Current Pharmaceutical Design

ISSN: 1381-6128

Current Pharmaceutical Design
Volume 15, Number 33, 2009

Contents


The Hypoxia-Inducible Factor (HIF) Pathway as a Target for Prevention and Treatment of Clinical Manifestations
Executive Editor: Thomas Kietzmann


Editorial: Pp. 3837-3838 [PMID: 19925432 PubMed - indexed for MEDLINE]


HIF-1 Inhibitors for Cancer Therapy: From Gene Expression to Drug Discovery Pp. 3839-3843
G. L. Semenza
[Abstract] [Purchase Article] [PMID: 19671047 PubMed - indexed for MEDLINE]


Regulation of HIF-1α at the Transcriptional Level Pp. 3844-3852
Agnes Görlach
[Abstract] [Purchase Article] [PMID: 19671046 PubMed - indexed for MEDLINE]


Factors Interacting with HIF-1α mRNA: Novel Therapeutic Targets Pp. 3853-3860
Stefanie Galbán and Myriam Gorospe
[Abstract] [Purchase Article] [PMID: 19671045 PubMed - indexed for MEDLINE]


Emerging Roles of microRNAs in the Molecular Responses to Hypoxia Pp. 3861-3866
Meredith E. Crosby, Cecilia M. Devlin, Peter M. Glazer, George A. Calin, and Mircea Ivan
[Abstract] [Purchase Article] [PMID: 19925433 PubMed - indexed for MEDLINE]


Kinases as Upstream Regulators of the HIF System: Their Emerging Potential as Anti-Cancer Drug Targets Pp. 3867-3877
Elitsa Y. Dimova, Carine Michiels and Thomas Kietzmann
[Abstract] [Purchase Article] [PMID: 19671044 PubMed - indexed for MEDLINE]


HIF Prolyl 4-Hydroxylases and their Potential as Drug Targets Pp. 3878-3885
Johanna Myllyharju
[Abstract] [Purchase Article] [PMID: 19671043 PubMed - indexed for MEDLINE]


HIF Prolyl-4-hydroxylase Interacting Proteins: Consequences for Drug Targeting Pp. 3886-3894
Roland H. Wenger, Gieri Camenisch, Daniel P. Stiehl and Dörthe M. Katschinski
[Abstract] [Purchase Article] [PMID: 19671040 PubMed - indexed for MEDLINE]


The VHL Tumor Suppressor: Master Regulator of HIF Pp. 3895-3903
Volker H. Haase
[Abstract] [Purchase Article] [PMID: 19671042 PubMed - indexed for MEDLINE]


Signalling Cross Talk of the HIF System: Involvement of the FIH Protein Pp. 3904-3907
M.L. Coleman and P.J. Ratcliffe
[Abstract] [Purchase Article] [PMID: 19671041 PubMed - indexed for MEDLINE]


General Articles


Potential Role of Dental Stem Cells in the Cellular Therapy of Cerebral Ischemia Pp. 3908-3916
Mehmet E. Yalvac, Albert A. Rizvanov, Ertugrul Kilic, Fikrettin Sahin, Marat A. Mukhamedyarov, Rustem R. Islamov and András Palotás
[Abstract] [Purchase Article]




Abstracts


[Back to top] [PMID: 19925432 PubMed - indexed for MEDLINE]
Editorial: The Hypoxia-Inducible Factor (HIF) Pathway as a Target for Prevention and Treatment of Clinical Manifestations

Within the last decade it became evident that a family of hypoxia-inducible transcription factors (HIFs) are key players in the cellular response to limited O₂ supply. From the three HIFs known today, HIF-1 is the best characterized and it appears to be of special interest since it activates transcription of glycolytic enzymes and glucose transporters as well as angiogenic growth factors to improve nutrient and blood supply to meet the energy demands of the cell. Thus, HIFs appear to have a central role in the adaptive cellular program responding to hypoxia in normal tissues.

HIFs are heterodimers composed of α- and ß-subunits, the latter also known as the ARNT proteins. While the levels of the β-subunits seem to be quite stable, the α-subunit levels are low under normoxia and upregulated under hypoxia. Thereby transcriptional, posttranscriptional and posttranslational processes are involved. From the latter, the oxygen-dependent hydroxylation of proline 402 and 564 followed by the binding of the von Hippel-Lindau (VHL) protein initiates degradation of HIF-1a by the ubiquitin-proteasome system whereas hydroxylation at asparagine 803 initiates loss of the coactivator p300/CBP.

