Protection by green tea, black tea, and indole-3-carbinol against 2-amino-3-methylimidazo[4,5-f]quinoline-induced DNA adducts and colonic aberrant crypts in the F344 rat

Epigenetic Regulation of NRF2/KEAP1 by Phytochemicals

Epigenetics has provided a new dimension to our understanding of nuclear factor erythroid 2–related factor 2/Kelch-like ECH-associated protein 1 (human NRF2/KEAP1 and murine Nrf2/Keap1) signaling. Unlike the genetic changes affecting DNA sequence, the reversible nature of epigenetic alterations provides an attractive avenue for cancer interception. Thus, targeting epigenetic mechanisms in the corresponding signaling networks represents an enticing strategy for therapeutic intervention with dietary phytochemicals acting at transcriptional, post-transcriptional, and post-translational levels. This regulation involves the interplay of histone modifications and DNA methylation states in the human NFE2L2/KEAP1 and murine Nfe2l2/Keap1 genes, acetylation of lysine residues in NRF2 and Nrf2, interaction with bromodomain and extraterminal domain (BET) acetyl “reader” proteins, and non-coding RNAs such as microRNA (miRNA) and long non-coding RNA (lncRNA). Phytochemicals documented to modulate NRF2 signaling act by reversing hypermethylated states in the CpG islands of NFE2L2 or Nfe2l2, via the inhibition of DNA methyltransferases (DNMTs) and histone deacetylases (HDACs), through the induction of ten-eleven translocation (TET) enzymes, or by inducing miRNA to target the 3′-UTR of the corresponding mRNA transcripts. To date, fewer than twenty phytochemicals have been reported as NRF2 epigenetic modifiers, including curcumin, sulforaphane, resveratrol, reserpine, and ursolic acid. This opens avenues for exploring additional dietary phytochemicals that regulate the human epigenome, and the potential for novel strategies to target NRF2 signaling with a view to beneficial interception of cancer and other chronic diseases.

Enhancement of colon carcinogenesis by the combination of indole-3 carbinol and synbiotics in hemin-fed rats

The risk of developing colorectal cancer (CRC) could be associated with red and processed meat intake. Experimental data supports that hemin iron, found abundantly in red meat, promotes CRC in mice and rats, while indole-3 carbinol (I3C) and synbiotics (syn) exert anti-carcinogenic activities in most studies of colon carcinogenesis. This study aimed to investigate the modifying effects of I3C and syn (inulin + Bifidobacterium lactis), given separately or together, on dimethylhidrazine (DMH)-induced colon carcinogenesis in hemin-fed rats. All animals were given four subcutaneous DMH injections and then, two weeks after carcinogen exposure, they began a basal diet containing hemin, hemin + I3C, hemin + syn, or hemin + I3C + syn for 23 weeks. The combination of I3C + syn significantly increased fecal water genotoxicity, tumor volume and invasiveness when compared to the hemin-fed control group. The groups fed I3C or syn alone had a significant reduction in the number of preneoplastic aberrant crypt foci (ACF) lesions compared to the hemin-fed group. Dietary I3C also reduced fecal water genotoxicity. Gene expression analysis of colorectal tumors demonstrated that the combination of dietary I3C + syn increased transcript levels for Raf1 and decreased tumor progression and invasiveness related to the genes Cdh1 and Appl1. This analysis also revealed that the Tnf and Cdh1 genes were significantly up- and down-regulated, respectively, in tumors of rats that received I3C, in comparison with the hemin-fed group. These findings reveal that the joint administration of I3C and syn enhanced the development of colon tumors induced by DMH in hemin-fed rats, while they potentially reduced ACF development when given alone.

Green Tea Polyphenols Inhibit Colorectal Tumorigenesis in Azoxymethane-Treated F344 Rats

In studying the cancer-preventive activities of green tea polyphenols, we previously demonstrated that dietary administration of polyphenon E (PPE) inhibited the formation of aberrant crypt foci (ACF) in the colon of azoxymethane (AOM)-treated F344 rats. Herein, we reported cancer-preventive activity of PPE using colorectal cancer as an end point. F344 rats were given two weekly injections of AOM, and then maintained on a 20% high-fat diet with or without 0.24% PPE for 34 wk. In the control group, 83% of rats developed colorectal tumors. Dietary PPE treatment significantly increased the plasma and colonic levels of tea polyphenols, and decreased tumor multiplicity and tumor size. Histological analysis indicated that PPE significantly decreased the incidence of adenocarcinoma, and the multiplicity of adenocarcinoma as well as the multiplicity of adenoma. PPE treatment significantly decreased plasma levels of proinflammatory eicosanoids, prostaglandin E2, and leukotriene B4. It also decreased β-catenin nuclear expression, induced apoptosis, and increased expression levels of RXRα, β, and γ in adenocarcinomas. In conclusion, our results convincingly demonstrated the inhibitory effects of orally administered PPE on colon carcinogenesis in AOM-treated rats and suggested possible biomarkers for the biological effects of green tea polyphenols.

Consumption of the total Western diet differentially affects the response to green tea in rodent models of chronic disease compared to the AIN93G diet

SCOPE

In pre-clinical studies investigating bioactive components, the efficacy of the bioactive is likely influenced by the basal diet provided to rodents. In this study, we hypothesized that a model bioactive, green tea extract (GTE), would have different effects on colon carcinogenesis, body composition, and lipid metabolism in mice fed a basal diet formulated to promote animal health and growth (AIN93G) as compared to a Western diet that emulates typical American intakes of micro- and macronutrients, the total Western diet (TWD).

METHODS AND RESULTS

Mice were fed either AIN93G or TWD, with or without GTE added to drinking water for 18 weeks. Aberrant crypt foci (ACF) in azoxymethane-initiated mice was nearly three times greater in mice fed TWD compared to AIN93G. Consumption of GTE suppressed ACF development only in mice fed the TWD. Similarly, supplementation with GTE suppressed weight gain and fasted glucose only in mice fed TWD, while GTE suppressed fat mass in mice fed either diet. Irrespective of diet, GTE supplementation increased cecum weight and decreased cecal SCFA concentration.

CONCLUSION

Collectively, these observations indicate that the TWD influences the bioactivity of GTE in rodent models of obesity, metabolism, and carcinogenesis.

Intranasal delivery of liposomal indole-3-carbinol improves its pulmonary bioavailability

Indole-3-carbinol (I3C), a constituent of commonly consumed Brassica vegetables, has been shown to have anticancer effects in a variety of preclinical models of lung cancer. However, it has shown only limited efficacy in clinical trials, likely due to its poor oral bioavailability. Intranasal administration of I3C has the potential to enhance the pulmonary accumulation of the drug, thereby improving its availability at the target site of action. In this study, we developed a liposomal formulation of I3C and evaluated its lung delivery and chemopreventive potential in tobacco smoke carcinogen [4-(methylnitro-samino)-1-(3-pyridyl)-1-butanone (NNK)]-treated mice. Intranasal administration of I3C liposomes led to a ∼100-fold higher lung exposure of I3C than the oral route of administration. Further, intranasal delivery of liposomal I3C led to a significant reduction (37%; p<0.05) in the levels of the DNA adduct formation induced by NNK treatment. Liposomal I3C also significantly increased (by 10-fold) the expression of CYP1A1, a cytochrome P450 enzyme known to increase the detoxification of chemical carcinogens by enhancing their metabolism. Overall, our findings demonstrate that intranasal administration of liposomal I3C has the potential to significantly improve the efficacy of I3C for lung cancer chemoprevention.

