Ginsenoside Rd stimulates the differentiation and mineralization of osteoblastic MC3T3-E1 cells by activating AMP-activated protein kinase via the BMP-2 signaling pathway

Abstract

As part of our search for biologically active anti-osteoporotic agents that enhance the differentiation and mineralization of osteoblastic MC3T3-E1 cells, we identified the ginsenoside Rd as the most active compound among ginsenosides. In this study, we showed that Rd stimulates osteoblastic differentiation and mineralization, manifested by the up-regulation of differentiation markers (alkaline phosphatase and osteogenic genes) and von Kossa/Alizarin Red staining, respectively. Rd induces the mRNA expression of bone morphogenetic protein-2 (BMP-2) and the secretion of the corresponding protein into media in a concentration-dependent manner. The mRNA expression and enzyme activity of alkaline phosphatase (ALP) were suppressed when MC3T3-E1 cells were exposed to noggin, a BMP-2 antagonist. The level of phosphorylated AMP-activated protein kinase (pAMPK) protein was also up-regulated by Rd in a time- and concentration-dependent manner. Rd-induced ALP activity, mineralization, and BMP-2 production were all inhibited by either Ara-A (AMPK inhibitor) or siRNA targeting AMPK. In addition, we investigated whether Rd-induced BMP-2 transduces signals through the Smad signaling pathways. Rd induced a significant level of phosphorylation of Smad1/5, and this effect was blocked when the cells were transfected with siRNA targeting Smad4, indicating that Smad1/5 must form complex with Smad4 to translocate into the nucleus and regulate the transcription of osteogenic genes. In summary, these results indicate that Rd induces the differentiation and mineralization of MC3T3-E1 cells through the activation of the AMPK/BMP-2/Smad signaling pathways. These findings provide a molecular basis for the osteogenic effect of Rd in MC3T3-E1 cells.