The immunometabolite itaconate inhibits heme synthesis and remodels cellular metabolism in erythroid precursors

As part of the inflammatory response by macrophages, Irg1 is induced resulting in millimolar quantities of itaconate being produced. This immunometabolite remodels the macrophage metabolome and acts as an antimicrobial agent when excreted. Itaconate is not synthesized within the erythron, but instead may be acquired from central macrophages within the erythroid island. Previously we reported that itaconate inhibits hemoglobinzation of developing erythroid cells. Herein we demonstrate that this is accomplished by inhibition of tetrapyrrole synthesis. In differentiating erythroid precursors, cellular heme and protoporphyrin IX synthesis are reduced by itaconate at an early step in the pathway. In addition, itaconate causes global alterations in cellular metabolite pools resulting in elevated levels of succinate, 2-hydroxyglutarate, pyruvate, glyoxylate, and intermediates of glycolytic shunts. Itaconate taken up by the developing erythron can be converted to itaconyl-CoA by the enzyme succinyl-CoA:glutarate-CoA transferase. Propionyl-CoA, propionyl-carnitine, methylmalonic acid, heptadecanoic acid and nonanoic acid, as well as the aliphatic amino acids threonine, valine, methionine, and isoleucine are increased, likely due to the impact of endogenous itaconyl-CoA synthesis. We further show that itaconyl-CoA is a competitive inhibitor of the erythroid-specific 5-aminolevulinate synthase (ALAS2), the first and rate-limiting step in heme synthesis. These findings strongly support our hypothesis that the inhibition of heme synthesis observed in chronic inflammation is mediated not only by iron limitation, but also by limitation of tetrapyrrole synthesis at the point of ALAS2 catalysis by itaconate. Thus, we propose that macrophage-derived itaconate promotes anemia during an inflammatory response in the erythroid compartment.