Cell Cycle Regulation of IRP2 Phosphorylation During Hematopoiesis

ABSTRACT Iron is essential for cellular growth due to its role as a cofactor in proteins involved in DNA synthesis, mitochondrial respiration and hemoglobin production. Vertebrate iron metabolism is controlled post- transcriptionally by iron-regulatory protein 2 (Irp2). Irp2 binds to iron-responsive elements (IREs) in the mRNAs of proteins involved in iron uptake (transferrin receptor 1), sequestration (ferritin) and export (ferroportin), and regulates the translation or stability of these mRNAs. During iron deficiency, Irp2 binds IREs to regulate mRNA translation or stability, whereas during iron sufficiency, Irp2 is degraded by the FBXL5-SCF ubiquitin ligase. Here we show that Irp2 is regulated by iron-independent phosphorylation of Ser157 during G2/M that disrupts its interaction with ferritin IRE mRNA during mitosis and depresses ferritin translation. Expression of Irp2-S157A in Irp2KO-MEFs causes a G2/M delay and slows proliferation. The significance of S157 phosphorylation was investigated in mice where Ser157 was mutated to Ala157 (Irp2A/A)). Irp2A/A mice exhibit macrocytic anemia, defective erythroid terminal differentiation, splenomegaly, and dysregulated systemic iron metabolism. We propose that cell-cycle regulation of ferritin and other IRE-mRNAs may provide a mechanism to modulate the cellular labile iron pool during the cell cycle. Our overall goal is to determine how loss of S157 phosphorylation perturbs erythropoiesis and causes anemia. Our aims are to 1) determine the cell-autonomous role for Irp2- S157 phosphorylation in erythropoiesis by bone marrow (BM) transplantation and flow cytometric analysis of cell-cycle status, proliferation, iron and other parameters, 2) identify mechanisms underlying dysregulated erythropoiesis in Irp2A/A mice using a mouse erythroblast in vitro differentiation system with WT and Irp2A/A BM cells, and proteomic analysis of erythroblast populations to identify protein changes between WT and Irp2A/A cells, and 3) generate an erythroid-specific Irp2A/A knockin mouse model to study erythroid terminal differentiation. We anticipate that our studies will provide new insights of normal and dysregulated erythropoiesis.