PHD2: from hypoxia regulation to disease progression

Tumor promoting effect of PDLIM2 downregulation involves mitochondrial ROS, oncometabolite accumulations and HIF-1α activation

BACKGROUND

Cancer is characterized by dysregulated cellular metabolism. Thus, understanding the mechanisms underlying these metabolic alterations is important for developing targeted therapies. In this study, we investigated the pro-tumoral effect of PDZ and LIM domain 2 (PDLIM2) downregulation in lung cancer growth and its association with the accumulation of mitochondrial ROS, oncometabolites and the activation of hypoxia-inducible factor-1 (HIF-1) α in the process.

METHODS

Databases and human cancer tissue samples were analyzed to investigate the roles of PDLIM2 and HIF-1α in cancer growth. DNA microarray and gene ontology enrichment analyses were performed to determine the cellular functions of PDLIM2. Seahorse assay, flow cytometric analysis, and confocal microscopic analysis were employed to study mitochondrial functions. Oncometabolites were analyzed using liquid chromatography-mass spectrometry (LC-MS). A Lewis lung carcinoma (LLC) mouse model was established to assess the in vivo function of PDLIM2 and HIF-1α.

RESULTS

The expression of PDLIM2 was downregulated in lung cancer, and this downregulation correlated with poor prognosis in patients. PDLIM2 highly regulated genes associated with mitochondrial functions. Mechanistically, PDLIM2 downregulation resulted in NF-κB activation, impaired expression of tricarboxylic acid (TCA) cycle genes particularly the succinate dehydrogenase (SDH) genes, and mitochondrial dysfunction. This disturbance contributed to the accumulation of succinate and other oncometabolites, as well as the buildup of mitochondrial reactive oxygen species (mtROS), leading to the activation of hypoxia-inducible factor 1α (HIF-1α). Furthermore, the expression of HIF-1α was increased in all stages of lung cancer. The expression of PDLIM2 and HIF-1α was reversely correlated in lung cancer patients. In the animal study, the orally administered HIF-1α inhibitor, PX-478, significantly reduces PDLIM2 knockdown-promoted tumor growth.

CONCLUSION

These findings shed light on the complex action of PDLIM2 on mitochondria and HIF-1α activities in lung cancer, emphasizing the role of HIF-1α in the tumor-promoting effect of PDLIM2 downregulation. Additionally, they provide new insights into a strategy for precise targeted treatment by suggesting that HIF-1α inhibitors may serve as therapy for lung cancer patients with PDLIM2 downregulation.

From Vessels to Neurons-The Role of Hypoxia Pathway Proteins in Embryonic Neurogenesis

Embryonic neurogenesis can be defined as a period of prenatal development during which divisions of neural stem and progenitor cells give rise to neurons. In the central nervous system of most mammals, including humans, the majority of neocortical neurogenesis occurs before birth. It is a highly spatiotemporally organized process whose perturbations lead to cortical malformations and dysfunctions underlying neurological and psychiatric pathologies, and in which oxygen availability plays a critical role. In case of deprived oxygen conditions, known as hypoxia, the hypoxia-inducible factor (HIF) signaling pathway is activated, resulting in the selective expression of a group of genes that regulate homeostatic adaptations, including cell differentiation and survival, metabolism and angiogenesis. While a physiological degree of hypoxia is essential for proper brain development, imbalanced oxygen levels can adversely affect this process, as observed in common obstetrical pathologies such as prematurity. This review comprehensively explores and discusses the current body of knowledge regarding the role of hypoxia and the HIF pathway in embryonic neurogenesis of the mammalian cortex. Additionally, it highlights existing gaps in our understanding, presents unanswered questions, and provides avenues for future research.

Novel furan chalcone modulates PHD-2 induction to impart antineoplastic effect in mammary gland carcinoma

Normoxic inactivation of prolyl hydroxylase-2 (PHD-2) in tumour microenvironment paves the way for cancer cells to thrive under the influence of HIF-1α and NF-κB. Henceforth, the present study is aimed to identify small molecule activators of PHD-2. A virtual screening was conducted on a library consisting of 265,242 chemical compounds, with the objective of identifying molecules that exhibit structural similarities to the furan chalcone scaffold. Further, PHD-2 activation potential of screened compound was determined using in vitro 2-oxoglutarate assay. The cytotoxic activity and apoptotic potential of screened compound was determined using various staining techniques, including 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide, 4',6-diamidino-2-phenylindole (DAPI), 1,1',3,3'-tetraethylbenzimi-dazolylcarbocyanine iodide (JC-1), and acridine orange/ethidium bromide (AO/EB), against MCF-7 cells. 7,12-Dimethylbenz[a]anthracene (DMBA) model of mammary gland cancer was used to study the in vivo antineoplastic efficacy of screened compound. [(E)-1-(4-fluorophenyl)-3-(furan-2-yl) prop-2-en-1-one] (BBAP-7) was screened and validated as a PHD-2 activator by an in vitro 2-oxo-glutarate assay. The IC50 of BBAP-7 on MCF-7 cells is 18.84 µM. AO/EB and DAPI staining showed nuclear fragmentation, blebbing and condensation in MCF-7 cells following BBAP-7 treatment. The red-to-green intensity ratio of JC-1 stained MCF-7 cells decreased after BBAP-7 treatment, indicating mitochondrial-mediated apoptosis. DMBA caused mammary gland dysplasia, duct hyperplasia and ductal carcinoma in situ. Carmine staining, histopathology, and scanning electron microscopy demonstrated that BBAP-7, alone or with tirapazamine, restored mammary gland surface morphology and structural integrity. Additionally, BBAP-7 therapy significantly reduced oxidative stress and glycolysis. The findings reveal that BBAP-7 activates PHD-2, making it a promising anticancer drug.

Intertwined regulators: hypoxia pathway proteins, microRNAs, and phosphodiesterases in the control of steroidogenesis

Oxygen sensing is of paramount importance for maintaining cellular and systemic homeostasis. In response to diminished oxygen levels, the hypoxia-inducible factors (HIFs) orchestrate various biological processes. These pivotal transcription factors have been identified as key regulators of several biological events. Notably, extensive research from our group and others has demonstrated that HIF1α exerts an inverse regulatory effect on steroidogenesis, leading to the suppression of crucial steroidogenic enzyme expression and a subsequent decrease in steroid levels. These steroid hormones occupy pivotal roles in governing a myriad of physiological processes. Substantial or prolonged fluctuations in steroid levels carry detrimental consequences across multiple organ systems and underlie various pathological conditions, including metabolic and immune disorders. MicroRNAs serve as potent mediators of multifaceted gene regulatory mechanisms, acting as influential epigenetic regulators that modulate a broad spectrum of gene expressions. Concomitantly, phosphodiesterases (PDEs) play a crucial role in governing signal transduction. PDEs meticulously manage intracellular levels of both cAMP and cGMP, along with their respective signaling pathways and downstream targets. Intriguingly, an intricate interplay seems to exist between hypoxia signaling, microRNAs, and PDEs in the regulation of steroidogenesis. This review highlights recent advances in our understanding of the role of microRNAs during hypoxia-driven processes, including steroidogenesis, as well as the possibilities that exist in the application of HIF prolyl hydroxylase (PHD) inhibitors for the modulation of steroidogenesis.

