Metformin and Mesenchymal Stem Cell Osteogenic Differentiation: Role of Organic Cation Transporters

Abstract

Objective: The long-term goal of these studies is to develop novel tissue engineering strategies to enhance craniofacial bone regeneration by combining human umbilical cord derived mesenchymal stem cells (UC-MSCs) and other potential MSC with either systemically or locally delivered metformin. Metformin is a first line, well-tolerated antidiabetic drug with potential osteogenic actions most likely mediated by the activation of the AMP-activated protein kinase (AMPK) signaling pathway. As a highly hydrophilic cationic drug, metformin requires active intracellular uptake via polyspecific cell membrane organic cation transporters (OCTs) encoded by the SLC22A gene family. Despite their critical involvement in hepatic and renal cellular transport, the role played by OCTs in metformin-induced AMPK pathway activation and osteogenic differentiation in UC-MSCs, remains largely unexplored. Here, we hypothesize that to effectively induce AMPK activation and osteogenic differentiation, metformin must gain intracellular access into functional OCT-expressing UC-MSCs. Methods: Immunoblotting was used to assess OCT expression in human-derived UCMSCs. UC-MSCs were treated in vitro with metformin to determine its intracellular uptake, AMPK pathway activation, mineralized nodule formation, and induction of osteogenic markers. Results: Immunoblotting and cellular uptake assays demonstrate that one or more of the OCT isoforms are highly expressed in UC-MSCs and mediate responses to metformin. Treatment of UC-MSCs with clinically relevant doses of metformin (10 μM) resulted in activation of the AMPK signaling pathway. Use of chemical inhibitors targeting OCT function (10 μM quinidine) or AMPK activation (10 μM compound C) markedly inhibited these responses. Metformin significantly enhanced UC-MSC mineralized nodule formation and increased expression and nuclear localization of the osteogenic transcription factor RUNX2. Collectively, these findings indicate that both OCTs and the AMPK signaling pathway play an important role in mediating metformin-induced UCMSC osteogenic differentiation. Conclusions: By gaining a mechanistic insight into the role played by OCTs on metformin-induced MSC osteogenic differentiation mediated by AMPK/RUNX2 signaling, our work may lead to future tissue engineering platforms where metformin together with functional, OCT expressing UC-MSCs may be used as a novel autogenous therapeutic option to enhance bone regeneration. In particular, these treatment strategies might benefit pediatric patients affected with congenital malformations that compromise orofacial skeletal tissues.