Generation of bile pigments by haem oxygenase: a refined cellular strategy in response to stressful insults

Heme Oxygenase 1 in Vertebrates: Friend and Foe

HO-1 is the inducible form of the enzyme heme-oxygenase. HO-1 catalyzes heme breakdown, reducing the levels of this important oxidant molecule and generating antioxidant, anti-inflammatory, and anti-apoptotic byproducts. Thus, HO-1 has been described as an important stress response mechanism during both physiologic and pathological processes. Interestingly, some findings are demonstrating that uncontrolled levels of HO-1 byproducts can be associated with cell death and tissue destruction as well. Furthermore, HO-1 can be located in the nucleus, influencing gene transcription, cellular proliferation, and DNA repair. Here, we will discuss several studies that approach HO-1 effects as a protective or detrimental mechanism in different pathological conditions. In this sense, as the major organs of vertebrates will deal specifically with distinct types of stresses, we discuss the HO-1 role in each of them, exposing the contradictions associated with HO-1 expression after different insults and circumstances.

Bilirubin as a Therapeutic Molecule: Challenges and Opportunities

There is strong evidence that serum free bilirubin concentration has significant effects on morbidity and mortality in the most significant health conditions of our times, including cardiovascular disease, diabetes, and obesity/metabolic syndrome. Supplementation of bilirubin in animal and experimental models has reproduced these protective effects, but several factors have slowed the application bilirubin as a therapeutic agent in human patients. Bilirubin is poorly soluble in water, and is a complex molecule that is difficult to synthesize. Current sources of this molecule are animal-derived, creating concerns regarding the risk of virus or prion transmission. However, recent developments in nanoparticle drug delivery, biosynthetic strategies, and drug synthesis have opened new avenues for applying bilirubin as a pharmaceutical agent. This article reviews the chemistry and physiology of bilirubin, potential clinical applications and summarizes current strategies for safe and efficient drug delivery.

Plasma metabolomic profiling in patients with rheumatoid arthritis identifies biochemical features predictive of quantitative disease activity

Background

Rheumatoid arthritis (RA) is a chronic, autoimmune disorder characterized by joint inflammation and pain. In patients with RA, metabolomic approaches, i.e., high-throughput profiling of small-molecule metabolites, on plasma or serum has thus far enabled the discovery of biomarkers for clinical subgroups, risk factors, and predictors of treatment response. Despite these recent advancements, the identification of blood metabolites that reflect quantitative disease activity remains an important challenge in precision medicine for RA. Herein, we use global plasma metabolomic profiling analyses to detect metabolites associated with, and predictive of, quantitative disease activity in patients with RA.

Methods

Ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was performed on a discovery cohort consisting of 128 plasma samples from 64 RA patients and on a validation cohort of 12 samples from 12 patients. The resulting metabolomic profiles were analyzed with two different strategies to find metabolites associated with RA disease activity defined by the Disease Activity Score-28 using C-reactive protein (DAS28-CRP). More specifically, mixed-effects regression models were used to identify metabolites differentially abundant between two disease activity groups (“lower”, DAS28-CRP ≤ 3.2; and “higher”, DAS28-CRP > 3.2) and to identify metabolites significantly associated with DAS28-CRP scores. A generalized linear model (GLM) was then constructed for estimating DAS28-CRP using plasma metabolite abundances. Finally, for associating metabolites with CRP (an indicator of inflammation), metabolites differentially abundant between two patient groups (“low-CRP”, CRP ≤ 3.0 mg/L; “high-CRP”, CRP > 3.0 mg/L) were investigated.

Results

We identified 33 metabolites differentially abundant between the lower and higher disease activity groups (P < 0.05). Additionally, we identified 51 metabolites associated with DAS28-CRP (P < 0.05). A GLM based upon these 51 metabolites resulted in higher prediction accuracy (mean absolute error [MAE] ± SD: 1.51 ± 1.77) compared to a GLM without feature selection (MAE ± SD: 2.02 ± 2.21). The predictive value of this feature set was further demonstrated on a validation cohort of twelve plasma samples, wherein we observed a stronger correlation between predicted and actual DAS28-CRP (with feature selection: Spearman’s ρ = 0.69, 95% CI: [0.18, 0.90]; without feature selection: Spearman’s ρ = 0.18, 95% CI: [−0.44, 0.68]). Lastly, among all identified metabolites, the abundances of eight were significantly associated with the CRP patient groups while controlling for potential confounders (P < 0.05).

Conclusions

We demonstrate for the first time the prediction of quantitative disease activity in RA using plasma metabolomes. The metabolites identified herein provide insight into circulating pro-/anti-inflammatory metabolic signatures that reflect disease activity and inflammatory status in RA patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-021-02537-4.

Hemoglobin stimulates vigorous growth of Streptococcus pneumoniae and shapes the pathogen's global transcriptome

Streptococcus pneumoniae (Spn) must acquire iron from the host to establish infection. We examined the impact of hemoglobin, the largest iron reservoir in the body, on pneumococcal physiology. Supplementation with hemoglobin allowed Spn to resume growth in an iron-deplete medium. Pneumococcal growth with hemoglobin was unusually robust, exhibiting a prolonged logarithmic growth, higher biomass, and extended viability in both iron-deplete and standard medium. We observed the hemoglobin-dependent response in multiple serotypes, but not with other host proteins, free iron, or heme. Remarkably, hemoglobin induced a sizable transcriptome remodeling, effecting virulence and metabolism in particular genes facilitating host glycoconjugates use. Accordingly, Spn was more adapted to grow on the human α − 1 acid glycoprotein as a sugar source with hemoglobin. A mutant in the hemoglobin/heme-binding protein Spbhp-37 was impaired for growth on heme and hemoglobin iron. The mutant exhibited reduced growth and iron content when grown in THYB and hemoglobin. In summary, the data show that hemoglobin is highly beneficial for Spn cultivation in vitro and suggest that hemoglobin might drive the pathogen adaptation in vivo. The hemoglobin receptor, Spbhp-37, plays a role in mediating the positive influence of hemoglobin. These novel findings provide intriguing insights into pneumococcal interactions with its obligate human host.

Therapeutic effects of CO-releaser/Nrf2 activator hybrids (HYCOs) in the treatment of skin wound, psoriasis and multiple sclerosis

Carbon monoxide (CO) produced by heme oxygenase-1 (HO-1) or delivered by CO-releasing molecules (CO-RMs) exerts anti-inflammatory action, a feature also exhibited by the nuclear factor erythroid 2-related factor 2 (Nrf2), a master regulator of the stress response. We have recently developed new hybrid molecules (HYCOs) consisting of CO-RMs conjugated to fumaric esters known to activate Nrf2/HO-1. Here we evaluated the biological activities of manganese (Mn) and ruthenium (Ru)-based HYCOs in human monocytes and keratinocytes in vitro as well as in vivo models of inflammation. The effects of HYCOs were compared to: a) dimethyl fumarate (DMF), a known fumaric ester used in the clinic; b) a CO-RM alone; or c) the combination of the two compounds. Mn–HYCOs donated CO and up-regulated Nrf2/HO-1 in vitro more efficiently than Ru–HYCOs. However, irrespective of the metal, a strong reduction in anti-inflammatory markers in monocytes stimulated by LPS was observed with specific HYCOs. This effect was not observed with DMF, CO-RM alone or the combination of the two, indicating the enhanced potency of HYCOs compared to the separate entities. Selected HYCOs given orally to mice accelerated skin wound closure, reduced psoriasis-mediated inflammation and disease symptoms equalling or surpassing the effect of DMF, and ameliorated motor dysfunction in a mouse model of multiple sclerosis. Thus, HYCOs have potent anti-inflammatory activities that are recapitulated in disease models in which inflammation is a prominent component. Prolonged daily administration of HYCOs (up to 40 days) is well tolerated in animals. Our results clearly confirm that HYCOs possess a dual mode of action highlighting the notion that simultaneous Nrf2 targeting and CO delivery could be a clinically relevant application to combat inflammation.

