赵世光
癌细胞葡萄糖代谢重编程的分子基础*
赵世光
赵世光 教授,主任医师,博士生导师,现任哈尔滨医科大学附属第一医院神经外科主任,黑龙江省高校神经外科重点实验室主任,国家临床重点专科建设项目负责人;兼任中国抗癌协会神经肿瘤专业委员会主任委员,中国康复医学会创伤康复专业委员会副主任委员,中国神经科学学会神经创伤与修复分会副主任委员。先后获得“龙江学者”、“卫生部有突出贡献中青年专家”等奖项。主要从事胶质瘤的分子基础和临床研究,其中在荧光引导胶质瘤切除术方面的贡献受到了国内外专家的认可,该技术被写入中华医学会《中国中枢神经系统胶质瘤诊断和治疗指南》。担任《Brain Tumor Pathology》国际编委,《中国神经肿瘤杂志》副主编。主持国家级、省级课题20余项,获教育部、省政府二等奖等成果奖励20余项。发表学术论文150余篇,其中SCI收录52篇。应邀在国内外学术会议上做大会报告50余次,12次担任国内外学术会议共同主席。
代谢的重编程是癌细胞的基本特征之一,其中葡萄糖代谢方式和途径的改变对癌症的发生和发展至关重要。即使在氧气足够充足的情况下,快速增殖的癌细胞生长所需的能量主要由糖酵解而非氧化磷酸化提供,癌细胞这种特殊的糖代谢现象被称为Warburg效应。这种特有的能量获取方式已在多种癌细胞中得到验证,以癌细胞对葡萄糖高摄取率和利用增加为原理的18F-FDG PET/CT显像已广泛应用于临床的癌症诊断。但癌细胞为何利用有氧酵解获取能量以及有氧酵解进行的分子基础目前尚不明确,本文围绕调控癌细胞糖酵解进程中的直接调控酶、癌基因及致癌代谢小分子进行分析和综述。
癌 葡萄糖代谢 Warburg效应
近年来,细胞代谢重编程在癌症中的重要性受到学者的广泛关注,特别是葡萄糖代谢方式的改变。然而癌细胞葡萄糖代谢的分子机制尚不明确。本文在前期研究的基础上针对癌细胞糖代谢重编程中的分子机制进行综述,希望为癌症的治疗提供一定的理论依据。
葡萄糖是细胞获取能量的最主要来源。在氧气充足条件下,正常或分化的细胞通过三羧酸循环(tricarboxylicacidcycle,TCA)将葡萄糖代谢并生成二氧化碳,在氧化磷酸化的过程中,每摩尔葡萄糖产生30或32 moL的三磷酸腺苷(adenosine triphosphate,ATP)和少量乳酸;仅在缺氧条件下,正常或分化的细胞才会通过无氧酵解的方式产生大量乳酸。与之截然不同的是,无论在氧气充足还是缺少的情况下,癌细胞均会优先利用糖酵解的方式产生大量的乳酸,并且每摩尔葡萄糖仅产生2 moL的ATP,有研究[1-2]认为这是由于癌细胞内部发生在线粒体上的氧化磷酸化途径被永久性破坏所致。但是随后的多项研究表明线粒体的功能在大多数癌细胞内并未被破坏,那么癌细胞因何利用如此低效率的糖代谢方式,目前对此的解释是癌细胞高强度的糖酵解对癌细胞的生长有以下两种优点。1)有氧酵解虽然导致每摩尔葡萄糖产生的ATP量较少,但是却能够充分利用细胞外的营养物质和葡萄糖来产生足够的ATP,这是因为在糖酵解底物充足和循环效率足够高的情况下,糖酵解产生的ATP超过氧化磷酸化所产生的ATP[1,3]。2)糖代谢方式的改变为细胞的合成代谢提供丰富的底物,如为核酸合成提供核糖、为脂类合成提供甘油和柠檬酸及大量的非必需氨基酸等[1,4]。这些特点被认为是癌细胞利用有氧糖酵解的原因。
癌细胞有氧酵解的进行依赖多个糖酵解关键限速酶、致癌/抑癌基因及其他多条癌症相关通路的调控。这些基因的位点突变、转录前/后修饰、异常表达及继发性癌症相关通路的激活形成的分子调控网络共同导致并维持了癌细胞内有氧酵解的进行、癌细胞的存活和生长。
HK2是HK家族的重要成员之一,也是糖酵解第一个关键限速酶。在多种癌组织中HK2的表达水平均显著升高,HK2的沉默能有效降低有氧糖酵解的水平,促使癌细胞的代谢方式由有氧酵解向氧化磷酸化转变[5-7]。目前研究[7-14]显示HK2的转录、表达及其在细胞内的分布与缺氧诱导因子-1α(hypoxia inducible factor-1α,HIF-1α)、AKT活化状态、葡萄糖浓度及miR-143的表达密切相关。HK2在癌细胞中的高表达及其复杂的调控网络直接促进了癌细胞有氧糖酵解水平的提高,满足了快速增殖细胞对能量的需求,进而维持癌细胞在低氧、低糖及酸性微环境中生存。
