Abstract
表观遗传学主要关注DNA甲基化、组蛋白修饰、染色质重塑,以及非编码RNA等超越DNA序列的基因调控机制。表观遗传机制参与了个体发育、细胞命运决定和肿瘤发生等众多生物学过程。其中表观遗传信息以各种染色质修饰和高级结构的形式存储于基因组中,它的建立和维持与细胞代谢紧密相关。肿瘤细胞中存在的代谢改变包括有氧糖酵解、葡萄糖摄取量增加、谷氨酰胺代谢异常活跃、利用非主要供能物质供能等,这些改变满足了肿瘤发生发展过程中旺盛的能量和物质需求,帮助细胞适应缺氧的肿瘤微环境,进而为肿瘤增殖、侵袭、迁移等生物活动提供支持。肿瘤细胞的表观遗传修饰与代谢之间存在复杂的相互关系,一方面肿瘤细胞中的代谢产物作为表观修饰酶的辅因子、修饰供体或拮抗分子影响表观修饰景观;另一方面表观遗传修饰可以直接改变代谢酶和转运蛋白的表达或通过影响信号转导和转录因子的表达调控细胞代谢。本文综述了不同表观遗传学过程与肿瘤细胞代谢之间的相互作用,并展望两者在肿瘤治疗中的潜在应用前景。
Abstract
Epigenetics concerns gene regulatory mechanisms beyond DNA sequence,such as DNA methylation,histone modification,chromatin remodeling,and non-coding RNA. Epigenetic mechanisms play a key role in development,cell fate decision and tumorigenesis. Chromatin modifications and its high order structure across our genome are major forms of epigenetic information,and its establishment and maintenance are closely related to cell metabolism. Metabolic changes in cancer cells include aerobic glycolysis,increased glucose uptake,abnormally active glutamine metabolism,and the use of non-conventional energy supply. These changes meet the vigorous energy and matter needs for the development and spread of cancer,and help tumor cells adapt to hypoxia microenvironment for their survival,proliferation,invasion and migration. There is a complex relationship between epigenetic modifications and cell metabolism in tumor. On the one hand,metabolites in tumor cells may act as cofactors,modification donors or antagonists of epigenetic enzymes,thus modulating the epigenetic landscape. On the other hand,epigenetic modifications can directly regulate the expression of metabolic enzymes,transporters,signaling pathway and transcription factors to affect cell metabolism. This article reviews the crosstalk between epigenetics and cancer metabolism,to explore their potential future applications in the treatment of tumors.
Keywords: Epigenetics, Metabolism, Tumor, DNA methylation, Histone modification, Non-coding RNA, Epigenetic landscape, Review
