Safe and effective transport and utilization of iron are achieved by tight regulation at the level of both individual cells and the organism as a whole. The expression of a number of proteins that play critical roles in iron metabolism is regulated by the intracellular concentration of iron. This is achieved through a consensus stem loop sequence in the messenger ribonucleic acids (mRNAs) that encode these proteins. When iron is scarce, two iron regulatory proteins (IRPs) bind specifically to this stem loop and modify either the stability or rate of translation of the mRNAs, whereas when intracellular iron is abundant, the IRPs assume a conformation that precludes mRNA binding. Systemic iron metabolism is regulated by the circulating polypeptide hormone hepcidin, which controls both dietary iron absorption from the gut and release of recycled iron from macrophages. These two modes of regulation are described in detail later in this chapter.
The dietary sources of iron vary considerably according to geographic location, cultural tastes, and economic status. Iron in food consists of inorganic salts and organic complexes derived from plants as well as heme from animal sources. Digestion of grains, vegetables, and fruits in the stomach and duodenum results in the release of ferric iron. As depicted in Figure 5-1, normal individuals absorb only 1 to 2 mg of iron per day, primarily at the villous tips of duodenal enterocytes. A ferrireductase at this site reduces the iron to its ferrous form, allowing it to enter the cell through a luminal transmembrane channel, the divalent metal transporter DMT1 (Fig. 5-2). A portion of the iron that has entered the enterocyte may be stored within a porous multimeric protein cage called ferritin. Ferrous iron exits from the cell through the transport protein ferroportin, which is localized within the plasma membrane on the abluminal or basolateral side of the enterocyte. Here the iron is rapidly oxidized to the ferric form that binds transferrin, the plasma protein responsible for iron transport throughout the circulation. As shown in Figure 5-2, the export of iron from the duodenal enterocyte can be suppressed by hepcidin, a small polypeptide hormone produced in the liver. The binding of hepcidin to its receptor, ferroportin, triggers the latter's internalization and subsequent degradation. As a result, the rate of egress of iron from the enterocyte is markedly dampened.
Iron homeostasis and distribution within the body: absorption, transfer to Fe-transferrin, incorporation into heme in red cell hemoglobin and muscle myoglobin, storage in the liver and reticuloendothelial system, and reutilization. These values pertain to a 70-kg adult male.
Absorption and egress of iron in the duodenal enterocyte. Iron (Fe3+) within the duodenal lumen is reduced to Fe2+ which enters the cell through divalent metal transporter (DMT1) and exits the cell on the other (abluminal) side through ferroportin and is oxidized to Fe3+. The binding of the master regulator hepcidin to ferroportin (middle) causes rapid internalization and degradation (right) of ferroportin, thereby blocking further egress of iron from the cell.
Among individuals from North and South America as well as Europe, a substantial portion of dietary iron is in the form of hemoglobin and myoglobin within meat and other animal sources. Following proteolytic degradation of these proteins in the stomach and proximal intestine, heme is released and absorbed intact into the enterocyte, where it is degraded by heme oxygenase, resulting in the release of iron that is either stored as ferritin or exits from the cell via ferroportin. The luminal heme channel/importer has not yet been identified.
The plasma protein transferrin binds ferric iron at two sites with extraordinarily high affinity. As a result of such tight binding, the concentration of free iron in the plasma is too low to be easily measured. Thus transferrin effectively protects tissues and cells from the toxicity of "free" iron. As shown in Figure 5-1, transferrin picks up iron either from duodenal enterocytes or from macrophages, which (as discussed later in the "Iron Recycling" section) accumulate iron recycled from hemoglobin in senescent red cells.
All cells except the duodenal enterocyte take up iron via the binding of Fe-transferrin to transferrin receptors on the plasma membrane. In keeping with their obligation to produce high levels of hemoglobin, erythroid precursor cells express much higher levels of transferrin receptors than do other cells. As illustrated in Figure 5-3, the expression of transferrin receptor is regulated in an iron-dependent manner by the binding of IRPs to earlier-mentioned consensus stem loops at the 3′ untranslated region of transferrin receptor mRNA. In iron deficiency, when there is a premium on optimizing entry of iron into cells, the binding of IRPs to these stem loops greatly enhances the stability of the transferrin receptor mRNA and thereby increases protein expression. Up-regulation of the transport protein DMT1 in iron deficiency also depends on enhanced mRNA stability via binding of IRPs to 3′ mRNA stem loops.
