Multidrug resistance in Candida albicans: disruption of the BENr gene

The anti-staphylococcal fusidic acid as an efflux pump inhibitor combined with fluconazole against vaginal candidiasis in mouse model

BACKGROUND

Candida albicans is the most common fungus that causes vaginal candidiasis in immunocompetent women and catastrophic infections in immunocompromised patients. The treatment of such infections is hindered due to the increasing emergence of resistance to azoles in C. albicans. New treatment approaches are needed to combat candidiasis especially in the dwindled supply of new effective and safe antifungals. The resistance to azoles is mainly attributed to export of azoles outside the cells by means of the efflux pump that confers cross resistance to all azoles including fluconazole (FLC).

OBJECTIVES

This study aimed to investigate the possible efflux pump inhibiting activity of fusidic acid (FA) in C. albicans resistant isolates and the potential use of Fusidic acid in combination with fluconazole to potentiate the antifungal activity of fluconazole to restore its activity in the resistant C. albicans isolates.

METHODS

The resistance of C. albicans isolates was assessed by determination of minimum inhibitory concentration. The effect of Fusidic acid at sub-inhibitory concentration on efflux activity was assayed by rhodamine 6G efflux assay and intracellular accumulation. Mice model studies were conducted to evaluate the anti-efflux activity of Fusidic acid and its synergistic effects in combination with fluconazole. Impact of Fusidic acid on ergosterol biosynthesis was quantified. The synergy of fluconazole when combined with Fusidic acid was investigated by determination of minimum inhibitory concentration. The cytotoxicity of Fusidic acid was tested against erythrocytes. The effect of Fusidic acid on efflux pumps was tested at the molecular level by real-time PCR and in silico study. In vivo vulvovaginitis mice model was used to confirm the activity of the combination in treating vulvovaginal candidiasis.

RESULTS

Fusidic acid showed efflux inhibiting activity as it increased the accumulation of rhodamine 6G, a substrate for ABC-efflux transporter, and decreased its efflux in C. albicans cells. The antifungal activity of fluconazole was synergized when combined with Fusidic acid. Fusidic acid exerted only minimal cytotoxicity on human erythrocytes indicating its safety. The FA efflux inhibitory activity could be owed to its ability to interfere with efflux protein transporters as revealed by docking studies and downregulation of the efflux-encoding genes of both ABC transporters and MFS superfamily. Moreover, in vivo mice model showed that using fluconazole-fusidic acid combination by vaginal route enhanced fluconazole antifungal activity as shown by lowered fungal burden and a negligible histopathological change in vaginal tissue.

CONCLUSION

The current findings highlight FA's potential as a potential adjuvant to FLC in the treatment of vulvovaginal candidiasis.

A state-of-the-art review and prospective therapeutic applications of prenyl flavonoids as chemosensitizers against antifungal multidrug resistance in Candida albicans

Multidrug resistance (MDR) in the opportunistic pathogen Candida albicans is defined as non-susceptibility to at least one agent in two or more drug classes. This phenomenon has been increasingly reported since the rise in the incidence of fungal infections in immunocompromised patients at the end of the last century. After the discovery of efflux pump overexpression as a principal mechanism causing MDR in Candida strains, drug discovery targeting fungal efflux transporters has had a growing impact. Chemosensitization aims to enhance azole intracellular concentrations through combination therapy with transporter inhibitors. Consequently, the use of drug efflux inhibitors combined with the antifungal agent will sensitize the pathogen. As a result, the use of lower drug concentrations will reduce possible adverse effects on the host. Through an extensive revision of the literature, this review aims to provide an exhaustive and critical analysis of the studies carried out in the past two decades, regarding the chemosensitization strategy to cope with multidrug resistance in C. albicans. This work provides a deep analysis of the research about the inhibition of drug-efflux membrane transporters by prenylated flavonoids and the interactions of these phytocompounds with azole antifungals as an approach to chemosensitize multidrug-resistant C. albicans strains. We highlight the importance of prenylflavonoids and their particular chemical and pharmacological characteristics that make them excellent candidates with therapeutic potential as chemosensitizers. Finally, we propose the need for further research of prenyl flavonoids as inhibitors of drug-efflux mediated fungal resistance.

Directed Mutational Strategies Reveal Drug Binding and Transport by the MDR Transporters of Candida albicans

Multidrug resistance (MDR) transporters belonging to either the ATP-Binding Cassette (ABC) or Major Facilitator Superfamily (MFS) groups are major determinants of clinical drug resistance in fungi. The overproduction of these proteins enables the extrusion of incoming drugs at rates that prevent lethal effects. The promiscuity of these proteins is intriguing because they export a wide range of structurally unrelated molecules. Research in the last two decades has used multiple approaches to dissect the molecular basis of the polyspecificity of multidrug transporters. With large numbers of drug transporters potentially involved in clinical drug resistance in pathogenic yeasts, this review focuses on the drug transporters of the important pathogen Candida albicans. This organism harbors many such proteins, several of which have been shown to actively export antifungal drugs. Of these, the ABC protein CaCdr1 and the MFS protein CaMdr1 are the two most prominent and have thus been subjected to intense site-directed mutagenesis and suppressor genetics-based analysis. Numerous results point to a common theme underlying the strategy of promiscuity adopted by both CaCdr1 and CaMdr1. This review summarizes the body of research that has provided insight into how multidrug transporters function and deliver their remarkable polyspecificity.

An Azole-Tolerant Endosomal Trafficking Mutant of Candida albicans Is Susceptible to Azole Treatment in a Mouse Model of Vaginal Candidiasis

We recently reported that a Candida albicans endosomal trafficking mutant continues to grow after treatment with the azole antifungals. Herein, we report that the vps21Δ/Δ mutant does not have a survival advantage over wild-type isolates after fluconazole treatment in a mouse model of vaginal candidiasis. Furthermore, loss of VPS21 does not synergize with established mechanisms of azole resistance, such as overexpression of efflux pumps or of Erg11p, the target enzyme of the azoles. In summary, although loss of VPS21 function enhances C. albicans survival after azole treatment in vitro, it does not seem to affect azole susceptibility in vivo.

Endosomal Trafficking Defects Can Induce Calcium-Dependent Azole Tolerance in Candida albicans

The azole antifungals arrest fungal growth through inhibition of ergosterol biosynthesis. We recently reported that a Candida albicans vps21Δ/Δ mutant, deficient in membrane trafficking through the late endosome/prevacuolar compartment (PVC), continues to grow in the presence of the azoles despite the depletion of cellular ergosterol. Here, we report that the vps21Δ/Δ mutant exhibits less plasma membrane damage upon azole treatment than the wild type, as measured by the release of a cytoplasmic luciferase reporter into the culture supernatant. Our results also reveal that the vps21Δ/Δ mutant has abnormal levels of intracellular Ca²⁺ and, in the presence of fluconazole, enhanced expression of a calcineurin-responsive RTA2-GFP reporter. Furthermore, the azole tolerance phenotype of the vps21Δ/Δ mutant is dependent upon both extracellular calcium levels and calcineurin activity. These findings underscore the importance of endosomal trafficking in determining the cellular consequences of azole treatment and indicate that this may occur through modulation of calcium- and calcineurin-dependent responses.

MFS transporters of Candida species and their role in clinical drug resistance

ABC (ATP-binding cassette) and MFS (major facilitator superfamily) exporters, belonging to two different superfamilies, are one of the most prominent contributors of multidrug resistance (MDR) in yeast. While the role of ABC efflux pump proteins in the development of MDR is well documented, the MFS transporters which are also implicated in clinical drug resistance have not received due attention. The MFS superfamily is the largest known family of secondary active membrane carriers, and MFS exporters are capable of transporting a host of substrates ranging from small molecules, including organic and inorganic ions, to complex biomolecules, such as peptide and lipid moieties. A few of the members of the drug/H(+) antiporter family of the MFS superfamily function as multidrug transporters and employ downhill transport of protons to efflux their respective substrates. This review focuses on the recent developments in MFS of Candida and highlights their role in drug transport by using the example of the relatively well characterized promiscuous Mdr1 efflux pump of the pathogenic yeast C. albicans.

