Inhibitors of farnesyl protein transferase and MEK1,2 induce apoptosis in fibroblasts transformed with farnesylated but not geranylgeranylated H-Ras

Farnesyl protein transferase inhibitors (FTIs) reverse the transformed phenotype of fibroblasts expressing activated H-Ras and block anchorage-independent growth and tumorigenesis of tumor cell lines independent of their Ras mutational status. FTIs induce significant tumor regression accompanied by apoptosis in several transgenic mouse tumor models. FTI treatment of tumor cells in vitro is proapoptotic under certain cell culture conditions. Induction of apoptosis by FTIs in vitro generally requires a second death-promoting signal. To better understand FTI-induced apoptosis we analyzed the effect of SCH 66336, a tricyclic FTI, on apoptosis of Ras-transformed Rat2 fibroblasts. Treatment of H-Ras-CVLS-transformed fibroblasts with MEK1,2 inhibitors provides a pharmacological second signal to enhance FTI-induced apoptosis. Simultaneous treatment of these cells with a MEK1,2 inhibitor markedly enhanced caspase-3 activity and the apoptotic response to SCH 66336. The combination treatment resulted in a more complete and sustained inhibition of MAPK pathway activity than observed with either drug alone. Surprisingly, after treatment with either agent alone or in combination, no apoptotic response was observed in Rat2 cells transformed with a geranylgeranylated form of H-Ras (H-Ras-CVLL). Differences were also observed when SCH 66336 treatment was combined with forced suspension growth or serum withdrawal, in that an increase in drug-induced apoptosis was observed in H-Ras-CVLS-transformed Rat2 cells but not H-Ras-CVLL-transformed Rat2 cells. The lack of apoptotic effect of SCH 66336 and MEK inhibitor, alone or in combination, in H-Ras-CVLL-transformed cells suggests a difference in the reliance of cells transformed with farnesylated and geranylgeranylated forms of H-Ras on the MAPK signal transduction cascade for survival. K-Ras-transformed cells underwent apoptosis upon MEK1,2 inhibition but not in response to SCH 66336 treatment. The apoptotic response induced by MEK1,2 inhibitors is much greater in magnitude in H-Ras-transformed cells than in K-Ras-transformed cells, also pointing to differences in pathway utilization and/or dependence for these two Ras isoforms.