Molecular mechanisms of action and therapeutic uses of pharmacological inhibitors of HIF-prolyl 4-hydroxylases for treatment of ischemic diseases

Hypoxia-Inducible Factor 1-Alpha (HIF-1α) and Cancer: Mechanisms of Tumor Hypoxia and Therapeutic Targeting

Hypoxia-inducible factor 1-alpha (HIF-1α) is necessary for cells to adapt to low oxygen levels often present in the tumor microenvironment. HIF-1α triggers a transcriptional program that promotes invasion, angiogenesis, metabolic reprogramming, and cell survival when it is active in hypoxic environments. These processes together lead to the growth and spread of tumors. This review article examines the molecular mechanisms by which HIF-1α contributes to tumor progression, including its regulation by oxygen-dependent and independent pathways, interactions with oncogenic signaling networks, and impact on the tumor microenvironment. Additionally, we explore current therapeutic strategies targeting HIF-1α, such as small molecule inhibitors, RNA interference, and immunotherapy approaches. Understanding the multifaceted roles of HIF-1α in cancer biology not only elucidates the complexities of tumor hypoxia but also opens avenues for developing novel and more effective cancer therapies.

The Role of Oxygen Homeostasis and the HIF-1 Factor in the Development of Neurodegeneration

Understanding the molecular underpinnings of neurodegeneration processes is a pressing challenge for medicine and neurobiology. Alzheimer's disease (AD) and Parkinson's disease (PD) represent the most prevalent forms of neurodegeneration. To date, a substantial body of experimental evidence has strongly implicated hypoxia in the pathogenesis of numerous neurological disorders, including AD, PD, and other age-related neurodegenerative conditions. Hypoxia-inducible factor (HIF) is a transcription factor that triggers a cell survival program in conditions of oxygen deprivation. The involvement of HIF-1α in neurodegenerative processes presents a complex and sometimes contradictory picture. This review aims to elucidate the current understanding of the interplay between hypoxia and the development of AD and PD, assess the involvement of HIF-1 in their pathogenesis, and summarize promising therapeutic approaches centered on modulating the activity of the HIF-1 complex.

Protective Effect of Cardiomyocyte-Specific Prolyl-4-Hydroxylase 2 Inhibition on Ischemic Injury in a Mouse MI Model

BACKGROUND

Our earlier studies showed that inhibiting prolyl-4-hydroxylase enzymes (PHD-1 and PHD-3) improves angiogenesis, heart function, and limb perfusion in mouse models via stabilizing hypoxia-inducible transcription factor-alpha (HIF-1α). The present study explored the effects of the prolyl-4-hydroxylase enzyme, PHD-2, on ischemic heart failure using cardiac-specific PHD-2 gene knockout (KO) mice (PHD2-/-).

STUDY DESIGN

Adult wild-type (WT) and PHD2-/- mice, 8-12 weeks old, were subjected to myocardial infarction (MI) by irreversibly ligating the left anterior descending (LAD) coronary artery. All sham group mice underwent surgery without LAD ligation. Animals were divided into four groups 1) Wild-type Sham (WTS); Wild-Type myocardial infarction (WTMI); 3) PHD2KO Sham (PHD2-/-S); 4) PHD2KO myocardial infarction (PHD2-/-MI). Left ventricular tissue samples collected at various time points following surgery were used for microRNA expression profiling, Western blotting, immunohistochemical, and echocardiographic analysis.

RESULTS

Volcano plot analysis revealed 19 differentially expressed miRNAs in the PHD2-/-MI compared to the WTMI group. Target analysis using Ingenuity Pathway Analysis showed several differentially regulated miRNAs targeting key signaling pathways such as Akt, VEGF, Ang-1, PTEN, apoptosis, and hypoxia pathways. Compared to the WTMI group, Western blot analysis showed increased HIF-1α, VEGF, phospho-AKT, and β-catenin expression and reduced Bax expression for the PHD2-/-MI group post-MI. Echocardiographic analysis showed preserved heart functions, and picrosirius red staining revealed decreased fibrosis in PHD2-/-MI compared to the WTMI group.

CONCLUSION

PHD2 inhibition showed preserved heart function, enhanced angiogenic factor expression, and decreased apoptotic markers after MI. Overall, PHD2 gene inhibition is a promising candidate for managing cardiovascular diseases.

Near-Infrared Fluorescent Heptamethine Cyanine Dyes for COX-2 Targeted Photodynamic Cancer Therapy

We designed and synthesized two heptamethine cyanine based theranostic probes that aimed to target COX-2 in cancer cells. One is I-IR799-CXB which I-IR799 was conjugated to COX-2 specific inhibitor, celecoxib, and another is I-IR799-IMC , where the non-selective COX inhibitor, indomethacin, was used. I-IR799 is a heptamethine cyanine derivative that can be activated by near infrared light for photodynamic therapy (PDT) purposes. I-IR799-CXB and I-IR799-IMC were tested for their cancer targeting and photodynamic efficiency towards liver hepatocellular carcinoma cells (HepG2) compared to normal liver cell, alpha mouse liver 12 cells (AML12). Interestingly, after conjugation, I-IR799-IMC exhibited superior tumour targetability and PDT efficiency than I-IR799-CXB .

