Targeting of HIF-alpha to the von Hippel-Lindau ubiquitylation complex by O2-regulated prolyl hydroxylation

Hypoxia affects expression of circadian genes PER1 and CLOCK in mouse brain

The key elements of circadian clockwork and oxygen homeostasis are the PAS protein family members PER and CLOCK and hypoxia-inducible factor 1alpha (HIF-1alpha). The PAS domain serves as an interface for protein-protein interactions. We asked whether a cross-talk exists between the PAS components of hypoxic and circadian pathways. We found several isoforms of PER1 protein that exhibit tissue-specific size differences. In the mouse brain, a predominantly nuclear 48 kDa isoform that followed a daily rhythm was observed. The 48 kDa form was found in the nuclear fractions derived from mouse liver, Swiss3T3 fibroblasts, and N2A neuroblastoma cells. In mouse kidney and human 293 kidney cells, a 55 kDa PER1 form was detected. CLOCK was observed as a predicted 100 kDa protein in rat-1 cells and in all analyzed mouse tissues including brain, liver, kidney, and spleen. In contrast to PER1, CLOCK protein expression was not rhythmic. Exposure to hypoxia led to increased PER1 and CLOCK protein levels in mice. Based on coimmunoprecipitation experiments that showed protein-protein interaction between PER1 and the alpha subunit of HIF-1, we suggest that these hypoxic effects may be modulated by HIF-1alpha.-Chilov, D., Hofer, T., Bauer, C., Wenger, R. H., Gassmann, M. Hypoxia affects expression of circadian genes PER1 and CLOCK in mouse brain.

Structure of proline 3-hydroxylase. Evolution of the family of 2-oxoglutarate dependent oxygenases

Iron (II)/2-oxoglutarate (2-OG)-dependent oxygenases catalyse oxidative reactions in a range of metabolic processes including the hydroxylation of proline and lysine residues during the post-translational modification of collagen. 2-OG oxygenases commonly require ascorbate for full activity. In the vitamin C deficient disease, scurvy, reduced activity of 2-OG oxygenases results in impaired formation of collagen. Here we report the crystal structure of bacterial proline 3-hydroxylase from Streptomyces sp., an enzyme which hydroxylates proline at position 3, the first of a 2-OG oxygenase catalysing oxidation of a free alpha-amino acid. Structures were obtained for the enzyme in the absence of iron (to 2.3A resolution, R=20.2%, Rfree=25.3%) and that complexed to iron (II) (to 2.4A resolution, R=19.8%, Rfree=22.6%). The structure contains conserved motifs present in other 2-OG oxygenases including a 'jelly roll' beta strand core and residues binding iron and 2-oxoglutarate, consistent with divergent evolution within the extended family. The structure differs significantly from many other 2-OG oxygenases in possessing a discrete C-terminal helical domain. Analysis of the structure suggests a model for proline binding and a mechanism for uncoupling of proline and 2-OG turnover.

Novel protein domains and repeats in Drosophila melanogaster: insights into structure, function, and evolution

Sequence database searching methods such as BLAST, are invaluable for predicting molecular function on the basis of sequence similarities among single regions of proteins. Searches of whole databases however, are not optimized to detect multiple homologous regions within a single polypeptide. Here we have used the prospero algorithm to perform self-comparisons of all predicted Drosophila melanogaster gene products. Predicted repeats, and their homologs from all species, were analyzed further to detect hitherto unappreciated evolutionary relationships. Results included the identification of novel tandem repeats in the human X-linked retinitis pigmentosa type-2 gene product, repeated segments in cystinosin, associated with a defect in cystine transport, and 'nested' homologous domains in dysferlin, whose gene is mutated in limb girdle muscular dystrophy. Novel signaling domain families were found that may regulate the microtubule-based cytoskeleton and ubiquitin-mediated proteolysis, respectively. Two families of glycosyl hydrolases were shown to contain internal repetitions that hint at their evolution via a piecemeal, modular approach. In addition, three examples of fruit fly genes were detected with tandem exons that appear to have arisen via internal duplication. These findings demonstrate how completely sequenced genomes can be exploited to further understand the relationships between molecular structure, function, and evolution.

