Isopentenoid synthesis in isolated embryonic Drosophila cells. Farnesol catabolism and omega-oxidation

Differential Regulation of Gene Expression by Cholesterol Biosynthesis Inhibitors That Reduce (Pravastatin) or Enhance (Squalestatin 1) Nonsterol Isoprenoid Levels in Primary Cultured Mouse and Rat Hepatocytes

Squalene synthase inhibitors (SSIs), such as squalestatin 1 (SQ1), reduce cholesterol biosynthesis but cause the accumulation of isoprenoids derived from farnesyl pyrophosphate (FPP), which can modulate the activity of nuclear receptors, including the constitutive androstane receptor (CAR), farnesoid X receptor, and peroxisome proliferator-activated receptors (PPARs). In comparison, 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (e.g., pravastatin) inhibit production of both cholesterol and nonsterol isoprenoids. To characterize the effects of isoprenoids on hepatocellular physiology, microarrays were used to compare orthologous gene expression from primary cultured mouse and rat hepatocytes that were treated with either SQ1 or pravastatin. Compared with controls, 47 orthologs were affected by both inhibitors, 90 were affected only by SQ1, and 51 were unique to pravastatin treatment (P < 0.05, ≥1.5-fold change). When the effects of SQ1 and pravastatin were compared directly, 162 orthologs were found to be differentially coregulated between the two treatments. Genes involved in cholesterol and unsaturated fatty acid biosynthesis were up-regulated by both inhibitors, consistent with cholesterol depletion; however, the extent of induction was greater in rat than in mouse hepatocytes. SQ1 induced several orthologs associated with microsomal, peroxisomal, and mitochondrial fatty acid oxidation and repressed orthologs involved in cell cycle regulation. By comparison, pravastatin repressed the expression of orthologs involved in retinol and xenobiotic metabolism. Several of the metabolic genes altered by isoprenoids were inducible by a PPARα agonist, whereas cytochrome P450 isoform 2B was inducible by activators of CAR. Our findings indicate that SSIs uniquely influence cellular lipid metabolism and cell cycle regulation, probably due to FPP catabolism through the farnesol pathway.

Nonsterol Isoprenoids Activate Human Constitutive Androstane Receptor in an Isoform-Selective Manner in Primary Cultured Mouse Hepatocytes

Our laboratory previously reported that accumulation of nonsterol isoprenoids following treatment with the squalene synthase inhibitor, squalestatin 1 (SQ1) markedly induced cytochrome P450 (CYP)2B1 mRNA and reporter activity in primary cultured rat hepatocytes, which was dependent on activation of the constitutive androstane receptor (CAR). The objective of the current study was to evaluate whether isoprenoids likewise activate murine CAR (mCAR) or one or more isoforms of human CAR (hCAR) produced by alternative splicing (SPTV, hCAR2; APYLT, hCAR3). We found that SQ1 significantly induced Cyp2b10 mRNA (∼3.5-fold) in primary hepatocytes isolated from both CAR-wild-type and humanized CAR transgenic mice, whereas the 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor pravastatin had no effect. In the absence of CAR, basal Cyp2b10 mRNA levels were reduced by 28-fold and the effect of SQ1 on Cyp2b10 induction was attenuated. Cotransfection with an expression plasmid for hCAR1, but not hCAR2 or hCAR3, mediated SQ1-induced CYP2B1 and CYP2B6 reporter activation in hepatocytes isolated from CAR-knockout mice. This effect was also observed following treatment with the isoprenoid trans,trans-farnesol. The direct agonist CITCO increased interaction of hCAR1, hCAR2, and hCAR3 with steroid receptor coactivator-1. However, no significant effect on coactivator recruitment was observed with SQ1, suggesting an indirect activation mechanism. Further results from an in vitro ligand binding assay demonstrated that neither farnesol nor other isoprenoids are direct ligands for hCAR1. Collectively, our findings demonstrate that SQ1 activates CYP2B transcriptional responses through farnesol metabolism in an hCAR1-dependent manner. Further, this effect probably occurs through an indirect mechanism.

Statins inhibit blastocyst formation by preventing geranylgeranylation

STUDY HYPOTHESIS

Statins, inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase of the mevalonate pathway and prescription drugs that treat hypercholesterolemia, compromise preimplantation mouse development via modulation of HIPPO signaling.

STUDY FINDING

HMG-CoA reductase activity is required for trophectoderm specification, namely blastocyst cavity formation and Yes-associated protein (YAP) nuclear localization, through the production of isoprenoid geranylgeranyl pyrophosphate (GGPP) and the action of geranylgeranyl transferase.

WHAT IS KNOWN ALREADY

Previous studies have shown that treatment of mouse embryos with mevastatin prevents blastocyst formation, but how HMG-CoA reductase is involved in preimplantation development is unknown. HIPPO signaling regulates specification of the trophectoderm lineage of the mouse blastocyst by controlling the nuclear localization of YAP. In human cell lines, the mevalonate pathway regulates YAP to mediate self-renewal and survival through geranylgeranylation of RHO proteins. These studies suggest that in preimplantation development, statins may act through HIPPO pathway to interfere with trophectoderm specification and thereby inhibit blastocyst formation.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Eight-cell stage (E2.5) mouse embryos were treated in hanging drop culture with chemical agents, namely statins (lovastatin, atorvastatin, cerivastatin and pravastatin), mevalonic acid (MVA), cholesterol, squalene, farnesyl pyrophosphate (FPP), geranylgeranyl pyrophosphate (GGPP), geranylgeranyltransferase inhibitor GGTI-298, RHO inhibitor I, and squalene synthase inhibitor YM-53601, up to the late blastocyst stage (E4.5). Efficiency of blastocyst formation was assessed based on gross morphology and the measurement of the cavity size using an image analysis software. Effects on cell lineages and HIPPO signaling were analyzed using immunohistochemistry with confocal microscopy based on the expression patterns of the lineage-specific markers and the nuclear accumulation of YAP. Effects on cell lineages were also examined by quantitative RT-PCR based on the transcript levels of the lineage-specific marker genes. Data were analyzed using one-way ANOVA and two-sample t-test.

