Alphaviruses induce apoptosis in Bcl-2-overexpressing cells: evidence for a caspase-mediated, proteolytic inactivation of Bcl-2

Bcl-2 oncogene expression plays a role in the establishment of persistent viral infection by blocking virus-induced apoptosis, This might be achieved by preventing virus-induced activation of caspase-3, an IL-1 beta-converting enzyme (ICE)-like cysteine protease that has been implicated in the death effector phase of apoptosis, Contrary to this model, rye show that three cell types highly overexpressing functional Bcl-2 displayed caspase-3 activation and underwent. apoptosis in response to infection with alphaviruses Semliki Forest and Sindbis as efficiently as vector control counterparts. In all three cell types, overexpressed 26 kDa Bcl-2 was cleaved into a 23 kDa protein, Antibody epitope mapping revealed that cleavage occurred at one or two target sites for caspases within the amino acid region YEWD(31)down arrow AGD(34)down arrow A, removing the N-terminal BH4 region known to be essential for the death-protective activity of Bcl-2, Preincubation of cells with the caspase inhibitor Z-VAD prevented Bcl-2 cleavage and partially restored the protective activity of Bcl-2 against virus-induced apoptosis, Moreover, a murine Bcl-2 mutant having Asp31, Asp34 and Asp36 substituted by Glu was resistant to proteolytic cleavage and abrogated apoptosis following virus infection, These findings indicate that alphaviruses can trigger a caspase-mediated inactivation of Bcl-2 in order to evade the death protection imposed by this survival factor.