陳娜,蘇萍
鎘對雄(男)性生殖系統(tǒng)毒性的研究進(jìn)展
陳娜,蘇萍△
鎘(Cadmium)是有毒重金屬,其對肝、腎、肺、骨骼、生殖系統(tǒng)及血液系統(tǒng)均有毒性,雄性生殖系統(tǒng)對鎘的毒性更加敏感。鎘進(jìn)入機(jī)體后抑制類固醇激素合成急性調(diào)節(jié)蛋白(steroidogenic acute regulatory protein,StAR)、膽固醇側(cè)鏈裂解酶(P450scc)、3β-羥類固醇脫氫酶(3β-HSD)以及17β-HSD等睪酮合成關(guān)鍵酶活性,使睪酮合成受到抑制,血漿睪酮水平下降。鎘可誘導(dǎo)生殖細(xì)胞凋亡,引起睪丸細(xì)胞和細(xì)胞器的超微結(jié)構(gòu)變化,最終損害雄性生殖系統(tǒng)的功能。內(nèi)分泌紊亂、氧化應(yīng)激失衡和凋亡通路的激活在鎘毒性機(jī)制中發(fā)揮著重要作用,某些抗氧化劑和抗凋亡藥物可在一定程度上拮抗鎘的生殖毒性。綜述鎘對雄(男)性生殖能力的影響和毒性機(jī)制。
鎘;生殖器疾病,男(雄)性;細(xì)胞凋亡;氧化性應(yīng)激
鎘(Cadmium)污染是最嚴(yán)重的環(huán)境污染之一[1]。鎘主要通過受污染的食物和水進(jìn)入人體,在人體的半衰期長達(dá)20~30年[2]。大量研究證實,鎘進(jìn)入機(jī)體后可導(dǎo)致機(jī)體急性或慢性損傷,如肝、腎功能不全,睪丸病變及其他系統(tǒng)疾病[3-4]。隨著工業(yè)化水平不斷提高,工業(yè)廢物和城市垃圾排放量增大以及農(nóng)業(yè)生產(chǎn)大量使用化學(xué)肥料,使得空氣、土壤以及農(nóng)作物中鎘含量不斷上升。在過去的一個世紀(jì)中,工業(yè)鎘應(yīng)用的增加導(dǎo)致環(huán)境污染,使得人暴露于鎘污染,有研究表明,大約有2.3%的美國人尿鎘水平升高[5-6]。人生殖系統(tǒng)對鎘污染更加敏感,近年調(diào)查發(fā)現(xiàn),鎘可降低男性精液質(zhì)量,導(dǎo)致男性生殖能力下降甚至不育[7-9]。
鎘對雄性生殖能力影響,主要表現(xiàn)在鎘可致睪丸內(nèi)分泌功能下降和精子生成障礙,影響精子成熟,造成生育力損害。
1.1對生殖內(nèi)分泌功能的影響鎘對雄性生殖內(nèi)分泌功能的影響是潛在而顯著的,鎘通過降低睪酮合成相關(guān)酶活性進(jìn)而降低機(jī)體睪酮含量。Manna等[10]研究顯示,一定劑量鎘暴露會明顯降低Wistar大鼠睪丸類固醇合成限速酶3β-羥基類固醇脫氫酶(3β-HSD)和17β-HSD活性,且血漿睪酮水平、精子數(shù)量均明顯降低。重金屬作為一大類環(huán)境內(nèi)分泌干擾物,可影響機(jī)體內(nèi)分泌,越來越多的研究著眼于探索重金屬鎘對機(jī)體不同發(fā)育階段的生殖毒性作用。青春期鎘暴露可導(dǎo)致成年雄(男)性生殖功能障礙。研究發(fā)現(xiàn)青春期小鼠鎘暴露后,小鼠體內(nèi)血漿睪酮水平和睪酮合成酶活性均下降,睪酮水平的下降可能影響青春期睪丸正常發(fā)育和精子發(fā)生,致使成年小鼠生殖功能障礙[11]。