Requirement of IFI16 for the Maximal Activation of p53 Induced by Ionizing Radiation

Abstract

IFI16 is a member of the PYRIN superfamily that has been implicated in BRCA1-mediated apoptosis and inflammation signaling pathways. Here we report that most breast cancer cell lines examined expressed decreased mRNA and protein levels of IFI16, although IFI16 is expressed in human primary normal mammary epithelial cells. Significantly, immunohistochemical analysis of tissues from 25 breast cancer patients demonstrated that carcinoma cells showed negative or weaker staining of IFI16 compared with positive nuclear staining in normal mammary duct epithelium. si-RNA-mediated reduction of IFI16 resulted in perturbation of p53 activation when treated with ionizing radiation (IR). Expression of IFI16 enhanced p53 transcriptional activity in cells exposed to IR. Adenovirus expression of IFI16 in IFI16-deficient MCF7 induced apoptosis, which was enhanced by radiomimetic neocarcinostatin treatment. Tetracycline-regulated IFI16 also induced apoptosis when coexpressed with p53 in p53-deficient EJ cells subjected to IR, suggesting that IFI16 is involved in p53-mediated transmission of apoptosis signaling. Consistent with these results, expression of IFI16 enhanced activation of the known p53 target genes, including p21, Hdm2, and bax in MCF7 cells. These results suggest that loss of IFI16 results in deregulation of p53-mediated apoptosis, leading to cancer development.

Expression of an IFN-Inducible Cellular Senescence Gene, IFI16, Is Up-Regulated by p53

“…Additionally, the IFI16 protein binds to p53 (27) and increased expression of the IFI16 protein in cells potentiates p53-mediated transcription of p21-luc-reporter (27). These observations (25)(26)(27) provide support for the idea that increased levels of the IFI16 protein in old cells, by potentiating p53-mediated transcription of cell cycle inhibitory genes, such as p21, contribute to senescence-associated permanent cell growth arrest. Although these observations suggest a role for IFI16 protein in p53-mediated cellular senescence, signaling pathways that contribute to transcriptional activation of the IFI16 gene during cellular senescence remain to be elucidated.…”

“…Studies have provided evidence that increased levels of the IFI16 protein in cells potentiate p53-mediated transcription of target genes (25)(26)(27). Moreover, it has been shown that IFI16 protein is needed for p53-mediated apoptosis in certain cells following ionizing radiation -induced apoptosis (26).…”

“…Interestingly, increased coexpression of the IFI16 and p53 proteins in MCF-7 cells enhanced the transcription of the known p53 target genes, such as p21, Hdm2, and Bax (26). Importantly, knockdown of IFI16 expression in MCF-7 cells results in decreased phosphorylation of p53 on Ser 15 following ionizing radiation treatment and decreased p53-mediated apoptosis (26). Additionally, the IFI16 protein binds to p53 (27) and increased expression of the IFI16 protein in cells potentiates p53-mediated transcription of p21-luc-reporter (27).…”

IFI16 in Human Prostate Cancer

“…Interestingly, a study found that the expression of IFI16 protein is relatively low in human breast cancer cell lines and tumors (compared with normal breast epithelial cells and tissues). Furthermore, treatment of these breast cancer cell lines with 5-aza-dC resulted in up-regulation of IFI16 expression (16). Consequently, we considered the possibility that either our immunoblotting conditions to detect relatively low levels of IFI16 protein in prostate cancer cell lines were not optimum or the expression of IFI16 gene is silenced in prostate cancer cell lines.…”

“…Although treatment of human breast cancer cell lines with 5-aza-dC has been reported to induce the expression of IFI16 gene (16), the presence of a CpG island has not been shown in the 5 ¶-regulatory region of IFI16 gene. Consistent with a role for IFI16 protein in cellular senescence-associated cell growth arrest, we have noted (5) that the expression of IFI16 protein was either very low or undetectable in most human prostate cancer cell lines tested (including LNCaP and DU-145 cell lines).…”