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 persist ent 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 imp licated in the death effector phase of apoptosis, Contrary to this mod el, rye show that three cell types highly overexpressing functional Bc l-2 displayed caspase-3 activation and underwent. apoptosis in respons e to infection with alphaviruses Semliki Forest and Sindbis as efficie ntly as vector control counterparts. In all three cell types, overexpr essed 26 kDa Bcl-2 was cleaved into a 23 kDa protein, Antibody epitope mapping revealed that cleavage occurred at one or two target sites fo r 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 t he caspase inhibitor Z-VAD prevented Bcl-2 cleavage and partially rest ored the protective activity of Bcl-2 against virus-induced apoptosis, Moreover, a murine Bcl-2 mutant having Asp31, Asp34 and Asp36 substit uted by Glu was resistant to proteolytic cleavage and abrogated apopto sis following virus infection, These findings indicate that alphavirus es can trigger a caspase-mediated inactivation of Bcl-2 in order to ev ade the death protection imposed by this survival factor.