MAIN RESULTS AND THE ROLE OF CHANCE

All four statins examined inhibited blastocyst formation. The adverse impact of statins was rescued by supplementation of MVA (P < 0.01) or GGPP (P < 0.01) but not squalene nor cholesterol. Blastocyst formation was also prevented by GGTI-298 (P < 0.01). These results indicate that HMG-CoA reductase activity is required for blastocyst formation mainly through the production of GGPP but not cholesterol. Inhibition of RHO proteins, known targets of geranylgeranylation, impaired blastocyst formation, which was not reversed by GGPP supplementation. Nuclear localization of YAP was diminished by statin treatment but fully restored by supplementation of MVA (P < 0.01) or GGPP (P < 0.01). This suggests that HIPPO signaling is regulated by GGPP-dependent mechanisms, possibly geranylgeranylation of RHO, to enable trophectoderm formation. YM-53601 prevented blastocyst formation (P < 0.01), but its adverse impact was not rescued by supplementation of squalene or cholesterol, suggesting that squalene synthesis inhibition was not the cause of blastocyst defects.

LIMITATIONS, REASONS FOR CAUTION

Analyses were conducted on embryos cultured ex vivo, but they enable the determination of specific concentrations that impair embryo development which can be compared with drug concentrations in the reproductive tract when testing in vivo impact of statins through animal experimentations. Also, analyses were conducted in only one species, the mouse. Epidemiological studies on the effects of various types of statins on the fertility of women are necessary.

WIDER IMPLICATIONS OF THE FINDINGS

Our study reveals how the mevalonate pathway is required for blastocyst formation and intersects with HIPPO pathway to provide a mechanistic basis for the embryotoxic effect of statins. This bears relevance for women who are taking statins while trying to conceive, since statins have potential to prevent the conceptus from reaching the blastocyst stage and to cause early conceptus demise.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This study was supported by grants from the George F. Straub Trust of the Hawaii Community Foundation (13ADVC-60315 to V.B.A.) and the National Institutes of Health, USA (P20GM103457 to V.B.A.). The authors have no conflict of interest to declare.

Discovery of DF-461, a Potent Squalene Synthase Inhibitor

We report the development of a new trifluoromethyltriazolobenzoxazepine series of squalene synthase inhibitors. Structure-activity studies and pharmacokinetics optimization on this series led to the identification of compound 23 (DF-461), which exhibited potent squalene synthase inhibitory activity, high hepatic selectivity, excellent rat hepatic cholesterol synthesis inhibitory activity, and plasma lipid lowering efficacy in nonrodent repeated dose studies.

Endogenous sterol biosynthesis is important for mitochondrial function and cell morphology in procyclic forms of Trypanosoma brucei

Sterol biosynthesis inhibitors are promising entities for the treatment of trypanosomal diseases. Insect forms of Trypanosoma brucei, the causative agent of sleeping sickness, synthesize ergosterol and other 24-alkylated sterols, yet also incorporate cholesterol from the medium. While sterol function has been investigated by pharmacological manipulation of sterol biosynthesis, molecular mechanisms by which endogenous sterols influence cellular processes remain largely unknown in trypanosomes. Here we analyse by RNA interference, the effects of a perturbation of three specific steps of endogenous sterol biosynthesis in order to dissect the role of specific intermediates in proliferation, mitochondrial function and cellular morphology in procyclic cells. A decrease in the levels of squalene synthase and squalene epoxidase resulted in a depletion of cellular sterol intermediates and end products, impaired cell growth and led to aberrant morphologies, DNA fragmentation and a profound modification of mitochondrial structure and function. In contrast, cells deficient in sterol methyl transferase, the enzyme involved in 24-alkylation, exhibited a normal growth phenotype in spite of a complete abolition of the synthesis and content of 24-alkyl sterols. Thus, the data provided indicates that while the depletion of squalene and post-squalene endogenous sterol metabolites results in profound cellular defects, bulk 24-alkyl sterols are not strictly required to support growth in insect forms of T. brucei in vitro.

Roles of rat and human aldo-keto reductases in metabolism of farnesol and geranylgeraniol

Farnesol (FOH) and geranylgeraniol (GGOH) with multiple biological actions are produced from the mevalonate pathway, and catabolized into farnesoic acid and geranylgeranoic acid, respectively, via the aldehyde intermediates (farnesal and geranylgeranial). We investigated the intracellular distribution, sequences and properties of the oxidoreductases responsible for the metabolic steps in rat tissues. The oxidation of FOH and GGOH into their aldehyde intermediates were mainly mediated by alcohol dehydrogenases 1 (in the liver and colon) and 7 (in the stomach and lung), and the subsequent step into the carboxylic acids was catalyzed by a microsomal aldehyde dehydrogenase. In addition, high reductase activity catalyzing the aldehyde intermediates into FOH (or GGOH) was detected in the cytosols of the extra-hepatic tissues, where the major reductase was identified as aldo-keto reductase (AKR) 1C15. Human reductases with similar specificity were identified as AKR1B10 and AKR1C3, which most efficiently reduced farnesal and geranylgeranial among seven enzymes in the AKR1A-1C subfamilies. The overall metabolism from FOH to farnesoic acid in cultured cells was significantly decreased by overexpression of AKR1C15, and increased by addition of AKR1C3 inhibitors, tolfenamic acid and R-flurbiprofen. Thus, AKRs (1C15 in rats, and 1B10 and 1C3 in humans) may play an important role in controlling the bioavailability of FOH and GGOH.

Synthetic arylquinuclidine derivatives exhibit antifungal activity against Candida albicans, Candida tropicalis and Candida parapsilopsis

BACKGROUND

Sterol biosynthesis is an essential pathway for fungal survival, and is the biochemical target of many antifungal agents. The antifungal drugs most widely used to treated fungal infections are compounds that inhibit cytochrome P450-dependent C14α-demethylase (CYP51), but other enzymes of this pathway, such as squalene synthase (SQS) which catalyses the first committed step in sterol biosynthesis, could be viable targets. The aim of this study was to evaluate the antifungal activity of SQS inhibitors on Candida albicans, Candida tropicalis and Candida parapsilopsis strains.

METHODS

Ten arylquinuclidines that act as SQS inhibitors were tested as antiproliferative agents against three ATCC strains and 54 clinical isolates of Candida albicans, Candida tropicalis and Candida parapsilopsis. Also, the morphological alterations induced in the yeasts by the experimental compounds were evaluated by fluorescence and transmission electron microscopy.

RESULTS

The most potent arylquinuclidine derivative (3-[1'-{4'-(benzyloxy)-phenyl}]-quinuclidine-2-ene) (WSP1267) had a MIC50 of 2 μg/ml for all species tested and MIC90 varying from 4 μg/ml to 8 μg/ml. Ultrathin sections of C. albicans treated with 1 μg/ml of WSP1267 showed several ultrastructural alterations, including (a) loss of cell wall integrity, (b) detachment of the plasma membrane from the fungal cell wall, (c) accumulation of small vesicles in the periplasmic region, (d) presence of large electron-dense vacuoles and (e) significantly increased cell size and cell wall thickness. In addition, fluorescence microscopy of cells labelled with Nile Red showed an accumulation of lipid droplets in the cytoplasm of treated yeasts. Nuclear staining with DAPI revealed the appearance of uncommon yeast buds without a nucleus or with two nuclei.

