HIF prolyl hydroxylase inhibition prior to transient focal cerebral ischaemia is neuroprotective in mice

Recent Advances and Therapeutic Implications of 2-Oxoglutarate-Dependent Dioxygenases in Ischemic Stroke

Ischemic stroke is a common disease with a high disability rate and mortality, which brings heavy pressure on families and medical insurance. Nowadays, the golden treatments for ischemic stroke in the acute phase mainly include endovascular therapy and intravenous thrombolysis. Some drugs are used to alleviate brain injury in patients with ischemic stroke, such as edaravone and 3-n-butylphthalide. However, no effective neuroprotective drug for ischemic stroke has been acknowledged. 2-Oxoglutarate-dependent dioxygenases (2OGDDs) are conserved and common dioxygenases whose activities depend on O2, Fe2+, and 2OG. Most 2OGDDs are expressed in the brain and are essential for the development and functions of the brain. Therefore, 2OGDDs likely play essential roles in ischemic brain injury. In this review, we briefly elucidate the functions of most 2OGDDs, particularly the effects of regulations of 2OGDDs on various cells in different phases after ischemic stroke. It would also provide promising potential therapeutic targets and directions of drug development for protecting the brain against ischemic injury and improving outcomes of ischemic stroke.

Hypoxia and Hypoxia Mimetic Agents As Potential Priming Approaches to Empower Mesenchymal Stem Cells

Mesenchymal stem cells (MSC) exhibit self-renewal capacity and multilineage differentiation potential, making them attractive for research and clinical application. The properties of MSC can vary depending on specific micro-environmental factors. MSC resides in specific niches with low oxygen concentrations, where oxygen functions as a metabolic substrate and a signaling molecule. Conventional physical incubators or chemically hypoxia mimetic agents are applied in cultures to mimic the original low oxygen tension settings where MSC originated. This review aims to focus on the current knowledge of the effects of various physical hypoxic conditions and widely used hypoxia-mimetic agents-PHD inhibitors on mesenchymal stem cells at a cellular and molecular level, including proliferation, stemness, differentiation, viability, apoptosis, senescence, migration, immunomodulation behaviors, as well as epigenetic changes.

A comprehensive review of stroke-related signaling pathways and treatment in western medicine and traditional Chinese medicine

This review provides insight into the complex network of signaling pathways and mechanisms involved in stroke pathophysiology. It summarizes the historical progress of stroke-related signaling pathways, identifying potential interactions between them and emphasizing that stroke is a complex network disease. Of particular interest are the Hippo signaling pathway and ferroptosis signaling pathway, which remain understudied areas of research, and are therefore a focus of the review. The involvement of multiple signaling pathways, including Sonic Hedgehog (SHH), nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE), hypoxia-inducible factor-1α (HIF-1α), PI3K/AKT, JAK/STAT, and AMPK in pathophysiological mechanisms such as oxidative stress and apoptosis, highlights the complexity of stroke. The review also delves into the details of traditional Chinese medicine (TCM) therapies such as Rehmanniae and Astragalus, providing an analysis of the recent status of western medicine in the treatment of stroke and the advantages and disadvantages of TCM and western medicine in stroke treatment. The review proposes that since stroke is a network disease, TCM has the potential and advantages of a multi-target and multi-pathway mechanism of action in the treatment of stroke. Therefore, it is suggested that future research should explore more treasures of TCM and develop new therapies from the perspective of stroke as a network disease.

Chrysin protects against cerebral ischemia-reperfusion injury in hippocampus via restraining oxidative stress and transition elements

Chrysin is a natural flavonoid compound that has antioxidant and neuroprotective effects. Cerebral ischemia reperfusion (CIR) is closely connected with increased oxidative stress in the hippocampal CA1 region and homeostasis disorder of transition elements such as iron (Fe), copper (Cu) and zinc (Zn). This exploration was conducted to elucidate the antioxidant and neuroprotective effects of chrysin based on transient middle cerebral artery occlusion (tMCAO) in rats. Experimentally, sham group, model group, chrysin (50.0 mg/kg) group, Ginaton (21.6 mg/kg) group, Dimethyloxallyl Glycine (DMOG, 20.0 mg/kg) + chrysin group and DMOG group were devised. The rats in each group were performed to behavioral evaluation, histological staining, biochemical kit detection, and molecular biological detection. The results indicated that chrysin restrained oxidative stress and the rise of transition element levels, and regulated transition element transporter levels in tMCAO rats. DMOG activated hypoxia-inducible factor-1 subunit alpha (HIF-1α), reversed the antioxidant and neuroprotective effects of chrysin, and increased transition element levels. In a word, our findings emphasize that chrysin plays a critical role in protecting CIR injury via inhibiting HIF-1α against enhancive oxidative stress and raised transition metal levels.

Clinical Potential of Hypoxia Inducible Factors Prolyl Hydroxylase Inhibitors in Treating Nonanemic Diseases

Hypoxia inducible factors (HIFs) and their regulatory hydroxylases the prolyl hydroxylase domain enzymes (PHDs) are the key mediators of the cellular response to hypoxia. HIFs are normally hydroxylated by PHDs and degraded, while under hypoxia, PHDs are suppressed, allowing HIF-α to accumulate and transactivate multiple target genes, including erythropoiesis, and genes participate in angiogenesis, iron metabolism, glycolysis, glucose transport, cell proliferation, survival, and so on. Aiming at stimulating HIFs, a group of small molecules antagonizing HIF-PHDs have been developed. Of these HIF-PHDs inhibitors (HIF-PHIs), roxadustat (FG-4592), daprodustat (GSK-1278863), vadadustat (AKB-6548), molidustat (BAY 85-3934) and enarodustat (JTZ-951) are approved for clinical usage or have progressed into clinical trials for chronic kidney disease (CKD) anemia treatment, based on their activation effect on erythropoiesis and iron metabolism. Since HIFs are involved in many physiological and pathological conditions, efforts have been made to extend the potential usage of HIF-PHIs beyond anemia. This paper reviewed the progress of preclinical and clinical research on clinically available HIF-PHIs in pathological conditions other than CKD anemia.

