DISLODGMENT AND ACCELERATED DEGRADATION OF RAS

Membrane anchorage of Ras oncoproteins, required for transforming acti vity, depends on their carboxy-terminal farnesylcysteine. We previousl y showed that S-trans,trans-farnesylthiosalicylic acid (FTS), a synthe tic farnesylcysteine mimetic, inhibits growth of ErbB2- and Ras-transf ormed cells, but not of v-Raf-transformed cells, suggesting that FTS i nterferes specifically with Ras functions. Here we demonstrate that FT S dislodges Ras from membranes of H-Ras-transformed (EJ) cells, facili tating its degradation and decreasing total cellular Ras. The dislodge d Ras that was transiently present in the cytosol was degraded relativ ely rapidly, causing a decrease of up to 80% in total cellular Ras. Th e half-life of Ras was 10 +/- 4 h in FTS-treated EJ cells and 27 +/- 4 h in controls. The dislodgment of membrane Ras and decrease in total cellular Ras were dose-dependent: 50% of the effects occurred at 10-15 mu M, comparable to concentrations (7-10 mu M) required for 50% growt h inhibition in EJ cells. Higher concentrations of FTS (25-50 mu M) we re required to dislodge Ras from Rat-1 cell membranes expressing norma l Ras, suggesting some selectivity of FTS toward oncogenic Ras. Membra ne localization of the prenylated G beta gamma of heterotrimeric G pro teins was not affected by FTS in EJ cells. An FTS-related compound, N- acetyl-S-farnesyl-L-cysteine, which does not inhibit EJ cell growth, d id not affect Ras. FTS did not inhibit growth of Rat-1 cells transform ed by N-myristylated H-Ras and did not reduce the total amount of this Ras isoform. The results suggest that FTS affects docking of Ras in t he cell membrane in a rather specific manner, rendering the protein su sceptible to proteolytic degradation.