Laia Vilà

Impairment of hepatic Stat‐3 activation and reduction of PPARα activity in fructose‐fed rats

Fructose makes up a significant proportion of energy intake in westernized diets; its increased consumption has paralleled the growing prevalence of obesity and metabolic syndrome over the past two decades. In the current study, we demonstrate that fructose administration (10% wt/vol) in the drinking water of rats reduces the trans‐activating and trans‐repressing activity of the hepatic peroxisome proliferator‐activated receptor α (PPARα). As a consequence, fructose decreases hepatic fatty oxidation and increases pro‐inflammatory transcription factor nuclear factor κB (NF‐κB) activity. These changes were not observed in glucose‐administered rats (10% wt/vol), although both carbohydrates produced similar changes in plasma adiponectin and in the hepatic expression of transcription factors and enzymes involved in fatty acid synthesis. Fructose‐fed, but not glucose‐fed, rats were hyperleptinemic and exhibited increased tyrosine phosphorylation of the signal transducer and activator of transcription‐3 (STAT‐3) transcription factor, although they did not present a similar increase in the serine phosphorylation of nuclear STAT3. Thus, an impairment in the hepatic transduction of the leptin signal could be responsible for the observed alterations in PPARα activity in fructose‐fed rats. Because PPARα activity is lower in human than in rodent liver, fructose ingestion in humans should cause even worse effects, which would partly explain the link between increased consumption of fructose and widening epidemics of obesity and metabolic syndrome. Conclusion: Hypertriglyceridemia and hepatic steatosis induced by fructose ingestion result from a reduction in the hepatic catabolism of fatty acids driven by a state of leptin resistance. (HEPATOLOGY 2007;45:778–788.)

Reduction of liver fructokinase expression and improved hepatic inflammation and metabolism in liquid fructose-fed rats after atorvastatin treatment

Consumption of beverages that contain fructose favors the increasing prevalence of metabolic syndrome alterations in humans, including non-alcoholic fatty liver disease (NAFLD). Although the only effective treatment for NAFLD is caloric restriction and weight loss, existing data show that atorvastatin, a hydroxymethyl-glutaryl-CoA reductase inhibitor, can be used safely in patients with NAFLD and improves hepatic histology. To gain further insight into the molecular mechanisms of atorvastatins therapeutic effect on NAFLD, we used an experimental model that mimics human consumption of fructose-sweetened beverages. Control, fructose (10% w/v solution) and fructose+atorvastatin (30 mg/kg/day) Sprague-Dawley rats were sacrificed after 14 days. Plasma and liver tissue samples were obtained to determine plasma analytes, liver histology, and the expression of liver proteins that are related to fatty acid synthesis and catabolism, and inflammatory processes. Fructose supplementation induced hypertriglyceridemia and hyperleptinemia, hepatic steatosis and necroinflammation, increased the expression of genes related to fatty acid synthesis and decreased fatty acid β-oxidation activity. Atorvastatin treatment completely abolished histological signs of necroinflammation, reducing the hepatic expression of metallothionein-1 and nuclear factor kappa B binding. Furthermore, atorvastatin reduced plasma (x 0.74) and liver triglyceride (x 0.62) concentrations, decreased the liver expression of carbohydrate response element binding protein transcription factor (x 0.45) and its target genes, and increased the hepatic activity of the fatty acid β-oxidation system (x 1.15). These effects may be related to the fact that atorvastatin decreased the expression of fructokinase (x 0.6) in livers of fructose-supplemented rats, reducing the metabolic burden on the liver that is imposed by continuous fructose ingestion.

Metabolic Alterations and Increased Liver mTOR Expression Precede the Development of Autoimmune Disease in a Murine Model of Lupus Erythematosus

Although metabolic syndrome (MS) and systemic lupus erythematosus (SLE) are often associated, a common link has not been identified. Using the BWF1 mouse, which develops MS and SLE, we sought a molecular connection to explain the prevalence of these two diseases in the same individuals. We determined SLE- markers (plasma anti-ds-DNA antibodies, splenic regulatory T cells (Tregs) and cytokines, proteinuria and renal histology) and MS-markers (plasma glucose, non-esterified fatty acids, triglycerides, insulin and leptin, liver triglycerides, visceral adipose tissue, liver and adipose tissue expression of 86 insulin signaling-related genes) in 8-, 16-, 24-, and 36-week old BWF1 and control New-Zealand-White female mice. Up to week 16, BWF1 mice showed MS-markers (hyperleptinemia, hyperinsulinemia, fatty liver and visceral adipose tissue) that disappeared at week 36, when plasma anti-dsDNA antibodies, lupus nephritis and a pro-autoimmune cytokine profile were detected. BWF1 mice had hyperleptinemia and high splenic Tregs till week 16, thereby pointing to leptin resistance, as confirmed by the lack of increased liver P-Tyr-STAT-3. Hyperinsulinemia was associated with a down-regulation of insulin related-genes only in adipose tissue, whereas expression of liver mammalian target of rapamicyn (mTOR) was increased. Although leptin resistance presented early in BWF1 mice can slow-down the progression of autoimmunity, our results suggest that sustained insulin stimulation of organs, such as liver and probably kidneys, facilitates the over-expression and activity of mTOR and the development of SLE.

