Heme and the endothelium. Effects of nitric oxide on catalytic iron and heme degradation by heme oxygenase

Deletion of cox7c Results in Pan-Azole Resistance in Aspergillus fumigatus

In Aspergillus fumigatus, the most prevalent resistance to azoles results from mutational modifications of the azole target protein Cyp51A, but there are non-cyp51A mutants resistant to azoles, and the mechanisms underlying the resistance of these strains remain to be explored. Here, we identified a novel cytochrome c oxidase, cox7c (W56*), nonsense mutation in the laboratory and found that it caused reduced colony growth and resistance to multiantifungal agents. Meanwhile, we revealed that cold storage is responsible for increased tolerance of conidia to itraconazole (ITC) stress, which further advances azole-resistant mutations (cryopreservation→ITC tolerance→azole resistance). The deletion or mutation of cox7c results explicitly in resistance to antifungal-targeting enzymes, including triazoles, polyenes, and allylamines, required for ergosterol synthesis, or resistance to fungal ergosterol. A high-performance liquid chromatography (HPLC) assay showed that the cox7c knockout strain decreased intracellular itraconazole concentration. In addition, the lack of Cox7c resulted in the accumulation of intracellular heme B. We validated that an endogenous increase in, or the exogenous addition of, heme B was capable of eliciting azole resistance, which was in good accordance with the phenotypic resistance analysis of cox7c mutants. Furthermore, RNA sequencing verified the elevated transcriptional expression levels of multidrug transport genes. Additionally, lower itraconazole-induced reactive oxygen species generation in mycelia of a cox7c-deletion strain suggested that this reduction may, in part, contribute to drug resistance. These findings increase our understanding of how A. fumigatus’s direct responses to azoles promote fungal survival in the environment and address genetic mutations that arise from patients or environments.

Expressed Soybean Leghemoglobin: Effect on Escherichia coli at Oxidative and Nitrosative Stress

Leghemoglobin (Lb) is an oxygen-binding plant hemoglobin of legume nodules, which participates in the symbiotic nitrogen fixation process. Another way to obtain Lb is its expression in bacteria, yeasts, or other organisms. This is promising for both obtaining Lb in the necessary quantity and scrutinizing it in model systems, e.g., its interaction with reactive oxygen (ROS) and nitrogen (RNS) species. The main goal of the work was to study how Lb expression affected the ability of Escherichia coli cells to tolerate oxidative and nitrosative stress. The bacterium E. coli with the embedded gene of soybean leghemoglobin a contains this protein in an active oxygenated state. The interaction of the expressed Lb with oxidative and nitrosative stress inducers (nitrosoglutathione, tert-butyl hydroperoxide, and benzylviologen) was studied by enzymatic methods and spectrophotometry. Lb formed NO complexes with heme-nitrosylLb or nonheme iron-dinitrosyl iron complexes (DNICs). The formation of Lb-bound DNICs was also detected by low-temperature electron paramagnetic resonance spectroscopy. Lb displayed peroxidase activity and catalyzed the reduction of organic peroxides. Despite this, E. coli-synthesized Lb were more sensitive to stress inducers. This might be due to the energy demand required by the Lb synthesis, as an alien protein consumes bacterial resources and thereby decreases adaptive potential of E. coli.

Nitric oxide and heme-NO stimulate superoxide production by NADPH oxidase 5

Nitric oxide (NO) is a ubiquitous cell signaling molecule which mediates widespread and diverse processes in the cell. These NO dependent effects often involve activation (e.g. NO binding to the heme group of soluble guanylyl cyclase for cGMP production) or inactivation (e.g. S-nitrosation) of protein targets. We studied the effect of NO and heme-NO on the transmembrane signaling enzyme NADPH oxidase 5 (NOX5), a heme protein which produces superoxide in response to increases in intracellular calcium. We found that treatment with NO donors increases NOX5 activity through heme-dependent effects, and that this effect could be recapitulated by the addition of heme-NO. This work adds to our understanding of NOX5 regulation in the cell but also provides a framework for understanding how NO could cause widespread changes in hemeprotein activity based on different affinities for heme v. heme-NO, and helps explain the opposing roles NO plays in activation and inactivation of hemeprotein targets.

New Insights into the Chemical Reactivity of Dry-Cured Fermented Sausages: Focus on Nitrosation, Nitrosylation and Oxidation

Nitrite and nitrate are added to cured meat for their bacteriological, technological and sensorial properties. However, they are suspected to be involved in the formation of nitroso compounds (NOCs), such as potentially mutagenic nitrosamines, nitrosylheme and nitrosothiols. Controlling the sanitary and sensorial qualities of cured meat products by reducing these additives requires elucidating the mechanisms involved in the formation of NOCs. To this end, we studied the dose-response relationship of added sodium nitrite and/or sodium nitrate (0/0, 80/80, 0/200, and 120/120 ppm) on the formation of NOCs in dry cured fermented sausages. The results showed a basal heme iron nitrosylation in the absence of NaNO2/NaNO3 due to starter cultures. This reaction was promoted by the addition of NaNO2/NaNO3 in the other conditions. Reducing the dose to 80/80 ppm still limits lipid oxidation without the formation of non-volatile nitrosamines. Conversely, the addition of NO2/NO3 slightly increases protein oxidation through higher carbonyl content. The use of 80/80 ppm could be a means of reducing these additives in dry-cured fermented meat products.

Chemical reactivity of nitrite and ascorbate in a cured and cooked meat model implication in nitrosation, nitrosylation and oxidation

Nitrite, added to cured meat for its bacteriological and technological properties, is implicated in the formation of nitroso compounds (NOCs), such as nitrosylheme, nitrosamines and nitrosothiols, suspected to have a potential impact on human health. The mechanisms involved in NOC formation are studied in regard with the dose-response relationship of added nitrite and its interaction with ascorbate on NOC formation in a cured and cooked meat model. The impact of a second cooking stage on nitrosation was evaluated. The addition of nitrite in the cured and cooked model promoted heme iron nitrosylation and S-nitrosation but not N-nitrosation. Nitrite reduced lipid oxidation without an additional ascorbate effect. The second cooking sharply increased the nitrosamine content while the presence of ascorbate considerably lowered their levels and protected nitrosothiols from degradation. This study gives new insights on the chemical reactivity of NOCs in a cured meat model.

