SELECTIVE-INHIBITION OF RAS-DEPENDENT CELL-GROWTH BY FARNESYLTHIOSALICYLIC ACID

S-trans,trans-FarnesylthiosalicyIic acid (FTS) is a novel farnesylated rigid carboxylic acid derivative, In cell-free systems, it acts as a potent competitive inhibitor (K-i = 2.6 mu M) of the enzyme prenylated protein methyltransferase (PPMTase), which methylates the carboxyl-te rminal S-prenylcysteine in a large number of prenylated proteins inclu ding has, In such systems, FTS inhibits Ras methylation but not Ras fa rnesylation. Inhibition of the PPMTase by FTS in homogenates or membra nes of a variety of tissues and cell lines is inferred from a block in the methylation of exogenously added substrates such as N-acetyl-S-tr ans,trans-farnesyl-L-cysteine and of endogenous substrates including s mall GTP-binding proteins, FTS can also inhibit methylation of these p roteins in intact cells (e.g. in Rat-1 fibroblasts, Ras transformed Ra t-1, and B16 melanoma cells), Unlike in cell-free systems, however, re latively high concentrations of FTS (50-100 mu M) are required for par tial blocking (10-40%) of protein methylation in the intact cells, Thu s, FTS is a weak inhibitor of methylation in intact cells, Because met hylation is the last step in the processing of Ras and related protein s, FTS is not likely to affect steps that precede it, e.g. protein pre nylation. This may explain why the growth and gross morphology of a va riety of cultured cell types (including Chinese hamster ovary, NIH3T3, Rat1, B16 melanoma, and PC12) is not affected by up to 25 mu M FTS an d is consistent with the observed lack of FTS-induced cytotoxicity. Ne vertheless, FTS reduces the levels of Ras in cell membranes and can in hibit Ras-dependent cell growth in vitro, independently of methylation , It inhibits the growth of human Ha-ras-transformed cells (EJ cells) and reverses their transformed morphology in a dose-dependent manner ( 0.1-10 mu M). The drug does not interfere with the growth of cells tra nsformed by v-Raf or T-antigen but inhibits the growth of ErbB2-transf ormed cells and blocks the mitogenic effects of epidermal and basic fi broblast growth factors, thus implying its selectivity toward Ras grow th signaling, possibly via modulation of Ras-Raf communication, Taken together, the results raise the possibility that FTS may specifically interfere with the interaction of Ras with a farnesylcysteine recognit ion domain in the cell membrane, This drug, and perhaps other farnesyl ated rigid carboxylic acid analogs, may be used for in vitro character ization of the PPMTase and for the identification of a putative Ras fa rnesylcysteine recognition domain in cell membranes.