Characterization of a subset of the basic-helix-loop-helix-PAS superfamily that interacts with components of the dioxin signaling pathway

The HIF/VHL pathway: from oxygen sensing to innate immunity

In aerobic organisms, all cells have the capacity to respond to changes in oxygenation through the stabilization and transcriptional activation of hypoxia-inducible factor (HIF). At sites of tissue injury, oxygen delivery to individual cells may be compromised or insufficient due to increased metabolic demands, and it is to these areas that immune cells, including neutrophils, must migrate and operate effectively. In addition to the role of HIF to regulate the adaptive metabolic and survival responses of these cells at sites of reduced oxygenation, more complex interactions between HIF and pro-inflammatory pathways are now emerging. The mechanisms by which HIF modulates pro-inflammatory myeloid cell lifespan and function remain to be fully characterized, but roles for the oxygen-sensing hydroxylase enzymes through direct hydroxylation of NF-kappaB and its repressor protein IkappaBalpha have been suggested. The ability of HIF to modulate cellular glucose utilization is also thought to be important, with the maintenance of intracellular ATP pools linked to enhanced myeloid cell aggregation, motility, invasiveness, and bacterial killing. Additional non-hypoxia-mediated routes to up-regulate HIF are also now recognized. In this review we describe the role of HIF in the oxygen-sensing response, and the oxygen-dependent and -independent regulation of myeloid cell function and longevity. Understanding these processes and the role they play in regulating innate immune responses within inflamed sites, both hypoxic and normoxic, may offer new opportunities for therapeutic intervention.

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.

Disruption of clock gene expression alters responses of the aryl hydrocarbon receptor signaling pathway in the mouse mammary gland

The biological effects of many environmental toxins are mediated by genes containing Per-Arnt-Sim (PAS) domains, the aryl hydrocarbon receptor (AhR), and AhR nuclear translocator. Because these transcription factors interact with other PAS genes that form the circadian clockworks in mammals, we determined whether targeted disruption of the clock genes, Per1 and/or Per2, alters toxin-induced expression of known biological markers in the AhR signaling pathway. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a prototypical Ahr agonist, had an inductive effect on mammary gland expression of cytochrome P450, subfamily I, polypeptide 1 (Cyp1A1) mRNA regardless of genotype. However, TCDD-mediated Cyp1A1 induction in the mammary glands of Per1(ldc) and Per1(ldc)/Per2(ldc) mice was significantly (17.9- and 5.9-fold) greater than that in wild-type (WT) animals. In addition, TCDD-induced Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mammary glands was significantly increased relative to that in WT mice. Similar to in vivo observations, experiments using primary cultures of mammary gland tissue demonstrated that TCDD-induced Cyp1A1 and Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mutant cells was significantly greater than that in WT cultures. AhR mRNA levels were distinctively elevated in cells derived from all mutant genotypes, but they were commonly decreased in WT and mutant cultures after TCDD treatment. In WT mice, an interesting corollary is that the inductive effects of TCDD on mammary gland expression of Cyp1A1 and Cyp1B1 vary over time and are significantly greater during the night. These findings suggest that clock genes, especially Per1, may be involved in TCDD activation of AhR signaling pathways.

Iron-regulatory proteins limit hypoxia-inducible factor-2α expression in iron deficiency

Hypoxia stimulates erythropoiesis, the major iron-utilization pathway. We report the discovery of a conserved, functional iron-responsive element (IRE) in the 5' untranslated region of the messenger RNA encoding endothelial PAS domain protein-1, EPAS1 (also called hypoxia-inducible factor-2alpha, HIF2alpha). Via this IRE, iron regulatory protein binding controls EPAS1 mRNA translation in response to cellular iron availability. Our results uncover a regulatory link that permits feedback control between iron availability and the expression of a key transcription factor promoting iron utilization. They also show that an IRE that is structurally distinct from, for example, the ferritin mRNA IRE and that has been missed by in silico approaches, can mediate mechanistically similar responses.

CLOCK and NPAS2 have overlapping roles in the suprachiasmatic circadian clock

Heterodimers of CLOCK and BMAL1, bHLH-PAS transcription factors, are believed to be the major transcriptional regulators of the circadian clock mechanism in mammals. However, a recent study shows that CLOCK-deficient mice continue to exhibit robust behavioral and molecular rhythms. Here we report that the transcription factor NPAS2 (MOP4) is able to functionally substitute for CLOCK in the master brain clock in mice to regulate circadian rhythmicity.

Molecular cloning, characterization and expression of two hypoxia-inducible factor alpha subunits, HIF-1alpha and HIF-2alpha, in a hypoxia-tolerant marine teleost, Atlantic croaker (Micropogonias undulatus)

Alteration of gene expression is a crucial component of adaptation by animals to hypoxic conditions and is mediated by specific transcription factors, hypoxia-inducible factors (HIFs), which are composed of alpha and beta subunits. In this study, we report the cloning and characterization of two HIF-alpha subunits, HIF-1alpha and HIF-2alpha, and their expression in various tissues of a hypoxia-tolerant marine teleost, Atlantic croaker (Micropogonias undulatus). The full-length croaker HIF-1alpha (2805 bp) and HIF-2alpha (3205 bp) cDNAs contain open reading frames encoding proteins with 720 and 847 amino acids, respectively, which are highly homologous to the HIF-1alpha and HIF-2alpha proteins of other non-mammalian species. Croaker HIF-1 alpha shares only 43% sequence identity with the croaker HIF-2alpha subunit. However, the basic helix-loop-helix/Per-ARNT-Sim regions appear to be relatively well conserved between the two proteins, with identities of 75-83%. The core oxygen-dependent degradation domain regions in croaker HIFs are well conserved, suggesting a similar mechanism of HIF degradation to that in other vertebrate species. Northern blot analysis showed that croaker HIF-1alpha and HIF-2alpha mRNAs (transcript sizes 3.0-3.8 kb) are highly expressed in the brain, heart, liver, and gonads under hypoxic conditions, whereas muscle tissues show lower levels of expression. Short-term (1.7 mg/L dissolved oxygen, DO for 3 days to 1 week) and long-term (1.7, 2.7 and 3.7 mg/L DO for 3 weeks) hypoxia exposure caused significant increases in HIF-1alpha and HIF-2alpha mRNA expression in croaker ovaries compared to mRNA levels in fish held in normoxic conditions (DO: 6.5 mg/L). However, HIF transcript levels in hypoxia-exposed fish had returned to control values 24 h after the DO in the tanks was restored to normoxic levels. The results suggest that the upregulation of both HIF-1alpha and HIF-2alpha subunits at the transcriptional level is an important component of adaptation of croaker to chronic hypoxia and HIF-alphas are potentially useful molecular indicators of

Interplay of circadian clocks and metabolic rhythms

This review examines the connections between circadian and metabolic rhythms. Examples from a wide variety of well-studied organisms are used to illustrate some of the genetic and molecular pathways linking circadian timekeeping to metabolism. The principles underlying biological timekeeping by intrinsic circadian clocks are discussed briefly. Genetic and molecular studies have unambiguously identified the importance of gene expression feedback circuits to the generation of overt circadian rhythms. This is illustrated particularly well by the results of genome-wide expression studies, which have uncovered hundreds of clock-controlled genes in cyanobacteria, fungi, plants, and animals. The potential connections between circadian oscillations in gene expression and circadian oscillations in metabolic activity are a major focus of this review.

