Effect of okadaic acid in rat adipocytes: differential stimulation of glucose and lipid metabolism and induction of refractoriness to insulin and vanadate.

The insulin-like effects of okadaic acid (OKA) in rat adipocytes were further characterized. Okadaic acid did not alter insulin receptor function. This includes undisturbed insulin binding and receptor-mediated ligand internalization in OKA-treated cells. Also, the tyrosine kinase activity of the insulin receptor was not modified in a cell-free system. The stimulating effects of OKA were significantly increased by preincubating (40 min) the cells at 37 C. At lower temperatures (i.e. 26-30 C), OKA did not mimic insulin. Maximal stimulation of lipogenesis occurred at 0.5 microM and then declined at higher concentrations. The insulin-like effects of OKA on lipogenesis did not persist after removal of the agent by washing at 37 C. Okadaic acid maximally stimulated the incorporation of [1-14C]glucose into lipids and the oxidation of [6-14C]glucose into 14CO2, but unlike insulin, it had little if any effect of oxidizing [1-14C]glucose to 14CO2 or incorporating [6-14C]glucose into lipids. Okadaic acid was equivalent to insulin in stimulating 3-O-methyl-glucose uptake. Since the insulin-like effects of OKA did not persist after preincubation and washing, the effects of insulin in OKA-treated cells could be evaluated. The adipocytes were found to be fully refractory to the modulating actions of insulin. Thus, insulin did not stimulate glucose transport, its oxidation, or its incorporation into lipids, and failed to reverse lipolysis. Unresponsiveness was fully developed after 40-min preincubation at 37 C with 3 microM OKA and was half-maximal at 0.13 microM OKA. It persisted at least over a period of 150 min. The effect of OKA was restricted to the stimulating actions of insulin and vanadate. Basal activities were not altered, nor was the ability of the desensitized cells to respond to isoproterenol. The lack of an insulin-like effect of OKA on some metabolic pathways enabled us to demonstrate that OKA (0.25 microM) also rendered adipocytes fully unresponsive to insulin in the continuous presence of the agent. Western blotting of the 40,000 x g pellets with antibodies to phosphotyrosine revealed the appearance of a protein with an apparent mol wt of 43,000 in OKA-desensitized cells. In summary, OKA mimics some of insulin bioeffects, but concomitantly renders the cells tolerant to the modulating action of the hormone itself.