Gliotoxin and related epipolythiodioxopiperazines

The evidence-based multifaceted roles of hepatic stellate cells in liver diseases: A concise review

Hepatic stellate cells (HSCs) play central roles in liver disease pathogenesis, spanning steatosis to cirrhosis and hepatocellular carcinoma. These cells, located in the liver's sinusoidal space of Disse, transition from a quiescent, vitamin A-rich state to an activated, myofibroblast-like phenotype in response to liver injury. This activation results from a complex interplay of cytokines, growth factors, and oxidative stress, leading to excessive collagen deposition and liver fibrosis, a hallmark of chronic liver diseases. Recently, HSCs have gained recognition for their dynamic, multifaceted roles in liver health and disease. Attention has shifted toward their involvement in various liver conditions, including acute liver injury, alcoholic and non-alcoholic fatty liver disease, and liver regeneration. This review aims to explore diverse functions of HSCs in these acute or chronic liver pathologies, with a focus on their roles beyond fibrogenesis. HSCs exhibit a wide range of actions, including lipid storage, immunomodulation, and interactions with other hepatic and extrahepatic cells, making them pivotal in the hepatic microenvironment. Understanding HSC involvement in the progression of liver diseases can offer novel insights into pathogenic mechanisms and guide targeted therapeutic strategies for various liver conditions.

A Timeline of Biosynthetic Gene Cluster Discovery in Aspergillus fumigatus: From Characterization to Future Perspectives

In 1999, the first biosynthetic gene cluster (BGC), synthesizing the virulence factor DHN melanin, was characterized in Aspergillus fumigatus. Since then, 19 additional BGCs have been linked to specific secondary metabolites (SMs) in this species. Here, we provide a comprehensive timeline of A. fumigatus BGC discovery and find that initial advances centered around the commonly expressed SMs where chemical structure informed rationale identification of the producing BGC (e.g., gliotoxin, fumigaclavine, fumitremorgin, pseurotin A, helvolic acid, fumiquinazoline). Further advances followed the transcriptional profiling of a ΔlaeA mutant, which aided in the identification of endocrocin, fumagillin, hexadehydroastechrome, trypacidin, and fumisoquin BGCs. These SMs and their precursors are the commonly produced metabolites in most A. fumigatus studies. Characterization of other BGC/SM pairs required additional efforts, such as induction treatments, including co-culture with bacteria (fumicycline/neosartoricin, fumigermin) or growth under copper starvation (fumivaline, fumicicolin). Finally, four BGC/SM pairs were discovered via overexpression technologies, including the use of heterologous hosts (fumicycline/neosartoricin, fumihopaside, sphingofungin, and sartorypyrone). Initial analysis of the two most studied A. fumigatus isolates, Af293 and A1160, suggested that both harbored ca. 34-36 BGCs. However, an examination of 264 available genomes of A. fumigatus shows up to 20 additional BGCs, with some strains showing considerable variations in BGC number and composition. These new BGCs present a new frontier in the future of secondary metabolism characterization in this important species.

Applications and Opportunities in Using Disulfides, Thiosulfinates, and Thiosulfonates as Antibacterials

Sulfur-containing molecules have a long history of bioactivity, especially as antibacterial agents in the fight against infectious pathogens. Organosulfur compounds from natural products have been used to treat infections throughout history. Many commercially available antibiotics also have sulfur-based moieties in their structural backbones. In the following review, we summarize sulfur-containing antibacterial compounds, focusing on disulfides, thiosulfinates, and thiosulfonates, and opportunities for future developments in the field.

Targeting HIF-1α by Natural and Synthetic Compounds: A Promising Approach for Anti-Cancer Therapeutics Development

Advancement in novel target detection using improved molecular cancer biology has opened up new avenues for promising anti-cancer drug development. In the past two decades, the mechanism of tumor hypoxia has become more understandable with the discovery of hypoxia-inducible factor-1α (HIF-1α). It is a major transcriptional regulator that coordinates the activity of various transcription factors and their downstream molecules involved in tumorigenesis. HIF-1α not only plays a crucial role in the adaptation of tumor cells to hypoxia but also regulates different biological processes, including cell proliferation, survival, cellular metabolism, angiogenesis, metastasis, cancer stem cell maintenance, and propagation. Therefore, HIF-1α overexpression is strongly associated with poor prognosis in patients with different solid cancers. Hence, pharmacological targeting of HIF-1α has been considered to be a novel cancer therapeutic strategy in recent years. In this review, we provide brief descriptions of natural and synthetic compounds as HIF-1α inhibitors that have the potential to accelerate anticancer drug discovery. This review also introduces the mode of action of these compounds for a better understanding of the chemical leads, which could be useful as cancer therapeutics in the future.

Integration of In Silico and In Vitro Analysis of Gliotoxin Production Reveals a Narrow Range of Producing Fungal Species

Gliotoxin is a fungal secondary metabolite with impact on health and agriculture since it might act as virulence factor and contaminate human and animal food. Homologous gliotoxin (GT) gene clusters are spread across a number of fungal species although if they produce GT or other related epipolythiodioxopiperazines (ETPs) remains obscure. Using bioinformatic tools, we have identified homologous gli gene clusters similar to the A. fumigatus GT gene cluster in several fungal species. In silico study led to in vitro confirmation of GT and Bisdethiobis(methylthio)gliotoxin (bmGT) production in fungal strain cultures by HPLC detection. Despite we selected most similar homologous gli gene cluster in 20 different species, GT and bmGT were only detected in section Fumigati species and in a Trichoderma virens Q strain. Our results suggest that in silico gli homology analyses in different fungal strains to predict GT production might be only informative when accompanied by analysis about mycotoxin production in cell cultures.

