Iron in Cell Metabolism and Disease

Abstract

Iron is the trace element. We get the iron from the dietary sources. The enterocytes lining the upper duodenal of the intestine absorb the dietary iron through a divalent metal transporter (DMT1). The absorbed ferrous iron is oxidized to ferric iron in the body. This ferric iron from the blood is carried to different tissues by an iron transporting protein, transferrin. The cells in the tissues take up this ferric form of iron by internalizing the apo transferrin with its receptors on them. The apo transferrin complex in the cells get dissociated resulting in the free iron in cell which is utilized for cellular purposes or stored in the bound form to an iron storage protein, ferritin. The physiological levels of iron are critical for the normal physiology and pathological outcomes, hence the iron I rightly called as double-edged sword. This chapter on iron introduces the readers basic information of iron, cellular uptake, metabolism, and its role cellular physiology and provides the readers with the scope and importance of research on iron that hold the great benefit for health care and personalized medicine or diseases specific treatment strategies, blood transfusions and considerations.

1. Introduction

Iron is a chemical element with symbol Fe and atomic number 26 (Figure 1). Classified as a transition metal, iron is a solid at room temperature. The symbol “Fe” is derived from the Latin ferrum for “firmness”, iron exists in many different forms in nature. Iron makes up 5% and the chief constituent of earth’s crust. It is the second abundant metal on earth and is most abundant as an alloy. Although its most abundance, it is required in very small amounts and hence is also called a trace element. The readers should also appreciate the meaning of “trace” here, in other sense that its trace amounts are very critical, and any more amounts of this element is as dangerous as its normal functions in the cell. Hence, the iron is called to be a double-edged sword [1]. Iron is a metal which belong to the transition metals group or VIIIB elements group of periodic table (Figure 2). Iron exists in different forms in nature [2]. The pure metal is very reactive chemically and rapidly corrodes, especially in moist air or at elevated temperatures. It has four allotropic forms or ferrites, known as alpha, beta, gamma, and omega, with transition points at 700, 928, and 1530C. The alpha form is magnetic, but when transformed into the beta form, the magnetism disappears although the lattice remains unchanged. The relations of these forms are peculiar. Pig iron is an alloy containing about 3 percent carbon with varying amounts of sulfur, silicon, manganese, and phosphorus [3].

Figure 1.

Properties of iron (source: adapted from Encyclopædia Britannica, Inc).

Figure 2.

Periodic table of elements representing element properties and their categories based on the similarities in physicochemical properties and atomic number.

Iron is a hard, brittle and can molded into many different forms. Iron is used to produce other alloys, including steel. It is the cheap, abundant, useful, and important metal. Wrought iron contains only a few tenths of a percent of carbon, is tough, malleable, less fusible, and usually has a “fibrous” structure. However, Carbon steel is an alloy of iron with small amounts of Mn, S, P, and Si. Alloy steels are carbon steels with other additives such as nickel, chromium, vanadium, etc.

2. Iron in cellular milieu (forms, states, and importance)

2.1 Iron forms

Cell is the structural and functional unit of life which performs many different physiological and biochemical processes that are essential for the survival of an organism, either unicellular or multicellular. The cells depend on food, essential micro and macro nutrients, vitamins, etc., to perform these biochemical processes in the tissues and organs of the body, and iron is one such essential requirement for cellular processes [4]. Living beings acquire the iron from the surrounding environment either by consumption (animals) or by absorption (prokaryotes, plants, fungi etc.,). The iron absorbed or taken through food exists in two important forms in the biological systems, the highly insoluble oxidized nonabsorbable form, Fe3+ and the readily absorbable reduced soluble form, Fe2+ [5]. These two forms also exist as free and bound forms and the levels of free and bound form is very critical for the normal functioning of the cells, opportunistic pathogen infections and the physiological state of cells or the iron related diseases of the organism [6, 7, 8]. For example, higher levels of free iron can help in bacterial infection and survival as in the case of Mycobacterium Tuberculosis [9] or the hemochromatosis (iron overload, a condition where body stores too much of iron which cause serious damage to the vital organs) [10]. The different optimum levels of iron in the body are illustrated in the Figure 3. So, the reduced Fe2+ form is the form that the cells can take up from the blood and is transported to many tissues where it is required [11]. The excess iron in the blood is absorbed by the cells and tissues is stored in the tissues or cells in a bound form, meaning that the iron is either incorporated into the enzymes, as cofactor, or can be stored in the tissues and cells in the body by special proteins called “Ferritins”, or transported to the cells in bound form to iron transporters in the cells called “Transferrin” [12, 13].

