The first suggestion that physical exercise results in free radical-mediated damage to tissues appeared in 1978, and the past three decades have resulted in a large growth of knowledge regarding exercise and oxidative stress. Although the sources of oxidant production during exercise continue to be debated, it is now well established that both resting and contracting skeletal muscles produce reactive oxygen species and reactive nitrogen species. Importantly, intense and prolonged exercise can result in oxidative damage to both proteins and lipids in the contracting myocytes. Furthermore, oxidants can modulate a number of cell signaling pathways and regulate the expression of multiple genes in eukaryotic cells. This oxidant-mediated change in gene expression involves changes at transcriptional, mRNA stability, and signal transduction levels. Furthermore, numerous products associated with oxidant-modulated genes have been identified and include antioxidant enzymes, stress proteins, DNA repair proteins, and mitochondrial electron transport proteins. Interestingly, low and physiological levels of reactive oxygen species are required for normal force production in skeletal muscle, but high levels of reactive oxygen species promote contractile dysfunction resulting in muscle weakness and fatigue. Ongoing research continues to probe the mechanisms by which oxidants influence skeletal muscle contractile properties and to explore interventions capable of protecting muscle from oxidant-mediated dysfunction.

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Exercise-induced oxidative stress: cellular mechanisms and impact on muscle force production

https://escholarship.org/content/qt3w59t47c/qt3w59t47c.pdf?t=pzvu93

Cachexia: a new definition.

https://www.cell.com/cell-metabolism/pdf/S1550-4131(07)00339-7.pdf

FoxO3 Coordinately Activates Protein Degradation by the Autophagic/Lysosomal and Proteasomal Pathways in Atrophying Muscle Cells

Background The combination of complete diaphragm inactivity and mechanical ventilation (for more than 18 hours) elicits disuse atrophy of myofibers in animals. We hypothesized that the same may also occur in the human diaphragm. Methods We obtained biopsy specimens from the costal diaphragms of 14 brain-dead organ donors before organ harvest (case subjects) and compared them with intraoperative biopsy specimens from the diaphragms of 8 patients who were undergoing surgery for either benign lesions or localized lung cancer (control subjects). Case subjects had diaphragmatic inactivity and underwent mechanical ventilation for 18 to 69 hours; among control subjects diaphragmatic inactivity and mechanical ventilation were limited to 2 to 3 hours. We carried out histologic, biochemical, and gene-expression studies on these specimens. Results As compared with diaphragm-biopsy specimens from controls, specimens from case subjects showed decreased cross-sectional areas of slow-twitch and fast-twitch fibers of 57% (P = 0.001) and 53% (P = 0.01), respectively, decreased glutathione concentration of 23% (P = 0.01), increased active caspase-3 expression of 100% (P = 0.05), a 200% higher ratio of atrogin-1 messenger RNA (mRNA) transcripts to MBD4 (a housekeeping gene) (P = 0.002), and a 590% higher ratio of MuRF-1 mRNA transcripts to MBD4 (P = 0.001). Conclusions The combination of 18 to 69 hours of complete diaphragmatic inactivity and mechanical ventilation results in marked atrophy of human diaphragm myofibers. These findings are consistent with increased diaphragmatic proteolysis during inactivity.

https://www.nejm.org/doi/pdf/10.1056/NEJMoa070447

Rapid disuse atrophy of diaphragm fibers in mechanically ventilated humans.

All intracellular proteins and many extracellular proteins are continually “turning over;” i.e. , they are being hydrolyzed to their constituent amino acids and replaced by new synthesis. Although the continual destruction of cell proteins might seem wasteful, this process serves several

Protein Degradation by the Ubiquitin–Proteasome Pathway in Normal and Disease States

With trauma, sepsis, cancer, or uremia, animals or patients experience accelerated degradation of muscle protein in the ATP-ubiquitin-proteasome (Ub-P'some) system. The initial step in myofibrillar proteolysis is unknown because this proteolytic system does not break down actomyosin complexes or myofibrils, even though it degrades monomeric actin or myosin. Since cytokines or insulin resistance are common in catabolic states and will activate caspases, we examined whether caspase-3 would break down actomyosin. We found that recombinant caspase-3 cleaves actomyosin, producing a characteristic, approximately 14-kDa actin fragment and other proteins that are degraded by the Ub-P'some. In fact, limited actomyosin cleavage by caspase-3 yields a 125% increase in protein degradation by the Ub-P'some system. Serum deprivation of L6 muscle cells stimulates actin cleavage and proteolysis; insulin blocks these responses by a mechanism requiring PI3K. Cleaved actin fragments are present in muscles of rats with muscle atrophy from diabetes or chronic uremia. Accumulation of actin fragments and the rate of proteolysis in muscle stimulated by diabetes are suppressed by a caspase-3 inhibitor. Thus, in catabolic conditions, an initial step resulting in loss of muscle protein is activation of caspase-3, yielding proteins that are degraded by the Ub-P'some system. Therapeutic strategies could be designed to prevent these events.

/pdf/activation-of-caspase-3-is-an-initial-step-triggering-4btcvlyxda.pdf

Activation of caspase-3 is an initial step triggering accelerated muscle proteolysis in catabolic conditions

Protein degradation is a critical process for the growth and function of cells. Proteolysis eliminates abnormal proteins, controls many cellular regulatory processes, and supplies amino acids for cellular remodeling. When substrates of proteolytic pathways are poorly recognized or there is mistiming of proteolysis, profound changes in cell function can occur. Based on these potential problems, it is not surprising that alterations in proteolytic enzymes/cofactors or in the structure of protein substrates that render them more or less susceptible to degradation are responsible for disorders associated with kidney cell malfunctions. Multiple pathways exist for protein degradation. The best-described proteolytic system is the ubiquitin-proteasome pathway, which requires ATP and degrades the bulk of cellular and some membrane proteins. This review will survey examples of renal abnormalities that are associated with defective protein degradation involving the ubiquitin-proteasome pathway. Loss of muscle mass associated with chronic renal failure, von Hippel-Lindau disease, Liddle syndrome, and ischemic acute renal failure will be discussed. These examples are indicative of the diverse roles of the ubiquitin-proteasome system in renal-associated pathological conditions.

Richard Debigare

Papers

Activation of caspase-3 is an initial step triggering accelerated muscle proteolysis in catabolic conditions

Proteolysis, the ubiquitin-proteasome system, and renal diseases