Chaetominine, a cytotoxic alkaloid produced by endophytic Chaetomium sp. IFB-E015

Biodiversity in production of antibiotics and other bioactive compounds

Microbes continue to play a highly considerable role in the drug discovery and development process. Nevertheless, the number of new chemical entities (NCEs) of microbial origin that has been approved by the Food and Drug Administration (FDA) has been reduced in the past decade. This scarcity can be partly attributed to the redundancy in the discovered molecules from microbial isolates, which are isolated from common terrestrial ecological units. However, this situation can be partly overcome by exploring rarely exploited ecological niches as the source of microbes, which reduces the chances of isolating compounds similar to existing ones. The use of modern and advanced isolation techniques, modification of the existing fermentation methods, genetic modifications to induce expression of silent genes, analytical tools for the detection and identification of new chemical entities, use of polymers in fermentation to enhance yield of fermented compounds, and so on, have all aided in enhancing the frequency of acquiring novel compounds. These compounds are representative of numerous classes of diverse compounds. Thus, compounds of microbial origin and their analogues undergoing clinical trials continue to demonstrate the importance of compounds from microbial sources in modern drug discovery.

Inducing secondary metabolite production by the endophytic fungus Fusarium tricinctum through coculture with Bacillus subtilis

Coculturing the fungal endophyte Fusarium tricinctum with the bacterium Bacillus subtilis 168 trpC2 on solid rice medium resulted in an up to 78-fold increase in the accumulation in constitutively present secondary metabolites that included lateropyrone (5), cyclic depsipeptides of the enniatin type (6-8), and the lipopeptide fusaristatin A (9). In addition, four compounds (1-4) including (-)-citreoisocoumarin (2) as well as three new natural products (1, 3, and 4) were not present in discrete fungal and bacterial controls and only detected in the cocultures. The new compounds were identified as macrocarpon C (1), 2-(carboxymethylamino)benzoic acid (3), and (-)-citreoisocoumarinol (4) by analysis of the 1D and 2D NMR and HRMS data. Enniatins B1 (7) and A1 (8), whose production was particularly enhanced, inhibited the growth of the cocultivated B. subtilis strain with minimal inhibitory concentrations (MICs) of 16 and 8 μg/mL, respectively, and were also active against Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis with MIC values in the range 2-8 μg/mL. In addition, lateropyrone (5), which was constitutively present in F. tricinctum, displayed good antibacterial activity against B. subtilis, S. aureus, S. pneumoniae, and E. faecalis, with MIC values ranging from 2 to 8 μg/mL. All active compounds were equally effective against a multiresistant clinical isolate of S. aureus and a susceptible reference strain of the same species.

Discovery of a new class of immunosuppressants from Trichothecium roseum co-inspired by cross-kingdom similarity in innate immunity and pharmacophore motif

The limited selection of immunosuppressants in the clinic hampers the efficient management of immune disorders such as rejections after organ transplantations. However, the search for new immunosuppressive compounds remains random and creates inevitably financial and laborious wastes. Herein, we present an immunity-inspired discovery strategy that rationally allows an efficient identification of immunosuppressive compounds from the endophyte culture, as exemplified by the new peptide trichomide A. This compound exerts its immunosuppressive action more selectively than cyclosporin A. It was found that trichomide A decreases the expression of Bcl-2, increases the expression of Bax, and has a small or negligible effect on the expressions of p-Akt, CD25, and CD69. Our study strengthens the idea that the cross-kingdom similarity in immunity among living things could provide a shorter route towards the identification of natural products valuable for the development of new immunosuppressants.

Chemical Constituents from an Endophytic Fungus Chaetomium Globosum Z1

A new ergosterol, 15β-hydroxyl-(22 E,24 R)-ergosta-3,5,8,22-tetraen-one (1), along with three known ergosterols, two known cytochalasins, and two known azapholines were isolated from Chaetomium globosum Z1. The structures of these compounds were elucidated on the basis of spectroscopic methods (HR-ESI-MS, 1D NMR, and 2D NMR). Compound 6 showed significant cytotoxic activity against A-549 and MG-63 cell lines with IC50 values of 6.96 and 1.73 μg/mL, respectively.

