Two Epstein–Barr virus (EBV) oncoproteins cooperate to repress expression of the proapoptotic tumour-suppressor Bim: clues to the pathogenesis of Burkitt's lymphoma

Abstract

Epstein-Barr virus (EBV) contributes to the development of several human cancers including the endemic form of Burkitt's lymphoma (BL). In culture, EBV induces the continuous proliferation of primary B cells as lymphoblastoid cell lines (LCLs) and if EBV-negative BL-derived cells are infected with EBV, latency-associated viral factors confer resistance to various inducers of apoptosis. Nuclear proteins EBNA3A and EBNA3C (but not EBNA3B) are necessary to establish LCLs and their expression may be involved in the resistance of BL cells to cytotoxic agents. We have therefore created recombinant EBVs from which each of the EBNA3 genes has been independently deleted, and revertant viruses in which the genes have been re-introduced into the viral genome. Infection of EBVnegative BL cells with this panel of EBVs and challenge with various cytotoxic drugs showed that EBNA3A and EBNA3C cooperate as the main determinants of both drug resistance and the downregulation of the proapoptotic Bcl-2-family member Bcl-2-interacting mediator of cell death (Bim). The regulation of Bim is predominantly at the level of RNA, with little evidence of post-translational Bim stabilization by EBV. In the absence of Bim, EBNA3A and EBNA3C appear to provide no survival advantage. The level of Bim is a critical regulator of B cell survival and reduced expression is a major determinant of lymphoproliferative disease in mice and humans; moreover, Bim is uniquely important in the pathogenesis of BL. By targeting this tumour-suppressor for repression, EBV significantly increases the likelihood of B lymphomagenesis in general, and BL in particular. Our results may also explain the selection pressure that gives rise to a subset of BL that retain expression of the EBNA3 proteins.

A novel Bim-BH3-derived Bcl-XL inhibitor: Biochemical characterization, in vitro, in vivo and ex-vivo anti-leukemic activity

“…25,26 Homozigous deletion of Bim is associated with mantle cell lymphoma 27 and survival of autoreactive T and B cells, 28,29 and repression of Bim expression by EBV has a significant role in Burkitt lymphoma. 30 Several antitumor chemotherapies were demonstrated to kill neoplastic cells through Bim-activated pathways, including a Raf kinase inhibitor in acute myeloid cells, 31 glucocorticoids in leukaemia cell lines and primary leukemia cells, 32,33 ABT-737 for chronic lymphocytic leukaemia cells and myeloma cell lines 18 and paclitaxel in xenografts of epithelial cancer cells. 34 Altogether, Bim can be classified as a powerful tumour suppressor, and its strong pro-apoptotic activity is regulated both at transcriptional and post-translational levels.…”

Epstein-Barr virus nuclear antigens 3C and 3A maintain lymphoblastoid cell growth by repressing p16 INK4A and p14 ARF expression

“…EBNA3C or EBNA3A have many specific and potentially significant interactions with other transcription factors or modifiers, including PU.1, Spi-B, HDAC1, DP103, prothymosin-α, p300, Nm23-H1, SUMO1, and SUMO3, cyclin A, SCF SKP2 ubiquitin ligase, pRb, Chk2, Mdm2, and MRS18-2, and these interactions could be relevant to CDKN2A p16 INK4A or p14 ARF regulation and LCL growth (27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38). Furthermore, EBNA3C upregulates TCL1A and ITGA4, down-regulates JAG1 and NCALD RNAs, and cooperates with EBNA3A in repressing Bim, a proapoptotic Bcl-2 family protein (24,(39)(40)(41). Indeed, EBNA3C and EBNA3A may cooperatively repress the p16 INK4A and p14 ARF tumor suppressors to allow cell cycle progression, as is required for MYC conversion of tissue cells to stem cells, for HPV16-and HPV18-induced cervical cancer, and for other enforced cell proliferations (42)(43)(44).…”

Epstein-Barr virus interactions with the Bcl-2 protein family and apoptosis in human tumor cells

“…Most reports show that BIM is a key regulator of life and death decisions (Anderton et al, 2008;Paschos et al, 2009). Thus, it is not surprising that EBV BHRF1 binds to the BH3-only peptide BIM and interacts strongly with its protein (Flanagan and Letai, 2008).…”

“…Using recombinant EBVs established with a bacterial artificial chromosome (BAC) system, it was revealed that EBNA3A and EBNA3C cooperate to inhibit the activation of the tumor-suppressor gene BIM. The inhibition appears to be predominantly directed at the regulation of BIM mRNA levels (Anderton et al, 2008). This process may involve an epigenetic mechanism that can be initiated or maintained by interactions between EBNA3A and EBNA3C and suppressive marks on local chromatin.…”

“…Clybouw et al (2005) showed that EBV infection leads to the downregulation of BIM protein expression, depending mainly on BIMEL expression. Most data indicate that EBNA3A and EBNA3C regulate the expression of BIM at the level of transcription (Anderton et al, 2008). Strong evidence was provided by the fact that EBNA3A and EBNA3C together inhibit the initiation of BIM transcripts (Paschos et al, 2012).…”