ABSTRACT
Lipocalin-2 (LCN2) belongs to the superfamily of lipocalins and plays critical roles in the control of cellular homeostasis during inflammation and in responses to cellular stress or injury. In the liver, LCN2 triggers protective effects following acute or chronic injury, and its expression is a reliable indicator of liver damage. However, little is known about LCN2's functions in the homeostasis and metabolism of hepatic lipids or in the development of steatosis. In this study, we fed wild type (WT) and LCN2-deficient (Lcn2(-/-)) mice a methionine- and choline-deficient (MCD) diet as a nutritional model of non-alcoholic steatohepatitis, and compared intrahepatic lipid accumulation, lipid droplet formation, mitochondrial content, and expression of the Perilipin proteins that regulate cellular lipid metabolism. We found that Lcn2(-/-) mice fed an MCD diet accumulated more lipids in the liver than WT controls, and that the basal expression of the lipid droplet coat protein Perilipin 5 (PLIN5, also known as OXPAT) was significantly reduced in these animals. Similarly, the overexpression of LCN2 and PLIN5 were also found in animals that were fed with a high fat diet. Furthermore, the loss of LCN2 and/or PLIN5 in hepatocytes prevented normal intracellular lipid droplet formation both in vitro and in vivo. Restoration of LCN2 in Lcn2(-/-) primary hepatocytes by either transfection or adenoviral vector infection induced PLIN5 expression and restored proper lipid droplet formation. Our data indicate that LCN2 is a key modulator of hepatic lipid homeostasis that controls the formation of intracellular lipid droplets by regulating PLIN5 expression. LCN2 may therefore represent a novel therapeutic drug target for the treatment of liver diseases associated with elevated fat accumulation and steatosis.
ABSTRACT
Lipocalin 2 (LCN2) belongs to the superfamily of lipocalins which represent a group of small secreted proteins classified as extracellular transport proteins expressed in many tissues. LCN2 is strongly increased in experimental models of acute and chronic liver injuries. To investigate the function of LCN2 in normal liver homeostasis and under conditions of inflammatory liver injury, we comparatively analyzed hepatic extracts taken from Lcn2-deficient and wild type mice under basal conditions and after stimulation with lipopolysaccharides. Liver was chemically and mechanically lysed and extracts were subjected to 2-D-DIGE after minimal labeling (G200 and G300 dyes) using an appropriate internal standard (G100). Afterwards MALDI TOF MS and MS/MS were used to identify differentially expressed proteins. Proteins that were identified to be differentially expressed include for example the chloride intracellular channel protein 4 (CLIC4), aminoacylase 1 and transketolase. The altered expression of respective genes was confirmed by Western blot analysis and further validated by quantitative real time PCR. Altogether, the complex expression alterations in mice lacking LCN2 under normal conditions and after exposure to inflammatory stimuli reveal that LCN2 has essential function in liver homeostasis and in the onset of inflammatory responses in which LCN2 expression dramatically increases.