Erythropoietin regulates signaling pathways associated with neuroprotective events

Erythropoietin is a cytokine that binds to the Erythropoietin receptor and regulates the formation of erythroid cells during erythropoiesis in the bone marrow. However, many other organs and tissues express Erythropoietin and its receptor, such as the Nervous System, which principally regulates tissue protection. In the Central Nervous System, Erythropoietin is principally expressed by astrocytes, while neurons mainly express Erythropoietin receptors. Moreover, Erythropoietin acts as a pleiotropic molecule with neuroprotective effects, and its mechanisms of signal transduction pathways are defined, and there is a growing interest in its therapeutic potential. This review focuses on the role of Erythropoietin and its relationship with HIF1, PI3/Akt, GSK3B, JAK/STAT, and MAPKs signaling pathways that leads to cell survival after injury in the Central Nervous System. Knowledge of these signaling systems comprehensively could better guide EPO treatment to restoring different SNC alterations mediated by different insults.

The Effect of Erythropoietin and Its Derivatives on Ischemic Stroke Therapy: A Comprehensive Review

Numerous studies explored the therapeutic effects of erythropoietin (EPO) on neurodegenerative diseases. Few studies provided comprehensive and latest knowledge of EPO treatment for ischemic stroke. In the present review, we introduced the structure, expression, function of EPO, and its receptors in the central nervous system. Furthermore, we comprehensively discussed EPO treatment in pre-clinical studies, clinical trials, and its therapeutic mechanisms including suppressing inflammation. Finally, advanced studies of the therapy of EPO derivatives in ischemic stroke were also discussed. We wish to provide valuable information on EPO and EPO derivatives’ treatment for ischemic stroke for basic researchers and clinicians to accelerate the process of their clinical applications.

Structure-Activity Relationships and Transcriptomic Analysis of Hypoxia-Inducible Factor Prolyl Hydroxylase Inhibitors

To evaluate the differences in action of commercially available 2-oxoglutarate mimetics and “branched-tail” oxyquinoline inhibitors of hypoxia-inducible factor prolyl hydroxylase (HIF PHD), the inhibitors’ IC₅₀ values in the activation of HIF1 ODD-luciferase reporter were selected for comparative transcriptomics. Structure–activity relationship and computer modeling for the oxyquinoline series of inhibitors led to the identification of novel inhibitors, which were an order of magnitude more active in the reporter assay than roxadustat and vadadustat. Unexpectedly, 2-methyl-substitution in the oxyquinoline core of the best HIF PHD inhibitor was found to be active in the reporter assay and almost equally effective in the pretreatment paradigm of the oxygen-glucose deprivation in vitro model. Comparative transcriptomic analysis of the signaling pathways induced by HIF PHD inhibitors showed high potency of the two novel oxyquinoline inhibitors (#4896-3249 and #5704-0720) at 2 μM concentrations matching the effect of 30 μM roxadustat and 500 μM dimethyl oxalyl glycine in inducing HIF1 and HIF2-linked pathways. The two oxyquinoline inhibitors exerted the same activation of HIF-triggered glycolytic pathways but opposite effects on signaling pathways linked to alternative substrates of HIF PHD 1 and 3, such as p53, NF-κB, and ATF4. This finding can be interpreted as the specificity of the 2-methyl-substitute variant for HIF PHD2.

Pericyte, but not astrocyte, hypoxia inducible factor-1 (HIF-1) drives hypoxia-induced vascular permeability in vivo

BACKGROUND

Ways to prevent disease-induced vascular modifications that accelerate brain damage remain largely elusive. Improved understanding of perivascular cell signalling could provide unparalleled insight as these cells impact vascular stability and functionality of the neurovascular unit as a whole. Identifying key drivers of astrocyte and pericyte responses that modify cell-cell interactions and crosstalk during injury is key. At the cellular level, injury-induced outcomes are closely entwined with activation of the hypoxia-inducible factor-1 (HIF-1) pathway. Studies clearly suggest that endothelial HIF-1 signalling increases blood-brain barrier permeability but the influence of perivascular HIF-1 induction on outcome is unknown. Using novel mouse lines with astrocyte and pericyte targeted HIF-1 loss of function, we herein show that vascular stability in vivo is differentially impacted by perivascular hypoxia-induced HIF-1 stabilization.

METHODS

To facilitate HIF-1 deletion in adult mice without developmental complications, novel Cre-inducible astrocyte-targeted (GFAP-CreER^T2; HIF-1α^fl/fl and GLAST-CreER^T2; HIF-1α^fl/fl) and pericyte-targeted (SMMHC-CreER^T2; HIF-1α^fl/fl) transgenic animals were generated. Mice in their home cages were exposed to either normoxia (21% O₂) or hypoxia (8% O₂) for 96 h in an oxygen-controlled humidified glove box. All lines were similarly responsive to hypoxic challenge and post-Cre activation showed significantly reduced HIF-1 target gene levels in the individual cells as predicted.

RESULTS

Unexpectedly, hypoxia-induced vascular remodelling was unaffected by HIF-1 loss of function in the two astrocyte lines but effectively blocked in the pericyte line. In correlation, hypoxia-induced barrier permeability and water accumulation were abrogated only in pericyte targeted HIF-1 loss of function mice. In contrast to expectation, brain and serum levels of hypoxia-induced VEGF, TGF-β and MMPs (genes known to mediate vascular remodelling) were unaffected by HIF-1 deletion in all lines. However, in agreement with the permeability data, immunofluorescence and electron microscopy showed clear prevention of hypoxia-induced tight junction disruption in the pericyte loss of function line.

CONCLUSION

This study shows that pericyte but not astrocyte HIF-1 stabilization modulates endothelial tight junction functionality and thereby plays a pivotal role in hypoxia-induced vascular dysfunction. Whether the cells respond similarly or differentially to other injury stimuli will be of significant relevance.

