Tumor Suppressor pVHL Research Articles

Induction of the Mdm2 gene and protein by kinase signaling pathways is repressed by the pVHL tumor suppressor

The tumor suppressor von Hippel-Lindau, pVHL, is a multifaceted protein. One function is to dock to the hypoxia-inducible transcription factor (HIF) and recruit a larger protein complex that destabilizes HIF via ubiquitination, preventing angiogenesis and tumor development. pVHL also binds to the tumor suppressor p53 to activate specific p53 target genes. The oncogene Mdm2 impairs the formation of the p53-pVHL complex and activation of downstream genes by conjugating nedd8 to pVHL. While Mdm2 can impact p53 and pVHL, how pVHL may impact Mdm2 is unclear. Like p53 somatic mutations, point mutations are evident in pVHL that are common in renal clear cell carcinomas (RCC). In patients with RCC, Mdm2 levels are elevated, and we examined whether there was a relationship between Mdm2 and pVHL. TCGA and DepMap analysis revealed that mdm2 gene expression was elevated in RCC with vhl point mutations or copy number loss. In pVHL reconstituted or deleted isogenetically match RCC or MEF cell lines, Mdm2 was decreased in the presence of pVHL. Furthermore, through analysis using genetic and pharmacological approaches, we show that pVHL represses Mdm2 gene expression by blocking the MAPK-Ets signaling pathway and blocks Akt-mediated phosphorylation and stabilization of Mdm2. Mdm2 inhibition results in an increase in the p53-p21 pathway to impede cell growth. This finding shows how pVHL can indirectly impact the function of Mdm2 by regulating signaling pathways to restrict cell growth.

Abstract PR011: Downregulation of the HIF2a target cyclin D1 underlies the efficacy of the HIF2a inhibitor belzutifan in kidney cancer

Abstract Abstract: Most clear cell renal cell carcinomas (ccRCCs) are caused by loss of the pVHL tumor suppressor function. Inactivation of pVHL leads to increased accumulation of HIF2a, which drives ccRCC in preclinical models. HIF2a interacts with ARNT and functions as a heterodimeric transcription factor. HIF2a can activate hundreds of downstream target genes, including many that are suspected of playing a role in ccRCC pathogenesis based on their biological functions. Peloton Therapeutics developed a specific allosteric HIF2a inhibitor named PT2399, which disrupts the HIF2a-ARNT interaction and silences the HIF2a transcription program. Furthermore, PT2399 suppresses the growth of ccRCC cells in in-vitro and in-vivo assays. A more advanced version of PT2399, Belzutifan, was recently approved for treating tumors in patients with germline VHL mutations (VHL disease) and is in Phase 3 testing for sporadic ccRCCs. Since HIF2a is a transcriptional activator, we hypothesized that PT2399’s ability to suppress ccRCC is linked to repression of one or more HIF2a target genes. To identify these genes, we performed a pooled whole genome CRISPR activation (CRISPRa) screen in the presence or absence of PT2399. This screen identified multiple genes (“hits”) that, when activated, likely conferred resistance to PT2399. To further differentiate between direct vs indirect HIF2a targets that confer resistance to PT2399, we performed nascent RNA-seq (Pro-seq) after acute treatment of ccRCC cells with PT2399. Comparison of the CRISPRa and Pro-Seq data, together with secondary validation assays, identified CCND1, which encodes Cyclin D1, as a direct target gene of HIF2a that, when activated, confers complete resistance to PT2399 in multiple ccRCC cell lines. Additionally, we showed that wild-type Cyclin D1 (WT), but not a mutant (KE) that fails to activate CDK4/6, confers resistance to PT2399. Interestingly, Cyclin D1 overrides the growth inhibitory effect of PT2399 in ccRCC cells irrespective of pRB status. To identify the relevant Rb-independent substrates of Cyclin D1, we performed IP-MS experiments in RB1-/- ccRCC cells under substrate trapping conditions. Our preliminary data from these experiments suggests that RBL1 (p107) and RBL2 (p130) substitute for pRB in RB1-/- ccRCC cells. Our findings provide further support for CDK4/6 as a therapeutic target in ccRCC. Citation Format: Nitin H. Shirole, Devishi Kesar, William R. Sellers, William G. Kaelin Jr. Downregulation of the HIF2a target cyclin D1 underlies the efficacy of the HIF2a inhibitor belzutifan in kidney cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Kidney Cancer Research; 2023 Jun 24-27; Austin, Texas. Philadelphia (PA): AACR; Cancer Res 2023;83(16 Suppl):Abstract nr PR011.

