Regulation of 15N Urea Isotopomers Production

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DESCRIPTION (provided by applicant): Impaired urea synthesis and consequent hyperammonemia (HA) are common occurrences in disorders of congenital defects of the urea cycle and of fatty acid oxidation (FAO), nonalcoholic fatty liver disease (NAFLD) and/or "Metabolic Syndrome" (MS). Still unknown are the biochemical and metabolic mechanisms by which defective FAO and fatty liver impair ureagenesis. Nor is an effective treatment available. During the current funding period we found that agmatine (AGM), the product of arginine decarboxylase, elevates hepatic [cAMP] and stimulates both ureagenesis and FAO. Our preliminary data demonstrate that AGM or 5-aminoimidazole-4-carboxamide-1-¿-D-ribofuranoside (AICAR), an activator of AMP-activated protein kinase (AMPK), enhances ureagenesis in a rat model of NAFLD or MS. Together, the observations strongly suggest that AGM has many of the effects expected for an activator of AMPK. In this renewal proposal our overall aim is to elucidate the mechanisms by which AGM or AICAR regulates hepatic glutamine metabolism and urea synthesis in NAFLD or MS. A long-term objective is to develop a clinically applicable pharmacotherapeutic intervention to improve ureagenesis in patients with NAFLD and/or MS. We propose to explore two related Specific Aims/Hypotheses: (i) Impaired ureagenesis in NAFLD is a consequence of mitochondrial dysfunction and a resultant decrease in synthesis of N-acetylglutamate (NAG), an obligatory activator of carbamoyl phosphate synthetase-I (CPS-I), the initial and the rate-limiting step of ureagenesis. AGM and AICAR augment FAO, thereby triggering a metabolic cascade that attenuates the metabolic derangements associated with NAFLD and MS. The result is an augmentation of ureagenesis; and (ii) NAFLD decreases hepatic uptake and metabolism of glutamine. This would limit mitochondrial [glutamate] and NAG synthesis. The net result is a failure of activation of CPS-I. AGM and AICAR stimulate FAO, improve glutamine uptake and permit more glutamate to be available for NAG synthesis, and thus, greater CPS-I activity. Based on these hypotheses, questions to be addressed include: (1) Is the action of AGM on FAO and ureagenesis mediated via activation of AMPK and/or cAMP-PKA? (2) How does acute or chronic treatment with AGM or AICAR affect metabolic coordination between hepatic FAO, the TCA cycle and ureagenesis in NAFLD or MS?; and (3) How does treatment with AGM or AICAR and subsequent activation of AMPK and/or cAMP-PKA affect hepatic glutamine uptake and metabolism, whole-body protein turnover and ureagenesis in NAFLD or MS. Experiments will be performed using a rat model of fatty liver and/or MS and various systems, including (a) isolated hepatocytes; (b) a liver perfusion system; (c) isolated mitochondria; and (d) in vivo study. We will use 15N and/or 13C labeled precursors and gas chromatography-mass spectrometry (GC-MS), MALDI-TOF-mass spectrometry, nuclear magnetic resonance (NMR) and molecular biology. Combining these methodologies provides a superb tool to pinpoint the primary mechanism(s) of AGM or AICAR action. Data obtained will provide new and pivotal information to elucidate the role of AGM or AICAR in the regulation of hepatic glutamine metabolism and ureagenesis. This knowledge may lead to development of a novel pharmacotherapeutic intervention to improve urea synthesis in NAFLD or MS.