The effect of bisphosphonates on human periodontal-ligament- fibroblast-mediated osteoclastogenesis

Bisphosphonates (BPs) such as pamidronate are used to treat bone destructive diseases and act by inhibiting bone resorption by osteoclasts, thereby improving bone quality. In contrast, BPs were shown to stimulate osteoclast formation and resorption at the mouse molar root. Periodontal ligament (PDL) fibroblasts could be seen as the mediators of osteoclast formation at this location. Here, we investigated whether BP exposure of PDL fibroblasts isolated from molar roots alters osteoclast formation. Human PDL fibroblasts were isolated from healthy donors, and subjected to pamidronate (PAM; 1-100 µM) in vitro for 24 hours, after which cell viability was measured. PAM was removed, and freshly isolated peripheral blood mononuclear cells (PBMCs) were added to the PAM-treated or vehicle-treated fibroblasts. Osteoclasts were counted after 21 days of co-culture, and osteoclast-related gene expression was measured. Cell viability was not affected after 24 hours of treatment with any concentration of PAM. However, in the long term, transient exposure to 100 µM PAM was toxic to PDL fibroblasts. Osteoclast formation was induced by the vehicle-treated PDL fibroblasts, and this was not affected by pre-treatment with 1 or 10 µM PAM. Also, gene expression of the osteoclast marker tartrate-resistant acid phosphatase was unaffected by pre-treatment with PAM. Yet, 100 µM completely blocked osteoclast formation, probably due to the absence of PDL fibroblasts. Moreover, fewer PBMCs survived in this condition than in a mono-culture of PBMCs. In conclusion, BPs inhibited periodontal-ligament-fibroblast-mediated osteoclast formation, probably due to a toxic effect on PDL fibroblasts, however, there was no effect on osteoclastogenesis with a non-toxic BP concentration.