胃微生态与胃癌的研究进展
2017-03-25韩渭丽钱美睿聂勇战
韩渭丽,曹 莹,钱美睿,聂勇战
胃微生态与胃癌的研究进展
韩渭丽,曹 莹,钱美睿,聂勇战
胃微生态平衡是人体健康的重要前提,幽门螺杆菌(Helicobacter pylori, Hp)是目前已发现的与胃癌相关的关键病原体之一,普遍存在于人胃黏膜上皮。Hp感染可引起胃内其他菌群的改变,还可引起长期慢性的胃黏膜损伤,导致一系列胃黏膜上皮恶性进展和胃癌的发生。本文就胃微生态与Hp感染的关系、Hp感染在胃癌发生中的作用、胃内其他菌群在胃癌发生中的作用及微生态制剂在胃癌治疗的作用进行综述。进一步揭示Hp感染对胃微生态平衡的影响,胃微生态平衡和Hp感染在胃癌发生发展中的作用及微生态制剂在胃癌治疗中的意义。
胃微生态;幽门螺杆菌;胃癌;毒力因子;微生态制剂
人体微生物与多种疾病存在密切关系,其在与宿主共进化过程中形成共生关系,具有调节宿主消化吸收、代谢、免疫的重要作用[1]。人胃肠道菌群结构复杂,胃内菌群平衡是人类胃肠道健康的基础和前提,幽门螺杆菌(Helicobacter pylori, Hp)是目前发现的胃微生态菌群中与胃癌发生密切相关的病原体,Hp毒力因子一直被认为是胃癌发生的重要因素之一[2]。胃癌的发生是一个多因素、多阶段进行性发展的过程,其发生受多种因素的影响,如老年、男性、消化道肿瘤家族史阳性、吸烟、不良饮食、Hp感染等,其中,Hp感染者胃癌的发病风险将增加75%以上[3]。世界人群50%存在不同程度的Hp感染,我国人群具有Hp高感染率和胃癌高发的特点[4]。目前,胃微生态和Hp感染在胃癌发生发展过程中的作用尚不十分明确。近年来,越来越多的研究发现Hp感染可诱导胃肠菌群失衡。本文主要总结胃微生态与Hp感染的关系,Hp感染在胃癌发生中的作用,胃内其他菌群在胃癌发生中的作用及胃微生态制剂在胃癌治疗的作用。
1 胃微生态与Hp
Hp可定植于人类胃黏膜,引发胃肠道菌群失衡,形成慢性活动性胃炎,并可进一步发展成为消化性溃疡或胃上皮黏膜恶性病变,影响胃黏膜上皮细胞免疫功能[5-6]。有研究对86例成人和儿童进行16S rRNA基因测序发现,无Hp感染的儿童和成人胃内菌群组成相似,仅在低丰度类菌群存在细微差异;而Hp感染的儿童胃内高丰度菌群比例与非感染的同龄人相比,差异显著;并且相对于Hp感染的成人,Hp感染儿童有较高水平的IL-10和TGF-β表达;上述表明感染Hp后胃内菌群结构发生明显改变,且对儿童机体免疫具有一定影响[7]。Hp感染还可能影响胃内菌群活性,成人上消化道(胃、十二指肠和口腔)菌群活性检测发现, Hp阳性患者胃黏膜和胃液活性菌群中Hp占主导地位,而其他菌群活性明显不同,因此Hp感染可对宿主胃活性菌群造成一定影响[8]。此外,对Hp阳性患者胃内菌群分析表明,Hp感染增加了变形杆菌、螺旋菌和酸杆菌,同时减少了放线菌、拟杆菌和厚壁菌,明显改变胃内细菌丰度[9]。再者,对双胞胎胃内菌群分析发现,Hp阴性双胞胎胃内菌群结构及组成无明显差异;然而,同一对双胞胎Hp阳性者胃内菌群结构相较于Hp阴性者发生巨大改变,可见Hp感染对胃内菌群的影响大于遗传效应[10]。
对Hp阳性雌性小鼠模型研究发现,Hp定植雌性小鼠胃内,使硬壁菌门、拟杆菌、变形菌等丰度明显降低,厚壁菌门等丰度增加[11]。有研究发现,通过16S rDNA检测发现蒙古沙鼠胃黏膜感染Hp后,可检测到Hp和乳酸杆菌的混合菌,且乳酸杆菌处主导地位。表明乳酸杆菌可以抑制胃内Hp[12]。
胃内微生物与Hp能够相互作用,Hp感染可影响胃内其他菌群结构、丰度和活菌活性,能特定上调或下调某些菌群,其对胃内菌群结构的影响大于遗传作用;其次,胃内其他菌群如乳酸杆菌等益生菌可以抑制胃内Hp活性。
2 Hp感染在胃癌中的作用
