Treatment with CO-RMs during cold storage improves renal function at reperfusion.

Low concentrations of carbon monoxide (CO) can protect tissues against ischemia-reperfusion (I-R) injury. We have recently identified a novel class of compounds, CO-releasing molecules (CO-RMs), which exert important pharmacological activities by carrying and delivering CO to biological systems. Here, we examined the possible beneficial effects of CO liberated from CO-RMs on the damage inflicted by cold storage and I-R in isolated perfused kidneys. Hemodynamic and biochemical parameters as well as mitochondrial respiration were measured in isolated perfused rabbit kidneys that were previously flushed with CO-RMs and stored at 4 degrees C for 24 h. Two water-soluble CO-RMs were tested: (1) sodium boranocarbonate (CORM-A1), a boron-containing carbonate that releases CO at a slow rate, and (2) tricarbonylchloro(glycinato)ruthenium(II) (CORM-3), a transition metal carbonyl that liberates CO very rapidly in solution. Kidneys flushed with Celsior solution supplemented with CO-RMs (50 microM) and stored at 4 degrees C for 24 h displayed at reperfusion a significantly higher perfusion flow rate (PFR), glomerular filtration rate, and sodium and glucose reabsorption rates compared to control kidneys flushed with Celsior solution alone. Addition of 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ), a guanylate cyclase inhibitor, prevented the increase in PFR mediated by CO-RMs. The respiratory control index from kidney mitochondria treated with CO-RMs was also markedly increased. Notably, renal protection was lost when kidneys were flushed with Celsior containing an inactive compound (iCO-RM), which had been deliberately depleted of CO. CO-RMs are effective therapeutic agents that deliver CO during kidney cold preservation and can be used to ameliorate vascular activity, energy metabolism and renal function at reperfusion.

Reviewing the use of carbon monoxide-releasing molecules (CO-RMs) in biology: implications in endotoxin-mediated vascular dysfunction.

The inducible stress protein heme oxygenase-1 (HO-1) has been linked to tissue and organ protection against the deleterious actions of many pathological conditions, including endotoxin challenge. Similar protection can be achieved by the main products of heme oxygenase activity, namely bilirubin and carbon monoxide (CO). Since the identification of novel chemical compounds that liberate CO in biological systems (CO-releasing molecules or CO-RMs), our group and others have had access to a convenient and simple pharmacological tool that enables to study the role of CO in physiological functions. This article will review the scientific literature published to date on CO-RMs, with emphasis on the in vitro, ex vivo and in vivo experimental models employed to determine the contribution of CO to cellular mechanisms. In addition, we will report on the effect of heme oxygenase-related substances, such as bilirubin, CORM-3 and hemin, in a model of endotoxin-induced hypotension. Among the three different approaches examined, CORM-3 proved the most effective agent in reducing the fall in blood pressure caused by endotoxin. Furthermore, heme oxygenase-related substances affected the endotoxin-stimulated induction and distribution of hepatic HO-1 and inducible nitric oxide synthase (iNOS). Thus, it emerges that CO-RMs could exert important biological actions in the context of endotoxic-mediated dysfunction.

Carbon monoxide-releasing molecules (CO-RMs) modulate respiration in isolated mitochondria.

Emerging evidence reveals that heme oxygenase-1 (HO-1) and its product carbon monoxide (CO) can exert diverse biological and cytoprotective effects. Our group has recently identified a new class of compounds (CO-releasing molecules or CO-RMs) that can carry and deliver CO to biological systems and can be used to examine the physiological properties of CO. Here, we evaluated the influence of endogenously-generated CO (via HO-1 induction by hemin) and CO liberated from exogenously supplied CO-RMs on mitochondrial function. Renal mitochondria were isolated either from rats with increased HO-1 or from untreated animals, the latter being exposed to different concentrations of CO-RMs (10-100 microM). We found that mitochondrial oxygen uptake was significantly reduced in kidneys after HO-1 induction and, in a similar fashion, CO-RMs inhibited mitochondrial function in a concentration-dependent manner. Specifically, a marked depression of state 3 was observed resulting in a significant decrease in respiratory control index (RCI) values. When mitochondria were incubated with the inactive forms of CO-RMs, which are devoid of CO, the respiratory parameters remained unchanged. In summary, the results indicate that HO-1 induction and enhanced CO decrease renal oxygen consumption and alter mitochondrial function suggesting that CO could be a physiological regulator of mitochondrial oxidative phosphorylation.

