Hypoxia plays a major role in the induction of angiogenesis during tumor development. One mechanism by which tumor cells respond to a reduced oxygen level is via the activation of hypoxia-inducible factor-1 (HIF-1). HIF-1 is an oxygen-dependent transcriptional activator that plays crucial roles in the angiogenesis of tumors and mammalian development. HIF-1 consists of a constitutively expressed HIF-1β subunit and the highly regulated HIF-1α subunits. The stability and activity of HIF-1α are regulated by various post-translational modifications, hydroxylation, acetylation, phosphorylation and sumoyaltion. Therefore, HIF-1α interacts with several protein factors including PHD, pVHL, ARD-1, SUMO and p300/CBP. Under normoxia, the HIF-1α subunit is rapidly degraded via the von Hippel-Lindau tumor suppressor gene product (pVHL)-mediated ubiquitin/proteasome pathway. The association of pVHL and HIF-1α under normoxic conditions is triggered by the hydroxylation of prolines and the acetylation of lysine within a polypeptide segment known as the oxygen-dependent degradation (ODD) domain. On the contrary, under the hypoxia condition, the HIF-1α subunit becomes stable and interacts with coactivators such as p300/CBP to modulate its transcriptional activity. Under hypoxic conditions, HIF-1 eventually acts as a master regulator of numerous hypoxia-inducible genes. The target genes of HIF-1 are especially related to angiogenesis, cell proliferation and survival, and to glucose and iron metabolism. Moreover, it was reported that the activation of HIF-1α is closely associated with a variety of tumors and oncogenic pathways. Hence, the blocking of HIF-1α itself or the blocking of HIF-1α interacting proteins inhibits tumor growth. Based on these findings, HIF-1 can be a prime target for anticancer therapies. Therefore, this review summarizes the molecular mechanism of HIF-1α stability, the biological functions of HIF-1 and its potential applications for cancer therapies.
ARD1; Angiogenesis; Anticancer therapy; Cell proliferation/survival; Glucose metabolism; HIF-1; Iron metabolism; PHD; SUMO; pVHL; p300/CBP; Transcription factor
Our previous research found that structural changes of the microtubule network influence glycolysis in cardiomyocytes by regulating the hypoxia-inducible factor (HIF)-1α during the early stages of hypoxia. However, little is known about the underlying regulatory mechanism of the changes of HIF-1α caused by microtubule network alternation. The von Hippel-Lindau tumor suppressor protein (pVHL), as a ubiquitin ligase, is best understood as a negative regulator of HIF-1α.
In primary rat cardiomyocytes and H9c2 cardiac cells, microtubule-stabilization was achieved by pretreating with paclitaxel or transfection of microtubule-associated protein 4 (MAP4) overexpression plasmids and microtubule–depolymerization was achieved by pretreating with colchicine or transfection of MAP4 siRNA before hypoxia treatment. Recombinant adenovirus vectors for overexpressing pVHL or silencing of pVHL expression were constructed and transfected in primary rat cardiomyocytes and H9c2 cells. With different microtubule-stabilizing and -depolymerizing treaments, we demonstrated that the protein levels of HIF-1α were down-regulated through overexpression of pVHL and were up-regulated through knockdown of pVHL in hypoxic cardiomyocytes. Importantly, microtubular structure breakdown activated p38/MAPK pathway, accompanied with the upregulation of pVHL. In coincidence, we found that SB203580, a p38/MAPK inhibitor decreased pVHL while MKK6 (Glu) overexpression increased pVHL in the microtubule network altered-hypoxic cardiomyocytes and H9c2 cells.
This study suggests that pVHL plays an important role in the regulation of HIF-1α caused by the changes of microtubular structure and the p38/MAPK pathway participates in the process of pVHL change following microtubule network alteration in hypoxic cardiomyocytes.
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.