Non-canonical chemical feedback self-limits nitric oxide-cyclic GMP signaling in health and disease

Nitric oxide (NO)-cyclic GMP (cGMP) signaling is a vasoprotective pathway therapeutically targeted, for example, in pulmonary hypertension. Its dysregulation in disease is incompletely understood. Here we show in pulmonary artery endothelial cells that feedback inhibition by NO of the NO receptor, the cGMP forming soluble guanylate cyclase (sGC), may contribute to this. Both endogenous NO from endothelial NO synthase and exogenous NO from NO donor compounds decreased sGC protein and activity. This effect was not mediated by cGMP as the NO-independent sGC stimulator, or direct activation of cGMP-dependent protein kinase did not mimic it. Thiol-sensitive mechanisms were also not involved as the thiol-reducing agent N-acetyl-L-cysteine did not prevent this feedback. Instead, both in-vitro and in-vivo and in health and acute respiratory lung disease, chronically elevated NO led to the inactivation and degradation of sGC while leaving the heme-free isoform, apo-sGC, intact or even increasing its levels. Thus, NO regulates sGC in a bimodal manner, acutely stimulating and chronically inhibiting, as part of self-limiting direct feedback that is cGMP independent. In high NO disease conditions, this is aggravated but can be functionally recovered in a mechanism-based manner by apo-sGC activators that re-establish cGMP formation.

The Role of Hydrogen Peroxide in Redox-Dependent Signaling: Homeostatic and Pathological Responses in Mammalian Cells

Hydrogen peroxide (H₂O₂) is an important metabolite involved in most of the redox metabolism reactions and processes of the cells. H₂O₂ is recognized as one of the main molecules in the sensing, modulation and signaling of redox metabolism, and it is acting as a second messenger together with hydrogen sulfide (H₂S) and nitric oxide (NO). These second messengers activate in turn a cascade of downstream proteins via specific oxidations leading to a metabolic response of the cell. This metabolic response can determine proliferation, survival or death of the cell depending on which downstream pathways (homeostatic, pathological, or protective) have been activated. The cells have several sources of H₂O₂ and cellular systems strictly control its concentration in different subcellular compartments. This review summarizes research on the role played by H₂O₂ in signaling pathways of eukaryotic cells and how this signaling leads to homeostatic or pathological responses.

Ethnopharmacological Approaches for Therapy of Jaundice: Part I

Jaundice is a very common symptom especially in the developing countries. It is associated with several hepatic diseases which are still major causes of death. There are many different approaches to jaundice treatment and the growing number of ethnomedicinal studies shows the plant pharmacology as very promising direction. Many medicinal plants are used for the treatment of jaundice, however a comprehensive review on this subject has not been published. The use of medicinal plants in drug discovery is highly emphasized (based on their traditional and safe uses in different folk medicine systems from ancient times). Many sophisticated analytical techniques are emerging in the pharmaceutical field to validate and discover new biologically active chemical entities derived from plants. Here, we aim to classify and categorize medicinal plants relevant for the treatment of jaundice according to their origin, geographical location, and usage. Our search included various databases like Pubmed, ScienceDirect, Google Scholar. Keywords and phrases used for these searches included: "jaundice," "hyperbilirubinemia," "serum glutamate," "bilirubin," "Ayurveda." The first part of the review focuses on the variety of medicinal plant used for the treatment of jaundice (a total of 207 medicinal plants). In the second part, possible mechanisms of action of biologically active secondary metabolites of plants from five families for jaundice treatment are discussed.

Ethnopharmacological Approaches for Therapy of Jaundice: Part I

Jaundice is a very common symptom especially in the developing countries. It is associated with several hepatic diseases which are still major causes of death. There are many different approaches to jaundice treatment and the growing number of ethnomedicinal studies shows the plant pharmacology as very promising direction. Many medicinal plants are used for the treatment of jaundice, however a comprehensive review on this subject has not been published. The use of medicinal plants in drug discovery is highly emphasized (based on their traditional and safe uses in different folk medicine systems from ancient times). Many sophisticated analytical techniques are emerging in the pharmaceutical field to validate and discover new biologically active chemical entities derived from plants. Here, we aim to classify and categorize medicinal plants relevant for the treatment of jaundice according to their origin, geographical location, and usage. Our search included various databases like Pubmed, ScienceDirect, Google Scholar. Keywords and phrases used for these searches included: “jaundice,” “hyperbilirubinemia,” “serum glutamate,” “bilirubin,” “Ayurveda.” The first part of the review focuses on the variety of medicinal plant used for the treatment of jaundice (a total of 207 medicinal plants). In the second part, possible mechanisms of action of biologically active secondary metabolites of plants from five families for jaundice treatment are discussed.

Mechanisms of nitric oxide crosstalk with reactive oxygen species scavenging enzymes during abiotic stress tolerance in plants

Nitric oxide (NO) acts in a concentration and redox-dependent manner to counteract oxidative stress either by directly acting as an antioxidant through scavenging reactive oxygen species (ROS), such as superoxide anions (O(2)(-)*), to form peroxynitrite (ONOO(-)) or by acting as a signaling molecule, thereby altering gene expression. NO can interact with different metal centres in proteins, such as heme-iron, zinc-sulfur clusters, iron-sulfur clusters, and copper, resulting in the formation of a stable metal-nitrosyl complex or production of varied biochemical signals, which ultimately leads to modification of protein structure/function. The thiols (ferrous iron-thiol complex and nitrosothiols) are also involved in the metabolism and mobilization of NO. Thiols bind to NO and transport it to the site of action whereas nitrosothiols release NO after intercellular diffusion and uptake into the target cells. S-nitrosoglutathione (GSNO) also has the ability to transnitrosylate proteins. It is an NO˙ reservoir and a long-distance signaling molecule. Tyrosine nitration of proteins has been suggested as a biomarker of nitrosative stress as it can lead to either activation or inhibition of target proteins. The exact molecular mechanism(s) by which exogenous and endogenously generated NO (or reactive nitrogen species) modulate the induction of various genes affecting redox homeostasis, are being extensively investigated currently by various research groups. Present review provides an in-depth analysis of the mechanisms by which NO interacts with and modulates the activity of various ROS scavenging enzymes, particularly accompanying ROS generation in plants in response to varied abiotic stress.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Astrocyte-neuron interactions in neurological disorders

Astrocytes have long been considered as just providing trophic support for neurons in the central nervous system, but recently several studies have highlighted their importance in many functions such as neurotransmission, metabolite and electrolyte homeostasis, cell signaling, inflammation, and synapse modulation. Astrocytes are, in fact, part of a bidirectional crosstalk with neurons. Moreover, increasing evidence is stressing the emerging role of astrocyte dysfunction in the pathophysiology of neurological disorders, including neurodegenerative disease, stroke, epilepsy, migraine, and neuroinflammatory diseases.

