Conformational characterization of human angiogenin by limited proteolysis

Crystal structures of murine angiogenin-2 and -3-probing 'structure--function' relationships amongst angiogenin homologues

Angiogenin (Ang) is a potent inducer of neovascularization. Point mutations in human Ang have been linked to cancer progression and two neurodegenerative diseases: amyotrophic lateral sclerosis and Parkinson's disease. Intensive structural and functional analyses of Ang have been paramount in assigning functions to this novel homologue of bovine pancreatic RNase A. However, inhibitor-binding studies with crystalline Ang (for designing potential anti-cancer drugs) have been hampered as a result of the inaccessibility of the active site. Experiments with the murine homologues of Ang have not only overcome the obvious practical limitations encountered when studying the role of a human protein in healthy individuals, but also the crystal structures of murine angiogenins (mAng and mAng-4) have revealed themselves to have greater potential for the visualization of small-molecule inhibitor binding at the active site. In the present study, we report the crystal structures of two more murine Ang paralogues, mAng-2 and mAng-3, at 1.6 and 1.8 Å resolution, respectively. These constitute the first crystal structures of an Ang with a zinc ion bound at the active site and provide some insight into the possible mode of inhibition of the ribonucleolytic activity of the enzyme by these divalent cations. Both structures show that the residues forming the putative P₁, B₁ and B₂ subsites occupy positions similar to their counterparts in human Ang and are likely to have conserved roles. However, a less obtrusive conformation of the C-terminal segment in mAng-3 and the presence of a sulfate ion in the B₁ subsite of mAng-2 suggest that these proteins have the potential to be used for inhibitor-binding studies. We also discuss the biological relevance of the structural similarities and differences between the different Ang homologues.

Internalization and processing of human angiogenin by cultured aortic smooth muscle cells

Human angiogenin is a 14-kDa plasma protein with angiogenic and ribonucleolytic activities. Angiogenin binds specifically to aortic smooth muscle cells, activates second messenger pathways, and inhibits their proliferation. Human and bovine aortic smooth muscle cells were used to study the internalization and intracellular fate of human angiogenin at 37 degrees C. Using a specific antibody against angiogenin, we found that the internalized native protein was localized in the perinuclear region at 30 min and then dispersed throughout the cytoplasm. In conditions favoring receptor-mediated endocytosis, internalization of iodinated angiogenin showed a first peak at 5 min and then further increased for up to 24 h. The half-life of the molecule, calculated as 12 h in chase experiments, could contribute to its intracellular accumulation. In cell extracts, in addition to the 14-kDa protein, a 8.7-kDa fragment was observed at 24 h, and three fragments with molecular mass of 10.5, 8.7, and 6. 1 kDa were detected at 48 h. Our data point to a specific internalization and processing of human angiogenin by aortic smooth muscle cells.

Limited proteolysis of angiogenin by elastase is regulated by plasminogen

Human neutrophil elastase cleaves angiogenin at the Ile-29/Met-30 peptide bond to produce two major disulfide-linked fragments with apparent molecular weights of 10,000 and 4000, respectively. Elastase-cleaved angiogenin has slightly increased ribonucleolytic activity, but has lost its ability to undergo nuclear translocation in endothelial cells, a process essential for angiogenic activity. Cleavage appears to alter the cell-binding properties of angiogenin, despite the fact that it occurs some distance from the putative receptor-binding site, since the elastase-cleaved protein fails to compete with its native counterpart for nuclear translocation in endothelial cells. Plasminogen specifically accelerates elastase proteolysis of angiogenin. It does not enhance elastase activity toward ribonuclease A or the synthetic peptide substrate MeOSuc-Ala-Ala-Pro-Val-pNA. Plasminogen-accelerated inactivation of angiogenin by elastase might be a significant event in the process of angiogenin-induced angiogenesis since (i) angiogenin and plasminogen circulate in plasma at high concentrations, (ii) angiogenin, especially when bound to actin, activates tissue plasminogen activator to generate plasmin from plasminogen, and (iii) elastase cleaves plasminogen to produce angiostatin, a potent inhibitor of angiogenesis and metastasis. Interrelationships among angiogenin, plasminogen, plasminogen activators, elastase, and angiostatin may provide a sensitive regulatory system to balance angiogenesis and antiangiogenesis.

