TIE-2 (Angiopoietin-1 receptor, Tyrosine-protein kinase receptor TEK)

The tyrosine kinase receptor TIE-2 was initially identified as a specific endothelial growth factor receptor that mediated several properties of endothelial cells under both physiologcal and pathological conditions. Angiopoietins are the natural ligands of TIE-2 and they induce TIE-2- depending signalling, including survival and apoptosis of endothelial cells, vascular permeability and regulate capillary sprouting. In general, the outcome of tie receptor signalling depends on which vascular bed is involved, and crosstalk between different VEGFs has an important modulating effect on the properties of the ligands. TIE-2 plays an important role in several vascular diseases, known as vascular malformation (e.g. venous malformation). Independent of angiogenesis and its involvement in lymphangiogenesis, TIE-2 maintains a long-term, quiescent population of hematopoietc stems cells in the bone marrow. In a tumor model, a subset of monocytes was found to be positve for TIE-2 and that they have an important function in paracrine support of nascent blood vessels. During cancer formation and spreading, TIE-2 was found to be overexpressed in tumor vessels. However, outsite the vascular compartment TIE-2 is expressed in several types of cancer including leukemia and in gastric tumors, breast tumors and gliomas. In gliomas, TIE-2 expression in the neoplastic glial cells was significantly associated with progression from a lower to a higher grade where it seems to regulate glioma cell adhesion to extracellular matrix. Together with the reported fact that malignant gliomas epress high levels of Ang1, suggest the existence of an autocrine loop for cell matrix interaction. The possible role of TIE-2 in tumors is more currently under investigation. Because it is expressed in several cellular lineages insite the tumor, the receptor may be an attractive target for cancer therapy. A natural occuring soluble TIE-2 receptor fragment of 75 kDa (sTIE-2) is generated by shedding. Soluble TIE-2 inhibits angiopoietin-mediated TIE-2 phosphorylation and anti-apoptosis. TIE-2 shedding is mediated by PI2K/Akt and p38 MAPK. The serum levels of sTIE-2 in patients is in the lower ng/ml range and dependent of the disease. Soluble TIE-2 was also measured in preterm infants including the ROP syndrome (retinopathy of prematurity). Elevated plasma levels in active ROP patients were observed for sTIE-2. In experimental tumor mouse models sTIE-2 can be used for tumor regression and prolonged the tumor-free survival in 80% of the animals.