Targeting the HIF2-VEGF axis in renal cell carcinoma.

Insights into the role of the tumor suppressor pVHL in oxygen sensing motivated the testing of drugs that target the transcription factor HIF or HIF-responsive growth factors, such as VEGF, for the treatment of cancers caused by VHL inactivation, such as clear-cell renal cell carcinoma (ccRCC). Multiple VEGF inhibitors are now approved for the treatment of ccRCC, and a HIF2α inhibitor has advanced to phase 3 development for this disease. These inhibitors are now also increasingly combined with immune-checkpoint blockers. In this Perspective, we describe the understanding of the mechanisms of oxygen sensing and hypoxia signaling that resulted in the development of HIF2α-targeted therapies for patients with VHL-associated tumors. We also present future directions for extending the use of these therapies to other cancers.

EU-OPENSCREEN: A Novel Collaborative Approach to Facilitate Chemical Biology

Compound screening in biological assays and subsequent optimization of hits is indispensable for the development of new molecular research tools and drug candidates. To facilitate such discoveries, the European Research Infrastructure EU-OPENSCREEN was founded recently with the support of its member countries and the European Commission. Its distributed character harnesses complementary knowledge, expertise, and instrumentation in the discipline of chemical biology from 20 European partners, and its open working model ensures that academia and industry can readily access EU-OPENSCREEN’s compound collection, equipment, and generated data. To demonstrate the power of this collaborative approach, this perspective article highlights recent projects from EU-OPENSCREEN partner institutions. These studies yielded (1) 2-aminoquinazolin-4(3H)-ones as potential lead structures for new antimalarial drugs, (2) a novel lipodepsipeptide specifically inducing apoptosis in cells deficient for the pVHL tumor suppressor, (3) small-molecule-based ROCK inhibitors that induce definitive endoderm formation and can potentially be used for regenerative medicine, (4) potential pharmacological chaperones for inborn errors of metabolism and a familiar form of acute myeloid leukemia (AML), and (5) novel tankyrase inhibitors that entered a lead-to-candidate program. Collectively, these findings highlight the benefits of small-molecule screening, the plethora of assay designs, and the close connection between screening and medicinal chemistry within EU-OPENSCREEN.

How hypoxia controls the kinase Akt

Signal Transduction The protein kinase Akt controls cell survival and proliferation. In human cells in culture, Guo et al. found that Akt was modified by the prolyl hydroxylase EglN1 (see the Perspective by Voulgarelis and Tsichlis). Such prolyl hydroxylation suppressed enzymatic activity of Akt. EglN1 is sensitive to oxygen concentrations, and in cells experiencing hypoxia, EglN1 activity decreased and Akt became activated. This activation was associated with decreased binding of Akt to the pVHL tumor suppressor protein, which bound preferentially to prolyl-hydroxylated Akt and inhibited its activity. These effects could promote growth and survival of tumor cells exposed to hypoxia. Science , this issue p. [929][1]; see also p. [870][2] [1]: /lookup/doi/10.1126/science.aad5755 [2]: /lookup/doi/10.1126/science.aah6254

WSB1 promotes tumor metastasis by inducing pVHL degradation.

