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

Iron regulatory proteins increase neuronal vulnerability to hydrogen peroxide

Iron regulatory protein (IRP)-1 and IRP2 inhibit ferritin synthesis by binding to an iron responsive element in the 5'-untranslated region of its mRNA. The present study tested the hypothesis that neurons lacking these proteins would be resistant to hydrogen peroxide (H(2)O(2)) toxicity. Wild-type cortical cultures treated with 100-300microM H(2)O(2) sustained widespread neuronal death, as measured by lactate dehydrogenase assay, and a significant increase in malondialdehyde. Both endpoints were reduced by over 85% in IRP2 knockout cultures. IRP1 gene deletion had a weaker and variable effect, with approximately 20% reduction in cell death at 300microM H(2)O(2). Ferritin expression after H(2)O(2) treatment was increased 1.9- and 6.7-fold in IRP1 and IRP2 knockout cultures, respectively, compared with wild-type. These results suggest that iron regulatory proteins, particularly IRP2, increase neuronal vulnerability to oxidative injury. Therapies targeting IRP2 binding to ferritin mRNA may attenuate neuronal loss due to oxidative stress.

Heme oxygenase 2 deficiency increases brain swelling and inflammation after intracerebral hemorrhage

Intracerebral hemorrhage (ICH) remains a major medical problem and currently has no effective treatment. Hemorrhaged blood is highly toxic to the brain, and catabolism of the pro-oxidant heme, mainly released from hemoglobin, is critical for the resolution of hematoma after ICH. The degradation of the pro-oxidant heme is controlled by heme oxygenase (HO). We have previously reported a neuroprotective role for HO2 in early brain injury after ICH; however, in vivo data that specifically address the role of HO2 in brain edema and neuroinflammation after ICH are absent. Here, we tested the hypothesis that HO2 deletion would exacerbate ICH-induced brain edema, neuroinflammation, and oxidative damage. We subjected wild-type (WT) and HO2 knockout ((-/-)) mice to the collagenase-induced ICH model. Interestingly, HO2(-/-) mice had enhanced brain swelling and neuronal death, although HO2 deletion did not increase collagenase-induced bleeding; the exacerbation of brain injury in HO2(-/-) mice was also associated with increases in neutrophil infiltration, microglial/macrophage and astrocyte activation, DNA damage, peroxynitrite production, and cytochrome c immunoreactivity. In addition, we found that hemispheric enlargement was more sensitive than brain water content in the detection of subtle changes in brain edema formation in this model. Combined, these novel findings extend our previous observations and demonstrate that HO2 deficiency increases brain swelling, neuroinflammation, and oxidative damage. The results provide additional evidence that HO2 plays a critical protective role against ICH-induced early brain injury.

Exacerbated corneal inflammation and neovascularization in the HO-2 null mice is ameliorated by biliverdin

Heme oxygenase (HO-1 and HO-2) represents an intrinsic cytoprotective and anti-inflammatory system based on its ability to modulate leukocyte migration and to inhibit expression of inflammatory cytokines and proteins. HO-2 deletion leads to unresolved corneal inflammation and chronic inflammatory complications including ulceration, perforation and neovascularization. We examined the consequences of HO-2 deletion on hemangiogenesis and lymphangiogenesis in the model of suture-induced inflammatory neovascularization. An 8.0 silk suture was placed at the corneal apex of wild type and HO-2 null mice. Neovascularization was assessed by vital microscopy and quantified by image analysis. Hemangiogenesis and lymphangiogenesis were determined by immunofluorescence staining using anti-CD31 and anti-LYVE-1 antibodies, respectively. Inflammation was quantified by histology and myeloperoxidase activity. The levels of HO-1 expression and inflammatory cytokines were determined by real time PCR and ELISA, respectively. Corneal sutures produced a consistent inflammatory response and a time-dependent neovascularization. The response in HO-2 null mice was associated with a greater increase compared to the wild type in the number of leukocytes (827,600+/-129,000 vs. 294,500+/-57,510; p<0.05), neovessels measured by vital microscopy (21.91+/-1.05 vs. 12.77+/-1.55 mm; p<0.001) 4 days after suture placement. Hemangiogenesis but not lymphangiogenesis was more pronounced in HO-2 null mice compared to wild type mice. Induction of HO-1 in sutured corneas was greatly attenuated in HO-2 null corneas and treatment with biliverdin diminished the exaggerated inflammatory and neovascular response in HO-2 null mice. The demonstration that the inflammatory responses, including expression of proinflammatory proteins, inflammatory cell influx and hemangiogenesis are exaggerated in HO-2 knockout mice strongly supports the notion that the HO system is critical for controlling the inflammatory and neovascular response in the cornea. Hence, pharmacological amplification of this system may constitute a novel therapeutic strategy for the treatment of corneal disorders associated with excessive inflammation and neovascularization.