The von Hippel-Lindau tumor suppressor protein (pVHL) is inactivated in the hereditary cancer syndrome von Hippel-Lindau disease and in the majority of sporadic renal carcinomas. pVHL is the substrate-binding subunit of the CBCVHL ubiquitin ligase complex that negatively regulates cell growth by promoting the degradation of hypoxia-inducible transcription factor subunits (HIF1/2α). Proteomics-based identification of novel pVHL substrates is hampered by their short half-life and low abundancy in mammalian cells. The usefulness of yeast two-hybrid (Y2H) approaches, on the other hand, has been limited by the failure of pVHL to adopt its native structure and by the absence of prolylhydroxylase activity critical for pVHL substrate recognition. Therefore, we modified the Y2H system to faithfully reconstitute the physical interaction between pVHL and its substrates. Our approach relies on the coexpression of pVHL with the cofactors Elongin B and Elongin C and with HIF1/2α prolylhydroxylases. In a proof-of-principle Y2H screen, we identified the known substrates HIF1/2α and new candidate substrates including diacylglycerol kinase iota, demonstrating that our strategy allows detection of stable interactions between pVHL and otherwise elusive cellular targets. Additional future applications may include structure/function analyses of pVHL-HIF1/2α binding and screens for therapeutically relevant compounds that either stabilize or disrupt this interaction.
Hypoxia inducible factor-1 (HIF-1) is the master regulator of metabolic
adaptation to hypoxia. It is appreciated that HIF-1α accumulation is
achieved under normoxic conditions by e.g., nitric oxide. We determined
molecular mechanisms of HIF-1α accumulation under the impact of
S-nitrosoglutathione (GSNO). In human embryonic kidney cells GSNO
provoked nuclear accumulation of HIF-1α. This appeared unrelated to gene
transcription and protein translation, thus pointing to inhibition of
HIF-1α degradation. Indeed, GSNO as well as the hypoxia mimic
CoCl2 decreased ubiquitination of HIF-1α and GSNO-induced
HIF-1α failed to coimmunoprecipitate with pVHL (von Hippel Lindau
protein). Considering that HIF-1α-pVHL interactions require prolyl
hydroxylation of HIF-1α, we went on to demonstrate inhibition of
HIF-1α prolyl hydroxylases (PHDs) by GSNO. In vitro HIF-1α-pVHL
interactions revealed that GSNO dose-dependently inhibits PHD activity but not
the interaction of a synthetic peptide resembling the hydroxylated
oxygen-dependent degradation domain of HIF-1α with pVHL. We conclude
that GSNO-attenuated prolyl hydroxylase activity accounts for HIF-1α
accumulation under conditions of NO formation during normoxia and that PHD
activity is subject to regulation by NO.
The Copper Metabolism MURR1 Domain containing 1 protein COMMD1 has been associated with copper homeostasis, NF-κB signaling, and sodium transport. Recently, we identified COMMD1 as a novel protein in HIF-1 signaling. Mouse embryos deficient for Commd1 have increased expression of hypoxia/HIF-regulated genes i.e. VEGF, PGK and Bnip3. Hypoxia-inducible factors (HIFs) are master regulators of oxygen homeostasis, which control angiogenesis, erythropoiesis, glycolysis and cell survival/proliferation under normal and pathologic conditions. Although HIF activity is mainly controlled by ubiquitination and protein degradation by the von Hippel Lindau (pVHL) tumor suppressor gene other mechanisms have recently been identified that regulate HIF signaling independently of pVHL.
Here we characterized the mechanism by which COMMD1 regulates HIF-1α protein degradation. We show that COMMD1 competes with the chaperone heat shock protein HSP90β for binding to the NH2-terminal DNA-binding and heterodimerization domain of HIF-1α to regulate HIF-1α stability together with HSP70. Inhibition of HSP90 activity with 17-Allylamino-17-demethoxygeldanamycin (17-AAG) increased COMMD1-mediated HIF-1α degradation independent of ubiquitin and pVHL.
These data reveal a novel role for COMMD1 in conjunction with HSP90β/HSP70 in the ubiquitin and O2-independent regulation of HIF-1α.
