Endocrine Abstracts

Synthetic peroxisome proliferator-activated receptor gamma (PPARγ) ligands are used clinically to improve insulin sensitivity. Although designed as specific ligands for PPARγ, there is evidence for some ‘off target’ effects being mediated by a non-classical, non-PPARγ mechanism. Recent evidence further suggests that some of these effects may be glucocorticoid receptor (GR)-dependent.

To analyse activation of GR following exposure to PPARγ ligands, we have investigated the effect of the thiazolidinedione, Rosiglitazone in human cells (A549 and HeLa cells which express endogenous GR, and a GR deficient U20S cell line).

GR trafficking studies in HeLa cells and U20S cells expressing GR-GFP demonstrate that GR translocates to the nucleus in a significant percentage of cells (control 4%; Rosi 32% nuclear GR) following 30 min incubation with Rosiglitazone, with almost exclusive nuclear GR localisation after 120 min (94% nuclear GR); an effect comparable to that induced by the synthetic glucocorticoid Dexamethasone (Dex). Interestingly, whilst Rosiglitazone induces GR phosphorylation (ser211), treatment with Dex induces both GR phosphorylation and repression.

Transfection with a simple glucocorticoid responsive reporter gene (TAT3-luc) suggests that Rosiglitazone activates GR-dependent transcription in both HeLa and A549 cells (34-fold and 4-fold induction respectively). Furthermore, studies in U20S cells show this effect to be dependent on GR expression.

Finally, we show that the antiproliferative action of Rosiglitazone is potentiated in U20S cells that overexpress GR, further suggesting GR dependence.

These novel findings show that Rosiglitazone activates the GR and promotes nuclear translocation to mediate GR dependent cellular effects. This is an unexpected mechanism for Rosiglitazone action in human cells that may explain some of the PPARγ-independent activities of the drug observed in vivo. This mechanism may be important for the design of more specific PPARγ ligands and may be exploited for new therapeutics targeting the glucocorticoid receptor itself.