Alismanin A, a Triterpenoid with a C34 Skeleton from Alisma orientale as a Natural Agonist of Human Pregnane X Receptor

Inhibition effect of 1-acetoxy-6α-(2-methylbutyryl)eriolanolide toward soluble epoxide hydrolase: Multispectral analysis, molecular dynamics simulation, biochemical, and in vitro cell-based studies

Soluble epoxide hydrolase (sEH) serves as a potential target in inflammation-related diseases. Based on the bioactivity-guided separation, a new sesquiterpenoid inulajaponoid A (1) was isolated from Inula japonica with a sEH inhibitory effect, together with five known compounds, such as 1-O-acetyl-6-O-isobutyrylbritannilactone (2), 6β-hydroxytomentosin (3), 1β,8β-dihydroxyeudesma-4(15),11(13)-dien-12,6α-olide (4), (4S,6S,7S,8R)-1-O-acetyl-6-O-(3-methylvaleryloxy)-britannilactone (5), and 1-acetoxy-6α-(2-methylbutyryl)eriolanolide (6). Among them, compounds 1 and 6 were assigned as mixed and uncompetitive inhibitors, respectively. The result of immunoprecipitation (IP)-MS demonstrated the specific binding of compound 6 to sEH in the complex system, which was further confirmed by the fluorescence-based binding assay showing its equilibrium dissociation constant (K_d = 2.43 μM). The detail molecular stimulation revealed the mechanism of action of compound 6 with sEH through the hydrogen bond of amino acid residue Gln384. Furthermore, this natural sEH inhibitor (6) could suppress the MAPK/NF-κB activation to regulate inflammatory mediators, such as NO, TNF-α, and IL-6, which confirmed the anti-inflammatory effect of inhibition of sEH by 6. These findings provided a useful insight to develop sEH inhibitors upon the sesquiterpenoids.

Metabolites isolated from the human intestinal fungus Penicillium oxalicum SL2 and their agonistic effects on PXR and FXR

Intestinal commensal fungi are vital to human health, and their metabolites play a key role in the reciprocal relationship. In the present work, eighteen alkaloids and seven monoterpenoids were isolated from the fermentation of the human intestinal fungus Penicillium oxalicum SL2, including seven undescribed alkaloids (penicilloxalines A-G), three undescribed monoterpenoids (penicilloxalines H-J), and fifteen reported compounds. The structures of the isolated compounds were identified by HRESIMS, 1D and 2D NMR, electronic circular dichroism spectra and quantum chemical calculations. Some metabolites displayed moderate agonistic effects against the pregnane X receptor (PXR), whereas (6R)3,7-dimethyl-6,7-dihydroxy-2(Z)-octenoic acid displayed a significant agonistic effect against the farnesoid X receptor (FXR) with an EC₅₀ value of 0.43 μM, which was verified by investigating FXR downstream target genes and proteins, such as small heterodimer partner 1 (SHP1), fibroblast growth factor (FGF), and bile salt export pump (BSEP).

Dietary phytochemicals as modulators of human pregnane X receptor

As a promiscuous xenobiotic sensor, pregnane X receptor (PXR) plays a crucial role in drug metabolism. Since dietary phytochemicals exhibit the potential to modulate human PXR, this review aims to summarize the plant-derived PXR modulators, including agonists, partial agonists, and antagonists. The crystal structures of the apo and ligand-bound forms of PXR especially that of PXR complexed with binary mixtures are summarized, in order to provide the structural basis for PXR binding promiscuity and synergistic activation of PXR by composite ligands. Furthermore, this review summarizes the characterized agonists, partial agonists, and antagonists of human PXR from botanical source. Contrary to PXR agonists, there are only a few antagonists obtained from botanical source due to the promiscuity of PXR. It is worth noting that trans-resveratrol and a series of methylindoles have been identified as partial agonists of PXR, both in activating PXR function, but also inhibiting the effect of other PXR agonists. Since antagonizing PXR function plays a crucial role in the prevention of drug-drug interactions and improvement of therapeutic efficacy, further research is necessary to screen more plant-derived PXR antagonists in the future. In summary, this review may contribute to understanding the roles of phytochemicals in food-drug and herb-drug interactions.

