Abstract 4389: E2F7 regulates sensitivity to the cytotoxic actions of anthracyclins in squamous cell carcinoma.
Authors: Hazar Rethinam Mehlika , Saunders Nicholas , Simpson Fiona 
Published: August 2014

Squamous cell carcinoma (SCC) of the skin and head and neck region is a common and potentially fatal malignancy. Current chemotherapy treatments are generally not curative. Therefore, there is considerable need to develop new and targeted therapies to treat SCC. There is abundant data to show that E2Fs play an important role in many KC functions such as differentiation, growth and apoptosis. However, the identification of two new inhibitory forms of E2F, E2F7 and E2F8 has caused a rethink of our previous models. In this study, we examined the effects of E2F1, E2F7 and E2F8 on cytotoxic responses to chemotherapeutic agents.

For in vitro experiments, we generated murine KCs from E2F7 fl/fl or E2F8 fl/fl mice that were made deficient in E2F7 and E2F8 following infection with Cre expressing adenoviral constructs. In addition, we had KCs generated from E2F1 KO mice. Our results showed that E2F8 deficient KCs displayed unaltered responses to 3 structurally different classes of chemotherapeutics represented by doxorubicin (anthracycline), cisplatin (platinum) or etoposide (topoisomerase II inhibitor). Similarly, E2F1 and E2F7-deficient KCs retained apoptotic responses to etoposide and cisplatin. In contrast, E2F7-deficient murine KCs were selectively sensitised to the cytotoxic actions of doxorubicin whilst E2F1-deficiency protected against doxorubicin. We showed that the sensitivity to doxorubicin was E2F7 specific since the reintroduction of E2F7 in E2F7 deficient KCs had a clear protective effect against doxorubicin. These results indicate that E2F7 regulate cytotoxic responses of SCC to doxorubicin, and doxorubicin toxicity is mediated, in part, by an E2F1-dependent mechanism. We also looked at the possible mechanism to drive E2F7-mediated doxorubicin induced cell death. Our results showed activation of caspase-3 in E2F7 deficient KCs mediates the doxorubicin-induced reduction in cell viability. Since E2F7 modulates sensitivity to doxorubicin, we looked at different SCC cells in their response to cytotoxic action of doxorubicin. The results suggested that SCC cells vary in their sensitivity to doxorubicin and this correlates with E2F7/E2F1 protein expression ratio in these SCC cells. The most sensitive SCC cell lines, KJD, had a very low E2F7/E2F1 ratio whilst the most insensitive SCC cell line, SCC25, had a very high E2F7/E2F1 ratio. These data showed that E2F7/E2F1 ratio is disrupted in SCC and this contributes to the insensitivity to cytotoxic actions of doxorubicin.

Combined, these data indicate the existence of a novel, non-redundant and isoform-specific activities of E2F7 in KCs that have implications for our understanding of chemotherapeutic sensitivity in SCC. Our findings would suggest that if we sensitise SCC to the effects of anthracyclines, this would have a significant clinical value and add to therapeutic options available to treat SCC patients where there is a critical need.

Citation Format: Mehlika Hazar Rethinam, Nicholas Saunders, Fiona Simpson. E2F7 regulates sensitivity to the cytotoxic actions of anthracyclins in squamous cell carcinoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4389. doi:10.1158/1538-7445.AM2013-4389