Inactivation of the Tumor Suppressor CYLD Sensitizes Mice to Breast Cancer Development

BACKGROUND/AIM

Breast cancer is a leading cause of cancer-related deaths among women. Down-regulation of the tumor suppressor gene Cyld in breast cancer has been linked to a poor prognosis. This study investigated the role of Cyld in breast cancer using conditional mutant mouse models carrying a Cyld mutation, which inactivates the deubiquitinating activity of its protein product CYLD in mammary epithelial cells.

MATERIALS AND METHODS

We examined the potential of CYLD inactivation to induce mammary tumors spontaneously or modify the susceptibility of mice to mammary tumorigenesis by DMBA treatment or ErbB2 over-expression.

RESULTS

CYLD inactivation significantly increased susceptibility to breast cancer induced by either DMBA treatment or ErbB2 over-expression. Moreover, while CYLD inactivation alone did not lead to spontaneous mammary tumorigenesis, it did contribute to the formation of multifocal hyperplastic lesions in virgin mice of predominantly FVB/NJ background.

CONCLUSION

Our study demonstrates the tumor enhancing potential of CYLD inactivation in mammary tumorigenesis in vivo and establishes novel relevant mouse models that can be exploited for developing prognostic and therapeutic protocols.

Inactivation of Tumor Suppressor CYLD Inhibits Fibroblast Reprogramming to Pluripotency

CYLD is a tumor suppressor gene coding for a deubiquitinating enzyme that has a critical regulatory function in a variety of signaling pathways and biological processes involved in cancer development and progression, many of which are also key modulators of somatic cell reprogramming. Nevertheless, the potential role of CYLD in this process has not been studied. With the dual aim of investigating the involvement of CYLD in reprogramming and developing a better understanding of the intricate regulatory system governing this process, we reprogrammed control (CYLDWT/WT) and CYLD DUB-deficient (CYLDΔ9/Δ9) mouse embryonic fibroblasts (MEFs) into induced pluripotent stem cells (iPSCs) through ectopic overexpression of the Yamanaka factors (Oct3/4, Sox2, Klf4, c-myc). CYLD DUB deficiency led to significantly reduced reprogramming efficiency and slower early reprogramming kinetics. The introduction of WT CYLD to CYLDΔ9/Δ9 MEFs rescued the phenotype. Nevertheless, CYLD DUB-deficient cells were capable of establishing induced pluripotent colonies with full spontaneous differentiation potential of the three germ layers. Whole proteome analysis (Data are available via ProteomeXchange with identifier PXD044220) revealed that the mesenchymal-to-epithelial transition (MET) during the early reprogramming stages was disrupted in CYLDΔ9/Δ9 MEFs. Interestingly, differentially enriched pathways revealed that the primary processes affected by CYLD DUB deficiency were associated with the organization of the extracellular matrix and several metabolic pathways. Our findings not only establish for the first time CYLD's significance as a regulatory component of early reprogramming but also highlight its role as an extracellular matrix regulator, which has profound implications in cancer research.

Deubiquitination of NLRP6 by Cyld critically regulates colonic inflammation

The inflammasome NLRP6 plays a crucial role in regulating colonic inflammation and cancer. However, the molecular mechanisms by which NLRP6 function is inhibited to prevent excessive inflammation remain unclear. Here, we demonstrate that the deubiquitinase Cyld prevents excessive interleukin 18 (IL-18) production in the colonic mucosa by deubiquitinating NLRP6. We show that deubiquitination inhibited the NLRP6-ASC inflammasome complex and regulated the maturation of IL-18. Cyld deficiency in mice resulted in elevated levels of active IL-18 and severe colonic inflammation and increased colon cancer. Further, in patients with ulcerative colitis, the concentration of active IL-18 was inversely correlated with CYLD expression. Thus, we have identified a novel regulatory mechanism that inhibits the NLRP6-IL-18 pathway in colonic inflammation. (Nat Immunol. 2020. 21(6):626–635. PMID: 32424362)

HDAC1/2 Inhibitor Romidepsin Suppresses DEN-Induced Hepatocellular Carcinogenesis in Mice

Background

Hepatocellular carcinoma (HCC) is a frequently diagnosed cancer and a leading cause of cancer-related death worldwide. Its rapid progression, combined with the limited treatment options at late stages, imposes the need for early detection and aggressive intervention. Based on the knowledge that hepatocarcinogenesis is significantly influenced by histone acetylation, we directed our search for novel HCC therapeutics among histone deacetylation inhibitors (HDACi). The aim of the present study was to investigate the effect of HDAC1/2 inhibitor Romidepsin in the well-established mouse model of diethylnitrosamine (DEN)-induced HCC.

