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

Antifungal activity of redox-active benzaldehydes that target cellular antioxidation

Background

Disruption of cellular antioxidation systems should be an effective method for control of fungal pathogens. Such disruption can be achieved with redox-active compounds. Natural phenolic compounds can serve as potent redox cyclers that inhibit microbial growth through destabilization of cellular redox homeostasis and/or antioxidation systems. The aim of this study was to identify benzaldehydes that disrupt the fungal antioxidation system. These compounds could then function as chemosensitizing agents in concert with conventional drugs or fungicides to improve antifungal efficacy.

Methods

Benzaldehydes were tested as natural antifungal agents against strains of Aspergillus fumigatus, A. flavus, A. terreus and Penicillium expansum, fungi that are causative agents of human invasive aspergillosis and/or are mycotoxigenic. The yeast Saccharomyces cerevisiae was also used as a model system for identifying gene targets of benzaldehydes. The efficacy of screened compounds as effective chemosensitizers or as antifungal agents in formulations was tested with methods outlined by the Clinical Laboratory Standards Institute (CLSI).

Results

Several benzaldehydes are identified having potent antifungal activity. Structure-activity analysis reveals that antifungal activity increases by the presence of an ortho-hydroxyl group in the aromatic ring. Use of deletion mutants in the oxidative stress-response pathway of S. cerevisiae (sod1Δ, sod2Δ, glr1Δ) and two mitogen-activated protein kinase (MAPK) mutants of A. fumigatus (sakAΔ, mpkCΔ), indicates antifungal activity of the benzaldehydes is through disruption of cellular antioxidation. Certain benzaldehydes, in combination with phenylpyrroles, overcome tolerance of A. fumigatus MAPK mutants to this agent and/or increase sensitivity of fungal pathogens to mitochondrial respiration inhibitory agents. Synergistic chemosensitization greatly lowers minimum inhibitory (MIC) or fungicidal (MFC) concentrations. Effective inhibition of fungal growth can also be achieved using combinations of these benzaldehydes.

Conclusions

Natural benzaldehydes targeting cellular antioxidation components of fungi, such as superoxide dismutases, glutathione reductase, etc., effectively inhibit fungal growth. They possess antifungal or chemosensitizing capacity to enhance efficacy of conventional antifungal agents. Chemosensitization can reduce costs, abate resistance, and alleviate negative side effects associated with current antifungal treatments.

Screening for effectors that modify multidrug resistance in yeast

The yeast transcription factors Pdr1p and Pdr3p regulate the expression of several genes that encode energy-dependent efflux pumps involved in multidrug resistance. They recognize specific pleiotropic drug resistance elements in the promoters of the target gene such as PDR5 coding for a major multidrug transporter. Gain-of-function mutations in Pdr1p/Pdr3p result in over-expression of transporter genes and establishment of multidrug resistance. We developed a novel yeast-based screening procedure designed to detect compounds that specifically modify multidrug resistance due to an interference with the expression of drug efflux transporter genes. The screening is based on the ability to abrogate the growth defect of cells suffering from the galactose induced Pdr3p driven over-expression of a dominant-lethal allele of the PMA1 gene placed under the control of the PDR5 promoter. Validation of the assay was achieved by showing that growth inhibition was relieved by mutant Pdr3p devoid of activation domain. This screening system may also be used to select the loss-of-function pdr3 (or pdr1) mutants and to identify specific gene(s) whose over-expression or deletion will suppress the expression of multidrug transporters and increase the susceptibility of yeast cells to antifungals.

Cross-resistance to strobilurin fungicides in mitochondrial and nuclear mutants ofSaccharomyces cerevisiae

In yeast the resistance to kresoxim-methyl and azoxystrobin, like the resistance to strobilurin A (mucidin) is under the control of both mitochondrial cob gene and the PDR network of nuclear genes involved in multidrug resistance. The mucidin-resistant mucl (G137R) and muc2 (L275S) mutants of Saccharomyces cerevisiae containing point mutations in mtDNA were found to be cross-resistant to kresoxim-methyl and azoxystrobin. Cross-resistance to all three strobilurin fungicides was also observed in yeast transformants containing gain-of-function mutations in the nuclear PDR3 gene. On the other hand, nuclear mutants containing disrupted chromosomal copies of the PDR1 and PDR3 genes or the PDR5 gene alone were hypersensitive to kresoxim-methyl, azoxystrobin and strobilurin A. The frequencies of spontaneous mutants selected for resistance either to kresoxim-methyl, azoxystrobin or strobilurin A were similar and resulted from mutations both in mitochondrial and nuclear genes. The results indicate that resistance to strobilurin fungicides, differing in chemical structure and specific activity, can be caused by the same molecular mechanism involving changes in the structure of apocytochrome b and/or increased efflux of strobilurins from fungal cells.

