Bisphosphonates are potent antiresorptive drugs which have antifracture efficacy by reducing bone turnover rate and increasing bone mineral density. In addition to inhibiting osteoclast function, bisphosphonates have been reported to also promote survival of osteocyte and osteoblast via an anti-apoptotic effect, mediated by opening of hemi-gap junction channels formed by connexin43 (Cx43). In this study, we investigated the effect of risedronate, one amino-bisphosphonate, on osteoblast differentiation and Cx43 expression using the mesenchymal cell line C2C12. Risedronate dose-dependently increased the activity of osterix (OSE)-luciferase containing Runx2 response element with highest activity at 50 The activity of osteocalcin (OC)- and bone sialoprotein (BSP)-luciferase reporters, markers of osteoblast differentiation, were also increased by risedronate. When risedronate and BMP2 were used in combination, alkaline phosphatase (ALP) activity increased to a larger extent than when BMP2 was used alone. Risedronate as well as the pro-osteogenic transcription factors, Runx2, Osterix or Dlx5, increased transcriptional activity of the Cx43 promoter in a dose-dependent manner. In the presence of Runx2 or Dlx5, risedronate had an additive effect on Cx43 promoter activity. Accordingly, risedronate increased protein expression of Cx43, Runx2, Osterix, and Dlx5. These results suggest that risedronate promotes osteoblastic differentiation and positively regulates Cx43 gene transcription. (C) 2013 Elsevier Inc. All rights reserved.