Characterization of TEK receptor tyrosine kinase and its ligands, Angiopoietins, in human hematopoietic progenitor cells

TEK, or TIE-2, is a receptor tyrosine kinase (RTK) that is known as a functioning molecule of vascular endothelial cells. TEK comprises a subfamily of RTK with TIE, and these two receptors play critical roles in vascular maturation, maintenance of integrity and remodeling. We generated mAb against the extracellular domain of human TEK protein to elucidate its expression pattern in human hematopoietic cells. Flow cytometric analysis of bone marrow cells revealed that TEK was expressed in 27% of CD34⁺ cells, 20% of c-KIT⁺ cells and 26% of CD34⁺CD38^- cells, indicating that TEK is expressed in a subset of primitive hematopoietic stem cells (HSC). TEK was also expressed in 20% of CD19⁺ B lymphocytes but not in other lineage-committed cells. Progenitor assays in methylcellulose culture showed that CD34⁺TEK⁺ cells formed significantly less BFU-E and CFU-Mix than CD34⁺TEK^- cells, but there was no difference in the number of CFU-GM between these two populations. Two recently identified TEK ligands, termed Angiopoietin-1 and -2, bound to TEK with similar affinities, and Angiopoietin-1 effectively induced TEK phosphorylation in hematopoietic cells. Angiopoietin-2 also induced a low level of TEK phosphorylation and weakened the phosphorylation induced by Angiopoietin-1, suggestive of an elaborate regulator of the TEK-TEK ligand signaling pathway. Although neither ligands affected the proliferation of TEK-transfected hematopoietic cells or the colony formation of CD34⁺TEK⁺ bone marrow cells, both promoted the adhesion of TEK-transfected hematopoietic cells to a collagen matrix or a layer of bone marrow stromal cells. These findings indicate that the TEK-TEK ligand signaling pathway is regulated in a refined manner and is involved in hematopoietic cell-microenvironment interaction.