Extracellular calcium chronically-induced human osteoblasts effects: Specific modulation of osteocalcin and collagen type XV.

Fluctuation in extracellular calcium (Ca2+) concentration occurs during bone remodeling. Free ionised Ca2+ plays a critical role in regulating osteoblast functions. We analyzed the effects of different concentrations of free ionised Ca2+ (0.5, 1.3, 2.6mM) on human osteoblasts and we evaluated osteoblastic phenotype (marker expression and cell morphology) and functions (osteogenic differentiation, cell proliferation and cell signaling). Our data show human osteoblasts that chronically-stimulated with 0.5, 1.3, or 2.6mM Ca2+ significantly increase intracellular content of alkaline phosphatase, collagen type I, osteocalcin and bone sialoprotein, whereas collagen type XV was down-modulated and RUNX2 expression was not affected. We also found a Ca2+ concentration-dependent increase in osteogenic differentiation and cell proliferation, associated to an increase of signaling protein PLC²1 and p-ERK. Human osteoblast morphology was affected by Ca2+ as seen by the presence of numerous nucleoli, cells in mitosis, cell junctions and an increased number of vacuoles. In conclusion, our data show a clear phenotypical and functional effect of extracellular Ca2+ on human osteoblasts and support the hypothesis of a direct role of this cation in the bone remodeling processes. J. Cell. Physiol. Copyright 2011 Wiley Periodicals, Inc.