∆Np63α inhibits Rac1 activation and cancer cell invasion through suppression of PREX1.

ΔNp63α, a member of the p53 family of transcription factors, plays a critical role in maintaining the proliferative potential of stem cells in the stratified epithelium. Although ΔNp63α is considered an oncogene and is frequently overexpressed in squamous cell carcinoma, loss of ΔNp63α expression is associated with increased tumor cell invasion and metastasis. We recently identified a ΔNp63α/miR-320a/PKCγ signaling axis that regulates cancer cell invasion by inhibiting phosphorylation of the small GTPase Rac1, a master switch of cell motility that positively regulates cell invasion in multiple human cancers. In this study, we identified a novel mechanism by which ΔNp63α negatively regulates Rac1 activity, by inhibiting the expression of the Rac-specific Guanine Exchange Factor PREX1. ΔNp63α knockdown in multiple squamous cell carcinoma cell lines leads to increased Rac1 activation, which is abrogated by treatment with the Rac1 inhibitor NSC23766. Furthermore, ΔNp63α negatively regulates PREX1 transcript and protein levels. Using a Rac-GEF activation assay, we also showed that ΔNp63α reduces the levels of active PREX1. The inhibition of the PREX1-Rac1 signaling axis by ΔNp63α leads to impaired cell invasion, thus establishing the functional relevance of this link. Our results elucidated a novel molecular mechanism by which ΔNp63α negatively affects cancer cell invasion and identifies the ΔNp63α/Rac1 axis as a potential target for metastasis.

TIP60 up-regulates ΔNp63α to promote cellular proliferation.

An estimated 5.4 million cases of nonmelanoma skin cancer are reported in the United States at an associated cost of $4.8 billion. ΔNp63α, a proto-oncogene in the p53 family of transcription factors, is overexpressed in squamous cell carcinoma (SCC) and associated with poor prognosis and survival. ΔNp63α elicits its tumorigenic effects in part by promoting cellular proliferation and cell survival. Despite its importance in SCC, the upstream regulation of ΔNp63α is poorly understood. In this study, we identify TIP60 as a novel upstream regulator of ΔNp63α. Using a combination of overexpression, silencing, stable expression, and pharmacological approaches in multiple cell lines, we showed that TIP60 up-regulates ΔNp63α expression. Utilizing cycloheximide treatment, we showed that TIP60 catalytic activity is required for stabilization of ΔNp63α protein levels. We further showed that TIP60 coexpression inhibits ΔNp63α ubiquitination and proteasomal degradation. Stabilization of ΔNp63α protein was further associated with TIP60-mediated acetylation. Finally, we demonstrated that TIP60-mediated regulation of ΔNp63α increases cellular proliferation by promoting G2/M progression through MTS assays and flow cytometry. Taken together, our findings provide evidence that TIP60 may contribute to SCC progression by increasing ΔNp63α protein levels, thereby promoting cellular proliferation.