Biochemical investigations of polyphenol degradation enzymes in the phototrophic bacterium Rubrivivax gelatinosus.

Phloroglucinol (1,3,5-trihydroxybenzene) is an important intermediate in the degradation of flavonoids and tannins by anaerobic bacteria. Recent studies have shed light on the enzymatic mechanism of phloroglucinol degradation in butyrate-forming anaerobic bacteria, including environmental and intestinal bacteria such as Clostridium and Flavonifractor sp. Phloroglucinol degradation gene clusters have also been identified in other metabolically diverse bacteria, although the polyphenol metabolism of these microorganisms remain largely unexplored. Here, we describe biochemical studies of polyphenol degradation enzymes found in the purple non-sulfur bacterium Rubrivivax gelatinosus IL144, an anaerobic photoheterotroph reported to utilize diverse organic compounds as carbon sources for growth. In addition to the phloroglucinol reductase and dihydrophloroglucinol cyclohydrolase that catalyze phloroglucinol degradation, we characterize a Mn2+-dependent phloretin hydrolase that catalyzes the cleavage of phloretin into phloroglucinol and phloretic acid. We also report a Mn2+-dependent decarboxylase (DeC) that catalyzes the reversible decarboxylation of 2,4,6-trihydroxybenzoate to form phloroglucinol. A bioinformatics search led to the identification of DeC homologs in diverse soil and gut bacteria, and biochemical studies of a DeC homolog from the human gut bacterium Flavonifractor plautii demonstrated that it is also a 2,4,6-trihydroxybenzoate decarboxylase. Our study expands the range of enzymatic mechanisms for phloroglucinol formation, and provides further biochemical insight into polyphenol metabolism in the anaerobic biosphere.

Phloretin suppresses intestinal inflammation and maintained epithelial tight junction integrity by modulating cytokines secretion in in vitro model of gut inflammation.

Ulcerative colitis is a type of inflammatory bowel disease which in long run can lead to colorectal cancer (CRC). Chronic inflammation can be a key factor for the occurrence of CRC thus mitigating an inflammation can be a preventive strategy for the occurrence of CRC. In this study we have explored the anti-inflammatory potential of phloretin, in in vitro gut inflammation model, developed by co-culture of Caco2 (intestinal epithelial) cells and RAW264.7 macrophages (immune cells). Phloretin is a dihydrochalcone present in apple, pear and strawberries. An anti-inflammatory effect of phloretin in reducing LPS induced inflammation and maintenance of transepithelial electric resistance (TEER) in Caco2 cells was examined. Paracellular permeability assay was performed using Lucifer yellow dye to evaluate the effect of phloretin in inhibiting gut leakiness caused by inflammatory mediators secreted by activated macrophages. Phloretin attenuated LPS induced nitric oxide levels, oxidative stress, depolarization of mitochondrial membrane potential in Caco2 cells as evidenced by reduction in reactive oxygen species (ROS), and enhancement of MMP, and decrease in inflammatory cytokines IL8, TNFα, IL1ß and IL6. It exhibited anti-inflammatory activity by inhibiting the expression of NFκB, iNOS and Cox2. Phloretin maintained the epithelial integrity by regulating the expression of tight junction proteins ZO1, occludin, Claudin1 and JAM. Phloretin reduced LPS induced levels of Cox2 along with the reduction in Src expression which further regulated an expression of tight junction protein occludin. Phloretin in combination to sodium pyruvate exhibited potential anti-inflammatory activity via targeting NFkB signaling. Our findings paved a way to position phloretin as nutraceutical in preventing the occurrence of colitis and culmination of disease into colitis associated colorectal cancer.

Crystal Structures of the C-Glycosyltransferase UGT708C1 from Buckwheat Provide Insights into the Mechanism of C-Glycosylation.

