Brain-specific knock-out of hypoxia-inducible factor-1alpha reduces rather than increases hypoxic-ischemic damage

Effect of propofol post-treatment on blood-brain barrier integrity and cerebral edema after transient cerebral ischemia in rats

Although propofol has been reported to offer neuroprotection against cerebral ischemia injury, its impact on cerebral edema following ischemia is not clear. The objective of this investigation is to evaluate the effects of propofol post-treatment on blood-brain barrier (BBB) integrity and cerebral edema after transient cerebral ischemia and its mechanism of action, focusing on modulation of aquaporins (AQPs), matrix metalloproteinases (MMPs), and hypoxia inducible factor (HIF)-1α. Cerebral ischemia was induced in male Sprague-Dawley rats (n = 78) by occlusion of the right middle cerebral artery for 1 h. For post-treatment with propofol, 1 mg kg(-1) min(-1) of propofol was administered for 1 h from the start of reperfusion. Nineteen rats undergoing sham surgery were also included in the investigation. Edema and BBB integrity were assessed by quantification of cerebral water content and extravasation of Evans blue, respectively, following 24 h of reperfusion. In addition, the expression of AQP-1, AQP-4, MMP-2, and MMP-9 was determined 24 h after reperfusion and the expression of HIF-1α was determined 8 h after reperfusion. Propofol post-treatment significantly reduced cerebral edema (P < 0.05) and BBB disruption (P < 0.05) compared with the saline-treated control. The expression of AQP-1, AQP-4, MMP-2, and MMP-9 at 24 h and of HIF-1α at 8 h following ischemia/reperfusion was significantly suppressed in the propofol post-treatment group (P < 0.05). Propofol post-treatment attenuated cerebral edema after transient cerebral ischemia, in association with reduced expression of AQP-1, AQP-4, MMP-2, and MMP-9. The decreased expression of AQPs and MMPs after propofol post-treatment might result from suppression of HIF-1α expression.

The role of HIF in cobalt-induced ischemic tolerance

Understanding the endogenous survival pathways induced by ischemic tolerance may yield targets for neuroprotection from stroke. One well-studied pathway reported to be evoked by preconditioning stimuli is the transcription factor HIF (hypoxia-inducible factor). However, whether HIF induction by ischemic insults is neuroprotective or toxic is still unclear. We examined the ability of three prolyl-hydroxylase inhibitors, which induce HIF, to protect hippocampal cultures from oxygen-glucose deprivation. Hippocampal cultures were exposed to ischemic preconditioning or various concentrations of cobalt chloride, deferoxamine (DFO) or dimethyloxylalyglycine (DMOG), prior to lethal oxygen-glucose deprivation (OGD). Cell survival of neurons and astrocytes was determined with dual-label immunocytochemistry. The induction of HIF targets was assessed in mixed as well as astrocyte-enriched cultures. Ischemic preconditioning, as well as low concentrations of cobalt and DFO, enhanced the survival of neurons following OGD. However, DMOG exacerbates OGD-induced neuronal death. At low concentrations, all three prolyl-hydroxylase (PHD) inhibitors increased the survival of astrocytes. Neuroprotective concentrations of cobalt induced the transcription of the cytokine erythropoietin (EPO) in astrocyte cultures. In addition, pretreatment with recombinant human erythropoietin (rH-EPO) also protected neurons from OGD. Our data suggest that HIF-induced EPO, released from astrocytes, protects neurons from OGD. However, the three PHD inhibitors each exhibited different neuroprotective profiles at low concentrations, suggesting that not all PHD inhibitors are created equal. The protective effects at low doses is reminiscent of HIF involvement in ischemic tolerance, in which sub-lethal insults induce HIF pathways resulting in neuroprotection, whereas the high-dose toxicity suggests that over-activation of HIF is not always protective. Therefore, the choice of inhibitor and dose may determine the clinical utility of these compounds. Deferoxamine exhibited little toxicity even at higher doses, and therefore appears a promising candidate for clinical use.

Changes of hypoxia-inducible factor-1 signaling and the effect of cilostazol in chronic cerebral ischemia

Hypoxia-inducible factor-1 and its specific target gene heme oxygenase-1, are involved in acute cerebral ischemia. However, very few studies have examined in detail the changes in the hypoxia-inducible factor-1/heme oxygenase-1 signaling pathway in chronic cerebral ischemia. In this study, a rat model of chronic cerebral ischemia was established by permanent bilateral common carotid artery occlusion, and these rats were treated with intragastric cilostazol (30 mg/kg) for 9 weeks. Morris water maze results showed that cognitive impairment gradually worsened as the cerebral ischemia proceeded. Immunohistochemistry, semi-quantitative PCR and western blot analysis showed that hypoxia-inducible factor-1α and heme oxygenase-1 expression levels increased after chronic cerebral ischemia, with hypoxia-inducible factor-1α expression peaking at 3 weeks and heme oxygenase-1 expression peaking at 6 weeks. These results suggest that the elevated levels of hypoxia-inducible factor-1α may upregulate heme oxygenase-1 expression following chronic cerebral ischemia and that the hypoxia-inducible factor-1/heme oxygenase-1 signaling pathway is involved in the development of cognitive impairment induced by chronic cerebral ischemia. Cilostazol treatment alleviated the cognitive impairment in rats with chronic cerebral ischemia, decreased hypoxia-inducible factor-1α and heme oxygenase-1 expression levels, and reduced apoptosis in the frontal cortex. These findings demonstrate that cilostazol can protect against cognitive impairment induced by chronic cerebral ischemic injury through an anti-apoptotic mechanism.

Subarachnoid hemorrhage and cerebral vasospasm - literature review

Subarachnoid hemorrhage represents a serious disease with high mortality and morbidity. Two important areas are becoming the central research interest of subarachnoid hemorrhage: cerebral vasospasm and early brain injury. The authors have reviewed the major contributions in experimental subarachnoid hemorrhage documented in the medical literature in the past 5 years. Treatments interfering with nitric oxide - or endothelin-pathways continue to show antispasmotic effects in experimental models of subarachnoid hemorrhage. Inflammation and oxidative stress play a vital role in the pathophysiology of cerebral vasospasm. Apoptosis, a relevant cause of early brain injury after subarachnoid hemorrhage, also underline the etiology of cerebral vasospasm. Future research studies will continue to elucidate the pathophysiological pathways and treatment modalities targeting cerebral vasospasm and early brain injury, enabling an improvement in outcome for patients with subarachnoid hemorrhage.

The alteration of hypoxia inducible factor-1 (HIF-1) and its target genes in mood disorder patients

Recent studies suggest that the dysfunction of neural plasticity is associated with mood disorders. Hypoxia-inducible factor-1 (HIF-1), which is a transcriptional activator of vascular endothelial growth factor (VEGF), activates the cellular response to hypoxia. HIF-1 is ubiquitously expressed in all cells, including peripheral leukocytes. However, little is known about the role of HIF-1 in mood disorder. In the present study, we investigated the mRNA expression levels of HIF-1 (α and β) and its target genes (VEGF, GLUT1, PGK1, PFKFB3, and LDHA) in the peripheral white blood cells of patients with major depressive disorder (MDD) and bipolar disorder (BPD). We found increased expression of HIF- 1α and HIF-1β mRNA, as well as the target genes, VEGF, and PFKFB3 in both MDD and BPD patients in a depressive state compared to healthy control subjects. Furthermore, the mRNA expression levels of GLUT1, PGK1, and LDHA were increased in MDD patients in a depressive state compared to healthy control subjects. We also found increased expression of HIF-1α and LDHA mRNA in MDD patients in a remissive state, whereas the mRNA expression levels of other genes in a remissive state were comparable to those in healthy control subjects. There was no significant difference in mRNA expression levels of the genes examined among patients receiving any type of antidepressant or mood stabilizer. Our data suggest that altered expression of HIF-1 and its target genes mRNA in peripheral blood cells are associated-mainly in a state-dependent manner-with mood disorders (especially with MDD). In addition, altered expression of HIF-1 and its target genes may be associated with the pathophysiology of depression.