Nitric Oxide-Dependent Pathways as Critical Factors in the Consequences and Recovery after Brain Ischemic Hypoxia

Brain ischemia is one of the leading causes of disability and mortality worldwide. Nitric oxide (NO^•), a molecule that is involved in the regulation of proper blood flow, vasodilation, neuronal and glial activity constitutes the crucial factor that contributes to the development of pathological changes after stroke. One of the early consequences of a sudden interruption in the cerebral blood flow is the massive production of reactive oxygen and nitrogen species (ROS/RNS) in neurons due to NO^• synthase uncoupling, which leads to neurotoxicity. Progression of apoptotic or necrotic neuronal damage activates reactive astrocytes and attracts microglia or lymphocytes to migrate to place of inflammation. Those inflammatory cells start to produce large amounts of inflammatory proteins, including pathological, inducible form of NOS (iNOS), which generates nitrosative stress that further contributes to brain tissue damage, forming vicious circle of detrimental processes in the late stage of ischemia. S-nitrosylation, hypoxia-inducible factor 1α (HIF-1α) and HIF-1α-dependent genes activated in reactive astrocytes play essential roles in this process. The review summarizes the roles of NO^•-dependent pathways in the early and late aftermath of stroke and treatments based on the stimulation or inhibition of particular NO^• synthases and the stabilization of HIF-1α activity.

Hypoxia-inducible factor-1: Regulatory mechanisms and drug development in stroke

Stroke is an acute cerebrovascular disease caused by sudden rupture of blood vessels in the brain or blockage of blood vessels, which has now become one of the main causes of adult death. During stroke, hypoxia-inducible factor-1 (HIF-1), as an important regulator under hypoxia conditions, is involved in the pathological process of stroke by regulating multi-pathways, such as glucose metabolism, angiogenesis, erythropoiesis, cell survival. However, the roles of HIF-1 in stroke are still controversial, which are related with ischemic time and degree of ischemia. The regulatory mechanisms of HIF-1 in stroke include inflammation, autophagy, oxidative stress, apoptosis and energy metabolism. The potential drugs targeting HIF-1 have attracted more attention, such as HIF-1 inhibitors, HIF-1 stabilizers and natural products. Based on the role of HIF-1 in stroke, HIF-1 is expected to be a potential target for stroke treatment. Resolving when and what interventions for HIF-1 to take during stroke will provide novel strategies for stroke treatment.

Pericyte hypoxia-inducible factor-1 (HIF-1) drives blood-brain barrier disruption and impacts acute ischemic stroke outcome

Pericytes play essential roles in blood-brain barrier integrity and their dysfunction is implicated in neurological disorders such as stroke although the underlying mechanisms remain unknown. Hypoxia-inducible factor-1 (HIF-1), a master regulator of injury responses, has divergent roles in different cells especially during stress scenarios. On one hand HIF-1 is neuroprotective but on the other it induces vascular permeability. Since pericytes are critical for barrier stability, we asked if pericyte HIF-1 signaling impacts barrier integrity and injury severity in a mouse model of ischemic stroke. We show that pericyte HIF-1 loss of function (LoF) diminishes ischemic damage and barrier permeability at 3 days reperfusion. HIF-1 deficiency preserved barrier integrity by reducing pericyte death thereby maintaining vessel coverage and junctional protein organization, and suppressing vascular remodeling. Importantly, considerable improvements in sensorimotor function were observed in HIF-1 LoF mice indicating that better vascular functionality post stroke improves outcome. Thus, boosting vascular integrity by inhibiting pericytic HIF-1 activation and/or increasing pericyte survival may be a lucrative option to accelerate recovery after severe brain injury.

Astrocyte-specific hypoxia-inducible factor 1 (HIF-1) does not disrupt the endothelial barrier during hypoxia in vitro

BACKGROUND

Astrocytes (AC) are essential for brain homeostasis. Much data suggests that AC support and protect the vascular endothelium, but increasing evidence indicates that during injury conditions they may lose their supportive role resulting in endothelial cell activation and BBB disturbance. Understanding the triggers that flip this switch would provide invaluable information for designing new targets to modulate the brain vascular compartment. Hypoxia-inducible factor-1 (HIF-1) has long been assumed to be a culprit for barrier dysfunction as a number of its target genes are potent angiogenic factors. Indeed AC themselves, reservoirs of an array of different growth factors and molecules, are frequently assumed to be the source of such molecules although direct supporting evidence is yet to be published. Being well known reservoirs of HIF-1 dependent angiogenic molecules, we asked if AC HIF-1 dependent paracrine signaling drives brain EC disturbance during hypoxia.

METHODS

First we collected conditioned media from control and siRNA-mediated HIF-1 knockdown primary rat AC that had been exposed to normoxic or hypoxic conditions. The conditioned media was then used to culture normoxic and hypoxic (1% O₂) rat brain microvascular EC (RBE4) for 6 and 24 h. Various activation parameters including migration, proliferation and cell cycling were assessed and compared to untreated controls. In addition, tight junction localization and barrier stability per se (via permeability assay) was evaluated.

RESULTS

AC conditioned media maintained both normoxic and hypoxic EC in a quiescent state by suppressing EC metabolic activity and proliferation. By FACs we observed reduced cell cycling with an increased number of cells in G0 phase and reduced cell numbers in M phase compared to controls. EC migration was also blocked by AC conditioned media and in correlation hypoxic tight junction organization and barrier functionality was improved. Surprisingly however, AC HIF-1 deletion did not impact EC responses or barrier stability during hypoxia.

CONCLUSIONS

This study demonstrates that AC HIF-1 dependent paracrine signaling does not contribute to AC modulation of EC barrier function under normoxic or hypoxic conditions. Thus other cell types likely mediate EC permeability in stress scenarios. Our data does however highlight the continuous protective effect of AC on the barrier endothelium. Exploring these protective mechanisms in more detail will provide essential insight into ways to prevent barrier disturbance during injury and disease.

Neuroprotective Effect of HIF Prolyl Hydroxylase Inhibition in an In Vitro Hypoxia Model

A novel potent analog of the branched tail oxyquinoline group of hypoxia-inducible factor (HIF) prolyl hydroxylase inhibitors, neuradapt, has been studied in two treatment regimes in an in vitro hypoxia model on murine primary hippocampal cultures. Neuradapt activates the expression of HIF1 and HIF2 target genes and shows no toxicity up to 20 μM, which is more than an order of magnitude higher than its biologically active concentration. Cell viability, functional activity, and network connectivity between the elements of neuronal networks have been studied using a pairwise correlation analysis of the intracellular calcium fluctuations in the individual cells. An immediate treatment with 1 μM and 15 μM neuradapt right at the onset of hypoxia not only protects from the death, but also maintains the spontaneous calcium activity in nervous cells at the level of the intact cultures. A similar neuroprotective effect in the post-treatment scenario is observed for 15 μM, but not for 1 μM neuradapt. Network connectivity is better preserved with immediate treatment using 1 μM neuradapt than with 15 μM, which is still beneficial. Post-treatment with neuradapt did not restore the network connectivity despite the observation that neuradapt significantly increased cell viability at 1 μM and functional activity at 15 μM. The preservation of cell viability and functional activity makes neuradapt promising for further studies in a post-treatment scenario, since it can be combined with other drugs and treatments restoring the network connectivity of functionally competent cells.

