Characterization of a cDNA-encoding rabbit brain heme oxygenase-2 and identification of a conserved domain among mammalian heme oxygenase isozymes: possible heme-binding site?

Identification of heme oxygenase-1 from golden pompano (Trachinotus ovatus) and response of Nrf2/HO-1 signaling pathway to copper-induced oxidative stress

Heme oxygenase-1(HO-1) is a stress-inducible enzyme that mediates antioxidative and cytoprotective effects to maintain cellular redox homeostasis. In the present study, the full sequence of HO-1 was cloned from golden pompano(Trachinotus ovatus) by RT-PCR and RACE-PCR. The full cDNA sequence of HO-1 was 1349 bp in length which comprised of a 726 bp open reading frame (ORF) preceded by 262 bp 5'-untranslated region (UTR), and followed by a 360 bp 3'UTR, encoding 241 amino acid residues. Phylogenetic analysis revealed that HO-1 showed highest similarity to that of Takifugu rubripes. Tissue distribution analysis showed that the expression level of HO-1 was relatively high in heart, liver and spleen. A trial was conducted to investigate the response of Nrf2/HO-1 signaling pathway to oxidative stress induced by copper. The results showed that mRNA expression of NF-E2-related nuclear factor2 (Nrf2), Kelch-like-ECH-associated protein1 (keap1), superoxide dismutase (SOD), catalase (CAT), HO-1, NAD(P)H quinone oxidoreductase 1 (NQO1) and Glutathione peroxidase (GSH-PX) all significantly increased in copper treated group than that in the control group. This work provides new insight into the molecular mechanism underlying the Nrf2/HO-1 pathway in oxidative response in T. ovatus.

Heme oxygenase-1/carbon monoxide as modulators of autophagy and inflammation

Heme oxygenase-1 (HO-1) catalyzes heme degradation to generate biliverdin-IXα, carbon monoxide (CO), and iron. The HO-1/CO system confers cytoprotection in animal models of organ injury and disease, via modulation of inflammation and apoptosis. Recent studies have uncovered novel anti-inflammatory targets of HO-1/CO including regulation of the autophagy and inflammasome pathways. Autophagy is a lysosome-dependent program for the turnover of cellular organelles such as mitochondria, proteins, and pathogens; which may downregulate inflammatory processes. Therapeutic modulation of autophagy by CO has been demonstrated in models of sepsis. The nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3) inflammasome regulates the maturation of pro-inflammatory cytokines. CO can regulate NLRP3 inflammasome activation and associated pro-inflammatory cytokines production and promote the resolution of inflammation by upregulating the synthesis of specialized pro-resolving mediators (SPMs). Mitochondria may represent a proximal target of HO-1/CO action. HO-1 may localize to mitochondria in response to stress, while CO can moderate mitochondrial dysfunction and regulate mitochondrial autophagy (mitophagy) and biogenesis. The interplay between mitochondrial autophagy, mitochondrial dysfunction, and the regulation and resolution of inflammation may make important contributions to the protection afforded by HO-1/CO in cellular and organ injury models. Recent studies have continued to explore the potential of CO for clinical applications.

Heme Oxygenase-2 (HO-2) as a therapeutic target: Activators and inhibitors

Heme oxygenase (HO) enzymes are involved in heme catabolism and several physiological functions. Among the different HO isoforms, HO-2 stands out for its neuroprotective properties and modulatory activity in male reproduction. However, unlike the HO-1 ligands, the potential therapeutic applications of HO-2 inhibitors/activators have not been extensively explored yet. Moreover, the physiological role of HO-2 is still unclear, mostly due to the lack of highly selective HO-2 chemical probes. To boost the interest on this intriguing target, the present review updates the knowledge on the structure-activity relationships of HO-2 inhibitors and activators, as well as their potential therapeutic applications. To the best of our knowledge, among HO-2 inhibitors, clemizole derivatives are the most selective HO-2 inhibitors reported so far (IC₅₀ HO-1 >100 μM, IC₅₀ HO-2 = 3.4 μM), while the HO-2 nonselective inhibitors described herein possess IC₅₀ HO-2 values ≤ 10 μM. Furthermore, the development of HO-2 activators, such as menadione analogues, helped to understand the critical moieties required for HO-2 activation. Recent advances in the potential therapeutic applications of HO-2 inhibitors/activators cover the fields of neurodegenerative, cardiovascular, inflammatory, and reproductive diseases further stimulating the interest towards this target.

Differential inhibition of rat and mouse microsome heme oxygenase by derivatives of imidazole and benzimidazole

Metalloporphyrin heme oxygenase (HO) inhibitors have made an important contribution to elucidating the role of HO in physiological processes. Nevertheless, their off-target effects have drawn substantial criticism, which prompted us to develop non-porphyrin, azole-based inhibitors of HO. These second-generation HO inhibitors were evaluated using spleen and brain microsomes from rats as native sources of HO-1 and HO-2, respectively. Recently, the use of azole-based inhibitors of HO has been extended to other mammalian species and, as a consequence, it will be important to characterize the inhibitors in these species. The goal of this study was to compare the inhibitory profile of imidazole- and benzimidazole-based inhibitors of HO in a breast-cancer-implanted mouse to that of an untreated rat. For spleen and brain microsomes from both species, HO protein expression was determined by Western blotting and concentration-response curves for imidazole- and benzimidazole-derivative inhibition of HO activity were determined using a headspace gas-chromatographic assay. It was found that the effects on HO activity by imidazole and benzimidazole derivatives were different between the 2 species and were not explained by differences in HO expression. Thus, the HO inhibitory profile should be determined for azole derivatives before they are used in mammalian species other than rats.