Glycogen synthase kinase 3 phosphorylates hypoxia-inducible factor 1alpha and mediates its destabilization in a VHL-independent manner

Hypoxia-inducible transcription factor 1alpha (HIF-1alpha) is a key player in the response to hypoxia. Additionally, HIF-1alpha responds to growth factors and hormones which can act via protein kinase B (Akt). However, HIF-1alpha is not a direct substrate for this kinase. Therefore, we investigated whether the protein kinase B target glycogen synthase kinase 3 (GSK-3) may have an impact on HIF-1alpha. We found that the inhibition or depletion of GSK-3 induced HIF-1alpha whereas the overexpression of GSK-3beta reduced HIF-1alpha. These effects were mediated via three amino acid residues in the oxygen-dependent degradation domain of HIF-1alpha. In addition, mutation analyses and experiments with von Hippel-Lindau (VHL)-defective cells indicated that GSK-3 mediates HIF-1alpha degradation in a VHL-independent manner. In line with these observations, the inhibition of the proteasome reversed the GSK-3 effects, indicating that GSK-3 may target HIF-1alpha to the proteasome by phosphorylation. Thus, the direct regulation of HIF-1alpha stability by GSK-3 may influence physiological processes or pathophysiological situations such as metabolic diseases or tumors.

Identification and characterization of genes susceptible to transcriptional cross-talk between the hypoxia and dioxin signaling cascades

The aryl hydrocarbon receptor (AHR) and hypoxia inducible factors (HIFs) are transcription factors that control the adaptive response to toxicants such as dioxins and decreases in available oxygen, respectively. The AHR and HIFs utilize the same heterodimeric partner, the aryl hydrocarbon nuclear translocator (ARNT) for proper function. This requirement raises the possibility that cross-talk exists between these critical signaling systems. Single gene and reporter assays have yielded conflicting results regarding the nature of the competition for ARNT. Therefore, to determine the extent of cross-talk between the AHR and HIFs, a comprehensive analysis was performed using global gene expression analysis. The results identified 767 and 430 transcripts that are sensitive to cobalt chloride and 2,3,7,8-tetrachlorodibenzo-rho-dioxin (TCDD) stimulation, respectively, with 308 and 176, respectively, exhibiting sensitivity to cross-talk. The overlap between these two sets consists of 33 unique transcripts, including the classic target genes CYP1A1, carbonic anhydrase IX, and those involved in lipid metabolism and coagulation. Computational analysis of the regulatory region of these genes identified complex relationships between HIFs, AHR, and their respective response elements as well as other DNA motifs, including the SRF, Sp-1, NF-kB, and AP-2 binding sites. These results suggest that HIF-AHR cross-talk is limited to genes with regulatory regions that contain specific motifs and architectures.

Hypoxia, drug therapy and toxicity

Hypoxia is defined as a decrease in available oxygen reaching the tissues of the body. It is linked to the pathology of cancer, cardiovascular disease, and stroke, the leading causes of death in the United States. Cells under hypoxic stress either induce an adaptive response that includes increasing the rates of glycolysis and angiogenesis or undergo cell death by promoting apoptosis or necrosis. The ability of cells to maintain a balance between adaptation and cell death is regulated by a family of transcription factors called the hypoxia inducible factors (HIF). HIF1, the most widely studied HIF, is essential for regulating the expression of a battery of hypoxia-responsive genes involved in the adaptive and cell death responses. The ability of HIF1 to balance these 2 responses likely lies in the regulation of HIF1alpha stability and transcriptional activity by post-translational hydroxylation and its ability to respond to other cellular factors including key metabolites and growth factors. Targeting HIF1 signaling for therapeutics, therefore, requires an understanding of how these various signals converge upon HIF1 and regulate its role in maintaining the balance between adaptation and cell death. In addition, one must understand how this balance can be perturbed during toxicant-induced tissue damage. This review will summarize our current understanding of hypoxia signaling as it applies to drug therapy and toxicity and describe how these processes can influence the HIF-mediated balance between adaptation and cell death.

IOP1, a novel hydrogenase-like protein that modulates hypoxia-inducible factor-1alpha activity

A central means by which mammalian cells respond to low oxygen tension is through the activation of the transcription factor HIF-1 (hypoxia-inducible factor-1). Under normoxic conditions, HIF-1alpha (the alpha subunit of HIF-1) is targeted for rapid degradation by the ubiquitin-proteasome pathway. Under hypoxic conditions, this degradation is inhibited, thereby leading to the stabilization and activation of HIF-1alpha. Here, we report the identification of IOP1 (iron-only hydrogenase-like protein 1), a protein homologous with enzymes present in anaerobic organisms that contain a distinctive iron-sulfur cluster. IOP1 is present in a broad range of cell types. Knockdown of IOP1 using siRNA (small interfering RNA) in mammalian cells increases protein levels of HIF-1alpha under both normoxic and hypoxic conditions, and augments hypoxia-induced HRE (hypoxia response element) reporter gene and endogenous HIF-1alpha target gene expressions. We find that IOP1 knockdown up-regulates HIF-1alpha mRNA levels, thereby providing a mechanism by which knockdown induces the observed effects. The results collectively provide evidence that IOP1 is a component of the protein network that regulates HIF-1alpha in mammalian cells.

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.

Molecular cloning and characterization of theXenopus hypoxia-inducible factor 1α (xHIF1α)

We report the molecular cloning and the characterization of the Xenopus homolog of mammalian hypoxia-inducible factor 1alpha (HIF1alpha), a member of the bHLH/PAS transcription factor family. Searches in Xenopus genome sequences and phylogenetic analysis reveal the existence of HIF1alpha and HIF2alpha paralogs in the Xenopus laevis species. Sequence data analyses indicate that the organization of protein domains in Xenopus HIF1alpha (xHIF1alpha) is strongly conserved. We also show that xHIF1alpha heterodimerizes with the Xenopus Arnt1 protein (xArnt1) with the proteic complex being mediated by the HLH and PAS domains. Subcellular analysis in a Xenopus XTC cell line using chimeric GFP constructs show that over-expression of xHIF1alpha and xArnt1 allows us to detect the xHIF1alpha/xArnt1 complex in the nucleus, but only in the presence of both partners. Further analyses in XTC cell line show that over-producing xHIF1alpha and xArnt1 mediates trans-activation of the hypoxia response element (HRE) reporter. The trans-activation level can be increased in hypoxia conditions. Interestingly such trans-activation properties can be also observed when human Arnt1 is used together with the xHIF1alpha.

Three circadian clock genes Per2, Arntl, and Npas2 contribute to winter depression

BACKGROUND

Multiple lines of evidence suggest that the circadian clock contributes to the pathogenesis of winter depression or seasonal affective disorder (SAD). We hypothesized that sequence variations in three genes, including Per2, Arntl, and Npas2, which form a functional unit at the core of the circadian clock, predispose to winter depression.

METHODS

In silico analysis of the biological effects of allelic differences suggested the target single-nucleotide polymorphisms (SNPs) to be analyzed in a sample of 189 patients and 189 matched controls. The most relevant SNP in each gene was identified for the interaction analysis and included in the multivariate assessment of the combined effects of all three SNPs on the disease risk.

RESULTS

SAD was associated with variations in each of the three genes in gene-wise logistic regression analysis. In combination analysis of variations of Per2, Arntl, and Npas2, we found additive effects and identified a genetic risk profile for the disorder. Carriers of the risk genotype combination had the odds ratio of 4.43 of developing SAD as compared with the remaining genotypes, and of 10.67 as compared with the most protective genotype combination.

