Muscle Cachexia: Current Concepts of Intracellular Mechanisms and Molecular Regulation

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Abstract
To review present knowledge of intracellular mechanisms and molecular regulation of muscle cachexia. Muscle cachexia, mainly reflecting degradation of myofibrillar proteins, is an important clinical feature in patients with severe injury, sepsis, and cancer. The catabolic response in skeletal muscle may result in muscle wasting and weakness, delaying or preventing ambulation and rehabilitation in these patients and increasing the risk for pulmonary complications. Muscle cachexia, induced by severe injury, sepsis, and cancer, is associated with increased gene expression and activity of the calcium/calpain- and ubiquitin/proteasome-proteolytic pathways. Calcium/calpain-regulated release of myofilaments from the sarcomere is an early, and perhaps rate-limiting, component of the catabolic response in muscle. Released myofilaments are ubiquitinated in the N-end rule pathway, regulated by the ubiquitin-conjugating enzyme E214k and the ubiquitin ligase E3α, and degraded by the 26S proteasome. An understanding of the mechanisms regulating muscle protein breakdown is important for the development of therapeutic strategies aimed at treating or preventing muscle cachexia in patients with severe injury, sepsis, cancer, and perhaps other catabolic conditions as well.

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