Mechanism, origin, and evolution of anoxia tolerance in animals

Organisms vary widely in their tolerance to conditions of limiting oxygen supply to their cells and tissues. A unifying framework of hypoxia tolerance is now available that is based on information from cell-level models from highly anoxia-tolerant species, such as the aquatic turtle, and from other more hypoxia-sensitive systems. The response of hypoxia-tolerant systems to oxygen lack occurs in two (defense and rescue) phases. The first lines of defense against hypoxia include a drastic, if balanced, suppression of ATP demand and supply pathways; this regulation allows ATP levels to remain constant, even while ATP turnover rates greatly decline. The ATP requirements of ion pumping are down-regulated by generalized 'channel' arrest in hepatocytes and by the arrest of specific ion channels in neurons. In hepatocytes, the ATP demands of protein synthesis are down-regulated on exposure to hypoxia by an immediate global blockade of the process (probably through translational arrest caused by complexing between polysomes and elongation factors). In hypoxia-sensitive cells, this translational arrest seems irreversible, but hypoxia-tolerant systems activate 'rescue' mechanisms if the period of oxygen lack is extended by preferentially regulating the expression of several proteins. In these cells, a cascade of processes underpinning hypoxia rescue and defense begins with an oxygen sensor (a heme protein) and a signal transduction pathway that leads to the specific activation of some genes (increased expression of several proteins) and to specific down-regulation of other genes (decreased expression of several other proteins). The functional roles of the oxygen-sensing and signal-transduction system include significant gene-based metabolic reprogramming - the rescue process - with maintained down-regulation of energy demand and supply pathways in metabolism throughout the hypoxic period. We consider that, through this recent work, it is becoming evident how normoxic-maintenance ATP turnover rates can be down-regulated by an order of magnitude or more - to a new hypometabolic steady state, which is prerequisite for surviving prolonged hypoxia or anoxia. Because the phylogenies of the turtles and of fishes are well known, we are now in an excellent position to assess conservative vs. adaptable features in the evolution of the above hypoxia-response physiology in these two specific animal lineages.

APC2 Cullin protein and APC11 RING protein comprise the minimal ubiquitin ligase module of the anaphase-promoting complex

In mitosis, the anaphase-promoting complex (APC) regulates the onset of sister-chromatid separation and exit from mitosis by mediating the ubiquitination and degradation of the securin protein and mitotic cyclins. With the use of a baculoviral expression system, we have reconstituted the ubiquitin ligase activity of human APC. In combination with Ubc4 or UbcH10, a heterodimeric complex of APC2 and APC11 is sufficient to catalyze the ubiquitination of human securin and cyclin B1. However, the minimal APC2/11 ubiquitin ligase module does not possess substrate specificity, because it also ubiquitinates the destruction box deletion mutants of securin and cyclin B1. Both APC11 and UbcH10 bind to the C-terminal cullin homology domain of APC2, whereas Ubc4 interacts with APC11 directly. Zn(2+)-binding and mutagenesis experiments indicate that APC11 binds Zn(2+) at a 1:3 M ratio. Unlike the two Zn(2+) ions of the canonical RING-finger motif, the third Zn(2+) ion of APC11 is not essential for its ligase activity. Surprisingly, with Ubc4 as the E2 enzyme, Zn(2+) ions alone are sufficient to catalyze the ubiquitination of cyclin B1. Therefore, the Zn(2+) ions of the RING finger family of ubiquitin ligases may be directly involved in catalysis.

Dissecting hypoxia-dependent and hypoxia-independent steps in the HIF-1alpha activation cascade: implications for HIF-1alpha gene therapy

The heterodimeric hypoxia-inducible factor (HIF)-1 is a master transcriptional regulator of oxygen homeostasis and a possible target for gene therapy of ischemic disease. Although the role of oxygen concentration in HIF-1a protein stabilization is well established, it is less clear whether and how oxygen-regulated mechanisms contribute to HIF-1a protein modifications, nuclear translocation, heterodimerization with the b-subunit, recruitment of cofactors, and gene trans-activation. Because the HIF-1a protein is proteolytically degraded under normoxic conditions, we established two HeLa Tet-Off cell lines (HT42 and HT43), which inducibly overexpress high levels of HIF-1a under normoxic conditions, allowing to distinguish hypoxia-dependent from hypoxia-independent activation mechanisms. Using these cells, we found that normoxically induced HIF-1a is localized to the nucleus, binds DNA, and trans-activates reporter and endogenous target genes. The levels of p53 expression remained unaffected. The MAP kinase inhibitor PD98059 attenuated HIF-1a protein modifications and trans-activation ability but not protein stabilization and DNA-binding activity. Because overexpressed HIF-1a is fully localized to the nucleus but displays only partial DNA-binding and trans-activation activity, mitogen-activated protein kinase-dependent phosphorylation might be required for full HIF-1 activation. HIF-1a protein was also overexpressed in vivo, following the transplantation of HT42 cells into nude mice, demonstrating the feasibility of HIF-1a gene transfer.

