The Candida albicans lanosterol 14-alpha-demethylase (ERG11) gene promoter is maximally induced after prolonged growth with antifungal drugs

The Endocytosis Adaptor Sla1 Facilitates Drug Susceptibility and Fungal Pathogenesis Through Sla1-Efg1 Regulating System in Candida albicans

Introduction

The role of endocytosis in Candida albicans drug-resistance and pathogenicity remains poorly understood, despite its importance as a fundamental component of intracellular trafficking.

Objective

In order to understand the role of endocytosis in Candida albicans cell wall integrity, drug resistance, and virulence.

Methods

Detection of intracellular endocytosis by FM4-64 staining; Scanning electron microscopy is used to detect cell wall components; Spot assay for detecting drug sensitivity; Co-ip is used to detect protein interactions.

Results

In this study, we found the functions of Sla1 in regulating endocytosis is conserved among pathogenic fungi. Our results also revealed that the deletion of the SLA1 gene altered cell wall properties, composition, and gene expression. In addition, we showed that C. albicans Sla1 was responsible for hyphal development in vitro and for fungal pathogenicity in a murine infection model. Intriguingly, sla1∆/∆ mutant demonstrated enhanced drug resistance, and Sla1 was found to interact with the transcription factor Efg1; the relationship between Sla1 and Efg1 impacts the expression of genes encoding components of the ergosterol biosynthesis pathway, including ERG1, EGR11, and ERG25.

Discussion

These findings have expanded our knowledge of the capabilities of Sla1 beyond its role as an endocytosis adapter and provided insights into a potential new therapeutic target for the treatment of fungal infections.

Molecular Evaluation of the mRNA Expression of the ERG11, ERG3, CgCDR1, and CgSNQ2 Genes Linked to Fluconazole Resistance in Candida glabrata in a Colombian Population

INTRODUCTION

The study of Candida glabrata genes associated with fluconazole resistance, from a molecular perspective, increases the understanding of the phenomenon with a view to its clinical applicability.

OBJECTIVE

We sought to establish the predictive molecular profile of fluconazole resistance in Candida glabrata by analyzing the ERG11, ERG3, CgCDR1, and CgSNQ2 genes.

METHOD

Expression was quantified using RT-qPCR. Metrics were obtained through molecular docking and Fisher discriminant functions. Additionally, a predictive classification was made against the susceptibility of C. glabrata to fluconazole.

RESULTS

The relative expression of the ERG3, CgCDR1, and CgSNQ2 genes was higher in the fluconazole-resistant strains than in the fluconazole-susceptible, dose-dependent strains. The gene with the highest relative expression in the fluconazole-exposed strains was CgCDR1, and in both the resistant and susceptible, dose-dependent strains exposed to fluconazole, this was also the case. The molecular docking model generated a median number of contacts between fluconazole and ERG11 that was lower than the median number of contacts between fluconazole and ERG3, -CgCDR1, and -CgSNQ2. The predicted classification through the multivariate model for fluconazole susceptibility achieved an accuracy of 73.5%.

CONCLUSION

The resistant strains had significant expression levels of genes encoding efflux pumps and the ERG3 gene. Molecular analysis makes the identification of a low affinity between fluconazole and its pharmacological target possible, which may explain the lower intrinsic susceptibility of the fungus to fluconazole.

Recent developments in membrane targeting antifungal agents to mitigate antifungal resistance

Fungal infections cause severe and life-threatening complications especially in immunocompromised individuals. Antifungals targeting cellular machinery and cell membranes including azoles are used in clinical practice to manage topical to systemic fungal infections. However, continuous exposure to clinically used antifungal agents in managing the fungal infections results in the development of multi-drug resistance via adapting different kinds of intrinsic and extrinsic mechanisms. The unique chemical composition of fungal membranes presents attractive targets for antifungal drug discovery as it is difficult for fungal cells to modify the membrane targets for emergence of drug resistance. Here, we discussed available antifungal drugs with their detailed mechanism of action and described different antifungal resistance mechanisms. We further emphasized structure-activity relationship studies of membrane-targeting antifungal agents, and classified membrane-targeting antifungal agents on the basis of their core scaffold with detailed pharmacological properties. This review aims to pique the interest of potential researchers who could explore this interesting and intricate fungal realm.

Cell Wall Proteome Profiling of a Candida albicans Fluconazole-Resistant Strain from a Lebanese Hospital Patient Using Tandem Mass Spectrometry-A Pilot Study

Candida albicans is an opportunistic pathogenic fungus responsible for high mortality rates in immunocompromised individuals. Azole drugs such as fluconazole are the first line of therapy in fungal infection treatment. However, resistance to azole treatment is on the rise. Here, we employ a tandem mass spectrometry approach coupled with a bioinformatics approach to identify cell wall proteins present in a fluconazole-resistant hospital isolate upon drug exposure. The isolate was previously shown to have an increase in cell membrane ergosterol and cell wall chitin, alongside an increase in adhesion, but slightly attenuated in virulence. We identified 50 cell wall proteins involved in ergosterol biosynthesis such as Erg11, and Erg6, efflux pumps such as Mdr1 and Cdr1, adhesion proteins such as Als1, and Pga60, chitin deposition such as Cht4, and Crh11, and virulence related genes including Sap5 and Lip9. Candidial proteins identified in this study go a long way in explaining the observed phenotypes. Our pilot study opens the way for a future large-scale analysis to identify novel proteins involved in drug-resistance mechanisms.

Lactate Like Fluconazole Reduces Ergosterol Content in the Plasma Membrane and Synergistically Kills Candida albicans

Candida albicans is an opportunistic pathogen that induces vulvovaginal candidiasis (VVC), among other diseases. In the vaginal environment, the source of carbon for C. albicans can be either lactic acid or its dissociated form, lactate. It has been shown that lactate, similar to the popular antifungal drug fluconazole (FLC), reduces the expression of the ERG11 gene and hence the amount of ergosterol in the plasma membrane. The Cdr1 transporter that effluxes xenobiotics from C. albicans cells, including FLC, is delocalized from the plasma membrane to a vacuole under the influence of lactate. Despite the overexpression of the CDR1 gene and the increased activity of Cdr1p, C. albicans is fourfold more sensitive to FLC in the presence of lactate than when glucose is the source of carbon. We propose synergistic effects of lactate and FLC in that they block Cdr1 activity by delocalization due to changes in the ergosterol content of the plasma membrane.

