Glucuronides of tea catechins: enzymology of biosynthesis and biological activities

Green tea (Camellia sinensis) catechins and vascular function

The health benefits of green tea (Camellia sinensis) catechins are becoming increasingly recognised. Amongst the proposed benefits are the maintenance of endothelial function and vascular homeostasis and an associated reduction in atherogenesis and CVD risk. The mounting evidence for the influential effect of green tea catechins on vascular function from epidemiological, human intervention and animal studies is subject to review together with exploration of the potential mechanistic pathways involved. Epigallocatechin-3-gallate, one of the most abundant and widely studied catechin found in green tea, will be prominent in the present review. Since there is a substantial inconsistency in the published data with regards to the impact of green tea catechins on vascular function, evaluation and interpretation of the inter- and intra-study variability is included. In conclusion, a positive effect of green tea catechins on vascular function is becoming apparent. Further studies in animal and cell models using physiological concentrations of catechins and their metabolites are warranted in order to gain some insight into the physiology and molecular basis of the observed beneficial effects.

Green tea extract weakens the antibacterial effect of amoxicillin in methicillin-resistant Staphylococcus aureus infected mice

Tea (Camellia sinensis) has been known for its modulation of resistance of methicillin-resistant Staphylococcus aureus (MRSA) to beta-lactam antibiotics in vitro. This study aimed to confirm the in vitro effect of green tea extracts with beta-lactams and to determine whether green tea extracts can reduce the minimum inhibitory concentrations (MICs) of amoxicillin in MRSA-infected mice. The catechins in the test tea that account for the reduced resistance to beta-lactams were quantitatively determined by high-performance liquid chromatography. The MICs of the ampicillin, cefazolin, amoxicillin, oxacillin, tea extract alone and tea extract in combination with beta-lactams were determined. Proportions of tea extracts and amoxicillin-tea extract combinations were administered to groups of mice enterally. The in vitro experiment showed that the MICs of four beta-lactams were greatly decreased in the presence of 0.25% tea extract. However, in an in vivo experiment, amoxicillin in combination with 5% tea extract conferred a higher ED(50) than that of antibiotic alone. Green tea extract, alone or in combination with amoxicillin, does not have protective benefits in MRSA-infected mice. This study concluded that tea extract weakened the antibacterial effect of amoxicillin in MRSA infected mice. Tea drinking is not recommended in combination with amoxicillin treatment.

Bioavailability of procyanidin dimers and trimers and matrix food effects in in vitro and in vivo models

Among procyanidins (PC), monomers, such as catechin and epicatechin, have been widely studied, whereas dimer and trimer oligomers have received much less attention, despite their abundance in our diet. Recent studies have showed that as dimers and trimers could be important in determining the biological effects of procyanidin-rich food, understanding their bioavailability and metabolism is fundamental. The purpose of the present work is to study the stability of PC under digestion conditions, the metabolism and the bioavailability by using a combination of in vitro and in vivo models. Simultaneously, the matrix effect of a carbohydrate-rich food on the digestibility and bioavailability of PC is investigated. The results show a high level of stability of PC under gastric and duodenal digestion conditions. However, the pharmacokinetic study revealed limited absorption. Free forms of dimers and trimers have been detected in rat plasma, reaching the maximum concentration 1 h after oral intake of a grape seed extract.

Hepatotoxicity of high oral dose (-)-epigallocatechin-3-gallate in mice

The tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) has been studied for chronic disease preventive effects, and is marketed as part of many dietary supplements. However, case-reports have associated the use of green tea-based supplements with liver toxicity. We studied the hepatotoxic effects of high dose EGCG in male CF-1 mice. A single dose of EGCG (1500 mg/kg, i.g.) increased plasma alanine aminotransferase (ALT) by 138-fold and reduced survival by 85%. Once-daily dosing with EGCG increased hepatotoxic response. Plasma ALT levels were increased 184-fold following two once-daily doses of 750 mg/kg, i.g. EGCG. Moderate to severe hepatic necrosis was observed following treatment with EGCG. EGCG hepatotoxicity was associated with oxidative stress including increased hepatic lipid peroxidation (5-fold increase), plasma 8-isoprostane (9.5-fold increase) and increased hepatic metallothionein and gamma-histone 2AX protein expression. EGCG also increased plasma interleukin-6 and monocyte chemoattractant protein-1. Our results indicate that higher bolus doses of EGCG are hepatotoxic to mice. Further studies on the dose-dependent hepatotoxic effects of EGCG and the underlying mechanisms are important given the increasing use of green tea dietary supplements, which may deliver much higher plasma and tissue concentrations of EGCG than tea beverages.

Bioactive components of tea: cancer, inflammation and behavior

Tea is one of the most widely consumed beverages worldwide. Several studies have suggested that catechins and theaflavins found in tea may reduce the risk of various types of cancers. Major advances have been made to understand the molecular events leading to cancer prevention; however, the evidence is not conclusive. Evidence from pre-clinical and clinical studies also suggests that persistent inflammation can progress to cancer. Several possible mechanisms of action may explain the cancer preventive aspects of tea components specifically anti-inflammatory effects. In regards to brain health, green tea catechins have been recognized as multifunctional compounds for neuroprotection with beneficial effects on vascular function and mental performance. Theanine, a unique amino acid in tea, enhances cognition in humans and has neuroprotective effects. Human interventional studies with well characterized tea products are needed.

The challenge of developing green tea polyphenols as therapeutic agents

The health benefits of green tea and its main constituent (-)-epigallocatechin gallate [(-)-EGCG] have been widely supported by results from epidemiological, cell culture, animal and clinical studies. On the other hand, there are a number of issues, such as stability, bioavailability and metabolic transformations under physiological conditions, facing the development of green tea polyphenols into therapeutic agents. We previously reported that the synthetic peracetate of (-)-EGCG has improved stability and better bioavailability than (-)-EGCG itself and can act as pro-drug under both in vitro and in vivo conditions. Analogs of catechins have been synthesized and their structure activity relationship provides an understanding to the mechanism of proteasome inhibition. Metabolic methylation of catechins leading to methylated (-)-EGCG may alter the biological activities of these compounds.

Antioxidative and anti-carcinogenic activities of tea polyphenols

Tea (Camellia sinensis, Theaceace), a popular beverage consumed world-wide, has been studied for its preventive effects against cancer as well as cardiovascular, neurodegenerative, and other diseases. Most of the proposed beneficial effects have been attributed to the polyphenolic compounds in tea, but the nature of these activities and the molecular mechanisms of their actions remain unclear. Tea polyphenols are known to be strong antioxidants. Prevention of oxidative stress, modulation of carcinogen metabolism, and prevention of DNA damage have been suggested as possible cancer preventive mechanisms for tea and tea polyphenols. In this chapter, we discuss these topics in the light of biotransformation and bioavailability of tea polyphenols. We also review the preventive effects of tea polyphenols in animal models of carcinogenesis and some of the possible post-initiation mechanisms of action. Finally, we discuss the effects of tea consumption on cancer risk in humans. It is our aim to raise some of the unanswered questions regarding cancer prevention by tea and to stimulate further research in this area.

