Oxidative stress causes enhanced endothelial cell injury in human heme oxygenase-1 deficiency

The first known human case of heme oxygenase-1 (HO-1) deficiency is presented in this report. The patient is a six-year-old boy with severe growth retardation. He has been suffering from persistent hemolytic anemia characterized by marked erythrocyte fragmentation and intravascular hemolysis, with paradoxical increase of serum haptoglobin and low bilirubin. An abnormal coagulation/fibrinolysis system, associated with elevated thrombomodulin and von Willebrand factor, indicated the presence of severe, persistent endothelial damage. Electron microscopy of renal glomeruli revealed detachment of endothelium, with subendothelial deposition of an unidentified material. Iron deposition was noted in renal and hepatic tissue. Immunohistochemistry of hepatic tissue and immunoblotting of a cadmium-stimulated Epstein-Barr virus-transformed lymphoblastoid cell line (LCL) revealed complete absence of HO-1 production. An LCL derived from the patient was extremely sensitive to hemin-induced cell injury. Sequence analysis of the patient's HO-1 gene revealed complete loss of exon-2 of the maternal allele and a two-nucleotide deletion within exon3 of the paternal allele. Growth retardation, anemia, iron deposition, and vulnerability to stressful injury are all characteristics observed in recently described HO-1 targeted mice. This study presents not only the first human case of HO-1 deficiency but may also provide clues to the key roles played by this important enzyme in vivo.

Neutrophil activation by heme: implications for inflammatory processes

Heme, a ubiquitous iron-containing compound, is present in large amounts in many cells and is inherently dangerous, particularly when it escapes from intracellular sites. The release of heme from damaged cells and tissues is supposed to be higher in diseases such as malaria and hemolytic anemia or in trauma and hemorrhage. We investigated here the role of free ferriprotoporphyrin IX (hemin) as a proinflammatory molecule, with particular attention to its ability to activate neutrophil responses. Injecting hemin into the rat pleural cavity resulted in a dose-dependent migration of neutrophils, indicating that hemin is able to promote the recruitment of these cells in vivo. In vitro, hemin induced human neutrophil chemotaxis and cytoskeleton reorganization, as revealed by the increase of neutrophil actin polymerization. Exposure of human neutrophils to 3 microM hemin activated the expression of the chemokine interleukin-8, as demonstrated by quantitative reverse-transcription polymerase chain reaction, indicating a putative molecular mechanism by which hemin induces chemotaxis in vivo. Brief incubation of human neutrophils with micromolar concentrations of hemin (1-20 microM) triggered the oxidative burst, and the production of reactive oxygen species was directly proportional to the concentration of hemin added to the cells. Finally, we observed that human neutrophil protein kinase C was activated by hemin in vitro, with a K(1/2) of 5 microM. Taken together, these results suggest a role for hemin as a proinflammatory agent able to induce polymorphonuclear neutrophil activation in situations of clinical relevance, such as hemolysis or hemoglobinemia.

Amyloid precursor proteins inhibit heme oxygenase activity and augment neurotoxicity in Alzheimer's disease

Amyloid precursor protein (APP) generates the beta-amyloid peptide, postulated to participate in the neurotoxicity of Alzheimer's disease. We report that APP and APLP bind to heme oxygenase (HO), an enzyme whose product, bilirubin, is antioxidant and neuroprotective. The binding of APP inhibits HO activity, and APP with mutations linked to the familial Alzheimer's disease (FAD) provides substantially greater inhibition of HO activity than wild-type APP. Cortical cultures from transgenic mice expressing Swedish mutant APP have greatly reduced bilirubin levels, establishing that mutant APP inhibits HO activity in vivo. Oxidative neurotoxicity is markedly greater in cerebral cortical cultures from APP Swedish mutant transgenic mice than wild-type cultures. These findings indicate that augmented neurotoxicity caused by APP-HO interactions may contribute to neuronal cell death in Alzheimer's disease.