In silico pathway analysis: the final frontier towards completely rational drug design

Pathway and network analyses are rapidly becoming the mainstream tools for functional interpretation of high-throughput data and for drug discovery. Current scientific literature has plenty of examples on how pathway analysis tools are used across all steps of drug development pipeline. Pathway and network analyses already enable rational selection of drug targets based on the knowledge about disease biology. Pathway analysis tools are also popular for the analysis of drug action and validation of drug efficacy and toxicity. This article overviews current achievements of pathway analysis and suggests future directions for its application in drug development such as rational design of combinatorial therapy and personalized medicine.

An environmental analysis of genes associated with schizophrenia: hypoxia and vascular factors as interacting elements in the neurodevelopmental model

Investigating and understanding gene-environment interaction (G × E) in a neurodevelopmentally and biologically plausible manner is a major challenge for schizophrenia research. Hypoxia during neurodevelopment is one of several environmental factors related to the risk of schizophrenia, and links between schizophrenia candidate genes and hypoxia regulation or vascular expression have been proposed. Given the availability of a wealth of complex genetic information on schizophrenia in the literature without knowledge on the connections to environmental factors, we now systematically collected genes from candidate studies (using SzGene), genome-wide association studies (GWAS) and copy number variation (CNV) analyses, and then applied four criteria to test for a (theoretical) link to ischemia-hypoxia and/or vascular factors. In all, 55% of the schizophrenia candidate genes (n=42 genes) met the criteria for a link to ischemia-hypoxia and/or vascular factors. Genes associated with schizophrenia showed a significant, threefold enrichment among genes that were derived from microarray studies of the ischemia-hypoxia response (IHR) in the brain. Thus, the finding of a considerable match between genes associated with the risk of schizophrenia and IHR and/or vascular factors is reproducible. An additional survey of genes identified by GWAS and CNV analyses suggested novel genes that match the criteria. Findings for interactions between specific variants of genes proposed to be IHR and/or vascular factors with obstetric complications in patients with schizophrenia have been reported in the literature. Therefore, the extended gene set defined here may form a reasonable and evidence-based starting point for hypothesis-based testing of G × E interactions in clinical genetic and translational neuroscience studies.

Glutathione peroxidase overexpression causes aberrant ERK activation in neonatal mouse cortex after hypoxic preconditioning

BACKGROUND

Preconditioning of neonatal mice with nonlethal hypoxia (HPC) protects the brain from hypoxic-ischemic (HI) injury. Overexpression of human glutathione peroxidase 1 (GPx1), which normally protects the developing murine brain from HI injury, reverses HPC protection, suggesting that a certain threshold of hydrogen peroxide concentration is required for activation of HPC signaling.

METHODS

Activation (phosphorylation) of extracellular-regulated kinase (ERK) 1/2 and Akt, and induction of hypoxia-inducible factor (HIF)-1α were assessed in the cortex, one of the main structures affected by HI and protected by HPC, at different time points after reoxygenation in wild-type (WT) and GPx1-overexpressing animals.

RESULTS

GPx1 overexpression prevented both the global and nuclear increase in activated ERK at 0.5 h after HPC and caused a significant decrease in phospho-ERK (pERK)/ERK levels at 24 h after HPC. In contrast, HIF-1α induction at the end of hypoxia was unaffected by GPx1 overexpression. In the cortex of preconditioned WT animals, enhanced pERK staining was primarily observed in neurons and to a lower extent in astrocytes and endothelial cells, with a nuclear prominence.

CONCLUSION

Aberrant activation of ERK probably explains the paradoxical reversal of HPC protection by GPx1 overexpression. The results identify hydrogen peroxide as an important mediator of neuroprotective ERK signaling.

Transglutaminase 2 facilitates or ameliorates HIF signaling and ischemic cell death depending on its conformation and localization

Transglutaminase 2 (TG2) is a widely expressed and multifunctional protein that modulates cell death/survival processes. We have previously shown that TG2 binds to hypoxia inducible factor 1β (HIF1β) and decreases the upregulation of HIF responsive genes; however, the relationship between these observations was not investigated. In this study, we investigated whether endogenous TG2 is sufficient to suppress HIF activity and whether the interaction between TG2 and HIF1β is required for this suppression. shRNA-mediated silencing of TG2 significantly enhanced HIF activation in response to hypoxia. In addition, nuclear localization of TG2 is required for its suppressive effect on HIF activity, with TG2 being recruited to HIF responsive promoters in hypoxic conditions. These observations suggest that TG2 directly regulates hypoxic transcriptional machinery; however, its interaction with HIF1β was not required for this regulation. We also examined whether TG2's effect on cell death/survival processes in ischemia is due to its effects on HIF signaling. Our results indicate that TG2 mediated HIF suppression can be separated from TG2's effect on cell survival in hypoxic/hypoglycemic conditions. Lastly, here we show that nuclear TG2 in the closed conformation and non-nuclear TG2 in the open conformation have opposing effects on hypoxic/hypoglycemic cell death, which could explain previous controversial results. Overall, our results further clarify the role of TG2 in mediating the cellular response to ischemia and suggest that manipulating the conformation of TG2 might be of pharmacological interest as a therapeutic strategy for the treatment of ischemia-related pathologies.

Hypoxia markers are expressed in interneurons exposed to recurrent seizures

An early but transient decrease in oxygen availability occurs during experimentally induced seizures. Using pimonidazole, which probes hypoxic insults, we found that by increasing the duration of pilocarpine-induced status epilepticus (SE) from 30 to 120 min, counts of pimonidazole-immunoreactive neurons also increased (P < 0.01, 120 vs 60 and 30 min). All the animals exposed to SE were immunopositive to pimonidazole, but a different scenario emerged during epileptogenesis when a decrease in pimonidazole-immunostained cells occurred from 7 to 14 days, so that only 1 out of 4 rats presented with pimonidazole-immunopositive cells. Pimonidazole-immunoreactive cells robustly reappeared at 21 days post-SE induction when all animals (7 out of 7) had developed spontaneous recurrent seizures. Specific neuronal markers revealed that immunopositivity to pimonidazole was present in cells identified by neuropeptide Y (NPY) or somatostatin antibodies. At variance, neurons immunopositive to parvalbumin or cholecystokinin were not immunopositive to pimonidazole. Pimonidazole-immunopositive neurons expressed remarkable immunoreactivity to hypoxia-inducible factor 1α (HIF-1α). Interestingly, surgical samples obtained from pharmacoresistant patients showed neurons co-labeled by HIF-1α and NPY antibodies. These interneurons, along with parvalbumin-positive interneurons that were negative to HIF-1α, showed immunopositivity to markers of cell damage, such as high-mobility group box 1 in the cytoplasm and cleaved caspase-3 in the nucleus. These findings suggest that interneurons are continuously endangered in rodent and human epileptogenic tissue. The presence of hypoxia and cell damage markers in NPY interneurons of rats and patients presenting with recurrent seizures indicates a mechanism of selective vulnerability in a specific neuronal subpopulation.

