The contribution of VHL substrate binding and HIF1-alpha to the phenotype of VHL loss in renal cell carcinoma

Hypoxia-inducible factor 1 and cancer pathogenesis

Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator that mediates adaptive responses to hypoxia. HIF-1 activity is increased in the majority of human cancers as a result of genetic alterations and intratumoral hypoxia. HIF-1 activates the transcription of genes that increase O(2) availability by stimulating angiogenesis or that reprogram cellular metabolism to adapt to reduced O(2) availability. Proof of principle studies in mouse models suggests that inhibition of HIF-1 activity may have therapeutic effects, especially in combination with other anticancer drugs.

Role of the VHL (von Hippel-Lindau) gene in renal cancer: a multifunctional tumour suppressor

The VHL (von Hippel-Lindau) tumour-suppressor gene is inactivated in VHL disease and in sporadic cases of CCRCC [clear-cell RCC (renal cell carcinoma)]. pVHL (VHL protein) functions as part of an E3 ubiquitin ligase complex that targets proteins for proteasomal degradation. The best-characterized substrate is HIF-alpha (hypoxia-inducible factor-alpha). Loss of pVHL and subsequent up-regulation of HIF target genes has been attributed to the highly vascular nature of these neoplasms. However, pVHL does not just function as the executioner of HIF-alpha. Additional functions of pVHL that may be important in preventing CCRCC tumorigenesis have been identified, including primary cilium maintenance, assembly of the extracellular matrix and roles in the stabilization of p53 and Jade-1 (gene for apoptosis and differentiation in epithelia). Current evidence indicates that pVHL probably requires additional co-operating signalling pathways for CCRCC initiation and tumorigenesis.

The von Hippel-Lindau tumour suppressor protein: O2 sensing and cancer

The von Hippel-Lindau disease is caused by inactivating germline mutations of the VHL tumour suppressor gene and is associated with an increased risk of a variety of tumours in an allele-specific manner. The role of the heterodimeric transcription factor hypoxia-inducible factor (HIF) in the pathogenesis of VHL-defective tumours has been more firmly established during the past 5 years. In addition, there is now a greater appreciation of HIF-independent VHL functions that are relevant to tumour development, including maintenance of the primary cilium, regulation of extracellular matrix formation and turnover, and modulation of cell death in certain cell types following growth factor withdrawal or in response to other forms of stress.

A RNA antagonist of hypoxia-inducible factor-1alpha, EZN-2968, inhibits tumor cell growth

Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that plays a critical role in angiogenesis, survival, metastasis, drug resistance, and glucose metabolism. Elevated expression of the alpha-subunit of HIF-1 (HIF-1alpha), which occurs in response to hypoxia or activation of growth factor pathways, is associated with poor prognosis in many types of cancer. Therefore, down-regulation of HIF-1alpha protein by RNA antagonists may control cancer growth. EZN-2968 is a RNA antagonist composed of third-generation oligonucleotide, locked nucleic acid, technology that specifically binds and inhibits the expression of HIF-1alpha mRNA. In vitro, in human prostate (15PC3, PC3, and DU145) and glioblastoma (U373) cells, EZN-2968 induced a potent, selective, and durable antagonism of HIF-1 mRNA and protein expression (IC(50), 1-5 nmol/L) under normoxic and hypoxic conditions associated with inhibition of tumor cell growth. Additionally, down-regulation of HIF-1alpha protein by EZN-2968 led to reduction of its transcriptional targets and of human umbilical vein endothelial cell tube formation. In vivo, administration of EZN-2968 to normal mice led to specific, dose-dependent, and highly potent down-regulation of endogenous HIF-1alpha and vascular endothelial growth factor in the liver. The effect can last for days after administration of single dose of EZN-2968 and is associated with long residence time of locked nucleic acid in certain tissues. In efficacy studies, tumor reduction was found in nude mice implanted with DU145 cells treated with EZN-2968. Ongoing phase I studies of EZN-2968 in patients with advanced malignancies will determine optimal dose and schedule for the phase II program.

The ubiquitin ligase Siah2 regulates tumorigenesis and metastasis by HIF-dependent and -independent pathways

The ubiquitin ligase Siah2 has been shown to regulate prolyl hydroxylase 3 (PHD3) stability with concomitant effect on HIF-1alpha availability. Because HIF-1alpha is implicated in tumorigenesis and metastasis, we used SW1 mouse melanoma cells, which develop primary tumors with a propensity to metastasize, in a syngeneic mouse model to assess a possible role for Siah2 in these processes. Inhibiting Siah2 activity by expressing a peptide designed to outcompete association of Siah2-interacting proteins reduced metastasis through HIF-1alpha without affecting tumorigenesis. Conversely, inhibiting Siah2 activity by means of a dominant-negative Siah2 RING mutant primarily reduced tumorigenesis through the action of Sprouty 2, a negative regulator of Ras signaling. Consistent with our findings, reduced expression of PHD3 and Sprouty2 was observed in more advanced stages of melanoma tumors. Using complementary approaches, our data establish the role of Siah2 in tumorigenesis and metastasis by HIF-dependent and -independent mechanisms.

Cellular oxygen sensing in health and disease

To avoid localised problems resulting from excess or inadequate oxygen, all cells and tissues have the ability to sense and respond to changes in oxygen levels. Despite their rich blood supply, the kidneys have unique properties with respect to oxygen that enable them to act as specialised organs, sensing oxygen delivery as well as rendering them prone to hypoxic injury. Essential to normal growth and development, as well as the control of energy metabolism, angiogenesis and erythropoiesis, cellular oxygen homoeostasis is central to the pathophysiology of anaemia, ischaemia, inflammation and cancer, both within the kidney and more generally. A major transcriptional pathway, predominantly regulated by hypoxia-inducible factor (HIF), controls many hundreds of genes, either directly or indirectly, that serve to modulate both the supply and consumption of oxygen. Recent advances have illuminated the mechanisms underlying the regulation of HIF by oxygen and have defined novel therapeutic targets. The challenge now is for us to understand the complexities generated by multiple isoforms of the various components of oxygen sensing, the identification of additional levels of control, and the tissue specific responses to activation of the HIF pathway.

