Heme oxygenase-1 promotes granuloma development and protects against dissemination of mycobacteria

Pathophysiological Role of Vimentin Intermediate Filaments in Lung Diseases

Vimentin intermediate filaments, a type III intermediate filament, are among the most widely studied IFs and are found abundantly in mesenchymal cells. Vimentin intermediate filaments localize primarily in the cytoplasm but can also be found on the cell surface and extracellular space. The cytoplasmic vimentin is well-recognized for its role in providing mechanical strength and regulating cell migration, adhesion, and division. The post-translationally modified forms of Vimentin intermediate filaments have several implications in host-pathogen interactions, cancers, and non-malignant lung diseases. This review will analyze the role of vimentin beyond just the epithelial to mesenchymal transition (EMT) marker highlighting its role as a regulator of host-pathogen interactions and signaling pathways for the pathophysiology of various lung diseases. In addition, we will also examine the clinically relevant anti-vimentin compounds and antibodies that could potentially interfere with the pathogenic role of Vimentin intermediate filaments in lung disease.

A Dual Role of Heme Oxygenase-1 in Tuberculosis

Iron metabolism is vital for the survival of both humans and microorganisms. Heme oxygenase-1 (HO-1) is an essential stress-response enzyme highly expressed in the lungs, and catabolizes heme into ferrous iron, carbon monoxide (CO), and biliverdin (BV)/bilirubin (BR), especially in pathological conditions which cause oxidative stress and inflammation. Ferrous iron (Fe²⁺) is an important raw material for the synthesis of hemoglobin in red blood cells, and patients with iron deficiency are often associated with decreased cellular immunity. CO and BR can inhibit oxidative stress and inflammation. Thus, HO-1 is regarded as a cytoprotective molecule during the infection process. However, recent study has unveiled new information regarding HO-1. Being a highly infectious pathogenic bacterium, Mycobacterium tuberculosis (MTB) infection causes acute oxidative stress, and increases the expression of HO-1, which may in turn facilitate MTB survival and growth due to increased iron availability. Moreover, in severe cases of MTB infection, excessive reactive oxygen species (ROS) and free iron (Fe²⁺) due to high levels of HO-1 can lead to lipid peroxidation and ferroptosis, which may promote further MTB dissemination from cells undergoing ferroptosis. Therefore, it is important to understand and illustrate the dual role of HO-1 in tuberculosis. Herein, we critically review the interplay among HO-1, tuberculosis, and the host, thus paving the way for development of potential strategies for modulating HO-1 and iron metabolism.

Zebrafish Heme Oxygenase 1a Is Necessary for Normal Development and Macrophage Migration

Heme oxygenase function is highly conserved between vertebrates where it plays important roles in normal embryonic development and controls oxidative stress. Expression of the zebrafish heme oxygenase 1 genes is known to be responsive to oxidative stress suggesting a conserved physiological function. In this study, we generate a knockout allele of zebrafish hmox1a and characterize the effects of hmox1a and hmox1b loss on embryonic development. We find that loss of hmox1a or hmox1b causes developmental defects in only a minority of embryos, in contrast to Hmox1 gene deletions in mice that cause loss of most embryos. Using a tail wound inflammation assay we find a conserved role for hmox1a, but not hmox1b, in normal macrophage migration to the wound site. Together our results indicate that zebrafish hmox1a has clearly a partitioned role from hmox1b that is more consistent with conserved functions of mammalian Heme oxygenase 1.

Hypoxia‐inducible factor (HIF)‐1α‐induced regulation of lung injury in pulmonary aspiration is mediated through NF‐kB

Aspiration‐induced lung injury is a common grievance encountered in the intensive care unit (ICU). It is a significant risk factor for improving ventilator‐associated pneumonia (VAP) and acute respiratory distress syndrome (ARDS). Hypoxia‐inducible factor (HIF)‐1α is one of the primary transcription factors responsible for regulating the cellular response to changes in oxygen tension. Here, we sought to determine the role of HIF‐1α and specifically the role of type 2 alveolar epithelial cells in generating the acute inflammatory response following acid and particles (CASP) aspiration. Previous studies show HIF‐1 α is involved in regulating the hypoxia‐stimulated expression of MCP‐1 in mice and humans. The CASP was induced in C57BL/6, ODD‐Luc, HIF‐1α (+/+) control, and HIF‐1α conditional knockout (HIF‐1α (−/−) mice). Following an injury in ODD mice, explanted organs were subjected to IVIS imaging to measure the degree of hypoxia. HIF‐1α expression, BAL albumin, cytokines, and histology were measured following CASP. In C57BL/6 mice, the level of HIF‐1α was increased at 1 h after CASP. There were significantly increased levels of albumin and cytokines in C57BL/6 and ODD‐Luc mice lungs following CASP. HIF‐1α (+/+) mice given CASP demonstrated a synergistic increase in albumin leakage, increased pro‐inflammatory cytokines, and worse injury. MCP‐1 antibody neutralized HIF‐1α (+/+) mice showed reduced granuloma formation. The NF‐κB expression was increased substantially in the HIF‐1α (+/+) mice following CASP compared to HIF‐1α (−/−) mice. Our data collectively identify that HIF‐1α upregulation of the acute inflammatory response depends on NF‐κB following CASP.

The Role of NRF2 in Mycobacterial Infection

The incidence of pulmonary nontuberculous mycobacterial (NTM) infection is increasing worldwide, and its clinical outcomes with current chemotherapies are unsatisfactory. The incidence of tuberculosis (TB) is still high in Africa, and the existence of drug-resistant tuberculosis is also an important issue for treatment. To discover and develop new efficacious anti-mycobacterial treatments, it is important to understand the host-defense mechanisms against mycobacterial infection. Nuclear erythroid 2 p45-related factor-2 (NRF2) is known to be a major regulator of various antioxidant response element (ARE)-driven cytoprotective gene expressions, and its protective role has been demonstrated in infections. However, there are not many papers or reviews regarding the role of NRF2 in mycobacterial infectious disease. Therefore, this review focuses on the role of NRF2 in the pathogenesis of Mycobacterium tuberculosis and Mycobacterium avium infection.

