The cellular response to decreased iron availability, iron restriction, serves as the basis for anemia in over a billion people worldwide. This response impairs erythropoietin (Epo) driven development of bone marrow erythroid progenitors and contributes to anemias associated with chronic diseases, aging, as well as iron deficiency anemia (IDA). In aggregate, anemias associated with iron restriction have major health and economic impacts. Therefore, understanding the mechanisms underlying the iron restriction response and designing therapies to target this response are critically important. The erythroid iron restriction response involves lineage-selective inactivation of the aconitase enzymes involved in metabolism of citrate to isocitrate. Supplementation of iron-deprived cells with isocitrate strikingly rescues erythroid development in cell culture and animal models of anemia. Conversely, pharmacologic inhibition of aconitase suffices to block differentiation of erythroid progenitors, inducing anemia in normal mice and correcting erythrocytosis in mice with polycythemia vera. The regulation of erythropoiesis by aconitase activity arises through complex and poorly-understood interplay between metabolism and Epo signaling, affecting PKC, ERK, and AKT. The translational importance of this pathway was highlighted in our recent publication showing isocitrate treatment to ameliorate anemia of chronic inflammation in a rat arthritis model (J. Clin. Invest., 123:3614-23, 2013). These studies have led to NIH STTR funding for pre-clinical development of isocitrate as a novel therapy for human anemias of chronic disease and inflammation (ACDI). The therapeutic efficacy of isocitrate derives from its capacity to block the iron restriction response, which otherwise sensitizes erythroid progenitors to inhibition by inflammatory cytokines. This sensitization arises from superinduction of the transcription factor PU.1 by cooperative interplay between specific iron restriction and inflammatory signaling pathways. Normally downregulated early in erythroid development, PU.1 is a master regulator whose levels dictate myeloid versus erythroid cell fate in hematopoietic progenitors.
Aim 1 will characterize erythroid PU.1 dysregulation associated with clinically-relevant in vivo models of iron restricted anemia. Regarding proximal signaling abnormalities in erythroid iron restriction, exciting new data implicate the scaffold protein Scribble as a key target of aconitase activity. Scribble normally functions as a determinant of cell polarity, as well as a critical assembly platform for multiple phosphatases and kinases. We have found that aconitase inhibition perturbs the polar morphology of developing erythroblasts. Furthermore, signaling pathways controlled by Scribble are dysregulated by either aconitase inhibition or iron restriction. Most compellingly, aconitase inhibition induces dramatic downregulation of Scribble and of its chaperone SGT1. Iron restriction also strongly downregulates Scribble, and isocitrate treatment blocks this downregulation. Accordingly, Aim 2 will determine the mechanism and consequences of erythroid Scribble downregulation by aconitase inhibition and iron restriction.
Debilitating anemias often arise in chronic illnesses like kidney disease, cancer, and autoimmunity. Tremendous resources have been committed to the treatment of these anemias, but the available treatments have several drawbacks. These anemias result from impaired iron delivery to red cell precursors, erythroblasts, leading to a block in their development. Inflammation-associated signals also contribute to this developmental block. Our lab has begun to uncover the mechanisms by which erythroblasts sense and respond to iron deprivation. This information has led to the design of a novel treatment for anemias of chronic disease. The current project will delineate a newly identified pathway by which iron deprivation fundamentally reconfigures erythroblast development.
