Structural and physiological features of sterols necessary to satisfy bulk membrane and sparking requirements in yeast sterol auxotrophs

Lipid droplets control the negative effect of non-yeast sterols in membranes of Saccharomyces cerevisiae under hypoxic stress

The effectivity of utilization of exogenous sterols in the yeast Saccharomyces cerevisiae exposed to hypoxic stress is dependent on the sterol structure. The highly imported sterols include animal cholesterol or plant sitosterol, while ergosterol, typical of yeasts, is imported to a lesser extent. An elevated utilization of non-yeast sterols is associated with their high esterification and relocalization to lipid droplets (LDs). Here we present data showing that LDs and sterol esterification play a critical role in the regulation of the accumulation of non-yeast sterols in membranes. Failure to form LDs during anaerobic growth in media supplemented with cholesterol or sitosterol resulted in an extremely long lag phase, in contrast to normal growth in media with ergosterol or plant stigmasterol. Moreover, in hem1∆, which mimics anaerobiosis, neither cholesterol nor sitosterol supported the growth in an LD-less background. The incorporation of non-ergosterol sterols into the membranes affected fundamental membrane characteristics such as relative membrane potential, permeability, tolerance to osmotic stress and the formation of membrane domains. Our findings reveal that LDs assume an important role in scenarios wherein cells are dependent on the utilization of exogenous lipids, particularly under anoxia. Given the diverse lipid structures present in yeast niches, LDs fulfil a protective role, mitigating the risk of excessive accumulation of potentially toxic steroids and fatty acids in the membranes. Finally, we present a novel function for sterols in a model eukaryotic cell - alleviation of the lipotoxicity of unsaturated fatty acids.

Azoles activate type I and type II programmed cell death pathways in crop pathogenic fungi

Triazoles are widely used to control pathogenic fungi. They inhibit the ergosterol biosynthetic pathway, but the precise mechanisms leading to fungicidal activities in many fungal pathogens are poorly understood. Here, we elucidate the mode of action of epoxiconazole and metconazole in the wheat pathogen Zymoseptoria tritici and the rice blast fungus Magnaporthe oryzae. We show that both azoles have fungicidal activity and reduce fluidity, but not integrity, of the plasma membrane. This impairs localisation of Cdc15-like F-BAR proteins, resulting in defective actin ring assembly and incomplete septation. However, mutant studies and pharmacological experiments in vitro and in planta show that azole lethality is due to a combination of reactive oxygen species-induced apoptosis and macroautophagy. Simultaneous inhibition of both programmed cell death pathways abolishes azole-induced cell death. Other classes of ergosterol biosynthesis inhibitors also induce apoptosis and macroautophagy, suggesting that activation of these two cell death pathways is a hallmark of ergosterol synthesis-targeting fungicides. This knowledge will inform future crop protection strategies.

Cell cycle dependence on the mevalonate pathway: Role of cholesterol and non-sterol isoprenoids

The mevalonate pathway is responsible for the synthesis of isoprenoids, including sterols and other metabolites that are essential for diverse biological functions. Cholesterol, the main sterol in mammals, and non-sterol isoprenoids are in high demand by rapidly dividing cells. As evidence of its importance, many cell signaling pathways converge on the mevalonate pathway and these include those involved in proliferation, tumor-promotion, and tumor-suppression. As well as being a fundamental building block of cell membranes, cholesterol plays a key role in maintaining their lipid organization and biophysical properties, and it is crucial for the function of proteins located in the plasma membrane. Importantly, cholesterol and other mevalonate derivatives are essential for cell cycle progression, and their deficiency blocks different steps in the cycle. Furthermore, the accumulation of non-isoprenoid mevalonate derivatives can cause DNA replication stress. Identification of the mechanisms underlying the effects of cholesterol and other mevalonate derivatives on cell cycle progression may be useful in the search for new inhibitors, or the repurposing of preexisting cholesterol biosynthesis inhibitors to target cancer cell division. In this review, we discuss the dependence of cell division on an active mevalonate pathway and the role of different mevalonate derivatives in cell cycle progression.

Discovery of an ergosterol-signaling factor that regulates Trypanosoma brucei growth

Ergosterol biosynthesis and homeostasis in the parasitic protozoan Trypanosoma brucei was analyzed by RNAi silencing and inhibition of sterol C24β-methyltransferase (TbSMT) and sterol 14α-demethylase [TbSDM (TbCYP51)] to explore the functions of sterols in T. brucei growth. Inhibition of the amount or activity of these enzymes depletes ergosterol from cells at <6 fg/cell for procyclic form (PCF) cells or <0.01 fg/cell for bloodstream form (BSF) cells and reduces infectivity in a mouse model of infection. Silencing of TbSMT expression by RNAi in PCF or BSF in combination with 25-azalanosterol (AZA) inhibited parasite growth and this inhibition was restored completely by adding synergistic cholesterol (7.8 μM from lipid-depleted media) with small amounts of ergosterol (1.2 μM) to the medium. These observations are consistent with the proposed requirement for ergosterol as a signaling factor to spark cell proliferation while imported cholesterol or the endogenously formed cholesta-5,7,24-trienol act as bulk membrane components. To test the potential chemotherapeutic importance of disrupting ergosterol biosynthesis using pairs of mechanism-based inhibitors that block two enzymes in the post-squalene segment, parasites were treated with AZA and itraconazole at 1 μM each (ED50 values) resulting in parasite death. Taken together, our results demonstrate that the ergosterol pathway is a prime drug target for intervention in T. brucei infection.