信使RNA(messenger RNA,mRNA);长链非编码RNA(long non-coding RNA,lncRNA);环状RNA(circular RNA,circRNA);微RNA(micro RNA,miRNA,miR);S-腺苷甲硫氨酸(S-adenosyl methionine,SAM);S-腺苷-L-高半胱氨酸(S-adenosyl-L-homocysteine,SAH);烟酰胺腺嘌呤二核苷酸(nicotinamide adenine dinucleotide,NAD);DNA甲基化转移酶(DNA methyltransferase,DNMT);腺苷三磷酸(adenosine triphosphate,ATP);10/11易位蛋白家族(ten-eleven translocation family protein,TET);M2型丙酮酸激酶(pyruvate kinase subtype M2,PKM2);葡萄糖转运蛋白(glucose transporter,GLUT);果糖-1,6-二磷酸酶(fructose-1,6-diphosphate,FBP);磷脂酰肌醇3-激酶(phosphoinositide 3-kinase,PI3K);蛋白激酶B(protein kinase B,AKT);哺乳动物雷帕霉素靶蛋白(mammalian target of rapamycin,mTOR);缺氧诱导因子1α(hypoxia inducible factor-1α,HIF-1α);10号染色体缺失磷酸酶及张力蛋白同源物(phosphatase and tensin homolog deleted on chromosome ten,PTEN);希佩尔林道抑癌基因(Von Hippel-Lindau tumor suppressor ,VHL);肝激酶B(liver kinase B,LKB);腺苷一磷酸(adenosine monophosphate,AMP); AMP活化蛋白激酶(AMP-activated protein kinase,AMPK);腺苷二磷酸(adenosine diphosphate,ADP);组蛋白赖氨酸甲基转移酶(histone lysine methyltransferase,HKMT);沉默信息调节因子(silent information regulator,SIRT);神经前体细胞表达发育下调蛋白(neural precursor cell expressed develop- mentally down-regulated protein,NEDD);环磷酸腺苷反应元件结合蛋白的结合蛋白(CREB binding protein,CBP);蛋白质精氨酸甲基转移酶(protein arginine methyltransferase,PRMT);混合系白血病(mixed lineage leukemia,MLL);赖氨酸特异性蛋白质去甲基化酶家族(lysine-specific protein demethylases,LSD);ATP-柠檬酸裂合酶(ATP-citrate lyase,ACL);组蛋白脱乙酰酶(histone deacetylase,HDAC);G蛋白转导蛋白亚单位家族(guanine nucleotide binding protein alpha transducing activity polypeptide family,GNAT);短链家族酰基化辅酶A合成酶(acyl-CoA synthetase short chain family member,ACSS);6-甲基腺嘌呤(6-methyladenosine,m6A);甲基转移酶(methyltransferase-like,METTL);肥胖相关基因(fat mass and obesity associated,FTO);Alk B同源蛋白(Alk B homologue,ALKBH);酰基辅酶A合成酶长链家族成员(acyl-CoA synthetase long chain family member,ACSL);胆囊癌相关的丙酮酸羧化酶抑制剂(gall bladder cancer associated suppressor of pyruvate carboxylase lncRNA,GCASPC);
肿瘤代谢是近年来肿瘤生物学研究的热点之一,肿瘤细胞中受调控的代谢改变包括有氧糖酵解(瓦博格效应) [1] 、葡萄糖摄取量增加、谷氨酰胺代谢异常活跃 [2] 、利用非主要供能物质供能等,这些改变满足了肿瘤细胞快速增殖时旺盛的能量和物质需求,帮助细胞适应缺氧的肿瘤微环境,进而为肿瘤的增殖、侵袭、迁移等生物活动提供能量和物质支持。代谢改变在不同组织来源的肿瘤以及肿瘤发生发展的不同阶段广泛存在,实验证据表明多种机制参与了肿瘤代谢的驱动和调控,如肿瘤微环境、癌基因、代谢酶等 [3] 。
表观遗传是不改变基因的DNA序列而调控基因表达的可遗传或可继承机制的统称,包括DNA甲基化、组蛋白修饰、染色质重塑、非编码RNA等 [4] 。表观遗传参与调控细胞分化、个体发育、肿瘤发生和发展等众多生物学过程,是一种将环境压力和基因表达联系起来的重要机制 [5] 。肿瘤细胞中普遍存在着多种以染色质修饰等为内容的表观遗传景观变化,如基因组水平DNA低甲基化 [6] 、H4K16乙酰化水平显著降低等组蛋白修饰异常 [7] 。同时,作为广义的表观调控机制,mRNA甲基化修饰异常以及lncRNA、circRNA、miRNA等非编码RNA异常表达等也促进了肿瘤的发生。