Insight into the molecular sex dimorphism of ischaemic stroke in rat cerebral cortex: Focus on neuroglobin, sex steroids and autophagy

Including sex is of paramount importance in preclinical and clinical stroke researches, and molecular studies dealing in depth with sex differences in stroke pathophysiology are needed. To gain insight into the molecular sex dimorphism of ischaemic stroke in rat cerebral cortex, male and female adult rats were subjected to transient middle cerebral artery occlusion. The expression of neuroglobin (Ngb) and other functionally related molecules involved in sex steroid signalling (oestrogen and androgen receptors), steroidogenesis (StAR, TSPO and aromatase) and autophagic activity (LC3B-II/LC3B-I ratio, UCP2 and HIF-1α) was assessed in the ipsilateral ischaemic and contralateral non-ischaemic hemispheres. An increased expression of Ngb was detected in the injured female cerebral cortex. In contrast, increased expression of oestrogen receptor α, GPER, StAR, TSPO and UCP2, and decreased androgen receptor expression were detected in the injured male cortex. In both sexes, the ischaemic insult induced an upregulation of LC3B-II/-I ratio, indicative of increased autophagy. Therefore, the cerebral cortex activates both sex-specific and common molecular responses with neuroprotective potential after ischaemia-reperfusion, which globally results in similar stroke outcome in both sexes. Nonetheless, these different potential molecular targets should be taken into account when neuroprotective drugs aiming to reduce brain damage in ischaemic stroke are investigated.

Hypoxia-inducible factor (HIF) prolyl hydroxylase inhibitors induce autophagy and have a protective effect in an in-vitro ischaemia model

This study compared effects of five hypoxia-inducible factor (HIF) prolyl hydroxylases (PHD) inhibitors on PC12 cells and primary rat neurons following oxygen-glucose deprivation (OGD). At 100 µM, the PHD inhibitors did not cause cytotoxicity and apoptosis. MTT activity was only significantly reduced by FG4592 or Bayer 85-3934 in PC12 cells. The PHD inhibitors at 100 µM significantly increased the LC3-II/LC3-I expression ratio and downregulated p62 in PC12 cells, so did FG4592 (30 µM) and DMOG (100 µM) in neurons. HIF-1α was stabilised in PC12 cells by all the PHD inhibitors at 100 µM except for DMOG, which stabilised HIF-1α at 1 and 2 mM. In primary neurons, HIF-1α was stabilised by FG4592 (30 µM) and DMOG (100 µM). Pretreatment with the PHD inhibitors 24 hours followed by 24 hour reoxygenation prior to 6 hours OGD (0.3% O₂) significantly reduced LDH release and increased MTT activity compared to vehicle (1% DMSO) pretreatment. In conclusion, the PHD inhibitors stabilise HIF-1α in normoxia, induce autophagy, and protect cells from a subsequent OGD insult. The new class of PHD inhibitors (FG4592, FG2216, GSK1278863, Bay85-3934) have the higher potency than DMOG. The interplay between autophagy, HIF stabilisation and neuroprotection in ischaemic stroke merits further investigation.

Prolyl Hydroxylase Domain Inhibitor Protects against Metabolic Disorders and Associated Kidney Disease in Obese Type 2 Diabetic Mice

BACKGROUND

Prolyl hydroxylase domain (PHD) inhibitors, which stimulate erythropoietin production through the activation of hypoxia-inducible factor (HIF), are novel therapeutic agents used for treating renal anemia. Several PHD inhibitors, including enarodustat, are currently undergoing phase 2 or phase 3 clinical trials. Because HIF regulates a broad spectrum of genes, PHD inhibitors are expected to have other effects in addition to erythropoiesis, such as protection against metabolic disorders. However, whether such beneficial effects would extend to metabolic disorder-related kidney disease is largely unknown.

METHODS

We administered enarodustat or vehicle without enarodustat in feed to diabetic black and tan brachyury (BTBR) ob/ob mice from 4 to 22 weeks of age. To elucidate molecular changes induced by enarodustat, we performed transcriptome analysis of isolated glomeruli and in vitro experiments using murine mesangial cells.

RESULTS

Compared with BTBR ob/ob mice that received only vehicle, BTBR ob/ob mice treated with enarodustat displayed lower body weight, reduced blood glucose levels with improved insulin sensitivity, lower total cholesterol levels, higher adiponectin levels, and less adipose tissue, as well as a tendency for lower macrophage infiltration. Enarodustat-treated mice also exhibited reduced albuminuria and amelioration of glomerular epithelial and endothelial damage. Transcriptome analysis of isolated glomeruli revealed reduced expression of C-C motif chemokine ligand 2/monocyte chemoattractant protein-1 (CCL2/MCP-1) in enarodustat-treated mice compared with the vehicle-only group, accompanied by reduced glomerular macrophage infiltration. In vitro experiments demonstrated that both local HIF-1 activation and restoration of adiponectin by enarodustat contributed to CCL2/MCP-1 reduction in mesangial cells.

CONCLUSIONS

These results indicate that the PHD inhibitor enarodustat has potential renoprotective effects in addition to its potential to protect against metabolic disorders.

