Regulation of glucocorticoid receptor alpha and beta isoforms and type I 11 beta-hydroxysteroid dehydrogenase expression in human skeletal muscle cells: A key role in the pathogenesis of insulin resistance?

Glucocorticoid excess frequently results in obesity, insulin resistance, gl ucose intolerance, and hypertension and may be the product of altered gluco corticoid hormone action. Tissue sensitivity to glucocorticoid is regulated by the expression of glucocorticoid receptor isoforms (GR alpha and GR bet a) and 11 beta -hydroxysteroid dehydrogenase type I (11 beta HSD1)-mediated intracellular synthesis of active cortisol from inactive cortisone. We hav e analyzed the expression of GR alpha, GR beta, and 11 beta HSD1 and their hormonal regulation in skeletal myoblasts from men (n = 14) with contrastin g levels of adiposity and insulin resistance. Immunohistochemical, Northern blot, and Western blot analysis indicated abundant expression of GRa and 1 1 beta HSD1 under basal conditions. The apparent K-m and maximum velocity f or the conversion of cortisone to cortisol were 440 +/- 14 nmol/L and 75 +/ - 7 pmol/mg protein h and 437 +/- 16 nmol/L and 33 +/- 6 pmol/mg protein h (mean +/- SEM; n = 4) in the presence and absence of 20% serum. Incubation of myoblasts with increasing concentrations of glucocorticoid (50-1000 nmol /L) resulted in a dose-dependent decline in GR alpha expression and a dose- dependent increase in GR beta expression. 11 beta HSD1 activity was sensiti vely up-regulated by increasing concentrations of glucocorticoid (50-1000 n mol/L: P < 0.05). Abolition of these effects by the GR antagonist, RU38486, indicates that regulation of GR alpha, GRP, and 11 beta HSD1 expression is mediated exclusively by the GRa ligand-binding variant. In contrast, 11 be ta HSD1 was down-regulated by insulin (20-100 mU/mL: P < 0.01) in the prese nce of 20% serum, whereas incubation with insulin under serum-free conditio ns resulted in a dose-dependent increase in 11 beta HSD1 activity (P < 0.05 ). Incubation with insulin-like growth factor I resulted in a similar patte rn of 11<beta>HSD1 activity. Although neither testosterone nor androstenedi one (5-200 nmol/L) affected 11 beta HSD1 activity, incubation of myoblasts with dehydroepiandrosterone (500 nmol/L) resulted in a decline in 11 beta H SD1 activity (P < 0.05). These data suggest that glucocorticoid hormone act ion in skeletal muscle is determined principally by autoregulation of GR al pha, GR beta, and 11 beta HSD1 expression by the ligand-binding GR alpha is oform. Additionally, insulin and insulinlike growth factor I regulation of 11 beta HSD1 may represent a novel mechanism that maintains insulin sensiti vity in skeletal muscle tissue by diminishing glucocorticoid antagonism of insulin action.