p38MAPK Inhibits Neutrophil Development through Phosphorylation of C/EBPα on Serine 21

Many extracellular stimuli regulate growth, survival and differentiation responses through activation of the dual specificity kinase MAPK kinase 3 (MKK3) and its downstream effector p38 Mitogen-Activated Protein Kinase (MAPK). Using CD34⁺ hematopoietic progenitor cells, here we describe a novel role for MKK3-p38MAPK in the regulation of myelopoiesis. Inhibition of p38MAPK utilising the pharmacological inhibitor SB203580, enhanced neutrophil development ex-vivo, but conversely reduced eosinophil differentiation. In contrast, constitutive activation of MKK3 dramatically inhibited neutrophil differentiation. Transplantation of β2-microglobulin−/− NOD/SCID mice with CD34⁺ cells ectopically expressing constitutively active MKK3 resulted in reduced neutrophil differentiation in vivo, whereas eosinophil development was enhanced. Inhibitory phosphorylation of C/EBPα on serine 21 was induced upon activation of p38MAPK. Moreover, ectopic expression of a non-phosphorylatable C/EBPα mutant was sufficient to abrogate MKK3 induced inhibition of neutrophil development. Furthermore, treatment of CD34+ progenitors from patients with severe congenital neutropenia with SB203580 restored neutrophil development. These results establish a novel role for MKK3-p38MAPK in the regulation of lineage choices during myelopoiesis through modulation of C/EBPα activity. This signaling module may thus provide an important therapeutic target in the treatment of bone marrow failure.