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 ionized Ca2+ plays a critical role in regulating osteoblast functions. We analyzed the effects of different concentrations of free ionized Ca2+ (0.5, 1.3, and 2.6?mM) 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 2.6?mM 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 PLC beta 1 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. 227: 31513161, 2012. (c) 2011 Wiley Periodicals, Inc.