Decreased protein kinase C activation mediates inhibitory effect of norathyriol on serotonin-mediated endothelial permeability

Isolation and characterization of bioactive xanthones from Hippocratea africana (Willd.)Loes.ex Engl. (Celastraceae)

ETHNOPHARMACOLOGICAL RELEVANCE

Hippocratea africana root is used in African folk medicine for the treatment of several ailments, including pain and inflammation.

AIM OF THE STUDY

To isolate anti-inflammatory and analgesic compounds from the roots of H. africana, with accompanying antioxidant potentials.

MATERIALS AND METHODS

Dichloromethane, ethyl acetate, and aqueous fractions of H. africana roots, and isolated compounds from the bioactive ethyl acetate fraction were evaluated for anti-inflammatory and analgesic activities using the xylene induced oedema in mice and thermal induced pain models, respectively. The antioxidant potentials of isolated compounds were tested in 2,2-diphenyl-1-picryhydrazyl radical and ferric reducing antioxidant power assays. Structures were elucidated on the basis of spectroscopic analyses, including 1D and 2D NMR experiments, ionization mass spectrometry, and comparison with literature data.

RESULTS

Isoathyriol (1,3,7-trihydroxy-6-methoxyxanthone) and norathyriol (1,3,6,7-tetrahydroxyxanthone) were isolated from the potent anti-inflammatory and analgesic ethyl acetate fraction of H. africana roots. Isoathyriol and norathyriol demonstrated good anti-inflammatory, analgesic, and antioxidant properties compared with the standards used in each assay.

CONCLUSIONS

This study substantiates the use of H. africana root extract in the alleviation of inflammation and pain, and reports the characterization of secondary metabolites in H. africana and for the first time the presence of xanthones in Hippocratea genus.

Adipose tissue as a possible therapeutic target for polyphenols: A case for Cyclopia extracts as anti-obesity nutraceuticals

Obesity is a significant contributor to increased morbidity and premature mortality due to increasing the risk of many chronic metabolic diseases such as type 2 diabetes, cardiovascular disease and certain types of cancer. Lifestyle modifications such as energy restriction and increased physical activity are highly effective first-line treatment strategies used in the management of obesity. However, adherence to these behavioral changes is poor, with an increased reliance on synthetic drugs, which unfortunately are plagued by adverse effects. The identification of new and safer anti-obesity agents is thus of significant interest. In recent years, plants and their phenolic constituents have attracted increased attention due to their health-promoting properties. Amongst these, Cyclopia, an endemic South African plant commonly consumed as a herbal tea (honeybush), has been shown to possess modulating properties against oxidative stress, hyperglycemia, and obesity. Likewise, several studies have reported that some of the major phenolic compounds present in Cyclopia spp. exhibit anti-obesity effects, particularly by targeting adipose tissue. These phenolic compounds belong to the xanthone, flavonoid and benzophenone classes. The aim of this review is to assess the potential of Cyclopia extracts as an anti-obesity nutraceutical as underpinned by in vitro and in vivo studies and the underlying cellular mechanisms and biological pathways regulated by their phenolic compounds.

In Vitro Comparative Study of the Inhibitory Effects of Mangiferin and Its Aglycone Norathyriol towards UDP-Glucuronosyl Transferase (UGT) Isoforms

Mangiferin (MGF), the predominant constituent of extracts of the mango plant Mangifera Indica L., has been investigated extensively because of its remarkable pharmacological effects. In vitro recombinant UGTs-catalyzed glucuronidation of 4-methylumbelliferone (4-MU) was used to investigate the inhibition of mangiferin and aglycone norathyriol towards various isoforms of UGTs in our study, which evaluated the inhibitory capacity of MGF and its aglycone norathyriol (NTR) towards UDP-glucuronosyltransferase (UGT) isoforms. Initial screening experiment showed that deglycosylation of MGF into NTR strongly increased the inhibitory effects towards almost all the tested UGT isoforms at a concentration of 100 μM. Kinetic experiments were performed to further characterize the inhibition of UGT1A3, UGT1A7 and UGT1A9 by NTR. NTR competitively inhibited UGT1A3, UGT1A7 and UGT1A9, with an IC₅₀ value of 8.2, 4.4, and 12.3 μM, and a Ki value of 1.6, 2.0, and 2.8 μM, respectively. In silico docking showed that only NTR could dock into the activity cavity of UGT1A3, UGT1A7 and UGT1A9. The binding free energy of NTR to UGT1A3, 1A7, 1A9 were −7.4, −7.9 and −4.0 kcal/mol, respectively. Based on the inhibition evaluation standard ([I]/Ki < 0.1, low possibility; 0.1 < [I]/Ki < 1, medium possibility; [I]/Ki > 1, high possibility), an in vivo herb–drug interaction between MGF/NTR and drugs mainly undergoing UGT1A3-, UGT1A7- or UGT1A9-catalyzed metabolism might occur when the plasma concentration of NTR is above 1.6, 2.0 and 2.8 μM, respectively.

