Lipocalin 2 (LCN2)-deficient mice are more prone to hepatic steatosis: LCN2 and mitochondrial and peroxisomal integrity

Background: Lipocalin-2 (LCN2) or neutrophil gelatinase-associated lipocalin (NGAL) is a small secreted adipokine associated with transport of small hydrophobic molecules [1]. In the liver, it limits bacterial growth and modulates the inflammatory response by acting as a “help me” signal attracting circulating blood cells into the tissue [2]. We have previously demonstrated that LCN2 is involved in control of hepatic fat metabolism by regulating the expression of the intracellular lipid droplet protein PLIN5/O'X'PAT [3]. Methods: We here extended our work and performed a comparative proteome profiling of wild type and Lcn2-deficient mice that were fed either with a standard chow or a methionine- and choline-deficient diet. Histological scoring of steatosis was adapted from guidelines used in the scoring of non-alcoholic fatty liver disease in humans. Alterations in protein expression were confirmed by Western blot and real-time PCR analysis. Mitochondrial and peroxisomal integrity was evaluated by a large variety of methods. To estimate the biological significance of LCN2 in lipid homeostasis, we compared tissue sections of Lcn2-deficient mice or WT mice fed an MCD diet by Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight (MALDI-TOF) mass spectrometry imaging. Results: Gene ontology enrichment analysis performed with the web-based application Gene Ontology enRIchment anaLysis and visuaLizAtion (GOrilla) identified enriched GO terms that are associated with mitochondrial function and carboxylic acid metabolic processes. Comparative measurements of the mitochondrial iron pool, membrane potential, intracellular lipid peroxidation, and peroxisome staining confirmed that the presence of LCN2 impacts mitochondrial function and peroxisome quantities in primary hepatocytes. In regard to fat analysis, significant changes to m/z signal intensities for various sphingomyelins, triglycerides, and glycerophospholipid species were identified. In the liver, the loss of Lcn2 was associated with an increase in two arachidonic acid containing glycerophospholipids suggesting that LCN2 is critically involved in polyunsaturated fatty acid metabolism. Conclusions: In summary, our data shows that LCN2 is a versatile serum protein that impacts protein and fat homeostasis and further controls the integrity and function of mitochondria and peroxisomes. References: [1] Asimakopoulou A, Weiskirchen R. Lipocalin 2 in the pathogenesis of fatty liver disease and non-alcoholic steatohepatitis. Clin Lipidol. 2015;10:47 – 67. [2] Asimakopoulou A, Borkham-Kamphorst E, Tacke F, Weiskirchen R. Lipocalin-2 (NGAL/LCN2), a "help-me" signal in organ inflammation. Hepatology 2016;63:669 – 71. [3] Asimakopoulou A, Borkham-Kamphorst E, Henning M, Yagmur E, Gassler N, Liedtke C, Berger T, Mak TW, Weiskirchen R. Lipocalin-2 (LCN2) regulates PLIN5 expression and intracellular lipid droplet formation in the liver. Biochim Biophys Acta 2014;1842:1513 – 24.