Role of haem oxygenase-1 in microbial host defence

Does ambient CO have protective effect for COPD patient?

BACKGROUND

Existing studies found paradoxical effects of carbon monoxide (CO) on human health. Carbon monoxide (CO), at high concentrations, is a well-known toxicant, but recent studies suggest that CO at low concentrations may have protective health effects under certain conditions.

OBJECTIVES

To investigate the acute effect of ambient CO on hospital admission for chronic obstructive pulmonary disease (COPD) in Shanghai, China.

METHODS

Daily data on COPD admissions and CO concentrations between 2006 and 2008 were collected. We applied over-dispersed generalized additive Poisson models, adjusted for weather conditions, day of the week and public holidays, long-term and seasonal trends.

RESULTS

During the study period, the average CO concentration was 1.3mg/m(3), well below the international health-based standard. Negative associations were found between ambient CO concentration and daily COPD hospitalization. An interquartile range increase (0.6 mg/m(3)) in CO concentration at lag 3 day corresponded to -2.97% (95% confidence interval: -4.63%, -1.31%) change in COPD hospitalization. The negative associations were robust after adjustment for co-pollutants (PM10, NO2 and SO2). The protective effect of CO appeared to be more evident in the cool season.

CONCLUSION

Short-term exposure to CO at low ambient concentration may be associated with reduced risk of COPD hospitalization. Our results may contribute to a comprehensive understanding on the health effects of ambient CO.

Heme oxygenase-1 acts as an antiviral factor for porcine reproductive and respiratory syndrome virus infection and over-expression inhibits virus replication in vitro

Virus replication depends upon host-cell processes in infected cells, and this is true for porcine reproductive and respiratory syndrome virus (PRRSV), the causative agent of PRRS that is a worldwide threat to the swine industry. Heme oxygenase-1 (HO-1) is a ubiquitously expressed inducible isoform of the first and rate-limiting enzyme for heme degradation. Our previous research suggested that HO-1 may play an important role in PRRSV infection. However, the function of HO-1 in PRRSV infection is unclear. In the present study, Marc-145, PK-15(CD163) cell lines and porcine alveolar macrophages (PAMs) were used to evaluate the effects of HO-1 induction and over-expression on the replication of two different PRRSV strains. Induction of HO-1 markedly decreased the replication of PRRSV strains in the different cells. Similarly, adenoviral-mediated over-expression of HO-1 also greatly decreased the replication of PRRSV. In contrast, ablation of HO-1 using small interfering RNA concomitantly increased PRRSV replication. Therefore, the data were consistent with HO-1 acting as an antiviral factor and these findings suggested that over-expression or induction of HO-1 may provide a potential therapeutic strategy against PRRSV infection.

Ambient carbon monoxide associated with reduced risk of hospital admissions for respiratory tract infections

RATIONALE

Recent experimental and clinical studies suggest that exogenous carbon monoxide (CO) at lower concentrations may have beneficial effects under certain circumstances, whereas population-based epidemiologic studies of environmentally relevant CO exposure generated mixed findings.

OBJECTIVES

To examine the acute effects of ambient CO on respiratory tract infection (RTI) hospitalizations.

METHODS

A time series study was conducted. Daily emergency hospital admission and air pollution data in Hong Kong were collected from January 2001 to December 2007. Log-linear Poisson models were used to estimate the associations between daily hospital admissions for RTI and daily average concentrations of CO across three background air monitoring stations and three roadside stations, respectively, controlling for other traffic-related copollutants.

MEASUREMENTS AND MAIN RESULTS

CO concentrations were low during the study period with a daily average of 0.6 ppm in background stations and 1.0 ppm in roadside stations. Negative associations were found between ambient CO concentrations and daily hospital admissions for RTI. One ppm increase in background CO at lag 0-2 days was associated with -5.7% (95% confidence interval, -9.2 to -2.1) change in RTI admissions from the whole population according to single-pollutant model; the negative association became stronger when nitrogen dioxide or particulate matter with aerodynamic diameter less than 10 μm was adjusted for in two-pollutant models. The negative association seemed to be stronger in the adults than in the children and elderly.

