Entamoeba histolytica: cytopathogenicity, including serum effects on contact-dependent and toxin-induced lysis of hamster kidney cell monolayers

Extracellular Cysteine Proteases of Key Intestinal Protozoan Pathogens-Factors Linked to Virulence and Pathogenicity

Intestinal diseases caused by protistan parasites of the genera Giardia (giardiasis), Entamoeba (amoebiasis), Cryptosporidium (cryptosporidiosis) and Blastocystis (blastocystosis) represent a major burden in human and animal populations worldwide due to the severity of diarrhea and/or inflammation in susceptible hosts. These pathogens interact with epithelial cells, promoting increased paracellular permeability and enterocyte cell death (mainly apoptosis), which precede physiological and immunological disorders. Some cell-surface-anchored and molecules secreted from these parasites function as virulence markers, of which peptide hydrolases, particularly cysteine proteases (CPs), are abundant and have versatile lytic activities. Upon secretion, CPs can affect host tissues and immune responses beyond the site of parasite colonization, thereby increasing the pathogens' virulence. The four intestinal protists considered here are known to secrete predominantly clan A (C1- and C2-type) CPs, some of which have been characterized. CPs of Giardia duodenalis (e.g., Giardipain-1) and Entamoeba histolytica (EhCPs 1-6 and EhCP112) degrade mucin and villin, cause damage to intercellular junction proteins, induce apoptosis in epithelial cells and degrade immunoglobulins, cytokines and defensins. In Cryptosporidium, five Cryptopains are encoded in its genome, but only Cryptopains 4 and 5 are likely secreted. In Blastocystis sp., a legumain-activated CP, called Blastopain-1, and legumain itself have been detected in the extracellular medium, and the former has similar adverse effects on epithelial integrity and enterocyte survival. Due to their different functions, these enzymes could represent novel drug targets. Indeed, some promising results with CP inhibitors, such as vinyl sulfones (K11777 and WRR605), the garlic derivative, allicin, and purified amoebic CPs have been obtained in experimental models, suggesting that these enzymes might be useful drug targets.

Specific and reversible inhibition of Entamoeba histolytica cysteine-proteinase activities by Zn2+: implications for adhesion and cell damage

BACKGROUND

Cysteine-proteinases are thought to play an important role in E. histolytica pathogenicity. Although effective blockage of proteolytic activities can be obtained with several known inhibitors, the high cellular toxicity of most of the inhibitors precludes experimentation with live cells or animal models. Specific cysteine-proteinase inhibitors that could be utilized in studies of virulence are of great need in the field of amebiasis.

METHODS

Cysteine-proteinase activities were determined in trophozoite lysates by azocasein degradation and after PAGE and gelatin zymograms. Inhibition of the activities was assessed in the presence of 0.01-2.5 mM concentrations of divalent cations of the IIB and VIII series such as Zn, Cd, Hg, Ni, and Co. Reversibility was induced with 25 mM L-cysteine or 50 mM L-histidine and by metal chelation with 5 mM phenantroline. The inhibitory effect of ZnCl2 was tested with live cells in fibronectin-binding and cytotoxicity assays.

RESULTS

ZnCl2 specifically inhibited cysteine-proteinase activities in trophozoite lysates in a concentration-dependent manner. Additionally, 1.0-2.5 mM ZnCl2 blocked proteolysis in more than 70%. This inhibition was completely reverted by L-cysteine, L-histidine, or phenantroline. Similar results were obtained by analyzing individual cysteine-proteinase activities separated in gelatin zymograms. At these concentrations, ZnCl2 significantly interfered with trophozoite adhesion, thus making amebas deficient in substrate degradation and cell damage.

CONCLUSIONS

ZnCl2 is an effective inhibitor of amebic cysteine-proteinases. Its low toxicity at relatively high concentrations, high solubility, and low cost make it adequate for live cell experimentation and animal models of amebic virulence.

Destruction of normal human eosinophils by Entamoeba histolytica

We evaluated the in-vitro interaction of normal human eosinophil leucocytes and a virulent strain of Entamoeba histolytica (HM1-IMSS) in the presence of immune serum. At a 10:1 (eosinophil:amoeba) ratio a significant time-dependent destruction of eosinophils was found from the first hour onward, and a similar, albeit weaker, cytopathic effect was found in the 200:1 ratio mixtures, with some delay in the microscopic evidence of such effect. Results were unaffected by serum factors, and amoebae emerged virtually unharmed throughout these experiments, again regardless of the presence of serum factors. These results indicate that, as with neutrophil leucocytes, virulent E. histolytica is capable of destroying normal human eosinophils in vitro.

