張黎明，朱彩霞，蔡啟良

復(fù)旦大學(xué)基礎(chǔ)醫(yī)學(xué)院教育部/衛(wèi)生部醫(yī)學(xué)分子病毒學(xué)重點(diǎn)實(shí)驗(yàn)室，上海 200032

隨著腫瘤病因?qū)W的研究不斷深入，人們發(fā)現(xiàn)病毒感染與多種腫瘤的發(fā)生和發(fā)展密切相關(guān)。病毒入侵宿主細(xì)胞后常引發(fā)多個(gè)關(guān)鍵細(xì)胞信號(hào)通路的調(diào)控紊亂，其中對(duì)作為腫瘤微環(huán)境細(xì)胞應(yīng)激反應(yīng)關(guān)鍵信號(hào)通路之一的缺氧信號(hào)(hypoxia signaling)的調(diào)控尤為重要。現(xiàn)已證實(shí)，許多常見人類腫瘤病毒編碼的毒蛋白可篡改宿主細(xì)胞缺氧信號(hào)通路，從而促進(jìn)細(xì)胞生長(zhǎng)、抗凋亡，以及病毒自身的復(fù)制與繁殖。本文就各種常見腫瘤病毒如何通過(guò)調(diào)控缺氧誘導(dǎo)因子(hypoxia-inducible factor，HIF)信號(hào)通路而應(yīng)對(duì)缺氧微環(huán)境誘發(fā)腫瘤發(fā)生發(fā)展分子機(jī)制的最新研究進(jìn)展進(jìn)行綜述。

