SEARCH

SEARCH BY CITATION

Keywords:

  • angiogenesis;
  • cyclooxygenase;
  • drug reprofiling;
  • hypoxia-inducible factor-1;
  • intratumoral hypoxia;
  • prostaglandins;
  • sphingolipids

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that immortalizes tumors by inducing key genes in cancer biology, including angiogenesis, glycolysis, invasion, and metastasis. Overexpression of HIF-1α is thus associated with resistance to cancer chemotherapy and increased patient mortality in several cancer phenotypes. In the present review, we summarize the role of intratumoral hypoxia and bioactive lipids in enhancing HIF-1 activity, critically discussing the potential for HIF-1α inhibitors in cancer chemotherapy. Considering preclinical studies, HIF-1 inhibitors appear to have antitumor effects and thus represent a novel therapeutic strategy.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

Many human cancers contain regions of hypoxia because of rapid cell proliferation and the presence of intratumoral blood vessels that are structurally and functionally abnormal. Notably, the presence of intratumoral hypoxia is associated with an increased risk for invasion, metastasis, treatment failure, and patient mortality.[1] A principal mechanism by which cells adapt to hypoxia is through the transcription factor hypoxia-inducible factor-1 (HIF-1), a heterodimeric protein composed of an O2-regulated HIF-1α and a constitutively expressed HIF-1β subunit. Under normoxic conditions, HIF-1α is hydroxylated on proline residue 402 and/or 564, which is required for binding of the von Hippel–Lindau (VHL) protein, the recognition subunit of an E3 ubiquitin ligase that targets HIF-1α for proteasomal degradation[2] (Fig. 1). Hydroxylation however decreases under hypoxic conditions, enabling HIF-1α to accumulate and dimerize with HIF-1β. The functional transcription factor then binds at the core hypoxia response element 5'-RCGTG-3' to induce genes involved in angiogenesis, glycolysis, de-differentiation, invasion, and metastasis. Overexpression of HIF-1α is thus associated with an aggressive phenotype and increased mortality in many cancer types.[3]

figure

Figure 1. Mechanisms of hypoxia-inducible factor (HIF)-1α regulation under aerobic and hypoxic conditions. VHL, von Hippel–Lindau.

Download figure to PowerPoint

Viral infection in HIF-1 overexpression

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

Numerous transforming viruses linked to tumorigenesis also increase HIF-1 expression—examples include the hepatitis B virus (HBV) X protein,[4] human papillomavirus (HPV) E6/E7 oncoproteins,[5] and latent membrane protein 1 (LMP-1) from Epstein–Barr virus (EBV).[6] By increasing HIF-1α synthesis[7] and degradation of the prolyl hydroxylases,[6] LMP-1 promotes tumorigenicity through inducing an HIF-1-dependent de-differentiated phenotype characteristic of cancer progenitor cells.[8, 9] LMP-1 also upregulates glycolytic enzymes—rapidly proliferating cancer cells depend on the Warburg effect to generate sufficient glycolytic intermediates for anabolic metabolism.[10] LMP-1-mediated upregulation of matrix metalloproteinase 9 (MMP-9), which digests the basement membrane, further facilitates cellular invasion and metastasis.[11] Consequently, oncoviruses may cause HIF-1 overexpression and potentiate tumorigenesis.

Bioactive lipids in HIF-1 overexpression

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

In addition to hypoxia and infection by oncoviruses, such as EBV,[6] HBV[4], and HPV,[5] HIF-1 expression may also be upregulated by bioactive lipid mediators. The role of eicosanoids in carcinogenesis was first elucidated in epidemiological studies, whereby a reduced risk for colon cancer was observed in long-term users of non-steroidal anti-inflammatory drugs.[12] Derived from the enzymatic action of cyclooxygenase (COX) on arachidonic acid, levels of prostaglandin E2 (PGE2) were shown to be elevated in immunohistochemical analyses of gastric and colon cancer surgical biopsies.[12, 13] Subsequent exposure of human HCT116 colon and PC-3ML prostate cancer cells to PGE2 revealed that PGE2 induced vascular endothelial growth factor (VEGF) expression in an HIF-1α-dependent process.[14, 15] In a potential positive feedback loop, activation of the PGE2 receptor EP1 in HepG2 hepatocellular carcinoma cells upregulates normoxic expression of HIF-1α, which then binds to the hypoxia response element (HRE) within the COX-2 promoter to further enhance PGE2 production and promote tumorigenesis.[16, 17] In support of this hypothesis, increased cell proliferation and tube formation of human umbilical vein endothelial cells (HUVECs) was observed in an AGS gastric cancer cell line transfected with a COX-2 expression vector. Critically, the consequent HIF-1α protein accumulation and subsequent angiogenic effects on HUVECs were reduced by NS-398 (COX-2 inhibitor), SC19220 (PGE2 receptor antagonist), and antisense HIF-1α transfection.[13] Thus, the COX-2/PGE2/HIF-1α/VEGF pathway may be critical in angiogenesis and tumor progression.