Entamoeba histolytica: role of amebic proteinases and polymorphonuclear leukocytes in acute experimental amebiasis in the rat

The injection of 1 x 10(6) trophozoites of axenically grown Entamoeba histolytica strain HM-1 in the subcutaneous tissue of the rat results in an acute and self-limited inflammatory process, characterized by the early onset of conspicuous tissue necrosis and focal hemorrhage in the vicinity of the parasites, followed by infiltration with polymorphonuclear leukocytes. The process develops for 5-10 hr but during that period amebic trophozoites progressively disappear, leukocytes undergo degenerative changes, and the lesion tends to heal in 72-96 hr. In leukopenic animals (less than 1000 white blood cells/ml) tissue necrosis and hemorrhage are equally conspicuous in the neighborhood of amebas. Inhibition of amebic proteinase activity prior to injection by heat denaturation, p-hydroxy-mercuri-benzoate (PHMB), soybean trypsin inhibitor (STI), and human alpha-2-macroglobulin (alpha 2M), alone or in various combinations, results in absence or notorious decrease in tissue necrosis as well as in clearly diminished inflammatory reaction. This effect is particularly evident when cysteine proteinases are either specifically or generally inhibited. On the other hand, amebic proteinase inhibition with alpha 2M and STI does not interfere with the cell-killing capacity of trophozoites co-incubated in vitro for 2 hr with rat peritoneal cells enriched for macrophages. We conclude that in acute experimental amebiasis produced in the subcutaneous tissue of the rat, amebic cysteine (and perhaps other) proteinases are primarily responsible for necrosis and are also important, but not essential, for inflammation. We also suggest that in this model polymorphonuclear leukocytes are not required for tissue necrosis. Finally, in an in vitro model, the cell-killing capacity of amebas is not influenced by the proteinase activity of the parasite.

Entamoeba histolytica: virulence potential and sensitivity to metronidazole and emetine of four isolates possessing nonpathogenic zymodemes

The pathogenic potential of four Entamoeba histolytica isolates obtained from asymptomatic carriers and possessing nonpathogenic zymodemes was compared to four E. histolytica strains obtained from invasive cases of amebiasis and having pathogenic zymodemes. Both xenic and axenic cultures of a number of strains were tested. Determinations of cytopathogenicity were done in vitro by measuring the rates of destruction of tissue cultured monolayers of baby hamster kidney cells by intact amebae or by its cell-free extracts. The in vivo virulence was tested by assessing their capacity to form hepatic abscesses in hamsters or cecal ulcerations in rats. The results obtained show that two of the isolates from asymptomatic carriers (strains SAW 1734R clAR and WI:0385:191) were as virulent as three of the invasive ones (HM-1:IMSS, 200:NIH, and SAW 408). Two other isolates from asymptomatic carriers and one from a dysentery case were avirulent. All the E. histolytica isolates tested were similarly sensitive to metronidazole and emetine (IC50 1-10 micrograms/ml). The results indicate that the pathogenic potential of E. histolytica varies between isolates and can be affected by culture conditions and by the presence or absence of bacterial cells. These findings suggest that virulence does not necessarily correlate with a pathogenic zymodeme.

Pathogenic and non-pathogenic Entamoeba: pore formation and hemolytic activity

Pore-forming activity in planar lipid bilayers and liposomes of extracts from differentially pathogenic Entamoeba and the capacity of trophozoites and subcellular fractions to lyse human red blood cells (hrbc) were investigated. In all amebas studied, the two activities paralleled each other. They were high in E. histolytica irrespective of the virulence of the particular strain, but low in non-pathogenic E. histolytica-like amebas of human origin as well as in E. invadens, which is pathogenic for reptiles, and in E. moshkovskii isolated from sewage. We conclude that the capacities to insert pores and to lyse are not sufficient for virulence although they may be necessary. The subcellular distribution of the hemolytic activity of E. histolytica and its sensitivity to a variety of inhibitors and activators differ from those of other known amebic cytotoxic activities including pore formation. Therefore, there may be an additional constituent of E. histolytica involved in the cytotoxicity of the parasite.

