus latent membrane protein 1 induces expression of the epidermal growth factor receptor through effects on Bcl-3 and STAT3

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) activates multiple signaling pathways. Two regions, C-terminal-activating region 1 (CTAR1) and CTAR2, have been identified within the cytoplasmic carboxy terminal domain that activates NF-kappaB. CTAR2 activates the canonical NF-kappaB pathway, which includes p50/p65 complexes. CTAR1 can activate both the canonical and noncanonical pathways to produce multiple distinct NF-kappaB dimers, including p52/p50, p52/p65, and p50/p50. CTAR1 also uniquely upregulates the epidermal growth factor receptor (EGFR) in epithelial cells. Increased p50-Bcl-3 complexes have been detected by chromatin precipitation on the NF-kappaB consensus motifs within the egfr promoter in CTAR1-expressing epithelial cells and nasopharyngeal carcinoma cells. In this study, the mechanism responsible for the increase in Bcl-3 has been further investigated. The data indicate that LMP1-CTAR1 induces Bcl-3 mRNA and increases the nuclear translocation of both Bcl-3 and p50. LMP1-CTAR1 constitutively activates STAT3, and this activation was not due to the induction of interleukin 6 (IL-6). In LMP1-CTAR1-expressing cells, increased levels of activated STAT3 were detected by chromatin immunoprecipitation on STAT-binding sites located within both the promoter and the second intron of Bcl-3. A STAT3 inhibitor significantly reduced the activation of STAT3, as well as the CTAR1-mediated upregulation of Bcl-3 and EGFR. These data suggest that LMP1 activates distinct forms of NF-kappaB through multiple pathways. In addition to activating the canonical and noncanonical pathways, LMP1-CTAR1 constitutively activates STAT3 and increases Bcl-3. The increased nuclear Bcl-3 and p50 homodimer complexes positively regulate EGFR expression. These results indicate that LMP1 likely regulates distinct cellular genes by activating specific NF-kappaB pathways.     

The Epstein-Barr Virus Oncoprotein Latent Membrane Protein 1 Engages the Tumor Necrosis Factor Receptor-Associated Proteins TRADD and Receptor-Interacting Protein (RIP) but Does Not Induce Apoptosis or Require RIP for NF-κB Activation

A site in the Epstein-Barr virus (EBV) transforming protein LMP1 that constitutively associates with the tumor necrosis factor receptor 1 (TNFR1)-associated death domain protein TRADD to mediate NF-kappaB and c-Jun N-terminal kinase activation is critical for long-term lymphoblastoid cell proliferation. We now find that LMP1 signaling through TRADD differs from TNFR1 signaling through TRADD. LMP1 needs only 11 amino acids to activate NF-kappaB or synergize with TRADD in NF-kappaB activation, while TNFR1 requires approximately 70 residues. Further, LMP1 does not require TRADD residues 294 to 312 for NF-kappaB activation, while TNFR1 requires TRADD residues 296 to 302. LMP1 is partially blocked for NF-kappaB activation by a TRADD mutant consisting of residues 122 to 293. Unlike TNFR1, LMP1 can interact directly with receptor-interacting protein (RIP) and stably associates with RIP in EBV-transformed lymphoblastoid cell lines. Surprisingly, LMP1 does not require RIP for NF-kappaB activation. Despite constitutive association with TRADD or RIP, LMP1 does not induce apoptosis in EBV-negative Burkitt lymphoma or human embryonic kidney 293 cells. These results add a different perspective to the molecular interactions through which LMP1, TRADD, and RIP participate in B-lymphocyte activation and growth.