ORIGINAL ARTICLE The Consequences of Long-Term Glycogen Synthase Kinase-3 Inhibition on Normal and Insulin Resistant Rat Hearts T. B. Flepisi & Amanda Lochner & Barbara Huisamen Published online: 3 July 2013 # Springer Science+Business Media New York 2013 Abstract Background Glycogen synthase kinase-3 (GSK-3) is a ser- ine-threonine protein kinase, discovered as a regulator of glycogen synthase. GSK-3 may regulate the expression of SERCA-2a potentially affecting myocardial contractility. It is known to phosphorylate and inhibit IRS-1, thus disrupting insulin signalling. This study aimed to determine whether myocardial GSK-3 protein and its substrate proteins are dysregulated in obesity and insulin resistance, and whether chronic GSK-3 inhibition can prevent or reverse this. Methods Weight matched male Wistar rats were rendered obese by hyperphagia using a special diet (DIO) for 16 weeks and compared to chow fed controls. Half of each group was treated with the GSK-3 inhibitor CHIR118637 (30 mg/kg/day) from week 12 to16 of the diet period. Biometric and biochem- ical parameters were measured and protein expression determined by Western blotting and specific antibodies. Ca 2+ ATPase activity was determined spectrophotometrically. Cardiomyocytes were prepared by collagenase perfusion and insulin stimulated 2-deoxy-glucose uptake determined. Results DIO rats were significantly heavier than controls, as- sociated with increased intra-peritoneal fat and insulin resis- tance. GSK-3 inhibition did not affect weight but improved insulin resistance, also on cellular level. It had no effect on GSK-3 expression but elevated its phospho/total ratio and elevated IRS-2 expression. Obesity lowered SERCA-2a ex- pression and activity while GSK-3 inhibition alleviated this. The phospho/total ratio of phospholamban underscored inhibi- tion of SERCA-2a in obesity. In addition, signs of myocardial hypertrophy were observed in treated control rats. Conclusion GSK-3 inhibition could not reverse all the det- rimental effects of obesity but may be harmful in normal rat hearts. It regulates IRS-2, SERCA-2a and phospholamban expression but not IRS-1. Keywords Cardiomyopathy . GSK-3 . Insulin resistance . Obesity . GSK-3 inhibition Introduction Glycogen synthase kinase-3 (GSK-3) is a serine/threonine protein kinase that was discovered in the early 1980s as an enzyme involved in regulation of glucose metabolism [1, 2] This involvement increased interest and since then, it has been extensively researched, especially in the context of the metabolic actions of insulin [3, 4]. In humans, two GSK-3 isoforms have been identified, GSK-3α with a molecular weight of ~51 kDa and GSK-3β with a molecular weight of ~47 kDa [5, 6] These two isoforms are structurally similar, however, their expression patterns, substrate preferences and cellular functions are not identical [4, 7] GSK-3 is constitu- tively active in cells and can be acutely inactivated by insulin through phosphorylation by protein kinase B (PKB/Akt). PKB/Akt therefore directly regulates the activity of GSK-3 [8, 9]. Studies have shown that GSK-3 phosphorylates and inhibits glycogen synthase, therefore glycogen synthesis, as well as insulin receptor substrate-1 (IRS-1) activity thereby impairing insulin signalling [10]. In addition, it has been reported that GSK-3 protein plays an important role in the T. B. Flepisi Department of Medicine, Division of Pharmacology, Faculty of Health Sciences, University of Stellenbosch, Tygerberg 7505, South Africa B. Huisamen Diabetes Discovery Platform, Medical Research Council, Tygerberg 7505, South Africa A. Lochner : B. Huisamen (*) Department of Biomedical Sciences, Division of Medical Physiology, Faculty of Health Sciences, University of Stellenbosch, P.O. Box 19063, Tygerberg 7505, Republic of South Africa e-mail: bh3@sun.ac.za Cardiovasc Drugs Ther (2013) 27:381–392 DOI 10.1007/s10557-013-6467-8