1 缺氧信號(hào)通路

缺氧信號(hào)可分為缺氧微環(huán)境應(yīng)激及其效應(yīng)蛋白HIF信號(hào)，兩者在致病過(guò)程中相輔相成、密不可分。腫瘤細(xì)胞的快速生長(zhǎng)需新生血管以提供足夠氧氣。然而，新生血管的有效覆蓋往往遠(yuǎn)低于氧氣擴(kuò)散所需范圍，導(dǎo)致氧氣供應(yīng)不能滿足腫瘤細(xì)胞生長(zhǎng)和代謝的需要[1]。在缺氧條件下，腫瘤組織及其周邊的免疫細(xì)胞、內(nèi)皮細(xì)胞、上皮細(xì)胞、成纖維細(xì)胞等共同形成腫瘤內(nèi)缺氧微環(huán)境(intratumoral hypoxic microenvironment)[2-5]。細(xì)胞如何感應(yīng)缺氧微環(huán)境、何種信號(hào)通路參與細(xì)胞的缺氧應(yīng)答等一直是科學(xué)家關(guān)注的熱點(diǎn)。目前較清楚的分子機(jī)制是：細(xì)胞通過(guò)HIF及其信號(hào)通路應(yīng)答缺氧環(huán)境 (圖1)[6]。HIF主要以異二聚體形式存在，包括α和β兩個(gè)亞基。其中α亞基存在HIF-1α、HIF-2α和HIF-3α異構(gòu)體。HIF-1α在細(xì)胞中普遍表達(dá)，而HIF-2α和HIF-3α只在某些特定的細(xì)胞類型中表達(dá)[7]。HIF-1由相對(duì)分子質(zhì)量120 000左右的α亞單位(HIF-lα)和91 000～94 000的β亞單位﹝HIF-1β或芳香烴受體核轉(zhuǎn)位蛋白(aryl hydrocarbon receptor nuclear translocator，ARNT)﹞組成，兩者均屬堿性螺旋-環(huán)-螺旋(basic helix-loop-helix, bHLH)轉(zhuǎn)錄因子超家族中的PAS(Per-ARNT-Sim)亞族蛋白，該超家族中每個(gè)成員在其N端均含有與其他亞單位聚合所必需的bHLH-PAS結(jié)構(gòu)域[8]。HIF-1α與HIF-2α很多功能相同，但在某些特定細(xì)胞中功能截然相反[9]，HIF-3α可對(duì)HIF-1α和HIF-2α介導(dǎo)的基因轉(zhuǎn)錄活性起負(fù)調(diào)節(jié)作用[10]。HIF的轉(zhuǎn)錄活性主要是通過(guò)調(diào)節(jié)HIF-α的穩(wěn)定性來(lái)實(shí)現(xiàn)的，其中HIF-1α和HIF-2α已證實(shí)受脯氨酰羥化酶(prolyl hydroxylase，PHD)和HIF-1抑制因子(factor inhibiting HIF-1，F(xiàn)IH)的負(fù)調(diào)節(jié)。在常氧條件下(氧氣量>8%～10%)，PHD可誘發(fā)HIF-1α第402位和564位(HIF-2α第405和531位)的脯氨酸羥基化[7]。該羥基化修飾主要發(fā)生在HIF-1α的氧依賴性降解結(jié)構(gòu)域(oxygen-dependent degradation domain, ODDD)。羥基化修飾后的ODDD可與腫瘤抑制蛋白VHL (von-Hippel-Lindau)特異結(jié)合[6]。HIF-1α與VHL結(jié)合后，VHL通過(guò)募集延伸因子B、延伸因子C、Cul2和Rbx1形成E3泛素化連接酶，并與泛素偶聯(lián)酶(ubiquitin-conjugating enzyme，E2)和泛素激活酶(ubiquitin-activating enzyme，E1)形成蛋白酶復(fù)合體，介導(dǎo)HIF-1α的泛素化修飾，促使HIF-1α降解[11]。同時(shí)，F(xiàn)IH可使HIF-1α的C端反式激活結(jié)構(gòu)域(transactivation domain, TAD)的第803位天冬氨酸殘基羥基化，阻止轉(zhuǎn)錄輔助激活因子cAMP反應(yīng)元件結(jié)合蛋白的結(jié)合蛋白(cAMP response element binding protein binding protein，CBP)/P300與HIF-1α結(jié)合，抑制HIF-1α下游靶基因的轉(zhuǎn)錄激活表達(dá)[12]。相反，在缺氧條件下逃避PHD羥基化修飾的HIF-1α通過(guò)多種癌基因信號(hào)通路 (如Src、Ras、蛋白激酶C等) 介導(dǎo)磷酸化修飾而穩(wěn)定存在， 同時(shí)穩(wěn)定積累的HIF-1α進(jìn)入細(xì)胞核后與靶基因啟動(dòng)子的反式激活缺氧應(yīng)答元件(hypoxia response element，HRE；即5′-RACGTG-3′)結(jié)合，并在ARNT、CBP/P300和DNA聚合酶Ⅱ復(fù)合體等共同作用下激活血管內(nèi)皮生長(zhǎng)因子(vascular endothelial growth factor，VEGF)、葡萄糖轉(zhuǎn)運(yùn)蛋白1(glucose transporter 1，Glut1)等靶基因的轉(zhuǎn)錄表達(dá)[13]。

圖1 不同人類腫瘤病毒及其抗原篡改的缺氧信號(hào)靶點(diǎn)Fig.1 Hypoxia signaling targets deregulated by different oncogenic viruses and their antigens

2 病毒對(duì)HIF信號(hào)通路調(diào)控的分子機(jī)制

越來(lái)越多的研究表明，缺氧應(yīng)激在病毒誘發(fā)腫瘤發(fā)生中發(fā)揮重要作用。多種DNA 和RNA病毒對(duì)宿主細(xì)胞的缺氧信號(hào)及其缺氧微環(huán)境應(yīng)答均有調(diào)控作用，但病毒如何利用這種缺氧微環(huán)境篡改HIF信號(hào)通路以達(dá)到促進(jìn)病毒感染和誘發(fā)腫瘤發(fā)生發(fā)展等問題，目前尚不清楚。以下就已報(bào)道的病毒與缺氧信號(hào)相互調(diào)控的機(jī)制進(jìn)行綜述(圖1)。

