The role of AKT during IL-3-dependent survival signalling

© 2014 Dr. Benjamin GreenThe activation of AKT in response to cytokine receptor signalling promotes protein synthesis, cellular growth and proliferation. AKT is also thought to play a central role in maintaining cell viability by repressing apoptosis pathways, via direct or indirect regulation of the Bcl-2 protein family. To determine the role of AKT in interleukin-3 signalling, IL-3 dependent myeloid cell lines derived from mice lacking AKT1, AKT2 or AKT3 were generated. The viability of these cell lines indicated that each gene was redundant for both proliferation and survival pathways; however the deletion of AKT1 specifically, rendered cells less tolerant to low IL-3 concentrations. Conversely, the over-expression of constitutively active AKT1 or AKT2 in WT cells was sufficient to delay apoptosis in response to IL-3 withdrawal, however was not sufficient to induce proliferation or maintain long-term survival in the absence of IL-3. AKT1 prolonged survival of BIM- or BAD-deficient cells, but not cells lacking PUMA, indicating that AKT1-dependent repression of apoptosis was in part dependent on PUMA and independent of BIM or BAD. Furthermore, the over-expression of either AKT isoform required p53 but not FOXO3a to prolong viability after IL-3 withdrawal. Supporting data was observed whereby AKT over-expression is sufficient to phosphorylate and stabilise the p53 inhibitor MDM2. These results suggest that AKT1 promotes survival in response to IL-3 by phosphorylating and stabilising MDM2, thereby repressing p53 activity that would otherwise transcribe PUMA to promote apoptosis