EPO modulation of cell-cycle regulatory genes, and cell division, in primary bone marrow erythroblasts

Erythropoietin (EPO's) actions on erythroblasts are ascribed largely to survival effects. Certain studies, however, point to EPO-regulated proliferation. To investigate this problem in a primary system, Kit(pos)CD71(high) erythroblasts were prepared from murine bone marrow, and were first used in the array-based discovery of EPO-modulated cell-cycle regulators. Five cell-cycle progression factors were rapidly up-modulated: nuclear protein 1 (Nuprl), G1 to S phase transition 1 (Gspt1), early growth response 1 (Egr1), Ngfi-A binding protein 2 (Nab2), and cy- clin D2. In contrast, inhibitory cyclin G2, p27/Cdkn1b, and B-cell leukemia lymphoma 6 (Bci6) were sharply downmodulated. For CYCLIN G2, ectopic expression also proved to selectively attenuate EPO-dependent UT7epo cellcycle progression at S-phase. As analyzed in primary erythroblasts expressing minimal EPO receptor alleles, EPO repression of cyclin G2 and Bc16, and induction of cyclin D2, were determined to depend on PY343 (and Stat5) signals. Furthermore, erythroblasts expressing a on PY-null EPOR-HM allele were abnormally distributed in GO/G1. During differentiation divisions, EPOR-HM Ter119(POs)erythroblasts conversely accumulated in S-phase and faltered in an apparent EPO-directed transition to GO/G1. EPO/EPOR signals therefore control the expression of select cellcycle regulatory genes that are proposed to modulate stage-specific decisions for erythroblast cell-cycle progression.