Neurons lacking iron regulatory protein-2 are highly resistant to the toxicity of hemoglobin

The effect of iron regulatory protein-2 (IRP2) on ferritin expression and neuronal vulnerability to hemoglobin was assessed in primary cortical cell cultures prepared from wild-type and IRP2 knockout mice. Baseline levels of H and L-ferritin subunits were significantly increased in IRP2 knockout neurons and astrocytes. Hemoglobin was toxic to wild-type neurons in mixed neuron-astrocyte cultures, with an LC(50) near 3 microM for a 24 h exposure. Neuronal death was reduced by 85-95% in knockout cultures, and also in cultures containing knockout neurons plated on wild-type astrocytes. Protein carbonylation, reactive oxygen species formation, and heme oxygenase-1 expression after hemoglobin treatment were also attenuated by IRP2 gene deletion. These results suggest that IRP2 binding activity increases the vulnerability of neurons to hemoglobin, possibly by reducing ferritin expression. Therapeutic strategies that target this regulatory mechanism may be beneficial after hemorrhagic CNS injuries.

Carbon monoxide-releasing molecule tricarbonyldichlororuthenium (II) dimer induces concentration-dependent alterations in the electrophysiological properties of axons in mammalian spinal cord

Traumatic spinal cord injury (SCI) typically involves intraparenchymal hemorrhage and a cascade of inflammatory and cytotoxic processes leading to tissue necrosis and apoptosis. A consequence of the hemorrhage is the accumulation of deoxygenated heme proximal and distal to the epicenter of the lesion. The heme oxygenase (HO) system is an endogenous heme degradation system and is upregulated following neurotrauma. The breakdown of heme via HO activity yields the byproducts carbon monoxide (CO), biliverdin, and iron. CO has documented neuromodulatory properties; however, the effects of elevated concentrations of CO on axonal conduction in the spinal cord have not previously been studied. The present study tested the hypothesis that CO causes alterations in the electrophysiological properties of axons within the isolated guinea-pig spinal cord. Ex vivo spinal cord preparations were exposed to 100, 500, and 1000 microM concentrations of the carbon monoxide-releasing molecule (CORM) 2 for 30 min in a double sucrose gap electrophysiological recording system and the compound action potential (CAP) and membrane potential (CMP) were recorded continuously during pretreatment, CORM-2 treatment, and washout (30 min) with Krebs' solution. CAP amplitude and area were significantly (P<0.05) reduced following treatment with 500 and 1000 microM CORM-2 and did not recover during washout. No effect on CMP was observed, however, stimulus-peak latency did increase significantly (P<0.05) following CORM-2 treatment at these concentrations, and a decrease in the amplitude of the second CAP elicited by paired-pulse stimulation was also evident at interpulse intervals of 2 and 4 ms. These results are consistent with a CO-induced alteration in axonal conduction, possibly attributable to modified Na+ channel conductance. They also identify a new mechanism by which post-traumatic hemorrhage contributes to the neurological deficits observed following SCI.

Time course of increased heme oxygenase activity and expression after experimental intracerebral hemorrhage: correlation with oxidative injury

Heme oxygenase (HO) activity in tissue adjacent to an intracerebral hematoma may modulate cellular vulnerability to heme-mediated oxidative injury. Although HO-1 is induced after experimental intracerebral hemorrhage (ICH), the time course of this induction, its effect on tissue HO activity, and its association with oxidative injury markers has not been defined. We therefore quantified HO activity, HO-1 expression, tissue heme content, and protein carbonylation for 8 days after injection of autologous blood into the mouse striatum. Increased striatal HO-1 protein was observed within 24 h, peaked on day 5 at a level that was 10-fold greater than baseline, and returned to baseline by day 8; HO-2 expression was not altered. HO activity increased by only 1.6-fold at its peak on day 5, and had also returned to baseline by day 8. A significant increase in protein carbonylation was observed at 3-5 days, which also was markedly attenuated by 8 days, concomitant with a return of tissue heme to near-normal levels. These results suggest that the increase in HO activity in tissue surrounding an experimental ICH is considerably less than would be predicted based on an analysis of HO-1 expression per se. As HO-1 expression is temporally associated with increased tissue heme and increased protein carbonylation, it may be more useful as a marker of heme-mediated oxidative stress in ICH models, rather than as an index of HO activity.

Attenuation of oxidative injury after induction of experimental intracerebral hemorrhage in heme oxygenase-2 knockout mice

OBJECT

Experimental evidence suggests that hemoglobin degradation products contribute to cellular injury after intracerebal hemorrhage (ICH). Hemoglobin breakdown is catalyzed in part by the heme oxygenase (HO) enzymes. In the present study, the authors tested the hypothesis that HO-2 gene deletion is cytoprotective in an experimental ICH model.

