The histone deacetylase inhibitor trichostatin A blocks progesterone receptor-mediated transactivation of the mouse mammary tumor virus promoter in vivo.

Post-translational modifications of histones play an important role in modulating gene transcription within chromatin. We used the mouse mammary tumor virus (MMTV) promoter, which adopts an ordered nucleosomal structure, to investigate the impact of a specific inhibitor of histone deacetylase, trichostatin A (TSA), on progesterone receptor-activated transcription. TSA induced global histone hyperacetylation, and this effect occurred independently of the presence of hormone. Interestingly, chromatin immunoprecipitation analysis revealed no significant change in the level of acetylated histones associated with the MMTV promoter following high TSA treatment. In human breast cancer cells, in which the MMTV promoter adopts a constitutively "open" chromatin structure, treatment with TSA converted the MMTV promoter into a closed structure. Addition of hormone did not overcome this TSA-induced closure of the promoter chromatin. Furthermore, TSA treatment resulted in the eviction of the transcription factor nuclear factor-1 from the promoter and reduced progesterone receptor-induced transcription. Kinetic experiments revealed that a loss of chromatin-remodeling proteins was coincident with the decrease in MMTV transcriptional activity and the imposition of repressed chromatin architecture at the promoter. These results demonstrate that deacetylase inhibitor treatment at levels that induce global histone acetylation may leave specific regulatory regions relatively unaffected and that this treatment may lead to transcriptional inhibition by mechanisms that modify chromatin-remodeling proteins rather than by influencing histone acetylation of the local promoter chromatin structure.