促甲状腺激素（TSH）调控肝脏胆固醇合成限速酶信号网络的机制研究

Hypercholesterolemia is a common phenomenon in the state of hypothyroidism, which was used to being explained as the result of serum thyroxine (TH) deficency. However, elevation of serum total cholesterol(TC) has also been observed in patients with subclinical hypothyroidism(SCH), in which thyroid-stimulating hormone(TSH) is elevated but TH stays within its normal range. Thus, the development of dyslipidemia in SCH cannot be only responsible by the changes of TH, so does it for the secondary hypothyroidism, of which characterized by a reduction both in serum TSH and TH. This raises the question that elevated TSH might also plays an important role in the development of hypercholesterolemia. Our previous study suggested that TSH receptor (TSHR) is presented on hepatocyte membrane, through which TSH is mediated by the cyclic adenosine monophosphate/protein kinase A/cyclic adenosine monophosphate-responsive element binding protein (cAMP/PKA/CREB) signaling pathway, promotes the expression of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR), the rate-limiting enzyme in cholesterol synthesis, and ultimately increases the serum TC content. Additionally, our latest reports showed the changes of CREB-regulated transcription co-activator (CRTC2), Sterol regulatory element-binding protein(SREBP-2) and their related pathways in the regulation. Whether these pathways and networks also take part in the process of TSH regulates hepatic HMGCR and the exact functional mechanisms are still uncertain, and no one once had taken a relative investigation. Therefore, using both in vitro (TSH stimulation on liver cells, interference, blocking or activation of the key molecules) and in vivo (conditionally knockout of liver Tshr gene mouse model, TSHR-RNAi mouse model, thyroid total resection rat model) experiments and the relative key targets intervention approaches, we will elucidate the key molecules and major signaling networks about the effection of TSH on HMGCR in the liver, explore the pathophysioligical mechanism of the cholesterol metabolism abnormalities in thyroidal dysfunction situation and further provide newly theoretical evidence for its treatment strategy.

传统观点认为甲状腺功能异常时胆固醇水平改变是甲状腺激素（TH）作用的结果，但亚临床甲减（TSH升高，TH不变）胆固醇升高,而继发性甲减（TSH和TH降低）胆固醇降低均无法单纯用TH的作用解释。我们前期研究证明肝脏存在TSH受体,TSH通过该受体介导cAMP/PKA/CREB通路调节胆固醇合成限速酶（HMGCR），增加胆固醇含量；同时伴有CRTC2、SREBP-2及相关信号通路的变化，这些通路及其构成的网络是否也参与TSH调控HMGCR，具有怎样的作用？国内外未见研究报道。本项目拟采用体外（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（条件性敲除肝脏Tshr基因、TSHR-RNAi等小鼠模型、摘除甲状腺大鼠模型）相结合及干预关键靶点的研究策略，阐明TSH调控HMGCR的关键分子和主要信号网络，为揭示甲状腺疾病胆固醇代谢异常的病理生理机制和有效的干预治疗提供新的依据和策略。

亚临床甲状腺功能减退（亚甲减）是常见病（患病率16.&%）。患者常伴有高胆固醇血症，然而机制不清。课题组针对这一科学问题申请了该项目。研究中以血清TSH升高是亚甲减特征，以及肝脏是机体调节胆固醇代谢的重要器官为线索，以TSH 是否调节肝脏胆固醇代谢以及其主要机制为主要内容开展了研究。.利用分子生物学技术，以构建的多种小鼠模型（包括国际上首创建立了“药物性诱导亚甲减小鼠动物模型”）和多种肝细胞为主要研究对象，经过5年研究，获得结果主要有：1.肝脏TSH 效应是通过其受体（TSHR）实现。2.TSH通过多条信号通路增强肝脏胆固醇合成限速酶（HMGCR）转录表达，刺激新合成HMGCR增多（p<0.05）、新生胆固醇增加（p<0.05）。这个过程主要是激活TSHR/cAMP/PKA/CREB、 TSHR/βarrestin-1/PI3K/Akt/SREBP-2和CRTC2/FOXO1/SREBP-2，抑制TSHR/AMPK 信号通路。3.发现并论证TSHR/HNF-4α/CYP7A1轴是TSH抑制肝脏胆固醇转化为胆汁酸的主要信号通路（p<0.05）。通过建立的TSH调控肝脏胆固醇合成限速酶信号网络，明确了TSH诱导肝脏内胆固醇含量增加的分子机制。TSH正是通过诱发胆固醇合成，增加胆固醇来路；同时抑制胆固醇转化为胆汁酸，减少胆固醇去路这种异常的“协同作用”，导致亚甲减患者机体胆固醇代谢失衡，血清胆固醇升高。科学价值提示为临床决策亚甲减的治疗策略提供理论依据。应用价值体现在提示临床医生早期干预治疗亚临床甲减尤其轻度亚甲减的必要性；高胆固醇血症患者常规检查甲状腺功能，甲状腺功能异常患者常规检查血脂水平必要性。