Other eicosanoids implicated in HIF-1 upregulation include leukotrienes, lipoxins, and hydroxyeicosatetraenoic acids generated by lipoxygenases (LOXs). Specifically, PC-3 human prostate cancer cells with 12-LOX overexpression exhibited elevated nuclear HIF-1α levels, with the consequent increase in expression of VEGF and the glucose transporter 1 further enabling survival of tumor cells under hypoxic conditions.[18] Enhanced HIF-1α accumulation under normoxic conditions may alternatively be attributed to reactive oxygen species-dependent stabilization of HIF-1α by sphingosine-1-phosphate, an oncogenic bioactive lipid mediator that modulates angiogenesis, cell proliferation, and apoptosis.[19] Indeed, overexpression of sphingosine-1-kinase (SphK1) was observed in prostate PC-3, brain U87, and lung A549 cancer cells, and is correlated with poor prognosis in patients with glioblastoma.[20] Moreover, inhibition of SphK1 with short interfering RNA (siRNA) or pharmacological antagonists prevented the accumulation of HIF-1α and its downstream transcriptional activity.[19, 21] Thus, bioactive lipid mediators play an important role in HIF-1 overexpression and tumor progression.

HIF-1 as a therapeutic target

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

Numerous cancer chemotherapeutic agents target angiogenesis in tumors to reduce primary tumor growth. Response to anti-VEGF therapy has however been poor, as intratumoral hypoxia arising from impaired angiogenesis causes HIF-1-dependent metastasis and expansion of cancer stem cell pools.[22] The importance of HIF-1 in tumor growth is emphasized by experimental data in which overexpression of HIF-1α in HCT116 colorectal cancer[23] and PCI-10 pancreatic cancer cells[24] increased vessel density and tumor growth, respectively. Immunohistochemical analyses of oropharyngeal[25] and esophageal squamous cell carcinoma,[26] as well as laryngeal,[27] gastric,[28, 29] pancreatic,[30] colorectal,[31] and rectal carcinoma,[32] biopsies also revealed a positive correlation between HIF-1 overexpression, radiotherapy resistance, and increased patient mortality.

Critically, previous studies have demonstrated that HIF-1α null mutations severely impede tumor growth by reducing expression of glycolytic enzymes, with tumors unable to grow beyond 2 mm[3] unless supported by neovascularization mediated by hypoxic induction of HIF-1.[33, 34] In contrast with VEGF inhibitors, HIF-1 inhibitors were also shown to decrease breast cancer cell metastasis in mouse orthotopic transplantation models[35] and sensitize tumors to radiotherapy.[36] Additionally, antisense blockade of HIF-1α expression in gastric carcinoma cells reduced VEGF production in response to COX-2 overexpression,[13] with the introduction of HIF-1α siRNA into a glioma cell line downregulating MMP-2/MMP-9 to suppress cell migration and invasion into adjacent normal tissue.[37] As HIF-1 mediates multiple tumor survival mechanisms and its overexpression arises not only because of the actions of bioactive lipid mediators, viral infections, and intratumoral hypoxia but also through oncogene gain-of-function[38] and tumor suppressor gene[39] loss-of-function mutations, HIF-1 may represent a final common pathway in cancer pathogenesis. Hence, HIF-1 is an attractive target for cancer therapy.

