Commentary For reprint orders, please contact [email protected]
Can we target the a2-macroglobulin–hepcidin interaction to treat pathologic hypoferremia? “The advent of a new range of hepcidin therapeutics may now just be a matter of time.” Keywords: a2-macroglobulin n anemia of inflammation n hepcidin n iron metabolism n peptide mimetics
Hepcidin: the hormone of iron metabolism Hepcidin is a small disulfide-rich peptide (~2.8 kDa) that is produced by hepatocytes [4,5], and, at lower levels, by the kidney and heart [1]. Once secreted into the plasma, the major known biological activity of this peptide is to post-translationally decrease the expression of the cellular iron exporter, ferroportin 1 (Fpn1) [3]. This downregulation of Fpn1 is most significant in three major cell types: duodenal enterocytes, which are responsible for dietary iron absorption; mononuclear phagocytes, which are responsible for iron recycling from effete erythroc ytes; and hepatocytes, which are involved in iron storage [1]. Hepcidin decreases Fpn1 levels via lysosomal degradation [1], which reduces the amount of iron released by the affected cells. Therefore, hepcidin decreases circulating iron levels.
Hepcidin dysregulation & the anemia of inflamation The dysregulation of hepcidin is etiologically involved in many diseases of iron metabolism. Pathologically low hepcidin induces iron overload (e.g., in hereditary hemochromatosis types 1–3 [1]), while pathologically high hepcidin typically induces iron-limited anemia [1]. Importantly, the anemia of inflammation (AI), sometimes referred to as the ‘anemia of chronic disease’, is the second most common anemia after iron-deficiency anemia [6]. Indeed, AI is the most common form of anemia in hospitalized patients, and is most common in individuals with a chronically activated immune system. This can result from conditions such as chronic infections, inflammatory diseases, kidney disease, rejection following solid organ transplantation and various malignancies [6,7]. AI also includes the closely related ‘anemia of critical illness’. This typically occurs in critic ally ill patients soon after their admission into intensive care (e.g., following sepsis, surgery and major trauma) [8]. The major pathophysiological event in AI is the anemia resulting from iron-limited erythropoiesis, which leads to decreased red blood cell production. The iron limitation is primarily due to an increase in circulating levels of the inflammatory cytokine, IL-6, that upregulates expression of hepcidin, a type 2 acute-phase reactant [9,10]. An increase in circulating hepcidin downregulates Fpn1 expression in major iron-exporting cells, leading to increased intracellular iron retention, and, ultimately, results in decreased dietary iron absorption [6,7]. This sequestration of iron within the major ironrecycling cells greatly diminishes the body’s effective iron supply for erythropoiesis and is a major cause of the restricted iron availability to erythroid precursors [7]. It is now known that hepcidin dysregulation is a causal contributor to AI pathogenesis. Indeed,
10.4155/FMC.13.175 © 2014 Future Science Ltd
Future Med. Chem. (2014) 6(1), 13–16
Iron is an essential transition metal for life [1,2]. The ability of this metal to readily engage in single-electron oxidations and reductions between the ferrous (FeII) and the ferric (FeIII) states is largely responsible for its biological utility. Crucially, this characteristic of iron presents a ‘double-edged sword’ that poses significant oxidative risks to organisms (e.g., in promoting hydroxyl formation via the Fenton reaction). Over evolutionary time, the organism’s ‘attempt’ to minimize the cost–benefit ratio of iron’s promiscuous redox activity has resulted in a highly coordinated system for the regulation of cellular and systemic iron metabolism [1,2]. Much of the past decade of research in the iron metabolism field has focused on hepcidin, the ‘hormone of iron metabolism’, which appears to be the lynchpin of systemic iron homeostasis [3]. This 25-amino acid peptide is predominantly hepatically produced in a manner regulated by iron, inflammation, hypoxia and erythroid activity [3].
