The E2F4/p130 repressor complex cooperates with oncogenic ΔNp73α to promote cell survival in human papillomavirus 38 E6/E7-transformed keratinocytes and in cancer cells
Tumor suppressor p53 and its related proteins, p63 and p73, can be synthesized as multiple isoforms lacking part of the N- or C-terminal regions. Specifically, high expression of the {Delta}Np73 isoform is notoriously associated with various human malignancies characterized by poor prognosis. This isoform is also accumulated by oncogenic viruses such as Epstein-Barr virus (EBV), as well as genus beta human papillomaviruses (HPV) that appear to be involved in carcinogenesis. To gain additional insight into {Delta}Np73 mechanisms, we have performed proteomics analyses using human keratinocytes transformed by the E6 and E7 proteins of the beta-HPV type 38 virus as an experimental model (38HK). We find that {Delta}Np73 associates with the E2F4/p130 repressor complex through a direct interaction with E2F4. This interaction is favored by the N-terminal truncation of p73 characteristic of {Delta}Np73 isoforms. Moreover, it is independent of the C-terminal splicing status, suggesting that it could represent a general feature of {Delta}Np73 isoforms (, {beta}, {gamma}, {delta}, {varepsilon}, {zeta}, {theta}, {eta}, and {eta}1). We also show that the {Delta}Np73- E2F4/p130 complex inhibits the expression of specific genes, including genes encoding for negative regulators of proliferation, both in 38HK and in HPV-negative cancer-derived cell lines. Consistently, silencing of E2F4 in 38HK and in cancer cells results in induction of senescence. In conclusion, we have identified and characterized a novel transcriptional regulatory complex that exerts pro-survival functions in transformed cells. IMPORTANCEThe TP53 gene is mutated in about 50% of human cancers. In contrast, the TP63 and TP73 genes are rarely mutated but rather expressed as {Delta}Np63 and {Delta}Np73 isoforms in a wide range of malignancies, where they act as p53 antagonists. Accumulation of {Delta}Np63 and {Delta}Np73, which is associated with chemoresistance, can result from infection by oncogenic viruses such as EBV or HPV. Our study focuses on the highly carcinogenic {Delta}Np73 isoform and uses a viral model of cellular transformation. We unveil a physical interaction between {Delta}Np73 and the E2F4/p130 complex involved in cell cycle control, which rewires the E2F4/p130 transcriptional program. Consistently, we find that E2F4 gains pro-survival functions in transformed cells expressing {Delta}Np73. This report shows, for the first time, that {Delta}Np73 isoforms acquire novel protein-protein interactions with respect to the TAp73 tumor suppressor. This situation is analogous to the gain-of-function interactions of p53 mutants supporting cellular proliferation.