Fluconazole resistance in Candida albicans is induced by Pseudomonas aeruginosa quorum sensing

Microorganisms employ quorum sensing (QS) mechanisms to communicate with each other within microbial ecosystems. Emerging evidence suggests that intraspecies and interspecies QS plays an important role in antimicrobial resistance in microbial communities. However, the relationship between interkingdom QS and antimicrobial resistance is largely unknown. Here, we demonstrate that interkingdom QS interactions between a bacterium, Pseudomonas aeruginosa and a yeast, Candida albicans, induce the resistance of the latter to a widely used antifungal fluconazole. Phenotypic, transcriptomic, and proteomic analyses reveal that P. aeruginosa's main QS molecule, N-(3-Oxododecanoyl)-L-homoserine lactone, induces candidal resistance to fluconazole by reversing the antifungal's effect on the ergosterol biosynthesis pathway. Accessory resistance mechanisms including upregulation of C. albicans drug-efflux, regulation of oxidative stress response, and maintenance of cell membrane integrity, further confirm this phenomenon. These findings demonstrate that P. aeruginosa QS molecules may confer protection to neighboring yeasts against azoles, in turn strengthening their co-existence in hostile polymicrobial infection sites.

Palmitic Acid Inhibits the Virulence Factors of Candida tropicalis: Biofilms, Cell Surface Hydrophobicity, Ergosterol Biosynthesis, and Enzymatic Activity

Biofilm is the fortitude of Candida species infections which eventually causes candidiasis in human. C. tropicalis is one of the predominant Candida species commonly found in systemic infections, next to C. albicans. In Candida species, biofilm maturity initiates irreversible surface attachment of cells and barricades the penetration of conventional antifungals. Hence, the current study investigated the antifungal and antivirulence potency of palmitic acid (PA) against C. tropicalis mature biofilm and its associated virulence factors. In vitro results revealed an effective inhibition of biofilm in PA-treated C. tropicalis, compared to C. albicans and C. glabrata. Also, PA reduced C. tropicalis mature biofilm at various time points. Further, PA treatment triggered apoptosis in C. tropicalis through ROS mediated mitochondrial dysfunction as demonstrated by confocal microscopic observation of PI, DAPI and DCFDA staining. PA regulated other virulence factors such as cell surface hydrophobicity, ergosterol biosynthesis, protease and lipase after 48 h of treatment. Downregulation of ERG11 (Lanosterol 14-alpha demethylase) was contributed to the reduction of ergosterol in PA-treated C. tropicalis. However, enhanced hyphal growth was observed in PA-treated C. tropicalis through upregulation HWP1 (Hyphal wall protein) and EFG1 (Enhanced filamentous growth). This study highlighted the antibiofilm and antivirulence potency of PA against C. tropicalis. Hence, PA could be applied synergistically with other antifungal agents to increase the efficacy for regulating NCAC infections.

Repurposing approach identifies pitavastatin as a potent azole chemosensitizing agent effective against azole-resistant Candida species

The limited number of antifungals and the rising frequency of azole-resistant Candida species are growing challenges to human medicine. Drug repurposing signifies an appealing approach to enhance the activity of current antifungal drugs. Here, we evaluated the ability of Pharmakon 1600 drug library to sensitize an azole-resistant Candida albicans to the effect of fluconazole. The primary screen revealed 44 non-antifungal hits were able to act synergistically with fluconazole against the test strain. Of note, 21 compounds, showed aptness for systemic administration and limited toxic effects, were considered as potential fluconazole adjuvants and thus were termed as “repositionable hits”. A follow-up analysis revealed pitavastatin displaying the most potent fluconazole chemosensitizing activity against the test strain (ΣFICI 0.05) and thus was further evaluated against 18 isolates of C. albicans (n = 9), C. glabrata (n = 4), and C. auris (n = 5). Pitavastatin displayed broad-spectrum synergistic interactions with both fluconazole and voriconazole against ~89% of the tested strains (ΣFICI 0.05–0.5). Additionally, the pitavastatin-fluconazole combination significantly reduced the biofilm-forming abilities of the tested Candida species by up to 73%, and successfully reduced the fungal burdens in a Caenorhabditis elegans infection model by up to 96%. This study presents pitavastatin as a potent azole chemosensitizing agent that warrant further investigation.

A Crucial Role for Ergosterol in Plasma Membrane Composition, Localisation, and Activity of Cdr1p and H+-ATPase in Candida albicans

Candida albicans is an opportunistic fungal pathogen of humans. Treatment of C. albicans infections relies on azoles, which target the lanosterol 14α-demethylase (Erg11p) encoded by the ERG11 gene. Our results show that targeted gene disruption of ERG11 can result in resistance to ergosterol-dependent drugs (azoles and amphotericin B), auxotrophy and aerobically viable erg11Δ/Δ cells. Abnormal sterol deposition and lack of ergosterol in the erg11Δ/Δ strain leads to reduced plasma membrane (PM) fluidity, as well as dysfunction of the vacuolar and mitochondrial membranes, resulting respectively in defects in vacuole fusion and a reduced intracellular ATP level. The altered PM structure of the erg11Δ/Δ strain contributes to delocalisation of H⁺-ATPase and the Cdr1 efflux pump from the PM to vacuoles and, resulting in a decrease in PM potential (Δψ) and increased sensitivity to ergosterol-independent xenobiotics. This new insight into intracellular processes under Erg11p inhibition may lead to a better understanding of the indirect effects of azoles on C. albicans cells and the development of new treatment strategies for resistant infections.

Computational Tools: RNA Interference in Fungal Therapeutics

There is steady rise in the number of immunocompromised population due to increased use of potent immunosuppression therapies. This is associated with increased risk of acquiring fungal opportunistic infections in immunocompromised patients which account for high morbidity and mortality rates, if left untreated. The conventional antifungal drugs to treat fungal diseases (mycoses) are increasingly becoming inadequate due to observed varied susceptibility of fungi and their recurrent resistance. RNA interference (RNAi), sequence-specific gene silencing, is emerging as a promising new therapeutic approach. This chapter discusses various aspects of RNAi, viz., the fundamental RNAi machinery present in fungi, in silico siRNA features, designing guidelines and tools, siRNA delivery, and validation of gene knockdown for therapeutics against mycoses. Target gene identification is a crucial step in designing of gene-specific siRNA in addition to efficient delivery strategies to bring about effective inhibition of fungi. Subsequently, designed siRNA can be delivered effectively in vitro either by soaking fungi with siRNA or by transforming inverted repeat transgene containing plasmid into fungi, which ultimately generates siRNA(s). Finally, fungal inhibition can be verified at the RNA and protein levels by blotting techniques, fluorescence imaging, and biochemical assays. Despite challenges, several such in vitro studies have spawned optimism around RNAi as a revolutionary new class of therapeutics against mycoses. But, pharmacokinetic parameters need to be evaluated from in vivo studies and clinical trials to recognize RNAi as a novel treatment approach for mycoses.

