真核翻译起始因子5A在蛋白质合成中的功能及机制

在蛋白质合成过程中,除核糖体、氨酰 tRNA和mRNA外,还有多种翻译因子参与其中。真核翻译起始因子5A（eukaryotic translation initiation factor 5A, eIF5A）是维持细胞活性必不可少的翻译因子,在进化上高度保守。eIF5A是真核细胞中唯一含有羟腐胺赖氨酸（hypusine）的蛋白质,该翻译后修饰对eIF5A的活性至关重要。1978年,人们首次鉴定出eIF5A,认为它在翻译起始阶段促进第1个肽键的形成。直到2013年才证实它主要在翻译延伸阶段调控含多聚脯氨酸基序蛋白质的翻译。在经过四十多年研究后,人们对eIF5A的功能有了新的认识。近期基于核糖体图谱数据的分析表明,eIF5A能够缓解翻译延伸过程中核糖体在多种基序处的停滞,并不局限于多聚脯氨酸基序,并且它还能够通过促进肽链的释放增强翻译终止。此外,eIF5A还可以通过调控某些蛋白质的翻译,间接影响细胞内的各种生命活动。本文综述了eIF5A的多种翻译后修饰、在蛋白质合成和细胞自噬过程中的调控作用以及与人类疾病的关系,并与细菌及古细菌中的同源蛋白质进行了比较,探讨了该因子在进化中的保守性,以期为相关领域的研究提供一定的理论基础。     

eIF4A在帽依赖和IRES介导的翻译起始中的作用

生物体内翻译起始机制分为两类：帽依赖性翻译起始和内部核糖体进入位点(internal ribosome entry sites, IRES)介导的翻译起始。真核生物的翻译起始为经典的帽依赖性翻译起始模型,而大多数正链RNA病毒选择依赖于IRES的翻译机制。真核翻译起始因子4A (eukaryotic initiation factor 4A, eIF4A)是DEAD-box RNA解旋酶家族的成员,具有依赖于RNA的ATP酶活性和RNA解旋酶活性,而e IF4A具体的解旋机制至今仍不清晰。同时,eIF4A与其他翻译因子有着复杂而紧密的联系,在帽依赖性与IRES介导的翻译起始过程中扮演着至关重要的角色。本文主要对eIF4A的功能、结构以及eIF4A在帽依赖性与IRES介导的翻译起始过程中的机制作一综述。     

水稻eIF3大亚基(eIF3a)编码基因的克隆及其表达模式分析

真核生物翻译起始因子(eIFs)在蛋白合成中起关键作用,在已经鉴定的13个因子中eIF3(由8个或更多的亚基组成)是分子量最大的一个并在翻译起始过程中起着核心作用。eIF3a是eIF3中最大的亚基并介导了eIF3的大多数的功能的实现。利用荧光-差异显示PCR法对生长素处理后的水稻材料进行分析发现eIF3a的表达可被生长素诱导,进一步通过筛选cDNA文库分离出水稻编码eIF3a的全长cDNA(命名为OseIF3a1),OseIF3a1含3459碱基(含5’和3’非翻译区)并编码一个986氨基酸的蛋白,与基因组序列比较表明OseIF3a1基因存在有12个内含子。OSeIF3a1与玉米和烟草的同源蛋白一致性分别为82.4%和70.1%并与其他真核生物eIF3a蛋白具较高一致性。以组织材料进行的RT-PCR结果表明OseIF3a1在根、幼苗、幼穗、茎和叶中表达,启动子-报告基因的转基因结果进一步表明OseIF3a1在根尖及幼嫩叶片表达较高,进一步的RTPCR分析确证了生长素对OseIF3a1的诱导,表明生长素在调控植物生长时可能涉及了翻译水平上的调节。     

eIF4E抑制性蛋白翻译调控机制

真核生物mRNA的翻译调控,通常发生在起始阶段。异源三聚体复合物eIF4F中的eIF4E与mRNA5'端帽子结构的结合是该阶段的核心,而eIF4E抑制性蛋白正是通过与eIF4E的相互作用而调控着翻译起始过程,进而调控着翻译的速率。eIF4E抑制性蛋白对翻译的这种调控作用对细胞的生长、发育、癌症以及神经生物学方面有巨大影响,现主要就eIF4E抑制性蛋白的翻译调控机制进行综述。     

