CYTOSKELETAL INTEGRITY IS REQUIRED THROUGHOUT THE MITOGEN STIMULATIONPHASE OF THE CELL-CYCLE AND MEDIATES THE ANCHORAGE-DEPENDENT EXPRESSION OF CYCLIN D1

The proliferation of many nontransformed cells depends on cell adhesio n. We report here that disrupting the cytoskeleton in normal human fib roblasts causes the same cell cycle phenotype that is observed after b locking cell adhesion: suspended cells and cytochalasin D-treated mono layers fail to progress through G1 despite normal mitogen-induced expr ession of c-myc mRNA. Midway between G0 and the beginning of S-phase, cell cycle progression becomes independent of adhesion and the cytoske leton. At this stage, the cells are also mitogen independent. Molecula r analyses showed that Rb hyperphosphorylation and the induction of cy clin D1 occur slightly earlier than the transition to cytoskeleton ind ependence. Moreover, these molecular events are blocked by cytochalasi n D. Overall, our data indicate the following: 1) anchorage and cytosk eletal integrity are required throughout the mitogen-dependent part of G1; 2) mitogens and the cytoskeleton jointly regulate the phosphoryla tion of Rb; and 3) this interdependence is manifest in the regulation of cyclin D1.