Arginine decarboxylase inhibitors reduce the capacity of Trypanosoma cruzi to infect and multiply in mammalian host cells

Non-genetic mechanisms communicating antibiotic resistance: rethinking strategies for antimicrobial drug design

INTRODUCTION

Infections by multidrug-resistant bacteria are of great concern worldwide. In many cases, resistance is not due to the presence of specific antibiotic-modifying enzymes, but rather associated with a general impermeability of the bacterial cell envelope. The molecular bases of this intrinsic resistance are not completely understood. Moreover, horizontal gene transfers cannot solely explain the spread of intrinsic resistance among bacterial strains.

AREAS COVERED

This review focuses on the increased intrinsic antibiotic resistance mediated by small molecules. These small molecules can either be secreted from bacterial cells of the same or different species (e.g., indole, polyamines, ammonia, and the Pseudomonas quinolone signal) or be present in the bacterial cell milieu, whether in the environment, such as indole acetic acid and other plant hormones, or in human tissues and body fluids, such as polyamines. These molecules are metabolic byproducts that act as infochemicals and modulate bacterial responses toward antibiotics leading to increasing or decreasing resistance levels.

EXPERT OPINION

The non-genetic mechanisms of antibiotic response modulation and communication discussed in this review should reorient our thinking of the mechanisms of intrinsic resistance to antibiotics and its spread across bacterial cell populations. The identification of chemical signals mediating increased intrinsic antibiotic resistance will expose novel critical targets for the development of new antimicrobial strategies.

Trypanosoma cruzi Coexpressing Ornithine Decarboxylase and Green Fluorescence Proteins as a Tool to Study the Role of Polyamines in Chagas Disease Pathology

Polyamines are essential for Trypanosoma cruzi, the causative agent of Chagas disease. As T. cruzi behaves as a natural auxotrophic organism, it relies on host polyamines biosynthesis. In this paper we obtained a double-transfected T. cruzi parasite that expresses the green fluorescent protein (GFP) and a heterologous ornithine decarboxylase (ODC), used itself as a novel selectable marker. These autotrophic and fluorescent parasites were characterized; the ODC presented an apparent Km for ornithine of 0.51 ± 0.16 mM and an estimated V_max value of 476.2 nmoles/h/mg of protein. These expressing ODC parasites showed higher metacyclogenesis capacity than the auxotrophic counterpart, supporting the idea that polyamines are engaged in this process. This double-transfected T. cruzi parasite results in a powerful tool—easy to follow by its fluorescence—to study the role of polyamines in Chagas disease pathology and in related processes such as parasite survival, invasion, proliferation, metacyclogenesis, and tissue spreading.

Polyamine metabolism in Trypanosoma cruzi: studies on the expression and regulation of heterologous genes involved in polyamine biosynthesis

Biochemical studies have shown that Trypanosoma cruzi and Toxoplasma gondii are the only eukaryotic organisms so far described which are auxotrophic for polyamines. Both parasites are unable to carry out the de novo biosynthesis of putrescine, and therefore they need the addition of exogenous polyamines to the culture medium for their normal proliferation. Further investigations at the molecular level have demonstrated that the wild-type T. cruzi genome does not contain ornithine or arginine decarboxylase-like nucleic acid sequences, and that the corresponding genes have been presumably lost during evolution. Since T. cruzi behaves as a deletion mutant for ornithine decarboxylase (ODC) and arginine decarboxylase (ADC) genes, this parasite has been selected to study the regulation of the expression of heterologous genes involved in polyamine biosynthesis in other organisms. The resulting transgenic parasites have been useful tools to analyze the different stages of gene expression after transformation, as well as the mechanisms of drug resistance induction and the post-translational processing of enzyme precursors.

Insights into the redox biology of Trypanosoma cruzi: Trypanothione metabolism and oxidant detoxification

Trypanosoma cruzi is the etiologic agent of Chagas' disease, an infection that affects several million people in Latin America. With no immediate prospect of a vaccine and problems associated with current chemotherapies, the development of new treatments is an urgent priority. Several aspects of the redox metabolism of this parasite differ enough from those in the mammalian host to be considered targets for drug development. Here, we review the information about a trypanosomatid-specific molecule centrally involved in redox metabolism, the dithiol trypanothione, and the main effectors of cellular antioxidant defense. We focus mainly on data from T. cruzi, making comparisons with other trypanosomatids whenever possible. In these parasites trypanothione participates in crucial thiol-disulfide exchange reactions and serves as electron donor in different metabolic pathways, from synthesis of DNA precursors to oxidant detoxification. Interestingly, the levels of several enzymes involved in trypanothione metabolism and oxidant detoxification increase during the transformation of T. cruzi to its mammalian-infective form and the overexpression of some of them has been associated with increased resistance to macrophage-dependent oxidative killing. Together, the evidence suggests a central role of the trypanothione-dependent antioxidant systems in the infection process.

Physiological polyamines: simple primordial stress molecules

Physiological polyamines are ubiquitous polycations with pleiotropic biochemical activities, including regulation of gene expression, cell proliferation and modulation of cell signalling. Reports that the polyamines with cytoprotective activities were induced by diverse stresses raised the hypothesis that physiological polyamines may play a role in inducing stress response. In a wide range of organisms, physiological polyamines were not only induced by diverse stresses, such as reactive oxygen species (ROS), heat, ultraviolet (UV) and psychiatric stress but were able to confer beneficial effects for survival. Recent biochemical and genetic evidences show that polyamines can function as an ROS scavenger, acid tolerance factor and chemical chaperone, and positive regulators for expression of stress response genes which may explain their protective functions against diverse stresses. Taken together, these data suggest that physiological polyamines can function as primordial stress molecules in bacteria, plants and mammals, and may play an essential role in regulation of pathogen-host interactions.

