Glucocorticoid (GC) excess is characterized by increased adiposity, skeletal myopathy and insulin resistance. Despite the increasing use of GCs as therapeutic agents, the molecular mechanisms that underpin the GC mediated changes in insulin signalling are not clear. The majority of previous studies have used rodent models and have shown regulation at the level of the insulin receptor (IR), IRS-1 and PI3 kinase.
Primary cultures of human skeletal myocytes were grown to confluence and differentiated into myotubes in chemically defined media. The expression of key components of the insulin signalling cascade (IR, IRS1 and 2, PKB/akt 1 and 2), and genes involved in regulating GC response (glucocorticoid receptor (GR) and 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1)) was measured using real-time PCR. Functional impact was assessed by means of a tritiated glucose uptake assay. Experiments were performed across myocytes differentiation and following dexamethasone (Dex) treatment (1 μM, 24-hours).
Across myocyte differentiation, PKB/akt1 mRNA expression decreased 2.0-fold (P<0.05). In addition, 11β-HSD1 expression, a key enzyme regulating local GC availability to bind and activate GR, increased 4.2-fold. There was no significant change in GR expression. Dex increased 11β-HSD1 mRNA expression (1.9-fold) and interestingly, increased both IRS-1 (2.6-fold, P<0.05) and IRS-2 (4.3-fold, P<0.05) expression. However, insulin stimulated tritiated glucose uptake decreased in response to Dex treatment consistent with the induction of insulin resistance (1.9±0.2 [control] vs. 1.3±0.1 [Dex], P<0.05)
We have characterized changes in expression patterns of elements of the insulin signaling cascade and genes regulating GC response though myocyte differentiation and following GC treatment. Understanding the molecular mechanisms that mediate GC induced insulin resistance in muscle may facilitate the identification of novel therapeutic targets.