Sulfonylurea receptor 1 in central nervous system injury: a focused review

The sulfonylurea receptor 1 (Sur1)-regulated NC(Ca-ATP) channel is a nonselective cation channel that is regulated by intracellular calcium and adenosine triphosphate. The channel is not constitutively expressed, but is transcriptionally upregulated de novo in all cells of the neurovascular unit, in many forms of central nervous system (CNS) injury, including cerebral ischemia, traumatic brain injury (TBI), spinal cord injury (SCI), and subarachnoid hemorrhage (SAH). The channel is linked to microvascular dysfunction that manifests as edema formation and delayed secondary hemorrhage. Also implicated in oncotic cell swelling and oncotic (necrotic) cell death, the channel is a major molecular mechanism of 'accidental necrotic cell death' in the CNS. In animal models of SCI, pharmacological inhibition of Sur1 by glibenclamide, as well as gene suppression of Abcc8, prevents delayed capillary fragmentation and tissue necrosis. In models of stroke and TBI, glibenclamide ameliorates edema, secondary hemorrhage, and tissue damage. In a model of SAH, glibenclamide attenuates the inflammatory response due to extravasated blood. Clinical trials of an intravenous formulation of glibenclamide in TBI and stroke underscore the importance of recent advances in understanding the role of the Sur1-regulated NC(Ca-ATP) channel in acute ischemic, traumatic, and inflammatory injury to the CNS.

Hypoxia-inducible factor prolyl hydroxylase inhibition: robust new target or another big bust for stroke therapeutics?

A major challenge in developing stroke therapeutics that augment adaptive pathways to stress has been to identify targets that can activate compensatory programs without inducing or adding to the stress of injury. In this regard, hypoxia-inducible factor prolyl hydroxylases (HIF PHDs) are central gatekeepers of posttranscriptional and transcriptional adaptation to hypoxia, oxidative stress, and excitotoxicity. Indeed, some of the known salutary effects of putative 'antioxidant' iron chelators in ischemic and hemorrhagic stroke may derive from their abilities to inhibit this family of iron, 2-oxoglutarate, and oxygen-dependent enzymes. Evidence from a number of laboratories supports the notion that HIF PHD inhibition can improve histological and functional outcomes in ischemic and hemorrhagic stroke models. In this review, we discuss this evidence and highlight important gaps in our understanding that render HIF PHD inhibition a promising but not yet preclinically validated target for protection and repair after stroke.

Neurological diseases in relation to the blood-brain barrier

Disruption of the blood-brain barrier (BBB) has an important part in cellular damage in neurological diseases, including acute and chronic cerebral ischemia, brain trauma, multiple sclerosis, brain tumors, and brain infections. The neurovascular unit (NVU) forms the interface between the blood and brain tissues. During an injury, the cascade of molecular events ends in the final common pathway for BBB disruption by free radicals and proteases, which attack membranes and degrade the tight junction proteins in endothelial cells. Free radicals of oxygen and nitrogen and the proteases, matrix metalloproteinases and cyclooxgyenases, are important in the early and delayed BBB disruption as the neuroinflammatory response progresses. Opening of the BBB occurs in neurodegenerative diseases and contributes to the cognitive changes. In addition to the importance of the NVU in acute injury, angiogenesis contributes to the recovery process. The challenges to treatment of the brain diseases involve not only facilitating drug entry into the brain, but also understanding the timing of the molecular cascades to block the early NVU injury without interfering with recovery. This review will describe the molecular and cellular events associated with NVU disruption and potential strategies directed toward restoring its integrity.

Inhibition of MMP-9 by a selective gelatinase inhibitor protects neurovasculature from embolic focal cerebral ischemia

Background

Cerebral ischemia has been shown to induce activation of matrix metalloproteinases (MMPs), particularly MMP-9, which is associated with impairment of the neurovasculature, resulting in blood–brain barrier breakdown, hemorrhage and neurodegeneration. We previously reported that the thiirane inhibitor SB-3CT, which is selective for gelatinases (MMP-2 and −9), could antagonize neuronal apoptosis after transient focal cerebral ischemia.

Results

Here, we used a fibrin-rich clot to occlude the middle cerebral artery (MCA) and assessed the effects of SB-3CT on the neurovasculature. Results show that neurobehavioral deficits and infarct volumes induced by embolic ischemia are comparable to those induced by the filament-occluded transient MCA model. Confocal microscopy indicated embolus-blocked brain microvasculature and neuronal cell death. Post-ischemic SB-3CT treatment attenuated infarct volume, ameliorated neurobehavioral outcomes, and antagonized the increases in levels of proform and activated MMP-9. Embolic ischemia caused degradation of the neurovascular matrix component laminin and tight-junction protein ZO-1, contraction of pericytes, and loss of lectin-positive brain microvessels. Despite the presence of the embolus, SB-3CT mitigated these outcomes and reduced hemorrhagic volumes. Interestingly, SB-3CT treatment for seven days protected against neuronal laminin degradation and protected neurons from ischemic cell death.

Conclusion

These results demonstrate considerable promise for the thiirane class of selective gelatinase inhibitors as potential therapeutic agents in stroke therapy.

HIF-1-α and survivin involved in the anti-apoptotic effect of 2ME2 after global ischemia in rats

Survivin is an anti-apoptotic gene that decreases the apoptosis by depressing the expression of caspase-3. Hypoxia-inducible factor-1-alpha (HIF-1-alpha) is a transcription factor specifically activated by hypoxia. 2-methoxyestradiol (2ME2) is an estradiol derivative and a known HIF-1-alpha inhibitor. 2ME2 decreased apoptosis by inhibiting HIF-1-alpha. The aim of the present study was to investigate if survivin is involved in the anti-apoptotic effect of 2ME2. Male adult rats were used to make the global ischemia (GI) model. Ten minutes after GI, 2ME2 was injected intraperitoneally (16 mg/kg weight). Rats were killed at 6 hours, 12 hours, 24 hours, 48 hours, 96 hours, and 7 days. GI produced a marked increase in HIF-1-alpha expressions in the hippocampus at 6 hours and peaked at 48-96 hours. The expressions of survivin and caspase-3 were increased lightly in a similar time course. These molecular changes were accompanied by massive cell loss and apoptosis in the hippocampal regions. 2ME2 treatment reduced the expression of HIF-1-alpha, increased survivin expression, and decreased the expression of caspase-3. These results indicate that survivin and HIF-1-alpha were involved in the anti-apoptotic effect of 2ME2 treated following GI. 2ME2 may decrease the HIF-1-alpha expression, up-regulate the survivin expression, inhibit the expression of caspase-3, and finally reduce apoptosis after GI.

Systemic hypoxia differentially affects neurogenesis during early mouse brain maturation

BACKGROUND

Cerebral tissue oxygen level modifies crucial processes of neurogenesis, glial and neuronal development during physiological and hypoxic conditions. Whether hypoxia-sensitive factors such as doublecortin (DCX) and hypoxia-inducible transcription factor (HIF)-regulated CXCR4 and SDF-1 modify and activate adaptation to hypoxia in developing brain is not well understood. Present study investigated maturational regulation of oxygen-sensitive developmental genes and proteins in developing mouse brain in relation to the degree of hypoxia.

METHODS

Physiological expression of HIF-1, CXCR4, SDF-1 and DCX were analyzed in the brain of C57/BL6 mice (P0-P60). In addition, mice (P0, P7) were exposed to normoxia, acute (8% O(2), 6 h) or chronic hypoxia (10% O(2), 7 d) followed by reoxygenation. Gene expression was analyzed by quantitative PCR, proteins were quantified by Western blot analysis and immunohistochemistry.

RESULTS

Cerebral HIF-1α protein, CXCR4 and DCX mRNA levels showed maturational stage-related peak levels at P0/P1, whereas SDF-1 mRNA levels were highest at P17. CXCR4 and SDF-1 mRNA levels were not altered in response to hypoxia. Whereas DCX mRNA levels significantly increased during acute hypoxia, down-regulation of DCX transcripts was found in response to chronic hypoxia compared to controls, and these changes were related to specifically vulnerable brain regions.

CONCLUSIONS

Maturational stage-related dynamic changes of HIF-1α, CXCR4, SDF-1 and DCX may reflect involvement of hypoxia-regulated systems in important developmental regulatory processes of the developing brain. Extending the knowledge of differential effects of hypoxia on neurogenesis and dynamic regulatory networks present data provide a basis for future research on gestational age-specific neuroprotective options.

