Jackson B. Gibbs

Farnesyltransferase inhibition causes morphological reversion of ras-transformed cells by a complex mechanism that involves regulation of the actin cytoskeleton.

A potent and specific small molecule inhibitor of farnesyl-protein transferase, L-739,749, caused rapid morphological reversion and growth inhibition of ras-transformed fibroblasts (Rat1/ras cells). Morphological reversion occurred within 18 h of L-739,749 addition. The reverted phenotype was stable for several days in the absence of inhibitor before the transformed phenotype reappeared. Cell enlargement and actin stress fiber formation accompanied treatment of both Rat1/ras and normal Rat1 cells. Significantly, inhibition of Ras processing did not correlate with the initiation or maintenance of the reverted phenotype. While a single treatment with L-739,749 was sufficient to morphologically revert Rat1/ras cells, repetitive inhibitor treatment was required to significantly reduce cell growth rate. Thus, the effects of L-739,749 on transformed cell morphology and cytoskeletal actin organization could be separated from effects on cell growth, depending on whether exposure to a farnesyl-protein transferase inhibitor was transient or repetitive. In contrast, L-739,749 had no effect on the growth, morphology, or actin organization of v-raf-transformed cells. Taken together, the results suggest that the mechanism of morphological reversion is complex and may involve farnesylated proteins that control the organization of cytoskeletal actin.

Farnesyltransferase inhibitors versus Ras inhibitors

Over the past few years, the idea that farnesyl-protein transferase (FPTase) inhibitors might be effective antiproliferative/antitumor agents has been realized in studies of cultured cells and in rodent models of cancer. Most of the studies with FPTase inhibitors have focused on inhibiting the growth of ras-transformed cells in vitro or the growth of ras-dependent tumors in mice. More recently, it has been recognized that the antiproliferative effect of FPTase inhibitors may extend beyond ras-driven tumors. It now seems likely that the ability of FPTase inhibitors to reverse the malignant phenotype results, at least in part, from inhibiting the farnesylation of proteins other than Ras.

Pathways of Ras Function: Connections to the Actin Cytoskeleton

Publisher Summary Ras plays a central role in the cellular responses to many factors—namely, guanine nucleotide exchange factors (GEFs), guanine nucleotide dissociation stimulators (GDIs), and Guanine triphosphatase (GTPase)-activating proteins (GAPS). The recent advances in the biological and biochemical mechanisms of signal transduction involving Ras proteins are also reviewed in the chapter. The evidence connecting Ras to two intracellular signal transduction pathways that control transcription and cytoskeletal actin organization is also summarized in the chapter. Model for signal transduction in upstream and downstream pathways of Ras—mitogen-activated protein kinase (MAPK) pathway and Rho/Rac pathway—is presented in the chapter. MAPK pathway includes a set of cytoplasmic growth factor-regulated serine/threonine kinases that act on both ribosomal and nuclear factors, whereas the Rho/Rac pathway includes a set of Ras-related GTP-binding proteins that control the spatial pattern of polymerized actin in the cell. The model hypothesizes the sequential formation of multiprotein complexes at or near the plasma membrane, whose signaling capabilities lead to changes in nuclear processes and cytoskeletal actin organization. The nomenclatures of Ras signal transduction pathway molecules are also tabulated in the chapter. Perspectives on Ras function from studies on the phenotypic reversion of Ras-transformed cells are also considered in the chapter.

Imidazole-containing diarylether and diarylsulfone inhibitors of farnesyl-protein transferase

The design and syntheses of non-thiol inhibitors of farnesyl-protein transferase are described. Optimization of cysteine-substituted diarylethers led to highly potent imidazole-containing diarylethers and diarylsulfones. Polar diaryl linkers dramatically improved potency and gave highly cell active compounds.

