Influx of calcium through a redox-sensitive plasma membrane channel in thymocytes causes early necrotic cell death induced by the epipolythiodioxopiperazine toxins

The cellular and molecular toxicity of sporidesmin

The fungal metabolite sporidesmin is responsible for the hepatogenous photosensitising disease facial eczema in livestock. Toxicity is due to a sulfur-bridged epidithiodioxopiperazine ring that has wide biological reactivity. The ways in which the toxin causes hepatobiliary and other tissue damage have not been established. Hypotheses include direct interaction with cellular thiols including protein cysteine residues or production of reactive oxygen species resulting in oxidative stress. Comparison with the cellular effects of the structurally related compound gliotoxin suggests additional mechanisms including interaction with cell adhesion complexes and possible downstream consequences for regulated necrosis as a response to tissue injury. Revision of hypotheses of how sporidesmin affects cells has the potential to generate new strategies for control of facial eczema including through identification of proteins and genes that are associated with resistance to the disease.

Novel Action Targets of Natural Product Gliotoxin in Photosynthetic Apparatus

Gliotoxin (GT) is a fungal secondary metabolite that has attracted great interest due to its high biological activity since it was discovered by the 1930s. It exhibits a unique structure that contains a N-C = O group as the characteristics of the classical PSII inhibitor. However, GT's phytotoxicity, herbicidal activity and primary action targets in plants remain hidden. Here, it is found that GT can cause brown or white leaf spot of various monocotyledonous and dicotyledonous plants, being regarded as a potential herbicidal agent. The multiple sites of GT action are located in two photosystems. GT decreases the rate of oxygen evolution of PSII with an I 50 value of 60 µM. Chlorophyll fluorescence data from Chlamydomonas reinhardtii cells and spinach thylakoids implicate that GT affects both PSII electron transport at the acceptor side and the reduction rate of PSI end electron acceptors' pool. The major direct action target of GT is the plastoquinone QB-site of the D1 protein in PSII, where GT inserts in the QB binding niche by replacing native plastoquinone (PQ) and then interrupts electron flow beyond plastoquinone QA. This leads to severe inactivation of PSII RCs and a significant decrease of PSII overall photosynthetic activity. Based on the simulated modeling of GT docking to the D1 protein of spinach, it is proposed that GT binds to the-QB-site through two hydrogen bonds between GT and D1-Ser264 and D1-His252. A hydrogen bond is formed between the aromatic hydroxyl oxygen of GT and the residue Ser264 in the D1 protein. The 4-carbonyl group of GT provides another hydrogen bond to the residue D1-His252. So, it is concluded that GT is a novel natural PSII inhibitor. In the future, GT may have the potential for development into a bioherbicide or being utilized as a lead compound to design more new derivatives.

Novel epidithiodiketopiperazines as anti-viral zinc ejectors of the Feline Immunodeficiency Virus (FIV) nucleocapsid protein as a model for HIV infection

Focused libraries of multi-substituted epidithiodiketopiperazines (ETP) were prepared and evaluated for efficacy of inhibiting the nucleocapsid protein function of the Feline Immunodeficiency Virus (FIV) as a model for HIV. This activity was compared and contrasted to observed toxicity utilising an in-vitro cell culture approach. This resulted in the identification of several promising lead compounds with nanomolar potency in cells with low toxicity and a favorable therapeutic index.

Aspergillosis, a Natural Infection in Poultry: Mycological and Molecular Characterization and Determination of Gliotoxin in Aspergillus fumigatus Isolates

Aspergillosis affects all types of birds; it causes the loss of specimens with high ecologic value and also leads to significant economic losses within the poultry industry. The main etiologic agent is Aspergillus fumigatus , a filamentary fungus with multiple virulence factors, such as gliotoxin (GT), which is an immunosuppressive epipolythiodioxopiperazine molecule. Necropsy was performed on 73 poultry from different provenances, all of which presented with a respiratory semiology compatible with aspergillosis. A mycological culture was performed on the injured lungs of diseased birds, as was chloroform extraction of the GT, a thin-layer chromatography analysis (TLC), and a histopathology analysis with hematoxylin-eosin and Grocott stainings. The A. fumigatus identification was confirmed by PCR, where the ITS 1 5.1-5.8S-ITS 2 fragment of the rDNA complex was amplified. The in vitro GT production was studied by TLC in the recovered isolates from A. fumigatus . Seven isolates of A. fumigatus were obtained and in six of them, GT-like compounds were detected. In a lung sample, a compound with the same retention time (RF) as the reference GT was detected; whereas RF compounds different from the GT standard were observed in three lung samples.

