摘要
转录因子c-Myc广泛参与了正常细胞的增殖、分化和代谢等关键进程,在大多数肿瘤中,MYC原癌基因异常激活,过量表达的c-Myc蛋白可以直接调控关键代谢酶的表达,或通过抑制microRNA间接调控肿瘤相关的代谢通路,表现出营养吸收增加、糖酵解和谷氨酰胺代谢增强、脂肪酸和核苷酸合成增加等代谢失调特征。本文从c-Myc蛋白调控肿瘤糖酵解、谷氨酰胺代谢、三羧酸循环、脂质代谢及核苷酸代谢等研究进展进行综述,为研发靶向c-Myc的抗肿瘤靶点及药物提供理论参考。
致癌转录因子c-Myc(也称Myc)可直接结合基因启动子区的E-box调控多种基因的表达,通过调节多达15%的人类基因参与细胞周期以及细胞生长、凋亡、分化和代谢等关键进
大量研究表明,肿瘤发病风险增加与代谢异常密切相关,恶性肿瘤可发生代谢重编
c-Myc转录因子由MYC基因编码,该基因属于MYC原癌基因家族(除MYC基因外还包括编码N-Myc的MYCN基因和编码L-Myc的MYCL基因)。该转录因子可与另一种螺旋-环-螺旋亮氨酸拉链蛋白MAX进行二聚化,从而结合DNA并调控基因表

图1 c-Myc蛋白相关结构
A:c-Myc的bHLH-LZ结构域和MAX组成的异源二聚体与DNA的E-box形成复合物的晶体结构(Protein Data Bank,PDB code:1NKP);B:c- Myc蛋白结构域MAX: Myc相关因子X; MBⅠ、MBⅡ、MBⅢ、MBⅣ: c-Myc蛋白高度保守的4个Myc box(MB)结构域; PEST: c-Myc蛋白富含脯氨酸(P)、谷氨酸(E)、丝氨酸(S)和苏氨酸(T)的肽序列; bHLH-LZ: c-Myc蛋白无序的bHLH-LZ结构域,与MAX蛋白结合可形成螺旋结构
c-Myc作为一种转录因子,激活或抑制参与细胞过程(包括转录、翻译、染色质修饰和蛋白质降解等)的基因,在维持细胞稳态中发挥了关键作用。近年研究表明,c-Myc蛋白可与几乎所有的活性启动子和大多增强子结合,从而调控细胞生长进程中关键基因的表
代谢重编程是恶性肿瘤的一个标志,异常的代谢变化在肿瘤的发生发展过程中发挥重要作用。早在1920年研究发现肿瘤细胞存在能量代谢异常,表现为氧气充足的条件下糖酵解过度活跃、葡萄糖摄取率高、无氧代谢产物乳酸含量高,被称为有氧糖酵解或“瓦伯格效应(Warburg effect)
近年研究发现,乙肝相关肝癌与癌旁组织比较的特异性代谢差异,包括糖酵解、脂代谢和谷氨酰胺代谢途径的异常增
瓦伯格效应中,肿瘤细胞摄取大量的葡萄糖进行有氧糖酵解,以满足细胞快速生长和侵袭转移对ATP的高需

