神經(jīng)導(dǎo)向因子在腫瘤中的作用及其機(jī)制

楊潔，董振，崔紅娟

西南大學(xué) 家蠶基因組生物學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室，重慶 400716

隨著近幾年分子生物學(xué)、細(xì)胞生物學(xué)及表觀遺傳學(xué)等研究技術(shù)的不斷發(fā)展，各種生命活動(dòng)和病理進(jìn)程的分子機(jī)制逐漸被揭開(kāi)。Netrins作為一類自分泌蛋白或者膜結(jié)合蛋白，可以通過(guò)結(jié)合不同類型受體發(fā)揮不同的作用。它們最初被鑒定為參與中樞神經(jīng)系統(tǒng)的軸索指導(dǎo)，負(fù)責(zé)神經(jīng)細(xì)胞的遷移、分化和凋亡，可降低促炎介質(zhì)的血清水平，并穩(wěn)定血腦屏障，限制免疫細(xì)胞進(jìn)入中樞神經(jīng)系統(tǒng)[1]。除此之外，Netrins[2]也參與介導(dǎo)非神經(jīng)組織的黏附、遷移和分化等生理過(guò)程。目前的研究表明Netrins信號(hào)通路與重度抑郁癥[3]、阿爾茲海默癥[4]、心臟肥大和心力衰竭[5]、肥胖癥[6]等疾病均有密切聯(lián)系。特別是，Netrins家族成員促進(jìn)了多種腫瘤的進(jìn)展，因此，進(jìn)一步探索Netrins及其受體在腫瘤中的作用機(jī)制及防治意義成為在預(yù)防、治療腫瘤相關(guān)機(jī)制研究方面的熱點(diǎn)，并受到廣大科研學(xué)者的青睞。文中就Netrins在腫瘤中的最新研究成果，對(duì)Netrins在腫瘤中的研究進(jìn)展進(jìn)行總結(jié)，希望對(duì)其在腫瘤臨床、靶向治療的研發(fā)提供一定幫助。

1 Netrins及其受體簡(jiǎn)介

神經(jīng)導(dǎo)向因子 Netrins家族是一類與神經(jīng)軸突指導(dǎo)相關(guān)的蛋白，其結(jié)構(gòu)類似于層粘連蛋白。1990年，Netrins家族UNC6第一次在秀麗隱桿線蟲(chóng)Caenorhabditis elegans中被發(fā)現(xiàn)，證明其與神經(jīng)軸突導(dǎo)向及細(xì)胞遷移相關(guān)[7]。目前已識(shí)別出5個(gè)自分泌蛋白Netrin-1-5[8]和兩個(gè)膜結(jié)合蛋白NetrinG1、G2[9]。1994年，在脊椎動(dòng)物中，UNC6的同源物Netrin-1被發(fā)現(xiàn)[10]。人類的 Netrin-1基因定位于17p13.1，編碼 604個(gè)氨基酸大約 67.7 kDa；Netrin-2目前發(fā)現(xiàn)在鳥(niǎo)類和斑馬魚(yú)中表達(dá)，而Netrin-3則是Netrin-2在哺乳動(dòng)物的同源基因，定位于16p13.3，編碼580個(gè)氨基酸大約61 kDa；人類 Netrin-4[11]是中樞神經(jīng)系統(tǒng)中參與突起生長(zhǎng)和遷移的自分泌蛋白，基因定位于12q22-q23，編碼629個(gè)氨基酸蛋白大約70–84 kDa。如圖1所示，它們大多是具有N-末端層粘連蛋白結(jié)構(gòu)域 (包含3個(gè)EGF重復(fù)序列)、中樞層粘連蛋白結(jié)構(gòu)域和C末端結(jié)構(gòu)域 (NTR)的層粘連相關(guān)蛋白[12]。而新發(fā)現(xiàn)的家庭成員Netrin-5[13]則缺乏N末端結(jié)構(gòu)域和上皮生長(zhǎng)因子 (Epidermal growth factor，EGF) 重復(fù)序列。另外Netrin-GS[14]在其C末端具有糖基磷脂?；煎^ (Glycosylphosphatidylinositol，GPI)，并且通過(guò)結(jié)合糖基磷脂酰基醇實(shí)現(xiàn)細(xì)胞膜的結(jié)合，從而發(fā)揮其功能。

Netrins的功能與兩類受體相關(guān)，一類是結(jié)直腸癌缺失蛋白 (Deleted in colon cancer，DCC)[16]家族及其同系物再生蛋白 (Neogenin)[17]；另一類是UNC5S (Uncoordinated phenotype-5S)[18]家族和最新發(fā)現(xiàn)的受體唐氏綜合癥細(xì)胞黏附分子 (Down syndrome cell adhesion molecule，DSCAM)[19]、腺苷 A2b (Adenosine receptor A2b，A2b)[20]受體和整合素 (Integrin)[21]。DCC和Neogenin介導(dǎo)Netrin-1誘導(dǎo)軸突化學(xué)吸引和排斥[22]，在 DCC受體缺乏的情況下，UNC5家族的4個(gè)成員被證明介導(dǎo)Netrin-1誘導(dǎo)的軸突化學(xué)排斥[12,23]。

圖1 Netrins的結(jié)構(gòu)示意圖(改自Ylivinkka等[15])Fig. 1 Schematic illustration of the structure of netrins(adapted from Ylivinkka et al[15]). Different netrin domains are marked as follows: A: N-terminal domain related to laminin γ-chain; B: N-terminal domain related to laminin-chain; C: NTR domain; D: 3 EGF repeats; E:2 EGF repeats; F: GPI-anchor protein.

2 Netrins與腫瘤

近年來(lái)，越來(lái)越多的研究表明，Netrins在腫瘤的發(fā)生發(fā)展中也起著至關(guān)重要的作用，其作用形式也是一個(gè)復(fù)雜的生物學(xué)過(guò)程，這一過(guò)程中受到眾多腫瘤明星分子及多種信號(hào)通路的調(diào)節(jié)，如表 1所示。Netrins家族成員在腫瘤中的作用也不盡相同，絕大多數(shù)證據(jù)表明其具有促癌作用。比如，Netrin-1在結(jié)腸直腸癌、神經(jīng)母細(xì)胞瘤、膠質(zhì)母細(xì)胞瘤和轉(zhuǎn)移性乳腺癌等腫瘤中均表達(dá)上調(diào)，并且發(fā)現(xiàn)Netrin-1是癌細(xì)胞生存侵襲的促進(jìn)因子[15]。這一角色是Netrins通過(guò)結(jié)合一系列受體介導(dǎo)的，包括UNC5S和DCC家族。比如，Netrin-1可以通過(guò)結(jié)合受體DCC家族和UNC5家族直接與腫瘤細(xì)胞凋亡和血管再生相關(guān)，而且在某些腫瘤組織中促進(jìn)了癌基因的表達(dá)[24]。