Novel hydroxamates potentiated in vitro activity of fluconazole against Candida albicans

A set of 12 novel hydroxamate compounds (NHCs), structurally designed as inhibitors of histone deacetylase (HDAC) enzyme, were synthesized at our facility. These were adamantane derivatives with N-hydroxyacetamide as pharmacophore, and each of these compounds was tested for potentiating activity on fluconazole. The concentration of fluconazole which completely inhibited (concentration of complete inhibition [CCI]) the growth of Candida albicans ATCC 90028 and C. albicans ATCC 64550 was determined by micro-dilution method in the absence and presence of NHCs. The CCI of fluconazole without the NHC combination was 64 μg/ml and 1024 μg/ml against C. albicans ATCC 90028 and C. albicans ATCC 64550, respectively. The majority of the NHCs potentiated the fluconazole activity markedly as CCI of fluconazole against C. albicans ATCC 90028 reduced to 0.25 μg/ml. Similarly, CCI of fluconazole against C. albicans ATCC 64550 reduced to 4–8 μg/ml in combination with majority of NHCs while the best activity was displayed by the compound 1 with a reduction of CCI to 0.5 μg/ml. The study results revealed the potential usage of hydroxamate derivatives, structurally designed as HDAC inhibitors to enhance the activity of fluconazole against C. albicans.

Quantitation of ergosterol content and gene expression profile of ERG11 gene in fluconazole-resistant Candida albicans

Background and Purpose:

The frequency of opportunistic fungal infections in immunocompromised patients, especially by Candida species, has sharply increased in the last few decades. The objective of this study was to analyse the ergosterol content and gene expression profiling of clinical isolates of fluconazole-resistant Candida albicans.

Materials and Methods:

Sixty clinical samples were identified and collected from immunocompromised patients, namely recurrent oral, vaginal, and cutaneous candidiasis, during 2015-16. Antifungal susceptibility testing of fluconazole against clinical Candida species was performed according to Clinical and Laboratory Standards Institute guidelines. Ergosterol content and gene expression profiling of sterol 14α-demethylase (ERG11) gene in fluconazole-susceptible and –resistant C. albicans were investigated.

Results:

The specimens consisted of C. albicans (46.67%), Candida krusei (41.67%), and Candida tropicalis (11.67%). All the isolates were resistant to fluconazole. No significant reduction was noted in total cellular ergosterol content in comparison with untreated controls in terms of fluconazole-resistant C. albicans. The expressionlevel of ERG11 gene was down-regulated in fluconazole-susceptible C. albicans. Eventually, the expression pattern of ERG11 gene revealed no significant changes in fluconazole-resistant isolates compared to untreated controls. The results revealed no significant differences between fluconazole-susceptible and –resistant C. albicans sequences by comparison with ERG11 reference sequence.

Conclusion:

Our findings provide an insight into the mechanism of fluconazole resistance in C. albicans. The mechanisms proposed for clinical isolates of fluconazole-resistant C. albicans are alteration in sterol biosynthesis, analysis of expression level of ERG11 gene, and analysis of gene sequences. Nonetheless, further studies are imperative to find molecular mechanisms that could be targeted to control fluconazole resistance.

A Combination Fluorescence Assay Demonstrates Increased Efflux Pump Activity as a Resistance Mechanism in Azole-Resistant Vaginal Candida albicans Isolates

Candida albicans is a pathogenic fungus causing vulvovaginal candidiasis (VVC). Azole drugs, such as fluconazole, are the most common treatment for these infections. Recently, azole-resistant vaginal C. albicans isolates have been detected in patients with recurring and refractory vaginal infections. However, the mechanisms of resistance in vaginal C. albicans isolates have not been studied in detail. In oral and systemic resistant isolates, overexpression of the ABC transporters Cdr1p and Cdr2p and the major facilitator transporter Mdr1p is associated with resistance. Sixteen fluconazole-susceptible and 22 fluconazole-resistant vaginal C. albicans isolates were obtained, including six matched sets containing a susceptible and a resistant isolate, from individual patients. Using quantitative real-time reverse transcriptase PCR (qRT-PCR), 16 of 22 resistant isolates showed overexpression of at least one efflux pump gene, while only 1 of 16 susceptible isolates showed such overexpression. To evaluate the pump activity associated with overexpression, an assay that combined data from two separate fluorescent assays using rhodamine 6G and alanine β-naphthylamide was developed. The qRT-PCR results and activity assay results were in good agreement. This combination of two fluorescent assays can be used to study efflux pumps as resistance mechanisms in clinical isolates. These results demonstrate that efflux pumps are a significant resistance mechanism in vaginal C. albicans isolates.

Total Protein Profile and Drug Resistance in Candida albicans Isolated from Clinical Samples

This study was done to assess the antifungal susceptibility of clinical isolates of Candida albicans and to evaluate its total protein profile based on morphological difference on drug resistance. Hundred and twenty clinical isolates of C. albicans from various clinical specimens were tested for susceptibility against four antifungal agents, namely, fluconazole, itraconazole, amphotericin B, and ketoconazole. A significant increase of drug resistance in clinical isolates of C. albicans was observed. The study showed 50% fluconazole and itraconazole resistance at 32 μg mL(-1) with a MIC50 and MIC90 values at 34 and 47 and 36 and 49 μg mL(-1), respectively. All isolates were sensitive to amphotericin B and ketoconazole. The SDS-PAGE protein profile showed a prevalent band of ~52.5 kDa, indicating overexpression of gene in 72% strains with fluconazole resistance. Since the opportunistic infections of Candida spp. are increasing along with drug resistance, the total protein profile will help in understanding the evolutionary changes in drug resistance and also to characterize them.