Targeting the polyamine biosynthetic enzymes: a promising approach to therapy of African sleeping sickness, Chagas' disease, and leishmaniasis

Trypanosomatids depend on spermidine for growth and survival. Consequently, enzymes involved in spermidine synthesis and utilization, i.e. arginase, ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (AdoMetDC), spermidine synthase, trypanothione synthetase (TryS), and trypanothione reductase (TryR), are promising targets for drug development. The ODC inhibitor alpha-difluoromethylornithine (DFMO) is about to become a first-line drug against human late-stage gambiense sleeping sickness. Another ODC inhibitor, 3-aminooxy-1-aminopropane (APA), is considerably more effective than DFMO against Leishmania promastigotes and amastigotes multiplying in macrophages. AdoMetDC inhibitors can cure animals infected with isolates from patients with rhodesiense sleeping sickness and leishmaniasis, but have not been tested on humans. The antiparasitic effects of inhibitors of polyamine and trypanothione formation, reviewed here, emphasize the relevance of these enzymes as drug targets. By taking advantage of the differences in enzyme structure between parasite and host, it should be possible to design new drugs that can selectively kill the parasites.

Ornithine decarboxylase and S-adenosylmethionine decarboxylase in trypanosomatids

The production of polyamines has been shown to be an effective target for a drug against the West African form of sleeping sickness caused by Trypanosoma brucei gambiense. T. brucei belongs to the group of protozoan parasites classed as trypanosomatids. Parasitic species of this group are the causative agents of various tropical diseases besides African sleeping sickness, e.g. Chagas' disease (Trypanosoma cruzi), cutaneous (Lesihmania spp.) and visceral (Leishmania donovani) leishmaniasis. The metabolism of polyamines in the parasites is a potential target for the development of new drugs for treatment of these diseases. The key steps in polyamine synthesis are catalysed by ODC (ornithine decarboxylase) and AdoMetDC (S-adenosylmethionine decarboxylase). In the present paper, some of the available information on ODC and AdoMetDC in trypanosomatids will be described and discussed.

The ornithine decarboxylase gene of Trypanosoma brucei: Evidence for horizontal gene transfer from a vertebrate source

Kinetoplastid protozoans in the family Trypanosomatidae are parasites, many of them responsible for serious diseases in humans and domestic animals. Ornithine decarboxlyase (ODC), a protein at the core of polyamine metabolism, is a potential target for therapies to overcome these diseases. Eukaryotic phylogenies were constructed from full-length genes for ODC to determine the origin of ODC in the kinetoplastid protozoans. The Odc genes from Trypanosoma brucei and two other African trypanosomes, T. congolense and T. vivax, clustered with Odc genes from vertebrates rather than with Odc genes from other kinetoplastids and other protozoans, making this gene a candidate for horizontal gene transfer from a vertebrate source. This result is unique to the Odc gene from the African trypanosomes as four other genes produced phylogenies consistent with the expected taxonomic relationships for the organisms. Analysis of the genomic regions around the Odc genes in Leishmania major, T. brucei, and Trypanosoma cruzi supports the hypothesis of loss of the Odc gene in the Trypanosoma lineage followed by acquisition of a new copy from a vertebrate host in the African branch of the genus.

L-arginine metabolism during interaction of Trypanosoma cruzi with host cells

Trypanosoma cruzi invades a diversity of nucleated cells in the mammalian host. Macrophages are among the first cells to be parasitized and, after activation by inflammatory stimuli, they participate in the control of infection. However, some parasites manage to evade the immune response and establish a chronic infection in differentiated cells. L-arginine is located at the crossroads of divergent routes that produce metabolites, including nitric oxide and polyamines, which influence the outcome (i.e. resolution or progression) of infection. This article discusses the fate and actions of L-arginine-derived biomolecules formed both in the host and in the parasite during T. cruzi-host-cell interactions.

Targeting enzymes involved in spermidine metabolism of parasitic protozoa--a possible new strategy for anti-parasitic treatment

Sequencing data obtained from the Plasmodium, Anopheles gambiae and human genome projects provide a new basis for drug and vaccine development. One of the most characteristic features in the process of drug development against parasitic protozoa is target identification in a biological pathway. The next step must be a structure-based rational drug design if the target is not only present in the parasite. In mouse models of malaria, such drugs should be tested for efficacy of the new therapies. Here, we present data that pinpoint the existence of two enzymes of the polyamine pathway involved in spermidine metabolism in P. falciparum, i.e. deoxyhypusine synthase (DHS; EC 1.1.1.249) and homospermidine synthase (HSS; EC 2.5.1.45). Recent data obtained from the malaria genome databases showed that at least a putative gene encoding DHS is present in the parasite. Sequencing data from the P. falciparum genome project prove that the eukaryotic initiation factor eIF5A (the substrate for DHS) exists in P. falciparum. Here, we present the amino acid sequence of eIF5A from P. vivax, which causes tertiary malaria. EIF5A from P. vivax shows 82% nucleic acid and 97% amino acid identity to its homologue from P. falciparum. GC/MS data and inhibitor studies with agmatine prove that the triamine homospermidine occurs in the parasite. These data suggest a separate locus encoding HSS in P. falciparum. The hss gene recruits from the dhs gene in eukaryotes. Here, we present genomic DNA fragments obtained by amplification with primers of a conserved region (amino acid positions 550-1,043) between the putative P. falciparum DHS gene ( dhs) and the HSS gene ( hss) from the plant Senecio vulgaris (Asteraceae). The amplification product from different P. falciparum strains reveals differences in sequence identity, compared with the putative dhs gene from P. falciparum strain 3D7. Expression of the full-length clone and determination of HSS-specific activity will finally prove whether a separate region encoding HSS exists.