Cytopathogenicity of Entamoeba histolytica: the role of amebic adherence and contact-dependent cytolysis in pathogenesis

The mechanisms by which Entamoeba histolytica trophozoites adhere to and lyse target cells are reviewed from the perspective of pathogenesis. Adherence via the galactose and N-acetyl-galactosamine inhibitable amebic lectin and possible additional amebic adhesin molecules is followed by target cell death. Inhibition of the Gal/GalNAc lectin with GalNAc inhibits amebic cytolysis of target cells. Amebic activities implicated in the cytolytic event include vesicle exocytosis and maintenance of an acid pH, pore forming proteins, phospholipase A and proteases. Increased knowledge of the sequence of events leading to target cell lysis should lead to more effective treatment or prevention of infection by this enteric parasite and add to our basic understanding of eukaryotic cell-cell interactions.

The major neutral proteinase of Entamoeba histolytica

FPLC anion-exchange and chromatofocusing chromatography were used to purify the major neutral proteinase from secretions of axenically cultured Entamoeba histolytica trophozoites. HM-1 strain trophozoites, which were more proteolytically active than the less virulent HK-9 strain, were used for purification of the enzyme. It is a thiol proteinase with a subunit Mr of approximately 56,000, a neutral pH optimum, and a pI of 6. The importance of this enzyme in extraintestinal amoebiasis is suggested by its ability to degrade a model of connective tissue extracellular matrix as well as purified fibronectin, laminin, and type I collagen. The enzyme caused a loss of adhesion of mammalian cells in culture, probably because of its ability to degrade anchoring proteins. Experiments with a peptide substrate and inhibitors indicated that the proteinase preferentially binds peptides with arginine at P-1. It is also a plasminogen activator, and could thus potentiate host proteinase systems.

Inhibition of human monocyte locomotion by products of axenically grown E. histolytica

The supernatant fluid of axenically grown E. histolytica inhibits chemotaxis, chemokinesis and random mobility of human mononuclear phagocytes (MP) as measured in Boyden chambers. Human polymorphonuclear phagocytes (PMN) locomotion is apparently unaffected. The factor was found in comparable amounts in the supernatant fluid of axenic cultures of four E. histolytica strains that differed in their human pathogenicity and virulence, as well as in two entamoebas non-pathogenic for man. This dialysable and thermolabile MP-locomotion inhibiting entamoeba product (EP) can be absorbed out by incubation with MP, but not with lymphocytes, while partial absorption was observed using PMN. The MP-locomotion inhibitory effect of this EP was cancelled by inhibiting protein synthesis in the MP by means of cycloheximide. In vivo, this EP caused a delay in MP migration in Rebuck skin windows. The molecular weight of this EP lies between 478 and 765 by gel-sieve chromatography. Our results suggest a direct effect upon the cytoskeletal and locomotive apparatus of the MP. This MP-locomotion inhibiting EP could contribute to the paucity of the inflammatory reaction observed in the advanced stages of invasive amoebiasis and consequently also to the lack of scar tissue formation upon healing of amoebic lesions.

Entamoeba histolytica: purification of cathepsin B

A cytotoxic cysteine proteinase with a molecular weight of 16,000 was isolated from axenically grown trophozoites of Entamoeba histolytica. The enzyme was purified from frozen-thawed strain HM-1 by ion-exchange chromatography on DEAE-cellulose, organomercurial agarose affinity chromatography, and size-exclusion chromatography. The purified enzyme had proteinase activity that could be demonstrated on azocasein (pH 5), hemoglobin (pH 5), or carbobenzoxy-L-arginyl--L-arginyl-7-amino-4-trifluoromethylcoumarin++ + (Z-arg-arg-AFC), a substrate specific for cathepsin B. Enzyme activity was stable to high pH, but not to 40 C for 1 hr or 56 C for 0.5 hr. As typical of cysteine proteinases, inhibition of activity on Z-arg-arg-AFC by p-chloromercuribenzoate or mercury was reversed by free sulfhydryl groups. Both the proteinase and cytotoxic activities of the purified amoebal cathepsin B were inhibited by leupeptin and serum and activated by free sulfhydryl groups, supporting the hypothesis that both activities are characteristics of amoebal cathepsin B. Virulent strains of E. histolytica (HM-1 and Rahman) had significantly more cathepsin B activity per milligram protein than less virulent strains (HK-9, Laredo, and Huff). The correlation between higher levels of cathepsin B activity in strains with greater virulence could indicate a role for amoebal cathepsin B in the pathogenesis of amoebiasis.