The Epipolythiodiketopiperazine Gene Cluster in Claviceps purpurea: Dysfunctional Cytochrome P450 Enzyme Prevents Formation of the Previously Unknown Clapurines

Claviceps purpurea is an important food contaminant and well known for the production of the toxic ergot alkaloids. Apart from that, little is known about its secondary metabolism and not all toxic substances going along with the food contamination with Claviceps are known yet. We explored the metabolite profile of a gene cluster in C. purpurea with a high homology to gene clusters, which are responsible for the formation of epipolythiodiketopiperazine (ETP) toxins in other fungi. By overexpressing the transcription factor, we were able to activate the cluster in the standard C. purpurea strain 20.1. Although all necessary genes for the formation of the characteristic disulfide bridge were expressed in the overexpression mutants, the fungus did not produce any ETPs. Isolation of pathway intermediates showed that the common biosynthetic pathway stops after the first steps. Our results demonstrate that hydroxylation of the diketopiperazine backbone is the critical step during the ETP biosynthesis. Due to a dysfunctional enzyme, the fungus is not able to produce toxic ETPs. Instead, the pathway end-products are new unusual metabolites with a unique nitrogen-sulfur bond. By heterologous expression of the Leptosphaeria maculans cytochrome P450 encoding gene sirC, we were able to identify the end-products of the ETP cluster in C. purpurea. The thioclapurines are so far unknown ETPs, which might contribute to the toxicity of other C. purpurea strains with a potentially intact ETP cluster.

Gliotoxin promotes Aspergillus fumigatus internalization into type II human pneumocyte A549 cells by inducing host phospholipase D activation

The internalization of Aspergillus fumigatus into lung epithelial cells is critical for the infection process in the host. Gliotoxin is the most potent toxin produced by A. fumigatus. However, its role in A. fumigatus internalization into the lung epithelial cells is still largely unknown. In the present study, the deletion of the gliP gene regulating the production of gliotoxin in A. fumigatus suppressed the internalization of conidia into the A549 lung epithelial cells, and this suppression could be rescued by the exogenous addition of gliotoxin. At lower concentrations, gliotoxin enhanced the internalization of the conidia of A. fumigatus into A549 cells; in contrast, it inhibited the phagocytosis of J774 macrophages in a dose-dependent manner. Under a concentration of 100 ng/ml, gliotoxin had no effect on A549 cell viability but attenuated ROS production in a dose-dependent manner. Gliotoxin significantly stimulated the phospholipase D activity in the A549 cells at a concentration of 50 ng/ml. This stimulation was blocked by the pretreatment of host cells with PLD1- but not PLD2-specific inhibitor. Morphological cell changes induced by gliotoxin were observed in the A549 cells accompanying with obvious actin cytoskeleton rearrangement and a moderate alteration of phospholipase D distribution. Our data indicated that gliotoxin might be responsible for modulating the A. fumigatus internalization into epithelial cells through phospholipase D1 activation and actin cytoskeleton rearrangement.

Phytotoxic secondary metabolites and peptides produced by plant pathogenic Dothideomycete fungi

Many necrotrophic plant pathogenic fungi belonging to the class of Dothideomycetes produce phytotoxic metabolites and peptides that are usually required for pathogenicity. Phytotoxins that affect a broad range of plant species are known as non-host-specific toxins (non-HSTs), whereas HSTs affect only a particular plant species or more often genotypes of that species. For pathogens producing HSTs, pathogenicity and host specificity are largely defined by the ability to produce the toxin, while plant susceptibility is dependent on the presence of the toxin target. Non-HSTs are not the main determinants of pathogenicity but contribute to virulence of the producing pathogen. Dothideomycetes are remarkable for the production of toxins, particularly HSTs because they are the only fungal species known so far to produce them. The synthesis, regulation, and mechanisms of action of the most important HSTs and non-HSTs will be discussed. Studies on the mode of action of HSTs have highlighted the induction of programed cell death (PCD) as an important mechanism. We discuss HST-induced PCD and the plant hypersensitive response upon recognition of avirulence factors that share common pathways. In this respect, although nucleotide-binding-site-leucine-rich repeat types of resistance proteins mediate resistance against biotrophs, they can also contribute to susceptibility toward necrotrophs.