Development of a High-throughput NanoBRET Screening Platform to Identify Modulators of the RAS/RAF Interaction

Activating mutations in RAS are found in approximately 30% of human cancers, resulting in the delivery of a persistent signal to critical downstream effectors that drive tumorigenesis. RAS-driven malignancies respond poorly to conventional cancer treatments and inhibitors that target RAS directly are limited; therefore, the identification of new strategies and/or drugs to disrupt RAS signaling in tumor cells remains a pressing therapeutic need. Taking advantage of the live-cell bioluminescence resonance energy transfer (BRET) methodology, we describe the development of a NanoBRET screening platform to identify compounds that modulate binding between activated KRAS and the CRAF kinase, an essential effector of RAS that initiates ERK cascade signaling. Using this strategy, libraries containing synthetic compounds, targeted inhibitors, purified natural products, and natural product extracts were evaluated. These efforts resulted in the identification of compounds that inhibit RAS/RAF binding and in turn suppress RAS-driven ERK activation, but also compounds that have the deleterious effect of enhancing the interaction to upregulate pathway signaling. Among the inhibitor hits identified, the majority were compounds derived from natural products, including ones reported to alter KRAS nanoclustering (ophiobolin A), to impact RAF function (HSP90 inhibitors and ROS inducers) as well as some with unknown targets and activities. These findings demonstrate the potential for this screening platform in natural product drug discovery and in the development of new therapeutic agents to target dysregulated RAS signaling in human disease states such as cancer.

Structural and Mechanistic Insights into C-S Bond Formation in Gliotoxin

Glutathione‐S‐transferases (GSTs) usually detoxify xenobiotics. The human pathogenic fungus Aspergillus fumigatus however uses the exceptional GST GliG to incorporate two sulfur atoms into its virulence factor gliotoxin. Because these sulfurs are essential for biological activity, glutathionylation is a key step of gliotoxin biosynthesis. Yet, the mechanism of carbon−sulfur linkage formation from a bis‐hydroxylated precursor is unresolved. Here, we report structures of GliG with glutathione (GSH) and its reaction product cyclo[‐l‐Phe‐l‐Ser]‐bis‐glutathione, which has been purified from a genetically modified A. fumigatus strain. The structures argue for stepwise processing of first the Phe and second the Ser moiety. Enzyme‐mediated dehydration of the substrate activates GSH and a helix dipole stabilizes the resulting anion via a water molecule for the nucleophilic attack. Activity assays with mutants validate the interactions of GliG with the ligands and enrich our knowledge about enzymatic C−S bond formation in gliotoxin and epipolythiodioxopiperazine (ETP) natural compounds in general.

Genetic and Histopathological Alterations in Caco-2 and HuH-7 Cells Treated with Secondary Metabolites of Marine fungi

The present work aimed to study the activity of naturally derived fungal secondary metabolites as anticancer agents concerning their cytotoxicity, apoptotic, genetic, and histopathological profile. It was noticed that Aspergillus terreus, Aspergillus flavus, and Aspergillus fumigatus induced variable toxic potential that was cell type, secondary metabolite type, and concentration dependent. Human colonic adenocarcinoma cells (Caco-2) showed less sensitivity than hepatocyte-derived cellular carcinoma cells (HuH-7), and in turn, the half-maximal inhibitory concentration (IC₅₀) was variable. Also, the apoptotic potential of Aspergillus species-derived fungal secondary metabolites was proven via detection of up-regulated pro-apoptotic genes and down-regulation of anti-apoptotic genes. The expression level was cell type dependent. Concurrently, apoptotic profile was accompanied with cellular DNA accumulation at the G2/M phase, as well as an elevation in Pre-G1 phase but not during G0/G1 and S phases. Also, there were characteristic apoptotic features of treated cells presented as abnormal intra-nuclear eosinophilic structures, dead cells with mixed euchromatin and heterochromatin, ruptured cell membranes, apoptotic cells with irregular cellular and nuclear membranes, as well as peripheral chromatin condensation. It can be concluded that Aspergillus secondary metabolites are promising agents that can be used as supplementary agents to the currently applied anti-cancer drug regimen.

