Flavonoid apigenin inhibits motility and invasiveness of carcinoma cells in vitro.

Investigations of the mechanisms of the cancer-preventive activity of apigenin (4',5,7,-trihydroxyflavone), a plant-derived, anti-carcinogenic flavonoid, showed its interference with cell proliferation, survival, and gap junctional coupling. We used a model based on non-invasive HeLa wild-type cells and their connexin43 (Cx43) transfected counterparts to correlate the effect of apigenin on tumour cell invasiveness with its influence on cell motility. Both cell lines displayed similar motile properties in control conditions. Apigenin treatment resulted in a significant and reversible inhibition of translocation of both HeLa wild-type cells and HeLa Cx43 transfectants. The effect of apigenin on cell proliferation was less pronounced especially at low apigenin concentration, whereas its influence on cell motility correlated with the reduction of the invasive potential of HeLa Cx43 cells as shown by an invasion assay based on the confrontation of tumour cell spheroids with chick embryo heart fragments. HeLa Cx43 cells were highly invasive in controls, but did not invade the heart tissue at tumour cell aggregate-fibroblast capsule interfaces in the presence of apigenin and failed to fully engulf these heart fragments. Because the motility of chick heart fibroblasts was only slightly affected by apigenin, these observations indicate that apigenin exerts its anti-invasive effect on HeLa cells predominantly via a specific inhibition of tumour cell motility. This inhibitory effect of apigenin on tumour cell invasiveness in vitro demonstrates that apigenin may exert its anti-tumorigenic effect in vivo via inhibition of tumour cell penetration of the healthy tissue.

Hierarchy of carcinoma cell responses to apigenin: gap junctional coupling versus proliferation.

We discriminated the role of gap junctional communication and phenotypic constitution of tumour cells in determining their responsiveness to apigenin. Effects of apigenin on intercellular communication and proliferation of two lines of carcinoma cells, uncoupled HeLa cells and their coupled Cx43-transfected counterparts, were analysed and compared with the responses of highly coupled BICR/M1Rk cells. Dye transfer analyses demonstrated that apigenin decreases the degree of coupling in Cx43-coupled populations of HeLa cells but does not affect BICR/M1Rk cells. Similarly, no communication enhancement was observed in originally uncoupled HeLa cell populations. A G2-specific growth arrest paralleled by the induction of apoptosis was observed which was more pronounced and correlated with higher number of apoptotic events in coupled HeLa Cx43 transfectants than in parental cell line. On the other hand, apoptosis was not observed in highly coupled BICR/M1Rk cells, instead, these cells were only transiently blocked in G2 which might be a result of their ability to metabolise apigenin. These data demonstrate a hierarchy of systems determining cellular sensitivity to apigenin. Gap junctional coupling does not influence the quality of cell cycle-related responses to apigenin but modulates their magnitude. This modulating effect of gap junctional coupling depends, however, on a cellular context determined by specific cell phenotype and can be overcome by tissue-specific compensatory mechanisms.