2.1 DNA病毒介導(dǎo)HIF信號(hào)通路的紊亂調(diào)節(jié)

與缺氧相關(guān)的人類DNA病毒主要包括小DNA病毒﹝如腺病毒(adenovirus, AdV)、乙型肝炎病毒(hepatitis B virus，HBV)、人乳頭瘤病毒(human papillomavirus，HPV)等﹞和大DNA病毒﹝如單純皰疹病毒(herpes simplex virus，HSV)、人巨細(xì)胞病毒(human cytomegalovirus，HCMV)、EB病毒(Epstein-Barr virus，EBV)、卡波西肉瘤皰疹病毒(Kaposi’s sarcoma-associated herpesvirus，KSHV)等﹞。

2.1.1腺病毒腺病毒感染宿主細(xì)胞后可引起與缺氧應(yīng)激類似的染色體變化，并抵抗細(xì)胞凋亡。其主要機(jī)制是腺病毒感染后表達(dá)的E1A抗原能激活P53，P53作用于下游的P21，抑制細(xì)胞周期蛋白依賴性激酶(cyclin-dependent kinase，CDK)，導(dǎo)致細(xì)胞生長(zhǎng)阻滯；同時(shí)，P53積累使Puma和Noxa上調(diào)，從而誘導(dǎo)細(xì)胞凋亡[14]。但是，腺病毒編碼的E1B55K蛋白能抑制P53，促進(jìn)HIF-1α表達(dá)上調(diào)，并抑制P53介導(dǎo)的細(xì)胞生長(zhǎng)阻滯和凋亡；同時(shí)，E1B19K蛋白能結(jié)合Bak，阻止Bak激活、Bax/Bak復(fù)合體和多聚體形成，并抑制細(xì)胞凋亡，從而有利于腺病毒復(fù)制完成[15]。另一方面，在缺乏Bak時(shí)，Bax可代替Bak誘導(dǎo)細(xì)胞凋亡，但E1B19K亦能與Bax結(jié)合，從而全面阻止細(xì)胞凋亡[16]。

2.1.2HBVHBV X蛋白(HBV X protein，HBx)能在轉(zhuǎn)錄水平強(qiáng)烈誘導(dǎo)轉(zhuǎn)移相關(guān)蛋白1(metastasis-associated protein 1，MTA1)和組蛋白去乙?；?histone deacetylase，HDAC)基因的表達(dá)。在HBx共表達(dá)時(shí)，MTA1能介導(dǎo)HIF-1α蛋白ODDD的脫乙?；?，阻止HIF-1α與PHD及腫瘤抑制蛋白VHL的特異性結(jié)合，從而提高HIF-1α的穩(wěn)定性。累積的HIF-1α通過(guò)多種信號(hào)通路上調(diào)下游靶基因表達(dá)。此外，通過(guò)HBx誘導(dǎo)碳酸酐酶9(carbonic anhydrase 9，CA9)的表達(dá)，HBV能應(yīng)對(duì)缺氧微環(huán)境下的pH值變化，促進(jìn)感染細(xì)胞存活。此過(guò)程主要由HIF-1α與位于CA9基因轉(zhuǎn)錄起始位點(diǎn)上游-10/-3 bp的功能HRE結(jié)合并誘導(dǎo)其啟動(dòng)子的轉(zhuǎn)錄活性，以增加CA9的表達(dá)量[17,18]。

2.1.3HPV高危毒株HPV16的癌基因E6和E7在HPV自然感染中同時(shí)表達(dá)。E6蛋白通過(guò)抑制P53表達(dá)激活HIF-1α的轉(zhuǎn)錄活性[19]，E7蛋白則通過(guò)其N端的結(jié)構(gòu)域與HIF-1α結(jié)合并增強(qiáng)HIF-1α的轉(zhuǎn)錄活性[20]。雖然HDAC能穩(wěn)定和提高HIF-1α的轉(zhuǎn)錄功能，但HDAC可同時(shí)誘導(dǎo)HIF-1α下游靶基因去乙?；种破浒谢虻霓D(zhuǎn)錄表達(dá)。近期研究發(fā)現(xiàn)，E7蛋白能同時(shí)結(jié)合HDAC，從而阻止HDAC對(duì)HIF-1α下游靶基因轉(zhuǎn)錄表達(dá)的抑制作用[21]。

