Studies on the activity of the hypoxia-inducible-factor hydroxylases using an oxygen consumption assay

The activity and levels of the metazoan HIF (hypoxia-inducible factor) are regulated by its hydroxylation, catalysed by 2OG (2-oxoglutarate)- and Fe(II)-dependent dioxygenases. An oxygen consumption assay was developed and used to study the relationship between HIF hydroxylase activity and oxygen concentration for recombinant forms of two human HIF hydroxylases, PHD2 (prolyl hydroxylase domain-containing protein 2) and FIH (factor inhibiting HIF), and compared with two other 2OG-dependent dioxygenases. Although there are caveats on the absolute values, the apparent K(m) (oxygen) values for PHD2 and FIH were within the range observed for other 2OG oxygenases. Recombinant protein substrates were found to have lower apparent K(m) (oxygen) values compared with shorter synthetic peptides of HIF. The analyses also suggest that human PHD2 is selective for fragments of the C-terminal over the N-terminal oxygen-dependent degradation domain of HIF-1alpha. The present results, albeit obtained under non-physiological conditions, imply that the apparent K(m) (oxygen) values of the HIF hydroxylases enable them to act as oxygen sensors providing their in vivo capacity is appropriately matched to a hydroxylation-sensitive signalling pathway.     

HIF-1α peptide derivatives with modifications at the hydroxyproline residue as activators of HIF-1α

Hypoxia-inducible factor (HIF)-1α undergoes degradation under normoxia, which involves its proline hydroxylation and subsequent binding of proline-hydroxylated HIF-1α to the von Hippel-Lindau protein–Elongin B–Elongin C (VBC) complex. In this study, we designed and synthesized a series of peptides containing 556–575 residues of HIF-1α with modifications at the Pro-564 residue to inhibit the interaction between proline-hydroxylated HIF-1α and VBC. Employing a fluorescence polarization-based interaction assay, we evaluated inhibitory potency of these peptides and selected potent inhibitors. We then analyzed their effects in the cell level to show that the selected inhibitors induced HIF-1α stabilization in normoxic cells. Considering that proline hydroxylation of HIF-1α is routinely targeted for modulating the HIF pathway, our approach of using inhibitors against the interactions between HIF-1α and VBC would provide an alternative way of upregulating HIF-1 activity.     

Asparaginyl hydroxylation of the Notch ankyrin repeat domain by factor inhibiting hypoxia-inducible factor

The stability and activity of hypoxia-inducible factor (HIF) are regulated by the post-translational hydroxylation of specific prolyl and asparaginyl residues. We show that the HIF asparaginyl hydroxylase, factor inhibiting HIF (FIH), also catalyzes hydroxylation of highly conserved asparaginyl residues within ankyrin repeat (AR) domains (ARDs) of endogenous Notch receptors. AR hydroxylation decreases the extent of ARD binding to FIH while not affecting signaling through the canonical Notch pathway. ARD proteins were found to efficiently compete with HIF for FIH-dependent hydroxylation. Crystallographic analyses of the hydroxylated Notch ARD (2.35A) and of Notch peptides bound to FIH (2.4-2.6A) reveal the stereochemistry of hydroxylation on the AR and imply that significant conformational changes are required in the ARD fold in order to enable hydroxylation at the FIH active site. We propose that ARD proteins function as natural inhibitors of FIH and that the hydroxylation status of these proteins provides another oxygen-dependent interface that modulates HIF signaling.     

Enzyme–substrate reporters for evaluation of substrate specificity of HIF prolyl hydroxylase isoforms

An organism naturally responds to hypoxia via stabilization of hypoxia-inducible factor (HIF). There are three isoforms of HIFα subunits whose stability is regulated by three isozymes of HIF prolyl hydroxylase (PHD1-3). Despite intense studies on recombinant enzyme isoforms using homogeneous activity assay, there is no consensus on the PHD iso-form preference for the HIF isoform as a substrate. This work provides a new approach to the problem of substrate specificity using cell-based reporters expressing the enzyme and luciferase-labeled substrate pair encoded in the same expression vector. The cell is used as a microbioreactor for running the reaction between the overexpressed enzyme and substrate. Using this novel approach, no PHD3 activity toward HIF3 was demonstrated, indirectly pointing to the hydroxylation of the second proline in 564PYIP567 (HIF1) catalyzed by this isozyme. The use of “paired” enzyme–substrate reporters to evaluate the potency of “branched tail” oxyquinoline inhibitors of HIF PHD allows higher precision in revealing the optimal structural motif for each enzyme isoform.     

Ablation of endothelial prolyl hydroxylase domain protein‐2 promotes renal vascular remodelling and fibrosis in mice

Accumulating evidence demonstrates that hypoxia‐inducible factor (HIF‐α) hydroxylase system has a critical role in vascular remodelling. Using an endothelial‐specific prolyl hydroxylase domain protein‐2 (PHD2) knockout (PHD2^ECKO) mouse model, this study investigates the regulatory role of endothelial HIF‐α hydroxylase system in the development of renal fibrosis. Knockout of PHD2 in EC up‐regulated the expression of HIF‐1α and HIF‐2α, resulting in a significant decline of renal function as evidenced by elevated levels of serum creatinine. Deletion of PHD2 increased the expression of Notch3 and transforming growth factor (TGF‐β1) in EC, thus further causing glomerular arteriolar remodelling with an increased pericyte and pericyte coverage. This was accompanied by a significant elevation of renal resistive index (RI). Moreover, knockout of PHD2 in EC up‐regulated the expression of fibroblast‐specific protein‐1 (FSP‐1) and increased interstitial fibrosis in the kidney. These alterations were strongly associated with up‐regulation of Notch3 and TGF‐β1. We concluded that the expression of PHD2 in endothelial cells plays a critical role in renal fibrosis and vascular remodelling in adult mice. Furthermore, these changes were strongly associated with up‐regulation of Notch3/TGF‐β1 signalling and excessive pericyte coverage.     

