Type II Na+-Pi Cotransporters in Osteoblast Mineral Formation: Regulation by Inorganic Phosphate

Abstract

During calcification of bone, large amounts of phosphate (P_i) must be transported from the circulation to the osteoid. Likely candidates for osteoblast P_i transport are the type II sodium-phosphate cotransporters NaPi-IIa and NaPi-IIb that facilitate transcellular P_i flux in kidney and intestine, respectively. We have therefore determined the cotransporters'' expression in osteoblast-like cells. We have also studied the cotransporters'' regulation by P_i and during mineralization in vitro. Phosphate uptake and cotransporter protein expression was investigated at early, late and mineralizing culture stages of mouse (MC3T3-E1) and rat (UMR-106) osteoblast-like cells. Both NaPi-IIa and NaPi-IIb were expressed by both osteoblast-like cell lines. NaPi-IIa was upregulated in both cell lines one week after confluency. After 7 days in 3mM P_i NaPi-IIa was strongly upregulated in both cell lines. NaPi-IIb expression was unaffected by both culture stage and P_i supplementation. The expression of both cotransporters was unaffected by P_i deprivation. In vitro mineralization at 1.5mM P_i was preceded by a three-fold increase in osteoblast sodium-dependent P_i uptake and a corresponding upregulation of both NaPi-IIa and NaPi-IIb. Their expression thus seem regulated by phosphate in a manner consistent with their playing a role in transcellular P_i flux during mineralization.