The rate-limiting step of O2 activation in the α-ketoglutarate oxygenase factor inhibiting hypoxia inducible factor.
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ABSTRACT:
SUBMITTER: Hangasky JA
PROVIDER: S-EPMC4283935 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:α-Ketoglutarate (αKG) dependent oxygenases comprise a large superfamily of enzymes that activate O2 for varied reactions. While most of these enzymes contain a nonheme Fe bound by a His2(Asp/Glu) facial triad, a small number of αKG-dependent halogenases require only the two His ligands to bind Fe and activate O2. The enzyme "factor inhibiting HIF" (FIH) contains a His2Asp facial triad and selectively hydroxylates polypeptides; however, removal of the Asp ligand in the Asp201→Gly variant leads to a highly active enzyme, seemingly without a complete facial triad. Herein, we report on the formation of an Fe-Cl cofactor structure for the Asp201→Gly FIH variant using X-ray absorption spectroscopy (XAS), which provides insight into the structure of the His2Cl facial triad found in halogenases. The Asp201→Gly variant supports anion dependent peptide hydroxylation, demonstrating the requirement for a complete His2X facial triad to support O2 reactivity. Our results indicated that exogenous ligand binding to form a complete His2X facial triad was essential for O2 activation and provides a structural model for the His2Cl-bound nonheme Fe found in halogenases.
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Project description:Hypoxia-inducible factor 1α (HIF-1α), a major mediator of tumor physiology, is activated during tumor progression, and its abundance is correlated with therapeutic resistance in a broad range of solid tumors. The accumulation of HIF-1α is mainly caused by hypoxia or through the mutated succinate dehydrogenase A (SDHA) or fumarate hydratase (FH) expression to inhibit its degradation. However, its activation under normoxic conditions, termed pseudohypoxia, in cells without mutated SDHA or FH is not well documented. Here, we show that dimethyl-2-ketoglutarate (DKG), a cell membrane-permeable precursor of a key metabolic intermediate, α-ketoglutarate (α-KG), known for its ability to rescue glutamine deficiency, transiently stabilized HIF-1α by inhibiting activity of the HIF prolyl hydroxylase domain-containing protein, PHD2. Consequently, prolonged DKG-treatment under normoxia elevated HIF-1α abundance and up-regulated the expression of its downstream target genes, thereby inducing a pseudohypoxic condition. This HIF-1α stabilization phenotype is similar to that from treatment of cells with desferrioxamine (DFO), an iron chelator, or dimethyloxalyglycine (DMOG), an established PHD inhibitor, but was not recapitulated with other α-KG analogues, such as Octyl-2KG, MPTOM001 and MPTOM002. Our study is the first example of an α-KG precursor to increase HIF-1α abundance and activity. We propose that DKG acts as a potent HIF-1α activator, highlighting the potential use of DKG to investigate the contribution of PHD2-HIF-1α pathway to tumor biology.
| S-EPMC4242664 | biostudies-literature
Project description:Induction of a series of anti-hypoxic proteins protects cells during exposure to hypoxic conditions. Hypoxia-inducible factor-α (HIF-α) is a major transcription factor that orchestrates this protective effect. To activate HIF exogenously, without exposing cells to hypoxic conditions, many small-molecule inhibitors targeting prolyl hydroxylase domain-containing protein have been developed. In addition, suppression of factor inhibiting HIF-1 (FIH-1) has also been shown to have the potential to activate HIF-α. However, few small-molecule inhibitors of FIH-1 have been developed. In this study, we synthesized a series of furan- and thiophene-2-carbonyl amino acid derivatives having the potential to inhibit FIH-1. The inhibitory activities of these compounds were evaluated in SK-N-BE(2)c cells by measuring HIF response element (HRE) promoter activity. Several furan- and thiophene-2-carbonyl amino acid derivatives inhibited FIH-1 based on correlations among the docking score of the FIH-1 active site, the chemical structure of the compounds, and biological HIF-α/HRE transcriptional activity.
