Genetic ablations of iron regulatory proteins 1 and 2 reveal why iron regulatory protein 2 dominates iron homeostasis

Abstract

The two iron regulatory proteins IRP1 and IRP2 bind to transcripts of ferritin, transferrin receptor and other target genes to control the expression of iron metabolism proteins at the post-transcriptional level. Here we compare the effects of genetic ablation of IRP1 to IRP2 in mice. IRP1À/À mice misregulate iron metabolism only in the kidney and brown fat, two tissues in which the endogenous expression level of IRP1 greatly exceeds that of IRP2, whereas IRP2À/À mice misregulate the expression of target proteins in all tissues. Surprisingly, the RNA-binding activity of IRP1 does not increase in animals on a lowiron diet that is sufficient to activate IRP2. In animal tissues, most of the bifunctional IRP1 is in the form of cytosolic aconitase rather than an RNA-binding protein.Our findings indicate that the small RNA-binding fraction of IRP1, which is insensitive to cellular iron status, contributes to basal mammalian iron homeostasis, whereas IRP2 is sensitive to iron status and can compensate for the loss of IRP1 by increasing its binding activity. Thus, IRP2 dominates post-transcriptional regulation of iron metabolism in mammals.

Renal Iron Metabolism

“…Animals that lack IRP1 are unable to repress ferritin synthesis fully in the kidney under conditions of iron deficiency (33), demonstrating that IRP1 contributes significantly to regulation of iron metabolism in the kidney (Figures 5 and 6). In most other tissues, loss of IRP1-binding activity does not lead to misregulation of iron metabolism, because IRP2 levels increase in compensation (33,45). However, in kidney of IRP1Ϫ/Ϫ mice, although IRP2 levels increase in a compensatory manner, as expected, the increase in IRP2 is not sufficient to repress ferritin synthesis completely (Figures 5 and 6).…”

“…Kidney is the mouse tissue in which IRP1 is most highly expressed (33). Animals that lack IRP1 are unable to repress ferritin synthesis fully in the kidney under conditions of iron deficiency (33), demonstrating that IRP1 contributes significantly to regulation of iron metabolism in the kidney (Figures 5 and 6).…”

“…However, unlike IRP1, IRP2 undergoes iron-dependent ubiquitination and proteasomal degradation in iron-replete cells (reviewed in references [27,28]). IRP2 is expressed ubiquitously throughout the mouse tissues, including kidney ( Figure 5) (33). Animal studies have revealed that IRP2 has a very significant role in regulation of tissue iron metabolism, and complete loss of IRP2 results in microcytic anemia, elevated serum ferritin, and late-onset neurodegeneration (34 -36).…”

“…The vast majority of IRP1 is an active aconitase at physiologic oxygen concentrations (33), and the high expression levels of IRP1 in the proximal tubule may be explained by a need for cytosolic aconitase activity at this location. Proximal tubule cells reabsorb 75 to 90% of the citrate that enters the glomerular filtrate (48), and this resorbed citrate likely is metabolized by cytosolic aconitase to yield isocitrate, which in turn can be metabolized by cytosolic isocitrate dehydrogenase to yield 2-oxoglutarate, a carbon source that can replenish the mitochondrial citric acid cycle.…”

“…Localization of IRP1 in mouse kidney. In situ hybridization showed the IRP1 mRNA expression in sections of WT mouse kidney at various magnifications: ϫ20 (a), ϫ10 (b), and ϫ4 (c) (33). IRP1 mRNA was expressed in the proximal tubule of the kidney cortex (C) of WT mice, whereas there was little, if any, IRP1 mRNA in the kidney medulla (M).…”