DEP-1/CD148 is a receptor-like protein tyrosine phosphatase with antiproliferative and tumor-suppressive functions. Interestingly, it also positively regulates Src family kinases in hematopoietic and endothelial cells, where we showed it promotes VE-cadherin-associated Src activation and endothelial cell survival upon VEGF stimulation. However, the molecular mechanism involved and its biologic functions in endothelial cells remain illdefined. We demonstrate here that DEP-1 is phosphorylated in a Src-and Fyndependent manner on Y1311 and Y1320,
IntroductionDEP-1/CD148 (also called PTP or PTPRJ) is a receptor-like protein tyrosine phosphatase (PTP) expressed in several cell types including epithelial, endothelial, and hematopoietic cells. 1 It encompasses an extracellular domain containing 8 fibronectin-type IIIlike motifs, a transmembrane domain, a single intracellular catalytic domain, and a short C-terminal tail. 2 Initial studies demonstrated that its expression increases with cell density and suggested a function in cell contact-mediated growth inhibition. 2,3 Overexpression of DEP-1 in cancer cells was also reported to inhibit their growth, while thyroid cell transformation was associated with its decreased expression, indicative of a role for DEP-1 as a tumor suppressor. 4-8 DEP-1 was further identified as the gene associated with the mouse colon cancer susceptibility locus (Scc1), and was found to be frequently deleted and mutated in human cancers. 9 These growth inhibitory functions of DEP-1 are consistent with the nature of some of its reported substrates, which include the PDGF-, HGF (Met), and VEGF (VEGFR2) receptors as well as Src family kinases (SFKs), ERK1/2, and the p85 subunit of PI3K. 10-18 DEP-1 also dephosphorylates proteins from the cell-cell junctional complexes including p120catenin, occludin, and ZO-1, which might impact biologic functions dependent on the loosening/strengthening of intercellular contacts. 11,14,19 VEGFR2 is a potent activator of the angiogenic response and is the main mediator of the mitogenic, chemotactic, permeability, and survival effects of VEGF in normal and tumor-associated vessels. 20 VE-cadherin adhesion complexes are important sites of VEGFdependent signaling in confluent cells, as activated VEGFR2 associates to these complexes to mediate Akt activation and cell survival. 21 DEP-1 was shown to colocalize to these sites and to attenuate the phosphorylation of VEGFR2, resulting in the impaired activation of ERK1/2 and the contact inhibition of endothelial cell proliferation. 13,22 However, DEP-1 was also reported to positively regulate VE-cadherin-associated Src and Akt and to promote VEGF-dependent endothelial cell survival. 15 Consistent with these distinct in vitro functions, inactivation of DEP-1 in mice via the swapping of its catalytic domain and C-terminal tail with GFP revealed both positive and negative regulatory effects in vascular development, and resulted in defective vessel remodeling and branching in addition to increased endothelial cell proliferati...