Inhibition of prolyl hydroxylases increases erythropoietin production in ESRD

Hepcidin as a therapeutic target for anemia and inflammation associated with chronic kidney disease

INTRODUCTION

Anemia is a common manifestation of chronic kidney disease (CKD). The pathogenesis of CKD-associated anemia is multifactorial. Our understanding of the molecular control of iron metabolism has improved dramatically because of the discovery of hepcidin and attempts to introduce new drugs to stimulate erythropoiesis or affect the hepcidin-ferroportin pathway have recently emerged. Areas covered: We examine the possible role of hepcidin in iron metabolism and regulation and the potential therapeutic options involving hepcidin and hepcidin-ferroportin axis in renal anemia treatment. We focus on therapeutic targeting of hepcidin, the hepcidin-ferroportin axis and key molecules such as anti-hepcidin antibodies, spigelmers, and anticalins. We also discuss compounds affecting the bone morphogenetic protein receptor [BMP/BMPR] complex and molecules that influence hepcidin, such as hypoxia-inducible factor 1 stabilizers. Expert opinion: Hepcidin is a key regulator of iron availability and is a potential future therapeutic target for managing anemia that is associated with CKD. There are potential risks and benefits associated with novel sophisticated therapies and there are several novel options on the horizon; however, clinical data are currently limited and need development. Inhibition of hepcidin via various pathways might be a viable adjunctive therapeutic option in other clinical situations.

Hepcidin in chronic kidney disease anemia

Chronic kidney disease (CKD) is associated with several complications that worsen with progression of disease; anemia, disturbances in iron metabolism and inflammation are common features. Inflammatory response starts early, releasing pro-inflammatory cytokines, acute phase reactants and hepcidin. Hepcidin production is modulated by several factors, as hypoxia/anemia, erythropoietin and erythropoiesis products, transferrin saturation (TSAT) and liver iron levels, which are altered in CKD. Treatment of CKD anemia is based on pharmaceutical intervention, with erythropoietic stimulating agents and/or iron supplementation; however, in spite of the erythropoietic benefits, this therapy, on a regular basis, involves risks, namely iron overload. To overcome these risks, some therapeutic approaches are under study to target CKD anemia. Considering the actual alerts about risk of iron overload in dialysis patients, inhibition of hepcidin, the central key player in iron homeostasis, could be a pivotal strategy in the management of CKD anemia.

Source and microenvironmental regulation of erythropoietin in the kidney

PURPOSE OF REVIEW

Historically, the identity of O2-sensing renal erythropoietin (Epo)-producing (REP) cells was a matter of debate. This review summarizes how recent breakthroughs in transgenic mouse and in-situ hybridization techniques have facilitated sensitive and specific detection of REP cells and accelerated advancements in the understanding of the regulation of renal Epo production in health and disease.

RECENT FINDINGS

REP cells are a dynamically regulated unique subpopulation of tubulointerstitial cells with features of fibroblasts, pericytes and neurons. Under normal conditions, REP cells are located in the corticomedullary border region within a steep decrement in O2 availability. During the progression of chronic kidney disease (CKD), REP cells cease Epo production, dedifferentiate and contribute to the progression of renal fibrosis. However, CKD patients with renal anaemia still respond with elevated Epo production following treatment with hypoxia-mimicking agents.

SUMMARY

We hypothesize that REP cells are neuron-like setpoint providers and controllers, which integrate information about blood O2 concentration and local O2 consumption via tissue pO2, and combine these inputs with intrinsic negative feedback loops and perhaps tubular cross-talk, converging in Epo regulation.

Pharmacological inhibition of prolyl hydroxylase protects against inflammation-induced anemia via efficient erythropoiesis and hepcidin downregulation

Chronic inflammatory diseases are often associated with anemia. In such conditions, anemia is generally treated with erythropoiesis stimulating agents (ESAs) which are associated with potentially hazardous side effects and poor outcomes. Suboptimal erythropoiesis in chronic inflammation is believed to be caused by elevated hepcidin levels, which causes blockade of iron in tissue stores. In the current work using rodent models of inflammation, an orally available small molecule prolyl hydroxylase inhibitor desidustat was assessed as an effective treatment of anemia of inflammation. In BALB/c mice, a single dose treatment of desidustat attenuated the effect of lipopolysaccharide (LPS) - or turpentine oil-induced inflammation and increased serum erythropoietin (EPO), iron, and reticulocyte count, and decreased serum hepcidin levels. In turpentine oil-induced anemia in BALB/c mice, repeated dose desidustat treatment increased hemoglobin, RBC and hematocrit in a dose related manner. In female Lewis rats, treatment with desidustat markedly reduced PGPS-induced anemia and increased hemoglobin, red blood cell (RBC) and white blood cell (WBC) count, hematocrit, serum iron and spleen iron. These effects of desidustat were associated with reduction in hepcidin (HAMP) expression as well as reduction in serum hepcidin, and increased EPO expression in liver and kidneys. Desidustat treatment caused a significant increase in expression of Duodenal cytochrome B (DcytB), ferroportin (FPN1) and divalent metal transporter 1 (DMT1) in duodenum, and FPN1 and monocyte chemoattractant protein-1 (MCP-1) in liver suggesting an overall influence on iron metabolism. Thus, pharmacological inhibition of prolyl hydroxylase enzymes can be useful in treatment of anemia of inflammation.

A Fluorescent Benzo[g]isoquinoline-Based HIF Prolyl Hydroxylase Inhibitor for Cellular Imaging

Prolyl hydroxylation domain (PHD) enzymes catalyze the hydroxylation of the transcription factor hypoxia-inducible factor (HIF) and serve as cellular oxygen sensors. HIF and the PHD enzymes regulate numerous potentially tissue-protective target genes which can adapt cells to metabolic and ischemic stress. We describe a fluorescent PHD inhibitor (1-chloro-4-hydroxybenzo[g]isoquinoline-3-carbonyl)glycine which is suited to fluorescence-based detection assays and for monitoring PHD inhibitors in biological systems. In cell-based assays, application of the fluorescent PHD inhibitor allowed co-localization with a cellular PHD enzyme and led to live cell imaging of processes involved in cellular oxygen sensing.

HIF stabilizers in the management of renal anemia: from bench to bedside to pediatrics

Anemia is a common complication of chronic kidney disease (CKD) in adult and pediatric patients. It has traditionally been treated with erythropoietin therapy and iron supplementation, with great success. With the discovery of the major transcription factor hypoxia inducible factor (HIF) for the erythropoietin gene in 1992, molecules were created that inhibit the HIF prolyl-hydroxylase enzyme. This new class of drug-called HIF stabilizers, or HIF prolyl-hydroxylase inhibitors-prevents the proteasomal degradation of HIF-α, thereby inducing upregulation of the erythropoietin gene. This new strategy for treating CKD anemia is already in phase III clinical trials in adults, and the potential advantages of this therapy are that it is orally active (thereby avoiding injections), and patients are exposed to lower circulating levels of erythropoietin. The long-term safety of this strategy, however, requires elucidation in these trials, particularly since there are many other hypoxia-sensitive genes, notably, angiogenic factors such as vascular endothelial growth factors (VEGF), as well as glycolytic enzymes. As with all new therapies, it is only once a positive benefit: risk profile has been ascertained in adults that the treatment will translate across into pediatrics. Specific issues in the pediatric CKD population are discussed in this review.