Darius JR Lane Department of Pathology & Bosch Institute, Molecular Pharmacology & Pathology Program, Blackburn Building, University of Sydney, Sydney, New South Wales, 2006, Australia
Des R Richardson Author for correspondence: Department of Pathology & Bosch Institute, Molecular Pharmacology & Pathology Program, Blackburn Building, University of Sydney, Sydney, New South Wales, 2006, Australia Tel.: +61 2 9036 6548 Fax: +61 2 9036 6549 E-mail: [email protected]
ISSN 1756-8919
13
Commentary | Lane & Richardson Theurl and colleagues have recently demonstrated (using an animal model of AI) that the pharmacological blockade of endogenous hep cidin production leads to the release of ‘trapped’ iron from splenic macrophages (a type of mononuclear phagocyte), stimulation of erythropoiesis and an improvement of the anemia [11]. Hepcidin is transported in the blood by a2-macroglobulin As demonstrated by the proof-of-concept results of Theurl and colleagues [11], hepcidin-lowering therapeutics have the potential to play an important role in the treatment of AI. However, while their strategy was to target and downregulate the transcription of the hepcidin gene, other strat egies that target hepcidin transport and renal clearance may have equal or superior merit. Therefore, developing an in-depth understanding of hepcidin transport and excretion is paramount. The classical view of hepcidin transport is that it traverses the circulation as a free peptide. However, emerging evidence indicates that hepcidin is bound to plasma carrier proteins, such as a2-macroglobulin (a2-M) and albumin [12,13], and possibly to other proteins, such as a1-antitrypsin [14]. In 2009, Peslova and colleagues identified a2-M as the major specific hepcidin-binding partner in human plasma, accounting for up to 30% of protein-bound hepcidin [12]. They further demonstrated that a2-M in its native form possesses two highaffinity binding sites (K d = 177 ± 27 nM). Intriguingly, upon the well-described activation of a2-M, which normally occurs in vivo by the action of proteases, but can be mimicked in vitro by incubation with methylamine (i.e., a2-M-MA), further high-affinity binding sites that bind hepcidin with apparent allosteric cooperativity (K d = ~300 nM) were exposed [12]. Interestingly, although there was quantitatively more hepcidin bound to albumin, the binding of this peptide to albumin was of a far lower affinity (Kd = 1 mM) and nonsaturable [12]. Intriguingly, the complexation of hepcidin with a2-M led to a significantly greater decrease in Fpn1 levels in J744 cells than hepcidin alone or hepcidin in the presence of albumin [12]. In fact, the combination of albumin and hepcidin did not enhance Fpn1 downregulation further compared with hepcidin alone [12]. A recent study by Huang and colleagues determined, through in vivo studies, that hepcidin bound to either a2-M or a2-M-MA had a greater ability to lower serum iron than an equimolar 14
Future Med. Chem. (2014) 6(1)
concentration of hepcidin alone [13]. Interestingly, the ability of hepcidin bound to a2-M or a2-M-MA to downregulate Fpn1 levels was independent of the a2-M receptor, lipoprotein receptor-related protein 1 [13]. This suggests that endocytic uptake of the a2-M–hepcidin complex is not required for hepcidin to mediate its biochemical effect; a feature that may distinguish hepcidin from other a2-M peptide cargoes [15]. Notably, the complexation of hepcidin with a2-M led to a significant decrease in the appearance of hepcidin in the urine [13]. This is important because hepcidin is a small, lowMW peptide (~2.8 kDa) that would be expected to be readily cleared by the kidney. However, the binding of hepcidin to a large protein, such as a2-M (~725 kDa), would be expected to decrease the peptide’s renal clearance, which is precisely what was observed [13]. Taken together, these results suggest that the binding of hepcidin to a2-M increased its efficacy in downregulating Fpn1, and that this activity may have occurred due to decreased renal clearance of the peptide (i.e., an increase in its pharmacokinetic halflife). In addition, such results suggest that the translational outcome of targeting the binding of hepcidin by a2-M, for example by a competitive inhibitor, may prove to be an effective ‘hepcidin-lowering’ therapeutic strategy in hepcidindependent pathologic hypoferremia (e.g., AI). That is, decreased binding of hepcidin to a2-M would lead to increased renal excretion of this low-MW peptide. Evidence for the binding interaction between hepcidin and a2-M a2 -M has long been known to be a multi functional plasma protein that is capable of interacting