Low-density lipoprotein receptor-related protein 1 deficiency in cardiomyocytes reduces susceptibility to insulin resistance and obesity.

BACKGROUND: Low-density lipoprotein receptor-related protein 1 (LRP1) plays a key role in fatty acid metabolism and glucose homeostasis. In the context of dyslipemia, LRP1 is upregulated in the heart. Our aim was to evaluate the impact of cardiomyocyte LRP1 deficiency on high fat diet (HFD)-induced cardiac and metabolic alterations, and to explore the potential mechanisms involved. METHODS: We used TnT-iCre transgenic mice with thoroughly tested suitability to delete genes exclusively in cardiomyocytes to generate an experimental mouse model with conditional Lrp1 deficiency in cardiomyocytes (TNT-iCre+-LRP1flox/flox). FINDINGS: Mice with Lrp1-deficient cardiomyocytes (cm-Lrp1-/-) have a normal cardiac function combined with a favorable metabolic phenotype against HFD-induced glucose intolerance and obesity. Glucose intolerance protection was linked to higher hepatic fatty acid oxidation (FAO), lower liver steatosis and increased whole-body energy expenditure. Proteomic studies of the heart revealed decreased levels of cardiac pro-atrial natriuretic peptide (pro-ANP), which was parallel to higher ANP circulating levels. cm-Lrp1-/- mice showed ANP signaling activation that was linked to increased fatty acid (FA) uptake and increased AMPK/ ACC phosphorylation in the liver. Natriuretic peptide receptor A (NPR-A) antagonist completely abolished ANP signaling and metabolic protection in cm-Lrp1-/- mice. CONCLUSIONS: These results indicate that an ANP-dependent axis controlled by cardiac LRP1 levels modulates AMPK activity in the liver, energy homeostasis and whole-body metabolism.

Human hepatic lipase overexpression in mice induces hepatic steatosis and obesity through promoting hepatic lipogenesis and white adipose tissue lipolysis and fatty acid uptake.

Human hepatic lipase (hHL) is mainly localized on the hepatocyte cell surface where it hydrolyzes lipids from remnant lipoproteins and high density lipoproteins and promotes their hepatic selective uptake. Furthermore, hepatic lipase (HL) is closely associated with obesity in multiple studies. Therefore, HL may play a key role on lipid homeostasis in liver and white adipose tissue (WAT). In the present study, we aimed to evaluate the effects of hHL expression on hepatic and white adipose triglyceride metabolism in vivo. Experiments were carried out in hHL transgenic and wild-type mice fed a Western-type diet. Triglyceride metabolism studies included ß-oxidation and de novo lipogenesis in liver and WAT, hepatic triglyceride secretion, and adipose lipoprotein lipase (LPL)-mediated free fatty acid (FFA) lipolysis and influx. The expression of hHL promoted hepatic triglyceride accumulation and de novo lipogenesis without affecting triglyceride secretion, and this was associated with an upregulation of Srebf1 as well as the main genes controlling the synthesis of fatty acids. Transgenic mice also exhibited more adiposity and an increased LPL-mediated FFA influx into the WAT without affecting glucose tolerance. Our results demonstrate that hHL promoted hepatic steatosis in mice mainly by upregulating de novo lipogenesis. HL also upregulated WAT LPL and promoted triglyceride-rich lipoprotein hydrolysis and adipose FFA uptake. These data support the important role of hHL in regulating hepatic lipid homeostasis and confirm the broad cardiometabolic role of HL.

Análisis del proteoma hepático de ratones transgénicos de apo A-II humana: identificación de proteínas potencialmente implicadas en la regulación del metabolismo de triglicéridos y la respuesta a la insulina / Analysis of the liver proteome of human apolipoprotein A-II transgenic mice: identification of proteins potentially involved in the regulation of triglyceride metabolism and insulin response