Materials and Methods

C56BL/6 mice were treated with Romidepsin at the critical point of 10 months after DEN challenge and their livers were examined 2 months later using histopathology and morphometry. Protein levels were assessed in serum using ELISA and in liver tissues using Western blot and immunohistochemistry (in-situ detection). Gene expression was quantified using real-time PCR.

Results

Romidepsin suppressed cancer progression. This effect was associated with decreased proliferation and increased apoptosis of cancer cells. The cell cycle regulator CK2a, the anti-inflammatory molecule PPAR-γ, and the tumor suppressors PTEN and CYLD were upregulated in treated HCC. By contrast, the expression of PI3K, NF-κB p65 and c-Jun was reduced. In line with this result, the levels of two major apoptosis regulators, ie, BAD and the multifunctional protein c-Met, were lower in the blood serum of treated mice compared to the untreated mice with HCC.

Conclusion

These findings suggest that Romidepsin, a drug currently used in the treatment of lymphoma, could also be considered in the management of early-stage HCC.

Deubiquitination of NLRP6 inflammasome by Cyld critically regulates intestinal inflammation

The inflammasome NLRP6 plays a crucial role in regulating inflammation and host defense against microorganisms in the intestine. However, the molecular mechanisms by which NLRP6 function is inhibited to prevent excessive inflammation remain unclear. Here, we demonstrate that the deubiquitinase Cyld prevents excessive interleukin 18 (IL-18) production in the colonic mucosa by deubiquitinating NLRP6. We show that deubiquitination inhibited the NLRP6-ASC inflammasome complex and regulated the maturation of IL-18. Cyld deficiency in mice resulted in elevated levels of active IL-18 and severe colonic inflammation following Citrobacter rodentium infection. Further, in patients with ulcerative colitis, the concentration of active IL-18 was inversely correlated with CYLD expression. Thus, we have identified a novel regulatory mechanism that inhibits the NLRP6-IL-18 pathway in intestinal inflammation.

Functional analysis of the C. elegans cyld-1 gene reveals extensive similarity with its human homolog

The human cylindromatosis tumor suppressor (HsCyld) has attracted extensive attention due to its association with the development of multiple types of cancer. HsCyld encodes a deubiquitinating enzyme (HsCYLD) with a broad range of functions that include the regulation of several cell growth, differentiation and death pathways. HsCyld is an evolutionarily conserved gene. Homologs of HsCyld have been identified in simple model organisms such as Drosophila melanogaster and Caenorhabditis elegans (C. elegans) which offer extensive possibilities for functional analyses. In the present report we have investigated and compared the functional properties of HsCYLD and its C. elegans homolog (CeCYLD). As expected from the mammalian CYLD expression pattern, the CeCyld promoter is active in multiple tissues with certain gastrointestinal epithelia and neuronal cells showing the most prominent activity. CeCYLD is a functional deubiquitinating enzyme with similar specificity to HsCYLD towards K63- and M1-linked polyubiquiting chains. CeCYLD was capable of suppressing the TRAF2-mediated activation of NF-kappaB and AP1 similarly to HsCYLD. Finally, CeCYLD could suppress the induction of TNF-dependent gene expression in mammalian cells similarly to HsCYLD. Our results demonstrate extensively overlapping functions between the HsCYLD and CeCYLD, which establish the C. elegans protein as a valuable model for the elucidation of the complex activity of the human tumor suppressor protein.

Down-regulation of the Tumor Suppressor CYLD Enhances the Transformed Phenotype of Human Breast Cancer Cells

BACKGROUND/AIM

The cylindromatosis tumor suppressor (CYLD) has been implicated in the inhibition of human breast cancer development by virtue of the poor prognosis of patients with down-regulated CYLD expression. In order to investigate the mechanism of breast cancer suppression by CYLD, in the present study, cellular and molecular aspects of CYLD-dependent phenotypic regulation of different types of human breast cancer cell lines were analyzed.

