Selenium and the genesis of murine mammary tumors.

One hundred forty-seven female Sprague-Dawley rats were divided into 5 groups and at 60 days of age were treated i.g. with 5 mg of 7,12-dimethylbenzanthracene (DMBA) suspended in 1.0 ml of sesame oil. Selenium (Se), as selenium dioxide (SeO2), was administered in the drinking water to 4 of the 5 groups (30 rats/group) at 2 doses (2 and 4 mg/l) from 30-90 days of age (series 1) and from 90-150 days of age (series 2) prior to the onset of palpable mammary tumors. One group of rats (27 rats) served as controls. All rats were palpated weekly for mammary tumors and sacrificed 28 weeks after DMBA treatment. Total number of palpable mammary carcinomas which developed in each group were: controls, 60; series 1, 2 mg Se dose, 27, 4 mg Se dose, 29; series 2, 2 mg Se dose, 24, 4 mg Se dose, 32. Each dose level of Se in each series significantly (P less than 0.05) reduced the incidence of mammary carcinomas. These results provided evidence that Se can inhibit the early promoting phases of polycyclic hydrocarbon induced mammary carcinogenesis in female rats. Two hundred twenty-six nulliparous and 99 multiparous GR mice were treated daily with estrogen and progesterone for 13-16 weeks. Se (SeO2) was administered in the drinking water (2 mg/l) to one-half of these mice. Total number of mammary carcinomas in control nulliparous and multiparous mice were 119 and 90, respectively; in Se treated nulliparous and multiparous mice, 113 and 81, respectively. Se did not significantly effect mammary carcinoma incidence in hormone treated nulliparous and multiparous GR mice.