Pitfalls using metalloporphyrins in carbon monoxide research

Cadmium attenuates testosterone synthesis by promoting ferroptosis and blocking autophagosome-lysosome fusion

Ferroptosis is a newly defined programmed cell death pathway characterized by iron overload and lipid peroxidation. Increasing studies show that autophagy regulates testosterone synthesis and promotes ferroptosis. Testosterone is essential for sexual development and the maintenance of male characteristics. The deficiency of testosterone induced by cadmium (Cd) can severely affect male fertility. However, the underlying mechanism of testosterone reduction after Cd exposure remains blurry. In this study, we found that Cd affected iron homeostasis and elicited ferroptosis, ultimately reducing testosterone production. Mechanically, our findings revealed that Cd-induced ferroptosis depended upon the excessive activation of Heme oxygenase 1 (HMOX1) and the release of free iron from heme. Additionally, Cd exposure promoted autophagosome formation but blocked autophagosome-lysosome fusion, which attenuated the absorption of total cholesterol and triglycerides, further aggravating testosterone synthesis disorder. Collectively, Cd induced ferroptosis by iron homeostasis dysregulation, mediated by excessive activation of HMOX-1. The disruption of autophagy flow contributed to Cd-induced testicular dysfunction and attenuated testosterone synthesis.

Murine Bone Marrow Mesenchymal Stromal Cells Respond Efficiently to Oxidative Stress Despite the Low Level of Heme Oxygenases 1 and 2

AIMS

Mesenchymal stromal cells (MSCs) are heterogeneous cells from adult tissues that are able to differentiate in vitro into adipocytes, osteoblasts, or chondrocytes. Such cells are widely studied in regenerative medicine. However, the success of cellular therapy depends on the cell survival. Heme oxygenase-1 (HO-1, encoded by the Hmox1 gene), an enzyme converting heme to biliverdin, carbon monoxide, and Fe²⁺, is cytoprotective and can affect stem cell performance. Therefore, our study aimed at assessing whether Hmox1 is critical for survival and functions of murine bone marrow MSCs.

RESULTS

Both MSC Hmox1^+/+ and Hmox1^-/- showed similar phenotype, differentiation capacities, and production of cytokines or growth factors. Hmox1^+/+ and Hmox1^-/- cells showed similar survival in response to 50 μmol/L hemin even in increased glucose concentration, conditions that were unfavorable for Hmox1^-/- bone marrow-derived proangiogenic cells (BDMC). Hmox1^+/+ MSCs but not fibroblasts retained low ROS levels even after prolonged incubation with 50 μmol/L hemin, although both cell types have a comparable Hmox1 expression and similarly increase its levels in response to hemin. MSCs Hmox1^-/- treated with hemin efficiently induced expression of a vast panel of antioxidant genes, especially enzymes of the glutathione pathway. Innovation and Conclusion: Hmox1 overexpression is a popular strategy to enhance viability and performance of MSCs after the transplantation. However, murine MSCs Hmox1^-/- do not differ from wild-type MSCs in phenotype and functions. MSC Hmox1^-/- show better resistance to hemin than fibroblasts and BDMCs and rapidly react to the stress by upregulation of quintessential genes in antioxidant response. Antioxid. Redox Signal. 00, 000-000.

Could Heme Oxygenase-1 Be a New Target for Therapeutic Intervention in Malaria-Associated Acute Lung Injury/Acute Respiratory Distress Syndrome?

Malaria is a serious disease and was responsible for 429,000 deaths in 2015. Acute lung injury/acute respiratory distress syndrome (ALI/ARDS) is one of the main clinical complications of severe malaria; it is characterized by a high mortality rate and can even occur after antimalarial treatment when parasitemia is not detected. Rodent models of ALI/ARDS show similar clinical signs as in humans when the rodents are infected with murine Plasmodium. In these models, it was shown that the induction of the enzyme heme oxygenase 1 (HO-1) is protective against severe malaria complications, including cerebral malaria and ALI/ARDS. Increased lung endothelial permeability and upregulation of VEGF and other pro-inflammatory cytokines were found to be associated with malaria-associated ALI/ARDS (MA-ALI/ARDS), and both were reduced after HO-1 induction. Additionally, mice were protected against MA-ALI/ARDS after treatment with carbon monoxide- releasing molecules or with carbon monoxide, which is also released by the HO-1 activity. However, high HO-1 levels in inflammatory cells were associated with the respiratory burst of neutrophils and with an intensification of inflammation during episodes of severe malaria in humans. Here, we review the main aspects of HO-1 in malaria and ALI/ARDS, presenting the dual role of HO-1 and possibilities for therapeutic intervention by modulating this important enzyme.

Anti-apoptotic properties of carbon monoxide in porcine oocyte during in vitro aging

If fertilization of matured oocyte does not occur, unfertilized oocyte undergoes aging, resulting in a time-dependent reduction of the oocyte’s quality. The aging of porcine oocytes can lead to apoptosis. Carbon monoxide (CO), a signal molecule produced by the heme oxygenase (HO), possesses cytoprotective and anti-apoptotic effects that have been described in somatic cells. However, the effects of CO in oocytes have yet to be investigated. By immunocytochemistry method we detected that both isoforms of heme oxygenase (HO-1 and HO-2) are present in the porcine oocytes. Based on the morphological signs of oocyte aging, it was found that the inhibition of both HO isoforms by Zn-protoporphyrin IX (Zn-PP IX) leads to an increase in the number of apoptotic oocytes and decrease in the number of intact oocytes during aging. Contrarily, the presence of CO donors (CORM-2 or CORM-A1) significantly decrease the number of apoptotic oocytes while increasing the number of intact oocytes. We also determined that CO donors significantly decrease the caspase-3 (CAS-3) activity. Our results suggest that HO/CO contributes to the sustaining viability through regulation of apoptosis during in vitro aging of porcine oocytes.

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.

Proximal tubule-targeted heme oxygenase-1 in cisplatin-induced acute kidney injury

Heme oxygenase-1 (HO-1) is a cytoprotective enzyme that catalyzes the breakdown of heme to biliverdin, carbon monoxide, and iron. The beneficial effects of HO-1 expression are not merely due to degradation of the pro-oxidant heme but are also credited to the by-products that have potent, protective effects, including antioxidant, anti-inflammatory, and prosurvival properties. This is well reflected in the preclinical animal models of injury in both renal and nonrenal settings. However, excessive accumulation of the by-products can be deleterious and lead to mitochondrial toxicity and oxidative stress. Therefore, use of the HO system in alleviating injury merits a targeted approach. Based on the higher susceptibility of the proximal tubule segment of the nephron to injury, we generated transgenic mice using cre-lox technology to enable manipulation of HO-1 (deletion or overexpression) in a cell-specific manner. We demonstrate the validity and feasibility of these mice by breeding them with proximal tubule-specific Cre transgenic mice. Similar to previous reports using chemical modulators and global transgenic mice, we demonstrate that whereas deletion of HO-1, specifically in the proximal tubules, aggravates structural and functional damage during cisplatin nephrotoxicity, selective overexpression of HO-1 in proximal tubules is protective. At the cellular level, cleaved caspase-3 expression, a marker of apoptosis, and p38 signaling were modulated by HO-1. Use of these transgenic mice will aid in the evaluation of the effects of cell-specific HO-1 expression in response to injury and assist in the generation of targeted approaches that will enhance recovery with reduced, unwarranted adverse effects.

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.

