Effect of geraniol on fatty-acid and mevalonate metabolism in the human hepatoma cell line Hep G2.

Monoterpenes have multiple pharmacological effects on the metabolism of mevalonate. Geraniol, a dietary monoterpene, has in vitro and in vivo anti-tumor activity against several cell lines. We have studied the effects of geraniol on growth, fatty-acid metabolism, and mevalonate metabolism in the human hepatocarcinoma cell line Hep G2. Up to 100 micromol geraniol/L inhibited the growth rate and 3-hydroxymethylglutaryl coenzyme A reductase (HMG-CoA) reductase activity of these cells. At the same concentrations, it increased the incorporation of cholesterol from the medium in a dose-dependent manner. Geraniol-treated cells incorporated less 14C-acetate into nonsaponifiable lipids, inhibiting its incorporation into cholesterol but not into squalene and lanosterol. This is indicative of an inhibition in cholesterol synthesis at a step between lanosterol and cholesterol, a fact confirmed when cells were incubated with 3H-mevalonate. The incorporation of 3H-mevalonate into protein was also inhibited, whereas its incorporation into fatty acid increased. An inhibition of delta5 desaturase activity was demonstrated by the inhibition of the conversion of 14C-dihomo-gamma-linolenic acid into arachidonic acid. Geraniol has multiple effects on mevalonate and lipid metabolism in Hep G2 cells, affecting cell proliferation. Although mevalonate depletion is not responsible for cellular growth, it affects cholesterogenesis, protein prenylation, and fatty-acid metabolism.     

Prenylation of mammalian Ras protein in Xenopus oocytes.

Ras protein requires an intermediate of the cholesterol biosynthetic pathway for posttranslational modification and membrane anchorage. This step is necessary for biological activity. Maturation of Xenopus laevis oocytes induced by an oncogenic human Ras protein can be inhibited by lovastatin or compactin, inhibitors of the synthesis of mevalonate, an intermediate of cholesterol biosynthesis. This inhibition can be overcome by mevalonic acid or farnesyl diphosphate, a cholesterol biosynthetic intermediate downstream of mevalonate, but not by squalene, an intermediate after farnesyl pyrophosphate in the pathway. This study supports the idea that in Xenopus oocytes, the Ras protein is modified by a farnesyl moiety or its derivative. Furthermore, an octapeptide with the sequence similar to the C-terminus of the c-H-ras protein inhibits the biological activity of Ras proteins in vivo, suggesting that it competes for the enzyme or enzymes responsible for transferring the isoprenoid moiety (prenylation) in the oocytes. This inhibition of Ras prenylation by the peptide was also observed in vitro, using both Saccharomyces cerevisiae and Xenopus oocyte extracts. These observations show that Xenopus oocytes provide a convenient in vivo system for studies of inhibitors of the posttranslational modification of the Ras protein, especially for inhibitors such as peptides that do not penetrate cell membranes.     

High-content assay to study protein prenylation

The mevalonate pathway leads to synthesis of cholesterol and isoprenoid lipids. Prenyltransferases attach the isoprenoid lipids to the C-terminus of several small guanosine triphosphate-binding proteins. The prenyl groups are essential for the biological activity of these proteins. The prenyltransferases and other components of the mevalonate pathway are either present or potential drug targets for cancer, osteoporosis, restenosis, or high serum cholesterol level. Until recently, cellular assays to study protein prenylation have been tedious, low-throughput assays. The authors have developed a high-content imaging-based assay to study protein prenylation. The assay is based on a green fluorescent protein (GFP) reporter, which is tagged with the prenylation motif of human H-Ras. The C-terminus of H-Ras targets GFP to the plasma membrane. When protein prenylation is inhibited, the tagged GFP cannot be localized to plasma membrane but is soluble in the cells. The localization of the GFP reporter can be analyzed in the 96- or 384-well format using automated microscopy and automated image analysis. Information about cell number and nuclear intensity can be obtained from the same images. In compound screening, these readouts provide valuable information about the toxicity of the compounds. The authors have validated their assay using several inhibitors of the mevalonate pathway as well as siRNA against farnesyl pyrophosphate synthase, a critical enzyme in the synthesis of the isoprenoid lipids.     

