Modulation of apoptotic pathways in B cells by Epstein-Barr virus

Epstein-Barr virus (EBV) is a gammaherpesvirus associated with several human malignancies including Burkitt’s lymphoma (BL). Latent EBV infection protects BL cell lines from apoptosis induced by genotoxic drugs including cisplatin, roscovitine and nocodazole. This protection is dependent on the co-expression of EBNA3A and EBNA3C, which downregulate the pro-apoptotic Bim, but is independent of the tumour suppressor p53 (Anderton et at., Oncogene (2008) 27, 421-33). However, using the calcium ionophore ionomycin as the apoptosis-inducing agent, another pathway by which BL cells undergo apoptosis has been identified. This is independent of both Bim and p53, but can also be blocked by latent EBV. By assessing the response of a panel BL cells expressing restricted sets of viral latent genes to ionomycin, it was shown that EBNA1, EBNA2, LMP2A and LMP2B, full-length EBNA-LP, EBNA3A, EBNA3B, EBNA3C and EBERs are not required for ionomycin-resistance although LMP1 can provide partial protection. Recombinant and revertant EBV-BACs were constructed to investigate the contribution of the EBNA3 locus and the BHRF1 locus to ionomycin-resistance. It was determined that the EBNA3 locus is not involved in rescue from ionomycin. However, loss of the BHRF1 locus partially abrogates protection against ionomycin-induced apoptosis. The pro-apoptotic BH3-only protein Noxa is consistently induced at the level of protein and mRNA in BL cells sensitive to ionomycin irrespective of p53 status. Suppression of Noxa by shRNA-expressing lentiviruses attenuates ionomycin- induced apoptosis, indicating that EBV confers resistance at least in part by inhibiting the accumulation of Noxa through a yet to be identified mechanism. This Noxa- associated apoptotic pathway is also activated by the protein kinase inhibitor staurosporine. Further investigation using the DNA topoisomerase II inhibitor etoposide indicates that in addition EBNA3A and EBNA3C, EBV can also confer resistance against genotoxic agents through the BHRF1 locus in a Bim-independent manner.Open Acces