Hemin induces an iron-dependent, oxidative injury to human neuron-like cells

Hemin is released from hemoglobin after CNS hemorrhage and is present at high micromolar concentrations in intracranial hematomas. This highly reactive compound is potentially cytotoxic via a variety of oxidative and nonoxidative mechanisms. However, despite its clinical relevance, little is known of its effect on neuronal cells. In this study, we tested the hypotheses that hemin is toxic to human neurons at physiologically relevant concentrations and that its toxicity is iron dependent and oxidative. A homogeneous population of neuron-like cells was produced by sequential treatment of SH-SY5Y cells with retinoic acid and brain-derived neurotrophic factor, using the protocol of Encinas et al. Hemin exposure for 24 hr resulted in cell death that progressively increased between 3 and 30 microM (EC(50) approximately 10 microM); protoporphyrin IX, the iron-free congener of hemin, was not toxic. Cell death commenced at 14 hr and was preceded by a marked increase in cellular reactive oxygen species (ROS). Most injury and ROS production were prevented by concomitant treatment with an equimolar concentration of the lipid-soluble iron chelator phenanthroline; the water-soluble chelator deferoxamine was also effective at concentrations of 0.1 mM or higher. Heme oxygenase-2 was constitutively expressed by these cells, and heme oxygenase-1 was induced by hemin. Heme oxygenase inhibition attenuated ROS generation and reduced injury by about one-third. Cell death was also prevented with the sulfhydryl reducing agents glutathione and mercaptoethanol. Nuclear morphology in the hours prior to cell lysis revealed a predominantly homogenous staining pattern; the percentage of fragmented nuclei was increased only at 4 hr and then accounted for only 1.45% +/- 0.25% of cells. The general caspase inhibitor zVAD-fmk had no effect on cell viability. These results suggest that hemin is toxic to human neuron-like cells at concentrations that are less than 3% of those observed in intracranial hematomas. In this model, its toxicity is iron dependent, oxidative, and predominantly necrotic.

Heme: a determinant of life and death in renal tubular epithelial cells

Heme oxygenase-1 (HO-1) and p21 influence cell fate, and genetic HO-1 overexpression upregulates p21 and confers resistance to apoptosis. The present study examined the effects of heme, a metabolite incriminated in renal injury, on sensitivity to apoptosis and cell growth in conjunction with cellular expression of HO-1 and p21. Immortalized rat proximal tubular epithelial cells (IRPTCs) were exposed to hemin (10 microM) in serum-deplete media (0.1% FBS) and in standard cell culture media (5.0% FBS). In the presence of 0.1% FBS media, hemin induced p21 through an HO-dependent, p53-independent mechanism; certain products of HO activity (iron and carbon monoxide), but not others (ferritin, apoferritin, bilirubin), recapitulated these inductive effects on p21 expression. Along with this inductive effect on HO-1 and p21, hemin worsened apoptosis, the latter exacerbated by the inhibition of HO activity and loss of p21 expression. In IRPTCs maintained in 5% FBS, hemin induced HO-dependent p21 expression, provoked cell cycle arrest, and inhibited cell growth without inducing apoptosis; this inhibitory effect of hemin on cell growth was blocked by the concomitant inhibition of HO activity and loss of p21 expression. We conclude that hemin is a potent HO-dependent inducer of p21 and that hemin increases the sensitivity to apoptosis in serum-deplete conditions and decreases cell growth in serum-replete conditions; inhibiting HO activity and concomitantly ablating p21 expression exacerbate apoptosis and reverse the growth-inhibitory actions of hemin. We suggest that these effects of heme may influence the nature of, and recovery from, ischemic and nephrotoxic insults to the kidney.

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.

