Prostaglandin E2 Signaling through EP4 Receptor Promotes Hematopoietic Stem Cell Niche Regeneration and Enhances Hematopoietic Recovery

High-dose radiation treatment for hematological malignancies such as leukemia, lymphoma or multiple myeloma induces damage to the bone marrow microenvironment that limits hematopoietic regeneration. We previously demonstrated that single administration of dimethyl prostaglandin E2 (dmPGE2) at 24 hrs post lethal irradiation (796 cGys; LD70/30) in mice increases survival and accelerates hematopoietic recovery. Since bone marrow niches regulate hematopoiesis, we investigated whether the effects of PGE2 administration on hematopoietic recovery and survival were dependent on PGE2 signaling in bone marrow stromal cells. Total body irradiation (TBI) severely disrupts bone marrow niche components including MSC, osteoblasts and endothelial cells. Multivariate flow cytometry revealed that exposure  of mice to 650 cGy TBI reduced the total number of bone marrow MSC defined as CD45^-Ter119^-CD31^-PDGFR⁺CD51⁺MSC by 8 fold and reduced functional fibroblast colony formation (CFU-F) by 8.5 fold. Osteoblast (OB) and endothelial cell (EC) counts were reduced by 6.5 fold and 6 fold respectively. Treatment of mice with dmPGE2 at 24 hours post-irradiation substantially rescued MSC and EC, which were 3.5 and 2.2-fold higher compared to un-treated irradiated mice.  Histological and flow cytometric analysis indicated that total OB, OB precursors and mesenchymal progenitor cells (MPC)  in bone marrow were also enhanced by PGE2 administration. Interestingly, dmPGE2 failed to rescue MSC and OB from aged (24-27 mo/old) mice compared to young (4-6 mo/old) mice. Since PGE2 signals through four receptors (EP1-4), each with unique signaling pathways, we hypothesized that the hematopoietic niche regeneration was due to activation of PGE2 signaling via one or more EP receptors. Treatment of mice with the EP4 receptor agonist L-902,688 following irradiation enhanced the recovery of bone marrow MSC by 3.1 fold and EC by 2.0 fold compared to untreated irradiated control mice. EP1, EP2 and EP3 receptor agonists failed to enhance hematopoietic niche regeneration in irradiated mice. To further investigate the role of EP4 signaling in bone marrow niche reconstruction and hematopoietic recovery, we transplanted BM cells from wild-type mice into syngeneic wild-type or conditional inducible  EP4^-/- recipients after lethal irradiation, and analyzed stromal cells recovery and hematopoietic reconstitution. At 15 days post-transplantation, chimeric mice with EP4^-/- stroma displayed attenuated niche recovery and hematopoietic reconstitution compared to mice with wild-type stroma. PGE2 or EP4 agonist administration increased the expression of the endogenous anti-apoptotic protein Survivin, and enhanced survival of MSC and EC. In conclusion, our study suggests that PGE2 signaling through the EP4 receptor increases hemotopoietic recovery after TBI by enhancing the survival and expansion of MSC and endothelial cells. Furthermore, our data suggest the modulation/regulation PGE2 signaling in hematopoietic niche components could be beneficial to enhance HSC recovery following clinical hematopoietic transplantation.