Platelets can enhance vascular permeability

Platelets survey blood vessels, searching for endothelial damage and preventing loss of vascular integrity. However, there are circumstances where vascular permeability increases, suggesting that platelets sometimes fail to fulfill their expected function. Human inflammatory arthritis is associated with tissue edema attributed to enhanced permeability of the synovial microvasculature. Murine studies have suggested that such vascular leak facilitates entry of autoantibodies and may thereby promote joint inflammation. Whereas platelets typically help to promote microvascular integrity, we examined the role of platelets in synovial vascular permeability in murine experimental arthritis. Using an in vivo model of autoimmune arthritis, we confirmed the presence of endothelial gaps in inflamed synovium. Surprisingly, permeability in the inflamed joints was abrogated if the platelets were absent. This effect was mediated by platelet serotonin accumulated via the serotonin transporter and could be antagonized using serotonin-specific reuptake inhibitor antidepressants. As opposed to the conventional role of platelets to microvascular leakage, this demonstration that platelets are capable of amplifying and maintaining permeability adds to the rapidly growing list of unexpected functions for platelets.

Norathyriol suppresses skin cancers induced by solar ultraviolet radiation by targeting ERK kinases

Ultraviolet (UV) irradiation is the leading factor in the development of skin cancer, prompting great interest in chemopreventive agents for this disease. In this study, we report the discovery of norathyriol, a plant-derived chemopreventive compound identified through an in silico virtual screening of the Chinese Medicine Library. Norathyriol is a metabolite of mangiferin found in mango, Hypericum elegans, and Tripterospermum lanceolatum and is known to have anticancer activity. Mechanistic investigations determined that norathyriol acted as an inhibitor of extracellular signal-regulated kinase (ERK)1/2 activity to attenuate UVB-induced phosphorylation in mitogen-activated protein kinases signaling cascades. We confirmed the direct and specific binding of norathyriol with ERK2 through a cocrystal structural analysis. The xanthone moiety in norathyriol acted as an adenine mimetic to anchor the compound by hydrogen bonds to the hinge region of the protein ATP-binding site on ERK2. Norathyriol inhibited in vitro cell growth in mouse skin epidermal JB6 P+ cells at the level of G(2)-M phase arrest. In mouse skin tumorigenesis assays, norathyriol significantly suppressed solar UV-induced skin carcinogenesis. Further analysis indicated that norathyriol mediates its chemopreventive activity by inhibiting the ERK-dependent activity of transcriptional factors AP-1 and NF-κB during UV-induced skin carcinogenesis. Taken together, our results identify norathyriol as a safe new chemopreventive agent that is highly effective against development of UV-induced skin cancer.

Pharmacokinetic study of mangiferin in human plasma after oral administration

Mangiferin, an active component of traditional Chinese herbal medicine, although it is reported to have various pharmacological effects, the limited number of pharmacokinetic studies limit its wide application. To evaluate the pharmacokinetics of mangiferin in human, a sensitive high performance liquid chromatography-mass spectrometry (HPLC-MS) method for the determination of mangiferin in human plasma was developed. The proposed HPLC-MS method is selective, precise and accurate enough and enables the identification and quantification of mangiferin for the use in clinical studies. After single oral administration of 0.1, 0.3 and 0.9g mangiferin, respectively, the method was successfully applied for the pharmacokinetics of mangiferin in 21 healthy male Chinese volunteers. The pharmacokinetic of mangiferin was fit to the non-compartmental model. The pharmacokinetics parameters were calculated. Mangiferin concentration in plasma reached 38.64±6.75ng/mL about 1h after oral administration of 0.9g mangiferin and the the apparent elimination half-life (t1/2) was 7.85±1.72h. The absorption of mangiferin was increased with the administration of a large dose and it was concluded that the pharmacokinetics of mangiferin in human was nonlinear.

Mangiferin and hesperidin metabolites are absorbed from the gastrointestinal tract of pigs after oral ingestion of a Cyclopia genistoides (honeybush tea) extract