Low nuclear expression of HIF-hydroxylases PHD2/EGLN1 and PHD3/EGLN3 are associated with poor recurrence-free survival in clear cell renal cell carcinoma

BACKGROUND

Hypoxia inducible factors, HIF-1α and HIF-2α, and their main regulators, the prolyl hydroxylase domain proteins (PHDs), mediate cellular response to hypoxia and contribute to tumor progression in clear cell renal cell carcinoma (ccRCC). These biomarkers may improve the value of traditional histopathological features in predicting disease progression after nephrectomy for localized ccRCC and guide patient selection for adjuvant treatments.

PATIENTS AND METHODS

In this study, we analyzed the associations of PHD2 and PHD3 with histopathological tumor features and recurrence-free survival (RFS) in a retrospective cohort of 173 patients who had undergone surgery for localized ccRCC at Helsinki University Hospital (HUH), Finland. An external validation cohort of 191 patients was obtained from Turku University Hospital (TUH), Finland. Tissue-microarrays (TMA) were constructed using the primary tumor samples. Clinical parameters and follow-up information from 2006 to 2019 were obtained from electronic medical records. The cytoplasmic and nuclear expression of PHD2, and PHD3 were scored based on immunohistochemical staining and their associations with histopathological features and RFS were evaluated.

RESULTS

Nuclear PHD2 and PHD3 expression in cancer cells were associated with lower pT-stage and Fuhrman grade compared with negative nuclei. Patients with positive nuclear expression of PHD2 and PHD3 in cancer cells had favorable RFS compared with patients having negative tumors. The nuclear expression of PHD2 was independently associated with a decreased risk of disease recurrence or death from RCC in multivariable analysis. These results were observed in both cohorts.

CONCLUSIONS

The absence of nuclear PHD2 and PHD3 expression in ccRCC was associated with poor RFS and the nuclear expression of PHD2 predicted RFS regardless of other known histopathological prognostic factors. Nuclear PHD2 and PHD3 are potential prognostic biomarkers in patients with localized ccRCC and should be further investigated and validated in prospective studies.

AZD7648, a DNA-PKcs inhibitor, overcomes radioresistance in Hep3B xenografts and cells under tumor hypoxia

Radiation therapy is one of the most commonly used cancer treatments. However, it has important concerns such as damage to normal tissues around cancers and radioresistance. To overcome these problems, combination therapy using radiosensitizer and radiotherapy will be a good alternative. The present study investigated the effects of AZD7648 on overcoming radioresistance as well as radiosensitizing in Hep3B xenografts and cells. The results showed that AZD7648 enhanced ionizing radiation (IR)-induced tumor growth not only in radiosensitive but also radioresistant tumors. In particular, the combination of AZD7648 with radiation reduced the expression of hypoxia induce factor-1α (HIF-1α) in radioresistant tumors. In vitro studies, AZD7648 plus IR increased IR-induced G2/M arrest and regulated cell cycle checkpoints such as cyclinB1, p-cdc2 in normoxia but not in hypoxia. AZD7648 induced more radiation-mediated ROS than radiation only under normoxia, but these ROS were not altered by AZD7648 under hypoxia. Interestingly, AZD7648 downregulated HIF-1α expression level under CoCl2-treated hypoxic condition but not in normoxic condition. In conclusion, AZD7648 synergistically increased radiosensitivity through accumulating IR-induced G2/M arrest and further improved radioresistance via regulation of HIF-1α. The present data suggest that AZD7648 may be a strong radiosensitizer in radioresistant as well as radiosensitive cancers.

The complexity of nicotinamide adenine dinucleotide (NAD), hypoxic, and aryl hydrocarbon receptor cell signaling in chronic kidney disease

Early-stage detection of chronic kidney diseases (CKD) is important to treatment that may slow and occasionally halt CKD progression. CKD of diverse etiologies share similar histologic patterns of glomerulosclerosis, tubular atrophy, and interstitial fibrosis. Macro-vascular disease and micro-vascular disease promote tissue ischemia, contributing to injury. Tissue ischemia promotes hypoxia, and this in turn activates the hypoxia-inducible transcription factors (HIFs). HIF-1α and HIF-2α, share a dimer partner, HIF-1β, with the aryl hydrocarbon receptor (AHR) and are each activated in CKD and associated with kidney cellular nicotinamide adenine dinucleotide (NAD) depletion. The Preiss-Handler, salvage, and de novo pathways regulate NAD biosynthesis and gap-junctions regulate NAD cellular retention. In the Preiss-Handler pathway, niacin forms NAD. Niacin also exhibits crosstalk with HIF and AHR cell signals in the regulation of insulin sensitivity, which is a complication in CKD. Dysregulated enzyme activity in the NAD de novo pathway increases the levels of circulating tryptophan metabolites that activate AHR, resulting in poly-ADP ribose polymerase activation, thrombosis, endothelial dysfunction, and immunosuppression. Therapeutically, metabolites from the NAD salvage pathway increase NAD production and subsequent sirtuin deacetylase activity, resulting in reduced activation of retinoic acid-inducible gene I, p53, NF-κB and SMAD2 but increased activation of FOXO1, PGC-1α, and DNA methyltransferase-1. These post-translational responses may also be initiated through non-coding RNAs (ncRNAs), which are additionally altered in CKD. Nanoparticles traverse biological systems and can penetrate almost all tissues as disease biomarkers and drug delivery carriers. Targeted delivery of non-coding RNAs or NAD metabolites with nanoparticles may enable the development of more effective diagnostics and therapies to treat CKD.

A framework of transient hypercapnia to achieve an increased cerebral blood flow induced by nasal breathing during aerobic exercise

During exercise, cerebral blood flow (CBF) is expected to only increase to a maximal volume up to a moderate intensity aerobic effort, suggesting that CBF is expected to decline past 70 % of a maximal aerobic effort. Increasing CBF during exercise permits an increased cerebral metabolic activity that stimulates neuroplasticity and other key processes of cerebral adaptations that ultimately improve cognitive health. Recent work has focused on utilizing gas-induced exposure to intermittent hypoxia during aerobic exercise to maximize the improvements in cognitive function compared to those seen under normoxic conditions. However, it is postulated that exercising by isolating breathing only to the nasal route may provide a similar effect by stimulating a transient hypercapnic condition that is non-gas dependent. Because nasal breathing prevents hyperventilation during exercise, it promotes an increase in the partial arterial pressure of CO2. The rise in systemic CO2 stimulates hypercapnia and permits the upregulation of hypoxia-related genes. In addition, the rise in systemic CO2 stimulates cerebral vasodilation, promoting a greater increase in CBF than seen during normoxic conditions. While more research is warranted, nasal breathing might also promote benefits related to improved sleep, greater immunity, and body fat loss. Altogether, this narrative review presents a theoretical framework by which exercise-induced hypercapnia by utilizing nasal breathing during moderate-intensity aerobic exercise may promote greater health adaptations and cognitive improvements than utilizing oronasal breathing.

