Cloning and functional expression of the cDNA encoding rat lanosterol 14-alpha demethylase

Pre-cholesterol precursors in gametogenesis

Meiosis activating sterols (MAS) are biologically active post-lanosterol intermediates of cholesterol biosynthesis that are synthetized primarily in the gonads, including the sperm. MAS reinitiate the meiosis of oocytes in vitro while in vivo they seem to contribute to the oocyte quality and the progression of meiosis. The mRNAs for the MAS-producing enzyme lanosterol 14alpha-demethylase (CYP51) arise by alternative poly (A) signal selection. Only signals with low cleavage activity are used in the testis. Translation of mammalian CYP51s starts at one of the tandem in-frame ATGs. CYP51 protein of the bull is shorter compared to the human due to the usage of a more downstream translation start site. CYP51 proteins are post-translationally modified by glycosylations in the Golgi and on acrosomal membranes of the sperm. Green fluorescence protein-based ex vivo system has been developed to aid studying the intracellular transport of the MAS-producing CYP51. The influence of the post-translational modifications on MAS-synthesizing capacity is under investigation.

Cytotoxic effect of replication-competent adenoviral vectors carrying L-plastin promoter regulated E1A and cytosine deaminase genes in cancers of the breast, ovary and colon

Prodrug activating transcription unit gene therapy is one of several promising approaches to cancer gene therapy. Combining that approach with conditionally replication-competent viral vectors that are truly tumor specific has been an important objective of recent work. In this study, we report the construction of a new conditionally replication-competent bicistronic adenoviral vector in which the cytosine deaminase (CD) gene and the E1a gene are driven by the L-plastin tumor-specific promoter (AdLpCDIRESE1a). A similar vector driven by the CMV promoter has also been constructed (AdCMVCDIRESE1a) as a control. We have carried out in vitro cytotoxicity in carcinomas of the breast, ovary and colon, and in vivo efficacy studies with these vectors in an animal model of colon cancer. While the addition of the AdLpCDIRESE1a vector to established cancer cell lines showed significant cytotoxicity in tumor cells derived from carcinomas of the breast (MCF-7), colon (HTB-38) and ovary (Ovcar 5), no significant toxicity was seen in explant cultures of normal human mammary epithelial cells (HMEC) exposed to this vector. The addition of 5-fluorocytosine (5FC) significantly increased the cytotoxicity in an additive fashion of both the AdLpCDIRESE1a and AdCMVCDIRESE1a vectors as well as that of the AdLpCD replication incompetent vector to established tumor cell lines. However, no significant cytotoxicity was observed with the addition of 5FC to explant cultures of normal human mammary epithelial cells that had been exposed to the L-plastin-driven vectors. Studies with mixtures of infected and uninfected tumor cell lines showed that the established cancer cell lines infected with the AdLpCDIRESE1a vector generated significant toxicity to surrounding uninfected cells (the "bystander effect") even at a ratio of 0.25 of infected cells to infected + uninfected cells in the presence of 5FC. The injection of the AdLpCDIRESE1a vector into subcutaneous deposits of human tumor nodules in the nude mice was potentiated by administering 5-FC by intraperitoneal injection. This treatment resulted in a decreased tumor size and a decreased tumor cell growth rate. The mice treated with a combination of the AdLpCDIRESE1a vector intratumoral injection and intraperitoneal 5FC injections lived much longer than the other experimental groups exposed to the viral vector alone or to the combination of the intratumoral AdLpCD replication incompetent vector injections plus intraperitoneal 5-FC injections. These encouraging results with our newly constructed AdLpCDIRESE1a vector suggest a need for further study of its utility in a preclinical model of intracavitary therapy of pleural or peritoneal carcinomatosis.

Sterol regulatory element-binding proteins induce an entire pathway of cholesterol synthesis

To evaluate the effects of sterol regulatory element-binding proteins (SREBPs) on the expression of the individual enzymes in the cholesterol synthetic pathway, we examined expression of these genes in the livers from wild-type and transgenic mice overexpressing nuclear SREBP-1a or -2. As estimated by a Northern blot analysis, overexpression of nuclear SREBP-1a or -2 caused marked increases in mRNA levels of the whole battery of cholesterogenic genes. This SREBP activation covers not only rate-limiting enzymes such as HMG CoA synthase and reductase that have been well established as SREBP targets, but also all the enzyme genes in the cholesterol synthetic pathway tested here. The activated genes include mevalonate kinase, mevalonate pyrophosphate decarboxylase, isopentenyl phosphate isomerase, geranylgeranyl pyrophosphate synthase, farnesyl pyrophosphate synthase, squalene synthase, squalene epoxidase, lanosterol synthase, lanosterol demethylase, and 7-dehydro-cholesterol reductase. These results demonstrate that SREBPs activate every step of cholesterol synthetic pathway, contributing to an efficient cholesterol synthesis.