MATERIALS AND METHODS

CYLD expression was down-regulated by RNA interference in human breast cancer cell lines. Parental and CYLD-deficient cell lines were evaluated for their viability, migratory capacity, anchorage-independent growth and chemoresistance. Wild-type and mutated forms of CYLD were also evaluated for their ability to suppress the clonogenic potential of breast cancer cells.

RESULTS

CYLD down-regulation enhanced the survival and migratory properties of basal and luminal breast cancer cell lines. In addition, down-regulation of CYLD expression enhanced the ability of human breast cancer cells to grow in an anchorage-independent manner and could be associated with resistance to chemotherapeutic drugs. The growth-suppressive properties of CYLD on breast cancer cell lines were dependent on its de-ubiquitinating activity and its amino terminal cytoskeleton-interacting region.

CONCLUSION

Our results establish a broad range of tumor-suppressive properties that are conferred by CYLD in basal and luminal human breast cancer cells and support the significance of targeted de-ubiquitination by CYLD in breast cancer cell growth suppression.

The expression of tumor suppressor gene Cyld is upregulated by histone deacetylace inhibitors in human hepatocellular carcinoma cell lines

CYLD is a deubiquitinating enzyme that exerts a tumor suppressive function. Its downregulation or inactivation has been associated with the development of several types of malignancies including hepatocellular carcinoma (HCC). HCC cells display significantly lower Cyld expression compared to primary human hepatocytes, and Cyld downregulation can contribute to apoptotic resistance of HCC cells. Little is known about the mechanism of Cyld downregulation in human HCC cells. In the present study we explored the possible regulation of Cyld expression by histone deacetylases (HDACs) in human HCC cell lines. We demonstrated that the HDAC inhibitors suberoylanilide hydroxamic acid, sodium butyrate, and trichostatin A induced the upregulation of both mRNA and protein levels of CYLD in two different HCC cell lines, HepG2 and Huh7. Our results demonstrate the involvement of HDACs in the downregulation of Cyld expression in HCC cells and support and may improve the use of HDAC inhibitors for the treatment for HCC.

Epidermal CYLD inactivation sensitizes mice to the development of sebaceous and basaloid skin tumors

The deubiquitinase-encoding gene Cyld displays a dominant genetic linkage to a wide spectrum of skin-appendage tumors, which could be collectively designated as CYLD mutant-syndrome (CYLD^m-syndrome). Despite recent advances, little is understood about the molecular mechanisms responsible for this painful and difficult-to-treat skin disease. Here, we generated a conditional mouse model with epidermis-targeted expression of a catalytically deficient CYLD^m through K14-Cre-mediated deletion of exon 9 (hereafter refer to Cyld^EΔ9/Δ9 ). Cyld^EΔ9/Δ9 mice were born alive but developed hair and sebaceous gland abnormalities and dental defects at 100% and 60% penetrance, respectively. Upon topical challenge with DMBA/TPA, these animals primarily developed sebaceous and basaloid tumors resembling human CYLD^m-syndrome as opposed to papilloma, which is most commonly induced in WT mice by this treatment. Molecular analysis revealed that TRAF6-K63-Ubiquitination (K63-Ub), c-Myc-K63-Ub, and phospho-c-Myc (S62) were markedly elevated in Cyld^EΔ9/Δ9 skin. Topical treatment with a pharmacological c-Myc inhibitor induced sebaceous and basal cell apoptosis in Cyld^EΔ9/Δ9 skin. Consistently, c-Myc activation was readily detected in human cylindroma and sebaceous adenoma. Taken together, our findings demonstrate that Cyld^EΔ9/Δ9 mice represent a disease-relevant animal model and identify TRAF6 and c-Myc as potential therapeutic targets for CYLD^m-syndrome.

