Semmaphorin 3 A causes immune suppression by inducing cytoskeletal paralysis in tumour-specific CD8+ T cells

Semaphorin-3A (SEMA3A) functions as a chemorepulsive signal during development and can affect T cells by altering their filamentous actin (F-actin) cytoskeleton. The exact extent of these effects on tumour-specific T cells are not completely understood. Here we demonstrate that Neuropilin-1 (NRP1) and Plexin-A1 and Plexin-A4 are upregulated on stimulated CD8+ T cells, allowing tumour-derived SEMA3A to inhibit T cell migration and assembly of the immunological synapse. Deletion of NRP1 in both CD4+ and CD8+ T cells enhance CD8+ T-cell infiltration into tumours and restricted tumour growth in animal models. Conversely, over-expression of SEMA3A inhibit CD8+ T-cell infiltration. We further show that SEMA3A affects CD8+ T cell F-actin, leading to inhibition of immune synapse formation and motility. Examining a clear cell renal cell carcinoma patient cohort, we find that SEMA3A expression is associated with reduced survival, and that T-cells appear trapped in SEMA3A rich regions. Our study establishes SEMA3A as an inhibitor of effector CD8+ T cell tumour infiltration, suggesting that blocking NRP1 could improve T cell function in tumours.

Oncogenic cell tagging and single-cell transcriptomics reveal cell type-specific and time-resolved responses to Vhl inactivation in the kidney

Defining the initial events in oncogenesis and the cellular responses they entrain, even in advance of morphological abnormality, is a fundamental challenge in understanding cancer initiation. As a paradigm to address this, we longitudinally studied the changes induced by loss of the tumor suppressor gene von Hippel Lindau (VHL), which ultimately drives clear cell renal cell carcinoma. Vhl inactivation was directly coupled to expression of a tdTomato reporter within a single allele, allowing accurate visualization of affected cells in their native context and retrieval from the kidney for single-cell RNA-sequencing. This strategy uncovered cell-type specific responses to Vhl inactivation, defined a proximal tubular cell class with oncogenic potential, and revealed longer term adaptive changes in the renal epithelium and the interstitium. Oncogenic cell tagging also revealed markedly heterogeneous cellular effects including time-limited proliferation and elimination of specific cell types. Overall, this study reports an experimental strategy for understanding oncogenic processes in which cells bearing genetic alterations can be generated in their native context, marked, and analyzed over time. The observed effects of loss of Vhl in kidney cells provide insights into VHL tumor suppressor action and development of renal cell carcinoma.

Re-activation of HIF signalling pathway in the epicardium improves heart regeneration after myocardial infarction

Funding Acknowledgements

Type of funding sources: Foundation. Main funding source(s): British Heart Foundation

Background

In mouse, cardiac regenerative capacity is maintained for the first week after birth but lost thereafter. Reactivation of this process holds great therapeutic potential, however, the molecular pathways that might be targeted to extend neonatal regeneration remain elusive. Here, we explore a role for hypoxia inducible factor (HIF) family of transcription factors on the regulation of epicardial activity which is essential for cardiac response to injury.

Purpose

HIF signalling might be involved in the quiescence of the epicardium observed in the first week of life. Thus, HIF stabilisation may induce epicardial re-activation and extend the 7-day regenerative window in a neonate mouse model of myocardial infarction (MI).

Methods

We analysed epicardial expression of HIF signalling during embryonic development, and in a regenerative post-natal day 1 (P1) versus non-regenerative (P7) mouse model by single cell RNA-sequencing. Next, we established in vitro epicardial explants to confirm the role of HIF signalling in epicardial activation, and finally we induced MI by permanent ligation of the proximal left anterior descending (LAD) coronary artery in P7 mice to test weather HIF signalling re-activation improved heart regeneration in a non-regenerative model.

Results

Expression of both HIF-1α and HIF-2α is very pronounced at early stages of heart development and gradually decreases throughout gestation. The epicardium preferentially expresses HIF-1α by embryonic day (E) 16.5. Postnatally, GO term analysis showed an enrichment of hypoxia-related pathways in P1 compared to P7 hearts, largely in the epicardial cell population. Accordingly, expression of Phd2, encoding for the main suppressor of HIF signalling, was enriched in P7 derived epicardial cells. Notably, the increase in Phd2 levels coincided with a clear reduction in the expression of Wilms’ tumour 1 (Wt1) in P7 versus P1 hearts, a key player in the epicardial activation. In vitro studies on epicardial explants confirmed regulation of WT1 expression and migration capacity following genetical and pharmacological modulation of HIF signalling. Finally, in vivo targeting of PHD enzymes through pharmacological inhibition with clinically approved drugs in a neonatal mouse model of MI led to prolonged epicardial activation, increased vascularisation, augmented infarct resolution and preserved cardiac function up to 3 weeks after injury.

Conclusions

Together, these findings show that modulation of HIF signalling can regulate epicardial activity and improve heart regeneration capacity beyond the 7-day regenerative window and may represent a viable therapeutic strategy for treating ischaemic heart disease.

Hypoxia shapes the immune landscape in lung injury and promotes the persistence of inflammation

Hypoxemia is a defining feature of acute respiratory distress syndrome (ARDS), an often-fatal complication of pulmonary or systemic inflammation, yet the resulting tissue hypoxia, and its impact on immune responses, is often neglected. In the present study, we have shown that ARDS patients were hypoxemic and monocytopenic within the first 48 h of ventilation. Monocytopenia was also observed in mouse models of hypoxic acute lung injury, in which hypoxemia drove the suppression of type I interferon signaling in the bone marrow. This impaired monopoiesis resulted in reduced accumulation of monocyte-derived macrophages and enhanced neutrophil-mediated inflammation in the lung. Administration of colony-stimulating factor 1 in mice with hypoxic lung injury rescued the monocytopenia, altered the phenotype of circulating monocytes, increased monocyte-derived macrophages in the lung and limited injury. Thus, tissue hypoxia altered the dynamics of the immune response to the detriment of the host and interventions to address the aberrant response offer new therapeutic strategies for ARDS.

Accelerometer-measured physical activity and functional behaviours among people on dialysis

BACKGROUND

The feasibility of wrist-worn accelerometers, and the patterns and determinants of physical activity, among people on dialysis are uncertain.

METHODS

People on maintenance dialysis were fitted with a wrist-worn AxivityAX3 accelerometer. Subsets also wore a 14-day electrocardiograph patch (Zio^®PatchXT) and wearable cameras. Age-, sex- and season-matched UK Biobank control groups were derived for comparison.

RESULTS

Median (interquartile range) accelerometer wear time for the 101 recruits was 12.5 (10.4-13.5) days, of which 73 participants (mean age 66.5 years) had excellent wear on both dialysis and non-dialysis days. Mean (standard error) overall physical activity levels were 15.5 (0.7) milligravity units (mg), 14.8 (0.7) mg on dialysis days versus 16.2 (0.8) mg on non-dialysis days. This compared with 28.1 (0.5) mg for apparently healthy controls, 23.4 (0.4) mg for controls with prior cardiovascular disease (CVD) and/or diabetes mellitus and 22.9 (0.6) mg for heart failure controls. Each day, we estimated that those on dialysis spent an average of about 1 hour (h/day) walking, 0.6 h/day engaging in moderate-intensity activity, 0.7 h/day on light tasks, 13.2 h/day sedentary and 8.6 h/day asleep. Older age and self-reported leg weakness were associated with decreased levels of physical activity, but the presence of prior CVD, arrhythmias and listing for transplantation were not.

CONCLUSIONS

Wrist-worn accelerometers are an acceptable and reliable method to measure physical activity in people on dialysis and may also be used to estimate functional behaviours. Among people on dialysis, who are broadly half as active as general population controls, age and leg weakness appear to be more important determinants of low activity levels than CVD.

