Tissue-specific expression and regulation of GSK-3 in human skeletal muscle and adipose tissue

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Abstract
Glycogen synthase kinase-3 (GSK-3) is a ubiquitous kinase implicated in both insulin action and adipogenesis. To determine how these multiple roles may relate to insulin resistance, we studied the regulation of GSK-3 protein expression and phosphorylation in skeletal muscle and isolated adipocytes from nonobese healthy control (HC), obese control (OC), and obese type 2 diabetic (OT2D) subjects. At baseline there were no differences in the GSK-3 protein expression in adipocytes. OC subjects underwent a 6-mo caloric restriction resulting in a 7% decrease in body mass index (BMI) and a 21% improvement in insulin-stimulated whole body glucose disposal rate (GDR). GSK-3α and GSK-3β expression decreased in adipocytes ( P < 0.05), whereas GSK-3α protein expression increased in skeletal muscle ( P < 0.05). OT2D subjects were treated with troglitazone or metformin for 3–4 mo. After troglitazone treatment GDR improved ( P < 0.05) despite an increase in BMI ( P < 0.05), whereas metformin had no significant effect on GDR. There was no significant change in GSK-3 expression in adipocytes following troglitazone, whereas both GSK-3α and -β were decreased in skeletal muscle ( P < 0.05). Metformin treatment had no significant impact on GSK-3 protein expression in either adipocytes or skeletal muscle. Neither treatment influenced GSK-3 serine phosphorylation in skeletal muscle or adipocytes. These results suggest that there is tissue specificity for the regulation of GSK-3 in humans. In skeletal muscle GSK-3 plays a role in control of metabolism and insulin action, whereas the function in adipose tissue is less clear.

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