Bistheonellide A, an inhibitor of actin polymerization from the marine sponge Theonella sp., was introduced at a concentration of 100 nM into rat fibroblast of 2.4 x 10(4) cells/ml. Within 1 h, it disrupted stress fibers, accompanied by a marked change of the cell morphology, resulting in the formation of processes from the cell surface. Further incubation for 24 h in the presence of 100 nM
bistheonellide A led to binucleation in most cells and subsequent inhibition of cell cycle progression. When
bistheonellide A was withdrawn from the culture medium, binuclear cells began to grow again within 20 h and reverted to mononuclear morphology. Flow cytometric analysis fluorescence-activated cell sorting showed that 2C diploid
DNA content in G1 phase was changed into 4C content of
tetraploid for the
bistheonellide A treated-cells in G1 phase and into 8C content during G2 and M phase. Therefore, we suggested that the
bistheonellide A treatment inhibited cytokinesis, but not mitosis in M phase, and that treated cells were arrested at the early G1 phase. These effects of
bistheonellide A on the cell cycle progression of 3Y1 fibroblast were also observed more prominently in cells synchronized in S phase with
hydroxyurea. Cells in G0 phase were then activated by the addition of
fetal calf serum in the presence of 100 nM
bistheonellide A. Cell cycle progression of the
bistheonellide A-treated cells was obviously slowed down or completely inhibited during G1 phase. These results reveal that actin filaments are not only essential to cytokinesis but also for promoting the progression of cell cycle G1 to S phase.