Tie [tyrosine kinase with immunoglobulin (Ig) and epidermal growth factor (EGF) homology domains] subfamily members, Tie-1 and Tie-2, are among the many receptor tyrosine kinases (RTKs) expressed on endothelial cells.1 These unique RTKs have received great attention for their possible function in angiogenesis.1-10 Tie-1 is expressed on certain leukemic and endothelial cell lines as well as fresh mature and embryonic mouse and human endo thelial tissues.2-4 Both Tie-1 and Tie-2 are highly expressed and necessary for mouse embryonic development.7,8
The multiple gene family motifs that comprise the Tie RTKs (i.e. EGF-like, Ig-like, fibronectin-like, and tyrosine kinase domains) has led to the notion that Tie-1 and Tie-2 may play a role in hematopoietic cell differentiation and/or in blood/endothelial cell interactions.2, 9 Support for this hypothesis comes from studies illustrating Tie-1 expression on CD34+/CD38- hematopoietic stem cells, CD19+/CD20+ B cells as well as megakaryocytic K562, UT-7, and CMK cells.10 Some cell lines of erythrocytic lineage may also express Tie-1 and Tie-2.2,10 Tie-2 expression on hematopoietic stem cells and embryonic endothelial cells plays a critical role in the aggregation and adhesion of stem cells to endothelial cells allowing for their subsequent proliferation.9
The name angiopoietin reflects the role of this protein in angiogenesis12 and its potential role in hematopoiesis. Tie-1 and Tie-2 were both considered orphan receptors until angiopoietin-1 (Ang-1) was identified as a ligand for Tie-2.11 Ang-1 does not bind Tie-1. Targeted mutations of Ang-1 in mice are embryonically lethal with defects similar to those described for Tie-2-mutated mice.12
Utilizing mouse Ang-1 cDNA to probe both mouse uterine and human lung cDNA libraries, investigators isolated the homologous Ang-2 proteins.13 Mouse and human Ang-2 share 60% amino acid identity with their Ang-1 homologues and also bind Tie-2 with similar affinity (Kd ~3 nM). Ang-2 binding does not activate Tie-2 on endothelial cells, but serves as an effective antagonist for Ang-1 activation of Tie-2. Transgenic mice overexpressing Ang-2 die at an early embryonic stage mirroring the phenotypes of those with targeted mutations in Ang-1 or Tie-2.13
In vitro studies show that Ang-1 and Ang-2 are neither mitogenic nor anti-apoptotic for endothelial cells.14 Ang-1 is a chemotactic factor for endothelial cells and Ang-2 inhibits this response as expected for a natural antagonist.14 Tie-2-transfected fibroblasts exhibit strong chemotactic responses to both Ang-1 and Ang-214 consistent with observations that Ang-2 can activate Tie-2 receptors on non-endothelial cells.13 Ang-1 also promotes adhesion of hematopoietic stem cells to Tie-2-expressing endothelial cells and to fibronectin, thereby enhancing the proliferative response of stem cells.9 While Ang-2 antagonizes this Ang-1-mediated adhesion and proliferation, it is important for stem cell aggregation.9
PCR-based homology screenings and/or low-stringency hybridization screenings have identified mouse Ang-3, human Ang-3/4 and some angiopoietin-related proteins.15-17 Murine Ang-3 is a widely expressed antagonist capable of modulating Ang-1 activation of Tie-2.15 Human Ang-3/4 is a Tie-2 agonist highly expressed in the lung.15,16 Angiopoietin-related proteins are incapable of binding either of the Tie RTKs, presumably due to their lack of conservation of the CXCXC motif found within the fibrinogen-like domain of mouse and human angiopoietins.15,17
The exhaustive search of gene data banks using PCR-based homology screening and repeated low-stringency hybridization studies have not identified angiopoietins capable of binding Tie-1. Although Tie-1 and Tie-2 share unique structural characteristics, their extracellular domains are not very homologous; particularly the Ig-like domains and two of the three fibronectin type III-like domains.3 Suggestions for alternative ligands for Tie-1 have not been readily forthcoming. Expression of Tie-1 on cell types other than endothelial cells, particularly hematopoietic cells, may provide insights leading to the identification of ligands that function in hematopoiesis.