C-Glycosyltransferases (CGTs) catalyze the formation of C-glycosidic bonds for the biosynthesis of C-glycosides, but the underlying mechanism is unclear. This process improves the solubility and bioavailability of specialized metabolites, which play important roles in plant growth and development and represent rich resources for drug discovery. Here, we performed functional and structural studies of the CGT UGT708C1 from buckwheat (Fagopyrum esculentum). Enzymatic analysis showed that UGT708C1 is capable of utilizing both UDP-galactose and UDP-glucose as sugar donors. Our structural studies of UGT708C1 complexed with UDP-glucose and UDP identified the key roles of Asp382, Gln383, Thr151, and Thr150 in recognizing the sugar moiety of the donor substrate and Phe130, Tyr102, and Phe198 in binding and stabilizing the acceptor. A systematic site-directed mutagenesis study confirmed the important roles of these residues. Further structural analysis combined with molecular dynamics simulations revealed that phloretin binds to the acceptor binding pocket in a bent state with a precise spatial disposition and complementarity. These findings provide insights into a catalytic mechanism for CGTs.

Phloretin inhibits glucose transport and reduces inflammation in human retinal pigment epithelial cells.

During age-related macular degeneration (AMD), chronic inflammatory processes, possibly fueled by high glucose levels, cause a breakdown of the retinal pigment epithelium (RPE), leading to vision loss. Phloretin, a natural dihydroxychalcone found in apples, targets several anti-inflammatory signaling pathways and effectively inhibits transporter-mediated glucose uptake. It could potentially prevent inflammation and cell death of RPE cells through either direct regulation of inflammatory signaling pathways or through amelioration of high glucose levels. To test this hypothesis, ARPE-19 cells were incubated with or without phloretin for 1 h before exposure to lipopolysaccharide (LPS). Cell viability and the release of pro-inflammatory cytokines interleukin 6 (IL-6), IL-8 and vascular endothelial growth factor (VEGF) were measured. Glucose uptake was studied using isotope uptake studies. The nuclear levels of nuclear factor erythroid 2-related factor 2 (Nrf2) were determined alongside the phosphorylation levels of mitogen-activated protein kinases. Phloretin pretreatment reduced the LPS-induced release of IL-6 and IL-8 as well as VEGF. Phloretin increased intracellular levels of reactive oxygen species and nuclear translocation of Nrf2. It also inhibited glucose uptake into ARPE-19 cells and the phosphorylation of Jun-activated kinase (JNK). Subsequent studies revealed that Nrf2, but not the inhibition of glucose uptake or JNK phosphorylation, was the main pathway of phloretin's anti-inflammatory activities. Phloretin was robustly anti-inflammatory in RPE cells and reduced IL-8 secretion via activation of Nrf2 but the evaluation of its potential in the treatment or prevention of AMD requires further studies.

Phloretin Improves Ultrafiltration and Reduces Glucose Absorption during Peritoneal Dialysis in Rats.

BACKGROUND: Harmful glucose exposure and absorption remain major limitations of peritoneal dialysis (PD). We previously showed that inhibition of sodium glucose cotransporter 2 did not affect glucose transport during PD in rats. However, more recently, we found that phlorizin, a dual blocker of sodium glucose cotransporters 1 and 2, reduces glucose diffusion in PD. Therefore, either inhibiting sodium glucose cotransporter 1 or blocking facilitative glucose channels by phlorizin metabolite phloretin would reduce glucose transport in PD. METHODS: We tested a selective blocker of sodium glucose cotransporter 1, mizagliflozin, as well as phloretin, a nonselective blocker of facilitative glucose channels, in an anesthetized Sprague-Dawley rat model of PD. RESULTS: Intraperitoneal phloretin treatment reduced glucose absorption by >30% and resulted in a >50% higher ultrafiltration rate compared with control animals. Sodium removal and sodium clearances were similarly improved, whereas the amount of ultrafiltration per millimole of sodium removed did not differ. Mizagliflozin did not influence glucose transport or osmotic water transport. CONCLUSIONS: Taken together, our results and previous results indicate that blockers of facilitative glucose channels may be a promising target for reducing glucose absorption and improving ultrafiltration efficiency in PD.

Chloride channels with ClC-1-like properties differentially regulate the excitability of dopamine receptor D1- and D2-expressing striatal medium spiny neurons.