Selection of mutant CHO cells with constitutive activation of the HIF system and inactivation of the von Hippel-Lindau tumor suppressor

Hypoxia-inducible factor (HIF) mediates a widespread transcriptional response to hypoxia through binding to cis-acting DNA sequences termed hypoxia response elements (HREs). Activity of the transcriptional complex is suppressed in the presence of oxygen by processes that include the targeting of HIF-alpha subunits for ubiquitin-mediated proteolysis. To provide further insights into these processes we constructed Chinese hamster ovary (CHO) cells bearing stably integrated plasmids that expressed HRE-linked surface antigens and used these cells in genetic screens for mutants that demonstrated constitutive up-regulation of HRE activity. From mutagenized cultures, clones were isolated that demonstrated up-regulation of HRE activity and increased HIF-1alpha protein levels in normoxic culture. Transfection and cell fusion studies suggested that these cells possess recessive defects that affect one or more pathways involved in HIF-alpha proteolysis. Two lines were demonstrated to harbor truncating mutations in the von Hippel-Lindau (VHL) tumor suppressor gene. In these cells, defects in ubiquitylation of exogenous human HIF-1alpha in vitro could be complemented by wild type pVHL, and re-expression of a wild type VHL gene restored a normal pattern of HIF/HRE activity, demonstrating the critical dependence of HIF regulation on pVHL in CHO cells. In contrast, other mutant cells had no demonstrable mutation in the VHL gene, and ubiquitylated exogenous HIF-1alpha normally, suggesting that they contain defects at other points in the oxygen-regulated processing of HIF-alpha subunits.

The von Hippel-Lindau tumor suppressor protein mediates ubiquitination of activated atypical protein kinase C

The von Hippel-Lindau tumor-suppressor protein (pVHL) forms a protein complex (VCB-Cul2) with elongin C, elongin B, Cul-2, and Rbx1, which functions as a ubiquitin-protein ligase (E3). The alpha-subunits of the hypoxia-inducible factors have been identified as targets for the VCB-Cul2 ubiquitin ligase. However, a variety of cellular defects caused by the depletion of pVHL cannot be explained solely by the ubiquitin-mediated degradation of hypoxia-inducible factor-alpha. We show here that a member of the atypical protein kinase C (PKC) group, PKClambda, is ubiquitinated by the pVHL-containing E3 enzyme. An active PKClambda mutant is ubiquitinated more extensively than wild-type PKClambda in HEK293 cells, and the ubiquitination is further enhanced by the overexpression of pVHL. The activation of wild-type PKClambda by serum stimulation of cells enhances the ubiquitination of the protein, supporting the notion that active PKClambda is preferentially ubiquitinated by VCB-Cul2 ubiquitin ligase. Furthermore, we show that PKClambda can be ubiquitinated in vitro in a cell-free ubiquitination assay using purified recombinant components including VCB-Cul2. Given the known function of aPKC in the regulation of cell polarity and cell growth, PKClambda may be a target of pVHL in its function as a tumor suppressor.

Regulation of vascular endothelial growth factor expression by advanced glycation end products

Advanced glycation end products (AGEs) are generated during long term diabetes and are correlated with the development of diabetic complications, such as retinopathy. Diabetic retinopathy is characterized by an increased retinal neovascularization due to the action of the angiogenic factor, vascular endothelial growth factor (VEGF). In this report, we show that injection of insulin and glycated albumin (Alb-AGE) to mice increases VEGF mRNA expression in eyes. Insulin and Alb-AGE stimulate VEGF mRNA and protein expression in retinal epithelial cells (ARPE-19). Alb-AGE-induced VEGF expression is not modulated by the use of antioxidants, N-acetyl-l-cysteine or pyrrolidinedithiocarbamate, or by an inhibitor of phosphatidylinositol 3-kinase (PI3K), wortmannin. However, using an inhibitor of ERK activation, U0126, we show that Alb-AGE stimulates VEGF expression through an ERK-dependent pathway. Accordingly, we found that Alb-AGE activated mitogen-activate protein kinase, ERK1/2, JNK1/2, but not p38, and that Alb-AGE did not activate PI3K and PKB. Moreover, Alb-AGE activated the transcription factor, hypoxia inducible factor-1 (HIF-1) DNA binding activity. This activation is mediated by an increase in accumulation of the HIF-1alpha protein through an ERK-dependent pathway. Thus, stimulation of VEGF expression by Alb-AGE, through the activation of HIF-1, could play an important role in the development of diabetic retinopathy.

