Lipid biochemistry: functions of glycerolipids and sphingolipids in cellular signaling.

Products of glycerolipid and sphingolipid metabolism are now known to fulfill second messenger functions in a variety of cellular signaling pathways. Evidence for glycerolipid-derived second messengers was first obtained from the "phosphatidylinositol cycle," which involves a signal-dependent hydrolysis of phosphatidylinositol bisphosphate yielding diacylglycerol and inositol trisphosphate. The role of diacylglycerol in the regulation of protein kinase C activity and its site of interaction with PKC are now well known. Recently, another glycerolipid second messenger, phosphatidic acid, was found to interact with the protooncogenic Raf-1 kinase. In cultured cells, a signal-induced generation of phosphatidic acid was critical for Raf-1 translocation to the cell membrane. Thus, different glycerolipid second messengers appear to regulate distinct targets with exquisite specificity. Analogous to the PI cycle, a "sphingomyelin cycle" was also found to exist, generating sphingolipid second messengers. Ceramide, derived from the agonist-induced hydrolysis of sphingomyelin, is a potent biomolecule with effects in multiple cell signaling pathways. The steroid hormone progesterone stimulated sphingomyelin hydrolysis in Xenopus oocytes. Ceramide, derived from the "sphingomyelin cycle," was sufficient for meiotic cell cycle progression in the oocytes. These results demonstrate the various effects of lipid-derived second messengers and promise exciting discoveries into the roles of lipids in cell signaling.