Role of IFI 16 in cellular senescence of human fibroblasts

Abstract

Defects in interferon (IFN) signaling that result in loss of expression of IFN-inducible proteins are associated with cellular immortalization, an important early event in the development of human cancer. Here we report that loss of IFN-inducible IFI 16 expression in human fibroblasts allows bypass of cellular senescence. We found that levels of IFI 16 mRNA and protein were higher in human old versus young fibroblasts and immortalization of fibroblasts with telomerase resulted in decreased expression of IFI 16. Moreover, overexpression of IFI 16 in immortalized fibroblasts strongly inhibited cell proliferation. Interestingly, knockdown of IFI 16 expression in fibroblasts inhibited p53-mediated transcription, downregulated p21 WAF1 expression, and extended the proliferation potential. Importantly, treatment of immortal cell lines with 5-aza-2 0 -deoxycytidine, an inhibitor of DNA methyltransferase, resulted in upregulation of IFI 16. Our observations support the idea that increased levels of IFI 16 in older populations of human fibroblasts contribute to cellular senescence.

IFI16 in Human Prostate Cancer

“…As shown in Fig. 5D, consistent with our previous observations (18), nucleofection with small interfering RNA (siRNA) IFI16, but not siRNA control, resulted in >50% knockdown of IFI16 expression in TSA-treated LNCaP cells. Importantly, this reduced expression of IFI16 protein was associated with f50% increases in the AR protein levels and a decrease in the extent of the cleavage of PARP protein (Fig.…”

“…1A). Consistent with our previous observations (18), TSA-mediated increases in IFI16 protein levels in LNCaP cells were correlated well with increases in levels of IFI16 mRNA (Fig. 1B).…”

“…Furthermore, we have noted that increased expression of IFI16 protein in old (versus young) populations of normal human diploid fibroblasts (21) and in normal human prostate epithelial cells (5) is associated with cellular senescence-associated cell growth arrest, and immortalization of normal human cells is associated with down-regulation of IFI16 mRNA and protein (18). Moreover, consistent with a role for IFI16 protein as a scaffold protein, IFI16 protein binds to p53 (34), pRb and E2F1 (5), and BRCA1 (35) proteins.…”

Role of 2-5A-dependent RNase-L in senescence and longevity

“…The current challenge is to identify the molecular mediators of senescence, and determine how they are regulated in diverse oncogenic settings. IFN and ISGs are implicated in the senescent response (Kulaeva et al, 2003;Xin et al, 2004), and RNase-L is an established mediator of the antiproliferative activity of IFN (Hassel et al, 1993;Zhou et al, 1997), and functions as a tumor suppressor (Carpten et al, 2002;Casey et al, 2002); therefore, we examined the potential role for RNase-L in senescence.…”

“…These findings suggested a role for ISGs in cellular senescence, however the functional involvement in senescence has been demonstrated for only a few specific ISGs (e.g. IFI16 (Xin et al, 2004)). In light of the established antiproliferative activities of RNase-L and its role as a tumor suppressor, we investigated the possible involvement of RNase-L in cellular senescence.…”