Hemoglobin and hemin induce DNA damage in human colon tumor cells HT29 clone 19A and in primary human colonocytes

Epidemiological findings have indicated that red meat increases the likelihood of colorectal cancer. Aim of this study was to investigate whether hemoglobin, or its prosthetic group heme, in red meat, is a genotoxic risk factor for cancer. Human colon tumor cells (HT29 clone 19A) and primary colonocytes were incubated with hemoglobin/hemin and DNA damage was investigated using the comet assay. Cell number, membrane damage, and metabolic activity were measured as parameters of cytotoxicity in both cell types. Effects on cell growth were determined using HT29 clone 19A cells. HT29 clone 19A cells were also used to explore possible pro-oxidative effects of hydrogen peroxide (H2O2) and antigenotoxic effects of the radical scavenger dimethyl sulfoxide (DMSO). Additionally we determined in HT29 clone 19A cells intracellular iron levels after incubation with hemoglobin/hemin. We found that hemoglobin increased DNA damage in primary cells (> or =10 microM) and in HT29 clone 19A cells (> or =250 microM). Hemin was genotoxic in both cell types (500-1000 microM) with concomitant cytotoxicity, detected as membrane damage. In both cell types, hemoglobin and hemin (> or =100 microM) impaired metabolic activity. The growth of HT29 clone 19A cells was reduced by 50 microM hemoglobin and 10 microM hemin, indicating cytotoxicity at genotoxic concentrations. Hemoglobin or hemin did not enhance the genotoxic activity of H2O2 in HT29 clone 19A cells. On the contrary, DMSO reduced the genotoxicity of hemoglobin, which indicated that free radicals were scavenged by DMSO. Intracellular iron increased in hemoglobin/hemin treated HT29 clone 19A cells, reflecting a 40-50% iron uptake for each compound. In conclusion, our studies show that hemoglobin is genotoxic in human colon cells, and that this is associated with free radical mechanisms and with cytotoxicity, especially for hemin. Thus, hemoglobin/hemin, whether available from red meat or from bowel bleeding, may pose genotoxic and cytotoxic risks to human colon cells, both of which contribute to initiation and progression of colorectal carcinogenesis.

Vascular heme oxygenase-1 induction suppresses microvascular thrombus formation in vivo

OBJECTIVE

By heme degradation, heme oxygenase-1 (HO-1) provides endogenous carbon monoxide and bilirubin, both of which play major roles in vascular biology. The current study aimed to examine whether induction of HO-1 and its byproducts modulate the process of microvascular thrombus formation in vivo.

METHODS AND RESULTS

In individual microvessels of mouse cremaster muscle preparations, ferric chloride-induced thrombus formation was analyzed using intravital fluorescence microscopy. When mice were pretreated with an intraperitoneal injection of hemin, a HO-1 inducer, immunohistochemistry and Western blot protein analysis of cremaster muscle tissue displayed a marked induction of HO-1. In these animals, superfusion with ferric chloride solution induced arteriolar and venular thrombus formation, which, however, was significantly delayed when compared with thrombus formation in animals without HO-1 induction. The delay in thrombus formation in hemin-treated mice was completely blunted by tin protoporphyrin-IX, a HO-1 inhibitor, but not by copper protoporphyrin-IX, which does not inhibit the enzyme. Coadministration of the vitamin E analogue Trolox in HO-1-blocked animals almost completely restored the delay in thrombus formation, implying that, besides CO, the antioxidant HO pathway metabolite bilirubin mainly contributes to the antithrombotic property of HO-1. This was further supported by the fact that bilirubin was found as effective as hemin in delay of ferric chloride-induced thrombus formation. Animals with HO-1 induction revealed reduced P-selectin protein expression in cremaster muscle tissue, which most probably presented the molecular basis for delayed thrombus growth.

CONCLUSIONS

Local induction of HO-1 activity may be of preventive and therapeutic value for clinical disorders with increased risk of thrombotic events.

