Dual effect of inorganic polymeric phosphate/polyphosphate on osteoblasts and osteoclasts in vitro

Inorganic polymeric phosphate/polyphosphate (polyP) is a natural polymer existing in both pro- and eukaryotic systems. In the present study the effect of polyP as well as of polyP supplied in a stoichiometric ratio of 2m polyP:1m CaCl2 [polyP (Ca2+ complex)] on the osteoblast-like SaOS-2 cells and the osteoclast-like RAW 264.7 cells was determined. Both polymers are non-toxic for these cells up to a concentration of 100 mu m. In contrast to polyP, polyP (Ca2+ complex) significantly induced hydroxyapatite formation at a concentration >10 mu m, as documented by alizarin red S staining and scanning electron microscopic (SEM) inspection. Furthermore, polyP (Ca2+ complex) triggered in SaOS-2 cells transcription of BMP2 (bone morphogenetic protein 2), a cytokine involved in maturation of hydroxyapatite-forming cells. An additional activity of polyP (Ca2+ complex) is described by showing that this polymer impairs osteoclastogenesis. At concentrations>10 mu m polyP (Ca2+ complex) slows down the progression of RAW 264.7 cells to functional osteoclasts, as measured by the expression of TRAP (tartrate-resistant acid phosphatase). Finally, it is shown that 10-100 mu m polyP (Ca2+ complex) inhibited phosphorylation of IB by the respective kinase in RAW 264.7 cells. We concluded that polyP (Ca2+ complex) displays a dual effect on bone metabolizing cells. It promotes hydroxyapatite formation in SaOS-2 cells (osteoblasts) and impairs maturation of the osteoclast-related RAW 264.7 cells. Copyright (c) 2012 John Wiley & Sons, Ltd.