Natural soluble epoxide hydrolase inhibitors from Alisma orientale and their potential mechanism with soluble epoxide hydrolase

Inhibition of soluble epoxide hydrolase (sEH) is considered to be an effective treatment for inflammation-related diseases, and small molecules origin from natural products show promising activity against sEH. Two undescribed protostanes, 3β-hydroxy-25-anhydro-alisol F (1) and 3β-hydroxy-alisol G (2) were isolated from Alisma orientale and identified as new sEH inhibitors with IC₅₀ values of 10.06 and 30.45 μM, respectively. Potential lead compound 1 was determined as an uncompetitive inhibitor against sEH, which had a K_i value of 5.13 μM. In-depth molecular docking and molecular dynamics simulations revealed that amino acid residue Ser374 plays an important role in the inhibition of 1, which also provides an idea for the development of sEH inhibitors based on protostane-type triterpenoids.

Manniosides B-F, five new triterpenoid saponins from the leaves of Schefflera mannii (Hook.f.) Harms

Fifteen triterpenoid saponins including five new compounds (Mannioside B: 3β-[(β-d-glucopyranosyl)oxy]urs-12-en-28-oic acid α-l-rhamnopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 6)-β-d-glucopyranosyl ester (1), mannioside C: 3β-[(β-d-glucopyranosyl)23-dioxy]urs-12-en-28-oic acid α-l-rhamnopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 6)-β-d-glucopyranosyl ester (2), mannioside D: 3β,23-dihydroxyurs-12-en-28-oic acid β-d-glucopyranosyl-(1 → 6)- β-d-glucopyranosyl ester (3), mannioside E: 3β-hydroxy-23-oxolup-20(29)-en-28-oic acid α-l-rhamnopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 6)-β-d-glucopyranosyl ester (4) and mannioside F: (22S)-27β-[(β-d-glucopyranosyl)oxy]-22-hydroxyprotosta-12,24-dien-3β-yl β-d-glucopyranoside (5)) were isolated from the leaves of Schefflera mannii (Hook.f.) Harms. Their structures were established on the basis of 1D and 2D NMR data, mass spectrometry and chemical methods. The major isolated compounds were tested for their antiproliferative activity on human malignant epithelial (HeLa) cells but were not efficient at the concentration of 33 mM.

Investigation of the inhibitory effect of protostanes on human carboxylesterase 2 and their interaction: Inhibition kinetics and molecular stimulations

Carboxylesterase 2 (CES 2), plays a pivotal role in endobiotic homeostasis and xenobiotic metabolism. Protostanes, the major constituents of the genus Alisma, display a series of pharmacological activities. Despite the extensive studies of pharmacological activities, the investigation on inhibitory effects of protostanes against CES 2 is rarely reported. In this study, the inhibitory activities of a library of protostanes (1-25) against human CES 2 were investigated for the first time, using 6,8-dichloro-9,9-dimethyl-7-oxo-7,9-dihydroacridin-2-yl benzoate (DDAB) as the specific fluorescent probe for human CES 2. Compounds 1, 2, 7, 8, 12, 13, 18, 19, and 25 showed strong inhibitory effects towards CES 2. For the most potent compounds 1, 7, 13, and 25, the inhibition kinetics were further investigated, and these four protostanes were all uncompetitive inhibitors against human CES 2 with the inhibition constant (K_i) values ranging from 0.89 μM to 2.83 μM. In addition, molecular docking and molecular dynamics stimulation were employed to analyze the potential interactions between these protostanes and CES 2, and amino acid residue Gln422 was identified to play a crucial role in the strong inhibition of protostanes towards CES 2.

Alisma genus: Phytochemical constituents, biosynthesis, and biological activities

The genus Alisma contains 11 species distributed worldwide, of which at least two species (A. orientale [Sam.] Juzep. and A. plantago-aquatica Linn.) have been used as common herbal medicines. Secondary metabolites obtained from the genus Alisma are considered to be the material basis for the various biological functions and medicinal applications. In this review, we mainly focused on the recent investigations of secondary metabolites from plants of the genus Alisma and their biological activities, with the highlighting on the diversity of the chemical structures, the biosynthesis of interesting secondary metabolites, the biological activities, and the relationships between structures and bioactivities.

Triterpenoids From Alisma Species: Phytochemistry, Structure Modification, and Bioactivities

Plants from Alisma species belong to the genus of Alisma Linn. in Alismataceae family. The tubers of A. orientale (Sam.) Juzep, also known as Ze Xie in Chinese and Takusha in Japanese, have been used in traditional medicine for a long history. Triterpenoids are the main secondary metabolites isolated from Alisma species, and reported with various bioactive properties, including anticancer, lipid-regulating, anti-inflammatory, antibacterial, antiviral and diuretic activities. In this brief review, we aimed to summarize the phytochemical and pharmacological characteristics of triterpenoids found in Alisma, and discuss their structure modification to enhance cytotoxicity as well.