The von Hippel-Lindau tumor suppressor pVHL is an E3 ligase that targets hypoxia-inducible factors (HIFs). Mutation of VHL results in HIF up-regulation and contributes to processes related to tumor progression such as invasion, metastasis, and angiogenesis. However, very little is known with regard to post-transcriptional regulation of pVHL. Here we show that WD repeat and SOCS box-containing protein 1 (WSB1) is a negative regulator of pVHL through WSB1's E3 ligase activity. Mechanistically, WSB1 promotes pVHL ubiquitination and proteasomal degradation, thereby stabilizing HIF under both normoxic and hypoxic conditions. As a consequence, WSB1 up-regulates the expression of HIF-1α's target genes and promotes cancer invasion and metastasis through its effect on pVHL. Consistent with this, WSB1 protein level negatively correlates with pVHL level and metastasis-free survival in clinical samples. This work reveals a new mechanism of pVHL's regulation by which cancer acquires invasiveness and metastatic tendency.

The Tumor Suppressor pVHL Down-regulates Never-in-Mitosis A-related Kinase 8 via Hypoxia-inducible Factors to Maintain Cilia in Human Renal Cancer Cells

NEK8 (never in mitosis gene A (NIMA)-related kinase 8) is involved in cytoskeleton, cilia, and DNA damage response/repair. Abnormal expression and/or dysfunction of NEK8 are related to cancer development and progression. However, the mechanisms that regulate NEK8 are not well declared. We demonstrated here that pVHL may be involved in regulating NEK8. We found that CAK-I cells with wild-type vhl expressed a lower level of NEK8 than the cells loss of vhl, such as 786-O, 769-P, and A-498 cells. Moreover, pVHL overexpression down-regulated the NEK8 protein in 786-O cells, whereas pVHL knockdown up-regulated NEK8 in CAK-I cells. In addition, we found that the positive hypoxia response elements (HREs) are located in the promoter of the nek8 sequence and hypoxia could induce nek8 expression in different cell types. Consistent with this, down-regulation of hypoxia-inducible factors α (HIF-1α or HIF-2α) by isoform-specific siRNA reduced the ability of hypoxia inducing nek8 expression. In vivo, NEK8 and HIF-1α expression were increased in kidneys of rats subjected to an experimental hypoxia model of ischemia and reperfusion. Furthermore, NEK8 siRNA transfection significantly blocked pVHL-knockdown-induced cilia disassembling, through impairing the pVHL-knockdown-up-regulated NEK8 expression. These results support that nek8 may be a novel hypoxia-inducible gene. In conclusion, our findings show that nek8 may be a new HIF target gene and pVHL can down-regulate NEK8 via HIFs to maintain the primary cilia structure in human renal cancer cells.

Design and analysis of a Petri net model of the Von Hippel-Lindau (VHL) tumor suppressor interaction network.

Von Hippel-Lindau (VHL) syndrome is a hereditary condition predisposing to the development of different cancer forms, related to germline inactivation of the homonymous tumor suppressor pVHL. The best characterized function of pVHL is the ubiquitination dependent degradation of Hypoxia Inducible Factor (HIF) via the proteasome. It is also involved in several cellular pathways acting as a molecular hub and interacting with more than 200 different proteins. Molecular details of pVHL plasticity remain in large part unknown. Here, we present a novel manually curated Petri Net (PN) model of the main pVHL functional pathways. The model was built using functional information derived from the literature. It includes all major pVHL functions and is able to credibly reproduce VHL syndrome at the molecular level. The reliability of the PN model also allowed in silico knockout experiments, driven by previous model analysis. Interestingly, PN analysis suggests that the variability of different VHL manifestations is correlated with the concomitant inactivation of different metabolic pathways.

Candidate tumor suppressor and pVHL partner Jade-1 binds and inhibits AKT in renal cell carcinoma.