Importantly, hypoxia is also a major feature of a number of diseases and almost all solid tumors contain hypoxic regions in which O₂ concentrations are greatly reduced compared to the surrounding normal tissue. Further, a number of studies from various tumor entities showed that increased levels of HIFs, especially HIF-1α are associated with a poor prognosis. Thus, tumor cells survive and proliferate under extreme conditions where nutrients and oxygen are limited, and HIFs initiate the expression of various genes wich increase the risk of patient mortality. In addition, many studies focused on the understanding of cancer progression have shown that HIF α-subunits also respond to growth and coagulation factors, hormones, cytokines or stress factors under non-hypoxic conditions.

Altogether this increased the interest in HIF α-subunits, especially HIF-1α, as a drug target and new, specific therapeutic strategies targeting the HIF pathway are intesively investigated.

In this special issue, comprised of a series of review articles, important new findings on HIFs related to different aspects of HIF regulation, drug targeting as well as cell signalling events upstream and downstream of HIFs are highlighted.

In order to establish new potent therapeutic strategies, the review by Semenza summarizes recent aspects with respect to HIF-1 inhibitors and their potential use in cancer therapy [1]. Since also a number of hormones, cytokines and growth factors regulate the temporal expression of HIF-1α at the mRNA level, the detailed molecular mechanisms signalling pathways and transcription factors remained largely unknown. Here, the review by Görlach [2] gives an overview about positive and negative elements that influence HIF-1α gene expression. In the same line, the review by Galban and Gorospe summarizes factors interacting with HIF-1α mRNA and give a number of implications to use these factors as novel therapeutic targets [3].

In addition to the regulation of HIF-1α mRNA, recent studies have also established that microRNAs (miRs) can be regulated in response to hypoxia. In their review, Crosby and coworkers discuss the role of hypoxia-regulated miRs, and anticipate directions for clinical applications [4]. Further, other investigations pointed to a role of different kinase signalling pathways in the regulation of HIF α-subunit synthesis or stabilization of the protein. The review by Dimova and Kietzmann summarizes the knowledge about the kinase involvement and the potential use of kinase inhibitors to target the HIF system [5].

The stability of the HIF α-subunit is regulated by oxygen-dependent prolyl hydroxylation catalyzed by the HIF prolyl 4-hydroxylases (P4Hs) and these enzymes are considered as promising drug targets. In the review by Myllyharju the HIF-P4Hs features and studies with HIF-P4H inhibitors are summarized thus demonstrating efficacy of HIF-P4H inhibition in many clinical settings [6]. In line with this, more upstream acting HIF prolyl-4-hydroxylase interacting proteins may also represent another line of drug targets. The existing HIF prolyl-4-hydroxylase interacting proteins and the consequences for drug targeting are described in the review by Wenger and colleagues [7]. Proline hydroxylation of HIF-1α is the prerequisite for VHL binding and the review by Haase summarizes recent advances in VHL and HIF α-subunit associated diseases [8]. Further, asparaginyl hydroxylation influences the transcriptional activity of HIF due to the action of the factor inhibiting HIF (FIH) which may have alternative substrates. These issues are reviewed in the article from Coleman and Ratcliffe and the possible role of cross-talk between the FIH and HIF systems in fine tuning hypoxia responses are discussed [9].

Although a complete consistent picture with respect to the actions of different drugs used to target the HIF system cannot be reached so far and a number of mechanistic details need to be clarified, we think that the HIF field is still exciting and we hope that the contributions of this Special Issue will be useful for a number of readers in this or other areas of biomedical research.

