Microbial oxidation and glucosidation of echinocystic acid by Nocardia corallina

Terpenoid natural products exert neuroprotection via the PI3K/Akt pathway

PI3K/Akt, an essential signaling pathway widely present in cells, has been shown to be relevant to neurological disorders. As an important class of natural products, terpenoids exist in large numbers and have diverse backbones, so they have a great chance to be identified as neuroprotective agents. In this review, we described and summarized recent research for a range of terpenoid natural products associated with the PI3K/Akt pathway by classifying their basic chemical structures of the terpenes, identified by electronic searches on PubMed, Web of Science for research, and Google Scholar websites. Only articles published in English were included. Our discussion here concerned 16 natural terpenoids and their mechanisms of action, the associated diseases, and the methods of experimentation used. We also reviewed the discovery of their chemical structures and their derivatives, and some compounds have been concluded for their structure–activity relationships (SAR). As a result, terpenoids are excellent candidates for research as natural neuroprotective agents, and our content will provide a stepping stone for further research into these natural products. It may be possible for more terpenoids to serve as neuroprotective agents in the future.

Bioactive molecules from Nocardia: diversity, bioactivities and biosynthesis

Nocardia spp. are catalase positive, aerobic, and non-motile Gram-positive filamentous bacteria. Many Nocarida spp. have been reported as unusual causes of diverse clinical diseases in both humans and animals. Therefore, they have been studied for a long time, primarily focusing on strain characterization, taxonomic classification of new isolates, and host pathophysiology. Currently, there are emerging interests in isolating bioactive molecules from diverse actinobacteria including Nocardia spp. and studying their biosynthetic mechanisms. In addition, these species possess significant metabolic capacity, which has been utilized for generating diverse functionalized bioactive molecules by whole cell biotransformation. This review summarizes the structural diversity and biological activities of compounds biosynthesized or biotransformed by Nocardia spp. Furthermore, the recent advances on biosynthetic mechanisms and genetic engineering approaches for enhanced production or structural/functional modification are presented.

New Triterpenoid from Novel Triterpenoid 15-O-Glycosylation on Ganoderic Acid A by Intestinal Bacteria of Zebrafish

Functional bacteria that could biotransform triterpenoids may exist in the diverse microflora of fish intestines. Ganoderic acid A (GAA) is a major triterpenoid from the medicinal fungus Ganoderma lucidum. In studying the microbial biotransformation of GAA, dozens of intestinal bacteria were isolated from the excreta of zebrafish. The bacteria's ability to catalyze GAA were determined using ultra-performance liquid chromatography analysis. One positive strain, GA A07, was selected for functional studies. GA A07 was confirmed as Bacillus sp., based on the DNA sequences of the 16S rRNA gene. The biotransformed metabolite was purified with the preparative high-performance liquid chromatography method and identified as GAA-15-O-β-glucoside, based on the mass and nuclear magnetic resonance spectral data. The present study is the first to report the glycosylation of Ganoderma triterpenoids. Moreover, 15-O-glycosylation is a new microbial biotransformation of triterpenoids, and the biotransformed metabolite, GAA-15-O-β-glucoside, is a new compound.

Sequential Biotransformation of Antcin K by Bacillus subtilis ATCC 6633

The biotransformation of antcin K, a major ergostane triterpenoid from the fruiting bodies of Antrodia cinnamomea, by Bacillus subtilis (B. subtilis) ATCC 6633 was studied. Four metabolites from the biotransformation were isolated with preparative high-performance liquid chromatography and identified as 25S-antcin K 26-O-β-glucoside, 25R-antcin K 26-O-β-glucoside, 25S-antcin K 26-O-β-(6′-O-succinyl)-glucoside, and 25R-antcin K 26-O-β-(6′-O-succinyl)-glucoside with mass and nuclear magnetic resonance spectral analysis. By using either 25S-antcin K 26-O-β-glucoside or 25R-antcin K 26-O-β-glucoside as the biotransformation precursor, it was proven that 25S-antcin K 26-O-β-(6′-O-succinyl)-glucoside and 25R-antcin K 26-O-β-(6′-O-succinyl)-glucoside were biotransformed from 25S-antcin K 26-O-β-glucoside and 25R-antcin K 26-O-β-glucoside, respectively. To the best of our knowledge, this is the first study on the glycosylation of triterpenoids from A. cinnamomea, and the first time the succinylation of triterpenoid glycosides by microorganisms has been found. In addition, all four antcin K glucoside derivatives are new compounds.