CONCLUSIONS

Short-term exposure to ambient CO was associated with decreased risk of hospital admissions for RTI, suggesting some acute protective effects of low ambient CO exposure on respiratory infection.

The cytoprotective enzyme heme oxygenase-1 suppresses Ebola virus replication

Ebola virus (EBOV) is the causative agent of a severe hemorrhagic fever in humans with reported case fatality rates as high as 90%. There are currently no licensed vaccines or antiviral therapeutics to combat EBOV infections. Heme oxygenase-1 (HO-1), an enzyme that catalyzes the rate-limiting step in heme degradation, has antioxidative properties and protects cells from various stresses. Activated HO-1 was recently shown to have antiviral activity, potently inhibiting the replication of viruses such as hepatitis C virus and human immunodeficiency virus. However, the effect of HO-1 activation on EBOV replication remains unknown. To determine whether the upregulation of HO-1 attenuates EBOV replication, we treated cells with cobalt protoporphyrin (CoPP), a selective HO-1 inducer, and assessed its effects on EBOV replication. We found that CoPP treatment, pre- and postinfection, significantly suppressed EBOV replication in a manner dependent upon HO-1 upregulation and activity. In addition, stable overexpression of HO-1 significantly attenuated EBOV growth. Although the exact mechanism behind the antiviral properties of HO-1 remains to be elucidated, our data show that HO-1 upregulation does not attenuate EBOV entry or budding but specifically targets EBOV transcription/replication. Therefore, modulation of the cellular enzyme HO-1 may represent a novel therapeutic strategy against EBOV infection.

NOX4 induces oxidative stress and apoptosis through upregulation of caspases 3 and 9 and downregulation of TIGAR in HCV-infected Huh-7 cells

Aim: The present study was designed to determine the potential role of oxidative stress in the induction of apoptosis in a transient in vitro model of HCV-infected Huh-7 cells. Material & methods: A transient in vitro infectivity model of a Huh-7 cell line was established using serum from HCV genotype 3a patients. Quantitative expression of selected genes was measured using real-time PCR. Results: A test of the apoptotic responses of cells under stressful conditions showed a significant increase in selected oxidative stress and apoptotic markers, along with a significant decrease in expression of antioxidants following inoculation in a time-dependent manner. A significant decrease in TIGAR and a significant increase in p53 expression levels at day 6 suggested the possible role of p53 and TIGAR in the induction of apoptosis and oxidative stimuli in experimental Huh-7/HCV cell lines. Conclusion: Collectively, the findings of the current study suggest a role for p53 and TIGAR in HCV-induced apoptosis in the presence of oxidative stress in a Huh-7 cell line.

Heme oxygenase-1 induction alters chemokine regulation and ameliorates human immunodeficiency virus-type-1 infection in lipopolysaccharide-stimulated macrophages

We have elucidated a putative mechanism for the host resistance against HIV-1 infection of primary human monocyte-derived macrophages (MDM) stimulated with lipopolysaccharide (LPS). We show that LPS-activated MDM both inhibited HIV-1 entry into the cells and were refractory to post-entry productive viral replication. LPS-treated cells were virtually negative for mature virions as revealed by transmission electron microscopy. LPS activation of MDM markedly enhanced the expression of heme oxygenase-1 (HO-1), a potent inducible cytoprotective enzyme. Increased HO-1 expression was accompanied by elevated production of macrophage inflammatory chemokines (MIP1α and MIP1β) by LPS-activated MDM, significantly decreased surface chemokine receptor-5 (CCR-5) expression, and substantially reduced virus replication. Treatment of cells with HO-1 inhibitor SnPP IX (tin protoporphyrin IX) attenuated the LPS-mediated responses, HIV-1 replication and secretion of MIP1α, MIP1β, and LD78β chemokines with little change in surface CCR-5 expression. These results identify a novel role for HO-1 in the modulation of host immune response against HIV infection of MDM.