Deciphering the roles of aryl hydrocarbon receptor (AHR) in regulating carcinogenesis

Aryl hydrocarbon receptor (AHR) is a ligand-dependent receptor that belongs to the superfamily of basic helix-loop-helix (bHLH) transcription factors. The activation of the canonical AHR signaling pathway is known to induce the expression of cytochrome P450 enzymes, facilitating the detoxification metabolism in the human body. Additionally, AHR could interact with various signaling pathways such as epidermal growth factor receptor (EGFR), signal transducer and activator of transcription 3 (STAT3), hypoxia-inducible factor-1α (HIF-1α), nuclear factor ekappa B (NF-κβ), estrogen receptor (ER), and androgen receptor (AR) signaling pathways. Over the past 30 years, several studies have reported that various chemical, physical, or biological agents, such as tobacco, hydrocarbon compounds, industrial and agricultural chemical wastes, drugs, UV, viruses, and other toxins, could affect AHR expression or activity, promoting cancer development. Thus, it is valuable to overview how these factors regulate AHR-mediated carcinogenesis. Current findings have reported that many compounds could act as AHR ligands to drive the expressions of AHR-target genes, such as CYP1A1, CYP1B1, MMPs, and AXL, and other targets that exert a pro-proliferation or anti-apoptotic effect, like XIAP. Furthermore, some other physical and chemical agents, such as UV and 3-methylcholanthrene, could promote AHR signaling activities, increasing the signaling activities of a few oncogenic pathways, such as the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathways. Understanding how various factors regulate AHR-mediated carcinogenesis processes helps clinicians and scientists plan personalized therapeutic strategies to improve anti-cancer treatment efficacy. As many studies that have reported the roles of AHR in regulating carcinogenesis are preclinical or observational clinical studies that did not explore the detailed mechanisms of how different chemical, physical, or biological agents promote AHR-mediated carcinogenesis processes, future studies should focus on conducting large-scale and functional studies to unravel the underlying mechanism of how AHR interacts with different factors in regulating carcinogenesis processes.

A Novel PHD2/VHL-mediated Regulation of YAP1 Contributes to VEGF Expression and Angiogenesis

The transcriptional co-activator YAP1 is the major oncogenic component of the Hippo signaling pathway and contributes to the genesis and progression of various tumors, including non-small cell lung cancer (NSCLC). YAP1 levels are regulated by the canonical Hippo kinases, MST1/2 and LATS1/2, which modulate its cytoplasmic retention and proteasomal degradation. While non-canonical regulation of YAP1 has been reported, its role in hypoxic response is not fully elucidated. The studies presented here show that YAP1 levels and function are modulated by VHL and PHD2. YAP1 could regulate multiple genes involved in angiogenesis through E2F1; it also associates with HIF1α in cancer cells under hypoxic conditions, inducing the VEGF-A promoter. Under normoxic conditions, PHD2 associates with and hydroxylates specific proline residues on YAP1, facilitating its interaction with VHL and promoting ubiquitination and subsequent proteasomal degradation. Exposure to hypoxia dissociates YAP1 from PHD2 and VHL, elevating YAP1 levels and enhancing its association with HIF1α. YAP1-HIF1α interaction was higher in NSCLC and RCC samples, indicating a role for this interaction in the genesis of these cancers. Our results thus reveal a novel mode of regulation of YAP1 by PHD2 and VHL in normoxic cells, suggesting that YAP1-mediated induction of VEGF and other genes contributes to hypoxic response in tumors.

Regulation of Transactivation at C-TAD Domain of HIF-1α by Factor-Inhibiting HIF-1α (FIH-1): A Potential Target for Therapeutic Intervention in Cancer

Hypoxia-inducible factor-1alpha (HIF-1α) is a major transcription factor that adapts to low oxygen homeostasis and regulates the expression of several hypoxic genes, which aid in cancer survival and development. It has recently piqued the interest of translational researchers in the disciplines of cancer sciences. Hypoxia triggers an ample adaptive mechanism mediated via the HIF-1α transcriptional domain. Anaerobic glycolysis, angiogenesis, metastasis, and mitophagy are adaptive mechanisms that support tumor survival by promoting oxygen supply and regulating oxygen demand in hypoxic tumor cells. Throughout this pathway, the factor-inhibiting HIF-1α is a negative regulator of HIF-1α leading to its hydroxylation at the C-TAD domain of HIF-1α under normoxia. Thus, hydroxylated HIF-1α is unable to proceed with the transcriptional events due to interference in binding of C-TAD and CBP/p300. From this review, we can hypothesize that remodeling of FIH-1 activity is a unique mechanism that decreases the transcriptional activity of HIF-1α and, as a result, all of its hypoxic consequences. Hence, this review manuscript details the depth of knowledge of FIH-1 on hypoxia-associated cellular and molecular events, a potential strategy for targeting hypoxia-induced malignancies.

Hypoxia and Hypoxia-Inducible Factors in Lymphedema

Lymphedema is a chronic inflammatory disorder characterized by edema, fat deposition, and fibrotic tissue remodeling. Despite significant advances in lymphatic biology research, our knowledge of lymphedema pathology is incomplete. Currently, there is no approved pharmacological therapy for this debilitating disease. Hypoxia is a recognized feature of inflammation, obesity, and fibrosis. Understanding hypoxia-regulated pathways in lymphedema may provide new insights into the pathobiology of this chronic disorder and help develop new medicinal treatments.

The oxygen sensor Prolyl hydroxylase domain 2 regulates the in vivo suppressive capacity of regulatory T cells

The oxygen sensor prolyl hydroxylase domain 2 (PHD2) plays an important role in cell hypoxia adaptation by regulating the stability of HIF proteins (HIF1α and HIF2α) in numerous cell types, including T lymphocytes. The role of oxygen sensor on immune cells, particularly on regulatory T cell (Treg) function, has not been fully elucidated. The purpose of our study was to evaluate the role of PHD2 in the regulation of Treg phenotype and function. We demonstrate herein that selective ablation of PHD2 expression in Treg (PHD2^ΔTreg mice) leads to a spontaneous systemic inflammatory syndrome, as evidenced by weight loss, development of a rectal prolapse, splenomegaly, shortening of the colon, and elevated expression of IFN-γ in the mesenteric lymph nodes, intestine, and spleen. PHD2 deficiency in Tregs led to an increased number of activated CD4 conventional T cells expressing a Th1-like effector phenotype. Concomitantly, the expression of innate-type cytokines such as Il1b, Il12a, Il12b, and Tnfa was found to be elevated in peripheral (gut) tissues and spleen. PHD2^ΔTreg mice also displayed an enhanced sensitivity to dextran sodium sulfate-induced colitis and toxoplasmosis, suggesting that PHD2-deficient Tregs did not efficiently control inflammatory response in vivo, particularly those characterized by IFN-γ production. Further analysis revealed that Treg dysregulation was largely prevented in PHD2-HIF2α (PHD2-HIF2α^ΔTreg mice), but not in PHD2-HIF1α (PHD2-HIF1α^ΔTreg mice) double KOs, suggesting an important and possibly selective role of the PHD2-HIF2α axis in the control of Treg function. Finally, the transcriptomic analysis of PHD2-deficient Tregs identified the STAT1 pathway as a target of the PHD2-HIF2α axis in regulatory T cell phenotype and in vivo function.

