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

Insulin 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) beta-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 mu M or nickel 500 mu M inhibited insulin-induced PI 3-kinase activation by 85, 50 and 50%, respectively, whereas 200 mu M verapamil was without effect. In contrast, the insulin-induced tyrosine phosphorylation of IR beta-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 beta-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. (C) 2000 Elsevier Science B.V. All rights reserved.