Green tea and cardiovascular disease: from molecular targets towards human health

PURPOSE OF REVIEW

To summarize current knowledge of the protective effects of green tea and green tea constituents, particularly catechins, on the cardiovascular system.

RECENT FINDINGS

Consumption of green tea has been inversely associated with the development and progression of cardiovascular diseases and cardiovascular risk factors. Mechanisms that have been suggested as being involved in the antiatherosclerotic effects of green tea consumption primarily entail antioxidative, antiinflammatory, antiproliferative, and antithrombotic properties, as well as beneficial effects on endothelial function. Moreover, evidence exists for myocardial effects of tea constituents, including positive inotropic and antihypertrophic effects, and beneficial impact in myocardial ischaemia-reperfusion injury.

SUMMARY

Green tea represents a promising tool for the prevention of cardiovascular disorders.

Structural identification of novel glucoside and glucuronide metabolites of (-)-epigallocatechin-3-gallate in mouse urine using liquid chromatography/electrospray ionization tandem mass spectrometry

(-)-Epigallocatechin-3-gallate (EGCG), the most abundant and most biologically active polyphenolic compound in tea, has been proposed to have many health beneficial effects. The metabolic fate of EGCG, however, is not well understood. In the present study, we identified a novel EGCG metabolite, 7-O-beta-D-glucopyranosyl-EGCG-4''-O-beta-D-glucupyranoside, in a mouse urine sample using liquid chromatography/electrospray ionization tandem mass spectrometry. The structure of this metabolite was confirmed by analyzing the MSn (n = 1-4) spectra as well as comparing the MS/MS spectra of its product ions with those from EGCG and EGCG-4''-O-beta-D-glucupyranoside standards. To our knowledge, this is the first report of the identification of a glucoside metabolite of EGCG in mammals. Our results indicate that glucosidation represents a novel pathway in the metabolism of EGCG in mice.

Bioavailability issues in studying the health effects of plant polyphenolic compounds

Polyphenolic compounds are common in the diet and have been suggested to have a number of beneficial health effects including prevention of cancer, cardiovascular disease, diabetes, and others. For some dietary polyphenols, certain benficial effects are suggested by epidemiological studies, some are supported by studies in animal models, and still others are extrapolated from studies in vitro. Because of the relatively poor bioavailability of many of these compounds, the molecular basis of these beneficial effects is not clear. In the present review, we discuss the potential health benefits of dietary polyphenols from the point of view of bioavailability. Tea catechins, curcumin, and proanthocyanidins are used as examples to illustrate some of the problems that need to be resolved. Further research on both the biological activity and bioavailability of dietary polyphenols is needed to properly assess their usefulness for the prevention and treatment of disease.

Intestinal first-pass glucuronidation activities of selected dihydroxyflavones

Flavonoids have low bioavailabilities due to extensive intestinal first-pass metabolisms, especially glucuronidation. The present study aimed to evaluate the intestinal glucuronidation of dihydroxyflavones and provide more information on their structure-activity relationships. Seven dihydroxyflavones, namely 3,7-, 5,7-, 6,7-, 7,8-, 2',7-, 3',7-, and 4',7-dihydroxyflavone and a monohydroxyflavone, 7-hydroxyflavone, were investigated by incubating each hydroxyflavone at various concentrations with either human jejunum microsome or rat intestinal microsome. Two mono-glucuronides were identified for each dihydroxyflavone. For human jejunum microsome, most of the studied dihydroxyflavones demonstrated greater glucuronidation activities than that of 7-hydroxyflavone except for 3,7-dihydroxyflavone and 4',7-dihydroxyflavone. 3',7-dihydroxyflavone had the greatest intrinsic clearance which was at least seven times greater than that of all other dihydroxyflavones. In addition, species difference in glucuronidation activity was observed with human jejunum microsome higher than rat intestinal microsome for all hydroxyflavones except for 3,7-dihydroxyflavone. The results further demonstrated that the hydroxyl group positions do affect the intestinal glucuronidation activity of hydroxyflavones. Increasing the number of hydroxyl groups on A- or B-ring (except for 4'-OH) would enhance the glucuronidation activity of flavones, whereas adding a 3-OH on C-ring might not. Furthermore, existence of hydroxyl group at 3' position may enhance the glucuronidation activity of flavonoids.

Effect of genistein on the bioavailability and intestinal cancer chemopreventive activity of (-)-epigallocatechin-3-gallate

The green tea (Camellia sinensis) catechin, (-)-epigallocatechin-3-gallate (EGCG), has shown cancer-preventive activity in animal models. Previously, we have reported the bioavailability of EGCG in rats and mice. Here, we report that cotreatment of HT-29 human colon cancer cells with genistein (from soy) increased cytosolic EGCG by 2- to 5-fold compared with treatment with EGCG only. Inclusion of genistein, at non-cytotoxic concentrations, increased the growth inhibitory effects of EGCG against HT-29 cells (up to 2-fold at 20 microM genistein). Intragastric coadministration of EGCG (75 mg/kg) and genistein (200 mg/kg) to CF-1 mice resulted in an increase in plasma half-life (t(1/2) 148.7 +/- 16.4 versus 111.5 +/- 23.4 min) and exposure (AUC(0-->infinity) 183.9 +/- 20.2 versus 125.8 +/- 26.4 microg/ml x min) of EGCG. Cotreatment with genistein also increased the maximal concentration (C(max)), 6 h exposure (AUC(0-->360 min)), and t(1/2) of EGCG in the small intestine by 2.0-, 4.7- and 1.4-fold, respectively, compared with mice treated with EGCG only. Contrary to our expectations, the combination of 0.01% EGCG in the drinking fluid and 0.2% genistein in the diet enhanced intestinal tumorigenesis in male adenomatous polyposis coli (APC)(min/+) mice. This combination increased the multiplicity of tumors in the medial and distal small intestine: the largest increase was in tumors >2 mm in diameter (4.3-fold compared with controls). This study demonstrates that although genistein can enhance EGCG bioavailability and in vitro growth inhibitory activity, this combination enhances tumorigenesis in the APC(min/+) mouse. These results further show the need for careful cancer prevention studies in animal models and for caution when interpreting data from in vitro studies.