A conserved family of prolyl-4-hydroxylases that modify HIF

Mammalian cells respond to changes in oxygen availability through a conserved pathway that is regulated by the hypoxia-inducible factor (HIF). The alpha subunit of HIF is targeted for degradation under normoxic conditions by a ubiquitin-ligase complex that recognizes a hydroxylated proline residue in HIF. We identified a conserved family of HIF prolyl hydoxylase (HPH) enzymes that appear to be responsible for this posttranslational modification. In cultured mammalian cells, inappropriate accumulation of HIF caused by forced expression of the HIF-1alpha subunit under normoxic conditions was attenuated by coexpression of HPH. Suppression of HPH in cultured Drosophila melanogaster cells by RNA interference resulted in elevated expression of a hypoxia-inducible gene (LDH, encoding lactate dehydrogenase) under normoxic conditions. These findings indicate that HPH is an essential component of the pathway through which cells sense oxygen.

Mammalian PASKIN, a PAS-serine/threonine kinase related to bacterial oxygen sensors

The PAS domain is a versatile protein fold found in many archaeal, bacterial, and plant proteins capable of sensing environmental changes in light intensity, oxygen concentration, and redox potentials. The oxygen sensor FixL from Rhizobium species contains a heme-bearing PAS domain and a histidine kinase domain that couples sensing to signaling. We identified a novel mammalian PAS protein (PASKIN) containing a domain architecture resembling FixL. PASKIN is encoded by an evolutionarily conserved single-copy gene which is ubiquitously expressed. The human PASKIN and mouse Paskin genes show a conserved intron-exon structure and share their promoter regions with another ubiquitously expressed gene that encodes a regulator of protein phosphatase-1. The 144-kDa PASKIN protein contains a PAS region homologous to the FixL PAS domain and a serine/threonine kinase domain which might be involved in signaling. Thus, PASKIN is likely to function as a mammalian PAS sensor protein.

Auxin signaling: derepression through regulated proteolysis

Auxins are a class of phytohormones implicated in virtually every aspect of plant growth and development. Many early plant responses to auxin are apparently mediated by members of a family of Aux/IAA proteins that dimerize with and inhibit members of the auxin response factor (ARF) family of transcription factors. Aux/IAA proteins are unstable, and their degradation is triggered by a ubiquitin-protein ligase that is regulated by modification with a ubiquitin-related protein. Recent genetic and biochemical evidence indicates that auxin accelerates the degradation of the already short-lived Aux/IAA proteins to derepress transcription by ARF proteins. Several pieces of the auxin-signaling puzzle remain to be assembled, including the proteins that initially bind auxin, the proteins that convey this signal to the protein degradation machinery, and the targets of the transcriptional derepression.

The proteasome in cancer biology and treatment

During the last 30 years, investigation of the transcriptional and translational mechanisms of gene regulation has been a major focus of molecular cancer biology. More recently, it has become evident that cancer-related mutations and cancer-related therapies also can affect post-translational processing of cellular proteins and that control exerted at this level can be critical in defining both the cancer phenotype and the response to therapeutic intervention. One post-translational mechanism that is receiving considerable attention is degradation of intracellular proteins through the multicatalytic 26S proteasome. This follows growing recognition of the fact that protein degradation is a well-regulated and selective process that can differentially control intracellular protein expression levels. The proteasome is responsible for the degradation of all short-lived proteins and 70-90% of all long-lived proteins, thereby regulating signal transduction through pathways involving factors such as AP1 and NFKB, and processes such as cell cycle progression and arrest, DNA transcription, DNA repair/misrepair, angiogenesis, apoptosis/survival, growth and development, and inflammation and immunity, as well as muscle wasting (e.g. in cachexia and sepsis). In this review, we discuss the potential involvement of the proteasome in both cancer biology and cancer treatment.