Inactivation of mouse transmembrane prolyl 4-hydroxylase increases blood brain barrier permeability and ischemia-induced cerebral neuroinflammation

Hypoxia-inducible factor prolyl 4-hydroxylases (HIF-P4Hs) regulate the hypoxic induction of >300 genes required for survival and adaptation under oxygen deprivation. Inhibition of HIF-P4H-2 has been shown to be protective in focal cerebral ischemia rodent models, while that of HIF-P4H-1 has no effects and inactivation of HIF-P4H-3 has adverse effects. A transmembrane prolyl 4-hydroxylase (P4H-TM) is highly expressed in the brain and contributes to the regulation of HIF, but the outcome of its inhibition on stroke is yet unknown. To study this, we subjected WT and P4htm^−/− mice to permanent middle cerebral artery occlusion (pMCAO). Lack of P4H-TM had no effect on lesion size following pMCAO, but increased inflammatory microgliosis and neutrophil infiltration was observed in the P4htm^−/− cortex. Furthermore, both the permeability of blood brain barrier and ultrastructure of cerebral tight junctions were compromised in P4htm^−/− mice. At the molecular level, P4H-TM deficiency led to increased expression of proinflammatory genes and robust activation of protein kinases in the cortex, while expression of tight junction proteins and the neuroprotective growth factors erythropoietin and vascular endothelial growth factor was reduced. Our data provide the first evidence that P4H-TM inactivation has no protective effect on infarct size and increases inflammatory microgliosis and neutrophil infiltration in the cortex at early stage after pMCAO. When considering HIF-P4H inhibitors as potential therapeutics in stroke, the current data support that isoenzyme-selective inhibitors that do not target P4H-TM or HIF-P4H-3 would be preferred.

Plicosepalus acacia Extract and Its Major Constituents, Methyl Gallate and Quercetin, Potentiate Therapeutic Angiogenesis in Diabetic Hind Limb Ischemia: HPTLC Quantification and LC-MS/MS Metabolic Profiling

Plicosepalus acacia (Fam. Loranthaceae) has been reported to possess hypoglycemic, antioxidant, antimicrobial, and anti-inflammatory effects. Liquid chromatography combined with tandem mass spectrometry (LC-MS/MS) analysis revealed the presence of a high content of polyphenolic compounds that are attributed to the therapeutic effects of the crude extract. In addition, methyl gallate and quercetin were detected as major phytomedicinal agents at concentrations of 1.7% and 0.062 g%, respectively, using high-performance thin layer chromatography (HPTLC). The present study investigated the effect of the P. acacia extract and its isolated compounds, methyl gallate and quercetin, on hind limb ischemia induced in type 1 diabetic rats. Histopathological examination revealed that treatment with P. acacia extract, methyl gallate, and quercetin decreased degenerative changes and inflammation in the ischemic muscle. Further biochemical assessment of the hind limb tissue showed decreased oxidative stress, increased levels of nitric oxide and endothelial nitric oxide synthase (eNOS), and enhancement of the levels of heme oxygenase-1 (HO-1) and vascular endothelial growth factor (VEGF) in the groups treated with methyl gallate and quercetin. Expression levels of hypoxia inducible factor-1 alpha (HIF-1α), VEGF, fibroblast growth factor-2 (FGF-2), and miR-146a were upregulated in the muscle tissue of methyl gallate- and quercetin-treated groups along with downregulation of nuclear factor kappa B (NF-κB). In conclusion, P. acacia extract and its isolated compounds, methyl gallate and quercetin, mediated therapeutic angiogenesis in diabetic hind limb ischemia.

Targeting oxidative stress, a crucial challenge in renal transplantation outcome

Disorders characterized by ischemia/reperfusion (I/R) are the most common causes of debilitating diseases and death in stroke, cardiovascular ischemia, acute kidney injury or organ transplantation. In the latter example the I/R step defines both the amplitude of the damages to the graft and the functional recovery outcome. During transplantation the kidney is subjected to blood flow arrest followed by a sudden increase in oxygen supply at the time of reperfusion. This essential clinical protocol causes massive oxidative stress which is at the basis of cell death and tissue damage. The involvement of both reactive oxygen species (ROS) and nitric oxides (NO) has been shown to be a major cause of these cellular damages. In fact, in non-physiological situations, these species escape endogenous antioxidant control and dangerously accumulate in cells. In recent years, the objective has been to find clinical and pharmacological treatments to reduce or prevent the appearance of oxidative stress in ischemic pathologies. This is very relevant because, due to the increasing success of organ transplantation, clinicians are required to use limit organs, the preservation of which against oxidative stress is crucial for a better outcome. This review highlights the key actors in oxidative stress which could represent new pharmacological targets.

Physiological hypoxia restrains the senescence-associated secretory phenotype via AMPK-mediated mTOR suppression

Cellular senescence is a state of stable proliferative arrest triggered by damaging signals. Senescent cells persist during aging and promote age-related pathologies via the pro-inflammatory senescence-associated secretory phenotype (SASP), whose regulation depends on environmental factors. In vivo, a major environmental variable is oxygenation, which varies among and within tissues. Here, we demonstrate that senescent cells express lower levels of detrimental pro-inflammatory SASP factors in physiologically hypoxic environments, as measured in culture and in tissues. Mechanistically, exposure of senescent cells to low-oxygen conditions leads to AMPK activation and AMPK-mediated suppression of the mTOR-NF-κB signaling loop. Finally, we demonstrate that treatment with hypoxia-mimetic compounds reduces SASP in cells and tissues and improves strength in chemotherapy-treated and aged mice. Our findings highlight the importance of oxygen as a determinant for pro-inflammatory SASP expression and offer a potential new strategy to reduce detrimental paracrine effects of senescent cells.

