Apigenin-induced cell cycle arrest at G2/M in human colon cancer cells
A diet rich in fruits and vegetables is associated with a reduced incidence of many cancers, including colon cancer, and apigenin, a plant flavonoid, may contribute to this effect. Apigenin was previously shown to induce G2/M cell cycle arrest in three different human colon cancer cell lines that have APC mutations (SW480, HT29 and CaCo2). In this study, we assessed the importance of the APC tumor suppressor mutation in relation to the ability of apigenin to induce cell cycle arrest. The cell model system utilized for this study consisted of HT29-APC cells, which contain an inducible (methallothionine promoter) wild-type APC gene characterized by an increase in apoptosis in cultured HT29-APC cells treated with 100[Mu]M ZnCl2. HT29-GAL cells, containing β-galactosidase plasmid, served as the control. Apigenin treatment (from O[Mu]M to 80[Mu]M) for 48 hours resulted in a significant (P<0.05) reduction in the cell number concurrent with flow cytometry results showing a dose-dependent accumulation of cells in G2/M phase in uninduced HT29-APC and HT29-GAL cells. Terminal Deoxytransferase-Mediated Deoxyuridine NickEnd-Labeling (TUNEL) assay determined a significant (P<0.05) increase in the percentage of apoptotic HT29-APC cells when treated with O[Mu]M apigenin and 100[Mu]M of ZnCl2 after 120hr (induction of wild-type APC gene). Additionally, apigenin treatment (80[Mu]M) appeared to enhance the sensitivity of the HT29-APC cells to apoptosis when treated with ZnCl2. Alternatively, the percentage of apoptotic cells did not change in the HT29-APC cells when treated with apigenin alone or in the HT29-GAL cells when treated with either apigenin and/or ZnCl2. Flow cytometric analysis showed an increase (P<0.05) in the percentage of cells in G2/M when HT29-APC cells were treated with 80[Mu]M apigenin for 120 hr. This increase was not present in HT29-APC cells when treated with both 80[Mu]M apigenin and 100[Mu]iM ZnCl2 for 120hr. Nevertheless, the apigenin-treated HT29-GAL cells showed a significant (p<0.05) accumulation of cells in G2/M regardless of the presence of ZnCl2. Decreased cell numbers reflected the cell cycle arrest and apoptosis results. These results suggest that APC dysfunction may be critical for apigenin to induce cell cycle arrest in human colon cancer cell lines.