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.

Hypoxia-inducible factor1 (HIF-1) is an essential transcription factor for cellular adaptation to decreased oxygen availability. In normoxia the oxygen-sensitive α-subunit of HIF-1 is hydroxylated on Pro564 and Pro402 and thus targeted for proteasomal degradation. Three human oxygen-dependent HIF-1α prolyl hydroxylases (PHD1, PHD2, and PHD3) function as oxygen sensors in vivo. Furthermore, the asparagine hydroxylase FIH-1 (factor inhibiting HIF) has been found to hydroxylate Asp803 of the HIF-1 C-terminal transactivation domain, which results in the decreased ability of HIF-1 to bind to the transcriptional coactivator p300/CBP. We have fused these enzymes to the N-terminus of fluorescent proteins and transiently transfected the fusion proteins into human osteosarcoma cells (U2OS). Three-dimensional 2-photon confocal fluorescence microscopy showed that PHD1 was exclusively present in the nucleus, PHD2 and FIH-1 were mainly located in the cytoplasm and PHD3 was homogeneously distributed in cytoplasm and nucleus. Hypoxia did not influence the localisation of any enzyme under investigation. In contrast to FIH-1, each PHD inhibited nuclear HIF-1α accumulation in hypoxia. All hydroxylases suppressed activation of a cotransfected hypoxia-responsive luciferase reporter gene. Endogenous PHD2mRNA and PHD3mRNA were hypoxia-inducible, whereas expression of PHD1mRNA and FIH-1mRNA was oxygen independent. We propose that PHDs and FIH-1 form an oxygen sensor cascade of distinct subcellular localisation.

Hypoxia inducible factor-1 (HIF-1) is the master regulator of metabolic adaptation to hypoxia. It is appreciated that HIF-1␣ accumulation is achieved under normoxic conditions by e.g., nitric oxide. We determined molecular mechanisms of HIF-1␣ accumulation under the impact of S-nitrosoglutathione (GSNO). In human embryonic kidney cells GSNO provoked nuclear accumulation of HIF-1␣. This appeared unrelated to gene transcription and protein translation, thus pointing to inhibition of HIF-1␣ degradation. Indeed, GSNO as well as the hypoxia mimic CoCl 2 decreased ubiquitination of HIF-1␣ and GSNO-induced HIF-1␣ failed to coimmunoprecipitate with pVHL (von Hippel Lindau protein). Considering that HIF-1␣-pVHL interactions require prolyl hydroxylation of HIF-1␣, we went on to demonstrate inhibition of HIF-1␣ prolyl hydroxylases (PHDs) by GSNO. In vitro HIF-1␣-pVHL interactions revealed that GSNO dose-dependently inhibits PHD activity but not the interaction of a synthetic peptide resembling the hydroxylated oxygen-dependent degradation domain of HIF-1␣ with pVHL. We conclude that GSNO-attenuated prolyl hydroxylase activity accounts for HIF-1␣ accumulation under conditions of NO formation during normoxia and that PHD activity is subject to regulation by NO. INTRODUCTIONThe heterodimeric transcription factor hypoxia inducible factor-1 (HIF-1) plays a central role in adaptation to decreased oxygen availability (Semenza, 2002;Wenger, 2002;Brü ne and Zhou, 2003). HIF-1 is composed of the two basic helix-loop-helix-Per-Arnt-Sim (bHLH-PAS) proteins HIF-1␣ and the aryl hydrocarbon receptor nuclear translocator (ARNT), also known as HIF-1␤ (Wang and Semenza, 1995). In many cell types, the mRNA of both HIF-1␣ and HIF-1␤ appear permanently expressed and HIF-1␤ protein is constitutively present. However, HIF-1␣ protein is kept at a low or undetectable level under normoxia. In hypoxia, HIF-1␣ is strikingly induced, translocates to the nucleus, and dimerizes with HIF-1␤ to form HIF-1, which binds to hypoxiaresponsive elements (HRE) in regulatory regions of an impressive array of target genes involved in angiogenesis, erythropoiesis, vasomotor control, and energy metabolism as well as in cell survival decisions (for references, see Maxwell et al., 2001;Wenger, 2002; Zhu et al., 2002). This makes HIF-1 the master regulator of oxygen homeostasis to meet cell and tissue requirements in a situation of oxygen deficiency.In normoxia HIF-1␣ is bound to the von Hippel Lindau protein (pVHL) (Maxwell et al., 1999;Hon et al., 2002;Min et al., 2002), which is the substrate recognizing component of an E3 ubiquitin ligase complex (Cockman et al., 2000;Ohh et al., 2000;Tanimoto et al., 2000). Consequently, HIF-1␣ is polyubiquitinated and degraded by the 26S proteasome system, thus accounting for its low normoxic level of expression (Salceda and Caro, 1997;Huang et al., 1998;Kallio et al., 1999). The requirement of pVHL for HIF-1␣ degradation is underscored in cells that do not contain a functional pVHL, which leads to high levels of HIF-1␣ in normox...

