Abdessamad Debbab

Fungal endophytes from higher plants: a prolific source of phytochemicals and other bioactive natural products

Bioactive natural products from endophytic fungi, isolated from higher plants, are attracting considerable attention from natural product chemists and biologists alike as indicated by the steady increase of publications devoted to this topic during recent years (113 research articles on secondary metabolites from endophytic fungi in the period of 2008–2009, 69 in 2006–2007, 36 in 2004–2005, 14 in 2002–2003, and 18 in 2000–2001). This overview will highlight the chemical potential of endophytic fungi with focus on the detection of pharmaceutically valuable plant constituents, e.g. paclitaxel, camptothecin and podophyllotoxin, as products of fungal biosynthesis. In addition, it will cover new bioactive metabolites reported in recent years (2008–2009) from fungal endophytes of terrestrial and mangrove plants. The presented compounds are selected based on their antimicrobial, antiparasitic, cytotoxic as well as neuroprotective activities. Furthermore, possible factors influencing natural product production in endophytes cultivated in vitro and hence the success of bioprospecting from endophytes are likewise discussed in this review.

Fungal endophytes: unique plant inhabitants with great promises

Fungal endophytes residing in the internal tissues of living plants occur in almost every plant on earth from the arctic to the tropics. The endophyte–host relationship is described as a balanced symbiotic continuum ranging from mutualism through commensalism to parasitism. This overview will highlight selected aspects of endophyte diversity, host specificity, endophyte–host interaction and communication as well as regulation of secondary metabolite production with emphasis on advanced genomic methods and their role in improving our current knowledge of endophytic associations. Furthermore, the chemical potential of endophytic fungi for drug discovery will be discussed with focus on the detection of pharmaceutically valuable plant constituents as products of fungal biosynthesis. In addition, selected examples of bioactive metabolites reported in recent years (2008–2010) from fungal endophytes residing in terrestrial plants are presented grouped according to their reported biological activities.

Bioactive Compounds from Marine Bacteria and Fungi

Marine bacteria and fungi are of considerable importance as new promising sources of a huge number of biologically active products. Some of these marine species live in a stressful habitat, under cold, lightless and high pressure conditions. Surprisingly, a large number of species with high diversity survive under such conditions and produce fascinating and structurally complex natural products. Up till now, only a small number of microorganisms have been investigated for bioactive metabolites, yet a huge number of active substances with some of them featuring unique structural skeletons have been isolated. This review covers new biologically active natural products published recently (2007–09) and highlights the chemical potential of marine microorganisms, with focus on bioactive products as well as on their mechanisms of action.

Bioactive secondary metabolites from endophytes and associated marine derived fungi

Endophytes and fungi that are associated with marine invertebrates or with algae are of growing importance as promising sources of structurally unprecedented biologically active natural products. This review covers the literature published in 2010 and highlights new bioactive metabolites and known compounds for which hitherto novel biological activities have been reported. The compounds are grouped according to their reported biological activities which include anti-infective, cytotoxic, anti-parasitic, radical scavenging and anti-inflammatory activities. Overall, 106 fungal metabolites, including 87 new natural products, and 53 references are presented.

Bioactive Metabolites from the Endophytic Fungus Stemphylium globuliferum Isolated from Mentha pulegium

The endophytic fungus Stemphylium globuliferum was isolated from stem tissues of the Moroccan medicinal plant Mentha pulegium. Extracts of the fungus, which was grown on solid rice medium, exhibited considerable cytotoxicity when tested in vitro against L5178Y cells. Chemical investigation yielded five new secondary metabolites, alterporriol G (4) and its atropisomer alterporriol H (5), altersolanol K (11), altersolanol L (12), stemphypyrone (13), and the known compounds 6-O-methylalaternin (1), macrosporin (2), altersolanol A (3), alterporriol E (6), alterporriol D (7), alterporriol A (8), alterporriol B (9), and altersolanol J (10). The structures were determined on the basis of one- and two-dimensional NMR spectroscopy and mass spectrometry. Among the alterporriol-type anthranoid dimers, the mixture of alterporriols G and H (4/5) exhibited considerable cytotoxicity against L5178Y cells with an EC(50) value of 2.7 microg/mL, whereas the other congeners showed only modest activity. The compounds were also tested for kinase inhibitory activity in an assay involving 24 different kinases. Compounds 1, 2, 3, and the mixture of 4 and 5 were the most potent inhibitors, displaying EC(50) values between 0.64 and 1.4 microg/mL toward individual kinases.

Endophytes and associated marine derived fungi—ecological and chemical perspectives

The potential of endophytes and algal or invertebrate associated marine derived fungi as promising sources of structurally unprecedented bioactive natural products is undeniable and continues to attract broad attention. This review highlights new bioactive fungal metabolites reported in 2011 until April 2012, as well as known compounds for which novel biological activities have been disclosed. All compounds are grouped according to their reported biological activities which include cytotoxic, anti-infective, as well as radical scavenging, enzyme inhibition, anti-fouling and anti-parasitic activities. Overall, 178 fungal metabolites, including 138 new natural products are presented. Furthermore, new insights into fungal-host interaction, chemical communication, and chemo-ecological roles of fungal metabolites, as well as new strategies for bioprospecting are presented.

