Department of Biochemistry and Molecular Biology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania, United States of America.
PLoS One. 2011 Apr 20;6(4):e18966. doi: 10.1371/journal.pone.0018966.
Inappropriate activation of c-Myc (MYC) gene expression by the Wnt/ß-catenin signaling pathway is required for colorectal carcinogenesis. The elevated MYC levels in colon cancer cells are attributed in part to ß-catenin/TCF4 transcription complexes that are assembled at proximal Wnt/ß-catenin responsive enhancers (WREs). Recent studies suggest that additional WREs that control MYC expression reside far upstream of the MYC transcription start site. Here, I report the characterization of five novel WREs that localize to a region over 400 kb upstream from MYC. These WREs harbor nucleosomes with post-translational histone modifications that demarcate enhancer and gene promoter regions. Using quantitative chromatin conformation capture, I show that the distal WREs are aligned with the MYC promoter through large chromatin loops. The chromatin loops are not restricted to colon cancer cells, but are also found in kidney epithelial and lung fibroblast cell lines that lack de-regulated Wnt signaling and nuclear ß-catenin/TCF4 complexes. While each chromatin loop is detected in quiescent cells, the positioning of three of the five distal enhancers with the MYC promoter is induced by serum mitogens. These findings suggest that the architecture of the MYC promoter is comprised of distal elements that are juxtaposed through large chromatin loops and that ß-catenin/TCF4 complexes utilize this conformation to activate MYC expression in colon cancer cells.
Wnt/β-catenin 信号通路对 c-Myc（MYC）基因表达的异常激活是结直肠癌发生所必需的。结肠癌细胞中 MYC 水平的升高部分归因于β-catenin/TCF4 转录复合物，该复合物在近端 Wnt/β-catenin 反应增强子（WRE）处组装。最近的研究表明，控制 MYC 表达的其他 WRE 位于 MYC 转录起始位点的上游很远的位置。在这里，我报告了五个新的 WRE 的特征，这些 WRE 定位于 MYC 转录起始位点上游超过 400 kb 的区域。这些 WRE 含有具有翻译后组蛋白修饰的核小体，这些修饰标记了增强子和基因启动子区域。使用定量染色质构象捕获，我表明远端 WRE 通过大染色质环与 MYC 启动子对齐。染色质环不仅限于结肠癌细胞，也存在于缺乏失调的 Wnt 信号和核β-catenin/TCF4 复合物的肾上皮细胞和肺成纤维细胞系中。虽然在静止细胞中检测到每个染色质环，但五个远端增强子中的三个与 MYC 启动子的定位是由血清有丝分裂原诱导的。这些发现表明，MYC 启动子的结构由通过大染色质环并列的远端元件组成，β-catenin/TCF4 复合物利用这种构象在结肠癌细胞中激活 MYC 表达。
