Sex differences in the liver transcriptome are widespread in both mice and humans and are largely regulated by growth hormone (GH). The long-term goal of this project is to elucidate these sex differences to better understand the mechanistic underpinnings of the many clinically relevant sex differences impacted by GH; these include sex- differences in hepatic drug and steroid metabolism and lipid metabolic profiles, and in the incidence and severity of liver pathologies, such as non-alcoholic fatty liver disease (NAFLD) and liver fibrosis associated with development of hepatocellular carcinoma. Our recent studies in the mouse model revealed that GH acts through its sex-specific temporal patterns of pituitary secretion ? pulsatile in males and persistent in females ? and via GH- stimulated activation of liver STAT5, to establish a sex-differential epigenomic environment that enables the sex- specific actions of GH in the liver. We identified several thousand genomic regions marked as putative enhancers that have sex-biased binding sites for STAT5 and other essential GH-regulated liver transcription factors; and we showed that sex-specific deposition by Ezh1/Ezh2 of histone-H3 lysine 27 trimethyl marks (H3K27me3) is required specifically for the repression of many female-biased genes in male liver. Further, more than 200 sex-specific, GH- regulated and nuclear-enriched long, non-coding RNAs (lncRNAs) were discovered, and strong candidates for regulation of the sex-differential deposition of H3K27me3 and other chromatin marks at sex-specific genes and their enhancers were identified by analysis of a large panel of Diversity Outbred mouse livers. This project builds on these advances to elucidate fundamental mechanisms that underlie the transcriptional and epigenetic regulation by GH of sex-biased gene expression essential for normal liver function. The work proposed has two major aims: 1) to discover critical features that underpin sex-biased gene transcription associated with sex-biased liver disease by identifying functionally active sex-biased enhancers, which harbor the majority of genetic risk factors for fatty liver disease, and to elucidate their organization within chromatin loop domains and subdomains, and their interactions with sex-biased gene promoters; and 2) to discover the role of sex-specific, GH-regulated lncRNAs in establishing and maintaining the sex-differentiated chromatin states at sex-biased enhancers and genes to support sex differences in liver gene transcription, and then elucidate their contributions to the protective effects of GH- activated STAT5 against hepatic stresses that induce non-alcoholic fatty liver disease and other liver pathologies. Together, this work will identify key mechanistic features that enable GH, and its sex-dependent plasma patterns, to regulate the sex-biased expression of hundreds of genes that control liver metabolic processes with a major impact on human health and liver disease, and may link molecular features to pathophysiological outcomes. The results obtained will have a high impact on research in this field by shifting the mechanistic focus of GH action from correlation and inferred function to causality. These studies will also serve as a paradigm for the pulsatile hormone action of other endocrine factors that act through complex epigenetic mechanisms.
This project investigates how liver enzymes and liver metabolic function are controlled by growth hormone, a pituitary hormone, in ways that differ between males and females. The studies proposed will give a better understanding of how an individual's sex and hormonal status influence the liver and its ability to respond to steroids, drugs, toxicants and other hepatic stresses. This research will increase our understanding of the factors and mechanisms that underlie sex-differences in liver function and its pathologies, and may suggest new ways to prevent, diagnose or treat a variety of liver diseases that disproportionately affect members of one sex.