In vivo validation of PAX2 as a target for renal cancer therapy

PAX genes are frequently overexpressed in human cancer tissue and appear to contribute to the tumor phenotype, suggesting that they may be potential targets for cancer therapy. In particular, aberrant PAX2 expression has been reported in a high proportion of primary tumors, including the majority of renal cell carcinomas (RCC). We recently demonstrated that PAX2 suppresses cisplatin-induced apoptosis in cultured RCC cells. We hypothesized that silencing of PAX2 expression might partially overcome the notorious resistance of renal cell carcinomas to chemotherapy in vivo. In this report, we show that a PAX2 shRNA successfully knocks down PAX2 mRNA and protein levels in an RCC cell line (ACHN). ACHN cells stably transfected with shRNAs targeted against the PAX2 homeodomain are 3-6-fold more susceptible to cisplatin-induced caspase-3 activation than control ACHN cells line. Furthermore, growth of subcutaneous ACHN/shPAX2 xenografts in nude mice is significantly more responsive to cisplatin therapy than control ACHN cell tumors. Our observations validate PAX2 as a potential therapeutic gene target in renal cancer and suggest that adjunctive PAX2 knockdown may enhance the efficacy of other chemotherapeutic agents.

Suppression of renal cell carcinoma growth by inhibition of Notch signaling in vitro and in vivo

Loss of the tumor suppressor gene von Hippel-Lindau (VHL) plays a key role in the oncogenesis of clear cell renal cell carcinoma (CCRCC). The loss leads to stabilization of the HIF transcription complex, which induces angiogenic and mitogenic pathways essential for tumor formation. Nonetheless, additional oncogenic events have been postulated to be required for the formation of CCRCC tumors. Here, we show that the Notch signaling cascade is constitutively active in human CCRCC cell lines independently of the VHL/HIF pathway. Blocking Notch signaling resulted in attenuation of proliferation and restrained anchorage-independent growth of CCRCC cell lines. Using siRNA targeting the different Notch receptors established that the growth-promoting effects of the Notch signaling pathway were attributable to Notch-1 and that Notch-1 knockdown was accompanied by elevated levels of the negative cell-cycle regulators p21 Cip1 and/or p27 Kip1. Treatment of nude mice with an inhibitor of Notch signaling potently inhibited growth of xenotransplanted CCRCC cells. Moreover, Notch-1 and the Notch ligand Jagged-1 were expressed at significantly higher levels in CCRCC tumors than in normal human renal tissue, and the growth of primary CCRCC cells was attenuated upon inhibition of Notch signaling. These findings indicate that the Notch cascade may represent a novel and therapeutically accessible pathway in CCRCC.

mTOR pathway in renal cell carcinoma

After decades of therapeutic nihilism in the treatment of advanced renal cell carcinoma, remarkable therapeutic strides have been made over the last few years. Early forays into molecularly targeted therapy for this difficult-to-treat disease were based around the inhibition of gene products of the hypoxia-inducible factor (HIF) transcription factor (i.e., VEGF). Recent data suggest that inhibition of mTOR results in clinical benefit in patients with poor prognostic features, and in preclinical models this therapeutic effect involves downregulation of HIF. Intriguingly, patients with nonclear cell histology appeared to obtain clinical benefit when treated with mTOR inhibitors. This review will highlight the mTOR pathway, its relevance to both clear cell and nonclear cell renal cell carcinoma, and its place in the host of quickly expanding treatment options.

The specific contribution of hypoxia-inducible factor-2alpha to hypoxic gene expression in vitro is limited and modulated by cell type-specific and exogenous factors

Cellular integrity in hypoxia is dependent on molecular adaptations dominated by the heterodimeric transcription factor hypoxia-inducible factor (HIF). The HIF complex contains one of two alternative oxygen-regulated alpha-subunits considered to play distinct roles in the hypoxia response. Although HIF-2alpha may be more important in tumour biology and erythropoiesis, the spectrum of individual target genes is still insufficiently characterized. We therefore performed an Affymetrix gene array on Hep3B cells stimulated with a hypoxia-mimetic and transfected with either HIF-1alpha or HIF-2alpha siRNA. 271 transcripts were found to be induced HIF-dependently, including most previously identified HIF targets and a number of novel genes. Most were influenced by HIF-1alpha knock-down, whereas a smaller number were regulated by HIF-2alpha. Validation of a selection of genes by RNase protection confirmed the hypoxic regulation and HIF-1alpha- or HIF-2alpha-dependency in most cases, with the latter showing a lower amplitude. Many HIF-2alpha targets also responded to HIF-1alpha knock-down. Interestingly, regulation by HIF-2alpha was markedly influenced not only by cell type, but also by cell culture conditions, features that were not shared with HIF-1alpha-regulated genes. Thus, HIF-2alpha effects are modulated by a number of intrinsic and extrinsic factors which may be most relevant in tumour cells.

Targeting the mitochondria for cancer therapy: regulation of hypoxia-inducible factor by mitochondria

As tumors develop, they outgrow the vascular network that supplies cells with oxygen and nutrients needed for survival. In response to decreased oxygen levels, the tumor cells initiate a program of adaptation by inducing the transcription of multiple genes via the activation of the transcription factor hypoxia-inducible factor (HIF). Proteins encoded by a subset of genes induced by HIF promote tumorigenesis by acting directly on both the tumor cells and the microenvironment in which the tumor cells reside. The mechanism(s) by which hypoxia activates HIF is a subject of intensive research. Understanding how hypoxia activates HIF will provide targets for the development of therapies that could specifically target growing tumors by not allowing adequate adaptation to hypoxia, which is necessary for cancer progression. Here we outline how mitochondria regulate the activity of HIF during hypoxia.

Beyond the hypoxia-inducible factor-centric tumour suppressor model of von Hippel-Lindau

PURPOSE OF REVIEW

To provide an overview of the recent advances in the understanding of the molecular mechanisms governing the tumour suppressor functions of the von Hippel-Lindau protein.

RECENT FINDINGS

von Hippel-Lindau is a vital component of an E3 ubiquitin ligase complex involved in the oxygen-dependent targeting of hypoxia-inducible factor for ubiquitin-mediated destruction. Recent reports have linked von Hippel-Lindau to the regulation of diverse biological processes including cell adhesion, extracellular matrix assembly and ciliogenesis in a manner dependent and/or independent of hypoxia-inducible factor.