Atorvastatin ameliorates early brain injury after subarachnoid hemorrhage via inhibition of pyroptosis and neuroinflammation

Subarachnoid hemorrhage (SAH) is a subtype of stroke with high mortality and morbidity due to the lack of effective therapy. Atorvastatin has been reported to alleviate early brain injury (EBI) following subarachnoid hemorrhage (SAH) via reducing reactive oxygen species, antiapoptosis, regulated autophagy, and neuroinflammation. Which was the related to the pyroptosis? Pyroptosis can be defined as a highly specific inflammatory programmed cell death, distinct from classical apoptosis and necrosis. However, the precise role of pyroptosis in atorvastatin-mediated neuroprotection following SAH has not been confirmed. The present study aimed to investigate the neuroprotection and potential molecular mechanisms of atorvastatin in the SAH-induced EBI via regulating neural pyroptosis using the filament perforation model of SAH in male C57BL/6 mice, and the hemin-induced neuron damage model in HT-22. Atorvastatin or vehicle was administrated 2 h after SAH and hemin-induced neuron damage. The mortality, neurological score, brain water content, and neuronal death were evaluated. The results show that the atorvastatin treatment markedly increased survival rate, neurological score, greater survival of neurons, downregulated the protein expression of NLRP1, cleaved caspase-1, interleukin-1β (IL-1β), and IL-18, which indicated that atorvastatin-inhibited pyroptosis and neuroinflammation, ameliorated neuron death in vivo/vitro subjected to SAH. Taken together, this study demonstrates that atorvastatin improved the neurological outcome in rats and reduced the neuron death by against neural pyroptosis and neuroinflammation.

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.

Destabilisation and subsequent lysis of human erythrocytes induced by Plasmodium falciparum haem products

In falciparum malaria, both infected and uninfected red cells have structural and functional alterations. To investigate the mechanisms of these modifications, we studied the effects of two Plasmodium falciparum haem products (haematin and malaria pigment in the synthetic form beta-haematin) on isolated human red blood cells (RBCs) and purified RBC ghosts. A dose- and time-dependent incorporation of haematin into RBC ghosts and intact cells was observed, which was in proportion to the extent of haematin- induced haemolysis. RBCs pre-incubated with haematin were more sensitive to haemolysis induced by hypotonic shock, low pH, H2O2 or haematin itself. Haemolysis was not related to membrane lipid peroxidation and only partially to oxidation of protein sulphydryl groups and it could not be prevented by scavengers of lipid peroxidation or hydroperoxide groups. N-acetylcysteine partly protected the oxidation of SH groups and significantly reduced haemolysis. In contrast, beta-haematin was neither haemolytic nor oxidative towards protein sulphydryl groups. Beta-haematin did destabilise the RBC membrane, but to a lesser extent than haematin, inducing increased susceptibility to lysis caused by hypotonic medium, H2O2 or haematin. This study suggests that the destabilising effect of haematin and, to a much less extent, beta-haematin on the RBC membrane does not result from oxidative damage of membrane lipids but from direct binding or incorporation which may affect the reciprocal interactions between the membrane and cytoskeleton proteins. These changes could contribute to the reduced red cell deformability associated with severe malaria.

Activation of extracellular signal-regulated kinases potentiates hemin toxicity in astrocyte cultures

Hemin is present in intracranial hematomas in high micromolar concentrations and is a potent, lipophilic oxidant. Growing evidence suggests that heme-mediated injury may contribute to the pathogenesis of CNS hemorrhage. Extracellular signal-regulated kinases (ERKs) are activated by oxidants in some cell types, and may alter cellular vulnerability to oxidative stress. In this study, the effect of hemin on ERK activation was investigated in cultured murine cortical astrocytes, and the consequence of this activation on cell viability was quantified. Hemin was rapidly taken up by astrocytes, and generated reactive oxygen species (ROS) within 30 min. Increased immunoreactivity of dually phosphorylated ERK1/2 was observed in hemin-treated cultures at 30-120 min, without change in total ERK. Surprisingly, ERK activation was not attenuated by concomitant treatment with antioxidants (U74500A or 1,10-phenanthroline) at concentrations that blocked ROS generation. Cell death commenced after 2 h of hemin exposure and was reduced by antioxidants and by the caspase inhibitor Z-VAD-FMK. Cytotoxicity was also attenuated by MEK inhibition with PD98059 or U0126 at concentrations that were sufficient to prevent ERK activation. Whereas the effect of Z-VAD-FMK on cell survival was transient, the effect of MEK inhibitors was long-lasting. MEK inhibitors had no effect on cellular hemin uptake or subsequent ROS generation. The present results suggest that hemin activates ERK in astrocytes via a mechanism that is independent of ROS generation. This activation sensitizes astrocytes to hemin-mediated oxidative 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.

