Pleiotrophin Improves Survival Following Radiation-Induced Myelosuppression and Mediates HSC Expansion Via Induction Of Ras Signaling

Discovery of the mechanisms through which the bone marrow microenvironment stimulates hematopoietic regeneration following myelosuppression could lead to therapies to accelerate hematopoietic reconstitution in patients receiving chemotherapy, total body irradiation and stem cell transplantation. We have previously shown that treatment with pleiotrophin (PTN), a heparin-binding growth factor which is secreted by BM endothelial cells (ECs), causes a 10-fold expansion of murine long term-HSCs in culture (Himburg et al. Nat Med 2010). More recently, we demonstrated that PTN-deficient mice have a >10-fold deficit in LT-HSCs and hematopoietic regenerative capacity compared to PTN+/+ mice, suggesting an important role for PTN in maintaining the HSC pool in vivo (Himburg et al. Cell Reports 2012). In keeping with this, 100% of PTN-deficient mice died prior to day +30 following 700 cGy total body irradiation (TBI) compared to 30% mortality in irradiated, PTN+/+ mice (P<0.0001). In order to determine the therapeutic potential of PTN as a systemically deliverable agent to accelerate hematopoietic recovery following myelosuppression, we irradiated C57Bl6 mice with 700 cGy TBI followed by every other day dosing with PTN or saline through day +14. At day +30, 80% of the PTN-treated mice were alive compared to 30% survival in the irradiated, saline-treated group (P=0.03). Interestingly, systemic administration of PTN was equally effective at promoting the survival of irradiated mice when administered as late as 96 hrs post-TBI, suggesting that PTN promotes HSC regeneration after injury. Mechanistically, PTN treatment significantly decreased HSC apoptosis following ionizing radiation exposure (P=0.007), which may explain, at least in part, the mitigative effects of PTN treatment on survival from radiation injury. Furthermore, we have discovered that PTN treatment strongly induces elements of the Ras/MEK/ERK signaling cascade in HSCs, including phosphorylation of Grb2 and ERK1/2, while also increasing levels of the ERK1/2 target genes, phospho-Erf1 and Fra-1. Consistent with this finding, PTN-deficient mice have significantly decreased levels of phospho-ERK1/2 in BM HSCs, suggesting that deficient Ras/MEK/ERK signaling may explain, at least in part, the HSC deficit observed in PTN-deficient mice. Importantly, pharmacologic inhibition of Ras or MEK1/2 proteins or genetic inhibition of KRas in BM KSL cells significantly abrogated PTN-mediated expansion of BM KSL cells and CFU-GEMMs in culture (P<0.01 for total KSL cells and CFU-GEMMs). Taken together, these results suggest that PTN-mediated expansion of HSCs may be dependent upon activation of the Ras/MEK/ERK pathway and provide the basis for further studies to delineate the role of this pathway in mediating PTN effects on the HSC pool in vitro and in vivo.