with and ‘trapping’ a wide range of proteases [16]. The proteases (specifically endopeptidases) cleave within a ‘bait’ region near the middle of the polypeptide chain, which leads to a unique conformational change that entraps the protease [17]. This protease-mediated process activates a2-M, which generates novel binding sites for cargo molecules, such as cytokines and peptides [15]. Indeed, a wide variety of hormones or hormone complexes are known to circulate bound to a2-M [18,19]. Certain peptides, such as the human defensin, HNP-1, are specifically bound to activated a2-M [20]. Notably, HNP-1 belongs to the same family of highly disulfidebonded peptides as hepcidin. Therefore, the discovery that hepcidin is bound specifically to a2-M should not be entirely surprising. future science group
Can we target the a2-macroglobulin–hepcidin interaction to treat pathologic hypoferremia? In the case of HNP-1 binding by activated a2-M, it was observed that the binding of the peptide was ablated by treatment of samples with dithiothreitol (a disulfide-reducing agent), and that pretreatment of samples with iodoacetamide (a thiol-alkylating agent) greatly diminished the extent of binding [20]. Similarly, Huang and colleagues observed that the binding of hepcidin by a2-M was disrupted by dithiothreitol and partially prevented by iodoacetamide [13]. In the case of hepcidin, it remains unclear whether there are intermolecular disulfides formed between hepcidin and a2-M, or whether the effect of thiol-modifying agents could be explained by indirect effects on protein structure and hepcidin binding. However, what is apparent is that a pool of hepcidin bound to a2-M is labile, as demonstrated by ultrafiltration analyses, and that the binding of hepcidin to a2-M is exquisitely sensitive to small alterations in pH [13]. For instance, total loss of hepcidin binding occurs upon reducing the pH from 7.4 to 6 [13]. Can we target a2-macroglobulin with hepcidin mimetics? The knowledge that hepcidin is bound to a2-M in plasma provides the distinct possibility that we may be able to lower plasma hepcidin in certain disease states, such as AI, by disrupting the binding of hepcidin to a2 -M, which
| Commentary
could be used to increase the renal excretion of the peptide into urine. Theoretically, this may be best achieved by developing hepcidin mimetics (e.g., competitor peptides) that can be used to outcompete endogenous hepcidin for binding to a2-M. Importantly, such mimetics would need to effectively bind to a2 -M, but without promoting Fpn1 downregulation. In order to proceed rationally with such a quest, a greater depth of knowledge of the hepcidin binding sites on a2 -M and binding mechanisms are required. The advent of a new range of hepcidin-lowering therapeutics that could provide a major defense against wide-spread pathologic hypoferremias such as AI may now just be a matter of time. Financial & competing interests disclosure DJR Lane would like to thank the Cancer Institute New South Wales for an Early Career Fellowship [10/ECF/2– 18] and the National Health and Medical Research Council (NHMRC) of Australia for an Early Career Postdoctoral Fellowship [1013810]. DR Richardson thanks the NHMRC for a Senior Principal Research Fellowship and project grants. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.
References 1
2
3
4
5
Lawen A, Lane DJR. Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action. Antioxid. Redox Signal. 18, 2473–2507 (2013). Richardson DR, Lane DJR, Becker EM et al. Mitochondrial iron trafficking and the integration of iron metabolism between the mitochondrion and cytosol. Proc. Natl Acad. Sci. USA 107(24), 10775–10782 (2010). Ganz T. Hepcidin and iron regulation, 10 years later. Blood 117(17), 4425–4433 (2011). Krause A, Neitz S, Magert HJ et al. LEAP-1, a novel highly disulfide-bonded human peptide, exhibits antimicrobial activity. FEBS Lett. 480(2–3), 147–150 (2000). Park CH, Valore EV, Waring AJ, Ganz T. Hepcidin, a urinary antimicrobial peptide synthesized in the liver. J. Biol. Chem. 276(11), 7806–7810 (2001).
future science group
6
Ganz T. Molecular pathogenesis of anemia of chronic disease. Pediatr. Blood Cancer 46(5), 554–557 (2006).
7
Weiss G. Iron metabolism in the anemia of chronic disease. Biochim. Biophys. Acta 1790(7), 682–693 (2009).
8
Corwin HL. The role of erythropoietin therapy in the critically ill. Transfus. Med. Rev. 20(1), 27–33 (2006).
9
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. 113(9), 1271–1276 (2004).
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 101(7), 2461–2463 (2003). 11 Theurl I, Schroll A, Sonnweber T et al.
