MicroRNAs (miRNA) mediate distinct gene regulatory pathways triggered by epidermal growth factor receptor (EGFR) activation, which occurs commonly in lung cancers with poor prognosis. In this study, we report the discovery and mechanistic characterization of the miRNA miR-7 as an oncogenic "oncomiR" and its role as a key mediator of EGFR signaling in lung cancer cells. EGFR activation or ectopic expression of Ras as well as c-Myc stimulated miR-7 expression in an extracellular signal-regulated kinase (ERK)-dependent manner, suggesting that EGFR induces miR-7 expression through a Ras/ERK/Myc pathway. In support of this likelihood, c-Myc bound to the miR-7 promoter and enhanced its activity. Ectopic miR-7 promoted cell growth and tumor formation in lung cancer cells, significantly increasing the mortality of nude mice hosts, which were orthotopically implanted with lung cancers. Quantitative proteomic analysis revealed that miR-7 decreased levels of the Ets2 transcriptional repression factor ERF, the coding sequence of which was found to contain a miR-7 complementary sequence. Indeed, ectopic miR-7 inhibited production of ERF messages with a wild-type but not a silently mutated coding sequence, and ectopic miR-7 rescued growth arrest produced by wild-type but not mutated ERF. Together, these results identified that ERF is a direct target of miR-7 in lung cancer. Our findings suggest that miR-7 may act as an important modulator of EGFRmediated oncogenesis, with potential applications as a novel prognostic biomarker and therapeutic target in lung cancer. Cancer Res; 70(21); 8822-31. ©2010 AACR.

T cell factor 4 (TCF4) interacts with ␤-catenin in the WNT signaling pathway and transactivates downstream target genes involved in cancer progression. To identify proteins that regulate TCF4-mediated biological responses, we performed a yeast two-hybrid screen to search for a TCF4-binding protein(s) and found that MAD2B interacts with TCF4. We confirmed that MAD2B is a TCF4-binding protein by co-immunoprecipitation. Using the TOPFLASH reporter assay, we found that MAD2B blocks TCF4-mediated transactivation. The MAD2B binding regions of TCF4 were identified by TCF4 deletion mapping and electrophoretic mobility shift assay analysis. TCF4 and MAD2B interactions abolished the DNA binding ability of TCF4. Knockdown of MAD2B in SW480 colorectal cancer cells led to the conversion of epithelial cells to a mesenchymal fibroblastoid phenotype (epithelial-mesenchymal transdifferentiation). An E-cadherin promoter reporter analysis showed that MAD2B modulates TCF4-mediated E-cadherin expression. MAD2B knockdown blocked E-cadherin expression and significantly induced mesenchymal markers, such as N-cadherin and vimentin. Mesenchymal induction was accompanied by F-actin redistribution and the appearance of a fibroblastoid phenotype. MAD2B knockdown also increased both mRNA and protein levels of Slug, a known TCF4-induced E-cadherin transcriptional repressor. A chromatin immunoprecipitation assay showed that MAD2B silencing enhances the ability of TCF4 to bind the Slug promoter. Thus, MAD2B is a novel TCF4-interacting protein. This study provides the first evidence for the involvement of MAD2B in TCF4-mediated epithelial-mesenchymal transdifferentiation.The WNT signaling cascade is a crucial cell development and growth regulatory pathway (1-3). The key event of WNT signaling is the activation of the T cell factor (TCF) 2 -␤-catenin complex. WNT binding to transmembrane Frizzled receptors is followed by phosphorylation and functional deactivation of glycogen synthase kinase-3␤. Then membrane-associated ␤-catenin is released and stabilized in the cytoplasm. Accumulated ␤-catenin associates with TCFs and translocates into the nucleus, leading to transcriptional activation of downstream target genes (3, 4). Inappropriate activation of the WNT signaling pathway, such as by mutation of adenomatous polyposis coli and glycogen synthase kinase-3␤, is often involved in early premalignant lesions of the intestine and tumorigenesis of several cell types (5). Human germ line mutations in the adenomatous polyposis coli result in familial adenomatous polyposis, which is characterized by multiple intestinal adenomas (6). Up to 80% of colorectal cancers (CRCs) show mutations in both adenomatous polyposis coli alleles (7). Accumulation of ␤-catenin protein and transactivation of a certain set of TCF4 target genes by accumulated ␤-catenin are crucial for colorectal carcinogenesis (8).Within the TCF family proteins, TCF4, which is encoded by the Tcf7L2 gene, is predominantly expressed in CRC cells. TCF4 binds directly to DNA through its high mobil...