TRAF6 is a critical regulator of LMP1 functions in vivo

Epstein-Barr virus-driven B cell lymphoma mediated by a direct LMP1-TRAF6 complex

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) drives viral B cell transformation and oncogenesis. LMP1's transforming activity depends on its C-terminal activation region 2 (CTAR2), which induces NF-κB and JNK by engaging TNF receptor-associated factor 6 (TRAF6). The mechanism of TRAF6 recruitment to LMP1 and its role in LMP1 signalling remains elusive. Here we demonstrate that TRAF6 interacts directly with a viral TRAF6 binding motif within CTAR2. Functional and NMR studies supported by molecular modeling provide insight into the architecture of the LMP1-TRAF6 complex, which differs from that of CD40-TRAF6. The direct recruitment of TRAF6 to LMP1 is essential for NF-κB activation by CTAR2 and the survival of LMP1-driven lymphoma. Disruption of the LMP1-TRAF6 complex by inhibitory peptides interferes with the survival of EBV-transformed B cells. In this work, we identify LMP1-TRAF6 as a critical virus-host interface and validate this interaction as a potential therapeutic target in EBV-associated cancer.

Characterization of target gene regulation by the two Epstein-Barr virus oncogene LMP1 domains essential for B-cell transformation

IMPORTANCE

Epstein-Barr virus (EBV) causes multiple human cancers, including B-cell lymphomas. In cell culture, EBV converts healthy human B-cells into immortalized ones that grow continuously, which model post-transplant lymphomas. Constitutive signaling from two cytoplasmic tail domains of the EBV oncogene latent membrane protein 1 (LMP1) is required for this transformation, yet there has not been systematic analysis of their host gene targets. We identified that only signaling from the membrane proximal domain is required for survival of these EBV-immortalized cells and that its loss triggers apoptosis. We identified key LMP1 target genes, whose abundance changed significantly with loss of LMP1 signals, or that were instead upregulated in response to switching on signaling by one or both LMP1 domains in an EBV-uninfected human B-cell model. These included major anti-apoptotic factors necessary for EBV-infected B-cell survival. Bioinformatics analyses identified clusters of B-cell genes that respond differently to signaling by either or both domains.

Characterization of Target Gene Regulation by the Two Epstein-Barr Virus Oncogene LMP1 Domains Essential for B-cell Transformation

The Epstein-Barr virus (EBV) oncogene latent membrane protein 1 (LMP1) mimics CD40 signaling and is expressed by multiple malignancies. Two LMP1 C-terminal cytoplasmic tail regions, termed transformation essential sites (TES) 1 and 2, are critical for EBV transformation of B lymphocytes into immortalized lymphoblastoid cell lines (LCL). However, TES1 versus TES2 B-cell target genes have remained incompletely characterized, and whether both are required for LCL survival has remained unknown. To define LCL LMP1 target genes, we profiled transcriptome-wide effects of acute LMP1 CRISPR knockout (KO) prior to cell death. To then characterize specific LCL TES1 and TES2 roles, we conditionally expressed wildtype, TES1 null, TES2 null or double TES1/TES2 null LMP1 alleles upon endogenous LMP1 KO. Unexpectedly, TES1 but not TES2 signaling was critical for LCL survival. The LCL dependency factor cFLIP, which plays obligatory roles in blockade of LCL apoptosis, was highly downmodulated by loss of TES1 signaling. To further characterize TES1 vs TES2 roles, we conditionally expressed wildtype, TES1 and/or TES2 null LMP1 alleles in two Burkitt models. Systematic RNAseq analyses revealed gene clusters that responded more strongly to TES1 versus TES2, that respond strongly to both or that are oppositely regulated. Robust TES1 effects on cFLIP induction were again noted. TES1 and 2 effects on expression of additional LCL dependency factors, including BATF and IRF4, and on EBV super-enhancers were identified. Collectively, these studies suggest a model by which LMP1 TES1 and TES2 jointly remodel the B-cell transcriptome and highlight TES1 as a key therapeutic target.