Study on hypoxia-inducible factor and its roles in immune system

The Hypoxia-Inducible Factor-1 (HIF-1) is a dimeric protein complex that plays a significant role in responding to low oxygen or hypoxia concentrations. Chronic inflammation is one of the immune system responses and can increase HIF expression in involved tissues through lowering the oxygen and hypoxia. The HIF factor has many critical roles in immunity, and thus, we reviewed the crucial roles of this factor in the immune system. The results showed various key roles on the immune system, including physical defenses, innate immune (neutrophils apoptosis, macrophages) and inflammatory responses (pyrexia and local heat, iron access, etc.), upregulation in response to microbial infections, cytokines expression (IL-1, IL-2, IL-6, IL-8, IL-12, IL-18, TNF, etc.), drug targeting, etc. The HIF roles in the acquired immune system include: enhance the adaptation of cells (dendritic cells) to new conditions and triggering the signal pathways. The findings of the present review demonstrated that the HIF has important roles in the immune system, including physical defense, innate immune as well as acquired immunity; therefore, it may be considered as a potent drug targeting several diseases such as cancers, infectious diseases, etc.

The non-canonical functions of HIF prolyl hydroxylases and their dual roles in cancer

The hypoxia-inducible factor (HIF) prolyl hydroxylases (PHDs) are dioxygenases using oxygen and 2-oxoglutarate as co-substrates. Under normoxia, PHDs hydroxylate the conserved prolyl residues of HIFα, leading to HIFα degradation. In hypoxia PHDs are inactivated, which results in HIFα accumulation. The accumulated HIFα enters nucleus and initiates gene transcription. Many studies have shown that PHDs have substrates other than HIFα, implying that they have HIF-independent non-canonical functions. Besides modulating protein stability, the PHDs-mediated prolyl hydroxylation affects protein-protein interaction and protein activity for alternative substrates. Increasing evidence indicates that PHDs also have hydroxylase-independent functions. They influence protein stability, enzyme activity, and protein-protein interaction in a hydroxylase-independent manner. These findings highlight the functional diversity and complexity of PHDs. Due to having inhibitory activity on HIFα, PHDs are proposed to act as tumor suppressors. However, research shows that PHDs exert either tumor-promoting or tumor-suppressing features. Here, we try to summarize the current understanding of PHDs hydroxylase-dependent and -independent functions and their roles in cancer.

Targeting immunometabolism as an anti-inflammatory strategy

The growing field of immunometabolism has taught us how metabolic cellular reactions and processes not only provide a means to generate ATP and biosynthetic precursors, but are also a way of controlling immunity and inflammation. Metabolic reprogramming of immune cells is essential for both inflammatory as well as anti-inflammatory responses. Four anti-inflammatory therapies, DMF, Metformin, Methotrexate and Rapamycin all work by affecting metabolism and/or regulating or mimicking endogenous metabolites with anti-inflammatory effects. Evidence is emerging for the targeting of specific metabolic events as a strategy to limit inflammation in different contexts. Here we discuss these recent developments and speculate on the prospect of targeting immunometabolism in the effort to develop novel anti-inflammatory therapeutics. As accumulating evidence for roles of an intricate and elaborate network of metabolic processes, including lipid, amino acid and nucleotide metabolism provides key focal points for developing new therapies, we here turn our attention to glycolysis and the TCA cycle to provide examples of how metabolic intermediates and enzymes can provide potential novel therapeutic targets.

Circadian-Hypoxia Link and its Potential for Treatment of Cardiovascular Disease

Throughout the evolutionary time, all organisms and species on Earth evolved with an adaptation to consistent oscillations of sunlight and darkness, now recognized as 'circadian rhythm.' Single-cellular to multisystem organisms use circadian biology to synchronize to the external environment and provide predictive adaptation to changes in cellular homeostasis. Dysregulation of circadian biology has been implicated in numerous prevalent human diseases, and subsequently targeting the circadian machinery may provide innovative preventative or treatment strategies. Discovery of 'peripheral circadian clocks' unleashed widespread investigations into the potential roles of clock biology in cellular, tissue, and organ function in healthy and diseased states. Particularly, oxygen-sensing pathways (e.g. hypoxia inducible factor, HIF1), are critical for adaptation to changes in oxygen availability in diseases such as myocardial ischemia. Recent investigations have identified a connection between the circadian rhythm protein Period 2 (PER2) and HIF1A that may elucidate an evolutionarily conserved cellular network that can be targeted to manipulate metabolic function in stressed conditions like hypoxia or ischemia. Understanding the link between circadian and hypoxia pathways may provide insights and subsequent innovative therapeutic strategies for patients with myocardial ischemia. This review addresses our current understanding of the connection between light-sensing pathways (PER2), and oxygen-sensing pathways (HIF1A), in the context of myocardial ischemia and lays the groundwork for future studies to take advantage of these two evolutionarily conserved pathways in the treatment of myocardial ischemia.

