- Email: [email protected]
Breast cancer and organochlorines: a marker for susceptibility?
Surgical Oncology 7 (1999) 1}4
Review
Breast cancer and organochlorines: a marker for susceptibility? M.A. Musgrave , K.J. Aronson, S. Narod, W. Hanna, A.B. Miller , D.R. McCready * Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada Department of Community Health and Epidemiology, Queen+s University, Kingston, Ontario, Canada Breast Cancer Research, Women+s College Hospital, University of Toronto, Toronto, Ontario, Canada Department of Pathology, Women+s College Hospital, Toronto, Ontario, Canada Surgical Oncology, Rm 4-320, Princess Margaret Hospital, 610 University Avenue, Toronto, Ontario, Canada M5G 2M9
Abstract The incidence of breast cancer is increasing and despite extensive research e!orts, the etiology of this disease is largely unknown. Most women exhibit no known risk factors except for their age and sex. It has recently been postulated that the increased breast cancer incidence might be attributed to exposure to environmental carcinogens such as the organochlorine compounds. In this article, the scienti"c literature with respect to this possibility is reviewed and alternative hypotheses, which may in part explain the possible role of organochlorine compounds in the etiology of breast cancer, are presented. 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Breast cancer; Metabolism; Organochlorines; Pesticides; Risk factor
1. Introduction Breast cancer is a signi"cant cause of morbidity and death in North American women, yet relatively little is known about the etiology of this disease. Inherited susceptibility, as de"ned by a mutation in the breast cancer genes BRCA-1 and BRCA-2, accounts for only about ca 5% of cases. Risk factors known to increase susceptibility (early menarche, late menopause, nulliparity, or late age at "rst birth) may play a role in another one-third of cases [1]. This leaves the majority of women who develop breast cancer with dietary, socioeconomic, and other factors as possible causes. The steady increase in the incidence of breast cancer during the last several years has stimulated further concerns among breast cancer researchers and the general public. Some of the recorded increase in the disease can be attributed to better and earlier detection, but this does not completely explain the recent pattern [2, 3]. Environmental factors such as organochlorine compounds are receiving particular attention for their potential role in the etiology of breast cancer.
* Corresponding author. Tel.: (416) 946-6510; Fax: (416) 946-6590.
The organochlorines are a diverse group of ca 15 000 compounds which include DDT [1,1,1-trichloro-2,2bis( p-chlorophenyl)ethane], its metabolite DDE [1,1dichloro-2,2-bis(p-chlorophenyl)ethylene] and PCBs (polychlorinated biphenyls). Developed over 40 years ago, they include such chemicals as #ame retardants and pesticides, many of which have been banned since the early 1970s but are still found as persistent food chain contaminants because they are very stable and highly lipophilic. They "nd their way into human adipose tissue largely through the consumption of dietary fat. Although air and water pollution may contribute to the overall body burden of these compounds, contaminated food, primarily as "sh and animal fat, accounts for the bulk of human exposure. The slow elimination of these chemicals from the body has raised concerns with respect to the possibility that long-term, low-level exposure to these compounds via the food chain can result in large body accumulations and possible toxicity. While there has been speculation that organochlorine exposure results in direct genotoxicity, recent research has focused on the ability of these compounds to act as xenoestrogens [4]. This is an important concern considering that estrogen can promote breast cell proliferation and many of the established risk factors for breast cancer can be linked to total lifetime exposure to bioavailable estrogen.
