Macrophage Heme Research Articles

Distinct metabolic responses to heme in inflammatory human and mouse macrophages – Role of nitric oxide

Activation of inflammation is tightly associated with metabolic reprogramming in macrophages. The iron-containing tetrapyrrole heme can induce pro-oxidant and pro-inflammatory effects in murine macrophages, but has been associated with polarization towards an anti-inflammatory phenotype in human macrophages. In the current study, we compared the regulatory responses to heme and the prototypical Toll-like receptor (TLR)4 ligand lipopolysaccharide (LPS) in human and mouse macrophages with a particular focus on alterations of cellular bioenergetics. In human macrophages, bulk RNA-sequencing analysis indicated that heme led to an anti-inflammatory transcriptional profile, whereas LPS induced a classical pro-inflammatory gene response. Co-stimulation of heme with LPS caused opposing regulatory patterns of inflammatory activation and cellular bioenergetics in human and mouse macrophages. Specifically, in LPS-stimulated murine, but not human macrophages, heme led to a marked suppression of oxidative phosphorylation and an up-regulation of glycolysis. The species-specific alterations in cellular bioenergetics and inflammatory responses to heme were critically dependent on the availability of nitric oxide (NO) that is generated in inflammatory mouse, but not human macrophages. Accordingly, studies with an inducible nitric oxide synthase (iNOS) inhibitor in mouse, and a pharmacological NO donor in human macrophages, reveal that NO is responsible for the opposing effects of heme in these cells. Taken together, the current findings indicate that NO is critical for the immunomodulatory role of heme in macrophages.

The clinical relevance of heme detoxification by the macrophage heme oxygenase system.

Heme degradation by the heme oxygenase (HMOX) family of enzymes is critical for maintaining homeostasis and limiting heme-induced tissue damage. Macrophages express HMOX1 and 2 and are critical sites of heme degradation in healthy and diseased states. Here we review the functions of the macrophage heme oxygenase system and its clinical relevance in discrete groups of pathologies where heme has been demonstrated to play a driving role. HMOX1 function in macrophages is essential for limiting oxidative tissue damage in both acute and chronic hemolytic disorders. By degrading pro-inflammatory heme and releasing anti-inflammatory molecules such as carbon monoxide, HMOX1 fine-tunes the acute inflammatory response with consequences for disorders of hyperinflammation such as sepsis. We then discuss divergent beneficial and pathological roles for HMOX1 in disorders such as atherosclerosis and metabolic syndrome, where activation of the HMOX system sits at the crossroads of chronic low-grade inflammation and oxidative stress. Finally, we highlight the emerging role for HMOX1 in regulating macrophage cell death via the iron- and oxidation-dependent form of cell death, ferroptosis. In summary, the importance of heme clearance by macrophages is an active area of investigation with relevance for therapeutic intervention in a diverse array of human diseases.

Vascular and Macrophage Heme Oxygenase-1 in Hypertension: A Mini-Review.

Hypertension is one predictive factor for stroke and heart ischemic disease. Nowadays, it is considered an inflammatory disease with elevated cytokine levels, oxidative stress, and infiltration of immune cells in several organs including heart, kidney, and vessels, which contribute to the hypertension-associated cardiovascular damage. Macrophages, the most abundant immune cells in tissues, have a high degree of plasticity that is manifested by polarization in different phenotypes, with the most well-known being M1 (proinflammatory) and M2 (anti-inflammatory). In hypertension, M1 phenotype predominates, producing inflammatory cytokines and oxidative stress, and mediating many mechanisms involved in the pathogenesis of this disease. The increase in the renin–angiotensin system and sympathetic activity contributes to the macrophage mobilization and to its polarization to the pro-inflammatory phenotype. Heme oxygenase-1 (HO-1), a phase II detoxification enzyme responsible for heme catabolism, is induced by oxidative stress, among others. HO-1 has been shown to protect against oxidative and inflammatory insults in hypertension, reducing end organ damage and blood pressure, not only by its expression at the vascular level, but also by shifting macrophages toward the anti-inflammatory phenotype. The regulatory role of heme availability for the synthesis of enzymes involved in hypertension development, such as cyclooxygenase or nitric oxide synthase, seems to be responsible for many of the beneficial HO-1 effects; additionally, the antioxidant, anti-inflammatory, antiapoptotic, and antiproliferative effects of the end products of its reaction, carbon monoxide, biliverdin/bilirubin, and Fe2+, would also contribute. In this review, we analyze the role of HO-1 in hypertensive pathology, focusing on its expression in macrophages.