Pharmacologic inhibition of hepcidin expression reverses anemia of chronic
www.future-science.com
disease in rats. Blood 118(18), 4977–4984 (2011). 12 Peslova G, Petrak J, Kuzelova K et al.
Hepcidin, the hormone of iron metabolism, is bound specifically to a-2-macroglobulin in blood. Blood 113(24), 6225–6236 (2009). 13 Huang ML, Austin CJ, Sari MA et al.
Hepcidin bound to a-2-macroglobulin reduces ferroportin-1 expression and enhances its activity at reducing serum iron levels. J. Biol Chem. 288(35), 25450–25465 (2013). 14 Pandur E, Nagy J, Poor VS et al. a-1
antitrypsin binds preprohepcidin intracellularly and prohepcidin in the serum. FEBS J. 276(7), 2012–2021 (2009). 15 Borth W. a2-macroglobulin, a
multifunctional binding protein with targeting characteristics. FASEB J. 6(15), 3345–3353 (1992). 16 Barrett AJ, Starkey PM. The interaction of
a-2-macroglobulin with proteinases.
15
Commentary | Lane & Richardson Characteristics and specificity of the reaction, and a hypothesis concerning its molecular mechanism. Biochem. J. 133(4), 709–724 (1973). 17 Rehman AA, Ahsan H, Khan FH.
a-2-macroglobulin: a physiological guardian. J. Cell. Physiol. 228(8), 1665–1675 (2013).
16
18 Birkenmeier G, Kampfer I, Kratzsch J,
Schellenberger W. Human leptin forms complexes with a-2-macroglobulin which are recognized by the a-2-macroglobulin receptor/ low density lipoprotein receptor-related protein. Eur. J. Endocrinol. 139(2), 224–230 (1998). 19 Westwood M, Aplin JD, Collinge IA, Gill A,
White A, Gibson JM. a-2-macroglobulin:
Future Med. Chem. (2014) 6(1)
a new component in the insulin-like growth factor/insulin-like growth factor binding protein-1 axis. J. Biol. Chem. 276(45), 41668–41674 (2001). 20 Panyutich A, Ganz T. Activated a2-
macroglobulin is a principal defensinbinding protein. Am. J. Respir. Cell Mol. Biol. 5(2), 101–106 (1991).
future science group
Can we target the a2-macroglobulin–hepcidin interaction to treat pathologic hypoferremia? “The advent of a new range of hepcidin therapeutics may now just be a matter of time.” Keywords: a2-macroglobulin n anemia of inflammation n hepcidin n iron metabolism n peptide mimetics
Hepcidin: the hormone of iron metabolism Hepcidin is a small disulfide-rich peptide (~2.8 kDa) that is produced by hepatocytes [4,5], and, at lower levels, by the kidney and heart [1]. Once secreted into the plasma, the major known biological activity of this peptide is to post-translationally decrease the expression of the cellular iron exporter, ferroportin 1 (Fpn1) [3]. This downregulation of Fpn1 is most significant in three major cell types: duodenal enterocytes, which are responsible for dietary iron absorption; mononuclear phagocytes, which are responsible for iron recycling from effete erythroc ytes; and hepatocytes, which are involved in iron storage [1]. Hepcidin decreases Fpn1 levels via lysosomal degradation [1], which reduces the amount of iron released by the affected cells. Therefore, hepcidin decreases circulating iron levels.