The PP4R1 subunit of protein phosphatase PP4 targets TRAF2 and TRAF6 to mediate inhibition of NF-κB activation

TRAFs constitute a family of proteins that have been implicated in signal transduction by immunomodulatory cellular receptors and viral proteins. TRAF2 and TRAF6 have an E3-ubiquitin ligase activity, which is dependent on the integrity of their RING finger domain and it has been associated with their ability to activate the NF-κB and AP1 signaling pathways. A yeast two-hybrid screen with TRAF2 as bait, identified the regulatory subunit PP4R1 of protein phosphatase PP4 as a TRAF2-interacting protein. The interaction of TRAF2 with PP4R1 depended on the integrity of the RING finger domain of TRAF2. PP4R1 could interact also with the TRAF2-related factor TRAF6 in a RING domain-dependent manner. Exogenous expression of PP4R1 inhibited NF-κB activation by TRAF2, TRAF6, TNF and the Epstein-Barr virus oncoprotein LMP1. In addition, expression of PP4R1 downregulated IL8 induction by LMP1, whereas downregulation of PP4R1 by RNA interference enhanced the induction of IL8 by LMP1 and TNF. PP4R1 could mediate the dephosphorylation of TRAF2 Ser11, which has been previously implicated in TRAF2-mediated activation of NF-κB. Finally, PP4R1 could inhibit TRAF6 polyubiquitination, suggesting an interference with the E3 ubiquitin ligase activity of TRAF6. Taken together, our data identify a novel mechanism of NF-κB pathway inhibition which is mediated by PP4R1-dependent targeting of specific TRAF molecules.

Protein kinase N1, a cell inhibitor of Akt kinase, has a central role in quality control of germinal center formation

Germinal centers (GCs) are specialized microenvironments in secondary lymphoid organs where high-affinity antibody-producing B cells are selected based on B-cell antigen receptor (BCR) signal strength. BCR signaling required for normal GC selection is uncertain. We have found that protein kinase N1 (PKN1, also known as PRK1) negatively regulates Akt kinase downstream of the BCR and that this regulation is necessary for normal GC development. PKN1 interacted with and inhibited Akt1 kinase and transforming activities. Pkn1(-/-) B cells were hyperresponsive and had increased phosphorylated Akt1 levels upon BCR stimulation. In the absence of immunization or infection, Pkn1(-/-) mice spontaneously formed GCs and developed an autoimmune-like disease with age, which was characterized by autoantibody production and glomerulonephritis. More B cells, with fewer somatic BCR gene V region hypermutations were selected in Pkn1(-/-) GCs. These results indicate that PKN1 down-regulation of BCR-activated Akt activity is critical for normal GC B-cell survival and selection.

Inactivation of the deubiquitinase CYLD in hepatocytes causes apoptosis, inflammation, fibrosis, and cancer

The tumor suppressor cylindromatosis (CYLD) inhibits the NFκB and mitogen-activated protein kinase (MAPK) activation pathways by deubiquitinating upstream regulatory factors. Here we show that liver-specific disruption of CYLD triggers hepatocyte cell death in the periportal area via spontaneous and chronic activation of TGF-β activated kinase 1 (TAK1) and c-Jun N-terminal kinase (JNK). This is followed by hepatic stellate cell and Kupffer cell activation, which promotes progressive fibrosis, inflammation, tumor necrosis factor (TNF) production, and expansion of hepatocyte apoptosis toward the central veins. At later stages, compensatory proliferation results in the development of cancer foci featuring re-expression of oncofetal hepatic and stem cell-specific genes. The results demonstrate that, in the liver, CYLD acts as an important regulator of hepatocyte homeostasis, protecting cells from spontaneous apoptosis by preventing uncontrolled TAK1 and JNK activation.

Genomic analysis reveals a novel nuclear factor-κB (NF-κB)-binding site in Alu-repetitive elements

The transcription factor NF-κB is a critical regulator of immune responses. To determine how NF-κB builds transcriptional control networks, we need to obtain a topographic map of the factor bound to the genome and correlate it with global gene expression. We used a ChIP cloning technique and identified novel NF-κB target genes in response to virus infection. We discovered that most of the NF-κB-bound genomic sites deviate from the consensus and are located away from conventional promoter regions. Remarkably, we identified a novel abundant NF-κB-binding site residing in specialized Alu-repetitive elements having the potential for long range transcription regulation, thus suggesting that in addition to its known role, NF-κB has a primate-specific function and a role in human evolution. By combining these data with global gene expression profiling of virus-infected cells, we found that most of the sites bound by NF-κB in the human genome do not correlate with changes in gene expression of the nearby genes and they do not appear to function in the context of synthetic promoters. These results demonstrate that repetitive elements interspersed in the human genome function as common target sites for transcription factors and may play an important role in expanding the repertoire of binding sites to engage new genes into regulatory networks.

