Influence of titanium ion on mineral formation and properties of osteoid nodules in rat calvaria cultures

The effect of the addition of titanium ion (Ti) on osteoblast function and the mineralization of osteoid nodules in rat calvaria cultures was characte rized. Concentrations of 10 ppm of Ti or more inhibited cell proliferation; 5 ppm or less either had no effect or stimulated proliferation. The number of nodules formed was not influenced by 5 ppm of Ti, but mineral depositio n in nodules was suppressed, as revealed by von Kossa staining. Likewise, 5 ppm of Ti inhibited the incorporation of [Ca-45] in cultures during nodule formation even if the Ti was withdrawn from the medium when mineralization was initiated. In order to test whether the synthesis of osteoid component s was affected, the expression of osteonectin (OSN), osteopontin (OPN), ost eocalcin (OSC), and alkaline phosphatase (ALP) mRNAs as well as ALP enzyme activity was analyzed. The expression of OSN and OPN mRNAs was reduced dram atically, but OSC mRNA was little affected by 5 ppm of Ti. Ti delayed the d evelopment of ALP mRNA expression and enzyme activity relative to the contr ols. Thus Ti treatment changed the proportional composition of cellular mRN A contributing the osteoblast phenotype. (C) 1999 John Wiley & Sons, Inc.