Mechanisms of survival and maintenance of Hematopoietic Stem Cells and Multipotent Progenitor Cells

Sammanfattning: Hematopoietic stem cells and progenitor cells are maintained in the bone marrow microenvironment, where factors including soluble and membrane-bound cytokines influence the processes of self-renewal, proliferation and differentiation. Two nonreduntant cytokines with effects on hematopoiesis, Kit Ligand (KL) and Flt3 Ligand (FL), signal via related tyrosine kinase receptors, c-kit and Flt3. In this thesis, we have studied which signaling pathways that are activated by these cytokines and are important for survival. We have shown that the serine-threonine kinase PKB (also known as Akt) is activated by both cytokines, but the upstream target PI-3 kinase is only necessary for the survival induced by KL and not FL. Also, KL could not prevent apoptosis in cells overexpressing a dominant negative form of PKB. The forkhead transcription factor FoxO3, which is phosphorylated and inactivated by PKB, was further shown to be of central significance for KL-mediated survival. The survival signals mediated by FL was shown to include antiapoptotic Bcl-2 family proteins. When overexpressed in progenitor cell lines, Bcl-2 was more effective in blocking apoptosis than PKB. Although both Bcl-2 and PKB could prevent a decrease in the mitochondrial membrane potential, Bcl-2 was superior with this respect. Some enhancement in survival was also seen when both proteins were overexpressed simultaneously. Finally, we studied the survival of bone marrow-derived multipotent hematopoietic progenitors in the presence of low levels of oxygen, hypoxia. We showed that survival was enhanced by hypoxia and improved the expansion of primitive colony forming cells.