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Project description:Notch plays a critical role in the transition from proliferation to differentiation in the epidermis and corneal epithelium. Furthermore, aberrant Notch signaling is a feature of diseases like psoriasis, eczema, nonmelanoma skin cancer, and melanoma where differentiation and proliferation are impaired. Whereas much is known about the downstream events following Notch signaling, factors responsible for negatively regulating Notch receptor signaling after ligand activation are incompletely understood. Notch can undergo hydroxylation by factor-inhibiting hypoxia-inducible factor 1 (FIH-1); however, the biological significance of this phenomenon is unclear. Here we show that FIH-1 expression is up-regulated in diseased epidermis and corneal epithelium. Elevating FIH-1 levels in primary human epidermal keratinocytes (HEKs) and human corneal epithelial keratinocytes (HCEKs) impairs differentiation in submerged cultures and in a "three-dimensional" organotypic raft model of human epidermis, in part, via a coordinate decrease in Notch signaling. Knockdown of FIH-1 enhances keratinocyte differentiation. Loss of FIH-1 in vivo increased Notch activity in the limbal epithelium, resulting in a more differentiated phenotype. microRNA-31 (miR-31) is an endogenous negative regulator of FIH-1 expression that results in keratinocyte differentiation, mediated by Notch activation. Ectopically expressing miR-31 in an undifferentiated corneal epithelial cell line promotes differentiation and recapitulates a corneal epithelium in a three-dimensional raft culture model. Our results define a previously unknown mechanism for keratinocyte fate decisions where Notch signaling potential is, in part, controlled through a miR-31/FIH-1 nexus.
| S-EPMC3435188 | biostudies-literature
Project description:Non-heme Fe(II)/α-ketoglutarate (αKG)-dependent oxygenases catalyze a wide array of reactions through coupling oxidative decarboxylation of αKG to substrate oxygenation. This class of enzymes follows a sequential mechanism in which O2 reacts only after binding primary substrate, raising questions over how protein structure tailors molecular access to the Fe(II) cofactor. The enzyme "factor inhibiting hypoxia inducible factor" (FIH) senses pO2 in human cells by hydroxylating the C-terminal transactivation domain (CTAD), suggesting that structural elements limiting molecular access to the active site may limit the pO2 response. In this study, we tested the impact of a solvent-accessible tunnel in FIH on molecular access to the active site in FIH. The size of the tunnel was increased through alanine point mutagenesis (Y93A, E105A, and Q147A), followed by a suite of mechanistic and spectroscopic probes. Steady-state kinetics varying O2 or CTAD indicated that O2 passage through the tunnel was not affected by Ala substitutions, allowing us to conclude that this narrow tunnel did not impact pO2 sensing by FIH. Steady-state kinetics with varied αKG concentrations revealed increased substrate inhibition for the Ala variants, suggesting that a second αKG molecule may bind near the active site of FIH. If this solvent-accessible tunnel is the O2 entry tunnel, it may be narrow in order to permit O2 access while preventing metabolic intermediates, such as αKG, from inhibiting FIH under physiological conditions.
| S-EPMC5726895 | biostudies-literature
Project description:The asparaginyl hydroxylase, factor-inhibiting hypoxia-inducible factor (HIF), is central to the oxygen-sensing pathway that controls the activity of HIF. Factor-inhibiting HIF (FIH) also catalyzes the hydroxylation of a large set of proteins that share a structural motif termed the ankyrin repeat domain (ARD). In vitro studies have defined kinetic properties of FIH with respect to different substrates and have suggested FIH binds more tightly to certain ARD proteins than HIF and that ARD hydroxylation may have a lower K(m) value for oxygen than HIF hydroxylation. However, regulation of asparaginyl hydroxylation on ARD substrates has not been systematically studied in cells. To address these questions, we employed isotopic labeling and mass spectrometry to monitor the accrual, inhibition, and decay of hydroxylation under defined conditions. Under the conditions examined, hydroxylation was not reversed but increased as the protein aged. The extent of hydroxylation on ARD proteins was increased by addition of ascorbate, whereas iron and 2-oxoglutarate supplementation had no significant effect. Despite preferential binding of FIH to ARD substrates in vitro, when expressed as fusion proteins in cells, hydroxylation was found to be more complete on HIF polypeptides compared with sites within the ARD. Furthermore, comparative studies of hydroxylation in graded hypoxia revealed ARD hydroxylation was suppressed in a site-specific manner and was as sensitive as HIF to hypoxic inhibition. These findings suggest that asparaginyl hydroxylation of HIF-1 and ARD proteins is regulated by oxygen over a similar range, potentially tuning the HIF transcriptional response through competition between the two types of substrate.
