Surface-exposed amino acids of eosinophil cationic protein play a critical role in the inhibition of mammalian cell proliferation

Exploring the RNase A scaffold to combine catalytic and antimicrobial activities. Structural characterization of RNase 3/1 chimeras

Design of novel antibiotics to fight antimicrobial resistance is one of the first global health priorities. Novel protein-based strategies come out as alternative therapies. Based on the structure-function knowledge of the RNase A superfamily we have engineered a chimera that combines RNase 1 highest catalytic activity with RNase 3 unique antipathogen properties. A first construct (RNase 3/1-v1) was successfully designed with a catalytic activity 40-fold higher than RNase 3, but alas in detriment of its anti-pathogenic activity. Next, two new versions of the original chimeric protein were created showing improvement in the antimicrobial activity. Both second generation versions (RNases 3/1-v2 and -v3) incorporated a loop characteristic of RNase 3 (L7), associated to antimicrobial activity. Last, removal of an RNase 1 flexible loop (L1) in the third version enhanced its antimicrobial properties and catalytic efficiency. Here we solved the 3D structures of the three chimeras at atomic resolution by X-ray crystallography. Structural analysis outlined the key functional regions. Prediction by molecular docking of the protein chimera in complex with dinucleotides highlighted the contribution of the C-terminal region to shape the substrate binding cavity and determine the base selectivity and catalytic efficiency. Nonetheless, the structures that incorporated the key features related to RNase 3 antimicrobial activity retained the overall RNase 1 active site conformation together with the essential structural elements for binding to the human ribonuclease inhibitor (RNHI), ensuring non-cytotoxicity. Results will guide us in the design of the best RNase pharmacophore for anti-infective therapies.

Structure-Based Design of an RNase Chimera for Antimicrobial Therapy

Bacterial resistance to antibiotics urges the development of alternative therapies. Based on the structure-function of antimicrobial members of the RNase A superfamily, we have developed a hybrid enzyme. Within this family, RNase 1 exhibits the highest catalytic activity and the lowest cytotoxicity; in contrast, RNase 3 shows the highest bactericidal action, alas with a reduced catalytic activity. Starting from both parental proteins, we designed a first RNase 3/1-v1 chimera. The construct had a catalytic activity much higher than RNase 3, unfortunately without reaching an equivalent antimicrobial activity. Thus, two new versions were created with improved antimicrobial properties. Both of these versions (RNase 3/1-v2 and -v3) incorporated an antimicrobial loop characteristic of RNase 3, while a flexible RNase 1-specific loop was removed in the latest construct. RNase 3/1-v3 acquired both higher antimicrobial and catalytic activities than previous versions, while retaining the structural determinants for interaction with the RNase inhibitor and displaying non-significant cytotoxicity. Following, we tested the constructs' ability to eradicate macrophage intracellular infection and observed an enhanced ability in both RNase 3/1-v2 and v3. Interestingly, the inhibition of intracellular infection correlates with the variants' capacity to induce autophagy. We propose RNase 3/1-v3 chimera as a promising lead for applied therapeutics.

Mechanisms of pathogenic effects of eosinophil cationic protein and eosinophil-derived neurotoxin on human keratinocytes

Cutaneous deposition of eosinophil degranulation proteins is a major feature of eosinophil-rich cutaneous diseases including bullous pemphigoid (BP). We sought to better understand the effect of two of these proteins - eosinophil cationic protein (ECP) and eosinophil-derived neurotoxin (EDN), on human keratinocytes using the Het-1A cell line. To evaluate expression of key cytokines and chemokines observed in BP as well as metal metalloprotease 9 (MMP9), we performed qPCR and in-cell Western assays on cells treated with either ECP or EDN. We further evaluated the effect of ECP and EDN on keratinocyte survival, generation of reactive oxygen species (ROS) and apoptosis. Lastly, we assessed ECP and EDN's ability to induce keratinocyte detachment from provisional matrix. Treatment of keratinocytes with ECP and EDN resulted in significant increases in IL-5, eotaxin-1 and CCL5 (RANTES) expression at both mRNA and protein levels, but not IL-17 or IL-31. ECP and EDN also upregulate MMP9 production. Inhibiting MMP9, we confirmed that keratinocyte expression of IL-5, eotaxin-1 and RANTES was independent from MMP9. Both ECP and EDN were cytotoxic to keratinocytes, inducing ROS formation and apoptosis through a mitochondrion-dependent pathway as evidenced by results of terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) and cytochrome c release assays, respectively. ECP but not EDN led to significant keratinocyte detachment from provisional matrix. These findings demonstrate that the pathogenic effects of ECP and EDN in BP may result from their direct action on keratinocytes, and as such may became a target for future therapies in eosinophil-rich cutaneous diseases.

