Carbon monoxide promotes Fas/CD95-induced apoptosis in Jurkat cells

On the Question of CO's Ability to Induce HO-1 Expression in Cell Culture: A Comparative Study Using Different CO Sources

With the recognition of the endogenous signaling roles and pharmacological functions of carbon monoxide (CO), there is an increasing need to understand CO's mechanism of actions. Along this line, chemical donors have been introduced as CO surrogates for ease of delivery, dosage control, and sometimes the ability to target. Among all of the donors, two ruthenium-carbonyl complexes, CORM-2 and -3, are arguably the most commonly used tools for about 20 years in studying the mechanism of actions of CO. Largely based on data using these two CORMs, there has been a widely accepted inference that the upregulation of heme oxygenase-1 (HO-1) expression is one of the key mechanisms for CO's actions. However, recent years have seen reports of very pronounced chemical reactivities and CO-independent activities of these CORMs. We are interested in examining this question by conducting comparative studies using CO gas, CORM-2/-3, and organic CO donors in RAW264.7, HeLa, and HepG2 cell cultures. CORM-2 and CORM-3 treatment showed significant dose-dependent induction of HO-1 compared to "controls," while incubation for 6 h with 250-500 ppm CO gas did not increase the HO-1 protein expression and mRNA transcription level. A further increase of the CO concentration to 5% did not lead to HO-1 expression either. Additionally, we demonstrate that CORM-2/-3 releases minimal amounts of CO under the experimental conditions. These results indicate that the HO-1 induction effects of CORM-2/-3 are not attributable to CO. We also assessed two organic CO prodrugs, BW-CO-103 and BW-CO-111. BW-CO-111 but not BW-CO-103 dose-dependently increased HO-1 levels in RAW264.7 and HeLa cells. We subsequently studied the mechanism of induction with an Nrf2-luciferase reporter assay, showing that the HO-1 induction activity is likely due to the activation of Nrf2 by the CO donors. Overall, CO alone is unable to induce HO-1 or activate Nrf2 under various conditions in vitro. As such, there is no evidence to support attributing the HO-1 induction effect of the CO donors such as CORM-2/-3 and BW-CO-111 in cell culture to CO. This comparative study demonstrates the critical need to consider possible CO-independent effects of a chemical CO donor before attributing the observed biological effects to CO. It is also important to note that such in vitro results cannot be directly extrapolated to in vivo studies because of the increased level of complexity and the likelihood of secondary and/or synergistic effects in the latter.

The Triple Crown: NO, CO, and H2S in cancer cell biology

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are three endogenously produced gases with important functions in the vasculature, immune defense, and inflammation. It is increasingly apparent that, far from working in isolation, these three exert many effects by modulating each other's activity. Each gas is produced by three enzymes, which have some tissue specificities and can also be non-enzymatically produced by redox reactions of various substrates. Both NO and CO share similar properties, such as activating soluble guanylate cyclase (sGC) to increase cyclic guanosine monophosphate (cGMP) levels. At the same time, H2S both inhibits phosphodiesterase 5A (PDE5A), an enzyme that metabolizes sGC and exerts redox regulation on sGC. The role of NO, CO, and H2S in the setting of cancer has been quite perplexing, as there is evidence for both tumor-promoting and pro-inflammatory effects and anti-tumor and anti-inflammatory activities. Each gasotransmitter has been found to have dual effects on different aspects of cancer biology, including cancer cell proliferation and apoptosis, invasion and metastasis, angiogenesis, and immunomodulation. These seemingly contradictory actions may relate to each gas having a dual effect dependent on its local flux. In this review, we discuss the major roles of NO, CO, and H2S in the context of cancer, with an effort to highlight the dual nature of each gas in different events occurring during cancer progression.

Role of Butylphthalide in Immunity and Inflammation: Butylphthalide May Be a Potential Therapy for Anti-Inflammation and Immunoregulation

Inflammation and immunity play an essential role in disease pathogenesis. 3-N-Butylphthalide (NBP), a group of compounds extracted from seeds of Apium graveolens (Chinese celery), has been demonstrated as an efficient and effective therapy for ischemic stroke. The amount of research on NBP protective effect is increasing at pace, such as microcircular reconstruction, alleviating inflammation, ameliorating brain edema and blood-brain barrier (BBB) damage, mitochondrial function protection, antiplatelet aggregation, antithrombosis, decreasing oxidative damage, and reducing neural cell apoptosis. There has been increasing research emphasizing the association between NBP and immunity and inflammation in the past few years. Hence, it is aimed at reviewing the related literature and summarizing the underlying anti-inflammatory and immunoregulatory function of NBP in various disorders.

Carbon monoxide and a change of heart

The relationship between carbon monoxide and the heart has been extensively studied in both clinical and preclinical settings. The Food and Drug Administration (FDA) is keenly focused on the ill effects of carbon monoxide on the heart when presented with proposals for clinical trials to evaluate efficacy of this gasotransmitter in a various disease settings. This review provides an overview of the rationale that examines the actions of the FDA when considering clinical testing of CO, and contrast that with the continued accumulation of data that clearly show not only that CO can be used safely, but is potently cardioprotective in clinically relevant small and large animal models. Data emerging from Phase I and Phase II clinical trials argues against CO being dangerous to the heart and thus it needs to be redefined and evaluated as any other substance being proposed for use in humans. More than twenty years ago, the belief that CO could be used as a salutary molecule was ridiculed by experts in physiology and medicine. Like all agents designed for use in humans, careful pharmacology and safety are paramount, but continuing to hinder progress based on long-standing dogma in the absence of data is improper. Now, CO is being tested in multiple clinical trials using innovative delivery methods and has proven to be safe. The hope, based on compelling preclinical data, is that it will continue to be evaluated and ultimately approved as an effective therapeutic.

Light-Activated Carbon Monoxide Prodrugs Based on Bipyridyl Dicarbonyl Ruthenium(II) Complexes

Two photoactivatable dicarbonyl ruthenium(II) complexes based on an amide-functionalised bipyridine scaffold (4-position) equipped with an alkyne functionality or a green-fluorescent BODIPY (boron-dipyrromethene) dye have been prepared and used to investigate their light-induced decarbonylation. UV/Vis, FTIR and 13 C NMR spectroscopies as well as gas chromatography and multivariate curve resolution alternating least-squares analysis (MCR-ALS) were used to elucidate the mechanism of the decarbonylation process. Release of the first CO molecule occurs very quickly, while release of the second CO molecule proceeds more slowly. In vitro studies using two cell lines A431 (human squamous carcinoma) and HEK293 (human embryonic kidney cells) have been carried out in order to characterise the anti-proliferative and anti-apoptotic activities. The BODIPY-labelled compound allows for monitoring the cellular uptake, showing fast internalisation kinetics and accumulation at the endoplasmic reticulum and mitochondria.