CONCLUSION

Taken together, our data demonstrate that arylquinuclidine derivatives could be useful as lead compounds for the rational synthesis of new antifungal drugs.

Farnesylcysteine lyase is involved in negative regulation of abscisic acid signaling in Arabidopsis

The Arabidopsis FCLY gene encodes a specific farnesylcysteine (FC) lyase, which is responsible for the oxidative metabolism of FC to farnesal and cysteine. In addition, fcly mutants with quantitative decreases in FC lyase activity exhibit an enhanced response to ABA. However, the enzymological properties of the FCLY-encoded enzyme and its precise role in ABA signaling remain unclear. Here, we show that recombinant Arabidopsis FC lyase expressed in insect cells exhibits high selectivity for FC as a substrate and requires FAD and molecular oxygen for activity. Arabidopsis FC lyase is also shown to undergo post-translational N-glycosylation. FC, which is a competitive inhibitor of isoprenylcysteine methyltransferase (ICMT), accumulates in fcly mutants. Moreover, the enhanced response of fcly mutants to ABA is reversed by ICMT overexpression. These observations support the hypothesis that the ABA hypersensitive phenotype of fcly plants is the result of FC accumulation and inhibition of ICMT.

Sterol Biosynthesis Pathway as Target for Anti-trypanosomatid Drugs

Sterols are constituents of the cellular membranes that are essential for their normal structure and function. In mammalian cells, cholesterol is the main sterol found in the various membranes. However, other sterols predominate in eukaryotic microorganisms such as fungi and protozoa. It is now well established that an important metabolic pathway in fungi and in members of the Trypanosomatidae family is one that produces a special class of sterols, including ergosterol, and other 24-methyl sterols, which are required for parasitic growth and viability, but are absent from mammalian host cells. Currently, there are several drugs that interfere with sterol biosynthesis (SB) that are in use to treat diseases such as high cholesterol in humans and fungal infections. In this review, we analyze the effects of drugs such as (a) statins, which act on the mevalonate pathway by inhibiting HMG-CoA reductase, (b) bisphosphonates, which interfere with the isoprenoid pathway in the step catalyzed by farnesyl diphosphate synthase, (c) zaragozic acids and quinuclidines, inhibitors of squalene synthase (SQS), which catalyzes the first committed step in sterol biosynthesis, (d) allylamines, inhibitors of squalene epoxidase, (e) azoles, which inhibit C14α-demethylase, and (f) azasterols, which inhibit Δ²⁴⁽²⁵⁾-sterol methyltransferase (SMT). Inhibition of this last step appears to have high selectivity for fungi and trypanosomatids, since this enzyme is not found in mammalian cells. We review here the IC50 values of these various inhibitors, their effects on the growth of trypanosomatids (both in axenic cultures and in cell cultures), and their effects on protozoan structural organization (as evaluted by light and electron microscopy) and lipid composition. The results show that the mitochondrial membrane as well as the membrane lining the protozoan cell body and flagellum are the main targets. Probably as a consequence of these primary effects, other important changes take place in the organization of the kinetoplast DNA network and on the protozoan cell cycle. In addition, apoptosis-like and autophagic processes induced by several of the inhibitors tested led to parasite death.

Kinetic studies of AKR1B10, human aldose reductase-like protein: endogenous substrates and inhibition by steroids

A human member of the aldo-keto reductase (AKR) superfamily, AKR1B10, was identified as a biomarker of lung cancer, exhibiting high sequence identity with human aldose reductase (AKR1B1). Using recombinant AKR1B10 and AKR1B1, we compared their substrate specificity for biogenic compounds and inhibition by endogenous compounds and found the following unique features of AKR1B10. AKR1B10 efficiently reduced long-chain aliphatic aldehydes including farnesal and geranylgeranial, which are generated from degradation of prenylated proteins and metabolism of farnesol and geranylgeraniol derived from the mevalonate pathway. The enzyme oxidized aliphatic and aromatic alcohols including 20alpha-hydroxysteroids. In addition, AKR1B10 was inhibited by steroid hormones, bile acids and their metabolites, showing IC(50) values of 0.03-25 microM. Kinetic analyses of the alcohol oxidation and inhibition by the steroids and tolrestat, together with the docked model of AKR1B10-inhibitor complex, suggest that the inhibitory steroids and tolrestat bind to overlapping sites within the active site of the enzyme-coenzyme complex. Thus, we propose a novel role of AKR1B10 in controlling isoprenoid homeostasis that is important in cholesterol synthesis and cell proliferation through salvaging isoprenoid alcohols, as well as its metabolic regulation by endogenous steroids.

d-δ-Tocotrienol-mediated suppression of the proliferation of human PANC-1, MIA PaCa-2, and BxPC-3 pancreatic carcinoma cells

OBJECTIVE

The rate-limiting activity of the mevalonate pathway, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, provides intermediates essential for growth. Competitive inhibitors of HMG CoA reductase, such as the statins, and down-regulators of reductase, such as the tocotrienols, suppress tumor growth. We evaluated the impact of d-delta-tocotrienol, the most potent vitamin E isomer, on human MIA PaCa-2 and PANC-1 pancreatic carcinoma cells and BxPC-3 pancreatic ductal adenocarcinoma cells.

METHODS

Cell proliferation was measured by using CellTiter 96 Aqueous One Solution (Promega, Madison, Wis). Cell cycle distribution was determined by flow cytometry. Apoptosis was evaluated by Annexin V staining and fluorescence microscopy after dual staining with acridine orange and ethidium bromide.

RESULTS

d-delta-Tocotrienol induced concentration-dependent suppression of cell proliferation with 50% inhibitory concentrations of 28 (6) micromol/L (MIA PaCa-2), 35 (7) micromol/L (PANC-1), and 35 (8) microL (BxPC-3), respectively. These effects are attributable to cell cycle arrest at the G1 phase and apoptosis. Mevalonate attenuated d-delta-tocotrienol-mediated growth inhibition. A physiologically attainable blend of d-delta-tocotrienol and lovastatin synergistically suppressed the proliferation of MIA PaCa-2 cells.