Mitigation of peroxynitrite-mediated nitric oxide (NO) toxicity as a mechanism of induced adaptive NO resistance in the CNS

During CNS injury and diseases, nitric oxide (NO) is released at a high flux rate leading to formation of peroxynitrite (ONOO(*)) and other reactive nitrogenous species, which nitrate tyrosines of proteins to form 3-nitrotyrosine (3NY), leading to cell death. Previously, we have found that motor neurons exposed to low levels of NO become resistant to subsequent cytotoxic NO challenge; an effect dubbed induced adaptive resistance (IAR). Here, we report IAR mitigates, not only cell death, but 3NY formation in response to cytotoxic NO. Addition of an NO scavenger before NO challenge duplicates IAR, implicating reactive nitrogenous species in cell death. Addition of uric acid (a peroxynitrite scavenger) before cytotoxic NO challenge, duplicates IAR, implicating peroxynitrite, with subsequent 3NY formation, in cell death, and abrogation of this pathway as a mechanism of IAR. IAR is dependent on the heme-metabolizing enzyme, heme oxygenase-1 (HO1), as indicated by the elimination of IAR by a specific HO1 inhibitor, and by the finding that neurons isolated from HO1 null mice have increased NO sensitivity with concomitant increased 3NY formation. This data indicate that IAR is an HO1-dependent mechanism that prevents peroxynitrite-mediated NO toxicity in motor neurons, thereby elucidating therapeutic targets for the mitigation of CNS disease and injury.

Differential sensitivity of oligodendrocytes and motor neurons to reactive nitrogen species: implications for multiple sclerosis

Depending on its concentration, nitric oxide (NO) has beneficial or toxic effects. In pathological conditions, NO reacts with superoxide to form peroxynitrite, which nitrates proteins forming nitrotyrosine residues (3NY), leading to loss of protein function, perturbation of signal transduction, and cell death. 3NY immunoreactivity is present in many CNS diseases, particularly multiple sclerosis. Here, using the high flux NO donor, spermine-NONOate, we report that oligodendrocytes are resistant to NO, while motor neurons are NO sensitive. Motor neuron sensitivity correlates with the NO-dependent formation of 3NY, which is significantly more pronounced in motor neurons when compared with oligodendrocytes, suggesting peroxynitrite as the toxic molecule. The heme-metabolizing enzyme, heme-oxygenase-1 (HO1), is necessary for oligodendrocyte NO resistance, as demonstrated by loss of resistance after HO1 inhibition. Resistance is reinstated by peroxynitrite scavenging with uric acid further implicating peroxynitrite as responsible for NO sensitivity. Most importantly, differential sensitivity to NO is also present in cultures of primary oligodendrocytes and motor neurons. Finally, motor neurons cocultured with oligodendrocytes, or oligodendrocyte-conditioned media, become resistant to NO toxicity. Preliminary studies suggest oligodendrocytes release a soluble factor that protects motor neurons. Our findings challenge the current paradigm that oligodendrocytes are the exclusive target of multiple sclerosis pathology.

Dinitrosyl iron complexes bind with hemoglobin as markers of oxidative stress

Prooxidant and antioxidant properties of nitric oxide (NO) during oxidative stress are mostly dependent on its interaction with reactive oxygen species, Fe ions, and hemoproteins. One form of NO storage and transportation in cells and tissues is dinitrosyl iron complexes (DNIC), which can bind with both low-molecular-weight thiols and proteins, including hemoglobin. It was shown that dinitrosyl iron complexes bound with hemoglobin (Hb-DNIC) were formed in rabbit erythrocytes after bringing low-molecular-weight DNIC with thiosulfate into blood. It was ascertained that Hb-DNIC intercepted free radicals reacting with hemoglobin SH-groups and prevented oxidative modification of this protein caused by hydrogen peroxide. Destruction of Hb-DNIC can take place in the presence of both hydrogen peroxide and tert-butyl hydroperoxide. Hb-DNIC can also be destroyed at the enzymatic generation of superoxide-anion radical in the xanthine-xanthine oxidase system. If aeration in this system was absent, formation of the nitrosyl R-form of hemoglobin could be seen during the process of Hb-DNIC destruction. Study of Hb-DNIC interaction with reactive oxygen metabolites is important for understanding NO and Hb roles in pathological processes that could result from oxidative stress.

NO* chemistry: a diversity of targets in the cell

NO(*) alone is a poorly reactive species; however, it is able to undergo secondary reactions to form highly oxidizing and nitrating species, NO(2)(*), N(2)O(3), and ONOO(-). These secondary reactive nitrogen species (RNS) are capable of modifying a diversity of biomolecular structures in the cell. The chemical properties of individual RNS will be discussed, along with their ability to react with amino acids, metal cofactors, lipids, cholesterol, and DNA bases and sugars. Many of the identified RNS-induced modifications have been observed both in vitro and in vivo. Several of these chemical modifications have been attributed with a functional role in the cell, such as the modulation of enzyme activity. Other areas in the field will be discussed, including the ability of RNS to react with metabolites, RNA, and substrates in the mitochondrion, and the cellular removal/repair of RNS-modified structures.

Hemoglobin binding to A beta and HBG2 SNP association suggest a role in Alzheimer's disease

From a normal human brain phage display library screen we identified the gamma (A)-globin chain of fetal hemoglobin (Hb F) as a protein that bound strongly to A beta1-42. We showed the oxidized form of adult Hb (metHb A) binds with greater affinity to A beta1-42 than metHb F. MetHb is more toxic than oxyhemoglobin because it loses its heme group more readily. Free Hb and heme readily damage vascular endothelial cells similar to Alzheimer's disease (AD) vascular pathology. The XmnI polymorphism (C-->T) at -158 of the gamma (G)-globin promoter region can contribute to increased Hb F expression. Using family-based association testing, we found a significant protective association of this polymorphism in the NIMH sibling dataset (n=489) in families, with at least two affected and one unaffected sibling (p=0.006), with an age of onset >50 years (p=0.010) and >65 years (p=0.013), and families not homozygous for the APOE4 allele (p=0.041). We hypothesize that Hb F may be less toxic than adult Hb in its interaction with A beta and may protect against the development of AD.

HO‐1 is located in liver mitochondria and modulates mitochondrial heme content and metabolism

This study investigated whether inducible heme oxygenase-1[corrected] (HO-1) [corrected] is targeted to mitochondria and its putative effects on oxidative metabolism in rat liver. Western blot and immune-electron microscopy in whole purified and fractionated organelles showed basal expression of HO-1 protein in both microsomes and mitochondria (inner membrane), accompanied by a parallel HO activity. Inducers of HO-1 increased HO-1 targeting to the inner mitochondrial membrane, which also contained biliverdin reductase, supporting that both enzymes are in the same compartmentalization. Induction of mitochondrial HO-1 was associated with a decrease of mitochondrial heme content and selective reduction of protein expression of cytochrome oxidase (COX) subunit I, which is coded by the mitochondrial genome and synthesized in the mitochondria depending on heme availability; these changes resulted in decreased COX spectrum and activity. Mitochondrial HO-1 induction was also associated with down-regulation of mitochondrial-targeted NO synthase expression and activity, resulting in a reduction of NO-dependent mitochondrial oxidant yield; inhibition of HO-1 activity reverted these effects. In conclusion, we demonstrated for the first time localization of HO-1 protein in mitochondria. It is surmised that mitochondrial HO-1 has important biological roles in regulating mitochondrial heme protein turnover and in protecting against conditions such as hypoxia, neurodegenerative diseases, or sepsis, in which substantially increased mitochondrial NO and oxidant production have been implicated.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Vitamin E inhibits hemolysis induced by hemin as a membrane stabilizer

