Caspase 的活化机制

Caspase是一类与凋亡密切相关的蛋白水解酶家族,以Caspase前体酶原的形式存在大多后生动物的细胞中。Caspase在凋亡信号的作用下首先激活启动型Caspase引发Caspase级联反应,然后通过活化的执行型Caspase裂解特异性底物导致细胞凋亡。Caspase的活化是导致细胞凋亡的中心环节,位于Caspase级联反应上游的启动型Caspase的和下游的执行型Caspase有着明显不同的活化机制。     

Caspase的结构与功能

Caspase是执行细胞凋亡的主要酶类,绝大部分细胞凋亡依赖于Caspase的存在。目前已鉴定的哺乳动物Caspase有14种,它们具有很多共同的特点。Caspase酶原包括3个部分：原域、大亚单位和小亚单位。酶原本身的活性很低,但可以通过不同的方式被激活。一旦原域以及大、小亚单位之间的连结被切除,大小亚单位之间的连结被切除,大小亚单位之间相互作用形成一异二聚体,其中包含一个活性位点;两个异二聚体形成一个四聚体,成为Caspase的活性形式,活化的Caspase是细胞凋亡过程中的关键酶类,通过特异性的裂解底物而发挥其执行细胞凋亡的功能。此外,Caspase还在细胞因子成熟过程中发挥重要作用。     

黄芩苷干预甲型H1N1流感病毒感染诱导的A549细胞周期分布及凋亡

以甲型H1N1流感病毒作为刺激因素,在人肺腺癌A549细胞培养内采用MTT比色法和细胞病变(CPE)法观察黄芩苷不同作用时间的抗病毒效率;以碘化丙啶(Propidium iodide,PI)单染流式细胞仪分析细胞周期中各时期的细胞百分数和对细胞增殖的影响,以Hoechst33258染色荧光显微镜观察细胞凋亡的形态学变化,并采用免疫荧光实验测定膜受体通路Caspase 8和线粒体通路Caspase 9及作为细胞凋亡标志的Caspase 3的活性。结果显示:流感病毒感染36h后宿主细胞周期阻滞于S期(P<0.05),并在G0期细胞峰前出现一个亚二倍体细胞峰(凋亡细胞峰)。A549细胞中Caspase 8/3活性明显升高,但标记Caspase 9活性的荧光亮度增强不明显。黄芩苷对甲型流感病毒感染诱导的细胞损伤有较好的保护作用,最大剂量的黄芩苷100μg/mL无毒,可抑制病毒细胞病变的产生,50~100μg/mL治疗组可明显调节流感病毒感染诱导的细胞周期分布(P<0.05),细胞凋亡现象明显减少,100μg/mL黄芩苷治疗组Caspase 8/3活性显著降低,接近正常对照组细胞水平,有剂量依赖性。实验说明:黄芩苷可拮抗甲型流感病毒H1N1细胞病变,调控细胞周期分布,并通过抑制Caspase 8的激活,进一步抑制Caspase 3活性,从而发挥抗病毒作用。     

溶瘤腺病毒CD55-TRAIL-IETD-MnSOD联合阿霉素抑制肝癌细胞HepG2生长

目的:研究溶瘤腺病毒CD55-TRAIL-IETD-MnSOD与阿霉素联合使用对肝癌细胞HepG2生长的影响。方法:MTT法检测经CD55-TRAIL-IETD-MnSOD与阿霉素联合处理后HepG2细胞生存率的变化,Hoechst33342染色及流式细胞术检测联合处理诱导细胞凋亡时细胞形态变化及凋亡细胞数量,Western印迹检测联合处理所引发的凋亡通路。结果:MTT实验显示,联合使用CD55-TRAIL-IETD-MnSOD和阿霉素能更有效地抑制HepG2细胞的生长,且阿霉素与CD55-TRAIL-IETD-MnSOD是协同作用;Hochest染色实验和流式细胞术实验结果均显示联合使用CD55-TRAIL-IETD-MnSOD和阿霉素增强了诱导细胞凋亡的作用;Western印迹表明CD55-TRAIL-IETD-MnSOD和阿霉素联合使用激活了Caspase细胞凋亡途径。结论:阿霉素能促进溶瘤腺病毒CD55-TRAIL-IETD-MnSOD的复制,联合阿霉素和溶瘤腺病毒CD55-TRAIL-IETD-MnSOD可更有效激活Caspase细胞凋亡通路,抑制肝癌细胞HepG2生长。     

