Organ-dependent modifying effects of caffeine, and two naturally occurring antioxidants alpha-tocopherol and n-tritriacontane-16,18-dione, on 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced mammary and colonic carcinogenesis in female F344 rats

Animal Models of Colorectal Cancer: From Spontaneous to Genetically Engineered Models and Their Applications

Colorectal cancer is one of the most common gastrointestinal malignancies in humans, affecting approximately 1.8 million people worldwide. This disease has a major social impact and high treatment costs. Animal models allow us to understand and follow the colon cancer progression; thus, in vivo studies are essential to improve and discover new ways of prevention and treatment. Dietary natural products have been under investigation for better and natural prevention, envisioning to show their potential. This manuscript intends to provide the readers a review of rodent colorectal cancer models available in the literature, highlighting their advantages and disadvantages, as well as their potential in the evaluation of several drugs and natural compounds’ effects on colorectal cancer.

Dietary natural methylxanthines and colorectal cancer: a systematic review and meta-analysis

Some evidence suggests that caffeine, theophylline, and theobromine, as natural methylxanthines (MTXs), possess anti-cancer effects.

Drinking for protection? Epidemiological and experimental evidence on the beneficial effects of coffee or major coffee compounds against gastrointestinal and liver carcinogenesis

Recent meta-analyses indicate that coffee consumption reduces the risk for digestive tract (oral, esophageal, gastric and colorectal) and, especially, liver cancer. Coffee bean-derived beverages, as the widely-consumed espresso and "common" filtered brews, present remarkable historical, cultural and economic importance globally. These drinks have rich and variable chemical composition, depending on factors that vary from "seeding to serving". The alkaloids caffeine and trigonelline, as well as the polyphenol chlorogenic acid, are some of the most important bioactive organic compounds of these beverages, displaying high levels in both espresso and common brews and/or increased bioavailability after consumption. Thus, we performed a comprehensive literature overview of current knowledge on the effects of coffee beverages and their highly bioavailable compounds, describing: 1) recent epidemiological and experimental findings highlighting the beneficial effects against gastrointestinal/liver carcinogenesis, and 2) the main molecular mechanisms in these in vitro and in vivo bioassays. Findings predominantly address the protective effects of coffee beverages and their most common/bioavailable compounds individually on gastrointestinal and liver cancer development. Caffeine, trigonelline and chlorogenic acid modulate common molecular targets directly implicated in key cancer hallmarks, what could stimulate novel translational or population-based mechanistic investigations.

Natural Forms of Vitamin E as Effective Agents for Cancer Prevention and Therapy

Initial research on vitamin E and cancer has focused on α-tocopherol (αT), but recent clinical studies on cancer-preventive effects of αT supplementation have shown disappointing results, which has led to doubts about the role of vitamin E, including different vitamin E forms, in cancer prevention. However, accumulating mechanistic and preclinical animal studies show that other forms of vitamin E, such as γ-tocopherol (γT), δ-tocopherol (δT), γ-tocotrienol (γTE), and δ-tocotrienol (δTE), have far superior cancer-preventive activities than does αT. These vitamin E forms are much stronger than αT in inhibiting multiple cancer-promoting pathways, including cyclo-oxygenase (COX)- and 5-lipoxygenase (5-LOX)-catalyzed eicosanoids, and transcription factors such as nuclear transcription factor κB (NF-κB) and signal transducer and activator of transcription factor 3 (STAT3). These vitamin E forms, but not αT, cause pro-death or antiproliferation effects in cancer cells via modulating various signaling pathways, including sphingolipid metabolism. Unlike αT, these vitamin E forms are quickly metabolized to various carboxychromanols including 13'-carboxychromanols, which have even stronger anti-inflammatory and anticancer effects than some vitamin precursors. Consistent with mechanistic findings, γT, δT, γTE, and δTE, but not αT, have been shown to be effective for preventing the progression of various types of cancer in preclinical animal models. This review focuses on cancer-preventive effects and mechanisms of γT, δT, γTE, and δTE in cells and preclinical models and discusses current progress in clinical trials. The existing evidence strongly indicates that these lesser-known vitamin E forms are effective agents for cancer prevention or as adjuvants for improving prevention, therapy, and control of cancer.