The role of hypoxia-inducible factor stabilizers in the treatment of anemia in patients with chronic kidney disease

INTRODUCTION

The purpose of this study was to analyze the effects of hypoxia-inducible factor (HIF) stabilizers on anemia in non-dialysis-dependent (NDD) and dialysis-dependent (DD) chronic kidney disease (CKD) patients.

METHODS

Published studies were extracted from PubMed, China Biological Medicine Database (CBM), Wanfang database, and Cochrane Library on March 10, 2018, and relevant studies were pooled and included in a meta-analysis. Data on hemoglobin (Hb), ferritin, and hepcidin levels, total iron-binding capacity (TIBC), and incidence of adverse events (AEs) were extracted and pooled using Review Manager Version 5.3.

RESULTS

Data from nine selected studies were extracted. Meta-analysis of the included studies showed that HIF stabilizers reduced ferritin and hepcidin levels and increased Hb level and TIBC in NDD-CKD patients. However, HIF stabilizers only increased TIBC, and did not affect ferritin, hepcidin, and Hb levels in DD-CKD patients. Furthermore, no notable differences in AEs and severe AEs between NDD-CKD and DD-CKD patients were detected.

CONCLUSION

HIF stabilizers are effective for the treatment of anemia in NDD-CKD patients and safe for short-term use.

Novel Compound Induces Erythropoietin Secretion through Liver Effects in Chronic Kidney Disease Patients and Healthy Volunteers

BACKGROUND

TP0463518 is a novel hypoxia-inducible factor prolyl hydroxylase inhibitor developed to aid in the treatment of anemia associated with chronic kidney disease (CKD) and is expected to increase erythropoietin (EPO) derived from liver. Two phase I studies were conducted in healthy volunteers (HV) and CKD patients undergoing hemodialysis (i.e., HD patients) or those not undergoing dialysis (i.e., ND patients).

METHODS

Pharmacokinetics, pharmacodynamics, and safety profiles of TP0463518 were assessed. Forty HV received single oral doses of TP0463518 at 3, 6, 11, 20, and 36 mg or placebo. Twenty ND patients received single doses of TP0463518 at 1, 6, and 11 mg and 9 HD patients received TP0463518 at 1 and 11 mg doses. To identify the source organ of EPO, glycosylation patterns were determined using percentage migrated isoform (PMI) values.

RESULTS

Declining renal function slowed elimination of TP0463518 and increased the mean AUC0-∞. ∆Emax of serum EPO in 11-mg groups of HV, ND patients, and HD patients were 24.37 ± 11.37, 201.57 ± 130.34, and 1,324.76 ± 1,189.24 mIU/mL respectively. A strong correlation was -observed between logarithm conversions of ∆Emax and AUC0-∞ with correlation coefficients of 0.945. PMI values of blood after TP0463518 administration were elevated to similar or higher levels in comparison with those of umbilical cord blood, which mainly contains liver-derived EPO.

CONCLUSIONS

TP0463518 induced dose-dependent EPO production, mainly derived from the liver in HV and CKD patients. These results suggest that TP0463518 is a new strategy for treating anemia in CKD, which can be used regardless of renal functions.

HIF-1α promotes cyst progression in a mouse model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is mainly caused by mutations of the PKD1 gene and characterized by growth of bilateral renal cysts. Cyst growth is accompanied by regional hypoxia and induction of hypoxia-inducible factor (HIF)-1α in cyst-lining epithelial cells. To determine the relevance of HIF-1α for cyst growth in vivo we used an inducible kidney epithelium-specific knockout mouse to delete Pkd1 at postnatal day 20 or 35 to induce polycystic kidney disease of different severity and analyzed the effects of Hif-1α co-deletion and HIF-1α stabilization using a prolyl-hydroxylase inhibitor. HIF-1α expression was enhanced in kidneys with progressive cyst growth induced by early Pkd1 deletion, but unchanged in the milder phenotype induced by later Pkd1 deletion. Hif-1α co-deletion significantly attenuated cyst growth in the severe, but not in the mild, phenotype. Application of a prolyl-hydroxylase inhibitor resulted in severe aggravation of the mild phenotype with rapid loss of renal function. HIF-1α expression was associated with induction of genes that mediate calcium-activated chloride secretion. Thus, HIF-1α does not seem to play a role in early cyst formation, but accelerates cyst growth during progressive polycystic kidney disease. This novel mechanism of cyst growth may qualify as a therapeutic target.

Prolyl Hydroxylase Inhibitors: A Breakthrough in the Therapy of Anemia Associated with Chronic Diseases

Chronic kidney disease, cancer, chronic inflammatory disorders, nutritional, and genetic deficiency can cause anemia. Hypoxia causes induction of hypoxia-inducible factor (HIF), which stimulates erythropoietin (EPO) synthesis. Prolyl hydroxylase domain (PHD) enzyme inhibition can stabilize hypoxia-inducible factor (HIF). HIF stabilization also decreases hepcidin, a hormone of hepatic origin, which regulates iron homeostasis. PHD inhibitors represent a novel pharmacological treatment of anemia associated with chronic diseases. Many orally active PHD inhibitors like roxadustat, molidustat, vadadustat, and desidustat are in late phase clinical trials. This review discusses the role of PHD inhibitors in the treatment of anemia associated with chronic diseases.

Erythropoiesis and chronic kidney disease-related anemia: From physiology to new therapeutic advancements

Erythropoiesis is triggered by hypoxia and is strictly regulated by hormones, growth factors, cytokines, and vitamins to ensure an adequate oxygen delivery to all body cells. Abnormalities in one or more of these factors may induce different kinds of anemia requiring different treatments. A key player in red blood cell production is erythropoietin. It is a glycoprotein hormone, mainly produced by the kidneys, that promotes erythroid progenitor cell survival and differentiation in the bone marrow and regulates iron metabolism. A deficit in erythropoietin synthesis is the main cause of the normochromic normocytic anemia frequently observed in patients with progressive chronic kidney disease. The present review summarizes the most recent findings about each step of the erythropoietic process, going from the renal oxygen sensing system to the cascade of events induced by erythropoietin through its own receptor in the bone marrow. The paper also describes the new class of drugs designed to stabilize the hypoxia-inducible factor by inhibiting prolyl hydroxylase, with a discussion about their metabolism, disposition, efficacy, and safety. According to many trials, these drugs seem able to simulate tissue hypoxia and then stimulate erythropoiesis in patients affected by renal impairment. In conclusion, the in-depth investigation of all events involved in erythropoiesis is crucial to understand anemia pathophysiology and to identify new therapeutic strategies, in an attempt to overcome the potential side effects of the commonly used erythropoiesis-stimulating agents.