0960-7404/99/$ - see front matter 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 7 4 0 4 ( 9 8 ) 0 0 0 1 0 - 3
2
M.A. Musgrave et al. / Surgical Oncology 7 (1999) 1}4
2. Epidemiological evidence Some epidemiological evidence exists that suggests a link between organochlorine exposure and breast cancer. In New York City, Wol! et al. [5] conducted a nested case-control study with a cohort of 14 000 women enrolled in a prospective study of breast cancer screening. This study determined the blood levels of organochlorine compounds in women who developed breast cancer and compared them to healthy matched controls. Cases were found to have signi"cantly higher levels of DDE (a DDT metabolite) in their blood compared to the controls. Another nested case-control study was conducted by Kreiger et al. [6] in the San Francisco Bay area. Serum samples from a random sample of 50 breast cancer cases from three racial groups were compared to an equal number of race-matched controls. While the overall data did not initially support a relationship between DDE and PCB exposure and the risk of breast cancer, further analysis that separated subjects by race found a positive relationship for both blacks and whites [7]. Furthermore, Dewailly et al. [8] conducted a study in which they analyzed breast tissue for the presence of organochlorines from 41 women undergoing biopsy. Patients, who upon pathological review were determined to have estrogen-responsive breast cancers, demonstrated a higher concentration of DDE and PCBs in their breast tissue. Finally, Aronson and Miller [9] conducted a pilot case-control study of 59 patients undergoing breast biopsy in Toronto. Breast adipose tissue was analyzed for the presence of 20 di!erent organochlorine compounds. Each of the mean body burdens of DDT, hexachlorobenzene (HCB), total PCBs, and "ve PCB congeners were higher (P(0.05) in the 26 breast cancer cases than in controls. After controlling for several potential confounders by logistic regression, these results suggest that breast cancer risk is associated with increased tissue levels of several organochlorines, including some PCB congeners. However, the small population size of this study necessitates caution in interpretation, and was therefore the motivation for a much larger study currently underway in Toronto funded by the Canadian Breast Cancer Research Initiative. Thus, while there is some epidemiological evidence to support an association between breast cancer and the organochlorine compounds, these studies do not suggest the mechanism by which these chemicals could possibly alter the disease process, if, in fact, they do.
3. Mechanism of action In order for organochlorine compounds to be considered etiological factors in breast cancer, a mechanism by which these compounds produce carcinogenesis must
be demonstrated. One mechanism by which these compounds may ultimately produce their e!ects is direct genotoxicity at the DNA level. Evidence that these compounds may act as genotoxins comes from animal studies that show organochlorine-induced carcinogenesis in a dose-response and tissue-dependent manner [10}14]. However, estimates of human risk have been primarily based on extrapolation from these animal studies and the organochlorine levels required to induce cancer are substantially higher than those typical of environmental exposure. Gold et al. [15] concluded that when viewed against the large background of naturally occurring carcinogens in typical portions of food, including bread, co!ee and mangos, the residues of synthetic pesticides or environmental pollutants ranked low. Overall, this study cast some doubt on the relative importance of low-dose exposure to synthetic chemicals with respect to human cancers. Another plausible mechanism in which organochlorine compounds could act as an etiological factor for breast cancer lies in their ability to act as xenoestrogens [4]. This is related to their estrogen receptor agonist activity, and to the fact that they are metabolized in a manner similar to the primary endogenous estrogen, estradiol. Estradiol is metabolized via one of two pathways that produce products with di!erent estrogenicity. The "rst pathway yields a product known as 2-hydroxyestrone, a compound with minimal estrogenic activity thought to be an inactive and benign metabolite. The second metabolic route yields 16-a-hydroxyestrone, a potent estrogen. It has been postulated that by either inhibiting the second pathway, ultimately reducing the amount of 16-a-hydroxyestrone, or by enhancing the "rst pathway of estrogen, which produces the benign metabolite, breast cancer risk could be reduced. The converse may also be true in that increasing estradiol metabolism through the pathway that produces the active metabolite (16-a-hydroxyestrone), or decreasing metabolism through the pathway that produces the inactive metabolite (2-hydroxyestrone), could increase cancer risk. Compounds acting as xenoestrogens may act in several di!erent ways to interrupt or alter estrogen functioning and metabolism. These compounds could displace endogenous estrogen from its receptor thereby altering the stimulating/inhibiting e!ects of estrogen at the level of the cell or tissue. They could also dominate the metabolic pathways of catabolism resulting in production of alternative compounds, which could also be estrogenic. The xenoestrogens may also alter the predominant metabolism of endogenous compounds resulting in increased usage of alternative pathways, and production of metabolic products with varying degrees of estrogenic activity. Xenoestrogens which a!ect the relative functioning of the two previously described pathways could be responsible for shifting the overall estrogenic balance and hence modifying the risk of breast cancer.