Involvement of heme oxygenase-1 in suppression of T cell activation by quercetin

AIM

Acute rejection is still a major problem in transplantation and one of the most important causes of late graft loss. Cyclosporine and tacrolimus are widely used for suppression of T cell function to avoid graft rejection, but long-term use of these compounds is associated with serious toxicities. Quercetin, a flavonoid found in fruits and vegetables, has been demonstrated to exhibit cytoprotective effects through the induction of heme oxygenase (HO) -1, an enzyme involved in heme catabolism. We hypothesized that quercetin induces HO-1 in T cells and suppresses T cell function via HO-1. In the present study, we showed that quercetin suppressed the A23187-mediated expression of interleukin (IL) -2 in T cells.

METHODS

Mouse splenocytes, enriched T cells, and EL4 cells, a mouse T cell line, were treated with quercetin, and then stimulated with A23187, a calcium ionophore, concanavalin A, or anti-CD3ε and anti-CD28 antibodies. Cell proliferation, expression of IL-2, calcium mobilization, apoptosis, cell cycle, and phosphorylation of extracellular signal-regulated kinase (ERK) were investigated.

RESULTS

Quercetin induced HO-1, and this induction of HO-1 was implicated in the suppression of IL-2 production. Furthermore, the induction of HO-1 by quercetin suppressed the influx of calcium ions, a known trigger of IL-2 production. Additionally, quercetin suppressed T cell proliferation through promotion of cell cycle arrest via HO-1 induction, but quercetin did not induce apoptosis. To investigate the role of the signal transduction pathway in quercetin's effect on cell proliferation, we evaluated the phosphorylation of ERK in T cells. Quercetin suppressed the A23187-mediated stimulation of ERK, an effect that was mediated through HO-1. These results suggested that HO-1 is involved in the suppressive effects of quercetin on T cell activation and proliferation.

CONCLUSION

Our findings indicate that the quercetin may be a promising candidate for inducing HO-1 in T cells, thereby facilitating immunosuppressive effects.

HO-1 and CD39: It Takes Two to Protect the Realm

Cellular protective mechanisms exist to ensure survival of the cells and are a fundamental feature of all cells that is necessary for adapting to changes in the environment. Indeed, evolution has ensured that each cell is equipped with multiple overlapping families of genes that safeguard against pathogens, injury, stress, and dysfunctional metabolic processes. Two of the better-known enzymatic systems, conserved through all species, include the heme oxygenases (HO-1/HO-2), and the ectonucleotidases (CD39/73). Each of these systems generates critical bioactive products that regulate the cellular response to a stressor. Absence of these molecules results in the cell being extremely predisposed to collapse and, in most cases, results in the death of the cell. Recent reports have begun to link these two metabolic pathways, and what were once exclusively stand-alone are now being found to be intimately interrelated and do so through their innate ability to generate bioactive products including adenosine, carbon monoxide, and bilirubin. These simple small molecules elicit profound cellular physiologic responses that impact a number of innate immune responses, and participate in the regulation of inflammation and tissue repair. Collectively these enzymes are linked not only because of the mitochondria being the source of their substrates, but perhaps more importantly, because of the impact of their products on specific cellular responses. This review will provide a synopsis of the current state of the field regarding how these systems are linked and how they are now being leveraged as therapeutic modalities in the clinic.

Regulatory role of heme oxygenase-1 in silica-induced lung injury

Background

Silicosis, a progressive inflammatory lung disease attributed mainly to occupational exposure to silica dust, shows loss of lung function even after cessation of exposure. In addition to conventional evaluation methods such as chest X-ray, computed tomography, and spirometry, we identified heme oxygenase (HO)-1, an inducible antioxidant, as a potential biomarker to identify at-risk patients. We found that HO-1 was critical in attenuating the disease progression of silicosis; however, the key signaling pathway has not yet been elucidated. Here, we report the critical pathway after silica exposure, focusing on the role of silica-derived reactive oxygen species (ROS) signaling and its attenuation, which is mediated by HO-1 induction, in vivo and in vitro.

Methods

Normal bronchial epithelial cells and a macrophage cell line, as well as a murine silicosis model generated by intratracheal administration of 2.5 mg of crystalline silica, were used in this study. The pathways activated in response to silica exposure, including the mitogen-activated protein kinase (MAPK) signaling pathway, were examined and compared with or without super-induction of HO-1.

Results

The murine silicosis model was first assessed for the evaluation of activated pathways after silica exposure, focusing on ROS-MAPK activation. In the murine model, increased expression of HO-1 in the lungs was observed after silica-instillation. Moreover, silica-medicated activation of extracellular signal-regulated kinase (ERK) in the lungs was attenuated in response to silica-induced HO-1 upregulation. Activation of other MAPKs, such as p38 and c-Jun N-terminal kinase pathways, after silica exposure was not significantly different irrespective of HO-1 induction. Further in vitro studies showed that 1) silica-induced HO-1 was significantly attenuated by inhibiting ERK activation, and 2) carbon monoxide and bilirubin as final byproducts of HO-1 could inhibit ERK activation. Taken together, silica-induced HO-1 upregulation was mediated by ERK activation, and HO-1 further regulates ERK activation via its final byproducts, carbon monoxide and bilirubin.

Conclusions

This is the first study to demonstrate the regulatory role of HO-1 in silicosis. This finding could contribute to the development of a treatment strategy of monitoring HO-1 levels as a marker of therapeutic intervention.

Electronic supplementary material

The online version of this article (10.1186/s12931-018-0852-6) contains supplementary material, which is available to authorized users.

Gasotransmitters and the immune system: Mode of action and novel therapeutic targets

Gasotransmitters are a group of gaseous molecules, with pleiotropic biological functions. These molecules include nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO). Abnormal production and metabolism of these molecules have been observed in several pathological conditions. The understanding of the role of gasotransmitters in the immune system has grown significantly in the past years, and independent studies have shed light on the effect of exogenous and endogenous gasotransmitters on immune responses. Moreover, encouraging results come from the efficacy of NO-, CO- and H₂S -donors in preclinical animal models of autoimmune, acute and chronic inflammatory diseases. To date, data on the influence of gasotransmitters in immunity and immunopathology are often scattered and partial, and the scarcity of clinical trials using NO-, CO- and H₂S -donors, reveals that more effort is warranted. This review focuses on the role of gasotransmitters in the immune system and covers the evidences on the possible use of gasotransmitters for the treatment of inflammatory conditions.

