Overexpression of Evi-1 oncoprotein represses TGF-β signaling in colorectal cancer.

Human colorectal cancer (CRC) cells are resistant to the anti-proliferative effect of transforming growth factor-β (TGF-β), suggesting that disruption of TGF-β signaling plays an important role in colorectal carcinogenesis. Ecotropic virus integration site-1 (Evi-1) oncoprotein represses TGF-β signaling by interacting with Smads, but its role in CRC has not been established. The purpose of this study is to determine whether Evi-1 plays role(s) in CRCs and to characterize Evi-1 transcript(s) in CRCs. Evi-1 was overexpressed in 53% of human CRC samples, 100% of colon adenoma samples, and 100% of human colon cancer cell lines tested. Using 5' RACE, we cloned a novel Evi-1 transcript (Evi-1e) from a human CRC tissue and found that this novel transcript was expressed at a higher level in CRC tissues than in normal tissues and was the major Evi-1 transcript in CRCs. Transient Evi-1 transfection inhibited TGF-β-induced transcriptional activity and reversed the growth inhibitory effect of TGF-β in MC-26 mouse colon cancer cells. In conclusion, we have identified overexpression of Evi-1 oncoprotein as a novel mechanism by which a subset of human CRCs may escape TGF-β regulation. We have also identified a novel Evi-1 transcript, Evi-1e, as the major Evi-1 transcript expressed in human CRCs.