ROLE OF RHO-PROTEIN IN LOVASTATIN-INDUCED BREAKDOWN OF ACTIN CYTOSKELETON

The Rho GTPases are involved in actin cytoskeleton organization and si gnal transduction. They need polyisoprenylation for membrane associati on and activation. Lovastatin, a hydroxymethylglutaryl coenzyme A inhi bitor, prevents isoprene synthesis and thereby lipid modification of t he Rho protein carboxy terminus. Because lovastatin causes rounding up of cultured cells, we investigated whether the compound acts on the a ctin cytoskeleton through Rho proteins. Lovastatin treatment decreased F-actin content in a time- and concentration-dependent manner. G-acti n content remained unchanged. In lovastatin-treated NIH 3T3 cells, the amount of Rho protein which was ADP-ribosylated by Clostridium botuli num exoenzyme C3 decreased in membranes and increased in the cytosol f raction. Cycloheximide prevented lovastatin-induced rounding up of cel ls. However, after microinjection or direct application of exoenzyme C 3, cells treated with cycloheximide and lovastatin rounded up again. O n the contrary, lovastatin-treated, round Swiss 3T3 cells reverted to a flat morphology when microinjected with dominant active RhoA (Val14R hoA). Escherichia coli cytotoxic necrotizing factor (CNF1) which activ ates Rho proteins caused flattening of round, lovastatin-treated NIH 3 T3 cells. These results suggest that lovastatin affects the actin cyto skeleton through inactivation of Rho proteins.