Food-drug interaction of (-)-epigallocatechin-3-gallate on the pharmacokinetics of irinotecan and the metabolite SN-38

The aim of the present study was to investigate the effect of (-)-epigallocatechin-3-gallate (EGCG) on the pharmacokinetics of irinotecan (CPT-11) and its metabolite SN-38. EGCG was potentially used to modulate the ATPase activity of P-glycoprotein (P-gp). Experimental Sprague-Dawley rats were treated with EGCG (20mg/kg, i.v.) 10min before CPT-11 (10mg/kg, i.v.) administration, whereas the control group received CPT-11 (10mg/kg, i.v.) only. The biological samples were prepared by the protein precipitation and detected by HPLC-fluorescence detection which provided a good separation of CPT-11 and SN-38 within 10min. The pharmacokinetic data indicate that the area under the plasma concentration-time curves (AUC) of CPT-11 and SN-38 were increased by 57.7 and 18.3%, and AUC in bile were decreased by 15.8 and 46.8%, respectively, for the group pretreated with EGCG. The blood to bile distribution ratio (AUC(bile)/AUC(blood)) was significantly reduced after group coadministration of EGCG, it can be seen that the bile efflux transport system of CPT-11 and SN-38 may be markedly reduced by the treatment of EGCG which plays the role of P-gp inhibitor. In conclusion, EGCG was found to inhibit the transport of CPT-11 and SN-38 into the biliary elimination and their half-lives in plasma could be substantially prolonged. Based on the food-drug interaction, persons taking daily nutritional supplements should be warned of this interaction possibility.

Human urinary metabolite profile of tea polyphenols analyzed by liquid chromatography/electrospray ionization tandem mass spectrometry with data-dependent acquisition

Tea is rich in polyphenols and has a variety of biological activities. In order to better understand the biological effects of tea constituents on human health, markers for their exposure and their metabolic fates are needed. Previously, we have characterized several catechin metabolites in the blood and urine, but more information on the metabolite profile of tea polyphenols is needed. In the present study, the human urinary metabolite profile of tea polyphenols was investigated using liquid chromatography/electrospray ionization tandem mass spectrometry with data-dependent acquisition. With data-dependent MS/MS analysis by collecting the MS2 and MS3 spectra of the most intense ions in the sample, we identified more than twenty metabolites of tea polyphenols from human urine samples. (-)-Epigallocatechin (EGC) glucuronide, methylated EGC glucuronide, methylated EGC sulfate, (-)-epicatechin (EC) glucruronide, EC sulfate, methylated EC sulfate, as well as the glucuronide and sulfate metabolites of the ring-fission metabolites of tea catechins, 5-(3',4',5'-trihydroxyphenyl)-gamma-valerolactone (M4), 5-(3',4'-dihydroxyphenyl)-gamma-valerolactone (M6) and 5-(3',5'-dihydroxyphenyl)-gamma-valerolactone (M6'), were the major human urinary metabolites of tea polyphenols. To our knowledge, this is the first report of the direct simultaneous analysis of the human urinary metabolite profile of tea polyphenols using single sample analysis. This method can also be used for thorough investigations of the metabolite profiles of many other dietary constituents.

(-)Epigallocatechin-gallate (EGCG) prevents mitochondrial damage in isoproterenol-induced cardiac toxicity in albino Wistar rats: a transmission electron microscopic and in vitro study

Altered mitochondrial function and free radical-mediated tissue damage have been suggested as important pathological events in isoproterenol (ISO)-induced cardiotoxicity. This study was undertaken to know the preventive effect of (-)epigallocatechin-gallate (EGCG) on mitochondrial damage in ISO-induced cardiotoxicity in male Wistar rats. Rats were pretreated with EGCG (30 mg/kg) orally using an intragastric tube daily for a period of 21 days. After that, ISO (100mg/kg) was subcutaneously injected to rats at intervals of 24h for 2 days. ISO-induced rats showed significant increase in mitochondrial lipid peroxidation products (thiobarbituric acid reactive substances and lipid hydroperoxides) and significant decrease in mitochondrial antioxidants (superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferase, glutathione reductase and reduced glutathione). Also, significantly decreased activities of tricarboxylic acid cycle enzymes such as isocitrate, succinate, malate and alpha-ketoglutarate dehydrogenases and respiratory chain marker enzymes such as NADH-dehydrogenase and cytochrome-c-oxidase were observed in mitochondrial heart of myocardial infarcted rats. Prior treatment with EGCG (30mg/kg body weight) significantly prevented these alterations and restored normal mitochondrial function. Transmission electron microscopic findings also correlated with these biochemical parameters. In vitro studies on the effect of EGCG on scavenging 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2'-azinobis-(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS(+)), superoxide anion (O(-)), and hydroxyl (OH) radicals also confirmed the free radical scavenging and antioxidant activity of EGCG. Thus, the observed effects are due to the free radical scavenging and antioxidant potential of EGCG. Thus, this study confirmed the preventive effect of EGCG on isoproterenol-induced mitochondrial damage in experimentally induced myocardial infarction in Wistar rats.

Subcutaneous injection, from birth, of epigallocatechin-3-gallate, a component of green tea, limits the onset of muscular dystrophy in mdx mice: a quantitative histological, immunohistochemical and electrophysiological study

Dystrophic muscles suffer from enhanced oxidative stress. We have investigated whether administration of an antioxidant, epigallocatechin-3-gallate (EGCG), a component of green tea, reduces their oxidative stress and pathophysiology in mdx mice, a mild phenotype model of human Duchenne-type muscular dystrophy. EGCG (5 mg/kg body weight in saline) was injected subcutaneously 4x a week into the backs of C57 normal and dystrophin-deficient mdx mice for 8 weeks after birth. Saline was injected into normal and mdx controls. EGCG had almost no observable effects on normal mice or on the body weights of mdx mice. In contrast, it produced the following improvements in the blood chemistry, muscle histology, and electrophysiology of the treated mdx mice. First, the activities of serum creatine kinase were reduced to normal levels. Second, the numbers of fluorescent lipofuscin granules per unit volume of soleus and diaphragm muscles were significantly decreased by about 50% compared to the numbers in the corresponding saline-treated controls. Third, in sections of diaphragm and soleus muscles, the relative area occupied by histologically normal muscle fibres increased significantly 1.5- to 2-fold whereas the relative areas of connective tissue and necrotic muscle fibres were substantially reduced. Fourth, the times for the maximum tetanic force of soleus muscles to fall by a half increased to almost normal values. Fifth, the amount of utrophin in diaphragm muscles increased significantly by 17%, partially compensating for the lack of dystrophin expression.

Comparative examination of anti-proliferative activities of (-)-epigallocatechin gallate and (--)-epigallocatechin against HCT116 colorectal carcinoma cells

We compared anti-proliferative activities of (-)-epigallocatechin gallate (EGCG) and (-)-epigallocatechin (EGC) against HCT116 colorectal carcinoma cells. These catechins inhibited cell growth to nearly the same extent at low cell confluency in plates. However, their inhibitory effect grew weaker as cell confluence increased, and this tendency was more conspicuous for EGC than for EGCG. Both EGCG and EGC activated the phosphorylation of the major MAPKs, ERK, JNK, and p38, in the HCT116 cells as in many other established human cancer cells though to different extents. Cell cycle analyses, DNA fragmentation assays, and TUNEL assays as well as Western blot assays suggested that these catechins inhibited cell growth through mitogen-activated protein kinase (MAPK)-mediated apoptosis rather than cell cycle regulation.