M.A. Musgrave et al. / Surgical Oncology 7 (1999) 1}4
Experimental evidence from animal studies has demonstrated that exposure to estrogens such as the organochlorines leads to increased levels of 16a-hydroxyestrone and that mice which naturally acquire spontaneous breast tumours seem to have more of this metabolite [16]. As well, several experiments have demonstrated that organochlorines, including PCBs, bind to the estrogen receptor and mimic the e!ect of another potent metabolite, 17-b-estradiol, with respect to induction of enzyme activity, cell proliferation and di!erentiation. Furthermore, reproductive anomalies have been found in animals born into ecosystems that are polluted by xenoestrogens, especially those compounds that are environmentally persistent [16]. While this evidence is striking, it is important to note that xenoestrogens are at least 1000-fold less potent than the endogenous, naturally occurring estrogens in a woman's body. Given this discrepancy and the relatively small amounts of xenoestrogens detected in body tissues, it is hard to reconcile the ability of these compounds to so drastically alter estrogen metabolism compared to the more potent and abundant endogenous estrogens. Hence, it seems unlikely that mere environmental exposure to organochlorines could signi"cantly shift the lifetime estrogen balance and increase breast cancer risk by a xenoestrogenic mechanism.
3
particular, CYP 1A1. Equally important is the fact that this same gene also codes for enzymes involved in the hydroxylation of estrogen. It may be that the presence of organochlorine compounds in the breast tissue signals an underlying susceptibility to altered metabolism for many compounds including endogenous estrogens and exogenous xenoestrogens. Altered products of metabolism may in essence be the culprits responsible for disease either through overproduction of reactive intermediates which themselves are genotoxic, or through a failure of secondary metabolism to appropriately detoxify these genotoxic compounds. Since the organochlorines induce the P450 enzyme system and serve as substrates for metabolic activation, individuals with speci"c P450 and GST genotypes may be at particular risk for breast cancer and the pathway could potentially be hormonally driven. Organochlorines could contribute to breast tumour development by altering estrogen levels within the breast, increasing the total lifetime exposure to bioavailable estrogen, or as just postulated, by interacting with certain genetic polymorphisms to produce metabolic byproducts which may be carcinogenic or genotoxic. The body burden of organochlorines may be a marker of a genetic polymorphism that signals the production of alternative estrogenic metabolites, which are, in turn, carcinogenic. Thus, organochlorine compounds found in breast tissue may be markers for susceptibility to breast cancer.
4. An alternate hypothesis It may be that organochlorines have neither direct (genotoxic) nor indirect (xenoestrogenic) e!ects with respect to breast cancer. Perhaps the organochlorine levels in women's bodies re#ect di!erences in endogenous estrogen metabolism and are thus only markers of altered risk or susceptibility. It is known that the cytochrome P450 enzymes play a role in the metabolism of various environmental agents. These enzymes act on aromatic hydrocarbons to produce intermediates that may react with DNA and/or induce cellular proliferation [17]. Detoxi"cation of these intermediates is largely through glucuronide conjugation mediated by the enzyme glutathione-S-transferase (GST). Human genetic polymorphisms for both P450 and GST enzymes appear to account for a substantial proportion of inter-individual variation in susceptibility to environmentally-induced cancers of several tissues including lung and bladder [17}19]. Individuals with speci"c alleles appear to be at an increased cancer risk. The P450 genes, CYP 1A1 and CYP 1A2, as well as the genotype responsible for the absence of the detoxifying GST enzyme, GSTM1 null, appear to be particularly important. Previous studies of GSTM1 polymorphism in breast cancer found that ca 50% of tumours are GSTM1 null [20}22]. Organochlorines have been shown to induce the cytochrome P450s by an up-regulation of gene activity, in
5. Conclusions It is evident that an integrated approach in examining the role of environmental compounds and risk of breast cancer is needed to help resolve the current literature controversy as well as the public debate on the role of organochlorines in breast cancer. Previous studies in the area of environmental factors and breast cancer have largely been based on epidemiological data which provided evidence for a role of organochlorines in breast cancer but did not attempt to detail the mechanisms of action of these compounds. Most associations are correlational and inconsistent and cannot provide a basis for any de"nitive statement on the role of environmental compounds in the development of breast cancer. Recent advances in molecular biology may allow these correlational studies to be taken one step further. By relating environmental observations to the alterations in metabolic function at the level of the gene, the puzzle linking environmental agents and breast cancer could possibly be solved. Whether this will lead to further progress with respect to the biology of breast cancer remains to be seen. Organochlorines, per se, may not be the ultimate carcinogens with respect to breast cancer. Rather, an increased body burden of organochlorines may be
4
M.A. Musgrave et al. / Surgical Oncology 7 (1999) 1}4
a marker of &at risk' individuals who have an underlying genetic predisposition which renders them susceptible to breast cancer unrelated to the organochlorine compounds. In this scenario, the genetic polymorphism may in#uence the tissue levels of the products of organochlorine or estrogen metabolism from either exogenous or endogenous sources. It is possible that these endogenous metabolic byproducts are the ultimate carcinogens. Thus, increased organochlorine levels may re#ect an underlying genetic susceptibility for breast cancer in individuals within a phenotypic subgroup.
