doi: 10.1161/01.RES.0000249616.10603.d6
© 2006 American Heart Association, Inc.
Editorials |
CD163-Mediated Hemoglobin-Heme Uptake Activates Macrophage HO-1, Providing an Antiinflammatory Function
From the New York Medical College, Valhalla, NY.
Correspondence to Nader G. Abraham, New York Medical College, Basic Science Building, Valhalla, NY 10595. E-mail nader_abraham{at}nymc.edu
See related article, pages 943–950
Key Words: CD163 • heme oxygenase • antiinflammatory • carbon monoxide • bilirubin antioxidants
A study by Schaer et al,1 which appears in this issue of Circulation Research, provides evidence suggesting that CD163 acts as a hemoglobin (Hb) transporter and, via activation of macrophages, heme oxygenase-1 (HO-1) provides antiinflammatory activity, presumably by an increase in ferritin. Induction of HO-1 was observed to occur when the cultured macrophages, endothelial cells or tubular cells were exposed to heme or hemoglobin (for a review see Abraham and Kappas2). Heme and denatured hemoglobin toxicity play an important role in a broad spectrum of pathological circumstances such as myocardial ischemia, hypertension, cardiomyopathy, reperfusion, organ transplantation, pulmonary disorders, and inflammation among others.3 In the cell free system, hemoglobin is bound to haptoglobin before clearance by the macrophage hemoglobin scavenger receptor CD163. Schaer et al1 demonstrated that CD163-hemoglobin transport is regarded as a critical step in the hemoglobin clearance pathway in the macrophage, especially under conditions of extreme hemoglobin release resulting from hemolysis. Binding hemoglobin-haptoglobin to CD163 cells also elicits IL-10 secretion, which contributes to the induction of HO-1.4 Thus, CD163 has a functional role as an antioxidant by enhancing HO-1, the major cytoprotective response. This links heme transport directly to a known receptor with well characterized endocytic properties and signaling functions.5 Heme-hemopexin is also taken up by LRP/CD91 and, as such, may inhibit inflammatory functions in phagocytic and antiinflammatory macrophages; however, a link to functional HO-1 was not described.
Schaer et al1 demonstrated that induction of HO-1 by Hb is functionally linked to CD163 and, using gene array analysis, were able to show that Hb elicits a noninflammatory transcriptional response in macrophages, presumably by the coordinated increase in ferritin synthesis following the activation of HO-1. This response was mediated by CD163 and did not involve the depletion of reduced gluthathione. No involvement of protein phosphorylation-dependent CD163 signaling was observed. The authors concluded that CD163 acted as a heme transporter, which undergoes "constitutive and ligand independent internalization and recycling between the cell surface and the early endosomal compartment". Schaer et al clearly demonstrate that this pathway is active in vivo and conclude that this explains the antiinflammatory action associated with CD163 macrophages. This report confirms that induction of HO-1 is directly involved in the downregulation of inflammation, and suggests that this effect is because of ferritin synthesis as a result of induction of HO-1.
To better comprehend the authors’ conclusions, one must examine the heme-HO system and its role in cellular protection after oxidative injury (Figure 1). Heme is the prosthetic group of numerous enzymes and, as such, is crucial in the regulation of the activity of soluble guanylate cyclase (sGC), nitric oxide synthase (NOS), cytochrome P450 (CYP450) monooxygenases, cyclooxygenases (COX), catalase thromboxane synthase, mitochondrial cytochrome, and tryptophane pyrrolase, among other heme proteins (for a review see Abraham and Kappas, 2005). HO is the rate-limiting step in heme degradation, which degrades heme to carbon monoxide (CO), biliverdin, and iron by the stereospecific cleavage of the alpha-methene bridge. HO exists as the isoenzymes HO-1 (inducible form) and HO-2 (constitutive form). HO-1 and HO-2 catalytic activities are identical in terms of the mechanism of heme catabolism, and cofactor and substrate requirements.2 It is well documented that HO-1 is markedly induced by its substrate heme and by oxidant stress. This, in conjunction with the robust ability of HO-1 to protect against oxidative insult, has stimulated research on the antioxidant nature of HO-1/-2. The biliverdin formed is rapidly and stereospecifically reduced to bilirubin by biliverdin reductase. Both biliverdin and bilirubin are antioxidants. Subjects with elevated bilirubin, between 2 to 4 mg/d, have been found less likely to have vascular disease.6 HO-1–derived bilirubin was observed to have activity that is potentially associated with decreasing NOX oxidase activation by regulating p47 phox-dependent activity,7 which could contribute to the antiinflammatory response of Hb–HO-1. Induction of HO-1 increases heme degradation but does not cause depletion of microsomal heme under normal circumstances because of increase of heme synthesis (in vivo). Control of heme synthesis is vested in the first step, which catalysis condensations of mitochondrial succinyl-COA and glycine with the formation of delta aminolevulinic acid (ALA) by ALA synthase. Under basal conditions, ALA synthase activity is very low but sufficient to maintain an adequate supply of heme needed for normal physiological function. When heme itself was found in the mitochondria in concentrations of 10–7 M or greater, it caused inhibition of ALA synthase. Although mitochondrial heme levels undoubtedly exceed those of cytosol, where levels of may reach 10–6 to 10–7 M,8 the concentration of mitochondrial heme may cause an inhibition of ALA synthase not yet known. As seen in Figure 2, when there is an excess of heme, either because of an increased rate of synthesis or a decreased rate of removal by CD163 and other binding proteins, the excess of heme will inhibit ALA synthase or activity. Schaer et al have elegantly shown that macrophage CD163, by binding to circulating hemoglobin, will result in activation of HO-1 and thus prevent the accumulation of hemoglobin/heme and prooxidants, as well as attenuate reactive oxygen species (ROS) and cell damage. Therefore, CD163 plays an important role in regulating heme degradation by HO-1. It should be noted that a CD163-mediated increase in HO-1 does not decrease the constitutive heme proteins, such as COX-1, eNOS or mitochondrial proteins, but it will decrease those heme proteins with a rapid turnover, ie, inducible enzymes such as iNOS, COX-2, and tryptophane pyrrolase. Therefore, the CD163-mediated transport of hemoglobin to macrophages will increase HO-1 at the site needed, such as the vascular system, whereas decreasing iNOS and COX-2 and increasing ferritin, CO, and bilirubin.
