Relationship between antifungal activity and inhibition of sterol biosynthesis in miconazole, clotrimazole, and 15-azasterol

Effect of Miconazole and Terbinafine on Artemisinin Content of Shooty Teratoma of Artemisia annua

The effects produced by the addition of sterol synthesis inhibitors on the artemisinin content of the transgenic organ culture (A. tumefaciens ATCC 33970 or 15955) of Artemisia annua are presented. The transgenic tissue produced 3-4 fold higher levels of artemisinin 0.84% (56.3 mg/L) within a short culture period compared with field grown plants (0.23%). The addition of the sterol synthesis inhibitors, miconazole and terbinafine, to these transgenic cultures resulted in enhanced artemisinin content up to 1.15% and 1.44%, respectively. Further enhancement of artemisinin content was achieved by varying the addition time of the sterol synthesis inhibitor to the cultures. The best artemisinin content (2.62%) was observed after terbinafine (10 mg/L) addition on the sixteenth day of the culture period.

Farnesol biosynthesis in Candida albicans: cellular response to sterol inhibition by zaragozic acid B

The dimorphic fungus Candida albicans produces farnesol as a quorum-sensing molecule that regulates cellular morphology. The biosynthetic origin of farnesol has been resolved by treating these cells with zaragozic acid B, a potent inhibitor of squalene synthase in the sterol biosynthetic pathway. Treatment with zaragozic acid B leads to an eightfold increase in the amount of farnesol produced by C. albicans. Furthermore, C. albicans cell extracts contain enzymatic activity to convert [(3)H]farnesyl pyrophosphate to [(3)H]farnesol. Many common antifungal antibiotics (e.g., zaragozic acids, azoles, and allylamines) target steps in sterol biosynthesis. We suggest that the fungicidal activity of zaragozic acid derives in large part from the accumulation of farnesol that accompanies the inhibition of sterol biosynthesis.

Cytochrome P450 lanosterol 14α-demethylase (CYP51): insights from molecular genetic analysis of the ERG11 gene in Saccharomyces cerevisiae

Eukaryotes characteristically express a cytochrome P450-catalyzed sterol 14α-methyl demethylase as an essential step in the production of membrane sterols. Lanosterol 14α-demethylase of Saccharomyces cerevisiae is the best characterized representative of these enzymes among fungi and provides a model system for the molecular genetic analysis of the reaction. The gene for this P450 and the gene for the S. cerevisiae NADPH-cytochrome P450 reductase have been examined by mutational inactivation and for their regulation of expression. Our results have contributed to a better understanding of sterol biosynthesis in relation to mechanisms of resistance to fungicidal demethylase inhibitors, and promote the rationale for using S. cerevisiae in the further characterization of structure function relationships among sterol 14α-demethylases.

Cloning, sequencing, and disruption of the gene encoding sterol C-14 reductase in Saccharomyces cerevisiae

A sterol C-14 reductase (erg24-1) mutant of Saccharomyces cerevisiae was selected in a fen1, fen2, suppressor background on the basis of nystatin resistance and ignosterol (ergosta-8,14-dienol) production. The erg24-1 allele segregated genetically as a single, recessive gene. The wild-type ERG24 gene was cloned by complementation onto a 12-kb fragment from a yeast genomic library, and subsequently subcloned onto a 2.4-kb fragment. This was sequenced and found to contain an open reading frame of 1,314 bp, predicting a polypeptide of 438 amino acids (M(r) 50,612). A 1,088-bp internal region of the ERG24 gene was excised, replaced with a LEU2 gene, and integrated into the chromosome of the parental strain, FP13D (fen1, fen2) by gene replacement. The ERG24 null mutant produced ergosta-8,14-dienol as the major sterol, indicating that the delta 8-7 isomerase, delta 5-desaturase and the delta 22-desaturase were inactive on sterols with the C14 = 15 double bond.

