PPARgamma as an architectural regulator of gene expression in endocrine signaling

项目概要 核激素受体 (NHR) 的经典观点是它们是配体激活的转录 作为 DNA 结合、脂质控制的基因组开关的因子。一组 NHR 作为异二聚体 与专性异二聚体伴侣视黄酸 X 受体 (RXR)。 RXR 的主要合作伙伴之一是 过氧化物酶体增殖物激活受体 γ (PPAR)。 PPAR 被视为一种修饰脂肪酸， 前列腺素调节转录因子对于脂肪细胞分化至关重要并有助于脂质代谢 在包括巨噬细胞在内的许多细胞类型中诱导转录调节。使用传统的以配体为中心的 通过这些方法，我们和其他人能够识别 PPAR- 靶基因和配体调节过程，包括 胆固醇的摄取和流出，抑制炎症反应，调节胰岛素敏感性并促进 巨噬细胞的组织修复。这些发现使该受体成为代谢的关键内分泌调节剂 以及这种细胞类型的炎症。先前研究的一个主要弱点是它仅关注配体 受体的依赖性活性，并且没有考虑配体独立的活性。因此，有一个 我们对受体生物学和作用的理解存在巨大差距。最近的全基因组测定 结合位点在脂肪细胞中鉴定出至少两个数量级以上的基因组结合位点 巨噬细胞作为调节基因是必要的。这促使我们探索 PPAR:RXR 顺反子 交替极化的巨噬细胞。我们收集到的初步证据表明 (1) 巨噬细胞 PPARγ：RXR 顺反子在 STAT6 指导的 IL-4 极化后大大延伸； (2) 在交替极化的巨噬细胞中，染色质结合的 PPARγ 主要对配体不敏感； (3) PPARγ:RXR 异二聚体是基因组的结构元件，并且 (4) 异二聚体赋予 通过保留的基因增强子循环的转录记忆。这使我们能够提出新的假设 如下：大部分 PPAR:RXR 异二聚体对配体不敏感； (a) 对表观基因组的贡献 IL-4 极化巨噬细胞，并利用环化提供增强子和启动子之间的连接 粘连蛋白复合物和/或CTCF； (b) 建立转录细胞记忆和 (C) 有助于 巨噬细胞组织特异性和转录适应。我们建议在三个层面上进行测试。 在染色质和基因组折叠水平上，我们将确定 PPAR 的表观基因组作用。在蜂窝 我们将确定 PPAR 在染色质开放和转录记忆中发挥的作用。终于在 在体内水平上，将探讨PPAR-对肺巨噬细胞功能和对病毒感染的反应的贡献。 这些结果可能会为巨噬细胞表观基因组编程的研究开辟新途径 慢性炎症和代谢疾病以及组织再生的转化潜力。

PROJECT SUMMARY The canonical view of Nuclear Hormone Receptors (NHRs) is that these are ligand-activated transcription factors acting as DNA-bound, lipid-controlled genomic switches. A group of NHRs are acting as heterodimers with the obligate heterodimeric partner Retinoid X Receptor (RXR). One of the key partners of RXR is the Peroxisome Proliferator-Activated Receptor gamma (PPAR). PPAR is viewed as a modified fatty acid and prostanoid regulated transcription factor essential for fat cell differentiation and contributing to lipid metabolite induced transcriptional regulation in many cell types including macrophages. Using traditional ligand centric approaches, we and others were able to identify PPAR target genes and ligand regulated processes including cholesterol uptake and efflux, inhibition of inflammatory responses, regulating insulin sensitivity and promoting tissue repair in macrophages cells. These findings make this receptor a key endocrine regulator of metabolism and inflammation in this cell type. A major weakness of prior research was that it focused solely on ligand- dependent activities of the receptors and did not consider ligand-independent ones. Therefore, there is a substantial gap in our understanding of receptor biology and action. Recent genome-wide determinations of binding sites identified at least two orders of magnitude more genomic binding sites in fat cells and macrophages as regulated genes would necessitate. This prompted us to explore PPAR:RXR cistromes in alternatively polarized macrophages. We have gathered preliminary evidence to suggest that (1) The macrophage PPARγ:RXR cistrome is greatly extended upon IL-4 polarization directed by STAT6; (2) Chromatin-bound PPARγ is predominantly ligand-insensitive in alternatively polarized macrophages; (3) PPARγ:RXR heterodimers are architectural elements of the genome, and (4) the heterodimer confers transcriptional memory via retained gene-enhancer looping. This allows us to formulate new hypotheses as follows: A large fraction of PPAR:RXR heterodimers is ligand-insensitive; (a) Contributing to the epigenome of IL-4 polarized macrophages and provides links between enhancers and promoters via looping utilizing the cohesin complex and/or CTCF; (b) Establishes transcriptional cellular memory and (C) Contributes to macrophage tissue-specification and transcriptional adaptation. We are proposing to test these at three levels. At the chromatin and genome folding level we will determine the epigenomic roles for PPAR. At the cellular level we will determine the role PPAR plays in opening of chromatin and transcriptional memory. Finally at the in vivo level, contribution of PPAR to lung macrophage function and response to viral infection will be explored. The results will likely open up new avenues of research into the epigenomic programming of macrophages with translational potential in chronic inflammatory and metabolic disease and tissue regeneration.