Differential modulation of apoptosis sensitivity in CD95 type I and type II cells

We have recently identified two different pathways of CD95-mediated apoptos is (Scaffidi, C., Fulda, S., Srinivasan, A., Feng, L., Friesen, C., Tomasel li, K, J,, Debatin, K.-M,, Krammer, P. H., and Peter, M.E. (1998) EMBO J. 1 7, 1675-1687), CD95-mediated apoptosis in type I cells is initiated by larg e amounts of active caspase-8 formed at the death-inducing signaling comple x (DISC) followed by direct cleavage of caspase-3, In contrast, in type II cells very little DISC and small amounts of active caspase-8 sufficient to induce the apoptogenic activity of mitochondria are formed causing a profou nd activation of both caspase-8 and caspase-3, Only in type II cells can ap optosis be blocked by overexpressed Bcl-2 or Bcl-x(L). We now show that a n umber of apoptosis-inhibiting or -inducing stimuli only affect apoptosis in type II cells, indicating that they act on the mitochondrial branch of the CD95 pathway. These stimuli include the activation of protein kinase C, wh ich inhibits CD95-mediated apoptosis resulting in a delayed cleavage of BID , and the induction of apoptosis by the ceramide analog C-2-ceramide, In ad dition, we have identified the CD95 high expressing cell line Boe(R) as a C D95 apoptosis-resistant type II cell that can be sensitized by treatment wi th cycloheximide without affecting formation of the DISC, This also places the effects of cycloheximide in the mitochondrial branch of the type II CD9 5 pathway. In contrast, c-FLIP was found to block CD95-mediated apoptosis i n both type I and type II cells, because it acts directly at the DISC of bo th types of cells.