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 su bfamily of RTK with TIE, and these two receptors play critical roles i n vascular maturation, maintenance of integrity and remodeling. We gen erated mAb against the extracellular domain of human TEK protein to el ucidate its expression pattern in human hematopoietic cells. Flow cyto metric analysis of bone marrow cells revealed that TEK was expressed i n 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 hemat opoietic stem cells (HSC), TEK was also expressed in 20% of CD19(+) B lymphocytes but not in other lineage-committed cells, progenitor assay s in methylcellulose culture showed that CD34(+)TEK(+) cells formed si gnificantly less BFU-E and CFU-Mix than CD34(+)TEK(-) cells, but there was no difference in the number of CFU-GM between these two populatio ns. 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 als o induced a low level of TEK phosphorylation and weakened the phosphor ylation induced by Angiopoietin-1, suggestive of an elaborate regulato r of the TEK-TEK ligand signaling pathway. Although neither ligands af fected the proliferation of TEK-transfected hematopoietic cells or the colony formation of CD34(+)TEK(+) bone marrow cells, both promoted th e 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.