Investigational hypoxia-inducible factor prolyl hydroxylase inhibitors (HIF-PHI) for the treatment of anemia associated with chronic kidney disease

INTRODUCTION

In the last decade, concerns have been raised around the use of erythropoiesis-stimulating agents (ESAs) and intravenous iron in chronic kidney disease (CKD) patients, especially when given at high doses. Moreover, treatment with ESA is expensive.

AREAS COVERED

We searched PubMed for original articles, reviews, and editorials having as a topic anemia, CKD, hypoxia inducible factor, hepcidin, iron, and hypoxia-inducible factor prolyl hydroxylase inhibitors (HIF-PHI). HIF-PHI are a new class of small molecules activating HIF-alfa isoforms (the main mediators of the effects of hypoxia on the body). This causes the secretion of endogenous erythropoietin and increased iron availability. Differing from ESA, HIF-PHI are administered orally. Preliminary data from phase-II clinical studies have shown their efficacy and safety in the short term.

EXPERT OPINION

HIF-PHI are a new promising class of drugs. The results of large, phase-III clinical studies are awaited to prove their efficacy and safety on cardiovascular events and cancer development in the long term. Their capability of penetrating the ESA market in the future will be influenced also by their selling price. The oral administration of HIF-PHI will be weighed to the 'intra-lines' infusion of ESA in hemodialysis or to the infrequent subcutaneous injections of long-acting ESA.

Non-competitive cyclic peptides for targeting enzyme-substrate complexes

Affinity reagents are of central importance for selectively identifying proteins and investigating their interactions. We report on the development and use of cyclic peptides, identified by mRNA display-based RaPID methodology, that are selective for, and tight binders of, the human hypoxia inducible factor prolyl hydroxylases (PHDs) - enzymes crucial in hypoxia sensing. Biophysical analyses reveal the cyclic peptides to bind in a distinct site, away from the enzyme active site pocket, enabling conservation of substrate binding and catalysis. A biotinylated cyclic peptide captures not only the PHDs, but also their primary substrate hypoxia inducible factor HIF1-α. Our work highlights the potential for tight, non-active site binding cyclic peptides to act as promising affinity reagents for studying protein-protein interactions.

First-in-man-proof of concept study with molidustat: a novel selective oral HIF-prolyl hydroxylase inhibitor for the treatment of renal anaemia

AIMS

Insufficient erythropoietin (EPO) synthesis is a relevant cause of renal anaemia in patients with chronic kidney disease. Molidustat, a selective hypoxia-inducible factor prolyl hydroxylase (HIF-PH) inhibitor, increases endogenous EPO levels dose dependently in preclinical models. We examined the pharmacokinetics, safety, tolerability and effect on EPO levels of single oral doses of molidustat in healthy male volunteers.

METHODS

This was a single-centre, randomized, single-blind, placebo-controlled, group-comparison, dose-escalation study. Molidustat was administered at doses of 5, 12.5, 25, 37.5 or 50 mg as a polyethylene glycol-based solution.

RESULTS

In total, 45 volunteers received molidustat and 14 received placebo. Molidustat was absorbed rapidly, and the mean maximum plasma concentration and area under the concentration-time curve increased dose dependently. The mean terminal half-life was 4.64-10.40 h. A significant increase in endogenous EPO was observed following single oral doses of molidustat of 12.5 mg and above. Geometric mean peak EPO levels were 14.8 IU l^-1 (90% confidence interval 13.0, 16.9) for volunteers who received placebo and 39.8 IU l^-1 (90% confidence interval: 29.4, 53.8) for those who received molidustat 50 mg. The time course of EPO levels resembled the normal diurnal variation in EPO. Maximum EPO levels were observed approximately 12 h postdose and returned to baseline after approximately 24-48 h. All doses of molidustat were well tolerated and there were no significant changes in vital signs or laboratory safety parameters.

CONCLUSIONS

Oral administration of molidustat to healthy volunteers elicited a dose-dependent increase in endogenous EPO. These results support the ongoing development of molidustat as a potential new treatment for patients with renal anaemia.

Oxygen sensors as therapeutic targets in kidney disease

Hypoxia is a common clinical problem that has profound effects on renal homeostasis. Prolyl-4-hydroxylases PHD1, 2 and 3 function as oxygen sensors and control the activity of hypoxia-inducible factor (HIF), an oxygen-sensitive transcription factor that regulates a multitude of hypoxia responses, which help cells and tissues to adapt to low oxygen environments. This review provides an overview of the molecular mechanisms that govern these hypoxia responses and discusses clinical experience with compounds that inhibit prolyl-4-hydroxylases to harness HIF responses for therapy in nephrology.

Physiology and pathophysiology of renal erythropoietin-producing cells

Anemia is a common complication and contributes to increased morbidity and mortality in chronic kidney disease (CKD) patients. Whereas there has been a significant improvement of understanding the underlying mechanism of erythropoiesis, the treatment of renal anemia is still restricted to erythropoietin (EPO)-stimulating agents. The purpose of this article is to review the physiology of erythropoiesis, functional role of EPO and underlying molecular and cellular basis that regulate EPO production. Regulation of EPO production is at mRNA level. When anemia or hypoxia occurs, transcriptional factor, hypoxia-inducible factor (HIF), binds to EPO 5' hypoxic response element and EPO gene transcription increases. The renal EPO is mainly produced by pericytes. In CKD, pericytes transdifferentiate to myofibroblasts, and subsequently the ability of EPO production decreases, leading to renal anemia. Recent experimental and clinical studies show the promising efficacy of prolyl hydroxylase inhibitors in renal anemia through increasing EPO production by stabilizing HIF. Recent advances on epigenetics create a new field to study EPO gene expression at chromatin level. We will discuss the role of demethylating agent on restoring EPO expression, providing a novel approach to the treatment of renal anemia.

A Chinese herbal decoction, Jian-Pi-Yi-Shen, regulates the expressions of erythropoietin and pro-inflammatory cytokines in cultured cells

Background

A Chinese herbal formula, namely Jian-Pi-Yi-Shen (JPYS), has been clinically prescribed for patients with chronic kidney disease associated-anemia, and which can improve the patient’s immunological system. However, the mechanisms of JPYS involved in anemia and immune response have not been investigated. To study the role of JPYS in regulating hematopoietic and immunological functions, we investigated its activities on the expressions of erythropoietin and pro-inflammatory cytokines in cultured cells.