Structural basis of substrate specificity in porcine RNase 4

RNase 4, a member of the RNase A superfamily with substrate preference for uridine, has roles in host defence, angiogenesis and neurodegenerative diseases. It also exhibits the highest interspecies amino acid sequence similarity amongst RNase A family members. However, compared to other members of the RNase A family, including eosinophil-derived neurotoxin, eosinophil cationic protein and angiogenin, little is known about the molecular basis of substrate specificity in RNase 4. Here we report high to medium resolution structures of native porcine RNase 4 (PL3), a 'substrate-specificity' determining mutant D80A and their respective complexes with deoxyuridine 5'-monophosphate (dUMP) and deoxycytidine 5'-monophosphate (dCMP). These structures provide insight into the structural basis of the uridine versus cytosine substrate specificity in RNase 4: in the D80A mutant (D80A•dCMP), the side chain of Arg101 is positioned further away from the substrate-binding pocket due to the loss of the Asp80 side chain, reducing the repulsion force on the less favoured dCMP from Arg101 and allowing the ligand to occupy the binding pocket. This can also explain the observation that the ligand in the D80A•dCMP complex is stabilized only by a small number of hydrogen bonds. Compared to the previously reported structure of the human RNase 4•2'-deoxyuridine 3'-phosphate complex, the structure of PL3•dUMP complex shows additional hydrogen bonds between the ligand and the protein. In addition, the interaction between Arg101 and the dUMP ligand is absent. These observed differences are probably the result of the flexibility and different 'positioning' of the phosphate group among the mononucleotide ligands.

DATABASE

The atomic coordinates and structure factors for PL3 (5AR6), D80A (5ARJ), PL3∙dUMP (5ARK) and D80A∙dCMP (5ARL) complexes have been deposited with the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ, USA (http://www.rcsb.org/).

Effect of eosinophil cationic protein on human oral squamous carcinoma cell viability

The exact function of eosinophils in cancer, particularly in oral squamous cell carcinoma (OSCC), has not yet been elucidated and the possible antitumor effect of these leukocytes is associated with the release of cytotoxic proteins, particularly eosinophil cationic protein (ECP). The aim of this study was to evaluate the effect of ECP on human OSCC lines and to provide novel insights into the role of eosinophils in these tumors. The viability of the SCC-4 and SCC-25 OSCC cell lines was assessed by colorimetric assay using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). The cells were plated into 96-well plates in Dulbecco's modified Eagle's medium/Ham's nutrient mixture F12 supplemented with 10% fetal bovine serum. After 24 h, the indicated concentration of ECP (0-10 µ M) was added to each sample. The plate was read using a microplate reader at a wavelength of 570 nm. The association between variables was estimated by linear regression analysis. There was a significant inverse association between ECP concentrations with SCC-4 (β=0.16, P=0.019) and SCC-25 cell viability (β=0.24, P=0.006). To the best of our knowledge, the present study was the first to investigate the effects of ECP on OSCCs and to demonstrate a significant inverse association between ECP concentrations with SCC-4 and SCC-25 cell viability.

Asparagine-linked glycans determine the cytotoxic capacity of eosinophil cationic protein (ECP)

Eosinophil cationic protein (ECP) is a toxic, granule-stored protein of the eosinophil granulocyte. It is a heterogeneous protein; molecular weights can differ from 15 to 22 kDa, due to glycosylations. We purified high molecular weight ECP from blood donors with the ECP434GG (rs2073342) genotype, with the aim of examining whether removal of carbohydrates could enhance the cytotoxic capacity. The cytotoxic activity of the ECP pools was tested against the NCI-H69 cell line, before and after enzymatic deglycosylation. ECP was also analysed by SELDI-TOF MS to monitor the changes in molecular mass after deglycosylation. Five high molecular weight pools of ECP (HMW-ECP I-V) with decreasing degrees of glycosylation were tested at concentrations ranging from 0.02 to 0.6 μM. The activity ranged from EC50 of >0.6 μM to 0.04 μM; HMW-ECP II had the lowest activity and HMW-ECP V the highest. After deglycosylation with N-glycosidase F, pools HMW-ECP I-III were reduced to the same molecular weight of 15.78 kDa and acquired potent cytotoxic activities. HMW-ECP IV and V with molecular species at 16.3 and 16.1 kDa were highly cytotoxic as such and were only partially deglycosylated, with slight enhancement of the toxic properties. The results suggest the presence of several HMW-ECP molecular species with differences in their post-translational modifications and cytotoxic properties. We conclude that a fraction of native ECP is stored in a non-cytotoxic form, which can be converted into a cytotoxic form by N-deglycosylation, whereas another fraction is stored as a highly cytotoxic form carrying different post-translational modifications.

Insights into the glycosaminoglycan-mediated cytotoxic mechanism of eosinophil cationic protein revealed by NMR

