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

在大多数人类肿瘤中观察到Myc的不调节表达，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的不受调控的表达引发了转化细胞的多种特征，并有助于肿瘤的发生。令人惊讶的是，原代细胞中Myc的异位表达可诱导细胞凋亡，这被认为是保护生物体免受肿瘤发生的主要失效保护机制。目前的概念表明，只有继发性突变阻断myc诱导的细胞凋亡时，肿瘤才会出现。尽管进行了许多研究，但允许细胞区分生理和超生理水平的Myc的分子机制仍未得到解决。Myc蛋白是与基因组中大多数开放启动子结合的转录因子。一些模型假设所有的结合位点在功能上是相同的，并将Myc蛋白视为转录的通用“放大器”。这与显示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与锌指蛋白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的目标启动子结合了激活和抑制复合物的混合物。我们已经确定了一致的Miz1结合序列，并表明Myc和Miz1相互招募到它们的同源目标位点。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视为转录的特定调节因子，而不是一般的转录放大器。具有一致的Miz1结合基序位点的基因不是Myc抑制的靶标。被抑制基因的启动子的特点是缺乏一致的e -box和Miz1结合基序，并且结合这两种蛋白质的量相对较低，这表明它们的占用随着Myc水平的不同而变化。在另一项平行研究中，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，这些基因被选择性地调节以响应超生理水平的Myc。总的来说，这些数据表明Myc/Miz1复合物与不同目标启动子的相对亲和力允许细胞区分生理水平和超生理水平的Myc。在这里，我们建议验证这一假设，并研究潜在的生化机制。我们相信这项研究将为生化和最终定量理解myc诱导的细胞凋亡开辟道路。

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.