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Experimental evidence shows cross-talk in mammary cells between estrogen, insulin-like growth factor I (IGF-I) and their respective receptors and possible synergistic effects of estrogen receptor (ER) activation and increased IGF-I signaling with regard to breast tumor development, and epidemiological evidence suggests that circulating IGF-I levels may be related more to the risk of ER-positive than ER-negative breast cancer. Using a case-control study nested within the prospective European EPIC cohort (938 breast cancer cases and 1,394 matched control subjects), we analyzed the relationships of prediagnostic serum IGF-I levels with the risk of estrogen and progesterone receptor-positive and -negative breast tumors. IGF-I levels were positively associated with the risk of ER+ breast tumors overall (pre- and postmenopausal women combined, odds ratio (OR)Q4-Q1 = 1.41 [95% confidence interval (CI) 1.01-1.98] for the highest vs. lowest quartile; OR = 1.17 [95% CI 1.04-1.33] per 1-standard deviation (SD) increase in IGF-I, ptrend = 0.01) and among women who were diagnosed with breast cancer at 50 years or older (ORQ3-Q1 = 1.38 [95% CI 1.01-1.89]; OR = 1.19 [95% CI 1.04-1.36] per 1-SD increase in IGF-I, ptrend = 0.01) but not with receptor-positive disease diagnosed at an earlier age. No statistically significant associations were observed for ER- breast tumors overall and by age at diagnosis. Tests for heterogeneity by receptor status of the tumor were not statistically significant, except for women diagnosed with breast cancer at 50 years or older (phet = 0.03 for ER+/PR+ vs. ER-/PR- disease). Our data add to a global body of evidence indicating that higher circulating IGF-I levels may increase risk specifically of receptor-positive, but not receptor-negative, breast cancer diagnosed at 50 years or older. What's new Both estrogen and insulin-like growth factor (IGF-I) promote breast cancer formation, and evidence suggests the two may work together. Some breast tumors express estrogen receptor, others don't. Does the presence of estrogen receptor allow IGF-I to stimulate tumor formation To address this question, the authors compared women's IGF-I levels before diagnosis with their risk of developing breast cancer, with or without estrogen receptor. They found a direct relationship between IGF levels and risk of ER-positive breast tumors diagnosed after age 50. They found no association with ER-positive tumors diagnosed at an earlier age, nor with ER-negative tumors. © 2013 UICC.

Results from prospective studies on premenopausal serum hormone levels in relation to breast cancer risk have been inconclusive, especially with regard to tumor subtypes. Using a case-control study nested within the prospective European Prospective Investigation into Cancer and Nutrition (EPIC) cohort (801 breast cancer cases and 1,132 matched control subjects), we analyzed the relationships of prediagnostic serum estradiol, free estradiol, progesterone, testosterone, free testosterone and sex hormone-binding globulin (SHBG) levels with the risk of breast cancer by estrogen and progesterone receptor-positive and -negative breast tumors and by age at diagnoses. Higher prediagnostic serum levels of testosterone and free testosterone were associated with an increased overall risk of breast cancer [ORQ4-Q1 = 1.56 (95% CI 1.15-2.13), ptrend = 0.02 for testosterone and ORQ4-Q1 = 1.33 (95% CI 0.99-1.79), ptrend = 0.04 for free testosterone], but no significant risk association was observed for estradiol, free estradiol, progesterone and SHBG. Tests for heterogeneity between receptor-positive and -negative tumors were not significant. When analysis were stratified by age at tumor diagnosis, the odds ratios observed for estradiol were stronger and borderline significant for breast cancer diagnosed at age less than 50 [ORQ4-Q1 = 1.32 (95% CI 0.87-2.01), ptrend = 0.05] compared to breast cancer diagnosed at age 50 or above [ORQ4-Q1 = 0.94 (95% CI 0.60-1.47), ptrend = 0.34, phet = 0.04]. In conclusion, our data indicate that higher premenopausal circulating testosterone levels are associated with an increased risk of developing breast cancer, but do not show a significant association of estradiol or progesterone with breast cancer risk, overall, by menstrual cycle phase or by tumor receptor status, although a possible risk increase with higher estradiol levels for tumors diagnosed before age 50 was seen.