Prevention and treatment of cancers by immune modulating nutrients

Epidemiological and laboratory data support the protective effects of bioactive nutrients in our diets for various diseases. Along with various factors, such as genetic history, alcohol, smoking, exercise, and dietary choices play a vital role in affecting an individual's immune responses toward a transforming cell, by either preventing or accelerating a neoplastic transformation. Ample evidence suggests that dietary nutrients control the inflammatory and protumorigenic responses in immune cells. Immunoprevention is usually associated with the modulation of immune responses that help in resolving the inflammation, thus improving clinical outcome. Various metabolic pathway-related nutrients, including glutamine, arginine, vitamins, minerals, and long-chain fatty acids, are important components of immunonutrient mixes. Epidemiological studies related to these substances have reported different results, with no or minimal effects. However, several studies suggest that these nutrients may have immune-modulating effects that may lower cancer risk. Preclinical studies submit that most of these components may provide beneficial effects. The present review discusses the available data, the immune-modulating functions of these nutrients, and how these substances could be used to study immune modulation in a neoplastic environment. Further research will help to determine whether the mechanistic signaling pathways in immune cells altered by nutrients can be exploited for cancer prevention and treatment.

Cancer prevention by different forms of tocopherols

Many epidemiological studies have suggested that a low vitamin E nutritional status is associated with increased cancer risk. However, several recent large-scale human trials with high doses of α-tocopherol (α-T) have produced disappointing results. This points out the need for a better understanding of the biological activities of the different forms of tocopherols. Using a naturally occurring tocopherol mixture (γ-TmT) that is rich in γ-T, we demonstrated the inhibition of chemically induced lung, colon, and mammary cancer formation as well as the growth of xenograft tumors derived from human lung and prostate cancer cells. This broad anticancer activity of γ-TmT has been attributed mainly to the trapping of reactive oxygen and nitrogen species and inhibition of arachidonic acid metabolism. Activation of peroxisome proliferator-activated receptor γ (PPARγ) and the inhibition of estrogen signaling have also been observed in the inhibition of mammary cancer development. δ-T has been shown to be more active than γ-T in inhibiting the growth of human lung cancer cells in a xenograft tumor model and the development of aberrant crypt foci in azoxymethane-treated rats, whereas α-T is not effective in these models. The higher inhibitory activities of δ-T and γ-T (than α-T) are proposed to be due to their trapping of reactive nitrogen species and their capacity to generate side-chain degradation products, which retain the intact chromanol ring structure and could have cancer preventive activities.

Cancer-preventive activities of tocopherols and tocotrienols

The cancer-preventive activity of vitamin E has been studied. Whereas some epidemiological studies have suggested a protective effect of vitamin E against cancer formation, many large-scale intervention studies with alpha-tocopherol (usually large doses) have not demonstrated a cancer-preventive effect. Studies on alpha-tocopherol in animal models also have not demonstrated robust cancer prevention effects. One possible explanation for the lack of demonstrable cancer-preventive effects is that high doses of alpha-tocopherol decrease the blood and tissue levels of delta-tocopherols. It has been suggested that gamma-tocopherol, due to its strong anti-inflammatory and other activities, may be the more effective form of vitamin E in cancer prevention. Our recent results have demonstrated that a gamma-tocopherol-rich mixture of tocopherols inhibits colon, prostate, mammary and lung tumorigenesis in animal models, suggesting that this mixture may have a high potential for applications in the prevention of human cancer. In this review, we discuss biochemical properties of tocopherols, results of possible cancer-preventive effects in humans and animal models and possible mechanisms involved in the inhibition of carcinogenesis. Based on this information, we propose that a gamma-tocopherol-rich mixture of tocopherols is a very promising cancer-preventive agent and warrants extensive future research.

A gamma-tocopherol-rich mixture of tocopherols inhibits colon inflammation and carcinogenesis in azoxymethane and dextran sulfate sodium-treated mice