Physiological effects of fenpropimorph on wild-type Saccharomyces cerevisiae and fenpropimorph-resistant mutants

Fenpropimorph-resistant mutants of Saccharomyces cerevisiae were isolated by a gradient selection procedure. The mutants were cross-resistant to other morpholines (fenpropidin, dodemorph, tridemorph) and 15-azasterol, but were susceptible to azoles (miconazole, clotrimazole, ketoconazole) and nystatin. In the absence of fenpropimorph, the major sterol produced by the mutants and the parental strain was ergosterol. In the presence of fenpropimorph, ignosterol (ergosta-8,14-dien-3 beta-ol) was the major sterol produced by the mutants and the parental strain. The resistance to fenpropimorph involves two recessive genes, each of which allows a semiresistance, when they are isolated apart from one another. Strain JR4 (erg3 erg11), which produces 14-methylfecosterol [14 alpha-methyl-ergosta-8,24(28)-dien- 3-beta-ol) as the major sterol in the presence or absence of fenpropimorph, was also found to be resistant to the drug. The growth inhibitory effect of fenpropimorph on wild-type cells appears to be linked to the production of ignosterol. The uptake of exogenous sterol by wild-type cells was greatly enhanced in the presence of fenpropimorph. The growth inhibition caused by fenpropimorph could only be overcome with bulk levels of exogenous C-5,6-unsaturated sterols.

Cloning, disruption and sequence of the gene encoding yeast C-5 sterol desaturase

The ERG3 gene from Saccharomyces cerevisiae has been cloned by complementation of an erg3-2 mutation. ERG3 is the putative gene encoding the C-5 sterol desaturase required for ergosterol biosynthesis. The functional gene has been localized on a 2.5-kb HindIII-BamHI fragment containing an open reading frame comprising 365 amino acids. Gene disruption resulting from a deletion/substitution demonstrates that ERG3 is not essential for cell viability or the sparking function.

In vivo effects of fenpropimorph on the yeast Saccharomyces cerevisiae and determination of the molecular basis of the antifungal property

The effects of fenpropimorph on sterol biosynthesis and growth of Saccharomyces cerevisiae were examined to pinpoint the mode of action of fungicides that inhibit ergosterol biosynthesis. Taking advantage of sterol auxotrophy and sterol permeability in mutant strains, we show that growth inhibition is strongly correlated with inhibition of sterol biosynthesis. We confirm that in vivo and at low concentrations, fenpropimorph inhibits delta 8----delta 7-sterol isomerase, and in addition, when it is used at higher concentrations, it inhibits delta 14-sterol reductase. We show also that the fungistatic effect of fenpropimorph is not due to the accumulation of abnormal sterols in treated cells but is linked to the specific inhibition of ergosterol biosynthesis, leading to the arrest of cell proliferation in the unbudded G1 phase of the cell cycle.

Membrane fluidity alterations in a cytochrome P-450-deficient mutant of Candida albicans

A cytochrome P450-deficient mutant of the pathogenic fungus, Candida albicans, which accumulates exclusively 14 alpha-methylsterols in place of the normal end product sterol, ergosterol, was examined for alterations in membrane fluidity by electron paramagnetic resonance. The results using four nitroxyl spin labels indicated that exponential phase cultures of the mutant strain, D10, had a uniformly more rigid membrane than similarly grown wild type. Since D10 shows a sterol spectrum similar to that of wild type cells treated with imidazole and triazole antifungal agents, many of the physiological effects reported as the result of azole application may be the result of alterations in membrane fluidity.

Genetic and physiological analysis of azole sensitivity in Saccharomyces cerevisiae

Ketoconazole and fluconazole are azole antifungal agents which inhibit cytochrome P-450 mediated sterol C14 demethylation during ergosterol biosynthesis. We report on the activity of these antifungals on a variety of Saccharomyces cerevisiae strains grown under differing conditions known to affect cyt P-450 levels. Only slight increases in resistance to azoles were observed under conditions which induce the yeast cyt P-450 from undetectable levels. Strain variation was observed, with some strains exhibiting a fungicidal, and others a fungistatic response. Two cyt P-450 deficient mutants examined exhibited resistance to treatment with fluconazole and ketoconazole. This was attributed, at least in part, to an additional defect in sterol delta 5,6 desaturation and possibly to reduced cellular levels of azole drug.