Targeting heme oxygenase-1 and carbon monoxide for therapeutic modulation of inflammation

The heme oxygenase-1 (HO-1) enzyme system remains an attractive therapeutic target for the treatment of inflammatory conditions. HO-1, a cellular stress protein, serves a vital metabolic function as the rate-limiting step in the degradation of heme to generate carbon monoxide (CO), iron, and biliverdin-IXα (BV), the latter which is converted to bilirubin-IXα (BR). HO-1 may function as a pleiotropic regulator of inflammatory signaling programs through the generation of its biologically active end products, namely CO, BV and BR. CO, when applied exogenously, can affect apoptotic, proliferative, and inflammatory cellular programs. Specifically, CO can modulate the production of proinflammatory or anti-inflammatory cytokines and mediators. HO-1 and CO may also have immunomodulatory effects with respect to regulating the functions of antigen-presenting cells, dendritic cells, and regulatory T cells. Therapeutic strategies to modulate HO-1 in disease include the application of natural-inducing compounds and gene therapy approaches for the targeted genetic overexpression or knockdown of HO-1. Several compounds have been used therapeutically to inhibit HO activity, including competitive inhibitors of the metalloporphyrin series or noncompetitive isoform-selective derivatives of imidazole-dioxolanes. The end products of HO activity, CO, BV and BR may be used therapeutically as pharmacologic treatments. CO may be applied by inhalation or through the use of CO-releasing molecules. This review will discuss HO-1 as a therapeutic target in diseases involving inflammation, including lung and vascular injury, sepsis, ischemia-reperfusion injury, and transplant rejection.

Overview of heme degradation pathway

Heme oxygenase 1 and 2 activities are responsible for initiating most of the degradation of heme, although other enzyme pathways play a role as well. The degradation pathway also includes biliverdin reductase, the activity of which is coupled to oxidation of NADH and NADPH. This overview discusses the pathways and enzymes involved in heme degradation.

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.

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.

Expression and characterization of full-length human heme oxygenase-1: the presence of intact membrane-binding region leads to increased binding affinity for NADPH cytochrome P450 reductase

Heme oxygenase-1 (HO-1) is the chief regulatory enzyme in the oxidative degradation of heme to biliverdin. In the process of heme degradation, HO-1 receives the electrons necessary for catalysis from the flavoprotein NADPH cytochrome P450 reductase (CPR), releasing free iron and carbon monoxide. Much of the recent research involving heme oxygenase has been done using a 30 kDa soluble form of the enzyme, which lacks the membrane binding region (C-terminal 23 amino acids). The goal of this study was to express and purify a full-length human HO-1 (hHO-1) protein; however, due to the lability of the full-length form, a rapid purification procedure was required. This was accomplished by use of a glutathione-s-transferase (GST)-tagged hHO-1 construct. Although the procedure permitted the generation of a full-length HO-1, this form was contaminated with a 30 kDa degradation product that could not be eliminated. Therefore, attempts were made to remove a putative secondary thrombin cleavage site by a conservative mutation of amino acid 254, which replaces arginine with lysine. This mutation allowed the expression and purification of a full-length hHO-1 protein. Unlike wild type (WT) HO-1, the R254K mutant could be purified to a single 32 kDa protein capable of degrading heme at the same rate as the WT enzyme. The R254K full-length form had a specific activity of approximately 200-225 nmol of bilirubin h-1 nmol-1 HO-1 as compared to approximately 140-150 nmol of bilirubin h-1 nmol-1 for the WT form, which contains the 30 kDa contaminant. This is a 2-3-fold increase from the previously reported soluble 30 kDa HO-1, suggesting that the C-terminal 23 amino acids are essential for maximal catalytic activity. Because the membrane-spanning domain is present, the full-length hHO-1 has the potential to incorporate into phospholipid membranes, which can be reconstituted at known concentrations, in combination with other endoplasmic reticulum resident enzymes.

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.

Selectivity of imidazole-dioxolane compounds for in vitro inhibition of microsomal haem oxygenase isoforms

Haem oxygenases (HO) are involved in the catalytic breakdown of haem to generate carbon monoxide (CO), iron and biliverdin. It is widely accepted that products of haem catabolism are involved in biological signaling in many physiological processes. Conclusions to most studies in this field have gained support from the judicious use of synthetic metalloporphyrins such as chromium mesoporphyrin (CrMP) to selectively inhibit HO. However, metalloporphyrins have also been found to inhibit other haem-dependent enzymes, such as nitric oxide synthase (NOS), cytochromes P-450 (CYPs) and soluble guanylyl cyclase (sGC), induce the expression of HO-1 or exhibit varied toxic effects. To obviate some of these problems, we have been examining non-porphyrin HO inhibitors and the present study describes imidazole-dioxolane compounds with high selectivity for inhibition of HO-1 (rat spleen microsomes) compared to HO-2 (rat brain microsomes) in vitro. (2R,4R)-2-[2-(4-chlorophenyl)ethyl]-2-[(1H-imidazol-1-yl)methyl]-4-methyl-1,3-dioxolane hydrochloride) was identified as the most selective inhibitor with a concentration of 0.6 microM inhibiting HO-1(inducible) by 50% compared with 394 microM for HO-2 (constitutive). These compounds were found to have no effects on the catalytic activities of rat brain NOS and lung sGC, but were potent inhibitors of microsomal CYP2E1 and CYP3A1/3A2 activities. In conclusion, we have identified imidazole-dioxolanes that are able to inhibit microsomal HO in vitro with high selectivity for HO-1 compared to HO-2, and little or no effect on the activities of neuronal NOS and sGC. These molecules could be used to facilitate studies on the elucidation of physiological roles of HO/CO in biological systems.

