Aculeacin A resistant mutants of Candida albicans

Characterization of a lipopeptide-resistant strain of Candida albicans

Lipopeptides are antifungal agents that inhibit cell wall beta-(1,3)-glucan biosynthesis in fungal organisms. A mutant resistant to lipopeptides was generated by UV mutagenesis and characterized. The Candida albicans mutant (LP3-1) was stable and showed resistance specificity to a broad range of lipopeptides and certain glycolipid inhibitors. Other antifungal agents with diverse modes of action had a normal minimum inhibitory concentration profile for LP3-1 compared with the wild-type strain (CCH 442). In the in vitro beta-(1,3)-glucan synthase assay, both the lipopeptides and papulacandin-related agents had considerably higher 50% inhibitory concentration values in the LP3-1 strain than in the wild-type strain. In reconstitution assays, the resistance factor was associated with the integral membrane pellet rather than the peripheral GTP-binding protein. The LP3-1 strain had a membrane lipid profile similar to that of the parent strain and was virulent in a murine model of systemic candidiasis. Taken together, these results indicate that the resistance factor is associated with the integral membrane component of beta-(1,3)-glucan synthase. Lipopeptides are common antifungal agents encountered during screening of natural products. The LP3-1 strain was resistant to natural product extracts known to contain various lipopeptides. Thus, LP3-1 can be used in a dereplication assay.

Characterization of echinocandin-resistant mutants of Candida albicans: genetic, biochemical, and virulence studies

The pneumocandins are potent antifungal agents of the echinocandin class which are under development for use as broad-spectrum antimycotic therapy. One important consideration for any new therapeutic class for treating serious fungal infections is the potential for drug resistance development. In this study we have isolated and characterized four independent spontaneous Candida albicans mutants resistant to the potent semisynthetic pneumocandin L-733,560. These mutants have many of the properties of FKS1/ETG1 echinocandin-resistant mutants of Saccharomyces cerevisiae, including (i) cross-resistance to other 1,3-beta-D-glucan synthase inhibitors, such as papulacandin and echinocandins, but no change in sensitivity to other antifungal agents; (ii) in vitro glucan synthase activity that is more resistant to pneumocandins than the wild-type parent enzyme; and (iii) semidominant drug resistance in spheroplast fusion strains. The mutants were compared with C. albicans echinocandin-resistant mutants isolated by mutagenesis by L. Beckford and D. Kerridge (mutant M-2) (abstr. PS3.11, in Proceedings of the XI Congress of the International Society for Human and Animal Mycology, Montreal, Canada, 1992) and by A. Cassone, R. E. Mason, and D. Kerridge (mutant CA-2) (Sabouraudia 19:97-110, 1981). All of the strains had resistant enzyme activity in vitro. M-2 grew poorly and had low levels of enzyme activity. In contrast, CA-2 and the spontaneous mutants grew as well as the parents and had normal levels of glucan synthase activity. These results suggest that these resistant mutants may have alterations in glucan synthase. CA-2 was unable to form germ tubes, an ability retained by the spontaneous mutants. The virulence of the spontaneous mutants was unimpaired in a mouse model of disseminated candidiasis, while M-2 and CA-2 were 2 orders of magnitude less virulent than their parent strains. Significantly, mice challenged with the spontaneous mutant CAI4R1 responded therapeutically to lower levels of L-733,560 than would he predicted by the increase in in vitro susceptibility.

Inhibition of yeast (1,3)-beta-glucan synthase by phospholipase A2 and its reaction products

Fungal (1,3)-beta-glucan synthases are sensitive to a wide range of lipophilic inhibitors and it has been proposed that enzyme activity is highly sensitive to perturbations of the membrane environment. Yeast membranes were exposed to phospholipases and various lipophilic compounds, and the resultant effects on glucan synthase activity were ascertained. Glucan synthase from Saccharomyces cerevisiae was rapidly inactivated by phospholipase A2 (PLA2), and to a lesser extent by phospholipase C. Inactivation was time and dose-dependent and was protected against by EDTA and fatty-acid binding proteins (bovine and human serum albumins). Albumins also partially protected against inhibition by papulacandin B. PLA2 reaction products were structurally characterized and it was shown that fatty acids and lysophospholipids were the inhibitory moieties, with no novel inhibitory compounds apparent. Glucan synthase was inhibited by a range of fatty acids, monoglycerides and lysophospholipids. Inhibition by fatty acids was non-competitive, and progressive binding of [14C]oleic acid correlated with activity loss. Fluorescence anisotropy studies using diphenylhexatriene (DPH) confirm that fatty acids increase membrane fluidity. These results are consistent with proposals suggesting that glucan synthase inhibition is due in part to non-specific detergent-like disruption of the membrane environment, in addition to direct interactions of lipophilic inhibitors with specific target sites on the enzyme complex.