HO-1/CO system in tumor growth, angiogenesis and metabolism - Targeting HO-1 as an anti-tumor therapy

Heme oxygenase-1 (HO-1, hmox-1) catalyzes the rate-limiting step in the heme degradation processes. Out of three by-products of HO-1 activity, biliverdin, iron ions and carbon monoxide (CO), the latter was mostly shown to mediate many beneficial HO-1 effects, including protection against oxidative injury, regulation of apoptosis, modulation of inflammation as well as contribution to angiogenesis. Mounting evidence suggests that HO-1/CO systemmay be of special benefit in protection inmany pathological conditions, like atherosclerosis or myocardial infarction. By contrast, the augmented expression of HO-1 in tumor tissues may have detrimental effect as HO-1 accelerates the formation of tumor neovasculature and provides the selective advantage for tumor cells to overcome the increased oxidative stress during tumorigenesis and during treatment. The inhibition of HO-1 has been proposed as an anti-cancer therapy, however, because of non-specific effects of known HO-1 inhibitors, the discovery of ideal drug lowering HO-1 expression/activity is still an open question. Importantly, in several types of cancer HO-1/CO system exerts opposite activities, making the possible treatment more complicated. All together indicates the complex role for HO-1/CO in various in vitro and in vivo conditions.

How does binding of imidazole-based inhibitors to heme oxygenase-1 influence their conformation? Insights combining crystal structures and molecular modelling

Heme oxygenase-1 (HO-1) inhibition is associated with antitumor activity. Imidazole-based analogues show effective and selective inhibitory potency of HO-1. In this work, five single-crystal structures of four imidazole-based compounds are presented, with an in-depth structural analysis. In order to study the influence of the conformation of the ligands on binding to protein, conformational data from crystallography are compared with quantum mechanics analysis and molecular docking studies. Molecular docking of imidazole-based analogues in the active site of HO-1 is in good agreement with the experimental structures. Inhibitors interact with the heme cofactor and a hydrophobic pocket (Met34, Phe37, Val50, Leu147 and Phe214) in the HO-1 binding site. An alternate binding mode can be hypothesized for some inhibitors in the series.

Leucine-rich repeat kinase 2 deficiency is protective in rhabdomyolysis-induced kidney injury

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common known genetic cause of Parkinson's disease, and LRRK2 is also linked to Crohn's and Hansen's disease. LRRK2 is expressed in many organs in mammals but is particularly abundant in the kidney. We find that LRRK2 protein is predominantly localized to collecting duct cells in the rat kidney, with much lower expression in other kidney cells. While genetic knockout (KO) of LRRK2 expression is well-tolerated in mice and rats, a unique age-dependent pathology develops in the kidney. The cortex and medulla of LRRK2 KO rat kidneys become darkly pigmented in early adulthood, yet aged animals display no overt signs of kidney failure. Accompanying the dark pigment we find substantial macrophage infiltration in LRRK2 KO kidneys, suggesting the presence of chronic inflammation that may predispose to kidney disease. Unexpectedly, the dark kidneys of the LRRK2 KO rats are highly resistant to rhabdomyolysis-induced acute kidney injury compared with wild-type rats. Biochemical profiling of the LRRK2 KO kidneys using immunohistochemistry, proteomic and lipidomic analyses show a massive accumulation of hemoglobin and lipofuscin in renal tubules that account for the pigmentation. The proximal tubules demonstrate a corresponding up-regulation of the cytoprotective protein heme oxygenase-1 (HO-1) which is capable of mitigating acute kidney injury. The unusual kidney pathology of LRRK2 KO rats highlights several novel physiological roles for LRRK2 and provides indirect evidence for HO-1 expression as a protective mechanism in acute kidney injury in LRRK2 deficiency.

PI3K/Akt-independent NOS/HO activation accounts for the facilitatory effect of nicotine on acetylcholine renal vasodilations: modulation by ovarian hormones

We investigated the effect of chronic nicotine on cholinergically-mediated renal vasodilations in female rats and its modulation by the nitric oxide synthase (NOS)/heme oxygenase (HO) pathways. Dose-vasodilatory response curves of acetylcholine (0.01–2.43 nmol) were established in isolated phenylephrine-preconstricted perfused kidneys obtained from rats treated with or without nicotine (0.5–4.0 mg/kg/day, 2 weeks). Acetylcholine vasodilations were potentiated by low nicotine doses (0.5 and 1 mg/kg/day) in contrast to no effect for higher doses (2 and 4 mg/kg/day). The facilitatory effect of nicotine was acetylcholine specific because it was not observed with other vasodilators such as 5′-N-ethylcarboxamidoadenosine (NECA, adenosine receptor agonist) or papaverine. Increases in NOS and HO-1 activities appear to mediate the nicotine-evoked enhancement of acetylcholine vasodilation because the latter was compromised after pharmacologic inhibition of NOS (L-NAME) or HO-1 (zinc protoporphyrin, ZnPP). The renal protein expression of phosphorylated Akt was not affected by nicotine. We also show that the presence of the two ovarian hormones is necessary for the nicotine augmentation of acetylcholine vasodilations to manifest because nicotine facilitation was lost in kidneys of ovariectomized (OVX) and restored after combined, but not individual, supplementation with medroxyprogesterone acetate (MPA) and estrogen (E2). Together, the data suggests that chronic nicotine potentiates acetylcholine renal vasodilation in female rats via, at least partly, Akt-independent HO-1 upregulation. The facilitatory effect of nicotine is dose dependent and requires the presence of the two ovarian hormones.

Heme oxygenase-1 regulates the progression of K/BxN serum transfer arthritis

Background

Heme oxygenase-1 (HO-1) is induced in many cell types as a defense mechanism against stress. We have investigated the possible role of endogenous HO-1 in the effector phase of arthritis using the K/BxN serum transfer model of arthritis in HO-1 heterozygous and homozygous knock-out mice.

Methodology/Principal Findings

Arthritis was induced in C57/Black-6 xFVB (HO-1^+/+, HO-1^+/− and HO-1^−/−) mice by intraperitoneal injection of 150 µl serum from arthritic K/BxN mice at days 0 and 2. Blood was collected and animals were sacrificed at day 10. Histological analysis was performed in ankle sections. The levels of inflammatory mediators were measured in serum and paw homogenates by enzyme-linked immunosorbent assay or Multiplex technology. The incidence of arthritis was higher in HO-1^+/− and HO-1^−/− groups compared with HO-1^+/+. The inflammatory response was aggravated in HO-1^+/− mice as shown by arthritic score and the migration of inflammatory cells that could be related to the enhancement of CXCL-1 production. In addition, the HO-1^+/− group showed proteoglycan depletion significantly higher than HO-1^+/+ mice. Serum levels of matrix metalloproteinase-3, monocyte chemotactic protein-1, plasminogen activator inhibitor-1, E-selectin and intercellular adhesion molecule-1 were increased in arthritic HO-1^−/− mice, whereas vascular endothelial growth factor and some cytokines such as interferon-γ showed a reduction compared to HO-1^+/+ or HO-1^+/− mice. In addition, down-regulated gene expression of ferritin, glutathione S-reductase A1 and superoxide dismutase-2 was observed in the livers of arthritic HO-1^+/− animals.

Conclusion/Significance

Endogenous HO-1 regulates the production of systemic and local inflammatory mediators and plays a protective role in K/BxN serum transfer arthritis.

Structural insights into human heme oxygenase-1 inhibition by potent and selective azole-based compounds

The development of heme oxygenase (HO) inhibitors, especially those that are isozyme-selective, promises powerful pharmacological tools to elucidate the regulatory characteristics of the HO system. It is already known that HO has cytoprotective properties and may play a role in several disease states, making it an enticing therapeutic target. Traditionally, the metalloporphyrins have been used as competitive HO inhibitors owing to their structural similarity with the substrate, heme. However, given heme's important role in several other proteins (e.g. cytochromes P450, nitric oxide synthase), non-selectivity is an unfortunate side-effect. Reports that azalanstat and other non-porphyrin molecules inhibited HO led to a multi-faceted effort to develop novel compounds as potent, selective inhibitors of HO. This resulted in the creation of non-competitive inhibitors with selectivity for HO, including a subset with isozyme selectivity for HO-1. Using X-ray crystallography, the structures of several complexes of HO-1 with novel inhibitors have been elucidated, which provided insightful information regarding the salient features required for inhibitor binding. This included the structural basis for non-competitive inhibition, flexibility and adaptability of the inhibitor binding pocket, and multiple, potential interaction subsites, all of which can be exploited in future drug-design strategies.

