Involvement of phospholipids in the mechanism of insulin action in HEPG2 cells.

The mechanism of action by which insulin increases phosphatidic acid (PA) and diacylglycerol (DAG) levels was investigated in cultured hepatoma cells (HEPG2). Insulin stimulated phosphatidylcholine (PC) and phosphatidyl-inositol (PI) degradation through the activation of specific phospholipases C (PLC). The DAG increase appears to be biphasic. The early DAG production seems to be due to PI breakdown, probably through phosphatidyl-inositol-3-kinase (PI3K) involvement, whereas the delayed DAG increase is derived directly from the PC-PLC activity. The absence of phospholipase D (PLD) involvement was confirmed by the lack of PC-derived phosphatidylethanol production. Experiments performed in the presence of R59022, an inhibitor of DAG-kinase, indicated that PA release is the result of the DAG-kinase activity on the DAG produced in the early phase of insulin action.