Truncation of the deubiquitinating domain of CYLD in myelomonocytic cells attenuates inflammatory responses

The cylindromatosis tumor suppressor (CYLD) is a deubiquitinating enzyme that has been implicated in various aspects of adaptive and innate immune responses. Nevertheless, the role of CYLD in the function of specific types of immune cells remains elusive. In this report we have used conditional gene targeting in mice to address the role of the deubiquitinating activity of CYLD in the myelomonocytic lineage. Truncation of the deubiquitinating domain of CYLD specifically in myelomonocytic cells impaired the development of lethal LPS-induced endotoxic shock and the accumulation of thioglycollate-elicited peritoneal macrophages. Our data establish CYLD as a regulator of monocyte-macrophage activation in response to inflammatory stimuli and identify it as a potential target for therapeutic intervention in relevant inflammatory disorders in humans.

Glutaredoxin-1 regulates TRAF6 activation and the IL-1 receptor/TLR4 signalling

Glutaredoxin-1 (GRX-1) is a cytoplasmic enzyme that highly contributes to the antioxidant defense system. It catalyzes the reversible reduction of glutathione-protein mixed disulfides, a process called deglutathionylation. Here, we investigated the role of GRX-1 in the pathway triggered by interleukin-1/Toll-like receptor 4 (IL-1R/TLR4) by using RNA interference (RNAi) in HEK293 and HeLa cells. TNF receptor-associated factor 6 (TRAF6) is an intermediate signalling molecule involved in the signal transduction by members of the interleukin-1/Toll-like receptor (IL-1R/TLR) family. TRAF6 has an E3 ubiquitin ligase activity which depends on the integrity of an amino-terminal really interesting new gene (RING) finger motif. Upon receptor activation, TRAF6 undergoes K63-linked auto-polyubiquitination which mediates protein-protein interactions and signal propagation. Our data showed that IL-1R and TLR4-mediated NF-κB induction was severely reduced in GRX-1 knockdown cells. We found that the RING-finger motif of TRAF6 is S-glutathionylated under normal conditions. Moreover, upon IL-1 stimulation TRAF6 undergoes deglutathionylation catalyzed by GRX-1. The deglutathionylation of TRAF6 is essential for its auto-polyubiquitination and subsequent activation. Taken together, our findings reveal another signalling molecule affected by S-glutathionylation and uncover a crucial role for GRX-1 in the TRAF6-dependent activation of NF-κB by IL-1R/TLRs.

A Drosophila ortholog of the human cylindromatosis tumor suppressor gene regulates triglyceride content and antibacterial defense

The cylindromatosis (CYLD) gene is mutated in human tumors of skin appendages. It encodes a deubiquitylating enzyme (CYLD) that is a negative regulator of the NF-kappaB and JNK signaling pathways, in vitro. However, the tissue-specific function and regulation of CYLD in vivo are poorly understood. We established a genetically tractable animal model to initiate a systematic investigation of these issues by characterizing an ortholog of CYLD in Drosophila. Drosophila CYLD is broadly expressed during development and, in adult animals, is localized in the fat body, ovaries, testes, digestive tract and specific areas of the nervous system. We demonstrate that the protein product of Drosophila CYLD (CYLD), like its mammalian counterpart, is a deubiquitylating enzyme. Impairment of CYLD expression is associated with altered fat body morphology in adult flies, increased triglyceride levels and increased survival under starvation conditions. Furthermore, flies with compromised CYLD expression exhibited reduced resistance to bacterial infections. All mutant phenotypes described were reversible upon conditional expression of CYLD transgenes. Our results implicate CYLD in a broad range of functions associated with fat homeostasis and host defence in Drosophila.

