Secondary Metabolites of Osmanthus fragrans: Metabolism and Medicinal Value

Abstract

Osmanthus fragrans (scientific name: Osmanthus fragrans (Thunb.) Lour.) is a species of the Osmanthus genus in the family Oleaceae, and it has a long history of cultivation in China. O. fragrans is edible and is well known for conferring a natural fragrance to desserts. This flowering plant has long been cultivated for ornamental purposes. Most contemporary literature related to O. fragrans focuses on its edible value and new species discovery, but the functional use of O. fragrans is often neglected. O, fragrans has many properties that are beneficial to human health, and its roots, stems, leaves, flowers and fruits have medicinal value. These characteristics are recorded in the classics of traditional Chinese medicine. Studies on the metabolites and medicinal value of O. fragrans published in recent years were used in this study to evaluate the medicinal value of O. fragrans. Using keywords such as metabolites and Osmanthus fragrans, a systematic and nonexhaustive search of articles, papers and books related to the medicinal use of Osmanthus fragrans metabolites was conducted. Fifteen metabolites were identified through this literature search and classified into three categories according to their properties and structure: flavonoids, terpenes and phenolic acids. It was found that the pharmacological activities of these secondary metabolites mainly include antioxidant, anticancer, anti-inflammatory and antibacterial activities and that these metabolites can be used to treat many human diseases, such as cancer, skin diseases, cardiovascular diseases, and neurological diseases. Most of the reports that are currently available and concern the secondary metabolites of Osmanthus fragrans have limitations. Some reports introduce only the general classification of compounds in Osmanthus fragrans, and some reports introduce only a single compound. In contrast, the introduction section of this paper includes both the category and the functional value of each compound. While reviewing the data for this study, the authors found that the specific action sites of these compounds and their mechanisms of action in plants are relatively weak, and in the future, additional research should be conducted to investigate this topic further.

Keywords: Osmanthus fragrans; flavonoids; medicinal compound; phenolic acids; secondary metabolites; terpenoids.

FIGURE 1

Summary of the literature according to keywork and the year published over the last 10 years. (A) Bubble chart showing the number of publications in 5 years from the past 10 years. The horizontal axis is the publication year, and the vertical axis is the relevant keyword used to search the literature. Correlative studies are relatively weak, and studies of various secondary metabolites are clearly differentiated. (B) Heatmap showing the prevalence of keywords in publications over the past 10 years. The figure shows that the research statuses of the three major categories of secondary metabolites selected in this paper are very different, with research on flavonoids being the most extensive and research on phenolic acids being the least extensive. (The basic principle of the author’s preliminary background investigation before writing this manuscript is to first conduct a general search on the research progress of O. fragrans, select compounds extracted from O. fragrans, and then screen out relatively important secondary metabolites of O. fragrans).

FIGURE 2

Representative medicinal compounds in O. fragrans and their biosynthetic pathways. (A) The main synthesis pathway containing naringenin, luteolin and DHQ in the flavonoid biosynthesis pathway is presented. Naringenin can produce luteolin and DHQ through reactions catalysed by different enzymes. (B) Loganic acid is a terpenoid. The synthetic pathway of loganic acid in the monoterpene biosynthesis pathway. Loganic acid can be produced from geraniol through reactions catalysed by a series of enzymes. (C) SA, PCA and Ty are synthesized via three different phenolic acid biosynthesis pathways, but the synthetic pathway of SA can be connected to the synthetic pathways of PCA and Ty. The double dotted arrow in the figure represents the connection between the pathways. (The relevant O. fragrans gene information in this biosynthetic pathway map comes from an article about O. fragrans published by) (Yang, et al., 2018).

FIGURE 3

Representative medicinal compounds in O. fragrans and their pharmacological mechanisms of action. (A) Pathways by which Dio affects MH7A cell apoptosis. Dio inhibits the production of inflammatory mediators and cytokines (TNF-α, IL-1β, IL-6, etc.) induced by D-GalN and can also regulate the Akt/NF-kB pathway. Dio can also inhibit the PI3K/Akt pathway. PI3k is phosphorylated to generate Akt, and Akt is a key molecule in the activation of the NF-κB signalling pathway. Inhibition of this pathway reduces cell proliferation and inflammatory cytokine production in MH7A cells and promotes apoptosis. (B) Effects of DHQ on the mTORC2/Akt signalling pathway under high-glucose conditions. Reactive oxygen species (ROS) can activate the protein kinase PI3K signalling pathway. There are two downstream pathways of PI3k. In the one presented here, the PI3k-Akt signalling pathway affects mTORC2 and mTORC1. PI3k generates PIP3, PIP3 further activates PDK1, PDK1 activates Akt through phosphorylation, and Akt further activates mTORC2. DHQ has an inhibitory effect on ROS production. After the addition of DHQ, the activity of ROS is reduced, the regulatory effect of ROS on the PI3k pathway is weakened, the expression of Akt, which is downstream of PI3k, is decreased, and the expression of mTORC2, which is downstream of Akt, is inhibited. Akt phosphorylation can regulate GSK3 and reduce cell survival to ameliorate diabetic nephropathy in rats. (Red indicates increase/promotion, green indicates decrease/inhibition, and yellow indicates promotion of the original pathway but inhibition of this pathway.)