Study on the anti-tumor metabolites of an endophytic fungus Aspergillus terreus TZS-201607 in Panax ginseng
Plant endophytes refer to a group of microorganisms that live within healthy plant tissues or at a certain stage of their life cycle, and do not cause significant disease symptoms to the host plant. In recent decades, endophytic fungi have been shown to have the ability to synthesize unique secondary metabolites, making them a hot topic in the field of natural products research. The secondary metabolites of endophytic fungi in plants have the characteristics of diverse structures and significant activity. At present, secondary metabolites with antibacterial, insecticidal, anti-tumor, antioxidant and other activities have been discovered from endophytic fungi, covering various structural types such as polyketones (macrolides, quinones, anthraquinones, etc.), terpenes, alkaloids, peptides, etc. Endophytic fungi can produce active ingredients that are the same or similar to their host plants during long-term coevolution. Therefore, the study of endophytic fungi in medicinal plants has attracted increasing attention. In particular, numerous anti-tumor lead compounds or drugs have been discovered from endophytic fungi in plants, such as paclitaxel, podophyllotoxin, camptothecin, etc., indicating that there is great potential to explore new anti-tumor active lead compounds from medicinal plant endophytic fungi.

Taizi Shen is the dried root of Pseudostellaria heterophylla (Miq) Pax, a plant in the Caryophyllaceae family. It is a commonly used traditional Chinese medicine in China and has the effects of nourishing qi and spleen, generating fluids and moistening the lungs. It is used to treat symptoms such as dry cough, spontaneous sweating and thirst, spleen deficiency and fatigue, and loss of appetite. Compared with other medicinal plants, there is relatively little research on the isolation and identification of endophytic fungi in Panax ginseng. Cai et al. isolated 18 endophytic fungi from Panax ginseng, among which 6 strains had significant anti-tumor activity in their fermentation products, and 3 strains had good antioxidant activity in their fermentation products. However, there is currently a lack of in-depth research on the secondary metabolites of endophytic fungal activity in Panax ginseng. In order to study the unique microbial resource of endophytic fungi, we isolated 12 strains of endophytic fungi from Panax ginseng and screened and analyzed the diversity and activity of their secondary metabolites. The secondary metabolites of strain Aspergillus terreus TZS-201607 exhibit rich chemical structural diversity. In the screening of anti-tumor activity, the fermentation product of this strain of potato glucose broth (PDB) showed certain inhibitory activity against HeLa cells (48 hour inhibition rate>50%). To obtain secondary metabolites with anti-tumor activity from this strain, further research was conducted on its chemical composition and activity.

Medicinal plants and their endophytic fungi have formed a unique mutualistic symbiotic relationship through long-term coevolution. Among them, host plants provide growth, protection, and nutritional environment for fungi, while endophytic fungi promote plant growth and defense by synthesizing secondary metabolites. Due to the significant biological activity of these secondary metabolites and their great potential for new drug development, the endophytic fungal resources in medicinal plants have been proven to be a treasure trove that urgently needs to be developed. In the past 30 years, a large number of endophytic fungi have been isolated and purified from hundreds of medicinal plants, such as Ningqianhu, Chuanxiong, oleander, etc., indicating that medicinal plants contain abundant endophytic fungal resources. As a commonly used traditional Chinese medicine in China, there is limited research on the isolation and identification of endophytic fungi in Taizishen. Currently, only 18 strains of endophytic fungi have been isolated from it. Therefore, the endophytic fungal resources of Panax ginseng need to be further explored.

Endophytic fungi in medicinal plants can produce structurally diverse active secondary metabolites, of which a considerable portion have anti-tumor pharmacological activity and demonstrate potential application value. The structural types of these metabolites include alkaloids, steroids, terpenes, flavonoids, anthraquinones, etc. Anthraquinone is the most abundant and important class of natural quinone compounds, with significant biological activity, especially anti-tumor activity. It has the characteristics of natural low toxicity and high efficiency, which has attracted widespread attention from researchers. In order to study the endophytic fungi of Panax ginseng and obtain significantly active secondary metabolites, especially compounds with anti-tumor activity, this study used various chromatographic techniques to isolate and purify 16 compounds from the endophytic fungus A. terreus TZS-201607 of Panax ginseng, including ketones (compounds 1-4), anthraquinones (compounds 5-7), and sterols (compounds 8-16). The compounds 1-7, 12, 14-16 reported in this study were isolated for the first time from the endophytic fungus A. terreus. In vitro anti-tumor activity tests showed that compounds 6, 7, and 14 exhibited strong cytotoxic activity (IC50<10 μ M) against human tumor cell lines A549, BT-549, HeLa, and THP-1. This study enriches the diversity of endophytic fungal resources in Panax ginseng, and to a certain extent enriches the chemical structure types and biological activities of secondary metabolites of fungus A. terreus. At the same time, this study discovered anthraquinone compounds with significant anti-tumor activity, providing compound entities for the discovery and research of subsequent drug lead compounds.