Hepcidin

Abstract

The hepcidin is antimicrobial peptide has antimicrobial effects discover before more than a thousand years; it has a great role in iron metabolism and innate immunity. Hepcidin is a regulator of iron homeostasis. Its production is increased by iron excess and inflammation and decreased by hypoxia and anemia. Iron-loading anemias are diseases in which hepcidin is controlled by ineffective erythropoiesis and concurrent iron overload impacts. Hepcidin reacts with ferroportin. The ferroportin is found in spleen, duodenum, placenta, if the ferroportin decrease, it results in the reduced iron intake and macrophage release of iron, and using the iron which stores in the liver. Gene of human hepcidin is carried out by chromosome 19q13.1. It consists of (2637) nucleated base. HAMP gene was founded in the liver cells, in brain, trachea, heart, tonsils, and lung. Changing in the HAMP gene will produce a change in hepcidin function. The hepcidin is made many stimulators are included opposing effects exerted by pathological and physiological conditions. Hepcidin is essential for iron metabolism, understanding stricter and genetic base of hepcidin is crucial step to know iron behavior and reactions to many health statuses.

1. Introduction

The hepcidin is antimicrobial peptide has antimicrobial effects discover before more than a thousand years; it has a great role in iron metabolism and innate immunity [1]. It’s a peptide hormone produced by the liver that acts as an iron regulator. Hepcidin is an iron homeostasis regulator. Iron deficiency and inflammation boost its production, while hypoxia and anemia diminish it. Hepcidin prevents iron from duodenal enterocytes absorbing dietary iron, macrophages recycling iron from senescent erythrocytes, and iron-storing hepatocytes from entering the bloodstream. Hepcidin is controlled by inefficient erythropoiesis and concurrent iron overload consequences in iron-loading anemias [2].

Human urine and blood, particularly plasma after filtration, were used to isolate hepcidin. Macrophages, adipocytes, neutrophils, lymphocytes, kidney cells, and -cells all make hepcidin. The studies experiment on mice used for determination hepcidin regulation, expression, function, and structure. Severe iron overload is occurring due to the gene responsible with hepcidin production; the gen has the role of iron function. However, decreased and iron increased hepcidin expression in transgenic animals. Hepcidin serves a variety of purposes, including inflammation, hypoxia, and iron storage [3].

Ferroportin reacts with hepcidin. The ferroportin is located in the spleen, duodenum, and placenta; a decrease in ferroportin results in reduced iron intake and iron release by macrophages, as well as the use of iron stored in the liver [4].

2. The HAMP gene and structure of hepcidin

The human hepcidin gene is located on chromosome 19q13.1. It is made up of 263 nucleated bases. The HAMP gene was discovered in liver cells, brain cells, trachea, heart, tonsils, and lung cells [5].

Hepcidin comes in three forms: 25 aa, 22 aa, and 20 aa peptide. The HAMP gene encodes preprohepcidin, which has 84 amino acids. The structure of hepcidin25 (Figure 1), which consists of (8) cysteine linked by a disulfide bond, is detected in urine, while 25 and 20 are found in human serum. The structure of hepcidin is studied using NMR spectroscopy; it has four disulfide links [7].

Figure 1.

Molecule structure of human synthetic hepcidin-25. Background: Hepcidin-25. Front: Showing the general structure of hepcidin-25. Gray arrows are distorted β-sheets, and colored gray is peptide backbone. Colored yellow is a disulfide bond, blue is indicate to positive residues of lysine and arginine, red indicates to the negative residue of aspartic acid, and colored green indicates to histidine which containing amino-terminal [6].

3. Hepcidin gene regulation

Location of the HAMP gene is at 19q13 chromosome mRNA. Several genetic factors affect on iron concentration, hypoxia, inflammation, erythropoiesis, and anemia. All these factors have two pathways on the gene. The first signaling is by bone proteins and the second Janus kinase/signal related to inflammation [8].

