No abstract

Depending on the cellular context, Ras can activate characteristic effectors by mechanisms still poorly understood. Promotion by galectin-1 of Ras activation of Raf-1 but not of phosphoinositide 3-kinase (PI3-K) is one such mechanism. In this report, we describe a mechanism controlling selectivity of K-Ras4B (K-Ras), the most important Ras oncoprotein. We show that galectin-3 acts as a selective binding partner of activated K-Ras. Galectin-3 co-immunoprecipitated significantly better with KRas-GTP than with K-Ras-GDP, H-Ras, or N-Ras and colocalized with green fluorescent protein-K-Ras(G12V), not with green fluorescent protein-H-Ras(G12V), in the cell membrane. Co-transfectants of K-Ras/galectin-3, but not of H-Ras/galectin-3, exhibited enhanced and prolonged epidermal growth factor-stimulated increases in Ras-GTP, Raf-1 activity, and PI3-K activity. Extracellular signal-regulated kinase (ERK) activity, however, was attenuated in K-Ras/galectin-3 and in K-Ras(G12V)/galectin-3 co-transfectants. Galectin-3 antisense RNA inhibited the epidermal growth factor-stimulated increase in K-Ras-GTP but enhanced ERK activation and augmented K-Ras(G12V) transformation activity. Thus, unlike galectin-1, which prolongs Ras activation of ERK and inhibits PI3-K, K-Ras-GTP/galectin-3 interactions promote, in addition to PI3-K and Raf-1 activation, a third inhibitory signal that attenuates active ERK. These experiments established a novel and specific mechanism controlling the duration and selectivity of signals of active K-Ras, which is extremely important in many human tumors.

This demonstrates a novel mechanism controlling the duration and selectivity of the Ras signal. Ras gains selectivity when it is associated with galectin-1, mimicking the selectivity of Ras(T35S), which activates Raf-1 but not PI3K.

Membrane anchorage of Ras oncoproteins, required for transforming activity, depends on their carboxy-terminal farnesylcysteine. We previously showed that S-trans,trans-farnesylthiosalicylic acid (FTS), a synthetic farnesylcysteine mimetic, inhibits growth of ErbB2- and Ras-transformed cells, but not of v-Raf-transformed cells, suggesting that FTS interferes specifically with Ras functions. Here we demonstrate that FTS dislodges Ras from membranes of H-Ras-transformed (EJ) cells, facilitating its degradation and decreasing total cellular Ras. The dislodged Ras that was transiently present in the cytosol was degraded relatively rapidly, causing a decrease of up to 80% in total cellular Ras. The 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 microM, comparable to concentrations (7-10 microM) required for 50% growth inhibition in EJ cells. Higher concentrations of FTS (25-50 microM) were required to dislodge Ras from Rat-1 cell membranes expressing normal Ras, suggesting some selectivity of FTS toward oncogenic Ras. Membrane localization of the prenylated G beta gamma of heterotrimeric G proteins 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, did not affect Ras. FTS did not inhibit growth of Rat-1 cells transformed 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 the cell membrane in a rather specific manner, rendering the protein susceptible to proteolytic degradation.