Hypoxia-inducible factor-1α has a key role in hypoxic preconditioning

Neuroprotective Efficacy of Betulinic Acid Hydroxamate, a B55α/PP2A Activator, in Acute Hypoxia-Ischemia-Induced Brain Damage in Newborn Rats

There is an increasing evidence of the neuroprotective effects of hypoxia inducing factor prolyl-hydroxylase inhibitors (HIF-PHDi) after hypoxic-ischemic (HI) brain damage (HIBD). We studied the neuroprotective effects of betulinic hydroxamate (BAH), a novel B55α/PP2A activator that dephosphorylates and inhibits PHD2 activity, in a rat model of neonatal HIBD. Seven-day-old (P7) Wistar rats were exposed to hypoxia after left carotid artery electrocoagulation and then received vehicle (HI + VEH) or BAH 3 mg/kg i.p. 30 min post-insult. Brain damage was assessed by magnetic resonance imaging (MRI) and neurobehavioral studies testing motor and cognitive performance at P14 and P37, as well as immunohistochemical studies (TUNEL and myelin basic protein (MBP) signal) at P37. Mechanisms of damage were assessed at P14 determining excitotoxicity (glutamate/N-acetylaspartate ratio by H⁺-magnetic resonance spectroscopy), oxidative stress (protein nitrosylation by Oxyblot), and inflammation (cytokine and chemokine concentration). BAH reduced brain damage volume and cell death, preventing the development of motor and working memory deficits. BAH showed a robust protective effect on myelination, restoring MBP expression at P37. BAH modulated excitotoxicity, oxidative stress, and inflammation. Most neuroprotective effects were still present despite BAH administration was delayed for 12 h, whereas beneficial effects on motor strength at P14 and on cell death and myelination at P37 were preserved even when BAH administration was delayed for 24 h. In conclusion, BAH appears as an effective neuroprotective treatment for neonatal HIBD in a manner associated with the modulation of excitotoxicity, oxidative stress, and inflammation, showing a broad therapeutic window.

Aged Mouse Hippocampus Exhibits Signs of Chronic Hypoxia and an Impaired HIF-Controlled Response to Acute Hypoxic Exposures

Altered hypoxia-inducible factor-alpha (HIF-α) activity may have significant consequences in the hippocampus, which mediates declarative memory, has limited vascularization, and is vulnerable to hypoxic insults. Previous studies have reported that neurovascular coupling is reduced in aged brains and that diseases which cause hypoxia increase with age, which may render the hippocampus susceptible to acute hypoxia. Most studies have investigated the actions of HIF-α in aging cortical structures, but few have focused on the role of HIF-α within aged hippocampus. This study tests the hypothesis that aging is associated with impaired hippocampal HIF-α activity. Dorsal hippocampal sections from mice aged 3, 9, 18, and 24 months were probed for the presence of HIF-α isoforms or their associated gene products using immunohistochemistry and fluorescent in situ hybridization (fISH). A subset of each age was exposed to acute hypoxia (8% oxygen) for 3 h to investigate changes in the responsiveness of HIF-α to hypoxia. Basal mean intensity of fluorescently labeled HIF-1α protein increases with age in the hippocampus, whereas HIF-2α intensity only increases in the 24-month group. Acute hypoxic elevation of HIF-1α is lost with aging and is reversed in the 24-month group. fISH reveals that glycolytic genes induced by HIF-1α (lactose dehydrogenase-a, phosphoglycerate kinase 1, and pyruvate dehydrogenase kinase 1) are lower in aged hippocampus than in 3-month hippocampus, and mRNA for monocarboxylate transporter 1, a lactose transporter, increases. These results indicate that lactate, used in neurotransmission, may be limited in aged hippocampus, concurrent with impaired HIF-α response to hypoxic events. Therefore, impaired HIF-α may contribute to age-associated cognitive decline during hypoxic events.

Translational insights into stem cell preconditioning: From molecular mechanisms to preclinical applications

Cell-based therapy (CBT) is a revolutionary approach for curing a variety of degenerative diseases. Stem cell-based regenerative medicine is a novel strategy for treating tissue damages regarding stem cells unique properties such as differentiation potential, paracrine impacts, and self-renewal ability. However, the current cell-based treatments encounter considerable challenges to be translated into clinical practice, including low cell survival, migration, and differentiation rate of transplanted stem cells. The poor stem cell therapy outcomes mainly originate from the unfavorable condition of damaged tissues for transplanted stem cells. The promising method of preconditioning improves cell resistance against the host environment's stress by imposing certain conditions similar to the harsh microenvironment of the damaged tissues on the transplanted stem cells. Various pharmacological, biological, and physical inducers are able to establish preconditioning. In addition to their known pharmacological effects on tissues and cells, these preconditioning agents improve cell biological aspects such as cell survival, proliferation, differentiation, migration, immunomodulation, paracrine impacts, and angiogenesis. This review focuses on different protocols and inducers of preconditioning along with underlying molecular mechanisms of their effects on stem cell behavior. Moreover, preclinical applications of preconditioned stem cells in various damaged organs such as heart, lung, brain, bone, cartilage, liver, and kidney are discussed with prospects of their translation into the clinic.

