Role of serine 249 of ezrin in the regulation of sodium-dependent phosphate transporter NaPi-IIa activity in renal proximal tubular cells.

Type IIa sodium-dependent phosphate transporter (NaPi-IIa) is responsible for renal phosphate reabsorption and maintenance of systemic phosphate homeostasis in mammals. Macromolecular complex formation of NaPi-IIa with sodium-proton exchanger related factor-1 (NHERF-1) and ezrin is important for apical membrane localization in the proximal tubular cells. Here, we investigated the interactions of the ezrin phosphomimetic mutation of serine to aspartic acid at 249 with NHERF-1 and the inhibition of apical membrane localization of NaPi-IIa. In vitro phosphorylation analysis revealed that serine 249 of human ezrin serves as a phosphorylation site for protein kinase A. The N-terminal half of ezrin had a dominant negative effect on the phosphate transport activity and inhibited the apical localization of NaPi-IIa in renal proximal tubular cells. We found that the phosphomimetic S249D mutant interfered with the inhibitory effects of the dominant negative mutant on the transport and localization of NaPi-IIa. The S249D mutant also inhibited the interaction with NHERF-1. Therefore, serine 249 of ezrin can play important roles in the regulation of the complex formation and membrane localization of NaPi-IIa.

Clodronate stimulates bone formation as well as inhibits bone resorption and increases bone mineral density in rats fed a low-calcium diet.

The pharmacological actions of bisphosphonates are due to the inhibitory effects on bone resorption, but little is known about the bisphosphonate action on bone formation. The purpose of this study is to elucidate the actions of bisphosphonates, clodronate, on bone formation in the experimental in vivo and in vitro rat models. The bone mineral density (BMD) was decreased in the rats fed a low-calcium diet (0.05% Ca) for 6 days compared with the rats fed a normal-calcium diet (0.5% Ca). The decrease in BMD was suppressed in the 2 mgP/day and the 4 mgP/day clodronate administrations. Bone formation rate (BFR) in rats fed a low-calcium diet was significantly increased compared with the rats fed a normal-calcium diet, and the 2 mgP clodronate administration further increased the BFR. In the cultured rat bone marrow cells, the area of mineralized nodules was significantly increased at 10(-7) and 10(-6) M clodronate, but high concentration of clodronate decreased the area. From these results, it is concluded that clodronate stimulates bone formation when the drug was given to a rat with a relatively lower dose that is sufficient to prevent bone resorption and that this effect may be due to the stimulatory effect on the differentiation process of osteoblasts.