肿瘤的表观遗传与代谢之间有着复杂的相互关系,一方面肿瘤细胞中代谢小分子常常作为表观修饰酶的辅因子、修饰供体、激动或拮抗分子而影响表观修饰酶的活性,如SAM/SAH比值可以影响甲基转移酶的活性进而调控DNA和组蛋白的甲基化 [8] ,乙酰辅酶A和NAD +影响组蛋白乙酰化 [ 9- 10] 等;另一方面表观遗传修饰可以直接改变代谢酶、转运蛋白、信号转导、转录因子等的表达来调控细胞代谢。本文主要讨论不同表观遗传学过程影响和调控肿瘤代谢的机制,包括DNA甲基化、组蛋白修饰、mRNA修饰和非编码RNA等,并总结和展望针对肿瘤代谢的表观遗传学手段在肿瘤治疗中的实践与前景。
1DNA甲基化与肿瘤代谢
在基因组DNA中存在着5′碳原子甲基化修饰的5-甲基胞嘧啶残基,这种由DNMT催化的共价修饰被称为DNA甲基化。在哺乳动物细胞的基因组中,CpG序列出现的频率相比其他二核苷酸序列低,然而在基因的5′端启动子附近存在着长度约为1kb的CpG序列富集的区域,被称为CpG岛 [11] 。CpG岛上的DNA甲基化常导致基因沉默,而这种甲基化状态可在DNA复制时由DNMT传递到新合成的DNA单链,进而完成世代间的遗传 [12] 。DNMT的三种亚型DNMT1、DNMT3A和DNMT3B均在肿瘤中过表达 [13] 。
SAM是DNMT常见的活化甲基供体,而其去甲基后生成的SAH可以抑制DNMT活性,故细胞内的DNA甲基化水平受SAM/SAH比值调控。SAM是一碳代谢循环的产物,由甲硫氨酸和ATP在甲硫氨酸腺苷基转移酶催化下合成 [8] 。证据表明,肿瘤细胞可以通过增强一碳代谢来提高SAM的供应,如通过大型中性氨基酸转运蛋白1/4的过表达直接增加甲硫氨酸的摄取 [14] ,而SAM的过量供应可能会导致CpG位点的DNA超甲基化和异常的基因沉默,并与肿瘤发生相关。研究显示,小鼠 GNMT基因敲除使肝脏SAM升高40倍以上,抑癌基因 RASSF1和 SOCS2的启动子甲基化水平升高,肝细胞癌的发病率随之上升 [15] 。
与DNA甲基化相对的是DNA去甲基化,两种过程的动态平衡决定了DNA甲基化水平。5-甲基胞嘧啶的去甲基过程可分为两步,分别由TET和胸腺嘧啶DNA糖苷酶催化 [ 16- 17] 。TET是潜在的肿瘤抑制因子,其在多种人类肿瘤中表达下调 [ 18- 19] ,故肿瘤细胞启动子区域高甲基化可能是由甲基化作用增强和去甲基化作用减弱协同调控的。TET是α酮戊二酸依赖性双加氧酶,需要α酮戊二酸作为辅因子,并被三羧酸循环中间产物竞争性抑制。代谢酶基因如异柠檬酸脱氢酶1/2、琥珀酸脱氢酶、延胡索酸酶发生突变的癌细胞可能会积累过量的三羧酸循环中间产物,如2-羟基戊二酸酯、琥珀酸酯和延胡索酸酯以抑制TET的活性,进而通过影响DNA甲基化修饰调控肿瘤的发生和发展 [20] 。
由于上述DNA甲基化和DNA去甲基化过程的异常,相较于癌旁组织,肿瘤细胞中普遍存在着全基因组范围的DNA甲基化水平下降以及部分基因和miRNA的CpG岛甲基化水平上升 [21] ,可直接调节代谢相关基因的表达,或通过相关信号通路和转录因子间接调控肿瘤代谢过程。
研究发现,基因组DNA低甲基化状态使得多种肿瘤中的PKM2表达上调 [22] 。在糖酵解中,其限速酶PKM2可催化磷酸烯醇式丙酮酸生成丙酮酸。PKM2以四聚体、二聚体和单体三种不同形式稳定存在,而PKM2的四种同工酶通常以四聚体的形式存在。PKM2单体酶活性低,且可定位于细胞核,这与瓦博格效应的产生和维持密切相关 [23] 。在动物肿瘤模型和多种人类肿瘤样本中都有癌基因低甲基化现象,如肝细胞癌 C-MYC基因第三外显子低甲基化 [24] 、慢性淋巴细胞白血病 BCL2基因低甲基化 [25] 。
部分基因启动子区域DNA高甲基化介导的基因沉默也在肿瘤代谢调控中发挥了作用。FBP1、FBP2可催化果糖-1,6-二磷酸生成果糖-6-磷酸,是糖异生的限速酶 [26] ,可以拮抗糖酵解。在肺癌、乳腺癌、肝癌等组织中发现了FBP1的启动子区域高甲基化,其表达也随之下调 [ 27- 29] 。CpG岛高甲基化还使得内质网相关降解蛋白3的表达量下降,GLUT1通过蛋白酶体的分解减少,表达量上升,导致葡萄糖摄取量增加 [30] 。PI3K/AKT/mTOR信号通路在肿瘤葡萄糖代谢的调控中发挥了核心作用,该通路激活可增加HIF-1α以及GLUT1和糖酵解相关代谢酶的表达 [ 31- 32] 。有报道表明,肿瘤组织中存在着PTEN [33] 、VHL [34] 、脯氨酸羟化酶 [35] 等的启动子甲基化,这些抑癌基因的沉默使得PI3K/AKT/mTOR信号通路激活,有氧糖酵解增强。而在LKB1/AMPK/mTOR通路中,LKB1通过激活AMPK抑制mTOR [36] 。研究者在20.16%的肺癌组织中发现了LKB1启动子甲基化,并使其表达量降低以解除对mTOR的抑制 [37] 。FBP2可以上调细胞内AMP/ADP水平,激活AMPK以抑制mTOR,故FBP2的启动子甲基化也参与了有氧糖酵解的调控 [38] 。
2组蛋白修饰与肿瘤代谢
组蛋白是真核细胞染色质中高度保守的结构蛋白。构成染色质的基本结构单元核小体由核心组蛋白H2A、H2B、H3、H4各两个拷贝形成的八聚体与接头组蛋白H1结合147bp的DNA构成。核心组蛋白氨基端肽链上的氨基酸残基可在多种组蛋白表观修饰酶的作用下被共价修饰,修饰的类型包括甲基化、磷酸化、酰基化、泛素化等 [39] 。为精确调控“组蛋白密码”,细胞采用了一系列酶或结合蛋白来产生、消除或识别这些翻译后修饰,这些蛋白被称为书写器、擦除器和阅读器。这些书写器和擦除器往往具有修饰位点、修饰类型的特异性,同时可以招募相关蛋白和转录因子至基因组的特定位置以调控相关基因的表达 [ 40- 41] 。