Constitutive CD40 signaling phenocopies the transforming function of the Epstein-Barr virus oncoprotein LMP1 in vitro

The oncoprotein LMP1 mimics an activated CD40 receptor, yet it is not known whether constitutive CD40 signaling, like LMP1, is sufficient to transform cells. Here we demonstrate that constitutive activation of the CD40 pathway by a chimeric LMP1CD40 molecule resembles the transforming function of LMP1 in inducing loss of contact inhibition and anchorage independent growth of Rat1 fibroblasts. Rat1 transformation correlates with the expression level of LMP1CD40 and depends on its ability to oligomerize. Our data provide direct evidence for the oncogenic potential of the CD40 signaling pathway, which is also established as a model-mechanism for LMP1-induced transformation.

Human ubiquitin specific protease 31 is a deubiquitinating enzyme implicated in activation of nuclear factor-κB

TRAF2 mediates activation of the transcription factors NF-kappaB and AP1 by TNF. A yeast two-hybrid screen of a human cDNA library identified a ubiquitin specific protease homologue (USP31) as a TRAF2-interacting protein. Two cDNAs encoding for USP31 were identified. One cDNA encodes a 1035-amino acid long isoform of USP31 (USP31, long isoform) and the other a 485-amino acid long isoform of USP31 (USP31S1, short isoform). USP31 and USP31S1 share a common amino terminal region with homology to the catalytic region of known deubiquitinating enzymes. Enzymatic assays demonstrated that USP31 but not USP31S1 possess deubiquitinating activity. Furthermore, it was shown that USP31 has a higher activity towards lysine-63-linked as compared to lysine-48-linked polyubiquitin chains. Overexpression of USP31 in HEK 293T cells inhibited TNFalpha, CD40, LMP1, TRAF2, TRAF6 and IKKbeta-mediated NF-kappaB activation, but did not inhibit Smad-mediated transcription activation. In addition, both USP31 isoforms interact with p65/RelA. Our data support a role for USP31 in the regulation of NF-kappaB activation by members of the TNF receptor superfamily.

The BRG1- and hBRM-associated factor BAF57 induces apoptosis by stimulating expression of the cylindromatosis tumor suppressor gene

Mutation of BRG1, hBRM, and their associated factors, INI1 and BAF57, in primary human tumors has suggested that inactivation of human SWI/SNF (hSWI/SNF) complexes may be involved in neoplastic transformation. BT549 is an invasive human breast carcinoma cell line that lacks expression of BAF57, a key hSWI/SNF subunit that mediates interaction with transcriptional activators and corepressors. In this study we investigated the role of BAF57 in suppressing tumorigenesis by establishing BT549 stable cell lines that expresses full-length BAF57 protein. BT549 clones expressing BAF57 demonstrated marked phenotypic changes, slow growth kinetics, and restoration of contact inhibition. Altered growth was found to be due in part to cell cycle arrest and induction of apoptosis. Furthermore, microarray analysis revealed that BAF57-mediated cell death was associated with up-regulation of proapoptotic genes including the tumor suppressor familial cylindromatosis (CYLD), which was found to be a direct target of BAF57 as determined by chromatin immunoprecipitation analysis. Increased expression of CYLD in BT549 cells induced apoptosis, while its suppression by small interfering RNA inhibited cell death in BAF57 expressing BT549 cells. These findings demonstrate the importance of BAF57 in cell growth regulation and provide a novel link between hSWI/SNF chromatin remodelers and apoptosis.

Induction of apoptosis by rewiring the signal transduction of Epstein-Barr virus oncoprotein LMP1 toward caspase activation

The Epstein-Barr virus latent membrane protein 1 (LMP1) is an oncoprotein which mimics activated tumor necrosis factor receptor family members. Here we demonstrate the principle that an inducible association of the LMP1 cytoplasmic carboxyl terminus with caspase-8 by a heterodimerizing agent causes apoptosis. This process depends on the catalytic activity of caspase-8 and the ability of LMP1 to oligomerize constitutively at the plasma membrane. Our data indicate that chemical inducers of the association of the LMP1 carboxyl terminus with caspase-8 can kill LMP1-expressing cells selectively. Such compounds could be used as chemotherapeutic agents for LMP1-associated malignancies.