Attenuation of Antioxidant Capacity in Human Breast Cancer Cells by Carbon Monoxide through Inhibition of Cystathionine β-Synthase Activity: Implications in Chemotherapeutic Drug Sensitivity

Drug resistance is a major impediment to effective treatment of breast cancer. Compared to normal cells, cancer cells have an increased antioxidant potential due to an increased ratio of reduced to oxidized glutathione (GSH/GSSG). This is known to confer therapeutic resistance. Here, we have identified a mechanism, unique to breast cancer cells, whereby cystathionine β-synthase (CBS) promotes elevated GSH/GSSG. Lentiviral silencing of CBS in human breast cancer cells attenuated GSH/GSSG, total GSH, nuclear factor erythroid 2-related factor 2 (Nrf2), and processes downstream of Nrf2 that promote GSH synthesis and regeneration of GSH from GSSG. Carbon monoxide (CO) reduced GSH/GSSG in three breast cancer cell lines by inhibiting CBS. Furthermore, CO sensitized breast cancer cells to doxorubicin. These results provide insight into mechanism(s) by which CBS increases the antioxidant potential and the ability for CO to inhibit CBS activity to alter redox homeostasis in breast cancer, increasing sensitivity to a chemotherapeutic.

Heme oxygenase-1 inhibits basophil maturation and activation but promotes its apoptosis in T helper type 2-mediated allergic airway inflammation

The anti-inflammatory role of heme oxygenase-1 (HO-1) has been studied extensively in many disease models including asthma. Many cell types are anti-inflammatory targets of HO-1, such as dendritic cells and regulatory T cells. In contrast to previous reports that HO-1 had limited effects on basophils, which participate in T helper type 2 immune responses and antigen-induced allergic airway inflammation, we demonstrated in this study, for the first time, that the up-regulation of HO-1 significantly suppressed the maturation of mouse basophils, decreased the expression of CD40, CD80, MHC-II and activation marker CD200R on basophils, blocked DQ-ovalbumin uptake and promoted basophil apoptosis both in vitro and in vivo, leading to the inhibition of T helper type 2 polarization. These effects of HO-1 were mimicked by exogenous carbon monoxide, which is one of the catalytic products of HO-1. Furthermore, adoptive transfer of HO-1-modified basophils reduced ovalbumin-induced allergic airway inflammation. The above effects of HO-1 can be reversed by the HO-1 inhibitor Sn-protoporphyrin IX. Moreover, conditional depletion of basophils accompanying hemin treatment further attenuated airway inflammation compared with the hemin group, indicating that the protective role of HO-1 may involve multiple immune cells. Collectively, our findings demonstrated that HO-1 exerted its anti-inflammatory function through suppression of basophil maturation and activation, but promotion of basophil apoptosis, providing a possible novel therapeutic target in allergic asthma.

Low intensity-pulsed ultrasound induced apoptosis of human hepatocellular carcinoma cells in vitro

The present study was conducted to determine whether low intensity-pulsed ultrasound (LIPUS) could induce apoptosis of human hepatocellular carcinoma cells, SMMC-7721, and to define the mechanism of ultrasound-induced apoptosis, in vitro. MTT assay was used to measure cell proliferation. Apoptosis was investigated by multiple methods such as flow cytometry, DNA fragmentation, Ca(2+) mobilizations, pro- and anti-apoptotic protein expression, and light as well as ultramicroscopic morphology. The results provide evidence that LIPUS induced a dose-dependent effect on cell viability and apoptosis of SMMC-7721 cells. Specifically, exposure of cells to >0.5 W/cm(2) intensity significantly increased cell apoptosis, caused shifts in cell cycle phase, and induced structural changes. Ultrasound significantly increased intracellular Ca(2+) concentrations and modulated expression of caspase-3, Bcl-2 and Bax. The findings suggest that this novel technology can be used to induce SMMC-7721 apoptosis via the Ca(2+)/mitochondrial pathway and could potentially be of clinical use for the treatment of hepatocellular carcinoma (SMMC-7721 cell line) and other cancers.

Carbon monoxide in the treatment of sepsis

Carbon monoxide (CO), a low-molecular-weight gas, is endogenously produced in the body as a product of heme degradation catalyzed by heme oxygenase (HO) enzymes. As the beneficial roles of HO system have been elucidated in vitro and in vivo, CO itself has also been reported as a potent cytoprotective molecule. Whereas CO represents a toxic inhalation hazard at high concentration, low-dose exogenous CO treatment (~250-500 parts per million) demonstrates protective functions including but not limited to the anti-inflammatory and antiapoptotic effects in preclinical models of human diseases. Of note, CO exposure confers protection in animal models of sepsis by inhibiting inflammatory responses and also enhancing bacterial phagocytosis in leukocytes. These unique functions of CO including both dampening inflammation and promoting host defense mechanism are mediated by multiple pathways such as autophagy induction or biosynthesis of specialized proresolving lipid mediators. We suggest that CO gas may represent a novel therapy for patients with sepsis.

Synthesis and assessment of CO-release capacity of manganese carbonyl complexes derived from rigid α-diimine ligands of varied complexity

Four manganese carbonyl complexes of the type [MnBr(CO)₃(NˆN)] (NˆN = α-diimine ligands) namely [MnBr(CO)₃(bpy)] (1), [MnBr(CO)₃(phen)] (2), [MnBr(CO)₃(dafo)] (3) and [MnBr(CO)₃(pyzphen)] (4) (where bpy = bipyridine, phen = 1,10-phenanthroline, dafo = 4,5-diazafluoren-9-one and pyzphen = pyrazino[2,3-f][1,10]-phenanthroline) have been synthesized and structurally characterized. These four complexes containing the fac-[Mn(CO)₃] motif release CO upon illumination with low power visible and UV light. The CO release rates and the absorption maxima of the complexes are however very similar despite systematic increase in structural complexity in the rigid α-diimine ligand frames. This is quite in contrary to manganese carbonyl complexes derived from α-diimine ligands in which at least one of the imine functions is not part of the rigid ring systems. Results of this study will provide help in the future design of ligand frames suitable for the syntheses of photoCORMs to deliver CO to biological targets under the control of light.

Salvianolic acids attenuate rat hippocampal injury after acute CO poisoning by improving blood flow properties

Carbon monoxide (CO) poisoning causes the major injury and death due to poisoning worldwide. The most severe damage via CO poisoning is brain injury and mortality. Delayed encephalopathy after acute CO poisoning (DEACMP) occurs in forty percent of the survivors of acute CO exposure. But the pathological cause for DEACMP is not well understood. And the corresponding therapy is not well developed. In order to investigate the effects of salvianolic acid (SA) on brain injury caused by CO exposure from the view point of hemorheology, we employed a rat model and studied the dynamic of blood changes in the hemorheological and coagulative properties over acute CO exposure. Compared with the groups of CO and 20% mannitol + CO treatments, the severe hippocampal injury caused by acute CO exposure was prevented by SA treatment. These protective effects were associated with the retaining level of hematocrit (Hct), plasma viscosity, fibrinogen, whole blood viscosities and malondialdehyde (MDA) levels in red blood cells (RBCs). These results indicated that SA treatment could significantly improve the deformation of erythrocytes and prevent the damage caused by CO poisoning. Meanwhile, hemorheological indexes are good indicators for monitoring the pathological dynamic after acute CO poisoning.

