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Vanadate Augments Insulin Binding and Prolongs Insulin Action in Rat Adipocytes^*

I. GEORGE FANTUS, FERHAAN AHMAD and GUY DERAGON

Protein and Polypeptide Hormone Laboratory, Department of Medicine, Royal Victoria Hospital and McGill University Montreal, Quebec, Canada

Address all correspondence and requests for reprints to: Dr. I. G. Fantus, Protein and Polypeptide Hormone Laboratory, Room 2-11, Strathcona Medical Building, McGill University, 3640 University Street, Montreal, Quebec, H3A 2B2 Canada.

Abstract

Vanadate has been documented to inhibit tyrosine phosphatase activity and to have insulin-mimetic effects. However, oral administration to hypoinsulinemic diabetic rats in vivo lowers blood glucose at serum concentrations of vanadate that have minimal insulin-like effects in vitro. We, therefore, investigated the effect of low concentrations of vanadate on insulin binding, processing, and action. Preincubation of rat adipocytes for 2 h at 37 C with 10-200 µM vanadate resulted in a dose-dependent increase in [125I]insulin binding at 37 C to a maximum of 45% above the control value. Total cell-associated radioactivity and internalized (acid-resistant) hormone were similarly increased. Binding studies at 15 C in the presence of potassium cyanide revealed that this effect was associated with an increase in insulin receptor affinity. Consistent with these results, vanadate affected binding at 37 C only at low concentrations of insulin. Preloading adipocytes for 8 min with 0.4 ng/ml [¹²⁵I]insulin revealed that vanadate slowed the rate of release of internalized hormone (50% release; 9.0 min vs. 12.5 min). The proportion of [¹²⁵I]insulin released in intact form (trichloroacetic acid precipitable) was significantly increased by vanadate up to 15 min.

Preincubation of adipocytes with vanadate resulted in an apparent increased sensitivity, with a shift to the left in the dose-response curve of insulin-stimulated lipogenesis (ED6o, 0.2 _vs. 0.08 ng/ml). Furthermore, vanadate maintained maximum insulin-stimulated lipogenesis after extensive washing to remove insulin. These effects could not be accounted for by the insulinmimetic effect of vanadate alone.

We conclude that 1) low concentrations of vanadate (<200 µM) increase insulin receptor affinity and consequent insulin uptake in rat adipocytes; 2) the excess cell-associated insulin exists largely as intact hormone; and 3) the increased binding at low insulin concentrations results in an apparent increase in insulin sensitivity. Vanadate at low concentrations also prolongs insulin action. Whether tyrosine phosphatase inhibition is the basic biochemical mechanism remains to be determined. (Endocrinology 127: 2716–2725,1990)

Footnotes

^* This work was supported by the Medical Research Council of Canada (Grant MA-7658).

Recipient of a Chercheur Boursier Scholarship from the Fonds de la Recherche en Sante du Québec.

Supported by a Medical Research Council of Canada, McGill University Faculty of Medicine Summer Student Award.

Received June 17, 1990.

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