Secretion of ribonucleases by normal and immortalized cells grown in serum-free culture conditions

The requirement of serum in cell culture is a major limitation for studies on secreted ribonucleases (RNases) because serum contains a high amount of ribonucleolytic activity. Defined culture condition is thus of interest to improve our knowledge of the RNase biology. We report here that cells from three different types and origins, Chinese hamster lung fibroblasts, bovine smooth muscle cells, and human endothelium-derived EA.hy926 cells, proliferate consistently in the presence of a basal medium supplemented with bovine serum albumin, high-density lipoproteins, basic fibroblast growth factor, insulin, and transferrin. Using a new quantitative radio-RNase inhibitor assay, two distinct ribonucleolytic assays, and a radioimmunoassay against angiogenin, it is shown that RNases became apparent in media conditioned by cell monolayers. Both the hamster lung fibroblast and the EA.hy926 cell lines secreted larger amounts of RNase inhibitor-interacting factors and RNase activity than normal smooth muscle cells. The serum-free medium represents an alternative way to grow these cells and allows investigation of biosynthesis and functions of RNases in culture. It should be useful to identify and quantitate unambiguously specific members of the RNase family secreted by normal versus tumor cells in culture.

Interaction of heparin with human angiogenin

HT-29 human colon adenocarcinoma cells adhere rapidly to human angiogenin (Ang) via interactions with cell-surface heparan sulfate moieties (Soncin, F., Shapiro, R., and Fett, J. W. (1994) J. Biol. Chem. 269, 8999-9005). Soluble heparin inhibits adhesion, and Ang itself binds tightly to heparin-Sepharose. In the present study, the interaction of Ang with heparin has been further characterized. The basic cluster Arg-31/Arg-32/Arg-33 has been identified as an important component of the heparin binding site. Mutations of these residues, and of Arg-70 as well, decrease both the affinity of Ang for heparin-Sepharose and the capacity of Ang to support cell adhesion. Replacements of four other basic residues do not affect heparin binding. Heparin partially protects Ang from cleavage by trypsin at Lys-60, suggesting that heparin also binds to the region of Ang that contains this residue. The map here determined indicates that the heparin recognition site on Ang lies outside the catalytic center; indeed, heparin has no significant effect on the ribonucleolytic activity of Ang. It also does not influence the angiogenic activity of this protein. Light scattering measurements on Ang-heparin mixtures suggest that 1 heparin chain (mass of 16.5 kDa) can accommodate approximately 9 Ang molecules. The minimum size required for a heparin fragment to effectively inhibit HT-29 cell adhesion to Ang was determined to be 6 disaccharide units. The implications of these findings for inhibition of Ang-mediated tumor establishment in vivo are discussed.

The widespread expression of angiogenin in different human cells suggests a biological function not only related to angiogenesis

Angiogenin is a secreted polypeptide that induces neovascularization in vivo. The expression of angiogenin by human cells in culture was investigated by using a specific radioimmunoassay and by cDNA hybridization. Angiogenin immunoreactivity was widely but differentially produced by anchorage-dependent growing cells including vascular endothelial cells from saphenous and umbilical veins, aortic smooth muscle cells, fibroblasts (from embryos, new-borns and adults), and tumour cells. Endothelial cells from saphenous veins and the endothelium-derived EA.hy926 cell line released immunoreactivity whatever the stage of the culture, including release at the lag phase, during exponential growth and at the confluent phase. However, the rate of accumulation of angiogenin varied as a function of EA.hy926 cell density. As compared to anchored cells, normal peripheral blood cells and tumour cells of myelomonocytic and megakaryocytic origin did not noticeably secrete angiogenin except at low levels. A myeloma cell line supernatant contained as much angiogenin cross-reactivity as did anchored cells, while four tumour T-cell lines expressed the cross-reactivity at different levels, i.e. from undetectable levels to a high level. A 0.9-kb angiogenin messenger RNA was detected by Northern-blot analyses in a variety of representative cells correlating with the presence of immunoreactivity in the cell-culture media. The widespread expression pattern of angiogenin suggests a physiological function that is not restricted to the neovascularization process.