SUMMARY

The tumour suppressor function of von Hippel-Lindau has remained hypoxia-inducible factor-centric since the discovery of von Hippel-Lindau as a bona fide negative regulator of the ubiquitous oxygen-sensing pathway. Emerging evidence supports this hypothesis with the elucidation of fundamental cellular processes deregulated upon the inactivation of the von Hippel-Lindau-hypoxia-inducible factor pathway, but has also proved compelling on the hypoxia-inducible factor-independent tumour suppressor role of von Hippel-Lindau. These and continuing studies into the molecular pathways and mechanisms governing the tumour suppressor functions of von Hippel-Lindau will ultimately afford new avenues for anticancer strategies for the improved treatment of a diverse array of cancers.

Biology of hypoxia-inducible factor-2alpha in development and disease

The transcriptional response to hypoxia is primarily mediated by two hypoxia-inducible factors--HIF-1alpha and HIF-2alpha. While these proteins are highly homologous, increasing evidence suggests they have unique transcriptional targets and differential impact on tumor growth. Furthermore, non-transcriptional effects of the HIF-alpha subunits, including effects on the Notch and c-Myc pathways, contribute to their distinct functions. HIF-2alpha transcriptional targets include genes involved in erythropoiesis, angiogenesis, metastasis, and proliferation. Therefore, HIF-2alpha contributes significantly to both normal physiology as well as tumorigenesis. Here, we summarize the function of HIF-2alpha during development as well as its contribution to pathologic conditions, such as tumors and vascular disease.

pVHL acts as an adaptor to promote the inhibitory phosphorylation of the NF-kappaB agonist Card9 by CK2

The VHL tumor suppressor protein (pVHL) is part of an E3 ubiquitin ligase that targets HIF for destruction. pVHL-defective renal carcinoma cells exhibit increased NF-kappaB activity but the mechanism is unclear. NF-kappaB affects tumorigenesis and therapeutic resistance in some settings. We found that pVHL associates with the NF-kappaB agonist Card9 but does not target Card9 for destruction. Instead, pVHL serves as an adaptor that promotes the phosphorylation of the Card9 C terminus by CK2. Elimination of these sites markedly enhanced Card9's ability to activate NF-kappaB in VHL(+/+) cells, and Card9 siRNA normalized NF-kappaB activity in VHL(-/-) cells and restored their sensitivity to cytokine-induced apoptosis. Furthermore, downregulation of Card9 in VHL(-/-) cancer cells reduced their tumorigenic potential. Therefore pVHL can serve as an adaptor for both a ubiquitin conjugating enzyme and a kinase. The latter activity, which promotes Card9 phosphorylation, links pVHL to control of NF-kappaB activity and tumorigenesis.

Mitochondrial complex III regulates hypoxic activation of HIF

Decreases in oxygen levels are observed in physiological processes, such as development, and pathological situations, such as tumorigenesis and ischemia. In the complete absence of oxygen (anoxia), mammalian cells are unable to generate sufficient energy for survival, so a mechanism for sensing a decrease in the oxygen level (hypoxia) before it reaches a critical point is crucial for the survival of the organism. In response to decreased oxygen levels, cells activate the transcription factors hypoxia-inducible factors (HIFs), which lead to metabolic adaptation to hypoxia, as well as to generate new vasculature to increase oxygen supply. How cells sense decreases in oxygen levels to regulate HIF activation has been hotly debated. Emerging evidence indicates that reactive oxygen species (ROS) generated by mitochondrial complex III are required for hypoxic activation of HIF. This review examines the current knowledge about the role of mitochondrial ROS in HIF activation, as well as implications of ROS-level regulation in pathological processes such as cancer.

The VHL tumor suppressor and HIF: insights from genetic studies in mice

The von Hippel-Lindau tumor suppressor gene product, pVHL, functions as the substrate recognition component of an E3-ubiquitin ligase, which targets the oxygen-sensitive alpha-subunit of hypoxia-inducible factor (HIF) for rapid proteasomal degradation under normoxic conditions and as such plays a central role in molecular oxygen sensing. Mutations in pVHL can be found in familial and sporadic clear cell carcinomas of the kidney, hemangioblastomas of the retina and central nervous system, and pheochromocytomas, underscoring its gatekeeper function in the pathogenesis of these tumors. Tissue-specific gene targeting of VHL in mice has demonstrated that efficient execution of pVHL-mediated HIF proteolysis under normoxia is fundamentally important for survival, proliferation, differentiation and normal physiology of many cell types, and has provided novel insights into the biological function of individual HIF transcription factors. In this review, we discuss the role of HIF in the development of the VHL phenotype.

Hypoxia-inducible factor-1alpha and hypoxia-inducible factor-2alpha are expressed in kaposi sarcoma and modulated by insulin-like growth factor-I

PURPOSE

Neoangiogenesis is essential for tumor development. Hypoxia-inducible factor (HIF), a transcriptional factor composed of two subunits (alpha and beta), plays a key role in this process, activating proangiogenic factors such as vascular endothelial growth factor (VEGF). The HIF alpha subunits are critically regulated by oxygen and are also modulated by growth factors. Kaposi sarcoma (KS) is a highly vascular tumor that releases large amounts of VEGF and for which we have recently described an essential role for the insulin-like growth factor (IGF) system. We therefore investigated the expression of HIF alpha subunits in biopsies from KS tumors and their modulation by IGF-I in KSIMM, a KS cell line.

RESULTS

Both HIF-1alpha and HIF-2alpha were expressed in KS biopsies in all tumoral stages. HIF-1alpha immunopositivity increased through the tumor development with highest expression in the late nodular stages. In KSIMM cells, IGF-I induced accumulation of both HIF alpha subunits. The induction suggests a translation mechanism as documented by cycloheximide chase experiment coupled with constant RNA levels as evaluated by quantitative real-time PCR. IGF-I-induced HIF alpha accumulation was followed by an increase in HIF function as assessed both by reporter gene assay and by induction of endogenous target gene expression (VEGF-A). Specific blockade of IGF-I receptor with alphaIR3 antibody or with picropodophyllin, a specific IGF-IR tyrosine kinase inhibitor, diminishes the basal and IGF-I-dependent induction of both HIF alpha congeners.

CONCLUSION

These novel findings show the coupling between the IGF and HIF signaling in KS and suggest a coordinated contribution by these pathways to the characteristic vascular phenotype of this tumor.

