HIGH-FAT FEEDING IMPAIRS INSULIN-STIMULATED GLUT4 RECRUITMENT VIA AN EARLY INSULIN-SIGNALING DEFECT

Glucose transport in skeletal muscle can be mediated by two separate p athways, one stimulated by insulin and the other by muscle contraction . High-fat feeding impairs glucose transport in muscle, but the mechan ism remains unclear. FVB mice (3 weeks old) were fed a high-fat diet ( 55% fat, 24% carbohydrate, 21% protein) or standard chow for 3-4 weeks or 8 weeks. Insulin-stimulated glucose transport, assessed with eithe r 2-deoxyglucose or 3-O-methylglucose was decreased 35-45% (P < 0.001) in isolated soleus muscle, regardless of diet duration. Similarly, gl ucose transport stimulated by okadaic acid, a serine/threonine phospha tase inhibitor, was also 45% lower with high-fat feeding, but the gluc ose transport response to hypoxia or -(6-aminohexyl)-5-chloro-1-naphth alene-sulfonamide (W-7) (which are stimulators of the ''contraction pa thway'') was intact. Hexokinase I, II, and total activity were normal in soleus muscle from high-fat-fed mice. GLUT4 expression in soleus mu scle from the high-fat-fed mice was also normal, but the insulin-stimu lated cell surface recruitment of GLUT4 assessed by exofacial photolab eling with [H-3]-ATB bis-mannose was reduced by 50% (P < 0.001). Insul in-receptor substrate 1 (IRS-1) associated phosphatidylinositol (PI) 3 -kinase activity stimulated by insulin was also reduced by 36% (P < 0. 001), and expression of p85 and p110(b) subunits of PI 3-kinase was no rmal. In conclusion, high-fat feeding selectively impairs insulin-stim ulated, but not contraction-pathway-mediated, glucose transport by red ucing GLUT4 translocation to the plasma membrane. This appears to resu lt from an acquired defect in insulin activation of PI 3-kinase. Since effects of okadaic acid on glucose transport are independent of PI 3- kinase, a second signaling defect may also be induced.