Resveratrol is efficiently glucuronidated by UDP-glucuronosyltransferases in the human gastrointestinal tract and in Caco-2 cells

Gut Microbiota-Derived Resveratrol Metabolites, Dihydroresveratrol and Lunularin, Significantly Contribute to the Biological Activities of Resveratrol

Although resveratrol (RES) is barely detectable in the plasma and tissues upon oral consumption, collective evidence reveals that RES presents various bioactivities in vivo, including anti-inflammation and anti-cancer. This paradox necessitates further research on profiling and characterizing the biotransformation of RES, as its metabolites may contribute profound biological effects. After 4-week oral administration, 11 metabolites of RES were identified and quantified in mice by HPLC-MS/MS, including dihydro-resveratrol (DHR), lunularin (LUN), and conjugates (sulfates and glucuronides) of RES, DHR and LUN. Importantly, DHR, LUN, and their conjugates were much more abundantly distributed in tissues, gastrointestinal tract (GIT), and biological fluids compared to RES and its conjugates. Moreover, we established that DHR and LUN were gut bacteria-derived metabolites of RES, as indicated by their depletion in antibiotic-treated mice. Furthermore, the biological activities of RES, DHR, and LUN were determined at physiologically relevant levels. DHR and LUN exhibited stronger anti-inflammatory and anti-cancer effects than RES at the concentrations found in mouse tissues. In summary, our study profiled the tissue distribution of the metabolites of RES after its oral administration in mice and uncovered the important role of gut microbial metabolites of RES in the biological activities of RES in vivo.

PD-1/PD-L1 Checkpoints and Resveratrol: A Controversial New Way for a Therapeutic Strategy

Simple Summary

Over the last decade, immunotherapies using antibodies targeting the programmed cell death 1 (PD-1) checkpoint or its ligand, programmed death ligand 1 (PD-L1), have emerged as promising therapeutic strategies against cancer. However, some current limitations include a relatively low rate of “responders”, the high cost of the treatment, and a rare risk of hyper-progression. Currently, the main challenge is, therefore, to improve these therapies, for instance, by using combined approaches. Here, we summarize the accumulating evidence that resveratrol (RSV) plays a role in the modulation of the PD-1/PD-L1 axis in cancer cells, suggesting the potential of therapeutic strategies combining RSV with PD-L1 or anti-PD-1 inhibitors. We then discuss the therapeutic potential of polyphenols such as RSV to be used in combination with PD-L1 or PD-1 inhibitors for the management of cancer patients.

Abstract

Immune checkpoints refer to a range of immunoregulatory molecules that modulate the immune response. For example, proteins expressed at the surface of T-cells (including PD-1 and CTLA-4) and their ligands (PD-L1 and B7-1/B7-2, respectively), expressed by cancer cells and antigen-presenting cells, are needed to prevent excessive immune responses. However, they dampen anti-tumor immunity by limiting T-cell activity, making them promising therapeutic targets in cancer. Although immunotherapies using checkpoint blocking/neutralizing antibodies targeting PD-L1 or PD-1 have proven their superiority over conventional chemotherapies or targeted therapies by enhancing T-cell-mediated anti-tumor immunity, some limitations have emerged. These include a relatively low rate of “responders” (<50%; irrespective of cancer type), the high cost of injections, and a rare risk of hyper-progression. For clinicians, the current challenge is thus to improve the existing therapies, potentially through combinatory approaches. Polyphenols such as resveratrol (RSV), a trihydroxystilbene found in various plants and an adjuvant in numerous nutraceuticals, have been proposed as potential therapeutic targets. Beyond its well-known pleiotropic effects, RSV affects PD-L1 and PD-1 expression as well as PD-L1 subcellular localization and post-translational modifications, which we review here. We also summarize the consequences of PD-1/PD-L1 signaling, the modalities of their blockade in the context of cancer, and the current status and limitations of these immunotherapies. Finally, we discuss their potential use in combination with chemotherapies, and, using RSV as a model, we propose polyphenols as adjuvants to enhance the efficacy of anti-PD-1/anti-PD-L1 immunotherapies.

Towards resolving the enigma of the dichotomy of resveratrol: cis- and trans-resveratrol have opposite effects on TyrRS-regulated PARP1 activation

Unlike widely perceived, resveratrol (RSV) decreased the average lifespan and extended only the replicative lifespan in yeast. Similarly, although not widely discussed, RSV is also known to evoke neurite degeneration, kidney toxicity, atherosclerosis, premature senescence, and genotoxicity through yet unknown mechanisms. Nevertheless, in vivo animal models of diseases and human clinical trials demonstrate inconsistent protective and beneficial effects. Therefore, the mechanism of action of RSV that elicits beneficial effects remains an enigma. In a previously published work, we demonstrated structural similarities between RSV and tyrosine amino acid. RSV acts as a tyrosine antagonist and competes with it to bind to human tyrosyl-tRNA synthetase (TyrRS). Interestingly, although both isomers of RSV bind to TyrRS, only the cis-isomer evokes a unique structural change at the active site to promote its interaction with poly-ADP-ribose polymerase 1 (PARP1), a major determinant of cellular NAD+-dependent stress response. However, retention of trans-RSV in the active site of TyrRS mimics its tyrosine-bound conformation that inhibits the auto-poly-ADP-ribos(PAR)ylation of PARP1. Therefore, we proposed that cis-RSV-induced TyrRS-regulated auto-PARylation of PARP1 would contribute, at least in part, to the reported health benefits of RSV through the induction of protective stress response. This observation suggested that trans-RSV would inhibit TyrRS/PARP1-mediated protective stress response and would instead elicit an opposite effect compared to cis-RSV. Interestingly, most recent studies also confirmed the conversion of trans-RSV and its metabolites to cis-RSV in the physiological context. Therefore, the finding that cis-RSV and trans-RSV induce two distinct conformations of TyrRS with opposite effects on the auto-PARylation of PARP1 provides a potential molecular basis for the observed dichotomic effects of RSV under different experimental paradigms. However, the fact that natural RSV exists as a diastereomeric mixture of its cis and trans isomers and cis-RSV is also a physiologically relevant isoform has not yet gained much scientific attention.

