Endocrine Abstracts

The Estrogen Receptor (ER) is a well-studied, ligand dependent transcription factor. We have analysed, by kinetic chromatin immunoprecipitation (ChIP) assays, the mechanisms by which the ER modulates the permissiveness and transcriptional activity of the PS2 gene. Sequential ChIP assays were used to define the cyclical association of approximately 50 different proteins to the pS2 promoter. Key results from these studies are (i) that unliganded and liganded ER direct cycling on target promoters, with only liganded ER directing the cyclical association and activation of polymerase II; (ii) that there are two phases to cyclical activity, an association phase where the intermediate and basal transcriptional machinery become recruited to the promoter and a clearance phase in which transcription factors generally regarded as repressive, such as histone deacetylases (HDAC’s) and chromatin remodelling complexes, act to reset the promoter to allow a subsequent cycle to commence.

This interplay between the ER and chromatin is further underlined by experiments that evaluate the effects of HDAC inhibitors on estrogen signaling. As suggested by the involvement of HDAC activity in transcriptional cycling, inhibition of HDAC activity induced a profound anti-estrogenic effect on gene expression profiles. This notwithstanding, analysis of promoters repressed in response to inhibition of deacetylase activity showed that local histones were in fact deacetylated. This effect is likely to result from the action of NAD dependent deacetylase activity (the sirtuin class of deacetylases).

We also showed that CpG islands associated with the transcriptional start site become methylated in response to deacetylase inhibition. In summary, ER induces transcriptional cycling and this defines additional targets that can be exploited to modulate estrogen signaling. Proof of concept that this model can indeed by used to inhibit estrogen signaling by novel mechanistic strategies have been obtained using inhibitors of Zn dependent deacetylase activity.