Isoflavone genistein inhibits the initiation and promotion of two-stage skin carcinogenesis in mice

Isoflavone genistein is a specific inhibitor of protein tyrosine kinase (PTK) and has been shown to have a variety of anticancer activities in cultured cells and animal models. We report here that genistein significantly inhibits 7,12-dimethylbenz[α]anthracene (DMBA)-initiated and 12-O-tetradecanoyl phorbol-13-acetate (TPA)-promoted skill tumorigenesis in a two-stage carcinogenesis model. In an initiation study, 10 μmol genistein was applied daily to female SENCAR mouse skin for 1 week, followed by initiation with 10 nmol DMBA. Mice were then treated with twice weekly 4 μg TPA. Genistein was shown to reduce tumor incidence and multiplicity in DMBA-initiated skill tumors by ~20 (P < 0.05) and 50% (P < 0.01), respectively. Two promotion studies were conducted using CD-1 and SENCAR mice. In experiment 1, CD-1 mice were initiated with 100 nmol DMBA and followed by a twice weekly regimen of 1 and 5 μmol genistein/4 μg TPA. In experiment 2, SENCAR mice were initiated with 10 nmol DMBA and followed by a regimen of 5, 10 and 20 μmol genistein/2 μg TPA. Both studies consistently showed that genistein substantially inhibited TPA-promoted skin tumorigenesis by reducing the tumor multiplicity by ~60 and 75%, respectively (P < 0.01). However, the tumor incidence appeared to be less affected. Mechanistic studies showed that genistein inhibited DMBA-induced bulky DNA adduct formation and substantially suppressed TPA-stimulated H₂O₂ and inflammatory responses in mouse skin by > 60% (P < 0.01). In contrast, genistein only exhibited a moderate inhibition of TPA-induced ornithine decarboxylase activity (P > 0.05). Our results suggest that genistein exerts its anti-initiational and anti-promotional effects on skin carcinogenesis probably through blockage of DNA adduct formation and inhibition of oxidative and inflammatory events in vivo.