Unliganded progesterone receptor-mediated targeting of an RNA-containing repressive complex silences a subset of hormone-inducible genes

Abstract

A close chromatin conformation precludes gene expression in eukaryotic cells. Genes activated by external cues have to overcome this repressive state by locally changing chromatin structure to a more open state. Although much is known about hormonal gene activation, how basal repression of regulated genes is targeted to the correct sites throughout the genome is not well understood. Here we report that in breast cancer cells, the unliganded progesterone receptor (PR) binds genomic sites and targets a repressive complex containing HP1g (heterochromatin protein 1g), LSD1 (lysine-specific demethylase 1), HDAC1/2, CoREST (corepressor for REST [RE1 {neuronal repressor element 1} silencing transcription factor]), KDM5B, and the RNA SRA (steroid receptor RNA activator) to 20% of hormone-inducible genes, keeping these genes silenced prior to hormone treatment. The complex is anchored via binding of HP1g to H3K9me3 (histone H3 tails trimethylated on Lys 9). SRA interacts with PR, HP1g, and LSD1, and its depletion compromises the loading of the repressive complex to target chromatinpromoting aberrant gene derepression. Upon hormonal treatment, the HP1g-LSD1 complex is displaced from these constitutively poorly expressed genes as a result of rapid phosphorylation of histone H3 at Ser 10 mediated by MSK1, which is recruited to the target sites by the activated PR. Displacement of the repressive complex enables the loading of coactivators needed for chromatin remodeling and activation of this set of genes, including genes involved in apoptosis and cell proliferation. These results highlight the importance of the unliganded PR in hormonal regulation of breast cancer cells.

More help than hindrance

“…1). 7 Already one minute after hormone treatment pPR interacts with and activates the CyclinA/CDK2 complex, which is recruited to regulated promoters and phosphorylates histone H1. 8 Hormone activated CDK2 also phosphorylates poly (ADP-ribose) polymerase 1 (PARP1), which converts NAD+ to poly(ADP-ribose) (PAR) attached to itself and to linker and core histones, facilitating displacement of histone H1.…”

RNAs — Physical and functional modulators of chromatin reader proteins

“…Similarly, RNA-mediated recruitment of HP1γ to specific gene loci has been described in human cells [38]. Together with progesterone receptor (PR) and LSD1, HP1γ associates with a specific RNA known as steroid receptor RNA activator (RNA SRA) to repress a large set of hormone-inducible genes in the absence of ligand.…”

“…and a reader of the histone modification (e.g. Cbx7, HP1) in cis or in trans thereby recruiting these proteins to specific gene loci [16,33,38]. (C) Drosophila and/or mammalian HP1 binds to nascent mRNA of transcriptionally induced gene loci and/or directly interacts with active RNA polymerase II.…”

Nuclear receptors and chromatin: an inducible couple

“…It will also be important to determine the role of unliganded NRs in development as well as disease. An intriguing study has found that unliganded PR forms a repressor complex with a non-coding RNA and several epigenetic silencers including histone demethylases LSD1, KDM5B and histone deacetylases HDAC1/2 (Vicent et al 2013). In other words, unactivated PR maintains repressed chromatin structures at progesterone target gene enhancers, while the epigenetic landscape is reversed upon the interaction of PR with its ligand (Vicent et al 2013).…”