Apoptosis-resistant mitochondria in T cells selected for resistance to Fassignaling

Jurkat leukemic T cells are highly sensitive to the extrinsic pathways of a poptosis induced via the death receptor Fas or tumor necrosis factor-relate d apoptosis-inducing ligand as well as to the intrinsic/mitochondrial pathw ays of death induced by VP-16 or staurosporin, We report here that clonal J urkat cell lines selected for resistance to Fas-induced apoptosis were cros s-resistant to VP-16 or staurosporin. Each of the apoptotic pathways was bl ocked at an apical phase, where common regulators of apoptosis have not yet been defined The Fas pathway was blocked at the level of caspase-8, wherea s the intrinsic pathway was blocked at the mitochondria No processing or ac tivity of caspases was detected in resistant cells in response to either Fa s-cross-linking or VP-16 treatment. Also, no apoptosis-associated alteratio ns in the mitochondrial inner membrane, outer membrane, or matrix were dete cted in resistant Jurkat cells treated with VP-16, Thus, no changes in perm eability transition, loss in inner membrane cardiolipin, generation of reac tive oxygen species, or release of cytochrome c were observed in resistant cells treated with VP-16. Further, unlike purified mitochondria from wild t ype cells, those obtained from resistant cells did not release cytochrome c or apoptosis-inducing factor in response to recombinant Bax or truncated B id. These results identify a defect in mitochondria ability to release inte rmembrane proteins in response to Bid or Bax as a mechanism of resistance t o chemotherapeuetic drugs. Further, the selection of VP-16-resistant mitoch ondria via elimination of Fas-susceptible cells may suggest the existence o f a shared regulatory component between the extrinsic and intrinsic pathway s of apoptosis.