Angiopoietin-1 (Ang-1) is a secreted glycoprotein that plays a critical role in the development and maintenance of the vascular system. The angiopoietin (ANGPT)-TIE2/TEK signaling pathway is essential for blood and lymphatic vascular homeostasis. ANGPT1 is a potent TIE2 activator, whereas ANGPT2 functions as a context-dependent agonist/antagonist. Angiopoietin-1 can protect against pulmonary arterial hypertension, reduce the extent of fibrosis and remodeling in infarcted diabetic myocardium, and enhance tumor progression and metastasis.
Angiopoietin-2 (Ang-2; also ANGPT2) is a secreted glycoprotein that plays a complex role in angiogenesis and inflammation. Both Ang-2 and the related Angiopoietin-1 (Ang-1) are ligands for the receptor tyrosine kinase Tie-2. While Ang-1 is a potent Tie 2 agonist, Ang-2 may act as either a Tie-2 antagonist or agonist, depending upon its state of multimerization. The higher the order of oligomer, the more effective Ang-2 becomes as a Tie-2 agonist. The short isoform appears to block the binding of either Ang-1 or full-length Ang-2 to Tie-2. Ang-2 functions as a pro-angiogenic factor, although it can also induce EC death and vessel regression. Upon its release from quiescent EC, it regulates vascular remodeling by promoting EC survival, proliferation, and migration and destabilizing the interaction between EC and perivascular cells. In addition, ANG-2 is strongly expressed in the vasculature of many tumors and it has been suggested that ANG-2 may act synergistically with other cytokines such as vascular endothelial growth factor to promote tumor-associated Angiogenesis and tumor progression.
Angiopoietin-like 3 (ANGPTL3) is a secreted glycoprotein that is structurally related to the angiopoietins. Mature human ANGPTL3 contains an N-terminal coiled coil domain and a C-terminal fibrinogen-like domain. ANGPTL3 is expressed in the liver from early in development through adulthood. Full length ANGPTL3 circulates in the plasma as do the proteolytically separated N-and C-terminal segments containing the coiled coil domain and fibrinogen-like domains,respectively. ANGPTL3 directly inhibits lipoprotein lipase (LPL) and endothelial lipase (EL), enzymes responsible for hydrolyzing circulating triglycerides and HDL phospholipids. ANGPTL3 promotes an increase in circulating triglyceride levels without altering VLDL or HDL secretion or uptake. ANGPTL3 expression in vivo is up-regulated by LXR agonists and down-regulated by insulin, leptin, and agonists of TRβ or PPARβ.
Angiopoietin-related protein 4（ANGPTL4）is a secreted protein and contains 1 fibrinogen C-terminal domain. The protein may act as a regulator of angiogenesis and modulate tumorigenesis. It inhibits proliferation, migration, and tubule formation of endothelial cells and reduces vascular leakage. ANGPTL4 may exert a protective function on endothelial cells through an endocrine action. It is directly involved in regulating glucose homeostasis, lipid metabolism, and insulin sensitivity (By similarity). In response to hypoxia, the unprocessed form of the protein accumulates in the subendothelial extracellular matrix (ECM). The matrix-associated and immobilized unprocessed form limits the formation of actin stress fibers and focal contacts in the adhering endothelial cells and inhibits their adhesion. It also decreases motility of endothelial cells and inhibits the sprouting and tube formation.
Angiotensinogen is involved in maintaining blood pressure and in the pathogenesis of essential hypertension and preeclampsia. Genetic variations in Angiotensinogen are associated with susceptibility to essential hypertension, and can cause renal tubular dysgenesis, a severe disorder of renal tubular development. Defects in the encoding gene have also been associated with non-familial structural atrial fibrillation, and inflammatory bowel disease.
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