Modifications of diketopiperazines assembled by cyclodipeptide synthases with cytochrome P450 enzymes

2,5-Diketopiperazines are the smallest cyclic peptides comprising two amino acids connected via two peptide bonds. They can be biosynthesized in nature by two different enzyme families, either by nonribosomal peptide synthetases or by cyclodipeptide synthases. Due to the stable scaffold of the diketopiperazine ring, they can serve as precursors for further modifications by different tailoring enzymes, such as methyltransferases, prenyltransferases, oxidoreductases like cyclodipeptide oxidases, 2-oxoglutarate-dependent monooxygenases and cytochrome P450 enzymes, leading to the formation of intriguing secondary metabolites. Among them, cyclodipeptide synthase-associated P450s attracted recently significant attention, since they are able to catalyse a broader variety of astonishing reactions than just oxidation by insertion of an oxygen. The P450-catalysed reactions include hydroxylation at a tertiary carbon, aromatisation of the diketopiperazine ring, intramolecular and intermolecular carbon-carbon and carbon-nitrogen bond formation of cyclodipeptides and nucleobase transfer reactions. Elucidation of the crystal structures of three P450s as cyclodipeptide dimerases provides a structural basis for understanding the reaction mechanism and generating new enzymes by protein engineering. This review summarises recent publications on cyclodipeptide modifications by P450s.Key Points• Intriguing reactions catalysed by cyclodipeptide synthase-associated cytochrome P450s• Homo- and heterodimerisation of diketopiperazines• Coupling of guanine and hypoxanthine with diketopiperazines.

In Vitro and In Vivo Antibacterial Activity of Gliotoxin Alone and in Combination with Antibiotics against Staphylococcus aureus

Multidrug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) is one of the major causes of hospital-acquired and community infections and pose a challenge to the human health care system. Therefore, it is important to find new drugs that show activity against these bacteria, both in monotherapy and in combination with other antimicrobial drugs. Gliotoxin (GT) is a mycotoxin produced by Aspergillus fumigatus and other fungi of the Aspergillus genus. Some evidence suggests that GT shows antimicrobial activity against S. aureus in vitro, albeit its efficacy against multidrug-resistant strains such as MRSA or vancomycin-intermediate S. aureus (VISA) strainsis not known. This work aimed to evaluate the antibiotic efficacy of GT as monotherapy or in combination with other therapeutics against MRSA in vitro and in vivo using a Caenorhabditis elegans infection model.

Research Progress of the Biosynthesis of Natural Bio-Antibacterial Agent Pulcherriminic Acid in Bacillus

Pulcherriminic acid is a cyclic dipeptide found mainly in Bacillus and yeast. Due to the ability of pulcherriminic acid to chelate Fe3+ to produce reddish brown pulcherrimin, microorganisms capable of synthesizing pulcherriminic acid compete with other microorganisms for environmental iron ions to achieve bacteriostatic effects. Therefore, studying the biosynthetic pathway and their enzymatic catalysis, gene regulation in the process of synthesis of pulcherriminic acid in Bacillus can facilitate the industrial production, and promote the wide application in food, agriculture and medicine industries. After initially discussing, this review summarizes current research on the synthesis of pulcherriminic acid by Bacillus, which includes the crystallization of key enzymes, molecular catalytic mechanisms, regulation of synthetic pathways, and methods to improve efficiency in synthesizing pulcherriminic acid and its precursors. Finally, possible applications of pulcherriminic acid in the fermented food, such as Chinese Baijiu, applying combinatorial biosynthesis will be summarized.

Chemical repertoire and biosynthetic machinery of the Aspergillus flavus secondary metabolome: A review

Filamentous fungi represent a rich source of extrolites, including secondary metabolites (SMs) comprising a great variety of astonishing structures and interesting bioactivities. State-of-the-art techniques in genome mining, genetic manipulation, and secondary metabolomics have enabled the scientific community to better elucidate and more deeply appreciate the genetic and biosynthetic chemical arsenal of these microorganisms. Aspergillus flavus is best known as a contaminant of food and feed commodities and a producer of the carcinogenic family of SMs, aflatoxins. This fungus produces many SMs including polyketides, ribosomal and nonribosomal peptides, terpenoids, and other hybrid molecules. This review will discuss the chemical diversity, biosynthetic pathways, and biological/ecological role of A. flavus SMs, as well as their significance concerning food safety and security.

Targeting hypoxia-inducible factor-1, for cancer treatment: Recent advances in developing small-molecule inhibitors from natural compounds

Rapid progress in molecular cancer biology coupled with the discovery of novel oncology drugs has opened new horizons for cancer target discovery. As one of the crucial signaling pathways related to tumorigenesis, hypoxia-inducible factor-1 (HIF-1) coordinates the activity of many transcription factors and their downstream molecules that impact tumor growth and metastasis. Accumulating evidence suggests that the transcriptional responses to acute hypoxia are mainly attributable to HIF-1α. Moreover, the overexpression of HIF-1α in several solid cancers has been found to be strongly associated with poor prognosis. Thus, pharmacological targeting of the HIF-1 signaling pathways has been considered as a new strategy for cancer therapy in the recent years. Although over the past decade, tremendous efforts have been made in preclinical studies to develop new HIF-1 inhibitors from natural products (reservoirs of novel therapeutic agents), to date, these efforts have not been successfully translated into clinically available treatments. In this review, we provide new insights into the bio-pharmacological considerations for selecting natural compounds as potential HIF-1 inhibitors to accelerate anti-cancer drug development. In addition, we highlighted the importance of assessing the dependency of cancer on HIF1A to shortlist cancer types as suitable disease models. This may subsequently lead to new paradigms for discovering more HIF-1 inhibitors derived from natural products and facilitate the development of potent therapeutic agents targeting specific cancer types.

n→π* Interactions Modulate the Disulfide Reduction Potential of Epidithiodiketopiperazines

Epithiodiketopiperazines (ETPs) are a structurally complex class of fungal natural products with potent anticancer activity. In ETPs, the diketopiperazine ring is spanned by a disulfide bond that is constrained in a high-energy eclipsed conformation. We employed computational, synthetic, and spectroscopic methods to investigate the physicochemical attributes of this atypical disulfide bond. We find that the disulfide bond is stabilized by two n→π* interactions, each with large energies (3-5 kcal/mol). The n→π* interactions in ETPs make disulfide reduction much more difficult, endowing stability in physiological environments in a manner that could impact their biological activity. These data reveal a previously unappreciated means to stabilize a disulfide bond and highlight the utility of the n→π* interaction in molecular design.

