Polycyclic aromatic hydrocarbons and digestive tract cancers: a perspective

Cancers of the colon are most common in the Western world. In majority of these cases, there is no familial history and sporadic gene damage seems to play an important role in the development of tumors in the colon. Studies have shown that environmental factors, especially diet, play an important role in susceptibility to gastrointestinal (GI) tract cancers. Consequently, environmental chemicals that contaminate food or diet during preparation become important in the development of GI cancers. Polycyclic aromatic hydrocarbons (PAHs) are one such family of ubiquitous environmental toxicants. These pollutants enter the human body through consumption of contaminated food, drinking water, inhalation of cigarette smoke, automobile exhausts, and contaminated air from occupational settings. Among these pathways, dietary intake of PAHs constitutes a major source of exposure in humans. Although many reviews and books on PAHs and their ability to cause toxicity and breast or lung cancer have been published, aspects on contribution of diet, smoking and other factors toward development of digestive tract cancers, and strategies to assess risk from exposure to PAHs have received much less attention. This review, therefore, focuses on dietary intake of PAHs in humans, animal models, and cell cultures used for GI cancer studies along with epidemiological findings. Bioavailability and biotransformation processes, which influence the disposition of PAHs in body and the underlying causative mechanisms of GI cancers, are also discussed. The existing data gaps and scope for future studies is also emphasized. This information is expected to stimulate research on mechanisms of sporadic GI cancers caused by exposure to environmental carcinogens.

Bioaccessibility of polycyclic aromatic hydrocarbons: relevance to toxicity and carcinogenesis

INTRODUCTION

Bioaccessibility is a growing area of research in the field of risk assessment. As polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants, they are the toxicants of focus to establish cancer risks in humans. Orally ingested PAHs also cause toxicity and even affect the pharmacokinetic behavior of some therapeutic agents. Toward this end, bioaccessibility is being used as a tool to assess the risk of PAHs via dietary exposures.

AREAS COVERED

This review covers some in vitro bioaccessibility models for PAHs that have been used for the past one-and-a-half decade. This review also considers the factors that influence bioaccessibility and debates the merits and limitations of using a bioaccessibility concept for estimating risk from ingestion of PAH-contaminated soil and food. Finally, the authors discuss the implications of bioaccessibility for PAH-induced toxicity and cancers in the context of risk assessment.

EXPERT OPINION

So far, much of the focus on PAH bioaccessibility is centered on soil as a preferential matrix. However, ingestion of PAHs through diet far exceeds the amount accidentally ingested through soil. Therefore, bioaccessibility could be exploited as a tool to assess the relative risk of various dietary ingredients tainted with PAHs. While bioaccessibility is a promising approach for assessing PAH risk arising from various types of contaminated soils, none of the models proposed appears to be valid. Bioaccessibility values, derived from in vitro studies, still require validation from in vivo studies.

Chemoprevention of benzo(a)pyrene-induced colon polyps in ApcMin mice by resveratrol

Human dietary exposure to benzo(a)pyrene (BaP) has generated interest with regard to the association of BaP with gastrointestinal carcinogenesis. Since colon cancer ranks third among cancer-related mortalities, it is necessary to evaluate the effect of phytochemicals on colon cancer initiation and progression. In this study, we investigated the preventive effects of resveratrol (RVT) on BaP-induced colon carcinogenesis in Apc(Min) mouse model. For the first group of mice, 100 μg BaP/kg body weight was administered to mice in peanut oil via oral gavage over a 60-day period. For the second group, RVT was coadministered with BaP at a dose of 45 μg/kg. For the third group, RVT was administered for 1 week prior to BaP exposure for 60 days. Jejunum, colon and liver were collected at 60 days post BaP and RVT exposure; adenomas in jejunum and colon were counted and subjected to histopathology. RVT reduced the number of colon adenomas in BaP+RVT-treated mice significantly compared to that in mice that received BaP alone. While dysplasia of varying degrees was noted in colon of BaP-treated mice, the dysplasias were of limited occurrence in RVT-treated mice. To ascertain whether the tumor inhibition is a result of altered BaP-induced toxicity of tumor cells, growth, apoptosis and proliferation of adenocarcinoma cells were assessed posttreatment with RVT and BaP. Cotreatment with RVT increased apoptosis and decreased cell proliferation to a greater extent than with BaP alone. Overall, our observations reveal that RVT inhibits colon tumorigenesis when given together with BaP and holds promise as a therapeutic agent.

