Effect of zoledronic acid on oral fibroblasts and epithelial cells: a potential mechanism of bisphosphonate-associated osteonecrosis

Osteonecrosis of the jaw secondary to bisphosphonate infusion (zoledronic acid-ZA) is assumed to be a bone disease. This study investigated the effects of ZA on soft tissues using oral mucosal cells as an in vitro model of soft tissue cell death in the pathogenesis of bone necrosis. Human gingival fibroblast and keratinocyte cell lines were exposed to different concentrations of ZA (0.25-3 mu mol/l), using 1 mu mol/l as the expected baseline concentration. A dose-response effect on apoptosis and cell proliferation [Terminal deoxynucleotidyl transferase-mediated dUTP-Biotin End Labelling and Annexin V or Coulter counter and 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium), respectively] was observed with increasing ZA concentrations; both reversed using siRNA against caspase 3 or 9. Gene expression analysis using RT(2) Profiler polymerase chain reaction Arrays demonstrated the differential expression of multiple genes involved in apoptosis including those that encode TNF, BCL-2, Caspase, IAP, TRAF and Death Domain families. Western blot analysis confirmed the presence of activated forms of caspase 3 and 9 and underexpression of survivin protein expression. This study demonstrated that low concentrations of ZA rapidly and directly affected the oral mucosal tissues though the induction of a gene-regulated apoptotic process. These findings support the potential for soft tissue injury as an initiating/potentiating event for osteonecrosis.