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HYCOs are dual activity molecules targeting the Nrf2/HO-1 axis and delivering carbon monoxide.

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Mn- and Ru-based HYCOs were compared in in vitro and in vivo models of inflammation.

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HYCOs reduce inflammation in monocytes and increase proliferation in keratinocytes.

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Oral administration of HYCOs accelerates skin wound healing and ameliorates psoriasis and multiple sclerosis.

New Arylethanolimidazole Derivatives as HO-1 Inhibitors with Cytotoxicity against MCF-7 Breast Cancer Cells

In this paper, a novel series of imidazole-based heme oxygenase-1 (HO-1) inhibitors is reported. These compounds were obtained by modifications of previously described high potent and selective arylethanolimidazoles. In particular, simplification of the central linker and repositioning of the hydrophobic portion were carried out. Results indicate that a hydroxyl group in the central region is crucial for the potency as well as the spatial distribution of the hydrophobic portion. Docking studies revealed a similar interaction of the classical HO-1 inhibitors with the active site of the protein. The most potent and selective compound (5a) was tested for its potential cytotoxic activity against hormone-sensitive and hormone-resistant breast cancer cell lines (MCF-7 and MDA-MB-231).

Heme Oxygenase-1 Inhibition Sensitizes Human Prostate Cancer Cells towards Glucose Deprivation and Metformin-Mediated Cell Death

High levels of heme oxygenase (HO)-1 have been frequently reported in different human cancers, playing a major role in drug resistance and regulation of cancer cell redox homeostasis. Metformin (MET), a drug widely used for type 2 diabetes, has recently gained interest for treating several cancers. Recent studies indicated that the anti-proliferative effects of metformin in cancer cells are highly dependent on glucose concentration. The present work was directed to determine whether use of a specific inhibitor of HO-1 activity, alone or in combination with metformin, affected metastatic prostate cancer cell viability under different concentrations of glucose. MTT assay and the xCELLigence system were used to evaluate cell viability and cell proliferation in DU145 human prostate cancer cells. Cell apoptosis and reactive oxygen species were analyzed by flow cytometry. The activity of HO-1 was inhibited using a selective imidazole-based inhibitor; genes associated with antioxidant systems and cell death were evaluated by qRT-PCR. Our study demonstrates that metformin suppressed prostate cancer growth in vitro and increased oxidative stress. Disrupting the antioxidant HO-1 activity, especially under low glucose concentration, could be an attractive approach to potentiate metformin antineoplastic effects and could provide a biochemical basis for developing HO-1-targeting drugs against solid tumors.

Progress in the development of selective heme oxygenase-1 inhibitors and their potential therapeutic application

Heme oxygenases (HOs) are a family of enzymes involved in the selective catabolism of free circulating heme. While HO-2 is constitutively expressed, HO-1 is strongly overexpressed under stressful stimuli (e.g., oxidative stress). Under these conditions, HO-1 exerts its strong cytoprotective activities and plays a crucial role in stimulating cell survival by removing the pro-oxidant heme and by producing carbon monoxide and biliverdin (promptly reduced to bilirubin). Unfortunately, the broad spectrum of HO-1 cytoprotective effects has been well experimentally documented both in normal and tumor cells, where the enzyme can be overexpressed, making it an exciting target in the management of some type of tumors. Development of non-competitive HO-1 inhibitors dates back in 2002 with the discovery of Azalanstat. Since then, many efforts have been devoted to the identification of selective HO-1 and HO-2 inhibitors and to unravel the molecular determinants responsible for selectivity. Molecular modeling studies supported the identification of chemical features involved in the recognition and inhibition of these enzymes. Herein, medicinal chemistry aspects and in silico studies related to the development of HO inhibitors will be discussed. The purpose of this review is to highlight recent advances in the development of new selective HO-1 and HO-2 inhibitors and covers the last six years (2013-2018).

Differentiation impairs Bach1 dependent HO-1 activation and increases sensitivity to oxidative stress in SH-SY5Y neuroblastoma cells

Neuronal adaptation to oxidative stress is crucially important in order to prevent degenerative diseases. The role played by the Nrf2/HO-1 system in favoring cell survival of neuroblastoma (NB) cells exposed to hydrogen peroxide (H₂O₂) has been investigated using undifferentiated or all-trans retinoic acid (ATRA) differentiated SH-SY5Y cells. While undifferentiated cells were basically resistant to the oxidative stimulus, ATRA treatment progressively decreased cell viability in response to H₂O₂. HO-1 silencing decreased undifferentiated cell viability when exposed to H₂O₂, proving the role of HO-1 in cell survival. Conversely, ATRA differentiated cells exposed to H₂O₂ showed a significantly lower induction of HO-1, and only the supplementation with low doses of bilirubin (0,5-1 μM) restored viability. Moreover, the nuclear level of Bach1, repressor of HO-1 transcription, strongly decreased in undifferentiated cells exposed to oxidative stress, while did not change in ATRA differentiated cells. Furthermore, Bach1 was displaced from HO-1 promoter in undifferentiated cells exposed to H₂O₂, enabling the binding of Nrf2. On the contrary, in ATRA differentiated cells treated with H₂O₂, Bach1 displacement was impaired, preventing Nrf2 binding and limiting HO-1 transcription. In conclusion, our findings highlight the central role of Bach1 in HO-1-dependent neuronal response to oxidative stress.

Bilirubin modulated cytokines, growth factors and angiogenesis to improve cutaneous wound healing process in diabetic rats

Bilirubin has shown cutaneous wound healing potential in some preliminary studies. Here we hypothesize that bilirubin facilitates wound healing in diabetic rats by modulating important healing factors/candidates and antioxidant parameters in a time-dependent manner. Diabetes was induced in male Wistar rats by streptozotocin. In all diabetic rats wounds were created under pentobarbitone anesthesia. All the rats were divided into two groups, of which one (control) was treated with ointment base and other with bilirubin ointment (0.3%). Wound closer measurement and tissue collection were done on days 3, 7, 14 and 19 post-wounding. The relative expressions of hypoxia inducible factor-1 alpha (HIF-1α), vascular endothelial growth factor (VEGF), stromal cell-derived factor-1 alpha (SDF-1α), transforming growth factor- beta1 (TGF-β1()), tumor necrosis factor-α (TNF-α) and interlukin-10 (IL-10) mRNA and proteins and the mRNA of interlukin-1 beta (IL-1β) and matrix metalloprteinase-9 (MMP-9) were determined in the wound tissues. CD-31 staining and collagen content were evaluated by immunohistochemistry and picrosirius red staining, respectively. Histopathological changes were assessed by H&E staining. The per cent wound closer was significantly higher from day 7 onwards in bilirubin-treated rats. HIF-1α, VEGF, SDF-1α, TGF-β1, IL-10 mRNA and protein levels were significantly higher on days 3, 7 and 14 in bilirubin-treated rats. The mRNA expression and protein level of TNF-α and the mRNA of IL-1β and MMP-9 were progressively and markedly reduced in bilirubin-treated rats. The collagen deposition and formation of blood vessels were greater in bilirubin-treated rats. Bilirubin markedly facilitated cutaneous wound healing in diabetic rats by modulating growth factors, cytokines, neovasculogenesis and collagen contents to the wound site. Topical application of bilirubin ointment might be of great use in cutaneous wound healing in diabetic patients.

Biliary tract external drainage increases the expression levels of heme oxygenase-1 in rat livers

Background

Heme oxygenase-1 (HO-1) protects cells by anti-oxidation, maintaining normal microcirculation and anti-inflammatory under stress. This study investigated the effects of biliary tract external drainage (BTED) on the expression levels of HO-1 in rat livers.

Methods

Biliary tract external drainage was performed by inserting a cannula into the bile duct. Sixty Sprague–Dawley rats were randomized to the following groups: sham 1 h group; BTED 1 h group; bile duct ligation (BDL) 1 h group; sham 6 h group and BTED 6 h group. The expression levels of HO-1 mRNA were analyzed using real-time RT-PCR. The expression levels of HO-1 were analyzed using immunohistochemistry.