The immunological significance of tumor necrosis factor receptor-associated factors (TRAFs)

The tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family of molecules are intracellular signaling adaptors and control diverse signaling pathways mediated not only by the TNFR superfamily and the Toll-like receptor/interleukin-1 receptor superfamily but also by unconventional cytokine receptors such as IL-6 and IL-17 receptors. There are seven family members, TRAF1 to TRAF7, in mammals. Exaggerated immune responses induced through TRAF signaling downstream of these receptors often lead to inflammatory and autoimmune diseases including rheumatoid arthritis, inflammatory bowel disease, psoriasis and autoinflammatory syndromes, and thus those signals are major targets for therapeutic intervention. For this reason, it has been very important to understand signaling mechanisms regulated by TRAFs that greatly impact on life/death decisions and the activation, differentiation and survival of cells of the innate and adaptive immune systems. Accumulating evidence suggests that dysregulated cellular expression and/or signaling of TRAFs causes overproduction of proinflammatory cytokines, which facilitates aberrant activation of immune cells. In this review, I will explain the structural and functional aspects that are responsible for the cellular activity and disease outcomes of TRAFs, and summarize the findings of recent studies on TRAFs in terms of how individual TRAF family molecules regulates biological and disease processes in the body in both positive and negative ways. This review also discusses how TRAF mutations contribute to human disease.

MicroRNA miR-146a-5p inhibits the inflammatory response and injury of airway epithelial cells via targeting TNF receptor-associated factor 6

Bronchial asthma is a common respiratory disease, which is characterized by airway inflammation, remodeling and hyperresponsiveness. MicroRNAs (miRNAs), as reported, are implicated in the pathogenesis of many diseases, but how miRNAs-146a-5p (miR-146a-5p) works in asthma remains inconclusive. In this work, we proved that miR-146a-5p expression was inhibited in asthma patients’ plasma and platelet activating factor (PAF)-induced human small airway epithelial cells (HSAECs). MiR-146a-5p up-regulation ameliorated the inflammatory reaction and cell barrier damage of HSAECs induced by PAF, and inhibited the apoptosis; besides, miR-146a-5p down-regulation functioned oppositely. In addition, miR-146a-5p could target TNF receptor-associated factor 6 (TRAF6) and negatively regulate its expression. TRAF6 overexpression could counterract the impact of miR-146a-5p up-regulation on PAF-induced inflammation, cell barrier damage and apoptosis of HSAECs. Collectively, miR-146a-5p may protect airway epithelial cells and inhibit the pathogenesis of asthma via targeting TRAF6.

Epstein-Barr Functional Mimicry: Pathogenicity of Oncogenic Latent Membrane Protein-1 in Systemic Lupus Erythematosus and Autoimmunity

Systemic lupus erythematosus (SLE) and other autoimmune diseases are propelled by immune dysregulation and pathogenic, disease-specific autoantibodies. Autoimmunity against the lupus autoantigen Sm is associated with cross-reactivity to Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1). Additionally, EBV latent membrane protein-1 (LMP1), initially noted for its oncogenic activity, is an aberrantly active functional mimic of the B cell co-stimulatory molecule CD40. Mice expressing a transgene (Tg) for the mCD40-LMP1 hybrid molecule (containing the cytoplasmic tail of LMP1) have mild autoantibody production and other features of immune dysregulation by 2-3 months of age, but no overt autoimmune disease. This study evaluates whether exposure to the EBV molecular mimic, EBNA-1, stimulates antigen-specific and concurrently-reactive humoral and cellular immunity, as well as lupus-like features. After immunization with EBNA-1, mCD40-LMP1 Tg mice exhibited enhanced, antigen-specific, cellular and humoral responses compared to immunized WT congenic mice. EBNA-1 specific proliferative and inflammatory cytokine responses, including IL-17 and IFN-γ, were significantly increased (p<0.0001) in mCD40-LMP1 Tg mice, as well as antibody responses to amino- and carboxy-domains of EBNA-1. Of particular interest was the ability of mCD40-LMP1 to drive EBNA-1 associated molecular mimicry with the lupus-associated autoantigen, Sm. EBNA-1 immunized mCD40-LMP1 Tg mice exhibited enhanced proliferative and cytokine cellular responses (p<0.0001) to the EBNA-1 homologous epitope PPPGRRP and the Sm B/B' cross-reactive sequence PPPGMRPP. When immunized with the SLE autoantigen Sm, mCD40-LMP1 Tg mice again exhibited enhanced cellular and humoral immune responses to both Sm and EBNA-1. Cellular immune dysregulation with EBNA-1 immunization in mCD40-LMP1 Tg mice was accompanied by enhanced splenomegaly, increased serum blood urea nitrogen (BUN) and creatinine levels, and elevated anti-dsDNA and antinuclear antibody (ANA) levels (p<0.0001 compared to mCD40 WT mice). However, no evidence of immune-complex glomerulonephritis pathology was noted, suggesting that a combination of EBV and genetic factors may be required to drive lupus-associated renal disease. These data support that the expression of LMP1 in the context of EBNA-1 may interact to increase immune dysregulation that leads to pathogenic, autoantigen-specific lupus inflammation.