Carnosine Supplementation Enhances Post Ischemic Hind Limb Revascularization

High (millimolar) concentrations of the histidine containing dipeptide - carnosine (β-alanine-L-histidine) are present in the skeletal muscle. The dipeptide has been shown to buffer intracellular pH, chelate transition metals, and scavenge lipid peroxidation products; however, its role in protecting against tissue injury remains unclear. In this study, we tested the hypothesis that carnosine protects against post ischemia by augmenting HIF-1α angiogenic signaling by Fe2+ chelation. We found that wild type (WT) C57BL/6 mice, subjected to hind limb ischemia (HLI) and supplemented with carnosine (1g/L) in drinking water, had improved blood flow recovery and limb function, enhanced revascularization and regeneration of myocytes compared with HLI mice placed on water alone. Carnosine supplementation enhanced the bioavailability of carnosine in the ischemic limb, which was accompanied by increased expression of proton-coupled oligopeptide transporters. Consistent with our hypothesis, carnosine supplementation augmented HIF-1α and VEGF expression in the ischemic limb and the mobilization of proangiogenic Flk-1+/Sca-1+ cells into circulation. Pretreatment of murine myoblast (C2C12) cells with octyl-D-carnosine or carnosine enhanced HIF-1α protein expression, VEGF mRNA levels and VEGF release under hypoxic conditions. Similarly pretreatment of WT C57/Bl6 mice with carnosine showed enhanced blood flow in the ischemic limb following HLI surgery. In contrast, pretreatment of hypoxic C2C12 cells with methylcarcinine, a carnosine analog, lacking Fe2+ chelating capacity, had no effect on HIF-1α levels and VEGF release. Collectively, these data suggest that carnosine promotes post ischemic revascularization via augmentation of pro-angiogenic HIF-1α/VEGF signaling, possibly by Fe2+ chelation.

Dimethyloxalylglycine (DMOG) and the Caspase Inhibitor "Ac-LETD-CHO" Protect Neuronal ND7/23 Cells of Gluocotoxicity

It well known that long-lasting hyperglycaemia disrupts neuronal function and leads to neuropathy and other neurodegenerative diseases. The α-ketoglutarate analogue (DMOG) and the caspase-inhibitor "Ac-LETD-CHO are potential neuroprotective molecules. Whether their protections may also extend glucotoxicity-induced neuropathy is not known. Herein, we evaluated the possible cell-protective effects of DMOG and Ac-LETD-CHO against hyperglycaemia-induced reactive oxygen species and apoptosis in ND7/23 neuronal cells. The impact of glucotoxicity on the expression of HIF-1α and a panel of micro-RNAs of significance in hyperglycaemia and apoptosis was also investigated.ND7/23 cells cultured under hyperglycaemic conditions showed decreased cell viability and elevated levels of ROS production in a dose- and time-dependent manner. However, presence DMOG (500 µM) and/or Ac-LETD-CHO (50 µM) counteracted this effect and increase cell viability concomitant with reduction in ROS production, DNA damage and apoptosis. AcLETD-CHO suppressed hyperglycaemia-induced caspase 3 activation in ND7/23 cells. Both DMOG and Ac-LETD-CHO increased HIF-1α expression paralleled with the suppression of miR-126-5p, miR-128-3p and miR-181 expression and upregulation of miR-26b, 106a-5p, 106b-5p, 135a-5p, 135b-5p, 138-5p, 199a-5p, 200a-3p and 200c-3p expression.We demonstrate a mechanistic link for the DMOG and Ac-LETD-CHO protection against hyperglycaemia-induced neuronal dysfunction, DNA damage and apoptosis and thereby propose that pharmacological agents mimicking these effects may represent a promising novel therapy for the hyperglycaemia-induced neuropathy.

Local pharmacological induction of angiogenesis: Drugs for cells and cells as drugs

The past decades have seen significant advances in pro-angiogenic strategies based on delivery of molecules and cells for conditions such as coronary artery disease, critical limb ischemia and stroke. Currently, three major strategies are evolving. Firstly, various pharmacological agents (growth factors, interleukins, small molecules, DNA/RNA) are locally applied at the ischemic region. Secondly, preparations of living cells with considerable bandwidth of tissue origin, differentiation state and preconditioning are delivered locally, rarely systemically. Thirdly, based on the notion, that cellular effects can be attributed mostly to factors secreted in situ, the cellular secretome (conditioned media, exosomes) has come into the spotlight. We review these three strategies to achieve (neo)angiogenesis in ischemic tissue with focus on the angiogenic mechanisms they tackle, such as transcription cascades, specific signalling steps and cellular gases. We also include cancer-therapy relevant lymphangiogenesis, and shall seek to explain why there are often conflicting data between in vitro and in vivo. The lion's share of data encompassing all three approaches comes from experimental animal work and we shall highlight common technical obstacles in the delivery of therapeutic molecules, cells, and secretome. This plethora of preclinical data contrasts with a dearth of clinical studies. A lack of adequate delivery vehicles and standardised assessment of clinical outcomes might play a role here, as well as regulatory, IP, and manufacturing constraints of candidate compounds; in addition, completed clinical trials have yet to reveal a successful and efficacious strategy. As the biology of angiogenesis is understood well enough for clinical purposes, it will be a matter of time to achieve success for well-stratified patients, and most probably with a combination of compounds.

