PKCδ-induced PU.1 phosphorylation promotes hematopoietic stem cell differentiation to dendritic cells.

Human CD34(+) hematopoietic stem cells (HSCs) exhibit the potential to differentiate into a variety of specialized blood cells. The distinct intracellular mechanisms that control cell fate and lineage commitment of these multipotent cells are not well defined. In this study, we investigate and modulate the signaling processes during HSC differentiation toward myeloid dendritic cells (mDCs). DC differentiation induced by the cytokines Granulocyte macrophage colony-stimulating factor (GM-CSF) and Interleukin-4 (IL-4) led to activation of the Extracellular-signal-regulated kinase (ERK), protein kinase C (PKC), and Janus kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) but not the SAPK/c-Jun NH(2) -terminal kinase and p38 mitogen-activated protein kinase signaling pathways. From the activated signaling pathways the PKC isoform δ was found to phosphorylate the transcription factor PU.1, which is described as one of the key factors for myeloid HSC differentiation. On molecular level, PKCδ regulated PU.1 activity by affecting its transactivation activity, whereas its DNA binding activity remained unaffected. This was accompanied by PKCδ-induced phosphorylation of the PU.1 transactivation domain. Furthermore, treatment with PKC- and ERK1/2-specific signaling inhibitors impaired both HSC differentiation toward mDCs as well as phosphorylation-mediated transactivation activity of PU.1. Taken together, these results provide new insights into the molecular mechanisms promoting the differentiation process of HSCs toward mDCs and introduce the PKC isoform δ as critical mediator.