Comparative analysis of signal transduction by CD40 and the Epstein-Barr virus oncoprotein LMP1 in vivo

There is much evidence, based primarily on in vitro studies, indicating that the Epstein-Barr virus oncoprotein latent membrane protein 1 (LMP1) mimics an activated CD40 receptor. In order to investigate the extent of similarity between LMP1 and CD40 functions in vivo, we analyzed the cytoplasmic signaling properties of LMP1 and CD40 in B cells in a directly comparable manner. For this purpose, we generated transgenic mice expressing either LMP1 or a chimeric LMP1CD40 molecule, which constitutively activates the CD40 pathway, under the control of the CD19 promoter. LMP1 and LMP1CD40 were expressed at similar levels in a B-lymphocyte-specific manner. Similar to LMP1, LMP1CD40 suppressed germinal center (GC) formation and antibody production in response to thymus-dependent antigens, albeit to a greater extent than LMP1. Furthermore, the avidity of the antibodies produced against thymus-dependent antigens was lower for LMP1CD40 transgenic mice than for wild-type and LMP1 transgenic mice. GC suppression was linked to the ability of LMP1CD40 and LMP1 to downregulate mRNA and protein levels of BCL6 and to suppress the activity of the BCL6 promoter. In contrast to LMP1, LMP1CD40 caused an upregulation of CD69, CD80, and CD86 in B cells and a dramatic increase in serum immunoglobulin M. In addition, LMP1CD40 but not LMP1 transgenic mice had elevated numbers of marginal-zone B cells and increased populations of polymorphonuclear cells and/or neutrophils. Consistent with these findings, LMP1CD40 but not LMP1 transgenic mice showed signs of spontaneous inflammatory reactions and the potential for autoimmunity.

NF-κB Is Essential for Induction of CYLD, the Negative Regulator of NF-κB

The transcription factor NF-kappaB regulates genes involved in inflammatory and immune responses, tumorigenesis, and apoptosis. In contrast to the pleiotropic stimuli that lead to its positive regulation, the known signaling mechanisms that underlie the negative regulation of NF-kappaB are very few. Recent studies have identified the tumor suppressor CYLD, loss of which causes a benign human syndrome called cylindromatosis, as a key negative regulator for NF-kappaB signaling by deubiquitinating tumor necrosis factor (TNF) receptor-associated factor (TRAF) 2, TRAF6, and NEMO (NF-kappaB essential modulator, also known as IkappaB kinase gamma). However, how CYLD is regulated remains unknown. The present study revealed a novel autoregulatory feedback pathway through which activation of NF-kappaB by TNF-alpha and bacterium nontypeable Haemophilus influenzae (NTHi) induces CYLD that in turn leads to the negative regulation of NF-kappaB signaling. In addition, TRAF2 and TRAF6 appear to be differentially involved in NF-kappaB-dependent induction of CYLD by TNF-alpha and NTHi. These findings provide novel insights into the autoregulation of NF-kappaB activation.

Epstein-Barr virus latent membrane protein 1 activation of NF-kappaB through IRAK1 and TRAF6

Epstein-Barr virus latent membrane protein 1 (LMP1) activation of NF-kappaB is critical for Epstein-Barr virus-infected B lymphocyte survival. LMP1 activates the IkappaB kinase complex and NF-kappaB through two cytoplasmic signaling domains that engage tumor necrosis factor receptor-associated factor (TRAF)1/2/3/5 or TRADD and RIP. We now use cells lacking expression of TRAF2, TRAF5, TRAF6, IKKalpha, IKKbeta, IKKgamma, TAB2, IL-1 receptor-associated kinase (IRAK)1, or IRAK4 to assess their roles in LMP1-mediated NF-kappaB activation. LMP1-induced RelA nuclear translocation was similar in IKKalpha knockout (KO) and WT murine embryo fibroblasts (MEFs) but substantially deficient in IKKbeta KO MEFs. NF-kappaB-dependent promoter responses were also substantially deficient in IKKbeta KO MEFs but were hyperactive in IKKalpha KO MEFs. More surprisingly, NF-kappaB responses were near normal in TRAF2 and TRAF5 double-KO MEFs, IKKgamma KO MEFs, TAB2 KO MEFs, and IRAK4 KO MEFs but were highly deficient in TRAF6 KO MEFs and IRAK1 KO HEK293 cells. Consistent with the importance of TRAF6, LMP1-induced NF-kappaB activation in HEK293 cells was inhibited by expression of dominant-negative TAB2 and Ubc13 alleles. These data extend a role for IKKalpha in IKKbeta regulation, identify an unusual IKKbeta-dependent and IKKgamma-independent NF-kappaB activation, and indicate that IRAK1 and TRAF6 are essential for LMP1-induced NF-kappaB activation.