Haem oxygenase limits Mycobacterium marinum infection-induced detrimental ferrostatin-sensitive cell death in zebrafish

Iron homeostasis is essential for both sides of the host-pathogen interface. Restricting access of iron slows bacterial growth while iron is also a necessary cofactor for host immunity. Haem oxygenase 1 (HMOX1) is a critical regulator of iron homeostasis that catalyses the liberation of iron during degradation of haem. It is also a stress-responsive protein that can be rapidly upregulated and confers protection to the host. Although a protective role of HMOX1 has been demonstrated in a variety of diseases, the role of HMOX1 in Mycobacterium tuberculosis infection is equivocal across experiments with different host-pathogen combinations. Here, we use the natural host-pathogen pairing of the zebrafish-Mycobacterium marinum infection platform to study the role of zebrafish haem oxygenase in mycobacterial infection. We identify zebrafish Hmox1a as the relevant functional paralog of mammalian HMOX1 and demonstrate a conserved role for Hmox1a in protecting the host from M. marinum infection. Using genetic and chemical tools, we show zebrafish Hmox1a protects the host against M. marinum infection by reducing infection-induced iron accumulation and ferrostatin-sensitive cell death.

Donor-defined mesenchymal stem cell antimicrobial potency against nontuberculous mycobacterium

Chronic nontuberculous mycobacterial infections with Mycobacterium avium and Mycobacterium intracellulare complicate bronchiectasis, chronic obstructive airway disease, and the health of aging individuals. These insidious intracellular pathogens cause considerable morbidity and eventual mortality in individuals colonized with these bacteria. Current treatment regimens with antibiotic macrolides are both toxic and often inefficient at providing infection resolution. In this article, we demonstrate that human marrow‐derived mesenchymal stem cells are antimicrobial and anti‐inflammatory in vitro and in the context of an in vivo sustained infection of either M. avium and/or M. intracellulare.

Nrf2 Regulates Granuloma Formation and Macrophage Activation during Mycobacterium avium Infection via Mediating Nramp1 and HO-1 Expressions

Nontuberculous mycobacteria (NTM) are an important cause of morbidity and mortality in pulmonary infections. Among them, Mycobacterium avium complex (MAC) is the most common cause of pulmonary NTM disease worldwide.

Nrf2 is a redox-sensitive transcription factor that is thought to be important in protection against intracellular pathogens. To determine the protective role of Nrf2 in the host defense against Mycobacterium avium complex (MAC), both wild-type and Nrf2-deficient mice were intranasally infected with MAC bacteria. Nrf2-deficient mice were highly susceptible to MAC bacteria compared with wild-type mice. There were no significant changes in the levels of oxidative stress and Th1 cytokine production between genotypes. Comprehensive transcriptome analysis showed that the expressions of Nramp1 and HO-1 were much lower in the infected lungs, and the expression of Nramp1 was especially lower in alveolar macrophages of Nrf2-deficient mice than of wild-type mice. Electron microscopy showed that many infected alveolar macrophages from Nrf2-deficient mice contained a large number of intracellular MAC bacteria with little formation of phagolysosomes, compared with those from wild-type mice. Treatment with sulforaphane, an activator of Nrf2, increased resistance to MAC with increased lung expression of Nramp1 and HO-1 in wild-type mice. These results indicate that Nramp1 and HO-1, regulated by Nrf2, are essential in defending against MAC infection due to the promotion of phagolysosome fusion and granuloma formation, respectively. Thus, Nrf2 is thought to be a critical determinant of host resistance to MAC infection.

Heme Oxygenase-1 as a Pharmacological Target for Host-Directed Therapy to Limit Tuberculosis Associated Immunopathology

Excessive inflammation and tissue damage are pathological hallmarks of chronic pulmonary tuberculosis (TB). Despite decades of research, host regulation of these clinical consequences is poorly understood. A sustained effort has been made to understand the contribution of heme oxygenase-1 (HO-1) to this process. HO-1 is an essential cytoprotective enzyme in the host that controls inflammation and oxidative stress in many pathological conditions. While HO-1 levels are upregulated in animals and patients infected with Mycobacterium tuberculosis (Mtb), how it regulates host responses and disease pathology during TB remains unclear. This lack of clarity is due in part to contradictory studies arguing that HO-1 induction contributes to both host resistance as well as disease progression. In this review, we discuss these conflicting studies and the role of HO-1 in modulating myeloid cell functions during Mtb disease progression. We argue that HO-1 is a promising target for host-directed therapy to improve TB immunopathology.

Modulation of Inflammation and Immune Responses by Heme Oxygenase-1: Implications for Infection with Intracellular Pathogens

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme molecules releasing equimolar amounts of biliverdin, iron and carbon monoxide. Its expression is induced in response to stress signals such as reactive oxygen species and inflammatory mediators with antioxidant, anti-inflammatory and immunosuppressive consequences for the host. Interestingly, several intracellular pathogens responsible for major human diseases have been shown to be powerful inducers of HO-1 expression in both host cells and in vivo. Studies have shown that this HO-1 response can be either host detrimental by impairing pathogen control or host beneficial by limiting infection induced inflammation and tissue pathology. These properties make HO-1 an attractive target for host-directed therapy (HDT) of the diseases in question, many of which have been difficult to control using conventional antibiotic approaches. Here we review the mechanisms by which HO-1 expression is induced and how the enzyme regulates inflammatory and immune responses during infection with a number of different intracellular bacterial and protozoan pathogens highlighting mechanistic commonalities and differences with the goal of identifying targets for disease intervention.