Hypoxia: the tumor's gateway to progression along the angiogenic pathway

Decreased aerobic (hypoxic) conditions in tumors induce the release of cytokines that promote vascularization and thereby enhance tumor growth and metastasis. Recent major advances have provided insight into the role hypoxia plays in cancer biology. The domain structure of the hypoxia-inducible factor 1alpha (HIF-1alpha) has been elucidated, as has the mechanism by which stabilization of HIF-1alpha leads to initiation of the transcription of target genes involved in growth of blood vessels.

Regulation of the hypoxia-inducible factor 1alpha by the inflammatory mediators nitric oxide and tumor necrosis factor-alpha in contrast to desferroxamine and phenylarsine oxide

Hypoxic/ischemic conditions provoke activation of the hypoxia-inducible factor-1 (HIF-1), which functions as a transcription factor. HIF-1 is composed of the HIF-1alpha and -beta subunits, and stability regulation occurs via accumulation/degradation of HIF-1alpha with the notion that a prolyl hydroxylase accounts for changes in protein level. In addition, there is evidence that HIF-1 is up-regulated by diverse agonists during normoxia. We investigated the impact of inflammatory mediators nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) on HIF-1alpha regulation. For comparison, LLC-PK(1) cells were exposed to hypoxia, stimulated with desferroxamine (DFX, known to mimic hypoxia), and the thiol-cross-linking agent phenylarsine oxide (PAO). Although all stimuli elicited HIF-1alpha stabilization with differences in the time-dependent accumulation pattern, significant variations appeared with regard to signaling. With the use of a superoxide anion (O(2-)) generator, we established an O(2-)-sensitive pathway that blocked HIF-1alpha stabilization in response to NO and TNF-alpha while DFX- and PAO-evoked HIF-1alpha stabilization appeared O(2-)-insensitive. NO and TNF-alpha signaling required phosphorylation events, especially activation of the phosphatidylinositol 3-kinase/Akt, which is in contrast to DFX and PAO. Based on HIF-1-dependent luciferase reporter gene analysis, it was found that, in contrast to NO and TNF-alpha, PAO resembled a stimulus that induced a dysfunctional HIF-1 complex. These data indicate that diverse agonists activate HIF-1alpha under normoxic conditions by employing different signaling pathways.

FIH-1: a novel protein that interacts with HIF-1alpha and VHL to mediate repression of HIF-1 transcriptional activity

Hypoxia-inducible factor 1 (HIF-1) is a master regulator of oxygen homeostasis that controls angiogenesis, erythropoiesis, and glycolysis via transcriptional activation of target genes under hypoxic conditions. O(2)-dependent binding of the von Hippel-Lindau (VHL) tumor suppressor protein targets the HIF-1alpha subunit for ubiquitination and proteasomal degradation. The activity of the HIF-1alpha transactivation domains is also O(2) regulated by a previously undefined mechanism. Here, we report the identification of factor inhibiting HIF-1 (FIH-1), a protein that binds to HIF-1alpha and inhibits its transactivation function. In addition, we demonstrate that FIH-1 binds to VHL and that VHL also functions as a transcriptional corepressor that inhibits HIF-1alpha transactivation function by recruiting histone deacetylases. Involvement of VHL in association with FIH-1 provides a unifying mechanism for the modulation of HIF-1alpha protein stabilization and transcriptional activation in response to changes in cellular O(2) concentration.

C. elegans EGL-9 and mammalian homologs define a family of dioxygenases that regulate HIF by prolyl hydroxylation

HIF is a transcriptional complex that plays a central role in mammalian oxygen homeostasis. Recent studies have defined posttranslational modification by prolyl hydroxylation as a key regulatory event that targets HIF-alpha subunits for proteasomal destruction via the von Hippel-Lindau ubiquitylation complex. Here, we define a conserved HIF-VHL-prolyl hydroxylase pathway in C. elegans, and use a genetic approach to identify EGL-9 as a dioxygenase that regulates HIF by prolyl hydroxylation. In mammalian cells, we show that the HIF-prolyl hydroxylases are represented by a series of isoforms bearing a conserved 2-histidine-1-carboxylate iron coordination motif at the catalytic site. Direct modulation of recombinant enzyme activity by graded hypoxia, iron chelation, and cobaltous ions mirrors the characteristics of HIF induction in vivo, fulfilling requirements for these enzymes being oxygen sensors that regulate HIF.

HIF-1, O(2), and the 3 PHDs: how animal cells signal hypoxia to the nucleus

Hypoxia-inducible factor 1 (HIF-1) is a global regulator of cellular and systemic O(2) homeostasis in animals. A molecular basis for O(2)-regulated expression of the HIF-1 alpha subunit has now been determined, providing a mechanism for changes in gene expression in response to changes in cellular oxygenation.

