Nitric oxide-donating aspirin inhibits β-catenin T cell factor (TCF) signaling in SW480 colon cancer cells by disrupting the nuclear β-catenin-TCF association

Dysregulation of the Writ pathway and altered beta-catenin expression are central early events in colorectal carcinogenesis. We studied the ortho, meta, and para (o-, m-, and p-) positional isomers of NO-donating aspirin (NO-ASA), a chemopreventive agent against colon cancer, for their effect on beta-catenin/T cell factor (TCF) signaling. In human SW480 colon carcinoma cells, cell-growth inhibition by NO-ASA [IC50 values for p-, o-, and m- were 48.1 +/- 4.3 (mean +/- SEM), 60.4 +/- 2.1, and 900 +/- 50 muM, respectively] was accompanied by significant inhibition of beta-catenin signaling. We determined beta-catenin-dependent TCF-4 transcriptional activity by measuring the activity of the luciferase gene placed under the control of TCF-4 regulatory sequences. The IC50 values for beta-catenin/TCF-4-signaling inhibition by NO-ASA were: o-, 2.6 +/- 0.4; m-, 15 +/- 5; p-, 1.1 +/- 0.1 muM; and for ASA, >5,000 muM. Total or nuclear levels of beta-catenin and its distribution in the cell were not altered by NO-ASA, as judged by protein expression levels and semiquantitative immunofluorescence analysis. NO-ASA disrupted the association of beta-catenin and TCF-4 in the nucleus, whereas ASA did not affect it. NO-ASA reduced the expression of cyclin D1, a downstream target gene that plays an important role in colon carcinogenesis. In contrast, a structural analog of NO-ASA lacking the-NO2 moiety did not affect TCF-4 transcriptional activity. Thus, NO-ASA inhibits beta-catenin-mediated TCF activity by preventing the formation of the beta-catenin/TCF-4 complex. This effect, occurring at NO-ASA concentrations far below those required to inhibit cell growth, may be a critical early event in the chemopreventive activity of NO-ASA against colon cancer.