CONCLUSION

Variations in the three circadian clock genes Per2, Arntl, and Npas2 are associated with the disease, supporting the hypothesis that the circadian clock mechanisms contribute to winter depression.

Dissecting the functions of the mammalian clock protein BMAL1 by tissue-specific rescue in mice

The basic helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) domain transcription factor BMAL1 is an essential component of the mammalian circadian pacemaker. Bmal1-/- mice lose circadian rhythmicity but also display tendon calcification and decreased activity, body weight, and longevity. To investigate whether these diverse functions of BMAL1 are tissue-specific, we produced transgenic mice that constitutively express Bmal1 in brain or muscle and examined the effects of rescued gene expression in Bmal1-/- mice. Circadian rhythms of wheel-running activity were restored in brain-rescued Bmal1-/- mice in a conditional manner; however, activity levels and body weight were lower than those of wild-type mice. In contrast, muscle-rescued Bmal1-/- mice exhibited normal activity levels and body weight yet remained behaviorally arrhythmic. Thus, Bmal1 has distinct tissue-specific functions that regulate integrative physiology.

Unexpected diversity of aryl hydrocarbon receptors in non-mammalian vertebrates: insights from comparative genomics

Ligand-activated receptors are well-known targets of environmental chemicals that disrupt endocrine signaling. Genomic approaches are providing new opportunities to understand the comparative biology and molecular evolution of these receptors. One example of this is the aryl hydrocarbon receptor (AHR), a basic-helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) transcription factor through which planar aromatic hydrocarbons cause altered gene expression and toxicity. In contrast to humans and other mammals, which possess a single AHR, teleosts such as the Atlantic killifish (Fundulus heteroclitus) have at least two AHRs (AHR1 and AHR2). Analysis of sequenced genomes has revealed additional, unexpected AHR diversity in non-mammalian vertebrates, including the chicken Gallus gallus (three predicted AHR genes), bony fishes such as the pufferfish Takifugu (formerly Fugu) rubripes (five AHR genes) and zebrafish Danio rerio (three AHR genes), and cartilaginous fishes such as the spiny dogfish Squalus acanthias (three AHR genes). In contrast, invertebrates appear to possess single AHRs that do not bind typical ligands of vertebrate AHRs. We suggest that AHR diversity in vertebrates arose through both gene and whole-genome duplications combined with lineage-specific gene loss, and that sensitivity to the developmental toxicity of planar aromatic hydrocarbons may have had its origin in the evolution of the ligand-binding capacity of the AHR in the chordate lineage. Comparative molecular and genomic studies are providing new insights into AHR diversity and function in non-mammalian species, revealing additional complexity in mechanisms by which environmental chemicals interfere with receptor-dependent signaling.

Effects of tryptophan photoproducts in the circadian timing system: searching for a physiological role for aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) mediates adverse effects of dioxins, but its physiological role remains ambiguous. The similarity between AhR and canonical circadian clock genes suggests potential involvement of AhR in regulation of circadian timing. Photoproducts of tryptophan (TRP), including 6-formylindolo[3,2-b]carbazole (FICZ), have high affinity for AhR and are postulated as endogenous ligands. Although TRP photoproducts activate AhR signaling in vitro, their effects in vivo have not been investigated in mammals. Because TRP photoproducts may act as transducers of light, we examined their effects on the circadian clock. Intraperitoneal injection of TRP photoproducts or FICZ to C57BL/6J mice dose dependently induced AhR downstream targets, cytochrome P4501A1 (CYP1A1) and cytochrome P4501B1 mRNA expression, in liver. c-fos mRNA, a commonly used marker for light responses, was also induced with FICZ, and all responses were AhR dependent. A rat-immortalized suprachiasmatic nucleus (SCN) cell line, SCN 2.2, was used to examine the direct effect of TRP photoproducts on the molecular clock. Both TRP photoproducts and FICZ-increased CYP1A1 expression and prolonged FICZ incubation altered the circadian expression of clock genes (Per1, Cry1, and Cry2) in SCN 2.2 cells. Furthermore, FICZ inhibited glutamate-induced phase shifting of the mouse SCN electrical activity rhythm. Circadian light entrainment is critical for adjustment of the endogenous rhythm to environmental light cycle. Our results reveal a potential for TRP photoproducts to modulate light-dependent regulation of circadian rhythm through triggering of AhR signaling. This may lead to further understanding of toxicity of dioxins and the role of AhR in circadian rhythmicity.

Development of novel therapeutic strategies that target HIF-1

Activity of hypoxia-inducible factor 1 (HIF-1) is increased in human cancers as a result of the physiological induction of HIF-1alpha in response to intratumoural hypoxia and as a result of genetic alterations that activate oncogenes and inactivate tumour suppressor genes. In many cancer types, increased HIF-1alpha expression is associated with increased risk of patient mortality. HIF-1 plays important roles in every major aspect of cancer biology through the transcriptional regulation of hundreds of genes. The efficacy of many novel anticancer agents that target signal transduction pathways may be due in part to their indirect inhibition of HIF-1. Several novel compounds with anticancer activity have been shown to inhibit HIF-1 and may be useful as components of individualised multidrug therapeutic regimens chosen based on molecular analyses of tumour biopsies.

The role of aryl hydrocarbon receptor in the pathogenesis of cardiovascular diseases

Numerous experimental and epidemiological studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are major constituents of cigarette tobacco tar, are strongly involved in the pathogenesis of the cardiovascular diseases (CVDs). Knowing that PAH-induced toxicities are mediated by the activation of a cytosolic receptor, aryl hydrocarbon receptor (AhR), which regulates the expression of a group of xenobiotic metabolizing enzymes (XMEs) such as CYP1A1, CYP1A2, CYP1B1, NQO1, and GSTA1, suggests a direct link between AhR-regulated XMEs and CVDs. Therefore, identifying the localization and expression of the AhR and its regulated XMEs in the cardiovascular system (CVS) is of major importance in understanding their physiological and pathological roles. Generally, it was believed that the levels of AhR-regulated XMEs are lower in the CVS than in the liver; however, it has been shown that similar or even higher levels of expression are demonstrated in the CVS in a tissue- and species-specific manner. Moreover, most, if not all, AhR-regulated XMEs are differentially expressed in most of the CVS, particularly in the endothelium cells, aorta, coronary arteries, and ventricles. Although the exact mechanisms of PAH-mediated cardiotoxicity are not fully understood, several mechanisms are proposed. Generally, induction of CYP1A1, CYP1A2, and CYP1B1 is considered cardiotoxic through generating reactive oxygen species (ROS), DNA adducts, and endogenous arachidonic acid metabolites. However the cardioprotective properties of NQO1 and GSTA1 are mainly attributed to the antioxidant effect by decreasing ROS and increasing the levels of endogenous antioxidants. This review provides a clear understanding of the role of AhR and its regulated XMEs in the pathogenesis of CVDs, in which imbalance in the expression of cardioprotective and cardiotoxic XMEs is the main determinant of PAH-mediated cardiotoxicity.

