Structure of human FIH-1 reveals a unique active site pocket and interaction sites for HIF-1 and von Hippel-Lindau

Changing story of the receptor for phosphatidylserine-dependent clearance of apoptotic cells

The phosphatidylserine receptor (PSR) was originally described as the putative receptor for phosphatidylserine, which is displayed on the outer membrane leaflet of apoptotic cells as a so-called 'eat me' signal. Since then, contradictory findings about this protein have been published. A common characteristic of all PSR loss-of-function experiments in vertebrates has been neonatal lethality accompanied by severe developmental defects. However, impairment of phagocytosis has only been detected in some of these experiments. Furthermore, several groups have shown that PSR localizes to the nucleus. Structural in silico analysis of PSR indicates that it has a JumonjiC domain, and the molecular features characteristic of Fe(II)-dependent and 2-oxoglutarate-dependent oxygenases. This review summarizes the current status of research on the PSR protein.

Protein kinase C-mediated modulation of FIH-1 expression by the homeodomain protein CDP/Cut/Cux

Under normoxia, FIH-1 (factor inhibiting HIF-1) inhibits the transcriptional activity of hypoxia-inducible factor (HIF); however, under such conditions, we observed a significant level of HIF activity in renal cell carcinoma (RCC). This phenomenon could be attributed to a decrease in the level of functional FIH that has been identified in our previous work. Nonetheless, the molecular mechanism of FIH regulation in cancer, in particular RCC, was unclear until now. In this communication, we have demonstrated that in RCC, the Cut-like homeodomain protein (CDP/Cut) is involved in FIH transcriptional regulation and is controlled by a specific signaling event involving protein kinase C (PKC) zeta. Furthermore, we have defined a unique CDP/Cut binding site on the FIH promoter. With chromatin immunoprecipitation assays, we show that CDP binds to the FIH-1 promoter in vivo and that this binding is PKC zeta dependent. Moreover, we have also defined a potential phosphorylation site in CDP (serine 987) that modulates FIH expression. CDP/Cut is a transcriptional repressor that decreases FIH-1 expression and subsequently leads to a decrease in the repressor activity of FIH-1. Without this repression, HIF activity increases, allowing for the increased transcription of the genes it regulates, such as the vascular endothelial growth factor and GLUT-1 genes. Both CDP and HIF levels are increased in several cancers and are responsible for the metastatic progression of the tumors. Taken together, our results suggest for the first time a potential connection between CDP and FIH that could lead to the development of future therapeutic interventions.

The diverse and pervasive chemistries of the alpha-keto acid dependent enzymes

The number of identified and confirmed alpha-keto acid dependent oxygenases is increasing rapidly. All of these enzymes have a relatively simple liganding arrangement for a single ferrous ion but collectively conduct a highly diverse set of chemistries. While hydroxylations and a variety of oxidation reactions have been most commonly observed, new reactions involving dealkylations, epimerizations and halogenations have recently been discovered. In this minireview we present what is known of the alpha-keto acid dependent enzymes and offer an argument that the chemistry that is unique to each enzyme occurs only after the production of a pivotal ferryl-oxo intermediate.

Non-heme dioxygenases: cellular sensors and regulators jelly rolled into one?

Members of the Fe(II)- and 2-oxoglutarate-dependent family of dioxygenases have long been known to oxidize several amino acids in various protein targets to facilitate protein folding. However, in recent years investigators have characterized several such hydroxylation modifications that serve a regulatory, rather than structural, purpose. Furthermore, the responsible enzymes seem to function directly as sensors of the cellular environment and metabolic state. For example, a cellular response pathway to low oxygen (hypoxia) is orchestrated through the actions of prolyl and asparaginyl hydroxylases that govern both the oxygen-dependent stability and transcriptional activity of the hypoxia-inducible transcription factor. Recently, a different subfamily of Fe(II)- and 2-oxoglutarate-dependent dioxygenases has been shown to carry out histone demethylation. The discovery of protein regulation via hydroxylation raises the possibility that other Fe(II)- and 2-oxoglutarate-dependent dioxygenases might also serve in a similar capacity.

Mechanistic and structural basis of stereospecific Cbeta-hydroxylation in calcium-dependent antibiotic, a daptomycin-type lipopeptide

Non-ribosomally synthesized lipopeptide antibiotics of the daptomycin type are known to contain unnatural beta-modified amino acids, which are essential for bioactivity. Here we present the biochemical and structural basis for the incorporation of 3-hydroxyasparagine at position 9 in the 11-residue acidic lipopeptide lactone calcium-dependent antibiotic (CDA). Direct hydroxylation of l-asparagine by AsnO, a non-heme Fe(2+)/alpha-ketoglutarate-dependent oxygenase encoded by the CDA biosynthesis gene cluster, was validated by Fmoc derivatization of the reaction product and LC/MS analysis. The 1.45, 1.92, and 1.66 A crystal structures of AsnO as apoprotein, Fe(2+) complex, and product complex, respectively, with (2S,3S)-3-hydroxyasparagine and succinate revealed the stereoselectivity and substrate specificity of AsnO. The comparison of native and product-complex structures of AsnO showed a lid-like region (residues F208-E223) that seals the active site upon substrate binding and shields it from sterically demanding peptide substrates. Accordingly, beta-hydroxylated asparagine is synthesized prior to its incorporation into the growing CDA peptide. The AsnO structure could serve as a template for engineering novel enzymes for the synthesis of beta-hydroxylated amino acids.

Modulation of hypoxia-inducible factor-1 alpha in cultured primary cells by intracellular ascorbate

Control of the transcription factor hypoxia inducible factor (HIF)-1 is mediated by hydroxylation by proline and asparagine hydroxylases. These enzymes require ascorbate for optimal activity, but little attention has been given to the effect of ascorbate on HIF-1 activation. Furthermore, cells in culture are ascorbate deficient. We investigated the effect of intracellular ascorbate on HIF-1alpha protein levels and on HIF-1-mediated gene expression in two human primary cell lines (umbilical vein endothelial cells and skin fibroblasts) and one human cancer cell line (A431 epithelial cells). Under normal culture conditions the cells contained no ascorbate and adding ascorbate to the medium increased intracellular concentrations in a dose-dependent manner. A basal level of HIF-1alpha detected in nonsupplemented cells under normoxic conditions disappeared when 10 microM ascorbate was added to the medium. Induction of HIF-1alpha by hypoxia (1% O(2)) or by CoCl(2) was markedly inhibited by ascorbate and loading with physiological levels resulted in almost complete reversal of HIF-1alpha stabilisation. Gene expression was similarly affected, with VEGF mRNA and GLUT-1 up-regulation being inhibited by ascorbate. Hence intracellular ascorbate is a major regulator of the hypoxic response in normal cells and optimal levels of this vitamin will have a profound effect on HIF-1-regulated processes.

