Mutation of the adenomatous polyposis coli (APC) gene is associated with the earliest stages of colorectal tumorigenesis and appears to be responsible for the hereditary condition familial adenomatous polyposis (FAP). Evidence indicates that cyclooxygenase-2 (COX-2) is induced and at elevated levels in human colorectal cancers and in the polyps of mouse FAP models. We have used HT-29 cells, a human colorectal carcinoma cell line with a mutant carboxy-truncated APC gene, in which intact APC gene has been introduced under the control of an inducible promoter. These HT-29-APC cells provide a suitable model system to examine how COX-2 expression becomes dysregulated after loss of APC function. Induction of full-length APC causes the HT-29-APC cells to undergo apoptosis. However, differentiation, as measured by alkaline phosphatase activity, is not induced upon expression of full-length APC. Full-length APC protein has been shown to bind the intracellular protein beta-catenin and, as a result, the Lef/Tcf transcription factors are down-regulated. Analysis of APC immunoprecipitates demonstrate a time-dependent increase of beta-catenin interacting with full-length APC. Thus, the Lef/Tcf signaling pathway is intact at this point in these cells. Furthermore, upon expression of full-length APC, COX-2 protein expression is down-regulated while COX-2 mRNA levels remain the same. These data indicate that APC plays a role, either directly or indirectly, in the translational regulation of COX-2. Treatment of the HT-29-APC cells with sodium butyrate, an inducer of apoptosis, does not alter COX-2 protein expression. Thus, COX-2 down-regulation appears to be APC specific and not just due to apoptotic induction. APC appears to uniquely regulate COX-2 expression. The mechanism by which COX-2 protein expression is down-regulated in the HT-29-APC cells is under investigation.