Caspase的非凋亡性作用CSCD

Caspase是一类胱天蛋白酶家族。大多数Caspase以半胱氨酸作为裂解底物的亲核基团,通过切割底物蛋白引起细胞凋亡。但是,研究发现Caspase存在非凋亡性作用,即Caspase活化后并不引发细胞凋亡,而是通过剪切不同底物或蛋白质互作,介导其他生物学事件,包括细胞增殖、分化、迁移、存活、形态重塑等。部分细胞的分化依赖于Caspase瞬时或位点局限的激活,细胞形态呈现类似于凋亡(不完全凋亡)的改变。Caspase对信号分子、转录因子的剪切灭活,可发挥转换细胞命运和分化进程的作用。在组织损伤时,应激细胞Caspase活化后,可提高自身防御能力,并通过旁分泌来指导邻近细胞进行补偿修复。因此,Caspase的活化不仅仅是一种细胞死亡信号,它的特定活化还是更改细胞形态、行为、命运的重要应激信号和效应分子。     

Caspase的非凋亡性作用

Caspase是一类胱天蛋白酶家族。大多数Caspase以半胱氨酸作为裂解底物的亲核基团,通过切割底物蛋白引起细胞凋亡。但是,研究发现Caspase存在非凋亡性作用,即Caspase活化后并不引发细胞凋亡,而是通过剪切不同底物或蛋白质互作,介导其他生物学事件,包括细胞增殖、分化、迁移、存活、形态重塑等。部分细胞的分化依赖于Caspase瞬时或位点局限的激活,细胞形态呈现类似于凋亡(不完全凋亡)的改变。Caspase对信号分子、转录因子的剪切灭活,可发挥转换细胞命运和分化进程的作用。在组织损伤时,应激细胞Caspase活化后,可提高自身防御能力,并通过旁分泌来指导邻近细胞进行补偿修复。因此,Caspase的活化不仅仅是一种细胞死亡信号,它的特定活化还是更改细胞形态、行为、命运的重要应激信号和效应分子。     

JNK激酶介导mGluR1对细胞凋亡的不同效应

代谢型谷氨酸受体1（mGluR1）可以通过激活多条信号通路促进或抑制细胞凋亡.然而,导致这种生理功能差异的机制尚不明确.本研究选用两种细胞系,即大鼠神经胶质瘤细胞系（C6）和人胚胎肾细胞（HEK293）分别研究内源性和外源转染的mGluR1的激活对细胞凋亡的影响及其调节机制. 结果显示,内源性mGluR1的活化能够激活PI3K/ERK/JNK通路,抑制凋亡试剂STS诱导的细胞凋亡;而外源转染的mGluR1的活化能够分别激活PI3K/ERK和JNK通路,同时促进STS诱导的应激损伤. HEK293细胞中,应用JNK通路抑制剂SP600125,能够部分抑制由mGluR1激活介导的caspase 3的剪切和细胞凋亡;而在C6细胞中阻断JNK通路,则加剧了由mGluR1活化而引起的细胞凋亡. 本文结果提示：mGluR1通过不同信号通路影响细胞凋亡,其中JNK通路可能是调控细胞凋亡的关键途径.本文为受体激活对细胞凋亡能够产生不同的调控作用提供了相应的证据.     

