Involvement of the rapamycin-sensitive pathway in the insulin regulation of muscle protein synthesis in streptozotocin-diabetic rats

Abstract

Insulin resistance in 3-day streptozotocin (STZ)-treated rats was manifested by the lack of antiproteolytic action of insulin as well as by a reduction of its stimulatory effect on protein synthesis (-60% compared with the control group) in epitrochlearis muscle incubated in vitro. In the present study, we have investigated the diabetes-associated alterations in the insulin signalling cascade, especially the phosphatidylinositol-3 kinase (PI-3 kinase)/p70 S6 kinase (p70(S6K)) pathway, in rat skeletal muscle. LY 294002, a specific inhibitor of PI-3 kinase, markedly decreased the basal rate of protein synthesis and completely prevented insulin-mediated stimulation of this process both in control and diabetic rats. Thus, PI-3 kinase is required for insulin-stimulated muscle protein synthesis in diabetic rats as in the controls. Rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), had no effect on the basal rate of protein synthesis in either of the experimental groups. In control rats, the stimulatory action of insulin on muscle protein synthesis was diminished by 36% in the presence of rapamycin, whereas in diabetic muscles this reduction amounted to 68%. The rapamycin-sensitive pathway makes a relatively greater contribution to the stimulatory effect of insulin on muscle protein synthesis in diabetic rats compared with the controls, due presumably to the preferential decrease in the rapamycin-insensitive component of protein synthesis. Neither basal nor insulin-stimulated p70(S6K) activity, a signalling element lying downstream of mTOR, were modified by STZ-diabetes.