METHODS

After anesthesia was induced with isoflurane, 3- to 6-month-old HO-2 knockout and wild-type mice were stereotactically injected with 15 microl autologous blood and a group of control mice were injected with an equal volume of sterile saline. Striatal protein and lipid oxidation were quantified 72 hours later using carbonyl and malondialdehyde assays. Cell viability was determined by performing a 3(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide (MTT) assay. Following blood injection, the investigators found a 3.4-fold increase in protein carbonylation compared with that in the contralateral striatum in wild-type mice; in knockout mice, the investigators found a twofold increase. The mean malondialdehyde concentration in injected striata was increased twofold in wild-type mice at this time, compared with 1.5-fold in knockout mice. Cell viability, as determined by MTT reduction, was reduced in injected striata to 38 +/- 4% of that in the contralateral striata in wild-type mice, compared with 66 +/- 5% in HO-2 knockout mice. Baseline striatal HO-1 protein expression was similar in wild-type and HO-2 knockout mice, but was induced more rapidly in the former after blood injection.

CONCLUSIONS

Deletion of HO-2 attenuates oxidative cell injury after whole-blood injection into the mouse striatum. Therapies that specifically target HO-2 may improve outcome after ICH.

Astrocyte-specific heme oxygenase-1 hyperexpression attenuates heme-mediated oxidative injury

In prior studies, we have observed that HO activity protects astrocytes from heme-mediated injury, but paradoxically increases neuronal injury. In this study, we tested the hypothesis that an adenovirus encoding the human HO-1 gene driven by an enhanced glial fibrillary acidic protein promoter (Ad-GFAP-HO-1) would increase HO-1 expression selectively in astrocytes, and provide cytoprotection. Treatment with 100 MOI Ad-GFAP-HO-1 for 24 h resulted in HO-1 expression that was 6.4-fold higher in cultured primary astrocytes than in neurons. Astrocyte HO activity was increased by approximately fourfold over baseline, which was sufficient to reduce cell death after 24-h hemin exposure by 60%, as assessed by both MTT and LDH release assays. A similar reduction in cell protein oxidation, quantified by carbonyl assay, was also observed. These results suggest that HO-1 transgene expression regulated by an enhanced GFAP promoter selectively increases HO-1 expression in astrocytes, and is cytoprotective. Further investigation of this strategy in vivo is warranted.

Interdependence of lipoxin A4 and heme-oxygenase in counter-regulating inflammation during corneal wound healing

In the immune-privileged cornea, epithelial wounds heal rapidly with almost no scarring and, unlike in most other tissues, acute inflammation in the absence of infection is beneficial to healing. Molecular mechanisms, which account for this striking property, remain to be clearly defined, but they likely include autacoids that control leukocyte activation. Two prominent enzymes, 12/15-lipoxygenase (LOX), which generates antiinflammatory lipid autacoids, and heme-oxygenase (HO), which generates antioxidants and carbon monoxide, are highly expressed in human and mouse corneas. LXA4, an endogenous 12/15-LOX product, proved to be a potent inhibitor of exacerbated inflammation and significantly increased re-epithelialization in corneal wounds. In vivo deletion of 12/15-LOX correlated with exacerbated inflammation and impaired wound healing in 12/15-LOX(-/-) mice, a phenotype that was rescued by treatment with LXA4. More importantly, 12/15-LOX(-/-) mice demonstrated impaired induction of HO-1 in both acute and exacerbated inflammation. Topical LXA4 restored HO-1 expression in 12/15-LOX(-/-) mice and amplified HO-1 gene expression in human corneal epithelial cells. HO-2(-/-) mice, which fail to induce HO-1, also demonstrated exacerbated inflammation in response to injury, a phenotype that, notably, correlated with a 50% reduction in endogenous LXA4 formation. Collectively, results demonstrate a critical role for LXA4 in inflammatory/reparative responses and provide the first evidence that 12/15-LOX and HO systems function in concert to control inflammation.

Heme oxygenase-2 is a critical determinant for execution of an acute inflammatory and reparative response

Heme oxygenase (HO) represents an intrinsic anti-inflammatory system based on its ability to regulate leukocyte function and inhibit expression of proinflammatory cytokines. This anti-inflammatory function is linked to the inducible isoform HO-1; the role of the constitutive isoform HO-2 is unknown. The current study was undertaken to investigate the role of HO-2 in the regulation of the acute inflammatory and reparative response by using HO-2-null mice and well-established animal models of epithelial injury and antigen-induced peritonitis. Here we show that in vivo deletion of HO-2 disables execution of the acute inflammatory and reparative response after epithelial injury and leads to an exaggerated inflammatory response in antigen-induced peritonitis. HO-2 deletion was associated with impaired HO-1 induction, indicating that HO-2 is critical for HO-1 expression and that the subsequent failure to up-regulate the HO system may contribute to unresolved inflammation and the development of chronic inflammatory conditions. Indeed, supplementation with the HO bioactive product, biliverdin, rescued the acute inflammatory and reparative response in HO-2-null mice. Thus, HO-2 sets in place a basal tone of anti-inflammatory signals that may be a prerequisite for the ordered execution of an inflammatory and reparative response.

