The Insulin-Like Effect of Vanadate on Lipolysis in Rat Adipocytes Is Not Accompanied by an Insulin-Like Effect on Tyrosine Phosphorylation*

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Abstract
Tyrosine phosphorylation of the insulin receptor and other intracellular proteins in rat adipocytes was examined using an immunoblot technique with antiphosphotyrosine antibody. Insulin at 10-7 M increased the tyrosine phosphorylation of the 95K subunit of the insulin receptor (15-fold) and proteins of 180K (7-fold) and 60K (23-fold). Increases in insulin-dependent phosphorylation of the three proteins were detectable at 10-10 M insulin and attained steady state within 30 sec of insulin (10-7 M) addition. Small effects of insulin (< 30% increases) were observed on proteins of 120K and 53K. In contrast to insulin, the effect of vanadate on tyrosine phosphorylation were small and nonspecific. Vanadate increased tyrosine phosphorylation of the 95K insulin receptor .beta.-subunit and the 120K and 60K proteins similarity, with increases of 1.5- to 3-fold at 1 mM and 2-fold or less at 200 and 50 .mu.M. Vanadate-dependent tyrosine phosphorylation of the 180K protein increased to a maximum of only 30% at 200 .mu.M. Tyrosine phosphorylation of the 53K protein was somewhat larger, approaching 4-fold at 1 mM vanadate. The concentrations of insulin and vanadate that inhibited isoproterenol-dependent lipolysis were not comparable to those that increased tyrosine phosphorylation. Vanadate at 1 mM was more potent as an antilipolytic agent than 10-9 M insulin (93% vs. 81%), yet increased tyrosine phosphorylation of the 95K insulin receptor .beta.-subunit only as effectively as 10-10 M insulin (which inhibited lipolysis only 42%). The dissimilar responses were even more pronounced when antilipolysis was compared to tyrosine phosphorylation of the 180K and 60K proteins. For example, insulin at 10-9 M increased tyrosine phosphorylation of the 180K protein 2.9-fold, while 1 mM vanadata had a negligible effect (10% increase). Thus, vanadate exerts an insulin-like effect on lipolysis, yet its effects on tyrosine phosphorylation differ from those of insulin.

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