Molecular mechanisms involved in enhancing HO-1 expression: de-repression by heme and activation by Nrf2, the "one-two" punch

Heme oxygenase (HO)-1 is a stress response protein, which confers cytoprotection against oxidative injury and provides a vital function in maintaining tissue homeostasis. Molecular mechanisms involved in the inducible transcription of ho-1 occurring in response to numerous and diverse stressful conditions have remained elusive. Since the discovery of E1 and E2, the two upstream enhancers regulating induction of ho-1 transcription in 1989, there have been many studies dealing with molecular mechanisms involved in enhancing HO-1 expression. In this commentary, recent advances in our understanding of the mechanisms involved in the induction of HO-1 expression in mammalian cells are summarized with some supportive results reported by others. Currently available data indicate that activation of ho-1 transcription involves both the heme (native substrate)-dependent selective alleviation of repressor and the oxidative stress-dependent activation of transcriptional activator. The stress-released free-heme (HO-1 substrate) from hemoproteins involved in causing oxidative stress itself appears to act as a molecular switch controlling the repressor- activator antagonism on the enhancer sequences of ho-1. Thus, induction of HO-1 appears to operate in a manner like a simple feedback loop. dox Signal. 7, 1674-1687.

Heme regulation in traumatic brain injury: relevance to the adult and developing brain

Intracranial bleeding is one of the most prominent aspects in the clinical diagnosis and prognosis of traumatic brain injury (TBI). Substantial amounts of blood products, such as heme, are released because of traumatic subarachnoid hemorrhages, intraparenchymal contusions, and hematomas. Despite this, surprisingly few studies have directly addressed the role of blood products, in particular heme, in the setting of TBI. Heme is degraded by heme oxygenase (HO) into three highly bioactive products: iron, bilirubin, and carbon monoxide. The HO isozymes, in particular HO-1 and HO-2, exhibit significantly different expression patterns and appear to have specific roles after injury. Developmentally, differences between the adult and immature brain have implications for endogenous protection from oxidative stress. The aim of this paper is to review recent advances in the understanding of heme regulation and metabolism after brain injury and its specific relevance to the developing brain. These findings suggest novel clinical therapeutic options for further translational study.

30 some years of heme oxygenase: from a "molecular wrecking ball" to a "mesmerizing" trigger of cellular events

In the beginning, the microsomal HO system was presumed to be made of one isozymes, now known as HO-1, which was cytP450-dependent; and, was thought to be of physiological significance solely in the context of catalysis of hemoglobin heme to bile pigments and CO. A succession of discoveries including characterization of the system as an independent mono-oxygenase, identification of a second form, called HO-2, free radical quenching activity of bile pigments, analogous function of CO in cell signaling to NO, and characterization of the system as HSP32 cognates has led to such an impressive expansion in the number of reports dealing with the HO system that surpass anyone's expectation. This review is a compilation of certain older findings and recent events that together ensure placement of the HO system in the mainstream research for decades to come.

Developmental expression of heme oxygenase in the rat lung

Heme oxygenase (HO), the rate-limiting enzyme in the formation of bilirubin, is expressed in the lung and may serve as an antioxidant. This enzyme results in the formation of antioxidant bile pigments and the degradation of pro-oxidant heme. We wanted to evaluate the differences in expression of HO-1, the inducible form, and HO-2, the constitutive isoenzyme, during lung maturation and document whether lung HO expression was similar to that of other antioxidant enzymes. Lung total HO activity and HO-1 and HO-2 proteins as well as HO-1 and HO-2 mRNA were evaluated in animals from 16 d of gestation (e(16.5)) to 2 mo of age. Heme content was also evaluated because heme is the substrate of the reaction. HO-1 mRNA was maximal at e(19.5) and e(20.5), whereas HO-2 mRNA was not changed throughout maturation. Lung HO-1 protein was highest on the first days of life and lowest in adults, whereas HO-2 protein was maximally expressed at postnatal d 5 and then declined to reach adult values. As to HO activity, there was a prenatal peak at e(20.5), a second lesser peak at d 5, and thereafter a decline to adult values. Lung heme content was inversely correlated with HO activity or protein as the highest heme values were seen in adults with the lowest HO activity. In response to hyperoxia, HO-1 mRNA was induced only in the adult lungs. A better understanding of the maturational regulation of lung HO will define a role for HO in newborns at risk for oxygen toxicity.

Regulatory heme and trichloroethylene intoxication: A possible explanation of the case of "A Civil Action"

In 1998, a amovie entitled "A Civil Action" was released. The movie described the Woburn case, begun in 1982 and concluded in 1990, one of the most famous cases of trichloroethylene pollution. In a small town near Boston, twelve children died of leukemia, which seemed attributable to trichloroethylene contamination of the drinking water. The victims, however, could not win the case, since evidence that the identified chemicals could cause leukemia and other human illnesses was rather sketchy. There have been many cases of trichloroethylene pollution in industrial nations including Japan, therefore, we reconsidered the missing link. Our conclusion is that the disease occurred not by a direct effect of the chemical hazard on biological macromolecules but by an indirect effect through the physiological system such as signal transduction and transcriptional regulation. In 1984, we reported a marked reduction in the regulatory heme pool by trichloroethylene exposure, however, the biological significance was not well understood. Recently, we found that the DNA binding activity of Bach1, a negative regulator of genes, is controlled by heme, the regulation of which seems to explain how leukemia develops. The heterodimer of Bach1 with MafK recognizes Maf recognition elements (MAREs) competing with the erythroid type positive regulator, a complex of NF-E2 with MafK. Bach1/MafK occupies MAREs under lower heme conditions, whereas MAREs are open to NF-E2/MafK along with increasing heme concentration. Since the NF-E2/MafK function is closely related to normal erythroid differentiation, of which disorders such as sideroblastic anemia are often related to neoplasia; i.e., a clonal disorder that can progress to leukemia. Thus, a marked decline in regulatory heme by trichloroethylene intoxication could be one of the pathways to leukemia.