Spiro fused diterpene-indole alkaloids from a creek-bottom-derived Aspergillus terreus

Four metabolites, teraspiridoles A-D (2-5), formed from the merger of a diterpene and modified indole scaffold were obtained from an Aspergillus terreus isolate. The structures and absolute configurations of these natural products were established using NMR, mass spectrometry, Marfey's method, VCD, and ECD data. Teraspiridole B (3) exhibited weak inhibition of planaria regeneration/survival.

Aniquinazolines A-D, four new quinazolinone alkaloids from marine-derived endophytic fungus Aspergillus nidulans

Four new quinazolinone alkaloids, namely, aniquinazolines A–D (1–4), were isolated and identified from the culture of Aspergillus nidulans MA-143, an endophytic fungus obtained from the leaves of marine mangrove plant Rhizophora stylosa. The structures of the new compounds were elucidated by spectroscopic analysis, and their absolute configurations were determined on the basis of chiral HPLC analysis of the acidic hydrolysates. The structure for 1 was confirmed by single-crystal X-ray diffraction analysis. All these compounds were examined for antibacterial and cytotoxic activity as well as brine shrimp (Artemia salina) lethality.

Short pathways to complexity generation: fungal peptidyl alkaloid multicyclic scaffolds from anthranilate building blocks

Complexity generation in naturally occurring peptide scaffolds can occur either by posttranslational modifications of nascent ribosomal proteins or through post assembly line tailoring of nonribosomal peptides. Short enzymatic pathways utilizing bimodular and trimodular nonribosomal peptide synthetase (NRPS) assembly lines, followed by tailoring oxygenases and/or prenyltransferases, efficiently construct complex fungal peptidyl alkaloid scaffolds in Aspergilli, Neosartorya, and Penicillium species. Use of the nonproteinogenic amino acid anthranilate as chain-initiating building block and chain-terminating intramolecular nucleophile leads efficiently to peptidyl alkaloid scaffolds with two to seven fused rings.

Photoassisted synthesis of enantiopure alkaloid mimics possessing unprecedented polyheterocyclic cores

Enantiopure alkaloid mimics are synthesized via high yielding intramolecular cycloadditions of photogenerated azaxylylenes tethered to pyrroles, with further growth of molecular complexity via post-photochemical transformations of primary photoproducts. This expeditious access to structurally unprecedented polyheterocyclic cores is being developed in the context of diversity-oriented synthesis, as the modular design allows for rapid "pre-assembly" of diverse photoprecursors from simple building blocks/diversity inputs.

Complexity generation in fungal peptidyl alkaloid biosynthesis: a two-enzyme pathway to the hexacyclic MDR export pump inhibitor ardeemin

Ardeemins are hexacyclic peptidyl alkaloids isolated from Aspergillus fischeri as agents that block efflux of anticancer drugs by MultiDrug Resistance (MDR) export pumps. To evaluate the biosynthetic logic and enzymatic machinery for ardeemin framework assembly, we sequenced the A. fischeri genome and identified the ardABC gene cluster. Through both genetic deletions and biochemical characterizations of purified ArdA and ArdB we show this ArdAB enzyme pair is sufficient to convert anthranilate (Ant), L-Ala, and L-Trp to ardeemin. ArdA is a 430 kDa trimodular nonribosomal peptide synthase (NRPS) that converts the three building blocks into a fumiquinazoline (FQ) regioisomer termed ardeemin FQ. ArdB is a prenyltransferase that takes tricyclic ardeemin FQ and dimethylallyl diphosphate to the hexacyclic ardeemin scaffold via prenylation at C2 of the Trp-derived indole moiety with intramolecular capture by an amide NH of the fumiquinazoline ring. The two-enzyme ArdAB pathway reveals remarkable efficiency in construction of the hexacyclic peptidyl alkaloid scaffold.

Alkaloids Produced by Endophytic Fungi: A Review

In recent years, a number of alkaloids have been discovered from endophytic fungi in plants, which exhibited excellent biological properties such as antimicrobial, insecticidal, cytotoxic, and anticancer activities. This review mainly deals with the research progress on endophytic fungi for producing bioactive alkaloids such as quinoline and isoquinoline, amines and amides, indole derivatives, pyridines, and quinazolines. The biological activities and action mechanisms of these alkaloids from endophytic fungi are also introduced. Furthermore, the relationships between alkaloid-producing endophytes and their host plants, as well as their potential applications in the future are discussed.