Comparison of sterol import under aerobic and anaerobic conditions in three fungal species, Candida albicans, Candida glabrata, and Saccharomyces cerevisiae

Sterol import has been characterized under various conditions in three distinct fungal species, the model organism Saccharomyces cerevisiae and two human fungal pathogens Candida glabrata and Candida albicans, employing cholesterol, the sterol of higher eukaryotes, as well as its fungal equivalent, ergosterol. Import was confirmed by the detection of esterified cholesterol within the cells. Comparing the three fungal species, we observe sterol import under three different conditions. First, as previously well characterized, we observe sterol import under low oxygen levels in S. cerevisiae and C. glabrata, which is dependent on the transcription factor Upc2 and/or its orthologs or paralogs. Second, we observe sterol import under aerobic conditions exclusively in the two pathogenic fungi C. glabrata and C. albicans. Uptake emerges during post-exponential-growth phases, is independent of the characterized Upc2-pathway and is slower compared to the anaerobic uptake in S. cerevisiae and C. glabrata. Third, we observe under normoxic conditions in C. glabrata that Upc2-dependent sterol import can be induced in the presence of fetal bovine serum together with fluconazole. In summary, C. glabrata imports sterols both in aerobic and anaerobic conditions, and the limited aerobic uptake can be further stimulated by the presence of serum together with fluconazole. S. cerevisiae imports sterols only in anaerobic conditions, demonstrating aerobic sterol exclusion. Finally, C. albicans imports sterols exclusively aerobically in post-exponential-growth phases, independent of Upc2. For the first time, we provide direct evidence of sterol import into the human fungal pathogen C. albicans, which until now was believed to be incapable of active sterol import.

Alteration study of lipids and phospholipids compositions of Pachysolen tannophilus membrane with aqueous ethanol media

The variations in lipid compositions of Pachysolen tannophilus membrane from aqueous to 5% aqueous ethanol solutions are examined. The decreases in phospholipids, sterols and unsaturated fatty acid contents and slight increase in glycolipids content are reported.

A genomewide screen reveals a role of mitochondria in anaerobic uptake of sterols in yeast

The mechanisms that govern intracellular transport of sterols in eukaryotic cells are not well understood. Saccharomyces cerevisiae is a facultative anaerobic organism that becomes auxotroph for sterols and unsaturated fatty acids in the absence of oxygen. To identify pathways that are required for uptake and transport of sterols, we performed a systematic screen of the yeast deletion mutant collection for genes that are required for growth under anaerobic conditions. Of the approximately 4800 nonessential genes represented in the deletion collection, 37 were essential for growth under anaerobic conditions. These affect a wide range of cellular functions, including biosynthetic pathways for certain amino acids and cofactors, reprogramming of transcription and translation, mitochondrial function and biogenesis, and membrane trafficking. Thirty-three of these mutants failed to grow on lipid-supplemented media when combined with a mutation in HEM1, which mimics anaerobic conditions in the presence of oxygen. Uptake assays with radio- and fluorescently labeled cholesterol revealed that 17 of the 33 mutants strongly affect uptake and/or esterification of exogenously supplied cholesterol. Examination of the subcellular distribution of sterols in these uptake mutants by cell fractionation and fluorescence microscopy indicates that some of the mutants block incorporation of cholesterol into the plasma membrane, a presumably early step in sterol uptake. Unexpectedly, the largest class of uptake mutants is affected in mitochondrial functions, and many of the uptake mutants show electron-dense mitochondrial inclusions. These results indicate that a hitherto uncharacterized mitochondrial function is required for sterol uptake and/or transport under anaerobic conditions and are discussed in light of the fact that mitochondrial import of cholesterol is required for steroidogenesis in vertebrate cells.

Characterizing Sterol Defect Suppressors Uncovers a Novel Transcriptional Signaling Pathway Regulating Zymosterol Biosynthesis

erg26-1ts cells harbor defects in the 4alpha-carboxysterol-C3 dehydrogenase activity necessary for conversion of 4,4-dimethylzymosterol to zymosterol. Mutant cells accumulate toxic 4-carboxysterols and are inviable at high temperature. A genetic screen aimed at cloning recessive mutations remediating the temperature sensitive growth defect has resulted in the isolation of four complementation groups, ets1-4 (erg26-1ts temperature sensitive suppressor). We describe the characterization of ets1-1 and ets2-1. Gas chromatography/mass spectrometry analyses demonstrate that erg26-1ts ets1-1 and erg26-1ts ets2-1 cells do not accumulate 4-carboxysterols, rather these cells have increased levels of squalene and squalene epoxide, respectively. ets1-1 and ets2-1 cells accumulate these same sterol intermediates. Chromosomal integration of ERG1 ERG7 at their loci in erg26-1ts ets1-1 and erg26-1ts and ets2-1 mutants, respectively, results in the loss of accumulation of squalene and squalene epoxide, re-accumulation of 4-carboxysterols and cell inviability at high temperature. Enzymatic assays demonstrate that mutants harboring the ets1-1 allele have decreased squalene epoxidase activity, while those containing the ets2-1 allele show weakened oxidosqualene cyclase activity. Thus, ETS1 and ETS2 are allelic to ERG1 and ERG7, respectively. We have mapped mutations within the erg1-1/ets1-1 (G247D) and erg7-1/ets2-1 (D530N, V615E) alleles that suppress the inviability of erg26-1ts at high temperature, and cause accumulation of sterol intermediates and decreased enzymatic activities. Finally using erg1-1 and erg7-1 mutant strains, we demonstrate that the expression of the ERG25/26/27 genes required for zymosterol biosynthesis are coordinately transcriptionally regulated, along with ERG1 and ERG7, in response to blocks in sterol biosynthesis. Transcriptional regulation requires the transcription factors, Upc2p and Ecm22p.

A comparison of the behavior of cholesterol and selected derivatives in mixed sterol-phospholipid Langmuir monolayers: a fluorescence microscopy study

Eukaryotic cells require sterols to achieve normal structure and function of their plasma membranes, and deviations from normal sterol composition can perturb these features and compromise cellular and organism viability. The Smith-Lemli-Opitz syndrome (SLOS) is a hereditary metabolic disease involving cholesterol (CHOL) deficiency and abnormal accumulation of the CHOL precursor, 7-dehydrocholesterol (7DHC). In this study, the interactions of CHOL and the related sterols desmosterol (DES) and 7DHC with l-alpha-dipalmitoylphosphatidylcholine (DPPC) monolayers were compared. Pressure-area isotherms and fluorescence microscopy were used to study DPPC monolayers containing 0, 10, 20, or 30 mol% sterol. Similar behavior was noted for CHOL- and DES-containing DPPC monolayers with both techniques. However, while 7DHC gave isotherms similar to those obtained with the other sterols, microscopy indicated limited domain formation with DPPC, indicating that 7DHC packs somewhat differently in DPPC membranes compared to CHOL and DES. These results are discussed in relation to SLOS pathobiology.

