通过调控肠道菌群治疗慢性疾病

洪斌 ,  蒋建东

工程(英文) ›› 2022, Vol. 18 ›› Issue (11) : 17 -20.

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工程(英文) ›› 2022, Vol. 18 ›› Issue (11) : 17 -20. DOI: 10.1016/j.eng.2021.08.015

通过调控肠道菌群治疗慢性疾病

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洪斌,蒋建东. 通过调控肠道菌群治疗慢性疾病[J]. 工程(英文), 2022, 18(11): 17-20 DOI:10.1016/j.eng.2021.08.015

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慢性疾病(简称慢病)包括多种异常状况和疾病,对患者的身体和(或)精神造成损害,并持续较长时间。慢病很大程度上是一种当代瘟疫,是发达国家和一些发展中国家发病率和死亡率的主要原因[1‒2]。这些致残和致命的疾病包括心脑血管疾病和代谢疾病,如肥胖症、糖尿病和代谢综合征,以及退行性神经疾病,如帕金森病和阿尔茨海默病。慢病的特点之一是发病机理多样并伴有相关的特异症状,其疾病进程可通过药物治疗和(或)生活方式的改变在某种程度上得到延迟或缓解。与这些疾病相关的生化指标异常,如糖尿病的高血糖、心血管疾病的高胆固醇、脂肪肝病的肝脏高脂肪含量等,是与临床表现(症状或体征,即中医所说的“标”)相关的生物学表型。虽然个体的遗传背景、不健康的生活方式、饮食和环境因素在慢病的发病机制中发挥作用,但大多数慢病的根本发病原因(即中医所说的“本”)仍不清楚。
近年来,越来越多的研究表明,肠道菌群及其代谢产物在慢病的发生和发展中起着重要作用。事实上,两千多年前,被称为西方医学之父的希波克拉底就指出:“所有疾病都始于肠道。”中国东汉时期学者王充(距今近两千年)也提出肠道与人类长寿有密切关系(如“欲得长生,肠中常清”的观点)。中医认为有毒物质从粪便进入循环系统可引起各种疾病(粪毒入血,百病蜂起)。目前,粪菌移植已成为一些难治性疾病的有效治疗方法。
肠道微生物在慢病的生理病理过程中的独特作用及分子机制尚未得到完全解析。最近几十年来,高通量测序技术的快速发展和应用揭示大量的微生物存在于人体的不同生态系统(即人体菌群)中。人类肠道中有数万亿的微生物细胞,与人体细胞的数量级相当[3],而在肠道微生物的基因组集合(即肠道微生物组,也称为“人类第二基因组”)中,基因数量至少比人类基因组高两个数量级[4]。肠道菌群包括细菌、古菌、真核生物和病毒/噬菌体,其中,细菌占绝大多数。每个人肠道菌群的组成是由宿主的遗传背景和肠道的生理环境决定的,而肠道的生理环境在很大程度上受到食物的影响[5]。为了强调菌群与宿主的共进化,诺贝尔生理学或医学奖得主Joshua Lederberg创造了“超级有机体”(superorganism)一词[6]。
肠道微生物组成的改变(即肠道菌群失调)与人类的一系列慢病具有相关性。具体来说,肠道菌群组成的有害特征可以被认为是慢病发病根本原因的组成部分,这些疾病包括肥胖症和糖尿病[7]、心血管疾病[8]、帕金森病[9]和阿尔茨海默病[10]等。肠道菌群可产生各种小分子代谢产物,如短链脂肪酸、三甲胺(TMA)、次级胆酸等,这些化合物可以进入血液循环,对全身的健康起到有益或有害的作用[11]。短链脂肪酸,主要是乙酸、丙酸和丁酸,是微生物将人体不能消化的碳水化合物进行发酵的主要产物。许多研究已经证实短链脂肪酸对代谢性疾病的影响,表明短链脂肪酸可能与糖尿病、动脉粥样硬化、高血压等疾病有因果关系[12‒16]。短链脂肪酸可以结合不同的细胞受体,作为信号分子调节宿主代谢,如通过肠-脑轴来控制食欲[17‒21]。微生物代谢物TMA来源于胆碱、肉碱等营养物质,可被肝酶氧化生成三甲胺N-氧化物(TMAO)[22]。尽管对TMAO在动脉粥样硬化进展中的作用(或作为生物标志物的意义)还存在一些争论[23‒25],但血浆TMAO水平被认为是除血浆胆固醇水平和慢性炎症外,动脉粥样硬化的一个新的独立危险因素。TMAO的潜在作用机制包括抑制胆固醇逆转运[26]、增强巨噬细胞泡沫细胞形成[27]、促进高血糖[28]、诱导血管炎症[29]、增加血小板的高反应性[30]等。与胆固醇代谢相关的胆汁酸代谢也是由肠道菌群参与并调节的。肝脏合成的初级胆汁酸被释放到小肠中,促进脂质和脂溶性维生素的消化和吸收。一小部分初级胆汁酸从肠道再吸收过程逃逸后,被小肠远端和结肠的肠道菌群转化为一系列不同结构的次级胆汁酸[31]。一些胆汁酸分子被认为是内分泌样信号分子,通过与各种宿主受体结合来调节宿主生理过程,如类法尼醇X受体(FXR)和Takeda G-蛋白偶联受体5(TGR5)[32‒33]。这些受体参与调节糖脂代谢和能量消耗的信号通路[33‒36]。因此,阐明肠道微生物来源代谢产物的作用机制使肠道菌群可能“作为药靶”,为通过调控肠道菌群治疗慢病提供了新的可能[37]。因此,可操纵细菌代谢的药物可能通过调控肠道菌群来治疗慢病。
