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1 Department of Medicine 9111G, University of California, San Diego, La Jolla,, California, United States
2 VA San Diego Healthcare System, San Diego, California, United States
* To whom correspondence should be addressed. E-mail: tciaraldi{at}popmail.ucsd.edu.
Glycogen synthase kinase 3 (GSK-3) is a ubiquitous kinase implicated in both insulin action and adipogenesis. In order 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 non-obese 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-month caloric restriction resulting in a 7% decrease in BMI and 21% improvement in insulin-stimulated whole body glucose disposal rate (GDR). GSK-3
and GSK-3
expression decreased in adipocytes (p<0.05), while GSK-3 protein expression increased in skeletal muscle (p<0.05). OT2D subjects were treated with troglitazone or metformin for 3-4 months. After troglitazone treatment GDR improved (p<0.05) despite an increase in the BMI (p<0.05), while metformin had no significant effect on GDR. There was no significant change in GSK-3 expression in adipocytes following troglitazone, while both GSK-3 and - 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, while the function in adipose tissue is less clear.
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