References [1] Sideman H, Stellman SD, Mushinski M. A di!erent perspective on breast cancer risk factors: some implications of the nonattributable risk. CA: A Cancer Journal for Clinicians 1982;32:301}13. [2] Feurer EJ, Wun L-M. How much of the recent rise in breast cancer can be explained by increases in mammography utilization? A dynamic population model approach. American Journal of Epidemiology 1992;136:1423}36. [3] Hahn RA, Moolgackar S. Nulliparity, decade of "rst birth, and breast cancer in Connecticut cohorts, 1855}1945: an ecological study. American Journal of Public Health 1989;79:1503}7. [4] Davis DL, Bradlow HL, Wol! MS, Woodru! T, Hoel, DG, Anton-Culver H. Medical hypothesis: Xenoestrogens as preventable causes of breast cancer. Environmental Health Perspectives 1993;101:372}7. [5] Wol! MS, Toniolo PG, Lee EW, Rivera M, Dubin N. Blood levels of organochlorine residues and risk of breast cancer. Journal of the National Cancer Institute 1993;85:648}52. [6] Kreiger N, Wol! MS, Hiatt RA, Rivera M, Vogelman J, Orentreich N. Breast cancer and serum organochlorines: a prospective study among white, black and asian women. Journal of the National Cancer Institute 1994;86:589}99. [7] Millikan R, DeVoto E, Newman B, Savitz D. Studying environmental in#uences and breast cancer risk: suggestions for an integrated population-based approach. Breast Cancer Research and Treatment 1995;35:79}89. [8] Dewailly E, Dodin S, Verreault R, Ayotte P, Sauve L, Morin J, Brisson J. High organochlorine body burden in women with estrogen receptor-positive breast cancer. Journal of the National Cancer Institute 1994;86:232}4. [9] Aronson KJ, Miller AB. Environmental factors for breast cancer. Epidemiology 1996;7(4):S38.
[10] Poland A, Knutson JC. 2,3,7,8-Tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons: examination of the mechanism of toxicity. Annual Review of Pharmacology and Toxicology 1982;22:517}54. [11] Safe S. Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and related compounds: environmental and mechanistic consideration which support the development of toxic equivalency factors. Critical Reviews in Toxicology 1990;21:51}88. [12] Goldstein JA, Safe S. Mechanism of action and structure}activity relationship for the chlorinated dibenzo-p-dioxins and related compounds. In: Halogenated Biphenyls, Terphenyls, Napthalenes, Dibenzodioxins and Related Compounds, Kimborough RD, Jensen A, editors. Elsevier, 1989:239}93. [13] International Agency for Research on Cancer. Monographs on the evaluation of the carcinogenic risk of chemicals to humans, Suppl. 7. Overall Evaluations of Carcinogenicity: An Updating of IARC Monographs, vols 1}42. International Agency for Research on Cancer, Lyon, 1987. [14] International Agency for Research on Cancer. Monographs on the evaluation of the carcinogenic risk of chemicals to humans, vol. 53. Occupational Exposures to Insecticide Application, and Some Pesticides. International Agency for Research on Cancer, Lyon, 1991. [15] Gold LS, Slone TH, Stern BR, Manley NB, Ames B. Rodent carcinogens: setting priorities. Science 1992;258:261}5. [16] Davis DL, Bradlow HL. Can environmental estrogens cause breast cancer?. Scienti"c American 1995;273:167}72. [17] Guengerich F. Cytochrome P450 enzymes. American Scientist 1993;81:440}7. [18] Bell D, Taylor J, Paulson D, Robertson D, Mohler J, Lucier G. Genetic risk and carcinogen-metabolism gene glutathioneS-transferase M1 (GSTM1) that increase susceptibility to bladder cancer. Journal of the National Cancer Institute 1993;85: 1159}64. [19] Nakachi K, Imai K, Hayashi S, Kawajiri K. Polymorphisms of the CYP 1A1 and glutathione-S-transferase genes associated with susceptibility to lung cancer in relation to cigarette dose in a Japanese population. Cancer Research 1993;53:2994}9. [20] Forrester L, Hayes J, Millis R, Barnes D, Harris A, Schlarger J, Powis G, Wolf C. Expression of glutathione-S-transferases and cytochrome P450 in normal and tumour breast tissue. Carcinogenesis 1990;11:2163}70. [21] Shea T, Cla"n G, Comstock K, et al. Glutathione transferase activity and isoenzyme composition in primary human breast cancers. Cancer Research 1990;50:6848}53. [22] Zhong S, Wyllie A, Barnes D, Wolf C, Spurr N. Relationship between the GSTM1 genetic polymorphism and susceptibility to bladder, breast and colon cancer. Carcinogenesis 1993;12:1821}4.
