The Emerging Role of Hepcidin in Iron Metabolism
Research in recent years has increased knowledge about hepcidin and its integrated role in the absorption and movement of iron in the body – a breakthrough which has started to provide a more functional view of iron metabolism.
“There has been a revolution in the understanding of iron metabolism and regulation.” said Dr. Joshua Zaritsky, Assistant Professor in the Division of Pediatric Nephrology at UCLA. “Where before we used to think it was just how much iron was on board, we recognize it’s more complicated than that and involves many intricate processes.”
The discovery of hepcidin and further understanding of how it inhibits the movement of iron and is itself regulated may eventually help clinicians better evaluate a patient’s iron status and may assist in more effective, efficient treatment for anemia of chronic disease.
Predicting Results of Iron Therapy
Ferritin and transferrin saturation (TSAT) tests, though imperfect, are commonly used as markers of iron status by reflecting the body’s iron stores and serum iron availability, respectively. In patients with absolute iron deficiency, both of these values are low, and oral iron and parenteral iron supplementation can alleviate iron deficiency and anemia. Similarly, for patients with chronic inflammatory conditions – like chronic kidney disease (CKD), cancer, inflammatory bowel disease and rheumatoid arthritis – maintaining healthy iron levels is a key step in anemia management.
Yet, in the presence of inflammation, the results of these standard tests of iron storage and circulating iron are not the best predictors to how a patient will respond to iron therapy. For example, one study of CKD patients on hemodialysis with an adequate ferritin (>500 ng/mL) and a low TSAT (<20%) exhibited a wide variability of response rates to iron therapy, with neither ferritin nor TSAT predicting response.1 In many circumstances like these, without more information there is no guarantee stored iron in the body or additional iron given via supplements will be incorporated into new, healthy RBCs.
Iron Regulator: Hepcidin
The discovery of hepcidin in 2001 and subsequent research on the peptide has helped to provide a more functional view of iron metabolism and elucidate the mechanisms affecting iron status in patients with chronic inflammation and anemia. Produced in the liver, hepcidin has been shown to prevent the absorption of iron from the digestive tract and also inhibit the release of stored iron from macrophages and hepatocytes.2
Each of these effects is mediated by hepcidin causing the rapid internalization and degradation of the main iron export channel ferroportin, located in cellular membranes.3 Through this effect, “Hepcidin allows the body to precisely control the level of iron in the body much like a thermostat would precisely regulate the temperature of a house,” added Dr. Zaritsky.
Natural Hepcidin Regulation & Consequences
As a regulator, changes in hepcidin levels keep iron absorption and movement appropriate to the need to replenish iron so that it is available for erythropoiesis. To understand the body’s regulation of hepcidin, research has examined the complex feedback mechanism involving iron loading, the body’s natural inflammatory response, and the intensity of erythropoiesis as each interact with hepcidin production.
Mediated by the hepatocytes’ bone morphogenetic protein receptor complex, serum iron levels likely influence how much new hepcidin is produced in the liver – low iron levels decreasing hepcidin production and high iron levels encouraging its production.4 When hepcidin levels are low, the body’s cells and tissues can become overloaded with iron because the digestive tract cannot inhibit absorption and the rest of the body does not have a way to excrete excess iron.
On the other hand, an elevated hepcidin level will reduce iron absorption in the digestive tract which could lead to iron deficiency over time or reduce the effectiveness of oral iron supplementation. Elevated hepcidin levels in the reticuloendothelial system can also inhibit the movement of stored iron by degrading ferroportin in macrophages and hepatocytes. Reduced availability of iron in the presence of adequate iron stores is often referred to as functional iron deficiency. A more serious progression of inhibited iron stores is known as reticuloendothelial blockade in which iron supplementation would be ineffective and could potentially lead to iron overload.2
- Iron loading – Serum iron levels are believed to influence how much hepcidin is produced in the liver, with low iron levels decreasing hepcidin production and high iron levels encouraging its production.
- Inflammation – Circulating cytokine, likely IL-6, is believed to increase hepcidin levels and possibly cause reticuloendothelial blockade.
- Erythropoiesis activity – Increased production of red blood cells has been speculated to decrease production of hepcidin, functionally increasing the absorption and movement of iron in the body.
Increased hepcidin has also been associated with elevated cytokine levels related to the body’s own inflammatory response to a chronic condition. Circulating cytokine, likely IL-6, is believed to induce production by binding a transcription activator to the hepcidin promoter.5,6 In addition, inflammation appears to cause reticuloendothelial blockade, likely mediated through the upregulation of hepcidin.
The discovery of hepcidin regulation by inflammatory mechanisms has greatly increased the understanding of anemia which affects many patients with chronic inflammatory ailments. As knowledge of these mechanisms progresses, additional therapeutic interventions may become available for clinicians treating patients with anemia of chronic disease.
By means of an unknown signal transduction pathway, increased erythropoiesis in the bone marrow has been speculated to decrease production of hepcidin. Functionally, this regulation reinforces the idea that if the body is boosting red blood cell production it will simultaneously need to increase iron movement and replenish iron stores, by means of a lower level of circulating hepcidin.
Indirectly, hepcidin may also be an important factor in determining if patients will respond to administration of erythropoiesis-stimulating agents (ESAs) for anemia treatment. In this situation, high hepcidin levels caused by increased inflammation may reduce iron movement and act like an emergency brake on RBC production, effectively counteracting the signal to decrease hepcidin during the accelerated erythropoiesis induced by ESA treatment. It is hypothesized that severely halting this flow of iron into RBC production can lead to patients needing larger ESA doses or can render them unresponsive to ESA treatment entirely.