Results

The expression levels of HO-1 mRNA in the liver of the BTED group increased significantly compared with the sham group 1 and 6 h after surgery (p < 0.05).The expression levels of HO-1 in the BTED group increased significantly compared with the sham group 1 and 6 h after surgery. The expression levels of HO-1 mRNA in the liver in the BDL group decreased significantly compared with the sham group 1 h after surgery (p < 0.05).The expression levels of HO-1 in the BDL group decreased significantly compared with the sham group at this time.

Conclusion

Biliary tract external drainages increase the expression levels of HO-1 in the liver.

A modern literature review of carbon monoxide poisoning theories, therapies, and potential targets for therapy advancement

The first descriptions of carbon monoxide (CO) and its toxic nature appeared in the literature over 100 years ago in separate publications by Drs. Douglas and Haldane. Both men ascribed the deleterious effects of this newly discovered gas to its strong interaction with hemoglobin. Since then the adverse sequelae of CO poisoning has been almost universally attributed to hypoxic injury secondary to CO occupation of oxygen binding sites on hemoglobin. Despite a mounting body of literature suggesting other mechanisms of injury, this pathophysiology and its associated oxygen centric therapies persists. This review attempts to elucidate the remarkably complex nature of CO as a gasotransmitter. While CO's affinity for hemoglobin remains undisputed, new research suggests that its role in nitric oxide release, reactive oxygen species formation, and its direct action on ion channels is much more significant. In the course of understanding the multifaceted character of this simple molecule it becomes apparent that current oxygen based therapies meant to displace CO from hemoglobin may be insufficient and possibly harmful. Approaching CO as a complex gasotransmitter will help guide understanding of the complex and poorly understood sequelae and illuminate potentials for new treatment modalities.

Pro-healing effects of bilirubin in open excision wound model in rats

Bilirubin, a by-product of heme degradation, has an important role in cellular protection. Therefore, we speculated that bilirubin could be of potential therapeutic value in wound healing. To validate the hypothesis, we used a full-thickness cutaneous wound model in rats. Bilirubin (30 mg/kg) was administered intraperitoneally every day for 9 days. The surface area of the wound was measured on days 0, 2, 4, 7 and 10 after the creation of the wound. The granulation tissue was collected on day 10 post-wounding for analysing various parameters of wound healing. Bilirubin treatment accelerated wound contraction and increased hydroxyproline and glucosamine contents. mRNA expression of pro-inflammatory factors such as intercellular cell adhesion molecule-1 (ICAM-1) and tumour necrosis factor-α (TNF-α) were down-regulated and that of anti-inflammatory cytokine interleukin-10 (IL-10) was up-regulated. The findings suggest that bilirubin could be a new agent for enhancing cutaneous wound healing.

Bilirubin participates in protecting of heme oxygenase-1 induction by quercetin against ethanol hepatotoxicity in cultured rat hepatocytes

To attenuate alcohol liver disease (ALD) is extremely urgent since ALD has been emerged as a major liver disease. The aim of the present study is to investigate the hepatoprotective effect against ethanol-induced injury of bilirubin, a product of heme metabolism degradation via HO and biliverdin reductase catalysis. Ethanol-incubated primary rat hepatocytes (100 mmol/L) were treated by quercetin, bilirubin, inflammatory factors, and/or HO-1 inducer/inhibitor for 24 h, and the cellular damage was assayed. Quercetin lowered ethanol-induced glutathione depletion and superoxide dismutase inactivation, inhibited the overproduction of malondialdehyde and reactive oxygen species, and decreased the leakage of cellular aspartate aminotransferase and lactate dehydrogenase, accompanying the normalization of bilirubin level. The effect of quercetin was mimicked by exogenous bilirubin in a dose-dependent manner to some extent (within 25 μmol/L) and pharmacological HO-1 inducer hemin, but abolished by HO-1 inhibitor zinc protoporphyrin-IX. Inflammatory challenge of TNF-α plus IL-6 further aggravated ethanol-induced oxidative damage, which was also attenuated by bilirubin in part. These findings shed a light on the anti-oxidative and anti-inflammatory role of bilirubin released from quercetin/HO-1 and biliverdin reductase pathway against ethanol hepatotoxicity and highlight a prospective strategy of nutritional intervention for ALD by naturally occurring quercetin to induce HO-1 with the release of bioactive end-products.

Bile pigments in pulmonary and vascular disease

The bile pigments, biliverdin, and bilirubin, are endogenously derived substances generated during enzymatic heme degradation. These compounds have been shown to act as chemical antioxidants in vitro. Bilirubin formed in tissues circulates in the serum, prior to undergoing hepatic conjugation and biliary excretion. The excess production of bilirubin has been associated with neurotoxicity, in particular to the newborn. Nevertheless, clinical evidence suggests that mild states of hyperbilirubinemia may be beneficial in protecting against cardiovascular disease in adults. Pharmacological application of either bilirubin and/or its biological precursor biliverdin, can provide therapeutic benefit in several animal models of cardiovascular and pulmonary disease. Furthermore, biliverdin and bilirubin can confer protection against ischemia/reperfusion injury and graft rejection secondary to organ transplantation in animal models. Several possible mechanisms for these effects have been proposed, including direct antioxidant and scavenging effects, and modulation of signaling pathways regulating inflammation, apoptosis, cell proliferation, and immune responses. The practicality and therapeutic-effectiveness of bile pigment application to humans remains unclear.

Therapeutic potential of heme oxygenase-1/carbon monoxide in lung disease

Heme oxygenase (HO), a catabolic enzyme, provides the rate-limiting step in the oxidative breakdown of heme, to generate carbon monoxide (CO), iron, and biliverdin-IXα. Induction of the inducible form, HO-1, in tissues is generally regarded as a protective mechanism. Over the last decade, considerable progress has been made in defining the therapeutic potential of HO-1 in a number of preclinical models of lung tissue injury and disease. Likewise, tissue-protective effects of CO, when applied at low concentration, have been observed in many of these models. Recent studies have expanded this concept to include chemical CO-releasing molecules (CORMs). Collectively, salutary effects of the HO-1/CO system have been demonstrated in lung inflammation/acute lung injury, lung and vascular transplantation, sepsis, and pulmonary hypertension models. The beneficial effects of HO-1/CO are conveyed in part through the inhibition or modulation of inflammatory, apoptotic, and proliferative processes. Recent advances, however, suggest that the regulation of autophagy and the preservation of mitochondrial homeostasis may serve as additional candidate mechanisms. Further preclinical and clinical trials are needed to ascertain the therapeutic potential of HO-1/CO in human clinical disease.

Lack of association between the c.544G>A polymorphism of the heme oxygenase-2 gene and age-related macular degeneration

Background

Age-related macular degeneration (AMD) is a primary cause of blindness among the elderly in developed countries. The nature of AMD is complex and includes both environmental and hereditary factors. Oxidative stress is thought to be essential in AMD pathogenesis. Iron is suggested to be implicated in the pathogenesis of AMD through the catalysis of the production of reactive oxygen species, which can damage the retina. Heme oxygenase-2 is capable of degradation of heme producing free iron ions, thus, diversity in heme oxygenase-2 gene may contribute to AMD. In the present work we analyzed the association between the c.544G>A polymorphism of the heme oxygenase-2 gene (HMOX2) (rs1051308) and AMD.

Material/Methods

This study enrolled 276 AMD patients and 105 sex- and age-matched controls. Genotyping of the polymorphism was performed with restriction fragment length polymorphism polymerase chain reaction (RFLP-PCR) on DNA isolated from peripheral blood.

Results

We did not find any association between the genotypes of the c.544G>A polymorphism and the occurrence of AMD. This lack of association was independent of potential AMD risk factors: tobacco smoking, sex and age. Moreover, we did not find any association between AMD and smoking in our study population.

Conclusions

The results suggest that the c.544G>A polymorphism of the heme oxygenase-2 gene is not associated with AMD in this Polish subpopulation.