The von Hippel-Lindau (VHL) tumor suppressor pVHL is lost in the majority of clear-cell renal cell carcinomas (RCC). Activation of the PI3K/AKT/mTOR pathway is also common in RCC, with PTEN loss occurring in approximately 30% of the cases, but other mechanisms responsible for activating AKT at a wider level in this setting are undefined. Plant homeodomain protein Jade-1 (PHF17) is a candidate renal tumor suppressor stabilized by pVHL. Here, using kinase arrays, we identified phospho-AKT1 as an important target of Jade-1. Overexpressing or silencing Jade-1 in RCC cells increased or decreased levels of endogenous phospho-AKT/AKT1. Furthermore, reintroducing pVHL into RCC cells increased endogenous Jade-1 and suppressed endogenous levels of phospho-AKT, which colocalized with and bound to Jade-1. The N-terminus of Jade-1 bound both the catalytic domain and the C-terminal regulatory tail of AKT, suggesting a mechanism through which Jade-1 inhibited AKT kinase activity. Intriguingly, RCC precursor cells where Jade-1 was silenced exhibited an increased capacity for AKT-dependent anchorage-independent growth, in support of a tumor suppressor function for Jade-1 in RCC. In support of this concept, an in silico expression analysis suggested that reduced Jade-1 expression is a poor prognostic factor in clear-cell RCC that is associated with activation of an AKT1 target gene signature. Taken together, our results identify 2 mechanisms for Jade-1 fine control of AKT/AKT1 in RCC, through loss of pVHL, which decreases Jade-1 protein, or through attenuation in Jade-1 expression. These findings help explain the pathologic cooperativity in clear-cell RCC between PTEN inactivation and pVHL loss, which leads to decreased Jade-1 levels that superactivate AKT. In addition, they prompt further investigation of Jade-1 as a candidate biomarker and tumor suppressor in clear-cell RCC.

The von Hippel-Lindau Protein pVHL Inhibits Ribosome Biogenesis and Protein Synthesis

pVHL, the product of von Hippel-Lindau (VHL) tumor suppressor gene, functions as the substrate recognition component of an E3-ubiquitin ligase complex that targets hypoxia inducible factor α (HIF-α) for ubiquitination and degradation. Besides HIF-α, pVHL also interacts with other proteins and has multiple functions. Here, we report that pVHL inhibits ribosome biogenesis and protein synthesis. We find that pVHL associates with the 40S ribosomal protein S3 (RPS3) but does not target it for destruction. Rather, the pVHL-RPS3 association interferes with the interaction between RPS3 and RPS2. Expression of pVHL also leads to nuclear retention of pre-40S ribosomal subunits, diminishing polysomes and 18S rRNA levels. We also demonstrate that pVHL suppresses both cap-dependent and cap-independent protein synthesis. Our findings unravel a novel function of pVHL and provide insight into the regulation of ribosome biogenesis by the tumor suppressor pVHL.

Abstract 846: ROS-mediated activation of AKT induces apoptosis via pVHL in prostate cancer cells .

Abstract Reactive oxygen species (ROS) play a central role in oxidative stress, which leads to the onset of diseases, such as cancer. Furthermore, ROS contributes to the delicate balance between tumor cell survival and death. However, the mechanisms by which tumor cells decide to elicit survival or death signals during oxidative stress are not completely understood. We have previously reported that ROS enhanced tumorigenic functions in prostate cancer cells, such as cell migration and invasion, which depended on CXCR4 and AKT signaling. Here, we report a novel mechanism by which ROS facilitated cell death through activation of AKT. We initially observed that ROS increased expression of phosphorylated AKT (p-AKT) in 22Rv1 human prostate cancer cells. The tumor suppressor PTEN, a negative regulator of AKT signaling, was rendered catalytically inactive through oxidation by ROS, although the expression levels remained consistent. Despite these events, cells still underwent apoptosis. Further investigation into apoptosis revealed that expression of the tumor suppressor pVHL increased, with concomitant decreased expression of HIF1α. Further investigation into pVHL during apoptosis revealed that the tumor suppressor becomes phosphorylated at S111 prior to activity; S111 is within a target motif (RXXS) forphosphorylation by AKT kinase. We then determined that pVHL and p-AKT associated in vitro via co-immunoprecipitation assay. Moreover, knockdown of pVHL by siRNA rescued HIF1α protein expression and rescued cells from apoptosis, which was determined by Annexin-V and cleaved-PARP. Collectively, our study suggests that in the context of oxidative stress, activatedAKT facilitated apoptosis by inducing pVHL function. Moreover, our studies suggest a novel paradigm for AKT-that it may participate in tumor suppressor rather than oncogenesis. Citation Format: Mahandranauth A. Chetram, Kia J. Jones, Christopher Coke, Ayesha Don-Salu-Hewage, Danaya A. Bethea, Cimona V. Hinton. ROS-mediated activation of AKT induces apoptosis via pVHL in prostate cancer cells . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 846. doi:10.1158/1538-7445.AM2013-846