Mutational Analysis of Intracellular Loops Identify Cross Talk with Nucleotide Binding Domains of Yeast ABC Transporter Cdr1p

The ABC transporter Cdr1 protein (Cdr1p) of Candida albicans, which plays a major role in antifungal resistance, has two transmembrane domains (TMDs) and two nucleotide binding domains (NBDs) that are interconnected by extracellular (ECLs) and intracellular (ICLs) loops. To examine the communication interface between the NBDs and ICLs of Cdr1p, we subjected all four ICLs to alanine scanning mutagenesis, replacing each of the 85 residues with an alanine. The resulting ICL mutant library was analyzed by biochemical and phenotypic mapping. Only 18% of the mutants from this library displayed enhanced drug susceptibility. Most of the drug-susceptible mutants displayed uncoupling between ATP hydrolysis and drug transport. The two drug-susceptible ICL1 mutants (I574A and S593A) that lay within or close to the predicted coupling helix yielded two chromosomal suppressor mutations that fall near the Q-loop of NBD2 (R935) and in the Walker A motif (G190) of NBD1. Based on a 3D homology model and kinetic analysis of drug transport, our data suggest that large distances between ICL residues and their respective chromosomal suppressor mutations rule out a direct interaction between them. However, they impact the transport cycle by restoring the coupling interface via indirect downstream signaling.

Trafficking through the late endosome significantly impacts Candida albicans tolerance of the azole antifungals

The azole antifungals block ergosterol biosynthesis by inhibiting lanosterol demethylase (Erg11p). The resulting depletion of cellular ergosterol and the accumulation of "toxic" sterol intermediates are both thought to compromise plasma membrane function. However, the effects of ergosterol depletion upon the function of intracellular membranes and organelles are not well described. The purpose of this study was to characterize the effects of azole treatment upon the integrity of the Candida albicans vacuole and to determine whether, in turn, vacuolar trafficking influences azole susceptibility. Profound fragmentation of the C. albicans vacuole can be observed as an early consequence of azole treatment, and it precedes significant growth inhibition. In addition, a C. albicans vps21Δ/Δ mutant, blocked in membrane trafficking through the late endosomal prevacuolar compartment (PVC), is able to grow significantly more than the wild type in the presence of several azole antifungals under standard susceptibility testing conditions. Furthermore, the vps21Δ/Δ mutant is able to grow despite the depletion of cellular ergosterol. This phenotype resembles an exaggerated form of "trailing growth" that has been described for some clinical isolates. In contrast, the vps21Δ/Δ mutant is hypersensitive to drugs that block alternate steps in ergosterol biosynthesis. On the basis of these results, we propose that endosomal trafficking defects may lead to the cellular "redistribution" of the sterol intermediates that accumulate following inhibition of ergosterol biosynthesis. Furthermore, the destination of these intermediates, or the precise cellular compartments in which they accumulate, may be an important determinant of their toxicity and thus ultimately antifungal efficacy.

MFS multidrug transporters in pathogenic fungi: do they have real clinical impact?

Infections caused by opportunistic fungal pathogens have reached concerning numbers due to the increase of the immunocrompromised human population and to the development of antifungal resistance. This resistance is often attributed to the action of multidrug efflux pumps, belonging to the ATP-binding cassette (ABC) superfamily and the major facilitator superfamily (MFS). Although many studies have focused on the role of ABC multidrug efflux transporters, little is still known on the part played by the Drug:H⁺ Antiporter (DHA) family of the MFS in this context. This review summarizes current knowledge on the role in antifungal drug resistance, mode of action and phylogenetic relations of DHA transporters, from the model yeast S. cerevisiae to pathogenic yeasts and filamentous fungi. Through the compilation of the predicted DHA transporters in the medically relevant Candida albicans, C. glabrata, C. parapsilosis, C. lusitaniae, C. tropicalis, C. guilliermondii, Cryptococcus neoformans, and Aspergillus fumigatus species, the fact that only 5% of the DHA transporters from these organisms have been characterized so far is evidenced. The role of these transporters in antifungal drug resistance and in pathogen-host interaction is described and their clinical relevance discussed. Given the knowledge gathered for these few DHA transporters, the need to carry out a systematic characterization of the DHA multidrug efflux pumps in fungal pathogens, with emphasis on their clinical relevance, is highlighted.

Candida glabrata drug:H+ antiporter CgTpo3 (ORF CAGL0I10384g): role in azole drug resistance and polyamine homeostasis

OBJECTIVES

The ability of opportunistic pathogenic Candida species to persist and invade specific niches in the human host depends on their resistance to natural growth inhibitors and antifungal therapy. This work describes the role of the Candida glabrata drug:H(+) antiporter CgTpo3 (ORF CAGL0I10384g) in this context.

METHODS

Deletion and cloning of CgTPO3 was achieved using molecular biology tools. C. glabrata strain susceptibility was assayed based on growth in liquid and solid media and through MIC determination. Radiolabelled compound accumulation or HPLC were used for the assessment of the role of CgTpo3 as a drug or polyamine transporter. Quantitative RT-PCR was used for expression analysis.

RESULTS

CgTpo3 was found to confer resistance to azole drugs in C. glabrata. This protein was found to be localized to the plasma membrane and to decrease the intracellular accumulation of [(3)H]clotrimazole, playing a direct role in its extrusion from pre-loaded C. glabrata cells. CgTPO3 was further found to confer resistance to spermine, complementing the susceptibility phenotypes exhibited by the deletion of its Saccharomyces cerevisiae homologue, TPO3. In spermine-stressed C. glabrata cells, CgTPO3 is transcriptionally activated in a CgPdr1-dependent manner, contributing to a decrease in the intracellular concentration of this polyamine. Clotrimazole exposure was found to lead to the intracellular accumulation of spermine, and pre-exposure to this polyamine was found consistently to lead to increased clotrimazole resistance.

CONCLUSIONS

Altogether, these results point to a significant role for CgTpo3 in azole drug resistance and in the tolerance to high polyamine concentrations, such as those found in the urogenital tract.

Insight into pleiotropic drug resistance ATP-binding cassette pump drug transport through mutagenesis of Cdr1p transmembrane domains

The fungal ATP-binding cassette (ABC) transporter Cdr1 protein (Cdr1p), responsible for clinically significant drug resistance, is composed of two transmembrane domains (TMDs) and two nucleotide binding domains (NBDs). We have probed the nature of the drug binding pocket by performing systematic mutagenesis of the primary sequences of the 12 transmembrane segments (TMSs) found in the TMDs. All mutated proteins were expressed equally well and localized properly at the plasma membrane in the heterologous host Saccharomyces cerevisiae, but some variants differed significantly in efflux activity, substrate specificity, and coupled ATPase activity. Replacement of the majority of the amino acid residues with alanine or glycine yielded neutral mutations, but about 42% of the variants lost resistance to drug efflux substrates completely or selectively. A predicted three-dimensional homology model shows that all the TMSs, apart from TMS4 and TMS10, interact directly with the drug-binding cavity in both the open and closed Cdr1p conformations. However, TMS4 and TMS10 mutations can also induce total or selective drug susceptibility. Functional data and homology modeling assisted identification of critical amino acids within a drug-binding cavity that, upon mutation, abolished resistance to all drugs tested singly or in combinations. The open and closed Cdr1p models enabled the identification of amino acid residues that bordered a drug-binding cavity dominated by hydrophobic residues. The disposition of TMD residues with differential effects on drug binding and transport are consistent with a large polyspecific drug binding pocket in this yeast multidrug transporter.