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.

Secondary Metabolites from the Culture of the Marine-derived Fungus Paradendryphiella salina PC 362H and Evaluation of the Anticancer Activity of Its Metabolite Hyalodendrin

High-throughput screening assays have been designed to identify compounds capable of inhibiting phenotypes involved in cancer aggressiveness. However, most studies used commercially available chemical libraries. This prompted us to explore natural products isolated from marine-derived fungi as a new source of molecules. In this study, we established a chemical library from 99 strains corresponding to 45 molecular operational taxonomic units and evaluated their anticancer activity against the MCF7 epithelial cancer cell line and its invasive stem cell-like MCF7-Sh-WISP2 counterpart. We identified the marine fungal Paradendryphiella salina PC 362H strain, isolated from the brown alga Pelvetia caniculata (PC), as one of the most promising fungi which produce active compounds. Further chemical and biological characterizations of the culture of the Paradendryphiella salina PC 362H strain identified (-)-hyalodendrin as the active secondary metabolite responsible for the cytotoxic activity of the crude extract. The antitumor activity of (-)-hyalodendrin was not only limited to the MCF7 cell lines, but also prominent on cancer cells with invasive phenotypes including colorectal cancer cells resistant to chemotherapy. Further investigations showed that treatment of MCF7-Sh-WISP2 cells with (-)-hyalodendrin induced changes in the phosphorylation status of p53 and altered expression of HSP60, HSP70 and PRAS40 proteins. Altogether, our study reveals that this uninvestigated marine fungal crude extract possesses a strong therapeutic potential against tumor cells with aggressive phenotypes and confirms that members of the epidithiodioxopiperazines are interesting fungal toxins with anticancer activities.

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.

Synthesis of Symmetrically 1,4-disubstituted piperazine-2,5-diones: A New Class of Antileishmanial Agents

A series of 1,4-diphenyl-2,5-dioxopiperazine derivatives were synthesised in one pot sequence. The compounds demonstrated appreciable cytotoxic activity against Leishmania donovani on both forms of the parasite, and the results suggested that some derivatives (4, 11 and 12) could be exploited as antileishmanial agents.

Gliotoxin from Aspergillus fumigatus Abrogates Leukotriene B4 Formation through Inhibition of Leukotriene A4 Hydrolase

The epidithiodioxopiperazine gliotoxin is a virulence factor of Aspergillus fumigatus, the most important airborne fungal pathogen of humans. Gliotoxin suppresses innate immunity in invasive aspergillosis, particularly by compromising neutrophils, but the underlying molecular mechanisms remain elusive. Neutrophils are the first responders among innate immune cells recruited to sites of infection by the chemoattractant leukotriene (LT)B₄ that is biosynthesized by 5-lipoxygenase and LTA₄ hydrolase (LTA₄H). Here, we identified gliotoxin as inhibitor of LTA₄H that selectively abrogates LTB₄ formation in human leukocytes and in distinct animal models. Gliotoxin failed to inhibit the formation of other eicosanoids and the aminopeptidase activity of the bifunctional LTA₄H. Suppression of LTB₄ formation by gliotoxin required the cellular environment and/or reducing conditions, and only the reduced form of gliotoxin inhibited LTA₄H activity. Conclusively, gliotoxin suppresses the biosynthesis of the potent neutrophil chemoattractant LTB₄ by direct interference with LTA₄H thereby impairing neutrophil functions in invasive aspergillosis.

Fragment-Based Covalent Ligand Screening Enables Rapid Discovery of Inhibitors for the RBR E3 Ubiquitin Ligase HOIP

Modification of proteins with polyubiquitin chains is a key regulatory mechanism to control cellular behavior and alterations in the ubiquitin system are linked to many diseases. Linear (M1-linked) polyubiquitin chains play pivotal roles in several cellular signaling pathways mediating immune and inflammatory responses and apoptotic cell death. These chains are formed by the linear ubiquitin chain assembly complex (LUBAC), a multiprotein E3 ligase that consists of 3 subunits, HOIP, HOIL-1L, and SHARPIN. Herein, we describe the discovery of inhibitors targeting the active site cysteine of the catalytic subunit HOIP using fragment-based covalent ligand screening. We report the synthesis of a diverse library of electrophilic fragments and demonstrate an integrated use of protein LC–MS, biochemical ubiquitination assays, chemical synthesis, and protein crystallography to enable the first structure-based development of covalent inhibitors for an RBR E3 ligase. Furthermore, using cell-based assays and chemoproteomics, we demonstrate that these compounds effectively penetrate mammalian cells to label and inhibit HOIP and NF-κB activation, making them suitable hits for the development of selective probes to study LUBAC biology. Our results illustrate the power of fragment-based covalent ligand screening to discover lead compounds for challenging targets, which holds promise to be a general approach for the development of cell-permeable inhibitors of thioester-forming E3 ubiquitin ligases.

