Date Published: August 31, 2010
Publisher: Hindawi Publishing Corporation
Author(s): Hong Ye, Tracey A. Rouault.
Erythropoiesis in animals is a synchronized process of erythroid cell differentiation that depends on successful acquisition of iron. Heme synthesis depends on iron through its dependence on iron sulfur (Fe-S) cluster biogenesis. Here, we review the relationship between Fe-S biogenesis and heme synthesis in erythropoiesis, with emphasis on the proteins, GLRX5, ABCB7, ISCA, and C1orf69. These Fe-S biosynthesis proteins are highly expressed in erythroid tissues, and deficiency of each of these proteins has been shown to cause anemia in zebrafish model. GLRX5 is involved in the production and ABCB7 in the export of an unknown factor that may function as a gauge of mitochondrial iron status, which may indirectly modulate activity of iron regulatory proteins (IRPs). ALAS2, the enzyme catalyzing the first step in heme synthesis, is translationally controlled by IRPs. GLRX5 may also provide Fe-S cofactor for ferrochelatase, the last enzyme in heme synthesis. ISCA and C1orf69 are thought to assemble Fe-S clusters for mitochondrial aconitase and for lipoate synthase, the enzyme producing lipoate for pyruvate dehydrogenase complex (PDC). PDC and aconitase are involved in the production of succinyl-CoA, a substrate for heme biosynthesis. Thus, many steps of heme synthesis depend on Fe-S cluster assembly.
Erythropoiesis, the manufacture of red blood cells (or erythrocytes), mainly occurs within bone marrow in human adults, for review see . In erythropoiesis, there is a stepwise differentiation of cell types, beginning with multipotent hematopoietic stem cells which successively mature into common myeloid progenitor cells, proerythroblasts, erythroblasts, and finally into mature erythrocytes . Erythropoiesis is stimulated by the hormone, erythropoietin (EPO), for review see , which enhances proliferation and differentiation of the erythroid cells by blocking apoptosis of erythroid progenitors, as is reviewed elsewhere, for review see [4–8].Hemoglobinization results from the production of hemoglobin, which requires synthesis of heme. Heme is synthesized by an eight step enzyme-catalyzed pathway,in which the final step is the insertion of an iron into protoporphyrin IX to form a protoheme, for review see [9, 10]. The substantial manufacture of heme for hemoglobin in red blood cells consumes 70% of body iron in humans. Iron homeostasis during erythropoiesis is highly regulated to synchronize synthesis of heme and globin and to avoid the potential toxicity caused by accumulation of excess iron or heme.
Iron in food is absorbed in the duodenum, from which it is released into the circulation via ferroportin, the iron exporter on basolateral membranes of duodenal enterocytes. Most of the daily iron supply in the human body comes from phagocytosis of senescent red blood cells by macrophages in the spleen, liver, and bone marrow. Macrophages recycle iron by metabolizing heme and releasing the free iron into the circulation via the membrane-bound ferrous iron transporter, ferroportin [11–13]. The ferroportin-mediated release of iron is therefore a key regulation point of systemic iron metabolism. Hepcidin is a small peptide synthesized mainly in the liver that modulates the abundance of ferroportin at the cellular membrane of cells that release iron, for review see [14–16]. Hepcidin is the master regulator of systemic iron homeostasis: low levels of hepcidin increase iron release into plasma, whereas high hepcidin levels decrease iron release into plasma. The transcription of hepcidin is complex and is finely tuned by a number of different signal transduction pathways, for review see [14, 17–19]. To coordinate iron metabolism to meet the demands of erythropoiesis, hepcidin expression is regulated by EPO, the erythropoiesis stimulator, and also possibly by growth differentiation factor 15 (GDF15) and twisted gastrulation (TWSG1), soluble peptides which are directly produced by erythroblasts [20, 21]. In cultured liver cells (primary hepatocytes and HepG2), hepcidin transcription is regulated by EPO, which mediates its effect through EPO receptor signaling and C/EBP transcription factor . GDF15 secretion from maturing erythroblasts may inhibit hepcidin mRNA expression in hepatocytes, which would therefore allow more release of iron into plasma from the duodenum and macrophages to support erythropoiesis. However, this potential role of GDF15 remains unproven, as GDF15 has failed to suppress hepcidin expression in cellular models [23, 24]. In thalassemia syndromes, GDF15 is overexpressed, and its proposed repression of hepcidin expression leads to iron overload . TWSG1 protein, which is also expressed by erythroblast cells, may regulate hepcidin expression together with GDF15 by interfering with BMP-mediated hepcidin expression , or it may act independently of GDF15.
Cellular iron homeostasis in mammals is primarily regulated by the IRP/IRE system, which operates mainly at the posttranscriptional level. Mammalian cells express two iron regulatory proteins (IRPs), including IRP1 (annotated as Aco1 in genome) and IRP2 (annotated as Ireb2 in genome, but commonly referred to as IRP2), for review see [25, 26]. IRP1 protein functions as a cytosolic aconitase when it ligates a [4Fe-4S] cluster whereas it is activated to bind to RNA stem-loops known as iron-responsive elements (IREs) when it lacks an Fe-S cluster. IRP2 is degraded in iron replete conditions whereas it is stabilized in cells that are iron-depleted, and stabilized IRP2 acts as a second IRE-binding protein. The iron-responsive element (IRE) consists of a conserved loop (5′-CAGUGN-3′) at the end of a base-paired helical stem that is interrupted by an unpaired “bulge” cytosine. IREs are usually found in the untranslated regions (UTR) of various mRNAs. When IRP proteins bind to an IRE in 5′UTR they inhibit translation whereas then they bind IREs in 3′UTR of the TfR1 transcript, they stabilize the mRNA.
Iron sulfur clusters (Fe-S) are synthesized in human cells by a mitochondrial machinery and also by an independent cytosolic machinery, which involve at least 20 proteins in total, for review see . In mitochondria, ISCS and ISD11 form a complex of cysteine desulfurase to provide the sulfur needed for initial Fe-S formation. It is thought that frataxin provides the iron by binding iron loosely to an acidic ridge . Fe-S clusters are assembled upon scaffold proteins, which include ISCU , NFU , and ISCA . In cytosol, the cytosolic forms of ISCS and ISD11 (c-ISCS and c-ISD11) provide sulfur , and iron may be acquired from the cytosolic iron pool, perhaps aided by a chaperone or cytosolic frataxin [45, 46]. In the cytosol, clusters are assembled upon various scaffolds including c-ISCU, c-NFU, c-ISCA, IOP1, for review see [39, 47], and NBP35 . Under conditions that impair mitochondrial Fe-S cluster synthesis, iron is imported into mitochondria with high priority, which in turn results in cytosolic iron deficiency and impairment in cytosolic Fe-S cluster synthesis [41, 44, 49].