Abstract 15940: Calcium and Insulin Regulate the Novel Glucose Transporter 8 (GLUT8) in the Healthy and Diabetic Myocardium

Abstract

Diabetes, an epidemic disease, is associated with the development of diabetic cardiomyopathy. Glucose uptake from the bloodstream into the cell is the rate-limiting step in whole body glucose utilization and is regulated by a family of membrane proteins called the glucose transporters (GLUT). The translocation of GLUT4, the main isoform, to the cell surface is insulin and contraction/calcium dependent in striated muscle. Contraction is regulated by the sarcoplasmic reticulum calcium ATPase (SERCA) pump. We recently demonstrated that diabetes impairs cardiac GLUT4 trafficking due to alterations of insulin signaling pathway and SERCA pump activity. However, the role of other GLUTs in the heart is still not elucidated. Although GLUT8 is highly expressed in the heart and could play a key role in cardiac glucose transport, its function and regulation are unknown. We hypothesized that, as for GLUT4, insulin and calcium will regulate GLUT8 translocation to the myocardial cell surface, which will be impaired during diabetes. Active cell surface GLUT (4 & 8) content was quantified by a biotinylated photolabeled assay in both intact perfused myocardium and isolated ventricular myocytes. To our knowledge, we are the first to apply this technique to GLUT8. Total GLUT (4 & 8) protein expression was measured by Western blot. Diabetes was induced by streptozotocin injection in wild type (WT) and transgenic (TG) mice overexpressing the SERCA pump in the heart and compared to their respective healthy littermates (n=16/group). In healthy myocytes, GLUT8 was primarily located intracellularly under basal condition, as for GLUT4. Insulin, calcium and isoproterenol stimulation (a beta-adrenergic agonist) significantly increased the translocation of both GLUT-4 and -8 to the cell surface. In addition, total GLUT-4 and -8 protein content was increased by 30 and 40% (P<0.05), respectively, in TG healthy mice overexpressing the SERCA pump compared to WT mice. As for GLUT4, active cell surface GLUT8 was decreased by 80% (P<0.05) in the diabetic ventricle but was restored by SERCA overexpression. Overall, our data suggest that, as for GLUT4, GLUT8 is a novel insulin-sensitive GLUT isoform in the heart, and its translocation is regulated by both insulin and calcium dependent pathways.