Latent membrane protein of Epstein-Barr virus induces cellular phenotypes independently of expression of Bcl-2.

The stable expression of the Epstein-Barr virus (EBV) latent membrane protein (LMP) in certain EBV-negative Burkitt's lymphoma cell lines correlates with an increased expression of the oncogene Bcl-2 (S. Henderson, M. Rowe, C. Gregory, D. Croom-Carter, F. Wang, R. Longnecker, E. Kieff, and A. Rickinson, Cell 65:1107-1115, 1991). This finding is consistent with a model in which Bcl-2 contributes to the immortalization of B cells mediated by EBV. We therefore asked whether the expression of Bcl-2 protein correlates with the induction of three cellular phenotypes induced by or associated with LMP. The expression of Bcl-2 in primary B cells infected with the B95-8 strain of EBV varied between 1 and 1.8 times that in uninfected cells when 50% of the cells were infected, expressed LMP, and incorporated 20-fold more [3H]thymidine than did uninfected cells. This finding indicates that induced proliferation of these primary cells is not sufficient to induce Bcl-2. We found that BALB/c 3T3 cells and their derivatives transformed by LMP do not express Bcl-2 detectably. The expression of LMP at high levels in lymphoid cells is cytotoxic and correlates with an increased expression of Bcl-2 following stable selection for the introduced LMP gene; 2 days after transfection, control vector- and LMP-transfected populations, however, express equal levels of Bcl-2 protein. We also analyzed transient expression of LMP in an EBV-negative Burkitt's lymphoma cell line. Infection of BJAB cells with the B95-8 strain of EBV results in an increase in Bcl-2 expression with a time course similar to that of LMP expression, and LMP alone transiently induces an increase in Bcl-2 expression in these cells. We interpret these observations to indicate that increased expression of Bcl-2 is unlikely to contribute to the ability of EBV to immortalize primary B cells and that both the transformation of rodent cells and the cytotoxicity mediated by LMP are independent of Bcl-2.