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.

Optical analysis of the HIF‐1 complex in living cells by FRET and FRAP

Hypoxia-inducible factor-1 (HIF-1) coordinates the cellular response to a lack of oxygen by controlling the expression of hypoxia-inducible genes that ensure an adequate energy supply. Assembly of the HIF-1 complex by its oxygen-regulated subunit HIF-1alpha and its constitutive beta subunit also known as ARNT is the key event of the cellular genetic response to hypoxia. By two-photon microscopy, we studied HIF-1 assembly in living cells and the mobility of fluorophore-labeled HIF-1 subunits by fluorescence recovery after photobleaching. We found a significantly slower nuclear migration of HIF-1alpha than of HIF-1beta, indicating that each subunit can move independently. We applied fluorescence resonance energy transfer to calculate the nanometer distance between alpha and beta subunits of the transcriptionally active HIF-1 complex bound to DNA. Both N termini of the fluorophore-labeled HIF-1 subunits were localized as close as 6.2 nm, but even the N and C terminus of the HIF-1 complex were not further apart than 7.4 nm. Our data suggest a more compact 3-dimensional organization of the HIF complex than described so far by 2-dimensional models.

JmjC-domain-containing proteins and histone demethylation

Histone methylation has important roles in regulating gene expression and forms part of the epigenetic memory system that regulates cell fate and identity. Enzymes that directly remove methyl marks from histones have recently been identified, revealing a new level of plasticity within this epigenetic modification system. Here we analyse the evolutionary relationship between Jumonji C (JmjC)-domain-containing proteins and discuss their cellular functions in relation to their potential enzymatic activities.

Reactive oxygen species in the control of hypoxia-inducible factor-mediated gene expression

Reactive oxygen species (ROS) have long been considered as cytotoxic. However, recent evidence indicates a prominent role of ROS as signaling molecules in the response to hormones, growth and coagulation factors, cytokines and other factors as well as to changes in oxygen tension. The hypoxia-inducible transcription factors (HIFs) are key players in the cellular response to changes in oxygen tension. Recently, HIFs have also been shown to respond to the above-mentioned non-hypoxic stimuli. In this article, the role of ROS in the regulation of HIF-1 under hypoxic and non-hypoxic conditions is summarized.

Cellular oxygen sensing: Crystal structure of hypoxia-inducible factor prolyl hydroxylase (PHD2)

Cellular and physiological responses to changes in dioxygen levels in metazoans are mediated via the posttranslational oxidation of hypoxia-inducible transcription factor (HIF). Hydroxylation of conserved prolyl residues in the HIF-alpha subunit, catalyzed by HIF prolyl-hydroxylases (PHDs), signals for its proteasomal degradation. The requirement of the PHDs for dioxygen links changes in dioxygen levels with the transcriptional regulation of the gene array that enables the cellular response to chronic hypoxia; the PHDs thus act as an oxygen-sensing component of the HIF system, and their inhibition mimics the hypoxic response. We describe crystal structures of the catalytic domain of human PHD2, an important prolyl-4-hydroxylase in the human hypoxic response in normal cells, in complex with Fe(II) and an inhibitor to 1.7 A resolution. PHD2 crystallizes as a homotrimer and contains a double-stranded beta-helix core fold common to the Fe(II) and 2-oxoglutarate-dependant dioxygenase family, the residues of which are well conserved in the three human PHD enzymes (PHD 1-3). The structure provides insights into the hypoxic response, helps to rationalize a clinically observed mutation leading to familial erythrocytosis, and will aid in the design of PHD selective inhibitors for the treatment of anemia and ischemic disease.

The oxygen sensing signal cascade under the influence of reactive oxygen species

Structural and functional integrity of organ function profoundly depends on a regular oxygen and glucose supply. Any disturbance of this supply becomes life threatening and may result in severe loss of organ function. Particular reductions in oxygen availability (hypoxia) caused by respiratory or blood circulation irregularities cannot be tolerated for longer periods due to an insufficient energy supply by anaerobic glycolysis. Complex cellular oxygen sensing systems have evolved to tightly regulate oxygen homeostasis. In response to variations in oxygen partial pressure (PO2), these systems induce adaptive and protective mechanisms to avoid or at least minimize tissue damage. These various responses might be based on a range of oxygen sensing signal cascades including an isoform of the neutrophil NADPH oxidase, different electron carrier units of the mitochondrial chain such as a specialized mitochondrial, low PO2 affinity cytochrome c oxidase (aa3) and a subfamily of 2-oxoglutarate dependent dioxygenases termed HIF (hypoxia inducible factor) prolyl-hydroxylase and HIF asparaginyl hydroxylase called factor-inhibiting HIF (FIH-1). Thus, specific oxygen sensing cascades involving reactive oxygen species as second messengers may by means of their different oxygen sensitivities, cell-specific and subcellular localization help to tailor various adaptive responses according to differences in tissue oxygen availability.

