“SREBP2-胆固醇代谢-Treg细胞分化”轴调控IDO1抑制剂对结肠癌的化学预防作用及机制研究

Chronic inflammation and immune escape are the key factors that lead to tumorigenesis. Clinical trials of IDOl inhibitors have demonstrated that their treatment of tumors alone does not benefit patients clinically. We found that IDO1 inhibitor 1-L-MT can effectively prevent the occurrence of AOM/DSS-induced mouse colon adenomas and impair the immunosuppressive effect of Treg cells. However，in the AOM-cholesterol-induced colon cancer mouse model, 1-L-MT promoted the growth of intestinal tumors and increased the numbers of Treg cells. This effect may be related to the abnormal activation of cholesterol metabolism pathway mediated by SREBP2. Studies at home and abroad have found that SREBP2-mediated cholesterol metabolism is closely related to Treg cell differentiation and CTLs activation, but its regulatory mechanism is unknown. This project intends to, combined with co-cultured cell models and Srebf2fl/fl, IDO1-/- transgenic mouse colon cancer models, study the role of SREBP2-cholesterol metabolism in colon cancer tumorigenesis using siRNA, EMSA, co-IP and other techniques, elucidate the regulatory mechanism of SREBP2 on Treg cell differentiation and CTLs activation, and verify the efficacy of SREBP2 inhibitors combined with IDO1 inhibitors in the prevention and treatment of colon cancer. It provides new ideas for the clinical application of immune-modulators to prevent

慢性炎症和免疫逃逸是导致肿瘤发生的关键要素。IDO1抑制剂的临床试验证实其单独治疗未使肿瘤患者获益。本课题组首先发现IDO1抑制剂1-L-MT能有效预防AOM/DSS诱导的小鼠结肠腺瘤发生，削弱Treg细胞免疫抑制效应；而AOM-胆固醇诱导的小鼠结肠癌模型中，1-L-MT却促进了结肠肿瘤生长，可能与SREBP2介导的胆固醇代谢异常激活有关；体外实验也证实1-L-MT能上调SREBP2的表达，提示SREBP2-胆固醇代谢-Treg细胞分化参与调控结肠炎癌转化。国内外研究发现SREBP2与Treg细胞分化、CTLs活化密切相关，具体调控机制不详。本项目旨在研究SREBP2-胆固醇代谢在结肠炎癌转化中的作用，阐明SREBP2对Treg细胞分化及CTLs活化的调控机制，研究SREBP2抑制剂联合阻断IDO1对结肠癌防治效果，为临床合理应用免疫调节剂防治肿瘤研究提供新思路。

IDO1介导的色氨酸/犬尿氨酸代谢参与调控肿瘤浸润T细胞的抗肿瘤免疫效应，与结肠炎相关性结肠癌的发生发展密切相关。SREBP2介导的胆固醇代谢途径也被证实与黑色素瘤、肺癌等多种肿瘤浸润T细胞免疫调节密切相关。本项目旨在研究IDO1介导的色氨酸/犬尿氨酸代谢与SREBP2介导的胆固醇代谢途径可能存在的交互关系，阐明SREBP2介导的胆固醇代谢影响IDO1抑制剂对结肠癌治疗作用的可能机制。结果证实：（1）IDO1能够通过激活AKT/mTOR/SREBP2信号通路，调控SREBP2下游胆固醇合成酶相关基因转录，增加结肠癌细胞内胆固醇含量，上调肿瘤细胞PD-L1的蛋白表达水平，诱导肿瘤组织中Treg细胞比例增加，抑制CD8+效应性T细胞的抗肿瘤免疫效应，促进AOM/DSS诱导的野生型小鼠结肠腺瘤的生长。然而IDO1能保护APCmin/+小鼠结肠腺瘤的生长，这种差异可能与IDO1介导的谷氨酰胺代谢异常增加了结肠肿瘤细胞的铁死亡有关。（2）高胆固醇饮食并不能显著增加结肠腺瘤组织中胆固醇的含量，反而代偿性地抑制SREBP2介导的胆固醇合成，保护AOM/DSS诱导的C57小鼠结肠腺瘤生长。（3）SREBP2抑制剂Fatostatin能够有效抑制肿瘤浸润T淋巴细胞内SREBP2介导的胆固醇合成，降低胞内胆固醇蓄积造成的内质网应激，减少肿瘤组织内Treg细胞以及CD8+T细胞凋亡和衰竭的比例，激活T细胞的抗肿瘤免疫效应，抑制MC38结肠癌、B16黑色素瘤、Lewis肺癌移植瘤的生长。（4）APCmin/+小鼠自发结肠腺瘤模型中，IDO1抑制剂INCB能够有效抑制IDO1介导的色氨酸/犬尿氨酸代谢，减少结肠腺瘤内Treg细胞的比例，保护APCmin/+小鼠结肠腺瘤的生长，然而高胆固醇饮食通过诱导肠道菌群紊乱，削弱了INCB的保护作用。（5）三七总皂苷能够抑制IFN-γ/Stat1/IDO1信号通路的激活，降低IDO1的蛋白表达，发挥抗肿瘤免疫调节作用，保护AOM/DSS诱导的结肠炎癌转化的过程。本项目阐明了IDO1作为信号转导分子能够调控SREBP2介导的胆固醇代谢途径，调控肿瘤细胞PD-L1介导的免疫逃逸效应；证实了抑制SREBP2介导的胆固醇代谢途径可以作为抗肿瘤免疫的有效治疗手段。