Mitochondrial Reactive Oxygen Species Participate in Signaling Triggered by Heme in Macrophages and upon Hemolysis

Hemolysis causes an increase of intravascular heme, oxidative damage, and inflammation in which macrophages play a critical role. In these cells, heme can act as a prototypical damage-associated molecular pattern, inducing TLR4-dependent cytokine production through the MyD88 pathway, independently of TRIF. Heme promotes reactive oxygen species (ROS) generation independently of TLR4. ROS and TNF production contribute to heme-induced necroptosis and inflammasome activation; however, the role of ROS in proinflammatory signaling and cytokine production remains unknown. In this study, we demonstrate that heme activates at least three signaling pathways that contribute to a robust MAPK phosphorylation and cytokine expression in mouse macrophages. Although heme did not induce a detectable Myddosome formation, the TLR4/MyD88 axis was important for phosphorylation of p38 and secretion of cytokines. ROS generation and spleen tyrosine kinase (Syk) activation induced by heme were critical for most proinflammatory signaling pathways, as the antioxidant N-acetyl-l-cysteine and a Syk inhibitor differentially blocked heme-induced ROS, MAPK phosphorylation, and cytokine production in macrophages. Early generated mitochondrial ROS induced by heme was Syk dependent, selectively promoted the phosphorylation of ERK1/2 without affecting JNK or p38, and contributed to CXCL1 and TNF production. Finally, lethality caused by sterile hemolysis in mice required TLR4, TNFR1, and mitochondrial ROS, supporting the rationale to target these pathways to mitigate tissue damage of hemolytic disorders.

Human Umbilical Cord Mesenchymal Stromal Cells Attenuate Systemic Sepsis in Part by Enhancing Peritoneal Macrophage Bacterial Killing via Heme Oxygenase-1 Induction in Rats.

Mesenchymal stromal cells have therapeutic potential in sepsis, but the mechanism of action is unclear. We tested the effects, dose-response, and mechanisms of action of cryopreserved, xenogeneic-free human umbilical cord mesenchymal stromal cells in a rat model of fecal peritonitis, and examined the role of heme oxygenase-1 in protection. Separate in vivo experiments evaluated mesenchymal stromal cells in fecal sepsis, established dose response (2, 5, and 10 million cells/kg), and the role of heme oxygenase-1 in mediating human umbilical cord-derived mesenchymal stromal/stem cell effects. Ex vivo studies utilized pharmacologic blockers and small inhibitory RNAs to evaluate mechanisms of mesenchymal stromal cell enhanced function in (rodent, healthy and septic human) macrophages. Human umbilical cord mesenchymal stromal cells reduced injury and increased survival (from 48%, 12 of 25 to 88%, 14 of 16, P = 0.0033) in fecal sepsis, with dose response studies demonstrating that 10 million cells/kg was the most effective dose. Mesenchymal stromal cells reduced bacterial load and peritoneal leukocyte infiltration (from 9.9 ± 3.1 × 10/ml to 6.2 ± 1.8 × 10/ml, N = 8 to 10 per group, P < 0.0001), and increased heme oxygenase-1 expression in peritoneal macrophages, liver, and spleen. Heme oxygenase-1 blockade abolished the effects of mesenchymal stromal cells (N = 7 or 8 per group). Mesenchymal stromal cells also increased heme oxygenase-1 expression in macrophages from healthy donors and septic patients. Direct ex vivo upregulation of macrophage heme oxygenase-1 enhanced macrophage function (phagocytosis, reactive oxygen species production, bacterial killing). Blockade of lipoxin A4 production in mesenchymal stromal cells, and of prostaglandin E2 synthesis in mesenchymal stromal cell/macrophage cocultures, prevented upregulation of heme oxygenase-1 in macrophages (from 9.6 ± 5.5-fold to 2.3 ± 1.3 and 2.4 ± 2.3 respectively, P = 0.004). Knockdown of heme oxygenase-1 production in macrophages ablated mesenchymal stromal cell enhancement of macrophage phagocytosis. Human umbilical cord mesenchymal stromal cells attenuate systemic sepsis by enhancing peritoneal macrophage bacterial killing, mediated partly via upregulation of peritoneal macrophage heme oxygenase-1. Lipoxin A4 and prostaglandin E2 play key roles in the mesenchymal stromal cell and macrophage interaction.

