Regulation of Expression of MHC Class I Genes

MHC I 类基因表达的调节

• 批准号: 7965151
• 负责人: Dinah S. Singer
• 金额: $ 58.65万
• 依托单位: DIVISION OF BASIC SCIENCES - NCI
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/7965151
• 关键词: Area; Autoantigens; Autoimmune Diseases; B-Lymphocytes; Be++ element; Beryllium; Binding Sites; Cell Line; Cell Surface Receptors; Complex; Data; Disease; Elements; Enhancers; Ensure; Event; Failure; Fibroblasts; Gene Expression; Gene Targeting; General Transcription Factors; Genes; Genetic Transcription; Goals; HIV; Homeostasis; Hormones; Immune; Immune response; Immunologic Surveillance; In Vitro; Indium; Inflammation; Investigation; L Cells; Lead; MHC Class I Genes; Major Histocompatibility Complex; Mediating; Molecular; Mutate; Pathway interactions; Pattern; Peptides; Play; Promoter Regions; Recruitment Activity; Regulation; Regulatory Element; Reporter Genes; Reporting; Research; Role; Signal Pathway; Signal Transduction; Site; T-Lymphocyte; TAF1 gene; Time; Tissues; Transcription Initiation; Transcription Initiation Site; Transgenic Mice; Tumor Antigens; carcinogenesis; cell type; cytokine; extracellular; in vivo; interest; novel; pathogen; programs; promoter; transcription factor; tumor

Transcription of major histocompatibility complex (MHC) class I genes is regulated by both tissue-specific (basal) and hormone/cytokine (activated) mechanisms. Although promoter-proximal regulatory elements have been characterized extensively, the roles of the core promoter and downstream elements in mediating regulation have been largely undefined. Basal and activated transcriptions of an MHC class I gene target distinct core promoter domains, nucleate distinct transcription initiation complexes and initiate at distinct sites within the promoter. Basal and activated transcription pathways recruit distinct transcription factor complexes to the core promoter elements and target distinct transcription initiation sites. Basal transcription is completely dependent upon the general transcription factor TAF1 whereas activated transcription is TAF1 independent. To further characterize regulation of class I gene expression, we have undertaken to characterize core promoter elements in vivo and to identify novel downstream promoter elements. To understand these mechanisms in more detail, we have characterized elements within the core promoter of the MHC class I gene. The minimal class I core promoter has been localized to a segment between -50 bp and +14 bp. Within this segment are sequences similar to canonical TATA and Inr promoter elements and an Sp1 binding site; however, no single element is necessary for transcription, although each provides some level of regulation in different cell types. For instance, the Inr element is a negative regulator in fibroblasts, but a positive regulator in T and B cell lines. Conversely, the TATA-like element is a positive regulator in T cell lines, but a negative regulator in B cell and fibroblast lines. Importantly, the roles of these core promoter elements has been examined and verified in vivo in transgenic mice bearing MHC class I genes mutated in the relevant promoter elements. Of particular interest, the downstream region of the MHC class I promoter region, between +1 and +32 bp, contains two novel regulatory elements. Under constitutive conditions, the two elements act in concert to down-regulate promoter activity, preferentially suppressing the use of TSS located at the 5 end of the cluster of multiple start sites. These elements repress constitutive, TAF1-dependent transcription. However, under activated TAF1-independent conditions, one element functions independently as an enhancer. Thus, the downstream regulatory elements associated with the class I promoter function to fine-tune and integrate both intracellular and extracellular signaling pathways to ensure the appropriate level of MHC class I transcription to maintain immune homeostasis. These novel downstream elements are functionally and mechanistically distinct from previously described downstream elements in mammalian promoters. Importantly, we have now identified a second promoter element within the core promoter region. This novel promoter element alone is capable of supporting transcription of a reporter gene in the presence or a heterologous enhancer. Furthermore, it is capable of supporting CIITA-activated transcription of a reporter gene. Most significantly, this novel promoter drives the expression of the native MHC class I gene both in stably transfected L cells and in transgenic mice. Thus, this promoter element, like the first, is capable of supporting both constitutive and activated transcription in vitro and in vivo. The roles of the two distinct promoters in regulating MHC class I transcription is an active area of investigation. Finally, novel regulatory elements that serve to establish tissue specific patterns of regulation have been identified by in vivo analysis and localized to intronic regions.

主要组织相容性复合物（MHC）I类基因的转录受组织特异性（基础）和激素/细胞因子（激活）机制调节。尽管启动子促进元的调节元素已被广泛表征，但核心启动子和下游元素在中介调节中的作用在很大程度上是不确定的。 MHC I类靶标不同核心启动子结构域的基础和激活的转录，对不同的转录起始复合物有核并在启动子内的不同位点启动。基础和激活的转录途径将不同的转录因子复合物募集到核心启动子元素，并靶向不同的转录起始位点。基础转录完全取决于一般转录因子TAF1，而活化的转录独立于TAF1。为了进一步表征对I类基因表达的调节，我们已承诺表征体内核心启动子元素并确定新型的下游启动子元素。为了更详细地了解这些机制，我们在MHC I类基因的核心启动子中表征了元素。最小的I类核心启动子已定位到-50 bp和+14 bp之间。在该段中，类似于规范tata和INR启动子元素以及SP1结合位点的序列。但是，尽管每个细胞类型都提供了一定程度的调节，但没有单一的元素才能进行转录。例如，INR元素是成纤维细胞中的负调节剂，但在T和B细胞系中是阳性调节剂。相反，TATA样元素是T细胞系中的一个正调节剂，但是B细胞和成纤维细胞系中的负调节剂。重要的是，这些核心启动子元素的作用已在体内进行了研究和验证，该小鼠在相关启动子元素中突变的具有MHC I类基因的转基因小鼠。特别有趣的是，MHC I类启动子区域的下游区域+1至+32 bp之间包含两个新型的调节元素。在本构条件下，这两个元素共同起作用以下调启动子活性，优先抑制位于多个起始位点群集的5端的TSS的使用。这些元素抑制构成型，依赖TAF1的转录。但是，在激活的TA​​F1独立条件下，一个元素作为增强子独立起作用。因此，与I类启动子功能相关的下游调节元素可以微调和整合细胞内和细胞外信号传导途径，以确保适当的MHC I类转录水平以维持免疫稳态。这些新型的下游元素在功能和机械上与先前描述的哺乳动物启动子中的下游元素不同。重要的是，我们现在已经确定了核心启动子区域内的第二个启动子元素。单独的启动子元素能够在存在或异源增强子中支持报告基因的转录。此外，它能够支持记者基因的CIITA激活转录。最重要的是，这种新型启动子在稳定转染的L细胞和转基因小鼠中驱动天然MHC I类基因的表达。因此，该启动子元素像第一个一样，能够在体外和体内支持本构和激活的转录。两个不同启动子在调节MHC I类转录中的作用是一个活跃的研究领域。最后，已经通过体内分析确定了用于建立组织特定调节模式的新型调节元素，并将其定位于内含子区域。