Iron Export through the Transporter Ferroportin 1 Is Modulated by the Iron Chaperone PCBP2

Abstract

Ferroportin 1 (FPN1) is an iron export protein found in mammals. FPN1 is important for the export of iron across the basolateral membrane of absorptive enterocytes and across the plasma membrane of macrophages. The expression of FPN1 is regulated by hepcidin, which binds to FPN1 and then induces its degradation. Previously, we demonstrated that divalent metal transporter 1 (DMT1) interacts with the intracellular iron chaperone protein poly(rC)-binding protein 2 (PCBP2). Subsequently, PCBP2 receives iron from DMT1 and then disengages from the transporter. In this study, we investigated the function of PCBP2 in iron export. Mammalian genomes encode four PCBPs (i.e. PCBP1-4). Here, for the first time, we demonstrated using both yeast and mammalian cells that PCBP2, but not PCBP1, PCBP3, or PCBP4, binds with FPN1. Importantly, ironloaded, but not iron-depleted, PCBP2 interacts with FPN1. The PCBP2-binding domain of FPN1 was identified in its C-terminal cytoplasmic region. The silencing of PCBP2 expression suppressed FPN1-dependent iron export from cells. These results suggest that FPN1 exports iron received from the iron chaperone PCBP2. Therefore, it was found that PCBP2 modulates cellular iron export, which is an important physiological process.Iron is an essential but potentially hazardous bio-metal (1). Because of its ability to readily accept or donate electrons, iron is a valuable cofactor that is used in oxygen transport, electron transfer, and DNA synthesis (2, 3). However, iron is potentially toxic because it catalyzes the generation of reactive oxygen species. The ensuing oxidative stress is associated with damage to cellular molecules, injury to tissues, and disease via processes including hydroxyl radical formation, glutathione depletion, protein aggregation, lipid peroxidation, and nucleic acid modification (4, 5).Despite its high abundance in nature, ferric iron is poorly bioavailable due to its exceedingly low solubility at physiological pH. Thus, the acquisition and usage of iron presents a considerable challenge to cells and organisms, which have evolved sophisticated mechanisms to satisfy their metabolic needs and concomitantly minimize the risk of toxicity.It has long been known that there is very little, yet potentially toxic, low-molecular-weight iron in rapidly metabolizing cells (6). However, the molecular mechanisms involved in intracellular iron transport have remained elusive (7). Recently, it was revealed that one of the mechanisms of iron transport and metabolism involves intracellular chaperone proteins (8). It was reported that poly(rC)-binding protein 1 (PCBP1), 3 also referred to as ␣-CP1 or hnRNP E1, is a cytosolic iron chaperone protein that delivers iron to ferritin (9). PCBP1 binds to iron with micromolar affinity at a molar ratio of 3:1 Fe:PCBP1. Notably, PCBP1 has been shown previously to function as an RNAand DNA-binding protein and has been identified as a member of a family of four homologous proteins containing three heterogeneous nuclear ribonucleoprotein K-homology (KH) do...

Intersection of Iron and Copper Metabolism in the Mammalian Intestine and Liver

“…PCBP1 facilitated iron loading onto ferritin (270). More recently, PCBP2 was shown to interact with DMT1 and FPN1 in hepatocytes, thus possibly functioning in a similar manner to copper chaperones (319, 320). The in vivo significance of these observations is not clear, and whether these proteins function similarly in enterocytes has not yet been investigated.…”

Molecular regulation of cellular iron balance

“…In fact, after the first report showing the delivery of iron to Ft by PCBP1 and 2 , it has been shown that these chaperones also deliver iron to both dinuclear (deoxyhypusine hydroxylase) and mononuclear (PHD) iron enzymes . Moreover, PCBP2 interacts with DMT1 and FPN , thus forming a pipeline for iron flux through the cell. Therefore, the identified PCBP partners/targets cover iron transporters, iron storing proteins, and proteins representative of the most important categories of iron‐dependent enzymes.…”

Iron transport proteins: Gateways of cellular and systemic iron homeostasis

“…PCBP1 and PCBP2 have been shown to function as iron chaperones that deliver cytosolic iron to ferritin for storage (30,31). Studies in HEp-2 cells, a human epithelial cell line, have found that PCBP2 (but not PCBP1) physically interacts with DMT1 and ferroportin (32,33), but whether this occurs in enterocytes or other cell types remains to be determined. Iron directed for storage in ferritin would serve to retain iron in the enterocyte and prevent its absorption into the body.…”