Transcriptional Regulation of Hepatocyte Differentiation and Function

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DESCRIPTION (provided by applicant): A critical, unresolved issue in gene regulation is the means by which chromatin that packages developmentally regulated genes is accessed and remodeled to orchestrate cell-type specific gene expression programs informing tissue differentiation and function. This poses a critical dilemma in cells of both the developing embryo, in which much of the chromatin is still compacted, and the adult organism, where alterations in gene expression in response to signaling cascades directing essential physiological functions necessitate rapid alterations in the chromatin landscape. Failure to properly navigate either of these biological challenges can have grave biomedical consequences in the form of congenital malformations, disease, and neoplasia in humans. Based on the ability of the initial "pioneer" chromatin binding transcription factor FoxO1 to remodel histone:DNA contacts within compacted chromatin in vitro, its recruitment of liver-enriched regulatory factors and RNA pol II to regulatory targets in hepatocytes, and regulation of these activities by posttranslational modifications linked to signaling pathways with proven roles in maintenance of glucose, bile acid and cholesterol homeostasis in humans we hypothesize that FoxO1 cooperates with closely juxtaposed liver- enriched transcription factors to carry out chromatin structural alterations necessary for developmental activation of hepatic genes crucial to human hepatocyte function. We argue that posttranslational modification of the FoxO1 DNA binding domain enables hormonal regulation of FoxO1 target genes in the human liver by disrupting this relationship. This proposal addresses two key questions that stem from this hypothesis: 1) How does FoxO1 cooperate with other liver-enriched regulatory factors to carry out chromatin remodeling and modification necessary for developmental activation of hepatic genes? and 2) Is this cooperation necessary for hormonal regulation of transcriptionally active chromatin encoding essential metabolic functions in the human liver? To accomplish this goal, we will use a novel hiPS (human induced pluripotent stem) cell human hepatocyte differentiation system to uncover basic molecular mechanisms informing the function and regulation of FoxO1 under conditions that recapitulate hepatocyte specification, differentiation, and function in humans. We will also use state of the art molecular techniques that enable the assembly and analysis of defined nucleoprotein structures to test mechanisms implicated by our observations. FoxO1 is now established as a key transcriptional regulator integrating hepatic glucose, cholesterol, bile acid, and lipid metabolism in humans; mutant FoxO1 alleles are recently demonstrated in diabetes and hepatic steatosis and fibrosis in human patients. Our hypothesis assumes that the chromatin binding and remodeling capabilities attributed to FoxO1 are essential for the assembly of the nucleoprotein structures tasked with activating FoxO1 target genes and mediating their response to diverse signaling cascades. Subversion of these gene regulatory events is a likely contributor to metabolic derangements and hepatic disease, making it vital that we uncover the key mechanisms and players.