Synthesis and Characterization of a "Turn-On" photoCORM for Trackable CO Delivery to Biological Targets

A designed photoactive CO releasing molecule (photoCORM), namely, fac-[MnBr(CO)3(pbt)] (1, pbt = 2-(2-pyridyl)benzothiazole), promotes CO-induced death of MDA-MB-231 human breast cancer cells upon illumination with broadband visible light. The CO release from this photoCORM can be tracked by rise in fluorescence within the cellular matrix due to deligation of the pbt ligand. The results of this study suggest the potential of 1 in eradication of cancer cells through CO delivery.

Photodelivery of CO by designed PhotoCORMs: correlation between absorption in the visible region and metal-CO bond labilization in carbonyl complexes

The therapeutic potential of photoactive CO-releasing molecules (photoCORMs) have called for close examination of the roles of the ligand(s) and the central metal atoms on the overall photochemical labilization of the metal-CO bonds. Along this line, we have synthesized four metal complexes, namely, [MnBr(azpy)(CO)3 ] (1), [Mn(azpy)(CO)3 (PPh3 )]ClO4 (2), [ReBr(azpy)(CO)3 ] (3), and [Re(azpy)(CO)3 (PPh3 )]ClO4 (4), derived from 2-phenylazopyridine. These complexes were characterized by spectroscopic and crystallographic studies. Although both 1 and 3 exhibit strong metal-to-ligand charge-transfer bands in the 500-600 nm region, only 1 photoreleases CO upon illumination with visible light. Results of theoretical studies were used to gain insight into this surprising difference. Strong spin-orbit coupling (prominent in heavy metals) appears to promote intersystem crossing to a triplet state in 3, a step that discourages CO release upon illumination with visible light. Slow release of CO from 2 and 4 also indicates that strong σ-donating ligands, such as Br(-) , accelerate the rate of CO photorelease relative to π-acid ligands, such as PPh3 .

Delayed inhaled carbon monoxide mediates the regression of established neointimal lesions

OBJECTIVE

Intimal hyperplasia (IH) contributes to the failure of vascular interventions. While many investigational therapies inhibit the development of IH in animal models, few of these potential therapies can reverse established lesions. Inhaled carbon monoxide (CO) dramatically inhibits IH in both rats and pigs when given perioperatively. It also prevented the development of pulmonary arterial hypertension in rodents. Interestingly, CO could reverse pulmonary artery structural changes and right heart hemodynamic changes when administered after the establishment of pulmonary hypertension. Thus, we hypothesize that inhaled CO may mediate the regression of established neointimal lesions.

METHODS

Rats underwent carotid artery balloon angioplasty injury. Carotid arteries were collected at 2 and 4 weeks after injury for morphometric analysis of the neointima. Another group was treated with inhaled CO (250 parts per million) for 1 hour daily from week 2 until week 4. Additional rats were sacrificed 3 days after initiating CO treatment, and the carotid arteries were examined for apoptosis by terminal deoxynucleotidyl transferase dUTP nick end-labeling, proliferation by Ki67 staining, and autophagy by microtubule-associated protein light chain 3 I/II staining.

RESULTS

At 2 weeks following injury, sizable neointimal lesions had developed (intimal/media = 0.92 ± 0.22). By 4 weeks, lesion size remained stable (0.80 ± 0.09). Delayed inhaled CO treatment greatly reduced neointimal lesion size vs the 2- and 4-week control mice (0.38 ± 0.05; P < .05). Arteries from the CO-treated rats exhibited significantly reduced apoptosis compared with control vessels (3.18% ± 1.94% vs 16.26% ± 5.91%; P = .036). Proliferation was also dramatically reduced in the CO-treated animals (2.98 ± 1.55 vs 10.37 ± 2.80; P = .036). No difference in autophagy between control and CO-treated rats was detected.

CONCLUSIONS

Delayed administration of inhaled CO reduced established neointimal lesion size. This effect was mediated by the antiproliferative effect of CO on medial and intimal smooth muscle cells without increases in arterial wall apoptosis or autophagy. Future studies will examine additional time points to determine if there is temporal variation in the rates of apoptosis and autophagy.

Gaseous therapeutics in acute lung injury

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) remain major causes of morbidity and mortality in critical care medicine despite advances in therapeutic modalities. ALI can be associated with sepsis, trauma, pharmaceutical or xenobiotic exposures, high oxygen therapy (hyperoxia), and mechanical ventilation. Of the small gas molecules (NO, CO, H₂S) that arise in human beings from endogenous enzymatic activities, the physiological significance of NO is well established, whereas that of CO or H₂S remains controversial. Recent studies have explored the potential efficacy of inhalation therapies using these small gas molecules in animal models of ALI. NO has vasoregulatory and redox-active properties and can function as a selective pulmonary vasodilator. Inhaled NO (iNO) has shown promise as a therapy in animal models of ALI including endotoxin challenge, ischemia/reperfusion (I/R) injury, and lung transplantation. CO, another diatomic gas, can exert cellular tissue protection through antiapoptotic, anti-inflammatory, and antiproliferative effects. CO has shown therapeutic potential in animal models of endotoxin challenge, oxidative lung injury, I/R injury, pulmonary fibrosis, ventilator-induced lung injury, and lung transplantation. H₂S, a third potential therapeutic gas, can induce hypometabolic states in mice and can confer both pro- and anti-inflammatory effects in rodent models of ALI and sepsis. Clinical studies have shown variable results for the efficacy of iNO in lung transplantation and failure for this therapy to improve mortality in ARDS patients. No clinical studies have been conducted with H₂S. The clinical efficacy of CO remains unclear and awaits further controlled clinical studies in transplantation and sepsis.

