Recent evidence suggests that constitutive activation of NF?B is associated with more aggressive estrogen receptor (ER) positive tumors, the development of tamoxifen resistance, and progression to estrogen-independent growth. To date, the major mode of crosstalk between ER and NF?B that has been described is mutual transrepression, where ER antagonizes NF?B activity and NF?B antagonizes ER activity, but whether transrepression contributes to breast tumor progression is not known. Our preliminary data suggest that it is the rapid, robust and synergistic up-regulation of multiple genes by ER and NF?B acting in a synergistic rather than an antagonistic manner that may be the major mechanism of crosstalk between these two factors in breast cancer cells. Furthermore, our findings suggest that ER and NF?B interaction may play an essential role in breast cancer cell survival. Our overall objective is to understand the functional and mechanistic significance of synergistic gene regulation by ER and NF?B in breast cancer. Our hypothesis is that activation of NF?B in ER+ breast tumors leads to synergistic up-regulation of pro-survival and drug resistance genes, which contribute to breast tumor progression. To explore this hypothesis, we propose to examine the effect of NF?B activation and inhibition on ER+ breast cancer cell survival and tumor growth in response to therapeutic drugs (Aim 1). In these studies, we will focus our attention on the role of one synergistically regulated, cell survival gene in cancer cell drug response and its expression in human ER positive breast tumors. To investigate the mechanism by which ER and NF?B synergistically regulate gene transcription, we will first examine whether a unique combination of response elements in the 5' flanking region of synergistically regulated genes contributes to synergy through cooperative ER and NF?B DNA binding and enhanced RNA Pol II recruitment and activation (Aim 2). In addition, we will focus on how the gene specific recruitment of SRC/p160 coactivators, and other known histone acetyltransferases, contributes to synergistic gene transcription through enhanced histone acetylation (Aim 3). Our transcriptional studies will be coupled with survival assays to determine if the same underlying mechanisms are essential for both. Taken together these studies are designed to provide insight into the molecular mechanisms of synergistic crosstalk between ER and NF?B and the importance of this crosstalk in the progression of hormone-dependent breast cancer.