Abstract While the molecular mechanisms by which the progesterone receptor (PR) regulates breast tumor growth and metastasis have not been fully elucidated, recent studies highlight the fact that PR has a complex role in breast cancer progression that may be dependent on ligand dose. For example, data from the Women's Health Initiative trial suggest that low-dose progestins used in combination with estrogen are associated with an increased risk of invasive breast cancer compared to estrogen alone. On the other hand, high-dose progestin therapy has been used as an effective front-line and second-line treatment for advanced, metastatic breast cancer. To investigate this paradox, we undertook studies to identify aspects of PR biology that respond differently to treatment with low or high doses of progestins. Interestingly, we found that treatment of T47D breast cancer cells with low-dose progestins generates a significantly more robust induction of a subset of PR target genes, including the cell cycle regulators cyclin D1 and E2F1, than treatment with high-dose progestins. Furthermore, we show that varying doses of progestins have differential effects on the receptor itself, including the subcellular localization, phosphorylation and subsequent turnover of PR. At the level of transcriptional regulation, our ChIP analyses have led us to hypothesize that the strength of PR activation of target gene expression may hinge upon the ratio of phospho-PR versus unmodified PR that is recruited to enhancer elements. This dose-dependent effect may result in the formation of different PR transcriptional complexes and explain the diverse downstream biologies that are regulated by varying levels of progestins throughout the female reproductive cycle and in diseases such as breast cancer. Given the wide range of medical therapies that utilize progestins, our findings may have important clinical implications. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4542. doi:10.1158/1538-7445.AM2011-4542