ROS-mediated activation of AKT induces apoptosis via pVHL in prostate cancer cells

Reactive oxygen species (ROS) play a central role in oxidative stress, which leads to the onset of diseases, such as cancer. Furthermore, ROS contributes to the delicate balance between tumor cell survival and death. However, the mechanisms by which tumor cells decide to elicit survival or death signals during oxidative stress are not completely understood. We have previously reported that ROS enhanced tumorigenic functions in prostate cancer cells, such as transendothelial migration and invasion, which depended on CXCR4 and AKT signaling. Here, we report a novel mechanism by which ROS facilitated cell death through activation of AKT. We initially observed that ROS enhanced the expression of phosphorylated AKT (p-AKT) in 22Rv1 human prostate cancer cells. The tumor suppressor PTEN, a negative regulator of AKT signaling, was rendered catalytically inactive through oxidation by ROS, although the expression levels remained consistent. Despite these events, cells still underwent apoptosis. Further investigation into apoptosis revealed that expression of the tumor suppressor pVHL increased, and contains a target site for p-AKT phosphorylation. pVHL and p-AKT associated in vitro, and knockdown of pVHL rescued HIF1α expression and the cells from apoptosis. Collectively, our study suggests that in the context of oxidative stress, p-AKT facilitated apoptosis by inducing pVHL function.

The Roles of VHL-Dependent Ubiquitination in Signaling and Cancer

The function of tumor suppressor VHL is compromised in the vast majority of clear cell renal cell carcinoma, and its mutations or loss of expression was causal for this disease. pVHL was found to be a substrate recognition subunit of an E3 ubiquitin ligase, and most of the tumor-derived mutations disrupt this function. pVHL was found to bind to the alpha subunits of hypoxia-inducible factor (HIF) and promote their ubiquitination and proteasomal degradation. Proline hydroxylation on key sites of HIFα provides the binding signal for pVHL E3 ligase complex. Beside HIFα, several other VHL targets have been identified, including activated epidermal growth factor receptor (EGFR), RNA polymerase II subunits RPB1 and hsRPB7, atypical protein kinase C (PKC), Sprouty2, β-adrenergic receptor II, and Myb-binding protein p160. HIFα is the most well studied substrate and has been proven to be critical for pVHL’s tumor suppressor function, but the activated EGFR and PKC and other pVHL substrates might also be important for tumor growth and drug response. Their regulations by pVHL and their relevance to signaling and cancer are discussed.

Von Hippel-Lindau gene product directs cytokinesis: a new tumor suppressor function

One of the mechanisms of tumorigenesis is that the failure of cell division results in genetically unstable, multinucleated cells. Here we show that pVHL, a tumor suppressor protein that has been implicated in the pathogenesis of renal cell carcinoma (RCC), plays an important role in regulation of cytokinesis. We found that pVHL-deficient RCC 786-O cells were multinucleated and polyploid. Reintroduction of wild-type pVHL into these cells rescued the diploid cell population, whereas the mutant pVHL-K171G failed to do so. We demonstrate that lysine 171 of pVHL is important for the final step of cytokinesis: the midbody abscission. The pVHL-K171G caused failure to localize the ESCRT-1 interacting protein Alix and the v-SNARE complex component Endobrevin to the midbody in 786-O cells, leading to defective cytokinesis. Moreover, SUMOylation of pVHL at lysine 171 might modulate its function as a cytokinesis regulator. pVHL tumor suppressor function was also disrupted by the K171G mutation, as evidenced by the xenograft tumor formation when 786-O clones expressing pVHL-K171G were injected into mice. Most RCC cell lines show a polyploid chromosome complement and consistent heterogeneity in chromosome number. Thus, this study offers a way to explain the chromosome instability in RCC and reveals a new direction for the tumor suppressor function of pVHL, which is independent of its E3 ubiquitin ligase activity.

