Submitted on June 27, 2006
Accepted on August 16, 2006
Unique uptake and efflux systems of inorganic phosphate in osteoclast-like cells
Mikiko Ito1, Sakiko Haito2, Mari Furumoto3, Yoko Uehata4, Aya Sakurai5, Hiroko Segawa6, Sawako Tatsumi5, MASASHI KUWAHATA7, and Ken-ichi Miyamoto8*
1 Inst. of Health Biosciences, Dept. of Molecular Nutrition, The university of Tokushima Graduate School, Tokushima, Japan
2 Molecular nutrituion, Inst of Health Biosciences, Tokushima Graduate School, Tokushima, Tokushima, Japan
3 Molecular Nutrition, Inst of Health Biosciences, Tokushima Graduate school, Tokushima, Tokushima, Japan
4 Molecular nutrition, Inst of Health Biosciences,Tokushima Graduate School, Tokushima, Tokushima, Japan
5 Molecular Nutrition, Inst of Health Biosciences, Tokushima Graduate School, Tokushima, Tokushima, Japan
6 Department of Molecular Nutrition, Institution of Health Biosciences, The University of Tokushima Graduate School, Tokushima, Tokushima, Japan
7 Nutritional Science, Tokushima University, Tokushima, Japan
8 Nutrional Science, Department of Nutrition, School of Medicine, Tokushima University, Tokushima, Japan
* To whom correspondence should be addressed. E-mail: miyamoto{at}nutr.med.tokushima-u.ac.jp.
During bone resorption, a large amount of inorganic phosphate (Pi) is generated within the osteoclast hemivacuole. The mechanisms involved in the disposal of this Pi are not clear. In the present study, we investigated the efflux of Pi from osteoclast-like cells. Pi efflux was activated by acidic conditions in osteoclast-like cells derived by the treatment of RAW264.7 cells with receptor activator of nuclear factor kappa B. Acid-induced Pi influx was not observed in renal proximal tubule-like OK cells, osteoblast-like MC3T3-E1 cells, or untreated RAW264.7 cells. Furthermore, Pi efflux was stimulated by extracellular Pi and several Pi analogues (phosphonoformic acid [PFA], phosphonoacetic acid [PAA], arsenate, and pyrophosphate). Pi efflux was time-dependent, with 50% released into the medium after 10 min. The efflux of Pi was increased by various inhibitors that block Pi uptake, and extracellular Pi did not affect the transport of [14C]-PFA into the osteoblast-like cells. Preloading of cells with Pi did not stimulate Pi efflux by PFA, indicating that the effect of Pi was not due trans-stimulation of Pi transport. Pi uptake was also enhanced under acidic conditions. Agents that prevent increases in cytosolic free Ca2+ concentration, including acetoxymethyl ester of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM), 2-aminoethyl diphenylborinate, and bongkrekic acid significantly inhibited Pi uptake in the osteoclast-like cells, suggesting that Pi uptake is regulated by calcium signaling in the endoplasmic reticulum and mitochondria of osteoclast-like cells. These results suggest that osteoclast-like cells have a unique Pi uptake/efflux system and can prevent Pi accumulation within osteoclast hemivacuoles.