Lack of Arginine Decarboxylase in Trypanosoma cruzi Epimastigotes

The presence of arginine decarboxylase (ADC) enzymatic activity in Trypanosoma cruzi epimastigotes is still a matter of controversy due to conflicting results published during the last few years. We have investigated whether arginine might indeed be a precursor of putrescine via agmatine in these parasites. We have shown that wild-type T. cruzi epimastigotes cultivated in a medium almost free of polyamines stopped their growth after several repeated passages of cultures in the same medium, and that neither arginine nor omithine were able to support or reinitiate parasite multiplication. In contrast, normal growth was quickly resumed after adding exogenous putrescine or spermidine. The in vivo labelling of parasites with radioactive arginine showed no conversion of this amino acid into agmatine, and attempts to detect ADC activity measured by the release of CO2 under different conditions in T. cruzi extracts gave negligible values for all strains assayed. The described data clearly indicate that wild-type T. cruzi epimastigotes lack ADC enzymatic activity.

Spermidine metabolism in parasitic protozoa--a comparison to the situation in prokaryotes, viruses, plants and fungi

Targeting polyamines of parasitic protozoa in chemotherapy has attracted attention because polyamines might reveal novel drug targets for antiparasite therapies (Müller et al. 2001). The biological function of the triamine spermidine in parasitic protozoa has not been studied in great detail although the results obtained mainly imply three different functions, i.e., cell proliferation, cell differentiation, and biosynthesis of macromolecules. Sequence information from the malaria genome project databases and inhibitor studies provide evidence that the current status of spermidine research has to be extended since enzymes of spermidine metabolism are present in the parasite (Kaiser et al. 2001). Isolation and characterisation of these enzymes, i.e., deoxyhypusine synthase (EC 1.1.1.249) (DHS) and homospermidine synthase (EC 2.5.1.44) (HSS) might lead to valuable new targets in drug therapy. Currently research on spermidine metabolism is based on the deposition of the deoxyhypusine synthase nucleic acid sequence in GenBank while the activity of homospermidine synthase was deduced from inhibitor studies. Spermidine biosynthesis is catalyzed by spermidine synthase (EC 2.5.1.16) which transfers an aminopropyl moiety from decarboxylated S-adenosylmethionine to putrescine. Spermidine is also an important precursor in the biosynthesis of the unusual amino acid hypusine (Wolff et al. 1995) and the uncommon triamine homospermidine in eukaryotes, in particular in pyrrolizidine alkaloid-producing plants (Ober and Hartmann 2000). Hypusine is formed by a two-step enzymatic mechanism starting with the transfer of an aminobutyl moiety from spermidine to the epsilon-amino group of one of the lysine residues in the precursor protein of eukaryotic initiation factor eIF5A by DHS (Lee and Park 2000). The second step of hypusinylation is completed by deoxyhypusine hydroxylase (EC 1.14.9929) (Abbruzzese et al. 1985). Homospermidine formation in eukaryotes parallels deoxyhypusine formation in the way that in an NAD(+)-dependent reaction an aminobutyl moiety is transferred from spermidine. In the case of homospermidine synthase, however the acceptor is putrescine. Thus the triamine homospermidine consists of two symmetric aminobutyl moieties while there is one aminobutyl and one aminopropyl moiety present in spermidine. Here, we review the metabolism of the triamine spermidine with particular focus on the biosynthesis of hypusine and homospermidine in parasitic protozoa, i.e., Plasmodium, Trypanosoma and Leishmania, compared to that in prokaryotes i.e., Escherichia coli, a phytopathogenic virus and pyrrolizidine alkaloid-producing plants (Asteraceae) and fungi.

L-arginine-dependent suppression of apoptosis in Trypanosoma cruzi: contribution of the nitric oxide and polyamine pathways

Until recently, a capacity for apoptosis and synthesis of nitric oxide *NO) were viewed as exclusive to multicellular organisms. The existence of these processes in unicellular parasites was recently described, with their biological significance remaining to be elucidated. We have evaluated L-arginine metabolism in Trypanosoma cruzi in the context of human serum-induced apoptotic death. Apoptosis was evidenced by the induction of DNA fragmentation and the inhibition of [3H]thymidine incorporation, which were inhibited by the caspase inhibitor Ac-Asp-Glu-Val-aspartic acid aldehyde (DEVD-CHO). In T. cruzi exposed to death stimuli, supplementation with L-arginine inhibited DNA fragmentation, restored [3H]thymidine incorporation, and augmented parasite *NO production. These effects were inhibited by the *NO synthase inhibitor N(omega)-nitroarginine methyl ester (L-NAME). Exogenous *NO limited DNA fragmentation but did not restore proliferation rates. Because L-arginine is also a substrate for arginine decarboxylase (ADC), and its product agmatine is a precursor for polyamine synthesis, we evaluated the contribution of polyamines to limiting apoptosis. Addition of agmatine, putrescine, and the polyamines spermine and spermidine to T. cruzi sustained parasite proliferation and inhibited DNA fragmentation. Also, the ADC inhibitor difluoromethylarginine inhibited L-arginine-dependent restoration of parasite replication rates, while the protection from DNA fragmentation persisted. In aggregate, these results indicate that T. cruzi epimastigotes can undergo programmed cell death that can be inhibited by L-arginine by means of (i) a *NO synthase-dependent *NO production that suppresses apoptosis and (ii) an ADC-dependent production of polyamines that support parasite proliferation.

