Chemical Investigations Of Five Strains Of Endophytic Fungi And Their Biological Activities

Fungi are well-known producers of bioactive secondary metabolites due to the magnitude of their diversities in species and habitats. Chemical studies of fungi have led to the discovery of many medically important natural products that have been/could be developed into leads for therapeutics. However, it has been increasingly difficult to identify new bioactive secondary metabolites from common fungi. Fungi from special ecological niches are more likely to produce structurally unique and biologically active compounds, presumably due to the hostile natural environment and poor nutritional conditions. Endophytic fungi are microorganisms that inhabit normal tissues of hosts without causing apparent pathogenic symptoms. The relationship between an endophyte and its host may range from latent phytopathogenesis to mutualistic symbiosis. In a symbiotic association, the host provides nutrition for the endophyte, which in turn produces bioactive metabolites to stimulate growth, to increase disease resistance, and to improve the host’s ability to withstand environmental stresses. Endophytic fungi have been considered as important sources of sustainable, bioactive, and chemically novel natural products and attracted widespread attention. Chemical investigations of the endophytic fungi from some special species, such as bryophytes and lichens, have a greater potential for development.In this study, the endophytic fungi isolated from bryophytes or lichens were cultured using PDA medium to prepare the crude extracts. By the initial bioassay and chemical analysis, five endophytic fungi (one from liverwort and four from lichens) were selected for further grown in a solid-substrate fermentation culture in larger scale. The ethyl acetate crude extracts obtained from the fungal species above were fractionated using column chromatography (CC) on silica gel, ODS, Sephadex LH-20, as well as reversed-phase high performance liquid chromatography (RP HPLC). A total of 83 secondary metabolites,28 of which were new ones, were obtained following separation and purification. The structures of these natural products were determined by mass spectrometry (MS), nuclear magnetic resonance (NMR) techniques, X-ray diffraction analysis, chemical derivatizations, together with the circular dichroism (CD) analysis. Plausible biogenetic pathways for some important metabolites were proposed. These isolates were evaluated for their inhibitory activity against the fungus Candida albicans and their cytotoxicity against several human tumor cell lines. Some metabolites showed strong activities against one or more targets in our bioassays. Further analysis revealed that compound 37 induced cell membrane damage and ROS accumulation resulted in the death of Candida albicans cells, and compound 58 induced G0/G1 cell cycle arrest and apoptosis in A549 (human lung carcinoma) cells. Besides, antifungal perylenequinones (compounds 77 and 78) from an endolichenic fungus Phaeosphaeria sp. and the crude extracts of this fungus were transformed into inactive derivatives when they was exposed to visible or ultraviolet (UV) light. This photo-driven detoxification could be taken as a self-resistant strategy for the producer.Chemical studies of the selected five strains of endophytic fungi (isolated from liverwort or lichens) have produced numerous new metabolites, and some of them possess significant antifungal and cytotoxic activities. These preliminary results imply that the endophytic fungi from liverworts and lichens could be another important source of bioactive natural products and have an expansive foreground to research.