Mechanism(s) Involved in Carbon Monoxide-releasing Molecule-2-mediated Cardioprotection During Ischaemia-reperfusion Injury in Isolated Rat Heart

The purpose of the present study was to determine the mechanism(s) involved in carbon monoxide-releasing molecule-2, carbon monoxide-releasing molecule-2-induced cardioprotection. We used the transition metal carbonyl compound carbon monoxide-releasing molecule-2 that can act as carbon monoxide donor in cardiac ischaemia-reperfusion injury model using isolated rat heart preparation. Langendorff's perfused rat hearts when treated with carbon monoxide-releasing molecule-2 (50 μM) for 10 min before global ischaemia exhibited significant reduction in postischaemic levels of myocardial injury markers, creatine kinase and lactate dehydrogenase in coronary effluent. Similarly, pretreatment with carbon monoxide-releasing molecule-2 showed significantly improved postischaemic recovery of heart rate, coronary flow rate, cardiodynamic parameters and reduced infarct size as compared to vehicle control hearts. Perfusion with p38 mitogen-activated protein kinase inhibitor, SB203580, a specific inhibitor of α and β isoform, before and concomitantly with carbon monoxide-releasing molecule-2 treatment abolished carbon monoxide-releasing molecule-2-induced cardioprotection. However, p38 mitogen-activated protein kinase alpha inhibitor, SCIO-469, was unable to inhibit the cardioprotective effect of carbon monoxide-releasing molecule-2. Furthermore, protective effect of carbon monoxide-releasing molecule-2 was significantly inhibited by the protein kinase C inhibitor, chelerythrine, when added before and concomitantly with carbon monoxide-releasing molecule-2. It was also observed that, perfusion with phosphatidylinositol 3-kinase inhibitor, wortmannin, before and concomitantly with carbon monoxide-releasing molecule-2 was not able to inhibit carbon monoxide-releasing molecule-2-induced cardioprotection. Interestingly, we observed that wortmannin perfusion before ischaemia and continued till reperfusion significantly inhibited carbon monoxide-releasing molecule-2-mediated cardioprotection. Our findings suggest that the carbon monoxide-releasing molecule-2 treatment may activate the p38 mitogen-activated protein kinase β and protein kinase C pathways before ischaemia and phosphatidylinositol 3-kinase pathway during reperfusion which may be responsible for the carbon monoxide-releasing molecule-2-mediated cardioprotective effect.

Activation of calcium-sensing receptors is associated with apoptosis in a model of simulated cardiomyocytes ischemia/reperfusion

Objective

Calcium-sensing receptors (CaSRs) are G-protein coupled receptors which maintain systemic calcium homeostasis and participate in hormone secretion, activation of ion channels, cell apoptosis, proliferation, and differentiation. Previous studies have shown that CaSRs induce apoptosis in isolated adult rat heart and in normal neonatal rat cardiomyocytes by G-protein-PLC-IP₃ signaling transduction. However, little knowledge is presently available concerning the role of CaSRs in the apoptosis induced by ischemia and reperfusion in neonatal cardiomyocytes.

Methods

Primary neonatal rat ventricular cardiomyocytes were incubated in ischemiamimetic solution for 2 h, and then re-incubated in normal culture medium for 24 h to establish a model of simulated ischemia/reperfusion (I/R). Cardiomyocyte apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). The expression of CaSRs mRNA was detected by real-time reverse transcription polymerase chain reaction (RT-PCR). In addition, the expressions of caspase-3 and Bcl-2 were analyzed by western blot.

Results

The simulated I/R enhanced the expression of CaSRs and cardiomyocyte apoptosis. GdCl₃, a specific activator of CaSRs, further increased the expression of CaSRs and cardiomyocyte apoptosis, along with up-regulation of caspase-3 and down-regulation of Bcl-2.

Conclusion

CaSRs are associated with I/R injury and apoptosis in neonatal rat ventricular cardiomyocytes via suppressing Bcl-2 and promoting caspase-3 expression.

Inflammation and Gliomagenesis: Bi-Directional Communication at Early and Late Stages of Tumor Progression

Inflammation has been closely linked to various forms of cancer. Less is known about the role of inflammation in glioma, especially at the initiation stage. In this review, we first describe the unique features of the immune system in the brain. We then discuss the current understanding of the mechanisms by which glioma cells modulate the immune system, especially how bi-directional communications between immune cells and glioma cells create an immunosuppressed microenvironment that promotes tumor survival and growth. We also address the potential tumor-initiating roles of inflammation in glioma. Finally, we describe several immunotherapy approaches currently being developed to reverse these interactions and stimulate the immune system to eliminate glioma cells.

The immunomodulatory role of carbon monoxide during transplantation

The number of organ and tissue transplants has increased worldwide in recent decades. However, graft rejection, infections due to the use of immunosuppressive drugs and a shortage of graft donors remain major concerns. Carbon monoxide (CO) had long been regarded solely as a poisonous gas. Ultimately, physiological studies unveiled the endogenous production of CO, particularly by the heme oxygenase (HO)-1 enzyme, recognizing CO as a beneficial gas when used at therapeutic doses. The protective properties of CO led researchers to develop uses for it, resulting in devices and molecules that can deliver CO in vitro and in vivo. The resulting interest in clinical investigations was immediate. Studies regarding the CO/HO-1 modulation of immune responses and their effects on various immune disorders gave rise to transplantation research, where CO was shown to be essential in the protection against organ rejection in animal models. This review provides a perspective of how CO modulates the immune system to improve transplantation and suggests its use as a therapy in the field.

The social network of carbon monoxide in medicine

Networking between cells is critical for proper functioning of the cellular milieu and is mediated by cascades of highly regulated and overlapping signaling molecules. The enzyme heme oxygenase-1 (HO-1) generates three separate signaling molecules through the catalysis of heme - carbon monoxide (CO), biliverdin, and iron - each of which acts via distinct molecular targets to influence cell function, both proximally and distally. This review focuses on state-of-the art developments and insights into the impact of HO-1 and CO on the innate immune response, the effects of which are responsible for an ensemble of functions that help regulate complex immunological responses to bacterial sepsis and ischemia/reperfusion injury. HO-1 exemplifies an evolutionarily conserved system necessary for the cellular milieu to adapt appropriately, function properly, and ensure survival of the organism.

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

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

CO metabolism, sensing, and signaling

CO is a colorless and odorless gas produced by the incomplete combustion of hydrocarbons, both of natural and anthropogenic origin. Several microorganisms, including aerobic and anaerobic bacteria and anaerobic archaea, use exogenous CO as a source of carbon and energy for growth. On the other hand, eukaryotic organisms use endogenous CO, produced during heme degradation, as a neurotransmitter and as a signal molecule. CO sensors act as signal transducers by coupling a "regulatory" heme-binding domain to a "functional" signal transmitter. Although high CO concentrations inhibit generally heme-protein actions, low CO levels can influence several signaling pathways, including those regulated by soluble guanylate cyclase and/or mitogen-activated protein kinases. This review summarizes recent insights into CO metabolism, sensing, and signaling.

