Effect of hypoxic exposure on iron absorption in heterozygous hypotransferrinaemic mice

Roxadustat (FG-4592) treatment for anemia in dialysis-dependent (DD) and not dialysis-dependent (NDD) chronic kidney disease patients: A systematic review and meta-analysis

The effect of roxadustat (FG-4592) on individuals with chronic kidney diseases (CKD) patients receiving or not receiving the dialysis was unclear. The aim of this study was to evaluate the efficacy of roxadustat for the treatment of anemia in patients who are dialysis dependent (DD) or dialysis independent (NDD) CKD. We performed a systematic review of randomised controlled trials (RCTs) comparing treatment with roxadustat versus placebo or epoetin alfa up to November 2019. We investigated the efficacy of roxadustat in the levels of hemoglobin and other clinical parameters in renal anemia in patients with NDD and DD-CKD. We estimated weighted-mean difference (WMD) using random effect models. We included six RCTs comprising 1001 patients of whom 70.6 % were treated with roxadustat and 294 controls. The control group for studies of NDD-CKD patients was placebo whereas an active control of epoetin-alfa was used in studies of DD-CKD patients. Median follow-up time was 8 weeks. All trials were industry-sponsored. Overall, roxadustat increased hemoglobin levels by 1.20 g/dl (95 % CI:0.66, 1.75,P < 0.0001,I² = 99.3 %). Hemoglobin levels increased by 1.99 g/dl in NDD-CKD patients versus placebo and 0.52 g/dl in DD-CKD patients versus epoetin-alfa. Roxadustat was associated with a decrease the levels of hepcidin by -49.3 ng/dl (-38.5 ng/dl in NDD patients versus placebo and -27.7 ng/dl in DD patients versus epoetin alfa), a decrease in ferritin of -49.7 μmol/l (-52.2 μmol/l in NDD patients versus placebo and -7.3 μmol/l in DD patients versus epoetin alfa), and increase in total iron-binding capacity of 32.2 μmol/l (14.1 μmol/l in NDD patients versus placebo and 13.6 μmol/l in DD patients versus epoetin alfa). The percentage change in the transferrin saturation levels was -2.07 % (-6%, NDD patients versus placebo, and +3.7 % in DD patients versus epoetin alfa) in anemia associated CKD patients. This review found roxadustast increases the levels of hemoglobin, serum transferrin, intestinal iron absorption, and reduces hepcidin in both NDD and DD-CKD patients. Safety data is still emerging.

Metabolic Regulation of Hypoxia-Inducible Transcription Factors: The Role of Small Molecule Metabolites and Iron

Hypoxia-inducible transcription factors (HIFs) facilitate cellular adaptations to low-oxygen environments. However, it is increasingly recognised that HIFs may be activated in response to metabolic stimuli, even when oxygen is present. Understanding the mechanisms for the crosstalk that exists between HIF signalling and metabolic pathways is therefore important. This review focuses on the metabolic regulation of HIFs by small molecule metabolites and iron, highlighting the latest studies that explore how tricarboxylic acid (TCA) cycle intermediates, 2-hydroxyglutarate (2-HG) and intracellular iron levels influence the HIF response through modulating the activity of prolyl hydroxylases (PHDs). We also discuss the relevance of these metabolic pathways in physiological and disease contexts. Lastly, as PHDs are members of a large family of 2-oxoglutarate (2-OG) dependent dioxygenases that can all respond to metabolic stimuli, we explore the broader role of TCA cycle metabolites and 2-HG in the regulation of 2-OG dependent dioxygenases, focusing on the enzymes involved in chromatin remodelling.