猪丁型冠状病毒诱导的细胞线粒体凋亡

猪丁型冠状病毒(Porcine deltacoronavirus,PDCoV)是一种新型的猪肠道致病性冠状病毒,可引起猪群剧烈腹泻及呕吐,但致病机制尚不清楚。本研究检测了PDCoV感染诱导的细胞凋亡。Caspase酶活性检测显示,在PDCoV感染的细胞中,caspase 3、caspase 8和caspase 9的活性随病毒感染量的增多而显著提高,类似的现象未能在紫外灭活病毒感染的细胞中观察到,表明PDCoV感染可同时激活内源性与外源性细胞凋亡通路,并暗示细胞凋亡的诱导依赖于病毒复制。为深入探究PDCoV诱导的内源性细胞凋亡,分别检测胞浆和线粒体中细胞色素C与凋亡诱导因子。结果显示,与正常细胞相比,PDCoV感染细胞从线粒体释放到胞浆的细胞色素C显著增多,且其释放量随着感染时间的延长而增多,而凋亡诱导因子始终定位于线粒体,提示PDCoV感染通过促使线粒体膜间隙的细胞色素C进入胞浆而启动caspase依赖的线粒体凋亡通路。本研究初步揭示了PDCoV诱导细胞凋亡的机制。     

Caspase信号通路在双歧杆菌脂磷壁酸诱导结肠癌细胞凋亡中的作用

目的探讨Caspase信号通路在双歧杆菌脂磷壁酸（LTA）诱导结肠癌细胞凋亡中的作用。方法RT-PCR检测经双歧杆菌LTA处理后,结肠癌Lovo细胞中MyD88和FADD mRNA的表达变化;AnnexinV检测经Caspase通用抑制剂（Z-Val-Ala-Asp-FMK）预先处理后,双歧杆菌LTA诱导结肠癌Lovo细胞凋亡率的变化;荧光法检测经双歧杆菌LTA处理后,Lovo细胞中Caspase-8活性的变化。结果经双歧杆菌LTA处理后,Lovo细胞中MyD88的mRNA表达明显升高（P〈0．05）,而FADD信号分子的mRNA表达无明显变化;双歧杆菌LTA能够增强Lovo细胞中Caspase-8的活性（P〈0．05）,且其诱导Lovo细胞凋亡的作用能够被Caspase抑制剂所抑制（P〈0．05）。结论MyD88信号分子在双歧杆菌LTA诱导Lovo细胞凋亡中可能起着承接上游分子TLRs与下游信号分子FADD的作用;而Caspase信号通路可能是双歧杆菌LTA诱导结肠癌Lovo细胞凋亡的主要信号传导途径。     

Caspase激活与调控的分子机制

Caspases是一类天冬氨酸特异性的半胱氨酸蛋白酶(IL-1β转化酶相关蛋白酶).迄今,在哺乳动物至少已发现13种caspase成员.Caspases在胞内通常以无活性的酶原形式存在,在其内部特定的天冬氨酸残基部位蛋白质裂解加工后可导致酶原激活,引发细胞凋亡.作为效应子的caspase在绝大多数细胞的凋亡过程中具有十分重要的作用.随着线虫死亡程序及某些死亡受体介导敏感细胞凋亡的信号机制的阐明,人们对caspase激活与调控在细胞凋亡中的机制研究已获得重大进展.     

Caspase activation, inhibition, and reactivation: a mechanistic view

Caspases, a unique family of cysteine proteases, execute programmed cell death (apoptosis). Caspases exist as inactive zymogens in cells and undergo a cascade of catalytic activation at the onset of apoptosis. The activated caspases are subject to inhibition by the inhibitor-of-apoptosis (IAP) family of proteins. This inhibition can be effectively removed by diverse proteins that share an IAP-binding tetrapeptide motif. Recent structural and biochemical studies have revealed the underlying molecular mechanisms for these processes in mammals and in Drosophila. This paper reviews these latest advances.     

The kinder side of killer proteases: Caspase activation contributes to neuroprotection and CNS remodeling

Caspases are a family of cysteine proteases that are expressed as inactive zymogens and undergo proteolytic maturation in a sequential manner in which initiator caspases cleave and activate the effector caspases 3, 6 and 7. Effector caspases cleave structural proteins, signaling molecules, DNA repair enzymes and proteins which inhibit apoptosis. Activation of effector, or executioner, caspases has historically been viewed as a terminal event in the process of programmed cell death. Emerging evidence now suggests a broader role for activated caspases in cellular maturation, differentiation and other non-lethal events. The importance of activated caspases in normal cell development and signaling has recently been extended to the CNS where these proteases have been shown to contribute to axon guidance, synaptic plasticity and neuroprotection. This review will focus on the adaptive roles activated caspases in maintaining viability, the mechanisms by which caspases are held in check so as not produce apoptotic cell death and the ramifications of these observations in the treatment of neurological disorders.     