JAK kinases promote invasiveness in VHL-mediated renal cell carcinoma by a suppressor of cytokine signaling-regulated, HIF-independent mechanism

von Hippel-Lindau (VHL) disease is a cancer syndrome, which includes renal cell carcinoma (RCC), and is caused by VHL mutations. Most, but not all VHL phenotypes are due to failure of mutant VHL to regulate constitutive proteolysis of hypoxia-inducible factors (HIFs). Janus kinases (JAK1, 2, 3, and TYK2) promote cell survival and proliferation, processes tightly controlled by SOCS proteins, which have sequence and structural homology to VHL. We hypothesized that in VHL disease, RCC pathogenesis results from enhanced SOCS1 degradation, leading to upregulated JAK activity. We find that baseline JAK2, JAK3, and TYK2 activities are increased in RCC cell lines, even after serum deprivation or coincubation with cytokine inhibitors. Furthermore, JAK activity is sustained in RCC stably expressing HIF2α shRNA. Invasion through Matrigel and migration in wound-healing assays, in vitro correlates of metastasis, are significantly greater in VHL mutant RCC compared with wild-type cells, and blocked by dominant-negative JAK expression or JAK inhibitors. Finally, we observe enhanced SOCS2/SOCS1 coprecipitation and reduced SOCS1 expression due to proteasomal degradation in VHL-null RCC compared with wild-type cells. The data support a new HIF-independent mechanism of RCC metastasis, whereby SOCS2 recruits SOCS1 for ubiquitination and proteasome degradation, which lead to unrestricted JAK-dependent RCC invasion. In addition to commonly proposed RCC treatment strategies that target HIFs, our data suggest that JAK inhibition represents an alternative therapeutic approach.

Chronic oxidative stress causes amplification and overexpression of ptprz1 protein tyrosine phosphatase to activate beta-catenin pathway

Ferric nitrilotriacetate induces oxidative renal tubular damage via Fenton-reaction, which subsequently leads to renal cell carcinoma (RCC) in rodents. Here, we used gene expression microarray and array-based comparative genomic hybridization analyses to find target oncogenes in this model. At the common chromosomal region of amplification (4q22) in rat RCCs, we found ptprz1, a tyrosine phosphatase (also known as protein tyrosine phosphatase zeta or receptor tyrosine phosphatase beta) highly expressed in the RCCs. Analyses revealed genomic amplification up to eightfold. Despite scarcity in the control kidney, the amounts of PTPRZ1 were increased in the kidney after 3 weeks of oxidative stress, and mRNA levels were increased 16 approximately 552-fold in the RCCs. Network analysis of the expression revealed the involvement of the beta-catenin pathway in the RCCs. In the RCCs, dephosphorylated beta-catenin was translocated to nuclei, resulting in the expression of its target genes cyclin D1, c-myc, c-jun, fra-1, and CD44. Furthermore, knockdown of ptprz1 with small interfering RNA (siRNA), in FRCC-001 and FRCC-562 cell lines established from the induced RCCs, decreased the amounts of nuclear beta-catenin and suppressed cellular proliferation concomitant with a decrease in the expression of target genes. These results demonstrate that chronic oxidative stress can induce genomic amplification of ptprz1, activating beta-catenin pathways without the involvement of Wnt signaling for carcinogenesis. Thus, iron-mediated persistent oxidative stress confers an environment for gene amplification.

Erythropoietin gene expression in renal carcinoma is considerably more frequent than paraneoplastic polycythemia

Signalling by erythropoietin (EPO) is increasingly recognised as a relevant mechanism in tumour biology, potentially leading to enhanced proliferation, angiogenesis and therapy resistance. Paraneoplastic polycythemia by cancerous overproduction of EPO is a rare event, but most frequently seen in patients with renal cell carcinoma (RCC). The majority of clear cell RCC displays a strong activation of the transcription factor regulating EPO, the Hypoxia-inducible Factor (HIF). Therefore, it is unclear why only a small minority of patients develop polycythemia. We studied 70 RCC for EPO gene and HIFalpha isoform expression. 34% of all RCC showed expression of EPO mRNA in RNase protection assays, which were almost exclusively of the clear cell type. Only 1 patient presented with polycythemia. In situ hybridisation revealed that expression of EPO was in the tumour cells. Expression of EPO mRNA was always associated with activation of HIF, which could involve HIF-1alpha and/or HIF-2alpha. The frequency of EPO gene expression in RCC is therefore much higher than the prevalence of polycythemia. Furthermore, activation of HIF appears necessary for EPO gene expression in RCC, but is clearly not the only determinant. Further to the reported expression of EPO receptors in tumour tissues, the finding of widespread expression of EPO in RCC supports the recent notion of an involvement of this system in paracrine or autocrine effects of tumour cells.

A yeast two-hybrid system reconstituting substrate recognition of the von Hippel-Lindau tumor suppressor protein

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 CBC^VHL 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 and dysregulated c-Myc cooperatively induce vascular endothelial growth factor and metabolic switches hexokinase 2 and pyruvate dehydrogenase kinase 1

Hypoxia is a pervasive microenvironmental factor that affects normal development as well as tumor progression. In most normal cells, hypoxia stabilizes hypoxia-inducible transcription factors (HIFs), particularly HIF-1, which activates genes involved in anaerobic metabolism and angiogenesis. As hypoxia signals a cellular deprivation state, HIF-1 has also been reported to counter the activity of MYC, which encodes a transcription factor that drives cell growth and proliferation. Since many human cancers express dysregulated MYC, we sought to determine whether HIF-1 would in fact collaborate with dysregulated MYC rather countering its function. Here, using the P493-6 Burkitt's lymphoma model with an inducible MYC, we demonstrate that HIF-1 cooperates with dysregulated c-Myc to promote glycolysis by induction of hexokinase 2, which catalyzes the first step of glycolysis, and pyruvate dehydrogenase kinase 1, which inactivates pyruvate dehydrogenase and diminishes mitochondrial respiration. We also found the collaborative induction of vascular endothelial growth factor (VEGF) by HIF-1 and dysregulated c-Myc. This study reports the previously unsuspected collaboration between HIF-1 and dysregulated MYC and thereby provides additional insights into the regulation of VEGF and the Warburg effect, which describes the propensity for cancer cells to convert glucose to lactate.

