In this prospective study of postmenopausal women who were not taking hormone therapy, higher serum levels of testosterone were associated with a lower risk of ER-negative breast cancer, independent of putative breast cancer risk factors and serum estradiol level. In addition, a higher level of both testosterone and estradiol was associated with an increased risk of ER-positive breast cancer, but the association between testosterone level and ER-positive breast cancer was partially explained by estradiol level.
To our knowledge, this is the first prospective study to report a statistically significant inverse association between serum testosterone level and the risk of ER-negative breast cancer. Indeed, only a few reports (7
) have evaluated associations between sex hormones and ER-negative breast cancer separately from ER-positive breast cancer. The Nurses’ Health Study (8
) reported that women in the higher quartiles of testosterone level had a lower risk ER- and PR-negative breast tumors compared with those in the lowest quartile (relative risk [RR] for Q2 vs Q1 = 0.4, 95% CI = 0.2 to 1.1; RR for Q3 vs Q1 = 0.6, 95% CI = 0.3 to 1.6; RR for Q4 vs Q1 = 0.7, 95% CI = 0.3 to 1.6); however, the associations were not statistically significant, perhaps because of the small sample size of ER-negative tumors (n = 38). The two other studies, which included 30 (7
) and 23 case subjects (13
), also observed no statistically significant associations between testosterone (7
) or estradiol levels (7
) and ER-negative breast cancer.
Our results regarding the associations between testosterone and estradiol levels and ER-positive breast cancer confirm previous findings. For example, the 2002 meta-analysis of data from nine prospective cohort studies by Key et al. (5
) reported strong associations between higher levels of estradiol and testosterone and the overall risk of breast cancer. Of the several studies published subsequently (3
), all but one (26
) observed statistically significant associations between higher sex hormone levels and higher risk of breast cancer, particularly for ER-positive tumors. In an updated report from the Nurses’ Health Study (8
), women whose testosterone and estradiol levels were in the highest quartiles had a twofold to threefold higher risk of ER- and PR-positive breast cancer compared with women in the lowest quartile. In the Study of Osteoporotic Fractures (3
), testosterone was more strongly associated with the risk of ER-positive breast cancer than estradiol. Having a testosterone concentration in the highest two quintiles conferred a fourfold higher risk of ER-positive cancer compared with having a testosterone concentration in the lowest quintile, independent of estradiol level. However, estradiol level was not statistically significantly associated with the risk of ER-positive breast cancer after adjustment for testosterone level. By contrast, results of our study indicate that estradiol level is more strongly associated with ER-positive breast cancer risk than testosterone level, and further suggest that the effect of testosterone is partially explained by estradiol.
Our finding that endogenous testosterone level has opposite associations with ER-negative and ER-positive breast cancer risks is biologically plausible. The role of testosterone in the development of breast cancer has been largely attributed to its conversion to estrogen by aromatase and the resultant stimulation of mammary cell proliferation via ER activation. However, preclinical evidence indicates that testosterone also has an antiproliferative effect on mammary cell growth that is regulated by the androgen receptor (11
). For example, in vitro, testosterone inhibits the proliferation of breast cancer cell lines in a dose-dependent manner and is a more potent inhibitor of proliferation in ER-negative cell lines compared with ER-positive cell lines (27
). Furthermore, adding physiological doses of testosterone to estrogen therapy markedly inhibited estrogen-induced mammary cell proliferation in ovariectomized rhesus monkeys and rats (28
). Given this dual role of testosterone as both a stimulator and an inhibitor of mammary cell growth, the net effect of testosterone on the development of breast cancer could depend on the ER status of the tumor. In ER-positive breast tumors, the androgen receptor–mediated inhibitory effect of testosterone on proliferation may be countered by the ER-mediated proliferative effect. However, in ER-negative tumors, testosterone could have an overall antiproliferative effect.
Whereas preclinical studies have suggested that testosterone supplementation may have a protective effect against breast cancer, the clinical evidence for such an effect is limited. Combined estrogen and testosterone use was associated with an increased risk of breast cancer in the Nurse’s Health Study (HR for current vs never users = 2.5, 95% CI = 1.53 to 4.04) (31
) and in the WHI-OS (HR for current users vs nonusers = 1.42, 95% CI = 0.95 to 2.11) (32
), but in only the former study was the association statistically significant. A clinical trial investigating the effect of testosterone patch use without estrogen on sexual dysfunction in postmenopausal women produced inconclusive results regarding the effects on breast cancer risk (33
). By contrast, a retrospective analysis of 631 postmenopausal women who received testosterone in addition to conventional hormone therapy observed a breast cancer incidence rate close to that expected in never-users of hormone therapy, a finding that based on only 12 breast cancer cases (34
). Although none of these studies evaluated associations by tumor ER status, it is likely that the majority of breast cancers that occurred in those studies were ER positive, given that the study populations consisted of postmenopausal women.
The strengths of this study include the rigorous centralized adjudication of breast cancer cases in a large and well-characterized cohort of postmenopausal women, and the large number of incident ER-negative case subjects with endogenous testosterone and estradiol measurements. The study limitations include the assessment of sex hormone concentrations at a single point in time and the fact that circulating levels of testosterone and estradiol may not reflect the local hormonal milieu of the breast tissue. In addition, it is possible that women who were diagnosed with breast cancer during follow-up may have had subclinical disease at baseline. However, excluding women who were diagnosed with breast cancer in the first year after their enrollment in the study did not affect the strength or statistical significance of the observed associations. Moreover, we did not conduct analyses by joint categories of tumor ER and PR status because only a small percentage of cancers were positive for one receptor but negative for the other. Finally, our results may not be generalizable to postmenopausal women who use hormone therapy, racial groups other than non-Hispanic whites, and premenopausal women.
In conclusion, this study provides evidence that higher levels of bioavailable testosterone are associated with lower risks of ER-negative breast cancer in postmenopausal women. These results shed new light on the etiology of ER-negative breast cancer and further reinforce the need to assess risk separately for ER-positive and ER-negative breast cancers. In addition, we observed that higher levels of bioavailable testosterone and estradiol are associated with an increased risk of ER-positive breast cancer. These findings confirm previous reports and further suggest that the effect of testosterone on ER-positive breast cancer is partially explained by that of estradiol.