Marked and rapid effects of pharmacological HIF-2α antagonism on hypoxic ventilatory control

Hypoxia-inducible factor (HIF) is strikingly upregulated in many types of cancer, and there is great interest in applying inhibitors of HIF as anticancer therapeutics. The most advanced of these are small molecules that target the HIF-2 isoform through binding the PAS-B domain of HIF-2α. These molecules are undergoing clinical trials with promising results in renal and other cancers where HIF-2 is considered to be driving growth. Nevertheless, a central question remains as to whether such inhibitors affect physiological responses to hypoxia at relevant doses. Here, we show that pharmacological HIF-2α inhibition with PT2385, at doses similar to those reported to inhibit tumor growth, rapidly impaired ventilatory responses to hypoxia, abrogating both ventilatory acclimatization and carotid body cell proliferative responses to sustained hypoxia. Mice carrying a HIF-2α PAS-B S305M mutation that disrupts PT2385 binding, but not dimerization with HIF-1β, did not respond to PT2385, indicating that these effects are on-target. Furthermore, the finding of a hypomorphic ventilatory phenotype in untreated HIF-2α S305M mutant mice suggests a function for the HIF-2α PAS-B domain beyond heterodimerization with HIF-1β. Although PT2385 was well tolerated, the findings indicate the need for caution in patients who are dependent on hypoxic ventilatory drive.

Recent advances in the biology of tumour hypoxia with relevance to diagnostic practice and tissue-based research

In this review article, we examine the importance of low levels of oxygen (hypoxia) in cancer biology. We provide a brief description of how mammalian cells sense oxygen. The hypoxia-inducible factor (HIF) pathway is currently the best characterised oxygen-sensing system, but recent work has revealed that mammals also use an oxygen-sensing system found in plants to regulate the abundance of some proteins and peptides with an amino-terminal cysteine residue. We discuss how the HIF pathway is affected during the growth of solid tumours, which develop in microenvironments with gradients of oxygen availability. We then introduce the concept of 'pseudohypoxia', a state of constitutive, oxygen-independent HIF system activation that occurs due to oncogenic stimulation in a number of specific tumour types that are of immediate relevance to diagnostic histopathologists. We provide an overview of the different methods of quantifying tumour hypoxia, emphasising the importance of pre-analytic factors in interpreting the results of tissue-based studies. Finally, we review recent approaches to targeting hypoxia/HIF system activation for therapeutic benefit, the application of which may require knowledge of which hypoxia signalling components are being utilised by a given tumour. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Structure-Activity Relationship and Crystallographic Studies on 4-Hydroxypyrimidine HIF Prolyl Hydroxylase Domain Inhibitors

The 2-oxoglutarate-dependent hypoxia inducible factor prolyl hydroxylases (PHDs) are targets for treatment of a variety of diseases including anaemia. One PHD inhibitor is approved for use for the treatment of renal anaemia and others are in late stage clinical trials. The number of reported templates for PHD inhibition is limited. We report structure-activity relationship and crystallographic studies on a promising class of 4-hydroxypyrimidine-containing PHD inhibitors.

Fumarate Hydratase Deletion in Pancreatic β Cells Leads to Progressive Diabetes

We explored the role of the Krebs cycle enzyme fumarate hydratase (FH) in glucose-stimulated insulin secretion (GSIS). Mice lacking Fh1 in pancreatic β cells (Fh1βKO mice) appear normal for 6–8 weeks but then develop progressive glucose intolerance and diabetes. Glucose tolerance is rescued by expression of mitochondrial or cytosolic FH but not by deletion of Hif1α or Nrf2. Progressive hyperglycemia in Fh1βKO mice led to dysregulated metabolism in β cells, a decrease in glucose-induced ATP production, electrical activity, cytoplasmic [Ca²⁺]_i elevation, and GSIS. Fh1 loss resulted in elevated intracellular fumarate, promoting succination of critical cysteines in GAPDH, GMPR, and PARK 7/DJ-1 and cytoplasmic acidification. Intracellular fumarate levels were increased in islets exposed to high glucose and in islets from human donors with type 2 diabetes (T2D). The impaired GSIS in islets from diabetic Fh1βKO mice was ameliorated after culture under normoglycemic conditions. These studies highlight the role of FH and dysregulated mitochondrial metabolism in T2D.

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Fh1 loss in β cells causes progressive Hif1α-independent diabetes

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Fh1 loss in β cells impairs ATP generation, electrical activity, and GSIS

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Elevated fumarate is a feature of diabetic murine and human islets

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“Normoglycemia” restores GSIS in Fh1βKO islets

PHD2 inactivation in Type I cells drives HIF-2α-dependent multilineage hyperplasia and the formation of paraganglioma-like carotid bodies

KEY POINTS

The carotid body is a peripheral arterial chemoreceptor that regulates ventilation in response to both acute and sustained hypoxia. Type I cells in this organ respond to low oxygen both acutely by depolarization and dense core vesicle secretion and, over the longer term, via cellular proliferation and enhanced ventilatory responses. Using lineage analysis, the present study shows that the Type I cell lineage itself proliferates and expands in response to sustained hypoxia. Inactivation of HIF-2α in Type I cells impairs the ventilatory, proliferative and cell intrinsic (dense core vesicle) responses to hypoxia. Inactivation of PHD2 in Type I cells induces multilineage hyperplasia and ultrastructural changes in dense core vesicles to form paraganglioma-like carotid bodies. These changes, similar to those observed in hypoxia, are dependent on HIF-2α. Taken together, these findings demonstrate a key role for the PHD2-HIF-2α couple in Type I cells with respect to the oxygen sensing functions of the carotid body.

ABSTRACT

The carotid body is a peripheral chemoreceptor that plays a central role in mammalian oxygen homeostasis. In response to sustained hypoxia, it manifests a rapid cellular proliferation and an associated increase in responsiveness to hypoxia. Understanding the cellular and molecular mechanisms underlying these processes is of interest both to specialized chemoreceptive functions of that organ and, potentially, to the general physiology and pathophysiology of cellular hypoxia. We have combined cell lineage tracing technology and conditionally inactivated alleles in recombinant mice to examine the role of components of the HIF hydroxylase pathway in specific cell types within the carotid body. We show that exposure to sustained hypoxia (10% oxygen) drives rapid expansion of the Type I, tyrosine hydroxylase expressing cell lineage, with little transdifferentiation to (or from) that lineage. Inactivation of a specific HIF isoform, HIF-2α, in the Type I cells was associated with a greatly reduced proliferation of Type I cells and hypoxic ventilatory responses, with ultrastructural evidence of an abnormality in the action of hypoxia on dense core secretory vesicles. We also show that inactivation of the principal HIF prolyl hydroxylase PHD2 within the Type I cell lineage is sufficient to cause multilineage expansion of the carotid body, with characteristics resembling paragangliomas. These morphological changes were dependent on the integrity of HIF-2α. These findings implicate specific components of the HIF hydroxylase pathway (PHD2 and HIF-2α) within Type I cells of the carotid body with respect to the oxygen sensing and adaptive functions of that organ.

Molecular and cellular mechanisms of HIF prolyl hydroxylase inhibitors in clinical trials

Inhibition of the human 2-oxoglutarate (2OG) dependent hypoxia inducible factor (HIF) prolyl hydroxylases (human PHD1-3) causes upregulation of HIF, thus promoting erythropoiesis and is therefore of therapeutic interest. We describe cellular, biophysical, and biochemical studies comparing four PHD inhibitors currently in clinical trials for anaemia treatment, that describe their mechanisms of action, potency against isolated enzymes and in cells, and selectivities versus representatives of other human 2OG oxygenase subfamilies. The 'clinical' PHD inhibitors are potent inhibitors of PHD catalyzed hydroxylation of the HIF-α oxygen dependent degradation domains (ODDs), and selective against most, but not all, representatives of other human 2OG dependent dioxygenase subfamilies. Crystallographic and NMR studies provide insights into the different active site binding modes of the inhibitors. Cell-based results reveal the inhibitors have similar effects on the upregulation of HIF target genes, but differ in the kinetics of their effects and in extent of inhibition of hydroxylation of the N- and C-terminal ODDs; the latter differences correlate with the biophysical observations.