Protein Hydroxylation by Hypoxia-Inducible Factor (HIF) Hydroxylases: Unique or Ubiquitous?

All metazoans that utilize molecular oxygen (O₂) for metabolic purposes have the capacity to adapt to hypoxia, the condition that arises when O₂ demand exceeds supply. This is mediated through activation of the hypoxia-inducible factor (HIF) pathway. At physiological oxygen levels (normoxia), HIF-prolyl hydroxylases (PHDs) hydroxylate proline residues on HIF-α subunits leading to their destabilization by promoting ubiquitination by the von-Hippel Lindau (VHL) ubiquitin ligase and subsequent proteasomal degradation. HIF-α transactivation is also repressed in an O₂-dependent way due to asparaginyl hydroxylation by the factor-inhibiting HIF (FIH). In hypoxia, the O₂-dependent hydroxylation of HIF-α subunits by PHDs and FIH is reduced, resulting in HIF-α accumulation, dimerization with HIF-β and migration into the nucleus to induce an adaptive transcriptional response. Although HIFs are the canonical substrates for PHD- and FIH-mediated protein hydroxylation, increasing evidence indicates that these hydroxylases may also have alternative targets. In addition to PHD-conferred alterations in protein stability, there is now evidence that hydroxylation can affect protein activity and protein/protein interactions for alternative substrates. PHDs can be pharmacologically inhibited by a new class of drugs termed prolyl hydroxylase inhibitors which have recently been approved for the treatment of anemia associated with chronic kidney disease. The identification of alternative targets of HIF hydroxylases is important in order to fully elucidate the pharmacology of hydroxylase inhibitors (PHI). Despite significant technical advances, screening, detection and verification of alternative functional targets for PHDs and FIH remain challenging. In this review, we discuss recently proposed non-HIF targets for PHDs and FIH and provide an overview of the techniques used to identify these.

Medicinal chemistry of metal chelating fragments in metalloenzyme active sites: A perspective

Numerous metal-containing enzymes (metalloenzymes) have been considered as drug targets related to diseases such as cancers, diabetes, anemia, AIDS, malaria, bacterial infection, fibrosis, and neurodegenerative diseases. Inhibitors of the metalloenzymes have been developed independently, most of which are mimics of substrates of the corresponding enzymes. However, little attention has been paid to the interactions between inhibitors and active site metal ions. This review is focused on different metal binding fragments and their chelating properties in the metal-containing active binding pockets of metalloenzymes. We have enumerated over one hundred of inhibitors targeting various metalloenzymes and identified over ten kinds of fragments with different binding patterns. Furthermore, we have investigated the inhibitors that are undergoing clinical evaluation in order to help looking for more potential scaffolds bearing metal binding fragments. This review will provide deep insights for the rational design of novel inhibitors targeting the metal-containing binding sites of specific proteins.

Possible Role of PHD Inhibitors as Hypoxia-Mimicking Agents in the Maintenance of Neural Stem Cells' Self-Renewal Properties

Hypoxia is the most critical factor for maintaining stemness. During embryonic development, neural stem cells (NSCs) reside in hypoxic niches, and different levels of oxygen pressure and time of hypoxia exposure play important roles in the development of NSCs. Such hypoxic niches exist in adult brain tissue, where the neural precursors originate. Hypoxia-inducible factors (HIFs) are key transcription heterodimers consisting of regulatory α-subunits (HIF-1α, HIF-2α, HIF-3α) and a constitutive β-subunit (HIF-β). Regulation of downstream targets determines the fate of NSCs. In turn, the stability of HIFs-α is regulated by prolyl hydroxylases (PHDs), whose activity is principally modulated by PHD substrates like oxygen (O₂), α-ketoglutarate (α-KG), and the co-factors ascorbate (ASC) and ferrous iron (Fe²⁺). It follows that the transcriptional activity of HIFs is actually determined by the contents of O₂, α-KG, ASC, and Fe²⁺. In normoxia, HIFs-α are rapidly degraded via the ubiquitin-proteasome pathway, in which PHDs, activated by O₂, lead to hydroxylation of HIFs-α at residues 402 and 564, followed by recognition by the tumor suppressor protein von Hippel–Lindau (pVHL) as an E3 ligase and ubiquitin labeling. Conversely, in hypoxia, the activity of PHDs is inhibited by low O₂ levels and HIFs-α can thus be stabilized. Hence, suppression of PHD activity in normoxic conditions, mimicking the effect of hypoxia, might be beneficial for preserving the stemness of NSCs, and it is clinically relevant as a therapeutic approach for enhancing the number of NSCs in vitro and for cerebral ischemia injury in vivo. This study will review the putative role of PHD inhibitors on the self-renewal of NSCs.

Re-education begins at home: an overview of the discovery of in vivo-active small molecule modulators of endogenous stem cells

INTRODUCTION

Degenerative diseases, such as Alzheimer's disease, heart disease and arthritis cause great suffering and are major socioeconomic burdens. An attractive treatment approach is stem cell transplantation to regenerate damaged or destroyed tissues. However, this can be problematic. For example, donor cells may not functionally integrate into the host tissue. An alternative methodology is to deliver bioactive agents, such as small molecules, directly into the diseased tissue to enhance the regenerative potential of endogenous stem cells. Areas covered: In this review, the authors discuss the necessity of developing these small molecules to treat degenerative diseases and survey progress in their application as therapeutics. They describe both the successes and caveats of developing small molecules that target endogenous stem cells to induce tissue regeneration. This article is based on literature searches which encompass databases for biomedical research and clinical trials. These small molecules are also categorized per their target disease and mechanism of action. Expert opinion: The development of small molecules targeting endogenous stem cells is a high-profile research area. Some compounds have made the successful transition to the clinic. Novel approaches, such as modulating the stem cell niche or targeted delivery to disease sites, should increase the likelihood of future successes in this field.