DCC和UNC5S屬于依賴性受體，都具有所謂的死亡結(jié)構(gòu)域[25]。當(dāng)受體不與Netrins結(jié)合時(shí)，死亡結(jié)構(gòu)域暴露和Caspase-9結(jié)合，導(dǎo)致Caspase-3被激活促進(jìn)細(xì)胞凋亡[26]。這類受體通過(guò)細(xì)胞內(nèi)信號(hào)誘導(dǎo)腫瘤細(xì)胞凋亡，包括結(jié)直腸癌、乳腺癌、卵巢癌、宮頸癌、胃癌、肺癌或腎癌等多種癌癥，這種特征賦予這些受體腫瘤抑制活性，所以這一類受體也被視為抑癌因子，在腫瘤的發(fā)生發(fā)展中扮演了重要的角色。而當(dāng)Netrins存在時(shí)，受體指導(dǎo)細(xì)胞存活、遷移和增殖，這可能與Netrins引發(fā)的受體 DCC和 UNC5S多聚化相關(guān)[27]。并且，UNC5B的表達(dá)由 p53直接調(diào)節(jié)，在沒(méi)有Netrins的情況下，UNC5B介導(dǎo)p53依賴性途徑誘導(dǎo)細(xì)胞凋亡。相反，在 Netrin-1的存在下，Netrin-1和DCC/UNC5S之間相互作用激活的信號(hào)通路抑制p53誘導(dǎo)的細(xì)胞凋亡，同時(shí)防止Caspase-3級(jí)聯(lián)激活，導(dǎo)致抗凋亡能力增強(qiáng)[28]，如圖 2所示。另一方面，有研究顯示Netrin-4在結(jié)直腸癌[29]、肝癌[30]中也扮演著抑癌因子的角色，但目前還不是很清楚 Netrins及其受體在不同腫瘤中引起差異的原因，Netrins及其受體在腫瘤的發(fā)展過(guò)程中扮演的精確角色還有待進(jìn)一步研究。

因此，研究Netrins及其受體在腫瘤發(fā)生發(fā)展中的作用機(jī)制及其調(diào)控意義對(duì)人類腫瘤的治療具有重要意義，關(guān)注Netrins在癌癥、腫瘤細(xì)胞發(fā)生發(fā)展過(guò)程中的生物學(xué)作用已成為焦點(diǎn)。由于腫瘤是一類疾病的總稱，不同腫瘤的遺傳背景差異很大。因此，下面將從Netrins在不同腫瘤中的研究現(xiàn)狀進(jìn)行總結(jié)。

2.1 Netrins與結(jié)直腸癌

結(jié)直腸癌 (Colorectal cancer，CRC) 是嚴(yán)重影響人類健康的消化系統(tǒng)惡性腫瘤之一，具有較高的發(fā)病率、死亡率[31]。

在對(duì)結(jié)直腸癌生物學(xué)研究過(guò)程中發(fā)現(xiàn) Netrin-1的受體 DCC、UNC5C在結(jié)直腸癌及其他腫瘤組織中高頻缺失，暗示 Netrin-1受體缺失途徑在癌癥中的重要性。而 Netrin-1通過(guò)抑制細(xì)胞凋亡，在結(jié)直腸癌發(fā)生過(guò)程中發(fā)揮著促癌作用[32]，通過(guò)其跨膜受體DCC和UNC5S參與Hippo信號(hào)通路的調(diào)節(jié)[33]，使細(xì)胞核中的 YAP水平升高，增強(qiáng)YAP的信號(hào)積累，從而促進(jìn)癌細(xì)胞增殖和遷移。此外，在腫瘤發(fā)生期間，Netrin-1受體分子改變的時(shí)間并不是隨機(jī)的，UNC5C的失活發(fā)生在腫瘤早期，而DCC缺失發(fā)生在多級(jí)結(jié)腸直腸癌的后期階段[34]。但與野生型UNC5C相比，突變體UNC5CA628K卻能減少細(xì)胞凋亡，促進(jìn)腫瘤惡化[35]，因此，探究Netrin-1和受體之間的作用對(duì)治療結(jié)直腸癌具有潛在意義。

除了Netrin-1外，Netrin-4與結(jié)直腸癌的關(guān)系也被逐步確定。Xu等研究發(fā)現(xiàn)，在結(jié)直腸癌肝轉(zhuǎn)移中發(fā)現(xiàn)血管內(nèi)皮生長(zhǎng)因子 (Vascular endothelial cell growth factor，VEGF) 和選擇素 (E-selectin) 上調(diào)，而Netrin-4和LK-68則被抑制[36]，表明Netrin-4與腫瘤血管生成有著密切聯(lián)系。Eveno等進(jìn)一步研究發(fā)現(xiàn)，Netrin-4可以通過(guò)與其受體 Neogenin結(jié)合而表現(xiàn)出抗血管生成活性，但Netrin-4不影響體外VEGF誘導(dǎo)的血管通透性急性增加[37]。此外，通過(guò)比對(duì)原發(fā)性結(jié)直腸癌和肝、肺部轉(zhuǎn)移的腫瘤，發(fā)現(xiàn)過(guò)表達(dá)Netrin-4既能抑制異種移植的原發(fā)腫瘤生長(zhǎng)，也能顯著降低結(jié)直腸癌肝轉(zhuǎn)移模型腫瘤數(shù)量和肝轉(zhuǎn)移體積[29]，表明Netrin-4能降低結(jié)直腸癌肺轉(zhuǎn)移。這些結(jié)果表明 Netrin-4具有抑制結(jié)直腸癌生長(zhǎng)和主要轉(zhuǎn)移部位的潛力。

2.2 Netrins與胰腺導(dǎo)管腺癌

胰腺導(dǎo)管腺癌 (Pancreatic ductal adenocarcinoma，PDAC) 具有發(fā)病隱匿、進(jìn)展迅速、預(yù)后極差等特點(diǎn)，素有癌中之王之稱[38]。近年來(lái)，研究人員發(fā)現(xiàn) Netrin-1也在胰腺導(dǎo)管腺癌的進(jìn)展中發(fā)揮著重要的作用。

Ramesh[39]和 Yildirim[40]等發(fā)現(xiàn) Netrin-1在胰腺癌、乳腺癌、前列腺癌、肝癌等腫瘤組織中高表達(dá)，而且，在胰腺癌患者中，沒(méi)有表達(dá)或者很少表達(dá)Netrin-1的中位生存期為10個(gè)月，而中等強(qiáng)度或者高表達(dá) Netrin-1的患者中位生存期僅僅為 4個(gè)月[41]。除此之外，Netrin-1及其受體UNC5H3表達(dá)對(duì)患有分化不良的腫瘤患者的總生存率有顯著影響[41]。進(jìn)一步研究發(fā)現(xiàn)，Netrin-1在體外賦予腫瘤和內(nèi)皮細(xì)胞凋亡抗性[42]，為腫瘤細(xì)胞提供粘附底物，誘導(dǎo)其侵襲，其可能機(jī)制是Netrin-1通過(guò)MDM2 (Murine double minute 2) 下調(diào)p53的表達(dá)，促進(jìn)腫瘤細(xì)胞體外增殖[43]。但也有報(bào)道顯示，Netrin-1在體內(nèi)可以抑制胰腺導(dǎo)管腺癌細(xì)胞的生長(zhǎng)，其與受體UNC5B相互作用激活黏著斑激酶 (Focal adhesion kinase，F(xiàn)AK)，刺激一氧化氮 (Nitric oxide，NO) 的產(chǎn)生，增強(qiáng)蛋白磷酸酶(Protein phosphatase 2A，PP2A) 的表達(dá)，抑制Mek/Erk/c-Jun信號(hào)通路進(jìn)而下調(diào)ITGB4的表達(dá)，從而抑制胰腺導(dǎo)管腺癌的生長(zhǎng)[44]。這可能是不同遺傳背景下的胰腺癌中Netrin-1介導(dǎo)的不同受體有關(guān)，其具體的效用和機(jī)制還需要進(jìn)一步評(píng)價(jià)。