Dynamic chloride (Cl-) regulation is critical for synaptic inhibition. In mature neurons, Cl- influx and extrusion are primarily controlled by ligand-gated anion channels (GABAA and glycine receptors) and the potassium chloride cotransporter K+-Cl- cotransporter 2 (KCC2), respectively. Here, we report for the first time, to our knowledge, a presence of a new source of Cl- influx in striatal neurons with properties similar to chloride voltage-gated channel 1 (ClC-1). Using whole cell patch-clamp recordings, we detected an outwardly rectifying voltage-dependent current that was impermeable to the large anion methanesulfonate (MsO-). The anionic current was sensitive to the ClC-1 inhibitor 9-anthracenecarboxylic acid (9-AC) and the nonspecific blocker phloretin. The mean fractions of anionic current inhibition by MsO-, 9-AC, and phloretin were not significantly different, indicating that anionic current was caused by active ClC-1-like channels. In addition, we found that Cl- current was not sensitive to the transmembrane protein 16A (TMEM16A; Ano1) inhibitor Ani9 and that the outward Cl- rectification was preserved even at a very high intracellular Ca2+ concentration (2 mM), indicating that TMEM16B (Ano2) did not contribute to the total current. Western blotting and immunohistochemical analyses confirmed the presence of ClC-1 channels in the striatum mainly localized to the somata of striatal neurons. Finally, we found that 9-AC decreased action potential firing frequencies and increased excitability in medium spiny neurons (MSNs) expressing dopamine type 1 (D1) and type 2 (D2) receptors in the brain slices, respectively. We conclude that ClC-1-like channels are preferentially located at the somata of MSNs, are functional, and can modulate neuronal excitability.

Antibiotic Pyrrolomycin as an Efficient Mitochondrial Uncoupler.

Pyrrolomycins C (Pyr_C) and D (Pyr_D) are antibiotics produced by Actinosporangium and Streptomyces. The mechanism of their antimicrobial activity consists in depolarization of bacterial membrane, leading to the suppression of bacterial bioenergetics through the uncoupling of oxidative phosphorylation, which is based on the protonophore action of these antibiotics [Valderrama et al., Antimicrob. Agents Chemother. (2019) 63, e01450]. Here, we studied the effect of pyrrolomycins on the isolated rat liver mitochondria. Pyr_C was found to be more active than Pyr_D and uncoupled mitochondria in the submicromolar concentration range, which was observed as the mitochondrial membrane depolarization and stimulation of mitochondrial respiration. In the case of mitoplasts (isolated mitochondria with impaired outer membrane integrity), the difference in the action of Pyr_C and Pyr_D was significantly less pronounced. By contrast, in inverted submitochondrial particles (SMPs), Pyr_D was more active as an uncoupler, which caused collapse of the membrane potential even at the nanomolar concentrations. The same ratio of the protonophoric activity of Pyr_D and Pyr_C was obtained by us on liposomes loaded with the pH indicator pyranine. The protonophore activity of Pyr_D in the planar bilayer lipid membranes (BLMs) was maximal at ~pH 9, i.e., at pH values close to pKa of this compound. Pyr_D functions as a typical anionic protonophore; its activity in the BLM could be reduced by the addition of the dipole modifier phloretin. The difference between the protonophore activity of Pyr_C and Pyr_D in the mitochondria and BLMs can be attributed to a higher ability of Pyr_C to penetrate the outer mitochondrial membrane.

[Enzymatic characterization of lignan glucosyltransferase of Isatis indigotica].