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B, Nrf-2, and AP-1, and activate HO-1 expression. In conditions in which proinflammatory molecules, hypoxia and oxidants are increased, NADPH oxidase is activated and there is an increase in ONOO, which is toxic. Induction of HO-1 decreases the inducible heme-dependent enzymes, such as iNOS, COX-2, and NADPH-oxidase, and attenuates inflammation.Carbon monoxide, which can act as both a messenger and a signaling molecule, is not an antioxidant per se, but can cause the induction of antioxidant genes,2 decrease the levels of superoxide (O2–),9 increase reduced gluthathione (GSH) levels (needed to enhance the redox state) and cyclic guanosine monophosphate (cGMP) (a vasodilator).2 CO has an antiapoptotic effect (for review see Ryter et al).10 The third product of heme degradation, iron, is rapidly bound to ferritin as manifest by a rapid increase in the levels of ferritin as Schaer et al have described. Thus, the induction of HO-1, the primary response to oxidant stress, results in a multifaceted defense against oxidative damage in the cell. Toxic levels of the hemoglobin/heme are removed by HD-1 and released iron sequestered by ferritin, also capable of diminishing formation of lipid peroxidation through hemoglobin-mediated generation of free radicals. In addition, CO is generated and biliverdin/bilirubin is produced, the former acting as an antioxidant through its role as a messenger and signaling molecule, the latter through their potent antioxidant properties.
As noted in Figure 3, the ability of bilirubin to prevent the oxidant-mediated vasoconstrictive actions of tumor necrosis factor and angiotensin II has been reported.2 Bilirubin, in low concentrations, is a scavenger of ROS in vitro, reduces oxidant-induced cellular injury and attenuates oxidant stress in vivo.11–14 It would be remiss not to point out that the above beneficial protective effects of CO, biliverdin/bilirubin and iron occur at physiological concentrations. They, like heme, are toxic when present in the cell at high levels, eg, kernicterus in newborn infants and iron storage diseases.
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The known effects of HO-1 activity will decrease heme, a powerful oxidant, and increase CO and bilirubin, thus preventing endothelial cell dysfunction and death by conserving NO,9,15 which is normally lost because of its binding to hemoglobin. Thus, CD163 transport of hemoglobin to macrophages with the subsequent induction of HO-1 by Hb may function as a protective mechanism to preserve NO-mediated vascular function. Increasing the macrophage expression of HO-1 in vivo, as described by Schaer et al, may provide a protective mechanism in the preservation of NO in HO-dependent mechanisms. HO-1–derived CO and bilirubin are crucial not only for the increase in cGMP and but also for restoration of vascular functions in diseases such as diabetes and hypertension where NOS is impaired. HO-1 product activity is associated with upregulation of other important antioxidant systems that protect the vasculature, such as extracellular SOD (EC-SOD), plasma catalase activity, and decreased superoxide production.9,15 Increased EC-SOD and decreased O2– formation which are essential factors in preserving the levels of NO necessary for other functions, including endothelial cell progenitor function (Figure 3).
For the first time, Schaer et al demonstrate a link between noninflammatory effect of hemoglobin clearance via increase in HO-1 expression and HO activity. This is an important initial step in the elucidation of the antiinflammatory activity associated with CD163 positive macrophages. They have ruled out the possibility that the hemoglobin CD163 pathway acts via an oxidative stress-signaling pathway. There remain numerous other potential avenues of exploration, including CO- and bilirubin-mediated protective pathways, signaling pathways, etc. CD163 represents a major pathway for the uptake of extracellular hemoglobin and free heme in the circulation. This pathway requires further examination to elucidate the mechanism of removal of hemoglobin and heme from circulation. The article by Schaer et al is an important first step in elucidating the relationship between CD163 and HO-1 induction and the mechanisms that are involved in noninflammatory hemoglobin clearance, opens up new strategic approaches for the effective management of Hb for a number of clinical disorders.
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Acknowledgments |
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We thank Jennifer Brown for her excellent editorial and secretarial assistance.
Sources of Funding
This work was supported by NIH grants DK068134, HL55601, and HL34300 (N.G.A.).
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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Related Article:
- Constitutive Endocytosis of CD163 Mediates Hemoglobin-Heme Uptake and Determines the Noninflammatory and Protective Transcriptional Response of Macrophages to Hemoglobin
- Christian A. Schaer, Gabriele Schoedon, Alexander Imhof, Michael O. Kurrer, and Dominik J. Schaer
Circ. Res. 2006 99: 943-950.[Abstract] [Full Text] [PDF]
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