Caffeic acid phenethyl ester and curcumin: a novel class of heme oxygenase-1 inducers.

Heme oxygenase-1 (HO-1) is a redox-sensitive inducible protein that provides efficient cytoprotection against oxidative stress. Curcumin, a polyphenolic natural compound that possesses anti-tumor and anti-inflammatory properties, has been reported recently to induce potently HO-1 expression in vascular endothelial cells (Free Rad Biol Med 28:1303-1312, 2000). Here, we extend our previous findings by showing that caffeic acid phenethyl ester (CAPE), another plant-derived phenolic agent, markedly increases heme oxygenase activity and HO-1 protein in astrocytes. The effect seems to be related to the peculiar chemical structures of curcumin and CAPE, because analogous antioxidants containing only portions of these two molecules were totally ineffective. At a final concentration of 30 microM, both curcumin and CAPE maximally up-regulated heme oxygenase activity while promoting marked cytotoxicity at higher concentrations (50-100 microM). Similar results were obtained with Curcumin-95, a mixture of curcuminoids commonly used as a dietary supplement. Incubation of astrocytes with curcumin or CAPE at concentrations that promoted maximal heme oxygenase activity resulted in an early increase in reduced glutathione followed by a significant elevation in oxidized glutathione contents. A curcumin-mediated increase in heme oxygenase activity was not affected by the glutathione precursor and thiol donor N-acetyl-L-cysteine. These data suggest that regulation of HO-1 expression by polyphenolic compounds is evoked by a distinctive mechanism which is not necessarily linked to changes in glutathione but might depend on redox signals sustained by specific and targeted sulfydryl groups. This study identifies a novel class of natural substances that could be used for therapeutic purposes as potent inducers of HO-1 in the protection of tissues against inflammatory and neurodegenerative conditions.

Interaction of heme with nitroxyl or nitric oxide amplifies heme oxygenase-1 induction: involvement of the transcription factor Nrf2.

Heme oxygenase-1 (HO-1) is a cytoprotective enzyme, the expression of which is highly sensitive to induction by pro-oxidant stimuli including the substrate heme and reactive oxygen species. Conceptually, the perception that HO-1 plays a key role in response to oxidative damage is paralleled by evidence showing high expression of HO-1 in a variety of cell systems challenged with nitric oxide (NO) or NO-derivatives, thus revealing a potential biological function for HO-1 against nitrosative stress. In this study, we report that exposure of cardiac cells to hemin (5-20 microM) in combination with compounds that liberate nitroxyl (HNO/NO-) or release NO significantly potentiates HO-1 mRNA and protein expression leading to a remarkable increase in heme oxygenase activity under both normoxic and hypoxic conditions. The amplification of the heme oxygenase pathway appears to involve a direct interaction between heme and the NO groups, as the ability of both NO(-)- and NO-releasing agents to induce HO-1 is totally lost by their pre-incubation for 1 hr in complete medium prior to cell treatment but is highly preserved by addition of hemin during the preincubation step. In addition, we show that the redox-sensitive transcription factor Nrf2 is highly expressed in the nuclear fraction of cells exposed to the NO- generator and that this effect is totally abolished by the presence of N-acetyl-L-cysteine. Interestingly, the expression of Nrf2 is gradually intensified by treating cells with a combination of the NO- releaser and increasing concentrations of hemin. Thus, a strict parallelism exists between the extent of HO-1 induction and expression of Nrf2 elicited by the heme-NO interaction. We propose that modification of the iron protoporphyrin centers by NO groups to modulate HO-1 expression might be regarded as a molecular switch to maximize heme oxygenase enzymatic activity and consequently mitigate the redox imbalance imposed by oxidative and nitrosative stress.

Homocysteine attenuates endothelial haem oxygenase-1 induction by nitric oxide (NO) and hypoxia.