The lignan glycosyltransferase UGT236(belonging to the UGT71 B family) from Isatis indigotica can catalyze the production of phloridzin from phloretin in vitro. UGT236 shares high identity with P2'GT from apple. In this study, the recombinant plasmid pET28 a-MBP-UGT236 was transferred into Escherichia coli Rosetta(DE3) cells and induced by isopropyl-ß-D-thiogalactoside(IPTG). The purified UGT236 protein was used for enzymatic characterization with phloretin as substrate. The results showed that UGT236 had the optimal reaction temperature of 40 ℃ and the optimal pH 8(Na_2HPO_4-NaH_2PO_4 system). The UGT236 activity was inhibited by Ni~(2+) and Al~(3+), enhanced by Fe~(2+), Co~(2+), and Mn~(2+), and did not affected by Mg~(2+), Ca~(2+), Li~+, Na~+, or K~+. The K_m, K_(cat), and K_(cat)/K_m of phloretin were 61.03 µmol·L~(-1), 0.01 s~(-1), and 157.11 mol~(-1)·s~(-1)·L, and those of UDPG were 183.6 µmol·L~(-1), 0.01 s~(-1), and 51.91 mol~(-1)·s~(-1)·L, respectively. The possible active sites were predicted by homologous modeling and molecular docking. By mutagenisis and catalytic activity detection, three key active sites, Glu391, His15, and Thr141, were identified, while Phe146 was related to product diversity. In summary, we found that the lignan glycosyltransferase UGT236 from I.indigotica could catalyze the reaction of phloretin into phloridzin. Several key amino acid residues were identified by structure prediction, molecular docking, and site-mutagenesis, which provided a basis for studying the specificity and diversity of phloretin glycoside products. This study can provide a reference for artificially producing glycosyltransferase elements with high efficiency and specific catalysis.

Biosynthesis of the Dihydrochalcone Sweetener Trilobatin Requires Phloretin Glycosyltransferase2.

Epidemics of obesity and type 2 diabetes drive strong consumer interest in plant-based low-calorie sweeteners. Trilobatin is a sweetener found at high concentrations in the leaves of a range of crabapple (Malus) species, but not in domesticated apple (Malus × domestica) leaves, which contain trilobatin's bitter positional isomer phloridzin. Variation in trilobatin content was mapped to the Trilobatin locus on LG 7 in a segregating population developed from a cross between domesticated apples and crabapples. Phloretin glycosyltransferase2 (PGT2) was identified by activity-directed protein purification and differential gene expression analysis in samples high in trilobatin but low in phloridzin. Markers developed for PGT2 cosegregated strictly with the Trilobatin locus. Biochemical analysis showed PGT2 efficiently catalyzed 4'-o-glycosylation of phloretin to trilobatin as well as 3-hydroxyphloretin to sieboldin. Transient expression of double bond reductase, chalcone synthase, and PGT2 genes reconstituted the apple pathway for trilobatin production in Nicotiana benthamiana Transgenic M. × domestica plants overexpressing PGT2 produced high concentrations of trilobatin in young leaves. Transgenic plants were phenotypically normal, and no differences in disease susceptibility were observed compared to wild-type plants grown under simulated field conditions. Sensory analysis indicated that apple leaf teas from PGT2 transgenics were readily discriminated from control leaf teas and were perceived as significantly sweeter. Identification of PGT2 allows marker-aided selection to be developed to breed apples containing trilobatin, and for high amounts of this natural low-calorie sweetener to be produced via biopharming and metabolic engineering in yeast.

A Holistic Evolutionary and 3D Pharmacophore Modelling Study Provides Insights into the Metabolism, Function, and Substrate Selectivity of the Human Monocarboxylate Transporter 4 (hMCT4).

Monocarboxylate transporters (MCTs) are of great research interest for their role in cancer cell metabolism and their potential ability to transport pharmacologically relevant compounds across the membrane. Each member of the MCT family could potentially provide novel therapeutic approaches to various diseases. The major differences among MCTs are related to each of their specific metabolic roles, their relative substrate and inhibitor affinities, the regulation of their expression, their intracellular localization, and their tissue distribution. MCT4 is the main mediator for the efflux of L-lactate produced in the cell. Thus, MCT4 maintains the glycolytic phenotype of the cancer cell by supplying the molecular resources for tumor cell proliferation and promotes the acidification of the extracellular microenvironment from the co-transport of protons. A promising therapeutic strategy in anti-cancer drug design is the selective inhibition of MCT4 for the glycolytic suppression of solid tumors. A small number of studies indicate molecules for dual inhibition of MCT1 and MCT4; however, no selective inhibitor with high-affinity for MCT4 has been identified. In this study, we attempt to approach the structural characteristics of MCT4 through an in silico pipeline for molecular modelling and pharmacophore elucidation towards the identification of specific inhibitors as a novel anti-cancer strategy.