Comparison of enzymically glucuronidated flavonoids with flavonoid aglycones in an in vitro cellular model of oxidative stress protection

This study modeled, in vitro, the potential effect of conjugative (phase II) metabolism on the cytoprotective capacity of fruit flavonoids against oxidative stress. Flavonoid aglycones were compared with their corresponding isomeric mixtures of glucuronides for their ability to enhance the survival of cultured human Jurkat T and neuroblastoma cells stressed with hydrogen peroxide. Various polyphenolic compounds were tested as substrates in vitro for an ovine liver glucuronyl transferase preparation. Flavonoids and their glycoside derivatives were found to be good substrates, whereas phenolic acids were either poor or nonsubstrates. Five common flavonoids were glucuronidated to prepare mixtures for bioassay testing. Glucuronidation generally weakened the cytoprotective capacities of flavonoids (in the presence of H(2)O(2)), but some compounds were weakened much more than others. The concentration that halved cell death was well below 0.5 microM for most flavonoids tested, but glucuronidation increased median effective concentration values to a range of 1-16 microM. This compares with the generally accepted physiological range (0.1-10 microM) for circulating dietary polyphenolics detected in the body. Therefore, some flavonoids may retain a reduced cytoprotective capacity in vitro, after glucuronidation, whereas others may be effectively inactivated.

The pharmokinetic limitations of antioxidant treatment for COPD

COPD is one of the leading causes of death worldwide and the age-adjusted mortality for this disease has risen significantly over the past 30 years. Current pharmacological treatments do not effectively address the inflammatory and apoptotic mechanisms that are critical in the development of this disease. Thus, despite therapy, patients typically experience a continued deterioration of their clinical status. Markers of oxidative stress are increased in the lungs of COPD patients and epidemiologic and animal studies indicate that antioxidants can protect the lungs from the damaging effects of cigarette smoke. To date, however, clinical trials of antioxidants for COPD have yielded disappointing results. This review discusses the pharmokinetic factors that limit the use of exogenous antioxidants as a treatment for this disease. In addition, it addresses strategies to overcome these limitations so that the beneficial properties of antioxidants can be translated into effective therapies for COPD patients.

Validation of green tea polyphenol biomarkers in a phase II human intervention trial

Health benefits of green tea polyphenols (GTPs) have been reported in many animal models, but human studies are inconclusive. This is partly due to a lack of biomarkers representing green tea consumption. In this study, GTP components and metabolites were analyzed in plasma and urine samples collected from a phase II intervention trial carried out in 124 healthy adults who received 500- or 1000-mg GTPs or placebo for 3 months. A significant dose-dependent elevation was found for (-)-epicatechin-3-gallate (ECG) (p<0.001, trend test) and (-)-epigallocatechin-3-gallate (EGCG) (p<0.05, trend test) concentrations in plasma at both 1-month and 3-months after intervention with GTPs. No significant increase of (-)-epicatechin (EC) or (-)-epigallocatechin (EGC) was observed in plasma after GTP intervention. A mixed-effects model indicated significant effects of dose (EGCG) and dose by time interaction (ECG), but not for EC and EGC. Analysis of phase 2 metabolic conjugates revealed a predominance of free GTPs in plasma, up to 85% for EGCG, while a majority of GTPs in urine were sulfated and glucuronidated conjugates (up to 100% for EC and 89% for EGC). These results suggest that plasma ECG and EGCG concentrations are reliable biomarkers for green tea consumption at the population level.

Comparison of the relative recovery of polyphenolics in two fruit extracts from a model of degradation during digestion and metabolism

To simulate the effects of digestion and metabolism on the survival of different polyphenolic compounds, extracts of blueberry and apple were deglycosylated by acid hydrolysis, followed by enzymic glucuronidation under neutral conditions, yielding approximately 5% overall recovery of polyphenolics. The major polyphenolics before and after the treatment were compared, to estimate which species are likely to be present in the intestinal lumen, undegraded and available for absorption, after consumption of the fruit. Whereas blueberry extract consisted predominantly of anthocyanins, epicatechin and caffeoyl quinate esters, the major components of the treated extract were quercetin glucuronides and (unglucuronidated) caffeoyl quinates, with only traces of anthocyanidin derivatives. In apple extract, compositional changes were less marked, but caffeoyl quinates, procyanidins and quercetin were enriched at the expense of caffeic acid, epicatechin and catechin. Hydrophobic compounds like phloretin and quercetin were extensively glucuronidated, whereas caffeic acid and caffeoyl quinate were not. These results suggest that the major polyphenolic components of a fruit are not necessarily the most important contributors to any health benefits because the polyphenolic composition in the intestinal lumen and consequently, in the circulation, may be considerably different.

Autoxidative quinone formation in vitro and metabolite formation in vivo from tea polyphenol (-)-epigallocatechin-3-gallate: studied by real-time mass spectrometry combined with tandem mass ion mapping

(-)-Epigallocatechin-3- gallate (EGCG), the most abundant and biologically active compound in tea, has been proposed to have beneficial health effects, including prevention of cancer and heart disease. Based mainly on studies in cell-line systems, in which EGCG is not stable, different mechanisms of action of EGCG have been proposed. It has been proposed also that oxidation of EGCG and its production of reactive oxygen species are responsible for biological activities such as receptor inactivation and telomerase inhibition. It is unclear, however, whether this phenomenon occurs in vivo. In the present study, the stability of EGCG and product formation in Tris-HCl buffer was investigated using real- time mass spectrometry combined with tandem mass ion mapping. With real-time mass data acquisition, we demonstrate for the first time the formation of EGCG quinone, EGCG dimer quinone, and other related compounds. The structural information of the major appearing ions was provided by tandem mass analysis of each ion. A mechanism for the autoxidation of EGCG based on the structural information of these ions was proposed. None of these oxidation products were observed in the plasma samples of mice after treatment with 50 mg/kg EGCG, i.p. daily for 3 days. Instead, the methylated and conjugated metabolites of EGCG were observed. Therefore the roles of EGCG autoxidation in the biological activities of this compound in vivo remain to be investigated further.

Possible role for green tea in ovarian cancer prevention

Ovarian cancer is the leading cause of death from gynecologic cancer. Tea, especially green tea, has shown promise in the prevention of several cancers. Green tea contains a number of compounds, including polyphenols, that have chemopreventive properties. There is much evidence from in vitro and animal studies suggesting that components of tea are associated with decreased risk or progression of ovarian cancer. However, epidemiologic studies have generated inconsistent results. Recent research conducted in China reported reduced risk of ovarian cancer and increased survival post diagnosis with green tea consumption. This review presents emerging evidence and the authors' perspectives on the role of green tea in ovarian cancer prevention.

Green tea extract thermogenesis-induced weight loss by epigallocatechin gallate inhibition of catechol-O-methyltransferase

Epidemiological studies have shown that intake of tea catechins is associated with a lower risk of cardiovascular disease. The antioxidative activity of tea-derived catechins has been extensively studied. Reports have shown that green tea extract intake is associated with increased weight loss due to diet-induced thermogenesis, which is generally attributed to the catechin epigallocatechin gallate. That catechin-polyphenols are known to be capable of inhibiting catechol-O-methyltransferase (the enzyme that degrades norepinephrine) is a possible explanation for why the green tea extract is effective in stimulating thermogenesis by epigallocatechin gallate to augment and prolong sympathetic stimulation of thermogenesis. Knowledge about thermogenesis-induced weight loss produced by green tea's epigallocatechin gallate and its ability to inhibit catechol-O-methyltransferase is important for health benefits and for prolonging the action of norepinephrine in the synaptic cleft.