PKM2是糖酵解调控的关键限速酶之一。大量研究显示PKM2在胃癌、肠癌、胶质瘤及肺癌等组织中的表达异常增高,正常或分化成熟的细胞主要表达PKM1而非PKM2,PKM2的表达水平与多种癌细胞的致瘤能力呈正相关,与癌症患者的预后呈负相关[15-17]。为保证PKM2的高表达,癌细胞内选择性剪切蛋白(hnRNP)可促使糖酵解的限速酶丙酮酸激酶从M1型向M2型转变[18]。有研究[19-20]表明PKM2过表达能对抗凋亡因子对癌细胞的杀伤作用,促进癌细胞增殖;PKM2的敲除则抑制癌细胞的增殖、侵袭以及体内癌组织的生长[21]。PKM2在细胞内有二聚体和四聚体两种存在形式,有研究[22-24]证实二聚体在有氧酵解的发生过程中扮演重要角色,而四聚体则主要通过TCA循环为细胞提供能量,两者的动态平衡依赖致癌基因和抑癌基因所组成的复杂分子网络的精细调控,共同维持癌细胞的致瘤能力。
PDH是线粒体内部氧化磷酸化进行的关键调控因子之一,其可使丙酮酸转化为乙酰辅酶A,PDH的表达下降间接使癌细胞内的糖酵解增强、乳酸生成增多[4,25-26]。PDH受线粒体基质蛋白丙酮酸脱氢酶激酶(pyruvate dehydrogenase kinase,PDK)的调控,PDK可以通过磷酸化PDH的E1α亚基抑制癌细胞氧化磷酸化的进行[27]。另外,PDH复合体还受细胞内乙酰辅酶A和NAD/NADH比率的影响[4,28]。DCA是PDK的抑制剂,其具有抑制PDK进而增强PDH、降低乳酸外流及促进氧化磷酸化的功能。目前研究[29-30]显示DCA主要通过促进线粒体膜的去极化、提高癌细胞内活性氧(reactive oxygen species,ROS)水平及上调电压依赖性钾离子通道等多种途径导致继发性的凋亡途径激活,进而导致癌细胞凋亡。
在实体癌组织内,细胞生长在一个缺氧的微环境中。HIF-1α是细胞应对缺氧的感受器,虽然常氧状态下HIF-1α通过与VHL形成的复合体依赖泛素化的途径很快被降解,但是缺氧能够通过激活ROS抑制泛素化的调控作用,促使HIF-1α稳定及其在细胞内的累积[31-32]。HIF-1α蛋白的异常堆积能够显著降低氧化磷酸化的效率,这在一定程度上促使癌细胞转向利用糖酵解来获取能量维持自身的生存;糖酵解会产生大量的乳酸、丙酮酸等代谢产物,这些代谢分子在细胞内积聚,进一步促使HIF-1α表达及活性的增加,最终形成了缺氧促进癌细胞糖酵解和大量乳酸的产生,乳酸反过来促进HIF-1α表达及活性增加的恶性循环[33-34]。HIF-1α对癌细胞此种糖酵解的影响主要依赖于其对糖代谢过程中酶的调控。有研究[35-39]证实HIF-1α作为重要的转录因子对参与葡萄糖转运的膜受体GLUT1及GLUT3,己糖激酶(HK)、葡萄糖-6-磷酸脱氢酶(G6PD)、乳酸脱氢酶A(LDHA)和单羧酸转运蛋白4(MCT4)等的表达均有重要影响。虽然上述证据表明HIF-1α对癌细胞的糖酵解进行及维持缺氧状态下的细胞存活至关重要,但是也有学者[40-42]指出HIF-1α通过调控PDK1和PDH酶的表达或活性使糖酵解产物丙酮酸进入三羧酸循环的数量显著减少,该过程显著减少癌细胞生长和增殖所需要的用于细胞内生物合成的中间产物产生。除了缺氧可以激活HIF-1α之外,一些生长因子比如胰岛素样-1(IGF-1)、EGF也可以激活HIF系统,刺激并促进癌细胞生长,一定程度上弥补由于缺氧所导致的生物合成过程中中间产物的缺乏,这些证据是对HIF-1α调控癌细胞糖酵解的重要补充[43-44]。