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

The hypoxia-inducible factor (HIF) system orchestrates cellular responses to hypoxia in animals. HIF is an α/β-heterodimeric transcription factor that regulates the expression of hundreds of genes in a tissue context-dependent manner. The major hypoxia-sensing component of the HIF system involves oxygen-dependent catalysis by the HIF hydroxylases; in humans there are three HIF prolyl hydroxylases (PHD1-3) and an asparaginyl hydroxylase (factor-inhibiting HIF (FIH)). PHD catalysis regulates HIFα levels, and FIH catalysis regulates HIF activity. How differences in HIFα hydroxylation status relate to variations in the induction of specific HIF target gene transcription is unknown. We report studies using small molecule HIF hydroxylase inhibitors that investigate the extent to which HIF target gene expression is induced by PHD or FIH inhibition. The results reveal substantial differences in the role of prolyl and asparaginyl hydroxylation in regulating hypoxia-responsive genes in cells. PHD inhibitors with different structural scaffolds behave similarly. Under the tested conditions, a broad-spectrum 2-oxoglutarate dioxygenase inhibitor is a better mimic of the overall transcriptional response to hypoxia than the selective PHD inhibitors, consistent with an important role for FIH in the hypoxic transcriptional response. Indeed, combined application of selective PHD and FIH inhibitors resulted in the transcriptional induction of a subset of genes not fully responsive to PHD inhibition alone. Thus, for the therapeutic regulation of HIF target genes, it is important to consider both PHD and FIH activity, and in the case of some sets of target genes, simultaneous inhibition of the PHDs and FIH catalysis may be preferable.

Structural basis for oxygen degradation domain selectivity of the HIF prolyl hydroxylases

The response to hypoxia in animals involves the expression of multiple genes regulated by the αβ-hypoxia-inducible transcription factors (HIFs). The hypoxia-sensing mechanism involves oxygen limited hydroxylation of prolyl residues in the N- and C-terminal oxygen-dependent degradation domains (NODD and CODD) of HIFα isoforms, as catalysed by prolyl hydroxylases (PHD 1-3). Prolyl hydroxylation promotes binding of HIFα to the von Hippel-Lindau protein (VHL)-elongin B/C complex, thus signalling for proteosomal degradation of HIFα. We reveal that certain PHD2 variants linked to familial erythrocytosis and cancer are highly selective for CODD or NODD. Crystalline and solution state studies coupled to kinetic and cellular analyses reveal how wild-type and variant PHDs achieve ODD selectivity via different dynamic interactions involving loop and C-terminal regions. The results inform on how HIF target gene selectivity is achieved and will be of use in developing selective PHD inhibitors.

Using an Integrated -Omics Approach to Identify Key Cellular Processes That Are Disturbed in the Kidney After Brain Death

In an era where we are becoming more reliant on vulnerable kidneys for transplantation from older donors, there is an urgent need to understand how brain death leads to kidney dysfunction and, hence, how this can be prevented. Using a rodent model of hemorrhagic stroke and next-generation proteomic and metabolomic technologies, we aimed to delineate which key cellular processes are perturbed in the kidney after brain death. Pathway analysis of the proteomic signature of kidneys from brain-dead donors revealed large-scale changes in mitochondrial proteins that were associated with altered mitochondrial activity and morphological evidence of mitochondrial injury. We identified an increase in a number of glycolytic proteins and lactate production, suggesting a shift toward anaerobic metabolism. Higher amounts of succinate were found in the brain death group, in conjunction with increased markers of oxidative stress. We characterized the responsiveness of hypoxia inducible factors and found this correlated with post-brain death mean arterial pressures. Brain death leads to metabolic disturbances in the kidney and alterations in mitochondrial function and reactive oxygen species generation. This metabolic disturbance and alteration in mitochondrial function may lead to further cellular injury. Conditioning the brain-dead organ donor by altering metabolism could be a novel approach to ameliorate this brain death-induced kidney injury.

Regulation of ventilatory sensitivity and carotid body proliferation in hypoxia by the PHD2/HIF-2 pathway

Ventilatory sensitivity to hypoxia increases in response to continued hypoxic exposure as part of acute acclimatisation. Although this process is incompletely understood, insights have been gained through studies of the hypoxia-inducible factor (HIF) hydroxylase system. Genetic studies implicate these pathways widely in the integrated physiology of hypoxia, through effects on developmental or adaptive processes. In keeping with this, mice that are heterozygous for the principal HIF prolyl hydroxylase, PHD2, show enhanced ventilatory sensitivity to hypoxia and carotid body hyperplasia. Here we have sought to understand this process better through comparative analysis of inducible and constitutive inactivation of PHD2 and its principal targets HIF-1α and HIF-2α. We demonstrate that general inducible inactivation of PHD2 in tamoxifen-treated Phd2(f/f);Rosa26(+/CreERT2) mice, like constitutive, heterozygous PHD2 deficiency, enhances hypoxic ventilatory responses (HVRs: 7.2 ± 0.6 vs. 4.4 ± 0.4 ml min(-1) g(-1) in controls, P < 0.01). The ventilatory phenotypes associated with both inducible and constitutive inactivation of PHD2 were strongly compensated for by concomitant inactivation of HIF-2α, but not HIF-1α. Furthermore, inducible inactivation of HIF-2α strikingly impaired ventilatory acclimatisation to chronic hypoxia (HVRs: 4.1 ± 0.5 vs. 8.6 ± 0.5 ml min(-1) g(-1) in controls, P < 0.0001), as well as carotid body cell proliferation (400 ± 81 vs. 2630 ± 390 bromodeoxyuridine-positive cells mm(-2) in controls, P < 0.0001). The findings demonstrate the importance of the PHD2/HIF-2α enzyme-substrate couple in modulating ventilatory sensitivity to hypoxia.

Betti reaction enables efficient synthesis of 8-hydroxyquinoline inhibitors of 2-oxoglutarate oxygenases

There is interest in developing potent, selective, and cell-permeable inhibitors of human ferrous iron and 2-oxoglutarate (2OG) oxygenases for use in functional and target validation studies. The 3-component Betti reaction enables efficient one-step C-7 functionalisation of modified 8-hydroxyquinolines (8HQs) to produce cell-active inhibitors of KDM4 histone demethylases and other 2OG oxygenases; the work exemplifies how a template-based metallo-enzyme inhibitor approach can be used to give biologically active compounds.

Potent and Selective Triazole-Based Inhibitors of the Hypoxia-Inducible Factor Prolyl-Hydroxylases with Activity in the Murine Brain

As part of the cellular adaptation to limiting oxygen availability in animals, the expression of a large set of genes is activated by the upregulation of the hypoxia-inducible transcription factors (HIFs). Therapeutic activation of the natural human hypoxic response can be achieved by the inhibition of the hypoxia sensors for the HIF system, i.e. the HIF prolyl-hydroxylases (PHDs). Here, we report studies on tricyclic triazole-containing compounds as potent and selective PHD inhibitors which compete with the 2-oxoglutarate co-substrate. One compound (IOX4) induces HIFα in cells and in wildtype mice with marked induction in the brain tissue, revealing that it is useful for studies aimed at validating the upregulation of HIF for treatment of cerebral diseases including stroke.