Existing HIF-1 inhibitors

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

Small molecule inhibitors of HIF-1 are highly desirable because of its central role in tumorigenesis. Broadly, small molecules may inhibit HIF-1 by decreasing its protein levels, DNA-binding, or transactivation (Table 1). Examples include PX-478, which inhibits translation via a VHL- and p53-independent mechanism,[40] and the RNA antagonist EZN-2968, which inhibits expression of HIF-1α messenger RNA (mRNA).[41] Notably, PX-478 and EZN-2968 cause dose-dependent reductions in levels of HIF-1α, VEGF secretion, and tumor size in xenograft models and were both well tolerated in phase I clinical trials.[40, 41] Other modulators identified to date include geldanamycin, which reduces heat shock protein 90 binding to HIF-1α to destabilize folding and increase proteasomal degradation,[42] and the mammalian target of rapamycin inhibitor rapamycin (mTOR). Indirect actions through upstream or downstream signaling pathways however cause many side-effects.

Table 1. HIF-1 inhibitors by mechanism of action and drug target
MechanismDrug
  1. COX-2, cyclooxygenase 2; HIF-1, hypoxia-inducible factor-1; mRNA, messenger RNA.

Decreased HIF-1α mRNA levelsAminoflavone
Decreased HIF-1α protein synthesisCardiac glycosides: digoxin
COX-2 inhibitors: ibuprofen
mTOR inhibitors: rapamycin
RNA interference: EZN-2968
Topoisomerase II: mitoxantrone
Decreased HIF-1α protein stabilizationHSP90 inhibitors: geldanamycin
Unknown: PX-478
Decreased HIF-1α: HIF-1β dimerizationAcriflavine
Rolitetracycline
Decreased HIF-1: DNA-bindingEchinomycin
Doxorubicin
Daunorubicin
Decreased HIF-1 transactivationBortezomib

In an alternate approach, echinomycin and anthracycline antibiotics inhibit HIF-1 : DNA binding via DNA intercalation at the HRE.[43, 44] DNA intercalators however show limited sequence specificity, thus causing off-target effects. Indeed, echinomycin cannot be used in cancer chemotherapy because of its secondary action on Sp1 (5'-CCGCCC-3'), which ultimately increases HIF-1α expression under normoxic conditions. The development of novel specific HIF-1 inhibitors is thus needed.

Searching for new HIF-1 inhibitors

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

On average, it costs over $1 billion and takes 15 years to create a new drug. Traditional drug discovery using high-throughput screening in a chemical space with over 1060 compounds is not only expensive but also time-consuming, and plagued by artificial hits. Virtual screening is thus increasingly used in de novo rational drug design to screen chemical libraries in silico, limiting the number of compounds for physical screening to a subset more likely to yield “hits.” Despite selection for highly drug-like structures that follow Lipinski's Rule of Five,[45] 90% of drug candidates nonetheless fail clinical testing because of safety concerns.

The Nobel Laureate James Black famously once stated, “The most fruitful basis for the discovery of a new drug is to start with an old drug.” In drug reprofiling, the molecular mechanisms of existing drugs with known safety and pharmacokinetic profiles are re-examined for novel therapeutic indications based on the concept of polypharmacology, where the average drug acts on five different targets.[46] Numerous successes have been achieved with reprofiling, with the most striking example being thalidomide. Originally introduced as an anti-emetic for morning sickness and later withdrawn for teratogenicity, the Food and Drug Administration has recently approved thalidomide for erythema nodosum leprosum and multiple myeloma. Other noteworthy reprofiling successes include sildenafil for erectile dysfunction and finasteride for male pattern baldness.

Drug reprofiling libraries (Table 2), containing small molecules with history of use in human clinical trials, may similarly be screened for HIF-1 inhibitors. Indeed, cardiac glycosides[47] and mitoxantrone[48] were found to inhibit HIF-1α mRNA protein translation in a topoisomerase-II-dependent and -independent manner, respectively, using this approach. The antiseptic acriflavine was also discovered to significantly decrease tumor size and the number of circulating angiogenic cells in tumor bearing mice via inhibition of HIF-1α : HIF-1β dimerization.[49] Critically, cardiac glycosides, mitoxantrone, and acriflavine have all been clinically used, thus enabling immediate commencement of phase II clinical trials for efficacy and the move to clinic potentially rapid.

Table 2. Examples of drug reprofiling libraries
LibraryTotal drugs
  1. FDA, Food and Drug Administration; NIH, National Institutes of Health.

