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.
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 transcriptional and post‐transcriptional regulation of gene expression. Here, we describe and experimentally demonstrate one such regulatory motif, termed feed‐forward loop (FFL), which involves the TF E2F1, a set of miRNAs, and their common targets (Figure 8). In this FFL, E2F1, a key regulator of cell‐cycle progression, transcriptionally activates a family of 15 miRNAs that are organized in three paralogous polycistrons on three different chromosomes. These miRNAs silence a group of antiproliferative regulators including the pocket proteins pRb and p130 and the CDK inhibitors p21 and p57. Importantly, these genes are themselves transcriptional targets of E2F1. Thus, a TF activates a set of genes as well as a set of miRNAs, which in turn post‐transcriptionally regulate that set of genes. Increasing the complexity of this regulatory FFL, many of the shared targets of E2F1 and the miRNAs function as regulators of the cell cycle; some negatively regulate E2F itself. For example, the pocket proteins pRB and p130 are the major components that regulate the activity of E2F family members throughout the phases of the cell cycle through direct protein–protein interaction. The TF p53 is regarded as one of the key proteins that prevent malignant transformation (Ryan et al, 2001), and deactivating mutations of this tumor suppressor are highly common in a wide variety of tumors (Hussain and Harris, 1999). A hallmark activity of p53 is the inhibition of proliferation and the induction of cellular senescence on diverse types of stress signals with oncogenic potential, including DNA damage, telomere shortening and oncogene activation. There are several known mechanisms by which p53 negatively regulates proliferation, the key one being the transcriptional activation of the CDK inhibitor p21, which indirectly inhibits the activity of E2F family members. Another recently discovered mechanism for inhibiting proliferation by p53 is the induction of miRNAs from the miR‐34 family, which also modulate the E2F pathway (He et al, 2007; Tarasov et al, 2007; Tazawa et al, 2007; Kumamoto et al, 2008). Additionally, direct and indirect transcriptional repression by p53 is considered important for its ability to inhibit proliferation (Ho and Benchimol, 2003). Using miRNA microarrays, we discovered that p53 activation during cellular senescence in primary human fibroblasts leads to a decrease in the expression of the above‐mentioned family of miRNAs, including members of the miR‐17‐92, miR‐106b/93/25 and miR‐106a‐92 polycistronic miRNA clusters. A similar decrease in miRNA expression was observed in human breast cancer specimens that harbor wild‐type p53 as compared with those that harbor mutant forms of p53. We further investigated the mechanism by which p53 represses the expression of this group of miRNAs, and found that activation of p53 leads to a dramatic reduction of E2F1 mRNA, protein and activity levels, which in turn leads to a decrease in the E2F1‐dependent transcriptional activation of these miRNAs. To study the consequence of deregulation of this FFL and importance of its inhibition by p53, we ectopically expressed representative members from the set of p53‐repressed miRNAs, namely the miR‐106b/93/25 polycistron, in primary human fibroblasts. Consequently, these cells acquired an enhanced proliferative phenotype manifested by increased growth rate, increased colony formation efficiency and delayed entry into replicative senescence. These results position the repression of this set of miRNAs as a novel mechanism by which p53 inhibits proliferation and controls cell fate.

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• Standort: Deutsche Nationalbibliothek Frankfurt am Main
• Umfang: Online-Ressource
• Sprache: Englisch

Verweise und Beziehungen

• Erschienen in: p53‐repressed miRNAs are involved with E2F in a feed‐forward loop promoting proliferation ; volume:4 ; number:1 ; year:2008 ; extent:15 ; Molecular systems biology ; 4, Heft 1 (2008) (gesamt 15)

Beteiligte, Orts- und Zeitangaben

• Urheber: Brosh, Ran ; Shalgi, Reut ; Liran, Atar ; Landan, Gilad ; Korotayev, Katya ; Nguyen, Giang Huong ; Enerly, Espen ; Johnsen, Hilde ; Buganim, Yosef ; Solomon, Hilla ; Goldstein, Ido ; Madar, Shalom ; Goldfinger, Naomi ; Børresen‐Dale, Anne‐Lise ; Ginsberg, Doron ; Harris, Curtis C. ; Pilpel, Yitzhak ; Oren, Moshe ; Rotter, Varda

Weitere Informationen

• DOI: 10.1038/msb.2008.65
• URN: urn:nbn:de:101:1-2023073105545213727753
• Rechteinformation: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.