BCL-X(L) FUNCTIONS DOWNSTREAM OF CASPASE-8 TO INHIBIT FAS-NECROSIS-FACTOR AND TUMOR-NECROSIS-FACTOR RECEPTOR-1-INDUCED APOPTOSIS OF MCF7 BREAST-CARCINOMA CELLS

Stimulation of the Fas or tumor necrosis factor receptor 1 (TNFR1) cel l surface receptors leads to the activation of the death effector prot ease, caspase-8, and subsequent apoptosis. In some cells, Bcl-x(L) ove rexpression can inhibit anti-Fas-and tumor necrosis factor (TNF)-alpha -induced apoptosis, To address the effect of Bcl-x(L) on caspase-8 pro cessing, Fas-and TNFR1-mediated apoptosis were studied in the MCF7 bre ast carcinoma cell line stably transfected with human Fas cDNA (MCF7/F ) or double transfected with Fas and human Bcl-x(L) cDNAs (MCF7/FB). B cl-x(L) strongly inhibited apoptosis induced by either anti-Fas or TNF -alpha. In addition, Bcl-x(L) prevented the change in cytochrome c imm unolocalization induced by anti-Fas or TNF-alpha treatment, Using anti bodies that recognize the p20 and p10 subunits of active caspase-8, pr oteolytic processing of caspase-8 was detected in MCF7/F cells followi ng anti-Fas or TNF-alpha, but not during UV-induced apoptosis, In MCF7 /FB cells, caspase-8 was processed normally while processing of the do wnstream caspase-7 was markedly attenuated, Moreover, apoptosis induce d by direct microinjection of recombinant, active caspase-8 was comple tely inhibited by Bcl-x(L). These data demonstrate that Bcl-x(L) can e xert an anti-apoptotic function in cells in which caspase-8 is activat ed, Thus, at least in some cells, caspase-8 signaling in response to F as or TNFR1 stimulation is regulated by a Bcl-x(L)-inhibitable step.