MOLECULAR ANALYSIS OF TRANSCRIPTIONAL REPRESSION BY TGIF

由转化生长因子β家族信号激活的细胞反应是许多发育和增殖的基础。 事件，包括哺乳动物细胞的中胚层诱导、背化和抗增殖反应。 TGIF是一种转录抑制因子，它招募了包括mSin3和msin3在内的一系列通用辅阻遏子。 CTBP。TGIF与转化生长因子β激活的Smads相互作用并响应转化生长因子β信号抑制 转化生长因子β激活的基因表达。此外，TGIF还招募了泛素连接酶(TiuH)， 导致Smad2泛素化和降解，Smad2是转化生长因子β信号转导的关键介质。一起， TGIF的这些功能设定了细胞对转化生长因子β的最大转录反应。TGIF还 通过依赖于视黄酸受体(RXR)的维甲酸反应元件抑制基因表达。 然而，这种抑制的机制和受影响的基因范围仍有待确定。 人类TGIF基因突变导致全前脑畸形，这是头面部发育的严重缺陷， 其中的主要缺陷是腹侧前脑发育失败。转化生长因子β和维甲酸 已知信号调节前脑发育，这两条途径都与HPE有关。它不是 已知TGIF突变是否通过干扰转化生长因子β信号或维甲酸信号导致HPE。我们将测试 假设TGIF是抑制RXRα依赖基因的RXRα特异性辅抑制子 在没有配基的情况下表达。RXRα是几个核受体的伙伴，除了 维甲酸受体，这样一种RXR功能的特异性抑制剂就会调节许多核受体 小路。我们将确定哪些RXRα依赖的核受体反应被抑制 TGIF以及辅阻遏子招募和核受体泛素化在这种抑制中的作用。我们会 确定TGIF和TIUL是否优先通过特定的靶向RXR-PPAR伽马复合体 Tiull与PPAR伽马的相互作用。最后，我们将测试TGIF是否通过 阻断转化生长因子β介导的生长抑制，或通过其他非转化生长因子β途径。在许多细胞类型中， 包括上皮细胞和淋巴样细胞在内，转化生长因子β信号阻止了细胞周期，并导致突变 转化生长因子β反应的缺失与人类癌症的发生有关。TGIF在食道肿瘤中扩增， 对转化生长因子β介导的生长抑制具有更强的抵抗力，这表明TGIF在肿瘤发生中发挥了作用。

The cellular responses activated by TGF beta family signaling underlie many developmental and proliferative events, including mesoderm induction, dorsalization and antiproliferative responses in mammalian cells. TGIF is a transcriptional represser which recruits a complex of general corepressors, including mSin3 and CtBP. TGIF interacts with TGF beta activated Smads and in response to TGF beta signaling represses expression of genes which are activated by TGF beta. Additionally, TGIF recruits a ubiquitin ligase (TiuH), resulting in ubiquitination and degradation of Smad2,the critical mediator of TGF beta signaling. Together, these functions of TGIF act to set the maximal transcriptional response of a cell to TGF beta. TGIF also inhibits gene expression via a specific retinoid X receptor (RXR)dependent retinoic acid response element. However, the mechanism of this repression and the range of genes affected remain to be determined. Mutations in the human TGIF gene result in holoprosencephaly, a severe defect of craniofacial development, in which the primary defect is a failure of ventral forebrain development. Both TGF beta and retinoic acid signaling are known to regulate forebrain development, and both pathways are implicated in HPE. It is not known whether TGIF mutations cause HPE by disrupting TGF beta signals or retinoid signaling. We will test the hypothesis that TGIF is an RXR alpha specific corepressor that inhibits RXR alpha dependent gene expression in the absence of ligand. RXR alpha is a partner for several nuclear receptors in addition to retinoic acid receptors, such that a specific inhibitor of RXRfunction will regulate many nuclear receptor pathways. We will determine which RXR alpha dependent nuclear receptor responses are repressed by TGIF and the role of corepressor recruitment and nuclear receptor ubiquitination in this repression. We will determine whether TGIF and Tiull preferentially target RXR-PPARgamma complexes via a specific interaction of Tiull with PPAR gamma. Finally, we will test whether TGIF regulates cell cycle progression by blocking TGF beta mediated growth inhibition, or by other TGF beta independent means. In many cell types, including epithelial and lymphoid cells, TGF beta signaling arrests the cell cycle, and mutations which result in loss of TGF beta responses contribute to human cancer. TGIF is amplified in esophageal tumors, which are more resistant to TGF beta mediated growth inhibition, suggesting a role for TGIF in tumorigenesis.