研究发现,相较于正常代谢的细胞,肿瘤细胞中存在组蛋白修饰异常,如部分位点的组蛋白甲基化水平升高或乙酰化缺失等。这些异常存在的组蛋白修饰常作为某些肿瘤的标志物 [7] 。组蛋白修饰在一些肿瘤细胞中通过调节相关基因表达从而直接或间接影响细胞内的代谢过程,进而发挥相应的调节作用。
2.1组蛋白甲基化与肿瘤代谢
组蛋白甲基化是指主要发生在核心组蛋白H3和H4氨基端上的赖氨酸或精氨酸残基的甲基化修饰,包括H3K4、H3K9、H3K36和H4K20等位点。与DNA甲基化类似,组蛋白甲基化由组蛋白甲基化转移酶催化,受SAM/SAH比值影响。其中,赖氨酸可在 HKMT的催化下被单甲基化、二甲基化或三甲基化,精氨酸可在PRMT的催化下被单甲基化或二甲基化 [42] ,而这些酶同时也可以甲基化非组蛋白底物。甲基化的位点、水平和类型的多样性使其可编码更多的信息,从而参与异染色质形成、DNA修复、基因组印迹等细胞的生物学活动 [43] 。一般认为,H3K4、H3K36、H3K79的三甲基化与基因表达激活相关,而H3K9、H3K27的三甲基化则多与基因表达沉默相关 [44] 。
HKMT表达异常可导致多个位点的组蛋白赖氨酸甲基化水平改变,与肿瘤的发生和发展相关。如H3K4甲基化主要由 MLL基因维持, MLL基因如果在血液细胞中发生易位,会与其他蛋白基因生成融合蛋白导致H3K4高度甲基化,并因此改变基因的正常表达,诱导促癌基因表达,使得血液细胞分化异常,造成血液系统肿瘤的发生 [45] 。又如多梳抑制复合物介导的组蛋白异常的甲基化状态可引起细胞癌变。多梳抑制复合物的核心组成蛋白包括EZH2/EZH1、EED、SUZ12、RBAP48和AEBP2。EZH2/EZH1与EED、SUZ12结合后具有H3K27的甲基化活性。研究表明,许多肿瘤细胞中EZH2过表达 [46] 。此外,EZH2的催化活性中心氨基酸突变可引起H3K27甲基化水平上升和受多梳抑制复合物调控的下游基因沉默,导致淋巴瘤细胞增殖 [ 47- 48] 。已有研究发现,PRMT在多种肿瘤细胞中过度表达,但其对疾病发生的影响研究仍处于相对早期阶段 [49] 。如乳腺癌细胞中PRMT1过表达使肿瘤细胞更易于生存和侵入 [50] 。PRMT1和MLL1形成复合物并一定程度上导致了白血病的发生 [51] 。这些研究表明,组蛋白甲基化水平和甲基化酶的活性受肿瘤代谢的影响,又反过来对肿瘤细胞生理代谢具有不可忽视的作用,提示其作为相关药物靶点的价值。
与甲基转移酶相对应,去甲基化转移酶在肿瘤发生中也具有重要作用。组蛋白赖氨酸的去甲基化由组蛋白赖氨酸去甲基化酶介导,包含α酮戊二酸依赖性JmjC结构域蛋白家族和黄素腺嘌呤二核苷酸依赖性LSD [52] 。JmjC结构域蛋白家族去甲基化酶的调控与TET类似,可以被α酮戊二酸激活,并被2-羟基戊二酸酯、琥珀酸和延胡索酸抑制,能够介导三甲基化赖氨酸甲基化消除。LSD是另一类去甲基化酶,与单胺氧化酶同源,由于催化化学机制的限制,只能催化单甲基化和二甲基化赖氨酸的甲基消除。其中LSD1是一个被广泛研究的去甲基化酶,它能够与许多蛋白质直接发生相互作用,参与复合物的调节功能。如LSD1是MLL1复合物的一个亚基,可对H3K4me2和H3K9me2进行动态可逆催化,相关位点甲基化水平的变化可影响血液细胞的分化和血液肿瘤的生长 [53] 。
2.2组蛋白乙酰化与肿瘤代谢
组蛋白乙酰化主要发生在组蛋白尾部的赖氨酸残基上。组蛋白乙酰基转移酶和组蛋白去乙酰化酶共同调控了乙酰化修饰的建立与去除,两者的功能异常与疾病的发生密切相关 [54] 。一般而言,组蛋白的乙酰化可改变局部染色质的电荷性质及微环境,使得染色质结构开放,利于转录因子结合,促进基因转录,而去乙酰化则抑制转录 [55] 。相关研究表明,肿瘤细胞内的组蛋白乙酰化水平往往较高,这与其旺盛的转录活动相适应。组蛋白乙酰化水平主要受乙酰辅酶A含量的调控。ACL可以将柠檬酸转化为乙酰辅酶A,在肿瘤细胞中旺盛的糖酵解增强了底物柠檬酸的供给,同时ACL的表达和活性均上调,这使得乙酰辅酶A合成增加,组蛋白乙酰化水平升高 [56] 。C-MYC也可以通过上调组蛋白乙酰基转移酶正调控组蛋白乙酰化 [57] 。
研究发现,在实体瘤和血液系统疾病患者的细胞中,组蛋白乙酰基转移酶的基因有遗传突变,如p300/CBP基因失活、突变等 [58] 。但肿瘤的发生是否直接来源于组蛋白乙酰基转移酶突变从而导致细胞内异常的乙酰化水平还莫衷一是。动物实验结果表明,突变的乙酰基转移酶基因会导致白血病发生,但这一观点仍需要更多的证据支持。最近的研究结果揭示,组蛋白乙酰基转移酶还能够乙酰化C-MYC、P53、PTEN等非组蛋白等,说明组蛋白乙酰基转移酶发挥活性的分子机制对肿瘤有直接的影响 [59] 。
