Inhibition of MEK/ERK signalling pathway promotes erythroid differentiation and reduces HSCs engraftment in ex vivo expanded haematopoietic stem cells.

The MEK/ERK pathway is found to be important in regulating different biological processes such as proliferation, differentiation and survival in a wide variety of cells. However, its role in self-renewal of haematopoietic stem cells is controversial and remains to be clarified. The aim of this study was to understand the role of MEK/ERK pathway in ex vivo expansion of mononuclear cells (MNCs) and purified CD34+ cells, both derived from human umbilical cord blood (hUCB). Based on our results, culturing the cells in the presence of an inhibitor of MEK/ERK pathway-PD0325901 (PD)-significantly reduces the expansion of CD34+ and CD34+  CD38- cells, while there is no change in the expression of stemness-related genes (HOXB4, BMI1). Moreover, in vivo analysis demonstrates that PD reduces engraftment capacity of ex vivo expanded CD34+ cells. Notably, when ERK pathway is blocked in UCB-MNCs, spontaneous erythroid differentiation is promoted, found in concomitant with increasing number of burst-forming unit-erythroid colony (BFU-E) as well as enhancement of erythroid glycophorin-A marker. These results are in total conformity with up-regulation of some erythroid enhancer genes (TAL1, GATA2, LMO2) and down-regulation of some erythroid repressor genes (JUN, PU1) as well. Taken together, our results support the idea that MEK/ERK pathway has a critical role in achieving the correct balance between self-renewal and differentiation of UCB cells. Also, we suggest that inhibition of ERK signalling could likely be a new key for erythroid induction of UCB-haematopoietic progenitor cells.