Freshly isolated adult rat ventricular cardiomyocytes have been used to characterize the action profile of the new thiazolidinedione antidiabetic drug MCC-555. Preincubation of cells with the compound (100 μM for 30 min or 10 μM for 2 h) did not modify basal 3-O-methylglucose transport, but produced a marked sensitizing effect (2-to 3-fold increase in insulin action at 3 x 10-11 M insulin) and a further enhancement of maximum insulin action (1.8-fold). MCC-555 did not modulate autophosphorylation of the insulin receptor and tyrosine phosphorylation of insulin receptor substrate- 1 (IRS-1). However, insulin action (10-10 and 10-7 M) on IRS-1- associated phosphatidyl-inositol (PI) 3-kinase activity was enhanced 2-fold in the presence of MCC-555. Association of the p85 adapter subunit of PI 3- kinase to IRS-1 was not modified by the drug. Immunoblotting experiments demonstrated expression of the peroxisomal proliferator-activated receptor- γ in cardiomyocytes reaching about 30% of the abundance observed in adipocytes. The insulin-sensitizing effect of MCC-555 was lost after inhibition of protein synthesis by preincubation of the cells with cycloheximide (1 mM; 30 min). Cardiomyocytes from obese Zucker rats exhibited a completely blunted response of glucose transport at 3 x 10-11 M insulin. MCC-555 ameliorates this insulin resistance, producing a 2-fold stimulation of glucose transport, with maximum insulin action being 1.6-fold higher than that in control cells. This drug effect was paralleled by a significant dephosphorylation of IRS-1 on Ser/Thr. In conclusion, MCC-555 rapidly sensitizes insulin-stimulated cardiac glucose uptake by enhancing insulin signaling resulting from increased intrinsic activity of PI 3-kinase. Acute activation of protein expression leading to a modulation of the Ser/Thr phosphorylation state of signaling proteins such as IRS-1 may be underlying this process. It is suggested that MCC-555 may provide a causal therapy of insulin resistance by targeted action on the defective site in the insulin signaling cascade.
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