Effects of green tea compounds on irinotecan metabolism

The effects of green tea compounds on the metabolism of irinotecan have never been investigated. We aimed to study whether catechins [(-)-epigallocatechin gallate (EGCG), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC), (-)-epicatechin] affect the inactivation metabolism of irinotecan into 7-ethyl-10-[4-N-(1-piperidino)-1-amino]carbonyloxycamptothecin (NPC) (by CYP3A4) and 7-ethyl-10-hydroxycamptothecin (SN-38) into 7-ethyl-10-hydroxycamptothecin glucuronide (SN-38G) (by UGT1A1). Human liver microsomes, hepatocytes and Hep G2 cells were incubated with catechins and treated with irinotecan and/or SN-38. NPC and SN-38G formation was measured by high-performance liquid chromatography. UGT1A1 mRNA levels were measured by real-time polymerase chain reaction. In human liver microsomes, a concentration-dependent decrease in the formation of NPC and SN-38G was observed. In human hepatocytes, a significant increase in SN-38G production was observed in 33% (EGCG), 44% (ECG), and 44% (EGC) of the hepatocyte preparations. Phenobarbital increased the formation of SN-38G in 100% of the same hepatocyte preparations. In Hep G2 cells, no increase in SN-38G formation was observed. With the exception of ECG in one liver, catechins did not increase UGT1A1 mRNA levels. NPC production was also significantly increased in 40% of the hepatocyte preparations for each catechin. However, the production of 6beta-hydroxytestosterone remained unaffected in other hepatocyte preparations. At pharmacologically relevant concentrations, catechins are unlikely to inhibit the formation of irinotecan inactive metabolites when administered concomitantly. The induction effect of catechins on UGT1A1 seems to be modest and highly variable. Catechins do not induce CYP3A4 activity. The effect of acute and prolonged use of green tea on the pharmacokinetics of irinotecan in patients remains to be evaluated.

Transdermal delivery of (-)-epigallocatechin-3-gallate, a green tea polyphenol, in mice

Epigallocatechin-3-gallate (EGCG) is the most studied catechin in green tea (Camellia sinensis). EGCG and green tea are cancer preventive in many animal models, and numerous mechanisms have been proposed in cell lines. EGCG is poorly bioavailable in man and rodents. We hypothesized that transdermal delivery of EGCG could result in improved bioavailability. Following application of EGCG transdermal gel (50 mg kg(-1), t.d.) to SKH-1 mice, EGCG was observed in the epidermis (1365.7-121.0 ng g(-1)) and dermis (411.2-42.6 ng g(-1)). The maximum plasma concentration (Cmax) of EGCG was 44.5 ng mL(-1). The t(1/2) (94.4 h) and AUC(0-->24 h) (881.5 ng mL(-1) h) of EGCG were greater than values previously reported for oral EGCG. The t(1/2) and area under the concentration-time curve up to 24 h (AUC(0-->24 h)) in the liver, small intestine and colon were 21.3-74.6 h and 715-2802 ng g(-1)h, respectively. Stability studies showed that the transdermal formulation was stable at 4 degrees C and had a half-life (t(1/2)) of 47.1 and 20.2 h at 25 degrees C and 37 degrees C, respectively. These data indicate that transdermal EGCG is useful for delivering prolonged levels of EGCG to plasma and tissues, and may provide an alternative to tea consumption as a dosage form of EGCG.

Peracetylation as a means of enhancing in vitro bioactivity and bioavailability of epigallocatechin-3-gallate

(-)-Epigallocatechin-3-gallate (EGCG) is the widely studied catechin in green tea (Camellia sinensis). Previously, we have reported the low bioavailability of EGCG in rats and mice. As a means of improving the bioavailability of EGCG, we have prepared a peracetylated EGCG derivative (AcEGCG) and herein report its growth inhibitory activity and cellular uptake in vitro, as well as bioavailability in mice. AcEGCG exhibited enhanced growth inhibitory activity relative to EGCG in both KYSE150 human esophageal (IC50 = 10 versus 20 microM) and HCT116 human colon cancer cells (IC50 = 32 versus 45 microM). AcEGCG was rapidly converted to EGCG by HCT116 cells, and treatment of cells with AcEGCG resulted in a 2.8- to 30-fold greater intracellular concentration of EGCG as compared with treatment with EGCG. AcEGCG was also more potent than EGCG at inhibiting nitric oxide production (4.4-fold) and arachidonic acid release (2.0-fold) from lipopolysaccharide-stimulated RAW264.7 murine macrophages. Intragastric administration of AcEGCG to CF-1 mice resulted in higher bioavailability compared with administration of equimolar doses of EGCG. The plasma area under the curve from 0 to infinity (AUC0-->infinity) of total EGCG was 465.0 and 194.6 [(microg/ml) . min] from the administration of AcEGCG and EGCG, respectively. The t1/2 of EGCG was also increased following administration of AcEGCG compared with EGCG (441.0 versus 200.3 min). The AUC0-->infinity and t1/2 were also increased in small intestinal (2.8- and 4.3-fold, respectively) and colonic tissues (2.4- and 6.0-fold, respectively). These data suggest that acetylation represents a means of increasing the biological potency in vitro, increasing the bioavailability of EGCG in vivo, and may improve cancer-preventive activity.

Possible drug-metabolism interactions of medicinal herbs with antiretroviral agents

Herbal medicines are widely used by HIV patients. Several herbal medicines have been shown to interact with antiretroviral drugs, which might lead to drug failure. We have aimed to provide an overview of the modulating effects of Western and African herbal medicines on antiretroviral drug-metabolizing and transporting enzymes, focusing on potential herb-antiretroviral drug interactions. Echinacea, garlic, ginkgo, milk thistle, and St. John's wort have the potential to cause significant interactions. In vitro and in vivo animal studies also indicated other herbs with a potential for interactions; however, most evidence is based on in vitro studies. Further pharmacokinetic studies to unveil potential Western and especially African herb-antiretroviral drug interactions are urgently required, and the clinical significance of these interactions should be assessed.

