Synthesis of imidazole-containing analogues of farnesyl pyrophosphate and evaluation of their biological activity on protein farnesyltransferase

With the aim of creating new bisubstrate inhibitors of protein farnesyltransferase (FTase), new carboxylic farnesyl pyrophosphate analogues have been designed and synthesized. The original structures are built around three elements: a prenyl moiety, a 1,4-diacid motif and an imidazole ring. All the compounds were evaluated for their ability to inhibit FTase and compared with the corresponding derivatives lacking the imidazole ring, synthesized for that purpose. These new compounds are not bisubstrate inhibitors probably because the imidazole ring is not in the right position to interact with the zinc atom. However these derivatives display FPP competitive inhibition with a good activity in the carboxylic farnesyl pyrophosphate analogues series.     

Towards the synthesis of bisubstrate inhibitors of protein farnesyltransferase: Synthesis and biological evaluation of new farnesylpyrophosphate analogues

Protein farnesyltransferase (FTase) has recently appeared as a new target of parasitic diseases, a field poor in drugs in development. With the aim of creating new bisubstrate inhibitors of FTase, new farnesyl pyrophosphate analogues have been studied. Farnesyl analogues with a malonic acid function exhibited the best inhibitory activity on FTase. This group was introduced into our imidazole-containing model leading to new compounds with submicromolar activities. Kinetic experiments have been realized to determine their binding mode to the enzyme.     

Inhibition of purified p21ras farnesyl:protein transferase by Cys-AAX tetrapeptides

We report the identification, purification, and characterization of a farnesyl:protein transferase that transfers the farnesyl moiety from farnesyl pyrophosphate to a cysteine in p21ras proteins. The enzyme was purified approximately 60,000-fold from rat brain cytosol through use of a chromatography step based on the enzyme's ability to bind to a hexapeptide containing the consensus sequence (Cys-AAX) for farnesylation. The purified enzyme migrated on gel filtration chromatography with an apparent molecular weight of 70,000-100,000. High resolution SDS-polyacrylamide gels showed two closely spaced approximately 50 kd protein bands in the final preparation. The enzyme was inhibited competitively by peptides as short as 4 residues that contained the Cys-AAX motif. These peptides acted as alternative substrates that competed with p21H-ras for farnesylation. Effective peptides included the COOH-terminal sequences of all known p21ras proteins as well as those of lamin A and B.     

Dexamethasone-induced decrease in HMG-CoA reductase and protein-farnesyl transferase activities does not impair ras processing in AR 4-2 J cells

Rat pancreatic acinar cells AR 4-2J respond to dexamethasone by differentiation and a decreased proliferation rate. Protein labelling by [³H]-mevalonolactone, used as a precursor of farnesyl and geranylgeranyl isoprenoid groups, was increased in the presence of dexamethasone. In these same conditions, dexamethasone decreased HMG-CoA reductase activity, leading to a diminished isotopic dilution of the mevalonate precursor. As ras proteins, known to be involved in the regulation of proliferation and differentiation, need to be farnesylated for full biological function, we also measured the level of farnesyl transferase activity and found a dose-dependent decrease in dexamethasone treated cells. Despite these negative effects of dexamethasone on mevalonate pathway, there was no appearance of non-isoprenylated forms of ras, indicating that the level of isoprenoid precursors and farnesyl transferase activity were not limiting in this model.     

Oxytocin analogues with amide groups substituted by tetrazole groups in position 4, 5 or 9

Eleven oxytocin analogues substituted in position 4, 5 or 9 by tetrazole analogues of amino acids were prepared using solid-phase peptide synthesis method and tested for rat uterotonic in vitro and pressor activities, as well as for their affinity to human oxytocin receptor. The tetrazolic group has been used as a bioisosteric substitution of carboxylic, ester or amide groups in structure-activity relationship studies of biologically active compounds. Replacement of the amide groups of Gln(4) and Asn(5) in oxytocin by tetrazole analogues of aspartic, glutamic and alpha-aminoadipic acids containing the tetrazole moiety in the side chains leads to analogues with decreased biological activities. Oxytocin analogues in which the glycine amide residue in position 9 was substituted by tetrazole analogues of glycine had diminished activities as well. The analysis of differences in rat uterotonic activity and in the affinity to human oxytocin receptors of analogues containing either an acidic 5-substituted tetrazolic group or a neutral 1,5- or 2,5-tetrazole nucleus makes it possible to draw some new conclusions concerning the role of the amide group of amino acids in positions 4, 5 and 9 of oxytocin for its activity. The data suggest that the interaction of the side chain of Gln(4) with the oxytocin receptor is influenced mainly by electronic effects and the hydrogen bonding capacity of the amide group. Steric effects of the side chain are minor. Substitution of Asn(5) by its tetrazole derivative gave an analogue of very low activity. The result suggests that in the interaction between the amide group of Asn(5) and the binding sites of oxytocic receptor hydrogen bonds are of less importance than the spatial requirements for this group.     

