AMPK 在促甲状腺激素(TSH)调控肝脏胆固醇合成中的作用机制

The patients with the subclinical hypothyroidism often have the elevated TSH level and hypercholesterolemia.The epidemiological survey has showed that TSH level is positively related with the total cholesterol. Our previous research confirmed that the functional TSH receptor (TSHR) expresses in the membrane of the liver cells, and TSH could up-regulate the activity of HMGCR,the rate-limiting enzyme in cholesterol synthesis.TSH elevates the cholesterol level by the cAMP / PKA /CREB signaling pathway.It is accompanied by the decreasing the activity of AMPK,which is the energy receptor of the glucose and lipid metabolism.However,there is no any report to elaborate the effects of TSH on AMPK in and abroad. In order to explore the molecular mechanism of AMPK in TSH regulating HMGCR in the liver, we will focus on the key targets using different methods containing the in vitro and in vivo approaches. In vitro, we will use the means by TSH stimulating the liver cells and disturbing, blocking or activating the expression of the key molecules,while in vivo, we would like to set up the animal models as the knockout of the hepatic Tshr genes and the interference of siRNA in AMPKα,respectively. It will help us go deep into understanding the mechanism of hypercholesterolemia in the subclinical hypothyroidism and provide the new ideas and the therapeutic targets for the clinical treatments.

亚临床甲减患者常表现为TSH升高、高胆固醇血症；流行病学调查发现，TSH与总胆固醇呈正相关性。我们前期研究证实，肝细胞膜有TSH受体（TSHR）表达，TSH通过cAMP/PKA/CREB信号通路上调胆固醇合成限速酶HMGCR，增加胆固醇合成；同时伴有糖脂代谢能量感受器- - AMPK活性降低。目前国内外尚无TSH对AMPK影响的研究。本研究拟采用体外（TSH刺激肝细胞及干扰、阻断或激活关键分子表达等手段）和体内（敲除肝脏Tshr基因、siRNA干扰AMPKα等动物模型）相结合及干预关键靶点的研究策略，探讨AMPK在TSH调控肝脏HMGCR中的分子作用机制。这将丰富亚临床甲减高胆固醇血症的发病理论机制，为临床治疗提供新的思路和治疗靶点。

3-羟基-3-甲基戊二酰辅酶A还原酶（HMGCR）是胆固醇合成限速酶，在多个水平上受到调节，包括转录水平、翻译水平、酶的降解、磷酸化/去磷酸化修饰和产物反馈抑制等，参与维持胆固醇稳态。 本项目发现肝细胞内AMPK磷酸化HMGCR 871位点的丝氨酸，继而增加磷酸化 HMGCR 与非磷酸HMGCR的比值，抑制该酶的活性；AMPK也能直接磷酸化 SREBP-2 前体及核内活性形式的苏氨酸位点，但不影响其丝氨酸位点磷酸化，磷酸化后的 SREBP-2 活性降低。SREBP-2 是调节 HMGCR的关键蛋白，HMGCR在细胞液中经蛋白激酶催化发生磷酸化丧失活性，而在磷酸酶作用下又可以脱去磷酸从而恢复酶活性，调节胆固醇的合成。TSH通过TSHR抑制肝脏AMPK 172位点的苏氨酸磷酸化，减少 AMPK 活性，从而分别解除对HMGCR和SREBP-2蛋白活性抑制， 增强HMGCR活性，最终增加胆固醇合成。