Vollum Institute and Division of Hematology and Medical Oncology, Oregon Health & Science University, Portland, OR 97239, USA.
Proc Natl Acad Sci U S A. 2010 Jan 5;107(1):145-50. doi: 10.1073/pnas.0912294107. Epub 2009 Dec 4.
Aberrant MYC gene expression by the Wnt/beta-catenin pathway is implicated in colorectal carcinogenesis. Wnt/beta-catenin signaling stimulates association of the beta-catenin coactivator complex with two Wnt responsive enhancers (WREs) located in close proximity to MYC gene boundaries. Each enhancer directly binds members of the TCF/Lef family of transcription factors that, in turn, recruit beta-catenin. In a previous report, we showed that the downstream MYC enhancer (MYC 3' WRE) cooperated with the upstream enhancer (MYC 5' WRE) to activate expression of a heterologous reporter gene in response to Wnt/beta-catenin and mitogen signaling. Here we use chromatin conformation capture (3C) to show that the MYC 5' and 3' WREs are juxtaposed at the genomic MYC locus during active transcription. This MYC 5'3' chromatin loop is present in HCT116 human colorectal cancer cells that contain high levels of nuclear beta-catenin and is absent in HEK293 cells that contain trace amounts of nuclear beta-catenin. Depletion of functional beta-catenin/TCF complexes blocks formation of the MYC 5'3 chromatin loop. Furthermore, we find that the chromatin loop is absent in quiescent cells, but is rapidly and transiently induced by serum mitogens in a beta-catenin-dependent manner. Thus, we propose that a distinct chromatin architecture coordinated by beta-catenin/TCF-bound WREs accompanies transcriptional activation of MYC gene expression.
Wnt/β-catenin 信号通路中异常的 MYC 基因表达与结直肠癌的发生有关。Wnt/β-catenin 信号刺激β-catenin 共激活复合物与位于 MYC 基因边界附近的两个 Wnt 反应增强子（WRE）结合。每个增强子直接结合 TCF/Lef 转录因子家族的成员，而这些成员反过来又招募β-catenin。在之前的报告中，我们表明下游 MYC 增强子（MYC 3' WRE）与上游增强子（MYC 5' WRE）合作，以响应 Wnt/β-catenin 和有丝分裂信号激活异源报告基因的表达。在这里，我们使用染色质构象捕获（3C）显示，在活跃转录期间，MYC 5' 和 3' WRE 位于基因组 MYC 基因座上并置。这种 MYC 5'3'染色质环存在于含有高水平核β-catenin 的 HCT116 人结直肠癌细胞中，并且不存在于仅含有痕量核β-catenin 的 HEK293 细胞中。功能性β-catenin/TCF 复合物的缺失会阻止 MYC 5'3 染色质环的形成。此外，我们发现静止细胞中不存在染色质环，但在血清有丝分裂原的作用下，以β-catenin 依赖性方式迅速且短暂地诱导。因此，我们提出，由β-catenin/TCF 结合的 WRE 协调的独特染色质结构伴随着 MYC 基因表达的转录激活。
