Bile acids, principally deoxycholic acid (DCA), have been implicated in the promotion of colon tumorigenesis in both animals and humans. Increasing evidence suggests that bile acids may exert their tumor promoting activity by modulating intracellular signaling and altering gene expression. In this study we have investigated the effect of bile acids on the tumor suppressor p53 using the human colon tumor cell line HCT116, which retains the wild-type p53 gene and functional p53 signaling in response to DNA damage. We found that exposure of the cells to elevated concentrations of DCA suppressed accumulation of p53 protein as well as p53 transactivation and impaired the p53 response of the cells to DNA damaging agents, such as ionizing radiation. Neither ursodeoxycholic acid, a putative chemopreventive agent, nor cholic acid, which is biologically inert, had any effect on p53 protein level and transactivation activity. Further examination revealed that instead of inhibition, DCA induced p53 mRNA in a dose-dependent manner, indicating that the inhibitory effect of DCA on p53 protein is mediated by a post-transcriptional mechanism. Both lactacystin, a specific inhibitor of the 26S proteasome, and leptomycin B, a specific inhibitor of the nuclear export protein CRM1, could block the effect that DCA had on p53 protein levels, suggesting that DCA suppressed p53 by stimulating the process of proteasome-mediated degradation of p53. Significantly, blocking extracellular signal-regulated kinase (ERK) signaling, but not protein kinase C (PKC), blunted suppression by DCA of p53 protein levels and transactivation activity, suggesting that DCA suppressed p53, in part, by stimulating the ERK signaling pathway. Both ERK and PKC signaling have been previously demonstrated to be stimulated by DCA. These results suggest a novel signaling mechanism of bile acids that may play an important role in colon tumor promotion mediated by bile acids.