Dysregulated expression or function of the oncogenic transcription factor Myc occurs frequently in human malignancies. Through the positive and negative regulation of an expansive network of target genes, Myc globally reprograms cells to drive proliferation and, in some settings, to induce cell death1-4
. Myc uses distinct mechanisms for activating and repressing gene expression. When inducing transcription, Myc dimerizes with its binding partner Max and binds to genomic DNA directly upstream or within the first introns of target genes5,6
. When repressing transcription, Myc does not seem to contact DNA directly. Rather, Myc is recruited through protein-protein interactions to core promoters, where it antagonizes the activity of positive regulators of transcription7
. For example, Myc can bind to and inhibit the activity of the transcription factor Myc-interacting zinc finger protein 1 (Miz1), thereby preventing Miz1 from activating transcription of the CDKN1A
(also known as p21WAF1/CIP1
) and CDKN2B
) cell cycle-inhibitory genes8,9
. Repression of other Myc targets is probably mediated through the ability of Myc to interact with and to antagonize the activity of additional transcriptional regulators, including Sp1, Smad2 and NF-Y (refs. 10-12
miRNAs are a diverse family of RNA molecules, typically ~18-24 nucleotides in length, that have emerged as a class of Myc-regulated transcripts13
. miRNAs regulate the stability and translational efficiency of partially complementary target mRNAs. miRNAs are initially transcribed by RNA polymerase II as long primary transcripts (pri-miRNAs) that are capped, polyadenylated and frequently spliced14,15
. The mature miRNA sequences are located in introns or exons of pri-miRNAs, within regions that fold into hairpin structures of ~60-80 nucleotides. Although most pri-miRNAs are noncoding transcripts, some miRNAs are located within introns of protein-coding genes16
. miRNA maturation requires a series of endonuclease reactions in which miRNA hairpins are excised from pri-miRNAs, the terminal loop of the hairpin is removed, and one strand of the resulting duplex is selectively loaded into the RNA-induced silencing complex (RISC)17
. This miRNA-programmed RISC is the effector complex that carries out regulation of target mRNAs.
A large body of evidence has documented nearly ubiquitous dysregulation of miRNA expression in cancer cells18
. These changes in miRNA expression are highly informative for the classification and prognosis of cancer19,20
. In addition, altered expression of specific miRNAs has been shown to promote tumorigenesis. For example, a group of six co-transcribed miRNAs known as the miR-17-92 cluster is amplified in lymphoma and solid tumors21
. These miRNAs are frequently overexpressed in tumors, promote proliferation in cell lines, and accelerate angiogenesis and tumorigenesis in mouse models of Myc-induced colon cancer and lymphoma22-25
. Although select miRNAs are upregulated in cancer cells, global miRNA abundance seems to be generally reduced in tumors20
. Downregulation of miRNA probably contributes to neoplastic transformation by allowing an increased expression of proteins with oncogenic potential26
. Evidence suggests that a block in the first step of miRNA processing may contribute to the reduced abundance of select miRNAs in cancer cells27
. Additional mechanisms of miRNA downregulation, including direct transcriptional repression, have not been investigated.
We previously showed that Myc directly activates transcription of the miR-17-92 cluster13
. Through the analysis of human and mouse models of Myc-mediated lymphomagenesis, we have now identified a large set of additional Myc-regulated miRNAs. Unexpectedly, induction of Myc results primarily in widespread downregulation of miRNA expression. Chromatin immunoprecipitation (ChIP) studies indicate that Myc binds directly to promoters or conserved regions upstream of the miRNAs that it represses. We also show that expression of Myc-repressed miRNAs markedly impedes lymphoma cell growth in vivo
. These observations suggest that repression of tumor-suppressing miRNAs is a fundamental component of the Myc tumorigenic program.