Heme oxygenase-1 regulates the immune response to influenza virus infection and vaccination in aged mice

Underlying mechanisms of individual variation in severity of influenza infection and response to vaccination are poorly understood. We investigated the effect of reduced heme oxygenase-1 (HO-1) expression on vaccine response and outcome of influenza infection. HO-1-deficient and wild-type (WT) mice (kingdom, Animalia; phylum, Chordata; genus/species, Mus musculus) were infected with influenza virus A/PR/8/34 with or without prior vaccination with an adenoviral-based influenza vaccine. A genome-wide association study evaluated the expression of single-nucleotide polymorphisms (SNPs) in the HO-1 gene and the response to influenza vaccination in healthy humans. HO-1-deficient mice had decreased survival after influenza infection compared to WT mice (median survival 5.5 vs. 6.5 d, P=0.016). HO-1-deficient mice had impaired production of antibody following influenza vaccination compared to WT mice (mean antibody titer 869 vs. 1698, P=0.02). One SNP in HO-1 and one SNP in the constitutively expressed isoform HO-2 were independently associated with decreased antibody production after influenza vaccination in healthy human volunteers (P=0.017 and 0.014, respectively). HO-1 deficient mice were paired with sex- and age-matched WT controls. HO-1 affects the immune response to both influenza infection and vaccination, suggesting that therapeutic induction of HO-1 expression may represent a novel adjuvant to enhance influenza vaccine effectiveness.

Heme oxygenase-1 promotes the persistence of Leishmania chagasi infection

Visceral leishmaniasis (VL) remains a major public health problem worldwide. This disease is highly associated with chronic inflammation and a lack of the cellular immune responses against Leishmania. It is important to identify major factors driving the successful establishment of the Leishmania infection to develop better tools for the disease control. Heme oxygenase-1 (HO-1) is a key enzyme triggered by cellular stress, and its role in VL has not been investigated. In this study, we evaluated the role of HO-1 in the infection by Leishmania infantum chagasi, the causative agent of VL cases in Brazil. We found that L. chagasi infection or lipophosphoglycan isolated from promastigotes triggered HO-1 production by murine macrophages. Interestingly, cobalt protoporphyrin IX, an HO-1 inductor, increased the parasite burden in both mouse and human-derived macrophages. Upon L. chagasi infection, macrophages from Hmox1 knockout mice presented significantly lower parasite loads when compared with those from wild-type mice. Furthermore, upregulation of HO-1 by cobalt protoporphyrin IX diminished the production of TNF-α and reactive oxygen species by infected murine macrophages and increased Cu/Zn superoxide dismutase expression in human monocytes. Finally, patients with VL presented higher systemic concentrations of HO-1 than healthy individuals, and this increase of HO-1 was reduced after antileishmanial treatment, suggesting that HO-1 is associated with disease susceptibility. Our data argue that HO-1 has a critical role in the L. chagasi infection and is strongly associated with the inflammatory imbalance during VL. Manipulation of HO-1 pathways during VL could serve as an adjunctive therapeutic approach.

Regulation of inducible heme oxygenase and cyclooxygenase isozymes in a mouse model of spotted fever group rickettsiosis