Strategies for the real-time detection of Ca2+ channel events of single cells: recent advances and new possibilities

Ca(2+) ion channels play key roles in cell physiology and they are important drug targets. The Ca(2+) channel events are mainly measurable by fluorescent and patch clamp methods. This review summarizes the recent advances of these techniques for the detection of Ca(2+) channel events and the prospect of their new directions in the near future. Conventional bulk fluorescent methods are amenable to high-throughput applications, but they are not real-time single-cell measurements, which provide kinetic data on individual cells and offer unparalleled sensitive data for rare cells. Recent advances on real-time single-cell fluorescent measurements are conducted on microfluidic chips with scalable cell-retention sites, integrated with electrical stimulation and fluorescent measuring features. Patch clamp techniques are real-time measurements conducted on single cells, but the measurements are of low throughput. Recent advances are conducted on microfluidic patch clamp chips for high-throughput applications. Future real-time single-cell Ca(2+) channel event measurements will be conducted in a multiparametric manner in an integrated and automated microfluidic chip.

Redox-directed cancer therapeutics: molecular mechanisms and opportunities

Redox dysregulation originating from metabolic alterations and dependence on mitogenic and survival signaling through reactive oxygen species represents a specific vulnerability of malignant cells that can be selectively targeted by redox chemotherapeutics. This review will present an update on drug discovery, target identification, and mechanisms of action of experimental redox chemotherapeutics with a focus on pro- and antioxidant redox modulators now in advanced phases of preclinal and clinical development. Recent research indicates that numerous oncogenes and tumor suppressor genes exert their functions in part through redox mechanisms amenable to pharmacological intervention by redox chemotherapeutics. The pleiotropic action of many redox chemotherapeutics that involves simultaneous modulation of multiple redox sensitive targets can overcome cancer cell drug resistance originating from redundancy of oncogenic signaling and rapid mutation.Moreover, some redox chemotherapeutics may function according to the concept of synthetic lethality (i.e., drug cytotoxicity is confined to cancer cells that display loss of function mutations in tumor suppressor genes or upregulation of oncogene expression). The impressive number of ongoing clinical trials that examine therapeutic performance of novel redox drugs in cancer patients demonstrates that redox chemotherapy has made the crucial transition from bench to bedside.

Phytotoxins, Elicitors and Other Secondary Metabolites from Phytopathogenic “Blackleg” Fungi: Structure, Phytotoxicity and Biosynthesis

The metabolites produced by the fungal species Leptosphaeria maculans and L. biglobosa under different culture conditions, together with their phytotoxic activities are reviewed. In addition, the biosynthetic studies of blackleg metabolites carried out to date are described and suggestions for species reclassification are provided.

Role of intracellular calcium and S-glutathionylation in cell death induced by a mixture of isothiazolinones in HL60 cells

Previously we reported that brief exposure of HL60 cells to a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI) shifts the cells into a state of oxidative stress that induces apoptosis and necrosis. In this study, flow cytometric analysis showed that CMI/MI induces early perturbation of calcium homeostasis, increasing cytosolic and mitochondrial calcium and depleting the intracellular endoplasmic reticulum (ER) stores. The calcium chelator BAPTA-AM reduced necrosis and secondary necrosis, the loss of DeltaPsim and S-glutathionylation induced by necrotic doses of CMI/MI, but did not protect against CMI/MI-induced apoptosis, mitochondrial calcium uptake and mitochondrial hyperpolarization. This indicates that increased cytoplasmic calcium does not have a causal role in the induction of apoptosis, while cross-talk between the ER and mitochondria could be responsible for the induction of apoptosis. GSH-OEt pretreatment, which enhances cellular GSH content, reduced S-glutathionylation and cytosolic and mitochondrial calcium levels, thus protecting against both apoptosis and necrosis shifting to apoptosis. Therefore, the degree of GSH depletion, paralleled by the levels of protein S-glutathionylation, may have a causal role in increasing calcium levels. The mitochondrial calcium increase could be responsible for apoptosis, while necrosis is associated with cytoplasmic calcium overload. These findings suggest that S-glutathionylation of specific proteins acts as a molecular linker between calcium and redox signalling.

