Active efflux by multidrug transporters as one of the strategies to evade chemotherapy and novel practical implications of yeast pleiotropic drug resistance

New Heterocyclic Compounds from Oxazol-5(4H)-one and 1,2,4-Triazin-6(5H)-one Classes: Synthesis, Characterization and Toxicity Evaluation

This paper describes the synthesis of new heterocycles from oxazol-5(4H)-one and 1,2,4-triazin-6(5H)-one classes containing a phenyl-/4-bromophenylsulfonylphenyl moiety. The oxazol-5(4H)-ones were obtained via condensation of 2-(4-(4-X-phenylsulfonyl)benzamido)acetic acids with benzaldehyde/4-fluorobenzaldehyde in acetic anhydride and in the presence of sodium acetate. The reaction of oxazolones with phenylhydrazine, in acetic acid and sodium acetate, yielded the corresponding 1,2,4-triazin-6(5H)-ones. The structures of the compounds were confirmed using spectral (FT-IR, ¹H-NMR, ¹³C-NMR, MS) and elemental analysis. The toxicity of the compounds was evaluated on Daphnia magna Straus crustaceans and on the budding yeast Saccharomyces cerevisiae. The results indicate that both the heterocyclic nucleus and halogen atoms significantly influenced the toxicity against D. magna, with the oxazolones being less toxic than triazinones. The halogen-free oxazolone had the lowest toxicity, and the fluorine-containing triazinone exhibited the highest toxicity. The compounds showed low toxicity against yeast cells, apparently due to the activity of plasma membrane multidrug transporters Pdr5 and Snq2. The predictive analyses indicated an antiproliferative effect as the most probable biological action. The PASS prediction and CHEMBL similarity studies show evidence that the compounds could inhibit certain relevant oncological protein kinases. These results correlated with toxicity assays suggest that halogen-free oxazolone could be a good candidate for future anticancer investigations.

Searching for the Mechanism that Mediates Mefenoxam-Acquired Resistance in Phytophthora infestans and How It Is Regulated

Phytophthora infestans, the causal agent of late blight disease of potatoes, is mainly controlled by the use of fungicides. Isolates that are resistant to commonly used fungicides have been reported. Also, several studies show that originally mefenoxam-sensitive isolates acquire resistance to this fungicide when exposed to sublethal concentrations. This phenomenon, termed "mefenoxam-acquired resistance," has been observed in different Phytophthora species and seems to be unique to mefenoxam. In this study, we aimed to elucidate the molecular mechanism mediating this type of resistance as well as a possible regulatory process behind it. A combination of computational analyses and experimental approaches was used to identify differentially expressed genes with a potential association to the phenomenon. These genes were classified into seven functional groups. Most of them seem to be associated with a pleiotropic drug resistance (PDR) phenotype, typically involved in the expulsion of diverse metabolites, drugs, or other substances out of the cell. Despite the importance of RNA Polymerase I for the constitutive resistance of P. infestans to mefenoxam, our results indicate no clear interaction between this protein and the acquisition of mefenoxam resistance. Several small non-coding RNAs were found to be differentially expressed and specifically related to genes mediating the PDR phenotype, thus suggesting a possible regulatory process. We propose a model of the molecular mechanisms acting within the cell when P. infestans acquires resistance to mefenoxam after exposed to sublethal concentrations of the fungicide. This study provides important insights into P. infestans' cellular and regulatory functionalities.

Saccharomyces cerevisiae as a model system to study the response to anticancer agents

The development of new strategies for cancer therapeutics is indispensable for the improvement of standard protocols and the creation of other possibilities in cancer treatment. Yeast models have been employed to study numerous molecular aspects directly related to cancer development, as well as to determine the genetic contexts associated with anticancer drug sensitivity or resistance. The budding yeast Saccharomyces cerevisiae presents conserved cellular processes with high homology to humans, and it is a rapid, inexpensive and efficient compound screening tool. However, yeast models are still underused in cancer research and for screening of antineoplastic agents. Here, the employment of S. cerevisiae as a model system to anticancer research is discussed and exemplified. Focusing on the important determinants in genomic maintenance and cancer development, including DNA repair, cell cycle control and epigenetics, this review proposes the use of mutant yeast panels to mimic cancer phenotypes, screen and study tumor features and synthetic lethal interactions. Finally, the benefits and limitations of the yeast model are highlighted, as well as the strategies to overcome S. cerevisiae model limitations.

Antifungal drug resistance mechanisms in fungal pathogens from the perspective of transcriptional gene regulation

Fungi are primitive eukaryotes and have adapted to a variety of niches during evolution. Some fungal species may interact with other life forms (plants, insects, mammals), but are considered as pathogens when they cause mild to severe diseases. Chemical control strategies have emerged with the development of several drugs with antifungal activity against pathogenic fungi. Antifungal agents have demonstrated their efficacy by improving patient health in medicine. However, fungi have counteracted antifungal agents in several cases by developing resistance mechanisms. These mechanisms rely on drug resistance genes including multidrug transporters and drug targets. Their regulation is crucial for the development of antifungal drug resistance and therefore transcriptional factors critical for their regulation are being characterized. Recent genome-wide studies have revealed complex regulatory circuits involving these genetic and transcriptional regulators. Here, we review the current understanding of the transcriptional regulation of drug resistance genes from several fungal pathogens including Candida and Aspergillus species.