Metabolism of cis- and trans-Resveratrol and Dihydroresveratrol in an Intestinal Epithelial Model

Trans-resveratrol, a well-known plant phenolic compound, has been intensively investigated due to its association with the so-called French paradox. However, despite its high pharmacological potential, trans-resveratrol has shown relatively low bioavailability. Trans-resveratrol is intensively metabolized in the intestine and liver, yielding metabolites that may be responsible for its high bioactivity. The aim of this study was to investigate and compare the metabolism of trans-resveratrol (tRes), cis-resveratrol (cRes) and dihydroresveratrol (dhRes) in an in vitro epithelial model using Caco-2 cell lines. Obtained metabolites of tRes, cRes and dhRes were analyzed by LC/MS Q-TOF, and significant differences in the metabolism of each compound were observed. The majority of tRes was transported unchanged through the Caco-2 cells, while cRes was mostly metabolized. The main metabolite of both cis- and trans-resveratrol observed as a result of colon microbial metabolism, dhRes, was metabolized almost completely, with only traces of the unchanged molecule being found. A sulphate conjugate was identified as the main metabolite of tRes in our model, while a glucuronide conjugate was the major metabolite of cRes and dhRes. Since metabolism of simple phenolics and polyphenols plays a crucial role in their bioavailability, detailed knowledge of their transformation is of high scientific value.

Insights on the Effects of Resveratrol and Some of Its Derivatives in Cancer and Autoimmunity: A Molecule with a Dual Activity

In recent years, the interest in natural compounds exerting immunoregulatory effects has enormously increased. Among these, the polyphenol resveratrol, found in a variety of foods and beverages, including red grapes and red wine, has been demonstrated to exert both in vitro and in vivo biological activities. More specifically, it has antiaging, cardioprotective, antioxidant, immunomodulatory, anti-inflammatory and chemopreventive activities. Due to its anti-proliferative, pro-apoptotic and immunoregulatory effects, resveratrol has gained substantial attention for the treatment of cancer or autoimmunity, which represent frequently diagnosed diseases with important consequences for the health of the patients affected. The aim of the present review is to focus on the role of resveratrol in the modulation of cancer as well as of several organ-specific or systemic autoimmune diseases, including autoimmune hepatitis, type 1 diabetes mellitus, inflammatory bowel disease, rheumatoid arthritis, systemic lupus erythematosus and multiple sclerosis.

Comparison of the inhibition potential of parthenolide and micheliolide on various UDP-glucuronosyltransferase isoforms

Parthenolide (PTL) and micheliolide (MCL) are sesquiterpene lactones with similar structures, and both of them have been reported to exhibit multiple biochemical and pharmacological activities. This study aims to investigate the inhibition of these two compounds on the activity of UDP-glucuronosyltransferases (UGTs). In vitro incubation mixture for recombinant UGTs-catalyzed glucuronidation metabolism of 4-methylumbelliferone (4-MU) was utilized to investigate the inhibition potential. Inhibition kinetics (including inhibition type and parameters) were determined, and in silico docking was employed to elucidate the inhibition difference between PTL and MCL on UGT1A1. MCL showed no inhibition toward all the UGT isoforms, and PTL showed strong inhibition toward UGT1A1. The half-maximal inhibitory concentration (IC₅₀) of PTL on the activity of UGT1A1 was determined to be 64.4 μM. Inhibition kinetics determination showed that PTL exerted noncompetitive inhibition toward UGT1A1, and the inhibition kinetic constant (K_i) was determined to be 12.1 μM. In silico docking method has been employed to show that hydrogen bonds between PTL and the activity cavity of UGT1A1 contributed to the stronger inhibition of PTL on the activity of UGT1A1 than MCL. In conclusion, PTL can more easily induce drug-drug interaction (DDI) with clinical drugs mainly undergoing UGT1A1-catalyzed glucuronidation.

Quantification of trans-resveratrol and its metabolites in human plasma using ultra-high performance liquid chromatography tandem quadrupole-orbitrap mass spectrometry

Trans-resveratrol is a stilbene polyphenol with a large spectrum of biological activities. This is why it is widely studied in terms of activities, bioavailability and quantitation in different foods, beverages and biological matrices. Different analytical methods are employed for its quantitation. In this study a quadrupole-orbitrap tandem mass spectrometer coupled to a reverse phase ultra-high performance liquid chromatography is applied to a quantitation of trans-resveratrol and its metabolites trans-resveratrol-3-O-β-d-glucuronide, trans-resveratrol-4'-O-β-d-glucuronide, trans-resveratrol-3-O-sulfate, a,b-dihydroresveratrol, a,b-dihydroresveratrol-glucuronide, a,b-dihydroresveratrol-glucuronide-sulfate, a,b-dihydroresveratrol-sulfate, trans-resveratrol-3,5-O-β-d-diglucuronide, trans-resveratrol-3,4'-O-d-β-diglucuronide, trans-resveratrol-3-O-β-d-glucuronide-sulfate and trans-resveratrol-4'-O-β-d-glucuronide-sulfate in human plasma. MS/MS experiments coupled to a high resolving power and accurate mass measurements as well as the use of labeled internal standards enabled the achievement of linear calibration curves across the four orders of magnitude concentration ranges. The method was validated in terms of specificity and selectivity, accuracy and precision, sensitivity and matrix effect and can be now applied to pharmacokinetic studies or routine analysis. In addition, the application of quadrupole-orbitrap mass spectrometer to the quantitation of trans-resveratrol and its metabolites provides acquisition of full collision induced dissociation spectra of analyzed compounds giving place to the structural characterization and sensitivity and linear concentration ranges respecting the accuracy and precision, specificity and selectivity requirements.

