PROTEIN-TYROSINE-PHOSPHATASE ACTIVITY REGULATES OSTEOCLAST FORMATION AND FUNCTION - INHIBITION BY ALENDRONATE

Alendronate (ALN), an aminobisphosphonate used in the treatment of ost eoporosis, is a potent inhibitor of bone resorption, Its molecular tar get is still unknown. This study examines the effects of ALN on the ac tivity of osteoclast protein-tyrosine phosphatase (PTP; protein-tyrosi ne phosphate phosphohydrolase, EC 3.1.3.48), called PTP epsilon, Using osteoclast-like cells generated by coculturing mouse bone marrow cell s with mouse calvaria osteoblasts, we found by molecular cloning and R NA blot hybridization that PTP epsilon Is highly expressed in osteocla stic cells, A purified fusion protein of PTP epsilon expressed in bact eria was inhibited by ALN with an IC50 of 2 mu M Other PTP inhibitors- orthovanadate and phenylarsine oxide (PAO)-inhibited PTP epsilon with IC50 values of 0.3 mu M and 18 mu M, respectively, ALN and another bis phosphonate, etidronate, also inhibited the activities of other bacter ially expressed PTPs such as PTP sigma and CD45 (also called leukocyte common antigen), The PTP inhibitors ALN, orthovanadate, and PAO suppr essed irt vitro formation of multinucleated osteoclasts from osteoclas t precursors and in vitro bone resorption by isolated rat osteoclasts (pit formation) with estimated IC50 values of 10 mu M, 3 mu M, and 0.0 5 mu M, respectively, These findings suggest that tyrosine phosphatase activity plays an Important role in osteoclast formation and function and is a putative molecular target of bisphosphonate action.