Inhibition of p38 MAPK Promotes Hematopoietic Stem Cells Self-Renewal and Ex Vivo Expansion.

Abstract 34

Activation of the p38 mitogen-activated protein kinase (p38 MAPK) is implicated in the inhibitory effects of TNF-α, TGF-β, interferons and reactive oxygen species (ROS) on hematopoiesis and self-renewal of hematopoietic stem cells (HSCs). Clinically, overactivated p38 MAPK contributes to the pathogenesis of myelodysplastic syndromes (MDS) and Fanconi anemia. Inhibition of p38 MAPK with pharmacological agents improves hematopoietic progenitors' function in MDS. However, it has yet to be determined if p38 MAPK plays a role in regulation of normal HSC self-renewal and whether inhibition of p38 MAPK can improve HSC ex vivo expansion. In the present study, we found that sorted mouse bone marrow HSCs (Lin⁻ Sca1⁺ c-kit⁺ cells or LSK⁺ cells) exhibited specific activation of p38 MAPK after seven days culture in serum-free medium supplemented with stem cell growth factors (SCF, Tpo and Flt3 ligand). The activation of p38 MAPK was associated with rapid differentiation of HSCs and induction of cellular senescence. Addition of SB203580 (SB, a specific p38 MAPK inhibitor) to the culture abrogated the activation of p38 MAPK, inhibited the induction of cellular senescence and increased the expression of several HSC self-renewing genes (such as CXCR4, HoxB4 and Gfi1). Moreover, HSCs cultured with SB resulted in a significantly greater HSC expansion than HSCs cultured without SB as assessed by flow cytometry and cobblestone area-forming cell (CAFC) assay. Finally, competitive repopulation assays revealed that HSCs expanded with SB produced a dramatic increase in donor-derived engraftments after transplantation to irradiated recipients. These findings suggest that p38 MAPK plays an important role in the regulation of HSC self-renewal and its inhibitors (e.g. SB203580) may be clinically useful in the ex vivo expansion of HSCs.