The putative oncogene GASC1 demethylates tri- and dimethylated lysine 9 on histone H3

Methylation of lysine and arginine residues on histone tails affects chromatin structure and gene transcription. Tri- and dimethylation of lysine 9 on histone H3 (H3K9me3/me2) is required for the binding of the repressive protein HP1 and is associated with heterochromatin formation and transcriptional repression in a variety of species. H3K9me3 has long been regarded as a 'permanent' epigenetic mark. In a search for proteins and complexes interacting with H3K9me3, we identified the protein GASC1 (gene amplified in squamous cell carcinoma 1), which belongs to the JMJD2 (jumonji domain containing 2) subfamily of the jumonji family, and is also known as JMJD2C. Here we show that three members of this subfamily of proteins demethylate H3K9me3/me2 in vitro through a hydroxylation reaction requiring iron and alpha-ketoglutarate as cofactors. Furthermore, we demonstrate that ectopic expression of GASC1 or other JMJD2 members markedly decreases H3K9me3/me2 levels, increases H3K9me1 levels, delocalizes HP1 and reduces heterochromatin in vivo. Previously, GASC1 was found to be amplified in several cell lines derived from oesophageal squamous carcinomas, and in agreement with a contribution of GASC1 to tumour development, inhibition of GASC1 expression decreases cell proliferation. Thus, in addition to identifying GASC1 as a histone trimethyl demethylase, we suggest a model for how this enzyme might be involved in cancer development, and propose it as a target for anti-cancer therapy.

Regulation of angiogenesis by hypoxia-inducible factor 1

Hypoxia is an imbalance between oxygen supply and demand that occurs in cancer and in ischemic cardiovascular disease. Hypoxia-inducible factor 1 (HIF-1) was originally identified as the transcription factor that mediates hypoxia-induced erythropoietin expression. More recently, the delineation of molecular mechanisms of angiogenesis has revealed a critical role for HIF-1 in the regulation of angiogenic growth factors. In this review, we discuss the role of HIF-1 in developmental, adaptive and pathological angiogenesis. In addition, potential therapeutic interventions involving modulation of HIF-1 activity in ischemic cardiovascular disease and cancer will be discussed.

The zinc chelator, N,N,N′,N′-tetrakis (2-pyridylmethyl) ethylenediamine, increases the level of nonfunctional HIF-1α protein in normoxic cells

The hypoxia-inducible factor-1alpha (HIF-1alpha) subunit is activated in response to lack of oxygen. HIF-1alpha-specific prolyl hydroxylase and factor inhibiting HIF-1alpha (FIH-1) catalyze hydroxylation of the proline and asparagine residues of HIF-1alpha, respectively. The hydroxyproline then interacts with ubiquitin E3 ligase, the von Hippel-Lindau protein, leading to degradation of HIF-1alpha by ubiquitin-dependent proteasomes, while the hydroxylation of the asparagine residue prevents recruitment of the coactivator, cAMP-response element-binding protein (CBP), thereby decreasing the transactivation ability of HIF-1alpha. We found that the Zn-specific chelator, N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), enhances the activity of HIF-1alpha-proline hydroxylase 2 but the level of HIF-1alpha protein does not fall because TPEN also inhibits ubiquitination. Since the Zn chelator does not prevent FIH-1 from hydroxylating the asparagine residue of HIF-1alpha, its presence leads to the accumulation of HIF-1alpha that is both prolyl and asparaginyl hydroxylated and is therefore nonfunctional. In hypoxic cells, TPEN also prevents HIF-1alpha from interacting with CBP, so reducing expression of HIF-1alpha target genes. As a result, Zn chelation causes the accumulation of nonfunctional HIF-1alpha protein in both normoxia and hypoxia.

Reversal of histone lysine trimethylation by the JMJD2 family of histone demethylases

Histone methylation regulates chromatin structure, transcription, and epigenetic state of the cell. Histone methylation is dynamically regulated by histone methylases and demethylases such as LSD1 and JHDM1, which mediate demethylation of di- and monomethylated histones. It has been unclear whether demethylases exist that reverse lysine trimethylation. We show the JmjC domain-containing protein JMJD2A reversed trimethylated H3-K9/K36 to di- but not mono- or unmethylated products. Overexpression of JMJD2A but not a catalytically inactive mutant reduced H3-K9/K36 trimethylation levels in cultured cells. In contrast, RNAi depletion of the C. elegans JMJD2A homolog resulted in an increase in general H3-K9Me3 and localized H3-K36Me3 levels on meiotic chromosomes and triggered p53-dependent germline apoptosis. Additionally, other human JMJD2 subfamily members also functioned as trimethylation-specific demethylases, converting H3-K9Me3 to H3-K9Me2 and H3-K9Me1, respectively. Our finding that this family of demethylases generates different methylated states at the same lysine residue provides a mechanism for fine-tuning histone methylation.

Structural studies on 2-oxoglutarate oxygenases and related double-stranded β-helix fold proteins

Mononuclear non-heme ferrous iron dependent oxygenases and oxidases constitute an extended enzyme family that catalyze a wide range of oxidation reactions. The largest known sub-group employs 2-oxoglutarate as a cosubstrate and catalysis by these and closely related enzymes is proposed to proceed via a ferryl intermediate coordinated to the active site via a conserved HXD/E...H motif. Crystallographic studies on the 2-oxoglutarate oxygenases and related enzymes have revealed a common double-stranded beta-helix core fold that supports the residues coordinating the iron. This fold is common to proteins of the cupin and the JmjC transcription factor families. The crystallographic studies on 2-oxoglutarate oxygenases and closely related enzymes are reviewed and compared with other metallo-enzymes/related proteins containing a double-stranded beta-helix fold. Proposals regarding the suitability of the active sites and folds of the 2-oxoglutarate oxygenases to catalyze reactions involving reactive oxidizing species are described.

Natural products to drugs: daptomycin and related lipopeptide antibiotics

Daptomycin (Cubicin) is a lipopeptide antibiotic approved in the USA in 2003 for the treatment of skin and skin structure infections caused by Gram-positive pathogens. It is a member of the 10-membered cyclic lipopeptide family of antibiotics that includes A54145, calcium-dependent antibiotic (CDA), amphomycin, friulimicin, laspartomycin, and others. This review highlights research on this class of antibiotics from 1953 to 2005, focusing on more recent studies with particular emphasis on the interplay between structural features and antibacterial activities; chemical modifications to improve activity; the genetic organization and biosynthesis of lipopeptides; and the genetic engineering of the daptomycin biosynthetic pathway to produce novel derivatives for further chemical modification to develop candidates for clinical evaluation.