Figure 3.

Absorption of iron, transport, and the levels in the body. (ref. adapted from Jiten P Kothadia et al., 2016).

2.2 Levels and the distribution of iron in the body

The levels of iron in the blood, body tissue, and its distribution are very critical for the proper function, physiology, immune functions, and the determinant of the opportunistic infections in the body [14, 15, 16]. Hence, the levels of iron are critically regulated by the body tissues and cells. Here we shall investigate different levels and the distribution of iron, which control the tissue or the cellular absorption of the iron, by looking at the levels of iron in different tissues and the absorption of iron in human beings [17]. The normal levels of iron in the body are approximately 35 and 45 mg/kg body weight in adult women and men. Of the total iron we get form the food, about 60–70% is present in hemoglobulin in circulating RBCs, 10% is present in myoglobins (hemoglobin in the muscle), cytochromes, and iron containing enzymes accounting for not more than 4 mg–8 mg [18]. In healthy individual, 20–30% of iron is stored in the ferritins (an iron storage protein) and hemosiderins (iron acquisition protein) in hepatocytes and reticuloendothelial macrophages as shown in the Figures 4 and 5. Transferrin, another iron holding protein in the body, contains less than 1% (approx. 4 mg) of the total iron stores of the body and has the significant and highest turnover (25 mg/day). Transferrin transports 80% the iron in its bound form to majorly to the bone marrow for synthesis of hemoglobin in the development erythrocytes (Ibrahim Mustafa, 2011, Ph.D. thesis, http://hdl.handle.net/2429/36590) Thus, it transports the iron from blood to the bone marrow regularly and hence its high turnover. Most of the cellular iron is obtained from the dead RBCs and the iron released into the blood by reticuloendothelial cell macrophages in the liver where the senescence red blood cells are degraded after the completion of 120 days lifespan. So, the iron from the blood is regularly transported to all the tissue types or organelles by the transferrin molecules to deliver to the iron metalloprotein in those tissues. For example, the iron transported to mitochondria is incorporated into protoporphyrin IX, an important component of oxygen transport during oxidative phosphorylation. The iron which is lost due to wear and tear of the tissues and regularly during menstrual bleeding in females is replenished from the from dietary iron.

Figure 4.

Iron absorption and distribution in the body (adapted from Mustafa, 2011).

Figure 5.

Iron uptake by the cells or tissues and the regulation of iron uptake in the body (ref. Mustafa, 2011).

Iron is critical for oxygen transport and one of the most abundant metals in the human body, plays an important role in cellular processes such as the synthesis of DNA, RNA, and proteins that hep in the electron transport, cellular respiration, cell proliferation, differentiation, and regulation of gene expression [19]. Iron metabolism takes place in brain, testes, intestine, placenta, and skeletal muscles and high levels of iron is found in liver, brain, red blood cells, and macrophages [20]. Thus, the iron homeostasis is critical for the proper functioning of these organs and the altered levels results in the tissues results disturbed tissue functions and/or pathological or clinical conditions [11, 17]. Hence, it is very critical even during the blood transfusion to consider the levels of iron whether it is in bound or free form [21].

