OR19-07 Metformin Inhibits Activation of the Melanocortin Receptor 2 and 3 in Vitro, a Possible Mechanism for Its Anti-Androgenic and Weight Balancing Effects in Vivo

Metformin is recommended as one of the first-line drugs for the treatment of type 2 diabetes and the metabolic syndrome. In addition to its insulin sensitizing effects, it has been shown to attenuate androgen excess in women with polycystic ovary syndrome (PCOS) or congenital adrenal hyperplasia (CAH), as well as to ameliorate obesity. The mechanisms of metformin action seem manifold. Preclinical studies suggest that it inhibits the cellular stress response at the level of the mitochondrial OXPHOS system and through AMPK dependent and independent mechanisms. Recent studies have shown that metformin decreases ACTH secretion from pituitary and reduces ACTH-stimulated adrenal secretion. In this study we investigated the effect of metformin through its specific melanocortin receptor 2 (MC2R) on signaling targeting adrenal steroidogenesis. To assess this effect, we used mouse adrenal OS3 cells, which do not express the MC2R. Cells were transfected with the human melanocortin receptor 2 and stimulated by ACTH. Downstream cyclic AMP production was then assessed by a co-transfected cAMP-responsive vector producing luciferase that was measured by a dual luciferase assay. The amount of luciferase produced in this assay corresponds to the amount of receptor activation with varying amount of ACTH. The effect of metformin was then tested in this system. We found a significant inhibition of ACTH induced MC2R activation and signaling with 10 mM metformin. The ACTH concentration response curve (CRC) was half-log shifted indicating antagonism. This effect was dose dependent with an IC(50) of 4.2 mM. Metformin did not affect cell viability and basal cAMP level under used conditions. We also tested the effect of metformin on homologous receptors (MCRs). No significant effect was found on MC1R and MC4R activity. However, a 2-log shift in ACTH EC(50) was observed with MC3R. In conclusion, metformin seems to act on MC2R and MC3R signaling directly. The role of MC2R for steroidogenesis is well established. MC3R is involved in energy balance and seems to act as a rheostat when the metabolism is challenged. Our study may explain how metformin attenuates the excess response to ACTH and helps in weight loss and improving androgen excess in PCOS and CAH.