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Fas stimulation induces RB dephosphorylation and proteolysis that is blocked by inhibitors of the ICE protease family

Fas antigen is a member of the tumor necrosis factor/nerve growth factor receptor family. Stimulation of Fas by Fas ligand or agonistic antibodies results in the activation of interleukin‐1β converting enzyme‐like (ICE‐like) proteases, and proteolytic cleavage of poly(ADP‐ribose) polymerase (PARP). Ultimately, Fas activation leads to apoptotic cell death. The importance of PARP cleavage to the death process remains unclear. We have hypothesized that the cleavage of other cellular substrates may be important for Fas‐mediated apoptosis. Here we show that stimulation of Fas results in significant alterations of retinoblastoma protein (RB). Treatment of Jurkat cells, a human leukemic T cell line, with anti‐Fas induces dephosphorylation of RB, followed by proteolytic cleavage. These events precede internucleosomal DNA fragmentation. Dephosphorylation and cleavage of RB are inhibited by a specific tetrapeptide inhibitor of ICE‐like proteases or by expression of cowpox virus CrmA protein or the Bcl‐2 oncoprotein. Inhibition of these RB changes correlates with inhibition of apoptosis. We propose that cleavage of RB may represent an important step in the pathway of Fas‐mediated apoptotic cell death. J. Cell. Biochem. 64:586–594. © 1997 Wiley‐Liss, Inc.