The dual role of candida glabrata drug:H+ antiporter CgAqr1 (ORF CAGL0J09944g) in antifungal drug and acetic acid resistance

Opportunistic Candida species often have to cope with inhibitory concentrations of acetic acid, in the acidic environment of the vaginal mucosa. Given that the ability of these yeast species to tolerate stress induced by weak acids and antifungal drugs appears to be a key factor in their persistence and virulence, it is crucial to understand the underlying mechanisms. In this study, the drug:H⁺ antiporter CgAqr1 (ORF CAGL0J09944g), from Candida glabrata, was identified as a determinant of resistance to acetic acid, and also to the antifungal agents flucytosine and, less significantly, clotrimazole. These antifungals were found to act synergistically with acetic acid against this pathogen. The action of CgAqr1 in this phenomenon was analyzed. Using a green fluorescent protein fusion, CgAqr1 was found to localize to the plasma membrane and to membrane vesicles when expressed in C. glabrata or, heterologously, in Saccharomyces cerevisiae. Given its ability to complement the susceptibility phenotype of its S. cerevisiae homolog, ScAqr1, CgAqr1 was proposed to play a similar role in mediating the extrusion of chemical compounds. Significantly, the expression of this gene was found to reduce the intracellular accumulation of ³H-flucytosine and, to a moderate extent, of ³H-clotrimazole, consistent with a direct role in antifungal drug efflux. Interestingly, no effect of CgAQR1 deletion could be found on the intracellular accumulation of ¹⁴C-acetic acid, suggesting that its role in acetic acid resistance may be indirect, presumably through the transport of a still unidentified physiological substrate. Although neither of the tested chemicals induces changes in CgAQR1 expression, pre-exposure to flucytosine or clotrimazole was found to make C. glabrata cells more sensitive to acetic acid stress. Results from this study show that CgAqr1 is an antifungal drug resistance determinant and raise the hypothesis that it may play a role in C. glabrata persistent colonization and multidrug resistance.

Candida glabrata drug:H+ antiporter CgQdr2 confers imidazole drug resistance, being activated by transcription factor CgPdr1

The widespread emergence of antifungal drug resistance poses a severe clinical problem. Though predicted to play a role in this phenomenon, the drug:H(+) antiporters (DHA) of the major facilitator superfamily have largely escaped characterization in pathogenic yeasts. This work describes the first DHA from the pathogenic yeast Candida glabrata reported to be involved in antifungal drug resistance, the C. glabrata QDR2 (CgQDR2) gene (ORF CAGL0G08624g). The expression of CgQDR2 in C. glabrata was found to confer resistance to the antifungal drugs miconazole, tioconazole, clotrimazole, and ketoconazole. By use of a green fluorescent protein (GFP) fusion, the CgQdr2 protein was found to be targeted to the plasma membrane in C. glabrata. In agreement with these observations, CgQDR2 expression was found to decrease the intracellular accumulation of radiolabeled clotrimazole in C. glabrata and to play a role in the extrusion of this antifungal from preloaded cells. Interestingly, the functional heterologous expression of CgQDR2 in the model yeast Saccharomyces cerevisiae further confirmed the role of this gene as a multidrug resistance determinant: its expression was able to complement the susceptibility phenotype exhibited by its S. cerevisiae homologue, QDR2, in the presence of imidazoles and of the antimalarial and antiarrhythmic drug quinidine. In contrast to the findings reported for Qdr2, CgQdr2 expression does not contribute to the ability of yeast to grow under K(+)-limiting conditions. Interestingly, CgQDR2 transcript levels were seen to be upregulated in C. glabrata cells challenged with clotrimazole or quinidine. This upregulation was found to depend directly on the transcription factor CgPdr1, the major regulator of multidrug resistance in this pathogenic yeast, which has also been found to be a determinant of quinidine and clotrimazole resistance in C. glabrata.

2-Aminothiophenes as building blocks in heterocyclic synthesis: synthesis and antimicrobial evaluation of a new class of pyrido[1,2-a]thieno[3,2-e]pyrimidine, quinoline and pyridin-2-one derivatives

Multisubstituted 2-aminothiophenes 1a-c can be readily cyanoacylated via reaction with cyanoacetic acid in presence of acetic anhydride under a microwave irradiation to form the corresponding cyanoacetamides 2a-c, which condensed with DMF-DMA to form the corresponding enamines 4 that reacted with hydrazine hydrate to yield the aminopyrazoles 5. Moreover the cyanoacetamides 2a-c reacted with a variety of arylidenmalononitrile to afford a novel pyrido[1,2-a]thieno[3,2-e]pyrimidine derivatives 12a-o. In addition the enamines 4a,b reacted with malononitrile to afford the pyrido[1,2-a]thieno[3,2-e]pyrimidine derivatives 19a,b. The cyanoacetamides 2a,b reacted also with salicylaldehyde to afford the quinoline derivatives 24a,b. Moreover the cyanoacetamides 2a,b reacted with the enaminones 25a-c to form the corresponding Pyridin-2-one derivatives 29a-c. Reactions of 2a,c with bezenediazonium chloride afford the arylhydrazones 30a,b that reacted with chloroacetonitrile to form the acyclic product 31 which could not be further cyclized to the corresponding 4-aminopyrazole. The X-ray crystallographic analyses of seven products could be obtained thus establishing with certainty the proposed structures in this work. Most of the synthesized compounds in this investigation were tested and evaluated as antimicrobial agents.

Efflux in fungi: la pièce de résistance

Pathogens must be able to overcome both host defenses and antimicrobial treatment in order to successfully infect and maintain colonization of the host. One way fungi accomplish this feat and overcome intercellular toxin accumulation is efflux pumps, in particular ATP-binding cassette transporters and transporters of the major facilitator superfamily. Members of these two superfamilies remove many toxic compounds by coupling transport with ATP hydrolysis or a proton gradient, respectively. Fungal genomes encode a plethora of members of these families of transporters compared to other organisms. In this review we discuss the role these two fungal superfamilies of transporters play in virulence and resistance to antifungal agents. These efflux transporters are responsible not only for export of compounds involved in pathogenesis such as secondary metabolites, but also export of host-derived antimicrobial compounds. In addition, we examine the current knowledge of these transporters in resistance of pathogens to clinically relevant antifungal agents.

MFS transportome of the human pathogenic yeast Candida albicans

BACKGROUND

The major facilitator superfamily (MFS) is one of the two largest superfamilies of membrane transporters present ubiquitously in bacteria, archaea, and eukarya and includes members that function as uniporters, symporters or antiporters. We report here the complete transportome of MFS proteins of a human pathogenic yeast Candida albicans.

RESULTS

Computational analysis of C. albicans genome enabled us to identify 95 potential MFS proteins which clustered into 17 families using Saier's Transport Commission (TC) system. Among these SP, DHA1, DHA2 and ACS represented major families consisting of 22, 22, 9 and 16 members, respectively. Family designations in C. albicans were validated by subjecting Saccharomyces cerevisiae genome to TC system. Based on the published available genomics/proteomics data, 87 of the putative MFS genes of C. albicans were found to express either at mRNA or protein levels. We checked the expression of the remaining 8 genes by using RT-PCR and observed that they are not expressed under basal growth conditions implying that either these 8 genes are expressed under specific growth conditions or they may be candidates for pseudogenes.

CONCLUSION

The in silico characterisation of MFS transporters in Candida albicans genome revealed a large complement of MFS transporters with most of them showing expression. Considering the clinical relevance of C. albicans and role of MFS members in antifungal resistance and nutrient transport, this analysis would pave way for identifying their physiological relevance.

Identification of promoter elements responsible for the regulation of MDR1 from Candida albicans, a major facilitator transporter involved in azole resistance

Upregulation of the MDR1 (multidrug resistance 1) gene is involved in the development of resistance to antifungal agents in clinical isolates of the pathogen Candida albicans. To better understand the molecular mechanisms underlying the phenomenon, the cis-acting regulatory elements present in the MDR1 promoter were characterized using a beta-galactosidase reporter system. In an azole-susceptible strain, transcription of this reporter is transiently upregulated in response to either benomyl or H(2)O(2), whereas its expression is constitutively high in an azole-resistant strain (FR2). Two cis-acting regulatory elements within the MDR1 promoter were identified that are necessary and sufficient to confer the same transcriptional responses on a heterologous promoter (CDR2). One, a benomyl response element (BRE), is situated at position -296 to -260 with respect to the ATG start codon. It is required for benomyl-dependent MDR1 upregulation and is also necessary for constitutive high expression of MDR1. A second element, termed H(2)O(2) response element (HRE), is situated at position -561 to -520. The HRE is required for H(2)O(2)-dependent MDR1 upregulation, but dispensable for constitutive high expression. Two potential binding sites (TTAG/CTAA) for the bZip transcription factor Cap1p (Candida AP-1 protein) lie within the HRE. Moreover, inactivation of CAP1 abolished the transient response to H(2)O(2). Cap1p, which has been previously implicated in cellular responses to oxidative stress, may thus play a trans-acting and positive regulatory role in the H(2)O(2)-dependent transcription of MDR1. A minimal BRE (-290 to -273) that is sufficient to detect in vitro sequence-specific binding of protein complexes in crude extracts prepared from C. albicans was also defined. Interestingly, the sequence includes a perfect match to the consensus binding sequence of Mcm1p, raising the possibility that MDR1 may be a direct target of this MADS box transcriptional activator. In conclusion, while the identity of the trans-acting factors that bind to the BRE and HRE remains to be confirmed, the tools developed during this characterization of the cis-acting elements of the MDR1 promoter should now serve to elucidate the nature of the components that modulate its activity.