Ras family small GTPase-mediated neuroprotective signaling in stroke

Selective neuronal cell death is one of the major causes of neuronal damage following stroke, and cerebral cells naturally mobilize diverse survival signaling pathways to protect against ischemia. Importantly, therapeutic strategies designed to improve endogenous anti-apoptotic signaling appear to hold great promise in stroke treatment. While a variety of complex mechanisms have been implicated in the pathogenesis of stroke, the overall mechanisms governing the balance between cell survival and death are not well-defined. Ras family small GTPases are activated following ischemic insults, and in turn, serve as intrinsic switches to regulate neuronal survival and regeneration. Their ability to integrate diverse intracellular signal transduction pathways makes them critical regulators and potential therapeutic targets for neuronal recovery after stroke. This article highlights the contribution of Ras family GTPases to neuroprotective signaling cascades, including mitogen-activated protein kinase (MAPK) family protein kinase- and AKT/PKB-dependent signaling pathways as well as the regulation of cAMP response element binding (CREB), Forkhead box O (FoxO) and hypoxiainducible factor 1(HIF1) transcription factors, in stroke.

Nitric oxide modulates hypoxia-inducible factor-1 and poly(ADP-ribose) polymerase-1 cross talk in response to hypobaric hypoxia

The physiological response to hypobaric hypoxia represents a complex network of biochemical pathways in which the nitrergic system plays an important role. Previous studies have provided evidence for an interplay between the hypoxia-inducible factor-1 (HIF-1) and poly(ADP-ribose) polymerase-1 (PARP-1) under hypoxia. Here, we evaluate the potential involvement of nitric oxide (NO) in the cross talk between these two proteins. With this aim, we studied comparatively the effect of pharmacological inhibitors of NO production or PARP activity in the response of the mouse cerebral cortex to 4 h of exposure to a simulated altitude of 31,000 ft. Particularly, we analyzed the NO and reactive oxygen species production, the expression of NO synthase (NOS) isoforms, PARP-1 activity, HIF-1α expression and HIF-1 transcriptional activity, the protein level of the factor inhibiting HIF, and, finally, beclin-1 and fractin expression, as markers of cellular damage. Our results demonstrate that the reduction of NO level did not affect reactive oxygen species production but significantly 1) dampened the posthypoxic increase in neuronal NOS and inducible NOS expression without altering endothelial NOS protein level; 2) prevented PARP activation; 3) decreased HIF-1α response to hypoxia; 4) achieved a higher long-term HIF-1 transcriptional activity by reducing factor inhibiting HIF expression; and 5) reduced hypoxic damage. The pharmacological inhibition of PARP reproduced the NOS expression pattern and the HIF-1α response observed in NOS-inhibited mice, supporting its involvement in the NO-dependent regulation of hypoxia. As a whole, these results provide new data about the molecular mechanism underlying the beneficial effects of controlling NO production under hypobaric hypoxic conditions.

Sequential activation of hypoxia-inducible factor 1 and specificity protein 1 is required for hypoxia-induced transcriptional stimulation of Abcc8

Cerebral ischemia causes increased transcription of sulfonylurea receptor 1 (SUR1), which forms SUR1-regulated NC(Ca-ATP) channels linked to cerebral edema. We tested the hypothesis that hypoxia is an initial signal that stimulates transcription of Abcc8, the gene encoding SUR1, via activation of hypoxia-inducible factor 1 (HIF1). In the brain microvascular endothelial cells, hypoxia increased SUR1 abundance and expression of functional SUR1-regulated NC(Ca-ATP) channels. Luciferase reporter activity driven by the Abcc8 promoter was increased by hypoxia and by coexpression of HIF1α. Surprisingly, a series of luciferase reporter assays studying the Abcc8 promoter revealed that binding sites for specificity protein 1 (Sp1), but not for HIF, were required for stimulation of Abcc8 transcription by HIF1α. Luciferase reporter assays studying Sp1 promoters of three species, and chromatin immunoprecipitation analysis in rats after cerebral ischemia, indicated that HIF binds to HIF-binding sites on the Sp1 promoter to stimulate transcription of the Sp1 gene. We conclude that sequential activation of two transcription factors, HIF and Sp1, is required to stimulate transcription of Abcc8 following cerebral ischemia. Sequential gene activation in cerebral ischemia provides a plausible molecular explanation for the prolonged treatment window observed for inhibition of the end-target gene product, SUR1, by glibenclamide.

Ischemic tolerance in the brain: endogenous adaptive machinery against ischemic stress

Although more than 100 drugs have been examined clinically, tissue plasminogen activator remains the only drug approved for the treatment of acute ischemic stroke. Since the discovery of ischemic tolerance, it has been widely recognized that the brain possesses an endogenous protective machinery to protect against ischemic stress. Recent studies have clarified that both the upregulation of neuroprotective signaling and the downregulation of inflammatory or apoptotic pathways are involved equally in the acquisition of ischemic tolerance. The triggering stimuli for ischemic stresses are divided into hypoxic, oxidant/inflammatory, and glutamate stress. Glutamate stress, particularly the synaptic stimulation of the N-methyl-D-aspartate receptor, leads to activation of the cAMP response element-binding protein, which could subsequently induce gene expression of several neuroprotective molecules. Gene reprogramming and metabolic downregulation are intimately involved in ischemic tolerance as well as in hibernation and hypothermia. Micro-RNAs may be a key player for tuning the level of gene expression in ischemic tolerance. Future research should be performed to investigate the most effective combination for brain protection, enhancement of cell survival signaling, and inhibition of the inflammatory or apoptotic pathways.

Hypoxia inducible factor-1α is involved in the neurodegeneration induced by isoflurane in the brain of neonatal rats

More and more data show isoflurane, a commonly used volatile anesthetic has dual effects on neuron fate. However, the underlying mechanisms that can explain the apparent paradox are poorly understood. Hypoxia inducible factor (HIF)-1α, a transcription factor, has been found regulating both prosurvival and prodeath pathways in the CNS. Previously, we found that isoflurane can activate HIF-1α under normoxic conditions in vitro and HIF-1α has been found to be involved in the pre-conditioning effect of isoflurane in various organs. Here, we investigated whether HIF-1α is a contributing factor in the neurodegenration in rodent primary cultured neurons and in developing rat brain. Isoflurane dose-dependently induced apoptotic neurodegeneration in neonatal rats as assessed by S100β, cleaved caspase 3 and poly-(ADP-ribose) polymerase (PARP), respectively. Notably, isoflurane up-regulates HIF-1α protein levels in vivo and in vitro during induction of neurodegeneration. Likewise, isoflurane resulted in a significant elevation of cytosonic calcium levels in neuron cultures. Furthermore, knockdown of HIF-1α expression in cultured neurons attenuated isoflurane-induced neurotoxicity. Finally, Morris water maze (MWM) test showed neonatal exposure to isoflurane impaired juvenile learning and memory ability in rats. These findings indicate that HIF-1α is involved in the neurodegeneration induced by isoflurane in the brain of neonatal rats, suggesting HIF-1α may be a candidate for the dual effects of isoflurane on neuron fate.