The perihydroxylated perylene quinone hypericin has been reported to possess potent anti-metastatic and antiangiogenic activities, generated by targeting diverse crossroads of cancer-promoting processes via unique mechanisms. Hypericin is the only known exogenous reagent that can induce forced poly-ubiquitination and accelerated degradation of heat shock protein 90 (Hsp90) in cancer cells. Hsp90 client proteins are thereby destabilized and rapidly degraded. Hsp70 client proteins may potentially be also affected via preventing formation of hsp90-hsp70 intermediate complexes. We show here that hypericin also induces enhanced degradation of hypoxia-inducible factor 1α (HIF-1α) in two human tumor cell lines, U87-MG glioblastoma and RCC-C2VHL−/− renal cell carcinoma and in the non-malignant ARPE19 retinal pigment epithelial cell line. The hypericin-accelerated turnover of HIF-1α, the regulatory precursor of the HIF-1 transcription factor which promotes hypoxic stress and angiogenic responses, overcomes the physiologic HIF-1α protein stabilization which occurs in hypoxic cells. The hypericin effect also eliminates the high HIF-1α levels expressed constitutively in the von-Hippel Lindau protein (pVHL)-deficient RCC-C2VHL−/− renal cell carcinoma cell line. Unlike the normal ubiquitin-proteasome pathway-dependent turnover of HIF-α proteins which occurs in normoxia, the hypericin-induced HIF-1α catabolism can occur independently of cellular oxygen levels or pVHL-promoted ubiquitin ligation of HIF-1α. It is mediated by lysosomal cathepsin-B enzymes with cathepsin-B activity being optimized in the cells through hypericin-mediated reduction in intracellular pH. Our findings suggest that hypericin may potentially be useful in preventing growth of tumors in which HIF-1α plays pivotal roles, and in pVHL ablated tumor cells such as renal cell carcinoma through elimination of elevated HIF-1α contents in these cells, scaling down the excessive angiogenesis which characterizes these tumors.
pVHL, product of von Hippel-Lindau (VHL) tumor suppressor gene, functions as the substrate recognition component of an E3-ubiquitin ligase that targets proteins for ubiquitination and proteasomal degradation. Hypoxia-inducible factor α (HIFα) is the well-known substrate of pVHL. Besides HIFα, pVHL also binds to many other proteins and has multiple functions. In this manuscript, we report that the nuclear clusterin (nCLU) is a target of pVHL. We found that pVHL had a direct interaction with nCLU. nCLU bound to pVHL at pVHL's β domain, the site for recognition of substrate, indicating that nCLU might be a substrate of pVHL. Interestingly, pVHL bound to nCLU but did not lead to nCLU destruction. Further studies indicated that pVHL mediated K63-linked ubiquitination of nCLU and promoted nCLU nuclear translocation. In summary, our results disclose a novel function of pVHL that mediates K63-linked ubiquitination and identify nCLU as a new target of pVHL.
Inactivation of von Hippel-Lindau tumor suppressor protein (pVHL) is associated with von Hippel-Lindau disease, an inherited cancer syndrome, as well as the majority of patients with sporadic clear cell renal carcinoma (RCC). While the involvement of pVHL in oxygen sensing through targeting HIFα subunits to ubiquitin-dependent proteolysis has been well documented, less is known about pVHL regulation under both normoxic and hypoxic conditions. We found that pVHL levels decreased in hypoxia and that hypoxia-induced cell cycle arrest is associated with pVHL expression in RCC cells. pVHL levels fluctuate during the cell cycle, paralleling cyclin B1 levels, with decreased levels in mitosis and G1. pVHL contains consensus Destruction box sequences, and pVHL associates with Cdh1, an activator of the anaphase promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. We show that pVHL has a decreased half-life in G1, Cdh1 downregulation results in increased pVHL expression, while Cdh1 overexpression results in decreased pVHL expression. Taken together these results suggest that pVHL is a novel substrate of APC/CCdh1. Destruction box-independent pVHL degradation was also detected, indicating that other ubiquitin ligases are also activated for pVHL degradation.
Von Hippel-Lindau protein; renal cell carcinoma; anaphase promoting complex/cyclosome; hypoxia; cell cycle
Tumor hypoxia plays a crucial role in tumorigenesis. Under hypoxia, hypoxia-inducible factor 1α (HIF-1α) regulates activation of genes promoting malignant progression. Under normoxia, HIF-1α is hydroxylated on prolines 402 and 564 and is targeted for ubiquitin-mediated degradation by interacting with the von Hippel-Lindau protein complex (pVHL). We have developed a novel method of studying the interaction between HIF-1α and pVHL using the split firefly luciferase complementation-based bioluminescence system in which HIF-1α and pVHL are fused to amino-terminal and carboxy-terminal fragments of the luciferase, respectively. We demonstrate that hydroxylation-dependent interaction between the HIF-1α and pVHL leads to complementation of the two luciferase fragments, resulting in bioluminescence in vitro and in vivo. Complementation-based bioluminescence is diminished when mutant pVHLs with decreased affinity for binding HIF-1α are used. This method represents a new approach for studying interaction of proteins involved in the regulation of protein degradation.