Isoprenoids and protein prenylation: implications in the pathogenesis and therapeutic intervention of Alzheimer’s disease

The mevalonate–isoprenoid–cholesterol biosynthesis pathway plays a key role in human health and disease. The importance of this pathway is underscored by the discovery that two major isoprenoids, farnesyl and geranylgeranyl pyrophosphate, are required to modify an array of proteins through a process known as protein prenylation, catalyzed by prenyltransferases. The lipophilic prenyl group facilitates the anchoring of proteins in cell membranes, mediating protein–protein interactions and signal transduction. Numerous essential intracellular proteins undergo prenylation, including most members of the small GTPase superfamily as well as heterotrimeric G proteins and nuclear lamins, and are involved in regulating a plethora of cellular processes and functions. Dysregulation of isoprenoids and protein prenylation is implicated in various disorders, including cardiovascular and cerebrovascular diseases, cancers, bone diseases, infectious diseases, progeria, and neurodegenerative diseases including Alzheimer’s disease (AD). Therefore, isoprenoids and/or prenyltransferases have emerged as attractive targets for developing therapeutic agents. Here, we provide a general overview of isoprenoid synthesis, the process of protein prenylation and the complexity of prenylated proteins, and pharmacological agents that regulate isoprenoids and protein prenylation. Recent findings that connect isoprenoids/protein prenylation with AD are summarized and potential applications of new prenylomic technologies for uncovering the role of prenylated proteins in the pathogenesis of AD are discussed.     

Murine guanylate-binding protein: incomplete geranylgeranyl isoprenoid modification of an interferon-gamma-inducible guanosine triphosphate-binding protein

Farnesylation of Ras proteins is necessary for transforming activity. Although farnesyl transferase inhibitors show promise as anticancer agents, prenylation of the most commonly mutated Ras isoform, K-Ras4B, is difficult to prevent because K-Ras4B can be alternatively modified with geranylgeranyl (C20). Little is known of the mechanisms that produce incomplete or inappropriate prenylation. Among non-Ras proteins with CaaX motifs, murine guanylate-binding protein (mGBP1) was conspicuous for its unusually low incorporation of [(3)H]mevalonate. Possible problems in cellular isoprenoid metabolism or prenyl transferase activity were investigated, but none that caused this defect was identified, implying that the poor labeling actually represented incomplete prenylation of mGBP1 itself. Mutagenesis indicated that the last 18 residues of mGBP1 severely limited C20 incorporation but, surprisingly, were compatible with farnesyl modification. Features leading to the expression of mutant GBPs with partial isoprenoid modification were identified. The results demonstrate that it is possible to alter a protein's prenylation state in a living cell so that graded effects of isoprenoid on function can be studied. The C20-selective impairment in prenylation also identifies mGBP1 as an important model for the study of substrate/geranylgeranyl transferase I interactions.     

Control of RAS mRNA level by the mevalonate pathway.

The ability of Ras proteins to initiate eukaryotic cell proliferation requires the post-translational attachment of a farnesyl group, an isoprenoid lipid moiety derived from mevalonate, to the carboxyl-terminus of the protein. This modification is essential for the subsequent processing and intracellular targeting of the Ras protein. Here we report that mevalonate is also required for the efficient synthesis of Ras proteins in Saccharomyces cerevisiae. Depletion of intracellular mevalonate resulted in decreased steady-state levels of Ras1p and Ras2p, an effect that was mediated at the level of mRNA accumulation. The sequences controlling the response of RAS2 mRNA level to mevalonate availability, mapped to the coding region of the RAS2 gene. Mevalonate starvation also had a significant effect on the expression of some, but not all, genes encoding prenylated proteins. The regulatory effect on RAS2 mRNA did not require a functional farnesyl transferase. These results uncover a novel regulatory role for mevalonate-derived products and expand the potential for inhibitors of mevalonate metabolism as anti-cancer agents.     