Coffee and tea consumption, genotype-based CYP1A2 and NAT2 activity and colorectal cancer risk-results from the EPIC cohort study

Coffee and tea contain numerous antimutagenic and antioxidant components and high levels of caffeine that may protect against colorectal cancer (CRC). We investigated the association between coffee and tea consumption and CRC risk and studied potential effect modification by CYP1A2 and NAT2 genotypes, enzymes involved in the metabolization of caffeine. Data from 477,071 participants (70.2% female) of the European Investigation into Cancer and Nutrition (EPIC) cohort study were analyzed. At baseline (1992-2000) habitual (total, caffeinated and decaffeinated) coffee and tea consumption was assessed with dietary questionnaires. Cox proportional hazards models were used to estimate adjusted hazard ratio's (HR) and 95% confidence intervals (95% CI). Potential effect modification by genotype-based CYP1A2 and NAT2 activity was studied in a nested case-control set of 1,252 cases and 2,175 controls. After a median follow-up of 11.6 years, 4,234 participants developed CRC (mean age 64.7 ± 8.3 years). Total coffee consumption (high vs. non/low) was not associated with CRC risk (HR 1.06, 95% CI 0.95-1.18) or subsite cancers, and no significant associations were found for caffeinated (HR 1.10, 95% CI 0.97-1.26) and decaffeinated coffee (HR 0.96, 95% CI 0.84-1.11) and tea (HR 0.97, 95% CI 0.86-1.09). High coffee and tea consuming subjects with slow CYP1A2 or NAT2 activity had a similar CRC risk compared to non/low coffee and tea consuming subjects with a fast CYP1A2 or NAT2 activity, which suggests that caffeine metabolism does not affect the link between coffee and tea consumption and CRC risk. This study shows that coffee and tea consumption is not likely to be associated with overall CRC.

Glucosinolates from pak choi and broccoli induce enzymes and inhibit inflammation and colon cancer differently

Feeding a glucosinolate-enriched pak choi diet reduced colitis and tumor numbers. No effects were observed by a glucosinolate-enriched broccoli diet.

Antigenotoxic and apoptotic activity of green tea polyphenol extracts on hexavalent chromium-induced DNA damage in peripheral blood of CD-1 mice: analysis with differential acridine orange/ethidium bromide staining

This study was conducted to investigate the modulating effects of green tea polyphenols on genotoxic damage and apoptotic activity induced by hexavalent chromium [Cr (VI)] in CD-1 mice. Animals were divided into the following groups: (i) injected with vehicle; (ii) treated with green tea polyphenols (30 mg/kg) via gavage; (iii) injected with CrO₃ (20 mg/kg) intraperitoneally; (iv) treated with green tea polyphenols in addition to CrO₃. Genotoxic damage was evaluated by examining micronucleated polychromatic erythrocytes (MN-PCEs) obtained from peripheral blood at 0, 24, 48, and 72 h after treatment. Induction of apoptosis and cell viability were assessed by differential acridine orange/ethidium bromide (AO/EB) staining. Treatment of green tea polyphenols led to no significant changes in the MN-PCEs. However, CrO₃ treatment significantly increased MN-PCEs at 24 and 48 h after injection. Green tea polyphenols treatment prior to CrO₃ injection led to a decrease in MN-PCEs compared to the group treated with CrO₃ only. The average of apoptotic cells was increased at 48 h after treatment compared to control mice, suggesting that apoptosis could contribute to eliminate the DNA damaged cells induced by Cr (VI). Our findings support the proposed protective effects of green tea polyphenols against the genotoxic damage induced by Cr (VI).

Prospective study of the relationship between coffee and tea with colorectal cancer risk: the PLCO Cancer Screening Trial. Br J Cancer. 2013;109:1352-1359. [PMID: 23907431 PMCID: PMC3778290 DOI: 10.1038/bjc.2013.434]

Background:

Coffee and tea are commonly consumed and carry potential anticancer components that could reduce the risk of colorectal cancer; however, their relationships with colorectal cancer risk remain inconsistent.

Methods:

A prospective analysis was carried out to examine the relationships of coffee and tea intake with colorectal cancer risk in 57 398 men and women in the intervention arm of the National Cancer Institute-Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial, a national screening study that limits differential detection biases. Coffee and tea intakes were assessed by food frequency questionnaire.

Results:

Six hundred and eighty-one incident colorectal cancer cases were ascertained during a median follow-up of 11.4 years. Greater coffee intake was not associated with risk of colorectal cancer (relative risk (RR)=1.08, 95% confidence interval (CI)=0.79–1.48, P_trend=0.23). Stratifying by cancer site (P_{heterogeneity}=0.48) or stage (P_{heterogeneity}=0.83) did not alter the relationship. Associations remained unchanged in subsets of participants for either caffeinated or decaffeinated coffee or when stratifying by several colorectal cancer risk factors. Similarly, greater tea intake was not associated with colorectal cancer risk overall (RR=0.77, 95% CI=0.55–1.09, P_trend=0.17) or by cancer site (P_{heterogeneity}=0.14) or stage (P_{heterogeneity}=0.60). These associations were not modified by several colorectal cancer risk factors.

Conclusion:

The findings of this study do not provide evidence to suggest that drinking coffee or tea is beneficial in protecting against colorectal cancer.

Heterocyclic Aromatic Amines Heterocomplexation with Biologically Active Aromatic Compounds and Its Possible Role in Chemoprevention

Food-borne heterocyclic aromatic amines (HCAs) are known mutagens and carcinogens present especially in Western population diet, which contains large amount of meat and its products. HCAs are capable of interacting with DNA directly through the formation of covalent adducts, however this process requires biological activation in liver, mainly by cytochrome P450 enzymes. This process may produce mutations and in consequence may contribute to the development of cancer. However, there are many studies showing that several biologically active aromatic compounds (BACs) may protect against genotoxic effects of HCAs. Direct interactions and noncovalent heterocomplexes formation may be one of the most important mechanisms of such protection. This work describes several BACs present in human diet, which are capable of molecular complexes formation with HCAs and protect cells as well as whole organisms against HCAs action.

Mechanistic issues concerning cancer prevention by tea catechins

The cancer preventive activities of tea (Camellia sinensis, Theaceae) have been demonstrated in animal models for cancers at different organ sites and suggested by some epidemiological studies. Many mechanisms for cancer prevention have been proposed based on studies in cell lines, which demonstrated the modulation of signal transduction and metabolic pathways by (-)-epigallocatechin-3-gallate (EGCG), the most abundant and active polyphenol in green tea. These molecular events may result in cellular changes, such as enhancement of apoptosis, suppression of cell proliferation, and inhibition of angiogenesis. Nevertheless, it is not known whether these are the molecular mechanisms of inhibition of carcinogenesis in animals and humans. This article discusses the key issues involved in extrapolating results from cell line studies to mechanistic information in vivo and in translating animal studies to human cancer prevention.

Cancer prevention by tea: Evidence from laboratory studies. Pharmacol Res. 2011;64:113-122. [PMID: 21397027 DOI: 10.1016/j.phrs.2011.03.001]

The cancer preventive activities of tea (Camellia sinensis Theaceae) have been studied extensively. Inhibition of tumorigenesis by green tea extracts and tea polyphenols has been demonstrated in different animal models, including those for cancers of the skin, lung, oral cavity, esophagus, stomach, small intestine, colon, bladder, liver, pancreas, prostate, and mammary glands. Many studies in cell lines have demonstrated the modulation of signal transduction and metabolic pathways by (-)-epigallocatechin-3-gallate (EGCG), the most abundant and active polyphenol in green tea. These molecular events can result in cellular changes, such as enhancement of apoptosis, suppression of cell proliferation, and inhibition of angiogenesis. Nevertheless, the molecular mechanisms of inhibition of carcinogenesis in animals and humans remain to be further investigated. Future research directions in this area are discussed.