2.3 Iron transport in the cells

The dietary iron exists in two different forms, a haem form found in animal source foods which is the F2+ iron complexed with organic compounds and a non-haem form present in plant foods which is also called inorganic form. The non-haem iron or the Fe3+ iron is the major form of dietary iron, and this iron is not absorbed by the cells in the body, meaning that this Fe3+ needs to be reduced to Fe2+ to be taken up by the cells. This reduction is carried out the enzyme, ferric reductase also called ferric reductase duodenal cytochrome B (DCYTB or CYBRD1), which is found on the apical brush border membrane of intestinal epithelial cells in the duodenum and upper Jejunum [12]. The reduced Fe2+ is now transported or taken up by the apical surface of enterocytes lining the duodenal surface with the help of a 12 transmembrane divalent metal transporter protein1 (DMLT1). The Fe2+ iron transported to the enterocytes enters systemic circulation from the basolateral surface of the cells by the only known iron transporter, ferroportin another 12 transmembrane protein encoded by the Solute Carrier Family 40 Member 1(SLC40A1) gene. This transporter is also expressed in other cell types, particularly macrophages where it is highly expressed thus serves as iron uptake machinery which is utilized by the Mycobacterium Tuberculin during tuberculosis infection. Ferroportin also helps in the release of iron form the stores from the hepatocytes assisted by the cupper containing ferroxidase enzyme ceruloplasmin or the membrane bound counterpart hephaestin, a membrane associated ferroxidase in the intestine (Figure 6). The enzymes oxidize the iron from Fe2+ form to Fe3+ form before it binds to transport protein transferrin which transports the iron to different tissues from the blood which will be discussing in the iron uptake by the cells or tissues [12].

Figure 6.

Distribution of iron in the body for different function in the different tissues (picture taken from JBC MINIREVIEWS| VOLUME 292, ISSUE 31, P12735–743).

3. Iron metabolism in cells (uptake, storage, transport, and component of cellular macromolecules)

Iron that is transported or absorbed into the system circulation is bound to the transporting protein, transferrin, present in the blood. Transferrin as the name suggests transfers the iron to all the tissues in the body. Iron also exists in the blood as non-transferrin bound form, especially when the serum levels iron is high, and the transferrin is completely saturated during hereditary haemochromatosis (HH) or any other iron overload conditions such as cancer, irregular heartbeat, and cirrhosis of the liver [22].

3.1 Uptake of iron by the cells

The transferrin bound iron in the blood is taken by the cells and tissues in the body. The transferrin along with its bound iron (holotransferrin) binds to the transferrin receptor1 (TFR1) that is expressed ubiquitously on the cell surfaces. The complex of iron-tranferrin-TFR1 is endocytosed by the cells. The endocytosed vesicles in the cell are acidified resulting in the opening of the DMT1 present in the vessels that releases the iron into the cells. The vesicle with the transferrin-TFR1 complex is recycled back to the membrane of the cells where it is reincorporated and thus the transferrin is released into the blood is available for the next round of iron transport basing on the levels iron in the blood. Iron is also transported to the cells through non-transferrin bound means as non-transferrin bound Iron (NTBI) which involves the zinc transporter ZRT/IRT-like protein 14 (ZIP14) a member of the SLC39A zinc transporter family. The iron that is transported to the cells and tissues is used for the cellular functions such as oxygen transport in RBCs by complexed with hemoglobin and myoglobin, cofactor for enzymes which are majorly involved in oxidation reduction reactions or stored as iron pools to serve the cell requirements when needed [5, 13].

3.2 Storage of iron in the tissues

Liver is the major site for iron storage in the body. The excess iron in the cells is stored in the iron storage proteins which contains iron holding pockets thousands in number to hold the iron. The iron storage proteins in the cells are ferritin and hemosiderin. Ferritin is the major iron storage protein at the cellular and organismal level. It stores 30% of the total storage iron in the cells and thus also sequesters the very reactive toxic fe2+ iron that generates reactive oxygen species (ROS) by Fenton reaction or subsequent reaction in the body. Ferritin is a spherical shell made up of 24 subunit proteins and has a centrally located iron holding cavity which can accommodate 4500 iron molecules in the Fe3 (III) complexed state. In this iron bound form ferritin not only stores the iron but also regulates the iron levels in the tissues by slow release (Figure 7). Hemosiderin is another protein which stores the iron in cells and tissues in the body. It is an iron storage complex of digested ferritin and lysosomes. Hemosiderin also forms when the ferritin is completely saturated with iron, the excess iron in the cells and tissues forms complex with phosphate and hydroxide forms. However, if the body burden of iron increases beyond normal levels, excess hemosiderin is deposited in the liver and heart. This can reach the point that the function of these organs is impaired, and death ensues [23].

Figure 7.