Effect of heme oxygenase-1 on the vulnerability of astrocytes and neurons to hemoglobin

The heme oxygenase (HO) enzymes catalyze the rate-limiting step of heme breakdown. Prior studies have demonstrated that the vulnerability of neurons and astrocytes to hemoglobin is modified in cells lacking HO-2, the constitutive isoform. The present study assessed the effect of the inducible isoform, HO-1. Wild-type astrocytes treated for 3-5 days with 3-30 microM hemoglobin sustained no loss of viability, as quantified by LDH and MTT assays. The same treatment resulted in death of 25-50% of HO-1 knockout astrocytes, and a 4-fold increase in protein oxidation. Cell injury was attenuated by transfer of the HO-1 gene, but not by bilirubin, the antioxidant heme breakdown product. Conversely, neuronal protein oxidation and cell death after hemoglobin exposure were similar in wild-type and HO-1 knockout cultures. These results suggest that HO-1 induction protects astrocytes from the oxidative toxicity of Hb, but has no effect on neuronal injury.

Glutamate induces oxidative stress and apoptosis in cerebral vascular endothelial cells: contributions of HO-1 and HO-2 to cytoprotection

In cerebral circulation, epileptic seizures associated with excessive release of the excitatory neurotransmitter glutamate cause endothelial injury. Heme oxygenase (HO), which metabolizes heme to a vasodilator, carbon monoxide (CO), and antioxidants, biliverdin/bilirubin, is highly expressed in cerebral microvessels as a constitutive isoform, HO-2, whereas the inducible form, HO-1, is not detectable. Using cerebral vascular endothelial cells from newborn pigs and HO-2-knockout mice, we addressed the hypotheses that 1) glutamate induces oxidative stress-related endothelial death by apoptosis, and 2) HO-1 and HO-2 are protective against glutamate cytotoxicity. In cerebral endothelial cells, glutamate (0.1–2.0 mM) increased formation of reactive oxygen species, including superoxide radicals, and induced major keystone events of apoptosis, such as NF-κB nuclear translocation, caspase-3 activation, DNA fragmentation, and cell detachment. Glutamate-induced apoptosis was greatly exacerbated in HO-2 gene-deleted murine cerebrovascular endothelial cells and in porcine cells with pharmacologically inhibited HO-2 activity. Glutamate toxicity was prevented by superoxide dismutase, suggesting apoptotic changes are oxidative stress related. When HO-1 was pharmacologically upregulated by cobalt protoporphyrin, apoptotic effects of glutamate in cerebral endothelial cells were completely prevented. Glutamate-induced reactive oxygen species production and apoptosis were blocked by a CO-releasing compound, CORM-A1 (50 μM), and by bilirubin (1 μM), consistent with the antioxidant and cytoprotective roles of the end products of HO activity. We conclude that both HO-1 and HO-2 have anti-apoptotic effects against oxidative stress-related glutamate toxicity in cerebral vascular endothelium. Although HO-1, when induced, provides powerful protection, HO-2 is an essential endogenous anti-apoptotic factor against glutamate toxicity in the cerebral vascular endothelium.

Inhibition of the ERK/MAP kinase pathway attenuates heme oxygenase-1 expression and heme-mediated neuronal injury

Hemin is an oxidant that accumulates in intracranial hematomas. Its neurotoxicity is increased by its breakdown, which is catalyzed by the heme oxygenase (HO) enzymes. In this study we tested the hypothesis that inhibiting signaling events mediating HO-1 induction would protect cultured cortical neurons from hemin. A fivefold increase in HO-1 expression was observed in mixed neuron-astrocyte cultures 4h after hemin exposure. This was markedly reduced by the ERK pathway inhibitor U0126. The JNK inhibitor SP600125 had a weak but statistically significant effect, while the p38 inhibitor SB239063 was ineffective. Hemin neurotoxicity, as assessed by LDH release, propidium iodide staining, and malondialdehyde assay, was also prevented by U0126 but not by SB239063; SP600125 had little or no effect. Consistent with reduced iron release, ferritin expression was also attenuated by U0126, while cell hemin accumulation was increased. These results suggest that targeting the ERK pathway may prevent HO-1 induction in response to hemin, and reduce neuronal injury.