Resveratrol glucuronidation in vitro: potential implications of inhibition by probenecid

OBJECTIVES

Resveratrol is a naturally occurring antioxidant with therapeutic potential in prevention and treatment of neoplastic disease and other human disorders. However, net clearance of resveratrol in humans is very high, mainly due to glucuronide conjugation. This leads to extensive presystemic extraction and low plasma concentrations after oral dosage. The present study evaluated the effect of probenecid, an inhibitor of glucuronide conjugation, on resveratrol metabolism in vitro.

METHODS

Biotransformation of resveratrol to its 3-O-glucuronide and 4'-O-glucuronide conjugates was studied in vitro using human liver microsomal preparations. The mechanism and inhibitory potency of probenecid were evaluated based on a mixed competitive-noncompetitive inhibition model.

KEY FINDINGS

Probenecid inhibition of resveratrol 3-O-glucuronidation was predominantly noncompetitive, with an inhibition constant (K_i ) averaging 3.1 mm.

CONCLUSIONS

The ratio of in vivo maximum concentration of probenecid [I] during usual clinical use to the in vitro K_i value ([I]/K_i ) exceeds the boundary value of 0.1, used by regulatory agencies to identify the possibility of clinical drug interactions. This finding, together with the known property of probenecid as an inhibitor of glucuronide conjugation in humans, suggests that probenecid could serve as a pharmacokinetic boosting agent to enhance systemic exposure to resveratrol in humans.

A focus on resveratrol and ocular problems, especially cataract: From chemistry to medical uses and clinical relevance

Low vision and blindness are important health problems that affect millions of people throughout the world. The most common and important pathologies are diabetic retinopathy, age-related macular degeneration, glaucoma as well as cataracts. The latter consists of an opacification of the lens of the eye which impedes the passage of light and represents one of the most important causes of vision loss. Among the risk factors for cataract development, there are life-style factors such as the use of tobacco, abuse of alcohol and unhealthy diet. In light of this, dietary components that possess anti-oxidant activity, such as polyphenols for instance, can be considered good candidates for human studies in the prevention and or treatment of such diseases. Among dietary components, the antioxidant capacity of certain polyphenols is well known, and these could be good candidates. In this review we focus our attention on the current scientific literature regarding to the effects of resveratrol on cataracts and other ocular diseases, along with its potential mechanism/s of action. A large number of preclinical studies support the involvement of resveratrol in clinical trials for the prevention and treatment of eye diseases induced by oxidative stress and inflammation, such as age-related cataract.

Natural polyphenols based new therapeutic avenues for advanced biomedical applications

Polyphenols are naturally occurring, synthetic or semisynthetic organic compounds that offer a vast array of advanced biomedical applications. The mostly researched polyphenolic compounds are resveratrol and flavanols, notably (-)-epicatechin. The ongoing research on clinically important resveratrol and flavanols has revealed their potentials as extremely efficient drug agents that can be leveraged for new therapeutic designs for combating stroke related injuries, cancer and renal failures. Here, we have highlighted recent developments in this area with an emphasis on the biomedical applications of polyphenols. Also, a perspective on the future research directions has been discussed. We believe that this review would facilitate further research and development of polyphenols as a therapeutic avenue in medical science.

Involvement of the inhibition of intestinal glucuronidation in enhancing the oral bioavailability of resveratrol by labrasol containing nanoemulsions

Nanoemulsions have been developed for the oral delivery of poorly bioavailable phenolic compounds that are sensitive to intestinal glucuronidation. However, little is known about the contribution of UDP-glucuronosyltransferase (UGT) inhibitory excipients in nanoemulsions toward the inhibition of intestinal glucuronidation and the consequent enhanced bioavailability. In this study, Labrasol but not poloxamer 188 (F68) was found to inhibit the glucuronidation of resveratrol (RES), a model phenolic compound, in an inhibition assay with rat microsomes. Subsequently, two nanoemulsions, Lab-N and F68-N, were prepared with similar particle size distribution, zeta potentials, and entrapment efficiency by coemulsifying with Labrasol or F68, respectively. Although Lab-N exhibited inferior or comparable profiles of in vitro release, cellular uptake in Caco-2 cells, and lymphatic transport in rats to F68-N, the in vitro absorption study with everted sacs suggested that Labrasol containing formulations significantly and dose-dependently increased the transport of RES relative to free RES or F68-N by decreasing the amount of permeated metabolite, RES-3-glucuronide (RES-G). The in vivo pharmacokinetic experiments indicated that Lab-N exhibited increments in the maximum plasma concentration and the bioavailability of RES by 1098% and 560%, respectively, and significant decreases in those of RES-G, compared to F68-N. The overall results demonstrated that the improved oral bioavailability of RES by Lab-N was mainly attributable to the inhibition of intestinal glucuronidation by the presence of UGT inhibitory excipient.

The Nutraceutical Bioavailability Classification Scheme: Classifying Nutraceuticals According to Factors Limiting their Oral Bioavailability

The oral bioavailability of a health-promoting dietary component (nutraceutical) may be limited by various physicochemical and physiological phenomena: liberation from food matrices, solubility in gastrointestinal fluids, interaction with gastrointestinal components, chemical degradation or metabolism, and epithelium cell permeability. Nutraceutical bioavailability can therefore be improved by designing food matrices that control their bioaccessibility (B*), absorption (A*), and transformation (T*) within the gastrointestinal tract (GIT). This article reviews the major factors influencing the gastrointestinal fate of nutraceuticals, and then uses this information to develop a new scheme to classify the major factors limiting nutraceutical bioavailability: the nutraceutical bioavailability classification scheme (NuBACS). This new scheme is analogous to the biopharmaceutical classification scheme (BCS) used by the pharmaceutical industry to classify drug bioavailability, but it contains additional factors important for understanding nutraceutical bioavailability in foods. The article also highlights potential strategies for increasing the oral bioavailability of nutraceuticals based on their NuBACS designation (B*A*T*).