Sterol stringency of proliferation and cell cycle progression in human cells

Cholesterol is a major component of the plasma membrane in mammalian cells, where it acts as a modulator of bulk physical state and integrity. In addition to its structural role, cholesterol is essential for proliferation and other cell processes. The present study was undertaken to explore the stringency of the requirement for cholesterol as a regulator of proliferation and cell cycle progression. Comparisons were made between cholesterol and other sterol analogs that differ from cholesterol in three specific elements: the presence of a Delta5 double bond in ring B, the hydroxyl group at C-3, and the presence of an aliphatic side chain. The human leukemia cells HL-60 and MOLT-4 were cultured in cholesterol-free medium and treated with different sterols in the presence or absence of SKF 104976, a competitive inhibitor of lanosterol 14alpha-demethylase that allows the synthesis of isoprenoid derivatives but not cholesterol. Our results show that the beta-hydroxyl group at C-3 and the unsaturated bond at Delta5 are necessary for cell proliferation and cell cycle progression. The sterol analog 5alpha-cholestan-3beta-ol (dihydrocholesterol), which is saturated at Delta5 and has an A/B ring junction in the trans configuration, was also able to support cell growth. However, 5beta-cholestan-3beta-ol and 5beta-cholestan-3alpha-ol, both of which have an A/B ring junction in the cis configuration, were totally ineffective in supporting cell growth. Indeed, they produced an inhibition of cell proliferation and arrested the cell cycle specifically in the G2/M phase. These effects of 5beta-cholestanols were abrogated by cholesterol in a concentration-dependent manner. Moreover, 5beta-cholestanols potently inhibited cholesterol biosynthesis and transcription driven by the sterol response element. In addition to providing a description of the structural features of sterols associated with their supporting action on cell proliferation in mammalian cells, the present results demonstrate that selected cholesterol analogs may act as cytostatic agents, interrupting cell cycle progression specifically in the G2/M phase.

Effects of distal cholesterol biosynthesis inhibitors on cell proliferation and cell cycle progression

Cholesterol is a major lipid component of the plasma membrane in animal cells. In addition to its structural requirement, cholesterol is essential in cell proliferation and other cell processes. The aim of the present study was to elucidate the stringency of the requirement for cholesterol as a regulator of proliferation and cell cycle progression, compared with other sterols of the cholesterol biosynthesis pathway. Human promyelocytic HL-60 cells were cultured in cholesterol-free medium and treated with different distal inhibitors of cholesterol biosynthesis (zaragozic acid, SKF 104976, SR 31747, BM 15766, and AY 9944), which allow the synthesis of isoprenoid derivatives and different sets of sterol intermediates, but not cholesterol. The results showed that only the inhibition of sterol Delta7-reductase was compatible with cell proliferation. Blocking cholesterol biosynthesis upstream of this enzyme resulted in the inhibition of cell proliferation and cell cycle arrest selectively in G2/M phase.

Molecular cloning and biochemical characterization of Candida albicans acyl-CoA:sterol acyltransferase, a potential target of antifungal agents

To determine whether Candida albicans acyl CoA:sterol acyltransferase (ASAT) can be a potential target enzyme for the protoberberine derivative (HWY-289), we have isolated a gene encoding Ca-ASAT and examined inhibitory effects of HWY-289 on the overexpressed Ca-ASAT. HWY-289 specifically inhibits Ca-ASAT in a non-competitive manner in vitro (IC(50) [9.2microM], K(i) [5.15microM]). The cloned CaARE2 gene (1830 nucleotides [nt]) encodes active Ca-ASAT protein that exhibits a calculated molecular mass of 71.3kDa. The amino acid sequence of CaAre2p is 33.4% and 35.1% identical to those of Saccharomyces cerevisiae ScAre1p and ScAre2p homologues, respectively. Recombinant and endogenous Ca-ASAT displayed identical patterns of inhibition upon exposure to HWY-289 and a preference for cholesterol and oleoyl-CoA as substrates. Northern blot analysis showed that CaARE2 was activated by HWY-289, but not by CI-976 (a human acyl-coenzyme A:cholesterol acyltransferase inhibitor), in a dose-dependent manner (up to 5mg/L), suggesting different selectivities of action between HWY-289 and CI-976 on Ca-ASAT activity.

Yeast genome-wide expression analysis identifies a strong ergosterol and oxidative stress response during the initial stages of an industrial lager fermentation

Genome-wide expression analysis of an industrial strain of Saccharomyces cerevisiae during the initial stages of an industrial lager fermentation identified a strong response from genes involved in the biosynthesis of ergosterol and oxidative stress protection. The induction of the ERG genes was confirmed by Northern analysis and was found to be complemented by a rapid accumulation of ergosterol over the initial 6-h fermentation period. From a test of the metabolic activity of deletion mutants in the ergosterol biosynthesis pathway, it was found that ergosterol is an important factor in restoring the fermentative capacity of the cell after storage. Additionally, similar ERG10 and TRR1 gene expression patterns over the initial 24-h fermentation period highlighted a possible interaction between ergosterol biosynthesis and the oxidative stress response. Further analysis showed that erg mutants producing altered sterols were highly sensitive to oxidative stress-generating compounds. Here we show that genome-wide expression analysis can be used in the commercial environment and was successful in identifying environmental conditions that are important in industrial yeast fermentation.