REFERENCES

[1] Semenza GL. HIF-1 inhibitors for cancer therapy: From gene expression to drug discovery. Curr Pham Des 2009; 15(33): 3839-3843.
[2] Görlach A. Curr Pham Des 2009; Regulation of HIF-1α at the transcriptional level. Curr Pham Des 2009; 15(33): 3844-3852.
[3] Galbán S, Gorospe M. Factors interacting with HIF-1α mRNA: Novel therapeutic targets. Curr Pham Des 2009; 15(33): 3853-3860.
[4] Crosby ME, Devlin CM, Glazer PM, Calin GA, Ivan M. Emerging roles of microRNAs in the molecular responses to hypoxia. Curr Pham Des 2009; 15(33): 3861-3866.
[5] Dimova EY, Kietzmann T. Kinases as upstream regulators of the HIF system: Their emerging potential as anti-cancer drug targets. Curr Pham Des 2009; 15(33): 3867-3877.
[6] Myllyharju J. HIF Prolyl 4-Hydroxylases and their potential as drug targets. Curr Pham Des 2009; 15(33): 3878-3885.
[7] Wenger RH, Camenisch G, Stiehl DP, Katschinski DM. HIF prolyl-4-hydroxylase interacting proteins: consequences for drug targeting. Curr Pham Des 2009; 15(33): 3886-3894.
[8] Haase V. The VHL Tumor Suppressor: Master Regulator of HIF Curr Pham Des 2009; 15(33): 3895-3903.
[9] Coleman ML, Ratcliffe PJ. Signalling cross talk of the HIF system: Involvement of the FIH protein. Curr Pham Des 2009; 15(33): 3904-3907.

Thomas Kietzmann
University of Kaiserslautern
Faculty of Chemistry, Department Biochemistry
Erwin-Schrödinger-Str. 54
67663 Kaiserslautern
Germany
Tel: +496312054953
Fax: +49-6312053419
E-mail: tkietzm@gwdg.de


[Back to top] [Purchase Article] [PMID: 19671047 PubMed - indexed for MEDLINE]
HIF-1 Inhibitors for Cancer Therapy: From Gene Expression to Drug Discovery
G. L. Semenza

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric protein composed of HIF-1α and HIF-1α subunits, which is activated in response to reduced O₂ availability. HIF-1 transactivates genes encoding proteins that are involved in key aspects of the cancer phenotype, including cell immortalization and de-differentiation, stem cell maintenance, genetic instability, glucose uptake and metabolism, pH regulation, autocrine growth/survival, angiogenesis, invasion/metastasis, and resistance to chemotherapy. Increased HIF-1α levels, as determined by immunohistochemical analysis of tumor biopsy specimens, is associated with increased mortality in many human cancers. Drugs that inhibit HIF-1 activity and have anti-cancer effects in vivo have been identified and clinical trials are warranted to establish the contexts in which addition of such agents to therapy protocols will result in increased patient survival.


[Back to top] [Purchase Article] [PMID: 19671046 PubMed - indexed for MEDLINE]
Regulation of HIF-1α at the Transcriptional Level
Agnes Görlach

The hypoxia-inducible factor HIF-1 has been shown to be mandatory for the cellular adaptation to hypoxia. In addition, evidence has been provided that HIF-1 can mediate various stress responses and that it may play an important role under inflammatory conditions even independently of hypoxia. HIF-1 is a heterodimer consisting of an α-subunit which is subject to tight regulation, and a β-subunit, also termed ARNT, which appears to be constitutively expressed. In addition to the complex network controlling the cellular content of HIF-1α at the level of protein stability, recent evidence showed that HIF-1α levels can also be regulated at the mRNA level. In fact, transcriptional regulatory networks seem to be an additional way of controlling HIF-1α levels and may open new therapeutic options to modulate HIF-1α also under non-hypoxic conditions.


[Back to top] [Purchase Article] [PMID: 19671045 PubMed - indexed for MEDLINE]
Factors Interacting with HIF-1α mRNA: Novel Therapeutic Targets
Stefanie Galbán and Myriam Gorospe

The heterodimeric transcription factor HIF-1 (hypoxia-inducible factor-1) induces angiogenesis, a process that is aberrantly elevated in cancer. The HIF-1β subunit is constitutively expressed, but the levels of the HIF-1α subunit are robustly regulated, increasing under hypoxic conditions and decreasing in normoxia. These changes result from rapid alterations in the rates of HIF-1α production and degradation. While the regulation of HIF-1α degradation is understood in significant detail, much less is known about the regulation of HIF-1α biosynthesis. Here, we review recent evidence that HIF-1α production is effectively controlled by post-transcriptional mechanisms. We focus on the RNA-binding proteins (RBPs) and the non-coding RNAs that interact with the HIF-1α mRNA and influence its half-life and translation rate. HIF-1α mRNA-binding factors are emerging as promising pharmacological targets to control HIF-1α production selectively and efficiently.