Knockdown of heme oxygenase-2 impairs corneal epithelial cell wound healing

Heme oxygenase (HO) represents an intrinsic cytoprotective system based on its anti-oxidative and anti-inflammatory properties mediated via its products biliverdin/bilirubin and carbon monoxide (CO). We showed that deletion of HO-2 results in impaired corneal wound healing with associated chronic inflammatory complications. This study was undertaken to examine the role of HO activity and the contribution of HO-1 and HO-2 to corneal wound healing in an in vitro epithelial scratch injury model. A scratch wound model was established using human corneal epithelial (HCE) cells. These cells expressed both HO-1 and HO-2 proteins. Injury elicited a rapid and transient increase in HO-1 and HO activity; HO-2 expression was unchanged. Treatment with biliverdin or CORM-A1, a CO donor, accelerated wound closure by 10% at 24 h. Inhibition of HO activity impaired wound closure by more than 50%. However, addition of biliverdin or CORM-A1 reversed the effect of HO inhibition on wound healing. Moreover, knockdown of HO-2 expression, but not HO-1, significantly impaired wound healing. These results indicate that HO activity is required for corneal epithelial cell migration. Inhibition of HO activity impairs wound healing while amplification of its activity restores and accelerates healing. Importantly, HO-2, which is highly expressed in the corneal epithelium, appears to be critical for the wound healing process in the cornea. The mechanisms by which it contributes to cell migration in response to injury may reside in the cytoprotective properties of CO and biliverdin.

New light on the anti-colitic actions of therapeutic aminosalicylates: the role of heme oxygenase

Although a variety of pharmaceutical preparations of aminosalicylate are commonly used in the clinic for the control of inflammatory bowel disease, the mechanisms underlying their therapeutic actions remain unclear. Recent in vivo and in vitro studies have demonstrated that 5-aminosalicylic acid (5-ASA), regarded as the active moiety in aminosalicylate preparations such as sulfasalazine, can induce the heat shock protein, heme oxygenase-1 (HO-1) and up-regulate HO enzyme activity in the colon. As HO-1 can produce endogenous anti-oxidant and anti-inflammatory moieties such as bilirubin and carbon monoxide (CO), these findings suggest a novel mechanism of action for aminosalicylates, acting as anti-colitic agents through the up-regulation of HO-1 enzyme expression and activity.

Biliverdin amides reveal roles for propionate side chains in bilin reductase recognition and in holophytochrome assembly and photoconversion

Linear tetrapyrroles (bilins) perform important antioxidant and light-harvesting functions in cells from bacteria to humans. To explore the role of the propionate moieties in bilin metabolism, we report the semisynthesis of mono- and diamides of biliverdin IXalpha and those of its non-natural XIIIalpha isomer. Initially, these were examined as substrates of two types of NADPH-dependent biliverdin reductase, BVR and BvdR, and of the representative ferredoxin-dependent bilin reductase, phycocyanobilin:ferredoxin oxidoreductase (PcyA). Our studies indicate that the NADPH-dependent biliverdin reductases are less accommodating to amidation of the propionic acid side chains of biliverdin IXalpha than PcyA, which does not require free carboxylic acid side chains to yield its phytobilin product, phycocyanobilin. Bilin amides were also assembled with BV-type and phytobilin-type apophytochromes, demonstrating a role for the 8-propionate in the formation of the spectroscopically native P(r) dark states of these biliprotein photosensors. Neither ionizable propionate side chain proved to be essential to primary photoisomerization for both classes of phytochromes, but an unsubstituted 12-propionate was required for full photointerconversion of phytobilin-type phytochrome Cph1. Taken together, these studies provide insight into the roles of the ionizable propionate side chains in substrate discrimination by two bilin reductase families while further underscoring the mechanistic differences between the photoconversions of BV-type and phytobilin-type phytochromes.

Metalloporphyrins in the management of neonatal hyperbilirubinemia

Neonatal jaundice in the first week of life is a common problem in newborns. It is due to an imbalance of bilirubin production and its elimination, which can lead to significantly elevated levels of circulating bilirubin or hyperbilirubinemia. Use of phototherapy and/or exchange transfusion are the current modes for treating neonatal hyperbilirubinemia and preventing any neurologic damage. These strategies, however, only remove bilirubin that has already been formed. Preventing the production of excess bilirubin may be a more logical approach. Synthetic heme analogs, metalloporphyrins, are competitive inhibitors of heme oxygenase, the rate-limiting enzyme in bilirubin production, and their use has been proposed as an attractive alternative strategy for preventing or treating severe hyperbilirubinemia.

Could carbon monoxide and bilirubin be friends as well as foes of the body?

Endogenous carbon monoxide (CO) production was first described 60 years ago. CO is a by-product of the metabolism of haeme to biliverdin. This, in turn, becomes bilirubin. During the past 15 years epidemiological studies and animal experiments have identified bilirubin as a molecule at the crossroads of the protection of the body against reactive oxygen species (ROS). The studies have focused on bilirubin as a biomarker of arterial disease. Recently the potential of CO as a therapeutic agent has been explored. This review assesses the current state of evidence and sets the data in the context of whether CO is an endogenous signalling molecule, a marker of vascular disease and, whether, together with bilirubin, CO could be a potential therapeutic agent.

Gases in the mitochondria

Gasomodulators - nitric oxide, carbon monoxide and hydrogen sulphide - are important physiological mediators that have been implicated in disorders such as neurodegeneration and sepsis. Some of their biological functions involve the mitochondria. In particular, their inhibition of cytochrome c oxidase has received much attention as this can cause energy depletion and cytotoxicity. However, reports that cellular energy production and cell survival are maintained even in the presence of gasomodulators are not uncommon. In both cases, modulation of mitochondrial targets by the gasomodulators appears to be an important event. We provide an overview of the effects of the gasomodulators on the mitochondria.

Carbon monoxide in biology and microbiology: surprising roles for the "Detroit perfume"

Carbon monoxide (CO) is a colorless, odorless gas with a reputation for being an anthropogenic poison; there is extensive documentation of the modes of human exposure, toxicokinetics, and health effects. However, CO is also generated endogenously by heme oxygenases (HOs) in mammals and microbes, and its extraordinary biological activities are now recognized and increasingly utilized in medicine and physiology. This review introduces recent advances in CO biology and chemistry and illustrates the exciting possibilities that exist for a deeper understanding of its biological consequences. However, the microbiological literature is scant and is currently restricted to: 1) CO-metabolizing bacteria, CO oxidation by CO dehydrogenase (CODH) and the CO-sensing mechanisms that enable CO oxidation; 2) the use of CO as a heme ligand in microbial biochemistry; and 3) very limited information on how microbes respond to CO toxicity. We demonstrate how our horizons in CO biology have been extended by intense research activity in recent years in mammalian and human physiology and biochemistry. CO is one of several "new" small gas molecules that are increasingly recognized for their profound and often beneficial biological activities, the others being nitric oxide (NO) and hydrogen sulfide (H2S). The chemistry of CO and other heme ligands (oxygen, NO, H2S and cyanide) and the implications for biological interactions are briefly presented. An important advance in recent years has been the development of CO-releasing molecules (CO-RMs) for aiding experimental administration of CO as an alternative to the use of CO gas. The chemical principles of CO-RM design and mechanisms of CO release from CO-RMs (dissociation, association, reduction and oxidation, photolysis, and acidification) are reviewed and we present a survey of the most commonly used CO-RMs. Amongst the most important new applications of CO in mammalian physiology and medicine are its vasoactive properties and the therapeutic potentials of CO-RMs in vascular disease, anti-inflammatory effects, CO-mediated cell signaling in apoptosis, applications in organ preservation, and the effects of CO on mitochondrial function. The very limited literature on microbial growth responses to CO and CO-RMs in vitro, and the transcriptomic and physiological consequences of microbial exposure to CO and CO-RMs are reviewed. There is current interest in CO and CO-RMs as antimicrobial agents, particularly in the control of bacterial infections. Future prospects are suggested and unanswered questions posed.