Lead, chemical porphyria, and heme as a biological mediator

One of the most well-characterized symptoms of lead poisoning is porphyria. The biochemical signs of lead intoxication related to porphyria are delta-aminolevulinic aciduria, coproporphyrinuria, and accumulation of free and zinc protoporphyrin in erythrocytes. From the 1970s to the early 80s, almost all of the enzymes in the heme pathway had been purified and characterized, and it was demonstrated that delta-aminolevulinic aciduria is due to inhibition of delta-aminolevulinate dehydratase by lead. Lead also inhibits purified ferrochelatase; however, the magnitude of inhibition was essentially nil even under pathological conditions. Further study proved the disturbance of iron-reducing activity by moderate lead exposure. Far different from these two enzymes, lead failed to inhibit purified coproporphyrinogen oxidase, i.e., the mechanism of coproporphyrinuria has not yet been understood. During the 80s to the 90s, the effects of environmental hazards including lead were elucidated through stress proteins, indicating the induction of some heme pathway enzymes as stress proteins. At that time, gene environment interaction was another focus of toxicology, since gene carriers of porphyrias are considered to be a high-risk group to chemical pollutants. Toxicological studies from the 70s to the 90s focused on the direct effect of hazards on biological molecules, such as the heme pathway enzymes, and many environmental pollutants were proved to affect cytosolic heme. Recently, we demonstrated the mechanism of the heme-controlled transcription system, which suggests that the indirect effects of environmental hazards are also important for elucidating toxicity, i.e., the hazards can affect cell functions through such biological mediators as regulatory heme. It is, therefore, probable that toxicology in the future will focus on biological systems such as gene regulation and signal transduction systems.

Heme oxygenase-2 interaction with metalloporphyrins: function of heme regulatory motifs

Heme oxygenase-2 (HO-2) degrades heme [Fe-protoporphyrin IX (Fe-PP)] to CO and bilirubin. The enzyme is a hemoprotein and interacts with nitric oxide. HO-2 has two copies of heme regulatory motif (HRM) with a conserved core of Cys264-Pro265 and Cys281-Pro282. We examined interaction of HO-2 HRMs with Fe-PP, Zn-protoporphyrin IX (Zn-PP; HO-2 inhibitor), and protoporphyrin IX (PP IX). Spectral analyses, using 1:4 or 1:1 molar ratio of the heme to 10-residue peptides, corresponding to HRM containing HO-2 sequences, revealed specific interactions as indicated by a shift in the absorption spectrum of heme. Five residue peptides qualitatively produced similar results. Substitution of cysteine with alanine in either peptide eliminated interactions, and substitution of proline with alanine reduced the peptides' affinity for heme. Neither Zn-PP nor PP IX absorption spectrum was affected by HRM peptides. The circular dichroism spectra confirmed heme-HRM peptides interactions. An astounding 4,000-6,000-fold higher concentrations of KCN were required at pH 7.5 to displace HRM peptides from heme. Data suggest (a) each HRM can contribute to HO-2-heme interaction, (b) heme iron interacts with cysteine thiol, (c) charged residues upstream of Cys264-Pro265 result in its high-affinity heme binding, and (d) inhibition of HO-2 activity by synthetic metalloporphyrins does not involve HRMs. We suggest that heme bound to HRMs may serve as a binding site/reservoir for gaseous signal molecules.

Heme mediates derepression of Maf recognition element through direct binding to transcription repressor Bach1

Heme controls expression of genes involved in the synthesis of globins and heme. The mammalian transcription factor Bach1 functions as a repressor of the Maf recognition element (MARE) by forming antagonizing hetero-oligomers with the small Maf family proteins. We show here that heme binds specifically to Bach1 and regulates its DNA-binding activity. Deletion studies demonstrated that a heme-binding region of Bach1 is confined within its C-terminal region that possesses four dipeptide cysteine-proline (CP) motifs. Mutations in all of the CP motifs of Bach1 abolished its interaction with heme. The DNA-binding activity of Bach1 as a MafK hetero-oligomer was markedly inhibited by heme in gel mobility shift assays. The repressor activity of Bach1 was lost upon addition of hemin in transfected cells. These results suggest that increased levels of heme inactivate the repressor Bach1, resulting in induction of a host of genes with MARES:

Heme oxygenase-2 immunoreactivity in developing and mature bovine olfactory epithelium

In the present study the localization of heme oxygenase-2 (HO-2) in developing and mature olfactory epithelium of the bovine is investigated using immunohistochemistry and post embedding immunogold labelling. HO-2 immunoreactivity is first seen in epithelial cells localized along the luminal surface of the olfactory pit. Up to midgestation the number of HO-2 immunoreactive cells increases throughout all layers of the developing olfactory epithelium. From midgestation through adulthood immunostaining is restricted to perinuclear cytoplasm and axons of mature olfactory receptor neurons localized in intermediate epithelial regions. The temporal and spatial expression patterns of HO-2 immunohistochemistry support the notion that CO plays a role in neuronal differentiation while its presence in mature neurons might be functionally related to olfactory transduction.

Heme oxygenase-2 is present in the sarcolemma region of skeletal muscle fibers and is non-continuously co-localized with nitric oxide synthase-1

There is increasing evidence that the heme oxygenase-2 (HO-2)/carbon monoxide (CO) pathway and the nitric oxide synthase (NOS)/nitric oxide (NO) pathway functionally cross-talk. Therefore, we investigated the appearance of HO-2 in mammalian skeletal muscles where NOS-1 is known to be expressed in high quantities. Immunoblotting of rat hind limb extensor muscles extracts revealed a single 36 kDa band demonstrating the existence of HO-2 in skeletal muscle and indicating the monospecifity of the antibody that was applied. Immunohistochemistry on healthy rat extensor hind limb muscles showed that HO-2 is present in satellite cells, endothelial cells of the vascular system, fibrocytes/fibroblasts but also fiber type-independently in extrafusal myofibers either in association with the non-junctional sarcolemma region, or in a subsarcolemmal network or, less prominently, in cross-striated stripes connected to longitudinally running lines. Combined HO-2 immunohistochemistry and NOS-1 histochemistry revealed an apparent co-localization of both molecules only in the non-junctional sarcolemma region of extrafusal type II myofibers outside costameres. In diseased muscles of mdx mice, HO-2 expression was not changed. In patients suffering from Duchenne's muscular dystrophy, it was absent in the sarcolemma region. In conclusion, the HO-2/CO system is present in mammalian skeletal muscle where it is non-continuously co-localized with the NOS-1/NO-system. This finding implicates an optionally functional cross-talk between both gaseous signaling pathways.