Metabolism of the environmental toxicant benzo(a)pyrene by subcellular fractions of human ovary

Knowledge of the ability of the female reproductive system to metabolize environmental chemicals is critical not only from the standpoint of toxicity but also from infertility risk assessment. Benzo(a)pyrene (BaP) is a toxicant that is released into the environment from automobile exhausts, cigarette smoke, burning of refuse, industrial emissions, and hazardous waste sites. In exposed animals, BaP becomes activated to reactive metabolites that interfere with target organ function and as a consequence cause toxicity. Studies on animal models conducted in our laboratories and those of others have shown that BaP possess endocrine disrupting properties. Thus, this chemical has the potential to cause infertility and cancers in the female genital tract. An understanding of BaP metabolism in the female reproductive system will be of importance in the diagnosis and management of female fertility as well as cancers in the reproductive tissues. Therefore, the objective of our study was to examine the metabolism of BaP by human ovarian subcellular fractions. Human ovary samples (eight individuals) were obtained from postoperative tissue removed from subjects with uterine tumors. Subcellular fractions (nuclear, cytosolic, mitochondrial, and microsomal) were prepared by differential centrifugation. BaP (1 μM and 3 μM) was individually incubated with individual subcellular fractions for 15 min and the products were analyzed by high-performance liquid chromatography. Among the different fractions tested, microsomal BaP metabolism was higher than the rest of the fractions. The BaP metabolites identified were as follows: BaP-9,10-diol, BaP-4,5-diol, BaP-7,8-diol, 9(OH) BaP, 3(OH) BaP, BaP-1,6-dione, BaP-3,6-dione, and BaP-6,12-dione. Of interest was the presence of DNA-reactive metabolites such as BaP-3,6-dione, BaP-6,12-dione, and BaP 7,8-diol, which have been implicated in the causation of infertility and cancer. Our results indicate that women who are exposed to BaP via cigarette smoke, occupational settings, and diet are more likely at a larger risk of this toxicant-induced infertility and cancer than others.

Comparative metabolism of benzo(a)pyrene by ovarian microsomes of various species

Knowledge of the ability of the female reproductive system to metabolize polycyclic aromatic hydrocarbons (PAHs) is critical to the diagnosis and management of female infertility and for risk assessment purposes. The PAHs are a family of widespread pollutants that are released into the environment from automobile exhausts, cigarette smoke, burning of refuse, industrial emissions, and hazardous waste sites. In exposed animals, PAHs become activated to reactive metabolites that interfere with target organ function and as a consequence cause toxicity. The extent of susceptibility to PAH exposure may depend on the ability of animals to metabolize these chemicals. The present study has been undertaken to assess whether any differences exist among mammals in the metabolism of benzo(a)pyrene (BaP), a prototypical PAH compound. Microsomes isolated from the liver and ovaries of rats, mice, goats, sheep, pigs, and cows were incubated with 5 microM BaP. Postincubation, samples were extracted with ethyl acetate and analyzed for BaP/metabolites by reverse-phase HPLC with fluorescence detection. The rate of metabolism (pmol of metabolite/min/mg protein) was found to be more in liver than in ovary in all the species studied (P < 0.05). The differences in metabolite concentrations were statistically significant (P < 0.0001) among the various species in both organs studied. Multiple species comparison also revealed that the differences were statistically significant (P < 0.001) between rodents (rat and mouse) and higher mammals (ewe, sow, and cow). Even among the higher mammals, in a majority of the cases, the differences in metabolite concentrations were significantly different (P < 0.001) both in ovary and liver. The BaP metabolites identified were 4,5-diol; 7,8-diol; 9,10-diol; 3-hydroxy BaP; and 9-hydroxy BaP. The rodent microsomes produced considerably higher proportion of BaP 4,5-diol and 9,10-diol than did cow, sow, goat, and sheep. However, microsomes from higher mammals converted a greater proportion of BaP to 3-hydroxy and 9-hydroxy BaP, the detoxification products of BaP. Overall, our results revealed a great variation among species to metabolize BaP.