Showing your ID: intrinsic disorder as an ID for recognition, regulation and cell signaling

Regulation, recognition and cell signaling involve the coordinated actions of many players. To achieve this coordination, each participant must have a valid identification (ID) that is easily recognized by the others. For proteins, these IDs are often within intrinsically disordered (also ID) regions. The functions of a set of well-characterized ID regions from a diversity of proteins are presented herein to support this view. These examples include both more recently described signaling proteins, such as p53, alpha-synuclein, HMGA, the Rieske protein, estrogen receptor alpha, chaperones, GCN4, Arf, Hdm2, FlgM, measles virus nucleoprotein, RNase E, glycogen synthase kinase 3beta, p21(Waf1/Cip1/Sdi1), caldesmon, calmodulin, BRCA1 and several other intriguing proteins, as well as historical prototypes for signaling, regulation, control and molecular recognition, such as the lac repressor, the voltage gated potassium channel, RNA polymerase and the S15 peptide associating with the RNA polymerase S-protein. The frequent occurrence and the common use of ID regions in important protein functions raise the possibility that the relationship between amino acid sequence, disordered ensemble and function might be the dominant paradigm for the molecular recognition that serves as the basis for signaling and regulation by protein molecules.

Regulation of hypoxia-inducible factor 1 by prolyl and asparaginyl hydroxylases

Hypoxia-inducible factor 1 (HIF-1) functions as a master regulator of oxygen homeostasis by mediating a wide range of cellular and systemic adaptive physiological responses to reduced oxygen availability. In this review, we will summarize recent progress in elucidating the molecular mechanisms of HIF-1 activation, focusing on the role of oxygen-dependent prolyl and asparaginyl hydroxylases in hypoxia signal transduction.

Dioxygenases as O2-dependent regulators of the hypoxic response pathway

A ubiquitous pathway by which mammalian cells sense and respond to changes in oxygen availability relies upon the hypoxic induction of a transcription factor, HIF. HIF in turn activates the expression of an assemblage of genes promoting compensatory shifts in the capacity for anaerobic metabolism, O2 delivery, and other adaptive processes. The stability and activity of HIF are each regulated as a function of O2. Both mechanisms are directly mediated by posttranslational modification of this transcription factor: hydroxylation of proline and asparagine residues, respectively. These modifications are performed by members of the Fe(II)- and 2-oxoglutarate-dependent dioxygenase family whose activities are directly and indirectly dependent on cellular O2 levels. As such, these oxygenases fill a role as environmental and metabolic sensors, a paradigm that may extend to other biological pathways.

Studies on the specificity of unprocessed and mature forms of phytanoyl-CoA 2-hydroxylase and mutation of the iron binding ligands

The mature form of phytanoyl-coenzyme A 2-hydroxylase (PAHX), a nonheme Fe(II)- and 2-oxoglutarate-dependent oxygenase, catalyzes the alpha-hydroxylation of phytanoyl-CoA within peroxisomes. Mutations in PAHX result in some forms of adult Refsum's disease. Unprocessed PAHX (pro-PAHX) contains an N-terminal peroxisomal targeting sequence that is cleaved to give mature PAHX (mat-PAHX). Previous studies have implied a difference in the substrate specificity of the unprocessed and mature forms of PAHX. We demonstrate that both forms are able to hydroxylate a range of CoA derivatives, but under the same assay conditions, the N-terminal hexa-His-tagged unprocessed form is less active than the nontagged mature form. Analyses of the assay conditions suggest a rationale for the lack of activity previously reported for some substrates (e.g. isovaleryl-CoA) for the (His)6pro-PAHX. Site-directed mutagenesis was used to support proposals for the identity of the iron binding ligands (His-175, Asp-177, His-264) of the 2-His-1-carboxylate motif of PAHX. Mutation of other histidine residues (His-213, His-220, His-259) suggested that these residues were not involved in Fe(II) binding.

Disruption of dimerization and substrate phosphorylation inhibit factor inhibiting hypoxia-inducible factor (FIH) activity

HIF (hypoxia-inducible factor) is an alphabeta transcription factor that modulates the hypoxic response in many animals. The cellular abundance and activity of HIF-alpha are regulated by its post-translational hydroxylation. The hydroxylation of HIF is catalysed by PHD (prolyl hydroxylase domain) enzymes and FIH (factorinhibiting HIF), all of which are 2-oxoglutarate- and Fe(II)-dependent dioxygenases. FIH hydroxylates a conserved asparagine residue in HIF-alpha (Asn-803), which blocks the binding of HIF to the transcriptional co-activator p300, preventing transcription of hypoxia-regulated genes under normoxic conditions. In the present paper, we report studies on possible mechanisms for the regulation of FIH activity. Recently solved crystal structures of FIH indicate that it is homodimeric. Site-directed mutants of FIH at residues Leu-340 and Ile-344, designed to disrupt dimerization, were generated in order to examine the importance of the dimeric state in determining FIH activity. A single point mutant, L340R (Leu-340-->Arg), was shown to be predominantly monomeric and to have lost catalytic activity as measured by assays monitoring 2-oxoglutarate turnover and asparagine hydroxylation. In contrast, the I344R (Ile-344-->Arg) mutant was predominantly dimeric and catalytically active. The results imply that the homodimeric form of FIH is required for productive substrate binding. The structural data also revealed a hydrophobic interaction formed between FIH and a conserved leucine residue (Leu-795) on the HIF substrate, which is close to the dimer interface. A recent report has revealed that phosphorylation of Thr-796, which is adjacent to Leu-795, enhances the transcriptional response in hypoxia. Consistent with this, we show that phosphorylation of Thr-796 prevents the hydroxylation of Asn-803 by FIH.

Intrinsically unstructured proteins and their functions

Many gene sequences in eukaryotic genomes encode entire proteins or large segments of proteins that lack a well-structured three-dimensional fold. Disordered regions can be highly conserved between species in both composition and sequence and, contrary to the traditional view that protein function equates with a stable three-dimensional structure, disordered regions are often functional, in ways that we are only beginning to discover. Many disordered segments fold on binding to their biological targets (coupled folding and binding), whereas others constitute flexible linkers that have a role in the assembly of macromolecular arrays.

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.

