c-CBL (CBL) encodes a member of the Cbl family of proteins, which functions as an E3 ubiquitin ligase. We describe a dominant developmental disorder resulting from germline missense CBL mutations, which is characterized by constitutional anomalies that include impaired growth, developmental delay, cryptorchidism, and a predisposition to juvenile myelomonocytic leukemia (JMML). Some individuals experienced spontaneous regression of their JMML but developed vasculitis later in life. Importantly, JMML specimens from affected children show loss of the normal CBL allele through acquired isodisomy. Consistent with these genetic data, the common p.Y371H mutant Cbl protein induces cytokine-independent growth and constitutive phosphorylation of ERK, AKT, and S6 only in hematopoietic cells in which normal Cbl expression is reduced by RNA interference. We conclude that germline CBL mutations have developmental, tumorigenic, and functional consequences that are reminiscent of disorders that are caused by hyperactive Ras/Raf/MEK/ERK signaling and include neurofibromatosis type 1, and Noonan, Costello, cardiofaciocutaneous, and Legius syndromes.
Substrate engagement by F-box proteins promotes NEDD8 modification of cullins, which is necessary for the activation of cullin-RING E3 ubiquitin ligases (CRLs). However, the mechanism by which substrate recruitment triggers cullin neddylation remains unclear. Here, we identify DCNL1 (defective in cullin neddylation 1-like 1) as a component of CRL2 called ECV (elongins BC/CUL2/VHL) and show that molecular suppression of DCNL1 attenuates CUL2 neddylation. DCNL1 via its DAD patch binds to CUL2 but is also able to bind VHL independent of CUL2 and the DAD patch. The engagement of the substrate hypoxia-inducible factor 1α (HIF1α) to the substrate receptor VHL increases DCNL1 binding to VHL as well as to CUL2. Notably, an engineered mutant form of HIF1α that associates with CUL2, but not DCNL1, fails to trigger CUL2 neddylation and retains ECV in an inactive state. These findings support a model in which substrate engagement prompts DCNL1 recruitment that facilitates the initiation of CUL2 neddylation and define DCNL1 as a “substrate sensor switch” for ECV activation.
Granulocyte-macrophage colony-stimulating factor (GM-CSF) and the related cytokines interleukin (IL)-3 and IL-5 regulate the production and functional activation of hematopoietic cells. GM-CSF acts on monocytes/macrophages and granulocytes, and several chronic inflammatory diseases and a number of haematological malignancies such as Juvenile myelomonocytic leukaemia (JMML) are associated with deregulated GM-CSF receptor (GMR) signaling. The downregulation of GMR downstream signaling is mediated in part by the clearance of activated GMR via the proteasome, which is dependent on the ubiquitylation of βc signaling subunit of GMR via an unknown E3 ubiquitin ligase. Here, we show that suppressor of cytokine signaling 1 (SOCS-1), best known for its ability to promote ubiquitin-mediated degradation of the non-receptor tyrosine kinase Janus kinase 2 (JAK2), also targets GMRβc for ubiquitin-mediated degradation and attenuates GM-CSF-induced downstream signaling.
A characteristic of cancer cells is the generation of lactate from glucose in spite of adequate oxygen for oxidative phosphorylation. This property – known as the “Warburg effect” or aerobic glycolysis – contrasts with anaerobic glycolysis, which is triggered in hypoxic normal cells. The Warburg effect is thought to provide a means for cancer cells to survive under conditions where oxygen is limited and to generate metabolites necessary for cell growth. The shift from oxidative phosphorylation to glycolysis in response to hypoxia is mediated by the production of hypoxia-inducible factor (HIF) – a transcription factor family that stimulates the expression of proteins involved in glucose uptake and glycolysis. We reported previously that elevated phospholipase D (PLD) activity in renal and breast cancer cells is required for the expression of the α subunits of HIF1 and HIF2. We report here that the aerobic glycolysis observed in human breast and renal cancer cells is dependent on the elevated PLD activity. Intriguingly, the effect of PLD on the Warburg phenotype was dependent on the mammalian target of rapamycin complex 1 (mTORC1) in the breast cancer cells and on mTORC2 in the renal cancer cells. These data indicate that elevated PLD-mTOR signaling, which is common in human cancer cells, is critical for the metabolic shift to aerobic glycolysis.
