Glucocorticoid Receptor Beta Enhances Myoblast Proliferation and Myotube Formation, and Reduces Dexamethasone‐Induced Atrogene Expression

Abstract

BackgroundChronic elevations in glucocorticoids from endogenous production or exogenously due to disease treatment, cause skeletal muscle proteolysis and loss of muscle mass. The alpha isoform of the glucocorticoid receptor (GRα) but not the beta isoform (GRβ), has a glucocorticoid binding domain, implicating GRα as the mediator of glucocorticoid‐induced proteolysis. Because GRβ is known to cause glucocorticoid resistance, targeting GRβ may be beneficial in impairing muscle loss as a result of GRα activity. To date, there is a significant lack of research on the role of GRβ in the regulation of glucocorticoid‐induced skeletal muscle atrophy.PurposeThe purpose of this study was to determine how the overexpression of GRβ affects myotube formation and the responsiveness of atrogenes to dexamethasone.MethodsMouse GRβ cDNA was overexpressed in C2C12 mouse myoblasts using lentivirus. Changes in cell proliferation were quantified using an MTT assay. Myosin heavy chain staining of day 4 myotubes was used to quantify the effects of GRβ overexpression on indices of myotube formation. Real‐time PCR was used to investigate changes in mRNA expression of glucocorticoid responsive genes and western blot was used to quantify changes in protein expression in response to GRβ overexpression.ResultsGRβ protein expression in C2C12 mouse myoblasts was unaffected by dexamethasone, however, increased in response to insulin. Lentiviral‐mediated GRβ overexpression increased mRNA levels of MyoD and myogenin in confluent myoblast cultures. Furthermore, GRβ overexpression led to an enhanced proliferation of myoblasts associated with reduced mRNA levels of phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Following four days of differentiation, GRβ overexpressing myotubes had increased total number of nuclei within myotubes, nuclei per myotube, the number of myotubes, and a greater fusion index. Lastly, when treated with dexamethasone for 24 hours, GRβ overexpressing myotubes had a blunted MAFbx and MuRF1 atrogene response.ConclusionsOverexpression of GRβ in skeletal muscle may reduce the catabolic effects of glucocorticoids on skeletal muscle by reducing atrogene responsiveness, primarily causing a state of glucocorticoid resistance. Increasing GRβ levels may stabilize muscle mass and preserve metabolic and physical function during exposure to high doses of glucocorticoids. Future work is warranted to determine the effects of GRβ overexpression in vivo in response to physical, social, and pharmacological models of glucocorticoid excess.