Bile acids are physiological agents that facilitate absorption, transport and disposal of steroids, nutrients, metabolites and xenobiotics. Bile acids also are signaling molecules that activate nuclear receptors to regulate lipid and glucose homeostasis. Dysregulation of bile acid metabolism causes dyslipidemia, gallstone, liver, and cardiovascular diseases, and diabetes. Bile acids, insulin and cytokines inhibit CYP7A1, the first and rate-limiting enzyme of the bile acid synthesis pathway in the liver. The expression of CYP7A1 is mainly regulated at gene transcriptional level. Bile acid activates the FXR/SHP pathway, and several cell signaling pathways to inhibit CYP7A1 gene transcription. The mechanism of nuclear receptors and signaling pathways regulating CYP7A1 gene transcription remain to be elucidated. Most studies on CYP7A1 gene regulation have been performed in animal models. We have observed marked differences in regulation of the human and rodent CYP7A1 genes. Our central hypothesis is that as an acute phase response to inflammation and liver injury, bile acids and other stimuli activate the PI3K/AKT and MAP kinase signaling pathways, and signal crosstalk regulates nuclear receptors and co-regulators to remodel CYP7A1 and CYP8B1 chromatins.
The specific aim 1 will study nuclear receptor and co-regulator modulation of CYP7A1 and CYP8B1 chromatin. The roles of HNF4? and FTP, and their co-regulators Prox1, PGC-1?, histone deacetylase (HDAC) and silencing information regulator (SIRT1) will be studied. HepG2 and primary human hepatocytes will be used for chromatin immunoprecipitation (ChIP), co-IP and mammalian two-hybrid assays to study DNA-protein and protein-protein interaction.
Aim 2 will study the crosstalk of bile acid, cytokine, insulin, glucagon and PMA signaling pathways in regulating CYP7A1/CYP8B1 gene transcription. Real time PCR, siRNA, and microarrays will be used to identify genes involved in signaling crosstalk. Am 3 will study hepatocyte growth factor (HGF) regulation of CYP7A1 gene transcription to identify signaling pathway and crosstalk with insulin signaling. The objective of this research is to elucidate the molecular mechanism of nuclear receptor and signaling pathway regulation of bile acid synthesis. This study could contribute to understanding the mechanisms of metabolic diseases caused by dysregulation of bile acid synthesis and develop therapeutic agents to treat dyslipidemia, liver diseases, obesity and type II diabetes.
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