Hepatic gluconeogenesis a very important component at glucose homeostasis. Glucose synthesis and secretion by the liver is induced during fasting; it is strongly suppressed by insulin and is inappropriately activated in both type 1 and type 2 diabetes. Hepatic gluconeogenesis is a major contributor to the hyperglycemia of diabetes, particularly in the fasted state. We have recently shown that key gluconeogenic agents/hormones - cyclic AMP and glucocorticoids - induce the transcriptional coactivator PGC-1. When expressed at physiological levels in primary hepatocytes or liver in vivo, PGC-1 activates the entire program of gluconeogenesis including the secretion of glucose. These data suggest that PGC-1 is a central target of the gluconeogenic hormones, linking these agents to the transcriptional activation of PEPCK and other key genes.
For Specific Aims, we will first examine in detail how PGC-1 regulates the PEPCK promoter. In particular, we will utilize a variety of hormonal conditions and the PEPCK promoter to determine the key transcriptional factor targets for PGC-1 docking. Interactions between PGC-1 and HNF-4alpha glucocorticoid receptor and/or other components will he studied in close molecular detail, investigating critical amino acids residues that serve to form the coactivator/transcription factor pairing.
Our second aim will he to critically analyze the genetic role of PGC-1 in gluconeogenesis through construction of liver-specific transgenic mice expressing PGC-1, and a liver-specific PGC-1 knock-out. Mice will he studied for the activation of key genes of gluconeogenesis and metabolism of glucose in vivo, utilizing glucose clamp techniques and tracers. These mice will also be valuable to study the genetic relationship between PGC-1 and potential transcription factor targets, utilizing crosses between the PGC-1 transgenic mice and transcription factor knock-out mice. Our last major aim will be to study how gluconeogenic hormones regulate PGC-1 transcription, utilizing transfection of PGC-1 promoters into hepatocytes treated with insulin, cyclic AMP and glucocorticoids. We are especially interested in determining whether insulin suppresses PGC-1 expression through FKHR, CREB or potentially novel factors. Novel and important components regulating the PGC-1 promoter will be isolated, cloned and characterized. These studies together should provide critical insights into mechanisms controlling the process of gluconeogenesis. This also has the clear potential to lead to the development of new anti-diabetes drugs.
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