世界生命科學前沿動態(tài)周報（五十三）

（8.15-8.21/2011）
美寶國際集團:陶國新  

    主要內(nèi)容：揭示基因組不穩(wěn)定性的新機制；腫瘤細胞群的平衡可由其任意亞群細胞恢復(fù)；線粒體中的血管緊張素系統(tǒng)；Fh突變導致的新的能量代謝方式；應(yīng)用藥物重新定位開發(fā)新藥；干細胞和癌細胞調(diào)控的新認識。

    焦點動態(tài)：腫瘤細胞群的平衡可由其任意亞群細胞恢復(fù)。

1. 揭示基因組不穩(wěn)定性的新機制

【動態(tài)】

由于復(fù)制速度遠快于轉(zhuǎn)錄速度，復(fù)制體和RNA聚合酶之間不可避免地會頻繁出現(xiàn)同向碰撞。美國科學家在大腸桿菌中發(fā)現(xiàn)此類碰撞的后果依賴于轉(zhuǎn)錄延長復(fù)合物（ECs）的生產(chǎn)狀態(tài)。與回溯的ECs的同向碰撞導致DNA雙鏈斷裂，而正面相撞不會。他們提出了一個機械模型予以解釋。進一步研究顯示細菌利用多種策略來避免復(fù)制體與回溯的RNA聚合酶發(fā)生碰撞，最常見的策略就是通過翻譯防止RNA聚合酶回溯。如果取消翻譯，DNA雙鏈斷裂被延長因子（抑制回溯或重新激活回溯的ECs）所抑制。最終，終止因子也通過去除被抑制的ECs對基因組穩(wěn)定性作出貢獻。其研究結(jié)果確立了RNA聚合酶的回溯是染色體完整性的內(nèi)在危險因素，提示了活性核糖體和其他的抗回溯機制是基因組穩(wěn)定性的維持因素。

【點評】
該研究發(fā)現(xiàn)了大腸桿菌通常產(chǎn)生染色體斷裂的細胞機制，進一步探明了基因組不穩(wěn)定性的原因。

【參考論文】
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. 腫瘤細胞群的平衡可由其任意亞群細胞恢復(fù)

【動態(tài)】

在單個的腫瘤中癌細胞經(jīng)常存在具有不同功能屬性的不同表型。癌細胞群通常在不同狀態(tài)的細胞構(gòu)成上表現(xiàn)出獨特的平衡，其發(fā)生機制還很不明了。美國科學家研究了人乳腺癌細胞系中表型比例的動態(tài)變化。發(fā)現(xiàn)純化出來的特定表型的細胞亞群隨時間推移又回到原來平衡態(tài)。這些觀察結(jié)果可以用Markov模型解釋，即細胞在不同狀態(tài)間隨機轉(zhuǎn)變。這一模型預(yù)測一，確定特定條件，隨時間推移任何亞群細胞都會回到平衡態(tài)表型比例。預(yù)測二，乳腺癌干細胞樣細胞從非干細胞樣細胞全新產(chǎn)生。這些發(fā)現(xiàn)有益于我們理解腫瘤異質(zhì)性并揭示單個細胞行為的隨機性促進了癌細胞群的表型平衡。

【點評】
同一腫瘤中癌細胞并非都是一樣的，更像一個復(fù)雜社會，由具有不同功能的不同類型癌細胞形成獨特的平衡。而腫瘤干細胞也可能是有由其他類型癌細胞轉(zhuǎn)變來的。
【參考論文】
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. 線粒體中的血管緊張素系統(tǒng)
【動態(tài)】
腎素-血管緊張素（Ang）系統(tǒng)通過Ang II 1型和2型受體調(diào)節(jié)多種生理功能。以往的研究提示在激活表面膜Ang受體時細胞內(nèi)的Ang II池可能以自分泌模式釋放。作為選擇，提出了一種細胞內(nèi)的腎素-血管緊張素系統(tǒng)，主要焦點在核Ang受體。美國科學家研究了線粒體的血管緊張素系統(tǒng)，發(fā)現(xiàn)功能性的Ang II 2型受體出現(xiàn)在線粒體內(nèi)膜上與內(nèi)源性的Ang定位相同。他們證明了線粒體的血管緊張素系統(tǒng)的激活是藕連于線粒體一氧化氮的產(chǎn)生并能調(diào)節(jié)呼吸作用。此外，他們還提出了線粒體血管緊張素受體表達中年齡相關(guān)的變化的證據(jù)，即長期進行Ang II 1型受體抑制劑氯沙坦治療可以逆轉(zhuǎn)升高的線粒體Ang II 1型受體和降低的2型受體密度。在人線粒體中存在功能性的血管緊張素系統(tǒng)為理解線粒體和慢性疾病狀態(tài)之間相互作用提供了基礎(chǔ)，并揭示了用于優(yōu)化線粒體功能和減少衰老帶來的慢性病負擔的潛在治療靶點。

