title: Characterization of heme as activator of Toll-like receptor 4.
abstract: Heme is an ancient and ubiquitous molecule present in organisms of all kingdoms, composed of an atom of iron linked to four ligand groups of porphyrin. A high amount of free heme, a potential amplifier of the inflammatory response, is a characteristic feature of diseases with increased hemolysis or extensive cell damage. Here we demonstrate that heme, but not its analogs/precursors, induced tumor necrosis factor-alpha (TNF-alpha) secretion by macrophages dependently on MyD88, TLR4, and CD14. The activation of TLR4 by heme is exquisitely strict, requiring its coordinated iron and the vinyl groups of the porphyrin ring. Signaling of heme through TLR4 depended on an interaction distinct from the one established between TLR4 and lipopolysaccharide (LPS) since anti-TLR4/MD2 antibody or a lipid A antagonist inhibited LPS-induced TNF-alpha secretion but not heme activity. Conversely, protoporphyrin IX antagonized heme without affecting LPS-induced activation. Moreover, heme induced TNF-alpha and keratinocyte chemokine but was ineffective to induce interleukin-6, interleukin-12, and interferon-inducible protein-10 secretion or co-stimulatory molecule expression. These findings support the concept that the broad ligand specificity of TLR4 and the different activation profiles might in part reside in its ability to recognize different ligands in different binding sites. Finally, heme induced oxidative burst, neutrophil recruitment, and heme oxygenase-1 expression independently of TLR4. Thus, our results presented here reveal a previous unrecognized role of heme as an extracellular signaling molecule that affects the innate immune response through a receptor-mediated mechanism.
authors.affiliation: Departamento de Imunologia, Instituto de Microbiologia, Universidade Federal do Rio de Janeiro, 21941-590 Rio de Janeiro, RJ, Brazil.
authors.forename: Rodrigo T
authors.initials: RT
authors.lastname: Figueiredo
authors.forename: Patricia L
authors.initials: PL
authors.lastname: Fernandez
authors.forename: Diego S
authors.initials: DS
authors.lastname: Mourao-Sa
authors.forename: Bárbara N
authors.initials: BN
authors.lastname: Porto
authors.forename: Fabianno F
authors.initials: FF
authors.lastname: Dutra
authors.forename: Letícia S
authors.initials: LS
authors.lastname: Alves
authors.forename: Marcus F
authors.initials: MF
authors.lastname: Oliveira
authors.forename: Pedro L
authors.initials: PL
authors.lastname: Oliveira
authors.forename: Aurélio V
authors.initials: AV
authors.lastname: Graça-Souza
authors.forename: Marcelo T
authors.initials: MT
authors.lastname: Bozza
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D016207:Cytokines / Q000276:immunology; D004336:Drug Antagonism; D015971:Gene Expression Regulation, Enzymologic; D006418:Heme / Q000037:antagonists & inhibitors / Q000276:immunology* / Q000494:pharmacology; D051547:Heme Oxygenase-1 / Q000276:immunology; D006461:Hemolysis / Q000235:genetics / Q000276:immunology; D007113:Immunity, Innate / Q000187:drug effects / Q000235:genetics / Q000276:immunology*; D007249:Inflammation / Q000235:genetics / Q000276:immunology; D018950:Lipopolysaccharide Receptors / Q000276:immunology; D008070:Lipopolysaccharides / Q000494:pharmacology; D008262:Macrophage Activation / Q000187:drug effects / Q000235:genetics / Q000276:immunology*; D008264:Macrophages / Q000276:immunology*; D051379:Mice; D018345:Mice, Knockout; D053594:Myeloid Differentiation Factor 88 / Q000276:immunology; D009504:Neutrophils / Q000276:immunology; D017319:Photosensitizing Agents / Q000037:antagonists & inhibitors / Q000494:pharmacology; D011524:Protoporphyrins / Q000037:antagonists & inhibitors / Q000494:pharmacology; D016897:Respiratory Burst / Q000187:drug effects / Q000235:genetics / Q000276:immunology; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics / Q000276:immunology*; D051197:Toll-Like Receptor 4 / Q000172:deficiency / Q000276:immunology*
chemical_list: D016207:Cytokines; D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D017319:Photosensitizing Agents; D011524:Protoporphyrins; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4; D006418:Heme; C028025:protoporphyrin IX; D051547:Heme Oxygenase-1