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

Thermodynamics of the Association of Aminoglycoside Antibiotics with Human Angiogenin

BACKGROUND

The body needs to maintain a firm balance between the inducers and inhibitors of angiogenesis, the process of proliferation of blood vessels from pre-existing ones. Human angiogenin (hAng), being a potent inducer of angiogenesis, is a cause of tumor cell proliferation, therefore its inhibition becomes a vital area of research. Aminoglycosides are linked ring systems consisting of amino sugars and an aminocyclitol ring and are in use in clinical practices for a long time. These compounds have found clinical uses as antibacterial agents that inhibit bacterial protein synthesis.

OBJECTIVE

Gentamycin C1, Kanamycin A, Neomycin B, Paromomycin I, and Streptomycin A are commonly used aminoglycoside antibiotics that have been used for the present study. Among these, Neomycin has reported inhibitory activity against angiogenin-induced angiogenesis on the chicken chorioallantoic membrane. This study focuses on the thermodynamic parameters involved in the interactions of these antibiotics with hAng.

METHODS

Agarose gel-based assay, Fluorescence quenching studies and Docking studies.

RESULTS

Anti-ribonucleolytic effect of the antibiotics was observed qualitatively using an agarose gelbased assay, which shows that Neomycin exhibits the most efficient inhibition of hAng. Fluorescence quenching studies at different temperatures, using Stern-Volmer and van't Hoff equations provide information about the thermodynamics of binding, which furthermore highlights the higher binding constant of Neomycin. Docking studies showed that the antibiotics preferably interact with the nuclear translocation site, except Streptomycin, which shows affinity towards the ribonucleolytic site of the protein with very less affinity value.

CONCLUSION

The study has shown the highly spontaneous formation of Neomycin-hAng complex, giving an exothermic reaction with increase in the degree of freedom of the protein-ligand complex.

Dimerization of Human Angiogenin and of Variants Involved in Neurodegenerative Diseases

Human Angiogenin (hANG, or ANG, 14.1 kDa) promotes vessel formation and is also called RNase 5 because it is included in the pancreatic-type ribonuclease (pt-RNase) super-family. Although low, its ribonucleolytic activity is crucial for angiogenesis in tumor tissues but also in the physiological development of the Central Nervous System (CNS) neuronal progenitors. Nevertheless, some ANG variants are involved in both neurodegenerative Parkinson disease (PD) and Amyotrophic Lateral Sclerosis (ALS). Notably, some pt-RNases acquire new biological functions upon oligomerization. Considering neurodegenerative diseases correlation with massive protein aggregation, we analyzed the aggregation propensity of ANG and of three of its pathogenic variants, namely H13A, S28N, and R121C. We found no massive aggregation, but wt-ANG, as well as S28N and R121C variants, can form an enzymatically active dimer, which is called ANG-D. By contrast, the enzymatically inactive H13A-ANG does not dimerize. Corroborated by a specific cross-linking analysis and by the behavior of H13A-ANG that in turn lacks one of the two His active site residues necessary for pt-RNases to self-associate through the three-dimensional domain swapping (3D-DS), we demonstrate that ANG actually dimerizes through 3D-DS. Then, we deduce by size exclusion chromatography (SEC) and modeling that ANG-D forms through the swapping of ANG N-termini. In light of these novelties, we can expect future investigations to unveil other ANG determinants possibly related with the onset and/or development of neurodegenerative pathologies.

Targeted human cytolytic fusion proteins at the cutting edge: harnessing the apoptosis-inducing properties of human enzymes for the selective elimination of tumor cells

Patient-specific targeted therapy represents the holy grail of anti-cancer therapeutics, allowing potent tumor depletion without detrimental off-target toxicities. Disease-specific monoclonal antibodies have been employed to bind to oncogenic cell-surface receptors, representing the earliest form of immunotherapy. Targeted drug delivery was first achieved by means of antibody-drug conjugates, which exploit the differential expression of tumor-associated antigens as a guiding mechanism for the specific delivery of chemically-conjugated chemotherapeutic agents to diseased target cells. Biotechnological advances have expanded the repertoire of immunology-based tumor-targeting strategies, also paving the way for the next intuitive step in targeted drug delivery: the construction of recombinant protein drugs consisting of an antibody-based targeting domain genetically fused with a cytotoxic peptide, known as an immunotoxin. However, the most potent protein toxins have typically been derived from bacterial or plant virulence factors and commonly feature both off-target toxicity and immunogenicity in human patients. Further refinement of immunotoxin technology thus led to the replacement of monoclonal antibodies with humanized antibody derivatives, including the substitution of non-human toxic peptides with human cytolytic proteins. Preclinically tested human cytolytic fusion proteins (hCFPs) have proven promising as non-immunogenic combinatory anti-cancer agents, however they still require further enhancement to achieve convincing candidacy as a single-mode therapeutic. To date, a portfolio of highly potent human toxins has been established; ranging from microtubule-associated protein tau (MAP tau), RNases, granzyme B (GrB) and death-associated protein kinase (DAPk). In this review, we discuss the most recent findings on the use of these apoptosis-inducing hCFPs for the treatment of various cancers.

Allicin improves the function of cardiac microvascular endothelial cells by increasing PECAM-1 in rats with cardiac hypertrophy

OBJECTIVE

Cardiac microvascular damage is significantly associated with the development of cardiac hypertrophy (CH). Researchers found that allicin could inhibit CH, but the relationship between cardiac microvessel and the inhibition of allicin on CH has not been reported. We aimed to investigate the effect of allicin on the function of cardiac microvascular endothelial cells (CMECs) in CH rat.

MATERIALS AND METHODS

The hemodynamic parameters were measured by BL-420F biological function experimental system and the indicators of the ventricular structure and function were measured by echocardiographic system. MTT assay was performed to assess the cell viability. Nitrite detection was performed to detect nitric oxide content. The morphology and molecular characteristics were detected by electron micrographs, immunofluorescence, quantitative real-time polymerase chain reaction (qRT-PCR), western blot. Wound healing experiment, analysis of tube formation and shear adaptation were performed to assess CMECs migration ability, angiogenesis and shear-responsiveness respectively.

RESULT

Our findings have identified that microvascular density was decreased by observing the expression of platelet endothelial cell adhesion molecule-1 (PECAM-1) in CH rats. Interestingly, allicin improved the distribution and expression of PECAM-1. Meanwhile, allicin enhanced the migration and angiogenesis ability of CMECs, activated PECAM-1-PI3K-AKT-eNOS signaling pathway, however, the role of allicin was disappear after PECAM-1 was silenced. Allicin decreased the expression of caspase-3 and receptor interacting protein 3 (RIP3), inhibited necroptosis, and increased the levels of Angiopoietin-2 (Ang-2) and platelet-derived growth factor receptor-β (PDGFR-β). Under 10 dyn/cm² condition, allicin advanced the modification ability of CMECs's shear-adaptation by activating PECAM-1.

