Transcriptional profiles of progestogen effects in the postmenopausal breast

Estrogen plus progestin hormone therapy has been associated with increased breast proliferation, breast density, and breast cancer risk in postmenopausal women, beyond that seen with estrogen alone. The goal of this study was to evaluate progestogen effects on gene expression profiles in the breast contributing to this promotional effect. Twenty-five ovariectomized adult female cynomolgus monkeys were given the following treatments (expressed as equivalent doses for women) in a randomized crossover design: (1) placebo; (2) oral estradiol (E2, 1 mg/day); (3) E2 + micronized progesterone (P4, 200 mg/day); and (4) E2 + medroxyprogesterone acetate (MPA, 2.5 mg/day). Treatments were given for two months, and breast biopsies were taken after each treatment period. On microarray analysis E2 + MPA treatment resulted in a greater number of significantly regulated genes compared to E2 + P4 and E2 alone (P < 0.0001). Treatment with E2 alone induced modest effects on select genes related to epidermal growth factor receptor (EGFR) activity which were augmented by the addition of MPA but not P4, consistent with patterns of epithelial cell proliferation. Genes induced by E2 + MPA included the EGFR ligands EGF, TGFA, and AREG, and downstream targets such as STAT5A, STAT5B, SRC, EIF4EBP1, and MYC. Progestogens showed mixed antagonistic effects on E2-induced genes which tended to be greater for P4 than MPA. These findings suggest that a standard dose of oral E2 + MPA has a more pronounced effect on gene expression in the breast compared to E2 alone or E2 + P4 and that promotional effects of E2 + MPA may be mediated in part by increased EGFR activity.