【點評】
新發(fā)現(xiàn)線粒體中存在血管緊張素系統(tǒng)，并隨年齡而衰減但能用藥物逆轉(zhuǎn)。
【參考論文】
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突變導致的新的能量代謝方式
【動態(tài)】
延胡索酸水合酶（FH）是三羧酸循環(huán)中催化延胡索酸水解為蘋果酸的酶。FH的遺傳突變導致遺傳性平滑肌瘤病和腎細胞癌。已經(jīng)證明FH缺乏會導致延胡索酸積累，在正常氧含量時激活缺氧誘導因子。然而，迄今沒有機制能解釋細胞在沒有功能性的三羧酸循環(huán)時存活的能力。一個由多國科學家共同完成的最新研究用去除Fh1的基因改造老鼠腎細胞結(jié)合新開發(fā)的這些細胞代謝的計算模型來預(yù)測和實驗驗證一條始于谷氨酸鹽攝取止于膽紅素分泌的線性代謝途徑。這一牽涉血紅素生物合成和降解的途徑，使得Fh1缺陷細胞能夠利用累積的三羧酸循環(huán)代謝產(chǎn)物并允許線粒體生產(chǎn)部分NADH。他們預(yù)測并確認了以此途徑為目標能使Fh1缺陷細胞無法存活而不影響野生型的含有Fh1的細胞。

【點評】
這一研究說明關(guān)閉某些發(fā)生特殊突變改變了能量代謝途徑的腎癌細胞的能量工廠可以殺死這些癌細胞而不影響正常細胞。
【參考論文】
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. 應(yīng)用藥物重新定位開發(fā)新藥
【動態(tài)】
藥物重新定位，即將已有藥物分子應(yīng)用到新的適應(yīng)癥，比傳統(tǒng)的藥物開發(fā)有數(shù)種優(yōu)勢，包括更少的開發(fā)費用和更短的審批歷程。最新的藥物重新定位技術(shù)使用高通量實驗方法評價化合物的潛在治療能力。美國科學家整合了100種疾病的基因表達測量值和164種藥物分子的基因表達測量值，形成對這些藥物的預(yù)測治療作用。他們實驗驗證了一個預(yù)測，即抗?jié)兯幬鬟涮娑∧軌蛑委煼蜗侔⒂卯惙N移植老鼠模型證明了它的體內(nèi)體外活性。

【點評】
通過最先進的計算模型和生物學信息數(shù)據(jù)分析來充分挖掘現(xiàn)有藥物分子的治療作用的潛力不失是一種更有效更經(jīng)濟的新藥開發(fā)途徑。
【參考論文】
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. 干細胞和癌細胞調(diào)控的新認識
【動態(tài)】
細胞自我更新或分化的決定源于多種調(diào)節(jié)網(wǎng)絡(luò)的整合和相互調(diào)和。Notch 和Wnt/β-catenin 信號經(jīng)常交匯于干細胞和祖細胞并相互轉(zhuǎn)錄調(diào)節(jié)。當細胞經(jīng)歷分化的各階段時，每種途徑的生物學效應(yīng)常常依賴于環(huán)境和時機。美國科學家最近發(fā)現(xiàn)在干細胞和結(jié)腸癌細胞中膜結(jié)合的Notch常常與未磷酸化的（活性的）β-catenin有物理性的聯(lián)系并負向調(diào)節(jié)活性β-catenin蛋白在翻譯后的堆積。Notch依賴性的β-catenin蛋白的調(diào)節(jié)不需要配體參與的Notch與膜解離或者糖原合成酶激酶3β依賴的β-catenin消除復(fù)合體的活性，但它確實需要內(nèi)吞噬轉(zhuǎn)接蛋白Numb和溶酶體活性。

【點評】
對于干細胞和癌細胞更深入的了解，可能最終會導致調(diào)節(jié)細胞生長環(huán)境來控制細胞的命運。 

【參考論文】
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.