2.3 Netrins和胃癌

胃癌是一種嚴(yán)重威脅人類健康的惡性腫瘤之一，其發(fā)病率位居第四，死亡率位居第二[45]。研究發(fā)現(xiàn) Netrins在人類胃癌細(xì)胞組織及病人血清中表達(dá)上調(diào)，暗示 Netrins可能與胃癌相關(guān)[46]。進(jìn)一步的研究發(fā)現(xiàn)，Netrin-1[47]和Netrin-4[48]通過(guò)其受體Neogenin引起致瘤途徑 (Jak/Stat、PI3K/Akt和Erk/MAPK) 的激活，從而促進(jìn)胃癌細(xì)胞的增殖和侵襲能力；當(dāng)敲低Netrins及其受體能顯著抑制胃癌細(xì)胞增殖和侵襲，同時(shí)引起 Stat3、Erk、Akt和p38的磷酸化水平降低。除此之外，同結(jié)直腸癌類似，Netrins受體缺陷也是胃癌的常見(jiàn)特征，并且在不同時(shí)期，受體表達(dá)量具有明顯差異。在胃癌發(fā)生的早期進(jìn)程中，DCC和UNC5C基因異常甲基化[49-50]，但在晚期胃癌中這種現(xiàn)象又逐漸消失，并且 Netrin-1濃度升高[51]，表明 Netrin-1受體的累積變化是胃癌進(jìn)展的晚期事件。在晚期的胃癌中，這種動(dòng)態(tài)的變化也許有另外的因子參與了重要調(diào)節(jié)，導(dǎo)致受體的失活，具體的機(jī)制還需要進(jìn)一步研究。

2.4 Netrins與乳腺癌

乳腺癌是世界各地婦女癌癥死亡的主要原因，在我國(guó)，乳腺癌是女性中發(fā)病率第一、致死率第六的惡性腫瘤，并且近年來(lái)發(fā)病率逐漸上升[52]。而腫瘤轉(zhuǎn)移是乳腺癌患者死亡的主要原因，通常在多個(gè)部位發(fā)生[53]，常見(jiàn)于肺[54]、肝[55]、腦[56]和骨[57]等。

在研究乳腺癌轉(zhuǎn)移的機(jī)制過(guò)程中，發(fā)現(xiàn)與人類非轉(zhuǎn)移性乳腺癌不同的是，大部分轉(zhuǎn)移性乳腺癌高表達(dá)Netrin-1[58]。在Netrin-1高表達(dá)的乳腺轉(zhuǎn)移性腫瘤細(xì)胞中，降低Netrin-1表達(dá)或者加入其可溶性受體會(huì)發(fā)生凋亡[32]。因此大部分人轉(zhuǎn)移性乳腺癌中，高表達(dá)的Netrin-1具有腫瘤細(xì)胞存活的選擇性優(yōu)勢(shì)。在針對(duì)乳腺癌的治療研究中，對(duì)于 Netrin-1靶點(diǎn)治療研究表明，Netrin-1的沉默、干擾Netrin-1/DRs (Dependence receptors) 的相互作用、地西他濱和抗Netrin-1中和抗體的組合可以有效地減少不同的腫瘤生長(zhǎng)和轉(zhuǎn)移[25,32,59-62]。而在非轉(zhuǎn)移性的乳腺癌中，顯示Netrin-1和DAPK1依賴DNA甲基化表達(dá)的缺失[59]，DAPK1是一種絲氨酸蘇氨酸激酶，能夠轉(zhuǎn)導(dǎo)Netrin-1依賴性受體促細(xì)胞凋亡，DAPK1的缺失導(dǎo)致了腫瘤細(xì)胞的進(jìn)一步惡化。

另一個(gè)Netrins家族成員Netrin-4與侵襲性的乳腺癌預(yù)后相關(guān)[63]，Oncomine數(shù)據(jù)也顯示，與正常乳腺組織相比，乳腺癌中Netrin-4降低。Netrin-4過(guò)表達(dá)能誘導(dǎo)黏著蛋白 (N-cadherin) 和波形蛋白 (Vimentin) 下調(diào)，從而減弱細(xì)胞遷移和侵襲[64]。但也有報(bào)道顯示，Netrin-4在乳腺癌或乳腺癌滲出液中表達(dá)很高[65]，并且Netrin-4和α6β1共同整合在特定的血管床上，促進(jìn)了腫瘤血管的生成作用[20]。此外，Netrin-4也可以激活小GTP酶和Src家族激酶/FAK來(lái)刺激體外和體內(nèi)淋巴通透性，誘導(dǎo)轉(zhuǎn)基因小鼠皮膚和乳腺腫瘤中淋巴管和血管的生長(zhǎng)、遷移，導(dǎo)致轉(zhuǎn)移增強(qiáng)[66]。這些研究表明，Netrin-4與乳腺癌的轉(zhuǎn)移密切相關(guān)；然而，究竟Netrin-4是促進(jìn)乳腺癌轉(zhuǎn)移還是抑制其轉(zhuǎn)移，還需要進(jìn)一步評(píng)價(jià)。

2.5 Netrins與卵巢癌

卵巢癌是死亡率最高的婦科惡性腫瘤之一，5年生存率不到45%[67]。據(jù)報(bào)道，在卵巢癌中，Netrin-1隨卵巢癌的惡化而表達(dá)升高[68]，并且與SOX6 (Sex-determining region Y box 6) 表達(dá)也有相關(guān)性[69]。已經(jīng)證實(shí)，SOX6[70]是腫瘤抑制因子，在細(xì)胞增殖、分化中起重要作用，包括卵巢癌、食管鱗狀細(xì)胞癌、肝癌和慢性骨髓性白血病。而SOX6與Netrin-1在卵巢腫瘤中表達(dá)成反比關(guān)系[69]，SOX6對(duì)卵巢癌細(xì)胞體內(nèi)增殖、細(xì)胞侵襲和異種移植生長(zhǎng)具有抑制作用，而Netrin-1的過(guò)表達(dá)會(huì)顯著地消除這一現(xiàn)象?？傊@些研究表明Netrin-1在調(diào)節(jié)腫瘤生長(zhǎng)中具有重要的作用，對(duì)于開(kāi)發(fā)針對(duì)卵巢癌的治療藥物具有潛在意義。

