Matrix regulation of skeletal cell apoptosis - Role of calcium and phosphate ions

Previously, we noted that inorganic phosphate (P-i), a major component of b one extracellular matrix, induced osteoblast apoptosis (Meleti, Z., Shapiro , I. M., and Adams, C. S. (2000) Bolts (NY) 27, 359-366). Since Ca2+ along with P-i is released from bone during the resorption process, we advanced t he hypothesis that Ca2+ modulates P-i-mediated osteoblast apoptosis. To tes t this hypothesis, osteoblasts were incubated with both ions, and cell deat h was determined. We noted that a modest increase in the medium Ca2+ concen trations ([Ca2+](e)) of 0.1-1 mM caused a profound and rapid enhancement in P-i-dependent death of cultured osteoblasts. An elevation in [Ca2+](e) alo ne had no effect on osteoblast viability, whereas Ca2+ channel blockers fai led to inhibit killing of ion pair-treated cells. These results indicated t hat P-i-mediated cell death is not dependent on a sustained increase in the cytosolic Ca2+ concentration. Terminal dUTP nick-end labeling analysis and measurement of caspase-3 activity of the ion pair-treated cells suggested that death was apoptotic; Apoptosis was confirmed using caspase-3 and endon uclease inhibitors. The mitochondrial membrane potential and cytosolic Ca2 status of the treated cells were evaluated. After incubation with [Ca2+](e ) and P-i, a decrease in mitochondrial fluorescence was noted, suggesting t hat the ions decreased the mitochondrial transmembrane potential. Subsequen t to the fall in mitochondrial membrane potential, there was a transient el evation in the cytosolic Ca2+ concentration. Results of the study suggest t hat the ion pair conspire at the level of the plasma membrane to induce int racellular changes that result in loss of mitochondrial function. The subse quent increase in the cytosolic Ca2+ concentration may trigger downstream e vents that transduce osteoblast apoptosis.