Hemin is a potential cytolytic agent. To test the effect of vitamin E on hemin-mediated permeability in cell membranes, sheep erythrocytes were chosen as an appropriate model to study hemolysis induced by hemin. Hemin-induced hemolysis but did not elicit lipid peroxidation in sheep erythrocytes. Vitamin E was effective in inhibiting hemin-mediated hemolysis. Both chromanol ring and the isoprenoid side chain of tocopherols were essential for inhibition of hemin-induced hemolysis. There was a strong correlation between the inhibitory effects of tocopherols on hemin-induced erythrocyte hemolysis and their effects on fluorescence anisotropy of cell membranes. Our results suggested that, in contrast to its antioxidant activity, vitamin E inhibits hemolysis induced by hemin as a membrane stabilizing agent.

Carbon monoxide and hydrogen sulfide: gaseous messengers in cerebrovascular circulation

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

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.

Regulation and expression of heme oxygenase enzymes in aged-rat brain: age related depression in HO-1 and HO-2 expression and altered stress-response

The heme oxygenase isozymes, HO-1 and HO-2, oxidatively cleave the heme molecule to produce biliverdin and the gaseous messenger, CO. The cleavage results in the release of iron, a regulator of transferrin, ferritin, and nitric oxide (NO) synthase gene expression. Biliverdin reductase (BVR) then catalyzes the reduction of biliverdin, generating the potent intracellular antioxidant, bilirubin. We report an age-related decrease in HO-1 and HO-2 expression present in select brain regions including the hippocampus and the substantia nigra, that are involved in the high order cognitive processes of learning and memory. The age-related loss of monoxide-producing potential in select regions of the brain was not specific to the HO system but was also observed in neuronal NO-generating system. Furthermore, compared to 2-month old rats, the ability of aged brain tissue to respond to hypoxic/hyperthermia was compromised at both the protein and the transcription levels as judged by attenuated induction of HO-1 immunoreactive protein and its 1.8 Kb transcript. Neotrofin (AIT), a cognitive-enhancing and neuroprotective drug, caused a robust increase in HO-1 immunoreactive protein in select neuronal regions and increased the expression of HO-2 transcripts. The potential interplay between regulation of HO-2 gene expression and the serum levels of the adrenal steroids is discussed. We suggest the search for therapeutic agents that reverse the decline and aberrant stress response of HO enzymes may lead to effective treatment regimens for age-associated neuronal deficits.

Nitric oxide increases carbon monoxide production by piglet cerebral microvessels

Carbon monoxide (CO) and nitric oxide (NO) can be involved in the regulation of cerebral circulation. Inhibition of production of either one of these gaseous intercellular messengers inhibits newborn pig cerebral arteriolar dilation to the excitatory amino acid glutamate. Glutamate can increase NO production. Therefore, the present study tests the hypothesis that NO, which is increased by glutamate, stimulates the production of CO by cerebral microvessels. Experiments used freshly isolated cerebral microvessels from piglets that express only heme oxygenase-2 (HO-2). CO production was measured by gas chromatography-mass spectrometry. Although inhibition of nitric oxide synthase (NOS) with N(omega)-nitro-l-arginine (l-NNA) did not alter basal HO-2 catalytic activity or CO production, l-NNA blocked glutamate stimulation of HO-2 activity and CO production. Furthermore, the NO donor sodium nitroprusside mimicked the actions of glutamate on HO-2 and CO production. The action of NO appears to be via cGMP because 8-bromo-cGMP mimics and 1H-[1,2,4]oxadiazole-[4,3-a]quinoxalin-1-one (ODQ) blocks glutamate stimulation of CO production and HO-2 catalytic activity. Inhibitors of neither casein kinase nor phosphotidylinositol 3-kinase altered HO-2 catalytic activity. Conversely, inhibition of calmodulin with calmidazolium chloride blocked glutamate stimulation of CO production and reduced HO-2 catalytic activity. These data suggest that glutamate may activate NOS producing NO that leads to CO synthesis via a cGMP-dependent elevation of HO-2 catalytic activity. These results are consistent with the findings in vivo that either HO or NOS inhibition blocks cerebrovascular dilation to glutamate in piglets.

A key role for heme oxygenase-1 in nitric oxide resistance in murine motor neurons and glia

Nitric oxide is utilized at low levels for intercellular signaling, and at high levels as a cytotoxic weapon during inflammation. Cellular NO resistance can be increased by prior exposure to sublethal NO levels to induce defense gene expression (adaptive NO resistance), which has been correlated with increased expression of heme oxygenase-1 (HO1) and was blocked by a heme oxygenase inhibitor. However, the possibility remained that other activities were affected by the inhibitor. To address this question, we conducted a genetic study of the HO1 role. We show here that primary cultures of spinal motor neurons and glia from homozygous HO1-null mice are strikingly more sensitive to NO cytotoxicity than are cells expressing HO1. Following an exposure to NO, the HO1-deficient cells were much more prone to apoptosis than were HO1-expressing cells with either one or two copies of a functional HO1 gene. These results confirm the in vivo role of HO1 as a front-line defense against NO toxicity in neuronal cells.

Adaptation of the endothelium to fluid flow: in vitro analyses of gene expression and in vivo implications

Biomechanical forces generated by blood flow play an important role in the pathogenesis of vascular disease. For example, regions exposed to non-uniform shear stresses develop early atherosclerotic lesions while areas exposed to uniform shear stresses are protected. A variety of in vitro flow apparatuses have been created to apply well-characterized flow patterns to endothelial cells in an effort to dissect the cellular and molecular pathways involved in these distinct processes. Recent advances in biotechnology have permitted large-scale transcriptional profiling techniques to replace candidate gene screens and have allowed the genome-wide examination of biomechanical force-induced endothelial gene expression profiles. This review provides an overview of biomechanical force-induced modulation of endothelial phenotype. It examines the effect of sustained laminar shear stress (LSS), a type of uniform shear stress, on in vitro endothelial gene expression by synthesizing data from the early candidate gene and differential display polymerase chain reaction (PCR) approaches to the numerous, recent, high throughput functional genomic analyses. These studies demonstrate that prolonged LSS regulates the expression of only a small percentage (approximately 1-5%) of endothelial genes, and this transcriptional profile produces an endothelial phenotype that is quiescent, being protected from apoptosis, inflammation and oxidative stress. These observations provide a possible molecular mechanism for the strong correlation between patterns of blood flow and the occurrence of vascular pathologies, such as atherosclerosis, in vivo.