慢性阻塞性肺疾病患者肺小血管低氧诱导因子1α与其羟化酶表达

目的:通过观察慢性阻塞性肺疾病(COPD)患者肺小动脉内低氧诱导因子α亚基(HIF-1α)及HIF脯氨酸羟化酶(PHD)、HIF抑制因子的表达,探讨其在肺血管重塑中的可能作用。方法:选因肺肿瘤行肺叶切除者,COPD组(12例),对照组(14例)。取2组患者的肺组织,原位杂交和免疫组化检测HIF-1α、PHD1、PHD2、PHD3、FIH的mRNA及蛋白表达水平。观察并计算肺小动脉管壁厚度(PAMT)及肺小动脉管壁面积与血管总面积的比值(WA%)。结果:①COPD组PAMT(40μm±5μm)、WA%(50%±9%)均较对照组(分别为(31μm±4μm,39%±6%)高(均P<0.01);②COPD组肺小血管HIF-1αmRNA和蛋白表达(吸光度(A)值)(0.230±0.036,0.275±0.039)较对照组(0.174±0.029,0.102±0.015)增强(均P<0.01),蛋白质表达增高更明显。COPD组肺小血管PHD1 mRNA表达(0.180±0.030)与对照组(0.191±0.029)比无明显改变(P>0.05)。COPD组PHD2、PHD3 mRNA表达(0.245±0.044,0.252±0.023)较对照组(0.182±0.028,0.127±0.017)明显增高(均P<0.01)。PHD1蛋白质表达(0.104±0.015)较对照组(0.209±0.023)降低(P<0.01)。PHD2蛋白质表达(0.274±0.044)较对照组(0.219±0.043)增高(P<0.01)。PHD3蛋白质表达(0.161±0.023)较对照组(0.146±0.021)略增高,但差异无统计学意义(P>0.05)。两组间FIHmRNA和蛋白质表达差异均无显著性(P>0.05);③相关分析表明HIF-1α蛋白质水平与WA%,PAMT及PHD2、PHD3 mRNA及PHD2蛋白质呈正相关,与PHD1蛋白质呈负相关。结论:PHDs可能通过调节HIF-1α表达参与COPD患者的肺血管重塑。     

Overexpression of the HIF hydroxylases PHD1, PHD2, PHD3 and FIH are individually and collectively unfavorable prognosticators for NSCLC survival

Introduction

Hypoxia induced factors (HIFs) are at the heart of the adaptive mechanisms cancer cells must implement for survival. HIFs are regulated by four hydroxylases; Prolyl hydroxylase (PHD)-1,-2,-3 and factor inhibiting HIF (FIH). We aimed to investigate the prognostic impact of these oxygen sensors in NSCLC.

Methods

Tumor tissue samples from 335 resected stages I to IIIA NSCLC patients was obtained and tissue microarrays (TMAs) were constructed. Hydroxylase expression was evaluated by immunohistochemistry.

Principal Findings

There was scorable expression for all HIF hydroxylases in tumor cells, but not in stroma. In univariate analyses, high tumor cell expression of all the HIF hydroxylases were unfavorable prognosticators for disease-specific survival (DSS); PHD1 (P = 0.023), PHD2 (P = 0.013), PHD3 (P = 0.018) and FIH (P = 0.033). In the multivariate analyses we found high tumor cell expression of PHD2 (HR = 2.03, CI 95% 1.20–3.42, P = 0.008) and PHD1 (HR = 1.45, CI 95% 1.01–2.10, P = 0.047) to be significant independent prognosticators for DSS. Besides, there was an additive prognostic effect by the increasing number of highly expressed HIF hydroxylases. Provided none high expression HIF hydroxylases, the 5-year survival was 80% vs. 23% if all four were highly expressed (HR = 6.48, CI 95% 2.23–18.8, P = 0.001).

Conclusions

HIF hydroxylases are, in general, poor prognosticators for NSCLC survival. PHD1 and PHD2 are independent negative prognostic factors in NSCLC. Moreover, there is an additive poor prognostic impact by an increasing number of highly expressed HIF hydroxylases.     

Induction of hypoxia inducible factor (HIF-1α) in rat kidneys by iron chelation with the hydroxypyridinone, CP94

Hypoxia inducible factor (HIF-1α) is a master regulator of tissue adaptive responses to hypoxia whose stability is controlled by an iron containing prolyl hydroxylase domain (PHD) protein. A catalytic redox cycle in the PHD's iron center that results in the formation of a ferryl (Fe(+4)) intermediate has been reported to be responsible for the hydroxylation and subsequent degradation of HIF-1α under normoxia. We show that induction of HIF-1α in rat kidneys can be achieved by iron reduction by the hydroxypyridin-4 one (CP94), an iron chelator administered intraperitoneally in rats. The extent of HIF protein stabilization as well as the expression of HIF target genes, including erythropoietin (EPO), in kidney tissues was comparable to those induced by known inhibitors of the PHD enzyme, such as desferrioxamine (DFO) and cobalt chloride (CoCl(2)). In human kidney cells and in vitro PHD activity assay, we were able to show that the HIF-1α protein can be stabilized by addition of CP94. This appears to inactivate PHD; and thus prevents the hydroxylation of HIF-1α. In conclusion, we have identified the inhibition of iron-binding pocket of PHD as an underlying mechanism of HIF induction in vivo and in vitro by a bidentate hydroxypyridinone.