PECAM-1 (CD31) is an immunoreceptor tyrosine-based inhibitory motif 
(ITIM)-containing surface glycoprotein expressed on various hematopoietic cells 
as well as on endothelial cells. PECAM-1 has been shown to play roles in 
regulation of adhesion, migration and apoptosis. The BCR/ABL fusion tyrosine 
kinase is expressed in chronic myeloid leukemia and Philadelphia-positive (Ph+) 
acute lymphoblastic leukemia cells, and its inhibition by the clinically used 
tyrosine kinase inhibitors imatinib or dasatinib induces apoptosis of these 
cells. In the present study, we demonstrate that PECAM-1 is tyrosine 
phospho-rylated in its ITIM motifs in various BCR/ABL-expressing cells including 
primary leukemia cells. Studies using imatinib and dasatinib as well as 
transient expression experiments in 293T cells revealed that PECAM-1 was 
phosphorylated directly by BCR/ABL, which was enhanced by the imatinib-resistant 
E255K and T315I mutations, or partly by the Src family tyrosine kinases, 
including Lyn, which were activated dependently or independently on BCR/ABL. We 
also demonstrate by using a substrate trapping mutant of SHP2 that tyrosine 
phosphorylated PECAM-1 binds SHP2 and is a major substrate for this tyrosine 
phosphatase in BCR/ABL-expressing cells. Overexpression of PECAM-1 in 
BCR/ABL-expressing cells, including K562 human leukemia cells, enhanced cell 
adhesion and partially inhibited imatinib-induced apoptosis involving 
mitochondria depolarization and caspase-3 cleavage, at least partly, in an 
ITIM-independent manner. These data suggest that PECAM-1 may play a role in 
regulation of apoptosis as well as adhesion of BCR/ABL-expressing cells to 
modulate their imatinib sensitivity and would be a possible candidate for 
therapeutic target in Ph+ leukemias.