Exisulind and its analogues are inhibitors of cyclic GMP phosphodiesterases (PDEs) that have been shown to activate and induce protein kinase G, resulting in the induction of apoptosis in colon cancer cells. These drugs also reduce β-catenin protein levels and decrease cyclin D1 mRNA levels in SW480 cells. Herein we report on studies pertaining to exisulind regulation of β-catenin levels and activity in colon tumor cells. Exisulind and its higher-affinity PDE analogues, (Z)-5-fluoro-2-methyl-(4-pyridylidene)-3-(N-benzyl)-indenylacetamide hydrochloride (CP461) and (Z)-1H-indene-3-acetamide, 5-fluoro-2-methyl-N-(phenylmethyl)-1-[(3,4,5-trimethoxyphenyl)methylene] (CP248), reduced β-catenin, including the nuclear β-catenin in SW480 cells (ec 5 0 ~200μM, 1μM, and <1μM, respectively). The 50% reduction of β-catenin was seen in 8-14hr. There was no change in β-catenin mRNA. Exisulind-induced β-catenin reduction was blocked by the proteasomal inhibitor MG132 (Z-leu-Leu-Leu-CHO), indicating that the effect of exisulind involved ubiquitin-proteasomal degradation. A consequence of reduced β-catenin in SW480 cells was that exisulind, CP461, and CP248 caused a concentration- and time-dependent decrease in cyclin D1 levels (ec 5 0 ~300μM, 1μM, and <1μM, respectively) in 4hr. The effect was via decreased cyclin D1 mRNA levels. Exisulind-induced degradation of β-catenin was not blocked by the inhibition of caspase-3 activity and/or apoptosis, and some SW480 cells showed a reduction in β-catenin levels before the appearance of early apoptosis indicators. Expression of the N-terminal 170 amino acid fragment of β-catenin reduced the effects of β-catenin degradation, cyclin D1 reduction, and the apoptosis response to exisulind. These results indicate that exisulind-induced β-catenin degradation precedes the induction of apoptosis and that the down-regulation of inappropriate β-catenin-activated genes accounts in part for the pro-apoptotic effects of exisulind and CP461 in colon tumor cells.