Neurons overexpressing heme oxygenase-1 resist oxidative stress-mediated cell death

This is the first report on the protective effect of heme oxygenase-1 (HO-1) overexpression against oxidative stress-mediated neuronal cell death and demonstration of a decreased production of oxygen free radicals when HO-1 levels are increased. HO-1 is the heat shock/stress cognate of the heat shock protein 32 family of proteins. A known function of these proteins is alpha-meso bridge-specific cleavage of the heme molecule. For the present study, we used cerebellar granular neurons (CGNs) isolated from homozygous transgenic (Tg) mice that overexpress HO-1 under neuron-specific enolase control and nontransgenic (Ntg) littermates. The Tg mouse CGNs were characterized by increased levels of HO-1 mRNA and protein, a lower resting intracellular calcium concentration, and a reduced HO-1 transcriptional response to glutamate-mediated oxidative stress. Compared with the Ntg neurons, when exposed to glutamate (30 microM or 3 mM), the magnitude of cell viability was increased and the number of cells exhibiting membrane permeability and chromatin condensation were significantly decreased in the Tg CGN cultures. The population of neurons surviving glutamate toxicity decreased when HO-1 activity was inhibited by a peptide inhibitor. The neuroprotective effect by HO-1 was extended to H(2)O(2)-induced cell death. The mechanism of protection may involve in part a reduced production of reactive oxygen species upon exposure to glutamate. We suggest that induction of HO-1 by pharmacological means may be a novel approach to amelioration of oxidative insults to neurons.

Heme oxygenase-2 and nitric oxide synthase immunoreactivity of bovine olfactory receptor neurons and a comparison with the distribution of NADPH-diaphorase staining

It has recently been suggested that, in addition to nitric oxide (NO), carbon monoxide (CO) is an important gaseous messenger which might be involved in vertebrate olfactory transduction because its effects include activation of guanylyl cyclase and the formation of cGMP. As there is no information regarding the presence of heme oxygenase-2 -- the constitutive isoform of the heme oxygenase system -- in olfactory neurons of non-rodent species, we have investigated the distribution pattern of heme oxygenase-2 in the olfactory epithelium of the bovine, a representative of macrosmatics. Localization of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity of the olfactory epithelium was compared with heme oxygenase-2 and NO synthase (NOS) immunoreactivities in order to obtain possible hints at functional significance. NADPH-d activity was particularly intense in apical dendrites of receptor neurons. It was also found in Bowman glands and intraepithelial duct cells. Less intense, discrete NADPH-d activity was present also at intermediate and basal levels of the olfactory epithelium, corresponding to the layer of receptor neuron somata and basal cells. While heme oxygenase-2 activity mainly occurred in neuronal perikarya, a very intense NOS immunoreactivity, exclusively for the inducible isoform, was detected in the apical dendrites. Ultrastructurally, NADPH-d histochemistry showed distinct labelling of membranes, in particular of endoplasmic reticulum, mitochondria and nucleus. The coincident localization of the moderate NADPH-d activity and heme oxygenase-2 immunoreactivity in receptor cell perikarya suggest a functional association between NADPH-cytochrome P450 reductase and heme oxygenase-2. In contrast, dendritic localization of NADPH-d activity is topically and possibly functionally related to the presence of the inducible isoform of NOS. The results suggest that both CO and NO may be generated in bovine receptor neurons and thus involved in odorant stimulation. Based on immunocytochemical localization of synthesizing enzymes, NO might be regarded as a direct regulator of transduction related processes while CO might act as a modulator of the initial signal.

Interaction of heme oxygenase-2 with nitric oxide donors. Is the oxygenase an intracellular 'sink' for NO?

Heme oxygenase-2 (HO-2) is the constitutive cognate of the heat-shock protein-32 family of proteins. These proteins catalyze oxidative cleavage of heme to CO and biliverdin, and release Fe. HO-2 is a hemoprotein and binds heme at heme regulatory motifs (HRMs) with a conserved Cys-Pro pair; two copies of HRM are present in HO-2 (Cys264 and Cys281). The HO-2 HRMs are not present in HO-1 and are not involved in HO-2 catalytic activity. Optical CD, and spectral and activity analyses were used to examine reactivity of HO isozymes with NO species produced by NO donors. Purified Escherichia coli-expressed HO preparations, wild-type HO-2, Cys264/Cys281 --> Ala/Ala HO-2-mutant (HO-2-mut) and HO-1 preparations were used. A type II change (red shift) of the Soret band (405 nm --> 413-419 nm) was observed when wild-type HO-2 was treated with sodium nitroprusside (SNP), S-nitroglutathione (GSNO), S-nitroso-N-acetylpenicillamine (SNAP) or 3-morpholinosydnonimine (SIN-1); the NO scavenger, hydroxocobalamin (HCB) prevented the shift. Only SIN-1, which produces peroxynitrite by generating both NO and superoxide anion, decreased the Soret region absorption and the pyridine hemochromogen spectrum of HO-2; superoxide dismutase (SOD) blocked the decrease. Binding of heme to HO-2 protein was required for shift and/or decrease in absorption of the Soret band. NO donors significantly inhibited HO-2 activity, with SNP being the most potent inhibitor (> 40%). Again, trapping NO with HCB blocked HO-2 inactivation. HO-1 and HO-2-mut were not inactivated by NO donors. CD data suggest that the decrease in HO-2 activity was not related to change by NO species of the secondary structure of HO-2. Western blot analysis suggests that NO donors did not cause HO-1 protein loss and Northern blot analysis of HeLa cells treated with SIN-1 and SNP indicates that, unlike HO-1 mRNA, which is remarkably responsive to the treatments, HO-2 mRNA levels were modestly increased ( approximately two to threefold) by NO donors. The data are consistent with the possibility that NO interaction with HO-2-bound heme effects electronic interactions of residues involved in substrate binding and/or oxygen activation. The findings permit the hypothesis that HO-2 and NO are trans-inhibitors, whereby biological activity of NO is attenuated by interaction with HO-2, serving as an intracellular 'sink' for the heme ligand, and NO inhibits HO-2 catalytic activity. As such, the cellular level of both signaling molecules, CO and NO would be moderated.

