Activation of cannabinoid receptor CB2 regulates osteogenic and osteoclastogenic gene expression in human periodontal ligament cells

Background and Objective: Cannabinoid receptor CB2, expressed in osteoblasts and osteoclasts, plays a crucial role in the regulation of bone metabolism. Since periodontal ligament (PDL) cells can differentiate into osteoblasts, this study was undertaken to investigate CB2 expression and the effect of CB2 activation on osteogenic differentiation of PDL cells. Material and Methods: Human PDL (hPDL) cells were obtained from extracted teeth of periodontally healthy subjects. Expression of CB2 was observed in hPDL cells by RT-PCR, Western blotting and immunofluorescence assay. Then hPDL cells were treated with a CB2-specific agonist, HU-308 (10-7 m), for 12, 24, 48 or 72 h. The mRNA expressions of osteogenic genes, such as runt-related transcription factor 2 (Runx2), bone sialoprotein (BSP), osteopontin (OPN), alkaline phosphatase (ALP), osteocalcin (OC) and collagen type I (COL I), and osteoclastogenic genes, including osteoprotegerin (OPG) and receptor activator of NF-kappa B ligand (RANKL), were examined using quantitative real-time PCR analysis. A mineralization assay was performed in hPDL cells in mineralization conditions with or without HU-308. Results: Expression of CB2 mRNA and protein was detected in hPDL cells. HU-308 enhanced the mRNA levels of the above osteogenic genes. Expression of the OPG gene was up-regulated, whereas RANKL gene expression was down-regulated, contributing to the elevated OPG/RANKL ratio. Accelerated mineralization was observed in hPDL cells in mineralization conditions with HU-308. Conclusion: Our findings demonstrate that activation of CB2 is able to enhance osteogenic differentiation of hPDL cells and potentially create a favorable osteogenic microenvironment. This implies that CB2 might play an important role in alveolar bone metabolism.