Estrogen Inhibition of Mammary Tumor Growth in Rats; Counteraction by Prolactin

Abstract

Summary Daily injection of 20 μg of estradiol benzoate for 20 days into female Sprague-Dawley rats with DMBA-induced mammary adenocarcinomas, produced a decrease in average number and mean total tumor diameter per rat. Control rats not treated with estrogen showed an increase in average number and total tumor diameter per rat. When rats were injected daily with both estrogen and 1 mg of ovine prolactin for 20 days, there was no reduction in average number of tumors and a significant increase in average total tumor diameter per rat. These results suggest that large doses of estrogen directly inhibit stimulation by prolactin of mammary tumor growth.