M. tuberculosis Reprograms Hematopoietic Stem Cells to Limit Myelopoiesis and Impair Trained Immunity

A greater understanding of hematopoietic stem cell (HSC) regulation is required for dissecting protective versus detrimental immunity to pathogens that cause chronic infections such as Mycobacterium tuberculosis (Mtb). We have shown that systemic administration of Bacille Calmette-Guérin (BCG) or β-glucan reprograms HSCs in the bone marrow (BM) via a type II interferon (IFN-II) or interleukin-1 (IL1) response, respectively, which confers protective trained immunity against Mtb. Here, we demonstrate that, unlike BCG or β-glucan, Mtb reprograms HSCs via an IFN-I response that suppresses myelopoiesis and impairs development of protective trained immunity to Mtb. Mechanistically, IFN-I signaling dysregulates iron metabolism, depolarizes mitochondrial membrane potential, and induces cell death specifically in myeloid progenitors. Additionally, activation of the IFN-I/iron axis in HSCs impairs trained immunity to Mtb infection. These results identify an unanticipated immune evasion strategy of Mtb in the BM that controls the magnitude and intrinsic anti-microbial capacity of innate immunity to infection.

Macrophage Signaling Pathways in Pulmonary Nontuberculous Mycobacteria Infections

The incidence and prevalence of nontuberculous mycobacteria (NTM) lung disease is rising worldwide and accounts for most clinical cases of NTM disease. NTM infections occur in both immunocompetent and immunocompromised hosts. Macrophages are the primary host cells that initiate an immune response to NTM. Defining the molecular events that govern the control of infection within macrophages is fundamental to understanding the pathogenesis of NTM disease. Here, we review key macrophage host signaling pathways that contribute to the host immune response to pulmonary NTM infections. In this review, we focus primarily on NTM that are known to cause lung disease, including Mycobacterium avium intracellulare, M. abscessus, and M. kansasii.

Autoantibody-Mediated Erythrophagocytosis Increases Tuberculosis Susceptibility in HIV Patients

Macrophage dysfunction is associated with increased tuberculosis (TB) susceptibility in patients with human immunodeficiency virus (HIV) infection. However, the mechanisms underlying how HIV infection impairs macrophage function are unclear. Here, we found that levels of autoantibodies against red blood cells (RBCs) were significantly elevated in patients with HIV as determined by direct antiglobulin test (DAT). DAT positivity was significantly associated with TB incidence in both univariate and multivariate analyses (odds ratio [OR] = 11.96 [confidence interval {CI}, 4.68 to 30.93] and 12.65 [3.33 to 52.75], respectively). Ex vivo analysis showed that autoantibodies against RBCs enhanced erythrophagocytosis and thus significantly impaired macrophage bactericidal function against intracellular Mycobacterium tuberculosis Mechanistically, autoantibody-mediated erythrophagocytosis increased heme oxygenase-1 (HO-1) expression, which inhibited M. tuberculosis-induced autophagy in macrophages. Silencing ATG5, a key component for autophagy, completely abrogated the effect of erythrophagocytosis on macrophage bactericidal activity against M. tuberculosis In conclusion, we have demonstrated that HIV infection increases autoantibody-mediated erythrophagocytosis. This process impairs macrophage bactericidal activity against M. tuberculosis by inhibiting HO-1-associated autophagy. These findings reveal a novel mechanism as to how HIV infection increases TB susceptibility.IMPORTANCE HIV infection significantly increases TB susceptibility due to CD4 T-cell loss and macrophage dysfunction. Although it is relatively clear that CD4 T-cell loss represents a direct effect of HIV infection, the mechanism underlying how HIV infection dampens macrophage function is unknown. Here, we show that HIV infection enhances autoantibody-mediated erythrophagocytosis, which dampens macrophage bactericidal activity against TB by inhibiting HO-1-associated autophagy. Our findings reveal a novel mechanism explaining how HIV infection increases susceptibility to TB. We propose that DAT could be a potential measure to identify HIV patients who are at high TB risk and who would be suitable for anti-TB chemotherapy preventive treatment.

Microanatomic Distribution of Myeloid Heme Oxygenase-1 Protects against Free Radical-Mediated Immunopathology in Human Tuberculosis

Heme oxygenase-1 (HO-1) is a cytoprotective enzyme that controls inflammatory responses and redox homeostasis; however, its role during pulmonary tuberculosis (TB) remains unclear. Using freshly resected human TB lung tissue, we examined the role of HO-1 within the cellular and pathological spectrum of TB. Flow cytometry and histopathological analysis of human TB lung tissues showed that HO-1 is expressed primarily in myeloid cells and that HO-1 levels in these cells were directly proportional to cytoprotection. HO-1 mitigates TB pathophysiology by diminishing myeloid cell-mediated oxidative damage caused by reactive oxygen and/or nitrogen intermediates, which control granulocytic karyorrhexis to generate a zonal HO-1 response. Using whole-body or myeloid-specific HO-1-deficient mice, we demonstrate that HO-1 is required to control myeloid cell infiltration and inflammation to protect against TB progression. Overall, this study reveals that zonation of HO-1 in myeloid cells modulates free-radical-mediated stress, which regulates human TB immunopathology.

Cinnamaldehyde protects VSMCs against ox-LDL-induced proliferation and migration through S arrest and inhibition of p38, JNK/MAPKs and NF-κB

Cinnamaldehyde (Cin), as a traditional flavor constituent isolated from the bark of Cinnamonum cassia Presl, has been commonly used for - digestive, cardiovascular and immune system diseases. The pathology of vascular smooth muscle cells (VSMCs) accelerated the progression of atherosclerosis. In our study, we found that cinnamaldehyde significantly suppressed ox-LDL-induced VSMCs proliferation, migration and inflammatory cytokine overproduction, as well as foam cell formation in VSMCs and macrophages. Moreover, cinnamaldehyde inhibited the phosphorylation of p38, JNK and p65 NF-κB and increased heme oxygenase-1 (HO-1) activity. In addition, cinnamaldehyde reduced monocyte chemotactic protein-1 (MCP-1), matrix metalloproteinase-2 (MMP-2) and lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) expression. Furthermore, cinnamaldehyde arrested cell cycle in S phase. Thus, results indicated that cinnamaldehyde antagonized the ox-LDL-induced VSMCs proliferation, migration, inflammation and foam cell formation through regulation of HO-1, MMP-2, LOX-1 and blockage of cell cycle, and - suppression of p38, JNK/MAPK and NF-κB signaling pathways.

