Development of colon cancer is a multistep process frequently involving mutations in both the APC and p53 tumor suppressor genes. In this study we treated the HCT-116 colon cancer cell line with alkylating agents including N-methyl-N'-nitro-N-nitrosoguanidine (MNNG),which is known to cause colon cancer in animals, and examined the expression of both APC and p53 genes. Exposure of cells with MNNG caused an 8-12-fold increase in the level of APC mRNA and protein. APC induction was shown to result from increased nuclear transcription of the APC gene and correlated with a concomitant increase in the p53 protein level after MNNG treatment. A necessary role for p53 in APC gene regulation is supported by the failure of MNNG to induce APC expression in cell lines either expressing very low levels of p53 (HeLa cells) or no p53 (K562 erythroleukemia cells). The overexpression of wild-type p53 gene into HCT-116 cells mimicked the effect of MNNG-induced expression of APC mRNA. A direct causal role for p53 in APC gene regulation was further evaluated by transfecting the wild-type p53 gene into K562 cells and observing a 5-fold increase in the APC gene expression. These results support a model featuring a direct link between p53 and APC in response to alkylation-induced DNA damage and suggest a novel role for p53 in a stress-response pathway involving APC.