Functional Characterization and Structural Basis of an Efficient Di-C-glycosyltransferase from Glycyrrhiza glabra.

A highly efficient di-C-glycosyltransferase GgCGT was discovered from the medicinal plant Glycyrrhiza glabra. GgCGT catalyzes a two-step di-C-glycosylation of flopropione-containing substrates with conversion rates of >98%. To elucidate the catalytic mechanisms of GgCGT, we solved its crystal structures in complex with UDP-Glc, UDP-Gal, UDP/phloretin, and UDP/nothofagin, respectively. Structural analysis revealed that the sugar donor selectivity was controlled by the hydrogen-bond interactions of sugar hydroxyl groups with D390 and other key residues. The di-C-glycosylation capability of GgCGT was attributed to a spacious substrate-binding tunnel, and the G389K mutation could switch di- to mono-C-glycosylation. GgCGT is the first di-C-glycosyltransferase with a crystal structure, and the first C-glycosyltransferase with a complex structure containing a sugar acceptor. This work could benefit the development of efficient biocatalysts to synthesize C-glycosides with medicinal potential.

Phloretin attenuates STAT-3 activity and overcomes sorafenib resistance targeting SHP-1-mediated inhibition of STAT3 and Akt/VEGFR2 pathway in hepatocellular carcinoma.

BACKGROUND: Hepatocellular carcinoma (HCC) is the most common primary liver malignancy. Phloretin (PH) possesses anticancer, antitumor, and hepatoprotective effects, however, the effects and potential mechanisms of phloretin remain elusive. METHODS: Five HCC cells were tested in vitro for sensitivity to PH, Sorafenib (Sor) or both and the apoptosis, signal transduction and phosphatase activity were analyzed. To validate the role of SHP-1, we used PTP inhibitor III and SHP-1 siRNA. Further, we used purified SHP-1 proteins or HCC cells expressing deletion N-SH2 domain or D61A point mutants to study the PH efficacy on SHP-1. The `in vivo studies were conducted using HepG2 and SK-Hep1 and Sor resistant HepG2SR and Huh7SR xenografts. Molecular docking was done with Swiss dock and Auto Dock Vina. RESULTS: PH inhibited cell growth and induced apoptosis in all HCC cells by upregulating SHP-1 expression and downregulating STAT3 expression and further inhibited pAKT/pERK signaling. PH activated SHP-1 by disruption of autoinhibition of SHP-1, leading to reduced p-STAT3Tyr705 level. PH induced apoptosis in two Sor-resistant cell lines and overcome STAT3, AKT, MAPK and VEGFR2 dependent Sor resistance in HCCs. PH potently inhibited tumor growth in both Sor-sensitive and Sor-resistant xenografts in vivo by impairing angiogenesis, cell proliferation and inducing apoptosis via targeting the SHP-1/STAT3 signaling pathway. CONCLUSION: Our data suggest that PH inhibits STAT3 activity in Sor-sensitive and -resistant HCCs via SHP-1-mediated inhibition of STAT3 and AKT/mTOR/JAK2/VEGFR2 pathway. Our results clearly indicate that PH may be a potent reagent for hepatocellular carcinoma and a noveltargeted therapy for further clinical investigations.

Silencing a phloretin-specific glycosyltransferase perturbs both general phenylpropanoid biosynthesis and plant development.

