Hypoxia alters progression of the erythroid program

Hypoxia can induce erythropoiesis through regulated increase of erythropoietin (Epo) production. We investigated the direct influence of oxygen tension (pO2) in the physiologic range (2-8%) on erythroid progenitor cell differentiation using cultures of adult human hematopoietic progenitor cells exposed to decreasing (20% to 2%) pO2 and independent of variation in Epo levels. Decreases in hemoglobin (Hb)-containing cells were observed at the end of the culture period with decreasing pO2. This is due, in part, to a reduction in cell growth and, at 2% O2, a marked increase in cell toxicity. Analysis of the kinetics of cell differentiation showed an increase in the proportion of cells with glycophorin-A expression and Hb accumulation at physiologic pO2. Cells were characterized by an early induction of γ-globin expression and a delay and reduction in peak levels of β-globin expression. Overall, fetal Hb and γ-globin expression were increased at physiologic pO2, but these increases were reduced at 2% O2 as cultures become cytotoxic. At reduced pO2, induction of Epo-receptor (Epo-R) by Epo was decreased and delayed, analogous to the delay in β-globin induction. The oxygen-dependent reduction of Epo-R can account for the associated cytotoxicity at 2% O2. Epo induction of erythroid transcription factors, EKLF, GATA-1, and SCL/Tal-1, was also delayed and decreased at reduced pO2, consistent with lower levels of Epo-R and resultant Epo signaling. These changes in Epo-R and globin gene expression raise the possibility that the early increase of γ-globin is a consequence of reduced Epo signaling and a delay in induction of erythroid transcription factors.