Aromatase (product of CYP19 gene), the critical enzyme in estrogen biosynthesis, is upregulated in 70% of all breast cancers and is highly correlated with cyclooxygenase 2 (COX-2), the rate-determining enzyme in prostanoid biosynthesis. Expression of COX-2 also is correlated with the oncogene HER-2/neu. The efficacy of current endocrine therapies for breast cancer is predicted only if the tumor contains significant amounts of estrogen receptor (ER). Since the progesterone receptor (PR) is an estrogen-induced target gene, it has been suggested that its presence may serve as an indicator of ER functional capacity and the differentiation state of the tumor. In the present study, we tested the hypothesis that PR serves a crucial protective role by antagonizing inflammatory response pathways in the breast. We observed that progesterone antagonized the stimulatory effects of cAMP and IL-1 on aromatase, COX-2 and HER-2/neu expression in T47D breast cancer cells. These actions of progesterone were associated with increased expression of the NF-B inhibitor, IB. In 28 breast cancer cell lines, IB expression was positively correlated with PR mRNA levels; overexpression of a phosphorylation-defective mutant of IB inhibited expression of aromatase, COX-2 and HER-2/neu. Moreover, in breast cancer cell lines cultured in the absence of progesterone, upregulation of endogenous PR caused decreased expression of aromatase, COX-2 and HER-2/neu expression, while downregulation of endogenous PR resulted in a marked induction of aromatase and HER-2/neu mRNA. Collectively, these findings suggest that PR plays an important anti-inflammatory role in breast cancer cells via ligand-dependent and ligand-independent mechanisms.