RAPID COMMUNICATION Erythropoietin Can Promote Erythroid Progenitor Survival by Repressing Apoptosis Through Bc l-X, and Bcl-2

Erythropoietin (Epo), the hormone that is the principal regu-lator of red blood cell production, interacts with high-affinity receptors on the surface of erythroid progenitor cells and maintains their survival. Epo has been shown to promote cell viability by repressing apoptosis; however, the molecu-lar mechanism involved is unclear. In the present studies we have examined whether Epo acts as a survival factor through the regulation of the bel-2 family of apoptosis-regulatory genes. We addressed this issue in HCD-57, a murine ery-throid progenitor cell line that requires Epo for proliferation and survival. When HCD-57 cells were cultured in the ab-sence of Epo, Bel-2 and Bcl-XL but not Bax were downregu-lated, and the cells underwent apoptotic cell death. HCD-57 cells infected with a retroviral vector encoding human Bcl-XL or Bcl-2 rapidly stopped proliferating but remained viabl