The endovanilloid/endocannabinoid system in human osteoclasts: possible involvement in bone formation and resorption

Recent studies suggest a role for the endocannabinoid/endovanilloid anandamide in the regulation of bone resorption/formation balance in mice. Here, we examined the co-expression of the transient receptor potential vanilloid type 1 (TRPV1) and the cannabinoid CB1/CB2 receptors together with N-acylphosphatidylethanolamine-hydrolizing phospholipase D (NAPE-PLD) and fatty acid amide hydrolase (FAAH), the two enzymes responsible of the synthesis and catabolism of anandamide respectively, in human osteoclasts. Co-expression of TRPV1, CB1/CB2, NAPE-PLD and FAAH was found in both human osteoclast cultures and in native osteoclasts from human bone biopsies. Moreover, agonist-evoked calcium entry indicated that the TRPV1 receptor is functionally active in vitro. Consistently, biomolecular and functional experiments showed that resiniferatoxin (RTX), a selective TRPV1 receptor agonist, increased the expression and the activity of TRAP and cathepsin K, two specific osteoclast biomarkers. The evidence that cannabinoid and vanilloid receptors are co-expressed in human osteoclasts suggests that they might cross-talk to modulate the intrinsic balance of bone mineralization and resorption by different actions of anandamide through TRPV1 and cannabinoid receptors. The presence of the endocannabinoid/endovanilloid proteins in human osteoclasts will likely have implications for the management of bone demineralization associated syndrome (i.e. osteoporosis).