Mitochondrial oxygen sensing: regulation of hypoxia-inducible factor by mitochondrial generated reactive oxygen species

Decreased oxygen availability (hypoxia) promotes physiological processes such as energy metabolism, angiogenesis, cell proliferation and cell viability through the transcription factor HIF (hypoxia-inducible factor). Activation of HIF can also promote pathophysiological processes such as cancer and pulmonary hypertension. The mechanism(s) by which hypoxia activates HIF are the subject of intensive research. In this chapter we outline the model in which mitochondria regulate the stability of HIF through the increased production of ROS (reactive oxygen species) during hypoxia.

Pseudohypoxic pathways in renal cell carcinoma

Mutations of the von Hippel-Lindau (VHL) or fumarate hydratase (FH) genes lead to morphologically different renal cell carcinomas with distinct clinical courses and outcomes. The VHL protein is a part of an ubiquitin ligase complex that targets proteins for proteosomal degradation. FH is one of the mitochondrial enzymes of the Kreb's cycle. Despite two different functionalities and cellular locations, loss of either VHL or FH products has been shown to alter expression levels of hypoxia-inducible factors (HIF-1alpha and HIF-2alpha) and their downstream targets. HIF proteins are key regulators of oxygen homeostasis. Tight regulation of HIF allows for cell survival and growth at the time of hypoxic stress. HIF acts via transcriptional regulation of vascular endothelial growth factor, platelet derived growth factor, endothelial growth factor receptor, glucose transporter protein 1, erythropoietin, and transforming growth factor-alpha. Loss of VHL or FH is thought to result in a pseudohypoxic state so that cellular response pathways mediated by HIF are activated despite normal oxygen conditions. Understanding of these pseudohypoxic pathways has provided a better appreciation of the molecular mechanisms of carcinogenesis in addition to providing a rationale for targeted therapeutic approaches.

Association of microRNA-34a overexpression with proliferation is cell type-dependent

Recently Welch et al. reported that microRNA (miRNA)-34a functions as a potential tumor suppressor in neuroblastoma cells (Oncogene 26: 5017-22, 2007). Here, we conversely show that miRNA-34a supports cell proliferation in rat oxidative stress-induced renal carcinogenesis and is overexpressed in various types of human cancers. While searching for genetically unstable chromosomal areas in rat renal carcinogenesis, we found the miRNA-34 family reciprocally overexpressed in chromosomal areas with frequent allelic loss. By in situ hybridization and reverse transcription-polymerase chain reaction, cerebral neurons and Purkinje cells showed the highest expression of a major type, miRNA-34a, followed by a variety of endocrine cells and proliferating cells including germinal center lymphocytes and mouse embryonic fibroblasts and stem cells. In contrast, normal renal tubules, hepatocytes and myocardial cells showed faint expression. After 3 weeks of ferric nitrilotriacetate (Fe-NTA)-induced oxidative stress, regenerating renal proximal tubular cells showed high miRNA-34a expression. All of the Fe-NTA-induced rat renal carcinomas and an array of human cancers (151 positive cases of 177) showed high expression of miRNA-34a. Furthermore, knockdown of miRNA-34a with small interfering RNA significantly suppressed proliferation not only of renal carcinoma cells but also of HeLa and MCF7 cells. These results indicate that miRNA-34a overexpression, an acquired trait during carcinogenesis, supports cell proliferation in the majority of cancers suggesting an unexpected link in the cellular metabolism between cancer and neuronal and/or endocrine cells, which warrants further investigation.

Evaluation of HIF-1 inhibitors as anticancer agents

Hypoxia-inducible factor 1 (HIF-1) regulates the transcription of many genes involved in key aspects of cancer biology, including immortalization, maintenance of stem cell pools, cellular dedifferentiation, genetic instability, vascularization, metabolic reprogramming, autocrine growth factor signaling, invasion/metastasis, and treatment failure. In animal models, HIF-1 overexpression is associated with increased tumor growth, vascularization, and metastasis, whereas HIF-1 loss-of-function has the opposite effect, thus validating HIF-1 as a target. In further support of this conclusion, immunohistochemical detection of HIF-1alpha overexpression in biopsy sections is a prognostic factor in many cancers. A growing number of novel anticancer agents have been shown to inhibit HIF-1 through a variety of molecular mechanisms. Determining which combination of drugs to administer to any given patient remains a major obstacle to improving cancer treatment outcomes.

The N-terminal transactivation domain confers target gene specificity of hypoxia-inducible factors HIF-1alpha and HIF-2alpha

The basic helix-loop-helix-Per-ARNT-Sim-proteins hypoxia-inducible factor (HIF)-1alpha and HIF-2alpha are the principal regulators of the hypoxic transcriptional response. Although highly related, they can activate distinct target genes. In this study, the protein domain and molecular mechanism important for HIF target gene specificity are determined. We demonstrate that although HIF-2alpha is unable to activate multiple endogenous HIF-1alpha-specific target genes (e.g., glycolytic enzymes), HIF-2alpha still binds to their promoters in vivo and activates reporter genes derived from such targets. In addition, comparative analysis of the N-terminal DNA binding and dimerization domains of HIF-1alpha and HIF-2alpha does not reveal any significant differences between the two proteins. Importantly, replacement of the N-terminal transactivation domain (N-TAD) (but not the DNA binding domain, dimerization domain, or C-terminal transactivation domain [C-TAD]) of HIF-2alpha with the analogous region of HIF-1alpha is sufficient to convert HIF-2alpha into a protein with HIF-1alpha functional specificity. Nevertheless, both the N-TAD and C-TAD are important for optimal HIF transcriptional activity. Additional experiments indicate that the ETS transcription factor ELK is required for HIF-2alpha to activate specific target genes such as Cited-2, EPO, and PAI-1. These results demonstrate that the HIF-alpha TADs, particularly the N-TADs, confer HIF target gene specificity, by interacting with additional transcriptional cofactors.

