Bioactive anthraquinone dimers from the leafhopper pathogenic fungus Torrubiella sp. BCC 28517

Diverse host-associated fungal systems as a dynamic source of novel bioactive anthraquinones in drug discovery: Current status and future perspectives

BACKGROUND

Despite, a large number of bioactive anthraquinones (AQs) isolated from host-living fungi, only plant-derived AQs were introduced in the global consumer markets. Host-living fungi represents renewable and extendible resources of diversified metabolites to be exploited for bioactives production. Unique classes of AQs from fungi include halogenated and steroidal AQs, and absent from planta are of potential to explore for biological activity against urging diseases such as cancer and multidrug-resistant pathogens. The structural diversity of fungal AQs, monomers, dimers, trimers, halogenated, etc… results in a vast range of pharmacological activities.

AIM OF REVIEW

The current study capitalizes on uncovering the diversity and distribution of host-living fungal systems producing AQs in different terrestrial ecosystems ranging from plant endophytes, lichens, animals and insects. Furthermore, the potential bioactivities of fungal derived AQs i.e., antibacterial, antifungal, antiviral (anti-HIV), anticancer, antioxidant, diuretic and laxative activities are assembled in relation to their structure activity relationship (SAR). Analyzing for structure-activity relationship among fungal AQs may facilitate bioengineering of more potential analogues. Withal, elucidation of AQs biosynthetic pathways in fungi is discussed from different fungal hosts to open up new possibilities for potential biotechnological applications. Such comprehensive review unravels terrestrial host-living fungal systems as a treasure trove in drug discovery, in addition to future perspectives and trends for their exploitation in pharmaceutical industries.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Such comprehensive review unravels terrestrialhost-living fungal systems as a treasure trove in drug discovery, in addition to future perspectives and trends for their exploitation in pharmaceutical industries.

Exploration of Potent Cytotoxic Molecules from Fungi in Recent Past to Discover Plausible Anticancer Scaffolds

Fungi are known to produce diverse scaffolds possessing unique biological activities, however, to date, no molecule discovered from a fungal source has reached the market as an anti-cancer drug. Every year number of cytotoxic molecules of fungal origin are getting published and critical analysis of those compounds is necessary to identify the potent ones. A review mentioning the best cytotoxic fungal metabolites and their status in the drug development was published in 2014. In this report, we have included 176 cytotoxic molecules isolated from fungi after 2014 and categorized them according to their potencies such as IC₅₀ values below 1 μM, 1-5 μM, and 5-10 μM. The emphasis was given to those 42 molecules which have shown IC₅₀ less than 1 μM and discussed to a great extent. This review shall provide potent scaffolds of fungal origin which can be given priority in the development as a drug candidate for cancer therapeutics.

Recent total syntheses of anthraquinone-based natural products

The scientific community has found deep interest in anthraquinone-based compounds due to their therapeutic properties and challenging structural elements. Various architecturally beautiful natural products have been successfully synthesized in recent decades utilizing two main strategies: either an early-stage synthesis of the anthraquinone and further elongation of the system, or a late-stage introduction of the anthraquinone ring moiety. Select syntheses of complex anthraquinone monomers and dimers within the past 20 years are described with an emphasis on the retrosynthetic disconnections that shape the anthraquinone-installation strategy.

Synthetic Studies of the Rubellin Natural Products: Development of a Stereoselective Strategy and Total Synthesis of (+)-Rubellin C

This manuscript describes our studies of the class of natural products known as the rubellins, culminating in the total synthesis of (+)-rubellin C. These anthraquinone-based natural products contain a variety of stereochemical and architectural motifs, including a 6-5-6-fused ring system, 5 stereogenic centers, and a central quaternary center. Herein, we report our development of a strategy to target the stereochemically dense core and anthraquinone nucleus, including approaches such as a bifunctional allylboron and vinyl triflate reagent, an anthraquinone benzylic metalation strategy, and a late-stage anthraquinone introduction strategy. Our studies culminate in a successful route to highly functionalized anthraquinone-based natural product scaffolds and a stereoselective total synthesis of (+)-rubellin C. These strategies and outcomes will aid in synthetic planning toward anthraquinone-based natural products of high interest.

