Regulation of Glucocorticoid Receptor a and b Isoforms and Type I 11b-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, glucose intolerance, and hypertension and may be the product of altered glucocorticoid hormone action. Tissue sensitivity to glucocorticoid is regulated by the expression of glucocorticoid receptor isoforms (GRa and GRb) and 11b-hydroxysteroid dehydrogenase type I (11bHSD1)-mediated intracellular synthesis of active cortisol from inactive cortisone. We have analyzed the expression of GRa, GRb, and 11bHSD1 and their hormonal regulation in skeletal myoblasts from men (n 5 14) with contrasting levels of adiposity and insulin resistance. Immunohistochemical, Northern blot, and Western blot analysis indicated abundant expression of GRa and 11bHSD1 under basal conditions. The apparent Km and maximum velocity for the conversion of cortisone to cortisol were 440 6 14 nmol/L and 75 6 7 pmol/mg proteinzh and 437 6 16 nmol/L and 33 6 6 pmol/mg proteinzh (mean 6 SEM; n 5 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 GRa expression and a dose-dependent increase in GRb expression. 11bHSD1 activity was sensitively up-regulated by increasing concentrations of glucocorticoid (50–1000 nmol/L: P , 0.05). Abolition of these effects by the GR antagonist, RU38486, indicates that regulation of GRa, GRb, and 11bHSD1 expression is mediated exclusively by the GRa ligandbinding variant. In contrast, 11bHSD1 was down-regulated by insulin (20–100 mU/mL: P , 0.01) in the presence of 20% serum, whereas incubation with insulin under serum-free conditions resulted in a dose-dependent increase in 11bHSD1 activity (P , 0.05). Incubation with insulin-like growth factor I resulted in a similar pattern of 11bHSD1 activity. Although neither testosterone nor androstenedione (5–200 nmol/L) affected 11bHSD1 activity, incubation of myoblasts with dehydroepiandrosterone (500 nmol/L) resulted in a decline in 11bHSD1 activity (P , 0.05). These data suggest that glucocorticoid hormone action in skeletal muscle is determined principally by autoregulation of GRa, GRb, and 11bHSD1 expression by the ligand-binding GRa isoform. Additionally, insulin and insulinlike growth factor I regulation of 11bHSD1 may represent a novel mechanism that maintains insulin sensitivity in skeletal muscle tissue by diminishing glucocorticoid antagonism of insulin action. (J Clin Endocrinol Metab 86: 2296–2308, 2001) C SYNDROME and glucocorticoid excess due to exposure to supraphysiological doses of glucocorticoid frequently result in a spectrum of clinical features distinguished by obesity, insulin resistance, glucose intolerance, and hypertension (1). These characteristics bear marked similarities to key features of the metabolic syndrome and type II diabetes (2, 3) and may be the product of increased glucocorticoid antagonism of insulin action. Indeed, glucocorticoids promote gluconeogenesis and glycogen synthesis (4, 5), inhibit glycogenolysis, and reduce the disposal of glucose to the intracellular compartment as a consequence of inhibition of the translocation of the glucose transporter, GLUT-4, to the cell membrane (6–8). Moreover, glucocorticoids promote the differentiation of preadipocytes into mature fat cells (9), diminish glucose uptake, and stimulate lipoprotein lipase activity in adipose tissue, which result in an increase in lipid mobilization and triglyceride sequestration in visceral fat depots (7, 8). Additionally, glucocorticoids inhibit the activity of lipoprotein lipase in skeletal muscle and diminish the uptake of circulating triglyceride, which contribute to the clinical and atherogenic features of dislipidemia that frequently accompany glucocorticoid excess and insulin resistance (10). Importantly, the close parallel between the clinical features of the metabolic syndrome and glucocorticoid excess suggest that abnormalities of glucocorticoid hormone action may contribute to the pathogenesis of key features of the metabolic syndrome (11) and the development of premature atherosclerosis and cardiovascular dis-