Hypoxia-inducible factor-1 (HIF-1)

Adaptation to low oxygen tension (hypoxia) in cells and tissues leads to the transcriptional induction of a series of genes that participate in angiogenesis, iron metabolism, glucose metabolism, and cell proliferation/survival. The primary factor mediating this response is the hypoxia-inducible factor-1 (HIF-1), an oxygen-sensitive transcriptional activator. HIF-1 consists of a constitutively expressed subunit HIF-1beta and an oxygen-regulated subunit HIF-1alpha (or its paralogs HIF-2alpha and HIF-3alpha). The stability and activity of the alpha subunit of HIF are regulated by its post-translational modifications such as hydroxylation, ubiquitination, acetylation, and phosphorylation. In normoxia, hydroxylation of two proline residues and acetylation of a lysine residue at the oxygen-dependent degradation domain (ODDD) of HIF-1alpha trigger its association with pVHL E3 ligase complex, leading to HIF-1alpha degradation via ubiquitin-proteasome pathway. In hypoxia, the HIF-1alpha subunit becomes stable and interacts with coactivators such as cAMP response element-binding protein binding protein/p300 and regulates the expression of target genes. Overexpression of HIF-1 has been found in various cancers, and targeting HIF-1 could represent a novel approach to cancer therapy.

Geldanamycin treatment reduces neovascularization in a mouse model of retinopathy of prematurity

BACKGROUND

The benzoquinoid antibiotic 17-allylaminogeldanamycin (17-AAG) inhibits the Ras/Raf/MEK and PI3-Kinase signaling pathways and down-regulates vascular endothelial factor expression. Here we use a mouse model of oxygen-induced retinopathy to investigate the effect of 17-AAG on retinal neovascularization and vascular recovery.

MATERIAL AND METHODS

C57BL/6 mice were exposed to 75% oxygen from postnatal day 7 (P7) to P12 and recovered in room air thereafter. Beginning with P12 mice were treated for 5 days by daily IP injection of 17-AAG (12.5 mg/kg body weight) micro dispersed in an emulsion of 4% Lipoid EPC, 5% sucrose, and 0.9% NaCl or Wortmannin (100 microg/kg body weight). On P17, the retinal vascular and avascular area, neovascular blood vessel tufts, and main vessel tortuosity were quantified after perfusion of the mice with FITC-Concanavalin A. The mRNA levels of VEGF, angiopoietin 1 and 2 were quantified by real-time RT-PCR.

RESULTS

After 17-AAG treatment, a reduction of the vascular area was measured from 37.8 +/- 5.2% to 30.8 +/- 5.7% (P = 0.005), and an increase of the avascular area from 10.8 +/- 5.6% to 20.3 +/- 6.6% (P = 0.001). No alteration of the vascular pattern, the number of blood vessel tufts and the main vessel tortuosity was achieved by treatment with the PI-3 kinase inhibitor Wortmannin. After treatment with 17-AAG, the numbers of tufts (127.9 33.2) were different from the controls (173.7 +/- 55.2, P = 0.035), but not the main vessel tortuosity. No significant change in VEGF and angiopoietin 1 mRNA expression could be achieved with either of the treatments. Wortmannin treatment also did not change the angiopoietin 2 mRNA level, whereas the level was reduced in 17-AAG treated mice retina from 436-fold (+/- 64) to 200-fold (+/-55) (P = 0.035).

CONCLUSION

An IP injection of 17-AAG is able to reduce angioproliferative retinopathy in a mouse model for oxygen-induced retinopathy. Our data indicate that the mechanism does not involve a direct or indirect reduction of the VEGF mRNA level, but acts downstream of the VEGF pathway. Thus, 17-AAG probably does not work by PI-3 kinase inhibition but via the Ras/Raf/MEK pathway. These data underline the potential utility of tyrosine kinase inhibitors in hypoxia induced neovascularization.

HIFing the brakes: therapeutic opportunities for treatment of human malignancies

The unfortunate ability of tumor cells to survive and expand in an uncontrolled manner has captivated the attention of clinicians and basic scientists alike. The molecular mechanisms that tumor cells use to grow are the very same pathways used in normal cell growth and differentiation. One important pathway conferring a growth advantage on tumor cells is the epidermal growth factor receptor (EGFR) pathway. Signaling through the EGFR leads to activation of the phosphatidylinositol 3-kinase and Akt pathway and to increased activity of multiple effectors, including hypoxia-inducible factors (HIFs), which are cellular transcription factors involved in environmental stress response. The target genes that HIF members stimulate that are relevant to tumor growth include transcriptional activators and repressors and cytokines and growth factors, as well as their receptors. In this Perspective, findings from several recent studies are discussed in terms of their effect on the signal transducers, target genes, and tumor properties that are ultimately affected during EGFR-stimulated HIF signaling in cancer cells.

The GCN4 bZIP can bind to noncognate gene regulatory sequences

We show that a minimalist basic region/leucine zipper (bZIP) hybrid, comprising the yeast GCN4 basic region and C/EBP leucine zipper, can target mammalian and other gene regulatory sequences naturally targeted by other bZIP and basic/helix-loop-helix (bHLH) proteins. We previously reported that this hybrid, wt bZIP, is capable of sequence-specific, high-affinity binding of DNA comparable to that of native GCN4 to the cognate AP-1 and CRE DNA sites. In this work, we used DNase I footprinting and electrophoretic mobility shift assay to show that wt bZIP can also specifically target noncognate gene regulatory sequences: C/EBP (CCAAT/enhancer binding protein, 5'-TTGCGCAA), XRE1 (Xenobiotic response element, 5'-TTGCGTGA), HRE (HIF response element, 5'-GCACGTAG), and the E-box (Enhancer box, 5'-CACGTG). Although wt bZIP still targets AP-1 with strongest affinity, both DNA-binding specificity and affinity are maintained with wt bZIP binding to noncognate gene regulatory sequences: the dissociation constant for wt bZIP in complex with AP-1 is 13 nM, while that for C/EBP is 120 nM, XRE1 240 nM, and E-box and HRE are in the microM range. These results demonstrate that the bZIP possesses the versatility to bind various sequences with varying affinities, illustrating the potential to fine-tune a designed protein's affinity for its DNA target. Thus, the bZIP scaffold may be a powerful tool in design of small, alpha-helical proteins with desired DNA recognition properties.

A clock shock: mouse CLOCK is not required for circadian oscillator function

The circadian clock mechanism in the mouse is composed of interlocking transcriptional feedback loops. Two transcription factors, CLOCK and BMAL1, are believed to be essential components of the circadian clock. We have used the Cre-LoxP system to generate whole-animal knockouts of CLOCK and evaluated the resultant circadian phenotypes. Surprisingly, CLOCK-deficient mice continue to express robust circadian rhythms in locomotor activity, although they do have altered responses to light. At the molecular and biochemical levels, clock gene mRNA and protein levels in both the master clock in the suprachiasmatic nuclei and a peripheral clock in the liver show alterations in the CLOCK-deficient animals, although the molecular feedback loops continue to function. Our data challenge a central feature of the current mammalian circadian clock model regarding the necessity of CLOCK:BMAL1 heterodimers for clock function.