Reducción en la actividad de transactivación y transrepresión de PPARα en un modelo experimental de síndrome metabólico por fructosa dietética

Introduccion Dietas ricas en fructosa producen, en roedores, hipertrigliceridemia y esteatosis hepatica. En el presente estudio, quisimos valorar como afectaba la administracion de este hidrato de carbono a la actividad hepatica de transactivacion y transrepresion del receptor activado por proliferadores peroxisomicos α (PPARα). Material y metodos Se aleatorizo a 24 ratas Sprague-Dawley en 3 grupos de tratamiento: control, fructosa y glucosa (10% peso/volumen [p/v] hidrato de carbono en agua de bebida, durante 14 dias). En plasma se valoraron los valores de trigliceridos, acidos grasos libres, glucosa, insulina y leptina. En tejido hepatico se determino: contenido de trigliceridos, actividad de betaoxidacion de los acidos grasos, valores de expresion (ARN mensajero, proteina y actividad de union proteina-ADN) de los genes objeto de nuestro estudio. Resultados La administracion de fructosa produjo un incremento de 1,9 y 1,8 veces en los valores de trigliceridos plasmaticos y hepaticos, respectivamente. Las concentraciones de leptina se incrementaron 2,4 veces en estos animales, sin que apareciera hiperglucemia ni resistencia a la insulina. La actividad de transactivacion de PPARα se redujo, asi como la actividad de betaoxidacion hepatica de los acidos grasos (48%). La actividad de transrepresion de PPARα tambien se redujo en estos animales, y se produjo un incremento del 30% en la actividad del factor NF-κB en las ratas fructosa frente a animales control. Estos resultados no se observaron tras la administracion de glucosa; no obstante, los 2 hidratos de carbono modificaron de forma similar la expresion hepatica de factores de transcripcion y enzimas clave implicados en la sintesis de acidos grasos. Conclusiones La hipertrigliceridemia y la esteatosis hepatica inducida por la ingesta de fructosa son el resultado de una reduccion en el catabolismo hepatico de los acidos grasos asociado a un deficit de actividad PPARα.

Resistencia a la leptina: eje MAPK-AMPK y fosforilación de STAT-3 en Ser727 en rata alimentada con fructosa *

Introduccion En ratas, dietas ricas en fructosa producen hipertrigliceridemia y esteatosis hepatica, como consecuencia de una reduccion en la actividad transcripcional del receptor activado por proliferadores peroxisomicos alfa, junto con un estado de resistencia parcial a la leptina. En el presente trabajo, hemos estudiado como la fructose afecta a la via de senalizacion de la leptina. Material y metodos Se distribuyeron de forma aleatorizada 16 ratas Sprague-Dawley macho en 2 grupos: control y fructosa (10% peso/volumen en agua de bebida, durante 14 dias). Se valoraron los valores plasmaticos de trigliceridos, glucosa, leptina, insulina y adiponectina. En el higado se determinaron: contenido de trigliceridos, actividad de β-oxidacion, actividad AMPK (del ingles AMP-activated protein kinase) y valores de expresion proteica y de acido ribonucleico mensajero de diferentes genes de la via de senalizacion de la leptina. Resultados Las ratas fructosa presentaron un incremento en los valores de leptina plasmatica (1.9X) y de la expresion proteica de STAT-3-PTyr705 (1.9X) respecto las ratas control. No se observaron diferencias en las formas fosforilada y activas de STAT-3-PSer727 y de AMPK. Estas 2 fosforilaciones estan controladas por la via de las MAPK (del ingles mitogen activated protein kinase), que tampoco presentaba modificaciones entre los 2 grupos de estudio. El deficit de fosforilacion es consecuencia de una alteracion en la senalizacion del receptor a la leptina (ObRL) que no se fosforila en el residuo Tyr985, ni fosforila a JAK-2 (del ingles Janus-activated kinase 2), posiciones que controlan la via de las MAPK. SOCS-3 (del ingles supresor of cytokine signaling 3), que presenta un incremento en su expresion proteica (2.8X) en las ratas fructosa, puede ser la causa de la falta de activacion de ObRL-PTyr985 y de JAK-2. Conclusiones La administracion de fructosa provoca un incremento en la expresion proteica de SOCS-3. Como consecuencia, se observa la falta de fosforilacion de los residuos Tyr985 de ObRL y de JAK-2, que parecen ser la causa de la falta de actividad de la via de las MAPK y, por lo tanto, del deficit de actividad de AMPK y de STAT-3.