Circulating Estrogen Metabolites and Risk of Breast Cancer among Postmenopausal Women in the Nurses' Health Study

Estradiol and estrone are well-established risk factors for postmenopausal breast cancer. Experimental evidence suggests that specific estrogen metabolites, produced via irreversible hydroxylation of estrone and estradiol at position 2 or 16, may independently influence carcinogenesis.

We performed a nested case-control study of breast cancer (328 breast cancer cases; 639 controls) among postmenopausal women within the Nurses' Health Study to examine the role of estrogens and estrogen metabolites (jointly referred to as EM). Plasma concentrations of each EM (unconjugated + conjugated forms) were measured by LC-MS/MS. Multivariable conditional logistic regression, adjusting for breast cancer risk factors, estimated relative risks (RR) and 95% confidence intervals of breast cancer across quintiles of individual EM, EM pathways, and pathway ratios. Associations by estrogen receptor (ER) and progesterone receptor (PR) status were analyzed by unconditional logistic regression.

Estradiol and estrone were strongly associated with increased breast cancer risk [estradiol: RRQ5 vs. Q1 (95% confidence interval) = 2.64 (1.64-4.26); estrone: 2.78 (1.74-4.45); both P-trends <0.001]. The 2-hydroxylation pathway was strongly associated with risk [RRQ5 vs. Q1 = 3.09 (1.81-5.27); P-trend <0.001] and remained so after adjusting for unconjugated estradiol [RRQ5 vs. Q1 = 2.23 (1.25-3.96); P-trend = 0.01]. Although the 16-hydroxylation pathway was modestly associated with risk [RRQ5 vs. Q1 = 1.62 (1.03-2.54); P-trend = 0.01], the association was attenuated after unconjugated estradiol adjustment [RRQ5 vs. Q1 = 1.24 (0.77-1.99); P-trend = 0.19]. Similar positive associations with the 2-pathway and 16-pathway were observed for ER+/PR+ and ER-/PR- tumors.

In this cohort of postmenopausal women, 2-hydroxylation of estrone and estradiol was associated with increased breast cancer risk, independent of unconjugated estradiol.

These results highlight the need to revisit the role of estrogen metabolism in breast cancer etiology and prevention. See related In the Spotlight, p. 367.