2.1.4HSVHSV感染細(xì)胞后，雙鏈RNA依賴性蛋白激酶(double-stranded RNA-dependent protein kinase，PKR)被磷酸化，并活化真核起始因子2α(eukaryotic initiation factor 2α，eIF-2α)蛋白磷酸化，從而使相關(guān)蛋白合成關(guān)閉，導(dǎo)致HSV-1復(fù)制停止。在缺氧環(huán)境中HSV感染細(xì)胞后，由于缺氧能逐級(jí)激活細(xì)胞外信號(hào)調(diào)節(jié)激酶1/2(extracellular signal regulated kinase 1/2，ERK1/2)和絲裂原活化蛋白激酶的激酶(mitogen-activated protein kinase kinase，MEK)磷酸化，并抑制PKR磷酸化，使相關(guān)蛋白合成打開，增強(qiáng)HSV-1的復(fù)制[22]。

2.1.5HCMVHCMV感染細(xì)胞后，可導(dǎo)致宿主細(xì)胞中HIF-1α表達(dá)增高。經(jīng)紫外線輻射而失活的HCMV與宿主細(xì)胞孵育后，也會(huì)導(dǎo)致宿主細(xì)胞中HIF-1α特異RNA水平增高，說(shuō)明HIF-1α高表達(dá)與毒基因表達(dá)無(wú)關(guān)。但是，經(jīng)紫外線輻射的HCMV誘導(dǎo)產(chǎn)生的HIF-1α不如正常條件下HCMV感染后細(xì)胞產(chǎn)生的HIF-1α穩(wěn)定，表明HCMV感染細(xì)胞后并不是單純提高HIF-1α表達(dá)，而是同時(shí)增強(qiáng)HIF-1α穩(wěn)定性。此外，研究表明 HCMV感染細(xì)胞9 h后才出現(xiàn)HIF-1α表達(dá)升高，顯示其感染宿主細(xì)胞后并不直接誘導(dǎo)HIF-1α表達(dá)[23]。隨后通過(guò)特異性的HCMV感染抑制劑實(shí)驗(yàn)進(jìn)一步證明，HIF-1α的激活與磷脂酰肌醇3激酶(phosphatidylinositol 3 kinase，PI3K)/蛋白激酶B(protein kinase B，Akt)/哺乳類雷帕霉素靶蛋白(mammalian target of rapamycin，mTOR)和PI3K激酶(PI3K kinase，PIKK)/Akt/mTOR兩個(gè)信號(hào)通路有關(guān)，其中Akt的激活對(duì)HIF-1α的誘導(dǎo)是必需的[24]。

2.1.6EBV在缺氧微環(huán)境中，缺氧應(yīng)激可誘導(dǎo)EBV感染細(xì)胞進(jìn)入G0/G1期。細(xì)胞進(jìn)入G0/G1期后，EBV即刻早期基因BZLF1的轉(zhuǎn)錄將被激活，表達(dá)其相應(yīng)的ZEBRA蛋白(即Zta蛋白)，同時(shí)驅(qū)動(dòng)EBV從潛伏期進(jìn)入裂解復(fù)制期[25]。相反，EBV在Ⅱ型或Ⅲ型潛伏感染中，潛伏性膜蛋白1(latent membrane protein 1，LMP1)表達(dá)可顯著上調(diào)HIF-1α的表達(dá)量，LMP1通過(guò)抑制P42/P44絲裂原活化蛋白激酶(mitogen-activated protein kinase，MAPK)通路及促進(jìn)細(xì)胞內(nèi)活性氧(reactive oxygen species，ROS)生成來(lái)降低H2O2含量和誘導(dǎo)HIF-1α表達(dá)[26]。此外，在鼻咽癌上皮細(xì)胞中亦發(fā)現(xiàn)LMP1可通過(guò)提高泛素化通路中E3泛素蛋白連接酶Siah1的穩(wěn)定性，提高HIF-1α的表達(dá)及活性。主要機(jī)制是EBV感染細(xì)胞中高水平表達(dá)的Siah1蛋白可誘導(dǎo)PHD1/PHD3復(fù)合體降解，阻礙VHL/HIF-1α復(fù)合體形成，促使HIF-1α與其他輔助蛋白形成聚合物，并結(jié)合其下游靶基因啟動(dòng)子HRE，以激活VEGF、胰島素樣生長(zhǎng)因子2(insulin-like growth factor 2，ILGF2)等細(xì)胞因子表達(dá)，引發(fā)宿主細(xì)胞的轉(zhuǎn)化[27]。