[Is Oxygen Deficiency Always Harmful?]

Is Oxygen Deficiency Always Harmful? Abstract. The role of the cardiovascular circulation is to supply tissue with oxygen and nutrients. Oxygen deficiency (hypoxia) is considered life-threatening, since cells die, either through apoptotic or necrotic processes. Tissue tries to counteract this by means of evolutionary signalling pathways, such as the nuclear hypoxia-inducible factor, which protects the tissue by promoting cell survival strategies and simultaneously intervening in angiogenesis, haematogenesis and metabolic processes. Recent findings indicate that these conserved signalling pathways can also function as therapeutic approaches in wound healing of bones and skin, as well as in the regeneration of tissues, e.g. in the liver, and in the hematopoietic system.

Loss of PHD3 in myeloid cells dampens the inflammatory response and fibrosis after hind-limb ischemia

Macrophages are essential for the inflammatory response after an ischemic insult and thereby influence tissue recovery. For the oxygen sensing prolyl-4-hydroxylase domain enzyme (PHD) 2 a clear impact on the macrophage-mediated arteriogenic response after hind-limb ischemia has been demonstrated previously, which involves fine tuning a M2-like macrophage population. To analyze the role of PHD3 in macrophages, we performed hind-limb ischemia (ligation and excision of the femoral artery) in myeloid-specific PHD3 knockout mice (PHD3^−/−) and analyzed the inflammatory cell invasion, reperfusion recovery and fibrosis in the ischemic muscle post-surgery. In contrast to PHD2, reperfusion recovery and angiogenesis was unaltered in PHD3^−/− compared to WT mice. Macrophages from PHD3^−/− mice showed, however, a dampened inflammatory reaction in the affected skeletal muscle tissues compared to WT controls. This was associated with a decrease in fibrosis and an anti-inflammatory phenotype of the PHD3^−/− macrophages, as well as decreased expression of Cyp2s1 and increased PGE2-secretion, which could be mimicked by PHD3^−/− bone marrow-derived macrophages in serum starvation.

Trimodal rescue of hind limb ischemia with growth factors, cells, and nanocarriers: fundamentals to clinical trials

Peripheral artery disease is a severe medical condition commonly characterized by critical or acute limb ischemia. Gradual accumulation of thrombotic plaques in peripheral arteries of the lower limb may lead to intermittent claudication or ischemia in muscle tissue. Ischemic muscle tissue with lesions may become infected, resulting in a non-healing wound. Stable progression of the non-healing wound associated with severe ischemia might lead to functional deterioration of the limb, which, depending on the severity, can result in amputation. Immediate rescue of ischemic muscles through revascularization strategies is considered the gold standard to treat critical limb ischemia. Growth factors offer multiple levels of protection in revascularization of ischemic tissue. In this review, the basic mechanism through which growth factors exert their beneficial properties to rescue the ischemic limb is extensively discussed. Moreover, clinical trials based on growth factor and stem cell therapy to treat critical limb ischemia are considered. The clinical utility of stem cell therapy for the treatment of limb ischemia is explained and recent advances in nanocarrier technology for selective growth factor and stem cell supplementation are summarized.

Hypoxia-regulated mechanisms in the pathogenesis of obesity and non-alcoholic fatty liver disease

The pandemic rise in obesity has resulted in an increased incidence of metabolic complications. Non-alcoholic fatty liver disease is the hepatic manifestation of the metabolic syndrome and has become the most common chronic liver disease in large parts of the world. The adipose tissue expansion and hepatic fat accumulation characteristics of these disorders compromise local oxygen homeostasis. The resultant tissue hypoxia induces adaptive responses to restore oxygenation and tissue metabolism and cell survival. Hypoxia-inducible factors (HIFs) function as master regulators of this hypoxia adaptive response, and are in turn hydroxylated by prolyl hydroxylases (PHDs). PHDs are the main cellular oxygen sensors and regulate HIF proteasomal degradation in an oxygen-dependent manner. HIFs and PHDs are implicated in numerous physiological and pathological conditions. Extensive research using genetic models has revealed that hypoxia signaling is also a key mechanism in adipose tissue dysfunction, leading to adipose tissue fibrosis, inflammation and insulin resistance. Moreover, hypoxia affects liver lipid metabolism and deranges hepatic lipid accumulation. This review summarizes the molecular mechanisms through which the hypoxia adaptive response affects adipocyte and hepatic metabolism, and the therapeutic possibilities of modulating HIFs and PHDs in obesity and fatty liver disease.

