Characterization of bile salt-induced apoptosis in colon cancer cell lines

Bile salts have been shown to be involved in the etiology of colorectal cancer. Although there is a large body of evidence for bile salts as a cocarcinogen in azoxymethane-induced colorectal cancer, bile salt-induced apoptosis of colorectal cancer cells has not yet been studied in detail. Therefore, we investigated the effects of different bile salts on apoptosis and apoptotic signaling in colon cancer cell lines. Incubation of colorectal cancer cell lines with physiological concentrations of deoxycholic acid led to a dramatic induction of apoptosis. Caspase cleavage and caspase activation occurred as early as 30 min after the addition of deoxycholate. Caspase-2 (Ich-1, Nedd2), caspase-3 (CPP-32, YAMA, Apopain), caspase-7 (Mch-3, ICE-LAP-3), and caspase-8 (FLICE, Mach-1, Mch5) are activated in HT-29, whereas caspase-1 (ICE) remained intact. Caspase activation and cellular apoptosis induced by bile salts were reversed by broad spectrum and selective caspase inhibitors. As opposed to hepatocyte death mediated by bile acids, CD95 was not involved in deoxycholate-induced apoptosis. The cytoprotective effect of ursodeoxycholic acid in hepatocytes or other tumor cell lines, which is mediated by inhibiting the mitochondrial permeability transition, was not observed in colon cancer cell lines as well. This points to distinct intracellular functions of ursodeoxycholate in different cancer cell types. Here we describe the specificity of bile salt-induced apoptosis in colon cancer cell lines. Differences from hepatocytes are shown. Bile acid-specific caspase activation is part of the apoptotic pathway induced by bile salts in colon cancer cell lines. Furthermore, a lack of cytoprotective function of ursodeoxycholate in these cells is demonstrated. Our data raise questions as to the role of bile salts in colorectal carcinogenesis.