Role of meiosis-activating sterols in rat oocyte maturation: effects of specific inhibitors and changes in the expression of lanosterol 14alpha-demethylase during the preovulatory period

In vitro studies on mouse oocytes have shown that two closely related sterols, subsequently named meiosis-activating sterols (MAS), can overcome the inhibitory effect of hypoxanthine on the resumption of meiosis. These sterols are synthesized by cytochrome P(450) lanosterol 14alpha-demethylase (LDM), a key enzyme in cholesterol biosynthesis. We have used specific inhibitors of LDM, azalanstat (RS-21607) and RS-21745, to test whether MAS is an obligatory mediator in the resumption of meiosis in the rat. Addition of azalanstat and RS-21745 (1-200 microM) to culture medium of rat isolated cumulus-enclosed oocyte and preovulatory follicle-enclosed oocyte stimulated by LH/hCG did not allow separation between their inhibition of the resumption of meiosis and the degeneration of oocytes. In both models, doses of the drug that inhibited oocyte maturation also increased oocyte degeneration. The inhibitors only partially suppressed follicular progesterone production. We have examined by reverse transcriptase-polymerase chain reaction, Western blotting, and immunocytochemistry the ovarian expression of LDM mRNA and protein during the preovulatory period. We did not find evidence for the stimulation of this enzyme by LH/hCG. The strongest staining by LDM antiserum was obtained in primordial and primary oocytes, and the staining was reduced with oocyte growth. In addition, strong LDM staining could be observed in some of the granulosa cells, especially of the corona radiata localized in close proximity to the oocyte. In conclusion, our results with specific inhibitors and molecular approaches do not reveal evidence to support the hypothesis that MAS is an obligatory step in the stimulation of the resumption of meiosis. Specific inhibitors of MAS synthesis did not prevent spontaneous or LH-stimulated meiosis at doses that have previously been shown to effectively suppress LDM activity. Much higher concentrations of the inhibitors, which affected meiosis, were detrimental to oocytes, leading to their degeneration. The timing of LDM expression in the ovary was incompatible with a role for MAS in meiosis. Finally, the preferential localization of LDM protein to the oocytes suggests MAS production in oocytes rather than its transport from the somatic compartment as implied by the proposed role of MAS as a cumulus-oocyte signal molecule.

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.

Characteristics of the heterologously expressed human lanosterol 14alpha-demethylase (other names: P45014DM, CYP51, P45051) and inhibition of the purified human and Candida albicans CYP51 with azole antifungal agents

Human and Candida albicans CYP51 were purified to homogeneity after GAL10-based heterologous expression in yeast in order to resolve the basis for the selective inhibition of the fungal enzyme over the human orthologue by the azole drugs ketoconazole and itraconazole, used in the treatment of systemic fungal infection. The purified proteins have similar spectral characteristics, both giving a maximum at 448 nm in reduced carbon monoxide difference spectra. Substrate affinity constants of 20.8 and 29.4 microM and Vmax of 0. 15 and 0.47 nmol/min/nmol were observed for C. albicans and human enzymes, respectively, in reconstituted enzymatic assays, using an intermediate of the demethylation reaction [32-3H]-3beta-hydroxylanost-7-en-32-ol as the substrate. Both enzymes gave similar type II spectra on titration with drugs, but a reduced affinity was observed for human CYP51 using the ability of carbon monoxide to displace the drug as a ligand and by calculation of IC50. However, although the results indicate higher affinity of the drugs for their target CYP51 in the major fungal pathogen C. albicans, when compared directly to CYP51 from humans, the difference was less than 10-fold. This difference is an order of magnitude lower than previously reported data based on measurements using unpurified human CYP51 enzyme preparations. Consequently, increased azole doses to combat resistant candidaemia may well inhibit endogenous human CYP51 and the potential consequences are discussed.

