世界生命科学前沿动态周报（五十三）

（8.15-8.21/2011）
美宝国际集团:陶国新  

    主要内容：揭示基因组不稳定性的新机制；肿瘤细胞群的平衡可由其任意亚群细胞恢复；线粒体中的血管紧张素系统；Fh突变导致的新的能量代谢方式；应用药物重新定位开发新药；干细胞和癌细胞调控的新认识。

    焦点动态：肿瘤细胞群的平衡可由其任意亚群细胞恢复。

1. 揭示基因组不稳定性的新机制

【动态】

由于复制速度远快于转录速度，复制体和RNA聚合酶之间不可避免地会频繁出现同向碰撞。美国科学家在大肠杆菌中发现此类碰撞的后果依赖于转录延长复合物（ECs）的生产状态。与回溯的ECs的同向碰撞导致DNA双链断裂，而正面相撞不会。他们提出了一个机械模型予以解释。进一步研究显示细菌利用多种策略来避免复制体与回溯的RNA聚合酶发生碰撞，最常见的策略就是通过翻译防止RNA聚合酶回溯。如果取消翻译，DNA双链断裂被延长因子（抑制回溯或重新激活回溯的ECs）所抑制。最终，终止因子也通过去除被抑制的ECs对基因组稳定性作出贡献。其研究结果确立了RNA聚合酶的回溯是染色体完整性的内在危险因素，提示了活性核糖体和其他的抗回溯机制是基因组稳定性的维持因素。

【点评】
该研究发现了大肠杆菌通常产生染色体断裂的细胞机制，进一步探明了基因组不稳定性的原因。

【参考论文】
Cell, Volume 146, Issue 4, 533-543, 19 August 2011 DOI:10.1016/j.cell.2011.07.034
Linking RNA Polymerase Backtracking to Genome Instability in E. coli
Dipak Dutta, Konstantin Shatalin, Vitaly Epshtein, et al. 
Frequent codirectional collisions between the replisome and RNA polymerase (RNAP) are inevitable because the rate of replication is much faster than that of transcription. Here we show that, in E. coli, the outcome of such collisions depends on the productive state of transcription elongation complexes (ECs). Codirectional collisions with backtracked (arrested) ECs lead to DNA double-strand breaks (DSBs), whereas head-on collisions do not. A mechanistic model is proposed to explain backtracking-mediated DSBs. We further show that bacteria employ various strategies to avoid replisome collisions with backtracked RNAP, the most general of which is translation that prevents RNAP backtracking. If translation is abrogated, DSBs are suppressed by elongation factors that either prevent backtracking or reactivate backtracked ECs. Finally, termination factors also contribute to genomic stability by removing arrested ECs. Our results establish RNAP backtracking as the intrinsic hazard to chromosomal integrity and implicate active ribosomes and other anti-backtracking mechanisms in genome maintenance.

2. 肿瘤细胞群的平衡可由其任意亚群细胞恢复

【动态】

在单个的肿瘤中癌细胞经常存在具有不同功能属性的不同表型。癌细胞群通常在不同状态的细胞构成上表现出独特的平衡，其发生机制还很不明了。美国科学家研究了人乳腺癌细胞系中表型比例的动态变化。发现纯化出来的特定表型的细胞亚群随时间推移又回到原来平衡态。这些观察结果可以用Markov模型解释，即细胞在不同状态间随机转变。这一模型预测一，确定特定条件，随时间推移任何亚群细胞都会回到平衡态表型比例。预测二，乳腺癌干细胞样细胞从非干细胞样细胞全新产生。这些发现有益于我们理解肿瘤异质性并揭示单个细胞行为的随机性促进了癌细胞群的表型平衡。

【点评】
同一肿瘤中癌细胞并非都是一样的，更像一个复杂社会，由具有不同功能的不同类型癌细胞形成独特的平衡。而肿瘤干细胞也可能是有由其他类型癌细胞转变来的。
【参考论文】
Cell, 2011; 146 (4): 633-644 DOI: 10.1016/j.cell.2011.07.026
Stochastic State Transitions Give Rise to Phenotypic Equilibrium in Populations of Cancer Cells
Piyush B. Gupta, Christine M. Fillmore, Guozhi Jiang, et al. 
Cancer cells within individual tumors often exist in distinct phenotypic states that differ in functional attributes. While cancer cell populations typically display distinctive equilibria in the proportion of cells in various states, the mechanisms by which this occurs are poorly understood. Here, we study the dynamics of phenotypic proportions in human breast cancer cell lines. We show that subpopulations of cells purified for a given phenotypic state return towards equilibrium proportions over time. These observations can be explained by a Markov model in which cells transition stochastically between states. A prediction of this model is that, given certain conditions, any subpopulation of cells will return to equilibrium phenotypic proportions over time. A second prediction is that breast cancer stem-like cells arise de novo from non-stem-like cells. These findings contribute to our understanding of cancer heterogeneity and reveal how stochasticity in single-cell behaviors promotes phenotypic equilibrium in populations of cancer cells.