Non-peptidic, non-prenylic bisubstrate farnesyltransferase inhibitors. Part 3: structural requirements of the central moiety for farnesyltransferase inhibitory activity

Recently, we have described non-peptidic, non-prenylic bisubstrate analogues as a novel type of farnesyltransferase inhibitor composed of a farnesyl-mimetic, a linker and an AAX-peptidomimetic substructure. With this study, we showed that the amide function connecting the farnesyl-mimetic and the linking substructures of our inhibitors is crucial for their activity. We suggest that the amide is bound to the essential zinc ion in the farnesyltransferases active center. We identified succinic and glutaric acid, respectively, in addition to the initially used 1-alanyl moiety as suitable linking structures. Glycine can also be used in this function provided the distance between the alpha-amide group and the center of the peptidomimetic substructure is enlarged by introduction of an additional methylene unit into the peptidomimetic substructure.     

A CAAX or a CAAL motif and a second signal are sufficient for plasma membrane targeting of ras proteins.

Mutational analysis of p21ras has shown that plasma membrane targeting requires the combination of a CAAX motif with a polybasic domain of six lysine residues or a nearby palmitoylation site. However, it is not known from these studies whether these signals alone target p21ras to the plasma membrane. We now show that these C-terminal sequences are sufficient to target a heterologous cytosolic protein to the plasma membrane. Interestingly, the key feature of the p21K-ras(B) polybasic domain appears to be a positive charge, since a polyarginine domain can function as a plasma membrane targeting motif in conjunction with the CAAX box and p21K-ras(B) with the polylysine domain replaced by arginines is biologically active. Since some ras-related proteins are modified by geranylgeranyl rather than farnesyl we have investigated whether modification of p21ras with geranylgeranyl affects its subcellular localization. Geranylgeranyl can substitute for farnesyl in combining with a polybasic domain to target p21K-ras(B) to the plasma membrane, but such geranylgeranylated proteins are more tightly bound to the membrane. This increased avidity of binding is presumably due to the extra length of the geranylgeranyl alkyl chain.     

Posttranslational modification of proteins by isoprenoids in mammalian cells

Isoprenylation is a posttranslational modification that involves the formation of thioether bonds between cysteine and isoprenyl groups derived from pyrophosphate intermediates of the cholesterol biosynthetic pathway. Numerous isoprenylated proteins have been detected in mammalian cells. Those identified include K-, N-, and H-p21ras, ras-related GTP-binding proteins such as G25K (Gp), nuclear lamin B and prelamin A, and the gamma subunits of heterotrimeric G proteins. The modified cysteine is located in the fourth position from the carboxyl terminus in every protein where this has been studied. For p21ras, the last three amino acids are subsequently removed and the exposed cysteine is carboxylmethylated. Similar processing events may occur in lamin B and G protein gamma subunits, but the proteolytic cleavage in prelamin A occurs upstream from the modified cysteine. Lamin B and p21ras are modified by C15 farnesyl groups, whereas other proteins such as the G protein gamma subunits are modified by C20 geranylgeranyl chains. Separate enzymes may catalyze these modifications. The structural features that govern the ability of particular proteins to serve as substrates for isoprenylation by C15 or C20 groups are not completely defined, but studies of the p21ras modification using purified farnesyl:protein transferase suggest that the sequence of the carboxyl-terminal tetrapeptide is important. Isoprenylation plays a critical role in promoting the association of p21ras and the lamins with the cell membrane and nuclear envelope, respectively. Future studies of the role of isoprenylation in the localization and function of ras-related GTP-binding proteins and signal-transducing G proteins should provide valuable new insight into the link between isoprenoid biosynthesis and cell growth.     