Quinazolin-4-one coupled with pyrrolidin-2-iminium alkaloids from marine-derived fungus Penicillium aurantiogriseum

Three new alkaloids, including auranomides A and B (1 and 2), a new scaffold containing quinazolin-4-one substituted with a pyrrolidin-2-iminium moiety, and auranomide C (3), as well as two known metabolites auranthine (4) and aurantiomides C (5) were isolated from the marine-derived fungus Penicillium aurantiogriseum. The chemical structures of compounds 1–3 were elucidated by extensive spectroscopic methods, including IR, HRESIMS and 2D NMR spectroscopic analysis. The absolute configurations of compounds 1–3 were suggested from the perspective of a plausible biosynthesis pathway. Compounds 1–3 were subjected to antitumor and antimicrobial screening models. Auranomides A–C exhibited moderate cytotoxic activity against human tumor cells. Auranomides B was the most potent among them with an IC₅₀ value of 0.097 μmol/mL against HEPG2 cells.

Synthesis of 3-substituted and 2,3-disubstituted quinazolinones via Cu-catalyzed aryl amidation

CuI/4-hydroxy-L-proline catalyzed coupling of N-substituted o-bromobenzamides with formamide takes place at 80 °C, affording 3-substituted quinazolinones directly. Under these conditions other amides that were tested only provided simple coupling products, which can be converted into 2,3-disubstituted quinazolinones via HMDS/ZnCl(2) mediated condensative cyclization.

Xylarenones C-E from an endophytic fungus isolated from Alibertia macrophylla

Xylarenones C-E (2-4), three new eremophilane sesquiterpenes, have been isolated from solid substrate cultures of a Camarops-like endophytic fungus isolated from Alibertia macrophylla. The structures were elucidated by analysis of spectroscopic data. Compounds were evaluated in subtilisin and pepsin protease assays, and compound 2 showed potent inhibitory activity against both proteases.

Visible light-induced intramolecular cyclization reactions of diamines: a new strategy to construct tetrahydroimidazoles

A new and efficient synthesis of highly substituted tetrahydroimidazole derivatives by means of visible light-induced intramolecular cyclization reactions has been described. This photoredox catalytic reaction exhibited high diastereoselectivity and afforded the desired products in good yields.

Fungal indole alkaloid biosynthesis: genetic and biochemical investigation of the tryptoquialanine pathway in Penicillium aethiopicum

Tremorgenic mycotoxins are a group of indole alkaloids which include the quinazoline-containing tryptoquivaline (2) that are capable of eliciting intermittent or sustained tremors in vertebrate animals. The biosynthesis of this group of bioactive compounds, which are characterized by an acetylated quinazoline ring connected to a 6-5-5 imidazoindolone ring system via a 5-membered spirolactone, has remained uncharacterized. Here, we report the identification of a gene cluster (tqa) from P. aethiopicum that is involved in the biosynthesis of tryptoquialanine (1), which is structurally similar to 2. The pathway has been confirmed to go through an intermediate common to the fumiquinazoline pathway, fumiquinazoline F, which originates from a fungal trimodular nonribosomal peptide synthetase (NRPS). By systematically inactivating every biosynthetic gene in the cluster, followed by isolation and characterization of the intermediates, we were able to establish the biosynthetic sequence of the pathway. An unusual oxidative opening of the pyrazinone ring by an FAD-dependent berberine bridge enzyme-like oxidoreductase has been proposed based on genetic knockout studies. Notably, a 2-aminoisobutyric acid (AIB)-utilizing NRPS module has been identified and reconstituted in vitro, along with two putative enzymes of unknown functions that are involved in the synthesis of the unnatural amino acid by genetic analysis. This work provides new genetic and biochemical insights into the biosynthesis of this group of fungal alkaloids, including the tremorgens related to 2.