Impaired lymphocyte reconstitution after autologous transplant is associated with apoptosis of CD8+ T cells and adverse clinical outcome

We noticed that the lymphocyte counts, after autologous hematopoietic stem cell transplantation, oscillated during the first 4 post-transplant months. Thereafter, the lymphocyte counts stabilized and segregated the patients into two groups, those who normalized their lymphocyte counts and those with prolonged lymphopenia. In both groups, the CD4 counts remained low for at least 6 months. However, in approximately half of the patient, the CD8 counts increased to normal or above normal values. Patients with prolonged lymphopenia had higher rates of lymphocytes' spontaneous apoptosis and the lymphocytes in patients who restored their counts expressed the intracellular CD14-derived MO2 epitope that protects the cells from apoptosis. These findings were translated to longer disease-free survival and overall survival in patients who restored the CD8 counts. Collectively, our data show that post-transplant lymphocytes that express intracellular CD14-MO2 epitope have survival advantage.

Global analysis of prokaryotic tRNA-derived cyclodipeptide biosynthesis

BACKGROUND

Among naturally occurring small molecules, tRNA-derived cyclodipeptides are a class that have attracted attention for their diverse and desirable biological activities. However, no tools are available to link cyclodipeptide synthases identified within prokaryotic genome sequences to their chemical products. Consequently, it is unclear how many genetically encoded cyclodipeptides represent novel products, and which producing organisms should be targeted for discovery.

RESULTS

We developed a pipeline for identification and classification of cyclodipeptide biosynthetic gene clusters and prediction of aminoacyl-tRNA substrates and complete chemical structures. We leveraged this tool to conduct a global analysis of tRNA-derived cyclodipeptide biosynthesis in 93,107 prokaryotic genomes, and compared predicted cyclodipeptides to known cyclodipeptide synthase products and all known chemically characterized cyclodipeptides. By integrating predicted chemical structures and gene cluster architectures, we created a unified map of known and unknown genetically encoded cyclodipeptides.

CONCLUSIONS

Our analysis suggests that sizeable regions of the chemical space encoded within sequenced prokaryotic genomes remain unexplored. Our map of the landscape of genetically encoded cyclodipeptides provides candidates for targeted discovery of novel compounds. The integration of our pipeline into a user-friendly web application provides a resource for further discovery of cyclodipeptides in newly sequenced prokaryotic genomes.

Palladium-catalyzed intramolecular transfer hydrogenation & cycloaddition of p-quinamine-tethered alkylidenecyclopropanes to synthesize perhydroindole scaffolds

A facile Pd-catalyzed intramolecular transfer hydrogenation and cycloaddition of p-quinamine-tethered alkylidenecyclopropanes has been developed, giving rigid tricyclic perhydroindole skeletons in moderate to good yields.

Quantitative Modeling of Bis(pyridine)silver(I) Permanganate Oxidation of Hydantoin Derivatives: Guidelines for Predicting the Site of Oxidation in Complex Substrates

The bis(pyridine)silver(I) permanganate promoted hydroxylation of diketopiperazines has served as a pivotal transformation in the synthesis of complex epipolythiodiketopiperazine alkaloids. This late-stage C-H oxidation chemistry is strategically critical to access N-acyl iminium ion intermediates necessary for nucleophilic thiolation of advanced diketopiperazines en route to potent epipolythiodiketopiperazine anticancer compounds. In this study, we develop an informative mathematical model using hydantoin derivatives as a training set of substrates by relating the relative rates of oxidation to various calculated molecular descriptors. The model prioritizes Hammett values and percent buried volume as key contributing factors in the hydantoin series while correctly predicting the experimentally observed oxidation sites in various complex diketopiperazine case studies. Thus, a method is presented by which to use simplified training molecules and resulting correlations to explain and predict reaction behavior for more complex substrates.

Cyclodipeptides: An Overview of Their Biosynthesis and Biological Activity

Cyclodipeptides (CDP) represent a diverse family of small, highly stable, cyclic peptides that are produced as secondary functional metabolites or side products of protein metabolism by bacteria, fungi, and animals. They are widespread in nature, and exhibit a broad variety of biological and pharmacological activities. CDP synthases (CDPSs) and non-ribosomal peptide synthetases (NRPSs) catalyze the biosynthesis of the CDP core structure, which is further modified by tailoring enzymes often associated with CDP biosynthetic gene clusters. In this review, we provide a comprehensive summary of CDP biosynthetic pathways and modifying enzymes. We also discuss the biological properties of some known CDPs and their possible applications in metabolic engineering.

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.

