Abstract
The role of hematopoietic cytokines in lineage commitment remains uncertain. To gain insight into the contribution of cytokine signaling to myeloid lineage specification, we compared granulocyte colony-stimulating factor (G-CSF) and macrophage colony-stimulating factor (M-CSF) signaling in Ba/F3 cells expressing both the G-CSF and M-CSF receptors and in lineage-negative murine marrow cells. G-CSF and M-CSF serve as prototypes for additional cytokines that also influence immature myeloid cells. G-CSF specifically activated signal transducer and activator of transcription 3 and induced Src homology region 2 domain-containing phosphatase 2 (SHP2) phosphorylation, whereas M-CSF preferentially activated phospholipase Cgamma2, and thereby extracellular signal-regulated kinase (ERK), to stabilize c-Fos and stimulate CCAAT/enhancer-binding protein (C/EBP)alpha(S21) phosphorylation. In contrast, activation of Jun kinase or c-Jun was similar in response to either cytokine. Inhibition of ERK prevented induction of c-Fos by M-CSF and reduced C/EBPalpha phosphorylation and formation of colony-forming unit-monocytes. SHP2 inhibition reduced ERK activation in G-CSF, but not M-CSF, and reduced colony-forming unit-granulocytes, underscoring divergent pathways to ERK activation. Phorbol ester mimicked the effect of M-CSF, activating ERK independent of SHP2. In summary, M-CSF activates ERK more potently than G-CSF, and thereby induces higher levels of c-Fos and phospho-C/EBPalpha(S21), which may directly interact to favor monopoiesis, whereas G-CSF activates signal transducer and activator of transcription 3 and SHP2, potentially shifting the balance to granulopoiesis via gene induction by C/EBPalpha homodimers and via effects of SHP2 on regulators besides ERK.