Abstract

The oncostatic effects of melatonin on the mammary gland have been studied in transgenic mice carrying the N-ras proto-oncogene under the control of the MMTV-LTR. Female (4-week-old) virgin mice with positive transgenic pedigrees were injected with melatonin (200 micrograms/mouse/ day, five times a week) or vehicle late in the evening. After 5 months of treatment, animals were sacrificed and the mammary glands were dissected for whole mounts, histology, and immunohistochemical analysis with a mouse monoclonal antibody specific for N-ras protein. Mammary glands of control transgenic mice showed different densities of hyperplastic alveolar nodules (HANs) consisting primarily of dysplastic epithelial cells with nuclear atypia and prominent nucleoli. The epithelial cells of HANs showed a high expression of N-ras while no immunostaining was detected in the unaffected mammary parenchyma. Only one (10%) of the control transgenic mice presented an infiltrating ductal carcinoma with the neoplastic cells overexpressing N-ras protein. The mammary glands of melatonin treated mice had a lower density of HANs, absence of epithelial dysplastic cells, and weak immunostaining of N-ras protein in comparison to the vehicle-treated group. None of the melatonin treated animals developed mammary carcinomas during the observation period. The lymph nodes of the inguinal mammary glands of all the vehicle-treated transgenic mice presented hyperplasia and two animals even had lymphomas, whereas in melatonin-treated animals there was less hyperplasia (two cases were atrophic) and a lack of lymphomas. We conclude that in the mammary glands of MMTV-LTR/N-ras transgenic female virgin mice, melatonin a) reduces the incidence of HANs and the expression of N-ras protein in focal hyperplastic lesions, b) completely prevents the development of epithelial cell atypia and mammary adenocarcinomas, and c) also reduces the hyperplasia of the mammary lymphoid tissue and prevents the development of lymphomas.