Structure and function analysis of CaMdr1p, a major facilitator superfamily antifungal efflux transporter protein of Candida albicans: identification of amino acid residues critical for drug/H+ transport

We have cloned and overexpressed multidrug transporter CaMdr1p as a green fluorescent protein-tagged protein to show its capability to extrude drug substrates. The drug extrusion was sensitive to pH and energy inhibitors and displayed selective substrate specificity. CaMdr1p has a unique and conserved antiporter motif, also called motif C [G(X6)G(X3)GP(X2)GP(X2)G], in its transmembrane segment 5 (TMS 5). Alanine scanning of all the amino acids of the TMS 5 by site-directed mutagenesis highlighted the importance of the motif, as well as that of other residues of TMS 5, in drug transport. The mutant variants of TMS 5 were placed in four different categories. The first category had four residues, G244, G251, G255, and G259, which are part of the conserved motif C, and their substitution with alanine resulted in increased sensitivity to drugs and displayed impaired efflux of drugs. Interestingly, first category mutants, when replaced with leucine, resulted in more dramatic loss of drug resistance and efflux. Notwithstanding the location in the core motif, the second category included residues which are part of the motif, such as P260, and those which were not part of the motif, such as L245, W248, P256, and F262, whose substitution with alanine resulted in a severe loss of drug resistance and efflux. The third category included G263, which is a part of motif C, but unlike other conserved glycines, its replacement with alanine or leucine showed no change in the phenotype. The replacement of the remaining 11 residues of the fourth category did not result in any change. The putative helical wheel projection showed clustering of functionally critical residues to one side and thus suggests an asymmetric nature of TMS 5.

Transcriptional regulation of MDR1, encoding a drug efflux determinant, in fluconazole-resistant Candida albicans strains through an Mcm1p binding site

Constitutive, high-level transcription of the gene encoding the drug efflux facilitator Mdr1p is commonly observed in laboratory and clinical strains of Candida albicans that are resistant to the antifungal drug fluconazole (FLC). In five independently isolated FLC(R) laboratory strains, introduction of a wild-type MDR1 promoter fragment fused to the yeast enhanced green fluorescent protein (yEGFP) reporter gene resulted in high-level expression of GFP, demonstrating that overexpression of MDR1 is dependent on a trans-acting factor. This study identified a 35-bp MDR1 promoter element, termed the MDRE, that mediates high-level MDR1 transcription. When inserted into a heterologous promoter, the MDRE was sufficient to mediate high-level expression of the yEGFP reporter gene specifically in MDR1 trans-activation strains. The MDRE promoted transcription in an orientation-independent and dosage-dependent manner. Deletion of the MDRE in the full-length promoter did not abolish MDR1 trans-activation, indicating that elements upstream of the MDRE also contribute to transcription of MDR1 in these overexpression strains. Analysis of the MDRE sequence indicated that it contains an Mcm1p binding site very similar in organization to the site seen upstream of the Saccharomyces cerevisiae MFA1 and STE2 genes. Electrophoretic mobility shift analysis demonstrated that both wild-type, FLC-sensitive and MDR1 trans-activated, FLC-resistant strains contain a factor that binds the MDRE. Depletion of Mcm1p, by use of a strain in which MCM1 expression is under the control of a regulated promoter (44), resulted in a loss of MDRE binding activity. Thus, the general transcription factor Mcm1p participates in the regulation of MDR1 expression.

Overexpression of the MDR1 gene is sufficient to confer increased resistance to toxic compounds in Candida albicans

Overexpression of MDR1, which encodes a membrane transport protein of the major facilitator superfamily, is one mechanism by which the human fungal pathogen Candida albicans can develop increased resistance to the antifungal drug fluconazole and other toxic compounds. In clinical C. albicans isolates, constitutive MDR1 overexpression is accompanied by the upregulation of other genes, but it is not known if these additional alterations are required for Mdr1p function and drug resistance. To investigate whether MDR1 overexpression is sufficient to confer a drug-resistant phenotype in C. albicans, we expressed the MDR1 gene from the strong ADH1 promoter in C. albicans laboratory strains that did not express the endogenous MDR1 gene as well as in a fluconazole-resistant clinical C. albicans isolate in which the endogenous MDR1 alleles had been deleted and in a matched fluconazole-susceptible isolate from the same patient. Forced MDR1 overexpression resulted in increased resistance to the putative Mdr1p substrates cerulenin and brefeldin A, and this resistance did not depend on the additional alterations which occurred during drug resistance development in the clinical isolates. In contrast, artificial expression of the MDR1 gene from the ADH1 promoter did not enhance or only slightly enhanced fluconazole resistance, presumably because Mdr1p expression levels in the transformants were considerably lower than those observed in the fluconazole-resistant clinical isolate. These results demonstrate that MDR1 overexpression in C. albicans is sufficient to confer resistance to some toxic compounds that are substrates of this efflux pump but that the degree of resistance depends on the Mdr1p expression level.

Drug-induced regulation of the MDR1 promoter in Candida albicans

Resistance of Candida albicans to azole antifungal drugs is mediated by two types of efflux pumps, encoded by the MDR1 gene and the CDR gene family. MDR1 mRNA levels in a susceptible clinical isolate are induced by benomyl (BEN) but not by other drugs previously shown to induce MDR1. To monitor MDR1 expression under several conditions, the MDR1 promoter was fused to the Renilla reniformis luciferase reporter gene (RLUC). The promoter was monitored for its responses to four oxidizing agents, five toxic hydrophobic compounds, and an alkylating agent, all shown to induce major facilitator pumps in other organisms. Deletion constructs of the MDR1 promoter were used to analyze the basal transcription of the promoter and its responses to the toxic compound BEN and the oxidizing agent tert-butyl hydrogen peroxide (T-BHP). The cis-acting elements in the MDR1 promoter responsible for induction by BEN were localized between -399 and -299 upstream of the start codon. The cis-acting elements responsible for MDR1 induction by T-BHP were localized between -601 and -500 upstream of the start codon. The T-BHP induction region contains a sequence that resembles the YAP1-responsive element (YRE) in Saccharomyces cerevisiae. This Candida YRE was placed upstream of a noninducible promoter in the luciferase construct, resulting in an inducible promoter. Inversion or mutation of the 7-bp YRE eliminated induction. Many of the drugs used in this analysis induce the MDR1 promoter at concentrations that inhibit cell growth. These analyses define cis-acting elements responsible for drug induction of the MDR1 promoter.

Regulated overexpression of CDR1 in Candida albicans confers multidrug resistance

OBJECTIVES

Information on the function of Candida albicans ATP-binding cassette (ABC) membrane transporter Cdr1p has come from studying the effect of gene inactivation in C. albicans and from heterologous Cdr1p expression in the yeast Saccharomyces cerevisiae. These approaches, however, give only an indirect indication of Cdr1p function in C. albicans itself. The objective of this study was to determine Cdr1p function in C. albicans by induced overexpression of Cdr1p in a C. albicans CDR1-deleted strain.

METHODS

The C. albicans CDR1 open reading frame was fused to the C. albicans HEX1 promoter and used to complement a CDR1-null mutant to create strain FL3. The effect of inducing the FL3 HEX1 promoter, by growth on medium containing N-acetylglucosamine (GlcNAc) as the carbon source, on CDR1 expression and drug susceptibility was determined.

RESULTS

C. albicans FL3 cells grown on medium containing GlcNAc overexpressed CDR1 mRNA and a 170 kDa plasma membrane protein that reacted with anti-Cdr1p antibodies. Overexpression of Cdr1p in C. albicans FL3 conferred resistance to structurally unrelated chemicals such as terbinafine, brefeldin A, cerulenin and nigericin as well as to azole antifungal agents, but not resistance to polyene antibiotics. FK506, ascomycin and ciclosporin A chemosensitized FL3 to fluconazole. FL3 cells grown on GlcNAc effluxed 5.3 times as much Cdr1p substrate rhodamine 6G, over a 10 min period, as FL3 cells grown on glucose, and this rhodamine 6G efflux was inhibited by including fluconazole in the assay.

