title: MYC protein inhibits transcription of the microRNA cluster MC-let-7a-1~let-7d via noncanonical E-box.
abstract: The human microRNA cluster MC-let-7a-1∼let-7d, with three members let-7a-1, let-7f-1, and let-7d, is an important cluster of the let-7 family. These microRNAs play critical roles in regulating development and carcinogenesis. Therefore, precise control of MC-let-7a-1∼let-7d level is critical for cellular functions. In this study, we first showed that the expression of these three members was significantly reduced in human hepatocellular carcinoma HepG2 cells as compared with the immortalized human liver L02 cells. We demonstrated that the MC-let-7a-1∼let-7d cluster was encoded by a single polycistronic transcript driven by a 10-kb upstream promoter, with two MYC-binding sites. Importantly, MYC inhibited MC-let-7a-1∼let-7d promoter activity via binding to the noncanonical E-box 3 downstream of the transcription start sites, whereas it enhanced promoter activity by binding to the canonical E-box 2 upstream of the transcription start sites. We found that although the binding affinity of MYC to E-box 2 was stronger than E-box 3, the binding quantum of MYC to E-box 3 was significantly higher in cancerous HepG2 cells as compared with the noncancerous L02 cells. In addition, forced expression of let-7 could reverse the MYC-mediated cell proliferation. These findings suggested that in L02 cells with a low level of MYC, MYC binds mainly to E-box 2 to enhance MC-let-7a-1∼let-7d expression. However, in HepG2 cells with an elevated MYC, the extra MYC could bind to E-box 3 to suppress the transcription of MC-let-7a-1∼let-7d and thus enable HepG2 cells to maintain a high level of MYC and a low level of let-7 microRNAs simultaneously.
authors: Laboratory of Integrated Biosciences, School of Life Sciences, Sun Yat-sen University, Guangzhou, China.
authors: Zifeng
authors: Z
authors: Wang
authors: Sheng
authors: S
authors: Lin
authors: Julia Jun
authors: JJ
authors: Li
authors: Zhenhua
authors: Z
authors: Xu
authors: Hong
authors: H
authors: Yao
authors: Xiao
authors: X
authors: Zhu
authors: Dan
authors: D
authors: Xie
authors: Zan
authors: Z
authors: Shen
authors: Johnny
authors: J
authors: Sze
authors: Kui
authors: K
authors: Li
authors: Gang
authors: G
authors: Lu
authors: Danny Tat-Ming
authors: DT
authors: Chan
authors: Wai Sang
authors: WS
authors: Poon
authors: Hsiang-fu
authors: HF
authors: Kung
authors: Marie Chia-mi
authors: MC
authors: Lin
mesh_terms: D057809:HEK293 Cells; D056945:Hep G2 Cells; D006801:Humans; D035683:MicroRNAs / Q000096:biosynthesis* / Q000235:genetics; D005810:Multigene Family; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D020218:Response Elements; D014158:Transcription, Genetic
chemical_list: C489427:MYC protein, human; D035683:MicroRNAs; D016271:Proto-Oncogene Proteins c-myc; C487931:mirnlet7 microRNA, human