TLR4 activation alters labile heme levels to regulate BACH1 and heme oxygenase-1 expression in macrophages

Heme oxygenase (HO)-1, a stress-inducible enzyme that converts heme into carbon monoxide (CO), iron and biliverdin, exerts important anti-inflammatory effects in activated macrophages. HO-1 expression is mainly governed by a mutual interplay between the transcriptional factor NRF2 and the nuclear repressor BTB and CNC homology 1 (BACH1), a heme sensor protein. In the current study we hypothesized that alterations in the levels of intracellular labile heme in macrophages stimulated by lipopolysaccharide (LPS), a prototypical pro-inflammatory Toll-like receptor (TLR)4 agonist, are responsible for BACH1-dependent HO-1 expression. To this end, labile heme was determined in both mouse bone marrow-derived macrophages (mBMDMs) and human monocyte-derived macrophages (hMDMs) using an apo-horseradish peroxidase-based assay. We found that LPS raised the levels of labile heme, depressed BACH1 protein and up-regulated HO-1 in mBMDMs. In contrast, in hMDMs LPS decreased labile heme levels while increasing BACH1 expression and down-regulating HO-1. These effects were abolished by the TLR4 antagonist TAK-242, suggesting that TLR4 activation triggers the signaling cascade leading to changes in the labile heme pool. Studies using mBMDMs from BACH1−/− and NRF2−/− mice revealed that regulation of HO-1 and levels of labile heme after LPS stimulation are strictly dependent on BACH1, but not NRF2. A strong interplay between BACH1-mediated HO-1 expression and intracellular levels of labile heme was also confirmed in hMDMs with siRNA knockdown studies and following inhibition of de novo heme synthesis with succinylacetone. Finally, CORM-401, a compound that liberates CO, counteracted LPS-dependent down-regulation of HO-1 and restored levels of labile heme in hMDMs. In conclusion, alterations of labile heme levels in macrophages following TLR4 stimulation play a crucial role in BACH1-mediated regulation of HO-1 expression.

Heme and metformin coordinate human and murine macrophage heme oxygenase 1 expression with foam cell resistance partly via adenosine monophosphate kinase and activating transcription factor 1 (AMPK-ATF1)

Rationale: Intraplaque hemorrhage (IPH) is a key driver of the progression of atherosclerotic plaques. We recently characterised Mhem as a novel macrophage phenotype that limits the atherogenicity of IPH. Mhem are directed by activating transcription factor 1 (ATF1), which is activated by phosphorylation. A better understanding of the counter-atherogenic ATF1-Mhem pathway may facilitate anti-atherosclerotic therapies.

Lymphotoxin-β receptor signalling links steatohepatitis, hyper-lipidemia and atherosclerosis

Lymphotoxin-β receptor signalling links steatohepatitis, hyper-lipidemia and atherosclerosis

Effect of carbon monoxide on Mycobacterium tuberculosis pathogenesis

The intracellular pathogen Mycobacterium tuberculosis (Mtb) is exposed to multiple host antimicrobial pathways, including toxic gases such as superoxide, nitric oxide and carbon monoxide (CO). To survive, mycobacteria evolved mechanisms to resist the toxic environment, and in this review we focus on a relatively new field, namely, the role of macrophage heme oxygenase and its enzymatic product CO in Mtb pathogenesis. In particular, we focus on (i) the induction of heme oxygenase during Mtb infection and its relevance to Mtb pathogenesis, (ii) the ability of mycobacteria to catabolize CO, (iii) the transcriptional reprogramming of Mtb by exposure to CO, (iv) the general antimicrobial properties of CO and (v) new genetic evidence characterizing the ability of Mtb to resist CO toxicity. Developing a complete molecular and genetic understanding of the pathogenesis of Mtb is essential to its eventual eradication.

Kinetics and Specificity of Feline Leukemia Virus Subgroup C Receptor (FLVCR) Export Function and Its Dependence on Hemopexin

The feline leukemia virus subgroup C receptor (FLVCR) is a heme export protein that is required for proerythroblast survival and facilitates macrophage heme iron recycling. However, its mechanism of heme export and substrate specificity are uncharacterized. Using [(55)Fe]heme and the fluorescent heme analog zinc mesoporphyrin, we investigated whether export by FLVCR depends on the availability and avidity of extracellular heme-binding proteins. Export was 100-fold more efficient when the medium contained hemopexin (K(d) < 1 pm) compared with albumin (K(d) = 5 nm) at the same concentration and was not detectable when the medium lacked heme-binding proteins. Besides heme, FLVCR could export other cyclic planar porphyrins, such as protoporphyrin IX and coproporphyrin. However, FLVCR has a narrow substrate range because unconjugated bilirubin, the primary breakdown product of heme, was not transported. As neither protoporphyrin IX nor coproporphyrin export improved with extracellular hemopexin (versus albumin), our observations further suggest that hemopexin, an abundant protein with a serum concentration (6.7-25 mum) equivalent to that of the iron transport protein transferrin (22-31 mum), by accepting heme from FLVCR and targeting it to the liver, might regulate macrophage heme export and heme iron recycling in vivo. Final studies show that hemopexin directly interacts with FLVCR, which also helps explain why FLVCR, in contrast to some major facilitator superfamily members, does not function as a bidirectional gradient-dependent transporter. Together, these data argue that hemopexin has a role in assuring systemic iron balance during homeostasis in addition to its established role as a scavenger during internal bleeding or hemolysis.

