Increase in liver cytosolic lipases activities and VLDL-TAG secretion rate do not prevent the non-alcoholic fatty liver disease in cafeteria diet-fed rats

Highlights

• Cafeteria diet increased serum insulin and liver TAG and cholesterol content.

• Cafeteria diet did not change the sympathetic nervous system activity to liver.

• Cafeteria diet increased the liver lipogenesis.

• Cafeteria diet increased the cytosolic lipases activities and VLDL-TAG secretion.

Abstract

We have previously shown that the cafeteria diet increases body fat mass, plasma triacylglycerol (TAG) and insulin levels, glucose uptake by white and brown adipose tissues, as well as the sympathetic activity to both adipose tissues in Wistar rats. The metabolic pathways responsible for the development of non-alcoholic fatty liver disease (NAFLD) were examined in cafeteria diet-fed rats. After 3 weeks offering cafeteria diet, we evaluated: (i) activity of the sympathetic nervous system by norepinephrine turnover rates; (ii) de novo fatty acid synthesis in vivo using ³H₂O; (iii) secretion of very low density lipoprotein (VLDL)-TAG secretion measuring serum TAG levels after administration of lipase lipoprotein inhibitor, (iv) liver cytosolic lipases activities and (v) liver mRNA expression of enzymes involved in lipids secretion and oxidation by RT-PCR. The cafeteria diet induced an increase in TAG (120%) and cholesterol (30%) liver contents. Cafeteria diet did not change the sympathetic nervous system activity to liver, but induced a marked increase in the lipogenesis (approximately four-fold) and significant increase in cytosolic lipases activities (46%) and VLDL-TAG secretion (22%) compared to control diet-fed rats. The cafeteria diet also increased the microsomal triglyceride transfer protein (30%) and carnitine palmitoyltransferase I (130%) mRNA expression but decreased the apolipoprotein B100 (26%) mRNA expression. Our findings demonstrate that the increase in the cytosolic lipases activities and VLDL-TAG secretion rates were not able to compensate for the increased lipogenesis rates induced by the cafeteria diet, resulting in NAFLD.

Introduction

Non-alcoholic fatty liver disease (NAFLD) is now considered to be one of the most common forms of chronic liver disease in the Western world. NAFLD refers to a clinical-pathological spectrum of conditions ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), involving inflammation and some evidence of liver cell damage, and in some cases, cirrhosis [1]. In the United States, the prevalence of NAFLD is approximately 30%, while the incidence of NASH is about 3% [1,2]. The NAFLD prevalence increases among overweight (58%) and non-diabetic obese (98%) individuals [3,4].

There is considerable traffic of both non-esterified fatty acids (NEFAs) and triacylglycerols (TAGs) in the liver, independent of food state [5]. In the postprandial state, the liver synthesizes fatty acids (FAs) and takes up chylomicron remnants from the bloodstream [5]. NEFAs may be converted into other lipid species, such as glycerolipids, glycerophospholipids, and sterols, which can be packaged into very low density lipoprotein (VLDL) particles and secreted from the liver into the plasma [5]. Under fasting conditions, NEFAs are released from adipose tissue and returned to the liver, where they are oxidized in the mitochondria to provide energy or converted into TAGs and packaged in VLDL particles that are secreted into the plasma [5]. Thus, the TAG content in the cytoplasm of hepatocytes is influenced by the balance of all these metabolic pathways.

Nutritional, hormonal, and neural mechanisms regulate the lipid metabolism. Fructose increases the generation of acetyl-CoA and glycerol-3-phosphate because fructokinase is not regulated [6]. Insulin stimulates lipogenic enzymes and inhibits FA oxidation [7], while the sympathetic nervous system increases the rate of VLDL-TAG secretion [8] and mobilization of FAs from adipose tissue [9]. Several studies have described an increase in VLDL secretion in models of insulin resistance, characterized by hyperinsulinemia and hyperglycemia, such as type 2 diabetics individuals [10], high-fructose diet-fed hamsters [11], high-fat diet-fed mice [12], tumor necrosis factor-α-treated hamsters [13], and high-fat diet-fed and LDL receptor-null mice [14]. However, ob/ob mice, despite hyperinsulinemia and hyperglycemia, have similar VLDL-TAG secretion to lean mice [15]. Also, acute hyperinsulinemia reduces the VLDL-TAG production in these groups, showing a more pronounced effect in lean mice than ob/ob mice [15]. Thus, insulin effects on VLDL secretion are not fully clarified. The regulation of VLDL secretion is frequently associated with apolipoprotein B (ApoB) synthesis and secretion. It has been suggested that either the inhibitory effects of insulin signaling on ApoB secretion is very short-lived or the hepatic TAGs are the dominant regulator of ApoB secretion [16].

Furthermore, studies using adipose triglyceride lipase (ATGL)- and hormone-sensitive lipase (HSL)-deficient mice have suggested that these enzymes can contribute to the development of hepatic steatosis [17,18], although HSL and ATGL are responsible for only approximately 40% of the total hydrolases activity in the hepatic tissue [18,19]. Recently, we demonstrated that the high-fructose diet induces severe liver steatosis accompanied by a decrease in cytosolic lipases activities [20]. Here, our main objective was to investigate the biochemical pathways involved in the development of NAFLD induced by a cafeteria diet. Thus, we evaluated the effect of chronic treatment with a cafeteria diet on the liver lipid accumulation, rates of de novo FA synthesis, cytosolic lipases activities, VLDL-TAG secretion rates, and mRNA expression of genes involved in lipid oxidation and secretion, as well as the possible involvement of the sympathetic nervous system activity.