Use of cilofungin as direct fluorescent probe for monitoring antifungal drug-membrane interaction

Cilofungin is an antifungal cyclopeptide which inhibits cell wall (1,3)-beta-glucan biosynthesis in fungal organisms, and its action against Candida albicans (1,3)-beta-glucan synthase has been widely studied. Since glucan synthase inactivation is thought to partially result from perturbations of the membrane lipid environment, the interaction of cilofungin with fungal membranes and phosphatidylcholine membrane vesicles was studied. Cilofungin, which contains two independent aromatic groups, has an excitation maximum of 270 nm and an emission maximum of 317 nm in aqueous solution. Comparison of the fluorescence properties of cilofungin with those of the analogs pneumocandin B0, N-acetyl-tyrosinamide, and 4-hydroxybenzamide indicated that the emission of cilofungin largely derived from the p-octyloxybenzamide side chain. Microsomal membranes from Saccharomyces cerevisiae, C. albicans, and phosphatidylcholine membrane vesicles induced a blue shift in the cilofungin emission spectrum and increased the cilofungin steady-state emission anisotropy, providing direct evidence for a cilofungin-membrane interaction. Cilofungin interacted more strongly with membranes of C. albicans than with those of S. cerevisiae, correlating with previous findings that C. albicans is far more susceptible than S. cerevisiae to the action of cilofungin. These findings support the hypothesis that drug-induced inhibition of the (1,3)-beta-glucan synthesis results from the perturbation of the membrane environment and the interaction with the glucan synthase complex combined. The study demonstrated ways in which the fluorescence properties of drugs can be used to directly evaluate drug-membrane interactions and structure-activity relationships.

Isolation and characterization of Saccharomyces cerevisiae mutants resistant to aculeacin A

Aculeacin A is a lipopeptide that inhibits beta-glucan synthesis in yeasts. A number of Saccharomyces cerevisiae mutants resistant to this antibiotic were isolated, and four loci (ACR1, ACR2, ACR3, and ACR4) whose products are involved in the sensitivity to aculeacin A of yeast cells were defined. Mutants containing mutations in the four loci were also resistant to echinocandin B, another member of this lipopeptide family of antibiotics. In contrast, acr1, acr3, and acr4 mutants were resistant to papulacandin B (an antibiotic containing a disaccharide linked to two fatty acid chains that also inhibits beta-glucan synthesis), but acr2 mutants were susceptible to this antibiotic. This result defines common and specific steps in the entry and action of aculeacin A and papulacandin B. The analysis of double mutants revealed an epistatic effect of the acr2 mutation on the other three mutations. Cell walls of the four different mutants did not show significant alterations in composition with respect to the parental strain, and in vitro glucan synthase activity was also unaffected. However, cell surface hydrophobicity in three of the mutants was considerably decreased with respect to the parental strain.

Activity of cilofungin (LY 121019), a new lipopeptide antibiotic, on the cell wall and cytoplasmic membrane of Candida albicans. Structural modifications in scanning and transmission electron microscopy

Cilofungin, a new biosemisynthetic analog of echinocandin B, inhibits the synthesis of beta-(1,3)-glucan resulting in severe modifications of the cell wall and cytoplasmic membrane of sensitive organisms. The morphological modifications to budding yeast cells, pseudomycelium, mycelium and germ tubes of Candida albicans were studied by scanning and transmission electron microscopy after 3 and 16 h exposure to cilofungin. Changes in yeast cell morphology were apparent after 3 h in 0.1 microgram ml-1 cilofungin but were more marked in 1 and 10 micrograms ml-1 cilofungin. Most of the yeasts failed to separate and formed aggregates. Cracks and discontinuities were present in the cell wall and the cell membrane became undulated and fractured. Inclusions into the periplasmalemma space were observed, along with a release of cellular components. An important inhibition of germ tube formation was noted and the structure of true mycelium and pseudomycelium was severely modified. The budding area of yeast cells was particularly susceptible to damage by cilofungin.