The polyphenol profile of apple (Malus × domestica) is dominated by the dihydrochalcone glycoside phloridzin, but its physiological role is yet to be elucidated. Biosynthesis of phloridzin occurs as a side branch of the main phenylpropanoid pathway, with the final step mediated by the phloretin-specific glycosyltransferase UGT88F1. Unexpectedly, given that UGTs are sometimes viewed as 'decorating enzymes', UGT88F1 knockdown lines were severely dwarfed, with greatly reduced internode lengths, narrow lanceolate leaves, and changes in leaf and fruit cellular morphology. These changes suggested that auxin transport had been altered in the knockdown lines, which was confirmed in assays showing that auxin flux from the shoot apex was increased in the transgenic lines. Metabolite analysis revealed no accumulation of the phloretin aglycone, as well as decreases in many non-target phenylpropanoid compounds. This decreased accumulation of metabolites appeared to be mediated by the repression of the phenylpropanoid pathway via a reduction in key transcript levels (e.g. phenylalanine ammonia lyase, PAL) and enzyme activities (PAL and chalcone synthase). Application of exogenous phloridzin to the UGT88F1 knockdown lines in tissue culture enhanced axial leaf growth and partially restored some aspects of 'normal' apple leaf growth. Together, our results strongly implicate dihydrochalcones as critical compounds in modulating phenylpropanoid pathway flux and establishing auxin patterning early in apple development.

C-Glycosyltransferases catalyzing the formation of di-C-glucosyl flavonoids in citrus plants.

Citrus plants accumulate many kinds of flavonoids, including di-C-glucosyl flavonoids, which have attracted considerable attention due to their health benefits. However, the biosynthesis of di-C-glucosyl flavonoids has not been elucidated at the molecular level. Here, we identified the C-glycosyltransferases (CGTs) FcCGT (UGT708G1) and CuCGT (UGT708G2) as the primary enzymes involved in the biosynthesis of di-C-glucosyl flavonoids in the citrus plants kumquat (Fortunella crassifolia) and satsuma mandarin (Citrus unshiu), respectively. The amino acid sequences of these CGTs were 98% identical, indicating that CGT genes are highly conserved in the citrus family. The recombinant enzymes FcCGT and CuCGT utilized 2-hydroxyflavanones, dihydrochalcone, and their mono-C-glucosides as sugar acceptors and produced corresponding di-C-glucosides. The Km and kcat values of FcCGT toward phloretin were <0.5 µm and 12.0 sec-1 , and those toward nothofagin (3'-C-glucosylphloretin) were 14.4 µm and 5.3 sec-1 , respectively; these values are comparable with those of other glycosyltransferases reported to date. Transcripts of both CGT genes were found to concentrate in various plant organs, and particularly in leaves. Our results suggest that di-C-glucosyl flavonoid biosynthesis proceeds via a single enzyme using either 2-hydroxyflavanones or phloretin as a substrate in citrus plants. In addition, Escherichia coli cells expressing CGT genes were found to be capable of producing di-C-glucosyl flavonoids, which is promising for commercial production of these valuable compounds.

Phloretin reduces cell injury and inflammation mediated by Staphylococcus aureus via targeting sortase B and the molecular mechanism.

Sortase B (SrtB) is a vital virulence factor that plays a critical role in Staphylococcus aureus (S. aureus) infections, indicating that it could be a latent target for S. aureus infections. In this study, phloretin, a natural compound that primarily exists in the pericarp and velamen of apples and pears, shows little anti-S. aureus activity, but significantly inhibited SrtB activity in vitro. The results of lactate dehydrogenase release and live/dead cell assays suggested that phloretin reduced human alveolar epithelial cell damage caused by S. aureus. Additionally, an adhesion assay confirmed that phloretin lowered the colony count of S. aureus in human alveolar cells. Phloretin treatment significantly attenuated the inflammatory response in macrophage cells (J774) co-cultured with S. aureus as determined by an enzyme-linked immune-sorbent assay. Furthermore, the results of molecular dynamics simulation, site-directed mutagenesis, and fluorescence spectroscopy quenching indicated that phloretin was directly located in the active pocket of SrtB and blocked substrate binding, leading to the loss of SrtB activity. These results indicate that phloretin is a possible candidate for treatment of S. aureus infections.

Anticancer Activity of Phloretin Against Human Gastric Cancer Cell Lines Involves Apoptosis, Cell Cycle Arrest, and Inhibition of Cell Invasion and JNK Signalling Pathway.

