Novel insights into the pharmacological modulation of human periodontal ligament stem cells by the amino-bisphosphonate Alendronate.

Alendronate (ALN) is a second-generation bisphosphonate widely used for osteoporosis and cancer-induced bone lesions. Many studies have confirmed a strong relationship between osteonecrosis of the jaws (ONJ) development and oral bisphosphonates, especially ALN, although the molecular mechanisms underlying this pathology have not yet been elucidated. The reduction in bone turnover and vascularization usually observed in ONJ are the result of ALN action on different cell types harboured in oral microenvironment, such as osteoclasts, endothelial cells, and periodontal ligament stem cells (PDLSCs). In this perspective, the present study aims to investigate the effects of different ALN concentrations (2 μM, 5 μM, 10 μM, 25 μM, 50 μM) on the phenotype and functional properties of human PDLSCs (hPDLSCs). hPDLSCs showed a decrease in cell viability (MTT assay) only when treated with ALN concentration of 10 μM or larger for 48 h and 72 h. Cell cycle analysis revealed a moderate increase in proportion of S-phase cells after exposure to low ALN concentration (2-5 μM), an effect that was reverted after exposure to 10-50 μM ALN. Conversely, cell death was evidenced via Annexin V/PI assay at very high concentration of ALN (50 μM) after 4 days of treatment. In addition, we explored whether the effects of ALN on hPDLSCs growth and survival can be mediated by its ability to modulate oxidative stress. To this, we quantified the intracellular ROS amount and lipid peroxidation by using DCF probe and Bodipy staining, respectively. Flow cytometry analysis showed that ALN induced a dose-dependent reduction of intracellular oxidative stress and lipid peroxidation upon treatment with low concentrations at both 48 h and 72 h. Increased levels of oxidative stress was reported at 50 μM ALN and was also confirmed via TEM analysis. Despite the stability of the cellular immunophenotype, hPDLSCs showed impaired mobility after ALN exposure. Chronic exposure (7-14 days) to ALN in the range of 2-10 μM significantly decreased the expression of the differentiation-related factors ALP, RUNX2, COLI, and OPN as well as the osteogenic ability of hPDLSCs compared with untreated cells. Conversely, higher doses were found to be neutral. Our findings indicated that the effects of ALN on hPDLSCs behavior are dose-dependent and suggest a role for oxidative stress in ALN-induced cell death that may lead to novel therapeutic approaches for ONJ.