Anxiolytic properties of green tea polyphenol (−)-epigallocatechin gallate (EGCG)

Naturally occurring polyphenols are potent antioxidants. Some of these compounds are also ligands for the GABA(A) receptor benzodiazepine site. This feature endows them with sedative properties. Here, the anxiolytic activity of the green tea polyphenol (-)-epigallocatechin gallate (EGCG) was investigated after acute administration in mice, using behavioral tests (elevated plus-maze and passive avoidance tests) and by electrophysiology on cultured hippocampal neurons. Patch-clamp experiments revealed that EGCG (1-10 muM) had no effect on GABA currents. However, EGCG reversed GABA(A) receptor negative modulator methyl beta-carboline-3-carboxylate (beta-CCM) inhibition on GABA currents in a concentration dependent manner. This was also observed at the level of synaptic GABA(A) receptors by recording spontaneous inhibitory synaptic transmission. In addition, EGCG consistently inhibited spontaneous excitatory synaptic transmission. Behavioral tests indicated that EGCG exerted both anxiolytic and amnesic effects just like the benzodiazepine drug, chlordiazepoxide. Indeed, EGCG in a dose-dependent manner both increased the time spent in open arms of the plus-maze and decreased the step-down latency in the passive avoidance test. GABA(A) negative modulator beta-CCM antagonized EGCG-induced amnesia. Finally, state-dependent learning was observable after chlordiazepoxide and EGCG administration using a modified passive avoidance procedure. Optimal retention was observed only when animals were trained and tested in the same state (veh-veh or drug-drug) and significant retrieval alteration was observed in different states (veh-drug or drug-veh). Moreover, EGCG and chlordiazepoxide fully generalized in substitution studies, indicating that they induced indistinguishable chemical states for the brain. Therefore, our data support that EGCG can induce anxiolytic activity which could result from an interaction with GABA(A) receptors.

Tea polyphenols and theaflavins are present in prostate tissue of humans and mice after green and black tea consumption

Green and black tea have shown promise in the chemoprevention of prostate cancer. The objective of this study was to determine the bioavailability and bioactivity of tea polyphenols (PP) and theaflavins in human serum and human and mouse tissues. A decaffeinated black tea diet was administered to C57BL/6 mice. PPs and theaflavins were found in the small and large intestine, liver, and prostate in conjugated and free forms. The relative prostate bioavailability of theaflavin was 70% higher than that of epigallocatechin gallate (EGCG). In the second mouse study, a green tea (GT) diet was administered followed by the control diet for 1-5 d. Epicatechin (EC), EGCG, and epicatechin gallate (ECG) concentrations in prostate tissue were significantly decreased after 1 d of consuming the control diet. Epigallocatechin gallate (EGC), however, did not decrease significantly. For the human study, 20 men scheduled for surgical prostatectomy were randomly assigned to consume 1.42 L daily of GT, BT, or a caffeine-matched soda control (SC) for 5 d before radical prostatectomy. Tea PPs were greater in prostate samples from men consuming BT and GT than in men consuming SC (P = 0.0025). Although tea PP were not detectable in serum, ex vivo LNCaP prostate cancer cell proliferation was less when cells were grown in media containing patient serum collected after BT (P < 0.001) and GT (P = 0.025) consumption relative to baseline serum This is the first human study to show that tea polyphenols and theaflavins are bioavailable in the prostate where they may be active in the prevention of prostate cancer.

Pharmacokinetic studies of green tea catechins in maternal plasma and fetuses in rats

We carried out a pharmacokinetic study to determine the levels and profiles of catechins in pregnant rats and their fetuses after ingestion of green tea extract (GTE). We measured total catechin levels after enzyme digestions. Dams, at 15.5 days of gestation, were fed with GTE and catechins were measured in the maternal plasma, placenta, and fetus 0, 0.5, 1, 2, 3, 5, 8, 10, 12, 16, and 20 h after maternal GTE intake. The pharmacokinetic changes were analyzed by non compartmental models. We found that maternal plasma concentrations of catechins were about 10 times higher than in placenta and 50-100 times higher than in the fetus. AUC and Cmax levels of (-)-epicatechin (EC) were the highest in plasma while the levels of (-)-epigallocatechin gallate (EGCG) were the highest in the placenta and the fetus. The exposure level of catechin derivatives was higher than the gallate derivatives in maternal plasma after normalization but reversed in the placenta and fetus. The absorption of epi-isomers in plasma was found to be more favorable than their non epi-isomer counterparts. EGCG had the highest level of exposure (AUC) and the highest Cmax in the fetus, implying it may have potential for in utero antioxidant protection.

Uridine diphosphoglucuronosyltransferase pharmacogenetics and cancer

The uridine diphosphoglucuronosyltransferases (UGTs) belong to a superfamily of enzymes that catalyse the glucuronidation of numerous endobiotics and xenobiotics. Several human hepatic and extrahepatic UGT isozymes have been characterized with respect to their substrate specificity, tissue expression and gene structure. Genetic polymorphisms have been identified for almost all the UGT family members. A wide variety of anticancer drugs, dietary chemopreventives and carcinogens are known to be conjugated by members of both UGT1A and UGT2B subfamilies. This review examines in detail each UGT isozyme known to be associated with cancer and carcinogenesis. The cancer-related substrates for several UGTs are summarized, and the functionally relevant genetic polymorphisms of UGTs are reviewed. A number of genotype-phenotype association studies have been carried out to characterize the role of UGT pharmacogenetics in several types of cancer, and these examples are discussed here. In summary, this review focuses on the role of the human UGT genetic polymorphisms in carcinogenesis, chemoprevention and cancer risk.

In vitro phase II metabolism of xanthohumol by human UDP-glucuronosyltransferases and sulfotransferases

Xanthohumol (XN) is the principal prenylated flavonoid of the hop plant and has recently gained considerable interest due to its potential cancer-chemopreventive effects. However, the metabolism of XN has not yet been investigated in detail. Therefore, we studied the in vitro phase II metabolism of XN using nine human recombinant UDP-glucuronosyltransferases (UGT) and five sulfotransferases (SULT). The identification of the metabolites formed was elucidated using HPLC with diode array detection as well as HPLC/API-ES MS. XN was efficiently glucuronidated by UGT 1 A 8, 1 A 9, and 1 A 10; further important UGTs were UGT 1 A 1, 1 A 7, and 2 B 7. With respect to the sulfation reaction, SULT 1 A 1*2, 1 A 2, and 1 E 1 were the most active SULT forms. UGT 1 A 3, 1 A 4, and 1 A 6 as well as SULT 1 A 3 and 2 A 1 were of minor importance for the conjugation of XN. Three mono-glucuronides as well as three mono-sulfates were identified. Considering the tissue distribution of the tested UGT and SULT enzyme forms, these findings suggest a prominent role for the glucuronidation and sulfation of XN in the liver as well as in the gastrointestinal tract.