Heme oxygenase-1/carbon monoxide: from metabolism to molecular therapy

Heme oxygenase-1 (HO-1), a ubiquitous inducible stress-response protein, serves a major metabolic function in heme turnover. HO activity cleaves heme to form biliverdin-IXalpha, carbon monoxide (CO), and iron. Genetic experiments have revealed a central role for HO-1 in tissue homeostasis, protection against oxidative stress, and in the pathogenesis of disease. Four decades of research have witnessed not only progress in elucidating the molecular mechanisms underlying the regulation and function of this illustrious enzyme, but also have opened remarkable translational applications for HO-1 and its reaction products. CO, once regarded as a metabolic waste, can act as an endogenous mediator of cellular signaling and vascular function. Exogenous application of CO by inhalation or pharmacologic delivery can confer cytoprotection in preclinical models of lung/vascular injury and disease, based on anti-apoptotic, anti-inflammatory, and anti-proliferative properties. The bile pigments, biliverdin and bilirubin, end products of heme degradation, have also shown potential as therapeutics in vascular disease based on anti-inflammatory and anti-proliferative activities. Further translational and clinical trials research will unveil whether the HO-1 system or any of its reaction products can be successfully applied as molecular medicine in human disease.

The use of synthetic linear tetrapyrroles to probe the verdin sites of human biliverdin-IXalpha reductase and human biliverdin-IXbeta reductase

Many vertebrate species express two enzymes that are capable of catalysing the reduction of various isomers of biliverdin. Biliverdin-IXalpha reductase (BVR-A) is most active with its physiological substrate biliverdin-IXalpha, but can also reduce the three other biliverdin isomers IXbeta, IXdelta and IXgamma. Biliverdin-IXbeta reductase (BVR-B) catalyses the reduction of only the IXbeta, IXdelta and IXgamma isomers of biliverdin. Therefore, the activity of BVR-A can be measured using biliverdin-IXalpha as a specific substrate. We now show that the dimethyl esters of biliverdin-IXbeta and biliverdin-IXdelta are substrates for BVR-B, but not for BVR-A. This provides a useful method for specifically assaying the activity of both BVR-A and BVR-B in crude mixtures, using biliverdin-IXalpha for BVR-A and the dimethyl ester of either biliverdin-IXbeta or biliverdin-IXdelta for BVR-B. Human BVR-A has been suggested as a pharmacological target for neonatal jaundice. Because of the absence of a crystal structure with biliverdin bound to BVR-A, we have investigated indirect ways of examining tetrapyrrole binding. In the present study, we report that a number of sterically locked conformers of 18-ethylbiliverdin-IXalpha are substrates for human BVR-A, and discuss the implications for the biliverdin binding site. The oxidation of bilirubin-IXalpha ditaurate to biliverdin-IXalpha ditaurate is also described. We show that biliverdin-IXalpha ditaurate is a substrate for human BVR-A and discuss the possibility of using a competing substrate, which is reduced to a water soluble and excretable rubin, as a prototypic inhibitor of BVR-A.

Heme oxygenase-1, a critical arbitrator of cell death pathways in lung injury and disease

Increases in cell death by programmed (i.e., apoptosis, autophagy) or nonprogrammed mechanisms (i.e., necrosis) occur during tissue injury and may contribute to the etiology of several pulmonary or vascular disease states. The low-molecular-weight stress protein heme oxygenase-1 (HO-1) confers cytoprotection against cell death in various models of lung and vascular injury by inhibiting apoptosis, inflammation, and cell proliferation. HO-1 serves a vital metabolic function as the rate-limiting step in the heme degradation pathway and in the maintenance of iron homeostasis. The transcriptional induction of HO-1 occurs in response to multiple forms of chemical and physical cellular stress. The cytoprotective functions of HO-1 may be attributed to heme turnover, as well as to beneficial properties of its enzymatic reaction products: biliverdin-IXalpha, iron, and carbon monoxide (CO). Recent studies have demonstrated that HO-1 or CO inhibits stress-induced extrinsic and intrinsic apoptotic pathways in vitro. A variety of signaling molecules have been implicated in the cytoprotection conferred by HO-1/CO, including autophagic proteins, p38 mitogen-activated protein kinase, signal transducer and activator of transcription proteins, nuclear factor-kappaB, phosphatidylinositol 3-kinase/Akt, and others. Enhanced HO-1 expression or the pharmacological application of HO end-products affords protection in preclinical models of tissue injury, including experimental and transplant-associated ischemia/reperfusion injury, promising potential future therapeutic applications.

Iron behaving badly: inappropriate iron chelation as a major contributor to the aetiology of vascular and other progressive inflammatory and degenerative diseases

BACKGROUND

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular 'reactive oxygen species' (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation.

REVIEW

We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation).The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible.This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, since in some circumstances (especially the presence of poorly liganded iron) molecules that are nominally antioxidants can actually act as pro-oxidants. The reduction of redox stress thus requires suitable levels of both antioxidants and effective iron chelators. Some polyphenolic antioxidants may serve both roles.Understanding the exact speciation and liganding of iron in all its states is thus crucial to separating its various pro- and anti-inflammatory activities. Redox stress, innate immunity and pro- (and some anti-)inflammatory cytokines are linked in particular via signalling pathways involving NF-kappaB and p38, with the oxidative roles of iron here seemingly involved upstream of the IkappaB kinase (IKK) reaction. In a number of cases it is possible to identify mechanisms by which ROSs and poorly liganded iron act synergistically and autocatalytically, leading to 'runaway' reactions that are hard to control unless one tackles multiple sites of action simultaneously. Some molecules such as statins and erythropoietin, not traditionally associated with anti-inflammatory activity, do indeed have 'pleiotropic' anti-inflammatory effects that may be of benefit here.

CONCLUSION

Overall we argue, by synthesising a widely dispersed literature, that the role of poorly liganded iron has been rather underappreciated in the past, and that in combination with peroxide and superoxide its activity underpins the behaviour of a great many physiological processes that degrade over time. Understanding these requires an integrative, systems-level approach that may lead to novel therapeutic targets.

A cytoprotective role for the heme oxygenase-1/CO pathway during neural differentiation of human mesenchymal stem cells

The inducible protein heme oxygenase-1 (HO-1) catalyzes the oxidation of heme to carbon monoxide (CO) and biliverdin, which play a concerted action in cytoprotection against oxidative stress and in the modulation of cell proliferation and differentiation. Here we report that both HO-1 expression and activity can be highly increased in undifferentiated human mesenchymal stem cells (MSCs) treated with hemin, a known HO-1 inducer. However, HO-1 mRNA and protein expression gradually decrease when MSCs undergo neural differentiation in vitro, making them extremely susceptible to glutamate-mediated cytotoxicity. A time course for HO-1 revealed that this protein is markedly down-regulated after 2 days and returns to control levels 6 days after differentiation. Treatment with glutamate (250 microM) after 2 days of neural differentiation resulted in a more pronounced lactate dehydrogenase release, a marker of cell injury, compared with undifferentiated cells. Notably, cells pretreated with hemin (50 microM) or compounds that release small amounts of CO (10 microM CORM-3 and CORM-A1) rendered cells more resistant to glutamate-induced toxicity; this effect was evident in both undifferentiated and differentiated MSCs. Our findings indicate that MSCs become more vulnerable to oxidative injury during the early stages of differentiation via mechanisms that involve a temporary inhibition of HO-1 expression. Thus, overexpression of HO-1 and CO-releasing molecules could provide a possible therapeutic strategy to improve cell viability during neural differentiation in applications that use stem cell technology.

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.