Enzymatic processing of fumiquinazoline F: a tandem oxidative-acylation strategy for the generation of multicyclic scaffolds in fungal indole alkaloid biosynthesis

Aspergillus fumigatus Af293 is a known producer of quinazoline natural products, including the antitumor fumiquinazolines, of which the simplest member is fumiquinazoline F (FQF) with a 6-6-6 tricyclic core derived from anthranilic acid, tryptophan, and alanine. FQF is the proposed biological precursor to fumiquinazoline A (FQA) in which the pendant indole side chain has been modified via oxidative coupling of an additional molecule of alanine, yielding a fused 6-5-5 imidazoindolone. We recently identified fungal anthranilate-activating nonribosomal peptide synthetase (NRPS) domains through bioinformatics approaches. One domain previously identified is part of the trimodular NRPS Af12080, which we predict is responsible for FQF formation. We now show that two adjacent A. fumigatus ORFs, a monomodular NRPS Af12050 and a flavoprotein Af12060, are necessary and sufficient to convert FQF to FQA. Af12060 oxidizes the 2',3'-double bond of the indole side chain of FQF, and the three-domain NRPS Af12050 activates l-Ala as the adenylate, installs it as the pantetheinyl thioester on its carrier protein domain, and acylates the oxidized indole for subsequent intramolecular cyclization to create the 6-5-5 imidazolindolone of FQA. This work provides experimental validation of the fumiquinazoline biosynthetic cluster of A. fumigatus Af293 and describes an oxidative annulation biosynthetic strategy likely shared among several classes of polycyclic fungal alkaloids.

Scalable total syntheses of N-linked tryptamine dimers by direct indole-aniline coupling: psychotrimine and kapakahines B and F

This report details the invention of a method to enable syntheses of psychotrimine (1) and the kapakahines F and B (2, 3) on a gram scale and in a minimum number of steps. Mechanistic inquiries are presented for the key enabling quaternization of indole at the C3 position by electrophilic attack of an activated aniline species. Excellent chemo-, regio-, and diastereoselectivities are observed for reactions with o-iodoaniline, an indole cation equivalent. Additionally, the scope of this reaction is broad with respect to the tryptamine and aniline components. The anti-cancer profiles of 1-3 have also been evaluated.

Cytotoxic and antibacterial activities of endophytic fungi isolated from plants at the National Park, Pahang, Malaysia

BACKGROUND

Endophytes, microorganisms which reside in plant tissues, have potential in producing novel metabolites for exploitation in medicine. Cytotoxic and antibacterial activities of a total of 300 endophytic fungi were investigated.

METHODS

Endophytic fungi were isolated from various parts of 43 plants from the National Park Pahang, Malaysia. Extracts from solid state culture were tested for cytotoxicity against a number of cancer cell lines using the MTT assay. Antibacterial activity was determined using the disc diffusion method.

RESULTS

A total of 300 endophytes were isolated from various parts of plants from the National Park, Pahang. 3.3% of extracts showed potent (IC50 < 0.01 microg/ml) cytotoxic activity against the murine leukemic P388 cell line and 1.7% against a human chronic myeloid leukemic cell line K562. Sporothrix sp. (KK29FL1) isolated from Costus speciosus showed strong cytotoxicity against colorectal carcinoma (HCT116) and human breast adenocarcinoma (MCF7) cell lines with IC50 values of 0.05 microg/ml and 0.02 microg/ml, respectively. Antibacterial activity was demonstrated for 8% of the extracts.

CONCLUSION

Results indicate the potential for production of bioactive agents from endophytes of the tropical rainforest flora.

A straightforward total synthesis of (-)-chaetominine

A total synthesis of the tripeptide alkaloid (-)-chaetominine (1) was achieved in 9.3% overall yield starting from commercially available D-tryptophan methyl ester, based on a short and straightforward (nine steps) sequence. The early stage introduction (first step) of the quinazolinone moiety and the late stage introduction (penultimate step) of the hydroxy group allowed a synthetic strategy devoid of protective groups. The key step of the process is the a-c tricyclic ring construction via an unprecedented NCS-mediated N-acyl cyclization on an indole ring to give tetrahydro-1H-pyrido[2,3-b]indole 11. In the penultimate step, oxidation of the tetracyclic intermediate 14 with oxaziridine 15 gave only one of the four possible diastereoisomers, the cis-diastereoisomer 16 resulting from the attack of the oxaziridine to the double bond face opposite to the c-d ring substituents. In the last step, the complete stereocontrol of the Et(3)SiH/TFA reduction of compound 16, probably involving a N-acyliminium ion, can be attributed to ring constrain, which forces the b-c ring junction in the more stable cis-orientation. (-)-Chaetominine (1) showed a negligible inhibitory activity on several cancer cell lines.