Caspase-dependent apoptotic pathways in CNS injury

Recent studies have suggested a role for neuronal apoptosis in cell loss following acute CNS injury as well as in chronic neurodegeneration. Caspases are a family of cysteine requiring aspartate proteases with sequence similarity to Ced-3 protein of Caenorhabditis elegans. These proteases have been found to contribute significantly to the morphological and biochemical manifestations of apoptotic cell death. Caspases are translated as inactive zymogens and become active after specific cleavage. Of the 14 identified caspases, caspase-3 appears to be the major effector of neuronal apoptosis induced by a variety of stimuli. A role for caspase-3 in injury-induced neuronal cell death has been established using semispecific peptide caspase inhibitors. This article reviews the current literature relating to pathways regulating caspase activation in apoptosis associated with acute and chronic neurodegeneration, and suggests that identification of critical upstream caspase regulatory mechanisms may permit more effective treatment of such disorders.     

Caspase inhibitors

Caspases are the key effector molecules of the physiological death process known as apoptosis, although some are involved in activation of cytokines, rather than cell death. They exist in most of our cells as inactive precursors (zymogens) that kill the cell once activated. Caspases can be controlled in two ways. The processing and activation of a caspase can be regulated by molecules such as FADD, APAF-1, Bcl-2 family members, FLIP and IAPs. Active caspases can be controlled by a variety of inhibitors that directly interact with the protease. This review describes the later direct caspase inhibitors that have been identified, products of both viral and cellular genes, and artificial caspase inhibitors that have been developed both as research tools and as pharmaceutical agents to inhibit cell death in vivo.     

Protein complexes activate distinct caspase cascades in death receptor and stress-induced apoptosis

Caspases play a central role in the execution phase of apoptosis and are responsible for many of the morphological features normally associated with this form of cell death. Caspases can activate one another and consequently can initiate specific caspase cascades. Caspases-8 and -9 appear to be the apical caspases activated in death receptor- and mitochondrial stress-induced apoptosis, respectively. The role of large protein complexes in mediating these pathways is discussed.     

Proteolytic signaling by TNFalpha: caspase activation and IkappaB degradation

Following binding its death receptor on the plasma membrane, tumor necrosis factor (TNF) induces the receptor trimerization and recruits a number of death domain-containing molecules to form the receptor complex. The complex promotes activation of downstream caspase cascade and induces degradation of IkappaBalpha. Caspases are activated using mechanisms of oligomeration and 'self-controlled proteolysis'. According to their structures and functions, apoptosis related caspases can be divided into upstream and downstream caspases. In general, upstream caspases cleave and activate downstream caspases by proteolysis of the Asp-X site. Activated caspases then cleaved target substrates. To date, more than 70 proteins have been identified to be substrates of caspases in mammalian cells. Caspases can alter the function of their target proteins by destroying structural components of the cytoskeleton and nuclear scaffold or by removing their regulatory domains. Activation of NF-kappaB is dependent on the degradation of IkappaBalpha. IkappaB kinase (IKK) phosphorylates IkappaBalpha at the residues 32 and 36 followed by polyubiquitination at lysine 21 and 22 and subsequent degradation of the molecules by 26S proteasome. There is extensive crosstalk between the apoptotic and NF-kappaB signaling pathways that emanate from TNF-R1. On the one hand, activation of NF-kappaB can inactivate caspases; on the other hand, activated caspases can inhibit the activation of NF-kappaB. Both processes involve in proteolysis. This crosstalk may be important for maintaining the balance between the two pathways and for determining whether a cell should live or die.     

