δ-tocotrienol protects mouse and human hematopoietic progenitors from γ-irradiation through extracellular signal-regulated kinase/mammalian target of rapamycin signaling

Background Exposure to gamma-radiation causes rapid hematopoietic cell apoptosis and bone marrow suppression. However, there are no approved radiation countermeasures for the acute radiation syndrome. In this study, we demonstrated that natural delta-tocotrienol, one of the isomers of vitamin E, significantly enhanced survival in total body lethally irradiated mice. We explored the effects and mechanisms of delta-tocotrienol on hematopoietic progenitor cell survival after gamma -irradiation in both in vivo and in vitro experiments. Design and Methods CD2F1 mice and human hematopoietic progenitor CD34(+) cells were treated with delta-tocotrienol or vehicle control 24 h before or 6 h after gamma-irradiation. Effects of delta-tocotrienol on hematopoietic progenitor cell survival and regeneration were evaluated by clonogenicity studies, flow cytometry, and bone marrow histochemical staining. delta-tocotrienol and gamma-irradiation-induced signal regulatory activities were assessed by immunofluorescence staining, immunoblotting and short-interfering RNA assay. Results delta-tocotrienol displayed significant radioprotective effects. A single injection of delta-tocotrienol protected 100% of CD2F1 mice from total body irradiation-induced death as measured by 30-day post-irradiation survival. delta-tocotrienol increased cell survival, and regeneration of hematopoietic microfoci and lineage(-)/Sca-1(+)/ckit(+) stem and progenitor cells in irradiated mouse bone marrow, and protected human CD34(+) cells from radiation-induced damage. delta-tocotrienol activated extracellular signal-related kinase 1/2 phosphorylation and significantly inhibited formation of DNA-damage marker gamma-H2AX foci. In addition, delta-tocotrienol up-regulated mammalian target of rapamycin and phosphorylation of its downstream effector 4EBP-1. These alterations were associated with activation of mRNA translation regulator eIF4E and ribosomal protein S6, which is responsible for cell survival and growth. Inhibition of extracellular signal-related kinase 1/2 expression by short interfering RNA abrogated delta-tocotrienol-induced mammalian target of rapamycin phosphorylation and clonogenicity, and increased gamma-H2AX foci formation in irradiated CD34(+) cells. Conclusions Our data indicate that delta-tocotrienol protects mouse bone marrow and human CD34+ cells from radiation-induced damage through extracellular signal-related kinase activation-associated mammalian target of rapamycin survival pathways.