Actions of "antioxidants" in the protection against atherosclerosis

This review addresses the role of oxidative processes in atherosclerosis and its resulting cardiovascular disease by focusing on the outcome of antioxidant interventions. Although there is unambiguous evidence for the presence of heightened oxidative stress and resulting damage in atherosclerosis, it remains to be established whether this represents a cause or a consequence of the disease. This critical question is complicated further by the increasing realization that oxidative processes, including those related to signaling, are part of normal cell function. Overall, the results from animal interventions suggest that antioxidants provide benefit neither generally nor consistently. Where benefit is observed, it appears to be achieved at least in part via modulation of biological processes such as increase in nitric oxide bioavailability and induction of protective enzymes such as heme oxygenase-1, rather than via inhibition of oxidative processes and lipid oxidation in the arterial wall. Exceptions to this may be situations of multiple/excessive stress, the relevance of which for humans is not clear. This interpretation is consistent with the overall disappointing outcome of antioxidant interventions in humans and can be rationalized by the spatial compartmentalization of cellular oxidative signaling and/or damage, complex roles of oxidant-producing enzymes, and the multifactorial nature of atherosclerosis.

Up-regulation of the inflammatory response by ovariectomy in collagen-induced arthritis. effects of tin protoporphyrin IX

We have studied the influence of ovariectomy on the inflammatory response and bone metabolism on CIA as a model of postmenopausal arthritis as well as the effects of tin protoporphyrin IX (SnPP), a heme oxygenase inhibitor. Ovariectomy in non-arthritic mice produced increased serum PGD2 levels and up-regulated the expression of COX-2, h-PGDS, l-PGDS, and HO-1 in the joints. In CIA, ovariectomy potentiated the inflammatory response with higher levels of serum IL-6 and MMP-3, local PGD2 and MMP-3 as well as trabecular bone erosion. In OVX-CIA, SnPP decreased the serum levels of IL-6, MMP-3, and PGD2; down-regulated TNFα, COX-2, hPGDS, PGD2, PGE2, and MMP-3 in joint tissues; and also decreased focal bone loss in the inflamed joint. Ovariectomy up-regulates inflammatory mediators in non-arthritic and in arthritic animals. In the OVX-CIA model, SnPP exerts anti-inflammatory effects which are not associated with the prevention of systemic bone loss.

Metalloporphyrins - an update

Metalloporphyrins are structural analogs of heme and their potential use in the management of neonatal hyperbilirubinemia has been the subject of considerable research for more than three decades. The pharmacological basis for using this class of compounds to control bilirubin levels is the targeted blockade of bilirubin production through the competitive inhibition of heme oxygenase (HO), the rate-limiting enzyme in the bilirubin production pathway. Ongoing research continues in the pursuit of identifying ideal metalloporphyrins, which are safe and effective, by defining therapeutic windows and targeted interventions for the treatment of excessive neonatal hyperbilirubinemia.

A novel, "double-clamp" binding mode for human heme oxygenase-1 inhibition

The development of heme oxygenase (HO) inhibitors is critical in dissecting and understanding the HO system and for potential therapeutic applications. We have established a program to design and optimize HO inhibitors using structure-activity relationships in conjunction with X-ray crystallographic analyses. One of our previous complex crystal structures revealed a putative secondary hydrophobic binding pocket which could be exploited for a new design strategy by introducing a functional group that would fit into this potential site. To test this hypothesis and gain further insights into the structural basis of inhibitor binding, we have synthesized and characterized 1-(1H-imidazol-1-yl)-4,4-diphenyl-2-butanone (QC-308). Using a carbon monoxide (CO) formation assay on rat spleen microsomes, the compound was found to be ∼15 times more potent (IC₅₀ = 0.27±0.07 µM) than its monophenyl analogue, which is already a potent compound in its own right (QC-65; IC₅₀ = 4.0±1.8 µM). The crystal structure of hHO-1 with QC-308 revealed that the second phenyl group in the western region of the compound is indeed accommodated by a definitive secondary proximal hydrophobic pocket. Thus, the two phenyl moieties are each stabilized by distinct hydrophobic pockets. This “double-clamp” binding offers additional inhibitor stabilization and provides a new route for improvement of human heme oxygenase inhibitors.

Heme oxygenase-1 overexpression fails to attenuate hypertension when the nitric oxide synthase system is not fully operative

Heme oxygenase (HO) is an enzyme that is involved in numerous secondary actions. One of its products, CO, seems to have an important but unclear role in blood pressure regulation. CO exhibits a vasodilator action through the activation of soluble guanylate cyclase and the subsequent production of cyclic guanosine monophosphate (cGMP). The aim of the present study was to determine whether pathological and pharmacological HO-1 overexpression has any regulatory role on blood pressure in a renovascular model of hypertension. We examined the effect of zinc protoporyphyrin IX (ZnPP-IX) administration, an inhibitor of HO activity, on mean arterial pressure (MAP) and heart rate in sham-operated and aorta-coarcted (AC) rats and its interaction with the nitric oxide synthase (NOS) pathway. Inhibition of HO increased MAP in normotensive rats with and without hemin pretreatment but not in hypertensive rats. Pretreatment with NG-nitro-L-arginine methyl ester blocked the pressor response to ZnPP-IX, suggesting a key role of NOS in the cardiovascular action of HO inhibition. In the same way, AC rats, an experimental model of hypertension with impaired function and low expression of endothelial NOS (eNOS), did not show any cardiovascular response to inhibition or induction of HO. This finding suggests that eNOS was necessary for modulating the CO response in the hypertensive group. In conclusion, the present study suggests that HO regulates blood pressure through CO only when the NOS pathway is fully operative. In addition, chronic HO induction fails to attenuate the hypertensive stage induced by coarctation as a consequence of the impairment of the NOS pathway.

Tin protoporphyrin provides protection following cerebral hypoxia-ischemia: involvement of alternative pathways

The contribution of heme oxygenase (HO)-linked pathways to neurodegeneration following cerebral hypoxia-ischemia (HI) remains unclear. We investigated whether HO modulators affected HI-induced brain damage and explored potential mechanisms involved. HI was induced in 26-day-old male Wistar rats by left common carotid artery ligation, followed by exposure to a humidified atmosphere of 8% oxygen for 1 hr. Tin protoporphyrin (SnPP; an HO inhibitor), ferriprotoporphyrin (FePP; an HO inducer), or saline was administered intraperitoneally once daily from 1 day prior to HI until sacrifice at 3 days post-HI. SnPP reduced (P < 0.05) infarct volume compared with saline-treated animals, but FePP had no effect on brain injury. SnPP did not significantly inhibit HO activity at 3 days post-HI, but SnPP increased (P < 0.001) total nitric oxide synthase (NOS) activity compared with HI + saline. Both inducible NOS and cyclooxygenase activities were attenuated (P < 0.05) by SnPP, whereas mitochondrial complex I and V activities were augmented (P < 0.05) by SnPP. SnPP had no effect on NMDA receptor currents. Overall, like other HO inhibitors, SnPP produced many nonselective effects, such as attenuation of inflammatory enzymes and increased mitochondrial respiratory function, which were associated with a protective response 3 days post-HI.

Haem oxygenase-1 and cardiovascular disease: mechanisms and therapeutic potential

Cardiovascular disease remains the leading cause of death worldwide. Despite progress in management, there remain a significant number of patients who are not eligible for current treatment options. Traditionally, HO-1 (haem oxygenase-1), one of two isoenzymes that initiate haem catabolism, was thought to only play a metabolic role. However, HO-1 is now recognized to have additional protective activities in states of heightened noxious stimuli or stress such as acute coronary syndromes. The present review article provides an overview of the mode of action of HO-1 in vascular protection, with particular emphasis on its atheroprotective, anti-inflammatory and antioxidative properties, as well as its role in vascular repair. Furthermore, we present evidence for the protective effects of HO-1 in CVD (cardiovascular disease) in both animal and human studies. Given its potential in vascular protection and repair, strategies aimed at inducing HO-1 emerge as a novel and alternative therapeutic target in the management of CVD.

