p53‐repressed miRNAs are involved with E2F in a feed‐forward loop promoting proliferation

Normal cell growth is governed by a complicated biological system, featuring multiple levels of control, often deregulated in cancers. The role of microRNAs (miRNAs) in the control of gene expression is now increasingly appreciated, yet their involvement in controlling cell proliferation is still not well understood. Here we investigated the mammalian cell proliferation control network consisting of transcriptional regulators, E2F and p53, their targets and a family of 15 miRNAs. Indicative of their significance, expression of these miRNAs is downregulated in senescent cells and in breast cancers harboring wild‐type p53. These miRNAs are repressed by p53 in an E2F1‐mediated manner. Furthermore, we show that these miRNAs silence antiproliferative genes, which themselves are E2F1 targets. Thus, miRNAs and transcriptional regulators appear to cooperate in the framework of a multi‐gene transcriptional and post‐transcriptional feed‐forward loop. Finally, we show that, similarly to p53 inactivation, overexpression of representative miRNAs promotes proliferation and delays senescence, manifesting the detrimental phenotypic consequence of perturbations in this circuit. Taken together, these findings position miRNAs as novel key players in the mammalian cellular proliferation network. Synopsis Precise regulation of gene expression is crucial for maintaining homeostasis in healthy tissues and for the execution of cellular programs such as proliferation, differentiation and cell death. In the last decade, microRNAs (miRNAs) have been uncovered as an expanding family of gene expression regulators. These short non‐coding RNAs regulate gene expression at the post‐transcriptional level by promoting translational inhibition or mRNA degradation (Bartel, 2004 ). Similar to protein‐coding genes, the expression of miRNAs is also regulated by transcription factors (TFs), and induction or repression of miRNAs has been demonstrated to play a role in physiological processes such as immune response (Thai et al , 2007 ) and apoptosis (Chang et al , 2007 ; Raver‐Shapira et al , 2007 ). Accordingly, deregulation of miRNAs is associated with diverse types of diseases, including a variety of cancers (Esquela‐Kerscher and Slack, 2006 ; Volinia et al , 2006 ). In an earlier computational study, we predicted the presence of several types of regulatory network motifs that involve TFs and miRNAs (Shalgi et al , 2007 ), and may provide a mechanism for fine‐tuned coordination between transcri