PXR: a center of transcriptional regulation in cancer

Pregnane X receptor (PXR, NR1I2) is a prototypical member of the nuclear receptor superfamily. PXR can be activated by both endobiotics and xenobiotics. As a key xenobiotic receptor, the cellular function of PXR is mostly exerted by its binding to the regulatory gene sequences in a ligand-dependent manner. Classical downstream target genes of PXR participate in xenobiotic responses, such as detoxification, metabolism and inflammation. Emerging evidence also implicates PXR signaling in the processes of apoptosis, cell cycle arrest, proliferation, angiogenesis and oxidative stress, which are closely related to cancer. Here, we discussed, in addition to the characterization of PXR per se, the biological function and regulatory mechanism of PXR signaling in cancer, and its potential for the targeted prevention and therapeutics.

Protostane-type triterpenoids as natural soluble epoxide hydrolase inhibitors: Inhibition potentials and molecular dynamics

The inhibition of soluble epoxide hydrolase (sEH) is a promising therapeutic approach to treat inflammation and other disorders. In our present investigation on searching for sEH inhibitors from traditional Chinese medicines, we found that Alisma orientale displayed inhibition of sEH. We constructed a small library of protostane-type triterpenoids (1-25) isolated from A. orientale, and screened their inhibitory activities. Alismanin B (1), 11-deoxy-25-anhydro alisol E (4), 11-deoxy alisol B (5), and 25-O-ethyl alisol A (15) displayed concentration-dependently inhibitory activities against sEH with IC₅₀ values from 3.40 ± 0.57 μM to 9.57 ± 0.88 μM. 11-Deoxy-25-anhydro alisol E (4) and 11-deoxy alisol B (5) were defined as mixed-type competitive inhibitors with Ki values of 12.6 and 3.48 μM, respectively, based on the result of inhibition kinetics. The potential interaction mechanism of 11-deoxy alisol B (5) with sEH was analyzed by molecular docking and molecular dynamics, revealing that amino acid residues Trp336 and Tyr466 were vital for its inhibitory activity.

Uncarialins A-I, Monoterpenoid Indole Alkaloids from Uncaria rhynchophylla as Natural Agonists of the 5-HT1A Receptor

Nine new monoterpenoid indole alkaloids, uncarialins A-I (1-9), were isolated from Uncaria rhynchophylla as well as 14 known analogues (10-23). Their structures were determined by HRESIMS, 1D and 2D NMR, and experimental and calculated electronic circular dichroism data. Compounds 5, 7, 15, and 22 displayed significant agonistic effects against the 5-HT_1A receptor with EC₅₀ values of 2.2 ± 0.1, 0.1 ± 0.1, 1.6 ± 0.3, and 2.0 ± 0.5 μM, respectively. The mechanisms of action of these four compounds with the 5-HT_1A receptor were investigated by molecular docking, and the results suggested that amino acid residues Asp116, Thr196, Asn386, and Tyr390 played critical roles in the observed activity of the above-mentioned compounds.

Novel C-17 spirost protostane-type triterpenoids from Alisma plantago-aquatica with anti-inflammatory activity in Caco-2 cells

Twenty-one protostane-type triterpenoids with diverse structures, including nine new compounds (1-9), were isolated from the of Alisma plantago-aquatica Linn. Structurally, alisolides A‒F (1-6), composed of an oxole group coupled to a five-membered ring, represent unusual C-17 spirost protostane-type triterpenoids. Alisolide H (8) is a novel triterpenoid with an unreported endoperoxide bridge. Alisolide I (9) represents the first example of 23,24-acetal triterpenoid. Their structures were elucidated based on spectroscopic analysis, wherein the absolute configurations of 4‒6, 8 were further confirmed by the Mo2(OAc)4-induced ECD method. Furthermore, all isolates were evaluated for their inhibitory effects on LPS-induced NO production in Caco-2 cells, and all the compounds showed remarkable inhibitory activities, with IC50 values in the range of 0.76-38.20 μmol/L.

