Oral contraceptives and breast cancer. In vitro effect of contraceptive steroids on human mammary cell growth.

The proliferative response of human mammary epithelial cells cultured in medium containing oral contraceptive steroids, singly and in combination, was measured. Cells came from 59 normal, nonmalignant atypical, and malignant breast tissue specimens. Growth of most cultures was stimulated by the estrogens 17 beta-estradiol and ethinyl estradiol, and any combination containing ethinyl estradiol. Most (75%) malignant specimens were stimulated by one or more of the progestins, whereas none of the nonmalignant cells were. For two of the estrogen/progestin combinations found in many birth control pills, malignant cells were stimulated to grow faster than nonmalignant cells. It was concluded that oral contraceptive steroids can stimulate the growth of mammary epithelia in cell culture and might do the same in vivo. These data corroborate epidemiologic data which suggest oral contraceptives might act as tumor promoters, especially in the older user, who is more likely to have malignant cells already present in the breast.