2.6 Netrins與膠質(zhì)母細(xì)胞瘤

膠質(zhì)母細(xì)胞瘤 (Glioblastoma，GBM) 是顱內(nèi)最常見(jiàn)的原發(fā)性惡性腫瘤，中位生存期只有一年多，也沒(méi)有標(biāo)準(zhǔn)的二線治療[71]。

目前，在臨床GBM病理標(biāo)本中發(fā)現(xiàn)Netrin-1高表達(dá)，并且Netrin-1的表達(dá)與干細(xì)胞標(biāo)記物巢蛋白 (Nestin) 相關(guān)，導(dǎo)致干細(xì)胞球體的形成增加[72]。研究表明，Netrin-1[73]通過(guò)激活Notch信號(hào)，與干細(xì)胞結(jié)合，改變非侵襲性 GBM 細(xì)胞表型，促使擴(kuò)散入侵并增加 GBM 干細(xì)胞標(biāo)記物巢蛋白的表達(dá)。并且，Netrin-1與細(xì)胞增殖、腫瘤惡性分級(jí)呈正相關(guān)[74]，可在體外和體內(nèi)介導(dǎo) UNC5A誘導(dǎo)NF-κB p65ser536磷酸化和c-Myc表達(dá)，促進(jìn)GBM細(xì)胞增殖[75]。并且研究發(fā)現(xiàn)靶向抑制Netrin-1的表達(dá)，特別是干細(xì)胞樣細(xì)胞浸潤(rùn)生長(zhǎng)受到顯著抑制[72]，腫瘤細(xì)胞增殖、存活率和侵襲能力也顯著減弱[76]。Netrin-1作為GBM干細(xì)胞干性和運(yùn)動(dòng)的重要調(diào)節(jié)因子，為探索治療膠質(zhì)母細(xì)胞瘤的藥物靶點(diǎn)提供有力支持。

而另一個(gè)成員 Netrin-4可能作為致癌基因，與ITGB4共同作用激活A(yù)kt-mToR信號(hào)通路促進(jìn)成膠質(zhì)母細(xì)胞瘤增殖；而且發(fā)現(xiàn)抑制 Netrin-4或者ITGB4都會(huì)增強(qiáng)膠質(zhì)母細(xì)胞瘤對(duì)藥物替莫唑胺(Temozolomide，TMZ) 的敏感性，外源性添入Netrin-4和ITGB4會(huì)減弱TMZ誘導(dǎo)的細(xì)胞凋亡[77]。但也有研究顯示，Netrin-4也能在膠質(zhì)母細(xì)胞瘤內(nèi)激活Erk-MAPK信號(hào)通路，從而抑制GBM細(xì)胞的增殖[78]?？傊?，研究顯示，Netrin-4在膠質(zhì)母細(xì)胞瘤的研究中有重要的作用，但為何形成這種差異還需要進(jìn)一步探究。

2.7 Netrins與神經(jīng)母細(xì)胞瘤

神經(jīng)母細(xì)胞瘤 (Neuroblastoma，NB) 是兒童早期最常見(jiàn)的實(shí)體瘤之一，在新生兒中的發(fā)病率為 1/10 000–1/8 000[79]。Netrin-1 通過(guò)阻斷 UNC5S的促凋亡活性，在侵襲性 NB的生長(zhǎng)和傳播中傳遞了選擇性優(yōu)勢(shì)[80]。神經(jīng)母細(xì)胞瘤之所以侵襲性強(qiáng)，是因?yàn)樾TP酶Rac1活性是神經(jīng)軸突生長(zhǎng)所必需的，UNC5A和DCC通過(guò)激活Rac1引發(fā)細(xì)胞皮層肌動(dòng)蛋白重組，而肌動(dòng)蛋白的裝配會(huì)引起腫瘤細(xì)胞的侵襲轉(zhuǎn)移，恰巧Netrin-1能加強(qiáng)刺激UNC5A和DCC導(dǎo)致Rac1激活[81-82]。而另外一個(gè)受體UNC5D是p53家族成員直接靶向的依賴性受體[83]。在 NB中，神經(jīng)生長(zhǎng)因子 (Nerve growth factor，NGF) 大大加強(qiáng)了UNC5D、E2F1和p53的調(diào)控，誘導(dǎo)的UNC5D被天冬氨酸蛋白水解酶2/3 (Caspase2/3) 裂解，釋放的細(xì)胞內(nèi)片段轉(zhuǎn)移到細(xì)胞核中并與E2F1相互作用，選擇性地反式激活前凋亡靶基因，UNC5D與p53和E2F1形成正反饋回路，以促進(jìn)神經(jīng)母細(xì)胞瘤回歸過(guò)程中 NGF依賴介導(dǎo)的程序性細(xì)胞死亡[84]，但Netrin-1的表達(dá)強(qiáng)烈抑制UNC5D的裂解及其誘導(dǎo)的凋亡[85]，以此來(lái)抑制神經(jīng)母細(xì)胞瘤的程序性死亡。

Netrins的另外一個(gè)受體Neogenin-1也是一種在胚胎發(fā)育和成體內(nèi)穩(wěn)態(tài)期間，參與軸突引導(dǎo)、血管生成、神經(jīng)元細(xì)胞遷移和細(xì)胞死亡的跨膜受體[86-89]。Villanueva[90]等發(fā)現(xiàn)在NB中，在其配體不存在的情況下，Neogenin-1也能促進(jìn)細(xì)胞死亡，但敲低 Neogenin-1會(huì)減少細(xì)胞遷移。Neogenin-1及其配體Netrin-1和Netrin-4在原代神經(jīng)母細(xì)胞瘤中的表達(dá)較高，預(yù)示著患者存活率差[90]。在神經(jīng)母細(xì)胞瘤中，DCC、UNC5S和Neogenin-1的表達(dá)預(yù)示著腫瘤的遷移侵襲能力增強(qiáng)，而它們卻都能促進(jìn)細(xì)胞的凋亡，一方面發(fā)揮著腫瘤抑制的角色，一方面又促進(jìn)了腫瘤的發(fā)展。這些數(shù)據(jù)表明 Netrins及其受體可能作為治療神經(jīng)母細(xì)胞瘤特定的靶標(biāo)因子，具有極其重要的作用。

表1 Netrins在不同腫瘤中的表達(dá)情況、功能及其介導(dǎo)的下游信號(hào)通路Table 1 Summary of the netrins expression, suggested functions and novel pathways in different cancers

圖2 Netrins及其受體在腫瘤中發(fā)生發(fā)展中的作用機(jī)制(改自Kefeli等[24]，Arakawa[28])Fig. 2 Diagram of the mechanisms of netrins and its receptors in tumor initiation and progression (adapted from Kefeli et al[24], Arakawa[28]).