Effects of flavonoids extracted from Scutellaria baicalensis Georgi on hemin–nitrite–H2O2 induced liver injury

Hemin-nitrite-H2O2 system may play a role in liver oxidative injury in some pathological events. In this paper, the effects of the three active components of the root of Scutellaria baicalensis Georgi, i.e. baicalin, baicalein and wogonin, on hemin-nitrite-H2O2 induced liver injury were studied in liver homogenate, liver microsome and human hepatoblastoma cell line HepG2 cells. It was found that hemin-nitrite-H2O2 could induce liver homogenate protein nitration, lipid peroxidation and liver microsome protein oxidation; it also caused a decrease of HepG2 cells viability. Baicalein, baicalin and wogonin could inhibit protein nitration and lipid peroxidation in liver homogenate as well as in HepG2 cells in a dose-dependent manner, the inhibition order was baicalein>baicalin>>wogonin. These three flavonoids also inhibited the oxidation of protein in liver microsome, the decrease of cell viability and the content of GSH in HepG2 cells, among which baicalin represented the most inhibitory effect. Besides, hemin-H2O2 induced cell injury could be augmented with the existence of nitrite, indicating protein nitration involved in hemin-nitrite-H2O2 induced liver injury. These results demonstrated hemin-nitrite-H2O2 could induce liver injury through oxidizing or nitrating different biomolecules. Baicalein, baicalin and wogonin could inhibit hemin-nitrite-H2O2 induced liver injury in dose-dependent manners by inhibiting oxidation and nitration.

Increasing expression of heme oxygenase-1 by proteasome inhibition protects astrocytes from heme-mediated oxidative injury

Hemin is released from hemoglobin after CNS hemorrhage, and may contribute to cell loss in surrounding tissue. Heme oxygenase-1 (HO-1) is induced by these injuries, and may have an effect on cell viability. In a prior study, we reported that increasing HO-1 expression by adenoviral gene transfer prior to hemin exposure attenuated oxidative stress and cell death in astrocytes. However, rapid gene transfer to the CNS may not be feasible. HO-1 expression is controlled by a stress-responsive transcription factor, Nrf2, which is a labile protein that is subject to proteasomal degradation. In this study, we hypothesized that preventing degradation of Nrf2 with a lipid-soluble proteasome inhibitor would increase HO-1 expression and protect astrocytes from hemin. Treatment of cortical astrocyte cultures with 1 microM MG-132 resulted in a rapid increase in Nrf2, to a level that was five-fold that of vehicle-treated cultures by 2 h. This was followed by a three to six-fold increase in HO-1 expression that persisted through the 16 h observation period. Exposure of cultures to 30 microM or 60 microM hemin for 8 h resulted in death, as measured by LDH release, of 39+/-3.0 or 67.5+/-5.9% of astrocytes. Pre-treatment with MG-132 prevented approximately half of this injury. Cytoprotection persisted at 24 h, and was also observed when injury was assessed via the MTT assay. Astrocyte protein oxidation produced by hemin was also significantly attenuated by MG-132 pre-treatment. These results suggest that increasing HO-1 expression with a proteasome inhibitor protects astrocytes from heme-mediated oxidative injury. This pharmacological approach may provide a mechanism for rapidly upregulating HO-1 in astrocytes 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.