The HIF pathway as a therapeutic target

Hypoxia-inducible factor (HIF) is an alpha,beta-heterodimeric transcription factor that mediates cellular responses to low oxygen concentration via the transcriptional activation of specific genes involved in both tumorogenesis and angiogenesis. Manipulation of the HIF pathway has potential use for the treatment of ischemic disease and cancer. Unlike HIF-beta, which is constitutively expressed, the levels and activity of the HIF-alpha subunit are regulated by processes involving posttranslational hydroxylation, catalyzed by Fe(II)- and 2-oxoglutarate-dependent oxygenases. This review focuses on the HIF pathway as a therapeutic target.

Many Amino Acid Substitutions in a Hypoxia-inducible Transcription Factor (HIF)-1α-like Peptide Cause Only Minor Changes in Its Hydroxylation by the HIF Prolyl 4-Hydroxylases

Three human prolyl 4-hydroxylases (P4Hs) regulate the hypoxia-inducible transcription factors (HIFs) by hydroxylating a Leu-Xaa-Xaa-Leu-Ala-Pro motif. We report here that the two leucines in the Leu-Glu-Met-Leu-Ala-Pro core motif of a 20-residue peptide corresponding to the sequence around Pro(564) in HIF-1alpha can be replaced by many residues with no or only a modest decrease in its substrate properties or in some cases even a slight increase. The glutamate and methionine could be substituted by almost any residue, eight amino acids in the former position and four in the latter being even better for HIF-P4H-3 than the wild-type residues. Alanine was by far the strictest requirement, because no residue could fully substitute for it in the case of HIF-P4H-1, and only serine or isoleucine, valine, and serine did this in the cases of HIF-P4Hs 2 and 3. Peptides with more than one substitution, having the core sequences Trp-Glu-Met-Val-Ala-Pro, Tyr-Glu-Met-Ile-Ala-Pro, Ile-Glu-Met-Ile-Ala-Pro, Trp-Glu-Met-Val-Ser-Pro, and Trp-Glu-Ala-Val-Ser-Pro were in most cases equally as good or almost as good substrates as the wild-type peptide. The acidic residues present in the 20-residue peptide also played a distinct role, but alanine substitution for any six of them, and in some combinations even three of them, had no negative effects. Substitution of the proline by 3,4-dehydroproline or l-azetidine-2-carboxylic acid, but not any other residue, led to a high rate of uncoupled 2-oxoglutarate decarboxylation with no hydroxylation. The data obtained for the three HIF-P4Hs in various experiments were in most cases similar, but in some cases HIF-P4H-3 showed distinctly different properties.

Factor inhibiting hypoxia-inducible factor (FIH) and other asparaginyl hydroxylases

FIH (Factor inhibiting hypoxia-inducible factor), an asparaginyl β-hydroxylase belonging to the super-family of 2-oxoglutarate and Fe(II)-dependent dioxygenases, catalyses hydroxylation of Asn-803 of hypoxia-inducible factor, a transcription factor that regulates the mammalian hypoxic response. Only one other asparaginyl β-hydroxylase, which catalyses hydroxylation of both aspartyl and asparaginyl residues in EGF (epidermal growth factor)-like domains, has been characterized. In the light of recent crystal structures of FIH, we compare FIH with the EGFH (EGF β-hydroxylase) and putative asparagine/asparaginyl hydroxylases. Sequence analyses imply that EGFH does not contain the HXD/E iron-binding motif characteristic of most of the 2-oxoglutarate oxygenases.

Conformational Flexibility of the C Terminus with Implications for Substrate Binding and Catalysis Revealed in a New Crystal Form of Deacetoxycephalosporin C Synthase

Deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus catalyses the oxidative ring expansion of the penicillin nucleus into the nucleus of cephalosporins. The reaction requires dioxygen and 2-oxoglutarate as co-substrates to create a reactive iron-oxygen intermediate from a ferrous iron in the active site. The active enzyme is monomeric in solution. The structure of DAOCS was determined earlier from merohedrally twinned crystals where the last four C-terminal residues (308-311) of one molecule penetrate the active site of a neighbouring molecule, creating a cyclic trimeric structure in the crystal. Shortening the polypeptide chain from the C terminus by more than four residues diminishes activity. Here, we describe a new crystal form of DAOCS in which trimer formation is broken and the C-terminal arm is free. These crystals show no signs of twinning, and were obtained from DAOCS labelled with an N-terminal His-tag. The modified DAOCS is catalytically active. The free C-terminal arm protrudes into the solvent, and the C-terminal domain (residues 268-299) is rotated by about 16 degrees towards the active site. The last 12 residues (300-311) are disordered. Structures for various enzyme-substrate and enzyme-product complexes in the new crystal form confirm overlapping binding sites for penicillin and 2-oxoglutarate. The results support the notion that 2-oxoglutarate and dioxygen need to react first to produce an oxidizing iron species, followed by reaction with the penicillin substrate. The position of the penicillin nucleus is topologically similar in the two crystal forms, but the penicillin side-chain in the new non-twinned crystals overlaps with the position of residues 304-306 of the C-terminal arm in the twinned crystals. An analysis of the interactions between the C-terminal region and residues in the active site indicates that DAOCS could also accept polypeptide chains as ligands, and these could bind near the iron.

Crystal Structure and Mechanistic Implications of 1-Aminocyclopropane-1-Carboxylic Acid Oxidase—The Ethylene-Forming Enzyme

The final step in the biosynthesis of the plant signaling molecule ethylene is catalyzed by 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO). ACCO requires bicarbonate as an activator and catalyzes the oxidation of ACC to give ethylene, CO2, and HCN. We report crystal structures of ACCO in apo-form (2.1 A resolution) and complexed with Fe(II) (2.55 A) or Co(II) (2.4 A). The active site contains a single Fe(II) ligated by three residues (His177, Asp179, and His234), and it is relatively open compared to those of the 2-oxoglutarate oxygenases. The side chains of Arg175 and Arg244, proposed to be involved in binding bicarbonate, project away from the active site, but conformational changes may allow either or both to enter the active site. The structures will form a basis for future mechanistic and inhibition studies.

