Heat shock protein 72 (Hsp72) inhibits apoptosis induced by some stresses that trigger the intrinsic apoptosis pathway. However, with the exception of TNFalpha-induced apoptosis, a role for Hsp72 in modulating the extrinsic pathway of apoptosis has not been clearly established. In this study, it was demonstrated that Hsp72 could inhibit Fas-mediated apoptosis of type II CCRF-CEM cells, but not type I SW480 or CH1 cells. Similar results were obtained when Fas ligand or an agonistic Fas antibody initiated the Fas apoptosis pathway. In CCRF-CEM cells, Hsp72 inhibited mitochondrial membrane depolarization and cytochrome c release but did not alter surface Fas expression or processing of caspase-8 and Bid, indicating that Hsp72 acts upstream of the mitochondria to inhibit Fas-mediated apoptosis. Thus, the ability of Hsp72 to inhibit Fas-mediated apoptosis is limited to type II cells where involvement of the intrinsic pathway is required for efficient effector caspase activation.
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