Host heme oxygenase-1: Friend or foe in tackling pathogens?

Infectious diseases are a major challenge in management of human health worldwide. Recent literature suggests that host immune system could be modulated to ameliorate the pathogenesis of infectious disease. Heme oxygenase (HMOX1) is a key regulator of cellular signaling and it could be modulated using pharmacological reagents. HMOX1 is a cytoprotective enzyme that degrades heme to generate carbon monoxide (CO), biliverdin, and molecular iron. CO and biliverdin (or bilirubin derived from it) can restrict the growth of a few pathogens. Both of these also induce antioxidant pathways and anti-inflammatory pathways. On the other hand, molecular iron can induce proinflammatory pathway besides making the cellular environment oxidative in nature. Since microbial infections often induce oxidative stress in host cells/tissues, role of HMOX1 has been analyzed in the pathogenesis of number of infections. In this review, we have described the role of HMOX1 in pathogenesis of bacterial infections caused by Mycobacterium species, Salmonella and in microbial sepsis. We have also provided a succinct overview of the role of HMOX1 in parasitic infections such as malaria and leishmaniasis. In the end, we have also elaborated the role of HMOX1 in viral infections such as AIDS, hepatitis, dengue, and influenza. © 2018 IUBMB Life, 70(9):869-880, 2018.

Antimycobacterial effect of IFNG (interferon gamma)-induced autophagy depends on HMOX1 (heme oxygenase 1)-mediated increase in intracellular calcium levels and modulation of PPP3/calcineurin-TFEB (transcription factor EB) axis

IFNG (interferon gamma)-induced autophagy plays an important role in the elimination of intracellular pathogens, such as Mycobacterium tuberculosis (Mtb). However, the signaling cascade that leads to the increase in autophagy flux in response to IFNG is poorly defined. Here, we demonstrate that HMOX1 (heme oxygenase 1)-generated carbon monoxide (CO) is required for the induction of autophagy and killing of Mtb residing in macrophages in response to immunomodulation by IFNG. Interestingly, IFNG exposure of macrophages induces an increase in intracellular calcium levels that is dependent on HMOX1 generated CO. Chelation of intracellular calcium inhibits IFNG-mediated autophagy and mycobacterial clearance from macrophages. Moreover, we show that IFNG-mediated increase in intracellular calcium leads to activation of the phosphatase calcineurin (PPP3), which dephosphorylates the TFEB (transcription factor EB) to induce autophagy. PPP3-mediated activation and nuclear translocation of TFEB are critical in IFNG-mediated mycobacterial trafficking and survival inside the infected macrophages. These findings establish that IFNG utilizes the PPP3-TFEB signaling axis for inducing autophagy and regulating mycobacterial growth. We believe this signaling axis could act as a therapeutic target for suppression of growth of intracellular pathogens.

Th17-Inducing Cytokines IL-6 and IL-23 Are Crucial for Granuloma Formation during Experimental Paracoccidioidomycosis

Paracoccidioidomycosis (PCM), a chronic granulomatous disease caused by the thermally dimorphic fungus Paracoccidioides brasiliensis and Paracoccidioides lutzii, has the highest mortality rate among systemic mycosis. The T helper 1-mediated immunity is primarily responsible for acquired resistance during P. brasiliensis infection, while susceptibility is associated with Th2 occurrence. Th17 is a population of T CD4⁺ cells that, among several chemokines and cytokines, produces IL-17A and requires the presence of IL-1, IL-6, and TGF-β for differentiation in mice and IL-23 for its maintenance. Th17 has been described as an arm of the immune system that enhances host protection against several bacterial and fungal infections, as Pneumocystis carinii and Candida albicans. In this study, we aimed to evaluate the Th17 immune response and the role of Th17-associated cytokines (IL-6, IL-23, and IL-17A) during experimental PCM. First, we observed that P. brasiliensis infection [virulent yeast strain 18 of P. brasiliensis (Pb18)] increased the IL-17A production in vitro and all the evaluated Th17-associated cytokines in the lung tissue from C57BL/6 wild-type mice. In addition, the deficiency of IL-6, IL-23, or IL-17 receptor A (IL-17RA) impaired the compact granuloma formation and conferred susceptibility during infection, associated with reduced tumor necrosis factor-α, IFN-γ, and inducible nitric oxide synthase enzyme expression. Our data suggest that IL-6 production by bone marrow-derived macrophages (BMDMs) is important to promote the Th17 differentiation during Pb18 infection. In accordance, the adoptive transfer of BMDMs from C57BL/6 to infected IL-6^-/- or IL-17RA^-/- mice reduced the fungal burden in the lungs compared to nontransferred mice and reestablished the pulmonary granuloma formation. Taken together, these results suggest that Th17-associated cytokines are involved in the modulation of immune response and granuloma formation during experimental PCM.