Targeting heme oxygenase-1 in vascular disease

Heme oxygenase-1 (HO-1) metabolizes heme to generate carbon monoxide (CO), biliverdin, and iron. Biliverdin is subsequently metabolized to bilirubin by biliverdin reductase. HO-1 has recently emerged as a promising therapeutic target in the treatment of vascular disease. Pharmacological induction or gene transfer of HO-1 ameliorates vascular dysfunction in animal models of atherosclerosis, post-angioplasty restenosis, vein graft stenosis, thrombosis, myocardial infarction, and hypertension, while inhibition of HO-1 activity or gene deletion exacerbates these disorders. The vasoprotection afforded by HO-1 is largely attributable to its end products: CO and the bile pigments, biliverdin and bilirubin. These end products exert potent anti-inflammatory, antioxidant, anti-apoptotic, and anti-thrombotic actions. In addition, CO and bile pigments act to preserve vascular homeostasis at sites of arterial injury by influencing the proliferation, migration, and adhesion of vascular smooth muscle cells, endothelial cells, endothelial progenitor cells, or leukocytes. Several strategies are currently being developed to target HO-1 in vascular disease. Pharmacological induction of HO-1 by heme derivatives, dietary antioxidants, or currently available drugs, is a promising near-term approach, while HO-1 gene delivery is a long-term therapeutic goal. Direct administration of CO via inhalation or through the use of CO-releasing molecules and/or CO-sensitizing agents provides an attractive alternative approach in targeting HO-1. Furthermore, delivery of bile pigments, either alone or in combination with CO, presents another avenue for protecting against vascular disease. Since HO-1 and its products are potentially toxic, a major challenge will be to devise clinically effective therapeutic modalities that target HO-1 without causing any adverse effects.

Heme oxygenase inhibition by 1-aryl-2-(1h-imidazol-1-yl/1h-1,2,4-triazol-1-yl)ethanones and their derivatives

Previous studies by our research group have been concerned with the design of selective inhibitors of heme oxygenases (HO-1 and HO-2). The majority of these were based on a four-carbon linkage of an azole, usually an imidazole, and an aromatic moiety. In the present study, we designed and synthesized a series of inhibition candidates containing a shorter linkage between these groups, specifically, a series of 1-aryl-2-(1H-imidazol-1-yl/1H-1,2,4-triazol-1-yl)ethanones and their derivatives. As regards HO-1 inhibition, the aromatic moieties yielding best results were found to be halogen-substituted residues such as 3-bromophenyl, 4-bromophenyl, and 3,4-dichlorophenyl, or hydrocarbon residues such as 2-naphthyl, 4-biphenyl, 4-benzylphenyl, and 4-(2-phenethyl)phenyl. Among the imidazole-ketones, five (36-39, and 44) were found to be very potent (IC(50)<5 muM) toward both isozymes. Relative to the imidazole-ketones, the series of corresponding triazole-ketones showed four compounds (54, 55, 61, and 62) having a selectivity index >50 in favor of HO-1. In the case of the azole-dioxolanes, two of them (80 and 85), each possessing a 2-naphthyl moiety, were found to be particularly potent and selective HO-1 inhibitors. Three non-carbonyl analogues (87, 89, and 91) of 1-(4-chlorophenyl)-2-(1H-imidazol-1-yl)ethanone were found to be good inhibitors of HO-1. For the first time in our studies, two azole-based inhibitors (37 and 39) were found to exhibit a modest selectivity index in favor of HO-2. The present study has revealed additional candidates based on inhibition of heme oxygenases for potentially useful pharmacological and therapeutic applications.

The effects of azole-based heme oxygenase inhibitors on rat cytochromes P450 2E1 and 3A1/2 and human cytochromes P450 3A4 and 2D6

Heme oxygenases (HOs) catalyze the degradation of heme to biliverdin, carbon monoxide (CO), and free iron. The two major isoforms, HO-1 (inducible) and HO-2 (constitutive), are involved in a variety of physiological functions, including inflammation, apoptosis, neuromodulation, and vascular regulation. Major tools used in exploring these actions have been metalloporphyrin analogs of heme that inhibit the HOs. However, these tools are limited by their lack of selectivity; they affect other heme-dependent enzymes, such as cytochromes P450 (P450s), soluble guanylyl cyclase (sGC), and nitric-oxide synthase (NOS). Our laboratory has successfully synthesized a number of nonporphyrin azole-based HO inhibitors (QC-xx) that had little or no effect on sGC and NOS activity. However, their effects on various P450 isoforms have yet to be fully elucidated. To determine the effects of the QC-xx inhibitors on P450 enzyme activity, microsomal preparations of two rat P450 isoforms (2E1 and 3A1/3A2) and two human P450 supersome isoforms (3A4 and 2D6) were incubated with varying concentrations of HO inhibitor, and the activity was determined by spectrophotometric or fluorometric analysis. Results indicated that some QC compounds demonstrated little to no inhibition of the P450s, whereas others did inhibit these P450 isoforms. Four structural regions of QC-xx were analyzed, leading to the identification of structures that confer a decreased effect on both rat and human P450 isoforms studied while maintaining an inhibitory effect on the HOs.

Isocyanides inhibit human heme oxygenases at the verdoheme stage

Heme oxygenases (HO) catalyze the oxidative cleavage of heme to generate biliverdin, CO, and free iron. In humans, heme oxygenase-1 (hHO-1) is overexpressed in tumor tissues, where it helps to protect cancer cells from anticancer agents, while HOs in fungal pathogens, such as Candida albicans, function as the primary means of iron acquisition. Thus, HO can be considered a potential therapeutic target for certain diseases. In this study, we have examined the equilibrium binding of three isocyanides, isopropyl, n-butyl, and benzyl, to the two major human HO isoforms (hHO-1 and hHO-2), Candida albicans HO (CaHmx1), and human cytochrome P450 CYP3A4 using electronic absorption spectroscopy. Isocyanides coordinate to both ferric and ferrous HO-bound heme, with tighter binding by the more hydrophobic isocyanides and 200-300-fold tighter binding to the ferrous form. Benzyl isocyanide was the strongest ligand to ferrous heme in all the enzymes. Because the dissociation constants (KD) of the ligands for ferrous heme-hHO-1 were below the limit of accuracy for equilibrium titrations, stopped-flow kinetic experiments were used to measure the binding parameters of the isocyanides to ferrous hHO-1. Steady-state activity assays showed that benzyl isocyanide was the most potent uncompetitive inhibitor with respect to heme with a KI = 0.15 microM for hHO-1. Importantly, single turnover assays revealed that the reaction was completely stopped by coordination of the isocyanide to the verdoheme intermediate rather than to the ferric heme complex. Much tighter binding of the inhibitor to the verdoheme intermediate differentiates it from inhibition of, for example, CYP3A4 and offers a possible route to more selective inhibitor design.

Heme reversibly damps PERIOD2 rhythms in mouse suprachiasmatic nucleus explants

The hypothalamic suprachiasmatic nucleus (SCN), which in mammals serves as the master circadian pacemaker by synchronizing autonomous clocks in peripheral tissues, is composed of coupled single-cell oscillators that are driven by interlocking positive/negative transcriptional/translational feedback loops. Several studies have suggested that heme, a common prosthetic group that is synthesized and degraded in a circadian manner in the SCN, may modulate the function of several feedback loop components, including the REV-ERB nuclear receptors and PERIOD2 (PER2). We found that ferric heme (hemin, 3-100 microM) dose-dependently and reversibly damped luminescence rhythms in SCN explants from mice expressing a PER2::LUCIFERASE (PER2::LUC) fusion protein. Inhibitors of heme oxygenases (HOs, which degrade heme to biliverdin, carbon monoxide, and iron) mimicked heme's effects on PER2 rhythms. In contrast, heme and HO inhibition did not damp luminescence rhythms in thymus and esophagus explants and had only a small effect on PER2::LUC damping in spleen explants, suggesting that heme's effects are tissue-specific. Analysis of the effects of heme's degradation products on SCN PER2::LUC rhythms indicated that they probably were not responsible for heme's effects on rhythms. The heme synthesis inhibitor N-methylprotoporphyrinIX (NMP) lengthened the circadian period of SCN PER2::LUC rhythms by about an hour. These data are consistent with an important role for heme in the circadian system.

