The safety concerns which cast doubt on the prudence of using synthetic COX-2 inhibitors as cancer chemopreventive agents have invigorated the search for novel chemopreventive agents from the plant kingdom. Tricin (4’,5,7-trihydroxy-3’,5’-dimethoxyflavone) contained in grass species and apigenin (4’,5,7-trihydroxyflavone) from leafy vegetables are congeneric flavones with putative cancer chemopreventive properties, which are thought to engage mechanisms targeted at COX enzymes. Here the hypothesis was tested that COX activity and levels are affected by tricin and apigenin in a similar fashion. Experiments were conducted with both flavones in three different systems: i) to explore their effect on COX activity they were incubated with purified COX-1 and COX-2 enzymes from sheep seminal vesicles and placenta, respectively, in a cell free-system; activity was assessed using a commercially available chemiluminescent enzyme activity kit, which measures COX peroxidase function. ii) Their ability to interfere with inducible COX-2 expression in intact cells was studied in human colon-derived epithelial HCEC cells; enzyme expression was induced by phorbol myristoyl acetate, after which cells were exposed to the flavones for 6 or 24 h. iii) Their effect on inducible prostaglandin E-2 (PGE-2) levels was assessed by competitive immunoassay in HCEC cells. Tricin inhibited COX-1 and COX-2 activity in the cell-free system similarly with an IC50 of ∼1 μM, whilst apigenin up to 25 μM did not affect enzyme activity. Tricin decreased inducible COX-2 protein expression significantly; densitometry readings showed that after 24 h incubation 10 μM tricin decreased COX-2 protein by 48±16% (mean±ΣΔ, ν=3) compared to control. Apigenin reduced COX-2 levels after incubation with cells for 6 h in a concentration-dependent fashion, at 20 μM by 55±13% (mean±SD, n=3) compared to control. In contrast, exposure of cells for 24 h to apigenin increased COX-2 expression, at 20 μM by 2.1 fold over control. Both tricin and apigenin decreased PGE-2 generation in HCEC cells. After 24 h incubation with 20 μM flavone, tricin reduced PGE-2 by 52±10% (mean±SD, n=5) compared to cells unexposed to flavone, whilst apigenin abolished measurable PGE-2 almost completely. These results show that tricin and apigenin regulate COX-enzymes in different ways, tricin being a potent COX inhibitor in vitro, whilst apigenin, which was devoid of enzyme-inhibitory properties, reduced cellular PGE-2 levels more strongly than tricin.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]