Fatty acid and sterol metabolism: potential antimicrobial targets in apicomplexan and trypanosomatid parasitic protozoa

Current treatments for diseases caused by apicomplexan and trypanosomatid parasites are inadequate due to toxicity, the development of drug resistance and an inability to eliminate all life cycle stages of these parasites from the host. New therapeutics agents are urgently required. It has recently been demonstrated that type II fatty acid biosynthesis occurs in the plastid of Plasmodium falciparum and Toxoplasma gondii and inhibitors of this pathway such as triclosan and thiolactomycin restrict their growth. Furthermore, Trypanosoma brucei has recently been demonstrated to use type II fatty acid biosynthesis for myristate synthesis and to be susceptible to thiolactomycin. As this pathway is absent from mammals, it may provide an excellent target for novel antimicrobial agents to combat these diverse parasites. Leishmania and Trypanosoma parasites produce ergosterol-related sterols by a biosynthetic pathway similar to that operating in pathogenic fungi and their growth is susceptible to sterol biosynthesis inhibitors. Thus, inhibition of squalene 2,3-epoxidase by terbinafine, 14alpha-methylsterol 14-demethylase by azole and triazole compounds and delta(24)-sterol methyl transferase by azasterols all cause a depletion of normal sterols and an accumulation of abnormal amounts of sterol precursors with cytostatic or cytoxic consequences. However, Leishmania parasites can survive with greatly altered sterol profiles induced by continuous treatment with low concentrations of some inhibitors and they also have some ability to utilise and metabolise host sterol. These properties may permit the parasites to evade treatment with sterol biosynthesis inhibitors in some clinical situations and need to be taken into account in the design of future drugs.

A specific structural requirement for ergosterol in long-chain fatty acid synthesis mutants important for maintaining raft domains in yeast

Fungal sphingolipids contain ceramide with a very-long-chain fatty acid (C26). To investigate the physiological significance of the C26-substitution on this lipid, we performed a screen for mutants that are synthetically lethal with ELO3. Elo3p is a component of the ER-associated fatty acid elongase and is required for the final elongation cycle to produce C26 from C22/C24 fatty acids. elo3delta mutant cells thus contain C22/C24- instead of the natural C26-substituted ceramide. We now report that under these conditions, an otherwise nonessential, but also fungal-specific, structural modification of the major sterol of yeast, ergosterol, becomes essential, because mutations in ELO3 are synthetically lethal with mutations in ERG6. Erg6p catalyzes the methylation of carbon atom 24 in the aliphatic side chain of sterol. The lethality of an elo3delta erg6delta double mutant is rescued by supplementation with ergosterol but not with cholesterol, indicating a vital structural requirement for the ergosterol-specific methyl group. To characterize this structural requirement in more detail, we generated a strain that is temperature sensitive for the function of Erg6p in an elo3delta mutant background. Examination of raft association of the GPI-anchored Gas1p and plasma membrane ATPase, Pma1p, in the conditional elo3delta erg6(ts) double mutant, revealed a specific defect of the mutant to maintain raft association of preexisting Pma1p. Interestingly, in an elo3delta mutant at 37 degrees C, newly synthesized Pma1p failed to enter raft domains early in the biosynthetic pathway, and upon arrival at the plasma membrane was rerouted to the vacuole for degradation. These observations indicate that the C26 fatty acid substitution on lipids is important for establishing raft association of Pma1p and stabilizing the protein at the cell surface. Analysis of raft lipids in the conditional mutant strain revealed a selective enrichment of ergosterol in detergent-resistant membrane domains, indicating that specific structural determinants on both sterols and sphingolipids are required for their association into raft domains.

Differential effects of ergosterol and cholesterol on Cdk1 activation and SRE-driven transcription

Cholesterol is essential for cell growth and division, but whether this is just a consequence of its use in membrane formation or whether it also elicits regulatory actions in cell cycle machinery remains to be established. Here, we report on the specificity of this action of cholesterol in human cells by comparing its effects with those of ergosterol, a yeast sterol structurally similar to cholesterol. Inhibition of cholesterol synthesis by means of SKF 104976 in cells incubated in a cholesterol-free medium resulted in cell proliferation inhibition and cell cycle arrest at G2/M phase. These effects were abrogated by cholesterol added to the medium but not by ergosterol, despite that the latter was used by human cells and exerted similar homeostatic actions, as the regulation of the transcription of an SRE-driven gene construct. In contrast to cholesterol, ergosterol was unable to induce cyclin B1 expression, to activate Cdk1 and to resume cell cycle in cells previously arrested at G2. This lack of effect was not due to cytotoxicity, as cells exposed to ergosterol remained viable and, upon supplementing with UCN-01, an activator of Cdk1, they progressed through mitosis. However, in the presence of suboptimal concentrations of cholesterol, ergosterol exerted synergistic effects on cell proliferation. This is interpreted on the basis of the differential action of these sterols, ergosterol contributing to cell membrane formation and cholesterol being required for Cdk1 activation. In summary, the action of cholesterol on G2 traversal is highly specific and exerted through a mechanism different to that used for cholesterol homeostasis, reinforcing the concept that cholesterol is a specific regulator of cell cycle progression in human cells.

Specific sterols required for the internalization step of endocytosis in yeast

Sterols are major components of the plasma membrane, but their functions in this membrane are not well understood. We isolated a mutant defective in the internalization step of endocytosis in a gene (ERG2) encoding a C-8 sterol isomerase that acts in the late part of the ergosterol biosynthetic pathway. In the absence of Erg2p, yeast cells accumulate sterols structurally different from ergosterol, which is the major sterol in wild-type yeast. To investigate the structural requirements of ergosterol for endocytosis in more detail, several erg mutants (erg2Delta, erg6Delta, and erg2Deltaerg6Delta) were made. Analysis of fluid phase and receptor-mediated endocytosis indicates that changes in the sterol composition lead to a defect in the internalization step. Vesicle formation and fusion along the secretory pathway were not strongly affected in the ergDelta mutants. The severity of the endocytic defect correlates with changes in sterol structure and with the abundance of specific sterols in the ergDelta mutants. Desaturation of the B ring of the sterol molecules is important for the internalization step. A single desaturation at C-8,9 was not sufficient to support internalization at 37 degrees C whereas two double bonds, either at C-5,6 and C-7,8 or at C-5,6 and C-8,9, allowed internalization.

