Abstract
In the etiology of breast cancer, estrogens and its metabolites play a key role as tumor initiators and promoters. Co-expression of estrogen synthesizing enzymes (aromatase and steroid sulfatase) and estrogen metabolizing enzymes (CYP1A1 and CYP1B1) in breast tissue makes it plausible that locally formed estrogens and estrogen metabolites may reach
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concentrations sufficiently high to evoke tumorigenic effects. Exposure to exogenous compounds can alter estrogen metabolism by affecting enzyme levels or catalytic activity. In addition, several functional polymorphisms have been described for the estrogen metabolizing enzymes that can result in different catechol estrogen levels.
The aim of this thesis was to study altered estrogen metabolism and the implications for breast carcinogenesis. Phase I metabolism of estrogens is described in chapter two. We describe the influence of several dioxin-like compounds on estrogen 2-hydroxylase (CYP1A1) and estrogen 4-hydroxylase (CYP1B1) activity. Special emphasis is placed on the effects on the estrogen 4-/2-hydroxylation ratio, which is suggested to provide a marker for neoplastic breast tissue. A tumorigenic (MCF-7) and a non-tumorigenic (MCF-10A) human mammary epithelial cell line were used in this study and the usefulness of the estrogen 4-/2-hydroxylation ratio as biomarker for elevated breast cancer risk is discussed. Chapter three and four focus on the phase II metabolism of estrogens by COMT. In chapter three, we investigated the effects of several naturally occuring compounds (phytochemicals) on COMT activity in cytosol from healthy mammary tissues obtained from reduction mammoplasty. Some phytochemicals have a catechol structure, which makes them potential substrates for the COMT enzyme. As a result, competition between catechol estrogens and phytochemicals with a catechol structure, and decreased inactivation of catechol estrogens can occur. Implications of decreased COMT activity caused by phytochemicals for catechol estrogen-induced DNA damage were studied in the MCF-7 cell line. Chapter four describes the effects of impaired COMT activity on cell growth, catechol estrogen-induced DNA damage and cell cycle status in the MCF-7 and MCF-10A cell lines. COMT appears to play a crucial role in determining the genotoxic potential of the catechol estrogens. We showed that this effect is largely attributed to 2-MeOE2,
which is known to exert a variety of anti-tumor actions.
Another aim of this thesis was to investigate the use of CYP1A1 and CYP1B1 in human peripheral blood lymphocytes as biomarkers of exposure to dioxins and dioxin-like compounds. This is described in the last two chapters. In chapter five, we studied the interindividual differences in constitutive and induced catalytic activity and gene expression of CYP1A1 and CYP1B1 in human lymphocytes of ten healthy female volunteers. Lymphocytes were cultured and exposed to TCDD or the less potent dioxin-like PCB126. In addition, the possible influence of the CYP1A1*2A and CYP1B1 Val432Leu polymorphisms was studied. Furthermore, the suitability of CYP1A1 and CYP1B1 expression levels in human lymphocytes as biomarkers of exposure to environmental factors was addressed by comparing the effects we found in vitro, with the concentrations of dioxins and dioxin-like compounds that can be found in human blood. In chapter six we studied the use of CYP1B1 as biomarker of exposure in a human population as part of a European project (PCBRISK). This study reports the effect of the CYP1B1 Val432Leu polymorphism on the correlation between CYP1B1 gene expression level in peripheral lymphocytes and exposure to environmental factors within a human population exposed to environmental pollution as a consequence of the 25-year long production of PCBs
in eastern Slovakia.
A summary and general discussion of the results and implications of these studies is given in chapter seven.
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