人体中共有两大类组蛋白去乙酰化酶,其中锌离子依赖的HDAC表达在不同的肿瘤细胞中被检测到的差异很大。如在前列腺癌和胃癌中HDAC1高表达,在结肠癌、宫颈癌和胃癌中HDAC2高表达 [60] 。尽管不同亚型的HDAC在不同肿瘤中发挥的功能不尽相同,但这些蛋白的异常表达都可能在某种程度上促进肿瘤细胞的增殖和生存。在某种意义上,HDAC也可作为相关药物的靶点。Sirtuins成员构成另一类组蛋白或非组蛋白(尤其是线粒体蛋白)去乙酰化酶家族,其酶活性的发挥依赖于代谢产物NAD +,参与抗衰老途径并与许多疾病尤其是肿瘤的发生有关。如SIRT6可以直接与HIF-1α和C-MYC相互作用,并通过组蛋白去乙酰化作用抑制其转录,在结肠癌、胰腺癌和肝癌中均检测到了SIRT6缺失 [61] ;SIRT7对H3K18Ac去甲基化作用具有选择性催化活性,也可以直接抑制C-MYC [62] ;与SIRT6/7相反,SIRT2通过间接稳定C-MYC参与肿瘤代谢调控,其对H4K16Ac去甲基化作用有催化活性,导致泛素蛋白连接酶NEDD4表达受到抑制,NEDD4可以通过泛素化途径负调控C-MYC,而SIRT2本身在肿瘤细胞系中被MYC上调,构成一个正反馈循环,通过MYC依赖性转录正调控糖酵解和谷氨酰胺代谢,促进肿瘤发生 [63] 。
2.3组蛋白非乙酰酰基化与肿瘤代谢
近年来随着研究的深入和质谱技术的发展,组蛋白非乙酰的酰基化修饰被鉴定出来,包括但不限于组蛋白的甲酰化、丙酰化、丁酰化、巴豆酰化、2-羟基异丁酰化、琥珀酰化、苯甲酰化、β-羟基丁酰化、豆蔻酰化等 [ 64- 67] 。这些新型修饰的发现丰富了组蛋白修饰的内容,说明组蛋白翻译后修饰的调控层次更加丰富多样,有望为更深入地探究组蛋白修饰的生物学意义提供新的研究资料。但新型组蛋白酰基化在前期研究鉴定过程中因一些酰基化类型的结构较为相似,在质谱上可能区分度不高。其解决方法一是质谱样品准备时采用特异性抗体进行富集,二是对质谱数据核心算法进行优化处理。经过多轮探索和深入研究,多种新型酰基化修饰才被鉴定出来。
组蛋白翻译后修饰的本质都是对氨基酸进行化学改变,而酰基化修饰一般发生在组蛋白尾部的赖氨酸上,其建立和去除是一个动态调节的过程,依赖于细胞内的代谢环境和执行催化反应的酶等相应蛋白表达。针对非乙酰化酰基化特异性的酰基催化酶并没有鉴定出来,但之前鉴定的乙酰基转移酶具有催化其他酰基酰化的能力,包括GNAT、MYST(Moz-Ybfz/Sas3-Sas2-Tipbo)乙酰基转移酶家族、p300/CBP家族等 [ 68- 69] 。其中,p300是目前关注最多的酰基转移酶,它除了可以催化乙酰化外,还可以催化组蛋白的丙酰化、丁酰化、巴豆酰化、羟基丁酰化、琥珀酰化和戊二酰化 [ 70- 72] ;GNAT和MYST家族成员却对非乙酰类的酰基催化具有更严苛的条件,如乙酰化酶氨合成通用控制蛋白5、p300/CBP相关因子、赖氨酸乙酰基转移酶5可以在体外催化丙酰化的完成,通用控制蛋白5和p300/CBP相关因子可以在体外催化丁酰化,但酶的催化效率较乙酰化降低 [ 73- 75] 。
各种组蛋白酰基化的供体为酰基辅酶A,酰基转移酶似乎对于各种酰基化辅酶A并没有选择性,因此推测组蛋白酰基化的类型和丰富程度取决于周围环境中酰基辅酶A的相对浓度,各种不同比例酰基辅酶A则存在相应的竞争关系。如上所述,在多细胞生物中,乙酰辅酶A主要是三羧酸循环中伴随ATP生成过程产生的,同时乙酰辅酶A是组蛋白酰基的主要供体。当 ACL基因被敲除后,组蛋白乙酰化水平降低,巴豆酰化水平增高 [72] ,与此同时,组蛋白巴豆酰化水平提高可作为一个信号,提示细胞在代谢过程中增加乙酰辅酶A的产出,以维持细胞正常的生理代谢过程。这不仅说明酰基辅酶A之间可能存在潜在的竞争关系,也说明细胞能根据周边动态变化的营养环境,动态运用组成相似或相近的化学分子来影响相关基因表达调控,从而维持细胞生存的稳态。
细胞中非乙酰化酰基辅酶A的浓度很大程度上取决于细胞中脂肪酸代谢的活跃程度。如在细胞培养基中加入标记的短链脂肪酸,后续可以在组蛋白修饰上发现标记的放射性,说明短链的脂肪酸可以转变成为非乙酰酰基辅酶A,从而对组蛋白进行酰基化修饰 [ 67, 76] 。ACSS2是产生巴豆酰辅酶A重要的酶,敲除 ACSS2会导致组蛋白巴豆酰化水平下降 [77] 。有趣的是,在众多肿瘤中,ACSS2表达量大幅度上升,与肿瘤的发生程度呈正相关 [ 78- 79] 。但ACSS2表达量的变化对于非乙酰酰化组蛋白的影响还没有彻底揭示出来。且ACSS2的活性可被乙酰化抑制、被SIRT1介导的去乙酰化过程激活 [80] ,说明细胞对于ACSS2的调控是复杂、多层次的。
与乙酰化类似,非乙酰酰基化修饰的去除主要由HDAC和Sirtuins两类擦除体完成。Sirtuins家族有7个成员,具有广泛的去酰基化修饰的能力,然而各个成员对于不同碳链长度的非乙酰化酰基化修饰去除能力则不尽相同。如SIRT3可以去除巴豆酰化和β-羟基丁酰化修饰,SIRT5可以催化去除琥珀酰化、戊二酰化和丙二酰化,但其酶活性都很低 [ 81- 84] 。相较于Sirtuins家族,对于HDAC 家族去除蛋白酰基化尤其是非乙酰酰基化的能力及特异性知之甚少,其作用的底物范围、位点特异性和酶动力学参数等均有待深入研究。
近来研究表明,组蛋白巴豆酰化修饰不仅是细胞对于周围营养变化的一种应急反应机制,同时也是一种良好的肿瘤标志物。如巴豆酰化的表达水平在肝癌、胃癌和肾癌中下调,而在甲状腺癌、食管癌、结肠癌、胰腺癌和肺癌中上调,这表 明巴豆酰化在不同肿瘤中可能表现出不同的特性 [ 85- 89] 。而巴豆酰化水平的变化则很可能与各种肿瘤细胞的代谢途径和代谢水平变化有关,关于这一过程的具体调控机制研究仍不充分。