Hamartin: An Endogenous Neuroprotective Molecule Induced by Hypoxic Preconditioning

Hypoxic/ischemic preconditioning (HPC/IPC) is an innate neuroprotective mechanism in which a number of endogenous molecules are known to be involved. Tuberous sclerosis complex 1 (TSC1), also known as hamartin, is thought to be one such molecule. It is also known that hamartin is involved as a target in the rapamycin (mTOR) signaling pathway, which functions to integrate a variety of environmental triggers in order to exert control over cellular metabolism and homeostasis. Understanding the role of hamartin in ischemic/hypoxic neuroprotection will provide a novel target for the treatment of hypoxic-ischemic disease. Therefore, the proposed molecular mechanisms of this neuroprotective role and its preconditions are reviewed in this paper, with emphases on the mTOR pathway and the relationship between the expression of hamartin and DNA methylation.

Acquired Resilience: An Evolved System of Tissue Protection in Mammals

This review brings together observations on the stress-induced regulation of resilience mechanisms in body tissues. It is argued that the stresses that induce tissue resilience in mammals arise from everyday sources: sunlight, food, lack of food, hypoxia and physical stresses. At low levels, these stresses induce an organised protective response in probably all tissues; and, at some higher level, cause tissue destruction. This pattern of response to stress is well known to toxicologists, who have termed it hormesis. The phenotypes of resilience are diverse and reports of stress-induced resilience are to be found in journals of neuroscience, sports medicine, cancer, healthy ageing, dementia, parkinsonism, ophthalmology and more. This diversity makes the proposing of a general concept of induced resilience a significant task, which this review attempts. We suggest that a system of stress-induced tissue resilience has evolved to enhance the survival of animals. By analogy with acquired immunity, we term this system 'acquired resilience'. Evidence is reviewed that acquired resilience, like acquired immunity, fades with age. This fading is, we suggest, a major component of ageing. Understanding of acquired resilience may, we argue, open pathways for the maintenance of good health in the later decades of human life.

Hypoxic Preconditioning Augments the Therapeutic Efficacy of Bone Marrow Stromal Cells in a Rat Ischemic Stroke Model

Transplantation of bone marrow stromal cells (BMSCs) is a promising therapy for ischemic stroke, but the poor oxygen environment in brain lesions limits the efficacy of cell-based therapies. Here, we tested whether hypoxic preconditioning (HP) could augment the efficacy of BMSC transplantation in a rat ischemic stroke model and investigated the underlying mechanism of the effect of HP. In vitro, BMSCs were divided into five passage (P0, P1, P2, P3, and P4) groups, and HP was applied to the groups by incubating the cells with 1% oxygen for 0, 4, 8, 12, and 24 h, respectively. We demonstrated that the expression of hypoxia-inducible factor-1α (HIF-1α) was increased in the HP-treated BMSCs, while their viability was unchanged. We also found that HP decreased the apoptosis of BMSCs during subsequent simulated ischemia-reperfusion (I/R) injury, especially in the 8-h HP group. In vivo, a rat transient focal cerebral ischemia model was established. These rats were administered normal cultured BMSCs (N-BMSCs), HP-treated BMSCs (H-BMSCs), or DMEM cell culture medium (control) at 24 h after the ischemic insult. Compared with the DMEM control group, the two BMSC-transplanted groups exhibited significantly improved functional recovery and reduced infarct volume, especially the H-BMSC group. Moreover, HP decreased neuronal apoptosis and enhanced the expression of BDNF and VEGF in the ischemic brain. Survival and differentiation of transplanted BMSCs were also increased by HP, and the quantity of engrafted BMSCs was significantly correlated with neurological function improvement. These results suggest that HP may enhance the therapeutic efficacy of BMSCs in an ischemic stroke model. The underlying mechanism likely involves the inhibition of caspase-3 activation and an increasing expression of HIF-1α, which promotes angiogenesis and neurogenesis and thereby reduces neuronal death and improves neurological function.

