Myc-dependent apoptosis as a tumor-suppressive mechanism: how do cells discriminate between physiological and oncogenic levels of Myc expression?

Myc 依赖性细胞凋亡作为肿瘤抑制机制：细胞如何区分 Myc 表达的生理水平和致癌水平？

• 批准号: 244461114
• 负责人: Professor Dr. Martin Eilers
• 金额: --
• 依托单位: Lehrstuhl für Biochemie und Molekularbiologie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2013
• 资助国家: 德国
• 起止时间: 2012-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/244461114?language=en
• 关键词: Myc; dependent; apoptosis; tumor; suppressive

Deregulated expression of Myc is observed in the majority of all human tumors and is universal in some tumor entities. A plethora of tissue culture and transgenic experiments documents that deregulated expression of Myc elicits multiple characteristics of transformed cells and contributes to tumorigenesis. Surprisingly, ectopic expression of Myc in primary cells induces apoptosis and this is thought to be a major failsafe mechanism that protects organisms from tumorigenesis. Current concepts suggest that tumors can only arise when secondary mutations block Myc-induced apoptosis. Despite many studies, the molecular mechanisms that allow cells to discriminate physiological from supra-physiological levels of Myc remain unresolved.Myc proteins are transcription factors that bind to the majority of open promoters in the genome. Some models assume that all binding sites are functionally equivalent and see Myc proteins as universal "amplifiers" of transcription. This contrasts with experiments showing that Myc activates and represses specific genes.We have shown previously that Myc forms a complex with the Zn-finger protein Miz1 and inhibits Miz1-dependent transcription. This is critical for Myc-induced tumorigenesis. We have now performed extensive ChIp-sequencing and found that Miz1 co-binds the majority of Myc target promoters; hence, target promoters of Myc bind a mixture of activating and repressive complexes. We have identified the consensus Miz1 binding sequence and shown that Myc and Miz1 recruit each other to their cognate target sites. As a result, the topology, relative proportion of Myc and Miz1 bound, and the response to Myc differs among promoters. The resulting model sees Myc as a specific regulator of transcription rather than a general transcriptional amplifier.Genes with consensus Miz1 binding motifs sites are not targets for repression by Myc. Promoters of repressed genes are characterized by the absence of consensus E-boxes and Miz1 binding motifs and by binding relatively low amounts of both proteins, suggesting that their occupancy changes with varying Myc levels. In a parallel work, we have found that induction of apoptosis by Myc in epithelial cells depends on association of Myc with Miz1 and identified a group of genes that are regulated by Myc in a Miz1-dependent manner. These genes are required for epithelial cell integrity and Myc specifically suppresses growth of epithelial, but not of stem cells. Strikingly, these genes are selectively regulated in response to supra-physiological levels of Myc. Collectively, the data suggest that the relative affinities of Myc/Miz1 complexes to different target promoters allow cells to discriminate physiological from supra-physiological levels of Myc. Here we propose to test this hypothesis and study the underlying biochemical mechanisms. We believe that this study will open the way for a biochemical and, ultimately, quantitative understanding of Myc-induced apoptosis.

在大多数人类肿瘤中观察到MYC的放松调节表达，并且在某些肿瘤实体中是普遍的。大量的组织培养和转基因实验证明，放大MYC表达的调节，引起了转化细胞的多种特征，并导致肿瘤发生。出乎意料的是，MYC在原代细胞中的异位表达诱导凋亡，这被认为是保护生物免受肿瘤发生的主要故障安全机制。当前的概念表明，只有当次要突变阻断MYC诱导的细胞凋亡时才能产生肿瘤。尽管进行了许多研究，但允许细胞将生理学与MYC上生理学水平区分开的分子机制尚未解决。MyC蛋白是与基因组中大多数开放启动子结合的转录因子。一些模型假设所有结合位点在功能上都是等效的，并且将MYC蛋白视为转录的通用“放大器”。这与表明MYC激活并抑制特定基因的实验形成对比。我们先前表明MYC与Zn Finger蛋白Miz1形成复合物，并抑制MIZ1依赖性转录。这对于MYC诱导的肿瘤发生至关重要。现在，我们已经进行了广泛的芯片测序，发现MIZ1共同结合了大多数MYC目标启动子。因此，MYC的靶启动子结合了激活和抑制性复合物的混合物。我们已经确定了共有的MIZ1结合序列，并表明MYC和MIZ1相互募集到其同源目标位点。结果，MYC和MIZ1结合的拓扑，相对比例以及对MYC的反应在启动子之间有所不同。所得模型将MYC视为转录的特定调节剂，而不是一般的转录放大器。具有共识MIZ1结合基序位点位点的基因不是MYC抑制的目标。被抑制基因的启动子的特征是没有共识E盒和Miz1结合基序以及两种蛋白质相对较低的结合，表明它们的占用率随着MYC水平的变化而变化。在平行的工作中，我们发现MYC在上皮细胞中诱导凋亡取决于MYC与MIZ1的关联，并确定了一组由MYC以MIZ1依赖性方式调节的基因。这些基因是上皮细胞完整性所必需的，MYC专门抑制上皮的生长，但不抑制干细胞的生长。令人惊讶的是，这些基因根据MYC的上生物学水平有选择地调节。总体而言，数据表明，MYC/MIZ1复合物与不同靶启动子的相对亲和力使细胞可以区分生理学与MYC的上生物学水平。在这里，我们建议检验这一假设并研究潜在的生化机制。我们认为，这项研究将为MYC诱导的细胞凋亡的生化，最终定量理解开辟道路。

项目成果

Repression of SRF target genes is critical for Myc‐dependent apoptosis of epithelial cells

• DOI: 10.15252/embj.201490467
• 发表时间: 2015-06
• 期刊: The EMBO Journal
• 作者: Katrin E. Wiese;Heidi M. Haikala;B. Eyss;E. Wolf;C. Esnault;A. Rosenwald;R. Treisman;J. Klefström;M. Eilers