The role of the actin cytoskeleton in cell transition through the middle of the G1 phase of the mitotic cycle

: The influence of dihydrocylochalasin B (H2CB), which selectively disrupts the actin cytoskeleton structure, on G1 phase progression after stimulation of quiescent Swiss 3T3 cells by epidermal growth factor (EGF) was investigated. H2CB was added to the culture medium (10 mg/ml) for different periods of time after cell cycle induction by EGF (10 ng/ml). Efficiency of mitogenic stimulation was estimated by 14C-thymidine uptake 21-23 h after EGF addition. It is shown that the main actin-mediated step is in the middle of G1, from 8 to 12 h after stimulation. Disorganization of actin cytoskeleton structure only during this time led to complete and irreversible block of cell entry into the S phase. On the contrary, the same effect of H2CB during the early period of G1 (first 6 h) led to the increase in 14C-thymidine incorporation in DNA. The specificity of actin-dependent period was proven in experiments with another inhibitor of cell proliferation--dancylcadaverine, whose effect was revealed at the earlier time--4-6 h after cell stimulation. Inhibition of protein synthesis during actin-dependent period (8-12 h) led to the same block of cell progression through G1 as it was seen after actin structure disruption. These data suggest that actin-dependent block is associated with the appearance and functioning of such specific regulators of G1 as cyclins (D, E) and their complexes with Cdk's (G1 kinases) which phosphorylate Rb protein (p1 10) associated with transcription factors E2F.