Background: Estradiol and estrone are well-established risk factors for postmenopausal breast cancer (BC). Experimental evidence suggests that specific estrogen metabolites, produced via irreversible hydroxylation of estrone and estradiol at the 2- or 16-position may independently influence carcinogenesis.
Methods: We performed a nested case-control study of BC (328 BC cases; 639 controls) among postmenopausal women within the Nurses' Health Study (NHS)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 liquid chromatography-tandem mass spectrometry. Multivariable conditional logistic regression, adjusting for BC risk factors, estimated relative risks (RR) and 95% confidence intervals (CI) of BC across quintiles of individual EM, EM pathways, and pathway ratios. Associations by estrogen/progesterone receptor (ER/PR) status were analyzed by unconditional logistic regression.
Results: Estradiol and estrone were strongly associated with increased BC risk [estradiol: RRQ5v.Q1 (95% CI)=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 [RRQ5v.Q1=3.09 (1.81-5.27), p-trend<0.001], and remained so after adjusting for unconjugated estradiol [RRQ5v.Q1=2.23 (1.25-3.96), p-trend=0.01]. While the 16-hydroxylation pathway was modestly associated with risk [RRQ5v.Q1=1.62 (1.03-2.54), p-trend=0.01], the association was attenuated after unconjugated estradiol adjustment [RRQ5v.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.
Conclusions: In this cohort of postmenopausal women, 2-hydroxylation of estrone and estradiol was associated with increased BC risk, independent of unconjugated estradiol.
Impact: These results highlight the need to revisit the role of estrogen metabolism in BC etiology and prevention.
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