Vascular endothelial cells (ECs) lining the blood vessels are the preferred primary targets of pathogenic Rickettsia species in the host. In response to oxidative stress triggered by infection, ECs launch defense mechanisms such as expression of heme oxygenase-1 (HO-1). Previous evidence from an established animal model of Rocky Mountain spotted fever also suggests selective modulation of anti-oxidant enzyme activities in the target host tissues. In this study, we have examined the expression profiles of HO-1 and COX-2 in different tissues during Rickettsia conorii infection of susceptible C3H/HeN mice. RNA hybridization with murine HO-1 and COX-2-specific complementary DNA probes revealed increased HO-1 expression in the liver and brain of mice infected with three different doses of R. conorii ranging from 2.25×10(3) to 2.25×10(5) pfu, relatively non-remarkable changes in the lungs, and a trend for down-regulation in the spleen. The most prominent HO-1 response was evident in the liver with ∼4-fold increase on day 4 post-infection, followed by a decline on day 7. HO-1 expression in the brain, however, peaked with significantly higher levels on day 7. Following infection with both sub-lethal as well as lethal doses of infection, the transcript encoding COX-2 also displayed a pattern of increased expression in the liver and brain. Although immunohistochemical staining revealed increased abundance of HO-1 protein in the liver of infected mice, adjoining serial sections did not exhibit positive staining for COX-2 in infected tissues. The levels of monocyte chemoattractant protein-1 (MCP-1) and keratinocyte-derived cytokine (KC) were significantly higher in the sera of infected mice and corresponded with the onset and severity of the disease. Treatment of infected animals with anti-oxidants α-lipoic acid and N-acetylcysteine and HO inhibitor stannous protoporphyrin (SnPPIX) showed only selective beneficial effects on HO-1 and COX-2 expression in the liver and spleen and serum levels of KC and MCP-1. R. conorii infection of susceptible mice, therefore, results in selective regulation of the expression of HO-1 and COX-2 in a manner dependent on the target host tissue's cellular environment and the propensity of infection with rickettsiae.

Down-regulation of heme oxygenase-1 by SVCV infection

Heme oxygenase (HO-1) is a cytoprotective enzyme that plays a critical role in defending the body against oxidant-induced injury during inflammatory processes. In mammalian systems, viral infection or antigen expression can down-regulate the expression of HO-1. In turn, the induction of HO-1 or overexpression of HO-1 results in potent and direct antiviral activity that targets the replication of several mammalian viruses. In this study, the HO-1 gene of Cyprinus carpio was cloned, and the expression profile of HO-1 was investigated during spring viremia of carp virus (SVCV) infection. The results demonstrate that the expression of HO-1 was down-regulated during SVCV infection in the EPC cells and in common carp. These results indicated that SVCV infection could induce host oxidative stress, which may contribute to tissue injury in affect fish.

Curcumin: an anti-inflammatory molecule from a curry spice on the path to cancer treatment

Oxidative damage and inflammation have been pointed out in preclinical studies as the root cause of cancer and other chronic diseases such as diabetes, hypertension, Alzheimer's disease, etc. Epidemiological and clinical studies have suggested that cancer could be prevented or significantly reduced by treatment with anti-oxidant and anti-inflammatory drugs, therefore, curcumin, a principal component of turmeric (a curry spice) showing strong anti-oxidant and anti-inflammatory activities, might be a potential candidate for the prevention and/or treatment of cancer and other chronic diseases. However, curcumin, a highly pleiotropic molecule with an excellent safety profile targeting multiple diseases with strong evidence on the molecular level, could not achieve its optimum therapeutic outcome in past clinical trials, largely due to its low solubility and poor bioavailability. Curcumin can be developed as a therapeutic drug through improvement in formulation properties or delivery systems, enabling its enhanced absorption and cellular uptake. This review mainly focuses on the anti-inflammatory potential of curcumin and recent developments in dosage form and nanoparticulate delivery systems with the possibilities of therapeutic application of curcumin for the prevention and/or treatment of cancer.

Heme oxygenase-1-mediated autophagy protects against hepatocyte cell death and hepatic injury from infection/sepsis in mice