Targeting Hypoxia-Inducible Factors for the Treatment of Anemia in Chronic Kidney Disease Patients

BACKGROUND

Anemia, a common complication of chronic kidney disease (CKD), has previously been attributed primarily to decreased production of erythropoietin. More recently, it has become apparent that the etiology of anemia involves several other factors, most notably dysfunctional iron metabolism, mediated via increased hepcidin activity and reduced clearance. Current management of anemia in patients with advanced CKD is based on erythropoiesis-stimulating agents and iron supplementation, along with red blood cell transfusions when necessary; however, safety considerations associated with these therapies highlight the need to pursue alternative treatment options targeting other mechanisms such as hypoxia-inducible factors (HIFs) that act as central regulators of erythropoiesis by coordinating a series of graded hypoxic responses.

SUMMARY

This review discusses the discovery of the HIF pathway and its regulation via HIF prolyl hydroxylase enzymes in the context of erythropoiesis and iron metabolism. The rationale for targeting this pathway and the clinical development of HIF prolyl hydroxylase inhibitors are reviewed, with a commentary on the potential implications of this class of agents in CKD anemia management. Key Messages: Pharmacologic activation of the HIF pathway results in a transient pseudo-hypoxic state that stimulates erythropoiesis in CKD patients with anemia. Results from clinical studies of a number of HIF prolyl hydroxylase inhibitors are increasingly available and provide support for the continued evaluation of the risk-benefit ratio of this novel therapeutic approach to the treatment of anemia in CKD.

Modulation of Dcytb (Cybrd 1) expression and function by iron, dehydroascorbate and Hif-2α in cultured cells

BACKGROUND

Duodenal cytochrome b (Dcytb) is a mammalian plasma ferric reductase enzyme that catalyses the reduction of ferric to ferrous ion in the process of iron absorption. The current study investigates the relationship between Dcytb, iron, dehydroascorbate (DHA) and Hif-2α in cultured cell lines.

METHODS

Dcytb and Hif-2α protein expression was analysed by Western blot technique while gene regulation was determined by quantitative PCR. Functional analyses were carried out by ferric reductase and (59)Fe uptake assays.

RESULTS

Iron and dehydroascorbic acid treatment of cells inhibited Dcytb mRNA and protein expression. Desferrioxamine also enhanced Dcytb mRNA level after cells were treated overnight. Dcytb knockdown in HuTu cells resulted in reduced mRNA expression and lowered reductase activity. Preloading cells with DHA (to enhance intracellular ascorbate levels) did not stimulate reductase activity fully in Dcytb-silenced cells, implying a Dcytb-dependence of ascorbate-mediated ferrireduction. Moreover, Hif-2α knockdown in HuTu cells led to a reduction in reductase activity and iron uptake.

CONCLUSIONS

Taken together, this study shows the functional regulation of Dcytb reductase activity by DHA and Hif-2α.

GENERAL SIGNIFICANCE

Dcytb is a plasma membrane protein that accepts electrons intracellularly from DHA/ascorbic acid for ferrireduction at the apical surface of cultured cells and enterocytes.

Effect of hypoxia on the expression of iron regulatory proteins 1 and the mechanisms involved

Iron is essential for many biological processes, including oxygen delivery, and its supply is tightly regulated. Iron regulatory proteins (IRPs, IRP1 and IRP2) are master regulators of cellular iron metabolism. Hypoxia triggers a broad range of gene responses that are primarily mediated by hypoxia-inducible factor-1 (HIF-1). In this study, we have shown that hypoxia could not only upregulate the expression of hypoxia inducible factor-1 but also downregulate the expression of IRP1. However, the molecular mechanisms that govern the IRP1 response to hypoxia are not known. Herein we suggested that HIF/HRE system was an essential link between IRP1 and hypoxia. The HRE of IRP1 5'-regulation regions could combine with HIF-1 in vitro. Dual-luciferase reporter assay showed that IRP1 was directly downregulated by HIF/HRE system.