FDA-approved640
John Hopkins Clinical Compound Library 1.01514
NIH Clinical Collections I and II727
Prestwick Chemical Library1200

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References

HIF-1 is widely recognized as the central mediator to cellular hypoxia, inducing key genes in angiogenesis, dedifferentiation, glycolysis, invasion, and metastasis to promote tumorigenesis. Overexpression of HIF-1, which may arise due to the actions of bioactive lipid mediators, as well as viral infections, hypoxia, and or mutations to oncogenes and tumor suppressor genes, is thus associated with poor prognosis in many cancer types. Hence, we anticipate that the identification of novel selective HIF-1 inhibitors will prove to be of significant therapeutic value.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Viral infection in HIF-1 overexpression
  5. Bioactive lipids in HIF-1 overexpression
  6. HIF-1 as a therapeutic target
  7. Existing HIF-1 inhibitors
  8. Searching for new HIF-1 inhibitors
  9. Conclusion
  10. References
  • 1
    Semenza GL. Evaluation of HIF-1 inhibitors as anticancer agents. Drug Discov. Today 2007; 12: 853859.
  • 2
    Kaelin WG, Ratcliffe PJ. Oxygen sensing by metazoans: the central role of the HIF hydroxylase pathway. Mol. Cell 2008; 30: 393402.
  • 3
    Zhong H, De Marzo AM, Laughner E et al. Overexpression of hypoxia-inducible factor 1alpha in common human cancers and their metastases. Cancer Res. 1999; 59: 58305835.
  • 4
    Kondo S, Seo SY, Yoshizaki T et al. EBV latent membrane protein 1 up-regulates hypoxia-inducible factor 1alpha through Siah1-mediated down-regulation of prolyl hydroxylases 1 and 3 in nasopharyngeal epithelial cells. Cancer Res. 2006; 66: 98709877.
  • 5
    Yoo YG, Cho S, Park S, Lee MO. The carboxy-terminus of the hepatitis B virus X protein is necessary and sufficient for the activation of hypoxia-inducible factor-1alpha. FEBS Lett. 2004; 577: 121126.
  • 6
    Nakamura M, Bodily JM, Beglin M, Kyo S, Inoue M, Laimins LA. Hypoxia-specific stabilization of HIF-1alpha by human papillomaviruses. Virology 2009; 387: 442448.
  • 7
    Wakisaka N, Kondo S, Yoshizaki T, Murono S, Furukawa M, Pagano JS. Epstein-Barr virus latent membrane protein 1 induces synthesis of hypoxia-inducible factor 1 alpha. Mol. Cell. Biol. 2004; 24: 52235234.
  • 8
    Kondo S, Wakisaka N, Muramatsu M et al. Epstein-Barr virus latent membrane protein 1 induces cancer stem/progenitor-like cells in nasopharyngeal epithelial cell lines. J. Virol. 2011; 85: 1125511264.
  • 9
    Helczynska K, Kronblad A, Jögi A et al. Hypoxia promotes a dedifferentiated phenotype in ductal breast carcinoma in situ. Cancer Res. 2003; 63: 14411444.
  • 10
    Darekar S, Georgiou K, Yurchenko M et al. Epstein-Barr virus immortalization of human B-cells leads to stabilization of hypoxia-induced factor 1 alpha, congruent with the Warburg effect. PLoS ONE 2012; 7: e42072.
  • 11
    Wakisaka N, Pagano JS. Epstein-Barr virus induces invasion and metastasis factors. Anticancer Res. 2003; 23: 21332138.
  • 12
    Rigas B, Goldman IS, Levine L. Altered eicosanoid levels in human colon cancer. J. Lab. Clin. Med. 1993; 122: 518523.
  • 13
    Huang SP, Wu MS, Shun CT et al. Cyclooxygenase-2 increases hypoxia-inducible factor-1 and vascular endothelial growth factor to promote angiogenesis in gastric carcinoma. J. Biomed. Sci. 2005; 12: 229241.
  • 14
    Fukuda R, Kelly B, Semenza GL. Vascular endothelial growth factor gene expression in colon cancer cells exposed to prostaglandin E2 is mediated by hypoxia-inducible factor 1. Cancer Res. 2003; 63: 23302334.