Uptake of apoptotic cells drives the growth of a pathogenic trypanosome in macrophages

After apoptosis, phagocytes prevent inflammation and tissue damage by the uptake and removal of dead cells. In addition, apoptotic cells evoke an anti-inflammatory response through macrophages. We have previously shown that there is intense lymphocyte apoptosis in an experimental model of Chagas' disease, a debilitating cardiac illness caused by the protozoan Trypanosoma cruzi. Here we show that the interaction of apoptotic, but not necrotic T lymphocytes with macrophages infected with T. cruzi fuels parasite growth in a manner dependent on prostaglandins, transforming growth factor-beta (TGF-beta) and polyamine biosynthesis. We show that the vitronectin receptor is critical, in both apoptotic-cell cytoadherence and the induction of prostaglandin E2/TGF-beta release and ornithine decarboxylase activity in macrophages. A single injection of apoptotic cells in infected mice increases parasitaemia, whereas treatment with cyclooxygenase inhibitors almost completely ablates it in vivo. These results suggest that continual lymphocyte apoptosis and phagocytosis of apoptotic cells by macrophages have a role in parasite persistence in the host, and that cyclooxygenase inhibitors have potential therapeutic application in the control of parasite replication and spread in Chagas' disease.

Ornithine decarboxylase gene deletion mutants of Leishmania donovani

A knockout strain of Leishmania donovani lacking both ornithine decarboxylase (ODC) alleles has been created by targeted gene replacement. Growth of Deltaodc cells in polyamine-deficient medium resulted in a rapid and profound depletion of cellular putrescine pools, although levels of spermidine were relatively unaffected. Concentrations of trypanothione, a spermidine conjugate, were also reduced, whereas glutathione concentrations were augmented. The Deltaodc L. donovani exhibited an auxotrophy for polyamines that could be circumvented by the addition of the naturally occurring polyamines, putrescine or spermidine, to the culture medium. Whereas putrescine supplementation restored intracellular pools of both putrescine and spermidine, exogenous spermidine was not converted back to putrescine, indicating that spermidine alone is sufficient to meet the polyamine requirement, and that L. donovani does not express the enzymatic machinery for polyamine degradation. The lack of a polyamine catabolic pathway in intact parasites was confirmed radiometrically. In addition, the Deltaodc strain could grow in medium supplemented with either 1,3-diaminopropane or 1, 5-diaminopentane (cadaverine), but polyamine auxotrophy could not be overcome by other aliphatic diamines or spermine. These data establish genetically that ODC is an essential gene in L. donovani, define the polyamine requirements of the parasite, and reveal the absence of a polyamine-degradative pathway.

Structural, ultrastructural studies and evolution of Trypanosoma cruzi-infected mice treated with thioridazine

Thioridazine (THI) is a tricyclic drug that belongs to the phenothiazine series. THI had a lethal effect upon epimastigotes in culture medium in a concentration of 0.5 microM. Trypanocidal effect upon trypomastigotes of Trypanosoma cruzi was observed above 0.5 mM of THI. Ultrastructural studies showed intracellular vacuoles in both parasite forms and mitochondrion reorganization. To analyze the use of THI as a therapy, male mice were inoculated with 7.10(4) trypomastigotes of T. cruzi, Tulahuen strain, and treated with THI for 3 days, with 80 mg/kg/day. Survival and parasitemia of mice treated with lower doses were similar to those observed in the control group (14 days postinfection). THI treatment modified parasitemia levels. They were negative by day 20 p.i. Hearts from control untreated mice presented typical chagasic myocarditis. Hearts from THI-treated mice sacrificed by day 30 p.i. showed inflammatory infiltrates without amastigote nests. Three months postinfection a mild infiltrate located in the interventricular septum was observed. Survival of this group was 9 months. Present results show that THI had a direct effect upon parasitemia, improved survival of treated mice, and modified the evolution of experimental Chagas disease.

Trypanosoma cruzi has not lost its S-adenosylmethionine decarboxylase: characterization of the gene and the encoded enzyme

All attempts to identify ornithine decarboxylase in the human pathogen Trypanosoma cruzi have failed. The parasites have instead been assumed to depend on putrescine uptake and S-adenosylmethionine decarboxylase (AdoMetDC) for their synthesis of the polyamines spermidine and spermine. We have now identified the gene encoding AdoMetDC in T. cruzi by PCR cloning, with degenerate primers corresponding to conserved amino acid sequences in AdoMetDC proteins of other trypanosomatids. The amplified DNA fragment was used as a probe to isolate the complete AdoMetDC gene from a T. cruzi genomic library. The AdoMetDC gene was located on chromosomes with a size of approx. 1.4 Mbp, and contained a coding region of 1110 bp, specifying a sequence of 370 amino acid residues. The protein showed a sequence identity of only 25% with human AdoMetDC, the major differences being additional amino acids present in the terminal regions of the T. cruzi enzyme. As expected, a higher sequence identity (68-72%) was found in comparison with trypanosomatid AdoMetDCs. When the coding region was expressed in Escherichia coli, the recombinant protein underwent autocatalytic cleavage, generating a 33-34 kDa alpha subunit and a 9 kDa beta subunit. The encoded protein catalysed the decarboxylation of AdoMet (Km 0.21 mM) and was stimulated by putrescine but inhibited by the polyamines, weakly by spermidine and strongly by spermine. Methylglyoxal-bis(guanylhydrazone) (MGBG), a potent inhibitor of human AdoMetDC, was a poor inhibitor of the T. cruzi enzyme. This differential sensitivity to MGBG suggests that the two enzymes are sufficiently different to warrant the search for compounds that might interfere with the progression of Chagas' disease by selectively inhibiting T. cruzi AdoMetDC.

