Bcl-xL functions downstream of caspase-8 to inhibit Fas- and tumor necrosis factor receptor 1-induced apoptosis of MCF7 breast carcinoma cells

Stimulation of the Fas or tumor necrosis factor receptor 1 (TNFR1) cell surface receptors leads to the activation of the death effector protease, caspase-8, and subsequent apoptosis. In some cells, Bcl-x(L) overexpression can inhibit anti-Fas-and tumor necrosis factor (TNF)-alpha-induced apoptosis, To address the effect of Bcl-x(L) on caspase-8 processing, Fas-and TNFR1-mediated apoptosis were studied in the MCF7 breast carcinoma cell line stably transfected with human Fas cDNA (MCF7/F) or double transfected with Fas and human Bcl-x(L) cDNAs (MCF7/FB). Bcl-x(L) strongly inhibited apoptosis induced by either anti-Fas or TNF-alpha. In addition, Bcl-x(L) prevented the change in cytochrome c immunolocalization induced by anti-Fas or TNF-alpha treatment, Using antibodies that recognize the p20 and p10 subunits of active caspase-8, proteolytic processing of caspase-8 was detected in MCF7/F cells following anti-Fas or TNF-alpha, but not during UV-induced apoptosis, In MCF7/FB cells, caspase-8 was processed normally while processing of the downstream caspase-7 was markedly attenuated, Moreover, apoptosis induced by direct microinjection of recombinant, active caspase-8 was completely inhibited by Bcl-x(L). These data demonstrate that Bcl-x(L) can exert an anti-apoptotic function in cells in which caspase-8 is activated, Thus, at least in some cells, caspase-8 signaling in response to Fas or TNFR1 stimulation is regulated by a Bcl-x(L)-inhibitable step.