CONCLUSION

This study provides the first direct demonstration of Cdr1p pump activity in C. albicans.

Chemosensitization of fluconazole resistance in Saccharomyces cerevisiae and pathogenic fungi by a D-octapeptide derivative

Hyperexpression of the Saccharomyces cerevisiae multidrug ATP-binding cassette (ABC) transporter Pdr5p was driven by the pdr1-3 mutation in the Pdr1p transcriptional regulator in a strain (AD/PDR5(+)) with deletions of five other ABC-type multidrug efflux pumps. The strain had high-level fluconazole (FLC) resistance (MIC, 600 microg ml(-1)), and plasma membrane fractions showed oligomycin-sensitive ATPase activity up to fivefold higher than that shown by fractions from an isogenic PDR5-null mutant (FLC MIC, 0.94 microg ml(-1)). In vitro inhibition of the Pdr5p ATPase activity and chemosensitization of cells to FLC allowed the systematic screening of a 1.8-million-member designer D-octapeptide combinatorial library for surface-active Pdr5p antagonists with modest toxicity against yeast cells. Library deconvolution identified the 4-methoxy-2,3,6-trimethylbenzensulfonyl-substituted D-octapeptide KN20 as a potent Pdr5p ATPase inhibitor (concentration of drug causing 50% inhibition of enzyme activity [IC(50)], 4 microM) which chemosensitized AD/PDR5(+) to FLC, itraconazole, and ketoconazole. It also inhibited the ATPase activity of other ABC transporters, such as Candida albicans Cdr1p (IC(50), 30 microM) and Cdr2p (IC(50), 2 microM), and chemosensitized clinical isolates of pathogenic Candida species and S. cerevisiae strains that heterologously hyperexpressed either ABC-type multidrug efflux pumps, the C. albicans major facilitator superfamily-type drug transporter Ben(R)p, or the FLC drug target lanosterol 14 alpha-demethylase (Erg11p). Although KN20 also inhibited the S. cerevisiae plasma membrane proton pump Pma1p (IC(50), 1 microM), the peptide concentrations required for chemosensitization made yeast cells permeable to rhodamine 6G. KN20 therefore appears to indirectly chemosensitize cells to FLC by a nonlethal permeabilization of the fungal plasma membrane.

Two membrane proteins located in the Nag regulon of Candida albicans confer multidrug resistance

Pathogenic fungus Candida albicans can efficiently utilize the aminosugar N-acetylglucosamine (GlcNAc) as energy source. Since the mucosal membrane, the site of infection is rich in amino sugars, this specific adaptation is important for the establishment of infection. The genes encoding for the enzymes of the GlcNAc catabolic pathway, GlcNAc kinase (HXK1), GlcNAc-6-phosphate deacetylase (DAC1), and glucosamine-6-phosphate deaminase (NAG1), are present in a cluster, the Nag regulon, which is associated with virulence. In this study, we have characterized two genes, TMP1 and TMP2, present within the Nag regulon, upstream to DAC1. They encode two membrane associated sugar transporters of the major facilitator superfamily (MFS). The null mutant of TMP1 and TMP2 is able to grow in GlcNAc, implying that they are not involved in GlcNAc transport. However, it shows increased susceptibility to a number of unrelated antifungal compounds such as cycloheximide, 4-nitroquinoline-N-oxide, and 1-10 phenanthroline. Northern blot analysis revealed that TMP1 and TMP2 are upregulated in response to these drugs, suggesting that they function as multiple drug efflux pumps.

The genetic basis of fluconazole resistance development in Candida albicans

Infections by the opportunistic fungal pathogen Candida albicans are widely treated with the antifungal agent fluconazole that inhibits the biosynthesis of ergosterol, the major sterol in the fungal plasma membrane. The emergence of fluconazole-resistant C. albicans strains is a significant problem after long-term treatment of recurrent oropharyngeal candidiasis (OPC) in acquired immunodeficiency syndrome (AIDS) patients. Resistance can be caused by alterations in sterol biosynthesis, by mutations in the drug target enzyme, sterol 14alpha-demethylase (14DM), which lower its affinity for fluconazole, by increased expression of the ERG11 gene encoding 14DM, or by overexpression of genes coding for membrane transport proteins of the ABC transporter (CDR1/CDR2) or the major facilitator (MDR1) superfamilies. Different mechanisms are frequently combined to result in a stepwise development of fluconazole resistance over time. The MDR1 gene is not or barely transcribed during growth in vitro in fluconazole-susceptible C. albicans strains, but overexpressed in many fluconazole-resistant clinical isolates, resulting in reduced intracellular fluconazole accumulation. The activation of the gene in resistant isolates is caused by mutations in as yet unknown trans-regulatory factors, and the resulting constitutive high level of MDR1 expression causes resistance to other toxic compounds in addition to fluconazole. Disruption of both alleles of the MDR1 gene in resistant C. albicans isolates abolishes their resistance to these drugs, providing genetic evidence that MDR1 mediates multidrug resistance in C. albicans.

The SCR1 gene from Schwanniomyces occidentalis encodes a highly hydrophobic polypeptide, which confers ribosomal resistance to cycloheximide

In Saccharomyces cerevisiae, the SCR1 gene from Schwanniomyces occidentalis is known to induce ribosomal resistance to cycloheximide (cyh). A 2.8 kb DNA fragment encoding this gene was sequenced. Its EMBL Accession No. is AJ419770. It disclosed a putative tRNA(Asn) (GUU) sequence located downstream of an open reading frame (ORF) of 1641 nucleotides. This ORF was shown to correspond to SCR1. It would encode a highly hydrophobic polypeptide (SCR1) with 12 transmembrane domains. SCR1 is highly similar to a variety of yeast proteins of the multidrug-resistance (MDR) family. However, SCR1 only conferred resistance to cyh but not to benomyl or methotrexate. The cyh-resistance phenotype induced by SCR1 was confirmed in several S. cerevisiae strains that expressed this gene to reside at the ribosomal level. In contrast, a beta-galacosidase-tagged SCR1 was found to be integrated in the endoplasmic reticulum (ER). It is proposed that the ribosomes of yeast cells expressing SCR1 undergo a conformational change during their interaction with the ER, which lowers their affinity for cyh-binding. If so, these findings would disclose a novel ribosomal resistance mechanism.

Resistance mechanisms in clinical isolates of Candida albicans

Resistance to azole antifungals continues to be a significant problem in the common fungal pathogen Candida albicans. Many of the molecular mechanisms of resistance have been defined with matched sets of susceptible and resistant clinical isolates from the same strain. Mechanisms that have been identified include alterations in the gene encoding the target enzyme ERG11 or overexpression of efflux pump genes including CDR1, CDR2, and MDR1. In the present study, a collection of unmatched clinical isolates of C. albicans was analyzed for the known molecular mechanisms of resistance by standard methods. The collection was assembled so that approximately half of the isolates were resistant to azole drugs. Extensive cross-resistance was observed for fluconazole, clotrimazole, itraconazole, and ketoconazole. Northern blotting analyses indicated that overexpression of CDR1 and CDR2 correlates with resistance, suggesting that the two genes may be coregulated. MDR1 overexpression was observed infrequently in some resistant isolates. Overexpression of FLU1, an efflux pump gene related to MDR1, did not correlate with resistance, nor did overexpression of ERG11. Limited analysis of the ERG11 gene sequence identified several point mutations in resistant isolates; these mutations have been described previously. Two of the most common point mutations in ERG11 associated with resistance, D116E and E266D, were tested by restriction fragment length polymorphism analysis of the isolates from this collection. The results indicated that the two mutations occur frequently in different isolates of C. albicans and are not reliably associated with resistance. These analyses emphasize the diversity of mechanisms that result in a phenotype of azole resistance. They suggest that the resistance mechanisms identified in matched sets of susceptible and resistant isolates are not sufficient to explain resistance in a collection of unmatched clinical isolates and that additional mechanisms have yet to be discovered.