Optimized expression and catalytic properties of a wheat obtusifoliol 14alpha-demethylase (CYP51) expressed in yeast. Complementation of erg11Delta yeast mutants by plant CYP51

CYP51s form the only family of P450 proteins conserved in evolution from prokaryotes to fungi, plants and mammals. In all eukaryotes, CYP51s catalyse 14alpha-demethylation of sterols. We have recently isolated two CYP51 cDNAs from sorghum [Bak, S., Kahn, R.A., Olsen, C. E. & Halkier, B.A. (1997) Plant J. 11, 191-201] and wheat [Cabello-Hurtado, F., Zimmerlin, A., Rahier, A., Taton, M., DeRose, R., Nedelkina, S., Batard, Y., Durst, F., Pallett, K.E. & Werck-Reichhart, D. (1997) Biophys. Biochem. Res. Commun. 230, 381-385]. Wheat and sorghum CYP51 proteins show a high identity (92%) compared with their identity with their fungal and mammalian orthologues (32-39%). Data obtained with plant microsomes have previously suggested that differences in primary sequences reflect differences in sterol pathways and CYP51 substrate specificities between animals, fungi and plants. To investigate more thoroughly the properties of the plant CYP51, the wheat enzyme was expressed in yeast strains overexpressing different P450 reductases as a fusion with either yeast or plant (sorghum) membrane targeting sequences. The endogenous sterol demethylase gene (ERG11) was then disrupted. A sorghum-wheat fusion protein expressed with the Arabidopsis thaliana reductase ATR1 showed the highest level of expression and activity. The expression induced a marked proliferation of microsomal membranes so as to obtain 70 nmol P450.(L culture)-1, with CYP51 representing 1.5% of microsomal protein. Without disruption of the ERG11 gene, the expression level was fivefold reduced. CYP51 from wheat complemented the ERG11 disruption, as the modified yeasts did not need supplementation with exogenous ergosterol and grew normally under aerobic conditions. The fusion plant enzyme catalysed 14alpha-demethylation of obtusifoliol very actively (Km,app = 197 microm, kcat = 1.2 min-1) and with very strict substrate specificity. No metabolism of lanosterol and eburicol, the substrates of the fungal and mammalian CYP51s, nor metabolism of herbicides and fatty acids was detected in the recombinant yeast microsomes. Surprisingly lanosterol (Ks = 2.2 microM) and eburicol (Ks = 2.5 microm) were found to bind the active site of the plant enzyme with affinities higher than that for obtusifoliol (Ks = 289 microM), giving typical type-I spectra. The amplitudes of these spectra, however, suggested that lanosterol and eburicol were less favourably positioned to be metabolized than obtusifoliol. The recombinant enzyme was also used to test the relative binding constants of two azole compounds, LAB170250F and gamma-ketotriazole, which were previously reported to be potent inhibitors of the plant enzyme. The Ks of plant CYP51 for LAB170250F (0.29 microM) and gamma-ketotriazole (0.40 microM) calculated from the type-II sp2 nitrogen-binding spectra were in better agreement with their reported effects as plant CYP51 inhibitors than values previously determined with plant microsomes. This optimized expression system thus provides an excellent tool for detailed enzymological and mechanistic studies, and for improving the selectivity of inhibitory molecules.

Human lamin B receptor exhibits sterol C14-reductase activity in Saccharomyces cerevisiae

Lamin B receptor (LBR), a nuclear protein of avian and mammalian cells, contains an hydrophobic domain that shares extensive structural similarities with the members of the sterol reductase family. To test if the sterol-reductase-like domain of LBR could be enzymatically competent, several sterol reductase-defective strains of Saccharomyces cerevisiae were transformed with a human-LBR expressing vector. LBR production did not change the ergosterol biosynthesis defect in an erg4 mutant impaired in sterol C24(28) reductase. In contrast, the sterol C14 reduction step and ergosterol prototrophy were restored in LBR-producing erg24 transformants which lack endogenous sterol C14 reductase. To test the effects of C14 reductase inhibitors on LBR activity, we constructed EMY54, an ergosterol-requiring strain that is devoid of both sterol C8-C7 isomerase and sterol C14 reductase activities. EMY54 cells recovered the capability of synthesizing ergost-8-en-3beta-ol upon transformation with a vector that expressed either yeast sterol C14 reductase or hLBR. In addition, growth in sterol-free medium was restored in these transformants. Sterol biosynthesis and proliferation of LBR-producing cells were found to be highly susceptible to fenpropimorph and tridemorph, but only moderately susceptible to SR 31747. Our results strongly suggest that hLBR is a sterol C14 reductase.