Substrates and modulators of the multidrug transporter Cdr1p of Candida albicans in antifungal extracts of medicinal plants

The effective treatment of infections caused by the most frequent human fungal pathogens Candida albicans and Candida glabrata is hindered by a limited number of available antifungals and development of resistance. In this study, we identified new extracts of medicinal plants inhibiting the growth of C. glabrata, a species generally showing low sensitivity to azoles. The methanolic extract of Anacardium occidentalis with an MIC of 80 microg ml(-1) proved to be the most active. In contrast to higher azole sensitivity, C. albicans showed increased resistance to several extracts. Investigation of the possible contribution of the multidrug transporter of the ATP-binding cassette superfamily Cdr1p of C. albicans to extract tolerance revealed a differential response upon overproduction of this protein in Saccharaomyces cerevisiae. Whereas the growth inhibitory activity of many extracts was not affected by CDR1 overexpression, increased sensitivity to some of them was observed. In contrast, extracts showing no detectable anticandidal activity including the ethyl acetate extract of Trichilia emetica were detoxified by Cdr1p. The presence of a non-toxic Cdr1p-mediated ketoconazole resistance modulator accompanying growth-inhibitory Cdr1p substrates in this extract was revealed by further fractionation experiments.

Efflux-mediated antifungal drug resistance

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.

Deletion of yeast CWP genes enhances cell permeability to genotoxic agents

We have previously reported the development of a novel genotoxic testing system based on the transcriptional response of the yeast RNR3-lacZ reporter gene to DNA damage. This system appears to be more sensitive than other similar tests in microorganisms, and is comparable with the Ames test. In an effort to further enhance detection sensitivity, we examined the effects of altering major cell wall components on cell permeability and subsequent RNR3-lacZ sensitivity to genotoxic agents. Although inactivation of single CWP genes encoding cell wall mannoproteins had little effect, the simultaneous inactivation of both CWP1 and CWP2 had profound effects on the cell wall structure and permeability. Consequently, the RNR3-lacZ detection sensitivity is markedly enhanced, especially to high molecular weight compounds such as 4-nitroquinoline-N-oxide (> sevenfold) and phleomycin (> 13-fold). In contrast, deletion of genes encoding representative membrane components or membrane transporters had minor effects on cell permeability. We conclude that the yeast cell wall mannoproteins constitute the major barrier to environmental genotoxic agents and that their removal will significantly enhance the sensitivity of RNR-lacZ as well as other yeast-based genotoxic tests.

Candida albicans zinc cluster protein Upc2p confers resistance to antifungal drugs and is an activator of ergosterol biosynthetic genes

The human pathogen Candida albicans is responsible for a large proportion of infections in immunocompromised individuals, and the emergence of drug-resistant strains is of medical concern. Resistance to antifungal azole compounds is often due to an increase in drug efflux or an alteration of the pathway for synthesis of ergosterol, an important plasma membrane component in fungi. However, little is known about the transcription factors that mediate drug resistance. In Saccharomyces cerevisiae, two highly related transcriptional activators, Upc2p and Ecm22p, positively regulate the expression of genes involved in ergosterol synthesis (ERG genes). We have identified a homologue in C. albicans of the S. cerevisiae UPC2/ECM22 genes and named it UPC2. Deletion of this gene impaired growth under anaerobic conditions and rendered cells highly susceptible to the antifungal drugs ketoconazole and fluconazole. Conversely, overexpression of Upc2p increased resistance to ketoconazole, fluconazole, and fluphenazine. Azole-induced expression of the ERG genes was abolished in a Delta upc2 strain, while basal levels of these mRNAs remained unchanged. Importantly, the purified DNA binding domain of Upc2p bound in vitro to putative sterol response elements in the ERG2 promoter, suggesting that Upc2p increases the expression of the ERG genes by directly binding to their promoters. These results provide an important link between changes in the ergosterol biosynthetic pathway and azole resistance in this opportunistic fungal species.

Identification of putative Plasmodium falciparum mefloquine resistance genes

Mefloquine is an effective antimalarial drug; however, resistant strains of the human malarial pathogen, Plasmodium falciparum, are beginning to arise. The yeast Saccharomyces cerevisiae is sensitive to mefloquine, enabling a screen for P. falciparum genes involved in resistance. Yeast were transformed with a P. falciparum expression library, followed by selection on mefloquine plates and sequencing of plasmids that conferred resistance. We characterized the four genes that conferred the strongest mefloquine-resistant phenotype in yeast. All four (PFD0090c, PFI0195c, PF10_0372 and PF14_0649) are uncharacterized P. falciparum genes from distinct chromosomes (4, 9, 10 and 14, respectively). The mefloquine-resistant phenotype was dependent on induction of the P. falciparum gene and independent of vector context. PFI0195c, which likely encodes a small GTPase activator (GAP), also conferred resistance to cycloheximide and halofantrine in yeast. Immunolocalization of the encoded protein to the Golgi complex in yeast is consistent with potential GAP function. The other three candidate proteins localized to the cytoplasm and plasma membrane (PF14_0649), nuclear envelope/ER (PF10_0372) and Golgi (PFD0090c) of yeast. Analysis of mefloquine-resistant P. falciparum strains and the mefloquine-sensitive strain, W2, by sequencing and semi-quantitative RT-PCR identified no relevant mutations in the resistant strains but showed that PFI0195c was upregulated in two out of three resistant strains and PF14_0649 was upregulated in all resistant strains tested.