Effects of a prolyl hydroxylase inhibitor on kidney and cardiovascular complications in a rat model of chronic kidney disease

Cardiovascular disease (CVD) is the main cause of death in patients with kidney disease. Hypoxia plays a crucial role in the progression of chronic kidney disease (CKD) and cardiovascular disease, which is associated with fibrosis, inflammation, and oxidative injury. Previous studies have indicated that prolyl hydroxylase (PHD) inhibitors, stabilizers of hypoxia-inducible factors (HIFs), can be used to treat acute organ injuries such as renal ischemia-reperfusion, myocardial infarction, and, in some contexts, CKD. However, the effects of PHD inhibitors on cardiovascular complications in CKD remain unknown. In the present study, we investigated whether HIF activation has a beneficial effect on kidney and cardiovascular outcomes in the remnant kidney model. We used the 5/6 nephrectomy model with the nitric oxide synthase inhibitor N^ω-nitro-l-arginine (20 mg/L in the drinking water). Rats received diet with 0.005% enarodustat (PHD inhibitor) or vehicle for 8 wk starting 2 wk before 5/6 nephrectomy. Activation of HIF by the PHD inhibitor reduced cardiac hypertrophy and ameliorated myocardial fibrosis in association with restored capillary density and improvement in mitochondrial morphology. With regard to kidneys, enarodustat ameliorated fibrosis in association with reduced proinflammatory cytokine expression, reduced apoptosis, and restored capillary density, even though renal endpoints such as proteinuria and serum creatinine levels were not significantly affected by enarodustat, except for blood urea nitrogen levels at 4 wk. In addition, cardiac hypertrophy marker genes, including atrial natriuretic peptide, were suppressed in P19CL6 cells treated with enarodustat. These findings suggest that PHD inhibitors might show beneficial effects in cardiovascular complications caused by CKD.

Angioneurins - Key regulators of blood-brain barrier integrity during hypoxic and ischemic brain injury

The loss of blood-brain barrier (BBB) integrity leading to vasogenic edema and brain swelling is a common feature of hypoxic/ischemic brain diseases such as stroke, but is also central to the etiology of other CNS disorders. In the past decades, numerous proteins, belonging to the family of angioneurins, have gained increasing attention as potential therapeutic targets for ischemic stroke, but also other CNS diseases attributed to BBB dysfunction. Angioneurins encompass mediators that affect both neuronal and vascular function. Recently, increasing evidence has been accumulated that certain angioneurins critically determine disease progression and outcome in stroke among others through multifaceted effects on the compromised BBB. Here, we will give a concise overview about the family of angioneurins. We further describe the most important cellular and molecular components that contribute to structural integrity and low permeability of the BBB under steady-state conditions. We then discuss BBB alterations in ischemic stroke, and highlight underlying cellular and molecular mechanisms. For the most prominent angioneurin family members including vascular endothelial growth factors, angiopoietins, platelet-derived growth factors and erythropoietin, we will summarize current scientific literature from experimental studies in animal models, and if available from clinical trials, on the following points: (i) spatiotemporal expression of these factors in the healthy and hypoxic/ischemic CNS, (ii) impact of loss- or gain-of-function during cerebral hypoxia/ischemia for BBB integrity and beyond, and (iii) potential underlying molecular mechanisms. Moreover, we will highlight novel therapeutic strategies based on the activation of endogenous angioneurins that might improve BBB dysfuntion during ischemic stroke.

Hypoxia Mimetic Agents for Ischemic Stroke

Every year stroke claims more than 6 million lives worldwide. The majority of them are ischemic stroke. Small molecule-based therapeutics for ischemic stroke has attracted a lot of attention, but none has been shown to be clinically useful so far. Hypoxia-inducible factor-1 (HIF-1) plays a crucial role in the transcriptional adaptation of cells to hypoxia. Small molecule-based hypoxia-mimetic agents either stabilize HIF-1α via HIF-prolyl hydroxylases (PHDs) inhibition or through other mechanisms. In both the cases, these agents have been shown to confer ischemic neuroprotection in vitro and in vivo. The agents which act via PHD inhibition are mainly classified into iron chelators, iron competitors, and 2 oxoglutarate (2OG) analogs. This review discusses HIF structure and key players in the HIF-1 degradation pathway as well as the genes, proteins and chemical molecules that are connected to HIF-1 and how they affect cell survival following ischemic injury. Furthermore, this review gives a summary of studies that used PHD inhibitors and other HIF-1α stabilizers as hypoxia-mimetic agents for the treatment of ischemic injury.

Prolyl hydroxylase domain inhibitors: can multiple mechanisms be an opportunity for ischemic stroke?

Stroke and cerebrovascular disease are now the fifth most common cause of death behind other diseases such as heart, cancer and respiratory disease and accounts for approximately 40-50 fatalities per 100,000 people each year in the United States. Currently the only therapy for acute stroke, is intravenous administration of tissue plasminogen activator which was approved in 1996 by the FDA. Surprisingly no new treatments have come on the market since, although endovascular mechanical thrombectomy is showing promising results in trials. Recently focus has shifted towards a preventative therapy rather than trying to reverse or limit the amount of damage occurring following stroke onset. During one of the components of ischemia, hypoxia, a number of physiological changes occur within neurons which include the stabilization of hypoxia-inducible factors. The activity of these proteins is regulated by O₂, Fe²⁺, 2-OG and ascorbate-dependant hydroxylases which contain prolyl-4-hydroxylase domains (PHDs). PHD inhibitors are capable of pharmacologically activating the body's own endogenous adaptive response to low levels of oxygen and have therefore become an attractive therapeutic target for treating ischemia. They have been widely used in the periphery and have been shown to have a preconditioning and protective effect against a later and more severe ischemic insult. Currently there are a number of these agents in phase 1, 2 and 3 clinical trials for the treatment of anemia. In this review we assess the neuroprotective effects of PHD inhibitors, including dimethyloxalylglycine and deferoxamine and suggest that not all of their effects in the CNS are HIF-dependent. Unravelling new roles and a better understanding of the function of PHD inhibitors in the CNS may be of great benefit especially when investigating their use in the treatment of stroke and other ischemic diseases.