Mitochondrial reactive oxygen species trigger hypoxia-inducible factor-dependent extension of the replicative life span during hypoxia

Physiological hypoxia extends the replicative life span of human cells in culture. Here, we report that hypoxic extension of replicative life span is associated with an increase in mitochondrial reactive oxygen species (ROS) in primary human lung fibroblasts. The generation of mitochondrial ROS is necessary for hypoxic activation of the transcription factor hypoxia-inducible factor (HIF). The hypoxic extension of replicative life span is ablated by a dominant negative HIF. HIF is sufficient to induce telomerase reverse transcriptase mRNA and telomerase activity and to extend replicative life span. Furthermore, the down-regulation of the von Hippel-Lindau tumor suppressor protein by RNA interference increases HIF activity and extends replicative life span under normoxia. These findings provide genetic evidence that hypoxia utilizes mitochondrial ROS as signaling molecules to activate HIF-dependent extension of replicative life span.

HIF2 alpha reduces growth rate but promotes angiogenesis in a mouse model of neuroblastoma

BACKGROUND

HIF2alpha/EPAS1 is a hypoxia-inducible transcription factor involved in catecholamine homeostasis, vascular remodelling, physiological angiogenesis and adipogenesis. It is overexpressed in many cancerous tissues, but its exact role in tumour progression remains to be clarified.

METHODS

In order to better establish its function in tumourigenesis and tumour angiogenesis, we have stably transfected mouse neuroblastoma N1E-115 cells with the native form of HIF2alpha or with its dominant negative mutant, HIF2alpha (1-485) and studied their phenotype in vitro and in vivo.

RESULTS

In vitro studies reveal that HIF2alpha induces neuroblastoma cells hypertrophy and decreases their proliferation rate, while its inactivation by the HIF2alpha (1-485) mutant leads to a reduced cell size, associated with an accelerated proliferation. However, our in vivo experiments show that subcutaneous injection of cells overexpressing HIF2alpha into syngenic mice, leads to the formation of tumour nodules that grow slower than controls, but that are well structured and highly vascularized. In contrast, HIF2alpha (1-485)-expressing neuroblastomas grow fast, but are poorly vascularized and quickly tend to extended necrosis.

CONCLUSION

Together, our data reveal an unexpected combination between an antiproliferative and a pro-angiogenic function of HIF2alpha that actually seems to be favourable to the establishment of neuroblastomas in vivo.

HIF-2alpha downregulation in the absence of functional VHL is not sufficient for renal cell differentiation

BACKGROUND

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-alpha) subunits for ubiquitination and subsequent degradation. Using an RNA interference approach to lower levels of HIF-2alpha in two different renal cell lines that lack functional pVHL, we have tested the contribution of HIF-2alpha toward cellular pVHL activities.

RESULTS

Knockdown of HIF-2alpha resulted in cell cycle arrest of renal cells that were grown on collagen I, indicating that this pVHL function is dependent on HIF-2alpha regulation. However, cellular morphological changes and downregulation of integrins alpha5 and beta1, which were seen upon pVHL replacement, were not faithfully phenocopied by HIF-2alpha reduction. Moreover, fibronectin deposition and expression of renal cell differentiation markers were observed in cells containing replaced pVHL, but not in HIF-2alpha knockdown cells, indicating that these pVHL functions may occur independently of HIF-2alpha downregulation.

CONCLUSION

These results indicate that HIF-2alpha regulation is not sufficient for pVHL-induced renal cell differentiation. We hypothesize that in addition to HIF-2alpha dysregulation, abrogation of additional pVHL functions is required for the initiation of renal carcinogenesis.

Hypoxia-inducible factor linked to differential kidney cancer risk seen with type 2A and type 2B VHL mutations

Clear cell carcinoma of the kidney is a major cause of mortality in patients with von Hippel-Lindau (VHL) disease, which is caused by germ line mutations that inactivate the VHL tumor suppressor gene. Biallelic VHL inactivation, due to mutations or hypermethylation, is also common in sporadic clear cell renal carcinomas. The VHL gene product, pVHL, is part of a ubiquitin ligase complex that targets the alpha subunits of the heterodimeric transcription factor hypoxia-inducible factor (HIF) for destruction under well-oxygenated conditions. All VHL mutations linked to classical VHL disease compromise this pVHL function although some missense mutations result in a low risk of kidney cancer (type 2A VHL disease) while others result in a high risk (type 2B VHL disease). We found that type 2A mutants were less defective than type 2B mutants when reintroduced into VHL-/- renal carcinoma cells with respect to HIF regulation. A stabilized version of HIF2alpha promoted tumor growth by VHL-/- cells engineered to produce type 2A mutants, while knock-down of HIF2alpha in cells producing type 2B mutants had the opposite effect. Therefore, quantitative differences with respect to HIF deregulation are sufficient to account for the differential risks of kidney cancer linked to VHL mutations.

The VHL tumor suppressor inhibits expression of the IGF1R and its loss induces IGF1R upregulation in human clear cell renal carcinoma

Clear cell renal cell cancer (CC-RCC) is a highly chemoresistant tumor characterized by frequent inactivation of the von Hippel-Lindau (VHL) gene. The prognosis is reportedly worse in patients whose tumors express immunoreactive type I insulin-like growth factor receptor (IGF1R), a key mediator of tumor cell survival. We aimed to investigate how IGF1R expression is regulated, and found that IGF1R protein levels were unaffected by hypoxia, but were higher in CC-RCC cells harboring mutant inactive VHL than in isogenic cells expressing wild-type (WT) VHL. IGF1R mRNA and promoter activities were significantly lower in CC-RCC cells expressing WT VHL, consistent with a transcriptional effect. In Sp1-null Drosophila Schneider cells, IGF1R promoter activity was dependent on exogenous Sp1, and was suppressed by full-length VHL protein (pVHL) but only partially by truncated VHL lacking the Sp1-binding motif. pVHL also reduced the stability of IGF1R mRNA via sequestration of HuR protein. Finally, IGF1R mRNA levels were significantly higher in CC-RCC biopsies than benign kidney, confirming the clinical relevance of these findings. Thus, we have identified a new hypoxia-independent role for VHL in suppressing IGF1R transcription and mRNA stability. VHL inactivation leads to IGF1R upregulation, contributing to renal tumorigenesis and potentially also to chemoresistance.