[Back to top] [Purchase Article] [PMID: 19925433 PubMed - indexed for MEDLINE]
Emerging Roles of microRNAs in the Molecular Responses to Hypoxia
Meredith E. Crosby, Cecilia M. Devlin, Peter M. Glazer, George A. Calin, and Mircea Ivan

Recent studies have established that the regulation of microRNAs (miRs) is a feature of the hypoxic response. In this review, we discuss the role of hypoxia-regulated miRs, with an emphasis on miR-210 and miR-373, and anticipate directions for clinical applications. The induction of miR-210 and miR-373 is dependent upon hypoxia inducible factor (HIF), and their up-regulation has been detected in a variety of solid tumors. Both miRs have been associated with adverse prognosis and metastatic potential. The increased expression of miR-210 is linked to an in vivo hypoxic signature. MiR-210 also participates in endothelial and neuronal cells’ response to oxygen deprivation and may possess a role in the regulation of angiogenesis. A variety of miR-210 and miR-373 targets that may be relevant to hypoxia have been validated or proposed. Very recently, targets of these miRs that are implicated in DNA repair have been identified, thus establishing an additional link between the hypoxic tumor microenvironment and DNA damage. Extending beyond cancer biology, some of miR-210 targets are likely involved in the regulation of angiogenesis, and neuronal cell survival. Inactivation of miRs affected by hypoxia presents a promising therapeutic strategy in the case of difficult-to-treat cancers, as well as in other non-cancer-related diseases.


[Back to top] [Purchase Article] [PMID: 19671044 PubMed - indexed for MEDLINE]
Kinases as Upstream Regulators of the HIF System: Their Emerging Potential as Anti-Cancer Drug Targets
Elitsa Y. Dimova, Carine Michiels and Thomas Kietzmann

The hypoxia-inducible factor-1 (HIF-1) is a key regulator in the mammalian response to oxygen deficiency under both physiological and pathological conditions such as cancer. A number of studies indicated an association between tumor hypoxia, increased hypoxia-inducible factor (HIF-1α) levels and a poor prognosis. The HIF-1α regulation in response to hypoxia occurs primarily on the level of protein stability due to posttranslational hydroxylation. However, HIF α-subunits also respond to various growth factors, hormones, or cytokines under non-hypoxic conditions implicating the involvement of different kinase pathways in their regulation thereby increasing the interest in HIF-1α as a new drug target. Herein, we review current knowledge about phosphorylation-dependent HIF-1α regulation, HIF-1α protein-protein interactions and subcellular localization with emphasis on new therapeutic strategies targeting the HIF pathway.


[Back to top] [Purchase Article] [PMID: 19671043 PubMed - indexed for MEDLINE]
HIF Prolyl 4-Hydroxylases and their Potential as Drug Targets
Johanna Myllyharju

Hypoxia-inducible transcription factor (HIF) is the master regulator of hypoxia-inducible genes involved in the mediation of survival and adaptive responses to insufficient oxygen availability, such as genes involved in hematopoesis, angiogenesis, iron transport, glucose utilization, resistance to oxidative stress, cell proliferation, survival and apoptosis, extracellular matrix homeostasis, and tumor progression. The stability of the HIFα subunit is regulated by oxygen-dependent prolyl 4-hydroxylation catalyzed by the HIF prolyl 4-hydroxylases (P4Hs). The 4-hydroxyproline residues generated in normoxic conditions facilitate binding of HIFα to the von Hippel-Lindau E3 ubiquitin ligase complex resulting in the attachment of ubiquitin molecules and subsequent rapid proteasomal degradation of HIFα. In hypoxia this oxygen-requiring hydroxylation event is inhibited, HIFα escapes degradation and can translocate to the nucleus and form a functional dimer with HIFβ that triggers the hypoxic response. HIF-P4Hs are considered as promising drug development targets in the treatment of diseases such as myocardial infarction, stroke, peripheral vascular disease, inflammation, diabetes and severe anemias. Studies with HIF-P4H inhibitors in various animal models and ongoing clinical trials support this hypothesis by demonstrating efficacy in many applications.


