鈉-葡萄糖協同轉運蛋白2抑制劑對糖尿病腎病保護機制的研究進展

郭詩哲(綜述)　張朝云(審校)

(復旦大學附屬華山醫院內分泌科-復旦大學內分泌糖尿病研究所　上海　200040)

糖尿病腎病(diabetic kidney disease,DKD)是糖尿病最嚴重的慢性并發癥之一,是導致終末期腎病(end-stage renal disease,ESRD)的主要原因[1]。高血壓、高血糖、遺傳易感性和人種等多個因素與DKD的發展相關[2]。全球范圍內,糖尿病患者的微量白蛋白尿發生率高達39%,蛋白尿的年發生率為3.1%[3]。在中國約30%的1型糖尿病患者和20%的2型糖尿病患者最終發展為DKD[4]。

近年來,一種新型降糖藥鈉-葡萄糖協同轉運蛋白2 (sodium-glucose cotransporter 2,SGLT2)抑制劑受到越來越多的關注,其作為一類新型的口服降糖藥,作用于腎小管,通過抑制腎小管對葡萄糖的重吸收而發揮降糖作用。此外,臨床試驗發現SGLT2抑制劑在腎臟保護方面也有作用,本文將就其基礎及臨床研究進展進行綜述。

SGLT2抑制劑自從全球首個SGLT2抑制劑達格列凈(Dapagliglozin)于2012年11月被歐盟批準上市以來,已經有多個SGLT2抑制劑相繼上市。2013年3月,美國食品藥品管理局(FDA)批準上市首個SGLT2抑制劑坎格列凈(Canagliflozin),用于治療2型糖尿病[6]。2014年FDA又批準了2個SGLT2抑制劑,分別是達格列凈和恩格列凈(Empagliflozin)。伊格列凈(Ipragliflozin)、托格列凈(Tofogliflozin)和魯格列凈(Luseogliglozin)在日本已批準上市[7]。Remogliflozin etabonate(BHV091009)和Ertugliflozin(PF-04971729/MK-8835)等SGLT2抑制劑已處于臨床試驗階段。

作用機制體內葡萄糖轉運經葡萄糖轉運體實現,分為兩類,即葡萄糖轉運蛋白(glucose transporter,GLUT)和SGLT[8]。GLUT主要負責葡萄糖的被動轉運,而SGLT則負責葡萄糖的主動轉運。體內的SGLT被SLC5家族編碼,分為5種:SGLT1～SGLT5,其中最主要的是SGLT1和SGLT2[9]。在腎臟中,SGLT1主要位于腎臟近端小管的S3段,經腎小球濾過的葡萄糖有10%經SGLT1重吸收,而SGLT2主要位于腎臟近端小管S1～S2段,90%葡萄糖的重吸收由SGLT2完成[10]。在1型和2型糖尿病狀態下,腎臟近端小管的SGLT2表達增加,這一點在糖尿病患者與各種嚙齒動物的糖尿病模型一致[11-15]。SGLT2表達水平升高,導致葡萄糖重吸收增加,這形成了糖尿病狀態下一種代償適應。而SGLT2 抑制劑通過抑制SGLT2、減少腎小管對葡萄糖的重吸收,使體內的葡萄糖從尿中排出,從而降低血糖水平。

SGLT2抑制劑對DKD的保護作用2016年發表于NEnglJMed的一項大型臨床研究顯示[16],SGLT2抑制劑恩格列凈可以明顯延緩DKD進展,這是除RAAS系統阻滯劑藥物外,首項經循證醫學證明可延緩DKD進展的藥物。該研究對7 020例有高心腦血管風險的2型糖尿病患者進行隨機分組,分為恩格列凈治療組與安慰劑治療組,并進行了長達37個月的隨訪,結果發現與安慰劑治療組相比,恩格列凈治療組的腎臟病發病率或惡化風險明顯降低,尿白蛋白肌酐比值(urine albumin-to-creatinine,UACR)翻倍的相對危險率下降,腎臟替代治療的發生率也明顯減少[16]。Heerspink等[17]根據CANATA-SU研究結果報道,在1 450位使用二甲雙胍以及RAAS系統阻滯劑作為基礎治療的2型糖尿病患者中,隨機分為加用坎格列凈和格列美脲治療,并進行為期2年的隨訪。研究發現相較于格列美脲組,坎格列凈組GFR估測值(estimated glomerular filtration rate,eGFR)明顯降低,從而減緩DKD的發生發展。盡管如此,關于SGLT2抑制劑對于DKD的療效及其安全性仍需要更多的臨床證據來支持。目前注冊的SGLT2抑制劑影響腎臟功能的臨床研究已有數十項,例如臨床試驗NCT02065791正在招募2型糖尿病合并DKD的患者,旨在揭示坎格列凈安慰劑是否可以降低DKD患者ESRD的發生率。