Iron-dependent regulation of the rate of translation of ferritin messenger ribonucleic acid (mRNA) and the stability of transferrin receptor (TfR) mRNA. When the intracellular iron level is low, iron regulatory proteins (IRPs) bind to a consensus stem loop iron regulatory element in the respective mRNAs, resulting in enhancement of transferrin receptor mRNA levels and suppression of ferritin translation. When the intracellular iron level is high, the IRPs do not bind to the stem loops. Transferrin receptor mRNA is degraded (yellow arrows), and translation of ferritin is unimpeded.
Transferrin molecules loaded with two iron atoms have much greater affinity for the receptor than do those with either a single atom or none. As shown in Figure 5-4, the iron-transferrin/transferrin receptor complex is rapidly internalized as a plasma membrane microvesicle that includes the iron transport protein DMT1. A proton pump docks onto this microvesicle and acidifies its interior, thereby releasing iron, which then exits via DMT1. Once in the cytosol, this iron either goes to mitochondria for heme biosynthesis or is stored as ferritin.
The uptake of transferrin-bound iron (Fe2-Tf) into a cell via the transferrin receptor (TfR). Microvesicles containing the Fe2-Tf/TfR complex fuse with a proton pump, which acidifies the interior of the microvesicle, triggering the release of iron, which then exits via divalent metal transporter (DMT1). Once the iron atoms are released, the TfR releases apotransferrin (Apo-Tf). When the vesicle fuses with the plasma membrane, apotransferrin is returned to the plasma.
IRON UTILIZATION IN ERYTHROPOIESIS
In vivo studies using radiolabeled iron have shown that more than 90% of transferrin-bound iron in the plasma goes to erythroid cells in the bone marrow and is incorporated into heme in hemoglobin. Normally, this process is very efficient. In contrast, in patients with ineffective erythropoiesis or marrow hypoplasia, only a small proportion of circulating Fe-transferrin is incorporated into circulating red cells.
Normal red cells survive in the blood for about 120 days. When they reach their dotage, they are recognized and taken up primarily by splenic macrophages (Fig. 5-1), where the hemoglobin is degraded with the release of its iron into ferritin stores. Similar to the mechanism of its exit from the duodenal enterocyte (Fig. 5-2), iron exits from the macrophage via ferroportin and is bound to transferrin in the plasma. The flux of iron from macrophages back to the bone marrow is about 20 mg per day, a much greater proportion of the daily utilization than the influx of iron from the duodenum (about 1-2 mg/day). Thus in vivo iron homeostasis invests heavily in a high-capacity, high-throughput cycle in which iron derived from senescent red cells is delivered to the bone marrow for incorporation into new red cells.
Because free iron is so toxic to cells and tissues, it is important that whatever iron is not being used for synthesis of heme or for other purposes is sequestered within the cell as a storage depot that can be tapped as needed. The challenge of safe and highly bioavailable dynamic storage of iron is met by ferritin, a protein that assembles into a 24-subunit cage surrounding an inner core of iron hydroxide. The production of ferritin is exquisitely attuned to the cell's needs. As shown in Figure 5-3, when iron is scarce, ferritin production is halted because the binding of IRPs to the stem loop iron in the 5′ end of ferritin mRNA blocks translation. In contrast, when iron is abundant, the IRPs can no longer bind to the stem loop, and translation is unimpeded. With increasing accumulation of cellular iron, some of the ferritin becomes denatured and is converted to hemosiderin, from which iron is less readily mobilized. The preponderance of body iron is stored as ferritin and hemosiderin in two sites shown in Figure 5-1: about 600 mg in reticuloendothelial macrophages that include hepatic Kupffer cells and about 1000 mg in hepatic parenchymal cells. These estimates pertain to men of all ages. Women in the childbearing age group have less stored iron because of blood loss from menses and iron usurpation by the fetus during pregnancy. Because the accumulation of iron stores is a very slow process, children also have low levels of liver and macrophage iron.
No physiologic mechanisms have evolved to enhance the rate of iron loss from the body in those who are overloaded with iron or to retard this rate in iron deficient individuals. In all individuals there is a steady-state release of iron by shedding of cells from the skin, hair, intestinal mucosa, and urinary tract, amounting to about 1 mg per day. Women shed additional iron from menstruation amounting to roughly 1 mg per day.