ΔNp73 Enhances Promoter Activity of TGF-β Induced Genes

The p53 homolog p73 is frequently overexpressed in cancers. Especially the transactivation domain truncated isoform ΔNp73 has oncogenic properties and its upregulation is associated with poor patient survival. It has been shown that ΔNp73 has an inhibitory effect on the transactivation capacity of p53 and other p73 isoforms. Here, we confirm this finding but surprisingly find that ΔNp73 may also stimulate the expression of TGF-β signaling targets. Promoter-reporter analysis indicated that the presence of Smad Binding Elements (SBE) in the promoter is sufficient for stimulation of gene expression by ΔNp73. TGF-β signaling was less efficient in ΔNp73 downregulated cells, whereas tetracycline induced ΔNp73 increased expression of endogenous TGF-β regulated genes PAI-1 and Col1a1. Pull-down assays with SBE DNA suggest that ΔNp73 enhances smad3/4 binding to SBEs, thereby stimulating TGF-β signaling. Chromatin immunoprecipitation assays confirmed a direct interaction between ΔNp73 and SBE. Given the role of TGF-β signaling in carcinogenesis, tumor invasion and metastasis via targets like PAI-1 and Col1a1, our data suggest a model on how this effect of ΔNp73 could be a contributing factor in cancer progression.

p53同源蛋白p73（p53 homolog p73）在多种癌症中常发生过表达。其中，截短反式激活结构域的异构体ΔNp73（ΔNp73）具有致癌特性，其表达上调与患者不良生存预后显著相关。已有研究证实，ΔNp73可抑制p53及其他p73异构体的反式激活能力。本研究验证了上述结论，但意外发现ΔNp73还可促进转化生长因子β（TGF-β）信号通路靶基因的表达。启动子-报告基因分析显示，启动子区域存在Smad结合元件（Smad Binding Elements, SBE）即可介导ΔNp73对基因表达的激活作用。在ΔNp73表达下调的细胞中，TGF-β信号通路的激活效率显著降低；而经四环素诱导ΔNp73表达后，细胞内源性TGF-β调控基因PAI-1与Col1a1的表达水平明显升高。针对SBE DNA的下拉实验结果表明，ΔNp73可增强Smad3/4与SBE的结合能力，进而激活TGF-β信号通路。染色质免疫沉淀实验（chromatin immunoprecipitation assays）证实，ΔNp73与SBE之间存在直接相互作用。鉴于TGF-β信号通路可通过调控PAI-1、Col1a1等靶基因参与癌变、肿瘤侵袭与转移过程，本研究结果提出了ΔNp73通过该机制促进癌症进展的潜在模型。