  • 15
    Liu XH, Kirschenbaum A, Lu M et al. Prostaglandin E2 induces hypoxia-inducible factor-1alpha stabilization and nuclear localization in a human prostate cancer cell line. J. Biol. Chem. 2002; 277: 5008150086.
  • 16
    Ji R, Chou CL, Xu W, Chen XB, Woodward DF, Regan JW. EP1 prostanoid receptor coupling to G i/o up-regulates the expression of hypoxia-inducible factor-1 alpha through activation of a phosphoinositide-3 kinase signaling pathway. Mol. Pharmacol. 2010; 77: 10251036.
  • 17
    Kaidi A, Qualtrough D, Williams AC, Paraskeva C. Direct transcriptional up-regulation of cyclooxygenase-2 by hypoxia-inducible factor (HIF)-1 promotes colorectal tumor cell survival and enhances HIF-1 transcriptional activity during hypoxia. Cancer Res. 2006; 66: 66836691.
  • 18
    Krishnamoorthy S, Jin R, Cai Y et al. 12-Lipoxygenase and the regulation of hypoxia-inducible factor in prostate cancer cells. Exp. Cell Res. 2010; 316: 17061715.
  • 19
    Ader I, Brizuela L, Bouquerel P, Malavaud B, Cuvillier O. Sphingosine kinase 1: a new modulator of hypoxia inducible factor 1alpha during hypoxia in human cancer cells. Cancer Res. 2008; 68: 86358642.
  • 20
    Yoshida Y, Nakada M, Harada T et al. The expression level of sphingosine-1-phosphate receptor type 1 is related to MIB-1 labeling index and predicts survival of glioblastoma patients. J. Neurooncol 2010; 98: 4147.
  • 21
    French KJ, Schrecengost RS, Lee BD et al. Discovery and evaluation of inhibitors of human sphingosine kinase. Cancer Res. 2003; 63: 59625969.
  • 22
    Conley SJ, Gheordunescu E, Kakarala P et al. Antiangiogenic agents increase breast cancer stem cells via the generation of tumor hypoxia. Proc. Natl Acad. Sci. U. S. A. 2010; 109: 27842789.
  • 23
    Ravi R, Mookerjee B, Bhujwalla ZM et al. Regulation of tumor angiogenesis by p53-induced degradation of hypoxia-inducible factor 1alpha. Genes Dev. 2000; 14: 3444.
  • 24
    Akakura N, Kobayashi M, Horiuchi I et al. Constitutive expression of hypoxia-inducible factor-1alpha renders pancreatic cancer cells resistant to apoptosis induced by hypoxia and nutrient deprivation. Cancer Res. 2001; 61: 65486554.
  • 25
    Aebersold DM, Burri P, Beer KT et al. Expression of hypoxia-inducible factor-1alpha: a novel predictive and prognostic parameter in the radiotherapy of oropharyngeal cancer. Cancer Res. 2001; 61: 29112916.
  • 26
    Tzao C, Lee SC, Tung HJ et al. Expression of hypoxia-inducible factor (HIF)-1alpha and vascular endothelial growth factor (VEGF)-D as outcome predictors in resected esophageal squamous cell carcinoma. Dis. Markers 2008; 25: 141148.
  • 27
    Schrijvers ML, van der Laan BF, de Bock GH et al. Overexpression of intrinsic hypoxia markers HIF1alpha and CA-IX predict for local recurrence in stage T1-T2 glottic laryngeal carcinoma treated with radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 2008; 72: 161169.
  • 28
    Griffiths EA, Pritchard SA, Valentine HR et al. Hypoxia-inducible factor-1alpha expression in the gastric carcinogenesis sequence and its prognostic role in gastric and gastro-oesophageal adenocarcinomas. Br. J. Cancer 2007; 96: 95103.
  • 29
    Takahashi R, Tanaka S, Hiyama T et al. Hypoxia-inducible factor-1alpha expression and angiogenesis in gastrointestinal stromal tumor of the stomach. Oncol. Rep. 2003; 10: 797802.
  • 30
    Sun HC, Qiu ZJ, Liu J et al. Expression of hypoxia-inducible factor-1 alpha and associated proteins in pancreatic ductal adenocarcinoma and their impact on prognosis. Int. J. Oncol. 2007; 30: 13591367.
