Nutritional and Hormonal Regulation of Hepatic Genes

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Mammals regulate the expression of their genetic information in response to nutritional and metabolic signals; however, little is currently known regarding the molecular mechanisms involved in these processes. The long range goal of this proposal is to understand one system of metabolic regulation: the transcriptional induction of lipogenic enzyme genes that occurs in the hepatocyte in response to increased glucose metabolism. Over fifteen enzymes and proteins involved in the process of triglyceride formation are induced in response to elevated glucose in the hepatocyte. We have previously defined a specific conserved regulatory sequence in the 5'- flanking region of several of these genes that is necessary and sufficient for this regulation and designated this sequence the carbohydrate response element (ChoRE). Recently, we have detected by electrophoretic mobility shift assay a nuclear protein(s) from liver that forms a complex with the ChoRE. This complex is detected with a variety of functionally active ChoREs, but not with mutants that have lost the ability to support the glucose response. We postulate that this protein(s), which we will designate as ChoRF for carbohydrate responsive factor, is the end-target of a novel glucose-signaling pathway in the hepatocyte. To unravel this novel pathway, we will first analyze the molecular nature of the novel ChoRF complex that binds to glucose regulatory sequences of the lipogenic enzyme genes. Preliminary evidence suggests that members of the basic/helix- loop-helix/PAS family of transcription factors are involved in regulation. We will purify and clone candidate factors and evaluate their role in the formation of the ChoRF complex and glucose regulation of lipogenic gene expression. We will also evaluate the respective roles of ChoRF and SREBP, a second transcription factor involved in lipogenic control, in mediating the actions of glucose and insulin in the hepatocyte. These two signals work together to coordinate the response of the animal to carbohydrate diet. Finally, we will explore the molecular mechanism responsible for ChoRF regulation in response to altered glucose metabolism in the hepatocyte. These studies should permit us to decipher the signaling process by which hepatocytes can 'sense' increased glucose metabolism and respond by changing its transcriptional program.