Effects Of Lipopolysaccharide On Hepatic Lipid Metabolism In Mice

Abstract Objective Non-alcoholic fatty liver disease (NAFLD) is a common chronicliver disease. In recent years, the incidence rate of NAFLD showed an increasingtrend in the population. Thus, the pathogenesis mechanisms of NAFLD have becomean increasing concern. Clinical and animal studies have shown that inflammation andinfection play an important role in NAFLD pathogenesis, but the specific mechanismthat inflammation and infection disturbed hepatic lipid metabolism has not beenelucidated. Hepatic lipid metabolism is involved in the process of lipid synthesis,transfer and decomposition. Some studies found recently that the transcription factorssterol regulatory element binding protein-1c (SREBP-1c) and carbohydrate responseelement binding protein (ChREBP) signaling pathway play an important role in theliver lipid synthesis pathway, fatty acid translocation enzyme (CD36) is an importanttransporter in the liver fatty acid transport, the nuclear receptor peroxisomeproliferator-activated receptor γ (PPARγ) play an important role in the liver lipidsynthesis and transport, the nuclear receptor peroxisome proliferator-activatedreceptor alpha (PPARα) play an important role in the regulation of key enzymes ofliver lipid decomposition. This study established the model of bacteriallipopolysaccharide (LPS) induce inflammation and infection in mice, observed effectsof LPS on hepatic fatty acid and triglyceride (TG) biosynthesis, transport anddecomposition pathways, in order to further studied the role of transcription factorsSREBP-1c and ChREBP signaling pathway, CD36, nuclear receptors PPARγ andPPARα in LPS disturbed the hepatic lipid metabolism, focused to analyze molecularmechanisms of LPS disturbed the hepatic lipid metabolism and clarified molecularmechanisms of inflammation and infection-induced NAFLD.Methods The present study included two separate experiments. In experiment1, toinvestigate the effects of LPS on hepatic lipid synthesis, transfer and catabolism in mice, sixteen male ICR mice were randomly divided into two groups, which werecontrol group and LPS12h group. LPS12h group mice were administered with LPS(2.0mg/kg) by intraperitoneally(ip) injection after fasted for2h, and control groupmice were administered with an equal volume of saline by ip injection. All mice werefasted and sacrificed after12h. Blood serum was collected for serum TGmeasurement. Liver was collected for qPCR, Western blot and hepatic TGmeasurement, or fixed in neutral-buffered formalin for histological examination, orfrozen-fixed in OCT mounting media for Oil red O staining. Serum biochemicalparameters in mice were detected used automatic biochemical analyzer, pathologicalchanges in liver tissue were observed by HE staining, liver lipid were measured by oilred O staining, and the levels of transcription factor SREBP-1c and its downstreamtarget genes, transporter CD36and the nuclear receptor PPARα downstream targetgene mRNA were analyzed by qPCR. In experiment2, thirty-two male ICR micewere randomly divided into four groups, which were control group, LPS2h group,LPS6h group and LPS12h group, and analyzed the protein levels of keytranscription factors SREBP-1c and ChREBP, nuclear receptors PPARγ and PPARα.LPS2h, LPS6h and LPS12h groups were respectively administered with LPS (2.0mg/kg) by ip injection after fasted12h,8h and2h. And control group wereadministered with an equal volume of saline by ip injection after2h. All mice weresacrificed after LPS treatment (2h,6h and12h) and control group for12h, livertissue was collected to detect the protein levels of transcription factors SREBP-1c andChREBP and nuclear receptors PPARγ and PPARα by Western blot measurement.Results The first experiment results showed that serum ALT and AST content weresignificantly higher in mice treated by LPS for12h compared with the control group,and hepatocytes were swollen, fat and inflammatory cells infiltration in theintracellular by hematoxylin and Eosin staining, which suggested that LPS triggeredhepatic inflammatory response and liver injury in mice treated by LPS for12h. Thelevel of serum and hepatic TG was significantly increased in mice treated by LPS for 12h. An obvious hepatic lipid accumulation, which was determined by Oil Red Ostaining, was observed in mice treated by LPS for12h. Compared with the controlgroup, the levels of hepatic srebp-1c and its downsteam target genes fas mRNA wereincreased in mice treated by LPS for12h, and no significant difference on the level ofacc mRNA was observed, but scd-1was decreased. Compared with the control group,LPS significantly downregulated the levels of hepatic PPARα downstream targetgenes fatty acid oxidation-relatedenzyme (cpt1α, cyp4a10and cyp4a14) mRNA, buthad no significant affected the level of cd36mRNA. The second experiment resultsfurther showed that the nuclear protein level of hepatic SREBP-1c in mice treated byLPS for6h and12h was significantly increased and showed a significant time–effectrelationship compared with the control group, but the nuclear protein level of hepaticChREBP had no significant difference, PPARγ was significantly increased in micetreated by LPS for12h, and the nuclear protein level of hepatic PPARα wassignificantly reduced in different times(2h,6h and12h) treated by LPS in mice, andthe most significant effect in LPS6h group.Conclusion LPS evoked transcription factor SREBP-1c, which induced itsdownstream target genes lead to increase hepatic TG synthesis, and LPS inhibitednuclear receptor PPARα activation, which downregulated its downstream target genesmRNA level, lead to decrease the hepatic free fatty acid oxidation, finally resulted inhepatic lipid accumulation and NAFLD.