Protein-glycosaminoglycan interactions are essential in many biological processes and human diseases, yet how their recognition occurs is poorly understood. Eosinophil cationic protein (ECP) is a cytotoxic ribonuclease that interacts with glycosaminoglycans at the cell surface; this promotes the destabilization of the cellular membrane and triggers ECP's toxic activity. To understand this membrane destabilization event and the differences in the toxicity of ECP and its homologues, the high resolution solution structure of the complex between full length folded ECP and a heparin-derived trisaccharide (O-iPr-α-D-GlcNS6S-α(1-4)-L-IdoA2S-α(1-4)-D-GlcNS6S) has been solved by NMR methods and molecular dynamics simulations. The bound protein retains the tertiary structure of the free protein. The (2)S(0) conformation of the IdoA ring is preferably recognized by the protein. We have identified the precise location of the heparin binding site, dissected the specific interactions responsible for molecular recognition, and defined the structural requirements for this interaction. The structure reveals the contribution of Arg7, Gln14, and His15 in helix α1, Gln40 in strand β1, His64 in loop 4, and His128 in strand β6 in the recognition event and corroborates the previously reported participation of residues Arg34-Asn39. The participation of the catalytic triad (His15, Lys38, His128) in recognizing the heparin mimetic reveals, at atomic resolution, the mechanism of heparin's inhibition of ECP's ribonucleolytic activity. We have integrated all the available data to propose a molecular model for the membrane interaction process. The solved NMR complex provides the structural model necessary to design inhibitors to block ECP's toxicity implicated in eosinophil pathologies.

Chemoattraction of macrophages by secretory molecules derived from cells expressing the signal peptide of eosinophil cationic protein

Background

Eosinophil cationic protein is a clinical asthma biomarker that would be released into blood, especially gathered in bronchia. The signal peptide of eosinophil cationic protein (ECPsp) plays an important role in translocating ECP to the extracellular space. We previously reported that ECPsp inhibits microbial growth and regulates the expression of mammalian genes encoding tumor growth factor-α (TGF-α) and epidermal growth factor receptor (EGFR).

Results

In the present study, we first generated a DNA microarray dataset, which showed that ECPsp upregulated proinflammatory molecules, including chemokines, interferon-induced molecules, and Toll-like receptors. The levels of mRNAs encoding CCL5, CXCL10, CXCL11, CXCL16, STAT1, and STAT2 were increased in the presence of ECPsp by 2.07-, 4.21-, 7.52-, 2.6-, 3.58-, and 1.67-fold, respectively. We then constructed a functional linkage network by integrating the microarray dataset with the pathway database of Kyoto Encyclopedia of Genes and Genomes (KEGG). Follow-up analysis revealed that STAT1 and STAT2, important transcriptional factors that regulate cytokine expression and release, served as hubs to connect the pathways of cytokine stimulation (TGF-α and EGFR pathways) and inflammatory responses. Furthermore, integrating TGF-α and EGFR with the functional linkage network indicated that STAT1 and STAT2 served as hubs that connect two functional clusters, including (1) cell proliferation and survival, and (2) inflammation. Finally, we found that conditioned medium in which cells that express ECPsp had been cultured could chemoattract macrophages. Experimentally, we also demonstrated that the migration of macrophage could be inhibited by the individual treatment of siRNAs of STAT1 or STAT2. Therefore, we hypothesize that ECPsp may function as a regulator for enhancing the migration of macrophages through the upregualtion of the transcriptional factors STAT1 and STAT2.

Conclusion

The increased expression and release of various cytokines triggered by ECPsp may attract macrophages to bronchia to purge damaged cells. Our approach, involving experimental and computational systems biology, predicts pathways and potential biological functions for further characterization of this novel function of ECPsp under inflammatory conditions.

Eosinophil cationic protein: overview of biological and genetic features

The eosinophil cationic protein (ECP) is a small polypeptide that originates from activated eosinophil granulocytes. A wide range of stimuli has been shown to induce the secretion of ECP. The gene that encodes the human ECP is located on chromosome 14, and the protein shares the overall three-dimensional structure and the RNase active-site residues with other proteins in the RNase A superfamily. Several single-nucleotide polymorphisms in the human ECP gene have been currently described. ECP has many biological functions, including an immunoregulatory function, the regulation of fibroblast activity, and the induction of mucus secretion in the airway. Additionally, the protein is a potent cytotoxic molecule and has the capacity to kill mammalian and nonmammalian cells. The purpose of this article was to review the known biological and genetic characteristics of ECP that contribute to the understanding of this protein's role in the development and progression of a wide variety of diseases.

Antimicrobial activity of human eosinophil granule proteins: involvement in host defence against pathogens

Eosinophils have been associated with the pathophysiology of various allergic diseases and asthma. Eosinophils secrete a number of granule proteins that have been identified as effector molecules responsible for many of the actions of eosinophils. The four major eosinophil granule proteins, major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil derived neurotoxin (EDN) and eosinophil peroxidase have been shown to be involved in a number of eosinophil associated functions. EDN possesses antiviral activity against single stranded RNA viruses like respiratory syncytial virus, Hepatitis and HIV, whereas ECP and MBP have antibacterial and antiparasitic properties. This review summarizes the studies on antipathogenic activities of eosinophil granule proteins against bacteria, viruses, protozoans and helminths.

