Rhesus monkeys frequently develop obesity and insulin resistance followed by type 2 diabetes when allowed free access to chow. This insulin resistance is partly due to defective glucose transport into skeletal muscle. In this study, we examined signaling factors required for insulin-stimulated glucose transport in muscle biopsies taken during euglycemic-hyperinsulinemic clamps in nondiabetic, obese prediabetic, and diabetic monkeys. Insulin increased activities of insulin receptor substrate (IRS)-1-dependent phosphatidylinositol (PI) 3-kinase and its downstream effectors, atypical protein kinase Cs (aPKCs) (ζ/λ/ι) and protein kinase B (PKB) in muscles of nondiabetic monkeys. Insulin-induced increases in glucose disposal and aPKC activity diminished progressively in prediabetic and diabetic monkeys. Decreases in aPKC activation appeared to be at least partly due to diminished activation of IRS-1-dependent PI 3-kinase, but direct activation of aPKCs by the PI 3-kinase lipid product PI-3,4,5-(PO4)3 was also diminished. In conjunction with aPKCs, PKB activation was diminished in prediabetic muscle but, differently from aPKCs, seemed to partially improve in diabetic muscle. Interestingly, calorie restriction and avoidance of obesity largely prevented development of defects in glucose disposal and aPKC activation. Our findings suggest that defective activation of aPKCs contributes importantly to obesity-dependent development of skeletal muscle insulin resistance in prediabetic and type 2 diabetic monkeys.
Address correspondence and reprint requests to Robert V. Farese, ACOS-151, James A. Haley Veterans Administration Hospital, 13000 Bruce B. Downs Blvd., Tampa, FL 33612. E-mail:.
Received for publication 7 December 2001 and accepted in revised form 16 July 2002.
aPKC, atypical protein kinase C; IRS, insulin receptor substrate; Kglu, glucose disappearance rate; PDK-1, 3-phosphoinositide-dependent protein kinase-1; PI, phosphatidylinositol; PIP3, PI-3,4,5-(PO4)3; PKB, protein kinase B.