The effects of [beta]-hydroxy-[beta]-methylbutyrate (HMB) and leucine on cellular signaling pathways controlling protein synthesis and degradation during sedentary and post-exercise recovery in skeletal muscle
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Recent research suggests that [beta]-hydroxy-[beta]-methylbutyrate (HMB), a metabolite of leucine (Leu), increases muscle mass and attenuates muscle damage during resistance training. Although Leu acts as a potent stimulator of protein synthesis, HMB, but not Leu, has been reported to be effective in suppressing proteolysis in skeletal muscle. However, mechanisms for the effects of HMB on cell signaling pathways controlling muscle protein turnover during rest and after endurance exercise are still poorly understood. Furthermore, the effects of HMB on cell signaling pathways controlling protein synthesis and degradation under normal in vivo conditions warrant further investigation. For optimal gains in muscle mass, the appropriate type and amount of protein (PRO) is required for positive protein balance to occur in skeletal muscle. Therefore, this dissertation was designed to determine the effect of HMB, PRO and Leu, individually and in combination, on the regulation of cellular signaling pathways controlling muscle protein turnover during sedentary and post-exercise conditions. Study 1 demonstrated that, compared with HMB and PRO alone, the combination of HMB and PRO was more effective in activating the mTOR signaling pathway, which controls protein synthesis, and inhibiting FOXO3A, a major regulator of the ubiquitin-proteasome proteolytic signaling pathway. Study 2 demonstrated that, compared with its individual components, a novel HMB/PRO/Leu supplement better activated protein-synthetic signals and inhibited proteolytic signals in skeletal muscle, and these effects were better sustained. Finally, Study 3 demonstrated that adding Leu to PRO-enriched mixtures after exercise additively activated protein-synthetic signals in a fiber type-specific manner, and adding HMB clearly inhibited proteolytic signaling proteins. Furthermore, provision of an HMB/PRO/Leu supplement after exercise was found to favorably modulate signaling pathways controlling both protein synthesis and degradation. Taken together, the results of these studies suggest that a novel nutrient supplement, composed of HMB, Leu and PRO, additively enhances the intracellular signaling proteins controlling protein synthesis and attenuates signaling proteins controlling proteolysis in skeletal muscle during sedentary and post-exercise recovery. Therefore, such a supplement may be beneficial for both athletic and therapeutic purposes.