Research mechanisms of Thyrotropin Receptor Expression

在之前的一个项目中，我们已经鉴定了大鼠促甲状腺激素(TSH)受体基因启动子区域的结构和功能，并鉴定了几个调节该基因在甲状腺中表达的顺式作用元件和反式作用因子。一个值得注意的发现是，转录因子TSEP-1/YB-1作为抑制物作用于主要组织相容性复合体II类基因，也作为TSH受体启动子的抑制物发挥作用。在此基础上，我们进一步分析了大鼠TSH受体启动子区域及其相互作用因素，进一步克隆并鉴定了大鼠TSH受体启动子的-4.2kb区域。-4.2kb区的启动子活性低于我们已经分析过的最小启动子区域的启动子活性，支持最小启动子区域对活性的重要性。在上游区域有许多可能的TSEP-1/YB-1结合位点，它们具有抑制因子的功能。在自身免疫性甲状腺疾病(AITD)中，甲状腺细胞表达MHC-II类抗原对自身免疫反应的诱导和持续具有重要意义。TSH受体本身也是一种重要的自身抗原。为了调控TSH受体和MHC-II类基因在甲状腺细胞上的表达，我们重点研究了TSEP-1/YB-1对这两个基因的作用。我们发现烟酰胺增强了这两个基因的启动子活性，这归因于烟酰胺诱导的TSEP-1/YB-1表达的剂量依赖性降低。这一结果并不直接表明烟酰胺的治疗价值，但表明存在可能同时调节这两个基因表达的药物。如能抑制甲状腺细胞表面TSH受体和MHC-II类分子的表达，可能有利于Graves病等AITD的治疗。我们正在未来的项目中探索这种可能性。

In a previous project, we have characterized structure and function of the promoter region of the rat thyrotropin (TSH) receptor gene, and identified several cis-acting elements and trans-acting factors which regulate the expression of the gene in thyroid. One of notable findings is that a transcription factor TSEP-1/YB-1, which acts on major histocompatibility complex class II gene as a repressor, also functions as a repressor on the TSH receptor promoter. Based on these studies, we further analyzed the promoter region and factors interacting with it in the present project.We further cloned and characterized -4.2 kb region of the rat TSH receptor promoter. The -4.2 kb region exhibited less promoter activity than that of the minimal promoter region which we already analyzed, supporting the importance of the minimal promoter region on the activity. There are many putative TSEP-1/YB-1 binding sites, which function as a repressor, in the upstream region. These elements may suppress the promoter activity constitutively.Aberrant expression of MHC class II antigen on thyroid cells is important for induction and perpetuation of autoimmune reaction in autoimmune thyroid diseases (AITD). TSH receptor itself is also important as an autoantigen. To control the expression of TSH receptor and MHC class II on thyroid cells, we focused on the action of TSEP-1/YB-1 on both genes. We showed that nicotinamide potentiated promoter activity of both genes, and this was attributed a dose-dependent reduction of TSEP-1/YB-1 expression induced by nicotinamide. This result does not directly implicate a therapeutic value of nicotinamide, but suggests the presence of agents which may regulate the expression of both genes simultaneously. If there were such agent which could repress the expression of TSH receptor and MHC class II on thyroid cells, it could be beneficial for treatment of AITD such aS Graves' disease. We are pursuing this possibility in the future project.