| S-EPMC3190818 | biostudies-literature
Project description:Protein dynamics are crucial for the mechanistically ordered enzymes to bind to their substrate in the correct sequence and perform catalysis. Factor-inhibiting HIF-1 (FIH) is a nonheme Fe(II) α-ketoglutarate-dependent oxygenase that is a key hypoxia (low pO2) sensor in humans. As these hypoxia-sensing enzymes follow a multistep chemical mechanism consuming α-ketoglutarate, a protein substrate that is hydroxylated, and O2, understanding protein flexibility and the order of substrate binding may aid in the development of strategies for selective targeting. The primary substrate of FIH is the C-terminal transactivation domain (CTAD) of hypoxia-inducible factor 1α (HIF) that is hydroxylated on the side chain of Asn803. We assessed changes in protein flexibility connected to metal and αKG binding, finding that (M+αKG) binding significantly stabilized the cupin barrel core of FIH as evidenced by enhanced thermal stability and decreased protein dynamics as assessed by global amide hydrogen/deuterium exchange mass spectrometry and limited proteolysis. Confirming predictions of the consensus mechanism, (M+αKG) increased the affinity of FIH for CTAD as measured by titrations monitoring intrinsic tryptophan fluorescence. The decreased protein dynamics caused by (M+αKG) enforces a sequentially ordered substrate binding sequence in which αKG binds before CTAD, suggesting that selective inhibition may require inhibitors that target the binding sites of both αKG and the prime substrate. A consequence of the correlation between dynamics and αKG binding is that all relevant ligands must be included in binding-based inhibitor screens, as shown by testing permutations of M, αKG, and inhibitor.
| S-EPMC7003530 | biostudies-literature
Project description: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.
| S-EPMC1133735 | biostudies-literature
Project description:The hypoxia-inducible factor (HIF) system orchestrates cellular responses to hypoxia in animals. HIF is an α/β-heterodimeric transcription factor that regulates the expression of hundreds of genes in a tissue context-dependent manner. The major hypoxia-sensing component of the HIF system involves oxygen-dependent catalysis by the HIF hydroxylases; in humans there are three HIF prolyl hydroxylases (PHD1-3) and an asparaginyl hydroxylase (factor-inhibiting HIF (FIH)). PHD catalysis regulates HIFα levels, and FIH catalysis regulates HIF activity. How differences in HIFα hydroxylation status relate to variations in the induction of specific HIF target gene transcription is unknown. We report studies using small molecule HIF hydroxylase inhibitors that investigate the extent to which HIF target gene expression is induced by PHD or FIH inhibition. The results reveal substantial differences in the role of prolyl and asparaginyl hydroxylation in regulating hypoxia-responsive genes in cells. PHD inhibitors with different structural scaffolds behave similarly. Under the tested conditions, a broad-spectrum 2-oxoglutarate dioxygenase inhibitor is a better mimic of the overall transcriptional response to hypoxia than the selective PHD inhibitors, consistent with an important role for FIH in the hypoxic transcriptional response. Indeed, combined application of selective PHD and FIH inhibitors resulted in the transcriptional induction of a subset of genes not fully responsive to PHD inhibition alone. Thus, for the therapeutic regulation of HIF target genes, it is important to consider both PHD and FIH activity, and in the case of some sets of target genes, simultaneous inhibition of the PHDs and FIH catalysis may be preferable.
| S-EPMC5034057 | biostudies-literature
Project description:Oxygen homeostasis plays a critical role in angiogenesis, erythropoiesis, and cell metabolism. Oxygen homeostasis is set by the hypoxia inducible factor-1α (HIF-1α) pathway, which is controlled by factor inhibiting HIF-1α (FIH). FIH is a non-heme Fe(II), α-ketoglutarate (αKG)-dependent dioxygenase that inhibits HIF-1α by hydroxylating the C-terminal transactivation domain (CTAD) of HIF-1α at HIF-Asn(803). A tight coupling between CTAD binding and O2 activation is essential for hypoxia sensing, making changes in the coordination geometry of Fe(II) upon CTAD encounter a crucial feature of this enzyme. Although the consensus chemical mechanism for FIH proposes that CTAD binding triggers O2 activation by causing the Fe(II) cofactor to release an aquo ligand, experimental evidence of this has been absent. More broadly, this proposed coordination change at Fe(II) has not been observed during steady-state turnover in any αKG oxygenase to date. In this work, solvent isotope effects (SIEs) were used as a direct mechanistic probe of substrate-triggered aquo release in FIH, as inverse SIEs (SIE < 1) are signatures for pre-equilibrium aquo release from metal ions. Our mechanistic studies of FIH have revealed inverse solvent isotope effects in the steady-state rate constants at limiting concentrations of CTAD or αKG [(D2O)kcat/KM(CTAD) = 0.40 ± 0.07, and (D2O)kcat/KM(αKG) = 0.32 ± 0.08], providing direct evidence of aquo release during steady-state turnover. Furthermore, the SIE at saturating concentrations of CTAD and αKG was inverse ((D2O)kcat = 0.51 ± 0.07), indicating that aquo release occurs after CTAD binds. The inverse kinetic SIEs observed in the steady state for FIH can be explained by a strong Fe-OH2 bond. The stable Fe-OH2 bond plays an important part in FIH's regulatory role over O2 homeostasis in humans and points toward a strategy for tightly coupling O2 activation with CTAD hydroxylation that relies on substrate triggering.
| S-EPMC4059518 | biostudies-literature