Transcriptional profiling identifies a role for BrlA in the response to nitrogen depletion and for StuA in the regulation of secondary metabolite clusters in Aspergillus fumigatus

Conidiation (asexual sporulation) is a key developmental process in filamentous fungi. We examined the gene regulatory roles of the Aspergillus fumigatus developmental transcription factors StuAp and BrlAp during conidiation. Conidiation was completely abrogated in an A. fumigatus DeltabrlA mutant and was severely impaired in a DeltastuA mutant. We determined the full genome conidiation transcriptomes of wild-type and DeltabrlA and DeltastuA mutant A. fumigatus and found that BrlAp and StuAp governed overlapping but distinct transcriptional programs. Six secondary metabolite biosynthetic clusters were found to be regulated by StuAp, while only one cluster exhibited BrlAp-dependent expression. The DeltabrlA mutant, but not the DeltastuA mutant, had impaired downregulation of genes encoding ribosomal proteins under nitrogen-limiting, but not carbon-limiting, conditions. Interestingly, inhibition of the target of rapamycin (TOR) pathway also caused downregulation of ribosomal protein genes in both the wild-type strain and the DeltabrlA mutant. Downregulation of these genes by TOR inhibition was associated with conidiation in the wild-type strain but not in the DeltabrlA mutant. Therefore, BrlAp-mediated repression of ribosomal protein gene expression is not downstream of the TOR pathway. Furthermore, inhibition of ribosomal protein gene expression is not sufficient to induce conidiation in the absence of BrlAp.

Real-time detection of the early event of cytotoxicity of herbal ingredients on single leukemia cells studied in a microfluidic biochip

A microfluidic approach has been developed for the real-time detection of drug effects, based on the quantitative measurement of calibrated cytosolic calcium ([Ca(2+)](i)) on single cancer cells. This microfluidic method is rapid by detecting the early event of cytotoxicity of drug candidates on cancer cells, without waiting for a couple of days needed for cell seeding and drug treatment by conventional assays. The miniaturized biochip consists of a V-shaped structure for the single-cell selection and retention. Various test reagents such as the chemotherapy drug (daunorubicin), an ionophore (ionomycin), and herbal ingredients from licorice (isoliquiritigenin or IQ) were investigated for their abilities to stimulate sustained cellular [Ca(2+)](i) elevations. The microfluidic results obtained in hours have been confirmed by conventional cytotoxicity assays which take days to complete. Moreover, any color or chemical interference problems found in the conventional assays of herbal compounds could be resolved. Using the microfluidic approach, IQ (50 microM) has been found to cause a sustained [Ca(2+)](i) elevation and cytotoxic effects on leukemia cells. The microfluidic single-cell analysis not only reduces reagent cost, and demands less cells, but also reveals some phenomena due to cellular heterogeneity that cannot be observed in bulk analysis.

Production of the toxin sirodesmin PL by Leptosphaeria maculans during infection of Brassica napus

SUMMARY Sirodesmin PL is a non-host-selective phytotoxin produced by Leptosphaeria maculans, which causes blackleg disease of canola (Brassica napus). Previous studies have shown that sirodesmin PL biosynthesis involves a cluster of 18 co-regulated genes and that disruption of the two-module non-ribosomal peptide synthetase gene (sirP) in this cluster prevents the production of sirodesmin PL. Loss of sirodesmin PL did not affect the growth or fertility of the sirP mutant in vitro, but this mutant had less antibacterial and antifungal activity than the wild-type. When the sirP mutant was inoculated on to cotyledons of B. napus, it caused similar-sized lesions on cotyledons as the wild-type isolate, but subsequently caused fewer lesions and was half as effective as the wild-type in colonizing stems, as shown by quantitative PCR analyses. However, no significant difference was observed in size of lesions when either wild-type or mutant isolates were injected directly into the stem. The expression of two cluster genes, sirP and an ABC transporter, sirA, was studied in planta. Fungal isolates containing fusions of the green fluorescent protein gene with the promoters of these genes fluoresced after 10 days post-inoculation (dpi). Transcripts of sirP and sirA were detected after 11 dpi in cotyledons by reverse transcriptase PCR, and expression of both genes increased dramatically in stem tissue. This expression pattern was consistent with the distribution of sirodesmin PL in planta as revealed by mass spectrometry experiments.

