Imbalance between apoptosis and cell proliferation during early stages of mammary gland carcinogenesis in ACI rats

Estrogen and ionizing radiation are well-documented human breast carcinogens, yet the exact mechanisms of their deleterious effects on mammary gland remain to be discerned. Here we analyze the balance between cellular proliferation and apoptosis in the mammary glands of rats exposed to estrogen and X-ray radiation and the combined action of these carcinogenic agents.For the first time, we show that combined exposure to estrogen and radiation has a synergistic effect on cell proliferation in the mammary glands of ACI rats, as evidenced by a substantially greater magnitude of cell proliferation, especially after 12 and 18 weeks of treatment, when compared to mammary glands of rats exposed to estrogen or radiation alone. We also demonstrate that an imbalance between cell proliferation and apoptosis, rather than enhanced cell proliferation or apoptosis suppression alone, may be a driving force for carcinogenesis. Our studies further suggest that compromised functional activity of p53 may be one of the mechanisms responsible for the proliferation/apoptosis imbalance.In sum, the results of our study indicate that evaluation of the extent of cell proliferation and apoptosis before the onset of preneoplastic lesions may be a potential biomarker of breast cancer risk after exposure to breast carcinogens.