3mRNA的m6A修饰与肿瘤代谢
与DNA修饰和组蛋白修饰类似,近年来RNA的各种修饰及其功能引起了研究者们的广泛关注,以mRNA修饰为代表的表观转录组研究成为了广义表观遗传学领域又一个新兴热点。在各类RNA修饰中,甲基化是最主要的修饰形式,m6A和5-甲基胞嘧啶是其中具有代表性的两种修饰类型,尤其以丰度最高的m6A研究最为深入。m6A修饰发现较早,但其催化酶和识别结合蛋白的发现和鉴定则是近些年的进展。其中,METTL3、METTL4和WATP复合体可以催化m6A形成 [ 90- 91] ,而FTO和ALKBH5可以去除甲基化 [ 92- 93] ,m6A修饰的结合识别蛋白一般包含YTH结构域。m6A和m6A结合蛋白的发现证明了这种修饰类型是可逆的,细胞根据其所处的发育时期、生理状态的不同完成动态的调控过程。
FTO催化m6A去甲基化的过程要经历复杂的中间反应步骤,而ALKBH5可直接催化m6A为腺嘌呤。研究表明,ALKBH5倾向于结合特异的m6A修饰的单链RNA,从而催化m6A去甲基化。此外,识别结合这种修饰的蛋白主要是含有YTH结构域的蛋白家族,包括YTHDF1、YTHDF2、YTHDF3、YTHDC1和YTHDC2等 [ 94- 97] 。代谢中间产物对RNA m6A修饰的调控与DNA甲基化和去甲基化修饰基本相同,如SAM/SAH比值影响m6A修饰,2-羟基戊二酸酯、琥珀酸和延胡索酸抑制FTO的m6A去甲基化作用。
最近研究发现,FTO在急性髓细胞白血病中高表达,锚蛋白重复序列、细胞信号抑制因子盒蛋白2和视黄酸受体α基因抑制白血病细胞生长或介导药物应答的基因在急性髓细胞白血病样本中受到抑制,并且这些基因表达的mRNA受到FTO m6A去甲基化酶的调节 [98] 。而ALKBH5能够维持恶性胶质瘤细胞的干性,在恶性胶质瘤细胞中,ALKBH5通过lncRNA FOXM1-AS介导识别并催化转录因子FOXM1上m6A修饰的去甲基化,从而使其处于较高的表达水平 [99] 。此外,YTHDC2在多种肿瘤细胞系中的表达显著高于正常的干细胞 [100] ,这表明m6A的水平变化是细胞生理状态的一个重要指标,失调的m6A信号会影响细胞的生理代谢进程,导致正常的生理过程发生紊乱。之前的研究表明,异柠檬酸脱氢酶在神经胶质瘤和白血病中有较高的突变率从而使得α酮戊二酸转变为致癌的代谢物2-羟基戊二酸酯,导致DNA和组蛋白的高度甲基化,从而诱发肿瘤 [86] 。最新的研究显示,虽然2-羟基戊二酸酯的累积会导致肿瘤,但同时也上调了 MYC、 CEPBPA等基因的mRNA的m6A修饰,后者被YTHDF2识别后降解,抑制肿瘤的葡萄糖和氨基酸代谢 [101] 。
mRNA的m6A修饰可以在转录后水平微调基因表达,促进翻译并缓冲miRNA的抑制作用,进而通过信号通路参与肿瘤细胞代谢调控。在一些类型的白血病中,METTL3可以增强BCL2和PTEN的m6A修饰和表达水平,影响糖代谢并产生致癌作用 [102] 。在子宫内膜癌中METTL3下调和METTL4突变导致了AKT的激活,进而增强有氧糖酵解和细胞增殖速度 [103] 。非编码RNA的m6A修饰也可以影响其对基因表达的调控。METTL3可以增强原始miRNA的m6A修饰,并加速其成熟 [104] 。m6A修饰的circRNA具有蛋白编码能力,单个m6A修饰位点就可以推进翻译的开始 [105] 。大量lncRNA具有m6A修饰,这些修饰可以影响lncRNA和RNA结合蛋白、特定DNA位点的相互作用。lncRNA MALAT1具有一系列的m6A修饰位点,这些修饰可以改变它的结构、细胞定位以及与一些蛋白的亲和力,影响有氧糖酵解的调控 [106] 。
4非编码RNA与肿瘤代谢
非编码RNA是指不编码蛋白质序列,不能通过翻译生成蛋白质的RNA,它们在生命活动中也有着重要的功能。其中直接参与细胞生命活动的称为“管家”非编码RNA,主要包括核糖体RNA、转运RNA等;而参与基因表达调控的称为调控性非编码RNA,主要包括miRNA、lncRNA、circRNA等 [107] 。相对于癌旁组织,肿瘤中存在着差异表达的非编码RNA,它们参与调节肿瘤细胞的代谢。与其他表观遗传机制相似,非编码RNA可以调控关键癌基因、抑癌基因的表达,进而通过肿瘤中的信号通路调控细胞代谢,非编码RNA也可以直接调控代谢酶和转运蛋白的表达。
4.1miRNA与肿瘤代谢
miRNA是一类单链非编码小RNA,由内源基因编码,长度约为22bp,可以靶向mRNA并进行碱基配对,导致转录后抑制 [108] 。
