title: Reciprocal regulation of microRNA-122 and c-Myc in hepatocellular cancer: role of E2F1 and transcription factor dimerization partner 2.
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.
  
  
  CONCLUSIONS
  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).
authors.affiliation: Department of Molecular and Cellular Biochemistry, Ohio State University, Columbus, OH; Molecular, Cellular and Developmental Biology Program, Ohio State University, Columbus, OH.
authors.forename: Bo
authors.initials: B
authors.lastname: Wang
authors.forename: Shu-hao
authors.initials: SH
authors.lastname: Hsu
authors.forename: Xinmei
authors.initials: X
authors.lastname: Wang
authors.forename: Huban
authors.initials: H
authors.lastname: Kutay
authors.forename: Hemant Kumar
authors.initials: HK
authors.lastname: Bid
authors.forename: Jianhua
authors.initials: J
authors.lastname: Yu
authors.forename: Ramesh K
authors.initials: RK
authors.lastname: Ganju
authors.forename: Samson T
authors.initials: ST
authors.lastname: Jacob
authors.forename: Mariia
authors.initials: M
authors.lastname: Yuneva
authors.forename: Kalpana
authors.initials: K
authors.lastname: Ghoshal
mesh_terms: D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D004195:Disease Models, Animal; D015536:Down-Regulation / Q000235:genetics / Q000502:physiology; D050687:E2F1 Transcription Factor / Q000235:genetics / Q000502:physiology*; D015972:Gene Expression Regulation, Neoplastic / Q000235:genetics / Q000502:physiology*; D022781:Hepatocytes / Q000378:metabolism / Q000473:pathology; D006801:Humans; D008113:Liver Neoplasms / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D035683:MicroRNAs / Q000235:genetics / Q000502:physiology*; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000502:physiology*; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D015854:Up-Regulation / Q000235:genetics / Q000502:physiology
chemical_list: D004268:DNA-Binding Proteins; D050687:E2F1 Transcription Factor; C493753:E2F1 protein, human; C493754:E2f1 protein, mouse; C531907:MIRN122 microRNA, human; C489427:MYC protein, human; D035683:MicroRNAs; C544354:Mirn122 microRNA, mouse; C489428:Myc protein, mouse; D016271:Proto-Oncogene Proteins c-myc; C094103:TFDP2 protein, human; C490579:Tfdp2 protein, mouse; D014157:Transcription Factors
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