2.1.7KSHVKSHV又稱人皰疹病毒8型(human herpesvirus 8，HHV-8)，在感染宿主細(xì)胞中存在2個(gè)生活時(shí)期——潛伏期和裂解復(fù)制期。缺氧環(huán)境可促使?jié)摲腥镜腒SHV從潛伏期轉(zhuǎn)向細(xì)胞裂解復(fù)制期。缺氧誘導(dǎo)產(chǎn)生的KSHV相關(guān)蛋白使HIF-1α表達(dá)增多，KSHV的rta基因啟動(dòng)子激活，導(dǎo)致Rta大量表達(dá)并促使KSHV進(jìn)入細(xì)胞裂解期[28]。另外， Dalton-Griffin和Wilson等證明X-盒結(jié)合蛋白1(X-box binding protein 1，XBP1)對(duì)rta(ORF50)基因啟動(dòng)子亦起激活作用。rta基因啟動(dòng)子內(nèi)存在XBP1相關(guān)反應(yīng)元件(XBP1-related response element，XRE)，在急性缺氧條件下被激活而使XBP1高水平表達(dá)，驅(qū)使KSHV進(jìn)入細(xì)胞裂解期[29]。研究還表明，KSHV基因組中ORF34和ORF50基因啟動(dòng)子區(qū)均存在HIF-1α的特異結(jié)合元件HRE，HRE與HIF-1α或HIF-2α結(jié)合即可激活毒蛋白R(shí)ta表達(dá)并誘導(dǎo)KSHV進(jìn)入裂解復(fù)制期[30]。此外，本課題組前期研究結(jié)果表明，缺氧環(huán)境中KSHV編碼的潛伏相關(guān)核抗原(latency-associated nuclear antigen，LANA)能在轉(zhuǎn)錄和翻譯水平提高HIF-1α表達(dá)，而HIF-1α與ORF50啟動(dòng)子區(qū)域的HRE結(jié)合使Rta表達(dá)，從而使KSHV進(jìn)入細(xì)胞裂解期[31,32]。LANA能上調(diào)Aurora A的表達(dá)水平，而高表達(dá)的Aurora A可誘導(dǎo)P53磷酸化，增強(qiáng)LANA與P53的相互作用，促進(jìn)LANA介導(dǎo)的P53泛素化和降解，從而影響HIF-1α蛋白翻譯后的穩(wěn)定性[33]；另外，LANA還可通過(guò)EC5S泛素化修飾作用，促使腫瘤抑制蛋白P53和VHL降解，而低水平的P53和VHL蛋白不能有效抑制HIF-1α[34]，從而促進(jìn)宿主細(xì)胞存活和快速生長(zhǎng)。

