Expression of an IFN-inducible cellular senescence gene, IFI16, is up-regulated by p53.

IFN-inducible IFI16 protein (encoded by IFI16 gene at 1q23.1) is the human member of the IFN-inducible structurally related p200 family proteins. Increased expression of the IFI16 protein, a positive modulator of p53-mediated transcription, in normal old human diploid fibroblasts (HDF) is associated with cellular senescence-mediated cell growth arrest. However, the underlying mechanisms that contribute to transcriptional activation of the IFI16 gene in old HDFs remain to be elucidated. Here, we reported that functional activation of p53 in normal young HDFs and p53-null Saos2 cell line resulted in transcriptional activation of the IFI16 gene. We identified a potential p53 DNA-binding site (indicated as IFI16-p53-BS) in the 5'-regulatory region of the IFI16 gene. Importantly, p53 bound to IFI16-p53-BS in a sequence-specific manner in gel-mobility shift assays. Furthermore, p53 associated with the 5'-regulatory region of the IFI16 gene in chromatin immunoprecipitation assays. Interestingly, p53 associated with the regulatory region of the IFI16 gene only on treatment of cells with DNA-damaging agents or in the old, but not in the young, HDFs. Importantly, our promoter-reporter assays, which were coupled with site-directed mutagenesis of IFI16-p53-BS, showed that p53 activates transcription of the IFI16 gene in HDFs through the p53 DNA-binding site. Together, our observations provide support for the idea that up-regulation of IFI16 expression by p53 and functional interactions between IFI16 protein and p53 contribute to cellular senescence.