Introducción. La función de la apolipoproteína A-II (apo A-II) en el metabolismo lipoproteico y su relación con la arteriosclerosis es poco conocida. Los estudios realizados en humanos y ratones modificados genéticamente han demostrado un efecto directo de la apo A-II en el metabolismo de los triglicéridos y los ácidos grasos libres (AGL) y la sensibilidad a la insulina. El objetivo principal de este estudio es la identificación de proteínas diferencialmente expresadas en el hígado de ratones transgénicos de apo A-II humana (h) y su potencial relación con el metabolismo de los triglicéridos y la glucosa. Material y métodos. Se realizaron estudios metabólicos de las lipoproteínas ricas en triglicéridos, la betaoxidación hepática y la prueba de tolerancia a la glucosa en ratones transgénicos de apo A-IIh y en controles en situación de ayunas y tras una carga oral de aceite de oliva. Los cambios en el perfil de expresión proteica se analizaron mediante el análisis comparativo de geles bidimensionales y la identificación de proteínas mediante espectrometría de masas MALDI-TOF. Resultados. Los ratones transgénicos de apo A-IIh presentaban un incremento del colesterol y los triglicéridos de las lipoproteínas que contienen apo B, hipertrigliceridemia aumentada tras la carga oral de ácido oleico, así como un aclaramiento acelerado de la glucosa tras la prueba de sobrecarga de glucosa. Estos cambios estaban asociados a una reducción en el catabolismo de las lipoproteínas ricas en triglicéridos y la tasa de betaoxidación hepática, pero sin cambios significativos en la actividad de la lipoproteína lipasa. De las más de 1.000 manchas resueltas en el rango pH 3 a 10, se identificaron 55 alteraciones significativas en los ratones transgénicos en comparación con los ratones controles, 16 de las cuales estaban relacionadas directamente con el metabolismo de los AGL y los carbohidratos. Conclusiones. La sobreexpresión apo A-IIh en ratones transgénicos induce hipertrigliceridemia posprandial debido, al menos en parte, a un defecto en el catabolismo de las lipoproteínas ricas en triglicéridos. La aproximación proteómica nos ha permitido detectar y caracterizar diferencias en el proteoma hepático de los ratones transgénicos de apo A-IIh y establecer proteínas potencialmente involucradas en el metabolismo de los AGL. Se requieren más estudios bioquímicos y moleculares para investigar el significado funcional de los cambios encontrados (AU)

Introduction. The role of apolipoprotein A-II (apo A-II) in lipoprotein metabolism and its relationship with atherosclerosis is poorly understood. Several studies both in humans and genetically modified mice have shown a direct effect of apo A-II on triglyceride and free fatty acid (FFA) metabolism and on insulin sensitivity. The aim of this study was to identify the proteins differentially expressed in the livers of human apo A-II transgenic mice and their potential relationship with triglyceride and glucose metabolism. Matherial and methods. Metabolic studies of triglyceride-rich lipoproteins, liver beta-oxidation and the glucose tolerance test were conducted in C57BL/6 control mice and human apo A-II transgenic mice in both fasting and postprandial states. The changes in protein expression patterns were determined by analysis of two-dimensional polyacrylamide gel electrophoresis while protein identification was performed by mass spectrometry (MALDI-TOF). Results. Human apo A-II transgenic mice showed an increase in cholesterol and triglycerides from apo B-containing lipoproteins, a higher response after oral fat test with oleic acid and higher glucose clearance after the glucose test. These changes were associated with a reduction in the clearance of triglyceride-rich lipoproteins and in the rate of liver beta-oxidation, but there were no significant changes in lipoprotein lipase activity. Within the pH 3-10 range, 55 out of more than 1,000 spots were identified to be significantly altered in human apo A-II transgenic mice compared with control mice, and 16 of these spots were directly related with FFA and carbohydrate metabolism. Conclusions. Overexpression of human apo A-II in transgenic mice induces postprandial hypertriglyceridemia. This finding is at least partly due to reduced catabolism of triglyceride-rich lipoproteins. We have been able to detect and characterize differences in the liver proteome of human apo A-II transgenic mice and to identify proteins potentially related to FFA metabolism. Further biochemical and molecular studies are required to investigate the functional significance of the changes found (AU)

Paradoxical exacerbation of combined hyperlipidemia in human apolipoprotein A-II transgenic mice treated with fenofibrate.

Apolipoprotein (apo) A-II has been biochemically and genetically linked to familial combined hyperlipidemia. Human ApoA-II transgenic mice and peroxisome proliferator-activated receptor alpha (PPARalpha)-deficient mice share some similar phenotypic characteristics. The aim of this study was to determine whether a fibrate-induced PPARalpha activation corrects the combined hyperlipidemia present in human apoA-II transgenic mice. ApoA-II transgenic mice were treated with fenofibrate (250 mg/kg) for 13 days. After this period, they presented a remarkable 8-fold increase in plasma triglycerides. This was concomitant with a 4-fold increase in non-high-density lipoprotein (non-HDL) cholesterol, a quantitatively similar decrease in HDL cholesterol and a severe reduction in mouse plasma apoA-I and apoA-II. Fenofibrate stimulated liver fatty acid beta-oxidation, increased the transcriptional expression of carnitine palmitoyltransferase 1 and phospholipid transfer protein, and decreased expression of apoA-I and apoC-III. However, very-low-density lipoprotein (VLDL)-triglyceride production and lipoprotein lipase (LPL) activities and the expression of other PPARalpha target genes were similar in mice treated with vehicle and fenofibrate. Further, fenofibrate-treated mice presented decreased in vivo [3H]VLDL catabolism and decreased VLDL-triglyceride hydrolysis by exogenous LPL. Therefore, the paradoxical enhancement of hyperlipidemia in fenofibrate-treated apoA-II transgenic mice is mainly due to decreased VLDL catabolism and, also, to a partial impairment in PPARalpha-signaling.