Role for BH3-Only Protein NOXA In Growth-Factor Deprivation and Early Erythropoiesis

Abstract 4235

Red blood cell production is a strictly regulated process and homeostatic maintenance of the erythropoietic system requires equilibrium between the rate of erythroid cell production and red blood cell destruction. Hematopoietic cytokines play a crucial role in regulating expansion, differentiation and survival of erythrocyte progenitors. Shortage of growth factors triggers the mitochondrial apoptosis pathway, which is critically dependent on Bcl-2 family members. However, the contribution of this mechanism in the regulation of erythropoiesis remains ill-defined. This prompted us to screen for candidate genes involved in this process in erythroid progenitors. We found that the expression of Noxa, a pro-apoptotic Bcl-2 family member, is upregulated during erythroid differentiation and following cytokine-withdrawal in erythroid progenitor cells. Knockdown or deletion of Noxa in IL-3 dependent human and murine erythroid progenitor cell lines increased Mcl-1 levels, which correlated with markedly decreased apoptosis following cytokine withdrawal. Importantly, Noxa ablation in mice increased extra-medullary erythropoiesis, resulting in enhanced numbers of early splenic erythroblasts and circulating reticulocytes. Noxa-deficient hematopoietic progenitors were more resistant to apoptosis induced by growth factor deprivation and displayed increased colony-forming potential. In addition, combined loss of Noxa and Bim resulted in enhanced resistance of erythroid progenitors to cytokine withdrawal compared to WT or single Bim knockouts, suggesting a non-redundant role for Noxa and Bim in regulating survival of erythroid progenitors in response to cytokine deprivation. Finally, in a model of acute haemolytic anaemia, deletion of Noxa enhanced subsequent hematocrit recovery. Together, these findings identify a non-redundant role for BH3-only protein Noxa in the regulation of erythroblast survival during early erythropoiesis. Therefore, Noxa may be a novel component to control red blood cell numbers and modulation of this pathway could be envisaged in therapeutic options for treatment of anaemia.