Flavonoids as human carboxylesterase 2 inhibitors: Inhibition potentials and molecular docking simulations

In our search for natural human carboxylesterase 2 (hCE 2) inhibitors from natural products, we investigated inhibitory effects and mechanisms of flavonoids (1-16) against hCE 2. The results demonstrated that kurarinone (1), baicalein (2), 2-[(2'-(1-hydroxy-1-methylethyl)-7'-(3-methyl-2-butenyl)-2',3'-dihydrobenzofuran)-5-yl]-7-hydroxy-8-(3-methyl-2-butenyl)chroman-4-one (5), luteolin (6), kushenol X (9), and kushenol C (11) displayed significantly inhibitory effects against hCE 2 with IC₅₀ values of 1.46 ± 0.43, 5.22 ± 0.89, 1.13 ± 0.19, 9.78 ± 0.98, 3.05 ± 0.46, and 2.61 ± 0.52 μM, respectively. Compounds 1, 5, 6, 9, and 11 were all uncompetitive inhibitors with Ki values of 1.73, 1.59, 16.89, 1.72, and 0.79 μM, respectively, and their Km values ranged from 2.08 μM to 5.41 μM. Furthermore, molecular docking was conducted for investigating mechanisms of compounds 1, 5, 6, 9, and 11 with hCE 2. These results suggested that compounds 1, 5, 6, 9, and 11 could be served as lead compounds for the development of novel hCE 2 inhibitors.

Phytochemical constituents from Uncaria rhynchophylla in human carboxylesterase 2 inhibition: Kinetics and interaction mechanism merged with docking simulations

BACKGROUND

Carboxylesterases (CEs) belong to the serine hydrolase family, and are in charge of hydrolyzing chemicals with carboxylic acid ester and amide functional groups via Ser-His-Glu. Uncaria rhynchophylla (Miq.) Miq. ex Havil. is a famous traditional Chinese medicine used in managing hyperpyrexia, epilepsy, preeclampsia, and hypertension in China.

HYPOTHESIS/PURPOSE

To discover the potential natural human carboxylesterase 2 (hCE 2) inhibitors from U. rhynchophylla.

METHODS

Compounds were obtained from the hooks of U. rhynchophylla by silica gel and preparative HPLC. Their structures were elucidated by using HRESIMS, 1D and 2D NMR spectra. Their inhibitory activeties and inhibition kinetics against hCE 2 were assayed by the fluorescent probe, and potential mechanisms were also investigated by molecular docking.

RESULTS

Twenty-three compounds, including a new phenolic acid uncariarhyine A (1), eight known triterpenoids (2-9), and ten known aromatic derivatives (10, 13-16, and 19-23), were isolated from U. rhynchophylla. Compounds 1-5, 7, 9, and 15 showed significant inhibitory activities against hCE 2 with IC₅₀ values from 4.01  ± 0.61 µM to 18.60 ± 0.21 µM, and their inhibition kinetic analysis results revealed that compounds 1, 5, 9, and 15 were non-competitive; compounds 3 and 4 were mixed-type, and compounds 2 and 7 were uncompetitive. Molecular docking studies indicated inhibition mechanisms of compounds 1-5, 7, 9, and 15 against hCE 2.

CONCLUSION

Our present findings highlight potential natural hCE 2 inhibitors from U. rhynchophylla.

Survey of natural products reported by Asian research groups in 2017

The new natural products reported in 2017 in peer-reviewed articles in journals with good reputations were reviewed and analyzed. The advances made by Asian research groups in the field of natural products chemistry in 2017 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.

Xylarianins A-D from the endophytic fungus Xylaria sp. SYPF 8246 as natural inhibitors of human carboxylesterase 2

Eighteen secondary metabolites were isolated from the fermentation broth of the endophytic fungus Xylaria sp. SYPF 8246, including four new compounds, xylarianins A-D (1-4), three new natural products, 6-methoxycarbonyl-2'-methyl-3,5,4',6'-tetramethoxy-diphenyl ether (5), 2-chlor-6-methoxycarbonyl-2'-rnethyl-3,5,4',6'-tetramethoxy-diphenyl ether (6), and 2-chlor-4'-hydroxy-6-methoxy carbonyl-2'-methyl-3,5,6'-trimethoxy-diphenyl ether (7), and eleven known compounds (8-18). Their structural elucidations were conducted by using 1D and 2D NMR, HRESIMS, and Rh₂(OCOCF₃)₄-induced electronic circular dichroism (ECD) spectra analyses. The integrated ¹H and ¹³C NMR data of three new natural products 5-7 were reported for the first time. All the isolated compounds were assayed for their inhibitory activities against human carboxylesterase 2 (hCE 2). Compounds 1, 5-9, and 18 displayed significant inhibitory activities against hCE 2 with IC₅₀ values of 10.43 ± 0.51, 6.69 ± 0.85, 12.36 ± 1.27, 18.25 ± 1.78, 29.78 ± 0.48, 18.86 ± 1.87, and 20.72 ± 1.51 µM, respectively. The interactions between compounds 1 and 5 with hCE 2 were anaylzed by molecular docking.