Alteration of benzo(a)pyrene biotransformation by resveratrol in ApcMin/+ mouse model of colon carcinogenesis

Epidemiological surveys have revealed that environmental and dietary factors contribute to most of the human cancers. Our earlier studies have shown that resveratrol (RVT), a phytochemical reduced the tumor number, size and incidence of dysplasias induced by benzo(a)pyrene (BaP), an environmental toxicant in the Apc^Min/+ mouse model of colon cancer. In this study we investigated to ascertain whether the preventive effects of RVT on BaP-induced colon carcinogenesis is a result of altered BaP biotransformation by RVT. For the first group of mice, 100 μg BaP/kg bw was administered in peanut oil via oral gavage over a 60 day period. For the second group, 45 μg RVT/kg bw was co-administered with BaP. For the third group, RVT was administered for 1 week prior to BaP exposure. Blood, colon and liver were collected from control and BaP/RVT-treated mice at 60 days post-BaP & RVT exposure. We have assayed activities and expression (protein & mRNA) of drug metabolizing enzymes such as cytochrome P4501A1 (CYP1A1), CYP1B1, and glutathione-S-transferase (GST) in colon and liver samples from the treatment groups mentioned above. An increased expression of CYP1A1 in liver and colon and of CYP1B1 in liver of BaP-treated mice was seen, while RVT inhibited the extent of biotransformation mediated by these enzymes in the respective tissue samples. In the case of GST, an increased expression in colon of BaP alone-treated mice was noted when RVT was administered prior to BaP or simultaneously with BaP. However, there is no change in liver GST expression between BaP and RVT treatment groups. The concentrations of BaP aqueous (phase II) metabolites were found to be greater than the organic (phase I) metabolites, suggesting that RVT slows down the phase I metabolism (metabolic activation) of BaP, while enhancing phase II metabolism (detoxification). Additionally, the BaP-DNA adduct concentrations measured in colon and liver of BaP + RVT-treated mice were low relative to their BaP counterparts. Taken together, our findings strongly suggest that RVT alleviates BaP-induced colon carcinogenesis by impairing biotransformation pathways and DNA adduct formation, and therefore holds promise as a chemopreventive agent.

Effect of benzo(a)pyrene exposure on fluoranthene metabolism by mouse adipose tissue microsomes

The present study has been undertaken to examine whether exposure to benzo(a)pyrene (BaP), a polycyclic aromatic hydrocarbon (PAH) compound, influences the metabolism of fluoranthene (FLA), another PAH compound. Microsomes were isolated from the adipose tissue of mice that received 50 microg/kg BaP and incubated with FLA (3 microM) alone; FLA in combination with BaP at equimolar concentrations, and a control group that received nothing. Post-incubation, samples were extracted with ethyl acetate and analyzed for FLA metabolites by reverse-phase HPLC with fluorescence detection. The rate of FLA metabolism (pmol of metabolite/min/mg protein) was increased when microsomes from BaP-treated mice were exposed to FLA alone and FLA in combination with BaP, compared to controls. On the other hand, the difference in FLA metabolic rate between microsomes that were exposed to FLA + BaP was higher than the ones that received FLA. The microsomes from BaP-pre-treated mice produced a considerably higher proportion of FLA 2, 3-diol, and 2, 3 D FLA when microsomes were incubated with FLA. There were no differences in the FLA metabolite types formed when BaP-pre-treated mice were co-incubated with BaP and FLA than with FLA alone. The enhanced biotransformation of FLA as a result of prior and concomitant exposure to BaP may have implications for assessment of risks arising from human exposure to PAH mixtures.

