• hydroxyurea;
  • human erythroid cells;
  • GATA;
  • death receptor;
  • egr-1

Summary. Hydroxyurea (HU) has been shown to increase the proportion of fetal haemoglobin (HbF) in most sickle cell patients. A low-dosage regimen increased total haemoglobin (Hb) levels in some thalassaemia intermedia patients by preferentially increasing β-globin biosynthesis. To further characterize these apparent dose-dependent effects of HU, we examined erythroid cells exposed to HU (5–100 µmol/l) in two-phase liquid culture. Low doses (from 5 to 25 µmol/l) increased Hb levels by up to 2·7-fold, and a high dose (100 µmol/l) increased Hb levels when added at d 3–6 of phase II, with no significant changes in response to HU during the late stage of phase II culture (≥ 9 d). HU exposure during d 0–3 of phase II culture increased the number of erythroid colonies to a maximum of fivefold at 5 µmol/l HU. GATA-1 mRNA was downregulated at a high dose and GATA-2 was dose dependently upregulated over a lower dosage range. Treatment with 100 µmol/l HU dramatically upregulated the death receptor DR-5, caspase 3, as determined by cDNA microarray analysis. In contrast, 10 µmol/l HU modestly upregulated mRNA levels of the early growth response gene. Our results suggest that HU exerts concentration-dependent effects on HbF production and erythropoiesis and that these two effects are mediated by distinct molecular mechanisms.