Evidence for glucose/hexosamine in vivo regulation of insulin/IGF-I hybridreceptor assembly

Hybrid receptors composed of an insulin alpha beta-hemireceptor and a type 1 IGF alpha beta-hemireceptor are formed in tissues expressing both molecul es. We recently reported an increased hybrid receptor expression in skeleta l muscle of type 2 diabetic patients that is inversely correlated with in v ivo insulin sensitivity. It is unclear whether these changes were due to pr imary abnormalities or to secondary derangements acting in vivo, such as hy perglycemia. To address this, we determined abundance of hybrids in skeleta l muscle from three groups of rats: controls, diabetic (90% pancreatectomy) , and diabetic treated with phlorizin to normalize plasma glucose levels. W e found that the abundance of hybrid receptors was higher in diabetic rats compared with control and phlorizin-treated diabetic rats (percentage of I- 125-insulin bound versus total added radioactivity [B/T] 1.8 +/- 0.11, 0.4 +/- 0.01 and 0.32 +/- 0.04, respectively; P < 0.0001). Fasting plasma gluco se levels were positively correlated with hybrids abundance (r = 0.77, P < 0.002). Hybrid receptor protein content, assessed by immunoblotting, was 2. 4-fold higher in diabetic rats as compared with control and phlorizin-treat ed diabetic rats. Because it has been shown that some of the regulatory eff ects of glucose may be mediated by the glucosamine pathway, we subsequently determined the effect of an in vivo glucosamine infusion on hybrid recepto r formation. We found that abundance of hybrids was significantly higher in muscle from glucosamine-treated rats compared with control rats (B/T = 0.1 7 +/- 0.02 and 0.11 +/- 0.01, respectively; P < 0.009). Quantitation of hyb rid content by immunoblotting revealed that their abundance was 1.9-fold hi gher in glucosamine-treated rats. The results demonstrate that I) elevated glucose levels in diabetic rats are associated with increased expression of hybrid receptors in muscle, 2) correction of hyperglycemia with phlorizin completely reverses increased expression of hybrids, and 3) glucosamine inf used into control rats mimics the effects of hyperglycemia on hybrid recept or formation. Thus, the results support the hypothesis that glucose acting, at least in part, through the glucosamine pathway may play an important ro le in regulating hybrid receptor assembly in vivo.