CONCLUSION

Allicin provided cardioprotection for CH rats by improving the function of CMECs through increasing the expression of PECAM-1.

Evidence for Novel Action at the Cell-Binding Site of Human Angiogenin Revealed by Heteronuclear NMR Spectroscopy, in silico and in vivo Studies

A member of the ribonuclease A superfamily, human angiogenin (hAng) is a potent angiogenic factor. Heteronuclear NMR spectroscopy combined with induced-fit docking revealed a dual binding mode for the most antiangiogenic compound of a series of ribofuranosyl pyrimidine nucleosides that strongly inhibit hAng's angiogenic activity in vivo. While modeling suggests the potential for simultaneous binding of the inhibitors at the active and cell-binding sites, NMR studies indicate greater affinity for the cell-binding site than for the active site. Additionally, molecular dynamics simulations at 100 ns confirmed the stability of binding at the cell-binding site with the predicted protein-ligand interactions, in excellent agreement with the NMR data. This is the first time that a nucleoside inhibitor is reported to completely inhibit the angiogenic activity of hAng in vivo by exerting dual inhibitory activity on hAng, blocking both the entrance of hAng into the cell and its ribonucleolytic activity.

Proteomic Analysis of Human Angiogenin Interactions Reveals Cytoplasmic PCNA as a Putative Binding Partner

Human Angiogenin (hAng) is a member of the ribonuclease A superfamily and a potent inducer of neovascularization. Protein interactions of hAng in the nucleus and cytoplasm of the human umbilical vein cell line EA.hy926 have been investigated by mass spectroscopy. Data are available via ProteomeXchange with identifiers PXD006583 and PXD006584. The first gel-free analysis of hAng immunoprecipitates revealed many statistically significant potential hAng-interacting proteins involved in crucial biological pathways. Surprisingly, proliferating cell nuclear antigen (PCNA), was found to be immunoprecipitated with hAng only in the cytoplasm. The hAng-PCNA interaction and colocalization in the specific cellular compartment was validated with immunoprecipitation, immunoblotting, and immunocytochemistry. The results revealed that PCNA is predominantly localized in the cytoplasm, while hAng is distributed both in the nucleus and in the cytoplasm. hAng and PCNA colocalize in the cytoplasm, suggesting that they may interact in this compartment.

A Boronic Acid Conjugate of Angiogenin that Shows ROS-Responsive Neuroprotective Activity

Angiogenin (ANG) is a human ribonuclease that is compromised in patients with amyotrophic lateral sclerosis (ALS). ANG also promotes neovascularization, and can induce hemorrhage and encourage tumor growth. The causal neurodegeneration of ALS is associated with reactive oxygen species, which are also known to elicit the oxidative cleavage of carbon-boron bonds. We have developed a synthetic boronic acid mask that restrains the ribonucleolytic activity of ANG. The masked ANG does not stimulate endothelial cell proliferation but protects astrocytes from oxidative stress. By differentiating between the two dichotomous biological activities of ANG, this strategy could provide a viable pharmacological approach for the treatment of ALS.

Structural insights into human angiogenin variants implicated in Parkinson's disease and Amyotrophic Lateral Sclerosis

Mutations in Angiogenin (ANG), a member of the Ribonuclease A superfamily (also known as RNase 5) are known to be associated with Amyotrophic Lateral Sclerosis (ALS, motor neurone disease) (sporadic and familial) and Parkinson’s Disease (PD). In our previous studies we have shown that ANG is expressed in neurons during neuro-ectodermal differentiation, and that it has both neurotrophic and neuroprotective functions. In addition, in an extensive study on selective ANG-ALS variants we correlated the structural changes to the effects on neuronal survival and the ability to induce stress granules in neuronal cell lines. Furthermore, we have established that ANG-ALS variants which affect the structure of the catalytic site and either decrease or increase the RNase activity affect neuronal survival. Neuronal cell lines expressing the ANG-ALS variants also lack the ability to form stress granules. Here, we report a detailed experimental structural study on eleven new ANG-PD/ALS variants which will have implications in understanding the molecular basis underlying their role in PD and ALS.

NMR study of Met-1 human Angiogenin: (1)H, (13)C, (15)N backbone and side-chain resonance assignment

Here, we report the high yield expression and preliminary structural analysis via solution hetero-nuclear NMR spectroscopy of the recombinant Met-1 human Angiogenin. The analysis reveals a well folded as well as, a monomeric polypeptide. Τhe sequence-specific assignment of its (1)H, (15)N and (13)C resonances at high percentage was obtained. Also, using TALOS+ its secondary structure elements were determined.

The ammonium sulfate inhibition of human angiogenin

In this study, we investigate the inhibition of human angiogenin by ammonium sulfate. The inhibitory potency of ammonium sulfate for human angiogenin (IC50 = 123.5 ± 14.9 mm) is comparable to that previously reported for RNase A (119.0 ± 6.5 mm) and RNase 2 (95.7 ± 9.3 mm). However, analysis of two X-ray crystal structures of human angiogenin in complex with sulfate anions (in acidic and basic pH environments, respectively) indicates an entirely distinct mechanism of inhibition. While ammonium sulfate inhibits the ribonucleolytic activity of RNase A and RNase 2 by binding to the active site of these enzymes, sulfate anions bind only to peripheral substrate anion-binding subsites of human angiogenin, and not to the active site.

Transition State Charge Stabilization and Acid-Base Catalysis of mRNA Cleavage by the Endoribonuclease RelE

The bacterial toxin RelE is a ribosome-dependent endoribonuclease. It is part of a type II toxin-antitoxin system that contributes to antibiotic resistance and biofilm formation. During amino acid starvation, RelE cleaves mRNA in the ribosomal A-site, globally inhibiting protein translation. RelE is structurally similar to microbial RNases that employ general acid-base catalysis to facilitate RNA cleavage. The RelE active site is atypical for acid-base catalysis, in that it is enriched with positively charged residues and lacks the prototypical histidine-glutamate catalytic pair, making the mechanism of mRNA cleavage unclear. In this study, we use a single-turnover kinetic analysis to measure the effect of pH and phosphorothioate substitution on the rate constant for cleavage of mRNA by wild-type RelE and seven active-site mutants. Mutation and thio effects indicate a major role for stabilization of increased negative change in the transition state by arginine 61. The wild-type RelE cleavage rate constant is pH-independent, but the reaction catalyzed by many of the mutants is strongly dependent on pH, suggestive of general acid-base catalysis. pH-rate curves indicate that wild-type RelE operates with the pK(a) of at least one catalytic residue significantly downshifted by the local environment. Mutation of any single active-site residue is sufficient to disrupt this microenvironment and revert the shifted pK(a) back above neutrality. pH-rate curves are consistent with K54 functioning as a general base and R81 as a general acid. The capacity of RelE to effect a large pK(a) shift and facilitate a common catalytic mechanism by uncommon means furthers our understanding of other atypical enzymatic active sites.

