Running title: A critical role of PI 3-kinase in hematopoietic stem cells

The Steel Factor (SF) and its receptor c-Kit play a critical role for various cell types at different levels in the hematopoietic hierarchy. Whether similar or distinct signaling pathways are employed upon c-Kit activation in different cell types within the hematopoietic hierarchy is not known. To study c-Kit signaling pathways in the hematopoietic system we have compared c-Kit downstream signaling events in SFdependent HSC-like cell lines to that of mast cells. Both Erk and Protein Kinase B (PKB)/Akt are activated by ligand-induced activation of the c-Kit receptor in the HSC-like cell lines. Surprisingly, PI-3-kinase inhibitors block not only PKB/Akt activation, but also activation of Raf and Erk. SF induced activation of Ras is not affected by inhibition of PI-3-kinase. In mast cells and other more committed hematopoietic precursors the activation of Erk by SF is not PI-3-kinase dependent. Our results suggest that a molecular signaling switch occurs during differentiation in the hematopoietic system whereby immature hematopoietic progenitor/stem cells employ a PI-3-kinase sensitive pathway in the activation of both Erk and PKB/Akt, which is then switched upon differentiation to the more commonly described PI-3kinase independent MAP-kinase pathway. Word count 186 For personal use only. on October 17, 2017. by guest www.bloodjournal.org From