The hypoxia-inducible factor 1α (HIF-1α) is the master regulator of the cellular response to hypoxia. A key regulator of HIF-1α is von Hippel-Lindau protein (pVHL), which mediates the oxygen-dependent, proteasomal degradation of HIF-1α in normoxia. Here, we describe a new regulator of HIF-1α, the hypoxia-associated factor (HAF), a novel E3-ubiquitin ligase that binds HIF-1α leading to its proteasome-dependent degradation irrespective of cellular oxygen tension. HAF, a protein expressed in proliferating cells, binds and ubiquitinates HIF-1α in vitro, and both binding and E3 ligase activity are mediated by HAF amino acids 654 to 800. Furthermore, HAF overexpression decreases HIF-1α levels in normoxia and hypoxia in both pVHL-competent and -deficient cells, whereas HAF knockdown increases HIF-1α levels in normoxia, hypoxia, and under epidermal growth factor stimulation. In contrast, HIF-2α is not regulated by HAF. In vivo, tumor xenografts from cells overexpressing HAF show decreased levels of HIF-1α accompanied by decreased tumor growth and angiogenesis. Therefore, HAF is the key mediator of a new HIF-1α-specific degradation pathway that degrades HIF-1α through a new, oxygen-independent mechanism.
Inactivating mutations within the von Hippel-Lindau (VHL) tumor suppressor gene predispose patients to develop a variety of highly vascularized tumors. pVHL targets α subunits of the heterodimeric transcription factor hypoxia-inducible factor (HIF), a critical regulator of energy metabolism, angiogenesis, hematopoiesis, and oxygen (O2) delivery, for ubiquitin-mediated degradation in an O2-dependent manner. To investigate the role of Vhl in cellular proliferation and tumorigenesis, we utilized mouse embryonic fibroblasts (MEFs), a common tool for analyzing cell cycle regulation, and generated Vhl−/− MEF-derived fibrosarcomas. Surprisingly, growth of both Vhl−/− MEFs and fibrosarcomas was impaired, although tumor vascularity was increased. Decreased proliferation of Vhl−/− MEFs was correlated with an overexpression of cyclin kinase inhibitors (CKIs) p21 and p27. The transcription of p21 and p27 is inhibited by c-Myc; therefore, the induction of CKIs was attributed to the ability of HIF to antagonize c-Myc activity. Indeed, p21 mRNA levels were elevated under normoxia in Vhl−/− MEFs, while c-Myc transcriptional activity was markedly reduced. Gene silencing of HIF-1α by small interfering RNA reduced p21 and p27 protein and mRNA levels in Vhl−/− MEFs. The induction of p21 and p27, mediated by constitutive activation of the HIF pathway, provides a mechanism for the decreased proliferation rates of Vhl−/− MEFs and fibrosarcomas. These results demonstrate that a loss of pVHL can induce growth arrest in certain cells types, which suggests that additional genetic mutations are necessary for VHL-associated tumorigenesis.
The hypoxia-inducible factor-1α (HIF-1α) is a transcription factor that mediates adaptive cellular responses to decreased oxygen availability (hypoxia). At normoxia, HIF-1α is targeted by the von Hippel-Lindau tumor suppressor protein (pVHL) for degradation by the ubiquitin-proteasome pathway. In the present study we have observed distinct cell-type-specific differences in the ability of various tested pVHL-interacting subfragments to stabilize HIF-1α and unmask its function at normoxia. These properties correlated with differences in subcellular compartmentalization and degradation of HIF-1α. We observed that the absence or presence of nuclear localization or export signals differently affected the ability of a minimal HIF-1α peptide spanning residues 559 to 573 of mouse HIF-1α to stabilize endogenous HIFα and induce HIF-driven reporter gene activity in two different cell types (primary mouse endothelial and HepG2 hepatoma cells). Degradation of HIF-1α occurred mainly in the cytoplasm of HepG2 cells, whereas it occurs with equal efficiency in nuclear and cytoplasmic compartments of primary endothelial cells. Consistent with these observations, green fluorescent protein-HIF-1α is differently distributed during hypoxia and reoxygenation in hepatoma and endothelial cells. Consequently, we propose that differential compartmentalization of degradation of HIF-1α and the subcellular distribution of HIF-1α may account for cell-type-specific differences in stabilizing HIF-1α protein levels under hypoxic conditions.