Phospholipase D; Warburg effect; glycolysis; metabolic transformation; hypoxia-inducible factor
Chuvash polycythemia (CP) is a rare congenital form of polycythemia caused by homozygous R200W and H191D mutations in the von Hippel-Lindau (VHL) gene whose gene product is the principal negative regulator of hypoxia-inducible factor. However, the molecular mechanisms underlying some of the hallmark features of CP such as hypersensitivity to erythropoietin are unclear. Here, we show that VHL directly binds suppressor of cytokine signalling 1 (SOCS1) to form a heterodimeric E3 ligase that targets phosphorylated (p)JAK2 for ubiquitin-mediated destruction. In contrast, CP-associated VHL mutants have altered affinity for SOCS1 and fail to engage and degrade pJAK2. Systemic administration of a highly selective JAK2 inhibitor, TG101209, reverses the disease phenotype in vhlR200W/R200W knock-in mice, a model that faithfully recapitulates human CP. These results reveal VHL as a SOCS1-cooperative negative regulator of JAK2 and provide compelling biochemical and preclinical evidence for JAK2- targeted therapy in CP patients.
VHL; SOCS; JAK2; STAT5; polycythemia
p53 mutations are rarely detected in clear-cell renal cell carcinoma (CCRCC), but paradoxically these tumors remain highly resistant to chemotherapy and death receptor-induced death. Here, we show that the accumulation of HIF2α, a critical oncogenic event in CCRCC upon the loss of von Hippel-Lindau (VHL) tumor suppressor protein, leads to Hdm2-mediated suppression of p53. Primary CCRCC specimens exhibiting strong hypoxic signatures show increased levels of activated nuclear phospho-Hdm2(Ser166) that is concomitant with low p53 expression. The abrogation of Hdm2/p53 interaction using a small molecule Hdm2 inhibitor nutlin-3 or the downregulation of HIF2α via HIF2α-specific shRNA or wild-type VHL reconstitution restores p53 function and reverses the resistance of CCRCC cells to Fas-mediated and chemotherapy-induced cell death. These findings unveil a mechanistic link between HIF2α and p53 and provide a rationale for combining Hdm2 antagonists with chemotherapy for the treatment of CCRCC.
HIF2α; p53; Hdm2; VHL; RCC
Apoptosis is a fundamental host defence mechanism against invading microbes. Inactivation of NF-κB attenuates encephalomyocarditis virus (EMCV) virulence by triggering rapid apoptosis of infected cells, thereby pre-emptively limiting viral replication. Recent evidence has shown that hypoxia-inducible factor (HIF) increases NF-κB-mediated anti-apoptotic response in clear-cell renal cell carcinoma (CCRCC) that commonly exhibit hyperactivation of HIF due to the loss of its principal negative regulator, von Hippel–Lindau (VHL) tumour suppressor protein. Here, we show that EMCV challenge induces a strong NF-κB-dependent gene expression profile concomitant with a lack of interferon-mediated anti-viral response in VHL-null CCRCC, and that multiple established CCRCC cell lines, as well as early-passage primary CCRCC cultured cells, are acutely susceptible to EMCV replication and virulence. Functional restoration of VHL or molecular suppression of HIF or NF-κB dramatically reverses CCRCC cellular susceptibility to EMCV-induced killing. Notably, intratumoural EMCV treatment of CCRCC in a murine xenograft model rapidly regresses tumour growth. These findings provide compelling pre-clinical evidence for the usage of EMCV in the treatment of CCRCC and potentially other tumours with elevated HIF/NF-κB-survival signature.
EMCV; HIF; NF-κB; RCC; VHL
Solid tumours invariably exhibit regions of hypoxia and up-regulation of receptor tyrosine kinases (RTKs) that trigger multiple signal pathways, including those that govern cell proliferation, survival and motility, ultimately contributing to oncogenesis. Although past studies have shown hypoxia-dependent transcriptional and translational induction of several RTK expression and their respective ligands, recent evidence suggests that hypoxia regulates RTK signalling through endocytosis, a major deactivation pathway of RTKs. Hypoxia-mediated endocytosis is also thought to modulate the activity of a growing list of other membrane-associated proteins such as integrins and Na,K-ATPase. These recent discoveries underscore the emergence of endocytosis as an important hypoxia-mediated regulatory process in cancer.
