TIP48/Reptin and H2A.Z Requirement for Initiating Chromatin Remodeling in Estrogen-Activated Transcription

Abstract

Histone variants, including histone H2A.Z, are incorporated into specific genomic sites and participate in transcription regulation. The role of H2A.Z at these sites remains poorly characterized. Our study investigates changes in the chromatin environment at the Cyclin D1 gene ( CCND1) during transcriptional initiation in response to estradiol in estrogen receptor positive mammary tumour cells. We show that H2A.Z is present at the transcription start-site and downstream enhancer sequences of CCND1 when the gene is poorly transcribed. Stimulation of CCND1 expression required release of H2A.Z concomitantly from both these DNA elements. The AAA+ family members TIP48/reptin and the histone variant H2A.Z are required to remodel the chromatin environment at CCND1 as a prerequisite for binding of the estrogen receptor (ERα) in the presence of hormone. TIP48 promotes acetylation and exchange of H2A.Z, which triggers a dissociation of the CCND1 3′ enhancer from the promoter, thereby releasing a repressive intragenic loop. This release then enables the estrogen receptor to bind to the CCND1 promoter. Our findings provide new insight into the priming of chromatin required for transcription factor access to their target sequence. Dynamic release of gene loops could be a rapid means to remodel chromatin and to stimulate transcription in response to hormones.

Author Summary

Our study investigates changes in the chromatin environment at the Cyclin D1 gene that are a prerequisite for transcriptional initiation in response to estradiol. Gene expression is under control of chromatin structure. Histone variants, including histone H2A.Z, are incorporated into specific genomic sites and participate in transcription regulation. We show that H2A.Z is present at the transcription start-site and downstream enhancer sequences of CCND1 when the gene is poorly transcribed. Stimulation of CCND1 expression required release of H2A.Z concomitantly from both these DNA elements. The TIP48/reptin protein, which is part of several chromatin remodeling complexes, also associated with the CCND1 regulatory elements. Here, TIP48 promotes exchange of H2A.Z, which triggers a dissociation of the CCND1 enhancer from the promoter, thereby releasing a repressive intragenic loop. This release then enables estrogen receptor binding to the CCND1 promoter. Acetylation of H2A.Z is required for these processes. Our findings provide new insight into the priming of chromatin required for transcription factor access to their target sequence. Hence, we propose a new model for early events in transcription activation that were not shown before. Specifically, release of looping could be a rapid means to activate transcription efficiently in response to stimuli, in particular estrogen.