Role of RhoA, mDia, and ROCK in cell shape-dependent control of the Skp2-p27^kip-1 pathway and the G₁/S transition

Cell shape-dependent control of cell-cycle progression underlies the spatial differentials of growth that drive tissue morphogenesis, yet little is known about how cell distortion impacts the biochemical signaling machinery that is responsible for growth control. Here we show that the Rho family GTPase, RhoA, conveys the "cell shape signal" to the cell-cycle machinery in human capillary endothelial cells. Cells accumulating p27^kip1 and arrested in mid G₁ phase when spreading were inhibited by restricted extracellular matrix adhesion, whereas constitutively active RhoA increased expression of the F-box protein Skp2 required for ubiquitination-dependent degradation of p27^kip1 and restored G₁ progression in these cells. Studies with dominant-negative and constitutively active forms of radial, a downstream effector of RhoA, and with a pharmacological inhibitor of ROCK, another RhoA target, revealed that RhoA promoted G₁ progression by altering the balance of activities between these two downstream effectors. These data indicate that signaling proteins such as radial and ROCK, which are thought to be involved primarily in cytoskeletal remodeling, also mediate cell growth regulation by coupling cell shape to the cell-cycle machinery at the level of signal transduction.