Metabolism of skin-absorbed resveratrol into its glucuronized form in mouse skin

Resveratrol (RESV) is a plant polyphenol, which is thought to have beneficial metabolic effects in laboratory animals as well as in humans. Following oral administration, RESV is immediately catabolized, resulting in low bioavailability. This study compared RESV metabolites and their tissue distribution after oral uptake and skin absorption. Metabolomic analysis of various mouse tissues revealed that RESV can be absorbed and metabolized through skin. We detected sulfated and glucuronidated RESV metabolites, as well as dihydroresveratrol. These metabolites are thought to have lower pharmacological activity than RESV. Similar quantities of most RESV metabolites were observed 4 h after oral or skin administration, except that glucuronidated RESV metabolites were more abundant in skin after topical RESV application than after oral administration. This result is consistent with our finding of glucuronidated RESV metabolites in cultured skin cells. RESV applied to mouse ears significantly suppressed inflammation in the TPA inflammation model. The skin absorption route could be a complementary, potent way to achieve therapeutic effects with RESV.

Novel resveratrol-based substrates for human hepatic, renal, and intestinal UDP-glucuronosyltransferases

Trans-Resveratrol (tRes) has been shown to have powerful antioxidant, anti-inflammatory, anticarcinogenic, and antiaging properties; however, its use as a therapeutic agent is limited by its rapid metabolism into its conjugated forms by UDP-glucuronosyltransferases (UGTs). The aim of the current study was to test the hypothesis that the limited bioavailability of tRes can be improved by modifying its structure to create analogs which would be glucuronidated at a lower rate than tRes itself. In this work, three synthetic stilbenoids, (E)-3-(3-hydroxy-4-methoxyphenyl)-2-(3,4,5-trimethoxyphenyl)acrylic acid (NI-12a), (E)-2,4-dimethoxy-6-(4-methoxystyryl)benzaldehyde oxime (NI-ST-05), and (E)-4-(3,5-dimethoxystyryl)-2,6-dinitrophenol (DNR-1), have been designed based on the structure of tRes and synthesized in our laboratory. UGTs recognize and glucuronidate tRes at each of the 3 hydroxyl groups attached to its aromatic rings. Therefore, each of the above compounds was designed with the majority of the hydroxyl groups blocked by methylation and the addition of other novel functional groups as part of a drug optimization program. The activities of recombinant human UGTs from the 1A and 2B families were examined for their capacity to metabolize these compounds. Glucuronide formation was identified using HPLC and verified by β-glucuronidase hydrolysis and LC-MS/MS analysis. NI-12a was glucuronidated at both the -COOH and -OH functions, NI-ST-05 formed a novel N-O-glucuronide, and no product was observed for DNR-1. NI-12a is primarily metabolized by the hepatic and renal enzyme UGT1A9, whereas NI-ST-05 is primarily metabolized by an extrahepatic enzyme, UGT1A10, with apparent Km values of 240 and 6.2 μM, respectively. The involvement of hepatic and intestinal UGTs in the metabolism of both compounds was further confirmed using a panel of human liver and intestinal microsomes, and high individual variation in activity was demonstrated between donors. In summary, these studies clearly establish that modified, tRes-based stilbenoids may be preferable alternatives to tRes itself due to increased bioavailability via altered conjugation.

Substrate selectivity of human intestinal UDP-glucuronosyltransferases (UGTs): in silico and in vitro insights

The current drug development process aims to produce safe, effective drugs within a reasonable time and at a reasonable cost. Phase II metabolism (glucuronidation) can affect drug action and pharmacokinetics to a considerable extent and so its studies and prediction at initial stages of drug development are very imperative. Extensive glucuronidation is an obstacle to oral bioavailability because the first-pass glucuronidation [or premature clearance by UDP-glucuronosyltransferases (UGTs)] of orally administered agents frequently results in poor oral bioavailability and lack of efficacy. Modeling of new chemical entities/drugs for UGTs and their kinetic data can be useful in understanding the binding patterns to be used in the design of better molecules. This review concentrates on first-pass glucuronidation by intestinal UGTs, including their topology, expression profile, and pharmacogenomics. In addition, recent advances are discussed with respect to substrate selectivity at the binding pocket, structural requirements, and mechanism of enzyme actions.

Bioavailability of wine-derived phenolic compounds in humans: a review

Phenolic compounds are produced in the seeds and skins of grapes, and are transferred into wine during the fermentation process. Phenolic compounds can also be imparted into wine from maturation and storage in oak wood barrels after fermentation. The consumption of wine, an alcoholic beverage, has been observed in epidemiological studies to reduce the risk of cardiovascular disease and certain cancers, as well as diabetes and dementia, in a J-shaped relationship between amount consumed and level of risk. The bioactivity of wine primarily observed in vitro and ex vivo, may result from wine's relatively high content of phenolic compounds, which is similar to that observed in fruits and vegetables; a Mediterranean fruit and vegetable rich-diet is also associated with a reduced risk of cardiovascular disease and cancers. If the wine-derived phenolic compounds or their active metabolites are not absorbed in sufficient amounts and in a readily available form for cells, however, then they are less likely to have any significant in vivo activity. This review considers and discusses the available data to date on the bioavailability of the different wine-derived phenolic compounds in humans.

