Apoptosis inhibitor 5 (API-5; AAC-11; FIF) is upregulated in human carcinomas in vivo.

Apoptosis inhibitor 5 (API-5) is a 55 kDa nuclear protein with potent anti-apoptotic signaling in tumor cells in vitro. In this study, we analyzed the expression of the API-5 protein in vivo in a broad spectrum of human carcinomas, including those of the colon, lung, liver, kidney, pancreas, stomach and esophagus using tumor tissues obtained during tumor resection. The results showed significant upregulation of API-5 expression in biopsies of lung (23%, n=13) and colorectal tumors (33%, n=27) in comparison with biopsies from the adjacent normal tissue. Colon cancer biopsies were used to study the cell populations with an upregulated level of expression of API-5 more closely. Using a magnetic bead-based selection for the epithelial cell marker EpCAM, we purified epithelial cells from the tumor and control tissues and analyzed these cells for API-5 expression by western immunoblotting. We observed that EpCAM-positive tumor cells expressed API-5 in all three colorectal cancer cases tested, in contrast to the control EpCAM-positive and EpCAM-negative cells isolated from the control or tumor tissues. These data suggest that the expression of the API-5 protein is upregulated in tumor epithelial cells and may serve as a prognostic marker in colorectal cancer.