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Abstract

Persistent infections with high-risk human papillomavirus (HPV) types, such as HPV16 and HPV18, are the major risk factor for the development of cervical cancer. The global incidence of cervical cancers is expected to rise in the next decades. Therefore, a better understanding of critical parameters determining the malignant phenotype of HPV-positive cancer cells, as well as the development and improvement of therapeutic strategies for cervical cancer treatment, is highly warranted. The extent of hypoxia in solid cancers can critically affect tumor aggressiveness and their therapy response. Besides chronic hypoxia, cycling hypoxia (cycH) is a prevalent but much less investigated form of tumor hypoxia, in which cancer cells are exposed to recurrent phases of hypoxia and reoxygenation. Importantly, cycH may lead to a particularly aggressive and therapy-resistant cellular phenotype. The effects of cycH in cervical cancer cells were thus far unknown. The results of this thesis reveal distinct effects of cycH on the phenotype of HPV-positive cancer cells. Unlike under chronic hypoxia, cervical cancer cells under cycH maintain HPV E6/E7 oncogene expression and function. In addition, they exhibit an increased resistance to radiotherapy, as well as to pro-senescent and pro-apoptotic chemotherapy. Interestingly, the resistance of cervical cancer cells to the pro-apoptotic effects of Cisplatin, the key chemotherapeutic drug for cervical cancer treatment, is particularly pronounced in cells treated under cycH, exceeding even their increased resistance under chronic hypoxia. Proteome analyses reveal that cervical cancer cells exposed to cycH exhibit a unique proteome signature, which includes a significant downregulation of luminal lysosomal proteins, such as the potentially pro-apoptotic Cathepsins B and L. Additionally, a reduction in autophagic flux under cycH is detectable. Comprehensive analyses of the differential susceptibility of cervical cancer cells to Cisplatin treatment under different oxygen conditions were performed. Whereas they did not provide evidence that this phenomenon is linked to the observed lysosomal alterations, they revealed that the Caspase 8-linked activation of the pro-apoptotic BH3 interacting domain death agonist (BID) protein plays a crucial role. Under cycH, Cisplatin-treated cells generate much less truncated BID protein, the active form of BID, resulting in impaired mitochondrial outer membrane permeabilization and pro-apoptotic Cytochrome c release. In summary, these results show that cycH profoundly influences the phenotype of HPV-positive cancer cells, including the virus/host cell crosstalk, the proteomic signature, and key cellular processes such as the composition of lysosomal contents or autophagy regulation. Furthermore, cycH critically alters the therapeutic susceptibility in that cervical cancer cells exposed to cycH are especially resistant to Cisplatin and thus could represent a particularly challenging cellular subpopulation for cervical cancer chemotherapy.