Selective inactivation of glutaredoxin by sporidesmin and other epidithiopiperazinediones

Glutaredoxin (thioltransferase) is a thiol-disulfide oxidoreductase that displays efficient and specific catalysis of protein-SSG deglutathionylation and is thereby implicated in homeostatic regulation of the thiol-disulfide status of cellular proteins. Sporidesmin is an epidithiopiperazine-2,5-dione (ETP) fungal toxin that disrupts cellular functions likely via oxidative alteration of cysteine residues on key proteins. In the current study sporidesmin inactivated human glutaredoxin in a time- and concentration-dependent manner. Under comparable conditions other thiol-disulfide oxidoreductase enzymes, glutathione reductase, thioredoxin, and thioredoxin reductase, were unaffected by sporidesmin. Inactivation of glutaredoxin required the reduced (dithiol) form of the enzyme, the oxidized (intramolecular disulfide) form of sporidesmin, and molecular oxygen. The inactivated glutaredoxin could be reactivated by dithiothreitol only in the presence of urea, followed by removal of the denaturant, indicating that inactivation of the enzyme involves a conformationally inaccessible disulfide bond(s). Various cysteine-to-serine mutants of glutaredoxin were resistant to inactivation by sporidesmin, suggesting that the inactivation reaction specifically involves at least two of the five cysteine residues in human glutaredoxin. The relative ability of various epidithiopiperazine-2,5-diones to inactivate glutaredoxin indicated that at least one phenyl substituent was required in addition to the epidithiodioxopiperazine moiety for inhibitory activity. Mass spectrometry of the modified protein is consistent with formation of intermolecular disulfides, containing one adducted toxin per glutaredoxin but with elimination of two sulfur atoms from the detected product. We suggest that the initial reaction is between the toxin sulfurs and cysteine 22 in the glutaredoxin active site. This study implicates selective modification of sulfhydryls of target proteins in some of the cytotoxic effects of the ETP fungal toxins and their synthetic analogues.

The mitochondrial protein Bak is pivotal for gliotoxin-induced apoptosis and a critical host factor of Aspergillus fumigatus virulence in mice

Aspergillus fumigatus infections cause high levels of morbidity and mortality in immunocompromised patients. Gliotoxin (GT), a secondary metabolite, is cytotoxic for mammalian cells, but the molecular basis and biological relevance of this toxicity remain speculative. We show that GT induces apoptotic cell death by activating the proapoptotic Bcl-2 family member Bak, but not Bax, to elicit the generation of reactive oxygen species, the mitochondrial release of apoptogenic factors, and caspase-3 activation. Activation of Bak by GT is direct, as GT triggers in vitro a dose-dependent release of cytochrome c from purified mitochondria isolated from wild-type and Bax- but not Bak-deficient cells. Resistance to A. fumigatus of mice lacking Bak compared to wild-type mice demonstrates the in vivo relevance of this GT-induced apoptotic pathway involving Bak and suggests a correlation between GT production and virulence. The elucidation of the molecular basis opens new strategies for the development of therapeutic regimens to combat A. fumigatus and related fungal infections.

A new and rapid bioassay for the detection of gliotoxin and related epipolythiodioxopiperazines produced by fungi

Gliotoxin is an immunosuppressive cytotoxin produced by numerous environmental or pathogenic fungal species. For this reason, it is one of the mycotoxins which must be systematically searched for in samples for biological control. In this study, a new, rapid and sensitive method for detecting gliotoxin has been developed. This bioassay is based on the induction of morphological changes in cultured cells (human KB cell line) by gliotoxin. Interpretation of the assay can be carried out after 1 h of incubation, either by direct microscopic observation, or with an automated microplate-reader at 630 nm. The limit of detection is 18-20 ng of gliotoxin in the well, depending on the used observation method. A high degree of specificity of the detection is brought about by the ability of the reducing reactant dithiothreitol to inhibit the biological activities of epipolythiodioxopiperazines (ETPs), such as gliotoxin, by reducing their polysulfide bridge. The bioassay allows a rapid primary screening of samples and a semi-quantitative evaluation of the gliotoxin concentration in extracts. The method has been used to study the gliotoxin production by different fungal strains, allowing to highlight 3 strains of Aspergillus fumigatus producing gliotoxin in various extracts.