Health-promoting properties such as antioxidative, anticarcinogenic, and cholesterol-lowering effects are described for mangiferin and hesperidin, the major phenolic compounds present in Cyclopia genistoides (honeybush). However, knowledge of their metabolic fate and their absorption from the gastrointestinal tract is very limited. The aim of this study was to determine the concentrations of mangiferin, hesperidin, and their metabolites in plasma, urine, and feces samples from pigs consuming an extract of Cyclopia genistoides. Pigs were administered up to 74 mg mangiferin per kilogram of body weight and 1 mg hesperidin per kilogram of body weight per day for 11 days. Plasma samples were collected at various time points on days 9 and 11 of the study and days 1 and 2 after termination of extract administration. Urine and feces were collected in fractions for 24 hours. In the plasma samples, the aglycone of mangiferin (norathyriol) was detected. Mean plasma concentrations ranged from 7.8 to 11.8 mumol/L. Six metabolites of mangiferin and hesperidin were detected in the urine, including methyl mangiferin, norathyriol, its monoglucuronide, hesperetin, hesperetin monoglucuronide, and eriodictyol monoglucuronide. Between 26.0% and 30.8% of the administered dose of hesperidin and only between 1.4% and 1.6% of mangiferin could be detected in the urine on days 9 and 11 of the study. Approximately 8.2% of the administered dose of mangiferin was determined in the feces. The main metabolite was norathyriol. Neither hesperidin nor metabolites ascribed to hesperidin intake were detected. The results suggest that formation of norathyriol from mangiferin occurs in vivo, and specific metabolites were identified in blood and excretion products in urine and feces. This study will aid in investigating the physiological functions of the parent compounds in vivo.

Identification and determination of four metabolites of mangiferin in rat urine

Four metabolites of mangiferin were firstly isolated and identified from rat urine. The structures of the four metabolites were determined to be 1,3,7-trihydroxyxanthone (M-1), 1,3,6,7-tetrahydroxyxanthone (M-2), 1,3,6-trihydroxy-7-methoxyxanthone (M-3) and 1,7-dihydroxyxanthone (M-4), respectively. A simple and specific analytical method for determination of the four metabolites in rat urine was developed by high performance liquid chromatography (HPLC). Quercetin was employed as an internal standard. The correlation coefficients of the calibration curves were higher than 0.997, both intra- and inter-day precision of four metabolites were determined and their R.S.D. did not exceed 10%. The accuracy and linear range had been investigated in detail. The cumulative urinary excretions of the four metabolites were measured and the possible metabolic pathway of the metabolites was discussed.

Pharmacological Effects of Xanthones as Cardiovascular Protective Agents

Many epidemiological studies indicate that consumption of dietary polyphenolic compounds is beneficial in the prevention of cardiovascular diseases. Xanthones are a class of polyphenolic compounds that commonly occur in plants and have been shown to have extensive biological and pharmacological activities. Recently, the pharmacological properties of xanthones in the cardiovascular system have attracted great interest. Xanthones and xanthone derivatives have been shown to have beneficial effects on some cardiovascular diseases, including ischemic heart disease, atherosclerosis, hypertension and thrombosis. The protective effects of xanthones in the cardiovascular system may be due to their antioxidant, antiinflammatory, platelet aggregation inhibitory, antithrombotic and/or vasorelaxant activities. In particular, the antagonism of endogenous nitric oxide synthase inhibitors by xanthones may represent the basis for improved endothelial function and for reduction of events associated with atherosclerosis.

Isolation of a human intestinal bacterium that transforms mangiferin to norathyriol and inducibility of the enzyme that cleaves a C-glucosyl bond

The C-glucosyl bond of C-glucosides generally tolerates acid and enzymatic hydrolysis. Many C-glucosides are cleaved by human intestinal bacteria. We isolated the specific bacterium involved in the metabolism of mangiferin (2-beta-D-glucopyranosyl-1,3,6,7-tetrahydroxyxanthone), C-glucosyl xanthone, from a mixture of human fecal bacteria. The anaerobic Bacteroides species named MANG, transformed mangiferin to the aglycone, norathyriol, suggesting cleavage of a C-glucosyl bond. However, B. sp. MANG cleaved C-glucosyl in a dose- and time-dependent manner only when cultivated in the presence of mangiferin. Cleavage was abolished by inhibitors of RNA and protein syntheses, such as rifampicin and chloramphenicol, respectively, indicating that the enzyme that cleaves C-glucosyl is induced by mangiferin. In contrast, mangiferin did not affect bacterial alpha- and beta-glucosidase activities under any conditions. The C-glucosyl-cleavage in cell-free extracts was not altered by potent glucosidase inhibitors such as 1-deoxynojirimycin and gluconolactone. Therefore, the C-glucosyl-cleaving enzyme substantially differs from known glucosidases that cleave O-glucosides. This is the first description of a specific intestinal bacterium that is involved in the metabolism of mangiferin and which produces a novel and inducible C-glucosyl-cleaving enzyme.