Overexpression and mutation as a genetic mechanism of fluconazole resistance in Candida albicans isolated from human immunodeficiency virus patients in Indonesia

Fluconazole is the standard treatment for oropharyngeal candidiasis, which is the third most common opportunistic infection in human immunodeficiency virus (HIV)/AIDS patients in Indonesia. Overuse of this drug could lead to the emergence of resistance. The objective of this study was to analyse the role of ERG11, CDR1, CDR2 and MDR1 gene overexpression and mutations in the ERG11 gene as a genetic mechanism of fluconazole resistance in Candida albicans isolated from HIV patients in Indonesia. Overexpression of ERG11, CDR1, CDR2 and MDR1 was analysed by real-time reverse transcription PCR, while ERG11 gene mutation analysis was performed using sequencing methods. Seventeen isolates out of 92 strains of C. albicans isolated from 108 HIV patients were found to be resistant to azole antifungals. The highest gene overexpression of ERG11 was found in C. albicans resistant to single fluconazole, while the highest gene overexpression of CDR2 was detected in all isolates of C. albicans resistant to multiple azoles. Amino acid substitutions were observed at six positions, i.e. D116E, D153E, I261V, E266D, V437I and V488I. The amino acid substitution I261V was identified in this study and was probably associated with fluconazole resistance. The combination of overexpression of CDR2 and ERG11 and mutation in the ERG11 gene was found to be a genetic mechanism of fluconazole resistance in C. albicans isolated from HIV patients in Indonesia.

Stepwise emergence of azole, echinocandin and amphotericin B multidrug resistance in vivo in Candida albicans orchestrated by multiple genetic alterations

OBJECTIVES

The objective of this study was to characterize the underlying molecular mechanisms in consecutive clinical Candida albicans isolates from a single patient displaying stepwise-acquired multidrug resistance.

METHODS

Nine clinical isolates (P-1 to P-9) were susceptibility tested by EUCAST EDef 7.2 and Etest. P-4, P-5, P-7, P-8 and P-9 were available for further studies. Relatedness was evaluated by MLST. Additional genes were analysed by sequencing (including FKS1, ERG11, ERG2 and TAC1) and gene expression by quantitative PCR (CDR1, CDR2 and ERG11). UV-spectrophotometry and GC-MS were used for sterol analyses. In vivo virulence was determined in the insect model Galleria mellonella and evaluated by log-rank Mantel-Cox tests.

RESULTS

P-1 + P-2 were susceptible, P-3 + P-4 fluconazole resistant, P-5 pan-azole resistant, P-6 + P-7 pan-azole and echinocandin resistant and P-8 + P-9 MDR. MLST supported genetic relatedness among clinical isolates. P-4 harboured four changes in Erg11 (E266D, G307S, G450E and V488I), increased expression of ERG11 and CDR2 and a change in Tac1 (R688Q). P-5, P-7, P-8 and P-9 had an additional change in Erg11 (A61E), increased expression of CDR1, CDR2 and ERG11 (except for P-7) and a different amino acid change in Tac1 (R673L). Echinocandin-resistant isolates harboured the Fks1 S645P alteration. Polyene-resistant P-8 + P-9 lacked ergosterol and harboured a frameshift mutation in ERG2 (F105SfsX23). Virulence was attenuated (but equivalent) in the clinical isolates, but higher than in the azole- and echinocandin-resistant unrelated control strain.

CONCLUSIONS

C. albicans demonstrates a diverse capacity to adapt to antifungal exposure. Potentially novel resistance-inducing mutations in TAC1, ERG11 and ERG2 require independent validation.

Induction of Candida albicans drug resistance genes by hybrid zinc cluster transcription factors

The pathogenic yeast Candida albicans can develop resistance to azole antifungal drugs by overexpressing ERG11, which encodes the drug target, or the multidrug efflux pumps MDR1 and CDR1/CDR2. The constitutive upregulation of these genes is usually caused by gain-of-function mutations in the zinc cluster transcription factors Upc2, Mrr1, and Tac1, respectively. These transcription factors are also required for the induction of their target genes in drug-susceptible strains in the presence of specific stimuli. By swapping the DNA-binding domains of Mrr1, Tac1, and Upc2 we investigated if the hybrid transcription factors could activate their new target genes in response to the same signals. When Tac1 was targeted to the MDR1 and ERG11 promoters, the expression of these genes became inducible by fluphenazine. Similarly, MDR1 and CDR2 were strongly upregulated by fluconazole when Upc2 was fused to the DNA-binding domains of Mrr1 and Tac1, respectively. In contrast, Mrr1 was unable to promote gene expression in response to benomyl when it was targeted to the CDR2 and ERG11 promoters instead of the MDR1 promoter. These results suggest that Tac1 and Upc2 themselves are activated by the inducers fluphenazine and fluconazole, respectively, whereas benomyl does not activate Mrr1 itself but a coregulatory factor that is present at the promoters of Mrr1 target genes. Strains in which the expression levels of Mrr1 and Tac1 target genes were controlled by Upc2 exhibited increased fluconazole resistance, demonstrating that the ability to efficiently upregulate the expression of efflux pumps in the presence of the drug results in enhanced intrinsic fluconazole resistance.

Ascorbic acid inhibition of Candida albicans Hsp90-mediated morphogenesis occurs via the transcriptional regulator Upc2

Morphogenetic transitions of the opportunistic fungal pathogen Candida albicans are influenced by temperature changes, with induction of filamentation upon a shift from 30 to 37°C. Hsp90 was identified as a major repressor of an elongated cell morphology at low temperatures, as treatment with specific inhibitors of Hsp90 results in elongated growth forms at 30°C. Elongated growth resulting from a compromised Hsp90 is considered neither hyphal nor pseudohyphal growth. It has been reported that ascorbic acid (vitamin C) interferes with the yeast-to-hypha transition in C. albicans. In the present study, we show that ascorbic acid also antagonizes the morphogenetic change caused by hampered Hsp90 function. Further analysis revealed that Upc2, a transcriptional regulator of genes involved in ergosterol biosynthesis, and Erg11, the target of azole antifungals, whose expression is in turn regulated by Upc2, are required for this antagonism. Ergosterol levels correlate with elongated growth and are reduced in cells treated with the Hsp90 inhibitor geldanamycin (GdA) and restored by cotreatment with ascorbic acid. In addition, we show that Upc2 appears to be required for ascorbic acid-mediated inhibition of the antifungal activity of fluconazole. These results identify Upc2 as a major regulator of ascorbic acid-induced effects in C. albicans and suggest an association between ergosterol content and elongated growth upon Hsp90 compromise.