Abstract C259: pVHL function is involved in cell death induced by YC-1 in renal cell carcinoma

Abstract Renal cell carcinoma (RCC) is quite resistant to chemotherapy, whereas immunotherapeutic agents, including IL-2 and IFN-α, are only modestly effective with response rates of 10–20%. Novel and more effective therapeutic agents are aspired to alter the natural history of advanced RCCs. Since the protein level of von Hippel-Lindau tumor suppressor (pVHL) was extremely low in 75% of RCCs, importance of small molecules that selectively target pVHL-deficient RCC has been highlighted recently. In this study, we evaluated several known anti-cancer agents to find small molecules that induce growth inhibition according to pVHL level in RCC cells. Our screening efforts identified YC-1, known as an anti-cancer agent that has two distinct biological actions, elevation of intracellular cGMP and inhibition of transcription factor HIF-α. In RCC cell lines we examined, YC-1 showed more effective growth inhibition to pVHL-positive RCC cells as compared to RCC cells expressing reduced level of pVHL. Cell cycle analyses revealed that YC-1 caused delay of S-phase progression and subsequent induction of apoptosis more effectively in pVHL-positive RCC cells. Under the treatment of YC-1, phosphorylation of S-phase checkpoint proteins ATM and CHK2 were elevated dramatically, and cleavages of caspase-3/9 and PARP-1, molecular hallmarks of cells undergoing apoptotic events, were promoted. However, no obvious DNA damage was observed when the cells were exposed to YC-1 at the sufficiently high concentration. This YC-1-induced preferential apoptotic cell death in pVHL-positive RCC cells did not result from its known effects, assessed by sensitivity to the other soluble guanylate cyclase activators and by HIF-1α level. No difference was observed in growth rates of RCC cells in spite of pVHL level when the cells were exposed to BAY41-2272, a YC-1 analog, at the concentration enough to up-regulate intracellular cGMP level. In addition, as pVHL plays a key role in down-regulation of HIF-1α protein, pVHL-deficient RCC cells usually showed less sensitivity to YC-1, even though they contain remarkably higher expression of HIF-1α. Our findings demonstrated that VHL expression sensitizes RCC cells to YC-1. Taken together with the previous report that pVHL functions as an inhibitor to RCC cell apoptosis induced by cellular stress, the results imply that YC-1 targets the pVHL-dependent survival pathway by unknown mechanisms. Thus, YC-1 is considered as a new lead compound of anticancer agents for target therapy of pVHL-positive RCCs. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C259.

U19/Eaf2 Binds to and Stabilizes von Hippel-Lindau Protein

Studies have firmly established a key regulatory role for the tumor suppressor pVHL in the regulation of the vascular system and normal spermatogenesis. Here, we report that knockout of the newly identified tumor suppressor U19/Eaf2 also caused vascular system abnormalities and aspermatogenesis, suggesting a potential link between U19/Eaf2 and pVHL. Coimmunoprecipitation and in vitro binding assays showed an association between U19/Eaf2 and pVHL, whereas deletion mutagenesis revealed the requirement of the NH(2) terminus of U19/Eaf2 and both the alpha and beta domains of pVHL for this binding. U19/Eaf2 stabilizes pVHL, as shown by protein stability and pulse-chase studies. Testes and mouse embryonic fibroblasts (MEF) derived from U19/Eaf2 knockout mice expressed reduced levels of pVHL, indicating that full in vivo expression of pVHL indeed requires U19/Eaf2. As expected, U19/Eaf2 knockout MEF cells exhibited an increased level and activity of hypoxia-inducible factor 1alpha (HIF1alpha), a protein typically regulated via a pVHL-mediated degradation pathway. Furthermore, angiogenesis in a Matrigel plug assay was significantly increased in U19/Eaf2 knockout mice. The above observations argue that U19/Eaf2 can modulate HIF1alpha and angiogenesis, possibly via direct binding and stabilization of pVHL.