Hydroxylation of HIF-1: oxygen sensing at the molecular level

The ability to sense and respond to changes in oxygenation represents a fundamental property of all metazoan cells. The discovery of the transcription factor HIF-1 has led to the identification of protein hydroxylation as a mechanism by which changes in PO2 are transduced to effect changes in gene expression.

Hypoxia upregulates the expression of the NDRG1 gene leading to its overexpression in various human cancers

Background

The expression of NDRG1 gene is induced by nickel, a transition metal sharing similar physical properties to cobalt. Nickel may create hypoxia-like conditions in cells and induce hypoxia-responsive genes, as does cobalt. Therefore NDRG1 is likely to be another gene induced by hypoxia. HIF-1 is a transcription factor which has a major role in the regulation of hypoxia-responsive genes, and thus it could be involved in the transcriptional regulation of NDRG1 gene. Hypoxia is such a common feature of solid tumours that it is of interest to investigate the expression of Ndrg1 protein in human cancers.

Results

Hypoxia and its mimetics induce in vitro expression of NDRG1 gene and cause the accumulation of Ndrg1 protein. Protein levels remain high even after cells revert to normoxia. Although HIF-1 is involved in the regulation of NDRG1, long term hypoxia induces the gene to some extent in HIF-1 knock-out cells. In the majority of human tissues studied, Ndrg1 protein is overexpressed in cancers compared to normal tissues and also reflects tumour hypoxia better than HIF-1 protein.

Conclusions

Hypoxia is an inducer of the NDRG1 gene, and nickel probably causes the induction of the gene by interacting with the oxygen sensory pathway. Hypoxic induction of NDRG1 is mostly dependent on the HIF-1 transcription factor, but HIF-1 independent pathways are also involved in the regulation of the gene during chronic hypoxia. The determination of Ndrg1 protein levels in cancers may aid the diagnosis of the disease.

Molecular genetic analysis of FIH-1, FH, and SDHB candidate tumour suppressor genes in renal cell carcinoma

BACKGROUND

Overexpression of the hypoxia inducible factor 1 (HIF-1) and HIF-2 transcription factors and the consequent upregulation of hypoxia inducible mRNAs is a feature of many human cancers and may be unrelated to tissue hypoxia. Thus, the VHL (von Hippel-Lindau) tumour suppressor gene (TSG) regulates HIF-1 and HIF-2 expression in normoxia by targeting the alpha subunits for ubiquitination and proteolysis. Inactivation of the VHL TSG in VHL tumours and in sporadic clear cell renal cell carcinoma (RCC) results in overexpression of HIF-1 and HIF-2. However, RCC without VHL inactivation may demonstrate HIF upregulation, suggesting that VHL independent pathways for HIF activation also exist. In RCC, three candidate HIF activating genes exist-FIH-1 (factor inhibiting HIF), SDHB, and FH-which may be dependent or independent of VHL inactivation.

AIMS

To investigate FIH-1, SDHB, and FH for somatic mutations in sporadic RCC.

METHODS

Gene mutation was analysed in primary RCCs (clear cell RCCs, papillary RCCs, and oncocytomas) and RCC cell lines. SDHB mutation analysis was performed by denaturing high performance liquid chromatography followed by direct sequencing of aberrant PCR products. FH and FIH-1 mutation analysis were performed by single stranded conformational polymorphism and direct sequencing of PCR products.

RESULTS

No mutations were identified in the three genes investigated.

CONCLUSIONS

There was no evidence to suggest that somatic mutations occur in the FH, FIH-1, or SDHB TSGs in sporadic RCCs.

HIF hydroxylation and cellular oxygen sensing

Hypoxia-inducible factor (HIF) is a transcriptional complex that mediates a broad range of cellular and systemic responses to hypoxia. Analysis of HIF-alpha subunits has demonstrated that its activity is regulated by a series of oxygen-dependent enzymatic hydroxylations at specific prolyl and asparaginyl residues. Combined structural/genetic approaches have identified the relevant enzymes as members of the 2-oxoglutarate-dependent dioxygenase superfamily, possessing a beta-barrel 'jelly-roll' conformation that aligns a 2-histidine/1-carboxylate iron co-ordination motif at the catalytic centre. HIF prolyl hydroxylation is performed by a closely related set of isoenzymes (PHD1-3) that differ in abundance and subcellular localisation. Hydroxylation of either human HIF-1alpha Pro402 or Pro564 promotes interaction with the von Hippel-Lindau tumour suppressor protein (pVHL). In oxygenated cells this process targets HIF-alpha for rapid proteasomal destruction. HIF asparaginyl hydroxylation is performed by a protein termed factor inhibiting HIF (FIH). In oxygenated cells hydroxylation of human HIF-1alpha Asn803 prevents interaction with the p300 transcriptional co-activator, providing a second mechanism by which HIF-mediated transcription is inactivated. Genetic studies demonstrate a critical function for both types of enzyme in regulating the HIF transcriptional cascade. Limitation of activity in hypoxia supports a central role of these hydroxylases in cellular oxygen sensing. Regulation of the amount of hydroxylase protein, and the supply of other co-substrates and co-factors, particularly the cellular availability of iron, also contribute to tuning the physiological response to hypoxia.

The phosphatidylserine receptor from Hydra is a nuclear protein with potential Fe(II) dependent oxygenase activity

BACKGROUND

Apoptotic cell death plays an essential part in embryogenesis, development and maintenance of tissue homeostasis in metazoan animals. The culmination of apoptosis in vivo is the phagocytosis of cellular corpses. One morphological characteristic of cells undergoing apoptosis is loss of plasma membrane phospholipid asymmetry and exposure of phosphatidylserine on the outer leaflet. Surface exposure of phosphatidylserine is recognised by a specific receptor (phosphatidylserine receptor, PSR) and is required for phagocytosis of apoptotic cells by macrophages and fibroblasts.

RESULTS

We have cloned the PSR receptor from Hydra in order to investigate its function in this early metazoan. Bioinformatic analysis of the Hydra PSR protein structure revealed the presence of three nuclear localisation signals, an AT-hook like DNA binding motif and a putative 2-oxoglutarate (2OG)-and Fe(II)-dependent oxygenase activity. All of these features are conserved from human PSR to Hydra PSR. Expression of GFP tagged Hydra PSR in hydra cells revealed clear nuclear localisation. Deletion of one of the three NLS sequences strongly diminished nuclear localisation of the protein. Membrane localisation was never detected.

