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Abstract

Hypoxia is a hallmark of solid tumors, including HPV-positive cervical cancers, and it poses a significant challenge to cancer treatment, as hypoxic tumor regions are often associated with increased therapy resistance and poor patient prognosis. One possible contributing factor to this resistance is the ability of hypoxic cancer cells to evade cellular senescence – a process leading to an irreversible growth arrest. While the repression of the HPV E6/E7 oncogenes in normoxic HPV-positive cells efficiently induces senescence, hypoxic HPV-positive cells, despite efficiently shutting off E6/E7 expression, evade senescence and induce a reversible growth arrest that can be overcome by reoxygenation. The overall aim of this thesis was to uncover mechanisms that enable hypoxic HPV-positive cancer cells to evade senescence. Through comparative gene and protein expression analyses following E6/E7 repression under normoxic and hypoxic conditions, I identified distinct regulatory patterns of key cell cycle regulatory factors. Notably, B-MYB emerged as a critical factor differentially regulated under these conditions. Its strong downregulation in normoxic cells following E6/E7 knockdown was associated with the onset of senescence, while its sustained expression under hypoxia correlated with senescence evasion, suggesting a role for B-MYB in controlling this process. To further investigate the functional role of B-MYB, I performed knockdown experiments and assessed the phenotypic consequences using various cellular assays. Interestingly, B-MYB depletion led to a partial senescence induction under normoxia. I further showed that this limited senescence response was due to the compensatory induction of a B-MYB paralog, A-MYB, which could counteract the pro-senescent effects of B-MYB loss. Consequently, the combined repression of both B-MYB and A-MYB was needed for efficient senescence induction under normoxia. Strikingly, I found that this compensatory mechanism is disrupted under hypoxia, rendering hypoxic cancer cells particularly susceptible to the pro-senescent effects of B-MYB loss. This increased vulnerability of hypoxic cells could be reversed by ectopic A-MYB overexpression. Further, my studies revealed that the B-MYB/A-MYB axis also plays a key role for the senescence response of normoxic and hypoxic HPV-negative cancer cells of various histological backgrounds, indicating that this newly discovered mechanism could be broadly conserved across different tumor types. Collectively, my findings provide novel insights into the regulation of senescence in normoxic and hypoxic cancer cells and identify B-MYB and A-MYB as central factors in these processes. Further, they reveal a particular vulnerability of hypoxic cancer cells to B-MYB inhibition that could be exploited to develop new strategies to target this therapeutically challenging tumor cell population.