Prevention of farnesylation of c‐Ha‐Ras protein enhances synergistically the cytotoxic action of doxorubicin in cycling but not in quiescent cells

Ras, the product of a proto‐oncogene, is a GTP‐hydrolyzing enzyme found mutated in approximately 50% of human cancers. “Gain of function” mutations of Ras lead to an escape of transformed cells from cell‐cycle control, rendering them independent to stimulation by growth factors, giving them almost unlimited proliferation capacity. The cytosolic precursor isoform of Ras is biologically inactive. After several post‐translational modifications, Ras is anchored to the plasma membrane and, thereby, the protein becomes activated. The finding that lipid modifications of Ras protein, particularly farnesylation, are essential for its signal transduction activity, gave rise to the concept that blocking farnesyl protein transferase (FPTase), the enzyme catalyzing the first step in the Ras modification cascade, would prevent proper membrane anchoring and provide an improved approach in the cure of tumors harboring Ras mutations. In the present study we used transformed rat cells overexpressing a temperature‐sensitive p53 protein, adopting wt conformation at 32°C and mutant conformation at 37°C. We treated the cells growing at 32 or 37°C with doxorubicin alone, or in combination with inhibitors of FPTase. Combined treatment was more efficient and the same inhibition of cell proliferation was reached at lower DOX concentrations. The treatment strongly affected the growth rate of tumor cells but only negligibly of normal cells. However, the inhibitors of FPTase prevented the membrane anchoring in both situations. These results show two striking advantages of the combined treatment: the desired cytostatic effect on tumor cells at lower drug concentrations and clearly reduced adverse effects on quiescent cells. J. Cell. Biochem. 99: 1664–1676, 2006. © 2006 Wiley‐Liss, Inc.