Somatic mutations or reduced expression of the von Hippel-Lindau (VHL) tumor suppressor occurs in the majority of the clear cell renal cell carcinoma (ccRCC) and is a causal factor for the pathogenesis of ccRCC. pVHL was reported to suppress the oncogenic activity of Epidermal Growth Factor Receptor (EGFR) by reducing the expression of the EGFR agonist TGF-α and by reducing the translation efficiency of EGFR itself. Furthermore, it was reported that pVHL down-regulates activated EGFR by promoting efficient lysosomal degradation of the receptor. These modes of negative regulation of EGFR by pVHL were dependent on Hypoxia Inducible Factor (HIF). In this study, we report that HIF was not the only factor stabilizing the activated EGFR in VHL-deficient ccRCC cells. Down-regulation of endogenous HIF in these cells had little effect on the turnover rates of the activated EGFR. Furthermore, neither pretreatment with lysomomal inhibitors pretreatment nor down-regulation of c-Cbl, a major E3 ubiquitin ligase that targets the activated EGFR for lysosomal degradation, significantly increased the stabilities of EGFR in VHL-expressing ccRCC cells. In contrast, pretreatment with proteasomal inhibitors extended EGFR lifetime and led to similar EGFR half-lives in VHL-expressing and VHL-deficient ccRCC cells. Down-regulation of c-Cbl in VHL-deficient ccRCC cells revealed that the c-Cbl and pVHL collaborated to down-regulate the activated EGFR. Finally, we found that pVHL promoted the poly-ubiquitylation of the activated EGFR, and this function was c-Cbl-independent. Thus these results indicate that pVHL limits EGFR signaling by promoting c-Cbl-independent poly-ubiquitylation of the activated receptor, which likely results in its degradation by proteasome.
Mutational inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene has been linked to hereditary as well as sporadic clear cell renal carcinomas. The product of the VHL gene, pVHL, acts to target hypoxia-inducible factor alpha (HIF-α) subunits for ubiquitination and subsequent degradation. Using an RNA interference approach to lower levels of HIF-2α in two different renal cell lines that lack functional pVHL, we have tested the contribution of HIF-2α toward cellular pVHL activities.
Knockdown of HIF-2α resulted in cell cycle arrest of renal cells that were grown on collagen I, indicating that this pVHL function is dependent on HIF-2α regulation. However, cellular morphological changes and downregulation of integrins α5 and β1, which were seen upon pVHL replacement, were not faithfully phenocopied by HIF-2α reduction. Moreover, fibronectin deposition and expression of renal cell differentiation markers were observed in cells containing replaced pVHL, but not in HIF-2α knockdown cells, indicating that these pVHL functions may occur independently of HIF-2α downregulation.
These results indicate that HIF-2α regulation is not sufficient for pVHL-induced renal cell differentiation. We hypothesize that in addition to HIF-2α dysregulation, abrogation of additional pVHL functions is required for the initiation of renal carcinogenesis.
In this study, we investigated the role of Nur77, an orphan nuclear receptor, in HIF-α transcriptional activity. We found that Nur77 associates and stabilizes HIF-1α via indirect interaction. Nur77 was found to interact with pVHL in vivo via the α-domain of pVHL. By binding to pVHL, Nur77 competed with elongin C for pVHL binding. Moreover, Nur77-binding to pVHL inhibited the pVHL-mediated ubiquitination of HIF-1α and ultimately increased the stability and transcriptional activity of HIF-1α. The ligand-binding domain of Nur77 was found to interact with pVHL and the expression of this ligand-binding domain was sufficient to stabilize and transactivate HIF-1α. Under the conditions that cobalt chloride was treated or pVHL was knocked down, Nur77 could not stabilize HIF-α. Moreover, Nur77 could not further stabilize HIF-2α in A498/VHL stable cells, which is consistent with our finding that Nur77 indirectly stabilizes HIF-α by binding to pVHL. Thus, our results suggest that an orphan nuclear receptor Nur77 binds to pVHL, thereby stabilizes and increases HIF-α transcriptional activity under the non-hypoxic conditions.
