BcR-induced apoptosis involves differential regulation of C₁₆ and C₂₄-ceramide formation and sphingolipid-dependent activation of the proteasome

In this study, we describe an ordered formation of long- and very long-chain ceramide species in relation to the progression of B-cell receptor (BcR) triggering induced apoptosis. An early and caspase-independent increase in long-chain ceramide species, in which C₁₆-ceramide predominated, was observed 6 h after BcR triggering. In contrast, very long-chain ceramide species were generated later, 12-24 h after BcR triggering. The formation of these very long-chain ceramide species, in which C₂₄-ceramide predominated, required the activation of effector caspases. BcR-induced formation of long-chain ceramide species resulted in proteasomal activation and degradation of XIAP and subsequent activation of effector caspases, demonstrating an important cell-biological mechanism through which long-chain ceramides may be involved in the progression of BeR triggering induced apoptosis and subsequent formation of very long-chain ceramide species. BcR-induced activation of the proteasome was blocked with ISP-1/myriocin, a potent and selective inhibitor of serine palmitoyl transferase that catalyzes the first and rate-limiting step in the de novo formation of ceramide. Both ISP-1 and clasto-lactacystin β-lactone, an irreversible inhibitor of the proteasome, prevented BcR cross-linking-induced XIAP degradation. Also, a mutant XIAP lacking the ubiquitin-ligating ring finger motif was completely resistant to proteasome-mediated degradation, and Ramos cells overexpressing XIAP became highly resistant to BcR cross-linking-induced activation of caspases. The formation of C₁₆-ceramide in response to BcR cross-linking was found unaltered in XIAP overexpressing Ramos cells, whereas C₂₄-ceramide formation was completely abolished. These results demonstrate how de novo generated long-chain ceramide species may be involved in the activation of downstream effector caspases and subsequent formation of very long-chain ceramide species. As such, these results provide novel and important insights into the significance of specific ceramide species in defined stages of apoptosis.