In vitro bioactivity of 17alpha-estradiol

A miniaturised short-term in vitro assay based on the activation of the human estrogen receptor alpha and genetically modified yeast (Saccharomyces cerevisiae) cells was performed to explore the capacity of this system to monitor the bioactivity of estrogenic compounds, particularly 17alpha- and 17beta-estradiol. Together with the human estrogen receptor (hER)-alpha plasmid, the reporter plasmid containing a yeast-optimised version of the green fluorescent protein (yEGFP) linked to three repeats of the cis-acting estrogen hormone-responsive element (ERE) were expressed in a strain being deleted in the pleiotropic drug resistance transporters Pdr5, Snq2 and Yor1, known to facilitate efflux of organic compounds including steroids and chemotherapeutics. Agonists that bind to hER in vitro trigger estrogen receptor-mediated transcriptional activation of the GFP reporter gene monitored by fluorescence emission at 535 nm. The sensitivity of the assay was tested with various 17alpha- and 17beta-estradiol concentrations, yielding a detection limit of 5 pg/ml (0.018 nM) for the agonist 17beta-E2 in solvent and in human charcoal-stripped serum using a S. cerevisiae pdr5, snq2 and yor1 mutant strain. For 17alpha-estradiol only, at approximately 1500 pg/ml a similar fluorescence response compared to 100 pg/ml 17beta-E2 was observed implicating a much weaker potency of this stereoisomer. The specificity of the system was tested by expression of a truncated hER lacking the ligand-binding domain E and by administration of the androgen, 4-androsten 3,17 dione. Both controls did not yield an increase in fluorescence emission. This fluorescence emission assay enables detection of estrogenic biological activity induced by direct agonists, such as 17beta-E2 at concentrations similar to those found in human sera or by estrogen-like chemicals.

Fluconazole binding and sterol demethylation in three CYP51 isoforms indicate differences in active site topology

14alpha-Demethylase (CYP51) is a key enzyme in all sterol biosynthetic pathways (animals, fungi, plants, protists, and some bacteria), catalyzing the removal of the C-14 methyl group following cyclization of squalene. Based on mutations found in CYP51 genes from Candida albicans azole-resistant isolates obtained after fluconazole treatment of fungal infections, and using site-directed mutagenesis, we have found that fluconazole binding and substrate metabolism vary among three different CYP51 isoforms: human, fungal, and mycobacterial. In C. albicans, the Y132H mutant from isolates shows no effect on fluconazole binding, whereas the F145L mutant results in a 5-fold increase in its IC(50) for fluconazole, suggesting that F145 (conserved only in fungal 14alpha-demethylases) interacts with this azole. In C. albicans, F145L accounts, in part, for the difference in fluconazole sensitivity reported between mammals and fungi, providing a basis for treatment of fungal infections. The C. albicans Y132H and human Y145H CYP51 mutants show essentially no effect on substrate metabolism, but the Mycobacterium tuberculosis F89H CYP51 mutant loses both its substrate binding and metabolism. Because these three residues align in the three isoforms, the results indicate that their active sites contain important structural differences, and further emphasize that fluconazole and substrate binding are uncoupled properties.

Genome-wide expression profile of steroid response in Saccharomyces cerevisiae

The response of the yeast Saccharomyces cerevisiae to human steroid hormone progesterone was studied by genomic expression profiling. The transcription profile data revealed that steroid response was a global phenomenon wherein a host of genes were affected. For example, 163 genes were upregulated and 40 genes were downregulated, by at least more than twofold. The major categories of upregulated genes included protein destination (15%), metabolism (14%), transport facilitation (12%), cell growth, cell division, and DNA synthesis (8%), and transcription (7%), while metabolism (22%), transcription (11%), intracellular transport (10%), cell growth, cell division, and DNA synthesis (10%), energy (8%), cell rescue, defense, and cell death (6%), and protein synthesis (6%) encoding genes were downregulated. Notwithstanding the fact that yeast cells do not possess commonly occurring steroid response cascade similar to higher eukaryotes, our results demonstrate that a short-term exposure to progesterone results in differential regulation of predominantly stress responsive genes.

A yeast-based method for the detection of cyto and genotoxicity

A miniaturized short-term in vivo genotoxicity screening assay based on genetically modified yeast (Saccharomyces cerevisiae) cells was performed to explore the capacity of this eukaryotic organism to detect the presence of genotoxic compounds. An increased general sensitivity of yeast cells to toxic compounds was obtained by using a strain being deleted in the prominent pleiotropic drug resistance mediating efflux transporters PDR5, SNQ2 and YOR1. In order to detect genotoxic effects, a yeast optimized version of the green fluorescent protein (GFP) was fused to the RAD54 promoter that is activated upon DNA damage. Various model substances including the oxygenated fuel additive methyl tertiary-butyl ether (MTBE) and the direct acting genotoxins methyl-N-nitro-N-nitrosoguanidine (MNNG) and 4-nitroquinoline-1-oxide (4-NQO) were tested. All model substances were in parallel examined for chronic cytotoxicity. The results point out the sufficiency of both the sensitivity of the yeast cells to detect chronic cytotoxicity and the intensity of the fluorescence signal for the assessment of genotoxic effects. Thus, the test enables simultaneous detection of cytotoxic and genotoxic effects. By partial automation and implementation of the test in the microtitre scale this bioassay allows parallel sensitive pre-screening of numerous samples.