We investigated the effects of a gamma-tocopherol-rich mixture of tocopherols (gamma-TmT, containing 57% gamma-T, 24% delta-T, and 13% alpha-T) on colon carcinogenesis in azoxymethane (AOM)/dextran sulfate sodium (DSS)-treated mice. In experiment 1, 6-week-old male CF-1 mice were given a dose of AOM (10 mg/kg body weight, i.p.), and 1 week later, 1.5% DSS in drinking water for 1 week. The mice were maintained on either a gamma-TmT (0.3%)-enriched or a standard AIN93M diet, starting 1 week before the AOM injection, until the termination of experiment. In the AOM/DSS-treated mice, dietary gamma-TmT treatment resulted in a significantly lower colon inflammation index (52% of the control) on day 7 and number of colon adenomas (9% of the control) on week 7. gamma-TmT treatment also resulted in higher apoptotic index in adenomas, lower prostaglandin E2, leukotriene B4, and nitrotyrosine levels in the colon, and lower prostaglandin E2, leukotriene B4, and 8-isoprostane levels in the plasma on week 7. Some of the decreases were observed even on day 7. In experiment 2 with AOM/DSS- treated mice sacrificed on week 21, dietary 0.17% or 0.3% gamma-TmT treatment, starting 1 week before the AOM injection, significantly inhibited adenocarcinoma and adenoma formation in the colon (to 17-33% of the control). Dietary 0.3% gamma-TmT that was initiated after DSS treatment also exhibited a similar inhibitory activity. The present study showed that gamma-TmT effectively inhibited colon carcinogenesis in AOM/DSS-treated mice, and the inhibition may be due to the apoptosis-inducing, anti-inflammatory, antioxidative, and reactive nitrogen species-trapping activities of tocopherols.

Protective versus promotional effects of white tea and caffeine on PhIP-induced tumorigenesis and beta-catenin expression in the rat

A 1 year carcinogenicity bioassay was conducted in rats treated with three short cycles of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)/high-fat (HF) diet, followed by 2% white tea (wt/vol), 0.05% epigallocatechin-3-gallate (EGCG) or 0.065% caffeine as sole source of fluid intake. Thirty-two percent of the PhIP/HF controls survived to 1 year, compared with 50, 48.7 and 18.2% in groups given white tea, EGCG and caffeine, respectively. After 1 year, PhIP/HF controls had tumors in the colon, skin, small intestine, Zymbal's gland, salivary gland and pancreas. For all sites combined, excluding the colon, tumor incidence data were as follows: PhIP/HF 69.5%, PhIP/HF + EGCG 48.7%, PhIP/HF + white tea 46.9% and PhIP/HF + caffeine 13.3%. Unexpectedly, a higher incidence of colon tumors was detected in rats post-treated with white tea (69%) and caffeine (73%) compared with the 42% incidence in PhIP/HF controls. In the colon tumors, beta-catenin mutations were detected at a higher frequency after caffeine posttreatment, and there was a shift toward more tumors harboring substitutions of Gly34 with correspondingly high protein and messenger RNA expression seen for both beta-catenin and c-Myc. c-Myc expression exhibited concordance with tumor promotion, and there was a concomitant increase in cell proliferation versus apoptosis in colonic crypts. A prior report described suppression of PhIP-induced colonic aberrant crypts by the same test agents, but did not incorporate a HF diet. These findings are discussed in the context of epidemiological data which do not support an adverse effect of tea and coffee on colon tumor outcome-indeed, some such studies suggest a protective role for caffeinated beverages.

Inhibition of intestinal tumorigenesis in Apcmin/+ mice by (-)-epigallocatechin-3-gallate, the major catechin in green tea

The present study was designed to investigate the effects of two main constituents of green tea, (-)-epigallocatechin-3-gallate (EGCG) and caffeine, on intestinal tumorigenesis in Apc(min/+) mice, a recognized mouse model for human intestinal cancer, and to elucidate possible mechanisms involved in the inhibitory action of the active constituent. We found that p.o. administration of EGCG at doses of 0.08% or 0.16% in drinking fluid significantly decreased small intestinal tumor formation by 37% or 47%, respectively, whereas caffeine at a dose of 0.044% in drinking fluid had no inhibitory activity against intestinal tumorigenesis. In another experiment, small intestinal tumorigenesis was inhibited in a dose-dependent manner by p.o. administration of EGCG in a dose range of 0.02% to 0.32%. P.o. administration of EGCG resulted in increased levels of E-cadherin and decreased levels of nuclear beta-catenin, c-Myc, phospho-Akt, and phospho-extracellular signal-regulated kinase 1/2 (ERK1/2) in small intestinal tumors. Treatment of HT29 human colon cancer cells with EGCG (12.5 or 20 micromol/L at different times) also increased protein levels of E-cadherin by 27% to 58%, induced the translocation of beta-catenin from nucleus to cytoplasm and plasma membrane, and decreased c-Myc and cyclin D1 (20 micromol/L EGCG for 24 hours). These results indicate that EGCG effectively inhibited intestinal tumorigenesis in Apc(min/+) mice, possibly through the attenuation of the carcinogenic events, which include aberrant nuclear beta-catenin and activated Akt and ERK signaling.

