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.

Myc的非调控表达在大多数人类肿瘤中都能观察到，并且在某些肿瘤实体中普遍存在。大量的组织培养和转基因实验证明，Myc的表达失控会引起转化细胞的多种特征，并有助于肿瘤的发生。令人惊讶的是，在原代细胞中异位表达Myc会诱导细胞凋亡，这被认为是保护生物免受肿瘤发生的主要失败保护机制。目前的概念表明，只有当次级突变阻止Myc诱导的细胞凋亡时，才会发生肿瘤。尽管有许多研究，但允许细胞区分生理和超生理水平的Myc的分子机制仍未解决。Myc蛋白是一种转录因子，与基因组中大多数开放启动子结合。一些模型假设所有的结合位点在功能上是等价的，并将Myc蛋白视为转录的通用“放大器”。这与实验表明Myc激活和抑制特定基因形成对比。我们之前已经证明，Myc与锌指蛋白Miz1形成复合体，并抑制Miz1依赖的转录。这对Myc诱导的肿瘤发生至关重要。我们现在已经进行了广泛的芯片测序，发现Miz1共结合了大多数Myc靶启动子；因此，Myc靶启动子结合了激活和抑制复合体。我们已经鉴定了共识的Miz1结合序列，并表明Myc和Miz1相互招募到它们的同源靶点。因此，不同启动子的拓扑结构、Myc和Miz1结合的相对比例以及对Myc的反应都不同。由此产生的模型将Myc视为特定的转录调节因子，而不是一般的转录放大器。具有共识的Miz1结合基序的基因不是Myc抑制的目标。被抑制基因的启动子的特征是缺乏共同的E-box和Miz1结合基序，并且结合相对较少的这两种蛋白，这表明它们的占有率随着Myc水平的不同而变化。在一项平行的工作中，我们发现Myc诱导上皮细胞凋亡依赖于Myc与Miz1的关联，并确定了一组受Myc以Miz1依赖的方式调节的基因。这些基因是上皮细胞完整性所必需的，Myc特异性地抑制上皮细胞的生长，但不抑制干细胞的生长。值得注意的是，这些基因是根据Myc的超生理水平进行选择性调控的。总之，这些数据表明，Myc/Miz1复合体对不同靶标启动子的相对亲和力允许细胞区分生理和超生理水平的Myc。在这里，我们建议检验这一假说，并研究其潜在的生化机制。我们相信，这项研究将为对Myc诱导的细胞凋亡进行生化和最终的定量理解开辟道路。

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