p53是对抗癌细胞生长的重要保护蛋白,能够抑制多种致癌因素驱动下的癌细胞增殖、生长并促进其凋亡。越来越多的证据表明p53通过多种方式调控癌细胞的代谢进而影响癌细胞的生物学行为。糖酵解是癌细胞获取能量的主要来源,TP53诱导的糖酵解和凋亡调节因子(TP53-induced glycolysis and apoptosis regulator,TIGAR)能够改变癌细胞对葡萄糖利用的多条通路,增强磷酸戊糖途径,致使细胞内还原型烟酰胺腺嘌呤二核苷酸磷酸(nicotinamide-adenine dinucleotide phosphate,NADPH)的积聚,进而对抗癌细胞内部的ROS,促进癌细胞存活;由于TIGAR启动子上有两个可以结合p53的位点,因此p53能够靶向结合TIGAR,抑制其转录进而降低癌细胞的糖酵解,抑制癌细胞生长[45-47]。除了抑制糖酵解之外,p53也能提高癌细胞氧化磷酸化的能力,研究证实p53直接结合致SCO2的启动子,促进该蛋白的转录及合成,SCO2主要的功能在于将铜离子转移致Cox复合体上,是细胞线粒体氧化磷酸化进行的重要调控因子之一,在癌细胞内过表达SCO2增加氧化磷酸化的水平,p53缺乏的细胞内SCO2蛋白表达及氧化磷酸化水平均显著降低,而糖酵解的水平却明显增强[47-48]。以上研究结果提示p53的缺失或突变导致的SCO2蛋白表达降低破坏了氧化磷酸化的呼吸链,最终导致细胞所需ATP的产生由氧化磷酸化向有氧酵解转变。综上所述,这些证据表明p53的激活通过抑制糖酵解,促进线粒体内的氧化磷酸化调控癌细胞的糖代谢;然而p53如何通过复杂的分子调控网络同时协调这两个方面的功能有待进一步研究。
c-Myc的表达增强在肠癌、乳腺癌、前列腺癌和膀胱癌等恶性肿瘤中均比较常见,c-Myc蛋白对癌细胞的作用主要依赖其作为转录因子调控细胞周期的作用。随着学者们对该蛋白认识的不断深入,越来越多的证据提示c-Myc与癌细胞的糖代谢进程密切相关,其中c-Myc调控LDHA的证据被最先发现[49],然后c-Myc刺激GLUT1的表达,促进葡萄糖向细胞内的转运,c-Myc诱导HK2、PKM2等糖酵解关键酶在癌细胞内的表达并加速糖酵解的作用被陆续发现[50-52]。这些研究结果提示c-Myc通过促进糖酵解的各个环节,加快乳酸和ATP的生成,增强癌细胞的致瘤能力。研究还发现c-Myc除了直接对糖酵解代谢途径产生影响之外,还通过调控miR-23a/miR-23b间接调控GLS,后者是谷氨酰胺转化为谷氨酸的第一个酶,使得癌细胞内部有充足的谷氨酸转化为α-酮戊二酸进入TCA循环,因此c-Myc高表达的细胞对谷氨酰胺的需求增加,而谷氨酰胺的剥夺显著减少了三羧酸循环的中间代谢产物并诱导癌细胞的凋亡,癌细胞对谷氨酰胺的这种高度依赖被认为与c-Myc活性显著相关[53]。另外,有研究显示缺氧诱导因子HIF-1与c-Myc具有协同促进糖酵解关键酶表达的作用,两者分别在缺氧和常氧条件下调控糖酵解酶的表达,促进癌细胞在缺氧环境中生存及癌组织的生长[54-55]。
异柠檬酸脱氢酶1(isocitrate dehydrogenase 1,IDH1)的突变在癌细胞中普遍存在,该突变在癌症进展的早期阶段扮演重要角色,IDH1的突变导致异柠檬酸向酮戊二酸转化的能力减弱进而使2-HG在细胞内的聚集增加[56-57]。在正常细胞中2-HG是三羧酸循环的副产物,表达水平较低,一般不具有生物学功能,但在癌组织中2-HG通过竞争性抑制的机制参与对恶性肿瘤细胞生物学行为的调控,主要表现为2-HG的累积可以抑制包括人体内“组蛋白去甲基化酶”与“脯氨酸羟化酶”在内的多个重要双加氧酶的活力,从而改变细胞的增殖和生长方式,防止HIF-1的泛素化降解,促进HIF-1积聚,维持癌细胞在低氧环境下的生存,促进正常细胞向癌细胞的转化进并诱发恶性肿瘤[58-59]。2-HG还可通过调控组蛋白H3K9和H3K27的甲基化抑制癌细胞的分化[60]。
癌细胞糖代谢方式的改变不是由单一的基因异常造成,而是多基因、多通路共同作用使细胞内部多个生物代谢化学反应通路继发性减弱或增强的结果,其目的是为了满足癌细胞对营养物质和能量的需求,维持自身的存活和增殖。然而目前对癌细胞代谢重编程的分子机制研究仍然处于初级阶段,随着组学技术特别是代谢组学技术在癌领域的利用和快速发展,相信更多癌代谢相关的通路和分子会被发现,针对癌代谢的分子靶向治疗也将成为未来研究的热点。