The disrupted metabolism of homocysteine (Hcy) causes hyperhomocysteinemia, a condition associated with the impairment of nitric oxide (NO) bio-availability, tissue hypoxia and increased risk of vascular disease. Here, we examined how Hcy modulates the induction of the stress protein haem oxygenase-1 (HO-1) evoked by NO releasing agents and hypoxia in vascular endothelial cells. We found that Hcy (0.5 mM) markedly reduced the increase in haem oxygenase activity and HO-1 protein expression induced by sodium nitroprusside (SNP, 0.5 mM) but did not affect HO-1 activation mediated by S-nitroso-N-acetyl-penicillamine. Cells pre-treated with Hcy followed by addition of fresh medium containing SNP still exhibited an augmented haem oxygenase activity. Interestingly, high levels of Hcy were also able to abolish hypoxia-mediated HO-1 expression in a concentration-dependent manner. These novel findings indicate that hyperhomocysteinemia interferes with crucial signaling pathways required by cells to respond and adapt to stressful conditions.

Role of heme oxygenase-1 in hypoxia-reoxygenation: requirement of substrate heme to promote cardioprotection.

Heme oxygenase-1 (HO-1) catalyzes the enzymatic degradation of heme to carbon monoxide, bilirubin, and iron. All three products possess biological functions; bilirubin, in particular, is a potent free radical scavenger of which its antioxidant property is enhanced at low oxygen tension. Here, we investigated the effect of severe hypoxia and reoxygenation on HO-1 expression in cardiomyocytes and determined whether HO-1 and its product, bilirubin, have a protective role against reoxygenation damage. Hypoxia caused a time-dependent increase in both HO-1 expression and heme oxygenase activity, which gradually declined during reoxygenation. Reoxygenation of hypoxic cardiomyocytes produced marked injury; however, incubation with hemin or bilirubin during hypoxia considerably reduced the damage at reoxygenation. The protective effect of hemin is attributable to increased availability of substrate for heme oxygenase activity, because hypoxic cardiomyocytes generated very little bilirubin when incubated with medium alone but produced substantial bile pigment in the presence of hemin. Interestingly, incubation with hemin also maintained high heme oxygenase activity levels during the reoxygenation period. Reactive oxygen species generation was enhanced after hypoxia, and hemin and bilirubin were capable once again to attenuate this effect. These results indicate that the HO-1-bilirubin pathway can effectively defend hypoxic cardiomyocytes against reoxygenation injury and highlight the issue of heme availability in the cytoprotective action afforded by HO-1.

Protective role of heme oxygenases against endotoxin-induced diaphragmatic dysfunction in rats.

Reactive oxygen species are strongly implicated in diaphragmatic dysfunction during sepsis. We investigated whether the heme oxygenase (HO) pathway, which is a powerful protective cellular system, protects the diaphragm against oxidative stress and contractile failure during sepsis. A basal expression of both the inducible and constitutive HO protein isoforms (HO-1 and HO-2, respectively) was found in the diaphragm. Enhanced HO-1 expression in diaphragmatic myocytes was observed 24 h after Escherichia coli endotoxin (lipopolysaccharide, LPS) inoculation and remained elevated for at least 96 h. Enhanced HO-1 expression was also observed in the rectus abdominis and soleus muscles and in the left ventricular myocardium of endotoxemic animals. Diaphragmatic HO-2 expression was not modified by endotoxin. Diaphragmatic HO activity exhibited a biphasic time course characterized by a transient decrease during the first 12 h followed by a significant increase at 24 h, corresponding to HO-1 induction. Diaphragmatic force was significantly reduced 24 h after LPS, concomitantly with muscular oxidative stress. Administation of an inhibitor of heme oxygenase activity, zinc protoporphyrin IX (ZnPP-IX), further impaired muscular oxidative stress and contractile failure. By contrast, increased levels of HO-1 expression obtained by pretreatment of rats with hemin, a powerful inducer of HO-1, completely prevented LPS-mediated diaphragmatic oxidative stress and contractile failure. This protective effect was reversed by ZnPP-IX. These results show an important protective role for the HO pathway against sepsis-induced diaphragmatic dysfunction, which could be related to its antioxidant properties.

Curcumin, an antioxidant and anti-inflammatory agent, induces heme oxygenase-1 and protects endothelial cells against oxidative stress.