CONCLUSIONS

Suppression of mevalonate pathway activities, be it by modulators of HMG CoA reductase (statins, tocotrienols, and farnesol), farnesyl transferase (farnesyl transferase inhibitors), and/or mevalonate pyrophosphate decarboxylase (phenylacetate) activity, may have a potential in pancreatic cancer chemotherapy.

Quantitative determination of farnesyl and geranylgeranyl diphosphate levels in mammalian tissue

Farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP) are branch point intermediates of isoprenoid biosynthesis. Inhibitors of isoprenoid biosynthesis, such as the statins and bisphosphonates, are widely used therapeutic agents. However, little is known about the degree to which they alter levels of upstream and downstream isoprenoids, including FPP and GGPP. Therefore, we developed a method to isolate and quantify FPP and GGPP from mammalian tissues. Tissues from mice were collected, snap frozen in liquid nitrogen, and stored at -80 degrees C. FPP and GGPP were isolated by a combined homogenization and extraction procedure and were purified with a C18 solid phase extraction column. Farnesyl protein transferase (FTase) or geranylgeranyl protein transferase I (GGTase I) were used to conjugate FPP and GGPP with fluorescent dansylated peptides. FPP and GGPP were quantified by high-performance liquid chromatography (HPLC). The respective concentrations of FPP and GGPP are as follows: 0.355+/-0.030 and 0.827+/-0.082 units of nmol/g wet tissues in brain, 0.320+/-0.019 and 0.293+/-0.035 units of nmol/g wet tissues in kidney, 0.326+/-0.064 and 0.213+/-0.029 units of nmol/g wet tissues in liver, and 0.364+/-0.015 and 0.349+/-0.023 units of nmol/g wet tissues in heart (means+/-SEM). This method allows for determination of FPP and GGPP concentrations in any tissue type and is sensitive enough to detect changes following treatment with inhibitors of isoprenoid biosynthesis.

Arabidopsis thaliana plants possess a specific farnesylcysteine lyase that is involved in detoxification and recycling of farnesylcysteine

In plants, prenylated proteins are involved in actin organization, calcium-mediated signal transduction, and many other biological processes. Arabidopsis thaliana mutants lacking functional protein prenyltransferase genes have also revealed roles for prenylated proteins in phytohormone signaling and meristem development. However, to date, the turnover of prenylated plant proteins and the fate of the prenylcysteine (PC) residue have not been described. We have detected an enzyme activity in Arabidopsis plants that metabolizes farnesylcysteine (FC) to farnesal, which is subsequently reduced to farnesol. Unlike its mammalian ortholog, Arabidopsis FC lyase exhibits specificity for FC over geranylgeranylcysteine (GGC), and recognizes N-acetyl-FC (AFC). FC lyase is encoded by a gene on chromosome 5 of the Arabidopsis genome (FCLY, At5g63910) and is ubiquitously expressed in Arabidopsis tissues and organs. Furthermore, T-DNA insertions into the FCLY gene cause significant decreases in FC lyase activity and an enhanced response to abscisic acid (ABA) in seed germination assays. The effects of FCLY mutations on ABA sensitivity are even greater in the presence of exogenous FC. These data suggest that plants possess a specific FC detoxification and recycling pathway.

Enzymatic characteristics of an aldo-keto reductase family protein (AKR1C15) and its localization in rat tissues

A member of the aldo-keto reductase superfamily, AKR1C15, was isolated via cDNA cloning, but its physiological function remains unknown. Here, we show that recombinant AKR1C15 is an NADPH-dependent reductase with broad substrate specificity for aromatic, alicyclic and aliphatic carbonyl compounds, including acetoin, 2,5-hexanedione, methylglyoxal, farnesal, retinals, 17-ketosteroids and monosaccharides. Especially, all-trans-retinal, alpha-diketones and lipid-derived aldehydes including 4-hydroxynonenal were excellent substrates showing low K(m) values (0.3-5.5 microM). Immunohistochemical and reverse transcription-PCR analyses revealed that AKR1C15 is highly expressed in rat bronchiolar Clara cells, type II alveolar cells, gastric parietal cells, the epithelial cells of the stomach and colon, and the brown adipocytes. The enzyme was not detected in cells of other rat tissues, but is consistently expressed in the vascular endothelial cells. These results suggest that AKR1C15 plays a role in retinoid, steroid, isoprenoid and carbohydrate metabolism, as well as a defense system, protecting against reactive carbonyl compounds.

Antiproliferative and ultrastructural effects of BPQ-OH, a specific inhibitor of squalene synthase, on Leishmania amazonensis

Parasites of the Leishmania genus require for the growth and viability the de novo synthesis of specific sterols as such as episterol and 5-dehydroepisterol because cholesterol, which is abundant in their mammalian hosts, does not fulfill the parasite sterol requirements. Squalene synthase catalyzes the first committed step in the sterol biosynthesis and has been studied as a possible target for the treatment of high cholesterol levels in humans. In this work we investigated the antiproliferative and ultrastructural effects induced by 3-(biphenyl-4-yl)-3-hydroxyquinuclidine (BPQ-OH), a specific inhibitor of squalene synthase, on promastigote and amastigote forms of Leishmania amazonensis. BPQ-OH had a potent dose-dependent growth inhibitory effect against promastigotes and amastigotes, with IC(50) values 0.85 and 0.11 microM, respectively. Ultrastructural analysis of the treated parasites revealed several changes in the morphology of promastigote forms. The main ultrastructural change was found in the plasma membrane, which showed signs of disorganization, with the concomitant formation of elaborated structures. We also observed alterations in the mitochondrion-kinetoplast complex such as mitochondrial swelling, rupture of its internal membrane and an abnormal compaction of the kinetoplast. Other alterations included the appearance of multivesicular bodies, myelin-like figures, alterations of the flagellar membrane and presence of parasites with two or more nuclei and kinetoplasts. We conclude that the BPQ-OH was a potent growth inhibitor of L. amazonensis, which led to profound changes of the parasite's ultrastructure and might be a valuable lead compound for the development of novel anti-Leishmania agents.

