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Date of release: 21 December, 2009

Estrogen metabolites and breast cancer


It has been proposed that a shift toward 2-hydroxyestrone (2-OHE1) from the 16α-hydroxyestrone (16-OHE1) metabolic pathway may be inversely associated with breast cancer risk because 2-OHE1 is thought to be less genotoxic and estrogenic than 16-OHE1. In the recently published study of Arslan and colleagues [1], serum levels and the ratio of the estradiol metabolites 2-OHE1 and 16-OHE1 were investigated in 377 incident premenopausal women and 377 matched controls and the risk of breast cancer was calculated. No significant associations between breast cancer risk and estradiol metabolite concentrations were observed.

Comment

Discussion is ongoing whether the natural estrogen, 17β-estradiol (E2) per se acts as a carcinogenic compound, especially in regard to breast cancer development. Research in the last 10 years has indicated that estradiol metabolites may play a more important role than their parent substance in the development of breast cancer [2].
 
These metabolites are not, as was previously assumed, inactive metabolic products destined for excretion, but may have important functions to fulfil, for example in the cardiovascular system and in malignant diseases [3, 4]. The main metabolites formed by A-ring metabolism are 2-OHE1 and 4-hydroxyestrone (4-OHE1), and those formed by D-ring metabolism are 16-OHE1 and estriol. In vitro experiments have demonstrated that 2-OHE1 may act in a neutral way or even in an anticarcinogenic way when converted into 2-methoxyestradiol, whereas 16-OHE1 appears to have potent estrogenic properties and thus may act in a carcinogenic way. Certain estradiol metabolites have stronger activities on proliferation and apoptosis than their parent substance [5].
 
Using primary cultures of cells from the terminal duct lobular unit, measurements were carried out in low-risk patients, i.e. patients undergoing mammary reduction, and in high-risk patients, i.e. patients with mammary carcinomas. The synthesis of 16-OHE1 was only slightly increased in low-risk patients, but was markedly increased in high-risk patients [6]. Studies of estrogen metabolism showed that, in mice with a high risk of mammary carcinoma, 16-hydroxylation was significantly increased compared to mice with a low tumor risk [7]. 2-Hydroxylation showed no risk-dependent differences.
 
Ongoing from these results, it was hypothesized that the ratio of 2-OHE1 to 16-OHE1 may determine a woman’s risk of developing breast cancer. Several observational trials have investigated this 2-OHE1 : 16-OHE1 ratio in pre- and postmenopausal women. However, existing studies have had different designs, have been conducted in different populations, and some have only small sample sizes. Thus, their results are inconsistent, with some showing a strong inverse association of increasing values of the 2-OHE1 : 16-OHE1 ratio with breast cancer, some showing a modest association, and still others showing no association. The highest consistency was found for postmenopausal women. However, in premenopausal women, the proposal is still under scrutiny, since here only two prospective studies are available so far. 
 
The results from Arslan and colleagues [1] further underline a questionable relationship between the 2-OHE1 : 16-OHE1 ratio and breast carcinogenesis in premenopausal women. However, our own investigations have indicated that statistical analysis using absolute values and the ratio of the estradiol metabolites as calculated by Arslan and colleagues may have disadvantages as compared to using the relative changes of the metabolites [8]. Therefore, it would also be of interest to calculate the relative changes. In addition, we were able to demonstrate that certain factors such as body mass index can strongly influence the ratio [8]. It can be assumed that other, as yet not defined, confounding factors may exist.
 
Whether the 2-OHE1 : 16-OHE1 ratio may play a role in the development of breast cancer still remains unclear. Perhaps further elucidation might be available if studies are conducted using sophisticated laboratory methods to measure especially the 4-hydroxyestrogens, which appear to have a more potent carcinogenic property as compared to 16-OHE1, according to in vitro and in vivo experiments [4, 5].
 
However, there are indications that additional factors such as certain genetic polymorphisms and excessive oxidative stress are necessary to influence the synthesis of intermediate aggressive metabolites, which may already have gene toxicity at low concentrations and can destruct DNA structures and/or have the potency to influence carcinogenesis by intensive proliferative actions [9]. It remains doubtful whether physiological estradiol per se can be causally carcinogenic by proliferative processes. 
 
A comprehensive statement on the carcinogenicity of combined hormone therapy and hormonal contraceptives, as classified by the World Health Organization [10], has been published after discussions during the World Congress of Menopause in 2005 (Buenos Aires) [11].

Comentario

Alfred O. Mueck
Head of the Department of Endocrinology and Menopause; Head of the Center of Womens Health, University Womens Hospital, Tuebingen, Germany

    References

  1. Arslan AA, Shore RE, Afanasyeva Y, et al. Circulating estrogen metabolites and risk for breast cancer in premenopausal women. Cancer Epidemiol Biomarkers Prev 2009;18:22739. Published August, 2009.
    http://www.ncbi.nlm.nih.gov/pubmed/19661086

  2. Yager JD, Davidson NE. Estrogen carcinogenesis in breast cancer. N Engl J Med 2006;354:270-82.
    http://www.ncbi.nlm.nih.gov/pubmed/16421368

  3. Lippert TH, Seeger H, Mueck AO. Estrogens and the cardiovascular system: role of estradiol metabolites in hormone replacement therapy. Climacteric 1998;1:296-301.
    http://www.ncbi.nlm.nih.gov/pubmed/11907937

  4. Mueck AO, Seeger H, Lippert TH. Estradiol metabolism and malignant disease. Maturitas 2002;43:1-10.
    http://www.ncbi.nlm.nih.gov/pubmed/12270576

  5. Seeger H, Wallwiener D, Krmer E, et al. Comparison of possible carcinogenic estradiol metabolites: effects on proliferation, apoptosis and metastasis of human breast cancer cells. Maturitas 2006;54:72-77.
    http://www.ncbi.nlm.nih.gov/pubmed/16213115

  6. Osborne MP, Bradlow HL, Wong GY, et al. Upregulation of estradiol C16 alpha-hydroxylation in human breast tissue: a potential biomarker of breast cancer risk. J Natl Cancer Inst 1993;85:1917-1920.
    http://www.ncbi.nlm.nih.gov/pubmed/8230281

  7. . Bradlow HL, Hershcopf RJ, Martucci CP, et al. Estradiol 16alpha-hydroxylation in the mouse correlates with tumor incidence and presence of murine mammary tumor virus: a possible model for the hormonal etiology of breast cancer in humans. Proc Natl Acad Sci USA 1985;82:6295-9.
    http://www.ncbi.nlm.nih.gov/pubmed/2994069

  8. Seeger H, Zimmermann B, Brunner S, et al. Estradiol metabolites in postmenopausal women with and without breast cancer a case control study. Senologie 2008;5:211.


  9. Mueck AO, Seeger H. Breast cancer: are estrogen metabolites carcinogenic? Climacteric 2007;10(Suppl2):62-5.
    http://www.ncbi.nlm.nih.gov/pubmed/17882675

  10. Cogliano V, Grosse Y, Baan R, et al.; WHO International Agency for Research on Cancer. Carcinogenicity of combined oestrogen-progestagen contraceptives and menopausal treatment. Lancet Oncol 2005;6:552-3.
    http://www.ncbi.nlm.nih.gov/pubmed/16094770

  11. Schneider HP, Mueck AO, Kuhl H. IARC Monographs on carcinogenicity of combined hormonal contraceptives and menopausal therapy. Climacteric 2005;8:311-16.
    http://www.ncbi.nlm.nih.gov/pubmed/16390765