Non-Coding RNAs in the Crosstalk between Breast Cancer Cells and Tumor-Associated Macrophages

Non-coding RNAs (ncRNAs) play a pivotal role in regulating the tumor microenvironment (TME) by controlling gene expression at multiple levels. In tumors, ncRNAs can mediate the crosstalk between cancer cells and other cells in the TME, such as immune cells, stromal cells, and endothelial cells, influencing tumor development and progression. Tumor-associated macrophages (TAMs) are among the most abundant inflammatory cells infiltrating solid cancers that promote tumorigenesis, and their infiltration correlates with a poor prognosis in many tumors. Cancer cells produce different ncRNAs that orchestrate TAM recruitment and polarization toward a tumor-promoting phenotype. Tumor-reprogrammed macrophages shape the TME by promoting angiogenesis and tissue remodeling, and suppressing the anti-tumor activity of adaptive immune cells. TAMs can also produce ncRNA molecules that boost cancer cell proliferation and direct their phenotype and metabolic changes facilitating cancer progression and metastasis. This review will focus on the crosstalk between cancer cells and TAMs mediated by microRNAs and long non-coding RNAs during breast cancer (BC) initiation and progression.

Lactate-Loaded Nanoparticles Induce Glioma Cytotoxicity and Increase the Survival of Rats Bearing Malignant Glioma Brain Tumor

A glioblastoma is an aggressive form of a malignant glial-derived tumor with a poor prognosis despite multimodal therapy approaches. Lactate has a preponderant role in the tumor microenvironment, playing an immunoregulatory role as well as being a carbon source for tumor growth. Lactate homeostasis depends on the proper functioning of intracellular lactate regulation systems, such as transporters and enzymes involved in its synthesis and degradation, with evidence that an intracellular lactate overload generates metabolic stress on tumor cells and tumor cell death. We propose that the delivery of a lactate overload carried in nanoparticles, allowing the intracellular release of lactate, would compromise the survival of tumor cells. We synthesized and characterized silica and titania nanoparticles loaded with lactate to evaluate the cellular uptake, metabolic activity, pH modification, and cytotoxicity on C6 cells under normoxia and chemical hypoxia, and, finally, determined the survival of an orthotopic malignant glioma model after in situ administration. A dose-dependent reduction in metabolic activity of treated cells under normoxia was found, but not under hypoxia, independent of glucose concentration. Lactated-loaded silica nanoparticles were highly cytotoxic (58.1% of dead cells) and generated significant supernatant acidification. In vivo, lactate-loaded silica nanoparticles significantly increased the median survival time of malignant glioma-bearing rats (p = 0.005) when administered in situ. These findings indicate that lactate-loaded silica nanoparticles are cytotoxic on glioma cells in vitro and in vivo.

PHD2 deletion in endothelial or arterial smooth muscle cells reveals vascular cell type-specific responses in pulmonary hypertension and fibrosis

Hypoxia plays an important regulatory role in the vasculature to adjust blood flow to meet metabolic requirements. At the level of gene transcription, the responses are mediated by hypoxia-inducible factor (HIF) the stability of which is controlled by the HIF prolyl 4-hydroxylase-2 (PHD2). In the lungs hypoxia results in vasoconstriction, however, the pathophysiological relevance of PHD2 in the major arterial cell types; endothelial cells (ECs) and arterial smooth muscle cells (aSMCs) in the adult vasculature is incompletely characterized. Here, we investigated PHD2-dependent vascular homeostasis utilizing inducible deletions of PHD2 either in ECs (Phd2^∆ECi) or in aSMCs (Phd2^∆aSMC). Cardiovascular function and lung pathologies were studied using echocardiography, Doppler ultrasonography, intraventricular pressure measurement, histological, ultrastructural, and transcriptional methods. Cell intrinsic responses were investigated in hypoxia and in conditions mimicking hypertension-induced hemodynamic stress. Phd2^∆ECi resulted in progressive pulmonary disease characterized by a thickened respiratory basement membrane (BM), alveolar fibrosis, increased pulmonary artery pressure, and adaptive hypertrophy of the right ventricle (RV). A low oxygen environment resulted in alterations in cultured ECs similar to those in Phd2^∆ECi mice, involving BM components and vascular tone regulators favoring the contraction of SMCs. In contrast, Phd2^∆aSMC resulted in elevated RV pressure without alterations in vascular tone regulators. Mechanistically, PHD2 inhibition in aSMCs involved  actin polymerization -related tension development via activated cofilin. The results also indicated that hemodynamic stress, rather than PHD2-dependent hypoxia response alone, potentiates structural remodeling of the extracellular matrix in the pulmonary microvasculature and respiratory failure.

Reprogramming of glucocorticoid receptor function by hypoxia

Here, we investigate the impact of hypoxia on the hepatic response of glucocorticoid receptor (GR) to dexamethasone (DEX) in mice via RNA-sequencing. Hypoxia causes three types of reprogramming of GR: (i) much weaker induction of classical GR-responsive genes by DEX in hypoxia, (ii) a number of genes is induced by DEX specifically in hypoxia, and (iii) hypoxia induces a group of genes via activation of the hypothalamic-pituitary-adrenal (HPA) axis. Transcriptional profiles are reflected by changed GR DNA-binding as measured by ChIP sequencing. The HPA axis is induced by hypothalamic HIF1α and HIF2α activation and leads to GR-dependent lipolysis and ketogenesis. Acute inflammation, induced by lipopolysaccharide, is prevented by DEX in normoxia but not during hypoxia, and this is attributed to HPA axis activation by hypoxia. We unfold new physiological pathways that have consequences for patients suffering from GC resistance.

Hypoxia Pathway Proteins and Their Impact on the Blood Vasculature

Every cell in the body requires oxygen for its functioning, in virtually every animal, and a tightly regulated system that balances oxygen supply and demand is therefore fundamental. The vascular network is one of the first systems to sense oxygen, and deprived oxygen (hypoxia) conditions automatically lead to a cascade of cellular signals that serve to circumvent the negative effects of hypoxia, such as angiogenesis associated with inflammation, tumor development, or vascular disorders. This vascular signaling is driven by central transcription factors, namely the hypoxia inducible factors (HIFs), which determine the expression of a growing number of genes in endothelial cells and pericytes. HIF functions are tightly regulated by oxygen sensors known as the HIF-prolyl hydroxylase domain proteins (PHDs), which are enzymes that hydroxylate HIFs for eventual proteasomal degradation. HIFs, as well as PHDs, represent attractive therapeutic targets under various pathological settings, including those involving vascular (dys)function. We focus on the characteristics and mechanisms by which vascular cells respond to hypoxia under a variety of conditions.

Molecular Pathways Involved in the Development of Congenital Erythrocytosis

Patients with idiopathic erythrocytosis are directed to targeted genetic testing including nine genes involved in oxygen sensing pathway in kidneys, erythropoietin signal transduction in pre-erythrocytes and hemoglobin-oxygen affinity regulation in mature erythrocytes. However, in more than 60% of cases the genetic cause remains undiagnosed, suggesting that other genes and mechanisms must be involved in the disease development. This review aims to explore additional molecular mechanisms in recognized erythrocytosis pathways and propose new pathways associated with this rare hematological disorder. For this purpose, a comprehensive review of the literature was performed and different in silico tools were used. We identified genes involved in several mechanisms and molecular pathways, including mRNA transcriptional regulation, post-translational modifications, membrane transport, regulation of signal transduction, glucose metabolism and iron homeostasis, which have the potential to influence the main erythrocytosis-associated pathways. We provide valuable theoretical information for deeper insight into possible mechanisms of disease development. This information can be also helpful to improve the current diagnostic solutions for patients with idiopathic erythrocytosis.