2.2 RNA病毒介導(dǎo)HIF信號(hào)通路的紊亂調(diào)節(jié)

目前與人類腫瘤相關(guān)并篡改缺氧信號(hào)的RNA病毒主要有3種。

2.2.1人T細(xì)胞白血病病毒1型人T細(xì)胞白血病病毒1型 (human T-cell leukaemia virus type 1，HTLV-1)感染可引起成熟T細(xì)胞失活，導(dǎo)致相關(guān)白血病。HTLV-1編碼的Tax蛋白可激活PI3K/Akt信號(hào)途徑，該信號(hào)途徑的激活與HTLV-1感染T細(xì)胞的存活密切相關(guān)[35]。其中HIF-1α作為該信號(hào)途徑中的關(guān)鍵蛋白，在HTLV-1感染T細(xì)胞中受Tax蛋白正調(diào)控而激活。Tax不僅促進(jìn)HIF-1α的大量表達(dá)和累積，還可提高HIF-1α的DNA結(jié)合效率。在PI3K/Akt突變T細(xì)胞中，HTLV-1感染后表達(dá)的Tax蛋白未能有效激活HIF-1α的表達(dá)及其DNA結(jié)合能力[36]。因此，Tax主要激活PI3K/Akt信號(hào)途徑，繼而激活HIF-1，以促進(jìn)HTLV-1相關(guān)腫瘤的發(fā)生發(fā)展。

2.2.2丙型肝炎病毒大量數(shù)據(jù)表明，丙型肝炎病毒(hepatitis C virus，HCV)感染可導(dǎo)致慢性肝炎、肝硬化和肝癌。在HCV陽(yáng)性患者肝癌組織中可檢測(cè)到HCV負(fù)鏈RNA，且通常與HBV合并感染。Papaevangelous等發(fā)現(xiàn)，缺氧應(yīng)激不僅顯著上調(diào)抗缺氧應(yīng)激、糖代謝和細(xì)胞分裂的基因表達(dá)，還大大提高HCV在人體肝臟細(xì)胞內(nèi)的復(fù)制效率。但是，該過(guò)程并不影響病毒入侵宿主細(xì)胞及其RNA轉(zhuǎn)錄翻譯[37]。此外，Abe等亦發(fā)現(xiàn)在缺氧應(yīng)激作用下，HCV核心蛋白可通過(guò)調(diào)節(jié)核因子κB (nuclear factor κB，NF-κB)和HIF-1α轉(zhuǎn)錄活性而上調(diào)VEGF的表達(dá)。表明缺氧應(yīng)激與HCV復(fù)制及其致瘤性密切相關(guān)[38]。

2.2.3人類免疫缺陷病毒轉(zhuǎn)錄激活因子4(activating transcription factor 4，ATF4)是細(xì)胞感受外界各種應(yīng)激(包括代謝和缺氧)并向胞內(nèi)傳遞信號(hào)的重要調(diào)控因子。人類免疫缺陷病毒1型(human immunodeficiency virus type 1，HIV-1)可上調(diào)ATF4的表達(dá)，并通過(guò)ATF4改變胞內(nèi)蛋白的折疊狀態(tài)，傳遞各種應(yīng)激信號(hào)。同時(shí)，HIV編碼的反式轉(zhuǎn)錄激活因子(trans-activator of transcription，Tat) 與ATF4直接作用，以加強(qiáng)HIV-1在細(xì)胞內(nèi)的復(fù)制，繼而激活長(zhǎng)末端重復(fù)序列(long terminal repeat，LTR)[39]。此外，HIV感染可誘發(fā)缺氧應(yīng)激引起的肺動(dòng)脈高壓(pulmonary hypertension，PH)。隨后機(jī)制研究發(fā)現(xiàn)，HIV蛋白酶抑制劑可通過(guò)干擾Akt磷酸化途徑阻斷HIV在缺氧條件下誘發(fā)的PH現(xiàn)象[40]。