The relationship between symptoms and blood pressure during maintenance hemodialysis

Intradialytic hypotension (IDH) is a detrimental complication of maintenance hemodialysis, but how it is defined and reported varies widely in the literature. European Best Practice Guideline and Kidney Disease Outcomes Quality Initiative guidelines require symptoms and a mitigating intervention to fulfill the diagnosis, but morbidity and mortality outcomes are largely based on blood pressure alone. Furthermore, little is known about the incidence of asymptomatic hypotension, which may be an important cause of hypoperfusion injury and impaired outcome. Seventy-seven patients were studied over 456 dialysis sessions. Blood pressure was measured at 15-minute intervals throughout the session and compared with post-dialysis symptom questionnaire results using mixed modeling to adjust for repeated measures in the same patient. The frequency of asymptomatic hypotension was estimated by logistic regression using a variety of commonly cited blood pressure metrics that describe IDH. In 113 sessions (25%) where symptoms were recorded on the questionnaire, these appear not to have been reported to dialysis staff. When symptoms were reported (293 sessions [64%]), an intervention invariably followed. Dizziness and cramp were strongly associated with changes in systolic blood pressure (SBP), but not diastolic blood pressure. Nausea occurred more frequently in younger patients but was not associated with falls in blood pressure. Thresholds that maximized the probability of an intervention rather than a session remaining asymptomatic were SBP <100 mmHg or a 20% reduction in SBP from baseline. The probability of SBP falling to <100 mmHg in an asymptomatic session was 0.23. Symptoms are frequently not reported by patients who are hypotensive during hemodialysis, which leads to an underestimation of IDH if symptom-based definitions are used. A revised definition of IDH excluding patient-reported symptoms would be in line with literature reporting morbidity and mortality outcomes and include sessions in which potentially detrimental asymptomatic hypotension occurs.

The role of hypoxia-inducible factors in organ donation and transplantation: the current perspective and future opportunities

Hypoxia-inducible factors are the universal cellular oxygen-sensitive transcription factors that activate a number of hypoxia responsive genes, some of which are responsible for protective cellular functions. During organ donation, allografts are exposed to significant periods of hypoxia and ischemia. Exploiting this pathway during donor management and organ preservation could prevent and reduce allograft injury and improve the outcomes of organ transplantation. We review the evidence on this pathway in organ preservation, drawing on experimental studies on donor management and ischemia reperfusion injury focusing on kidney, liver, cardiac and lung transplantation. We review the major technical and experimental challenges in exploring this pathway and suggest potential future avenues for research.

Selective small molecule probes for the hypoxia inducible factor (HIF) prolyl hydroxylases

The hypoxia inducible factor (HIF) system is central to the signaling of low oxygen (hypoxia) in animals. The levels of HIF-α isoforms are regulated in an oxygen-dependent manner by the activity of the HIF prolyl-hydroxylases (PHD or EGLN enzymes), which are Fe(II) and 2-oxoglutarate (2OG) dependent oxygenases. Here, we describe biochemical, crystallographic, cellular profiling, and animal studies on PHD inhibitors including selectivity studies using a representative set of human 2OG oxygenases. We identify suitable probe compounds for use in studies on the functional effects of PHD inhibition in cells and in animals.

Carotid body hyperplasia and enhanced ventilatory responses to hypoxia in mice with heterozygous deficiency of PHD2

Oxygen-dependent prolyl hydroxylation of hypoxia-inducible factor (HIF) by a set of closely related prolyl hydroxylase domain enzymes (PHD1, 2 and 3) regulates a range of transcriptional responses to hypoxia. This raises important questions about the role of these oxygen-sensing enzymes in integrative physiology. We investigated the effect of both genetic deficiency and pharmacological inhibition on the change in ventilation in response to acute hypoxic stimulation in mice. Mice exposed to chronic hypoxia for 7 days manifest an exaggerated hypoxic ventilatory response (HVR) (10.8 ± 0.3 versus 4.1 ± 0.7 ml min⁻¹ g⁻¹ in controls; P < 0.01). HVR was similarly exaggerated in PHD2^+/− animals compared to littermate controls (8.4 ± 0.7 versus 5.0 ± 0.8 ml min⁻¹ g⁻¹; P < 0.01). Carotid body volume increased (0.0025 ± 0.00017 in PHD2^+/− animals versus 0.0015 ± 0.00019 mm³ in controls; P < 0.01). In contrast, HVR in PHD1^−/− and PHD3^−/− mice was similar to littermate controls. Acute exposure to a small molecule PHD inhibitor (PHI) (2-(1-chloro-4-hydroxyisoquinoline-3-carboxamido) acetic acid) did not mimic the ventilatory response to hypoxia. Further, 7 day administration of the PHI induced only modest increases in HVR and carotid body cell proliferation, despite marked stimulation of erythropoiesis. This was in contrast with chronic hypoxia, which elicited both exaggerated HVR and cellular proliferation. The findings demonstrate that PHD enzymes modulate ventilatory sensitivity to hypoxia and identify PHD2 as the most important enzyme in this response. They also reveal differences between genetic inactivation of PHDs, responses to hypoxia and responses to a pharmacological inhibitor, demonstrating the need for caution in predicting the effects of therapeutic modulation of the HIF hydroxylase system on different physiological responses.

Roles of individual prolyl-4-hydroxylase isoforms in the first 24 hours following transient focal cerebral ischaemia: insights from genetically modified mice

This study investigated the function of each of the hypoxia inducible factor (HIF) prolyl-4-hydroxylase enzymes (PHD1–3) in the first 24 h following transient focal cerebral ischaemia by using mice with each isoform genetically suppressed. Male, 8- to 12-week old PHD1−/−, PHD2+/− and PHD3−/− mice and their wild-type (WT) littermate were subjected to 45 min of middle cerebral artery occlusion (MCAO). During the experiments, regional cerebral blood flow (rCBF) was recorded by laser Doppler flowmetry. Behaviour was assessed at both 2 h and 24 h after reperfusion with a common neuroscore. Infarct volumes, blood–brain barrier (BBB) disruption, cerebral vascular density, apoptosis, reactive oxygen species (ROS), HIF1α, and glycogen levels were then determined using histological and immunohistochemical techniques. When compared to their WT littermates, PHD2+/− mice had significantly increased cerebral microvascular density and more effective restoration of CBF upon reperfusion. PHD2+/− mice showed significantly better functional outcomes and higher activity rates at both 2 h and 24 h after MCAO, associated with significant fewer apoptotic cells in the penumbra and less BBB disruption; PHD3−/− mice had impaired rCBF upon early reperfusion but comparable functional outcomes; PHD1−/− mice did not show any significant changes following the MCAO. Production of ROS, HIF1α staining and glycogen content in the brain were not different in any comparison. Life-long genetic inhibition of PHD enzymes produces different effects on outcome in the first 24 h after transient cerebral ischaemia. These need to be considered in optimizing therapeutic effects of PHD inhibitors, particularly when isoform specific inhibitors become available.

Photoplethysmographic derivation of respiratory rate: a review of relevant physiology

An abnormal respiratory rate is often the earliest sign of critical illness. A reliable estimate of respiratory rate is vital in the application of remote telemonitoring systems, which may facilitate early supported discharge from hospital or prompt recognition of physiological deterioration in high-risk patient groups. Traditional approaches use analysis of respiratory sinus arrhythmia from the electrocardiogram (ECG), but this phenomenon is predominantly limited to the young and healthy. Analysis of the photoplethysmogram (PPG) waveform offers an alternative means of non-invasive respiratory rate monitoring, but further development is required to enable reliable estimates. This review conceptualizes the challenge by discussing the effect of respiration on the PPG waveform and the key physiological mechanisms that underpin the derivation of respiratory rate from the PPG.

