PREEXPOSURE TO GLUCOSAMINE INDUCES INSULIN-RESISTANCE OF GLUCOSE-TRANSPORT AND GLYCOGEN-SYNTHESIS IN ISOLATED RAT SKELETAL-MUSCLES - STUDY OF MECHANISMS IN MUSCLE AND IN RAT-1 FIBROBLASTS OVEREXPRESSING THE HUMAN INSULIN-RECEPTOR

Increased routing of glucose through the hexosamine-biosynthetic pathw ay has been implicated in the development of glucose-induced insulin r esistance of glucose transport in cultured adipocytes. Because both gl ucosamine and glucose enter this pathway as glucosamine-6-phosphate, w e examined the effects of preincubation with glucosamine in isolated r at diaphragms and in fibroblasts overexpressing the human insulin rece ptor (HIR-cells). In muscles, pre-exposure to glucosamine inhibited su bsequent basal and, to a greater extent, insulin-stimulated glucose tr ansport in a time- and dose-dependent manner and abolished the stimula tion by insulin of glycogen synthesis. Insulin receptor number, activa tion of the insulin receptor tyrosine kinase in situ and after solubil ization, and the total pool of glucose transporters (GLUT4) were unaff ected, and glycogen synthase was activated by glucosamine pretreatment . In HIR-cells, which express GLUT1 and not GLUT4, basal and insulin-s timulated glucose trans rt were unaffected by glucosamine, but glycoge n synthesis was markedly inhibited. Insulin-stimulated activation of p rotein kinases (MAP and S6) was unaffected, and the fractional velocit y and apparent total activity of glycogen synthase was increased in gl ucosamine-treated HIR-cells. In pulse-labeling studies, addition of gl ucosamine during the chase prolonged processing of insulin proreceptor s to receptors and altered the electrophoretic mobility of proreceptor s and processed alpha-subunits, consistent with altered glycosylation. Glucosamine-induced insulin resistance of glucose transport appears t o be restricted to GLUT4-expressing cells, i.e., skeletal muscle and a dipocytes; it may reflect impaired translocation of GLUT4 to the plasm alemma. The glucosamine-induced imbalance in UDP sugars, i.e., increas ed UDP-N-acetylhexosamines and decreased UDP-glucose, may alter glycos ylation of critical proteins and limit the flux of glucose into glycog en.