Reduced Skeletal Muscle Inhibitor of B Content Is Associated With Insulin Resistance in Subjects With Type 2 Diabetes Reversal by Exercise Training

Skeletal muscle insulin resistance plays a key role in the pathogenesis of type 2 diabetes. It recently has been hypothesized that excessive activity of the inhibitor of B (I B)/nuclear factor B (NF B) inflammatory pathway is a mechanism underlying skeletal muscle insulin resistance. However, it is not known whether I B/NF B signaling in muscle from subjects with type 2 diabetes is abnormal. We studied I B/NF B signaling in vastus lateralis muscle from six subjects with type 2 diabetes and eight matched control subjects. Muscle from type 2 diabetic subjects was characterized by a 60% decrease in I B protein abundance, an indicator of increased activation of the I B/NF B pathway. I B abundance directly correlated with insulin-mediated glucose disposal (Rd) during a hyperinsulinemic (40 mU m 2 min )-euglycemic clamp (r 0.63, P 0.01), indicating that increased I B/NF B pathway activity is associated with muscle insulin resistance. We also investigated whether reversal of this abnormality could be a mechanism by which training improves insulin sensitivity. In control subjects, 8 weeks of aerobic exercise training caused a 50% increase in both I B and I B protein. In subjects with type 2 diabetes, training increased I B and I B protein to levels comparable with that of control subjects, and these increments were accompanied by a 40% decrease in tumor necrosis factor muscle content and a 37% increase in insulin-stimulated glucose disposal. In summary, subjects with type 2 diabetes have reduced I B protein abundance in muscle, suggesting excessive activity of the I B/NF B pathway. Moreover, this abnormality is reversed by exercise training. Diabetes 55:760–767, 2006 Evidence has accumulated in recent years indicating that type 2 diabetes and other insulinresistant disorders are characterized by chronic inflammation (1,2). Specifically, it has been postulated that excessive activity of the inhibitor B (I B)/ nuclear factor B (NF B) inflammatory pathway may be an important molecular mechanism responsible for skeletal muscle insulin resistance (1,3–7). NF B is a family of transcription factors that regulate the expression of proinflammatory genes. In unstimulated cells, NF B is predominantly localized in the cytoplasm, associated with an inhibitory protein, I B. Several stimuli, including cytokines (1), reactive oxygen species, hyperglycemia, and free fatty acids (FFAs) (5), activate I B kinase (IKK), the upstream kinase of I B. Upon activation by inflammatory factors, IKK phosphorylates I B, causing rapid I B polyubiquitination and degradation by proteosomes. Following release from I B, NF B translocates from the cytoplasm to the nucleus, where it binds to target genes to stimulate transcription of inflammatory mediators such as tumor necrosis factor (TNF ), interleukin (IL)-1 , and IL-6 (1,8,9). I Bs are members of a gene family that contain seven mammalian members, including I B , I B , I Bε, I B , Bcl-3, and the precursor Rel proteins p100 and p105. I B and I B share common properties to interact with NF B and inhibit DNA binding. NF B proteins consist of five members, including p65, p50, p52, RelB, and c-Rel. Dimerization of two NF B family members is necessary for their DNA-binding properties. The predominant activating NF B dimer in skeletal muscle is the p50-p65 heterodimer (10). NF B p65 contains a COOH-terminal transcriptional domain that is crucial for its ability to activate inflammatory gene expression (9). Interventions aimed at blocking the I B/NF B pathway, such as genetic deletion of IKK in mice (4) and the administration of salicylates, which inhibit IKK , to subjects with type 2 diabetes (11) significantly improve peripheral insulin sensitivity. These findings strongly suggest that activation of the I B/NF B pathway plays an important role in the pathogenesis of insulin resistance. However, it is not known whether the I B/NF B pathway is excessively active in skeletal muscle from subjects with type 2 diabetes. The goal of the present study was to From the Diabetes Division, University of Texas Health Science Center at San Antonio, San Antonio, Texas; and the Texas Diabetes Institute, San Antonio, Texas. Address correspondence and reprint requests to Nicolas Musi, MD, 701 S. Zarzamora, MS 10-5, San Antonio, TX 78207. E-mail: [email protected]. Received for publication 26 May 2005 and accepted in revised form 18 November 2005. AMPK, AMP-activated protein kinase; FFA, free fatty acid; GCRC, General Clinical Research Center; I B, inhibitor of B; IKK, I B kinase; IL, interleukin; NF B, nuclear factor B; TNF , tumor necrosis factor . © 2006 by the American Diabetes Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.