Mechanism by which TRAF6 Participates in the Immune Regulation of Autoimmune Diseases and Cancer

Tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), an E3 ubiquitin ligase, is a signal transduction molecule shared by the interleukin-1 receptor (IL-1R)/Toll-like receptor (TLR) family and the TNFR superfamily. TRAF6 has a unique TRAF domain and RING finger domain that mediate intracellular signaling events. In the immune system, TRAF6-mediated signaling has been shown to be critical for the development, homeostasis, and activation of a variety of immune cells, including B cells, T cells, dendritic cells, and macrophages. Although the pathogenesis and etiology of autoimmune diseases and cancer are not fully understood, it is worth noting that existing studies have shown that TRAF6 is involved in the pathogenesis and development of a variety of these diseases. Herein, we reviewed the role of TRAF6 in certain immune cells, as well as the function and potential effect of TRAF6 in autoimmune diseases and cancer. Our review indicates that TRAF6 may be a novel target for autoimmune diseases and cancer.

Bystander inhibition of humoral immune responses by Epstein-Barr virus LMP1

Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1), which mimics a constitutively active receptor, is required for viral transformation of primary B cells. LMP1 is expressed in EBV-infected germinal center (GC) B cells of immunocompetent individuals, suggesting that it may contribute to persistent EBV infection. In this study, we generated and analyzed mice that expressed LMP1 under the control of the CD19 or activation-induced cytidine deaminase (AID) promoter. Expression of LMP1 induced activation of B cells but severely inhibited their differentiation into antibody-secreting cells (ASCs) in vitro and GC B cells in vivo. LMP1-expressing (LMP1+) B cells not only suppressed the functions of wild-type (WT) B cells in in vitro co-culture, but also blocked differentiation of WT B cells into GC B cells and ASCs in immunized bone marrow chimeric mice. Microarray analysis revealed that the gene encoding indoleamine 2,3-dioxygenase 1 (IDO1), a major enzyme involved in the tryptophan metabolic process, was highly induced by LMP1. Either inhibition of IDO1 activity by methyl-l-tryptophan or knockout of Ido1 in LMP1+ B cells could rescue WT B cells from such suppression. IDO1-induced tryptophan consumption and production of tryptophan metabolites appeared to be responsible for inhibition of B-cell function. We conclude that LMP1 expression in antigen-committed B cells not only directly impairs GC B-cell differentiation, but also indirectly inhibits the functions of neighboring B cells, resulting in suppression of humoral immune responses. Such bystander inhibition by LMP1+ B cells may contribute to immune evasion by EBV.

The Epstein-Barr virus LMP1 interactome: biological implications and therapeutic targets

The oncogenic potential of Epstein-Barr virus (EBV) is mostly attributed to latent membrane protein 1 (LMP1), which is essential and sufficient for transformation of fibroblast and primary lymphocytes. LMP1 expression results in the activation of multiple signaling cascades like NF-ΚB and MAP kinases that trigger cell survival and proliferative pathways. LMP1 specific signaling events are mediated through the recruitment of a number of interacting proteins to various signaling domains. Based on these properties, LMP1 is an attractive target to develop effective therapeutics to treat EBV-related malignancies. In this review, we focus on LMP1 interacting proteins, associated signaling events, and potential targets that could be exploited for therapeutic strategies.

Transgenic Mouse Models of Tumor Virus Action

Genetically engineered mice (GEMs) have provided valuable insights into the carcinogenic properties of various human tumor viruses, which, in aggregate, are etiologically associated with over 15% of all human cancers. This review provides an overview of seminal discoveries made through the use of GEM models for human DNA tumor viruses. Emphasis is placed on the discoveries made in the study of human papillomaviruses, Merkel cell carcinoma-associated polyomavirus, Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus, because GEMs have contributed extensively to our understanding of how these DNA tumor viruses directly contribute to human cancers.