The Aryl Hydrocarbon Receptor Signaling Pathway Is Modified through Interactions with a Kelch Protein

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor with important roles in metabolic adaptation, dioxin toxicology, and vascular development. To understand the details of this signal transduction pathway, we have used the yeast two-hybrid system to identify proteins that physically interact with the AHR in a ligand-dependent manner. Using this strategy, we identified a novel modifier of the AHR signaling pathway that we named Ah-receptor associated protein 3 (ARA3). Coexpression of ARA3 with an AHR chimera in yeast and mammalian cells enhances signaling in response to agonists. The human full-length cDNA previously was described as influenza virus nonstructural protein-1 binding protein (NS1BP). This protein contains four apparent domains-a "broad complex/tramtrack/bric-a-brac" (BTB) domain, a "kelch" domain, a "BTB and C-terminal kelch" (BACK) domain, and an intervening region (IVR). The carboxyl terminus of the AHR "Per-ARNT-Sim" (periodicity/AHR nuclear translocator/simple-minded) domain and the BACK/IVR domains of ARA3 mediate the AHR-ARA3 interaction. The BACK/IVR domains of ARA3 also are sufficient to modify AHR signaling in yeast and mammalian cells. In an effort to provide a preliminary model of NS1BP activity in AHR signaling, we demonstrate that NS1BP regulates the concentration of functional AHR in mammalian cells.

A family with erythrocytosis establishes a role for prolyl hydroxylase domain protein 2 in oxygen homeostasis

The number of red blood cells is normally tightly regulated by a classic homeostatic mechanism based on oxygen sensing in the kidney. Decreased oxygen delivery resulting from anemia induces the production of erythropoietin, which increases red cell production and hence oxygen delivery. Investigations of erythropoietin regulation identified the transcription factor hypoxia-inducible factor (HIF). HIF is now recognized as being a key regulator of genes that function in a comprehensive range of processes besides erythropoiesis, including energy metabolism and angiogenesis. HIF itself is regulated through the alpha-subunit, which is hydroxylated in the presence of oxygen by a family of three prolyl hydroxylase domain proteins (PHDs)/HIF prolyl hydroxylases/egg-laying-defective nine enzymes. Hydroxylation allows capture by the von Hippel-Lindau tumor suppressor gene product, ubiquitination, and destruction by the proteasome. Here we describe an inherited mutation in a mammalian PHD enzyme. We show that this mutation in PHD2 results in a marked decrease in enzyme activity and is associated with familial erythrocytosis, identifying a previously unrecognized cause of this condition. Our findings indicate that PHD2 is critical for normal regulation of HIF in humans.

The aryl hydrocarbon receptor and light

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that has been intensively studied with respect to the toxicity of xenobiotics. However, its function in response to light has never been summarized. Here, we provide an overview of AhR activation by light with a focus on the role of tryptophan in light-induced AhR activation. We discuss the involvement of the AhR in different biological rhythms and speculate on the possible role of the AhR in UV-induced responses in skin. Furthermore, this review points out future research needs in this field.

Hypoxic regulation of VEGF, HIF-1(alpha) in coronary collaterals development

BACKGROUND

The interindividual variability for the development of collaterals in coronary artery disease is dependent on the hypoxic induction level of VEGF. To determine whether the hypoxic induction of VEGF is controlled by the transcription of HIF-1 (alpha), the VEGF and HIF-1 (alpha) m-RNA levels were correlated to hypoxia in monocytes harvested from patients with coronary artery disease.

METHODS

The collateral scoring system used was modified from the TIMI system. The mononuclear cell layer of the patients' blood was cultured in hypoxia (1% O2, 5% CO2, 94%N2) and normoxia (5% CO2, 95% room air) for 17 hours. The VEGF and HIF-1 (alpha) mRNA levels were measured using a RT-PCR technique. We calculated the fold inductionsof VEGF, HIF-1 (alpha) mRNA with hypoxia by dividing the hypoxic and the normoxic values.

RESULTS

We found significantly higher hypoxic inductions of VEGF m-RNA in patients with collaterals compared to patients with no collaterals. However, there was no differencein the hypoxic inductions of HIF-1 (alpha) between the two groups (VEGF m-RNA mean fold inductions 3.71 +/- 3.30 versus 1.65 +/- 0.62, p=0.012, HIF-1(alpha) mRNA 1.42 +/- 0.58 versus 1.20 +/- 0.39, p=0.165).

CONCLUSIONS

We concluded that the interindividual variability in the hypoxic inductions of VEGF m-RNA in monocytes in patients is not controlled by the transcriptional levels of HIF-1 (alpha) with hypoxia. These findings suggest that a mechanism such as the post-transcriptional modification of HIF-1(alpha) is involved in the hypoxic inductions of VEGF.

Sim1 and Sim2 are required for the correct targeting of mammillary body axons

The mammillary body (MB), and its axonal projections to the thalamus (mammillothalamic tract, MTT) and the tegmentum (mammillotegmental tract, MTEG), are components of a circuit involved in spatial learning. The bHLH-PAS transcription factors SIM1 and SIM2 are co-expressed in the developing MB. We have found that MB neurons are generated and that they survive at least until E18.5 in embryos lacking both Sim1 and Sim2 (Sim1(-/-);Sim2(-/-)). However, the MTT and MTEG are histologically absent in Sim1(-/-);Sim2(-/-) embryos, and are reduced in embryos lacking Sim1 but bearing one or two copies of Sim2, indicating a contribution of the latter to the development of MB axons. We have generated, by homologous recombination, a null allele of Sim1 (Sim1(tlz)) in which the tau-lacZ fusion gene was introduced, allowing the staining of MB axons. Consistent with the histological studies, lacZ staining showed that the MTT/MTEG is barely detectable in Sim1(tlz/tlz);Sim2(+/-) and Sim1(tlz/tlz);Sim2(-/-) brains. Instead, MB axons are splayed and grow towards the midline. Slit1 and Slit2, which code for secreted molecules that induce the repulsion of ROBO1-producing axons, are expressed in the midline at the level of the MB, whereas Robo1 is expressed in the developing MB. The expression of Rig-1/Robo3, a negative regulator of Slit signalling, is upregulated in the prospective MB of Sim1/Sim2 double mutants, raising the possibility that the growth of mutant MB axons towards the midline is caused by a decreased sensitivity to SLIT. Finally, we found that Sim1 and Sim2 act along compensatory, but not hierarchical, pathways, suggesting that they play similar roles in vivo.

Aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) expression in Baikal seal (Pusa sibirica) and association with 2,3,7,8-TCDD toxic equivalents and CYP1 expression levels

Most toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAHs) are mediated by ligand-activated aryl hydrocarbon receptor (AHR) signaling pathway. To understand the regulation mechanism of AHR and AHR nuclear translocator (ARNT) expression in wild Baikal seal (Pusa sibirica) population contaminated by PHAHs, the present study investigated hepatic mRNA expression levels of AHR and its heterodimer, ARNT genes, in association with biological index (age, gender and body weight), PHAH accumulation and expression levels of cytochrome P450 (CYP) 1A and 1B. While there was no gender difference, the AHR mRNA expression levels were increased with ages (p = 0.014) and body weights (p = 0.015), indicating that AHR expression might be affected by these biological factors. The AHR mRNA expression levels exhibited significant positive correlations with total TEQs and most of individual congener TEQs derived from polychorinated dibenzo-p-dioxins, dibenzofurans and non-ortho coplanar polychorinated biphenyls (PCBs), indicating the transcriptional up-regulation of AHR expression by these congeners. On the other hand, there was no significant correlation between individual TEQs from mono-ortho coplanar PCBs and AHR expression. These results imply the structure-related transcriptional activity of AHR among PHAHs congeners. AHR mRNA levels showed positive correlations with both CYP1A protein (p = 0.039) and CYP1A1 mRNA expression levels (p = 0.046). In contrast to AHR expression, neither the total nor individual congener TEQs influenced ARNT at the transcriptional level. ARNT mRNA showed significant negative correlations with CYP1A/1B protein (p = 0.027 and p = 0.006) and CYP1A1 mRNA expression levels (p = 0.039), implying the existence of different transcriptional regulation between AHR and ARNT genes and negative regulation by CYP1A/1B-mediated signaling pathways. The present findings may render significant insight on the basic mechanisms underlying regulation of AHR and ARNT expressions associated with biological factors and PHAH exposure in wild mammalian populations.

