Cytokine signalling in haematopoietic cells

Abstract

Cytokine receptor signalling is essential for cell survival, proliferation and subsequent differentiation of haematopoietic stem cells (HSCs). Cytokines control development of haematopoietic progenitors into cells of the myeloid, lymphoid and erythroid lineages by stimulating cell cycle progression, proliferation and differentiation as well as by inhibiting apoptosis. My work focusses on Interleukin-3 (IL-3) and Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF), two essential cytokines in haematopoiesis. Binding of a cytokine to its specific receptor leads to the activation of multiple kinase signalling pathways, including the JAK/STAT, Ras-MAP kinase (MAPK) and PI3-kinase/AKT pathways. In this signalling network, the IkappaB Kinase (IKK) complex plays an important role as a downstream signalling hub. My research investigates how IL-3 mediated IKK activation promotes the survival of myeloid cells and what role this process may play in the development of related diseases, such as myeloproliferative disorders. Using immortalised growth factor (IL-3 or GM-CSF) dependent myeloid progenitor cells (FDMs), as well as employing various in vivo mouse models of haematopoietic development, I was able to show that 1) IKK is a major signalling hub linking IL-3, TNFR1 and p53 signalling to control the survival in haematopoietic cells by describing for the first time a role for the E3 ubiquitin ligase MDM2 downstream of IL-3- or TNFalpha-mediated IKK2 activation, suggesting crosstalk between the NF-kB signalling and p53 signalling pathways; 2) IKK regulates cellular metabolism through activation of NF-kB- and p53-dependent metabolic target genes by showing that deletion of IKK2 but not IKK1 in hematopoietic cells significantly alters cellular metabolism, impairing oxidative phosphorylation and upregulating glycolysis due to altered expression of p53-dependent metabolic target genes; 3) IKK plays a crucial role during haematopoietic development, regulating myeloid cell proliferation, lineage commitment and survival, showing that deletion of IKK2 but not IKK1 in haematopoietic progenitor cells severely affects haematopoietic development by skewing lineage commitment in vivo, resulting in neutrophilia, elevated circulating interleukin-6 and lethality due to severe gastrointestinal inflammation. The work presented in this thesis provides new important insights into the role of IKK in haematopoietic cells and haematopoietic development and clearly demonstrate that IKK1 and IKK2, the two catalytic subunits of the IKK complex, have distinct functions depending on the context of activation. In the future, this fact could be exploited to develop novel targeted therapies to specifically target a subunit in disease settings such as haematopoietic malignancies where aberrant NF-kappaB activity is frequently observed.