Sp1 binds to the external promoter of the p73 gene and induces the expression of TAp73γ in lung cancer

The p73 gene possesses an extrinsic P1 promoter and an intrinsic P2 promoter, resulting in TAp73 and Delta Nu p73 isoforms, respectively. The ultimate effect of p73 in oncogenesis is thought to depend on the apoptotic TA to antiapoptotic Delta N isoforms' ratio. This study was aimed at identifying novel transcription factors that affect TA isoform synthesis. With the use of bioinformatics tools, in vitro binding assays, and chromatin immunoprecipitation analysis, a region extending -233 to -204 bp upstream of the transcription start site of the human p73 P1 promoter, containing conserved Sp1-binding sites, was characterized. Treatment of cells with Sp1 RNAi and Sp1 inhibitor functionally suppress TAp73 expression, indicating positive regulation of P1 by the Sp1 protein. Notably Sp1 inhibition or knockdown also reduces Delta Nu p73 protein levels. Therefore, Sp1 directly regulates TAp73 transcription and affects Delta Nu p73 levels in lung cancer. TAp73 gamma was shown to be the only TA isoform overexpressed in several lung cancer cell lines and in 26 non-small cell lung cancers, consistent with Sp1 overexpression, thereby questioning the apoptotic role of this specific p73 isoform in lung cancer.