Cancer prevention with natural compounds

Botanical and nutritional compounds have been used for the treatment of cancer throughout history. These compounds also may be useful in the prevention of cancer. Population studies suggest that a reduced risk of cancer is associated with high consumption of vegetables and fruits. Thus, the cancer chemopreventive potential of naturally occurring phytochemicals is of great interest. There are numerous reports of cancer chemopreventive activity of dietary botanicals, including cruciferous vegetables such as cabbage and broccoli, Allium vegetables such as garlic and onion, green tea, Citrus fruits, soybeans, tomatoes, berries, and ginger, as well as medicinal plants. Several lead compounds, such as genistein (from soybeans), lycopene (from tomatoes), brassinin (from cruciferous vegetables), sulforaphane (from asparagus), indole-3-carbinol (from broccoli), and resveratrol (from grapes and peanuts) are in preclinical or clinical trials for cancer chemoprevention. Phytochemicals have great potential in cancer prevention because of their safety, low cost, and oral bioavailability. In this review, we discuss potential natural cancer preventive compounds and their mechanisms of action.

Chemopreventive effects of Coltect, a novel dietary supplement, alone and in combination with 5-aminosalicylic acid in 1,2-dimethylhydrazine-induced colon cancer in rats

OBJECTIVES

Coltect is a novel dietary supplement containing curcumin, green tea and selenomethionine. Previous reports have suggested that these agents can prevent colorectal cancer (CRC). The present study examined the chemopreventive effect of Coltect alone or combined with 5-aminosalicylic acid (5-ASA) using the 1,2-dimethylhydrazine (DMH) model in rats.

METHODS

The effect of Coltect was examined on HT-29 CRC cells by growth inhibition assay. Apoptosis was determined by annexin V-FITC/PI staining. Male rats were injected with DMH in vivo and treated with Coltect 150 mg/kg, 5-ASA 50 mg/kg or their combination, by oral gavage. Aberrant crypt foci (ACF) were identified by methylene blue staining.

RESULTS

HT-29 cells exhibited a dose-dependent response to Coltect. Part of the growth inhibition can be explained by the induction of mild-moderate apoptosis in cancer cells (28%) compared with the untreated cells (10%). In the in vivo model, the average number of ACF was divided into small (1-3 crypts) or large (≥4 crypts). The Coltect compound reduced the number of small and large ACF similarly to 5-ASA (40% reduction). This reduction was amplified by combining the two agents (70% reduction).

CONCLUSION

Coltect inhibits the growth of colon cancer cells, induces apoptosis and inhibits ACF development. Furthermore, it augments the growth inhibitory effect of 5-ASA in vivo. This may be clinically important since this safe dietary supplement-drug combination can be administered as a chemopreventive regimen for the treatment of CRC.

Molecular mechanisms of green tea polyphenols

Tea, next to water, is the most popular beverage in the world. It has been suggested that tea consumption has the cancer-preventive effects. Epidemiological studies have indicated decreased cancer occurrence in people who regularly drink green tea. Research has also discovered numerous mechanisms of action to explain the biological effects of tea. The most abundant and popular compound studied in tea research is (-)-epigallocatechin-3-gallate or (-)-EGCG, which is a powerful antioxidant and can inhibit a number of tumor cell proliferation and survival pathways. Tea polyphenols are known to inhibit metaloproteonases, various protein kinases, and proteins that regulate DNA replication and transformation. We also reported that ester bond-containing tea polyphenols, for example, (-)-EGCG, potently and specifically inhibited the tumor proteasomal activity. We further demonstrated that methylation on green tea polyphenols under physiological conditions decreased their proteasome-inhibitory activity, contributing to decreased cancer-preventive effects of tea consumption. Since (-)-EGCG is unstable under physiological conditions, we also developed the peracetate-protected or prodrug form of (-)-EGCG, Pro-EGCG (1), and showed that Pro-EGCG (1) increases the bioavailability, stability, and proteasome-inhibitory and anticancer activities of (-)-EGCG in human breast cancer cells and tumors, demonstrating its potential use for cancer prevention and treatment.

Molecular mechanisms of the physiological functions of the aryl hydrocarbon (dioxin) receptor, a multifunctional regulator that senses and responds to environmental stimuli

The aryl hydrocarbon receptor (AhR) was originally identified as a ligand-activated transcription factor that is involved in the induction of xenobiotic-metabolizing Cytochrome P4501A1 (CYP1A1). For several decades, AhR has been studied in relation to toxicology and pharmacology. With recent discoveries on novel AhR functions, AhR research has expanded into multiple aspects of physiology, such as reproduction, innate immunity and tumor suppression. In this review, we summarize and discuss recent progress in mechanistic and functional studies on AhR with particular emphasis on physiological processes.

Glucosinolates in Brassica vegetables: the influence of the food supply chain on intake, bioavailability and human health

Glucosinolates (GLSs) are found in Brassica vegetables. Examples of these sources include cabbage, Brussels sprouts, broccoli, cauliflower and various root vegetables (e.g. radish and turnip). A number of epidemiological studies have identified an inverse association between consumption of these vegetables and the risk of colon and rectal cancer. Animal studies have shown changes in enzyme activities and DNA damage resulting from consumption of Brassica vegetables or isothiocyanates, the breakdown products (BDP) of GLSs in the body. Mechanistic studies have begun to identify the ways in which the compounds may exert their protective action but the relevance of these studies to protective effects in the human alimentary tract is as yet unproven. In vitro studies with a number of specific isothiocyanates have suggested mechanisms that might be the basis of their chemoprotective effects. The concentration and composition of the GLSs in different plants, but also within a plant (e.g. in the seeds, roots or leaves), can vary greatly and also changes during plant development. Furthermore, the effects of various factors in the supply chain of Brassica vegetables including breeding, cultivation, storage and processing on intake and bioavailability of GLSs are extensively discussed in this paper.

Aryl hydrocarbon receptor suppresses intestinal carcinogenesis in ApcMin/+ mice with natural ligands. Proc Natl Acad Sci U S A. 2009;106:13481-13486. [PMID: 19651607 DOI: 10.1073/pnas.0902132106]

Intestinal cancer is one of the most common human cancers. Aberrant activation of the canonical Wnt signaling cascade, for example, caused by adenomatous polyposis coli (APC) gene mutations, leads to increased stabilization and accumulation of beta-catenin, resulting in initiation of intestinal carcinogenesis. The aryl hydrocarbon receptor (AhR) has dual roles in regulating intracellular protein levels both as a ligand-activated transcription factor and as a ligand-dependent E3 ubiquitin ligase. Here, we show that the AhR E3 ubiquitin ligase has a role in suppression of intestinal carcinogenesis by a previously undescribed ligand-dependent beta-catenin degradation pathway that is independent of and parallel to the APC system. This function of AhR is activated by both xenobiotics and natural AhR ligands, such as indole derivatives that are converted from dietary tryptophan and glucosinolates by intestinal microbes, and suppresses intestinal tumor development in Apc(Min/+) mice. These findings suggest that chemoprevention with naturally-occurring and chemically-designed AhR ligands can be used to successfully prevent intestinal cancers.

Aryl hydrocarbon receptor suppresses intestinal carcinogenesis in ApcMin/+ mice with natural ligands. Proc Natl Acad Sci U S A. 2009;106:13481-13486. [PMID: 19651607 DOI: 10.1073/2fpnas.0902132106]

Intestinal cancer is one of the most common human cancers. Aberrant activation of the canonical Wnt signaling cascade, for example, caused by adenomatous polyposis coli (APC) gene mutations, leads to increased stabilization and accumulation of beta-catenin, resulting in initiation of intestinal carcinogenesis. The aryl hydrocarbon receptor (AhR) has dual roles in regulating intracellular protein levels both as a ligand-activated transcription factor and as a ligand-dependent E3 ubiquitin ligase. Here, we show that the AhR E3 ubiquitin ligase has a role in suppression of intestinal carcinogenesis by a previously undescribed ligand-dependent beta-catenin degradation pathway that is independent of and parallel to the APC system. This function of AhR is activated by both xenobiotics and natural AhR ligands, such as indole derivatives that are converted from dietary tryptophan and glucosinolates by intestinal microbes, and suppresses intestinal tumor development in Apc(Min/+) mice. These findings suggest that chemoprevention with naturally-occurring and chemically-designed AhR ligands can be used to successfully prevent intestinal cancers.