Computational and experimental studies on the catalytic mechanism of biliverdin-IXbeta reductase

BVR-B (biliverdin-IXbeta reductase) also known as FR (flavin reductase) is a promiscuous enzyme catalysing the pyridine-nucleotide-dependent reduction of a variety of flavins, biliverdins, PQQ (pyrroloquinoline quinone) and ferric ion. Mechanistically it is a good model for BVR-A (biliverdin-IXalpha reductase), a potential pharmacological target for neonatal jaundice and also a potential target for adjunct therapy to maintain protective levels of biliverdin-IXalpha during organ transplantation. In a commentary on the structure of BVR-B it was noted that one outstanding issue remained: whether the mechanism was a concerted hydride transfer followed by protonation of a pyrrolic anion or protonation of the pyrrole followed by hydride transfer. In the present study we have attempted to address this question using QM/MM (quantum mechanics/molecular mechanics) calculations. QM/MM potential energy surfaces show that the lowest energy pathway proceeds with a positively charged pyrrole intermediate via two transition states. These initial calculations were performed with His(153) as the source of the proton. However site-directed mutagenesis studies with both the H153A and the H153N mutant reveal that His(153) is not required for catalytic activity. We have repeated the calculation with a solvent hydroxonium donor and obtain a similar energy landscape indicating that protonation of the pyrrole is the most likely first step followed by hydride transfer and that the required proton may come from bulk solvent. The implications of the present study for the design of inhibitors of BVR-A are discussed.

Improved myocardial function after cold storage with preservation solution supplemented with a carbon monoxide-releasing molecule (CORM-3)

BACKGROUND

Carbon monoxide-releasing molecules (CO-RMs) are pharmacologically active as they protect against cardiac graft rejection and cold ischemia-mediated renal dysfunction. We investigated the cardioprotective role of carbon monoxide (CO) released from CORM-3 against cold ischemia-mediated injury in the heart and evaluated its potential application in the clinical setting of cardiac transplantation.

METHODS

Isolated rat hearts underwent cold ischemic storage for 4 or 6 hours using St Thomas Hospital solution that was supplemented with either CORM-3 (50 mumol/liter) or its inactive counterpart (iCORM-3), which does not release CO. Hearts were then reperfused. Both functional parameters and release of cardiac enzymes were assessed.

RESULTS

Addition of CORM-3 to the preservation solution resulted in a significant improvement in systolic and diastolic function as well as coronary flow when compared with hearts treated with iCORM-3. In addition, lower levels of the cardiac enzymes creatine kinase and lactate dehydrogenase were measured in the perfusate of hearts stored with CORM-3.

CONCLUSIONS

The improved functional recovery and reduced enzyme release after cardiac cold storage with CORM-3, but not iCORM-3, indicate that CO is the main mediator of myocardial protection. Thus, CO-RMs can be used as adjuvants to improve the preservation of hearts for transplantation.

Pharmacological and clinical aspects of heme oxygenase

This review is intended to stimulate interest in the effect of increased expression of heme oxygenase-1 (HO-1) protein and increased levels of HO activity on normal and pathological states. The HO system includes the heme catabolic pathway, comprising HO and biliverdin reductase, and the products of heme degradation, carbon monoxide (CO), iron, and biliverdin/bilirubin. The role of the HO system in diabetes, inflammation, heart disease, hypertension, neurological disorders, transplantation, endotoxemia and other pathologies is a burgeoning area of research. This review focuses on the clinical potential of increased levels of HO-1 protein and HO activity to ameliorate tissue injury. The use of pharmacological and genetic probes to manipulate HO, leading to new insights into the complex relationship of the HO system with biological and pathological phenomena under investigation, is reviewed. This information is critical in both drug development and the implementation of clinical approaches to moderate and to alleviate the numerous chronic disorders in humans affected by perturbations in the HO system.

Use of carbon monoxide as a therapeutic agent: promises and challenges

As a by-product of heme catabolism by the heme oxygenase system, carbon monoxide (CO) has been neglected for many years, and only recently has its role as an essential signaling molecule been appreciated. In the past decade, the use of CO gas in pre-clinical experimental models of disease has produced some remarkable data indicating that its therapeutic delivery to mammals could alleviate inflammatory processes and cardiovascular disorders. However, the inherent toxic nature of CO cannot be ignored, knowing that inhalation of uncontrolled amounts of this gas can ultimately lead to serious systemic complications and neuronal derangements. From a clinical perspective, a key question is whether a safe and therapeutically effective threshold of CO can be reached locally in organs and tissues without delivering potentially toxic amounts through the lung. The advent of CO-releasing molecules (CO-RMs), a group of compounds capable of carrying and liberating controlled quantities of CO in cellular systems, appears a plausible alternative in the attempt to overcome the limitations of CO gas. Although in its infancy and far from being used for clinical applications, the CO-RMs technology is supported by very encouraging biological results and reflected by the chemical versatility of these compounds and their endless potential to be transformed into CO-based pharmaceuticals.

The involvement of heme oxygenase-1 activity in the therapeutic actions of 5-aminosalicylic acid in rat colitis

The mechanism of action of 5-aminosalicylic acid (5-ASA), the active therapeutic moiety of a number of clinically used anti-colitic agents, is unclear. The present study investigates whether the beneficial effects in vivo could involve induction of the heat shock protein, heme oxygenase-1 (HO-1), known to provide endogenous anti-oxidant and anti-inflammatory moieties which can modulate colonic inflammation. The effects of 5-ASA on the colonic expression and activity of HO-1 along with its effect on the inflammatory damage have been evaluated in the colitis provoked by instillation of trinitrobenzene sulphonic acid (TNBS) over 48 h in the rat. Intracolonic administration of 5-ASA (8, 25 and 75 mg/kg/day) dose-dependently reduced the TNBS-provoked macroscopic colonic inflammatory injury, myeloperoxidase (MPO) activity and TNF-alpha levels, while also dose-dependently increasing colonic heme oxygenase enzyme activity. Colonic HO-1 protein expression, determined by Western blot analysis in this colitis model, was likewise further induced by 5-ASA. Intracolonic administration of 5-ASA alone under unchallenged conditions also induced colonic HO-1 protein expression and stimulated heme oxygenase enzyme activity. Administration of zinc protoporphyrin (50 micromol/kg/day, s.c.), which prevented the increase in colonic heme oxygenase activity, abolished the anti-colitic effect of 5-ASA. These results suggest that 5-ASA may exert its colonic anti-oxidant and anti-inflammatory effects in vivo in part through the up-regulation of HO-1 enzyme expression and activity.

Hemoglobin induces cytotoxic damage of glycine-preserved renal tubules

In isolated tubular segments (ITS) of rat kidney cortex, we studied the effect of hemoglobin (Hb) on reoxygenation damage. All tubules were suspended in Ringer's solution containing 5-mm glycine and oxygenated for 30 min with 95% O(2):5% CO(2), followed by a 30-min period with 95% N(2):5% CO(2), and final reoxygenation for 60 min. Untreated tubules served as controls. Different concentrations of free Hb and equivalent amounts of intact erythrocytes were added to the incubation medium. Secondly, we added deferoxamine (DFO) to Hb and erythrocytes. Membrane leakage and lipid peroxidation were measured by lactate dehydrogenase and glutamate dehydrogenase and the development of thiobarbituric acid reactive substances. Cell function was quantified by gluconeogenesis and intracellular potassium accumulation. Hb exerted concentration-dependent cytotoxic effects indicated by significantly increased enzyme leakage rates, lipid peroxidation and a significantly decreased cell function (P < 0.05), in ITS during hypoxia, and subsequent reoxygenation. Moreover, we found that toxicity of both Fe(2+) and Fe(3+) ions increased with rising concentration. However, Fe(2+) showed a higher tissue toxicity than Fe(3+). DFO reduced significantly the reoxygenation damage of free Hb and iron ions. Our data clearly demonstrate a pronounced cytotoxic effect of free Hb in ITS, which critically depended on the reduction state of the iron ions.

