This study examined potential mechanisms contributing to the inhibition of protein synthesis in skeletal muscle and heart after administration of tumor necrosis factor (TNF)-α. Rats had vascular catheters implanted, and TNF-α was infused continuously for 24 h. TNF-α decreased in vivo-determined rates of global protein synthesis in gastrocnemius (39%) and heart (25%). The TNF-α-induced decrease in protein synthesis in the gastrocnemius involved a reduction in the synthesis of both myofibrillar and sarcoplasmic proteins. To identify potential mechanisms responsible for regulating mRNA translation, we examined several eukaryotic initiation factors (eIFs) and elongation factors (eEFs). TNF-α decreased the activity of eIF-2B in muscle (39%) but not in heart. This diminished activity was not caused by a reduction in the content of eIF-2Bε or the content and phosphorylation state of eIF-2α. Skeletal muscle and heart from TNF-α-treated rats demonstrated 1) an increased binding of the translation repressor 4E-binding protein-1 (4E-BP1) with eIF-4E, 2) a decreased amount of eIF-4E associated with eIF-4G, and 3) a decreased content of the hyperphosphorylated γ-form of 4E-BP1. In contrast, the infusion of TNF-α did not alter the content of eEF-1α or eEF-2, or the phosphorylation state of eEF-2. In summary, these data suggest that TNF-α impairs skeletal muscle and heart protein synthesis, at least in part, by decreasing mRNA translational efficiency resulting from an impairment in translation initiation associated with alterations in eIF-4E availability.

Prolonged sepsis and exposure to an inflammatory milieu decreases muscle protein synthesis and reduces muscle mass. As a result of its ability to integrate diverse signals, including hormones and nutrients, the mammalian target of rapamycin (mTOR) is a dominant regulator in the translational control of protein synthesis. Under postabsorptive conditions, sepsis decreases mTOR kinase activity in muscle, as evidenced by reduced phosphorylation of both eukaryotic initiation factor (eIF)4E-binding protein (BP)-1 and ribosomal S6 kinase (S6K)1. These sepsis-induced changes, along with the redistribution of eIF4E from the active eIF4E·eIF4G complex to the inactive eIF4E·4E-BP1 complex, are preventable by neutralization of tumor necrosis factor (TNF)-α but not by antagonizing glucocorticoid action. Although the ability of mTOR to respond to insulin-like growth factor (IGF)-I is not disrupted by sepsis, the ability of leucine to increase 4E-BP1 and S6K1 phosphorylation is greatly attenuated. This “leucine resistance” results from a cooperative interaction between both TNF-α and glucocorticoids. Finally, although septic animals are not IGF-I resistant, the anabolic actions of IGF-I are nonetheless reduced because of the development of growth hormone resistance, which decreases both circulating and muscle IGF-I. Herein, we highlight recent advances in the mTOR signaling network and emphasize their connection to the atrophic response observed in skeletal muscle during sepsis. Although many unanswered questions remain, understanding the cellular basis of the sepsis-induced decrease in translational activity will contribute to the rational development of therapeutic interventions and thereby minimize the debilitating affects of the atrophic response that impairs patient recovery.

Many preclinical studies in critical care medicine and related disciplines rely on hypothesis-driven research in mice. The underlying premise posits that mice sufficiently emulate numerous pathophysiologic alterations produced by trauma/sepsis and can serve as an experimental platform for answering clinically relevant questions. Recently, the lay press severely criticized the translational relevance of mouse models in critical care medicine. A series of provocative editorials were elicited by a highly publicized research report in the Proceedings of the National Academy of Sciences (PNAS; February 2013), which identified an unrecognized gene expression profile mismatch between human and murine leukocytes following burn/trauma/endotoxemia. Based on their data, the authors concluded that mouse models of trauma/inflammation are unsuitable for studying corresponding human conditions. We believe this conclusion was not justified. In conjunction with resulting negative commentary in the popular press, it can seriously jeopardize future basic research in critical care medicine. We will address some limitations of that PNAS report to provide a framework for discussing its conclusions and attempt to present a balanced summary of strengths/weaknesses of use of mouse models. While many investigators agree that animal research is a central component for improved patient outcomes, it is important to acknowledge known limitations in clinical translation from mouse to man. The scientific community is responsible to discuss valid limitations without overinterpretation. Hopefully, a balanced view of the strengths/weaknesses of using animals for trauma/endotoxemia/critical care research will not result in hasty discount of the clear need for using animals to advance treatment of critically ill patients.

