Abstract
Ceramides are known to have a regulatory function in apoptosis, including the release of cytochrome c and other proapoptotic factors from the mitochondrial intermembrane space. Ceramides can form large, stable channels in the outer mitochondrial membrane, leading to the proposal that ceramide channels are the pathway through which these proteins are released. Here, we report that sphingosine, a product of ceramide hydrolysis by ceramidase, is capable of destabilizing ceramide channels, leading to their disassembly. Sphingosine is directly responsible for the disassembly of ceramide channels in planar membrane experiments and markedly reduces the ability of ceramide to induce the release of intermembrane space proteins from mitochondria in vitro. Low concentrations of both L and D sphingosine potentiate the release of intermembrane space proteins by long-chain ceramide and channel formation in liposomes. These results provide evidence for a mechanism by which the disassembly of ceramide channels, as initiated by ceramidase, could be accelerated by the direct interaction of the hydrolysis product with the ceramide channels themselves. This mechanism therefore could form a positive feedback loop for rapid shut-down of ceramide channels. However, potentiation of ceramide channel formation is also possible and thus both effects could influence the propensity for mitochondria-mediated apoptosis.