Peroxynitrite-mediated inactivation of heme oxygenases

Background

Endogenous nitric oxide (NO) and carbon monoxide (CO) are generated by nitric oxide synthase and heme oxygenase, respectively. Like NO, CO has been accepted as an important cellular signaling molecule in biological systems. An up-regulation in both gene and protein expression of heme oxygenase-1 (HO-1) under oxidative/nitrosative stress has been well documented, and the protective role of HO-1 and HO-2 against oxidative damage is proposed. However, data on the direct effect of reactive oxygen/nitrogen species (ROS/RNS) on HO function is incomplete. Using gas chromatography to quantify carbon monoxide (CO) formation from heme oxidation, we investigated the effects of peroxynitrite (ONOO^-) on the in vitro catalytic activity of rat spleen (HO-1) and brain (HO-2) microsomal heme oxygenases.

Results

Exposure to ONOO^- led to concentration-dependent but reversible decreases in the activity of microsomal rat spleen and brain HO activity. Spleen HO activity was 100-fold more sensitive to ONOO^--dependent inactivation compared to that of the brain, with IC₅₀ values of 0.015 ± 0.005 mM and 1.25 ± 0.25 mM respectively. Inhibition of both rat spleen and brain microsomal HO activity was also observed with tetra-nitromethane, a tyrosine nitrating agent, as well as two NO donors, S-nitrosoglutathione (GSNO) and diethylamine NONOate (DEA-NONOate). However, no additive effect was found following the application of NO donors and ONOO^- together.

Conclusion

These results indicate that ONOO^- may regulate HO-1 and HO-2 activities by mechanisms that involve different interactions with these proteins. It is suggested that while nitration of tyrosine residues and oxidation of sulfhydryl groups may be involved, consideration should be given to other facets of ONOO^- chemistry. This inhibition of HO activity offers a mechanism for cross talk between the nitric oxide synthase and HO systems.

Nitric oxide metabolism in mammalian cells: substrate and inhibitor profiles of a NADPH-cytochrome P450 oxidoreductase-coupled microsomal nitric oxide dioxygenase

Human intestinal Caco-2 cells metabolize and detoxify NO via a dioxygen- and NADPH-dependent, cyanide- and CO-sensitive pathway that yields nitrate. Enzymes catalyzing NO dioxygenation fractionate with membranes and are enriched in microsomes. Microsomal NO metabolism shows apparent KM values for NO, O2, and NADPH of 0.3, 9, and 2 microM, respectively, values similar to those determined for intact or digitonin-permeabilized cells. Similar to cellular NO metabolism, microsomal NO metabolism is superoxide-independent and sensitive to heme-enzyme inhibitors including CO, cyanide, imidazoles, quercetin, and allicin-enriched garlic extract. Selective inhibitors of several cytochrome P450s and heme oxygenase fail to inhibit the activity, indicating limited roles for a subset of microsomal heme enzymes in NO metabolism. Diphenyleneiodonium and cytochrome c(III) inhibit NO metabolism, suggesting a role for the NADPH-cytochrome P450 oxidoreductase (CYPOR). Involvement of CYPOR is demonstrated by the specific inhibition of the NO metabolic activity by inhibitory anti-CYPOR IgG. In toto, the results suggest roles for a microsomal CYPOR-coupled and heme-dependent NO dioxygenase in NO metabolism, detoxification, and signal attenuation in mammalian cells.

Nitric oxide and iron metabolism in exercised rat with L-arginine supplementation

The present study was designed to investigate whether L-arginine (Arg) supplementation in exercise affects nitric oxide (NO) synthesis in tissues and thus iron metabolism. Rats were assigned to one of four groups: EG (Exercise), SG (Sedentary), EAG (Exercise + Arg), and SAG (Sedentary + Arg). Both EG and EAG swam 2 h/day for 3 months. Both SAG and EAG received 3% Arg supplementation in their drinking water. The results showed that Arg supplementation in exercise (EAG) significantly increased nitrite and nitrate (NOx) concentration in the kidney and BMC, rather than in the liver, spleen and heart. Arg supplementation significantly increased both nonheme iron (NHI) and catalytic iron (CI) content in the kidney, to the extent that the ratio of CI/NHI or storage iron (SI)/NHI was not significantly affected, and significantly decreased NHI content and increased CI content in BMC, to the extent that SI content or SI/NHI was significantly decreased. These findings suggest that Arg supplementation in exercise, possibly through increasing NO synthesis, may change CI formation in the kidney and BMC, and affect iron storage in BMC rather than in the kidney.

Nitric Oxide Synthesis Inhibition Promotes Renal Production of Carbon Monoxide

We tested the hypothesis that the status of NO synthesis influences the renal heme-heme oxygenase system. Studies were conducted in untreated rats and rats treated with the NO synthesis inhibitor N G -nitro- l -arginine methyl ester for 2 days. Treated and untreated rats were contrasted in terms of renal expression of heme oxygenase-1 and -2, renal carbon monoxide (CO)-generating activity, and urinary CO concentration and excretion rate. Heme oxygenase-1 and -2 proteins were similarly expressed in the kidneys of untreated and treated rats. In contrast, the NADPH-dependent component of the CO-generating activity of renal homogenates incubated with heme (a measure of heme oxygenase activity) was higher ( P <0.05) in kidneys from rats treated with the NO synthesis inhibitor relative to corresponding data in untreated rats (1015±95 versus 379±111 pmol CO/mg per hour). Similarly, relative to corresponding data in untreated rats, rats treated with the NO synthesis inhibitor displayed increased ( P <0.05) urinary CO concentration (920±174 versus 2286±472 pmol/mL) and urinary CO excretion (4.7±0.4 versus 14.3±2.7 pmol/min). This study demonstrates that NO synthesis inhibition upregulates the urinary concentration and excretion rate of CO, and the HO-dependent generation of CO by renal homogenates, without affecting the expression of renal heme oxygenase isoforms. Our findings imply that endogenous NO is an inhibitory regulator of renal CO generation by HO.

The effects of nitric oxide on prostanoid production and release by human umbilical vein endothelial cells

Human umbilical vein endothelial cells (HUVEC) express and synthesize both constitutive and inducible nitric oxide synthase (NOS) and cyclo-oxygenase (COX) enzymes, and have been extensively used as an in vitro model to investigate the role of these enzymes in the patho-physiology of placenta-fetal circulation. In this study we investigated the role of NO in regulating prostanoid production and release from HUVEC. Both untreated and IL-1beta-treated HUVEC were exposed to various NOS inhibitors and NO donors in short-term (1 or 3 hours) experiments, and the effects on prostanoid production were evaluated through the measurement of prostaglandins (PG) I2, E2 and F2alpha released in the incubation medium. We found that the inhibition of inducible NOS but not endothelial NOS antagonizes the IL-1beta-induced increase in PGI2 release. However, NOS inhibitors do not modify baseline PGI2 production. Pharmacological levels of NO, obtained with various NO donors, inhibit basal and IL-1beta-stimulated PG release.

