Two CD95 (APO-1/Fas) signaling pathways

We have identified two cell types, each using almost exclusively one of two different CD95 (APO-1/Fas) signaling pathways. In type I cells, caspase-8 was activated within seconds and caspase-3 within 30 min of receptor engagement, whereas in type II cells cleavage of both caspases was delayed for similar to 60 min, However, both type I and type II cells showed similar kinetics of CD95-mediated apoptosis and loss of mitochondrial transmembrane potential (Delta Psi(m)). Upon CD95 triggering, all mitochondrial apoptogenic activities were blocked by Bcl-2 or Bcl-x(L) overexpression in both cell types, However, in type II but not type I cells, overexpression of Bcl-2 or Bcl-x(L) blocked caspase-8 and caspase-3 activation as well as apoptosis, In type I cells, induction of apoptosis was accompanied by activation of large amounts of caspase-8 by the death-inducing signaling complex (DISC), whereas in type IZ cells DISC formation was strongly reduced and activation of caspase-8 and caspase-3 occurred following the loss of Delta Psi(m). Overexpression of caspase-3 in the caspase-3-negative cell line MCF7-Fas, normally resistant to CD95-mediated apoptosis by overexpression of Bcl-x(L), converted these cells into true type I cells in which apoptosis was no longer inhibited by Bcl-x(L). In summary, in the presence of caspase-3 the amount of active caspase-8 generated at the DISC determines whether a mitochondria-independent apoptosis pathway is used (type I cells) or not (type II cells).