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The ARF Tumor-Suppressor Controls Drosha Translation to Prevent Ras-Driven Transformation

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The ARF Tumor-Suppressor Controls Drosha Translation to Prevent Ras-Driven Transformation Oncogene (2013), 1–8 & 2013 Macmillan Publishers Limited All rights reserved 0950-9232/13 www.nature.com/onc ORIGINAL ARTICLE The ARF tumor-suppressor controls Drosha translation to prevent Ras-driven transformation MJ Kuchenreuther1,2 and JD Weber1,2,3 ARF is a multifunctional tumor suppressor that acts as both a sensor of oncogenic stimuli and as a key regulator of ribosome biogenesis. Recently, our group established the DEAD-box RNA helicase and microRNA (miRNA) microprocessor accessory subunit, DDX5, as a critical target of basal ARF function. To identify other molecular targets of ARF, we focused on known interacting proteins of DDX5 in the microprocessor complex. Drosha, the catalytic core of the microprocessor complex, has a critical role in the maturation of specific non-coding RNAs, including miRNAs and ribosomal RNAs (rRNAs). Here, we report that chronic or acute loss of Arf enhanced Drosha protein expression. This induction did not involve Drosha mRNA transcription or protein stability but rather relied on the increased translation of existing Drosha mRNAs. Enhanced Drosha expression did not alter global miRNA production but rather modified expression of a subset of miRNAs in the absence of Arf. Elevated Drosha protein levels were required to maintain the increased rRNA synthesis and cellular proliferation observed in the absence of Arf. Arf-deficient cells transformed by oncogenic RasV12 were dependent on increased Drosha expression as Drosha knockdown was sufficient to inhibit Ras-dependent cellular transformation. Thus, we propose that ARF regulates Drosha mRNA translation to prevent aberrant cell proliferation and Ras-dependent transformation. Oncogene advance online publication, 14 January 2013; doi:10.1038/onc.2012.601 Keywords: ARF; Drosha; translation; tumor suppression; rRNA INTRODUCTION expression by inhibiting translation and/or inducing degradation 14 Cancer initiation and progression are hallmarked by the loss of of specific target mRNAs. Similar to rRNAs, most miRNAs initially regulatory mechanisms that control cellular growth and prolifera- exist as long primary transcripts (pri-miRNAs) that are processed tion. The CDKN2A (Ink4a/Arf) locus encodes two distinct tumor through a series of enzymatic cleavage steps to generate mature 15–17 suppressors, p16INK4a and p19ARF (p14ARF in humans).1 ARF has miRNAs. Drosha forms two different complexes, a small classically been regarded as an activator of p53 through its ability microprocessor complex that contains only Drosha and DGCR8 to bind and sequester Mdm2, the E3 ubiquitin ligase for p53, in and a larger complex that contains accessory proteins, including 18,19 the nucleolus.2–5 Recently, numerous p53-independent functions DDX5. Although both complexes are capable of processing have been attributed to ARF,6 including its regulation of ribosomal pri-miRNAs, only the latter has demonstrated the ability to process 20 RNA (rRNA) synthesis and ribosome biogenesis.7 ARF pre-rRNAs. accomplishes this, at least in part, through its repressive Abnormalities in mature miRNA levels as well as in the interaction with the ribosome chaperone nucleophosmin expression of miRNA processing enzymes have been linked with 21 (NPM) thereby limiting steady-state ribosome biogenesis and various types of disease. Although some studies suggest that growth.1,8–10 there are correlations between decreased Drosha expression and Recently, a nucleolar proteomic screen expanded on the tumor incidence and prognosis, other studies have shown that mechanism through which ARF controls rRNA synthesis by reducing Drosha expression impairs rRNA processing and cellular identifying a novel relationship between ARF and the DEAD-box proliferation.22–26 Despite Drosha’s apparent link to human RNA helicase, DDX5.11 Basal ARF proteins restrict DDX5 access to diseases, little is known about its regulation. Previous studies the nucleolus and antagonize the DDX5–NPM interaction. This have shown that Drosha is stabilized through its protein–protein interference is sufficient to disrupt DDX5’s role in rRNA transcrip- interaction with DGCR8, but outside of this, no other forms of tion and rRNA processing. An assessment of other proteins that transcriptional or post-transcriptional regulation are known.27 are involved in ribosome biogenesis, especially those that Given ARF’s multifaceted involvement in mediating ribosome associate with NPM and/or DDX5 could reveal additional modes biogenesis and its newly characterized relationship with DDX5, a through which ARF acts as a potent tumor suppressor. component of the Drosha processing complex, we sought to Drosha is a RNase III endonuclease