Ataxia-telangiectasia mutated gene product (ATM) is a 350 kDa Serine/Threonine kinase belonging to the family of Phosphatidylinositol-3 kinase like kinases. ATM functions as a key element in DNA Damage Response (DDR), a mechanism that maintains genomic integrity within the cells. ATM is activated after double stranded DNA damage and initiates signalling cascades that determine the process of decision-making of cell fate and involves the participation of multiple proteins. This vital protein acts first by sensing double stranded DNA breaks and second by transducing the signal and activating other downstream proteins of the repair pathway via its kinase function. This provides an important link between signals generated after DNA damage, the cell cycle pathway and apoptotic machinery. This function is crucial for mammalian cells which are constantly challenged by genotoxic agents from a variety of sources and therefore require a robust sensing and repair mechanism to maintain cell vitality. Cells lacking ATM are hypersensitive to cytotoxic insults, particularly genotoxic stress, induced through radiation or radiomimetic drugs. This thesis describes the discovery and characterisation of novel autoregulatory feedback loops of ATM kinase in human cells. Firstly, I have discovered that inhibition of ATM kinase activity causes induction of ATM protein expression followed by time dependent oscillations. This novel autoregulatory mechanism was demonstrated in cell cycle independent manner and both in the absence and presence of DNA damage. ATM promoter assay revealed that this autoregulation was governed at the transcriptional level. Furthermore, this autoregulatory induction of ATM was also accompanied by a transient upregulation of P53, pATR and E2F1 levels. Elucidation of the underlying trafficking mechanism of ATM during such autoregulation and in DDR also revealed a novel ATM sub-cellular trafficking mechanism which was dependent on its own kinase activity. This trafficking mechanism involved DNA damage induced Golgi to nuclear transport of phosphorylated ATM S-1981 to elicit DDR. This was found to be a conserved pathway required during the initiation of DDR and was demonstrated in multiple cell lines. Further studies into the sub-cellular transport machinery revealed the involvement of β-COPI coatomer protein in this mechanism of ATM trafficking, which was found to be autoregulated by ATM kinase, and required 387-388 ATM di-Lysine motif. The discovery of these functionally important autoregulatory mechanisms of ATM were further utilised to develop Luciferase reporter based biosensor of DNA damage and single cell fluorescence based ATM inhibition assay to screen for ATM inhibitors. Finally, following the discovery and characterisation of these functional spatio-temporal autoregulations of ATM, quantitative estimations of the kinetics of signalling cascades initiated by it during DDR and its overall outcome on cellular fate were determined to study ATM pathway systematically for employing a quantitative systems biology approach. These novel findings have immensely increased our understanding of ATM regulation and function. Elucidation of the mechanisms of novel autoregulations of ATM provide new dimensions through which DDR pathway could be manipulated, and as such could be utilised for achieving targeted cellular sensitivity in therapeutic intervention of cancer.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:650533 |
| Date | January 2012 |
| Creators | Khalil, Hilal Shahid |
| Contributors | Zhelev, Nikolai Z. |
| Publisher | Abertay University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10373/2179 |
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