Overexpression of heme oxygenase-1 is neuroprotective in a model of permanent middle cerebral artery occlusion in transgenic mice

Heme oxygenase-1 (HO-1, HSP32) is an early gene that is responsive to an array of pathological conditions including, but not limited to, hypoxia and cerebral ischemia. HO-1 cleaves the heme molecule and produces carbon monoxide (CO) and biliverdin (an antioxidant) and is essential for iron homeostasis. The purpose of this study was to investigate, using transgenic (Tg) mice, whether overexpression of HO-1 in the brain augments or attenuates cellular injury caused by ischemic stroke. Homozygous HO-1 Tg mice that overexpress HO-1 under the control of the neuron-specific enolase promoter (characterized previously) were used. Under halothane anesthesia and normothermic conditions, wild-type nontransgenic (nTg; n = 22) and HO-1 Tg (n = 24) mice were subjected to middle cerebral artery occlusion (MCAo). Six hours after induction of ischemia, Tg and nTg mice developed infarcts that were 39 +/- 6 and 63 +/- 9 mm3, respectively (p < 0.01). No significant difference between the two strains was observed in the values of brain edema (11.3 +/- 4% in Tg vs. 14.6 +/- 5% in nTg; p < 0.1). At 24 h after MCAo, Tg mice exhibited significant neuroprotection as determined by the stroke volumes (41 +/- 2 mm3 in Tg vs. 74 +/- 5 mm3 in nTg; p < 0.01) and values of ischemic cerebral edema (21 +/- 6% in Tg vs. 35 +/- 11% in nTg; p < 0.01). Data suggest that neuroprotection in Tg mice was, at least in part, related to the following findings: (a) constitutively up-regulated cyclic GMP and bcl-2 levels in neurons; (b) inhibition of nuclear localization of p53 protein; and (c) antioxidant action of HO-1, as detected by postischemic neuronal expression of ferritin, and decreases in iron staining and tissue lipid peroxidation. We suggest that pharmacological stimulation of HO-1 activity may constitute a novel therapeutic approach in the amelioration of ischemic injury during the acute period of stroke.

Heme oxygenase isozymes in bone: induction of HO-1 mRNA following physiological levels of mechanical loading in vivo

Heme oxygenases (HO) are responsible for the production of carbon monoxide, which has been suggested to act similarly to nitric oxide as a signaling molecule. Inducible HO-1 and constitutive HO-2 were located in sections of weight-bearing ulnae of the rat by immunocytochemistry. Intense HO-1 localization was restricted to peri- and endosteal sites, whereas HO-2 staining occurred in osteoblasts and osteocytes throughout the cortex. Northern blot hybridization of mRNA levels for HO-1 and HO-2 extracted from bones was also performed. Six hours after a single 10 min period of noninvasive mechanical loading of the ulna in vivo, generating physiological levels of strain sufficient to initiate an osteogenic response, the level of mRNA for the inducible HO-1 isoform was increased, but that of HO-2 was unchanged. The presence of a constitutive and strain-related upregulation of an inducible enzyme capable of producing carbon monoxide suggests that carbon monoxide may participate not only in bone cells' basal metabolism but also in their adaptive response to mechanical load.

Phytobilin biosynthesis: cloning and expression of a gene encoding soluble ferredoxin-dependent heme oxygenase from Synechocystis sp. PCC 6803

The phytobilin chromophores of phycobiliproteins and phytochromes are biosynthesized from heme in a pathway that begins with the opening of the tetrapyrrole macrocycle of protoheme to form biliverdin IX alpha, in a reaction catalyzed by heme oxygenase. A gene containing an open reading frame with a predicted polypeptide that has a sequence similar to that of a conserved region of animal microsomal heme oxygenases was identified in the published genomic sequence of Synechocystis sp. PCC 6803. This gene, named ho1, was cloned and expressed in Escherichia coli under the control of the lacZ promoter. Cells expressing the gene became green colored due to the accumulation of biliverdin IX alpha. The size of the expressed protein was equal to the predicted size of the Synechocystis gene product, named HO1. Heme oxygenase activity was assayed in incubations containing extract of transformed E. coli cells. Incubations containing extract of induced cells, but not those containing extract of uninduced cells, had ferredoxin-dependent heme oxygenase activity. With mesoheme as the substrate, the reaction product was identified as mesobiliverdin IX alpha by spectrophotometry and reverse-phase HPLC. Heme oxygenase activity was not sedimented by centrifugation at 100, 000 g. Expression of HO1 increased several-fold during incubation of the cells for 72 h in iron-deficient medium.