PVHL: A Multipurpose Adaptor Protein

Mutations in the von Hippel-Lindau tumor suppressor gene VHL occur in various inherited and sporadically occurring tumors. The protein encoded by VHL--pVHL--bears no known enzymatic activities but interacts with numerous protein partners. With the identification of distinct pVHL-containing multiprotein complexes, a refined portrait of pVHL tumor suppressor function has arisen. In general, pVHL acts as a multipurpose adaptor protein that controls a diverse array of gene expression programs, as well as extracellular matrix assembly and microtubule-based processes, by linking various target proteins to appropriate enzymatic activities. These findings provide an evermore complex but coherent view of how pVHL functions molecularly and of the consequences of dysregulation of these diverse molecular activities on tumor formation.

Hypoxia-inducible factor-2 regulates vascular tumorigenesis in mice

The von Hippel-Lindau tumor suppressor pVHL regulates the stability of hypoxia-inducible factors (HIF)-1 and -2, oxygen-sensitive basic helix-loop-helix transcription factors, which mediate the hypoxic induction of angiogenic growth factors such as vascular endothelial growth factor. Loss of pVHL function results in constitutive activation of HIF-1 and HIF-2 and is associated with the development of highly vascularized tumors in multiple organs. We have used a conditional gene-targeting approach to investigate the relative contributions of HIF-1 and HIF-2 to VHL-associated vascular tumorigenesis in a mouse model of liver hemangiomas. Here we demonstrate genetically that conditional inactivation of HIF-2alpha suppressed the development of VHL-associated liver hemangiomas and that angiogenic gene expression in hepatocytes is predominantly regulated by HIF-2 and not by HIF-1. These findings suggest that HIF-2 is the dominant HIF in the pathogenesis of VHL-associated vascular tumors and that pharmacologic targeting of HIF-2 may be an effective strategy for their treatment.

Interaction of Hydroxylated Collagen IV with the von Hippel-Lindau Tumor Suppressor

The von Hippel-Lindau tumor suppressor (pVHL) targets hydroxylated alpha-subunits of hypoxia-inducible factor (HIF) for ubiquitin-mediated proteasomal destruction through direct interaction with the hydroxyproline binding pocket in its beta-domain. Although disruption of this process may contribute to VHL-associated tumor predisposition by up-regulation of HIF target genes, genetic and biochemical analyses support the existence of additional functions, including a role in the assembly of extracellular matrix. In an attempt to delineate these pathways, we searched for novel pVHL-binding proteins. Here we report a direct, hydroxylation-dependent interaction with alpha-chains of collagen IV. Interaction with pVHL was also observed with fibrillar collagen chains, but not the folded collagen triple helix. The interaction was suppressed by a wide range of tumor-associated mutations, including those that do not disturb the regulation of HIF, supporting a role in HIF-independent tumor suppressor functions.

Hypoxia-inducible factor–2 (HIF-2) regulates hepatic erythropoietin in vivo

Erythropoiesis is critically dependent on erythropoietin (EPO), a glycoprotein hormone that is regulated by hypoxia-inducible factor (HIF). Hepatocytes are the primary source of extrarenal EPO in the adult and express HIF-1 and HIF-2, whose roles in the hypoxic induction of EPO remain controversial. In order to define the role of HIF-1 and HIF-2 in the regulation of hepatic EPO expression, we have generated mice with conditional inactivation of Hif-1alpha and/or Hif-2alpha (Epas1) in hepatocytes. We have previously shown that inactivation of the von Hippel-Lindau tumor suppressor pVHL, which targets both HIFs for proteasomal degradation, results in increased hepatic Epo production and polycythemia independent of Hif-1alpha. Here we show that conditional inactivation of Hif-2alpha in pVHL-deficient mice suppressed hepatic Epo and the development of polycythemia. Furthermore, we found that physiological Epo expression in infant livers required Hif-2alpha but not Hif-1alpha and that the hypoxic induction of liver Epo in anemic adults was Hif-2alpha dependent. Since other Hif target genes such phosphoglycerate kinase 1 (Pgk) were Hif-1alpha dependent, we provide genetic evidence that HIF-1 and HIF-2 have distinct roles in the regulation of hypoxia-inducible genes and that EPO is preferentially regulated by HIF-2 in the liver.