Heme oxygenase-1 accelerates cutaneous wound healing in mice

Heme oxygenase-1 (HO-1), a cytoprotective, pro-angiogenic and anti-inflammatory enzyme, is strongly induced in injured tissues. Our aim was to clarify its role in cutaneous wound healing. In wild type mice, maximal expression of HO-1 in the skin was observed on the 2(nd) and 3(rd) days after wounding. Inhibition of HO-1 by tin protoporphyrin-IX resulted in retardation of wound closure. Healing was also delayed in HO-1 deficient mice, where lack of HO-1 could lead to complete suppression of reepithelialization and to formation of extensive skin lesions, accompanied by impaired neovascularization. Experiments performed in transgenic mice bearing HO-1 under control of keratin 14 promoter showed that increased level of HO-1 in keratinocytes is enough to improve the neovascularization and hasten the closure of wounds. Importantly, induction of HO-1 in wounded skin was relatively weak and delayed in diabetic (db/db) mice, in which also angiogenesis and wound closure were impaired. In such animals local delivery of HO-1 transgene using adenoviral vectors accelerated the wound healing and increased the vascularization. In summary, induction of HO-1 is necessary for efficient wound closure and neovascularization. Impaired wound healing in diabetic mice may be associated with delayed HO-1 upregulation and can be improved by HO-1 gene transfer.

Inhibitors of the heme oxygenase - carbon monoxide system: on the doorstep of the clinic?

The past decade has seen substantial developments in our understanding of the physiology, pathology, and pharmacology of heme oxygenases (HO), to the point that investigators in the field are beginning to contemplate therapies based on administration of HO agonists or HO inhibitors. A significant amount of our current knowledge is based on the judicious application of metalloporphyrin inhibitors of HO, despite their limitations of selectivity. Recently, imidazole-based compounds have been identified as potent and more selective HO inhibitors. This 'next generation' of HO inhibitors offers a number of desirable characteristics, including isozyme selectivity, negligible effects on HO protein expression, and physicochemical properties favourable for in vivo distribution. Some of the applications of HO inhibitors that have been suggested are treatment of hyperbilirubinemia, neurodegenerative disorders, certain types of cancer, and bacterial and fungal infections. In this review, we address various approaches to altering HO activity with a focus on the potential applications of second-generation inhibitors of HO.

Modulation of heme oxygenase/carbon monoxide system affects the inhibitory neurotransmission involved in gastrointestinal motility of streptozotocin-treated diabetic rats

Alterations in gastrointestinal motility of diabetic patients have been linked to degenerative changes induced by glucose abnormalities in the peripheral nervous system. The heme oxygenase/carbon monoxide (HO/CO) signalling represents one of the non-adrenergic/non-cholinergic (NANC) neurotransmission pathways involved in regulation of physiological peristalsis. To investigate the role of HO/CO system in intestinal motility under diabetic conditions, the response to electrical field stimulation (EFS) and western blot analysis of HO/CO pathway components were studied on duodenum longitudinal smooth muscle strips isolated from streptozotocin (STZ)-treated diabetic rats (65 mg kg(-1), i.p.) and respective controls (CTRL), 6 weeks after the onset of diabetes. When compared to CTRL, the ability of CO releasing molecule (CORM-3) (100-400 micromol L(-1)) to enhance NANC relaxation was significantly impaired in STZ-treated rats (P < 0.05). Conversely, in vitro incubation with the HO inhibitor ZnPPIX (10 micromol L(-1), 60 min) significantly reduced EFS-induced relaxation in CTRL (P < 0.05), but not in STZ-treated rats. Interestingly, the ability of ZnPPIX to inhibit EFS-induced relaxation was partially restored in STZ-treated rats co-administered in vivo with the HO-1 inducer cobalt protoporphyrin IX (CoPPIX) (0.5 mg per 100 g body weight weekly). Expression of inducible HO-1 protein was increased in homogenates from STZ-treated rats (vs CTRL, P < 0.01), and further increased in STZ-treated rats receiving CoPPIX (P < 0.05). Taken together, our data underline the essential role of HO/CO system in regulation of inhibitory NANC neurotransmission in the duodenum and suggest that dysregulation of HO/CO activity may represent one mechanism by which gastrointestinal motility is altered in diabetes.

Heme oxygenase-1 prevents airway mucus hypersecretion induced by cigarette smoke in rodents and humans

We investigated the role of heme oxygenase-1 (HO-1), a powerful anti-inflammatory and anti-oxidant enzyme, in modulating cigarette smoke (CS)-induced mucus secretion. In both rats and mice, 5-day CS exposure increased HO-1 expression and activity, mucus secretion, MUCIN 5AC (MUC5AC) gene and protein expression, and local inflammation, along with up-regulation of dual oxidase 1 gene expression and both the activity and phosphorylation of the epidermal growth factor receptor, which is involved in MUC5AC induction. Pharmacological induction of HO-1 prevented these actions and inhibition of HO-1 expression by a specific siRNA potentiated them. In French participants to the European Community Respiratory Health Survey II (n = 210, 30 to 53 years of age, 50% males) exposed to CS, a significant increase in the percentage of participants with chronic sputum was observed in those harboring at least one allele with a long (GT)(n) in the HO-1 promoter gene (>33 repeats), which is associated with a low level of HO-1 protein expression, compared with those with a short number of (GT)n repeats (21.7% versus 8.6%, P = 0.047). No such results were observed in those who had never smoked (n = 297). We conclude that HO-1 has a significant protective effect against airway mucus hypersecretion in animals and humans exposed to CS.

Investigation of a possible interaction between the heme oxygenase/biliverdin reductase and nitric oxide synthase pathway in murine gastric fundus and jejunum

This study investigated the possible interaction between the heme oxygenase (HO)/biliverdin reductase (BVR) and nitric oxide synthase (NOS) pathway in murine gastric fundus and jejunum, since previous studies have shown that both HO-2 and BVR are expressed in interstitial cells of Cajal (ICCs) and co-localized with neuronal NOS in a large proportion of myenteric neurons along the gastrointestinal tract. Neither HO inhibition by chromium mesoporphyrin (CrMP) nor co-incubation with CO or biliverdin/bilirubin affected nitrergic neurotransmission - i.e. relaxations induced by non-adrenergic non-cholinergic (NANC) nerve stimulation or exogenous NO - under normal physiological conditions. However, biliverdin/bilirubin reversed the inhibitory effect of the superoxide generator LY83583 on exogenous NO-induced relaxations in both tissues. When gastric fundus muscle strips were depleted of the endogenous antioxidant Cu/Zn superoxide dismutase (SOD) by the Cu-chelator DETCA, electrically induced NANC relaxations were also affected by LY82583; however, biliverdin/bilirubin could not substitute for the loss of Cu/Zn SOD when this specific antioxidant enzyme was depleted. In jejunal muscle strips, the combination DETCA plus LY83583 nearly abolished contractile phasic activity and, hence, did not allow studying nitrergic relaxation in these experimental conditions. In conclusion, this study does not establish a role for HO/CO in inhibitory NANC neurotransmission in murine gastric fundus and jejunum under normal physiological conditions. However, the antioxidants biliverdin/bilirubin might play an important role in the protection of the nitrergic neurotransmitter against oxidative stress.

