Background: The mammalian target of rapamycin (mTOR) is an important regulator of hematopoietic stem cell (HSC) self-renewal and its overactivation contributes to HSC premature exhaustion in part via induction of HSC senescence. Inhibition of mTOR with rapamycin has the potential to promote long-term hematopoiesis of ex vivo expanded HSCs to facilitate the clinical application of HSC transplantation for various hematologic diseases.
Methods: A well-established ex vivo expansion system for mouse bone marrow HSCs was used to investigate whether inhibition of overactivated mTOR with rapamycin can promote long-term hematopoiesis of ex vivo expanded HSCs and to elucidate the mechanisms of action of rapamycin.
Results: HSC-enriched mouse bone marrow LSK cells exhibited a time-dependent activation of mTOR after ex vivo expansion in a serum-free medium supplemented with stem cell factor, thrombopoietin, and Flt3 ligand. The overactivation of mTOR was associated with induction of senescence but not apoptosis in LSK cells and a significant reduction in the ability of HSCs to produce long-term hematopoietic reconstitution. Inhibition of overactivated mTOR with rapamycin promoted ex vivo expansion and long-term hematopoietic reconstitution of HSCs. The increase in long-term hematopoiesis of expanded HSCs is likely attributable in part to rapamycin-mediated up-regulation of Bmi1 and down-regulation of p16, which prevent HSCs from undergoing senescence during ex vivo expansion.
Conclusions: These findings suggest that mTOR plays an important role in the regulation of HSC self-renewal in vitro and inhibition of mTOR hyperactivation with rapamycin may represent a novel approach to promote ex vivo expansion and their long-term hematopoietic reconstitution of HSCs.