Mechanisms of primary cancer prevention by butyrate and other products formed during gut flora-mediated fermentation of dietary fibre

Dietary fibres are indigestible food ingredients that reach the colon and are then fermented by colonic bacteria, resulting mainly in the formation of short-chain fatty acids (SCFA) such as acetate, propionate, and butyrate. Those SCFA, especially butyrate, are recognised for their potential to act on secondary chemoprevention by slowing growth and activating apoptosis in colon cancer cells. Additionally, SCFA can also act on primary prevention by activation of different drug metabolising enzymes. This can reduce the burden of carcinogens and, therefore, decrease the number of mutations, reducing cancer risk. Activation of GSTs by butyrate has been studied on mRNA, protein, and enzyme activity level by real-time RT-PCR, cDNA microarrays, Western blotting, or photometrical approaches, respectively. Butyrate had differential effects in colon cells of different stages of cancer development. In HT29 tumour cells, e.g., mRNA GSTA4, GSTP1, GSTM2, and GSTT2 were induced. In LT97 adenoma cells, GSTM3, GSTT2, and MGST3 were induced, whereas GSTA2, GSTT2, and catalase (CAT) were elevated in primary colon cells. Colon cells of different stages of carcinogenesis differed in post-transcriptional regulatory mechanisms because butyrate increased protein levels of different GST isoforms and total GST enzyme activity in HT29 cells, whereas in LT97 cells, GST protein levels and activity were slightly reduced. Because butyrate increased histone acetylation and phosphorylation of ERK in HT29 cells, inhibition of histone deacetylases and the influence on MAPK signalling are possible mechanisms of GST activation by butyrate. Functional consequences of this activation include a reduction of DNA damage caused by carcinogens like hydrogen peroxide or 4-hydroxynonenal (HNE) in butyrate-treated colon cells. Treatment of colon cells with the supernatant from an in vitro fermentation of inulin increased GST activity and decreased HNE-induced DNA damage in HT29 cells. Additional animal and human studies are needed to define the exact role of dietary fibre and butyrate in inducing GST activity and reducing the risk of colon cancer.

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.

A New Medium-term Rat Colorectal Bioassay Applying Neoplastic Lesions as End Points for Detection of Carcinogenesis Modifiers Effects with Weak or Controversial Modifiers

We have established a two-stage, medium-term rat colorectal carcinogenesis model featuring induction of neoplastic lesions within ten weeks. In the present study, we examined the ability of this model to detect weak modifiers. F344 male rats were given three subcutaneous (sc) injections of 1,2-dimethyl-hydrazine (DMH, 40 mg/kg b.w.) in one week followed by drinking water containing 1% dextran sodium sulfate (DSS) for a second week. One week after this regimen, basal diet alone, or diets containing 10% perilla oil, 10% corn oil, 10% dextrin, or 0.1% indole-3-carbinol (I3C) were supplied. The perilla oil and corn oil groups did not show significant differences in the numbers of aberrant crypt foci (ACF) and incidences or multiplicity of proliferative lesions as compared to the controls at either time point. In the dextrin group, the total number of ACF at week ten was significantly increased. With I3C, the total number of ACF and incidence and multiplicities of adenocarcinomas at week ten and the incidence of invasive tumors at week twenty were significantly increased. These data essentially correspond with earlier reported results, except in the vegetable oil cases. Thus, the system is suitable for detection of colorectal carcinogenesis modifiers with advantages over previous models using ACF alone as end points.

Phytochemical regulation of UDP-glucuronosyltransferases: implications for cancer prevention

Uridine 5'-diphospho-glucuronosyltransferases (UGTs) are Phase II biotransformation enzymes that metabolize endogenous and exogenous compounds, some of which have been associated with cancer risk. Many phytochemicals have been shown to induce UGTs in humans, rodents, and cell culture systems. Because UGTs maintain hormone balance and facilitate excretion of potentially carcinogenic compounds, regulation of their expression and activity may affect cancer risk. Phytochemicals regulate transcription factors such as the nuclear factor-erythroid 2-related factor 2 (Nrf2), aryl hydrocarbon, and pregnane X receptors as well as proteins in several signal transduction cascades that converge on Nrf2 to stimulate UGT expression. This induction can be modified by several factors, including phytochemical dose and bioavailability and interindividual variation in enzyme expression. In this review, we summarize the knowledge of dietary modulation of UGTs, particularly by phytochemicals, and discuss the potential mechanisms by which phytochemicals regulate UGT transcription.

Comparison of white tea, green tea, epigallocatechin-3-gallate, and caffeine as inhibitors of PhIP-induced colonic aberrant crypts

There is growing interest in the possible health benefits of tea. We reported previously on the inhibition by white tea of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced colonic aberrant crypt foci (ACF) in the rat (4). To distinguish between blocking and suppressing effects, and thus provide mechanistic insights into prevention during the initiation versus post-initiation phases of carcinogenesis, white tea, and green tea were administered at 2% (w/v) as the sole source of drinking fluid either 2 wk before and 2 wk during PhIP dosing (100 mg/kg, every other day by oral gavage), or starting 1 wk after the carcinogen and continued until the study was terminated at 16 wk. In the former protocol, each tea produced marginal inhibition of colonic ACF, despite evidence for changes in several hepatic enzymes involved in heterocyclic amine metabolism. Post-initiation, however, the data were as follows (ACF/colon, mean +/- SE): PhIP/water 12.2 +/- 1.5; PhIP/white tea 5.9 +/- 0.9 (** P < 0.01); PhIP/caffeine 5.9 +/- 1.5 (** P < 0.01); PhIP/EGCG 3.5 +/- 0.8 (***P < 0.001); PhIP/green tea 8.9 +/- 1.2 (P = 0.22, not significant). In the latter study, apoptosis was determined using in situ oligo ligation and cleaved caspase-3 assays, whereas cell proliferation was assessed via bromodeoxyuridine (BrdU) incorporation. No consistent changes were seen in apoptosis assays, but BrdU labeling was as follows (percent of cells positive/colonic crypt, mean +/- SE): PhIP/water 10.4 +/- 0.6; PhIP/white tea 8.6 +/- 0.2 (*P < 0.05); PhIP/EGCG 6.0 +/- 0.85 (** P < 0.01); PhIP/caffeine 8.75 +/- 0.45 (*P < 0.05); PhIP/green tea 9.5 +/- 0.4 (P > 0.05, not significant). The data imply that white tea, caffeine, and EGCG may be most effective post-initiation, via the inhibition of cell proliferation in the colon and through the suppression of early lesions.

Inhibition of carcinogenesis by tea constituents

The possible cancer preventive activity of tea has received much attention in recent years. The inhibitory activities of tea and tea constituents against carcinogenesis at different organ sites have been demonstrated in many animal models. The effect of tea consumption on human cancers, however, remains inconclusive. The mechanisms of action of tea polyphenols, especially EGCG, the most abundant and active catechin, have been extensively investigated. Most of the studies, however, were based on cell culture systems, and these mechanisms need to be evaluated and verified in animal models or humans in order to gain more understanding on the effect of tea consumption on human cancer. Human intervention trials are warranted to determine the possible prevention of cancer of specific sites by preparation of tea constituents.