Methods

The standardized herbal extracts of JPYS (0–30 μg/ml) were applied onto cultured cells for 24–48 h. Total RNA was collected from the treated cells and subjected to real-time quantitative PCR analysis. Cultured HEK293T cells, transfected with a construct composed of hypoxia response element tagged with a luciferase gene, i.e. pHRE-Luc, were treated with JPYS extracts (1–30 μg/ml) for 24 h. The cell lysates were subjected to luciferase assay.

Results

The treatment with JPYS extract onto cultured HEK293T cells induced erythropoietin expression in a dose-dependent manner, having the highest response by ~ 50% of increase. In parallel, application of JPYS extract for 24 h stimulated expressions of interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α in cultured RAW 264.7 macrophages. In contrast, the pretreatment with JPYS extract suppressed expressions of IL-1β, IL-6, and TNF-α in lipopolysaccharide-induced macrophages.

Conclusions

These results confirmed the hematopoietic function of JPYS in regulating erythropoietin expression, as well as the bidirectional immune-modulatory roles of JPYS by regulating the expression of pro-inflammatory cytokines in cultures.

Roxadustat in the treatment of anaemia in chronic kidney disease

INTRODUCTION

Anaemia is one of the hallmarks of advanced chronic kidney disease (CKD); it correlates with a lower quality of life and increased cardiovascular risk. Currently its management is based on iron and erythropoiesis-stimulating agents (ESAs) therapy. Given safety issues on ESA therapy and excessive iron use, anaemia management is still suboptimal. Areas covered: The inhibitors of the prolyl-hydroxylases domain (PHD) are oral drugs which activate the hypoxia-inducible factors (HIF) and stimulate the production of endogenous erythropoietin. Roxadustat (FG-4592) is a second-generation PHD inhibitor; it is undergoing now phase-III clinical development. Expert opinion: Phase-II clinical trials have shown that roxadustat is effective and save in the short term in either non-dialysis or dialysis CKD patients. Roxadustat is a chemical drug and thus has the potential of being cheaper than traditional ESAs. Given that the peaks of endogenous EPO are much lower than those observed with traditional ESA, it is possible to speculate the roxadustat (and more in general PHD inhibitors) will be safer than ESA on cardiovascular safety end-points. Considering that HIFs are involved in different pathways, with possible promotion of relevant side effects, their safety must be proven in long-term studies.

Discovery of JTZ-951: A HIF Prolyl Hydroxylase Inhibitor for the Treatment of Renal Anemia

Inhibition of hypoxia inducible factor prolyl hydroxylase (PHD) represents a promising strategy for the discovery of a next generation treatment for renal anemia. We identified several 5,6-fused ring systems as novel scaffolds of the PHD inhibitor on the basis of pharmacophore analysis. In particular, triazolopyridine derivatives showed potent PHD2 inhibitory activities. Examination of the predominance of the triazolopyridines in potency by electrostatic calculations suggested favorable π-π stacking interactions with Tyr310. Lead optimization to improve the efficacy of erythropoietin release in cells and in vivo by improving cell permeability led to the discovery of JTZ-951 (compound 14), with a 5-phenethyl substituent on the triazolopyridine group, which increased hemoglobin levels with daily oral dosing in rats. Compound 14 was rapidly absorbed after oral administration and disappeared shortly thereafter, which could be advantageous in terms of safety. Compound 14 was selected as a clinical candidate.

HIF-prolyl hydroxylases as therapeutic targets in erythropoiesis and iron metabolism

A classic response to systemic hypoxia is the increase in red blood cell production. This response is controlled by the prolyl hydroxylase domain/hypoxia-inducible factor (HIF) pathway, which regulates a broad spectrum of cellular functions. The discovery of this pathway as a key regulator of erythropoiesis has led to the development of small molecules that stimulate the production of endogenous erythropoietin and enhance iron metabolism. This review provides a concise overview of the cellular and molecular mechanisms that govern HIF-induced erythropoietic responses and provides an update on clinical experience with compounds that target HIF-prolyl hydroxylases for anemia therapy.

A central role for hypoxia-inducible factor (HIF)-2α in hepatic glucose homeostasis

Hepatic glucose production is regulated by hormonal and dietary factors. At fasting, 80% of glucose released into the circulation is derived from the liver, among which gluconeogenesis accounts for 55% and the rest by glycogenolysis. Studies suggest a complex mechanism involved in the regulation of hepatic glucose metabolism during fasting and post-absorptive phase. Oxygen plays a key role in numerous metabolic pathways such as TCA cycle, gluconeogenesis, glycolysis and fatty acid oxidation. Oxygenation of the gastrointestinal tract including liver and intestine is dynamically regulated by changes in the blood flow and metabolic activity. Cellular adaptation to low oxygen is mediated by the transcription factors HIF-1α and HIF-2α. HIF-1α regulates glycolytic genes whereas HIF-2α is known to primarily regulate genes involved in cell proliferation and iron metabolism. This review focuses on the role of the oxygen sensing signaling in the regulation of hepatic glucose output with an emphasis on hypoxia inducible factor (HIF)-2α. Recent studies have established a metabolic role of HIF-2α in systemic glucose homeostasis. Understanding the HIF-2α dependent mechanism in hepatic metabolism will greatly enhance our potential to utilize the oxygen sensing mechanisms to treat metabolic diseases.

New options for the anemia of chronic kidney disease

Anemia is a common complication of chronic kidney disease. Use of erythropoiesis-stimulating agents (ESA) has been a mainstay of treatment since 1990. A series of large trials demonstrated that ESAs have serious safety problems, including increasing cardiovascular and thrombotic events, and death. Analyses suggest high pharmacologic doses of ESAs, rather than the highly achieved hemoglobin, may mediate harm. Hypoxia-inducible factor (HIF) activators stimulate endogenous erythropoietin production and enhance iron availability. In early clinical trials, these oral agents appear to be capable of replacing ESA therapy and minimizing the need for i.v. iron therapy for chronic kidney disease–related anemia, while having other potentially advantageous actions. Large phase 3 trials are underway with several HIF activators. This commentary reviews trends in anemia management, the safety issues related to our present therapies, the role of HIF in regulating erythropoiesis, and the diverse actions of HIF activators.

The hypoxia-adenosine link during inflammation

Hypoxic tissue conditions occur during a number of inflammatory diseases and are associated with the breakdown of barriers and induction of proinflammatory responses. At the same time, hypoxia is also known to induce several adaptive and tissue-protective pathways that dampen inflammation and protect tissue integrity. Hypoxia-inducible factors (HIFs) that are stabilized during inflammatory or hypoxic conditions are at the center of mediating these responses. In the past decade, several genes regulating extracellular adenosine metabolism and signaling have been identified as being direct targets of HIFs. Here, we discuss the relationship between inflammation, hypoxia, and adenosine and that HIF-driven adenosine metabolism and signaling is essential in providing tissue protection during inflammatory conditions, including myocardial injury, inflammatory bowel disease, and acute lung injury. We also discuss how the hypoxia-adenosine link can be targeted therapeutically in patients as a future treatment approach for inflammatory diseases.