Epstein-Barr Virus LMP1-Mediated Oncogenicity

Epstein-Barr virus latent membrane protein 1 (LMP1) is expressed in multiple human malignancies, including nasopharyngeal carcinoma and Hodgkin and immunosuppression-associated lymphomas. LMP1 mimics CD40 signaling to activate multiple growth and survival pathways, in particular, NF-κB. LMP1 has critical roles in Epstein-Barr virus (EBV)-driven B-cell transformation, and its expression causes fatal lymphoproliferative disease in immunosuppressed mice. Here, we review recent developments in studies of LMP1 signaling, LMP1-induced host dependency factors, mouse models of LMP1 lymphomagenesis, and anti-LMP1 immunotherapy approaches.

Down-regulation of increased TRAF6 expression in the peripheral mononuclear cells of patients with primary Sjögren's syndrome by an EBV-EBER1-specific synthetic single-stranded complementary DNA molecule

AIM

We described earlier a simultaneously increased that the increased expression of miRNA-146a/b was accompanied by an increase in the expression of and TRAF6 and a decrease in the expression of IRAK1 genes in the peripheral mononuclear cells (PBMCs) of patients with primary Sjogren's syndrome (pSS) patients. Recently, the expression of EBV encoded. RNA (EBER) was published in the B cells of salivary glands of in pSS. In the present study, we applied an EBV-EBER1 specific synthetic single stranded complementary DNA molecule (EBV-EBER1-cDNA) to test whether any EBER1 related effect exists also in PBMCs of pSS patients.

METHODS

In the PBMCs of pSS patients and healthy controls, we investigated in vitro the effects of a synthetic single stranded EBV-EBER1-cDNA molecule, synthetic double-stranded (ds)RNA polyinosinic-polycytidylic acid [poly (I:C)] and polyadenylic acid potassium salt poly-adenylic acid [poly-(A)] on the expression of TRAF6 gene tested by qRTPCR. The release of interferon -α was detected by ELISA.

RESULTS

EBV-EBER1-cDNA resulted in a significant reduction in the expression of TRAF6 in the cells of patients, but in the healthy controls not, whereas the treatments with poly (I:C) and poly-(A) could not reduce the TRAF6 over-expression. No release of EBER1 could be observed in the culture supernatants of patients with pSS. Only the treatment with poly (I:C) resulted in a significant increase of interferon -α release, and only in the heathy controls. No release of EBER1 molecules took place during the culturing of cells. EBV-EBER- cDNA acted functionally on the cells of patients only.

CONCLUSION

These findings give a further evidence of the linkage between EBV and pSS, furthermore, they show the possible role of EBV-EBER1 in the induction of increased TRAF6 expression in the peripheral B cells of Sjögren's patients.

Animal models of tumorigenic herpesviruses--an update

Any one model system, be it culture or animal, only recapitulates one aspect of the viral life cycle in the human host. By providing recent examples of animal models for Epstein-Barr virus and Kaposi sarcoma-associated herpesvirus, we would argue that multiple animal models are needed to gain a comprehensive understanding of the pathogenesis associated with human oncogenic herpesviruses. Transgenic mice, homologous animal herpesviruses, and tumorgraft and humanized mouse models all complement each other in the study of viral pathogenesis. The use of animal model systems facilitates the exploration of novel anti-viral and anti-cancer treatment modalities for diseases associated with oncogenic herpesviruses.

Tumor necrosis factor receptor- associated factor 6 (TRAF6) regulation of development, function, and homeostasis of the immune system

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is an adapter protein that mediates a wide array of protein-protein interactions via its TRAF domain and a RING finger domain that possesses non-conventional E3 ubiquitin ligase activity. First identified nearly two decades ago as a mediator of interleukin-1 receptor (IL-1R)-mediated activation of NFκB, TRAF6 has since been identified as an actor downstream of multiple receptor families with immunoregulatory functions, including members of the TNFR superfamily, the Toll-like receptor (TLR) family, tumor growth factor-β receptors (TGFβR), and T-cell receptor (TCR). In addition to NFκB, TRAF6 may also direct activation of mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), and interferon regulatory factor pathways. In the context of the immune system, TRAF6-mediated signals have proven critical for the development, homeostasis, and/or activation of B cells, T cells, and myeloid cells, including macrophages, dendritic cells, and osteoclasts, as well as for organogenesis of thymic and secondary lymphoid tissues. In multiple cellular contexts, TRAF6 function is essential not only for proper activation of the immune system but also for maintaining immune tolerance, and more recent work has begun to identify mechanisms of contextual specificity for TRAF6, involving both regulatory protein interactions, and messenger RNA regulation by microRNAs.