Insulin-induced Ca2+ entry in hepatocytes is important for PI 3-kinase activation, but not for insulin receptor and IRS-1 tyrosine phosphorylation

AbstractInsulin produces an influx of Ca2+ into isolated rat hepatocyte couplets that is important to couple its tyrosine kinase receptor to MAPK activity (Benzeroual et al., Am. J. Physiol. 272, (1997) G1425–G1432. In the present study, we have examined the implication of Ca2+ in the phosphorylation state of the insulin receptor (IR) β-subunit and of insulin receptor substrate-1 (IRS-1), as well as in the stimulation of PI 3-kinase activity in cultured hepatocytes. External Ca2+ chelation (EGTA 4 mM) or administration of Ca2+ channel inhibitors gadolinium 50 μM or nickel 500 μM inhibited insulin-induced PI 3-kinase activation by 85, 50 and 50%, respectively, whereas 200 μM verapamil was without effect. In contrast, the insulin-induced tyrosine phosphorylation of IR β-subunit and of IRS-1 was not affected by any of the experimental conditions. Our data demonstrate that the stimulation of PI 3-kinase activity by the activated insulin receptor, but not the phosphorylation of IR β-subunit and IRS-1, requires an influx of Ca2+. Ca2+ thus appears to play an important role as a second messenger in insulin signaling in liver cells