Mammalian ferrochelatase, the terminal enzyme in the heme biosynthetic pathway, possesses an iron-sulfur [2Fe-2S] cluster that does not participate in catalysis. We investigated ferrochelatase expression in iron-deficient erythropoietic tissues of mice lacking iron regulatory protein 2, in iron-deficient murine erythroleukemia cells, and in human patients with ISCU myopathy. Ferrochelatase activity and protein levels were dramatically decreased in Irp2 ؊/؊ spleens, whereas ferrochelatase mRNA levels were increased, demonstrating posttranscriptional regulation of ferrochelatase in vivo. Translation of ferrochelatase mRNA was unchanged in iron-depleted murine erythroleukemia cells, and the stability of mature ferrochelatase protein was also unaffected. However, the stability of newly formed ferrochelatase protein was dramatically decreased during iron deficiency. Ferrochelatase was also severely depleted in muscle biopsies and cultured myoblasts from patients with ISCU myopathy, a disease caused by deficiency of a scaffold protein required for Fe-S cluster assembly. Together, these data suggest that decreased Fe-S cluster availability because of cellular iron depletion or impaired Fe-S cluster assembly causes reduced maturation and stabilization of apoferrochelatase, providing a direct link between Fe-S biogenesis and completion of heme biosynthesis. We propose that decreased heme biosynthesis resulting from impaired Fe-S cluster assembly can contribute to the pathogenesis of diseases caused by defective Fe-S cluster biogenesis. (Blood. 2010;115:860-869)
IntroductionHeme, the iron-containing tetrapyrrole cofactor of hemoproteins, is indispensable for many cellular and organismal metabolic processes because of its ability to confer gas transport and electron transfer functionalities to countless enzymes. In animals, heme biosynthesis begins in the mitochondrion with the conjugation of succinyl-coenzyme A and glycine by ␦-aminolevulinic acid synthase (ALAS) to form ␦-aminolevulinic acid (ALA). 1 ALA is transported into the cytoplasm where it serves as the building block for the tetrapyrrole macrocycle via a series of well-characterized reactions, 2 and the process is concluded in the mitochondrion with insertion of iron into protoporphyrin IX (PPIX) by ferrochelatase to form heme. 3 Large amounts of iron and heme are required by developing erythroblasts to supply millimolar quantities of hemoglobin in erythrocytes. An iron-responsive element (IRE) present in the 5Ј untranslated region (UTR) of the mRNA that encodes erythroidspecific isoform of ALAS (Alas2) 4 allows for binding of iron regulatory proteins 1 and 2 (IRP1/2) during iron deficiency, resulting in decreased synthesis of aminolevulinic acid synthase 2 (ALAS2) protein and restricted initiation of heme biosynthesis. In contrast, IRE sequences in the 3Ј-UTR of transferrin receptor 1 (Tfr1) confer protection from nuclease degradation on IRP binding, 5 thus increasing TfR1 protein expression during iron deficiency. Accordingly, Irp2 Ϫ/Ϫ erythroblasts express les...