Two Proteins, Mn2+, and Low Molecular Cofactor Are Required for C-Glucosyl-Cleavage of Mangiferin

C-Glucosides, in which sugars are attached to the aglycone by carbon-carbon bonds, are generally resistant to acid and enzyme hydrolysis. The C-glucosyl bond of mangiferin, a xanthone C-glucoside, was cleaved by anaerobic incubation with a human intestinal bacterium, Bacteroides sp. MANG, to give norathyriol. A cell-free extract obtained by sonication of B. sp. MANG demonstrated cleaving activity for mangiferin to norathyriol by adding NADH, diaphorase, and dithiothreitol. Both high molecular weight (>10 k) and low molecular weight (<10 k) fractions obtained from the cell-free extract were required for the activity. MnCl2 was necessary for the activity, but other metal ions were not. By purification of the high molecular weight fraction using DEAE-cellulose and Phenyl Sepharose column chromatography, two fractions, designated as proteins A and B, were separated and required for the activity. Neither protein A nor protein B alone showed any activity. This is the first report describing a C-glucosyl-cleaving enzyme from human intestinal bacterium that seems to involve a novel enzyme mechanism.

The effect of elevated extracellular glucose on adherens junction proteins in cultured rat heart endothelial cells

Vascular permeability is a proof of vascular endothelial cell dysfunction induced by diabetes. Vascular permeability is directly related to the width of intercellular endothelial cells junctions, which may become permeable to macromolecules as a result of a change in endothelial cell shape. To determine the role of hyperglycemia in endothelial cell shape, the study examined the effect of high concentrations of glucose on the shape of cultured rat heart endothelial cells. This result indicated that the high-glucose-induced changes in the morphology of endothelial cells, via the glucose-mediated reorganization of F-actin. In endothelial cells, the actin cytoskeleton is tethered to the zonula adherens and focal adhesions, which mediate cell-cell and cell-matrix interactions respectively. The present study demonstrated that the high-glucose-induced changes in the actin-binding protein such as filamin, zonula adherens proteins such as alpha-, beta-, and gamma-catenin, focal adhesions proteins such as focal adhesion kinase, paxillin, and tyrosine phosphorylation of paxillin. It appears that differences in expression of adherens junctions molecules on rat heart endothelial cells in response to high glucose reflect endothelial glucose toxicity, which may also induce endothelial dysfunction in diabetes.

Serotonin-induced protein kinase C activation in cultured rat heart endothelial cells

This study examined whether serotonin can activate protein kinase C in rat heart endothelial cells. Protein kinase C isozyme translocation was examined by Western blot analysis with isozyme-specific anti-protein kinase C antibody. In this study, only alpha protein kinase C isozyme was found to be translocated from the cytosolic to the particulate fractions after serotonin stimulation. The effect of serotonin on the incorporation of 32P from [gamma-32P]ATP into peptide substrate was studied as another indicator of protein kinase C activation. The experiments in this study demonstrated that the Ca(2+)-phospholipid-dependent protein kinase, protein kinase C, was activated by serotonin. By investigating [3H]phorbol 12,13-dibutyrate binding to protein kinase C and trypsin-treated protein kinase C activity, we demonstrated that the site of action of serotonin is probably the regulatory domain of protein kinase C. Finally, we also demonstrated that serotonin had no effect on the intracellular concentration of cyclic nucleotides (cAMP, cGMP). These findings support the hypothesis that protein kinase C may be an important participant in serotonin-induced endothelial cell contraction and barrier dysfunction.

Serotonin-stimulated increase in cytosolic Ca(2+) in cultured rat heart endothelial cells

This study was designed to investigate the effects of serotonin on changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) in cultured rat heart endothelial cells. Serotonin stimulated a biphasic change in cytosolic Ca(2+) of rat heart endothelial cells: an initial transient increase, which primarily reflects the release of Ca(2+) from internal stores, followed by a slow rise in [Ca(2+)](i) during the incubation with serotonin. Our study also demonstrated that the pattern of the serotonin-induced increase in [Ca(2+)](i) was different from that induced by thrombin in rat heart endothelial cells. In this study, the role of [Ca(2+)](i) on endothelial paracellular barrier function was also investigated. Serotonin induced an increase in endothelial permeability which paralleled the rise in [Ca(2+)](i) and was blocked by the 5-HT(2) receptor antagonist cyproheptadine. Therefore, the serotonin-stimulated increase in cytosolic Ca(2+) and macromolecular permeability was receptor-mediated in rat heart endothelial cells. Further experiments demonstrated that the serotonin-induced increase in [Ca(2+)](i) was inhibited by the phospholipase C inhibitors, neomycin and [6-[[17beta-3-methoxyestra-1,3, 5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122). Experiments involving the rapid depletion of intracellular Ca(2+) stores and Ca(2+)-free medium demonstrated that the biphasic response of endothelial Ca(2+) to serotonin was related to the release of Ca(2+) from intracellular stores and to the influx of extracellular Ca(2+). We also suggest that serotonin-induced changes in [Ca(2+)](i) are related to Ca(2+) channels sensitive to voltage-operated and inorganic Ca(2+) channel blockers.