Role of human UDP-glucuronosyltransferases in the biotransformation of the triazoloacridinone and imidazoacridinone antitumor agents C-1305 and C-1311: highly selective substrates for UGT1A10

5-Diethylaminoethylamino-8-hydroxyimidazoacridinone, C-1311 (NSC-645809), is an antitumor agent shown to be effective against breast cancer in phase II clinical trials. A similar compound, 5-dimethylaminopropylamino-8-hydroxytriazoloacridinone, C-1305, shows high activity against experimental tumors and is expected to have even more beneficial pharmacological properties than C-1311. Previously published studies showed that these compounds are not substrates for cytochrome P450s; however, they do contain functional groups that are common targets for glucuronidation. Therefore, the aim of this work was to identify the human UDP-glucuronosyltransferases (UGTs) able to glucuronidate these two compounds. High-performance liquid chromatography analysis was used to examine the activities of human recombinant UGT1A and UGT2B isoforms and microsomes from human liver [human liver microsomes (HLM)], whole human intestinal mucosa [human intestinal microsomes (HIM)], and seven isolated segments of human gastrointestinal tract. Recombinant extrahepatic UGT1A10 glucuronidated 8-hydroxyl groups with the highest catalytic efficiency compared with other recombinant UGTs, V(max)/K(m) = 27.2 and 8.8 μl · min⁻¹ · mg protein⁻¹, for C-1305 and C-1311, respectively. In human hepatic and intestinal microsomes (HLM and HIM, respectively), high variability in UGT activities was observed among donors and for different regions of intestinal tract. However, both compounds underwent UGT-mediated metabolism to 8-O-glucuronides by microsomes from both sources with comparable efficiency; V(max)/K(m) values were from 4.0 to 5.5 μl · min⁻¹ · mg protein⁻¹. In summary, these studies suggest that imid azoacridinone and triazoloacridinone drugs are glucuronidated in human liver and intestine in vivo and may form the basis for future translational studies of the potential role of UGTs in resistance to these drugs.

In vitro pharmacokinetic characterization of mulberroside A, the main polyhydroxylated stilbene in mulberry (Morus alba L.), and its bacterial metabolite oxyresveratrol in traditional oral use

Mulberroside A (MulA) is one of the main bioactive constituents in mulberry (Morus alba L.). This study examined the determining factors for previously reported oral pharmacokinetic profiles of MulA and its bacterial metabolite oxyresveratrol (OXY) on in vitro models. When incubated anaerobically with intestinal bacteria, MulA underwent rapid deglycosylation and generated two monoglucosides and its aglycone OXY sequentially. MulA exhibited a poor permeability and predominantly traversed Caco-2 cells via passive diffusion; yet, the permeation of OXY across Caco-2 cells was much more rapid and involved efflux (both p-glycoprotein and MRPs)-mediated mechanisms. Moreover, OXY underwent extensive hepatic glucuronidation; yet, the parent MulA was kept intact in liver subcellular preparations. There was insignificant species difference in intestinal bacterial conversion of MulA and the extent of OXY hepatic glucuronidation between humans and rats, while OXY exhibited a distinct positional preference of glucuronidation in the two species. Overall, these findings revealed a key role of intestinal bacterial conversion in absorption and systemic exposure of MulA and its resultant bacterial metabolite OXY in oral route in humans and rats and warranted further investigational emphasis on OXY and its hepatic metabolites for understanding the benefits of mulberry.

Natural prenylated resveratrol analogs arachidin-1 and -3 demonstrate improved glucuronidation profiles and have affinity for cannabinoid receptors

RATIONALE

The therapeutic promise of trans-resveratrol (tRes) is limited by poor bioavailability following rapid metabolism. We hypothesise that trans-arachidin-1 (tA1) and trans-arachidin-3 (tA3), peanut hairy root-derived isoprenylated analogs of tRes, will exhibit slower metabolism/enhanced bioavailability and retain biological activity via cannabinoid receptor (CBR) binding relative to their non-prenylated parent compounds trans-piceatannol (tPice) and tRes, respectively.

RESULTS

The activities of eight human UDP-glucuronosyltransferases (UGTs) toward these compounds were evaluated. The greatest activity was observed for extrahepatic UGTs 1A10 and 1A7, followed by hepatic UGTs 1A1 and 1A9. Importantly, an additional isoprenyl and/or hydroxyl group in tA1 and tA3 slowed overall glucuronidation. CBR binding studies demonstrated that all analogs bound to CB1Rs with similar affinities (5-18 µM); however, only tA1 and tA3 bound appreciably to CB2Rs. Molecular modelling studies confirmed that the isoprenyl moiety of tA1 and tA3 improved binding affinity to CB2Rs. Finally, although tA3 acted as a competitive CB1R antagonist, tA1 antagonised CB1R agonists by both competitive and non-competitive mechanisms.

CONCLUSIONS

Prenylated stilbenoids may be preferable alternatives to tRes due to increased bioavailability via slowed metabolism. Similar structural analogs might be developed as novel CB therapeutics for obesity and/or drug dependency.

Metabolites and tissue distribution of resveratrol in the pig

SCOPE

trans-Resveratrol (RES) and/(or) its metabolites exert many effects in vivo. Our aim was to study the metabolism and tissue distribution of RES using the pig, a mammal physiologically close to humans.

METHODS AND RESULTS

Forty-seven tissues, organs and fluids were analyzed 6 h after intragastric RES administration (5.9 mg/kg body weight) using HPLC-MS/MS. Twelve RES and seven dihydroresveratrol (DH-RES) metabolites were detected. DH-RES was the main metabolite in cecum, colon and rectum, whereas RES-3-O-glucuronide was the most abundant one in fluids and organs. Approximately 74.5% of the total RES administered was recovered in the form of RES, DH-RES and derived metabolites (65.1% along the gastrointestinal tract, 7.7% in urine, 1.2% in bile and 0.5% in organs). We report here, for the first time, the occurrence of RES ribosyl-sulfate derivative, DH-RES diglucuronide, DH-RES sulfoglucuronide and DH-RES disulfate as well as the metabolic profile of RES and DH-RES in the aorta, lymph, lymph node, ovaries, uterus, cerebellum, pancreas, urinary bladder tissue, fat and muscle.