Amyloidogenic variant of apolipoprotein A-I elicits cellular stress by attenuating the protective activity of angiogenin

Amyloidogenic ‘gain-of-function' mutations in apolipoprotein A-I (ApoA-I) gene (APOA1) result in systemic amyloidosis characterized by aggregate deposition and eventually cell death. However, how amyloidogenic variants of ApoA-I induce cell death is unknown. Here we report that one of the mechanisms by which amyloidogenic ApoA-I induces cell death is through attenuating anti-stress activity of angiogenin (ANG), a homeostatic protein having both pro-growth and pro-survival functions. Under growth conditions, ANG is located in nucleolus where it promotes ribosomal RNA (rRNA) transcription thereby stimulating cell growth. In adverse conditions, ANG is relocated to cytoplasm to promote damage repairs and cell survival. We find that in cells overexpressing the L75P-APOA1 mutant ANG expression is decreased and normal cellular localization of ANG is altered in response to stress and growth signals. In particular, ANG does not relocate to cytoplasm under stress conditions but is rather retained in the nucleolus where it continues promoting rRNA transcription, thus imposing a ribotoxic effect while simultaneously compromising its pro-survival activity. Consistently, we also find that addition of exogenous ANG protects cells from L75P-ApoA-I-induced apoptosis.

Bacterial toxin RelE: a highly efficient ribonuclease with exquisite substrate specificity using atypical catalytic residues

The toxin RelE is a ribosome-dependent endoribonuclease implicated in diverse cellular processes, including persistence. During amino acid starvation, RelE inhibits translation by cleaving ribosomal A-site mRNA. Although RelE is structurally similar to other microbial endoribonucleases, the active-site amino acid composition differs substantially and lacks obvious candidates for general acid-base functionality. Highly conserved RelE residues (Lys52, Lys54, Arg61, Arg81, and Tyr87) surround the mRNA scissile phosphate, and specific 16S rRNA contacts further contribute to substrate positioning. We used a single-turnover kinetic assay to evaluate the catalytic importance of individual residues in the RelE active site. Within the context of the ribosome, RelE rapidly cleaves A-site mRNA at a rate similar to those of traditional ribonucleases. Single-turnover rate constants decreased between 10(2)- and 10(6)-fold for the RelE active-site mutants of Lys52, Lys54, Arg61, and Arg81. RelE may principally promote catalysis via transition-state charge stabilization and leaving-group protonation, in addition to achieving in-line substrate positioning in cooperation with the ribosome. This kinetic analysis complements structural information to provide a foundation for understanding the molecular mechanism of this atypical endoribonuclease.

Kaposi's sarcoma-associated herpesvirus-positive primary effusion lymphoma tumor formation in NOD/SCID mice is inhibited by neomycin and neamine blocking angiogenin's nuclear translocation

Angiogenin (ANG) is a 14-kDa multifunctional proangiogenic secreted protein whose expression level correlates with the aggressiveness of several tumors. We observed increased ANG expression and secretion in endothelial cells during de novo infection with Kaposi's sarcoma-associated herpesvirus (KSHV), in cells expressing only latency-associated nuclear antigen 1 (LANA-1) protein, and in KSHV latently infected primary effusion lymphoma (PEL) BCBL-1 and BC-3 cells. Inhibition of phospholipase Cγ (PLCγ) mediated ANG's nuclear translocation by neomycin, an aminoglycoside antibiotic (not G418-neomicin), resulted in reduced KSHV latent gene expression, increased lytic gene expression, and increased cell death of KSHV(+) PEL and endothelial cells. ANG detection in significant levels in KS and PEL lesions highlights its importance in KSHV pathogenesis. To assess the in vivo antitumor activity of neomycin and neamine (a nontoxic derivative of neomycin), BCBL-1 cells were injected intraperitoneally into NOD/SCID mice. We observed significant extended survival of mice treated with neomycin or neamine. Markers of lymphoma establishment, such as increases in animal body weight, spleen size, tumor cell spleen infiltration, and ascites volume, were observed in nontreated animals and were significantly diminished by neomycin or neamine treatments. A significant decrease in LANA-1 expression, an increase in lytic gene expression, and an increase in cleaved caspase-3 were also observed in neomycin- or neamine-treated animal ascitic cells. These studies demonstrated that ANG played an essential role in KSHV latency maintenance and BCBL-1 cell survival in vivo, and targeting ANG function by neomycin/neamine to induce the apoptosis of cells latently infected with KSHV is an attractive therapeutic strategy against KSHV-associated malignancies.

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.

Ribonuclease 4 protects neuron degeneration by promoting angiogenesis, neurogenesis, and neuronal survival under stress

Altered RNA processing is an underlying mechanism of amyotrophic lateral sclerosis (ALS). Missense mutations in a number of genes involved in RNA function and metabolisms are associated with ALS. Among these genes is angiogenin (ANG), the fifth member of the vertebrate-specific, secreted ribonuclease superfamily. ANG is an angiogenic ribonuclease, and both its angiogenic and ribonucleolytic activities are important for motor neuron health. Ribonuclease 4 (RNASE4), the fourth member of this superfamily, shares the same promoters with ANG and is co-expressed with ANG. However, the biological role of RNASE4 is unknown. To determine whether RNASE4 is involved in ALS pathogenesis, we sequenced the coding region of RNASE4 in ALS and control subjects and characterized the angiogenic, neurogenic, and neuroprotective activities of RNASE4 protein. We identified an allelic association of SNP rs3748338 with ALS and demonstrated that RNASE4 protein is able to induce angiogenesis in in vitro, ex vivo, and in vivo assays. RNASE4 also induces neural differentiation of P19 mouse embryonal carcinoma cells and mouse embryonic stem cells. Moreover, RNASE4 not only stimulates the formation of neurofilaments from mouse embryonic cortical neurons, but also protects hypothermia-induced degeneration. Importantly, systemic treatment with RNASE4 protein slowed weight loss and enhanced neuromuscular function of SOD1 (G93A) mice.

Motoneurons secrete angiogenin to induce RNA cleavage in astroglia

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disorder affecting motoneurons. Mutations in angiogenin, encoding a member of the pancreatic RNase A superfamily, segregate with ALS. We previously demonstrated that angiogenin administration shows promise as a neuroprotective therapeutic in studies using transgenic ALS mice and primary motoneuron cultures. Its mechanism of action and target cells in the spinal cord, however, are largely unknown. Using mixed motoneuron cultures, motoneuron-like NSC34 cells, and primary astroglia cultures as model systems, we here demonstrate that angiogenin is a neuronally secreted factor that is endocytosed by astroglia and mediates neuroprotection in paracrine. We show that wild-type angiogenin acts unidirectionally to induce RNA cleavage in astroglia, while the ALS-associated K40I mutant is also secreted and endocytosed, but fails to induce RNA cleavage. Angiogenin uptake into astroglia requires heparan sulfate proteoglycans, and engages clathrin-mediated endocytosis. We show that this uptake mechanism exists for mouse and human angiogenin, and delivers a functional RNase output. Moreover, we identify syndecan 4 as the angiogenin receptor mediating the selective uptake of angiogenin into astroglia. Our data provide new insights into the paracrine activities of angiogenin in the nervous system, and further highlight the critical role of non-neuronal cells in the pathogenesis of ALS.