Echinocystic Acid Facilitates Neurite Outgrowth in Neuroblastoma Neuro2a Cells and Enhances Spatial Memory in Aged Mice

Aging leads to functional changes in the brain and decreases ability of learning and memory. Neurite outgrowth is important in learning and memory, therefore regulation of neurite outgrowth might be a candidate for treating aged brain. Echinocystic acid (EA), a pentacyclic triterpene, has shown to exert various neurological effects. However, the effect of EA on neurite outgrowth has not been studied. In this study, we examined if EA is effective on neurite outgrowth and memory in aged mice. The effect of EA on neurite outgrowth was observed by examining neurite processes of Neuro2a cells treated with EA. Western blot analysis was conducted to examine possible mechanisms. Morris water maze test was used to examine the effect of EA on learning and memory in aged mice. Immunohistochemistry was conducted to observe the effect of EA on neurite outgrowth in the hippocampus. EA was shown to induce neurite outgrowth in a concentration dependent manner without affecting cell viability. Moreover, EA treatment increased phosphorylation of c-jun N-terminal kinase (JNK) and JNK inhibitor, SP600125, blocked the effect of EA on neurite outgrowth. These results demonstrated that EA treatment promotes neurite outgrowth through the JNK signaling pathway. In in vivo experiments, EA treatment increased neurite outgrowth in aged mouse hippocampus. Moreover, EA treatment enhanced spatial learning and memory in aged mice. These results suggest that EA can be developed as a new, naturally occurring drug to treat ageing-related neurological diseases.

New derivatives of ursolic acid through the biotransformation by Bacillus megaterium CGMCC 1.1741 as inhibitors on nitric oxide production

Microbial transformation of ursolic acid (1) by Bacillus megaterium CGMCC 1.1741 was investigated and yielded five metabolites identified as 3-oxo-urs-12-en-28-oic acid (2); 1β,11α-dihydroxy-3-oxo-urs-12-en-28-oic acid (3); 1β-hydroxy-3-oxo-urs-12-en-28, 13-lactoe (4); 1β,3β, 11α-trihydroxyurs-12-en-28-oic acid (5) and 1β,11α-dihydroxy-3-oxo-urs-12-en-28-O-β-d-glucopyranoside (6). Metabolites 3, 4, 5 and 6 were new natural products. Their nitric oxide (NO) production inhibitory activity was assessed in lipopolysaccharide (LPS) - stimulated RAW 264.7 cells. Compounds 3 and 4 exhibited significant activities with the IC50 values of 1.243 and 1.711μM, respectively. A primary structure-activity relationship was also discussed.

Microbial hydroxylation and glycosylation of pentacyclic triterpenes as inhibitors on tissue factor procoagulant activity

To discover new inhibitors on tissue factor procoagulant activity, 20 pentacyclic triterpenes were semi-synthetized through microbial transformation and assayed on the model of human THP-1 cells stimulated by lipopolysaccharide. In the biotransformation two types of reactions were observed, regio-selective hydroxylation and glycosylation. The bioassay results showed that most of tested compounds were significant effective on this model and two of the biotransformation products 23-hydroxy-28-O-β-d-glucopyranosyl betulinic acid (3d) and 28-O-β-d-glucopyranosyl oleanic acid (1a) exhibited most potential activities with the IC₅₀ values of 0.028, 0.035nM respectively. The preliminary structure and activity relationship analysis revealed that the aglycones with single free hydroxyl group on the skeleton (1, 1j) were less effective than that with more free hydroxyl groups (1d, 1f, 2), mono-glycosylation can significantly enhance their inhibitory effects. Our findings also provide some potential leading compounds for tissue factor-related diseases, such as cancer and cardiovascular diseases.

Echinocystic acid inhibits RANKL-induced osteoclastogenesis by regulating NF-κB and ERK signaling pathways

Receptor activator of nuclear factor-κB ligand (RANKL) is a key factor in the differentiation and activation of osteoclasts. Echinocystic acid (EA), a pentacyclic triterpene isolated from the fruits of Gleditsia sinensis Lam, was reported to prevent reduction of bone mass and strength and improve the cancellous bone structure and biochemical properties in ovariectomy rats. However, the molecular mechanism of EA on the osteoclast formation has not been reported. The purpose of this study was to investigate the effects and mechanism of EA on RANKL-induced osteoclastogenesis. Our results showed that EA inhibited the formation of osteoclast, as well as the expression of osteoclastogenesis-related marker proteins in bone marrow macrophages (BMMs). At molecular levels, EA inhibited RANKL-induced NF-κB activation and ERK phosphorylation in BMMs. In conclusion, the present study demonstrated that EA can suppress osteoclastogenesis in vitro. Moreover, we clarified that these inhibitory effects of EA occur through suppression of NF-κB and ERK activation. Therefore, EA may be a potential agent in the treatment of osteoclast-related diseases such as osteoporosis.