Modulation of duodenal iron uptake by hypoxia and fasting in the rat

The effect of hypoxic exposure on invitroduodenal Fe uptake kinetics was studied in tissue fragments from rats that were fed or fasted overnight before study. Hypoxic exposure was for 3 d at 0·5 atm and fasting was for the last 18-24 h before Fe uptake determinations. The non-permeable Fe2+chelator 3-(2-pyridyl)-5,6-bis-(4-phenyl-sulphonic acid)-l,2,4-triazine (ferrozine), and medium deoxygenation inhibited uptake in all experimental groups. Ferrozine sensitivity and mucosal Fe3+reductase activity were greatest in hypoxic animals. Fe uptake was inhibited by membrane depolarization only after fasting or hypoxic exposure of the rats. The data demonstrated that Fe uptake by rat duodenal fragments involves at least two mechanisms: a membrane-potentialindependent mechanism which is not responsive to hypoxia and a second mechanism, induced by fasting or hypoxia, which is inhibited by membrane depolarization. Uptake is partially dependent on reduction of Fe3+to Fe2+and this is primarily associated with the second mechanism for uptake. These properties have been reported also in mouse and human Fe uptake, suggesting that the rat is a useful model for the study of basic mechanisms of Fe absorption

Animal models with enhanced erythropoiesis and iron absorption

The regulation of iron absorption is of considerable interest in mammals since excretion is minimal. Recent advances in iron metabolism have expounded the molecular mechanisms by which iron absorption is attuned to the physiological demands of the body. The pinnacle was the discovery and identification of hepcidin, a hepatic antimicrobial peptide that regulates absorption to maintain iron homeostasis. While the intricacies of its expression and regulation by HFE, transferrin receptor 2 and hemojuvelin are still speculative, hepcidin responsiveness has correlated negatively with iron absorption in different models and disorders of iron metabolism. Consequently, hepcidin expression is repressed to enhance iron absorption during stimulated erythropoiesis even in situations of elevated iron stores. Animal models have been crucial to the advances in understanding iron metabolism and the present review focuses on phenylhydrazine treated and hypotransferrinaemic rodents. These, respectively, experimental and genetic models of enhanced erythropoiesis highlight the shifting focus of iron absorption regulation from the marrow to the liver.

Production and storage of nitric oxide in adaptation to hypoxia

Adaptation to hypobaric hypoxia is known to exert multiple protective effects related with nitric oxide (NO). However the effect of adaptation to hypoxia on NO metabolism has remained unclear in many respects. In the present work we studied the interrelation between NO production and storage in the process of adaptation to hypoxia. The NO production was determined by the total nitrite/nitrate concentration in rats plasma. The volume of NO store was evaluated in vitro by the magnitude of isolated aorta relaxation to diethyldithiocarbamate. It was shown that both the nitrite/nitrate level and the NO store increased as adaptation to hypoxia developed. Furthermore, the NO store volume significantly correlated with plasma nitrite/nitrate. Therefore, adaptation to hypoxia stimulates NO production and storage and these effects can potentially underlie NO-dependent beneficial effects of adaptation.

Oxygen-regulated transferrin expression is mediated by hypoxia-inducible factor-1

Transferrin (Tf) is a liver-derived iron transport protein whose plasma concentration increases following exposure to hypoxia. Here, we present a cell culture model capable of expressing Tf mRNA in an oxygen-dependent manner. A 4-kilobase pair Tf promoter/enhancer fragment as well as the 300-base pair liver-specific Tf enhancer alone conveyed hypoxia responsiveness to a heterologous reporter gene construct in hepatoma but not HeLa cells. Within this enhancer, a 32-base pair hypoxia-responsive element was identified, which contained two hypoxia-inducible factor-1 (HIF-1) binding sites (HBSs). Mutation analysis showed that both HBSs function as oxygen-regulated enhancers in Tf-expressing as well as in non-Tf-expressing cell lines. Mutation of both HBSs was necessary to completely abolish hypoxic reporter gene activation. Transient co-expression of the two HIF-1 subunits HIF-1alpha and aryl hydrocarbon receptor nuclear translocator (ARNT)/HIF-1beta resulted in enhanced reporter gene expression even under normoxic conditions. Overexpression of a dominant-negative ARNT/HIF-1beta mutant reduced hypoxic activation. DNA binding studies using nuclear extracts from the mouse hepatoma cell line Hepa1 and the ARNT/HIF-1beta-deficient subline Hepa1C4, as well as antibodies raised against HIF-1alpha and ARNT/HIF-1beta confirmed that HIF-1 binds the Tf HBSs. Mutation analysis and competition experiments suggested that the 5' HBS was more efficient in binding HIF-1 than the 3' HBS. Finally, hypoxic induction of endogenous Tf mRNA was abrogated in Hepa1C4 cells, confirming that HIF-1 confers oxygen regulation of Tf gene expression by binding to the two HBSs present in the Tf enhancer.