UPC2A is required for high-level azole antifungal resistance in Candida glabrata

Candida glabrata, the second most common cause of Candida infections, is associated with high rates of mortality and often exhibits resistance to the azole class of antifungal agents. Upc2 and Ecm22 in Saccharomyces cerevisiae and Upc2 in Candida albicans are the transcriptional regulators of ERG11, the gene encoding the target of azoles in the ergosterol biosynthesis pathway. Recently two homologs for these transcription factors, UPC2A and UPC2B, were identified in C. glabrata. One of these, UPC2A, was shown to influence azole susceptibility. We hypothesized that due to the global role for Upc2 in sterol biosynthesis in S. cerevisiae and C. albicans, disruption of UPC2A would enhance the activity of fluconazole in both azole-susceptible dose-dependent (SDD) and -resistant C. glabrata clinical isolates. To test this hypothesis, we constructed mutants with disruptions in UPC2A and UPC2B alone and in combination in a matched pair of clinical azole-SDD and -resistant isolates. Disruption of UPC2A in both the SDD and resistant isolates resulted in increased susceptibility to sterol biosynthesis inhibitors, including a reduction in fluconazole MIC and minimum fungicidal concentration, enhanced azole activity by time-kill analysis, a decrease in ergosterol content, and downregulation of baseline and inducible expression of several sterol biosynthesis genes. Our results indicate that Upc2A is a key regulator of ergosterol biosynthesis and is essential for resistance to sterol biosynthesis inhibitors in C. glabrata. Therefore, the UPC2A pathway may represent a potential cotherapeutic target for enhancing azole activity against this organism.

Enhanced antifungal efficacy of tebuconazole using gated pH-driven mesoporous nanoparticles

pH-sensitive gated mesoporous silica nanoparticles have been synthesized. Increased extracellular pH and internalization into living yeast cells triggered molecular gate aperture and cargo release. Proper performance of the system was demonstrated with nanodevices loaded with fluorescein or with the antifungal agent tebuconazole. Interestingly, nanodevices loaded with tebuconazole significantly enhanced tebuconazole cytotoxicity. As alterations of acidic external pH are a key parameter in the onset of fungal vaginitis, this nanodevice could improve the treatment for vaginal mycoses.

Endoplasmic reticulum localized PerA is required for cell wall integrity, azole drug resistance, and virulence in Aspergillus fumigatus

GPI-anchoring is a universal and critical post-translational protein modification in eukaryotes. In fungi, many cell wall proteins are GPI-anchored, and disruption of GPI-anchored proteins impairs cell wall integrity. After being synthesized and attached to target proteins, GPI anchors undergo modification on lipid moieties. In spite of its importance for GPI-anchored protein functions, our current knowledge of GPI lipid remodelling in pathogenic fungi is limited. In this study, we characterized the role of a putative GPI lipid remodelling protein, designated PerA, in the human pathogenic fungus Aspergillus fumigatus. PerA localizes to the endoplasmic reticulum and loss of PerA leads to striking defects in cell wall integrity. A perA null mutant has decreased conidia production, increased susceptibility to triazole antifungal drugs, and is avirulent in a murine model of invasive pulmonary aspergillosis. Interestingly, loss of PerA increases exposure of β-glucan and chitin content on the hyphal cell surface, but diminished TNF production by bone marrow-derived macrophages relative to wild type. Given the structural specificity of fungal GPI-anchors, which is different from humans, understanding GPI lipid remodelling and PerA function in A. fumigatus is a promising research direction to uncover a new fungal specific antifungal drug target.

Functional characterization of Candida albicans Hos2 histone deacetylase

Candida albicans is a mucosal commensal organism capable of causing superficial (oral and vaginal thrush) infections in immune normal hosts, but is a major pathogen causing systemic and mucosal infections in immunocompromised individuals. Azoles have been very effective anti-fungal agents and the mainstay in treating opportunistic mold and yeast infections. Azole resistant strains have emerged compromising the utility of this class of drugs. It has been shown that azole resistance can be reversed by the co-administration of a histone deacetylase (HDAC) inhibitor, suggesting that resistance is mediated by epigenetic mechanisms possibly involving Hos2, a fungal deacetylase. We report here the cloning and functional characterization of  HOS2 (High Osmolarity  Sensitive) , a gene coding for fungal histone deacetylase from  C. albicans. Inhibition studies showed that Hos2 is susceptible to pan inhibitors such as trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA), but is not inhibited by class I inhibitors such as MS-275. This  in  vitro enzymatic assay, which is amenable to high throughput could be used for screening potent fungal Hos2 inhibitors that could be a potential anti-fungal adjuvant. Purified Hos2 protein consistently deacetylated tubulins, rather than histones from TSA-treated cells. Hos2 has been reported to be a putative NAD+ dependent histone deacetylase, a feature of sirtuins. We assayed for sirtuin activation with resveratrol and purified Hos2 protein and did not find any sirtuin activity.

Mutual co-regulation between GPI-N-acetylglucosaminyltransferase and ergosterol biosynthesis in Candida albicans

A novel co-regulation exists between the first step of GPI (glycosylphosphatidylinositol) anchor biosynthesis and the rate-determining step of ergosterol biosynthesis in Candida albicans. Depleting CaGpi19p, an accessory subunit of the enzyme complex that initiates GPI biosynthesis, down-regulates ERG11, altering ergosterol levels and drug response. This effect is specific to CaGpi19p depletion and is not due to cell wall defects or GPI deficiency. Additionally, down-regulation of ERG11 down-regulates CaGPI19 and GPI biosynthesis.

Azole drugs are imported by facilitated diffusion in Candida albicans and other pathogenic fungi

Despite the wealth of knowledge regarding the mechanisms of action and the mechanisms of resistance to azole antifungals, very little is known about how the azoles are imported into pathogenic fungal cells. Here the in-vitro accumulation and import of Fluconazole (FLC) was examined in the pathogenic fungus, Candida albicans. In energized cells, FLC accumulation correlates inversely with expression of ATP-dependent efflux pumps. In de-energized cells, all strains accumulate FLC, suggesting that FLC import is not ATP-dependent. The kinetics of import in de-energized cells displays saturation kinetics with a K_m of 0.64 uM and V_max of 0.0056 pmol/min/10⁸ cells, demonstrating that FLC import proceeds via facilitated diffusion through a transporter rather than passive diffusion. Other azoles inhibit FLC import on a mole/mole basis, suggesting that all azoles utilize the same facilitated diffusion mechanism. An analysis of related compounds indicates that competition for azole import depends on an aromatic ring and an imidazole or triazole ring together in one molecule. Import of FLC by facilitated diffusion is observed in other fungi, including Cryptococcus neoformans, Saccharomyces cerevisiae, and Candida krusei, indicating that the mechanism of transport is conserved among fungal species. FLC import was shown to vary among Candida albicans resistant clinical isolates, suggesting that altered facilitated diffusion may be a previously uncharacterized mechanism of resistance to azole drugs.

