Genetic response to farnesyltransferase inhibitors: proapoptotic targets of RhoB.

Knockout mouse studies have established that the transformation-selective death program triggered by farnesyltransferase inhibitor (FTI) requires a gain-of-function in the stress-regulated small GTPase RhoB. To gain insight into this death program, we compared the genetic response of cells with different RhoB genotypes to FTI treatment. The microarray hybridization strategy we employed focused specifically on events preceding the execution of RhoB-dependent apoptosis, which is crucial for effective antineoplastic responses in mouse, rather than on other aspects of the FTI response mediated by RhoB gain-of-function (e.g., growth inhibition). Genes that control cell adhesion and cell shape were represented prominently among upregulated targets, as were genes that control signal transduction, vesicle dynamics, transcription, and immunity. Genes that control cell cycle checkpoints and progression through S phase and mitosis were among the major downregulated targets. In support of the concept of RhoB as a negative regulator of Ras signaling pathways, the most strongly downregulated gene scored was farnesyl pyrophosphate synthetase, the enzyme that produces the substrate used by FT to farnesylate Ras proteins. Gene clustering revealed modules for MAPK signaling, cell cycle progression, and immune response as proapoptotic targets of RhoB. This report identifies genes that pertain to the transformation-selective apoptotic program triggered by FTI. Further study of this program may yield insights into the dramatic differences in efficacy and apoptotic prowess of most FTIs in human cancers, versus transgenic mouse models.

Transformation-selective apoptotic program triggered by farnesyltransferase inhibitors requires Bin1.

Neoplastic transformation sensitizes many cells to apoptosis. This phenomenon may underlie the therapeutic benefit of many anticancer drugs, but its molecular basis is poorly understood. We have used a selective and potent farnesyltransferase inhibitor (FTI) to probe a mechanism of apoptosis that is peculiarly linked to neoplastic transformation. While nontoxic to untransformed mouse cells, FTI triggers a massive RhoB-dependent, p53-independent apoptosis in mouse cells that are neoplastically transformed. Here we offer evidence that the BAR adapter-encoding tumor suppressor gene Bin1 is required for this transformation-selective death program. Targeted deletion of Bin1 in primary mouse embyro fibroblasts (MEFs) transformed by E1A+Ras did not affect FTI-induced reversion, actin fiber formation, or growth inhibition, but it abolished FTI-induced apoptosis. The previously defined requirement for RhoB in these effects suggests that Bin1 adapter proteins act downstream or in parallel to RhoB in cell death signaling. The death defect in Bin1 null cells was significant insofar as it abolished FTI efficacy in tumor xenograft assays. p53 deletion did not phenocopy the effects of Bin1 deletion. However, MEFs transformed by SV40 large T antigen+Ras were also resistant to apoptosis by FTI, consistent with other evidence that large T inhibits Bin1-dependent cell death by a p53-independent mechanism. Taken together, the results define a function for Bin1 in apoptosis that is conditional on transformation stress. This study advances understanding of the functions of BAR adapter proteins, which are poorly understood, by revealing genetic interactions with an Rho small GTPase that functions in stress signaling. The frequent losses of Bin1 expression that occur in human breast and prostate cancers may promote tumor progression and limit susceptibility to FTI or other therapeutic agents that exploit the heightened sensitivity of neoplastic cells to apoptosis.