New bioactive metabolites from the marine-derived fungus Dichotomomyces cejpii and their pharmacological relevance

Abstract

Natural products from fungal sources provide an astonishing diversity of bioactive molecules. The secondary metabolite production in fungi is strongly affected by environmental influences and the isolation of fungal strains from uncommon, less investigated habitats, such as the marine environement is considered as promising approach for the discovery of novel bioactive compounds with a high potential for drug discovery. <br /> The main goal of this study was the isolation, identification and biological evaluation of secondary metabolites from the marine-derived fungus <em>Dichotomomyces cejpii</em>. This fungus was chosen for intensive investigation on the basis of promising results from spectroscopic analysis and positive antibiotic agar diffusion tests of fungal crude extracts. Furthermore analysis of available literature had shown that this fungus had been scarcely chemically investigated so far. <br /> The main goal of this investigation was the isolation, identification and biological evaluation of secondary metabolites from the marine-derived fungus <em>Dichotomomyces cejpii</em> with emphasis on compounds exhibiting activities toward cannabinoid and type II nuclear receptors. Aβ-42 lowering activity potentially useful to treat Alzheimer dementia was also assayed. Of further interest were secondary metabolites with antibiotic activities. In order to achieve this goal a classical approach for natural product research was envisaged, which was in case of antbiotic activity, supported by bioactivity guided fractionation.<br /> For initial bioassays four different agar-based media were chosen for small scale cultivation, following the idea of the so-called OSMAC (One Strain Many Compounds) approach. On the basis of the chemical and biological results obtained with these extracts, two cultivation methods which differed greatly in their nitrogen and carbohydrate supply sources were selected for large scale cultivation. <br /> Detailed chemical investigation of the resulting two extracts provided in total fifteen complex natural products. Seven of these have new structures (1-3, 9-11 and 14) and two had only been obtained semi-synthethically before (15-16).<br /> The new emindole SB beta-mannoside (1) and 27-O-methylasporyzin C (2) belong to the class of indoloditerpenes. They were obtained together with another new indoloditerpene, referred to as emindole SB-formate (3) and three further structurally related metabolites (4-6). All investigated indoloditerpenes display activities toward cannabinoid and related receptors, whereat compound 1 shows selective antagonistic activity toward cannabinoid receptor subtype 2 (CB²: Ki 10.6 μM), while compound 2 has selective antagonistic activity toward the cannabinoid related receptor GPR18 (IC₅₀ 13.4 μM). Due to the potent physiological responses, such as, e.g. immunomodulatory effects, both receptors are targets for drug development. The new natural indole derivatives 1 and 2 may thus serve as lead structures for the development of potent and selectiveCB² or GPR18 receptor-blocking drugs, respectively.<br /> Further, we identified three new sterols with an untypical pattern of carbon-carbon double bounds (9-11) and reisolated the isocyanid xanthocillin X dimethylether (12). The new sterol 16-O-desmethylasporyergosterol-β-D-mannoside (9) and xanthocillin X dimethylether (12) display moderate Aβ-42 lowering activity. Given that Aβ-42 lowering agents are, inter alia, considered as therapeutics for Alzheimer disease, these results suggest further investigations.<br /> Additionally, three non-cytotoxic gliotoxin derivatives were isolated (14-16). The new 6- acetylmonodethiogliotoxin (14) is characterized by a naturally occurring very rare transannulated monosulfide bridge. This characteristic moiety seems to be essential for the observed selective agonistic activity toward nuclear liver X receptor α (LXRα). Targeting LXR has demonstrated to possess therapeutic potential in the treatment of atherosclerosis, diabetes, cancer, cardiovascular diseases, autoimmune disorders, and Alzheimer disease, whereat LXR α/β subtype selective ligands are desired. Therefore compound 14 may serve as lead for the development of more potent and selective ligands which interact with this therapeutically interesting receptor.<br /> In conclusion the marine-derived strain of <i>Dichotomomyces cejpii</i> has been identified as powerful producer of highly diverse and bioactive secondary metabolites.