Interspecies metabolism of heterocyclic aromatic amines and the uncertainties in extrapolation of animal toxicity data for human risk assessment

Heterocyclic aromatic amines (HAAs) are potent bacterial mutagens that are formed in cooked meats, tobacco smokes condensate, and diesel exhaust. Many HAAs are carcinogenic in experimental animal models. Because of their wide-spread occurrence in the diet and environment, HAAs may contribute to some common types of human cancers. The extrapolation of animal toxicity data on HAAs to asses human health risk has many uncertainties, which can lead to tenuous risk assessment estimates. Perhaps the most critical and variable parameters in interspecies extrapolation are the effects of dose, species differences in catalytic activities of xenobiotic metabolism enzymes (XMEs), human XME polymorphisms that lead to interindividual differences in carcinogen metabolism and dietary constituents that may either augment or diminish the carcinogenic potency of these genotoxins. The impact of these parameters on the metabolism and toxicological properties of HAAS and uncertainties in extrapolation of animal toxicity data for human risk assessment are presented in this article.

Cancer prevention by natural compounds

Increasing attention is being paid to the possibility of applying cancer chemopreventive agents for individuals at high risk of neoplastic development. For this purpose by natural compounds have practical advantages with regard to availability, suitability for oral application, regulatory approval and mechanisms of action. Candidate substances such as phytochemicals present in foods and their derivatives have been identified by a combination of epidemiological and experimental studies. Plant constituents include vitamin derivatives, phenolic and flavonoid agents, organic sulfur compounds, isothiocyanates, curcumins, fatty acids and d-limonene. Examples of compounds from animals are unsaturated fatty acids and lactoferrin. Recent studies have indicated that mechanisms underlying chemopreventive potential may be combinations of anti-oxidant, anti-inflammatory, immune-enhancing, and anti-hormone effects, with modification of drug-metabolizing enzymes, influence on the cell cycle and cell differentiation, induction of apoptosis and suppression of proliferation and angiogenesis playing roles in the initiation and secondary modification stages of neoplastic development. Accordingly, natural agents are advantageous for application to humans because of their combined mild mechanism. Here we review naturally occurring compounds useful for cancer chemprevention based on in vivo studies with reference to their structures, sources and mechanisms of action.

Modification of N‐Acetyltransferases and Glutathione S‐Transferases by Coffee Components: Possible Relevance for Cancer Risk

Enzymes of xenobiotic metabolism are involved in the activation and detoxification of carcinogens and can play a pivotal role in the susceptibility of individuals toward chemically induced cancer. Differences in such susceptibility are often related to genetically predetermined enzyme polymorphisms but may also be caused by enzyme induction or inhibition through environmental factors or in the frame of chemopreventive intervention. In this context, coffee consumption, as an important lifestyle factor, has been under thorough investigation. Whereas the data on a potential procarcinogenic effect in some organs remained inconclusive, epidemiology has clearly revealed coffee drinkers to be at a lower risk of developing cancers of the colon and the liver and possibly of several other organs. The underlying mechanisms of such chemoprotection, modifications of xenobiotic metabolism in particular, were further investigated in rodent and in vitro models, as a result of which several individual chemoprotectants out of the >1000 constituents of coffee were identified as well as some strongly metabolized individual carcinogens against which they specifically protected. This chapter discusses the chemoprotective effects of several coffee components and whole coffee in association with modifications of the usually protective glutathione-S-transferase (GST) and the more ambivalent N-acetyltransferase (NAT). A key role is played by kahweol and cafestol (K/C), two diterpenic constituents of the unfiltered beverage that were found to reduce mutagenesis/tumorigenesis by strongly metabolized compounds, such as 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine, 7,12-dimethylbenz[a]anthracene, and aflatoxin B(1), and to cause various modifications of xenobiotic metabolism that were overwhelmingly beneficial, including induction of GST and inhibition of NAT. Other coffee components such as polyphenols and K/C-free coffee are also capable of increasing GST and partially of inhibiting NAT, although to a somewhat lesser extent.

