Nitrogen Monoxide Inhibits Heme Synthesis In Reticulocytes

Abstract 4252

Anemia of chronic disease (ACD) is a condition that often manifests in patients with chronic immune activation due to chronic infections, autoimmune disorders, cancer and other diseases. The pathogenesis of ACD is complex and involves inefficient erythropoietin production, immune-mediated inhibition of erythropoiesis, and retention of iron in hemoglobin-processing macrophages. During their development, erythroid cells are closely associated with macrophages. In inflammatory conditions, activated macrophages generate large quantities of the gaseous molecule, nitric oxide (NO), which has numerous effects on iron metabolism. In this study, we explored the possibility that NO affects iron metabolism in erythroid cells. We treated reticulocytes with the NO donors, sodium nitroprusside (SNP) and S-Nitroso-N-acetyl-D,L-penicillamine (SNAP). We show that NO inhibits ⁵⁹Fe incorporation from ⁵⁹Fe-transferrin into reticulocytes and their heme. Significantly, 5-aminolevulinic acid (ALA, the product of ALA synthase, which catalyzes the first step of heme synthesis) reversed the SNP-mediated decrease in ⁵⁹Fe incorporation into heme but not the cellular ⁵⁹Fe uptake. In addition, SNP treatment led to an increase in eIF2α phosphorylation (which is known to occur in heme-deficient cells) and decreased globin translation. Importantly, the addition of ALA to SNP-treated reticulocytes prevented the effect of SNP on eIF2α phosphorylation and reversed globin synthesis inhibition. This indicates that in SNP treated reticulocytes, the phosphorylation of eIF2α and inhibition of globin synthesis occur indirectly, via NO's effect on erythroid-specific ALA synthase (ALA-S2). These results led us to conclude that NO has two distinct effects on reticulocytes, namely: a decrease in ALA-S2 activity and a decrease in transferrin-mediated iron uptake. The profound impact of NO on heme synthesis, iron uptake and globin translation in reticulocytes raises the possibility that NO production by macrophages could also contribute to ACD.