Role of flavonoids in suppressing the enhancement of phospholipid metabolism by tumor promoters.

Quercetin, a ubiquitous flavonoid in plants, inhibited the incorporation of [32P]inorganic phosphate (32Pi) into phospholipid of HeLa cells enhanced by 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter. Among the flavonoids tested, luteolin was the most effective in inhibiting the action of TPA. Studies on structure-activity relationships demonstrated that more hydroxylated flavones and flavonols had stronger inhibitory effects. Quercetin and luteolin also inhibited enhancement of 32Pi-incorporation into phospholipid by dihydroteleocidin B, another potent tumor promoter. The inhibitory effect of quercetin on the biological action of tumor promoters is interesting in relation to the non-carcinogenicity of this flavonoid in animals, in spite of its mutagenicity.