Polyisoprenoid amphiphilic compounds as inhibitors of squalene synthesis and other microsomal enzymes.

Analogues of farnesyl pyrophosphate containing a farnesyl moiety and a variety of amine residues replacing the pyrophosphate have been synthesized. Most of these compounds were effective inhibitors of the synthesis of squalene and presqualene pyrophosphate from farnesyl pyrophosphate. 50% inhibition was obtained at concentrations between 50 and 100 micron. These analogues also inhibited other microsomal enzymes so they apparently function as general inhibitors of microsomal enzymes.     

Selective modification of CaaX peptides with ortho-substituted anilinogeranyl lipids by protein farnesyl transferase: competitive substrates and potent inhibitors from a library of farnesyl diphosphate analogues

Protein farnesyl transferase (FTase) catalyzes transfer of a 15-carbon farnesyl group from farnesyl diphosphate (FPP) to a conserved cysteine in the C-terminal Ca1a2X motif of a range of proteins ("C" refers to the cysteine, "a" to any aliphatic amino acid, and "X" to any amino acid), and the lipid chain interacts with, and forms part of, the Ca1a2X peptide binding site. Here, we employed a library of anilinogeranyl diphosphate (AGPP) derivatives to examine whether altering the interacting surface between the two substrates could be exploited to generate Ca1a2X peptide selective FPP analogues. Analysis of transfer kinetics to dansyl-GCVLS peptide revealed that AGPP analogues with substituents smaller than or equal in size to a thiomethyl group supported FTase function, while analogues with larger substituents did not. Analogues with small meta-substitutions on the aniline ring such as iodo and cyano increased reactivity with dansyl-GCVLS and provided analogues that were effective FPP competitors. Other analogues with ortho-substitutions on the aniline were potent dansyl-GCVLS modification FTase inhibitors (Ki in the 2.4-18 nM range). Both meta- and para-trifluoromethoxy-AGPP are transferred to dansyl-GCVLS while the ortho-substituted isomer was a potent farnesyl transferase inhibitor (FTI) with an inhibition constant Ki = 3.0 nM. In contrast, ortho-trifluoromethoxy-AGPP was efficiently transferred to dansyl-GCVIM. Competition for dansyl-GCVLS and dansyl-GCVIM peptides by FPP and ortho-trifluoromethoxy-AGPP gave both analogue and farnesyl modified dansyl-GCVIM but only farnesylated dansyl-GCVLS. We provide evidence that competitive modification of dansyl-GCVIM by ortho-trifluoromethoxy-AGPP stems from a prechemical step discrimination between the competing peptides by the FTase-analogue complex. These results show that subtle changes engineered into the isoprenoid structure can alter the reactivity and FPP competitiveness of the analogues, which may be important for the development of prenylated protein function inhibitors.     

Inhibition ofras oncogene: A novel approach to antineoplastic therapy

The most frequently detected oncogene alterations, both in animal and human cancers, are the mutations in the ras oncogene family. These oncogenes are mutated or overexpressed in many human tumors, with a high incidence in tumors of the pancreas, thyroid, colon, lung and certain types of leukemia. Ras is a small guanine nucleotide binding protein that transduces biological information from the cell surface to cytoplasmic components within cells. The signal is transduced to the cell nucleus through second messengers, and it ultimately induces cell division. Oncogenic forms of p21(ras) lead to unregulated, sustained signaling through downstream effectors. The ras family of oncogenes is involved in the development of both primary tumors and metastases making it a good therapeutic target. Several therapeutic approaches to cancer have been developed pointing to reducing the altered gene product or to eliminating its biological function: (1) gene therapy with ribozymes, which are able to break down specific RNA sequences, or with antisense oligonucleotides, (2) immunotherapy through passive or active immunization protocols, and (3) inhibition of p21(ras) farnesylation either by inhibition of farnesyl transferase or synthesis inhibition of farnesyl moieties.     