Overview of medically important antifungal azole derivatives

Fungal infections are a major burden to the health and welfare of modern humans. They range from simply cosmetic, non-life-threatening skin infections to severe, systemic infections that may lead to significant debilitation or death. The selection of chemotherapeutic agents useful for the treatment of fungal infections is small. In this overview, a major chemical group with antifungal activity, the azole derivatives, is examined. Included are historical and state of the art information on the in vitro activity, experimental in vivo activity, mode of action, pharmacokinetics, clinical studies, and uses and adverse reactions of imidazoles currently marketed (clotrimazole, miconazole, econazole, ketoconazole, bifonazole, butoconazole, croconazole, fenticonazole, isoconazole, oxiconazole, sulconazole, and tioconazole) and under development (aliconazole and omoconazole), as well as triazoles currently marketed (terconazole) and under development (fluconazole, itraconazole, vibunazole, alteconazole, and ICI 195,739).

Effect of antifungal agents on lipid biosynthesis and membrane integrity in Candida albicans

Eight antifungal agents were examined for effects on lipid biosynthesis and membrane integrity in Candida albicans. Lipids were labeled in vivo or in vitro with [14C]acetate and analyzed by thin-layer and gas chromatography. Membrane integrity was measured by a recently developed [14C]aminoisobutyric acid radiolabel release assay. The imidazole antifungal agents miconazole, econazole, clotrimazole, and ketoconazole, at concentrations inhibiting ergosterol biosynthesis (0.1 microM), decreased the ratio of unsaturated to saturated fatty acids in vivo but not in vitro. Similarly, naftifine, tolnaftate, and the azasterol A25822B, at concentrations inhibiting ergosterol biosynthesis (10, 100, and 1 microM, respectively), decreased the ratio of unsaturated to saturated fatty acids in vivo only. This suggests that the effect on fatty acids observed with ergosterol biosynthesis inhibitors may be secondary to the effect on ergosterol. With imidazoles, oleic acid antagonized inhibition of cell growth but not inhibition of ergosterol. This suggests that, with the C-14 demethylase inhibitors, decreased unsaturated fatty acids, rather than decreased ergosterol, are responsible for growth inhibition. Cerulenin, previously reported to be a potent inhibitor of both fatty acid and ergosterol biosynthesis, was found in the present study to inhibit the former (at 5 microM) but not the latter (up to 100 microM). Of the antifungal agents tested, econazole and miconazole (at 100 microM) produced complete release of [14C]aminoisobutyric acid, which is consistent with membrane damage.

Effect of oxiconazole and Ro 14-4767/002 on sterol pattern in Candida albicans

The effect of the imidazole oxiconazole and the morpholine derivative Ro 14-4767/002 on the sterol metabolism of Candida albicans was investigated at different periods of growth. Ergosterol, representing the main sterol component of control cells, was markedly reduced in oxiconazole-treated and Ro 14-4767/002-treated cells. However, the total sterol content of the cells treated with both drugs was increased due to accumulation of other sterols not present in control cells: in oxiconazole-treated cells 24-methenedihydrolanosterol, 4,14-dimethylfecosterol and 14-methylfecosterol accumulated, indicating an inhibition of C14-demethylation. This is in agreement with the mode of action described for other azoles in various pathogen fungi. In Ro 14-4767/002-treated cells the main sterol accumulated was ignosterol, indicating an inhibition of delta 14-sterol reductase and delta 8-delta 7-isomerase. This inhibition has not been described before in human pathogens although it has been previously found in plant pathogenic fungi treated with fenpropimorph.

Yeast sterols: yeast mutants as tools for the study of sterol metabolism

Yeast mutants defective in ergosterol synthesis are valuable tools for investigating sterol metabolism. Both sterol mutants and sterol auxotrophs have been utilized in determining what sterol structural features are required for yeast cell viability. Both types of mutants can also be studied to ascertain how changes in sterol structure affect membrane properties. Other aspects of sterol metabolism, such as the specificity of sterol esterification, have been elucidated by the sterol auxotrophs. In broader applications, interrelationships between sterol metabolism and other cellular functions (e.g., heme metabolism) may also be examined with these mutants. By analyzing the lipid composition of the sterol mutants, on the other hand, much of the ergosterol biosynthetic pathway has been delineated. The unusual sterols of the mutants can also be obtained to develop assays for the enzymes involved in ergosterol synthesis. Thus, by utilizing mutants, the simple eukaryotic system of yeast may be extended to explore the entire field of sterol metabolism and its relationship to cellular physiology.