EPO-independent activation of BCL-XL and BCL2 expression at discrete stages of (pro)erythroblast development. (A) In primary bone marrow erythroblasts, neither BCL-XL nor BCL2 expression is EPO-modulated. Kit^posCD71^highTer119^neg erythroblasts were expanded from wt-EPOR bone marrow preparations. Cells then were cultured for 5 hours in 50 μg/mL transferrin, 0.1% BSA, 15 ng/mL insulin, 0.1 mM 2-ME, IMDM. EPO was then added (2.5 U/mL), and at 0, 2.5, and 7.5 hours cell lysates were prepared. Lysates then were analyzed by Western blotting for levels of BCL-XL, BCL2, PIM1, and beta-tubulin. In parallel, possible EPO modulation of Bcl-x or Bcl2 transcripts also was analyzed, here at 0, 30, 90, and 270 minutes of EPO exposure. Values are mean-fold modulation plus or minus SE. (B) Mapping of EPOR subdomains to EPO-regulated survival factor circuits. Presently defined EPO-modulated transcriptional response circuits are outlined. An EPOR JAK2-only circuit mediates Foxo3a, Bim, and Trb2 repression. In parallel, an EPOR/PY343/STAT5 axis enhances Pim1 and Pim3 expression, and affords EPO induction of Irs2, S3G, and Trb3.