Equivocal impact of transplacental and lactational exposure to a food-derived carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, on prostate and colon lesion development in F344 rats

The carcinogenic potential of maternal dietary exposure to 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) was investigated with the focus on the prostate and mammary glands and colons of the offspring. PhIP-DNA adducts were immunohistochemically detected in all three organs of 3-week-old animals after dams had received 200 ppm PhIP in the diet from 4 weeks before mating to weaning. The sites were essentially the same as in adult rats, suggesting that the distribution of bioactivating enzymes for PhIP do not differ greatly. Ki-67 labeling indices were increased only in the colons of the female offspring at 3-weeks of age. Development of preneoplastic and neoplastic lesions in the prostate and colon was not enhanced when the rats were maintained on PhIP-free diet until 63 weeks of age, except for significant increase in multiplicity of aberrant crypt foci in the females. The present findings indicate that PhIP exposure via placenta and breast milk may be important but that high doses and long periods may be necessary for tumor development.

Tailoring cancer chemoprevention regimens to the individual

The present article, which is a tribute to the memory of Dr. Edward Bresnick, emphasizes the importance of environmental and life-style factors for cancer causation in the human population and points out approaches to cancer prevention. These approaches include vaccinations for the prevention of cancers that are caused by infectious agents as well as the use of cancer chemopreventive agents. The use of tamoxifen and letrozole to prevent breast cancer, finasteride to prevent prostate cancer, sunscreens or topical applications of 5-fluorouracil to prevent sunlight-induced skin cancer, and aspirin or calcium to prevent colon cancer are a few examples of cancer chemoprevention in high risk individuals and in the general population. An underdeveloped area of cancer chemoprevention is the use of combinations of agents that work by different mechanisms. It was pointed out that animal studies indicate that many cancer chemopreventive agents inhibit carcinogenesis under one set of experimental conditions but enhance carcinogenesis under another set of experimental conditions. These observations suggest that tailoring the chemopreventive regimen to the individual or to groups of individuals living under different environmental conditions or with different mechanisms of carcinogenesis may be an important aspect of cancer chemoprevention in human populations. How to tailor cancer chemoprevention regimens to the individual is an important challenge for the future.

Mutagenicity of food-derived carcinogens and the effect of antioxidant vitamins

The food-derived heterocyclic amines (HCAs) 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are mutagenic in the Ames test and produce tumors in laboratory animals, including monkeys. These HCAs have also been shown to induce gene mutations in vivo. To assess the antimutagenic effects of dietary antioxidant vitamins, beta-carotene, ascorbic acid (vitamin C), and alpha-tocopherol (vitamin E), on food-borne mutagenes/carcinogens, we evaluated the mutagenic activity of the compounds alone or combined with antioxidant vitamins. We utilized the rat lymphocyte mutation assay at the hypoxanthine guanine phosphoribosyl transferase (Hprt) locus. Female Fischer 344 rats treated with different doses (0, 2.5, 5.0, 25.0, and 50.0 mg/kg) of the carcinogens were sacrificed 5 wk after mutagen treatment. Although IQ and MeIQ slightly increased mutation frequency (MF) at some doses, a significant (P < 0.0009) increase in MF was found in animals exposed to MeIQx at 25 mg/kg. PhIP was the most mutagenic of the HCAs, with increases (P < 0.0001) in MF detected at all dose levels compared with controls. Because PhIP was the most mutagenic, it was selected for studies using the dietary antioxidant vitamins. Addition of antioxidant vitamins, singly or in a mixture, caused a significant (P < 0.0001) decrease in PhIP-induced Hprt MF. Vitamin E was the most effective at decreasing Hprt MF. In addition, we determined whether carcinogen metabolism would be affected by ingestion of vitamins. The activities of endogenous detoxification enzymes, glutathione S-transferase and glutathione peroxidase (GPx), were thus examined. Intake of beta-carotene and vitamin C without the carcinogen resulted in an increase (P < 0.05) in GPx activity. Also a modest increase in GPx activity was seen in animals that received the antioxidant mixture alone. Although the mechanisms of action of the antioxidants remain to be determined, the results indicate that dietary-derived HCA treatment induced MF in rat lymphocytes and suggest that antioxidants in food or taken as supplements could, in part, counteract such mutagenic activities.