Peniterpenoids A-C, new sesquiterpenoid metabolites from a wheat cyst nematode Penicillium janthinellum

Three new sesquiterpenoids, peniterpenoids A - C (1-3), together with six known metabolites (4-9) were isolated from an entomogenous fungus Penicillium janthinellum (LB1.20090001) collected from a wheat cyst nematode. The structures of the new compounds were elucidated based on NMR and HRESIMS spectroscopic analyses. The absolute configuration of the C-8 secondary alcohol of peniterpenoid B (2) was determined by [Rh₂(OCOCF₃)₄]-induced ECD experiment. Subsequently, the antimicrobial and DPPH scavenging activities were determined. Compounds 6-8 exhibited moderate antibacterial activities against Staphylococcus aureus (CGMCC1.2465) with MIC values of 25.0, 50.0 and 12.5 μg/mL, respectively.

Total Synthesis of (+)-Rubellin C

The rubellins are a family of stereochemically complex anthraquinoid heterodimers containing an unprecedented chemical scaffold. Although the rubellins have been known for over three decades, no total synthesis has been achieved since their discovery. Their topology is characterized by a 6-5-6 fused ring system, five neighboring stereocenters including a quaternary center all in a convoluted core, and an anthraquinone nucleus. The rubellin architecture has been shown to inhibit and reverse the aggregation of tau protein, a therapeutically relevant target for Alzheimer's disease. Herein, we describe the first stereoselective synthesis of a member of the family, (+)-rubellin C, in 16 steps. Strategic disconnections allow expedient construction of stereochemical and topological intricacy in a short sequence of borylative and transition metal-catalyzed steps.

Fungal Bioactive Anthraquinones and Analogues

This review, covering the literature from 1966 to the present (2020), describes naturally occurring fungal bioactive anthraquinones and analogues biosynthesized by the acetate route and concerning several different functionalized carbon skeletons. Hydrocarbons, lipids, sterols, esters, fatty acids, derivatives of amino acids, and aromatic compounds are metabolites belonging to other different classes of natural compounds and are generated by the same biosynthetic route. All of them are produced by plant, microorganisms, and marine organisms. The biological activities of anthraquinones and analogues comprise phytotoxic, antibacterial, antiviral, anticancer, antitumor, algicide, antifungal, enzyme inhibiting, immunostimulant, antiplatelet aggregation, cytotoxic, and antiplasmodium activities. The review also covers some practical industrial applications of anthraquinones.

Secondary metabolites from hypocrealean entomopathogenic fungi: novel bioactive compounds

Covering: 2014 up to the third quarter of 2019 Entomopathogens constitute a unique, specialized trophic subgroup of fungi, most of whose members belong to the order Hypocreales (class Sordariomycetes, phylum Ascomycota). These Hypocrealean Entomopathogenic Fungi (HEF) produce a large variety of secondary metabolites (SMs) and their genomes rank highly for the number of predicted, unique SM biosynthetic gene clusters. SMs from HEF have diverse roles in insect pathogenicity as virulence factors by modulating various interactions between the producer fungus and its insect host. In addition, these SMs also defend the carcass of the prey against opportunistic microbial invaders, mediate intra- and interspecies communication, and mitigate abiotic and biotic stresses. Thus, these SMs contribute to the role of HEF as commercial biopesticides in the context of integrated pest management systems, and provide lead compounds for the development of chemical pesticides for crop protection. These bioactive SMs also underpin the widespread use of certain HEF as nutraceuticals and traditional remedies, and allowed the modern pharmaceutical industry to repurpose some of these molecules as life-saving human medications. Herein, we survey the structures and biological activities of SMs described from HEF, and summarize new information on the roles of these metabolites in fungal virulence.

Selected Fungal Natural Products with Antimicrobial Properties

Fungal natural products and their effects have been known to humankind for hundreds of years. For example, toxic ergot alkaloids produced by filamentous fungi growing on rye poisoned thousands of people and livestock throughout the Middle Ages. However, their later medicinal applications, followed by the discovery of the first class of antibiotics, penicillins and other drugs of fungal origin, such as peptidic natural products, terpenoids or polyketides, have altered the historically negative reputation of fungal "toxins". The development of new antimicrobial drugs is currently a major global challenge, mainly due to antimicrobial resistance phenomena. Therefore, the structures, biosynthesis and antimicrobial activity of selected fungal natural products are described here.

