Insulin signaling and insulin sensitizing in muscle and liver of obese monkeys: peroxisome proliferator-activated receptor gamma agonist improves defective activation of atypical protein kinase C

Abstract

Obesity, the metabolic syndrome, and aging share several pathogenic features in both humans and non-human primates, including insulin resistance and inflammation. Since muscle and liver are considered key integrators of metabolism, we sought to determine in biopsies from lean and obese aging rhesus monkeys the nature of defects in insulin activation and, further, the potential for mitigation of such defects by an in vivo insulin sensitizer, rosiglitazone, and a thiazolidinedione activator of the peroxisome proliferator-activated receptor gamma. The peroxisome proliferator-activated receptor gamma agonist reduced hyperinsulinemia, improved insulin sensitivity, lowered plasma triglycerides and free fatty acids, and increased plasma adiponectin. In muscle of obese monkeys, previously shown to exhibit defective insulin signaling, the insulin sensitizer improved insulin activation of atypical protein kinase C (aPKC), the defective direct activation of aPKC by phosphatidylinositol (PI)-3,4,5-(PO₄)₃, and 5'-AMP-activated protein kinase and increased carnitine palmitoyltransferase-1 mRNA expression, but it did not improve insulin activation of insulin receptor substrate (IRS)-1-dependent PI 3-kinase (IRS-1/PI3K), protein kinase B, or glycogen synthase. We found that, although insulin signaling was impaired in muscle, insulin activation of IRS-1/PI3K, IRS-2/PI3K, protein kinase B, and aPKC was largely intact in liver and that rosiglitazone improved insulin signaling to aPKC in muscle by improving responsiveness to PI-3,4,5-(PO₄)₃.

FIG. 1.

Model of insulin signaling in skeletal muscle. IRS-1, insulin receptor substrate-1; PI 3-K, phosphatidylinositol 3-kinase; PDK-1, 3-phosphoinositide dependent protein kinase-1; PKB, protein kinase B; GSK, glycogen synthase kinase; GS, glycogen synthase; PP1, protein phosphatase-1; PKA, protein kinase A; GLUT 4, glucose transporter 4.

FIG. 2.

Comparison of insulin signaling to IRS-2-dependent PI3K (A), IRS-1-dependent PI3K (B), aPKCs (C), and PKB (D), and in liver of lean versus obese monkeys before (−) and during (+) the insulin stimulation of a euglycemic hyperinsulinemic clamp. Basal values are shown by clear bars, and insulin-stimulated values are shown by shaded bars. The number of determinations is shown in parentheses. In the liver, there were no significant differences between the effects of insulin in lean versus obese (insulin-resistant) monkeys (all p's > 0.05). aPKC, atypical protein kinase C.

FIG. 3.

Alterations in body weight (upper left), fasting plasma levels of insulin (upper middle), and insulin-mediated glucose disposal rate (whole body insulin sensitivity) (upper right), and of fasting triglycerides (lower left), NEFA (lower middle), and adiponectin levels (lower right) after RSGZ treatment of obese insulin-resistant monkeys. RSGZ, rosiglitazone; NEFA, nonesterified fatty acid.

FIG. 4.

Effects of RSGZ on basal and insulin-stimulated (A–C) or PIP₃-stimulated (D) activation of IRS-1-dependent PI3K (A), PKB (B), and aPKCs (C and D) in muscle of obese monkeys. Euglycemic clamps with muscle biopsies were performed before and at the end of 3 months of RSGZ treatment. Basal values are shown by clear bars, and insulin-stimulated values are shown by shaded bars. Note that PIP₃ was added only to assays of aPKCs immunoprecipitated from basal (noninsulin-treated) muscle. The number of determinations is shown in parentheses. RSGZ significantly increased aPKC activity (p < 0.05) and in basal (noninsulin-stimulated) samples RSGZ enhanced the response to PIP₃ (p < 0.05). PIP₃, PI-3,4,5-(PO₄)₃; RSGZ, rosiglitazone.

FIG. 5.

Failure of RSGZ to alter aPKC levels or insulin effects on serine-473 phosphorylation of PKB in muscle of obese monkeys. Shown here are representative blots from two monkeys.

FIG. 6.

Treatment of obese monkeys with RSGZ significantly increased AMPK activity in muscle (p < 0.001). The number of determinations is shown in parentheses. AMPK, 5′-AMP-activated protein kinase.

FIG. 7.

Effect of RSGZ to increase CPT-1 mRNA gene expression during the euglycemic hyperinsulinemic clamp in muscle of obese monkeys. The number of determinations is shown in parentheses. CPT-1, carnitine palmitoyltransferase-1.