Published online Feb 21, 2014. doi: 10.3748/wjg.v20.i7.1807
Revised: September 13, 2013
Accepted: October 14, 2013
Published online: February 21, 2014
Processing time: 208 Days and 14.1 Hours
AIM: To explore lipocalin-2 (LCN-2) expression and its possible role and mechanism(s) of production in rat models of diet-inducible fatty liver.
METHODS: Fatty liver was triggered in male Sprague-Dawley rats fed either with liquid Lieber-DeCarli (LDC) or LDC + 70% cal fructose (L-HFr) diet for 4 or 8 wk. Chow-nourished animals served as controls. Hepatic expression of LCN-2 and other metabolic and inflammatory mediators was assessed by quantitative reverse transcription polymerase chain reaction and Western blotting. Serum LCN-2, fasting leptin, and lipid profile were evaluated via Enzyme-Linked Immunosorbent Assay, Radioimmunoassay, and colorimetric assays, respectively. The localization of LCN-2 in the liver was detected by using immunofluorescence staining. Furthermore, HE stain was used to evaluate hepatic fat degeneration and inflammation.
RESULTS: Both LDC-fed and L-HFr-fed rat histologically featured fatty liver. In the liver, mRNA transcriptions of Mcp-1, a2-m, Il-8 and Glut5 were increased in the L-HFr group at both time points (P < 0.001), while the transcription of Tlr4, Inos, and Tnf-α was significantly up-regulated at week 4. Interestingly, hepatic Lcn-2 expression was 90-fold at week 4 and 507-fold at week 8 higher in L-HFr-subjected rats vs control (P < 0.001). In contrast to HDL-cholesterol, systemic levels of LCN-2, fasting leptin and triglycerides were elevated in the L-HFr regimen (P < 0.001). Moreover, protein expression of hepatic LCN-2, CD14, phospho-MAPK, caspase-9, cytochrome c and 4-hydroxynonenal was increased in the L-HFr group. Conversely, the hepatic expression of PGC-1α (a mitochondrial-biogenic protein) was reduced in the L-HFr category at week 8. The localization of LCN-2 in the liver was predominantly restricted to MPO+ granulocytes.
CONCLUSION: Fructose diet up-regulates hepatic LCN-2 expression, which correlates with the increased indicators of oxidative stress and mitochondrial dysfunction. The LCN-2 may be involved in liver protection.
Core tip: Both Lieber-DeCarli (LDC) and LDC + 70% cal fructose (L-HFr) models featured fatty liver. Fructose-enriched regimen induced metabolic syndrome in the corresponding rats. Lipocalin-2 (LCN-2) was strikingly increased in the liver and serum of the L-HFr group. In this group, the increase of LCN-2 synthesis was associated with inflammation at week 4, whereas the peak value of LCN-2 at week 8 was mainly accompanied by impairment of the mitochondrial function. Nevertheless, an interaction coexists between both processes. The indicators of stress conditions and apoptosis were elevated at both time points. Evidently, the expression of LCN-2 was correlated to inflammatory and metabolic processes.