Modulation of AP-1 by natural chemopreventive compounds in human colon HT-29 cancer cell line

Purpose. Activator protein-1 (AP-1) has been implicated as playing important roles in apoptosis and cancer development. In this work, we studied several natural chemopreventive compounds for their potential chemopreventive properties in the modulation of AP-1 signaling pathway in HT-29 colon cancer cells. Methods. The HT-29 cells were transfected with AP-1 -luciferase reporter gene, and one of the stable clones (C-4) was used for subsequent experiments. The HT-29 C-4 cells were treated for 1 h with various natural chemopreventive agents and challenged with AP-1 stimulators such as 12-O-tetradecanoylphorbol-13-acetate (TPA) or hydrogen peroxide ( H2O2) for 6 h. The c-Jun N-terminal kinase (JNK) was examined to understand the effect of these compounds on the upstream signaling activator of AP-1. The protein expression level of endogenous cyclin D1, a gene that is under the control of AP-1, was also analyzed after treatments with the agents. In addition, cell death induced by these compounds was evaluated by MTS assay [3-(4,5-dimethylthiazol-2-yl)-5-(3-arboxymethoxyphenyl)-2-(4-sulfophenyl)- 2H-tetrazolium, inner salt]. Results. TPA and H2O2 treatments strongly induced AP-1 - luciferase activity as expected. Phenethyl isothiocyanate, sulforaphane, curcumin, and resveratrol increased AP-1 - luciferase activity dose-dependently and then decreased at higher doses in the presence or absence of TPA. Allyl isothiocyanate and (-)- epigallocatechin-3-gallate ( EGCG) increased AP-1 - luciferase activity dose-dependently up to 50 and 100 muM. Other tea catechins and procyanidin dimers, however, had little or no effect on AP-1 - luciferase activity. The JNK activity was induced by the isothiocyanates and EGCG. Most of the chemopreventive compounds induced cell death in a dose-dependent manner, with the exception of epicatechin (EC) and the procyanidins, which had little effect. The expression of endogenous cyclin D1 protein was well correlated with those of AP-1 - luciferase assay. Conclusion. Taken together, these results suggest that natural chemopreventive compounds may have differential biological functions on the signal transduction pathways such as AP-1 in the intervention of colon cancer progression and carcinogenesis.