Review
Breast cancer and organochlorines: a marker for susceptibility? M.A. Musgrave , K.J. Aronson, S. Narod, W. Hanna, A.B. Miller , D.R. McCready * Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada Department of Community Health and Epidemiology, Queen+s University, Kingston, Ontario, Canada Breast Cancer Research, Women+s College Hospital, University of Toronto, Toronto, Ontario, Canada Department of Pathology, Women+s College Hospital, Toronto, Ontario, Canada Surgical Oncology, Rm 4-320, Princess Margaret Hospital, 610 University Avenue, Toronto, Ontario, Canada M5G 2M9
Abstract The incidence of breast cancer is increasing and despite extensive research e!orts, the etiology of this disease is largely unknown. Most women exhibit no known risk factors except for their age and sex. It has recently been postulated that the increased breast cancer incidence might be attributed to exposure to environmental carcinogens such as the organochlorine compounds. In this article, the scienti"c literature with respect to this possibility is reviewed and alternative hypotheses, which may in part explain the possible role of organochlorine compounds in the etiology of breast cancer, are presented. 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Breast cancer; Metabolism; Organochlorines; Pesticides; Risk factor
1. Introduction Breast cancer is a signi"cant cause of morbidity and death in North American women, yet relatively little is known about the etiology of this disease. Inherited susceptibility, as de"ned by a mutation in the breast cancer genes BRCA-1 and BRCA-2, accounts for only about ca 5% of cases. Risk factors known to increase susceptibility (early menarche, late menopause, nulliparity, or late age at "rst birth) may play a role in another one-third of cases [1]. This leaves the majority of women who develop breast cancer with dietary, socioeconomic, and other factors as possible causes. The steady increase in the incidence of breast cancer during the last several years has stimulated further concerns among breast cancer researchers and the general public. Some of the recorded increase in the disease can be attributed to better and earlier detection, but this does not completely explain the recent pattern [2, 3]. Environmental factors such as organochlorine compounds are receiving particular attention for their potential role in the etiology of breast cancer.
* Corresponding author. Tel.: (416) 946-6510; Fax: (416) 946-6590.
The organochlorines are a diverse group of ca 15 000 compounds which include DDT [1,1,1-trichloro-2,2bis( p-chlorophenyl)ethane], its metabolite DDE [1,1dichloro-2,2-bis(p-chlorophenyl)ethylene] and PCBs (polychlorinated biphenyls). Developed over 40 years ago, they include such chemicals as #ame retardants and pesticides, many of which have been banned since the early 1970s but are still found as persistent food chain contaminants because they are very stable and highly lipophilic. They "nd their way into human adipose tissue largely through the consumption of dietary fat. Although air and water pollution may contribute to the overall body burden of these compounds, contaminated food, primarily as "sh and animal fat, accounts for the bulk of human exposure. The slow elimination of these chemicals from the body has raised concerns with respect to the possibility that long-term, low-level exposure to these compounds via the food chain can result in large body accumulations and possible toxicity. While there has been speculation that organochlorine exposure results in direct genotoxicity, recent research has focused on the ability of these compounds to act as xenoestrogens [4]. This is an important concern considering that estrogen can promote breast cell proliferation and many of the established risk factors for breast cancer can be linked to total lifetime exposure to bioavailable estrogen.