ABC transporters of the wheat pathogen Mycosphaerella graminicola function as protectants against biotic and xenobiotic toxic compounds

We have studied the role of five ABC transporter genes (MgAtr to MgAtr5) from the wheat pathogen Mycosphaerella graminicola in multidrug resistance (MDR). Complementation of Saccharomyces cerevisiae mutants with the ABC transporter genes from M. graminicola showed that all the genes tested encode proteins that provide protection against chemically unrelated compounds, indicating that their products function as multidrug transporters with distinct but overlapping substrate specificities. Their substrate range in yeast includes fungicides, plant metabolites, antibiotics, and a mycotoxin derived from Fusarium graminearum (diacetoxyscirpenol). Transformants of M. graminicola in which individual ABC transporter genes were deleted or disrupted did not exhibit clear-cut phenotypes, probably due to the functional redundancy of transporters with overlapping substrate specificity. Independently generated MgAtr5 deletion mutants of M. graminicola showed an increase in sensitivity to the putative wheat defence compound resorcinol and to the grape phytoalexin resveratrol, suggesting a role for this transporter in protecting the fungus against plant defence compounds. Bioassays with antagonistic bacteria indicated that MgAtr2 provides protection against metabolites produced by Pseudomonas fluorescens and Burkholderia cepacia. In summary, our results show that ABC transporters from M. graminicola play a role in protection against toxic compounds of natural and artificial origin.

Three-dimensional reconstruction of the Saccharomyces cerevisiae multidrug resistance protein Pdr5p

Pdr5p, the major multidrug exporter in Saccharomyces cerevisiae, is a member of the ATP-binding cassette (ABC) superfamily. Pdr5p shares similar mechanisms of substrate recognition and transport with the human MDR1-Pgp, despite an inverted topology of transmembrane and ATP-binding domains. The hexahistidine-tagged Pdr5p multidrug transporter was highly overexpressed in yeast strains where other ABC genes have been deleted. After solubilization and purification, the 160-kDa recombinant Pdr5p has been reconstituted into a lipid bilayer. Controlled detergent removal from Pdr5p-lipid-detergent micelles allowed the production of peculiar square-shaped particles coexisting with liposomes and proteoliposomes. These particles having 11 nm in side were well suited for single particle analysis by electron microscopy. From such analysis, a computed volume has been determined at 25-A resolution, giving insight into the structural organization of Pdr5p. Comparison with the reported structures of different bacterial ABC transporters was consistent with a dimeric organization of Pdr5p in the square particles. Each monomer was composed of three subregions corresponding to a membrane region of about 50 A in height that joins two well separated protruding stalks of about 40 A in height, ending each one with a cytoplasmic nucleotide-binding domain (NBD) lobe of about 50-60 A in diameter. The three-dimensional reconstruction of Pdr5p revealed a close arrangement and a structural asymmetric organization of the two NBDs that appeared oriented perpendicularly within a monomer. The existence of different angular positions of the NBDs, with respect to the stalks, suggest rotational movements during the catalytic cycle.

Drug resistance in yeasts--an emerging scenario

In view of the increasing threat posed by fungal infections in immunocompromised patients and due to the non-availability of effective treatments, it has become imperative to find novel antifungals and vigorously search for new drug targets. Fungal pathogens acquire resistance to drugs (antifungals), a well-established phenomenon termed multidrug resistance (MDR), which hampers effective treatment strategies. The MDR phenomenon is spread throughout the evolutionary scale. Accordingly, a host of responsible genes have been identified in the genetically tractable budding yeast Saccharomyces cerevisiae, as well as in a pathogenic yeast Candida albicans. Studies so far suggest that, while antifungal resistance is the culmination of multiple factors, there may be a unifying mechanism of drug resistance in these pathogens. ABC (ATP binding cassette) and MFS (major facilitator superfamily) drug transporters belonging to two different superfamilies, are the most prominent contributors to MDR in yeasts. Considering the abundance of the drug transporters and their wider specificity, it is believed that these drug transporters may not exclusively export drugs in fungi. It has become apparent that the drug transporters of the ABC superfamily of S. cerevisiae and C. albicans are multifunctional proteins, which mediate important physiological functions. This review summarizes current research on the molecular mechanisms underlying drug resistance, the emerging regulatory circuits of MDR genes, and the physiological relevance of drug transporters.

Involvement of multidrug resistance proteins (MDR) in the modulation of glucocorticoid response

Glucocorticoid resistance is a problem in the treatment of many diseases. One possible factor involved in the modulation of a glucocorticoid response is the export of glucocorticoids out of the cell. It has been shown that multidrug resistance protein 1 (MDR1, ABCB1), a member of the ABC family, is capable of transporting some glucocorticoids. This paper uses a mouse cell line, LMCAT in which the glucocorticoid response can be modulated by inhibitors of multidrug resistance proteins. Glucocorticoids fall into three categories. Firstly, those that are transported by an Abcb1a/Abcb1b transporter and whose transport can be inhibited by inhibitors of ABCB1 activity. Functional Abcb1a/Abcb1b was detected by inhibition of rhodamine efflux by these drugs and mRNA for Abcb1a and Abcb1b were detected in these cells. Secondly, those that are not transported. Finally, those that are transported by an Abcc1a transporter. Calcein transport out of these cells was blocked by treatment with probenecid indicating a functional Abcc1a transporter. Abcc1a mRNA was also detected in these cells. Thus, this paper provides insight into the mechanisms of glucocorticoid transport in cells and demonstrates a diversity of two independent mechanisms of transport of glucocorticoids by Abcb1a/Abcb1b and Abcc1a with individual patterns of steroid specificity.

