Glucocorticoid (GC) excess is characterized by central obesity, insulin resistance and in some cases, type 2 diabetes mellitus. Whilst it is accepted that GCs cause insulin resistance, both insulin and GCs act synergistically to promote adipocyte differentiation. We have previously shown that GCs cause tissue specific changes in insulin sensitivity, enhancing insulin signalling in human adipose tissue in contrast to muscle.
TRB3, a mammalian homolog of Drosophila tribbles, functions as a negative modulator of insulin signalling in liver where it binds directly to PKB/akt, preventing phosphorylation and subsequent activation. We hypothesise that GC regulation of TRB3 may mediate changes in insulin signalling in human adipocytes in response to GC treatment.
Human subcutaneous adipocytes (Chub-S7 cells) were grown to confluence and differentiated in chemically defined media. Insulin stimulated PKB/akt phosphorylation was determined by western blotting and TRB3 expression by real-time PCR. In Chub-S7 cells, Dexamethasone (Dex) treatment augmented insulin signalling and decreased TRB3 mRNA expression; an effect blocked by the GC receptor antagonist, RU38486 (ctrl 1±0 vs. Dex 1uM 0.44±0.19 vs. Dex + RU486 2.4±0.64 P<0.05).
Chub-S7 cells were able to convert inactive cortisone to cortisol through the action of 11-β-hydroxysteroid dehydrogenase type 1 (11βHSD1) and activity was inhibited by glycyrrhetinic acid (GE) (ctrl 18.59%±3.1 vs. GE 0%±0 P<0.05). Both cortisol and cortisone increased insulin stimulated PKB/akt phosphorylation (ctrl 1±0 vs. cortisol, 2.5±0.49 P<0.05; ctrl 1±0 vs. cortisone 1.69±0.09, P<0.05). In addition, TRB3 mRNA expression decreased (ctrl 1±0 vs. cortisol 0.43±0.11 vs. cortisone 0.39±0.10 P<0.05). Co-incubation of cortisone with GE abolished the insulin stimulated PKB/akt phosphorylation (cortisone, 1.69±0.09 vs. cortisone + GE 1.14±0.14 P<0.05)
Our data demonstrates the importance of pre-receptor regulation of GCs through the action of 11β-HSD1 upon insulin signalling in human adipose tissue. In addition, we have shown that TRB-3 is expressed in human adipocytes and that regulation of expression may represent a key mechanism underpinning changes in insulin signalling in response to GC treatment.