Attenuating Protein Degradation and Enhancing Protein Synthesis in Skeletal Muscle in Stressed Animal Model Systems

Abstract

A dynamic balance between protein synthesis and degradation determines muscle protein content. In animal models, and some human trials, a leucine metabolite - 3-hydroxy-3-methylbutyrate (HMB) – has been shown to attenuate stress-induced muscle loss. PURPOSE: To determine the effects of HMB on the changes in proteinhomeostasis involved in cancer-induced muscle loss. METHODS: Experiments used the murine MAC-16 tumor model. After tumor implantation, test mice were treated by daily gavage of HMB while control mice recieved saline. After treatment (3+ days) mice were sacrificed, and skeletal muscle excised. Protein degradation rates (tyrosine release) and protein synthesis rates (uptake of radiolabeled phenylalanine) were determined in isolated muscles suspended in media. Components of the ubiquitin-proteosome system, and various signaling factors involved in the control of this system were assessed by Western blot and EMSA techniques. The effect of rapamycin on HMB treated animals was also assessed. RESULTS: The presence of a tumor stressor was confirmed to induce substantial weight loss, which included loss of muscle mass. This was a result of both increased protein synthesis (∼ 2X) and decreased protein breakdown (∼ 3X). Increased protein breakdown is caused by a dramatic up-regulation of the ubiquitin-proteosome system, in this case through a known signaling cascade triggered by muscle cell binding of proteolysis inducing factor (PIF). This signaling is interrupted by HMB feeding. Tumor-induced stress also reduces protein synthetic rates to very low levels. This effect is at least partially attenuated by HMB feeding. The effect of HMB on protein synthesis is completely prevented by treatment with rapamycin. CONCLUSIONS: Protein homeostasis in skeletal muscle is affected by a number of stressors, and both protein synthesis and degradation rates can be impacted. Orally administered HMB attenuates protein degradation in muscle by preventing induction of a proteolytic pathway as well as at least partially preventing protein synthesis shutdown. The impact on protein degradation involves disruption of a major proteolytic pathway. HMB feeding also prevented tumor-induced shutdown of protein synthesis, apparently through a pathway similar to leucine. Further work is needed to confirm this hypothesis, as well as to assess the impact of HMB on other examples of disruption of muscle protein homeostasis such as intense exercise bouts, cardiovascular and renal disease.