The antiapoptotic function of megakaryocyte growth and development factor (Peg-rmMGDF) Is disrupted in STAT5ab−/− and JAK2+/− mice

PEG-rmMGDF (60 μg/kg) given to mice immediately after lethal myelosuppression (Carboplatin 80 mg/kg + 750R Cs-137), prevents death and promotes multilineage hematopoietic reconstitution (Pestina et al., Exp Hematol 27, Suppl 1: 100, 1999). Our data from study of p53-deficient mice suggests that this myeloprotection results in part by preventing p53-dependent apoptosis of c-Mpl-expressing multipotential hematopoietic cells. To define molecules involved in the antiapoptotic signaling pathways induced by c-Mpl ligation, we performed in vivo studies using mice with specific gene deletions. Jak-2 tyrosine kinase is rapidly activated as a result of c-Mpl ligation, but its role in antiapoptotic signaling has not been examined in vivo. Because Jak2−/− is an embryonic lethal due to failed development of hematopoiesis, we examined whether adult Jak2+/− mice were protected from lethal myelosuppression by PEG-rmMGDF. The survival rate of Jak2+/− mice (68%) was lower than that of wild-type (WT) mice (91%), indicating that Jak2 activation plays a role in the myeloprotective signaling pathway in vivo. We next investigated the importance of the Jak2 substrates Stat5a and b in this pathway using mice with combined deficiency of these Stat proteins. Intact Stat5ab−/− mice had normal steady-state platelet counts and similar elevations in platelet numbers in response to PEG-rmMGDF to those of WT mice, suggesting that neither Stat5 protein is required during megakaryocytopoieses. However, Stat5−/− mice exposed to the myelosuppressive regimen were severely compromised in their ability to be protected by PEG-rmMGDF (19% survival vs. 100% for Stat5ab+/− and 90% for WT mice). These findings provide in vivo evidence that Jak2 and Stat5 play important roles in the myeloprotective signaling pathways initiated by c-Mpl ligation.