Feminization of male mouse liver by continuous growth hormone infusion or loss of EZH1/2: activation of sex-biased transcriptional networks and dynamic changes in chromatin states
Lau Corona, Dana
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The sex-dependent pituitary growth hormone (GH) secretory profiles, pulsatile in males and persistent in females, regulate sex-biased expression of hundreds of genes in mammalian liver, contributing to sex differences in hepatic metabolism and disease. The sex-biased GH actions in the liver are mediated by STAT5b and enhanced by a network of transcription factors including the male-biased BCL6 and the female-specific CUX2, acting in the context of sex-biased chromatin states. First, the transcriptional and epigenomic changes induced by continuous-GH infusion (cGH) in male mice, which rapidly feminizes the temporal profile of liver STAT5 activity, were examined. RNA-seq analysis determined that cGH repressed the majority of male-biased genes and induced most female-biased genes within 4-days; however, several highly female-specific genes showed partial feminization. Female-biased genes already in an active chromatin state in male liver were induced early; genes in an inactive chromatin state often responded late. Early cGH-responsive genes included Cux2 and Bcl6 and their targets. DNase-seq and ChIP-seq were used to identify changes in sex-specific chromatin accessibility and histone modifications accompanying these cGH-induced gene expression changes. H3-K27me3 is a key sex-biased repressive mark found preferentially at highly female-biased genes in male mouse liver. Consistently, induction of female-biased genes by cGH was associated with loss of H3-K27me3 at their gene bodies. H3K27 methylation is catalyzed by Polycomb Repressive Complex-2 (PRC2) through its homologous catalytic subunits EZH1 and EZH2. An Ezh1-knockout mouse model with a hepatocyte-specific knockout of Ezh2 (DKO) was used to further investigate the role of H3-K27me3 in repressing sex-biased genes in mouse liver. Loss of Ezh1/Ezh2 led to a significant decrease in sex-specific gene expression, with many female-biased genes induced and male-biased genes repressed. These gene responses were more extensive in male than female liver, as was the loss of H3K27me3 sites and the reciprocal increases in active histone marks. There was substantial up-regulation of liver cancer and liver fibrosis-related genes in male and female DKO-mouse liver, with a subset of genes preferentially up-regulated in females. Thus, GH regulated sex-biased liver physiology is dictated by transcription factors arranged in a hierarchical network and by dynamic sex-biased epigenetic states.