ARACHIDONATE ACTIVATION OF PROTEIN-KINASE-C MAY BE INVOLVED IN THE STIMULATION OF PROTEIN-SYNTHESIS BY INSULIN IN L6 MYOBLASTS

Insulin stimulated protein synthesis in L6 myoblasts but did not incre ase the labelling of DAG or the release of phosphocholine from phospha tidylcholine. The DAG lipase inhibitor, RHC 80267, more than doubled t he amount of label appearing in DAG but did not stimulate protein synt hesis. Even in the presence of the DAG lipase inhibitor insulin failed to have any effect on DAG labelling, and conversely RHC 80267 did not modify the insulin-induced increase in protein synthesis. These resul ts suggest that endogenous DAG production is not involved in the stimu lation of,protein synthesis by insulin. However, exogenous diacylglyce rols (1-oleoyl-2-acetyl glycerol and 1-stearoyl-2-arachidonoyl glycero l) both stimulated protein synthesis in L6 myoblasts. The efficacy of the former (arachidonate-free) DAG suggested that their action was by activation of protein kinase C rather than by arachidonate release and prostaglandin formation. Ibuprofen, an inhibitor of cyclo-oxygenase f ailed to block the effects of insulin whereas a second cyclo-oxygenase inhibitor, indomethacin had only a partial inhibitory effect. The pro tein kinase C (PKC) inhibitor, RO-31-8220, totally blocked the effect of insulin. Since indomethacin is also recognised to inhibit phospholi pase A(2), the data suggests that insulin acts on protein synthesis in myoblasts by arachidonate activation of PKC.