Temporal Quantitative Proteomics Reveals Proteomic and Phosphoproteomic Alterations Associated with Adaptive Response to Hypoxia in Melanoma Cells

Simple Summary

Most solid tumours, including melanoma (skin cancer), are riddled with areas lacking adequate oxygen supply due to insufficient vasculature. Cancer cells in these regions are resistant to therapies and contribute to cancer spread and poor treatment response in patients. Understanding the mechanisms by which cancer cells adapt to survive in such a hostile environment will provide novel avenues for treatment. In this study, we investigated mechanisms that melanoma cells use to adapt and survive in an oxygen-poor environment. We used four different melanoma cell lines and studied how protein levels and phosphorylation patterns on thousands of proteins change when the cells are exposed to poor oxygen conditions. This revealed potential mechanisms on which cancer cells are dependent for survival. These survival mechanisms can be potentially targeted to achieve durable response to therapy. We demonstrate this by targeting one such mechanism required for cancer cell survival.

Abstract

Hypoxia is a common feature in various solid tumours, including melanoma. Cancer cells in hypoxic environments are resistant to both chemotherapy and radiation. Hypoxia is also associated with immune suppression. Identification of proteins and pathways that regulate cancer cell survival in hypoxic environments can reveal potential vulnerabilities that can be exploited to improve the efficacy of anticancer therapies. We carried out temporal proteomic and phosphoproteomic profiling in melanoma cell lines to identify hypoxia-induced protein expression and phosphorylation changes. By employing a TMT-based quantitative proteomics strategy, we report the identification and quantitation of >7000 proteins and >10,000 phosphosites in melanoma cell lines grown in hypoxia. Proteomics data show metabolic reprogramming as one of the prominent adaptive responses in hypoxia. We identify several novel hypoxia-mediated phosphorylation changes that have not been reported before. They reveal kinase signalling pathways that are potentially involved in modulating cellular response to hypoxia. In addition to known protein expression changes, we identify several novel proteomic alterations associated with adaptive response to hypoxia. We show that cancer cells require the ubiquitin–proteasome system to survive in both normoxia and hypoxia. Inhibition of proteasome activity affects cell survival and may provide a novel therapeutic avenue to target cancer cells in hypoxia. Our study can serve as a valuable resource to pursue novel candidates to target hypoxia in cancers and improve the efficacy of anticancer therapies.

HIF-Prolyl Hydroxylase Domain Proteins (PHDs) in Cancer-Potential Targets for Anti-Tumor Therapy?

Simple Summary

In solid tumors, proliferation of cancer cells typically outpaces the growth of functional vessels. The net result is often an obstructed blood circulation and areas of deprived oxygen (hypoxia). To overcome this acute stress, hypoxia inducible factors (HIFs) stimulate the expression of numerous proteins that will support adaptation to this situation and stimulate further growth, differentiation, and even dissemination. The HIF-response is closely controlled by a class of enzymes known as the HIF prolyl hydroxylases (PHDs). They are true oxygen sensors and directly regulate the activity of HIFs. Although many studies are currently focusing on inhibiting the activity of HIFs in tumors, the role of hypoxia signaling is complex and regulating PHDs in a number of tumor settings might be beneficial. This review gives an overview of the literature on the nature of PHDs in tumor-associated cells and discusses available PHD inhibitors and their potential use as an anti-tumor therapy.

Abstract

Solid tumors are typically associated with unbridled proliferation of malignant cells, accompanied by an immature and dysfunctional tumor-associated vascular network. Consequent impairment in transport of nutrients and oxygen eventually leads to a hypoxic environment wherein cells must adapt to survive and overcome these stresses. Hypoxia inducible factors (HIFs) are central transcription factors in the hypoxia response and drive the expression of a vast number of survival genes in cancer cells and in cells in the tumor microenvironment. HIFs are tightly controlled by a class of oxygen sensors, the HIF-prolyl hydroxylase domain proteins (PHDs), which hydroxylate HIFs, thereby marking them for proteasomal degradation. Remarkable and intense research during the past decade has revealed that, contrary to expectations, PHDs are often overexpressed in many tumor types, and that inhibition of PHDs can lead to decreased tumor growth, impaired metastasis, and diminished tumor-associated immune-tolerance. Therefore, PHDs represent an attractive therapeutic target in cancer research. Multiple PHD inhibitors have been developed that were either recently accepted in China as erythropoiesis stimulating agents (ESA) or are currently in phase III trials. We review here the function of HIFs and PHDs in cancer and related therapeutic opportunities.

Functional significance of germline EPAS1 variants

Mosaic or somatic EPAS1 mutations are associated with a range of phenotypes including pheochromocytoma and/or paraganglioma (PPGL), polycythemia and somatostatinoma. The pathogenic potential of germline EPAS1 variants however is not well understood. We report a number of germline EPAS1 variants occurring in patients with PPGL, including a novel variant c.739C>A (p.Arg247Ser); a previously described variant c.1121T>A (p.Phe374Tyr); several rare variants, c.581A>G (p.His194Arg), c.2353C>A (p.Pro785Thr) and c.2365A>G (p.Ile789Val); a common variant c.2296A>C (p.Thr766Pro). We performed detailed functional studies to understand their pathogenic role in PPGL. In transient transfection studies, EPAS1/HIF-2α p.Arg247Ser, p.Phe374Tyr and p.Pro785Thr were all stable in normoxia. In co-immunoprecipitation assays, only the novel variant p.Arg247Ser showed diminished interaction with pVHL. A direct interaction between HIF-2α Arg247 and pVHL was confirmed in structural models. Transactivation was assessed by means of a HRE-containing reporter gene in transiently transfected cells, and significantly higher reporter activity was only observed with EPAS1/HIF-2α p.Phe374Tyr and p.Pro785Thr. In conclusion, three germline EPAS1 variants (c.739C>A (p.Arg247Ser), c.1121T>A (p.Phe374Tyr) and c.2353C>A (p.Pro785Thr)) all have some functional features in common with somatic activating mutations. Our findings suggest that these three germline variants are hypermorphic alleles that may act as modifiers to the expression of PPGLs.

Hypoxia Pathway Proteins are Master Regulators of Erythropoiesis

Erythropoiesis is a complex process driving the production of red blood cells. During homeostasis, adult erythropoiesis takes place in the bone marrow and is tightly controlled by erythropoietin (EPO), a central hormone mainly produced in renal EPO-producing cells. The expression of EPO is strictly regulated by local changes in oxygen partial pressure (pO₂) as under-deprived oxygen (hypoxia); the transcription factor hypoxia-inducible factor-2 induces EPO. However, erythropoiesis regulation extends beyond the well-established hypoxia-inducible factor (HIF)-EPO axis and involves processes modulated by other hypoxia pathway proteins (HPPs), including proteins involved in iron metabolism. The importance of a number of these factors is evident as their altered expression has been associated with various anemia-related disorders, including chronic kidney disease. Eventually, our emerging understanding of HPPs and their regulatory feedback will be instrumental in developing specific therapies for anemic patients and beyond.