hypoxia-inducible factor 1, alpha subunit; orphan nuclear receptor NGFI-B; ubiquitination; VHL protein, human
Hypoxia-inducible factor 1 (HIF-1) is controlled through stability regulation of its alpha subunit, which is expressed under hypoxia but degraded under normoxia. Degradation of HIF-1α requires association of the von Hippel Lindau protein (pVHL) to provoke ubiquitination followed by proteasomal digestion. Besides hypoxia, nitric oxide (NO) stabilizes HIF-1α under normoxia but destabilizes the protein under hypoxia. To understand the role of NO under hypoxia we made use of pVHL-deficient renal carcinoma cells (RCC4) that show a high steady state HIF-1α expression under normoxia. Exposing RCC4 cells to hypoxia in combination with the NO donor DETA-NO (2,2′-(hydroxynitrosohydrazono) bis-ethanimine), but not hypoxia or DETA-NO alone, decreased HIF-1α protein and attenuated HIF-1 transactivation. Mechanistically, we noticed a role of calpain because calpain inhibitors reversed HIF-1α degradation. Furthermore, chelating intracellular calcium attenuated HIF-1α destruction by hypoxia/DETA-NO, whereas a calcium increase was sufficient to lower the amount of HIF-1α even under normoxia. An active role of calpain in lowering HIF-1α amount was also evident in pVHL-containing human embryonic kidney cells when the calcium pump inhibitor thapsigargin reduced HIF-1α that was stabilized by the prolyl hydroxylase inhibitor dimethyloxalylglycine (DMOG). We conclude that calcium contributes to HIF-1α destruction involving the calpain system.
The Von Hippel-Lindau gene (VHL) is frequently deleted or mutated in human renal cell carcinoma (RCC) at the early stage. According to the well-established theory, pVHL acts as a tumor suppressor through its E3 ligase activity, which targets hypoxia-inducing factor-1α (HIF-1α). However, the elevated expression of HIF-1α did not promote cell proliferation, indicating that there would be another target, which could promote cell proliferation at the early cancer stage of RCC. In this study, we show that estrogen receptor-α (ER-α) is a novel proteasomal degradation target of the pVHL E3 ligase. Indeed, the overexpression of VHL suppresses exo- and endogenous ER-α expression, whereas si-pVHL can increase ER-α expression. The negative regulation of pVHL on ER-α expression is achieved by its E3 ligase activity. Thus, pVHL can promote the ER-α ubiquitinylation. In addition, we revealed that ER-α and HIF-1α are competitive substrates of pVHL. Thus, under normal conditions, ER-α overexpression can increase the transcription factor activity of HIF-1α. Under the hypoxic condition, where HIF-1α is not a suitable target of pVHL, ER-α is more rapidly degraded by pVHL. However, in VHL-deficient cells, the expression of ER-α and HIF-1α is retained, so that the hypoxic condition did not suppress cell proliferation obviously compared with cells that are expressing pVHL. Thus, blocking of ER-α using its inhibitor could suppress the proliferation of VHL-deficient cells as effectively as hypoxia-induced growth suppression. Considering our results, blocking of ER-α signaling in VHL-deficient cancer cells would be beneficial for cancer suppression. Indeed, we showed the anti-proliferative effect of Faslodex in VHL-deficient cells.
RCC; VHL; ER-alpha; hypoxia; cell proliferation
Biallelic inactivation of the von Hippel–Lindau tumor suppressor gene (VHL) is linked to the development of hereditary (VHL-associated) and sporadic clear-cell renal carcinomas as well as other abnormalities. The VHL gene product, pVHL, is part of an E3 ubiquitin ligase complex that targets the α subunits of the heterodimeric transcription factor HIF (hypoxia-inducible factor) for degradation in the presence of oxygen. Here we report that a HIF2α variant lacking both of its two prolyl hydroxylation/pVHL-binding sites prevents tumor inhibition by pVHL in a DNA-binding dependent manner. Conversely, downregulation of HIF2α with short hairpin RNAs is sufficient to suppress tumor formation by pVHL-defective renal carcinoma cells. These results establish that tumor suppression by pVHL is linked to regulation of HIF target genes.
