Abstract 29: Heme Levels Are Increased in the Hearts of Patients with End-Stage Heart Failure

Abstract

Introduction: Diseases of iron imbalance can cause cardiomyopathy, as observed in hereditary hemochromatosis or Friedreich’s ataxia. However, the iron status of a failing heart has not been systematically examined. Results: The iron levels were quantified in ten hearts from human patients with end-stage cardiomyopathy and ten unmatched healthy control hearts. Total cellular and cytosolic non-heme iron levels were significantly reduced in failing hearts; however, there was a 2-fold increase in cytosolic heme levels in heart failure, as measured by the total protoporphyrin IX (PPIX) content. The amount of iron-free PPIX was low and did not differ between the two groups, suggesting that the majority of PPIX is complexed with iron to form heme, and thus our assay accurately estimates heme levels in the heart. To study the mechanism for the increased heme content in failing hearts, we examined the expression of genes involved in heme and iron homeostasis. Consistent with elevated heme, there was a decrease in the levels of the cellular iron importer (transferrin receptor 1 orTfR1) and the first enzyme in heme synthesis (δ-aminolevulinic acid synthase 1 or ALAS1), both of which are negatively regulated by heme. Other genes involved in heme synthesis, iron incorporation into PPIX, and heme degradation were not affected. However, we noted a significant increase in the mRNA and protein levels of ALAS2 in the failing hearts. ALAS2 is normally expressed in hemoatopoietic cells and displays very low expression in the heart. Its induction in the setting of heart failure provides a possible explanation for increased heme levels. We then assessed whether increase in ALA and heme in failing hearts is associated with generation of reactive oxygen species (ROS). Addition of exogenous ALA led to a four-fold increase in ROS, while incubation with hemin have doubled ROS levels in these cells, suggesting that augmentation of heme synthesis may exacerbate heart failure by increasing the oxidative stress. Conclusions: Our results suggest that heme levels are increased in human hearts with end-stage cardiomyopathy, possibly through the induction of ALAS2 expression. The elevated heme levels may exacerbate heart failure by increased production of ROS.