Molecular genetics of the Smith-Lemli-Opitz syndrome and postsqualene sterol metabolism

The Smith-Lemli-Opitz syndrome is a disorder of morphogenesis resulting from an enzymatic defect in the last step of cholesterol metabolism (reduction of 7-dehydrocholesterol). Analysis of the defective gene and identification of mutations therein have paved the way for the study of the molecular genetics of the disorder which is caused by numerous different mutations. Future efforts should identify a postulated intracellular signalling activity of sterol intermediates, isolate proteins that govern the sterol traffic between intracellular compartments, structurally characterize the enzyme delta 7-sterol reductase defective in the Smith-Lemli-Opitz syndrome and investigate the pathomechanism of sterol depletion-induced dysmorphogenesis.

Biotransformation of 24 alpha-methylcholesterol and 24 beta-methylcholesterol by yeast mutant GL7

Incubation of 24 alpha- and 24 beta-methylcholesterols with yeast mutant GL7 afforded their corresponding C-22-desaturated products under the catalysis of sterol delta 22(23)-desaturase. The metabolites were identified to be 22-dehydro-24 alpha-methylcholesterol (2% yield from 24 alpha-methylcholesterol) and 22-dehydro-24 beta-methylcholesterol (51% yield from 24 beta-methylcholesterol) respectively on the basis of their chromatographic and spectral properties. It was concluded that the sterol delta 22(23)-desaturase prefers the 24 beta-methyl sterols and is highly stereospecific.

Synthesis of 6(alpha, beta)-aminocholestanols as ergosterol biosynthesis inhibitors

delta 7-5-Desaturase catalyses one of the last steps in ergosterol biosynthesis in fungi. Moreover delta 5-unsaturation is necessary for the sparking function. Synthesis of three pairs of C-6 epimeric cholestanol derivatives are described as potential growth inhibitors. Preliminary results suggest that 6 beta-aminocholestanol is a potent antifungal agent.

Biochemistry, cell biology and molecular biology of lipids of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a powerful experimental system to study biochemical, cell biological and molecular biological aspects of lipid synthesis. Most but not all genes encoding enzymes involved in fatty acid, phospholipid, sterol or sphingolipid biosynthesis of this unicellular eukaryote have been cloned, and many gene products have been functionally characterized. Less information is available about genes and gene products governing the transport of lipids between organelles and within membranes, turnover and degradation of complex lipids, regulation of lipid biosynthesis, and linkage of lipid metabolism to other cellular processes. Here we summarize current knowledge about lipid biosynthetic pathways in S. cerevisiae and describe the characteristic features of the gene products involved. We focus on recent discoveries in these fields and address questions on the regulation of lipid synthesis, subcellular localization of lipid biosynthetic steps, cross-talk between organelles during lipid synthesis and subcellular distribution of lipids. Finally, we discuss distinct functions of certain key lipids and their possible roles in cellular processes.

Role of ergosterol in growth inhibition of Saccharomyces cerevisiae by syringomycin E

The antifungal activity of the lipodepsipeptide syringomycin E from Pseudomonas syringae pv. syringae is modulated by sterols. To study the requirement of the predominant fungal sterol, ergosterol, in syringomycin E action, the sterol composition of Saccharomyces cerevisiae sterol auxotroph strain FY-14 was modified and sensitivity to syringomycin E examined. Cells containing solely ergosterol, cholesterol, beta-sitosterol or stigmasterol were sensitive to syringomycin E with the latter two being the most sensitive. Cells containing growth-promoting cholesterol were the most sensitive and those with growth-promoting ergosterol the least sensitive. It is concluded that sensitivity to syringomycin E is modulated by growth-promoting sterols and does not necessarily require ergosterol.

Requirement of heme to replace the sparking sterol function in the yeast Saccharomyces cerevisiae

At least four distinctive sterol functions have been defined in the yeast Saccharomyces cerevisiae. One of these functions, identified as sparking, has the lowest quantitative requirement for sterol, but has the greatest structural specificity. Based on studies utilizing a yeast strain auxotrophic for both heme and sterol biosynthesis, it had been reported that a delta 5-sterol was essential for the growth of the organism. We demonstrate here that heme, and not a heme precursor, can replace the delta 5-sparking sterol requirement of heme auxotrophic strains of yeast.

Synthesis and bioevaluation of delta 7-5-desaturase inhibitors, an enzyme late in the biosynthesis of the fungal sterol ergosterol

Ergosterol, the predominant sterol of fungi, is postulated to have many cellular functions which include a bulk membrane role and a regulatory role. Studies with sterol auxotrophs show that, even in the presence of sterols which can fulfill the bulk membrane requirements, a small concentration of ergosterol is absolutely necessary for growth. The delta 5-double bond appears to be required for the regulatory role of ergosterol; therefore, development of inhibitors of the enzyme that introduce this double bond, delta 7-sterol 5-desaturase (5-desaturase), may lead to effective antifungal agents. Within is the first reported synthesis of inhibitors of fungal 5-desaturase and the development of an in vitro tritium efficacy radioassay. The inhibitors were of the general structure 7,22(E)-ergostadien-3 beta-ol with alpha-face heteroatom substituents in the vicinity of C-5. They exhibited IC50 values of 47-149 microM.

