Pathogenesis of non-alcoholic fatty liver disease mediated by YAP
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This study aimed to investigate the mechanism of the interaction between Yes-associated protein (YAP) and transforming growth factor-β (TGF-β)/Smad signaling pathways in the development of non-alcoholic fatty liver disease (NAFLD).
Serum samples of monkeys with biopsy-proven NAFLD and healthy normal monkeys were used to measure fasting plasma glucose (FPG), low-density lipoprotein (LDL), high-density lipoprotein (HDL), triglyceride (TG) and albumin (ALB) with the BECKMAN CX5 PRO. Hematoxylin-eosin staining (H&E) was used for pathologic analysis, Masson trichrome staining was used to assess for fibrosis staging, and Oil Red O staining was used to detect lipid droplet deposition. According to an NAFLD activity score of < 4 points and > 4 points, the samples were divided into groups: the steatosis group and fibrosing NASH group. Furthermore, monkeys with a fibrosis stage < 2 were assigned to the mild fibrosis group, while monkeys with fibrosis stage ≥ 2 were assigned to the significant fibrosis group. Moreover, the fibrosis stage was subdivided as follows: stages 1a, 1c and 2–3. Immunohistochemistry and real-time quantitative PCR were used to quantify protein and gene expression, respectively.
In the present study, 54 monkeys with NAFLD and 23 normal monkeys were recruited. Serum FPG and TG levels were higher in fibrosing NASH monkeys compared with simple steatosis and normal monkeys, and differences between simple steatosis and normal monkeys were not statistically significant (p > 0.05). YAP increased in NAFLD, which mainly localized in the nuclei of hepatocytes, perivascular cells and bile duct cells; the accumulation of YAP correlated with the severity of hepatocyte injury. Compared with normal monkeys, the expression of TGF-β, α-smooth muscle actin (α-SMA), Drosophila mothers against decapentaplegic protein 3 (Smad3) and connective tissue growth factor (CTGF) in the liver of simple steatosis monkeys significantly increased (p < 0.01). Compared with simple steatosis monkeys, the expression of TGF-β, α-SMA, Smad3 and CTGF in fibrosing NASH significantly increased (p < 0.01). However, the expression of Drosophila mothers against decapentaplegic protein 7 (Smad7) in the liver of fibrosing NASH monkeys significantly decreased (p < 0.01). With the severity of liver fibrosis, the expression of TGF-β, α-SMA, Smad3 and CTGF gradually increased, and the difference was statistically significant (p < 0.01). However, there was no significant difference in the expression of Smad3 between fibrosis stage 1a and 1c. Compared with normal monkeys, the expression of Smad7 in the liver of monkeys with fibrosis significantly decreased (p < 0.01), but was significantly higher at fibrosis stage 1c than at fibrosis stage 1a and 2.
The YAP and TGF-β signaling pathways and the interaction between them promote the development and progression of NAFLD.
KeywordsNAFLD NASH Hepatic fibrosis YAP TGF-β
This work was supported by grants from the National Key Technology Support Program (2014BAI03B01) and National Key Scientific Instrument and Equipment Development Project of China (2013YQ49085906).
ZLC, CH and PC designed the experiments and interpreted the data. PC executed the in vivo study. WZ generated and administered the animal models. QHL, QG, JFC, LKL, BC and WTL executed the other experiments. ZLC and CH supervised the project.
Compliance with ethical standards
Conflict of interest
Ping Chen, Qihui Luo, Chao Huang, Qi Gao, Like Li, Jingfei Chen, Bing Chen, Wentao Liu, Wen Zeng and Zhengli Chen declare that they have no conflicts of interest.
All animal experiments were performed in accordance with the Animal Care and Use Committee Guidelines of Sichuan Agricultural University, China.
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