0960-7404/99/$ - see front matter 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 7 4 0 4 ( 9 8 ) 0 0 0 1 0 - 3
2
M.A. Musgrave et al. / Surgical Oncology 7 (1999) 1}4
2. Epidemiological evidence Some epidemiological evidence exists that suggests a link between organochlorine exposure and breast cancer. In New York City, Wol! et al. [5] conducted a nested case-control study with a cohort of 14 000 women enrolled in a prospective study of breast cancer screening. This study determined the blood levels of organochlorine compounds in women who developed breast cancer and compared them to healthy matched controls. Cases were found to have signi"cantly higher levels of DDE (a DDT metabolite) in their blood compared to the controls. Another nested case-control study was conducted by Kreiger et al. [6] in the San Francisco Bay area. Serum samples from a random sample of 50 breast cancer cases from three racial groups were compared to an equal number of race-matched controls. While the overall data did not initially support a relationship between DDE and PCB exposure and the risk of breast cancer, further analysis that separated subjects by race found a positive relationship for both blacks and whites [7]. Furthermore, Dewailly et al. [8] conducted a study in which they analyzed breast tissue for the presence of organochlorines from 41 women undergoing biopsy. Patients, who upon pathological review were determined to have estrogen-responsive breast cancers, demonstrated a higher concentration of DDE and PCBs in their breast tissue. Finally, Aronson and Miller [9] conducted a pilot case-control study of 59 patients undergoing breast biopsy in Toronto. Breast adipose tissue was analyzed for the presence of 20 di!erent organochlorine compounds. Each of the mean body burdens of DDT, hexachlorobenzene (HCB), total PCBs, and "ve PCB congeners were higher (P(0.05) in the 26 breast cancer cases than in controls. After controlling for several potential confounders by logistic regression, these results suggest that breast cancer risk is associated with increased tissue levels of several organochlorines, including some PCB congeners. However, the small population size of this study necessitates caution in interpretation, and was therefore the motivation for a much larger study currently underway in Toronto funded by the Canadian Breast Cancer Research Initiative. Thus, while there is some epidemiological evidence to support an association between breast cancer and the organochlorine compounds, these studies do not suggest the mechanism by which these chemicals could possibly alter the disease process, if, in fact, they do.
3. Mechanism of action In order for organochlorine compounds to be considered etiological factors in breast cancer, a mechanism by which these compounds produce carcinogenesis must
be demonstrated. One mechanism by which these compounds may ultimately produce their e!ects is direct genotoxicity at the DNA level. Evidence that these compounds may act as genotoxins comes from animal studies that show organochlorine-induced carcinogenesis in a dose-response and tissue-dependent manner [10}14]. However, estimates of human risk have been primarily based on extrapolation from these animal studies and the organochlorine levels required to induce cancer are substantially higher than those typical of environmental exposure. Gold et al. [15] concluded that when viewed against the large background of naturally occurring carcinogens in typical portions of food, including bread, co!ee and mangos, the residues of synthetic pesticides or environmental pollutants ranked low. Overall, this study cast some doubt on the relative importance of low-dose exposure to synthetic chemicals with respect to human cancers. Another plausible mechanism in which organochlorine compounds could act as an etiological factor for breast cancer lies in their ability to act as xenoestrogens [4]. This is related to their estrogen receptor agonist activity, and to the fact that they are metabolized in a manner similar to the primary endogenous estrogen, estradiol. Estradiol is metabolized via one of two pathways that produce products with di!erent estrogenicity. The "rst pathway yields a product known as 2-hydroxyestrone, a compound with minimal estrogenic activity thought to be an inactive and benign metabolite. The second metabolic route yields 16-a-hydroxyestrone, a potent estrogen. It has been postulated that by either inhibiting the second pathway, ultimately reducing the amount of 16-a-hydroxyestrone, or by enhancing the "rst pathway of estrogen, which produces the benign metabolite, breast cancer risk could be reduced. The converse may also be true in that increasing estradiol metabolism through the pathway that produces the active metabolite (16-a-hydroxyestrone), or decreasing metabolism through the pathway that produces the inactive metabolite (2-hydroxyestrone), could increase cancer risk. Compounds acting as xenoestrogens may act in several di!erent ways to interrupt or alter estrogen functioning and metabolism. These compounds could displace endogenous estrogen from its receptor thereby altering the stimulating/inhibiting e!ects of estrogen at the level of the cell or tissue. They could also dominate the metabolic pathways of catabolism resulting in production of alternative compounds, which could also be estrogenic. The xenoestrogens may also alter the predominant metabolism of endogenous compounds resulting in increased usage of alternative pathways, and production of metabolic products with varying degrees of estrogenic activity. Xenoestrogens which a!ect the relative functioning of the two previously described pathways could be responsible for shifting the overall estrogenic balance and hence modifying the risk of breast cancer.