3. 线粒体中的血管紧张素系统
【动态】
肾素-血管紧张素（Ang）系统通过Ang II 1型和2型受体调节多种生理功能。以往的研究提示在激活表面膜Ang受体时细胞内的Ang II池可能以自分泌模式释放。作为选择，提出了一种细胞内的肾素-血管紧张素系统，主要焦点在核Ang受体。美国科学家研究了线粒体的血管紧张素系统，发现功能性的Ang II 2型受体出现在线粒体内膜上与内源性的Ang定位相同。他们证明了线粒体的血管紧张素系统的激活是藕连于线粒体一氧化氮的产生并能调节呼吸作用。此外，他们还提出了线粒体血管紧张素受体表达中年龄相关的变化的证据，即长期进行Ang II 1型受体抑制剂氯沙坦治疗可以逆转升高的线粒体Ang II 1型受体和降低的2型受体密度。在人线粒体中存在功能性的血管紧张素系统为理解线粒体和慢性疾病状态之间相互作用提供了基础，并揭示了用于优化线粒体功能和减少衰老带来的慢性病负担的潜在治疗靶点。

【点评】
新发现线粒体中存在血管紧张素系统，并随年龄而衰减但能用药物逆转。
【参考论文】
Proceedings of the National Academy of Sciences, 2011; DOI:10.1073/pnas.1101507108
Identification and characterization of a functional mitochondrial angiotensin system
Peter M. Abadir, D. Brian Foster, Michael Crow, et al. 
The renin-angiotensin (Ang) system regulates multiple physiological functions through Ang II type 1 and type 2 receptors. Prior studies suggest an intracellular pool of Ang II that may be released in an autocrine manner upon stretch to activate surface membrane Ang receptors. Alternatively, an intracellular renin-Ang system has been proposed, with a primary focus on nuclear Ang receptors. A mitochondrial Ang system has not been previously described. Here we report that functional Ang II type 2 receptors are present on mitochondrial inner membranes and are colocalized with endogenous Ang. We demonstrate that activation of the mitochondrial Ang system is coupled to mitochondrial nitric oxide production and can modulate respiration. In addition, we present evidence of age-related changes in mitochondrial Ang receptor expression, i.e., increased mitochondrial Ang II type 1 receptor and decreased type 2 receptor density that is reversed by chronic treatment with the Ang II type 1 receptor blocker losartan. The presence of a functional Ang system in human mitochondria provides a foundation for understanding the interaction between mitochondria and chronic disease states and reveals potential therapeutic targets for optimizing mitochondrial function and decreasing chronic disease burden with aging.

4. Fh突变导致的新的能量代谢方式
【动态】
延胡索酸水合酶（FH）是三羧酸循环中催化延胡索酸水解为苹果酸的酶。FH的遗传突变导致遗传性平滑肌瘤病和肾细胞癌。已经证明FH缺乏会导致延胡索酸积累，在正常氧含量时激活缺氧诱导因子。然而，迄今没有机制能解释细胞在没有功能性的三羧酸循环时存活的能力。一个由多国科学家共同完成的最新研究用去除Fh1的基因改造老鼠肾细胞结合新开发的这些细胞代谢的计算模型来预测和实验验证一条始于谷氨酸盐摄取止于胆红素分泌的线性代谢途径。这一牵涉血红素生物合成和降解的途径，使得Fh1缺陷细胞能够利用累积的三羧酸循环代谢产物并允许线粒体生产部分NADH。他们预测并确认了以此途径为目标能使Fh1缺陷细胞无法存活而不影响野生型的含有Fh1的细胞。

【点评】
这一研究说明关闭某些发生特殊突变改变了能量代谢途径的肾癌细胞的能量工厂可以杀死这些癌细胞而不影响正常细胞。
【参考论文】
Nature, 2011; DOI:10.1038/nature10363
Haem oxygenase is synthetically lethal with the tumour suppressor fumarate hydratase
Christian Frezza, Liang Zheng, Ori Folger, et al.  
Fumarate hydratase (FH) is an enzyme of the tricarboxylic acid cycle (TCA cycle) that catalyses the hydration of fumarate into malate. Germline mutations of FH are responsible for hereditary leiomyomatosis and renal-cell cancer (HLRCC). It has previously been demonstrated that the absence of FH leads to the accumulation of fumarate, which activates hypoxia-inducible factors (HIFs) at normal oxygen tensions. However, so far no mechanism that explains the ability of cells to survive without a functional TCA cycle has been provided. Here we use newly characterized genetically modified kidney mouse cells in which Fh1 has been deleted, and apply a newly developed computer model of the metabolism of these cells to predict and experimentally validate a linear metabolic pathway beginning with glutamine uptake and ending with bilirubin excretion from Fh1-deficient cells. This pathway, which involves the biosynthesis and degradation of haem, enables Fh1-deficient cells to use the accumulated TCA cycle metabolites and permits partial mitochondrial NADH production. We predicted and confirmed that targeting this pathway would render Fh1-deficient cells non-viable, while sparing wild-type Fh1-containing cells. This work goes beyond identifying a metabolic pathway that is induced in Fh1-deficient cells to demonstrate that inhibition of haem oxygenation is synthetically lethal when combined with Fh1 deficiency, providing a new potential target for treating HLRCC patients.