Mechanisms of loss of functions of human angiogenin variants implicated in amyotrophic lateral sclerosis

Background

Mutations in the coding region of angiogenin (ANG) gene have been found in patients suffering from Amyotrophic Lateral Sclerosis (ALS). Neurodegeneration results from the loss of angiogenic ability of ANG (protein coded by ANG). In this work, we performed extensive molecular dynamics (MD) simulations of wild-type ANG and disease associated ANG variants to elucidate the mechanism behind the loss of ribonucleolytic activity and nuclear translocation activity, functions needed for angiogenesis.

Methodology/Principal Findings

MD simulations were carried out to study the structural and dynamic differences in the catalytic site and nuclear localization signal residues between WT-ANG (Wild-type ANG) and six mutants. Variants K17I, S28N, P112L and V113I have confirmed association with ALS, while T195C and A238G single nucleotide polymorphisms (SNPs) encoding L35P and K60E mutants respectively, have not been associated with ALS. Our results show that loss of ribonucleolytic activity in K17I is caused by conformational switching of the catalytic residue His114 by 99°. The loss of nuclear translocation activity of S28N and P112L is caused by changes in the folding of the residues ³¹RRR³³ that result in the reduction in solvent accessible surface area (SASA). Consequently, we predict that V113I will exhibit loss of angiogenic properties by loss of nuclear translocation activity and L35P by loss of both ribonucleolytic activity and nuclear translocation activity. No functional loss was inferred for K60E. The MD simulation results were supported by hydrogen bond interaction analyses and molecular docking studies.

Conclusions/Significance

Conformational switching of catalytic residue His114 seems to be the mechanism causing loss of ribonucleolytic activity and reduction in SASA of nuclear localization signal residues ³¹RRR³³ results in loss of nuclear translocation activity in ANG mutants. Therefore, we predict that L35P mutant, would exhibit loss of angiogenic functions, and hence would correlate with ALS while K60E would not show any loss.

New insights into the role of angiogenin in actin polymerization

Angiogenin is a potent stimulator of angiogenesis. It interacts with endothelial cells and induces a wide range of cellular responses initiating a process of blood vessel formation. One important target of angiogenin is endothelial cell-surface actin, and their interaction might be one of crucial steps in angiogenin-induced neovascularization. Recently, it was shown that angiogenin inhibits polymerization of G-actin and changes the physical properties of F-actin. These observations suggest that angiogenin may cause changes in the cell cytoskeleton. This chapter reviews the current state of the literature regarding angiogenin structure and function and discusses the relationship between the angiogenin and actin and possible functional roles of their interaction.

Vascular disruption and the role of angiogenic proteins after spinal cord injury

Spinal cord injuries (SCI) can result in devastating paralysis, for which there is currently no robustly efficacious neuroprotective/neuroregenerative treatment. When the spinal cord is subjected to a traumatic injury, the local vasculature is disrupted and the blood–spinal cord barrier is compromised. Subsequent inflammation and ischemia may then contribute to further secondary damage, exacerbating neurological deficits. Therefore, understanding the vascular response to SCI and the molecular elements that regulate angiogenesis has considerable relevance from a therapeutic standpoint. In this paper, we review the nature of vascular damage after traumatic SCI and what is known about the role that angiogenic proteins—angiopoietin 1 (Ang1), angiopoietin 2 (Ang2) and angiogenin—may play in the subsequent response. To this, we add recent work that we have conducted in measuring these proteins in the cerebrospinal fluid (CSF) and serum after acute SCI in human patients. Intrathecal catheters were installed in 15 acute SCI patients within 48 h of injury. CSF and serum samples were collected over the following 3–5 days and analysed for Ang1, Ang2 and angiogenin protein levels using a standard ELISA technique. This represents the first description of the endogenous expression of these proteins in an acute human SCI setting.

Mutational dynamics of murine angiogenin duplicates

Background

Angiogenin (Ang) is a protein involved in angiogenesis by inducing the formation of blood vessels. The biomedical importance of this protein has come from findings linking mutations in Ang to cancer progression and neurodegenerative diseases. These findings highlight the evolutionary constrain on Ang amino acid sequence. However, previous studies comparing human Angiogenin with homologs from other phylogenetically related organisms have led to the conclusion that Ang presents a striking variability. Whether this variability has an adaptive value per se remains elusive. Understanding why many functional Ang paralogs have been preserved in mouse and rat and identifying functional divergence mutations at these copies may explain the relationship between mutations and function. In spite of the importance of testing this hypothesis from the evolutionarily and biomedical perspectives, this remains yet unaccomplished. Here we test the main mutational dynamics driving the evolution and function of Ang paralogs in mammals.

Results

We analysed the phylogenetic asymmetries between the different Ang gene copies in mouse and rat in the context of vertebrate Ang phylogeny. This analysis shows strong evidence in support of accelerated evolution in some Ang murine copies (mAng). This acceleration is not due to non-functionalisation because constraints on amino acid replacements remain strong. We identify many of the amino acid sites involved in signal localization and nucleotide binding by Ang to have evolved under diversifying selection. Compensatory effects of many of the mutations at these paralogs and their key structural location in or nearby important functional regions support a possible functional shift (functional divergence) in many Ang copies. Similarities between 3D-structural models for mAng copies suggest that their divergence is mainly functional.

Conclusions

We identify the main evolutionary dynamics shaping the variability of Angiogenin in vertebrates and highlight the plasticity of this protein after gene duplication. Our results suggest functional divergence among mAng paralogs. This puts forward mAng as a good system candidate for testing functional plasticity of such an important protein while stresses caution when using mouse as a model to infer the consequences of mutations in the single Ang copy of humans.

Novel endoribonucleases as central players in various pathways of eukaryotic RNA metabolism

For a long time it has been assumed that the decay of RNA in eukaryotes is mainly carried out by exoribonucleases, which is in contrast to bacteria, where endoribonucleases are well documented to initiate RNA degradation. In recent years, several as yet unknown endonucleases have been described, which has changed our view on eukaryotic RNA metabolism. Most importantly, it was shown that the primary eukaryotic 3' --> 5' exonuclease, the exosome complex has the ability to endonucleolytically cleave its physiological RNA substrates, and novel endonucleases involved in both nuclear and cytoplasmic RNA surveillance pathways were discovered concurrently. In addition, endoribonucleases responsible for long-known processing steps in the maturation pathways of various RNA classes were recently identified. Moreover, one of the most intensely studied RNA decay pathways--RNAi--is controlled and stimulated by the action of different endonucleases. Furthermore, endoribonucleolytic cleavages executed by various enzymes are also the hallmark of RNA degradation and processing in plant chloroplasts. Finally, multiple context-specific endoribonucleases control qualitative and/or quantitative changes of selected transcripts under particular conditions in different eukaryotic organisms. The aim of this review is to discuss the impact of all of these discoveries on our current understanding of eukaryotic RNA metabolism.

