HSP27 inhibits cytochrome c-dependent activation of procaspase-9

We have previously shown that the small heat shock protein HSP27 inhibited apoptotic pathways triggered by a variety of stimuli in mammalian cells. Th e present study demonstrates that HSP27 overexpression decreases U937 human leukemic cell sensitivity to etoposide-induced cytotoxicity by preventing apoptosis. As observed for Bcl-2, HSP27 overexpression delays poly(ADP-ribo se)polymerase cleavage and procaspase-3 activation. In contrast with Bcl-2, HSP27 overexpression does not prevent etoposide-induced cytochrome c relea se from the mitochondria. In a cell-free system, addition of cytochrome c a nd dATP to cytosolic extracts from untreated cells induces the proteolytic activation of procaspase-3 in both control and bcl-2-transfected U937 cells but fails to activate procaspase-3 in HSP27-overexpressing cells. Immunode pletion of HSP27 from cytosolic extracts increases cytochrome c/dATP-mediat ed activation of procaspase-3. Overexpression of HSP27 also prevents procas pase-9 activation. In the cell-free system, immunodepletion of HSP27 increa ses LEDH-AFC peptide cleavage activity triggered by cytochrome c/dATP treat ment. We conclude that HSP27 inhibits etoposide-induced apoptosis by preven ting cytochrome c and dATP-triggered activity of caspase-9, downstream of c ytochrome c release.-Garrido, C., Bruey, J.-M., Fromentin, A., Hammann, A,, Arrigo, A. P., Solary, E. HSP27 inhibits cytochrome c-dependent activation of procaspase-9.