3 缺氧應(yīng)激對(duì)病毒感染及其致宿主細(xì)胞病變的作用

3.1 缺氧應(yīng)激與病毒應(yīng)答

缺氧微環(huán)境雖可影響病毒在宿主細(xì)胞內(nèi)的復(fù)制，但病毒同時(shí)啟動(dòng)一系列信號(hào)級(jí)聯(lián)反應(yīng)來(lái)應(yīng)對(duì)缺氧微環(huán)境的刺激。例如，缺氧應(yīng)激不僅引起EBV感染細(xì)胞的細(xì)胞周期G0/G1停滯，誘導(dǎo)EBV早期蛋白Zta表達(dá)，還可增加EBV基因組的拷貝數(shù)[25,41]，同時(shí)可啟動(dòng)EBV和KSHV的裂解復(fù)制期。另外，缺氧應(yīng)激亦可對(duì)病毒感染宿主的各個(gè)過(guò)程產(chǎn)生影響。以HBV為例，X區(qū)是HBV結(jié)構(gòu)和功能上重疊最多的區(qū)段，X區(qū)的變異可影響HBV的復(fù)制、表達(dá)及其致癌作用[42]。Lee等發(fā)現(xiàn)，在缺氧條件下培養(yǎng)HBV感染的肝細(xì)胞時(shí)，與肝癌發(fā)生密切相關(guān)的X基因表達(dá)升高、HBV增強(qiáng)子Ⅰ活性增強(qiáng)，進(jìn)而激活VEGF表達(dá)及誘導(dǎo)肝癌組織中血管生成[43]。Yoo等亦發(fā)現(xiàn)，X蛋白可通過(guò)激活MAPK磷酸化途徑，增加HIF-1α的穩(wěn)定性和轉(zhuǎn)錄活性。在X基因的轉(zhuǎn)基因鼠肝臟中，HIF-1α和VEGF的表達(dá)明顯增高，HIF-1α與X蛋白相互作用可影響HIF-1α靶基因的轉(zhuǎn)錄活性，與肝癌的發(fā)生有關(guān)[44]。另一個(gè)例子是，缺氧應(yīng)激對(duì)HPV感染宿主細(xì)胞的周期調(diào)控亦有影響。Amellem等發(fā)現(xiàn)，HPV18的E7蛋白可破壞缺氧應(yīng)激誘導(dǎo)的細(xì)胞周期停滯[45]。另外，雖然缺氧條件可增強(qiáng)HCV復(fù)制，但并不影響病毒入侵宿主細(xì)胞及其RNA翻譯[46]。在HCV感染患者體內(nèi)，與缺氧應(yīng)激相關(guān)的基因如金屬硫蛋白1H(metallothionein 1H，MT1H)、熱休克蛋白70/90(heat shock protein 70/90，Hsp70/90)表達(dá)明顯增高，而基質(zhì)金屬蛋白酶7(matrix metalloproteinase 7，MMP-7)表達(dá)降低，表明HCV感染宿主細(xì)胞過(guò)程與缺氧信號(hào)途徑相關(guān)[47]。更有意思的是，HCMV感染可抑制宿主細(xì)胞因缺氧應(yīng)激導(dǎo)致的細(xì)胞復(fù)制終止，其主要機(jī)制是HCMV感染可激活mTOR的2個(gè)效應(yīng)因子eIF4E結(jié)合蛋白(eIF4E binding protein，4E-BP)和eIF4G的磷酸化[23]。

3.2 缺氧應(yīng)激與染毒細(xì)胞轉(zhuǎn)化

缺氧應(yīng)激可通過(guò)多種形式作用于細(xì)胞，包括導(dǎo)致細(xì)胞生理特征和致病性等相關(guān)基因(如血管活性物質(zhì)和基質(zhì)蛋白)表達(dá)水平的變化。這些活性物質(zhì)和基質(zhì)蛋白可影響血管結(jié)構(gòu)和周圍組織的組成。從基因水平來(lái)看，缺氧可誘導(dǎo)與血管收縮神經(jīng)和平滑肌絲裂原相關(guān)的基因〔如血小板衍生生長(zhǎng)因子B(platelet-derived growth factor B，PDGF-B)、內(nèi)皮素1(endothelin 1)、VEGF、血小板反應(yīng)蛋白1(thrombospondin 1) 〕，以及基質(zhì)蛋白編碼基因〔如膠原酶Ⅳ(collagenase Ⅳ)、MMP-9、血小板反應(yīng)蛋白1〕的表達(dá)[48]。同時(shí)，缺氧也可抑制抗有絲分裂效應(yīng)因子內(nèi)皮型一氧化氮合酶(endothelial nitric oxide synthase，eNOS)的表達(dá)。缺氧應(yīng)激對(duì)細(xì)胞的作用有時(shí)間依賴性，細(xì)胞暴露于缺氧的時(shí)間不同，產(chǎn)生的影響也不同：短期暴露可導(dǎo)致生理性和可逆性的血管和周圍組織的結(jié)構(gòu)改變；長(zhǎng)期暴露則導(dǎo)致不可逆的血管結(jié)構(gòu)改變及平滑肌的增生和纖維化[49]。

