Isolation and characterization of a novel gene, hRFI, preferentially expressed in esophageal cancer

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Abstract
HTID1, a human homologue of Drosophila tumor suppressor, I(2)tid regulates the release of cytochrome c from mitochondria and subsequent alteration of caspase-3 activity on apoptosis induced by exogenous stimuli, such as tumor necrosis factor-α and mitomycin C. To search for an interacting molecule with hTid1, we applied two-hybrid yeast screening and isolated a novel gene, which encodes a 46 kDa protein of 373 residues. Within the deduced amino acid sequence, a region showing homology to the Ring Finger domain of X-linked inhibitor of apoptosis protein was identified and the gene was designated as hRFI, standing for human Ring Finger homologous to IAP type. A 2.0 kb hRFI transcript was ubiquitously expressed in all human tissues as well as several cancer cell lines examined. Northern blot analysis showed that in 70% (14 out of 20) of esophageal cancer patients, expression of hRFI in cancerous regions was two or more times higher than in the corresponding normal tissues. HeLa cells transfected with hRFI construct exhibited a tendency to resist TNF-α induced apoptosis, suggesting an anti-apoptotic function of the hRFI product. Finally, hRFI protein was shown to be cleaved within the DEDD sequence spanning residues 230–233 by caspase-3 during the apoptotic induction.