图2 c-Myc参与调控肿瘤糖酵解、谷氨酰胺代谢、三羧酸循环、脂质代谢、己糖胺及核苷酸生物合成等代谢进程
在肿瘤细胞糖酵解通路中,丙酮酸激酶的米氏常数较高,其与底物磷酸烯醇式丙酮酸具有较低的亲和力,导致糖酵解中间体6-磷酸葡萄糖(glucose-6-phosphate)和3-磷酸甘油酸(3-phosphoglycerate)的浓度升高,从而引发两条生物合成分支:戊糖磷酸途径(pentose phosphate pathway,PPP)和丝氨酸合成途径(serine synthesis pathway,SSP
戊糖磷酸途径(PPP)包括两个阶段:第一阶段利用6-磷酸葡萄糖氧化生成5-磷酸核酮糖,可用于核苷酸生物合成;第二阶段中5-磷酸核酮糖经过一系列转酮基及转醛基反应,最后生成3-磷酸甘油醛及6-磷酸果糖,二者还可重新进入糖酵解途径进行代
丝氨酸合成途径(SSP)是糖酵解通路的第2个重要分支,通常在肿瘤细胞中上调。糖酵解途径中的3-磷酸甘油酸经磷酸甘油酸脱氢酶催化,再途经中间产物后合成丝氨酸,在丝氨酸羟甲基转移酶(serine hydroxymethyltransferase,SHMT)催化下转化为甘氨酸,从而为一碳代谢(one-carbon metabolism)提供中间体,一碳代谢可以将氨基酸代谢与核苷酸及一些重要物质的生物合成联系起
谷氨酰胺是一种带有胺基基团的人体非必需氨基酸,在血液中含量最为丰富,几乎参与增殖细胞中的每个生物合成途径。在恶性肿瘤中,谷氨酰胺的一个重要作用是在谷氨酰胺酶(glutaminase,GLS)的催化下水解生成谷氨酸,通过谷氨酸脱氢酶(glutamate dehydrogenase,GDH)催化的直接脱氨作用以α-酮戊二酸(α-ketoglutarate,α-KG)的形式进入三羧酸循环,为细胞的恶性增殖提供能量,这种情况称之为“谷氨酰胺成瘾
由谷氨酰胺降解产生的谷氨酸除了以α-KG的形式进入三羧酸循环外,还可以在转氨酶的作用下,催化氨基转移到草酰乙酸或丙酮酸上,同时产生非必需氨基酸天冬氨酸和丙氨酸。在c-Myc失调引发的肾癌中可以检测到天冬氨酸和丙氨酸水平的升
值得注意的是,不同肿瘤的谷氨酰胺代谢特征具有显著的异质性。例如在肝癌等依赖于谷氨酰胺代谢的肿瘤细胞中,外源性的谷氨酰胺的缺乏会导致细胞的死
TCA循环是糖酵解、谷氨酰胺代谢及脂质代谢等重要代谢通路的枢纽,能够产生用于生物大分子合成的前体物质,维持细胞增殖的需求。TCA循环中的主要代谢物乙酰辅酶A(acetyl-CoA)是脂质代谢中的重要前体成分。随着脂质组学技术的发展。有研究表明,从头合成脂质可以为增殖的肿瘤细胞提供合成子细胞质膜和细胞器膜的磷脂成分从而为肿瘤细胞的恶性增殖创造条
细胞脂质代谢的重编程有助于肿瘤细胞的恶性转化和进
在HBP通路中,葡萄糖和谷氨酰胺可通过代谢产生乙酰氨基葡萄糖尿苷二磷酸(uridine 5'-diphosphate-N-acetylglu-cosamine,UDP-GlcNAc),经O-GlcNAc糖基转移酶(O-GlcNAc transferase,OGT)催化完成蛋白质丝氨酸或苏氨酸残基上的O-GlcNAc修
GLUT1:葡萄糖转运体1; HK2:己糖激酶2; PFK1:磷酸果糖激酶1; ENO1:烯醇化酶1; PK:丙酮酸激酶; LDHA:乳酸脱氢酶A; NAD:烟酰胺腺嘌呤二核苷酸; G6PD:葡萄糖-6-磷酸脱氢酶; TKT:转酮醇酶; OGT: O-GlcNAc糖基转移酶; SHMT:丝氨酸羟甲基转移酶; CPT:肉毒碱棕榈酰基转移酶; FAS:脂肪酸合成酶; ACC:乙酰辅酶A羧化酶; ACLY: ATP柠檬酸裂解酶; SDHA:琥珀酸脱氢酶复合体亚基A; GDH:谷氨酸脱氢酶; GLS:谷氨酰胺酶; GS:谷氨酰胺合成酶; P5CS: 1-吡咯啉-5-羧酸合成酶; PYCR:1-吡咯啉-5-羧酸还原酶; SLC1A5:溶质载体家族蛋白; PRPS2:磷酸核糖焦磷酸合成酶2; PPP:戊糖磷酸途径; HBP:己糖胺生物合成途径; SSP:丝氨酸合成途径
核苷酸是生物体内核酸的重要组成,参与了多种代谢和调节活动。在无限增殖的肿瘤细胞中,转录和复制活动更加频繁,核苷酸的生物合成也随之增强;c-Myc作为转录因子,在核苷酸生物合成方面同样具有协调促进作用。研究表
在正常细胞生长和分裂过程中,代谢是精确协调的过程,包含了许多种代谢反应以及代谢物,在体内与微环境相互作用形成功能性代谢网络,而代谢失调往往与多种肿瘤的发生发展有关。近年来,越来越多的科研工作者开始关注肿瘤中异常代谢的特征,发掘肿瘤发生发展过程中与代谢依赖性密切相关的靶点蛋白。c-Myc在正常细胞中受到严格调控,以响应营养供应和代谢应激,维持细胞的正常生理功能,一旦其编码基因发生突变,c-Myc蛋白持续积累,将促进包括肿瘤特异性基因在内的多种基因表达,参与肿瘤的发生与发展。致癌水平的c-Myc蛋白可与几乎所有的活性启动子结合,驱动葡萄糖代谢、谷氨酰胺代谢、脂肪酸合成、核苷酸合成及O-GlcNAc修饰等进程。大量研究证实c-Myc蛋白可以通过调控代谢通路中的关键酶,如G6PD、SHMT、FAS和GLS等,广泛参与到肿瘤细胞的代谢调控中。从此角度出发,可以考虑通过调节机体的c-Myc相关代谢酶水平来调控机体的异常代谢,改善机体的肿瘤微环境从而达到抑制肿瘤细胞生长的效果。此外,c-Myc本身是否在不同时间及不同的肿瘤内选择性激活也将是一个重要的探索领域。因此,深度挖掘c-Myc介导的肿瘤代谢改变中的激活和调控作用将为肿瘤的治疗及新型抗肿瘤药物的开发提供思路。

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