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XAF1 (XIAP-associated factor 1) is a newly identified protein that exerts anti-apoptotic activity by binding to and relocating XIAP from the cytoplasm to nucleus. We have identified XAF1 as an interferon IFN α (IFN α) induced gene in human skin-derived cells, using DNA microarrays. It has been reported that XAF1 is mainly expressed in normal tissues but is missing or present at low levels in most cancer cell lines, which implies a tumor-suppressing function. We describe the identification of a novel splice variant of human XAF1, designated XAF1C, which contains a cryptic exon. Incorporation of this exon (exon 4b) into the mRNA introduces an in-frame stop codon, resulting in a shortened open reading frame (ORF) of 495 nucleotides. The predicted 164 amino acid (AA) protein that lacks the C-terminal domain of the previously described XAF1(A), but contains a unique 24 AA carboxy terminus. Like XAF1(A), XAF1C mRNA expression was detected in a wide range of human cancer cell lines and also in normal human tissues. The ratio of XAF1(A) and XAF1C mRNA expression differs amongst the cell lines tested, suggesting differential mRNA stabilities and/or the existence of tissue- or cell type-specific splicing regulation. In transfected cells, xaf1c encodes a truncated protein of 18 KD, which is distributed primarily in the nucleus. In further studies we show that IFN α, at pharmacological concentrations, can significantly increase susceptibility to TRAIL-induced cell death in human breast cancer cell lines: MCF-7, MDA-MB-231. The induction of XAF1(A) and XAF1C expression by IFNa was confirmed by semi-quantitative RT-PCR for the breast cancer cell lines. Stable expression of XAF1C in MCF7 cells increases their sensitization to TRAIL-induced apoptosis by IFN α. Supported by funds from the Feist-Weiller Cancer Center and NCI CA78560.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]