Heme and lipid peroxides in hemoglobin-modified low-density lipoprotein mediate cell survival and adaptation to oxidative stress

Low-density lipoprotein (LDL) oxidation mediated by a variety of catalysts in atherosclerotic lesions plays a crucial role in the genesis and evolution of atherosclerotic plaques. In this study we focused on oxidative properties of hemoglobin (Hb)-modified LDL because Hb is present in atherosclerotic lesions. Under low oxygen tensions Hb was previously found to modify apolipoprotein B100 with covalent binding of Hb fragments and formation of electronegative LDL particles (LDL-). Here we show that HbLDL is highly susceptible to oxidation, but is not cytotoxic to vascular cells, as was found for LDL- isolated from human plasma. HbLDL and LDL- have similar levels of oxidized lipid products and low uptake rates; however, the virtual absence of HbLDL-induced toxicity depends on a marked adaptive oxidative stress response. This was evidenced by a time- and dose-dependent induction of heme oxygenase (HO-1). Cell survival was significantly decreased in the presence of HO-1 inhibitor, tin protoporphyrin (SnPPIX). HO-1 induction by HbLDL increased resistance of cells to toxic doses of hemin or t-BuOOH. The high sensitivity to oxidation and HO-1 induction was largely dependent on lipid hydroperoxides and heme associated with HbLDL. Reduction of pre-existing lipid peroxides using ebselen delayed HbLDL kinetics and inhibited HO-1 induction. Moreover, heme inactivation or its degradation inhibited HO-1 induction and provided an additive inhibitory effect to ebselen. We conclude that Hb-catalyzed reactions may modulate vascular cell survival and oxidative stress adaptation due to the presence of peroxides and heme, thus providing a possible mechanism for the evolution of atherosclerotic and hemorrhagic lesions.

Haem and nitric oxide: synergism in the modulation of the endothelial haem oxygenase-1 pathway

NO potently up-regulates vascular haem oxygenase-1 (HO-1), an inducible defensive protein that degrades haem to CO, iron and the antioxidant bilirubin. Since several pathological states are characterized by increased NO production and liberation of haem from haem-containing proteins, we examined how NO influences HO-1 induction mediated by haemin. Aortic endothelial cells treated with S-nitroso-N-acetylpenicillamine (SNAP), sodium nitroprusside (SNP) or diethylenetriamine-NONOate (DETA/NO) and haemin exhibited higher levels of haem oxygenase activity compared with cells exposed to NO donors or haemin alone. This was accompanied by a marked increase in bilirubin production and, notably, by a strong magnification of cellular haem uptake. A role for haem metabolites in modulating HO-1 expression by NO was assessed by exposing cells to SNAP, SNP or DETA/NO in medium derived from cells treated with haemin, which contained increased bilirubin levels. This treatment considerably potentiated HO-1 expression and haem oxygenase activity mediated by NO and the use of a haem oxygenase inhibitor abolished this effect. Both iron liberated during haem breakdown and the formation of nitroxyl anion from NO appeared to partially contribute to the amplifying phenomenon; in addition, medium from haemin-treated cells significantly augmented the release of NO by NO donors. Thus we have identified novel mechanisms related to the induction of HO-1 by NO indicating that the signalling actions of NO vary significantly in the presence of haem and haem metabolites, ultimately increasing the defensive abilities of the endothelium to counteract oxidative and nitrosative stress.

Effect of carbon monoxide on dopamine and glutamate uptake and cGMP levels in rat brain

After the recognition of nitric oxide (NO) as a messenger molecule in the nervous system, carbon monoxide (CO) has received attention with similar properties. The present study aims to elucidate the effects of CO on synaptosomal dopamine ((3)H-DA) and glutamate ((3)H-Glu) uptake and on cGMP levels; possible interaction between NO and CO systems was also evaluated. Our results provide evidence for the inhibition of DA and Glu uptake by CO in a time-, dose-, and temperature-dependent manner in rat striatum and hippocampus, respectively; the inhibition observed was sexually dimorphic with more pronounced effects in females. Basal cGMP levels were higher in female rats than males in the striatum and exogenous CO increased striatal cGMP levels only in males; no effect of CO was observed in the hippocampus. In vivo nitric oxide synthase (NOS) inhibition increased DA and Glu uptake; however, CO was still effective in inhibiting uptake following NOS inhibiton. Taken together, these findings suggest a role for CO in trans-synaptic regulation through modulation of DA and Glu transporters and of cGMP levels; the effect on cGMP levels is independent of NOS activity and appears to be sexually dimorphic and region specific.

Nitric oxide synthesis influences the renal vascular response to heme oxygenase inhibition

We studied the effects of the heme oxygenase (HO) inhibitor stannous mesoporphyrin (SnMP; 40 micromol/kg i.v.) on renal hemodynamics in anesthetized rats with and without 48-h pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthesis. SnMP decreased renal blood flow (RBF) and increased renal vascular resistance (RVR) in both groups. The SnMP-induced reduction of RBF in L-NAME-pretreated rats was more prominent than in rats without pretreatment (43 +/- 7 vs. 13 +/- 3%) as was the SnMP-induced elevation of RVR (87 +/- 31 vs. 14 +/- 5%). The renal vasoconstrictor effect of SnMP is linked, in part, to amplification of prevailing neurohormonal constrictor mechanisms, since in L-NAME-pretreated rats it was prevented by concurrent administration of prazosin or losartan. However, SnMP (15 micromol/l) also elicits vasoconstriction in isolated, pressurized renal interlobular arteries and the response is more intense in vessels obtained from L-NAME-pretreated rats than from rats without pretreatment. These data indicate that the status of NO synthesis conditions the vascular response to HO inhibition in the rat kidney.

Regulation of heme oxygenase-1 by redox signals involving nitric oxide

Heme oxygenase-1 (HO-1) is an inducible stress protein the expression of which can be markedly augmented in eukaryotes by a wide range of substances that cause a transient change in the cellular redox state. The importance of this protein in physiology and disease is underlined by the versatility of HO-1 inducers and the functional role attributed to HO-1 products (carbon monoxide and bilirubin) in conditions that are associated with moderate or severe cellular stress. An intriguing aspect is the recent evidence showing that nitric oxide, a ubiquitous signaling molecule, finely modulates the activation of HO-1 expression. As the effects of oxidative stress on the regulation of the HO-1 gene have been well established and characterized, this review will focus on the biological relevance of redox signals involving nitric oxide and reactive nitrogen species that lead to up-regulation of the HO-1 pathway, with particular emphasis on vascular tissues and the cardiovascular system.

