Title

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

Document Type

Article

Publication Date

2001

Abstract

Previously, we noted that inorganic phosphate (Pi), a major component of bone extracellular matrix, induced osteoblast apoptosis (Meleti, Z., Shapiro, I. M., and Adams, C. S. (2000) Bone (NY) 27, 359-366). Since Ca2+ along with Pi is released from bone during the resorption process, we advanced the hypothesis that Ca2+ modulates Pi-mediated osteoblast apoptosis. To test this hypothesis, osteoblasts were incubated with both ions, and cell death was determined. We noted that a modest increase in the medium Ca2+ concentrations ([Ca2+]e) of 0.1-1 mm caused a profound and rapid enhancement in Pi-dependent death of cultured osteoblasts. An elevation in [Ca2+]e alone had no effect on osteoblast viability, whereas Ca2+ channel blockers failed to inhibit killing of ion pair-treated cells. These results indicated that Pi-mediated cell death is not dependent on a sustained increase in the cytosolic Ca 2+ 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 endonuclease inhibitors. The mitochondrial membrane potential and cytosolic Ca2+ status of the treated cells were evaluated. After incubation with [Ca 2+]e and Pi, a decrease in mitochondrial fluorescence was noted, suggesting that the ions decreased the mitochondrial transmembrane potential. Subsequent to the fall in mitochondrial membrane potential, there was a transient elevation in the cytosolic Ca2+ concentration. Results of the study suggest that the ion pair conspire at the level of the plasma membrane to induce intracellular changes that result in loss of mitochondrial function. The subsequent increase in the cytosolic Ca 2+ concentration may trigger downstream events that transduce osteoblast apoptosis.

Publication Title

Journal of Biological Chemistry

Volume

276

Issue

23

First Page

20316

Last Page

20322

Comments

This article was published in Journal of Biological Chemistry, Volume 276, Issue 23, Pages 20316-20322.

The published version is available at http://dx.doi.org/10.1074/jbc.M006492200.

Copyright © 2001.

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