Gliotoxin induces caspase-dependent neurite degeneration and calpain-mediated general cytotoxicity in differentiated human neuroblastoma SH-SY5Y cells

In this study, a significant increase by 50% in intracellular free calcium concentration ([Ca(2+)](i)) was observed in differentiated human neuroblastoma (SH-SY5Y) cells after exposure to 0.25microM of the fungal metabolite gliotoxin for 72h. Further, the involvement of caspases and calpains was demonstrated to underlie the gliotoxin-induced cytotoxic and neurite degenerative effects. The caspase inhibitor Z-VAD-fmk almost completely reduced the neurite degeneration from 40% degeneration of neurites to 5% as compared to control. Inhibition of calpains with calpeptin significantly attenuated gliotoxin-induced cytotoxicity, determined as reduction in total cellular protein content, from 43% to 14% as compared to control cells. Western blot analyses of alphaII-spectrin breakdown fragments confirmed activity of the proteases, and that alphaII-spectrin was cleaved by caspases in gliotoxin-exposed cells. These results show that calpains and caspases have a role in the toxicity of gliotoxin in differentiated SH-SY5Y cells and that the process may be Ca(2+)-mediated.

Fungal metabolite gliotoxin blocks mast cell activation by a calcium- and superoxide-dependent mechanism: implications for immunosuppressive activities

Fungal secondary metabolites such as gliotoxin, an epipolythiodioxopiperazine toxin produced by pathogenic fungi like Candida and Aspergillus, possess immunosuppressive activities and have been thought to contribute to pathology of fungal infections in animals and humans. Since recent studies show that mast cell plays a crucial role in the front of host defense, we examined whether fungal secondary metabolites affected mast cell activation. We found that gliotoxin had suppressive effects on FcepsilonRI-dependent or -independent mast cell activation, including degranulation, leukotriene C4 secretion, and TNF-alpha and IL-13 production. Gliotoxin also suppressed intracellular Ca2+ rise through store-operated Ca2+ channels with a minimal effect on depletion of internal Ca2+ stores. Finally, gliotoxin induced intracellular production of superoxide possibly through a thiol redox cycling, which appeared to mediate suppressive effects on mast cell activation. These findings suggest that suppression of mast cell activation might contribute to the establishment of infections with gliotoxin-producing fungi.

Bioinformatic and expression analysis of the putative gliotoxin biosynthetic gene cluster ofAspergillus fumigatus

Gliotoxin is a secondary metabolite produced by several fungi including the opportunistic animal pathogen Aspergillus fumigatus. It is a member of the epipolythiodioxopiperazine (ETP) class of toxins characterised by a disulphide bridged cyclic dipeptide. A putative cluster of 12 genes involved in gliotoxin biosynthesis has been identified in A. fumigatus by a comparative genomics approach based on homology to genes from the sirodesmin (another ETP) biosynthetic gene cluster of Leptosphaeria maculans. The physical limits of the cluster in A. fumigatus have been defined by bioinformatics and by identifying the genes that are co-regulated and whose timing of expression correlates with the production of gliotoxin in culture.

Gliotoxin-induced cytotoxicity in three salmonid cell lines: cell death by apoptosis and necrosis

Epithelial (CHSE-214), fibroblast (RTG-2) and macrophage (RTS11) cell lines from Chinook salmon and rainbow trout were tested for their sensitivity to gliotoxin, a fungal metabolite. Gliotoxin treatment for 6 or 24 h caused cell viability to decrease in a dose-dependent manner, with effective concentrations (EC50s) being similar for the three cell lines but varying with exposure time. Under some exposure conditions, hallmarks of apoptosis were detected. Apoptosis was evaluated by the appearance of fragmented nuclei upon H33258 staining and of genomic DNA laddering into 180 bp oligomers. Gliotoxin induced cell detachment in RTG-2 and CHSE-214 cultures, under some conditions. These were the only cultures of these two cell lines in which apoptosis was detected, and apoptotic cells appeared more frequent in the detached population. At the highest concentration, 15 microM, the cells died by an alternative mode, likely necrosis. By contrast, in RTS11 cultures cell detachment was not observed, and apoptosis occurred over a wider concentration range, even 15 microM, reaching levels of over 90%. The preferential death by necrosis for epithelial cells (CHSE-214) and by apoptosis for macrophages (RTS11) could be a beneficial host response to gliotoxin-producing fungi, leading respectively to the development and then resolution of inflammation.

The epipolythiodioxopiperazine (ETP) class of fungal toxins: distribution, mode of action, functions and biosynthesis

Epipolythiodioxopiperazines (ETPs) are toxic secondary metabolites made only by fungi. The best-known ETP is gliotoxin, which appears to be a virulence factor associated with invasive aspergillosis of immunocompromised patients. The toxicity of ETPs is due to the presence of a disulphide bridge, which can inactivate proteins via reaction with thiol groups, and to the generation of reactive oxygen species by redox cycling. With the availability of complete fungal genome sequences and efficient gene-disruption techniques for fungi, approaches are now feasible to delineate biosynthetic pathways for ETPs and to gain insights into the evolution of such gene clusters.