Tumour hypoxia promotes the accumulation of the otherwise oxygen-labile hypoxia-inducible factor (HIF)-α subunit whose expression is associated with cancer progression, poor prognosis and resistance to conventional radiation and chemotherapy. The oxygen-dependent degradation of HIF-α is carried out by the von Hippel–Lindau (VHL) protein-containing E3 that directly binds and ubiquitylates HIF-α for subsequent proteasomal destruction. Thus, the cellular proteins involved in the VHL–HIF pathway have been recognized as attractive molecular targets for cancer therapy. However, the various compounds designed to inhibit HIF-α or HIF-downstream targets, although promising, have shown limited success in the clinic. In the present study, we describe the bioengineering of VHL protein that removes the oxygen constraint in the recognition of HIF-α while preserving its E3 enzymatic activity. Using speckle variance–optical coherence tomography (sv–OCT), we demonstrate the dramatic inhibition of angiogenesis and growth regression of human renal cell carcinoma xenografts upon adenovirus-mediated delivery of the bioengineered VHL protein in a dorsal skin-fold window chamber model. These findings introduce the concept and feasibility of ‘bio-tailored’ enzymes in the treatment of HIF-overexpressing tumours.
angiogenesis; ARNT; HIF; renal cell carcinoma; VHL
Chromosomal abnormalities, such as structural and numerical abnormalities, are a common occurrence in cancer. The close association of homologous chromosomes during interphase, a phenomenon termed somatic chromosome pairing, has been observed in cancerous cells, but the functional consequences of somatic pairing have not been established. Gene expression profiling studies revealed that somatic pairing of chromosome 19 is a recurrent chromosomal abnormality in renal oncocytoma, a neoplasia of the adult kidney. Somatic pairing was associated with significant disruption of gene expression within the paired regions and resulted in the deregulation of the prolyl-hydroxylase ELGN2, a key protein that regulates the oxygen-dependent degradation of hypoxia-inducible factor (HIF). Overexpression of ELGN2 in renal oncocytoma increased ubiquitin-mediated destruction of HIF and concomitantly suppressed the expression of several HIF-target genes, including the pro-death BNIP3L gene. The transcriptional changes that are associated with somatic pairing of chromosome 19 mimic the transcriptional changes that occur following DNA amplification. Therefore, in addition to numerical and structural chromosomal abnormalities, alterations in chromosomal spatial dynamics should be considered as genomic events that are associated with tumorigenesis. The identification of EGLN2 as a significantly deregulated gene that maps within the paired chromosome region directly implicates defects in the oxygen-sensing network to the biology of renal oncocytoma.
Together, renal oncocytoma and chromophobe renal cell carcinoma (RCC) account for approximately 10% of masses that are resected from the kidney. However, the molecular defects that are associated with the development of these neoplasias are not clear. Here, we take advantage of recent advances in genetics and computational analysis to screen for chromosomal abnormalities that are present in both renal oncocytoma and chromophobe RCC. We show that while chromophobe RCC cells contain an extra copy of chromosome 19, the renal oncoctyoma cells contain a rarely reported chromosomal abnormality. Both of these chromosomal abnormalities result in transcriptional disruptions of EGLN2, a gene that is located on chromosome 19 and is critical for the cellular response to changes in oxygen levels. Defects in oxygen sensing are found in other types of kidney tumors, and the identification of EGLN2 directly implicates defects in the oxygen-sensing network in these neoplasias as well. These findings are important because the chromosomal defect present in renal oncocytomas may also be present in other tumor cells. In addition, deregulation of EGLN2 reveals a unique way in which perturbations in oxygen-sensing are associated with disease.
The product of the von Hippel-Lindau gene (VHL) acts as the substrate-recognition component of an E3 ubiquitin ligase complex that ubiquitylates the catalytic α subunit of hypoxia-inducible factor (HIF) for oxygen-dependent destruction. Although emerging evidence supports the notion that deregulated accumulation of HIF upon the loss of VHL is crucial for the development of clear-cell renal cell carcinoma (CC-RCC), the molecular events downstream of HIF governing renal oncogenesis remain unclear. Here, we show that the expression of a homophilic adhesion molecule, E-cadherin, a major constituent of epithelial cell junctions whose loss is associated with the progression of epithelial cancers, is significantly down-regulated in primary CC-RCC and CC-RCC cell lines devoid of VHL. Reintroduction of wild-type VHL in CC-RCC (VHL−/−) cells markedly reduced the expression of E2 box-dependent E-cadherin-specific transcriptional repressors Snail and SIP1 and concomitantly restored E-cadherin expression. RNA interference-mediated knockdown of HIFα in CC-RCC (VHL−/−) cells likewise increased E-cadherin expression, while functional hypoxia or expression of VHL mutants incapable of promoting HIFα degradation attenuated E-cadherin expression, correlating with the disengagement of RNA polymerase II from the endogenous E-cadherin promoter/gene. These findings reveal a critical HIF-dependent molecular pathway connecting VHL, an established “gatekeeper” of the renal epithelium, with a major epithelial tumor suppressor, E-cadherin.
