Maturation specific sensitivity of osteocytes to calcium and phosphate ion pair‐induced apoptosis

Our previous work demonstrated the sensitivity of bone cells to microenvironmental apoptogens. As the lifespan of an osteocyte can extend into decades, osteocytic terminal differentiation must necessarily remove such sensitivity. Therefore, the goal of this study is to probe the relationship between osteocyte differentiation state and sensitivity to apoptosis. Murine Long bone‐derived osteocytes (MLO) A5 and Y4 cell lines, representative of early and late stage osteocytes, respectively, were seeded onto well plates, allowed to grow for 2, 7 or 14 days and subsequently exposed to increased levels of the calcium and phosphate ion pair for 24 h. Following exposure, MTT assays were performed to quantify cell death. Apoptosis was confirmed through caspase‐3 and TUNEL staining. Osteocytic phenotypes were confirmed through alkaline phosphatase staining. Results indicated that, when treated with increased levels of the calcium and phosphate ion pair, the MLO A5 cell line, representative of early osteocytes, exhibited greater sensitivity to apoptosis than the MLO Y4 cell line, representative of late osteocytes. These data suggest that sensitivity to calcium and phosphate induced apoptosis is a function of maturation level. This work was supported by a grant from the Center for Chronic Disorders of Aging at the Philadelphia College of Osteopathic Medicine.