Characterization and biological significance of immunosuppressive peptide D2702.75-84(E --> V) binding protein. Isolation of heme oxygenase-1

This is the first report on peptidic inhibitors of heme oxygenase. Such peptides were originally developed from the immunomodulatory peptide 2702.75-84 which corresponds to amino acid residues 75 to 84 of the alpha1-helix of HLA-B2702 (2702.75-84) and has been shown to be immunosuppressive in vitro and in vivo. In vitro, 2702.75-84 inhibited cytotoxic T- and natural killer cell- mediated target cell lysis, and in vivo peptide therapy resulted in prolongation of heart and skin allograft survival in mice. The peptide was also shown to bind to heat shock protein 70. However, D-enantiomers of 2702.75-84 and derivatives thereof, while still being immunosuppressive, did not bind to heat shock protein 70. This study was designed to identify proteins binding to peptide D2702.75-84(E --> V) (rvnlrialry) consisting of D-amino acids. Compared with 2702.75-84 (RENLRIALRY), glutamic acid residue 76 (E) was replaced with valine (V). Affinity chromatography using immobilized D2702.75-84(E --> V) and mouse and human cell extracts, resulted in the isolation of heme oxygenase-1 (HO-1). Peptide D2702.75-84 inhibited HO activity in vitro in a dose dependent manner. Similar to what has been observed with other inhibitors of HO, administration of peptide into mice resulted in an up-regulation of HO-1 mRNA and protein, as well as enzyme activity in liver, spleen and kidney. Other peptides derived from 2702.75-84 with similar immunomodulatory activity displayed similar effects. In contrast, inactive derivatives of 2702.75-84 had no effect on HO activity. Therefore, the immunosuppressive effects of the described immunomodulatory peptides are similar to those of cobalt-protoporphyrin, a known up-regulator of HO-1. Our results suggest that HO-1 modulation may be a novel mechanism of immunomodulation.

Isolation and characterization of a cDNA from the rat brain that encodes hemoprotein heme oxygenase-3

Two isozymes of heme oxygenase (HO), HO-1 or HSP32 and the constitutive form HO-2, have been characterized to date. We report the discovery of a third protein species and refer to it as HO-3. HO-3 is the product of a single transcript of approximately 2.4 kb and can encode a protein of approximately 33 kDa. The HO-3 transcript is found in the spleen, liver, thymus, prostate, heart, kidney, brain and testis and is the product of a single-copy gene. The predicted amino acid structure of HO-3 differs from both HO-1 (HSP32) and HO-2 but is closely related to HO-2 (approximately 90%). Escherichia coli expressed and purified HO-3 protein does not cross react with polyclonal antibodies to either rat HO-1 or HO-2, is a poor heme catalyst, and displays hemoprotein spectral characteristics. The predicted protein has two heme regulatory motifs that may be involved in heme binding. These motifs and the hemoprotein nature of HO-3 suggest a potential regulatory role for the protein in cellular processes which are heme-dependent.

Heme oxygenase-2 is a hemoprotein and binds heme through heme regulatory motifs that are not involved in heme catalysis

The heme oxygenase (HO) system degrades heme to biliverdin and CO and releases chelated iron. In the primary sequence of the constitutive form, HO-2, there are three potential heme binding sites: two heme regulatory motifs (HRMs) with the absolutely conserved Cys-Pro pair, and a conserved 24-residue heme catalytic pocket with a histidine residue, His151 in rat HO-2. The visible and pyridine hemochromogen spectra suggest that the Escherichia coli expressed purified HO-2 is a hemoprotein. The absorption spectrum, heme fluorescence quenching, and heme titration analysis of the wild-type protein versus those of purified double cysteine mutant (Cys264/Cys281 --> Ala/Ala) suggest a role of the HRMs in heme binding. While the His151 --> Ala mutation inactivates HO-2, Cys264 --> Ala and Cys281 --> Ala mutations individually or together (HO-2 mut) do not decrease HO activity. Also, Pro265 --> Ala or Pro282 --> Ala mutation does not alter HO-2 activity. Northern blot analysis of ptk cells indicates that HO-2 mRNA is not regulated by heme. The findings, together with other salient features of HO-2 and the ability of heme-protein complexes to generate oxygen radicals, are consistent with HO-2, like five other HRM-containing proteins, having a regulatory function in the cell.

The heme oxygenase system: a regulator of second messenger gases

The heme oxygenase (HO) system consists of two forms identified to date: the oxidative stress-inducible protein HO-1 (HSP32) and the constitutive isozyme HO-2. These proteins, which are different gene products, have little in common in primary structure, regulation, or tissue distribution. Both, however, catalyze oxidation of heme to biologically active molecules: iron, a gene regulator; biliverdin, an antioxidant; and carbon monoxide, a heme ligand. Finding the impressive heme-degrading activity of brain led to the suggestion that "HO in brain has functions aside from heme degradation" and to subsequent exploration of carbon monoxide as a promising and potentially significant messenger molecule. There is much parallelism between the biological actions and functions of the CO- and NO-generating systems; and their regulation is intimately linked. This review highlights the current information on molecular and biochemical properties of HO-1 and HO-2 and addresses the possible mechanisms for mutual regulatory interactions between the CO- and NO-generating systems.

Ontogeny of heme oxygenase activity in the hippocampus, frontal cerebral cortex, and cerebellum of the guinea pig

Heme oxygenase (HO) catalyzes the oxidation of heme to produce carbon monoxide (CO), biliverdin and iron. CO is considered to function as a novel neuronal messenger in the brain analogous to nitric oxide. The ontogeny of microsomal HO activity in the hippocampus, frontal cerebral cortex, and cerebellum of the immature fetal, mature fetal and adult guinea pig was determined using an optimized assay which quantitated heme-derived CO formation by a gas chromatographic method. There was a distinct developmental profile of HO activity that was similar for all three brain regions. In particular, HO activity was maximal in the mature fetus compared with the immature fetus and the adult. These data demonstrate that HO activity is developmentally regulated and that there is similar ontogeny of HO activity in the hippocampus, frontal cerebral cortex, and cerebellum of the guinea pig.