Enhanced in vitro neurotoxicity of artemisinin derivatives in the presence of haemin

The role of haem in the neurotoxicity of artemisinin derivatives has been studied in vitro by examining neurite outgrowth measured by image analysis and cellular metabolism of the tetrazolium salt MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) measured spectrophotometrically in the neuroblastoma cell line NB2a, and by examining binding of radiolabelled dihydroartemisinin to NB2a cell and rat brain proteins. In the cases of artemether, dihydroartemisinin, and arteether, haemin (ferriprotoporphyrin IX) significantly increased the dose-related inhibition of neurite outgrowth from differentiating NB2a cells and significantly increased the dose-dependent inhibition of MTT metabolism. Inhibition of neurite outgrowth and metabolism of MTT in the presence or absence of haemin ranged from 72% to 93% and from 27% to 49% at a drug concentration of 300 nM. Haemin also significantly increased the dose-related binding of radiolabelled dihydroartemisinin to proteins from NB2a cells approximately twofold and to rat brain between three- and sixfold. Haemin did not enhance the neurotoxicity of desoxyarteether, a structural analogue of arteether with an ether linkage in the place of the endoperoxide bridge. It is suggested that haemin may catalyse the transformation of these derivatives via an interaction with the endoperoxide bridge of the artemisinin derivative to produce free radicals or electrophilic intermediates that are toxic to neuronal cells.

Heme-mediated reactive oxygen species toxicity to retinal pigment epithelial cells is reduced by hemopexin

Catalysis of the formation of reactive oxygen species (RO2S) by low molecular weight complexes of iron has been implicated in several pathological conditions in the retina since photoreceptors and retinal pigment epithelial cells are likely to be especially sensitive to RO2S. Since protective proteins cannot cross the blood-retinal barrier, it is likely that the retina performs its own protective functions by synthesizing proteins that bind iron and nonprotein iron complexes, the major catalysts of RO2S generation. Investigations were carried out to determine whether pigment epithelial cells are themselves sensitive to iron-generated RO2S and whether apo-transferrin and apo-hemopexin, known to be made locally in the retina, can perform a protective function. In 51Cr release assays, the toxicity of exogenous RO2S including hydrogen peroxide or superoxide (generated by xanthine oxidase/hypoxanthine) to human retinal pigment epithelial cells was inhibited by the iron chelators, desferrioxamine and apo-transferrin. Free but not protein-bound ferric iron and heme exacerbated the toxic effect. The toxic effect of heme was abolished by the heme-scavenging, extracellular antioxidant, apo-hemopexin, and also by exogenous bovine serum albumin. In addition, heme toxicity was inhibited by a 3 h preincubation of cells with either heme, apo-hemopexin, or heme-hemopexin 24 h prior to the toxicity assay. It is concluded, first, that toxic effects of iron and heme can be prevented by apo-transferrin or apo-hemopexin and, second, that exposure of RPE cells to free heme or hemopexin sets in motion a series of biochemical events resulting in protection against oxidative stress. It is probable that these include heme oxygenase induction.

Uptake, metabolism and toxicity of hemin in cultured neurons

Following hemorrhagic stroke, red blood cells lyse and release neurotoxic hemin into the interstitial space. The present study investigates whether neurons can accumulate and metabolize hemin. We demonstrate that cultured neurons express the heme carrier protein 1 (HCP1), and that this transporter appears to contribute to the time- and concentration-dependent accumulation of hemin by neurons. Although exposure of neurons to hemin stimulates the synthesis of the iron storage protein ferritin, approximately 80% of the hemin accumulated by neurons remains intact. Within 24h of incubation, substantial neurotoxicity was observed that was not attenuated by the cell permeable, selective ferrous iron chelator, 1,10-phenanthroline. These results demonstrate that while neurons efficiently accumulate hemin they slowly degrade it, and they support the conclusion that intact hemin is more neurotoxic than the iron released from the breakdown of hemin. Further investigations are required to determine the basis of this neurotoxicity.

