Polyphosphate-Mediated Inhibition of Tartrate-Resistant Acid Phosphatase and Suppression of Bone Resorption of Osteoclasts

Inorganic polyphosphate (poly(P)) has recently been found to play an important role in bone formation. In this study, we found that tartrate-resistant acid phosphatase (TRAP), which is abundantly expressed in osteoclasts, has polyphosphatase activity that degrades poly(P) and yields Pi as well as shorter poly(P) chains. Since the TRAP protein that coprecipitated with anti-TRAP monoclonal antibodies exhibited both polyphosphatase and the original phosphatase activity, poly(P) degradation activity is dependent on TRAP and not on other contaminating enzymes. The ferrous chelator α, α’-bipyridyl, which inhibits the TRAP-mediated production of reactive oxygen species (ROS), had no effect on such poly(P) degradation, suggesting that the degradation is not dependent on ROS. In addition, shorter chain length poly(P) molecules were better substrates than longer chains for TRAP, and poly(P) inhibited the phosphatase activity of TRAP depending on its chain length. The IC50 of poly(P) against the original phosphatase activity of TRAP was 9.8 µM with an average chain length more than 300 phosphate residues, whereas the IC50 of poly(P) with a shorter average chain length of 15 phosphate residues was 8.3 mM. Finally, the pit formation activity of cultured rat osteoclasts differentiated by RANKL and M-CSF were markedly inhibited by poly(P), while no obvious decrease in cell number or differentiation efficiency was observed for poly(P). In particular, the inhibition of pit formation by long chain poly(P) with 300 phosphate residues was stronger than that of shorter chain poly(P). Thus, poly(P) may play an important regulatory role in osteoclastic bone resorption by inhibiting TRAP activity, which is dependent on its chain length.