Urinary biomarkers of tea polyphenols and risk of colorectal cancer in the Shanghai Cohort Study

There have been no studies on specific tea polyphenol biomarkers and risk of colorectal cancer in humans. We prospectively examined the associations between validated biomarkers of specific tea polyphenols and risk of developing colorectal cancer among a cohort of 18,244 men in Shanghai, China, with 16 years of follow-up. Epigallocatechin (EGC), 4'-O-methyl-epigallocatechin (4'-MeEGC) and epicatechin, and their metabolites in baseline urine samples were measured on 162 incident colorectal cancer cases and 806 matched controls. Individuals with high prediagnostic urinary EGC levels had a lower risk of colon cancer. Compared with undetectable EGC, odds ratios (95% confidence interval) for colon cancer in the lowest, intermediate and highest tertile of detectable EGC were 0.64 (0.33-1.24), 0.60 (0.30-1.20) and 0.40 (0.19-0.83), respectively (p for trend = 0.02). A similar inverse relation between 4'-MeEGC and colon cancer also was observed. Compared with the lowest quartile, odds ratios (95% confidence intervals) for colon cancer in the 2nd, 3rd and 4th quartiles of urinary 4'-MeEGC were 0.49 (0.25-0.96), 0.32 (0.16-0.67) and 0.41 (0.20-0.84), respectively (p for trend = 0.006). The strongest protective effect was seen for regular tea drinkers who showed high levels of urinary EGC and 4'-MeEGC. No association between urinary levels of epicatechin or its metabolite and colon cancer risk was observed. Urinary levels of tea polyphenols and their metabolites were not associated with rectal cancer risk. The present study supports the notion of tea catechins as chemopreventive agents against the development of colon cancer in humans.

Gene expression analysis during tumor enhancement by the dietary phytochemical, 3,3'-diindolylmethane, in rainbow trout

Indole-3-carbinol (I3C) and 3,3'-diindolylmethane (DIM), a primary I3C derivative, are known dietary chemopreventive agents also available as supplements. However, I3C has been found to act as a tumor promoter in rat (multi-organ) and trout (liver) models. I3C and DIM were previously found to be estrogenic in trout liver based on toxicogenomic profiles. In this study, we compare the post-initiation effects of DIM and 17beta-estradiol (E2) on aflatoxin B(1) (AFB(1))-induced hepatocarcinogenesis in trout. Trout were initiated as embryos with AFB(1) and juvenile fish were fed diets containing 0, 120 or 400 p.p.m. DIM or 5 p.p.m. E2 for 18 weeks. Tumor incidence was determined at 13 months and found to be significantly elevated in AFB(1)-initiated trout fed either 400 p.p.m. DIM or 5 p.p.m. E2 compared with control animals. To evaluate the mechanism of tumor enhancement, hepatic gene expression profiles were examined in animals fed promotional diets during the course of tumorigenesis and in hepatocellular carcinomas (HCCs) of initiated animals. We demonstrate that DIM alters gene expression profiles similar to E2 in liver samples during tumorigenesis and in HCC tumors. Further, HCCs from animals on DIM and E2 promotional diets had a transcriptional signature indicating decreased invasive or metastatic potential compared with HCCs from control animals. Overall, these findings are the first to demonstrate tumor promotion by DIM. They confirm the importance of estrogenic signaling in the mechanism of promotion by dietary indoles in trout liver and indicate a possible dual effect that enhances tumor incidence and decreases potential for metastasis.

Tea and cancer prevention: molecular mechanisms and human relevance

Tea made from the leaves of the plant Camellia sinensis is a popular beverage. The possible cancer-preventive activity of tea and tea polyphenols has been studied extensively. This article briefly reviews studies in animal models, cell lines, and possible relevance of these studies to the prevention of human cancer. The cancer-preventive activity of tea constituents have been demonstrated in many animal models including cancer of the skin, lung, oral cavity, esophagus, stomach, liver, pancreas, small intestine, colon, bladder, prostate, and mammary gland. The major active constituents are polyphenols, of which (-)-epigallocatechin-3-gallate (EGCG) is most abundant, most active, and most studied, and caffeine. The molecular mechanisms of the cancer-preventive action, however, are just beginning to be understood. Studies in cell lines led to the proposal of many mechanisms on the action of EGCG. However, mechanisms based on studies with very high concentrations of EGCG may not be relevant to cancer prevention in vivo. The autooxidation of EGCG in cell culture may also produce activities that do not occur in many internal organs. In contrast to the cancer prevention activity demonstrated in different animal models, no such conclusion can be convincingly drawn from epidemiological studies on tea consumption and human cancers. Even though the human data are inconclusive, tea constituents may still be used for the prevention of cancer at selected organ sites if sufficient concentrations of the agent can be delivered to these organs. Some interesting examples in this area are discussed.

Cruciferous vegetables and colo-rectal cancer

Cruciferous vegetables have been studied extensively for their chemoprotective effects. Although they contain many bioactive compounds, the anti-carcinogenic actions of cruciferous vegetables are commonly attributed to their content of glucosinolates. Glucosinolates are relatively biologically inert but can be hydrolysed to a range of bioactive compounds such as isothiocyanates (ITC) and indoles by the plant-based enzyme myrosinase, or less efficiently by the colonic microflora. A number of mechanisms whereby ITC and indoles may protect against colo-rectal cancer have been identified. In experimental animals cruciferous vegetables have been shown to inhibit chemically-induced colon cancer. However, the results of recent epidemiological cohort studies have been inconsistent and this disparity may reflect a lack of sensitivity of such studies. Possible explanations for the failure of epidemiological studies to detect an effect include: assessment of cruciferous vegetable intake by methods that are subject to large measurement errors; the interaction between diet and genotype has not been considered: the effect that post-harvest treatments may have on biological effects of cruciferous vegetables has not been taken into account.

Green tea, black tea and colorectal cancer risk: a meta-analysis of epidemiologic studies

Experimental studies have supported tea as a chemopreventive agent for colorectal cancer. No quantitative summary of the epidemiologic evidence on tea and colorectal cancer risk has ever been performed. The current meta-analysis included 25 papers conducted in 11 countries across three continents (North America, Asia and Europe). Summary odds ratios (ORs) for highest versus non/lowest tea consumption levels were calculated based on fixed and random effects models. The meta-regression and stratified methods were used to examine heterogeneity across studies. For green tea, the combined results from eight studies indicated a reduced risk of colorectal cancer with intake [summary OR = 0.82, 95% confidence interval (CI) = 0.69-0.98]. The protective effect is mainly found among the three case-control studies of colon cancer (summary OR = 0.74, 95% CI = 0.60-0.93). Results from studies of rectal cancer irrespective of study design (case-control versus cohort) (summary OR = 0.99, 95% CI = 0.71-1.37) and cohort studies of colon cancer (summary OR = 0.99, 95% CI = 0.79-1.24) were compatible with the null hypothesis. For black tea, the summary OR derived from 20 studies was 0.99 (95% CI = 0.87-1.13). There is wide divergence in results across the 20 individual studies; formal tests for homogeneity across studies revealed statistically significant differences in findings across all studies (P < 0.001), amongst the 7 cohort studies (P = 0.002), and amongst the 13 case-control studies (P < 0.001). Despite the strong evidence from in vitro and non-human in vivo studies in support of green and black tea as potential chemopreventive agents against colorectal cancer, available epidemiologic data are insufficient to conclude that either tea type may protect against colorectal cancer in humans.

