RESUMEN
Insulin resistance is often associated with obesity and can precipitate type 2 diabetes. To date, most known approaches that improve insulin resistance must be preceded by the amelioration of obesity and hepatosteatosis. Here, we show that this provision is not mandatory; insulin resistance and hyperglycemia are improved by the modification of hepatic fatty acid composition, even in the presence of persistent obesity and hepatosteatosis. Mice deficient for Elovl6, the gene encoding the elongase that catalyzes the conversion of palmitate to stearate, were generated and shown to become obese and develop hepatosteatosis when fed a high-fat diet or mated to leptin-deficient ob/ob mice. However, they showed marked protection from hyperinsulinemia, hyperglycemia and hyperleptinemia. Amelioration of insulin resistance was associated with restoration of hepatic insulin receptor substrate-2 and suppression of hepatic protein kinase C epsilon activity resulting in restoration of Akt phosphorylation. Collectively, these data show that hepatic fatty acid composition is a new determinant for insulin sensitivity that acts independently of cellular energy balance and stress. Inhibition of this elongase could be a new therapeutic approach for ameliorating insulin resistance, diabetes and cardiovascular risks, even in the presence of a continuing state of obesity.
Asunto(s)
Acetiltransferasas/metabolismo , Dieta Aterogénica , Grasas de la Dieta/farmacología , Resistencia a la Insulina , Obesidad/metabolismo , Acetiltransferasas/deficiencia , Acetiltransferasas/genética , Animales , Peso Corporal/efectos de los fármacos , Carcinoma Hepatocelular/patología , Línea Celular Tumoral , Grasas de la Dieta/administración & dosificación , Elongasas de Ácidos Grasos , Eliminación de Gen , Insulina/metabolismo , Proteínas Sustrato del Receptor de Insulina , Péptidos y Proteínas de Señalización Intracelular/fisiología , Neoplasias Hepáticas/patología , Masculino , Ratones , Ratones Noqueados , Obesidad/inducido químicamente , Obesidad/genética , Fosfoproteínas/fisiología , Fosforilación , Proteína Quinasa C-epsilon/antagonistas & inhibidores , Proteínas Proto-Oncogénicas c-akt/metabolismo , ARN Mensajero/metabolismo , Transducción de Señal , Factores de TiempoRESUMEN
Sterol-regulatory element binding protein-1c (SREBP-1c) is a transcription factor that controls lipogenesis in the liver. Hepatic SREBP-1c is nutritionally regulated, and its sustained activation causes hepatic steatosis and insulin resistance. Although regulation of SREBP-1c is known to occur at the transcriptional level, the precise mechanism by which insulin signaling activates SREBP-1c promoter remains to be elucidated. Here we show that protein kinase C beta (PKCbeta) is a key mediator of insulin-mediated activation of hepatic SREBP-1c and its target lipogenic genes. Activation of SREBP-1c in the liver of refed mice was suppressed by either adenoviral RNAi-mediated knockdown or dietary administration of a specific inhibitor of protein kinase C beta. The effect of PKCbeta inhibition was cancelled in insulin depletion by streptozotocin (STZ) treatment of mice. Promoter analysis indicated that PKCbeta activates SREBP-1c promoter through replacement of Sp3 by Sp1 for binding to the GC box in the sterol regulatory element (SRE) complex, a key cis-element of SREBP-1c promoter. Knockdown of Sp proteins demonstrated that Sp3 and Sp1 play reciprocally negative and positive roles in nutritional regulation of SREBP-1c, respectively. This new understanding of PKCbeta involvement in nutritional regulation of SREBP-1c activation provides a new aspect of PKCbeta inhibition as a potential therapeutic target for diabetic complications.
Asunto(s)
Insulina/metabolismo , Isoenzimas/metabolismo , Hígado/fisiología , Proteína Quinasa C/metabolismo , Proteína 1 de Unión a los Elementos Reguladores de Esteroles/metabolismo , Activación Transcripcional , Animales , Línea Celular , Diabetes Mellitus Experimental , Activación Enzimática , Humanos , Isoenzimas/antagonistas & inhibidores , Isoenzimas/genética , Masculino , Ratones , Ratones Endogámicos C57BL , Regiones Promotoras Genéticas , Proteína Quinasa C/antagonistas & inhibidores , Proteína Quinasa C/genética , Proteína Quinasa C beta , Proteína Quinasa C-epsilon/genética , Proteína Quinasa C-epsilon/metabolismo , Interferencia de ARN , Ratas , Ratas Sprague-Dawley , Proteína 1 de Unión a los Elementos Reguladores de Esteroles/genética , Factores de Transcripción/genética , Factores de Transcripción/metabolismoRESUMEN
The role of glomerular SREBP-1c in diabetic nephropathy was investigated. PEPCK-promoter transgenic mice overexpressing nuclear SREBP-1c exhibited enhancement of proteinuria with mesangial proliferation and matrix accumulation, mimicking diabetic nephropathy, despite the absence of hyperglycemia or hyperlipidemia. Isolated transgenic glomeruli had higher expression of TGFbeta-1, fibronectin, and SPARC in the absence of marked lipid accumulation. Gene expression of P47phox, p67phox, and PU.1 were also activated, accompanying increased 8-OHdG in urine and kidney, demonstrating that glomerular SREBP-1c could directly cause oxidative stress through induced NADPH oxidase. Similar changes were observed in STZ-treated diabetic mice with activation of endogenous SREBP-1c. Finally, diabetic proteinuria and oxidative stress were ameliorated in SREBP-1-null mice. Adenoviral overexpression of active and dominant-negative SREBP-1c caused consistent reciprocal changes in expression of both profibrotic and oxidative stress genes in MES13 mesangial cells. These data suggest that activation of glomerular SREBP-1c could contribute to emergence and/or progression of diabetic nephropathy.