Enniatins A1, B and B1 from an endophytic strain of Fusarium tricinctum induce apoptotic cell death in H4IIE hepatoma cells accompanied by inhibition of ERK phosphorylation.

Enniatins are mycotoxins which have important impact on human health, e.g. as contaminants of cereals, but also are discussed as possible anticancer agents. We investigated toxic effects of enniatins A1, B and B1 isolated from Fusarium tricinctum on different cancer cell lines. The enniatins showed moderate activity in HepG2 and C6 cells (EC(50)-values approximately 10-25 microM), but were highly toxic in H4IIE cells (EC(50)-values approximately 1-2.5 microM). In H4IIE cells, all enniatins increased caspase 3/7 activity and nuclear fragmentation as markers for apoptotic cell death. Enniatin A1, enniatin B1, and, to a lesser extent, also enniatin B decreased the activation of extracellular regulated protein kinase (ERK) (p44/p42), a mitogen-activated protein kinase which is associated with cell proliferation. Furthermore, enniatins A1 and B1, but not enniatin B were able to inhibit moderately tumor necrosis factor alpha (TNF-alpha)-induced NF-kappaB activation. Screening of 24 additional protein kinases involved in signal transduction pathways (cell proliferation, survival, angiogenesis and metastasis) showed no inhibitory activity of enniatins. We conclude that enniatins A1 and B1 and, to a lesser extent, enniatin B may possess anticarcinogenic properties by induction of apoptosis and disruption of ERK signalling pathway. Further analysis of these substances is necessary to analyse their usefulness for cancer therapy.

Protein Kinase and HDAC Inhibitors from the Endophytic Fungus Epicoccum nigrum

A chemical investigation of the endophytic fungus Epicoccum nigrum isolated from leaves of Mentha suaveolens collected in Morocco resulted in the isolation of five new polyketides, epicocconigrones A and B (1 and 2), 3-methoxyepicoccone B (3), 3-methoxyepicoccone (4), and 2,3,4-trihydroxy-6-(methoxymethyl)-5-methylbenzaldehyde (5), together with five known compounds (6-10). The structures of the new compounds were unambiguously determined by extensive analysis of the 1D and 2D NMR and mass spectroscopic data. Compounds 1 and 10 showed potent inhibition of at least 15 protein kinases with IC50 values ranging from 0.07 to 9.00 μM. Moreover, compounds 1 and 10 inhibited histone deacetylase (HDAC) activities with IC50 values of 9.8 and 14.2 μM, respectively. A preliminary structure-activity relationship is discussed. Interestingly, compounds 1 and 10 exert mainly cytostatic effects in human lymphoma RAJI and U-937 cell lines.

Atropisomeric Dihydroanthracenones as Inhibitors of Multiresistant Staphylococcus aureus

Two bisdihydroanthracenone atropodiastereomeric pairs, including homodimeric flavomannin A (1) and the previously unreported flavomannin B (2), two new unsymmetrical dimers (3 and 4), and two new mixed dihydroanthracenone/anthraquinone dimers (5 and 6) were isolated from Talaromyces wortmannii , an endophyte of Aloe vera . The structures of 2-6 were elucidated by extensive NMR and mass spectrometric analyses. The axial chirality of the biaryls was determined using TDDFT ECD and VCD calculations, the combination of which however did not allow the assignment of the central chirality elements of 1. The compounds exhibited antibacterial activity against Staphylococcus aureus , including (multi)drug-resistant clinical isolates. Reporter gene analyses indicated induction of the SOS response for some of the derivatives, suggesting interference with DNA structure or metabolism. Fluorescence microscopy demonstrated defective segregation of the bacterial chromosome and DNA degradation. Notably, the compounds showed no cytotoxic activity, encouraging their further evaluation as potential starting points for antibacterial drug development.

Pro-apoptotic and immunostimulatory tetrahydroxanthone dimers from the endophytic fungus Phomopsis longicolla.

Four tetrahydroxanthone dimers (1-4) and four biogenetically related monomers (5-8), including the new derivatives 4-6, were isolated from the endophyte Phomopsis longicolla. The absolute configurations of 2-4 were established for the first time by TDDFT electronic circular dichroism calculations, and that of phomoxanthone A (1) was revised by X-ray crystallography. Phomoxanthone A (1) showed the strongest pro-apoptotic activity when tested against a panel of human cancer cell lines, including cisplatin-resistant cells, whereas it was up to 100-fold less active against healthy blood cells. It was also the most potent activator of murine T lymphocytes, NK cells, and macrophages, suggesting an activation of the immune system in parallel to its pro-apoptotic activity. This dual effect in combating cancer cells could help in fighting resistance during chemotherapy. Preliminary structure-activity studies of isolated compounds and derivatives obtained by semisynthesis (9a-11) hinted at the location of the biaryl axis and the presence of acetyl groups as important structural elements for the biological activity of the studied tetrahydroxanthones.