在葡萄糖代谢中,miRNA参与调控葡萄糖转运蛋白以及糖酵解关键酶的表达。GLUT1、GLUT3 和GLUT4在多种肿瘤中过表达 [109] 。乳腺癌研究证明,miR-22可以直接靶向GLUT1 [110] ;而肾细胞癌样本miR-1291相比周围正常组织表达降低,进而导致GLUT1过表达 [111] ;GLUT3对葡萄糖的亲和力和转运能力都较高,在膀胱癌T24细胞系中,miR-195-5p可以通过调节GLUT3表达来抑制葡萄糖摄取和细胞增殖 [112] 。在糖酵解反应中,miR-143、miR-497可以直接靶向己糖激酶 [ 113- 114] ,miR-122可以直接靶向果糖1,6二磷酸醛缩酶 [115] ,miR-1256 可以直接靶向磷酸甘油酸激酶1 [116] ,miR-122、miR-326可以抑制PKM2的表达 [ 117- 118] ,miR-124靶向PKM的剪接体多聚嘧啶串结合蛋白1以诱导PKM亚型表达从PKM2转换为PKM1 [119] 。这些能够负调控糖酵解酶表达的miRNA作为抑癌因子在多种人类肿瘤组织中表达降低 [85] 。糖酵解途径生成的丙酮酸需要通过乳酸脱氢酶转化为乳酸并生成NAD +,以满足旺盛的能量和物质代谢需求。有研究表明,miR-34a、miR-34c、miR-369-3p和miR-374a具有靶向乳酸脱氢酶的潜力 [120] 。此外,miR-186和miR-145可以抑制HIF-1α [ 121- 122] ,miR-145和miR-34a可以在多种肿瘤中抑制C-MYC [ 123- 124] ,通过肿瘤信号通路参与调节葡萄糖代谢。
在脂质代谢中,miRNA参与了脂肪酸氧化、三酰甘油和胆固醇代谢的调控。肝脏是人体脂肪代谢的重要场所,食物中的三酰甘油和胆固醇酯通过乳糜微粒运输至肝细胞后,可在肝细胞中被水解。miR-122可在肝脏中调节脂肪和胆固醇代谢 [125] ,并在肝细胞癌中表达下调 [126] 。在体外和细胞实验中,miR-122可使肝细胞脂肪氧化增加,胆固醇和脂肪酸合成速率降低,AMP/ADP水平上升,AMPK激活 [127] 。在小鼠中敲除 miR-122a会出现脂肪性肝炎、肝硬化、肝细胞癌等病变 [125] 。ACSL1催化细胞中脂肪酸代谢的第一步反应,使其活化并参与三酰甘油的合成。miR-205可以直接靶向ACSL1,下调其表达以抑制三酰甘油合成 [128] 。肝癌中miR-205表达下调,使得肿瘤细胞脂质代谢异常 [129] 。
在肿瘤氨基酸代谢调控中,miR-23家族和 C-MYC发挥了核心作用。在谷氨酰胺分解代谢中,谷氨酰胺可在谷氨酰胺酶催化下脱氨基生成谷氨酸。类似地,在脯氨酸分解代谢中,脯氨酸也可以在脯氨酸氧化酶/脱氢酶的作用下转化为谷氨酸 [130] 。谷氨酸可进一步脱氨基生成α酮戊二 酸进入三羧酸循环,为细胞供能。在多种肿瘤中, C-MYC可通过下调miR-23a和miR-23b来增强两种miRNA直接靶向的谷氨酰胺酶表达,促进谷氨酰胺分解生成谷氨酸,进而为细胞供能 [131] ;同时C-MYC可通过上调miR-23b*抑制脯氨酸氧化酶/脱氢酶表达,进而抑制脯氨酸的分解代谢 [132] 。
4.2circRNA与肿瘤代谢
circRNA是由mRNA内含子或外显子环化形成的共价闭合单链RNA,在真核细胞中广泛存在 [133] 。circRNA通常不编码蛋白质,但可以通过多种机制调控基因表达。它可以与mRNA竞争性结合miRNA,减弱miRNA对靶基因表达的抑制,这一过程被称为miRNA海绵作用,是circRNA发挥生物学功能最主要的机制 [134] 。circRNA还可与lncRNA或RNA结合蛋白相互作用,参与转录的调节。
在糖酵解中,circRNA可以直接调节转运蛋白和代谢酶,或与信号通路和转录因子相互作用来参与调控有氧糖酵解。胰腺中circHIPK3含量很高,可以降低GLUT2的表达 [135] ,还可以通过对 miR-124的海绵作用诱导瓦博格效应 [136] 。在乳腺癌中HIF-1α可以上调circDENND4C并促进细胞增殖,但与有氧糖酵解间因果关系尚不明确 [137] 。在细胞实验中,缺氧条件下circ_0010729可以通过对过表达的miR-186的海绵作用解除其对靶向HIF-1α表达的抑制,增强糖酵解 [138] 。在多种肿瘤中,circRNA_001569和circBIRC6可以对miR-145产生海绵作用,circBIRC6还可以对miR-34a 产生海绵作用,解除两种miRNA对C-MYC的抑制,增强糖酵解 [ 139- 140] 。
然而,虽然有很多证据提示circRNA参与了氧化磷酸化、脂肪代谢和氨基酸代谢的调控,这一调控在人类肿瘤体系中的实验证据并不充分。
4.3lncRNA与肿瘤代谢
lncRNA是一类由RNA聚合酶Ⅱ转录生成的、不可翻译生成蛋白质的、长度在200个核苷酸以上的转录物。lncRNA可以根据表达来源分为六种类型:基因间lncRNA(位于两个可编码转录本之间)、双向lncRNA(蛋白质编码基因的启动子反向转录的产物)、有义/反义RNA(与蛋白质编码基因外显子或其反义链同源的片段)、内含子lncRNA(与蛋白质编码基因的内含子同源)、增强子RNA(增强子的转录本)。lncRNA可以通过多种可能的机制参与基因表达调控:指南作用(与转录调节因子结合,引导其靶向目标序列)、支架作用(在核糖核蛋白复合物的形成过程中充当支架)、诱饵作用(与转录调节因子结合,阻止其与靶基因的相互作用)、miRNA海绵、增强子RNA充当反式激活RNA [141] 。