Prolonged cigarette smoke exposure decreases heme oxygenase-1 and alters Nrf2 and Bach1 expression in human macrophages: Roles of the MAP kinases ERK 1/2 and JNK

Tobacco may be involved in the decreased macrophage heme oxygenase-1 (HO-1) expression described in smoking-induced severe emphysema, via the nuclear factor erythroid 2-related factor 2 (Nrf2)/Kelch-like ECH-associated protein 1 (Keap1)–BTB and CNC homology 1, basic leucine zipper transcription factor 1 (Bach1) pathway. We assessed in vitro effects of cigarette smoke condensate (CS) in the human monocyte/macrophage cell line (THP-1). CS exposure led to increased HO-1 and nuclear Nrf2 expression (6 h) followed by decreased HO-1 expression concomitantly with nuclear Nrf2/Bach1 ratio decrease (72 h). CS-induced mitogen-activated protein kinase (MAPK) phosphorylation. Extracellular-signal-regulated kinase 1/2 (ERK 1/2) and c-Jun NH2-terminal kinase (JNK) inhibition completely abrogated CS effects on HO-1 expression and nuclear Nrf2/Bach1 translocation. These results suggest that ERK 1/2 and JNK are involved in CS-induced biphasic HO-1 expression by a specific regulation of Nrf2/Keap1–Bach1.

A Heme Export Protein Is Required for Red Cell Differentiation and Iron Homeostasis.

Heme is required by all aerobic cells, yet toxic, so its intracellular concentration must be tightly regulated. We previously determined that the Feline Leukemia Virus, subgroup C, cell surface Receptor (FLVCR), is a heme export protein (Cell ; 118:6, 2004), and characterized the erythropoietic phenotype of inducible FLVCR null mice (Flvcrflox/flox;Mx-cre, ASH abstract 2006). Deleted mice develop a severe hyperchromic macrocytic anemia (HGB 3.8±0.2 g/dl, MCV 65.5±2.1 fL, MCH 22.7±0.6 pg vs. HGB 15.4±0.6, MCV 50.8±1.5, MCH 15.9±0.5). Morphologic, flow cytometric, and erythroid colony analyses of spleen and bone marrow cells showed a block in erythroid differentiation at the CFU-E/proerythroblast stage, a phenotype identical to that observed in cats with pure red cell aplasia (PRCA) due to retroviral inhibition of FLVCR. Mice transplanted with Flvcrflox/flox;Mx-cre marrow, and then treated with p(I)p(C) to delete Flvcr in hematopoietic cells, also developed PRCA. These studies show that FLVCR is required by CFU-E/proerythroblasts, likely to export excess heme and to ensure cell survival, and show that PRCA results from a lack of FLVCR in hematopoietic, and not microenvironmental cells. By western blot, FLVCR is highly expressed in tissues with high heme flux, like the placenta, uterus, duodenum, liver, and cultured macrophages, which suggests that it also has a role in heme-iron trafficking or in the prevention of heme toxicity in nonerythroid cells. To define its macrophage function, we exposed marrow-derived macrophages from deleted and control mice to ferric ammonium citrate (FAC) or IgG-coated red blood cells ± hepcidin and measured ferritin accumulation. Deleted and control macrophages exposed to FAC showed equivalent increases in ferritin which as predicted, increased further with hepcidin treatment. However, after exposure to opsonized red blood cells, deleted macrophages accumulated significantly more ferritin than controls (122.3±0.8 ng/mh protein vs. 67.3±1.3), which increased further with hepcidin treatment. Therefore, not all heme in macrophages is degraded to iron, but rather some traverses the cell intact via FLVCR. We next evaluated the role of FLVCR in iron homeostasis by examining all tissues in deleted mice. Within 5 weeks, deleted mice developed pronounced iron loading in hepatocytes and subsequently within duodenal enterocytes and splenic macrophages. By 7 months, there was swelling of hepatocytes lining bile canaliculi and bile stasis. In contrast, mice in which Flvcr was deleted only in hematopoietic cells showed no iron overload after 5–6 weeks. Liver hepcidin expression by quantitative RT-PCR was significantly increased in both deleted mice and in mice lacking FLVCR only in their hematopoietic cells. These data demonstrate that hepcidin expression alone does not account for the iron overload, and suggest that FLVCR exports heme from liver into bile, thus providing a mechanism for iron to exit the body. Since high hepcidin levels are in contrast to other iron loading anemias with ineffective erythropoiesis, the “erythropoietic regulator” of liver hepcidin expression must require cells more differentiated than proerythroblasts. Together, our work establishes that FLVCR is required for terminal red blood cell development and argues that systemic iron balance involves heme-iron trafficking via FLVCR, in addition to the well-described elemental iron pathways.