2.8 Netrins與其他腫瘤

除了上述腫瘤外，Netrins及其受體在其他腫瘤中目前研究還較少，但是也發(fā)揮著重要的作用。例如，阻斷中樞系統(tǒng)成神經(jīng)管細(xì)胞瘤中表達(dá)的內(nèi)源性Netrin-1、Neogenin或UNC5B能夠抑制其侵襲[91]。在多發(fā)性骨髓瘤[92]、前列腺癌細(xì)胞[93]中Netrin-1水平顯著升高，而其受體UNC5B表達(dá)受到抑制[93]。在黑色素瘤中，Netrin-1通過(guò)結(jié)合受體 UNC5B促進(jìn)黑色素瘤細(xì)胞侵襲和遷移，在惡性黑色素瘤的進(jìn)展中起重要作用[94]，隨著近年來(lái)抗體療法的出現(xiàn)，2C9作為UNC5B的單克隆特異性抗體，經(jīng)過(guò)2C9處理后的黑色素瘤細(xì)胞的遷移侵襲作用受到顯著抑制[95-96]。在晚期非小細(xì)胞肺癌組織中發(fā)現(xiàn)Netrin-1濃度隨化療而降低[40]，以及在彌漫性大B細(xì)胞淋巴瘤和套細(xì)胞淋巴瘤中[97]，也發(fā)現(xiàn)Netrin-1高表達(dá)，Netrin-1的功能干擾與DCC、UNC5H介導(dǎo)的細(xì)胞死亡有關(guān)，得益于受體作為抑癌因子的作用。在宮頸癌或晚期子宮內(nèi)膜異位癥(III-IV期) 中，miR-196和 miR-20a高表達(dá)，并通過(guò)靶向結(jié)合Netrin-4 mRNA 3’-UTR中的一個(gè)結(jié)合位點(diǎn)來(lái)抑制Netrin-4的表達(dá)，在這里，Netrin-4扮演著腫瘤抑制因子的角色[98-99]。在成纖維肉瘤HT1080細(xì)胞中，敲低Netrin-4能抑制該腫瘤細(xì)胞的增殖，但促進(jìn)了遷移、侵襲能力[100]。這些研究表明，Netrins家族成員，尤其是 Netrin-1和Netrin-4，與許多腫瘤的發(fā)生發(fā)展密切相關(guān)。然而，它們?cè)谀[瘤中的研究還比較局限，很多作用機(jī)制還不夠清楚，需要更多數(shù)據(jù)來(lái)支持。

3 總結(jié)與展望

文中總結(jié)了近年來(lái) Netrins及其受體在結(jié)直腸癌、胰腺癌、胃癌、肺癌、乳腺癌、卵巢癌、膠質(zhì)母細(xì)胞瘤、神經(jīng)母細(xì)胞瘤及其他腫瘤中的最新研究進(jìn)展，顯示了Netrin-1、Netrin-4在這些腫瘤中的重要意義。Kefeli等也總結(jié)了大量研究資料得出Netrin-1具有促癌作用，被認(rèn)為是一個(gè)非常重要的治療靶標(biāo)[24]。

大多數(shù)Netrins受體如DCC和UNC5S家族往往扮演著抑癌的角色。它們激活p53等凋亡途徑的凋亡活性，從而降低腫瘤細(xì)胞的存活能力。通過(guò)缺失受體、過(guò)表達(dá)配體或抑制下游信號(hào)傳導(dǎo)能夠抑制Netrins受體的抑癌作用。例如，依賴性受體 DNA的異常甲基化、突變都會(huì)使受體的凋亡活性喪失。

而Netrins，尤其是Netin-1和Netrin-4，可以通過(guò)結(jié)合這些受體，競(jìng)爭(zhēng)性地抑制其活性，封閉其抑癌作用，而促進(jìn)了腫瘤的發(fā)生發(fā)展。Netrins作為促癌基因?qū)τ谀[瘤細(xì)胞的存活是有利的，Netrins的表達(dá)激活了血管內(nèi)皮生長(zhǎng)因子的表達(dá)，以及 Mek/Erk、Akt/Stat3、Notch、mToR 和 NF-κB等信號(hào)途徑，并可以促進(jìn)原癌基因c-Myc的表達(dá)，從而促進(jìn)腫瘤的發(fā)展。因此，進(jìn)一步研究Netrins在腫瘤中的作用及其機(jī)制，有利于針對(duì) Netrins及其受體，開(kāi)發(fā)出新型的靶向治療抗體或者其特異性抑制。

另外，Netrins作用機(jī)制研究的不斷深入不僅有利于開(kāi)發(fā)新型的抗腫瘤療法，也會(huì)對(duì)進(jìn)一步理解Netrins在神經(jīng)系統(tǒng)及免疫過(guò)程中的作用機(jī)制提供參考。比如Netrins調(diào)控的mToR信號(hào)通路也廣泛參與了神經(jīng)細(xì)胞的調(diào)節(jié)和免疫細(xì)胞的活性[101-102]。

但是，目前對(duì)Netrin家族其余成員Netrin-2、Netrin-3、Netrin-5及 Netrin-GS在腫瘤發(fā)生發(fā)展中的研究尚少。因此，對(duì)Netrin家族其余成員在腫瘤細(xì)胞中是否起著關(guān)鍵作用也是值得探究的。這些研究對(duì)于實(shí)現(xiàn)腫瘤的靶向治療具有重要的意義，希望能對(duì)臨床用藥及其機(jī)制研究提供幫助。

REFERENCES

[1]Mulero P, Córdova C, Hernández M, et al. Netrin-1 and multiple sclerosis: A new biomarker for neuroinflammation? Eur J Neurol, 2017, 24(9):1108–1115.

[2]Yebra M, Diaferia GR, Montgomery AM, et al.Endothelium-derived netrin-4 supports pancreatic epithelial cell adhesion and differentiation through integrins α2β1 and α3β1. PLoS ONE, 2011, 6(7):e22750.

[3]Zeng Y, Navarro P, Fernandez-Pujals AM, et al. A combined pathway and regional heritability analysis indicates netrin1 pathway is associated with major depressive disorder. Biol Psychiatry, 2017, 81(4):336–346.

[4]Hashimoto Y, Toyama Y, Kusakari S, et al. An alzheimer disease-linked rare mutation potentiates netrin receptor uncoordinated-5c-induced signaling that merges with amyloid β precursor protein signaling. J Biol Chem, 2016, 291(23): 12282–12293.

[5]Wang N, Cao YS, Zhu Y. Netrin-1 prevents the development of cardiac hypertrophy and heart failure.Mol Med Rep, 2016, 13(3): 2175–2181.

[6]Ramkhelawon B, Hennessy EJ, Ménager M, et al.Netrin-1 promotes adipose tissue macrophage retention and insulin resistance in obesity. Nat Med, 2014,20(4): 377–384.

[7]Hedgecock EM, Culotti JG, Hall DH. The unc-5, unc-6,and unc-40 genes guide circumferential migrations of pioneer axons and mesodermal cells on the epidermis in C. Elegans. Neuron, 1990, 4(1): 61–85.

[8]Garrett AM, Jucius TJ, Sigaud LP, et al. Analysis of expression pattern and genetic deletion of netrin5 in the developing mouse. Front Mol Neurosci, 2016, 9: 3.

[9]Larrieu-Lahargue F, Thomas KR, Li DY. Netrin ligands and receptors: Lessons from neurons to the endothelium. Trends Cardiovasc Med, 2012, 22(2):44–47.

[10]Serafini T, Kennedy TE, Galko MJ, et al. The netrins define a family of axon outgrowth-promoting proteins homologous toC. elegansunc-6. Cell, 1994, 78(3):409–424.

