Characterization of Apt- cell lines exhibiting cross-resistance to glucocorticoid- and Fas-mediated apoptosis

Apoptosis induction by staurosporine, ceramide, and Fas stimulation was investigated in the mouse thymoma cell line W7.2 and a panel of dexamethasone (dex)-resistant W7.2 mutant cell lines, Apt3.8, Apt4.8 and Apt5.8, and a Bcl-2 transfected W7.2 cell line (Wbcl2). While W7.2 cells were found to be sensitive to these apoptosis inducers, the Apt- mutants and Wbcl2 cells were shown to be resistant to some or all of the treatments. Specifically, all three Apt- mutants and Wbcl2 cells were found to be resistant to ceramide and Fas-mediated apoptosis, whereas, Apt4.8 and Apt5.8 were sensitive to staurosporine-induced apoptosis under conditions in which Apt3.8 and Wbcl2 cells were resistant. Measurements of caspase activity and cytochrome c release in cytosolic extracts of dex and staurosporine-treated cells indicated that the recessive Apt- mutations effect steps upstream of mitochondrial dysfunction. Steady-state RNA levels of apoptosis-associated gene transcripts showed that the observed differential resistance of the Apt- cell lines could not be explained by altered expression of numerous Bcl-2 or Fas related genes. Transient transfection of human Fas gene coding sequences into the Apt- mutants and Wbcl2 cells did not induce apoptosis, even though these same cell lines were sensitive to ectopic expression of the FADD and caspase 8 genes. Taken together, these data provide genetic evidence for the existence of shared components in the dex- and Fas-mediated apoptotic pathways in W7.2 cells.