CONCLUSION

This study contributes to the clarification of the metabolism and tissue distribution of RES and could help to further understand the mechanisms underlying its effects.

First-pass metabolism via UDP-glucuronosyltransferase: a barrier to oral bioavailability of phenolics

Glucuronidation mediated by UDP-glucuronosyltransferases (UGTs) is a significant metabolic pathway that facilitates efficient elimination of numerous endobiotics and xenobiotics, including phenolics. UGT genetic deficiency and polymorphisms or inhibition of glucuronidation by concomitant use of drugs are associated with inherited physiological disorders or drug-induced toxicities. Moreover, extensive glucuronidation can be a barrier to oral bioavailability as the first-pass glucuronidation (or premature clearance by UGTs) of orally administered agents usually results in the poor oral bioavailability and lack of efficacies. This review focused on the first-pass glucuronidation of phenolics including natural polyphenols and pharmaceuticals. The complexity of UGT-mediated metabolism of phenolics is highlighted with species-, gender-, organ- and isoform-dependent specificity, as well as functional compensation between UGT1A and 2B subfamily. In addition, recent advances are discussed with respect to the mechanisms of enzymatic actions, including the important properties such as binding pocket size and phosphorylation requirements.

Colorectal cancer: chemopreventive role of curcumin and resveratrol

Colorectal cancer (CRC) is a second leading cause of cancer deaths in the Western world. Currently there is no effective treatment except resection at a very early stage with or without chemotherapy. Of various epithelial cancers, CRC in particular has a potential for prevention, since most cancers follow the adenoma-carcinoma sequence, and the interval between detection of an adenoma and its progression to carcinoma is usually about a decade. However no effective chemopreventive agent except COX-2 inhibitors, limited in their scope due to cardiovascular side effects, have shown promise in reducing adenoma recurrence. To this end, natural agents that can target important carcinogenic pathways without demonstrating discernible adverse effects would serve as ideal chemoprevention agents. In this review, we discuss merits of two such naturally occurring dietary agents-curcumin and resveratrol-for chemoprevention of CRC.

Effects of cell differentiation and assay conditions on the UDP-glucuronosyltransferase activity in Caco-2 cells

Cell differentiation increases UDP-glucuronosyltransferase (UGT) gene expression in Caco-2 cells. Glucuronidation of 13 UGT substrates, 1-naphthol, diclofenac, epitestosterone, estradiol, ethinylestradiol, indomethacin, oxazepam, R- and S-propranolol, propofol, testosterone, trifluoperazine, and zidovudine, were studied to derive a broad view on the effect of cell differentiation on the glucuronidation activities of different human UGTs. In parallel, the glucuronidation of these compounds in human liver microsomes (HLM) and human intestinal microsomes (HIM) was analyzed. Because many of the substrates are highly lipophilic, the effects of dimethyl sulfoxide (DMSO) concentrations in the reaction mixture on glucuronidation rates were tested, as well as the effect of alamethicin, a pore-forming peptide. Large differences were observed in the effects of DMSO and alamethicin between recombinant UGTs and Caco-2 cells and HLM and HIM, and, therefore, the activity assays were performed under multiple conditions. Regardless of the assay conditions, however, the results clearly indicated that although differentiation increases glucuronidation activity, the rates in Caco-2 cells are mostly very low, much lower than those in either HLM or HIM. One clear exception was observed: substrates of UGT1A6, such as 1-naphthol, were glucuronidated at very high rates in both undifferentiated and differentiated Caco-2 cells. It may thus be concluded that Caco-2 cells, even differentiated ones, do not provide a good model system to assess first-pass drug glucuronidation in the intestine.

Bioavailability challenges associated with development of anti-cancer phenolics

Phenolics including many polyphenols and flavonoids have the potentials to become chemoprevention and chemotherapy agents. However, poor bioavailability limits their biological effects in vivo. This paper reviews the factors that affect phenolics absorption and their bioavailabilities from the points of view of their physicochemical properties and disposition in the gastrointestinal tract. The up-to-date research data suggested that solubility and metabolism are the primary reasons that limit phenolic aglycones' bioavailability although stability and poor permeation may also contribute to the poor bioavailabilities of the glycosides. Future investigations should further optimize phenolics' bioavailabilities and realize their chemopreventive and chemotherapeutic effects in vivo.

trans-Resveratrol reduces precancerous colonic lesions in dimethylhydrazine-treated rats

trans-Resveratrol, a natural occurring polyphenol, has been described as an antiproliferative and proapoptotic agent in vitro. Here, we studied the effect of trans-resveratrol administered orally at a dose of 60 mg/kg for 49 days on early preneoplastic markers induced by the intraperitoneal injection of 1,2-dimethylhydrazine (20 mg/kg). We measured trans-resveratrol and its derivates by liquid-liquid extraction followed by high-performance liquid chromatography diode array detection analysis in colon contents. Dihydroresveratrol was the most abundant compound in the colon, followed by trans-resveratrol glucuronide and small amounts of trans-resveratrol and its sulfate. The administration of trans-resveratrol decreased aberrant crypt foci by 52%, and mucin depleted foci by 45% in colon. In conclusion, the correlation between the reduction of precancerous colonic lesions and the availability of trans-resveratrol in the colon provides a new insight into the therapeutical potential of this polyphenol and its metabolites.