Endogenous reactive oxygen species is an important mediator of miconazole antifungal effect

We investigated the significance of endogenous reactive oxygen species (ROS) produced by fungi treated with miconazole. ROS production in Candida albicans was measured by a real-time fluorogenic assay. The level of ROS production was increased by miconazole at the MIC (0.125 micro g/ml) and was enhanced further in a dose-dependent manner, with a fourfold increase detected when miconazole was used at 12.5 micro g/ml. This increase in the level of ROS production was completely inhibited by pyrrolidinedithiocarbamate (PDTC), an antioxidant, at 10 micro M. In a colony formation assay, the decrease in cell viability associated with miconazole treatment was significantly prevented by addition of PDTC. Moreover, the level of ROS production by 10 clinical isolates of Candida species was inversely correlated with the miconazole MIC (r = -0.8818; P < 0.01). These results indicate that ROS production is important to the antifungal activity of miconazole.

Mechanisms of resistance to fungicides in field strains of Botrytis cinerea

Field strains of Botrytis cinerea Pers ex Fr, the causal agent of grey mould diseases, were collected from French vineyards between 1993 and 2000. Several phenotypes have been characterized according to the inhibitory effects of fungicides towards germ-tube elongation and mycelial growth. Two types of benzimidazole-resistant strains (Ben R1 and Ben R2) could be detected; negative cross-resistance to phenylcarbamates (e.g. diethofencarb) was only found in Ben R1. Benzimidazole resistance was related to point mutations at codon 198 (Ben R1) or 200 (Ben R2) of the beta-tubulin gene. Most dicarboximide-resistant strains were also weakly resistant to aromatic hydrocarbon fungicides (e.g. dicloran) but remained sensitive to phenylpyrroles (e.g. fludioxonil). These resistant field strains (Imi R1) contained a single base pair mutation at position 365 in a two-component histidine kinase gene, probably involved in the fungal osmoregulation. Three anilinopyrimidine-resistant phenotypes have been identified. In the most resistant one (Ani R1), resistance was restricted to anilinopyrimidines, but no differences were observed in the amino-acid sequences of cystathionine beta-lyase (the potential target site of these fungicides) from Ani R1 or wild-type strains. In the two other phenotypes (Ani R2 and Ani R3), resistance extended to various other groups of fungicide, including dicarboximides, phenylpyrroles and sterol biosynthesis inhibitors. This multi-drug resistance was probably determined by over-production of ATP-binding cassette transporters. The hydroxyanilide fenhexamid is a novel botryticide whose primary target site is the 3-keto reductase involved in sterol C-4 demethylations. Apart from the multi-drug-resistant strain Ani R3, three other fenhexamid-resistant phenotypes have been recognized. For two of them (Hyd R1 and Hyd R2) fenhexamid-resistance seemed to result from P450-mediated detoxification. Reduced sensitivity of the target site could be the putative resistance mechanism operating in the third resistant phenotype (Hyd R3). Increased sensitivity to inhibitors of sterol 14 alpha-demethylase recorded in Hyd R1 strains was related to two amino-acid changes at positions 15 and 105 of this enzyme.

Quantitative analysis of the relative transcript levels of ABC transporter Atr genes in Aspergillus nidulans by real-time reverse transcription-PCR assay

The development of assays for quantitative analysis of the relative transcript levels of ABC transporter genes by real-time reverse transcription-PCR (RT-PCR) might provide important information about multidrug resistance in filamentous fungi. Here, we evaluate the potential of real-time RT-PCR to quantify the relative transcript levels of ABC transporter Atr genes from Aspergillus nidulans. The AtrA to AtrD genes showed different and higher levels in the presence of structurally unrelated drugs, such as camptothecin, imazalil, itraconazole, hygromycin, and 4-nitroquinoline oxide. We also verified the relative transcript levels of the Atr genes in the A. nidulans imazalil-resistant mutants. These genes displayed a very complex pattern in different ima genetic backgrounds. The imaB mutant has higher basal transcript levels of AtrB and -D than those of the wild-type strain. The levels of these two genes are comparable when the imaB mutant is grown in the presence and absence of imazalil. The imaC, -D, and -H mutants have higher basal levels of AtrA than that of the wild type. The same behavior is observed for the relative transcript levels of AtrB in the imaG mutant background.

