MiR-194, commonly repressed in colorectal cancer, suppresses tumor growth by regulating the MAP4K4/c-Jun/MDM2 signaling pathway

Tumor growth cascade is a complicated and multistep process with numerous obstacles. Until recently, evidences have shown the involvement of microRNAs (miRNAs) in tumorigenesis and tumor progression of various cancers, including colorectal cancer (CRC). In this study, we explored the role of miR-194 and its downstream pathway in CRC. We acquired data through miRNA microarray profiles, showing that the expression of miR-194 was significantly suppressed in CRC tissues compared with corresponding noncancerous tissues. Decreased miR-194 expression was obviously associated with tumor size and tumor differentiation, as well as TNM stage. Both Kaplan–Meier and multivariate survival analysis showed that downregulated miR-194 was associated with overall survival. Moreover, functional assays indicated that overexpression of miR-194 in CRC cell lines inhibited cell proliferation both <i>in vitro</i> and <i>in vivo</i>. In addition, using dual-luciferase reporter gene assay, we found MAP4K4 was the direct target of miR-194. Silencing of MAP4K4 resulted in similar biological behavior changes to that of overexpression of miR-194. We also observed through Human Gene Expression Array that MDM2 was one of the downstream targets of MAP4K4. Knockdown of MAP4K4 downregulated MDM2 expression through transcription factor c-Jun binding to the −1063 to -1057 bp of the promoter. These results suggest that miR-194, regulating the MAP4K4/c-Jun/MDM2 signaling pathway, might act as a tumor suppressor and serve as a novel target for CRC prevention and therapy.

肿瘤生长级联反应是一类复杂且多步骤的病理过程，伴随诸多阻碍。直至近期，诸多研究证据表明，微小RNA（microRNAs，miRNAs）参与包括结直肠癌（colorectal cancer，CRC）在内的多种癌症的发生与进展过程。本研究旨在探讨miR-194及其下游通路在结直肠癌中的作用。本研究通过miRNA微阵列芯片获取数据，结果显示，相较于配对的癌旁正常组织，结直肠癌组织中miR-194的表达水平显著下调。miR-194表达下调与肿瘤大小、肿瘤分化程度及TNM分期显著相关。卡普兰-迈耶（Kaplan–Meier）生存分析与多因素生存分析均证实，miR-194低表达与患者总生存期密切相关。此外，功能实验结果表明，在结直肠癌细胞系中过表达miR-194可在体外（in vitro）与体内（in vivo）均抑制细胞增殖。通过双荧光素酶报告基因实验，本研究发现MAP4K4是miR-194的直接靶基因。沉默MAP4K4所引发的细胞生物学行为改变，与miR-194过表达所产生的效应一致。本研究还通过人类基因表达芯片分析发现，MDM2是MAP4K4的下游靶基因之一。敲低MAP4K4可通过使转录因子c-Jun结合至MDM2基因启动子的-1063至-1057 bp区域，从而下调MDM2的表达。上述结果表明，miR-194通过调控MAP4K4/c-Jun/MDM2信号通路发挥肿瘤抑制因子的作用，有望成为结直肠癌防治的新型靶点。