The dynamics of prostaglandin H synthases. Studies with prostaglandin h synthase 2 Y355F unmask mechanisms of time-dependent inhibition and allosteric activation

Prostaglandin H synthases (PGHSs) catalyze the conversion of arachidonic acid to prostaglandins. In this report, we describe the effect of a PGHS2 Y355F mutation on the dynamics of PGHS2 catalysis and inhibition. Tyr355 is part of a hydrogen-bonding network located at the entrance to the cyclooxygenase active site. The Y355F mutant exhibited allosteric activation kinetics in the presence of arachidonic acid that was defined by a curved Eadie-Scatchard plot and a Hill coefficient of 1.36 +/- 0.05. Arachidonic acid-induced allosteric activation has not been directly observed with wild type PGHS2. The mutation also decreased the observed time-dependent inhibition by indomethacin, flurbiprofen, RS-57067, and SC-57666. Detailed kinetic analysis showed that the Y355F mutation decreased the transition state energy associated with slow-binding inhibition (EIdouble dagger) relative to the energy associated with catalysis (ESdouble dagger) by 1.33, 0.67, and 1.06 kcal/mol, respectively, for indomethacin, flurbiprofen, and RS-57067. These observations show Tyr355 to be involved in the molecular mechanism of time-dependent inhibition. We interpret these results to indicate that slow binding inhibitors and the Y355F mutant slow the rate and unmask intrinsic, dynamic events associated with product formation. We hypothesize that the dynamic events are the equilibrium between relaxed and tightened organizations of the hydrogen-bonding network at the entrance to the cyclooxygenase active site. It is these rearrangements that control the rate of substrate binding and ultimately the rate of prostaglandin formation.

Cloning and sequence analysis of the eburicol 14alpha-demethylase gene of the obligate biotrophic grape powdery mildew fungus

In order to obtain molecular data concerning field resistance of Uncinula necator, the causal agent of grape powdery mildew, to sterol demethylation inhibitors, a major group of fungicides, the gene encoding the target of these compounds (eburicol 14alpha-demethylase) was cloned and sequenced from this obligately biotrophic phytopathogenic fungus. This single-copy gene encodes a 524 amino acid protein which displays high similarity to other known sterol 14alpha-demethylases (CYP51s). The coding sequence is interrupted by two short introns at positions identical to introns in Penicillium italicum CYP51, which is the only other known CYP51 gene in which introns have been identified. Intron excision was verified by cDNA sequencing.

A mutation in the 14 alpha-demethylase gene of Uncinula necator that correlates with resistance to a sterol biosynthesis inhibitor

We investigated the molecular basis of resistance of the obligate biotrophic grape powdery mildew fungus Uncinula necator to sterol demethylation-inhibiting fungicides (DMIs). The sensitivity of 91 single-spore field isolates of U. necator to triadimenol was assessed by using a leaf disc assay. Resistance factors (RF) ranged from 1.8 to 26.0. The gene encoding the target of DMIs (eburicol 14 alpha-demethylase) from five sensitive and seven resistant isolates was cloned and sequenced. A single mutation, leading to the substitution of a phenylalanine residue for a tyrosine residue at position 136, was found in all isolates exhibiting an RF higher than 5. No mutation was found in sensitive or weakly resistant (RF, < 5) isolates. An allele-specific PCR assay was developed to detect the mutation. Among the 91 isolates tested, only isolates with RF higher than 5 carried the mutation. Three of the 19 resistant isolates and all sensitive and weakly resistant isolates did not possess the mutation. The mutation at codon 136 is thus clearly associated with high levels of resistance to triadimenol.