The glycerol channel Fps1p mediates the uptake of arsenite and antimonite in Saccharomyces cerevisiae

The Saccharomyces cerevisiae FPS1 gene encodes a glycerol channel protein involved in osmoregulation. We present evidence that Fps1p mediates influx of the trivalent metalloids arsenite and antimonite in yeast. Deletion of FPS1 improves tolerance to arsenite and potassium antimonyl tartrate. Under high osmolarity conditions, when the Fps1p channel is closed, wild-type cells show the same degree of As(III) and Sb(III) tolerance as the fps1Delta mutant. Additional deletion of FPS1 in mutants defective in arsenite and antimonite detoxification partially suppresses their hypersensitivity to metalloid salts. Cells expressing a constitutively open form of the Fps1p channel are highly sensitive to both arsenite and antimonite. We also show by direct transport assays that arsenite uptake is mediated by Fps1p. Yeast cells appear to control the Fps1p-mediated pathway of metalloid uptake, as expression of the FPS1 gene is repressed upon As(III) and Sb(III) addition. To our knowledge, this is the first report describing a eukaryotic uptake mechanism for arsenite and antimonite and its involvement in metalloid tolerance.

Will genomics revolutionize antimicrobial drug discovery?

Utilizing genome sequence data from bacterial and fungal pathogens for the discovery of new antimicrobial agents has received considerable attention, both practical and critical, from the pharmaceutical and biotechnological communities. Although no new drugs derived from genomics-based discovery have been reported to be in a development pipeline, the utilization of genomics has revolutionized many aspects of drug discovery. The application, utility, opportunity, and challenges afforded by many of these new approaches are discussed.

Antibiotic efflux pumps

Active efflux from procaryotic as well as eucaryotic cells strongly modulates the activity of a large number of antibiotics. Effective antibiotic transport has now been observed for many classes of drug efflux pumps. Thus, within the group of primary active transporters, predominant in eucaryotes, six families belonging to the ATP-binding cassette superfamily, and including the P-glycoprotein in the MDR (Multi Drug Resistance) group and the MRP (Multidrug Resistance Protein), have been recognized as being responsible for antibiotic efflux. Within the class of secondary active transporters (antiports, symports, and uniports), ten families of antibiotic efflux pumps have been described, distributed in five superfamilies [SMR (Small Multidrug Resistance), MET (Multidrug Endosomal Transporter), MAR (Multi Antimicrobial Resistance), RND (Resistance Nodulation Division), and MFS (Major Facilitator Superfamily)]. Nowadays antibiotic efflux pumps are believed to contribute significantly to acquired bacterial resistance because of the very broad variety of substrates they recognize, their expression in important pathogens, and their cooperation with other mechanisms of resistance. Their presence also explains high-level intrinsic resistances found in specific organisms. Stable mutations in regulatory genes can produce phenotypes of irreversible multidrug resistance. In eucaryotes, antibiotic efflux pumps modulate the accumulation of antimicrobials in phagocytic cells and play major roles in their transepithelial transport. The existence of antibiotic efflux pumps, and their impact on therapy, must now be taken fully into account for the selection of novel antimicrobials. The design of specific, potent inhibitors appears to be an important goal for the improved control of infectious diseases in the near future.

In vivo characterization of the drug resistance profile of the major ABC transporters and other components of the yeast pleiotropic drug resistance network

Multidrug resistance (MDR) mediated by broad specificity transporters is one of the most important strategies used by pathogens, including cancer cells, to evade chemotherapy. In the yeast Saccharomyces cerevisiae, a complex pleiotropic drug resistance (PDR) network of genes involved in MDR is composed of the transcriptional regulators Pdr1p and Pdr3p, which activate expression of the ATP-binding cassette (ABC) MDR transporters-encoding genes PDR5, SNQ2, and YOR1 as well as other not yet identified genes. We have screened 349 toxic compounds in isogenic S. cerevisiae strains deleted of PDRS, SNQ2, or YOR1 in different combinations as well as both PDR1 and PDR3. The screen revealed extremely promiscuous, yet limited, and to a large extent overlapping but distinct drug resistance profiles of Pdr5p, Snq2p, and Yor1p. These ABC-MDR transporters mediated resistance to most currently available classes of clinically and agriculturally important fungicides and also to many antibiotics, herbicides, and others. Several classes of compounds were identified for the first time in the drug resistance spectrum of MDR transporters. These are fungicides, such as anilinopyrimidines, benzimidazoles, benzenedicarbonitriles, dithiocarbamates, guanidines, imidothiazoles, polyenes, pyrimidynyl carbinols, and strobilurine analogues; the urea derivative and anilide herbicides; flavonoids, several membrane lipids resembling detergents; and newly synthesized lysosomotropic aminoesters; as well as many others. Identification of compounds showing Pdr1p, Pdr3p-dependent, but Pdr5p-, Snq2p-, and Yor1p-independent toxicity, reflected in the case of rhodamine 6G, by efflux alterations, suggests the involvement of new drug resistance genes and is a first step toward their identification. The highly increased toxicity of bile acids toward the PDR1, PDR3 double disruptant together with the decreased level of BAT1 promoter dependent beta-galactosidase activity suggest that the Bat1p ABC transporter is a new member of the PDR network. Our results may contribute to a better understanding of the mechanism of MDR, in particular in the pathogenic yeast Candida albicans. They also provide and indication of the physiological function of MDR transporters and suggest new approaches for the cloning of the mammalian bile acid transporters.

Role of the PDR gene network in yeast susceptibility to the antifungal antibiotic mucidin

Yeast strains disrupted in the PDR1, PDR3, or PDR5 gene, but not in SNQ2, exhibited higher sensitivity to mucidin (strobilurin A) than did the isogenic wild-type strains. Different gain-of-function mutations in the PDR1 and PDR3 genes rendered yeast mutants resistant to this antibiotic. Mucidin induced PDR5 expression, but the changes in the expression of SNQ2 were only barely detectable. The results indicate that PDR5 provides the link between transcriptional regulation by PDR1 and PDR3 and mucidin resistance of yeast.