DOSE-DEPENDENT LEVELS OF EPIGALLOCATECHIN-3-GALLATE IN HUMAN COLON CANCER CELLS AND MOUSE PLASMA AND TISSUES

Epigallocatechin-3-gallate (EGCG; molecular formula: C22H18011)is the most abundant catechin in green tea (Camellia sinensis Theaceae). Both EGCG and green tea have been shown to have cancer-preventive activity in a number of animal models, and numerous mechanisms have been proposed based on studies with human cell lines. EGCG has been shown to undergo extensive biotransformation to yield methylated and glucuronidated metabolites in mice, rats, and humans. In the present study, we determined the concentration-dependent uptake of EGCG by HT-29 human colon cancer cells (20-600 microM) and the dose dependence of EGCG plasma and tissue levels after a single dose of EGCG (50-2000 mg/kg i.g.) to male CF-1 mice. The cytosolic levels of EGCG were linear with respect to extracellular concentration of EGCG after treatment of HT-29 cells for 2 h (915.3-6851.6 microg/g). In vivo, EGCG exhibited a linear dose relationship in the plasma (0.03-4.17 microg/ml), prostate (0.01-0.91 microg/g), and liver (0.09-18.3 microg/g). In the small intestine and colon, however, the levels of EGCG plateaued between 500 and 2000 mg/kg i.g. These results suggest that absorption of EGCG from the small intestine is largely via passive diffusion; however, at high concentrations, the small intestinal and colonic tissues become saturated. The levels of 4''-O-methyl-EGCG and 4',4''-di-O-methyl-EGCG parallel those of EGCG with respect to dose. The present study provides information with respect to what concentrations of EGCG are achievable in mice and may guide dose selection for future cancer chemoprevention studies with EGCG.

Glucuronidation of the green tea catechins, (-)-epigallocatechin-3-gallate and (-)-epicatechin-3-gallate, by rat hepatic and intestinal microsomes

The flavonoids (-)-epigallocatechin-3-gallate (EGCg) and (-)-epicatechin-3-gallate (ECg) are major components of green tea and show numerous biological effects. We investigated the glucuronidation of these compounds and of quercetin by microsomes. Quercetin was almost fully glucuronidated by liver microsomes after 3 h, whereas ECg and ECGg were conjugated to a lesser extent (12.2 +/- 0.2 and 7.5 +/- 0.2%, respectively). The intestinal microsomes also glucuronidated quercetin much more efficiently than ECg and EGCg. Although the rates were lower than quercetin, intestinal microsomes exhibited higher activity on the galloyl group of ECg and EGCg compared to the flavonoid ring, whereas hepatic glucuronidation was higher on the flavonoid ring of EGCg and ECg compared to the galloyl groups. The low glucuronidation rates could partially explain why these flavanols are present in plasma as unconjugated forms.

Green tea (Camellia sinensis) extract does not alter cytochrome p450 3A4 or 2D6 activity in healthy volunteers

Green tea extract is a widely used dietary supplement. The objective of this study was to assess the influence of a decaffeinated green tea (DGT; Camellia sinensis) extract on the activity of the drug-metabolizing enzymes cytochrome P-450 2D6 and 3A4. Probe drugs dextromethorphan (30 mg, CYP2D6 activity) and alprazolam (ALPZ; 2 mg, CYP3A4 activity) were administered orally to healthy volunteers (n = 11) at baseline, and again after treatment with four DGT capsules/day for 14 days. Each DGT capsule contained 211 +/- 25 mg of green tea catechins and <1 mg of caffeine. Dextromethorphan metabolic ratios (DMRs) and alprazolam pharmacokinetics were determined at baseline and after DGT treatment. There were no significant differences in ALPZ pharmacokinetics at baseline and after DGT treatment (all P values >/= 0.05; maximum concentration in plasma, 33 +/- 8 versus 34 +/- 13 ng/ml; time to reach maximum concentration in plasma, 1.4 +/- 1.2 versus 1.4 +/- 1.2 h; area under the plasma concentration versus time curve, 480 +/- 119 versus 510 +/- 107 h. ng. ml(-1); half-life of elimination, 12.3 +/- 1.7 versus 13.1 +/- 3.4 h). The DMR was 0.053 +/- 0.045 at baseline and 0.041 +/- 0.032 after DGT supplementation (P > 0.05). The plasma concentration of the green tea flavonoid, (-)-epigallocatechin gallate, reached 1.3 +/- 1.8 microM 2 h after DGT treatment. Our results indicate that DGT is unlikely to alter the disposition of medications primarily dependent on the CYP2D6 or CYP3A4 pathways of metabolism.

Synthesis and biological activity of the tea catechin metabolites, M4 and M6 and their methoxy-derivatives

Syntheses are reported for metabolites M4 (1) and M6 (2) of the green tea polyphenols epicatechin (EC) and epigallocatechin (EGC) and their gallate derivatives. Several methoxy-derivatives of 1 and 2 were also prepared. Compounds 1 and 2 were evaluated for growth inhibitory activity against a panel of immortalized and malignant human cell lines with 1 being the more active compound. The possible antiinflammatory activity of 1 and its trimethoxy derivative was also evaluated. Neither compound inhibited the release of arachidonic acid, although 1 inhibited NO production by 50% at 20 microM.

(-)-Epigallocatechin-3-gallate promotes pro-matrix metalloproteinase-7 production via activation of the JNK1/2 pathway in HT-29 human colorectal cancer cells

Matrix metalloproteinase (MMP)-7 (matrilysin-1) plays significant roles in the growth, invasion, and metastasis of colorectal tumors, while (-)-epigallocatechin-3-gallate (EGCG), a green tea polyphenol with chemopreventive properties, has been shown to be an inhibitor of MMP-2 and MMP-9. In the present study, HT-29 human colorectal cancer cells were treated with EGCG to examine its effects on pro-MMP-7 induction and production using RT-PCR and western blot analyses. Surprisingly, EGCG (10-100 microM) treatment increased both intracellular and extracellular pro-MMP-7 protein levels (2.6-8.4-fold and 1.9-6.4-fold, respectively) in dose- and time-dependent manner, with a significant upregulation of its mRNA expression. EGCG also activated extracellular signal-regulated protein kinase (ERK)1/2, c-JUN NH2-terminal kinase (JNK)1/2 and p38 mitogen-activated protein kinase (MAPK), as previously reported. In addition, the polyphenol triggered the phosphorylation of c-JUN (Ser63 and Ser73) and induced c-JUN/c-FOS, thereby increasing the DNA binding activity of activator protein-1 (AP-1), as shown by an AP-1 luciferase reporter assay. Pharmacological blockade of MAPK activities suggested that pro-MMP-7 expression was induced via JNK1/2 activation, but not in the case of ERK1/2 or p38 MAPK. N-Acetyl-L-cysteine, superoxide (O2-) dismutase and catalase attenuated the EGCG-induced pro-MMP-7 production, suggesting an involvement of oxidative stress in these events. Conversely, EGCG spontaneously generated O2- in a cell-free system that utilized a cytochrome C reduction method. Further, (-)-epicatechin-3-gallate (25 and 100 microM) and green tea polyphenols (33 and 132 microg/ml) induced pro-MMP-7 expression, whereas (-)-epicatechin and (-)-epigallocatechin (100 microM each) did not. Induction of pro-MMP-7 expression by EGCG was also shown in another human colorectal adenocarcinoma cell line, Caco-2. Our results suggest that some green tea catechins induce pro-MMP-7 production via O2- production and the activation of JNK1/2, c-JUN, c-FOS and AP-1 in HT-29 cells.

