BCL-2 AND HSP27 ACT AT DIFFERENT LEVELS TO SUPPRESS PROGRAMMED CELL-DEATH

Apoptosis and necrosis, two morphologically distinct forms of cell dea th, can be induced by common stimuli depending on the doses and the ce ll type, This study compares the protective effect of oncoprotein Bcl- 2 and of the small stress protein Hsp27 on these two types of cell dea th, We use rat embryo fibroblasts conditionally immortalized by the ts A58 mutant of SV40 large T antigen as parental cells to develop cell l ines carrying inducible bcl-2 or hsp27 genes. Two apoptotic stimuli we re used: shift to the restrictive temperature that induced p53-mediate d apoptosis and treatment with low doses of hydrogen peroxide, Necrosi s was induced by high doses of hydrogen peroxide. Although Bcl-2 and H sp27 protect these cells from necrotic death, only Bcl-2 appears capab le of preventing apoptotic death, Bcl-2 protection is not mediated by a negative effect on the induction of the p53 responsive genes bax or waf1 but it slows down at least two stages of apoptosis: decrease of m itochondrial membrane potential and subsequent morphological changes, In contrast, although Hsp27 has been recently shown to inhibit apoptos is induced by various stimuli, its overexpression has no effect on apo ptosis in this cell system. It should be also noticed that the apoptot ic stimuli (temperature shift or hydrogen peroxide treatment) induce H sp27, but not Bcl-2 accumulation suggesting that, in parental cells, H sp27 might already provide some protection, However, taken together th ese results suggest that Hsp27, as well as Bcl-2, acts at several leve ls to inhibit cell death, but that their protective functions only par tially overlap.