Alcohol abuse, either by acute intoxication or prolonged excessive consumption, leads to pathological changes in many organs and tissues including skeletal muscle. As muscle protein serves not only a contractile function but also as a metabolic reserve for amino acids, which are used to support the energy needs of other tissues, its content is tightly regulated and dynamic. This review focuses on the etiology by which alcohol perturbs skeletal muscle protein balance and thereby over time produces muscle wasting and weakness. The preponderance of data suggest that alcohol primarily impairs global protein synthesis, under basal conditions as well as in response to several anabolic stimuli including growth factors, nutrients, and muscle contraction. This inhibitory effect of alcohol is mediated, at least in part, by a reduction in mTOR kinase activity via a mechanism that remains poorly defined but likely involves altered protein-protein interactions within mTOR complex 1. Furthermore, alcohol can exacerbate the decrement in mTOR and/or muscle protein synthesis present in other catabolic states. In contrast, alcohol-induced changes in muscle protein degradation, either global or via specific modulation of the ubiquitin-proteasome or autophagy pathways, are relatively inconsistent and may be model dependent. Herein, changes produced by acute intoxication versus chronic ingestion are contrasted in relation to skeletal muscle metabolism, and limitations as well as opportunities for future research are discussed. As the proportion of more economically developed countries ages and chronic illness becomes more prevalent, a better understanding of the etiology of biomedical consequences of alcohol use disorders is warranted.

: Mechanical unloading induces muscle atrophy and bone loss; however, the time course and interdependence of these effects is not well defined. We subjected 4-month-old C57BL/6J mice to hindlimb suspension (HLS) for 3 weeks, euthanizing 12 to 16 mice on day (D) 0, 7, 14, and 21. Lean mass was 7% to 9% lower for HLS versus control from D7–21. Absolute mass of the gastrocnemius (gastroc) decreased 8% by D7, and was maximally decreased 16% by D14 of HLS. mRNA levels of Atrogin-1 in the gastroc and quadriceps (quad) were increased 99% and 122%, respectively, at D7 of HLS. Similar increases in MuRF1 mRNA levels occurred at D7. Both atrogenes returned to baseline by D14. Protein synthesis in gastroc and quad was reduced 30% from D7–14 of HLS, returning to baseline by D21. HLS decreased phosphorylation of SK61, a substrate of mammalian target of rapamycin (mTOR), on D7–21, whereas 4E-BP1 was not lower until D21. Cortical thickness of the femur and tibia did not decrease until D14 of HLS. Cortical bone of controls did not change over time. HLS mice had lower distal femur bone volume fraction (−22%) by D14; however, the effects of HLS were eliminated by D21 because of the decline of trabecular bone mass of controls. Femur strength was decreased approximately 13% by D14 of HLS, with no change in tibia mechanical properties at any time point. This investigation reveals that muscle atrophy precedes bone loss during unloading and may contribute to subsequent skeletal deficits. Countermeasures that preserve muscle may reduce bone loss induced by mechanical unloading or prolonged disuse. Trabecular bone loss with age, similar to that which occurs in mature astronauts, is superimposed on unloading. Preservation of muscle mass, cortical structure, and bone strength during the experiment suggests muscle may have a greater effect on cortical than trabecular bone. © 2014 American Society for Bone and Mineral Research.