Heme-Oxygenase-1 Expression Contributes to the Immunoregulation Induced by Fasciola hepatica and Promotes Infection

Fasciola hepatica, also known as the liver fluke, is a trematode that infects livestock and humans causing fasciolosis, a zoonotic disease of increasing importance due to its worldwide distribution and high economic losses. This parasite immunoregulates the host immune system by inducing a strong Th2 and regulatory T immune response by immunomodulating dendritic cell (DC) maturation and alternative activation of macrophages. In this paper, we show that F. hepatica infection in mice induces the upregulation of heme-oxygenase-1 (HO-1), the rate-limiting enzyme in the catabolism of free heme that regulates the host inflammatory response. We show and characterize two different populations of antigen presenting cells that express HO-1 during infection in the peritoneum of infected animals. Cells that expressed high levels of HO-1 expressed intermediate levels of F4/80 but high expression of CD11c, CD38, TGFβ, and IL-10 suggesting that they correspond to regulatory DCs. On the other hand, cells expressing intermediate levels of HO-1 expressed high levels of F4/80, CD68, Ly6C, and FIZZ-1, indicating that they might correspond to alternatively activated macrophages. Furthermore, the pharmacological induction of HO-1 with the synthetic metalloporphyrin CoPP promoted F. hepatica infection increasing the clinical signs associated with the disease. In contrast, treatment with the HO-1 inhibitor SnPP protected mice from parasite infection, indicating that HO-1 plays an essential role during F. hepatica infection. Finally, HO-1 expression during F. hepatica infection was associated with TGFβ and IL-10 levels in liver and peritoneum, suggesting that HO-1 controls the expression of these immunoregulatory cytokines during infection favoring parasite survival in the host. These results contribute to the elucidation of the immunoregulatory mechanisms induced by F. hepatica in the host and provide alternative checkpoints to control fasciolosis.

Chronic signaling via the metabolic checkpoint kinase mTORC1 induces macrophage granuloma formation and marks sarcoidosis progression

Aggregation of hypertrophic macrophages constitutes the basis of all granulomatous diseases such as tuberculosis or sarcoidosis and is decisive for disease pathogenesis. However, macrophage-intrinsic pathways driving granuloma initiation and maintenance remain elusive. Here we show that activation of the metabolic checkpoint kinase mTORC1 in macrophages by deletion of Tsc2 was sufficient to induce hypertrophy and proliferation resulting in excessive granuloma formation in vivo. TSC2-deficient macrophages formed mTORC1-dependent granulomatous structures in vitro and showed constitutive proliferation mediated by the neo-expression of cyclin-dependent kinase 4 (CDK4). Moreover, mTORC1 promoted metabolic reprogramming via CDK4 towards increased glycolysis, while simultaneously inhibiting NF-κB signaling and apoptosis. Inhibition of mTORC1 induced apoptosis and completely resolved granulomas in myeloid TSC2-deficient mice. In human sarcoidosis patients mTORC1 activation, macrophage proliferation, and glycolysis were identified as hallmarks that correlated with clinical disease progression. Collectively, TSC2 maintains macrophage quiescence and prevents mTORC1-dependent granulomatous disease with clinical implications for sarcoidosis.

Attenuated heme oxygenase-1 responses predispose the elderly to pulmonary nontuberculous mycobacterial infections

Pulmonary infections with nontuberculous mycobacteria (P-NTM), such as by Mycobacterium avium complex (M. avium), are increasingly found in the elderly, but the underlying mechanisms are unclear. Recent studies suggest that adaptive immunity is necessary, but not sufficient, for host defense against mycobacteria. Heme oxygenase-1 (HO-1) has been recognized as a critical modulator of granuloma formation and programmed cell death in mycobacterial infections. Old mice (18-21 mo) infected with M. avium had attenuated HO-1 response with diffuse inflammation, high burden of mycobacteria, poor granuloma formation, and decreased survival (45%), while young mice (4-6 mo) showed tight, well-defined granuloma, increased HO-1 expression, and increased survival (95%). To further test the role of HO-1 in increased susceptibility to P-NTM infections in the elderly, we used old and young HO-1^+/+ and HO-1^-/- mice. The transcriptional modulation of the JAK/STAT signaling pathway in HO-1^-/- mice due to M. avium infection demonstrated similarities to infected wild-type old mice with upregulation of SOCS3 and inhibition of Bcl2. Higher expression of SOCS3 with downregulation of Bcl2 resulted in higher macrophage death via cellular necrosis. Finally, peripheral blood monocytes (PBMCs) from elderly patients with P-NTM also demonstrated attenuated HO-1 responses after M. avium stimulation and increased cell death due to cellular necrosis (9.69% ± 2.02) compared with apoptosis (4.75% ± 0.98). The augmented risk for P-NTM in the elderly is due, in part, to attenuated HO-1 responses, subsequent upregulation of SOCS3, and inhibition of Bcl2, leading to programmed cell death of macrophages, and sustained infection.

Macrophages and Iron Metabolism

Iron is a transition metal that due to its inherent ability to exchange electrons with a variety of molecules is essential to support life. In mammals, iron exists mostly in the form of heme, enclosed within an organic protoporphyrin ring and functioning primarily as a prosthetic group in proteins. Paradoxically, free iron also has the potential to become cytotoxic when electron exchange with oxygen is unrestricted and catalyzes the production of reactive oxygen species. These biological properties demand that iron metabolism is tightly regulated such that iron is available for core biological functions while preventing its cytotoxic effects. Macrophages play a central role in establishing this delicate balance. Here, we review the impact of macrophages on heme-iron metabolism and, reciprocally, how heme-iron modulates macrophage function.

The emerging role of gasotransmitters in the pathogenesis of tuberculosis

Mycobacterium tuberculosis (Mtb) is a facultative intracellular pathogen and the second largest contributor to global mortality caused by an infectious agent after HIV. In infected host cells, Mtb is faced with a harsh intracellular environment including hypoxia and the release of nitric oxide (NO) and carbon monoxide (CO) by immune cells. Hypoxia, NO and CO induce a state of in vitro dormancy where Mtb senses these gases via the DosS and DosT heme sensor kinase proteins, which in turn induce a set of ∼47 genes, known as the Mtb Dos dormancy regulon. On the contrary, both iNOS and HO-1, which produce NO and CO, respectively, have been shown to be important against mycobacterial disease progression. In this review, we discuss the impact of O2, NO and CO on Mtb physiology and in host responses to Mtb infection as well as the potential role of another major endogenous gas, hydrogen sulfide (H2S), in Mtb pathogenesis.