Human angiogenin presents neuroprotective and migration effects in neuroblastoma cells

Human angiogenin (ANG) has been highlighted as an angiogenic factor which supports primary and metastatic tumor growth. Recent genetic studies have shown that ANG is presented as a susceptibility gene for amyotrophic lateral sclerosis (ALS) and ALS-frontotemporal dementia (ALS-FTD). They found several missense mutations, including K40I, which present the weakest functional activity in ANG variants. In this study, we investigate whether human wild type ANG (wANG) and its variant K40I (mANG) maintain their divergent functional capacities in neuronal cells. To evaluate this, SH-SY5Y neuroblastoma cells were transfected with wANG and mANG DNA and identified both wild and mutant ANG are localized to nuclei and have no effects on proliferation. We have shown that human wANG prevented cell death under H(2)O(2)-induced oxidative stress in both SH-SY5Y and NSC-34 cells, tested by MTT assay. These effects were more enhanced in motor neuron cell NSC-34. wANG also played a role in cell migration, while mANG decreased these functional activities. Immunoblot analysis revealed that the intracellular signaling of ERK1/2 (at Thr183/Tyr185) was increased following transfection of the wANG gene, and significantly decreased by mANG in neuronal cells. These findings suggest that human ANG plays a critical role in cell protection and migration following alterations in ERK1/2 signaling in SH-SY5Y cells. This may provide the possible relationship between mutations in hANG and other neurodegenerative diseases as well as ALS.

Binding of human angiogenin inhibits actin polymerization

Angiogenin is a potent inducer of angiogenesis, a process of blood vessel formation. It interacts with endothelial and other cells and elicits a wide range of cellular responses including migration, proliferation, and tube formation. One important target of angiogenin is endothelial cell-surface actin and their interaction might be one of essential steps in angiogenin-induced neovascularization. Based on earlier indications that angiogenin promotes actin polymerization, we studied the binding interactions between angiogenin and actin in a wide range of conditions. We showed that at subphysiological KCl concentrations, angiogenin does not promote, but instead inhibits polymerization by sequestering G-actin. At low KCl concentrations angiogenin induces formation of unstructured aggregates, which, as shown by NMR, may be caused by angiogenin's propensity to form oligomers. Binding of angiogenin to preformed F-actin does not cause depolymerization of actin filaments though it causes their stiffening. Binding of tropomyosin and angiogenin to F-actin is not competitive at concentrations sufficient for saturation of actin filaments. These observations suggest that angiogenin may cause changes in the cell cytoskeleton by inhibiting polymerization of G-actin and changing the physical properties of F-actin.

Angiogenin protects motoneurons against hypoxic injury

Cells can adapt to hypoxia through the activation of hypoxia-inducible factor-1 (HIF-1), which in turn regulates the expression of hypoxia-responsive genes. Defects in hypoxic signaling have been suggested to underlie the degeneration of motoneurons in amyotrophic lateral sclerosis (ALS). We have recently identified mutations in the hypoxia-responsive gene, angiogenin (ANG), in ALS patients, and have shown that ANG is constitutively expressed in motoneurons. Here, we show that HIF-1alpha is sufficient and required to activate ANG in cultured motoneurons exposed to hypoxia, although ANG expression does not change in a transgenic ALS mouse model or in sporadic ALS patients. Administration of recombinant ANG or expression of wild-type ANG protected motoneurons against hypoxic injury, whereas gene silencing of ang1 significantly increased hypoxia-induced cell death. The previously reported ALS-associated ANG mutations (Q12L, K17I, R31K, C39W, K40I, I46V) all showed a reduced neuroprotective activity against hypoxic injury. Our data show that ANG plays an important role in endogenous protective pathways of motoneurons exposed to hypoxia, and suggest that loss of function rather than loss of expression of ANG is associated with ALS.

Ribonuclease inhibitor regulates neovascularization by human angiogenin

Human angiogenin (ANG) is a homologue of bovine pancreatic ribonuclease (RNase A) that induces neovascularization. ANG is the only human angiogenic factor that possesses ribonucleolytic activity. To stimulate blood vessel growth, ANG must be transported to the nucleus and must retain its catalytic activity. Like other mammalian homologues of RNase A, ANG forms a femtomolar complex with the cytosolic ribonuclease inhibitor protein (RI). To determine whether RI affects ANG-induced angiogenesis, we created G85R/G86R ANG, which possesses 10(6)-fold lower affinity for RI but retains wild-type ribonucleolytic activity. The neovascularization of rabbit corneas by G85R/G86R ANG was more pronounced and more rapid than by wild-type ANG. These findings provide the first direct evidence that RI serves to regulate the biological activity of ANG in vivo.

Control of motoneuron survival by angiogenin

Mutations in the hypoxia-inducible factor angiogenin (ANG) have been identified in Amyotrophic Lateral Sclerosis (ALS) patients, but the potential role of ANG in ALS pathogenesis was undetermined. Here we show that angiogenin promotes motoneuron survival both in vitro and in vivo. Angiogenin protected cultured motoneurons against excitotoxic injury in a PI-3-kinase/Akt kinase-dependent manner, whereas knock-down of angiogenin potentiated excitotoxic motoneuron death. Expression of wild-type ANG protected against endoplasmic reticulum (ER) stress-induced and trophic-factor-withdrawal-induced cell death in vitro, whereas the ALS-associated ANG mutant K40I exerted no protective activity and failed to activate Akt-1. In SOD1(G93A) mice angiogenin delivery increased lifespan and motoneuron survival, restored the disease-associated decrease in Akt-1 survival signaling, and reversed a pathophysiological increase in ICAM-1 expression. Our data demonstrate that angiogenin is a key factor in the control of motoneuron survival.

Angiogenin-stimulated rRNA transcription is essential for initiation and survival of AKT-induced prostate intraepithelial neoplasia

Angiogenin (ANG), originally identified as an angiogenic ribonuclease, has recently been shown to play a direct role in prostate cancer cell proliferation by mediating rRNA transcription. ANG is up-regulated in human prostate cancer and is the most significantly up-regulated gene in AKT-driven prostate intraepithelial neoplasia (PIN) in mice. Enhanced cell proliferation in the PIN lesions requires increased ribosome biogenesis, a multistep process involving an orchestrated production of ribosomal proteins and rRNA. AKT is known to enhance ribosomal protein production through the mammalian target of rapamycin pathway. However, it was unknown how rRNA is proportionally increased. Here, we report that ANG is essential for AKT-driven PIN formation and survival. We showed that up-regulation of ANG in the AKT-overexpressing mouse prostates is an early and lasting event. It occurs before PIN initiation and lasts beyond PIN is fully developed. Knocking down ANG expression by intraprostate injection of lentivirus-mediated ANG-specific small interfering RNA prevents AKT-induced PIN formation without affecting AKT expression and its signaling through the mammalian target of rapamycin pathway. Neomycin, an aminoglycoside that blocks nuclear translocation of ANG, and N65828, a small-molecule enzymatic inhibitor of the ribonucleolytic activity of ANG, both prevent AKT-induced PIN formation and reverse established PIN. They also decrease nucleolar organizer region, restore cell size, and normalize luminal architectures of the prostate despite continuous activation of AKT. All three types of the ANG inhibitor suppress rRNA transcription of the prostate luminal epithelial cells and inhibit AKT-induced PIN, indicating an essential role of ANG in AKT-mediated cell proliferation and survival.