Specific downregulation of the transcription factor HIF2α is sufficient to suppress tumor formation by cells lacking the functional tumor suppressor (pVHL), demonstrating that tumor suppression by pVHL is linked to regulation of HIF target genes
Hypoxia-inducible factors (HIFs) are heterodimeric oxygen-sensitive basic helix-loop-helix transcription factors that play central roles in cellular adaptation to low oxygen environments. The von-Hippel Lindau tumor suppressor (pVHL) is the substrate recognition component of an E3 ubiquitin ligase and functions as a master regulator of HIF activity by targeting the hydroxylated HIF-alpha subunit for ubiquitylation and rapid proteasomal degradation under normoxic conditions. Mutations in pVHL can be found in familial and sporadic hemangioblastomas, clear cell carcinomas of the kidney, pheochromocytomas and inherited forms of erythrocytosis, illustrating the importance of disrupted molecular oxygen sensing in the pathogenesis of these diseases. Tissue-specific gene targeting of pVHL in mice has demonstrated that efficient execution of HIF proteolysis is critically important for normal tissue physiology, and has provided novel insights into the functional consequences of HIF activation on the cellular and tissue level. Here we focus on the contribution of individual HIF transcription factors to the development of VHL phenotypes and discuss how the pVHL/HIF axis could be exploited pharmacologically.
von Hippel-Lindau (VHL) tumor suppressor; hypoxia-inducible factor (HIF); renal cell cancer; hemangioblastoma; erythropoietin; anemia; metabolism; kidney cysts; mouse model
The von Hippel-Lindau (VHL) tumor suppressor gene product is the recognition component of an E3 ubiquitin ligase and is inactivated in patients with VHL disease and in most sporadic clear cell renal carcinomas (RCC). pVHL controls oxygen-responsive gene expression at the transcriptional and post-transcriptional levels. The vascular endothelial growth factor A (VEGFA) mRNA contains AU-rich elements (AREs) in the 3' untranslated region, and mRNA stability or decay is determined through ARE-associated RNA binding factors. We show here that levels of the ARE binding factor, AUF1, are regulated by pVHL and by hypoxia. pVHL and AUF1 stably associate with each other in cells and AUF1 is a ubiquitylation target of pVHL. AUF1 and another RNA binding protein, HuR, bind to VEGFA ARE RNA. Ribonucleoprotein (RNP)-immunoprecipitations showed that pVHL associates indirectly with VEGFA mRNA through AUF1 and/or HuR, and this complex is associated with VEGFA mRNA decay under normoxic conditions. Under hypoxic conditions pVHL is downregulated, while AUF1 and HuR binding to VEGF mRNA is maintained, and this complex is associated with stabilized mRNA. These studies suggest that AUF1 and HuR bind to VEGFA ARE RNA under both normoxic and hypoxic conditions, and that a pVHL-RNP complex determines VEGFA mRNA decay. These studies further implicate the ubiquitin-proteasome system in ARE-mediated RNA degradation.
von Hippel-Lindau; hypoxia; VEGF; AUF1; HuR; hnRNP
Εnhanced expression of transcription factor hypoxia inducible factor HIF-1α is known to play a critical role in the modulation of cell metabolism and survival pathways as well as having stem-cell–like properties. Furthermore, accumulated data reveal the existence of cross-regulation between the oxygen-sensing and heat shock pathways contributing to the adaptation of cells under stressful conditions. Pterygium, a stem cell disorder with premalignant features, has been reported to demonstrate hypoxia. The purpose of this study was to investigate the co-expression patterns of transcription factor HIF-1α and von Hippel Lindau protein (pVHL)—which normally acts to keep levels of HIF-1α activity low under normoxic conditions—in pterygium and normal conjunctival human samples. Additionally, expression of HIF-1α compared to the activation of heat shock proteins (Hsp90, Hsp70, and Hsp27) was studied. Emphasis was placed on the detection of HIF-1α and Hsp90, which associates with and stabilizes HIF-1α to promote its transcriptional activity.
Semi-serial paraffin-embedded sections and tissue extracts from pterygium and normal conjunctival samples were studied by immunohistochemistry and western blot analysis, respectively, with the use of specific antibodies. Double labeling immunofluorescence studies on cryostat sections were also included.
