Bile acid-induced DNA damage and repair in bacterial and mammalian cells.

Abstract

Colon cancer is the second most common type of cancer in the United States. Its incidence is linked epidemiologically to high levels of bile acids in the feces. Bile acids have been implicated as promoters and cocarcinogens in the etiology of colon cancer and as comutagens and mutagens in bacteria. These observations suggest the hypothesis that bile acids may damage DNA. By using the DNA-damage inducible SOS system in Escherichia coli, this study shows that when bacteria are exposed to bile acids there is induction of the SOS repair system and preferential survival of cells undergoing repair. Additionally, differential killing assays using repair defective bacteria show strains defective in recombinational repair or excision repair have lower survival when treated with bile acids than their parental wild-type counterparts. Human fibroblasts were treated with bile acids and unscheduled DNA synthesis (UDS) was measured. UDS is considered to represent the DNA synthesis step in excision repair. UDS, measured by autoradiography, was found to significantly increase in human fibroblasts upon treatment with bile acids. In addition, differential cytotoxicity assays with Chinese Hamster Ovary cells showed that different DNA-repair pathway defective cells were sensitive to different bile acids. Introduction of DNA damage and induction of DNA-repair by bile acids implicates them as possible direct carcinogens in the etiology of colon cancer.