Induction of hypervascularity without leakage or inflammation in transgenic mice overexpressing hypoxia-inducible factor-1alpha

Hypoxia-inducible factor-1alpha (HIF-1alpha) transactivates genes required for energy metabolism and tissue perfusion and is necessary for embryonic development and tumor explant growth. HIF-1alpha is overexpressed during carcinogenesis, myocardial infarction, and wound healing; however, the biological consequences of HIF-1alpha overexpression are unknown. Here, transgenic mice expressing constitutively active HIF-1alpha in epidermis displayed a 66% increase in dermal capillaries, a 13-fold elevation of total vascular endothelial growth factor (VEGF) expression, and a six- to ninefold induction of each VEGF isoform. Despite marked induction of hypervascularity, HIF-1alpha did not induce edema, inflammation, or vascular leakage, phenotypes developing in transgenic mice overexpressing VEGF cDNA in skin. Remarkably, blood vessel leakage resistance induced by HIF-1alpha overexpression was not caused by up-regulation of angiopoietin-1 or angiopoietin-2. Hypervascularity induced by HIF-1alpha could improve therapy of tissue ischemia.

Independent function of two destruction domains in hypoxia-inducible factor-alpha chains activated by prolyl hydroxylation

Oxygen-dependent proteolytic destruction of hypoxia-inducible factor-alpha (HIF-alpha) subunits plays a central role in regulating transcriptional responses to hypoxia. Recent studies have defined a key function for the von Hippel-Lindau tumour suppressor E3 ubiquitin ligase (VHLE3) in this process, and have defined an interaction with HIF-1 alpha that is regulated by prolyl hydroxylation. Here we show that two independent regions within the HIF-alpha oxygen-dependent degradation domain (ODDD) are targeted for ubiquitylation by VHLE3 in a manner dependent upon prolyl hydroxylation. In a series of in vitro and in vivo assays, we demonstrate the independent and non-redundant operation of each site in regulation of the HIF system. Both sites contain a common core motif, but differ both in overall sequence and in the conditions under which they bind to the VHLE3 ligase complex. The definition of two independent destruction domains implicates a more complex system of pVHL-HIF-alpha interactions, but reinforces the role of prolyl hydroxylation as an oxygen-dependent destruction signal.

Hypoxia regulation of gene transcription

The adaptive responses to hypoxia include the transcriptional activation of various genes like those encoding for glycolytic enzymes, growth factors and vasoactive peptides that tend to ameliorate the damaging effect of the lack of oxygen. Most of these genes are regulated by the hypoxia-inducible factor 1 complex (HIF-1), a heterodimer protein complex that activates transcription through binding to specific hypoxic-responsive sequences (HRE) present in those genes. Hypoxia induces HIF-1 complex formation by stabilizing the HIF-1alpha sub-unit, which under normoxic conditions is degraded by the ubiquitin-proteasome system. The molecular mechanisms by which cells sense the hypoxic signal and transduce the hypoxic response are still not clear. The more accepted models for oxygen sensing involve the participation of heme-proteins sensors or are based in redox-reactions which may or not involve participation of mitochondria.

HIF-1alpha binding to VHL is regulated by stimulus-sensitive proline hydroxylation

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a global transcriptional regulator of the hypoxic response. Under normoxic conditions, HIF-1alpha is recognized by the von Hippel-Lindau tumor-suppressor protein (VHL), a component of an E3 ubiquitin ligase complex. This interaction thereby promotes the rapid degradation of HIF-1alpha. Under hypoxic conditions, HIF-1alpha is stabilized. We have previously shown that VHL binds in a hypoxia-sensitive manner to a 27-aa segment of HIF-1alpha, and that this regulation depends on a posttranslational modification of HIF-1alpha. Through a combination of in vivo coimmunoprecipitation assays using VHL and a panel of point mutants of HIF-1alpha in this region, as well as MS and in vitro binding assays, we now provide evidence that this modification, which occurs under normoxic conditions, is hydroxylation of Pro-564 of HIF-1alpha. The data furthermore show that this proline hydroxylation is the primary regulator of VHL binding.

