Effects of continuous and cyclic combined oral contraceptives on mouse mammary gland structure

The breast goes through cycles of expansion, differentiation, and regression with every estrous/menstrual cycle, driven by hormonal stimulation followed by hormone withdrawal. The number of cycles over a lifetime is positively associated with breast cancer risk; this increased risk may be the result of hormone-stimulated epithelial growth, inflammatory tissue remodeling processes during hormone withdrawal, or both. The use of cyclic combined oral contraceptive (cyclic COC) regimen, that incorporates a hormone withdrawal period in each cycle, is associated with increased risk of ER/PR negative breast cancer, particularly in current COC users, and women who started COC use early in life. In contrast, the effect of extended-use (i.e., continuous) OC on mammary gland structure or breast cancer risk has not been evaluated. The purpose of the current study was to test the hypothesis that extended OC provides protection against breast cancer, by removing the hormone withdrawal phase associated with both cyclic OC and normal cycling. Starting at day 50 of age (to mimic COC exposure in young women), BALB/C mice were fed a fresh liquid diet daily containing no OC, or ethinyl estradiol and levonorgestrel, either continuously for 28 days, or as a cyclic regimen (three days OC, followed by one day basal liquid diet). After 28 days, 10 mice per group were sacrificed and mammary glands dissected for whole mount and histologic analysis. The extended OC group showed an average increase in epithelial density of 65% compared to control; the cyclic group was increased by 13%. However this increased epithelial density in both OC regimens was associated with more differentiated state, with a decrease in terminal end buds, TEB, frequency balanced by an increase in alveolar buds, and a trend towards increased ductal secretions. Cyclic OC increased Ki-67 staining in small mammary ducts (epithelium plus stroma). These suggest that exposure to cyclic OC increase the percentage of apoptosis in the mammary gland cells, which could be related to an increase of breast cancer risk. Both OC regimens increased the number of cells infiltrating the mammary adipose stroma. These results suggest that short-term exposure to continuous COC can induce a more differentiated state in nonparous mouse mammary glands, with a higher differentiation in continuous OC than in cyclic OC. At this point there is no in vivo evidence that suggest that extended OC provides protection against breast cancer, but this data suggests that continuous OC may protect against carcinogenic exposure.