Growth Inhibition of Human Colon Cancer Cells by Nitric Oxide (NO) -Donating Aspirin Is Associated with Cyclooxygenase-2 Induction and β-Catenin/T-Cell Factor Signaling, Nuclear Factor-κB, and NO Synthase 2 Inhibition: Implications for Chemoprevention

Nitric oxide (NO)-releasing aspirin (ASA), consisting of a traditional ASA molecule to which a NO-donating moiety is covalently bound, is a promising colon cancer chemopreventive agent. NO-ASA inhibits colon cancer cell growth more potently than ASA by inhibiting cell proliferation and enhancing cell killing. We examined in cultured human colon cancer cells the effect of NO-ASA on the β-catenin/T-cell factor signaling pathway, nuclear factor-κB, and NO synthase 2 and on cyclooxygenase (COX) expression, all presumed to participate in colon carcinogenesis. Besides inhibiting cell growth, NO-ASA inhibited the β-catenin/T-cell factor signaling pathway (IC₅₀, 1.1 μM), nuclear factor-κB DNA binding (IC₅₀, 7.5 μM), and NO synthase 2 expression (IC₅₀, 2 μM). Interestingly, NO-ASA induced COX-2 expression, although it had no effect on COX-1. COX-2 induction was accompanied by increased prostaglandin E₂ production. These effects occurred at NO-ASA concentrations below or near its IC₅₀ for cell growth (IC₅₀, 2-50 μM). The metabolism of NO-ASA by these cells is characterized by a rapid deacetylation step and the formation of a conjugate with glutathione. NO-ASA had no effect on intracellular cyclic GMP concentrations. We propose a model incorporating the pleiotropic effects of NO-ASA on cell signaling and postulate that collectively these effects may contribute to its strong chemopreventive effect.