Adaptive responses to sepsis are necessary to prevent organ failure and death. Cellular signaling responses that limit cell death and structural damage allow a cell to withstand insult from sepsis to prevent irreversible organ dysfunction. One such protective pathway to reduce hepatocellular injury is the up-regulation of heme oxygenase-1 (HO-1) signaling. HO-1 is up-regulated in the liver in response to multiple stressors, including sepsis and lipopolysaccharide (LPS), and has been shown to limit cell death. Another recently recognized rudimentary cellular response to injury is autophagy. The aim of these investigations was to test the hypothesis that HO-1 protects against hepatocyte cell death in experimental sepsis in vivo or LPS in vitro via induction of autophagy. These data demonstrate that both HO-1 and autophagy are up-regulated in the liver after cecal ligation and puncture (CLP) in C57BL/6 mice or in primary mouse hepatocytes after treatment with LPS (100 ng/mL). CLP or LPS results in minimal hepatocyte cell death. Pharmacological inhibition of HO-1 activity using tin protoporphyrin or knockdown of HO-1 prevents the induction of autophagic signaling in these models and results in increased hepatocellular injury, apoptosis, and death. Furthermore, inhibition of autophagy using 3-methyladenine or small interfering RNA specific to VPS34, a class III phosphoinositide 3-kinase that is an upstream regulator of autophagy, resulted in hepatocyte apoptosis in vivo or in vitro. LPS induced phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), in part, via HO-dependent signaling. Moreover, inhibition of p38 MAPK prevented CLP- or LPS-induced autophagy.

CONCLUSION

Sepsis or LPS-induced autophagy protects against hepatocellular death, in part via an HO-1 p38 MAPK-dependent signaling. Further investigations are needed to elucidate how autophagic signaling prevents apoptosis and cell death.

Reductive stress in microbes: implications for understanding Mycobacterium tuberculosis disease and persistence

Mycobacterium tuberculosis (Mtb) is a remarkably successful pathogen that is capable of persisting in host tissues for decades without causing disease. Years after initial infection, the bacilli may resume growth, the outcome of which is active tuberculosis (TB). In order to establish infection, resist host defences and re-emerge, Mtb must coordinate its metabolism with the in vivo environmental conditions and nutrient availability within the primary site of infection, the lung. Maintaining metabolic homeostasis for an intracellular pathogen such as Mtb requires a carefully orchestrated series of oxidation-reduction reactions, which, if unbalanced, generate oxidative or reductive stress. The importance of oxidative stress in microbial pathogenesis has been appreciated and well studied over the past several decades. However, the role of its counterpart, reductive stress, has been largely ignored. Reductive stress is defined as an aberrant increase in reducing equivalents, the magnitude and identity of which is determined by host carbon source utilisation and influenced by the presence of host-generated gases (e.g. NO, CO, O(2) and CO(2)). This increased reductive power must be dissipated for bacterial survival. To recycle reducing equivalents, microbes have evolved unique electron 'sinks' that are distinct for their particular environmental niche. In this review, we describe the specific mechanisms that some microbes have evolved to dispel reductive stress. The intention of this review is to introduce the concept of reductive stress, in tuberculosis research in particular, in the hope of stimulating new avenues of investigation.

Inorganic arsenic alters expression of immune and stress response genes in activated primary human T lymphocytes

Inorganic arsenic, a carcinogenic environmental contaminant, exerts immunosuppressive effects on human T lymphocytes. In particular, interleukin-2 (IL2) secretion and T cell proliferation are reduced when peripheral blood mononuclear cells (PBMC) from individuals chronically exposed to arsenic are stimulated ex vivo with lectins such as phytohemaglutinin (PHA). However, it is not clear whether the metalloid directly acts on T cells or blocks monocyte-dependent accessory signals activated by PHA. We report that in vitro pre-treatment of PBMC with sodium arsenite (NaAs) reduces IL2 secretion and T cell proliferation induced by PHA, but does not prevent expression of monocyte-derived cytokines (IL1, IL6, TNFα) functioning as lymphocyte-activating factors. In addition, we found that NaAs delays induction of IL2 and IL2 receptor α chain (IL2RA) mRNA levels in human primary isolated T cells activated by PHA. Kinetic analysis showed that NaAs pre-treatment first inhibits, but thereafter markedly increases, induction of IL2 and IL2RA mRNA when T cells are stimulated with PHA for 8 h and 72 h, respectively. We conducted whole genome microarray-based analysis of gene expression in primary T cell cultures derived from independent donors. NaAs systematically and significantly up-regulated a set of 35 genes, including several immune and stress genes, such as IL13, granulocyte-macrophage colony stimulating factor, lymphotoxin α and heme oxygenase-1 (HO-1). Up-regulation of HO-1, a stress and immunosuppressive protein, was rapidly detectable, both in T cells and in PBMC treated with NaAs. Inhibition of the immunosuppressive activity of HO-1 in PBMC however failed to prevent NaAs-dependent inhibition of T cell proliferation induced by PHA. Our findings demonstrate that, at least in vitro, inorganic arsenic acts directly on human T cells and impairs their activity, probably independently of HO-1 expression and monocyte-related accessory signals.

