Cadmium induces apoptosis in the human osteoblast-like cell line Saos-2

Human exposure to the heavy metal cadmium has been associated with the development of bone diseases, including osteoporosis and osteomalacia. The mechanisms by which cadmium exerts a direct effect on bone remain unclear. Bone cells go through apoptosis for proper bone remodeling; therefore, it was hypothesized that cadmium disrupts this normal balance by inducing apoptosis. Human osteoblast- like cells ( Saos- 2) were treated with 10 - 200 mM cadmium chloride ( CdCl2) and evaluated by trypan blue staining and phase- contrast microscopy. Exposure to CdCl2 resulted in decreased cell viability and changes in cell morphology characteristic of apoptosis. The role of apoptosis in cadmium-induced toxicity was further evaluated using the fluorescent marker annexin V, which detects externalization of cell membrane phosphatidylserine. Nuclear changes associated with apoptosis were assessed by Hoechst staining and a DNA fragmentation assay. A significant increase in annexin V- positive cells was observed following CdCl2 treatment. Nuclear changes associated with apoptosis, including marginalization and condensing of chromatin and DNA fragmentation, were also observed following CdCl2 treatment. Cadmium- induced apoptosis in Saos- 2 cells was also accompanied by an increase in caspase- 3 activity. The addition of the caspase- 3 inhibitor N- acetyl- Asp- GluVal- Asp- aldehyde ( Ac- DEVD- CHO) or the known cadmium chelating agent potassium bis( 2- hydroxyethy) dithiocarbamate, ( K[bhedtc]), blocked caspase- 3 activation induced by cadmium. Collectively, this study has identified a role for apoptosis in cadmium-induced toxicity in bone cells, and provides insight for future studies on mechanisms underlying the disruption of apoptotic signaling cascades in bone and the relationship to bone disease.