In vivo effects of porphyrins on bacterial DNA

The DNA damage in intact Staphylococcus aureus and E. coli cells induced by photosensitized deuteroporphyrin or hemin is described. Treatment of S. aureus cultures with hemin or photosensitized deuteroporphyrin (Dp) caused time-dependent changes in the plasmidial DNA profiles. The major observation was the disappearance of the plasmid supercoiled fraction. The chromosomal DNA was also affected by hemin and by photosensitized Dp, since its degradation products were detected after exposing the bacterial cells to the porphyrin drugs. Photosensitization of E. coli cells, pretreated with Dp and polymyxin B nonapeptide (PMBNP), also resulted in plasmidial damage. No such damage occurred when E. coli cultures were treated with hemin and PMBNP. The above results can be tightly correlated with the antimicrobial action of porphyrins. Their damage to the bacterial DNA seems to reflect one of the in vivo effects of these porphyrins.

Neuroprotective effects of valproic acid against hemin toxicity: possible involvement of the down-regulation of heme oxygenase-1 by regulating ubiquitin-proteasomal pathway

During hemorrhagic stroke induced by intracerebral hemorrhage (ICH), brain injury occurs from the deleterious actions of hemoglobin byproducts; induction of heme oxygenase-1 (HO-1) also plays a critical role in the neurotoxicity in ICH. Valproic acid (VPA), which is a commonly used drug in the treatment of epilepsy, has been reported to have neuroprotective effects against various neuronal insults including ischemic stroke. We investigated the effect of VPA on HO-1-mediated neurotoxicity in an experimental model of ICH. We investigated the effects of VPA on HO-1 protein in primary cortical neurons: (1) the expression levels of HO-1 mRNA and protein measured by RT-PCR and Western blotting; (2) the cell viability and ROS generation by MTT reduction assay and ROS measurement; (3) the signal pathway regulated by VPA using IP-Western blotting; (4) the effects of VPA on hemin-induced cell death by hemin microinjection and immunohistochemistry in vivo. VPA treatment partially blocked cell death induced by hemin, which is released from hemoglobin during ICH, both in rat primary cortical neurons and rat brain. Treatment of VPA significantly decreased the expression of HO-1 protein both in vitro and in vivo. Hemin treatment induced HO-1 protein expression and this was partially blocked by pretreatment with VPA, which might be mediated by increased ubiquitination and degradation of HO-1 via ERK1/2 and JNK activation in primary cortical neurons. Our results indicate that VPA inhibits hemin toxicity by downregulating HO-1 protein expression, and provide a therapeutic strategy to attenuate intracerebral hemorrhagic injury.

The role of glutathione in the neurotoxicity of artemisinin derivatives in vitro

The role of antioxidants in the neurotoxicity of the antimalarial endoperoxides artemether and dihydroartemisinin was studied in vitro by quantitative image analysis of neurite outgrowth in the neuroblastoma cell line NB2a. Intracellular glutathione concentrations were measured by high performance liquid chromatography with fluorescence detection. Both dihydroartemisinin (1 microM) and a combination of artemether (0.3 microM) plus haemin (2 microM) significantly inhibited neurite outgrowth from differentiating NB2a cells to 11.5 +/- 11.0% (SD) and 19.6 +/- 15.2% of controls, respectively. The inhibition by artemether/haemin was prevented by the antioxidants superoxide dismutase (109.7 +/- 47.8% of control), catalase (107.0 +/- 29.3%) glutathione (123.8 +/- 12.4%), L-cysteine (88.0 +/- 6.3%), N-acetyl-L-cysteine (107.8 +/- 14.9%), and ascorbic acid (104.3 +/- 12.7%). Dihydroartemisinin-induced neurotoxicity was completely or partially prevented by L-cysteine (99.5 +/- 17.7% of control), glutathione (57.9 +/- 23.4% of control), and N-acetyl-L-cysteine (57.3 +/- 9.5%), but was not prevented by superoxide dismutase, catalase, or ascorbic acid. Buthionine sulphoximine, an inhibitor of gamma-glutamylcysteine synthetase, significantly increased the neurotoxic effect of non-toxic concentrations of artemether/haemin (0.1 microM/2 microM) and dihydroartemisinin (0.2 microM), suggesting that endogenous glutathione participates in the prevention of the neurotoxicity of artemether/haemin and dihydroartemisinin. Artemether/haemin completely depleted intracellular glutathione levels, whereas dihydroartemisinin had no effect. We conclude that although glutathione status is an important determinant in the neurotoxicity of endoperoxides, depletion of glutathione is not a prerequisite for their toxicity. This is consistent with their mechanisms of toxicity being free radical-mediated damage to redox-sensitive proteins essential for neurite outgrowth, or alteration of a redox-sensitive signalling system which regulates neurite outgrowth.