Muf1, a novel Elongin BC-interacting leucine-rich repeat protein that can assemble with Cul5 and Rbx1 to reconstitute a ubiquitin ligase

The heterodimeric Elongin BC complex has been shown to interact in vitro and in mammalian cells with a conserved BC-box motif found in a growing number of proteins including RNA polymerase II elongation factor Elongin A, SOCS-box proteins, and the von Hippel-Lindau (VHL) tumor suppressor protein. Recently, the VHL-Elongin BC complex was found to interact with a module composed of Cullin family member Cul2 and RING-H2 finger protein Rbx1 to reconstitute a novel E3 ubiquitin ligase that activates ubiquitylation by the E2 ubiquitin-conjugating enzymes Ubc5 and Cdc34. In the context of the VHL ubiquitin ligase, Elongin BC functions as an adaptor that links the VHL protein to the Cul2/Rbx1 module, raising the possibility that the Elongin BC complex could function as an integral component of a larger family of E3 ubiquitin ligases by linking alternative BC-box proteins to Cullin/Rbx1 modules. In this report, we describe identification and purification from rat liver of a novel leucine-rich repeat-containing BC-box protein, MUF1, which we demonstrate is capable of assembling with a Cullin/Rbx1 module containing the Cullin family member Cul5 to reconstitute ubiquitin ligase activity. In addition, we show that the additional BC-box proteins Elongin A, SOCS1, and WSB1 are also capable of assembling with the Cul5/Rbx1 module to reconstitute potential ubiquitin ligases. Taken together, our findings identify MUF1 as a new member of the BC-box family of proteins, and they predict the existence of a larger family of Elongin BC-based E3 ubiquitin ligases.

Regulation of hypoxia-inducible factor is preserved in the absence of a functioning mitochondrial respiratory chain

Hypoxia-inducible factor (HIF) mediates a large number of transcriptional responses to hypoxia and has an important role in processes that include angiogenesis and erythropoiesis. The HIF DNA binding complex consists of 2 basic-helix-loop-helix PAS proteins designated alpha and beta subunits. Regulation occurs principally through the alpha subunits, which are stabilized and activated in hypoxia. Although substantial evidence implicates reactive oxygen species (ROS) in the regulatory process, the precise mechanisms remain unclear. Mitochondria are an important source of ROS, and in one model it has been proposed that hypoxia increases the generation of ROS at complex III in the mitochondrion and that this signal acts through a transduction pathway to stabilize HIF-1alpha and to activate HIF. To test this model the induction of the HIF-1alpha subunit and the HIF target gene, glucose-transporter-1, was examined in a variety of mutant cells that lacked mitochondrial DNA (rho0) or had other genetic defects in mitochondrial respiration. HIF induction by hypoxia was essentially normal in all cells tested. Hydrogen peroxide production was measured by the luminol/peroxidase method and found to be reduced in rho0 versus wild-type cells and reduced by hypoxia in both rho0 and wild-type cells. Furthermore, concentrations of rotenone that maximally inhibited respiration did not affect HIF activation by hypoxia. These data do not support the model outlined above and indicate that a functional respiratory chain is not necessary for the regulation of HIF by oxygen.

Ubiquitination of a new form of alpha-synuclein by parkin from human brain: implications for Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive accumulation in selected neurons of protein inclusions containing alpha-synuclein and ubiquitin. Rare inherited forms of PD are caused by autosomal dominant mutations in alpha-synuclein or by autosomal recessive mutations in parkin, an E3 ubiquitin ligase. We hypothesized that these two gene products interact functionally, namely, that parkin ubiquitinates alpha-synuclein normally and that this process is altered in autosomal recessive PD. We have now identified a protein complex in normal human brain that includes parkin as the E3 ubiquitin ligase, UbcH7 as its associated E2 ubiquitin conjugating enzyme, and a new 22-kilodalton glycosylated form of alpha-synuclein (alphaSp22) as its substrate. In contrast to normal parkin, mutant parkin associated with autosomal recessive PD failed to bind alphaSp22. In an in vitro ubiquitination assay, alphaSp22 was modified by normal but not mutant parkin into polyubiquitinated, high molecular weight species. Accordingly, alphaSp22 accumulated in a non-ubiquitinated form in parkin-deficient PD brains. We conclude that alphaSp22 is a substrate for parkin's ubiquitin ligase activity in normal human brain and that loss of parkin function causes pathological alphaSp22 accumulation. These findings demonstrate a critical biochemical reaction between the two PD-linked gene products and suggest that this reaction underlies the accumulation of ubiquitinated alpha-synuclein in conventional PD.