Structure of ferritin and storage of iron. Fenton reaction to of generation of reactive of reactive oxygen species- ferritin sequesters excess iron and prevents in generating ROS.(ref: Antioxidants & Redox Signaling VOL. 10, NO. 6).

3.3 Iron as a component of cellular macromolecules

About 95% of the total dietary iron in the cell of the body is in the bound form to proteins (about 95%) either in the cell cytosol or compartmentalized in different cell organelles. The rest 5% is available in form of free form represented as a cytosolic labile iron (LCI) or the labile pool iron [24]. This labile pool is, though very less, is the major for most of the deleterious effects in the body as it is very reactive, freely available, exchangeable and chelate able form in the cells. This labile form can initiate ROS generation, induce peroxidation of lipids of cells, or changes the oxidation and reaction in cells and thus is very toxic to cells. Here in this topic, we will discuss the important cellular components of iron and their physiological significance.

3.4 Labile cell iron (LCI)

LCI is the generic term for generic to describe labile iron in the cell, or cell compartments which exists in either fe2+ or Fe3+ form. The levels of LCI greatly vary depending on the location or cellular component, metabolic state, and the chemical composition of the component. The LCI is very important in the cell physiology as it serves as a metal source for metabolism also an indicator of cellular iron levels [25]. Thus, cells balance the uptake of circulating total bound iron (TBI) and store in the ferritin shells as unutilized iron basing on the LCI levels. Hence, this can be used as a dynamic cell parameter as the LCI pools are likely to vary over time in response to chemical or biological stimuli as well as to metabolic demands/responses. Conversely, the LCI levels decrease in iron starvation, which results in stable or transient overexpression of cytosolic or mitochondrial ferritin, a common scene in some mitochondrial disorders of aberrant mitochondrial iron accumulation. Thus, LCI acts an important indicator in pharmacological and research settings [24, 26]. LCI also helps in important components of chaperone role for human poly (rC)-binding proteins 1–4 (PCBPs 1–4), members of members of the heterogenous nuclear ribonucleoprotein family comprised of PCBPs 1–4 RNA/DNA-binding proteins involved in diverse processes such as splicing, transcript stabilization and translational regulation [27].

3.5 Mitochondrial iron metabolism

Mitochondrial plays and critical role in cellular iron metabolism as these are the only sites for heme synthesis, essential component of RBCs, skeletal or cardiac muscle oxygen carrying function and Iron–Sulfur cluster (ISC) biogenesis which serves a plethora of functions in the cells. These iron complexes are very critical component of metabolic enzymes, and defect is mitochondrial iron metabolism results in severe diseases [28]. Iron transport in mitochondria can be mediated by the kiss and run process of iron containing vesicles with mitochondrial membrane or the receptor mediated endocytosis which is mediated by particularly by PCBP2 [29]. Binding of this protein to the mitochondrial membrane DMT1 results in the efflux or the influx of iron into and out of mitochondria. Mitochondria also express its specific ferritin (FTMT) to store the iron [30]. High levels of FTMT are also expressed in sideroblasts (i.e., erythroblasts with iron granules) of patients affected by sideroblastic anemia [31].

3.6 Iron as component of heme

Heme is the precursor of oxygen carrying protein of the body’s RBCs, skeletal and cardiac muscle, cytochromes, and many other enzymes. Heme contains 95% of functional iron in the human body, and two-thirds of the average person’s dietary iron intake in developed countries and major cause of many of the iron associated diseases due to consumption of iron rich sources, especially from the animal origin [32]. It is complexed with porphyrin ring of hemoglobulin and myoglobin proteins. Apart from the enormous importance of iron or heme in the body, it is also important to note that the polymerization of heme (polyheme) which arises because of the neutralization of gastric contents by pancreatic juice or the Hemozoin that in formed during the erythrocytic cycle of Plasmodium infection in human beings are toxic to the cells (Modern Blood banking and transfusion practices, 6th edition) [33]. Besides, loss of iron because of tear and wear of tissues or the menstrual loss in human results in anemia. The list of diseases which are associated with the iron intake can be found at this reference (Nutrients 2014, 6, 1080–1102; doi:10.3390/nu6031080, [34]).

