Fig. 2.

Proposed interplay between TAG, glycerophospholipids and sphingolipid metabolism. De novo synthesis of sphingolipids occurs via SPT in the endoplasmic reticulum. A ceramide transfer protein transports ceramide to the Golgi apparatus, where complex sphingolipids, like SM and glycosphingolipids (not shown) are synthesized. Complex sphingolipids are then transferred to the plasma membrane, which contains more then 75% of the total cellular sphingolipids.

The synthesis of both TAG and glycerophospholipid from activated fatty acids follows a common pathway to the formation of DAG. DAG can be used either for TAG synthesis, storage, and secretion or for synthesis of the two main glycerophospholipid classes, phosphatidylcholine (PC) or phosphatidylserine (PS). Metabolically, the sphingolipid, glycerophosoholipid and TAG pathways are linked through SM synthase, shown in the middle of the diagram. Exogenous palmitic acid partitions in a competitive fashion between sphingolipid and TAG pools.

DAG and ceramide (boxed in the middle) are bioactive metabolites implicated in down-regulation of insulin signaling cascade. As the diagram indicates, they are in the crossroad of mulltiple pathways and their levels are subject to complex regulation.