Cardiopulmonary function in two human disorders of the hypoxia-inducible factor (HIF) pathway: von Hippel-Lindau disease and HIF-2alpha gain-of-function mutation

The hypoxia-inducible factors (HIFs; isoforms HIF-1α, HIF-2α, HIF-3α) mediate many responses to hypoxia. Their regulation is principally by oxygen-dependent degradation, which is initiated by hydroxylation of specific proline residues followed by binding of von Hippel-Lindau (VHL) protein. Chuvash polycythemia is a disorder with elevated HIF. It arises through germline homozygosity for hypomorphic VHL alleles and has a phenotype of hematological, cardiopulmonary, and metabolic abnormalities. This study explores the phenotype of two other HIF pathway diseases: classic VHL disease and HIF-2α gain-of-function mutation. No cardiopulmonary abnormalities were detected in classic VHL disease. HIF-2α gain-of-function mutations were associated with pulmonary hypertension, increased cardiac output, increased heart rate, and increased pulmonary ventilation relative to metabolism. Comparison of the HIF-2α gain-of-function responses with data from studies of Chuvash polycythemia suggested that other aspects of the Chuvash phenotype were diminished or absent. In classic VHL disease, patients are germline heterozygous for mutations in VHL, and the present results suggest that a single wild-type allele for VHL is sufficient to maintain normal cardiopulmonary function. The HIF-2α gain-of-function phenotype may be more limited than the Chuvash phenotype either because HIF-1α is not elevated in the former condition, or because other HIF-independent functions of VHL are perturbed in Chuvash polycythemia.—Formenti, F., Beer, P. A., Croft, Q. P. P., Dorrington, K. L., Gale, D. P., Lappin, T. R. J., Lucas, G. S., Maher, E. R., Maxwell, P. H., McMullin, M. F., O'Connor, D. F., Percy, M. J., Pugh, C. W., Ratcliffe, P. J., Smith, T. G., Talbot, N. P., Robbins, P. A. Cardiopulmonary function in two human disorders of the hypoxia-inducible factor (HIF) pathway: von Hippel-Lindau disease and HIF-2α gain-of-function mutation.

Differential sensitivity of hypoxia inducible factor hydroxylation sites to hypoxia and hydroxylase inhibitors

Hypoxia inducible factor (HIF) is regulated by dual pathways involving oxygen-dependent prolyl and asparaginyl hydroxylation of its α-subunits. Prolyl hydroxylation at two sites within a central degradation domain promotes association of HIF-α with the von Hippel-Lindau ubiquitin E3 ligase and destruction by the ubiquitin-proteasome pathways. Asparaginyl hydroxylation blocks the recruitment of p300/CBP co-activators to a C-terminal activation domain in HIF-α. These hydroxylations are catalyzed by members of the Fe(II) and 2-oxoglutarate (2-OG) oxygenase family. Activity of the enzymes is suppressed by hypoxia, increasing both the abundance and activity of the HIF transcriptional complex. We have used hydroxy residue-specific antibodies to compare and contrast the regulation of each site of prolyl hydroxylation (Pro⁴⁰², Pro⁵⁶⁴) with that of asparaginyl hydroxylation (Asn⁸⁰³) in human HIF-1α. Our findings reveal striking differences in the sensitivity of these hydroxylations to hypoxia and to different inhibitor types of 2-OG oxygenases. Hydroxylation at the three sites in endogenous human HIF-1α proteins was suppressed by hypoxia in the order Pro⁴⁰² > Pro⁵⁶⁴ > Asn⁸⁰³. In contrast to some predictions from in vitro studies, prolyl hydroxylation was substantially more sensitive than asparaginyl hydroxylation to inhibition by iron chelators and transition metal ions; studies of a range of different small molecule 2-OG analogues demonstrated the feasibility of selectively inhibiting either prolyl or asparaginyl hydroxylation within cells.

Regulation of type II transmembrane serine proteinase TMPRSS6 by hypoxia-inducible factors: new link between hypoxia signaling and iron homeostasis

Hepcidin is a liver-derived hormone with a key role in iron homeostasis. In addition to iron, it is regulated by inflammation and hypoxia, although mechanisms of hypoxic regulation remain unclear. In hepatocytes, hepcidin is induced by bone morphogenetic proteins (BMPs) through a receptor complex requiring hemojuvelin (HJV) as a co-receptor. Type II transmembrane serine proteinase (TMPRSS6) antagonizes hepcidin induction by BMPs by cleaving HJV from the cell membrane. Inactivating mutations in TMPRSS6 lead to elevated hepcidin levels and consequent iron deficiency anemia. Here we demonstrate that TMPRSS6 is up-regulated in hepatic cell lines by hypoxia and by other activators of hypoxia-inducible factor (HIF). We show that TMPRSS6 expression is regulated by both HIF-1α and HIF-2α. This HIF-dependent up-regulation of TMPRSS6 increases membrane HJV shedding and decreases hepcidin promoter responsiveness to BMP signaling in hepatocytes. Our results reveal a potential role for TMPRSS6 in hepcidin regulation by hypoxia and provide a new molecular link between oxygen sensing and iron homeostasis.

Prolyl hydroxylase 3 (PHD3) is essential for hypoxic regulation of neutrophilic inflammation in humans and mice

The regulation of neutrophil lifespan by induction of apoptosis is critical for maintaining an effective host response and preventing excessive inflammation. The hypoxia-inducible factor (HIF) oxygen-sensing pathway has a major effect on the susceptibility of neutrophils to apoptosis, with a marked delay in cell death observed under hypoxic conditions. HIF expression and transcriptional activity are regulated by the oxygen-sensitive prolyl hydroxylases (PHD1-3), but the role of PHDs in neutrophil survival is unclear. We examined PHD expression in human neutrophils and found that PHD3 was strongly induced in response to hypoxia and inflammatory stimuli in vitro and in vivo. Using neutrophils from mice deficient in Phd3, we demonstrated a unique role for Phd3 in prolonging neutrophil survival during hypoxia, distinct from other hypoxia-associated changes in neutrophil function and metabolic activity. Moreover, this selective defect in neutrophil survival occurred in the presence of preserved HIF transcriptional activity but was associated with upregulation of the proapoptotic mediator Siva1 and loss of its binding target Bcl-xL. In vivo, using an acute lung injury model, we observed increased levels of neutrophil apoptosis and clearance in Phd3-deficient mice compared with WT controls. We also observed reduced neutrophilic inflammation in an acute mouse model of colitis. These data support what we believe to be a novel function for PHD3 in regulating neutrophil survival in hypoxia and may enable the development of new therapeutics for inflammatory disease.

Expression profiling in progressive stages of fumarate-hydratase deficiency: the contribution of metabolic changes to tumorigenesis

Hereditary leiomyomatosis and renal cell carcinoma (HLRCC) is caused by mutations in the Krebs cycle enzyme fumarate hydratase (FH). It has been proposed that "pseudohypoxic" stabilization of hypoxia-inducible factor-α (HIF-α) by fumarate accumulation contributes to tumorigenesis in HLRCC. We hypothesized that an additional direct consequence of FH deficiency is the establishment of a biosynthetic milieu. To investigate this hypothesis, we isolated primary mouse embryonic fibroblast (MEF) lines from Fh1-deficient mice. As predicted, these MEFs upregulated Hif-1α and HIF target genes directly as a result of FH deficiency. In addition, detailed metabolic assessment of these MEFs confirmed their dependence on glycolysis, and an elevated rate of lactate efflux, associated with the upregulation of glycolytic enzymes known to be associated with tumorigenesis. Correspondingly, Fh1-deficient benign murine renal cysts and an advanced human HLRCC-related renal cell carcinoma manifested a prominent and progressive increase in the expression of HIF-α target genes and in genes known to be relevant to tumorigenesis and metastasis. In accord with our hypothesis, in a variety of different FH-deficient tissues, including a novel murine model of Fh1-deficient smooth muscle, we show a striking and progressive upregulation of a tumorigenic metabolic profile, as manifested by increased PKM2 and LDHA protein. Based on the models assessed herein, we infer that that FH deficiency compels cells to adopt an early, reversible, and progressive protumorigenic metabolic milieu that is reminiscent of that driving the Warburg effect. Targets identified in these novel and diverse FH-deficient models represent excellent potential candidates for further mechanistic investigation and therapeutic metabolic manipulation in tumors.

