Multiple E2F-Induced MicroRNAs Prevent Replicative Stress in Response to Mitogenic Signaling

Abstract

Transcription of microRNAs (miRNAs) is thought to be regulated similarly to that of protein-coding genes. However, how miRNAs are regulated during the cell division cycle is not well understood. We have analyzed the transcription profiles of miRNAs in response to mitogenic stimulation in primary fibroblasts. About 33% of the miRNAs expressed in these cells are induced upon exit from quiescence. Many of these miRNAs are specifically induced by E2F1 or E2F3 during the G 1 /S transition and are repressed in E2F1/3-knockout cells. At least four miRNA clusters, let-7a-d, let-7i, mir-15b-16-2, and mir-106b-25, are direct targets of E2F1 and E2F3 during G 1 /S and are repressed in E2F1/3-null cells. Interestingly, these miRNAs do not contribute to E2F-dependent entry into S phase but rather inhibit the G 1 /S transition by targeting multiple cell cycle regulators and E2F targets. In fact, E2F1 expression results in a significant increase in S-phase entry and DNA damage in the absence of these microRNAs. Thus, E2F-induced miRNAs contribute to limiting the cellular responses to E2F activation, thus preventing replicative stress. Given the known function of E2F of inducing other oncogenic miRNAs, control of miRNAs by E2F is likely to play multiple roles in cell proliferation and in proliferative diseases such as cancer.MicroRNAs (miRNAs) are small (ϳ23-nucleotide [nt]) regulatory RNA molecules that exert posttranscriptional control of specific target mRNAs (3). More than half of the human protein-coding genes appear to have been under selective pressure to maintain pairing of their 3Ј untranslated regions (3Ј-UTRs) to miRNAs (5). This interaction is known to induce mRNA degradation or inhibition of translation through specific albeit imperfect base pairing (22,24). Several target genes have been validated, indicating that each individual miRNA can target a few or, possibly, multiple genes. These small miRNAs therefore regulate fundamental cellular processes such as proliferation, differentiation, or apoptosis during development (65). In addition, deregulation of miRNAs is frequently observed in a wide range of diseases, including cancer (19,21). The analysis of miRNA function and regulation in specific cellular processes has proven to be required for a full understanding of malignant transformation and to envision new therapeutic possibilities.Tumor development is accompanied by a variety of genetic and epigenetic alterations in protein-coding genes and small, noncoding RNA genes. By regulating specific oncogenes or tumor suppressor molecules, miRNAs may have profound effects on tumor development. The first report linking miRNAs and cancer showed a frequent deletion of miR-15a and miR-16-1 in patients with B-cell chronic lymphocytic leukemia (13). Further analysis of miRNA expression signatures has suggested a frequent involvement of miRNAs in the initiation and progression of human tumors (12). Although the critical targets for most cancer-associated miRNAs are unknown, some relevant targets have been characterized in sp...

miR-15 and miR-16 Are Direct Transcriptional Targets of E2F1 that Limit E2F-Induced Proliferation by Targeting Cyclin E

“…It was shown previously that E2F1 activates AKT via transcriptional regulation of the adaptor Gab2 and that this E2F1-dependent AKT activation serves to inhibit E2F1-mediated apoptosis (44). In addition, a recent study showed that in murine fibroblasts, E2F1 and E2F3 transcriptionally upregulate the expression of a number of miRNAs, which can, in turn, inhibit cell proliferation (45).…”

E2F1 and E2F2 prevent replicative stress and subsequent p53-dependent organ involution

“…[8][9][10] E2F1 and E2F2 are known to function both as transcriptional activators and as repressors of their target genes, including those coding for microRNAs. [11][12][13][14][15] Moreover, E2F1 and E2F2 perform positive as well as negative regulatory roles in cellular proliferation, which appear to be cell specific and context dependent. They have been reported to promote cellcycle entry and progression upon their ectopic expression.…”

The long intergenic non-coding RNA CCR492 functions as a let-7 competitive endogenous RNA to regulate c-Myc expression

“…miRNAs are short ncRNAs (~22 nucleotides long) that serve as guides for targeting the RNA to imperfectly complementary miRNA recognition elements (MREs) within target mRNAs inducing both translational repression, and mRNA decapping/deadenylation [4]. Several studies have shown that miRNAs can control the expression levels of genes involved in the cell-cycle regulatory machinery [5,6]. The let-7 miRNA was initially discovered in Caenorhabditis elegans [7], and has been shown to be highly evolutionarily conserved [8].…”