EVIDENCE THAT FARNESYLTRANSFERASE INHIBITORS SUPPRESS RAS TRANSFORMATION BY INTERFERING WITH RHO ACTIVITY

Small-molecule inhibitors of the housekeeping enzyme farnesyltransfera se (FT) suppress the malignant growth of Pas-transformed cells. Previo us work suggested that the activity of these compounds reflected effec ts on actin stress fiber regulation rather than Ras inhibition, Rho pr oteins regulate stress fiber formation, and one member of this family, RhoB, is farnesylated in vivo. Therefore, we tested the hypothesis th at interference with RhoB was the principal basis by which the peptido mimetic FT inhibitor L-739,749 suppressed Ras transformation. The half -life of RhoB was found to be similar to 2 h, supporting the possibili ty that it could be functionally depleted within the 18-h period requi red by L-739,749 to induce reversion. Cell treatment with L-739,749 di srupted the vesicular localization of RhoB but did not effect the loca lization of the closely related RhoA protein, Ras-transformed Rat1 cel ls ectopically expressing N-myristylated forms of RhoB (Myr-rhoB), who se vesicular localization was unaffected by L-739,749, were resistant to drug treatment. The protective effect of Myr-rhoB required the inte grity of the RhoB effector domain and was not due to a gain-of-functio n effect of myristylation on cell growth, In contrast, Rat1 cells tran sformed by a myristylated Ras construct remained susceptible to growth inhibition by L-739,749, We concluded that Rho is necessary for Ras t ransformation and that PT inhibitors suppress the transformed phenotyp e at least in part by direct or indirect interference with Rho, possib ly with RhoB itself.