STAT3-mediated differentiation and survival of myeloid cells in response to granulocyte colony-stimulating factor: role for the cyclin-degendent kinase inhibitor p27(Kip1)

The signal transducer and activator of transcription (STAT) proteins have b een implicated in cytokine-regulated proliferation, differentiation and cel l survival. Granulocyte colony-stimulating factor (G-CSF), a regulator of g ranulocytic differentiation, induces a robust and sustained activation of S TAT3. Here, we show that introduction of dominant negative (DN) forms of ST AT3 interferes with G-CSF-induced differentiation and survival in murine 32 D cells, G-CSF induces expression of the cyclin-dependent kinase (cdk) inhi bitor p27(Kip1) (but not p21(Cip1)), which is completely blocked by DN-STAT 3. The ability of tyrosine-to-phenylalanine substitution mutants of the G-C SF receptor to activate STAT3 strongly correlated with their capacity to in duce p27 expression and their ability to mediate differentiation and surviv al, suggesting a causal relationship between STAT3 activation, p27 expressi on and the observed cellular responses. We identified a putative STAT bindi ng site in the promoter region of p27 that showed both STAT3 binding in ele ctrophoretic mobility shift assays and functional activity in luciferase re porter assays, Finally, we studied G-CSF-induced responses in primary bone marrow and spleen cells of p27-deficient mice. Compared with wildtype, myel oid progenitors from p27-deficient mice showed significantly increased prol iferation and reduced differentiation in response to G-CSF. These findings indicate that STAT3 controls myeloid differentiation, at least partly, via upregulation of p27(Kip1).