Protective role of heme oxygenase-1 against inflammation in atherosclerosis

Heme oxygenase-1 (HO-1) catalyzes the first and rate-limiting step in the metabolism of free heme into equimolar amounts of ferrous iron, carbon monoxide (CO), and biliverdin. Biliverdin is subsequently converted to bilirubin by biliverdin reductase. HO-1 has recently been identified as a promising therapeutic target in the treatment of vascular inflammatory disease, including atherosclerosis. HO-1 represses inflammation by removing the pro-inflammatory molecule heme and by generating CO and the bile pigments, biliverdin and bilirubin. These HO-1 reaction products are capable of blocking innate and adaptive immune responses by modifying the activation, differentiation, maturation, and/or polarization of numerous immune cells, including endothelial cells, monocytes/macrophages, dendritic cells, T lymphocytes, mast cells, and platelets. These cellular actions by CO and bile pigments result in diminished leukocyte recruitment and infiltration, and pro-inflammatory mediator production within atherosclerotic lesions. This review highlights the mechanisms by which HO-1 suppresses vascular inflammation in atherosclerosis, and explores possible therapeutic modalities by which HO-1 and its reaction products can be employed to ameliorate vascular inflammatory disease.

Carbon monoxide inhibits Fas activating antibody-induced apoptosis in endothelial cells

Background

The extrinsic apoptotic pathway initiates when a death ligand, such as the Fas ligand, interacts with its cell surface receptor (ie., Fas/CD95), forming a death-inducing signaling complex (DISC). The Fas-dependent apoptotic pathway has been implicated in several models of lung or vascular injury. Carbon monoxide, an enzymatic product of heme oxygenase-1, exerts antiapoptotic effects at low concentration in vitro and in vivo.

Methods

Using mouse lung endothelial cells (MLEC), we examined the antiapoptotic potential of carbon monoxide against apoptosis induced by the Fas/CD95-activating antibody (Jo2). Carbon monoxide was applied to cell cultures in vitro. The expression and/or activation of apoptosis-related proteins and signaling intermediates were determined using Western Immunoblot and co-immunoprecipitation assays. Cell death was monitored by lactate dehydrogenase (LDH) release assays. Statistical significance was determined by student T-test and a value of P < 0.05 was considered significant.

Results

Treatment of MLEC with Fas-activating antibody (Jo2) induced cell death associated with the formation of the DISC, and activation of caspases (-8, -9, and -3), as well as the pro-apoptotic Bcl-2 family protein Bax. Exposure of MLEC to carbon monoxide inhibited Jo2-induced cell death, which correlated with the inhibition of DISC formation, cleavage of caspases-8, -9, and -3, and Bax activation. Carbon monoxide inhibited the phosphorylation of the Fas-associated death domain-containing protein, as well as its association with the DISC. Furthermore, carbon monoxide induced the expression of the antiapoptotic protein FLIP and increased its association with the DISC.

CO-dependent cytoprotection against Fas mediated apoptosis in MLEC depended in part on activation of ERK1/2-dependent signaling.

Conclusions

Carbon monoxide has been proposed as a potential therapy for lung and other diseases based in part on its antiapoptotic effects in endothelial cells. In vitro, carbon monoxide may inhibit both Fas/caspase-8 and Bax-dependent apoptotic signaling pathways induced by Fas-activating antibody in endothelial cells. Strategies to block Fas-dependent apoptotic pathways may be useful in development of therapies for lung or vascular disorders.

Carbon monoxide as an endogenous vascular modulator

Carbon monoxide (CO) is produced by heme oxygenase (HO)-catalyzed heme degradation to CO, iron, and biliverdin. HO has two active isoforms, HO-1 (inducible) and HO-2 (constitutive). HO-2, but not HO-1, is highly expressed in endothelial and smooth muscle cells and in adjacent astrocytes in the brain. HO-1 is expressed basally only in the spleen and liver but can be induced to a varying extent in most tissues. Elevating heme, protein phosphorylation, Ca(2+) influx, and Ca(2+)/calmodulin-dependent processes increase HO-2 activity. CO dilates cerebral arterioles and may constrict or dilate skeletal muscle and renal arterioles. Selected vasodilatory stimuli, including seizures, glutamatergic stimulation, hypoxia, hypotension, and ADP, increase CO, and the inhibition of HO attenuates the dilation to these stimuli. Astrocytic HO-2-derived CO causes glutamatergic dilation of pial arterioles. CO dilates by activating smooth muscle cell large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. CO binds to BK(Ca) channel-bound heme, leading to an increase in Ca(2+) sparks-to-BK(Ca) channel coupling. Also, CO may bind directly to the BK(Ca) channel at several locations. Endothelial nitric oxide and prostacyclin interact with HO/CO in circulatory regulation. In cerebral arterioles in vivo, in contrast to dilation to acute CO, a prolonged exposure of cerebral arterioles to elevated CO produces progressive constriction by inhibiting nitric oxide synthase. The HO/CO system is highly protective to the vasculature. CO suppresses apoptosis and inhibits components of endogenous oxidant-generating pathways. Bilirubin is a potent reactive oxygen species scavenger. Still many questions remain about the physiology and biochemistry of HO/CO in the circulatory system and about the function and dysfunction of this gaseous mediator system.

Role of dopamine D2 receptors in ischemia/reperfusion induced apoptosis of cultured neonatal rat cardiomyocytes

Background

Myocardial ischemia/reperfusion injury is the major cause of morbidity and mortality for cardiovascular diseases. Dopamine D₂ receptors are expressed in cardiac tissues. However, the roles of dopamine D₂ receptors in myocardial ischemia/reperfusion injury and cardiomyocyte apoptosis are unclear. Here we investigated the effects of both dopamine D₂ receptors agonist (bromocriptine) and antagonist (haloperidol) on apoptosis of cultured neonatal rat ventricular myocytes induced by ischemia/reperfusion injury.

Methods

Myocardial ischemia/reperfusion injury was simulated by incubating primarily cultured neonatal rat cardiomyocytes in ischemic (hypoxic) buffer solution for 2 h. Thereafter, these cells were incubated for 24 h in normal culture medium.

Results

Treatment of the cardiomyocytes with 10 μM bromocriptine significantly decreased lactate dehydrogenase activity, increased superoxide dismutase activity, and decreased malondialdehyde content in the culture medium. Bromocriptine significantly inhibited the release of cytochrome c, accumulation of [Ca²⁺]_i, and apoptosis induced by ischemia/reperfusion injury. Bromocriptine also down-regulated the expression of caspase-3 and -9, Fas and Fas ligand, and up-regulated Bcl-2 expression. In contrast, haloperidol (10 μM) had no significant effects on the apoptosis of cultured cardiomyocytes under the aforementioned conditions.

Conclusions

These data suggest that activation of dopamine D₂ receptors can inhibit apoptosis of cardiomyocytes encountered during ischemia/reperfusion damage through various pathways.

