Phosphorylation of PML is essential for activation of C/EBPe and PU.1 to accelerate granulocytic differentiation

Promyelocytic leukemia (PML) is a nuclear protein that functions as a regulator of transcription, cell proliferation, apoptosis and myeloid cell differentiation. PML is subjected to post-translational modifications such as sumoylation and phosphorylation. However, the physiological significance of these modifications, especially for myeloid cell differentiation, remains unclear. In this report, we found that four serine residues in the PML C-terminal region are highly phosphorylated in a myeloid cell line. Wild-type PML accelerated G-CSF-induced granulocytic differentiation, but a phosphorylation-deficient PML mutant failed. PML interacted with C/EBPe, a transcription factor essential for granulopoiesis, activated C/EBPe-mediated transcription in concert with p300 and accelerated C/EBPe-induced granulocytic differentiation. Phosphorylation of PML was required for stimulating C/EBPe-dependent transcription and accelerating C/EBPe-induced granulocytic differentiation. We also found that PML phosphorylation was required for stimulation of PU.1-dependent transcription and acceleration of PU.1-induced granulocytic differentiation. These results suggest that phosphorylation plays essential roles in the regulation of PML to accelerate granulocytic differentiation through multiple pathways.