A monoclonal antibody to human angiogenin. Inhibition of ribonucleolytic and angiogenic activities and localization of the antigenic epitope

A monoclonal antibody (mAb) to human angiogenin, a protein that induces formation of new blood vessels, was produced by somatic cell fusion techniques and designated as 26-2F. It is an IgGl kappa whose binding affinity, expressed as an IC50, is (1.6 +/- 0.1) x 10(-9) M as determined by a competition radioimmunoassay. mAb 26-2F neutralizes the ribonucleolytic activity of angiogenin as assessed by in vitro protein synthesis and tRNA degradation assays. It also effectively inhibits neovascularization induced by angiogenin on the chick chorioallantoic membrane. Epitope mapping indicates that the binding region of angiogenin recognized by mAb 26-2F is discontinuous and involves both Trp-89 and residues in the segment 38-41. This epitope is formed by two surface loops which are juxtaposed in the three-dimensional structure of human angiogenin recently determined by X-ray crystallography. Thus mAb 26-2F, along with similar antibodies under investigation, will facilitate structure/function studies of angiogenin, help define its physiological role, and lead to an understanding of the consequences of its inhibition in pathological situations in which angiogenin may be involved.

Dual site model for the organogenic activity of angiogenin

The residues that are indispensable for the ribonucleolytic activity of angiogenin are also known to be essential for its angiogenic activity. We now demonstrate that residues in another region of the protein, devoid of catalytic residues, are additionally required for angiogenesis. Endoproteinase Lys-C or a baby hamster kidney cell protease cleaves angiogenin at the peptide bond either between Lys-60 and Asn-61 or between Glu-67 and Asn-68, respectively. The two polypeptide fragments resulting from either cleavage remain linked by disulfide bonds. These two derivatives and des-(Asn61-Glu67)-angiogenin--in which both bonds are cleaved--retain their ribonucleolytic activities toward tRNA, 18S and 28S rRNA, and dinucleoside phosphates but are no longer angiogenic on the chicken embryo chorioallantoic membrane. Further, their capacity to elicit a second messenger response in endothelial cells is greatly decreased. Moreover, none of these three derivatives inhibit angiogenin-induced angiogenesis. This contrasts with two active site mutants of angiogenin. These results identify the residues from 60 to 68 as a region of angiogenin that is part of a cell-surface receptor binding site [see accompanying manuscript: Hu, G.-F., Chang, S.-I., Riordan, J.F. & Vallee, B.L. (1991) Proc. Natl. Acad. Sci. USA 88, 2227-2231] and serve as the basis for a dual site model of the organogenic activity of angiogenin.

A covalent angiogenin/ribonuclease hybrid with a fourth disulfide bond generated by regional mutagenesis