M.A. Musgrave et al. / Surgical Oncology 7 (1999) 1}4
Experimental evidence from animal studies has demonstrated that exposure to estrogens such as the organochlorines leads to increased levels of 16a-hydroxyestrone and that mice which naturally acquire spontaneous breast tumours seem to have more of this metabolite [16]. As well, several experiments have demonstrated that organochlorines, including PCBs, bind to the estrogen receptor and mimic the e!ect of another potent metabolite, 17-b-estradiol, with respect to induction of enzyme activity, cell proliferation and di!erentiation. Furthermore, reproductive anomalies have been found in animals born into ecosystems that are polluted by xenoestrogens, especially those compounds that are environmentally persistent [16]. While this evidence is striking, it is important to note that xenoestrogens are at least 1000-fold less potent than the endogenous, naturally occurring estrogens in a woman's body. Given this discrepancy and the relatively small amounts of xenoestrogens detected in body tissues, it is hard to reconcile the ability of these compounds to so drastically alter estrogen metabolism compared to the more potent and abundant endogenous estrogens. Hence, it seems unlikely that mere environmental exposure to organochlorines could signi"cantly shift the lifetime estrogen balance and increase breast cancer risk by a xenoestrogenic mechanism.
3
particular, CYP 1A1. Equally important is the fact that this same gene also codes for enzymes involved in the hydroxylation of estrogen. It may be that the presence of organochlorine compounds in the breast tissue signals an underlying susceptibility to altered metabolism for many compounds including endogenous estrogens and exogenous xenoestrogens. Altered products of metabolism may in essence be the culprits responsible for disease either through overproduction of reactive intermediates which themselves are genotoxic, or through a failure of secondary metabolism to appropriately detoxify these genotoxic compounds. Since the organochlorines induce the P450 enzyme system and serve as substrates for metabolic activation, individuals with speci"c P450 and GST genotypes may be at particular risk for breast cancer and the pathway could potentially be hormonally driven. Organochlorines could contribute to breast tumour development by altering estrogen levels within the breast, increasing the total lifetime exposure to bioavailable estrogen, or as just postulated, by interacting with certain genetic polymorphisms to produce metabolic byproducts which may be carcinogenic or genotoxic. The body burden of organochlorines may be a marker of a genetic polymorphism that signals the production of alternative estrogenic metabolites, which are, in turn, carcinogenic. Thus, organochlorine compounds found in breast tissue may be markers for susceptibility to breast cancer.
