Abstract 4415: Role for the protein farnesyltransferase in the regulation of microtubule dynamics

Abstract

Abstract The protein farnesyltransferase (FTase) is well characterized for its prenylation activity whereby a lipid farnesyl moiety is added to substrate proteins with a C-terminal CaaX motif. Farnesylation of the Ras oncogene was shown to be important for both its plasma membrane localization and its downstream tumorigenic signaling. Inhibitors of FTase (FTIs) were subsequently developed to inhibit Ras-dependent tumorigenesis, but soon it was found that Ras could be alternatively geranylgeranylated and remain active. However, it was also found that FTIs retain some inhibition of tumor growth activity even when Ras is alternatively geranylgeranylated, suggesting that FTIs exert their antitumor activity through other proteins than Ras. In preclinical studies, FTIs appeared to be very synergistic with microtubule-stabilizing agent such as the taxanes. In trying to understand the underlying mechanism for this synergy, we have demonstrated that FTIs increase tubulin acetylation by inhibiting the tubulin deacetylase HDAC6 and stabilizing microtubules, which in turn favors paclitaxel binding and eventually leads to mitotic arrest and cell death. We have found that FTase, microtubules and HDAC6 are part of the same protein complex and more interestingly that FTase directly bind microtubules. The interaction between FTase and microtubules was abolished in vivo upon treatment of cells with FTIs. The direct binding of FTase to microtubules requires the presence of the C-terminal end of tubulin, a known binding site for Microtubule-Associated Proteins (MAPs), and is reduced in presence of MAPs. Together with the observed reduction of microtubules dynamics in cells after treatment with FTIs, these results suggest a role for FTase in the regulation of microtubule dynamics. To test this hypothesis, we used an in vitro tubulin polymerization assay using purified bovine brain tubulin in presence or absence of recombinant purified FTase. Preliminary experiments showed that FTase greatly reduced both the rate and extent of tubulin polymerization when FTase was added directly to soluble tubulin dimers, but had no effect on preformed microtubules. Interestingly, tubulin is not a substrate for farnesylation as it does not contain a CaaX motif, and thus the effect of FTase on tubulin polymerization appears to reveal a new role for FTase in cells. Experiments are underway to determine whether the farnesyltransferase activity is required for the regulation of microtubules and to further elucidate the importance of this regulation in cells. A better understanding of this FTase-tubulin interaction and their role in vitro and in vivo might lead to the development of inhibitors that could specifically affect this interaction without altering farnesylated proteins. (Supported by NCI 5P01CA116676 and NCI 5F32CA134148-02) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4415.