Curcumin, a widely used spice and coloring agent in food, has been shown to possess potent antioxidant, antitumor promoting and anti-inflammatory properties in vitro and in vivo. The mechanism(s) of such pleiotropic action by this yellow pigment is unknown; whether induction of distinct antioxidant genes contributes to the beneficial activities mediated by curcumin remains to be investigated. In the present study we examined the effect of curcumin on endothelial heme oxygenase-1 (HO-1 or HSP32), an inducible stress protein that degrades heme to the vasoactive molecule carbon monoxide and the antioxidant biliverdin. Exposure of bovine aortic endothelial cells to curcumin (5-15 microM) resulted in both a concentration- and time-dependent increase in HO-1 mRNA, protein expression and heme oxygenase activity. Hypoxia (18 h) also caused a significant (P < 0.05) increase in heme oxygenase activity which was markedly potentiated by the presence of low concentrations of curcumin (5 microM). Interestingly, prolonged incubation (18 h) with curcumin in normoxic or hypoxic conditions resulted in enhanced cellular resistance to oxidative damage; this cytoprotective effect was considerably attenuated by tin protoporphyrin IX, an inhibitor of heme oxygenase activity. In contrast, exposure of cells to curcumin for a period of time insufficient to up-regulate HO-1 (1.5 h) did not prevent oxidant-mediated injury. These data indicate that curcumin is a potent inducer of HO-1 in vascular endothelial cells and that increased heme oxygenase activity is an important component in curcumin-mediated cytoprotection against oxidative stress.

Dynamics of haem oxygenase-1 expression and bilirubin production in cellular protection against oxidative stress.

The inducible isoform of haem oxygenase (HO-1) has been proposed as an effective system to counteract oxidant-induced cell injury. In several circumstances, this cytoprotective effect has been attributed to increased generation of the antioxidant bilirubin during haem degradation by HO-1. However, a direct implication for HO-1-derived bilirubin in protection against oxidative stress remains to be established. In the present study, we examined the dynamics of HO-1 expression and bilirubin production after stimulation of vascular smooth-muscle cells with hemin, a potent inducer of the HO-1 gene. We found that hemin-mediated increase in HO-1 protein expression and haem oxygenase activity is associated with augmented bilirubin levels. The majority of bilirubin production occurred early after exposure of cells to hemin. Hemin pre-treatment also resulted in high resistance to cell injury caused by an oxidant-generating system. Interestingly, this protective effect was manifest only when cells were actively producing bilirubin as a consequence of increased haem availability and utilization by HO-1. Tin protoporphyrin IX, an inhibitor of haem oxygenase activity, significantly reduced bilirubin generation and reversed cellular protection afforded by hemin treatment. Furthermore, addition of bilirubin to the culture medium markedly reduced the cytotoxicity produced by oxidants. Our findings provide direct evidence that bilirubin generated after up-regulation of the HO-1 pathway is cytoprotective against oxidative stress.

Endothelial heme oxygenase-1 induction by hypoxia. Modulation by inducible nitric-oxide synthase and S-nitrosothiols.

The stress protein heme oxygenase-1 (HO-1) is induced in endothelial cells exposed to nitric oxide (NO)-releasing agents, and this process is finely modulated by thiols (Foresti, R., Clark, J. E., Green, C. J., and Motterlini R. (1997) J. Biol. Chem. 272, 18411-18417). Here, we report that up-regulation of HO-1 in aortic endothelial cells by severe hypoxic conditions (pO(2)

Heme oxygenase-1-derived bilirubin ameliorates postischemic myocardial dysfunction.

Bilirubin is a potent antioxidant generated intracellularly during the degradation of heme by the enzyme heme oxygenase. The purpose of this study was to determine the role of increased cardiac bilirubin in protection against postischemic myocardial dysfunction. Rat hearts were isolated and perfused according to the Langendorff technique to evaluate the recovery of myocardial function after 30 min of global ischemia and 60 min of reperfusion. We found that upregulation of the inducible isoform of heme oxygenase (HO-1) by treatment of animals with hemin 24 h before ischemia ameliorated myocardial function and reduced infarct size (tetrazolium staining) on reperfusion of isolated hearts. Tin protoporphyrin IX, an inhibitor of heme oxygenase activity, completely abolished the improved postischemic myocardial performance observed after hemin-mediated HO-1 induction. Likewise, cardiac tissue injury was exacerbated by treatment with tin protoporphyrin IX. Increased cardiac HO-1 expression and heme oxygenase activity were associated with enhanced tissue bilirubin content and an increased rate of bilirubin release into the perfusion buffer. Furthermore, exogenously administered bilirubin at concentrations as low as 100 nanomolar significantly restored myocardial function and minimized both infarct size and mitochondrial damage on reperfusion. Our data provide strong evidence for a primary role of HO-1-derived bilirubin in cardioprotection against reperfusion injury.

