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NEDD8 Acts As a 'Molecular Switch' Defining the Functional Selectivity Of scientificscientificreport report NEDD8 acts as a ‘molecular switch’ defining the functional selectivity of VHL Ryan C. Russell & Michael Ohh+ Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada The von Hippel–Lindau (VHL) tumour suppressor protein is (HIF), which is the main transcription factor that activates the important in the E3 ubiquitin ligase ECV (Elongin B/C–CUL2– expression of many hypoxia-inducible genes to counter VHL)-mediated destruction of hypoxia-inducible factor and the the detrimental effects of compromised oxygen availability promotion of fibronectin (FN) extracellular matrix assembly. (Kaelin, 2002), and as a positive regulator of fibronectin (FN) Although the precise molecular mechanism controlling the extracellular matrix (ECM) assembly (Roberts & Ohh, 2008). selectivity of VHL function remains unknown, a failure in either ECV (Elongin B/C–CUL2–VHL) is an SCF (SKP1/CDC53 or process is associated with oncogenic progression. Here, we show CUL1/F-box protein)-like E3 ubiquitin ligase in which VHL acts as that VHL performs its FN-associated function independently of a substrate-conferring component that recruits the a-subunits of the ECV complex, highlighting the autonomy of these pathways. HIF. These have been modified with hydroxyl groups on Furthermore, we show that NEDD8, a ubiquitin-like molecule, conserved prolyl residues within the oxygen-dependent degrada- acts as a ‘molecular switch’ in which its covalent conjugation to tion domain (ODD) by a class of prolyl hydroxylases in an oxygen- VHL prohibits the engagement of the scaffold component CUL2 dependent manner (Kaelin, 2002). This mechanistic insight and, concomitantly, activates the association with FN. These explains why HIFa is stabilized under hypoxia to bind to findings provide the first mechanistic step in defining the constitutively expressed HIFb (also known as aryl hydrocarbon functional selectivity of VHL and explain a previously unrecognized receptor nuclear translocator) to form an active heterodimeric function of NEDD8. transcription factor. Concordantly, cells under hypoxia or tumour Keywords: CUL2; ECV; fibronectin; HIF; NEDD8; VHL cells devoid of VHL irrespective of oxygen tension have enhanced EMBO reports (2008) 9, 486–491. doi:10.1038/embor.2008.19 expression of HIF target genes, such as VEGF (vascular endothelial growth factor), GLUT1 (glucose transporter 1) and EPO (erythro- INTRODUCTION poietin; Roberts & Ohh, 2008). The overexpression of hypoxia- Inheritance of one faulty von Hippel–Lindau (VHL) allele is the inducible genes probably contributes to the hypervascular nature cause of VHL disease, which is characterized by the development of VHL disease-associated tumours, and supports the idea that of retinal, cerebellar and spinal haemangioblastomas, phaeochromo- constitutive stabilization of HIFa is a crucial oncogenic event cytoma and renal clear-cell carcinoma (RCC; Kaelin, 2002). following the loss of VHL (Roberts & Ohh, 2008). Tumorigenesis begins with the loss or inactivation of the VHL also binds to FN and this physical interaction represents a remaining wild-type VHL allele in a susceptible cell (Kaelin, requisite step in the promotion of correct ECM assembly (Ohh 2002). Biallelic inactivation of the VHL locus is also responsible et al, 1998). All tumour-causing VHL mutants tested so far show a for the development of most of the sporadic RCC, establishing striking failure in binding to and/or in the assembly of FN (Clifford VHL as a crucial ‘gatekeeper’ gene of the renal epithelium et al, 2001; Hoffman et al, 2001). Recently, VHL was also shown (Kaelin, 2002). to interact with collagen IV (COLIV) to promote its deposition in The phenotypic variation observed in relatives that carry the extracellular space (Grosfeld et al, 2007; Kurban et al, 2008). mutations in VHL (VHL kindred) are due to alterations in tissue- These findings underscore the significance of VHL-mediated ECM specific functions or in the severity with which certain VHL assembly in tumour suppression. Kurban et al (2006) showed that functions are altered. The two most well-characterized functions the loss of correct FN ECM promotes angiogenesis of RCC of VHL are as a negative regulator of hypoxia-inducible factor xenograft in an HIF-independent manner and that HIFa stabiliza- tion in the context of intact FN ECM results in tumours with low microvessel density despite the overexpression of VEGF. Tang et al Department of Laboratory Medicine and Pathobiology, University of Toronto, 1 King’s College Circle, Toronto, Ontario, Canada M5S 1A8 (2006) showed, in mice with conditional knockout of VHL in +Corresponding author. Tel: þ 1 416 946 7922; Fax: þ 1 416 978 5959; endothelial cells, that VHL has a crucial role in the vascular FN E-mail: [email protected] ECM assembly, independent of its role in regulating HIFa.In Received 8 October 2007; revised 10 January 2008; accepted 23 January 2008; addition, studies of genomic clustering in Caenorhabditis elegans published