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Bcl-xL overexpression restricts heat-induced apoptosis and influences hsp70, bcl-2, and Bax protein levels in FL5.12 cells.

Although several proteins have been identified that can inhibit stress-induced apoptosis, the cytoprotective potential of bcl-xL against heat shock and its ability to alter hsp70 induction is not known. The current study, using control and bcl-xL-overexpressing IL-3-dependent FL5.12 cells, compared the effects of 1 h of acute heat stress (42 degrees C) followed by 1, 4, and 8 h recovery (37 degrees C) on hsp70, bax, bcl-2, and bcl-xL protein levels and apoptosis. Less than 0.5% of untreated cells were apoptotic. There was significantly more apoptosis in control ( approximately 16%) as compared to bcl-xL cells ( approximately 3%) 8 h after heat stress. Immunoblotting revealed a time-dependent increase in hsp70 protein levels following 1 h of heat stress in control, but not bcl-xL-overexpressing cells. bcl-2 protein levels were lower in bcl-xL-overexpressing cells than in controls, but decreased in both cell lines after heat stress. bax protein levels in bcl-xL-overexpressing cells were decreased approximately 80% below baseline levels 1 h post heat shock. This decrease was maintained to 8 h. No change in bax protein was observed in control cells up to 8 h post heat shock. These data indicate that bcl-xL overexpression mitigates the effects of acute heat stress so that hsp70 induction is eliminated and apoptosis is prevented. The rapid loss of bax protein following heat stress in bcl-xL-overexpressing, but not control, cells may contribute to their resistance to apoptosis. Conversely, the loss of bcl-2 protein following heat stress in control cells may contribute to their susceptibility to apoptosis.