Lipocalin 2 in perpetuating hepatic lipid homeostasis challenged by high fructose diet

• Lipocalin 2 in der Erhaltung der hepatischen Lipidhomöostase bei fruktosereicher Ernährung

Lambertz, Jessica; Weiskirchen, Ralf (Thesis advisor); Hollert, Henner (Thesis advisor); van Dongen, Joost Thomas (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

Fructose is known as a lipogenic sugar, part of High Fructose Corn Syrup (HFCS) contributing to elevated fasting plasma free fatty acids and plasma triglycerides, redounding to the development of metabolic disturbances and diabetes. About 90% of ingested fructose is metabolized in the liver. Chronic fructose consumption can promote hepatic lipid synthesis and triosephosphate can provide further substrates for de novo lipogenesis. Furthermore, fructose was shown to be involved in the development and worsening of kidney damage going along with systemic inflammation, tubular injury and renal dysfunction. The intake of excess dietary fructose most often leads to non-alcoholic fatty liver disease (NAFLD). Regarding the development of NAFLD with the "two-hit theory", steatosis is the first hit and steatohepatitis, which requires the presence of factors such as oxidative stress, characterizes the second hit. Chronic consumption of fructose results in lipogenic gene expression in this organ. Nevertheless, how fructose is involved in NAFLD progression is still not fully understood, limiting therapy. Lipocalin 2 (LCN2) is a small secreted transport protein that binds to fatty acids, phospholipids, steroids, retinol, and pheromones. LCN2 regulates lipid and energy metabolism in obesity and is upregulated in response to insulin. It was previously discovered that LCN2 has hepatoprotective effects and upregulation is a reliable marker of liver damage and inflammation. To investigate if LCN2 has impact on the metabolism of fructose and thereby arising liver and kidney damage, we fed wild type and Lcn2-deficient mice for 4 and 8 weeks on diets that were enriched in fructose either by adding this sugar to the drinking water (30% (w/v)), or by feeding a chow containing 60% (w/w) fructose. Feeding high fructose was hypothesized to cause time-depended liver- and kidney damage in mice and more severe steatosis in mice lacking LCN2 after prolonged feeding of fructose was expected. In this study, male and female mice were analysed separately in order to investigate gender-specific differences during progression of disease. Body weight and daily intake of food and water of these mice was measured regularly during the study. Fat content in liver sections was visualized using Oil Red O-stain, and expression levels of genes involved in fat and sugar metabolism, as well as inflammatory marker genes were measured by qRT-PCR and Western blot analysis. Fructose-induced steatosis and liver damage was more prominent in female than in male mice, but the most severe hepatic damage occurred in female mice lacking LCN2. Slight kidney damage was only observed in Lcn2-deficient mice. Unexpectedly, consumption of elevated fructose did not induce de novo lipogenesis or inflammation. Instead, fructose appears to directly affect liver homeostasis, thereby manipulating fat metabolism. In conclusion, this study shows that LCN2 acts in a lipid-independent manner to protect the liver against fructose-induced damage. Female mice showed higher steatosis and damage after fructose treatment than their male counterparts, confirming the potential influence of estrogen on lipid homeostasis reported in previous studies. Fructose might disturb liver homeostasis by promoting lipid uptake into the liver, while LCN2 counteracts this lipid uptake.

Institutions

• Department of Biology [160000]
• Molecular Ecology of the Rhizosphere Group [163720]
• [526000-3]