  • 31
    Rajaganeshan R, Prasad R, Guillou PJ et al. The role of hypoxia in recurrence following resection of Dukes' B colorectal cancer. Int. J. Colorectal Dis. 2008; 23: 10491055.
  • 32
    Rasheed S, Harris AL, Tekkis PP et al. Hypoxia-inducible factor-1alpha and -2alpha are expressed in most rectal cancers but only hypoxia-inducible factor-1alpha is associated with prognosis. Br. J. Cancer 2009; 100: 16661673.
  • 33
    Ryan HE, Lo J, Johnson RS. HIF-1 alpha is required for solid tumor formation and embryonic vascularization. EMBO J. 1998; 17: 30053015.
  • 34
    Ma WW, Adjei AA. Novel agents on the horizon for cancer therapy. CA Cancer J. Clin. 2009; 5: 111137.
  • 35
    Zhang H, Wong CC, Wei H et al. HIF-1-dependent expression of angiopoietin-like 4 and L1CAM mediates vascular metastasis of hypoxic breast cancer cells to the lungs. Oncogene 2012; 31: 17571770.
  • 36
    Moeller BJ, Cao Y, Li CY, Dewhirst MW. Radiation activates HIF-1 to regulate vascular radiosensitivity in tumors: role of reoxygenation, free radicals, and stress granules. Cancer Cell 2004; 5: 429441.
  • 37
    Fujiwara S, Nakagawa K, Harada H et al. Silencing hypoxia-inducible factor-1alpha inhibits cell migration and invasion under hypoxic environment in malignant gliomas. Int. J. Oncol. 2007; 30: 793802.
  • 38
    Giatromanolaki A, Koukourakis MI, Simopoulos C et al. c-erbB-2 related aggressiveness in breast cancer is hypoxia inducible factor-1alpha dependent. Clin. Cancer Res. 2004; 10: 79727977.
  • 39
    Maxwell PH, Wiesener MS, Chang GW et al. The tumour suppressor protein VHL targets hypoxia-inducible factors for oxygen-dependent proteolysis. Nature 1999; 399: 271275.
  • 40
    Koh MY, Spivak-Kroizman T, Venturini S et al. Molecular mechanisms for the activity of PX-478, an antitumor inhibitor of the hypoxia-inducible factor-1A. Mol. Cancer Ther. 2008; 7: 90100.
  • 41
    Greenberger LM, Horak ID, Filpula D et al. A RNA antagonist of hypoxia-inducible factor-1alpha, EZN-2968, inhibits tumor cell growth. Mol. Cancer Ther. 2008; 7: 35983608.
  • 42
    Isaacs JS, Jung YJ, Mimnaugh EG, Martinez A, Cuttitta F, Neckers LM. Hsp90 regulates a von Hippel Lindau-independent hypoxia-inducible factor-1 alpha-degradative pathway. J. Biol. Chem. 2002; 277: 2993629944.
  • 43
    Kong D, Park EJ, Stephen AG et al. Echinomycin, a small-molecule inhibitor of hypoxia-inducible factor-1 DNA-binding activity. Cancer Res. 2005; 65: 90479055.
  • 44
    Lee K, Qian DZ, Rey S, Wei H, Liu JO, Semenza GL. Anthracycline chemotherapy inhibits HIF-1 transcriptional activity and tumor-induced mobilization of circulating angiogenic cells. Proc. Natl Acad. Sci. U. S. A. 2009; 106: 23532358.
  • 45
    Lipinski CA, Lombardo F, Dominy BW, Feeney PJ. Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv. Drug Deliv. Rev. 2001; 46: 326.
  • 46
    Overington JP, Al-Lazikani B, Hopkins AL. How many drug targets are there? Nat. Rev. Drug Discov. 2006; 5: 993996.
  • 47
    Zhang H, Qian DZ, Tan YS et al. Digoxin and other cardiac glycosides inhibit HIF-1alpha synthesis and block tumor growth. Proc. Natl Acad. Sci. U. S. A. 2008; 105: 1957919586.
  • 48
    Toh YM, Li TK. Mitoxantrone inhibits HIF-1α expression in a topoisomerase II-independent pathway. Clin. Cancer Res. 2011; 17: 50265037.
  • 49
    Lee K, Zhang H, Qian DZ, Rey S, Liu JO, Semenza GL. Acriflavine inhibits HIF-1 dimerization, tumor growth, and vascularization. Proc. Natl Acad. Sci. U. S. A. 2009; 106: 1791017915.