Synergistic tumor promoter effects of estrone and progesterone in methylnitrosourea-induced rat mammary cancer

Tumor promoter effects of steroid hormones were tested in ovariectomized rats treated with the tumor initiator, methylnitrosourea. Continuous treatment with low doses of estrone induced tumor in 55% of experimental animals while intact control animals had a 96–100% incidence. Although progesterone alone produced no tumors in ovariectomized, methylnitrosourea-treated animals, estrone and progesterone acted synergistically, inducing tumorigenesis in 88% of treated rats. These results point to an important interaction between estrogens and progestins in promoting mammary tumorigenesis.