Role of caspases and possible involvement of retinoblastoma protein duringTGF beta-mediated apoptosis of human B lymphocytes

In this study, we investigated the involvement of caspases in TGF beta-indu ced apoptosis in human B cells. Our results show that TGF beta-mediated nuc lear fragmentation, observed in the Epstein-Barr virus-negative Burkitt's L ymphoma cell line BL41, was abolished in the presence of the tripeptide cas pase inhibitor zVAD-fmk or the specific caspase-3 inhibitor DEVD-fmk. Other apoptotic manifestations such as cell shrinkage, surface phosphatidylserin e expression and chromatin condensation were strongly inhibited by zVAD-fmk but only partially by DEVD-fmk. This suggests that other caspases in addit ion to caspase-3 control these apoptotis-associated features. Specific acti vation of caspase-3 during TGF beta-induced apoptosis was demonstrated by t he DEVD-fmk-sensitive expression of the active p17 subunit of caspase-3 and by in vivo cleavage of PARP. In addition, TGP beta treatment of BL41 promo ted the expression of both dephosphorylated and truncated forms of the reti noblastoma protein. Inhibition of caspase-3 activity abolished both nuclear fragmentation and expression of the truncated retinoblastoma protein, with out modifying the G1 cell cycle arrest induced by TGF beta. Our data thus d emonstrate that TGF beta-induced apoptosis of lymphoma B lymphocytes is dep endent on caspase activation and involves caspase-dependent cleavage of the retinoblastoma protein.