miR-155靶向抑制色氨酸分解代谢限速酶TDO2，以维持肝细胞癌肿瘤干细胞的干性

Cancer stem cells (CSCs) were thought to be responsible for tumor formation, tumor recurrence and metastasis. In hepatocellular carcinoma (HCC), we have reported that EpCAM+ HCC cells were hepatic CSCs and that miR-155 was specifically highly expressed and functionally important in hepatic CSCs. However, it remains unknown of the mechanism for miR-155 maintaining the stemness of hepatic CSCs. Tumor metabolism plays an important role in HCC development and progression while little is discovered on CSC metabolism in HCC. In our preliminary data, we found that 1) TDO2, a rate-limiting enzyme of tryptophan metabolism, expressed at a significant low level in EpCAM+ hepatic CSCs compared to EpCAM+ normal hepatic stem cells as well as differentiated EpCAM- cells in normal liver and HCC; 2) TDO2 was a predicted target of miR-155 and suppressing miR-155 in HCC cells could increase the TDO2 expression. All these suggested that miR-155 might target TDO2 to regulate tryptophan metabolism, in order to maintain the stemness of hepatic CSCs. In this project, we plan to 1) further validate miR-155 targeting TDO2 and tryptophan metabolism; 2) examine the role of TDO2 and other important metabolites or enzymes of tryptophan metabolism pathway in regulating the miR-155 expression and the stemness of hepatic CSCs; 3) explore possible roles of the miR-155-TDO2 axis as a molecular biomarker or a therapeutic target in HCC. This project will increase our understanding on regulatory signaling pathways of hepatic CSCs and roles of tumor metabolism in regulating hepatic CSCs, and pave the way to specifically eliminate hepatic CSCs.

肿瘤干细胞被认为是肿瘤形成和复发转移的元凶。我们已报道EpCAM+肝癌细胞为肿瘤干细胞，miR-155在该肿瘤干细胞中特异高表达且功能重要，然机制尚不清楚。肿瘤代谢在肝细胞癌发生发展中功能重要，但目前对肝癌干细胞代谢鲜有研究。近期初步实验揭示，色氨酸分解代谢限速酶TDO2在肝癌干细胞中的表达，较肝脏分化的肿瘤细胞及正常干细胞和肝细胞显著低，在肝癌细胞系中TDO2可为miR-155靶向降低。提示在肝癌中，miR-155可能靶向抑制TDO2，调控色氨酸分解代谢途径而维持肝癌干细胞功能。本课题拟进一步验证miR-155对TDO2的靶向负调节及对色氨酸分解代谢的调节；检测TDO2及色氨酸分解代谢通路关键酶和代谢物在调控肝癌干细胞中的作用；探索miR155-TDO2轴在肝癌中可能的临床分子标志物及分子靶标作用。本研究将深化对肝癌干细胞信号通路及细胞代谢的理解，为靶向性消灭肝癌干细胞提供理论依据。

肿瘤干细胞与肿瘤的形成和复发转移密切相关。miR-155和miR-192均在多种不同肿瘤干细胞标志物阳性的肝癌中表达显著改变，本研究拟探究其通过调节肿瘤代谢而参与肿瘤干性调节的机制。miR-155在EpCAM阳性、CD44阳性和CD133阳性的肝癌干细胞中高表达，其可以直接靶向调节色氨酸代谢通路中的关键酶TDO2；然而TDO2对肝癌肿瘤细胞的恶性表型和干性调节则较弱，且miR-155与色氨酸代谢相关性较弱。miR-192在多种肝细胞癌肿瘤干细胞（EpCAM、CD133、CD90、CD44、CD24）中均表达降低，miR-192低表达的肝癌患者肿瘤糖酵解途径明显激活； miR-192敲除的肝癌细胞糖酵解显著增强；已发现miR-192靶向调节糖酵解通路中的两个关键调节蛋白：PFKFB3和GLUT1；miR-192还可以靶向调节c-Myc，且c-Myc可以转录抑制miR-192的表达，从而构成负反馈调节。miR-192敲除肝癌细胞由于糖酵解产生的大量乳酸还可以进一步选择性与肿瘤微环境作用，促进肿瘤干性。综上，miR-155靶向TDO2，然而与色氨酸代谢相惯性较弱；miR-192参与糖酵解途径的调节，并通过糖酵解途径进一步与肿瘤微环境作用，促进肝细胞癌肿瘤干性。我们的研究深化了对肝癌干细胞信号通路及细胞代谢的理解，为靶向性消灭肝癌干细胞提供了理论依据。