Hippo信号转录复合体YAP-TEAD与HNF4α组成双负反馈环在肝癌发生发展中的作用机制研究

Great progress has been achieved in the study of Hippo signaling in regulating tumorigenesis; however, the downstream molecular events that mediate this process have not been completely defined. Moreover, the regulation of Hippo signaling during tumorigenesis in hepatocellular carcinoma (HCC) remains largely unknown. Our previous experiments of this proposal indicated that HNF4α expression was negatively regulated by YAP1 in HCC cells via a ubiquitin proteasome pathway. By contrast, HNF4α was found to directly associate with TEAD4 to compete with YAP1 for binding to TEAD4, thus inhibiting the transcriptional activity of YAP-TEAD and the expression of Hippo target genes. Moreover, overexpression of HNF4α was found to significantly compromise YAP-TEAD-induced HCC cell proliferation and stem cell expansion. In this proposal, we will systematically investigate the mechanism and function of the Double-Negative Feedback Loop between YAP-TEAD and HNF4α, in molecular, cellular, rat/mouse tumor model and clinical tumor sample levels. As YAP acts as a dominant oncogene in HCC and plays a crucial role in stem cell homeostasis and tissue regeneration, manipulating the interaction between YAP, TEADs and HNF4α may provide a new approach for HCC treatment and regenerative medicine.

Hippo信号通路参与各项生理病理过程的研究已取得巨大进展，但其调节肿瘤发生发展的下游分子机制，和其在肝癌发生发展中所受的调控因素尚未完全明确，是当前关注的热点问题。本项目前期研究首次发现，Hippo信号转录复合体YAP-TEAD可通过泛素化降解途径抑制肝细胞核因子HNF4α的蛋白水平；相反地，HNF4α可与TEAD4直接相互作用，并破坏YAP与TEAD之间的结合从而抑制YAP-TEAD的转录激活功能及Hippo靶基因的表达。此外，过表达HNF4α可抑制YAP-TEAD激活所诱导的HCC细胞增殖和脱分化。基于此我们首次提出YAP-TEAD与HNF4α在肝癌发生发展过程中可互相抑制形成双负反馈作用环。本项目将从分子水平、细胞水平、大鼠和小鼠肝癌模型活体水平及临床标本病理水平，系统研究YAP-TEAD与HNF4α组成双负反馈环在肝癌发生发展过程中的作用机制，将为肝癌诊治和新药研发提供新切入点。

Hippo信号通路参与肿瘤疾病进程的各项研究已取得巨大进展，但其调节肿瘤发生发展的下游分子机制，和其在肝癌发生发展中所受的调控因素尚未完全明确，是当前关注的热点问题。本项目系统地探讨了Hippo信号通路下游转录激活复合体YAP-TEAD与肝细胞核因子HNF4α在肝细胞癌发生发展中的关系。项目研究结果表明，YAP1通过泛素化-蛋白酶体途径对肝癌细胞中HNF4α的蛋白表达具有负调控作用；相反地，HNF4α可与TEAD4直接相互作用，并破坏YAP1与TEAD4之间的结合，从而抑制YAP-TEAD复合体的转录激活功能及相关Hippo信号通路靶基因的表达。此外，HNF4α的过度表达显著抑制YAP-TEAD复合体激活所诱导的肝癌细胞增殖和干细胞扩增。最后，本项目系统检测了DEN诱导大鼠肝癌模型以及Mst1/Mst2 DKO小鼠肝癌模型等活体水平中YAP-TEAD和HNF4α的相互调控关系，重新表达HNF4α显著抑制Mst1/Mst2 DKO小鼠的肝脏肿大与肝癌发生，与体外实验结论一致。综上，本项目首次提出YAP-TEAD与HNF4α在肝细胞癌发生发展过程中可互相抑制形成双负反馈作用环，从而调节细胞的增殖与分化；并从分子水平、细胞水平、大鼠和小鼠肝癌模型活体水平及临床标本病理水平，系统阐明了YAP-TEAD与HNF4α组成双负反馈环在肝细胞癌发生发展过程中的作用机制。由于YAP1是肝癌疾病进程的关键癌基因，并在肝脏干细胞内稳态和组织再生中起着重要作用，因此靶向调控YAP-TEAD和HNF4α之间的相互作用可能为肝癌治疗和再生医学提供新的途径。