Reactive Oxygen Species Formation in the Brain at Different Oxygen Levels: The Role of Hypoxia Inducible Factors

Hypoxia inducible factor (HIF) is the master oxygen sensor within cells and is central to the regulation of cell responses to varying oxygen levels. HIF activation during hypoxia ensures optimum ATP production and cell integrity, and is associated both directly and indirectly with reactive oxygen species (ROS) formation. HIF activation can either reduce ROS formation by suppressing the function of mitochondrial tricarboxylic acid cycle (TCA cycle), or increase ROS formation via NADPH oxidase (NOX), a target gene of HIF pathway. ROS is an unavoidable consequence of aerobic metabolism. In normal conditions (i.e., physioxia), ROS is produced at minimal levels and acts as a signaling molecule subject to the dedicated balance between ROS production and scavenging. Changes in oxygen concentrations affect ROS formation. When ROS levels exceed defense mechanisms, ROS causes oxidative stress. Increased ROS levels can also be a contributing factor to HIF stabilization during hypoxia and reoxygenation. In this review, we systemically review HIF activation and ROS formation in the brain during hypoxia and hypoxia/reoxygenation. We will then explore the literature describing how changes in HIF levels might provide pharmacological targets for effective ischaemic stroke treatment. HIF accumulation in the brain via HIF prolyl hydroxylase (PHD) inhibition is proposed as an effective therapy for ischaemia stroke due to its antioxidation and anti-inflammatory properties in addition to HIF pro-survival signaling. PHD is a key regulator of HIF levels in cells. Pharmacological inhibition of PHD increases HIF levels in normoxia (i.e., at 20.9% O₂ level). Preconditioning with HIF PHD inhibitors show a neuroprotective effect in both in vitro and in vivo ischaemia stroke models, but post-stroke treatment with PHD inhibitors remains debatable. HIF PHD inhibition during reperfusion can reduce ROS formation and activate a number of cellular survival pathways. Given agents targeting individual molecules in the ischaemic cascade (e.g., antioxidants) fail to be translated in the clinic setting, thus far, HIF pathway targeting and thereby impacting entire physiological networks is a promising drug target for reducing the adverse effects of ischaemic stroke.

Differential regulation of angiogenesis in the developing mouse brain in response to exogenous activation of the hypoxia-inducible transcription factor system

Angiogenesis due to hypoxic-ischemic (HI) injury represents a crucial compensatory mechanism of the developing brain that is mainly regulated by hypoxia-inducible transcription factors (HIF). Pharmacological stimulation of HIF is suggested as a neuroprotective option, however, studies of its effects on vascular development are limited. We analyzed the influence of the prolyl-4-hydroxylase inhibitor (PHI), FG-4497, and erythropoietin (rhEPO) on post-hypoxic angiogenesis (angiogenic growth factors, vessel structures) in the developing mouse brain (P7) assessed after a regeneration period of 72 h. Exposure to systemic hypoxia (8% O₂, 6 h) was followed by treatment (i.p.) with rhEPO (2500/5000 IU/kg) at 0, 24 and 48 h or FG-4497 (60/100 mg/kg) compared to controls. In response to FG-4497 treatment cortical and hippocampal vessel area and branching were significantly increased compared to controls. This was associated with elevated ANGPT-2 as well as decreased ANGPT-1 and TIE-2 mRNA levels. In response to rhEPO, mildly increased angiogenesis was associated with elevated ANGPT-2 but also TIE-2 mRNA levels in comparison to controls. In conclusion, present data demonstrate a differential regulation of the angiopoietin/TIE-2 system in response to PHI and rhEPO in the post-hypoxic developing brain pointing to potential functional consequences for vascular regeneration and vessel development.

Treatment of renal anemia: Erythropoiesis stimulating agents and beyond

Anemia, complicating the course of chronic kidney disease, is a significant parameter, whether interpreted as subjective impairment or an objective prognostic marker. Renal anemia is predominantly due to relative erythropoietin (EPO) deficiency. EPO inhibits apoptosis of erythrocyte precursors. Studies using EPO substitution have shown that increasing hemoglobin (Hb) levels up to 10–11 g/dL is associated with clinical improvement. However, it has not been unequivocally proven that further intensification of erythropoiesis stimulating agent (ESA) therapy actually leads to a comprehensive benefit for the patient, especially as ESAs are potentially associated with increased cerebro-cardiovascular events. Recently, new developments offer interesting options not only via stimulating erythropoeisis but also by employing additional mechanisms. The inhibition of activin, a member of the transforming growth factor superfamily, has the potential to correct anemia by stimulating liberation of mature erythrocyte forms and also to mitigate disturbed mineral and bone metabolism as well. Hypoxia-inducible factor prolyl hydroxylase inhibitors also show pleiotropic effects, which are at the focus of present research and have the potential of reducing mortality. However, conventional ESAs offer an extensive body of safety evidence, against which the newer substances should be measured. Carbamylated EPO is devoid of Hb augmenting effects whilst exerting promising tissue protective properties. Additionally, the role of hepcidin antagonists is discussed. An innovative new hemodialysis blood tube system, reducing blood contact with air, conveys a totally different and innocuous option to improve renal anemia by reducing mechanical hemolysis.

Endothelial progenitor cells transplantation attenuated blood-brain barrier damage after ischemia in diabetic mice via HIF-1α

Background

Blood-brain barrier impairment is a major indicator of endothelial dysfunction in diabetes. Studies showed that endothelial progenitor cell (EPC) transplantation promoted angiogenesis and improved function recovery after hind limb ischemia in diabetic mice. The effect of EPC transplantation on blood-brain barrier integrity after cerebral ischemia in diabetic animals is unknown. The aim of this study is to explore the effect of EPC transplantation on the integrity of the blood-brain barrier after cerebral ischemia in diabetic mice.

Methods

EPCs were isolated by density gradient centrifugation and characterized by flow cytometry and immunostaining. Diabetes was induced in adult male C57BL/6 mice by a single injection of streptozotocin at 4 weeks before surgery. Diabetic mice underwent 90-minute transient middle cerebral artery occlusion surgery and received 1 × 10⁶ EPCs transplantation immediately after reperfusion. Brain infarct volume, blood-brain barrier permeability, tight junction protein expression, and hypoxia inducible factor-1α (HIF-1α) mRNA level were examined after treatment.

Results

We demonstrated that neurological deficits were attenuated and brain infarct volume was reduced in EPC-transplanted diabetic mice after transient cerebral ischemia compared to the controls (p < 0.05). Blood-brain barrier leakage and tight junction protein degradation were reduced in EPC-transplanted mice (p <0.05). EPCs upregulated HIF-1α expression while HIF-1α inhibitor PX-478 abolished the beneficial effect of EPCs.

Conclusions

We conclude that EPCs protected blood-brain barrier integrity after focal ischemia in diabetic mice through upregulation of HIF-1α signaling.