Dysregulation of hypoxia pathways in fumarate hydratase-deficient cells is independent of defective mitochondrial metabolism

Mutations in the gene encoding the Krebs cycle enzyme fumarate hydratase (FH) predispose to hereditary leiomyomatosis and renal cell cancer in affected individuals. FH-associated neoplasia is characterized by defective mitochondrial function and by upregulation of transcriptional pathways mediated by hypoxia-inducible factor (HIF), although whether and by what means these processes are linked has been disputed. We analysed the HIF pathway in Fh1-/- mouse embryonic fibroblasts (MEFs), in FH-defective neoplastic tissues and in Fh1-/- MEFs re-expressing either wild-type or an extra-mitochondrial restricted form of FH. These experiments demonstrated that upregulation of HIF-1alpha occurs as a direct consequence of FH inactivation. Fh1-/- cells accumulated intracellular fumarate and manifested severe impairment of HIF prolyl but not asparaginyl hydroxylation which was corrected by provision of exogenous 2-oxoglutarate (2-OG). Re-expression of the extra-mitochondrial form of FH in Fh1-/- cells was sufficient to reduce intracellular fumarate and to correct dysregulation of the HIF pathway completely, even in cells that remained profoundly defective in mitochondrial energy metabolism. The findings indicate that upregulation of HIF-1alpha arises from competitive inhibition of the 2-OG-dependent HIF hydroxylases by fumarate and not from disruption of mitochondrial energy metabolism.

Mutation analysis of hypoxia-inducible factors HIF1A and HIF2A in renal cell carcinoma

BACKGROUND

Inactivation of the Von Hippel-Lindau (VHL) tumour suppressor gene leading to overexpression of hypoxia-inducible transcription factors (HIF)-1alpha and -2alpha is a critical event in the pathogenesis of most clear cell renal cell carcinomas (RCC). HIF-1alpha and HIF-2alpha share significant homology and regulate overlapping repertoires of hypoxia-inducible target genes but may have differing effects on RCC cell growth. Loss of HIF-1alpha expression has been described in RCC cell lines and primary tumours. Whether mutations in the alpha-subunits of HIF-1alpha and HIF-2alpha contribute to renal tumourigenesis was investigated here.

MATERIALS AND METHODS

Mutation analysis of the complete coding sequence of HIF-1alpha and HIF-2alpha was carried out in primary RCC (n=40).

RESULTS

The analysis revealed a somatic HIF1A missense substitution, p.Val116Glu, in a single RCC. Functional studies demonstrated that p.Val116Glu impaired HIF-1alpha transcriptional activity. Genotyping of HIF1A variants p.Pro582Ser and p.Ala588Thr demonstrated no significant differences between RCC patients and controls.

CONCLUSION

The detection of a loss-of-function HIF1A mutation in a primary RCC is consistent with HIF-1 and HIF-2 having different roles in renal tumourigenesis, However, somatic mutations of HIF1A are not frequently implicated in the pathogenesis of RCC.

Expression of hypoxia-inducible factor (HIF)-1alpha and vascular endothelial growth factor (VEGF)-D as outcome predictors in resected esophageal squamous cell carcinoma

Hypoxia-inducible factor (HIF)-1alpha and vascular endothelial growth factor (VEGF) are important angiogenic factors in human cancers. Relative to VEGF-C, prognostic significance of VEGF-D expression and its association with HIF-1alpha expression remain elusive in esophageal squamous cell cancer (ESCC). We studied expression of HIF-1alpha and VEGF-D using immunohistochemistry in 85 resected ESCC specimens and correlated results with patients' clinicopathologic parameters and survival. Association between expression of HIF-1alpha and VEGF-D was investigated using a concordance analysis. High expression of HIF-1alpha and VEGF-D was observed in 52 (61.2%) and 56 (65.9%) patients, respectively. HIF-1alpha expression correlated well with tumor stage (P = 0.041), whereas VEGF-D expression correlated with tumor stage (P = 0.027) and N status (P = 0.019). Groups of high HIF-1alpha and VEGF-D showed worse survivals than those of low expression (P = 0.002 and 0.001, respectively). Multivariate analysis supported expression of HIF-1alpha and VEGF-D as significant survival predictors (P = 0.044 and 0.035, respectively). A concordance rate of 69.5% was observed between expression of HIF-1alpha and VEGF-D. In conclusion, protein expression of HIF-1alpha and VEGF-D are independent prognostic predictors. An association between expression of HIF-1alpha and VEGF-D suggests that these two angiogenic factors are essential in progression of ESCC.

Mutation of the von Hippel-Lindau gene alters human cardiopulmonary physiology

Intracellular responses to hypoxia are coordinated by the von Hippel-Lindau--hypoxia-inducible factor (VHL-HIF) transcriptional system. This study investigated the potential role of the VHL-HIF pathway in human systems-level physiology. Patients diagnosed with Chuvash polycythaemia, a rare disorder in which VHL signalling is specifically impaired, were studied during acute hypoxia and hypercapnia. Subjects breathed through a mouthpiece and ventilation was measured while pulmonary vascular tone was assessed echocardiographically. The patients were found to have elevated basal ventilation and pulmonary vascular tone, and ventilatory, pulmonary vasoconstrictive and heart rate responses to acute hypoxia were greatly increased, as were heart rate responses to hypercapnia. The patients also had abnormal pulmonary function on spirometry. This study's findings demonstrate that the VHL-HIF signalling pathway, which is so central to intracellular oxygen sensing, also regulates the organ systems upon which cellular oxygen delivery ultimately depends.

The VHL tumor suppressor inhibits expression of the IGF1R and its loss induces IGF1R upregulation in human clear cell renal carcinoma

Clear cell renal cell cancer (CC-RCC) is a highly chemoresistant tumor characterized by frequent inactivation of the von Hippel-Lindau (VHL) gene. The prognosis is reportedly worse in patients whose tumors express immunoreactive type I insulin-like growth factor receptor (IGF1R), a key mediator of tumor cell survival. We aimed to investigate how IGF1R expression is regulated, and found that IGF1R protein levels were unaffected by hypoxia, but were higher in CC-RCC cells harboring mutant inactive VHL than in isogenic cells expressing wild-type (WT) VHL. IGF1R mRNA and promoter activities were significantly lower in CC-RCC cells expressing WT VHL, consistent with a transcriptional effect. In Sp1-null Drosophila Schneider cells, IGF1R promoter activity was dependent on exogenous Sp1, and was suppressed by full-length VHL protein (pVHL) but only partially by truncated VHL lacking the Sp1-binding motif. pVHL also reduced the stability of IGF1R mRNA via sequestration of HuR protein. Finally, IGF1R mRNA levels were significantly higher in CC-RCC biopsies than benign kidney, confirming the clinical relevance of these findings. Thus, we have identified a new hypoxia-independent role for VHL in suppressing IGF1R transcription and mRNA stability. VHL inactivation leads to IGF1R upregulation, contributing to renal tumorigenesis and potentially also to chemoresistance.

Target gene selectivity of hypoxia-inducible factor-alpha in renal cancer cells is conveyed by post-DNA-binding mechanisms

Inactivation of the von Hippel–Lindau tumour suppressor in renal cell carcinoma (RCC) leads to failure of proteolytic regulation of the α subunits of hypoxia-inducible factor (HIF), constitutive upregulation of the HIF complex, and overexpression of HIF target genes. However, recent studies have indicated that in this setting, upregulation of the closely related HIF-α isoforms, HIF-1α and HIF-2α, have contrasting effects on tumour growth, and activate distinct sets of target genes. To pursue these findings, we sought to elucidate the mechanisms underlying target gene selectivity for HIF-1α and HIF-2α. Using chromatin immunoprecipitation to probe binding to hypoxia response elements in vivo, and expression of chimaeric molecules bearing reciprocal domain exchanges between HIF-1α and HIF-2α molecules, we show that selective activation of HIF-α target gene expression is not dependent on selective DNA-binding at the target locus, but depends on non-equivalent C-terminal portions of these molecules. Our data indicate that post-DNA binding mechanisms that are dissimilar for HIF-1α and HIF-2α determine target gene selectivity in RCC cells.