Ethyl 3,4-dihydroxybenzoate (EDHB): a prolyl hydroxylase inhibitor attenuates acute hypobaric hypoxia mediated vascular leakage in brain

Sudden exposure to altitude hypoxia is responsible for acute mountain sickness (AMS) in un-acclimatized persons. If not treated in time, AMS can worsen and leads to high altitude cerebral edema, which can be fatal. Present study explores the efficacy of ethyl 3,4-dihydroxybenzoate (EDHB), a prolyl hydroxylase enzyme inhibitor, in modulating adaptive responses to hypobaric hypoxia (HH) in rat brain. Male Sprague-Dawley rats treated with EDHB (75 mg/kg for 3 days), were subjected to acute HH exposure at 9144 m (30,000 ft) for 5 h. Animals were assessed for transvascular leakage and edema formation in brain and role of key inflammatory markers along with hypoxia responsive genes. HH stress increased transvascular permeability and edema formation in conjunction with upregulation of nuclear factor-κB (NF-κB) and its regulated proteins. There was surge in pro-inflammatory cytokines tumor necrosis factor-α, interleukin-6, interferon-γ, monocyte chemoattractant protein-1 and decrement in anti-inflammatory cytokine interleukin-10. Further, upregulation of vascular endothelial growth factor (VEGF), a vascular permeability marker and down-regulation of antioxidant and anti-inflammatory proteins hemoxygenase (HO-1) and metallothionein (MT-1) was also observed under hypoxia. EDHB supplementation effectively scaled down HH induced cerebral edema with concomitant downregulation of brain NF-κB expression. There was significant curtailment of pro-inflammatory cytokines and cell adhesion molecules. There was significant downregulation of permeability factor VEGF by EDHB with concomitant increment in hypoxia inducible factor (HIF1α) and anti-inflammatory proteins HO-1 and MT-1 compared to HH control thus accentuating the potential of EDHB as effective hypoxic preconditioning agent in ameliorating HH mediated injury in brain.

The Isoquinolone Derived Prolyl Hydroxylase Inhibitor ICA Is a Potent Substrate of the Organic Anion Transporters 1 and 3

OBJECTIVE

Many cellular responses to hypoxia are mediated by the transcription factor complex hypoxia-inducible factor (HIF). HIF stability is governed by a family of dioxygenases called HIF prolyl hydroxylases (PHDs). Isoquinolone-derived PHD inhibitors, like 2-(1-chloro-4-hydroxyisoquinoline-3-carboxamido) acetate (ICA), which stabilize the intracellular HIF-α have been suggested as a potentially beneficial therapeutic strategy for the treatment of disorders associated with ischemia. To stabilize HIF-α, ICA has to be taken up into proximal tubule cells (PCTs) across the basolateral membrane by one of the organic anion transporters 1, 2 or 3 (OAT1, OAT2 or OAT3). The release into the urine across the luminal membrane may be mediated by OAT4.

METHOD

To demonstrate interaction of ICA with human OAT1, OAT2, OAT3 and OAT4, ICA was tested on these transporters stably transfected in HEK293 cells by using p-aminohippurate (PAH), cGMP and estrone-3-sulfate (ES) as reference substrates, respectively.

RESULTS

Uptakes of PAH and ES in OAT1- and OAT3-transfected HEK293 cells were inhibited by ICA with half-maximal inhibition values of 0.29 ± 0.05 and 2.58 ± 0.16 µM, respectively. OAT2 was less sensitive to ICA. Efflux experiments identified ICA as an OAT1 and OAT3 substrate. Preloading OAT4-transfected HEK293 cells with ICA stimulated ES uptake by 18.3 ± 3.8%.

CONCLUSION

The uptake of ICA across the basolateral membrane of PCTs occurs mainly by OAT1 and the efflux into the tubular lumen by OAT4.

Targeting EPO and EPO receptor pathways in anemia and dysregulated erythropoiesis

INTRODUCTION

Recombinant human erythropoietin (rhEPO) is a first-line therapeutic for the anemia of chronic kidney disease, cancer chemotherapy, AIDS (Zidovudine therapy), and lower-risk myelodysplastic syndrome. However, rhEPO frequently elevates hypertension, is costly, and may affect cancer progression. Potentially high merit therefore exists for defining new targets for anti-anemia agents within erythropoietin (EPO) and EPO receptor (EPOR) regulatory circuits.

AREAS COVERED

EPO production by renal interstitial fibroblasts is subject to modulation by several regulators of hypoxia-inducible factor 2a (HIF2a) including Iron Response Protein-1, prolyl hydroxylases, and HIF2a acetylases, each of which holds potential as anti-anemia drug targets. The cell surface receptor for EPO (EPOR) preassembles as a homodimer, together with Janus Kinase 2 (JAK2), and therefore it remains attractive to develop novel agents that trigger EPOR complex activation (activating antibodies, mimetics, small-molecule agonists). Additionally, certain downstream transducers of EPOR/JAK2 signaling may be druggable, including Erythroferrone (a hepcidin regulator), a cytoprotective Spi2a serpin, and select EPOR-associated protein tyrosine phosphatases.

EXPERT OPINION

While rhEPO (and biosimilars) are presently important mainstay erythropoiesis-stimulating agents (ESAs), impetus exists for studies of novel ESAs that fortify HIF2a's effects, act as EPOR agonists, and/or bolster select downstream EPOR pathways to erythroid cell formation. Such agents could lessen rhEPO dosing, side effects, and/or costs.