PDCD4在炎症和肿瘤中的作用和机制

程序性细胞死亡因子4(programmed cell death 4, PDCD4)是在研究细胞凋亡机制中克隆出来的新基因.后来发现PDCD4广泛表达于多种组织和器官,但是在多种肿瘤中表达缺失或低表达,恢复PDCD4表达可明显抑制肿瘤的生长或迁移侵袭,是一种新的抑癌基因.进一步研究发现PDCD4不仅发挥抑癌基因的作用,而且参与炎性疾病的发生和发展,它是一把"双刃剑",在不同的疾病模型中表现出促进或抑制的作用,研究表明PDCD4敲除小鼠(Mus musculus)可以有效抵抗实验性自身免疫性脑脊髓炎、1型糖尿病、肥胖及动脉粥样硬化等慢性炎性疾病的发生,而PDCD4敲除后加重LPS(lipopolysaccharide)诱导的急性肝损伤和DSS(Dextran sulfate sodium salt)诱导的急性结肠炎. PDCD4的作用机制尚不完全清楚,目前认为PDCD4对下游靶基因的调控主要通过两种方式:翻译起始因子eIF4A依赖性和非依赖性的方式. eIF4A依赖性方式是指PDCD4通过与eIF4A直接相互作用抑制其解旋酶活性,进而抑制具有特定5′UTR结构m RNA的翻译; eIF4A非依赖方式是指PDCD4可以通过与其他蛋白质(如转录因子)结合,抑制其活性或干扰其磷酸化进而调控靶基因的转录及其下游一系列信号通路.本文对PDCD4的作用和机制进行综述,并对其成为肿瘤和多种疾病治疗的新靶点进行展望.     

白质消融性白质脑病致病基因EIF2B的突变功能研究

白质消融性白质脑病(leukoencephalopathy with vanishing white matter,VWM)是儿童最常见的遗传性白质脑病之一,是目前人类遗传性疾病中首个被确定由于mRNA翻译启动异常所致疾病,是由编码真核细胞翻译启动因子2B(eukaryotic translation initiation factor 2B,eIF2B)的五个亚单位(eIF2Bα、β、γ、δ、ε)的基因(EIF2B1-5)任一突变所致。eIF2B是一种鸟嘌呤核苷酸交换因子,调控全部mRNA的翻译起始过程。eIF2B突变功能研究尚处于起步阶段。EIF2B突变可能通过不同的途径影响eIF2B的功能。例如,通过影响eIF2B复合体的形成或其与底物的结合从而破坏eIF2B的鸟苷酸转移因子(GEF)活性或引起细胞应激反应异常。EIF2B突变是否影响胶质前体细胞的分化是VWM发病机制的另一个关键问题。     

水稻eIF3大亚基（eIF3a）编码基因的克隆及其表达模式分析

真核生物翻译起始因子（eIFs)在蛋白合成中起关键作用,在已经鉴定 的13个因子中eIF3（由8个或更多的亚基组成）是分子量最大的一个并在翻译起始过程中起着核心作用。eIF3a是eIF3中最大的亚基并介导了eIF3的大多数的功能的实现。利用荧光－差异显示PCR法对生长素处理后的水稻材料进行分析发现eIF3a的表达可被生长素诱导,进一步通过筛选cDNA文库分离出水稻编码eIF3a的全长cDNA（命名为OseIF3a1),OseIF3a1含3459碱基（含5‘和3‘非翻译区）并编码一个986氨基酸的蛋白,与基因组序列比较表明OseIF3a1基因存在有12个内含子。OseIF3a1与玉米和烟草的同源蛋白一致性分别为82．4％和70．1％并与其他真核生物eIF3a蛋白具较高一致性。以组织材料进行的RT－PCR结果表明OseIF3a1在根、幼苗、幼穗、茎和叶中表达,启动子－报告基因的转基因结果进一步表明OseIF3a1在根尖及幼嫩叶片表达较高,进一步的RT－PCR分析确证了生长素对OseIF3a1的诱导,表明生长素在调控植物生长时可能涉及了翻译水平上的调节。     