ω-Hydroxylation of farnesol by mammalian cytochromes P450

Studies have shown that mammalian cytochromes p450 participate in the metabolism of terpenes, yet their role in the biotransformation of farnesol, an endogenous 15-carbon isoprenol, is unknown. In this report, [(14)C]-farnesol was transformed to more polar metabolites by NADPH-supplemented mammalian microsomes. In experiments with microsomes isolated from acetone-treated animals, the production of one polar metabolite was induced, suggesting catalysis by CYP2E1. The metabolite was identified as (2E, 6E, 10E)-12-hydroxyfarnesol. In studies with purified CYP2E1, 12-hydroxyfarnesol was obtained as the major product of farnesol metabolism. Among a series of available human p450 enzymes, only CYP2C19 also produced 12-hydroxyfarnesol. However, in individual human microsomes, CYP2E1 was calculated to contribute up to 62% toward total 12-hydroxyfarnesol production, suggesting CYP2E1 as the major catalyst. Mammalian cells expressing CYP2E1 demonstrated further farnesol metabolism to alpha,omega-prenyl dicarboxylic acids. Since such acids were identified in animal urine, the data suggest that CYP2E1 could be an important regulator of farnesol homeostasis in vivo. In addition, CYP2E1-dependent 12-hydroxyfarnesol formation was inhibited by pharmacological alcohol levels. Given that farnesol is a signaling molecule implicated in the regulation of tissue and cell processes, the biological activity of ethanol may be mediated in part by interaction with CYP2E1-dependent farnesol metabolism.

Isoprenoids: Remarkable diversity of form and function

The isoprenoid biosynthetic pathway is the source of a wide array of products. The pathway has been highly conserved throughout evolution, and isoprenoids are some of the most ancient biomolecules ever identified, playing key roles in many life forms. In this review we focus on C-10 mono-, C-15 sesqui-, and C-20 diterpenes. Evidence for interconversion between the pathway intermediates farnesyl pyrophosphate and geranylgeranyl pyrophosphate and their respective metabolites is examined. The diverse functions of these molecules are discussed in detail, including their ability to regulate expression of the beta-HMG-CoA reductase and Ras-related proteins. Additional topics include the mechanisms underlying the apoptotic effects of select isoprenoids, antiulcer activities, and the disposition and degradation of isoprenoids in the environment. Finally, the significance of pharmacological manipulation of the isoprenoid pathway and clinical correlations are discussed.

Farnesyl-O-acetylhydroquinone and geranyl-O-acetylhydroquinone suppress the proliferation of murine B16 melanoma cells, human prostate and colon adenocarcinoma cells, human lung carcinoma cells, and human leukemia cells

Farnesyl-O-acetylhydroquinone (IC(50)=2.5 microM/l) suppressed the proliferation of murine B16F10 melanoma cells with a potency much greater than those of farnesol (IC(50)=45 microM/l) and farnesyl anthranilate (IC(50)=46 microM/l), its alcohol, and ester counterparts with proven anti-tumor activities in vivo. Geranyl-O-acetylhydroquinone (IC(50)=5.1 microM/l) also had a much-improved activity compared to geraniol (IC(50)=160 microM/l) and geranyl anthranilate (IC(50)=30 microM/l). The suppression by farnesyl-O-acetylhydroquinone was concentration- and time-dependent and was accompanied by arrest of cell cycle at G1 and G2/M phases as shown by flow cytometry. Farnesyl-O-acetylhydroquinone and lovastatin had additive impact on B16 cell proliferation. Farnesyl-O-acetylhydroquinone also suppressed the proliferations of human cancer cells HL-60, DU145, PC-3, LNCaP, Caco-2, and A549. Our results suggested that farnesyl derivatives, suppressors of tumor 3-hydroxy-3-methylglutaryl coenzyme A reductase activities, have potential as chemopreventive or chemotherapeutic agents.

Squalene synthase as a chemotherapeutic target in Trypanosoma cruzi and Leishmania mexicana

Trypanosoma cruzi and Leishmania parasites have a strict requirement for specific endogenous sterols (ergosterol and analogs) for survival and growth and cannot use the abundant supply of cholesterol present in their mammalian hosts. Squalene synthase (SQS, E.C. 2.5.1.21) catalyzes the first committed step in sterol biosynthesis and is currently under intense study as a possible target for cholesterol-lowering agents in humans, but it has not been investigated as a target for anti-parasitic chemotherapy. SQS is a membrane-bound enzyme in both T. cruzi epimastigotes and Leishmania mexicana promastigotes with a dual subcellular localization, being almost evenly distributed between glycosomes and mitochondrial/microsomal vesicles. Kinetic studies showed that the parasite enzymes display normal Michaelis-Menten kinetics and the values of the kinetic constants are comparable to those of the mammalian enzyme. We synthesized and purified 3-(biphenyl-4-yl)-3-hydroxyquinuclidine (BPQ-OH), a potent and specific inhibitor of mammalian SQS and found that it is also a powerful non-competitive inhibitor of T. cruzi and L. mexicana SQS, with K(i)'s in the range of 12-62 nM. BPQ-OH induced a dose-dependent reduction of proliferation the extracellular stages of these parasites with minimal growth inhibitory concentrations (MIC) of 10-30 microM. Growth inhibition and cell lysis induced by BPQ-OH in both parasites was associated with complete depletion of endogenous squalene and sterols, consistent with a blockade of de novo sterol synthesis at the level of SQS. BPQ-OH was able to eradicate intracellular T. cruzi amastigotes from Vero cells cultured at 37 degrees C, with a MIC of 30 microM with no deleterious effects on host cells. Taken together, these results support the notion that SQS inhibitors could be developed as selective anti-trypanosomatid agents.

Volatile isoprenoid constituents of fruits, vegetables and herbs cumulatively suppress the proliferation of murine B16 melanoma and human HL-60 leukemia cells

Substantial evidence from epidemiological studies supports the inverse association between the intake of fruits, vegetables and other plant products and cancer incidence. Cancer-preventive constituents of fruits and vegetables may inhibit carcinogen activation, enhance carcinogen detoxification, prevent carcinogens from interacting with critical target sites, or impede tumor progression. These activities, however, are achievable only when levels of individual bioactive constituents reach beyond those attainable from a normal balanced diet. Isoprenoids, a broad class of mevalonate-derived phytochemicals ubiquitous in the plant kingdom, suppress the proliferation of tumor cells and the growth of implanted tumors. A search for volatile isoprenoid constituents of food products spanning seven plant families identified 179 isoprenoids. Of these, 41 purchased from commercial sources were screened for efficacy in suppressing the proliferation of murine B16 melanoma cells. Individual isoprenoids suppressed the proliferation of B16 and HL-60 promyelocytic leukemia cells with varying degrees of potency. Cell cycle arrest at the G(0)-G(1) phase and apoptosis account, at least in part, for the suppression. Blends of isoprenoids suppressed B16 and HL-60 cell proliferation with efficacies equal to the sum of the individual impacts. These findings suggest that the cancer-protective property of fruits, vegetables, and related products is partly conferred by the cumulative impact of volatile isoprenoid constituents.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Farnesol blocks the L-type Ca2+ channel by targeting the alpha 1C subunit

