Tip27抑制TAK1 /p38MAPK/JNK信号途径改善肝脏脂毒性的分子机制研究

Liver plays an important role in gluco-lipid metabolism. Hepatic lipotoxicity may ensue when the hepatic capacity to utilize, store and export fatty acids as triglycerides is overwhelmed. Hepatic lipotoxicity is a key factor for hepatic Insulin resistance (IR). Hepatic IR may induce severe systematic IR, thus increasing incidence of metabolic related diseases. Understanding the mechanism of lipotoxicity induced hepatic IR may provide novel therapeutic target in the management and prevention of metabolic associated pathologies. TAK1 is a subclass of the nuclear receptor superfamily. Recent reports suggest that certain fatty acids and eicosanoids bind to and enhance the transcriptional activity of TAK1; thereby suggesting that TAK1 might function as a lipid sensor. TAK1-deficient mice are resistant to the development of age- and high-fat diet (HFD)-induced obesity and are protected against obesitylinked hepatic steatosis, white adipose tissue associated inflammation, and IR. Tip27 interacts specifically with TAK1 and acts as a strong repressor of TAK1. Our previous studies showed that Tip27 could improve gluco-lipid metabolism in hepa1-6 cells. And up-regulated Tip27 expression in ApoE-/- mice resulted reduced both hepatic and plasma lipids. Therefore, we hypothesized that Tip27 may play an important role in improving hepatic IR by inhibiting TAK1 signaling. To test this hypothesis, we will utilize high fat diet (HFD)fed Tip27 transgenic mice and HepG2 cells with fatty acid to explore the role of TAK1 pathway in the development of lipotoxicity induced hepatic IR as well as mechanism of Tip27 in improving hepatic lipotoxicity and IR. We would show novel data that Tip27 may inhibit fatty acid-induced hepatic lipotoxicity and IR through inactivating TAK1/JNK/p38 pathways.

脂毒性是肝胰岛素抵抗（IR）的重要病理学基础，肝IR可诱发严重的系统性IR。对脂毒性诱导的肝IR的机制研究将推进对代谢相关疾病发病机制的新认识及防治策略的革新。新近研究证实：1）TAK1信号通路参与细胞能量代谢；2）TAK1-/-小鼠可以抵抗高脂诱导的肝脏脂肪沉积及炎细胞浸润；3）Tip27可抑制TAK1活性。课题组前期研究发现Tip27参与调节肝细胞脂代谢相关基因，高表达Tip27的质粒可降低肥胖小鼠血脂，改善肝脂质沉积。因此Tip27可能在改善脂毒性诱导的IR的过程中起重要作用，并通过抑制TAK1信号通路改善肝脂质沉积及IR。为证实该推断，拟采用Tip27转基因小鼠、HepG2细胞等，通过体内外实验，利用特异性信号通路抑制剂及分子生物学方法，探讨TAK1信号通路异常激活在脂毒性诱导的肝IR发生发展中的作用及Tip27改善肝脂毒性及IR的分子机制，并为防治代谢相关性肝病提供新的分子靶点。

脂毒性是肝胰岛素抵抗（IR）的重要病理生理机制。本项目主要通过体内外实验探讨Tip27 改善糖脂代谢、肝脏脂毒性及IR的分子机制。高脂喂养Tip27/JAZF1-Tg转基因小鼠血浆FBG、INS、TC、及 ALT均低于高脂喂养的C57小鼠；高脂喂养C57小鼠肝脏组织TNF-α、MCP-1、IL-8的mRNA表达水平及P-JNK、P-p38MAPK、P-ERK的蛋白表达水平均显著高于野生对照组，然而IKBα蛋白表达低于野生普食对照组；而Tip27/JAZF1-Tg转基因组小鼠肝脏组织JAZF1 mRNA及蛋白表达均增高，而TNF-α、MCP-1及IL-8的基因表达及P-JNK及P-P38 MAPK的蛋白表达水平明显降低，炎症状态改善；小鼠肝脏糖异生相关的基因PEPCK and G6Pase mRNA 基因表达下调，脂肪酸合成相关酶如ACC及FAS mRNA表达亦显著降低，而胰岛素信号通路关键分子如IR, IRS-1, AMPK及 Akt 等磷酸化水平显著增加，而LY294002可以减弱Tip27抑制糖异生的作用，提示Tip27可能通过PI3K信号通路抑制肝脏糖异生及增加胰岛素敏感性。进一步体外研究也发现脂毒性状态下肝细胞TNF-α、MCP-1及IL-8的表达显著升高，并且P-JNK、P-p38MAPK、P-ERK的磷酸化水平显著增强， IKBα蛋白表达明显降低；而利用Ad-Tip/JAZF1腺病毒感染饱和脂肪酸作用的肝细胞后，TNF-α、MCP-1及IL-8的表达显著降低，并且P-JNK、P-P 38 MAPK的磷酸化水平降低，IKBα蛋白表达增高，P-ERK无明显变化，提示Tip27/JAZF1可抑制促炎因子的表达，缓解肝细胞脂肪变性的发生发展。因此。因此，综合体内外结果提示脂毒性（饱和脂肪酸或高脂喂养）通过激活JNK、p38MAPK及抑制胰岛素信号通路PI3K等信号通路增加胰岛素抵抗，而Tip27/JAZF1可能通过抑JNK、p38MAPK及NF-κB信号通路并增加PI3K磷酸化促进胰岛素信号传导，进而抑制促进肝脏炎症，减轻脂毒性，改善糖脂代谢及IR。