Interaction of hydroxylated collagen IV with the von hippel-lindau tumor suppressor

The von Hippel-Lindau tumor suppressor (pVHL) targets hydroxylated alpha-subunits of hypoxia-inducible factor (HIF) for ubiquitin-mediated proteasomal destruction through direct interaction with the hydroxyproline binding pocket in its beta-domain. Although disruption of this process may contribute to VHL-associated tumor predisposition by up-regulation of HIF target genes, genetic and biochemical analyses support the existence of additional functions, including a role in the assembly of extracellular matrix. In an attempt to delineate these pathways, we searched for novel pVHL-binding proteins. Here we report a direct, hydroxylation-dependent interaction with alpha-chains of collagen IV. Interaction with pVHL was also observed with fibrillar collagen chains, but not the folded collagen triple helix. The interaction was suppressed by a wide range of tumor-associated mutations, including those that do not disturb the regulation of HIF, supporting a role in HIF-independent tumor suppressor functions.

HIF prolyl hydroxylases in the rat; organ distribution and changes in expression following hypoxia and coronary artery ligation

Hypoxia-inducible factor (HIF) regulates expression of genes involved in adaptation to hypoxia and ischemia. Three prolyl hydroxylases (PHD1-3) underlie oxygen-regulated destruction of HIFalpha chains. We have investigated the organ distribution of the PHDs in the rat, their regulation by hypoxia and changes in local expression after experimental myocardial infarction using RNase protection assays, in situ hybridization and immunohistochemistry. mRNAs of all isoforms were detectable in heart, liver, kidney, brain, testis and lung. In normal animals, highest levels for PHD2 mRNA and PHD3 mRNA were found in myocardium, whereas PHD1 mRNA was detected predominantly in the testis. PHD1 mRNA was constitutively expressed. PHD2 mRNA was induced by hypoxia in the liver and PHD3 mRNA in liver, testis and heart. Overall our results show that PHD2 mRNA is ubiquitously expressed in normal animals, in keeping with a general role in oxygen sensing. PHD1 and 3 mRNA distributions suggest particular roles in testis and heart, respectively. In a model of myocardial infarction, in situ hybridization showed periischemic enhancement for PHD2 mRNA and PHD3 mRNA, but not PHD1 mRNA. Immunostaining of PHD2 and 3 in infarcted hearts showed enhanced protein expression, maximal 7 days after infarction. Levels were strongest in regions neighboring areas of HIF staining but also partially overlapped with these zones. Inducibility of PHD2 and 3 by hypoxia and ischemia in vivo has important implications both for the pathophysiology of conditions where oxygen supply is deranged and for attempts to manipulate the HIF system therapeutically.

Posttranslational hydroxylation of ankyrin repeats in IkappaB proteins by the hypoxia-inducible factor (HIF) asparaginyl hydroxylase, factor inhibiting HIF (FIH)

Studies on hypoxia-sensitive pathways have revealed a series of Fe(II)-dependent dioxygenases that regulate hypoxia-inducible factor (HIF) by prolyl and asparaginyl hydroxylation. The recognition of these unprecedented signaling processes has led to a search for other substrates of the HIF hydroxylases. Here we show that the human HIF asparaginyl hydroxylase, factor inhibiting HIF (FIH), also efficiently hydroxylates specific asparaginyl (Asn)-residues within proteins of the IkappaB family. After the identification of a series of ankyrin repeat domain (ARD)-containing proteins in a screen for proteins interacting with FIH, the ARDs of p105 (NFKB1) and IkappaBalpha were shown to be efficiently hydroxylated by FIH at specific Asn residues in the hairpin loops linking particular ankyrin repeats. The target Asn residue is highly conserved as part of the ankyrin consensus, and peptides derived from a diverse range of ARD-containing proteins supported FIH enzyme activity. These findings demonstrate that this type of protein hydroxylation is not restricted to HIF and strongly suggest that FIH-dependent ARD hydroxylation is a common occurrence, potentially providing an oxygen-sensitive signal to a diverse range of processes.

Novel Thioredoxin Inhibitors Paradoxically Increase Hypoxia-Inducible Factor-α Expression but Decrease Functional Transcriptional Activity, DNA Binding, and Degradation

PURPOSE

Hypoxia-inducible factor-alpha (HIF-alpha) is a transcription factor that regulates the response to hypoxia. HIF-alpha protein is found at high levels in many cancers, and the redox protein thioredoxin-1 (Trx-1) increases both aerobic and hypoxia-induced HIF-alpha. Therefore, Trx-1 and HIF-alpha are attractive molecular targets for novel cancer therapeutics.

EXPERIMENTAL DESIGN

We investigated whether two novel anticancer drugs AJM290 and AW464 (quinols), which inhibit Trx-1 function, can inhibit the HIF pathway.

RESULTS

Treatment of several cancer cell lines with AJM290 or AW464 prevented the hypoxia-induced increase of vascular endothelial growth factor (VEGF) at subtoxic concentrations. AJM290 and AW464 also decreased VEGF in pVHL mutant renal cell carcinoma cells that constitutively overexpress HIF-alpha protein. They surprisingly up-regulated HIF-alpha expression in breast cancer cell lines in normoxia and hypoxia as well as in pVHL mutant cells. In the MDA-MB-468 breast cancer cell line, the compounds inhibited RNA and protein expression of the HIF-alpha target genes, carbonic anhydrase IX, VEGF, and BNIP3, concordantly with HIF-alpha up-regulation. Both compounds specifically inhibited HIF-alpha-dependent induction of hypoxia regulatory element-luciferase and HIF-1alpha hypoxia regulatory element-DNA binding. To analyze the HIF-1alpha domain inhibited by AJM290, we transfected cells with plasmids expressing a fusion protein of Gal linked to HIF-1alpha or HIF-1alpha COOH-terminal transactivation domain (CAD) with a Gal4-responsive luciferase reporter gene. AJM290 inhibited both the full-length HIF-1alpha and HIF-1alpha CAD transcriptional activity.

CONCLUSIONS

AJM290 and AW464 are inhibitors of HIF-1alpha CAD transcription activity and DNA binding, but they also inhibit degradation of HIF, in contrast to other Trx inhibitors.

Mutation of von Hippel-Lindau tumour suppressor and human cardiopulmonary physiology

BACKGROUND

The von Hippel-Lindau tumour suppressor protein-hypoxia-inducible factor (VHL-HIF) pathway has attracted widespread medical interest as a transcriptional system controlling cellular responses to hypoxia, yet insights into its role in systemic human physiology remain limited. Chuvash polycythaemia has recently been defined as a new form of VHL-associated disease, distinct from the classical VHL-associated inherited cancer syndrome, in which germline homozygosity for a hypomorphic VHL allele causes a generalised abnormality in VHL-HIF signalling. Affected individuals thus provide a unique opportunity to explore the integrative physiology of this signalling pathway. This study investigated patients with Chuvash polycythaemia in order to analyse the role of the VHL-HIF pathway in systemic human cardiopulmonary physiology.

METHODS AND FINDINGS

Twelve participants, three with Chuvash polycythaemia and nine controls, were studied at baseline and during hypoxia. Participants breathed through a mouthpiece, and pulmonary ventilation was measured while pulmonary vascular tone was assessed echocardiographically. Individuals with Chuvash polycythaemia were found to have striking abnormalities in respiratory and pulmonary vascular regulation. Basal ventilation and pulmonary vascular tone were elevated, and ventilatory, pulmonary vasoconstrictive, and heart rate responses to acute hypoxia were greatly increased.

