Therapeutic metformin concentrations positively regulate proliferation in endometrial epithelial cells via mTOR activation and augmented mitochondrial strength

Abstract

Metformin, an antidiabetic drug, has recently been repositioned in the treatment of several nondiabetic disorders, including reproductive disorders such as polycystic ovarian syndrome, where it improves endometrial functions. In vitro studies employing supratherapeutic concentrations (5–20 mmol/L) of metformin have reported antiproliferative effects on endometrial epithelial and stromal cells. However, animal and human studies have revealed that therapeutic serum concentrations of metformin range between 20 and 70 µmol/L. In the present study, the effect of therapeutic concentrations of metformin was studied on endometrial epithelial cells (EECs). Therapeutic concentrations of metformin induced proliferation in Ishikawa and HEC-1A cells. The proliferation of EECs was found to be mammalian target of rapamycin (mTOR) dependent. Interestingly, therapeutic metformin concentrations were not able to activate the classical AMP-activated protein kinase (AMPK) signaling. On the contrary, supratherapeutic metformin concentration (10 mmol/L) inhibited mTOR and activated AMPK signaling. Microarray analysis of metformin-treated HEC-1A cells revealed dose-dependent differential effects on biological pathways associated with translation, ribosomal RNA processing, mitochondrial translation, and cell proliferation. Therapeutic concentrations of metformin upregulated mitochondrial number as demonstrated by increased MitoTracker™ Red staining and enhanced succinate dehydrogenase expression; however, higher concentration (10 mmol/L) abrogated the same. Our results suggest that therapeutic concentrations of metformin augment mitochondrial strength and induce mTOR-dependent endometrial cell proliferation.