Up-Regulation of Neuronal Nitric Oxide Synthase Expression by Cobalt Chloride Through a HIF-1α Mechanism in Neuroblastoma Cells

Nitric oxide (NO) plays a dual role in response to neural hypoxia. NO is synthesized by three isoforms of nitric oxide synthase (NOS), among which the neuronal NOS (nNOS) is predominant in the nervous system. Hypoxia-inducible factor-1α (HIF-1α) is a transcription factor that is induced under hypoxic conditions, but its correlation with nNOS remains unclear. In the present study, we aimed at clarifying the regulation pattern of the nNOS expression in response to cobalt chloride (CoCl2), a widely used chemical mimic of hypoxia, and the role of HIF-1α in this process in neuroblastoma cells. We found CoCl2 evidently increased the nNOS expression and NO production in human neuroblastoma SK-N-SH cells, but the effect of CoCl2 on NO was partially abrogated by 7-nitroindazole, a selective inhibitor for nNOS. Importantly, we identified a hypoxia response element (HRE) within the nNOS promoter, to which HIF-1α may bind, and CoCl2 greatly enhanced the HIF-1α expression and its binding to the HRE. Meanwhile, we demonstrated that this HRE was functionally important for the activation of the nNOS transcription, and CoCl2 increased the transcriptional activity of the nNOS promoter through this HRE. Taken together, our study shows that CoCl2 may induce the nNOS expression and NO production through a HIF-1α mechanism in neuroblastoma cells, which may provide a potential target for the treatment of neurological hypoxic disorders caused by NO dysregulation.

NPGPx modulates CPEB2-controlled HIF-1α RNA translation in response to oxidative stress

Non-selenocysteine-containing phospholipid hydroperoxide glutathione peroxidase (NPGPx or GPx7) is an oxidative stress sensor that modulates the antioxidative activity of its target proteins through intermolecular disulfide bond formation. Given NPGPx's role in protecting cells from oxidative damage, identification of the oxidative stress-induced protein complexes, which forms with key stress factors, may offer novel insight into intracellular reactive oxygen species homeostasis. Here, we show that NPGPx forms a disulfide bond with the translational regulator cytoplasmic polyadenylation element-binding protein 2 (CPEB2) that results in negative regulation of hypoxia-inducible factor 1-alpha (HIF-1α) RNA translation. In NPGPx-proficient cells, high oxidative stress that disrupts this bonding compromises the association of CPEB2 with HIF-1α RNA, leading to elevated HIF-1α RNA translation. NPGPx-deficient cells, in contrast, demonstrate increased HIF-1α RNA translation under normoxia with both impaired induction of HIF-1α synthesis and blunted HIF-1α-programmed transcription following oxidative stress. Together, these results reveal a molecular mechanism for how NPGPx mediates CPEB2-controlled HIF-1α RNA translation in a redox-sensitive manner.

Po2 cycling protects diaphragm function during reoxygenation via ROS, Akt, ERK, and mitochondrial channels

Po2 cycling, often referred to as intermittent hypoxia, involves exposing tissues to brief cycles of low oxygen environments immediately followed by hyperoxic conditions. After experiencing long-term hypoxia, muscle can be damaged during the subsequent reintroduction of oxygen, which leads to muscle dysfunction via reperfusion injury. The protective effect and mechanism behind Po2 cycling in skeletal muscle during reoxygenation have yet to be fully elucidated. We hypothesize that Po2 cycling effectively increases muscle fatigue resistance through reactive oxygen species (ROS), protein kinase B (Akt), extracellular signal-regulated kinase (ERK), and certain mitochondrial channels during reoxygenation. Using a dihydrofluorescein fluorescent probe, we detected the production of ROS in mouse diaphragmatic skeletal muscle in real time under confocal microscopy. Muscles treated with Po2 cycling displayed significantly attenuated ROS levels (n = 5; P < 0.001) as well as enhanced force generation compared with controls during reperfusion (n = 7; P < 0.05). We also used inhibitors for signaling molecules or membrane channels such as ROS, Akt, ERK, as well as chemical stimulators to close mitochondrial ATP-sensitive potassium channel (KATP) or open mitochondrial permeability transition pore (mPTP). All these blockers or stimulators abolished improved muscle function with Po2 cycling treatment. This current investigation has discovered a correlation between KATP and mPTP and the Po2 cycling pathway in diaphragmatic skeletal muscle. Thus we have identified a unique signaling pathway that may involve ROS, Akt, ERK, and mitochondrial channels responsible for Po2 cycling protection during reoxygenation conditions in the diaphragm.

