RESUMO
Nicotinamide adenine dinucleotide (NAD+) is a universal coenzyme regulating cellular energy metabolism in many cell types. Recent studies have demonstrated the close relationships between defective NAD+ metabolism and aging and age-associated metabolic diseases. The major purpose of the present study was to test the hypothesis that NAD+ biosynthesis, mediated by a rate-limiting NAD+ biosynthetic enzyme, nicotinamide phosphoribosyltransferase (NAMPT), is essential for maintaining normal adipose tissue function and whole body metabolic health during the aging process. To this end, we provided in-depth and comprehensive metabolic assessments for female adipocyte-specific Nampt knockout (ANKO) mice during aging. We first evaluated body fat mass in young (≤4-mo-old), middle aged (10-14-mo-old), and old (≥18-mo-old) mice. Intriguingly, adipocyte-specific Nampt deletion protected against age-induced obesity without changing energy balance. However, data obtained from the hyperinsulinemic-euglycemic clamp procedure (HECP) demonstrated that, despite the lean phenotype, old ANKO mice had severe insulin resistance in skeletal muscle, heart, and white adipose tissue (WAT). Old ANKO mice also exhibited hyperinsulinemia and hypoadiponectinemia. Mechanistically, loss of Nampt caused marked decreases in WAT gene expression of lipogenic targets of peroxisome proliferator-activated receptor gamma (PPAR-γ) in an age-dependent manner. In addition, administration of a PPAR-γ agonist rosiglitazone restored fat mass and improved metabolic abnormalities in old ANKO mice. In conclusion, these findings highlight the importance of the NAMPT-NAD+-PPAR-γ axis in maintaining functional integrity and quantity of adipose tissue, and whole body metabolic function in female mice during aging.NEW & NOTEWORTHY Defective NAD+ metabolism is associated with aging and age-associated metabolic diseases. In the present study, we provided in-depth metabolic assessments in female mice with adipocyte-specific inactivation of a key NAD+ biosynthetic enzyme NAMPT and revealed an unexpected role of adipose tissue NAMPT-NAD+-PPAR-γ axis in maintaining functional integrity and quantity of adipose tissue and whole body metabolic health during the aging process.
Assuntos
Adipócitos , Envelhecimento , NAD , Nicotinamida Fosforribosiltransferase , Animais , Feminino , Camundongos , Adipócitos/metabolismo , Envelhecimento/metabolismo , Citocinas/metabolismo , Metabolismo Energético/genética , Resistência à Insulina/genética , Camundongos Endogâmicos C57BL , Camundongos Knockout , NAD/metabolismo , Nicotinamida Fosforribosiltransferase/metabolismo , Nicotinamida Fosforribosiltransferase/genética , Obesidade/metabolismo , Obesidade/genética , Fenótipo , PPAR gama/metabolismo , PPAR gama/genéticaRESUMO
BACKGROUND & AIMS: Restoring hepatic and peripheral insulin sensitivity is critical to prevent or reverse metabolic syndrome and type 2 diabetes. Glucose homeostasis comprises in part the complex regulation of hepatic glucose production and insulin-mediated glucose uptake and oxidation in peripheral tissues. We previously identified hepatocyte arginase 2 (Arg2) as an inducible ureahydrolase that improves glucose homeostasis and enhances glucose oxidation in multiple obese, insulin-resistant models. We therefore examined structure-function determinants through which hepatocyte Arg2 governs systemic insulin action and glucose oxidation. METHODS: To do this, we generated mice expressing wild-type murine Arg2, enzymatically inactive Arg2 (Arg2H160F) and Arg2 lacking its putative mitochondrial targeting sequence (Arg2Δ1-22). We expressed these hepatocyte-specific constructs in obese, diabetic (db/db) mice and performed genetic complementation analyses in hepatocyte-specific Arg2-deficent (Arg2LKO) mice. RESULTS: We show that Arg2 attenuates hepatic steatosis, independent of mitochondrial localization or ureahydrolase activity, and that enzymatic arginase activity is dispensable for Arg2 to augment total body energy expenditure. In contrast, mitochondrial localization and ureahydrolase activity were required for Arg2-mediated reductions in fasting glucose and insulin resistance indices. Mechanistically, Arg2Δ1-22 and Arg2H160F failed to suppress glucose appearance during hyperinsulinemic-euglycemic clamping. Quantification of heavy-isotope-labeled glucose oxidation further revealed that mistargeting or ablating Arg2 enzymatic function abrogates Arg2-induced peripheral glucose oxidation. CONCLUSION: We conclude that the metabolic effects of Arg2 extend beyond its enzymatic activity, yet hepatocyte mitochondrial ureahydrolysis drives hepatic and peripheral oxidative metabolism. The data define a structure-based mechanism mediating hepatocyte Arg2 function and nominate hepatocyte mitochondrial ureahydrolysis as a key determinant of glucose oxidative capacity in mammals.
Assuntos
Arginase , Diabetes Mellitus Tipo 2 , Camundongos , Animais , Arginase/genética , Arginase/metabolismo , Glucose , Hepatócitos/metabolismo , Obesidade/metabolismo , Insulina , Mamíferos/metabolismoRESUMO
Urea cycle impairment and its relationship to obesity and inflammation remained elusive, partly due to the dramatic clinical presentation of classical urea cycle defects. We generated mice with hepatocyte-specific arginase 2 deletion (Arg2LKO) and revealed a mild compensated urea cycle defect. Stable isotope tracing and respirometry revealed hepatocyte urea and TCA cycle flux defects, impaired mitochondrial oxidative metabolism, and glutamine anaplerosis despite normal energy and glucose homeostasis during early adulthood. Yet during middle adulthood, chow- and diet-induced obese Arg2LKO mice develop exaggerated glucose and lipid derangements, which are reversible by replacing the TCA cycle oxidative substrate nicotinamide adenine dinucleotide. Moreover, serum-based hallmarks of urea, TCA cycle, and mitochondrial derangements predict incident fibroinflammatory liver disease in 106,606 patients nearly a decade in advance. The data reveal hierarchical urea-TCA cycle control via ARG2 to drive oxidative metabolism. Moreover, perturbations in this circuit may causally link urea cycle compromise to fibroinflammatory liver disease.