Cell-cycle arrest at G2/M and growth inhibition by apigenin in human coloncarcinoma cell lines

Apigenin, a common dietary flavonoid, has been shown to induce cell cycle a rrest in both epidermal and fibroblast cells and inhibit skin tumorigenesis in murine models. The present study assessed the influence of apigenin on cell growth and the cell cycle in the human colon carcinoma cell lines SW48 0, HT-29, and Caco-2. Treatment of each cell line with apigenin (0-80 mu M) resulted in a dose-dependent reduction in both cell number and cellular pr otein content, compared with untreated control cultures. DNA flow cytometri c analysis indicated that treatment with apigenin resulted in G2/M arrest i n alt three cell lines in a time- and dose-dependent manner. Apigenin treat ment (80 mu M) for 48 h produced maximum G2/M arrest of 64%, 42%, and 26% i n SW480 cells, HT-29 cells, and Caco-2 cells, respectively, in comparison w ith control cells (15%). The proportion of S-phase cells was not altered by apigenin treatment in each of the three cell lines. The G2/M arrest was re versible after 48 h of apigenin treatment in the most sensitive cell line S W480. The degree of G2/M arrest by apigenin was inversely correlated with t he corresponding inhibition of cell growth measurements in all three cell l ines (r = -0.626 to -0.917, P less than or equal to 0.005). Moreover, an im mune complex kinase assay demonstrated an inhibition of p34(cdc2) kinase ac tivity, a critical enzyme in GUM transition, in each cell fine after treatm ent with apigenin (50-80 mu M). Western blot analyses indicated that both p 34(cdc2) and cyclin B1 proteins were also decreased after apigenin treatmen t. These results indicate that apigenin inhibits colon carcinoma cell growt h by inducing a reversible G2/M arrest and that this arrest is associated, at least in part, with inhibited activity of p34(cdc2) kinase and reduced a ccumulation of p34(cdc2) and cyclin B1 proteins. Differences in induction o f G2/M arrest by apigenin in the three colon carcinoma cell lines suggest t hat dietary apigenin may be differentially effective against tumors with sp ecific mutational spectra. Mol. Carcinog. 28. 102-110, 2000. (C) 2000 Wiley -Liss, Inc.