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) escapes O(2)-driven proteasomal degradation irrespective of its subcellular localization: nucleus or cytoplasm

Eukaryotic cells sense oxygen and adapt to hypoxia by regulating a number of genes. Hypoxia-inducible factor 1 (HIF-1) is the 'master' in this pleiotypic response. HIF-1 comprises two members of the basic helix--loop--helix transcription factor family, HIF-1 alpha and HIF-1 beta. The HIF-1 alpha protein is subject to drastic O(2)-dependent proteasomal control. However, the signalling components regulating the 'switch' for 'escaping' proteasomal degradation under hypoxia are still largely unknown. The rapid nuclear translocation of HIF-1 alpha could represent an efficient way to escape from this degradation. We therefore asked, where in the cell is HIF-1 alpha degraded? To address this question, we trapped HIF-1 alpha either in the cytoplasm, by fusing HIF-1 alpha to the cytoplasmic domain of the Na(+)-H(+) exchanger (NHE-1), or in the nucleus, by treatment with leptomycin B. Surprisingly, we found that HIF-1 alpha is stabilized by hypoxia and undergoes O(2)-dependent proteasomal degradation with an identical half-life (5--8 min) in both cellular compartments. Therefore, HIF-1 alpha entry into the nucleus is not, as proposed, a key event that controls its stability. This result markedly contrasts with the mechanism that controls p53 degradation via MDM2.

The Caenorhabditis elegans hif-1 gene encodes a bHLH-PAS protein that is required for adaptation to hypoxia

Hypoxia-inducible factor, a heterodimeric transcription complex, regulates cellular and systemic responses to low oxygen levels (hypoxia) during normal mammalian development or tumor progression. Here, we present evidence that a similar complex mediates response to hypoxia in Caenorhabditis elegans. This complex consists of HIF-1 and AHA-1, which are encoded by C. elegans homologs of the hypoxia-inducible factor (HIF) alpha and beta subunits, respectively. hif-1 mutants exhibit no severe defects under standard laboratory conditions, but they are unable to adapt to hypoxia. Although wild-type animals can survive and reproduce in 1% oxygen, the majority of hif-1-defective animals die in these conditions. We show that the expression of an HIF-1:green fluorescent protein fusion protein is induced by hypoxia and is subsequently reduced upon reoxygenation. Both hif-1 and aha-1 are expressed in most cell types, and the gene products can be coimmunoprecipitated. We conclude that the mechanisms of hypoxia signaling are likely conserved among metazoans. Additionally, we find that nuclear localization of AHA-1 is disrupted in an hif-1 mutant. This finding suggests that heterodimerization may be a prerequisite for efficient nuclear translocation of AHA-1.

HIFalpha targeted for VHL-mediated destruction by proline hydroxylation: implications for O2 sensing

HIF (hypoxia-inducible factor) is a transcription factor that plays a pivotal role in cellular adaptation to changes in oxygen availability. In the presence of oxygen, HIF is targeted for destruction by an E3 ubiquitin ligase containing the von Hippel-Lindau tumor suppressor protein (pVHL). We found that human pVHL binds to a short HIF-derived peptide when a conserved proline residue at the core of this peptide is hydroxylated. Because proline hydroxylation requires molecular oxygen and Fe(2+), this protein modification may play a key role in mammalian oxygen sensing.

Signal transduction. How do cells sense oxygen?

How do organisms sense the amount of oxygen in the environment and respond appropriately when the amount of oxygen decreases (a condition called hypoxia)? In their Perspective, Zhu and Bunn discuss new findings (Ivan et al., Jaakkola et al.) that reveal how the HIF transcription factor, which switches on a group of hypoxia-response proteins, is itself regulated by changes in oxygen tension. The authors are in the Hematology Division of the Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115, USA. E-mail: zhu@calvin.bwh.harvard.edu, bunn@calvin.bwh.harvard.edu

Hypoxia and oxidative stress in breast cancer. Hypoxia signalling pathways

Hypoxia-inducible factor-1 (HIF), which is centrally involved in physiological oxygen homeostasis, is also activated in the majority of tumours. Activation of HIF can occur through genetic mechanisms or as a result of hypoxia within the tumour microenvironment. In some cases HIF activation appears to be intimately linked to the proliferative stimulus itself. HIF affects patterns of gene expression and tumour growth, although precise effects vary between tumour types. Modulation of HIF activity, if correctly applied, may be therapeutically beneficial in tumour therapy.