HIF-1 inhibits mitochondrial biogenesis and cellular respiration in VHL-deficient renal cell carcinoma by repression of C-MYC activity

Many cancer cells are characterized by increased glycolysis and decreased respiration, even under aerobic conditions. The molecular mechanisms underlying this metabolic reprogramming are unclear. Here we show that hypoxia-inducible factor 1 (HIF-1) negatively regulates mitochondrial biogenesis and O(2) consumption in renal carcinoma cells lacking the von Hippel-Lindau tumor suppressor (VHL). HIF-1 mediates these effects by inhibiting C-MYC activity via two mechanisms. First, HIF-1 binds to and activates transcription of the MXI1 gene, which encodes a repressor of C-MYC transcriptional activity. Second, HIF-1 promotes MXI-1-independent, proteasome-dependent degradation of C-MYC. We demonstrate that transcription of the gene encoding the coactivator PGC-1beta is C-MYC dependent and that loss of PGC-1beta expression is a major factor contributing to reduced respiration in VHL-deficient renal carcinoma cells.

Hypoxia-driven selection of the metastatic phenotype

Intratumoral hypoxia is an independent indicator of poor patient outcome and increasing evidence supports a role for hypoxia in the development of metastatic disease. Studies suggest that the acquisition of the metastatic phenotype is not simply the result of dysregulated signal transduction pathways, but instead is achieved through a stepwise selection process driven by hypoxia. Hypoxia facilitates disruption of tissue integrity through repression of E-cadherin expression, with concomitant gain of N-cadherin expression which allows cells to escape anoikis. Through upregulation of urokinase-type plasminogen activator receptor (uPAR) expression, hypoxia enhances proteolytic activity at the invasive front and alters the interactions between integrins and components of the extracellular matrix, thereby enabling cellular invasion through the basement membrane and the underlying stroma. Cell motility is increased through hypoxia-induced hepatocyte growth factor (HGF)-MET receptor signaling, resulting in cell migration towards the blood or lymphatic microcirculation. Hypoxia-induced vascular endothelial growth factor (VEGF) activity also plays a critical role in the dynamic tumor-stromal interactions required for the subsequent stages of metastasis. VEGF promotes angiogenesis and lymphangiogenesis in the primary tumor, providing the necessary routes for dissemination. VEGF-induced changes in vascular integrity and permeability promote both intravasation and extravasation, while VEGF-induced angiogenesis in the secondary tissue is essential for cell proliferation and establishment of metastatic lesions. Through regulation of these critical molecular targets, hypoxia promotes each step of the metastatic cascade and selects tumor cell populations that are able to escape the unfavorable microenvironment of the primary tumor.

Mammalian Tumor Suppressor Int6 Specifically Targets Hypoxia Inducible Factor 2α for Degradation by Hypoxia- and pVHL-independent Regulation

The hypoxia-inducible factors HIF-1 alpha and HIF-2 alpha are structurally similar as regards their DNA-binding and dimerization domains, but differ in their transactivation domains and, as is shown by experiments using hif-1 alpha(-/-) and hif-2 alpha(-/-) mice, in their functions. This implies that HIF-1 alpha and HIF-2 alpha may have unique target genes. To address this discrepancy and identify HIF-2 alpha-specific target genes, we performed yeast two-hybrid analysis and identified the tumor suppressor Int6/eIF3e/p48 as a novel target gene product involved in HIF-2 alpha regulation. The int6 gene was first identified from a screen in which the mouse mammary tumor virus was employed as an insertional mutagen to identify genes whose functions are critical for breast tumor formation. Here, by using two-hybrid analysis, immunoprecipitation in mammalian cells, and HRE-reporter assays, we report the specific interaction of HIF-2 alpha (but not HIF-1 alpha or HIF-3 alpha) with Int6. The results indicate that the direct interaction of Int6 induces proteasome inhibitor-sensitive HIF-2 alpha degradation. This degradation was clearly observed in renal cell carcinoma 786-O cells, and was found to be both hypoxia- and pVHL-independent. Furthermore, Int6 protein knockdown by int6-siRNA vectors or the dominant-negative mutant Int6-Delta C increased endogenous HIF-2 alpha expression, even under normoxia, and induced sets of critical angiogenic factors comprising vascular endoplasmic growth factor, angiopoietin, and basic fibroblast growth factor mRNA. These results indicate that Int6 is a novel and critical determinant of HIF-2 alpha-dependent angiogenesis as well as cancer formation, and that int6-siRNA transfer may be an effective therapeutic strategy in pathological conditions such as heart and brain ischemia, hepatic cirrhosis, and obstructive vessel diseases.

HIF-2alpha promotes hypoxic cell proliferation by enhancing c-myc transcriptional activity

HIF-2alpha promotes von Hippel-Lindau (VHL)-deficient renal clear cell carcinoma (RCC) tumorigenesis, while HIF-1alpha inhibits RCC growth. As HIF-1alpha antagonizes c-Myc function, we hypothesized that HIF-2alpha might enhance c-Myc activity. We demonstrate here that HIF-2alpha promotes cell-cycle progression in hypoxic RCCs and multiple other cell lines. This correlates with enhanced c-Myc promoter binding, transcriptional effects on both activated and repressed target genes, and interactions with Sp1, Miz1, and Max. Finally, HIF-2alpha augments c-Myc transformation of primary mouse embryo fibroblasts (MEFs). Enhanced c-Myc activity likely contributes to HIF-2alpha-mediated neoplastic progression following loss of the VHL tumor suppressor and influences the behavior of hypoxic tumor cells.

Target gene selectivity of hypoxia-inducible factor-alpha in renal cancer cells is conveyed by post-DNA-binding mechanisms

Inactivation of the von Hippel–Lindau tumour suppressor in renal cell carcinoma (RCC) leads to failure of proteolytic regulation of the α subunits of hypoxia-inducible factor (HIF), constitutive upregulation of the HIF complex, and overexpression of HIF target genes. However, recent studies have indicated that in this setting, upregulation of the closely related HIF-α isoforms, HIF-1α and HIF-2α, have contrasting effects on tumour growth, and activate distinct sets of target genes. To pursue these findings, we sought to elucidate the mechanisms underlying target gene selectivity for HIF-1α and HIF-2α. Using chromatin immunoprecipitation to probe binding to hypoxia response elements in vivo, and expression of chimaeric molecules bearing reciprocal domain exchanges between HIF-1α and HIF-2α molecules, we show that selective activation of HIF-α target gene expression is not dependent on selective DNA-binding at the target locus, but depends on non-equivalent C-terminal portions of these molecules. Our data indicate that post-DNA binding mechanisms that are dissimilar for HIF-1α and HIF-2α determine target gene selectivity in RCC cells.