3.7 Iron as component of proteins

Eukaryotic cells contain numerous iron-containing proteins, which can be mainly classified into three groups: Iron–sulfur (Fe-S) cluster proteins, hemoproteins, and non-heme/non-Fe-S proteins. Fe-S proteins are characterized by their different structures with variable oxidation states, ranging from [2Fe-2S] diamonds, [3Fe-4S] intermediates, to [4Fe-4S] cluster cubes. Examples of Fe-S proteins include DNA polymerases, DNA helicases, hydrogenases, nicotinamide adenine dinucleotide (NADH)-dehydrogenases, nitrogenases, ferredoxins, and aconitases [35, 36]. Hemoproteins have a heme prosthetic group that allows them to carry out oxidative functions. Examples of hemoproteins include cytochromes, hemoglobin, myoglobin, catalases, and peroxidases. Nonheme/non-Fe-S proteins can be further subgrouped into three classes: Mononuclear non-heme iron enzymes, diiron proteins, and proteins involved in ferric iron transport. This group of iron-containing proteins mainly includes the small subunit of ribonucleotide reductases (RNRs), superoxide dismutases (SODs), dioxygenases, pterin-dependent hydrolases, and lipoxygenases. One can find the list of different proteins and their functions in cells at this reference (Zhang: Iron-containing proteins in Arabidopsis) [37]. Iron is also stored or absorbed by the microbiome in the gut plays a major role that not only influence the metabolism and genome of the host but also required for the growth of good bacteria and the harmful opportunistic bacteria that may cause dreadful disease.

4. Importance of iron in cellular processes (mechanism of homeostasis)

Iron being the essential micronutrient and a major component of cellular respiration, metabolism, its distribution in body fluids and tissue and the diseases associated with the alterations in the levels of free or bound iron results in many diseases ranging from anemia, hemochromatosis, and infections. It is obvious that the regulation of iron levels in the body is an important parameter of cellular state or understanding the physiology of the system, but here we shall discuss with some of clinical examples to iterate the point that the levels of iron in the tissues or the cells changes the cellular state or the diseases causing capacity of an organism that enters the host [27, 38]. We will not be discussing the iron homeostasis or regulation of iron uptake or absorption in the body.

4.1 Effect of iron levels in blood transfusions

I believe that each one of us are aware importance of blood transfusion in human life in the present days, which is essential in case of major surgeries, accidental blood loss, leukemia’s, or the critical chronic blood infections. Blood group and Rh factor is the critical parameter during blood transfusions and the iron is no exception [39, 40]. It was found that the acute delivery of iron “predisposes patients to new infections, converts “benign” bacterial colonization into virulent infection, or enhances the virulence of existing infections” [41]. It should also be appreciated that the ease and the rapidity of the iron to interconvert between the ferric and ferrous forms interfere with the many of the cellular processes. The same excess iron in the blood also poses a potential threat to the cells as it initiates the production of reactive oxygen species (ROS) through Fenton reaction in the blood which would damage the cells or cause DNA damage in the delicate immune privileged organs, especially. Therefore, there is an active discussion on the levels of iron in the healthy blood donors, whether it is medically important and what steps to be taken while transfusions [42].

4.2 Effect of iron levels on the diseases causing pathogens

Iron availability to the parasites within the host is one of the critical factors that affects the disease-causing ability [43]. Mycobacterium tuberculosis is an intracellular pathogen that causes the life-threatening tuberculosis (TB) disease in host [44]. It has been found that the mutations that results in the macrophage iron overload results in the predisposition towards TB in HIV patients or increases the risk of TB.

It was also true for some of the commensal (microbiome), lactobacillus, Staphylococcus epidermidis or pathogens such as rabies can be deadly pathogens under right condition or the iron overload [45]. The reason is obvious as there is fierce competition between the host and the bacteria for iron under normal iron condition and this limits the iron for the bacteria to thrive to cause infection, but during the iron overload there is lot of iron available for these bacteria to convert into disease causing pathogens and innumerable pile up as we keep on the discussing this topic. Many more examples can be found in this editorial column (Iron: double edged sword).