CONCLUSIONS

Our results suggest that Hydra PSR is a nuclear 2-oxoglutarate (2OG)-and Fe(II)-dependent oxygenase. This is in contrast with the proposed function of Hydra PSR as a cell surface receptor involved in the recognition of apoptotic cells displaying phosphatidylserine on their surface. The conservation of the protein from Hydra to human infers that our results also apply to PSR from higher animals.

Substrate Requirements of the Oxygen-sensing Asparaginyl Hydroxylase Factor-inhibiting Hypoxia-inducible Factor

The hypoxia-inducible factor alpha subunits 1 and 2 (HIF-1alpha and HIF-2alpha) are subjected to oxygen-dependent asparaginyl hydroxylation, a modification that represses the carboxyl-terminal transactivation domain (CAD) at normoxia by preventing recruitment of the p300/cAMP-response element-binding protein coactivators. This hydroxylation is performed by the novel asparaginyl hydroxylase, factor-inhibiting HIF-1' (FIH-1), of which HIF-1alpha and HIF-2alpha are the only reported substrates. Here we investigated the substrate requirements of FIH-1 by characterizing its subcellular localization and by examining amino acids within the HIF-1alpha substrate for their importance in recognition and catalysis by FIH-1. Using immunohistochemistry, we showed that both endogenous and transfected FIH-1 are primarily confined to the cytoplasm and remain there under normoxia and following treatment with the hypoxia mimetic, dipyridyl. Individual alanine mutations of seven conserved amino acids flanking the hydroxylated asparagine in HIF-1alpha revealed the importance of the valine (Val-802) adjacent to the targeted asparagine. The HIF-1alpha CAD V802A mutant exhibited a 4-fold lower V(max) in enzyme assays, whereas all other mutants were hydroxylated as efficiently as the wild type HIF-1alpha CAD. Furthermore, in cell-based assays the transcriptional activity of V802A was constitutive, suggesting negligible normoxic hydroxylation in HEK293T cells, whereas the wild type and other mutants were repressed under normoxia. Molecular modeling of the HIF-1alpha CAD V802A in complex with FIH-1 predicted an alteration in asparagine positioning compared with the wild type HIF-1alpha CAD, providing an explanation for the impaired catalysis observed and confirming the importance of Val-802 in asparaginyl hydroxylation by FIH-1.

von Hippel–Lindau tumor suppressor: not only HIF's executioner

Loss of von Hippel-Lindau (VHL) protein function results in an autosomal-dominant cancer syndrome known as VHL disease, which manifests as angiomas of the retina, hemangioblastomas of the central nervous system, renal clear-cell carcinomas and pheochromocytomas. VHL tumor suppressor is a specific substrate-recognition component of the E3 ubiquitin complex, which regulates proteasomal degradation of the subunit of the hypoxia inducible transcription factor (HIF). Impaired VHL complex function leads to accumulation of HIF, overexpression of various HIF-induced gene products and formation of highly vascular neoplasia. However, the ubiquitylating role of the VHL complex extends beyond its function in regulating HIF, as it appears to regulate the stability of other proteins that might be involved in various steps of oncogenic processes.

Catalytic Properties of the Asparaginyl Hydroxylase (FIH) in the Oxygen Sensing Pathway Are Distinct from Those of Its Prolyl 4-Hydroxylases

The activity of hypoxia-inducible transcription factor HIF, an alphabeta heterodimer that has an essential role in adaptation to low oxygen availability, is regulated by two oxygen-dependent hydroxylation events. Hydroxylation of specific proline residues by HIF prolyl 4-hydroxylases targets the HIF-alpha subunit for proteasomal destruction, whereas hydroxylation of an asparagine in the C-terminal transactivation domain prevents its interaction with the transcriptional coactivator p300. The HIF asparaginyl hydroxylase is identical to a previously known factor inhibiting HIF (FIH). We report here that recombinant FIH has unique catalytic and inhibitory properties when compared with those of the HIF prolyl 4-hydroxylases. FIH was found to require particularly long peptide substrates so that omission of only a few residues from the N or C terminus of a 35-residue HIF-1alpha sequence markedly reduced its substrate activity. Hydroxylation of two HIF-2alpha peptides was far less efficient than that of the corresponding HIF-1alpha peptides. The K(m) of FIH for O(2) was about 40% of its atmospheric concentration, being about one-third of those of the HIF prolyl 4-hydroxylases but 2.5 times that of the type I collagen prolyl 4-hydroxylase. Several 2-oxoglutarate analogs were found to inhibit FIH but with distinctly different potencies from the HIF prolyl 4-hydroxylases. For example, the two most potent HIF prolyl 4-hydroxylase inhibitors among the compounds studied were the least effective ones for FIH. It should therefore be possible to develop specific small molecule inhibitors for the two enzyme classes involved in the hypoxia response.

The structural basis of cephalosporin formation in a mononuclear ferrous enzyme

Deacetoxycephalosporin-C synthase (DAOCS) is a mononuclear ferrous enzyme that transforms penicillins into cephalosporins by inserting a carbon atom into the penicillin nucleus. In the first half-reaction, dioxygen and 2-oxoglutarate produce a reactive iron-oxygen species, succinate and CO2. The oxidizing iron species subsequently reacts with penicillin to give cephalosporin and water. Here we describe high-resolution structures for ferrous DAOCS in complex with penicillins, the cephalosporin product, the cosubstrate and the coproduct. Steady-state kinetic data, quantum-chemical calculations and the new structures indicate a reaction sequence in which a 'booby-trapped' oxidizing species is formed. This species is stabilized by the negative charge of succinate on the iron. The binding sites of succinate and penicillin overlap, and when penicillin replaces succinate, it removes the stabilizing charge, eliciting oxidative attack on itself. Requisite groups of penicillin are within 1 A of the expected position of a ferryl oxygen in the enzyme-penicillin complex.

