The effects of oestrogens and progesterone on the chemical induction of mammary cancer in mice of the IF strain.

Abstract

This experiment tests the hypothesis that progesterone may be necessary in addition to estrogen for the induction of mammary cancer in mice treated with the carcinogen 20-methylcholanthrene. Groups of IF strain (a low breast cancer strain) mice receiving hormonal treatments (estradiol dipropionate progesterone in varying dose schedules) were given a standard limited dose of 20-methylcholanthrene applied to the skin. Breast tumors occurred in 38% of normal virgin females but in only 3% of normal males and 10% of ovariectomized females receiving estrogen as the only hormonal treatment. The progesterone-estrogen combination treatment significantly increased the incidence of mammary cancer in normal males (30%) and in ovariectomized females (82%). In the groups of mice with a high incidence of mammary cancer histological changes reported as precancerous were frequent even when the individual mouse had no tumor; these precancerous changes were rare or absent in groups with a low incidence of breast cancer even in the few animals with breast cancer. The findings suggest that progesterone when combined with estradiol dipropionate acts as a promoting agent in mammary carcinogenesis induced by 20-methylcholanthrene. It is also possible that progesterone is important in the development of breast tumors in mice possessing the genetic agent.