The association of mutant forms of Ras protein with a variety of human cancers has stimulated intense interest in therapies based on inhibiting oncogenic Ras signaling. Attachment of Ras proteins to the plasma membrane is required for effective Ras signaling and is initiated by the enzyme farnesyl protein transferase. We found that in the presence of potent farnesyl protein transferase inhibitors, Ras proteins in the human colon carcinoma cell line DLD-1 were alternatively prenylated by geranylgeranyl transferase-1. When H-Ras, N-Ras, K-Ras4A, and K-Ras4B were expressed individually in COS cells, H-Ras prenylation and membrane association were found to be uniquely sensitive to farnesyl transferase inhibitors; N-and K-Ras proteins incorporated the geranylgeranyl isoprene group and remained associated with the membrane fraction. The alternative prenylation of N-and K-Ras has significant implications for our understanding of the mechanism of action of farnesyl protein transferase inhibitors as anti-cancer chemotherapeutics.Newly synthesized Ras proteins are partitioned to the cytoplasmic face of the plasma membrane by a series of posttranslational modifications. The first step, catalyzed by the enzyme farnesyl protein transferase, is the addition of the 15-carbon isoprenyl group farnesyl to the sulfhydryl group of cysteine in the Ras carboxyl-terminal CAAX box (where C is cysteine, A is aliphatic, and X is typically Met or Ser) (1-3). Farnesylation is followed by proteolytic removal of the AAX amino acids and methylation of the carboxyl group of the farnesylated cysteine (4). Ras proteins at the plasma membrane cycle between an active GTP-bound state and an inactive GDPbound state. Mutations that stabilize the active GTP-bound state have been identified in over 30% of human tumors, with particularly high incidences in pancreatic (ϳ90%) and colon (ϳ50%) cancers. Four oncogenic Ras proteins have been described, H-Ras, N-Ras, K-Ras4A, and K-Ras4B. The majority of mutations associated with human cancer have been found in the K-Ras gene. The two K-Ras proteins are products of a single alternatively spliced transcript, with K-Ras4B the predominant isoform (Ͼ80%) (5, 6).Ras proteins that have been genetically modified so that they lack the isoprenylated cysteine do not associate with the plasma membrane and cannot transform fibroblasts (7). These genetic experiments provided the basis for the development of farnesyl transferase inhibitors (FTIs) 1 as anti-cancer agents. A number of reports have demonstrated that pharmacological inhibition of farnesyl protein transferase by CAAX analogs reduces anchorage-independent growth of Ras-transformed cells in soft agar (8) and slows growth of Ras-transformed cells in nude mice (9, 10). The FTIs appear relatively non-toxic in that they do not interfere with normal cell proliferation (11). This result was somewhat surprising because Ras function was shown to be necessary for normal growth factor signaling and cell proliferation (12). A mechanism through which cells may proliferate in...

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We are using a Rat-6 fibroblast cell line that stably overexpresses the P1 isozyme of protein kinase C (PKC)to study regulation of phospholipid hydrolysis by PKC.

Three flavonoid phospholipase A2 (PLA2) inhibitors were isolated from a MeOH extract of the bark of the Samoan medicinal plant Erythrina variegata. Two of these compounds, 4‘-hydroxy-3‘,5‘-diprenylisoflavonone (abyssinone V) (1) and 3,9-dihydroxy-2,10-diprenylpterocarp-6a-ene (erycrystagallin) (3), have been previously reported as antimicrobial agents from plants belonging to other Erythrina species. The isoflavonone 4‘-hydroxy-6,3‘,5‘-triprenylisoflavonone (2) is a new compound, and its structure was determined by spectroscopic techniques. The IC50 values for the PLA2 enzyme were 6, 10, and 3 μM for 1−3, respectively.