Farnesyl transferase inhibitors induce apoptosis of Ras-transformed cells denied substratum attachment.

Farnesyl transferase inhibitors (FTIs) are a novel class of antitumor drugs that block the oncogenic activity of Ras. Because FTIs lack significant cell toxicity in vitro and in vivo, a significant question is how they cause tumor regression. We now report that FTIs are in fact potent activators of apoptosis in Ras-transformed cells if attachment to substratum is prevented. When cultured at high density or on polyHEMA, a nonadherent substrate, Ras-transformed cells exhibited massive DNA degradation and cell death within 24 h of treatment with the FTI L-739,749. Death was p53-independent and was inhibited by the apoptosis suppressor BCL-XL. Furthermore, apoptosis was significantly attenuated by ectopic expression of a farnesyl-independent form of RhoB, a Rho protein previously implicated as a critical target for inhibition by FTIs. The findings suggest a link between FTIs and Rho-dependent adhesion signaling. Furthermore, our work indicates that FTIs revert cells to a state in which cell-substratum attachment is necessary for viability and suggests that apoptosis forms the basis for drug-induced tumor regression.