SLC39A6：鋅離子轉(zhuǎn)運蛋白，抗體偶聯(lián)藥物ADC新晉靶點！
日期：2022-12-02 15:34:12
近年來，抗體藥物偶聯(lián)物（ADC）新藥交易迎來爆發(fā)，更多企業(yè)開始進軍ADC領(lǐng)域。據(jù)Pharmsnap數(shù)據(jù)庫顯示，目前ADC新藥交易合作或授權(quán)的項目近100項，達成的交易金額超過400億美元。2022年迄今已達17項，SLC39A6成功上位，成為抗體藥ADC新晉靶點之一。
SLC39A6作為新發(fā)的癌癥相關(guān)因子，雖研究起步較晚，但近年來進展很快。早前，制藥巨擘默沙東（Merck）與全球ADC領(lǐng)域的龍頭公司Seagen達成協(xié)議，支付16億美元收購其靶向SLC39A6/LIV-1的ADC藥物（Ladiratuzumab Vedotin），用于轉(zhuǎn)移性三陰性乳腺癌臨床研究。默沙東還承諾將在研發(fā)的里程碑上投入高達26億美元。目前，越來越多的研究表明，SLC39A6在多種腫瘤細胞中高表達，但在正常組織有限表達，成為ADC治療有希望的候選者！今天我們一起來了解一下，抗體藥ADC新晉靶點--鋅轉(zhuǎn)運蛋白SLC39A6！
1. 什么是鋅轉(zhuǎn)運蛋白家族？
SLC39A家族和SLC30A家族為已發(fā)現(xiàn)的哺乳動物中的兩個鋅轉(zhuǎn)運蛋白家族，分別調(diào)節(jié)鋅離子轉(zhuǎn)進（influx）和轉(zhuǎn)出（efflux）細胞的運輸過程（圖1） [1, 6]。SLC39A家族（又稱ZIP家族），共有14個成員，即ZIP1-ZIP14 [2-3]。該家族多個成員已被證明可促進細胞外或細胞器內(nèi)的鋅離子轉(zhuǎn)運到細胞質(zhì)，具有鋅離子吸收（influx）攝取功能。而SLC30A家族（又稱ZIT家族），共有10個成員，與SLC39A/ZIP家族的功能相反，多個家族成員可協(xié)助鋅離子從細胞質(zhì)內(nèi)流出到細胞外或流進到細胞器內(nèi)，具有釋放（efflux）鋅離子功能 [2-3]。越來越多的證據(jù)表明，鋅轉(zhuǎn)運蛋白家族不僅直接參與細胞的鋅離子穩(wěn)態(tài)代謝，同時還通過復(fù)雜的機制影響著腫瘤和代謝疾病的發(fā)生和發(fā)展，其成員逐漸成為鋅營養(yǎng)和代謝疾病的研究熱點 [4-5]。
鋅轉(zhuǎn)運蛋白的兩大家族成員在各種亞細胞器、細胞及器官中的分布不同，如ZIP4/SLC39A4、ZIP5/SLC39A5、ZIP6/SLC39A6、ZIP10、ZIP14以及ZnT1/SLC30A1主要分布在細胞膜上，而其它眾多的鋅轉(zhuǎn)運蛋白主要分布在不同的細胞器膜上；在組織器官分布方面，ZIP4/SLC39A4和ZnT5分布小腸上皮細胞，ZIP10和ZnT1分布在腎臟上皮細胞，ZIP5、ZnT1及ZnT2分布在胰腺腺細胞，ZIP8及ZIP10分布在血細胞等等 [4-5]。
圖1. 鋅轉(zhuǎn)運蛋白示意圖 [6]
2. 什么是SLC39A6？
SLC39A6（又名ZIP6或LIV-1）屬于鋅轉(zhuǎn)運蛋白家族ZIP成員之一。根據(jù)蛋白結(jié)構(gòu)特點，哺乳動物ZIP蛋白又被分成4個亞家族，即亞家族I，II，LIV-1和gufA。SLC39A6是LIV-1亞家族，存在755個氨基酸和433個氨基酸兩種長度的異構(gòu)體。SLC39A6蛋白包括6-8個跨膜域，其氨基和羧基末端均位于細胞外或囊泡內(nèi)，第3和4跨膜域間存在富含組氨酸的結(jié)構(gòu)域，該區(qū)域具有結(jié)合鋅離子的功能，并具有金屬蛋白酶活性（圖2） [7]。SLC39A6的主要功能是將鋅離子從細胞外轉(zhuǎn)移至細胞內(nèi)，并且是酶、轉(zhuǎn)錄因子和信號分子發(fā)揮作用機制中的關(guān)鍵蛋白 [8-9]。
SLC39A6的表達較為廣泛，在胚盤、乳腺及前列腺中表達最為豐富。SLC39A6蛋白主要定位在細胞膜上，將鋅離子由胞外運至胞漿，或?qū)⒓毎鲀?nèi)的鋅離子轉(zhuǎn)運至胞漿。SLC39A6可通過調(diào)節(jié)鋅離子穩(wěn)衡代謝，在機體免疫反應(yīng)中發(fā)揮重要作用。近年來，諸多關(guān)于SLC39A6的研究表明，SLC39A6在許多癌癥中表達異常，如食管癌、宮頸癌、肺癌以及乳腺癌等等，提示SLC39A6蛋白在癌癥發(fā)生過程具有重要作用 [10-15]。