4.2.1 Effect of iron levels on immune system

Immune system in the body sequesters the iron in the body and thus prevents the iron dependent disease-causing pathogens, this is called the nutritional immunity [46]. In addition, nutritional immunity also modulates adaptive immune responses either in deficient or overload states [47].

4.2.2 Measuring or quantification methods of iron

Iron being the important element in different cellular processes, components of cells, enzymes, and proteins and the levels of iron are very well regulated in the body. The changes in the local concentrations in the cells and tissues results in many physiological and pathological conditions. Hence, estimating the concentrations acts as a good indicator to understand cellular physiological status and the pathological conditions in the body [48].

There are three main iron compartments in the and the alterations in the normal in each of the compartment is used as an indicator and the biochemical assessment is based on the iron levels in the serum. Thus offers an easy way to assess the iron concentrations in the serum that gives the readout about the infection state or the tissue physiology. Below are the irons indicators and the different understanding of the different physiological state of the body or tissues with respect to the iron alterations in the specific compartment (Table 1 and Figure 8) and the condition (Table 2).

	Men	Women
Functional iron
Hemoglobin	32	28
Myoglobin	5	4
Fe-containing enzymes	1-2	1-2
Storage iron
Ferritin and Hemosiderin	~ 11	~ 6
Transport iron
Transferrin	0.04	0.04

Table 1.

Iron distribution in the adults (mg Fe/kg body weight) (Ref. Stipanuk MH (2006).

Figure 8.

Iron compartmentalization in the body (http://en.wikipedia.org/wiki/File:lron_metabolism.svg).

Confounder	Indicator and direction of change	Comment
Inflammation	SF ↑	Ferritin is a positive acute-phase protein
	Transferrin ↓	Transferrin is a negative acute-phase protein
	Iron ↓	The release of cytokines leads to increased uptake and retention of iron in reticuloendothelial system cells, e.g., iron becomes sequestered and is not available for transport to the bone marrow for erythropoiesis
	EP ↑
	Hemoglobin ↓
Increased erythropoietic activity	EP, sTfR ↑	In thalassemia, sickle cell anemia, and hemoglobinopathies
Lead poisoning	EP ↑	Lead blocks the formation of heme and zinc protoporphyrin forms instead
Pregnancy	Hemoglobin ↓	Plasma volume expansion results in hemodilution
Dehydration	Hemoglobin ↑	The volume of fluid in blood drops and hemoglobin artificially rises
Smoking	Hemoglobin ↑	Compensation for decreased oxygen intake in heavy smokers
Altitude	Hemoglobin ↑	Compensation for decreased oxygen intake due at high altitude

Table 2.

Important confounders of iron status indicators (Ref. Am J Clin Nutr 2017;106(Suppl):1606S–14S).

EP, erythrocyte protoporphyrin; SF, serum ferritin; sTfR, soluble transferrin receptor;, increase in concentration;, decrease in concentration.

5. Conclusion

Iron is the essential micronutrient for the cell physiology and function that also points out not just the role in cellular function, but also a critical component in cellular infection, cytotoxicity, and the generation of reactive oxygen levels. This chapter provides the basic importance about the cellular iron, absorption, distribution, storage, critical concentration of tissue iron in the body and hints about the importance of these critical levels in the diseases and pathologies. Iron as an essential nutrient in the body and the increased levels or the free in iron in the body are more damaging to the cell and hence the iron is aptly called as double-edged sword. This chapter has discussed the pathogenesis of some normal gut microbes turning into pathogenic sps, macrophage iron levels and the infection of mycobacterium, haemochromatosis are the few examples that indicate the critical levels of iron in cell physiology and function. The critical importance of iron and this chapter provides the readers the importance of iron levels and points out to the fact that the iron has lot of scope in terms of understanding cell physiology, defining the cell function in diseases. Thus, the iron offers a huge scope for the research towards limiting the survival of pathogens in the body or could enhance the survival of good bacteria in the gut.

Besides, consideration regarding the blood transfusion, the irons levels (bound and unbound form in the donor blood would a major factor in blood transfusion, especially in terms of treating anemic patients, personalized medicine as in the cases of bacterial infection as to many species have differential sensitivity or pathogenicity to the iron levels are the areas of active debate and research.