肝肠循环胆红素代谢在炎症性肠病发生中的作用

Inflammatory bowel disease (IBD) can not be effectively cured and its detailed pathophysiological mechanisms remain largely unknown．As known to all, alteration of intestinal flora is closely related to enterohepatic cycle of bilirubin. The precise role of intestinal flora in the pathogenesis of IBD, and the efficacy of gut microbiota modulation by probiotics administration in the management of IBD are currently uncertain. In our previous study, we found that substantially lower serum bilirubin levels were detected in patients with IBD compared with controls, suggesting a negative relationship between serum bilirubin and IBD. Next，we unveiled that bilirubin could accumulate ubiquitinated proteins in human colon carcinoma cells, similar to classical proteasome inhibitor Velcade. We further confirmed that bilirubin inhibits the UPS by selectively targeting the proteasome-associated deubiquitinase UCHL5 and USP14. It is important that proteasome inhibition efficiently block NF-κB nuclear translocation, consequently inhibitors of proteasome is a potential treatment option for inflammatory conditions. Taken together, we propose a novel mechanism of IBD pathogenesis. To explore the protective role of endogenous bilirubin in IBD by inhibiting the UPS function or by antioxidant effect, we will establish enteritis models in hereditary hyperbilirubinemia (UTG1a1-/-) and hypoproteinemia mice and collect clinical data. The purpose of current study is to provide a novel perspective for the IBD therapy based on improvement of intestinal microecology.

目前炎症性肠病(IBD)发生机制不清，临床缺乏有效治疗手段。研究表明肠道菌群与肠道胆红素代谢密切相关；IBD病人肠道菌群紊乱，补充益生菌对其有积极作用。前期实验发现IBD患者血清总胆红素水平低于健康人，其与IBD发生负相关。胆红素不仅是重要的抗氧化剂，最近我们证实胆红素显著抑制泛素蛋白酶体系统(UPS)功能，其通过靶向蛋白酶体相关去泛素化酶USP14和UCHL5发挥作用。而抑制UPS可下调NF-κB，UPS抑制剂对炎症性疾病有治疗作用。据此我们提出了肝肠循环胆红素可能介导IBD发生的新机制。本研究拟通过在已构建的遗传性高胆红素血症和低蛋白血症小鼠上建立肠炎模型，结合临床病人样本分析，探讨内源性胆红素在肠道内的重吸收障碍致肠上皮细胞UPS功能相对活化及抗氧化失衡是否具有促IBD发生的作用，该研究不仅为改善肠道微生态治疗IBD提供新的理论依据，并提出纠正胆红素代谢失衡可能是IBD治疗的新策略。

近年来研究发现胆红素的肠肝循环是维持正常肠道功能的重要介质，同时在肠道疾病的发生发展中发挥作用。目前炎症性肠病（溃疡性结肠炎）发生机制不明，本研究在前期研究基础上证实了胆红素对溃疡性结肠炎具有一定的保护作用。我们的研究发现：①胆红素是一个特异性作用于26S蛋白酶体相关去泛素化酶UCHL5和USP14从而抑制蛋白酶体降解蛋白的功能。②炎症性肠病患者血清总胆红素水平低于健康人群，而炎症性肠病患者粪便中胆红素含量明显高于健康人群，炎症性肠病患者的疾病严重程度与血清胆红素水平不相关，排除患者因排泄增多而导致血清胆红素水平偏低的原因。③胆红素对UPS功能的抑制作用可以影响NF-κB通路，胆红素通过阻断IκB-α的降解从而抑制NF-κB通过的活化。④ 动物实验发现外源性胆红素能明显改善DSS诱导的小鼠肠炎的炎性症状和肠道菌群紊乱，胆红素处理小鼠后结肠组织对UCB重吸收增加，同时小鼠结肠组织中ROS含量减少、TAC水平升高和超氧阴离子水平减低。本研究首次阐明胆红素在病变局部的抗氧化特性，其作用机制可能是通过肝肠循环增强胆红素在肠道中的重吸收从而更好地清除超氧阴离子。通过本项目的研究，为肝肠循环胆红素参与溃疡性结肠炎发生发展提供理论依据。