DNA damage-induced cell death : The role of caspase-2

Activation of a family of cysteine proteases, called caspases, is an important event during apoptosis. In comparison to other caspases, less is known about regulatory functions of caspase-2. Previous studies from our group established caspase-2 as an essential apical regulator of apoptosis triggered by DNA damage. In addition, a primary role of caspase-2 has been implicated in DNA damage-induced mitotic catastrophe (MC). p53 family proteins have been suggested to play an important role in the activation of caspase-2, although the precise mechanism(s) is controversial. Despite considerable evidence indicating that caspase-2 in response to DNA damage engages a nuclear-mitochondrial pathway, assigning a distinct function to this protease has been difficult. Therefore, the main goal of this thesis was to investigate and understand the role of caspase-2 in different DNA damage-induced cell death scenarios. We address the question of potential caspase-2 regulators in DNA damage-induced cell death pathways and provide data concerning caspase-2 activation mechanisms. We show that the presence of functional p53 is needed in order to complete the apoptotic process mediated through the mitochondrial pathway. Both caspase-8 and caspase-2 act as bona fide initiator caspases and p53 is fundamental for their activation. While no direct interaction between p53 and caspase-2 was observed in the cell systems used, we clearly demonstrate that a functional connection between these two proteins is essential to initiate an apoptotic process. We further demonstrate the significance of p53 for caspase-2 activation in apoptotic cell death triggered by PRIMA-1METinduced mutant p53 reactivation. In addition, as suppression of caspase-2 expression affected the p53 protein level, possibilities of a reciprocal interaction between these proteins are discussed. Our results reveal the participation of endogenous caspase-2 with PIDD and RAIDD in the PIDDosome complex and the significance of this complex for caspase-2 activation in some cellular systems. However, our results also question its role as sole mediator of caspase-2 activation. Thus, we report that the latter is able to utilize the CD95-DISC as an activation platform. Our findings confirm a direct interaction between caspase-2 and -8 upstream of the mitochondria. Moreover, the ability of caspase-8 to cleave caspase-2 is demonstrated. Thus, the observed functional link between caspase-8 and -2 within the DISC complex represents an alternative mechanism to the PIDDosome for caspase-2 activation in response to DNA damage. Here, we also investigated the phenomenon of MC induced by DNA damage. A role for p53 as a negative regulator of MC is suggested, in a process where neither processing nor activation of caspase-2 is required. Instead caspase-2 and caspases in general, are essential for the termination of MC, suggesting that MC-related morphological changes are followed by activation of the apoptotic machinery. Apoptosis, however, is not always required for MC lethality since necrosis-like lysis of cells was also observed following MC. Thus, we propose that MC is not a specific type of cell death but rather a pre-stage preceding cell death. The latter is determined by the cellular protein profile involved in the regulation of the cell cycle, such as p53 and Chk2. As a result, the Nomenclature Committee on Cell Death (NCCD) recommends the use of terminology such as ‛cell death preceded by multinucleation or ‛cell death occurring during metaphase , when describing MC