HIF-1α is neuroprotective during the early phases of mild hypoxia in rat cortical neurons

Hypoxia-inducible factor 1α (HIF-1α) is a transcription factor that plays a key role in regulating the adaptive response to hypoxia. HIF-1α is stabilised during hypoxia and, after dimerisation with hypoxia-inducible factor 1β (HIF-1β), triggers the expression of various genes involved in cell cycle control and energy metabolism associated with cell survival. However, HIF-1α also regulates the expression of proapoptotic genes. The aim of this study was to ascertain the influence of HIF-1α on neurotoxicity evoked by hypoxia in rat cortical neurons. We found that mild hypoxia induces time-dependent neuronal death involving free radical production, mitochondrial depolarisation, cytochrome c release and caspase-3 activation. Lentivirus-mediated HIF-1α knockdown markedly strengthened all of these effects during the initial 24h of hypoxia, which suggests that HIF-1α plays a neuroprotective role in hypoxia-mediated neuronal death. After this initial period, the protective actions of HIF-1α disappeared over the course of the hypoxia-mediated HIF-1α stabilisation. Moreover, lentiviral-mediated overexpression of HIF-1α increased lactate dehydrogenase (LDH) A, one of the target genes for HIF-1α, but did not show protective actions on hypoxia-mediated neuronal death, indicating that the level of endogenous HIF-1α stabilisation achieved during hypoxia was already the maximum required for HIF-1α transcription activities. These results indicate that HIF-1α is neuroprotective in the early phases of hypoxia.

Cell-autonomous and non-cell-autonomous neuroprotective functions of RORα in neurons and astrocytes during hypoxia

There is increasing evidence to suggest that the neuronal response to hypoxia is regulated through their interactions with astrocytes. However, the hypoxia-induced molecular mechanisms within astrocytes which influence neuronal death have yet to be characterized. In this study, we investigated the roles of the nuclear receptor RORα (retinoid-related orphan receptor-α) respectively in neurons and astrocytes during hypoxia using cultures and cocultures of neurons and astrocytes obtained from RORα-deficient mice. We found that loss of RORα function in neuronal cultures increases neuronal death after hypoxia, suggesting a cell-autonomous neuroprotective effect of RORα. Moreover, wild-type neurons cocultured with RORα-deficient astrocytes are characterized by a higher death rate after hypoxia than neurons cocultured with wild-type astrocytes, suggesting that RORα also has a non-cell-autonomous action. By using cocultures of neurons and astrocytes of different genotypes, we showed that this neuroprotective effect of RORα in astrocytes is additive to its effect in neurons, and is mediated in part by cell-to-cell interactions between neurons and astrocytes. We also found that RORα is upregulated by hypoxia in both neurons and astrocytes. Furthermore, our data showed that RORα does not alter oxidative mechanisms during hypoxia but regulates hypoxic inducible factor 1α (HIF-1α) expression, a major regulator of hypoxia sensing, in a cell-specific manner. Indeed, the neuroprotective function of RORα in astrocytes correlates with a downregulation of HIF-1α selectively in these cells. Altogether, our results show that RORα is a key molecular player in hypoxia, protecting neurons through its dual action in neurons and astrocytes.

Differential effects of HIF-1 inhibition by YC-1 on the overall outcome and blood-brain barrier damage in a rat model of ischemic stroke

Hypoxia-inducible factor 1 (HIF-1) is a master regulator of cellular adaptation to hypoxia and has been suggested as a potent therapeutic target in cerebral ischemia. Here we show in an ischemic stroke model of rats that inhibiting HIF-1 and its downstream genes by 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) significantly increases mortality and enlarges infarct volume evaluated by MRI and histological staining. Interestingly, the HIF-1 inhibition remarkably ameliorates ischemia-induced blood-brain barrier (BBB) disruption determined by Evans blue leakage although it does not affect brain edema. The result demonstrates that HIF-1 inhibition has differential effects on ischemic outcomes and BBB permeability. It indicates that HIF-1 may have different functions in different brain cells. Further analyses show that ischemia upregulates HIF-1 and its downstream genes erythropoietin (EPO), vascular endothelial growth factor (VEGF), and glucose transporter (Glut) in neurons and brain endothelial cells and that YC-1 inhibits their expression. We postulate that HIF-1-induced VEGF increases BBB permeability while certain other proteins coded by HIF-1's downstream genes such as epo and glut provide neuroprotection in an ischemic brain. The results indicate that YC-1 lacks the potential as a cerebral ischemic treatment although it confers certain protection to the cerebral vascular system.

Oxygen Sensing, Cardiac Ischemia, HIF-1α and Some Emerging Concepts

Oxygen plays a critical role in the perpetuation and propagation of almost all forms of life. The primary site of cellular oxygen consumption is the mitochondrial electron transport chain and in addition, oxygen is also used as a substrate for various enzymes involved in cellular homeostasis. Although our knowledge of the biochemistry and physiology of oxygen transport is century old, recent development of sophisticated tools of biophysical chemistry revealed that tissue oxygenation and oxygen sensing is a highly evolved process, especially in mammals. Perturbation of normal oxygen supply is associated with diseases like tumorigenesis, myocardial infarction and stroke. Available information suggests that when tissue oxygen supply is limited, mitochondria emanate signals involving reactive oxygen species generation which in turn stabilizes oxygen sensing transcription factor HIF-1. Upon stabilization, HIF-1 elicits necessary genetic response to cope with the diminished oxygen level. In view of such critical role of HIF-1 in cellular oxygen sensing, recently there has been a heightened interest in understanding the biology of HIF-1 in the context of cardiovascular system. The following review describes some of the recent advances in this regard.

Inhibition of prolyl hydroxylases by dimethyloxaloylglycine after stroke reduces ischemic brain injury and requires hypoxia inducible factor-1α

Pathological oxygen deprivation inhibits prolyl hydroxylase (PHD) activity and stimulates a protective cellular oxygen-sensing response in part through the stabilization and activation of the Hypoxia Inducible Factor (HIF) 1α transcription factor. The present investigation tested the therapeutic potential of enhanced activation of oxygen-sensing pathways by competitive pharmacologic PHD inhibition after stroke, hypothesizing that post-ischemic PHD inhibition would reduce neuronal cell death and require the activation of HIF-1α. The PHD inhibitor dimethyloxaloylglycine (DMOG, 100 μM) reduced cell death by oxygen glucose deprivation (OGD), an in vitro model of ischemia, and the protection required HIF-1α. In vivo, DMOG (50 mg/kg, i.p.) administered 30 or 60 min after distal occlusion of the middle cerebral artery (MCA) in mice enhanced the activation of HIF-1α protein, enhanced transcription of the HIF-regulated genes vascular endothelial growth factor, erythropoietin, endothelial nitric oxide synthase, and pyruvate dehydrogenase kinase-1, reduced ischemic infarct volume and activation of the pro-apoptotic caspase-3 protein, reduced behavioral deficits after stroke, and reduced the loss of local blood flow in the MCA territory after stroke. Inhibition of HIF-1α in vivo by Digoxin or Acriflavine abrogated the infarct sparing properties of DMOG. These data suggest that supplemental activation of oxygen-sensing pathways after stroke may provide a clinically applicable intervention for the promotion of neurovascular cell survival after ischemia.

Regional genome transcriptional response of adult mouse brain to hypoxia

BACKGROUND

Since normal brain function depends upon continuous oxygen delivery and short periods of hypoxia can precondition the brain against subsequent ischemia, this study examined the effects of brief hypoxia on the whole genome transcriptional response in adult mouse brain.

RESULT

Pronounced changes of gene expression occurred after 3 hours of hypoxia (8% O(2)) and after 1 hour of re-oxygenation in all brain regions. The hypoxia-responsive genes were predominantly up-regulated in hindbrain and predominantly down-regulated in forebrain - possibly to support hindbrain survival functions at the expense of forebrain cognitive functions. The up-regulated genes had a significant role in cell survival and involved both shared and unshared signaling pathways among different brain regions. Up-regulation of transcriptional signaling including hypoxia inducible factor, insulin growth factor (IGF), the vitamin D3 receptor/retinoid X nuclear receptor, and glucocorticoid signaling was common to many brain regions. However, many of the hypoxia-regulated target genes were specific for one or a few brain regions. Cerebellum, for example, had 1241 transcripts regulated by hypoxia only in cerebellum but not in hippocampus; and, 642 (54%) had at least one hepatic nuclear receptor 4A (HNF4A) binding site and 381 had at least two HNF4A binding sites in their promoters. The data point to HNF4A as a major hypoxia-responsive transcription factor in cerebellum in addition to its known role in regulating erythropoietin transcription. The genes unique to hindbrain may play critical roles in survival during hypoxia.