Drechmerin H, a novel 1(2), 2(18)-diseco indole diterpenoid from the fungus Drechmeria sp. as a natural agonist of human pregnane X receptor

A novel 1(2), 2(18)-diseco indole diterpenoid, drechmerin H (1), was isolated from the fermentation broth of Drechmeria sp. together with a new indole diterpenoid, 2'-epi terpendole A (3), and a known analogue, terpendole A (2). Their structures were determined by HRESIMS, 1D and 2D NMR, ECD, and X-ray single crystal diffraction analyses as well as quantum chemical calculation. The abosulte configuration of terpendole A (2) was determined for the first time. Compound 1 displayed the significant agonistic effect on pregnane X receptor (PXR) with EC₅₀ value of 134.91 ± 2.01 nM, and its interaction with PXR was investigated by molecular docking. Meantime, a plausible biosynthetic pathway for compounds 1-3 is also discussed in the present work.

Indole diterpenoids from the endophytic fungus Drechmeria sp. as natural antimicrobial agents

A fungal strain, Drechmeria sp., was isolated from the root of Panax notoginseng. Totally, seven new indole diterpenoids, drechmerins A-G (1-7), were isolated from the fermentation broth of Drechmeria sp. together with four known analogues (8-11). Their structures were determined on the basis of 1D and 2D NMR and electronic circular dichroism (ECD) spectroscopic analyses as well as theoretical calculations. All the isolated compounds were evaluated for their antimicrobial activities against Candida albicans, Staphylococcus aureus, Bacillus cereus, B. subtillis, Pseudomonas aeruginosa, and Klebsiella pneumonia, respectively. Drechmerin B (2) displayed antimicrobial activity against C. albicans with an MIC value of 12.5 μg/mL. Molecular docking was used to investigate interactions of peptide deformylase with compounds 1-3, 5-7, 9, and 10.

Alisma orientale extract exerts the reversing cholestasis effect by activation of farnesoid X receptor

BACKGROUND

Cholestasis is a clinical syndrome of liver damage that is caused by accumulation of bile acids in the liver and systemic circulation. Farnesoid X receptor (FXR) can regulate synthesis, metabolism, and excretion of bile acids. The rhizomes of Alisma orientale is a well-known traditional Chinese medicine to treat edema, obesity, gonorrhea, leukorrhea, diarrhea, hyperlipidemia, and diabetes in China.

HYPOTHESIS/PURPOSE

We hypothesized Alisma orientale extract (AOE) to exert hepatoprotective effect against α-naphthylisothiocyanate (ANIT) induced cholestasis in rat. We aimed to investigate the mechanism of AOE.

STUDY DESIGN

Male Sprague Dawley rats with intrahepatic cholestasis induced by ANIT were treated with AOE (150, 300, or 600 mg/kg). Rats receiving vehicle (0.5% CMC-Na) served as control.

METHODS

48 h after ANIT administration, rats were sacrificed. Blood was collected to obtain serum and livers were removed for histopathology and protein preparation. Biochemical indicators in serum were determined using commercial kits and triterpenoids were determined by liquid chromatography tandem Qtrap mass spectrometry. Proteomics was analyzed by liquid chromatography tandem ion-trap mass spectrometry. The differently expressed proteins were analyzed via the network database and verified by western blotting. The interaction between triterpenoids and FXR were evaluated by luciferase assay and molecular docking.

RESULTS

AOE treatment significantly decreased the serum AST, ALT, TBIL, and intrahepatic TBA and improved the liver pathologic change induced by ANIT. Proteomics analysis indicated that AOE regulated proteins related to bile acid homeostasis via activating farnesoid X receptor (FXR) signaling pathway. Luciferase assay and molecular docking results indicated that triterpenoids could activate FXR, which resulting in ameliorative accumulation of bile acids in the liver by increase of metabolism and transportation for bile acids, and decrease of synthesis for bile acids.

CONCLUSION

AOE protected against rat liver injury and cholestasis induced by ANIT by activation of farnesoid X receptor, suggesting that A. orientale could be regarded as a potential hepatoprotective drug.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as illisimonin A from Illicium simonsii.