The pro-apoptotic B-cell lymphoma-2 (Bcl-2) Homology 3-only (BH3-only) proteins regulate the intrinsic apoptotic pathway. Two of these proteins are p53 up-regulated mediator of apoptosis (Puma) and Bcl-2 interacting mediator of cell death (Bim). Cell death stimuli such as growth factor deprivation result in the downstream activation of these proteins, ultimately leading to apoptosis.
We have explored in vitro how BH3-only proteins are regulated by growth factors. We used factor dependent myeloid cells (FDMs), which absolutely require the beta common receptor cytokines Interleukin-3 (IL-3) and Granulocyte Monocyte Colony-Stimulating Factor (GM-CSF) for survival and proliferation. Removal of IL-3 and GM-CSF from FDMs resulted in the up-regulation of Puma and Bim, leading to apoptosis. In contrast, if these cytokines were restored before the cells died, BH3-only protein stability was decreased. This occurred via a posttranslational mechanism that specifically involved phosphorylation by the IkappaB kinase (IKK). From this we have found that growth factors regulate BH3-only protein stability via a previously unknown link between the intrinsic (IL-3) and extrinsic (IKK) cell death pathway.
It is established in the literature that deregulation of BH3-only proteins results in tumour formation. We are currently investigating in vivo, the correlation between myeloid leukaemia (AML) and IKK kinase activity. Published literature suggests that IKK deletion reduces tumour formation. Our preliminary data show similar results to the published literature. This raises further questions as to whether we see a similar effect in other myeloid leukaemia models and whether we can treat patients with IKK inhibitors.