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(2014-03-31收稿)
(2014-04-30修回)
Molecular basis of glucose metabolic reprogramming in cancer cells
Shiguang ZHAO;E-mail:guangsz@hotmail.com
Department of Neurosurgery,The FirstAffiliated Hospital of Harbin Medical University,Harbin 150001,China
Cancer metabolic reprogramming is among the basic characteristics of cancer cells.Changes in glucose metabolism are essential for carcinogenesis and cancer development.Previous study indicated that energy is acquired mainly via glycolysis rather than oxidative phosphorylation in the presence of sufficient oxygen levels to promote the rapid proliferation of cells,such as cancer cells.This phenomenon is called the"Warburg effect."Furthermore,this unique approach of energy production in cancer cells has been validated in various types of cancer cells.On the basis of the characteristics of cancer cells with high glucose uptake and utilization,clinicians and medical practitioners extensively apply 18-fludeoxyglucose positron emission tomography in clinical diagnosis.Tumor cells undergo aerobic glycolysis to produce energy,but this metabolic pathway is poorly efficient;the molecular basis of aerobic glycolysis and the reason for these cells to undergo this metabolic pathway also remain unclear.In this article,glycolysis-related processes,including enzyme,oncogene,and oncometabolite regulation,in cancer cells are summarized.
cancer,glucose metabolism,Warburg effect
10.3969/j.issn.1000-8179.20140513
哈尔滨医科大学附属第一医院神经外科(哈尔滨市150001)
*本文课题受国家自然科学基金项目(编号:81272788)资助
赵世光 guangsz@hotmail.com
Shiguang ZHAO
This study was supported by the National Natural Science Foundation of China(No.81272788)
邢颖)