Human angiogenin is a blood vessel inducing protein whose primary structure displays 33% identity to that of bovine pancreatic ribonuclease A (RNase A). Angiogenin catalyzes limited cleavage of 18S and 28S ribosomal RNA and is several orders of magnitude less potent than RNase A toward conventional substrates. A striking structural difference between angiogenin and RNase is the virtual absence of sequence similarity within the region of RNase that contains the Cys-65--Cys-72 disulfide bond. Indeed, angiogenin lacks this disulfide linkage. The present report describes the use of regional mutagenesis to generate a covalent angiogenin/RNase hybrid protein, ARH-I, where residues 58-70 of angiogenin have been replaced by the corresponding segment of RNase A (residues 59-73). The protein expressed in Escherichia coli readily folds at pH 8.5 to form the four expected disulfide bonds. The in vivo angiogenic potency of ARH-I is markedly diminished compared with that of angiogenin when examined using the chick chorioallantoic membrane assay. In contrast, its enzymatic activity is dramatically increased. With high molecular weight wheat germ RNA and tRNA, ARH-I is 660- and 300-fold more active than angiogenin, respectively, while with poly(uridylic acid), poly(cytidylic acid), cytidylyl(3'----5')adenosine (CpA), and uridylyl(3'----5')adenosine (UpA) activity is enhanced by about 200-fold. In addition, the specificity of ARH-I toward dinucleoside 3',5'-phosphates is qualitatively similar to RNase A; while angiogenin prefers cytidylyl(3'----5')guanosine (CpG) to UpA, both RNase and the hybrid prefer UpA to CpG. ARH-I also displays greater than 10-fold enhanced activity toward rRNA in intact ribosomes, while abolishing the capacity of the ribosome to support cell-free protein synthesis. The enhanced enzymatic properties of ARH-I parallel a 2-fold increase in chemical reactivity of active-site lysine and histidine residues based on rates of chemical modification. The data indicate that introduction of a region of RNase A containing the Cys-65--Cys-72 disulfide bond into angiogenin dramatically increases RNase-like enzymatic activity while reducing its angiogenicity.

Role of lysines in human angiogenin: chemical modification and site-directed mutagenesis

The role of lysines in the ribonucleolytic and angiogenic activities of human angiogenin has been examined by chemical modification and site-directed mutagenesis. It was demonstrated previously [Shapiro, R., Weremowicz, S., Riordan, J.F., & Vallee, B.L. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8783-8787] that extensive treatment with lysine reagents markedly decreases the ribonucleolytic activity of angiogenin. In the present study, limited chemical modification with 1-fluoro-2,4-dinitrobenzene followed by C18 high-performance liquid chromatography yielded several (dinitrophenyl)angiogenin derivaties. The major derivative formed had slightly increased enzymatic activity compared with the unmodified protein. Tryptic peptide mapping demonstrated the site of modification to be Lys-50. A second derivative, modified at Lys-60, was 34% active. Analysis of a third derivative indicated that modification of Lys-82 did not decrease activity. Thus, Lys-50 and Lys-82 are unessential for enzymatic activity while Lys-60 may play a minor role. No pure derivative modified at Lys-40, corresponding to the active-site residue Lys-41 of the homologous protein ribonuclease A, could be obtained by chemical procedures. Therefore, we employed oligonucleotide-directed mutagenesis to replace this lysine with glutamine or arginine. The Gln-40 derivative had less than 0.05% enzymatic activity compared with the unmodified protein and substantially reduced angiogenic activity when examined with the chick embryo chorioallantoic membrane assay. These results suggest that the angiogenic activity of the protein is dependent on an intact enzymatic active site. The Arg-40 derivative had 2.2% ribonucleolytic activity compared with unmodified angiogenin. The effects of reductive methylation of this derivative indicate that no lysines other than Lys-40 are critical.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of Met-(-1) angiogenin in Escherichia coli: conversion to the authentic less than Glu-1 protein

A method for obtaining authentic human angiogenin utilizing an Escherichia coli recombinant expression system is described. A synthetic gene encoding angiogenin was placed into a vector for direct expression under the control of a modified E. coli trp promoter. The protein was produced by the bacteria in an insoluble form and purified to homogeneity by cation-exchange and reversed-phase HPLC following reduction/solubilization and reoxidation. The protein isolated was identified as Met-(-1) angiogenin by amino acid analysis and tryptic peptide mapping; the latter demonstrated that all three disulfide bonds had formed correctly. Both the enzymatic and angiogenic activities of the Met-(-1) protein were equivalent to those of native angiogenin. A Met-(-1) Leu-30 derivative of angiogenin was also isolated and found to be fully active. Conversion of Met-(-1) angiogenin to the authentic less than Glu-1 protein was achieved by treatment with Aeromonas aminopeptidase under conditions in which the new N-terminal glutamine readily cyclizes nonenzymatically. This aminopeptidase treatment may have more general applicability for removal of undesirable N-terminal methionine residues from foreign proteins expressed in bacteria.