Interchain proteolysis, in the absence of a dimerization stimulus, can initiate apoptosis-associated caspase-8 activation

Caspases coordinate the internal demolition of the cell that is seen during apoptosis. Proteolytic processing of caspases is observed during apoptosis, and this correlates with conversion of inactive caspase proenzymes into their active two-chain forms. However, recent studies have suggested that caspase-8 is activated through dimerization and that interchain proteolysis is not sufficient for activation of this caspase. This proposal casts doubt upon whether caspase-8 is productively activated by granzyme B during granule-dependent cytotoxic T lymphocyte or natural killer cell-mediated killing, for example. Contrary to the dimerization model, we show that direct proteolysis of caspase-8 by the cytotoxic T lymphocyte protease granzyme B, or by caspase-6, produces an active enzyme that displays robust proteolytic activity toward synthetic as well as natural caspase-8 substrates. These data suggest that enforced dimerization of caspase-8 zymogens by scaffold proteins such as Fas-associated protein with death domain (FADD), although important in certain contexts, is not a prerequisite for activation of this protease.     

What happened to plant caspases?

The extent of conservation in the programmed cell death pathways that are activated in species belonging to different kingdoms is not clear. Caspases are key components of animal apoptosis; caspase activities are detected in both animal and plant cells. Yet, while animals have caspase genes, plants do not have orthologous sequences in their genomes. It is 10 years since the first caspase activity was reported in plants, and there are now at least eight caspase activities that have been measured in plant extracts using caspase substrates. Various caspase inhibitors can block many forms of plant programmed cell death, suggesting that caspase-like activities are required for completion of the process. Since plant metacaspases do not have caspase activities, a major challenge is to identify the plant proteases that are responsible for the caspase-like activities and to understand how they relate, if at all, to animal caspases. The protease vacuolar processing enzyme, a legumain, is responsible for the cleavage of caspase-1 synthetic substrate in plant extracts. Saspase, a serine protease, cleaves caspase-8 and some caspase-6 synthetic substrates. Possible scenarios that could explain why plants have caspase activities without caspases are discussed.     

Caspases, IAPs and Smac/DIABLO: mechanisms from structural biology

Caspases are the central component of the apoptotic machinery that irreversibly commits a cell to die. Whereas all caspases are structurally similar, those involved in apoptosis can be categorized functionally as either initiator or effector caspases, which are activated by distinct mechanisms. The activated caspases are subject to inhibition by the inhibitor of apoptosis family of proteins. This inhibition can be removed by Smac/DIABLO during apoptosis. The underlying molecular mechanisms of caspase regulation are discussed in this article.     

Proteases for cell suicide: functions and regulation of caspases.

Caspases are a large family of evolutionarily conserved proteases found from Caenorhabditis elegans to humans. Although the first caspase was identified as a processing enzyme for interleukin-1beta, genetic and biochemical data have converged to reveal that many caspases are key mediators of apoptosis, the intrinsic cell suicide program essential for development and tissue homeostasis. Each caspase is a cysteine aspartase; it employs a nucleophilic cysteine in its active site to cleave aspartic acid peptide bonds within proteins. Caspases are synthesized as inactive precursors termed procaspases; proteolytic processing of procaspase generates the tetrameric active caspase enzyme, composed of two repeating heterotypic subunits. Based on kinetic data, substrate specificity, and procaspase structure, caspases have been conceptually divided into initiators and effectors. Initiator caspases activate effector caspases in response to specific cell death signals, and effector caspases cleave various cellular proteins to trigger apoptosis. Adapter protein-mediated oligomerization of procaspases is now recognized as a universal mechanism of initiator caspase activation and underlies the control of both cell surface death receptor and mitochondrial cytochrome c-Apaf-1 apoptosis pathways. Caspase substrates have bene identified that induce each of the classic features of apoptosis, including membrane blebbing, cell body shrinkage, and DNA fragmentation. Mice deficient for caspase genes have highlighted tissue- and signal-specific pathways for apoptosis and demonstrated an independent function for caspase-1 and -11 in cytokine processing. Dysregulation of caspases features prominently in many human diseases, including cancer, autoimmunity, and neurodegenerative disorders, and increasing evidence shows that altering caspase activity can confer therapeutic benefits.