HIF hydroxylase pathways in cardiovascular physiology and medicine

Hypoxia inducible factors (HIFs) are α/β heterodimeric transcription factors that direct multiple cellular and systemic responses in response to changes in oxygen availability. The oxygen sensitive signal is generated by a series of iron and 2-oxoglutarate-dependent dioxygenases that catalyze post-translational hydroxylation of specific prolyl and asparaginyl residues in HIFα subunits and thereby promote their destruction and inactivation in the presence of oxygen. In hypoxia, these processes are suppressed allowing HIF to activate a massive transcriptional cascade. Elucidation of these pathways has opened several new fields of cardiovascular research. Here, we review the role of HIF hydroxylase pathways in cardiac development and in cardiovascular control. We also consider the current status, opportunities, and challenges of therapeutic modulation of HIF hydroxylases in the therapy of cardiovascular disease.

Therapeutic inhibition of prolyl hydroxylase domain-containing enzymes in surgery: putative applications and challenges

Oxygen is essential for metazoans to generate energy. Upon oxygen deprivation adaptive and protective pathways are induced, mediated by hypoxia-inducible factors (HIFs) and prolyl hydroxylase domain-containing enzymes (PHDs). Both play a pivotal role in various conditions associated with prolonged ischemia and inflammation, and are promising targets for therapeutic intervention. This review focuses on aspects of therapeutic PHD modulation in surgically relevant disease conditions such as hepatic and intestinal disorders, wound healing, innate immune responses, and tumorigenesis, and discusses the therapeutic potential and challenges of PHD inhibition in surgical patients.

Signaling hypoxia by hypoxia-inducible factor protein hydroxylases: a historical overview and future perspectives

By the early 1900s, the close matching of oxygen supply with demand was recognized to be a fundamental requirement for physiological function, and multiple adaptive responses to environment hypoxia had been described. Nevertheless, the widespread operation of mechanisms that directly sense and respond to levels of oxygen in animal cells was not appreciated for most of the twentieth century with investigators generally stressing the regulatory importance of metabolic products. Work over the last 25 years has overturned that paradigm. It has revealed the existence of a set of “oxygen-sensing” 2-oxoglutarate dependent dioxygenases that catalyze the hydroxylation of specific amino acid residues and thereby control the stability and activity of hypoxia-inducible factor. The hypoxia-inducible factor hydroxylase pathway regulates a massive transcriptional cascade that is operative in essentially all animal cells. It transduces a wide range of responses to hypoxia, extending well beyond the classical boundaries of hypoxia physiology. Here we review the discovery and elucidation of these pathways, and consider the opportunities and challenges that have been brought into focus by the findings, including new implications for the integrated physiology of hypoxia and therapeutic approaches to ischemic/hypoxic disease.

Deletion of prolyl hydroxylase domain proteins (PHD1, PHD3) stabilizes hypoxia inducible factor-1 alpha, promotes neovascularization, and improves perfusion in a murine model of hind-limb ischemia

BACKGROUND

There is an emerging focus on investigating innovative therapeutic molecules that can potentially augment neovascularization in order to treat peripheral arterial disease (PAD). Although prolyl hydroxylase domain proteins 1 and 3 (PHD1 and PHD3) may modulate angiogenesis via regulation of hypoxia inducible factor-1α (HIF-1α), there has been no study directly addressing their roles in ischemia-induced vascular growth. We hypothesize that PHD1(-/-) or PHD3(-/-) deficiency might promote angiogenesis in the murine hind-limb ischemia (HLI) model.

STUDY DESIGN

Wild type (WT), PHD1(-/-) and PHD3(-/-) male mice aged 8-12weeks underwent right femoral artery ligation. Post-procedurally, motor function assessment and laser Doppler imaging were periodically performed. The mice were euthanized after 28days and muscles were harvested. Immunohistochemical analysis was performed to determine the extent of angiogenesis by measuring capillary and arteriolar density. VEGF expression was quantified by enzyme-linked immunosorbent assay (ELISA). Bcl-2 and HIF-1α were analyzed by immunofluorescence. Fibrosis was measured by picrosirius red staining.

RESULTS

PHD1(-/-) and PHD3(-/-) mice showed significantly improved recovery of perfusion and motor function score when compared to WT after femoral artery ligation. These mice also exhibited increased capillary and arteriolar density, capillary/myocyte ratio along with decreased fibrosis compared to WT. VEGF, Bcl-2 and HIF-1α expression increased in PHD1(-/-) and PHD3(-/-) mice compared to WT.

CONCLUSIONS

Taken together these results suggest that PHD1 and PHD3 deletions promote angiogenesis in ischemia-injured tissue, and may present a promising therapeutic strategy in treating PAD.

Hypoxia-inducible factors link iron homeostasis and erythropoiesis

Iron is required for efficient oxygen transport, and hypoxia signaling links erythropoiesis with iron homeostasis. Hypoxia induces a highly conserved signaling pathway in cells under conditions of low levels of O2. One component of this pathway, hypoxia-inducible factor (HIF), is a transcription factor that is highly active in hypoxic cells. The first HIF target gene characterized was EPO, which encodes erythropoietin-a glycoprotein hormone that controls erythropoiesis. In the past decade, there have been fundamental advances in our understanding of how hypoxia regulates iron levels to support erythropoiesis and maintain systemic iron homeostasis. We review the cell type-specific effects of hypoxia and HIFs in adaptive response to changes in oxygen and iron availability as well as potential uses of HIF modulators for patients with iron-related disorders.