Targeting angiogenin in therapy of amyotropic lateral sclerosis

BACKGROUND

Missense heterozygous mutations in the coding region of angiogenin (ANG) gene, encoding a 14 kDa angiogenic RNase, were recently found in patients of amyotropic lateral sclerosis (ALS). Functional analyses have shown that these are loss-of-function mutations, implying that angiogenin deficiency is associated with ALS pathogenesis and that increasing ANG expression or angiogenin activity could be a novel approach for ALS therapy.

OBJECTIVE

Review the evidence showing the involvement of angiogenin in motor neuron physiology and function, and provide a rationale for targeting angiogenin in ALS therapy.

METHODS

Review the current understanding of the mechanism of angiogenin action in connection with ALS genetics, pathogenesis and therapy.

CONCLUSION

ANG is the first gene whose loss-of-function mutations are associated with ALS pathogenesis. Therapeutic modulation of angiogenin level and activity in the spinal cord, either by systemic delivery of angiogenin protein or through retrograde transport of ANG-encoding viral particles, may be beneficial for ALS patients.

Mechanisms of action of angiogenin

Angiogenin induces angiogenesis by activating vessel endothelial and smooth muscle cells and triggering a number of biological processes, including cell migration, invasion, proliferation, and formation of tubular structures. It has been reported that angiogenin plays its functions mainly through four pathways: (1) exerting its ribonucleolytic activity; (2) binding to membrane actin and then inducing basement membrane degradation; (3) binding to a putative 170-kDa protein and subsequently transducing signal into cytoplasm; and (4) translocating into the nucleus of target cells directly and then enhancing ribosomal RNA transcription. Angiogenin can also translocate into the nucleus of cancer cells and induces the corresponding cell proliferation. Furthermore, angiogenin has neuroprotective activities in the central nervous system and the loss of its function may be related to amyotrophic lateral sclerosis. This review intends to conclude the mechanisms underlying these actions of angiogenin and give a perspective on future research.

Genetic selection for peptide inhibitors of angiogenin

The improper regulation of angiogenesis is implicit in a variety of diseases, including cancer. Angiogenin is unique among angiogenic factors in having ribonucleolytic activity. Inhibitors of this activity could serve as chemotherapeutics. The ribonucleolytic activity of angiogenin is toxic to the Origami strain of Escherichia coli. Herein, this cytotoxicity was used to identify inhibitors from a random nonapeptide library tethered to the C-terminus of human angiogenin. The selected sequences fell into three classes: (i) extremely hydrophobic, (ii) putative protease (ClpXP) substrates and (iii) slightly anionic. Two peptides corresponding to sequences in the last class were synthesized chemically and found to inhibit the ribonucleolytic activity of human angiogenin in vitro with micromolar values of Ki. Both peptides also inhibit bovine pancreatic ribonuclease, a homolog of angiogenin, though one exhibits selectivity for angiogenin. The affinity and selectivity of these peptides are comparable with the best known inhibitors of angiogenin. Moreover, the strategy used to identify them is general and could be applied to other cytotoxins.

Structural and functional implications of positive selection at the primate angiogenin gene

BACKGROUND

Angiogenesis, the formation of new blood vessels, is a primordial process in development and its dysregulation has a central role in the pathogenesis of many diseases. Angiogenin (ANG), a peculiar member of the RNase A superfamily, is a potent inducer of angiogenesis involved in many different types of cancer, amyotrophic lateral sclerosis and also with a possible role in the innate immune defense. The evolutionary path of this family has been a highly dynamic one, where positive selection has played a strong role. In this work we used a combined gene and protein level approach to determine the main sites under diversifying selection on the primate ANG gene and analyze its structural and functional implications.

RESULTS

We obtained evidence for positive selection in the primate ANG gene. Site specific analysis pointed out 15 sites under positive selection, most of which also exhibited drastic changes in amino acid properties. The mapping of these sites in the ANG 3D-structure described five clusters, four of which were located in functional regions: two in the active site region, one in the nucleolar location signal and one in the cell-binding site. Eight of the 15 sites under selection in the primate ANG gene were highly or moderately conserved in the RNase A family, suggesting a directed event and not a simple consequence of local structural or functional permissiveness. Moreover, 11 sites were exposed to the surface of the protein indicating that they may influence the interactions performed by ANG.

CONCLUSION

Using a maximum likelihood gene level analysis we identified 15 sites under positive selection in the primate ANG genes, that were further corroborated through a protein level analysis of radical changes in amino acid properties. These sites mapped onto the main functional regions of the ANG protein. The fact that evidence for positive selection is present in all ANG regions required for angiogenesis may be a good indication that angiogenesis is the process under selection. However, other possibilities to be considered arise from the possible involvement of ANG in innate immunity and the potential influence or co-evolution with its interacting proteins and ligands.

Genetic selection for critical residues in ribonucleases

Homologous mammalian proteins were subjected to an exhaustive search for residues that are critical to their structure/function. Error-prone polymerase chain reactions were used to generate random mutations in the genes of bovine pancreatic ribonuclease (RNase A) and human angiogenin, and a genetic selection based on the intrinsic cytotoxicity of ribonucleolytic activity was used to isolate inactive variants. Twenty-three of the 124 residues in RNase A were found to be intolerant to substitution with at least one particular amino acid. Twenty-nine of the 123 residues in angiogenin were likewise intolerant. In both RNase A and angiogenin, only six residues appeared to be wholly intolerant to substitution: two histidine residues involved in general acid/base catalysis and four cysteine residues that form two disulfide bonds. With few exceptions, the remaining critical residues were buried in the hydrophobic core of the proteins. Most of these residues were found to tolerate only conservative substitutions. The importance of a particular residue as revealed by this genetic selection correlated with its sequence conservation, though several non-conserved residues were found to be critical for protein structure/function. Despite voluminous research on RNase A, the importance of many residues identified herein was unknown, and those can now serve as targets for future work. Moreover, a comparison of the critical residues in RNase A and human angiogenin, which share only 35% amino acid sequence identity, provides a unique perspective on the molecular evolution of the RNase A superfamily, as well as an impetus for applying this methodology to other ribonucleases.