BACKGROUND Gastric cancer is one of most commonly diagnosed cancers and causes significant mortality worldwide. In this study, the antiproliferative and anticancer effects of Phloretin were evaluated against gastric cancer cell lines. MATERIAL AND METHODS MTT assay was used to assess the proliferation rate of gastric cancer cells. DAPI and annexin V/PI were used for detection of apoptotic cell death. Cell invasion was investigated by Transwell assays and the expression of the proteins was estimated by immunoblotting. RESULTS The results revealed that Phloretin exerts anticancer effects on all the gastric cancer cell lines used in this study. However, the anticancer effects were more significant (p<0.05) on the AGS cell line. Further, the effect of Phloretin on the viability of normal GES-1 cells was minimal. Apoptosis assays showed that Phloretin triggers apoptotic cell death in AGS gastric cancer cells. Phloretin could also cause the arrest of the AGS gastric cancer cells in the G2/M phase of the cell cycle and suppress their ability to migrate. Western blotting analysis revealed that Phloretin significantly decreased the expression of p-JNK and p-38. However, comparatively lower effects were observed on the expression of JNK and P38. CONCLUSIONS We showed that Phloretin is an important molecule for the treatment of gastric cancer.

Quercetin, Morin, Luteolin, and Phloretin Are Dietary Flavonoid Inhibitors of Monocarboxylate Transporter 6.

Monocarboxylate transporter 6 (MCT6; SLC16A5) has been recognized for its role as a xenobiotic transporter, with characterized substrates probenecid, bumetanide, and nateglinide. To date, the impact of commonly ingested dietary compounds on MCT6 function has not been investigated, and therefore, the objective of this study was to evaluate a variety of flavonoids for their potential MCT6-specific interactions. Flavonoids are a large group of polyphenolic phytochemicals found in commonly consumed plant-based products that have been recognized for their dietary health benefits. The uptake of bumetanide in human MCT6 gene-transfected Xenopus laevis oocytes was significantly decreased in the presence of a variety of flavonoids (e.g., quercetin, luteolin, phloretin, and morin), but was not significantly affected by flavonoid glycosides (e.g., naringin, rutin, phlorizin). The IC50 values of quercetin, phloretin, and morin were determined to be 25.3 ± 3.36, 17.3 ± 2.37, and 33.1 ± 3.29 µM, respectively. The mechanism of inhibition of phloretin was reversible and competitive, with a Ki value of 22.8 µM. Furthermore, typical MCT substrates were also investigated for their potential interactions with MCT6. Substrates of MCTs 1, 2, 4, 8, and 10 did not cause any significant decrease in MCT6-mediated bumetanide uptake, suggesting that MCT6 has distinct compound selectivity. In summary, these results suggest that dietary aglycon flavonoids may significantly alter the pharmacokinetics and pharmacodynamics of bumetanide and other MCT6-specific substrates, and may represent potential substrates for MCT6.

Enolate-Forming Phloretin Pharmacophores: Hepatoprotection in an Experimental Model of Drug-Induced Toxicity.

Drug-induced toxicity is often mediated by electrophilic metabolites, such as bioactivation of acetaminophen (APAP) to N-acetyl-p-benzoquinone imine (NAPQI). We have shown that APAP hepatotoxicity can be prevented by 2-acetylcyclopentanone (2-ACP). This 1,3-dicarbonyl compound ionizes to form an enolate nucleophile that scavenges NAPQI and other electrophilic intermediates. In this study, we expanded our investigation of enolate-forming compounds to include analyses of the phloretin pharmacophores, 2',4',6'-trihydroxyacetophenone (THA) and phloroglucinol (PG). Studies in a mouse model of APAP overdose showed that THA provided hepatoprotection when given either by intraperitoneal injection or oral administration, whereas PG was hepatoprotective only when given intraperitoneally. Corroborative research characterized the molecular pharmacology (efficacy, potency) of 2-ACP, THA, and PG in APAP-exposed isolated mouse hepatocytes. For comparative purposes, N-acetylcysteine (NAC) cytoprotection was also evaluated. Measurements of multiple cell parameters (e.g., cell viability, mitochondrial membrane depolarization) indicated that THA and, to a lesser extent, PG provided concentration-dependent protection against APAP toxicity, which exceeded that of 2-ACP or NAC. The enolate-forming compounds and NAC truncated ongoing APAP exposure and thereby returned intoxicated hepatocytes toward normal viability. The superior ability of THA to protect is related to multifaceted modes of action that include metal ion chelation, free radical trapping, and scavenging of NAPQI and other soft electrophiles involved in oxidative stress. The rank order of potency for the tested cytoprotectants was consistent with that determined in a parallel mouse model. These data suggest that THA or a derivative might be useful in treating drug-induced toxicities and other conditions that involve electrophile-mediated pathogenesis.

