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Regulation of Glucocorticoid Receptor and ß Isoforms and Type I 11ß-Hydroxysteroid Dehydrogenase Expression in Human Skeletal Muscle Cells: A Key Role in the Pathogenesis of Insulin Resistance?

Endocrinology and Metabolism Unit, Southampton General Hospital School of Medicine (C.B.W., S.J.D.), and Department of Chemical Pathology (P.J.W.) and Medical Research Council Environmental Epidemiology Unit (D.I.W.P.), Southampton General Hospital, Southampton, United Kingdom SO16 6YD; and Division of Biomedical Sciences, University of Portsmouth (S.J.D.), Portsmouth, United Kingdom P01 2UP

Address all correspondence and requests for reprints to: Dr Christopher B. Whorwood, Endocrinology and Metabolism Unit, Level D, South Block, Southampton General Hospital School of Medicine, Tremona Road, Southampton, United Kingdom SO16 6YD. E-mail: c.whorwood{at}soton.ac.uk

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 (GR and GRß) and 11ß-hydroxysteroid dehydrogenase type I (11ßHSD1)-mediated intracellular synthesis of active cortisol from inactive cortisone. We have analyzed the expression of GR, GRß, and 11ßHSD1 and their hormonal regulation in skeletal myoblasts from men (n = 14) with contrasting levels of adiposity and insulin resistance. Immunohistochemical, Northern blot, and Western blot analysis indicated abundant expression of GR and 11ßHSD1 under basal conditions. The apparent K_m and maximum velocity for 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 expression and a dose-dependent increase in GRß expression. 11ßHSD1 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 GR, GRß, and 11ßHSD1 expression is mediated exclusively by the GR ligand-binding variant. In contrast, 11ßHSD1 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 11ßHSD1 activity (P < 0.05). Incubation with insulin-like growth factor I resulted in a similar pattern of 11ßHSD1 activity. Although neither testosterone nor androstenedione (5–200 nmol/L) affected 11ßHSD1 activity, incubation of myoblasts with dehydroepiandrosterone (500 nmol/L) resulted in a decline in 11ßHSD1 activity (P < 0.05). These data suggest that glucocorticoid hormone action in skeletal muscle is determined principally by autoregulation of GR, GRß, and 11ßHSD1 expression by the ligand-binding GR isoform. Additionally, insulin and insulin-like growth factor I regulation of 11ßHSD1 may represent a novel mechanism that maintains insulin sensitivity in skeletal muscle tissue by diminishing glucocorticoid antagonism of insulin action.

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