The E2F4/p130 Repressor Complex Cooperates with Oncogenic ΔNp73α To Inhibit Gene Expression 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 DNp73a 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 Np73a 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 Np73a 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 DNp73 isoforms (alpha, beta, gamma, delta, epsilon, zeta, theta, eta, and eta 1). We show that the Delta Np73 alpha- 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. Such genes are not inhibited by E2F4/p130 in primary keratinocytes lacking DNp73a, indicating that the interaction with Delta Np73 alpha rewires the E2F4 transcriptional program. In conclusion, we have identified and characterized a novel transcriptional regulatory complex with potential implications in oncogenesis. IMPORTANCE The 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 Np73a isoform and uses a viral model of cellular transformation. We unveil a physical interaction between Delta Np73 alpha and the E2F4/p130 complex involved in cell cycle control, which rewires the E2F4/p130 transcriptional program. Our work shows that Delta Np73 isoforms can establish interactions with proteins that do not bind to the TAp73a tumor suppressor. This situation is analogous to the gain-of-function interactions of p53 mutants supporting cellular proliferation.