Interdonato等[12]通過招募111名12~14歲無抽煙史、祖籍起源相同的男性青少年,檢測其尿鎘水平和睪丸體積,發(fā)現(xiàn)生活在工業(yè)區(qū)的青少年尿鎘水平比遠(yuǎn)離工業(yè)區(qū)的青年高且睪丸體積較遠(yuǎn)離工業(yè)區(qū)的青年小,同時檢測血清激素水平發(fā)現(xiàn)工業(yè)區(qū)青少年體內(nèi)睪酮水平較非工業(yè)區(qū)的青少年低,認(rèn)為鎘的生殖毒性與年齡有關(guān),青少年期鎘暴露可能導(dǎo)致性成熟和睪丸發(fā)育延遲且鎘對青少年生殖系統(tǒng)的毒性作用較成年人更敏感。關(guān)于鎘的毒性和機(jī)制的研究中,多數(shù)采用的染鎘途徑是經(jīng)皮下或腹腔給藥,且染毒劑量偏高,這與自然條件下動物和人類鎘暴露的途徑和劑量是不一致的,未來還需更多、更加細(xì)化的研究來探索鎘與人類生殖健康的關(guān)系。
1.2對生精功能的影響鎘對雄性動物和人類生精功能具有明顯抑制作用。鎘對機(jī)體生精功能的抑制效應(yīng)呈劑量依賴性,一方面鎘可影響精子形態(tài),導(dǎo)致畸形精子數(shù)量增加,另一方面,其可抑制精子成熟,甚至隨著鎘濃度提高可導(dǎo)致精子發(fā)生完全停滯在精母細(xì)胞階段[13]。鎘不僅導(dǎo)致精子發(fā)生停滯在精母細(xì)胞階段,也可破壞精母細(xì)胞甚至是精原細(xì)胞的生長發(fā)育和正常形態(tài)結(jié)構(gòu),誘導(dǎo)生殖細(xì)胞凋亡,從而導(dǎo)致生精功能障礙[14]。近年研究表明,血清鎘水平增高致使男性精液質(zhì)量下降,甚至導(dǎo)致男性不育,具體表現(xiàn)為精液量減少、精子活力下降以及畸形率升高[9,15]。
鎘暴露可導(dǎo)致實驗動物睪丸萎縮,質(zhì)量下降,精曲小管纖維化以及生精細(xì)胞脫落等形態(tài)結(jié)構(gòu)上的改變。近年隨著電鏡技術(shù)的不斷發(fā)展,支持細(xì)胞、線粒體以及內(nèi)質(zhì)網(wǎng)等超微結(jié)構(gòu)逐漸被學(xué)者們所熟悉,因此鎘對睪丸結(jié)構(gòu)的影響研究不只是光鏡下的結(jié)果。研究發(fā)現(xiàn)一定劑量鎘暴露后可導(dǎo)致Balb/c小鼠支持細(xì)胞空泡化,支持細(xì)胞與精原細(xì)胞間緊密連接分離,生精細(xì)胞胞核出現(xiàn)異染色質(zhì)[4]。Lamas等[16]也證實鎘暴露后可導(dǎo)致電鏡下睪丸支持細(xì)胞內(nèi)形成大量脂滴,血睪屏障結(jié)構(gòu)紊亂,生殖細(xì)胞出現(xiàn)明顯的變性,精曲小管腔中的精子鞭毛大量黏附,線粒體形態(tài)和結(jié)構(gòu)異常。電鏡出現(xiàn)后學(xué)者們逐漸開始認(rèn)識到支持細(xì)胞以及支持細(xì)胞之間的血睪屏障結(jié)果對于維持生精細(xì)胞生殖發(fā)育的重要作用。有學(xué)者證實鎘對血睪屏障結(jié)構(gòu)的損傷早于對生殖細(xì)胞的損傷[17]。
目前尚不完全清楚鎘對雄性生殖毒性的作用機(jī)制。相對于機(jī)體其他組織、器官,鎘對睪丸的毒性影響較明顯。目前研究發(fā)現(xiàn)鎘對雄性生殖毒性存在以下幾種作用機(jī)制。