rAAV-mediated angiogenin gene transfer induces angiogenesis and modifies left ventricular remodeling in rats with myocardial infarction

In vitro studies have demonstrated that bovine angiogenin (ANG) significantly stimulates both the migration of endothelial cells and the formation of tubelike structures. The aim of this study was to explore whether ANG gene transfer could enhance vascularization, modify left ventricular remodeling, and attenuate cardiac dysfunction in rats with myocardial infarction (MI). We constructed a recombinant adeno-associated virus vector encoding the ANG gene (rAAV-ANG) and evaluated its angiogenic potential after regional transfection by intramyocardial injection immediately after left anterior descending artery ligation in rats. Four weeks after coronary artery ligation, rAAV-ANG transfection upregulated the myocardium ANG protein expression level in both normal and MI rats, and immunohistochemistry showed that the overexpressed ANG was distributed in the cytoplasm of cardiomyocytes. In rats with MI, rAAV-ANG treatment altered left ventricular remodeling, as indicated by a decrease in left ventricular end diastolic diameter, left ventricular end systolic diameter, cardiomyocyte diameter, ventricular weight to body weight ratio and interstitial fibrosis infiltration. We also found an increase in capillary density and partly restored cardiac function in the group receiving rAAV-ANG treatment. These results confirmed that in rats with MI, ANG gene transfer could induce angiogenesis, alter left ventricular remodeling, and attenuate cardiac dysfunction. This study provides a new choice of treatment for ischemic heart disease.

Angiogenin: a review of the pathophysiology and potential clinical applications

Angiogenin is a member of the ribonuclease (RNase) superfamily: enzymes of innate substrate specificity, but divergent functional capacities. Angiogenin is a normal constituent of the circulation and contained in a vasculature that rarely undergoes proliferation, but in some physiological and pathological conditions its levels increase in blood, promoting neovascularization. Hence, angiogenesis is a common pathophysiological attribute of angiogenin. In malignant disease, the most studied pathological state in regard to angionenin, abnormally high levels are seen, which may be of prognostic significance. Angiogenin has also been studied in other non-malignant pathological states. The aim of this review article is to provide an overview of the biochemistry and physiology of angiogenin, specifically in relation to the human pathological states where angiogenin has been implicated and finally, its potential clinical applications.

ANG mutations segregate with familial and 'sporadic' amyotrophic lateral sclerosis

We recently identified angiogenin (ANG) as a candidate susceptibility gene for amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder characterized by adult-onset loss of motor neurons. We now report the finding of seven missense mutations in 15 individuals, of whom four had familial ALS and 11 apparently 'sporadic' ALS. Our findings provide further evidence that variations in hypoxia-inducible genes have an important role in motor neuron degeneration.

3′-N-Alkylamino-3′-deoxy-ara-uridines: A new class of potential inhibitors of ribonuclease A and angiogenin

In this study, we report the inhibition of ribonuclease A (RNase A) by certain aminonucleosides. This is the first such instance of the use of this group of compounds to investigate the inhibitory activity of this protein. The compounds synthesized have been tested for their ability to inhibit the ribonucleolytic activity of RNase A by an agarose gel-based assay. A tRNA precipitation assay and inhibition kinetic studies with cytidine 2',3'-cyclic monophosphate as the substrate have also been conducted for two of the compounds. Results indicate substantial inhibitory activity with inhibition association constants in the micromolar range. The experimental studies have been substantiated by docking of the aminonucleoside ligands to RNase A using AutoDock. We find that the ligands preferentially bind to the active site of the protein molecule with a favorable free energy of binding. The study has been extended to a member of the ribonuclease superfamily, angiogenin, which is a potent inducer of blood vessel formation. We show that the aminonucleosides act as potent inhibitors of angiogenin induced angiogenesis.

Structure of murine angiogenin: features of the substrate- and cell-binding regions and prospects for inhibitor-binding studies

Angiogenin is an unusual member of the pancreatic ribonuclease superfamily that induces blood-vessel formation and is a promising anticancer target. The three-dimensional structure of murine angiogenin (mAng) has been determined by X-ray crystallography. Two structures are presented: one is a complex with sulfate ions (1.5 Angstroms resolution) and the other a complex with phosphate ions (1.6 Angstroms resolution). Residues forming the putative B(1), P(1) and B(2) subsites occupy positions similar to their hAng counterparts and are likely to play similar roles. The anions occupy the P(1) subsite, sulfate binding conventionally and phosphate adopting two orientations, one of which is novel. The B(1) subsite is obstructed by Glu116 and Phe119, with the latter assuming a less invasive position than its hAng counterpart. Hydrophobic interactions between the C-terminal segment and the main body of the protein are more extensive than in hAng and may underly the lower enzymatic activity of the murine protein. Elsewhere, the structure of the H3-B2 loop supports the view that hAng Asn61 interacts directly with cell-surface molecules and does not merely stabilize adjacent regions of the hAng structure. mAng crystals appear to offer small-molecule inhibitors a clear route to the active site and may even withstand a reorientation of the C-terminal segment that provides access to the cryptic B(1) subsite. These features represent considerable advantages over crystalline hAng and bAng.

Molecular recognition of human eosinophil-derived neurotoxin (RNase 2) by placental ribonuclease inhibitor

Placental ribonuclease inhibitor (RI) binds diverse mammalian RNases with dissociation constants that are in the femtomolar range. Previous studies on the complexes of RI with RNase A and angiogenin revealed that RI utilises largely distinctive interactions to achieve high affinity for these two ligands. Here we report a 2.0 angstroms resolution crystal structure of RI in complex with a third ligand, eosinophil-derived neurotoxin (EDN), and a mutational analysis based on this structure. The RI-EDN interface is more extensive than those of the other two complexes and contains a considerably larger set of interactions. Few of the contacts present in the RI-angiogenin complex are replicated; the correspondence to the RI-RNase A complex is somewhat greater, but still modest. The energetic contributions of various interface regions differ strikingly from those in the earlier complexes. These findings provide insight into the structural basis for the unusual combination of high avidity and relaxed stringency that RI displays.

Binding of non-natural 3'-nucleotides to ribonuclease A

2'-Fluoro-2'-deoxyuridine 3'-phosphate (dU(F)MP) and arabinouridine 3'-phosphate (araUMP) have non-natural furanose rings. dU(F)MP and araUMP were prepared by chemical synthesis and found to have three- to sevenfold higher affinity than uridine 3'-phosphate (3'-UMP) or 2'-deoxyuridine 3'-phosphate (dUMP) for ribonuclease A (RNase A). These differences probably arise (in part) from the phosphoryl groups of 3'-UMP, dU(F)MP, and araUMP (pK(a) = 5.9) being more anionic than that of dUMP (pK(a) = 6.3). The three-dimensional structures of the crystalline complexes of RNase A with dUMP, dU(F)MP and araUMP were determined at < 1.7 A resolution by X-ray diffraction analysis. In these three structures, the uracil nucleobases and phosphoryl groups bind to the enzyme in a nearly identical position. Unlike 3'-UMP and dU(F)MP, dUMP and araUMP bind with their furanose rings in the preferred pucker. In the RNase A.araUMP complex, the 2'-hydroxyl group is exposed to the solvent. All four 3'-nucleotides bind more tightly to wild-type RNase A than to its T45G variant, which lacks the residue that interacts most closely with the uracil nucleobase. These findings illuminate in atomic detail the interaction of RNase A and 3'-nucleotides, and indicate that non-natural furanose rings can serve as the basis for more potent inhibitors of catalysis by RNase A.