Azole antifungals are used to treat a wide variety of fungal infections of humans, animals and plants. A great deal is known about how the azoles interact with their target enzyme within fungal cells and how the azoles are exported from the fungal cell through various efflux pumps. Altered interactions with the target enzyme and altered efflux pump expression are common mechanisms of azole resistance in fungi. However, the mechanism by which azoles enter a fungal cell is not clear—many have assumed that azoles passively diffuse into the cell. This study demonstrates that azoles are not passively diffused, or actively pumped, into the cell. Instead, azoles are imported by facilitated diffusion, mediated by a transporter. Facilitated diffusion of azoles is saturable. All clinically important azoles, and many structurally related compounds, compete for FLC import, while structurally unrelated drugs do not compete. Azole import by facilitated diffusion is shown in four species of fungi, suggesting that it is common for most if not all fungi. Altered facilitated diffusion is observed in a collection of clinical isolates, suggesting that altered import is a previously uncharacterized mechanism of resistance.

The UPC2 promoter in Candida albicans contains two cis-acting elements that bind directly to Upc2p, resulting in transcriptional autoregulation

In Candida albicans, ergosterol biosynthetic genes, including ERG11, which encodes the target of azole antifungal drugs, are regulated by the transcriptional regulator Upc2p. To initially characterize the promoter of the UPC2 gene, 5' rapid amplification of cDNA ends was used to identify two transcriptional initiation sites upstream of the ATG codon. The regions within the UPC2 promoter required for azole regulation of the UPC2 promoter were then identified using nested deletions fused to a luciferase reporter which were tested for azole inducibility in wild-type (WT) and upc2Delta/upc2Delta strains. Two distinct regions important for azole induction were identified: a Upc2p-dependent region (UDR) between bp -450 and -350 upstream of the ATG codon and a Upc2p-independent region (UIR) between bp -350 and -250 upstream of the ATG codon. Within the UDR, loss or mutation of the sterol response element (SRE), so named because of homology to the Saccharomyces cerevisiae Upc2p binding site, resulted in a decrease in both basal and induced expression in the WT strain but did not affect azole inducibility in the upc2Delta/upc2Delta deletion strain. Gel shift analyses using the DNA binding domain of Upc2p confirmed binding of the protein to two SRE-related sequences within the UPC2 promoter, with strongest binding to the UDR SRE. Detailed gel shift analyses of the UDR SRE shows that Upc2p binds to a bipartite element within the UPC2 promoter, including the previously identified SRE and a new, adjacent element, the short direct repeat (SDR), with partial homology to the SRE.

Abc1p is a multidrug efflux transporter that tips the balance in favor of innate azole resistance in Candida krusei

Most Candida krusei strains are innately resistant to fluconazole (FLC) and can cause breakthrough candidemia in immunocompromised individuals receiving long-term prophylactic FLC treatment. Although the azole drug target, Erg11p, of C. krusei has a relatively low affinity for FLC, drug efflux pumps are also believed to be involved in its innate FLC resistance. We describe here the isolation and characterization of Abc1p, a constitutively expressed multidrug efflux pump, and investigate ERG11 and ABC1 expression in C. krusei. Examination of the ERG11 promoter revealed a conserved azole responsive element that has been shown to be necessary for the transcription factor Upc2p mediated upregulation by azoles in related yeast. Extensive cloning and sequencing identified three distinct ERG11 alleles in one of two C. krusei strains. Functional overexpression of ERG11 and ABC1 in Saccharomyces cerevisiae conferred high levels of resistance to azoles and a range of unrelated Abc1p pump substrates, while small molecule inhibitors of Abc1p chemosensitized C. krusei to azole antifungals. Our data show that despite the presence of multiple alleles of ERG11 in some, likely aneuploid, C. krusei strains, it is mainly the low affinity of Erg11p for FLC, together with the constitutive but low level of expression of the multidrug efflux pump Abc1p, that are responsible for the innate FLC resistance of C. krusei.

Cytoplasmic localization of sterol transcription factors Upc2p and Ecm22p in S. cerevisiae

Ergosterol homeostasis is a critical process for fungal cells. Paralogous zinc cluster transcription factors Upc2p and Ecm22p are major regulators of ergosterol biosynthesis in Saccharomyces cerevisiae. Upc2p and Ecm22p sense and respond to sterol depletion but their mechanism of activation has not been defined. Subcellular localization and functional expression of Upc2p-GFP and Ecm22p-GFP was monitored by fluorescence microscopy and flow cytometry in live yeast cells. Both fusion proteins localized to intracellular membranes and to perinuclear foci. Perinuclear localization of Upc2p-GFP and Ecm22p-GFP was increased when ergosterol biosynthesis was blocked by azole drug treatment. Nuclear localization in response to sterol depletion is consistent with the hypothesis that Upc2p and Ecm22p are trafficked from a membrane to the nucleus as a post-translational mechanism of sterol sensing.

Candida albicans UPC2 is transcriptionally induced in response to antifungal drugs and anaerobicity through Upc2p-dependent and -independent mechanisms

Many genes in the Candida albicans ergosterol biosynthetic pathway are controlled by the transcriptional activator Upc2p, which is upregulated in the presence of azole drugs and has been suggested to regulate its own transcription by an autoregulatory mechanism. The UPC2 promoter was cloned upstream of a luciferase reporter gene (RLUC). UPC2-RLUC activity was induced in response to ergosterol biosynthesis inhibitors and in response to anaerobicity. Under both conditions, induction correlates with the magnitude of sterol depletion. Azole inducibility in the parental strain was approximately 100-fold, and in a UPC2 homozygous deletion strain was 17-fold, suggesting that, in addition to autoregulation, UPC2 transcription is controlled by a novel, Upc2p-independent mechanism(s). Curiously, basal UPC2-RLUC activity was fivefold higher in the deletion strain, which may be an indirect consequence of the lower sterol level in this strain, or a direct consequence of repression by an autoregulatory mechanism. These results suggest that transcriptional regulation of UPC2 expression is important in the response to antifungal drugs, and that this regulation occurs through Upc2p-dependent as well as novel Upc2p-independent mechanisms.

