Metformin inhibits aromatase via an extracellular signal-regulated kinase-mediated pathway

Abstract

Metformin treatment, now widely prescribed in polycystic ovary syndrome, is aimed at correcting the associated insulin resistance, but it has also been shown to directly inhibit ovarian steroidogenesis. The mechanisms, however, by which metformin inhibits estradiol production in human granulosa cells remains unknown. Granulosa luteal cells were incubted with metformin, insulin, or combined metforminandinsulin treatment,andaromatasemRNAexpressionwasquantified using real-time RT-PCR. Enzyme activity was assessed by the conversion of 3H-androstenedione to estrone and estradiol. Metformin's effectonthe expression of specific untranslated first exon aromatase promoterswasanalyzed using semiquantitative PCR. The involvement ofMAPKkinase (MEK)/ERK pathwaywasinvestigated by immunoblotting for aromatase, phosphorylated, and total ERK-1,2 from cells cultured as above with/ without the MEK inhibitor PD98059. Metformin significantly inhibited basal and insulin-stimulated aromatase mRNA expression, with parallel results from the aromatase activity assay and protein assessment. This suppression was via down-regulation of aromatase promoter II, I.3, and 1.4 expression and was reversed by the addition of PD98059. Involvement of the ERK signaling pathway was demonstrated by the significant increase in phosphorylated ERK-1,2 with the combined metformin and insulin treatment. We have shown for the first time in human granulosa cells that metformin signficantly attenuated basal and insulin-stimulated P450 aromatase mRNA expression and activity, via silencing of key promoters. This occurred by activation of MEK/ERK pathway, which negatively regulated aromatase production. This is an important consideration given metformin's widespread use in polycystic ovary syndrome and may further support a possible therapeutic indication in estrogendependent breast tumors. Copyright © 2009 by The Endocrine Society.