[11]Koch M, Murrell JR, Hunter DD, et al. A novel member of the netrin family, β-netrin, shares homology with the β chain of laminin. J Cell Biol,2000, 151(2): 221–234.

[12]Cirulli V, Yebra M. Netrins: Beyond the brain. Nat Rev Mol Cell Biol, 2007, 8(4): 296–306.

[13]Yamagishi S, Yamada K, Sawada M, et al. Netrin-5 is highly expressed in neurogenic regions of the adult brain. Front Cell Neurosci, 2015, 9: 146.

[14]Nakashiba T, Nishimura S, Ikeda T, et al.Complementary expression and neurite outgrowth activity of netrin-g subfamily members. Mech Dev,2002, 111(1/2): 47–60.

[15]Ylivinkka I, Keski-Oja J, Hyyti?ainen M. Netrin-1: A regulator of cancer cell motility? Eur J Cell Biol, 2016,95(11): 513–520.

[16]Keino-Masu K, Masu M, Hinck L, et al. Deleted in colorectal cancer (dcc) encodes a netrin receptor. Cell,1996, 87(2): 175–185.

[17]Kolodziej PA, Timpe LC, Mitchell KJ, et al. Frazzled encodes a drosophila member of the dcc immunoglobulin subfamily and is required for cns and motor axon guidance. Cell, 1996, 87(2): 197–204.

[18]Leonardo ED, Hinck L, Masu M, et al. Vertebrate homologues ofC. elegansunc-5 are candidate netrin receptors. Nature, 1997, 386(6627): 833–838.

[19]Ly A, Nikolaev A, Suresh G, et al. Dscam is a netrin receptor that collaborates with dcc in mediating turning responses to netrin-1. Cell, 2008, 133(7):1241–1254.

[20]Corset V, Nguyen-Ba-Charvet KT, Forcet C, et al.Netrin-1-mediated axon outgrowth and camp production requires interaction with adenosine a2b receptor. Nature, 2000, 407(6805): 747–750.

[21]Li YN, Pinzón-Duarte G, Dattilo M, et al. The expression and function of netrin-4 in murine ocular tissues. Exp Eye Res, 2012, 96(1): 24–35.

[22]Culotti JG, Merz DC. Dcc and netrins. Curr Opin Cell Biol, 1998, 10(5): 609–613.

[23]Guijarro P, Simó S, Pascual M, et al. Netrin1 exerts a chemorepulsive effect on migrating cerebellar interneurons in a dcc-independent way. Mol Cell Neurosci, 2006, 33(4): 389–400.

[24]Kefeli U, Ucuncu Kefeli A, Cabuk D, et al. Netrin-1 in cancer: Potential biomarker and therapeutic target?Tumour Biol, 2017, 39(4): 1010428317698388.

[25]Delloye-Bourgeois C, Brambilla E, Coissieux MM, et al.Interference with netrin-1 and tumor cell death in non-small cell lung cancer. J Natl Cancer Inst, 2009,101(4): 237–247.

[26]Mazelin L, Bernet A, Bonod-Bidaud C, et al. Netrin-1 controls colorectal tumorigenesis by regulating apoptosis. Nature, 2004, 431(7004): 80–84.

[27]Mille F, Llambi F, Guix C, et al. Interfering with multimerization of netrin-1 receptors triggers tumor cell death. Cell Death Differ, 2009, 16(10): 1344–1351.

[28]Arakawa H. P53, apoptosis and axon-guidance molecules. Cell Death Differ, 2005, 12(8): 1057–1065.

[29]Eveno C, Contreres JO, Hainaud P, et al. Netrin-4 overexpression suppresses primary and metastatic colorectal tumor progression. Oncol Rep, 2013, 29(1):73–78.

[30]Feng M. miR-196a overexpress in liver cancer and promote hepatoma cell proliferation[D]. Changchun:Jilin University, 2015 (in Chinese).馮默. miR-196a在肝細(xì)胞肝癌中的表達(dá)和促增殖作用研究[D]. 長(zhǎng)春: 吉林大學(xué), 2015.

[31]Jemal A, Bray F, Center MM, et al. Global cancer statistics. CA Cancer J Clin, 2011, 61(2): 69–90.

[32]Fitamant J, Guenebeaud C, Coissieux MM, et al.Netrin-1 expression confers a selective advantage for tumor cell survival in metastatic breast cancer. Proc Natl Acad Sci USA, 2008, 105(12): 4850–4855.

[33]Qi Q, Li DY, Luo HR, et al. Netrin-1 exerts oncogenic activities through enhancing yes-associated protein stability. Proc Natl Acad Sci USA, 2015, 112(23):7255–7260.

[34]Shin SK, Nagasaka T, Jung BH, et al. Epigenetic and genetic alterations in netrin-1 receptors unc5c and dcc in human colon cancer. Gastroenterology, 2007,133(6): 1849–1857.

[35]Coissieux MM, Tomsic J, Castets M, et al. Variants in the netrin-1 receptor unc5c prevent apoptosis and increase risk of familial colorectal cancer.Gastroenterology, 2011, 141(6): 2039–2046.

[36]Xu B, Shen F, Cao J, et al. Angiogenesis in liver metastasis of colo-rectal carcinoma. Front Bioscience(Landmark Ed), 2013, 18: 1435–1443.

[37]Eveno C, Broqueres-You D, Feron JG, et al. Netrin-4 delays colorectal cancer carcinomatosis by inhibiting tumor angiogenesis. Ame J Pathol, 2011, 178(4):1861–1869.

[38]Chen WQ, Zheng RS, Baade PD, et al. Cancer statistics in China, 2015. CA: A Cancer J Clin, 2016,66(2): 115–132.

[39]Ramesh G, Berg A, Jayakumar C. Plasma netrin-1 is a diagnostic biomarker of human cancers. Biomarkers,2011, 16(2): 172–180.

[40]Yildirim ME, Kefeli U, Aydin D, et al. The value of plasma netrin-1 in non-small cell lung cancer patients as diagnostic and prognostic biomarker. Tumor Biology, 2016, 37(9): 11903–11907.

[41]Link BC, Reichelt U, Schreiber M, et al. Prognostic implications of netrin-1 expression and its receptors in patients with adenocarcinoma of the pancreas. Ann Surg Oncol, 2007, 14(9): 2591–2599.

[42]Dumartin L, Quemener C, Laklai H, et al. Netrin-1 mediates early events in pancreatic adenocarcinoma progression, acting on tumor and endothelial cells.Gastroenterology, 2010, 138(4): 1595–1606, 1606.e8.

[43]Huang Q, Hua HW, Jiang F, et al. Netrin-1 promoted pancreatic cancer cell proliferation by upregulation of mdm2. Tumour Biol, 2014, 35(10): 9927–9934.

[44]An XZ, Zhao ZG, Luo YX, et al. Netrin-1 suppresses the mek/erk pathway and itgb4 in pancreatic cancer.Oncotarget, 2016, 7(17): 24719–24733.

[45]Torre LA, Bray F, Siegel RL, et al. Global cancer statistics, 2012. CA Cancer J Clin, 2015, 65(2): 87–108.

