Fas-mediated apoptosis in T cells involves the dephosphorylation of the retinoblastoma protein by type 1 protein phosphatases

Apoptosis is triggered by a number of diferent stimuli including the activation of Fas antigen, a member of the TNF family, by the Fas ligand. The signal transduction events implicated in apoptosis are complex and remain only partially understood. In this study, we used calyculin A, a potent. inhibitor of serine/threonine (ser/thr) phosphatases types 1 and 2A, to investigate the role of ser/thr phosphatases in Fas-induced apoptosis. We showed that calyculin A inhibited Fas-induced DNA fragmentation and cytolysis in Jurkat cells and char this inhibition was not clue to the modulation of Fas. Okadaic acid also inhibited Fas-induced apoptosis of Jurkat cells, but at much higher concentrations (IJ-M level), thus implicating that type 1 phosphatases rather than type 2A are inhibited at. nM concentrations. Cross-linking Fas led ro the dephosphorylation of the retinoblastoma gene product (Rb) within 5 min, and to PARP cleavage within 2 h. Both events were inhibited by calyculin A indicating that apoptotic death triggered by Fas cross-linking involves the activation of type 1 ser/thr phosphatases. Human Immunolgoy 60, 1183-1194 (1999). (C) American Society Tor Histocompatibility and Immunogenetics, 1999 Published by Elsevier Science Inc.