An important regulator involved in oxygen-dependent gene expression is the transcription factor HIF (hypoxia-inducible factor), which is composed of an oxygen-sensitive alpha-subunit (HIF-1alpha or HIF-2alpha) and a constitutively expressed beta-subunit. In normoxia, HIF-1alpha is destabilized by post-translational hydroxylation of Pro-564 and Pro-402 by a family of oxygen-sensitive dioxygenases. The three HIF-modifying human enzymes have been termed prolyl hydroxylase domain containing proteins (PHD1, PHD2 and PHD3). Prolyl hydroxylation leads to pVHL (von-Hippel-Lindau protein)-dependent ubiquitination and rapid proteasomal degradation of HIF-1alpha. In the present study, we report that human PHD2 and PHD3 are induced by hypoxia in primary and transformed cell lines. In the human osteosarcoma cell line, U2OS, selective suppression of HIF-1alpha expression by RNA interference resulted in a complete loss of hypoxic induction of PHD2 and PHD3. Induction of PHD2 by hypoxia was lost in pVHL-deficient RCC4 cells. These results suggest that hypoxic induction of PHD2 and PHD3 is critically dependent on HIF-alpha. Using a VHL capture assay, we demonstrate that HIF-alpha prolyl-4-hydroxylase capacity of cytoplasmic and nuclear protein extracts was enhanced by prolonged exposure to hypoxia. Degradation of HIF-1alpha after reoxygenation was accelerated, which demonstrates functional relevance of the present results. We propose a direct, negative regulatory mechanism, which limits accumulation of HIF-1alpha in hypoxia and leads to accelerated degradation on reoxygenation after long-term hypoxia.

Hypoxia-inducible factor-1 (HIF-1) is a dimeric transcriptional complex that has been recognized primarily for its role in the maintenance of oxygen and energy homoeostasis. The HIF-1alpha subunit is O(2) labile and is degraded by the proteasome following prolyl-hydroxylation and ubiquitination in normoxic cells. The present review summarizes evidence that HIF-1 is also involved in immune reactions. Immunomodulatory peptides, including interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), stimulate HIF-1 dependent gene expression even in normoxic cells. Both the hypoxic and the cytokine-induced activation of HIF-1 involve the phosphatidylinositol- 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) signaling pathways. In addition, heat shock proteins (HSP) and other cofactors interact with HIF-1 subunits. HIF-1 increases the transcription of several genes for proteins that promote blood flow and inflammation, including vascular endothelial growth factor (VEGF), heme oxygenase-1, endothelial and inducible nitric oxide synthase (NOS) and cyclooxygenase-2 (COX-2). The pharmacologic activation of the HIF-1 complex can be desirable in ischemic and inflammatory disorders. In contrast, HIF-1 blockade may be beneficial to prevent tumor angiogenesis and tumor growth.