RNA病毒翻译调控元件—内部核糖体进入位点(IRES)

真核生物大多数蛋白质合成采用了依赖帽子结构的翻译起始方式.但一组缺乏帽子构的RNA病毒的蛋白质合成起始是依赖其5′端非翻译区（untranslated region,UTR）翻译调控的顺式作用元件——内部核糖体进入位点（internal ribosome entry site, IRES）.它 们能够在一些反式作用因子的辅助下,招募核糖体小亚基到病毒mRNA的翻译起始位点.前,依赖IRES元件翻译起始的RNA病毒在哺乳动物,无脊椎动物及植物中均有发现.因此,对RNA病毒IRES元件的深入研究,不仅有助于阐明相关疾病的发生机理,而且为工业应用和疾病治疗提供借鉴意义.本文对RNA病毒IRES元件发现、分类、结构与功能等作了综述.     

真核翻译起始因子2α激酶在肾脏疾病中的作用研究进展

肾脏疾病的防治一直是医学研究的重点。真核翻译起始因子2α激酶（eIF2α）是哺乳动物细胞中代谢应激反应的关键因子,可诱导整体蛋白质翻译抑制,并在不同的细胞代谢应激下控制细胞存活。eIF2α激酶在维持机体的正常生理功能方面及肿瘤、免疫和代谢相关疾病等的发生发展过程中发挥重要作用。研究提示eIF2α激酶可能参与多种肾脏疾病的病理过程,因此,本文对eIF2α激酶家族及其在肾脏疾病中的可能作用等方面的研究进展进行归纳总结,以期为肾脏疾病的防治提供新的参考和理论依据。     

真核翻译起始因子5A和多胺代谢及肿瘤的关系

真核翻译起始因子5A(eIF5A)是真核生物中普遍存在且高度保守的蛋白质,含有一个独特的氨基酸——羟腐胺赖氨酸,该氨基酸是在赖氨酸的基础上经翻译后修饰而成,该过程与多胺代谢密切相关。研究发现,eIF5A和多胺代谢以及肿瘤的发生发展密切相关,有望成为抗肿瘤靶向治疗新的分子靶点。现就eIF5A和多胺代谢及肿瘤关系的研究进展作一简要综述。     

Eukaryotic initiation factor 4E (eIF4E): A recap of the cap-binding protein

Eukaryotic initiation factor 4E (eIF4E), a fundamental effector and rate limiting element of protein synthesis, binds the 7-methylguanosine cap at the 5′ end of eukaryotic messenger RNA (mRNA) specifically as a constituent of eIF4F translation initiation complex thus facilitating the recruitment of mRNA to the ribosomes. This review focusses on the engagement of signals contributing to growth factor originated maxim and their role in the activation of eIF4E to achieve a collective influence on cellular growth, with a key focus on conjuring vital processes like protein synthesis. The review invites considerable interest in elevating the appeal of eIF4E beyond its role in regulating translation viz a viz cancer genesis, attributed to its phosphorylation state that improves the prospect for the growth of the cancerous cell. This review highlights the latest studies that have envisioned to target these pathways and ultimately the translational machinery for therapeutic intervention. The review also brings forward the prospect of eIF4E to act as a converging juncture for signaling pathways like mTOR/PI3K and Mnk/MAPK to promote tumorigenesis.     

The translational regulator eIF3a: The tricky eIF3 subunit!