Statistically significant increased expression of HIF-1α and Hsps (Hsp90, Hsp70, and Hsp27) in pterygia compared to normal conjunctiva was demonstrated (p<0.05). In contrast, no significant difference was detected for pVHL expression (p>0.05). Immunohistochemical findings revealed nuclear HIF-1α immunoreactivity in all the epithelial layers of 23/32 (71.8%) pterygium tissues. Furthermore, all epithelial layers of the majority (75%) of pterygium samples showed strong cytoplasmic immunoreactivity for Hsp27 while Hsp27 expression was detected in all pterygia (100%) examined. Hsp27 expression was not observed in the superficial layer of goblet cells. In some samples, focal basal epithelial cells exhibited weak Hsp27 expression or were Hsp27 immunonegative. Ιmmunoreactivity of phopsho-Hsp27 showed the same distribution pattern as Hsp27 did. Epithelium of all pterygia (100%) displayed moderate to strong Hsp90 cytoplasmic immunoreactivity. Furthermore, the majority of pterygia, specifically, 30/32 (93.7%) and 27/32 (84.3%) demonstrated, respectively, Hsp70 and pVHL cytoplasmic immunoreactivity. Hsp90, Hsp70, and pVHL immunoreactivity was mainly detected in basal and suprabasal epithelial layers even though strong immunoreactivity in all epithelial layers was also observed in some pterygia. Stroma vessels were immunopositive for Hsps (Hsp90, Hsp70, and Hsp27) and pVHL. A statistically significant correlation between the expression of HIF-1α and the activation status of Hsps (Hsp90, Hsp70, and Hsp27; p<0.05) was observed whereas HIF-1α expression did not correlate with pVHL expression (p>0.05). Double labeling immunofluorescence studies showed nuclear HIF-1α co-localization with cytoplasmic Hsp90 expression in cells distributed in the entire epithelium of pterygia, in contrast to, normal conjunctiva, which exhibited only a few scattered epithelial cells with cytoplasmic HIF-1α expression and basal epithelial cells with Hsp90 expression.
The upregulation of coordinated activation of HIF-1α and Hsps in pterygium may represent an adaptive process for the survival of cells under stressful conditions. The significance of the association of HIF-1α with Hsp90 with respect to the therapeutic approach of pterygium requires further evaluation.
Expression of carbonic anhydrase 9 (CA9) is associated with poor prognosis and increased tumor aggressiveness and does not always correlate with HIF-1α expression. Presently, we analyzed the regulation of CA9 expression during hypoxia by HIF-1α, Notch3, and the von Hippel-Lindau (VHL) in breast carcinoma cells. Both HIF-1α and Notch3 were absolutely required for the expression of CA9 mRNA, protein, and reporter. Reciprocal co-immunoprecipitation of HIF-1α, Notch3 intracellular domain (NICD3), and pVHL demonstrated their association. The presence of common consensus prolyl hydroxylation and pVHL binding motifs (L(XY)LAP);LLPLAP2191 suggested an oxygen-dependent regulation for NICD3. However, unlike the HIF-1α protein, NICD3 protein levels were not modulated with hypoxia or hypoxia-mimetic agents. Surprisingly, mutations of the common prolyl hydroxylation and pVHL binding domain lead to the loss of CA9 mRNA, protein, and reporter activity. Chromatin immunoprecipitation assay demonstrated the association of NICD3, HIF-1α, and pVHL at the CA9 promoter. Further, the NICD3 mutant defective in prolyl hydroxylation and subsequent pVHL binding caused a reduction in cell proliferation of breast carcinoma cells. We show here for the first time that the interaction of HIF-1α with NICD3 is important for the regulation of CA9 expression. These findings suggest that although CA9 is a hypoxia-responsive gene, its expression is modulated by the interaction of HIF-1α, Notch3, and VHL proteins. Targeting the expression of CA9 by targeting upstream regulators could be useful in cancer/stem cell therapy.
hypoxia; Notch3; CA9; PBX1; VHL
While missense mutations of von Hippel-Lindau disease (VHL) gene are the most common germline mutation underlying this heritable cancer syndrome, the mechanism of tumorigenesis is unknown. We found a quantitative reduction of missense mutant VHL protein (pVHL) in VHL-associated tumors associated with physiologic mRNA expression. While mutant pVHL is unstable and degraded contemporarily with translation, it retains its E3 ligase function, including hypoxia inducible factor degradation. The premature pVHL degradation is due to misfolding and imbalance of chaperonin binding. Histone deacetylase inhibitors (HDACis) can modulate this pathway by inhibiting the HDAC6-Hsp90 chaperone axis, stabilizing pVHL and restoring activity comparable to wild type protein in vitro and in animal models (786-O tumor xenografts). HDACi mediated stabilization of missense pVHL significantly attenuates the growth of 786-O rodent tumor model. These findings provide direct biologic insight into VHL-associated tumors and elucidate a new treatment paradigm for VHL.
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.