FARNESYLTRANSFERASE INHIBITORS BLOCK THE NEUROFIBROMATOSIS TYPE-I (NF1) MALIGNANT PHENOTYPE

Neurofibromatosis type I (NF1) is a hereditary tumor and developmental disorder whose defective gene was cloned previously. The protein prod uct of the NF1 gene, neurofibromin, contains a domain that shows signi ficant sequence homology to the known catalytic domains of mammalian R as GTPase-activating proteins (GAP) and the yeast IRA1 and IRA2 protei ns. This homologous region of neurofibromin has been shown to exhibit GAP activity toward Ras proteins. Malignant schwannoma cell lines from NF1 patients contain normal levels of GAP and nonmutated Pas proteins but barely detectable levels of neurofibromin, based on genetic mutat ions in the NF1 gene. Because these cells contain constitutively activ ated Ras.GTP, it has been proposed that neurofibromin may be the sole negative regulator of Ras in these cells. Overall, these results have implied an important role of the Ras signaling pathway in NF1 malignan t schwannomas. Recently, several laboratories have developed small mol ecule inhibitors of Ras function that inhibit the enzyme farnesyltrans ferase (FT). PT-mediated post-translational farnesylation of Ras prote ins is absolutely necessary for Ras function since this modification i s required for the anchoring of Ras proteins to the plasma cell membra ne. Although previous studies have shown that FT inhibitors can block the growth of tumor cells carrying mutant Ras proteins, it remained un clear how this class of inhibitors would affect tumor cells such as in NF1, whose malignant growth appears to be mediated by up-regulation o f wild-type Ras activity. Thus, in the current study, we investigated whether BMS-186511, a bisubstrate analogue inhibitor of FT, would inhi bit the malignant growth properties of a cell line established from ma lignant schwannoma of an NF1 patient. Our results indicate that the ma lignant growth properties of ST88-14 cells, the most malignant cell li ne among several well-characterized NF1 cells, are inhibited by BMS-18 6511 in a concentration-dependent manner. Following treatment with BMS -186511, ST88-14 cells became flat, nonrefractile, were contact-inhibi ted, and lost their ability to grow in soft agar. In the drug-exposed cells, Ras proteins were prevented from FT-mediated membrane associati on. BMS-186511 was found to specifically inhibit FT, but not geranylge ranyltransferase I, a closely related enzyme. Thus, it is conceivable that FT inhibitors may ultimately become the first generation of drugs against the malignant phenotype in NF1 based on rational insights int o the mechanism of action of neurofibromin.