Abstract A37: Disparities in exposures to volatile organic compounds among the marginalized communities of inner city Washington DC

The District of Columbia has one of the highest cancer incidence and mortality rates in the nation in conjunction with significant cancer-related geographic and racial disparities. Besides exposures to behavioral and occupational risk factors, characterizing potential exposures to environmental carcinogens in such high risk areas as well as understanding exposure disparities is of public health importance. It has been reported that individuals residing in over-populated and /or underserved areas of major cities are at increased risk of being exposed to various air pollutants, but no studies to date have explored this issue among such communities of inner city Washington DC. Environmental carcinogens, such as chloroform, and benzene, are major components of air pollution from vehicular exhaust, factory and refinery emissions, some building materials and industrial/household solvents. In this study we examined the levels of chloroform, benzene, toluene, and 1,2-dichlorobenzene in high risk areas (&lt;1 mile from highway/freeway) and compared it to those of low risk areas (&gt;3 miles from the highway/freeway). Levels of selected volatile organic compounds (VOCs) including benzene, chloroform, toluene and 1,2-dichlorobenzene were measured inside and outside of 16 homes in high risk areas and 14 homes in low risk areas over a 72 hour time period using an 3M Organic Vapor Monitor and analyzed using gas chromatograph mass spectrometer (GC/MS). The levels of all the four VOCs in the high risk areas were higher both inside and outside the homes compared to the low risk areas. Mean chloroform levels outside the residences in high risk areas were significantly higher (1.24 μg/m3) in comparison to low risk areas (0.43μg/m3), (P &lt; .05). Levels of all four compounds were higher inside the homes in both high and low risk areas than outside the homes, however, the inside-outside differences were significant for chloroform and toluene only. The mean exposure level of 1,2-dichlorobenzene was the highest (33.66μg/m3) inside the homes located in high risk areas as opposed to 1.48μg/m3 inside the homes in the low risk areas. This indicated a presence of additional indoor sources of this compounds including air fresheners, cleaning solvents, moth balls etc. in the high risk/low income communities. The results suggest that individuals living in high risk areas are more likely to be exposed to higher levels of indoor environmental carcinogens compared to low risk areas. Preventive efforts targeting indoor environmental carcinogens are warranted in efforts to reduce the cancer burden that disproportionately impacts African-Americans.

Citation Format: Ashley C. Huderson, Chiranjeev Dash, Everett Dodson, Amir Sapkota, Lucile Adams-Campbell. Disparities in exposures to volatile organic compounds among the marginalized communities of inner city Washington DC. [abstract]. In: Proceedings of the Twelfth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2013 Oct 27-30; National Harbor, MD. Philadelphia (PA): AACR; Can Prev Res 2013;6(11 Suppl): Abstract nr A37.

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Abstract 2864: Effect of resveratrol (RVT) on prior-, concurrent-, and post- benzo(a)pyrene (BaP) exposure in colon tumor development of ApcMin mice