Differential expression of heme oxygenase-1 in cultured cortical neurons and astrocytes determined by the aid of a new heme oxygenase antibody. Response to oxidative stress

Heme oxygenase exists as two isoenzymes designated heme oxygenase-1 (HO-1) and heme oxygenase-2 (HO-2). HO-2 is made constitutively in many cell types whereas HO-1 is a stress protein inducible by heat, heavy metals, ultraviolet irradiation, and oxidative stress. Recombinant rat HO-1 was expressed in bacteria and antiserum designated HO-1713 was raised against the purified protein. HO-1713 detected recombinant rat HO-1 and recombinant rat HO-2. In rat tissues it detected HO-1 and a second, unidentified band designated HO-L (heme oxygenase-like immunoreactivity) which was not HO-2. Cultured rat cortical neurons and forebrain astrocytes were exposed to hydrogen peroxide (0.14-0.7 micromolar for 30 or 60 min). Neurons which contained little detectable HO-1 and which were sensitive to hydrogen peroxide at the high end of the dose curve failed to induce HO-1 by Western blot analysis. In contrast, cultured rat forebrain astrocytes which contained HO-1 under normal culture conditions and which were resistant to injury by hydrogen peroxide, increased their content of immunoreactive HO-1 by 7-fold within 3 h after exposure. Our results support a protective role for HO-1 in oxidative injury and suggest that the relative inability of neurons to increase HO-1 after oxidative stress may contribute to their selective vulnerability vis-a-vis astrocytes. They also suggest that differential expression of heme oxygenase in studies utilizing CNS cultures may alter normal cell physiology and cell survival.

Heme oxygenase-2. Properties of the heme complex of the purified tryptic fragment of recombinant human heme oxygenase-2

Recombinant human microsomal heme oxygenase-2 was expressed in Escherichia coli. Tryptic digestion of the membrane fraction, in which the wild-type enzyme was localized, yielded a soluble tryptic peptide of 28 kDa, which retained the ability to accept electrons from NADPH-cytochrome P-450 reductase and the enzymatic activity for conversion of heme to biliverdin. The tryptic fragment, when purified to apparent homogeneity, bound one equivalent of heme to form a substrate-enzyme complex that had spectroscopic properties characteristic of heme proteins, such as myoglobin and hemoglobin. Optical absorption, Raman scattering, and EPR studies of the heme-tryptic fragment complex revealed that the ferric heme was six coordinate high spin at neutral pH and six coordinate low spin at alkaline pH, with a pK alpha value of 8.5. EPR and Raman scattering studies indicated that a neutral imidazole of a histidine residue served as the proximal ligand in the heme-heme oxygenase-2 fragment complex. The reaction with hydrogen peroxide converted the heme of the heme oxygenase-2 fragment complex into a verdoheme-like intermediate, while the reaction with m-chloroperbenzoic acid yielded a oxoferryl species. These spectroscopic properties are similar to those obtained for heme oxygenase-1, and thus the catalytic mechanism of heme oxygenase-2 appears to be similar to that of heme oxygenase-1.

Heme-heme oxygenase complex: structure and properties of the catalytic site from resonance Raman scattering

The resonance Raman spectra of ferric and ferrous forms of the heme-heme oxygenase (HO) complex (isoform 1) clarify several structural features of the catalytic active site. Isotopic substitution studies of the central iron atom of the heme demonstrate that the line at 218 cm-1 in the ferrous ligand-free form of the complex originates from the iron-histidine stretching mode. The presence of a Raman line at this frequency confirms that the fifth ligand coordinating to the heme is a neutral imidazole from a histidine residue. The modes associated with CO in the carboxy derivative of the ferrous enzyme complex have typical frequencies of histidine-bound heme proteins such as myoglobin. In the ferric form of the complex, at alkaline pH, hydroxide is identified as the bound exogenous ligand, and at neutral pH we infer that water is bound. Thus, the coordination of the heme-HO complex is the same as that in myoglobin. However, in a comparison of the low-frequency vibrational modes in the resonance Raman spectrum of the heme-HO complex to those of myoglobin, the spectra are found to be very different, indicating that the interactions between the heme and its amino acid pocket in these two proteins are quite different. The neutral imidazole may play several important roles in the physiological function of the heme-HO complex.

The structure, organization and differential expression of the gene encoding rat heme oxygenase-2

Overlapping phage lambda clones were utilized to determine the complete nucleotide (nt) sequence of the rat gene encoding HO-2, the major heme oxygenase isozyme in the brain. This isozyme is the constitutive cognate of HSP32 (HO-1). The 12,563-bp gene consists of five exons and four introns, the first two exons are separated by a large intron of 8429 nt. The minus strand of intron 1 contains a nested sequence of 1046 nt with 87% identity to the cDNAs encoding the mouse and human non-histone chromosomal protein, HMG-17. In addition to the coding region, the similarity includes 40 bp upstream from the putative start codon and 800 bp of 3' untranslated sequence. The HO-2 gene lacks a conventional TATA box, but a TATA-like sequence (TAACTA) is found 26 nt upstream from the major transcription start point (tsp), as determined by primer extension. Upstream of the tsp, only a glucocorticoid-response element is found. The structure of the regulatory region is consistent with the previously demonstrated refractory nature of this isozyme to common inducers of gene expression and its apparent response to developmental changes in the adrenal steroid hormone profile. HO-2 is encoded by two transcripts (approx. 1.3 and approx. 1.9 kb), the larger of which is translated less efficiently than the smaller. Presently, we show that the transcripts are the products of a single gene and differ in the use of the polyadenylation signal.(ABSTRACT TRUNCATED AT 250 WORDS)