Distinct sites of action of Bcl-2 and Bcl-xL in the ceramide pathway of apoptosis

We studied the inhibition of tumour necrosis factor α (TNFα)- and camptothecin-induced apoptosis by Bcl-2 and Bcl-x_L as they relate to the ceramide pathway. Expression of either Bcl-2 or Bcl-x_L provided significant protection from the apoptotic effects of TNFα or camptothecin. In contrast to Bcl-2, Bcl-x_L overexpression did not protect cells from ceramide-induced apoptosis. On the other hand, Bcl-x_L prevented the accumulation of endogenous ceramide in response to TNFα or camptothecin, whereas Bcl-2 showed little effect on ceramide formation. Moreover, Bcl-x_L, but not Bcl-2, totally inhibited a caspase-8-like activity in cell lysates stimulated with TNFα. These results identify a different mechanism of action for Bcl-x_L compared with Bcl-2 and they demonstrate that Bcl-x_L targets a point upstream of ceramide generation, whereas Bcl-2 functions downstream of ceramide in the TNFα- and camptothecin-activated pathways of apoptosis.