Evidence for negative feedback control of cholesterogenesis from mevalonate in liver: Absence in the intestine of guinea pigs fed A 0,5 % cholesterol diet

The in vitro rate of incorporation of [2-¹⁴C]-acetate and [2-¹⁴C]-mevalonate into cholesterol of liver, ileum and caecum was determined in guinea pigs. In control animals, contrary to the situation observed when acetate was used as precursor, the rate of conversion of mevalonate to cholesterol was higher in liver than in intestine. In this latter tissue, the cholesterogenesis varied depending on the portion tested. The distribution of radiolabel derived from mevalonate between esterified and unesterified cholesterol differed among the various tissues. In cholesterol-fed guinea pigs, the plasma, liver, intestine and aorta cholesterol contents increased significantly. In addition, a negative feedback control existed for hepatic cholesterol synthesis for mevalonate and acetate. This control was absent in intestinal tissues.     

Sites of control of hepatic cholesterol biosynthesis

An inhibition in the conversion of mevalonate to cholesterol has been demonstrated in liver of cholesterol-fed rats by both in vitro and in vivo methods. Synthesis decreased to 30% of the control value after 1 week and 20% after 1 month on a 1% cholesterol diet. After a year, synthesis from mevalonate was almost completely inhibited. The rate of conversion of squalene to cholesterol was not consistently decreased but that of farnesyl pyrophosphate to cholesterol was decreased considerably. The rate of conversion of mevalonate to farnesyl pyrophosphate by a soluble liver enzyme preparation was also decreased in cholesterol-fed animals. Sites of inhibition of cholesterol synthesis were detected before mevalonate, between mevalonate and farnesyl pyrophosphate, and after farnesyl pyrophosphate, probably at the conversion of farnesyl pyrophosphate to squalene. The inhibition of mevalonate conversion to cholesterol developed more slowly than that of acetate and appeared to be secondary to it. The maximum capacities of normal liver homogenates and slices to synthesize cholesterol from mevalonate were shown to be far greater than from acetate. Consequently, sites of inhibition after mevalonate probably do not have a significant effect on the over-all rate of cholesterol synthesis in the intact cholesterol-fed animal.     

Effects of peroxisome proliferators gemfibrozil and clofibrate on syntheses of dolichol and cholesterol in rat liver

The effects of two peroxisome proliferators, gemfibrozil and clofibrate, on syntheses of dolichol and cholesterol in rat liver were investigated. Gemfibrozil did not affect the overall content of dolichyl phosphate, but it changed the chain-length distribution of dolichyl phosphate, increasing the levels of species with shorter isoprene units. Gemfibrozil suppressed synthesis of dolichyl phosphate from [(3)H]mevalonate and [(3)H]farnesyl pyrophosphate in rat liver. In contrast, clofibrate increased the content of dolichol (free and acyl ester forms). It remarkably enhanced dolichol synthesis from mevalonate, but did not affect dolichol synthesis from farnesyl pyrophosphate. Gemfibrozil elevated cholesterol synthesis from [(14)C]acetate, but did not affect the synthesis from mevalonate. Clofibrate suppressed cholesterol synthesis from acetate, but did not affect cholesterol synthesis from mevalonate. These results suggest that gemfibrozil suppresses synthesis of dolichyl phosphate by inhibiting, at the least, the pathway from farnesyl pyrophosphate to dolichyl phosphate. As a result, the chain-length pattern of dolichyl phosphate may show an increase in shorter isoprene units. Clofibrate may increase the content of dolichol by enhancing dolichol synthesis from mevalonate. Gemfibrozil may increase cholesterol synthesis by activating the pathway from acetate to mevalonate. Unlike gemfibrozil, clofibrate may decrease cholesterol synthesis by inhibiting the pathway from acetate to mevalonate.     