2.1 Hp毒力因子在胃癌中的作用 Hp感染引起的胃疾病与Hp毒力因子、宿主胃黏膜状况和胃内微环境密切相关。其中Hp毒力因子是始动因素,主要包括Cag致病岛(cag pathogenicity island, Cag PAI)、空泡毒素(vacuolating cytotoxin gene A, VacA)和黏附素Baba[13]。有研究发现,160例Hp阳性患者中CagA、VacA和Baba阳性率分别为69%、100%和78%[14]。
2.1.1 Cag PAI Cag PAI是Hp的主要毒力因子之一,编码Ⅳ型分泌系统[15]。CagA能够通过多种分子机制影响至关重要的蛋白表达或功能,CagA可介导Wnt/β-catenin信号通路上调,Wnt /β-catenin信号通路是调节胚胎发育和成年组织稳态的关键途径,在胃肠道肿瘤中起着关键作用[16-17];此外,CagA能够激活PI3K/Akt及下游信号通路,诱导P53失活等导致肿瘤的发生[16]。CagA作为主要的毒力因子能够使β-连环蛋白及其靶向胃腺癌起始细胞标记的微小RNA(microRNA , miRNA)-320a和miR-4496表达下调,从而抑制体外细胞自我更新能力[18]。CagA与胃癌、消化性溃疡之间存在显著相关,胃癌组患者Hp感染率为72.8%,Hp阳性患者中CagA表达和抗CagA-IgG阳性率分别为63.4%和61.8%;与非溃疡性消化不良患者相比,二者在胃癌和消化性溃疡患者中显著增高[19]。此外,CagA表达阳性患者临床结局较差,CagA阳性的Hp感染患者胃癌风险高于Hp阴性患者[16]。
2.1.2 VacA VacA因具有使上皮细胞空泡化的功能而得名,VacA在Hp定植和生存中具有重要的作用[20]。VacA毒性还具有促进感染、调节淋巴细胞、改变膜通透性以及改变自噬细胞功能导致Hp在哺乳动物上皮细胞内存活等功能[21-22]。研究显示,不同VacA亚型,包括VacA m1、i1、d1、c1,与CagA的基因型显著相关,均可增加胃癌发生的风险,其OR值依次为4.29、6.11、3.18、15.53和2.59;多因素回归分析显示,调整性别和年龄,VacA c1型的胃癌发生风险可高达38.320倍;因此VacA c1有望成为预测55岁男性患者胃癌发生的重要指标[23]。此外,在89例Hp感染者中,VacA i1型占51.68%(46/89),在胃癌、消化性溃疡和慢性胃炎患者中分别占87.50%(21/24)、39.58%(19/48)和35.29%(6/17);与慢性胃炎患者相比,胃癌患者VacA i1型高达13.142倍,表明VacA i1与该地区胃癌发病风险显著相关,具有一定的指导意义[24]。
2.1.3 抗原结合黏附素Baba Baba是抗原结合黏附素,能够促使细菌黏附到胃黏膜表面,增强菌株毒力,从而成为识别消化性溃疡和胃癌的危险性的标志之一。Hp感染可使DL1等某些氨基酸替换,进而共同表达功能较多的Baba异构体[25]。研究发现,依赖于Baba黏附到胃黏膜上皮上的Hp能抑制细胞分化并受到ArsS调控[26],Baba等抗原结合黏附素能够使Hp在胃黏膜表层形成稳定的定植[27]。此外,Baba可能有除黏附蛋白外的其他功能,Baba的降低可能与适应性免疫或Toll样受体信号通路有关[28]。Baba2等基因表达在胃癌患者中明显高于胃溃疡性疾病和十二指肠疾病患者,Baba结合OipA、SabA等在精确有效地诊断Hp感染中具有重要意义[29-30]。