Phase 2 studies of oral hypoxia-inducible factor prolyl hydroxylase inhibitor FG-4592 for treatment of anemia in China

BACKGROUND

FG-4592 (roxadustat) is an oral hypoxia-inducible factor (HIF) prolyl hydroxylase inhibitor (HIF-PHI) promoting coordinated erythropoiesis through the transcription factor HIF. Two Phase 2 studies were conducted in China to explore the safety and efficacy of FG-4592 (USAN name: roxadustat, CDAN name: ), a HIF-PHI, in patients with anemia of chronic kidney disease (CKD), both patients who were dialysis-dependent (DD) and patients who were not dialysis-dependent (NDD).

METHODS

In the NDD study, 91 participants were randomized to low (1.1-1.75 mg/kg) or high (1.50-2.25 mg/kg) FG-4592 starting doses or to placebo. In the DD study, 87 were enrolled to low (1.1-1.8 mg/kg), medium (1.5-2.3 mg/kg) and high (1.7-2.3 mg/kg) starting FG-4592 doses or to continuation of epoetin alfa. In both studies, only oral iron supplementation was allowed.

RESULTS

In the NDD study, hemoglobin (Hb) increase ≥1 g/dL from baseline was achieved in 80.0% of subjects in the low-dose cohort and 87.1% in the high-dose cohort, versus 23.3% in the placebo arm (P < 0.0001, both). In the DD study, 59.1%, 88.9% (P = 0.008) and 100% (P = 0.0003) of the low-, medium- and high-dose subjects maintained their Hb levels after 5- and 6-weeks versus 50% of the epoetin alfa-treated subjects. In both studies, significant reductions in cholesterol were noted in FG-4592-treated subjects, with stability or increases in serum iron, total iron-binding capacity (TIBC) and transferrin (without intravenous iron administration). In the NDD study, hepcidin levels were significantly reduced across all FG-4592-treated arms as compared with no change in the placebo arm. In the DD study, hepcidin levels were also reduced in a statistically significant dose-dependent manner in the highest dose group as compared with the epoetin alfa-treated group. Adverse events were similar for FG-4592-treated and control subjects.

CONCLUSIONS

FG-4592 may prove an effective alternative for managing anemia of CKD. It is currently being investigated in a pivotal global Phase 3 program.

Hypoxia-inducible-factor-1 in trauma and critical care

HIF-1 is a ubiquitous signaling molecule constantly expressed by the body, but is degraded during normoxic conditions. In hypoxic conditions, it persists and is active. Hypoxia is often associated with trauma due to interrupted blood flow, inflammation or other reasons, causing HIF-1 to be active in signaling and recovery. In this review, the function of HIF-1 is examined, as well as its clinical significance with regard to trauma and critical care. Using this information, we then identify potential points of treatment and intervention.

The role of indoxyl sulfate in renal anemia in patients with chronic kidney disease

Renal anemia is a common complication in patients with advanced chronic kidney disease. In vitro studies have shown that indoxyl sulfate decreases erythropoietin production. Whether this effect is seen in vivo remains unclear. Our goal was to explore the role of indoxyl sulfate in renal anemia. We found serum indoxyl sulfate levels are significantly and negatively associated with erythropoietin levels in human. A multiple stepwise linear regression analyses after adjustment for other independent parameters revealed that free indoxyl sulfate, and total indoxyl sulfate were significantly associated with erythropoietin levels. In animal studies, erythropoietin gene and protein expression were markedly inhibited in rats with chronic kidney disease; however, this effect was significantly reversed by lowering serum indoxyl sulfate with AST-120. Indoxyl sulfate may also inhibit erythropoietin expression in animal models with chronic kidney disease. These findings further support the role of indoxyl sulfate in the development of renal anemia.

The effects of probiotic supplement on hemoglobin in chronic renal failure patients under hemodialysis: A randomized clinical trial

Background:

Chronic inflammation is one of the causes of anemia in chronic renal failure patients under hemodialysis. Probiotics probably establish a balance between pro- and anti-inflammatory cytokines. The study was conducted to determine the effects of probiotic supplementation on hemoglobin (Hb) in hemodialysis patients.

Materials and Methods:

A parallel clinical trial was conducted in which patients were randomly allocated into two groups. The intervention group (n = 18) was given a 500 mg probiotic supplement (a capsule) every day whereas the control group (n = 18) received placebo (a capsule), both for 3 months. Hb levels and C-reactive protein (CRP) levels were measured for three periods. The data were analyzed in SPSS-16 using statistical tests including the t-test and repeated-measures ANOVA.

Results:

In the probiotic supplementation group, the mean Hb was 9.22 ± 1.04 mg/dl before the intervention and reached 10.85 ± 1.177 mg/dl afterward, while in the placebo group, the mean Hb level was 9.38 ± 0.97 mg/dl before the intervention and reached 10.03 ± 1.97 mg/dl afterward (P > 0.05). During the study, the placebo caused to increase of Hb temporary, but in longer term, the effect of probiotic was more manifested. Hb levels increased in both groups although the change was not statistically significant (P > 0.05). The findings showed no significant differences between the two groups in either the pre- or post-intervention CRP levels (P = 0.239).

Conclusion:

Probiotic supplementation decreased Hb fluctuations in hemodialysis patients but did not result in a significant increase in Hb levels. Similar studies are therefore recommended to be conducted with a prolonged duration of the study or an increased probiotic dose with larger sample size to complete the results of the present study.

Subcellular Energetics and Metabolism: Potential Therapeutic Applications

Part I of this review discussed the similarities between embryogenesis, mammalian adaptions to hypoxia (primarily driven by hypoxia-inducible factor-1 [HIF-1]), ischemia-reperfusion injury (and its relationship with reactive oxygen species), hibernation, diving animals, cancer, and sepsis, and it focused on the common characteristics that allow cells and organisms to survive in these states. Part II of this review describes techniques by which researchers gain insight into subcellular energetics and identify potential future tools for clinicians. In particular, P nuclear magnetic resonance to measure high-energy phosphates, serum lactate measurements, the use of near-infrared spectroscopy to measure the oxidation state of cytochrome aa3, and the ability of the protoporphyrin IX-triplet state lifetime technique to measure mitochondrial oxygen tension are discussed. In addition, this review discusses novel treatment strategies such as hyperbaric oxygen, preconditioning, exercise training, therapeutic gases, as well as inhibitors of HIF-1, HIF prolyl hydroxylase, and peroxisome proliferator-activated receptors.

