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

Abstract

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 (Nupr1), G1 to S phase transition 1 (Gspt1), early growth response 1 (Egr1), Ngfi-A binding protein 2 (Nab2), and cyclin D2. In contrast, inhibitory cyclin G2, p27/Cdkn1b, and B-cell leukemia/ lymphoma 6 (Bcl6) 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 Bcl6, and induction of cyclin D2, were determined to depend on PY343 ( IntroductionThrough gene disruption experiments, 1 erythropoietin (EPO) and its dimeric single transmembrane receptor (EPOR) are known to be essential for erythroblast formation beyond a colony-forming unit-erythroid stage. In these erythroid progenitor cells, the EPOR plays a key survival role 2,3 and couples EPO to several antiapoptotic pathways. These include EPOR PY479 coupling to p85alpha PI3-kinase, thymoma viral proto-oncogene 1 (AKT), mammalian target of rapamycin (mTOR), and forkhead box O3a (FOXO3A) signal transduction factors. [4][5][6] A constitutively active AKT mutant also can rescue Janus kinase 2 (JAK2)-deficient fetal liver erythroblast development, 7 and ectopically expressed Bcl-2-like-1 (BCL-XL) can support erythroblast formation in the absence of Stat5 and/or EPO signaling. 8 In addition, an EPOR PY343 Stat5 binding site may promote Bcl-xl gene expression, 9,10 and recently this cytoplasmic phosphotyrosine site (and associated transduction pathways) have been shown to support stress erythropoiesis. 11 Erythroid progenitor cells are also sharply regulated within the contexts of both renewal and differentiation divisions. 12 Differentiation divisions occur during late-stage development, and exhibit a shortened G1-phase. 13 In such differentiating erythroblasts, E2F transcription factor 4 (E2F4) interestingly has been shown to selectively promote proliferation, and to possibly regulate several cell-cycle-associated genes (eg, Cyclin A2 [Ccna2], minichromosome maintenance deficient 2 mitotin (Mcm2) and proviral insertion Emi 1 [Emi1]). 14,15 E2F4-deficient peripheral erythrocytes also are macrocytic, implicating E2F4 effects on size control. Renewal erythroid divisions, in contrast, exhibit more standard cell-cycle properties, and are stimulated during hypoxic stress and anemia. 12 The nature of factors that exert primary control over erythroid progenitor renewal divisions, however, is less clear.Several previous studies (predominantly in cell line models) have suggested that EPO also may modulate erythroid progenitor proli...

Elevated Cyclin G2 Expression Intersects with DNA Damage Checkpoint Signaling and Is Required for a Potent G2/M Checkpoint Arrest Response to Doxorubicin

“…We previously showed that unscheduled CycG2 expression inhibits DNA synthesis, blunts CDK2 (but not CDK4) activity (13), and induces a p53-dependent G 1 -phase cell cycle arrest (15). Similar results have since been replicated by others using various epitope-tagged forms of CycG2 in several cell lines (10,13,14,16,24). Here we show that the potent cell cycle arrest response of HCT116 cells to exogenous CycG2 requires intact alleles for Chk2 and p53 (Fig.…”

“…We showed that overexpression of CycG2 inhibits CDK2 activity and that the CycG2-mediated G 1 /S-phase cell cycle arrest is p53-dependent (13,15). Subsequent studies determined that even moderate up-regulation of ectopic CycG2 expression inhibits cellular proliferation (10,16,24). We found that exogenous and endogenously expressed CycG2 is a CRM1-dependent nucleocytoplasmic shuttling protein that localizes to the cytoplasmic-cytoskeletal compartment of replicating cells where it associates with centrosomes via AKAP450 (15).…”

Mechanism of apoptosis involved in gastric mucosal lesions in Tibetans with high-altitude polycythemia

“…microcirculation disturbances and systemic disorders. However, effective prevention and control measures for HAPC have yet to be utilized (15,16). In addition, it has been reported that GML is the most frequent concomitant disease to HAPC.…”

β-thalassemia: a model for elucidating the dynamic regulation of ineffective erythropoiesis and iron metabolism

“…24 In turn, the phosphorylated form of JAK2 activates STAT5, 25 which modifies the expression of genes involved in proliferation, differentiation, and survival of erythroid precursors. 26,27 Erythropoiesis requires the coordinated biosynthesis of heme and globin chains to produce Hb during the differentiation process. Proteins that control iron intake and heme biosynthesis are reviewed in "Coordination of erythroid iron homeostasis, heme metabolism, and globin synthesis.…”