2.2 根除Hp在胃癌治疗中的作用 Hp是胃癌发生的关键致病菌,严重的萎缩、肠上皮化生、胃癌与Hp感染有关,根除Hp可能在预防胃癌发生和恶性进展中具有重要作用[31]。接受Hp根除治疗的胃癌患者病死率远低于安慰剂治疗患者(1.50% vs. 2.10%),死亡风险较低[32]。此外,根除Hp可降低老年患者病死率,并且能够降低胃肠上皮化生或异型增生患者胃癌发生率[33]。有研究对1000余例消化性溃疡患者根除Hp后平均随访约10年,发现Hp根除成功组患者患癌率显著低于根除失败组(0.21% vs. 0.45%),Hp根除成功组患者患癌的最长间隔是14.5年,而失败组为13.7年,表明Hp根除对胃癌的预防作用优于未根除[34]。在我国胃癌高发区进行1600余例Hp感染者随访研究,发现根除Hp可降低无萎缩等胃癌前病变患者胃癌发生率,疗效显著,高于未根除组(P=0.02),充分表明根除Hp在胃癌预防中的重要作用[35]。
由此可见,Hp感染是胃癌发生的重要细菌因素,根除Hp能够有效的在病因上预防胃癌发生,降低胃癌患者死亡风险。
3 胃内其他微生物与胃癌
与健康人群相比,消化系统疾病患者胃内微生物结构差异显著。微生物DNA微阵列分析显示,从非萎缩性胃炎到肠型胃癌,细菌多样性呈稳步下降[36]。此外,我国一项720例志愿者横断面研究结果同样表明,胃微生物多样性的减少与胃癌的发生相关,胃微生物丰度与胃蛋白酶原Ⅱ呈线性相关,降低胃微生物丰度,胃蛋白酶原Ⅰ/Ⅱ也降低,是慢性胃炎及胃癌易感性的标志之一[37]。与健康人群胃微生物群落对比发现,Hp阳性的胃癌患者梭状芽胞杆菌和普雷沃菌属较多,而丙酸杆菌、棒状杆菌和葡萄球菌明显减少[38]。虽然胃癌的发生与胃内某些特定菌群有关,包括乳酸杆菌、肺炎克雷伯菌等增加和普林单胞菌属、奈瑟菌属等的减少,但仍须进一步研究其在胃癌发生中的作用机制。
4 胃微生态与胃癌治疗
胃内微生态平衡是维持身体健康,预防胃癌的关键。胃内微生物多样性受食物多样性和药物等影响。Hp感染是与胃癌相关的最具代表性的细菌病原因素,因此胃癌的细菌学治疗主要为根除Hp。Hp根除治疗的关键是防止抗药性和二次耐药。然而,标准的三联疗法即质子泵抑制剂、阿莫西林和克拉霉素联用,对Hp感染的根除率在世界范围内呈下降趋势[39-40]。
近年来,各种微生态制剂,包括益生菌、益生元、合生元在生活中得到广泛应用,具有保护胃黏膜、增强免疫反应等作用。益生菌被定义为活的微生物,是由生理活菌或死菌组成,主要包括乳酸杆菌、双歧杆菌、肠球菌等,能够增强免疫反应、抑制细菌生长、调节免疫系统和改善肠道屏障功能[41]。益生元主要指能促进体内益生菌生长,而不被体内消耗的成分。益生元中龙胆低聚糖能够促进双歧杆菌生长并且对消化道肿瘤细胞具有明显的抑制作用,益生元蘑菇多糖能够通过刺激肠道有益菌生长,达到有效的保健作用[42-43]。合生元则是益生菌和益生元二者的混合成分。
一些益生菌菌株能够降低Hp活性,合并使用后能够提高Hp根除率;另一些则可以减少抗生素治疗不良反应的发生[44-45]。此外,在Hp阳性儿童中,使用特定的益生菌菌株同样能够减少Hp耐药,提高根除成功率,但须要进一步验证是否补充益生菌制剂有助于儿童Hp的根除[46]。在对克拉霉素耐药低的国家,根除Hp时添加铋和益生菌能够提高根除率至100%,同时,添加益生菌有助于减少Hp根除过程中不良反应的发生[47]。对650例Hp阳性受试者采用益生菌辅助三联疗法根除Hp的治疗结果分析显示,患者腹痛、腹胀等不良症状大部分优于安慰剂组,治疗效果明显提高,且减少不良症状和反应[48]。因此,益生菌在Hp根除中具有重要的作用,有助于提高胃癌疗效。
5 总结与展望
胃内微生态平衡是人体健康的前提和预防胃癌发生的关键,Hp是胃癌发生的关键病原体之一。Hp感染可对胃内菌群产生影响,影响机体免疫调节,改变胃内菌群活性和结构。根除Hp能够降低胃癌发生率,降低胃癌前病变发展为胃癌的风险和胃癌患者死亡风险。微生态制剂能够调节机体免疫功能,抑制Hp耐药,有效提高Hp根除率,在胃癌的预防和治疗中具有重要作用。
综上所述,维持胃内微生态平衡,根除Hp,合理使用益生菌等微生态制剂在胃癌的防治中具有重要的作用。然而,合理的Hp根除方式须要进一步探讨并进行大量的临床验证,以避免在根除Hp过程中可能对人体造成的损伤。