The von Hippel-Lindau tumor suppressor protein and clear cell renal carcinoma

Germ line VHL tumor suppressor gene loss-of-function mutations cause von Hippel-Lindau disease, which is associated with an increased risk of central nervous system hemangioblastomas, clear cell renal carcinomas, and pheochromocytomas. Somatic VHL mutations are also common in sporadic clear cell renal carcinomas. The VHL gene product, pVHL, is part of a ubiquitin ligase complex that targets the alpha-subunits of the heterodimeric transcription factor hypoxia-inducible factor (HIF) for polyubiquitylation, and hence, proteasomal degradation, when oxygen is available. pVHL-defective clear cell renal carcinomas overproduce a variety of mRNAs that are under the control of HIF, including the mRNAs that encode vascular endothelial growth factor, platelet-derived growth factor B, and transforming growth factor alpha. In preclinical models, down-regulation of HIF-alpha, especially HIF-2alpha, is both necessary and sufficient for renal tumor suppression by pVHL. These observations are probably relevant to the demonstrated clinical activity of vascular endothelial growth factor antagonists in clear cell renal carcinoma and form a foundation for the testing of additional agents that inhibit HIF, or HIF-responsive gene products, in this disease.

The hypoxia-inducible factor 2alpha N-terminal and C-terminal transactivation domains cooperate to promote renal tumorigenesis in vivo

Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor, consisting of an alpha subunit and a beta subunit, that controls cellular responses to hypoxia. HIFalpha contains two transcriptional activation domains called the N-terminal transactivation domain (NTAD) and the C-terminal transactivation domain (CTAD). HIFalpha is destabilized by prolyl hydroxylation catalyzed by EglN family members. In addition, CTAD function is inhibited by asparagine hydroxylation catalyzed by FIH1. Both hydroxylation reactions are linked to oxygen availability. The von Hippel-Lindau tumor suppressor protein (pVHL) is frequently mutated in kidney cancer and is part of the ubiquitin ligase complex that targets prolyl hydroxylated HIFalpha for destruction. Recent studies suggest that HIF2alpha plays an especially important role in promoting tumor formation by pVHL-defective renal carcinoma cells among the three HIFalpha paralogs. Here we dissected the relative contribution of the two HIF2alpha transactivation domains to hypoxic gene activation and renal carcinogenesis and investigated the regulation of the HIF2alpha CTAD by FIH1. We found that the HIF2alpha NTAD is capable of activating both artificial and naturally occurring HIF-responsive promoters in the absence of the CTAD. Moreover, we found that the HIF2alpha CTAD, in contrast to the HIF1alpha CTAD, is relatively resistant to the inhibitory effects of FIH1 under normoxic conditions and that, perhaps as a result, both the NTAD and CTAD cooperate to promote renal carcinogenesis in vivo.

HIF and fumarate hydratase in renal cancer

Hereditary leiomyomatosis and renal cell cancer is a recently described hereditary cancer syndrome in which affected individuals are predisposed to the development of leiomyomas of the skin and uterus. In addition, this clinical entity also can result in the development of biologically aggressive kidney cancer. Affected individuals harbour a germline mutation of the fumarate hydratase (FH) gene, which encodes an enzyme that catalyses conversion of fumarate to malate in the Kreb's cycle. Thus far, proposed mechanisms for carcinogeneis associated with this syndrome include aberrant apoptosis, oxidative stress, and pseudohypoxic drive. At this time, the majority of accumulating data support a role for pseudohypoxic drive in tumour development. The link between FH mutation and pseudohypoxic drive may reside in the biochemical alterations resulting from diminished/absent FH activity. These biochemical derangements may interfere with oxygen homeostasis and result in a cellular environment conducive to tumour formation.

Hypoxia-inducible factors: central regulators of the tumor phenotype

Low oxygen levels are a defining characteristic of solid tumors, and responses to hypoxia contribute substantially to the malignant phenotype. Hypoxia-induced gene transcription promotes characteristic tumor behaviors, including angiogenesis, invasion, metastasis, de-differentiation and enhanced glycolytic metabolism. These effects are mediated, at least in part, by targets of the hypoxia-inducible factors (HIFs). The HIFs function as heterodimers comprising an oxygen-labile alpha-subunit and a stable beta-subunit also referred to as ARNT. HIF-1alpha and HIF-2alpha stimulate the expression of overlapping as well as unique transcriptional targets, and their induction can have distinct biological effects. New targets and novel mechanisms of dysregulation place the HIFs in an ever more central role in tumor biology and have led to development of pharmacological inhibitors of their activity.

Regulation of angiogenesis by hypoxia and hypoxia-inducible factors

Maintenance of oxygen homeostasis is critical for the survival of multicellular organs. As a result, both invertebrates and vertebrates have developed highly specialized mechanisms to sense changes in oxygen levels and to mount adequate cellular and systemic responses to these changes. Hypoxia, or low oxygen tension, occurs in physiological situations such as during embryonic development, as well as in pathological conditions such as ischemia, wound healing, and cancer. A primary effector of the adaptive response to hypoxia in mammals is the hypoxia-inducible factor (HIF) family of transcription regulators. These proteins activate the expression of a broad range of genes that mediate many of the responses to decreased oxygen concentration, including enhanced glucose uptake, increased red blood cell production, and the formation of new blood vessels via angiogenesis. This latter process is dynamic and results in the establishment of a mature vascular system that is indispensable for proper delivery of oxygen and nutrients to all cells in both normal tissue and hypoxic regions. Angiogenesis is essential for normal development and neoplastic disease as tumors must develop mechanisms to stimulate vascularization to meet increasing metabolic demands. The link between hypoxia and the regulation of angiogenesis is an area of intense research and the molecular details of this connection are still being elaborated. This chapter will provide an overview of current knowledge and highlight new insights into the importance of HIF and hypoxia in angiogenesis in both physiological and pathophysiological conditions.

VHL promotes E2 box-dependent E-cadherin transcription by HIF-mediated regulation of SIP1 and snail

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 alpha 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 HIFalpha in CC-RCC (VHL(-/-)) cells likewise increased E-cadherin expression, while functional hypoxia or expression of VHL mutants incapable of promoting HIFalpha 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.