Towards understanding the gliotoxin detoxification mechanism: in vivo thiomethylation protects yeast from gliotoxin cytotoxicity

Gliotoxin (GT) is a mycotoxin produced by some species of ascomycete fungi including the opportunistic human pathogen Aspergillus fumigatus. In order to produce GT the host organism needs to have evolved a self-protection mechanism. GT contains a redox-cycling disulfide bridge that is important in mediating toxicity. Recently is has been demonstrated that A. fumigatus possesses a novel thiomethyltransferase protein called GtmA that has the ability to thiomethylate GT in vivo, which aids the organism in regulating GT biosynthesis. It has been suggested that thiomethylation of GT and similar sulfur-containing toxins may play a role in providing self-protection in host organisms. In this work we have engineered Saccharomyces cerevisiae, a GT-naïve organism, to express A. fumigatus GtmA. We demonstrate that GtmA can readily thiomethylate GT in yeast, which results in protection of the organism from exogenous GT. Our work has implications for understanding the evolution of GT self-protection mechanisms in organisms that are GT producers and non-producers.

Gliotoxin Inhibits Proliferation and Induces Apoptosis in Colorectal Cancer Cells

The discovery of new bioactive compounds from marine natural sources is very important in pharmacological research. Here we developed a Wnt responsive luciferase reporter assay to screen small molecule inhibitors of cancer associated constitutive Wnt signaling pathway. We identified that gliotoxin (GTX) and some of its analogues, the secondary metabolites from marine fungus Neosartorya pseufofischeri, acted as inhibitors of the Wnt signaling pathway. In addition, we found that GTX downregulated the β-catenin levels in colorectal cancer cells with inactivating mutations of adenomatous polyposis coli (APC) or activating mutations of β-catenin. Furthermore, we demonstrated that GTX induced growth inhibition and apoptosis in multiple colorectal cancer cell lines with mutations of the Wnt signaling pathway. Together, we illustrated a practical approach to identify small-molecule inhibitors of the Wnt signaling pathway and our study indicated that GTX has therapeutic potential for the prevention or treatment of Wnt dependent cancers and other Wnt related diseases.

A Unified Strategy to 6-5-6-5-6-Membered Epipolythiodiketopiperazines: Studies towards the Total Synthesis of Scabrosin Diacetate and Haematocin

The family of epipolythiodiketopiperazine (ETP) natural products consists of over 200 members possessing a wide diversity of structures and biological activity. Recently, the subgroup of 6-5-6-5-6-membered ETPs has gained substantial attention, which has resulted in several total syntheses. Despite all the efforts that have been invested into accessing these complex structures, no synthesis of scabrosin diacetate (1 a) and its related esters has been reported. Herein, our attempts towards scabrosin diacetate (1 a) and haematocin (3) starting from diketopiperazine 12 a as a late-stage intermediate are presented. Diketopiperazine 12 a can be conveniently accessed in multigram quantities from aldehyde 18 and diketopiperazine 21 and was envisioned to serve as a general platform for the synthesis of 6-5-6-5-6-membered ETPs.

Stereoselective construction of a key hydroindole precursor of epidithiodiketopiperazine (ETP) natural products

An asymmetric synthetic strategy for constructing the divergent-synthesis monomer of epidithiodiketopiperazine (ETP) natural products has been successfully developed. The functionalized 2,3,3a,4,7,7a-hexahydroindole scaffold was constructed by a diastereoselective inverse electron-demand Diels-Alder (IEDDA) reaction.

Gliotoxin suppresses NF-κB activation by selectively inhibiting linear ubiquitin chain assembly complex (LUBAC)

A linear ubiquitin chain, which consists of ubiquitin molecules linked via their N- and C-termini, is formed by a linear ubiquitin chain assembly complex (LUBAC) composed of HOIP, HOIL-1L, and SHARPIN, and conjugation of a linear ubiquitin chain on the NF-κB essential modulator (NEMO) is deeply involved in NF-κB activation induced by various signals. Since abnormal activation of NF-κB is associated with inflammatory disease and malignancy, we searched for an inhibitor of LUBAC by high-throughput screening (HTS) with a Tb(3+)-fluorescein FRET system. As a result, we found that the fungal metabolite gliotoxin inhibits LUBAC selectively by binding to the RING-IBR-RING domain of HOIP, the catalytic center of LUBAC. Gliotoxin has been well-known as an inhibitor of NF-κB activation, though its action mechanism has remained elusive. Here, we show that gliotoxin inhibits signal-induced NF-κB activation by selectively inhibiting LUBAC-mediated linear ubiquitin chain formation.

The dehydrogenase region of the NADPH oxidase component Nox2 acts as a protein disulfide isomerase (PDI) resembling PDIA3 with a role in the binding of the activator protein p67 (phox.)