CONCLUSION

Differences of forebrain and hindbrain hypoxia-responsive genes may relate to suppression of forebrain cognitive functions and activation of hindbrain survival functions, which may coordinately mediate the neuroprotection afforded by hypoxia preconditioning.

Neuroprotection by hypoxic preconditioning involves upregulation of hypoxia-inducible factor-1 in a prenatal model of acute hypoxia

The molecular pathways underlying the neuroprotective effects of preconditioning are promising, potentially drugable targets to promote cell survival. However, these pathways are complex and are not yet fully understood. In this study we have established a paradigm of hypoxic preconditioning based on a chick embryo model of normobaric acute hypoxia previously developed by our group. With this model, we analyzed the role of hypoxia-inducible factor-1α (HIF-1α) stabilization during preconditioning in HIF-1 signaling after the hypoxic injury and in the development of a neuroprotective effect against the insult. To this end, we used a pharmacological approach, based on the in vivo administration of positive (Fe(2+), ascorbate) and negative (CoCl(2)) modulators of the activity of HIF-prolyl hydroxylases (PHDs), the main regulators of HIF-1. We have found that preconditioning has a reinforcing effect on HIF-1 accumulation during the subsequent hypoxic injury. In addition, we have also demonstrated that HIF-1 induction during hypoxic preconditioning is necessary to obtain an enhancement in HIF-1 accumulation and to develop a tolerance against a subsequent hypoxic injury. We provide in vivo evidence that administration of Fe(2+) and ascorbate modulates HIF accumulation, suggesting that PHDs might be targets for neuroprotection in the CNS.

PACAP and a novel stable analog protect rat brain from ischemia: Insight into the mechanisms of action

Pituitary adenylate cyclase-activating polypeptide (PACAP) shows potent protective effects in numerous models of neurological insults. However, the use of PACAP as a clinically efficient drug is limited by its poor metabolic stability. By combining identification of enzymatic cleavage sites with targeted chemical modifications, a metabolically stable and potent PACAP38 analog was recently developed. The neuroprotective activity of this novel compound was for the first time evaluated and compared to the native peptide using a rat model of middle cerebral artery occlusion (MCAO). Our results show that as low as picomolar doses of PACAP38 and its analog strongly reduce infarct volume and improve neurological impairment induced by stroke. In particular, these peptides inhibit the expression of Bcl-2-associated death promoter, caspase 3, macrophage inflammatory protein-1α, inducible nitric oxide synthase 2, tumor necrosis factor-α mRNAs, and increase extracellular signal-regulated kinase 2, B-cell CLL/lymphoma 2 and interleukin 6 mRNA levels. These results indicate that the neuroprotective effect of PACAP after MCAO is not only due to its ability to inhibit apoptosis but also to modulate the inflammatory response. The present study highlights the potential therapeutic efficacy of very low concentrations of PACAP or its metabolically stable derivative for the treatment of stroke.

Adaptation to moderate hypoxia protects cortical neurons against ischemia-reperfusion injury and excitotoxicity independently of HIF-1α

Continuous exposure of cultured cortical neurons to moderate hypoxia (1% O(2)) elevates cellular accumulation of hypoxia-inducible factor-1α (HIF-1α) and improves basal survival of cultured cortical neurons. We examined the effects of adaptation to moderate hypoxia on the vulnerability of cultured neurons to the acute injury of simulated ischemia-reperfusion. Cortical neurons cultured continuously in 1% O(2) were markedly protected against simulated ischemia-reperfusion, with protection persisting through 72h after ischemia. Neurons from 1% O(2) conditions were also highly resistant to glutamate-induced NMDA receptor-dependent excitotoxic injury, despite expression of NMDA receptors at levels not significantly changed from controls. Inhibition of prolyl hydroxylase, mimicking cellular signaling effects of hypoxia including HIF-1α stabilization, also protected neurons against simulated ischemia-reperfusion injury. Nevertheless, genetic deletion of HIF-1α expression did not diminish the protection of neurons adapted to 1% O(2) from excitotoxicity or ischemia-reperfusion injury, nor did it prevent the protective effect of prolyl hydroxylase inhibition. We conclude that chronic exposure to moderate hypoxia, through HIF-1α-independent mechanisms, produces strong protective effects against excitotoxic and ischemia-reperfusion related injury.

Advances in experimental subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) remains to be a devastating disease with high mortality and morbidity. Two major areas are becoming the focus of the research interest of SAH: these are cerebral vasospasm (CVS) and early brain injury (EBI). This mini review will provide a broad summary of the major advances in experimental SAH during the last 3 years. Treatments interfering with nitric oxide (NO)- or endothelin-pathways continue to show antispasmotic effects in experimental SAH. HIF 1 may play both a detrimental and beneficial role in the setting of SAH, depending on its activation stage. Inflammation and oxidative stress contribute to the pathophysiology of both CVS and EBI. Apoptosis, a major component of EBI after SAH, also underlie the etiology of CVS. Since we recognize now that CVS and EBI are the two major contributors to the significant mortality and morbidity associated with SAH, ongoing research will continue to elucidate the underlying pathophysiological pathways and treatment strategies targeting both CVS and EBI may be more successful and improve outcome of patients with SAH.

Hypoxia-inducible factors and the response to hypoxic stress

Oxygen (O(2)) is an essential nutrient that serves as a key substrate in cellular metabolism and bioenergetics. In a variety of physiological and pathological states, organisms encounter insufficient O(2) availability, or hypoxia. In order to cope with this stress, evolutionarily conserved responses are engaged. In mammals, the primary transcriptional response to hypoxic stress is mediated by the hypoxia-inducible factors (HIFs). While canonically regulated by prolyl hydroxylase domain-containing enzymes (PHDs), the HIFα subunits are intricately responsive to numerous other factors, including factor-inhibiting HIF1α (FIH1), sirtuins, and metabolites. These transcription factors function in normal tissue homeostasis and impinge on critical aspects of disease progression and recovery. Insights from basic HIF biology are being translated into pharmaceuticals targeting the HIF pathway.

Late-gestational systemic hypoxia leads to a similar early gene response in mouse placenta and developing brain

Late-gestational intrauterine hypoxia represents a well-known risk factor of acquired perinatal brain injury. Cell type and age-specific sensitivity of hypoxia-responsive genes to low-oxygen partial pressure is to be considered in the screening for early indicators of fetoplacental tissue hypoxia. To identify early hypoxia-induced alterations in gene expression during late-gestational hypoxia (6% O2, 6 h; gestational day 20) we compared primary mouse placenta and brain transcriptomes using high-density oligonucleotide microarrays. Upregulation of candidate marker genes for hypoxia was confirmed by quantitative RT-PCR and immunohistochemistry. Both developing brain and placenta were highly responsive to systemic hypoxia at the level of gene expression involving hypoxia-inducible transcription factor (HIF)-dependent genes and immediate early genes (IEG) (Fos, Jun, Egr1, Bhlhb2), apoptosis-promoting factors (Bnip3, Dusp1, Ier3) that were all upregulated, and genes modulating RNA binding and translation (Rbm3, Thap2, Lig4, Rbm12b) that mainly were downregulated. Functional activity of the HIF system was obvious from elevated expression of various known HIF target genes (Adm, Vegf, Hk2, Pdk1, Bnip3, Ier3, Dusp-1), indicating immediate availability among early response to acute hypoxia. In addition, genes not yet described as being hypoxia related were identified that are involved in angiogenesis/cell differentiation (Gna13, Gab2), mRNA processing, and embryonic development. RT-PCR of placenta and brain tissues confirmed upregulation of selected HIF target genes and IEG. These data indicate that the early hypoxia-induced genomic response of the placenta mirrors that of developing brain in a temporally parallel manner. Our observations implicate future diagnostic options to identify fetal and cerebral tissue hypoxia.