Clinical aromatherapy and AIDS

Clinical aromatherapy is the use of essential oils for expected outcomes that are measurable and is a therapy that is used as part of nursing care in Switzerland, Germany, Australia, Canada, the United Kingdom, and, more recently, the United States. Essential oils are steam distillates obtained from aromatic plants. These volatile extracts have been used for many years by French hospitals against airborne bacteria and fungi. As antimicrobial agents, essential oils may be appropriate in HIV/AIDS for specific opportunistic infections. Aromatherapy can also alter perceptions of chronic pain, help maintain skin integrity, and is useful in stress management. Methods of application vary depending on the site of infection and the psychological profile of the patient and can include inhalation, compresses, baths, massage, and the "m" technique. This article will explore the potential use of essential oils in HIV/AIDS focusing on four opportunistic infections: Cryptococcus neoformans, Candida albicans, methicillin-resistant Staphylococcus aureus, and herpes simplex types I and II.

MDR1-mediated drug resistance in Candida dubliniensis

Candida dubliniensis is a recently described opportunistic fungal pathogen that is closely related to Candida albicans. Candida dubliniensis readily develops resistance to the azole antifungal agent fluconazole, both in vitro and in infected patients, and this resistance is usually associated with upregulation of the CdMDR1 gene, encoding a multidrug efflux pump of the major facilitator superfamily. To determine the role of CdMDR1 in drug resistance in C. dubliniensis, we constructed an mdr1 null mutant from the fluconazole-resistant clinical isolate CM2, which overexpressed the CdMDR1 gene. Sequential deletion of both CdMDR1 alleles was performed by the MPA(R)-flipping method, which is based on the repeated use of a dominant mycophenolic acid resistance marker for selection of integrative transformants and its subsequent deletion from the genome by FLP-mediated, site-specific recombination. In comparison with its parental strain, the mdr1 mutant showed decreased resistance to fluconazole but not to the related drug ketoconazole. In addition, we found that CdMDR1 confers resistance to the structurally unrelated drugs 4-nitroquinoline-N-oxide, cerulenin, and brefeldin A, since the enhanced resistance to these compounds of the parent strain CM2 compared with the matched susceptible isolate CM1 was abolished in the mdr1 mutant. In contrast, CdMDR1 inactivation did not cause increased susceptibility to amorolfine, terbinafine, fluphenazine, and benomyl, although overexpression of CdMDR1 in a hypersusceptible Saccharomyces cerevisiae strain had previously been shown to confer resistance to these compounds. The effect of CdMDR1 inactivation was identical to that seen in two similarly constructed C. albicans mdr1 mutants. Therefore, despite species-specific differences in the amino acid sequences of the Mdr1 proteins, overexpression of CaMDR1 and CdMDR1 in clinical C. albicans and C. dubliniensis strains seems to confer the same drug resistance profile in both species.

A novel multidrug efflux transporter gene of the major facilitator superfamily from Candida albicans (FLU1) conferring resistance to fluconazole

Azole resistance in Candida albicans can be mediated by several resistance mechanisms. Among these, alterations of the azole target enzyme and the overexpression of multidrug efflux transporter genes are the most frequent. To identify additional putative azole resistance genes in C. albicans, a genomic library from this organism was screened for complementation of fluconazole hypersusceptibility in Saccharomyces cerevisiae YKKB-13 lacking the ABC (ATP-binding cassette) transporter gene PDR5. Among the C. albicans genes obtained, a new gene was isolated and named FLU1 (fluconazole resistance). The deduced amino acid sequence of FLU1 showed similarity to CaMDR1 (formerly BEN(r)), a member of the major facilitator superfamily of multidrug efflux transporters. The expression of FLU1 in YKKB-13 mediated not only resistance to fluconazole but also to cycloheximide among the different drugs tested. The disruption of FLU1 in C. albicans had only a slight effect on fluconazole susceptibility; however, it resulted in hypersusceptibility to mycophenolic acid, thus suggesting that this compound could be a substrate for the protein encoded by FLU1. Disruption of FLU1 in a background of C. albicans mutants with deletions in several multidrug efflux transporter genes, including CDR1, CDR2 and CaMDR1, resulted in enhanced susceptibility to several azole derivatives. FLU1 expression did not vary significantly between several pairs of azole-susceptible and azole-resistant C. albicans clinical isolates. Therefore, FLU1 seems not to be required for the development of azole resistance in clinical isolates.

Enhanced resistance to experimental systemic candidiasis in tilorone-treated mice

Candida albicans is an increasingly important opportunistic fungal pathogen in immunocompromised patients. Natural killer (NK) cells constitute an important immune effector mechanism and are involved in the response to different pathological disorders. We wished to determine if this immune mechanism is involved in the specific response to C. albicans. Tilorone hydrochloride and related compounds have been described to display antiviral and antitumoral activity, as well as to enhance NK cell activity. In this study, we show the antimicrobial activity of different tilorone analogues and the enhanced resistance of tilorone-treated mice in experimental systemic candidiasis. We also present data suggesting that there is a correlation between NK cell activation and the resistance to experimental systemic candidiasis. Thus, it seems that the immunosurveillance of metastatic spread and the infection by C. albicans share some immune effector mechanisms, in particular activation of NK cells.

Evolution of microbial pathogens

Various genetic mechanisms including point mutations, genetic rearrangements and lateral gene transfer processes contribute to the evolution of microbes. Long-term processes leading to the development of new species or subspecies are termed macroevolution, and short-term developments, which occur during days or weeks, are considered as microevolution. Both processes, macro- and microevolution need horizontal gene transfer, which is particularly important for the development of pathogenic microorganisms. Plasmids, bacteriophages and so-called pathogenicity islands (PAIs) play a crucial role in the evolution of pathogens. During microevolution, genome variability of pathogenic microbes leads to new phenotypes, which play an important role in the acute development of an infectious disease. Infections due to Staphylococcus epidermidis, Candida albicans and Escherichia coli will be described with special emphasis on processes of microevolution. In contrast, the development of PAIs is a process involved in macroevolution. PAIs are especially important in processes leading to new pathotypes or even species. In this review, particular attention will be given to the fact that the evolution of pathogenic microbes can be considered as a specific example for microbial evolution in general.

Multiple amino acid substitutions in lanosterol 14alpha-demethylase contribute to azole resistance in Candida albicans

Lanosterol 14alpha-demethylase (14DM) is the target of the azole antifungals, and alteration of the 14DM sequence leading to a decreased affinity of the enzyme for azoles is one of several potential mechanisms for resistance to these drugs in Candida albicans. In order to identify such alterations the authors investigated a collection of 19 C. albicans clinical isolates demonstrating either frank resistance (MICs > or = 32 microg ml(-1)) or dose-dependent resistance (MICs 8-16 microg ml(-1)) to fluconazole. In cell-free extracts from four isolates, including the Darlington strain ATCC 64124, sensitivity of sterol biosynthesis to inhibition by fluconazole was greatly reduced, suggesting that alterations in the activity or affinity of the 14DM could contribute to resistance. Cloning and sequencing of the 14DM gene from these isolates revealed 12 different alterations (two to four per isolate) leading to changes in the deduced amino acid sequence. Five of these mutations have not previously been reported. To demonstrate that these alterations could affect fungal susceptibility to azoles, the 14DM genes from one sensitive and three resistant C. albicans strains were tagged at the carboxyl terminus with a c-myc epitope and expressed in Saccharomyces cerevisiae under control of the endogenous promoter. Transformants receiving 14DM genes from resistant strains had fluconazole MICs up to 32-fold higher than those of transformants receiving 14DM from a sensitive strain, although Western blot analysis indicated that the level of expressed 14DM was similar in all transformants. Amino acid substitutions in the 14DM gene from the Darlington strain also conferred a strong cross-resistance to ketoconazole. In conclusion, multiple genetic alterations in C. albicans 14DM, including several not previously reported, can affect the affinity of the enzyme for azoles and contribute to resistance of clinical isolates.