The effects of coffee on enzymes involved in metabolism of the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in rats

The effects of coffee on the metabolism and genotoxicity of the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) were investigated. Coffee diminished the bacterial mutagenicity of PhIP in the Ames reversion assay through inhibition of cytochrome P450 1A2 (CYP1A2), a key enzyme involved in the metabolic activation of PhIP. When given as part of the diet (0, 1 or 5% w/w) to male Fischer-344 rats for 2 weeks, coffee affected the expression of hepatic enzymes involved in PhIP metabolism. Coffee increased the expression of CYP1A2 by 16-fold in the 5% coffee-treated group, and approximately half of this inductive effect was attributed to caffeine. Coffee also increased the expression of enzymes involved in the detoxication of PhIP. A 2-fold increase in expression of glutathione S-transferase alpha was observed, UDP-glucuronosyl transferase (UGTs) activities of p-nitrophenol increased 2-fold, while N(2)-and N3-glucuronidation of the genotoxic metabolite 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP) increased by 1.3-fold in the 5% coffee-treated over the control group. The amount of PhIP (0.75 mg/kg, 24 h) eliminated in urine as the N(2)-and N3-glucuronide conjugates of HONH-PhIP increased by 1.8- and 2.5-fold, respectively, in the 5% coffee-treated group over control rats, suggesting either increased rates of N-oxidation of PhIP or N-glucuronidation of HONH-PhIP. Despite the strong induction of CYP1A2, there was no increase in PhIP-DNA adduct formation in colon and pancreas while liver adducts decreased by 50% over control animals. These data suggest that the effect of coffee on inhibition of PhIP N-oxidation and ensuing DNA damage is more important in vivo than its effect on induction of PhIP N-hydroxylation.

Mechanistic approach of contrasting modifying effects of caffeine on carcinogenesis in the rat colon and mammary gland induced with 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

Caffeine exerts potent chemopreventive action against 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced rat mammary gland carcinogenesis, but acts as a co-carcinogen in the colon. The present work was performed to clarify mechanisms underling these organ dependent actions. Female F344 rats were given PhIP and caffeine, PhIP alone, caffeine alone or no treatment for 4 weeks. PhIP-DNA adduct formation in the colon was significantly higher in the PhIP+caffeine than in the PhIP group, but levels in the mammary glands showed no inter-group differences. CYP1A2 mRNA expression in the livers of the PhIP+caffeine group tended to be higher than in either the PhIP or the caffeine alone groups. High mRNA expression for both N-acetyltransferase (NAT) 1 and NAT2 was observed in the colon, with less expression in the mammary gland. The levels of four DNA-repair enzymes were not influenced by the caffeine treatment. In conclusion, only increased level of DNA adducts in the colon partially related to the modifying effects of caffeine on PhIP-induced rat carcinogenesis. Thus, other unknown factors must be contributory.

The modulation by xanthines of the DNA-damaging effect of polycyclic aromatic agents. Part II. The stacking complexes of caffeine with doxorubicin and mitoxantrone

Recently accumulated statistical data indicate the protective effect of caffeine consumption against several types of cancer diseases. There are also reports about protective effect of caffeine and other xanthines against tumors induced by polycyclic aromatic hydrocarbons. One of the explanations of this phenomenon is based on biological activation of such carcinogens by cytochromes that are also known for metabolism of caffeine. In the accompanying paper [Kapuscinski et al., this issue] we provide evidence (flow cytometry and the cell cycle analysis) that the cytostatic effects of caffeine (CAF) on two DNA alkylating agents, which do not require the biological activation, depend on their ability to form stacking (pi-pi) complexes. In this study, we use physicochemical techniques (computer aided light absorption and microcalorimetry), and molecular modeling to examine previously published qualitative data. This is published both by our and other group's data, indicates that CAF is able to modify the cytotoxic and/or cytostatic action of the two well known antitumor drugs doxorubicin (DOX) and mitoxantrone (MIT). To obtain the quantitative results from the experimental data we used the statistical-thermodynamical model of mixed aggregation, to find the association constants K(AC) of the CAF-drug interaction (128+/-10 and 356+/-21M(-1) for DOX-CAF and MIT-CAF complex formation, respectively). In addition, the favorable enthalpy change of CAF-MIT (DeltaH=-11.3kcal/mol) was measured by microcalorimetry titration. The molecular modeling (semi-empirical and force field method) allowed us to obtain the geometry of these complexes, which indicated the favorable energy (DeltaE) of complex formation of the protonated drug's molecules in aqueous environment (-7.4 and -8.7kcal/mol for DOX-CAF.5H(2)O and MIT-CAF.8H(2)O complex, respectively). The molecular modeling calculation indicates the existence of CAF-drug complexes in which the MIT molecules are intercalated between two CAF molecules (DeltaE=-29.9kcal/mol). These results indicate that the attenuating effect of caffeine on cytotoxic or mutagenic effects of some polycyclic aromatic mutagens cannot be the result of metabolic activation in the cells, but simply is the physicochemical process of the sequestering of aromatic molecules (e.g. carcinogens or mutagens) by formation of the stacking complexes. The caffeine may then act as the "interceptor" of potential carcinogens (especially in the upper part of digesting track) where its concentration can reach the mM level). There is, however, no indication, both, in the literature or from our experiments, that the xanthines can reverse the damage to nucleic acids at the point when the damage to DNA has already occurred.