Differential gene up-regulation by hypoxia-inducible factor-1alpha and hypoxia-inducible factor-2alpha in HEK293T cells

Cells exposed to hypoxia respond by increasing the level of hypoxia-inducible factor-1 (HIF-1). This factor then activates a number of genes by binding to hypoxia response elements in their promoter regions. A second hypoxia-responsive factor, HIF-2, can activate many of the same genes as HIF-1. Overexpression of HIFs accompanies the pathogenesis of many tumors. It is unclear, however, as to the respective role of these factors in responsiveness to hypoxia and other stresses. To address this issue, we used microarray technology to study the genes activated in HEK293T cells by hypoxia or transfection with the alpha chain of HIF-1 (or mutant HIF-1 resistant to degradation) or HIF-2. Fifty-six genes were found to be up-regulated at least 3-fold by either hypoxia or transfection. Of these, 21 were elevated both by transfection with HIF-1alpha and with HIF-2alpha, and 14 were preferentially activated by HIF-1alpha including several involved in glycolysis. Ten genes were preferentially activated by HIF-2alpha, including two (CACNA1A and PTPRZ1) implicated in neurologic diseases. Interestingly, most HIF-2alpha-responsive genes were not substantially activated by hypoxia. An additional 10 genes were up-regulated by hypoxia but minimally activated by HIF-1alpha or HIF-2alpha transfection. Ten of the genes were studied by quantitative real-time PCR and/or by Northern blot and the results paralleled those found with microarray technology. Although confirmation in other systems will be necessary, these results indicate that whereas some genes are robustly activated by both HIF-1 and HIF-2, others can be preferentially activated by one or the other factor.

Hypoxia inducible factor (HIF) in rheumatology: low O2! See what HIF can do!

Maintenance of oxygen homoeostasis is the basic principle in cell proliferation, differentiation, survival, and function in all higher organisms. The transcription factor, HIF (hypoxia inducible factor) has a central role in oxygen homoeostasis, and is indispensably linked to energy metabolism. Abnormally reduced oxygen concentrations leading to dysfunctional cell metabolism are found in rheumatoid arthritis and hence, knowledge of the molecular adaptive responses to hypoxia and the involvement of HIF in the pathogenesis of RA are interesting.

The hypoxia-inducible factor and tumor progression along the angiogenic pathway

The hypoxia-inducible factor (HIF) is a transcription factor that plays a key role in the response of cells to oxygen levels. HIF is a heterodimer of alpha- and beta-subunits where the alpha-subunit is translated constitutively but has a very short half-life under normal oxygen concentrations. Negative regulation of the half-life and activity of the alpha-subunit is dependent on its posttranslational hydroxylation by hydroxylases that are dependent on oxygen for activity. Thus under low oxygen (hypoxic) conditions the hydroxylases are inactive and the alpha-subunit is stable and able to interact with the beta-subunit to bind and induce transcription of target genes. Hypoxic conditions are encountered in development and in disease states such as cancer. Tumors that have outstripped their blood supply become hypoxic and express high levels of HIF. HIF in turn targets genes that induce survival, glycolysis, and angiogenesis, a form of neovascularization, which ensures the tumor with a continued supply of oxygen and nutrients for further growth.

Crystal Structure and Interactions of the PAS Repeat Region of the Drosophila Clock Protein PERIOD

PERIOD proteins are central components of the Drosophila and mammalian circadian clock. Their function is controlled by daily changes in synthesis, cellular localization, phosphorylation, degradation, as well as specific interactions with other clock components. Here we present the crystal structure of a Drosophila PERIOD (dPER) fragment comprising two tandemly organized PAS (PER-ARNT-SIM) domains (PAS-A and PAS-B) and two additional C-terminal alpha helices (alphaE and alphaF). Our analysis reveals a noncrystallographic dPER dimer mediated by intermolecular interactions of PAS-A with PAS-B and helix alphaF. We show that alphaF is essential for dPER homodimerization and that the PAS-A-alphaF interaction plays a crucial role in dPER clock function, as it is affected by the 29 hr long-period perL mutation.

Contribution of NAD(P)H:quinone oxidoreductase 1 to protection against carcinogenesis, and regulation of its gene by the Nrf2 basic-region leucine zipper and the arylhydrocarbon receptor basic helix-loop-helix transcription factors

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a key enzyme involved in defence against reactive forms of oxygen and inhibition of neoplasia. Under conditions of oxidative stress, expression of NQO1 is induced, and the resulting increase in oxidoreductase protein provides the cell with multiple layers of protection against environmental insults. Firstly, the catalytic activity of NQO1 is directed towards the complete reduction and detoxication of highly reactive quinones. Secondly, the oxidoreductase maintains the endogenous lipid-soluble antioxidants, alpha-tocopherol-hydroquinone and ubiquinol in their reduced and active forms. Thirdly, NQO1 is required for the stabilisation of p53 protein in response to DNA-damaging stimuli, and it thereby influences cell fate decisions. In view of the anticarcinogenic actions of NQO1, an understanding of the mechanisms that govern its expression is desirable. The redox sensitivity of NQO1 transcription occurs through a cis-acting antioxidant response element (ARE) located within the regulatory region of the mouse, rat and human genes. This element recruits the positively acting basic leucine zipper (bZip) transcription factor NF-E2 p45-related factor 2 (Nrf2). Under normal constitutive conditions, Nrf2 associates with the cytoskeletal-binding protein Keap1, which regulates the subcellular distribution of the bZip factor and also targets it for proteasome-dependent degradation. Oxidative stress inhibits the Nrf2-Keap1 interaction, thus promoting nuclear accumulation of the transcription factor and transactivation of NQO1 and other ARE-driven genes. Mouse, rat and human NQO1 can also be induced by planar aromatic hydrocarbons through a cis-acting xenobiotic response element (XRE) located in their gene promoters. The XRE recruits the arylhydrocarbon receptor (AhR) and AhR nuclear translocator. Cross-talk may occur between Nrf2 and AhR, but the details of this process remain to be elucidated.

The biology of hypoxia: the role of oxygen sensing in development, normal function, and disease

The ability to sense and respond to changes in oxygen is essential for the survival of prokaryotic and eukaryotic organisms. Oxygen-sensing mechanisms have been developed to maintain cell and tissue homeostasis, as well as to adapt to the chronic low-oxygen conditions found in diseases such as cancer. This report on the first Keystone Meeting on the Biology of Hypoxia will summarize our current understanding of key genes and pathways involved in oxygen sensing that are required for normal development and that are dysregulated in disease states. It will also comment on future directions for this exciting field.