Peroxynitrite induces haem oxygenase-1 in vascular endothelial cells: a link to apoptosis.

Peroxynitrite (ONOO-) is a potent oxidizing agent generated by the interaction of nitric oxide (NO) and the superoxide anion. In physiological solution, ONOO- rapidly decomposes to a hydroxyl radical, one of the most reactive free radicals, and nitrogen dioxide, another species able to cause oxidative damage. In the present study we investigated the effect of ONOO- on the expression of haem oxygenase-1 (HO-1), an inducible protein that is highly up-regulated by oxidative stress. Exposure of bovine aortic endothelial cells to ONOO- (250-1000 microM) produced a concentration-dependent increase in haem oxygenase activity and HO-1 protein expression. This effect was completely abolished by the ONOO- scavengers uric acid and N-acetylcysteine, and partly attenuated by 1,3-dimethyl-2-thiourea, a scavenger of hydroxyl radicals. ONOO- also produced a concentration-dependent increase in apoptosis and cytotoxicity, which were considerably decreased by uric acid and N-acetylcysteine. A 70% decrease in apoptosis was observed when cells were exposed to ONOO- in the presence of 10 microM tin protoporphyrin IX (SnPPIX), an inhibitor of haem oxygenase activity. When SnPPIX was added 5 min after ONOO-, apoptosis decreased by only 40%, which suggests that an interaction between ONOO- and the protoporphyrin occurs in our system. Increased haem oxygenase activity by pretreatment of cells with haemin resulted in elevated bilirubin production and was associated with a substantial decrease (35%) in ONOO--mediated apoptosis. These results indicate the ability of ONOO- to modulate the expression of the stress protein HO-1 and suggest that the haem oxygenase pathway contributes to protection against the cytotoxic action of ONOO-.

The heme oxygenase pathway and its interaction with nitric oxide in the control of cellular homeostasis.

Heme oxygenase is the rate limiting enzyme in heme degradation to carbon monoxide (CO), iron and bilirubin. The inducible isoform of the protein, heme oxygenase-1 (HO-1), is susceptible to up-regulation by a diverse variety of conditions and agents in mammalian tissue, leading to the common conception that HO-1 is a stress related enzyme. However, as attempts are made to unravel the mechanisms by which HO-1 is induced and as we discover that CO, iron and bilirubin may be important effector molecules, we are learning to appreciate that heme oxygenases may be central to the regulation of many physiological and pathophysiological processes besides their established function in heme catabolism. One such process may be closely linked to nitric oxide (NO). It has been demonstrated that NO and NO donors are capable of inducing HO-1 protein expression, in a mechanism depending on the de novo synthesis of RNA and protein. Thus, it is postulated that NO may serve as a signaling molecule in the modulation of the tissue stress response. This review will highlight the current ideas on the role of CO-heme oxygenase and NO-nitric oxide synthase in cell signaling and discuss how the two systems are interrelated.

Heme oxygenase-1 induction in skeletal muscle cells: hemin and sodium nitroprusside are regulators in vitro.

The heat shock protein heme oxygenase-1 (HO-1) is regulated by a variety of physiological and pharmacological factors. In skeletal muscle tissue, HO-1 has been shown to be induced only by exercise and electrical stimulation in vivo. Both hemin and sodium nitroprusside (SNP) are potent inducers of HO-1 in other tissues. In this study, we examined the effects of these two agents on HO-1 induction in L6.G8 rat skeletal myoblast cells. Hemin and SNP increased cellular heme oxygenase activity in both a time- and concentration-dependent manner. Increases in the HO-1 mRNA level and protein expression accompanied changes in heme oxygenase activity. The ability of SNP to induce HO-1 in L6.G8 cells was reduced by coincubation with hydroxocobalamin, a known nitric oxide (NO) scavenger, suggesting that NO itself may be involved in HO-1 gene stimulation. These results indicate that HO-1 expression is sensitive to both hemin and SNP in skeletal myoblast cells and may indicate an important regulatory mechanism of heme catabolism in skeletal muscle tissue.

Heme oxygenase-1-derived carbon monoxide contributes to the suppression of acute hypertensive responses in vivo.

