A novel role for sphingolipid intermediates in activation-induced cell death in T cells

Activation-induced cell death (AICD), a process mediated by CD95 and CD95 ligand (CD95L), plays a critical role in regulating homeostasis of the immune system. Although the role of sphingolipids such as ceramides has been suggested to participate in CD95-mediated apoptosis, the exact role of these molecules in this process remains controversial. We employed myriocin, a specific inhibitor of serine palmitoyl-CoA transferase that mediates the first commitment step in sphingolipid synthesis. We found that myriocin could effectively block AICD in T-cell hybridomas and T-cell blasts. However, fumonisin B1, an inhibitor of the final step of ceramide synthesis, or inhibitors of sphingomyelinases did not prevent AICD. Furthermore, ceramide analogues, such as C2 and C6, could not reverse the inhibitory effect of myriocin. Interestingly, sphinganine, an intermediate of ceramide synthesis, completely reversed the inhibitory effect of myriocin, indicating a critical role of sphinganine. Myriocin did not modulate the expression of CD95 or CD95L, instead, it interfered with the early steps of CD95-mediated caspase activation. Therefore, we have uncovered a novel mechanism by which sphingolipid intermediates regulate CD95-mediated apoptosis.