A novel bisphosphonate inhibitor of squalene synthase combined with a statin or a nitrogenous bisphosphonate in vitro

Statins and nitrogenous bisphosphonates (NBP) inhibit 3-hydroxy-3-methylglutaryl-coenzyme-A reductase (HMGCR) and farnesyl diphosphate synthase (FDPS), respectively, leading to depletion of farnesyl diphosphate (FPP) and disruption of protein prenylation. Squalene synthase (SQS) utilizes FPP in the first committed step from the mevalonate pathway toward cholesterol biosynthesis. Herein, we have identified novel bisphosphonates as potent and specific inhibitors of SQS, including the tetrasodium salt of 9-biphenyl-4,8-dimethyl-nona-3,7-dienyl-1,1-bisphosphonic acid (compound 5). Compound 5 reduced cholesterol biosynthesis and lead to a substantial intracellular accumulation of FPP without reducing cell viability in HepG2 cells. At high concentrations, lovastatin and zoledronate impaired protein prenylation and decreased cell viability, which limits their potential use for cholesterol depletion. When combined with lovastatin, compound 5 prevented lovastatin-induced FPP depletion and impairment of protein farnesylation. Compound 5 in combination with the NBP zoledronate completely prevented zoledronate-induced impairment of both protein farnesylation and geranylgeranylation. Cotreatment of cells with compound 5 and either lovastatin or zoledronate was able to significantly prevent the reduction of cell viability caused by lovastatin or zoledronate alone. The combination of an SQS inhibitor with an HMGCR or FDPS inhibitor provides a rational approach for reducing cholesterol synthesis while preventing nonsterol isoprenoid depletion.     

Isoprenoid synthesis during the cell cycle. Studies of 3-hydroxy-3-methylglutaryl-coenzyme A synthase and reductase and isoprenoid labeling in cells synchronized by centrifugal elutriation

The activities of 3-hydroxy-3-methylglutaryl-coenzyme A synthase and reductase were assayed in exponentially growing LM fibroblasts and Friend murine erythroleukemia cells isolated at various stages of the cell cycle by centrifugal elutriation. The activities of these enzymes were similar in all phases of the cell cycle, regardless of whether the cells were cultured in the presence or absence of serum. These observations were confirmed in murine erythroleukemia cells synchronized by recultivation of pure populations of G1 cells. The incorporation of [14C]acetate or 3H2O into sterols decreased by 30-50% in later stages of the cell cycle, whereas the incorporation of [14C]acetate into ubiquinone increased as the cells progressed toward mitosis. Similar changes in the labeling of sterols compared to ubiquinone and dolichol were observed when [3H]mevalonate was used, suggesting that cell cycle-dependent alterations may occur in the flux of farnesyl pyrophosphate into the various branches of the isoprenoid pathway. Synchronized murine erythroleukemia cells incorporated [3H]mevalonate into protein-bound isoprenyl groups at all stages of the cell cycle, and there were no substantial changes in the electrophoretic profiles of these labeled polypeptides. The finding that the activities of the enzymes regulating mevalonate synthesis did not vary substantially during the cell cycle implies that changes in the endogenous mevalonate pool probably do not play a limiting role in regulating cell cycle traverse when cells are undergoing exponential growth. Although small cell cycle-dependent changes may occur in the relative activity of various post-mevalonate branches of the isoprenoid biosynthetic pathway, there is no evidence that synthesis of any major isoprenoid end product is confined exclusively to a specific phase of the cell cycle.     

Feedback inhibition of polyisoprenyl pyrophosphate synthesis from mevalonate in vitro. Implications for protein prenylation.