Cultured astrocytes from heme oxygenase-1 knockout mice are more vulnerable to heme-mediated oxidative injury

Hemin, the oxidized form of heme, is released from hemoglobin after CNS hemorrhage and may contribute to injury to surrounding tissue. The heme oxygenase (HO) enzymes catalyze the breakdown of hemin to biliverdin, carbon monoxide, and ferric iron. Although HO-2, the isoform expressed predominantly in neurons, accelerates heme-mediated neuronal injury, inhibitor studies suggest that HO-1 induction has a protective effect on astrocytes. In the present study, we directly compared the vulnerability of cultured HO-1 knockout and wild-type astrocytes to hemin. Consistent with prior observations, exposure of wild-type cultures to hemin for 24 hr resulted in protein carbonylation and concentration-dependent cell death between 10 and 60 microM, as determined by MTT and lactate dehydrogenase release assays. In cultures prepared from mice lacking the HO-1 gene, oxidative cell injury was approximately doubled. Both protein oxidation and cell death in HO-1 knockout astrocytes were significantly reduced by pretreating cultures with an adenovirus encoding the HO-1 gene prior to hemin exposure. HO-2 expression was observed in both knockout and wild-type cultures and was not altered by HO-1 gene deletion. Cell hemin accumulation after 20 hr hemin exposure was 4.7-fold higher in knockout cells. These results support the hypothesis that HO-1 protects astrocytes from heme-mediated oxidative injury. Selectively increasing its expression in astrocytes may be beneficial after hemorrhagic CNS injuries.

Heme oxygenase 2 is neuroprotective against intracerebral hemorrhage

Recent studies suggest a neuroprotective function for heme oxygenase 2 (HO2) in acute brain injury and ischemia. HO2, the main enzyme to degrade the pro-oxidant heme, was tested for its neuroprotective ability in postnatal neuronal cell cultures and in a model of collagenase-induced intracerebral hemorrhage. Genetic deletion of HO2 rendered cultured neurons 32% (P < 0.01) more vulnerable to hemin-induced toxicity, increased brain injury volume in mice by 30% (P < 0.05) at day 1 and by 67% (P < 0.05) at day 3, and worsened neurologic functions by 26% (P < 0.05) at day 1 and by 38% (P < 0.05) at day 3 following exposure to free heme liberated from hemorrhage. Together, these findings suggest that HO2 is a crucial neuroprotective enzyme in detoxifying high levels of heme from the brain and that further work is warranted to investigate potential therapeutic strategies that target HO2 in intracerebral hemorrhage.

Nitric oxide not carbon monoxide mediates nonadrenergic noncholinergic relaxation in the murine internal anal sphincter

BACKGROUND & AIMS

Inhibitory reflexes in the internal anal sphincter (IAS) are controlled by inhibitory nonadrenergic, noncholinergic innervation (i-NANC). We investigated the roles of 3 different neurohumoral agonists as possible i-NANC neurotransmitters: carbon monoxide (CO), nitric oxide (NO), and vasoactive intestinal peptide (VIP).

METHODS

IAS smooth muscle strips were isolated from wild-type (WT), heme oxygenase (HO)-2 knockout (HO-2-/-) and neuronal NO synthase (nNOS) knockout (nNOS-/-) mice. Relaxation of IAS was induced by CO, NO, VIP, and electrical field stimulation (EFS) in the presence and absence of neurohumoral inhibitors (tin protoporphyrin IX [SnPP IX] for CO synthesis, N(omega)-nitro-L-arginine [L-NNA] for NO synthesis, and VIP(10-28) for VIP receptor). Western blot and immunohistochemistry were used to test the presence and localization of HO (for CO synthesis) types 1 (HO-1) and 2 (HO-2), neuronal NO synthase (nNOS, for NO synthesis), and VIP.

RESULTS

All 3 neurohumoral agonists produced relaxation (with no difference between WT and HO-2-/- IAS), but CO was over 100 times less potent than NO and VIP. EFS produced relaxation in WT and HO-2-/- IAS with the same intensity. L-NNA and nNOS deletion (approximately 80%) and VIP(10-28) (approximately 15%) significantly inhibited the relaxations, whereas SnPP IX had no effect. Positive immunoreactivities for HO-2, nNOS, and VIP were found in the myenteric plexus of WT IAS. HO-2-/- IAS did not express immunoreactivity for HO-2.

CONCLUSIONS

i-NANC relaxations of mouse IAS are primarily mediated via NO (by nNOS activity) and partly via VIP. CO directly relaxes the mouse IAS but does not play any significant role in the i-NANC relaxation.