Antiplasmodial natural products: an update

Background

Malaria remains a significant public health challenge in regions of the world where it is endemic. An unprecedented decline in malaria incidences was recorded during the last decade due to the availability of effective control interventions, such as the deployment of artemisinin-based combination therapy and insecticide-treated nets. However, according to the World Health Organization, malaria is staging a comeback, in part due to the development of drug resistance. Therefore, there is an urgent need to discover new anti-malarial drugs. This article reviews the literature on natural products with antiplasmodial activity that was reported between 2010 and 2017.

Methods

Relevant literature was sourced by searching the major scientific databases, including Web of Science, ScienceDirect, Scopus, SciFinder, Pubmed, and Google Scholar, using appropriate keyword combinations.

Results and Discussion

A total of 1524 compounds from 397 relevant references, assayed against at least one strain of Plasmodium, were reported in the period under review. Out of these, 39% were described as new natural products, and 29% of the compounds had IC₅₀ ≤ 3.0 µM against at least one strain of Plasmodium. Several of these compounds have the potential to be developed into viable anti-malarial drugs. Also, some of these compounds could play a role in malaria eradication by targeting gametocytes. However, the research into natural products with potential for blocking the transmission of malaria is still in its infancy stage and needs to be vigorously pursued.

Chemoenzymatic reduction of citreorosein and its implications on aloe-emodin and rugulosin C (bio)synthesis

A chemoenzymatic reduction of citreorosein by the NADPH-dependent polyhydroxyanthracene reductase from Cochliobolus lunatus or MdpC from Aspergillus nidulans in the presence of Na₂S₂O₄ gave access to putative biosynthetic intermediates, (R)-3,8,9,10-tetrahydroxy-6-(hydroxymethyl)-3,4-dihydroanthracene-1(2H)-one and its oxidized form, (R)-3,4-dihydrocitreorosein. Herein, we discuss the implications of these results towards the (bio)synthesis of aloe-emodin and (+)-rugulosin C in fungi.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Perplexing Metabolomes in Fungal-Insect Trophic Interactions: A Terra Incognita of Mycobiocontrol Mechanisms

The trophic interactions of entomopathogenic fungi in different ecological niches viz., soil, plants, or insect themselves are effectively regulated by their maneuvered metabolomes and the plethora of metabotypes. In this article, we discuss a holistic framework of co-evolutionary metabolomes and metabotypes to model the interactions of biocontrol fungi especially with mycosed insects. Conventionally, the studies involving fungal biocontrol mechanisms are reported in the context of much aggrandized fungal entomotoxins while the adaptive response mechanisms of host insects are relatively overlooked. The present review asserts that the selective pressure exerted among the competing or interacting species drives alterations in their overall metabolomes which ultimately implicates in corresponding metabotypes. Quintessentially, metabolomics offers a most generic and tractable model to assess the fungal-insect antagonism in terms of interaction biomarkers, biosynthetic pathway plasticity, and their co-evolutionary defense. The fungi chiefly rely on a battery of entomotoxins viz., secondary metabolites falling in the categories of NRP's (non-ribosomal peptides), PK's (polyketides), lysine derive alkaloids, and terpenoids. On the contrary, insects overcome mycosis through employing different layers of immunity manifested as altered metabotypes (phenoloxidase activity) and overall metabolomes viz., carbohydrates, lipids, fatty acids, amino acids, and eicosanoids. Here, we discuss the recent findings within conventional premise of fungal entomotoxicity and the evolution of truculent immune response among host insect. The metabolomic frameworks for fungal-insect interaction can potentially transmogrify our current comprehensions of biocontrol mechanisms to develop the hypervirulent biocontrol strains with least environmental concerns. Moreover, the interaction metabolomics (interactome) in complementation with other -omics cascades could further be applied to address the fundamental bottlenecks of adaptive co-evolution among biological species.

Chemical constituents from the aerial parts of Melandrium firmum

Two new anthraquinones, melrubiellin C (1) and melrubiellin D (2), were isolated from the aerial parts of Melandrium firmum Rohrbach, together with eight known compounds (3-10). The structures of these compounds were elucidated using 1D and 2D NMR (COSY, HMQC, HMBC and NOESY) experiments. All isolated compounds were tested for their cytotoxicity against NCI-H460, Hep G2, MKN-28 and A-549 cells. Of these 10 compounds, 1 and 2 exhibited moderate cytotoxicity with IC50 values ranging from 9.54 to 32.41 μM.