Polyene susceptibility is dependent on nitrogen source in the opportunistic pathogen Candida albicans

OBJECTIVES

Polyene antifungal drugs, including amphotericin B or nystatin, target ergosterol in the fungal plasma membrane and are used to treat systemic, vaginal and oral fungal infections. In the oral cavity, the available nitrogen sources are primarily in the form of proteins, which are poor nitrogen sources. This study evaluates the effect of protein as a nitrogen source on drug susceptibilities.

METHODS

Candida albicans was grown in protein [bovine serum albumin (BSA) or casein (CSN)] as a sole nitrogen source, in ammonium sulphate (AS) as a nitrogen source, or in both protein and AS.

RESULTS

Cells grown in BSA or CSN were 4- to 16-fold less susceptible to amphotericin B and nystatin than those grown in AS. Similar results were observed for cycloheximide, but not for fluconazole or caspofungin, and were observed for many C. albicans clinical isolates. The results were observed in two different media, and in broth and on agar. Cells grown under these nitrogen-poor conditions have a reduction in ergosterol sterol levels and a reduction in overall sterol synthesis. Quantitative real-time reverse transcription-polymerase chain reaction analysis shows that some genes involved in sterol biosynthesis are induced under nitrogen-limiting conditions, consistent with the lower sterol levels.

CONCLUSIONS

The results demonstrate that nitrogen source has a significant effect on polyene susceptibilities. As these nitrogen-limiting conditions mimic oral nitrogen availability, they suggest that in vitro polyene susceptibilities may overestimate the in vivo susceptibilities to polyene drugs in the mouth.

An isochromosome confers drug resistance in vivo by amplification of two genes, ERG11 and TAC1

Acquired azole resistance is a serious clinical problem that is often associated with the appearance of aneuploidy and, in particular, with the formation of an isochromosome [i(5L)] in the fungal opportunist Candida albicans. Here we exploited a series of isolates from an individual patient during the rapid acquisition of fluconazole resistance (Flu(R)). Comparative genome hybridization arrays revealed that the presence of two extra copies of Chr5L, on the isochromosome, conferred increased Flu(R) and that partial truncation of Chr5L reduced Flu(R). In vitro analysis of the strains by telomere-mediated truncations and by gene deletion assessed the contribution of all Chr5L genes and of four specific genes. Importantly, ERG11 (encoding the drug target) and a hyperactive allele of TAC1 (encoding a transcriptional regulator of drug efflux pumps) made independent, additive contributions to Flu(R) in a gene copy number-dependent manner that was not different from the contributions of the entire Chr5L arm. Thus, the major mechanism by which i(5L) formation causes increased azole resistance is by amplifying two genes: ERG11 and TAC1.

Tetracycline alters drug susceptibility in Candida albicans and other pathogenic fungi

The tetracycline (TET) promoter has been used in several systems as an inducible regulator of gene expression. In control analyses, the standard Candida albicans laboratory strain SC5314 was found to have altered susceptibility to a variety of antifungal drugs in the presence of relatively high concentrations (50-200 microg ml(-1)) of TET. Altered susceptibility was most notable with exposure to amphotericin B (AMB), with a 32-fold increase in susceptibility, and terbinafine (TRB), with a 32-fold decrease in susceptibility. The TET/AMB synergy was observed in several clinical isolates of C. albicans and in the distantly related species Aspergillus fumigatus and Cryptococcus neoformans. The TET/AMB synergy is not related to efflux pump activity, as determined by FACS analyses and by analysis of a strain containing efflux pump deletions. Gene expression analyses by luciferase and by quantitative real-time reverse transcriptase PCR failed to identify significant alterations in expression of any genes associated with resistance. C. albicans grown with TET for 48 h does show a reduction in total cellular ergosterol. Analysis of growth curves suggests that the TET effect is associated with lack of a diauxic shift, which is related to a loss of mitochondrial function. MitoTracker fluorescent dye was used to demonstrate that TET has a direct effect on mitochondrial function. These results demonstrate the need for careful analysis of TET effects when using a TET-inducible promoter, especially in studies that involve antifungal drugs. This study defines some limits to the use of the TET-inducible promoter, and identifies effects on cells that are the result of TET exposure alone, not the result of expression of a targeted gene.

cis-Acting elements within the Candida albicans ERG11 promoter mediate the azole response through transcription factor Upc2p

The azole antifungal drugs are used to treat infections caused by Candida albicans and other fungi. These drugs interfere with the biosynthesis of ergosterol, the major sterol in fungal cells, by inhibiting an ergosterol biosynthetic enzyme, lanosterol 14 alpha-demethylase, encoded by the ERG11 gene. In vitro, these drugs as well as other ergosterol biosynthesis inhibitors increase ERG11 mRNA expression by activation of the ERG11 promoter. The signal for this activation most likely is the depletion of ergosterol, the end product of the pathway. To identify cis-acting regulatory elements that mediate this activation, ERG11 promoter fragments have been fused to the luciferase reporter gene from Renilla reniformis. Promoter deletions and linker scan mutations localized the region important for azole induction to a segment from bp -224 to -251 upstream of the start codon, specifically two 7-bp sequences separated by 13 bp. These sequences form an imperfect inverted repeat. The region is recognized by the transcription factor Upc2p and functions as an enhancer of transcription, as it can be placed upstream of a heterologous promoter in either direction, resulting in the azole induction of that promoter. The promoter constructs are not azole inducible in the upc2/upc2 homozygous deletion, demonstrating that Upc2p controls the azole induction of ERG11. These results identify an azole-responsive enhancer element (ARE) in the ERG11 promoter that is controlled by the Upc2p transcription factor. No other ARE is present in the promoter. Thus, this ARE and Upc2p are necessary and sufficient for azole induction of ERG11.

Defining Candida albicans stationary phase by cellular and DNA replication, gene expression and regulation

Stationary phase Candida albicans yeast cells harbour properties of better adherence, virulence and elevated drug resistance. C. albicans stationary phase is not well characterized in vitro either physiologically or molecularly. C. albicans yeast cells were grown in rich medium with 2% glucose. Based on growth and DNA profiles of cells, and by measurement of glucose and ethanol in the medium, we defined the timing of C. albicans entry into different growth transitions. We found that, compared with 24 h cells, mRNA content was less abundant in post-diauxic shift phase and even less in stationary phase C. albicans cells. Further analysis of the C. albicans transcriptome with oligonucleotide-based microarrays revealed that although the overall mRNA content had decreased, transcripts of many genes increased in post-diauxic shift phase as well as stationary phase. Genes involved in processes such as gluconeogenesis, stress resistance, adherence, DNA repair and ageing were expressed at higher levels at and beyond post-diauxic shift phase. Many C. albicans genes associated with virulence, drug resistance and cell-wall biosynthesis were expressed only at stationary phase. By screening 108 C. albicans transcription factor and cell-wall mutants we identified 17 genes essential for either entry or survival in stationary phase at 30 degrees C.