Sterol and steryl ester regulation of phospholipase A2 from the mosquito parasite Lagenidium giganteum

Lagenidium giganteum, a facultative parasite of mosquito larvae, cannot synthesize sterols, and requires an exogenous source of these lipids in order to enter its reproductive cycle. This parasite grows vegetatively in the absence of sterols, but requires cholesterol or structurally related compounds to produce motile zoospores, which are the only stage capable of infecting mosquitoes. Sterols structurally related to cholesterol and some steryl esters inhibited the activity of L. giganteum phospholipase A2 (PLA2), an enzyme that hydrolyzes fatty acids from the sn-2 position of glycerophospholipids. Sterols that induce reproduction inhibited L. giganteum PLA2 activity, while sterols and steroids that do not support sporulation had minimal effect. Most steryl esters had no effect on enzyme activity, but cholesteryl arachidonate (CA) was a potent inhibitor of parasite PLA2. Not all enzymes partly purified using a DEAE-Sephacel column were affected by these lipids, demonstrating selective inhibition of specific enzymes. Potency was enhanced by up to several orders of magnitude if epoxy fatty acids were esterified to the cholesterol nucleus. The steryl ester pool was dynamic during morphogenesis, and the fatty acid composition of the steryl esters did not mimic total cell or membrane (glycerophospholipid) fatty acid composition as L. giganteum proceeded through its growth cycle. Synthesis of CA and monoepoxy CA by the parasite was confirmed using electrospray mass spectrometry and collision-induced dissociation. Steryl derivatives selectively inhibited PLA2 enzymes from bovine pancreas, snake venom, and human cytoplasmic 85-kDa PLA2.

Cloning by metabolic interference in yeast and enzymatic characterization of Arabidopsis thaliana sterol delta 7-reductase

Reduction of the delta 7 double bond of sterols, a key biosynthetic step in higher eukaryotes, is lacking in lower eukaryotes like the yeast Saccharomyces cerevisiae, leading to terminal sterols with a delta 5,7-conjugated diene structure. Genes encoding two sterol reductases involved, respectively, in the reduction of sterol delta 14 and delta 24(28) double bonds have been cloned to date, but no sequence information was available on the enzyme responsible for delta 7-bond reduction. This study presents the cloning of the NADPH-sterol delta 7-reductase (delta 7-red) from Arabidopsis thaliana, based on a metabolic interference approach in yeast. The principle is the functional expression of a plant cDNA library in the yeast strain FY1679-28C tolerant to sterol modifications and the selection of clones resistant to the polyene fungicide nystatin. The toxicity of this compound is dependent on the presence of delta 5,7-unsaturated sterols in the yeast plasma membrane. One clone out of 10(5) transformants exhibits a cDNA-dependent alteration of cell sterol composition. The 1290-base pair cDNA open reading frame was isolated and sequenced. The corresponding protein presents a significant sequence similarity with yeast delta 14- and delta 24(28)-reductases and with human lamin B receptor. The coding sequence was extracted by polymerase chain reaction and inserted into a galactose-inducible yeast expression vector to optimize expression. Analysis using transformed wild type yeast or sterol altered mutants, indicated that delta 5,7-ergosta- and cholesta-sterols are efficiently reduced in vivo, regardless of the structural variations on the side chain. No reductase activity was observed toward the delta 14 or the delta 5 positions of sterols. In vivo extensive delta 7-reduction of the free and esterified pools of sterols was observed upon induction of the enzyme. Ergosterol present before induction was reduced into ergosta-5,22-dieneol, whereas ergosta-5-eneol is the new end product of sterol neosynthesis, indicating that the yeast delta 22 desaturase may be no longer active on C-7-saturated sterols. In vitro tests indicated that delta 7-reductase activity is preferentially associated with the endoplasmic reticulum membrane and confirmed the previous finding that NADPH is the reducing agent.

Identification and characterization of an S-adenosyl-L-methionine: delta 24-sterol-C-methyltransferase cDNA from soybean

In plants, the dominant sterols are 24-alkyl sterols, which play multiple roles in plant growth and development, i.e. as membrane constituents and as precursors to steroid growth regulators such as brassinosteroids. The initial step in the conversion of the phytosterol intermediate cycloartenol to the 24-alkyl sterols is catalyzed by S-adenosyl-L-methionine: delta 24-sterol-C-methyl-transferase (SMT), a rate-limiting enzyme for phytosterol biosynthesis. A cDNA clone (SMT1) encoding soybean SMT was isolated from an etiolated hypocotyl cDNA library by immunoscreening using an anti-(plasma membrane) serum. The deduced amino acid sequence of the SMT1 cDNA contained three conserved regions found in S-adenosyl-L-methionine-dependent methyltransferases. The overall structure of the polypeptide encoded by the SMT1 cDNA is most similar to the predicted amino acid sequence of the yeast ERG6 gene, the putative SMT structural gene. The polypeptide encoded by the SMT1 cDNA was expressed as a fusion protein in Escherichia coli and shown to possess SMT activity. The growing soybean vegetative tissues had higher levels of SMT transcript than mature vegetative tissues. Young pods and immature seeds had very low levels of the SMT transcript. The SMT transcript was highly expressed in flowers. The expression of SMT transcript was suppressed in soybean cell suspension cultures treated with yeast elicitor. The transcriptional regulation of SMT in phytosterol biosynthesis is discussed.

Cloning and characterization of the Saccharomyces cerevisiae C-22 sterol desaturase gene, encoding a second cytochrome P-450 involved in ergosterol biosynthesis

The ERG5 gene from Saccharomyces cerevisiae was cloned by complementation of an erg5-1 mutation using a negative selection protocol involving screening for nystatin-sensitive transformants. ERG5 is the putative gene encoding the C-22 sterol desaturase required in ergosterol biosynthesis. The functional gene was localized to a 2.15-kb SacI-EcoRI DNA fragment containing an open reading frame of 538 amino acids (aa). ERG5 contains a 10-aa motif consistent with its role as a cytochrome P-450 (CyP450) enzyme and is similar to a number of mammalian CyP450 enzymes. Gene disruption demonstrates that ERG5 is not essential for cell viability.

Isolation and characterization of the Saccharomyces cerevisiae SUT1 gene involved in sterol uptake

A new gene (SUT1) of Saccharomyces cerevisiae, implicated in sterol uptake, was isolated from a yeast genomic library constructed in a high-copy-number vector by virtue of conferring resistance to fenpropimorph in medium supplemented with ergosterol. The high expression of SUT1 in sterol auxotrophic mutant strains alleviates the requirement for accessory mutations affecting heme biosynthesis and allows sterol uptake in aerobiosis. Measurements of [14C]cholesterol uptake confirmed that SUT1 is involved in sterol absorption. Within the 4.1-kb insert isolated, the functional gene was localised on a 1.7-kb DNA fragment. The nucleotide sequence encodes a predicted protein of 299 amino acids. Northern blot analysis revealed that SUT1 is a new member of the hypoxic gene family. Gene disruption showed that SUT1 is not essential for aerobic or anaerobic yeast growth.