5. 应用药物重新定位开发新药
【动态】
药物重新定位，即将已有药物分子应用到新的适应症，比传统的药物开发有数种优势，包括更少的开发费用和更短的审批历程。最新的药物重新定位技术使用高通量实验方法评价化合物的潜在治疗能力。美国科学家整合了100种疾病的基因表达测量值和164种药物分子的基因表达测量值，形成对这些药物的预测治疗作用。他们实验验证了一个预测，即抗溃疡药西咪替丁能够治疗肺腺癌，并用异种移植老鼠模型证明了它的体内体外活性。

【点评】
通过最先进的计算模型和生物学信息数据分析来充分挖掘现有药物分子的治疗作用的潜力不失是一种更有效更经济的新药开发途径。
【参考论文】
Science Translational Medicine, 2011; 3 (96): 96ra77 DOI:10.1126/scitranslmed.3001318
Discovery and Preclinical Validation of Drug Indications Using Compendia of Public Gene Expression Data
Marina Sirota, Joel T. Dudley, Jeewon Kim, et al. 
The application of established drug compounds to new therapeutic indications, known as drug repositioning, offers several advantages over traditional drug development, including reduced development costs and shorter paths to approval. Recent approaches to drug repositioning use high-throughput experimental approaches to assess a compound’s potential therapeutic qualities. Here, we present a systematic computational approach to predict novel therapeutic indications on the basis of comprehensive testing of molecular signatures in drug-disease pairs. We integrated gene expression measurements from 100 diseases and gene expression measurements on 164 drug compounds, yielding predicted therapeutic potentials for these drugs. We recovered many known drug and disease relationships using computationally derived therapeutic potentials and also predict many new indications for these 164 drugs. We experimentally validated a prediction for the antiulcer drug cimetidine as a candidate therapeutic in the treatment of lung adenocarcinoma, and demonstrate its efficacy both in vitro and in vivo using mouse xenograft models. This computational method provides a systematic approach for repositioning established drugs to treat a wide range of human diseases.

6. 干细胞和癌细胞调控的新认识
【动态】
细胞自我更新或分化的决定源于多种调节网络的整合和相互调和。Notch 和Wnt/β-catenin 信号经常交汇于干细胞和祖细胞并相互转录调节。当细胞经历分化的各阶段时，每种途径的生物学效应常常依赖于环境和时机。美国科学家最近发现在干细胞和结肠癌细胞中膜结合的Notch常常与未磷酸化的（活性的）β-catenin有物理性的联系并负向调节活性β-catenin蛋白在翻译后的堆积。Notch依赖性的β-catenin蛋白的调节不需要配体参与的Notch与膜解离或者糖原合成酶激酶3β依赖的β-catenin消除复合体的活性，但它确实需要内吞噬转接蛋白Numb和溶酶体活性。

【点评】
对于干细胞和癌细胞更深入的了解，可能最终会导致调节细胞生长环境来控制细胞的命运。 

【参考论文】
Nature Cell Biology, 2011; DOI: 10.1038/ncb2313 
Notch post-translationally regulates β-catenin protein in stem and progenitor cells
Chulan Kwon, Paul Cheng, Isabelle N. King, et al. 
Cellular decisions of self-renewal or differentiation arise from integration and reciprocal titration of numerous regulatory networks. Notch and Wnt/β-catenin signalling often intersect in stem and progenitor cells and regulate each other transcriptionally. The biological outcome of signalling through each pathway often depends on the context and timing as cells progress through stages of differentiation. Here, we show that membrane-bound Notch physically associates with unphosphorylated (active) β-catenin in stem and colon cancer cells and negatively regulates post-translational accumulation of active β-catenin protein. Notch-dependent regulation of β-catenin protein did not require ligand-dependent membrane cleavage of Notch or the glycogen synthase kinase- 3β-dependent activity of the β-catenin destruction complex. It did, however, require the endocytic adaptor protein Numb and lysosomal activity. This study reveals a previously unrecognized function of Notch in negatively titrating active β-catenin protein levels in stem and progenitor cells.