online 7 March 2008 identified a discrete HIF-independent role of VHL in ECM function 486 EMBO reports VOL 9 | NO 5 | 2008 &2008 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION NEDD8 defines tumour suppressor function of VHL R.C. Russell & M. Ohh scientificreport (Bishop et al, 2004). Although these studies highlight the HIF- K C CUL2 Scram : siRNA independent FN assembly function of VHL, the mechanism O M IB: controlling the selectivity of VHL functions remained unknown. Anti-CUL2 Until recently, the ubiquitin-like molecule NEDD8 has been thought to conjugate exclusively to Cullins, in a manner Anti-vinculin analogous to ubiquitylation, to enhance the activity of SCF and 1 234567 ECV complexes (Pan et al, 2004; Sufan & Ohh, 2006). Emerging evidence has shown NEDD8 to be more versatile in substrate K C D/AR specificity and, importantly, several proteins involved in onco- O T /AR : P : IP H genesis, including VHL, p53, murine double minute 2 (MDM2), 4-M 4-W A C C -O C C D p73, epidermal growth factor receptor and the breast cancer- R R D –+ –+ : CUL2 siRNA Anti-H O associated protein 3, have now been shown to be targets of kDa NEDD8 modification (Stickle et al, 2004; Xirodimas et al, 2004; FN Gao et al, 2006; Oved et al, 2006; Watson et al, 2006). In 175 addition, the functional consequence of neddylation was found to 786-MOCK 786-WT 786-WT 786-MOCK kDa ∗ be more diverse than previously thought, extending beyond the FN 175 enhancement of the ubiquitin–proteasome pathway. Here, we provide evidence that NEDD8 acts as a ‘molecular switch’ in 86 CUL2 which its covalent conjugation to VHL precludes ECV formation by steric hindrance and, concomitantly, allows interaction with 86 FN, thereby providing the first mechanistic step in the definition of 47 the functional selectivity of VHL. 26 HA-VHL 47 RESULTS AND DISCUSSION 19 Elongin B Elongin C ECV- and FN-functions of VHL are mutually exclusive 26 HA-VHL The ability of VHL to regulate HIF is dependent on its ability to 1234 Anti-HA: IP/AR form a functional ECV. Therefore, we investigated whether the 19 Elongin B ability of VHL to bind to FN was also dependent on ECV. VHL- Elongin C 1234 associated FN was immunoprecipitated after short interfering RNA (siRNA)-mediated knockdown of CUL2, a scaffold component of Fig 1 | ECV- and fibronectin-associated functions of VHL are mutually ECV, in 35S-radiolabelled VHL-null RCC4 renal carcinoma cells exclusive. (A) Human embryonic kidney 293A cells were treated with ectopically expressing haemagglutinin (HA)-VHL (WT) or empty increasing amounts (10–100 nM) of CUL2 siRNA (lanes 2–4), scrambled plasmid (MOCK). The level of FN co-precipitating with VHL did (Scram) siRNA (lanes 5–7) or transfection reagent alone (MOCK; lane 1). not decrease despite a marked reduction in CUL2 expression (Fig 1; Equalized whole-cell lysates were resolved on SDS–polyacrylamide gel supplementary Fig S2E online). As expected, siRNA-mediated electrophoresis and immunoblotted (IB) with CUL2 (lower panel) or knockdown of CUL2 attenuated VHL-dependent ubiquitylation vinculin (upper panel) antibodies. (B) RCC4 cells stably transfected of HIF1aODD (supplementary Fig S2C online). Furthermore, with HA-VHL (WT) or empty plasmid (MOCK) were radiolabelled biotinylated HIF1aODD-OH peptides co-precipitated ECV com- (metabolically labelled with [35S]methionine). Cells were treated with ponents without the presence of FN, as compared with FN ( þ ) or without (À) CUL2 siRNA as indicated. Cell lysates were co-precipitated from 35S-radiolabelled 786-VHL RCC cells using immunoprecipitated with an HA antibody and the resolved proteins an HA antibody directed against HA-VHL (Fig 1C, compare lanes were visualized by autoradiography. (C) 786-WT and 786-MOCK cells 2 and 3). Although it is possible that HIF1aODD-OH peptides were metabolically labelled with [35S]methionine, lysed and might have displaced FN by competition for VHL, hypoxia or immunoprecipitated with an HA antibody (lanes 1,2) or pulled down hypoxia-mimetic (desferroxamine or CoCl2) treatment of 786-VHL with synthetic HIF1aODD-OH peptides (lanes 3,4). Bound proteins were cells did not increase VHL/FN interaction (supplementary Fig S3 resolved and visualized by autoradiography. The asterisk denotes online). These results argue against the idea that VHL binding to nonspecific protein bands. AR, autoradiography; ECV, Elongin FN is influenced by competition with HIFa, and suggest that ECV B/C–CUL2–VHL; FN, fibronectin; HA, haemagglutinin; IP, complex is not necessarily required for VHL to bind to FN. immunoprecipitation; RCC, renal clear cell; siRNA, short interfering RNA; VHL, von Hippel–Lindau. Disruption of NEDD8 pathway abrogates FN binding Recently, we have shown that mutations in VHL that disrupt a restrictive temperature as compared with cells under a NEDD8 conjugation lead to a failure in binding to FN (Stickle permissive temperature (Fig 2A), indicating that an intact NEDD8 et al, 2004). To investigate further whether VHL binding to FN is pathway is crucial for promoting VHL binding to FN.
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