We recently demonstrated that farnesol, a 15-carbon isoprenoid, blocks L-type Ca2+ channels in vascular smooth muscle cells. To elucidate farnesol's mechanism of action, we performed whole-cell and perforated-patch clamp experiments in rat aortic A7r5 cells and in Chinese hamster ovary (CHO) C9 cells expressing smooth muscle Ca2+ channel alpha 1C subunits. Farnesol dose-dependently and voltage-independently inhibited Ba2+ currents in both A7r5 and CHOC9 cells, with similar half-maximal inhibitions at 2.6 and 4.3 micromol/L, [corrected] respectively (P=NS). In both cell lines, current inhibition by farnesol was prominent over the whole voltage range without changes in the current-voltage relationship peaks. Neither intracellular infusion of the stable GDP analogue guanosine-5'-O-(2-thiodiphosphate) (100 micromol/L) [corrected] via the patch pipette nor strong conditioning membrane depolarization prevented the inhibitory effect of farnesol, which indicates G protein-independent inhibition of Ca2+ channels. In an analysis of the steady-state inactivation curve for voltage dependence, farnesol induced a significant, negative shift ( approximately 10 mV) of the potential causing 50% channel inactivation in both cell lines (P<0. 001). In contrast, the steepness factor characterizing the voltage sensitivity of the channels was unaffected. Unlike pharmacological Ca2+ channel blockers, farnesol blocked Ca2+ currents in the resting state: initial block was 63+/-8% in A7r5 cells and 50+/-9% in CHOC9 cells at a holding potential of -80 mV. We then gave 500 mg/kg body weight farnesol by gavage to Sabra hypertensive and normotensive rats and found that farnesol reduced blood pressure significantly in the hypertensive strain for at least 48 hours. We conclude that farnesol may represent an endogenous smooth muscle L-type Ca2+ channel antagonist. Because farnesol is active in cells expressing only the pore-forming alpha1 subunit, the data further suggest that this subunit represents the molecular target for farnesol binding and principal action. Finally, farnesol has a blood pressure-lowering action that may be relevant in vivo.

A cytochrome P450 terpenoid hydroxylase linked to the suppression of insect juvenile hormone synthesis

A cDNA encoding a cytochrome P450 enzyme was isolated from a cDNA library of the corpora allata (CA) from reproductively active Diploptera punctata cockroaches. This P450 from the endocrine glands that produce the insect juvenile hormone (JH) is most closely related to P450 proteins of family 4 and was named CYP4C7. The CYP4C7 gene is expressed selectively in the CA; its message could not be detected in the fat body, corpora cardiaca, or brain, but trace levels of expression were found in the midgut and caeca. The levels of CYP4C7 mRNA in the CA, measured by ribonuclease protection assays, were linked to the activity cycle of the glands. In adult females, CYP4C7 expression increased immediately after the peak of JH synthesis, reaching a maximum on day 7, just before oviposition. mRNA levels then declined after oviposition and during pregnancy. The CYP4C7 protein was produced in Escherichia coli as a C-terminal His-tagged recombinant protein. In a reconstituted system with insect NADPH cytochrome P450 reductase, cytochrome b5, and NADPH, the purified CYP4C7 metabolized (2E,6E)-farnesol to a more polar product that was identified by GC-MS and by NMR as (10E)-12-hydroxyfarnesol. CYP4C7 converted JH III to 12-trans-hydroxy JH III and metabolized other JH-like sesquiterpenoids as well. This omega-hydroxylation of sesquiterpenoids appears to be a metabolic pathway in the corpora allata that may play a role in the suppression of JH biosynthesis at the end of the gonotrophic cycle.

Massive production of farnesol-derived dicarboxylic acids in mice treated with the squalene synthase inhibitor zaragozic acid A

The zaragozic acids are potent inhibitors of squalene synthase. In vivo studies in mice confirmed our earlier observations that inhibition of squalene synthase by zaragozic acid A was accompanied by an increase in the incorporation of label from [3H]mevalonate into farnesyl-diphosphate (FPP)-derived isoprenoic acids (J. D. Bergstrom et al., 1993, Proc. Natl. Acad. Sci. USA 90, 80-84). Farnesyl-diphosphate-derived metabolites appear transiently in the liver. We were unable to detect any farnesol formation in the zaragozic acid-treated animals which indicates that FPP is readily converted to farnesoic acid and dicarboxylic acids in the liver. These metabolites were found to be produced only in the liver and not in the kidney. trans-3,7-Dimethyl-2-octaen-1,8-dioic acid and 3, 7-dimethyloctan-1,8-dioic acid were identified as the major end products of farnesyl-diphosphate metabolism in the urine of mice treated with zaragozic acid A. Quantitative analysis of these FPP-derived dicarboxylic acids by gas-liquid chromatography revealed that approximately 11 mg of total dicarboxylic acids is excreted per day into the urine of a mouse after 3 days of treatment with zaragozic acid A.

Gas chromatography-mass spectrometric method for quantitative determination in human urine of dicarboxylic (dioic) acids produced in the body as a consequence of cholesterol biosynthesis inhibition

A capillary gas chromatography-mass spectrometric (GC-MS) method in human urine has been developed and validated for the quantitative determination of dicarboxylic acids (dioic acids) which are produced in the body as a consequence of the administration of an inhibitor of the enzyme squalene synthase, which is involved in the biosynthesis of cholesterol. The standards and quality control (QC) samples were prepared by adding dioic acids into human urine. Internal standard (sebacic acid) was added to each urine sample (0.1 ml) and then dried by evaporation under nitrogen. The dried sample was reacted with pentafluorobenzyl (PFB) bromide under conditions that maximized the formation of the di-PFB ester (at the expense of the mono-PFB ester) of the dioic acids. After drying by evaporation, each sample residue was reconstituted in mesitylene and injected into a capillary GC-MS system via a splitless injection. The detection was by negative ion chemical ionization mass spectrometry with selected ion monitoring (SIM) of the [M-PFB]- of the analytes and the internal standard.

