Expression of Vascular Endothelial-Cadherin on Leukemic Cells Mediates Their Interaction with Vascular Endothelium.

It has already been established that subsets of leukemic cells express receptors for pro-angiogenic factors, such as vascular endothelial growth factor receptor-2 (VEGFR-2). Further studies have shown that these same leukemic cells also produce the ligand for these VEGF receptors, VEGF-A. This autocrine loop supports the invasion and proliferation of these particular leukemic cells. The VEGFR-2 signaling pathway is further dependent upon the co-activation of other pro-angiogenic factors, such as vascular endothelial (VE)-cadherin. VE-cadherin is an endothelial cell-specific transmembrane cellular adhesion protein that when bound results in the dephosphorylation of VEGFR-2 and contributes to neo-vessel formation. Recent studies have suggested that VE-cadherin may be expressed by a unique subset of hematopoietic cells, raising the possibility that leukemic cells may express VE-cadherin as well. We therefore sought to identify the expression of VE-cadherin on leukemic cell lines and primary samples, and further determine its role in the interaction with VE-cadherin-positive endothelial cells. Leukemic cell lines and primary leukemias were screened for the presence of VE-cadherin by both RT-PCR and Western blot analysis. Primary leukemic samples, as well as established cell lines for human erythroblastic leukemia (HEL) and acute myelogenous leukemia (KG-1a) were found to express VE-cadherin. VE-cadherin expression was further confirmed by flow cytometry and immunocytochemistry, which demonstrated that approximately 15% of the total population was VE-cadherin-positive. Proliferation and migration assays utilizing neutralizing monoclonal antibodies to VE-cadherin were performed with no obvious effects. However, immunofluorescent staining of a co-culture performed with the above leukemic cells grown on a layer of human umbilical vein endothelial cells (HUVECs) demonstrated that the leukemic cells and HUVECs interact via VE-cadherin. Furthermore, when these leukemic cells were injected into NOD-SCID mice subcutaneously, the VE-cadherin-positive cells localized around the vessels present within the leukemic chloroma. These data set forth the concept that a subset of leukemic cells expresses the protein VE-cadherin and that VE-cadherin is involved in the cell-to-cell interaction between a subset of leukemic cells and vascular endothelial cells. Heterotypic VE-cadherin interaction between leukemic cells and neo-vessels may increase the survival of leukemic cells and contribute to the generation of minimal residual disease. Therefore, inhibition of VEGFR-2 in conjunction with VE-cadherin may provide a novel strategy to eradicate minimal residual disease.