[46]Chen JM, Wang SH, Xu SX, et al. Netrin-1 protein expression in gastric cancer and its correlation with clinicopathologic features and prognosis. Sichuan Univ: Med Sci Ed, 2011, 42(6): 802–806 (in Chinese).陳金明, 王沈華, 徐善祥, 等. Netrin-1蛋白在胃癌組織中的表達(dá)與臨床病理及預(yù)后的關(guān)系. 四川大學(xué)學(xué)報(bào): 醫(yī)學(xué)版, 2011, 42(6): 802–806.

[47]Yin K, Wang LJ, Zhang X, et al. Netrin-1 promotes gastric cancer cell proliferation and invasion via the receptor neogenin through pi3k/akt signaling pathway.Oncotarget, 2017, 8(31): 51177–51189.

[48]Lv B, Song CH, Wu LJ, et al. Netrin-4 as a biomarker promotes cell proliferation and invasion in gastric cancer. Oncotarget, 2015, 6(12): 9794–9806.

[49]Hibi K, Sakata M, Sakuraba K, et al. Methylation of the dcc gene is lost in advanced gastric cancer.Anticancer Res, 2010, 30(1): 107–109.

[50]Hibi K, Sakata M, Sakuraba K, et al. Changes inUNC5Cgene methylation during human gastric carcinogenesis. Anticancer Res, 2009, 29(11):4397–4399.

[51]Kefeli U, Yildirim ME, Aydin D, et al. Netrin-1 concentrations in patients with advanced gastric cancer and its relation with treatment. Biomarkers, 2012,17(7): 663–667.

[52]Fan L, Strasser-Weippl K, Li JJ, et al. Breast cancer in China. Lancet Oncol, 2014, 15(7): e279–e289.

[53]Fidler IJ. The pathogenesis of cancer metastasis: The‘seed and soil’ hypothesis revisited. Nat Rev Cancer,2003, 3(6): 453–458.

[54]Hu J, Li G, Zhang PY, et al. A cd44v+subpopulation of breast cancer stem-like cells with enhanced lung metastasis capacity. Cell Death Dis, 2017, 8(3): e2679.

[55]Gordon AC, Salem R, Lewandowski RJ. Yttrium-90 radioembolization for breast cancer liver metastases. J Vasc Interv Radiol, 2016, 27(9): 1316–1319.

[56]Custódio-Santos T, Videira M, Brito MA. Brain metastasization of breast cancer. Biochim Biophys Acta Rev Cancer, 2017, 1868(1): 132–147.

[57]Jackson W, Sosnoski DM, Ohanessian SE, et al. Role of megakaryocytes in breast cancer metastasis to bone.Cancer Res, 2017, 77(8): 1942–1954.

[58]Gao YS, Wang CJ, Cai SP, et al. Expression of netrin-1 in invasive ductal breast cancer and its relationship with tumor metastasis. J Shandong Univ:Health Sci, 2015, 53(7): 39–42 (in Chinese).高永生, 王春健, 蔡淑萍, 等. Netrin-1在乳腺浸潤(rùn)性導(dǎo)管癌中的表達(dá)及與轉(zhuǎn)移的相關(guān)性. 山東大學(xué)學(xué)報(bào): 醫(yī)學(xué)版, 2015, 53(7): 39–42.

[59]Grandin M, Mathot P, Devailly G, et al. Inhibition of DNA methylation promotes breast tumor sensitivity to netrin-1 interference. EMBO Mol Med, 2016, 8(8):863–877.

[60]Delloye-Bourgeois C, Fitamant J, Paradisi A, et al.Netrin-1 acts as a survival factor for aggressive neuroblastoma. J Exp Med, 2009, 206(4): 833–847.

[61]Paradisi A, Creveaux M, Gibert B, et al. Combining chemotherapeutic agents and netrin-1 interference potentiates cancer cell death. EMBO Mol Med, 2013,5(12): 1821–1834.

[62]Grandin M, Meier M, Delcros JG, et al. Structural decoding of the netrin-1/unc5 interaction and its therapeutical implications in cancers. Cancer Cell,2016, 29(2): 173–185.

[63]Esseghir S, Kennedy A, Seedhar P, et al. Identification ofNTN4,TRA1, andSTC2as prognostic markers in breast cancer in a screen for signal sequence encoding proteins. Clin Cancer Res, 2007, 13(11): 3164–3173.

[64]Xu XP, Yan QY, Wang YA, et al. NTN4 is associated with breast cancer metastasis via regulation of emt-related biomarkers. Oncol Rep, 2017, 37(1):449–457.

[65]Yuan Y, Leszczynska M, Konstantinovsky S, et al.Netrin-4 is upregulated in breast carcinoma effusions compared to corresponding solid tumors. Diagnostic Cytopathol, 2011, 39(8): 562–566.

[66]Larrieu-Lahargue F, Welm AL, Thomas KR, et al.Netrin-4 induces lymphangiogenesisin vivo. Blood,2010, 115(26): 5418–5426.

[67]iegel R, Ward E, Brawley O, et al. Cancer statistics,2011: The impact of eliminating socioeconomic and racial disparities on premature cancer deaths. CA: A Cancer J Clin, 2011, 61(4): 212–236.

[68]Papanastasiou AD, Pampalakis G, Katsaros D, et al.Netrin-1 overexpression is predictive of ovarian malignancies. Oncotarget, 2011, 2(5): 363–367.

[69]Li Y, Xiao M, Guo FC. The role of sox6 and netrin-1 in ovarian cancer cell growth, invasiveness, and angiogenesis. Tumour Biol, 2017, 39(5):1010428317705508.

[70]Qin YR, Tang H, Xie FJ, et al. Characterization of tumor-suppressive function ofSOX6in human esophageal squamous cell carcinoma. Clin Cancer Res,2011, 17(1): 46–55.

[71]Carter T, Shaw H, Cohn-Brown D, et al. Ipilimumab and bevacizumab in glioblastoma. Clin Oncol, 2016,28(10): 622–626.

[72]Ylivinkka I, Sihto H, Tynninen O, et al. Motility of glioblastoma cells is driven by netrin-1 induced gain of stemness. J Experim Clin Cancer Res, 2017, 36(1): 9.

[73]Ylivinkka I, Hu YZ, Chen P, et al. Netrin-1-induced activation of notch signaling mediates glioblastoma cell invasion. J Cell Sci, 2013, 126(11): 2459–2469.

[74]Wan XL. Role of Netrin-1 in Human Glioma[D].Changchun: Northeast Normal University, 2013 (in Chinese).萬(wàn)茜淋. Netrin-1在人類神經(jīng)膠質(zhì)瘤中表達(dá)的研究[D].長(zhǎng)春: 東北師范大學(xué), 2013.

[75]Chen JY, He XX, Ma C, et al. Netrin-1 promotes glioma growth by activating nf-κB via UNC5A. Sci Rep, 2017, 7(1): 5454.

[76]Sanvoranart T, Supokawej A, Kheolamai P, et al.Targeting netrin-1 in glioblastoma stem-like cells inhibits growth, invasion, and angiogenesis. Tumour Biol, 2016, 37(11): 14949–14960.