A novel oxybis cresol verticilatin with highly varying degrees of biological activities from the insect pathogenic fungus Paecilomyces verticillatus

A novel oxybis cresol compound named verticilatin (1), together with two known compounds, 5-methylresorcinol (2) and 2,4-dihydroxy-3,6-dimethylbenzaldehyde (3), was isolated from cultures of the insect pathogenic fungi Paecilomyces verticillatus. The structures of compounds were determined by extensive spectroscopic analysis of HR-ESI-MS and 1D and 2D NMR including HSQC, HMBC, COSY, and ROESY. Fortunately, compound 1 exhibited significant inhibitory activities against CDC25B, cathepsin B, MEG2, and SHP2 enzyme, with IC50 values of 11.5, 3.5, 7.8, and 15 μg/ml, respectively.

Recent investigations of bioactive natural products from endophytic, marine-derived, insect pathogenic fungi and Thai medicinal plants

Living organisms in Thailand are very diverse due to the unique geographical location of Thailand. The diversity of Thai bioresources has proven to be a rich source of biologically active compounds. The present review covers bioactive substances from Thai endophytic, marine-derived, insect pathogenic fungi and medicinal plants. Many new compounds isolated from Thai bioresources have diverse skeletons belonging to various classes of natural products. These compounds exhibited an array of biological activities, and some are of pharmaceutical interest. Bioactive compounds from Thai bioresources have not only attracted organic chemists to develop strategies for total synthesis, but also attracted (chemical) biologists to investigate the mechanisms of action. The chemistry and biology of some selected compounds are also discussed in this review.

Torrubiellone E, an Antimalarial N-Hydroxypyridone Alkaloid from the Spider Pathogenic Fungus Torrubiella longissima BCC 2022

Torrubiellone E (1), a new N-hydroxypyridone alkaloid, was isolated from the spider pathogenic fungus Torrubiella longissima BCC 2022, together with the known compounds, torrubiellones A (2) and B (3), and JBIR-130 (4). Compound 1 exhibited antimalarial activity against Plasmodium falciparum K1 with an IC50 value of 3.2 μg/mL, while it also showed weak cytotoxic activities.

Cyclic lipodepsipeptides verlamelin A and B, isolated from entomopathogenic fungus Lecanicillium sp

Verlamelin and its new derivative (verlamelin B) were isolated from fermentation broth of entomopathogenic fungus Lecanicillium sp. HF627. As the structural elucidation of verlamelin so far was only preliminary, we studied and determined the absolute structure of these two compounds to be cyclo(5S-hydroxytetradecanoic acid-D-alloThr/Ser-D-Ala-L-Pro-L-Gln-D-Tyr-L-Val). This is the first study that precisely analyzed the structure of verlamelin.

Cytotoxic anthraquinone dimers from Melandrium firmum

Two new anthraquinone dimers, melrubiellin A (1) and melrubiellin B (2), were isolated from the aerial part of Melandrium firmum Rohrbach, along with seven known compounds (3-9). The structures of these compounds were elucidated by spectral analyses, including 1D and 2D NMR (COSY, HMQC, HMBC and NOESY) experiments. Compound 1 and 2 exhibited significant cytotoxicity towards HeLa, NCI-H460, Hep G2, Hep 3B and MKN-28 cell lines with IC50 values ranging from 5.26 to 81.16 μM.

Penostatin derivatives, a novel kind of protein phosphatase 1b inhibitors isolated from solid cultures of the entomogenous fungus Isaria tenuipes

Protein tyrosine phosphatase 1B (PTP1B) is implicated as a negative regulator of insulin receptor (IR) signaling and a potential drug target for the treatment of type II diabetes and other associated metabolic syndromes. Therefore, small molecular inhibitors of PTP1B can be considered as an attractive approach for the design of new therapeutic agents of type II diabetes diseases. In a continuing search for new protein phosphatase inhibitors from fungi, we have isolated a new compound, named penostatin J (1), together with three known ones, penostatin C (2), penostatin A (3), and penostatin B (4), from cultures of the entomogenous fungus Isaria tenuipes. The structure of penostatin J (1) was elucidated by extensive spectroscopic analysis. We also demonstrate for the first time that penostatin derivatives exhibit the best PTP1B inhibitory action. These findings suggest that penostatin derivatives are a potential novel kind of PTP1B inhibitors.