Effects of an azasterol inhibitor of sterol 24-transmethylation on sterol biosynthesis and growth of Leishmania donovani promastigotes

Leishmania donovani promastigotes were cultured in the presence of an azasterol (20-piperidin-2-yl-5 alpha-pregnane-3 beta,20-diol) to determine the effects on sterol biosynthesis and cell proliferation. Inhibition of growth increased gradually with azasterol concentrations up to 5 micrograms/ml; concentrations of azasterol exceeding 5 micrograms/ml were lethal. Sterol biosynthesis was affected by the azasterol when administered at concentrations as low as 100 pg/ml. The primary site of action was the alkylation at C-24 of a delta 24-sterol precursor. The 24-alkylated sterols [ergosta-5,7,24(24(1))-trien-3 beta-ol and ergosta-5,7,22-trien-3 beta-ol] of the protozoan were replaced by delta 24-cholesta-type sterols which then accumulated in the cells. Administration of the azasterol together with a bis-triazole inhibitor of the 14 alpha-methylsterol 14-demethylase reaction, which operates in sterol biosynthesis, resulted in depletion of 24-alkylsterols and their replacement with predominantly 14 alpha-methylsterols lacking a 24-alkyl group. Continuous subculture of promastigotes in the presence of the azasterol resulted in gradual depletion of 24-alkylsterols and their complete replacement by delta 24-cholesta-type sterols. Transfer of the azasterol-treated cells to medium lacking azasterol resulted in a gradual restoration, after several subcultures, of the normal 24-alkylsterol pattern. The results indicate that, although 24-alkylsterols are normally produced by the protozoan, it can nevertheless survive with sterols possessing only the cholestane skeleton. Thus there is no absolute requirement for 24-alkylsterols to fulfil some essential 'sparking' role associated with cell growth in promastigotes.

Biochemical and physiological effects of sterol alterations in yeast--a review

Considerable progress has been made in the selection and characterization of mutants that are defective in the synthesis of ergosterol in the yeast, Saccharomyces cerevisiae. Mutations in nearly every step of the yeast sterol biosynthetic pathway have been induced and selected. These mutants have been used to elucidate the sequential order of steps in sterol synthesis, to study the mode of action of antifungal agents and to determine the method of resistance of some pathogenic fungi, and to answer questions on the role of sterols in general cell biology. Physiological examination of ergosterol null mutants, lacking all biochemical activity attributed to the particular gene, supports a role for ergosterol in a number of critical functions in the organism. Among the physiological functions attributed to ergosterol are sparking and bulking requirements, involvement in amino acid and pyrimidine transport, resistance to antifungal agents and certain cations, and a requirement for respiratory activity. Those genetic null alleles discussed in this review are erg24, lacking the ability to reduce the delta 14 double bond; erg6, unable to methylate C-24; and erg3, defective in the C-5 desaturase. The different biochemical activities that are disrupted in the ergosterol mutants support a role for ergosterol in a number of critical functions in yeast.

Cloning of the late genes in the ergosterol biosynthetic pathway of Saccharomyces cerevisiae--a review

Research on the ergosterol biosynthetic pathway in fungi has focused on the identification of the specific sterol structure required for normal membrane structure and function and for completion of the cell cycle. The pathway and its end product are also the targets for a number of antifungal drugs. Identification of essential steps in ergo-sterol biosynthesis could provide new targets for the development of novel therapeutic agents. Nine of the eleven genes in the portion of the pathway committed exclusively to ergosterol biosynthesis have been cloned, and their essentiality for aerobic growth has been determined. The first three genes, ERG9 (squalene synthase), ERG1 (squalene epoxidase), and ERG7 (lanosterol synthase), have been cloned and found to be essential for aerobic viability since their absence would result in the cell being unable to synthesize a sterol molecule. The remaining eight genes encode enzymes which metabolize the first sterol, lanosterol, to ultimately form ergosterol. The two earliest genes, ERG11 (lanosterol demethylase) and ERG24 (C-14 reductase), have been cloned and found to be essential for aerobic growth but are suppressed by mutations in the C-5 desaturase (ERG3) gene and fen1 and fen2 mutations, respectively. The remaining cloned genes, ERG6 (C-24 methylase), ERG2 (D8AE7 isomerase), ERG3 (C-5 desaturase), and ERG4 (C-24(28) reductase), have been found to be nonessential. The remaining genes not yet cloned are the C-4 demethylase and the C-22 desaturase (ERG5).

Isolation of the ERG2 gene, encoding sterol delta 8-->delta 7 isomerase, from the rice blast fungus Magnaporthe grisea and its expression in the maize smut pathogen Ustilago maydis

The Magnaporthe grisea ERG2 gene, encoding delta 8-->delta 7 sterol isomerase, was isolated from a genomic library by heterologous hybridization to a fragment of the Ustilago maydis ERG2 gene. The isolated gene contained a reading frame of 745 bp which encoded a protein of 221 amino acids. The coding region was interrupted by a single putative 79-bp-long intron. The deduced amino-acid sequence exhibited similarity to the ERG2 gene products of U. maydis and of Saccharomyces cerevisiae, particularly in the central region of the proteins. The NH2-terminal of all three proteins contained a long stretch of amino acids that were strongly hydrophobic, suggesting that they may function by anchoring the protein to a membrane surface. The M. grisea ERG2 gene complemented a U. maydis deletion mutant in which the ERG2 gene had been removed using a one-step gene replacement procedure. The delta 8-->delta 7 sterol isomerase produced by the M. grisea ERG2 gene exhibited a level of sensitivity to the sterol biosynthesis inhibitor, tridemorph, similar to that of the enzyme derived from the U. maydis ERG2 gene.