Rhodamine 6G efflux for the detection of CDR1-overexpressing azole-resistant Candida albicans strains

We investigated the drug efflux mechanism in azole-resistant strains of Candida albicans using rhodamine 6G (R6G). No significant differences in R6G uptake were observed between azole-sensitive B2630 (9.02 +/- 0.02 nmol/10(8) cells) and azole-resistant B67081 (8.86 +/- 0.03 nmol/10(8) cells) strains incubated in glucose-free phosphate buffered saline. A significantly higher R6G efflux (2.0 +/- 0.21 nmol/10(8) cells) was noted in the azole-resistant strain (B67081) when glucose was added, compared with that in the sensitive strain B2630 (0.23 < or = 0.14 nmol/10(8) cells). A fluconazole-resistant strain C40 that expressed the benomyl resistance gene (CaMDR) also showed a low R6G efflux (0.16 +/- 0.06 nmol/10(8) cells) as did the sensitive strains. Accumulation of R6G in growing C. albicans cells was inversely correlated with the level of CDR1 mRNA expression. Our data also suggest that measurement of intracellular accumulation of R6G is a useful method for identification of azole-resistant strains due to CDR1-expressed drug efflux pump.

The bZip transcription factor Cap1p is involved in multidrug resistance and oxidative stress response in Candida albicans

Candida albicans is an opportunistic pathogenic yeast which frequently develops resistance to the antifungal agent fluconazole (FCZ) in patients undergoing long-term therapy. FCZ-resistant strains often display a reduced intracellular FCZ accumulation which correlates with the overexpression of the ATP-binding cassette transporters CDR1 and CDR2 or the major facilitator (MF) MDR1. We have recently cloned a C. albicans gene, named CAP1, which codes for a bZip transcription factor of the AP-1 family homologous to the Yap1 protein involved in multidrug resistance and response to oxidative stress in Saccharomyces cerevisiae. CAP1 was found to confer FCZ resistance in S. cerevisiae by transcriptionally activating FLR1, a gene coding for an MF homologous to the C. albicans MDR1 gene product (A.-M. Alarco, I. Balan, D. Talibi, N. Mainville, and M. Raymond, J. Biol. Chem. 272:19304-19313, 1997). To study the role of CAP1 in C. albicans, we constructed a CAI4-derived mutant strain carrying a homozygous deletion of the CAP1 gene (CJD21). We found that deletion of CAP1 did not affect the susceptibility of CJD21 cells to FCZ, cerulenin, brefeldin A, and diamide but caused hypersensitivity to cadmium, 4-nitroquinoline N-oxide, 1,10-phenanthroline, and hydrogen peroxide, an effect which was reverted by reintroduction of the CAP1 gene in these cells. Introduction of a hyperactive truncated allele of CAP1 (CAP1-TR) in CJD21 resulted in resistance of the cells to all of the above compounds except hydrogen peroxide. The hyperresistant phenotype displayed by the CJD21 CAP1-TR transformants was found to correlate with the overexpression of a number of potential CAP1 transcriptional targets such as MDR1, CaYCF1, CaGLR1, and CaTRR1. Taken together, our results demonstrate that CAP1 is involved in multidrug resistance and oxidative stress response in C. albicans. Finally, disruption of CAP1 in strain FR2, selected in vitro for FCZ resistance and constitutively overexpressing MDR1, did not suppress but rather increased the levels of MDR1 expression, demonstrating that CAP1 acts as a negative transcriptional regulator of the MDR1 gene in FR2 and is not responsible for MDR1 overexpression in this strain.

Multiple molecular mechanisms contribute to a stepwise development of fluconazole resistance in clinical Candida albicans strains

From each of two AIDS patients with oropharyngeal candidiasis, five Candida albicans isolates from recurrent episodes of infection which became gradually resistant against fluconazole during antimycotic treatment were analyzed for molecular changes responsible for drug resistance. In both patients, a single C. albicans strain was responsible for the recurrent infections, but the CARE-2 fingerprint pattern of the isolates exhibited minor genetic alterations, indicating that microevolution of the strains took place during fluconazole therapy. In the isolates from patient 1, enhanced mRNA levels of the MDR1 gene, encoding a multiple drug resistance protein from the superfamily of major facilitators, and constitutive high expression of the ERG11 gene, coding for the drug target enzyme sterol 14alpha-demethylase, correlated with a stepwise development of fluconazole resistance. The resistant strains exhibited reduced accumulation of fluconazole and, for the last in the series, a slight increase in drug needed to inhibit sterol 14alpha-demethylation in vitro. In the isolates from patient 2, increased MDR1 mRNA levels and the change from heterozygosity to homozygosity for a mutant form of the ERG11 gene correlated with continuously decreased drug susceptibility. In this series, reduced drug accumulation and increased resistance in the target enzyme activity, sterol 14alpha-demethylase, were observed. These results demonstrate that different molecular mechanisms contribute to a gradual development of fluconazole resistance in C. albicans.

Therapy of Candida infections: susceptibility testing, resistance, and therapeutic options

OBJECTIVE

Review the epidemiology of fungal infections, approved susceptibility testing methods, the scope of antifungal resistance, and advances in the treatment of fungal infections.

DATA SOURCES

MEDLINE databases (from 1966 to March 1998) were searched for literature pertaining to the epidemiology and management of fungal infections.

STUDY SELECTION AND DATA EXTRACTION

Articles were selected to assist in providing the reader an understanding of the epidemiology and management of fungal infections.

DATA SYNTHESIS

Fungi have emerged as an important class of pathogens. Even though fungi rank as the fourth most commonly encountered nosocomial bloodstream pathogen, and are associated with the highest mortality of commonly encountered pathogens, only within the past year have methods for conducting and guidelines for interpreting in vitro susceptibility tests been approved. Under the guidance of these standards, we have begun to understand important issues regarding fungi such as the scope and mechanisms of antifungal resistance. Although there has not been a significant addition to our antifungal armamentarium since 1992, advances in antifungal therapy have been realized with the reformulation of available agents and the delineation of the pharmacodynamic characteristics of several antifungals. Additionally, several new agents, including a new class of antifungals, probably will enter into clinical use within the next 5 years.

CONCLUSIONS

We have entered an era in which our understanding of fungi is increasing tremendously. Clinicians need to familiarize themselves with the current concepts surrounding the management of fungal infections in order to provide optimal care for their patients.

Decreased accumulation or increased isoleucyl-tRNA synthetase activity confers resistance to the cyclic beta-amino acid BAY 10-8888 in Candida albicans and Candida tropicalis

BAY 10-8888, a cyclic beta-amino acid, exerts its antifungal activity by inhibition of isoleucyl-tRNA synthetase activity after accumulation to a millimolar concentration inside the cell. We have selected and characterized BAY 10-8888-resistant Candida albicans mutants. Reduced BAY 10-8888 accumulation as well as increased isoleucyl-tRNA synthetase activity was observed in these mutants. Some of the mutants were cross-resistant to cispentacin, a structurally related beta-amino acid, while sensitivities to 5-fluorocytosine and fluconazole remained unchanged in all mutants. All except two in vitro-resistant mutants were pathogenic in a murine candidiasis model, and BAY 10-8888 failed to cure the infection. Furthermore, we have characterized BAY 10-8888 transport and isoleucyl-tRNA synthetase activity in several Candida tropicalis strains which showed MICs higher than those of other Candida strains. An analysis of the C. tropicalis strains revealed that intracellular concentrations of BAY 10-8888 were in the millimolar range, comparable to those for C. albicans. However, these isolates expressed isoleucyl-tRNA synthetase activities about fourfold higher than those for C. albicans. To test the possibility of resistance modeling, we determined the correlations between the intracellular concentration of BAY 10-8888, the specific activity of isoleucyl-tRNA synthetase, the number of free, i.e., noninhibited, isoleucyl-tRNA synthetase molecules/cell, and growth, assuming a linear relation. We found significant correlations between growth and the intracellular concentration of BAY 10-8888 and between growth and the number of free isoleucyl-tRNA synthetase molecules/cell, but not between growth and the specific activity of isoleucyl-tRNA synthetase.