Cytotoxic cytochalasin metabolites of endophytic Endothia gyrosa

In addition to the known metabolites cytochalasin H (1), cytochalasin J (2), and epoxycytochalasin H (3), two new 10-phenyl-(11)-cytochalasans, named cytochalasin Z10 and Z11 (4 and 5, resp.) were isolated from the solid substrate culture of Endothia gyrosa IFB-E023, an endophytic fungus residing inside the healthy leaf of Vatica mangachapo (Dipterocarpaceae). The structure determination of 4 and 5 was accomplished through correlative analyses of their spectral data (UV, ESI-MS, IR, (1)H- and (13)C-NMR, COSY, NOESY, HMQC, and HMBC). Metabolites 1-5 were demonstrated to be substantially cytotoxic to the human leukaemia K562 cell line with the IC(50) values of 10.1, 1.5, 24.5, 28.3, and 24.4 microM, respectively, which are comparable to that of 5-fluorouracil (33.0 microM), co-assayed as the positive reference.

Bioactive alkaloids from endophytic Aspergillus fumigatus

Two new alkaloids, named 9-deacetylfumigaclavine C (1) and 9-deacetoxyfumigaclavine C (2), along with 12 known compounds (3-14), were isolated from the culture of Aspergillus fumigatus. The structures of the new compounds were elucidated by comprehensive spectroscopic analyses. Compound 2 showed selectively potent cytotoxicity against human leukemia cells (K562) with an IC(50) value of 3.1 microM, which was comparable to that of doxorubicin hydrochloride, a presently prescribed drug for the treatment of leukemia. Furthermore, 14-norpseurotin (4) significantly induced neurite outgrowth of rat pheochromocytoma cells (PC12) at a 10.0 microM concentration.

New phytotoxic metabolites from Pestalotiopsis sp. HC02, a Fungus Residing in Chondracris rosee gut

Two new phytotoxic gamma-lactones, pestalotines A and B (1 and 2, resp.), along with 4-oxo-4H-pyran-3-acetic acid (3) and 6-hydroxyramulosin (=3,4,4a,5,6,7-hexahydro-6,8-dihydroxy-3-methyl-1H-2-benzopyran-1-one; 4), were isolateded from the culture of Pestalotiopsis sp. HC02, a fungus residing in the Chondracris rosee gut. Structures of the new metabolites were elucidated on the basis of their IR, NMR, and MS data. Pestalotines A and B (1 and 2, resp.) significantly inhibited the radical growth of Echinochloa crusgalli with IC(50) values of 1.85 x 10(-4) and 2.50 x 10(-4) M, respectively, comparable to that of 2-(2,4-dichlorophenoxy)acetic acid (0.94 x 10(-4) M) used as a positive control.

Chaetoglobins A and B, two unusual alkaloids from endophytic Chaetomium globosum culture

Chaetoglobins A (1) and B (2), two azaphilone alkaloid dimers with an unprecedented skeleton, were characterized from an endophytic fungus Chaetomium globosum with the former ascertained to be a significant cytotoxin valuable for anti-tumor drug discovery.

Penicidones A-C, three cytotoxic alkaloidal metabolites of an endophytic Penicillium sp

Along with the known secondary metabolites lumichrome, physcion, and emodin-1,6-dimethyl ether, three alkaloids named penicidones A-C (1-3) were isolated from the culture of Penicillium sp. IFB-E022, an endophytic fungal strain residing in the stem of Quercus variabilis (Fagaceae). The structures of penicidones A-C were established by a correlative interpretation of spectroscopic data including IR, UV and HR-ESI-MS, as well as by analysis of a set of 1D and 2D NMR experiments. The stereochemistry of compounds 1 and 2 was obtained by comparison of the optical rotation with those of vermistatin and its analogues. Penicidones A-C were the first group of natural products possessing a penicidone framework. Compounds 1-3 exhibited moderate cytotoxicity against four cancer cell lines.

Synthesis of (-)-chaetominine

The tricyclic hydroxy imidazolidinone was converted to chaetominine in seven steps in 22% overall yield. The key step was the construction of the delta-lactam by heating an amino ester with a catalytic amount of DMAP in toluene at reflux.