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Abstract

Oncogenic types of human papillomaviruses (HPVs) are major human carcinogens. The therapy of HPV-associated malignancies, such as cervical cancer, remains challenging and novel treatment options are urgently needed. This thesis aimed to gain a deeper understanding of the molecular mechanisms that sustain the proliferation of HPV-positive cancer cells and to identify new strategies to block their growth, thereby paving the way towards novel therapeutic approaches. My studies uncovered that the cellular ubiquitin ligase E6-associated protein (E6AP), an interaction partner of the HPV E6 oncoprotein, is essential for maintaining the proliferation of HPV-positive cancer cells by exerting a previously unrecognized anti-senescent function. RNA interference (RNAi)-mediated E6AP silencing, unlike E6 silencing, led to a potent induction of cellular senescence, an irreversible growth arrest. Notably, the pro-senescent effect of E6AP downregulation was specifically observed in HPV-positive cancer cells. Proliferation studies and cell cycle analyses revealed that the cellular growth inhibition and senescence induction upon E6AP repression critically depend on the activation of the p53-p21 axis as well as on the pocket proteins retinoblastoma protein (pRb) and p130. Additionally, my results showed that Metformin, which is being discussed for repurposing in cancer therapy, can also inhibit the proliferation of HPV-positive cancer cells. However, this effect is reversible and, further, was found to be independent of Metformin’s potential to downregulate the expression of the HPV E6 and E7 oncogenes. Moreover, my experiments demonstrated that Metformin exhibits complex functional interactions with chemotherapeutic agents in both HPV-positive and HPV-negative cancer cells. Whereas Metformin can protect against senescence induced by the prosenescent drug Etoposide, it can enhance the pro-apoptotic effects of Cisplatin, particularly after pre-treatment with Metformin. My mechanistic studies further revealed functional key roles for p53 and a truncated form of the BH3-interacting domain death agonist (BID) protein, t-BID, in mediating the cooperative effects between Metformin and Cisplatin. Collectively, my findings identify E6AP as a critical anti-senescent factor in HPV-positive cancer cells that is essential for their proliferation. E6AP might therefore serve as a new therapeutic target, with specificity for HPV-positive cancer cells. Moreover, my results uncover complex effects of Metformin on the phenotype of HPV-positive and HPV-negative cancer cells. Importantly, Metformin can substantially increase the anti-proliferative effects of Cisplatin, a drug that plays a key role in the treatment of HPV-positive cancers. This indicates that combinatorial treatments with Metformin and Cisplatin could potentially be of therapeutic benefit.