Quercetin inhibits the action of 12-O-tetradecanoylphorbol-13-acetate, a tumor promoter.

Quercetin, a mutagenic but noncarcinogenic flavonoid, inhibited the increased incorporation of inorganic phosphate into phospholipids of human embryo fibroblasts induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter. Quercetin also inhibited TPA-induced increases of sugar transport and RNA synthesis in chick embryo fibroblasts, and TPA-induced aggregation of human platelets. These results suggest that quercetin may have antitumor promoter activity, which provides a possible reason why quercetin does not develop malignant tumors despite its mutagenicity.