Current and future chemical therapies for treating anaemia in chronic kidney disease

INTRODUCTION

Erythropoiesis-stimulating agents (ESAs) are not perfect, since they have potential side effects. Iron therapy is also receiving growing attention in recent years. Areas covered: We performed a literature search on PubMed using the following key words: anemia, chronic kidney disease, HIF stabilisers, sotatercept, actin traps, iron, iron-containing phosphate binders, iron dialysate. We reviewed new drugs that are under clinical development to obtain better safety and activity and/or easier and cheaper manufacturing processes in comparison to available ESAs. We also considered new strategies to increase iron stores. Several phase 1 and 2 studies support the beneficial role of increasing Hypoxia Inducible factor (HIF) activity for stimulating endogenous erythropoiesis. Sotatercept and luspatercept, two activin traps, are undergoing clinical development mainly for indications other than CKD. They have the additional effect of improving osteoporosis. Iron-containing phosphate binders have become available recently. Expert opinion: Several medical needs are unmet with ESA. HIF stabilisers are the most appealing drugs undergoing clinical development. They expose patients to lower levels of EPO than ESA, possibly reducing unintended effects. Their long-term safety is still to be demonstrated. One new iron-containing phosphate binders has the potential of combining two indications: hyperphosphoremia and iron deficiency, possibly improving compliance.

Therapeutic targeting of the HIF oxygen-sensing pathway: Lessons learned from clinical studies

The oxygen-sensitive hypoxia-inducible factor (HIF) pathway plays a central role in the control of erythropoiesis and iron metabolism. The discovery of prolyl hydroxylase domain (PHD) proteins as key regulators of HIF activity has led to the development of inhibitory compounds that are now in phase 3 clinical development for the treatment of renal anemia, a condition that is commonly found in patients with advanced chronic kidney disease. This review provides a concise overview of clinical effects associated with pharmacologic PHD inhibition and was written in memory of Professor Lorenz Poellinger.

Hypoxia Inducible Factor (HIF) Hydroxylases as Regulators of Intestinal Epithelial Barrier Function

Human health is dependent on the ability of the body to extract nutrients, fluids, and oxygen from the external environment while at the same time maintaining a state of internal sterility. Therefore, the cell layers that cover the surface areas of the body such as the lung, skin, and gastrointestinal mucosa provide vital semipermeable barriers that allow the transport of essential nutrients, fluid, and waste products, while at the same time keeping the internal compartments free of microbial organisms. These epithelial surfaces are highly specialized and differ in their anatomic structure depending on their location to provide appropriate and effective site-specific barrier function. Given this important role, it is not surprising that significant disease often is associated with alterations in epithelial barrier function. Examples of such diseases include inflammatory bowel disease, chronic obstructive pulmonary disease, and atopic dermatitis. These chronic inflammatory disorders often are characterized by diminished tissue oxygen levels (hypoxia). Hypoxia triggers an adaptive transcriptional response governed by hypoxia-inducible factors (HIFs), which are repressed by a family of oxygen-sensing HIF hydroxylases. Here, we review recent evidence suggesting that pharmacologic hydroxylase inhibition may be of therapeutic benefit in inflammatory bowel disease through the promotion of intestinal epithelial barrier function through both HIF-dependent and HIF-independent mechanisms.

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.

Short-acting erythropoiesis-stimulating agents for anaemia in predialysis patients

BACKGROUND

The benefits of erythropoiesis-stimulating agents (ESA) for chronic kidney disease (CKD) patients have been previously demonstrated. However, the efficacy and safety of short-acting epoetins administered at larger doses and reduced frequency as well as of new epoetins and biosimilars remains uncertain.

OBJECTIVES

This review aimed to evaluate the benefits and harms of different routes, frequencies and doses of epoetins (epoetin alpha, epoetin beta and other short-acting epoetins) for anaemia in adults and children with CKD not receiving dialysis.

SEARCH METHODS

We searched the Cochrane Kidney and Transplant Specialised Register to 12 September 2016 through contact with the Information Specialist using search terms relevant to this review. Studies contained in the Specialised Register are identified through search strategies specifically designed for CENTRAL, MEDLINE, and EMBASE; handsearching conference proceedings; and searching the International Clinical Trials Register (ICTRP) Search Portal and ClinicalTrials.gov.

SELECTION CRITERIA

We included randomised control trials (RCTs) comparing different frequencies, routes, doses and types of short-acting ESAs in CKD patients.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed study eligibility and four authors assessed risk of bias and extracted data. Results were expressed as risk ratio (RR) or risk differences (RD) with 95% confidence intervals (CI) for dichotomous outcomes. For continuous outcomes the mean difference (MD) with 95% confidence intervals (CI) was used. Statistical analyses were performed using the random-effects model.

MAIN RESULTS

We identified 14 RCTs (2616 participants); nine studies were multi-centre and two studies involved children. The risk of bias was high in most studies; only three studies demonstrated adequate random sequence generation and only two studies were at low risk of bias for allocation concealment. Blinding of participants and personnel was at low risk of bias in one study. Blinding of outcome assessment was judged at low risk in 13 studies as the outcome measures were reported as laboratory results and therefore unlikely to be influenced by blinding. Attrition bias was at low risk of bias in eight studies while selective reporting was at low risk in six included studies.Four interventions were compared: epoetin alpha or beta at different frequencies using the same total dose (six studies); epoetin alpha at the same frequency and different total doses (two studies); epoetin alpha administered intravenously versus subcutaneous administration (one study); epoetin alpha or beta versus other epoetins or biosimilars (five studies). One study compared both different frequencies of epoetin alpha at the same total dose and at the same frequency using different total doses.Data from only 7/14 studies could be included in our meta-analyses. There were no significant differences in final haemoglobin (Hb) levels when dosing every two weeks was compared with weekly dosing (4 studies, 785 participants: MD -0.20 g/dL, 95% CI -0.33 to -0.07), when four weekly dosing was compared with two weekly dosing (three studies, 671 participants: MD -0.16 g/dL, 95% CI -0.43 to 0.10) or when different total doses were administered at the same frequency (four weekly administration: one study, 144 participants: MD 0.17 g/dL 95% CI -0.19 to 0.53).Five studies evaluated different interventions. One study compared epoetin theta with epoetin alpha and found no significant differences in Hb levels (288 participants: MD -0.02 g/dL, 95% CI -0.25 to 0.21). One study found significantly higher pain scores with subcutaneous epoetin alpha compared with epoetin beta. Two studies (165 participants) compared epoetin delta with epoetin alpha, with no results available since the pharmaceutical company withdrew epoetin delta for commercial reasons. The fifth study comparing the biosimilar HX575 with epoetin alpha was stopped after patients receiving HX575 subcutaneously developed anti-epoetin antibodies and no results were available.Adverse events were poorly reported in all studies and did not differ significantly within comparisons. Mortality was only detailed adequately in four studies and only one study included quality of life data.