Multidrug resistance proteins restrain the intestinal absorption of trans-resveratrol in rats

trans-Resveratrol, a natural antioxidant, has been described as a nutraceutic compound with important beneficial effects on health, but its low oral bioavailability hinders its therapeutic activity. Here, we studied the mechanisms of apical transport of trans-resveratrol in enterocytes and the role of ATP-binding cassette (ABC) transporters in the secretion of resveratrol glucuronide and sulfate resulting from the rapid intracellular metabolism. An intestinal perfusion method with recirculation in vivo was used in rats. Jejunal loops were perfused with increasing concentrations of trans-resveratrol and results showed that its uptake occurs by simple diffusion without the participation of a mediated transport. The apparent diffusion constant was 8.1 +/- 0.3 microL/(5 min.mg dry weight). The glycoprotein-P (Pgp, ABCB1), multidrug resistance-associated protein 2 (MRP2, ABCC2), and breast cancer resistance protein (BCRP, ABCG2) located in the apical membrane of enterocytes were investigated using specific inhibitors. The Pgp inhibitors verapamil (5 micromol/L) and cyclosporin A (5 micromol/L) did not affect the efflux of trans-resveratrol and its conjugates. The MRP2 inhibitors probenecid (2 mmol/L) and MK571 (10 micromol/L) reduced the efflux of glucuronide by 61 and 55%, respectively, and of sulfate by 43 and 28%, respectively. The BCRP inhibitor Ko143 (0.5 micromol/L) decreased the secretion of glucuronide by 64% and of sulfate by 46%. Our experiments identify MRP2 and BCRP as the 2 apical transporters involved in the efflux of resveratrol conjugates.

Metabolites are key to understanding health effects of wine polyphenolics

Phenolic compounds in grapes and wine are grouped within the following major classes: stilbenes, phenolic acids, ellagitannins, flavan-3-ols, anthocyanins, flavonols, and proanthocyanidins. Consumption of foods containing phenolic substances has been linked to beneficial effects toward chronic diseases such as coronary heart disease and colorectal cancer. However, such correlations need to be supported by in vivo testing and bioavailability studies are the first step in establishing cause and effect. Class members from all phenolic groups can be glucuronidated, sulfated, and/or methylated and detected at low concentrations in the bloodstream and in urine. But the majority of phenolic compounds from grapes and wine are metabolized in the gastrointestinal tract, where they are broken down by gut microflora. This typically involves deglycosylation, followed by breakdown of ring structures to produce phenolic acids and aldehydes. These metabolites can be detected in bloodstream, urine, and fecal samples by using sophisticated instrumentation methods for quantitation and identification at low concentrations. The health effects related to grape and wine consumption may well be due to these poorly understood phenolic acid metabolites. This review discusses the known metabolism of each major class of wine and grape phenolics, the means to measure them, and ideas for future investigations.

Dose-dependent absorption and metabolism of trans-polydatin in rats

There is considerable evidence that stilbenes provide health benefits. trans-Polydatin is one of the major stilbenoid compounds in red wine. The purpose of this study is to investigate the dose-dependent absorption and metabolism of trans-polydatin in rats. trans-Polydatin was administered to rats by gavage at three different doses (50, 100, and 300 mg x kg(-1)). Blood samples were then collected at different time points. In situ perfusion of rat small intestine and liver was used to investigate the first-pass metabolism of trans-polydatin. trans-Polydatin and its metabolites were detected by LC-MS/MS. trans-Polydatin and its metabolites resveratrol, glucuronidated resveratrol, and glucuronidated trans-polydatin were detected in plasma within 10 min following oral administration of trans-polydatin; in situ perfusion of the rat small intestine and liver with trans-polydatin yielded the same result. The AUC(0-infinity) of trans-polydatin and its metabolites increased in a dose dependent manner following oral administration of trans-polydatin. This indicates that in the rats, trans-polydatin is absorbed in a dose-dependent manner and undergoes extensive first-pass deglycosylation and glucuronidation. Orally administered trans-polydatin, therefore, is metabolized primarily to resveratrol in the small intestine and liver, where it is further metabolized to the glucuronidated resveratrol.

Safety studies conducted on high-purity trans-resveratrol in experimental animals

trans-Resveratrol is a naturally occurring polyphenolic compound found in a variety of foods, but predominantly in grapes. Safety studies were conducted on high-purity trans-resveratrol (Resvida), including skin and eye irritation, dermal sensitization, subchronic and reproductive toxicity, genotoxicity, and absorption, metabolism and excretion. Resvida was non-irritating to skin and eyes and non-sensitizing. It was non-mutagenic in a bacterial reverse mutation assay in Salmonella typhimurium and Escherichia coli, but exhibited clastogenic activity in a chromosomal aberration test in human lymphocytes. However, in an in vivo bone marrow micronucleus test in rats, Resvida was non-genotoxic. In a 28-day study, Resvida caused no adverse effects in rats at 50, 150 and 500 mg/kg bw/day. Similarly, in a 90-day study, Resvida did not cause any adverse effects in rats at up to 700 mg/kg bw/day; the highest dose tested. Resvida did not induce any adverse reproductive effects in an embryo-fetal toxicity study in rats at a dose of 750 mg/kg bw/day. Also, in vitro and in vivo absorption, metabolism, and excretion studies in Caco-2 cells, rat primary hepatocytes and male and female rats (in vivo) show that Resvida is readily absorbed, metabolized and excreted. These studies provide evidence that Resvida is well tolerated and non-toxic.

White wine phenolics are absorbed and extensively metabolized in humans

Despite the vast literature describing the biological effects of phenolic compounds, rather scarce data are available on their absorption from diet in humans. The present study focused on the absorption in humans of phenolic acids from white wine, particularly hydroxycinnamic acids and their esters with tartaric acid. The results obtained indicate that, following a single wine drink, hydroxycinnamic acids from white wine are absorbed from the gastrointestinal tract and circulate in the blood after being largely metabolized to the form of glucuronide and sulfate conjugates. Unmodified tartaric acid esters of hydroxycinnamic acids from wine are present in human plasma at low levels, if any. Wine hydroxycinnamic acids, although present in wine as conjugated forms, are still bioavailable to humans.

