All-trans retinoic acid induces XAF1 expression through an interferon regulatory factor-1 element in colon cancer
Background & Aims: X-linked inhibitor of apoptosis pro- tein (XIAP)-associated factor 1 (XAF1) is a novel tumor suppressor and interferon (IFN)-stimulated gene. All- trans retinoic acid (ATRA) exerts an antiproliferative ef- fect on tumor cells through up-regulation of IFN regula- tory factor 1 (IRF-1) and the downstream IFN-stimulated genes. The aim of this study was to determine the effect and mechanism of ATRA on XAF1 expression and the role of XAF1 in ATRA-induced growth inhibition in colon cancer. Methods: Gene expression is detected by re- verse-transcription polymerase chain reaction and im- munoblotting. The transcription activity of XAF1 pro- moter is examined by luciferase reporter assay. The activity of IFN regulatory factor binding element (IRF-E) is assessed by electrophoretic mobility shift assay and chromatin immunoprecipitation assay. Cell growth is evaluated by both in vitro and in vivo in nude mice xenografts. Results: IFN-alfa stimulates XAF1 promoter activity in the colon cancer cells Lovo and SW1116 dose-dependently. An IRF-1 binding element (IRF-E– XAF1) is found in the -30 to -38 nucleotide region up- stream of the ATG initiator codon of the XAF1 gene. Site-directed mutagenesis of IRF-E–XAF1 abrogates na- tive and IFN-induced promoter activity and binding ca- pacity. ATRA induces XAF1 expression both in vitro and in vivo through interaction with IRF-E–XAF1. Overexpres- sion of XAF1 increases cell susceptibility to ATRA-in- duced growth suppression both in vitro and in vivo. Furthermore, the effect of ATRA on XAF1 expression is independent of the promoter methylation and the sub- cellular distribution of XIAP. Conclusions: XAF1 partici- pates in ATRA-induced growth suppression through IRF- 1–mediated transcriptional regulation.