Abstract
Cancer is the result of a disturbed balance between cell division and growth on one hand, and programmed cell death on the other. Crucial cellular processes such as proliferation, apoptosis and differentiation are regulated by a complex network of signal transduction pathways. Therefore, cancer can be viewed as a disease
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of deregulated signal transduction: cancer cells grow when they should not and do not die (i.e. undergo apoptosis) when they should. Elucidation of disturbed signal transduction pathways involved in the carcinogenic process, both at the protein and the genomic level, is therefore critical for the identification of new targets for cancer treatment. The phosphoinositol-3 kinase (PI3K)-protein kinase B (PKB) signalling pathway is crucial to many aspects of cell growth and survival. Deregulation of PI3K and PKB occurs by a variety of mechanisms in a large percentage of human tumours. Furthermore, aberrant PKB signalling not only plays a role in cancer onset and progression but it has also been suggested to play a role in other pathological conditions such as obesity, diabetes and premature aging. Together, these facts have made the elucidation of the PI3K-PKB pathway the subject of many studies. In recent years, targeted deletion of specific isoforms of PKB in mice has proved to be a powerful tool for elucidating the physiological roles of PKB proteins, as well as the redundancy and specific functions of the different isoforms. These mice knockout studies support a role of PKB in metabolism, glycolysis, insulin signalling, growth, vascularisation and lipid metabolism. Some of the PKB substrates that mediate these processes have been identified. For example, it has been shown that PKB directly phosphorylates and inhibits members of the FOXO subfamily of forkhead transcription factors. Interestingly, broad somatic deletion of FOXO transcription factors results in a progressive cancer-prone condition, indicating that these proteins function as tumour suppressors. Regulation of the activity of FOXO transcription factors is thus a critical means through which PKB modulates cellular homeostasis. The continued elucidation of the role of the PI3K-PKB pathway in cell growth, survival, and proliferation has shed light on why regulation of this pathway is often altered within tumours. The prevalence of a hyperactivated PI3K-PKB pathway in human cancers suggests that cancer cells might be more sensitive to inhibitors of this pathway than normal cells. The continuing efforts to develop specific, high-affinity inhibitors against the PI3K-PKB pathway have the potential to yield new therapeutics to treat human cancer. The identification and characterisation of PI3K/PKB-mediated signalling events opens opportunities for development of previously unrecognised targets for anti-cancer therapeutics. Therefore in this thesis we have focussed on the identification and characterisation of novel signalling components downstream of PKB. We have specifically focussed on the role of PKB in cell survival and have performed phosphoproteomic and microarray analysis to identify novel downstream phosphorylation and transcriptional targets. This thesis is part of ongoing research into clarifying the specific role PKB plays in oncogenic transformation.
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