The prenylation of proteins utilizes the polyisoprenyl pyrophosphates (FPP) and geranylgeranyl pyrophosphate (GGPP) as prenyl donors. These polyisoprenoids are also precursors to ubiquinone and dolichol synthesis. We have previously described the geranylgeranylation of rab 1b from labeled mevalonate in rabbit reticulocyte lysates (Khosravi-Far, R., Lutz, R. J., Cox, A. D., Conroy, L., Bourne, J. R., Sinensky, M., Balch, W. E., Buss, J. C., and Der, C. J. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 6264-6268). We now directly demonstrate the incorporation of mevalonate into FPP and GGPP in rabbit reticulocyte cytosol. High pressure liquid chromatography analysis reveals that only all-trans-E,E,E-GGPP, the prenyl donor for in vivo protein geranylgeranylation, is synthesized. Incubations with recombinant H-ras and rab1b result in an increased synthesis of farnesyl and geranylgeranyl derivatives, respectively. The increase is wholly accounted for by protein-incorporated polyisoprenoids with no change in the polyisoprenyl pyrophosphate pools. Further, GGPP inhibits its own synthesis, without affecting FPP synthesis, with half-maximal inhibition at approximately 3 microM GGPP. Inhibition of FPP synthesis by the inhibition of isopentenyl isomerase causes a dramatic increase in isopentenyl pyrophosphate synthesis. FPP also inhibits conversion of mevalonate into FPP. These findings indicate that these polyisoprenyl pyrophosphates can down-regulate their own synthesis in vitro, and this regulation may control the levels of these polyisoprenoids in vivo.     

Farnesyl Acetate, a Derivative of an Isoprenoid of the Mevalonate Pathway, Inhibits DNA Replication in Hamster and Human Cells

Cells treated with compactin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the enzyme which catalyzes the rate-limiting step of the mevalonate pathway, are arrested prior to the DNA synthesis (S) phase of the cell cycle. Identification of a specific pathway product or products with a role in DNA replication, however, has remained elusive. In this report we demonstrate that farnesyl acetate, a derivative of the key isoprenoid pathway intermediate farnesyl pyrophosphate, inhibits DNA replication in both Chinese hamster ovary cells and human (HeLa) cells. This effect is revealed by measurement of DNA content using fluorescence-activated cell sorter analysis and by measurement of [³H]thymidine incorporation. We show that cells treated with farnesyl acetate retain protein synthesis capacity as DNA replication is inhibited and remain intact as viewed with the vital stain propidium iodide. The inhibition of DNA replication by farnesyl acetate occurs in cells treated with high levels of compactin and in cells lacking HMG-CoA reductase. These results indicate that farnesyl acetate action is not dependent on metabolism through the isoprenoid pathway and is not the result of the loss of a metabolite required for replication nor the accumulation of a metabolite which is inhibitory. In addition, cells treated with farnesyl acetate for over 6 h are irreversibly blocked from progressing through S phase, a phenomenon which differs sharply from the results with compactin, removal of which results in synchronous progression through S phase. Farnesyl acetate also blocks protein prenylation in cells, to a degree comparable to a known farnesylation inhibitor, BZA-5B. We propose that farnesyl acetate is acting in a manner quite different from the metabolic block caused by compactin, causing a rapid and irreversible block of DNA replication.     

Gamma-subunits of G proteins, but not their alpha- or beta-subunits, are polyisoprenylated. Studies on post-translational modifications using in vitro translation with rabbit reticulocyte lysates

Lipid modifications that may be introduced into several subunits of G proteins were explored by in vitro translation of recombinant mRNAs in reticulocyte lysates. In agreement with studies by others, myristic acid was incorporated into alpha i's and alpha o, but not alpha s, beta, or gamma's. In contrast, mevalonate (Mev) was incorporated only into gamma-subunits. Both, the gamma-subunit of transducin (gamma T) and that of other G proteins (gamma G) were modified by the lysates but with different characteristics. Labeled gamma T was unstable and was rapidly proteolyzed. Labeled gamma G was stable. The Mev-derivative in gamma G was sensitive to methyliodide and, after cleavage and chromatographic analysis, comigrated with the C20 polyisoprenol geranylgeraniol. This indicated that gamma G had been geranylgeranylated and that this polyisoprenoid was attached to the protein through a thioether linkage. It is thought that polyisoprenylation is defined by the COOH-terminal sequence Cys-A-A-X, where A is an aliphatic acid and X is any amino acid. Replacement by mutation of the Cys of the COOH-terminal -Cys-Ala-Ile-Leu sequence of gamma G with Ser abolished Mev incorporation, suggesting this Cys as the site of attachment of the geranylgeranyl moiety. Yet, Mev incorporation was less than 10% as much into gamma G with the Cys-A-A-X sequence -Cys-Ala-Ile-Trp. Consistent with geranylgeranylation, the C15 farnesyl moiety of farnesyl pyrophosphate was not incorporated into gamma G unless the incubations were fortified with Mev. In contrast, the farnesyl moiety was incorporated in an Mev-independent manner into gamma T (COOH terminus: -Cys-Val-Ile-Ser) and c-Ha-ras (COOH terminus: -Cys-Val-Leu-Ser) which are both farnesylated rather than geranylgeranylated. Thus, 1) separate enzymes appear to be involved in transferring farnesyl and geranylgeranyl groups to proteins, 2) structural factors other than the CAAX box contribute to the activity of the polyisoprenylating enzymes, and 3) this type of lipidation may be part of a proteolytic signaling system. Polyisoprenylation, which increases hydrophobicity of the derivatized protein, may play a role in anchoring not only ras but also G proteins to membranes.     