As more has become known about hepcidin’s role in iron metabolism, researchers have concurrently developed different methodologies and assays to accurately measure hepcidin levels and allow hepcidin research to flourish. With success, the inexpensive measurement of hepcidin could also greatly benefit clinicians diagnosing and treating patients with iron-related problems.
“Knowing a patient's hepcidin level may give you a functional view of iron – if they have the iron, are they able to utilize it and are they able to incorporate it into their red blood cells,” explained Dr. Zaritsky.
Researcher are exploring the potential clinical applications of measuring and analyzing a patient's hepcidin level, including:
- Early detection of iron deficiency in infants
- Identifying patients with anemia of chronic disease who are nonresponsive to ESAs
- Discovering patients who will require IV iron before oral iron proves ineffective
One early method included a urinary assay which measured hepcidin concentrations by antihepcidin antibody and chemiluminescence. This early assay relied on the unverified relationship between plasma hepcidin levels and normalized hepcidin level in the urine,7 which may be unequivocal for CKD patients with decreased kidney function and lowered ability to clear hepcidin from the blood stream.
More recent methodologies have set out to measure serum hepcidin, including mass spectrometry, which can supply quantitative serum and urine hepcidin measurements using a combination of weak cation exchange chromatography and time-of-flight mass spectrometry.8,9 Although it can measure three isoforms, hepcidin-20, -22, and -25, it has not become readily used due to the limited availability of the necessary equipment. A separate competitive enzyme-linked immunoassay was recently developed to detect serum hepcidin which has a lower limit of detection (5 ng/mL) and enough sensitivity to detect hepcidin fluctuations due to diurnal variation and in response to oral iron.10 This assay may also be supplied to clinicians inexpensively upon the development of a manageable kit.2
“Given that it has a known function in iron homeostasis, knowledge of a patient’s hepcidin level may give you useful information about a patient’s iron status and availability,” said Dr. Brian Young, Assistant Professor in the Department of Medicine at UCLA, and added that, “Developing an inexpensive hepcidin assay will be important as current markers of iron status, such as ferritin and TSAT, do have limitations.”
Being able to accurately measure hepcidin levels, researchers are exploring the benefits of knowing a patient's hepcidin level, including the possibilities to detect iron deficiency in infants, identify patients with anemia of chronic disease who are nonresponsive to ESAs, and discover patients who will require intravenous iron supplements before oral iron proves ineffective.10
“Knowing more about hepcidin will allow us to treat anemia of chronic disease more efficiently and proficiently,” stated Dr. Zaritsky. “Down the road, it could lead to therapeutic modulation of hepcidin levels and the way the body uses the iron stores it has.”
Although not measured readily at the bedside just yet, knowing a patient’s hepcidin level and what it means for their iron metabolism may be just what the doctor ordered to effectively and efficiently manage iron-related conditions and anemia of chronic disease.
Thanks to Dr. Brian Young and Dr. Joshua Zaritsky of UCLA for their assistance in producing this article.
- Coyne DW, Kapoian T, Suki W, Singh AK, Moran JE, Dahl NV, Rizkala AR; DRIVE Study Group. Ferric gluconate is highly efficacious in anemic hemodialysis patients with high serum ferritin and low transferrin saturation: results of the Dialysis Patients' Response to IV Iron with Elevated Ferritin (DRIVE) Study. J Am Soc Nephrol. 2007 Mar;18(3):975-84. Link.
- Young B, Zaritsky J. Hepcidin for Clinicians. Clin J Am Soc Nephrol. 2009 Jun 25. Link.
- Nemeth E, Tuttle MS, Powelson J, Vaughn MB, Donovan A, Ward DM, Ganz T, Kaplan J. Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization. Science. 2004 Dec 17;306(5704):2090-93. Link.
- Nemeth E, Rivera S, Gabayan V, Keller C, Taudorf S, Pedersen BK, Ganz T. IL-6 mediates hypoferremia of inflammation by inducing the synthesis of the iron regulatory hormone hepcidin. J Clin Invest. 2004 May;113(9):1271-76. Link.
- Schmidt PJ, Toran PT, Giannetti AM, Bjorkman PJ, Andrews NC. The transferrin receptor modulates Hfe-dependent regulation of hepcidin expression. Cell Metab. 2008 Mar;7(3):205-14. Link.
- Goswami T, Andrews NC. Hereditary hemochromatosis protein, HFE, interaction with transferrin receptor 2 suggests a molecular mechanism for mammalian iron sensing. J Biol Chem. 2006 Sep 29;281(39):28494-98. Link.
- Ganz T, Olbina G, Girelli D, Nemeth E, Westerman M. Immunoassay for human serum hepcidin. Blood. 2008 Nov 15;112(10):4292-97. Link.
- UMC St Radboud Onderzoek Klinische Chemie B.V. Hepcidin Analysis: Service in Mass Spectrometry: Method. Link. Accessed: July 9, 2009.
- Kemna E, Tjalsma H, Laarakkers C, Nemeth E, Willems H, Swinkels D. Novel urine hepcidin assay by mass spectrometry. Blood. 2005 Nov 1;106(9):3268-70. Link.
- Brugnara C. Blood. An immunoassay for human serum hepcidin at last: Ganz klar? 2008 Nov 15;112(10):3922-23. Link.