Hormonal stimulation causes a persistent enhancement of p53 responses to γ-radiaton in mammary epithelium. Free

A218

Pregnancy early in reproductive life reduces the incidence of breast cancer by up to 50 percent and is mimicked in rodents by treating with estrogen and progesterone (E+P). Acute stimulation with E+P dramatically enhances p53 responses to ionizing radiation. Therefore, we determined whether the timing of E+P treatment altered responsiveness of p53 and whether the effect was persistent. Methods: BALB/c-Trp53+/+ were treated with 14-day release pellets (Innovative Research) containing 50 μg estradiol and 3 mg progesterone. Treatments were administered to prepubertal females (10-24 days) or mature females (6-8 weeks). Parous mice were also examined. The tissues were allowed to regress, then mice were exposed to 5 Gy whole-body radiation to examine nuclear accumulation of p53 and apoptotic responses (TUNEL) by immunohistochemical staining. All treatments were replicated in Trp53-/- mice to assure responses were p53-dependent. Results: The accumulation of nuclear p53 and apoptosis were significantly greater in mammary tissues from mice treated with E+P compared to age-matched, nulliparous females. The responsiveness of p53 in E+P-treated mice was similar to that obtained in mammary tissues from parous females. The effect was similar regardless of the age at which the mice were treated with E+P. To determine if the protective effects of parity are due to alterations in the endocrine system, mammary glands from E+P-treated mice or virgin mice were maintained for 3 days in whole organ culture with or without E+P. The apoptotic responses to irradiation were retained by the E+P-treated glands in culture even in the absence of E+P. In vivo, the apoptotic response to irradiation was retained in the mammary gland after removal of endogenous E and P by ovariectomy. However, epithelial transplants in virgin hosts show that the protective alterations are not retained by mammary epithelial cells after outgrowth because the apoptotic response to irradiation in the epithelial transplants from parous and E+P-treated donors did not differ when compared to epithelial transplants from nulliparous donors. Conclusions: These results demonstrate that exposure to elevated levels of ovarian steroids cause a sustained enhancement of responses to radiation-induced DNA damage through p53-dependent pathways. As the effect on p53 activity persists even after withdrawal of the hormones, it may play a significant role in the protective effect of pregnancy on breast cancer risk.