3.1氧化應(yīng)激與男性生殖系統(tǒng)功能衰竭相關(guān)的研究表明,氧化應(yīng)激是一種主要的致病因素[18]。鎘離子是一種較強(qiáng)的脂質(zhì)過氧化誘導(dǎo)劑,可引起細(xì)胞脂質(zhì)過氧化,造成膜結(jié)構(gòu)和功能破壞,細(xì)胞線粒體受損,已證實鎘離子中毒后機(jī)體脂質(zhì)過氧化物顯著增多。另一方面,鎘通過抑制多種抗氧化酶活性、促進(jìn)活性氧族物質(zhì)和脂質(zhì)過氧化反應(yīng)產(chǎn)物產(chǎn)生,使細(xì)胞DNA出現(xiàn)氧化受損,導(dǎo)致細(xì)胞壞死[19-20]。鋅參與機(jī)體多種酶的代謝,對于維持細(xì)胞膜結(jié)構(gòu)的穩(wěn)定性不可或缺。鋅與鎘均屬于ⅡB族元素,理化特性相似,鋅是抗氧化劑超氧化物歧化酶(SOD)結(jié)構(gòu)的重要元件,對于維持SOD活性至關(guān)重要,鎘可競爭性結(jié)合睪丸中鋅結(jié)合蛋白的作用位點,導(dǎo)致睪丸氧化應(yīng)激失衡,致使睪丸組織結(jié)構(gòu)和功能受損,鋅可明顯拮抗鎘導(dǎo)致的睪丸氧化損傷[21]。最新的研究表明,孕期和哺乳期鎘暴露可影響睪丸的發(fā)育,導(dǎo)致35日齡幼鼠睪丸異常生精小管增多、睪丸質(zhì)量降低以及血漿睪酮水平下降,鎘可中斷鋅的代謝導(dǎo)致孕期和哺乳期母體內(nèi)鋅缺乏,并進(jìn)一步導(dǎo)致胎兒和哺乳期幼崽鋅缺乏[22]。而經(jīng)典的抗氧化劑如維生素E、維生素C等被證實可以拮抗鎘對睪丸的氧化損傷,無疑是對鎘具有氧化毒性的強(qiáng)有力證明。維生素E和β-胡蘿卜素是天然的抗氧化物,其通過抑制自由基活性而保護(hù)正常細(xì)胞不受傷害,抗氧化劑和活性氧清除劑對鎘毒性的抑制作用進(jìn)一步證實了鎘的作用機(jī)制與氧化失衡密切相關(guān)[23]。β-隱黃素(β-cryptoxanthin)是一種可以降低機(jī)體氧化應(yīng)激類胡蘿卜素,研究表明,β-隱黃素因可以顯著拮抗鎘引起的機(jī)體氧化應(yīng)激失衡,使得丙二醛(MDA)含量下降、SOD、過氧化氫酶(CAT)、谷胱甘肽(GSH)活性增加,從而有效抗衡了鎘導(dǎo)致的生殖細(xì)胞凋亡[24]。
3.2凋亡機(jī)制睪丸細(xì)胞如生精細(xì)胞的凋亡是鎘生殖毒性作用的重要環(huán)節(jié),睪丸細(xì)胞凋亡信號通路的激活被認(rèn)為是鎘毒性的重要機(jī)制。研究發(fā)現(xiàn)氧化應(yīng)激失衡和凋亡通路均在鎘致睪丸細(xì)胞的凋亡過程中發(fā)揮著作用[4,14,20]。半胱天冬酶-3(Caspase-3)和Caspase-9是經(jīng)典的導(dǎo)致細(xì)胞凋亡的蛋白酶,在組織細(xì)胞凋亡和腫瘤的發(fā)生中發(fā)揮重要作用。鎘暴露可導(dǎo)致華溪蟹睪丸組織氧化應(yīng)激失衡,表現(xiàn)為脂質(zhì)過氧化終產(chǎn)物MDA以及活性氧類物質(zhì)過氧化氫(H2O2)等含量增加,DNA碎片率增加,同時Caspase-3和Caspase-9活性明顯增加,睪丸細(xì)胞凋亡數(shù)也隨著鎘染毒濃度增加而增加,研究者認(rèn)為,鎘可以誘導(dǎo)睪丸氧化應(yīng)激和細(xì)胞凋亡,該凋亡過程可能與線粒體介導(dǎo)的Caspase-3和Caspase-9通路激活有關(guān)[20]。Kim等[25]發(fā)現(xiàn),鎘處理大鼠8 h可測得睪丸生殖細(xì)胞凋亡,脂質(zhì)過氧化終產(chǎn)物MDA含量增加;進(jìn)一步探索鎘凋亡作用機(jī)制時發(fā)現(xiàn)氧化應(yīng)激介導(dǎo)凋亡誘導(dǎo)因子(AIF)從線粒體到細(xì)胞核的轉(zhuǎn)運,抗壞血酸可抑制AIF從線粒體到細(xì)胞核的轉(zhuǎn)運,認(rèn)為細(xì)胞凋亡由氧化應(yīng)激引起,而鎘所致的氧化應(yīng)激可能與AIF從線粒體轉(zhuǎn)運到細(xì)胞核有關(guān)??沟蛲鏊幬锖涂寡趸瘎┛梢杂行м卓规k的生殖細(xì)胞凋亡率,進(jìn)一步證實了凋亡機(jī)制和氧化應(yīng)激失衡在鎘毒性中的作用[26-28]。