Induced expression of the Candida albicans multidrug resistance gene CDR1 in response to fluconazole and other antifungals

The Candida albicans CDR1 gene encodes a member of the ABC-type family of multidrug transporters which has been shown to be involved in azole resistance. Using an in-frame gene fusion between the CDR1 open reading frame and the green fluorescent protein allele yEGFP3, an optimized derivative for its use in C. albicans, we show here how the CDR1-yEGFP3 gene expression is induced in response to azoles as well as to other structurally unrelated drugs like cycloheximide. Moderate increases were observed for calcofluor, canavanine, 5'-fluorcytosine, cilofungin and caffeine, while no induction was found for the antifungals benomyl and amphotericin B or hydrogen peroxide at subinhibitory concentrations. The use of confocal microscopy enabled us to localize the Cdr1p fusion protein at the cell periphery, thus suggesting a cytoplasmic membrane localization. These results suggest deregulation of CDR1 gene as a putative mechanism for the generation of azole resistance in this clinically important pathogenic fungus.

Clinical, cellular, and molecular factors that contribute to antifungal drug resistance

In the past decade, the frequency of diagnosed fungal infections has risen sharply due to several factors, including the increase in the number of immunosuppressed patients resulting from the AIDS epidemic and treatments during and after organ and bone marrow transplants. Linked with the increase in fungal infections is a recent increase in the frequency with which these infections are recalcitrant to standard antifungal therapy. This review summarizes the factors that contribute to antifungal drug resistance on three levels: (i) clinical factors that result in the inability to successfully treat refractory disease; (ii) cellular factors associated with a resistant fungal strain; and (iii) molecular factors that are ultimately responsible for the resistance phenotype in the cell. Many of the clinical factors that contribute to resistance are associated with the immune status of the patient, with the pharmacology of the drugs, or with the degree or type of fungal infection present. At a cellular level, antifungal drug resistance can be the result of replacement of a susceptible strain with a more resistant strain or species or the alteration of an endogenous strain (by mutation or gene expression) to a resistant phenotype. The molecular mechanisms of resistance that have been identified to date in Candida albicans include overexpression of two types of efflux pumps, overexpression or mutation of the target enzyme, and alteration of other enzymes in the same biosynthetic pathway as the target enzyme. Since the study of antifungal drug resistance is relatively new, other factors that may also contribute to resistance are discussed.

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

Mankind is faced by the increasing emergence of resistant pathogens, including cancer cells. An overview of the different strategies adopted by a variety of cells to evade chemotherapy is presented, with a focus on the mechanisms of multidrug transport. In particular, we analyze the yeast network for pleiotropic drug resistance and assess the potentiality of this system for further understanding of the mechanism of broad specificity and for development of novel practical applications.

Update on antifungal resistance

Overuse of antifungal agents has resulted in the selection of naturally resistant Candida species, as well as expression of resistance from previously susceptible species resulting from genetic mutations and/or selection of resistant subpopulations. Strategies for the appropriate use of antifungal agents need to be developed to prevent further development of resistance.

AP1-mediated multidrug resistance in Saccharomyces cerevisiae requires FLR1 encoding a transporter of the major facilitator superfamily

We have isolated a Candida albicans gene that confers resistance to the azole derivative fluconazole (FCZ) when overexpressed in Saccharomyces cerevisiae. This gene encodes a protein highly homologous to S. cerevisiae yAP-1, a bZip transcription factor known to mediate cellular resistance to toxicants such as cycloheximide (CYH), 4-nitroquinoline N-oxide (4-NQO), cadmium, and hydrogen peroxide. The gene was named CAP1, for C. albicans AP-1. Cap1 and yAP-1 are functional homologues, since CAP1 expression in a yap1 mutant strain partially restores the ability of the cells to grow on toxic concentrations of cadmium or hydrogen peroxide. We have found that the expression of YBR008c, an open reading frame identified in the yeast genome sequencing project and predicted to code for a multidrug transporter of the major facilitator superfamily, is dramatically induced in S. cerevisiae cells overexpressing CAP1. Overexpression of either CAP1 or YAP1 in a wild-type strain results in resistance to FCZ, CYH, and 4-NQO, whereas such resistance is completely abrogated (FCZ and CYH) or strongly reduced (4-NQO) in a ybr008c deletion mutant, demonstrating that YBR008c is involved in YAP1- and CAP1-mediated multidrug resistance. YBR008c has been renamed FLR1, for fluconazole resistance 1. The expression of an FLR1-lacZ reporter construct is strongly induced by the overexpression of either CAP1 or YAP1, indicating that the FLR1 gene is transcriptionally regulated by the Cap1 and yAP-1 proteins. Taken collectively, our results demonstrate that FLR1 represents a new YAP1-controlled multidrug resistance molecular determinant in S. cerevisiae. A similar detoxification pathway is also likely to operate in C. albicans.

Increased mRNA levels of ERG16, CDR, and MDR1 correlate with increases in azole resistance in Candida albicans isolates from a patient infected with human immunodeficiency virus

Resistance to antifungal drugs, specifically azoles such as fluconazole, in the opportunistic yeast Candida albicans has become an increasing problem in human immunodeficiency virus (HIV)-infected individuals. The molecular mechanisms responsible for this resistance have only recently become apparent and can include alterations in the target enzyme of the azole drugs (lanosterol 14alpha demethylase [14DM]), or in various efflux pumps from both the ABC transporter and major facilitator gene families. To determine which of these possible mechanisms was associated with the development of drug resistance in a particular case, mRNA levels have been studied in a series of 17 clinical isolates taken from a single HIV-infected patient over 2 years, during which time the levels of fluconazole resistance of the strain increased over 200-fold. Using Northern blot analysis of steady-state levels of total RNA from these isolates, we observed increased mRNA levels of ERG16 (the 14DM-encoding gene), CDR1 (an ABC transporter), and MDR1 (a major facilitator) in this series. The timing of the increase in mRNA levels of each of these genes correlated with increases in fluconazole resistance of the isolates. Increased mRNA levels were not observed for three other ABC transporters, two other genes in the ergosterol biosynthetic pathway, or the NADPH-cytochrome P-450 oxidoreductase gene that transfers electrons from NADPH to 14DM. Increases in mRNA levels of ERG16 and CDR1 correlated with increased cross-resistance to ketoconazole and itraconazole but not to amphotericin B. A compilation of the genetic alterations identified in this series suggests that resistance develops gradually and is the sum of several different changes, all of which contribute to the final resistant phenotype.

Gamma interferon is not essential in host defense against disseminated candidiasis in mice

In vitro studies have suggested a role for interferon gamma (IFN-gamma) in host defense against disseminated candidiasis, but in vivo studies are inconclusive. We utilized homozygous IFN-gamma knockout (GKO) mice to determine if the cytokine is essential in host defense against this disease. Genotypes of mice were determined by PCR with specific primers for the normal or disrupted IFN-gamma gene. The GKO status of the mice was confirmed by an enzyme-linked immunosorbent assay, which showed no detectable IFN-gamma produced by their splenocytes stimulated by concanavalin A. To test the susceptibility of GKO mice to candidiasis, the animals were infected either intravenously (i.v.) or intragastrically (i.g.) with Candida albicans. GKO mice infected i.v. survived as long as wild-type (WT) mice and showed no difference in Candida CFU counts in liver, spleen, or kidneys compared to those for WT mice. When animals were given Candida i.g., at 3 h or at 10 or 21 days after infection, there was no dissemination of Candida to the lung, liver, spleen, or kidneys for either GKO or WT mice. There was no difference in Candida CFU counts recovered from the stomach or intestines between GKO and WT mice. Histological examination of the stomach cardial-atrium fold, where the fungus was located, showed that GKO mice did not have evidence of more tissue damage or fungal invasion than WT mice. Finally, the jejunum for both types of mice showed no evidence of tissue damage or fungal invasion. These studies indicate that IFN-gamma is not essential in host defense against C. albicans that originates from a mucosal site or that is given directly into the bloodstream in a mouse model.

Molecular mechanisms of azole resistance in fungi

This paper reviews the current status of our understanding of azole antifungal resistance mechanisms at the molecular level and explores their implications. Extensive biochemical studies have highlighted a significant diversity in mechanisms conferring resistance to azoles, which include alterations in sterol biosynthesis, target site, uptake and efflux. In stark contrast, few examples document the molecular basis of azole resistance. Those that do refer almost exclusively to mechanisms in laboratory mutants, with the exception of the role of multi-drug resistance proteins in clinical isolates of Candida albicans. It is clear that the technologies required to examine and define azole resistance mechanisms at the molecular level exist, but research appears distinctly lacking in this most important area.