圖2. SLC39A6結(jié)構(gòu)示意圖 [7]
3. SLC39A6相關(guān)的調(diào)節(jié)機制
鋅轉(zhuǎn)運體SLC39A6自發(fā)現(xiàn)以來一直受到廣泛關(guān)注，其參與腫瘤相關(guān)的機制正在深入研究中，如JAK/STAT、PI3K/AKT、EGFR-Ras-ERK、MAPK/ERK等信號通路。SLC39A6作為腫瘤相關(guān)的潛力分子，SLC39A6在腫瘤發(fā)生過程的分子機制并不是單一的，其復(fù)雜的作用機制有待進一步闡明。
3.1 SLC39A6與STAT3信號
信號轉(zhuǎn)導(dǎo)及轉(zhuǎn)錄激活因子３（STAT3）和鋅指蛋白（Snail）是上皮間質(zhì)轉(zhuǎn)化（EMT）通路的主要標(biāo)志物。SLC39A6被鑒定為STAT3的下游靶基因，促進鋅指蛋白Snail的入核。而Snail基因可降低與細胞黏附有關(guān)的基因表達，在上皮細胞轉(zhuǎn)化為間質(zhì)細胞的過程中起重要作用 [16-17]。因此，SLC39A6可通過影響Snail表達，調(diào)節(jié)細胞分化過程。
另有研究指出，在乳腺癌中，SLC39A6運輸鋅離子進入細胞，會抑制GSK3β的活性，失活的GSK3β不能磷酸化Snail，使得Snail留在核內(nèi)抑制細胞交聯(lián)基因E-cadherin的轉(zhuǎn)錄，促進細胞遷移 [18]。研究發(fā)現(xiàn)，在食管鱗癌中，SLC39A6是STAT3的下游靶基因，SLC39A6通過調(diào)控SNAIL2基因，參與食管鱗癌的EMT過程 [19]。
3.2 SLC39A6與EGFR信號
EGFR屬于膜受體酪氨酸激酶（receptor tyrosine kinase，RTK）家族，與表皮生長因子EGF、轉(zhuǎn)化生長因子α（TGFα）、肝素結(jié)合EGF樣生長因子（HBEGF）等配體結(jié)合后，可招募其他受體底物蛋白并激活一系列下游信號，如調(diào)控分裂增殖的Ras/MAPK、與癌癥相關(guān)的PI3K/AKT通路等。研究發(fā)現(xiàn)，腫瘤微環(huán)境中SLC39A6的表達會被例如EGF、IGF-1（insulinlike growth factor）等生長因子所激活 [20]。
在前列腺腫瘤細胞系中，過表達SLC39A6后會引起EGFR磷酸化和ERK磷酸化，其下游信號可誘導(dǎo)細胞的遷移和入侵行為 [21]。在食管癌中，過表達SLC39A6的細胞，可激活A(yù)KT和ERK通路，調(diào)節(jié)MMP1、MMP3、MYC和SLUG表達（圖3） [22]。目前，越來越多的研究揭示SLC39A6中可作為重要的癌相關(guān)基因，但SLC39A6在癌癥中具體的作用機制仍需要進一步深入地研究。
圖3. SLC39A6通過激活A(yù)KT和ERK途徑來促進ESCC [22]
4. SLC39A6在癌癥治療中的作用
在惡性腫瘤中，體內(nèi)鋅穩(wěn)態(tài)的紊亂與惡性腫瘤的進展相關(guān)，癌細胞通常具有上調(diào)的鋅轉(zhuǎn)運蛋白和鋅離子。因此，SLC39A6作為鋅的重要轉(zhuǎn)運蛋白在惡性腫瘤的進展具有重要作用。目前在消化系統(tǒng)腫瘤（如食管癌、肝癌及胰腺癌）、乳腺癌、膠質(zhì)瘤、前列腺癌、肺癌、膀胱癌等中發(fā)現(xiàn)SLC39A6均有異常表達。
4.1 SLC39A6與食管癌
在食管癌中，SLC39A6表達下調(diào)可抑制食管癌細胞的增殖和侵襲。高表達的SLC39A6與腫瘤侵襲性、細胞內(nèi)鋅水平和患者生存時間有關(guān)。進一步的分析表明，在SLC39A6表達上調(diào)的細胞株中，基質(zhì)金屬蛋白酶 1、基質(zhì)金屬蛋白酶 3、髓細胞增生原癌基因和表面免疫球蛋白呈現(xiàn)高表達。陸續(xù)的研究認為SLC39A6在食管鱗癌（ESCC）中具有促腫瘤作用，提示SLC39A6可作為高?；颊叩脑缙谔綔y器和預(yù)后的生物標(biāo)志物，因而SLC39A6的靶向治療可能是阻斷ESCC的一種潛在治療策略 [22, 23-24]。
4.2 SLC39A6與肝癌