Neuronal HIF-1α and HIF-2α deficiency improves neuronal survival and sensorimotor function in the early acute phase after ischemic stroke

Hypoxia-inducible factors mediate adaptive responses to ischemia, among others, by induction of anti- and pro-survival genes. Thus, the impact of HIF on neuronal survival upon stroke is controversial. Therefore, neuron-specific knockout mice deficient for Hif1a and Hif2a were exposed to inspiratory hypoxia or ischemia-reperfusion injury. Both Hif1a- and Hif2a-deficient mice showed no altered infarct and edema size, suggesting that both HIF-α subunits might compensate for each other. Accordingly, hypoxic HIF-target gene regulation was marginally affected with exception of anti-survival Bnip3 and pro-survival erythropoietin. In the early acute stage upon stroke, Hif1a/Hif2a double knockout mice exhibited significantly reduced expression of the anti-survival Bnip3, Bnip3L, and Pmaip1 Accordingly, global cell death and edema were significantly reduced upon 24 h but not 72 h reperfusion. Behavioral assessment indicated that Hif1a/Hif2a-deficient mice initially performed better, but became significantly more impaired after 72 h accompanied by increased apoptosis and reduced angiogenesis. Our findings suggest that in neurons HIF-1 and HIF-2 have redundant functions for cellular survival under ischemic conditions. By contrast, lack of anti-survival factors in Hif1a/Hif2a-deficient mice might protect from early acute neuronal cell death and neurological impairment, indicating a benefit of HIF-pathway inhibition in neurons in the very acute phase after ischemic stroke.

Neuronal prolyl-4-hydroxylase 2 deficiency improves cognitive abilities in a murine model of cerebral hypoperfusion

Episodes of cerebral hypoxia/ischemia increase the risk of dementia, which is associated with impaired learning and memory. Previous studies in rodent models of dementia indicated a favorable effect of the hypoxia-inducible factor (HIF) targets VEGF (vascular endothelial growth factor) and erythropoietin (Epo). In the present study we thus investigated whether activation of the entire adaptive HIF pathway in neurons by cell-specific deletion of the HIF suppressor prolyl-4-hydroxylase 2 (PHD2) improves cognitive abilities in young (3months) and old (18-28months) mice suffering from chronic brain hypoperfusion. Mice underwent permanent occlusion of the left common carotid artery, and cognitive function was assessed using the Morris water navigation task. Under conditions of both normal and decreased brain perfusion, neuronal PHD2 deficiency resulted in improved and faster spatial learning in young mice, which was preserved to some extent also in old animals. The loss of PHD2 in neurons resulted in enhanced hippocampal mRNA and protein levels of Epo and VEGF, but did not alter local microvascular density, dendritic spine morphology, or expression of synaptic plasticity-related genes in the hippocampus. Instead, better cognitive function in PHD2 deficient animals was accompanied by an increased number of neuronal precursor cells along the subgranular zone of the dentate gyrus. Overall, our current pre-clinical findings indicate an important role for the endogenous oxygen sensing machinery, encompassing PHDs, HIFs and HIF target genes, for proper cognitive function. Thus, pharmacological compounds affecting the PHD-HIF axis might well be suited to treat cognitive dysfunction and neurodegenerative processes.

On the pivotal role of PPARα in adaptation of the heart to hypoxia and why fat in the diet increases hypoxic injury

The role of peroxisome proliferator-activated receptor α (PPARα)-mediated metabolic remodeling in cardiac adaptation to hypoxia has yet to be defined. Here, mice were housed in hypoxia for 3 wk before in vivo contractile function was measured using cine MRI. In isolated, perfused hearts, energetics were measured using (31)P magnetic resonance spectroscopy (MRS), and glycolysis and fatty acid oxidation were measured using [(3)H] labeling. Compared with a normoxic, chow-fed control mouse heart, hypoxia decreased PPARα expression, fatty acid oxidation, and mitochondrial uncoupling protein 3 (UCP3) levels, while increasing glycolysis, all of which served to maintain normal ATP concentrations ([ATP]) and thereby, ejection fractions. A high-fat diet increased cardiac PPARα expression, fatty acid oxidation, and UCP3 levels with decreased glycolysis. Hypoxia was unable to alter the high PPARα expression or reverse the metabolic changes caused by the high-fat diet, with the result that [ATP] and contractile function decreased significantly. The adaptive metabolic changes caused by hypoxia in control mouse hearts were found to have occurred already in PPARα-deficient (PPARα(-/-)) mouse hearts and sustained function in hypoxia despite an inability for further metabolic remodeling. We conclude that decreased cardiac PPARα expression is essential for adaptive metabolic remodeling in hypoxia, but is prevented by dietary fat.-Cole, M. A., Abd Jamil, A. H., Heather, L. C., Murray, A. J., Sutton, E. R., Slingo, M., Sebag-Montefiore, L., Tan, S. C., Aksentijević, D., Gildea, O. S., Stuckey, D. J., Yeoh, K. K., Carr, C. A., Evans, R. D., Aasum, E., Schofield, C. J., Ratcliffe, P. J., Neubauer, S., Robbins, P. A., Clarke, K. On the pivotal role of PPARα in adaptation of the heart to hypoxia and why fat in the diet increases hypoxic injury.

Neuronal deficiency of HIF prolyl 4-hydroxylase 2 in mice improves ischemic stroke recovery in an HIF dependent manner

Hypoxia inducible factors (HIFs) mediate the endogenous adaptive responses to hypoxia. HIF prolyl 4-hydroxylase domain proteins (PHD) are important suppressors of the HIF pathway. Recently, we demonstrated that neuron-specific deletion of Phd2 reduces cerebral tissue damage in the very acute phase of ischemic stroke. In the present study, we investigated whether neuronal Phd2 ablation is likewise beneficial for stroke recovery, and aimed to identify underlying cellular mechanisms. Mice underwent permanent occlusion of the distal middle cerebral artery (pdMCAO) for either 7days (sub-acute stage) or 30days (chronic stage). One week after pdMCAO the infarct size of Phd2-deficient mice was significantly reduced as compared to wild-type (WT) mice. Accordingly, Phd2-deficient animals showed less impaired sensorimotor function. Neuronal loss of Phd2 upregulated vascular endothelial growth factor (VEGF) and significantly increased microvascular density along the infarct border in the sub-acute stage of stroke. Phd2-deficient mice showed reduced expression of pro-inflammatory cytokines and increased numbers of resting microglia/macrophages and reactive astrocytes within peri-infarct regions in comparison to WT littermates. Finally, brain tissue protection and increased angiogenesis upon sub-acute ischemic stroke was completely absent in Phd2 knockout mice that were additionally deficient for both Hif1a and Hif2a. Our findings suggest that lack of PHD2 in neurons improves histological and functional long-term outcome from ischemic stroke at least partly by amplifying endogenous adaptive neovascularization through activation of the HIF-VEGF axis.