Chemoprevention of heterocyclic amine-induced mammary carcinogenesis in rats

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is one of the most prevalent carcinogenic heterocyclic amines in the environment, targeting the colon, prostate, pancreas, and mammary gland in rodents. Chemopreventive effects of synthetic and naturally occurring compounds on PhIP-induced rat mammary carcinogenesis were investigated in a series of experiments. In a PhIP feeding model, groups of 20-21 female F344 rats each, were treated with 0.02% PhIP alone or PhIP plus 0.5% 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), 1% green tea catechins, 1% alpha-tocopherol, 0.1% ellagic acid, or 1% chlorophyllin, each in the diet, or 0.1% caffeine in drinking water for 52 weeks. To assess the mechanism of HTHQ and caffeine inhibition of PhIP-induced carcinogenesis, effects of these compound on the in vitro metabolic activation of PhIP were examined in the presence of S9 mix. In the next series of experiments, the PhIP intragastric dose model was applied to allow separate investigation of the effects of chemicals during the initiation and postinitiation periods. In these experiments, female Sprague-Dawley rats were given eight intragastric doses of 100 mg/kg body weight during the first 4-8 weeks for initiation. Either during initiation or after initiation, or only after initiation, animals were treated with either corn or perilla oil at doses of 5 and 20%, conjugated fatty acid derived from safflower oil (CFA-S) or perilla oil (CFA-P) at a dose of 1%, arctiin at doses of 0.02 and 0.2% in the diet, or sodium nitrite (NaNO(2)) at a dose of 0.2% in drinking water. In the PhIP feeding model, administration of PhIP alone for 52 weeks induced adenocarcinomas in 40% of rats, but the incidence was remarkably reduced to 5% by the simultaneous treatment with 0.5% HTHQ, a strong lipophilic phenolic antioxidant, or to 10% by 0.1% caffeine. Administration of 1% chlorophyllin exerted similar, albeit weaker, effects. alpha-Tocopherol at a dose of 0.5% only reduced the multiplicity of carcinomas, and 1% green tea catechins only the mean size of mammary tumors. In a metabolic activation study of PhIP, HTHQ and caffeine clearly inhibited the formation of metabolites. In the PhIP gastric dose model, among the naturally occurring compounds examined, a plant lignan arctiin, perilla oil, which contains a large amount of n-6 alpha-linolenic acid, and CFA-S or CFA-P inhibited mammary tumor development, particularly in the postinitiation period, although a clear dose response was not observed. Treatment with 0.2% NaNO(2) in the initiation period was found to lower the volume of mammary tumors. The present results indicate that a number of compounds may be candidate chemopreventive agents against PhIP-induced mammary carcinogenesis, acting through different mechanisms and depending on the stage of carcinogenesis.

Does lack of tocopherols and tocotrienols put women at increased risk of breast cancer?

Breast cancer is the leading site of new cancers in women and the second leading cause (after lung cancer) of cancer mortality in women. Observational studies that have collected data for dietary exposure to alpha-tocopherol with or without the other related tocopherols and tocotrienols have suggested that vitamin E from dietary sources may provide women with modest protection from breast cancer. However, there is no evidence that vitamin E supplements confer any protection whatever against breast cancer. Observational studies that have assessed exposure to vitamin E by plasma or adipose tissue concentrations of alpha-tocopherol have failed to provide consistent support for the idea that alpha-tocopherol provides any protection against breast cancer. In addition, evidence from studies in experimental animals suggest that alpha-tocopherol supplementation alone has little effect on mammary tumors. In contrast, studies in breast cancer cells indicate that alpha- gamma-, and delta-tocotrienol, and to a lesser extent delta-tocopherol, have potent antiproliferative and proapoptotic effects that would be expected to reduce risk of breast cancer. Many vegetable sources of alpha-tocopherol also contain other tocopherols or tocotrienols. Thus, it seems plausible that the modest protection from breast cancer associated with dietary vitamin E may be due to the effects of the other tocopherols and the tocotrienols in the diet. Additional studies will be required to determine whether this may be the case, and to identify the most active tocopherol/tocotrienol.