CONCLUSIONS

The features observed in this small group of patients with Chuvash polycythaemia are highly characteristic of those associated with acclimatisation to the hypoxia of high altitude. More generally, the phenotype associated with Chuvash polycythaemia demonstrates that VHL plays a major role in the underlying calibration and homeostasis of the respiratory and cardiovascular systems, most likely through its central role in the regulation of HIF.

Hypoxia-inducible factors: where, when and why?

Hypoxia-inducible factor (HIF) is a family of transcription factors that regulate the homeostatic response to oxygen deprivation during development, physiological adaptation, and pathological processes such as ischemia and neoplasia. Our understanding of the function of different HIF isoforms is being advanced by understanding the processes that regulate their activity, learning where and when they are expressed and what genes they regulate.

Use of novel monoclonal antibodies to determine the expression and distribution of the hypoxia regulatory factors PHD-1, PHD-2, PHD-3 and FIH in normal and neoplastic human tissues

AIMS

The cellular response to hypoxia includes the hypoxia inducible factor (HIF)-induced transcription of genes involved in diverse processes such as glycolysis, angiogenesis and the growth of experimental tumours. Regulation of the level of hypoxia inducible factors 1alpha and 2alpha (HIF-1alpha and HIF-2alpha) is a primary determinant of HIF activity. Recent biochemical and candidate gene approach studies have led to the discovery of three HIF-regulatory prolyl hydroxylases, PHD-1, -2 and -3 and an asparaginyl hydroxylase, also known as FIH (factor inhibiting HIF). In this study, we raised and characterized monoclonal antibodies against PHD-1, PHD-2, PHD-3 and FIH.

METHODS AND RESULTS

Immunohistochemistry of normal tissues with these monoclonal antibodies demonstrated a wide distribution in epithelial cells, stromal cells and leucocytes, with cytoplasmic staining predominating over nuclear staining. A preliminary study of tumours showed variable staining in tumour, stromal and inflammatory cells. While all tumour types showed some positive staining with each antibody, the overall pattern suggested a slight decrease in the amount of staining seen with PHD-1, -2 and -3 and an increase in FIH staining in neoplasia compared with corresponding normal tissues.

CONCLUSIONS

These monoclonal antibodies will allow further larger scale studies to determine the significance of PHD and FIH expression in neoplasia.

Contrasting properties of hypoxia-inducible factor 1 (HIF-1) and HIF-2 in von Hippel-Lindau-associated renal cell carcinoma

Defective function of the von Hippel-Lindau (VHL) tumor suppressor ablates proteolytic regulation of hypoxia-inducible factor alpha subunits (HIF-1alpha and HIF-2alpha), leading to constitutive activation of hypoxia pathways in renal cell carcinoma (RCC). Here we report a comparative analysis of the functions of HIF-1alpha and HIF-2alpha in RCC and non-RCC cells. We demonstrate common patterns of HIF-alpha isoform transcriptional selectivity in VHL-defective RCC that show consistent and striking differences from patterns in other cell types. We also show that HIF-alpha isoforms display unexpected suppressive interactions in RCC cells, with enhanced expression of HIF-2alpha suppressing HIF-1alpha and vice-versa. In VHL-defective RCC cells, we demonstrate that the protumorigenic genes encoding cyclin D1, transforming growth factor alpha, and vascular endothelial growth factor respond specifically to HIF-2alpha and that the proapoptotic gene encoding BNip3 responds positively to HIF-1alpha and negatively to HIF-2alpha, indicating that HIF-1alpha and HIF-2alpha have contrasting properties in the biology of RCC. In keeping with this, HIF-alpha isoform-specific transcriptional selectivity was matched by differential effects on the growth of RCC as tumor xenografts, with HIF-1alpha retarding and HIF-2alpha enhancing tumor growth. These findings indicate that therapeutic approaches to targeting of the HIF system, at least in this setting, will need to take account of HIF isoform-specific functions.

Determination and comparison of specific activity of the HIF-prolyl hydroxylases

Hypoxia-inducible factor (HIF) is a transcriptional complex that is regulated by oxygen sensitive hydroxylation of its alpha subunits by the prolyl hydroxylases PHD1, 2 and 3. To better understand the role of these enzymes in directing cellular responses to hypoxia, we derived an assay to determine their specific activity in both native cell extracts and recombinant sources of enzyme. We show that all three are capable of high rates of catalysis, in the order PHD2=PHD3>PHD1, using substrate peptides derived from the C-terminal degradation domain of HIF-alpha subunits, and that each demonstrates similar and remarkable sensitivity to oxygen, commensurate with a common role in signaling hypoxia.

Genetic analysis of pathways regulated by the von Hippel-Lindau tumor suppressor in Caenorhabditis elegans

The von Hippel-Lindau (VHL) tumor suppressor functions as a ubiquitin ligase that mediates proteolytic inactivation of hydroxylated α subunits of hypoxia-inducible factor (HIF). Although studies of VHL-defective renal carcinoma cells suggest the existence of other VHL tumor suppressor pathways, dysregulation of the HIF transcriptional cascade has extensive effects that make it difficult to distinguish whether, and to what extent, observed abnormalities in these cells represent effects on pathways that are distinct from HIF. Here, we report on a genetic analysis of HIF-dependent and -independent effects of VHL inactivation by studying gene expression patterns in Caenorhabditis elegans. We show tight conservation of the HIF-1/VHL-1/EGL-9 hydroxylase pathway. However, persisting differential gene expression in hif-1 versus hif-1; vhl-1 double mutant worms clearly distinguished HIF-1–independent effects of VHL-1 inactivation. Genomic clustering, predicted functional similarities, and a common pattern of dysregulation in both vhl-1 worms and a set of mutants (dpy-18, let-268, gon-1, mig-17, and unc-6), with different defects in extracellular matrix formation, suggest that dysregulation of these genes reflects a discrete HIF-1–independent function of VHL-1 that is connected with extracellular matrix function.

Besides its known function of inactivating hypoxia-inducible factor (HIF), genetically engineered worms clearly demonstrate that there exist HIF-independent effects of the von Hippel- Lindau tumor suppressor

Inhibition of endogenous HIF inactivation induces angiogenesis in ischaemic skeletal muscles of mice

Hypoxia-inducible factor (HIF) modulates transcriptional control of several genes involved in vascular growth and cellular metabolism. HIF activity can be enhanced by suppression of prolyl and asparaginyl hydroxylase activity by dimethyloxalylglycine (DMOG). We have compared the effects of DMOG treatment and femoral artery ligation individually or in combination on HIF-1alpha protein level, HIF-dependent gene expression and capillary-to-fibre ratio (C: F) in extensor digitorum longus and tibialis anterior muscles of mice. Immunohistochemical examination revealed that HIF-1alpha is present in non-ischaemic mouse skeletal muscles, but its amount increased profoundly in response to the combination of DMOG treatment and ischaemia. Combined treatment resulted in 39% increase in C: F in ischaemic muscles (P < 0.0001 versus controls) whereas individual treatments produced little effect under our conditions. Combined treatment led to a significant increase in endogenous HIF-1alpha protein (6.14 +/- 1.1 versus 1.17 +/- 0.2 in controls; P < 0.05) that was not apparent in mice treated with DMOG or femoral artery ligation alone. Ischaemia increased vascular endothelial growth factor (VEGF) protein production by 2.5-fold (P < 0.05 versus controls), irrespective of DMOG treatment. However, production of the VEGF receptor Flk-1 was more enhanced in ischaemic + DMOG-treated muscles (P < 0.001 and P < 0.05 compared with controls and untreated ischaemic muscles, respectively), which may explain the intensive growth of capillaries in those muscles. The findings indicate that treatment with DMOG has a potential therapeutic use in promoting angiogenesis in ischaemic diseases, and perhaps for improving muscle recovery after injury, exercise or training.