ROLE OF THE HUMAN HEAT-SHOCK-PROTEIN HSP70 IN PROTECTION AGAINST STRESS-INDUCED APOPTOSIS

Resistance to stress-induced apoptosis was examined in cells in which the expression of hsp70 was either constitutively elevated or inducibl e by a tetracycline-regulated transactivator. Heat-induced apoptosis w as blocked in hsp70-expressing cells, and this was associated with red uced cleavage of the common death substrate protein poly(ADP-ribose) p olymerase (PARP), Heat-induced cell death was correlated with the acti vation of the stress-activated protein kinase SAPK/JNK (c-Jun N-termin al kinase), Activation of SAPK/JNK was strongly inhibited in cells in which hsp70 was induced to a high level, indicating that hsp70 is able to block apoptosis by inhibiting signaling events upstream of SAPK/JN K activation, In contrast, SAPK/JNK activation was not inhibited by he at shock in cells with constitutively elevated levels of hsp70, Cells that constitutively overexpress hsp70 resist apoptosis induced by cera mide, a lipid signaling molecule that is generated by apoptosis-induci ng treatments and is linked to SAPK/JNK activation, Similar to heat st ress, resistance to ceramide-induced apoptosis occurs in spite of stro ng SAPK/JNK activation, Therefore, hsp70 is also able to inhibit apopt osis at some point downstream of SAPK/JNK activation, Since PARP cleav age is prevented in both cell lines, these results suggest that hsp70 is able to prevent the effector steps of apoptotic cell death, Process ing of the CED-3-related protease caspase-3 (CPP32/Yama/apopain) is in hibited in hsp70-expressing cells; however, the activity of the mature enzyme is not affected by hsp70 in vitro. Caspase processing may repr esent a critical heat-sensitive target leading to cell death that is i nhibited by the chaperoning function of hsp70, The inhibition of SAPK/ JNK signaling and apoptotic protease effector steps by hsp70 likely co ntributes to the resistance to stress-induced apoptosis seen in transi ently induced thermotolerance.