通過miRNA微陣列分析發(fā)現(xiàn)，在發(fā)生轉(zhuǎn)移的肝癌細胞系MHCC-97L，MHCC-97H和HCC-LM3中，miR-192在肝癌組織中表達下降，在肝癌患者血管細胞浸潤組的表達水平較非浸潤組下降。而SLC39A6是miR-192的直接和功能靶點，且SLC39A6與miR-192呈負相關(guān)，所以SLC39A6可能促進肝癌細胞的遷移和侵襲 [25-27]。
4.3 SLC39A6與胰腺癌
有研究揭示，在72例胰腺癌組織中，SLC39A6表達水平與腫瘤大小及淋巴浸潤有關(guān)。通過下調(diào)SLC39A6表達，構(gòu)建裸鼠模型，體內(nèi)和體外實驗的結(jié)果表明，抑制SLC39A6可抑制胰腺癌細胞增殖、遷移。由此說明，SLC39A6表達與胰腺癌腫瘤增殖相關(guān)，抑制SLC39A6可顯著減少胰腺癌細胞轉(zhuǎn)移或擴散 [28-29]。
4.4 SLC39A6與乳腺癌
SLC39A6最初在乳腺癌細胞系中被鑒定為雌激素誘導(dǎo)基因，故認為與乳腺癌的發(fā)生和發(fā)展密切相關(guān)。有關(guān)RNAseq分析的結(jié)果顯示，在乳腺癌luminal分型組中，鑒定出SLC39A6、PGR、ESR1、BCL2、GATA3和NAT1在轉(zhuǎn)移的腫瘤中出現(xiàn)下調(diào)，提示SLC39A6與乳腺腫瘤的分級、大小和分期可能有關(guān) [30-32]。目前，靶向SLC39A6的ADC藥物L(fēng)adiratuzumab vedotin（SGN-LIVI1），已在轉(zhuǎn)移性三陰性乳腺癌的臨床研究中與帕博利珠單抗（Keytruda）聯(lián)合使用 [18, 33]。
4.5 SLC39A6與其它腫瘤
此外，其它多種腫瘤中也發(fā)現(xiàn)了SLC39A6異常表達，包括膠質(zhì)瘤、卵巢癌、膀胱癌、前列腺癌、頭頸癌、肺癌、和胃癌 [15, 21, 34-38]。SLC39A6被認為處于細胞質(zhì)膜上，可導(dǎo)致細胞內(nèi)鋅水平增加，且SLC39A6在對類固醇激素敏感的組織如胎盤和前列腺中高表達 [1, 21, 39]。在胃癌中，SLC39A6的單核苷酸多態(tài)性rs1050631與手術(shù)切除胃腺瘤的患者預(yù)后相關(guān) [38]。在膠質(zhì)瘤中，SLC39A6高表達，并與級別呈顯著相關(guān) [34]。在非小細胞肺癌中，敲低SLC39A6可抑制非小細胞肺癌細胞的增殖、侵襲和遷移，并使細胞停留在G1期 [15, 40]。
5. SLC39A6的臨床應(yīng)用前景
來自Pharmsnap的數(shù)據(jù)顯示，已有1款基于SLC39A6的ADC抗體藥（Ladiratuzumab vedotin）處于臨床2期，用于腺癌、食管癌、乳腺癌等多種腫瘤治療。涉及SLC39A6靶點，有11項臨床試驗進行中，用于非小細胞肺癌和三陰性乳腺癌等實體瘤的治療。目前已上市的ADC產(chǎn)品共14款，已進入臨床的在研產(chǎn)品主要聚焦在HER2和TROP2，此外FRα、VEGF、CD19、CD22、CD30、EGFR、KAAG1、NaPi2b、c-MET、CLDN18.2、HER3、LIV-1/SLC39A6、Nectin-4、MSLN等靶點也紛紛入局。近年來，ADC賽道競爭白熱化，差異化布局，選擇其他潛力靶點已是眾多ADC研發(fā)的制勝法寶。SLC39A6作為新發(fā)腫瘤相關(guān)分子，新晉ADC靶點，有望在食管癌和乳腺癌等癌癥治療方面取得突破。
為鼎力協(xié)助各藥企針對SLC39A6在食管癌和乳腺癌等其它腫瘤在臨床中的研究，CUSABIO推出SLC39A6活性蛋白產(chǎn)品，（Code：CSB-BP621669HU1），助力您在SLC39A6機制方面的研究或其潛在臨床價值的探索。
Recombinant Human SLC39A6, partial (Active) (CSB-BP621669HU1)
The purity was greater than 95% as determined by SDS-PAGE.
Immobilized Human SLC39A6 at 1 μg/ml can bind Anti-SLC39A6 recombinant antibody (CSB-RA621669MA1HU), the EC50 is 0.6873-0.9010 ng/mL.
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