Defects in Management of Labile Iron and Oxidative Stress Underpin Red Cell Enucleation Defects in Rb Null Mice.

The Rb tumor suppressor is critically required for end-stage red cell maturation under conditions of oxidative stress, including in the developing fetal liver, in the bone marrow of aging mice, in the spleen and bone marrow of young mice treated with phenylhydrazine to induce hemolytic anemia, and in lethally irradiated mice reconstituted with donor tissue [1]. Loss of Rb resulted in a failure of end-stage red cells to enucleate, accumulation of red cells with a 4N DNA content and aberrant chromatin structure [1]. The molecular basis of these defects is not defined nor do we understand the reasons why pRb should be required under stress conditions, but not during normal “steady-state” erythropoiesis. The work presented will address both of these questions. In determining why pRb is critically required for stress erythropoiesis but not for steady-state erythropoiesis, we have demonstrated increased levels of reactive oxygen species (ROS) and labile iron in Rb null erythroblasts relative to wild-type control erythroblasts derived from E12.5 fetal liver. Furthermore, we show that quenching of ROS in Rb null erythroblasts by treatment of mice with the anti-oxidant N-acetyl cysteine (NAC) rescued aspects of the erythroid defect, including red cell enucleation and also extended the lifespan of Rb null mice. Similarly, chelation of labile iron with desferroxiamine restored enucleation capacity to Rb null erythroblasts. Furthermore, we show that the transferrin receptor (CD71) is transcriptionally repressed by pRb/E2F and examine whether deregulated expression of CD71 contributes to increased labile iron and oxidative stress in Rb null erythroblasts. These results suggest that loss of pRb limits the ability of erythroblasts to manage labile iron and oxidative stress, in part through deregulated expression of CD71, and that this contributes to the enucleation defect observed in Rb null mice. Given that pRb is itself regulated by ROS, we present a model in which the timely induction and repression of the CD71 receptor in differentiating erythroblasts is required to manage labile iron, oxidative stress and to coordinate cell cycle exit with end-stage maturation.