Quetiapine and venlafaxine synergically regulate heme oxygenase-2 protein expression in the hippocampus of stressed rats

HO-2 is a constitutive isoform of heme oxygenase (HO), a microsomal enzyme that catalyzes the cleavage of the heme ring to form ferrous iron, carbon monoxide, and biliverdin. In contrast to HO-1, which is inducible, HO-2 is not responsive to stimuli tested to date except for prolonged exposure to the adrenal glucocorticoids (GCs). Previous studies have shown that high GC concentrations or stress damage or kill hippocampal neurons. In the present study, it was found that chronic restraint stress decreased HO-2 protein levels in hippocampal neurons, as demonstrated by immunohistochemistry and Western blot analysis. Moreover, our results showed that the combination of 2.5mg/kg of venlafaxine and 5mg/kg of quetiapine effectively prevented the HO-2 protein decrease in hippocampal neurons of stressed rats, whereas either of the drugs alone did not show any effect. At higher dose levels, both quetiapine (10mg/kg) and venlafaxine (5mg/kg) produced significant effects comparable to that of their combination. Quetiapine is an atypical antipsychotic and venlafaxine an antidepressant. In previous studies, these two drugs have been shown to prevent or protect against the stress-induced decrease in hippocampal neurogenesis and BDNF expression. These data suggest that both quetiapine and venlafaxine share the hippocampus as their common target by enhancing hippocampal resilience, which may be impaired in patients with schizophrenia or depression.

Heme-oxygenase-1 promotes polychlorinated biphenyl mixture aroclor 1254-induced oxidative stress and dopaminergic cell injury

Dopaminergic (DAergic) systems have been identified as putative targets for polycholorinated biphenyl (PCB) actions. However, the precise mechanisms leading to neurotoxicity are unresolved. Reactive oxygen species (ROS) were recently shown to mediate injury in DAergic MN9D cells following exposure to Aroclor 1254 (A1254), a commercial PCB mixture. The oxidative stress response in DAergic cells included a persistent expression of heme oxygenase-1 (HO-1). This study tested the hypothesis that a sustained PCB-induced HO-1 response leads to abnormally high Fe levels, which generates ROS production and mediates death in the MN9D DAergic cell model. Accordingly, results indicated that A1254 augmented intracellular Fe levels in MN9D cells after 24 h. Fe chelation by desferoxamine or pharmacologic inhibition of HO activity with tin-protoporphyrin reduced Fe accumulation, ROS production, and cytotoxicity following A1254 exposure. HO-1 over-expression predisposed MN9D DAergic cells to enhanced ROS production and cell death in response to PCBs. Conversely, antisense inhibition of HO-1 expression prevented PCB-induced ROS production and cell death. These observations suggest that enhanced HO-1 catalytic activity and subsequent liberation of Fe participate in neurotoxic DAergic cell injury caused by A1254 exposure in vitro.

Effect of targeted deletion of the heme oxygenase-2 gene on hemoglobin toxicity in the striatum

The heme oxygenase (HO) enzymes catalyze the rate-limiting step in the breakdown of heme to iron, carbon monoxide, and biliverdin. A prior cell culture study demonstrated that deletion of HO-2, the isoform constitutively expressed in neurons, attenuated hemoglobin (Hb) neurotoxicity. The present study tested the hypothesis that HO-2 gene deletion is cytoprotective in a model of Hb toxicity in vivo. Stereotactic injection of 6 microL stroma-free Hb (SFHb) into the striatum significantly increased protein oxidation in wild-type mice at 24 to 72 h, as detected by an assay for carbonyl groups. At 72 h, carbonylation was increased 2.5-fold compared with that in the contralateral striatum. In HO-2 knockout mice, protein oxidation was not increased at 24 h, and was increased by only 1.7-fold at 72 h. Similarly, striatal lipid peroxidation, as detected by the malondialdehyde assay, was significantly greater in the SFHb-injected striata of wild-type mice than in knockout mice. Striatal cell viability, determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, was 45.0%+/-6.3% of that in contralateral striata in wild-type mice at 72 h; it was increased to 85%+/-8% in knockouts. Heme oxygenase-2 gene deletion did not alter weight loss or mortality after SFHb injection. Baseline striatal HO-1 expression was similar in knockout and wild-type mice; induction after SFHb injection occurred more rapidly in the latter. These results suggest that HO-2 gene deletion protects striatal cells from the oxidative toxicity of Hb in vivo. Pharmacologic or genetic strategies that target HO-2 may be beneficial after central nervous system hemorrhage, and warrant further investigation.

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.

The oxidative neurotoxicity of clioquinol

Clioquinol is a metal chelator that may attenuate beta-amyloid deposition and mitigate the progression of Alzheimer's disease. Its prior use as a systemic antibiotic was associated with a neurodegenerative syndrome, subacute myelo-optico-neuropathy (SMON), although a mechanistic link has not been precisely defined. While testing clioquinol in murine cortical cultures, it was observed to have a pro-oxidant effect. Exposure to 1-3 microM for 24 h increased malondialdehyde, and resulted in death of approximately 40% of neurons; a higher concentration (30 microM) was paradoxically less toxic. Both malondialdehyde production and cell death were attenuated by concomitant treatment with the antioxidants ascorbic acid and Trolox C, or with the lipid-soluble metal chelator 1,10-phenanthroline. In contrast, injury was increased in cultures prepared from mice lacking heme oxygenase-2, which protects against non-heme mediated oxidative injury to neurons. Addition of vitamin B12 to the culture medium was not cytoprotective. These results suggest that therapeutically relevant concentrations of clioquinol are toxic to cultured neurons by an oxidative mechanism that is unrelated to vitamin B12 deficiency. In vivo evaluation of the pro-oxidant effect of clioquinol seems warranted prior to further clinical trials.

