The farnesyltransferase inhibitor R115777 (ZARNESTRA®) enhances the pro-apoptotic activity of interferon-α through the inhibition of multiple survival pathways

interferon a (IFN alpha) induces an EGF-Ras -> Raf-1 -> Erk dependent survival pathway counteracting apoptosis induced by the cytokine. In this paper we have evaluated the effects of the combination between farnesyl-transferase inhibitor (FTI) R115777 and IFN alpha on the growth inhibition and apoptosis of cancer cells. Simultaneous exposure to R115777 and IFN alpha produced synergistic both antiproliferative and proapoptotic effects. In these experimental conditions, IFN alpha and R115777 completely antagonized the increased activity of both Ras and Erk-1/2 induced by IFN alpha and strongly reduced Akt activity. Furthermore, treatment with R115777 in combination with IFN alpha regimen induced tumor growth delay on established KB cell xenografts in nude mice, while the single agents were almost inactive. R115777 was again able to antagonize the Ras-dependent survival pathway induced by IFN alpha also in vivo. Raf-1, one of the downstream targets of Ras, has been reported to activate bcl-2 through displacement and/or phosphorylation of Bad. We have found that IFN alpha induced mitochondrial localization of Raf-1 that was antagonized by R115777. Moreover, IFN alpha increased Raf-1/bcl-2 immuno-conjugate formation and intracellular co-localization and enhanced phosphorylation of Bad at Ser 112 and again R115777 counteracted all these effects. Moreover, the use of plasmids encoding for dominant negative or dominant positive Raf-1 antagonized and potentiated, respectively, the co-immunoprecipitation between Raf-1 and bcl2. In conclusion, FTI R115777 strongly potentiates the antitumor activity of IFNa both in vitro and in vivo through the inhibition of different survival pathways that are dependent from isoprenylation of intracellular proteins such as ras. (c) 2007 Wiley-Liss, Inc.