Hypoxia, a novel inducer of acute phase gene expression in a human hepatoma cell line

Transcriptional regulation of gene expression by hypoxia is an important, but yet only marginally characterized mechanism by which organisms adapt to low oxygen concentrations. The human hepatoma cell line HepG2 is a widely used model for studying hypoxic induction of the hematopoietic growth factor erythropoietin. In an attempt to identify additional genes expressed in HepG2 cells during hypoxia, we differentially screened a cDNA library derived from hypoxic (1% O2) HepG2 cells using probes isolated from either normoxic (21% O2) or hypoxic cells. Two genes were identified, one encoding aldolase, a member of the glycolytic enzymes, and the other encoding alpha 1-antitrypsin which belongs to the family of the acute phase (AP) responsive proteins. Whereas hypoxic induction of glycolytic enzymes is well established, oxygen-dependent regulation of AP genes has not been reported so far. AP proteins are liver-derived plasma proteins whose production during inflammation is either up-regulated (positive AP reactants) or down-regulated (negative AP reactants). In the present study, we demonstrate that on the mRNA level hypoxic stimulation of HepG2 cells led to (i) an induction of the positive AP reactants alpha 1-antitrypsin, alpha 1-antichymotrypsin, complement C3, haptoglobin, and alpha 1-acid glycoprotein; (ii) a down-regulation of the negative AP reactant albumin; (iii) an up-regulation of the negative AP reactant transferrin; and (iv) unchanged levels of the positive AP reactants alpha- and beta-fibrinogen as well as hemopexin. Cycloheximide inhibited hypoxic up-regulation of AP mRNAs demonstrating that de novo protein synthesis is required for hypoxic induction. Nuclear run-on assays indicate that the hypoxic increase in AP mRNAs is mainly due to transcriptional regulation. The hypoxic response was compared to AP stimulation by interleukin 6. The results suggest that the adaptive response to hypoxia overlaps with, but is not identical with, the AP response mediated by interleukin 6.

Tissue iron loading and histopathological changes in hypotransferrinaemic mice

Tissue iron loading in hypotransferrinaemic (hpx/hpx) mice was investigated as a model for genetic (primary) haemochromatosis. Iron loading of liver preceded that in the pancreas and heart. One-year-old hpx/hpx mice showed iron staining in exocrine pancreas, liver parenchymal cells, and cardiac and intestinal smooth muscle cells. Iron-loaded macrophages were observed in all these tissues. Islets of Langerhans, biliary epithelial cells, and spleen were iron-free. The pancreas was fibrotic with massive macrophage infiltration and loss of secretory epithelium. Liver showed evidence of chronic inflammatory infiltration with increased collagen fibres in the parenchymal region but no cirrhosis. Serum aspartate aminotransferase activity and plasma glucose were increased in hpx/hpx compared with wild-type mice. Heavy iron loading with haemosiderin deposition in the liver could be demonstrated in hpx/hpx mice from 6 weeks of age. Heterozygous hypotransferrinaemic mice showed minor increases in liver iron stores at 6-12 weeks, but not at 1 year of age. Serum ferritin levels in heterozygous mice were also increased at 6-8 weeks of age. It was concluded that 1-year-old hpx/hpx mice showed evidence of liver and pancreatic damage secondary to tissue iron overload. The iron loading pattern and tissue damage showed some features which were distinct from those observed in haemochromatosis.