Iron Control of Erythropoiesis: A Novel Pathway in Which Aconitase Enzymes Couple Metabolic Flux with Cellular Development.

Erythroid iron deficiency, whether due to dimished body stores or histiocytic retention, diminishes marrow responsiveness to erythropoietin (Epo). Conversely, intravenous iron infusion augments marrow responsiveness to Epo, even in anemia patients with adequate pre-existing iron stores. Iron regulation of Epo-driven erythropoiesis affects proliferation and differentiation of early progenitors, prior to the commitment to heme production. Thus, while iron is essential for all cells, erythroid precursors manifest an exquisite sensitivity to iron deficiency, most likely as a rationing mechanism to protect other, more vital iron-dependent functions. Using primary human hematopoietic cultures with defined levels of transferrin saturation, we have confirmed the existence of a critical threshold of iron deprivation, at which erythroid progenitors display proliferative and maturation blockade while granulopoiesis and megakaryopoiesis remain unaffected. Extensive pharmacologic and genetic screening for components of this erythroid iron response pathway have identified the iron-sulfur cluster containing aconitase enzymes as a critical signaling node. Mitochondrial and cytosolic aconitase (mAcon & cAcon) interconvert citrate and isocitrate as a key step in the Krebs cycle. Firstly, treatment of iron deprived erythroid cultures with exogenous isocitrate, but not citrate, completely restored differentiation, as judged by glycophorin A (GPA) expression and hemoglobinization. By contrast, both citrate and isocitrate enhanced erythroid differentiation under iron replete conditions. Secondly, treatment of iron replete erythroid cultures with a specific aconitase inhibitor, fluorocitrate, induced a lineage-selective maturation blockade identical to that seen with iron deprivation. Thirdly, enzymography showed erythroid-lineage specific inactivation of both cAcon and mAcon in response to iron deprivation; immunoblotting showed no change mAcon protein levels as a function of either lineage or iron status. Fourthly, a retroviral genetic screen identified HBLD2, an iron-sulfur cluster assembly factor, as a protein whose overexpression reversed the erythroid maturation blockade associated with iron deprivation. Enzymography confirmed that overexpression of HBLD2 enhanced both cAcon and mAcon activities. Fifthly, administration to wild type, iron replete C57BL/6 mice of isocitrate at 200 mg/kg/day for 5 days caused a significant increase in peripheral red cell hemoglobinization, reflected by MCHC and Hb levels. Taken together, these results identify isocitrate as a positive regulator of Epo-mediated erythroid differentiation. In Epo-independent CD34+ cell culture systems, isocitrate did not enhance erythropoiesis. Therefore aconitase enzymes serve as a critical nexus in iron and Epo regulation of erythropoiesis, integrating cellular metabolism with developmental programming. The unique sensitivity of the erythroid lineage to iron deprivation appears to derive from a cellular milieu effect on the aconitase enzymes, promoting their inactivation under iron deprivation. Notably, IRP repression of mAcon translation does not contribute to the erythroid iron deprivation response pathway. This pathway may have relevance for future clinical approaches to Epo-refractory chronic anemias and polycythemia vera.