Barnase and binase: twins with distinct fates

RNases are enzymes that cleave RNAs, resulting in remarkably diverse biological consequences. Many RNases are cytotoxic. In some cases, they attack selectively malignant cells triggering an apoptotic response. A number of eukaryotic and bacterial RNase-based strategies are being developed for use in anticancer and antiviral therapy. However, the physiological functions of these RNases are often poorly understood. This review focuses on the properties of the extracellular RNases from Bacillus amyloliquefaciens (barnase) and Bacillus intermedius (binase), the characteristics of their biosynthesis regulation and their physiological role, with an emphasis on the similarities and differences. Barnase and binase can be regarded as molecular twins according to their highly similar structure, physical-chemical and catalytic properties. Nevertheless, the 'life paths' of these enzymes are not the same, as their expression in bacteria is controlled by diverse signals. Binase is predominantly synthesized under phosphate starvation, whereas barnase production is strictly dependent on the multifunctional Spo0A regulator controlling sporulation, biofilm formation and cannibalism. Barnase and binase also have some distinctions in practical applications. Barnase was initially suggested to be useful in research and biotechnology as a tool for studying protein-protein interactions, for RNA elimination from biological samples, for affinity purification of RNase fusion proteins, for the development of cloning vectors and for sterility acquisition by transgenic plants. Binase, as later barnase, was tested for antiviral, antitumour and immunogenic effects. Both RNases have found their own niche in cancer research as a result of success in targeted delivery and selectivity towards tumour cells.

Eosinophil cationic protein (ECP) can bind heparin and other glycosaminoglycans through its RNase active site

The eosinophil cationic protein (ECP) is an eosinophil-secreted RNase involved in the immune host defense, with a cytotoxic activity against a wide range of pathogens. During inflammation and eosinophilia disorders, ECP is secreted to the inflammation area, where it would contribute to the immune response. ECP secretion causes also severe damage to the host own tissues. ECP presents a high affinity for heparin and this property might be crucial for its immunomodulating properties, antipathogen action, and its toxicity against eukaryotic cells. ECP, also known as human RNase 3, belongs to the mammalian RNase A superfamily and its RNase activity is required for some of its biological properties. We have now proven that ECP heparin binding affinity depends on its RNase catalytic site, as the enzymatic activity is blocked by heparin. We have applied molecular modeling to analyze ECP binding to heparin representative probes, and identified protein residues at the catalytic and substrate binding sites that could contribute to the interaction. ECP affinity for heparin and other negatively charged glycosaminoglycans (GAGs) can explain not only its binding to the eukaryote cells glycocalix but also the reported high affinity for the specific carbohydrates at bacteria cell wall, promoting its antimicrobial action.

Refining the eosinophil cationic protein antibacterial pharmacophore by rational structure minimization

Sequence analysis of eosinophil cationic protein (ECP), a ribonuclease of broad antimicrobial activity, allowed identification of residues 1-45 as the antimicrobial domain. We have further dissected ECP(1-45) with a view to defining the minimal requirements for antimicrobial activity. Structure-based downsizing has focused on both α-helices of ECP(1-45) and yielded analogues with substantial potency against Gram-negative and -positive strains. Analogues ECP(8-36) and ECP(6-17)-Ahx-(23-36) (Ahx, 6-aminohexanoic acid) involve 36% and 40% size reduction relative to (1-45), respectively, and display a remarkably ECP-like antimicrobial profile. Both retain segments required for self-aggregation and lipolysaccharide binding, as well as the bacterial agglutination ability of parent ECP. Analogue (6-17)-Ahx-(23-36), in particular, is shown by NMR to preserve the helical traits of the native 8-16 (α1) and 33-36 (α2) regions and can be proposed as the minimal structure capable of reproducing the activity of the entire protein.

Analysing the eosinophil cationic protein--a clue to the function of the eosinophil granulocyte

Eosinophil granulocytes reside in respiratory mucosa including lungs, in the gastro-intestinal tract, and in lymphocyte associated organs, the thymus, lymph nodes and the spleen. In parasitic infections, atopic diseases such as atopic dermatitis and asthma, the numbers of the circulating eosinophils are frequently elevated. In conditions such as Hypereosinophilic Syndrome (HES) circulating eosinophil levels are even further raised. Although, eosinophils were identified more than hundred years ago, their roles in homeostasis and in disease still remain unclear. The most prominent feature of the eosinophils are their large secondary granules, each containing four basic proteins, the best known being the eosinophil cationic protein (ECP). This protein has been developed as a marker for eosinophilic disease and quantified in biological fluids including serum, bronchoalveolar lavage and nasal secretions. Elevated ECP levels are found in T helper lymphocyte type 2 (atopic) diseases such as allergic asthma and allergic rhinitis but also occasionally in other diseases such as bacterial sinusitis. ECP is a ribonuclease which has been attributed with cytotoxic, neurotoxic, fibrosis promoting and immune-regulatory functions. ECP regulates mucosal and immune cells and may directly act against helminth, bacterial and viral infections. The levels of ECP measured in disease in combination with the catalogue of known functions of the protein and its polymorphisms presented here will build a foundation for further speculations of the role of ECP, and ultimately the role of the eosinophil.

The role of eosinophils and eosinophil cationic protein in oral cancer: a review

Eosinophils are multifunctional leukocytes implicated in the pathogenesis of numerous inflammatory processes, such as allergies and parasitic infections. Increased number of these cells has been described in many human cancers, including oral squamous cell carcinoma, and its presence related to favourable as well as unfavourable prognosis. Although the exact role of eosinophils in tumours is not yet defined, the anti-tumour activity of these rare granulocytes has been associated with the release of their cytotoxic proteins, especially the eosinophil cationic protein, a single polypeptide chain with a molecular mass ranging from 15 to 22kDa encoded by the RNSE3 gene located on chromosome 14q11.2. Functional studies have implicated ECP in numerous processes, such as tissue remodelling in allergic inflammation; however its most striking function is the cytotoxic activity. The aim of this review is to summarise the role and functions of eosinophils and their granule-derived products in oral malignant tumours and the clinical value of the tumour-associated tissue eosinophilia for patients' prognosis.