Protein farnesyltransferase and geranylgeranyltransferase share a common alpha subunit

Mammalian farnesyltransferase, which attaches a 15 carbon isoprenoid, farnesyl, to a cysteine in p21ras proteins, contains two subunits, alpha and beta. The beta subunit is known to bind p21ras proteins. We show here that the alpha subunit is shared with another prenyltransferase that attaches 20 carbon geranylgeranyl to Ras-related proteins. Farnesyltransferase and geranylgeranyltransferase have similar molecular weights on gel filtration, but are separated by ion exchange chromatography. Both enzymes are precipitated and immunoblotted by multiple antibodies directed against the alpha subunit of farnesyltransferase. The two transferases have different specificities for the protein acceptor; farnesyltransferase prefers methionine or serine at the COOH-terminus and geranylgeranyltransferase prefers leucine. The current data indicate that both prenyltransferases are heterodimers that share a common alpha subunit with different beta subunits.     

Analogues of a potent oxytocin antagonist with truncated C-terminus or shorter amino acid side chain of the basic amino acid at position 8.

Twelve analogues were synthesized, their structure derived from modifications of [(S)Pmp1, D-Trp2, Pen6, Arg8]oxytocin, PA, in which (S)Pmp = beta,beta-(3-thiapentamethylene-beta-mercaptopropionic acid). PA is a potent antagonist of the uterotonic effect of oxytocin in the rat (uterotonic test in vitro, pA2 = 8.86) and in the baboon. Truncated analogues of PA from the C-terminus were systematically prepared ending in either the free acid or the amide, i.e. PA1-9 acid, PA1-8 acid, PA1-7 acid, PA1-6 acid, PA1-8 amide, PA1-7 amide and PA1-6 amide. PA1-8 amide was roughly as potent as PA in the rat uterotonic assay in vitro, and the shorter amides were only somewhat weaker antagonists. All four acid analogues were weaker antagonists than PA but still maintained rather high antagonistic potency. These findings suggest that, if these truncated acids form as metabolites in vivo, they may contribute to the overall biological effect of PA and their contribution should be taken into account. Furthermore, using these analogues, the radioimmunoassay measurements of PA may be standardized, as they may cross react with PA antibodies and interfere with the determination. In addition, five analogues were made by substituting Arg8 of PA with Lys, Orn8, Dab8, Dap8 and Cit8. All of these analogues maintained high potency as OTAs in the uterotonic assay, although their activity was only about 1.5-3 times lower than PA. The most potent analogue in the uterotonic assay, [Dap8]PA, pA2 = 8.53, had weak pressor activity (pA2 = 6.90) and no antidiuretic effect. The pressor activity was lower for all tested acids, and for PA1-6 acid it was even below the detection limit. Additionally, PA1-9 acid, PA1-7 acid and PA1-6 acid showed no antidiuretic activity. Hence, the PA1-6 acid is a potent OTA with pA2 = 8.27 and no measurable effect in the pressor or antidiuretic tests and thus it is a pure oxytocin antagonist. This fact makes it an attractive candidate for further studies on inhibition of OT biological effects and on preterm labour.     

A novel prenyltransferase for a small GTP-binding protein having a C-terminal Cys-Ala-Cys structure

smg p25A/rab3A p25 is a member of the small GTP-binding protein superfamily which is implicated in intracellular vesicle transport. smg p25A has a cDNA-predicted C-terminal structure of Cys-Ala-Cys. The protein purified from bovine brain membranes is geranylgeranylated at both the two cysteine residues and carboxyl-methylated at the C-terminal cysteine residue. Two types of prenyltransferase for small GTP-binding proteins have thus far been reported: ras p21 farnesyltransferase (ras p21 FT) and rhoA p21 geranylgeranyltransferase (rhoA p21 GGT). Neither of them geranylgeranylated smg p25A having a C-terminal Cys-Ala-Cys structure. In this paper, a smg p25A GGT was partially purified from bovine brain cytosol and separated from the ras p21 FT and rhoA p21 GGT by column chromatographies. smg p25A GGT transferred the geranylgeranyl moiety from geranylgeranyl pyrophosphate to both the two cysteine residues in the C-terminal Cys-Ala-Cys structure of smg p25A. smg p25A GGT did not use farnesyl pyrophosphate as a substrate and was also inactive on c-Ha-ras p21 and rhoA p21 with either farnesyl pyrophosphate or geranylgeranyl pyrophosphate as a substrate. These results indicate that there are at least three types of prenyltransferase for small GTP-binding proteins in mammalian tissues.     

