Bilodeau PA, Coyne ES, Wing SS. The ubiquitin proteasome system in atrophying skeletal muscle: roles and regulation. Am J Physiol Cell Physiol 311: C392-C403, 2016. First published August 10, 2016; doi:10.1152/ajpcell.00125.2016.-Muscle atrophy complicates many diseases as well as aging, and its presence predicts both decreased quality of life and survival. Much work has been conducted to define the molecular mechanisms involved in maintaining protein homeostasis in muscle. To date, the ubiquitin proteasome system (UPS) has been shown to play an important role in mediating muscle wasting. In this review, we have collated the enzymes in the UPS whose roles in muscle wasting have been confirmed through loss-of-function studies. We have integrated information on their mechanisms of action to create a model of how they work together to produce muscle atrophy. These enzymes are involved in promoting myofibrillar disassembly and degradation, activation of autophagy, inhibition of myogenesis as well as in modulating the signaling pathways that control these processes. Many anabolic and catabolic signaling pathways are involved in regulating these UPS genes, but none appear to coordinately regulate a large number of these genes. A number of catabolic signaling pathways appear to instead function by inhibition of the insulin/IGF-I/protein kinase B anabolic pathway. This pathway is a critical determinant of muscle mass, since it can suppress key ubiquitin ligases and autophagy, activate protein synthesis, and promote myogenesis through its downstream mediators such as forkhead box O, mammalian target of rapamycin, and GSK3, respectively. Although much progress has been made, a more complete inventory of the UPS genes involved in mediating muscle atrophy, their mechanisms of action, and their regulation will be useful for identifying novel therapeutic approaches to this important clinical problem.hormones; muscle atrophy SKELETAL MUSCLE SERVES TWO essential functions, a contractile function for locomotion/maintenance of posture and a metabolic function as the protein reservoir of the body. The myofibers that make up the muscle consist primarily of myofibrillar proteins. The ability of the body to maintain posture or to move arises from the precise organization of myofibrillar proteins into a contractile unit called the sarcomere. The sarcomere consists of thick filaments containing primarily myosin that can slide over thin filaments containing primarily actin. Both types of filaments contain additional regulatory proteins and are linked to the ␣-actinin-containing Z disk, the thin filaments directly so and the thick filaments via titin. Vimentin and desmin are intermediate filaments that serve to anchor sarcomeres properly at the Z disks. These myofibrillar proteins, as well as the sarcoplasmic proteins, also serve as the protein reservoir of the body. Upon fasting, once hepatic glycogen stores are depleted, muscle protein degradation must be activated to provide amino acids to the liver for gluconeogenesis. These amino a...