Inhibition of plasminogen activator inhibitor-1 expression in vascular smooth muscle cells by protoporphyrins through a heme oxygenase-independent mechanism

Heme oxygenase-1 (HO-1), a rate-limiting enzyme in heme catabolism, has been shown to play a regulatory role in the expression of plasminogen activator inhibitor-1 (PAI-1), a risk factor for vascular disease. Accordingly, we examined the effect of protoporphyrins, both HO inhibitors and activators, on PAI-1 expression in human vascular smooth muscle cells (VSMCs). Tin-protoporphyrin (SnPP) markedly inhibited the transforming growth factor beta1 (TGFbeta1)-induced expression of PAI-1 protein. Protoporphyrins, whether they are inhibitors or activators of HO, produced a similar inhibitory effect. However, SnPP had no effect on the level of PAI-1 mRNA transcripts. Knockdown of human HO-1 with a specific siRNA did not reduce the PAI-1 protein level in TGFbeta1-treated cells. In addition, the proteasome inhibitor lactacystin reversed the inhibitory effect of SnPP on PAI-1 protein expression. Both cobalt-protoporphyrin (CoPP) and CoCl2 markedly induced HO-1 expression. However, CoPP did not affect PAI-1 gene expression, whereas CoCl2 upregulated PAI-1 mRNA in a dose-dependent manner. Our results demonstrate that protoporphyrins can block the TGFbeta1-mediated induction of PAI-1 protein in VSMCs and that this inhibitory effect is independent of HO activity.

Pharmacologic induction of heme oxygenase 1 reduces acute inflammatory arthritis in mice

OBJECTIVE

To determine the consequences of pharmacologic up-regulation of heme oxygenase 1 (HO-1), and inhibition of HO-1 by injection of an anti-HO-1 small interfering RNA (siRNA), in vivo in the acute phase of a mouse model of nonautoimmune arthritis.

METHODS

In the K/BxN mouse serum transfer model, which mimics human inflammatory arthritis without lymphocyte influence, HO-1 was up-regulated by intraperitoneal injection of cobalt protoporphyrin IX (CoPP), a potent pharmacologic inducer, and was inhibited using a specific siRNA. The clinical progress of arthritis was monitored by measurement of paw thickness. Interleukin-1beta (IL-1beta), IL-6, tumor necrosis factor alpha (TNFalpha), serum antioxidant, and nitric oxide (NO) levels, prostaglandin E(2) (PGE(2)) production, and matrix metalloproteinase 9 (MMP-9) activity were measured in serum. At the end of the experiments, joints were examined for immunohistopathologic changes.

RESULTS

Intraperitoneal injection of CoPP alleviated disease symptoms, such as joint swelling, cartilage degradation, and proliferation of inflammatory tissue in joints, in the acute phase of inflammatory arthritis. The CoPP-induced expression of HO-1 in the joints and liver was associated with marked decreases in IL-1beta, IL-6, and TNFalpha levels, PGE(2) secretion, and MMP-9 activity in serum, and with a marked increase in systemic antioxidant activity. In contrast, NO production in serum and inducible NO synthase expression in chondrocytes were not affected by HO-1 induction. Specific inhibition of HO-1 by in vivo delivery of anti-HO-1 siRNA repressed the protective effects.

CONCLUSION

Our data provide the first evidence that pharmacologically induced up-regulation of HO-1 triggers a robust protective antiinflammatory response in a model of nonautoimmune arthritis in mice. This suggests that exogenously induced HO-1 may have potential as therapy in the acute phase of inflammatory arthritis in humans.

Renal hemodynamic, inflammatory, and apoptotic responses to lipopolysaccharide in HO-1-/- mice

Lipopolysaccharide (LPS) induces the stress-responsive gene heme oxygenase-1 (HO-1). The present study examined the significance of HO-1 in response to LPS. In HO-1(-/-) mice, as compared with HO-1(+/+) mice, LPS provoked a greater reduction in glomerular filtration rate and renal blood flow, increased renal cytokine expression, and increased activation of nuclear factor (NF)-kappaB. Conversely, HO-1-overexpressing renal epithelial cells, exposed to LPS, exhibited a blunted activation of NF-kappaB and less phosphorylation of its inhibitor, IkappaB. In HO-1(-/-) mice, as compared with HO-1(+/+) mice, LPS provoked markedly greater elevations in serum levels of Th1 cytokines, Th2 cytokines, chemokines, and cytokines that stimulate bone marrow progenitors. The liver, a major source of serum cytokines, showed an increased activation of NF-kappaB in LPS-treated HO-1(-/-) mice. In addition, LPS provoked widespread apoptosis of immune cells in the spleen and thymus in HO-1(-/-) mice but not in HO-1(+/+) mice. We conclude that HO-1 deficiency exhibits a heightened and dysregulated inflammatory response to LPS accompanied by greater impairment in renal hemodynamic response and widespread apoptosis of immune cells. Because polymorphisms in the HO-1 gene with diminished HO activity predispose to human disease, we speculate that our findings may be relevant to the clinical outcome in patients with sepsis syndromes.

Role of carbon monoxide in electrically induced non-adrenergic, non-cholinergic relaxations in the guinea-pig isolated whole trachea

BACKGROUND AND PURPOSE

Nitric oxide (NO) and vasoactive intestinal peptide (VIP) are considered transmitters of non-adrenergic, non-cholinergic (NANC) relaxations in guinea-pig trachea, whereas the role of carbon monoxide (CO) is unknown. This study was designed to assess the participation of CO, and to investigate the localization of haem oxygenase-2 (HO-2), the CO-producing enzyme, in tracheal neurons.

EXPERIMENTAL APPROACH

NANC responses to electrical field stimulation (EFS) at 3 and 10 Hz were evaluated in epithelium-free whole tracheal segments as intraluminal pressure changes. Drugs used were: L-nitroarginine methyl ester (L-NAME, 100 microM) to inhibit NO synthase (NOS), alpha-chymotrypsin (2 U ml(-1)) to inactivate VIP, zinc protoporphyrin-IX (ZnPP-IX, 10 microM) to inhibit HO-2, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM), a soluble guanylyl cyclase inhibitor. For immunohistochemistry, tissues were exposed to antibodies to PGP 9.5, a general neuronal marker, HO-2 and NOS, and processed with an indirect immunofluorescence method.

KEY RESULTS

alpha-Chymotrypsin did not affect NANC relaxations. ODQ inhibited NANC responses by about 60%, a value similar to that obtained by combining L-NAME and ZnPP-IX. The combination of ODQ, L-NAME and ZnPP-IX reduced the responses by 90%. Subpopulations of HO-2 positive neurons containing NOS were detected in tracheal sections.

CONCLUSIONS AND IMPLICATIONS

In the guinea-pig trachea, NANC inhibitory responses at 3 and 10 Hz use NO and CO as main transmitters. Their participation is revealed following inhibition of NOS, HO-2 and soluble guanylyl cyclase. The involvement of CO as a relaxing transmitter paves the way for novel therapeutic approaches in the treatment of airway obstruction.

Effect of heme oxygenase-1 on the vulnerability of astrocytes and neurons to hemoglobin

The heme oxygenase (HO) enzymes catalyze the rate-limiting step of heme breakdown. Prior studies have demonstrated that the vulnerability of neurons and astrocytes to hemoglobin is modified in cells lacking HO-2, the constitutive isoform. The present study assessed the effect of the inducible isoform, HO-1. Wild-type astrocytes treated for 3-5 days with 3-30 microM hemoglobin sustained no loss of viability, as quantified by LDH and MTT assays. The same treatment resulted in death of 25-50% of HO-1 knockout astrocytes, and a 4-fold increase in protein oxidation. Cell injury was attenuated by transfer of the HO-1 gene, but not by bilirubin, the antioxidant heme breakdown product. Conversely, neuronal protein oxidation and cell death after hemoglobin exposure were similar in wild-type and HO-1 knockout cultures. These results suggest that HO-1 induction protects astrocytes from the oxidative toxicity of Hb, but has no effect on neuronal injury.

