Sensitivity to Fas-mediated apoptosis is determined below receptor level in human vascular smooth muscle cells

Despite Fas expression, many cells resist Fas-induced apoptosis, Although d ifferences in surface Fas expression can explain Fas resistance, multiple p roteins below receptor level also inhibit Fas-induced apoptosis, To examine the mechanism of Fas resistance, we studied Fas-induced apoptosis in human medial vascular smooth muscle cells (VSMCs) from healthy coronary arteries . VSMCs showed marked heterogeneity to Fas-induced apoptosis, exhibiting bo th Fas-resistant (98.1+/-2.3% viable, n=4, P=NS) and Fas-sensitive (31.3+/- 2.6% viable, n=3, P<0.01) cells. Fas-resistant VSMCs expressed surface Pas and could recruit RIP, indicating that functional receptor complexes were f ormed. However, Fas-resistant cells showed reduced expression of FADD, Fas ligand, and caspases 3, 7, and 8 and increased expression of FLIP and c-IAP -1. Fas-induced apoptosis was associated with cleavage of caspase 3 and blo cked by inhibitors of caspase 3 or 8 but not caspase 1, 6, or 7, Selective inhibition of caspase 3 or 8 by antisense transfection inhibited Fas-induce d apoptosis, but their reexpression could not rescue the Fas-resistant phen otype, In vivo, medial VSMCs showed marked heterogeneity of expression of c aspase 3, We conclude that Pas sensitivity is determined not only by expres sion of surface Fas but by differential expression of Fas-signaling protein s below receptor level. Subpopulations of cells within the same tissue have different sensitivities to apoptosis, determined by expression of specific death-signaling proteins.