Human eosinophils exert TNF-α and granzyme A-mediated tumoricidal activity toward colon carcinoma cells

Peripheral blood and tissue eosinophilia is a prominent feature in allergic diseases and helminth infections. In cancer patients, tumor-associated tissue eosinophilia is frequently observed. Tumor-associated tissue eosinophilia can be associated with a favorable prognosis, notably in colorectal carcinoma. However, underlying mechanisms of eosinophil contribution to antitumor responses are poorly understood. We have in this study investigated the direct interactions of human eosinophils with Colo-205, a colorectal carcinoma cell line, and show that eosinophils induce apoptosis and directly kill tumor cells. Using blocking Abs, we found that CD11a/CD18 complex is involved in the tumoricidal activity. Coculture of eosinophils with Colo-205 led to the release of eosinophil cationic protein and eosinophil-derived neurotoxin as well as TNF-α secretion. Moreover, eosinophils expressed granzyme A, which was released upon interaction with Colo-205, whereas cytotoxicity was partially inhibited by FUT-175, an inhibitor of trypsin-like enzymatic activity. Our data present the first demonstration, to our knowledge, that granzyme A is a cytotoxic mediator of the eosinophil protein arsenal, exerting eosinophil tumoricidal activity toward Colo-205, and provide mechanistic evidence for innate responses of eosinophil against tumor cells.

Eosinophil-induced neurotoxicity: the role of eosinophil cationic protein/RNase 3

We analyze the effect of ECP on primary cultures of cerebellar granule cells (CGCs) and astrocytes in an effort to understand the role of ECP in the eosinophil-induced neurotoxicity. We have shown that ECP induces dose-dependent cell death in both CGCs and astrocytes. The effect of ECP action on cell morphology is consistent with apoptosis for both cell types. The apoptotic mechanism involves ECP binding on the cell surface and an increase in the free cytosolic Ca(2+) concentration. It is associated with the activation of caspase-3, -8 and -9, processes that are also involved in the apoptosis induced either by stroke or other neurodegenerative conditions. Our results open new insights to clarify the neurotoxic effects associated to ECP in the hypereosinophilic syndrome.

Inhibition of the interactions between eosinophil cationic protein and airway epithelial cells by traditional Chinese herbs

Background

The eosinophil cationic protein (ECP) is cytotoxic to bacteria, viruses, parasites and mammalian cells. Cells are damaged via processes of pore formation, permeability alteration and membrane leaking. Some clinical studies indicate that ECP gathers in the bronchial tract of asthma sufferers, damages bronchial and airway epithelial cells, and leads to in breathing tract inflammation; therefore, prevention of the cytotoxicity caused by ECP may serve as an approach to treat airway inflammation. To achieve the purpose, reduction of the ECP-cell interactions is rational. In this work, the Chinese herbal combinative network was generated to predict and identify the functional herbs from the pools of prescriptions. It was useful to select the node herbs and to demonstrate the relative binding ability between ECP and Beas-2B cells with or withour herb treatments.

Results

Eighty three Chinese herbs and prescriptions were tested and five effective herbs and six prescription candidates were selected. On the basis of effective single-herbal drugs and prescriptions, a combinative network was generated. We found that a single herb, Gan-cao, served as a node connecting five prescriptions. In addition, Sheng-di-huang, Dang-guei and Mu-tong also appeared in five, four and three kinds of prescriptions, respectively. The extracts of these three herbs indeed effectively inhibited the interactions between ECP and Beas-2B cells. According to the Chinese herbal combinative network, eight of the effective herbal extracts showed inhibitory effects for ECP internalizing into Beas-2B cells. The major components of Gang-cao and Sheng-di-huang, glycyrrhizic acid and verbascose, respectively, reduced the binding affinity between ECP and cells effectively.

Conclusions

Since these Chinese herbs reduced the binding affinity between ECP and cells and inhibited subsequent ECP entrance into cells, they were potential for mitigating the airway inflammation symptoms. Additionally, we mentioned a new concept to study the Chinese herbs using combinative network in the field of systems biology. The functional single herbs could be identified from the set of prescriptions.

NMR structural determinants of eosinophil cationic protein binding to membrane and heparin mimetics

Eosinophil cationic protein (ECP) is a highly stable, cytotoxic ribonuclease with the ability to enter and disrupt membranes that participates in innate immune defense against parasites but also kills human cells. We have used NMR spectroscopy to characterize the binding of ECP to membrane and heparin mimetics at a residue level. We believe we have identified three Arg-rich surface loops and Trp(35) as crucial for membrane binding. Importantly, we have provided evidence that the interaction surface of ECP with heparin mimetics is extended with respect to that previously described (fragment 34-38). We believe we have identified new sites involved in the interaction for the first time, and shown that the N-terminal alpha-helix, the third loop, and the first and last beta-strands are key for heparin binding. We have also shown that a biologically active ECP N-terminal fragment comprising the first 45 residues (ECP1-45) retains the capacity to bind membrane and heparin mimetics, thus neither the ECP tertiary structure nor its high conformational stability are required for cytotoxicity.