Pharmacological targeting of the HIF hydroxylases--A new field in medicine development

In human cells oxygen levels are 'sensed' by a set of ferrous iron and 2-oxoglutarate dependent dioxygenases. These enzymes regulate a broad range of cellular and systemic responses to hypoxia by catalysing the post-translational hydroxylation of specific residues in the alpha subunits of hypoxia inducible factor (HIF) transcriptional complexes. The HIF hydroxylases are now the subject of pharmaceutical targeting by small molecule inhibitors that aim to activate or augment the endogenous HIF transcriptional response for the treatment of anaemia and other hypoxic human diseases. Here we consider the rationale for this therapeutic strategy from the biochemical, biological and medical perspectives. We outline structural and mechanistic considerations that are relevant to the design of HIF hydroxylase inhibitors, including likely determinants of specificity, and review published reports on their activity in pre-clinical models and clinical trials.

Endurance training prevents negative effects of the hypoxia mimetic dimethyloxalylglycine on cardiac and skeletal muscle function

Hypoxic preconditioning is a promising strategy to prevent hypoxia-induced damages to several tissues. This effect is related to prior stabilization of the hypoxia-inducible factor-1α via inhibition of the prolyl-hydroxylases (PHDs), which are responsible for its degradation under normoxia. Although PHD inhibition has been shown to increase endurance performance in rodents, potential side effects of such a therapy have not been explored. Here, we investigated the effects of 1 wk of dimethyloxalylglycine (DMOG) treatment (150 mg/kg) on exercise capacity, as well as on cardiac and skeletal muscle function in sedentary and endurance-trained rats. DMOG improved maximal aerobic velocity and endurance in both sedentary and trained rats. This effect was associated with an increase in red blood cells without significant alteration of skeletal muscle contractile properties. In sedentary rats, DMOG treatment resulted in enhanced left ventricle (LV) weight together with impairment in diastolic function, LV relaxation, and pulse pressure. Moreover, DMOG decreased maximal oxygen uptake (state 3) of isolated mitochondria from skeletal muscle. Importantly, endurance training reversed the negative effects of DMOG treatment on cardiac function and restored maximal mitochondrial oxygen uptake to the level of sedentary placebo-treated rats. In conclusion, we provide here evidence that the PHD inhibitor DMOG has detrimental influence on myocardial and mitochondrial function in healthy rats. However, one may suppose that the deleterious influence of PHD inhibition would be potentiated in patients with already poor physical condition. Therefore, the present results prompt us to take into consideration the potential side effects of PHD inhibitors when administrated to patients.

Methylene blue-induced neuronal protective mechanism against hypoxia-reoxygenation stress

Brain ischemia and reperfusion (I/R) injury occurs in various pathological conditions, but there is no effective treatment currently available in clinical practice. Methylene blue (MB) is a century-old drug with a newly discovered protective function in the ischemic stroke model. In the current investigation we studied the MB-induced neuroprotective mechanism focusing on stabilization and activation of hypoxia-inducible factor-1α (HIF-1α) in an in vitro oxygen and glucose deprivation (OGD)-reoxygenation model.

METHODS

HT22 cells were exposed to OGD (0.1% O2, 6h) and reoxygenation (21% O2, 24h). Cell viability was determined with the calcein AM assay. The dynamic change of intracellular O2 concentration was monitored by fluorescence lifetime imaging microscopy (FLTIM). Glucose uptake was quantified using the 2-[N-(7-Nitrobenz-2-Oxa-1,3-Diazol-4-yl)Amino]-2-Deoxy-d-Glucose (2-NBDG) assay. ATP concentration and glycolytic enzyme activity were examined by spectrophotometry. Protein content changes were measured by immunoblot: HIF-1α, prolyl hydroxylase 2 (PHD2), erythropoietin (EPO), Akt, mTOR, and PIP5K. The contribution of HIF-1α activation in the MB-induced neuroprotective mechanism was confirmed by blocking HIF-1α activation with 2-methoxyestradiol-2 (2-MeOE2) and by transiently transfecting constitutively active HIF-1α.

RESULTS

MB increases cell viability by about 50% vs. OGD control. Compared to the corresponding control, MB increases intracellular O2 concentration and glucose uptake as well as the activities of hexokinase and G-6-PDH, and ATP concentration. MB activates the EPO signaling pathway with a corresponding increase in HIF-1α. Phosphorylation of Akt was significantly increased with MB treatment followed by activation of the mTOR pathway. Importantly, we observed, MB increased nuclear translocation of HIF-1α vs. control (about three folds), which was shown by a ratio of nuclear:cytoplasmic HIF-1α protein content.

CONCLUSION

We conclude that MB protects the hippocampus-derived neuronal cells against OGD-reoxygenation injury by enhancing energy metabolism and increasing HIF-1α protein content accompanied by an activation of the EPO signaling pathway.

Pharmacological inhibition of FTO

In 2007, a genome wide association study identified a SNP in intron one of the gene encoding human FTO that was associated with increased body mass index. Homozygous risk allele carriers are on average three kg heavier than those homozygous for the protective allele. FTO is a DNA/RNA demethylase, however, how this function affects body weight, if at all, is unknown. Here we aimed to pharmacologically inhibit FTO to examine the effect of its demethylase function in vitro and in vivo as a first step in evaluating the therapeutic potential of FTO. We showed that IOX3, a known inhibitor of the HIF prolyl hydroxylases, decreased protein expression of FTO (in C2C12 cells) and reduced maximal respiration rate in vitro. However, FTO protein levels were not significantly altered by treatment of mice with IOX3 at 60 mg/kg every two days. This treatment did not affect body weight, or RER, but did significantly reduce bone mineral density and content and alter adipose tissue distribution. Future compounds designed to selectively inhibit FTO’s demethylase activity could be therapeutically useful for the treatment of obesity.