Promoter and Cofactor Requirements for SERM-ER Activity

摘要

We aimed to understand the mechanisms by which Estrogen Receptor can regulate transcription both in a positive context after estrogen stimulation and in a negative context after tamoxifen. We originally planned to develop a methodology for specifically isolating chromatin to assess associated proteins but due to technical limitations we developed the ChiP-on-chip technique to map ER binding sites initially on a chromosome-wide scale but eventually on a genome-wide level. These experiments revealed surprising insight into ER acbon namely that ER rarely binds to promoter sequences but in a majority of cases binds to regions distant from transcription start sites. We also found an enrichment of motifs that gave us insight into the factors involved in augmenting ER activity. This led to the identification of FoxAl as a pioneer factor which we subsequently showed to be an essential component in recruiting ER to the chromatin. The whole-genome analysis revealed another class of cofactors including Oct-I C/EBPa and AP-I proteins. We used this information to identify the mechanisms by which ER can repress gene transcription. This included physiologic squelching at the early time point and direct repression via AP-I elements at the later time points. This study revealed exceptional information about ER function and provides the first complete list of the cis- regulatory elements that may be involved in breast cancer resistance to tamoxifen.