保護腎臟機制

降低血糖高血糖主要通過以下4種途徑引起DKD:多元醇通路的激活,晚期糖基化終末產物(advanced glycation end-product,AGE)的形成,蛋白激酶C (protein kinase C,PKC)活化和己糖胺通路的激活[18]。SGLT2抑制劑可以通過減少葡萄糖的重吸收降低血糖,大大減少了慢性高血糖導致的腎臟代謝紊亂。SGLT2抑制劑僅在血糖水平得到有效改善時,才能顯示出腎臟保護作用[15,19-23]。Komala等[24]用鏈脲佐菌素(streptozocin,STZ)誘導eNOS-/-的小鼠成為糖尿病模型,分別給予恩格列凈和替米沙坦治療,并用胰島素使各組的血糖水平均一化,不同于替米沙坦,恩格列凈治療組并未表現出腎臟保護作用,這項試驗表明DKD早期SGLT2抑制劑對腎臟的保護作用可能依賴于循環中葡萄糖水平的改善。

改善腎臟血流動力學穩態在很多糖尿病研究模型中,高灌注、腎小球內高壓以及代償升高的GFR都被證實是腎臟功能異常的早期表現[25]。在1型和2型糖尿病患者中,高灌注的發生率近50%[26],因此成為預測糖尿病最終是否發展為DKD的一項重要指標。腎小球的高灌注通常涉及神經血管機制和管球反射(tubuloglomeruler feedback,TGF)機制。COX-2抑制劑與ACEI都是通過調節腎臟血管節律即神經血管機制來改善高灌注的病理情況。然而,使用這些藥物并不能完全改善高灌注的現象,這提示應該關注管球反射機制在疾病進展過程中的作用。

在糖尿病患者慢性高血糖情況下,近曲小管SGLT2重吸收葡萄糖增加,隨之共轉運的Na+增加,在遠端尿液經過致密斑時Na+/K+/2Cl-濃度降低,致密斑就會接受等同于循環血量不足導致電解質濃度降低的信號調控,產生入球小動脈擴張的效應,引起腎小球高灌注、高球內壓及高濾過率[27]。在一項為期7天的研究中,給予STZ誘導的大鼠糖尿病模型以非選擇性抑制SGLT1和SGLT2受體的根皮甙(phlorizin)可以有效改善腎小球高灌注[28]。在STZ誘導的SGLT-/-小鼠糖尿病模型中腎小球高濾過的狀態也有明顯的改善[19]。另有實驗證明,特異性抑制SGLT2的藥物達格列凈可通過調節致密斑來降低糖尿病大鼠的高GFR[29]。

減輕氧化應激水平氧化應激是活性氧類(reactive oxygen species,ROS)和抗氧化防御之間的失衡,ROS的來源包括高糖誘導線粒體呼吸鏈來源的ROS產生增多、糖自動氧化及還原性煙酰胺腺嘌呤二核苷酸氧化酶氧化途徑等,而抗氧化系統的改變包括非酶性抗氧化分子減少和抗氧化酶的活性改變等[33]。在DKD中,ROS產生增加,清除減少,導致腎臟的損傷[34]。研究證明,AGE與氧化應激密切相關,AGE與巨噬細胞、血管內皮細胞、腎小球系膜細胞上的特異性受體(AGE recptor,RAGE)相結合,介導ROS產生等一系列反應。研究發現,STZ誘導大鼠產生高血糖和低體重的癥狀后,腎臟皮質和髓質中過氧化氫酶(catalase,CAT)的活性降低,谷胱甘肽過氧化物酶(glutathione peroxidase,GPx)活性上升,3-硝基酪氨酸(3-nitrotyrosine)相對于對照組有所增加,而根皮甙皮下注射有抗氧化應激的作用[35]。另一項研究發現,恩格列凈可以減少STZ誘導的糖尿病大鼠產生AGE、降低RAGE表達及氧化應激指標[36]。