[77]Li L, Hu YZ, Ylivinkka I, et al. Netrin-4 protects glioblastoma cells from temozolomide induced senescence. PLoS ONE, 2013, 8(11): e80363.

[78]Li L, Teng C, Liu JY, et al. Influence from Netrin-4 to ERK/MAPK signal pathway in GBM cell lines. Mod Oncol, 2017, 25(5): 697–700 (in Chinese).李里, 滕沖, 劉佳音, 等. Netrin-4對(duì)膠質(zhì)母細(xì)胞瘤細(xì)胞 erk/mapk信號(hào)通路的影響. 現(xiàn)代腫瘤醫(yī)學(xué),2017, 25(5): 697–700.

[79]Cheung NKV, Dyer MA. Neuroblastoma: Developmental biology, cancer genomics and immunotherapy. Nat Rev Cancer, 2013, 13(6): 397–411.

[80]Li L, Hu YZ, Ylivinkka I, et al. Netrin-4 protects glioblastoma cells from temozolomide induced senescence. PLoS ONE, 2013, 8(11): e80363.

[81]Picard M, Petrie RJ, Antoine-Bertrand J, et al. Spatial and temporal activation of the small gtpases rhoa and rac1 by the netrin-1 receptor unc5a during neurite outgrowth. Cellul Signall, 2009, 21(12): 1961–1973.

[82]Li XD, Saint-Cyr-Proulx E, Aktories K, et al. Rac1 and Cdc42 but not RhoA or rho kinase activities are required for neurite outgrowth induced by the netrin-1 receptor DCC (Deleted in Colorectal Cancer) in n1e-115 neuroblastoma cells. J Biol Chem, 2002,277(17): 15207–15214.

[83]Zhu YY, Li YY, Haraguchi S, et al. Dependence receptor unc5d mediates nerve growth factor depletion-induced neuroblastoma regression. J Clin Investigat, 2013, 123(7): 2935–2947.

[84]Wang H, Zhang B, Gu M, et al. Overexpression of the dependence receptor UNC5H4 inhibits cell migration and invasion, and triggers apoptosis in neuroblastoma cell. Tumour Biol, 2014, 35(6): 5417–5425.

[85]Wang H, Gu M, Li S, et al. Netrin-1 prevents apoptosis on SH-SH5Y cells induced by dependence receptor Unc5H4. Chin J Mod Appl Pharm, 2014, 31(12):1431–1434 (in Chinese).王弘, 顧敏, 李爽, 等. Netrin-1抑制受體unc5h4誘導(dǎo)人神經(jīng)母細(xì)胞瘤細(xì)胞凋亡. 中國(guó)現(xiàn)代應(yīng)用藥學(xué),2014, 31(12): 1431–1434.

[86]Milla LA, Arros A, Espinoza N, et al. Neogenin1 is a sonic hedgehog target in medulloblastoma and is necessary for cell cycle progression. Int J Cancer,2014, 134(1): 21–31.

[87]Lejmi E, Bouras I, Camelo S, et al. Netrin-4 promotes mural cell adhesion and recruitment to endothelial cells. Vasc Cell, 2014, 6(1): 1–13.

[88]Matsunaga E, Tauszig-Delamasure S, Monnier PP, et al.Rgm and its receptor neogenin regulate neuronal survival. Nat Cell Biol, 2004, 6(8): 749–755.

[89]O’Leary CJ, Bradford D, Chen M, et al. The netrin/rgm receptor, neogenin, controls adult neurogenesis by promoting neuroblast migration and cell cycle exit. Stem Cells, 2015, 33(2): 503–514.

[90]Villanueva AA, Falcón P, Espinoza N, et al. The netrin-4/Neogenin-1 axis promotes neuroblastoma cell survival and migration. Oncotarget, 2016, 8(6):9767–9782.

[91]Akino T, Han XZ, Nakayama H, et al. Netrin-1 promotes medulloblastoma cell invasiveness and angiogenesis, and demonstrates elevated expression in tumor tissue and urine of patients with pediatric medulloblastoma. Cancer Res, 2014, 74(14): 3716–3726.

[92]Huang ZQ, Yao HX, Lin LE, et al. Expression of Netrin-1 in multiple myeloma and its clinical significance. J Clin Exp Med, 2017, 15(21):2121–2124 (in Chinese).黃昭前, 姚紅霞, 林麗娥, 等. Netrin-1在多發(fā)性骨髓瘤患者骨髓細(xì)胞中的表達(dá)水平和臨床意義. 臨床和實(shí)驗(yàn)醫(yī)學(xué)雜志, 2017, 15(21): 2121–2124.

[93]Chen HW, Chen Q, Luo QD. Expression of netrin-1 by hypoxia contributes to the invasion and migration of prostate carcinoma cells by regulating yap activity.Exp Cell Res, 2016, 349(2): 302–309.

[94]Kaufmann S, Kuphal S, Schubert T, et al. Functional implication of netrin expression in malignant melanoma. Cell Oncol, 2009, 31(6): 415–422.

[95]Zuo YL, Yang JF, He J, et al. An uncoordinated-5 homolog b receptor monoclonal antibody regulates a375 melanoma cell migration. Monocl Antibod Immunod Immunoth, 2014, 33(4): 280–286.

[96]Zuo YL, Yang JF, He Y, et al. Preparation of a uncoordinated-5 homolog b monoclonal antibody and its effect on melanoma cell migrationin vitro. Chin J Cellul Mol Immunol, 2015, 31(9): 1259–1262 (in Chinese).左元玲, 楊劍峰, 何楊, 等. UNC5B單克隆抗體的制備及對(duì)黑素瘤細(xì)胞體外遷移能力的影響. 細(xì)胞與分子免疫學(xué)雜志, 2015, 31(9): 1259–1262.

[97]Broutier L, Creveaux M, Vial J, et al. Targeting netrin-1/dcc interaction in diffuse large B-cell and mantle cell lymphomas. Embo Mol Med, 2016, 8(2):96–104.

[98]Zhang J, Zheng FX, Yu G, et al. Mir-196a targets netrin 4 and regulates cell proliferation and migration of cervical cancer cells. Biochem Biophys Res Commun,2013, 440(4): 582–588.

[99]Zhao M, Tang QQ, Wu W, et al. Mir-20a contributes to endometriosis by regulating ntn4 expression. Mol Biol Rep, 2014, 41(9): 5793–5797.

[100]Jia Y. Roles and mechanism of Netrin4 and neogenin infibrosarcoma cell HT1080[D]. Xiamen: Xiamen University, 2014 (in Chinese).賈茵. Netrin4和neogenin在成纖維肉瘤細(xì)胞ht1080中的作用及其機(jī)制探討[D]. 廈門: 廈門大學(xué), 2014.

[101]Leibinger M, Andreadaki A, Fischer D. Role of mtor in neuroprotection and axon regeneration after inflammatory stimulation. Neurobiol Dis, 2012, 46(2): 314–324.

[102]Weichhart T, Hengstschl?ger M, Linke M. Regulation of innate immune cell function by mTOR. Nat Rev Immunol, 2015, 15(10): 599–614.