Anomalous renal effects of tin protoporphyrin in a murine model of sickle cell disease

In human and murine models of sickle cell disease (SCD), heme oxygenase-1 (HO-1) is induced in the kidney, an organ commonly involved in SCD. The present study assessed the role of HO-1 by using a competitive inhibitor of HO activity, tin protoporphyrin (SnPP), in protocols affording a composite, clinically relevant analysis of the kidney in SCD under unstressed and stressed conditions. Whereas short-term administration of SnPP exerted comparable renal hemodynamic effects in wild-type and sickle mice, chronic administration of SnPP exerted divergent effects: SnPP provoked tubulointerstitial inflammation and up-regulation of injury-related genes in wild-type mice, whereas in sickle mice SnPP reduced expression of injury-related genes and vascular congestion without provoking tubulointerstitial inflammation. SnPP also protected against the heightened sensitivity to renal ischemia observed in sickle mice, preventing ischemia-induced worsening of renal injury in sickle mice above that observed in wild-type mice. Effective and comparable inhibition of HO activity by SnPP in wild-type and sickle mice was confirmed. These findings suggest that induction of HO-1, at least as assessed by this approach, may contribute to renal injury in this murine model of SCD and uncover an experimental maneuver that protects the kidney in murine SCD.

Effect of heme and heme oxygenase-1 on vascular endothelial growth factor synthesis and angiogenic potency of human keratinocytes

BACKGROUND

Skin injury leads to the release of heme, a potent prooxidant which is degraded by heme oxygenase-1 (HO-1) to carbon monoxide, iron, and biliverdin, subsequently reduced to bilirubin. Recently the involvement of HO-1 in angiogenesis has been shown; however, the role of heme and HO-1 in wound healing angiogenesis has not been yet investigated.

RESULTS

Treatment of HaCaT keratinocytes with hemin (heme chloride) induced HO-1 expression and activity. The effect of heme on vascular endothelial growth factor (VEGF) synthesis is variable: induction is significant after a short, 6 h treatment with heme, while longer stimulation may attenuate its production. The involvement of HO-1 in VEGF synthesis was confirmed by inhibition of VEGF expression by SnPPIX, a blocker of HO activity and by attenuation of HO-1 mRNA expression with specific siRNA. Importantly, induction of HO-1 by hemin was able to overcome the inhibitory effect of high glucose on VEGF synthesis. Moreover, HO-1 expression was also induced in keratinocytes cultured in hypoxia, with concomitant augmentation of VEGF production, which was further potentiated by hemin stimulation. Accordingly, conditioned media from keratinocytes overexpressing HO-1 enhanced endothelial cell proliferation and augmented formation of capillaries in angiogenic assay in vitro.

CONCLUSIONS

HO-1 is involved in hemin-induced VEGF expression in HaCaT and may play a role in hypoxic regulation of this protein. HO-1 overexpression may be beneficial in restoring the proper synthesis of VEGF disturbed in diabetic conditions.

Cultured astrocytes from heme oxygenase-1 knockout mice are more vulnerable to heme-mediated oxidative injury

Hemin, the oxidized form of heme, is released from hemoglobin after CNS hemorrhage and may contribute to injury to surrounding tissue. The heme oxygenase (HO) enzymes catalyze the breakdown of hemin to biliverdin, carbon monoxide, and ferric iron. Although HO-2, the isoform expressed predominantly in neurons, accelerates heme-mediated neuronal injury, inhibitor studies suggest that HO-1 induction has a protective effect on astrocytes. In the present study, we directly compared the vulnerability of cultured HO-1 knockout and wild-type astrocytes to hemin. Consistent with prior observations, exposure of wild-type cultures to hemin for 24 hr resulted in protein carbonylation and concentration-dependent cell death between 10 and 60 microM, as determined by MTT and lactate dehydrogenase release assays. In cultures prepared from mice lacking the HO-1 gene, oxidative cell injury was approximately doubled. Both protein oxidation and cell death in HO-1 knockout astrocytes were significantly reduced by pretreating cultures with an adenovirus encoding the HO-1 gene prior to hemin exposure. HO-2 expression was observed in both knockout and wild-type cultures and was not altered by HO-1 gene deletion. Cell hemin accumulation after 20 hr hemin exposure was 4.7-fold higher in knockout cells. These results support the hypothesis that HO-1 protects astrocytes from heme-mediated oxidative injury. Selectively increasing its expression in astrocytes may be beneficial after hemorrhagic CNS injuries.

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.

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.

Pulmonary expression of inducible heme-oxygenase after ischemia/reperfusion of the lower extremities in rats

BACKGROUND

Expression of inducible heme-oxygenase (HO-1) has been shown to be increased in various inflammatory disorders, which may confer a protective role. The aim of our study was to assess pulmonary expression of HO-1 after ischemia/reperfusion (I/R) of the lower limbs in rats.

MATERIALS AND METHODS

We compared three groups of rats (n = 5/group): one Sham group, and two I/R groups (aorta cross-clamped for 2 h followed by 2 h of reperfusion), one of which pre-treated with Zn-protoporphyrin (Zn-PP), a competitive inhibitor of HO (50 micromol/kg, i.p.). At the end of experiment, lungs were harvested for determination of HO activity and HO-1 expression by Western blot and immunohistochemistry. Lung injury was assessed by bronchoalveolar lavage, histological study, and determination of the lung Evans Blue dye content, an index of microvascular permeability.

RESULTS

I/R of the lower limbs was responsible for acute lung injury (ALI), characterized by neutrophilic infiltration (87 +/- 20 x 10(3) neutrophils/mm(3), Sham group versus 191 +/- 38 x 10(3) neutrophils/mm(3), I/R group; P < 0.002) and an increase in lung Evans blue dye content: (74 +/- 6 microg/g, Sham group versus 122 +/- 48 microg/g, I/R group; P < 0.05). Pre-treatment with Zn-PP further increases the Evans Blue content (122 +/- 48 microg/g, I/R group versus 179 +/- 23 microg/g Zn-PP group P < 0.05) and the neutrophilic infiltration. Pulmonary heme-oxygenase activity, and HO-1 content were increased after I/R. (10.5 +/- 12 pmol bilirubin/mg protein/h, Sham group versus 101.2 +/- 66 pmol bilirubin/mg protein/h, I/R group; P < 0.02). Immunohistochemistry revealed that the expression of HO-1 was mainly localized to inflammatory cells.

CONCLUSIONS

ALI following I/R of the lower limbs in rats is associated with an increase of pulmonary expression of HO-1, inhibition of this expression increase the severity of ALI.

Inhibition of endogenous carbon monoxide production induces transient retention losses in the day-old chick when trained using a single trial passive avoidance learning task

Carbon monoxide (CO) is most often thought of as an exogenous toxin rather than as a possible endogenous nootrope. However, a limited number of studies have suggested that CO is necessary in memory processing for at least some tasks. While nitric oxide (NO) and CO are known activators of guanylyl cyclase (GC), only the effect of NO on GC has been extensively investigated as a mechanism underlying memory processing. The aim of the present study was to determine if inhibition of CO production would have an effect on memory processing. Using chicks trained on a single trial passive avoidance task, inhibition of CO production using zinc (II) deuteroporphyrin IX 2,4-bis ethylene glycol (ZnBG; 5 microM) resulted in two transient retention losses occurring at around 40 and 130 min post-training. The timing of these transient retention losses was similar to those observed following inhibition of GC, using the same species and task in a previous study. This supports the notion that CO is necessary in memory processing for this task and may act through a GC-dependent mechanism. As ZnBG also directly inhibits GC or nitric oxide synthase (NOS) at high concentrations, a second experiment was carried-out to confirm the specificity of ZnBG for heme oxygenase (HO) at the concentration used. The action of ZnBG was challenged with the HO agonist hemin (100 microM) and the transient deficits were abolished. This confirmed that the action of ZnBG on memory was through a CO-related mechanism rather than directly on GC or NOS. In this way the specificity of ZnBG (5 microM) for HO could be confirmed. The results support a role for endogenous CO in memory processing, possibly through activation of GC. In addition, the transient retention losses observed following administration of ZnBG suggest that CO may be necessary for memory retrieval and not formation as previously thought.