Hypobaric Preconditioning Modifies Group I mGluRs Signaling in Brain Cortex

The study assessed involvement of Ca(2+) signaling mediated by the metabotropic glutamate receptors mGluR1/5 in brain tolerance induced by hypoxic preconditioning. Acute slices of rat piriform cortex were tested 1 day after exposure of adult rats to mild hypobaric hypoxia for 2 h at a pressure of 480 hPa once a day for three consecutive days. We detected 44.1 ± 11.6 % suppression of in vitro anoxia-induced increases of intracellular Ca(2+) levels and a fivefold increase in Ca(2+) transients evoked by selective mGluR1/5 agonist, DHPG. Western blot analysis of cortical homogenates demonstrated a 11 ± 4 % decrease in mGluR1 immunoreactivity (IR), and in the nuclei-enriched fraction a 12 ± 3 % increase in IR of phospholipase Cβ1 (PLCβ1), which is a major mediator of mGluR1/5 signaling. Immunocytochemical analysis of the cortex revealed increase in the mGluR1/5 and PLCβ1 IR in perikarya, and a decrease in IR of the neuronal inositol trisphosphate receptors (IP3Rs). We suggest that enhanced expression of mGluR5 and PLCβ1 and potentiation of Ca(2+) signaling may represent pro-survival upregulation of Ca(2+)-dependent genomic processes, while decrease in mGluR1 and IP3R IR may be attributed to a feedback mechanism preventing excessive intracellular Ca(2+) release.

Hypoxic preconditioning protection is eliminated in HIF-1α knockout mice subjected to neonatal hypoxia-ischemia

BACKGROUND

Hypoxic preconditioning (HPc) protects the neonatal brain in the setting of hypoxia-ischemia (HI). The mechanisms of protection may depend on activation of hypoxia-inducible factor (HIF-1α). This study sought to clarify the role of HIF-1α after HPc and HI.

METHODS

To induce HPc, HIF-1α knockout and wild-type (WT) mice were exposed to hypoxia at postnatal day 6. At day 7, the mice underwent HI. Brain injury was determined by histology. HIF-1α, downstream targets, and markers of cell death were measured by western blot.

RESULTS

HPc protected the WT brain compared with WT without HPc, but did not protect the HIF-1α knockout brain. In WT, HIF-1α increased after hypoxia and after HI, but not with HPc. The HIF-1α knockout showed no change in HIF-1α after hypoxia, HI, or HPc/HI. After HI, spectrin 145/150 was higher in HIF-1α knockout, but after HPc/HI, it was higher in WT. Lysosome-associated membrane protein was higher in WT early after HI, but not later. After HPc/HI, lysosome-associated membrane protein was higher in HIF-1α knockout.

CONCLUSION

These results indicate that HIF-1α is necessary for HPc protection in the neonatal brain and may affect cell death after HI. Different death and repair mechanisms depend on the timing of HPc.

Preconditioning and post-treatment with cobalt chloride in rat model of perinatal hypoxic-ischemic encephalopathy

BACKGROUND

Hypoxia-ischemia (HI)-induced perinatal encephalopathy is a major cause of acute mortality and chronic neurologic morbidities such as cerebral palsy, mental retardation, and epilepsy. As the essential transcription factor for the activation of hypoxia-inducible genes, hypoxia-inducible factor 1 alpha (HIF-1α) plays an important role in the pathophysiological response to the stress of HI brain damage. Whether HIF-1α activation promotes neuroprotection in HI tissues is controversial.

METHODS

The left common carotid artery of rats aged 7days was ligated under anesthesia. The pups were then exposed to hypoxia in a normobaric chamber filled with 8% oxygen and 92% nitrogen for 2.5h. In the sham control group, the left common carotid artery was exposed but was not ligated or exposed to hypoxia. To assess the time window for effective treatment, the HIF-1α inducer cobalt chloride (CoCl2) was injected subcutaneously 1day before surgery, immediately or 1day after surgery. The brain tissues were harvested from the pups of each groups at 1, 2 and 7days after insult for HIF-1α protein ant its target genes expression and for investigating the injury. Morris water maze tests were performed at postnatal 7weeks.

RESULTS

HIF-1α protein levels and its target genes vascular endothelial growth factor, heme oxygenase-1, and insulin-like growth factor 1 were markedly increased after intraperitoneal injection of CoCl2 (60mg/kg). The target gene inducible nitric oxide synthase exhibited a biphasic time course. HI caused apoptosis and reduced capillary density, which were ameliorated by CoCl2. Both preconditioning with CoCl2 24h before HI and administration of CoCl2 24h after HI improved long-term reference memory compared with that in vehicle-injected littermate controls. Administration of CoCl2 immediately after HI did not improve spatial working memory.

CONCLUSIONS

CoCl2 activates HIF-1α and protects against brain damage in vivo. The time of administration could be used to manipulate the activity of HIF-1α pathways and promote recovery.

Dynamic aspects of cerebral hypoxic preconditioning measured in an in vitro model

Preconditioning increases the neurons' resistance to subsequent hypoxia. An in vitro study was conducted to explore kinetic aspects of hypoxic preconditioning. Hippocampal slices were exposed to one single or repeated episodes of oxygen and glucose deprivation (OGD). The interval between OGD episodes varied between 30 min and 180 min. OGD led to a significant reduction in the population spike amplitude. Subsequent episodes of OGD did not result in a further reduction in the population spike amplitude if the interval between the episodes was ca. 60 min, which demonstrated that there were preconditioning effects. In the experiment using an interval of 30 min, population spike amplitude decreased after each OGD episode. The set-up described is useful for detecting damaging effects of OGD as well as preconditioning effects. A time window of ca. 60 min is required to induce protective mechanisms.

Role of stress-inducible protein-1 in recruitment of bone marrow derived cells into the ischemic brains

Stress-inducible protein-1 (STI-1) is the proposed ligand for the cellular prion protein (PrP^C), which is thought to facilitate recovery following stroke. Whether STI-1 expression is affected by stroke and how its signalling facilitates recovery remain elusive. Brain slices from patients that died of ischemic stroke were collected for STI-1 immunohistochemistry. These findings were compared to results from cell cultures, mice with or without the PrP^C knockout, and rats. Based on these findings, molecular and pharmacological interventions were administered to investigate the underlying mechanisms and to test the possibility for therapy in experimental stroke models. STI-1 was upregulated in the ischemic brains from humans and rodents. The increase in STI-1 expression in vivo was not cell-type specific, as it was found in neurons, glia and endothelial cells. Likewise, this increase in STI-1 expression can be mimicked by sublethal hypoxia in primary cortical cultures (PCCs) in vitro, and appear to have resulted from the direct binding of the hypoxia inducible factor-1α (HIF-1α) to the STI-1 promoter. Importantly, this STI-1 signalling promoted bone marrow derived cells (BMDCs) proliferation and migration in vitro and recruitment to the ischemic brain in vivo, and augmenting its signalling facilitated neurological recovery in part by recruiting BMDCs to the ischemic brain. Our results thus identified a novel mechanism by which ischemic insults can trigger a self-protective mechanism to facilitate recovery.

This work identifies HIF-1α-mediated transcription of STI-1 and PrPc interaction as leading to BMDCs recruitment into ischemic brains following stroke in both patients and animal models of stroke, highlighting novel neuroprotective possibilities.

Transcriptional regulation of ncx1 gene in the brain

The ubiquitous sodium-calcium exchanger isoform 1 (NCX1) is a -bidirectional transporter that plays a relevant role under physiological and pathophysiological conditions including brain ischemia by regulating intraneuronal Ca(2+) and Na(+) homeostasis. Although changes in ncx1 protein and transcript expression have been detected during stroke, its transcriptional regulation is still largely unexplored. Here, we reviewed our recent findings on several transcription factors including cAMP response element-binding protein (CREB), nuclear factor kappa B (NF-κB), and hypoxia-inducible factor-1 (HIF-1) in the control of the ncx1 gene expression in neuronal cells.

CCL2 upregulation triggers hypoxic preconditioning-induced protection from stroke

Background

A brief exposure to systemic hypoxia (i.e., hypoxic preconditioning; HPC) prior to transient middle cerebral artery occlusion (tMCAo) reduces infarct volume, blood-brain barrier disruption, and leukocyte migration. CCL2 (MCP-1), typically regarded as a leukocyte-derived pro-inflammatory chemokine, can also be directly upregulated by hypoxia-induced transcription. We hypothesized that such a hypoxia-induced upregulation of CCL2 is required for HPC-induced ischemic tolerance.

Methods

Adult male SW/ND4, CCL2-null, and wild-type mice were used in these studies. Cortical CCL2/CCR2 message, protein, and cell-type specific immunoreactivity were determined following HPC (4 h, 8% O₂) or room air control (21% O₂) from 6 h through 2 weeks following HPC. Circulating leukocyte subsets were determined by multi-parameter flow cytometry in naïve mice and 12 h after HPC. CCL2-null and wild-type mice were exposed to HPC 2 days prior to tMCAo, with immunoneutralization of CCL2 during HPC achieved by a monoclonal CCL2 antibody.

Results

Cortical CCL2 mRNA and protein expression peaked at 12 h after HPC (both p < 0.01), predominantly in cortical neurons, and returned to baseline by 2 days. A delayed cerebral endothelial CCL2 message expression (p < 0.05) occurred 2 days after HPC. The levels of circulating monocytes (p < 0.0001), T lymphocytes (p < 0.0001), and granulocytes were decreased 12 h after HPC, and those of B lymphocytes were increased (p < 0.0001), but the magnitude of these respective changes did not differ between wild-type and CCL2-null mice. HPC did decrease the number of circulating CCR2⁺ monocytes (p < 0.0001) in a CCL2-dependent manner, but immunohistochemical analyses at this 12 h timepoint indicated that this leukocyte subpopulation did not move into the CNS. While HPC reduced infarct volumes by 27% (p < 0.01) in wild-type mice, CCL2-null mice subjected to tMCAo were not protected by HPC. Moreover, administration of a CCL2 immunoneutralizing antibody prior to HPC completely blocked (p < 0.0001 vs. HPC-treated mice) the development of ischemic tolerance.

Conclusions

The early expression of CCL2 in neurons, the delayed expression of CCL2 in cerebral endothelial cells, and CCL2-mediated actions on circulating CCR2⁺ monocytes, appear to be required to establish ischemic tolerance to focal stroke in response to HPC, and thus represent a novel role for this chemokine in endogenous neurovascular protection.

Transient ureteral obstruction prevents against kidney ischemia/reperfusion injury via hypoxia-inducible factor (HIF)-2α activation

Although the protective effect of transient ureteral obstruction (UO) prior to ischemia on subsequent renal ischemia/reperfusion (I/R) injury has been documented, the underlying molecular mechanism remains to be understood. We showed in the current study that 24 h of UO led to renal tubular hypoxia in the ipsilateral kidney in mice, with the accumulation of hypoxia-inducible factor (HIF)-2α, which lasted for a week after the release of UO. To address the functions of HIF-2α in UO-mediated protection of renal IRI, we utilized the Mx-Cre/loxP recombination system to knock out target genes. Inactivation of HIF-2α, but not HIF-1α blunted the renal protective effects of UO, as demonstrated by much higher serum creatinine level and severer histological damage. UO failed to prevent postischemic neutrophil infiltration and apoptosis induction in HIF-2α knockout mice, which also diminished the postobstructive up-regulation of the protective molecule, heat shock protein (HSP)-27. The renal protective effects of UO were associated with the improvement of the postischemic recovery of intra-renal microvascular blood flow, which was also dependent on the activation of HIF-2α. Our results demonstrated that UO protected the kidney via activation of HIF-2α, which reduced tubular damages via preservation of adequate renal microvascular perfusion after ischemia. Thus, preconditional HIF-2α activation might serve as a novel therapeutic strategy for the treatment of ischemic acute renal failure.

Hypoxic preconditioning attenuates global cerebral ischemic injury following asphyxial cardiac arrest through regulation of delta opioid receptor system

This study was designed to investigate whether delta opioid receptor (DOR) is involved in the neuroprotective effect induced by hypoxic preconditioning (HPC) in the asphyxial cardiac arrest (CA) rat model. Twenty-four hours after the end of 7-day HPC, the rats were subjected to 8-min asphyxiation and resuscitated with a standardized method. In the asphyxial CA rat model, HPC improved the neurological deficit score (NDS), inhibited neuronal apoptosis, and increased the number of viable hippocampal CA1 neurons at 24 h, 72 h, or 7 days after restoration of spontaneous circulation (ROSC); however, the above-mentioned neuroprotection of HPC was attenuated by naltrindole (a selective DOR antagonist). The expression of hypoxia-inducible factor-1α (HIF-1α) and DOR, and the content of leucine enkephalin (L-ENK) in the brain were also investigated after the end of 7-day HPC. HPC upregulated the neuronal expression of HIF-1α and DOR, and synchronously elevated the content of L-ENK in the rat brain. HIF-1α siRNA was used to further elucidate the relationship between HIF-1α and DOR in the HPC-treated brain. Knockdown of HIF-1α by siRNA markedly abrogated the HPC induced upregulation of HIF-1α and DOR. The present study demonstrates that the expression of DOR in the rat brain is upregulated by HIF-1α following exposure to 7-day HPC, at the same time, HPC also increases the production of endogenous DOR ligand L-ENK in the brain. DOR activation after HPC results in prolonged neuroprotection against subsequent global cerebral ischemic injury, suggesting a new mechanism of HPC-induced neuroprotection on global cerebral ischemia following CA and resuscitation.

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.

Endothelial-cardiomyocyte crosstalk enhances pharmacological cardioprotection

Endothelial cells (EC) serve a paracrine function to enhance signaling in cardiomyocytes (CM), and conversely, CM secrete factors that impact EC function. Understanding how EC interact with CM may be critically important in the context of ischemia-reperfusion injury, where EC might promote CM survival. We used isoflurane as a pharmacological stimulus to enhance EC protection of CM against hypoxia and reoxygenation injury. Triggering of intracellular signal transduction pathways culminating in the enhanced production of nitric oxide (NO) appears to be a central component of pharmacologically induced cardioprotection. Although the endothelium is well recognized as a regulator for vascular tone, little attention has been given to its potential importance in mediating cardioprotection. In the current investigation, EC-CM in co-culture were used to test the hypothesis that EC contribute to isoflurane-enhanced protection of CM against hypoxia and reoxygenation injury and that this protection depends on hypoxia-inducible factor (HIF1α) and NO. CM were protected against cell injury [lactate dehydrogenase (LDH) release] to a greater extent in the presence vs. absence of isoflurane-stimulated EC (1.7 ± 0.2 vs. 4.58 ± 0.8 fold change LDH release), and this protection was NO-dependent. Isoflurane enhanced release of NO in EC (1103 ± 58 vs. 702 ± 92 pmol/mg protein) and EC-CM in co-culture sustained NO release during reoxygenation. In contrast, lentiviral mediated HIF1α knockdown in EC decreased basal and isoflurane stimulated NO release in an eNOS dependent manner (517 ± 32 vs. 493 ± 38 pmol/mg protein) and prevented the sustained increase in NO during reoxygenation when co-cultured. Opening of mitochondrial permeability transition pore (mPTP), an index of mitochondrial integrity, was delayed in the presence vs. absence of EC (141 ± 2 vs. 128 ± 2.5 arbitrary mPTP opening time). Isoflurane stimulated an increase in HIF1α in EC but not in CM under normal oxygen tension (3.5 ± 0.1 vs. 0.79 ± 0.15 fold change density) and this action was blocked by pretreatment with the Mitogen-activated Protein/Extracellular Signal-regulated Kinase inhibitor U0126. Expression and nuclear translocation of HIF1α were confirmed by Western blot and immunofluorescence. Taken together, these data support the concept that EC are stimulated by isoflurane to produce important cardioprotective factors that may contribute to protection of myocardium during ischemia and reperfusion injury.

Fentanyl activates hypoxia-inducible factor 1 in neuronal SH-SY5Y cells and mice under non-hypoxic conditions in a μ-opioid receptor-dependent manner

Hypoxia-inducible factor 1 (HIF-1) is the main transcription factor responsible for hypoxia-induced gene expression. Perioperative drugs including anesthetics have been reported to affect HIF-1 activity. However, the effect of fentanyl on HIF-1 activity is not well documented. In this study, we investigated the effect of fentanyl and other opioids on HIF-1 activity in human SH-SY5Y neuroblastoma cells, hepatoma Hep3B cells, lung adenocarcinoma A549 cells and mice. Cells were exposed to fentanyl, and HIF-1 protein expression was examined by Western blot analysis using anti-HIF-1α and β antibodies. HIF-1-dependent gene expression was investigated by semi-quantitative real-time reverse transcriptase (RT)-PCR (qRT-PCR) and luciferase assay. Furthermore, fentanyl was administered intraperitoneally and HIF-1-dependent gene expression was investigated by qRT-PCR in the brains and kidneys of mice. A 10-μM concentration of fentanyl and other opioids, including 1 μM morphine and 4 μM remifentanil, induced HIF-1α protein expression and HIF-1 target gene expression in an opioid receptor-dependent manner in SH-SY5Y cells with activity peaking at 24h. Fentanyl did not augment HIF-1α expression during hypoxia-induced induction. HIF-1α stabilization assays and experiments with cycloheximide revealed that fentanyl increased translation from HIF-1α mRNA but did not stabilize the HIF-1α protein. Furthermore, fentanyl induced HIF-1 target gene expression in the brains of mice but not in their kidneys in a naloxone-sensitive manner. In this report, we describe for the first time that fentanyl, both in vitro and in vivo, induces HIF-1 activation under non-hypoxic conditions, leading to increases in expression of genes associated with adaptation to hypoxia.

Signal transducers and activators of transcription: STATs-mediated mitochondrial neuroprotection

Cerebral ischemia is defined as little or no blood flow in cerebral circulation, characterized by low tissue oxygen and glucose levels, which promotes neuronal mitochondria dysfunction leading to cell death. A strategy to counteract cerebral ischemia-induced neuronal cell death is ischemic preconditioning (IPC). IPC results in neuroprotection, which is conferred by a mild ischemic challenge prior to a normally lethal ischemic insult. Although many IPC-induced mechanisms have been described, many cellular and subcellular mechanisms remain undefined. Some reports have suggested key signal transduction pathways of IPC, such as activation of protein kinase C epsilon, mitogen-activated protein kinase, and hypoxia-inducible factors, that are likely involved in IPC-induced mitochondria mediated-neuroprotection. Moreover, recent findings suggest that signal transducers and activators of transcription (STATs), a family of transcription factors involved in many cellular activities, may be intimately involved in IPC-induced ischemic tolerance. In this review, we explore current signal transduction pathways involved in IPC-induced mitochondria mediated-neuroprotection, STAT activation in the mitochondria as it relates to IPC, and functional significance of STATs in cerebral ischemia.

NCX1 is a novel target gene for hypoxia-inducible factor-1 in ischemic brain preconditioning

BACKGROUND AND PURPOSE

The sodium-calcium exchanger-1 (NCX1) represents a key mediator for maintaining [Na(+)](i) and [Ca(2+)](i) homeostasis. Although changes in NCX1 protein and transcript expression have been detected during stroke, its transcriptional regulation is still unknown. Thus far, however, there is evidence that hypoxia-inducible factor-1 (HIF-1) is a nuclear factor required for transcriptional activation of several genes implicated in stroke. The main objective of this study was to investigate whether NCX1 gene might be a novel target of HIF-1 in the brain.

METHODS

Here we report that: (1) in neuronal cells, NCX1 increased expression after oxygen and glucose deprivation or cobalt-induced HIF-1 activation was prevented by silencing HIF-1; (2) the brain NCX1 promoter cloned upstream of the firefly-luciferase gene contained 2 regions of HIF-1 target genes called hypoxia-responsive elements that are sensitive to oxygen and glucose deprivation or cobalt chloride; (3) HIF-1 specifically bound hypoxia-responsive elements on brain NCX1, as demonstrated by band-shift and chromatin immunoprecipitation assays; (4) HIF-1α silencing prevented NCX1 upregulation and neuroprotection induced by ischemic preconditioning; and (5) NCX1 silencing partially reverted the preconditioning-induced neuroprotection in rats.

CONCLUSIONS

NCX1 gene is a novel HIF-1 target, and HIF-1 exerts its prosurvival role through NCX1 upregulation during brain preconditioning.

Chronic kidney disease: an inherent risk factor for acute kidney injury?

Epidemiologic evidence suggests that chronic kidney disease (CKD) is a risk factor for acute kidney injury (AKI) due to the prevalence of CKD in patients who have episodes of AKI. However, the high burden of comorbidities such as age, diabetes, peripheral vascular, cardiovascular, and liver disease accompanying CKD, and the difficulties of defining AKI in the setting of CKD make these observations difficult to interpret. These comorbidities not only could alter the course of AKI but also may be the driving force behind the epidemiologic association between CKD and AKI because of systemic changes and/or increased exposure to potential nephrotoxic risks. Here, we contend that studies suggesting that CKD is a risk factor for AKI may suffer from residual confounding and reflect an overall susceptibility to illness rather than biologic susceptibility of the kidney parenchyma to injury. In support of our argument, we discuss the clinical evidence from epidemiologic studies, and the knowledge obtained from animal models on the pathophysiology of AKI and CKD, demonstrating a preconditioning influence of the previously impaired kidneys against subsequent injury. We conclude that, under careful analysis, factors apart from the inherent pathophysiology of the diseased kidney may be responsible for the increased frequency of AKI in CKD patients, and the impact of CKD on the risk and severity of AKI needs further investigation. Moreover, certain elements in the pathophysiology of a previously injured kidney may, surprisingly, bear out to be protective against AKI.

The role and regulation of hypoxia-inducible factor-1alpha expression in brain development and neonatal hypoxic-ischemic brain injury

During neonatal hypoxic-ischemic brain injury, activation of transcription of a series of genes is induced to stimulate erythropoiesis, anti-apoptosis, apoptosis, necrosis and angiogenesis. A key factor mediating these gene transcriptions is hypoxia-inducible factor-1alpha (HIF-1alpha). During hypoxia, HIF-1alpha protein is stabilized and heterodimerizes with HIF-1beta to form HIF-1, subsequently regulating the expression of target genes. HIF-1alpha participates in early brain development and proliferation of neuronal precursor cells. Under pathological conditions, HIF-1alpha is known to play an important role in neonatal hypoxic-ischemic brain injury: on the one hand, HIF-1alpha has neuroprotective effects whereas it can also have neurotoxic effects. HIF-1alpha regulates the transcription of erythropoietin (EPO), which induces several pathways associated with neuroprotection. HIF-1alpha also promotes the expression of vascular endothelial cell growth factor (VEGF), which is related to neovascularization in hypoxic-ischemic brain areas. In addition, HIF-1alpha has an anti-apoptotic effect by increasing the expression of anti-apoptotic factors such as EPO during mild hypoxia. The neurotoxic effects of HIF-1alpha are represented by its participation in the apoptotic process by increasing the stability of the tumor suppressor protein p53 during severe hypoxia. Moreover, HIF-1alpha plays a role in cell necrosis, by interacting with calcium and calpain. HIF-1alpha can also exacerbate brain edema via increasing the permeability of the blood-brain barrier (BBB). Given these properties, HIF-1alpha has both neuroprotective and neurotoxic effects after hypoxia-ischemia. These events are cell type specific and related to the severity of hypoxia. Unravelling of the complex functions of HIF-1alpha may be important when designing neuroprotective therapies for hypoxic-ischemic brain injury.