Transgenic apple plants overexpressing the chalcone 3-hydroxylase gene of Cosmos sulphureus show increased levels of 3-hydroxyphloridzin and reduced susceptibility to apple scab and fire blight.

MAIN CONCLUSION: Overexpression of chalcone-3-hydroxylase provokes increased accumulation of 3-hydroxyphloridzin in Malus . Decreased flavonoid concentrations but unchanged flavonoid class composition were observed. The increased 3-hydroxyphlorizin contents correlate well with reduced susceptibility to fire blight and scab. The involvement of dihydrochalcones in the apple defence mechanism against pathogens is discussed but unknown biosynthetic steps in their formation hamper studies on their physiological relevance. The formation of 3-hydroxyphloretin is one of the gaps in the pathway. Polyphenol oxidases and cytochrome P450 dependent enzymes could be involved. Hydroxylation of phloretin in position 3 has high similarity to the B-ring hydroxylation of flavonoids catalysed by the well-known flavonoid 3'-hydroxylase (F3'H). Using recombinant F3'H and chalcone 3-hydroxylase (CH3H) from Cosmos sulphureus we show that F3'H and CH3H accept phloretin to some extent but higher conversion rates are obtained with CH3H. To test whether CH3H catalyzes the hydroxylation of dihydrochalcones in planta and if this could be of physiological relevance, we created transgenic apple trees harbouring CH3H from C. sulphureus. The three transgenic lines obtained showed lower polyphenol concentrations but no shift between the main polyphenol classes dihydrochalcones, flavonols, hydroxycinnamic acids and flavan 3-ols. Increase of 3-hydroxyphloridzin within the dihydrochalcones and of epicatechin/catechin within soluble flavan 3-ols were observed. Decreased activity of dihydroflavonol 4-reductase and chalcone synthase/chalcone isomerase could partially explain the lower polyphenol concentrations. In comparison to the parent line, the transgenic CH3H-lines showed a lower disease susceptibility to fire blight and apple scab that correlated with the increased 3-hydroxyphlorizin contents.

Transcriptome analysis reveals the inhibitory mechanism of phloretin on virulence expression of Staphylococcus aureus and its application in cooked chicken.

Staphylococcus aureus (S. aureus) enterotoxins have aroused great concern to food safety owing to its increased risk of food poisoning. The current research aimed to investigate the anti-virulence mechanisms of phloretin against S. aureus in terms of toxin activity and gene expression. The results indicated that phloretin could effectively inhibit the production of hemolysins and enterotoxins, and its anti-virulence effect was exerted in a concentration-dependent manner. Transcriptome results indicated that phloretin could downregulate the transcription level of majority virulence factors related genes (68 %) of S. aureus, including the quorum sensing-related genes (agrB, agrC, agrA, sspA, splF, splD and others) and bacterial secretion system-related genes (secDF, secY2, and yidC). In addition, it was speculated that phloretin was most likely to bind to the AgrA DNA binding domain, thereby affecting the expression of downstream virulence genes (hla, seb, spa, rot, geh, etc) based on molecular docking. Finally, the application in cooked chicken indicated that phloretin could effectively decrease the content of enterotoxins and improve the storage quality of cooked chicken. These findings not only evidenced the feasible anti-virulence activity of phloretin, but also provided a new strategy to prevent S. aureus food poisoning in cooked meat preservation.