Proteomic Analysis Reveals a Mitochondrial Remodeling of βTC3 Cells in Response to Nanotopography

Recently, using cluster-assembled zirconia substrates with tailored roughness produced by supersonic cluster beam deposition, we demonstrated that β cells can sense nanoscale features of the substrate and can translate these stimuli into a mechanotransductive pathway capable of preserveing β-cell differentiation and function in vitro in long-term cultures of human islets. Using the same proteomic approach, we now focused on the mitochondrial fraction of βTC3 cells grown on the same zirconia substrates and characterized the morphological and proteomic modifications induced by the nanostructure. The results suggest that, in βTC3 cells, mitochondria are perturbed by the nanotopography and activate a program involving metabolism modification and modulation of their interplay with other organelles. Data were confirmed in INS1E, a different β-cell model. The change induced by the nanostructure can be pro-survival and prime mitochondria for a metabolic switch to match the new cell needs.

Hypoxia-Driven Immune Escape in the Tumor Microenvironment

The tumor microenvironment is a complex ecosystem comprised of many different cell types, abnormal vasculature and immunosuppressive cytokines. The irregular growth kinetics with which tumors grow leads to increased oxygen consumption and, in turn, hypoxic conditions. Hypoxia has been associated with poor clinical outcome, increased tumor heterogeneity, emergence of resistant clones and evasion of immune detection. Additionally, hypoxia-driven cell death pathways have traditionally been thought of as tolerogenic processes. However, as researchers working in the field of immunotherapy continue to investigate and unveil new types of immunogenic cell death (ICD), it has become clear that, in some instances, hypoxia may actually induce ICD within a tumor. In this review, we will discuss hypoxia-driven immune escape that drives poor prognostic outcomes, the ability of hypoxia to induce ICD and potential therapeutic targets amongst hypoxia pathways.

Direct Peritoneal Resuscitation with Pyruvate Protects the Spinal Cord and Induces Autophagy via Regulating PHD2 in a Rat Model of Spinal Cord Ischemia-Reperfusion Injury

Direct peritoneal resuscitation with pyruvate (Pyr-PDS) has emerged as an interesting candidate to alleviate injury in diverse organs, while the potential mechanism has yet to be fully elucidated. To explore the effect of autophagy in the spinal cord ischemia-reperfusion (SCIR) injury and the underlying mechanism, we established a model of SCIR in vivo and in vitro. In vivo, male SD rats underwent aortic occlusion for 60 min and then followed by intraperitoneally infused with 20 mL of pyruvate or normal saline for 30 min, and the spinal cords were removed for analysis after 48 h of reperfusion. The functional and morphological results showed that Pyr-PDS alleviated SCIR injury; meanwhile, the expression of autophagy-related genes and transmission electron microscopy displayed autophagy was activated by SCIR injury, and Pyr-PDS treatment could further upregulate the degree of autophagy which plays a protective part in the SCIR injury, while there is no significant difference after treatment with saline. In addition, SCIR injury inhibited expression of PHD2, which results to activate its downstream HIF-1α/BNIP3 pathway to promote autophagy. In the Pyr-PDS, the results revealed PHD2 was further inhibited compared to the SCIR group, which could further activate the HIF-1α/BNIP3 signaling pathway. Additionally, oxygen-glucose deprivation and reoxygenation were applied to SH-SY5Y cells to mimic anoxic conditions in vitro, and the expression of autophagy-related genes, PHD2, and its downstream HIF-1α/BNIP3 pathway showed the same trend as the results in vivo. Besides, IOX2, a specific inhibitor of PHD2 was also treated to SH-SY5Y cells during reoxygenation, in which the result is as same as the pyruvate group. Then, we observed the expression of autophagy-related genes and the HIF-1α signal pathway in the process of reoxygenation; the results showed that as the reoxygenation goes, the expression of the HIF-1α signal pathway and degree of autophagy came to decrease gradually, while treated with pyruvate could maintain autophagy high and stable through keeping PHD2 at a lower level during reoxygenation, and the latter was observed downregulated during reoxygenation process from 0 to 24 hours in a time-effect way. The above results indicated that direct peritoneal resuscitation with pyruvate showed effective protection to ischemia-reperfusion of the spinal cord through activating autophagy via acting on PHD2 and its downstream HIF-1α/BNIP3 pathway.

Label-Free Interactome Analysis Revealed an Essential Role of CUL3-KEAP1 Complex in Mediating the Ubiquitination and Degradation of PHD2

Prolyl hydroxylase domain-containing protein 2 (PHD2/EGLN1) is a key regulatory enzyme that plays a fundamental role in the cellular hypoxic response pathway, mediating proline hydroxylation-dependent protein degradation of selected target proteins. However, the regulation of PHD2 homeostasis at the protein level is not well understood. Here, we perform label-free quantitative interactome analysis through immunoprecipitation coupled with mass spectrometry analysis. To minimize the side effects caused by ectopic overexpression, in HeLa cells, we stably overexpressed Flag-tagged PHD2 while suppressing the endogenous PHD2 by using an shRNA targeting its 3' UTR region. We identified and validated Cullin 3 as a novel PHD2 interactor in vivo. Through candidate screening, we further identified CUL3-KEAP1 E3 ubiquitin ligase complex as the major enzyme that regulates PHD2 degradation. Overexpression of either CUL3, KEAP1, or both significantly increases PHD2 ubiquitination and reduces PHD2 protein abundance. The knockdown of CUL3 or KEAP1 decreased PHD2 ubiquitination and inhibited PHD2 degradation. Accordingly, loss of the CUL3-KEAP1 complex under hypoxia promoted PHD2 stabilization and led to significantly reduced abundance of the PHD2 target, hypoxia-inducible factor 1A (HIF1A). Thus, CUL3-KEAP1 is an essential pathway that regulates PHD2 ubiquitination and degradation in cells.

Metabolic targeting of HIF-1α potentiates the therapeutic efficacy of oxaliplatin in colorectal cancer

Drug resistance is a major problem limiting the efficacy of chemotherapy in cancer treatment, and the hypoxia-induced stabilization of HIF-1α plays a role in this process. HIF-1α overexpression has been observed in a variety of human cancers, including colorectal cancer (CRC). Therefore, targeting HIF-1α is a promising strategy for overcoming chemoresistance to enhance the efficacy of chemotherapies in CRC. Here, we show that DNMT inhibitors can induce HIF-1α degradation to overcome oxaliplatin resistance and enhance anti-CRC therapy. We found that a low-toxicity DNMT inhibitor, zebularine, could downregulate HIF-1α expression and overcome hypoxia-induced oxaliplatin resistance in HCT116 cells and showed efficacy in HCT116 xenograft models and AOM/DSS-induced CRC mouse models. Zebularine could induce the degradation of HIF-1α protein through hydroxylation. LC-MS analysis showed a decrease in succinate in various CRC cells under hypoxia and in colon tissues of AOM/DSS-induced CRC mice. The decrease was reversed by zebularine. Tumor angiogenesis was also reduced by zebularine. Furthermore, zebularine potentiated the anticancer effect of oxaliplatin in AOM/DSS-induced CRC models. This finding provides a new strategy in which an increase in HIF-1α hydroxylation could overcome oxaliplatin resistance to enhance anti-CRC therapy.

