Silencing of the XAF1 gene by promoter hypermethylation in cancer cells and reactivation to TRAIL-sensitization by IFN-beta.

BACKGROUND: XIAP-associated factor 1 (XAF1) is a putative tumor suppressor that exerts its proapoptotic effects through both caspase-dependent and -independent means. Loss of XAF1 expression through promoter methylation has been implicated in the process of tumorigenesis in a variety of cancers. In this report, we investigated the role of basal xaf1 promoter methylation in xaf1 expression and assessed the responsiveness of cancer cell lines to XAF1 induction by IFN-beta. METHODS: We used the conventional bisulfite DNA modification and sequencing method to determine the methylation status in the CpG sites of xaf1 promoter in glioblastoma (SF539, SF295), neuroblastoma (SK-N-AS) and cervical carcinoma (HeLa) cells. We analysed the status and incidence of basal xaf1 promoter methylation in xaf1 expression in non-treated cells as well as under a short or long exposure to IFN-beta. Stable XAF1 glioblastoma knock-down cell lines were established to characterize the direct implication of XAF1 in IFN-beta-mediated sensitization to TRAIL-induced cell death. RESULTS: We found a strong variability in xaf1 promoter methylation profile and responsiveness to IFN-beta across the four cancer cell lines studied. At the basal level, aberrant promoter methylation was linked to xaf1 gene silencing. After a short exposure, the IFN-beta-mediated reactivation of xaf1 gene expression was related to the degree of basal promoter methylation. However, in spite of continued promoter hypermethylation, we find that IFN-beta induced a transient xaf1 expression, that in turn, was followed by promoter demethylation upon a prolonged exposure. Importantly, we demonstrated for the first time that IFN-beta-mediated reactivation of endogenous XAF1 plays a critical role in TRAIL-induced cell death since XAF1 knock-down cell lines completely lost their IFN-beta-mediated TRAIL sensitivity. CONCLUSION: Together, these results suggest that promoter demethylation is not the sole factor determining xaf1 gene induction under IFN-beta treatment. Furthermore, our study provides evidence that XAF1 is a crucial interferon-stimulated gene (ISG) mediator of IFN-induced sensitization to TRAIL in cancer.

Degradation of survivin by the X-linked inhibitor of apoptosis (XIAP)-XAF1 complex.

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a putative tumor suppressor in which expression is significantly reduced in human cancer cell lines and primary tumors. The proapoptotic effects of XAF1 have been attributed to both caspase-dependent and -independent means. In particular, XAF1 reverses the anti-caspase activity of XIAP, a physiological inhibitor of apoptosis. We further investigated the function of XAF1 by examining its relationship with other IAPs. Immunoprecipitation studies indicate that XAF1 binds to XIAP, cIAP1, cIAP2, Livin, TsIAP, and NAIP but not Survivin, an IAP that prevents mitotic catastrophe and in which antiapoptotic activity is exerted through direct XIAP interaction and stabilization. We found that overexpressed XAF1 down-regulates the protein expression of Survivin. Under these conditions, Survivin expression was restored in the presence of the proteasome inhibitor MG132 or a XIAP RING mutant that is defective in ubiquitin-protein isopeptide ligase (E3) activity, suggesting that XAF1 interaction activates E3 activity of XIAP and targets Survivin by direct ubiquitination. In addition, RNA interference targeting endogenous XIAP protected Survivin degradation by XAF1. Furthermore, interferon-beta-mediated XAF1 induction promoted formation of an endogenous XIAP-XAF1-Survivin complex. This complex facilitated Survivin degradation, which was prevented in XAF1(-/-) stable clones. Altogether, our study demonstrates that XAF1 mediates Survivin down-regulation through a complex containing XIAP, supporting dual roles for XAF1 in apoptosis and mitotic catastrophe.