Induction of Apoptosis in Skeletal Tissues: Phosphate-Mediated Chick Chondrocyte Apoptosis is Calcium Dependent

In an earlier study, we have shown that Pi induced apoptosis of terminally differentiated hypertrophic chondrocytes. To ascertain whether Ca²⁺ modulates Pi-induced cell death, we asked the following two questions: First, can we prevent Pi-induced apoptosis by removing Ca²⁺ from the culture medium; alternatively, can we potentiate cell death by increasing the Ca²⁺ concentration? Second, can we inhibit chondrocyte apoptosis by blocking Pi transport? We also explored the mechanism of apoptosis by evaluating mitochondrial activity and reactive oxygen species (ROS) generation in cells treated with the ion pair. We noted that EDTA and EGTA blocked Pi-induced apoptosis in a dose-dependent manner. While high levels of Ca²⁺ alone had little effect on chondrocyte viability, the cation enhanced Pi-dependent cell death and greatly increased Pi uptake. When Pi transport was blocked, there was complete inhibition of cell killing. The process of cell death was characterized by mitochondrial hyperpolarization; two hours following apoptogen treatment, there was a significant decrease in the mitochondrial membrane potential. Coincident with the changes in mitochondrial function, there was an increase in intracellular Ca²⁺ that was maintained throughout the experimental period. A raised Ca²⁺ signal was observed in blebs at the cell membrane. Finally, we noted that, 75 minutes after treatment with the ion pair, there was a six-fold elevation in ROS levels. This increase declined to baseline values after three hours. Based on these observations, we suggest that, at the cartilage mineralization front, an elevation in local environmental Ca²⁺ and Pi concentrations modulates oxidative metabolism, and triggers apoptosis of terminally differentiated chondrocytes.