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

• DOI: 10.1002/hep.26712
• Publication Date: 2013-08-28T16:17:41Z
• Authors: Bo Wang; Shu-hao Hsu; Xinmei Wang; Huban Kutay; Hemant Kumar Bid; Jianhua Yu; Ramesh K. Ganju; Samson T. Jacob; Mariia Yuneva; Kalpana Ghoshal
• Keywords: Mice, Knockout; 0301 basic medicine; 0303 health sciences; 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

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)