The superoxide (O(·-) 2)-generating NADPH oxidase of phagocytes consists of a membrane component, cytochrome b 558 (a heterodimer of Nox2 and p22 (phox) ), and four cytosolic components, p47 (phox) , p67 (phox) , p40 (phox) , and Rac. The catalytic component, responsible for O(·-) 2 generation, is Nox2. It is activated by the interaction of the dehydrogenase region (DHR) of Nox2 with the cytosolic components, principally with p67 (phox) . Using a peptide-protein binding assay, we found that Nox2 peptides containing a (369)CysGlyCys(371) triad (CGC) bound p67 (phox) with high affinity, dependent upon the establishment of a disulfide bond between the two cysteines. Serially truncated recombinant Nox2 DHR proteins bound p67 (phox) only when they comprised the CGC triad. CGC resembles the catalytic motif (CGHC) of protein disulfide isomerases (PDIs). This led to the hypothesis that Nox2 establishes disulfide bonds with p67 (phox) via a thiol-dilsulfide exchange reaction and, thus, functions as a PDI. Evidence for this was provided by the following: (1) Recombinant Nox2 protein, which contained the CGC triad, exhibited PDI-like disulfide reductase activity; (2) Truncation of Nox2 C-terminal to the CGC triad or mutating C369 and C371 to R, resulted in loss of PDI activity; (3) Comparison of the sequence of the DHR of Nox2 with PDI family members revealed three small regions of homology with PDIA3; (4) Two monoclonal anti-Nox2 antibodies, with epitopes corresponding to regions of Nox2/PDIA3 homology, reacted with PDIA3 but not with PDIA1; (5) A polyclonal anti-PDIA3 (but not an anti-PDIA1) antibody reacted with Nox2; (6) p67 (phox) , in which all cysteines were mutated to serines, lost its ability to bind to a Nox2 peptide containing the CGC triad and had an impaired capacity to support oxidase activity in vitro. We propose a model of oxidase assembly in which binding of p67 (phox) to Nox2 via disulfide bonds, by virtue of the intrinsic PDI activity of Nox2, stabilizes the primary interaction between the two components.

Epidithiodioxopiperazines. occurrence, synthesis and biogenesis

Epidithiodioxopiperazine alkaloids possess an astonishing array of molecular architecture and generally exhibit potent biological activity. Nearly twenty distinct families have been isolated and characterized since the seminal discovery of gliotoxin in 1936. Numerous biosynthetic investigations offer a glimpse at the relative ease with which Nature is able to assemble this class of molecules, while providing synthetic chemists inspiration for the development of more efficient syntheses. Herein, we discuss the isolation and characterization, proposed fungal biogeneses, and total syntheses of epidithiodioxopiperazines.

The tRNA-dependent biosynthesis of modified cyclic dipeptides

In recent years it has become apparent that aminoacyl-tRNAs are not only crucial components involved in protein biosynthesis, but are also used as substrates and amino acid donors in a variety of other important cellular processes, ranging from bacterial cell wall biosynthesis and lipid modification to protein turnover and secondary metabolite assembly. In this review, we focus on tRNA-dependent biosynthetic pathways that generate modified cyclic dipeptides (CDPs). The essential peptide bond-forming catalysts responsible for the initial generation of a CDP-scaffold are referred to as cyclodipeptide synthases (CDPSs) and use loaded tRNAs as their substrates. After initially discussing the phylogenetic distribution and organization of CDPS gene clusters, we will focus on structural and catalytic properties of CDPSs before turning to two recently characterized CDPS-dependent pathways that assemble modified CDPs. Finally, possible applications of CDPSs in the rational design of structural diversity using combinatorial biosynthesis will be discussed before concluding with a short outlook.

Role of gliotoxin in the symbiotic and pathogenic interactions of Trichoderma virens

Using a gene disruption strategy, we generated mutants in the gliP locus of the plant-beneficial fungus Trichoderma virens that were no longer capable of producing gliotoxin. Phenotypic assays demonstrated that the gliP-disrupted mutants grew faster, were more sensitive to oxidative stress and exhibited a sparse colony edge compared with the WT strain. In a plate confrontation assay, the mutants deficient in gliotoxin production were ineffective as mycoparasites against the oomycete, Pythium ultimum, and the necrotrophic fungal pathogen, Sclerotinia sclerotiorum, but retained mycoparasitic ability against Rhizoctonia solani. Biocontrol assays in soil showed that the mutants were incapable of protecting cotton seedlings from attack by P. ultimum, against which the WT strain was highly effective. The mutants, however, were as effective as the WT strain in protecting cotton seedlings against R. solani. Loss of gliotoxin production also resulted in a reduced ability of the mutants to attack the sclerotia of S. sclerotiorum compared with the WT. The addition of exogenous gliotoxin to the sclerotia colonized by the mutants partially restored their degradative abilities. Interestingly, as in Aspergillus fumigatus, an opportunistic human pathogen, gliotoxin was found to be involved in pathogenicity of T. virens against larvae of the wax moth, Galleria mellonella. The loss of gliotoxin production in T. virens was restored by complementation with the gliP gene from A. fumigatus. We have, thus, demonstrated that the putative gliP cluster of T. virens is responsible for the biosynthesis of gliotoxin, and gliotoxin is involved in mycoparasitism and biocontrol properties of this plant-beneficial fungus.

Synthesis of 3'-triazoyl-dinucleotides as precursors of stable Phe-tRNA(Phe) and Leu-tRNA(Leu) analogues

We report here the synthesis of stable Phe-tRNA(Phe) and Leu-tRNA(Leu) analogues containing a 1,2,3-triazole ring instead of the ribose-amino acid ester bond. The 1,2,3-triazole ring is generated by dipolar cycloaddition of alkyne Phe and Leu analogues to 3'-azido-3'-deoxyadenosine via the Cu(I)-catalysed Huisgen, Meldal, Sharpless 1,3-cycloaddition. The corresponding triazoyl pdCpA dinucleotides, obtained by classical phosphoramidite chemistry, were enzymatically ligated to 22-nt or 74-nt RNA generating stable Phe-tRNA(Phe) analogues containing the acceptor stem or full tRNA moieties, respectively. These molecules represent useful tools to study the contribution of the RNA and amino acid moieties in stabilization of aminoacyl-tRNA/protein complexes.