Regulation of gene expression is a fundamental step in cellular physiology as abnormalities in this process may lead to de-regulated growth and cancer. Translation of mRNA is mainly regulated at the rate-limiting initiation step, where many eukaryotic initiation factors (eIFs) are involved. The largest and most complex initiation factor is eIF3 which plays a role in translational regulation, cell growth and cancer. The largest subunit of eIF3 is eIF3a, although it is not required for the general function of eIF3 in translation initiation. However, eIF3a may play a role as a regulator of a subset of mRNAs and has been demonstrated to regulate the expression of p27^kip1, tyrosinated α-tubulin and ribonucleotide reductase M2 subunit. These molecules have a pivotal role in the regulation of the cell cycle. Moreover, the eIF3a mRNA is ubiquitously expressed in all tissues at different levels and is found elevated in a number of cancer types. eIF3a can modulate the cell cycle and may be a translational regulator for proteins important for entrance into S phase. The expression of eIF3a is decreased in differentiated cells in culture and the suppression of eIF3a expression can reverse the malignant phenotype and change the sensitivity of cells to cell cycle modulators. However, the role of eIF3a in cancer is still unclear. In fact, some studies have identified eIF3a to be involved in cancer development, while other results indicate that it could provide protection against evolution into higher malignancy. Together, these findings highlight the “tricky” and interesting nature of eIF3a.     

The tumor suppressive role of eIF3f and its function in translation inhibition and rRNA degradation

Deregulated translation plays an important role in human cancer. We previously reported decreased eukaryotic initiation factor 3 subunit f (eIF3f) expression in pancreatic cancer. Whether decreased eIF3f expression can transform normal epithelial cells is not known. In our current study, we found evidence that stable knockdown of eIF3f in normal human pancreatic ductal epithelial cells increased cell size, nuclear pleomorphism, cytokinesis defects, cell proliferation, clonogenicity, apoptotic resistance, migration, and formation of 3-dimensional irregular masses. Our findings support the tumor suppressive role of eIF3f in pancreatic cancer. Mechanistically, we found that eIF3f inhibited both cap-dependent and cap-independent translation. An increase in the ribosomal RNA (rRNA) level was suggested to promote the generation of cancer. The regulatory mechanism of rRNA degradation in mammals is not well understood. We demonstrated here that eIF3f promotes rRNA degradation through direct interaction with heterogeneous nuclear ribonucleoprotein (hnRNP) K. We showed that hnRNP K is required for maintaining rRNA stability: under stress conditions, eIF3f dissociates hnRNP K from rRNA, thereby preventing it from protecting rRNA from degradation. We also demonstrated that rRNA degradation occurred in non-P body, non-stress granule cytoplasmic foci that contain eIF3f. Our findings established a new mechanism of rRNA decay regulation mediated by hnRNP K/eIF3f and suggest that the tumor suppressive function of eIF3f may link to impaired rRNA degradation and translation.     

真核翻译起始因子3与肿瘤

真核翻译起始因子3是由多个亚基组成的,在真核翻译起始中发挥重要作用,近年来的研究表明其多个亚基在多种肿瘤细胞中存在异常表达的现象且与肿瘤的侵袭性、转移能力、分化程度及预后相关,使其有望成为肿瘤治疗的新靶点。     

A role of plakophilins in the regulation of translation

Plakophilins 1-3 are members of the p120^ctn-family of armadillo related proteins. They have been characterized as desmosomal plaque proteins, stabilizing desmosomal cadherins at the plasma membrane and interacting with the cytoskeletal linker protein desmoplakin. Loss of cell adhesion contributes to cancerogenesis. In agreement with this, some tumors were found to lack plakophilin expression. Surprisingly, in other tumors, plakophilins 1 and 3 are overexpressed. We have recently identified a function of plakophilins 1 and 3 in the regulation of protein synthesis. Plakophilin 1 was characterized as a component of the cap-binding translation initiation complex where it associates directly with the initiation factor eIF4A1. Plakophilin 1 not only stimulated the recruitment of eIF4A1 into the cap-complex but also its helicase activity. This pointed to a role of plakophilin 1 in the stimulation of proliferation. Given the importance of mRNA translation and protein synthesis in the development of cancer, we speculate that overexpressed plakophilin 1 could contribute to tumor formation. Thus, plakophilin's function in cancerogenesis could go both ways: while an increase of plakophilin could support cancerogenesis via the stimulation of translation and proliferation, loss of plakophilin could contribute to cancerogenesis and/or metastasis via loss of contact inhibition and increased motility. Elucidating the regulation of plakophilin's function in adhesion versus translation will help to understand this context-dependent phenomenon.     