Regulation of pleiotropic drug resistance in yeast

This review focuses on the molecular mechanisms involved in the regulation of multiple drug resistance in the model yeast Saccharomyces cerevisiae and the pathogenic fungus Candida albicans. Recent developments in the study of the transcription factors Pdr1p, Pdr3p and Yap1p are reported. Understanding the molecular basis leading to multiple drug resistance is a prerequisite for the development of new antifungal therapeutics. Copyright 1999 Harcourt Publishers Ltd.

CDR1, a multidrug resistance gene from Candida albicans, contains multiple regulatory domains in its promoter and the distal AP-1 element mediates its induction by miconazole

We previously demonstrated that the CDR1 gene, encoding a multidrug transporter in Candida albicans, is differentially upregulated by various drugs and steroids. In order to get an insight into the molecular basis of the induction of this gene we analyzed its promoter region. The transcription start site was mapped to 63 nucleotides upstream of the initiating ATG. Reporter assays revealed the presence of four upstream activating and four upstream repressing sequence domains along the entire promoter. Like the native gene, promoter-luciferase recombinants showed enhanced activity in response to various stresses like drugs, human steroid hormones and heavy metals. Mutational analysis demonstrated that while the proximal promoter (-345/+1) contains all the regulatory domains required for its induction by various other stresses, the miconazole response is mediated via the distal promoter (-857/-1147), harboring an AP-1 site. The involvement of the AP-1 element in mediating the latter effect was evident by an increase in AP-1 binding activity following miconazole treatment.

FLR1 gene (ORF YBR008c) is required for benomyl and methotrexate resistance in Saccharomyces cerevisiae and its benomyl-induced expression is dependent on pdr3 transcriptional regulator

In this work we report the disruption of a Saccharomyces cerevisiae ORF YBR008c (FLR1 gene) within the context of EUROFAN (EUROpean Functional Analysis Network) six-pack programme, using a PCR-mediated gene replacement protocol as well as the results of the basic phenotypic analysis of a deletant strain and the construction of a disruption cassette for inactivation of this gene in any yeast strain. We also show results extending the knowledge of the range of compounds to which FLR1 gene confers resistance to the antimitotic systemic benzimidazole fungicide benomyl and the antitumor agent methotrexate, reinforcing the concept that the FLR1 gene is a multidrug resistance (MDR) determinant. Our conclusions were based on the higher susceptibility to these compounds of flr1Delta compared with wild-type and on the increased resistance of both flr1Delta and wild-type strains upon increased expression of FLR1 gene from a centromeric plasmid clone. The present study also provides, for the first time, evidence that the adaptation of yeast cells to growth in the presence of benomyl involves the dramatic activation of FLR1 gene expression during benomyl-induced latency (up to 400-fold). Results obtained using a FLR1-lacZ fusion in a plasmid indicate that the activation of FLR1 expression in benomyl-stressed cells is under the control of the transcriptional regulator Pdr3p. Indeed, PDR3 deletion severely reduces benomyl-induced activation of FLR1 gene expression (by 85%), while the homologous Pdr1p transcription factor is apparently not involved in this activation.

Accumulation of 3-ketosteroids induced by itraconazole in azole-resistant clinical Candida albicans isolates

The effects of itraconazole on ergosterol biosynthesis were investigated in a series of 16 matched clinical Candida albicans isolates which had been previously analyzed for mechanisms of resistance to azoles (D. Sanglard, K. Kuchler, F. Ischer, J. L. Pagani, M. Monod, and J. Bille, Antimicrob. Agents Chemother., 39:2378-2386, 1995). Under control conditions, all isolates contained ergosterol as the predominant sterol, except two strains (C48 and C56). In isolates C48 and C56, both less susceptible to azoles than their parent, C43, substantial concentrations (20 to 30%) of 14alpha-methyl-ergosta-8,24(28)-diene-3beta,6alpha-dio l (3, 6-diol) were found. Itraconazole treatment of C43 resulted in a dose-dependent inhibition of ergosterol biosynthesis (50% inhibitory concentration, 2 nM) and accumulation of 3,6-diol (up to 60% of the total sterols) together with eburicol, lanosterol, obtusifoliol, 14alpha-methyl-ergosta-5,7,22,24(28)-tetraene-3betaol, and 14alpha-methyl-fecosterol. In strains C48 and C56, no further increase of 3,6-diol was observed after exposure to itraconazole. Ergosterol synthesis was less sensitive to itraconazole inhibition, as was expected for these azole-resistant isolates which overexpress ATP-binding cassette transporter genes CDR1 and CDR2. In addition to 3,6-diol, substantial amounts of obtusifolione were found after exposure to itraconazole. This toxic 3-ketosteroid was demonstrated previously to accumulate after itraconazole treatment in Cryptococcus neoformans and Histoplasma capsulatum but has not been reported in Candida isolates. Accumulation of obtusifolione correlated with nearly complete growth inhibition in these azole-resistant strains compared to that found in the susceptible parent strain, although the onset of growth inhibition only occurred at higher concentrations of itraconazole. ERG25 and ERG26 are the only genes assigned to the 4-demethylation process, of which the 3-ketoreductase is part. To verify whether mutations in these ERG25 genes contributed to obtusifolione accumulation, their nucleotide sequences were determined in all three related isolates. No mutations in ERG25 alleles of isolates C48 and C56 were found, suggesting that this gene is not involved in obtusifolione accumulation. The molecular basis for the accumulation of this sterol in these two strains remains to be established.

