Cytoskeletal integrity is required throughout the mitogen stimulation phase of the cell cycle and mediates the anchorage-dependent expression of cyclin D1.

The proliferation of many nontransformed cells depends on cell adhesion. We report here that disrupting the cytoskeleton in normal human fibroblasts causes the same cell cycle phenotype that is observed after blocking cell adhesion: suspended cells and cytochalasin D-treated monolayers fail to progress through G1 despite normal mitogen-induced expression of c-myc mRNA. Midway between G0 and the beginning of S-phase, cell cycle progression becomes independent of adhesion and the cytoskeleton. At this stage, the cells are also mitogen independent. Molecular analyses showed that Rb hyperphosphorylation and the induction of cyclin D1 occur slightly earlier than the transition to cytoskeleton independence. Moreover, these molecular events are blocked by cytochalasin D. Overall, our data indicate the following: 1) anchorage and cytoskeletal integrity are required throughout the mitogen-dependent part of G1; 2) mitogens and the cytoskeleton jointly regulate the phosphorylation of Rb; and 3) this interdependence is manifest in the regulation of cyclin D1.