Redox active calcium ion channels and cell death

Calcium plays a key role in both apoptotic and necrotic cell death. Emptying of intracellular calcium stores and/or alteration in intracellular calcium levels can modulate cell death in almost all cell types. These calcium fluxes are determined by the activity of membrane channels normally under tight control. The channels may be ligand activated or voltage dependent as well as being under the control of affector molecules such as calmodulin. It has become increasingly apparent that many calcium channels are affected by reactive oxygen or reactive nitrogen species; ROS/RNS. This may be part of the normal signaling pathways in the cell or by the action of exogenously generated ROS or RNS often by toxins. This review covers the recent literature on the activity of these redox active channels as related to cell death.

The sirodesmin biosynthetic gene cluster of the plant pathogenic fungus Leptosphaeria maculans

Sirodesmin PL is a phytotoxin produced by the fungus Leptosphaeria maculans, which causes blackleg disease of canola (Brassica napus). This phytotoxin belongs to the epipolythiodioxopiperazine (ETP) class of toxins produced by fungi including mammalian and plant pathogens. We report the cloning of a cluster of genes with predicted roles in the biosynthesis of sirodesmin PL and show via gene disruption that one of these genes (encoding a two-module non-ribosomal peptide synthetase) is essential for sirodesmin PL biosynthesis. Of the nine genes in the cluster tested, all are co-regulated with the production of sirodesmin PL in culture. A similar cluster is present in the genome of the opportunistic human pathogen Aspergillus fumigatus and is most likely responsible for the production of gliotoxin, which is also an ETP. Homologues of the genes in the cluster were also identified in expressed sequence tags of the ETP producing fungus Chaetomium globosum. Two other fungi with publicly available genome sequences, Magnaporthe grisea and Fusarium graminearum, had similar gene clusters. A comparative analysis of all four clusters is presented. This is the first report of the genes responsible for the biosynthesis of an ETP.

The ABC transporter gene in the sirodesmin biosynthetic gene cluster of Leptosphaeria maculans is not essential for sirodesmin production but facilitates self-protection

Epipolythiodioxopiperazine toxins are secreted by a range of fungi, including Leptosphaeria maculans, which produces sirodesmin, and Aspergillus fumigatus, which produces gliotoxin. The L. maculans biosynthetic gene cluster for sirodesmin includes an ABC transporter gene, sirA. Disruption of this gene led to increased secretion of sirodesmin into the medium and an altered ratio of sirodesmin to its immediate precursor. The transcription pattern of a peptide synthetase that catalyses an early step in sirodesmin biosynthesis was elevated in the sirA mutant by 47% over a 7-day period. This was consistent with the finding that the transporter mutant had elevated sirodesmin levels. Despite increased production of sirodesmin, the sirA mutant was more sensitive to both sirodesmin and gliotoxin. The putative gliotoxin transporter gene, gliA, (a major facilitator superfamily transporter) from A. fumigatus complemented the tolerance of the L. maculans sirA mutant to gliotoxin, but not to sirodesmin. The results indicate that SirA contributes to self-protection against sirodesmin in L. maculans and suggest a transporter other than SirA is primarily responsible for efflux of endogenously produced sirodesmin.

Mechanism of action of the antifibrogenic compound gliotoxin in rat liver cells

Gliotoxin has been shown to promote a reversal of liver fibrosis in an animal model of the disease although its mechanism of action in the liver is poorly defined. The effects of gliotoxin on activated hepatic stellate cells (HSCs) and hepatocytes have therefore been examined. Addition of gliotoxin (1.5 microM) to culture-activated HSCs resulted in its rapid accumulation, resulting in increased levels of glutathione and apoptosis without any evidence of oxidative stress. In contrast, although hepatocytes also rapidly sequestered gliotoxin, cell death only occurred at high (50-microM) concentrations of gliotoxin and by necrosis. At high concentrations, gliotoxin was metabolized by hepatocytes to a reduced (dithiol) metabolite and glutathione was rapidly oxidized. Fluorescent dye loading experiments showed that gliotoxin caused oxidative stress in hepatocytes. Antioxidants--but not thiol redox active compounds--inhibited both oxidative stress and necrosis in hepatocytes. In contrast, HSC apoptosis was not affected by antioxidants but was potently abrogated by thiol redox active compounds. The adenine nucleotide transporter (ANT) is implicated in mitochondrial-dependent apoptosis. HSCs expressed predominantly nonliver ANT isoform 1, and gliotoxin treatment resulted in a thiol redox-dependent alteration in ANT mobility in HSC extracts, but not hepatocyte extracts. In conclusion, these data suggest that gliotoxin stimulates the apoptosis of HSCs through a specific thiol redox-dependent interaction with the ANT. Further understanding of this mechanism of cell death will aid in finding therapeutics that specifically stimulate HSC apoptosis in the liver, a promising approach to antifibrotic therapy.