4. An alternate hypothesis It may be that organochlorines have neither direct (genotoxic) nor indirect (xenoestrogenic) e!ects with respect to breast cancer. Perhaps the organochlorine levels in women's bodies re#ect di!erences in endogenous estrogen metabolism and are thus only markers of altered risk or susceptibility. It is known that the cytochrome P450 enzymes play a role in the metabolism of various environmental agents. These enzymes act on aromatic hydrocarbons to produce intermediates that may react with DNA and/or induce cellular proliferation [17]. Detoxi"cation of these intermediates is largely through glucuronide conjugation mediated by the enzyme glutathione-S-transferase (GST). Human genetic polymorphisms for both P450 and GST enzymes appear to account for a substantial proportion of inter-individual variation in susceptibility to environmentally-induced cancers of several tissues including lung and bladder [17}19]. Individuals with speci"c alleles appear to be at an increased cancer risk. The P450 genes, CYP 1A1 and CYP 1A2, as well as the genotype responsible for the absence of the detoxifying GST enzyme, GSTM1 null, appear to be particularly important. Previous studies of GSTM1 polymorphism in breast cancer found that ca 50% of tumours are GSTM1 null [20}22]. Organochlorines have been shown to induce the cytochrome P450s by an up-regulation of gene activity, in
5. Conclusions It is evident that an integrated approach in examining the role of environmental compounds and risk of breast cancer is needed to help resolve the current literature controversy as well as the public debate on the role of organochlorines in breast cancer. Previous studies in the area of environmental factors and breast cancer have largely been based on epidemiological data which provided evidence for a role of organochlorines in breast cancer but did not attempt to detail the mechanisms of action of these compounds. Most associations are correlational and inconsistent and cannot provide a basis for any de"nitive statement on the role of environmental compounds in the development of breast cancer. Recent advances in molecular biology may allow these correlational studies to be taken one step further. By relating environmental observations to the alterations in metabolic function at the level of the gene, the puzzle linking environmental agents and breast cancer could possibly be solved. Whether this will lead to further progress with respect to the biology of breast cancer remains to be seen. Organochlorines, per se, may not be the ultimate carcinogens with respect to breast cancer. Rather, an increased body burden of organochlorines may be
4
M.A. Musgrave et al. / Surgical Oncology 7 (1999) 1}4
a marker of &at risk' individuals who have an underlying genetic predisposition which renders them susceptible to breast cancer unrelated to the organochlorine compounds. In this scenario, the genetic polymorphism may in#uence the tissue levels of the products of organochlorine or estrogen metabolism from either exogenous or endogenous sources. It is possible that these endogenous metabolic byproducts are the ultimate carcinogens. Thus, increased organochlorine levels may re#ect an underlying genetic susceptibility for breast cancer in individuals within a phenotypic subgroup.
References [1] Sideman H, Stellman SD, Mushinski M. A di!erent perspective on breast cancer risk factors: some implications of the nonattributable risk. CA: A Cancer Journal for Clinicians 1982;32:301}13. [2] Feurer EJ, Wun L-M. How much of the recent rise in breast cancer can be explained by increases in mammography utilization? A dynamic population model approach. American Journal of Epidemiology 1992;136:1423}36. [3] Hahn RA, Moolgackar S. Nulliparity, decade of "rst birth, and breast cancer in Connecticut cohorts, 1855}1945: an ecological study. American Journal of Public Health 1989;79:1503}7. [4] Davis DL, Bradlow HL, Wol! MS, Woodru! T, Hoel, DG, Anton-Culver H. Medical hypothesis: Xenoestrogens as preventable causes of breast cancer. Environmental Health Perspectives 1993;101:372}7. [5] Wol! MS, Toniolo PG, Lee EW, Rivera M, Dubin N. Blood levels of organochlorine residues and risk of breast cancer. Journal of the National Cancer Institute 1993;85:648}52. [6] Kreiger N, Wol! MS, Hiatt RA, Rivera M, Vogelman J, Orentreich N. Breast cancer and serum organochlorines: a prospective study among white, black and asian women. Journal of the National Cancer Institute 1994;86:589}99. [7] Millikan R, DeVoto E, Newman B, Savitz D. Studying environmental in#uences and breast cancer risk: suggestions for an integrated population-based approach. Breast Cancer Research and Treatment 1995;35:79}89. [8] Dewailly E, Dodin S, Verreault R, Ayotte P, Sauve L, Morin J, Brisson J. High organochlorine body burden in women with estrogen receptor-positive breast cancer. Journal of the National Cancer Institute 1994;86:232}4. [9] Aronson KJ, Miller AB. Environmental factors for breast cancer. Epidemiology 1996;7(4):S38.
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