Apoptosis: Sphingolipid cofactors
Nature Chemical Biology 8, 410 (May 2012)
Apoptosis depends upon BCL-2 proteins for mitochondrial outer membrane permeabilization (MOMP) and cytochrome c release. The mitochondrial outer membrane also interacts with other organelles at loci known to regulate lipid biosynthesis. Although sphingolipid metabolism is known to regulate apoptosis, the relationship between sphingolipids and BCL-2 proteins is not well understood. Chipuk et al. develop a biochemical protocol for isolating mitochondria from heavy membrane fractions (HMFs) and apply it to show that higher doses of an activator of BAK, a proapoptotic BCL-2 protein, are required to induce cytochrome c release from mitochondria than from HMFs. Adding back microsomal components removed during purification restored sensitivity. Peptide analysis of the microsomes revealed the presence of two neutral sphingomyelinases (N-SMases). Exogenous N-SMases cooperated with proapoptotic factors and mitochondria to promote MOMP. The authors confirmed that N-SMases were present in heterotypic mitochondria-associated membranes. Reconstitution of sphingolipid metabolism or inclusion of sphingolipid metabolites revealed that BAK cooperates with sphingosine-1-phosphate and that BAX cooperates with hexadecenal to promote cytochrome c release. The authors validated and extended these findings in a TNF–induced model of apoptosis in mammalian cells. Taken together, these data suggest that heterotypic mitochondrial membrane interactions are necessary to provide the appropriate sphingolipid environment for MOMP to proceed efficiently. Future studies will be needed to understand mechanistically how sphingolipids affect the activation of proapoptotic BCL-2 proteins.