The protein is regulated of hepcidin level depend on transferrin and interaction receptor. HFE is chanced from TfR1 [Tf-Fe3+] to promote its interaction with (TfR2). TfR2 and HFE link with the receptor of hemojuvelin by the BMP/Son for activating HAMP gene. This reaction stimuli phosphorylation of BMP receptor, and stimulating signals into the cell. The receptor of type II activates receptor of type I, then the signal transmits to the SMAD regulatory receiver, phosphorylating (SMAD-8, SMAD-5, and SMAD-1). The activated complex transfer to the nucleus for regulating gene transcription [9]. Matriptase-2 protein and SMAD-4 is a suppressor of BMP/SMAD. HJV is reacting with Matriptase-2 and causes fragmentation. Growth hormone and erythropoietin associate with the receptor, wherever, interferon and cytokines. The hepcidin wad produced in the liver, it increases if iron gets in liver cells. Hepcidin creates and released into the blood vessels and spread all the body. It interacts with other proteins in the liver, intestines, and WBC for iron storage when the hepcidin was produced at large amounts, increases the occurrence of liver tumor or chronic or acute hypoferremia. If the hepcidin is decreased in the production, results in mutations in the hemojuvelin gene, hepcidin gene, or transferrin receptor 2 [10].

4. HAMP gene mutation

Hepcidin function will be altered if the HAMP gene is altered. Exon 3 of the HAMP gene encodes proteins, and it is regarded the most critical and biggest section of the gene, containing several polymorphisms [10]. The HFE gene has more polymorphisms than the HAMP gene. There are approximately 16 forms of single nucleotide polymorphism that have been discovered in various investigations [8]. Mutations in the gene have been reported in a number of reports. People who have mutations in the HAMP gene develop juvenile hemochromatosis between the ages of 10 and 30. As the initial genetic change in the HAMP gene, microsatellite marker probes are utilized. After exchanging several amino acids in the active peptide, or replacing C78 with a tyrosine, C78T, the mutation occurs in c.233G > A at some point [11].

The mutation allows ferroportin to form bisulfite connections with hepcidin, resulting in an increase in iron absorption. C70R mutations result in cysteine bisulfite bond distortion. The arginine replaces the cysteine, which does not allow the creation of the bisulfite bridge between 3 and 6 in the hepcidin peptide. C to T substitutions were found at position (166) of the HAMP (166C-T), as well as arginine substitutions at position (56) for a halting codon (R56X), 193A to R56X. (T). Furthermore, the ferroprotein does not bind to hepcidin, resulting in the production of additional iron. In contrast, deleting guanine from exon two at location 93 causes an RNA mutation. The deletion of Met50del IVS21 from exon two causes a disruption in the active peptide’s expression as well as variations in reading frames. Met50 and (IVS + 1 (G)) are suppressed by the mutation. The reading frame is lengthened as a result of this mutation. Another mutation, G71D, causes a change in amino acid 71, which lies between 3 and 4 cysteine and precludes ferroprotein binding. In sickle cell disease patients, the HFE-H63D mutation is linked to the HAMP-G71D variant, which increases iron overload [12].

The polymorphism (G to A) occurs at the +14 position of the 5′-UTR region, resulting in a new initiation codon, a new aberrant protein, and a shift in the reading frame. After the mRNA is translated, an unstable protein will be produced, which will be analyzed. The related polymorphisms NC-582A > G and NC-1010C > T in the HAMP gene create a haplotype with ferritin concentrations greater than 300 g/L [13].

HFE gene polymorphisms are frequently linked to HAMP. With iron overload, there are various mixed clinical symptoms in some clinical instances. The variations C, 582A > G and C-153C > T reduce hepcidin expression, but the peptide’s mode of action remains same without transferrin saturation and increased ferritin levels. The patient who has HAMP gene mutations were cannot make hepcidin and unable to decrease iron absorption. The body organs become contain iron at large amounts such as heart and liver, and it will affect with damage. Any change in the HAMP gene could result in a faulty hepcidin protein, and it would have no effect. The accumulation of iron and ferritin in the organs contributes to the development of diseases in several organs, such as coronary artery disease, diabetes mellitus, HIV, HBV, and HCV, where reactive oxygen generates oxidative material that damages tissues. And some neurological illnesses, such as Alzheimer’s, Parkinson’s, and sclerosis, are linked to high levels of hepcidin in the blood [14].

5. The hepcidin clinical applications

The hepcidin is made many stimulators are included opposing effects exerted by pathological and physiological conditions. The response is usually rapid. The hepcidin production increases during few hours after inflammatory stimulation and iron administration. Several stimuli could associate with hepcidin. Such as in hepcidin production and severe ID with the inflammation [15, 16].

Several ineffective conditions, such as signals released by bone marrow and non-transfusion-dependent thalassemia. The results showed hepcidin suppression non-transfusion-dependent thalassemias other iron-loading anemias, and even in-thalassemia trait. Serum hepcidin in transfusion-dependent b-thalassemia showed increasing in blood transfusions and decreasing through inter-transfusion periods [16].

