Cellular response to stress: role of autophagy and cell death in caspase-9 deficient cells
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Chronic or unresolved stress can lead to cell death, and induction of apoptosis is crucial when the cellular adaptive mechanisms are not able to resolve persistent stress. Malfunction of the apoptotic process caused by blockade of the mitochondrial apoptotic pathway allows cells to resist stress-inducing agents and to therefore evade cell death. However, if the mitochondrial apoptotic pathway is impaired, certain stress stimuli can activate an alternative cell death pathway. Indeed, in cells in which some of the components of the mitochondrial death pathway, such as caspase-9, are deficient, a delayed mode of cell death associated with proteins involved in autophagy (ATG5) and apoptosis (caspase-8) has been observed upon prolonged stress. This alternate cell death pathway leads to the activation of caspase-8 in an autophagy-dependent mechanism. Caspase-8 acts as an initiator caspase in this model, activating executioner caspases that carry out cell death. Autophagy normally plays a pro-survival role and is involved in maintaining cellular homeostasis. However, in response to various stresses such as genotoxic and ER stress the level of autophagy is greatly increased and autophagy may contribute to cellular demise. Furthermore, while the expression of many autophagy-related genes (ATGs) is regulated downstream of eIF2α-ATF4, the precise underlying mechanisms are not yet fully explained. The aim of this study was to investigate stress-induced cell death under conditions of caspase-9 deficiency. In this thesis, I demonstrate that various stress stimuli induce cell death and caspase-8 activation. The same stress-inducing agents also activate autophagy and ATF4 through phosphorylation of eIF2α. Moreover, I elucidate a common, alternative cell death pathway activated in response to distinct forms of stress. Finally, I conclude that the integrated stress response (ISR) pathway is not involved in the regulation of autophagy-dependent stress-induced cell death. The work presented in this thesis provides a first comprehensive overview of an alternative cell death pathway and contributes to a better understanding of the mechanisms of cell death particularly in cells that are refractory to apoptosis. The thesis describes cellular responses to various stresses and highlights the importance of the pro-death role of autophagy in this novel death pathway.