Evidence that oocyte maturation induced by an oncogenic ras-p21 protein and insulin is mediated by overlapping yet distinct mechanisms.

We have recently shown that a peptide (residues 35-47) from a functional region of the ras p21 protein, thought to be involved in the binding of p21 to GTPase activating protein, the antibiotic azatyrosine, known to induce the ras-recision gene, and the selective protein kinase C inhibitor, CGP 41,251, all inhibit oncogenic p21 protein-induced maturation of oocytes in a dose-dependent manner. We now show that these three agents only partially inhibit insulin-induced oocyte maturation, known to be dependent on activation of cellular p21 protein. On the other hand, the anti-p21 protein antibody Y13-259 completely inhibits both insulin- and oncogenic p21 protein-induced maturation as does a tetrapeptide, CVIM, known to block the enzyme farnesyl transferase which covalently attaches the farnesyl moiety to the p21 protein allowing it to attach to the cell membrane. Our results suggest that while the oncogenic and insulin-activated normal p21 proteins share certain elements of their signal transduction pathways in common, these pathways diverge and allow for selective inhibition of the oncogenic pathway.     

Non-peptidic, non-prenylic inhibitors of the prenyl protein-specific protease Rce1

Several compounds designed as bisubstrate analogues of protein farnesyltransferase inhibited the prenyl protein-specific protease Rce1, qualifying them as lead structures for a novel class of non-peptidic, non-prenylic inhibitors of this protease.     

Synthesis of acyloxymethyl ester prodrugs of the transferable protein farnesyl transferase substrate farnesyl methylenediphosphonate

Three isoprenoid diphosphate analogues of farnesyl diphosphate (FPP) where the diphosphate has been replaced by methylene diphosphonate and the negative charges masked by frangible pivaloyloxymethyl (POM) esters were prepared. Farnesyl methylenediphosphonate is a sub-micromolar substrate for protein farnesyl transferase. The tripivaloyloxymethyl esters of isoprenoid methylenediphosphonate have significantly increased lipophilicity and may act as important farnesyl diphosphate prodrugs.     

Isoprenylation of the low molecular mass GTP-binding proteins rac 1 and rac 2: possible role in membrane localization

Ras proteins can be modified at their COOH-terminal cysteine in the motif Cys-Ali-Ali-Xaa by a farnesyl isoprenoid. This modification is essential for membrane association and biological activity of ras proteins. A similar COOH-terminal amino acid sequence, Cys-Xaa-Ali-Xaa, exists in the ras-related GTP-binding proteins rac 1 and rac 2. To determine whether these proteins were similarly modified, COS cells were transfected with rac 1 and rac 2 cDNA and expressed proteins were labeled with [3H]mevalonic acid. We report here that both rac 1 and rac 2 are post-translationally modified by addition of an isoprenoid group, the likely site of which is the COOH-terminal cysteine. Isoprenylation was found only in racs associated with particulate cell fractions, suggesting that this modification may be associated with membrane localization of the proteins. These data specifically identify mammalian low molecular mass GTP-binding proteins other than ras that undergo post-translational modification and further define the COOH-terminal consensus sequence, Cys-Ali-Ali-Xaa, as an isoprenylation signal. This sequence may identify a larger family of low molecular mass GTP-binding proteins which are isoprenylated.     

FMRFamide-related peptides (FaRPs): A new family of peptides from amphibian defensive skin secretions

Amphibian defensive skin secretions are known to contain a plethora of biologically-active peptides that are often structural and functional analogues of vertebrate neuropeptides. Here we report the structures of two invertebrate neuropeptide analogues, IPPQFMRF amide (IF-8 amide) and EGDEDEFLRF amide (EF-10 amide), from the defensive skin secretions of two different species of African hyperoliid frogs, Kassina maculata and Phylictimantis verrucosus, respectively. These represent the first canonical FMRF amide-related peptides (FaRPs) from a vertebrate source. The cDNA encoding IF-8 amide was cloned from a skin secretion library and found to contain a single copy of the peptide located at the C-terminus of a 58 amino acid residue open-reading frame. These data extend the potential targets of the defensive arsenal of amphibian tegumental secretions to parasitic/predatory invertebrates and the novel peptides described may represent the first vertebrate peptidic endectocides.