3.3內(nèi)分泌機(jī)制睪酮是男性體內(nèi)最重要的激素之一,睪酮可維持生精。動物實驗表明鎘可明顯降低機(jī)體睪酮水平,導(dǎo)致雄性生殖功能障礙。有研究表明,鎘導(dǎo)致睪丸血管壞死,影響了睪酮的合成,并最終導(dǎo)致血漿睪酮水平的下降[29]。類固醇合成酶是睪丸合成睪酮的必需元件,酶的改變會影響睪酮的合成。類固醇激素合成急性調(diào)節(jié)蛋白(steroidogenic acute regulatory protein,StAR)是細(xì)胞內(nèi)膽固醇快速轉(zhuǎn)運的調(diào)節(jié)因子,其與甾體激素的合成關(guān)系密切,疏水性的膽固醇從線粒體膜外轉(zhuǎn)運至線粒體內(nèi)是甾體激素的生成關(guān)鍵步驟。少量鎘可明顯降低雄性SD大鼠間質(zhì)細(xì)胞StAR蛋白水平,睪酮合成相關(guān)的其他關(guān)鍵蛋白酶膽固醇側(cè)鏈裂解酶(P450scc)、3β-HSD以及17β-HSD等均下降,同時生精細(xì)胞數(shù)量和精子細(xì)胞均下降[30]。Couto-Moraes等[31]研究發(fā)現(xiàn)給予一定劑量的氯化鎘可使大鼠血漿睪酮水平明顯下降,伴隨性行為受到影響,適量補(bǔ)充睪酮可逆轉(zhuǎn)鎘對實驗大鼠性行為的破壞。
近年在研究實驗動物鎘毒性機(jī)制的基礎(chǔ)上出現(xiàn)了大量的拮抗鎘毒性的方法和藥物。首先從氧化應(yīng)激方面來說,有效的抗氧化劑正在被不斷發(fā)現(xiàn),抗氧化藥物通過抑制鎘所致的氧化應(yīng)激失衡,清除脂質(zhì)過氧化物達(dá)到維持睪丸正常內(nèi)環(huán)境。鋅和硒是較為有效的拮抗鎘睪丸毒性的抗氧化劑[22,30]。經(jīng)典的抗氧化劑維生素E和維生素C也可以有效抑制鎘的睪丸毒性[23,32]。新型抗氧化劑如葡萄籽、β-隱黃素等在鎘睪丸毒性方面的研究正逐漸成熟[24,33]。在抗凋亡方面,抗凋亡藥物如咖啡酸苯乙酯、姜黃素、褪黑素等可以通過抑制凋亡信號通路的激活,從而抑制機(jī)體凋亡程序的啟動,抗凋亡藥物可以明顯降低鎘致生精細(xì)胞的凋亡,甚至在間質(zhì)細(xì)胞和支持細(xì)胞的凋亡方面也有抑制作用,且其在發(fā)揮抗凋亡作用的同時也可抑制機(jī)體氧化應(yīng)激的發(fā)生[4,26,34]。目前因為缺乏與臨床毒性相關(guān)聯(lián)的系統(tǒng)化的血鎘和尿鎘衡量的標(biāo)準(zhǔn),所以針對人類鎘中毒后規(guī)范化治療的研究很少。
鎘廣泛應(yīng)用于工業(yè)生產(chǎn),并通過食物、水、空氣、粉塵等經(jīng)消化道和呼吸道進(jìn)入人體。生殖系統(tǒng)是對鎘敏感的靶器官之一,生殖健康和生殖安全應(yīng)重視重金屬對生殖系統(tǒng)的毒性效應(yīng)。鎘對雄性生殖系統(tǒng)毒性主要表現(xiàn)在損傷睪丸、降低雄激素分泌和影響精子生成,最終使生殖能力下降甚至誘發(fā)生殖系統(tǒng)腫瘤發(fā)生。目前鎘對雄性生殖毒性和機(jī)制研究較明確,越來越多學(xué)者開始從事鎘與男性不育關(guān)系的研究,并取得了一定的成績。近年來針對雄性動物和人類生殖系統(tǒng)鎘損傷后的保護(hù)性藥物的探究也在逐漸成熟,國內(nèi)外大量文獻(xiàn)證實硒、黃芪、金屬硫蛋白、鋅、茶多酚等可拮抗鎘的毒性。對鎘研究的深入將有助于解決男性不育問題。
