Schnappauf Oskar, Beyes Sven, Dertmann Annika, Freihen Vivien, Frey Patrick, Jägle Sabine, Rose Katja, Michoel Tom, Grosschedl Rudolf, Hecht Andreas
Institute of Molecular Medicine and Cell Research, Faculty of Medicine, University of Freiburg, Stefan-Meier-Str. 17, D-79104 Freiburg, Germany; Department of Cellular and Molecular Immunology, Max Planck Institute of Immunobiology and Epigenetics, Stübeweg 51, D-79108 Freiburg, Germany; Faculty of Biology, University of Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany.
Institute of Molecular Medicine and Cell Research, Faculty of Medicine, University of Freiburg, Stefan-Meier-Str. 17, D-79104 Freiburg, Germany; Faculty of Biology, University of Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany.
Biochim Biophys Acta. 2016 Nov;1859(11):1353-1367. doi: 10.1016/j.bbagrm.2016.08.002. Epub 2016 Aug 5.
Transcriptional silencing is a major cause for the inactivation of tumor suppressor genes, however, the underlying mechanisms are only poorly understood. The EPHB2 gene encodes a receptor tyrosine kinase that controls epithelial cell migration and allocation in intestinal crypts. Through its ability to restrict cell spreading, EPHB2 functions as a tumor suppressor in colorectal cancer whose expression is frequently lost as tumors progress to the carcinoma stage. Previously we reported that EPHB2 expression depends on a transcriptional enhancer whose activity is diminished in EPHB2 non-expressing cells. Here we investigated the mechanisms that lead to EPHB2 enhancer inactivation. We show that expression of EPHB2 and SNAIL1 - an inducer of epithelial-mesenchymal transition (EMT) - is anti-correlated in colorectal cancer cell lines and tumors. In a cellular model of Snail1-induced EMT, we observe that features of active chromatin at the EPHB2 enhancer are diminished upon expression of murine Snail1. We identify the transcription factors FOXA1, MYB, CDX2 and TCF7L2 as EPHB2 enhancer factors and demonstrate that Snail1 indirectly inactivates the EPHB2 enhancer by downregulation of FOXA1 and MYB. In addition, Snail1 induces the expression of Lymphoid enhancer factor 1 (LEF1) which competitively displaces TCF7L2 from the EPHB2 enhancer. In contrast to TCF7L2, however, LEF1 appears to repress the EPHB2 enhancer. Our findings underscore the importance of transcriptional enhancers for gene regulation under physiological and pathological conditions and show that SNAIL1 employs a combinatorial mechanism to inactivate the EPHB2 enhancer based on activator deprivation and competitive displacement of transcription factors.
转录沉默是肿瘤抑制基因失活的主要原因，然而，其潜在机制仍知之甚少。EPHB2基因编码一种受体酪氨酸激酶，可控制上皮细胞在肠隐窝中的迁移和分布。通过其限制细胞扩散的能力，EPHB2在结直肠癌中发挥肿瘤抑制作用，随着肿瘤进展到癌阶段，其表达常常丢失。此前我们报道EPHB2的表达依赖于一个转录增强子，该增强子在不表达EPHB2的细胞中活性降低。在此，我们研究了导致EPHB2增强子失活的机制。我们发现，在结直肠癌细胞系和肿瘤中，EPHB2和上皮-间质转化（EMT）诱导因子SNAIL1的表达呈负相关。在Snail1诱导的EMT细胞模型中，我们观察到，小鼠Snail1表达后，EPHB2增强子处的活性染色质特征减弱。我们鉴定出转录因子FOXA1、MYB、CDX2和TCF7L2为EPHB2增强子因子，并证明Snail1通过下调FOXA1和MYB间接使EPHB2增强子失活。此外，Snail1诱导淋巴增强因子1（LEF1）的表达，LEF1竞争性地将TCF7L2从EPHB2增强子上取代。然而，与TCF7L2不同，LEF1似乎抑制EPHB2增强子。我们的研究结果强调了转录增强子在生理和病理条件下对基因调控的重要性，并表明SNAIL1采用一种组合机制，基于激活因子剥夺和转录因子的竞争性取代来使EPHB2增强子失活。
