3,3′-Diindolylmethane Enhances the Efficacy of Butyrate in Colon Cancer Prevention through Down-Regulation of Survivin

Butyrate is an inhibitor of histone deacetylase (HDAC) and has been extensively evaluated as a chemoprevention agent for colon cancer. We recently showed that mutations in the adenomatous polyposis coli (APC) gene confer resistance to HDAC inhibitor-induced apoptosis in colon cancers. Here, we show that APC mutation rendered colon cancer cells resistant to butyrate-induced apoptosis due to the failure of butyrate to down-regulate survivin in these cells. Another cancer-preventive agent, 3,3'-diindolylmethane (DIM), was identified to be able to down-regulate survivin in colon cancers expressing mutant APC. DIM inhibited survivin mRNA expression and promoted survivin protein degradation through inhibition of p34(cdc2)-cyclin B1-mediated survivin Thr(34) phosphorylation. Pretreatment with DIM enhanced butyrate-induced apoptosis in colon cancer cells expressing mutant APC. DIM/butyrate combination treatment induced the expression of proapoptotic Bax and Bak proteins, triggered Bax dimerization/activation, and caused release of cytochrome c and Smac proteins from mitochondria. Whereas overexpression of survivin blocked DIM/butyrate induced apoptosis, knocking down of survivin by small interfering RNA increased butyrate-induced apoptosis in colon cancer cells. We further showed that DIM was able to downregulate survivin and enhance the effects of butyrate in apoptosis induction and prevention of familial adenomatous polyposis in APC(min/+) mice. Thus, the combination of DIM and butyrate is potentially an effective strategy for the prevention of colon cancer.