Estrogen metabolism and risk of breast cancer in postmenopausal women

Abstract

Background: Estrogens are recognized causal factors in breast cancer. Interindividual variation in estrogen metabolism may also influence the risk of breast cancer and could provide clues to mechanisms of breast carcinogenesis. Long-standing hypotheses about how estrogen metabolism might influence breast cancer have not been adequately evaluated in epidemiological studies because of the lack of accurate, reproducible, and high-throughput assays for estrogen metabolites.

Methods: We conducted a prospective case-control study nested within the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO). Participants included 277 women who developed invasive breast cancer (case subjects) and 423 matched control subjects; at PLCO baseline, all subjects were aged 55-74 years, postmenopausal and not using hormone therapy, and provided a blood sample. Liquid chromatography-tandem mass spectrometry was used to measure serum concentrations of 15 estrogens and estrogen metabolites, in unconjugated and conjugated forms, including the parent estrogens, estrone and estradiol, and estrogen metabolites in pathways defined by irreversible hydroxylation at the C-2, C-4, or C-16 positions of the steroid ring. We calculated hazard ratios (HRs) approximating risk in highest vs lowest deciles of individual estrogens and estrogen metabolites, estrogens and estrogen metabolites grouped by metabolic pathways, and metabolic pathway ratios using multivariable Cox proportional hazards models. All statistical tests were two-sided.

Results: Nearly all estrogens, estrogen metabolites, and metabolic pathway groups were associated with an increased risk of breast cancer; the serum concentration of unconjugated estradiol was strongly associated with the risk of breast cancer (HR = 2.07, 95% confidence interval [CI] = 1.19 to 3.62). No estrogen, estrogen metabolite, or metabolic pathway group remained statistically significantly associated with the risk of breast cancer after adjusting for unconjugated estradiol. The ratio of the 2-hydroxylation pathway to parent estrogens (HR = 0.66, 95% CI = 0.51 to 0.87) and the ratio of 4-hydroxylation pathway catechols to 4-hydroxylation pathway methylated catechols (HR = 1.34, 95% CI = 1.04 to 1.72) were statistically significantly associated with the risk of breast cancer and remained so after adjustment for unconjugated estradiol.

Conclusions: More extensive 2-hydroxylation of parent estrogens is associated with lower risk, and less extensive methylation of potentially genotoxic 4-hydroxylation pathway catechols is associated with higher risk of postmenopausal breast cancer.

Figure 1

Pathways of estrogen metabolism. Adapted from Ziegler et al. (2) and reproduced with permission from Environmental Health Perspectives. The estrogen metabolites are formed by irreversible hydroxylation of the parent estrogens, estrone and estradiol, at the C-2, C-4, or C-16 positions of the steroid ring. The relative size of the chemical structure indicates the relative concentration of the estrogen or estrogen metabolite in serum of postmenopausal women. The structures are for the unconjugated forms of estrogens and estrogen metabolites.

Figure 2

Forest plots of hazard ratios (HRs) for invasive breast cancer and 95% confidence intervals (CIs) by quintiles of unconjugated estradiol, total estrogens and estrogen metabolites, and selected metabolic pathway ratios. HRs (black rectangles) are shown on a log scale. Gray lines represent 95% CIs. Quintiles are abbreviated Q1, Q2, Q3, Q4, and Q5 and were defined based on distributions of these measures among control subjects weighted to represent the study cohort. Models shown on the right-hand side were adjusted for log-transformed (to the base 1.2) continuous unconjugated estradiol. All models were adjusted for age at study entry: 55–59, 60–64, 65–69, 70–74 years; period of blood collection: January 26, 1994, to September 29, 1997, September 30, 1997, to October 17, 2001; age at menarche: <12, 12–13 or missing, ≥14 years; combined parity and age at birth of first child: nulliparous, ≥1 live birth and age <20 years, 1–2 live births and age 20–29 years, ≥3 live births and age 20–29 years or missing parity or missing age, ≥1 live birth and age ≥30 years; age at natural menopause: <45, 45–49, 50–54, ≥55 years, missing; type of menopause: natural menopause, surgical menopause with both ovaries removed, other or missing; first-degree family history of breast cancer: yes, no or missing; personal history of benign breast disease: yes, no or missing; previous use of menopausal hormone therapy: former, never. P for trend was based on two-sided Wald tests of the coefficient associated with a unit increase in log_1.2-transformed unconjugated estradiol, in log_1.2-transformed total estrogens and estrogen metabolites, or in log_1.04-transformed metabolic pathway ratios.

Figure 3

Comparison of estimated absolute risks of breast cancer (expressed as incidence rates, in cases per 10⁵ person-years) for each study control subject. Estimated risks were based on relative hazard (Cox) models for the estrogen and estrogen metabolite profile of each control subject. One model included unconjugated estradiol only (x-axis), and the other model included both unconjugated estradiol and an additional measure of interest (y-axis): total estrogens and estrogen metabolites (A); the ratio of the 2-hydroxylation pathway to parent estrogens (B); the ratio of the 2-hydroxylation pathway to the 16-hydroxylation pathway (C); and the ratio of 4-hydroxylation pathway catechols to 4-hydroxylation pathway methylated catechols (D). Absolute risk estimates were calibrated using breast cancer incidence rates for white women, aged 60–64 years, in the 2004–2006 Surveillance, Epidemiology, and End Results population (357 cases per 10⁵ person-years), as described in the “Methods” section. Each dot represents two estimated absolute risks for each control subject. If the two risks are equal, they will fall on a diagonal line. Upper and lower dashed lines demarcate risk predictions that differ by at least 50 cases per 10⁵ person-years. To estimate relative hazards from Cox models, estrogen and estrogen metabolism measures were log-transformed and the following covariates were included: age at study entry: 55–59, 60–64, 65–69, 70–74 years; period of blood collection: January 26, 1994, to September 29, 1997, September 30, 1997, to October 17, 2001; age at menarche: <12, 12–13 or missing, ≥14 years; combined parity and age at birth of first child: nulliparous, ≥1 live birth and age <20 years, 1–2 live births and age 20–29 years, ≥3 live births and age 20–29 years or missing parity or missing age, ≥1 live birth and age ≥30 years; age at natural menopause: <45, 45–49, 50–54, ≥55 years, missing; type of menopause: natural menopause, surgical menopause with both ovaries removed, other or missing; first-degree family history of breast cancer: yes, no or missing; personal history of benign breast disease: yes, no or missing; previous use of menopausal hormone therapy: former, never.