CYLD is a deubiquitinating enzyme that negatively regulates NF-kappaB activation by TNFR family members

Familial cylindromatosis is an autosomal dominant predisposition to tumours of skin appendages called cylindromas. Familial cylindromatosis is caused by mutations in a gene encoding the CYLD protein of previously unknown function. Here we show that CYLD is a deubiquitinating enzyme that negatively regulates activation of the transcription factor NF-kappaB by specific tumour-necrosis factor receptors (TNFRs). Loss of the deubiquitinating activity of CYLD correlates with tumorigenesis. CYLD inhibits activation of NF-kappaB by the TNFR family members CD40, XEDAR and EDAR in a manner that depends on the deubiquitinating activity of CYLD. Downregulation of CYLD by RNA-mediated interference augments both basal and CD40-mediated activation of NF-kappaB. The inhibition of NF-kappaB activation by CYLD is mediated, at least in part, by the deubiquitination and inactivation of TNFR-associated factor 2 (TRAF2) and, to a lesser extent, TRAF6. These results indicate that CYLD is a negative regulator of the cytokine-mediated activation of NF-kappaB that is required for appropriate cellular homeostasis of skin appendages.

Inhibition of transforming growth factor beta signaling and Smad-dependent activation of transcription by the Latent Membrane Protein 1 of Epstein-Barr virus

Inhibition of transforming growth factor beta (TGFbeta) signaling by the Epstein-Barr virus Latent Membrane Protein 1 (LMP1) may account, at least in part, for the oncogenic activity of LMP1. We found that LMP1 is a potent inhibitor of TGFbeta signaling and Smad-dependent activation of transcription in 293 epithelial cells and COS-7 fibroblasts. LMP1 strongly inhibited the uninduced and the Smad-inducible activity of the promoters of the human p21/WAF1/Cip1 gene and the mouse Smad7 gene. Inhibition of TGFbeta signaling and Smad-dependent activation of transcription by LMP1 was greatly reduced by deletion of both C-terminal activating regions 1 and 2 of LMP1 as well as by overexpression of a non-degradable form of IkappaB. In contrast, specific inhibitors of p38 kinase or MEK kinase did not reverse the inhibitory activity of LMP1. TGFbeta signaling was enhanced by overexpression of dominant negative forms of the LMP1 effectors TRAF2, NIK, and IKKbeta and was abolished by overexpression of p65/RelA or a p50/p65 fusion protein. Deletion of the transactivation domain of p65 abolished its inhibitory activity. Immunoblotting and immunofluorescence microscopy indicated that suppression of TGFbeta signaling and Smad transcriptional activity by LMP1 was not due to Smad degradation or cytoplasmic retention suggesting that LMP1 affects the nuclear function of Smad proteins. Our data are consistent with an essential role of NF-kappaB activation by LMP1 in the inhibition of TGFbeta signaling and Smad-mediated transcriptional responses.

Cellular signaling pathways engaged by the Epstein-Barr virus transforming protein LMP1

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is an essential component of the viral machinery that orchestrates cellular transformation and oncogenesis. The critical role of LMP1 in transformation has been established through recombinant genetic analysis of the EBV genome, ectopic expression in cell lines and transgenic mice and immunohistochemical analysis of EBV- associated tumor specimens. The principal mechanism of LMP1 function is based on mimicry of activated cell surface receptors of the tumor necrosis factor superfamily. LMP1 signaling culminates in the activation of transcription factors NF-kappaB, AP1 and STAT1/3, which have been tightly linked to prevention of apoptosis and malignant transformation. The molecular mechanisms of LMP1 function will be reviewed in this report.

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-κB, 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-κB 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-κB 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.

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.