Proteomic analysis of fructose-induced fatty liver in hamsters
Introduction
Fructose, which occurs naturally in honey and sweet fruits, is produced in crystalline and syrup forms for commercial use. The most commonly used corn syrup contains about 55% free fructose; and its use as a sweetener in processed foods and soft drinks has greatly increased by 20% to 30% over the past 20 years, a rate of increase similar to the incidence of obesity that has risen dramatically over the same period [1]. Preclinical studies indicate that high fructose consumption is associated with the development of metabolic syndrome, as manifested by glucose intolerance, hyperinsulinemia, hypertriglyceridemia, and whole-body insulin resistance [2], [3], [4], [5], [6]. In addition, there are some clinical data indicating that excessive fructose consumption for a limited period predisposes healthy subjects to body weight gain with concurrent elevation in plasma triglyceride (TG) and cholesterol levels, an atherogenic lipid profile that constitutes a major risk factor for clogging the artery and causing cardiovascular disease [7], [8], [9], [10]. Based on epidemiologic studies of obesity in relation to increased per capital consumption of high-fructose corn syrup from beverages, it is thought that excessive dietary intake of fructose is a confounding factor for the increased prevalence of overweight and morbid obesity in industrial countries [1]. There is evidence that frequent consumption of sugar-sweetened soft drinks is a potential contributing factor for childhood obesity [11], [12], [13], [14].
Such detrimental effect of fructose on health can be ascribed to the metabolic pathway in which fructose is metabolized after its dietary intake. In this regard, fructose differs from glucose in 3 fundamental ways. First, after absorption in the gastrointestinal track, fructose fluxes via the portal circulation into the liver, where it is almost completely metabolized [15]. Unlike glucose that enters hepatocytes through glucose transporter (Glut) 2, fructose enters hepatocytes via Glut5 independently of insulin [16]. Second, glucose breakdown is negatively regulated by phosphofructokinase, a hepatic enzyme that regulates glycolysis in liver, whereas fructose can evade this rate-limiting control mechanism and is metabolized into glycerol-3-phosphate and acetyl–coenzyme A. These 2 intermediate metabolites serve as substrates for glyceride synthesis, contributing to very low-density lipoprotein (VLDL)–TG production in liver [2], [3]. Third, fructose, as opposed to glucose, does not directly stimulate pancreatic insulin release because of the lack of Glut5 expression in β-cells [16]. Postprandial insulin secretion is instrumental for modulating glucose metabolism in peripheral tissues and regulating energy balance via the central nervous system through both direct and indirect mechanisms to control food intake and body weight gain [17], [18], [19], [20]. However, such an energy-balancing mechanism does not respond to dietary fructose uptake because of the inability of fructose to elicit insulin release. As a consequence, increased fructose flux into hepatocytes results in unrestrained production of intermediate metabolites, which favors energy storage by promoting de novo lipogenesis in liver.
High fructose consumption is associated with hepatic steatosis, but with poorly understood mechanisms [2], [3], [4]. To investigate the underlying mechanism of fructose-induced fatty liver, we used matrix-assisted laser desorption/ionization (MALDI)–based proteomics approach to identify candidate molecules that link high fructose consumption to the pathogenesis of hepatic steatosis. Syrian gold hamsters were fed a high-fructose diet (60% fructose, n = 6) or regular chow (n = 6) for 8 weeks. Hamsters fed on high-fructose diet, as opposed to control hamsters on regular chow, exhibited abnormal lipid profiles with increased fat deposition in liver. At the end of the 8-week treatment, hamsters were killed and liver tissues were subjected to MALDI-based proteomics. We show that high fructose feeding was associated with significant alterations in the expression of hepatic enzymes in multiple pathways. In addition to marked up-regulation of hepatic functions that promote TG synthesis and VLDL-TG production in liver, high fructose consumption resulted in perturbations in hepatic expression of antioxidant functions and molecular chaperones in protein folding. These data provide new insight into the molecular basis that links fructose-induced metabolic shift to aberrant hepatic metabolism in the pathogenesis of dyslipidemia and steatosis.
Section snippets
Animal studies
Male Syrian golden hamsters (5 weeks old; body weight, 81-90 g; Charles River Laboratory, Wilmington, MA) were fed with regular rodent chow or high-fructose diet (60% fructose, DYET 161506; Dyets, Bethlehem, PA) ad libitum in sterile cages with a 12-hour light/dark cycle for 8 weeks. Blood was collected from tail vein into capillary tubes precoated with potassium-EDTA (Sarstedt, Nümbrecht, Germany) for preparation of plasma or determination of blood glucose levels using Glucometer Elite (Bayer,
Characteristics of hamsters on regular chow vs high-fructose diet
To study the effect of high fructose consumption on glucose and lipid metabolism, we randomly assigned 5-week male hamsters into 2 groups (n = 6) to either regular chow or high-fructose diet. After 8-week feeding, we determined blood glucose and lipid parameters. As shown in Table 1, high-fructose–fed hamsters were associated with a slight body weight gain and a small increase in blood glucose levels. However, the differences in mean body weight and blood glucose levels between high-fructose
Conclusion
Excessive fructose consumption is associated with dyslipidemia, culminating in markedly elevated lipid levels in plasma and increased fat deposition in liver. Our studies provide insight into the underlying mechanism of fructose-induced hepatic steatosis and diabetic dyslipidemia. We show that high fructose feeding resulted in significant alterations in multiple pathways in hepatic metabolism. These include (1) functions in fatty acid transportation, VLDL-TG assembly, and cholesterol
Acknowledgment
We thank Drs Adama Kamagate and Sandra Slusher for critical reading of this manuscript. This study was supported in part by National Health Institute grants DK066301 (HHD) and Autoimmunity Centers of Excellence U19-AI056374-01 (SR and MT), and Department of Defense ERMS 00035010 (SR and MT).
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2015, Progress in Lipid ResearchCitation Excerpt :Proteomics techniques, such as tandem liquid chromatography–mass spectrometry and two-dimensional gel electrophoresis, can be used to study changes in proteins at large-scale, in casu in vitro and in vivo models of liver steatosis (see Table 3). Using such approaches, it has been found that hamsters fed a high-fructose diet abundantly produce many proteins, including fatty acid binding protein, carbamoyl-phosphate synthase, apolipoprotein A1, protein disulfide isomerase, proteins involved in anti-oxidation, such as peroxiredoxin 2 and heat shock protein 70, fructose-1,6-biphosphatase and glycerol kinase [281]. Upon feeding male C57BL/6J mice a HFD enhanced protein production of cytokeratine 8 and 18, vimentin and apolipoprotein E, but decreased quantities of glutathione peroxidase are observed [282].