Genistein arrests cell cycle progression of A549 cells at the G₂/M phase and depolymerizes interphase microtubules through binding to a unique site of tubulin

Abstract

Genistein (4',5,7-trihydroxyisoflavone), an isoflavone, is a major constituent of soyfoods. It has potential antiproliferative activity against several tumor types. We have examined the effect of genistein on cellular microtubules as well as its binding with purified tubulin in vitro. Cell viability experiments using human non-small lung epithelium carcinoma cells (A549) indicated that the IC₅₀ value for genistein is 72 μM. Flow cytometry experiments demonstrated that genistein arrested cell cycle progression at the G₂/M phase, but mitotic index data showed that genistein did not arrest cell cycle progression at mitosis. Immunofluorescence studies using an anti-α-tubulin antibody demonstrated a significant depolymerization of the interphase microtubules in a dose-dependent manner, and this was confirmed by the Western blot experiment using genistein-treated A549 cells. In vitro polymerization of purified tubulin into microtubules was inhibited by genistein with an IC₅₀ value of 87 μM. Genistein binding to tubulin quenched protein tryptophan fluorescence in a time- and concentration-dependent manner. Binding of genistein to tubulin was slow, taking ~45 min for equilibration at 37 °C. The association rate constant was 104.64 ± 20.63 M^-1 s^-1 at 37 °C. The stoichiometry of genistein binding to tubulin was nearly 1:1 (molar ratio) with a dissociation constant of 15 μM at 37 °C. It was interesting to note that genistein did not recognize either the colchicine site or the vinblastine binding site of tubulin. Surprisingly, genistein inhibited ANS binding and competed for its binding site of tubulin with a K_i of 20 μM as determined from a modified Dixon plot. Hence, we conclude that one of the mechanisms of antiproliferative activity of genistein is depolymerization of microtubules through binding of tubulin.