Guanosine stimulates neurite outgrowth in PC12 cells via activation of heme oxygenase and cyclic GMP

Undifferentiated rat pheochromocytoma (PC12) cells extend neurites when cultured in the presence of nerve growth factor (NGF). Extracellular guanosine synergistically enhances NGF-dependent neurite outgrowth. We investigated the mechanism by which guanosine enhances NGF-dependent neurite outgrowth. Guanosine administration to PC12 cells significantly increased guanosine 3-5-cyclic monophosphate (cGMP) within the first 24 h whereas addition of soluble guanylate cyclase (sGC) inhibitors abolished guanosine-induced enhancement of NGF-dependent neurite outgrowth. sGC may be activated either by nitric oxide (NO) or by carbon monoxide (CO). \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$N^{\omega } $$ \end{document}-Nitro-l-arginine methyl ester (l-NAME), a non-isozyme selective inhibitor of nitric oxide synthase (NOS), had no effect on neurite outgrowth induced by guanosine. Neither nNOS (the constitutive isoform), nor iNOS (the inducible isoform) were expressed in undifferentiated PC12 cells, or under these treatment conditions. These data imply that NO does not mediate the neuritogenic effect of guanosine. Zinc protoporphyrin-IX, an inhibitor of heme oxygenase (HO), reduced guanosine-dependent neurite outgrowth but did not attenuate the effect of NGF. The addition of guanosine plus NGF significantly increased the expression of HO-1, the inducible isozyme of HO, after 12 h. These data demonstrate that guanosine enhances NGF-dependent neurite outgrowth by first activating the constitutive isozyme HO-2, and then by inducing the expression of HO-1, the enzymes responsible for CO synthesis, thus stimulating sGC and increasing intracellular cGMP.

Heme oxygenase-1 modulates early inflammatory responses: evidence from the heme oxygenase-1-deficient mouse

Induction of heme oxygenase-1 (HO-1) is protective in tissue injury in models of allograft rejection and vascular inflammation through either prevention of oxidative damage or via immunomodulatory effects. To examine the specific role of HO-1 in modulating the immune response, we examined the differences in immune phenotype between HO-1 knockout (HO-1(-/-)) and wild-type (HO-1(+/+)) mice. Consistent with previous findings, marked splenomegaly and fibrosis were observed in HO-1(-/-) mice. The lymph nodes of HO-1-deficient mice demonstrated a relative paucity of CD3- and B220-positive cells, but no such abnormalities were observed in the thymus. Flow cytometric analysis of isolated splenocytes demonstrated no differences in the proportions of T lymphocytes, B lymphocytes or monocytes/macrophages between the HO-1(-/-) and HO-1(+/+) mice. Significantly higher baseline serum IgM levels were observed in HO-1(-/-) versus HO-1(+/+) mice. Under mitogen stimulation with either lipopolysaccharide or anti-CD3/anti-CD28, HO-1(-/-) splenocytes secreted disproportionately higher levels of pro-inflammatory Th1 cytokines as compared to those from HO-1(+/+) mice. These findings demonstrate significant differences in the immune phenotype between the HO-1(-/-) and the HO-1(+/+) mice. The absence of HO-1 correlates with a Th1-weighted shift in cytokine responses suggesting a general pro-inflammatory tendency associated with HO-1 deficiency.

Heme oxygenase-2 gene deletion attenuates oxidative stress in neurons exposed to extracellular hemin

Background

Hemin, the oxidized form of heme, accumulates in intracranial hematomas and is a potent oxidant. Growing evidence suggests that it contributes to delayed injury to surrounding tissue, and that this process is affected by the heme oxygenase enzymes. In a prior study, heme oxygenase-2 gene deletion increased the vulnerability of cultured cortical astrocytes to hemin. The present study tested the effect of HO-2 gene deletion on protein oxidation, reactive oxygen species formation, and cell viability after mixed cortical neuron/astrocyte cultures were incubated with neurotoxic concentrations of hemin.

Results

Continuous exposure of wild-type cultures to 1–10 μM hemin for 14 h produced concentration-dependent neuronal death, as detected by both LDH release and fluorescence intensity after propidium iodide staining, with an EC₅₀ of 1–2 μM; astrocytes were not injured by these low hemin concentrations. Cell death was consistently reduced by at least 60% in knockout cultures. Exposure to hemin for 4 hours, a time point that preceded cell lysis, increased protein oxidation in wild-type cultures, as detected by staining of immunoblots for protein carbonyl groups. At 10 μM hemin, carbonylation was increased 2.3-fold compared with control sister cultures subjected to medium exchanges only; this effect was reduced by about two-thirds in knockout cultures. Cellular reactive oxygen species, detected by fluorescence intensity after dihydrorhodamine 123 (DHR) staining, was markedly increased by hemin in wild-type cultures and was localized to neuronal cell bodies and processes. In contrast, DHR fluorescence intensity in knockout cultures did not differ from that of sham-washed controls. Neuronal death in wild-type cultures was almost completely prevented by the lipid-soluble iron chelator phenanthroline; deferoxamine had a weaker but significant effect.

Conclusions

These results suggest that HO-2 gene deletion protects neurons in mixed neuron-astrocyte cultures from heme-mediated oxidative injury. Selective inhibition of neuronal HO-2 may have a beneficial effect after CNS hemorrhage.

Circulatory effects and kinetics following acute administration of carbon monoxide in a porcine model

Carbon monoxide is produced in the endothelial cells and has possible vasodilator activity through three different pathways. The aim of this study was to demonstrate circulatory effects after administration of saturated carbon monoxide blood and to describe the pharmacokinetics of carbon monoxide. Six pigs were anesthetized and 150 ml blood was removed. This blood was bubbled with carbon monoxide until the carboxyhemoglobin (COHb) levels were 90-99%. A specific amount of this blood was then injected back to the animal. At predetermined times; arterial and mixed venous blood was drawn and analyzed for carbon monoxide. Systemic and pulmonary vascular resistance index (SVRi and PVRi) were measured and exhaled air was sampled and measured for carbon monoxide. Blood samples were gathered over 300 minutes along with measurements of invasive pressures, heart rate, cardiac output, oxygen saturation (SpO2), Hb, temperature and blood gases. We conclude that this type of exposure to carbon monoxide appears to have little or no effect on general vasomotor tone and, after correcting for basal levels of carbon monoxide, elimination occurs through the lungs as predicted by a single compartment model. The half-life of carbon monoxide was determined to be 60.5 minutes (SEM 4.7).

Carbon monoxide: to boldly go where NO has gone before

The discovery that nitric oxide (NO) has powerful vasoactive properties identical to those of endothelial-derived relaxing factor spawned a vast body of research investigating the physiological actions of small gas molecules. NO, which arises endogenously through the action of nitric oxide synthase (NOS) enzymes, is a highly reactive gas that plays important roles in the regulation of vascular and immune function. Carbon monoxide (CO), a similar yet much more chemically stable gas, occurs in nature as a product of the oxidation or combustion of organic materials. CO also arises in cells and tissues as a byproduct of heme oxygenase (HO) activity, which degrades heme to biliverdin-IXalpha. Like NO, CO acts as a vasorelaxant and may regulate other vascular functions such as platelet aggregation and smooth muscle proliferation. CO has also been implicated as a neurotransmitter in the central nervous system. HO-1, the inducible form of HO, confers cytoprotection against oxidative stress in vitro and in vivo. CO, when applied at low concentration, exerts potent cytoprotective effects mimicking those of HO-1 induction, including down-regulation of inflammation and suppression of apoptosis. Many of the effects of CO depend on the activation of guanylate cyclase, which generates guanosine 3',5'-monophosphate (cGMP), and the modulation of mitogen-activated protein kinase (MAPK) signaling pathways. This review highlights new advances in the interaction of CO with cellular signaling processes.