Ultraviolet radiation-induced non-melanoma skin cancer in the Crl:SKH1:hr-BR hairless mouse: augmentation of tumor multiplicity by chlorophyllin and protection by indole-3-carbinol

Over 1 million new cases of ultraviolet radiation-induced non-melanoma skin cancers (NMSC) per year now occur in the USA and the incidence of these diseases continues to increase. New preventative strategies are required. The hypothesis tested was that dietary administration of the putative cancer chemopreventatives sodium-copper-chlorophyllin (Chlor) or indole-3-carbinol (I3C) would inhibit UV-induced skin carcinogenesis in the Crl:SKH1:hr-BR hairless mouse. Groups of 20 mice were pre-fed isocaloric/isonutritive 20% corn-oil AIN-76a based diets that contained either Chlor (1.52 g%), I3C (5.08 g%) or no chemopreventative (control) for 2 weeks followed by exposure of their dorsal skin to a 10 week incremental, sub-erythemal, carcinogenic simulated solar UV exposure regime. Feeding was continued for the duration of the experiment. Matched non-UV exposed dietary groups were also included in the experimental design. The diets had no significant (p > 0.05) effect on body weight, feed consumption, cutaneous methanol-extractable UV photoprotective substances or on cutaneous UV-reflective characteristics. By day 180, UV-irradiated mice fed the Chlor had a significantly (p < 0.05) higher tumor multiplicity (33.6 +/- 4.72; mean +/- SEM) than UV-irradiated control animals (22.8 +/- 4.25). UV-irradiated mice fed I3C had a significantly (p < 0.001) lower tumor multiplicity (13.0 +/- 2.42) than that of both the UV-irradiated control and UV-irradiated Chlor-fed mice. The Chlor or I3C diets did not significantly (p > 0.05) affect UV-induced systemic suppression of contact hypersensitivity responses. These results demonstrate augmentation of the UV-induced cutaneous carcinogenic process by dietary chlorophyllin and protection from this carcinogenic process by indole-3-carbinol via mechanisms that do not involve changes in skin optical properties, modulation of photoimmunosuppression or caloric/nutrient effects.

Effects of indole-3-carbinol and phenethyl isothiocyanate on colon carcinogenesis induced by azoxymethane in rats

Indole-3-carbinol (I3C) and phenethyl isothiocyanate (PEITC) are breakdown products of the glucosinolates glucobrassicin and gluconasturtiin, respectively, and are thought to reduce carcinogen activation by P450 enzymes. To assess the effects of these compounds on colon cancer risk, rats were divided into five groups and fed the following diets: control diet (AIN-93G), or diets with PEITC or I3C added to the control diet: high-PEITC (3.37 mmols/kg diet-high level of PEITC), low-PEITC (0.67 mmols/kg-low level of PEITC), high-I3C (6.8 mmols/kg-high level of I3C) and low-I3C (1.36 mmols/kg-low level of I3C). Diets were fed for 2 weeks before and 10 weeks after administration of the colon carcinogen azoxymethane. Precancerous lesion (aberrant crypt foci, ACF) number in the distal colon was significantly lower in both high-I3C and low-I3C groups (6.9 +/- 0.8 and 5.9 +/- 0.59 per cm2, respectively) when compared with the control group (10.4 +/- 0.9). No significant difference in ACF number was found between the PEITC group and the control group. ACF expressing sialomucin, thought to indicate ACF more likely to progress to tumors, were greater in the high-PEITC group (13 +/- 3) than the control (5.6 +/- 2). Mucin-depleted ACF, suggested to have the greatest tumorigenic potential, tended to be lower in the low-I3C group (P < 0.06) compared with the control group. Mucosal apoptotic and cell proliferation labeling indices did not differ among groups, suggesting that reduction in the ACF number by I3C does not involve alterations in mucosal cell kinetics. No significant differences were found among the groups in hepatic cytochrome P450 2E1 (CYP2E1) activity, the first enzyme involved in activation of azoxymethane. However, there was increased activity of NADPH- and NADH reductases with high-I3C, which are the enzymes involved in the transfer of reducing equivalents to cytochrome P450. These results suggest that I3C lowers colon cancer risk through a mechanism not involving reduction of carcinogen activation by CYP2E1.

Inhibition of PhIP-induced mammary carcinogenesis in female rats by ingestion of freeze-dried beer

This study evaluated the modulating effect of non-alcoholic constituents of beer on 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced mammary carcinogenesis. Female Sprague-Dawley (SD) rats at 6 weeks of age were divided into four groups (n=26-30) and fed either a high fat diet or high fat diets containing 1, 2 or 4% freeze-dried beer (FD beer). One week after the start of feeding, rats received PhIP at a dose of 85 mg/kg by gavage four times weekly for 2 weeks. There were no differences in the body weights or diet intakes of rats between the control and the experimental groups. Weekly observation of palpable tumors indicated that tumor incidence and tumor multiplicity in the 2 and 4% FD beer groups were lower than in the control group throughout the experiment. Neoplastic lesions were pathologically examined at the end of the 22-weeks experiment. Tumor development was inhibited by FD beer intake in a dose-dependent manner. Tumor incidence (38.5%) and tumor multiplicity (0.8+/-0.4) for the group fed with a diet containing 4% FD were significantly reduced as compared with the control group (73.3% and 1.8+/-0.7). Supplementation with FD beer for 3 weeks together with the PhIP treatments resulted in increased liver GST activity, decreased liver CYP1A2 activity and a decrease in the number of DNA adducts in the mammary tissue, though these values were not significant. In conclusion, our results suggest that intake of FD beer may reduce the risk of carcinogenesis caused by heterocyclic amines.

Indole-3-carbinol suppresses NF-kappaB and IkappaBalpha kinase activation, causing inhibition of expression of NF-kappaB-regulated antiapoptotic and metastatic gene products and enhancement of apoptosis in myeloid and leukemia cells

Indole-3-carbinol, found in Brassica species vegetables (such as cabbage, cauliflower, and brussels spouts), exhibits antitumor effects through poorly defined mechanisms. Because several genes that regulate apoptosis, proliferation, and metastasis are regulated by nuclear factor-kappaB (NF-kappaB), we postulated that indole-3-carbinol must mediate its activity through NF-kappaB modulation. We demonstrated that indole-3-carbinol suppressed constitutive NF-kappaB activation and activation induced by tumor necrosis factor (TNF), interleukin-1beta (IL-1beta), phorbol 12-myristate 13-acetate (PMA), lipopolysaccharide (LPS), and cigarette smoke; the suppression was not cell type specific, because activation was inhibited in myeloid, leukemia, and epithelial cells. This activation correlated with the sequential suppression of the IkappaBalpha kinase, IkappaBalpha phosphorylation, IkappaBalpha ubiquitination, IkappaBalpha degradation, p65 phosphorylation, p65 nuclear translocation, p65 acetylation, and NF-kappaB-dependent reporter gene expression. The NF-kappaB-regulated gene products cyclin D1, cyclooxygenase-2 (COX-2), matrix metalloproteinase-9 (MMP-9), survivin, inhibitor-of-apoptosis protein-1 (IAP1), IAP2, X chromosome-linked IAP (XIAP), Bcl-2, Bfl-1/A1, TNF receptor-associated factor-1 (TRAF1), and Fas-associated death domain protein-like interleukin-1beta-converting enzyme inhibitory protein (FLIP) were all down-regulated by indole-3-carbinol. This down-regulation led to the potentiation of apoptosis induced by cytokines and chemotherapeutic agents. Indole-3-carbinol suppressed constitutive NF-kappaB activation in mononuclear cells derived from bone marrow of acute myelogenous leukemia patients, and this correlated with inhibition of cell growth. Overall, our results indicated that indole-3-carbinol inhibits NF-kappaB and NF-kappaB-regulated gene expression and that this mechanism may provide the molecular basis for its ability to suppress tumorigenesis.

Effect of Black Tea Intake on the Excretion of Mutagens in the Urine of Volunteers Taking a Beef Meal

The objective of this study was to investigate in a crossover study conducted in human volunteers whether black tea intake modulates the metabolism of heterocyclic amines, consumed in the form of well-cooked beefburgers, as exemplified by the excretion of mutagens in the urine. Mutagens were extracted from urine with blue rayon, and mutagenic activity was determined in the Ames test, in the presence of an activation system derived from Aroclor 1254–induced rats, and employing the Salmonella typhimurium O-acetylase over-expressing YG1024 bacterial strain. Volunteers consumed three well-cooked beefburgers, whereas a concurrently cooked fourth burger was analyzed for mutagenic activity. Following intake of the burgers, an increase in urinary mutagenic activity was observed, and mutagenic activity was completely excreted within 24 hours. A good correlation was obtained between the intake and excretion of mutagenic activity. The volunteers consumed the same burger meal on two different occasions, once following intake of 10 cups of strong black tea, and the second following intake of a corresponding volume of water. Urine was collected by each volunteer for 24 hours after the meal, and compliance was ascertained utilizing the excretion of p-aminobenzoic acid. The mutagenic ratio, defined as the ratio of urinary mutagenicity over the intake of mutagenicity, was not altered by the black tea, being 0.142 and 0.135 during the water and tea intake, respectively. It is inferred that short-term intake of high levels of black tea, as part of an otherwise normal diet, does not modulate the human metabolism of heterocyclic amines.