Lipocalin-7 is a matricellular regulator of angiogenesis

BACKGROUND

Matricellular proteins are extracellular regulators of cellular adhesion, signaling and performing a variety of physiological behaviors such as proliferation, migration and differentiation. Within vascular microenvironments, matricellular proteins exert both positive and negative regulatory cues to vascular endothelium. The relative balance of these matricellular cues is believed to be critical for vascular homeostasis, angiogenesis activation or angiogenesis resolution. However, our knowledge of matricellular proteins within vascular microenvironments and the mechanisms by which these proteins impact vascular function remain largely undefined. The matricellular protein lipocalin-7 (LCN7) is found throughout vascular microenvironments, and circumstantial evidence suggests that LCN7 may be an important regulator of angiogenesis. Therefore, we hypothesized that LCN7 may be an important regulator of vascular function.

METHODOLOGY AND PRINCIPAL FINDINGS

To test this hypothesis, we examined the effect of LCN7 overexpression, recombinant protein and gene knockdown in a series of in vitro and in vivo models of angiogenesis. We found that overexpression of LCN7 in MB114 and SVEC murine endothelial cell lines or administration of highly purified recombinant LCN7 protein increased endothelial cell invasion. Similarly, LCN7 increased angiogenic sprouting from quiescent endothelial cell monolayers and ex vivo aortic rings. Moreover, LCN7 increased endothelial cell sensitivity to TGF-β but did not affect sensitivity to other pro-angiogenic growth factors including bFGF and VEGF. Finally, morpholino based knockdown of LCN7 in zebrafish embryos specifically inhibited angiogenic sprouting but did not affect vasculogenesis within injected embryos.

CONCLUSIONS AND SIGNIFICANCE

No functional analysis has previously been performed to elucidate the function of LCN7 in vascular or other cellular processes. Collectively, our results show for the first time that LCN7 is an important pro-angiogenic matricellular protein of vascular microenvironments.

Selective inhibition of early--but not late--expressed HIF-1α is neuroprotective in rats after focal ischemic brain damage

The expression of hypoxia-inducible factor-1-alpha (HIF-1α) is upregulated in ischemic stroke, but its function is still unclear. In the present study, biphasic expression of HIF-1α was observed during 1-12 h and after 48 h in neurons exposed to ischemic stress in vitro and in vivo. Treating neurons with 2-methoxyestradiol (2ME2) 0.5 h after ischemic stress or pre-silencing HIF-1α with small interfering RNA (siRNA) decreased brain injury, brain edema and number of apoptotic cell, and downregulates Nip-like protein X (Nix) expression. Conversely, applying 2ME2 to neurons 8 h after ischemic stress or silencing the HIF-1α with siRNA 12 h after oxygen-glucose deprivation (OGD) increased neuron damage and decreased vascular endothelial growth factor (VEGF) expression. Taken together, these results demonstrate that HIF-1α induced by ischemia in early and late times leads cellular apoptosis and survival, respectively, and provides a new insight into the divergent roles of HIF-1α expression in neurons after ischemic stroke.

HIF-1 mediates pathogenic inflammatory responses to intestinal ischemia-reperfusion injury

Acute lung injury (ALI) and the development of the multiple organ dysfunction syndrome (MODS) are major causes of death in trauma patients. Gut inflammation and loss of gut barrier function as a consequence of splanchnic ischemia-reperfusion (I/R) have been implicated as the initial triggering events that contribute to the development of the systemic inflammatory response, ALI, and MODS. Since hypoxia-inducible factor (HIF-1) is a key regulator of the physiological and pathophysiological response to hypoxia, we asked whether HIF-1 plays a proximal role in the induction of gut injury and subsequent lung injury. Utilizing partially HIF-1α-deficient mice in a global trauma hemorrhagic shock (T/HS) model, we found that HIF-1 activation was necessary for the development of gut injury and that the prevention of gut injury was associated with an abrogation of lung injury. Specifically, in vivo studies demonstrated that partial HIF-1α deficiency ameliorated T/HS-induced increases in intestinal permeability, bacterial translocation, and caspase-3 activation. Lastly, partial HIF-1α deficiency reduced TNF-α, IL-1β, cyclooxygenase-2, and inducible nitric oxide synthase levels in the ileal mucosa after T/HS whereas IL-1β mRNA levels were reduced in the lung after T/HS. This study indicates that prolonged intestinal HIF-1 activation is a proximal regulator of I/R-induced gut mucosal injury and gut-induced lung injury. Consequently, these results provide unique information on the initiating events in trauma-hemorrhagic shock-induced ALI and MODS as well as potential therapeutic insights.

The science of stroke: mechanisms in search of treatments

This review focuses on mechanisms and emerging concepts that drive the science of stroke in a therapeutic direction. Once considered exclusively a disorder of blood vessels, growing evidence has led to the realization that the biological processes underlying stroke are driven by the interaction of neurons, glia, vascular cells, and matrix components, which actively participate in mechanisms of tissue injury and repair. As new targets are identified, new opportunities emerge that build on an appreciation of acute cellular events acting in a broader context of ongoing destructive, protective, and reparative processes. The burden of disease is great, and its magnitude widens as a role for blood vessels and stroke in vascular and nonvascular dementias becomes more clearly established. This review then poses a number of fundamental questions, the answers to which may generate new directions for research and possibly new treatments that could reduce the impact of this enormous economic and societal burden.

Inflammation and brain injury: acute cerebral ischaemia, peripheral and central inflammation

Inflammation is a classical host defence response to infection and injury that has many beneficial effects. However, inappropriate (in time, place and magnitude) inflammation is increasingly implicated in diverse disease states, now including cancer, diabetes, obesity, atherosclerosis, heart disease and, most relevant here, CNS disease. A growing literature shows strong correlations between inflammatory status and the risk of cerebral ischaemia (CI, most commonly stroke), as well as with outcome from an ischaemic event. Intervention studies to demonstrate a causal link between inflammation and CI (or its consequences) are limited but are beginning to emerge, while experimental studies of CI have provided direct evidence that key inflammatory mediators (cytokines, chemokines and inflammatory cells) contribute directly to ischaemic brain injury. However, it remains to be determined what the relative importance of systemic (largely peripheral) versus CNS inflammation is in CI. Animal models in which CI is driven by a CNS intervention may not accurately reflect the clinical condition; stroke being typically induced by atherosclerosis or cardiac dysfunction, and hence current experimental paradigms may underestimate the contribution of peripheral inflammation. Experimental studies have already identified a number of potential anti-inflammatory therapeutic interventions that may limit ischaemic brain damage, some of which have been tested in early clinical trials with potentially promising results. However, a greater understanding of the contribution of inflammation to CI is still required, and this review highlights some of the key mechanism that may offer future therapeutic targets.

Transglutaminase 2 protects against ischemic stroke

Transglutaminase 2 (TG2) is a multifunctional protein that modulates cell survival and death pathways. It is upregulated in numerous ischemic models, and protects primary neurons from oxygen and glucose deprivation. TG2 binds to the hypoxia inducible factor (HIF) 1beta and decreases the upregulation of hypoxic-induced proapoptotic genes. To investigate the role of TG2 in ischemic stroke in vivo, we used the murine, permanent middle cerebral artery (MCA) ligation model. TG2 mRNA levels are increased after MCA ligations, and transgenic mice that express human TG2 in neurons had significantly smaller infarct volumes than wild type littermates. Further, TG2 translocates into the nucleus within 2h post ligation. Nuclear-localized TG2 is also apparent in human stroke cases. TG2 suppressed the upregulation of the HIF-induced, proapoptotic gene, Noxa. The findings of this study indicate that TG2 plays a role in attenuating ischemic-induced cell death possibly by modulating hypoxic-induced transcriptional processes.