The eight human "canonical" ribonucleases: molecular diversity, catalytic properties, and special biological actions of the enzyme proteins

Human ribonucleases (RNases) are members of a large superfamily of rapidly evolving homologous proteins. Upon completion of the human genome, eight catalytically active RNases (numbered 1-8) were identified. These structurally distinct RNases, characterized by their various catalytic differences on different RNA substrates, constitute a gene family that appears to be the sole vertebrate-specific enzyme family. Apart from digestion of dietary RNA, a wide variety of biological actions, including neurotoxicity, angiogenesis, immunosuppressivity, and anti-pathogen activity, have been recently reported for almost all members of the family. Recent evolutionary studies suggest that RNases started off in vertebrates as host defence or angiogenic proteins.

TNF-alpha mediates eosinophil cationic protein-induced apoptosis in BEAS-2B cells

Background

Eosinophilic granulocytes are important for the human immune system. Many cationic proteins with cytotoxic activities, such as eosinophil cationic protein (ECP) and eosinophil-derived neurotoxin (EDN), are released from activated eosinophils. ECP, with low RNase activity, is widely used as a biomarker for asthma. ECP inhibits cell viability and induces apoptosis to cells. However, the specific pathway underlying the mechanisms of ECP-induced cytotoxicity remains unclear. This study investigated ECP-induced apoptosis in bronchial epithelial BEAS-2B cells and elucidated the specific pathway during apoptosis.

Results

To address the mechanisms involved in ECP-induced apoptosis in human BEAS-2B cells, investigation was carried out using chromatin condensation, cleavage of poly (ADP-ribose) polymerase (PARP), sub-G1 distribution in cell cycle, annexin V labeling, and general or specific caspase inhibitors. Caspase-8-dependent apoptosis was demonstrated by cleavage of caspase-8 after recombinant ECP treatment, accompanied with elevated level of tumor necrosis factor alpha (TNF-α). Moreover, ECP-induced apoptosis was effectively inhibited in the presence of neutralizing anti-TNF-α antibody.

Conclusion

In conclusion, our results have demonstrated that ECP increased TNF-α production in BEAS-2B cells and triggered apoptosis by caspase-8 activation through mitochondria-independent pathway.

The (1)H, (13)C, (15)N resonance assignment, solution structure, and residue level stability of eosinophil cationic protein/RNase 3 determined by NMR spectroscopy

Eosinophil cationic protein (ECP)/human RNase 3, a member of the RNase A family, is a remarkably cytotoxic protein implicated in asthma and allergies. These activities are probably due to ECP's ability to interact with and disrupt membranes and depend on two Trp, 19 Arg, and possibly an extremely high conformational stability. Here, we have used NMR spectroscopy to assign essentially all (1)H, (15)N, and backbone (13)C resonances, to solve the 3D structure in aqueous solution and to quantify its residue-level stability. The NMR solution structure was determined on the basis of 2316 distance constraints and is well-defined (backbone RMSD = 0.81 A). The N-terminus and the loop composed of residues 114-123 are relatively well-ordered; in contrast, conformational diversity is observed for the loop segments 17-22, 65-68, and 92-95 and most exposed sidechains. The side chain NH groups of the two Trp and 19 Arg showed no significant protection against hydrogen/deuterium exchange. The most protected NH groups belong to the first and last two beta-strands, and curiously, the first alpha-helix. Analysis of their exchange rates reveals a strikingly high global stability of 11.8 kcal/mol. This value and other stability measurements are used to better quantify ECP's unfolding thermodynamics.

The coding ECP 434(G>C) gene polymorphism determines the cytotoxicity of ECP but has minor effects on fibroblast-mediated gel contraction and no effect on RNase activity

Eosinophil cationic protein (ECP) is a secretory protein of the eosinophil granulocyte, a cell involved in innate immunity. Functional studies have implicated ECP in numerous processes, such as tissue remodeling in allergic inflammation and cytotoxicity toward a variety of pathogens. Recent genetic studies have suggested that the ECP 434(G>C) polymorphism resulting in an arg97thr substitution would alter the function of ECP in vivo. Functional (in vitro) studies of ECP up until now have either been conducted with native preparations containing an unknown mixture of the ECP(97arg) and ECP(97thr) variants, or with recombinant proteins. Therefore, we have now for the first time extracted the native ECP(97arg) and ECP(97thr) variants from healthy blood donors and tested them functionally in vitro. Our results show that the arg97thr shift dramatically alters the cytotoxic capacity of ECP in vitro; the tested ECP(97arg) variants were cytotoxic toward the small-cell lung cancer cell line NCI-H69, whereas ECP(97thr) was noncytotoxic. RNase activity was unaffected by the arg97thr substitution. Both ECP(97arg) and ECP(97thr) stimulated fibroblast-mediated collagen gel contraction, an experimental model, which depicts wound healing, in a dose-dependent manner. In conclusion, our results demonstrate that the ECP 434(G>C) gene polymorphism affects the functional properties of native ECP, but also that there is a dissociation between different biological activities; the arg97thr substitution impairs the cytotoxic potential of ECP but less the gel contraction and not at all the RNase activity.

