Influence of perinatal genistein exposure on the development of MNU-induced mammary carcinoma in female Sprague–Dawley rats

Genistein, a phytoestrogen, was subcutaneously (s.c.) injected to pregnant Sprague–Dawley CD rats on gestational days 16–20 at either 25 mg (Group 1) or 5 mg/day (Group 2). Female offspring of mothers not exposed to genistein during pregnancy received 12.5 mg genistein s.c. at neonatal days 15 and 18 (Group 3), or received vehicle only (Group 4). At 35 days of age, 4–9 female offspring from each group were autopsied to observe the influence of genistein, and remainder of female offspring received 50 mg/kg N-methyl-N-nitrosourea (MNU) intraperitoneally and were sacrificed when mammary tumors were larger than 1 cm in size or when they reached 35 weeks of age. Genistein treatment during the perinatal period resulted in lower body weight and lower relative uterine-ovarian weight at 35 days, and a prolonged estrus cycle with a long estrus phase at 12–16 weeks. However, at 35 days (time at MNU administration), mammary gland development, cell proliferation rate (PCNA labeling index), and the number of estrogen receptor (ER)- and progesterone receptor (PgR)-positive cells were similar between genistein-treated and untreated rats. Twenty-five or 5 mg genistein/day in utero (between days 16 and 20 of gestation) or 12.5 mg genistein/day on neonatal days 15 and 18 did not affect the incidence of mammary tumors >1 cm or the latency but did increase the number of mammary cancer lesions when MNU was administered at the time when the mammary gland growth in genistein-treated and untreated rats was similar. Thus, perinatal genistein is an endocrine disrupter and increases the multiplicity of MNU-induced mammary carcinoma in rats.