TP63 P2 promoter functional analysis identifies ß-catenin as a key regulator of ΔNp63 expression.

The ΔNp63 protein, a product of the TP63 gene that lacks the N-terminal domain, has a critical role in the maintenance of self renewal and progenitor capacity in several types of epithelial tissues. ΔNp63 is frequently overexpressed in squamous cell carcinoma (SCC) and in some other epithelial tumours. This overexpression may contribute to tumour progression through dominant-negative effects on the transcriptionally active (TA) isoforms of the p53 family (TAp63, TAp73 and p53), as well as through independent mechanisms. However, the molecular basis of ΔNp63 overexpression is not fully understood. Here, we show that the expression of ΔNp63 is regulated by the Wnt/ß-catenin pathway in human hepatocellular carcinoma (HCC) and SCC cell lines. This regulation operates in particular through TCF/LEF sites present in the P2 promoter of TP63. In addition, we show that ΔNp63 and ß-catenin are frequently coexpressed and accumulated in oesophageal SCC, but not in HCC. These results suggest that activation of the ß-catenin pathway may contribute to overexpression of ΔNp63 during tumour progression, in a cell type-specific manner.

Properties of the six isoforms of p63: p53-like regulation in response to genotoxic stress and cross talk with DeltaNp73.

TP63, a member of the TP53 gene family, encodes two groups of three isoforms (alpha, beta and gamma). The TAp63 isoforms act as transcription factors. The DeltaNp63 isoforms lack the main transcription activation domain and act as dominant-negative inhibitors of transactivation (TA) isoforms. To clarify the role of these isoforms and to better understand their functional overlap with p53, we ectopically expressed each p63 isoform in the p53-null hepatocellular carcinoma cell line Hep3B. All TA isoforms, as well as DeltaNp63alpha, had a half-life of <1 h when transiently expressed and were degraded by the proteasome pathway. The most stable form was DeltaNp63gamma, with a half-life of >8 h. As expected, TA isoforms differed in their transcriptional activities toward genes regulated by p53, TAp63gamma being the most active form. In contrast, DeltaNp63 isoforms were transcriptionally inactive on genes studied and inhibited TA isoforms in a dose-dependent manner. When stably expressed in polyclonal cell populations, TAp63beta and gamma isoforms were undetectable. However, when treated with doxorubicin (DOX), p63 proteins rapidly accumulated in the cells. This stabilization was associated with an increase in phosphorylation. Strikingly, in DOX-treated polyclonal populations, increase in TAp63 levels was accompanied by overexpression of DeltaNp73. This observation suggests complex regulatory cross talks between the different isoforms of the p53 family. In conclusion, p63 exhibits several transcriptional and stress-response properties similar to those of p53, suggesting that p63 activities should be taken into consideration in approaches to improve cancer therapies based on genotoxic agents.