The chemopreventive flavonoid apigenin induces G2/M arrest in keratinocytes.

Apigenin is a plant flavonoid which has been shown to significantly inhibit UV-induced mouse skin tumorigenesis when applied topically, and may represent an alternative sunscreen agent in humans. We have investigated the molecular mechanism(s) by which apigenin inhibits skin tumorigenesis. Initial studies examined the effects of apigenin on the cell cycle. DNA flow cytometric analysis indicated that culturing cells for 24 h in medium containing apigenin induced a G2/M arrest in two mouse skin derived cell lines, C50 and 308, as well as in human HL-60 cells. The G2/M arrest was fully reversible after an additional 24 h in medium without apigenin. We investigated the effects of apigenin on cyclin B1 and p34cdc2, since cyclin B1/p34cdc2 complexes regulate G2/M progression. Western blot and immune complex kinase assays using whole cell lysates from 308 and C50 cells treated for 24 h with 0-70 microM doses of apigenin demonstrated that apigenin treatment did not change the steady-state level of p34cdc2 protein, but did inhibit p34cdc2 H1 kinase activity in 308 cells. Western blot analysis showed that apigenin treatment of C50 cells and 308 cells inhibited the accumulation of cyclin B1 protein in a dose-dependent manner. The apigenin levels detected in cultured keratinocytes were relevant to those detected in epidermal cells of Sencar mice treated with tumor inhibitory doses of apigenin. In conclusion, we present evidence that apigenin induces a reversible G2/M arrest in cultured keratinocytes, the mechanism of which is in part due to inhibition of the mitotic kinase activity of p34cd2, and perturbation of cyclin B1 levels.