Characterization of Saccharomyces cerevisiae CYP51 and a CYP51 fusion protein with NADPH cytochrome P-450 oxidoreductase expressed in Escherichia coli

Saccharomyces cerevisiae CYP51, target of azole antifungal agents, and CYP51 fused with S. cerevisiae cytochrome P-450 oxidoreductase (FUS protein) were expressed in active forms in Escherichia coli by cloning into pET15b. The expression was monitored immunologically, catalytically, and by using reduced carbon monoxide difference and type II binding spectra. CYP51 and FUS enzymes were located in membranes and produced a Soret peak at 448 nm in the reduced CO difference spectrum. The cytochrome P-450 contents in the membrane fractions containing CYP51 and FUS proteins were 12.8 +/- 2.6 and 17.4 +/- 3.7 pmol/mg of protein, respectively. The NADPH cytochrome P-450 oxidoreductase (CPR) content was estimated to be 15.7 +/- 1.1 pmol/mg of protein in FUS membrane fractions. FUS protein catalyzed the demethylation of substrate at the 14alpha position, with a turnover number of 1.96 +/- 0.37 min(-1) in the presence of NADPH. No reductase activity was observed in membrane fractions containing CYP51, and therefore, CYP51 did not function catalytically in the presence of NADPH, but in the presence of an artificial electron donor, cumene hydroperoxide, activity was comparable to that of the FUS enzyme. Further support for a normal structure for the hemoproteins was obtained from type II binding spectra, in which the spectral response was saturated with an equimolar concentration of ketoconazole.

Cloning and functional expression in yeast of a cDNA coding for an obtusifoliol 14alpha-demethylase (CYP51) in wheat

Screening of a wheat cDNA library with an heterologous CYP81B1 probe from Helianthus tuberosus led to the isolation of a partial cDNA coding a protein with all the characteristics of a typical P450 with high homology (32-39% identity) to the fungal and mammalian CYP51s. Extensive screening of several wheat cDNA libraries isolated a longer cDNA (W516) coding a peptide of 453 amino acids. Alignment of W516 with other P450 sequences revealed that it was missing a segment corresponding to the N-terminal membrane anchor of the protein. The corresponding segment from the yeast lanosterol 14alpha-demethylase was linked to the partial wheat cDNA and the chimera expressed in Saccharomyces cerevisiae. Compared to microsomes from control yeasts, membranes of yeast expressing the chimera catalysed 14alpha-demethylation of obtusifoliol with an increased efficiency relative to lanosterol demethylase activity. W516 is thus a plant member of the most ancient and conserved P450 family, CYP51.

Isolation and reconstitution of the heme-thiolate protein obtusifoliol 14alpha-demethylase from Sorghum bicolor (L.) Moench

The heme-thiolate (cytochrome P450) enzyme which catalyzes the 14alpha-demethylation of obtusifoliol has been isolated from microsomes prepared from etiolated seedlings of Sorghum bicolor (L.) Moench. The obtusifoliol 14alpha-demethylase is a key enzyme in plant sterol biosynthesis and a target for the design of phyla-specific sterol 14alpha-demethylase inhibitors. Microsomal cytochrome P450s were solubilized by using the detergents Renex 690 and reduced Triton X-100, and the obtusifoliol 14alpha-demethylase was isolated by DEAE ion exchange and dye affinity column chromatography. The isolated enzyme has an absorption spectrum characteristic for low spin cytochrome P450s and produces a Type I binding spectrum with obtusifoliol as substrate. Binding spectra were not obtained with lanosterol, campesterol, sitosterol, or stigmasterol. Obtusifoliol 14alpha-demethylase has an apparent molecular mass of 53 kDa and is estimated to constitute approximately 20% of the total cytochrome P450 content of the microsomal membranes and about 0.2% of the total microsomal protein. Gas chromatography-mass spectrometry analysis of reconstitution experiments with dilauroylphosphatidylcholine micelles containing isolated obtusifoliol 14alpha-demethylase and sorghum NADPHcytochrome P450 oxidoreductase demonstrated the conversion of obtusifoliol (4alpha,14alpha-dimethyl-5alpha-ergosta-8, 24(28)-dien-3beta-ol) to 4alpha-methyl-5alpha-ergosta-8,14, 24(28)-trien3beta-ol, the 14alpha-demethylated product of obtusifoliol with a double bond introduced at the Delta14 position. The N-terminal amino acid sequence of the protein is MDLADIPQ/KQQRLMAGXALVV. Five internal sequences were obtained after endoproteinase Lys-C and Glu-C digestion. The fragment AAGAFSYISFGGGRH aligns with the unique heme binding domain of mammalian and yeast sterol 14alpha-demethylases which belong to the CYP51 family. Therefore it is conceivable that the obtusifoliol 14alpha-demethylase from plants also belongs to the CYP51 family, the only P450 family so far known to be conserved across the phyla.