Overexpression of Glucocorticoid Receptor ß Enhances Myogenesis and Reduces Catabolic Gene Expression.

ABSTRACT

Unlike the glucocorticoid receptor α (GRα), GR ß (GRß) has a truncated ligand-binding domain that prevents glucocorticoid binding, implicating GRα as the mediator of glucocorticoid-induced skeletal muscle loss. Because GRß causes glucocorticoid resistance, targeting GRß may be beneficial in impairing muscle loss as a result of GRα activity. The purpose of this study was to determine how the overexpression of GRß affects myotube formation and dexamethasone (Dex) responsiveness. We measured GR isoform expression in C2C12 muscle cells in response to Dex and insulin, and through four days of myotube formation. Next, lentiviral-mediated overexpression of GRß in C2C12 was performed, and these cells were characterized for cell fusion and myotube formation, as well as sensitivity to Dex via the expression of ubiquitin ligases. GRß overexpression increased mRNA levels of muscle regulatory factors and enhanced proliferation in myoblasts. GRß overexpressing myotubes had an increased fusion index. Myotubes overexpressing GRß had lower forkhead box O3 (Foxo3a) mRNA levels and a blunted muscle atrophy F-box/Atrogen-1 (MAFbx) and muscle ring finger 1 (MuRF1) response to Dex. We showed that GRß may serve as a pharmacological target for skeletal muscle growth and protection from glucocorticoid-induced catabolic signaling. Increasing GRß levels in skeletal muscle may cause a state of glucocorticoid resistance, stabilizing muscle mass during exposure to high doses of glucocorticoids.