Hemopexin: structure, function, and regulation

Hemopexin (HPX) is the plasma protein with the highest binding affinity to heme among known proteins. It is mainly expressed in liver, and belongs to acute phase reactants, the synthesis of which is induced after inflammation. Heme is potentially highly toxic because of its ability to intercalate into lipid membrane and to produce hydroxyl radicals. The binding strength between heme and HPX, and the presence of a specific heme-HPX receptor able to catabolize the complex and to induce intracellular antioxidant activities, suggest that hemopexin is the major vehicle for the transportation of heme in the plasma, thus preventing heme-mediated oxidative stress and heme-bound iron loss. In this review, we discuss the experimental data that support this view and show that the most important physiological role of HPX is to act as an antioxidant after blood heme overload, rather than to participate in iron metabolism. Particular attention is also put on the structure of the protein and on its regulation during the acute phase reaction.

Pharmacological interventions for the treatment of neonatal jaundice

In the neonate, hyperbilirubinaemia is usually due to a combination of an increased bilirubin load and decreased bilirubin elimination. Despite an understanding of the enzymatic pathways leading to bilirubin production and elimination, very few pharmacological interventions to prevent hyperbilirubinaemia are utilized and the mainstay of treatment remains phototherapy. Previously studied pharmacological agents such as D-penicillamine, phenobarbital and clofibrate may yet prove useful. Recent clinical trials using metalloporphyrins to inhibit heme catabolism and bilirubin production provides a novel pharmacological intervention for the treatment of neonatal jaundice. The safety and efficacy of these therapies will need to be confirmed prior to widespread use.

Cerebral vascular endothelial heme oxygenase: expression, localization, and activation by glutamate

Endogenous carbon monoxide (CO) contributes to vasodilator responses of cerebral microvessels in newborn pigs. We investigated the expression, intracellular localization, and activity of heme oxygenase (HO), the key enzyme in CO production, in quiescent cerebral microvascular endothelial cells (CMVEC) from newborn pigs. HO-1 and HO-2 isoforms were detected by RT-PCR, immunoblotting, and immunofluorescence. HO-1 and HO-2 are membrane-bound proteins that have a strong preference for the nuclear envelope and perinuclear area of the cytoplasm. Betamethasone (10(-6) to 10(-4) M for 48 h) was associated with upregulation of HO-2 protein by approximately 50% and inhibition of Cox-2 but did not alter HO-1 or endothelial nitric oxide synthase expression in CMVEC. In vivo betamethasone treatment of newborn pigs (0.2 and 5.0 mg/kg im for 48 h) upregulated HO-2 in cerebral microvessels by 30-60%. HO activity as (14)CO production from [(14)C]glycine-labeled endogenous heme was inhibited by chromium mesoporphyrin (10(-6) to 10(-4) M). L-Glutamate (0.3-1.0 mM) stimulated HO activity 1.5-fold. High-affinity specific binding sites for L-[(3)H]glutamate suggestive of the glutamate receptors were detected in CMVEC. Altogether, these data suggest that, in cerebral circulation of newborn pigs, endothelium-derived CO may contribute to basal vascular tone and to responses that involve glutamate receptor activation.

Cytochrome P450 4A11 expression in human keratinocytes: effects of ultraviolet irradiation

BACKGROUND

The skin is the major interface between the body and its environment. Directly and continuously exposed to a large variety of foreign agents and stimuli such as ultraviolet radiation (UVR), cutaneous cells are active sites of intense metabolism. The cytochromes P450 (P450) are a group of enzymes that play an important part in the protective role of the skin; they are a family of microsomal membrane-bound mono-oxygenases. These haem-containing proteins catalyse the insertion of an atom of molecular oxygen into the substrate. Although generally present at low levels, a certain number of these enzymes have now been characterized in mammalian skin as constitutive or inducible isoforms.

OBJECTIVES

To test the effects of UVR, a source of oxidative stress, on the expression of mRNA coding for several P450 isoforms (CYP), with particular reference to the CYP2E1 and CYP4A11 isoforms, which might play a role in lipid metabolism in human keratinocytes.

METHODS

Human keratinocytes were cultured, irradiated and mRNA expression was analysed by gel electrophoresis after reverse transcriptase polymerase chain reactions. CYP proteins were determined from keratinocyte microsomal fractions by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immunoperoxidase staining. Thin layer chromatography was used to detect (omega-1)- and (omega)-hydroxylation of lauric acid in the microsomal fractions.

RESULTS

mRNAs for CYP2E1, CYP1A1 and CYP3A5 were expressed in all the keratinocyte preparations tested; however, neither CYP3A4 nor CYP3A7 were detected, either in the presence or absence of UVR treatment. CYP19Aro, CYP2C19 and CYP26 were not expressed constitutively, although some induction of CYP19Aro was seen after combined UVB and UVA irradiation. CYP4A11 mRNA was not detected in any keratinocyte preparations either under control conditions or after UVB treatment. Nevertheless, in non-irradiated keratinocyte microsomes, two protein bands were immunoreactive with anti-CYP4A11 enzyme antibodies, one of which corresponds to CYP4A11 protein. UVA treatment of cultured keratinocytes induced CYP4A11 mRNA expression after 24 h, as well as an increase in immunoreactivity of the two protein bands. Although (omega-1)- and (omega)-hydroxylation of fatty acids is attributed to CYP2E1 and CYP4A11, respectively, in the liver or kidney, no omega-hydroxylation of lauric acid was observed in microsomal preparations from cultured keratinocytes.

CONCLUSIONS

However, CYP4A11 may participate in the defence mechanism against UVA-induced oxidative damage.

Methaemoglobin enhances the proliferation of transformed human epithelial cells: a possible outcome of neovascularisation and haemorrhage in tumours?

The effect of human methaemoglobin (metHb), possibly derived from extravasated red blood cells in tumours showing neovascularisation and haemorrhage, on the growth of transformed human epithelial cells was investigated. MetHb stimulated the growth of immortalised epithelial cells or transformed cells at precrisis stage (cells have bypassed M1, but not M2, the two mortality checkpoints). The stimulatory effect was due to the release of haemin from metHb that was isolated by a Sephadex column and identified by its characteristic light absorption spectrum. Although all the degradation products of haemin are currently known to be physiologically significant, only ferric iron derived from metHb or haemin could stimulate cell growth. High concentrations of metHb or haemin inhibited cell growth possibly due to the generation of high concentrations of bilirubin. However, bilirubin formed in the cells of human body is known to be transported to the liver for further processing and excretion. Haemoglobin oxidised to where tumours show neovascularisation and haemorrhage likely contributes significantly to the increased proliferation of cancerous cells.