Diamine auxotrophy may be a universal feature of Trypanosoma cruzi epimastigotes

Polyamines play an important and central role in normal cell growth and differentiation in many cells. In trypanosomatids, spermidine is also an essential precursor in the biosynthesis of the unique glutathione-spermidine conjugate, trypanothione. Our previous study has shown that the epimastigote stage of Trypanosoma cruzi (Silvio strain) is incapable of significant de novo synthesis of putrescine or cadaverine from their amino acid precursors [Hunter, Le Quesne and Fairlamb (1994) Eur. J. Biochem. 226, 1019-1027]. In this study we show that when grown to late log phase in medium containing trace amounts of putrescine (0.22 microM) and spermidine (0.63 microM), Y-strain epimastigotes contain low levels of polyamines with free glutathione as their principal low molecular mass thiol (> 97% of total glutathione). Following passage into fresh medium, trypanothione and glutathionylspermidine content increase to 46% of total glutathione by mid log phase but returns to less than 3% by late log phase. In contrast, when supplemented at inoculation with exogenous putrescine, glutathione-spermidine conjugates reach 80% of total glutathione by early log phase and remain elevated throughout growth. Supplementation with exogenous putrescine or spermidine during polyamine starvation (late log phase) results in increased conjugate levels (> 74% of total glutathione) and is associated with large increases in total putrescine and spermidine. Likewise, supplementation with exogenous cadaverine and aminopropylcadaverine results in similar increases in trypanothione analogues and total cadaverine and aminopropylcadaverine. In contrast, ornithine, arginine, lysine, agmatine and other amino acid precursors have no effect on polyamine or conjugate levels. No significant ornithine or arginine decarboxylase activities could be detected (< 0.8 pmol min-1 [mg protein]-1). Similar results were obtained for epimastigotes representing all the major zymodeme classes, providing evidence that diamine auxotrophy may be a universal feature of this stage of the life-cycle.

Characterization of the peptide substrate specificity of glutathionylspermidine synthetase from Crithidia fasciculata

Trypanothione, a metabolite specific to trypanosomatid parasites, is enzymatically synthesized from spermidine and glutathione by the consecutive action of the ATP-dependent carbon-nitrogen ligases, glutathionylspermidine synthetase and trypanothione synthetase. As part of our programme aimed at developing inhibitors of these enzymes, we have synthesized a series of analogues of glutathione (gamma-L-Glu-L-Cys-Gly) and tested them as substrates or inhibitors of glutathionylspermidine synthetase. Recognition at the gamma-glutamyl moiety appears to be essential, as any modification of this part of glutathione results in a total loss of activity as a substrate. Alkylation of the thiol side chain of cysteine with methyl, ethyl or propyl groups yields analogues with catalytic efficiencies (kcat/Km) as substrates equivalent to or better than glutathione. In contrast, the bulkier S-butyl analogue was a much poorer substrate. Substitution of L-Cys by amino acids with an alkyl side-chain is also well tolerated giving relative catalytic efficiencies of 1.1 and 1.5 for peptide analogues containing L-Val and L-Ile respectively. Other analogues, where the bulk of the alkyl chain is increased further (as in L-Leu or L-Phe) or where the glycine moiety is replaced with L-Ala, are inhibitors rather than substrates.

Regulation of a high-affinity diamine transport system in Trypanosoma cruzi epimastigotes

Trypanosoma cruzi epimastigotes take up exogenous [3H]putrescine and [3H]cadaverine by a rapid, high-affinity, transport system that exhibits saturable kinetics (putrescine K(m) 2.0 microM, V(max) 3.3 nmol/min per 10(8) cells; cadaverine K(m) 13.4 microM, V(max) 3.9 nmol/min per 10(8) cells). Putrescine transport is temperature dependent and requires the presence of a membrane potential and thiol groups for activity. Its activity is altered in response to extracellular putrescine levels and as the cells proceed through the growth cycle. This transporter shows high specificity for the diamines putrescine and cadaverine, but low specificity for the polyamines spermidine and spermine. The existence of rapid diamine/polyamine transport systems whose activity can be adjusted in response to the growth conditions is of particular importance, as they seem unable to synthesize their own putrescine [Hunter, Le Quesne and Fairlamb (1994) Eur. J. Biochem. 226, 1019-1027].

Agmatine, a bioactive metabolite of arginine. Production, degradation, and functional effects in the kidney of the rat

Until recently, conversion of arginine to agmatine by arginine decarboxylase (ADC) was considered important only in plants and bacteria. In the following, we demonstrate ADC activity in the membrane-enriched fraction of brain, liver, and kidney cortex and medulla by radiochemical assay. Diamine oxidase, an enzyme shown here to metabolize agmatine, was localized by immunohistochemistry in kidney glomeruli and other nonrenal cells. Production of labeled agmatine, citrulline, and ornithine from [3H]arginine was demonstrated and endogenous agmatine levels (10(-6)M) in plasma ultrafiltrate and kidney were measured by HPLC. Microperfusion of agmatine into renal interstitium and into the urinary space of surface glomeruli of Wistar-Frömter rats produced reversible increases in nephron filtration rate (SNGFR) and absolute proximal reabsorption (APR). Renal denervation did not alter SNGFR effects but prevented APR changes. Yohimbine (an alpha 2 antagonist) microperfusion into the urinary space produced opposite effects to that of agmatine. Microperfusion of urinary space with BU-224 (microM), a synthetic imidazoline2 (I2) agonist, duplicated agmatine effects on SNGFR but not APR whereas an I1 agonist had no effect. Agmatine effects on SNGFR and APR are not only dissociable but appear to be mediated by different mechanisms. The production and degradation of this biologically active substance derived from arginine constitutes a novel endogenous regulatory system in the kidney.

