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Wachter, Franziska (2014): Der konträre Einfluss von p53 auf die extrinsische und intrinsische Apoptoseinduktion in Tumorzellen. Dissertation, LMU München: Faculty of Medicine



In the past view decades the therapy of malignant diseases has improved significantly. However especially metastatic diseases and relapses still require the development of new therapeutic modalities. Most chemotherapeutic agents induce apoptosis. A better understanding of apoptosis signaling is needed in order to combine cytostatic drugs in a more efficient way and to develop new targeted therapies. Conventional cytotoxic drugs and new therapies target p53, a key regulator of apoptosis. p53 is one of the most intensively studied target proteins but the heterogeneity and diversity of the function of p53 remains unclear. We investigated the importance of p53 in the intrinsic and extrinsic apoptosis signaling, via a variety of molecular biological approaches (Wachter et al., 2013). Apoptosis is induced intrinsically by the release of pro-apoptotic proteins from the inter-membrane space of the mitochondrion (eg. doxorubicin) or via activation of death receptors on the surface of the cell (eg. TRAIL, Tumor necrosis factor Related Apoptosis Inducing Ligand). As described in the literature, wildtype p53 leaded to an increase in intrinsic cell death, whereas different p53 mutations reduced apoptosis. In our work we discovered that the influence of p53 status on extrinsic cell death induction was much more multifaceted. The presence of p53 in tumor cell lines and xenograft tumor cells resulted in augmented, unaffected or decreased cell death. With great interest we discovered that the replacement of wildtype p53 by mutant p53 did not disturb the extrinsic apoptosis inducing capacity. As a next step we studied the influence of cell cycle arrest induced by p53 activation on TRAIL apoptosis sensitivity (Ehrhardt/Wachter et al., 2013). Cell cycle arrest was induced in tumor cell lines and patient samples in G0, G1 or G2 with cytostatic drugs, phase-specific inhibitors or RNA interference against cyclinB and E. Molecular or biochemical arrest at any point of the cell cycle augmented the rate of apoptosis by TRAIL. Consequently, when cell cycle arrest was deactivated by adding caffeine, the antitumor activity of TRAIL was decreased. The extrinsic apoptosis by TRAIL was increased in senescent tumor cells. This is especially important for the therapy of minimal residual disease and for the prevention of relapse caused by resting tumor cells. Additionally, we studied the impact of p53 activation on the rate of apoptosis induced by combinations of cytostatic drugs (Ehrhardt, Schrembs, Moritz, Wachter et al., 2011). A single drug is not very effective in cancer therapy therefore malignant diseases are usually treated with a combination of cytostatic agents. Co-administration of anthracyclines and vinca alkaloids on the same day is a standard of care for hematopoietic malignancies. If both drugs are applied simultaneously, doxorubicin inhibits apoptosis induction of vincristine. While it is well understood that doxorubicin activates p53 and induces cell-cycle arrest, we observed that cell-cycle arrest disabled inactivation of anti-apoptotic Bcl-2-family members by vincristine. Therefore, it follows that vincristine was unable to activate downstream apoptosis signaling. Cycling cells are required for vincristine to kill cancer cells efficiently. In summary we have shown the heterogeneity of p53 in extrinsic and intrinsic apoptosis signaling. The impact of the p53 status of tumor cells on extrinsic apoptosis signaling should be considered, especially in the context of therapeutic approaches that aim to restore p53 function to enable cell death. p53 can act pro- and anti-apoptotic, depending on the individual patient tumor cell and on drug and signal interactions. Finally, this suggests that patients benefit from an individualized therapeutic regimen. Further investigation is required in order to better understand the therapeutic potential of these complex interactions.