Activation of biliverdin-IXalpha reductase by inorganic phosphate and related anions

The effect of pH on the initial-rate kinetic behaviour of BVR-A (biliverdin-IXalpha reductase) exhibits an alkaline optimum with NADPH as cofactor, but a neutral optimum with NADH as cofactor. This has been described as dual cofactor and dual pH dependent behaviour; however, no mechanism has been described to explain this phenomenon. We present evidence that the apparent peak of activity observed at neutral pH with phosphate buffer and NADH as cofactor is an anion-dependent activation, where inorganic phosphate apparently mimics the role played by the 2'-phosphate of NADPH in stabilizing the interaction between NADH and the enzyme. The enzymes from mouse, rat and human all exhibit this behaviour. This behaviour is not seen with BVR-A from Xenopus tropicalis or the ancient cyanobacterial enzyme from Synechocystis PCC 6803, which, in addition to being refractory to activation by inorganic phosphate, are also differentiated by an acid pH optimum with both nicotinamide nucleotides.

Beneficial effects of carbon monoxide-releasing molecules on post-ischemic myocardial recovery

There is increasing evidence corroborating a protective role of carbon monoxide releasing molecules (CORMs) in injured tissues. Carbon monoxide (CO) carriers have been recently developed as a pharmacological tool to simulate the effect of heme oxygenase-1-derived CO. The effects of CORM-3, a water-soluble CO releaser, on the incidence of reperfusion-induced ventricular fibrillation (VF) and tachycardia (VT) were studied in isolated rat hearts. Hearts were treated with different doses of CORM-3 before the induction of 30 min global ischemia followed by 120 min reperfusion. We found that at concentrations of 25 microM and 50 microM of CORM-3 promoted a significant reduction in the incidence of VF and VT. Thus, the incidence of VF was reduced by 67% (p<0.05) and 92% (p<0.05) with 25 microM and 50 microM of CORM-3, respectively. The protective effect of CORM-3 on the incidence of VT followed the same pattern. The antiarrhythmic protection was associated with a marked attenuation in infarct size, significant decreases in cellular Na(+) and Ca(2+) gains and K(+) loss. Consequently, the recovery of post-ischemic function was significantly improved. In conclusion, CORM-3 exerts beneficial effects against ischemia/reperfusion-induced injury through its abilities to release CO which mediates a cardioprotective action by regulating tissue Na(+), K(+), and Ca(2+) levels.

CO–metal interaction: vital signaling from a lethal gas

The past few years have witnessed intense research into the biological significance of carbon monoxide (CO) as an essential signaling mediator in cells and tissues. To transduce the signal properly, CO must react selectively with functional and structural proteins containing moieties that show preferred reactivity towards this gaseous molecule. This selectivity is exemplified by the interaction of CO with iron- and heme-dependent proteins, although systems containing other transition metals can potentially become a preferential target for CO. Notably, transition metal carbonyls, which carry and liberate CO, are also emerging as a pharmacological tool to mimic the bioactivity of endogenously generated CO. Thus, exploring how CO binding to metal complexes is translated into a cytoprotective function is a challenging task and might open up opportunities for therapeutic applications based on CO delivery.

Positive inotropic effects of carbon monoxide-releasing molecules (CO-RMs) in the isolated perfused rat heart

BACKGROUND AND PURPOSE

Carbon monoxide (CO) generated by the enzyme haeme oxygenase-1 (HO-1) during the breakdown of haeme is known to mediate a number of biological effects. Here, we investigated whether CO liberated from two water soluble carbon monoxide-releasing molecules (CO-RMs) exerts inotropic effects on the myocardium.

EXPERIMENTAL APPROACH

Rat isolated hearts perfused either at constant flow or constant pressure were used to test the effect of CO-RMs.

KEY RESULTS

CORM-3, a fast CO releaser, produced a direct positive inotropic effect when cumulative doses (3, 10 and 30 microg min(-1)) or a single dose (5 microM) were infused at either constant coronary pressure (CCP) or constant coronary flow (CCF). The inotropic effect mediated by CORM-3 was abolished by blockade of soluble guanylate cyclase or Na(+)/H(+) exchanger, but not by inhibitors of L-type Ca(2+) channels and protein kinase C. CORM-3 also caused a slight reduction in heart rate but did not alter coronary flow. In contrast, the slow CO releaser CORM-A1 produced significant coronary vasodilatation when given at the highest concentration (30 mug min(-1)) but exerted no effect on myocardial contractility or heart rate.

CONCLUSION AND IMPLICATIONS

A rapid CO release from CORM-3 exerts a direct positive inotropic effect on rat isolated perfused hearts, whereas CO slowly released by CORM-A1 had no effect on myocardial contractility but caused significant coronary vasodilatation. Both cGMP and Na(+)/H(+) exchange appear to be involved in this effect but further work is needed to determine the relative contribution of each pathway in CO-mediated inotropic effect.

Modulation of thrombin-induced neuroinflammation in BV-2 microglia by carbon monoxide-releasing molecule 3

Carbon monoxide-releasing molecules are emerging as a new class of pharmacological agents that regulate important cellular function by liberating CO in biological systems. Here, we examined the role of carbon monoxide-releasing molecule 3 (CORM-3) in modulating neuroinflammatory responses in BV-2 microglial cells, considering its practical application as a novel therapeutic alternative in the treatment of stroke. BV-2 microglia cells were incubated for 24 h in normoxic conditions with thrombin alone or in combination with interferon-gamma to simulate the inflammatory response. Cells were also subjected to 12 h of hypoxia and reoxygenated for 24 h in the presence of thrombin and interferon-gamma. In both set of experiments, the anti-inflammatory action of CORM-3 was evaluated by assessing its effect on nitric oxide production (nitrite levels) and tumor necrosis factor (TNF)-alpha release. CORM-3 (75 microM) did not show any cytotoxicity and markedly attenuated the inflammatory response to thrombin and interferon-gamma in normoxia and to a lesser extent in hypoxia as evidenced by a reduction in nitrite levels and TNF-alpha production. Inactive CORM-3, which does not liberate CO and is used as a negative control, failed to prevent the increase in inflammatory mediators. Blockade of endogenous CO production by tin protoporphyrin-IX did not change the anti-inflammatory activity of CORM-3, suggesting that CO liberated from the compound is responsible for the observed effects. In addition, inhibition of the mitogen-activated protein kinases phosphatidyl inositol 3 kinase and extracellular signal-regulated kinase amplified the anti-inflammatory effect of CORM-3. These results suggest that the anti-inflammatory activity of CORM-3 could be exploited to mitigate microglia activity in stroke and other neuroinflammatory diseases.

CO as a cellular signaling molecule

Many biological functions of heme oxygenase (HO), such as cytoprotection against oxidative stress, vasodilation, neurotransmission in the central or peripheral nervous systems, and anti-inflammatory, anti-apoptotic, or anti-proliferative potential, have been attributed to its enzymatic byproduct carbon monoxide (CO), although roles for biliverdin/bilirubin and iron have also been proposed. In addition to these well-characterized effects, recent findings reveal that HO-derived CO may act as an oxygen sensor and circadian modulator of heme biosynthesis. In lymphocytes, CO may participate in regulatory T cell function. A number of the known signaling effects of CO depend on stimulation of soluble guanylate cyclase and/or activation of mitogen-activated protein kinases (MAPK). Furthermore, modulation of caveolin-1 status may serve as an essential component of certain aspects of CO action, such as growth control. In this review, we summarize recent findings of the beneficial or detrimental effects of endogenous CO with an emphasis on the signaling pathways and downstream targets that trigger the action of this gas.

Heme oxygenase-1 mediates the anti-inflammatory actions of 2′-hydroxychalcone in RAW 264.7 murine macrophages

Chalcones are a group of plant-derived polyphenolic compounds that belong to the flavonoids family, and possess a wide variety of cytoprotective and modulatory functions. Chalcones exert their cytoprotective actions via activation of specific transcriptional factors and upregulation of endogenous defensive pathways, such as phase II enzymes and the stress protein heme oxygenase-1 (HO-1). In this study, we investigated the anti-inflammatory action of 2′-hydroxychalcone (2-HC) in a model of lipopolysaccharide (LPS)-induced inflammation in RAW 264.7 macrophages and examined the role of HO-1 in this process. Our results demonstrate that 2-HC potently induces HO-1 expression and markedly reduces LPS-mediated nitrite and TNF-α production. These effects are accompanied by inhibition of inducible nitric oxide synthase protein expression and abolished by blockade of heme oxygenase activity with either tin protoporphyrin IX or HO-1 small interfering RNA. By using a pharmacological approach and siRNA technology, we also found that phosphatidylinositol 3-kinase is a major cellular mediator in 2-HC-induced HO-1 expression. These findings strongly suggest that 2-HC exerts anti-inflammatory actions via activation of the HO-1 pathway and help to elucidate the mechanisms underlying the potential therapeutic value of chalcones.