Clinically relevant conditions include CKD, RBC transfusions, iron administration, replete iron stores, TMPRSS6 variants, infections/inflammatory disorders, ineffective erythropoiesis, hypoxia, erythropoietic stimulating agent administration, chronic liver diseases, alcohol abuse, HCV, hemochromatosis-related mutations, and testosterone estrogen administration. HCV, hereditary hemochromatosis; HH, iron deficiency; IDA, RBC, transmembrane protease serine 6, matriptase-2 encoding gene; CKD, glomerular filtration rate; GFR, hepatitis C virus; HCV, hereditary hemochromatosis; IDA, RBC, transmembrane protease serine 6, and matriptase-2 encoding [17].

6. Structure and location of HFE gene human

HFE protein was encoded by the HFE gene in humans. The gene lies at chromosome six—6p21.3. The protein is included membrane protein such as MHC class I-type and link with beta-2 microglobulin. HFE protein regulates iron uptake by transferrin HFE protein and the transferrin receptor which composed from (343) amino acid (Figure 2). Many other types of proteins such as a signal peptide, transferrin receptor-binding region, and immunoglobulin molecules. HFE is prominent in small intestinal absorptive cells, epithelial cells in stomach, macrophages, and granulocytes and monocytes [19].

Figure 2.

The HFE gene diagram. The image was changed after getting permission from the author. Cys282 -> Tyr282 exchanging mutation of C282Y and His63 -> Asp63 exchanging mutation of H63D [18].

7. Maintaining iron homeostasis by hepcidin

Hepcidin is regulated iron absorption. Pathway of iron is showed in (Figure 3), FPN1link with hepcidin is results in iron retention into the cell and do not allow of iron from getting in the plasma. Hepcidin is made and store in the liver cell [20].

Figure 3.

Hepcidin internalization and degradation.

Ferroportin binds to hepcidin results in degradation, wherever the reaction between ferroportin and hepcidin regulates and control on iron concentration. The hepcidin regulation is a very complex mechanism and depends on many transmembrane proteins. JAK-STAT activated HAMP expression in interleukin-6 (IL6) status and inflammation-mediated response [21].

Bone-morphogenetic protein is work as a key for the regulation of HAMP gene through SMAD signaling pathway.

Also, hemojuvelin is protein made in a lever membrane cell. If the iron becomes low, the hemojuvelin is activated wherever the sHJV inhibits HAMP level through lining with BMP. Regulation of iron is not understood although many proteins work to iron regulation we know it such as (TFR2) have great role in iron regulation [22].

8. Iron regulation by hepcidin

The function of the enterocytes has absorbed the iron, the iron store in the macrophages and hepatocytes and the process is controlled by hepcidin, the hepcidin is produced in the liver. Hepcidin consists of (84) amino acid, it is undergoing for several reactions to become (60) amino acid then transfer to (25) amino acid [23].

The hepcidin is hormone consist of four disulphide bonds and 32% beta-sheet. The function of the hepcidin is control on iron efflux by ferroportin wherever; the liver secretes iron in the plasma. After secretion, the hepcidin is bound with ferroportin; wherever, ferroportin is protein on the cell surface have transferred the iron inside the cell. If the ferroportin is reduced in expression, become the intracellular iron is less. Hepcidin is absorbed iron from the food and transfers to plasma, and the iron gets in the cell by binding hepcidin and ferroportin. Reduction of the ferroportin on the cell surface is mechanism unclear [9].

Figure 4 show how iron flows into plasma exclusively through the membrane channel, ferroportin. Macrophages, enterocyte’s hepatocytes are the principal cell types that express ferroportin and so export iron. The duodenum, spleen, and liver, which contain these cells, are important locations for controlling iron flux (blue arrows). Hepcidin, a 25-amino-acid hepatic hormone, regulates ferroportin levels. Endocytosis and proteolysis are triggered when hepcidin binds to ferroportin, preventing iron flow (red arrows) into the plasma from ferroportin-expressing tissues. Hepcidin production rises as iron stocks rise during infection (black arrows) and falls as erythropoiesis demands more iron (red arrows) [24].

Figure 4.

Regulation of iron balance.

9. Conclusion

Hepcidin is essential for iron metabolism, understanding stricter and genetic base of hepcidin is crucial step to know iron behavior and reactions to many health status. This chapter highlights on hepcidin structure and genetic information as well as its relation to iron metabolism.