AUTHORS' CONCLUSIONS

Epoetin alpha given at higher doses for extended intervals (two or four weekly) is non-inferior to more frequent dosing intervals in maintaining final Hb levels with no significant differences in adverse effects in non-dialysed CKD patients. However the data are of low methodological quality so that differences in efficacy and safety cannot be excluded. Further large, well designed, RCTs with patient-centred outcomes are required to assess the safety and efficacy of large doses of the shorter acting ESAs, including biosimilars of epoetin alpha, administered less frequently compared with more frequent administration of smaller doses in children and adults with CKD not on dialysis.

The VHL Tumor Suppressor Gene: Insights into Oxygen Sensing and Cancer

Mammalian cells sense changes in oxygen and transduce that information into adaptive changes in gene expression using a conserved pathway that converges on the heterodimeric transcription factor called hypoxia-inducible factor (HIF), which contains a labile alpha subunit and a stable beta subunit. In the presence of oxygen, the alpha subunit is hydroxylated on one (or both) of two highly conserved prolyl residues by an Egg-Laying Defective Nine (EglN) [also called Prolyl Hydroxylase Domain (PHD)] dioxygenase, which recruits an ubiquitin ligase complex containing the VHL tumor suppressor gene product. Germline VHL mutations cause von Hippel-Lindau (VHL) disease, which manifest as angiogenic tumors such as hemangioblastomas and kidney cancers. Somatic VHL inactivation and deregulation of HIF (especially HIF2α) drives sporadic kidney cancers and an HIF2α inhibitor is showing promise for this disease. VHL, EglN1, and HIF2α polymorphisms have been linked to familial polycythemia and adaptation to high altitude. Orally available EglN inhibitors are being developed for the treatment of anemia and ischemic diseases.

Hypoxia inducible factor stabilization improves defective ischemia-induced angiogenesis in a rodent model of chronic kidney disease

Chronic kidney disease (CKD) is associated with increased risk and worse prognosis of cardiovascular disease, including peripheral artery disease. An impaired angiogenic response to ischemia may contribute to poor outcomes of peripheral artery disease in patients with CKD. Hypoxia inducible factors (HIF) are master regulators of angiogenesis and therefore represent a promising target for therapeutic intervention. To test this we induced hind-limb ischemia in rats with CKD caused by 5/6 nephrectomy and administered two different treatments known to stabilize HIF protein in vivo: carbon monoxide and a pharmacological inhibitor of prolyl hydroxylation 2-(1-chloro-4- hydroxyisoquinoline-3-carboxamido) acetate (ICA). Expression levels of pro-angiogenic HIF target genes (Vegf, Vegf-r1, Vegf-r2, Ho-1) were measured by qRT-PCR. Capillary density was measured by CD31 immunofluorescence staining and HIF expression was evaluated by immunohistochemistry. Capillary density in ischemic skeletal muscle was significantly lower in CKD animals compared to sham controls. Rats with CKD showed significantly lower expression of HIF and all measured pro-angiogenic HIF target genes, including VEGF. Both HIF stabilizing treatments rescued HIF target gene expression in animals with CKD and led to significantly higher ischemia-induced capillary sprouting compared to untreated controls. ICA was effective regardless of whether it was administered before or after induction of ischemia and led to a HIF expression in skeletal muscle. Thus, impaired ischemia-induced angiogenesis in rats with CKD can be improved by HIF stabilization, even if started after onset of ischemia.

Hydroxylase inhibition regulates inflammation-induced intestinal fibrosis through the suppression of ERK-mediated TGF-β1 signaling. [corrected]

Fibrosis is a complication of chronic inflammatory disorders such as inflammatory bowel disease, a condition which has limited therapeutic options and often requires surgical intervention. Pharmacologic inhibition of oxygen-sensing prolyl hydroxylases, which confer oxygen sensitivity upon the hypoxia-inducible factor pathway, has recently been shown to have therapeutic potential in colitis, although the mechanisms involved remain unclear. Here, we investigated the impact of hydroxylase inhibition on inflammation-driven fibrosis in a murine colitis model. Mice exposed to dextran sodium sulfate, followed by a period of recovery, developed intestinal fibrosis characterized by alterations in the pattern of collagen deposition and infiltration of activated fibroblasts. Treatment with the hydroxylase inhibitor dimethyloxalylglycine ameliorated fibrosis. TGF-β1 is a key regulator of fibrosis that acts through the activation of fibroblasts. Hydroxylase inhibition reduced TGF-β1-induced expression of fibrotic markers in cultured fibroblasts, suggesting a direct role for hydroxylases in TGF-β1 signaling. This was at least in part due to inhibition of noncanonical activation of extracellular signal-regulated kinase (ERK) signaling. In summary, pharmacologic hydroxylase inhibition ameliorates intestinal fibrosis through suppression of TGF-β1-dependent ERK activation in fibroblasts. We hypothesize that in addition to previously reported immunosupressive effects, hydroxylase inhibitors independently suppress profibrotic pathways.

Oxygen Sensing and Homeostasis

The discovery of carotid bodies as sensory receptors for detecting arterial blood oxygen levels, and the identification and elucidation of the roles of hypoxia-inducible factors (HIFs) in oxygen homeostasis have propelled the field of oxygen biology. This review highlights the gas-messenger signaling mechanisms associated with oxygen sensing, as well as transcriptional and non-transcriptional mechanisms underlying the maintenance of oxygen homeostasis by HIFs and their relevance to physiology and pathology.

Tuning the Transcriptional Response to Hypoxia by Inhibiting Hypoxia-inducible Factor (HIF) Prolyl and Asparaginyl Hydroxylases

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.

Emerging drugs for the treatment of kidney disease-induced anemia

INTRODUCTION

Anemia has been remained one of the most characteristic and visible manifestations of chronic renal failure. Correction of anemia requires two main treatment strategies: increased stimulation of erythropoiesis, and maintenance of an adequate iron supply to the bone marrow.

AREAS COVERED

Erythropoiesis activating agents became a mainstay in the treatment of renal anemia for more than 25 years. Recently, there have been several attempts to introduce new drugs to stimulate erythropoiesis or affect the hepcidin-ferroportin pathway. Orally available hypoxia-inducible factor (HIF) stabilizing compounds are attractive alternatives. They not only increase hemoglobin, but also suppress hepcidin production and improve iron availability. Novel iron preparations, may also help to ameliorate anemia, with acceptable safety profile and other beneficial properties such a phosphate binding.

EXPERT OPINION

One should be aware of potential risks and benefits of novel sophisticated therapies and their role in the management of renal anemia remain to be established. In particular HIF stabilizers needs to be proven safe, or even safer than ESAs, in large long-term safety studies testing hard end points, due its ubiquitous nature and the regulation of variety of biological processes potentially leading to unexpected side effects. Besides safety, cost-effectiveness appears the major issue in the modern world, including nephrology.

