Abstract 863: Molecular alterations associated with the protective effect of short-term estradiol treatment against mammary carcinogenesis

Early childbirth is the only known physiological factor to have a protective effect against the development of breast cancer. Rats that have been exposed to carcinogens before or after a full-term pregnancy are protected from mammary carcinogenesis. Hormonal prevention strategies have used exogenous hormonal treatment to mimic the protective effect of early full-term pregnancy against breast cancer. We have previously demonstrated that short-term administration of pregnancy levels of estradiol alone or in combination with progesterone confers long-term protection against mammary carcinogenesis. In the present study our aim was to define a gene and protein expression profile that would serve as a signature for assessing the efficacy of the protective hormone treatment. Nine week old female Lewis rats were divided into the three following groups (n=15): (i) control, (ii) 10µg estradiol (non-protective dose), and (iii) 200µg estradiol (protective dose). Each treatment was given in the form of silastic capsule and continued for the length of gestation (3 weeks) in rats. At the end of the treatment, the silastic capsule was removed. The rats were terminated 8 and 16 weeks following removal of the hormone treatment. Mammary glands were removed, immediately snap frozen. Our data indicate that several genes like igfbp5, Rbp2, Egr1, igfbp2, Cd47, Prkra, Perp, Tdag, Spint2 and Hat1 involved in apoptosis, DNA repair and growth inhibition are upregulated in the mammary glands of rats that received pregnancy levels of estradiol treatment compared to the rats that received non-pregnancy levels of estradiol and untreated controls. By contrast, there were consistent decreases in genes involved in growth regulation (II18, THBS, Tshr), angiogenesis (MMp11), anti-apoptosis (Mt1a) and cell cycle regulation (Cdk2,Cdk4) in the mammary glands of pregnancy level estradiol treated rats compared to the non pregnancy level estradiol treated rats and untreated controls. These alterations were consistent at both time points indicating that these changes are constitutive. Furthermore, estrogen receptor pathway analysis revealed that ER-α is downregulated following administration of 10µg and 200µg estradriol. ER-β is upregulated in the 200 µg estradiol treatment and downregulated in the 10 µg estradiol treatment. In conclusion, short term treatment with pregnancy levels of estradiol leads to persistent down regulation of genes involved in, cell cycle, growth promotion, anti-apoptosis, o