Genetic Interaction between PGE2 and the Wnt/β-Catenin Signaling Pathway Regulates Definitive HSC Development and Homeostasis.

Hematopoietic stem cells (HSC) differentiate throughout the lifetime of the organism to produce each of the definitive blood lineages. Recently, we demonstrated that prostaglandin (PG) E2 regulates vertebrate HSC formation and homeostasis. The mechanism by which PGE2 influences HSCs is currently unknown. Through qPCR analysis and in situ hybridization, we determined that dimethyl (dm) PGE2 regulated expression of the classical wnt targets cyclinD1 and ephrinB2 during embryonic development. As the wnt pathway has been shown to regulate HSC self-renewal, we hypothesized that the two pathways interact genetically to control the production and homeostasis of HSCs. In TOP:dGFP wnt reporter fish, wnt activity was enhanced in vivo following the addition of dmPGE2, whereas indomethacin treatment resulted in the virtual absence of β-catenin/TCF signaling. Activation of wnt signaling in heat-shock (hs) inducible transgenic zebrafish (hs:wnt8) led to enhanced AGM HSC formation, while inhibition of wnt/β-catenin signaling at the membrane level (hs:dickkopf (dkk)), in the cytosol (hs:axin) or in the nucleus (hs:dominant negative (dn) TCF) reduced HSC numbers. Exposure to dmPGE2 rescued HSC inhibition mediated by dkk, but not by axin or dnTCF, implying that PGE2 interacts with the wnt signaling pathway at the level of the β-catenin destruction complex. Furthermore, indomethacin treatment diminished the effects of wnt induction on HSC formation, suggesting that PGE2 activity is required to mediate the full effects of wnt activation. We next sought to determine if PG signaling could regulate wnt-mediated control of hematopoietic recovery in the adult zebrafish following irradiation. As seen with dmPGE2 treatment, wnt induction during marrow recovery led to a significant increase in hematopoietic progenitors by day 10 post injury. Exposure to indomethacin blocked the wnt-mediated increase in kidney marrow precursors. Wnt activation has further been implicated in leukemogenesis. Repetitive activation of wnt signaling in adult fish induced a myeloproliferative disease. Significantly, treatment with indomethacin altered its development and progression. Finally, we investigated the effects of PGE2 modification of wnt activity on hematopoietic homeostasis in the mouse. Following myeloablative 5-FU treatment, PGE2 was capable of modifying wnt-mediated effects on hematopoietic recovery. Compared to wild-type control siblings APCMin mice exhibited elevated numbers of stem and progenitor cells after 2 weeks; treatment with indomethacin every other day prevented this regenerative advantage. These results suggest that pharmacological manipulation of wnt activity through modulation of PGE2 signaling will provide novel therapeutic approaches to regulate HSC homeostasis during bone marrow recovery and leukemogenesis.