Multidrug resistance as a dominant molecular marker in transformation of wine yeast

Pure wine yeast cultures are increasingly used in winemaking to perform controlled fermentations and produce wine of reproducible quality. For the genetic manipulation of natural wine yeast strains dominant selective markers are obviously useful. Here we demonstrate the successful use of the mutated PDR3 gene as a dominant molecular marker for the selection of transformants of prototrophic wine yeast Saccharomyces cerevisiae. The selected transformants displayed a multidrug resistance phenotype that was resistant to strobilurin derivatives and azoles used to control pathogenic fungi in agriculture and medicine, respectively. Random amplification of DNA sequences and electrophoretic karyotyping of the host and transformed strains after microvinification experiments resulted in the same gel electrophoresis patterns. The chemical and sensory analysis of experimental wines proved that the used transformants preserved all their useful winemaking properties indicating that the pdr3-9 allele does not deteriorate the technological properties of the transformed wine yeast strain.

Fungal ABC proteins: pleiotropic drug resistance, stress response and cellular detoxification

A number of prominent genetic diseases are caused by mutations in genes encoding ATP-binding cassette (ABC) proteins (Ambudkar, Gottesmann, 1998). Moreover, several mammalian ABC proteins such as P-glycoprotein (P-gp) (Gottesman et al., 1995) and multidrug-resistance-associated proteins (MRPs) (Cole, Deeley, 1998) have been implicated in multidrug resistance (MDR) phenotypes of tumor cells highly resistant to many different anticancer drugs. The characteristics of MDR phenomena include the initial resistance to a single anticancer drug, followed by the development of cross-resistance to many structurally and functionally unrelated drugs. Similar mechanisms of MDR exist in pathogenic fungi, including Candida and Aspergillus (Vanden Bossche et al., 1998), and also in parasites such as Plasmodium and Leishmania (Ambudkar, Gottesmann, 1998), as well as in many bacterial pathogens (Nikaido, 1998). To dissect the mechanisms of MDR development and to elucidate the physiological functions of ABC proteins, many efforts have been made during the past decade. Importantly, yeast orthologues of mammalian disease genes made this unicellular eukaryote an invaluable model system for studies on the molecular mechanisms of ABC proteins, in order to better understand and perhaps improve treatment of ABC gene-related disease. In this review, we provide an overview of ABC proteins and pleiotropic drug resistance in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. Furthermore, we discuss the role of ABC proteins in clinical drug resistance development of certain fungal pathogens.

Genome-wide expression patterns in Saccharomyces cerevisiae: comparison of drug treatments and genetic alterations affecting biosynthesis of ergosterol

Enzymes in the ergosterol-biosynthetic pathway are the targets of a number of antifungal agents including azoles, allylamines, and morpholines. In order to understand the response of Saccharomyces cerevisiae to perturbations in the ergosterol pathway, genome-wide transcript profiles following exposure to a number of antifungal agents targeting ergosterol biosynthesis (clotrimazole, fluconazole, itraconazole, ketoconazole, voriconazole, terbinafine, and amorolfine) were obtained. These profiles were compared to the transcript profiles of strains containing deletions of one of the late-stage ergosterol genes: ERG2, ERG5, or ERG6. A total of 234 genes were identified as responsive, including the majority of genes from the ergosterol pathway. Expression of several responsive genes, including ERG25, YER067W, and YNL300W, was also monitored by PCR over time following exposure to ketoconazole. The kinetics of transcriptional response support the conditions selected for the microarray experiment. In addition to ergosterol-biosynthetic genes, 36 mitochondrial genes and a number of other genes with roles related to ergosterol function were responsive, as were a number of genes responsive to oxidative stress. Transcriptional changes related to heme biosynthesis were observed in cells treated with chemical agents, suggesting an additional effect of exposure to these compounds. The expression profile in response to a novel imidazole, PNU-144248E, was also determined. The concordance of responsive genes suggests that this compound has the same mode of action as other azoles. Thus, genome-wide transcript profiles can be used to predict the mode of action of a chemical agent as well as to characterize expression changes in response to perturbation of a metabolic pathway.