title: Glucocorticoids differentially regulate degradation of MyoD and Id1 by N-terminal ubiquitination to promote muscle protein catabolism.
abstract: Accelerated protein degradation via the ubiquitin-proteasome pathway is the principal cause of skeletal muscle wasting associated with common human disease states and pharmacological treatment with glucocorticoids. Although many protein regulatory factors essential for muscle development and regeneration are degraded via the ubiquitin system, little is known about the mechanisms and regulation of this pathway that promote wasting muscle. Here, we demonstrate that, in differentiated myotubes, glucocorticoid, via the glucocorticoid receptor, selectively induces a decrease in protein abundance of MyoD, a master switch for muscle development and regeneration, but not that of its negative regulator Id1. This decrease in MyoD protein results from accelerated degradation after glucocorticoid exposure. Using MyoD and Id1 mutants deficient in either N terminus-dependent or internal lysine-dependent ubiquitination, we further show that these ubiquitination pathways of MyoD degradation are regulated differently from those of Id1 degradation. Specifically, glucocorticoid activates the N-terminal ubiquitination pathway in MyoD degradation in myotubes, without concomitant effects on Id1 degradation. This effect of glucocorticoid on MyoD and Id1 protein degradation is associated with the distinct cellular compartments in which their degradation occurs. Taken together, these results support a key role for the N terminus-dependent ubiquitination pathway in the physiology of muscle protein degradation.
authors.affiliation: The Edward Mallinckrodt Department of Pediatrics and Department of Developmental Biology, Washington University School of Medicine and St. Louis Children's Hospital, 660 South Euclid Avenue, St. Louis, MO 63110, USA.
authors.forename: Liping
authors.initials: L
authors.lastname: Sun
authors.forename: Julie S
authors.initials: JS
authors.lastname: Trausch-Azar
authors.forename: Louis J
authors.initials: LJ
authors.lastname: Muglia
authors.forename: Alan L
authors.initials: AL
authors.lastname: Schwartz
mesh_terms: D000818:Animals; D002451:Cell Compartmentation; D002460:Cell Line; D005938:Glucocorticoids / Q000502:physiology*; D006801:Humans; D051798:Inhibitor of Differentiation Protein 1 / Q000378:metabolism*; D051379:Mice; D018485:Muscle Fibers, Skeletal; D009124:Muscle Proteins; D009154:Mutation; D017570:MyoD Protein / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011965:Receptors, Glucocorticoid / Q000032:analysis; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: D005938:Glucocorticoids; D051798:Inhibitor of Differentiation Protein 1; D009124:Muscle Proteins; D017570:MyoD Protein; C057363:MyoD1 myogenic differentiation protein; D011965:Receptors, Glucocorticoid; D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex