Differential expression of heme oxygenase-1 in cultured cortical neurons and astrocytes determined by the aid of a new heme oxygenase antibody. Response to oxidative stress

Bilirubin: an endogenous scavenger of nitric oxide and reactive nitrogen species

Bilirubin is the final product of heme metabolism. Until recently, bilirubin was considered as a mere by-product of heme degradation but, in the last 20 years, many papers have appeared in the literature demonstrating that this bile pigment is endowed with a strong antioxidant activity, being able to counteract the cellular damage elicited by reactive oxygen species in many in vitro and in vivo experimental systems. Interestingly, compelling evidence has shown that BR can serve as an endogenous scavenger of both nitric oxide and reactive nitrogen species, thus widening the protective role of bilirubin to other reactive species originating within the cellular milieu. The aim of this paper is to give an overview of the interaction between bilirubin and nitric oxide/reactive nitrogen species; furthermore, the possible pathophysiological and clinical relevance of this interaction will be discussed.

Redox regulation of cellular stress response in aging and neurodegenerative disorders: role of vitagenes

Reduced expression and/or activity of antioxidant proteins lead to oxidative stress, accelerated aging and neurodegeneration. However, while excess reactive oxygen species (ROS) are toxic, regulated ROS play an important role in cell signaling. Perturbation of redox status, mutations favoring protein misfolding, altered glyc(osyl)ation, overloading of the product of polyunsaturated fatty acid peroxidation (hydroxynonenals, HNE) or cholesterol oxidation, can disrupt redox homeostasis. Collectively or individually these effects may impose stress and lead to accumulation of unfolded or misfolded proteins in brain cells. Alzheimer's (AD), Parkinson's and Huntington's disease, amyotrophic lateral sclerosis and Friedreich's ataxia are major neurological disorders associated with production of abnormally aggregated proteins and, as such, belong to the so-called "protein conformational diseases". The pathogenic aggregation of proteins in non-native conformation is generally associated with metabolic derangements and excessive production of ROS. The "unfolded protein response" has evolved to prevent accumulation of unfolded or misfolded proteins. Recent discoveries of the mechanisms of cellular stress signaling have led to new insights into the diverse processes that are regulated by cellular stress responses. The brain detects and overcomes oxidative stress by a complex network of "longevity assurance processes" integrated to the expression of genes termed vitagenes. Heat-shock proteins are highly conserved and facilitate correct protein folding. Heme oxygenase-1, an inducible and redox-regulated enzyme, has having an important role in cellular antioxidant defense. An emerging concept is neuroprotection afforded by heme oxygenase by its heme degrading activity and tissue-specific antioxidant effects, due to its products carbon monoxide and biliverdin, which is then reduced by biliverdin reductase in bilirubin. There is increasing interest in dietary compounds that can inhibit, retard or reverse the steps leading to neurodegeneration in AD. Specifically any dietary components that inhibit inappropriate inflammation, AbetaP oligomerization and consequent increased apoptosis are of particular interest, with respect to a chronic inflammatory response, brain injury and beta-amyloid associated pathology. Curcumin and ferulic acid, the first from the curry spice turmeric and the second a major constituent of fruit and vegetables, are candidates in this regard. Not only do these compounds serve as antioxidants but, in addition, they are strong inducers of the heat-shock response. Food supplementation with curcumin and ferulic acid are therefore being considered as a novel nutritional approach to reduce oxidative damage and amyloid pathology in AD. We review here some of the emerging concepts of pathways to neurodegeneration and how these may be overcome by a nutritional approach.

Complexity of astrocyte-motor neuron interactions in amyotrophic lateral sclerosis

Neurons and surrounding glial cells compose a highly specialized functional unit. In amyotrophic lateral sclerosis (ALS) astrocytes interact with motor neurons in a complex manner to modulate neuronal survival. Experiments using chimeric mice expressing ALS-linked mutations to Cu,Zn superoxide dismutase (SOD-1) suggest a critical modulation exerted by neighboring non-neuronal cell types on disease phenotype. When perturbed by primary neuronal damage, e.g. expression of SOD-1 mutations, neurons can signal astrocytes to proliferate and become reactive. Fibroblast growth factor-1 (FGF-1) can be released by motor neurons in response to damage to induce astrocyte activation by signaling through the receptor FGFR1. FGF-1 stimulates nerve growth factor (NGF) expression and secretion, as well as activity of the nuclear factor erythroid 2-related factor 2 (Nrf2) transcription factor. Nrf2 leads to the expression of antioxidant and cytoprotective enzymes such as heme oxygenase-1 and a group of enzymes involved in glutathione metabolism that prevent motor neuron degeneration. However, prolonged stimulation with FGF-1 or SOD-mediated oxidative stress in astrocytes may disrupt the normal neuron-glia interactions and lead to progressive neuronal degeneration. The re-expression of p75 neurotrophin receptor and neuronal NOS in motor neurons in parallel with increased NGF secretion by reactive astrocytes may be a mechanism to eliminate critically damaged neurons. Consequently, astrocyte activation in ALS may have a complex pathogenic role.

Up-regulation of heat shock protein HSP 20 in the hippocampus as an early response to hypoxia of the newborn

Hypoxia is an important challenge for newborn mammals. Stress generated at the brain level under low oxygenation conditions results in up-regulation of heat shock proteins (HSPs) and other stress proteins. The aim of the present work was to determine the effect of hypoxia in the newborn on some newly described small molecular weight HSPs (HSP 20 and B8) in the hippocampus, cortex and cerebellum of newborn piglets. These effects will be compared with those of other closely related proteins such as alphaB crystallin, HSP 27, heme oxygenase (HO)-1, HO-2, cyclooxygenase (COX)-1 and COX-2. The piglets were submitted to hypoxia (5% O(2); 95% N(2)) over either 1 or 4 h, with recovery periods ranging from 0 to 68 h. Western blot analysis showed that HSP 20 was rapidly induced only in the hippocampus, long before hypoxia-inducible transcription factor HIF-1alpha, while HSP 27 was rapidly induced in the cortex and cerebellum. Vascular epithelial growth factor was increased simultaneously in the three regions. Moreover, an increase in the expression of, respectively, HO-1 and COX-2 was observed later, but at the same time, in the three regions tested. It appears that HSP 20 can be an early marker of hypoxia in the hippocampus. The other small HSPs or stress proteins display different temporal patterns of up-regulation (HSP 27 and HO-1, COX-2) or do not show changes in their expressions (alphaB crystallin, HSP B8, HO-2 and COX-1).

Carbon monoxide and hydrogen sulfide: gaseous messengers in cerebrovascular circulation

This review focuses on two gaseous cellular messenger molecules, CO and H2S, that are involved in cerebrovascular flow regulation. CO is a dilatory mediator in active hyperemia, autoregulation, hypoxic dilation, and counteracting vasoconstriction. It is produced from heme by a constitutively expressed enzyme [heme oxygenase (HO)-2] expressed highly in the brain and by an inducible enzyme (HO-1). CO production is regulated by controlling substrate availability, HO-2 catalytic activity, and HO-1 expression. CO dilates arterioles by binding to heme that is bound to large-conductance Ca2+-activated K+ channels. This binding elevates channel Ca2+ sensitivity, that increases coupling of Ca2+ sparks to large-conductance Ca2+-activated K+ channel openings and, thereby, hyperpolarizes the vascular smooth muscle. In addition to dilating blood vessels, CO can either inhibit or accentuate vascular cell proliferation and apoptosis, depending on conditions. H2S may also function as a cerebrovascular dilator. It is produced in vascular smooth muscle cells by hydrolysis of l-cysteine catalyzed by cystathione gamma-lyase (CSE). H2S dilates arterioles at physiologically relevant concentrations via activation of ATP-sensitive K+ channels. In addition to dilating blood vessels, H2S promotes apoptosis of vascular smooth muscle cells and inhibits proliferation-associated vascular remodeling. Thus both CO and H2S modulate the function and the structure of circulatory system. Both the HO-CO and CSE-H2S systems have potential to interact with NO and prostanoids in the cerebral circulation. Much of the physiology and biochemistry of HO-CO and CSE-H2S in the cerebral circulation remains open for exploration.

Astrocytes protect neurons from ammonia toxicity

Ammonia is a neurotoxin that is implicated in the CNS dysfunction associated with hepatic encephalopathy, urea cycle disorders, Reye's syndrome and other neurological conditions. While in vivo studies suggest that astrocytes are the principal target of ammonia toxicity, recent in vitro investigations suggest that neurons may also be directly affected by ammonia. To further examine the issue of neural cell sensitivity to ammonia, pure rat cortical neuronal cultures, as well as co-cultures of neurons and astrocytes, were exposed to 5 mM NH4Cl for 48 h. Cultures were examined for morphological changes by light microscopy, measures of cell death, free radical production and changes in the mitochondrial inner membrane potential. Ammonia caused extensive degenerative changes in pure cultured neurons, while such neuronal changes were minor in the co-cultures. Similarly, processes of pure cultured neurons displayed a significant loss of the mitochondrial inner membrane potential, as compared to neurons in co-cultures. Cell death (LDH release) in ammonia-treated neuronal cultures was twice as great as untreated controls, while in co-cultures ammonia did not significantly increase cell death. Free radical production at 3 min was increased (69%, P<0.05) in pure neuronal cultures but not in co-cultures. The neuroprotective effects observed in co-cultures may have been mediated by the astrocyte's ability to scavenge free radicals, by their detoxification of ammonia and/or by their neurotrophic actions. The neuroprotective action of astrocytes may explain the failure to detect significant pathological changes in neurons in ammonia toxicity in vivo.

Heme regulation in traumatic brain injury: relevance to the adult and developing brain

Intracranial bleeding is one of the most prominent aspects in the clinical diagnosis and prognosis of traumatic brain injury (TBI). Substantial amounts of blood products, such as heme, are released because of traumatic subarachnoid hemorrhages, intraparenchymal contusions, and hematomas. Despite this, surprisingly few studies have directly addressed the role of blood products, in particular heme, in the setting of TBI. Heme is degraded by heme oxygenase (HO) into three highly bioactive products: iron, bilirubin, and carbon monoxide. The HO isozymes, in particular HO-1 and HO-2, exhibit significantly different expression patterns and appear to have specific roles after injury. Developmentally, differences between the adult and immature brain have implications for endogenous protection from oxidative stress. The aim of this paper is to review recent advances in the understanding of heme regulation and metabolism after brain injury and its specific relevance to the developing brain. These findings suggest novel clinical therapeutic options for further translational study.

Fibroblast Growth Factor-1 Induces Heme Oxygenase-1 via Nuclear FactorErythroid 2-related Factor 2 (Nrf2) in Spinal Cord Astrocytes

Fibroblast growth factor-1 (FGF-1) is highly expressed in motor neurons and can be released in response to sublethal cell injury. Because FGF-1 potently activates astroglia and exerts a direct neuroprotection after spinal cord injury or axotomy, we examined whether it regulated the expression of inducible and cytoprotective heme oxygenase-1 (HO-1) enzyme in astrocytes. FGF-1 induced the expression of HO-1 in cultured rat spinal cord astrocytes, which was dependent on FGF receptor activation and prevented by cycloheximide. FGF-1 also induced Nrf2 mRNA and protein levels and prompted its nuclear translocation. HO-1 induction was abolished by transfection of astrocytes with a dominant-negative mutant Nrf2, indicating that FGF-1 regulates HO-1 expression through Nrf2. FGF-1 also modified the expression of other antioxidant genes regulated by Nrf2. Both Nrf2 and HO-1 levels were increased and co-localized with reactive astrocytes in the degenerating lumbar spinal cord of rats expressing the amyotrophic lateral sclerosis-linked SOD1 G93A mutation. Overexpression of Nrf2 in astrocytes increased survival of co-cultured embryonic motor neurons and prevented motor neuron apoptosis mediated by nerve growth factor through p75 neurotrophin receptor. Taken together, these results emphasize the key role of astrocytes in determining motor neuron fate in amyotrophic lateral sclerosis.

Astroglial cytoprotection by erythropoietin pre-conditioning: implications for ischemic and degenerative CNS disorders

Erythropoietin (Epo) is a glycoprotein secreted by the kidney in response to hypoxia that stimulates erythropoiesis through interaction with cell surface Epo receptors. Pre-treatment with Epo has been shown to protect neurons in models of ischemic injury. The mechanism responsible for this neuroprotection and the effects of Epo on astroglial and other non-neuronal cell populations remain unknown. In the present study, we determined whether Epo pre-treatment protects neonatal rat astrocytes from apoptotic cell death resulting from treatment with nitric oxide, staurosporine (STS) and arsenic trioxide and possible mechanisms mediating Epo-related cytoprotection. Epo (5-20 U/mL) significantly attenuated multiple hallmarks of apoptotic cell death in astroglia exposed to nitric oxide and STS but not arsenic trioxide. Epo (20 U/mL) induced mild oxidative stress as shown by increases in heme oxygenase (HO)-1 mRNA and protein expression that could be suppressed by antioxidant coadministration. Moreover, coincubation with tin-mesoporphyrin, a competitive inhibitor of HO activity, abrogated the cytoprotective effects of Epo (20 U/mL) in the face of STS treatment. Thus, induction of the ho-1 gene may contribute to the glioprotection accruing from high-dose Epo exposure. Epo may augment astroglial resistance to certain chemical stressors by oxidative stress-dependent and -independent mechanisms.

Glial overexpression of heme oxygenase-1: a histochemical marker for early stages of striatal damage

The level of heme oxygenase-1 (HO-1) in the normal striatum is below the limit of immunodetection. However, HO-1 is overexpressed in both neural and non-neural cells in response to a wide range of lesions. We induced different types of lesions affecting the striatal cells or the main striatal afferent systems in rats to investigate if overexpression of HO-1 could be a useful histochemical marker of striatal damage. Thirty-six hours after intrastriatal or intraventricular injection of excitotoxins that affect striatal neurons (ibotenic acid) or of neurotoxins that affect striatal dopaminergic (6-hydroxydopamine) or serotonergic (5,7-dihydroxytriptamine) afferent terminals, or after surgical lesioning of cortico-striatal projections, there was intense induction of striatal HO-1 immunoreactivity (HO-1-ir). Double immunolabeling revealed that the HO-1-ir was located in glial cells. After intrastriatal injection of ibotenic acid, a central zone of neuronal degeneration contained numerous round and pseudopodic HO-1-ir cells, and was surrounded by a ring of HO-1-ir cells, most of which were immunoreactive for astroglial markers. Intraventricular injection of neurotoxins induced astroglial HO-1-ir cells which were more evenly distributed throughout the lesioned or denervated areas. HO-1-ir microglial cells were also observed in areas subjected to mechanical damage. The HO-1-ir was markedly lower or absent 1 week after lesion, and even more so 3 weeks after, although some HO-1-ir cells were still observed after intrastriatal injection of ibotenic acid or surgical corticostriatal deafferentation. The results indicate that determination of glial HO-1-ir is a useful histochemical marker for early stages of striatal damage.

Heme oxygenase expression in human central nervous system disorders

In the normal mammalian CNS, heme oxygenase-2 (HO-2) is constitutively, abundantly, and fairly ubiquitously expressed, whereas heme oxygenase-1 (HO-1) mRNA and protein are confined to small populations of scattered neurons and neuroglia. Unlike ho-2, the ho-1 gene in neural (and many systemic) tissues is exquisitely sensitive to upregulation by a host of pro-oxidant and other noxious stimuli. In Alzheimer disease, HO-1 immunoreactivity is significantly augmented in neurons and astrocytes of the hippocampus and cerebral cortex relative to age-matched, nondemented controls and colocalizes to senile plaques, neurofibrillary tangles, and corpora amylacea. In Parkinson disease, HO-1 decorates Lewy bodies of affected dopaminergic neurons and is highly overexpressed in astrocytes residing within the substantia nigra. The ho-1 gene is also upregulated in glial cells within multiple sclerosis plaques; in the vicinity of human cerebral infarcts, hemorrhages, and contusions; and in various other degenerative and nondegenerative human CNS disorders. The products of the heme oxygenase reaction, free ferrous iron, carbon monoxide, and biliverdin/bilirubin, are all biologically active molecules that may profoundly influence tissue redox homeostasis under a wide range of pathophysiological conditions. Evidence adduced from whole animal and in vitro studies indicates that enhanced HO-1 activity may either ameliorate or exacerbate neural injury, effects likely contingent upon the specific model employed, the duration and intensity of HO-1 induction, and the chemistry of the local redox microenvironment. HO-1 hyperactivity also promotes mitochondrial sequestration of nontransferrin iron in oxidatively challenged astroglia and may thereby contribute to the pathological iron deposition and bioenergetic failure amply documented in aging and degenerating human neural tissues.

Hemin induces heme oxygenase-1 in spinal cord vasculature and attenuates barrier disruption and neutrophil infiltration in the injured murine spinal cord

Heme oxygenase-1 (HO-1) has been shown to alter vascular function in part by attenuating inflammation. We induced HO-1 in blood vessels in the spinal cord by systemic administration of hemin. Twenty-four hours later, immediately prior to euthanasia, fluorescence conjugated Lycopersicon esculentum (tomato) lectin was given intravenously to label the vasculature. HO-1 was induced in blood vessels, particularly in the white matter, as evidenced by the immunolocalization of HO-1 in lectin positive vessels. Western blots confirmed the hemin-mediated induction of HO-1 in the uninjured spinal cord. We next examined the extent to which treatment with hemin or vehicle, 24 h prior to a moderate contusion injury, influenced early vascular dysfunction in the injured cord. All animals were euthanized 24 h after injury. Luciferase, a marker of barrier integrity, was given intravenously 30 min prior to euthanasia. The spinal cord was either prepared for quantification of luciferase activity or fixed by vascular perfusion and prepared for the immunolocalization of neutrophils. There was a significant attenuation of barrier permeability to luciferase and a significant reduction in the number of neutrophils in hemin treated animals as compared to the vehicle treated group. Together, these findings demonstrate that vascular induction of HO-1 modulates barrier function and neutrophil infiltration and suggest that this protein may be useful for limiting the early vascular dysfunction and inflammation that occurs in the acutely injured spinal cord.

Heme oxygenase and its products in the nervous system

Heme oxygenase (HO) cleaves the tetrapyrrolic ring of cellular heme moieties liberating carbon monoxide (CO) and equimolar amounts of free iron and biliverdin (BV). BV is in turn converted into bilirubin (BR) by the cytosolic enzyme BV reductase. Three HO isoforms have been described to date: HO-1, HO-2, and HO-3. All these isoforms are present in nervous tissue with different localizations and regulation. CO, the gaseous product of HO, exerts its biological effects through the activation of soluble guanylyl cyclase, but alternative signaling pathways, such as the activation of cyclooxygenase, have also been reported in the brain. In vitro and in vivo studies showed that CO, at the hypothalamic level, plays a key role in the modulation of stress response because it inhibits the release of antiinflammatory neuropeptides, such as corticotropin-releasing hormone and arginine vasopressin, and increases body temperature in rodents exposed to psychological stressors (stress fever). In the last few years, a new role of BR as an endogenously produced antioxidant has emerged, and several reports have shown that BR contributes to prevent cell damage mediated by reactive oxygen species, as well as nitric oxide and its congeners.

Differential induction of heme oxygenase and other stress proteins in cultured hippocampal astrocytes and neurons by inorganic lead

We examined the effects of exposure to inorganic lead (Pb2+) on the induction of stress proteins in cultured hippocampal neurons and astrocytes, with particular emphasis on the induction of heme oxygenase-1 (HO-1). In radiolabeled neuronal cultures, Pb2+ exposure had no significant effect on the synthesis of any protein at any concentration (up to 250 microM) or duration of exposure (up to 4 days). In radiolabeled astrocyte cultures, however, Pb2+ exposure (100 nM to 100 microM; 1-4 days) increased synthesis of proteins with approximate molecular weights of 23, 32, 45, 57, 72, and 90 kDa. Immunoblot experiments showed that Pb2+ exposure (100 nM to 10 microM, 1-14 days) induces HO-1 synthesis in astrocytes, but not in neurons; this is probably the 32-kDa protein. The other heme oxygenase isoform, HO-2, is present in both neurons and astrocytes, but is not inducible by Pb2+ at concentrations up to 100 microM. HO-1 can be induced by a variety of stimuli. We found that HO-1 induction in astrocytes is increased by combined exposure to Pb2+ and many other stresses, including heat, nitric oxide, H2O2, and superoxide. One of the stimuli that may induce HO-1 is oxidative stress. Lead exposure causes oxidative stress in many cell types, including astrocytes. Induction of HO-1 by Pb2+ is reduced by the hydroxyl radical scavengers dimethylthiourea (DMTU) and mannitol, but not by inhibitors of calmodulin, calmodulin-dependent protein kinases, protein kinase C, or extracellular signal-regulated kinases (ERK). Therefore, we conclude that oxidative stress is an important mechanism by which Pb2+ induces HO-1 synthesis in astrocytes.

Hematoma Removal, Heme, and Heme Oxygenase Following Hemorrhagic Stroke

The hemorrhagic strokes, intracerebral (ICH) and subarachnoid hemorrhage (SAH), often have poor outcomes. Indeed, the most common hemorrhagic stroke, ICH, has the highest mortality and morbidity rates of any stroke subtype. In this report, we discuss the evidence for the staging of red blood cell removal after ICH and the significance of control of this process. The protective effects of clinically relevant metalloporphyrin heme oxygenase inhibitors in experimental models of ICH and in superficial siderosis are also discussed. We also examine literature paradoxes related to both heme and heme oxygenase in various disorders of the central nervous system. Last, new data are presented that support the concept that heme, although primarily a pro-oxidant, can also have antioxidant properties.

Heme oxygenase-2 knockout neurons are less vulnerable to hemoglobin toxicity

When cortical neurons are exposed to hemoglobin, they undergo oxidative stress that ultimately results in iron-dependent cell death. Heme oxygenase (HO)-2 is constitutively expressed in neurons and catalyzes heme breakdown. Its role in the cellular response to hemoglobin is unclear. We tested the hypothesis that HO-2 attenuates hemoglobin neurotoxicity by comparing reactive oxygen species (ROS) formation and cell death in wild-type and HO-2 knockout cortical cultures. Consistent with prior observations, hemoglobin increased ROS generation, detected by fluorescence intensity after dihydrorhodamine 123 or dichlorofluorescin-diacetate loading, in wild-type neurons. This fluorescence was significantly attenuated in cultures prepared from HO-2 knockout mice, and cell death as determined by propidium iodide staining was decreased. In other experiments, hemoglobin exposure was continued for 19 h; cell death as quantified by LDH release was decreased in knockout cultures, and was further diminished by treatment with the HO inhibitor tin protoporphyrin IX. In contrast, HO-2 knockout neurons were more vulnerable than wild-type neurons to inorganic iron. HO-1, ferritin, and superoxide dismutase expression in HO-2 -/- cultures did not differ significantly from that observed in HO-2 +/+ cultures; cellular glutathione levels were slightly higher in knockout cultures. These results suggest that heme breakdown by heme oxygenase accelerates the oxidative neurotoxicity of hemoglobin, and may contribute to neuronal injury after CNS hemorrhage.

Endogenous heme oxygenase prevents impairment of cerebral vascular functions caused by seizures

In newborn pigs, the mechanism of seizure-induced cerebral hyperemia involves carbon monoxide (CO), the vasodilator product of heme catabolism by heme oxygenase (HO). We hypothesized that seizures cause cerebral vascular dysfunction when HO activity is inhibited. With the use of cranial window techniques, we examined cerebral vascular responses to endothelium-dependent (hypercapnia and bradykinin) and endothelium-independent (isoproterenol and sodium nitroprusside) dilators during the recovery from bicuculline-induced seizures in saline controls and in animals pretreated with a HO inhibitor, tin protoporphyrin (SnPP). SnPP (3 mg/kg iv) blocked dilation to heme and reduced the CO level in cortical periarachnoid cerebrospinal fluid, indicating HO inhibition in the cerebral microcirculation. In saline control piglets, seizures increased the CO level, which correlated with the time-dependent cerebral vasodilation; during the recovery (2 h after seizure induction), responses to all vasodilators were preserved. In SnPP-treated animals, cerebral vasodilation and the CO responses to seizures were greatly reduced, and cerebral vascular reactivity was severely impaired during the recovery. These findings suggest that HO in the cerebral microcirculation is rapidly activated during seizures and provides endogenous protection against seizure-induced vascular injury.

Heme oxygenase-2 protects against lipid peroxidation-mediated cell loss and impaired motor recovery after traumatic brain injury

After traumatic brain injury (TBI), substantial extracellular heme is released from hemoproteins during hemorrhage and cell injury. Heme oxygenase (HO) isozymes are thought to detoxify the pro-oxidant heme to the potent antioxidant, bilirubin. HO-1, the inducible isozyme, is expressed in glial populations after injury and may play a protective role. However, the role of HO-2, the predominant and constitutively expressed isozyme in the brain, remains unclear after TBI. We used a controlled cortical impact injury model to determine the extent and mechanism of damage between HO-2 knock-out (KO) (-/-) and wild-type (WT) (+/+) mice. The specific cellular and temporal expressions of HO-2 and HO-1 were characterized by immunocytochemistry and Western blots. HO-2 was immunolocalized in neurons both before and after TBI, whereas HO-1 was highly upregulated in glia only after TBI. HO activity determined by gas chromatography using brain sonicates from injured HO-2 KO mice was significantly less than that of HO-2 wild types, despite the induction of HO-1 expression after TBI. Cell loss was significantly greater in KO mice in areas including the cortex, the CA3 region of hippocampus, and the lateral dorsal thalamus. Furthermore, motor recovery after injury, as measured by the rotarod assay and an inclined beam-walking task, was compromised in the KO mice. Finally, brain tissue from injured HO-2 KO mice exhibited decreased ability to reduce oxidative stress, as measured with an Fe(2+)/ascorbic acid-mediated carbon monoxide generation assay for lipid peroxidation susceptibility. These findings demonstrate that HO-2 expression protects neurons against TBI by reducing lipid peroxidation via the catabolism of free heme.

Coordinate regulation of glutathione biosynthesis and release by Nrf2-expressing glia potently protects neurons from oxidative stress

Astrocytes have a higher antioxidant potential in comparison to neurons. Pathways associated with this selective advantage include the transcriptional regulation of antioxidant enzymes via the action of the Cap'n'Collar transcription factor Nrf2 at the antioxidant response element (ARE). Here we show that Nrf2 overexpression can reengineer neurons to express this glial pathway and enhance antioxidant gene expression. However, Nrf2-mediated protection from oxidative stress is conferred primarily by glia in mixed cultures. The antioxidant properties of Nrf2-overexpressing glia are more pronounced than those of neurons, and a relatively small number of these glia (< 1% of total cell number added) could protect fully cocultured naive neurons from oxidative glutamate toxicity associated with glutathione (GSH) depletion. Microarray and biochemical analyses indicate a coordinated upregulation of enzymes involved in GSH biosynthesis (xCT cystine antiporter, gamma-glutamylcysteine synthetase, and GSH synthase), use (glutathione S-transferase and glutathione reductase), and export (multidrug resistance protein 1) with Nrf2 overexpression, leading to an increase in both media and intracellular GSH. Selective inhibition of glial GSH synthesis and the supplementation of media GSH indicated that an Nrf2-dependent increase in glial GSH synthesis was both necessary and sufficient for the protection of neurons, respectively. Neuroprotection was not limited to overexpression of Nrf2, because activation of endogenous glial Nrf2 by the small molecule ARE inducer, tert-butylhydroquinone, also protected against oxidative glutamate toxicity.

Regulation of heme oxygenase-1 gene expression by anoxia and reoxygenation in primary rat hepatocyte cultures

Heme oxygenase (HO) catalyzes the rate-limiting enzymatic step of heme degradation and regulates the cellular heme content. Gene expression of the inducible isoform of HO, HO-1, is upregulated in response to various oxidative stress stimuli. To investigate the regulatory role of anoxia and reoxygenation (A/R) on hepatic HO-1 gene expression, primary cultures of rat hepatocytes were exposed after an anoxia of 4 hr to normal oxygen tension for various lengths of time. For comparison, gene expression of the noninducible HO isoform, HO-2, and that of the heat-shock protein 70 (HSP70) were determined. During reoxygenation, a marked increase of HO-1 and HSP70 steady-state mRNA levels was observed, whereas no alteration of HO-2 mRNA levels occurred. Corresponding to HO-1 mRNA, an increase of HO-1 protein expression was determined by Western blot analysis. The anoxia-dependent induction of HO-1 was prevented by pretreatment with the transcription inhibitor, actinomycin D, but not by the protein synthesis inhibitor, cycloheximide, suggesting a transcriptional regulatory mechanism. After exposure of hepatocytes to anoxia, the relative levels of oxidized glutathione increased within the first 40 min of reoxygenation. Pretreament of cell cultures with the antioxidant agents, beta-carotene and allopurinol, before exposure to A/R led to a marked decrease of HO-1 and HSP70 mRNA expression during reoxygenation. An even more pronounced reduction of mRNA expression was observed after exposure to desferrioxamine. Taken together, the data demonstrate that HO-1 gene expression in rat hepatocyte cultures after A/R is upregulated by a transcriptional mechanism that may be, in part, mediated via the generation of ROS and the glutathione system.

Hyperbilirubinemia protects against focal ischemia in rats

Heme oxygenase-1 (HO1) catalyzes oxidation of the heme molecule in concert with NADPH-cytochrome P450 reductase following the specific cleavage of heme into carbon monoxide, iron, and biliverdin, which is rapidly metabolized to bilirubin. HO1 is a stress-inducible protein that protects cells against oxidative injury, but its protective mechanism is not fully understood. The Eizai hyperbilirubinemic rat (EHBR), a mutant strain derived from the Sprague-Dawley rat (SDR), has a mutation in the gene for the canalicular multispecific organic anion transporter, which results in a phenotype of hyperbilirubinemia, and thus is a model of Dubin-Johnson syndrome in humans. In this study, we compared EHBR and SDR with regard to neuronal death induced by 2 hr of occlusion of the middle cerebral artery and reperfusion. In EHBR, the area that was immunoreactive for microtubule-associated protein-2 was significantly reduced, and the HO1-immunoreactive area was smaller than that in SDR. These results suggest that bilirubin has essentially a neuroprotective effect against focal ischemia and may participate in HO1-induced neuroprotection.

Level of haem oxygenase does not obligatorily reflect the sensitivity of PC12 cells to an oxidative shock induced by glutathione depletion

In order to investigate the function of haem oxygenase in neuronal cell death or survival, we have determined in PC12 cells whether induction of haem oxygenase mRNA and protein or inhibition of haem oxygenase activity may be able to modulate the cell response to an oxidative stress. Inhibition of glutathione biosynthesis by buthionine sulfoximine (BSO) has indeed been demonstrated, in this cell line, to decrease the intracellular content of glutathione and to trigger a gradual and programmed cell death. Inhibition of haem oxygenase by zinc protoporphyrin IX, a potent inhibitor of this enzyme, or by a recently described peptidic inhibitor, induced a significant decrease in the toxicity of BSO. This protective action was not due to an alteration in the metabolism of glutathione and was still observed when the protecting agent was added several hours after BSO treatment. Induction of haem oxygenase-1 mRNA and protein by either haemin or pyrrolidine dithiocarbamate was associated with no protection or a significant reduction in the toxicity of BSO respectively. Our results indicate that induction of haem oxygenase-1 is not obligatorily associated with an improved resistance towards oxidative stress and suggest that a byproduct of haem degradation may also become detrimental.

The influence of oxidative stress on catalase and MnSOD gene transcription in astrocytes

The brain is particularly vulnerable to oxygen free radicals, which have been implicated in the pathology of several neurological disorders. The antioxidant enzyme (AOE) system of the brain may play an important role in the protection against such oxidative stress. We investigated the influence of oxidative stress on the transcription of catalase and MnSOD mRNA. Primary rat astroglial cell cultures were treated either with hydrogen peroxide (H2O2), as a direct mediator of oxidative stress, or with the redox cycling compound paraquat. Both substances led to an increase of catalase and MnSOD mRNA levels. To further elucidate the mechanisms residing behind this increase, transfection experiments were performed. Transient transfection of primary astroglial cells with a reporter plasmid containing the upstream region of the catalase gene showed a decrease in reporter gene activity after exposure of transfected cells to either H2O2 or paraquat. In contrast, transfection experiments done with reporter plasmids for the MnSOD upstream region resulted in an increase of reporter gene activity after H2O2 as well as after paraquat treatment of transfected cells. These results indicate transcriptional regulation of MnSOD and post-transcriptional regulation of catalase gene expression after oxidative stress in primary rat astrocytes.

Gaseous neuromodulators in the control of neuroendocrine stress axis

The gaseous neuromodulator carbon monoxide has been shown to reduce the stimulated release of stress neuropeptides, such as vasopressin and oxytocin, from the rat hypothalamus in vitro, while evidence concerning corticotropin-releasing hormone is controversial. In vivo studies have been conducted in the rat, inhibiting heme oxygenase activity--and hence carbon monoxide biosynthesis--in the central nervous system by means of specific heme oxygenase blockers; these studies showed that basal heme oxygenase activity tends to oppose exaggerated increases in vasopressin secretion following immune-inflammatory challenges, whereas it favors the normal rise in circulating ACTH which follows footshock. Another gas normally produced in mammalian brains under basal conditions, hydrogen sulfide, also appears to play a role in the control of the hypothalamo-pituitary-adrenal axis. Indeed, increases in hydrogen sulfide levels within the hypothalamus, either obtained with hydrogen sulfide-enriched media or by the addition of the hydrogen sulfide precursor S-adenosyl-methionine, are associated with the inhibition of the stimulated release of corticotropin-releasing hormone from rat hypothalamic explants. Parellel in vivo experiments in the rat under resting conditions and after stress-induced adrenocortical activation show that S-adenosyl-methionine significantly reduces the rise in serum corticosterone levels caused by 1-h exposure to cold. These results demonstrate the pathophysiological importance of both carbon monoxide and hydrogen sulfide in the regulation of neuroendocrine function.

Brain vessels normally undergo cyclic activation and inactivation: evidence from tumor necrosis factor-alpha, heme oxygenase-1, and manganese superoxide dismutase immunostaining of vessels and perivascular brain cells

Studies of vascular biology during the past decade have identified an expanding list of agonists and antagonists that regulate local hemostasis, inflammation, and reactivity in blood vessels. Interactions at the blood-endothelial interface are intricate and complex and have been postulated to play a role in the initiation of stroke and the progression of brain injury during early hours of ischemia, particularly in conjunction with reperfusion injury (Hallenbeck, 1996). In the current study of normal and activated vessels in rat brain, immunoreactive tumor necrosis factor-alpha (TNF-alpha), heme oxygenase-1 (HO-1), and manganese superoxide dismutase (MnSOD) exhibit concentric perivascular rings involving vessel wall and surrounding parenchyma that appear to coincide with one another in serial sections. The ring patterns suggest periodic radial expansion of these molecules released through a process of cyclic activation and inactivation of brain vessel segments. In this process, the rings appear randomly scattered instead of affecting all vessels within a high power field (HPF) synchronously. The average number of vessels per HPF (mean +/- SD) with perivascular cuffs of immunoreactive MnSOD increased from 51 +/- 28 in Wistar, 72 +/- 46 in Wistar-Kyoto, and 84 +/- 30 in Sprague Dawley rats (no spontaneous strokes) to 184 +/- 72 in spontaneously hypertensive stroke-prone rats (spontaneous strokes). Perivascular immunoreactive cuffs are also increased in spontaneously hypertensive rats by induction of cytokine expression by lipopolysaccharide (64 +/- 15 vs. 131 +/- 32 /HPF). The patterns of TNF-alpha, HO-1, and MnSOD in naïve animals are interpreted to indicate that focal hemostatic balance normally fluctuates in brain vessels and influences surrounding parenchymal cells. Perivascular immunoreactive cuffs representing this process are more frequent in animals with lipopolysaccharide-induced endothelial activation or genetic stroke proneness.

The roles of carbon monoxide and nitric oxide in the control of the neuroendocrine stress response: complementary or redundant

There is widespread evidence in favour of nitric oxide (NO) acting as a gaseous neurotransmitter in the central nervous system, diffusing from its cells of origin and affecting surrounding neuronal tissue in evanescent three-dimensional waves. This is also true of the hypothalamus, where amongst other activities NO inhibits stimulation of corticotrophin-releasing hormone (CRH) and vasopressin release by inflammatory stressors, effects thought to be mediated by binding with soluble guanylate cyclase (sGC). Carbon monoxide is being increasingly recognised as another gaseous neuromodulator, but with principal effects on other hemoproteins such as cyclo-oxygenase, and a distinctly different profile of localisation.NO is predominantly a pro-inflammatory agent in the periphery while CO is often anti-inflammatory. In the hypothalamus, the actions of CO are also distinct from those of NO,with marked antagonistic effects on the inflammatory release of vasopressin, both in vitro and in vivo, but with little involvement in the regulation of CRH. Thus, it would appear that these apparently similar gases exert quite distinct and separate effects, although they cause broadly similar overall changes in the secretion of neuroendocrine stress hormones. We conclude that these two gases may play significant but different roles in the control of the neuroendocrine stress response, but one common feature may be attenuation of inflammation-induced release of stress hormones.

A functional link between heme oxygenase and cyclo-oxygenase activities in cortical rat astrocytes

Recent evidence shows that the activation of heme oxygenase (HO) within the CNS is associated with increased prostanoid production. In this study, we investigated whether changes in HO activity induced by pharmacological manipulation are associated with parallel variations in cyclo-oxygenase (COX) activity and prostaglandin production in an in vitro paradigm of CNS cells, i.e. primary cultures of rat cortical astrocytes. Pharmacological tools commonly used to induce changes in HO activity, namely the HO enhancers hemin and CoCl(2) as well as the HO inhibitor Sn-mesoporphyrin-9 (SnMP9), were tested in our model, and the variations in COX activity associated with the above treatments were monitored by measuring a COX end product, prostaglandin E2 (PGE2), released into the incubation medium. We found that the increase in HO activity induced by hemin and/or CoCl(2) was not consistently associated with increases in prostaglandin production, whereas HO inhibition by SnMP9 was normally followed by a decrease in PGE2 release. The above effect was observed after both acute (30 min) and prolonged (24 hr) incubations, suggesting that baseline HO activity contributes to the maintenance of normal PG production in this model. Experiments with the stable HO end products biliverdin and bilirubin suggest that these products may play a role in mediating HO-induced COX activation.

Astrocyte phenotype and prevention against oxidative damage in neurotoxicity

Astrocytes possess a potent array of protective systems. These are chiefly targeted against oxidised products and radicals, which are frequently present in increased amounts following exposure of nervous tissue to a range of toxic insults. Following exposure to the toxic chemicals astrocytes commonly respond by alteration in phenotype with upregulation of a large number of molecules, including those controlling the protective systems. This article summarizes evidence, largely obtained from in vitro studies, which supports the concept that some of the changes in astrocyte phenotype are associated with increased protection against the cytotoxicity caused by the oxidative damage that results from exposure to range of neurotoxicants.

Heme oxygenase-1: role in brain aging and neurodegeneration

The mechanisms responsible for excessive iron deposition and mitochondrial insufficiency in the aging and degenerating nervous system remain poorly understood. Heme oxygenase-1 (HO-1) is a 32kDa stress protein that degrades heme to biliverdin, free iron and carbon monoxide. Our laboratory has shown that cysteamine, dopamine, beta-amyloid, IL-1beta and TNF-alpha up-regulate HO-1 followed by mitochondrial sequestration of non-transferrin-derived 55Fe in cultured rat astroglia. In these cells and in rat astroglia transfected with the human HO-1 gene, mitochondrial iron trapping is abrogated by the HO-1 inhibitors, tin-mesoporphyrin and dexamethasone. We determined that HO-1 immunoreactivity is enhanced greatly in neurons and astrocytes of the hippocampus and cerebral cortex of Alzheimer subjects and co-localizes to senile plaques and neurofibrillary tangles (NFT). HO-1 staining is also augmented in astrocytes and decorates neuronal Lewy bodies in the Parkinson nigra. Collectively, our findings suggest that HO-1 over-expression contributes to the pathological iron deposition and mitochondrial damage documented in these aging-related neurodegenerative disorders. We recently observed that, paradoxically, HO-1 mRNA levels are markedly suppressed in peripheral lymphocytes of patients with early sporadic Alzheimer disease and may thus provide a useful biological marker of this condition.

Expression of antisense hsp70 is a major determining factor in heat-induced cell death of P-19 carcinoma cells

Overexpressed heat shock protein 70 (Hsp70) is known to be associated with thermoprotection in a number of cell lines and transgenic animals. We hypothesized that because overexpression of Hsp70 protects cells from lethal heat stress, inhibition of expression should make cells susceptible to heat stress. The model used for this study was a stably transfected P-19 carcinoma cell line expressing antisense hsp70 under the control of the hsp70b promoter. The results showed marked inhibition of Hsp70 expression after heat shock correlated with heat-induced cell death. Hsp90 and Hsc70 protein expression were not affected by the antisense construct. Unexpectedly, heme oxygenase (HO-1), another highly inducible heat shock protein, was not induced after heat shock in the antisense hsp70 cell line. Heat shock transcription factor-1 (HSF-1) was in a highly phosphorylated state in the antisense cell line before and after heat shock. This was in contrast to the untransfected control P-19 cells where HSF-1 was primarily highly phosphorylated after heat shock. A control cell line expressing only the vector, pMAMneo, without the antisense construct also showed partial loss of Hsp70 induction but not increased cell death after heat shock. The findings support the role of Hsp70 in thermoresistance.

Heme oxygenase-1 induction protects murine cortical astrocytes from hemoglobin toxicity

Exposure to micromolar concentrations of hemoglobin (Hb) results in the oxidative death of cultured cortical neurons, but glia are resistant. The role of heme oxygenase-1 (HO-1) induction on this glial resistance was investigated. Within two hours of exposure to 5 microM Hb, immunoblotting demonstrated an increase in HO-1 in confluent glial cultures. Consistent with prior observations, 23-30 h Hb exposure had little or no effect on glial viability, as assessed by lactate dehydrogenase release. Concomitant treatment with the HO inhibitors tin protoporphyrin IX or the D-amino acid peptide rvnlrialry resulted in release of 40-71% of glial lactate dehydrogenase; protein synthesis inhibition with cycloheximide produced a similar effect. These results are consistent with the hypothesis that HO-1 induction protects cortical astrocytes from Hb toxicity.

Inhibition of heme oxygenase in the central nervous system potentiates endotoxin-induced vasopressin release in the rat

Previous in vitro studies have shown that increases in endogenous carbon monoxide (CO) generation via activation of the enzyme heme oxygenase (HO) within the rat hypothalamus are associated with the reduced release of the neuropeptides, vasopressin (AVP) and oxytocin, while evidence concerning corticotrophin-releasing hormone (CRH) is controversial. The present study investigated whether there is also a functional relationship between the HO-CO pathway and AVP and corticosterone (Cort) in vivo. Male Wistar rats were challenged with bacterial lipopolysaccharide (LPS) at doses producing significant activation of the hypothalamo-pituitary-adrenal (HPA) axis. LPS was given alone or after pretreatment with the HO inhibitor Sn-protoporphyrin-9 (SnPP9). The latter was injected either intraperitoneally (i.p.) or by intracerebroventricular (i.c.v.) route. SnPP9 given i.p. failed to modify either basal or LPS-stimulated levels of AVP and Cort. On the contrary, i.c.v. SnPP9 strongly potentiated LPS-induced AVP release and significantly enhanced basal serum Cort levels, although it failed to potentiate stimulation by LPS. The LPS + i.c.v. SnPP9 also significantly reduced the hypothalamic stores of AVP compared to controls, correlating with increased circulating levels of AVP. Taken collectively, these data are in concordance with previous in vitro observations showing that the HO-CO pathway acts centrally to attenuate endotoxin-stimulated AVP release, while having less effects on the pituitary-adrenal axis.

Expanded expression of heme oxygenase-1 (HO-1) in the hypothalamic median eminence of aged as compared with young rats: an immunocytochemical study

This study was performed to examine the differences in expression of heme oxygenase protein with age using immunocytochemistry. We compared the contents of HO-1 and HO-2 between young and aged rats using immunocytochemical methods. Stronger HO-1 expression was detected in the internal layer of the median eminence (ME) of aged than of young rats. Moreover, the cells expressing HO-1 were larger in the aged than the young animals. Electron microscopy indicated these cells with HO-1-like immunoreactivity (HO-1-LI) to be astrocytes. These findings suggested that the expression of HO-1 increased in the ME with age. The significance of this increased expression of HO-1 with age will be discussed briefly.

Protective role of heme oxygenase-1 in oxidative stress-induced neuronal injury

Heme oxygenase-1 (HO-1) is a stress protein induced in response to a variety of oxidative challenges. After treatment of the hybrid septal cells SN 56 with beta-amyloid peptide (beta-AP1-40) or hydrogen peroxide (H2O2), we detected high levels of reactive oxygen species, accompanied by a significant elevation in HO-1 expression. Levels of HO-1 increased and then decreased following cell loss. Pretreatment of SN 56 cells with HO-1 antisense oligonucleotides dramatically decreased the immunoreactivity of HO-1 and significantly enhanced the cytotoxicity of beta-AP1-40 and H2O2. In contrast, pretreatment with hemin, an HO-1 inducer, increased the expression of HO-1 and decreased the beta-AP1-40- and H2O2-induced cytotoxicity. These findings support the importance of HO-1 in protecting neurons against oxidative stress-induced injury.

Bilirubin, formed by activation of heme oxygenase-2, protects neurons against oxidative stress injury

Heme oxygenase (HO) catalyzes the conversion of heme to carbon monoxide, iron, and biliverdin, which is immediately reduced to bilirubin (BR). Two HO active isozymes exist: HO1, an inducible heat shock protein, and HO2, which is constitutive and highly concentrated in neurons. We demonstrate a neuroprotective role for BR formed from HO2. Neurotoxicity elicited by hydrogen peroxide in hippocampal and cortical neuronal cultures is prevented by the phorbol ester, phorbol 12-myristate 13-acetate (PMA) via stimulation of protein kinase C. We observe phosphorylation of HO2 through the protein kinase C pathway with enhancement of HO2 catalytic activity and accumulation of BR in neuronal cultures. The neuroprotective effects of PMA are prevented by the HO inhibitor tin protoporphyrin IX and in cultures from mice with deletion of HO2 gene. Moreover, BR, an antioxidant, is neuroprotective at nanomolar concentrations.

The mosaic of brain glial hyperactivity during normal ageing and its attenuation by food restriction

Food restriction of adult rodents increases lifespan, with commensurate attenuation of age-related pathological lesions in many organs, as well as attenuation of normal ageing changes that are distinct from gross lesions. Previous work showed that chronic food restriction attenuated age-associated astrocyte and microglial hyperactivity in the hippocampal hilus, as measured by expression of glial fibrillary acidic protein and major histocompatibility complex II antigen (OX6). Here, we examined other markers of astrocyte and microglial activation in gray and white matter regions of ad libitum-fed (Brown Norway x Fischer 344) F1 male rats aged three and 24 months and chronic food-restricted rats aged 24 months. In situ hybridization and immunohistochemical techniques evaluated glial expression of glial fibrillary acidic protein, apolipoprotein E, apolipoprotein J (clusterin), heme oxygenase-1, complement 3 receptor (OX42), OX6 and transforming growth factor-beta1. All markers were elevated in the corpus callosum during ageing and were attenuated by food restriction, but other regions showed marked dissociation of the extent and direction of changes. Astrocytic activation, as measured with glial fibrillary acidic protein expression (coding and intron-containing RNA, immunoreactivity), increased with age in the corpus callosum, basal ganglia and hippocampus. Generally, food restriction attenuated the age-related increase in glial fibrillary acidic protein messenger RNA and immunoreactivity. Food restriction also reduced the age-related increase in apolipoprotein J and E messenger RNA and heme oxygenase-1 immunoreactivity in the basal ganglia and corpus callosum. However, astrocytes in the hilus of the hippocampus showed an age-related decrease in apolipoprotein J and E messenger RNA, which was further intensified by food restriction. The age-associated microglial activation measured by OX6 and OX42 immunoreactivity was reduced by food restriction in most subregions. The localized subsets of glial age changes and effects of food restriction comprise a mosaic of ageing consistent with the regional heterogeneity of ageing changes reported by others. In particular, age has a differential effect on astrocytic and microglial hyperactivity in gray versus white matter areas. The evident mosaic of glial ageing and responses to food restriction suggests that multiple mechanisms are at work during ageing.

Acute phencyclidine neurotoxicity in rat forebrain: induction of haem oxygenase-1 and attenuation by the antioxidant dimethylthiourea

Phencyclidine and other N-methyl-D-aspartate receptor antagonists are toxic to pyramidal neurons in the posterior cingulate/retrosplenial cortex of rat brain. Previous studies have shown induction of heat shock protein 70 in affected neurons. In this study, expression of haem oxygenase-1, a heat shock protein induced by oxidative stress, was examined in rat forebrain after administration of a single intraperitoneal dose of phencyclidine (50 mg/kg). Northern and Western blot analyses of brain tissue extracts from phencyclidine-treated rats revealed a marked induction of haem oxygenase-1 mRNA and protein, respectively. Immunohistochemistry studies revealed that phencyclidine increased haem oxygenase-1 immunoreactivity primarily in posterior cingulate/retrosplenial, piriform and entorhinal cortices, striatum and hippocampus. Haem oxygenase-1 protein was induced in non-neuronal cells, mainly astrocytes. Some microglia expressing haem oxygenase-1 protein were also found in the posterior cingulate/retrosplenial cortex. Haem oxygenase-1 immunoreactive astrocytes and microglia were present in close proximity to the heat shock protein 70-positive neurons in the posterior cingulate/retrosplenial cortex following phencyclidine. Pretreatment of rats with 1,3-dimethylthiourea, an antioxidant, significantly reduced haem oxygenase-1 protein induction by phencyclidine. Thus, induction of haem oxygenase-1 in glia by phencyclidine appears to be mediated mostly by oxidative stress. Experiments with the amino cupric silver stain for neuronal degeneration revealed phencyclidine-induced neurotoxicity in the posterior cingulate/retrosplenial cortex. The number of affected neurons was significantly reduced after 1,3-dimethylthiourea pretreatment. This suggests that the neurotoxicity of N-methyl-D-aspartate antagonists is due in part to the oxidative stress and may be amenable to therapeutic interventions.

Effect of arsenite on induction of CYP1A and CYP2H in primary cultures of chick hepatocytes

In earlier studies, treatment with sodium arsenite was shown to decrease total hepatic CYP in rats. A concomitant increase in heme oxygenase, the rate-limiting step in heme degradation to biliverdin, was considered responsible for the decrease in CYP. Here we investigated the effect of sodium arsenite on induction of CYP2H, CYP1A, and heme oxygenase in primary cultures of chicken embryo hepatocytes. When added simultaneously with inducer, arsenite inhibited phenobarbital-mediated increases in CYP2H and 3-methylcholanthrene-mediated increases in CYP1A, as measured enzymatically and immunochemically. Near maximal decreases were observed in these forms of CYP at a concentration of 2.5 microM sodium arsenite. The concentration-dependent decreases in CYP2H and CYP1A by sodium arsenite were concomitant with increases in heme oxygenase. Sodium arsenite was not toxic at concentrations as high as 10 microM, as indicated by protein synthesis and the reduction of MTT by intact cells. Sodium arsenite had no effect on induction of CYP2H1 mRNA, suggesting that the decreases in this form of CYP occurred post-transcriptionally. Treatment of cells with tin mesoporphyrin (SnMeso), an inhibitor of heme oxygenase, resulted in inhibition of arsenite-induced heme oxygenase. However, SnMeso did not alter the effect of arsenite to prevent phenobarbital-mediated increases in CYP2H protein. SnMeso alone inhibited phenobarbital-mediated increases in CYP2H. Inclusion of 2 or 5 microM exogenous heme with arsenite did not prevent the arsenite-mediated decrease in CYP2H. Combined treatment with heme and phenobarbital induced heme oxygenase to the same extent as treatment with heme, arsenite, and phenobarbital. However, CYP2H activity was decreased only when the treatment included arsenite. These results suggest that elevated levels of heme oxygenase alone are not responsible for arsenite-mediated decreases in CYP2H.

Heme oxygenase in the experimental ALS mouse

Heme oxygenase-1 (HO-1) is a stress protein inducible in some cells by oxidative stress. The status of heme oxygenase was investigated in a transgenic mouse model of amyotrophic lateral sclerosis (ALS) since oxidative mechanisms are postulated in neuronal injury. Three ALS mice [(SOD1-G93A)1Gur] and three controls [(SOD-1)2Gur] were obtained from The Jackson Laboratory. Behavioral differences suggestive of neurodegeneration in ALS mice developed at 4-5 months of age. All mice were killed at 7-8 months of age. Tissue vacuolation, cell loss, and the presence of GFAP+ cells were noted in the spinal cords of ALS mice. Spinal cord motor neurons in both control and ALS mice stained positive for heme oxygenase-2 (HO-2). While not precluding the presence of low levels of HO-1 neither immunohistochemical staining nor Western blot analysis provided evidence for significant HO-1 induction in degenerating spinal cord.

Targeted gene deletion of heme oxygenase 2 reveals neural role for carbon monoxide

Neuronal nitric oxide synthase (nNOS) generates NO in neurons, and heme-oxygenase-2 (HO-2) synthesizes carbon monoxide (CO). We have evaluated the roles of NO and CO in intestinal neurotransmission using mice with targeted deletions of nNOS or HO-2. Immunohistochemical analysis demonstrated colocalization of nNOS and HO-2 in myenteric ganglia. Nonadrenergic noncholinergic relaxation and cyclic guanosine 3',5' monophosphate elevations evoked by electrical field stimulation were diminished markedly in both nNOSDelta/Delta and HO-2(Delta)/Delta mice. In wild-type mice, NOS inhibitors and HO inhibitors partially inhibited nonadrenergic noncholinergic relaxation. In nNOSDelta/Delta animals, NOS inhibitors selectively lost their efficacy, and HO inhibitors were inactive in HO-2(Delta)/Delta animals.

Increased 3-nitrotyrosine and oxidative damage in mice with a human copper/zinc superoxide dismutase mutation

Mutations in copper/zinc superoxide dismutase (SOD1) cause a subset of cases of autosomal dominant familial amyotrophic lateral sclerosis (FALS). Transgenic mice that express these point mutations develop progressive paralysis and motor neuron loss thought to be caused by a gain-of-function of the enzyme. The gain-of-function may be an enhanced ability of the mutant SOD1 to generate .OH radicals or to facilitate peroxynitrite-mediated nitration of proteins. We found significant increases in concentrations of 3-nitrotyrosine, a marker of peroxynitrite-mediated nitration, in upper and lower spinal cord and in cerebral cortex of transgenic mice with the FALS-associated G93A mutation. Malondialdehyde, a marker of lipid peroxidation, was increased in cerebral cortex. 3-Nitrotyrosine-, heme oxygenase-1-, and malondialdehyde-modified protein immunoreactivities were increased throughout SOD1 transgenic mice spinal cord but particularly within motor neurons. These results suggest that the gain-of-function of at least one mutant SOD1 associated with FALS involves increased protein nitration and oxidative damage, which may play a role in neuronal degeneration.

Hypoxia-ischemia, but not hypoxia alone, induces the expression of heme oxygenase-1 (HSP32) in newborn rat brain

Heme oxygenase (HO) is the rate-limiting enzyme in the degradation of heme to produce bile pigments and carbon monoxide. The HO-1 isozyme is induced by a variety of agents such as heat, heme, and hydrogen peroxide. Evidence suggests that the bile pigments serve as antioxidants in cells with compromised defense mechanisms. Because hypoxia-ischemia (HI) increases the level of oxygen free radicals, the induction of HO-1 expression in the brain during ischemia could modulate the response to oxidative stress. To study the possible involvement of HO-1 in neonatal hypoxia-induced ischemic tolerance, we examined the brains of newborn rat pups exposed to 8% O2 (for 2.5 to 3 hours), and the brain of chronically hypoxic rat pups with congenital cardiac defects (Wistar Kyoto; WKY/ NCr). Heme oxygenase-1 immunostaining did not change after either acute or chronic hypoxia, suggesting that HO-1 is not a good candidate for explaining hypoxia preconditioning in newborn rat brain. To study the role of HO-1 in neonatal HI, 1-week-old rats were subjected to right carotid coagulation and exposure to 8% O2/92% N2 for 2.5 hours. Whereas HO enzymatic activity was unchanged in ipsilateral cortex and subcortical regions compared with the contralateral hemisphere or control brains, immunocytochemistry and Western blot analysis showed increased HO-1 staining in ipsilateral cortex, hippocampus, and striatum at 12 to 24 hours up to 7 days after HI. Double fluorescence immunostaining showed that HO-1 was expressed mostly in ED-1 positive macrophages. Because activated brain macrophages have been associated with the release of several cytotoxic molecules, the presence of HO-1 positive brain macrophages may determine the tissue vulnerability after HI injury.

Regulation of cyclic GMP levels in the rat frontal cortex in vivo: effects of exogenous carbon monoxide and phosphodiesterase inhibition

A microdialysis method combined with a sensitive radioimmunoassay was used to monitor cGMP release in the frontal cortex of the anesthetized rats in vivo. We assessed the relative contribution of endogenous nitric oxide (NO), and effects of exogenous carbon monoxide (CO) and phosphodiesterase activity, as possible regulators of cortical CGMP levels. Perfusion with CO-saturated aCSF (approximately 1 mM CO) failed to significantly stimulate cortical cGMP levels. For comparison, cerebellar cGMP levels increased by 2-fold during CO stimulation, followed by a prolonged response that was fully reversible with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). Cortical perfusion with zinc protopophyrin-IX (100 microM), a widely used inhibitor of the CO-generating enzyme heme oxygenase, suppressed cGMP levels by 50%, a response that spontaneously recovered in spite of the continuous presence of the metalloporphyrin. Perfusion with isobutylmethyl xanthine IBMX (1 mM) resulted in 5-fold increase in cortical cGMP levels, as compared to basal levels without IBMX. In the presence of IBMX, L-NAME suppressed basal cortical cGMP levels by 70% indicating that NO synthase activity generates the bulk of cGMP in this brain region, as previously shown for basal cGMP production in the hippocampus and the cerebellum. These data also emphasize a crucial role for phosphodiesterase activity in the maintenance of cGMP levels in vivo in the frontal cortex. The relatively weak responses to exogenous CO lend little support for a role of this gas in regulating basal cortical cGMP levels in vivo.

The heme oxygenase system: a regulator of second messenger gases

The heme oxygenase (HO) system consists of two forms identified to date: the oxidative stress-inducible protein HO-1 (HSP32) and the constitutive isozyme HO-2. These proteins, which are different gene products, have little in common in primary structure, regulation, or tissue distribution. Both, however, catalyze oxidation of heme to biologically active molecules: iron, a gene regulator; biliverdin, an antioxidant; and carbon monoxide, a heme ligand. Finding the impressive heme-degrading activity of brain led to the suggestion that "HO in brain has functions aside from heme degradation" and to subsequent exploration of carbon monoxide as a promising and potentially significant messenger molecule. There is much parallelism between the biological actions and functions of the CO- and NO-generating systems; and their regulation is intimately linked. This review highlights the current information on molecular and biochemical properties of HO-1 and HO-2 and addresses the possible mechanisms for mutual regulatory interactions between the CO- and NO-generating systems.

Long-term induction of haem oxygenase-1 (HSP-32) in astrocytes and microglia following transient focal brain ischaemia in the rat

Haem oxygenase-1 (HO-1) is a stress protein and a rate-limiting enzyme in haem degradation, generating ferrous iron, carbon monoxide and bile pigments. HO-1 has been suggested to be protective against oxidative stress. In the normal rodent brain the enzyme is localized in selected neuron populations, but heat shock, glutathione depletion in vivo and oxidative stress in vitro induce HO-1 predominantly in glial cells. We studied HO-1 expression in the brain following transient occlusion of the middle cerebral artery, and found increased mRNA levels in the ischaemic region from 4 h to 7 days after 90 min of ischaemia. The mRNA levels peaked at 12 h, and were localized perifocally. HO-1-immunoreactive astrocytes and microglial cells were seen in the perifocal area, in the ipsilateral and occasionally in the contralateral hippocampus. Some perifocal neurons were also HO-1-immunoreactive. In the infarcted area HO-1-positive microglia/macrophages were detected in double-labelling experiments. A microassay measuring the conversion of [14C]haem to [14C]bilirubin showed a two-fold increase in haem oxygenase activity in the infarcted core. These observations show a long-term induction of HO-1 protein and its activity following ischaemia-reperfusion brain injury, and indicate increased capacity for haem degradation and the generation of biologically active bile products, carbon monoxide and iron in astrocytes and some microglia/macrophages during focal brain ischaemia.

Induction of heme oxygenase-1 (HO-1) in glia after traumatic brain injury

In this study we examined the induction of heme oxygenase-1 (HO-1) in glia in the traumatized rat brain. HO-1 was immunolocalized in fixed sections of brain 3 h to 5 days after injury. Induction of this enzyme in astrocytes, microglia/macrophages, and oligodendrocytes was evaluated using immunofluorescent double labeling with monoclonal antibodies to glial fibrillary acidic protein, complement C3bi receptor, and myelin basic protein. Induction of HO-1 was apparent in the injured hemisphere and cerebellum as early as 24 h postinjury. The protein was likewise noted in similar regions of the brain at 72 h postinjury but appeared to be more widespread in its distribution. At 5 days postinjury, there was a notable decline in the degree of immunostaining for HO-1. HO-1 was typically induced in astrocytes in the cerebral cortex at the site of impact, in the deep cortical layers adjacent to the hemorrhagic lesions, and in the hippocampus. HO-1 was induced in Bergmann glia in the vermis of cerebellum. In addition, HO-1 was also induced in microglia/macrophages scattered throughout the ipsilateral cerebral cortex, cerebellum and subarachnoid space. These findings demonstrate prolonged glial induction of HO-1 in the traumatized brain. Such a response may reflect a protective role of these cells against secondary insults including oxidative stress.

Induction of neuronal apoptosis by camptothecin, an inhibitor of DNA topoisomerase-I: evidence for cell cycle-independent toxicity

Camptothecin is an S-phase-specific anticancer agent that inhibits the activity of the enzyme DNA topoisomerase-I (topo-I). Irreversible DNA double-strand breaks are produced during DNA synthesis in the presence of camptothecin, suggesting that this agent should not be toxic to nondividing cells, such as neurons. Unexpectedly, camptothecin induced significant, dose-dependent cell death of postmitotic rat cortical neurons in vitro; astrocytes were more resistant. Aphidicolin, an inhibitor of DNA polymerase alpha, did not prevent camptothecin-induced neuronal death, while death was prevented by actinomycin D and 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole as well as cycloheximide and anisomycin, inhibitors of RNA and protein synthesis, respectively. Camptothecin-induced neuronal death was apoptotic, as characterized by chromatin condensation, cytoplasmic shrinking, plasma membrane blebbing, and fragmentation of neurites. DNA fragmentation was also confirmed by the use of the in situ DNA end labeling assay. In addition, aurintricarboxylic acid, an inhibitor of the apoptotic endonuclease, partially protected against camptothecin-induced neuronal death. The toxicity of stereoisomers of a camptothecin analogue was stereospecific, demonstrating that toxicity was a result of inhibition of topo-I. The difference in sensitivity to camptothecin between neurons and astrocytes correlated with their transcriptional activity and level of topo-I protein expression. These data indicate important roles for topo-I in postmitotic neurons and suggest that topo-I inhibitors can induce apoptosis independent of DNA synthesis. We suggest a model based on transcriptionally mediated DNA damage, a novel mechanism of action of topo-I poisons.