Overexpression of long non-coding RNA NORAD promotes invasion and migration in malignant melanoma via regulating the MIR-205-EGLN2 pathway

Growing evidence suggests that long non-coding RNAs NORAD and miR-205 play a significant role in regulating cancer progression and metastasis. In this study, high expression of NORAD was firstly observed in melanoma tissues and human malignant melanoma cell lines, our aim was to study the interaction of them in the process of invasion and migration of malignant melanoma cells. NORAD, miR-205, and EGLN2 mRNA level in MM cells was detected by qRT-PCR. In situ hybridization (ISH) was performed to detect NORAD expression in MM tissues specimens. Effects of NORAD and miR-205 on Prolyl hydroxylase 2 (EGLN2) expression was explored by western blot in MM cells line. Dual-luciferase reporter assay was performed to verify the interaction relationship between NORAD and miR-205, as well as, miR-205 and EGLN2. Transwell assay was conducted to explore the effects of NORAD and miR-205 in vitro. Xenografts in nude mice experiment were used to confirm the role of NORAD and miR-205 in vivo. In vitro, NORAD knockdown significantly inhibited migration and invasion of malignant melanoma cells and elevated the expression of miR-205, there was an interaction between miR-205 and NORAD in the RNA-induced silencing complex. Upregulation of miR-205 induced significant inhibition of migratory and invasive ability compared with the scrambled control. However, downregulating NORAD largely reversed this effect. Furthermore, the regulatory effects of miR-205 on EGLN2 levels and the induction of endoplasmic reticulum stress were reversed by NORAD. In vivo, deletion of miR-205 induced tumor growth in nude mice. NORAD may play critical roles in tumorigenesis and progression of malignant melanoma by regulating of the miR-205-EGLN2 pathway, and may serve as a new therapeutic target.

Hypoxia Pathway Proteins in Normal and Malignant Hematopoiesis

The regulation of oxygen (O₂) levels is crucial in embryogenesis and adult life, as O₂ controls a multitude of key cellular functions. Low oxygen levels (hypoxia) are relevant for tissue physiology as they are integral to adequate metabolism regulation and cell fate. Hence, the hypoxia response is of utmost importance for cell, organ and organism function and is dependent on the hypoxia-inducible factor (HIF) pathway. HIF pathway activity is strictly regulated by the family of oxygen-sensitive HIF prolyl hydroxylase domain (PHD) proteins. Physiologic hypoxia is a hallmark of the hematopoietic stem cell (HSC) niche in the bone marrow. This niche facilitates HSC quiescence and survival. The present review focuses on current knowledge and the many open questions regarding the impact of PHDs/HIFs and other proteins of the hypoxia pathway on the HSC niche and on normal and malignant hematopoiesis.

Hypoxia preconditioning promotes bone marrow mesenchymal stem cells survival by inducing HIF-1α in injured neuronal cells derived exosomes culture system

Bone marrow derived stem cells (BMSCs) transplantation are viewed as a promising therapeutic candidate for spinal cord injury (SCI). However, the inflammatory microenvironment in the spinal cord following SCI limits the survival and efficacy of transplanted BMSCs. In this study, we investigate whether injured neuronal cells derived exosomes would influence the survival of transplanted BMSCs after SCI. In order to mimic the microenvironment in SCI that the neuronal cells or transplanted BMSCs suffer in vivo, PC12 cells conditioned medium and PC12 cell’s exosomes collected from H₂O₂-treated PC12 cell’s culture medium were cultured with BMSCs under oxidative stress in vitro. PC12 cells conditioned medium and PC12 cell’s exosomes significantly accelerated the apoptosis of BMSCs induced by H₂O₂. Moreover, the cleaved caspase-3, cytochrome (Cyt) C, lactate dehydrogenase (LDH) releases, and apoptotic percentage were increased, and the ratio of Bcl-2/Bax and cell viability were decreased. Inhibition of exosome secretion via Rab27a small interfering RNA prevented BMSCs apoptosis in vitro. In addition, hypoxia-preconditioned promoted the survival of BMSCs under oxidative stress both in vivo after SCI and in vitro. Our results also indicate that HIF-1α plays a central role in the survival of BMSCs in hypoxia pretreatment under oxidative stress conditions. siRNA-HIF-1α increased apoptosis of BMSCs; in contrast, HIF-1α inducer FG-4592 attenuated apoptosis of BMSCs. Taken together, we found that the injured PC12 cells derived exosomes accelerate BMSCs apoptosis after SCI and in vitro, hypoxia pretreatment or activating expression of HIF-1α to be important in the survival of BMSCs after transplantation, which provides a foundation for application of BMSCs in therapeutic potential for SCI.

MORG1+/- mice are protected from histological renal damage and inflammation in a murine model of endotoxemia

BACKGROUND

The MAPK-organizer 1 (MORG1) play a scaffold function in the MAPK and/or the PHD3 signalling paths. Recently, we reported that MORG1+/- mice are protected from renal injury induced by systemic hypoxia and acute renal ischemia-reperfusion injury via increased hypoxia-inducible factors (HIFs). Here, we explore whether MORG1 heterozygosity could attenuate renal injury in a murine model of lipopolysaccharide (LPS) induced endotoxemia.

METHODS

Endotoxemia was induced in mice by an intraperitoneal (i.p) application of 5 mg/kg BW LPS. The renal damage was estimated by periodic acid Schiff's staining; renal injury was evaluated by detection of urinary and plasma levels of neutrophil gelatinase-associated lipocalin and albumin/creatinine ratio via ELISAs. Renal mRNA expression was assessed by real-time PCR, whereas the protein expression was determined by immunohistochemistry or Western blotting.

RESULTS

LPS administration increased tubular injury, microalbuminuria, IL-6 plasma levels and renal TNF-α expression in MORG1 +/+ mice. This was accompanied with enhanced infiltration of the inflammatory T-cells in renal tissue and activation of the NF-κB transcription factors. In contrast, endotoxemic MORG1 +/- showed significantly less tubular injury, reduced plasma IL-6 levels, significantly decreased renal TNF-α expression and T-cells infiltration. In support, the renal levels of activated caspase-3 were lower in endotoxemic MORG1 +/- mice compared with endotoxemic MORG1 +/+ mice. Interestingly, LPS application induced a significantly higher accumulation of renal HIF-2α in the kidneys of MORG1+/- mice than in wild-type mice, accompanied with a diminished phosphorylation of IκB-α and IKK α,β and decreased iNOS mRNA in the renal tissues of the LPS-challenged MORG1+/- mice, indicating an inhibition of the NF-κB transcriptional activation.

CONCLUSIONS

MORG1 heterozygosity protects against histological renal damage and shows anti-inflammatory effects in a murine endotoxemia model through modulation of HIF-2α stabilisation and/or simultaneous inhibition of the NF-κB signalling. Here, we show for the first time that MORG1 scaffold could represent the missing link between innate immunity and inflammation.