Effect of common Brassica vegetables (Brussels sprouts and red cabbage) on the development of preneoplastic lesions induced by 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) in liver and colon of Fischer 344 rats

Aim of the present study was the investigation of effects of juices from commonly consumed Brassica vegetables (two cultivars of Brussels sprouts and two cultivars of red cabbage) on formation and development of preneoplastic lesions in colons (aberrant crypt foci, ACF) and livers (glutathione-S-transferase placental form, GST-P+) in male F344 rats. The foci were induced by 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), a widespread carcinogenic heterocyclic aromatic amine which is found in fried meats. Recently, we reported on pronounced protective effects in the two-organ foci model when the vegetable juices were given during the carcinogen treatment but several findings by other groups indicated that breakdown products of glucosinolates contained in Brassica vegetables cause tumour promotion in various organs of laboratory rodents. In the present study, the animals received the juices in the drinking water (5%) over a period of 20 days after treatment with IQ (100 mg/kg bw on 10 alternate days). To increase the foci yield (which facilitates the detection of modifying effects), the animals were fed with a modified (high fat, fibre free) AIN-76 diet. With exception of the sprout variety "Cyrus", all juices lowered the number of GST-P+ foci as well as the foci area in the liver, but none of these effects was statistically significant. In the colon, none of the juices had an impact on crypt multiplicity (number of crypts/focus), whereas the number of ACF was decreased; only with the sprout variety Maximus the protective effect was significant (reduction 49%). The present findings show that administration of vegetable juices to the animals after the carcinogen does not increase the number and size of IQ-induced preneoplastic lesions in liver and colon.

Heterocyclic amines: Mutagens/carcinogens produced during cooking of meat and fish

Research leading to the discovery of a series of mutagenic and carcinogenic heterocyclic amines (HCAs) was inspired by the idea that smoke produced during cooking of food, especially meat or fish, might be carcinogenic. More than ten kinds of HCAs, actually produced by cooking or heating of meat or fish, have now been isolated and their structures determined, most being previously unregistered compounds. They are highly mutagenic towards Salmonella typhimurium in the presence of S9 mix and are also mutagenic in vitro and in vivo toward mammalian cells. HCAs have now been chemically synthesized in quantity and subjected to long-term animal testing. When HCAs were fed in the diet, rodents developed cancers in many organs, including the colon, breast and prostate, and one HCA produced hepatomas in monkeys. The lesions exhibited alteration in genes including Apc, beta-catenin and Ha-ras, and these changes provide clues to the induction mechanisms. The HCAs are oxidized to hydroxyamino derivatives by cytochrome P450s, and further converted to ester forms by acetyltransferase and sulfotransferase. Eventually, they produce DNA adducts through the formation of N-C bonds at guanine bases. There are HCA-sensitive and resistant strains of rodents and a search for the responsible genes is now under way. While the content of HCAs in dishes consumed in ordinary life is low and not sufficient in itself to explain human cancer, the coexistence of many other mutagens/carcinogens of either autobiotic or xenobiotic type and the possibility that HCAs induce genomic instability and heightened sensitivity to tumor promoters suggest that avoidance of exposure to HCAs or reduction of HCAs' biological effects as far as possible are to be highly recommended. Usage of microwave ovens for cooking and supplementation of the diet, for example with soy-isoflavones, which have been found to suppress the occurrence of HCA-induced breast cancers, should be encouraged. Advice to the general public about how to reduce the carcinogenic load imposed by HCAs would be an important contribution to cancer prevention.

Inhibitory effects of beer on heterocyclic amine-induced mutagenesis and PhIP-induced aberrant crypt foci in rat colon

Anti-mutagenic and anti-carcinogenic effects of beer on heterocyclic amine (HCA)-induced carcinogenesis were studied in vitro and in vivo. Four commercial beers (two pilsner-type, black, and stout) showed inhibitory effects against five HCAs, 2-amino-3,8-dimethylimidazo [4,5-f]quinoxaline (MeIQx), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2), 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1) and 2-amino-3-methylimidazo[4,5-f]-quinoline (IQ), in the Ames assay using Salmonella typhimurium TA98 in the presence of rat S9 mix. The inhibitory effects of dark-colored beers (stout and black beer) were greater than those of pilsner-type beers. Dark-colored beers suppressed CYP1A2 activity in a dose-dependent manner, suggesting that inhibition of HCA activation is partly responsible for their strong anti-mutagenic effects. Anti-mutagenic effects were also observed when the pooled human S9 mix or activated IQ was used in the assay. The micronucleus test using Chinese hamster lung CHL/IU cells showed that the addition of freeze-dried samples of pilsner-type and stout beer to the culture medium significantly reduced the number of cells with micronuclei induced with PhIP or Trp-P-2. Single-cell gel electrophoresis assay (comet assay) revealed that oral ingestion of pilsner-type and stout beers for 1 week significantly inhibited DNA damage in the liver cells of male ICR mice exposed to MeIQx (13 mg/kg, i.p.). A decrease in the formation of DNA adducts was also observed using a 32P-postlabeling method. Male Fischer 344 rats orally received PhIP (75 mg/kg, five times a week for 2 weeks) and aberrant crypt foci (ACF) formation in the colon was analyzed after 5 weeks. The number of ACF was significantly reduced in rats fed a diet containing freeze-dried beer. These results suggest that beer inhibits the genotoxic effects of HCAs and may reduce the risk of carcinogenesis caused by food borne carcinogens.

Aberrant crypt foci and beta-catenin accumulated crypts; significance and roles for colorectal carcinogenesis

Preneoplastic or precancerous lesions in the large bowel have been characterized in terms of morphology and histochemical phenotype. However, the detailed histogenesis and relation of particular lesions to malignancies has not yet to be unequivocally clarified. Aberrant crypt foci (ACF), identified in whole-mount preparations of colonic mucosa in rodents and also recognized in human colon, are now frequently used as effective surrogate biomarkers for experimentally detection of chemopreventive agents against colorectal cancers, but the preneoplastic or precancerous nature of ACF in rodents and humans still remains inconclusive. Relatively recently, early appearing beta-catenin accumulated crypts (BCAC) have been described in en face preparations of colonic mucosa in rodents which differ from ACF in many features. BCAC are suggested to be premalignant rather than preneoplastic. The pathological significance of both lesions, including their advantages and disadvantages as surrogate end points for large bowel neoplasms, and roles in colorectal carcinogenesis are discussed here.

An improved method for the extraction of mutagens from human urine and cooked meat using blue rayon

A reproducible method has been developed and validated that allows the detection of mutagenic material in human urine following the intake of a meal containing pan-fried beef patties. The mutagens are extracted from the urine with blue rayon and eluted with methanol/ammonia (100:1). Using 14C-2-amino-3-methylimidazo[4,5-f]quinoline (14C-IQ) as a tracer, the extraction efficiency of heterocyclic amines was consistently found to exceed 90%. It was observed that the critical factor in attaining high extraction efficiencies was the concentration of blue rayon, which depended on the nature of the matrix. Constituents of human urine were demonstrated to compete with IQ for the binding sites on the blue rayon. For the determination of the mutagenic activity in the extracted urine, the O-acetylase over-expressing Salmonella typhimurium strain YG1024 was utilised, and the Ames test procedure was optimised with respect to S9 concentration and incorporation of a pre-incubation step. Using the developed procedure, a marked rise in mutagenic activity, determined both in the absence and presence of an activation system, was evident in human urine following the intake of a beef patty meal. Normal mutagenicity levels were restored within 24 h. Finally, a method is described, once again employing blue rayon, for the extraction of heterocyclic amines from cooked beef patties with a 92% efficiency, as exemplified by IQ.