Iron metabolism: microbes, mouse, and man

Recent advances in research on iron metabolism have revealed the identity of a number of genes, signal transduction pathways, and proteins involved in iron regulation in mammals. The emerging paradigm is a coordination of homeostasis within a network of classical iron metabolic pathways and other cellular processes such as cell differentiation, growth, inflammation, immunity, and a host of physiologic and pathologic conditions. Iron, immunity, and infection are intricately linked and their regulation is fundamental to the survival of mammals. The mutual dependence on iron by the host and invading pathogenic organisms elicits competition for the element during infection. While the host maintains mechanisms to utilize iron for its own metabolism exclusively, pathogenic organisms are armed with a myriad of strategies to circumvent these measures. This review explores iron metabolism in mammalian host, defense mechanisms against pathogenic microbes and the competitive devices of microbes for access to iron.

Salmonella enterica replication in hemophagocytic macrophages requires two type three secretion systems

Salmonella enterica serotype Typhimurium is a natural pathogen of mice, which acquire the bacteria orally and develop systemic acute infections that can become subacute to chronic infections. S. Typhimurium can reside within hemophagocytic macrophages (HMs) in SV129S6 mice, an Slc11a1/Nramp1(+/+) inbred strain. HMs are activated macrophages which have ingested viable hematopoietic cells and are a key characteristic of infectious and inflammatory diseases. Here we show that modest S. Typhimurium replication in HMs begins at 18 h postinfection, while activated macrophages kill the bacteria. For bacterial replication to occur, the phagocytosed viable cells must be grown to a low cell density and the multiplicity of infection must be low. HMs are able to kill phagocytosed Escherichia coli, produce reactive nitrogen species, and retain S. Typhimurium within membrane-bound vesicles. S. Typhimurium does not rescue E. coli upon coinfection of HMs. This indicates that S. Typhimurium does not cause HMs to become permissive for other microbes; rather, S. Typhimurium is especially equipped to survive within HMs. Two type three secretion systems (T3SS) encoded by S. Typhimurium are required for replication within HMs. While the T3SS within Salmonella pathogenicity island 2 (SPI-2) has been previously shown to be important for bacterial survival in cells, a role for SPI-1 in replication in macrophages has not been reported. The requirement for SPI-1 in HMs may help explain the role of SPI-1 during long-term colonization of mice.

Effect of proinflammatory cytokines on the expression and regulation of human beta-defensin 2 in human dental pulp cells

INTRODUCTION

Although the expression of human beta-defensin-2 (hBD-2) in odontoblasts from human dental pulp (HDP) has been reported, the production of hBD-2 and its regulation remains poorly understood. The aim of this study was to investigate the effect of cytokines on the induction of hBD-2 and its signaling mechanisms in HDP cells.

METHODS

After stimulation with tumor necrosis factor alpha (TNF-alpha) and interleukin 1 alpha (IL-1 alpha), reverse-transcriptase polymerase chain reaction, Western blot, and enzyme-linked immunosorbent assay experiments were performed to evaluate the effects of these cytokines on the production of hBD-2.

RESULTS

TNF-alpha and IL-1 alpha synergistically increased hBD-2 messenger RNA levels, protein expression, and activity. The up-regulation of hBD-2 by cytokines was attenuated by pretreatment with inhibitors of PKC, JNK, p38, ERK MAPK, nuclear factor-kappaB, and adenosine monophosphate-activated protein kinase (AMPK).

