Obesity, hormone therapy, estrogen metabolism and risk of postmenopausal breast cancer Francesmary Modugno 1,2 * , Kevin E. Kip 1 , Barbara Cochrane 3 , Lewis Kuller 1 , Thomas L. Klug 4 , Thomas E. Rohan 5 , Rowan T. Chlebowski 6 , Norman Lasser 7 and Marcia L. Stefanick 8 1 Department of Epidemiology, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, PA, USA 2 University of Pittsburgh Cancer Institute, Pittsburgh, PA, USA 3 Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA, USA 4 Immuna Care Corporation, Blue Bell, PA 19422, USA 5 Department of Epidemiology and Population Health, Albert Einstein College of Medicine, New York, NY, USA 6 Los Angeles Biomedical Research Institute, Torrance, CA, USA 7 University of Medicine and Dentistry of New Jersey, Newark, NJ, USA 8 Stanford University School of Medicine, Stanford, CA, USA Hormone therapy (HT) and body mass index (BMI) have been associated with postmenopausal breast cancer. Because estrogen metabolism may affect breast cancer risk and can be altered by weight and HT, it might play a role in the HT–BMI–breast cancer associations. We undertook a nested case-control study within the Observational Study of the Women’s Health Initiative. Baseline levels of 2- and 16a-hydroxy estrone (2-OHE1 and 16a-OHE1) were measured in 200 women who developed breast cancer during follow-up and 200 healthy controls matched to cases by ethnicity, enrollment date, clinic site, type of HT and years since menopause. Wilcoxon nonparametric tests were used to compare estrogen metabolite levels between cases and controls. Conditional logistic regression was used to assess the relationship between BMI, estro- gen metabolites and breast cancer risk. 16a-OHE1 levels were modestly but significantly higher in HT users among cases (median 356 pg/ml vs. 315 pg/ml) and controls (354 pg/ml vs. 298 pg/ml). 2-OHE1 levels were substantially and significantly higher in HT users among cases (369 pg/ml vs. 125 pg/ml) and con- trols (347 pg/ml vs. 134 pg/ml). For non-HT users only, greater BMI and higher 16a-OHE1 levels were individually and jointly associated with increased breast cancer risk (OR for women with high BMI and high 16a-OHE1 compared to those with low BMI and low 16a-OHE1 5 3.51, 95% CI 5 1.34–9.16). No associations between BMI, estrogen metabolism and breast cancer risk were found for HT users. Estrogen metabolism differs according to both BMI and HT use, potentially explaining the interaction between BMI and HT in relation to breast cancer risk. ' 2005 Wiley-Liss, Inc. Key words: breast neoplasms; hormone therapy; estrogen metabolism; body mass index; obesity The Women’s Health Initiative (WHI) clinical trial recently confirmed observational study findings 1,2 that progestin-contain- ing hormone therapy (HT) formulations are associated with an increased risk of postmenopausal breast cancer. 3 Elevated circu- lating estrogen levels may underlie this association, as substantial data, including markers of long-term estrogen exposure, 4 impli- cate estrogen in the etiology of postmenopausal breast cancer. However, for a given dose of estrogen, estrogen–only HT formula- tions appear to impart less risk than combined estrogen and pro- gestin therapies. 1,3,5–8 In particular, the WHI clinical trial reported no increase in risk associated with an estrogen-only formulation, 8 but a 24% increase in risk among women assigned to a combina- tion estrogen–progestin formulation compared to placebo. 3 Inter- estingly, this increase is less than expected, given the magnitude of the increase in circulating estrogens resulting from oral estro- gen preparations 9,10 and among studies that associate higher blood levels of estrogen with increased breast cancer risk. 11,12 Body size has also been implicated in postmenopausal breast cancer. 13,14 For example, risk increases by 3% for each kilogram per meter square increase in body mass index (BMI). 15 Again, cir- culating elevated estrogens levels, produced by the aromatization of androgens in adipose tissue, are believed to underlie this associ- ation, although other mechanisms, including effects of growth fac- tors and inflammation, have been proposed. 16 Interestingly, the breast cancer–BMI association appears limited to women not using HT, 13,17–20 and the mechanism mediating the BMI–HT interaction remains unknown. The fact that both HT use and high BMI elevate circulating estrogen levels suggests that estrogens might mediate the observa- tions described above. In the liver, estradiol, the estrogen most strongly associated with breast cancer, 21 is first (reversibly) con- verted to estrone, which is irreversibly converted to either 2- or 16a-hydroxy estrone in target cells. 22 Because the 2-OHE1 and 16a-OHE1 metabolic pathways compete for a limited substrate pool, an increase in metabolism of estrone via one pathway will reduce the amount of product in the competing pathway. 22 Both metabolites have estrogenic properties, although to varying degrees. 16a-OHE1 retains potent hormonal activities by binding strongly to the estrogen receptor. 22 Laboratory data 23–25 and some but not all epidemiologic studies 26–34 suggest a positive associa- tion between 16a-OHE1 levels and breast cancer risk. In contrast, 2-OHE1 binds to the estrogen receptor with reduced affinity 35 and because of O-methylation is cleared rapidly from circulation. 36 Data suggest that this metabolite does not increase breast cancer risk and, in fact, may impart some protection. 26–28,37,38 While genetic factors may play a role in determining whether the 2-OHE1 or the 16a-OHE1 pathway predominates, 39 body weight is also important. In thin women, the 2-OHE1 pathway predominates whereas in overweight women, the 16a-OHE1 path- way predominates. 40 Evidence also suggests that exogenous agents, including both pharmaceuticals and dietary factors, may alter the way by which estrogens are metabolized. 41,42 Of rele- vance here, HT regimens containing a progestin may cause a greater shift to the 16a-OHE1 pathway than estrogen-only regi- mens. 43 The observations that both weight and type of HT influen- ces estrogen metabolism led us to investigate whether estrogen metabolism may explain, in part, the less-than-expected increase in breast cancer risk associated with HT, the greater risk imparted by progestin-containing HT formulations compared to estrogen- only formulations and the interaction between BMI and HT in relation to breast cancer risk. Grant sponsors: National Heart, Lung, and Blood Institute, National Institutes of Health, Department of Health and Human Services; Grant sponsor: National Cancer Institute; Grant numbers: K07-CA80668, R21- CA95113, and R25-CA57703; Grant sponsor: Department of Defense; Grant number: DAMD17-02-1-0550. *Correspondence to: Department of Epidemiology, Graduate School of Public Health, 516A Parran Hall, 130 DeSoto Street, Pittsburgh, PA 15261, USA. Fax: 1412-383-2653. E-mail: modugno1@pitt.edu Received 24 May 2005; Accepted after revision 12 July 2005 DOI 10.1002/ijc.21487 Published online 13 September 2005 in Wiley InterScience (www. interscience.wiley.com). Int. J. Cancer: 118, 1292–1301 (2006) ' 2005 Wiley-Liss, Inc. Publication of the International Union Against Cancer