Low Cytosolic Non-Heme Iron Levels in Erythroid Cells Prevent IRP2-Mediated Ferritin Upregulation during Differentiation.

Erythroid cells are the major consumers of iron in the human body. Differentiating erythroid cells shuttle the metal with very high efficiency towards mitochondria for the formation of heme. To satisfy their high iron needs, developing red blood cells (RBC) have to sustain high expression of transferrin receptor 1 (TfR) despite increasing cellular iron concentration. Moreover, synthesis of ferritin must not be activated by incoming iron, since this would represent a counterproductive storage during the phase of high iron demand. Recently we have demonstrated that during terminal differentiation primary erythroid cells satisfy their exceptionally high requirements for iron by switching to a mode where the post-transcriptional, iron-dependent regulatory system, formed by iron responsive proteins (IRP1 and IRP2) and iron responsive elements (IREs), seems to sense a low-iron state. This occurs despite a massive net increase of iron import into the cell (Schranzhofer et al., Blood107:4159, 2006). To examine the hypothesis that erythroid cells have low non-heme iron levels in their cytosol, we experimentally increased the cytosolic iron pool by either inhibiting heme biosynthesis or overloading cells with iron. Both block of heme synthesis by either succinylacetone or isonicotinic acid hydrazide (INH) or administration of ferric ammonium citrate, resulted in a clear increase in ferritin levels. This increase was directly proportional to the increase in the cellular concentration of non-heme-iron. Moreover, the effect of INH, the inhibitor of 5-aminolevulinic acid (ALA) synthase, could be reversed by the addition of ALA. Strikingly, increases in ferritin expression upon perturbation of cellular iron homeostasis strongly correlated with the loss of IRE-binding activity of IRP2 but not IRP1, as determined by mobility shift assays. This suggests that IRP2 is the major regulator of ferritin expression in erythroid cells. To further elaborate on this observation, we cultured primary erythroblasts derived from IRP1−/− and IRP2−/− mice (kindly provided by Drs. M. Hentze and B. Galy). In agreement with the published phenotype of microcytic hypochromic anemia, only erythroblasts lacking IRP2 exhibited a reduction in hemoglobinization. Moreover, only IRP2−/− cells showed a significant increase in ferritin expression, whereas developing RBC lacking IRP1 had levels of ferritin protein equal to wild type cells. We conclude that in erythroid cells efficient shuttling of incoming iron towards mitochondria and its prompt use for heme formation is important to keep the cytosol in an iron-deprived state and consequently ferritin protein levels low. This translational repression seems to be mainly achieved by IRP2. Together with the observation that surface expression of TfR was reduced in IRP2−/− erythroblasts during self renewal but not during terminal differentiation, our results suggest that not only down-regulation of TfR, but also up-regulation of ferritin may be a major factor for the anemic phenotype observed in IRP2−/− mice.