Therapeutic applications of carbon monoxide-releasing molecules

Carbon monoxide (CO), which is formed in mammalian cells through the oxidation of haem by the enzyme haem oxygenase, actively participates in the regulation of key intracellular functions. Emerging evidence reveals that an increased generation of haem oxygenase-derived CO plays a critical role in the resolution of inflammatory processes and alleviation of cardiovascular disorders. The authors have identified a novel class of substances, CO-releasing molecules (CO-RMs), which are capable of exerting a variety of pharmacological activities via the liberation of controlled amounts of CO in biological systems. A wide range of CO carriers containing manganese (CORM-1), ruthenium (CORM-2 and -3), boron (CORM-A1) and iron (CORM-F3) are currently being investigated to tailor therapeutic approaches for the prevention of vascular dysfunction, inflammation, tissue ischaemia and organ rejection.

Fatty acid transduction of nitric oxide signaling: nitrolinoleic acid potently activates endothelial heme oxygenase 1 expression

Nitroalkenes are a class of cell signaling mediators generated by NO and fatty acid-dependent redox reactions. Nitrated fatty acids such as 10- and 12-nitro-9,12-octadecadienoic acid (nitrolinoleic acid, LNO(2)) exhibit pluripotent antiinflammatory cell signaling properties. Heme oxygenase 1 (HO-1) is up-regulated as an adaptive response to inflammatory mediators and oxidative stress. LNO(2) (1-10 microM) induced HO-1 mRNA and protein up to 70- and 15-fold, respectively, in human aortic endothelial cells. This induction of HO-1 occurred within clinical LNO(2) concentration ranges, far exceeded responses to equimolar amounts of linoleic acid and oxidized linoleic acid, and rivaled that induced by hemin. Ex vivo incubation of rat aortic segments with 25 microM LNO(2) resulted in a 40-fold induction of HO-1 protein that localized to endothelial and smooth muscle cells. Actinomycin D inhibited LNO(2) induction of HO-1 in human aortic endothelial cells, and LNO(2) activated a 4.5-kb human HO-1 promoter construct, indicating transcriptional regulation of the HO-1 gene. The peroxisome proliferator-activated receptor gamma (PPARgamma) receptor antagonist GW9662 did not inhibit LNO(2)-mediated HO-1 induction, and a methyl ester derivative of LNO(2) with diminished PPARgamma binding capability also induced HO-1, affirming a PPARgamma-independent mechanism. The NO scavengers 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and oxymyoglobin partially reversed induction of HO-1 by LNO(2), revealing that LNO(2) regulates HO-1 expression by predominantly NO-independent mechanisms. In summary, the metabolic and inflammatory signaling actions of nitroalkenes can be transduced by robust HO-1 induction.

The Interaction of Nitric Oxide with Distinct Hemoglobins Differentially Amplifies Endothelial Heme Uptake and Heme Oxygenase-1 Expression

Heme is a strong inducer and substrate of the stress protein heme oxygenase-1 (HO-1), which produces carbon monoxide, iron, and bilirubin. We have reported recently that nitric oxide (NO) augments the incorporation of free hemin in endothelial cells, resulting in amplified HO-1 expression and production of bilirubin. Here, we extend our studies by showing that both NO+ and NO- donors interacted with reduced (HbA0) or oxidized (metHb) hemoglobin, as well as hemoglobin from sickle cell disease (HbS), to strongly magnify HO-1, with a pattern of induction dependent on the oxidation state of the hemoglobin used. A corresponding enhancement of endothelial heme uptake was observed following exposure of HbA0 or HbS to the NO donors, which also increased the uptake of free hemin. We postulated that this effect may be caused by formation of heme-nitrosyl (H-NO) complexes, and indeed endothelial cells exposed to preformed H-NO showed greater heme incorporation than free hemin. Furthermore, NO donors directly affected the permeability of membranes to free hemin. In conclusion, our data indicate a novel role for NO in the modulation of heme transport and HO-1 induction in endothelial cells, which may be relevant for hematological disorders characterized by disruption of the heme-NO equilibrium.

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.

Reactive oxygen species and related haem pathway components as possible epigenetic modifiers in neurobehavioural pathology

The neuroendocrine response to stress utilizes several bio-communicative pathways which also play a role in neurodevelopmental plasticity. The mechanism of action of steroidal compounds includes DNA alteration by reactive oxygen species (ROS) arising through redox cycling of reactive hormone derivatives. ROS and reactive nitrogen species play a significant role in signaling networks affecting gene transcriptional regulation during normal as well as stress-induced responses. ROS-associated synaptic and regulatory region plasticity may have been important for normal brain evolution, but probably simultaneously lowered the threshold for inducing neuropathology. A shift from 'plasticity' to 'instability' is likely to be associated with the emergence of complex effects depending on the timing, duration and intensity of the ROS insult, and is suggested to include heritable epigenetic chromatin/regulatory region remodeling differentially influencing expression levels of significant neuropsychiatric genes and their variant alleles. Neurobehavioural disorder clinical manifestations have been linked with ROS effects. The concepts discussed here relate to ROS-associated instability of DNA regulatory region sequences and a proposal that it may play an important modifying role in brain and neuro-behaviourally related gene expression. Genes encoding key steps in mitochondrial, haem, iron and bilirubin ROS metabolic pathways have been used as examples to illustrate how ROS-modified regulatory networks could possibly alter the context within which (even ostensibly unrelated) neuropsychiatric gene candidates may sometimes be recruited. Furthermore, reactions of certain radicals release sufficient energy to generate UV-photons. DNA conformational changes accompanied by changes in photon emission suggest that functional neuroimaging findings probably reflect interaction on the level of ROS/biophoton/genome regulatory region domains rather than the signatures of individual neurobehavioural disorder candidate genes.

Protection against cisplatin-induced nephrotoxicity by a carbon monoxide-releasing molecule

Nephrotoxicity is one of the main side effects caused by cisplatin (CP), a widely used antineoplastic agent. Here, we examined the effect of a novel water-soluble carbon monoxide-releasing molecule (CORM-3) on CP-mediated cytotoxicity in renal epithelial cells and explored the potential therapeutic benefits of carbon monoxide in CP-induced nephrotoxicity in vivo. Exposure of LLC-PK(1) cells to CP (50 microM) caused significant apoptosis as evidenced by caspase-3 activation and an increased number of floating cells. Treatment with CORM-3 (1-50 microM) resulted in a remarkable and concentration-dependent decrease in CP-induced caspase-3 activity and cell detachment. This effect involved activation of the cGMP pathway as 1H-oxadiazole [4, 3-a] quinoxaline-1-ore (ODQ), a guanylate cyclase inhibitor, completely abolished the protection elicited by CORM-3. Using a rat model of CP-induced renal failure, we found that treatment with CP (7.5 mg/kg) caused a significant elevation in plasma urea (6.6-fold) and creatinine (3.1-fold) levels, which was accompanied by severe morphological changes and marked apoptosis in tubules at the corticomedullary junction. A daily administration of CORM-3 (10 mg/kg ip), starting 1 day before CP treatment and continuing for 3 days thereafter, resulted in amelioration of renal function as shown by reduction of urea and creatinine levels to basal values, a decreased number of apoptotic tubular cells, and an improved histological profile. A negative control (iCORM-3) that is incapable of liberating CO failed to prevent renal dysfunction mediated by CP, indicating that CO is directly involved in renoprotection. Our data demonstrate that CORM-3 can be used as an effective therapeutic adjuvant in the treatment of CP-induced nephrotoxicity.