Use of antioxidants to minimize the human health risk associated to mutagenic/carcinogenic heterocyclic amines in food

Heterocyclic amines (HAs) are mutagenic/carcinogenic compounds formed in meat during cooking. Several efforts have been made to minimize the risk associated to HA human exposure. Supplementation with antioxidants is considered a promising measure to reduce HA exposure because of their ability as inhibitors of HA formation or as blocking/suppressing agents on HA biotransformation/metabolism. The aim of this review is to present the current knowledge on the capability of synthetic and natural antioxidants to modulate HA-induced mutagenicity/carcinogenicity. Data show a general trend towards a reduction of HA formation both in model systems and in real foods as well as an effective modulation of biotransformation and metabolism. Phenolic compounds, particularly those from tea and olive oil, seem to be the most effective, although a great variability is observed because of the concentration-dependent pro- and antioxidant effects.

Antimutagenic activity of spearmint

The antimutagenic activity of spearmint (Mentha spicata), a popular food flavoring agent, was studied in the Salmonella assay. Spearmint leaves were brewed in hot water for 5 min at concentrations up to 5% (w/v), and the water extracts were tested against the direct-acting mutagens 4-nitro-1,2-phenylenediamine (NPD) and 2-hydroxyamino-3-methyl-3H-imidazo[4,5-f]quinoline (N-OH-IQ) using Salmonella typhimurium strain TA98. Nontoxic concentrations of spearmint extract inhibited the mutagenic activity of N-OH-IQ in a concentration-dependent fashion, but had no effect against NPD. These experiments by design focused on the water extract consumed commonly as an herbal tea, but chloroform and methanol extracts of spearmint also possessed antimutagenic activity against N-OH-IQ. Water extract of spearmint inhibited the mutagenic activity of the parent compound, 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline (IQ), in the presence of rat liver S9; however, the concentration for 50% inhibition (IC50) against IQ was approximately 10-fold higher than in assays with N-OH-IQ minus S9. At concentrations similar to those used in the Salmonella assays, spearmint extract inhibited two of the major enzymes that play a role in the metabolic activation of IQ, namely, cytochromes P4501A1 and 1A2, based on ethoxyresorufin O-deethylase and methoxyresorufin O-demethylase assays in vitro. In vivo, rats were given spearmint water extract (2%; w/v) as the sole source of drinking fluid before, during, and after 2-week treatment with IQ; colonic aberrant crypt foci were inhibited significantly at 8 weeks (P < 0.05, compared with rats given IQ alone). Collectively, these findings suggest that spearmint tea protects against IQ and possibly other heterocyclic amines through inhibition of carcinogen activation and via direct effects on the activated metabolite(s).

Antimutagenic activity of tea: role of polyphenols

PURPOSE OF REVIEW

Tea is considered to be one of the most promising dietary chemopreventive agents and, consequently, it is being studied extensively worldwide. Despite the fact that tea has proved very efficient in affording protection against chemical-induced cancer in animal models of the disease, epidemiological studies do not always support the laboratory findings, so that the value of tea as a human anticarcinogen may be considered as 'not proven'. A major mechanism of the anticarcinogenic activity of tea in animals is impairment of the interaction of carcinogens with DNA leading to mutations. The antimutagenic activity of tea as well as the underlying mechanisms will be reviewed, and the role of polyphenols, the postulated bioactive components, and caffeine will be critically evaluated.

RECENT FINDINGS

In rats, exposure to tea modulated the disposition of heterocyclic amines, a major group of food-borne carcinogens, stimulating the pathways that lead to deactivation, and this is concordant with the established ability of tea to modulate the carcinogen-metabolizing enzyme systems. These observations provide a rational mechanism for the anticarcinogenic activity of tea in animals.

SUMMARY

The beneficial activities of tea have always been attributed to the polyphenols, as these are present in tea at substantial concentrations and are endowed with antioxidant activity. It is becoming increasingly evident, however, that the bioavailability of these compounds is poor as a result of limited absorption and presystemic metabolism by mammalian and microbial enzymes. We propose that the biological activity of tea may be mediated by caffeine and microbial metabolites of polyphenols.

The disposition and metabolism of 2-amino-3-methylimidazo-[4,5-f]quinoline in the F344 rat at high versus low doses of indole-3-carbinol

Indole-3-carbinol (I3C), a compound found in cruciferous vegetables, inhibits the formation of DNA adducts, colonic aberrant crypts, and tumors in rats given heterocyclic amines, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Previous mechanism studies indicated that I3C induces cytochromes P4501A1 (CYP1A1) and CYP1A2, as well as phase 2 pathways, leading to enhanced metabolism and excretion of IQ. However, the chemopreventive activity is dependent on the dose of I3C, and at low doses which do not induce CYP1A activity, there is evidence for increased IQ-DNA adduct formation in vivo. The present study examined the fate of IQ in the rat and the profile of urinary metabolites across a broad range of I3C doses. Male F344 rats were given a single injection of I3C by oral gavage, at a dose equivalent to that received from a single daily exposure to 0, 5, 10, 25, 50, 100, 200, 500 or 1000 ppm I3C in the diet, or they were given the 1000-ppm-equivalent dose of I3C for 14 consecutive days. Subsequently, each rat was given 14C-labeled IQ (5 mg/kg; 0.1 mCi/kg) and the animal was sacrificed 8 h later. With increasing I3C, there was a dose-dependent decrease in IQ-associated radiolabel in several systemic tissues, and an increase in the radiolabel eliminated via the feces. In the urine, there was a dose-dependent increase in IQ-5-O-glucuronide and IQ-5-O-sulfate metabolites, and a concomitant decrease in the IQ-sulfamate at intermediate and high doses of I3C. However, 5- and 10 ppm-equivalent doses of I3C enhanced the levels of IQ-sulfamate compared with controls, possibly due to the high ratio of hepatic CYP1A2 versus CYP1A1 activities at these I3C doses. The possible significance of the low versus high dose effects are discussed in the context of ongoing clinical trials with I3C and the reported chemopreventive mechanisms in vivo.

Bax translocation to mitochondria is an important event in inducing apoptotic cell death by indole-3-carbinol (I3C) treatment of breast cancer cells

Indole-3-carbinol (I3C), a natural component of Brassica vegetables, has been found to be a promising cancer preventive agent. However, the precise molecular mechanism(s) by which I3C exerts its inhibitory effects on cancer cells has not been fully elucidated. We investigated the molecular mechanism of action of I3C during apoptotic processes in breast epithelial cells. Nontumorigenic and tumorigenic breast epithelial cells were exposed to I3C, and growth inhibition, apoptosis and expression of genes involved in apoptotic processes were measured. Translocation of Bax to the mitochondria was accessed by confocal imaging. Mitochondrial potential and cytochrome c release also were measured. We found that I3C inhibited the growth of breast cancer cells and induced apoptosis in these cells, concomitant with upregulation of Bax, and downregulation of Bcl-2. I3C induced translocation of Bax to the mitochondria in both tumorigenic and nontumorigenic cells, but concomitant loss of mitochondrial potential, release of cytochrome c and induction of apoptosis were observed only in cancer cells. In conclusion, I3C exerts its effects by regulating cell cycle and by altering the expression of genes involved in apoptotic pathway. The translocation of Bax to the mitochondria alone is not sufficient during I3C-induced apoptosis. Translocation of Bax followed by mitochondrial depolarization and cytochrome c release is necessary, which may be responsible for selective induction of apoptosis in cancer cells, supporting the potential preventive and/or therapeutic benefit of I3C against cancers.