小檗碱是黄连和小檗的主要药理活性成分,通过靶向肝脏和肌肉等主要能量代谢器官的糖脂代谢通路来降低糖脂水平,在治疗高脂血症和代谢综合征方面表现出临床疗效[38‒42]。然而,小檗碱的口服吸收很差,主要存在于肠道内容物中[43]。小檗碱调控肠道菌群的作用已被发现是小檗碱有益作用的重要机制。小檗碱能够改变肠道菌群的组成,不仅选择性抑制有害细菌,还能增加有益细菌的丰度,如双歧杆菌、乳酸菌、艾克曼氏菌等[44]。在高脂饮食喂养的大鼠中小檗碱被证明可以通过重塑肠道菌群、增加某些短链脂肪酸产生菌的丰度来增加短链脂肪酸水平;小檗碱促进丁酸的生成并在降低血浆胆固醇和血糖水平方面发挥作用[45‒46]。口服小檗碱后,可通过调节肠道菌群组成和微生物组的功能来抑制TMA和TMAO的产生,从而减轻胆碱诱导的动脉粥样硬化;通过将TMA产生菌移植给小鼠也证实了这一点[47]。小檗碱还能抑制肠道菌群的胆酸盐水解酶的活性,增加血清和粪便中结合胆汁酸的含量,而肠道FXR信号通路参与了小檗碱的降脂作用[48]。这些发现为肠道菌群与小檗碱抗动脉粥样硬化以及抗代谢综合征疗效之间的因果关系提供了新的关键的分子机制。
有趣的是,肠道菌群还可影响小檗碱的治疗效果。肠道菌群可通过细菌硝化还原酶将小檗碱转化为肠道可吸收的衍生物二氢小檗碱,然后在肠细胞中二氢小檗碱可被重新氧化为小檗碱并进入血液循环,从而增加小檗碱的血药浓度[49]。这样,小檗碱的降脂作用可能通过这种转化而实现双向调节:与健康人群相比,高脂血症患者粪便中硝基还原酶活性升高,从而使血液中小檗碱水平升高,以更好地发挥其治疗效果[50]。
另一方面,肠道微生物将小檗碱还原为二氢小檗碱可以促进酪氨酸羟化酶的辅酶四氢生物蝶呤的产生,而酪氨酸羟化酶是负责酪氨酸羟化生成左旋多巴的限速酶[51]。因此,口服小檗碱可加速肠道微生物产生左旋多巴,提高大脑多巴/多巴胺水平,从而改善帕金森病。将含有酪氨酸羟化酶的细菌移植给小鼠,结果表明作为酪氨酸羟化酶激动剂,小檗碱具有脑功能改善作用[51]。另一种由中草药制成的抗抑郁化合物芍药内酯苷同样难以被人体吸收,口服后无法在大脑中检测到。然而,研究表明,肠道菌群可将芍药内酯苷转化为苯甲酸,而苯甲酸可以透过血脑屏障,通过抑制脑中的D-氨基酸氧化酶而发挥抗抑郁作用[52]。这些发现为肠-脑轴的分子和化学机制提供了很好的例证,表明在治疗中枢神经系统疾病时,可以通过调节肠道菌群来改善脑功能。
事实上,一些从中草药中分离出来的天然产物虽然被独立的临床研究者证明具有疗效,但由于其作用机制尚不清楚,因此对中药的研究提出了挑战。了解其作用机制的主要困难之一是许多植物产物或化合物(如多糖、生物碱、黄酮、皂苷等)的口服生物利用度低。因此,问题在于,在非常低的血药浓度下,药物如何发挥其疗效? 实际上,通过肠道菌群将草药中的活性物质与它们在人体中的生物活性联系起来,可能为这类现象提供了一个科学解释。吸收不佳的物质在肠道中积累并与肠道菌群相互作用,在其中发生生物转化。到目前为止,肠道菌群与许多慢病建立了相关性。并且,越来越多的研究发现肠道菌群及其代谢产物在慢病中的致病性,即呈因果关系而非仅仅相关。这些发现为通过调节肠道菌群来治疗慢病提供了新的可能。能有效调控肠道菌群及其代谢产物的药物,尤其是中草药来源的药物,在治疗慢病方面将具有广阔的前景。
此外,一些药物既可以进入血液循环与宿主靶标相互作用,也可在肠道中与肠道菌群发生相互作用。例如,小檗碱可通过一种称为“药效云”(drug cloud)的作用模式治疗代谢紊乱[53],它既针对临床表征(如高胆固醇和高血糖),也针对根本发病原因(如肠道菌群失调和慢性炎症)。更具体地说,小檗碱可以作用于宿主器官中的低密度脂蛋白受体、胰岛素受体和腺苷一磷酸(AMP)活化蛋白激酶(AMPK)通路等,改善疾病的症状和体征,同时也可作用于肠道菌群来解决疾病的根本发病原因,即“标本兼治”。一些广泛应用于代谢性疾病或心血管疾病的药物,如二甲双胍(主要作用于宿主AMPK通路)和他汀类药物(主要作用于宿主低密度脂蛋白受体通路),也被发现可以通过改变肠道菌群及其代谢产物而起作用[36,54‒57]。目前,人们发现的药物对肠道菌群的调控机制仍只是冰山一角,探索调控肠道菌群对慢病防治结合的重要性,将加深我们对慢病“标本兼治”的理解。

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