Reactive oxygen species contribute to the presynaptic action of extracellular ATP at the frog neuromuscular junction

During normal cell metabolism the production of intracellular ATP is associated with the generation of reactive oxygen species (ROS), which appear to be important signalling molecules. Both ATP and ROS can be released extracellularly by skeletal muscle during intense activity. Using voltage clamp recording combined with imaging and biochemical assay of ROS, we tested the hypothesis that at the neuromuscular junction extracellular ATP generates ROS to inhibit transmitter release from motor nerve endings. We found that ATP produced the presynaptic inhibitory action on multiquantal end-plate currents. The inhibitory action of ATP (but not that of adenosine) was significantly reduced by several antioxidants or extracellular catalase, which breaks down H2O2. Consistent with these data, the depressant effect of ATP was dramatically potentiated by the pro-oxidant Fe2+. Exogenous H2O2 reproduced the depressant effects of ATP and showed similar sensitivity to anti- and pro-oxidants. While NO also inhibited synaptic transmission, inhibitors of the NO-producing cascade did not prevent the depressant action of ATP. The ferrous oxidation in xylenol orange assay showed the increase of ROS production by ATP and 2-MeSADP but not by adenosine. Suramin, a non-selective antagonist of P2 receptors, and pertussis toxin prevented the action of ATP on ROS production. Likewise, imaging with the ROS-sensitive dye carboxy-2',7'-dichlorodihydrofluorescein revealed increased production of ROS in the muscle treated with ATP or ADP while UTP or adenosine had no effect. Thus, generation of ROS contributed to the ATP-mediated negative feedback mechanism controlling quantal secretion of ACh from the motor nerve endings.

Polychlorinated biphenyl mixture aroclor 1254-induced oxidative stress plays a role in dopaminergic cell injury

Oxidative stress (OS) is thought to participate in the pathogenesis of neurodegenerative disorders, including Parkinson's disease (PD). Excessive reactive oxygen species (ROS) production can occur during the normal aging process or following exposure to environmental toxicants. Dopamine neurons, which degenerate during PD, are particularly sensitive to oxidative stress. Polychlorinated biphenyls (PCBs), persistent and widespread pollutants, have been shown to adversely impact dopaminergic (DAergic) pathways, but the role ROS play in neurotoxicity remains unclear. To test the hypothesis that PCB exposure compromises dopamine neurons by stimulating ROS production, the direct toxicity and oxidative stress response following PCB exposure was examined both in MN9D dopamine cells and primary mesencephalic cultures. PCBs induced a time- and concentration-dependent increase in ROS production, which preceded cytotoxicity. Whereas intracellular GSH depletion exacerbated PCB effects, antioxidant pretreatment attenuated ROS production and cell death. Coincident alterations in antioxidant defense enzymes also accompanied ROS production, including decreased MnSOD and increased CuZnSOD protein levels. The robust elevation in heme oxygenase-1 levels further support the activation of oxidative stress mechanisms following PCB exposure. Furthermore, PCBs produced concentration-dependent reductions in intracellular dopamine levels and elevated dopamine turnover. Although the intracellular source of ROS remains unknown, these results suggest that sublethal PCB concentrations activate an oxidative stress-related pathway, which potentially disrupts dopamine neuron function.

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.

The neurotoxic effect of sickle cell hemoglobin

A growing body of experimental evidence suggests that the oxidative neurotoxicity of hemoglobin A may contribute to neuronal loss after CNS hemorrhage. Several hemoglobin variants, including hemoglobin S, are more potent oxidants in cell-free systems. However, despite the increased incidence of hemorrhagic stroke associated with sickle cell disease, little is known of the effect of hemoglobin S on cells of neural origin. In the present study, its toxicity was quantified and directly compared with that of hemoglobin A in murine cortical cell cultures. Reactive oxygen species production, as assessed by cellular fluorescence after treatment with dihydrorhodamine 123, was significantly increased by exposure to 10 microM hemoglobin S for 2-4 h. Neuronal death, as measured by propidium iodide staining and lactate dehydrogenase release, commenced at 4 h; for a 20-h exposure, the EC50 was approximately 0.71 microm. Glial cells were not injured. Cell death was completely blocked by iron chelation with deferoxamine or phenanthroline. Direct comparison of sister cultures exposed to either hemoglobin A or hemoglobin S revealed a similar amount of cell injury in both groups. A significant difference was consistently observed only after treatment with 1 microM hemoglobin for 20 h, which resulted in death of approximately one third more neurons with hemoglobin S than with hemoglobin A. The results of this study suggest that sickle cell hemoglobin is neurotoxic at physiologically relevant concentrations. This toxicity is iron-dependent, oxidative, and quantitatively similar to that produced by hemoglobin A.

