Partial insulin resistance in the mouse BC3H-1 cell line: absent hexose-independent actions of insulin.

Abstract

We have studied the regulation of glycogen metabolism by insulin in the insulin-sensitive nonfusing muscle cell line BC3H-1. The basal percentage of glycogen synthase I activity was not altered by insulin alone at any concentration, time of exposure, or age of cells tested. The addition of glucose or 2-deoxyglucose to the glucose- and serum-free incubation medium caused a 2-fold increase in glycogen synthase I activity over basal levels, and the effect was enhanced to 3-fold if insulin was added to the medium. Glycogen phosphorylase a activity was not altered by incubation in the presence of insulin, but was lowered by the addition of 2-deoxyglucose. This effect was also enhanced in the presence of insulin. The effect of exogenously added sugar occurred only if a 6-phosphorylatable hexose was used. The effect seen with 2-deoxyglucose was stable to Sephadex G-25 desalting, suggesting that activation of glycogen synthase was the result of a stable (covalent) modification of the enzyme. We were also able to demonstrate the presence of glucose-6-phosphate-activatable glycogen synthase phosphatase activity in the myocytes. The effect of 2-deoxyglucose in the presence or absence of insulin could be completely reversed by including cytochalasin B in the medium, suggesting that both the effect of hexose and the insulin enhancement of its effect were entirely dependent on carrier-mediated hexose uptake. Four insulin-mimetic agents, H2O2 Concanavalin A, Na orthovanadate, and antiinsulin receptor B2 serum, were also tested. Despite different mechanisms of action, each agent qualitatively mimicked insulin in the myocytes. All stimulated hexose transport, glucose incorporation into glycogen, and hexose-dependent activation of glycogen synthase in a manner not additive with insulin, but none increased basal glycogen synthase I activity in the absence of hexose. These results suggest that although insulin is capable of regulating glycogen metabolism both by increasing the uptake of sugar and by altering the activation state of glycogen synthase and phosphorylase, these effects are entirely due to the stimulation of hexose uptake, and hexose-independent actions of insulin are absent in BC3H-1 cells.