Regulation of Apoptosis in Hematopoietic Progenitor Cells: Involvement of Different Signaling Pathways

Abstract: Proliferation, differentiation, and survival of hematopoietic stem cells and multipotent progenitor cells are regulated by cytokines and cell-cell interactions. Kit ligand (KL) and Flt3 ligand (FL) have pleiotropic effects, promotes survival, but are nonredundant. Using myeloid progenitor cell lines and mouse bone marrow-derived Lin- progenitors, we demonstrate that inhibition of phophatidylinositol (PI) 3-kinase abolish survival mediated by KL, whereas survival via FL is only partially affected. KL and FL both activate Akt, leading to inhibitory phosphorylation of the transcription factor FoxO3. Overexpression of constitutively active FoxO3, FoxO3(A3):ER, induced apoptosis even in the presence of KL or FL, indicating that inactivation of FoxO3 is crucial for signaling via both c-Kit and Flt3. Induction of FoxO3(A3):ER also inhibited myeloid and erythroid colony formation of Lin- progenitors. In addition FL, but not KL, induced expression of Bcl-2 and Bcl-xL. By overexpressing Akt and Bcl-2 we demonstrate that Bcl-2 is the better mediator of survival than Akt. However, Akt was crucial for KL-mediated survival since overexpression of dominant negative Akt induced apoptosis. We also conclude that Akt and Bcl-2 have synergistic effects since their coexpression was a far better mediator of survival than either one acting alone. In AML, Flt3 is commonly mutated via internal tandem duplications, rendering it constitutively active. Introducing Flt3-ITD into an IL-3 dependent progenitor cell line rendered it factor-independent. Both Akt and FoxO3 were phosphorylated in the absence of FL and several Bcl-2 family members were upregulated. In fact, Flt3-ITD activated additional Bcl-2 family members not activated by normal Flt3. Finally we found that PI3-kinase and an unidentified Src kinase were important for survival via Flt3-ITD. Signaling via normal Flt3 and Flt3-ITD differs in some aspects and disruption of specific Flt3-ITD signals may be potential targets for treatment.