Abstract
We have previously demonstrated that transcription of the luteinizing hormone receptor (LHR) gene is subject to repression by histone deacetylation at its promoter region, where a histone deacetylase (HDAC)/mSin3A complex is anchored at a proximal Sp1 site. The present studies have shown that epigenetic silencing and activation of the LHR gene is achieved through coordinated regulation at both the histone and DNA levels. The HDAC inhibitor trichostatin A (TSA) evoked robust but significantly lower activation of the LHR gene in JAR than in MCF-7 cells. This effect was localized to the 176-bp promoter region, which is highly methylated in JAR and lightly methylated in MCF-7 cells. Consequently, TSA and the DNA demethylating reagent 5-azacytidine (5-AzaC) caused marked synergistic activation of the LHR gene in JAR but not in MCF-7 cells. Multiple site-specific lysine acetylation of H3/H4 is associated with such LHR gene activation. Methylation or acetylation of H3 at K9 is present at the silenced and derepressed LHR promoter, respectively. While DNA methylation levels did not affect the histone code of the LHR gene promoter, demethylation of the promoter CpG sites was necessary for maximal stimulation of this gene. Mechanistically, the combined actions of TSA and 5-AzaC, but not either 5-AzaC or TSA alone, resulted in complete demethylation of the LHR gene promoter in JAR cells. Release of the repressive HDAC/mSin3A complex from the LHR gene promoter in both cell types required both TSA-induced changes of histone modifications and, concurrently, a demethylated promoter. Also, Dnmt1 was largely dissociated from the LHR gene promoter in the presence of TSA or TSA plus 5-AzaC, and binding of MBD2 in JAR cells was diminished upon conversion of the promoter to a demethylated state. Such changes induced a more permissive chromatin where recruitment of polymerase II and TFIIB to the promoter was significantly increased. The activated state of the LHR gene induced by TSA and 5-AzaC in JAR and MCF-7 cells was observed basally in LHR-expressing PLC cells, in which the promoter is unmethylated and associated with hyperacetylated histones. Consequently, PLC cells are unresponsive to drug treatment. These findings have elucidated a regulatory mechanism whereby concurrent dissociation of repressors and association of activators and basal transcriptional components, resulting from coordinated histone hyperacetylation and DNA demethylation, lead to derepression of the LHR gene expression.