Hypoxia Pathway Proteins As Central Mediators of Metabolism in the Tumor Cells and Their Microenvironment

Low oxygen tension or hypoxia is a determining factor in the course of many different processes in animals, including when tissue expansion and cellular metabolism result in high oxygen demands that exceed its supply. This is mainly happening when cells actively proliferate and the proliferating mass becomes distant from the blood vessels, such as in growing tumors. Metabolic alterations in response to hypoxia can be triggered in a direct manner, such as the switch from oxidative phosphorylation to glycolysis or inhibition of fatty acid desaturation. However, as the modulated action of hypoxia-inducible factors or the oxygen sensors (prolyl hydroxylase domain-containing enzymes) can also lead to changes in enzyme expression, these metabolic changes can also be indirect. With this review, we want to summarize our current knowledge of the hypoxia-induced changes in metabolism during cancer development, how they are affected in the tumor cells and in the cells of the microenvironment, most prominently in immune cells.

Tet1 facilitates hypoxia tolerance by stabilizing the HIF-α proteins independent of its methylcytosine dioxygenase activity

Because of the requirement of oxygen (O₂) to produce energy, aerobic organisms developed mechanisms to protect themselves against a shortage of oxygen in both acute status and chronic status. To date, how organisms tolerate acute hypoxia and the underlying mechanisms remain largely unknown. Here, we identify that Tet1, one member of the ten-eleven translocation (TET) family of methylcytosine dioxygenases, is required for hypoxia tolerance in zebrafish and mice. Tet1-null zebrafish and mice are more sensitive to hypoxic conditions compared with their wild-type siblings. We demonstrate that Tet1 stabilizes hypoxia-inducible factor α (HIF-α) and enhances HIF-α transcription activity independent of its enzymatic activity. In addition, we show that Tet1 modulates HIF-2α and HIF-1α through different mechanisms. Tet1 competes with prolyl hydroxylase protein 2 (PHD2) to bind to HIF-2α, resulting in a reduction of HIF-2α hydroxylation by PHD2. For HIF-1α, however, Tet1 has no effect on HIF-1α hydroxylation, but rather it appears to stabilize the C-terminus of HIF-1α by affecting lysine site modification. Furthermore, we found that Tet1 enhances rather than prevents poly-ubiquitination on HIF-α. Our results reveal a previously unrecognized function of Tet1 independent of its methylcytosine dioxygenase activity in hypoxia signaling.

Hypoxia-inducible factor 1-alpha does not regulate osteoclastogenesis but enhances bone resorption activity via prolyl-4-hydroxylase 2

Osteogenic-angiogenic coupling is promoted by the hypoxia-inducible factor 1-alpha (HIF-1α) transcription factor, provoking interest in HIF activation as a therapeutic strategy to improve osteoblast mineralization and treat pathological osteolysis. However, HIF also enhances the bone-resorbing activity of mature osteoclasts. It is therefore essential to determine the full effect(s) of HIF on both the formation and the bone-resorbing function of osteoclasts in order to understand how they might respond to such a strategy. Expression of HIF-1α mRNA and protein increased during osteoclast differentiation from CD14+ monocytic precursors, additionally inducing expression of the HIF-regulated glycolytic enzymes. However, HIF-1α siRNA only moderately affected osteoclast differentiation, accelerating fusion of precursor cells. HIF induction by inhibition of the regulatory prolyl-4-hydroxylase (PHD) enzymes reduced osteoclastogenesis, but was confirmed to enhance bone resorption by mature osteoclasts. Phd2^+/- murine osteoclasts also exhibited enhanced bone resorption, associated with increased expression of resorption-associated Acp5, in comparison with wild-type cells from littermate controls. Phd3^-/- bone marrow precursors displayed accelerated early fusion, mirroring results with HIF-1α siRNA. In vivo, Phd2^+/- and Phd3^-/- mice exhibited reduced trabecular bone mass, associated with reduced mineralization by Phd2^+/- osteoblasts. These data indicate that HIF predominantly functions as a regulator of osteoclast-mediated bone resorption, with little effect on osteoclast differentiation. Inhibition of HIF might therefore represent an alternative strategy to treat diseases characterized by pathological levels of osteolysis. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

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

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

Hypoxia-Induced Fibroblast Growth Factor 11 Stimulates Osteoclast-Mediated Resorption of Bone

Over-activation of osteoclasts is directly responsible for pathological bone loss in conditions such as rheumatoid arthritis and cancer metastasis to bone. Hypoxia is a common feature of these conditions, associated with poor prognosis, which also stimulates osteoclast-mediated bone resorption via induction of the hypoxia-inducible transcription factor HIF-1α. Here, we investigate the effects of fibroblast growth factor 11 (FGF11) on osteoclast function. FGF11 is an intracellular FGF that was induced both by hypoxia (2% O₂, p < 0.01) and by inhibition of the HIF-regulating prolyl hydroxylase enzymes (CoCl₂, p < 0.001) in osteoclasts. Isoform-specific siRNA demonstrated that the induction of Fgf11 mRNA expression by hypoxia is HIF-1α-dependent (p < 0.01). Hypoxic stimulation of bone resorption was inhibited in osteoclasts treated with siRNA targeting FGF11 (p < 0.05). This was at least partially due to reduced secretion of an unidentified pro-resorptive factor downstream of FGF11. FGF11 expression within hypoxic, resorbing osteoclasts co-localised with microtubule-associated alpha-tubulin. FGF11 was also abundantly expressed in osteoclasts within the rheumatoid synovium and in giant cell tumour of bone. This study suggests FGF11 as a novel factor driving pathological bone resorption in osteolytic disease and as a potential target for the development of new anti-resorptive therapeutic agents.

Prolyl Hydroxylase Domain-2 Inhibition Improves Skeletal Muscle Regeneration in a Male Murine Model of Obesity

Obesity leads to a loss of muscle mass and impaired muscle regeneration. In obese individuals, pathologically elevated levels of prolyl hydroxylase domain enzyme 2 (PHD2) limit skeletal muscle hypoxia-inducible factor-1 alpha and vascular endothelial growth factor (VEGF) expression. Loss of local VEGF may further impair skeletal muscle regeneration. We hypothesized that PHD2 inhibition would restore vigorous muscle regeneration in a murine model of obesity. Adult (22-week-old) male mice were fed either a high-fat diet (HFD), with 60% of calories derived from fat, or a regular diet (RD), with 10% of calories derived from fat, for 16 weeks. On day 5 following cryoinjury to the tibialis anterior muscle, newly regenerated muscle fiber cross-sectional areas were significantly smaller in mice fed an HFD as compared to RD, indicating an impaired regenerative response. Cryoinjured gastrocnemius muscles of HFD mice also showed elevated PHD2 levels (twofold higher) and reduced VEGF levels (twofold lower) as compared to RD. Dimethyloxalylglycine, a cell permeable competitive inhibitor of PHD2, restored VEGF levels and significantly improved regenerating myofiber size in cryoinjured mice fed an HFD. We conclude that pathologically increased PHD2 in the obese state drives impairments in muscle regeneration, in part by blunting VEGF production. Inhibition of PHD2 over activity in the obese state normalizes VEGF levels and restores muscle regenerative potential.