Glucose and Insulin Chronically Regulate Insulin Action via Different Mechanisms in BC₃H1 Myocytes: Effects on Glucose Transporter Gene Expression

We previously reported that, in primary cultured adipocytes, chronic exposure to glucose plus insulin impairs the insulin-responsive glucose transport system. In this study, we examined regulation of glucose transport in BC₃H1 myocytes as a model for muscle and found important differences between BC₃HI cells and adipocytes. In myocytes, chronic glucose exposure per se (25 mM) decreased basal glucose transport activity by 78% and insulin's acute ability to maximally stimulate transport by 68% (ED₅₀ ∼2.5 mM; T_½ ∼4 h). D-Mannose and 3-O-methylglucose diminished transport rates with ∼100 and 50% of the potency of D-glucose, respectively, wherea L-glucose, D-fructose, and D-galactose were inactive. Chronic glucose exposure also reduced cell surface insulin binding by 30% via an apparent decrease in receptor affinity, and this effect was associated with a comparable rightward shift in the insulin-glucose transport dose-response curve. In other studies, persistent stimulation with 15 nM insulin also decreased maximally stimulated glucose transport activity, which was independent and additive to the regulatory effect of glucose. Moreover, glucose and insulin-induced insulin resistance via different mechanisms. Glucose (25 mM) reduced the number of cellular glucose transporter proteins by 84% and levels of GLUT1 transporter mRNA by 50% (whether normalized to total RNA or CHO-B mRNA). In contrast, chronic insulin exposure led to a 2.1-fold increase in GLUT1 mRNA but did not alter cellular levels of transporter protein. Cotreatment with glucose prevented the insulin-induced rise in GLUT1 mRNA. BC₃HI cells did not express GLUT4 mRNA thatencodes the major transporter isoform in skeletal muscle. In conclusion, in BC₃H1 myocytes 1) glucose diminished insulin sensitivity by decreasing insulin receptor binding affinity and decreased basal and maximally insulin-stimulated glucose transport rates via cellular depletion of glucose transporters and suppression of GLUT1 mRNA; 2) chronic insulin exposure exerted an independent and additive effect to reduce maximal transport activity; however, insulin increased levels of GLUT1 mRNA and did not alter the cellular content of glucose transporters; and 3) although BC₃H1 cells are commonly used as a model for skeletal muscle, studies examining glucose transport should be interpreted cautiously due to the absence of GLUT4 expression. Nevertheless, the data generally support the idea that, in non-insulindependent diabetes mellitus, hyperglycemia and hyperinsulinemia can induce or exacerbate insulin resistance in target tissues.