Role of caspases and possible involvement of retinoblastoma protein during TGFβ-mediated apoptosis of human B lymphocytes

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Abstract
In this study, we investigated the involvement of caspases in TGFβ-induced apoptosis in human B cells. Our results show that TGFβ-mediated nuclear fragmentation, observed in the Epstein – Barr virus-negative Burkitt's Lymphoma cell line BL41, was abolished in the presence of the tripeptide caspase inhibitor ZVAD-fmk or the specific caspase-3 inhibitor DEVD-fmk. Other apoptotic manifestations such as cell shrinkage, surface phosphatidylserine expression and chromatin condensation were strongly inhibited by ZVAD-fmk but only partially by DEVD-fmk. This suggests that other caspases in addition to caspase-3 control these apoptotis-associated features. Specific activation of caspase-3 during TGFβ-induced apoptosis was demonstrated by the DEVD-fmk-sensitive expression of the active p17 subunit of caspase-3 and by in vivo cleavage of PARP. In addition, TGFβ treatment of BL41 promoted the expression of both dephosphorylated and truncated forms of the retinoblastoma protein. Inhibition of caspase-3 activity abolished both nuclear fragmentation and expression of the truncated retinoblastoma protein, without modifying the G1 cell cycle arrest induced by TGFβ. Our data thus demonstrate that TGFβ-induced apoptosis of lymphoma B lymphocytes is dependent on caspase activation and involves caspase-dependent cleavage of the retinoblastoma protein.