The enzyme heme oxygenase, which exists in inducible (HO-1) and constitutive (HO-2) isoforms, catalyzes the degradation of heme to biliverdin and CO in mammalian tissues. CO has been implicated in the control of vascular tone in a manner similar to that for NO. In the present study, we investigated the contribution of the heme oxygenase/CO pathway to the modulation of acute hypertensive responses in vivo induced by (1) alphaalphaHb, a chemically modified hemoglobin known to scavenge NO, and (2) NG-nitro-L-arginine methyl ester (L-NAME), a competitive NOS inhibitor. Experiments were carried out in conscious rats in which femoral arteries and veins were surgically catheterized 1 or 5 days before treatment with the vasoconstrictor agents. Intravenous infusion of alphaalphaHb (8% solution) or L-NAME (30 micromol/kg) [corrected] produced an acute and significant increase in mean arterial pressure (P<0.05) in rats at 5 days after catheter implantation. In contrast, no change in blood pressure was observed when alphaalphaHb or L-NAME was infused 1 day after the surgical intervention. The suppression of the hypertensive response observed at 1 day after surgery correlated with a significant (P<0.05) HO-1 expression in aorta, heart, and liver as well as increased aortic CO production and cGMP levels. At 1 day after surgery, pretreatment of animals with the heme oxygenase inhibitor zinc protoporphyrin IX (50 micromol/kg IP) markedly decreased aortic CO and cGMP levels and completely restored the vasoconstrictor effects of both alphaalphaHb and L-NAME. These results provide evidence for a crucial role of the heme oxygenase/CO pathway in the regulation of blood pressure under stress conditions in vivo.

Carbon monoxide is a major contributor to the regulation of vascular tone in aortas expressing high levels of haeme oxygenase-1.

The contribution of haeme oxygenase-derived carbon monoxide (CO) to the regulation of vascular tone in thoracic aorta was investigated following induction of the inducible isoform of haeme oxygenase (HO-1). Isometric smooth muscle contractions were recorded in isolated rat aortic ring preparations. Rings were incubated in the presence of the nitric oxide (NO) donor S-nitroso-N-acetyl penicillamine (SNAP, 500 microM) for 1 h, then repetitively washed and maintained for a further 4 h prior to producing a concentration-response curve to phenylephrine (PE, 1-3000 nM). Treatment with SNAP resulted in increased mRNA and protein expression of aortic HO-1 and was associated with a significant suppression of the contractile response to PE (P<0.05 vs control). Immunohistochemical staining procedures revealed marked HO-1 expression in the endothelial layer and, to a lesser extent, in smooth muscle cells. Induction of HO-1 in SNAP-treated rings was associated with a higher 14CO release compared to control, as measured by scintillation counting after incubation of aortas with [2-14C]-L-glycine, the precursor of haeme. Guanosine 3',5'-monophosphate (cyclic GMP) content was also greatly enhanced in aortas expressing high levels of HO-1. Incubation of aortic rings with the NO synthase inhibitor, NG-monomethyl-L-arginine (100 microM), significantly (P<0.05) increased the contractile response to PE in controls but failed to restore PE-mediated contractility in SNAP-treated rings. In contrast, the selective inhibitor of haeme oxygenase, tin protoporphyrin IX (SnPP-IX, 10 microM), restored the pressor response to PE in SNAP-treated rings whilst markedly reducing CO and cyclic GMP production. We conclude that up-regulation of the HO-1/CO pathway significantly contributes to the suppression of aortic contractility to PE. This effect appears to be mediated by the elevation of cyclic GMP levels and can be reversed by inhibition of the haeme oxygenase pathway.

Thiol compounds interact with nitric oxide in regulating heme oxygenase-1 induction in endothelial cells. Involvement of superoxide and peroxynitrite anions.

Thiols are very important antioxidants that protect cells against oxidative insults. Recently, a different and new physiological role has been defined for these compounds because of their involvement in nitric oxide (NO) binding and transport in biological systems. In view of these characteristics, we examined the effect of thiols and NO on the expression of the inducible form of heme oxygenase (HO-1), a stress protein that degrades heme to carbon monoxide and biliverdin. Cultured bovine aortic endothelial cells exposed to the NO donors sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) resulted in increased heme oxygenase activity and HO-1 expression. Co-incubation with N-acetylcysteine, a precursor of glutathione synthesis, significantly attenuated heme oxygenase induction by SNP and SNAP, and a reduction in heme oxygenase activity was also observed when cells were preincubated with N-acetylcysteine for 16 h prior to exposure to NO donors. This effect appears to be associated with NO stabilization by thiols through the formation of S-nitrosothiols. Hydroxocobalamin, a specific NO scavenger, significantly decreased endothelial heme oxygenase activity, indicating a direct involvement of NO released by NO donors to regulate the expression of this stress protein. Moreover, superoxide anion (O-2) and its reaction product with NO, peroxynitrite (ONOO-), were found to partially contribute to the observed NO-mediated activation of endothelial heme oxygenase. Thus, we suggest the existence of a dynamic equilibrium among free NO, O-2, and endogenous glutathione, which might constitute an interactive signaling mechanism modulating stress and adaptive responses in tissues.

