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 inducible by a tetracycline-regulated transactivator. Heat-induced apoptosis was blocked in hsp70-expressing cells, and this was associated with reduced cleavage of the common death substrate protein poly(ADP-ribose) polymerase (PARP). Heat-induced cell death was correlated with the activation of the stress-activated protein kinase SAPK/JNK (c-Jun N-terminal 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/JNK activation. In contrast, SAPK/JNK activation was not inhibited by heat shock in cells with constitutively elevated levels of hsp70. Cells that constitutively overexpress hsp70 resist apoptosis induced by ceramide, a lipid signaling molecule that is generated by apoptosis-inducing treatments and is linked to SAPK/JNK activation. Similar to heat stress, resistance to ceramide-induced apoptosis occurs in spite of strong SAPK/JNK activation. Therefore, hsp70 is also able to inhibit apoptosis at some point downstream of SAPK/JNK activation. Since PARP cleavage is prevented in both cell lines, these results suggest that hsp70 is able to prevent the effector steps of apoptotic cell death. Processing of the CED-3-related protease caspase-3 (CPP32/Yama/apopain) is inhibited in hsp70-expressing cells; however, the activity of the mature enzyme is not affected by hsp70 in vitro. Caspase processing may represent a critical heat-sensitive target leading to cell death that is inhibited by the chaperoning function of hsp70. The inhibition of SAPK/JNK signaling and apoptotic protease effector steps by hsp70 likely contributes to the resistance to stress-induced apoptosis seen in transiently induced thermotolerance.     

Presence of sauvagine-like epitopes in the interrenal gland of the bullfrog Rana catesbeiana

The objective of this study was to determine if a variety of hepatotoxicants could induce the level of heat shock protein 70I, and whether or not elevated levels of heat shock proteins (hsp's) could provide cytoprotection from those hepatotoxicants. Exposure of HepG2 cells to cytotoxic concentrations of bromobenzene, cadmium, cyclophosphamide, or diethylnitrosamine increased the level of hsp 70I protein and mRNA, while carbon tetrachloride and cocaine had no effect on hsp 70I or mRNA levels. To determine if induction of hsp 70I might afford protection against cytotoxicity, HepG2 cells were given a prior sublethal heat shock (sub-LHS) (43 degrees C for 1 hr) to induce hsp's and then challenged 24 hr later with the hepatotoxicants. Sub-LHS pretreatment diminished toxicity from bromobenzene, cadmium, cyclophosphamide, or diethyl-nitrosamine, but not carbon tetrachloride or cocaine. In cells treated with [14C]carbon tetrachloride or [3H]cocaine, no detectable covalent binding to proteins was observed; whereas, [14C]-bromobenzene treatment resulted in substantial covalent binding to cellular protein. The apparent absence of formation of reactive metabolite adducted proteins from cocaine and carbon tetrachloride may explain why no hsp 70I induction was observed with these agents. The correlation between hepatotoxicant induction of hsp 70I and cytoprotection afforded by sub-LHS pretreatment suggests that hsp 70I induction may represent an important cellular defense mechanism in the liver.