Among keratinocyte-derived squamous cell carcinoma (SCC), cutaneous SCC (CSCC) is the second most common cutaneous cancer type and the most common of the potentially fatal skin cancers. Approximately 90% of head and neck cancers are SCC (HNSCC) and HNSCC worldwide is the sixth most common cancer type afflicting mankind. While resectable disease may be treated by surgery with radiation or chemoradiation, there are still no curative options for advanced, unresectable disease. However, chemotherapy alone may offer a hope for unresectable, disseminated SCC, where 50-60% of patients have disease recurrence within 2 years and approximately 30% of these develop metastatic disease. The mainstay of chemotherapeutic treatment in SCC is the platinum-based drug, cisplatin, 5-fluorouracil (5FU), taxanes, and anti-EGFR monoclonal antibody such as Cetuximab. Nonetheless, despite advances in treatment techniques, the 5-year survival rate still remains at around 55%. Therefore, there remains a lack of options for recurrent or metastatic disease.
The E2F family of transcription factors has emerged as key regulators of proliferation, differentiation and response to stress and apoptotic stimuli in keratinocytes. Consistent with these roles, dysregulation of E2F expression/activity is a common occurrence in cancer and SCC in particular. Thus, better understanding of the E2F family of proteins is essential to establish how these processes are disrupted during SCC genesis. The E2F network exists as a complex map of interacting pathways, and the complete understanding of the E2F family will not be possible until physiological functions of newly identified inhibitory E2F proteins, E2F7 and E2F8, are fully revealed. I provide strong evidence, from in vitro experiments, that E2F7 plays a unique and non-redundant role in modulating UV- or chemotherapy-induced cytotoxicity whereas E2F8 has a unique and non-redundant capacity to regulate squamous differentiation. In addition, I report on a unique feedback loop between E2F1, E2F7 and E2F8 that highlights a previously unreported interdependent axis.
With respect to E2F7-specific functions, my work characterised two previously unidentified pathways as therapeutic targets, E2F7/Sphk1/S1P/AKT and E2F7/RacGAP1/AKT, by which the transcription factor E2F7 suppresses doxorubicin specific sensitivity in SCC cells. Targeting these pathways has the potential to expand the clinical activity of existing chemotherapeutics. I provide, in vitro, in vivo and patient data in this study that shows that E2F7-dependent repression of doxorubicin sensitivity is mediated via the induction of Sphingosine kinase 1 (Sphk1) and Rac GTPase activating protein 1 (RacGAP1). These are both novel findings and have significant implications for drug resistant SCC. Specifically, I showed that (i) E2F7 is overexpressed in patient SCCs and inhibits sensitivity to doxorubicin, (ii) that E2F7-dependent doxorubicin resistance is mediated via E2F-dependent induction of Sphk1 leading to overproduction of Sphingosine-1-phosphate (S1P) which in turn activates AKT, (iii) that pharmacological inhibition of Sphk1 or AKT sensitizes SCC cells to the cytotoxic actions of doxorubicin in vivo models of SCC resulting in profound tumour regression, iv) that induction RacGAP1 in SCC cells increases their sensitivity to doxorubicin and overexpression of RacGAP1 is linked to poor outcomes in SCC patients and v) that E2F7-dependent doxorubicin resistance is mediated via induction of RacGAP1 which in turn activates AKT-dependent pathways in vitro and in vivo. Since dysregulation of the E2F family of transcription factors is one of the most frequent targets in oncogenesis, it is likely that our findings will be relevant to many other cancer types. Moreover, our findings provide proof of concept for immediate translation to initiate a clinical trial using combinations of clinically available agents such as doxorubicin + AKT inhibitor or doxorubicin + an SK1 inhibitor.