鑒于腫瘤細(xì)胞的發(fā)生源于能促進(jìn)細(xì)胞快速生長(zhǎng)和存活的蛋白信號(hào)通路的紊亂調(diào)節(jié)，其中癌細(xì)胞的擴(kuò)散依賴于營(yíng)養(yǎng)成分和氧氣的供應(yīng)，而氧氣限量對(duì)控制新血管生成、葡萄糖代謝、細(xì)胞存活和腫瘤細(xì)胞轉(zhuǎn)移起關(guān)鍵作用[50]。缺氧微環(huán)境應(yīng)激對(duì)病毒介導(dǎo)腫瘤產(chǎn)生的影響主要是通過(guò)改變?nèi)毖跣盘?hào)所誘導(dǎo)的血管生成、厭氧代謝(anaerobic metabolism)，以及其他促腫瘤細(xì)胞存活信號(hào)通路的蛋白表達(dá)水平[51]。同時(shí)，缺氧和病毒感染共存情況不僅易導(dǎo)致細(xì)胞DNA損傷，誘導(dǎo)突變及遺傳不穩(wěn)定，還能導(dǎo)致DNA修復(fù)功能損失。由于DNA損傷和DNA修復(fù)抑制給染毒腫瘤細(xì)胞自我復(fù)制及永生而適應(yīng)應(yīng)激微環(huán)境提供了可能，任何有利于染毒細(xì)胞適應(yīng)缺氧微環(huán)境應(yīng)激的突變(如細(xì)胞周期阻滯、分化、抗凋亡和增加血管生成等)均具有一定優(yōu)勢(shì)。染毒細(xì)胞突變株的缺氧應(yīng)激選擇比非突變株具有更快的生長(zhǎng)能力和適應(yīng)能力，最后成為優(yōu)勢(shì)株而引發(fā)腫瘤的產(chǎn)生。由此可見，缺氧應(yīng)激和HIF 信號(hào)對(duì)細(xì)胞的廣泛損傷足以表明其在病毒誘導(dǎo)細(xì)胞轉(zhuǎn)化方面起關(guān)鍵作用。

4 結(jié)語(yǔ)

綜上所述，病毒入侵可誘導(dǎo)腫瘤的產(chǎn)生，但不同病毒入侵細(xì)胞的機(jī)制不同。缺氧應(yīng)激可通過(guò)改變病毒在體內(nèi)的復(fù)制翻譯及特定基因的表達(dá)情況而影響腫瘤的發(fā)生。另外，細(xì)胞在缺氧條件下會(huì)通過(guò)HIF信號(hào)通路作出一系列的生物學(xué)應(yīng)答來(lái)適應(yīng)缺氧條件。本文總結(jié)了缺氧信號(hào)、病毒感染與相關(guān)腫瘤發(fā)生發(fā)展的關(guān)系(包括缺氧應(yīng)激造成DNA損傷、DNA修復(fù)功能喪失等)、不同腫瘤病毒應(yīng)答缺氧應(yīng)激的分子機(jī)制，以及缺氧應(yīng)激對(duì)病毒入侵感染宿主細(xì)胞的影響等研究進(jìn)展?；谌毖跣盘?hào)通路中不同病毒誘發(fā)腫瘤的應(yīng)答途徑相似, 相應(yīng)的缺氧信號(hào)可能成為相關(guān)疾病診斷及治療的潛在標(biāo)簽、靶點(diǎn)和策略。

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