Driven to death: Inhibition of farnesylation increases Ras activity in osteosarcoma and promotes growth arrest and cell death

To improve cancer outcomes, investigators are turning increasingly to small-molecule medicines that disrupt vital signaling cascades, inhibit malignant growth, or induce apoptosis. One vital signaling molecule is Ras, and a key step in Ras activation is membrane anchoring of Ras through prenylation, the C-terminal addition of a lipid anchor. Small-molecule inhibitors of farnesyltransferase (FTI), the enzyme most often responsible for prenylating Ras, showed clinical promise, but development of FTIs such as tipifarnib has been stalled by uncertainty about their mechanism of action, because Ras seemed unimpeded in tipifarnib-treated samples. Interpretation was further complicated by the numerous proteins that may be farnesylated, as well as availability of an alternate prenylation pathway, geranylgeranylation. Initial observations of varied response by osteosarcoma cell lines to tipifarnib led us to evaluate the role of FTI in Ras signal alteration using osteosarcoma models. We describe our novel counterintuitive finding that endogenous Ras activity increases in osteosarcoma when farnesyltransferase is inhibited by either tipifarnib or short hairpin RNA. In response to tipifarnib, variable growth arrest and/or cell death correlated with levels of activated extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK). Sensitivity to tipifarnib treatment was shown by growth inhibition and by an increase in subdiploid cell numbers; cells with such sensitivity had increased activation of ERK and p38 MAPK. Because Ras must be prenylated to be active, our findings suggest that geranylgeranylated N-Ras or K-Ras B interacts differently with downstream effector proteins in sensitive osteosarcoma cells responding to tipifarnib, switching the balance from cell proliferation to growth inhibition.