Hypoxia-inducible factor (HIF) is a central regulator of cellular responses to hypoxia, and under normal oxygen tension the catalytic α subunit of HIF is targeted for ubiquitin-mediated destruction via the VHL-containing E3 ubiquitin ligase complex. Principally known for its association with oncogenesis, HIF has been documented to have a role in the antibacterial response. Interferons, cytokines with antiviral functions, have been shown to upregulate the expression of HIF-1α, but the significance of HIF in the antiviral response has not been established. Here, using renal carcinoma cells devoid of VHL or reconstituted with functional wild-type VHL or VHL mutants with various abilities to negatively regulate HIF as an ideal model system of HIF activity, we show that elevated HIF activity confers dramatically enhanced resistance to vesicular stomatitis virus (VSV)-mediated cytotoxicity. Inhibition of HIF activity using a small-molecule inhibitor, chetomin, enhanced cellular sensitivity to VSV, while treatment with hypoxia mimetic CoCl2 promoted resistance. Similarly, targeting HIF-2α by RNA interference also enhanced susceptibility to VSV. Expression profiling studies show that upon VSV infection, the induction of genes with known antiviral activity, such as that encoding beta interferon (IFN-β), is significantly enhanced by HIF. These results reveal a previously unrecognized role of HIF in the antiviral response by promoting the expression of the IFN-β gene and other genes with antiviral activity upon viral infection.
ECV is an E3 ubiquitin ligase complex, which is composed of elongins B and C, Rbx1, Cul2, and the substrate-conferring von Hippel-Lindau (VHL) tumorsuppressor protein that targets the catalytic α subunit of hypoxia-inducible factor (HIF) for oxygen-dependent ubiquitin-mediated destruction. Mutations in VHL that compromise proper HIFα regulation through ECV have been documented in the majority of renal cell carcinomas, underscoring the significance of the VHL-HIF pathway in renal epithelial oncogenesis. Recent evidence has shown that the modification of Cul2 by the ubiquitin-like molecule NEDD8 increases the activity of ECV to ubiquitylate HIFα. However, the underlying mechanism responsible for the NEDD8-mediated induction of ECV function is unknown. Here, we demonstrate that oxygen-dependent recognition of HIFα by VHL triggers Rbx1-dependent neddylation of Cul2, which preferentially engages the E2 ubiquitin-conjugating enzyme UbcH5a. These events establish a central role for the neddylation of Cul2 in a previously unrecognized, temporally coordinated activation of ECV with the recruitment of its substrate HIFα.
Cul2; NEDD8; UbcH5a; HIFα; VHL
The physiologic response to changes in cellular oxygen tension is ultimately governed by a heterodimeric transcription factor called hypoxia-inducible factor (HIF), which, in adaptation to compromised oxygen availability, transactivates a myriad of genes, including those responsible for de novo vascularization, production of oxygen-carrying red blood cells, and anaerobic metabolism. Accumulation of HIF is observed in most types of solid tumors and is frequently associated with poor prognosis and disease progression, underscoring the importance and relevance of HIF in cancer. The protein stability and, thereby, the activity of HIF are principally regulated by the von Hippel-Lindau (VHL) tumor-suppressor-containing E3 ubiquitin ligase complex (ECV) that targets the catalytic subunit HIFα for oxygen-dependent ubiquitin-mediated destruction. Individuals who inherit germline VHL mutation develop VHL disease, which is characterized by the development of hypervascular tumors in multiple yet specific organs. This review will examine recent progress in our understanding of the molecular mechanisms governing the function of ECV and the significance of consequential regulation of HIF in oncogenesis.
VHL; HIF; ECV; ubiquitin; NEDD8
Functional inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene is the cause of the familial VHL disease and most sporadic renal clear-cell carcinomas (RCC). pVHL has been shown to play a role in the destruction of hypoxia-inducible factor α (HIF-α) subunits via ubiquitin-mediated proteolysis and in the regulation of fibronectin matrix assembly. Although most disease-causing pVHL mutations hinder the regulation of the HIF pathway, every disease-causing pVHL mutant tested to date has failed to promote the assembly of the fibronectin matrix, underscoring its potential importance in VHL disease. Here, we report that a ubiquitin-like molecule called NEDD8 covalently modifies pVHL. A nonneddylateable pVHL mutant, while retaining its ability to ubiquitylate HIF, failed to bind to and promote the assembly of the fibronectin matrix. Expression of the neddylation-defective pVHL in RCC cells, while restoring the regulation of HIF, failed to promote the differentiated morphology in a three-dimensional growth assay and was insufficient to suppress the formation of tumors in SCID mice. These results suggest that NEDD8 modification of pVHL plays an important role in fibronectin matrix assembly and that in the absence of such regulation, an intact HIF pathway is insufficient to prevent VHL-associated tumorigenesis.