that was originally linked determine whether ARF impacted Drosha. In this report, we show with the processing of rRNAs and is now widely studied in the that Drosha is post-transcriptionally regulated in an ARF-depen- context of microRNA (miRNA) biogenesis.12,13 miRNAs represent a dent manner. We identify Drosha as a unique translational target class of short, endogenous, non-coding RNAs that control gene of the ARF tumor suppressor. Moreover, we show that the 1BRIGHT Institute, Washington University School of Medicine, St Louis, MO, USA; 2Department of Internal Medicine, Division of Molecular Oncology, Washington University School of Medicine, St Louis, MO, USA and 3Department of Cell Biology, Siteman Cancer Center, Washington University School of Medicine, St Louis, MO, USA. Correspondence: Dr JD Weber, BRIGHT Institute, Department of Internal Medicine, Division of Molecular Oncology, Washington University School of Medicine, 660 South Euclid Avenue, Campus 8069, St Louis, MO 63110, USA. E-mail: [email protected] Received 15 June 2012; revised 7 September 2012; accepted 7 November 2012 ARF regulates Drosha translation MJ Kuchenreuther and JD Weber 2 increased Drosha expression in the absence of Arf is required for levels following Arf loss, acute ARF knockdown, or ectopic efficient Ras-mediated cell transformation through Drosha’s ability overexpression of ARF as determined by quantitative reverse to regulate rRNA synthesis and cell proliferation. transcription–PCR (RT-PCR; Figures 1a–d, right; Supplementary Figure S1). Next, we treated WT and Arf À / À MEFs with Actinomycin D to evaluate Drosha mRNA stability and observed RESULTS similar rates of Drosha mRNA decay in the presence or absence of Loss of Arf induces Drosha expression Arf (Figure 2a). Taken together, these data indicate that ARF may Previous reports have demonstrated that ARF serves as a major post-transcriptionally regulate Drosha expression. regulator of ribosome biogenesis7 and that this is due, at least in Previous reports identified a positive relationship between 7,11 Drosha and its microprocessor partner, DGCR8, such that Drosha part, to its modulation of DDX5 localization and function. 27 Moreover, other studies have established a clear link between protein is stabilized when bound to DGCR8. To address potential changes in Drosha protein stability in response to Arf loss, Drosha DDX5 and Drosha in the processing of double-stranded RNAs, À / À including rRNAs.18,20 To test whether a relationship exists between protein expression was measured in WT and Arf MEFs Drosha and ARF, we first compared Drosha protein levels using following cycloheximide treatment. The rate of Drosha turnover wild-type (WT) and Arf À / À mouse embryonic fibroblasts (MEFs). remained unchanged in the presence or absence of Arf despite Higher levels of Drosha protein were observed in cells with the fact that we observed significantly more Drosha at the original genetic ablation of Arf exon 1b (Figure 1a, left). We explored this time of treatment in cells lacking Arf (Figures 2b and c). result further by acutely manipulating the expression of ARF using Furthermore, treatment of WT MEFs with the proteosomal inhibitor MG-132 failed to induce Drosha protein levels to those either a lentivirus encoding a short hairpin RNA (shRNA) targeting À / À Arf exon 1b, the ARF-specific exon of the CDKN2A locus, or an ARF observed in Arf MEFs (Figure 2d). These data suggest that the overexpressing retrovirus. Consistent with the previous finding, differences in Drosha protein expression were not caused by acute knockdown of basal ARF expression resulted in heightened altered protein stability that led us to hypothesize that Drosha Drosha expression (Figure 1b). Conversely, ectopic overexpression might be translationally regulated by ARF. of ARF lowered Drosha expression (Figure 1c). ARF-mediated Basal ARF negatively regulates multiple aspects of ribosome biogenesis, including rDNA transcription, rRNA processing, and regulation of Drosha protein expression was not MEF-specific as 7,9,28–30 similar trends were observed using Arf flox/flox mouse astrocytes ribosome nuclear export. Although it is possible that ARF’s infected with adenoviruses encoding Cre recombinase (Figure 1d). antagonizing role in these processes may disrupt the global translation of all mRNAs, existing data suggest that select genes The array of genetic techniques used to disrupt ARF activity 31 demonstrates a novel link between these two proteins and may be translationally regulated by ARF. Having ruled out ARF warranted a closer examination into the mechanism through regulation of Drosha transcription, mRNA, and protein stability, we which ARF suppresses Drosha. sought to determine the effects of Arf loss on the translation of existing Drosha mRNAs. We compared the percentage of Drosha mRNA transcripts associated with actively translating
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