Overexpression of wild-type RhoA produces growth arrest by disrupting actin cytoskeleton and microtubules

We have investigated the role of Rho GTPase in cell growth by generating st able cells that express the wild-type RhoA (RhoA(wt)) under the control of an inducible promoter. Induction of RhoA(wt) had a biphasic effect on the a ctin cytoskeleton. At low levels of expression, RhoA(wt) stimulated the ass embly of actin stress fibers without affecting cell growth. At high levels, there was a paradoxical disruption of the actin cytoskeleton accompanied b y a growth arrest. Cell cycle analysis revealed a dual block at the G(1)/S and G(2)/M checkpoints. The G(1)/S arrest correlated with the accumulation of p21(Cip1), resulting in the inhibition of cdk2 activity, whereas the G(2 )/M block correlated with the loss of microtubules. The cyclin B level and the cdc2 kinase activity, however, were increased, suggesting that the prog ression through mitosis rather than entry into the G(2)/M is defective when RhoA(wt) is overexpressed. Similar cell cycle defects and the loss of micr otubules were observed after a cytochalasin D treatment, indicating that th e ability of RhoA to regulate the integrity of actin cytoskeleton may be cr itical for the cell cycle transition through both the G(1)/S and M phase ch eckpoints,). Cell. Biochem. 80:229-24, 2000. (C) 2000 Wiley-Liss, Inc.