A mechanistic link between renal ischemia and fibrosis

Renal fibrosis is characterized by tubular cell atrophy and accumulation of extracellular matrix. Fibroblast activation becomes evident in areas surrounding atrophic tubules, with rarefaction of peritubular capillaries. Tubulointerstitial hypoxia is the final common pathway in progressive kidney disease. Hypoxia suppresses tubular epithelial growth and leads to failure of remodeling by facilitating dedifferentiation and apoptosis. Profibrotic factors such as transforming growth factor-β (TGF-β) mediate fibroblast activation, and recruited leukocytes, which appear in hypoxic areas, contribute to fibrosis. While resident renal cells adapt to the hypoxic environment via upregulation of relevant genes by hypoxia-inducible factor (HIF) family members, hypoxic adaptation via HIF may not be sufficient in chronic kidney disease (CKD) due to multiple factors. Thus, restoration of HIF-mediated responses may contribute to amelioration of CKD pathology. Studies to date have reported that HIF activation reduces inflammation and oxidative stress and ameliorates injury by decreasing tubular cell apoptosis and restoring peritubular capillary network. Prolyl hydroxylase domain (PHD) inhibitors that specifically activate HIF are currently evaluated for the treatment of renal anemia and may be effective for the treatment of CKD.

Immunologic Consequences of Hypoxia during Critical Illness

Hypoxia and immunity are highly intertwined at clinical, cellular, and molecular levels. The prevention of tissue hypoxia and modulation of systemic inflammation are cornerstones of daily practice in the intensive care unit. Potentially, immunologic effects of hypoxia may contribute to outcome and represent possible therapeutic targets. Hypoxia and activation of downstream signaling pathways result in enhanced innate immune responses, aimed to augment pathogen clearance. On the other hand, hypoxia also exerts antiinflammatory and tissue-protective effects in lymphocytes and other tissues. Although human data on the net immunologic effects of hypoxia and pharmacologic modulation of downstream pathways are limited, preclinical data support the concept of tailoring the immune response through modulation of the oxygen status or pharmacologic modulation of hypoxia-signaling pathways in critically ill patients.

[Mechanism of and Therapy for Kidney Fibrosis]

Fibrosis occurs in systemic tissues other than the brain and finally induces dysfunction of the fibrotic organ. Kidney fibrosis is related to scarring after acute kidney injury and the progression of chronic kidney disease. Kidney function decreases with the progression of kidney fibrosis. As fibrotic tissue cannot return to its original status, advanced kidney fibrosis requires the administration of dialysis or kidney transplantation. Thus, elucidation the mechanism of kidney fibrosis is an important research theme. The proliferation and activation of (myo) fibroblasts and the excessive production of an extracellular matrix are common mechanisms in fibrosis in many organs, but it seems that kidney fibrosis has specific pathways. Tubular epithelial, mesangial cells, and erythropoietin producing cells, which exist only in the kidney, participate in forming kidney fibrosis. This review highlights an understanding of the cells and their underlying mechanisms, which are specific to kidney fibrosis process: transforming growth factor-β (TGF-β), epithelial-mesenchymal transition, wingless/int-1 (WNT) signaling, renal anemia, and uremia. Finally, we describe potential therapies that focus on the mechanisms of kidney fibrosis: anti-TGF-β antibody and mammalian target of rapamycin (mTOR).

Distinct subpopulations of FOXD1 stroma-derived cells regulate renal erythropoietin

Renal peritubular interstitial fibroblast-like cells are critical for adult erythropoiesis, as they are the main source of erythropoietin (EPO). Hypoxia-inducible factor 2 (HIF-2) controls EPO synthesis in the kidney and liver and is regulated by prolyl-4-hydroxylase domain (PHD) dioxygenases PHD1, PHD2, and PHD3, which function as cellular oxygen sensors. Renal interstitial cells with EPO-producing capacity are poorly characterized, and the role of the PHD/HIF-2 axis in renal EPO-producing cell (REPC) plasticity is unclear. Here we targeted the PHD/HIF-2/EPO axis in FOXD1 stroma-derived renal interstitial cells and examined the role of individual PHDs in REPC pool size regulation and renal EPO output. Renal interstitial cells with EPO-producing capacity were entirely derived from FOXD1-expressing stroma, and Phd2 inactivation alone induced renal Epo in a limited number of renal interstitial cells. EPO induction was submaximal, as hypoxia or pharmacologic PHD inhibition further increased the REPC fraction among Phd2-/- renal interstitial cells. Moreover, Phd1 and Phd3 were differentially expressed in renal interstitium, and heterozygous deficiency for Phd1 and Phd3 increased REPC numbers in Phd2-/- mice. We propose that FOXD1 lineage renal interstitial cells consist of distinct subpopulations that differ in their responsiveness to Phd2 inactivation and thus regulation of HIF-2 activity and EPO production under hypoxia or conditions of pharmacologic or genetic PHD inactivation.

Four-Week Studies of Oral Hypoxia-Inducible Factor-Prolyl Hydroxylase Inhibitor GSK1278863 for Treatment of Anemia

Hypoxia-inducible factor prolyl hydroxylase inhibitors stabilize levels of hypoxia-inducible factor that upregulate transcription of multiple genes associated with the response to hypoxia, including production of erythropoietin. We conducted two phase 2a studies to explore the relationship between the dose of the hypoxia-inducible factor-prolyl hydroxylase inhibitor GSK1278863 and hemoglobin response in patients with anemia of CKD (baseline hemoglobin 8.5-11.0 g/dl) not undergoing dialysis and not receiving recombinant human erythropoietin (nondialysis study) and in patients with anemia of CKD (baseline hemoglobin 9.5-12.0 g/dl) on hemodialysis and being treated with stable doses of recombinant human erythropoietin (hemodialysis study). Participants were randomized 1:1:1:1 to a once-daily oral dose of GSK1278863 (0.5 mg, 2 mg, or 5 mg) or control (placebo for the nondialysis study; continuing on recombinant human erythropoietin for the hemodialysis study) for 4 weeks, with a 2-week follow-up. In the nondialysis study, GSK1278863 produced dose-dependent effects on hemoglobin, with the highest dose resulting in a mean increase of 1 g/dl at week 4. In the hemodialysis study, treatment with GSK1278863 in the 5-mg arm maintained mean hemoglobin concentrations after the switch from recombinant human erythropoietin, whereas mean hemoglobin decreased in the lower-dose arms. In both studies, the effects on hemoglobin occurred with elevations in endogenous erythropoietin within the range usually observed in the respective populations and markedly lower than those in the recombinant human erythropoietin control arm in the hemodialysis study, and without clinically significant elevations in plasma vascular endothelial growth factor concentrations. GSK1278863 was generally safe and well tolerated at the doses and duration studied. GSK1278863 may prove an effective alternative for managing anemia of CKD.