[Back to top] [Purchase Article] [PMID: 19671040 PubMed - indexed for MEDLINE]
HIF Prolyl-4-hydroxylase Interacting Proteins: Consequences for Drug Targeting
Roland H. Wenger, Gieri Camenisch, Daniel P. Stiehl and Dörthe M. Katschinski

Protein stability of hypoxia-inducible factor (HIF) α subunits is regulated by the oxygen-sensing prolyl-4-hydroxylase domain (PHD) enzymes. Under oxygen-limited conditions, HIFα subunits are stabilized and form active HIF transcription factors that induce a large number of genes involved in adaptation to hypoxic conditions with physiological implications for erythropoiesis, angiogenesis, cardiovascular function and cellular metabolism. Oxygen-sensing is regulated by the co-substrate-dependent activity and hypoxia-inducible abundance of the PHD enzymes which trigger HIFα stability even under low oxygen conditions. Because HIFα itself is notoriously reluctant to the development of antagonists, an increase in PHD activity would offer an interesting alternative to the development of drugs that interfere specifically with the HIF signalling pathway. Interestingly, among the recently discovered PHD interacting proteins were not only novel downstream targets but also upstream regulators of PHDs. Their PHD isoform-specific interaction offers the possibility to target distinct PHD isoforms and their non-identical downstream signalling pathways. This review summarizes our current knowledge on PHD interacting proteins, including upstream regulators, chaperonins, scaffolding proteins, and novel downstream transcription factors.


[Back to top] [Purchase Article] [PMID: 19671042 PubMed - indexed for MEDLINE]
The VHL Tumor Suppressor: Master Regulator of HIF
Volker H. Haase

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.


[Back to top] [Purchase Article] [PMID: 19671041 PubMed - indexed for MEDLINE]
Signalling Cross Talk of the HIF System: Involvement of the FIH Protein
M.L. Coleman and P.J. Ratcliffe

Cellular and systemic oxygen homeostasis is regulated by an oxygen-sensitive signalling pathway centred on a transcription factor known as Hypoxia Inducible Factor (HIF). Regulation of HIF activity and protein stability is mediated by a family of hydroxylases that act as oxygen sensors due to the dependence of the hydroxylation reaction on oxygen. The transcriptional activity of HIF is at least in part determined by asparaginyl hydroxylation by Factor Inhibiting HIF (FIH) of a C-terminal residue that regulates co-activator recruitment. The activity of FIH on HIF is limiting; emerging data suggest this may be due to competition from a large family of alternative FIH substrates that act as a ‘sink’ for FIH activity. These alternative substrates are targeted for hydroxylation at conserved Asn residues within a protein interaction domain known as the Ankyrin Repeat Domain (ARD). Many ARD-containing proteins bind to FIH more tightly than does HIF. Furthermore, ARD proteins are common within the proteome and in some cases are highly abundant. Since ARD substrates bind to FIH in a similar manner to HIF it is thought that these properties of the ARD family lead to competitive inhibition of FIH-dependent HIF hydroxylation. We summarise the current literature here and discuss the possible role of cross-talk between the FIH, HIF and ARD systems in fine tuning hypoxia responses.


[Back to top] [Purchase Article]
Potential Role of Dental Stem Cells in the Cellular Therapy of Cerebral Ischemia
Mehmet E. Yalvac, Albert A. Rizvanov, Ertugrul Kilic, Fikrettin Sahin, Marat A. Mukhamedyarov, Rustem R. Islamov and András Palotás

Stem cell based therapies for cerebral ischemia (CI) utilize different cell sources including embryonic stem cells (ESCs), neural stem cells (NSCs), umbilical cord blood cells (UCBCs), mesenchymal stem cells (MSCs), and some immortalized cell lines. To date, experimental studies showed that all of these cell sources have been successful to some extent in attenuating the ischemic damage and improving functional recovery after brain injury. Bone marrow derived MSCs seem to be the most widely used and well characterized cell source, which can be also employed for autologous transplantation. Currently, there are two main theories behind the therapeutic effect of stem cell transplantation for treating CIs. The first concept is cell replacement theory in which transplanted stem cells differentiate into progenitor and specialized somatic cells to supersede dying cells. The other hypothesis is based on immuno-modulatory, neuro-protective and neuro-trophic abilities of stem cells which help reducing stroke size and increasing the recovery of behavioral functions. Recent studies focusing on alternative stem cell sources have revealed that dental stem cells (DSCs), including dental pulp stem cells (DPSCs) and dental follicle cells (DFCs) possess properties of MSCs and NSCs. They differentiate into neural linage cells and some other cell types such as osteocytes, adipocytes, chondrocytes, muscle cells and hepatocytes. This review is intended to examine stem cell therapy approaches for CI and emphasize potential use of DSCs as an alternative cell source for the treatment of brain ischemia.