Inhibitory effect of hepatocyte growth factor on cardiomyocytes apoptosis is partly related to reduced calcium sensing receptor expression during a model of simulated ischemia/reperfusion

Calcium-sensing receptors (CaSR) are G-protein coupled receptors which maintain systemic calcium haemeostasis, participate in hormone secretion, activation of iron channel, cell apoptosis, proliferation and differentiation. Previous studies have show CaSR induce apoptosis in isolated rat adult heart and in normal rat neonatal cardiomyocytes by G-protein-PLC-IP3 signaling transinduction. A few of studies had demonstrated that CaSR induce apoptosis in cultured neonatal rat cardiomyocytes during ischemia/reperfusion. Hepatocyte growth factor (HGF), as a mesenchymally derived heterodimeric glycoprotein, play vital role in mitogenesis, angiogenesis, cellular motility and growth and anti-apoptosis after postinfarction heart failure via activation of transmembrane tyrosine kinase cell surface receptor c-Met. However, little knowledge exists about whether anti-apoptotic role of HGF in preventing cardiomyocytes injury induced by ischemia/reperfusion is associated with downregulation of CaSR expression. We incubated primary neonatal rat ventricular cardiomyocytes in ischemia-mimetic solution for 2 h, then reincubated them in normal culture medium for 24 h to establish a model of simulated ischemia/reperfusion (I/R). Cardiomyocyte apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. The expression of CaSR mRNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR). In addition, we analyzed the expression of Caspase-3, Bcl-2 and Phosphoinositide 3-kinase (PI3K) by Western blotting. The simulated I/R enhances the expression of CaSR and cardiomyocyte apoptosis. GdCl3, a specific activator of CaSR, further increase the expression of CaSR and Cardiomyocyte apoptosis, along with upregulation of Caspase-3, downregulation of Bcl-2 and inhibiting PI3K phosphorylation. Combination of GdCl3 with LY294002 (a selective PI3K inhibitor) increased Cardiomyocytes apoptosis but did not increased CaSR expression. Treatment of HGF decreased I/R- and GdCl3-induced apoptosis by suppressing Caspase-3 and promoting Bcl-2 and PI3K phosphorylation expression in accordance with downregulation of CaSR expression. HGF exerts protective role in I/R-induced apoptosis at least in part by inhibiting CaSR expression along with promoting Bcl-2, suppressing Caspase-3 expression and stimulating PI3K phosphorylation signaling pathway.

Challenges in clinical design of immunotherapy trials for malignant glioma

Glioblastoma multiforme (GBM) is the most common and lethal primary malignant brain tumor. The traditional treatments for GBM, including surgery, radiation, and chemotherapy, only modestly improve patient survival. Therefore, immunotherapy has emerged as a novel therapeutic modality. Immunotherapeutic strategies exploit the immune system's ability to recognize and mount a specific response against tumor cells, but not normal cells. Current immunotherapeutic approaches for glioma can be divided into 3 categories: immune priming (active immunotherapy), immunomodulation (passive immunotherapy), and adoptive immunotherapy. Immune priming sensitizes the patient's immune cells to tumor antigens using various vaccination protocols. In the case of immunomodulation, strategies are aimed at reducing suppressive cytokines in the tumor microenvironment or using immune molecules to specifically target tumor cells. Adoptive immunotherapy involves harvesting the patient's immune cells, followed by ex vivo activation and expansion before reinfusion. This article provides an overview of the interactions between the central nervous system and the immune system, and discusses the challenges facing current immunotherapeutic strategies.

Immunoregulatory effects of HO-1: how does it work?

The heme-catabolizing enzyme heme oxygenase-1 (HO-1; encoded by the Hmox1 gene) inhibits the pathogenesis of several immune-mediated inflammatory diseases. This unusually broad salutary effect is thought to rely on the immunoregulatory actions of HO-1, exerted on innate and adaptive immune cells. According to this notion, HO-1 'dampens' innate and adaptive immune responses, limiting immune-mediated tissue injury and thus suppressing the pathogenesis of immune-mediated inflammatory diseases. We will argue that the salutary effects of HO-1 are also exerted via its cytoprotective action, which sustains tissue function and prevents unfettered immune activation by endogenous proinflammatory ligands released from injured cells.

Cystathionine beta-synthase as a carbon monoxide-sensitive regulator of bile excretion

Carbon monoxide (CO) is a stress-inducible gas generated by heme oxygenase (HO) eliciting adaptive responses against toxicants; however, mechanisms for its reception remain unknown. Serendipitous observation in metabolome analysis in CO-overproducing livers suggested roles of cystathionine beta-synthase (CBS) that rate-limits transsulfuration pathway and H(2)S generation, for the gas-responsive receptor. Studies using recombinant CBS indicated that CO binds to the prosthetic heme, stabilizing 6-coordinated CO-Fe(II)-histidine complex to block the activity, whereas nitric oxide (NO) forms 5-coordinated structure without inhibiting it. The CO-overproducing livers down-regulated H(2)S to stimulate HCO(3) (-)-dependent choleresis: these responses were attenuated by blocking HO or by donating H(2)S. Livers of heterozygous CBS knockout mice neither down-regulated H(2)S nor exhibited the choleresis while overproducing CO. In the mouse model of estradiol-induced cholestasis, CO overproduction by inducing HO-1 significantly improved the bile output through stimulating HCO(3) (-) excretion; such a choleretic response did not occur in the knockout mice.

CONCLUSION

Results collected from metabolome analyses suggested that CBS serves as a CO-sensitive modulator of H(2)S to support biliary excretion, shedding light on a putative role of the enzyme for stress-elicited adaptive response against bile-dependent detoxification processes.

Environmental toxicity, oxidative stress and apoptosis: ménage à trois

Apoptosis is an evolutionary conserved homeostatic process involved in distinct physiological processes including organ and tissue morphogenesis, development and senescence. Its deregulation is also known to participate in the etiology of several human diseases including cancer, neurodegenerative and autoimmune disorders. Environmental stressors (cytotoxic agents, pollutants or toxicants) are well known to induce apoptotic cell death and to contribute to a variety of pathological conditions. Oxidative stress seems to be the central element in the regulation of the apoptotic pathways triggered by environmental stressors. In this work, we review the established mechanisms by which oxidative stress and environmental stressors regulate the apoptotic machinery with the aim to underscore the relevance of apoptosis as a component in environmental toxicity and human disease progression.

Heme oxygenase-1 and the vascular bed: from molecular mechanisms to therapeutic opportunities

Heme oxygenase-1, an enzyme degrading heme to carbon monoxide, iron, and biliverdin, has been recognized as playing a crucial role in cellular defense against stressful conditions, not only related to heme release. HO-1 protects endothelial cells from apoptosis, is involved in blood-vessel relaxation regulating vascular tone, attenuates inflammatory response in the vessel wall, and participates in blood-vessel formation by means of angiogenesis and vasculogenesis. The latter functions link HO-1 not only to cardiovascular ischemia but also to many other conditions that, like development, wound healing, or cancer, are dependent on neovascularization. The aim of this comprehensive review is to address the mechanisms of HO-1 regulation and function in cardiovascular physiology and pathology and to demonstrate some possible applications of the vast knowledge generated so far. Recent data provide powerful evidence for the involvement of HO-1 in the therapeutic effect of drugs used in cardiovascular diseases. Novel studies open the possibilities of application of HO-1 for gene and cell therapy. Therefore, research in forthcoming years should help to elucidate both the real role of HO-1 in the effect of drugs and the clinical feasibility of HO-1-based cell and gene therapy, creating the effective therapeutic avenues for this refined antioxidant system.

