it is unclear whether metformin, one of the anti-hyperglycemic agents commonly used for type 2 diabetes, could affect bone formation through activation of AMP-activated Protein kinase (AMPK). In Order to clarify this issue, we investigated the effects of metformin oil the differentiation and mineralization of osteoblastic MC3T3-E1 cells as well as intracellular signal transduction. Metformin (50 mu M) significantly increased collagen-I and osteocalcin mRNA expression, stimulated alkaline phosphatase activity, and enhanced cell mineralization. Moreover, metformin significantly activated AMPK in dose- and time-dependent manners, and induced endothelial nitric oxide synthase (eNOS) and bone morphogenetic protein-2 (BMP-2) expressions. Supplementation of Ara-A (0.1 mM), a specific AMPK inhibitor, significantly reversed the metformin-induced eNOS and BMP-2 expressions. Our findings Suggest that metformin can induce the differentiation and mineralization of osteoblasts via activation of the AMPK signaling pathway, and that this drug might be beneficial for not only diabetes but also osteoporosis by promoting bone formation. (C) 2008 Elsevier Inc. All rights reserved.