Transcription Factor Integration from Pathways Regulating Adult Blood Stem Cell Production and Self-Renewal.

Self-renewal of hematopoietic stem cells (HSCs) involves multiple signaling and transcription factors. We recently showed that prostaglandin (PG) E2 regulates the induction and engraftment of vertebrate HSC pathways. Yet, the targets and transcriptional output of the PGE2 pathway remains to be defined. The wnt pathway similarly affects HSC formation; activation of wnt signaling in heat-shock inducible transgenic zebrafish led to enhanced HSC formation, while inhibition of wnt/β-catenin signaling at the membrane level (dickkopf), in the cytosol (axin) or in the nucleus (dominant negative TCF), reduced HSC numbers. Using wnt transcriptional reporter zebrafish with multimerized TCF-binding sites. PGE2 was found to increase activity during embryonic development, demonstrating a direct interaction between these signaling pathways. The transcriptional output of the wnt pathway is linked to the interaction of TCF with β-catenin. Based on the transcriptional increase in multimerized TCF binding sites in vivo, we had hypothesized that prostaglandin signaling directly stimulates β-catenin activation. β-catenin is known to be phosphorylated on critical residues that modulate its activity. In marrow-derived cells, we demonstrate that prostaglandin increases a cyclic AMP pathway that leads to the PKA phosphorylation of β-catenin, thereby increasing its transcriptional activity. In HSCs, the interaction of multiple signaling pathways with transcriptional output is a method for modulating self-renewal in the stem cell pool as well as tissue differentiation.