Carbon monoxide, reactive oxygen signaling, and oxidative stress

The ubiquitous gas, carbon monoxide (CO), is of substantial biological importance, but apart from its affinity for reduced transition metals, particularly heme-iron, it is surprisingly nonreactive-as is the ferrous-carbonyl-in living systems. CO does form strong complexes with heme proteins for which molecular O2 is the preferred ligand and to which are attributed diverse physiological, adaptive, and toxic effects. Lately, it has become apparent that both exogenous and endogenous CO produced by heme oxygenase engender a prooxidant milieu in aerobic mammalian cells which initiates signaling related to reactive oxygen species (ROS) generation. ROS signaling contingent on CO can be segregated by CO concentration-time effects on cellular function, by the location of heme proteins, e.g., mitochondrial or nonmitochondrial sites, or by specific oxidation-reduction (redox) reactions. The fundamental responses to CO involve overt physiological regulatory events, such as activation of redox-sensitive transcription factors or stress-activated kinases, which institute compensatory expression of antioxidant enzymes and other adaptations to oxidative stress. In contrast, responses originating from highly elevated or protracted CO exposures tend to be nonspecific, produce untoward biological oxidations, and interfere with homeostasis. This brief overview provides a conceptual framework for understanding CO biology in terms of this physiological-pathological hierarchy.

Carbon monoxide in sepsis

Despite modern practices in critical care medicine, sepsis or systemic inflammatory response syndrome remains a leading cause of morbidity and mortality in the intensive care unit. Thus, the need to identify new therapeutic tools for the treatment of sepsis is urgent. In this context, carbon monoxide has become a promising therapeutic molecule that can potentially prevent uncontrolled inflammation in sepsis. In humans, carbon monoxide arises endogenously from the degradation of heme by heme oxygenase enzymes. Both endogenously synthesized and exogenously applied carbon monoxide can exert antiinflammatory and antiapoptotic effects in cells and tissues. Based on these properties, carbon monoxide, when applied at low concentration, conferred protection in a variety of cellular and rodent models of sepsis, and furthermore reduced morbidity and mortality in vivo. Therefore, application of carbon monoxide may have a major impact on the future of sepsis treatment. This review summarizes evidence for salutary effects of carbon monoxide in sepsis of various organs, including lung, heart, kidney, liver, and intestine, and discusses the potential translation of the data into human clinical trials.

Mechanisms of cell death in oxidative stress

Reactive oxygen or nitrogen species (ROS/RNS) generated endogenously or in response to environmental stress have long been implicated in tissue injury in the context of a variety of disease states. ROS/RNS can cause cell death by nonphysiological (necrotic) or regulated pathways (apoptotic). The mechanisms by which ROS/RNS cause or regulate apoptosis typically include receptor activation, caspase activation, Bcl-2 family proteins, and mitochondrial dysfunction. Various protein kinase activities, including mitogen-activated protein kinases, protein kinases-B/C, inhibitor-of-I-kappaB kinases, and their corresponding phosphatases modulate the apoptotic program depending on cellular context. Recently, lipid-derived mediators have emerged as potential intermediates in the apoptosis pathway triggered by oxidants. Cell death mechanisms have been studied across a broad spectrum of models of oxidative stress, including H2O2, nitric oxide and derivatives, endotoxin-induced inflammation, photodynamic therapy, ultraviolet-A and ionizing radiations, and cigarette smoke. Additionally ROS generated in the lung and other organs as the result of high oxygen therapy or ischemia/reperfusion can stimulate cell death pathways associated with tissue damage. Cells have evolved numerous survival pathways to counter proapoptotic stimuli, which include activation of stress-related protein responses. Among these, the heme oxygenase-1/carbon monoxide system has emerged as a major intracellular antiapoptotic mechanism.

Carbon monoxide and bilirubin: potential therapies for pulmonary/vascular injury and disease

Heme oxygenase (HO)-1, an inducible, low-molecular-weight stress protein, confers cellular and tissue protection in multiple models of injury and disease, including oxidative or inflammatory lung injury, ischemia/reperfusion (I/R) injuries, and vascular injury/disease. The tissue protection provided by HO-1 potentially relates to the endogenous production of the end products of its enzymatic activity: namely, biliverdin (BV)/bilirubin (BR), carbon monoxide (CO), and iron. Of these, CO and BV/BR show promise as possible therapeutic agents when applied exogenously in models of lung or vascular injury. CO activates intracellular signaling pathways that involve soluble guanylate cyclase and/or p38 mitogen-activated protein kinase. Although toxic at elevated concentrations, low concentrations of CO can confer antiinflammatory, antiapoptotic, antiproliferative, and vasodilatory effects. BV and BR are natural antioxidants that can provide protection against oxidative stress in cell culture and in plasma. Application of BV or BR protects against I/R injury in several organ models. Recent evidence has also demonstrated antiinflammatory and antiproliferative properties of these pigments. To date, evidence has accumulated for salutary effects of CO, BV, and/or BR in lung/vascular injury models, as well as in models of transplant-associated I/R injury. Thus, the exogenous application of HO end products may provide an alternative to pharmacologic or gene therapy approaches to harness the therapeutic potential of HO-1.

Distinct signaling pathways in TRAIL- versus tumor necrosis factor-induced apoptosis

Trimeric tumor necrosis factor (TNF) binding leads to recruitment of TRADD to TNFR1. In current models, TRADD recruits RIP, TRAF2, and FADD to activate NF-kappaB, Jun N-terminal protein kinase (JNK), and apoptosis. Using stable short-hairpin RNA (shRNA) knockdown (KD) cells targeting these adaptors, TNF death-inducing signaling complex immunoprecipitation demonstrates competitive binding of TRADD and RIP to TNFR1, whereas TRAF2 recruitment requires TRADD. Analysis of KD cells indicates that FADD is necessary for Fas-L- or TRAIL- but not TNF-induced apoptosis. Interestingly, TRADD is dispensable, while RIP is required for TNF-induced apoptosis in human tumor cells. TRADD is required for c-Jun phosphorylation upon TNF exposure. RIP KD abrogates formation of complex II following TNF exposure, whereas TRADD KD allows efficient RIP-caspase 8 association. Treatment with TRAIL also induces formation of a complex II containing FADD, RIP, IKKalpha, and caspase 8 and 10, leading to activation of caspase 8. Our data suggest that TNF triggers apoptosis in a manner distinct from that of Fas-L or TRAIL.

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.