We examined the biogenesis of the von Hippel-Lindau (VHL) tumor suppressor protein (pVHL) in vitro and in vivo. pVHL formed a complex with the cytosolic chaperonin containing TCP-1 (CCT or TRiC) en route to assembly with elongin B/C and the subsequent formation of the VCB-Cul2 ubiquitin ligase. Blocking the interaction of pVHL with elongin B/C resulted in accumulation of pVHL within the CCT complex. pVHL present in purified VHL-CCT complexes, when added to rabbit reticulocyte lysate, proceeded to form VCB and VCB-Cul2. Thus, CCT likely functions, at least in part, by retaining VHL chains pending the availability of elongin B/C for final folding and/or assembly. Tumor-associated mutations within exon II of the VHL syndrome had diverse effects upon the stability and/or function of pVHL-containing complexes. First, a pVHL mutant lacking the entire region encoded by exon II did not bind to CCT and yet could still assemble into complexes with elongin B/C and elongin B/C-Cul2. Second, a number of tumor-derived missense mutations in exon II did not decrease CCT binding, and most had no detectable effect upon VCB-Cul2 assembly. Many exon II mutants, however, were found to be defective in the binding to and subsequent ubiquitination of hypoxia-inducible factor 1α (HIF-1α), a substrate of the VCB-Cul2 ubiquitin ligase. We conclude that the selection pressure to mutate VHL exon II during tumorigenesis does not relate to loss of CCT binding but may reflect quantitative or qualitative defects in HIF binding and/or in pVHL-dependent ubiquitin ligase activity.
Inactivation of the von Hippel-Lindau (VHL) tumour suppressor
gene product pVHL is the cause of inherited VHL disease and is associated with sporadic kidney cancer. pVHL is found in a multiprotein complex with elongins B/C, Cul2, and Rbx1 forming an E3 ubiquitin ligase complex called VEC. This modular enzyme targets the α subunits of hypoxia-inducible factor (HIF) for ubiquitin-mediated destruction. Consequently, tumour cells lacking functional pVHL overproduce the products of HIF-target genes such as vascular endothelial growth factor (VEGF), which promotes angiogenesis. This likely accounts for the hypervascular nature of VHL-associated neoplasms. Although pVHL has been linked to the cell-cycle, differentiation, and the regulation of extracellular matrix assembly, microenvironment pH, and tissue invasiveness, this review will focus on the recent insights into the molecular mechanisms governing the E3 ubiquitin ligase function of VEC.
The von Hippel-Lindau tumor suppressor protein (pVHL) negatively regulates hypoxia-inducible mRNAs such as the mRNA encoding vascular endothelial growth factor (VEGF). This activity has been linked to its ability to form multimeric complexes that contain elongin C, elongin B, and Cul2. To understand this process in greater detail, we performed a series of in vitro binding assays using pVHL, elongin B, and elongin C variants as well as synthetic peptide competitors derived from pVHL or elongin C. A subdomain of elongin C (residues 17–50) was necessary and sufficient for detectable binding to elongin B. In contrast, elongin B residues required for binding to elongin C were not confined to a discrete colinear domain. We found that the pVHL (residues 157–171) is necessary and sufficient for binding to elongin C in vitro and is frequently mutated in families with VHL disease. These mutations preferentially involve residues that directly bind to elongin C and/or alter the conformation of pVHL such that binding to elongin C is at least partially diminished. These results are consistent with the view that diminished binding of pVHL to the elongins plays a causal role in VHL disease.
J. Clin. Invest. 104:1583–1591 (1999).
The von Hippel-Lindau tumor suppressor protein (pVHL) binds to elongins B and C and posttranscriptionally regulates the accumulation of hypoxia-inducible mRNAs under normoxic (21% O2) conditions. Here we report that pVHL binds, via elongin C, to the human homolog of the Caenorhabditis elegans Cul2 protein. Coimmunoprecipitation and chromatographic copurification data suggest that pVHL-Cul2 complexes exist in native cells. pVHL mutants that were unable to bind to complexes containing elongin C and Cul2 were likewise unable to inhibit the accumulation of hypoxia-inducible mRNAs. A model for the regulation of hypoxia-inducible mRNAs by pVHL is presented based on the apparent similarity of elongin C and Cul2 to Skp1 and Cdc53, respectively. These latter proteins form complexes that target specific proteins for ubiquitin-dependent proteolysis.