Hepcidin dysregulation & the anemia of inflamation The dysregulation of hepcidin is etiologically involved in many diseases of iron metabolism. Pathologically low hepcidin induces iron overload (e.g., in hereditary hemochromatosis types 1–3 [1]), while pathologically high hepcidin typically induces iron-limited anemia [1]. Importantly, the anemia of inflammation (AI), sometimes referred to as the ‘anemia of chronic disease’, is the second most common anemia after iron-deficiency anemia [6]. Indeed, AI is the most common form of anemia in hospitalized patients, and is most common in individuals with a chronically activated immune system. This can result from conditions such as chronic infections, inflammatory diseases, kidney disease, rejection following solid organ transplantation and various malignancies [6,7]. AI also includes the closely related ‘anemia of critical illness’. This typically occurs in critic ally ill patients soon after their admission into intensive care (e.g., following sepsis, surgery and major trauma) [8]. The major pathophysiological event in AI is the anemia resulting from iron-limited erythropoiesis, which leads to decreased red blood cell production. The iron limitation is primarily due to an increase in circulating levels of the inflammatory cytokine, IL-6, that upregulates expression of hepcidin, a type 2 acute-phase reactant [9,10]. An increase in circulating hepcidin downregulates Fpn1 expression in major iron-exporting cells, leading to increased intracellular iron retention, and, ultimately, results in decreased dietary iron absorption [6,7]. This sequestration of iron within the major ironrecycling cells greatly diminishes the body’s effective iron supply for erythropoiesis and is a major cause of the restricted iron availability to erythroid precursors [7]. It is now known that hepcidin dysregulation is a causal contributor to AI pathogenesis. Indeed,
10.4155/FMC.13.175 © 2014 Future Science Ltd
Future Med. Chem. (2014) 6(1), 13–16
Iron is an essential transition metal for life [1,2]. The ability of this metal to readily engage in single-electron oxidations and reductions between the ferrous (FeII) and the ferric (FeIII) states is largely responsible for its biological utility. Crucially, this characteristic of iron presents a ‘double-edged sword’ that poses significant oxidative risks to organisms (e.g., in promoting hydroxyl formation via the Fenton reaction). Over evolutionary time, the organism’s ‘attempt’ to minimize the cost–benefit ratio of iron’s promiscuous redox activity has resulted in a highly coordinated system for the regulation of cellular and systemic iron metabolism [1,2]. Much of the past decade of research in the iron metabolism field has focused on hepcidin, the ‘hormone of iron metabolism’, which appears to be the lynchpin of systemic iron homeostasis [3]. This 25-amino acid peptide is predominantly hepatically produced in a manner regulated by iron, inflammation, hypoxia and erythroid activity [3].
Darius JR Lane Department of Pathology & Bosch Institute, Molecular Pharmacology & Pathology Program, Blackburn Building, University of Sydney, Sydney, New South Wales, 2006, Australia
Des R Richardson Author for correspondence: Department of Pathology & Bosch Institute, Molecular Pharmacology & Pathology Program, Blackburn Building, University of Sydney, Sydney, New South Wales, 2006, Australia Tel.: +61 2 9036 6548 Fax: +61 2 9036 6549 E-mail: [email protected]
ISSN 1756-8919
13
Commentary | Lane & Richardson Theurl and colleagues have recently demonstrated (using an animal model of AI) that the pharmacological blockade of endogenous hep cidin production leads to the release of ‘trapped’ iron from splenic macrophages (a type of mononuclear phagocyte), stimulation of erythropoiesis and an improvement of the anemia [11]. Hepcidin is transported in the blood by a2-macroglobulin As demonstrated by the proof-of-concept results of Theurl and colleagues [11], hepcidin-lowering therapeutics have the potential to play an important role in the treatment of AI. However, while their strategy was to target and downregulate the transcription of the hepcidin gene, other strat egies that target hepcidin transport and renal clearance may have equal or superior merit. Therefore, developing an in-depth understanding of hepcidin transport and excretion is paramount. The classical view of hepcidin transport is that it traverses the circulation as a free peptide. However, emerging evidence indicates that hepcidin is bound to plasma carrier proteins, such as a2-macroglobulin (a2-M) and albumin [12,13], and possibly to other proteins, such as a1-antitrypsin [14]. In 2009, Peslova and colleagues identified a2-M as the major specific hepcidin-binding partner in human plasma, accounting for up to 30% of protein-bound hepcidin [12]. They further demonstrated that a2-M in its native form possesses two highaffinity binding sites (K d = 177 ± 27 nM). Intriguingly, upon the well-described activation of a2-M, which normally occurs in vivo by the action of proteases, but can be mimicked in vitro by incubation with methylamine (i.e., a2-M-MA), further high-affinity binding sites that bind hepcidin with apparent allosteric cooperativity (K d = ~300 nM) were exposed [12]. Interestingly, although there was quantitatively more hepcidin bound to albumin, the binding of this peptide to albumin was of a far lower affinity (Kd = 1 mM) and nonsaturable [12]. Intriguingly, the complexation of hepcidin with a2-M led to a significantly greater decrease in Fpn1 levels in J744 cells than hepcidin alone or hepcidin in the presence of albumin [12]. In fact, the combination of albumin and hepcidin did not enhance Fpn1 downregulation further compared with hepcidin alone [12]. A recent study by Huang and colleagues determined, through in vivo studies, that hepcidin bound to either a2-M or a2-M-MA had a greater ability to lower serum iron than an equimolar 14
