Ligand-dependent corepressor LCoR: a modulator of estrogen and progesterone target gene expression

Ligand-dependent corepressor LCoR was discovered almost ten years ago in the laboratory of Dr. John White through a yeast two-hybrid screen in which the ligand binding domain (LBD) of the estrogen receptor alpha (ERalpha) was used as bait. LCoR represses gene transcription by recruitment of the corepressor C-terminal binding proteins (CtBPs) and histone deacetylases (HDACs) through distinct domains. The action of HDACs strengthens the interactions of histones with negatively charged DNA resulting in stable nucleosomal and chromatin structures and repression of transcription. CtBPs interact with many transcriptional repressors through a common motif P/VLDLS/TXK/R, of which there are two in LCoR. We hypothesize that LCoR is a regulator of endogenous nuclear receptor target gene expression. Initial data characterized the function of LCoR as a nuclear receptor (NR) corepressor by using a series of in vitro and over expression assays. LCoR was also shown to have many protein-interacting domains that might contribute in its corepressor function. The goal of this thesis was to determine the role of LCoR in the control of endogenous estrogen and progesterone target gene expression. Through reporter gene assays with overexpression of LCoR and truncated forms of LCoR lacking the interacting domains, we have shown that both HDAC6 and CtBP1 contribute to transcriptional corepression observed with LCoR. To find out whether these cofactors are recruited to regulatory regions of endogenous ERalpha and PR target genes, ChIP and reChIP assays techniques were applied. These have shown a ligand-depenedent and orderly corecruitment of LCoR, HDAC6, CtBP1 and corresponding NR on estrogen and progesterone target genes. SiRNA-mediated gene silencing of LCoR, HDAC6 and CtBP1 in combination with reporter gene assay showed a transcriptional derepression in ERalpha- and PR-mediated gene transcription, confirming the transcriptional corepressor function of these coregulators. However, the in%%%%Il y a dix ans, le laboratoire de Dr. John White a decouvert un corepresseur nucleaire surnommer LCoR. La fonction du corepresseur est dependante de la presence d'agoniste. LCoR a ete decouvert grâce au systeme du double hybride de levure pour lequel le domaine de liaison du ligand (LBD) du recepteur estrogen alpha (ERalpha) a ete utilise comme appât. LCoR inhibe la transcription d'un gene en s'associant avec le corepresseur C-terminal binding proteins (CtBPs) et les histones deacetylases (HDACs). L'association de LCoR avec ces deux familles proteiques ce fait a travers deux domaines distincts. L'action enzymatique des HDACs stabilisent les structures du nucleosome et des histones en renforcant les liens entres ces derniers et l'ADN. Cette stabilite structurale a pour but d'inhiber la transcription genique. Les CtBPs interagissent avec un grand nombre de represseurs transcriptionnel en s'associant au motif d'interaction P/VLDLS/TXK/R, dont LCoR en possede deux. Nous emettons l'hypothese que LCoR est un regulateur transcriptionnel des genes cibles de…