We report here that quercetin, a naturally occurring bioflavonoid, is an effective blocker of insulin receptor tyrosine kinase-catalyzed phosphorylation of exogenous substrate. The ID50 was estimated to be 2 +/- 0.2-mu-M in cell-free experiments, using a partially purified insulin receptor and a random copolymer of glutamic acid and tyrosine as a substrate. Insulin-stimulated autophosphorylation of the receptor itself was not blocked by quercetin (up to 500-mu-M). In intact rat adipocytes, quercetin inhibited insulin-stimulating effects on glucose transport, oxidation, and its incorporation into lipids. Inhibition of lipogenesis (50%) occurred at 47 +/- 4-mu-M, whereas full inhibition was evident at 110 +/- 10-mu-M quercetin. In contrast, the effect of insulin in inhibiting lipolysis remained unaltered in quercetin-treated adipocytes. The inhibitor was devoid of general adverse cell affects. Basal activities and the ability of lipolytic agents to stimulate lipolysis were not affected. Inhibition by quercetin enabled us to evaluate which insulinomimetic agents are dependent on tyrosine phosphorylation of endogenous substrates for stimulating glucose metabolism. Quercetin blocked lipogenesis mediated by insulin, wheat germ agglutinin, and concanavalin A. The lipogenic effect of Zn2+ and Mn2+ was partially blocked, whereas that of vanadate was not affected at all. These results suggest that (a) insulin-activated glucose metabolism in rat adipocytes is dependent on receptor activity to phosphorylate endogenous substrates, (b) insulin receptor tyrosine kinase activity is not a prerequisite for insulin's lipolysis inhibiting effect, (c) insulinomimetic lectins need endogenous tyrosine phosphorylation to express their insulin-like effects, and (d) glucose metabolism can be activated in an alternative fashion (with vanadate ions), under conditions in which, presumably, the endogenous insulin receptor tyrosine transphosphorylation is fully arrested.
