Study On The Mechanism Of Oleuropein In Improving Hepatic Lipid Metabolism Disorders

Liver is a major site for lipid synthesis and oxidation.Excessive fatty acid synthesis and inhibited fatty acid oxidation are two main causes of hepatic lipid accumulation,which is an important risk of metabolic syndrome,diabetes mellitus,non-alcoholic fatty liver disease and other diseases.Therefore,inhibition of lipid synthesis and activation of mitochondrial oxidative metabolism-related pathways are considered as the main approaches to attenuate fatty liver and metabolism disorders.Our group previously found that the crude Jasmine granflorum L.flower has strong antiinflammatory,antioxidant,liver protection and anti-gastritis effects,but the major component and mechanism of its effect in improving metabolic syndrome remain to be further studied.Combining both in vitro and in vivo models with various techniques,the present study investigated the effect of oleuropein(Ole),a major active ingredient of Jasmine granflorum L.flower on liver lipid accumulation in a high-fat diet(HFD)mouse model and an in vitro hepatocyte model and the underlying molecular mechanism.The major findings are as follows.1.The effect and mechanism of Ole on HFD-induced metabolic syndrome in vivo.HFD containing 0.6%(w/w)Ole was fed to mice for 8 weeks.Compared with HFD-fed mice,the body weight and body fat content of the Ole-treated mice were significantly decreased,and the glucose tolerance was significantly improved without affecting the liver weight.H&E staining showed that Ole significantly reduced the area of lipid droplets in the liver of HFD-fed mice.Meanwhile,Ole significantly decreased fasting blood glucose,plasma cholesterol 、 low density lipoprotein cholesterol and alanine aminotransferase levels,and inhibited liver MCP-1 and CD68 m RNA expressions in HFD-fed mice.Further studies revealed that Ole significantly downregulated m RNA levels of key liver lipogenic enzymes(ACC and FAS)and their upstream transcription factor SREBP-1c.In the same time,Ole was found to significantly upregulate the m RNA expression of mitochondrial DNA and UCP2 in the liver.Furthermore,Ole remarkably increased the m RNA expression of mitochondrial gene promoter LKB1 and its downstream genes PGC-1α,Nrf2 and Tfam without altering AMPK.Ch IP assay showed that Ole significantly enhanced the binding of LKB1 to PGC-1α promoter,indicating that Ole could improve metabolic syndrome by LKB1-PGC-1α independent of AMPK activation.Collectively,these findings suggest that Ole is likely to be a critical component of Jasmine granflorum L.flower to exert the effects in attenuating fatty liver and associated metabolic disorders with the LKB1-PGC-1α as an critical upstream mechanism to mediate the inhibition of fatty acid synthesis and promotion of fatty acid oxidation.2.Mechanistic studies at the cellular level.This part was conducted in oleic acid(OA)-induced lipid accumulation in normal hepatocytes(AML-12).MTT assay showed that 10 ～ 160 μM Ole were able to significantly increase the cell activity and dose-dependently reduce the triglyceride content in 0.25 m M OA-induced AML-12 cells.Oil red O staining showed that the number and size of lipid droplets in OA-treated cells were significantly decreased after treatment with 80 μM Ole for24 h.Consistent with the finding in vivo,Ole significantly inhibited ACC m RNA expression and upregulated LKB1,PGC-1α and Tfam m RNA levels and increased the binding level of LKB1 to PGC-1α promoter without altering AMPK in OA-treated cells.Taken together,these results suggest that Ole may inhibit lipogenesis and activate mitochondrial metabolism by an AMP-independent LKB1-PGC-1α axis,thereby improving liver fat disorders.These findings not only reveal the mechanism underlying the therapeutic effect of Ole on HFD-induced hepatic lipid metabolism disorders,but also provide a new target for screening new drugs for fatty liver associated metabolic diseases.