ACTIVATION OF THE CD95 (APO-1 FAS) PATHWAY IN DRUG-INDUCED AND GAMMA-IRRADIATION-INDUCED APOPTOSIS OF BRAIN-TUMOR CELLS/

Chemotherapeutic agents and gamma-irradiation used in the treatment of brain tumors, the most common solid tumors of childhood, have been sh own to act primarily by inducing apoptosis. Here, we report that activ ation of the CD95 pathway was involved in drug- and gamma-irradiation- induced apoptosis of medulloblastoma and glioblastoma cells. Upon trea tment CD95 ligand (CD95-L) was induced that stimulated the CD95 pathwa y by crosslinking CD95 via an autocrine/paracrine loop. Blocking CD95- L/receptor interaction using F(ab')(2) anti-CD95 antibody fragments st rongly reduced apoptosis, Apoptosis depended on activation of caspases (interleukin 1 beta-converting enzyme/Ced-3 like proteases) as it was almost completely abrograted by the broad range caspase inhibitor ben zyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, Apoptosis was mediated by cleavage of the receptor proximal caspase FLICE/MACH (caspase-8) a nd the downstream caspase CPP32 (caspase-3, Apopain) resulting in clea vage of the prototype caspase substrate PARP, Moreover, CD95 was upreg ulated in wild-type p53 cells thereby increasing responsiveness toward s CD95 triggering. Since activation of the CD95 system upon treatment was also found in primary medulloblastoma cells ex vivo, these finding s may have implications to define chemosensitivity and to develop nove l therapeutic strategies in the management of malignant brain tumors.