Transient receptor potential vanilloid-1 regulates osteoprotegerin/RANKL homeostasis in human periodontal ligament cells

Background and Objective: Increasing evidence has shown the presence of transient receptor potential vanilloid-1 (TRPV1) in a variety of nonneuronal tissues; however, the function of TRPV1 in these cells is not well understood. In this study, we aimed to investigate the expression and function of TRPV1 in human periodontal ligament (HPDL) cells. As HPDL cells are known to play an important role in the bone-remodeling process, we hypothesized that TRPV1 might be implicated in the regulation of osteoprotegerin (OPG) and RANKL expression. Material and Methods: TRPV1 expression was examined by western blot analysis. The function of TRPV1 was studied using capsaicin, a well-known TRPV1 agonist. RT-PCR was performed to study the expression of OPG and RANKL mRNAs. The expression of OPG and RANKL proteins was analyzed by ELISA and western blotting, respectively. The mechanisms of capsaicin-induced OPG expression in HPDL cells were studied using inhibitors. Results: In this study we found that TRPV1 was present in HPDL cells. Treatment with capsaicin induced OPG expression in a dose-dependent manner but did not affect the expression of RANKL. The increase of the OPG/RANKL ratio was also found in human osteoblasts, but not in MC3T3-E1 cells, a mouse osteoblastic cell line, suggesting species specificity. Capsazepine, the competitive TRPV1 antagonist, significantly abolished the effect of capsaicin on OPG expression in HPDL cells. In addition, studies investigating the effects of a calcium chelator and a phospholipase C inhibitor indicated that calcium ions and phospholipase C were required for the induction. Interestingly, capsaicin was able to increase the OPG/RANKL ratio, even in the presence of prostaglandin E2, a potent inducer of RANKL. Conclusion: Our study demonstrates that activation of TRPV1 leads to an increase of the OPG/RANKL ratio in HPDL cells. These findings suggest the novel function of TRPV1 in periodontal tissues, at least, as the regulator of the OPG/RANKL axis.