O2-regulated gene expression: transcriptional control of cardiorespiratory physiology by HIF-1

The cardiovascular and respiratory systems play key roles in O(2) homeostasis. Physiological responses to hypoxia involve changes in gene expression that are mediated by the transcriptional activator hypoxia-inducible factor (HIF)-1. Analysis of mice heterozygous for a knockout allele at the locus encoding the O(2)-regulated HIF-1alpha or HIF-2alpha subunit has revealed that these proteins are required for multiple physiological responses to chronic hypoxia, including erythrocytosis and pulmonary vascular remodeling. In mice with partial HIF-2alpha deficiency, hypoxia-induced expression of endothelin-1 and norepinephrine is dramatically impaired, and the mice fail to develop pulmonary hypertension after 4 wk of exposure to 10% O(2). In mice with partial HIF-1alpha deficiency, the ability of the carotid body to sense and/or respond to acute or chronic hypoxia is lost. In wild-type mice, brief episodes of intermittent hypoxia are sufficient to induce production of erythropoietin (EPO), which protects the heart against apoptosis after ischemia-reperfusion, whereas in mice with partial HIF-1alpha deficiency, intermittent hypoxia does not induce EPO production or cardiac protection. Parenteral administration of EPO to rodents is sufficient to induce dramatic protection against ischemia-reperfusion injury in the heart. Thus HIF-1 mediates critical physiological responses to hypoxia, and the elucidation of these homeostatic mechanisms may lead to novel therapies for the most common causes of mortality in the US population.

The mammalian basic helix-loop-helix/PAS family of transcriptional regulators

Basic helix-loop-helix (bHLH)/PAS proteins are critical regulators of gene expression networks underlying many essential physiological and developmental processes. These include transcriptional responses to environmental pollutants and low oxygen tension, mediated by the aryl hydrocarbon (Dioxin) receptor and hypoxia inducible factors (HIF), respectively, and controlling aspects of neural development, mediated by the single minded (SIM) proteins. bHLH proteins must dimerise to form functional DNA binding complexes and bHLH/PAS proteins are distinguished from other members of the broader bHLH superfamily by the dimerisation specificity conferred by their PAS homology domains. bHLH/PAS proteins tend to be ubiquitous, latent signal-regulated transcription factors that often recognise variant forms of the classic E-box enhancer sequence bound by other bHLH proteins. Two closely related forms of each of the hypoxia inducible factors alpha and single minded proteins and the general partner protein, aryl hydrocarbon receptor nuclear translocator (ARNT), are present in many cell types. Despite high sequence conservation within their DNA binding and dimerisation domains, and having very similar DNA recognition specificities, the homologues are functionally non-redundant and biologically essential. While the mechanisms controlling partner choice and target gene activation that determine this functional specificity are poorly understood, interactions mediated by the PAS domains are essential. Information on structures and protein/protein interactions for members of the steroid hormone/nuclear receptor superfamily has contributed to our understanding of the way these receptors function and assisted the development of highly specific agonists and antagonists. Similarly, it is anticipated that developing a detailed mechanistic and structural understanding of bHLH/PAS proteins will ultimately facilitate drug design.

Perinatal asphyxia in the rat has lifelong effects on morphology, cognitive functions, and behavior

Perinatal asphyxia (PA) is a major determinant of neurological morbidity and mortality in the neonatal period. Many studies have been investigating neurological deficits following PA, including seizures, cerebral palsy, mental retardation, as well as psychiatric deficits. Most research performed so far has been focusing on acute or subacute sequelae and has uncovered a variety of morphological, neurochemical, behavioral, and cognitive changes following PA. However, information on long-term sequelae of animals that underwent a period of PA is scanty. Perinatally asphyxiated rats at the end of their life span present with immunohistochemical and synaptic changes as well as changes in brain protein expression. Furthermore, deficits in cognitive function tested in the Morris water maze and changes in social behavior were described. In this review, we are summarizing and discussing reported effects of global PA on morphology, cognitive functions, and behavior in rats at the end of their life span.

Use of 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin as a probe to determine the relative ligand affinity of human versus mouse aryl hydrocarbon receptor in cultured cells

The aryl hydrocarbon receptor (AhR) is a ligand-induced transcription factor that is activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other related compounds, leading to toxicity. There is considerable variation in the response to TCDD among different species, and this may be correlated to differences in the AhR. Variations in the structure of the AhR could result in altered biochemical properties of the receptor, such as ligand affinity or transactivation potential. The difference between the mouse AhR b-1 allele (mAhR(b-1)) and human AhR (hAhR), in terms of their relative affinity for a photoaffinity ligand (2-azido-3-[(125)I]iodo-7,8-dibromodibenzo-p-dioxin), was assessed using both in vitro assays and assays performed directly in cell culture. Results revealed that the hAhR has a lower affinity for the photoaffinity ligand compared with mAhR(b-1). In contrast with a previous study, we found that a single amino acid (valine 381) in hAhR is responsible for the lower ligand affinity, and mutating this residue to alanine results in restoration of high ligand affinity in hAhR. In vitro ligand binding assays are limited by the low concentrations of protein in the assays, and it is not appropriate to compare ligand affinities of different receptors using this method without performing a competition assay or increasing the protein concentration in the assay. Because of the limitation of the in vitro assay, the relative ligand occupancy of mAhR(b-1) and hAhR was compared most effectively within cells, revealing that mAhR(b-1) has a 10-fold higher relative ligand affinity in cells, whereas mAhR(d) has a 2-fold higher relative ligand affinity than hAhR.

The role of von Hippel-Lindau tumor suppressor protein and hypoxia in renal clear cell carcinoma

The majority of kidney cancers are caused by the mutation of the von Hippel-Lindau (VHL) tumor suppressor gene. VHL protein (pVHL) is part of an E3 ubiquitin ligase complex called VEC that is composed of elongin B, elongin C, cullin 2, NEDD8, and Rbx1. VEC targets a hypoxia-inducible factor (HIF) transcription factor for ubiquitin-mediated destruction selectively in the presence of oxygen. In the absence of wild-type pVHL, as in VHL patients or in the majority of sporadic clear cell renal cell carcinomas, HIF-responsive genes are inappropriately activated even under normoxia. Recent insights into the molecular mechanisms regulating the function of pVHL, and thereby HIF, in the context of kidney cancer are the focus of this review.

Oxygen-dependent and tissue-specific regulation of erythropoietin gene expression

Hypoxia-inducible expression of the gene encoding for the glycoprotein hormone erythropoietin (EPO) is the paradigm of oxygen-regulated gene expression. EPO is the main regulator of red blood cell production and more than 100 years of research on the regulation of EPO production have led to the identification of a widespread cellular oxygen sensing mechanism. Central to this signaling cascade is the transcription factor complex hypoxia-inducible factor-1 (HIF-1). Meanwhile, it is known that HIF-1 controls more than 50 oxygen-dependent genes and is now recognized as the main regulator of oxygen homoeostasis in the body. In addition to hypoxic induction, expression of the EPO gene is tightly regulated in a tissue-specific manner. During ontogeny, production of EPO required for erythropoiesis is switched from the fetal liver to the kidneys. Here EPO is mainly synthesized in adulthood. Production of EPO has also been found in organs where it has nonerythropoietic functions: EPO is important for development of the brain and is neuroprotective, whereas it stimulates angiogenesis in the reproductive tract and possibly in other organs. Understanding oxygen and tissue-specific regulation of EPO production is of high relevance for physiology. Moreover, this knowledge might be useful for new therapies to treat human diseases.