Extended ischemia prevents HIF1alpha degradation at reoxygenation by impairing prolyl-hydroxylation: role of Krebs cycle metabolites

Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor that activates the cellular response to hypoxia. The HIF1alpha subunit is constantly synthesized and degraded under normoxia, but degradation is rapidly inhibited when oxygen levels drop. Oxygen-dependent hydroxylation by prolyl-4-hydroxylases (PHD) mediates HIF1alpha proteasome degradation. Brain ischemia limits the availability not only of oxygen but also of glucose. We hypothesized that this circumstance could have a modulating effect on HIF. We assessed the separate involvement of oxygen and glucose in HIF1alpha regulation in differentiated neuroblastoma cells subjected to ischemia. We report higher transcriptional activity and HIF1alpha expression under oxygen deprivation in the presence of glucose (OD), than in its absence (oxygen and glucose deprivation, OGD). Unexpectedly, HIF1alpha was not degraded at reoxygenation after an episode of OGD. This was not due to impairment of proteasome function, but was associated with lower HIF1alpha hydroxylation. Krebs cycle metabolites fumarate and succinate are known inhibitors of PHD, while alpha-ketoglutarate is a co-substrate of the reaction. Lack of HIF1alpha degradation in the presence of oxygen was accompanied by a very low alpha-ketoglutarate/fumarate ratio. Furthermore, treatment with a fumarate analogue prevented HIF1alpha degradation under normoxia. In all, our data suggest that postischemic metabolic alterations in Krebs cycle metabolites impair HIF1alpha degradation in the presence of oxygen by decreasing its hydroxylation, and highlight the involvement of metabolic pathways in HIF1alpha regulation besides the well known effects of oxygen.

Prolyl hydroxylase domain inhibitors: a route to HIF activation and neuroprotection

Abstract Ischemic stroke is a major cause of death worldwide, and current therapeutic options are very limited. Preconditioning with an ischemic or hypoxic insult is beneficial in experimental models of ischemic stroke. Ischemia/hypoxia results in activation of numerous transcription factors, including hypoxia inducible factor (HIF), which is a master regulator of oxygen homeostasis. HIF activation induces a diverse range of target genes, encompassing a wide variety of cellular processes; including angiogenesis, energy metabolism, cell survival, radical production/scavenging, iron metabolism, stem cell homing, and differentiation. Inhibition of HIF prolyl hydroxylase domain (PHD) enzymes results in activation of HIF and is likely to mimic, at least in part, the effects of hypoxia preconditioning. A caveat is that not all consequences of HIF activation will be beneficial and some could even be deleterious. Nevertheless, PHD inhibitors may be therapeutically useful in the treatment of stroke. Prototype PHD inhibitors have shown promising results in preclinical models.

Hypoxia inducible factor 1alpha signaling in fractionated radiation-induced lung injury: role of oxidative stress and tissue hypoxia

To investigate the relationship of HIF1alpha signaling to oxidative stress, tissue hypoxia, angiogenesis and inflammation, female Fischer 344 rats were irradiated to the right hemithorax with a fractionated dose of 40 Gy (8 Gy x 5 days). The lung tissues were harvested before and at 4, 6, 10, 14, 18, 22 and 26 weeks after irradiation for serial studies of biological markers, including markers for hypoxia (HIF1alpha, pimonidazole and CA IX), oxidative stress (8-OHdG), and angiogenesis/capillary proliferation (VEGF/CD 105), as well as macrophage activation (ED-1) and cell signaling/fibrosis (NFkappaB, TGFbeta1), using immunohistochemistry and Western blot analysis. HIF1alpha staining could be observed as early as 4 weeks postirradiation and was significantly increased with time after irradiation. Importantly, HIF1alpha levels paralleled oxidative stress (8-OHdG), tissue hypoxia (pimonidazole and CA IX), and macrophage accumulation consistent with inflammatory response. Moreover, changes in HIF1alpha expression identified by immunohistochemistry assay parallel the changes in TGFbeta1, VEGF, NFkappaB and CD 105 levels in irradiated lungs. These results support the notion that oxidative stress and tissue hypoxia might serve as triggering signals for HIF1alpha activity in irradiated lungs, relating to radiation-induced inflammation, angiogenesis and fibrosis.

Protection by D609 through cell-cycle regulation after stroke

Expressions of cell-cycle regulating proteins are altered after stroke. Cell-cycle inhibition has shown dramatic reduction in infarction after stroke. Ceramide can induce cell-cycle arrest by up-regulation of cyclin-dependent kinase (Cdk) inhibitors p21 and p27 through activation of protein phosphatase 2A (PP2A). Tricyclodecan-9-yl-xanthogenate (D609)-increased ceramide levels after transient middle cerebral artery occlusion (tMCAO) in spontaneously hypertensive rat (SHR) probably by inhibiting sphingomyelin synthase (SMS). D609 significantly reduced cerebral infarction and up-regulated Cdk inhibitor p21 and down-regulated phospho-retinoblastoma (pRb) expression after tMCAO in rat. Others have suggested bFGF-induced astrocyte proliferation is attenuated by D609 due to an increase in ceramide by SMS inhibition. D609 also reduced the formation of oxidized phosphatidylcholine (OxPC) protein adducts. D609 may attenuate generation of reactive oxygen species and formation of OxPC by inhibiting microglia/macrophage proliferation after tMCAO (please also see note added in proof: D609 may prevent mature neurons from entering the cell cycle at the early reperfusion, however may not interfere with later proliferation of microglia/ macrophages that are the source of brain derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF-1) in offering protection). It has been proposed that D609 provides benefit after tMCAO by attenuating hypoxia-inducible factor-1alpha and Bcl2/adenovirus E1B 19 kDa interacting protein 3 expressions. Our data suggest that D609 provides benefit after stoke through inhibition of SMS, increased ceramide levels, and induction of cell-cycle arrest by up-regulating p21 and causing hypophosphorylation of Rb (through increased protein phosphatase activity and/or Cdk inhibition).

Preconditioning protects against oxidative injury involving hypoxia-inducible factor-1 and vascular endothelial growth factor in cultured astrocytes

Tolerance to brain injury involves hypoxia-inducible factor-1 (HIF-1) and its target genes as the key pathway mediating a cascade of events including cell survival, energetics, and angiogenesis. In this study, we established the treatment paradigms for an in vitro model of tolerance to oxidative injury in primary astrocytic cultures and further examined the roles for the HIF-1 signalling cascade. Isolated murine astrocytes were preconditioned with sub-toxic concentrations of HIF-1 inducers and subsequently exposed to a H(2)O(2) insult, where changes in cell viability and protein expression were determined. Preconditioning with non-damaging concentrations of desferrioxamine (DFO) and ethyl-3,4-dihydroxybenzoate (EDHB) significantly improved cellular viability after H(2)O(2) injury treatment. Time course studies revealed that DFO and EDHB treatments alone induced sequential activation of HIF-1 signal transduction where nuclear HIF-1alpha protein accumulation was detected as early as 2h, followed by downstream upregulation of intracellular and released VEGF from 4h and 8h onwards, respectively. The protective effects of DFO and EDHB preconditioning against H(2)O(2) injury were abolished by co-treatment with cycloheximide, an inhibitor of protein synthesis. Importantly, when the anti-HIF-1 compound, 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) was used, the cytoprotection and VEGF accumulation produced by DFO and EDHB preconditioning were diminished. These results indicate the essential role of the HIF-1 pathway in our model of tolerance against oxidative injury in cultured astrocytes, and suggest roles for astrocytic HIF-1 expression and VEGF release which may influence the function of surrounding cells and vasculature during oxidative stress-related brain diseases.