Angiogenin and Fragments

Definition
Angiogenin (ANG), a 14,124 Da protein that has been implicated in angiogenesis in tumor progression. It is secreted by tumor cells, a potent inducer of neovascularization1.

Discovery
In 1885, Fett et al., first isolated and characterized angiogenin from medium conditioned by HT-29 human colon adenocarcinoma cells1 and later from normal mammalian plasma2 and milk. It has been subsequently isolated from human, bovine, rabbit, pig & mouse sera and bovine milk. Angiogenin has ribonucleolytic activity with 33% sequence homology to pancreatic RNAse A3. A synthetic peptide ’H-Glu-Asn-Gly-Leu-Pro-Val-His-Leu-Asp-Gln-Ser-Ile-Phe-Arg-Arg-OH (108-122) corresponding to the C-terminal region of ANG inhibits the enzymatic and biological activities of angiogenin4. Several C-terminal synthetic peptides, including (Ang 108-123), significantly decreases angiogenin-induced neovascularization.

Structural characteristics
Acharya et al., determined the crystal structure of human antigenic at 2.4 0A. Overall Structure features a kidney shaped tertiary fold reminiscent of RNase A. The ribonucleolytic active center (His-13, His-114, and Lys-40) and the putative receptor binding site, both of which are critically involved in biological functions are distinct from Rnase A5. The central core of the molecule consists of P structure with a pair of antiparallel twisted forming the main topology with residues Ser-72 and Gly-99 at the apices. Two additional strands on either side of these central strands (residues 41-47; 111-116) complete the major sheet structure. There are 3 helices H1, residues 3-14, H2, residues 22-33 and H3, residues 49-58 present in the structure.

Mechanism of action
Four aspects of ANG have been discovered that is necessary for the process of ANG-induced angiogenesis,ANG exerts its ribonucleolytic activity-ANG has a very weak 105-106 lower ribonucleolytic activity than that of RNase A. This is because the pyrimidine binding site of ANG is “obstructed” by the glutamine (Gln)117 residue. However, ribonucleolytic activity of ANG is crucial for angiogenesis.ANG stimulates basement membrane degradation-ANG binds to a-actin on endothelial cell surface; ANG-actin complexes dissociate from the cell surface and accelerate tissue type plasminogen activator (tPA)-catalyzed generation of plasmin from plasminogen. ANG-actin complexes promote the degradation of basement membrane and extracellular matrix. This complex allows endothelial cells to penetrate and migrate into the perivascular tissue. Basement membrane degradation is an essential feature of angiogenesis. ANG activates signaling transduction- ANG binds to a 170-kDa receptor located on the endothelial cell surface and elicits second messenger systems. Binding of ANG to cell surface actin results in activation of a cell-associated protease system that promotes cell invasion. ERK1/2, protein kinase B/Akt1 pathways have been proposed to be activated by ANG stimulation. ANG nuclear translocation-Angiogenin undergoes nuclear translocation in endothelial cells via receptor-mediated endocytosis and nuclear localization sequence-assisted nuclear import. A nuclear localization signal (NLS), lies in 31-RRRGL-35 of the protein. Upon nuclear translocation it enhances rRNA transcription.

Functions
Functions of ANG in Angiogenesis-As a key angiogenic factor, ANG interacts with endothelial and smooth muscle cells to induce a wide range of cellular responses including cell migration, invasion, proliferation, and formation of tubular structures. ANG has also been reported to induce the proliferation of cancer cells directly. Recently, ANG gene was identified to be a potential amyotrophic lateral sclerosis (ALS) related gene6. ANG induces tumor growth in various types of human cancers, including breast, cervical, colon, colorectal, endometrial, gastric, liver, kidney, ovarian, pancreatic, prostate, and urothelial cancers, as well as astrocytoma, leukemia, lymphoma, melanoma, osteosarcoma, and Wilms’ tumor 6. ANG may be related with amyotrophic lateral sclerosis- Amyotrophic lateral sclerosis (ALS) is a progressive late onset neurodegenerative disorder affecting upper and lower motoneurons (MNs). Vascular endothelial growth factor was the first angiogenic factor shown to contribute to the pathogenesis of ALS 7.

References
1. Fett JW, Strydom DJ, Lobb RR, Alderman EM, Bethune JL, Riordan JF, Vallee BL (1985). Isolation and characterization of angiogenin, an antigenic protein from human carcinoma cells. Biochemistry, 24: 5480-5486.
2. Shapiro R, Strydom DJ, Olson KA, Vallee BL (1987). Isolation of angiogenin from normal human plasma. Biochemistry, 26: 5141-5146.
3. Strydom DJ, Fett JW, Lobb RR, Alderman EM, Bethune JL, Riordan JF, Vallee BL (1985). Amino acid sequence of human tumor derived angiogenin. Biochemistry, 24(20): 5486–5494.
4. Rybak SM, Auld DS, St Clair DK, Yao QZ, Fett JW (1989). C-terminal angiogenin peptides inhibit the biological and enzymatic activities of angiogenin. Biochem. Biophys. Res. Commun, 162: 535–543.
5. Acharya KR, Shapiro R, Allen SC, Riordan JF, Vallee BL (1994). Crystal structure of human angiogenin reveals the structural basis for its functional divergence from ribonuclease. Proc Natl Acad Sci USA, 91: 2915-2919.
6. Yoshioka N, Wang L, Kishimoto K, Tsuji T, Hu GF (2006). A therapeutic target for prostate cancer based on angiogenin-stimulated angiogenesis and cancer cell proliferation. Proc Natl Acad Sci USA, 103: 14519-14524.
7. Oosthuyse B, Moons L, Storkebaum E, Beck H, Nuyens D, Brusselmans K, Van Dorpe J, Hellings P, Gorselink M, Heymans S, Theilmeier G, Dewerchin M, Laudenbach V, Vermylen P, Raat H, Acker T, Vleminckx V, Van Den Bosch L, Cashman N, Fujisawa H, Drost MR, Sciot R, Bruyninckx F, Hicklin DJ, Ince C, Gressens P, Lupu F, Plate KH, Robberecht W, Herbert JM, Collen D, Carmeliet P(2001). Deletion of the hypoxia-response element in the vascular endothelial growth factor promoter causes motor neuron degeneration. Nat Gene, 28:131-138.