Antioxidant capacity of vegetables, spices and dressings relevant to nutrition

Vegetables are the most important sources of phenolics in the Mediterranean diet. Phenolics, especially flavonoids, are suggested as being essential bioactive compounds providing health benefits. In this study, twenty-seven vegetables, fifteen aromatic herbs and some spices consumed in Central Italy (the Marches region) were studied to reveal total phenolic, flavonoid and flavanol content as well as their antioxidant capacity measured by the oxygen radical absorbance capacity (ORAC) method. A comparison in terms of antioxidant capacity was made between different salads, as well as between salads to which aromatic herbs had been added. Lemon balm and marjoram at a concentration of 1.5 % w/w increased by 150 % and 200 % respectively the antioxidant capacity of a salad portion. A 200 g portion of a salad enriched with marjoram corresponded to an intake of 200 (SD 10) mg phenolics and 4000 (SD 300) ORAC units (micromol Trolox equivalents). Olive oils and wine or apple vinegars were the salad dressings that provided the highest increase in antioxidant capacity. Among the spices tested, cumin and fresh ginger made the most significant contribution to the antioxidant capacity. The results are useful in surveying the antioxidant parameters of vegetables, herbs and spices produced and consumed in our geographical area as well as in quantifying the daily intake of phenolics and ORAC units. The results can be used in public health campaigns to stimulate the consumption of vegetables able to provide significant health protection in order to prevent chronic diseases.

Toxicity of green tea extracts and their constituents in rat hepatocytes in primary culture

Recent reports on sporadic cases of liver disorders (acute hepatitis, icterus, hepatocellular necrosis) after ingestion of dietary supplements based on hydro-alcoholic extracts from green tea leaves led to restrictions of the marketing of such products in certain countries of the EU. Since green tea is considered to exert a number of beneficial health effects, and, therefore, green tea products are widely used as dietary supplements, we were interested in the possible mechanism of hepatotoxicity of green tea extracts and in the components involved in such effects. Seven hours after seeding on collagen, rat hepatocytes in primary culture were treated with various hydro-alcoholic green tea extracts (two different native 80% ethanolic dry extracts and an 80% ethanolic dry extract cleared from lipophilic compounds). Cells were washed, and reduction of resazurin, used as a viability parameter monitoring intact mitochondrial function, was determined. It was found that all seven green tea extracts examined enhanced resazurin reduction significantly at a concentration range of 100-500 microg/ml medium, while a significant decrease was observed at 1-3mg/ml medium. Decreased levels were concomitant with abundant necrosis as observed by microscopic inspection of the cultures and with increased leakage of lactate dehydrogenase activity from the cells. In a separate series of experiments, the green tea constituents (-)-epicatechin, (-)-epigallocatechin-3-gallate, caffeine and theanine were tested at concentrations reflecting their levels in a typical green tea extract. Synthetic (+)-epigallocatechin (200 microM) was used for comparison. Cytotoxicity was found with (-)-epigallocatechin-3-gallate only. The concomitant addition of 0.25 mM ascorbate/0.05 mM alpha-tocopherol had no influence on cytotoxicity. In conclusion, our results suggest that high concentrations of green tea extract can exert acute toxicity in rat liver cells. (-)-Epigallocatechin-3-gallate seems to be a key constituent responsible for this effect. The relatively low bioavailability of catechins reported after oral exposure to green tea argues, however, against a causal role of these constituents in the reported liver disorders.

A review of the health effects of green tea catechins in in vivo animal models

There is good evidence from in vitro studies that green tea catechins have a role in protection against degenerative diseases. However, the concentrations used in vitro are often higher than those found in animal or human plasma, and so in vivo evidence is required to demonstrate any protective effect of catechins. This article summarizes the most interesting in vivo animal studies on the protective effects of green tea catechins against biomarkers for cancer, cardiovascular disease, and other degenerative diseases. Generally, most studies using animal models show that consumption of green tea (catechins) provides some protection, although most studies have not examined dose response. Tea catechins could act as antitumorigenic agents and as immune modulators in immunodysfunction caused by transplanted tumors or by carcinogen treatment. Green tea has antiproliferative activity in hepatoma cells and hypolipidemic activity in hepatoma-treated rats, and some studies report that it prevents hepatoxicity. It could act as a preventive agent against mammary cancer postinitiation. Nevertheless, the implications of green tea catechins in preventing metastasis have not been clearly established. Long-term feeding of tea catechins could be beneficial for the suppression of high-fat diet-induced obesity by modulating lipid metabolism, could have a beneficial effect against lipid and glucose metabolism disorders implicated in type 2 diabetes, and could also reduce the risk of coronary disease. Further investigations on mechanisms, the nature of the active compounds, and appropriate dose levels are needed.

Piperine enhances the bioavailability of the tea polyphenol (-)-epigallocatechin-3-gallate in mice

(-)-Epigallocatechin-3-gallate (EGCG), from green tea (Camellia sinensis), has demonstrated chemopreventive activity in animal models of carcinogenesis. Previously, we reported the bioavailability of EGCG in rats (1.6%) and mice (26.5%). Here, we report that cotreatment with a second dietary component, piperine (from black pepper), enhanced the bioavailability of EGCG in mice. Intragastric coadministration of 163.8 micromol/kg EGCG and 70.2 micromol/kg piperine to male CF-1 mice increased the plasma C(max) and area under the curve (AUC) by 1.3-fold compared to mice treated with EGCG only. Piperine appeared to increase EGCG bioavailability by inhibiting glucuronidation and gastrointestinal transit. Piperine (100 micromol/L) inhibited EGCG glucuronidation in mouse small intestine (by 40%) but not in hepatic microsomes. Piperine (20 micromol/L) also inhibited production of EGCG-3"-glucuronide in human HT-29 colon adenocarcinoma cells. Small intestinal EGCG levels in CF-1 mice following treatment with EGCG alone had a C(max) = 37.50 +/- 22.50 nmol/g at 60 min that then decreased to 5.14 +/- 1.65 nmol/g at 90 min; however, cotreatment with piperine resulted in a C(max) = 31.60 +/- 15.08 nmol/g at 90 min, and levels were maintained above 20 nmol/g until 180 min. This resulted in a significant increase in the small intestine EGCG AUC (4621.80 +/- 1958.72 vs. 1686.50 +/- 757.07 (nmol/g.min)). EGCG appearance in the colon and the feces of piperine-cotreated mice was slower than in mice treated with EGCG alone. The present study demonstrates the modulation of the EGCG bioavailablity by a second dietary component and illustrates a mechanism for interactions between dietary chemicals.

Absorption and metabolism of flavonoids

The benefits of flavonoids as chemopreventive dietary or dietary supplemental agents are still only "potential." Much has been learned about possible mechanisms of action of these agents, but whether they can reach their multiple intended sites of action, particularly in humans, is largely unknown. The biological fate of the flavonoids, including their dietary glycoside forms, is highly complex, dependent on a large number of processes. This review is intended to bring some order into this complex area and deals with the fate of the naturally occurring glycosides, their enzymatic hydrolysis, as well as the resulting aglycones. The impact of membrane transporters as well as metabolic enzymes on the cellular availability of these phytochemicals is examined. A reevaluation of the concept of oral bioavailability applied to the dietary flavonoids is presented.