Endocrine Abstracts

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 GC mediated changes in insulin signalling are not clear. Within skeletal muscle, the microsomal enzyme, 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) converts inactive GC, 11-dehydrocorticosterone (A) to active corticosterone (B) and thus regulates GC availability to bind and activate the glucocorticoid receptor (GR). Our aim was to identify the molecular mechanisms that contribute to GC induced insulin resistance in skeletal muscle. Previous studies have shown that plasma free fatty acid induced insulin resistance is mediated by serine phosphorylation of insulin receptor substrate-1 (IRS-1) at position 307. This results in a reduced affinity of IRS-1 for the insulin receptor and also targets IRS-1 for proteosomal degradation.

Mouse C₂C₁₂ skeletal myocytes were grown to confluence and differentiated into myotubes in chemically defined media. The expression of key components of the insulin signalling cascade were measured using real-time PCR and western blot. Functional impact was assessed by means of tritiated glucose uptake. Experiments were performed using synthetic GC dexamethasone (DEX) and endogenous GC corticosterone (B). GC treatment induced insulin resistance in cultured skeletal myocytes as evidenced by decreased insulin stimulated glucose uptake (23%). DEX treatment decreased IRS-1 total protein and mRNA expression (0.5-fold, P<0.05 and 2-fold, P<0.05, respectively) and in addition increased serine 307 phosphorylation (4.5-fold, P<0.05). Following treatment with B, serine 307 phosphorylation increased in a dose and time dependent manner (3.9-fold (250 nM), 8.7-fold (500 nM), P<0.05; 1.3-fold (6 h), 2.0-fold (24 h)). Treatment with inactive GC, A (which relies on 11β-HSD1 activity for conversion to active B), decreased IRS-1 total protein expression (0.4-fold), an effect blocked by the 11β-HSD inhibitor, glycyrrhetinic acid (GE). In addition, serine 307 phosphorylation increased (1.9-fold). In summary, we have identified a novel action of GCs upon the insulin signalling cascade in skeletal muscle by increasing serine 307 phosphorylation of IRS-1 as well as reducing total protein expression. Furthermore, we have shown the importance of pre-receptor regulation through 11β-HSD type 1 in this response. GC induced phosphorylation of IRS-1 serine 307 may play a major role in the induction of insulin resistance seen in the skeletal muscle of patients with Cushing’s syndrome and type 2 diabetes.