Induction of cyclo-oxygenase-2 in human endothelial cells by SIN-1 in the absence of prostaglandin production

Nitric oxide (NO) regulates cyclo-oxygenase (COX) activity in various cell systems and reports conflict in regard to its stimulatory versus inhibitory role. Incubation of human umbilical vein endothelial cells (HUVEC) with SIN-1 (3-morpholinosydnonimine), a donor of NO, resulted in a rapid and dose-dependent increase in the expression of COX-2 as analysed by Western and Northern blotting. Incubation of HUVEC with SIN-1 and interleukine (IL)-1alpha resulted in increased induction of COX-2 compared with IL-1alpha alone and corresponded to an additive effect. The COX-2 induction was dependent on a de novo synthesis since cycloheximide, an inhibitor of protein synthesis, blocked the enzyme expression. The increase in COX-2 expression was not accompanied by a corresponding change in prostaglandin (PG) production. However, the COX activity was partially recovered when immunoprecipitated COX-2 was incubated with arachidonic acid and haematin. Peroxynitrite, a highly reactive nitrogen molecule derived from the interaction of NO and superoxide anion, significantly increased COX-2 expression. Under these conditions and within the limit of detection of the antibody, selective antibody for nitrotyrosine failed to detect nitrated COX-2 in immunoprecipitated COX-2 when cells where incubated with SIN-1 or SIN-1+IL-1alpha. Ro 31-8220, a specific inhibitor of protein kinase (PK) C, blocked the induction of COX-2. Also, SB203580, the selective inhibitor of p38 MAP kinase, strongly blocked the induction of COX-2 by SIN-1 in the presence or absence of IL-1alpha, whereas the MEK-1 inhibitor, PD 98059, affected it to a lesser extent. These data demonstrate that SIN-1 induces COX-2 in HUVEC in the absence of PG formation and suggest a complex regulation of COX-2 expression and PG formation by NO in endothelial cells.

Carbon monoxide from heme catabolism protects against hepatobiliary dysfunction in endotoxin-treated rat liver

BACKGROUND AND AIMS

Liver is a major organ for heme detoxification under disease conditions, but its self-protective mechanisms against the toxicity are unknown. This study aimed to examine roles of carbon monoxide (CO), the gaseous product of heme oxygenase (HO), in ameliorating hepatobiliary dysfunction during catabolism of heme molecules in endotoxemic livers.

METHODS

Vascular resistance and biliary flux of bilirubin-IXalpha, an index of HO-derived CO generation, were monitored in perfused livers of endotoxemic rats. Livers were perfused with HbO(2), which captures nitric oxide (NO) and CO, or metHb, a reagent trapping NO but not CO.

RESULTS

In endotoxin-pretreated livers where inducible NO synthase and HO-1 overproduced NO and CO, HbO(2) caused marked vasoconstriction and cholestasis. These changes were not reproduced by the NO synthase inhibitor aminoguanidine alone, but by coadministration of zinc protoporphyrin-IX, an HO inhibitor. CO supplementation attenuated the events caused by aminoguanidine plus zinc protoporphyrin-IX, suggesting that simultaneous elimination of these vasorelaxing gases accounts for a mechanism for HbO(2)-induced changes. This concept was supported by observation that metHb did not cause any cholestasis; the reagent captures NO but triggers CO overproduction through rapid degradation of the heme by HO-1.

CONCLUSIONS

These results suggest protective roles of CO against hepatobiliary dysfunction caused by heme overloading under stress conditions.

Expression of heme oxygenase in human airway epithelial cells

Elevated levels of carbon monoxide (CO) are found in the exhaled breath of patients with inflammatory diseases such as asthma and cystic fibrosis. Endogenous CO is derived from heme oxygenase (HO) (EC 1.14.99.3), which catabolizes heme-producing CO and biliverdin. There are three isoforms of HO: HO-1 is inducible by inflammatory cytokines and oxidants, including nitric oxide (NO), whereas HO-2 and HO-3 are expressed constitutively. Primary airway epithelial cells were treated with either 50 ng/ml interleukin-1 beta, tumor necrosis factor-alpha, and interferon-gamma (cytomix), or the NO donor NOC-18 for up to 24 h. Cytomix-induced HO-1 expression peaked at 4 h, returning to baseline by 24 h, whereas HO-2 expression remained unchanged. This increase in HO-1 expression could not be explained by an increase in NO production as inducible NO synthase expression increased between 12 and 24 h. However, the NO donor NOC-18 (500 microM) increased HO-1 expression twofold and HO activity 25-fold, whereas cytomix treatment increased HO activity eightfold. NO induction of HO-1 was not mediated via guanylyl cyclase and was not attenuated by 1 microM dexamethasone, although dexamethasone increased HO-2 protein. Therefore, airway epithelial cells express HO-2 and can express HO-1; thus, the epithelium may be a source of increased CO in airway diseases.

Self-cytoprotection against stress: feedback regulation of heme-dependent metabolism

This minireview provides insight into feedback regulation of heme-dependent metabolism as a defensive cellular response against stress. Interactions among heme-, iron-, porphyrin-, and CO/NO-dependent metabolic pathways during the stress-induced response are emphasized in the context of feedback regulation. The hypothetical model of the latter interactions is presented as tightly controlled feedback cycles.

Mechanisms underlying induction of heme oxygenase-1 by nitric oxide in renal tubular epithelial cells

We examined whether nitric oxide-generating agents influence expression of heme oxygenase-1 (HO-1) in renal proximal tubular epithelial cells, LLC-PK(1) cells, and the mechanisms underlying any such effects. In sublytic amounts, the nitric oxide donor sodium nitroprusside induced HO-1 mRNA and protein and HO activity in a dose-dependent and time-dependent fashion; this induction was specific for nitric oxide since the nitric oxide scavenger carboxy-2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide significantly reduced such induction. The induction of HO activity by sodium nitroprusside, or by another nitric oxide donor, spermine NONOate, was markedly reduced by the iron chelator deferoxamine. Two different thiol-containing agents, N-acetylcysteine and dithiothreitol, blunted such induction of HO by nitric oxide. Downstream products of nitric oxide, such as peroxynitrite or cGMP, were not involved in inducing HO. In higher concentrations (millimolar amounts), sodium nitroprusside induced appreciable cytotoxicity as assessed by lactate dehydrogenase (LDH) release and lipid peroxidation, and both of these effects were markedly reduced by deferoxamine. Inhibition of HO did not affect the cytotoxic effects (measured by LDH release) of sodium nitroprusside. We thus provide the novel description of the induction of HO-1 in renal proximal tubular epithelial cells exposed to nitric oxide donors and provide the first demonstration in kidney-derived cells for the involvement of a redox-based mechanism in such expression. We also demonstrate that, in LLC-PK(1) cells exposed to nitric oxide donors, chelatable iron is involved in eliciting the HO-1 response observed at lower concentrations of these donors, and in mediating the cytotoxic effects of these donors when present in higher concentrations.