Contribution of mutations in the cytochrome P450 14alpha-demethylase (Erg11p, Cyp51p) to azole resistance in Candida albicans

The cytochrome P450 14alpha-demethylase, encoded by the ERG11 (CYP51) gene, is the primary target for the azole class of antifungals. Changes in the azole affinity of this enzyme caused by amino acid substitutions have been reported as a resistance mechanism. Nine Candida albicans strains were used in this study. The ERG11 base sequence of seven isolates, of which only two were azole-sensitive, were determined. The ERG11 base sequences of the other two strains have been published previously. In these seven isolates, 12 different amino acid substitutions were identified, of which six have not been described previously (A149V, D153E, E165Y, S279F, V452A and G4655). In addition, 16 silent mutations were found. Two different biochemical assays, subcellular sterol biosynthesis and CO binding to reduced microsomal fractions, were used to evaluate the sensitivity of the cytochromes for fluconazole and itraconazole. Enzyme preparations from four isolates showed reduced itraconazole susceptibility, whereas more pronounced resistance to fluconazole was observed in five isolates. A three-dimensional model of C. albicans Cyp51p was used to position all 29 reported substitutions, 98 in total identified in 53 sequences. These 29 substitutions were not randomly distributed over the sequence but clustered in three regions from amino acids 105 to 165, from 266 to 287 and from 405 to 488, suggesting the existence of hotspot regions. Of the mutations found in the two N-terminal regions only Y132H was demonstrated to be of importance for azole resistance. In the C-terminal region three mutations are associated with resistance, suggesting that the non-characterized substitutions found in this region should be prioritized for further analysis.

In vitro activities of terbinafine against cutaneous isolates of Candida albicans and other pathogenic yeasts

Terbinafine is active in vitro against a wide range of pathogenic fungi, including dermatophytes, molds, dimorphic fungi, and some yeasts, but earlier studies indicated that the drug had little activity against Candida albicans. In contrast, clinical studies have shown topical and oral terbinafine to be active in cutaneous candidiasis and Candida nail infections. In order to define the anti-Candida activity of terbinafine, we tested the drug against 350 fresh clinical isolates and additional strains by using a broth dilution assay standardized according to the guidelines of the National Committee for Clinical Laboratory Standards (NCCLS) M27-A assay. Terbinafine was found to have an MIC of 1 microg/ml for reference C. albicans strains. For 259 clinical isolates, the MIC at which 50% of the isolates are inhibited (MIC50) of terbinafine was 1 microg/ml (fluconazole, 0.5 microg/ml), and the MIC90 was 4 microg/ml (fluconazole, 1 microg/ml). Terbinafine was highly active against Candida parapsilosis (MIC90, 0.125 microg/ml) and showed potentially interesting activity against isolates of Candida dubliniensis, Candida guilliermondii, Candida humicola, and Candida lusitaniae. It was not active against the Candida glabrata, Candida krusei, and Candida tropicalis isolates in this assay. Cryptococcus laurentii and Cryptococcus neoformans were highly susceptible to terbinafine, with MICs of 0.06 to 0.25 microg/ml. The NCCLS macrodilution assay provides reproducible in vitro data for terbinafine against Candida and other yeasts. The MICs for C. albicans and C. parapsilosis are compatible with the known clinical efficacy of terbinafine in cutaneous infections, while the clinical relevance of its activities against the other species has yet to be determined.

Beta-N-acetylhexosaminidase: a target for the design of antifungal agents

This review provides biochemical, analytical, and biological background information relating to beta-N-acetylhexosaminidase (HexNAc'ase; EC 3.2.1.52) as an emerging target for the design of low-molecular-weight antifungals. The article includes the following: (1) a biochemical description of HexNAc'ase (reaction catalyzed, nomenclature, and mechanism of action) that sets it apart from other, similar enzymes; (2) an overview and a critical evaluation of methods to assay the enzyme, including in crude extracts (photo- and fluorometric procedures with model substrates; HPLC/pulsed amperometric detection of N-acetylglucosamine and chito-oligomers; end-point vs. rate measurements); (3) a summary of some general characteristics of HexNAc'ases from fungi and organisms of other types (Km values, substrate preference, and glycoconjugation); (4) an hypothesis of a specific target function of wall-associated HexNAc'ase (a component of the assembly of surface-located enzymes effecting a continuous turnover and remodelling of the wall fabric through its combined hydrolytic and transglycosylating activities, and a mediator enzyme acting in concert with chitinase and chitin synthase to provide for the controlled lysis and synthesis of chitin during growth); (5) a tabulation of the structural formulae of reaction-based HexNAc'ase inhibitors with Ki values < or = 100 microM (some of them representing transition state mimics that could serve as leads for the development of new antifungals); and (6) an outline of approaches towards the establishment of a three-dimensional model of HexNAc'ase suitable for a truly rational design of antimycotics as well as agricultural fungicides.