Adenoviral transfer of the heme oxygenase-1 gene protects striatal astrocytes from heme-mediated oxidative injury

Heme oxygenase-1 (HO-1) is induced in the CNS after hemorrhage, and may have an effect on injury to surrounding tissue. Hemin, the preferred substrate of HO, is a neurotoxin that is present in intracranial hematomas. In a prior study, we observed that HO inhibitors increased the vulnerability of cultured cortical astrocytes to heme-mediated oxidative injury. To investigate the effect of HO more specifically, we used an adenoviral vector encoding the human HO-1 gene to specifically increase HO-1 expression. Incubation with 100 MOI of the HO-1 adenovirus (Adv-HHO-1) for 24 h increased both HO-1 protein and HO activity; a control adenovirus lacking the HO-1 gene had no effect. Using a DNA probe that was specific for human HO-1, 80.5 +/- 7.2% of astrocytes were observed to be infected by in situ hybridization. The cell death produced by 30-60 microM hemin was significantly reduced by pretreatment with 100 MOI Adv-HHO-1, as assessed by LDH release, propidium iodide exclusion, and MTT reduction assay. The threefold increase in cell protein oxidation produced by hemin was also attenuated in cultures pretreated with Adv-HHO-1. These results support the hypothesis that HO-1 protects astrocytes from heme-mediated oxidative injury. Specifically increasing astrocytic HO-1 by gene transfer may have a beneficial effect on hemorrhagic CNS injury.

Heme oxygenase-2 gene deletion attenuates oxidative stress in neurons exposed to extracellular hemin

Background

Hemin, the oxidized form of heme, accumulates in intracranial hematomas and is a potent oxidant. Growing evidence suggests that it contributes to delayed injury to surrounding tissue, and that this process is affected by the heme oxygenase enzymes. In a prior study, heme oxygenase-2 gene deletion increased the vulnerability of cultured cortical astrocytes to hemin. The present study tested the effect of HO-2 gene deletion on protein oxidation, reactive oxygen species formation, and cell viability after mixed cortical neuron/astrocyte cultures were incubated with neurotoxic concentrations of hemin.

Results

Continuous exposure of wild-type cultures to 1–10 μM hemin for 14 h produced concentration-dependent neuronal death, as detected by both LDH release and fluorescence intensity after propidium iodide staining, with an EC₅₀ of 1–2 μM; astrocytes were not injured by these low hemin concentrations. Cell death was consistently reduced by at least 60% in knockout cultures. Exposure to hemin for 4 hours, a time point that preceded cell lysis, increased protein oxidation in wild-type cultures, as detected by staining of immunoblots for protein carbonyl groups. At 10 μM hemin, carbonylation was increased 2.3-fold compared with control sister cultures subjected to medium exchanges only; this effect was reduced by about two-thirds in knockout cultures. Cellular reactive oxygen species, detected by fluorescence intensity after dihydrorhodamine 123 (DHR) staining, was markedly increased by hemin in wild-type cultures and was localized to neuronal cell bodies and processes. In contrast, DHR fluorescence intensity in knockout cultures did not differ from that of sham-washed controls. Neuronal death in wild-type cultures was almost completely prevented by the lipid-soluble iron chelator phenanthroline; deferoxamine had a weaker but significant effect.

Conclusions

These results suggest that HO-2 gene deletion protects neurons in mixed neuron-astrocyte cultures from heme-mediated oxidative injury. Selective inhibition of neuronal HO-2 may have a beneficial effect after CNS hemorrhage.

Heme oxygenase-2 gene deletion increases astrocyte vulnerability to hemin

In a prior study, we observed that heme oxygenase-2 gene deletion protected murine cortical neurons from heme-mediated injury. In the course of these studies, constitutive HO-2 expression was observed in astrocyte cultures. The present study tested the hypothesis that astrocytes lacking the HO-2 gene would be less vulnerable to heme. Contrary to this hypothesis, gene deletion resulted in a 50-75% increase in cell death after 6h exposure to 30 or 60microM hemin, as measured by LDH release. A similar effect was observed when cell viability was assessed with the MTT assay. HO-2 gene deletion did not alter cellular expression of HO-1. The increased sensitivity of knockout astrocytes to hemin was reversed by increasing HO-1 expression by adenoviral gene transfer. These results suggest that heme oxygenase protects astrocytes from heme-mediated oxidative injury and highlight the disparate effect of HO in neurons and astrocytes.