Reciprocal Regulation of MicroRNA-122 and c-Myc in Hepatocellular Cancer: Role of E2F1 and Transcription Factor Dimerization Partner 2

Mice, Knockout 0301 basic medicine Carcinoma, Hepatocellular Liver Neoplasms Down-Regulation Mice, Transgenic Up-Regulation 3. Good health DNA-Binding Proteins Gene Expression Regulation, Neoplastic Proto-Oncogene Proteins c-myc Disease Models, Animal Mice MicroRNAs 03 medical and health sciences Cell Line, Tumor Hepatocytes Animals Humans E2F1 Transcription Factor Transcription Factors
DOI: 10.1002/hep.26712 Publication Date: 2013-08-28T16:17:41Z
ABSTRACT
c-Myc is a well-known oncogene frequently up-regulated in different malignancies, whereas liver-specific microRNA (miR)-122, a bona fide tumor suppressor, is down-regulated in hepatocellular cancer (HCC). Here we explored the underlying mechanism of reciprocal regulation of these two genes. Real-time reverse-transcription polymerase chain reaction (RT-PCR) and northern blot analysis demonstrated reduced expression of the primary, precursor, and mature miR-122 in c-MYC -induced HCCs compared to the benign livers, indicating transcriptional suppression of miR-122 upon MYC overexpression. Indeed, chromatin immunoprecipitation (ChIP) assay showed significantly reduced association of RNA polymerase II and histone H3K9Ac, markers of active chromatin, with the miR-122 promoter in tumors relative to the c-MYC-uninduced livers, indicating transcriptional repression of miR-122 in c-MYC-overexpressing tumors. The ChIP assay also demonstrated a significant increase in c-Myc association with the miR-122 promoter region that harbors a conserved noncanonical c-Myc binding site in tumors compared to the livers. Ectopic expression and knockdown studies showed that c-Myc indeed suppresses expression of primary and mature miR-122 in hepatic cells. Additionally, Hnf-3β, a liver enriched transcription factor that activates miR-122 gene, was suppressed in c-MYC-induced tumors. Notably, miR-122 also repressed c-Myc transcription by targeting transcriptional activator E2f1 and coactivator Tfdp2, as evident from ectopic expression and knockdown studies and luciferase reporter assays in mouse and human hepatic cells. Conclusion : c-Myc represses miR-122 gene expression by associating with its promoter and by down-regulating Hnf-3β expression, whereas miR-122 indirectly inhibits c-Myc transcription by targeting Tfdp2 and E2f1. In essence, these results suggest a double-negative feedback loop between a tumor suppressor ( miR-122 ) and an oncogene ( c-Myc ). (Hepatology 2014;59:555–566)
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