Partial cloning and characterization of an arginine decarboxylase in the kidney

Using homology-based polymerase chain reaction (PCR) amplification, we demonstrate the presence of arginine decarboxylase mRNA in tissues involved in arginine metabolism (brain, kidney, gut, adrenal gland, and liver of the rat) but not in organs (lung, heart, and spleen) in which arginine metabolism is low or absent. The polymerase chain reaction product from the kidney had a nucleotide sequence 61% identical to that of the E. coli biosynthetic arginine decarboxylase. On a whole tissue basis, kidney homogenates were three times more active than brain homogenates at decarboxylating [1-14C]arginine. Subcellular fractionation localized the arginine decarboxylase activity of the kidney to the mitochondria fraction. Agmatine, one of the products of arginine decarboxylation, was found to inhibit nitric oxide formation by post-mitochondrial supernatants of the brain or kidney. We propose that arginine is metabolized to two structurally different signaling molecules, nitric oxide and agmatine. Furthermore, agmatine can influence the nitric oxide synthase pathway.

Identification and biosynthesis of N1,N9-bis(glutathionyl)aminopropylcadaverine (homotrypanothione) in Trypanosoma cruzi

Radiolabelling studies using tritiated ornithine, arginine and lysine, together with the relevant amino acid decarboxylase enzyme assays, indicate that the epimastigote stage of Trypanosoma cruzi is unable to synthesise significant amounts of putrescine and cadaverine de novo, compared to the amounts of these diamines scavenged from the growth medium. Radiolabelled putrescine is readily incorporated into spermidine, spermine and the trypanosomatid-specific polyamine-glutathione conjugate trypanothione (N1,N8-bis(glutathionyl)spermidine). Likewise, radiolabelled cadaverine is incorporated into the analogous polyamines aminopropylcadaverine, bis(aminopropyl)cadaverine and another major unidentified component. Subsequent studies showed this major component to be a novel polyamine-thiol conjugate whose structure was confirmed by chemical synthesis to be N1,N9-bis(glutathionyl)aminopropylcadaverine (homotrypanothione). Kinetic analyses using recombinant T. cruzi trypanothione reductase demonstrated that homotrypanothione disulphide is readily reduced by this enzyme with kinetic parameters similar to trypanothione disulphide, suggesting that it is a physiological substrate in vivo. Thus the epimastigote form of T. cruzi differs significantly from the African trypanosomes and Leishmania in (a) being unable to synthesise significant amounts of diamines de novo, (b) converting significant amounts of putrescine and cadaverine to spermine and bis(aminopropyl)cadaverine, respectively and (c) the ability to synthesise homotrypanothione as well as trypanothione. The implications of these findings with respect to the prospective chemotherapy of Chagas' disease are discussed.

Conduction defects and arrhythmias in Chagas' disease: possible role of gap junctions and humoral mechanisms

The protozoan parasite Trypanosoma cruzi causes Chagas' disease, a major cause of cardiac dysfunction in Latin Americans. Chagas' disease exhibits both acute and chronic phases, and each may be characterized by cardiac conduction disturbances. In acutely infected cultures of rodent heart cells, synchronized spontaneous beating becomes less regular, and coupling between cells is reduced. The basis of this decreased conduction is apparently in localization of the gap junction protein (Cx43) inside infected cells. Although total Cx43 is normal in infected cells, little is recognizable at appositional membranes. Electrophysiological properties are also altered by this infection. Action potentials are shortened, resting Ca2+ levels are elevated, and response to alpha-adrenergic agonists was altered, compared to controls. Humoral factors may contribute to the conduction defects in chronic Chagas' disease. Sera from chronically infected rabbits produced ECG abnormalities in Langendorff-perfused rabbit hearts. These findings indicate that chagasic infection may modify ion channel function in the heart, and we suggest that these changes may be manifested in the conduction disturbances that characterize this disease.

Inhibition of host cell invasion and intracellular replication of Trypanosoma cruzi by N,N'-bis(benzyl)-substituted polyamine analogs

We studied the effects of two N,N'-bis(benzyl)-substituted polyamine analogs on the capacities of Trypanosoma cruzi to invade and multiply within a mammalian host cell. At concentrations as low as 1 microM, these compounds reduced significantly the infectivity of the parasite for rat heart myoblasts in a time-dependent manner. Pretreatment of virulent T. cruzi trypomastigotes, but not myoblast pretreatment, reduced the level of infectivity. The inhibitory effects started to subside 3 h after removal of the drugs and were no longer detectable after 4 h. A significant decrease in the rate of intracellular amastigote multiplication was also seen when the drugs were added to myoblast cultures which had been previously infected with untreated T. cruzi. These results show that N,N'-bis(benzyl)-substituted polyamine analogs meet the two most important criteria for potential chemotherapeutic agents against T. cruzi infection, namely, inhibition of both host cell invasion and intracellular replication by this parasite.