體外研究證實,人腎臟近曲小管上皮細胞系HK-2在高糖環境(30 mmol/L)暴露后,ROS的產生顯著增加。用RNA干擾的方法敲除腎小管SGLT2基因后,高糖誘導的ROS的產生則受到抑制[37]。 SGLT2抑制劑托格列凈與抗氧化劑N-乙酰半胱氨酸產生相似作用,都可以減少高糖暴露下的HK-2細胞中ROS的產生[38]。

降低尿酸水平一項日本的研究顯示,在校正了年齡、性別、血壓及eGFR后,血尿酸水平高于7.2 mg/dL與腎血管壁增厚、透明變性及腎血管病相關[39]。一項中國的2型糖尿病橫斷面研究也顯示,高尿酸血癥和尿酸升高的人群中DKD發病率明顯增加[40]。流行病學資料顯示,無論在糖尿病還是非糖尿病人群中,尿酸都是腎臟疾病進展的獨立危險因素[40-41]。

在Ⅲ期臨床試驗中,與安慰劑相比,坎格列凈可顯著降低血尿酸水平[42]。一項Meta分析也顯示,達格列凈治療可以降低血尿酸水平[43]。一項試驗將魯格列凈給正常受試者服用,發現血中尿酸水平下降,尿中尿酸水平升高。研究發現,魯格列凈對尿酸在腎臟中的重吸收并沒有直接的作用[16]。腎臟中尿酸主要在近段腎小管重吸收,經尿酸轉移載體(uric acid transporter1,URAT1,SLC22A12)、有機陰離子轉移載體 4(organic anion transporter 4,OAT4,SLC22A11)及OAT10(SLC22A13)轉移入近段腎小管,經葡萄糖轉移載體9的亞型1(GLUT9 isoform 1)轉移入血[44-48]。最近,有細胞實驗發現高糖(葡萄糖10 mmol/L)可以增加近段腎小管尿酸經GLUT9亞型2外流。SGLT2抑制劑使腎小管局部葡萄糖濃度升高,使腎小管細胞頂端膜上發生的尿素-葡萄糖交換率提高,更多的尿酸從血液中排泄到尿液,最終降低了血尿酸水平[44,49]。這提示SGLT2抑制劑可通過降低尿酸發揮腎臟保護作用。

減輕腎小管間質損傷除了腎小球損傷,近年來腎小管損傷引起的腎臟功能惡化也受到越來越多的重視[50]。在2型糖尿病初期,腎小管的改變包括HK-2細胞肥大和基底膜增厚以及隨著病程進展引起的相應腎小管結構萎縮[50]。STZ誘導的糖尿病大鼠經過4周的恩格列凈治療,尿液中的腎小管間質損傷指標L-FABP下降,這提示SGLT2抑制劑可減輕腎小管損傷[36]。高糖環境下HK-2細胞中ROS生成增加,RAGE表達升高,當使用siRNA干擾SGLT2表達時可以減少腎小管細胞的凋亡[37]。同樣,恩格列凈也可以對高糖下HK-2起到抗氧化應激的作用[51]。

減輕腎臟炎癥及纖維化腎臟內的炎癥是糖尿病腎損傷的重要機制。在腎損傷早期,巨噬細胞浸潤并產生一系列的細胞毒性產物,包括促炎癥因子(如腫瘤壞死因子α、干擾素-γ、IL-1β)及促纖維化因子(如轉化生長因子β1)[52]。

在db/db 2型糖尿病小鼠中,用達格列凈治療12周后腎臟炎癥反應顯著減輕,減少巨噬細胞浸潤和促炎因子產生。同時,達格列凈還可以抑制促細胞凋亡的因子(如Caspase-12和Bax)的產生[23]。恩格列凈可通過抑制Akita小鼠的NF-κB從而抑制其下游激活轉錄的一系列炎癥因子(如CCL2、CD14、IL-6)[15]。

其他高血壓在2型糖尿病患者中十分常見,也是糖尿病微血管和大血管病變的重要的危險因素。因此,血壓控制對于腎臟保護必不可少[53-54]。SGLT2抑制劑增加腎臟對葡萄糖排泄的同時,也增加腎臟對水的排泄,從而降低體液容量,降低血壓[55]。因此,SGLT2可能通過有效調控血壓來保護腎臟。

結語根據現有的臨床和基礎研究,SGLT2抑制劑對于DKD病變保護的效果與機制尚不完全明確。SGLT2抑制劑作為一種新型降血糖藥物,除了改善血糖,還具有心臟、腎臟保護作用,還需要通過更多的長期臨床研究加以驗證。

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