The Anemia of Inflammation - An Iron Regulatory Hormone Weighs In

Nancy Andrews, M.D., PH.D.
Dr. Andrews is currently Leland Fikes Professor of Pediatrics and Associate Dean for Basic Sciences and Graduate Studies at Harvard Medical School, Senior Associate in Medicine at Children's Hospital Boston, and Distinguished Physician of the Dana-Farber Cancer Institute.

Anemia is a common complication in patients with chronic infections, malignancies, traumatic injuries, and other inflammatory conditions. Mind-boggling networks of cytokines and transcription factors have been invoked to explain the anemia of inflammation ("anemia of chronic disease"). Does it really have to be so complicated?

My favorite description of the cardinal features of the anemia of inflammation was published before we had any molecular understanding of this disorder.¹ As detailed by George Cartwright, clinical findings include decreased serum iron, increased stainable iron in macrophages, increased ferritin (likely reflecting increased macrophage stores), and hypoproliferative anemia. Serum iron levels are diminished by two mechanisms. First, macrophages that normally degrade old erythrocytes fail to return recycled iron to the circulation.  Second, intestinal iron absorption is impaired. These responses serve to sequester iron, keeping it away from invaders (microbes, tumors). But this is not without cost to the host. Less iron is available to erythroid precursor cells, accounting (at least in part) for the development of anemia.

When considered in this way, the anemia of inflammation could be described as a host defense that subjugates normal iron acquisition and distribution. Recently, a peptide hormone was discovered which seems to be responsible for this defense reaction.

Hepcidin (also known by its genetic name, HAMP) is a 20 to 25 amino acid protein produced in the liver.^2,3 It is similar to cysteine-rich "defensins," which act as anti-microbial peptides in innate immunity. But hepcidin has an extra role - it regulates iron homeostasis. Hepcidin production was revved up in response to induced iron overload in mice.³  An accidental "knockout" of the mouse hepcidin gene caused massive iron overload.⁴  Similarly, human patients lacking hepcidin developed early onset hemochromatosis.⁵ Deliberate overproduction of hepcidin resulted in iron deficiency.⁶ Taken together, these and other observations indicate that hepcidin normally acts to determine an iron "set-point,"  maintaining iron homeostasis.  Careful examination of mice lacking hepcidin suggested at least two regulatory activities of hepcidin - blocking intestinal iron absorption and interrupting macrophage iron recycling (Fig. 1).

A connection to the anemia of inflammation came from a study of patients with glycogen storage disease who were noted to have profound anemia that was indistinguishable from the anemia of inflammation. The anemia was invariably found in association with large hepatic adenomas. When the adenomas were removed, the anemia and iron abnormalities corrected.  It turned out that hepatocyte-like cells in the adenomas produced high levels of hepcidin.⁷ Thus, constitutive, high-level hepcidin production provided an explanation for their anemia, because of its role in interrupting macrophage iron recycling and intestinal absorption.

But hepatic adenomas are rare in patients with anemia of inflammation - this could not be the whole answer.  It turns out that inflammation, itself, stimulates hepcidin production. It was shown many years ago that treatment of mice with endotoxin causes hypoferremia (low serum iron).⁸ This can now be linked to recent observations that endotoxin induces hepcidin production.^3,9 Diverse patients with the anemia of inflammation have elevated levels of hepcidin excreted in the urine, suggesting that they, too, have responded by stimulating production of this peptide hormone.¹⁰

Does that mean that inflammatory cytokines are irrelevant in the anemia of inflammation?  Certainly not. Elegant studies from the Ganz lab have established that interleukin-6 (IL-6), a key inflammatory mediator, acts as an inducer of hepcidin production. IL-6 treatment stimulates hepcidin production in isolated hepatocytes and in hepatocyte-like cell lines.^10,11  Administration of IL-6 causes increased hepcidin production and hypoferremia in vivo.¹¹ Mice lacking IL-6 fail to induce hepcidin or become hypoferremic after treatment with endotoxin.11Taken together, these results leave little doubt that IL-6 links inflammation to hepcidin production (Fig. 2).¹²

An obvious prediction, therefore, would be that anemia and characteristic iron abnormalities would be found in other conditions associated with increased IL-6 production. This appears to be true. Patients with Castleman's disease have elevated IL-6 levels and anemia that has been difficult to explain. Patients with a variety of tumors, perhaps most notably multiple myeloma, also have elevated IL-6 levels and anemia. Finally, IL-6 levels are often increased in elderly patients, who frequently have unexplained anemia. It will be important to determine whether expression of hepcidin is increased in these disorders.

Is hepcidin the whole answer? Probably not - few disease processes are this easy. There remains a wealth of literature describing direct cytokine effects on erythroid precursors and macrophages, which must now be re-examined in light of the discovery of hepcidin. But it appears that induction of hepcidin can account for many of the clinical features of the anemia of inflammation, and that careful, therapeutic modulation of hepcidin activity might be useful in treatment of this disorder. Measurement of urinary hepcidin excretion, as pioneered by Ganz and colleagues, may be a valuable diagnostic method for determining when anemia is caused, at least in part, by inflammation.

References:

1. Cartwright GE. The anemia of chronic disorders. Semin Hematol. 1966;3:351-375.
2. Park CH, Valore EV, Waring AJ, Ganz T. Hepcidin, a urinary antimicrobial peptide synthesized in the liver. J Biol Chem. 2001;276:7806-7810.
3. Pigeon C, Ilyin G, Courselaud B, et al. A new mouse liver-specific gene, encoding a protein homologous to human antimicrobial peptide hepcidin, is overexpressed during iron overload. J Biol Chem. 2001;276:7811-7819.
4. Nicolas G, Bennoun M, Devaux I, et al. Lack of hepcidin gene expression and severe tissue iron overload in upstream stimulatory factor 2 (USF2) knockout mice. Proc Natl Acad Sci USA. 2001;98:8780-8785.
5. Roetto A, Papanikolaou G, Politou M, et al. Mutant antimicrobial peptide hepcidin is associated with severe juvenile hemochromatosis. Nat Genet. 2003;33:21-22.
6. Nicolas G, Bennoun M, Porteu A, et al. Severe iron deficiency anemia in transgenic mice expressing liver hepcidin. Proc Natl Acad Sci U S A. 2002;99:4596-4601.
7. Weinstein DA, Roy CN, Fleming MD, Loda MF, Wolfsdorf JI, Andrews NC. Inappropriate expression of hepcidin is associated with iron refractory anemia: implications for the anemia of chronic disease. Blood. 2002;100:3776-3781.
8. Cartwright GE, Lee GR. The anaemia of chronic disorders. Br J Haematol. 1971;21:147-152.
9. Nicolas G, Chauvet C, Viatte L, et al. The gene encoding the iron regulatory peptide hepcidin is regulated by anemia, hypoxia, and inflammation. J Clin Invest. 2002;110:1037-1044.
10. Nemeth E, Valore EV, Territo M, Schiller G, Lichtenstein A, Ganz T. Hepcidin, a putative mediator of anemia of inflammation, is a type II acute-phase protein. Blood. 2003;101:2461-2463.
11. Nemeth E, Rivera S, Gabayan V, et al. IL-6 mediates hypoferremia of inflammation by inducing the synthesis of the iron regulatory hormone hepcidin. J Clin Invest. 2004;113:1271-1276.
12. Andrews NC. Anemia of inflammation: the cytokine-hepcidin link. J Clin Invest. 2004;113:1251-1253.

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