Entamoeba histolytica and E. invadens: sulfhydryl-dependent proteolytic activity

Sodium dodecyl sulfate (SDS) and 2-mercaptoethanol (2-ME) activated proteolytic enzymes present in extracts of Entamoeba histolytica and E. invadens; SDS (0.5%) and 2-ME (1.4 and 715 mM) doubled the enzymatic activity when assayed on a stained insoluble substrate. Urea (4 M) did not reduce this activity, suggesting that amebic proteases are stable in the above denaturant conditions. Specific reagents for sulfhydryl (-SH) groups completely inhibited proteolytic activity regardless of pH. Inhibition with alkylating agents, such as N-ethylmaleimide and iodoacetamide, was reversed with 715 mM 2-ME as was also observed with papain. We conclude from these results that the main proteolytic enzymes contained in extracts of E. histolytica and E. invadens are dependent on free thiol groups.

Cytopathogenicity of Entamoeba histolytica

The lesions induced in man by Entamoeba histolytica are characterized by massive tissue injury in the absence of major local signs of a host immune response. The amoeba damages surrounding cells preferentially by contact-mediated cytolysis. Recently, a presumptive aetiological factor underlying this process has been identified. It is a protein, amoebapore, capable of spontaneous incorporation into host cell membranes. Therein it induces high conductance ion-channels which rapidly collapse the cellular transmembrane potential and lead to a prelytic state. Amoebapore is present within the amoeba in a highly aggregated state in a small, dense particle. It is shed into the medium in a particulate form by a stimulus-mediated process. Release is enhanced by addition of concanavalin A, lipopolysaccharide or the calcium ionophore A23187. Surface-labelling of intact amoeba, followed by fractionation of the homogenate in self-generating Percoll gradients, identified two labelled fractions, the plasma membrane and a particulate fraction sedimenting in the region of intracellular particulate amoebapore. This latter fraction appears to be material in the process of exocytosis. A highly immunogenic surface lipid has been identified and shown to be involved in the rapid surface redistribution of immune complexes, their shedding and endocytosis. The relevance of these findings to the immunoprophylaxis of amoebiasis is discussed.

Entamoeba histolytica: correlation between virulence and content of proteolytic enzymes

Intact trophozoites of the virulent Entamoeba histolytica strain HM-1:IMSS (HM-1) destroyed a monolayer of baby hamster kidney (BHK) cells at a higher rate and efficiency than trophozoites of the nonvirulent strain HK-9. The destructive effect could be partially attributed to the proteolytic activity of the amoeba, since quantitative differences were found in the enzymatic activity of the two strains tested. Crude extracts or secreted enzymes of HM-1 trophozoites digested Azocoll, as well as the bovine cold-insoluble globulin fraction, at a much higher rate than the corresponding preparations from HK-9. This proteolytic activity was found to be activated by free sulfhydryl groups. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the BHK cell proteins of pre- and postamoebic activities showed patterns similar to the trypsin effect on the same target cells. These enzymes were found to digest the proteins participating in the attachment of the target cells to the substrate and, consequently, cause detachment of these cells.

Correlation of virulence and collagenolytic activity in Entamoeba histolytica

The levels of collagenolytic activity of strains HM-1:1 MSS (HM-1), (HM-1), 200-NIH, and HK-9 of Entamoeba histolytica were compared. Collagen degradation was evaluated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Conditioned media as well as extracts of the highly virulent strain HM-1 effectively degraded native type I collagen. Significantly lower activity was found in the analogous fractions of strains 200-NIH and HK-9, which are not as virulent. The collagenolytic activity of strain HM-1 was associated with the isolated plasma membrane fraction and could be eluted from the membranes by buffers of high ionic strength, indicating that it is not an integral membrane protein. Unlike the vertebrate and the clostridial collagenases, the collagenolytic activity of E. histolytica HM-1 was enhanced in presence of dithiothreitol and was inhibited by N-ethylmaleimide. The correlation between virulence of the individual strains and their collagenolytic activity suggests that collagenase might play a role in pathogenesis of amoebiasis. The localization of the enzyme on the plasma membrane and its presence in the extracellular medium favor this view.