Neutrophil activation by heme: implications for inflammatory processes

Abstract

Heme, a ubiquitous iron-containing compound, is present in large amounts in many cells and is inherently dangerous, particularly when it escapes from intracellular sites. The release of heme from damaged cells and tissues is supposed to be higher in diseases such as malaria and hemolytic anemia or in trauma and hemorrhage. We investigated here the role of free ferriprotoporphyrin IX (hemin) as a proinflammatory molecule, with particular attention to its ability to activate neutrophil responses. Injecting hemin into the rat pleural cavity resulted in a dosedependent migration of neutrophils, indicating that hemin is able to promote the recruitment of these cells in vivo. In vitro, hemin induced human neutrophil chemotaxis and cytoskeleton reorganization, as revealed by the increase of neutrophil actin polymerization. Exposure of human neutrophils to 3 M hemin activated the expression of the chemokine interleukin-8, as demonstrated by quantitative reverse-transcription polymerase chain reaction, indicating a putative molecular mechanism by which hemin induces chemotaxis in vivo. Brief incubation of human neutrophils with micromolar concentrations of hemin (1-20 M) triggered the oxidative burst, and the production of reactive oxygen species was directly proportional to the concentration of hemin added to the cells. Finally, we observed that human neutrophil protein kinase C was activated by hemin in vitro, with a K 1/2 of 5 M. Taken IntroductionHeme released from hemeproteins has been shown to promote the formation of oxygen radicals, playing a role as a catalyst in the oxidation of lipids, proteins, and DNA. [1][2][3] In addition, free heme can promptly bind to and oxidize low-density lipoprotein, acting as a biologically relevant lipoprotein oxidant. 4 Hemoglobin (probably because of the release of free heme and heme iron) may contribute to the acute renal failure often seen after episodes of intravascular hemolysis. 5,6 In fact, it has been proposed that heme could be considered one causative agent in organ failure after ischemia-reperfusion because heme-oxygenase is induced in heart and kidney. 7 In normal conditions, diverse species produce avid hemebinding plasma proteins, such as hemopexin, that efficiently remove most of the heme produced intravascularly, 8 thus preventing nonspecific cellular heme uptake and heme-catalyzed oxidation reactions. However, pathologic situations of increased hemolysis can lead to very high levels of free heme, as in malaria, 9 sickle cell disease, 10 HELLP (hemolysis, elevated liver levels, and low platelet count) syndrome, 11 or regions with turbulent blood flow. 12 Very little work has been done to assess the consequences of the interaction of free heme with intact cells. It has been demonstrated that free heme is promptly incorporated into endothelial cells in vitro and that this association potentiates the oxidative damage induced by chemical agents. 13 It is interesting to note that patients suffering from sickle cell disease often exhibit a low-grade chronic inflammation...

Acute high-altitude sickness

“…It may be that sustained and increasing high pressures of sufficient duration in untreated HAPE can trigger inflammation [81], or it represents part of the healing process of a markedly disrupted alveolar-capillary barrier that occurs in the most severe cases of HAPE, especially with alveolar haemorrhage, since haem and other breakdown products of red cell haemoglobin are chemotactic for neutrophils [82]. Despite overwhelming evidence against a primary inflammatory alteration of the alveolar-capillary barrier in HAPE, it is nevertheless conceivable that any concurrent process altering the permeability of the alveolar-capillary barrier will lower the pressure required for formation of oedema.…”

Innate Immune Responses to Human Malaria: Heterogeneous Cytokine Responses to Blood-Stage Plasmodium falciparum Correlate with Parasitological and Clinical Outcomes

“…Intriguingly, the parasite metabolite hemozoin can mediate recruitment of monocytes (55), most likely via an IL-8-dependent mechanism (56), and NK cells have been shown to produce IL-8 in response to iRBC stimulation (52). In our in vitro experiments, IL-8 was among the earliest cytokines to be significantly up-regulated (at 3 h), whereas significant IL-8 increase could only be measured in two of the sporozoite-infected donors.…”

Heme and innate immunity: new insights for an old molecule

“…These prerogatives set neutrophil as a suitable cell type in order to investigate leukocyte responsiveness to free heme. On this study we reported that free heme acts as a prototypical proinflammatory molecule, able to induce neutrophil migration both in vivo and in vitro per se (Graça-Souza et al 2002). Interaction of free heme with human neutrophils leads to actin cytoskeleton reorganization and NADPH oxidase-dependent ROS generation through the induction of protein kinase C (PKC) activity.…”

“…It also increases interleukin (IL)-8 expression in a PKC-independent manner, providing the first clue of the pleithropic mechanisms of action of free heme on human neutrophils. This work was then the first evidence that heme is able to trigger an inflammatory response itself activating leukocyte responsiveness (Graça-Souza et al 2002).…”