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(2017-05-19收稿 2017-06-18修回)
(本文编辑 闫晶晶)
Research progress of gastric microecology and gastric cancer
HAN Wei-li, CAO Ying, QIAN Mei-rui, NIE Yong-zhan*
State Key Laboratory of Cancer Biology, Xijing Hospital of Digestive Diseases, The Fourth Military Medical University, Xi´an 710043, China *Corresponding author, E-mail: nieyongzhan@gmail.com
Gastric microecology is the critical premise of human health. Helicobacter pylori (Hp) is one of the pivotal pathogens related to gastric cancer, and Hp is commonly present in human gastric epithelial cells. Hp infection can give rise to changes of other bacterial flora in the stomach and long-term chronic gastric mucosal injuries, thus lead to a series of malignant progression of gastric mucosa, and even gastric cancer. In this review, we summarize the relationship of gastric microecology and Hp infection, the role of Hp infection and other gastric flora in gastric carcinogenesis, and the effect of microecologics in gastric cancer treatment. This review aims to further reveal the influence of Hp infection in gastric microecology, the role of gastric microecology and Hp infection in gastric carcinogenesis, and the significance of microecologics in the treatment of gastric cancer.
gastric microecology; Helicobacter pylori; gastric cancer; virulence factor; microecologics
R735.2;R37
A
1007-8134(2017)03-0144-04
10.3969/j.issn.1007-8134.2017.03.005
重大慢性非传染性疾病防控研究(2016YFC1303204);国家科技部支撑项目(2015BAI13B00)
710043 西安,第四军医大学西京消化病医院肿瘤生物学国家重点实验室(韩渭丽、曹莹、钱美睿),消化内科(聂勇战)
聂勇战,E-mail: nieyongzhan@gmail.com