The Roles of Stat3 and ERK in G-CSF Signaling.

G-CSF specifically stimulates the proliferation and differentiation of cells that are committed to the neutrophil-granulocyte lineage. We examined the role of stat3 in G-CSF signaling using the Cre-loxP recombination system. Although Stat3 was thought to be essential for the transduction of G-CSF-induced cell proliferation and differentiation signals, mice deficient for Stat3 in hematopoietic cells show neutrocytosis and infiltration of cells into the digestive tract. The number of progenitor cells in the neutrophil lineage is not changed, and G-CSF-induced proliferation of progenitor cells and prolonged neutrophil survival were observed in Stat3-deficient mice. In hematopoietic cells from Stat3-deficient mice, trace levels of SOCS3, a negative regulator of granulopoiesis, were observed, and SOCS3 expression was not induced by G-CSF stimulation. Stat3 null bone marrow cells displayed a significant activation of ERK1/2 under basal conditions, and the activation of ERK was enhanced and sustained by G-CSF stimulation. Furthermore, the augmented proliferation of Stat3-deficient bone marrow cells in response to G-CSF was dramatically decreased by addition of a MEK1 inhibitor. These results indicate that Stat3 functions as a negative regulator of G-CSF signaling by inducing SOCS3 expression, and ERK activation is the major factor responsible for inducing the proliferation of hematopoietic cells in response to G-CSF.