Extracellular Nucleoside Diphosphate Kinase NDPK/Nm23 Protein Isoforms A and B Support Erythropoiesis and Have a Burst-Promoting Activity.

Abstract

We previously demonstrated the in vitro hematopoietic effects of different nucleoside diphosphate kinase NDPK/Nm23 proteins, i.e. increase of burst-forming units erythroid (BFU-E) and decrease of colony-forming units macrophage (CFU-M) (Willems R et al. Experimental Hematology 2002; 30: 640–648). The effect was especially pronounced in the marrows that had a particular low BFU-E count. Since these experiments were performed under optimal erythroid growth factor stimulation including adequately high concentrations of erythropoietin (Epo) and stem cell factor (SCF 100 ng/mL), subtle effects of NDPK/Nm23 proteins on erythropoiesis could have been missed. In the present series of experiments on 7 normal bone marrow mononuclear cell suspensions, we deliberately created suboptimal in vitro conditions for early (BFU-E) erythropoiesis: i.e. serum free conditions with a limited number of cytokines (only Epo and SCF) and a low concentration of burst-promoting activity (SCF at 5 ng/ml). The results show that addition of NDPK A/Nm23-H1 and NDPK B/Nm23- H2 (but not of NDPK C) at high (40 μg/mL), but not low, extracellular concentration is sustaining and improving in vitro erythropoiesis in a statistically significant manner (P<0.05). Red cell colonies increased by a factor of 1.9 and 1.4 following addition of NDPK A/Nm23-H1 and NDPK B/Nm23- H2 respectively. Especially with NDPK A/Nm23-H1, this approaches levels attained under optimal conditions for erythropoiesis, i.e. at SCF concentration of 100 ng/mL, where a 2.1-fold increase was seen, as compared to cultures with low concentrations of SCF (5 ng/mL) without addition of extracellular NDPK/Nm23. In cultures without Epo no erythroid colonies were seen, even following addition of NDPK A/Nm23-H1 or NDPK B/Nm23-H2. This study strongly suggests that NDPK/Nm23 has a burst-promoting activity. The fairly high concentrations of NDPK/Nm23 constitutively present in plasma supports the physiological role of this protein in stimulating early erythropoiesis. Of the (known) cytokines or proteins with burst-promoting activity (eg. SCF, interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), NDPK/Nm23 (this study)), only SCF and NDPK/Nm23 are produced in a constitutive fashion, strongly supporting their role in the steady-state support of early erythropoiesis.