NO-mediated activation of heme oxygenase: endogenous cytoprotection against oxidative stress to endothelium.

We investigated the effect of nitric oxide (NO) on the induction of the stress protein heme oxygenase and its protective role in vascular endothelial cells exposed to hydrogen peroxide. Treatment of porcine aortic endothelial cells for 6 h with the NO-releasing compounds (0.1-1 mM) sodium nitroprusside (SNP), S-nitroso-N-acetylpenicillamine (SNAP), and 3-morpholinosydnonimine (SIN-1) resulted in a concentration-dependent increase in heme oxygenase activity. At 1 mM, the activity of heme oxygenase was augmented 8.5-fold with SNP, 5.8-fold with SNAP, and 5.7-fold with SIN-1 over the control value. In contrast, endothelial cells exposed to 100 microM S-bromoguanosine 3',5'-cyclic monophosphate, a tissue-permeable analogue that mimics the action of guanosine 3',5'-cyclic monophosphate, did not show any change in heme oxygenase activity. Activation of the inducible NO synthase by the synergistic action of bacterial lipopolysaccharide (250 ng/ml) and interferon-gamma (100 U/ml) also increased endothelial heme oxygenase activity by 3.2-fold (P < 0.05 vs control). Methylene blue (1 microM), an inhibitor of both NO synthase and guanylate cyclase activities, completely abolished this effect. Cells previously exposed to SNAP and SIN-1 exhibited a significant protection against the cytotoxicity mediated by hydrogen peroxide (250 microM) (P < 0.05). Conversely, SNP did not show any protective effects, possibly because of catalytic iron released during its chemical decomposition. In fact, the iron chelator deferoxamine (5 mM) completely suppressed the SNP-mediated cytotoxicity and partially attenuated the activity of heme oxygenase to a level equal to that mediated by SIN-1 and SNAP. These results indicate that NO is a determinant in the modulation of the activity of heme oxygenase leading to a major resistance of the endothelium to oxidative stress.

Oxidative-stress response in vascular endothelial cells exposed to acellular hemoglobin solutions.

We investigated the effect of different hemoglobins on the activation of endothelial heme oxygenase (HO), an inducible "stress" protein, which is responsible for heme catabolism, and we determined whether the propensity of hemoglobins to autoxidize correlates with endothelial heme uptake and cell injury. Porcine aortic endothelial cells were incubated for 6 h in the presence of 60 microM unmodified hemoglobin A0 (HbA0), hemoglobin cross-linked between the alpha-chains with bis-(3,5-dibromosalicyl)fumarate (alpha alpha Hb), or cyanomet-alpha alpha-hemoglobin (CNmet alpha alpha Hb). Endothelial HO activity augmented 4.1-fold in the presence of alpha alpha Hb, 2.7-fold with HbA0, and 1.8-fold with CNmet alpha alpha Hb over the control value. Deferoxamine, but not catalase or dimethylthiourea, partially attenuated the HO induction produced by alpha alpha Hb. The rates of methemoglobin formation exhibited a linear relationship over the time of incubation (r = 0.94), and the apparent rate constant was 1.8-fold higher for alpha alpha Hb (0.023 h-1) than for HbA0 (0.013 h-1). Endothelial heme content and lactate dehydrogenase (LDH) release, an index of cell injury, were also higher in alpha alpha Hb compared with HbA0 and CNmet alpha alpha Hb groups (P < 0.05). Deferoxamine but not catalase markedly reduced the release of LDH induced by alpha alpha Hb, whereas dimethylthiourea provided only a partial cytoprotection. These studies suggest that 1) the higher rate of oxidation of alpha alpha Hb contributes to the augmented endothelial HO activity, and 2) both heme release and iron-mediated oxygen radical formation are major contributors to endothelial oxidative stress and cytotoxicity generated by the cross-linked hemoglobin.