Future Med. Chem. (2014) 6(1)
concentration of hepcidin alone [13]. Interestingly, the ability of hepcidin bound to a2-M or a2-M-MA to downregulate Fpn1 levels was independent of the a2-M receptor, lipoprotein receptor-related protein 1 [13]. This suggests that endocytic uptake of the a2-M–hepcidin complex is not required for hepcidin to mediate its biochemical effect; a feature that may distinguish hepcidin from other a2-M peptide cargoes [15]. Notably, the complexation of hepcidin with a2-M led to a significant decrease in the appearance of hepcidin in the urine [13]. This is important because hepcidin is a small, lowMW peptide (~2.8 kDa) that would be expected to be readily cleared by the kidney. However, the binding of hepcidin to a large protein, such as a2-M (~725 kDa), would be expected to decrease the peptide’s renal clearance, which is precisely what was observed [13]. Taken together, these results suggest that the binding of hepcidin to a2-M increased its efficacy in downregulating Fpn1, and that this activity may have occurred due to decreased renal clearance of the peptide (i.e., an increase in its pharmacokinetic halflife). In addition, such results suggest that the translational outcome of targeting the binding of hepcidin by a2-M, for example by a competitive inhibitor, may prove to be an effective ‘hepcidin-lowering’ therapeutic strategy in hepcidindependent pathologic hypoferremia (e.g., AI). That is, decreased binding of hepcidin to a2-M would lead to increased renal excretion of this low-MW peptide. Evidence for the binding interaction between hepcidin and a2-M a2 -M has long been known to be a multi functional plasma protein that is capable of interacting with and ‘trapping’ a wide range of proteases [16]. The proteases (specifically endopeptidases) cleave within a ‘bait’ region near the middle of the polypeptide chain, which leads to a unique conformational change that entraps the protease [17]. This protease-mediated process activates a2-M, which generates novel binding sites for cargo molecules, such as cytokines and peptides [15]. Indeed, a wide variety of hormones or hormone complexes are known to circulate bound to a2-M [18,19]. Certain peptides, such as the human defensin, HNP-1, are specifically bound to activated a2-M [20]. Notably, HNP-1 belongs to the same family of highly disulfidebonded peptides as hepcidin. Therefore, the discovery that hepcidin is bound specifically to a2-M should not be entirely surprising. future science group
Can we target the a2-macroglobulin–hepcidin interaction to treat pathologic hypoferremia? In the case of HNP-1 binding by activated a2-M, it was observed that the binding of the peptide was ablated by treatment of samples with dithiothreitol (a disulfide-reducing agent), and that pretreatment of samples with iodoacetamide (a thiol-alkylating agent) greatly diminished the extent of binding [20]. Similarly, Huang and colleagues observed that the binding of hepcidin by a2-M was disrupted by dithiothreitol and partially prevented by iodoacetamide [13]. In the case of hepcidin, it remains unclear whether there are intermolecular disulfides formed between hepcidin and a2-M, or whether the effect of thiol-modifying agents could be explained by indirect effects on protein structure and hepcidin binding. However, what is apparent is that a pool of hepcidin bound to a2-M is labile, as demonstrated by ultrafiltration analyses, and that the binding of hepcidin to a2-M is exquisitely sensitive to small alterations in pH [13]. For instance, total loss of hepcidin binding occurs upon reducing the pH from 7.4 to 6 [13]. Can we target a2-macroglobulin with hepcidin mimetics? The knowledge that hepcidin is bound to a2-M in plasma provides the distinct possibility that we may be able to lower plasma hepcidin in certain disease states, such as AI, by disrupting the binding of hepcidin to a2 -M, which
| Commentary
could be used to increase the renal excretion of the peptide into urine. Theoretically, this may be best achieved by developing hepcidin mimetics (e.g., competitor peptides) that can be used to outcompete endogenous hepcidin for binding to a2-M. Importantly, such mimetics would need to effectively bind to a2 -M, but without promoting Fpn1 downregulation. In order to proceed rationally with such a quest, a greater depth of knowledge of the hepcidin binding sites on a2 -M and binding mechanisms are required. The advent of a new range of hepcidin-lowering therapeutics that could provide a major defense against wide-spread pathologic hypoferremias such as AI may now just be a matter of time. Financial & competing interests disclosure DJR Lane would like to thank the Cancer Institute New South Wales for an Early Career Fellowship [10/ECF/2– 18] and the National Health and Medical Research Council (NHMRC) of Australia for an Early Career Postdoctoral Fellowship [1013810]. DR Richardson thanks the NHMRC for a Senior Principal Research Fellowship and project grants. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.
