C-Myc-Responsive Long Non-Coding RNA LAST Regulates Cell Cycle Progression

The oncoprotein c-Myc plays a pivotal role in multiple cellular processes such as cell cycle progression,malignant transformation,differentiation suppression and apoptosis induction predominantly through its transcription activity.Indeed,as a master transcriptional factor,c-Myc regulates the expression of approximately 10-15%of genes in the genome including a variety of protein-coding gene,such as CDKN1A,CDKN2B,CCND1,CCND2,CDK4 and E2F2.Long noncoding RNAs(IncRNAs),which are defined as transcripts longer than 200 nucleotides lacking protein coding capacity,are emerging as important regulators of biological process,including regulation of gene expression at multiple levels,such as chromatin remodeling,transcription,and post-transcriptional modulation.Genome-wide studies have shown that c-Myc transcriptionally regulates many lncRNA genes such as PVT1,the CCAT family,and MYCLos,whereas a number of lncRNAs have been demonstrated to be important components of the c-Myc-mediated signal network.Up to now,researchers have found that a series of proteins and long non-coding RNAs regulated by c-Myc can participate in the regulation of c-Myc downstream biological functions,such as cell metabolism,tumor migration,and cell cycle.Although it has been reported that c-Myc can regulate various proteins through transcriptional regulation to affect cell cycle progression,it is rarely reported that c-Myc regulates cell cycle by transcribing IncRNA.To identify novel function of c-Myc-regulated long non-coding RNAs,doxycycline treated or untreated P493-6 cells carrying a c-Myc tet-off system were used to analyze lncRNA expression profile by performing long non-coding RNA microarray analysis,and a long non-coding RNA positively regulated by c-Myc was found in the microarray analysis,which can help stabilize the transcripts of some genes,so we named it LAST(LncRNA-Assisted Stabilization of Transcripts).LAST gene promoter region has two c-Myc binding sites.c-Myc can positive regulates LAST through this two sites.Knockdown of LAST caused a decrease in the percentage of cells in S and G2/M phases and an increase in G1 phase,indicating that LAST knockdown prevents cell passage from G1 into S phase.Then mRNA levels of G1-related cyclins and CDKs genes were selected for comparison in HCT116 cells before or after LAST gene knockdown,among all mRNAs examined,only CCND1 mRNA level was significantly decreased.Furthermore LAST cooperates with CNBP to regulate CCND1 mRNA stability and both LAST and CCND1 mRNA bind to CNBP through their G-rich motifs.In addition,data from CNBP RIP-seq and LAST RNA-seq showed CCND1 mRNA may not be the only target of both LAST and CNBP,three additional mRNAs(SOX9,NFE2L1,and PDF)could be the post-transcriptional targets for both LAST and CNBP.