Ion dependence of cytotoxicity of carmustine against PC12 cells

Cytotoxicity is a major complication of carmustine (1,3-bis(2-chloroethyl)-1-nitrosourea, BCNU) therapy for treatment of brain tumors and lymphomas. Using the lactate dehydrogenase (LDH) cell death assay in PC12 cells, we studied the role in this phenomenon of transmembrane ion fluxes that could be activated following inhibition by carmustine of glutathione reductase. The cytotoxic effect of carmustine developed during 4-6 h, with the EC50 of 27 microM. It depended on the extracellular Ca2+ concentration and substantially decreased upon Ca2+ removal. An almost complete suppression of toxicity was achieved when, additionally, monovalent cations were also replaced with impermeant organic cations. A similar loss of toxicity occurred in the presence of Ca2+ when extracellular Cl- was replaced with impermeable gluconate. Various blockers of cation and Cl- channels, as well as antioxidants also protected cells from carmustine. We conclude that carmustine toxicity against PC12 cells requires an influx of Ca2+ ions, supposedly through redox-sensitive cation channels.

Evidence that the lichen-derived scabrosin esters target mitochondrial ATP synthase in P388D1 cells

Scabrosin esters (SEs), which have been recently isolated from the lichen Xanthoparmelia scabrosa, belong to the epipolythiodioxopiperazine (ETP) class of secondary metabolites characterized by possession of a reactive disulfide bond. Colony forming assays show that these toxins are active against human tumor cell lines at nanomolar concentrations. Other members of the ETP class of toxins such as gliotoxin have been shown to induce apoptosis in cells, although the cellular target(s) of the ETP toxins is currently unknown. ETP toxins have been shown to inhibit a variety of enzymes via interaction with sensitive cysteine residues. Here we show that the typical scabrosin ester acetate butyrate induces early mitochondrial membrane hyperpolarization assessed by JC-1 staining accompanied by apoptotic cell death. The toxin lowers ATP in intact cells and inhibits the rate of ATP synthesis in permeabilzed cells. Comparison with the effects of the known ATP synthase inhibitor oligomycin B is consistent with ATP synthase as an early target in scabrosin ester-induced cell death.

Activation and inhibition of cellular calcium and tyrosine kinase signaling pathways identify targets of the HBx protein involved in hepatitis B virus replication

Human hepatitis B virus (HBV) HBx protein is a multifunctional protein that activates cellular signaling pathways and is thought to be essential for viral infection. Woodchuck HBV mutants that lack HBx are unable to replicate in vivo or are severely impaired. HBV replication in HepG2 cells, a human hepatoblastoma cell line, is stimulated 5- to 10-fold by HBx protein. We have utilized the HepG2, HBx-dependent HBV replication system to study the effects of activators and inhibitors of cytosolic calcium and tyrosine kinase signaling pathways on viral replication. By transfecting either a wild-type HBV genome or an HBV genome that does not express HBx and then treating transfected cells with activators or inhibitors of signaling pathways, we identified compounds that either impair wild-type HBV replication or rescue HBx-deficient HBV replication. Geldanamycin or herbimycin A, tyrosine kinase inhibitors, blocked HBV replication. Derivatives of cyclosporine, i.e., cyclosporine A, cyclosporine H, and SDZ NIM811, which block cytosolic calcium signaling and specifically the mitochondrial permeability transition pore (SDZ NIM811), also impaired HBV replication. Treatment of cells with compounds that increase cytosolic calcium levels by a variety of mechanisms rescued replication of an HBx-deficient HBV mutant. Transcription of viral RNA and production of viral capsids were only minimally affected by these treatments. These results define a functional signaling circuit for HBV replication that includes calcium signaling and activation of cytosolic signaling pathways involving Src kinases, and they suggest that these pathways are stimulated by HBx acting on the mitochondrial transition pore.