Dose-dependent effects of lovastatin on cell cycle progression. Distinct requirement of cholesterol and non-sterol mevalonate derivatives

The mevalonate pathway is tightly linked to cell proliferation. The aim of the present study is to determine the relationship between the inhibition of this pathway by lovastatin and the cell cycle. HL-60 and MOLT-4 human cell lines were cultured in a cholesterol-free medium and treated with increasing concentrations of lovastatin, and their effects on cell proliferation and the cell cycle were analyzed. Lovastatin was much more efficient in inhibiting cholesterol biosynthesis than protein prenylation. As a result of this, lovastatin blocked cell proliferation at any concentration used, but its effects on cell cycle distribution varied. At relatively low lovastatin concentrations (less than 10 microM), cells accumulated preferentially in G(2) phase, an effect which was both prevented and reversed by low-density lipoprotein cholesterol. At higher concentrations (50 microM), the cell cycle was also arrested at G(1) phase. In cells treated with lovastatin, those arrested at G(1) progressed through S upon mevalonate provision, whereas cholesterol supply allowed cells arrested at G(2) to traverse M phase. These results demonstrate the distinct roles of mevalonate, or its non-sterol derivatives, and cholesterol in cell cycle progression, both being required for normal cell cycling.     

Alterations of rat hepatic cholesterogenesis by heterologous lipoproteins.

Heterologous human lipoproteins were infused into rats in order to change acutely the lipoprotein pattern to a predominant kind and the effect on hepatic cholesterogenesis was subsequently observed. A 4-h intravenous infusion of human low density and very low density lipoproteins into rats produced a significant decrease in the incorporation of acetate into cholesterol in both liver slices and homogenates. An infusion of similar concentrations of human high density lipoprotein produced a significant increase in hepatic cholesterol synthesis. These infusions did not change mevalonate conversion to cholesterol in either the homogenates or slices. Concomitant with the changes in hepatic cholesterol synthesis were changes of similar magnitudes in the activity of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase. These alterations in hepatic cholesterol synthesis were associated with significant changes in microsomal cholesterol content. There was a significant increase in hepatic cholesterol synthesis with the infusion of apoproteins of high density lipoprotein. The apoproteins of very low density lipoprotein had no effect on hepatic cholesterogenesis. These studies indicate that circulating lipoproteins modify hepatic cholesterol synthesis and that the apoproteins of these lipoproteins may themselves be important for this action.     

Statins activate a mitochondria-operated pathway of apoptosis in breast tumor cells by a mechanism regulated by ErbB2 and dependent on the prenylation of proteins

Statins are inhibitors of the mevalonate synthesis pathway that induce apoptosis in tumor cells although the apoptotic mechanism activated by statins remains to be elucidated. We have examined the role of the mitochondria-operated pathway of apoptosis in the cell death induced by statins in breast tumor cells and its regulation by protein prenylation and ErbB2 overexpression. Lovastatin treatment down-regulates the expression of Bcl-2 and activates apoptosis through a mitochondria-operated, ErbB2- regulated mechanism. Apoptosis induced by statins is independent of their effects on cholesterol synthesis and involves protein prenylation. Our results indicate that prenylation of apoptosis-regulating proteins is a key event in the survival of breast tumor cells and this requirement could be circumvented in cells overexpressing the oncogene ErbB2.     