Genotypic evolution of azole resistance mechanisms in sequential Candida albicans isolates

TAC1 (for transcriptional activator of CDR genes) is critical for the upregulation of the ABC transporters CDR1 and CDR2, which mediate azole resistance in Candida albicans. While a wild-type TAC1 allele drives high expression of CDR1/2 in response to inducers, we showed previously that TAC1 can be hyperactive by a gain-of-function (GOF) point mutation responsible for constitutive high expression of CDR1/2. High azole resistance levels are achieved when C. albicans carries hyperactive alleles only as a consequence of loss of heterozygosity (LOH) at the TAC1 locus on chromosome 5 (Chr 5), which is linked to the mating-type-like (MTL) locus. Both are located on the Chr 5 left arm along with ERG11 (target of azoles). In this work, five groups of related isolates containing azole-susceptible and -resistant strains were analyzed for the TAC1 and ERG11 alleles and for Chr 5 alterations. While recovered ERG11 alleles contained known mutations, 17 new TAC1 alleles were isolated, including 7 hyperactive alleles with five separate new GOF mutations. Single-nucleotide-polymorphism analysis of Chr 5 revealed that azole-resistant strains acquired TAC1 hyperactive alleles and, in most cases, ERG11 mutant alleles by LOH events not systematically including the MTL locus. TAC1 LOH resulted from mitotic recombination of the left arm of Chr 5, gene conversion within the TAC1 locus, or the loss and reduplication of the entire Chr 5. In one case, two independent TAC1 hyperactive alleles were acquired. Comparative genome hybridization and karyotype analysis revealed the presence of isochromosome 5L [i(5L)] in two azole-resistant strains. i(5L) leads to increased copy numbers of azole resistance genes present on the left arm of Chr 5, among them TAC1 and ERG11. Our work shows that azole resistance was due not only to the presence of specific mutations in azole resistance genes (at least ERG11 and TAC1) but also to their increase in copy number by LOH and to the addition of extra Chr 5 copies. With the combination of these different modifications, sophisticated genotypes were obtained. The development of azole resistance in C. albicans is therefore a powerful instrument for generating genetic diversity.

[Antifungals cellular targets and mechanisms of resistance]

Antifungals of systemic use for the treatment of invasive fungal infections belong to four main chemical families which have globally three cellular targets in fungal cells: fluorinated pyrimidines act on deoxyribonucleic acid (DNA) replication and protein synthesis; polyenes and azoles are toxic for ergosterol and its biosynthetic pathway; lipopeptides inhibit the synthesis of cell wall beta glucans. The resistance mechanisms that are developed by some fungi begin to be well understood particularly in Candida yeasts. The underlying bases of these mechanisms are either mutations that modify the antifungal target, or that block access to the target, and, on the other hand, the overexpression of genes encoding the target, or some membrane proteins involved in the active efflux of antifungal drugs.

An update on antifungal targets and mechanisms of resistance in Candida albicans

Much progress has been made in the last decade in identifying genes responsible for antifungal resistance in Candida albicans. Attention has focused on five major C. albicans genes: ABC transporter genes CDR1 and CDR2, major facilitator efflux gene MDR1, and ergosterol biosynthesis genes ERG11 and ERG3. Resistance involves mutations in 14C-lanosterol demethylase, targeted by fluconazole (FLZ) and encoded by ERG11, and mutations that up-regulate efflux genes that probably efflux the antifungals. Mutations that affect ERG3 mutations have been understudied as mechanism resistance among clinical isolates. In vitro resistance in clinical isolates typically involves step-wise mutations affecting more than one of these genes, and often unidentified genes. Different approaches are needed to identify these other genes. Very little is understood about reversible adaptive resistance of C. albicans despite its potential clinical significance; most clinical failures to control infections other than oropharyngeal candidiasis (OPC) occur with in vitro susceptible strains. Tolerance of C. albicans to azoles has been attributed to the calcineurin stress-response pathway, offering new potential targets for next generation antifungals. Recent studies have identified genes that regulate CDR1 or ERG genes. The focus of this review is C. albicans, although information on Saccharomyces cerevisiae or Candida glabrata is provided in areas in where Candida research is underdeveloped. With the completion of the C. albicans genomic sequence, and new methods for high throughput gene overexpression and disruption, rapid progress towards understanding the regulation of resistance, novel resistance mechanisms, and adaptive resistance is expected in the near future.

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.

Role of Candida albicans transcription factor Upc2p in drug resistance and sterol metabolism

In Candida albicans, drug resistance to clinically important antifungal drugs may be regulated through the action of transcription factors in a manner that may or may not be similar to regulation in Saccharomyces cerevisiae. A search of the C. albicans genome identified a single homolog of the S. cerevisiae transcription factor genes UPC2 (ScUPC2) and ECM22 (ScECM22) that have been associated with regulation of ergosterol biosynthesis. Sequence analysis of this C. albicans UPC2 (CaUPC2) gene identifies two domains, an anchoring transmembrane domain and a transcription factor region containing multiple nuclear localization signals and a fungal Zn(2)-Cys(6) binuclear cluster domain. Heterozygous deletion, homozygous deletion, and reconstructed strains of CaUPC2 as well as the parental strain were tested against several antifungal drugs, including ergosterol biosynthesis inhibitors. The CaUPC2 homozygous deletion strain showed marked hypersusceptibility to most drugs, compared to the parental and reconstructed strains. The deletion strains accumulate significantly less radiolabeled cholesterol, suggesting reduced ergosterol scavenging in those strains. When grown under azole drug pressure, the parental, heterozygous deletion and reconstructed strains of CaUPC2 upregulate the ERG2 and ERG11 ergosterol biosynthesis genes, while the homozygous deletion strain shows no such upregulation. Consistent with these results, CaUPC2 deletion strains show reduced ergosterol levels, which may explain the increased susceptibilities of the CaUPC2 deletion strains. Thus, it appears that CaUPC2 acts as a transcription factor involved in the regulation of ergosterol biosynthetic genes and as a regulator of sterol uptake across the plasma membrane.