Synergism between tumor necrosis factor-alpha and interferon-gamma on macrophage activation for the killing of intracellular Trypanosoma cruzi through a nitric oxide-dependent mechanism

Intracellular replication of the protozoan parasite Trypanosoma cruzi inside macrophages is essential for the production of the disease and the development of the parasite. Two CD4+ T cell lines, A10 and A28, were established from T. cruzi-infected BALB/c mice which specifically proliferated to parasite antigens. The trypanocidal activity of BALB/c macrophages was induced upon culture with the A10, but not with the A28 T cell line. The cell-free supernatant from this A10 line, as well as from immune spleen cells stimulated with specific antigen or concanavalin A, but not from the A28 T cell line also activated the trypanocidal activity of peritoneal macrophages or of the J774 macrophage-like cell line. when the lymphokine content of the supernatants from both cell lines was analyzed, it was found that the A10 T cell line secreted interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha and interleukin 2, whereas the A28 line did not secrete IFN-gamma upon stimulation. Furthermore, the trypanocidal-inducing ability of A10 supernatant was completely abrogated by neutralizing anti-IFN-gamma antibodies and partially abrogated by neutralizing anti-TNF-alpha antibodies. When recombinant cytokines were added to J774 cells, IFN-gamma was able to induce significant trypanocidal activity whereas TNF-alpha was almost ineffective. However, TNF-alpha or lipopolysaccharide (LPS) showed a synergistic effect with IFN-gamma on macrophage activation. IFN-gamma triggered nitric oxide (NO) synthesis by J774 cells whereas TNF-alpha was almost ineffective. TNF-alpha and LPS were also synergistic with IFN-gamma in the NO production. Both the NO production and the trypanocidal activity in J774 cells induced by T cell supernatants or lymphokine combinations were inhibited by N-monomethyl-L-arginine, a competitive inhibitor of NO synthase activity. A good correlation between the levels of NO production and trypanocidal activity induced by different lymphokine preparations was found. Those results suggest that IFN-gamma and TNF-alpha, secreted by T. cruzi-immune T cells, are involved in the activation of the trypanocidal activity of mouse macrophages through an NO-dependent mechanism.

Binding of the specific ligand to Fc receptors on Trypanosoma cruzi increases the infective capacity of the parasite

The infective capacity of Trypanosoma cruzi was significantly increased after treatment with monoclonal IgG1 antibodies, whether or not specific for the parasite; minimal or no change in infectivity was seen after treatment with IgG2a, IgG2b or IgG3 monoclonal antibodies. The stimulatory effect was evidenced by elevated numbers of trypanosomes invading mammalian host cells in vitro compared to parasites treated with medium alone. Greater infectivity was also induced by pure human Fc, suggesting a role for Fc receptors on the organism. This inference received support in the fact that protein A inhibited the stimulatory effect of Fc. In addition, Fc-treated parasites incubated with fluorescein-labelled F(ab')2 from goat anti-human IgG exhibited fluorescence detectable by both ultraviolet microscopy and flow cytometry. 125I-Fc binding to T. cruzi was found to be saturable at 0 degrees and was inhibited by cold Fc but not by bovine serum albumin (BSA) or orosomucoid. Interestingly, 125I-Fc binding was greater at 37 degrees and it was not saturable with the concentrations that did saturate at 0 degrees. Possibly, Fc might up-regulate expression of its own receptor and greater endocytosis could take place at 37 degrees. Significant increases in infectivity were detectable after a 40 min pretreatment with Fc--hinting that Fc could trigger a chain of biochemical events underlying the phenomenon--and were reversible, becoming undetectable 2 hr after Fc removal. The average number of Fc receptors per parasite, determined at 0 degrees (at which binding saturation was possible), was estimated as 5 x 10(5), the dissociation constant was of the order of 10(-6)-10(7)M. The present results define an important biological role for an Fc-binding T. cruzi surface component and expose the capacity of this organism to exploit even elements of the immune system in its quest to attain intracellular localization, required for multiplication.

Regulation of Trypanosoma cruzi infectivity by alpha- and beta-adrenergic agonists: desensitization produced by prolonged treatments or increasing agonist concentrations

We previously reported that blood forms of Trypanosoma cruzi express alpha- and beta-adrenergic receptors and that binding of specific agonists to these receptors modifies the infective capacity of the parasite in vitro. The present study has revealed that the inhibitory effect of the beta-adrenergic agonist L-isoproterenol and the stimulatory effect of the alpha-adrenergic agonist L-phenylephrine are not produced when the parasite is subjected to prolonged exposure to otherwise effective doses of these agonists or when supraoptimal doses of these agonists are used. We refer to these phenomena as 'desensitization' because of their analogy with vertebrate cells becoming desensitized by prolonged exposure to, or relatively high concentrations of, adrenergic agonists. At a constant agonist concentration, T. cruzi desensitization was time-dependent and, when the time of parasite treatment with the agonists was not changed, the higher concentrations of the agonist tested were the most effective in producing desensitization. The reduced infectivity resulting from treatment with optimal doses of L-isoproterenol was accompanied by elevated levels of cyclic adenosine monophosphate (cAMP) which were not detectable when L-isoproterenol concentrations producing desensitization were used. This finding implicated cAMP as a likely second signal in the inhibitory mechanisms of this agonist. No significant change in cAMP was detectable in parasites treated with L-phenylephrine, leaving open the question about how optimal doses of this alpha-adrenergic agonist enhance T. cruzi infectivity. Parasite responsiveness to alpha- and beta-adrenergic agonists as well as the desensitization effects define a system which regulates infectivity and could be modified at the host tissue level by naturally occurring agonists.