Selectin-Mediated Interactions Regulate Cytokine Networks and Macrophage Heme Oxygenase-1 Induction in Cardiac Allograft Recipients

Host sensitization to major histocompatibility complex (MHC) antigens is among the most critical of problems facing heart transplantation. Selectins are postulated to mediate the early adhesive events in the recruitment of leukocytes at the allograft site. We investigated the significance of selectin–P-selectin glycoprotein ligand-1 (PSGL-1)–mediated in vivo interactions in the immune cascade leading to rejection of cardiac allografts in skin presensitized rats. Infusion of a soluble recombinant form of PSGL-1 (rPSGL-Ig) during skin graft–mediated sensitization prevented Day 1.0 ± 0.1 “accelerated” rejection in sensitized rat recipients, and prolonged cardiac allograft survival to Day 3.8 ± 1.0 (p < 0.001). This therapy significantly depressed serum IgM levels and decreased intragraft expression of Th1 type cytokines (IL-2 and IFN-γ) as well as of IL-1β and MCP-1, as compared with controls, without affecting the initial number of infiltrating mononuclear cells (MNC). A profound decrease in graft-infiltrating MNC was recorded at 24 hours in rPSGL-Ig–treated rats. The expression of heme oxygenase-1 (HO-1), an inducible heat shock protein 32 that protects against oxidative cell/tissue injury, was found in approximately 14-fold higher levels in the rPSGL-Ig–treated recipients as compared with controls. The HO-1 overexpression in rPSGL-Ig–treated hosts, primarily by infiltrating macrophages, was accompanied by virtual absence of myocardial infarcts and decreased frequency of TUNEL + cells at the graft site. Moreover, down-regulation of HO-1 expression by zinc protoporphyrin, an HO-1 antagonist, decreased expression of antiapoptotic Bag-1 molecule in recipients conditioned with rPSGL-Ig. Thus, the blockade of selectin–PSGL-1 interactions depresses intracardiac allograft expression of Th1 type cytokines, and might inhibit the differentiation of Th1 type cells. In addition, it up-regulates HO-1 expression and protects against myocardial infarction and apoptosis. Hence, this study reports on a previously unrecognized role of selectin–PSGL-1–mediated interactions after in vivo alloantigenic challenge.

Structural studies on bovine spleen heme oxygenase: Immunological and structural diversity among mammalian heme oxygenase enzymes

Heme oxygenase is an M r 32,000 microsomal enzyme which catalyzes the rate-limiting step in the oxidative catabolism of heme to yield equimolar quantities of biliverdin IXα, carbon monoxide, and iron. In the present investigation, evidence is presented suggesting that immunochemical and structural differences exist between bovine spleen heme oxygenase and heme oxygenase enzymes from other mammalian species. Using an antibody directed against bovine spleen heme oxygenase, enzyme-linked immunosorbent assays, Western blotting experiments, and cell-free translation immunoprecipitation studies showed that bovine spleen heme oxygenase is only weakly immunochemically related to heme oxygenase from rat spleen. This observation was supported by the fact that a rat spleen heme oxygenase cDNA probe did not hybridize significantly to bovine spleen heme oxygenase mRNA in Northern analyses nor to restriction fragments containing the bovine heme oxygenase gene in Southern analyses. Tryptic peptides were prepared from bovine spleen heme oxygenase and the amino acid sequences of nine peptides comprising 94 amino acid residues were determined, providing the first information on the primary structure of bovine spleen heme oxygenase. Comparison of the sequences of these tryptic peptides with regions of the deduced amino acid sequences of rat spleen and human macrophage heme oxygenase revealed sequence similarities ranging from 55 to 100%. Several peptides displaying the highest degree of sequence similarity were found to occur in regions of the heme oxygenase molecule postulated to contain the heme binding site, indicating that despite the immunochemical and apparent structural differences between bovine spleen heme oxygenase and the rat and human enzymes, functionally important amino acid residues have been conserved in the evolution of mammalian heme oxygenase genes.