Retinoic acid and PPAR-γ ligands synergistically inhibit ACTH secretion and proliferation in murine corticotroph tumor cells

In most cases of endogenous Cushing's syndrome increased production of cortisol by the adrenals is due to an ACTH-secreting pituitary adenoma. In a previous work we found that in the presence of retinoic acid (RA), the retinoid receptors (RAR and RXR) inhibited POMC transcription, ACTH production and proliferation in ACTH-secreting AtT-20 tumor cells. In the same line of investigation, the peroxisome proliferator activated receptor-γ (PPAR-γ) ligand, rosiglitazone, was shown by others to induce cell-cycle arrest, apoptosis, and suppression of ACTH secretion in corticotroph tumor cells. Like the RAR, PPAR-γ forms an heterodimer with the RXR protein. Recent studies in pancreatic and breast cancer cell lines have demonstrated that the RXR shares synergistic growth-inhibitory actions with PPAR-γ ligands.

We decided to examine the efficacy of a combination of PPAR-γ ligand (rosiglitazone) and retinoid receptor ligands (all trans RA for the RAR and 9-cis RA for the RXR), in corticotroph tumor cells. We first determined the lowest inhibitory dose of each, all trans RA, 9-cis RA, and rosiglitazone on cell proliferation and ACTH secretion as measured by the WST-1 assay and RIA, respectively. No or almost no inhibition of cell proliferation and ACTH secretion was observed at 10 nM all-trans RA, 1nM 9-cis RA and 5µM rosiglitazone. The combined treatment of the cells with these suboptimal doses of RA and rosiglitazone resulted in a synergistic enhancement of the inhibition of proliferation and ACTH production: a) Coincubation of rosiglitazone with 9-cis RA resulted in a 67% inhibition of cell proliferation and a 53% inhibition of ACTH secretion, b) Coincubation of rosiglitazone with all-trans RA inhibited cell proliferation by 58% and ACTH secretion by 57%.

These results suggest potential therapeutic uses for these compounds. Combining RA and rosiglitazone for the treatment of Cushing's disease might allow reduced dosages of both drugs and therefore prevent toxic effects.