Phosphorylation of farnesol in rat liver microsomes: properties of farnesol kinase and farnesyl phosphate kinase

As farnesol may serve as a nonsterol endogenous regulator of the mevalonate pathway, the possibility that a kinase specific for its phosphorylation is present in the rat liver was investigated. In the 10,000 g supernatant of rat liver, farnesyl monophosphate was synthesized in the presence of ATP. The Km value for farnesol was 2.3 microM. Various detergents inhibited the activity of the enzyme. The farnesol kinase was present in rough and in smooth I microsomes, but not in smooth II microsomes, mitochondria, peroxisomes, Golgi, or plasma membranes. The enzyme was associated with the inner, luminal surface of the vesicles. Further analyses have demonstrated that an enzymatic mechanism exists which catalyzes the phosphorylation of farnesyl-P to farnesyl-PP. Activity of the farnesyl phosphate kinase resulted in the phosphorylation of the monophosphate by CTP but not by ATP, GTP, or UTP. This enzyme is activated by low concentrations of detergents. Treatment with proteases and chemical probes indicate that this second phosphorylation reaction probably takes place on the outer, cytoplasmic surface of microsomal vesicles. These results demonstrate that rat liver microsomes contain two enzymes for the consecutive phosphorylation of farnesol to farnesyl-PP.

Farnesol as a regulator of HMG-CoA reductase degradation: characterization and role of farnesyl pyrophosphatase

We have recently reported that the isoprenoid compound farnesol accelerates degradation of the cholesterologenic enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, when added to cultured cells. We have thus proposed that farnesol is a required nonsterol regulator of this degradation event (T. E. Meigs, D. S. Roseman, and R. D. Simoni, 1996, J. Biol. Chem. 271, 7916-7922). In this report, we have studied the enzyme farnesyl pyrophosphatase (FPPase) in Chinese hamster ovary cells. We demonstrate that FPPase activity increases under conditions of increased metabolic flow through the isoprenoid pathway. Also, we show that a nonhydrolyzable analog of farnesyl pyrophosphate, an isoprenoid (phosphinylmethyl)phosphonate, inhibits FPPase in vitro, and when added to cells this inhibitor blocks the mevalonate-dependent, sterol-induced degradation of HMG-CoA reductase. Furthermore, exogenous farnesol overcomes the effect of this inhibitor. These results suggest an isoprenoid-mediated regulatory mechanism governing intracellular farnesol production and support the hypothesis that farnesol is a nonsterol regulator of reductase degradation.

Farnesol-derived dicarboxylic acids in the urine of animals treated with zaragozic acid A or with farnesol

Farnesyl diphosphate, the substrate for squalene synthase, accumulates in the presence of zaragozic acid A, a squalene synthase inhibitor. A possible metabolic fate for farnesyl diphosphate is its conversion to farnesol, then to farnesoic acid, and finally to farnesol-derived dicarboxylic acids (FDDCAs) which would then be excreted in the urine. Seven dicarboxylic acids were isolated by high performance liquid chromatography (HPLC) from urine of either rats or dogs treated with zaragozic acid A or rats fed farnesol. Their structures were determined by nuclear magnetic resonance analysis. Two 12-carbon, four 10-carbon, and one 7-carbon FDDCA were identified. The profile of urinary dicarboxylic acids from rats fed farnesol was virtually identical to that produced by treating with zaragozic acid A, establishing that these dicarboxylic acids are farnesol-derived. By feeding [1-14C]farnesol and comparing the mass of the dicarboxylic acids produced with the ultraviolet absorption of the HPLC peaks, a method to quantitate the ultraviolet-absorbing FDDCAs was devised. When rats were treated with zaragozic acid A, large amounts of FDDCAs were excreted in the urine. The high level of FDDCAs that were found suggests that their synthesis is the major metabolic fate for carbon diverted from cholesterol synthesis by a squalene synthase inhibitor. A metabolic pathway is proposed to explain the production of each of these FDDCAs.

Inhibition of squalene synthase in vitro by 3-(biphenyl-4-yl)-quinuclidine

Squalene synthase (SQS) catalyses a step following the final branch in the pathway of cholesterol biosynthesis. Inhibition of this enzyme, therefore, is an approach for the treatment of atherosclerosis with the potential for low side effects. We have characterised the inhibition of rat liver microsomal SQS by 3-(biphenyl-4-yl)quinuclidine (BPQ). BPQ follows slow binding kinetics in that the rate of accumulation of product decreases with time if the inhibitor is added when the assay is started. Preincubation of BPQ and SQS leads to a biphasic dose-response where accumulation of product is linear with time only for the sensitive phase. When the farnesyl pyrophosphate (FPP) substrate is present at 19.6 microM, approximately 77% of the SQS activity is sensitive to the inhibitor (vOs) and the remainder is insensitive (vOi). The apparent inhibition constants (K'i values) are respectively K'is = 4.5 nM and K'ii = 1300 nM. Similar biphasic behaviour is exhibited by other inhibitors and in microsomes prepared from human and marmoset liver. As the concentration of FPP is reduced below 19.6 microM, there is a decrease in the relative contribution from vOi. Conversely, the value of K'is for BPQ remains constant when the FPP concentration is changed, showing noncompetitive kinetics with respect to this substrate. Possible causes of the observed kinetics are discussed. Inhibition by BPQ is said to follow tight binding kinetics because the value of K'is is similar to the concentration of inhibitor binding sites. Thus, to avoid an artefactual variation in potency when the enzyme concentration is varied, it is necessary to allow for the effects of depletion of free inhibitor. Furthermore, estimates of potency that average activity across the two phases are influenced by the relative contributions of each phase. These contributions differ according to the FPP concentration and the species used as the source of microsomes. Thus, it is necessary to separate the phases to compare measurements made in different experiments. Our observations indicate that careful experimental design and data analysis are required to characterise the kinetics of SQS inhibitors.

Inhibition of squalene synthase of rat liver by novel 3' substituted quinuclidines

Squalene synthase (SQS) is a key enzyme in the biosynthetic pathway for cholesterol and is a target for improved agents to lower plasma levels of low-density lipoprotein (LDL). A series of novel 3' substituted quinuclidines have been discovered as inhibitors of the rat liver microsomal enzyme. In this study, we demonstrate the inhibitory effects in vitro and in vivo, of two examples of the series. When microsomes were preincubated with compounds, before addition of substrate, both 3-(biphenyl-4-yl)quinuclidine (BPQ) and 3-(biphenyl-4-yl)-3-hydroxyquinuclidine (BPQ-OH) were found to cause biphasic inhibition of the enzyme with apparent inhibition constants (K'i) for the sensitive phases of 12 nM and 15 nM, respectively. The K'i values for the insensitive phases were 1.8 microM and 2.9 microM, respectively. The two examples inhibited equally both steps of the SQS-catalysed reaction, as shown by parallel inhibition of 3H+ release and labelled squalene formation from [1-3H]farnesyl pyrophosphate (FPP). BPQ and BPQ-OH were shown to be inhibitors of hepatic sterol synthesis from mevalonate with ED50 values of 10.6 and 7.1 mg/kg, respectively, after acute oral administration to the rat. BPQ-OH was chosen for further study and, to determine its selectivity of effect on the mevalonate pathway in vivo, the effect of a dose of 70 mg/kg on the pattern of labelled mevalonate incorporation into the various lipid fractions of the rat liver was examined. As expected, the incorporation into squalene and sterol products was inhibited by about 70%. An appearance of label in fractions corresponding to farnesyl and geranylgeranylpyrophosphates, as well as the corresponding alcohols, was observed in treated but not control animals. In addition, the administration of compound resulted in the appearance of peaks of mevalonate-derived radioactivity in an acidic fraction believed to represent metabolites of farnesol. Such results are consistent with inhibition of the mevalonate pathway at, and not before, SQS. In contrast, there was a significant increase in the incorporation of labelled mevalonate into ubiquinone 10, and the synthesis of dolichols was apparently unchanged. The results suggest a specific effect of BPQ-OH on rat liver SQS. The compound is, therefore, an interesting lead for further investigation of this class of compounds.