Distal Hydrogen-bonding Interactions in Ligand Sensing and Signaling by Mycobacterium tuberculosis DosS

Mycobacterium tuberculosis DosS is critical for the induction of M. tuberculosis dormancy genes in response to nitric oxide (NO), carbon monoxide (CO), or hypoxia. These environmental stimuli, which are sensed by the DosS heme group, result in autophosphorylation of a DosS His residue, followed by phosphotransfer to an Asp residue of the response regulator DosR. To clarify the mechanism of gaseous ligand recognition and signaling, we investigated the hydrogen-bonding interactions of the iron-bound CO and NO ligands by site-directed mutagenesis of Glu-87 and His-89. Autophosphorylation assays and molecular dynamics simulations suggest that Glu-87 has an important role in ligand recognition, whereas His-89 is essential for signal transduction to the kinase domain, a process for which Arg-204 is important. Mutation of Glu-87 to Ala or Gly rendered the protein constitutively active as a kinase, but with lower autophosphorylation activity than the wild-type in the Fe(II) and the Fe(II)-CO states, whereas the E87D mutant had little kinase activity except for the Fe(II)-NO complex. The H89R mutant exhibited attenuated autophosphorylation activity, although the H89A and R204A mutants were inactive as kinases, emphasizing the importance of these residues in communication to the kinase core. Resonance Raman spectroscopy of the wild-type and H89A mutant indicates the mutation does not alter the heme coordination number, spin state, or porphyrin deformation state, but it suggests that interdomain interactions are disrupted by the mutation. Overall, these results confirm the importance of the distal hydrogen-bonding network in ligand recognition and communication to the kinase domain and reveal the sensitivity of the system to subtle differences in the binding of gaseous ligands.

Heme Oxygenase-1 Regulates Inflammation and Mycobacterial Survival in Human Macrophages during Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis, the causative agent of tuberculosis, is responsible for 1.5 million deaths annually. We previously showed that M. tuberculosis infection in mice induces expression of the CO-producing enzyme heme oxygenase (HO1) and that CO is sensed by M. tuberculosis to initiate a dormancy program. Further, mice deficient in HO1 succumb to M. tuberculosis infection more readily than do wild-type mice. Although mouse macrophages control intracellular M. tuberculosis infection through several mechanisms, such as NO synthase, the respiratory burst, acidification, and autophagy, how human macrophages control M. tuberculosis infection remains less well understood. In this article, we show that M. tuberculosis induces and colocalizes with HO1 in both mouse and human tuberculosis lesions in vivo, and that M. tuberculosis induces and colocalizes with HO1 during primary human macrophage infection in vitro. Surprisingly, we find that chemical inhibition of HO1 both reduces inflammatory cytokine production by human macrophages and restricts intracellular growth of mycobacteria. Thus, induction of HO1 by M. tuberculosis infection may be a mycobacterial virulence mechanism to enhance inflammation and bacterial growth.

Inhibition of ceramide de novo synthesis by myriocin produces the double effect of reducing pathological inflammation and exerting antifungal activity against A. fumigatus airways infection

BACKGROUND

Fungal infections develop in pulmonary chronic inflammatory diseases such as asthma, Chronic Obstructive Pulmonary Disease (COPD) and Cystic Fibrosis (CF). The available antifungal drugs may fail to eradicate fungal pathogens, that can invade the lungs and vessels and spread by systemic circulation taking advantage of defective lung immunity. An increased rate of sphingolipid de novo synthesis, leading to ceramide accumulation, was demonstrated in CF and COPD inflamed lungs. The inhibitor of sphingolipid synthesis myriocin reduces inflammation and ameliorates the response against bacterial airway infection in CF mice. Myriocin also inhibits sphingolipid synthesis in fungi and exerts a powerful fungistatic effect.

METHODS

We treated Aspergillus fumigatus infected airway epithelial cells with myriocin and we administered myriocin-loaded nanocarriers to A. fumigatus infected mice lung.

RESULTS

We demonstrate here that de novo synthesized ceramide mediates the inflammatory response induced by A. fumigatus infection in airway epithelia. CF epithelial cells are chronically inflamed and defective in killing internalized conidia. Myriocin treatment reduced ceramide increase and inflammatory mediator release whereas it upregulated HO1 and NOD2, allowing the recovery of a functional killing of conidia in these cells. Myriocin-loaded nanocarriers, intratracheally administered to mice, significantly reduced both the inflammatory response induced by A. fumigatus pulmonary challenge and fungal lung invasion.

CONCLUSIONS

We conclude that inhibition of sphingolipid synthesis can be envisaged as a dual anti-inflammatory and anti-fungal therapy in patients suffering from chronic lung inflammation with compromised immunity.

GENERAL SIGNIFICANCE

Myriocin represents a powerful agent for inflammatory diseases and fungal infection.

Heme as a danger molecule in pathogen recognition

Appropriate control of redox mechanisms are critical for and effective innate immune response, which employs multiple cell types, receptors and molecules that recognize danger signals when they reach the host. Recognition of pathogen-associated pattern molecules (PAMPs) is a fundamental host survival mechanism for efficient elimination of invading pathogens and resolution of the infection and inflammation. In addition to PAMPs, eukaryotic cells contain a plethora of intracellular molecules that are normally secured within the confines of the plasma membrane, but if liberated and encountered in the extracellular milieu can provoke rapid cell activation. These are known as Alarmins or Danger-Associated Molecular Patterns (DAMPs) and can be released actively by cells or passively as a result of sterile cellular injury after trauma, ischemia, or toxin-induced cell rupture. Both PAMPs and DAMPs are recognized by a series of cognate receptors that increase the generation of free radicals and activate specific signaling pathways that result in regulation of a variety of stress response, redox sensitive genes. Multiple mediators released, as cells die include, but are not limited to ATP, hydrogen peroxide, heme, formyl peptides, DNA or mitochondria provide the second signal to amplify immune responses. In this review, we will focus on how sterile and infective stimuli activate the stress response gene heme oxygenase-1 (Hmox1, HO-1), a master gene critical to an appropriate host response that is now recognized as one with enormous therapeutic potential. HO-1 gene expression is regulated in large part by redox-sensitive proteins including but not limited to nrf2. Both PAMPs and DAMPs increase the activation of nrf2 and HO-1. Heme is a powerful pro-oxidant and as such should be qualified as a DAMP. With its degradation by HO-1a molecule of carbon monoxide (CO) is generated that in turn serves as a bioactive signaling molecule. PAMPs such as bacterial endotoxin activate HO-1, and the CO that is generated diffuses into the extracellular milieu where it interacts with bacteria, altering their behavior to increase production of ATP, which then functions as a second signal danger molecule. This two-hit cycle scenario results in efficient and effective activation of host leukocytes to attack and clear bacteria in part via enhanced reactive oxygen species generation. We discuss this intimate communication that occurs between host and bacteria and how these molecules serve as critical regulators of the acute inflammatory response, the overall redox status of the cell, and survival of the host.

