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 表达失调，并且在某些肿瘤实体中普遍存在。大量组织培养和转基因实验证明，Myc 表达失调会引发转化细胞的多种特征，并导致肿瘤发生。令人惊讶的是，原代细胞中 Myc 的异位表达会诱导细胞凋亡，这被认为是保护生物体免受肿瘤发生的主要故障安全机制。目前的概念表明，只有当二次突变阻止 Myc 诱导的细胞凋亡时，肿瘤才会出现。尽管进行了许多研究，但允许细胞区分 Myc 生理水平和超生理水平的分子机制仍未解决。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，并将 Myc 蛋白视为通用的转录“放大器”。这与表明 Myc 激活和抑制特定基因的实验形成对比。我们之前已经证明 Myc 与 Zn 指蛋白 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。我们现在已经进行了广泛的 ChIp 测序，发现 Miz1 与大多数 Myc 靶启动子共同结合；因此，Myc 的靶启动子结合激活复合物和抑制复合物的混合物。我们已经确定了共有的 Miz1 结合序列，并表明 Myc 和 Miz1 相互招募到它们的同源靶位点。因此，启动子之间的拓扑结构、Myc 和 Miz1 结合的相对比例以及对 Myc 的响应都不同。由此产生的模型将 Myc 视为转录的特定调节因子，而不是一般的转录放大器。具有共有 Miz1 结合基序位点的基因不是 Myc 抑制的目标。被抑制基因的启动子的特点是缺乏共有的 E-box 和 Miz1 结合基序，并且结合相对少量的两种蛋白，表明它们的占据随着 Myc 水平的变化而变化。在一项平行工作中，我们发现上皮细胞中 Myc 诱导细胞凋亡依赖于 Myc 与 Miz1 的关联，并鉴定了一组以 Miz1 依赖性方式受 Myc 调节的基因。这些基因是上皮细胞完整性所必需的，Myc 特异性抑制上皮细胞的生长，但不抑制干细胞的生长。引人注目的是，这些基因根据 Myc 的超生理水平而受到选择性调节。总的来说，数据表明 Myc/Miz1 复合物对不同靶启动子的相对亲和力使细胞能够区分 Myc 的生理水平和超生理水平。在这里，我们建议检验这一假设并研究潜在的生化机制。我们相信这项研究将为 Myc 诱导的细胞凋亡的生化和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