Chaetomella acutiseta produces chaetomellic acids A and B which are reversible inhibitors of farnesyl-protien transferase

Chaetomellic acids A and B, isolated from Chaetomella acutiseta, are specific inhibitors of farnesyl-protein transferase that do not inhibit geranylgeranyl transferase type 1 or squalene synthase. Chaetomellic acids A and B are reversible inhibitors, resemble farnesyl diphosphate and probably inhibit FPTase by substituting for farnesyl diphosphate. Chaetomellic acid production appears to be widespread within the genus Chaetomella.

Farnesyltransferase inhibitors and anti-Ras therapy

SummaryThe oncoprotein encoded by mutantras genes is initially synthesized as a cytoplasmic precursor which requires posttranslational processing to attain biological activity; farnesylation of the cysteine residue present in the CaaX motif located at the carboxy-terminus of all Ras proteins is the critical modification. Once farnesylated and further modified, the mature Ras protein is inserted into the cells plasma membrane where it participates in the signal transduction pathways that control cell growth and differentiation. The farnesylation reaction that modifies Ras and other cellular proteins having an appropriate CaaX motif is catalyzed by a housekeeping enzyme termed farnesyl-protein transferase (FPTase). Inhibitors of this enzyme have been prepared by several laboratories in an effort to identify compounds that would block Ras-induced cell transformation and thereby function as Ras-specific anticancer agents. A variety of natural products and synthetic organic compounds were found to block farnesylation of Ras proteinsin vitro. Some of these compounds exhibit antiproliferative activity in cell culture, block the morphological alterations associated with Ras-transformation, and can block the growth of Ras-transformed cell lines in tumor colony-forming assays. By contrast, these compounds do not affect the growth or morphology of cells transformed by the Raf or Mos oncoproteins, which do not require farnesylation to achieve biological activity. The efficacy and lack of toxicity observed with FPTase inhibitors in an animal tumor model suggest that specific FPTase inhibitors may be useful for the treatment of some types of cancer.

Synthesis and biological activity of ras farnesyl protein transferase inhibitors. Tetrapeptide analogs with amino methyl and carbon linkages.

Replacement of the central amino methylene linkage of C[psi CH2NH]A[psi CH2NH]AX tetrapeptide inhibitors with carbon tethers led to compounds with potency in the nanomolar range. Some of the more potent olefinic compounds inhibit Ras processing in intact v-ras transformed NIH 3T3 cells with IC50 values in the 0.1 to 1 microM range, and inhibit selectively the anchorage-independent growth of H-ras transformed Rat1 cells at 10 microM.

Non-thiol 3-aminomethylbenzamide inhibitors of farnesyl-protein transferase.

The design and syntheses of non-thiol inhibitors of farnesyl-protein transferase are described. Substitutions on an imidazolylmethyl-AMBA-methionine template gave a highly potent and cell-active inhibitor.

Oxo-piperazine Derivatives of N-Arylpiperazinones as Inhibitors of Farnesyltransferase

The evaluation of SAR associated with the insertion of carbonyl groups at various positions of N-arylpiperazinone farnesyltransferase inhibitors is described herein. 1-Aryl-2,3-diketopiperazine derivatives exhibited the best balance of potency and pharmacokinetic profile relative to the parent 1-aryl-2-piperazinones.

Actinoplanic acids A and B as novel inhibitors of farnesyl-protein transferase

Actinoplanic acids A and B are macrocyclic polycarboxylic acids that are potent reversible inhibitors of farnesyl-protein transferase. Actinoplanic acids A and B were isolated from Actinoplanes sp. MA 7066 while actinoplanic acid B was isolated from both MA 7066 and Streptomyces sp. MA 7099. Actinoplanic acids A and B are competitive with respect to farnesyl diphosphate and are selective inhibitors of farnesyl-protein transferase because they do not inhibit geranylgeranyl-protein transferase type 1 or squalene synthase. MA 7066 is believed to be a novel species of actinomycetes while MA 7099 is believed to be a novel strain of Streptomyces violaceusniger on the basis of morphological, biochemical and chemotaxonomic characteristics as well as its production of actinoplanic acids.