Exposure of Aspergillus fumigatus to caspofungin results in the release, and de novo biosynthesis, of gliotoxin

Caspofungin is a member of the echinocandin class of antifungal agents that inhibit the synthesis of β 1,3 glucan thus disrupting fungal cell wall structure and function. Exposure of the Aspergillus fumigatus cultures to caspofungin (0.01, 0.1 or 1.0 μg/ml) resulted in a reduction in cell growth, but the production of the epipolythiodioxopiperazine toxin, gliotoxin, was comparable, or greater, in cultures exposed to caspofungin than untreated controls. Exposure of A. fumigatus hyphae to 1.0 μg/ml caspofungin for 4 h resulted in the release of amino acids (P = 0.01), protein (P = 0.002) and gliotoxin (P = 0.02). Cultures of A. fumigatus incubated in the presence of caspofungin for 4 or 24 h demonstrated enhanced gliotoxin release (P = 0.04 and 0.03, respectively) and biosynthesis (P = 0.04 and 0.03, respectively) compared to that by control cultures. The results presented here indicate that exposure of A. fumigatus to caspofungin results in increased cell permeability and an increase in the synthesis and release of gliotoxin. Since gliotoxin has well established immunosuppressive properties it is possible that exposure of A. fumigatus to caspofungin may potentiate the production of this toxin at the site of infection. Elevated gliotoxin biosynthesis may be an attempt by the fungus to restore the redox balance of the cell following exposure to the antifungal agent but the overall effect appears to be enhanced synthesis and release.

Antifungal and antihepatotoxic effects of sepia ink extract against oxidative stress as a risk factor of invasive pulmonary aspergillosis in neutropenic mice

BACKGROUND

There is a great need for novel strategies to overcome the high mortality associated with invasive pulmonary aspergillosis (IPA) in immunocompromised patients. To evaluate the antifungal and antihepatotoxic potentials of Sepia ink extract, its effect on liver oxidative stress levels was analyzed against IPA in neutropenic mice using amphotercin B as a reference drug.

MATERIALS AND METHODS

Eighty neutropenic infected mice were randomly assigned into four main groups. The 1(st) group was treated with saline, neutropenic infected (NI), the 2(nd) group was treated with ink extract (200 mg/kg) (IE) and the 3(rd) group was treated with amphotericin B (150 mg/kg) (AMB) and 4(th) group was treated with IE plus AMB. Treatment was started at 24 h after fungal inoculation (1×10(9) conidia/ml).

RESULTS

The present study revealed good in vitro and in vivo antifungal activity of IE against A. fumigatus. IE significantly reduced hepatic fungal burden and returns liver function and histology to normal levels. Compared with the untreated infected group, mice in the IE, AMB, and IE+ AMB groups had increased glutathione reduced (GSH) and superoxide dismutase (SOD) and significantly reduced malondialdehyde (MDA) levels at 24 and 72 h after inoculation with A. fumigatus conidia.

CONCLUSION

It is then concluded that in combination with antifungal therapy (AMB), IE treatment can reduce hepatic fungal burden, alleviate hepatic granulomatous lesions and oxidative stress associated with IPA in neutropenic mice.

Development and validation of an HPLC-MS/MS method for the early diagnosis of aspergillosis

Invasive aspergillosis is an opportunistic infection that is mainly caused by Aspergillus fumigatus, which is known to produce several secondary metabolites, including gliotoxin, the most abundant metabolite produced during hyphal growth. The diagnosis of invasive aspergillosis is often made late in the infection because of the lack of reliable and feasible diagnostic techniques; therefore, early detection is critical to begin treatment and avoid more serious complications. The present work reports the development and validation of an HPLC-MS/MS method for the detection of gliotoxin in the serum of patients with suspected aspergillosis. Chromatographic separation was achieved using an XBridge C18 column (150×2.1 mm id; 5 mm particle size) maintained at 25°C with the corresponding guard column (XBridge C18, 10×2.1 mm id, 5 mm particle size). The mobile phase was composed of a gradient of water and acetonitrile/water (95∶5 v/v), both containing 1 mM ammonium formate with a flow rate of 0.45 mL min⁻¹. Data from the validation studies demonstrate that this new method is highly sensitive, selective, linear, precise, accurate and free from matrix interference. The developed method was successfully applied to samples from patients suspected of having aspergillosis. Therefore, the developed method has considerable potential as a diagnostic technique for aspergillosis.

Strategies for mining fungal natural products

Fungi are well known for their ability to produce a multitude of natural products. On the one hand their potential to provide beneficial antibiotics and immunosuppressants has been maximized by the pharmaceutical industry to service the market with cost-efficient drugs. On the other hand identification of trace amounts of known mycotoxins in food and feed samples is of major importance to ensure consumer health and safety. Although several fungal natural products, their biosynthesis and regulation are known today, recent genome sequences of hundreds of fungal species illustrate that the secondary metabolite potential of fungi has been substantially underestimated. Since expression of genes and subsequent production of the encoded metabolites are frequently cryptic or silent under standard laboratory conditions, strategies for activating these hidden new compounds are essential. This review will cover the latest advances in fungal genome mining undertaken to unlock novel products.