Metabolic fate of L-arginine in relation to microbiostatic capability of murine macrophages

L-arginine is required for the fungistatic action of murine macrophages in vitro. To further investigate this requirement, L-arginine metabolism by macrophages was measured under conditions where fungistasis either succeeded or failed. Macrophage fungistasis correlated with metabolism of L-arginine to citrulline, nitrite, and nitrate. The metabolic rate was dependent on extracellular L-arginine concentration, reaching a maximum of 67 nmol nitrite/h per mg protein. It accounted for one-third of arginine consumed by fungistatic macrophages. Equimolar amounts of citrulline and total nitrite plus nitrate accumulated in medium. This was consistent with the hypothesis that one of the equivalent guanidino nitrogens of L-arginine was oxidized to both nitrite and nitrate leaving L-citrulline as the amino acid reaction product. The analogue, NG-mono-methyl-L-arginine, selectively inhibited nitrogen oxidation and it was shown previously that it inhibited fungistatic capability. Resident macrophages were not fungistatic and their nitrogen oxidation was low. Once macrophages began producing nitrite/nitrate, protein synthesis was not required during the next 8 h for either fungistasis or nitrogen oxidation. Two-thirds of L-arginine consumption was due to macrophage arginase yielding L-ornithine and urea, which accumulated in medium. This activity was dissociated from macrophage fungistasis. Nitrogen oxidation metabolism by macrophages is linked to a mechanism that inhibits proliferation of fungi. This may involve synthesis of an intermediate compound(s) that has antimicrobial properties.

Eflornithine. A new drug in the treatment of sleeping sickness

Eflornithine (alpha-difluoromethylornithine; DFMO) is a recently developed drug against African trypanosomiasis (sleeping sickness). After a short description of trypanosomiasis and the current treatment, the mechanism of action of eflornithine is discussed, some clinical data is given and attention is paid to recently discovered analogues of eflornithine. Some examples of combination therapy with eflornithine and possible applications are mentioned.

Novel biochemical pathways in parasitic protozoa

Throughout evolution, enzymes and their metabolites have been highly conserved. Parasites are no exception to this and differ most markedly by the absence of metabolic pathways that are present in the mammalian host. In general, parasites are metabolically lazy and rely on the metabolism of the host both for a supply of prefabricated components such as purines, fatty acids, sterols and amino acids and for the removal of end-products. Nonetheless, parasites are metabolically highly sophisticated in that (1) they retain the genetic capacity to induce many pathways, when needed, and (2) they have developed complex mechanisms for their survival in the host. Certain unique features of the metabolism of trypanosomes, leishmania, malaria and anaerobic protozoa will be discussed. This will include (1) glycolysis and electron transport with reference to the unique organelles: the glycosome and the hydrogenosome, (2) purine salvage, pyrimidine biosynthesis and folic acid metabolism and (3) polyamine and thiol metabolism with special reference to the role of the unique metabolite of trypanosomes and leishmanias, trypanothione.

DL-alpha-difluoromethyl[3,4-3H]arginine metabolism in tobacco and mammalian cells. Inhibition of ornithine decarboxylase activity after arginase-mediated hydrolysis of DL-alpha-difluoromethylarginine to DL-alpha-difluoromethylornithine

DL-alpha-Difluoromethylarginine (DFMA) is an enzyme-activated irreversible inhibitor of arginine decarboxylase (ADC) in vitro. DFMA has also been shown to inhibit ADC activities in a variety of plants and bacteria in vivo. However, we questioned the specificity of this inhibitor for ADC in tobacco ovary tissues, since ornithine decarboxylase (ODC) activity was strongly inhibited as well. We now show that [3,4-3H]DFMA is metabolized to DL-alpha-difluoromethyl[3,4-3H]ornithine [( 3,4-3H]DFMO), the analogous mechanism-based inhibitor of ODC, by tobacco tissues in vivo. Both tobacco and mammalian (mouse, bovine) arginases (EC 3.5.3.1) hydrolyse DFMA to DFMO in vitro, suggesting a role for this enzyme in mediating the indirect inhibition of ODC by DFMA in tobacco. These results suggest that DFMA may have other effects, in addition to the inhibition of ADC, in tissues containing high arginase activities. The recent development of potent agmatine-based ADC inhibitors should permit selective inhibition of ADC, rather than ODC, in such tissues, since agmatine is not a substrate for arginase.

Impairment of macrophage function by inhibitors of ornithine decarboxylase activity

The effects of irreversible inhibition of ornithine decarboxylase on the capacity of murine macrophages to take up a protozoan organism (Trypanosoma cruzi) or inert particles were investigated. Incubation of macrophage cultures with four different ornithine decarboxylase inhibitors, namely, DL-alpha-difluoromethylornithine (DFMO, 0.5 to 20 mM), delta-methyl-acetylenic putrescine (1 to 5 mM), monofluoromethyldehydroornithine ethyl ester (1 to 5 mM), and monofluoromethyldehydroornithine methyl ester (1 to 5 mM), before the addition of the parasites significantly reduced the percentage of macrophages with parasites, indicating that some of the host cells were no longer capable of binding or ingesting the parasite. The average number of trypanosomes per 100 macrophages was also diminished, denoting a lesser phagocytic capacity as a consequence of the treatments. These effects were reversible within 2 h after removal of excess DFMO. No alteration in parasite-macrophage interaction was seen when the trypanosomes were treated with DFMO. That the effects of DFMO on the macrophages probably resulted from a reduction in polyamine levels caused by inhibition of ornithine decarboxylase was indicated by the fact that these effects were not seen when the macrophages were incubated with DFMO in the presence of putrescine, the product of ornithine decarboxylation by ornithine decarboxylase. DFMO treatment of macrophages also inhibited the capacity of these cells to ingest killed parasites or latex beads and thus appeared to generally affect phagocytosis. An effect of DFMO on the susceptibility of macrophages to penetration by the parasites seemed less likely because no significant alteration in cell-parasite association occurred when myoblasts--which, not being phagocytic, can be infected only by membrane penetration--were treated with DFMO. Taken together, these results emphasize a role of ornithine decarboxylase activity and polyamine biosynthesis in macrophage function.