CDB15:0000055 ANGPT1 — TEK

ABSTRACT: TIE2 is a receptor-like tyrosine kinase expressed almost exclusively in endothelial cells and early hemopoietic cells and required for the normal development of vascular structures during embryogenesis. We report the identification of a secreted ligand for TIE2, termed Angiopoietin-1, using a novel expression cloning technique that involves intracellular trapping and detection of the ligand in COS cells. The structure of Angiopoietin-1 differs from that of known angiogenic factors or other ligands for receptor tyrosine kinases. Although Angiopoietin-1 binds and induces the tyrosine phosphorylation of TIE2, it does not directly promote the growth of cultured endothelial cells. However, its expression in close proximity with developing blood vessels implicates Angiopoietin-1 in endothelial developmental processes.

ABSTRACT: Vascular endothelial growth factor (VEGF), which acts via members of a family of endothelial-specific receptor tyrosine kinases, is the only factor that has been shown definitively to play a role in the formation of the embryonic vasculature. Only one other family of receptor tyrosine kinases, comprising TIE1 and TIE2, is largely endothelial cell specific. We have recently cloned a ligand for TIE2, termed Angiopoietin-1. Here we show that mice engineered to lack Angiopoietin-1 display angiogenic deficits reminiscent of those previously seen in mice lacking TIE2, demonstrating that Angiopoietin-1 is a primary physiologic ligand for TIE2 and that it has critical in vivo angiogenic actions that are distinct from VEGF and that are not reflected in the classic in vitro assays used to characterize VEGF. Angiopoietin-1 seems to play a crucial role in mediating reciprocal interactions between the endothelium and surrounding matrix and mesenchyme.

ABSTRACT: 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.