Structural analysis of metabolites of asiatic acid and its analogue madecassic acid in zebrafish using LC/IT-MSn

Although zebrafish has become a significant animal model for drug discovery and screening, drug metabolism in zebrafish remains largely unknown. Asiatic acid (AA) and madecassic acid (MA), two natural pentacyclic triterpenoids mainly obtained from Centella asiatica (L.) Urban, have been found to possess many pharmacological effects. This study is to probe the metabolic capability of zebrafish via investigation of the drug metabolism of AA and MA in zebrafish, using a sensitive LC/IT-MSn method. In addition, the main fragmentation pathways of AA and MA were reported for the first time. Nineteen metabolites of AA and MA were firstly identified after zebrafish was exposed to the drug, which all were the phase I metabolites and mainly formed from hydroxylation, dehydrogenation, hydroxylation and dehydrogenation, dihydroxylation and dehydrogenation, and dehydroxylation reaction. The results indicated that zebrafish possessed strong metabolic capacity, and the metabolites of AA and MA were formed via similar metabolic pathways and well matched with the known metabolic rules in vivo and in vitro, which supports the widely use of this system in drug metabolism research. This investigation would also contribute to the novel information on the structural elucidation, in vivo metabolites and metabolic mechanism of pentacyclic triterpenoids.

Microbial-catalyzed biotransformation of multifunctional triterpenoids derived from phytonutrients

Microbial-catalyzed biotransformations have considerable potential for the generation of an enormous variety of structurally diversified organic compounds, especially natural products with complex structures like triterpenoids. They offer efficient and economical ways to produce semi-synthetic analogues and novel lead molecules. Microorganisms such as bacteria and fungi could catalyze chemo-, regio- and stereospecific hydroxylations of diverse triterpenoid substrates that are extremely difficult to produce by chemical routes. During recent years, considerable research has been performed on the microbial transformation of bioactive triterpenoids, in order to obtain biologically active molecules with diverse structures features. This article reviews the microbial modifications of tetranortriterpenoids, tetracyclic triterpenoids and pentacyclic triterpenoids.

Microbial transformation of asiatic acid

Asiatic acid (1), a major pentacyclic triterpene of Centella asiatica, was subjected to transformation by Penicillium lilacinum ACCC 31890, Fusarium equiseti CGMCC 3.3658, and Streptomyces griseus CGMCC 4.18 strains. Incubation of asiatic acid with P. lilacinum ACCC 31890 and F. equiseti CGMCC 3.3658 gave an identical product: 2α,3β,15α,23-tetrahydroxyurs-12-en-28-oic acid (2). Biotransformation of asiatic acid by S. griseus CGMCC 4.18 resulted in three derivatives: 2α,3β,21β,23-tetrahydroxyurs-12-en-28-oic acid (3), 2α,3β,23-trihydroxyurs-12-en-28, 30-dioic acid (4), and 2α,3β,23,30-tetrahydroxyurs-12-en-28-oic acid (5). The structures of those derivatives were deduced from their spectral data. Products (2), (3), and (4) were new compounds. In addition, the in vitro cytotoxicities of those derivatives along with 1 were evaluated with several human cancer cell lines.

Multihydroxylation of ursolic acid by Pestalotiopsis microspora isolated from the medicinal plant Huperzia serrata

The structural modification of ursolic acid by an endophytic fungus Pestalotiopsis microspora, isolated from medicinal plant Huperzia serrata was reported for the first time. The structure diversity was very important for the SAR study of ursolic acid and its derivatives. Incubation of ursolic acid 1 with P. microspora afforded four metabolites: 3-oxo-15α, 30-dihydroxy-urs-12-en-28-oic acid (2), 3β, 15α-dihydroxy-urs-12-en-28-oic acid (3), 3β, 15α, 30- trihydroxy-urs-12-en-28-oic acid (4) and 3,4-seco-ursan-4,30-dihydroxy-12-en-3,28-dioic acid (5). All products were new compounds and their structures elucidation was mainly based on the spectroscopic data.

Biotransformation of ursolic acid by an endophytic fungus from medicinal plant Huperzia serrata

Endophytic fungi were used not only for their producing bioactive products but also for their ability to transform natural compounds. An endophytic fungus, isolated from medicinal plant Huperzia serrata, was identified as Umbelopsis isabellina based on the internal transcribed spacer of ribosomal DNA (rDNA-ITS) region. It was used to transform ursolic acid (1), a pentacyclic triterpene. Incubation of ursolic acid with U. isabellina afforded three products, 3β-hydroxy-urs-11-en-28,13-lactone (2), 3β,7β-dihydroxy-urs-11-en-28,13-lactone (3), 1β,3β-dihydroxy-urs-11-en-28,13-lactone (4). Although product 2 was a known compound, it was first obtained by microbial transformation. Products 3 and 4 were new compounds. The structural elucidation of the three compounds was achieved mainly by the 1D- and 2D-NMR, MS, IR data. The endophytic fungus U. isabellina can hydroxyate the C12-C13 double bond at position 13 of ursolic acid 1 and form a five-member lactone effectively. In the meantime, this fungus can also introduce the hydroxyl group at C-1 or C-7 of ursolic acid 1.