Bactericidal and membrane disruption activities of the eosinophil cationic protein are largely retained in an N-terminal fragment

ECP (eosinophil cationic protein) is an eosinophil secretion protein with antipathogen activities involved in the host immune defence system. The bactericidal capacity of ECP relies on its action on both the plasma membrane and the bacterial wall. In a search for the structural determinants of ECP antimicrobial activity, we have identified an N-terminal domain (residues 1-45) that retains most of ECP's membrane-destabilizing and antimicrobial activities. Two sections of this domain, ECP-(1-19) and ECP-(24-45), have also been evaluated. All three peptides bind and partially insert into lipid bilayers, inducing aggregation of lipid vesicles and leakage of their aqueous content. In such an environment, the peptides undergo conformational change, significantly increasing their alpha-helix content. The bactericidal activity of the three peptides against Escherichia coli and Staphylococcus aureus has been assessed at both the cytoplasmic membrane and the bacterial envelope levels. ECP-(1-45) and ECP-(24-45) partially retain the native proteins ability to bind LPS (lipopolysaccharides), and electron microscopy reveals cell damage by both peptides. Interestingly, in the E. coli cells agglutination activity of ECP is only retained by the longest segment ECP-(1-45). Comparative results suggest a task distribution, whereby residues 1-19 would contribute to membrane association and destabilization, while the 24-45 region would be essential for bactericidal action. Results also indicate that ECP cytotoxicity is not uniquely dependant on its membrane disruption capacity, and that specific interactions at the bacteria wall are also involved.

Characterization of molecular interactions between eosinophil cationic protein and heparin

Eosinophil cationic protein (ECP) is currently used as a biomarker for airway inflammation. It is a heparin-binding ribonuclease released by activated eosinophils. Its cytotoxicity toward cancer cell lines is blocked by heparin. The objective of this study was to locate the heparin binding site of ECP by site-directed mutagenesis and construction of a synthetic peptide derived from this region. Synthetic heparin with > or =5 monosaccharide units showed strong inhibition of ECP binding to the cell surface. Analysis of ECP mt1 (R34A/W35A/R36A/K38A) showed that these charged and aromatic residues were involved in ECP binding to heparin and the cell surface. A potential binding motif is located in the loop L3 region between helix alpha2 and strand beta1, outside the RNA binding domain. The synthetic peptide derived from the loop L3 region displayed strong pentasaccharide binding affinity and blocked ECP binding to cells. In addition, ECP mt1 showed reduced cytotoxicity. Thus, the tight interaction between ECP and heparin acts as the primary step for ECP endocytosis. These results provide new insights into the structure and function of ECP for anti-asthma therapy.

Eosinophil cationic protein high-affinity binding to bacteria-wall lipopolysaccharides and peptidoglycans

The eosinophil cationic protein (ECP) is an eosinophil-secreted RNase involved in the immune host defense, with a cytotoxic activity against a wide range of pathogens. The protein displays antimicrobial activity against both Gram-negative and Gram-positive strains. The protein can destabilize lipid bilayers, although the action at the membrane level can only partially account for its bactericidal activity. We have now shown that ECP can bind with high affinity to the bacteria-wall components. We have analyzed its specific association to lipopolysaccharides (LPSs), its lipid A component, and peptidoglycans (PGNs). ECP high-affinity binding capacity to LPSs and lipid A has been analyzed by a fluorescent displacement assay, and the corresponding dissociation constants were calculated using the protein labeled with a fluorophor. The protein also binds in vivo to bacteria cells. Ultrastructural analysis of cell bacteria wall and morphology have been visualized by scanning and transmission electron microscopy in both Escherichia coli and Staphylococcus aureus strains. The protein damages the bacteria surface and induces the cell population aggregation on E. coli cultures. Although both bacteria strain cells retain their shape and no cell lysis is patent, the protein can induce in E. coli the outer membrane detachment. ECP also activates the cytoplasmic membrane depolarization in both strains. Moreover, the depolarization activity on E. coli does not require any pretreatment to overcome the outer membrane barrier. The protein binding to the bacteria-wall surface would represent a first encounter step key in its antimicrobial mechanism of action.

A heparan sulfate-facilitated and raft-dependent macropinocytosis of eosinophil cationic protein

Eosinophil cationic protein (ECP), a human RNAseA superfamily member, highly implicated in asthma pathology, is toxic to bronchial epithelial cells following its endocytosis. The mechanism by which ECP is internalized into cells is poorly understood. In this study, we show that cell surface-bound heparan sulfate proteoglycans serve as the major receptor for ECP internalization. Removal of cell surface heparan sulfate by heparinases or reducing glycan sulfation by chlorate markedly decreased ECP binding to human bronchial epithelial Beas-2B cells. In addition, ECP uptake and associated cytotoxicity were reduced in glycosaminoglycan-defective cells compared with their wild-type counterparts. Furthermore, pharmacological treatment combined with siRNA knockdown identified a clathrin- and caveolin-independent endocytic pathway as the major route for ECP internalization. This pathway is regulated by Rac1 and ADP-ribosylating factor 6 GTPases. It requires cholesterol, actin cytoskeleton rearrangement and phosphatidylinositol-3-kinase activities, and is compatible with the characteristics of raft-dependent macropinocytosis. Thus, our results define the early events of ECP internalization and may have implications for novel therapeutic design for ECP-associated diseases.