High-resolution crystal structures of ribonuclease A complexed with adenylic and uridylic nucleotide inhibitors. Implications for structure-based design of ribonucleolytic inhibitors

The crystal structures of bovine pancreatic ribonuclease A (RNase A) in complex with 3',5'-ADP, 2',5'-ADP, 5'-ADP, U-2'-p and U-3'-p have been determined at high resolution. The structures reveal that each inhibitor binds differently in the RNase A active site by anchoring a phosphate group in subsite P1. The most potent inhibitor of all five, 5'-ADP (Ki = 1.2 microM), adopts a syn conformation (in contrast to 3',5'-ADP and 2',5'-ADP, which adopt an anti), and it is the beta- rather than the alpha-phosphate group that binds to P1. 3',5'-ADP binds with the 5'-phosphate group in P1 and the adenosine in the B2 pocket. Two different binding modes are observed in the two RNase A molecules of the asymmetric unit for 2',5'-ADP. This inhibitor binds with either the 3' or the 5' phosphate groups in subsite P1, and in each case, the adenosine binds in two different positions within the B2 subsite. The two uridilyl inhibitors bind similarly with the uridine moiety in the B1 subsite but the placement of a different phosphate group in P1 (2' versus 3') has significant implications on their potency against RNase A. Comparative structural analysis of the RNase A, eosinophil-derived neurotoxin (EDN), eosinophil cationic protein (ECP), and human angiogenin (Ang) complexes with these and other phosphonucleotide inhibitors provides a wealth of information for structure-based design of inhibitors specific for each RNase. These inhibitors could be developed to therapeutic agents that could control the biological activities of EDN, ECP, and ANG, which play key roles in human pathologies.

The crystal structure of human angiogenin in complex with an antitumor neutralizing antibody

The murine monoclonal antibody 26-2F neutralizes the angiogenic and ribonucleolytic activities of human angiogenin (ANG) and is highly effective in preventing the establishment and metastatic dissemination of human tumors in athymic mice. Here we report a 2.0 A resolution crystal structure for the complex of ANG with the Fab fragment of 26-2F that reveals the detailed interactions between ANG and the complementarity-determining regions (CDRs) of the antibody. Surprisingly, Fab binding induces a dramatic conformational change in the cell binding region of ANG at the opposite end of the molecule from the combining site; crosslinking experiments indicate that this rearrangement also occurs in solution. The ANG-Fab complex structure should be invaluable for designing maximally humanized versions of 26-2F for potential clinical use.

Inhibition of mammalian ribonucleases by endogenous adenosine dinucleotides

The most potent low molecular weight inhibitors of pancreatic RNase superfamily enzymes reported to date are synthetic derivatives of adenosine 5(')-pyrophosphate. Here we have investigated the effects of six natural nucleotides that also incorporate this moiety (NADP(+), NADPH, ATP, Ap(3)A, Ap(4)A, and Ap(5)A) on the activities of RNase A and two of its homologues, eosinophil-derived neurotoxin and angiogenin. With eosinophil-derived neurotoxin and angiogenin, Ap(5)A is comparable to the tightest binding inhibitors identified previously (K(i) values at pH 5.9 are 370 nM and 100 microM, respectively); it ranks among the strongest small antagonists of RNase A as well (K(i)=230 nM). The K(i) for NADPH with angiogenin is similar to that of Ap(5)A. These findings suggest that Ap(5)A and NADPH may serve as useful new leads for inhibitor design. Examination of inhibition under physiological conditions indicates that NADPH, ATP, and Ap(5)A may suppress intracellular RNase activity significantly in vivo.

Virtual screening to enrich hit lists from high-throughput screening: a case study on small-molecule inhibitors of angiogenin

"Hit lists" generated by high-throughput screening (HTS) typically contain a large percentage of false positives, making follow-up assays necessary to distinguish active from inactive substances. Here we present a method for improving the accuracy of HTS hit lists by computationally based virtual screening (VS) of the corresponding chemical libraries and selecting hits by HTS/VS consensus. This approach was applied in a case study on the target-enzyme angiogenin, a potent inducer of angiogenesis. In conjunction with HTS of the National Cancer Institute Diversity Set and ChemBridge DIVERSet E (approximately 18,000 compounds total), VS was performed with two flexible library docking/scoring methods, DockVision/Ludi and GOLD. Analysis of the results reveals that dramatic enrichment of the HTS hit rate can be achieved by selecting compounds in consensus with one or both of the VS functions. For example, HTS hits ranked in the top 2% by GOLD included 42% of the true hits, but only 8% of the false positives; this represents a sixfold enrichment over the HTS hit rate. Notably, the HTS/VS method was effective in selecting out inhibitors with midmicromolar dissociation constants typical of leads commonly obtained in primary screens.

A small-molecule inhibitor of the ribonucleolytic activity of human angiogenin that possesses antitumor activity

The results of previous preclinical and clinical studies have identified angiogenin (ANG) as a potentially important target for anticancer therapy. Here we report the design and implementation of a high-throughput screening assay to identify small molecules that bind to the ribonucleolytic active site of ANG, which is critically involved in the induction of angiogenesis by this protein. Screening of 18,310 compounds from the National Cancer Institute (NCI) Diversity Set and ChemBridge DIVERSet yielded 15 hits that inhibit the enzymatic activity of ANG with K(i) values <100 microM. One of these, NCI compound 65828 [8-amino-5-(4'-hydroxybiphenyl-4-ylazo)naphthalene-2-sulfonate; K(i) = 81 microM], was selected for more detailed studies. Minor changes in ANG or ligand structure markedly reduced potency, demonstrating that inhibition reflects active-site rather than nonspecific binding; these observations are consistent with a computationally generated model of the ANG.65828 complex. Local treatment with modest doses of 65828 significantly delayed the formation of s.c. tumors from two distinct human cancer cell types in athymic mice. ANG is the likely target involved because (i) a 65828 analogue with much lower potency against the enzymatic activity of ANG failed to exert any antitumor effect, (ii) tumors from 65828-treated mice had fewer interior blood vessels than those from control mice, and (iii) 65828 appears to have no direct effect on the tumor cells. Our findings provide considerable support for the targeting of the enzymatic active site of ANG as a strategy for developing new anticancer drugs.