Inhibition by white tea of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced colonic aberrant crypts in the F344 rat

There is growing interest in the potential health benefits of tea, including the anticarcinogenic properties. We report here that white tea, the least processed form of tea, is a potent inhibitor of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced colonic aberrant crypts in the rat. Male Fischer 344 rats were treated for 8 wk with white tea (2% wt/vol) or drinking water alone, and on alternating days in experimental Weeks 3 and 4 the animals were given PhIP (150 mg/kg body wt p.o.) or vehicle alone. At the end of the study there were 5.65 +/- 0.81 and 1.31 +/- 0.27 (SD) aberrant crypt foci per colon in groups given PhIP and PhIP + white tea, respectively (n = 12, P < 0.05). No changes were detected in N-acetyltransferase or arylsulfotransferase activities compared with controls, but there was marked induction of ethoxyresorufin O-deethylase, methoxyresorufin O-demethylase, and UDP-glucuronosyltransferase after treatment with white tea. Western blot revealed corresponding increases in cytochrome P-450 1A1 and 1A2 proteins. Enzyme assays and Western blot also revealed induction of glutathione S-transferase by white tea. There was less parent compound and 4'-hydroxy-PhIP but more PhIP-4'-O-glucuronide and PhIP-4'-O-sulfate in the urine from rats given PhIP + white tea than in urine from animals given carcinogen + drinking water. The results indicate that white tea inhibits PhIP-induced aberrant crypt foci by altering the expression of carcinogen-metabolizing enzymes, such that there is increased ring hydroxylation at the 4' position coupled with enhanced phase 2 conjugation.

Effects of arctiin on PhIP-induced mammary, colon and pancreatic carcinogenesis in female Sprague-Dawley rats and MeIQx-induced hepatocarcinogenesis in male F344 rats

Chemopreventive effects of arctiin, a lignan isolated from Arctium lappa (burdock) seeds, on the initiation or post initiation period of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induced mammary carcinogenesis in female rats and on 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)-associated hepatocarcinogenesis in male rats were examined. In experiment 1, female Sprague-Dawley (SD) rats were given intragastric doses of 100 mg/kg body wt of PhIP once a week for 8 weeks as initiation. Groups of 20 rats each were treated with 0.2 or 0.02% arctiin during or after PhIP initiation. Control rats were fed 0.2 or 0.02% arctiin, or basal diet alone during the experimental period. Animals were killed at the end of week 48. Although the incidence of mammary carcinomas did not significantly differ among the PhIP-treated groups, multiplicity was significantly decreased in rats given 0.2 (0.7+/-0.7, P<0.05) or 0.02% (1.0+/-1.1, P<0.05) arctiin after PhIP initiation as compared with the PhIP alone controls (2.1+/-2.5). The average number of colon aberrant crypt foci was also significantly decreased in these two groups. Pancreas acidophilic foci were induced in PhIP treated animals with slight decrease in the multiplicity with arctiin during the initiation phase. For liver carcinogenesis, groups of 15 male F344 rats were given a single intraperitoneal injection of diethylnitrosamine (DEN) and starting 2 weeks later, they were administered 0.03% MeIQx in the diet, MeIQx together with 0.5% arctiin, 0.1% arctiin or basal diet for 6 weeks. They were subjected to two-third partial hepatectomy 3 weeks after DEN initiation and killed at the end of week 8 for glutathione S-transferase placental form (GST-P) immunohistochemistry. The numbers and areas of preneoplastic GST-P positive foci were elevated by the treatment with MeIQx, and further increased by the simultaneous treatment with arctiin. These results indicate that arctiin has a protective effect on PhIP-induced carcinogenesis particularly in the mammary gland in the promotion period. On the other hand, it may have a weak co-carcinogenic influence on MeIQx-induced hepatocarcinogenesis. In addition, the results suggested that PhIP is a weak pancreatic carcinogen in female SD rats, targeting acinar cells.