The Iron age of host-microbe interactions

Microbes exert a major impact on human health and disease by either promoting or disrupting homeostasis, in the latter instance leading to the development of infectious diseases. Such disparate outcomes are driven by the ever-evolving genetic diversity of microbes and the countervailing host responses that minimize their pathogenic impact. Host defense strategies that limit microbial pathogenicity include resistance mechanisms that exert a negative impact on microbes, and disease tolerance mechanisms that sustain host homeostasis without interfering directly with microbes. While genetically distinct, these host defense strategies are functionally integrated, via mechanisms that remain incompletely defined. Here, we explore the general principles via which host adaptive responses regulating iron (Fe) metabolism impact on resistance and disease tolerance to infection.

Heme oxygenase-1 dysregulation in the brain: implications for HIV-associated neurocognitive disorders

Heme oxygenase-1 (HO-1) is a highly inducible and ubiquitous cellular enzyme that subserves cytoprotective responses to toxic insults, including inflammation and oxidative stress. In neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease and multiple sclerosis, HO-1 expression is increased, presumably reflecting an endogenous neuroprotective response against ongoing cellular injury. In contrast, we have found that in human immunodeficiency virus (HIV) infection of the brain, which is also associated with inflammation, oxidative stress and neurodegeneration, HO-1 expression is decreased, likely reflecting a unique role for HO-1 deficiency in neurodegeneration pathways activated by HIV infection. We have also shown that HO-1 expression is significantly suppressed by HIV replication in cultured macrophages which represent the primary cellular reservoir for HIV in the brain. HO-1 deficiency is associated with release of neurotoxic levels of glutamate from both HIV-infected and immune-activated macrophages; this glutamate-mediated neurotoxicity is suppressed by pharmacological induction of HO-1 expression in the macrophages. Thus, HO-1 induction could be a therapeutic strategy for neuroprotection against HIV infection and other neuroinflammatory brain diseases. Here, we review various stimuli and signaling pathways regulating HO-1 expression in macrophages, which could promote neuronal survival through HO-1-modulation of endogenous antioxidant and immune modulatory pathways, thus limiting the oxidative stress that can promote HIV disease progression in the CNS. The use of pharmacological inducers of endogenous HO-1 expression as potential adjunctive neuroprotective therapeutics in HIV infection is also discussed.

Induction of heme oxygenase-1 contributes to survival of Mycobacterium abscessus in human macrophages-like THP-1 cells

Mycobacterium abscessus (M.abs) is a rapidly growing mycobacterial species that infects macrophages, and is an important pathogen in patients with cystic fibrosis. We studied the early stages of M.abs infection of macrophages, with emphasis on the role of heme-oxygenase-1 (HO-1) in this infection. THP-1 cells were activated using TPA into macrophage-like cells and infected with M.abs for different time points. M.abs infection robustly induced HO-1 expression in the THP-1 cells. Production of HO-1 was p38 MAPK-dependent, as p38 inhibitors suppressed HO-1 induction. Pretreatment with HO-1 inhibitors tin-protoporphyrin (SnPP) significantly inhibited M.abs growth inside macrophages. Furthermore, inhibiting HO-1 using HO-1 siRNA or the HO-1 upstream signaling molecule; Nrf2 using Nrf2 siRNA resulted in similar inhibition of M.abs. In contrast, inducing HO-1 did not increase M.abs intracellular growth above control. Products of HO-1 metabolism of heme are bilirubin, biliverdin, carbon monoxide (CO) and iron. The addition of either bilirubin or biliverdin, but not CO, completely restored the SnPP inhibitory effect and partially that with HO-1 siRNA. To understand the mechanisms, we used Syto-62 labeled M.abs to infect macrophages. Interestingly, HO-1 inhibition promoted M.abs-containing phagosome fusion with lysosomes, which should enhance M.abs killing. M.abs infection enhanced THP-1 ROS production as demonstrated by increased DHE, DCF fluorescence, and EPR signal. HO-1 inhibition further increased ROS production in infected macrophages. Our results indicate that HO-1 induction is important for M.abs growth during the early stages of infection, and that the HO-1 products bilirubin and biliverdin, perhaps through modulation of intracellular ROS levels, may be involved.

•

HO-1 induction is important for Mycobacterium abscessus growth inside infected macrophages during the early stages of infection.

•

Reducing HO-1 products may enhance the ability of the macrophage to control Mycobacterium abscessus infection.

•

HO-1 inhibition increases phagosome–lysosome fusion and thus Mycobacterium abscessus killing.

In vitro and in vivo studies of a rapid and selective breath test for tuberculosis based upon mycobacterial CO dehydrogenase

One of the major hurdles in treating tuberculosis (TB) is the time-consuming and difficult methodology for diagnosis. Stable-isotope breath tests hold great potential for rapidly diagnosing an infectious disease, monitoring therapy, and determining a bacterial phenotype in a rapid, point-of-care manner that does not require invasive sampling. Here we describe the preclinical development of a potentially highly selective TB diagnostic breath test based upon the organism’s CO dehydrogenase activity. After development of the test in vitro, we were able to use the breath test to discriminate between infected and control rabbits, demonstrating that a diagnosis can potentially be made and also that a complex bacterial phenotype can be noninvasively and rapidly studied in the host.

