P062. Identification of the XAF1 tumor suppressor as a transcription target of tumor necrosis factor-α – NF-κB signaling in human colon cancer cells
J.-W. Kim, C.-K. Lee, H.J. Kim, Kyung Hee University Hospital, Internal Medicine, Seoul, Korea, Republic of
X-linked inhibitor of apoptosis protein (XIAP)-associated factor 1 (XAF1) antagonizes the anti-caspase activity of XIAP. Both the caspase inhibiting and the anti-apoptotic abilities of XIAP were found to be blocked by overexpressed XAF1. XAF1 messenger RNA is present in normal tissues but undetectable in various cancers and thus poses a potential tumor suppressor gene. The aim of this study was to identify the XAF1 as a transcriptional target of tumor necrosis factor (TNF)-α–NF-κB signaling in human colon cells.
Nine human colon cancer cell lines were used. XAF1 expression was detected by reverse-transcription polymerase chain reaction (PCR) and Western blot analysis. The effect of XAF1 in TNF-α-induced apoptosis was examined using flow cytometry, viability assay, and immunoblot assay. The signaling pathway involved in activation of XAF1 by TNF-α was assessed by the effects of the NF-κB inhibitor BAY11–7082, siRNA-mediated knockdown of p65/RelA, and p65/RelA expression. Activation of XAF1 promoter was detected by luciferase assay and chromatin immunoprecipitation assay.
TNF-α induce the expression of XAF1 at the transcription level in a dose-dependent manner. XAF1 elevation by TNF-α strongly enhances cellular sensitivity to TNF-α-induced apoptosis in colon cancer cells. TNF-α activation of XAF1 transcription occurs through the NF-κB signaling pathway. XAF1 promoter contains κB-binding elements and p65/RelA binds directly to the κB site.
Transcription of the XAF1 is activated through the NF-κB signaling pathway by TNF-α in human colorectal epithelial cells. These data suggest that epigenetic inactivation of XAF1 may confer tumor cell resistance to TNF-α during colorectal tumor progression. This study warrants further studies to define whether XAF1 could be a useful target for the early diagnosis, prognosis prediction and treatment of colorectal tumors and inflammatory bowel disease.