Colon cancer is one of the important cancers in terms of morbidity and mortality. In U.S. alone, around 60,000 lives/year are lost to colon cancer. Diet and environment are implicated in the development of sporadic colon cancers. Our laboratory has already reported the development of colon tumors in ApcMin mice orally exposed to BaP. In this study we investigated the timing of RVT administration on BaP-induced colon carcinogenesis in ApcMin mice. The mice were grouped under 5 treatment categories. For Group I, only BaP was administered (in peanut oil) at a dose of 100 μg/kg via oral gavage over a 60 day period. For Group II, RVT (in 10% ethanol + 90% deionized water) was co-administered with BaP at a dose of 45 μg/kg. For Group III, RVT was administered for 1 week prior to BaP dosing. For Group IV, RVT was administered for 1 week post BaP dosing. The Vth group received RVT only. Jejunum, colon and liver samples were collected at the end of exposure; adenomas in the jejunum and colon were counted and subjected to histopathology. RVT significantly inhibited the development of adenomas in the jejunum and colon by 45 and 40 % (P &lt; 0.001) respectively in Group II, compared to Group III (0 and 5%) and Group IV (0 and 2%). Though no significant difference was noted in the size of adenomas in jejunum of BaP + RVT-treated mice, compared to that of BaP-treated mice; the colon samples showed a different trend. RVT significantly reduced the size of colon adenomas in mice that received BaP & RVT simultaneously compared to mice that received BaP alone, RVT before BaP, and RVT post BaP. Though dysplasia of colon was observed in BaP-treated mice, it was of limited occurrence in RVT-treated mice. Our findings thus far reveal that the prevention colon tumorigenesis by RVT is effective only when given together with BaP, compared to prior and post BaP-exposure. Studies are in progress to see the underlying mechanisms for the differential preventive effects of RVT (funded by NIH grants 5T32 HL007735-14, RO3CA130112-01 and S11ES01415).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2864.

Abstract 5459: Benzo(a)pyrene biotransformation enzyme expression, activities and metabolite disposition in ApcMin mouse colon and liver is altered by resveratrol exposure

Statistics released by the American Cancer society indicates that colon cancer is the third leading cause of cancer cases and related deaths in America. In addition to dietary preferences and lifestyle habits, exposure to toxicants such as benzo(a)pyrene (BaP) is one of the major contributing factors to the development of sporadic colon cancer. Our laboratory has validated the ApcMin mouse model to study BaP-induced colon carcinogenesis. Ongoing studies in our laboratory are focused towards elucidating the anticarcinogenic effects of resveratrol (RVT) against colon tumors. Our studies thus far have shown a decrease in the incidence, size, and number of adenomas formed in the colon of mice exposed to BaP and RVT, compared to BaP exposure alone. Since biotransformation of toxicants is one of the key steps for initiating carcinogenesis, the objective of this study was to investigate whether RVT exposure simultaneously or prior to BaP treatment alters BaP biotransformation and bioavailability in ApcMin mice. The BaP treatment consisted of BaP-only administration (in peanut oil) at a dose of 100 μg/kg body weight (bw) via oral gavage over a 60 day period (group I); BaP (100 μg/kg bw) co-administered with RVT (in 10% ethanol + 90% deionized water) at a dose of 45 μg/kg bw (group II); RVT administered for 1 week prior to BaP dosing (group III). Blood, colon and liver samples were collected at the end of exposure period. The expression of BaP biotransformation enzymes (CYP1A1, CYP1B1 and GST) in liver and colon were assayed at the level of protein and enzyme activities. Plasma, liver and colon tissue samples were analyzed by reverse phase-HPLC for BaP metabolites. Resveratrol exposure both prior to- and concurrent with BaP exposure caused a decrease in the expression and activity of CYP1A1/1B1 enzymes and an increase in GST enzymes both in liver and colon. Additionally, our studies revealed a decrease in concentrations of organic (Phase I) metabolites in plasma, liver and colon in mice that received RVT + BaP compared to mice that received RVT alone. In contrast, an opposite trend was noted with aqueous (Phase II) metabolites registering an increase in mice that received RVT + BaP compared to mice that received RVT alone. Between the two RVT-treatment strategies, concurrent administration of RVT appeared to limit BaP bioactivation compared to RVT treatment prior to BaP exposure. In summary, our results suggest that RVT provides a preventive effect against BaP-induced colon cancer initiation and progression in ApcMin mice (funded by NIH grants 5T32HL007735-12, 1F31ES019432-01A1 and 5RO1CA142845-O2).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5459. doi:1538-7445.AM2012-5459

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[Figure: see text].