THE ETIOPATHOGENESIS OF BREAST-CANCER PREVENTION

Breast cancer, the most frequent malignancy diagnosed in women, contin ues to increase in incidence in all industrialized nations. The fact t hat this disease becomes incurable once it has spread to regional or d istant sites indicates that its complexity is beyond our present level of knowledge. A better understanding of the etiopathogenesis and biol ogy of breast cancer is required in order to develop a rational basis for its prevention and therapy. The observation that early parity redu ces the risk of developing breast cancer indicates that reproductive a nd hormonal conditions might play an important role in its prevention. The elucidation of the mechanisms mediating this protection requires the availability of adequate experimental models. The induction of rat mammary carcinomas with chemical carcinogens has proven to be useful for these purposes, especially since, in this model, full-term pregnan cy or treatment of virgin rats with a placental hormone, human chorion ic gonadotropin (hCG), prior to the administration of the carcinogen p rotects the mammary gland from tumor development. Since both pregnancy and hCG treatment induce differentiation of the mammary gland, this p rocess is considered to be essential for the inhibition of the neoplas tic process. The possibility of preventing breast cancer by treating y oung nulliparous females with hormones that mimic a full term pregnanc y is of practical interest to the human female population, but it requ ires a thorough knowledge of the development of the human breast. Our studies indicate that the breast of postpubertal nulliparous women is composed of lobular structures reflecting different stages of developm ent. Type I lobules are the most undifferentiated. Type 2 lobules evol ve from the previous ones; they are composed of a higher number of duc tular structures per lobule. They progress to lobules types 3 and 4, w hich are present in the breast during pregnancy and lactation. The typ e 1 lobule, considered to be the site of origin of ductal carcinomas, predominates in the breast of nulliparous women of all ages. In parous women, the type 3 lobule is the most frequent. Primary cultures deriv ed from breast tissues composed of type 1 lobules express phenotypes o f cell transformation not observed in cells derived from type 3 lobule s. These data acquire relevance in the light that women with a history of early pregnancy are at a lower risk of developing breast cancer th an nulliparous women, an effect attributed to differences in the degre e of differentiation of the breast. Pregnancy furthers the differentia tion of type 1 lobules to type 3, making them refractory to neoplastic transformation. These data provide useful intermediate endpoints for evaluating new strategies developed for breast cancer prevention.