No abstract

Three closely related fungal metabolites, The mammalian isoprenoid pathway not only produces sterols but also produces dolichol, ubiquinone, the farnesyl group of heme A, the farnesyl and geranylgeranyl groups of prenylated proteins, and the isopentenyl side chain of isopentenyl adenine. The pathways for the synthesis of these other isoprenoids diverge from the synthesis ofcholesterol either at or before the farnesyl diphosphate (FPP) branch point. Thus, squalene synthase, which catalyzes the reductive dimerization of 2 mol of FPP to 1 mol of squalene (2, 3), is the first committed step in sterol synthesis. A specific inhibitor of squalene synthase should serve to inhibit cholesterol synthesis and not adversely affect the synthesis of other isoprenoids. FPP, the substrate for squalene synthase, is water soluble and may be readily metabolized (4). Thus, squalene synthase offers a potential target for the safe and specific inhibition of cholesterol synthesis.In this report we describe the isolation, structure, physical characterization, and biological properties of three structurally similar fungal metabolites that are potent inhibitors of squalene synthase. These metabolites, zaragozic acid A (5-7), zaragozic acid B (8, 9), and zaragozic acid C (10-12), had been reported previously only in the patent literature; however, during the review process of this manuscript, three manuscripts (13-15) appeared on the squalestatins: squalestatin I is identical with zaragozic acid A, squalestatin II is des-4'-acetylzaragozic acid A, and squalestatin III is des-6-acylzaragozic acid A. This class of squalene synthase inhibitors has potential utility as cholesterol-lowering agents. MATERIALS AND METHODSZaragozic Acid A, Cultures, and Media. An unidentified sterile fungal culture, ATCC 20986, isolated from a water sample taken from the Jalon river in Zaragoza, Spain (hence the name zaragozic acids), was used to produce zaragozic acid A. The culture was maintained at 25°C on medium B agar slants composed of 4 g of yeast extract, 10 g of malt extract, 4 g of dextrose, and 20 g of agar per liter at pH 7.0.Zaragozic acid A was produced in a two-tiered fermentation process consisting of mycelial growth and development in medium A of ref. 1 and product formation in medium C. Medium C contained 5 g of malt extract, 1 g of peptone, 15 g of dextrose, 1 g of KH2PO4, and 0.5 g of MgSO4 7H20 per liter. Fermentations consisted of mycelial growth in medium A for 72 hr at 250C with agitation, followed by inoculation (5-10%) of medium C. Maximum product was obtained from 14-day agitated fermentations at 250C.Isolation of Zaragozic Acid A. To isolate zaragozic acid A, 23 liters of harvested broth was filtered through Celite, and the mycelial cake was extracted twice with 7 liters of 50%o aqueous methanol. The filtrate was combined with the extracts, diluted with water to a final composition of 25% methanol, and adsorbed on a 1.5-liter column of Mitsubishi HP-20 resin. After a column wash with 6 liters of4:6 (vol/vol) methanol/water, crud...

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 farnesolderived. By feeding [1-14 C]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.Squalene synthase is an attractive target for the development of a cholesterol synthesis inhibitor that could serve as a cholesterol lowering agent. Cholesterol synthesis inhibitors, such as lovastatin (1), a 3-hydroxy-3-methylglutaryl-coenzyme A inhibitor, are effective cholesterol-lowering agents in man and/or animals. Squalene synthase catalyzes the first committed step in cholesterol synthesis, and selective inhibition of this enzyme should result in inhibition of cholesterol synthesis without affecting the synthesis of other isoprenoids such as dolichol, ubiquinone, and the prenylated proteins. A novel class of fungal metabolites, known as zaragozic acids, has been recently discovered and characterized as potent inhibitors of squalene synthase (2-8). The zaragozic acids are subnanomolar inhibitors of squalene synthase in vitro, they inhibit cholesterol synthesis from acetate or mevalonate in cell culture and in animal models, and also have been shown to lower plasma cholesterol when administered orally in certain animal species (4, 5,8). Other classes of squalene synthase inhibitors have also been discovered (for a review, see Ref. 9).An important question to be answered for these compounds is the metabolic effect of inhibition of squalene synthase. The squalene synthase reaction consists of the reductive dimerization of two molecules of farnesyl diphosphate (FPP) 1 to form a molecule of squalene (10). The primary consequence of inhibition of this reaction would be an accumulation of FPP. Thus, the metabolic fate of FPP in the presence of a squalene synthase inhibitor is of interest. Previous wor...

No abstract

No abstract

No abstract

No abstract

A partial length cDNA encoding farnesyl pyrophosphate synthetase (hpt807) has been isolated from a human fetal liver cDNA library in lambda gt11. DNA sequence analysis reveals hpt807 is 1115 bp in length and contains an open reading frame coding for 346 amino acids before reaching a stop codon, a polyadenylation addition sequence, and the first 14 residues of a poly(A+) tail. Considerable nucleotide and deduced amino acid sequence homology is observed between hpt807 and previously isolated rat liver cDNAs for farnesyl pyrophosphate synthetase. Comparison with rat cDNAs suggests that hpt807 is about 20 bp short of encoding the initiator methionine of farnesyl pyrophosphate synthetase. The human cDNA was cloned into a prokaryotic expression vector and Escherichia coli strain DH5 alpha F'IQ was transformed. Clones were isolated that express an active fusion protein which can be readily observed on protein gels and specifically stained on immunoblots with an antibody raised against purified chicken farnesyl pyrophosphate phosphate synthetase. These data confirm the identity of hpt807 as encoding farnesyl pyrophosphate synthetase. Slot blot analyses of RNA isolated from Hep G2 cells show that the expression of farnesyl pyrophosphate synthetase mRNA is regulated. Lovastatin increases mRNA levels for farnesyl pyrophosphate synthetase 2.5-fold while mevalonic acid, low-density lipoprotein, and 25-hydroxycholesterol decrease mRNA levels to 40-50% of control values.