Mammalian antimicrobial proteins and peptides: overview on the RNase A superfamily members involved in innate host defence

The review starts with a general outlook of the main mechanisms of action of antimicrobial proteins and peptides, with the final aim of understanding the biological function of antimicrobial RNases, and identifying the key events that account for their selective properties. Although most antibacterial proteins and peptides do display a wide-range spectrum of action, with a cytotoxic activity against bacteria, fungi, eukaryotic parasites and viruses, we have only focused on their bactericidal activity. We start with a detailed description of the main distinctive structural features of the bacteria target and on the polypeptides, which act as selective host defence weapons.Following, we include an overview of all the current available information on the mammalian RNases which display an antimicrobial activity. There is a wealth of information on the structural, catalytic mechanism and evolutionary relationships of the RNase A superfamily. The bovine pancreatic RNase A (RNase A), the reference member of the mammalian RNase family, has been the main research object of several Nobel laureates in the 60s, 70s and 80s. A potential antimicrobial function was only recently suggested for several members of this family. In fact, the recent evolutionary studies indicate that this protein family may have started off with a host defence function. Antimicrobial RNases constitute an interesting example of proteins involved in the mammalian innate immune defence system. Besides, there is wealth of available information on the mechanism of action of short antimicrobial peptides, but little is known on larger polypeptides, that is, on proteins. Therefore, the identification of the mechanisms of action of antimicrobial RNases would contribute to the understanding of the proteins involved in the innate immunity.

Thermal unfolding of eosinophil cationic protein/ribonuclease 3: a nonreversible process

Eosinophil cationic protein (ECP)/ribonuclease 3 is a member of the RNase A superfamily involved in inflammatory processes mediated by eosinophils. ECP is bactericidal, helminthotoxic, and cytotoxic to tracheal epithelium cells and to several mammalian cell lines although its RNase activity is low. We studied the thermal stability of ECP by fourth-derivative UV absorbance spectra, circular dichroism, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The T (1/2) values obtained with the different techniques were in very good agreement (T (1/2) approximately 72 degrees C), and the stability was maintained in the pH range between 5 and 7. The ECP calorimetric melting curve showed, in addition to the main transition, a pretransitional conformational change with a T (1/2) of 44 degrees C. Both calorimetric transitions disappeared after successive re-heatings, and the ratio DeltaH versus DeltaH (vH) of 2.2 indicated a significant deviation from the two-state model. It was observed that the thermal unfolding was irreversible. The unfolding process gives rise to changes in the environment of aromatic amino acids that are partially maintained in the refolded protein with the loss of secondary structure and the formation of oligomers. From the thermodynamic analysis of ECP variants, the contribution of specific amino acids, such as Trp10 and the region 115-122, to thermal stability was also determined. The high thermal stability of ECP may contribute to its resistance to degradation when the protein is secreted to the extracellular medium during the immune response.

[Is the seric eosinophil cationic protein level a valuable tool of diagnosis in clinical practice?]

SCOPE

The eosinophil cationic protein (ECP) is one of the mediators released during eosinophil activation. These cells are effector cells taking part into the Th2-lymphocyte dependent allergic inflammation. Assaying ECP concentrations in blood and sputum may be useful in evaluating allergic inflammation (asthma and rhinitis). This summary considers the value of measuring ECP levels for the diagnosis of various diseases where an eosinophil-mediated tissue inflammation plays a role.

CURRENT SITUATION AND SALIENT POINTS

Levels of eosinophil cationic protein have been determined in nasal secretions, sputum, gastric secretions, feces and serum. They are increased during seasonal allergic rhinitis and perennial rhinitis, allergic asthma and atopic dermatitis. They are also increased in various gastro-intestinal disorders, some of which are associated with IgE: eosinophil intestinal diseases (esophagitis, gastro-enteritis and colitis), gastro-intestinal food allergy and intestinal parasitoses. Finally, they are increased in non IgE-dependent disorders: non allergic asthma with aspirin intolerance, respiratory infections, sinonasal polyposis, Churg-Strauss disease and idiopathic hyper-eosinophilia (HES) syndrome.

PERSPECTIVES

Assaying serum ECP could help in the diagnosis of several diseases. With parasitic disease the pathogenic progression may be accurately assessed, when serological tests are less indicative. ECP assay may point to non allergic asthma, either Fernand-Widal syndrome or Churg-Strauss disease. As for gastro-intestinal disorders, it indicates an eosinophilic tissue reaction. In the event of isolated hypereosinophilia, ECP assay may clarify whether it is benign or tending towards idiopathic HES. The assay of peroxidase and eosinophil-derived neurotoxin (EDN) should be also considered.