Evidence for the presence of "metabolic sterols" in Pneumocystis: identification and initial characterization of Pneumocystis carinii sterols

Mixed life cycle stages of rat-derived Pneumocystis carinii were isolated from host lungs and their sterols were compared with those present in lungs from normal and immunosuppressed uninfected rats. Gas-liquid chromatography consistently detected, resolved, and quantified 9, 10, and 20 sterol components in the total nonsaponifiable neutral lipid fraction of lungs from normal rats, lungs from immunosuppressed uninfected rats, and P. carinii preparations, respectively. In all samples, cholesterol was the most abundant sterol present, comprising 97%, 93%, and 78% of total sterols in lungs from normal rats, lungs from immunosuppressed uninfected rats, and P. carinii, respectively. Tentative identifications of several rat lung and P. carinii minor sterols were made based on gas-liquid chromatogram retention times and fragmentation patterns from mass spectral analyses. Campesterol (ergost-5-en-3-ol), cholest-5-en-3-one, and beta-sitosterol (stigmast-5-en-3-ol) were among the minor components present in both types of lung controls, and were also components of P. carinii sterols. In contrast to lung controls, the sterols of P. carinii were enriched in C28 and C29 sterols with one or two double bonds, and a hydroxyl group at C-3 (ergost-5-en-3-ol, ergost-7-en-3-ol, ergosta-dien-3-ol, stigmast-5-en-3-ol, stigmast-7-en-3-ol and stigmasta-dien-3-ol). Steryl esters of P. carinii, probably stored in cytoplasmic lipid droplets, were dominated by those present in the host lung.(ABSTRACT TRUNCATED AT 250 WORDS)

The ERG3 gene in Saccharomyces cerevisiae is required for the utilization of respiratory substrates and in heme-deficient cells

ERG3 is the structural gene in Saccharomyces cerevisiae for the sterol delta 5 desaturase that introduces the C5 = 6 unsaturation in ergosterol biosynthesis. The ERG3 gene has been mapped on chromosome XII, 13.7 centimorgans from GAL2 toward SPT8. The essentiality of the gene is dependent on the conditions used for the cultivation of the mutants. Insertionally inactivated mutants of ERG3 fail to grow without 'sparking' levels of delta 5 sterols in heme-deficient cells, and are unable to grow on the respiratory substrates glycerol and ethanol.

Sterol synthesis and viability of erg11 (cytochrome P450 lanosterol demethylase) mutations in Saccharomyces cerevisiae and Candida albicans

The identification of the precise structural features of yeast sterol molecules required for the essential "sparking" function has been a controversial area of research. Recent cloning and gene disruption studies in Saccharomyces cerevisiae have shown that C-24 methylation (ERG6), C-5 desaturation (ERG3) and delta 8-delta 7 isomerization (ERG2) are not required, while C-14 demethylation (ERG11) and C-14 reduction (ERG24) are each required for aerobic viability. Earlier observations had indicated that C-14 demethylase deficient strains could be restored to aerobic growth by suppressor mutations that caused a deficiency in C-5 desaturase. These strains were reported to synthesize some ergosterol, indicating that they contained leaky mutations in both ERG11 and ERG3, thereby making it impossible to determine whether the removal of the C-14 methyl group was required for aerobic viability. The availability of the ERG11 and ERG3 genes has been used in this study to construct strains that contain null mutants in both ERG11 and ERG3. Results show that these double disruption strains are viable and that spontaneously arising suppressors of the ERG11 disruption are erg3 mutants. The erg11 mutants of S. cerevisiae are compared to similar mutants of Candida albicans that are viable in the absence of the erg3 lesion.

Effect of sterol alterations on conjugation in Saccharomyces cerevisiae

Sterol auxotrophic strains of Saccharomyces cerevisiae were grown and allowed to conjugate on media supplemented with various sterols. The mating efficiency of the auxotrophs is perturbed by the replacement of the normal yeast sterol, ergosterol, with other sterols. After 4 h of mating, cells grown on ergosterol exhibited a 30-fold higher productive mating efficiency than those cells grown in stigmasterol. Aberrant budding by the conjugants was enhanced following incubation on stigmasterol and other non-ergosterol sterols. Using light and electron microscopy, we demonstrated that there is a reduced ability for stigmasterol-grown cells to undergo cytoplasmic fusion during conjugation. Many of the mated pairs remained adherent but prezygotic even after 12 h of incubation. The addition of ergosterol to cells previously grown on stigmasterol rescued the organisms, allowing for zygote formation and normal budding.

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.

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.

Involvement of heme components in sterol metabolism of Saccharomyces cerevisiae

There is an intimate association between sterol biosynthesis in yeast and aerobicity. Besides the requirement for molecular oxygen for the epoxidation of squalene, cytochrome hemoproteins are involved in demethylation and desaturation steps. Regulatory effects of hemes on sterol formation have been demonstrated using specifically defective mutants of yeast. Heme competency participates in a mechanism whereby wild-type cells are prevented from taking exogenous sterols from the growth media. The multiple interactions of hemes and sterols appear to be associated with the variously defined functions for sterols in the yeast cells.

Cloning and disruption of the yeast C-8 sterol isomerase gene

The yeast ERG2 gene codes for the C-8 sterol isomerase, an enzyme required for the isomerization of the delta 8 double bond to the delta 7 position in ergosterol biosynthesis. The ERG2 gene was cloned by complementation of a C-8 sterol isomerase mutant strain (erg2). The complementing region of DNA required to restore ergosterol synthesis to erg2 was limited to a 1.0 kb StuI-BglII fragment. In order to determine whether the ERG2 gene was essential for yeast viability, a LEU2 gene was inserted into the NdeI site (made blunt) of this 1.0 kb fragment. Transformation of a wild type diploid strain with the ERG2 substituted DNA resulted in the generation of viable haploids containing the erg2 null allele (erg2-4::Leu2). These results suggest that the C-8 sterol isomerase activity is not essential for yeast cell viability. This disruption represents the second ergosterol biosynthetic gene in the distal portion of the pathway to be disrupted without adversely affecting cell viability.

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.