The pathological effects of glucocorticoids (GC) are exemplified by patients with Cushings syndrome who develop central obesity, insulin resistance and in some cases, type 2 diabetes mellitus. It is generally accepted that GC cause insulin resistance, however, both insulin and GC increase adipocyte differentiation. The question therefore arises as to how GC stimulate adipocyte differentiation whilst apparently making adipocytes insulin resistant. We have hypothesized that GC cause tissue specific changes in insulin sensitivity, enhancing insulin signalling in human adipose tissue in contrast to liver, and that this may represent a novel mechanism of GC induced obesity.
Human subcutaneous adipocytes (Chub-S7 cells) were grown to confluence and differentiated in chemically defined media and treated with or without GC (dexamethasone and/or cortisol). Insulin stimulated PKB/akt phosphorylation was determined by western blotting and glucose transport measured by tritiated glucose uptake. Tissue specificity was examined using the human liver cell line, HepG2 cells.
In differentiated human adipocytes, GC induced a dose (control 1±0 vs 50 nM 1.2±0.08 vs 250 nM 2.25±0.24 vs 1000 nM 3.4±0.17, P<0.001) and time (control 1±0 vs 6 hr 4.11±1.7 vs 24 hr 9.36±4.15, P<0.05) dependent increase in insulin stimulated PKB/akt phosphorylation without alteration in total protein (western blotting) or mRNA (real-time PCR) expression. This effect was blocked by RU486 (GC receptor antagonist), and by wortmannin (PI3Kinase inhibitor). Furthermore, GC increased insulin stimulated glucose transport into differentiated human adipocytes in a PI3 kinase dependent manner (control 1.8±0.2 vs GC 2.7±0.3, P<0.05). In contrast, GC failed to regulate insulin stimulated PKB/akt phosphorylation in HepG2 cells.
We have demonstrated tissue specific regulation of insulin signalling by GC. In adipose tissue, GC enhances insulin signalling and we postulate that this represents a mechanism to increase adipocyte differentiation thereby contributing to GC induced obesity. Tissue specific regulation of GC action may therefore represent a novel therapeutic strategy.