Hypoxia-induced mitogenic factor has antiapoptotic action and is upregulated in the developing lung: coexpression with hypoxia-inducible factor-2alpha

Hypoxia-induced mitogenic factor (HIMF), also called FIZZ1 or RELMalpha, was a newly found cytokine. Hypoxia caused robust HIMF induction in the lung, and HIMF has potent pulmonary vasoconstrictive, proliferative, and angiogenic properties. To investigate the role of HIMF in lung development, we determined its spatial and temporal expression. From embryonic day (E)16 to postnatal day (P)28, HIMF was strongly expressed in the cytoplasm of bronchial epithelial cells, type II cells, endothelial cells, and primitive mesenchymal cells. Treatment with HIMF resulted in a significant reduction of apoptosis in cultured embryonic lung, thus revealing a previously unknown function of HIMF. Because HIMF gene is upregulated by hypoxia and contains a hypoxia-inducible transcription factor (HIF) binding site, we subsequently investigated whether HIMF was coexpressed with HIF-2alpha or HIF-1alpha. HIF-1alpha expression was temporally distinct from HIMF expression. In contrast, HIF-2alpha was present in endothelial cells, bronchial epithelial cells, and type II cells from E18 to P28. Thus, HIMF and HIF-2alpha were temporally and spatially coexpressed in the developing lung. These results indicate a role for HIMF in lung development, possibly under the control of HIF-2, and suggest that HIMF regulates apoptosis and may participate in lung alveolarization and maturation.

Hypoxia-inducible factor-1 in tumour angiogenesis

Hypoxia-inducible factor-1 (HIF-1), composed of HIF-α and HIF-β subunits, is a heterodimeric transcriptional activator. In response to hypoxia, stimulation of growth factors, and activation of oncogenes as well as carcinogens, HIF-1α is overexpressed and/or activated and targets those genes which are required for angiogenesis, metabolic adaptation to low oxygen and promotes survival. HIF-1 is critical for both physiological and pathological processes. Several dozens of putative direct HIF-1 target genes have been identified on the basis of one or more cis-acting hypoxia-response elements that contain an HIF-1 binding site. A variety of regulators including growth factors, genetic alterations, stress activators, and some carcinogens have been documented for regulation of HIF-1 in which several signaling pathways are involved depending on the stimuli and cell types. Activation of HIF-1 in combination with activated signaling pathways and regulators is implicated in tumour progression and prognosis. This review presents a summary of the structure and function of HIF-1α, and correlation among specific regulators and their signaling pathways.

HIFs and tumors--causes and consequences

For most organisms oxygen is essential fo life. When oxygen levels drop below those required to maintain the minimum physiological oxygen requirement of an organism or tissue it is termed hypoxia. To counter act possible deleterious effects of such a state, an immediate molecular response is initiated causing adaptation responses aimed at cell survival. This response is mediated by the hypoxia-inducible factor-1 (HIF-1), which is a heterodimer consisting of an alpha- and a beta-subunit. HIF-1 alpha protein is stabilized under hypoxic conditions and therefore confers selectivity to this response. Hypoxia is characteristic of tumors, mainly because of impaired blood supply resulting from abnormal growth. Over the past few years enormous progress has been made in the attempt to understand how the activation of the physiological response to hypoxia influences neoplastic growth. In this review some aspects of HIF-1 pathway activation in tumors and the consequences for pathophysiology and treatment of neoplasia are discussed.

Nur77 activated by hypoxia-inducible factor-1alpha overproduces proopiomelanocortin in von Hippel-Lindau-mutated renal cell carcinoma

Mutation in the von Hippel-Lindau (VHL) protein associated with renal cell carcinoma causes hypoxia-inducible factor (HIF) to stabilize and consequently to induce various HIF-targeting proteins. In this study, we found that proopiomelanocortin (POMC), an adrenocorticotropic hormone precursor, is up-regulated constitutively in VHL-mutated renal cell carcinoma. A critical transcription factor responsible for POMC overproduction was identified as Nur77, a member of the orphan steroid receptor superfamily. Little is known about how VHL mutation leads to activation of Nur77. We report that Nur77 is directly regulated by HIF. We show that HIF-1alpha, but not HIF-2alpha, binds to a putative HIF responsive element in the Nur77 promoter, activating the expression of Nur77. Mutation or deletion of the HIF binding site in the Nur77 promoter abrogates activation of a luciferase reporter gene under the control of Nur77 promoter by HIF-1alpha. The treatment of Nur77 antisense oligonucleotide reduces POMC transcription under hypoxic conditions. We confirmed that Nur77 and POMC are up-regulated in VHL-mutated renal cell carcinoma. In this study, we provide the first molecular evidence that Nur77 activated by HIF under hypoxic conditions regulates production of the peptide hormone precursor POMC.

Subcellular localization and regulation of hypoxia-inducible factor-2α in vascular endothelial cells

The hypoxia-inducible factors 1alpha (HIF-1alpha) and 2alpha (HIF-2alpha) have extensive structural homology and have been identified as transcription factors that mediate hypoxia-inducible gene expression through hypoxia-responsive element (HRE). They play critical roles not only in normal development, but also in tumor progression. Endothelial cells (EC) express both HIF-1alpha and -2alpha. In this study, we examined the subcellular localization of HIF-1alpha and -2alpha in bovine arterial EC (BAEC) by immunoblotting and immunocytostaining analysis and found that even under normoxic conditions, as with its heterodimeric partner ARNT, HIF-2alpha was stable, and was localized in the nucleus of BAEC differently than HIF-1alpha. HIF-2alpha might be regulated by a different mechanism than HIF-1alpha and might mediate the expression of some EC-specific genes under normoxic conditions. We further found that cardiovascular helix-loop-helix factor (CHF) 2, which had been identified as an ARNT-interacting protein, was expressed in BAEC and suppressed HRE-dependent gene expression both under normoxia and hypoxia. CHF2 might be one of the key regulators of HIF-2alpha-mediated gene expression in normoxic EC.

ARNT gene multiplicity in amphibians: characterization of ARNT2 from the frog Xenopus laevis

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a member of the Per-ARNT-Sim (PAS) protein superfamily, transcription factors that mediate the cellular responses to various developmental signals and environmental conditions. A beta-class ("partner") PAS protein, ARNT exhibits the capacity to form transcriptionally active heterodimers with several alpha-class ("sensor") proteins, including the aryl hydrocarbon receptors (AHRs), the hypoxia-inducible factors (HIFs), and the Single minded (Sim) proteins. Two genes encode different forms of ARNT in mammals: ARNT1, which is widely expressed, and ARNT2, which is limited to the brain and kidneys of adults and specific neural and branchial tissues of embryos. In contrast, fish apparently express only a single ARNT gene, although in different species, this may be either ARNT1 or ARNT2. In efforts to understand the evolution of ARNT proteins throughout the vertebrate lineage, we isolated an ARNT cDNA from early life stages of the amphibian Xenopus laevis. The encoded protein binds cognate DNA sequences in concert with mouse AHR. Phylogenetic analysis reveals that this sequence is orthologous to mammalian ARNT2 and paralogous to the recently reported X. laevis ARNT1. ARNT2 mRNA expression begins later than ARNT1 (stage 22 vs. stage 8), suggesting the two proteins play distinct roles during development. Hence, in the expression of two well-conserved ARNT paralogs with distinct expression patterns, X. laevis resembles mammals rather than fish. Diversity in the number and function of PAS proteins, including ARNT, may underlie significant species differences in developmental programming and biochemical response to environmental conditions. The identification of multiple amphibian ARNT paralogs represents an important step in the understanding of evolution and functional variation of ARNT in vertebrates.