Down-Regulation of eIF4GII by miR-520c-3p Represses Diffuse Large B Cell Lymphoma Development

Deregulation of the translational machinery is emerging as a critical contributor to cancer development. The contribution of microRNAs in translational gene control has been established however; the role of microRNAs in disrupting the cap-dependent translation regulation complex has not been previously described. Here, we established that elevated miR-520c-3p represses global translation, cell proliferation and initiates premature senescence in HeLa and DLBCL cells. Moreover, we demonstrate that miR-520c-3p directly targets translation initiation factor, eIF4GII mRNA and negatively regulates eIF4GII protein synthesis. miR-520c-3p overexpression diminishes cells colony formation and reduces tumor growth in a human xenograft mouse model. Consequently, downregulation of eIF4GII by siRNA decreases translation, cell proliferation and ability to form colonies, as well as induces cellular senescence. In vitro and in vivo findings were further validated in patient samples; DLBCL primary cells demonstrated low miR-520c-3p levels with reciprocally up-regulated eIF4GII protein expression. Our results provide evidence that the tumor suppressor effect of miR-520c-3p is mediated through repression of translation while inducing senescence and that eIF4GII is a key effector of this anti-tumor activity.     

Role of eIF3a in regulating cell cycle progression

Translational control is an essential process in regulation of gene expression, which occurs at the initiation step performed by a number of translation initiation factor complexes. eIF3a (eIF3 p170) is the largest subunit of the eIF3 complex. eIF3a has been suggested to play roles in regulating translation of a subset of mRNAs and in regulating cell cycle progression and cell proliferation. In this study, we examined the expression profile of eIF3a in cell cycle and its role in cell cycle progression. We found that eIF3a expression oscillated with cell cycle and peaked in S phase. Reducing eIF3a expression also reduced cell proliferation rate by elongating cell cycle but did not change the cell cycle distribution. However, eIF3a appears to play an important role in cellular responses to external cell cycle modulators likely by affecting synthesis of target proteins of these modulators.     

Lentivirus-mediated knockdown of eukaryotic translation initiation factor 3 subunit D inhibits proliferation of HCT116 colon cancer cells

Dysregulation of protein synthesis is emerging as a major contributory factor in cancer development. eIF3D (eukaryotic translation initiation factor 3 subunit D) is one member of the eIF3 (eukaryotic translation initiation factor 3) family, which is essential for initiation of protein synthesis in eukaryotic cells. Acquaintance with eIF3D is little since it has been identified as a dispensable subunit of eIF3 complex. Recently, eIF3D was found to embed somatic mutations in human colorectal cancers, indicating its importance for tumour progression. To further probe into its action in colon cancer, we utilized lentivirus-mediated RNA interference to knock down eIF3D expression in one colon cancer cell line HCT116. Knockdown of eIF3D in HCT116 cells significantly inhibited cell proliferation and colony formation in vitro. Flow cytometry analysis indicated that depletion of eIF3D led to cell-cycle arrest in the G2/M phase, and induced an excess accumulation of HCT116 cells in the sub-G1 phase representing apoptotic cells. Signalling pathways responsible for cell growth and apoptosis have also been found altered after eIF3D silencing, such as AMPKα (AMP-activated protein kinase alpha), Bad, PRAS40 [proline-rich Akt (PKB) substrate of 40 kDa], SAPK (stress-activated protein kinase)/JNK (c-Jun N-terminal kinase), GSK3β and PARP [poly(ADP-ribose) polymerase]. Taken together, these findings suggest that eIF3D might play an important role in colon cancer progression.