Defective Insulin-Induced GLUT4 Translocation in Skeletal Muscle of High Fat–Fed Rats Is Associated With Alterations in Both Akt/Protein Kinase B and Atypical Protein Kinase C (ζ/λ) Activities

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Abstract
The cellular mechanism by which high-fat feeding induces skeletal muscle insulin resistance was investigated in the present study. Insulin-stimulated glucose transport was impaired (∼40–60%) in muscles of high fat–fed rats. Muscle GLUT4 expression was significantly lower in these animals (∼40%, P < 0.05) but only in type IIa–enriched muscle. Insulin stimulated the translocation of GLUT4 to both the plasma membrane and the transverse (T)-tubules in chow-fed rats. In marked contrast, GLUT4 translocation was completely abrogated in the muscle of insulin-stimulated high fat–fed rats. High-fat feeding markedly decreased insulin receptor substrate (IRS)-1–associated phosphatidylinositol (PI) 3-kinase activity but not insulin-induced tyrosine phosphorylation of the insulin receptor and IRS proteins in muscle. Impairment of PI 3-kinase function was associated with defective Akt/protein kinase B kinase activity (−40%, P < 0.01) in insulin-stimulated muscle of high fat–fed rats, despite unaltered phosphorylation (Ser473/Thr308) of the enzyme. Interestingly, basal activity of atypical protein kinase C (aPKC) was elevated in muscle of high fat–fed rats compared with chow-fed controls. Whereas insulin induced a twofold increase in aPKC kinase activity in the muscle of chow-fed rats, the hormone failed to further increase the kinase activity in high fat–fed rat muscle. In conclusion, it was found that GLUT4 translocation to both the plasma membrane and the T-tubules is impaired in the muscle of high fat–fed rats. We identified PI 3-kinase as the first step of the insulin signaling pathway to be impaired by high-fat feeding, and this was associated with alterations in both Akt and aPKC kinase activities.

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