The expression of most UDP-glucuronosyltransferases (UGTs) is increased significantly during Caco-2 cell differentiation, whereas UGT1A6 is highly expressed also in undifferentiated cells

The human colon carcinoma cell line Caco-2 is often used as a model for intestinal drug absorption. To better understand xenobiotic glucuronidation in Caco-2 cells, we have examined the expression levels of different UDP-glucuronosyltransferases (UGTs) in them. The effects of two main factors were investigated, namely, passage number and cell differentiation. Hence, the mRNA levels of 15 human UGTs of subfamilies 1A and 2B were assessed in both undifferentiated and fully differentiated cells at four passage levels: P31, P37, P43, and P49. Quantitative reverse transcriptase-polymerase chain reaction was used to determine the mRNA levels of individual UGTs, and the values were normalized using beta-actin as a reference gene. The results indicate that although passage number in the tested range exerts a mild effect on the expression level of several UGTs, the contribution of cell differentiation is much larger. The expression of nearly all the UGTs that were examined in this study was significantly, sometimes greatly, increased during cell differentiation. UGT1A6 was a distinct exception to this rule, however, because it was already highly expressed in the undifferentiated cells. The mRNA findings were confirmed at the enzyme activity level by measuring the glucuronidation of 1-naphthol, a very good substrate for UGT1A6, as well as estradiol that is not glucuronidated by this enzyme. The results revealed that 1-naphthol glucuronidation activity was high in both the differentiated and undifferentiated cells, whereas estradiol glucuronidation was only detected in the differentiated cells. Thus, Caco-2 cell differentiation plays a major role in UGT expression and ensuing metabolic reactions.

Predicting the physiological relevance of in vitro cancer preventive activities of phytochemicals

There is growing interest in the ability of phytochemicals to prevent chronic diseases, such as cancer and heart disease. However, some of these agents have poor bioavailability and many of the in-depth studies into their mechanisms of action have been carried out in vitro using doses which are unachievable in humans. In order to optimize the design of chemopreventive treatment, it is important to determine which of the many reported mechanisms of action are clinically relevant. In this review we consider the physiologically achievable doses for a few of the best studied agents (indole-3-carbinol, diindolylmethane, curcumin, epigallocatechin-3-gallate and resveratrol) and summarize the data derived from studies using these low concentrations in cell culture. We then cite examples of in vitro effects which have been observed in vivo. Finally, the ability of agent combinations to act synergistically or antagonistically is considered. We conclude that each of the compounds shows an encouraging range of activities in vitro at concentrations which are likely to be physiologically relevant. There are also many examples of in vivo studies which validate in vitro observations. An important consideration is that combinations of agents can result in significant activity at concentrations where any single agent is inactive. Thus, for each of the compounds reviewed here, in vitro studies have provided useful insights into their mechanisms of action in humans. However, data are lacking on the full range of activities at low doses in vitro and the benefits or otherwise of combinations in vivo.

cis- and trans-Resveratrol are glucuronidated in rat brain, olfactory mucosa and cultured astrocytes

BACKGROUND/AIMS

Glucuronidation of cis- and trans-resveratrol (3,5,4'-trihydroxy-trans-stilbene), which is a naturally occurring phytoalexin known to exert a number of beneficial health effects, was investigated in rat brain, cultured astrocytes and olfactory mucosa.

METHODS

The isomers were incubated with tissue homogenates, microsomes, or rat liver recombinant UDP-glucuronosyltransferases in the presence of UDP-glucuronic acid. The glucuronides were separated by HPLC and quantitated. Astrocytes were exposed to lipopolysaccharide to promote inflammatory conditions.

RESULTS

All tissues were able to form resveratrol glucuronides although at a lower extent, when compared to the liver. The reaction was stereo- and regioselective. In brain tissue, trans-resveratrol 3-O-glucuronide was mainly formed, whereas the cis-isomer was glucuronidated at a lower rate on that position. No 4'-O-glucuronide was detected in brain. In olfactory mucosa homogenates, the cis 3-O-glucuronide was mainly formed, whereas the trans-isomer was glucuronidated only on the 3-position. In astrocytes, 3-O-glucuronides of the cis- and trans-resveratrol were only detected. The rat recombinant UGT1A6 and UGT2B1 isoforms were able to glucuronidate cis- and trans-resveratrol. Finally, in inflammatory conditions, trans-resveratrol glucuronidation was enhanced in astrocytes.

CONCLUSION

Brain tissues are effective in the glucuronidation of resveratrol isomers. This metabolism pathway is likely to modulate the concentration of these biologically active substances.

Intestinal UGTs as potential modifiers of pharmacokinetics and biological responses to drugs and xenobiotics

Uridine 5'-diphosphate-glucuronosyltransferases (UGTs) are the biological catalysts of glucuronidation, a major pathway of conjugative metabolism of drugs and xenobiotics. In addition to the liver and kidney, UGTs are highly expressed in the gastrointestinal tract, where they have the potential to influence the pharmacokinetics and biological effects of ingested drugs and xenobiotics. This paper reviews the current evidence for the contributions of intestinal UGTs to presystemic 'first-pass' metabolism and drug bioavailability, the extent of enterohepatic cycling and the clearance of drugs from plasma, as well as their influence on biological responses to drugs, including drug toxicity. The prediction of the effects of intestinal glucuronidation on these processes depends on knowledge of the types and amounts of UGTs expressed in the small intestine and their specific glucuronidating activities. Whereas the types of UGTs expressed in human gastrointestinal tract are well characterized, further research is needed to understand the absolute amounts of UGTs in the small intestine and the causes of observed high-interindividual variability in the intestinal expression of UGTs.