Phloretin Inhibits Phorbol Ester-Induced Tumor Promotion and Expression of Cyclooxygenase-2 in Mouse Skin: Extracellular Signal-Regulated Kinase and Nuclear Factor-κB as Potential Targets

The present study investigated the effect of phloretin [2',4',6'-trihydroxy-3-(4-hydroxyphenyl)-propiophenone] on 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced cyclooxygenase-2 (COX-2) expression and tumor promotion in mouse skin and explored the underlying molecular mechanisms. Topical application of phloretin significantly inhibited 7,12-dimethylbenz[a] anthracene-initiated and TPA-promoted mouse skin carcinogenesis. Pretreatment with phloretin on the dorsal skin of mice inhibited TPA-induced COX-2 expression in a dose-dependent manner. To elucidate the molecular mechanism underlying COX-2 inhibition by phloretin, we examined its effect on TPA-induced activation of nuclear factor-kappa B (NF-kappa B), a ubiquitous transcription factor responsible for TPA-induced COX-2 expression in mouse skin. Topically applied phloretin decreased the TPA-induced DNA binding of NF-kappa B. In addition, phloretin inhibited the phosphorylation as well as the catalytic activity of extracellular signal-regulated kinase (ERK), which was previously found to activate NF-kappa B and induce COX-2 expression in TPA-treated mouse skin. Taken together, the inhibitory effects of phloretin on TPA-induced NF-kappa B activation and COX-2 expression through the modulation of ERK signaling may partly account for its antitumor-promoting effect on mouse skin carcinogenesis.