Tuberculosis (TB) remains a major infectious cause of disease and death worldwide, and effective diagnosis and then treatment are the tools with which we fight TB. The more quickly and more specific the diagnosis can be made, the better, and this is also true of diagnosis being as close to the patient (point of care) as possible. Here we report our preclinical development of breath tests based upon specific mycobacterial metabolism that could, with development, allow rapid point-of-care diagnosis through measuring the mycobacterial conversion of labeled CO to labeled CO₂.

cor, a novel carbon monoxide resistance gene, is essential for Mycobacterium tuberculosis pathogenesis

Tuberculosis, caused by Mycobacterium tuberculosis, remains a devastating human infectious disease, causing two million deaths annually. We previously demonstrated that M. tuberculosis induces an enzyme, heme oxygenase (HO1), that produces carbon monoxide (CO) gas and that M. tuberculosis adapts its transcriptome during CO exposure. We now demonstrate that M. tuberculosis carries a novel resistance gene to combat CO toxicity. We screened an M. tuberculosis transposon library for CO-susceptible mutants and found that disruption of Rv1829 (carbon monoxide resistance, Cor) leads to marked CO sensitivity. Heterologous expression of Cor in Escherichia coli rescued it from CO toxicity. Importantly, the virulence of the cor mutant is attenuated in a mouse model of tuberculosis. Thus, Cor is necessary and sufficient to protect bacteria from host-derived CO. Taken together, this represents the first report of a role for HO1-derived CO in controlling infection of an intracellular pathogen and the first identification of a CO resistance gene in a pathogenic organism.

IMPORTANCE

Macrophages produce a variety of antimicrobial molecules, including nitric oxide (NO), hydrogen peroxide (H2O2), and acid (H+), that serve to kill engulfed bacteria. In addition to these molecules, human and mouse macrophages also produce carbon monoxide (CO) gas by the heme oxygenase (HO1) enzyme. We observed that, in contrast to other bacteria, mycobacteria are resistant to CO, suggesting that this might be an evolutionary adaptation of mycobacteria for survival within macrophages. We screened a panel of ~2,500 M. tuberculosis mutants to determine which genes are required for survival of M. tuberculosis in the presence of CO. Within this panel, we identified one such gene, cor, that specifically confers CO resistance. Importantly, we found that the ability of M. tuberculosis cells carrying a mutated copy of this gene to cause tuberculosis in a mouse disease model is significantly attenuated. This indicates that CO resistance is essential for mycobacterial survival in vivo.

Plasma heme oxygenase-1 levels distinguish latent or successfully treated human tuberculosis from active disease

BACKGROUND

Tuberculosis (TB) is associated with oxidative stress and the induction of host anti-oxidants to counteract this response. Heme oxygenase-1 (HO-1) is a critical promoter of cytoprotection in diverse disease models including mycobacterial infection. Nevertheless, the pattern of expression of HO-1 in human tuberculosis has not been studied. Here, we examine expression of HO-1 in M. tuberculosis-exposed and -infected individuals and test its ability to distinguish active from latent and successfully treated TB cases. In addition, we assess correlations between plasma levels of HO-1 and cytokines closely associated with the immunopathogenesis of TB.

METHODS

Cross-sectional and longitudinal analyses of levels of HO-1, acute phase proteins and pro-inflammatory cytokines were performed in plasma samples from individuals with active pulmonary, extra-pulmonary or latent TB infection and healthy controls as part of a prospective cohort study in South India.

RESULTS

Systemic levels of HO-1 were dramatically increased in individuals with active pulmonary and extra-pulmonary tuberculosis and particularly those with bilateral lung lesions and elevated bacillary loads in sputum. HO-1 levels effectively discriminated active from latent tuberculosis with higher predictive values than either C-reactive protein or serum amyloid protein. Moreover, there was a marked reduction in HO-1 levels in active TB cases following anti-tuberculous therapy but not in those who failed treatment. Pulmonary TB patients displaying the highest concentrations of HO-1 in plasma exhibited significantly elevated plasma levels of interleukin (IL)-10, interferon (IFN)-γ and IL-17 and diminished levels of tumor necrosis factor (TNF)-α.

CONCLUSION

These findings establish HO-1 levels as a potentially useful parameter for distinguishing active from latent or treated pulmonary tuberculosis, that is superior in this respect to the measurement of other acute inflammatory proteins.

Heme catabolism by heme oxygenase-1 confers host resistance to Mycobacterium infection. Infect Immun. 2013;81:2536-2545. [PMID: 23630967 DOI: 10.1128/iai.00251-13]

Heme oxygenases (HO) catalyze the rate-limiting step of heme degradation. The cytoprotective action of the inducible HO-1 isoform, encoded by the Hmox1 gene, is required for host protection against systemic infections. Here we report that upregulation of HO-1 expression in macrophages (M) is strictly required for protection against mycobacterial infection in mice. HO-1-deficient (Hmox1(-/-)) mice are more susceptible to intravenous Mycobacterium avium infection, failing to mount a protective granulomatous response and developing higher pathogen loads, than infected wild-type (Hmox1(+/+)) controls. Furthermore, Hmox1(-/-) mice also develop higher pathogen loads and ultimately succumb when challenged with a low-dose aerosol infection with Mycobacterium tuberculosis. The protective effect of HO-1 acts independently of adaptive immunity, as revealed in M. avium-infected Hmox1(-/-) versus Hmox1(+/+) SCID mice lacking mature B and T cells. In the absence of HO-1, heme accumulation acts as a cytotoxic pro-oxidant in infected M, an effect mimicked by exogenous heme administration to M. avium-infected wild-type M in vitro or to mice in vivo. In conclusion, HO-1 prevents the cytotoxic effect of heme in M, contributing critically to host resistance to Mycobacterium infection.