Role of Skeletal Muscle in Thiazolidinedione Insulin Sensitizer ( PPAR g Agonist ) Action

Thiazolidinedione (TZD) insulin sensitizers are specific agonists of peroxisome proliferator activated receptor (PPAR)g. However, their mechanism of action and the in vivo target tissue(s) that mediate insulin sensitization remain poorly defined. Although PPARg messenger RNA expression has been reported in skeletal muscle, the expression of PPARg within myocytes in intact muscle tissue has not been examined. An antipeptide PPARg antibody was generated; immunohistochemistry was then used to demonstrate that PPARg is present within nuclei of myocytes [in both skeletal (white and red fibers) and cardiac tissue (rodent and human)]. The effect of insulin sensitizer treatment on muscle insulin action was studied using ob/ob mice after 4 days dosing with a potent (6 nM PPARg Kd) TZD (10 mg/kgzday). 2-deoxyglucose (2-DOG) uptake was then assessed in freshly isolated soleus muscles from lean vs. ob/ob vs. TZD-treated ob/ob mice. In lean mouse muscles, 2-DOG uptake was stimulated by 82%, 95%, 165% (with 25, 100, 2000 mU/ml insulin); muscles from ob/ob were severely insulin resistant (,80% stimulation with 2000 mU/ml insulin). Muscles from TZD-treated ob/ob displayed a normal insulin response with 100 (71%) or 2000 (158%) mU/ml insulin. Additional studies were performed using ZDF rats treated with/without TZD for 7 days. In vivo 2-DOG glucose uptake into soleus, gastrocnemius, and diaphragm muscles was measured during euglycemichyperinsulinemic clamp. Compared with lean rats, muscle 2-DOG uptake in ZDF was reduced by 52% (soleus) or 71% (diaphragm). Partial (40–60%) normalization of the reduced 2-DOG uptake was evident in TZD-treated ZDF rats. In contrast to the effect of in vivo treatment on muscle insulin action, preincubation of isolated soleus muscles from naive lean or ob/ob mice for 5 h with 100 nM TZD did not affect insulin-stimulated 2-DOG uptake. We conclude: 1) PPARg is expressed in myocytes within skeletal and cardiac muscle. 2) In vivo activation of PPARg by treatment of insulin-resistant mice/rats with a potent TZD corrects impaired muscle insulin action. 3) The lack of a direct effect on muscle after 5 h in vitro TZD incubation suggests that changes in insulin action may require a longer duration of PPARg activation or that improved muscle insulin sensitivity may result from an indirect in vivo effect of PPARg activation (e.g. changes in systemic lipid metabolism). (Endocrinology 139: 5034–5041, 1998) T are a recently identified class of antidiabetic agents that improve glucose utilization without stimulating insulin release. These compounds function as insulin sensitizers and therefore have no potential to cause hypoglycemia (1). Many studies have shown that oncea-day oral administration of thiazolidinediones to obese animal models of insulin resistance and NIDDM (e.g. db/db, ob/ob mice and Zucker fatty rats) can result in substantial correction of marked hyperglycemia and/or hyperinsulinemia (2–5). These effects on blood glucose and insulin are also accompanied by substantial lowering of high circulating triglyceride and nonesterified free fatty acid (FFA) levels. At least three thiazolidinedione compounds have been shown to enhance insulin sensitivity and improve glucose homeostasis in insulin-resistant human subjects (6–8). Using cultured cell systems, thiazolidinediones have been shown to promote adipocyte differentiation and accelerate the induction of messenger RNAs (mRNAs) encoding adipocyte-specific genes such as fatty acid binding protein (aP2) (9, 10). These findings led to the hypothesis that these compounds might function as agonists for a specific nuclear receptor-peroxisome proliferator-activated receptor g (PPARg) that has been implicated in adipogenesis (11). Two related isoforms (PPARg1 and PPARg2), which differ by the addition of 30 N-terminal amino acids in PPARg2, occur as a result of alternative promoter usage and mRNA splicing