References 1
2
3
4
5
Lawen A, Lane DJR. Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action. Antioxid. Redox Signal. 18, 2473–2507 (2013). Richardson DR, Lane DJR, Becker EM et al. Mitochondrial iron trafficking and the integration of iron metabolism between the mitochondrion and cytosol. Proc. Natl Acad. Sci. USA 107(24), 10775–10782 (2010). Ganz T. Hepcidin and iron regulation, 10 years later. Blood 117(17), 4425–4433 (2011). Krause A, Neitz S, Magert HJ et al. LEAP-1, a novel highly disulfide-bonded human peptide, exhibits antimicrobial activity. FEBS Lett. 480(2–3), 147–150 (2000). Park CH, Valore EV, Waring AJ, Ganz T. Hepcidin, a urinary antimicrobial peptide synthesized in the liver. J. Biol. Chem. 276(11), 7806–7810 (2001).
future science group
6
Ganz T. Molecular pathogenesis of anemia of chronic disease. Pediatr. Blood Cancer 46(5), 554–557 (2006).
7
Weiss G. Iron metabolism in the anemia of chronic disease. Biochim. Biophys. Acta 1790(7), 682–693 (2009).
8
Corwin HL. The role of erythropoietin therapy in the critically ill. Transfus. Med. Rev. 20(1), 27–33 (2006).
9
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. 113(9), 1271–1276 (2004).
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 101(7), 2461–2463 (2003). 11 Theurl I, Schroll A, Sonnweber T et al.
Pharmacologic inhibition of hepcidin expression reverses anemia of chronic
www.future-science.com
disease in rats. Blood 118(18), 4977–4984 (2011). 12 Peslova G, Petrak J, Kuzelova K et al.
Hepcidin, the hormone of iron metabolism, is bound specifically to a-2-macroglobulin in blood. Blood 113(24), 6225–6236 (2009). 13 Huang ML, Austin CJ, Sari MA et al.
Hepcidin bound to a-2-macroglobulin reduces ferroportin-1 expression and enhances its activity at reducing serum iron levels. J. Biol Chem. 288(35), 25450–25465 (2013). 14 Pandur E, Nagy J, Poor VS et al. a-1
antitrypsin binds preprohepcidin intracellularly and prohepcidin in the serum. FEBS J. 276(7), 2012–2021 (2009). 15 Borth W. a2-macroglobulin, a
multifunctional binding protein with targeting characteristics. FASEB J. 6(15), 3345–3353 (1992). 16 Barrett AJ, Starkey PM. The interaction of
a-2-macroglobulin with proteinases.
15
Commentary | Lane & Richardson Characteristics and specificity of the reaction, and a hypothesis concerning its molecular mechanism. Biochem. J. 133(4), 709–724 (1973). 17 Rehman AA, Ahsan H, Khan FH.
a-2-macroglobulin: a physiological guardian. J. Cell. Physiol. 228(8), 1665–1675 (2013).
16
18 Birkenmeier G, Kampfer I, Kratzsch J,
Schellenberger W. Human leptin forms complexes with a-2-macroglobulin which are recognized by the a-2-macroglobulin receptor/ low density lipoprotein receptor-related protein. Eur. J. Endocrinol. 139(2), 224–230 (1998). 19 Westwood M, Aplin JD, Collinge IA, Gill A,
White A, Gibson JM. a-2-macroglobulin:
Future Med. Chem. (2014) 6(1)
a new component in the insulin-like growth factor/insulin-like growth factor binding protein-1 axis. J. Biol. Chem. 276(45), 41668–41674 (2001). 20 Panyutich A, Ganz T. Activated a2-
macroglobulin is a principal defensinbinding protein. Am. J. Respir. Cell Mol. Biol. 5(2), 101–106 (1991).
future science group