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.

Verotoxin (shiga toxin) sensitizes renal epithelial cells to increased heme toxicity: possible implications for the hemolytic uremic syndrome

Escherichia coli-derived verotoxins (VT; Shiga toxins) are causally related to the pathogenesis of enteropathic hemolytic uremic syndrome (HUS). Profound hemolysis is a defining feature of the disease, but it is not known whether the acute intravascular release of heme proteins contributes to HUS pathology. This study examined the biologic effects of hemin and VT by means of tubular epithelial-derived ACHN cells. Hemin at concentrations >/=200 microM caused cell rounding, spike formation, and detachment that was morphologically distinct from verocytotoxicity. VT caused apoptosis at concentrations >100 pM, as demonstrated by nuclear segmentation and poly(ADP-ribose) polymerase cleavage, whereas hemin-mediated injury of ACHN cells grown in serum-containing medium lacked attributes of programmed cell death. Pretreatment of ACHN monolayers with sublethal concentrations (1 to 10 pM) of VT for 12 to 18 h led to superadditive hemin-mediated cytotoxicity. This effect was not limited to ACHN cells, but was similarly noted in microvascular endothelial cells. Heme catabolism is regulated by (inducible) heme oxygenase-1 (HO-1). VT abrogated HO-1 expression in ACHN cells. Stimulation of cells for 6 h with CdCl(2), which markedly increased HO-1 expression before the addition of VT, blunted subsequent hemin injury. In conclusion, VT augments hemin-induced toxicity in renal tubular epithelial cells that can be reversed by prior induction of HO-1. It is proposed that VT subverts the physiologic defense against heme proteins by interfering with the regulated expression of HO-1 and that this mechanism contributes to the renal pathology in patients with Escherichia coli-associated HUS.

Ferriprotoporphyrin IX binding substances and the mode of action of chloroquine against malaria

Bovine serum albumin and preparations of cell sap from malaria parasites and normal erythrocytes were tested for ability to protect cellular membranes against the toxicity of ferriprotoporphyrin IX (FP) and a chloroquine-FP complex. Suspensions of Plasmodium berghei (approximately 7 X 10(6) parasites per ml, isolated from saponin-lysed, infected erythrocytes) were used as a test system. Toxicity was monitored by measuring changes in turbidity of these suspensions at 700 nm. Parasite cell sap (0.56 mg protein per ml) and albumin (1 mg per ml) completely prevented the toxicity of 40 micrometers FP. Erythrocyte cell sap (8.6 mg of hemoglobin per ml). Provided only partial protection from 40 micrometers FP. Neither the cell sap preparations nor albumin eliminated the toxicity of a chloroquine-FP complex formed from 20 micrometers chloroquine and 40 micrometers FP. These observations suggest that the cell sap preparations contain FP binding substances and that the mode of action of chloroquine may be to shunt FP away from a nontoxic complex with these substances and into a toxic chloroquine-FP complex.