CONCLUSION

These results suggest that TNF-alpha and IL-1 alpha up-regulate HBD-2 expression in HDP cells through the PKC, JNK MAPK, p38, ERK, NF-kappaB, and AMPK pathways. Thus, the induction of hBD-2 by proinflammatory cytokines might up-regulate the pulpal host immune defense system.

The IsdG-family of haem oxygenases degrades haem to a novel chromophore

Enzymatic haem catabolism by haem oxygenases is conserved from bacteria to humans and proceeds through a common mechanism leading to the formation of iron, carbon monoxide and biliverdin. The first members of a novel class of haem oxygenases were recently identified in Staphylococcus aureus (IsdG and IsdI) and were termed the IsdG-family of haem oxygenases. Enzymes of the IsdG-family form tertiary structures distinct from those of the canonical haem oxygenase family, suggesting that IsdG-family members degrade haem via a unique reaction mechanism. Herein we report that the IsdG-family of haem oxygenases degrade haem to the oxo-bilirubin chromophore staphylobilin. We also present the crystal structure of haem-bound IsdI in which haem ruffling and constrained binding of oxygen is consistent with cleavage of the porphyrin ring at the beta- or delta-meso carbons. Combined, these data establish that the IsdG-family of haem oxygenases degrades haem to a novel chromophore distinct from biliverdin.

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

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

Carbon monoxide is a poison... to microbes! CO as a bactericidal molecule

Inflammation and immunity result in a wide range of disease processes, including chronic obstructive pulmonary disease, ischemia-reperfusion injury, atherosclerosis, vascular thrombosis and sepsis. Heme oxygenase-1 (HO-1) is a key enzyme that is indispensable for the temporal and spatial regulation of host response and, together with its essential metabolite carbon monoxide (CO), is crucial for maintaining homeostasis, inhibition of inflammation and the preservation of function and life. The biology of HO-1 is being discussed in this review series by Soares and colleagues and thus will not be reviewed here. Rather we will complement the HO-1 overview with a comprehensive discussion of CO as perhaps the one product of HO-1 that has been most studied. Of the numerous physiologic effects observed with CO, in the past five years it has become apparent that CO has been ascribed an additional novel role as a 'bactericidal agent'. Its role in the maintenance of homeostasis remains intact; however, the designation necessitates the paradoxical induction of the inflammatory response and binding to hemoproteins in order to restore homeostasis and sustain life. In this article, we review and discuss reports that have propelled and challenged the paradoxical use of CO, once viewed as a toxic molecule, now as a host defense molecule agent against microbes.

Blockade of CD137 signaling counteracts polymicrobial sepsis induced by cecal ligation and puncture

Sepsis, a leading cause of death worldwide, involves proinflammatory responses and inefficient bacterial clearance. Previously, we have shown that CD137 (4-1BB), a member of the tumor necrosis factor receptor superfamily, plays critical roles in eradicating infective Listeria monocytogenes, a gram-positive bacterium, and that stimulation of CD137 protects mice from sepsis-induced death. In this study, we unexpectedly found that CD137 activation aggravated polymicrobial sepsis due to mixed gram-positive and gram-negative bacterial infection induced by cecal ligation and puncture (CLP). CD137-deficient (CD137(-/-)) mice showed significantly lower mortality than CD137-sufficient (CD137(+/+)) mice in the CLP model. Administration of an agonistic anti-CD137 monoclonal antibody (MAb) to CD137(+/+) mice decreased their survival in this infection model, while administration of a blocking anti-CD137 ligand MAb (TKS-1) to such mice increased their survival. CD137(-/-) mice and TKS-1-treated CD137(+/+) mice had lower levels of chemokines/proinflammatory cytokines (monocyte chemoattractant protein 1, interleukin-6 [IL-6], tumor necrosis factor alpha, IL-12) and an anti-inflammatory cytokine (IL-10), exhibited improved bacterial clearance in the peritoneum, liver, and blood, and had greater numbers of infiltrated peritoneal neutrophils and macrophages in the CLP model than control mice. Our data suggest that CD137 activation aggravates polymicrobial sepsis induced by CLP.