lncRNA在葡萄糖代谢中参与调控转运体蛋白和代谢酶的表达,并通过LKB1/AMPK、HIF等信号通路调节有氧糖酵解。多种lncRNA在肿瘤中调控GLUT家族的表达和亚细胞定位。在鼻咽癌中反义RNA ANRIL表达升高,并正向调控GLUT1的表达,增加葡萄糖摄取 [142] 。在肝细胞癌中HOX转录反义RNA可以诱导GLUT1并激活mTOR信号通路,增强糖酵解 [143] 。lncRNA MACC1-AS1在胃癌中高表达,可通过增加GLUT1在细胞膜上的分布来增加葡萄糖摄取 [144] 。lncRNA还可以正向调控己糖激酶2、PKM2、丙酮酸羧化酶等代谢酶,从而增强糖酵解。lncRNA PVT1在多种肿瘤中过表达,其可通过调节miR-497/己糖激酶2轴来增强葡萄糖代谢,进而产生致癌作用 [114] 。在肝细胞癌中,lncRNA H19可由miR-675通过激活早期生长反应蛋白1途径诱导而表达,进而提高下游PKM2的表达 [145] 。丙酮酸羧化酶参与调节多种代谢过程,包括葡萄糖代谢、糖异生和从头脂肪酸合成。lncRNA GCASPC可以与丙酮酸羧化酶相互作用,降低丙酮酸羧化酶的蛋白质稳定性。在胆囊癌中,GCASPC下调,并与肿瘤进展相关 [146] 。
LKB1是AMPK的上游调节剂, 可以磷酸化并激活AMPK,进而抑制mTOR的活性,调节细胞增殖和能量代谢。敲低 LKB1可促进肿瘤生长、葡萄糖摄取、ATP生产和大分子合成 [147] 。LINC00473在人非小细胞肺癌中上调,其表达量与LKB1的活性呈负相关 [148] 。lncRNA NBR2可在代谢应激下由LKB1/AMPK信号传导途径诱导表达。而NBR2可增强AMPK的活性,形成正反馈调节,降低糖酵解,发挥抑癌作用 [149] 。
多种lncRNA参与了HIF-1α对有氧糖酵解的调控。在缺氧条件下,lincRNA-p21被HIF-1α上调,并反过来增强HIF-1α的蛋白质稳定性,建立了正反馈调节 [150] 。在正常肝细胞系L02中,亚砷酸盐处理可上调葡萄糖转运蛋白、糖酵解相关代谢酶和lncRNA MALAT1的表达,其中MALAT1可以诱导HIF-1α与VHL解离,从而阻止VHL介导的HIF-1α泛素化,减少HIF-1α清除 [151] 。低氧条件可以诱导lncRNA H19表达,其参与了低氧诱导的癌细胞信号转导过程,从而调节葡萄糖的代谢 [152] 。LINK-A是一种在三阴性乳腺癌细胞的细胞质中表达的lncRNA,可诱导乳腺肿瘤激酶募集到表皮生长因子-糖蛋白nmb复合物中,并增强其活性。乳腺肿瘤激酶可在酪氨酸565处磷酸化HIF-1α,然后通过干扰脯氨酸564的羟基化来稳定HIF-1α [153] 。linc-ROR在低氧条件下表达,并通过对miR-145的分子海绵作用调节HIF-1α表达 [122] 。
细胞中p53缺失或突变可导致线粒体呼吸损伤并增加糖酵解。p53可调控葡萄糖转运蛋白(如GLUT1和GLUT4)以及一些糖酵解基因,包括磷酸甘油酸突变酶、6-磷酸果糖激酶1等的表达 [154] 。几种lncRNA直接或间接受p53调控。突变体p53(N340Q/L344R)可以通过lncRNA CUDR上调PKM2表达并促进肝癌的发展 [155] 。lncRNA母体表达基因3的过表达可诱导p53表达和p53下游靶基因的激活 [156] 。p53的反义转录物(wrap53)通过靶向p53的5′非编码区来调节p53的表达 [157] 。敲除成纤维细胞中的MALAT1刺激DNA损伤修复,可以导致p53及其下游靶基因激活 [158] 。DNA受损后,linc-ROR可以作为p53的阻遏物 [159] 。
5结语
表观遗传修饰的动态变化和代谢组之间的交叉会话是细胞适应外界营养条件变化和自身生命维持的一个重要方面。细胞中表观遗传修饰的建立依赖于代谢过程中产生的重要的中间产物,其修饰类型的变化作为响应细胞代谢变化的调控结果。另外,表观遗传修饰或者代谢通路中蛋白质的突变会导致某些重要基因表达的失调,反过来有助于肿瘤的发生。
在肿瘤细胞中,表观遗传修饰和细胞的代谢是高度交织的。一方面,癌化细胞中基因表达网络驱动的生理代谢过程通过影响DNA和组蛋白修饰酶等表观调控蛋白重塑表观遗传景观,使得肿瘤细胞具有独特的表观遗传信息谱。另一方面,表观遗传机制的改变导致了与细胞生理代谢相关基因表达的变化,从而影响整个代谢组的进程。这说明表观遗传修饰和代谢组之间相互交织的关系在肿瘤的发生和发展中扮演非常重要的角色,特别是在肿瘤细胞所显现出的无限增殖、多能性等方面。深刻理解表观遗传修饰与代谢组学之间的关系,有助于新分子靶标的发现,对开发针对性的药物具有极大的现实意义。
目前很多研究结果是基于细胞培养系统,在某种程度上很难代表细胞在体内的生长环境,而细胞的生长环境对于细胞代谢具有重要意义。未来的研究要更集中于肿瘤细胞微环境对于细胞代谢、表观遗传修饰的作用等方面,以更接近细胞在正常生理条件下的状态,这对于发现新的或更有效的分子靶标具有重要意义。
对于与表观遗传因素紧密相关肿瘤的治疗,一方面对于还不清楚致病机制的,需要进一步研究导致肿瘤发生的关键性因子,明确其所扮演的角色,有针对性地设计特异性药物,对相关靶点进行有目的性的治疗;另一方面,针对已经明确致病的表观遗传因子,可以在已有药物的基础上发展并完善相关治疗策略,特别是综合运用生物化学、免疫学等手段,更有针对性地治疗肿瘤。
Funding Statement
国家重点研发计划(2020YFA0803300)
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