Alendronate is a specific, nanomolar inhibitor of farnesyl diphosphate synthase

Alendronate, a nitrogen-containing bisphosphonate, is a potent inhibitor of bone resorption used for the treatment and prevention of osteoporosis. Recent findings suggest that alendronate and other N-containing bisphosphonates inhibit the isoprenoid biosynthesis pathway and interfere with protein prenylation, as a result of reduced geranylgeranyl diphosphate levels. This study identified farnesyl disphosphate synthase as the mevalonate pathway enzyme inhibited by bisphosphonates. HPLC analysis of products from a liver cytosolic extract narrowed the potential targets for alendronate inhibition (IC(50) = 1700 nM) to isopentenyl diphosphate isomerase and farnesyl diphosphate synthase. Recombinant human farnesyl diphosphate synthase was inhibited by alendronate with an IC(50) of 460 nM (following 15 min preincubation). Alendronate did not inhibit isopentenyl diphosphate isomerase or GGPP synthase, partially purified from liver cytosol. Recombinant farnesyl diphosphate synthase was also inhibited by pamidronate (IC(50) = 500 nM) and risedronate (IC(50) = 3.9 nM), negligibly by etidronate (IC50 = 80 microM), and not at all by clodronate. In osteoclasts, alendronate inhibited the incorporation of [(3)H]mevalonolactone into proteins of 18-25 kDa and into nonsaponifiable lipids, including sterols. These findings (i) identify farnesyl diphosphate synthase as the selective target of alendronate in the mevalonate pathway, (ii) show that this enzyme is inhibited by other N-containing bisphosphonates, such as risendronate, but not by clodronate, supporting a different mechanism of action for different bisphosphonates, and (iii) document in purified osteoclasts alendronate inhibition of prenylation and sterol biosynthesis.     

The prenylation of proteins.

The prenylated proteins represent a newly discovered class of post-translationally modified proteins. The known prenylated proteins include the oncogene product p21ras and other low molecular weight GTP-binding proteins, the nuclear lamins, and the gamma subunit of the heterotrimeric G proteins. The modification involves the covalent attachment of a 15-carbon (farnesyl) or 20-carbon (geranylgeranyl) isoprenoid moiety in a thioether linkage to carboxyl terminal cysteine. The nature of the attached substituent is dependent on specific sequence information in the carboxyl terminus of the protein. In addition, prenylation entrains other posttranslational modifications forming a reaction pathway. In this article, we review our current understanding of the biochemical reactions involved in prenylation and discuss the possible role of this modification in the control of cellular functions such as protein maturation and cell growth.     

F-244 specifically inhibits 3-hydroxy-3-methylglutaryl coenzyme A synthase

A beta-lactone isolated from Scopulariopsis sp. shows a potent inhibition of cholesterogenesis. The structure of this beta-lactone, termed F-244, is 3,5,7-trimethyl-12-hydroxy-13-hydroxymethyl-2,4-tetradecadiendioic acid 12,14-lactone. The inhibition site of F-244 in cholesterol synthesis was studied. The growth of Vero cells was inhibited at 6.25-12.5 micrograms/ml of F-244. The inhibition of growth was overcome by the addition of mevalonate to the culture medium, but not by the addition of acetate. In a rat liver enzyme system, the incorporations of [14C]acetate and [14C]acetyl-CoA into digitonin-precipitable sterol were 50% inhibited by 0.58 microgram/ml of F-244. The incorporation of [14C]mevalonate was not affected. Studies on the effects of F-244 on the three enzymes involved in mevalonate biosynthesis demonstrated that the drug specifically inhibits HMG-CoA synthase with IC50 value of 0.065 microgram/ml. The effect of analogs of F-244 on HMG-CoA synthase was also investigated.