Identification and characterization of a third human, rat, and mouse collagen prolyl 4-hydroxylase isoenzyme

Collagen prolyl 4-hydroxylases (C-P4Hs) catalyze the formation of 4-hydroxyproline by the hydroxylation of -X-Pro-Gly-triplets. The vertebrate enzymes are alpha 2 beta 2 tetramers, the beta-subunit being identical to protein-disulfide isomerase (PDI). Two isoforms of the catalytic alpha-subunit, which combine with PDI to form [alpha(I)]2 beta 2 and [alpha(II)]2 beta 2 tetramers, have been known up to now. We report here on the cloning and characterization of a third vertebrate C-P4H alpha-subunit isoform, alpha(III). The processed human, rat and mouse alpha(III) polypeptides consist of 520-525 residues, all three having signal peptides of 19-22 additional residues. The sequence of the processed human alpha(III) polypeptide is 35-37% identical to those of human alpha(I) and alpha(II), the highest identity being found within the catalytically important C-terminal region and all five critical residues at the cosubstrate binding sites being conserved. The sequence within a region corresponding to the peptide-substrate binding domain is less conserved, but all five alpha helices constituting this domain can be predicted to be located in identical positions in alpha(I), alpha(II), and alpha(III) and to have essentially identical lengths. The alpha(III) mRNA is expressed in many human tissues, but at much lower levels than the alpha(I) and alpha(II) mRNAs. In contrast to alpha(I) and alpha(II), no evidence was found for alternative splicing of the alpha(III) transcripts. Coexpression of a recombinant human alpha(III) polypeptide with PDI in human embryonic kidney cells led to the formation of an active enzyme that hydroxylated collagen chains and a collagen-like peptide and appeared to be an [alpha(III)]2 beta 2 tetramer. The catalytic properties of the recombinant enzyme were very similar to those of the type I and II C-P4Hs, with the exception that its peptide binding properties were intermediate between those of the type I and type II enzymes.

Hypoxia-induced gene expression occurs solely through the action of hypoxia-inducible factor 1alpha (HIF-1alpha): role of cytoplasmic trapping of HIF-2alpha

The hypoxia-inducible factors 1alpha (HIF-1alpha) and 2alpha (HIF-2alpha) have extensive structural homology and have been identified as key transcription factors responsible for gene expression in response to hypoxia. They play critical roles not only in normal development, but also in tumor progression. Here we report on the differential regulation of protein expression and transcriptional activity of HIF-1alpha and -2alpha by hypoxia in immortalized mouse embryo fibroblasts (MEFs). We show that oxygen-dependent protein degradation is restricted to HIF-1alpha, as HIF-2alpha protein is detected in MEFs regardless of oxygenation and is localized primarily to the cytoplasm. Endogenous HIF-2alpha remained transcriptionally inactive under hypoxic conditions; however, ectopically overexpressed HIF-2alpha translocated into the nucleus and could stimulate expression of hypoxia-inducible genes. We show that the factor inhibiting HIF-1 can selectively inhibit the transcriptional activity of HIF-1alpha but has no effect on HIF-2alpha-mediated transcription in MEFs. We propose that HIF-2alpha is not a redundant transcription factor of HIF-1alpha for hypoxia-induced gene expression and show evidence that there is a cell type-specific modulator(s) that enables selective activation of HIF-1alpha but not HIF-2alpha in response to low-oxygen stress.

Structural basis for negative regulation of hypoxia-inducible factor-1alpha by CITED2

Expression of hypoxia-responsive genes is mediated by the heterodimeric transcription factor hypoxia-inducible factor-1 (HIF-1) in complex with the p300/CREB-binding protein (p300/CBP) transcriptional coactivator. The protein CITED2, which binds p300/CBP, is thought to be a negative regulator of HIF-1 transactivation. We show that the CITED2 transactivation domain (TAD) disrupts a complex of the HIF-1alpha C-terminal TAD (C-TAD) and the cysteine-histidine-rich 1 (CH1) domain of p300/CBP by binding CH1 with high affinity. The high-resolution solution structure of the CITED2 TAD-p300 CH1 complex shows that the CITED2 TAD, like the HIF-1alpha C-TAD, folds on a helical, Zn2+-containing CH1 scaffold. The CITED2 TAD binds a different, more extensive surface of CH1 than does the HIF-1alpha C-TAD. However, a conserved 'LPXL' sequence motif in CITED2 and HIF-1alpha interacts with an overlapping binding site on CH1. Mutation of the LPEL sequence in full-length CITED2 abolishes p300 binding in vivo. These findings reveal that CITED2 regulates HIF-1 by competing for a hot spot on the p300 CH1 domain.

Oxygen sensing in cancer

Hypoxia is prevalent in many tumours and is prognostically important. A transcriptional pathway controlled by hypoxia-inducible factor-1 (HIF) is also commonly up-regulated in cancer, resulting in the induction of genes with both pro- and anti-tumourigenic properties. High HIF levels may arise as a response to the tumour micro-environment or because of genetic events, including mutations affecting the von Hippel-Lindau tumour suppressor protein. Recent elucidation of mechanisms underlying the regulation of HIF, via amino acid hydroxylases, suggests a role in balancing energy production, iron metabolism and oxygen supply. Co-selection of properties linked by the HIF pathway may explain the glycolytic phenotype of tumours and underlie tumour angiogenesis, which though benefiting the tumour as a whole is unlikely to be directly selected at the clonal level because it will not give one cell specific advantage over its neighbours.

HIF and oxygen sensing; as important to life as the air we breathe?

Molecular oxygen (O2)is a basic requirement for cellular growth and viability and many aspects of anatomy and physiology are dedicated to achieving reliable distribution. Recent work has identified a specific sensing and response system, centred around a transcription complex called Hypoxia-inducible Factor 1 (HIF-1), which forms the focus of this review. The HIF-system operates in all cell types and modulates a very broad range of cellular pathways, consistent with the broad importance of oxygen. It is implicated in a rapidly expanding range of developmental, physiological and pathological settings, and is potentially relevant to almost all areas of clinical medicine. Excitingly, the pathway can be activated with low molecular weight compounds which should offer therapeutic benefit, especially in diseases where oxygen supply is compromised.