Repression of Wnt/β-catenin response elements by p63 (TP63)

Area of Science:

• Molecular Biology
• Cancer Research
• Cell Signaling

Background:

• TP63 (p63) is a member of the TP53 (p53) gene family, crucial for keratinocyte stem cell function and squamous cell carcinoma differentiation.
• The role of p63 in Wnt/β-catenin signaling is controversial, with reports suggesting both activation and inhibition of target gene expression.
• The most abundant p63 isoform, ΔNp63α, has been implicated in regulating cellular processes, but its precise mechanism in Wnt signaling remains unclear.

Purpose of the Study:

• To elucidate the role of the TP63 (p63) gene, specifically the ΔNp63α isoform, in regulating Wnt/β-catenin signaling.
• To investigate the molecular mechanisms by which p63 influences the expression of Wnt target genes.
• To determine if ΔNp63α acts as an activator or repressor in the context of β-catenin-mediated transcription.

Main Methods:

• RNA silencing of p63 in squamous cell carcinoma lines to assess changes in Wnt target gene expression.
• Co-immunoprecipitation and cellular localization studies to examine interactions between ΔNp63α, PP2A, GSK-3β, and β-catenin.
• Reporter gene assays using Wnt response elements (WREs) in different cell lines (HEK293, SAOS-2) and chromatin immunoprecipitation to analyze p63 and TCF4 binding at target gene loci.

Main Results:

• p63 RNA-silencing led to varied effects on Wnt target gene expression, with some increasing and others decreasing.
• ΔNp63α did not alter GSK-3β phosphorylation or β-catenin nuclear localization but could enhance or inhibit β-catenin-dependent transcription depending on the cellular context and WREs.
• ΔNp63α directly repressed transcription from MMP7 regulatory regions and formed a complex with TCF4 at the MMP7 WRE, attenuating β-catenin recruitment.

Conclusions:

• The TP63 (p63) gene, particularly the ΔNp63α isoform, functions as a repressor of β-catenin-mediated gene expression.
• ΔNp63α interacts with TCF4 in the nucleus and at chromatin-level Wnt response elements, thereby modulating Wnt signaling output.
• These findings clarify the controversial role of p63 in Wnt signaling, establishing it as a key regulator of gene expression in processes like cellular differentiation and cancer.