An Osteoclastic Transmembrane Protein‐Tyrosine Phosphatase Enhances Osteoclast Activity in Part by Dephosphorylating EphA4 in Osteoclasts

We have previously shown that PTP‐oc is an enhancer of the functional activity of osteoclasts and that EphA4 is a suppressor. Here, we provide evidence that PTP‐oc enhances osteoclast activity in part through inactivation of EphA4 by dephosphorylating key phosphotyrosine (pY) residues of EphA4. We show that EphA4 was pulled down by the PTP‐oc trapping mutant but not by the wild‐type (WT) PTP‐oc and that transgenic overexpression of PTP‐oc in osteoclasts drastically decreased pY602 and pY779 residues of EphA4. Consistent with the previous findings that EphA4 deficiency increased pY173‐Vav3 level (Rac‐GTP exchange factor [GEF]) and enhanced bone resorption activity of osteoclasts, reintroduction of WT‐ Epha4 in Epha4 null osteoclasts led to ∼50% reduction in the pY173‐Vav3 level and ∼2‐fold increase in bone resorption activity. Overexpression of Y779F‐ Epha4 mutant in WT osteoclasts markedly increased in pY173‐Vav3 and reduced bone resorption activity, but overexpression of Y602F‐ Epha4 mutant had no effect, suggesting that pY779 residue plays an important role in the EphA4‐mediated suppression of osteoclast activity. Deficient EphA4 in osteoclasts has been shown to up‐regulate Rac‐GTPase and down‐regulate Rho‐GTPase. PTP‐oc overexpression in osteoclasts also increased the GTP‐Rac level to 300% of controls, but decreased the GTP‐Rho level to ∼50% of controls. PTP‐oc overexpression or deficient Epha4 each also reduced pY87‐Ephexin level, which is a Rho GEF. Thus, PTP‐oc may differentially regulate Rac signaling versus Rho signaling through dephosphorylation of EphA4, which has shown to have opposing effects on Rac‐GTPase versus Rho‐GTPase through differential regulation of Vav3 versus Ephexin. J. Cell. Biochem. 116: 1785–1796, 2015. © 2015 Wiley Periodicals, Inc.