Down-regulation of VE-PTP by siRNA, but not by anti–VE-PTP antibodies, enhances tyrosine phosphorylation of VE-cadherin–associated plakoglobin. (A) bEnd.5 cells were treated with polyclonal antibodies against VE-PTP (α-VE-PTP) or preimmune antibodies (control) for 1 h and subsequently immunoprecipitated for VE-cadherin, followed by immunoblotting with antibodies against phosphotyrosine (pTyr), VE-cadherin, or plakoglobin. Aliquots of cell lysates with identical protein content were directly immunoblotted for VE-PTP and VE-cadherin (bottom). (B) Similar as in A, except that antibody preincubation of cells was replaced by transfection with either control siRNA or VE-PTP siRNA.