Induction of geranyl pyrophosphate pyrophosphatase activity by cholesterol-suppressive isoprenoids

Diets supplemented (1 mmol/kg) with thymol, carvacrol, and beta-ionone significantly decreased the serum cholesterol levels of cockerels. These mevalonate-derived end products of plant secondary metabolism (isoprenoids) had no impact on two cytosolic prenyl alcohol (and ethanol) dehydrogenase activities; each treatment increased microsomal geranyl pyrophosphate pyrophosphatase activity by greater than twofold. The structural diversity of the isoprenoids which suppress cholesterol synthesis may be reconciled by their ability to increase pyrophosphatase activity, thus leading to the production of the endogenous, post-transcriptional regulator of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity.

Isopentenoid synthesis in isolated embryonic Drosophila cells: absolute mevalonic acid utilization and 3-hydroxy-3-methylglutaryl-coenzyme A reductase modulation

The relationship between absolute isopentenoidogenesis (total and specific) and 3-hydroxy-3-methylglutaryl-coenzyme A suppression in response to increased mevalonate availability is unknown. We determined absolute isopentenoidogenesis values for the nonsterologenic Drosophila Kc cell incubated (2 h) with increasing [3H]mevalonate concentrations. At least 80% of the maximum suppression of 3-hydroxy-3-methyl glutaryl-co-enzyme A activity was achieved when total isopentenoidogenesis was increased only 2-fold. However, a 12-fold increase in total isopentenoidogenesis was achieved at higher exogenous [3H]mevalonate concentrations. Thus, modulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity was coupled to physiological changes in mevalonate/nonsterol isopentenoid availability. In contrast, isopentenoid accumulation, oxidation, and secretion were enhanced with pharmacological increases in mevalonate availability. Furthermore, an apparent constancy of total isopentenoidogenesis values plus increased metabolism of exogenous mevalonate and a significant (35-45%) suppression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, in response to exogenous substrate concentrations (less than 150 microM), supported a partial or complete compensatory dimunition in endogenous substrate synthesis. Since these responses occurred within the 2-h study, earlier time periods must be assessed to (i) define the initial nonsterol-mediated regulatory response and (ii) to trap the nonsterol isopentenoid regulatory signal molecule(s).

Isopentenoid synthesis in isolated embryonic Drosophila cells: absolute, basal mevalonate synthesis rate determination

Embryonic Drosophila cells (Kc cells) and [5-3H]mevalonate (less than or equal to 10 microM) were used to determine the absolute basal in vivo rate of total mevalonic acid synthesis/utilization. An absolute in vivo mevalonic acid synthesis rate of 0.69 nmol/h/mg total cell protein was measured. Absolute mevalonate utilization was obtained by correcting for the extent of endogenous dilution of exogenous [3H]mevalonate at isotopic equilibrium. Cellular [3H]farnesol specific radioactivity was used as representative of a rapidly turning over isopentenoid pool. Although our previous Kc cell study (Havel, C. M., Rector, E. R. II, Watson, J. A., 1986, J. Biol. Chem. 261, 10,150-10,156) demonstrated that greater than or equal to 40% of the metabolized [3H]mevalonate appeared as 3H-labeled media water, this report established that t,t-3,7,11-[3H]trimethyl-2,6,10-dodecatriene-1,12 dioic acid was also secreted. Media accumulation of the C15-alpha,omega-prenyl dioic acid and 3H2O was related directly to [3H]mevalonic acid availability. This is the first mevalonate carbon balance study reported for a eukaryotic organism. It was concluded that (i) Kc cells synthesized more mevalonate than needed for normal growth and essential isopentenoids and (ii) excess mevalonate carbon accumulated intra- and extracellularly as isopentenoid compounds distal to C5 products. Finally, this study emphasized the need to measure total mevalonate utilization and not mevalonate conversion to a single isopentenoid end product in carbon balance investigations.

Cell-cycle-dependent, differential prenylation of proteins

Isoprenylated proteins related to cell growth have been detected during proliferation. Since cholesterogenesis (isoprenoid synthesis) is mandatory for cell proliferation, the observation of a temporally coordinated protein prenylation during the cell division cycle might constitute obligatory processes in the signalling pathway for initiating DNA replication and/or in maintaining the growing state. We have found such a definitive cell-cycle-phase-dependent pattern of prenylation for various classes of cytosolic and nuclear matrix proteins in synchronized HepG2 cells. Characteristic [3H]mevalonate incorporation began to increase during mid-to-late G1, just after cholesterol synthesis reached its apex, and peaked just prior to or coincident with mid S. Incorporation then declined subsequent to S (during G2) as cells approached mitosis. Prior to the rise in mevalonate incorporation into proteins, during early-to-mid G1, steady-state [14C]acetate incorporation into chromatographically resolved cholesterogenic lipid intermediates displayed a maximum only into cholesterol. However, during the late-G1/S interval, a singular peak of 14C incorporation was found for the farnesyl moiety (farnesol/nerolidol plus farnesyl diphosphate). Except for the farnesyl moiety, none of the other polyisoprenoids detected by our procedures showed any fluctuation in 14C incorporation subsequent to mid G1. These results support the proposal that subsequent to peak cholesterol synthesis in early-to-mid G1, the generation of a cholesterol-pathway-dependent set of post-translationally modified, polyisoprenylated proteins could constitute an obligatory step leading to the duplication of the cellular genome, thereby impelling transit through the cell cycle. The well known high flux through cholesterogenesis in tumors, which manifests an intrinsic lack of sensitivity to feedback inhibition and operates continuously, is consonant with this proposal.