Thyrotropin increases hepatic triglyceride content through upregulation of SREBP-1c activity

• Published in: Journal of hepatology 2014-12, Vol.61 (6), p.1358-1364
• Main Authors: Yan, Fang, Wang, Qi, Lu, Ming, Chen, Wenbin, Song, Yongfeng, Jing, Fei, Guan, Youfei, Wang, Laicheng, Lin, Yanliang, Bo, Tao, Zhang, Jie, Wang, Tingting, Xin, Wei, Yu, Chunxiao, Guan, Qingbo, Zhou, Xinli, Gao, Ling, Xu, Chao, Zhao, Jiajun
• Format: Article
• Language: English
• Subjects: Animals; Cyclic AMP - metabolism; Cyclic AMP-Dependent Protein Kinases - metabolism; Diet, High-Fat - adverse effects; Disease Models, Animal; Gastroenterology and Hepatology; Hep G2 Cells; Hepatic steatosis; Hepatocytes - drug effects; Hepatocytes - metabolism; Hepatocytes - pathology; Humans; In Vitro Techniques; Liver - metabolism; Liver - physiopathology; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; Non-alcoholic Fatty Liver Disease - epidemiology; Non-alcoholic Fatty Liver Disease - metabolism; Non-alcoholic Fatty Liver Disease - physiopathology; Peroxisome proliferator-activated receptor α; PPAR alpha - metabolism; Receptors, Thyrotropin - deficiency; Receptors, Thyrotropin - genetics; Receptors, Thyrotropin - metabolism; Risk Factors; Signal Transduction - physiology; Sterol Regulatory Element Binding Protein 1 - deficiency; Sterol Regulatory Element Binding Protein 1 - genetics; Sterol Regulatory Element Binding Protein 1 - metabolism; Sterol regulatory element-binding protein 1c; Thyrotropin (TSH); Thyrotropin - metabolism; Thyrotropin - pharmacology; Triglyceride; Triglycerides - metabolism; Up-Regulation - physiology
• ISSN: 0168-8278; 1600-0641
• DOI: 10.1016/j.jhep.2014.06.037

Background & Aims Hallmarks of non-alcoholic fatty liver disease (NAFLD) are increased triglyceride accumulation within hepatocytes. The prevalence of NAFLD increases steadily with increasing thyrotropin (TSH) levels. However, the underlying mechanisms are largely unknown. Here, we focused on exploring the effect and mechanism of TSH on the hepatic triglyceride content. Methods As the function of TSH is mediated through the TSH receptor (TSHR), Tshr−/− mice (supplemented with thyroxine) were used. Liver steatosis and triglyceride content were analysed in Tshr−/− and Tshr+/+ mice fed a high-fat or normal chow diet, as well as in Srebp-1c−/− and Tshr−/− Srebp-1c−/− mice. The expression levels of proteins and genes involved in liver triglyceride metabolism was measured. Results Compared with control littermates, the high-fat diet induced a relatively low degree of liver steatosis in Tshr−/− mice. Even under chow diet, hepatic triglyceride content was decreased in Tshr−/− mice. TSH caused concentration- and time-dependent effects on intracellular triglyceride contents in hepatocytes in vitro . The activity of SREBP-1c, a key regulator involved in triglyceride metabolism and in the pathogenesis of NAFLD, was significantly lower in Tshr−/− mice. In Tshr−/− Srebp-1c−/− mice, the liver triglyceride content showed no significant difference compared with Tshr+/+ Srebp-1c−/− mice. When mice were injected with forskolin (cAMP activator), H89 (inhibitor of PKA) or AICAR (AMPK activator), or HeG2 cells received MK886 (PPARα inhibitor), triglyceride contents presented in a manner dependent on SREBP-1c activity. The mechanism, underlying TSH-induced liver triglyceride accumulation, involved that TSH, through its receptor TSHR, triggered hepatic SREBP-1c activity via the cAMP/PKA/PPARα pathway associated with decreased AMPK, which further increased the expression of genes associated with lipogenesis. Conclusions TSH increased the hepatic triglyceride content, indicating an essential role for TSH in the pathogenesis of NAFLD.