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[本文編輯秦娟]
Toxicity Effect of Cadmium on Male Reproductive SystemCHEN Na,SU Ping.
Family Planning Research Institute,Tongji Medical College,Huazhong University of Science and Technology,Wuhan 430030,China
SU Ping,E-mail:suping24@126.com
Cadmium is a kind of toxic heavy metals and hazardous pollutant which affects many organs and systems,such as liver,kidney,lung,bones and reproductive system.The male reproductive system is more sensitive to cadmium toxicity than others.The decreased levels of StAR,P450scc,3β-HSD and 17β-HSD caused by the cadmium exposure lead to the inhibition of steroidogenesis and the reduction of testosterone level.Cadmium exposure induces the apoptosis of germ cells and the ultrastructure changes of organelles,which ultimately damage the function of male reproductive system.The endocrine disorder,imbalance of oxidative stress and the over-activation of apoptotic factors play an important role in the mechanism of cadmium toxicity.Some antioxidants and anti-apoptotic drugs could reduce the toxicity of cadmium to some extent.This paper discusses the effect of cadmium on the male reproductive system,as well as the treatment of cadmium exposure.
Cadmium;Genital diseases,male;Apoptosis;Oxidative stress(J Int Reprod Health/Fam Plan,2016,35:515-518)
430030武漢,華中科技大學(xué)同濟(jì)醫(yī)學(xué)院計劃生育研究所
蘇萍,E-mail:suping24@126.com
△審校者
(2016-09-10)