Hepatic Proteome and Lipid Profiling of Wild Type and Lipocalin-2-Deficient Mice in Experimental Steatosis

Background: Lipocalin-2 (LCN2) or neutrophil gelatinase-associated lipocalin (NGAL) is a small secreted adipokine belonging to the lipocalin family [1]. It binds and transports small hydrophobic molecules and limits bacterial growth by sequestering iron-containing siderophores. In the liver, LCN2 plays a protective role in inflammation, infection, and cellular stress. Recently, we demonstrated that LCN2 regulates lipid droplet protein Perilipin 5 (PLIN5) expression in primary hepatocytes and showed that LCN2 animals are more prone to hepatic inflammation and steatosis [2 – 4]. Methods: We here comparatively analyzed the proteome (label-free proteomics or 2D-DIGE protein expression profiling) of wild type and Lcn2-deficient mice fed either a standard-chow and a methionine- and choline-deficient (MCD) diet. The differential expression was confirmed by Western blot analysis and quantitative real-time PCR. We further employed comparative MALDI-TOF Imaging Mass Spectrometry to monitor the spatial distribution of a broad range of lipids in liver tissue sections of respective groups. Results: We identified a multitude of genes that are either upregulated during hepatic steatosis or differentially induced or repressed in mice lacking LCN2. Differentially expressed proteins were BRIT1/MCPH1, FABP5, HMGB1, HBB2, and L-FABP. In addition, we identified significantly altered m/z signal intensities for several sphingomyelins, triglycerides, and phospholipid species. Most notably, phosphatidylinositol phosphates were substantially elevated in MCD-fed mice, indicating chronic activation of phosphatidylinositol phosphate-dependent signaling pathways, and this alteration was unaffected by LCN2 deletion. Moreover, the abundance of some 20:4 lipids were elevated in the livers of Lcn2-deficient mice suggesting that this gene disruption might interfere with arachidonic acid and eicosanoid metabolism. Conclusion: In summary, our data indicate that LCN2 is a key switch influencing triglyceride balance, reactive oxidative stress formation, inflammatory response, and cellular apoptosis.