Emerging Role of Sphingolipids in Amphotericin B Drug Resistance

Molecular mechanism of host-yeast interactions and prevention by nanoformulation approaches

Fungal infections are a major source of morbidity and mortality in people with compromised immune systems, such as those with human immunodeficiency virus, cancer, organ transplant recipients, and patients undergoing chemotherapy in healthcare settings. According to a recent World Health Organization (WHO) fungal priority pathogens list, Cryptococcus spp., Candida spp., Aspergillus spp., and Candida auris cause severe invasive infections in human. These opportunistic pathogens cause a significant number of mycoses, which affect over a billion people annually. Around two million infections can be fatal, especially for those with compromised immune systems. To diagnose and treat mycoses, we need to understand the complex interactions between the fungus and the host during pathogenesis, the virulence-causing traits of the fungus, and how the host fights infection through the immune system. Although several antifungal drugs are available against fungal infections, their effectiveness is highly variable, with adverse effects. In addition, the increasing resistance to traditional antifungal treatments poses serious risks to the healthcare industry. Therefore, new therapeutic strategies are required to combat these potentially fatal fungal infections. Nanostructure-based formulations can improve the therapeutic efficacy of conventional medications by broadening their activities, decreasing toxicity, enhancing bioactivity, and improving biodistribution. The review highlights host and fungus interaction and how nanoformulations can be targeted against fungal infections.

The proteomic response of Aspergillus fumigatus to amphotericin B (AmB) reveals the involvement of the RTA-like protein RtaA in AmB resistance

The polyene antimycotic amphotericin B (AmB) and its liposomal formulation AmBisome belong to the treatment options of invasive aspergillosis caused by Aspergillus fumigatus. Increasing resistance to AmB in clinical isolates of Aspergillus species is a growing concern, but mechanisms of AmB resistance remain unclear. In this study, we conducted a proteomic analysis of A. fumigatus exposed to sublethal concentrations of AmB and AmBisome. Both antifungals induced significantly increased levels of proteins involved in aromatic acid metabolism, transmembrane transport, and secondary metabolite biosynthesis. One of the most upregulated proteins was RtaA, a member of the RTA-like protein family, which includes conserved fungal membrane proteins with putative functions as transporters or translocases. Accordingly, we found that RtaA is mainly located in the cytoplasmic membrane and to a minor extent in vacuolar-like structures. Deletion of rtaA led to increased polyene sensitivity and its overexpression resulted in modest resistance. Interestingly, rtaA expression was only induced by exposure to the polyenes AmB and nystatin, but not by itraconazole and caspofungin. Orthologues of rtaA were also induced by AmB exposure in A. lentulus and A. terreus. Deletion of rtaA did not significantly change the ergosterol content of A. fumigatus, but decreased fluorescence intensity of the sterol-binding stain filipin. This suggests that RtaA is involved in sterol and lipid trafficking, possibly by transporting the target ergosterol to or from lipid droplets. These findings reveal the contribution of RtaA to polyene resistance in A. fumigatus, and thus provide a new putative target for antifungal drug development.

Insights into the role of sterol metabolism in antifungal drug resistance: a mini-review

Sterols are essential for eukaryotic cells and are crucial in cellular membranes' structure, function, fluidity, permeability, adaptability to environmental stressors, and host-pathogen interactions. Fungal sterol, such as ergosterol metabolism, involves several organelles, including the mitochondria, lipid droplets, endoplasmic reticulum, and peroxisomes that can be regulated mainly by feedback mechanisms and transcriptionally. The majority of sterol transport in yeast occurs via non-vesicular transport pathways mediated by lipid transfer proteins, which determine the quantity of sterol present in the cell membrane. Pathogenic fungi Candida, Aspergillus, and Cryptococcus species can cause a range of superficial to potentially fatal systemic and invasive infections that are more common in immunocompromised patients. There is a significant risk of morbidity and mortality from these infections, which are very difficult to cure. Several antifungal drugs with different modes of action have received clinical approval to treat fungal infections. Antifungal drugs targeting the ergosterol biosynthesis pathway are well-known for their antifungal activity; however, an imbalance in the regulation and transport of ergosterol could lead to resistance to antifungal therapy. This study summarizes how fungal sterol metabolism and regulation can modulate sterol-targeting antifungal drug resistance.

Insights into the antifungal mechanism of Bacillus subtilis cyclic lipopeptide iturin A mediated by potassium ion channel

Fungal infections pose severe and potentially lethal threats to plant, animal, and human health. Ergosterol has served as the primary target for developing antifungal medications. However, many antifungal drugs remain highly toxic to humans due to similarity in cell membrane composition between fungal and animal cells. Iturin A, lipopeptide produced by Bacillus subtilis, efficiently inhibit various fungi, but demonstrated safety in oral administration, indicating the existence of targets different from ergosterol. To pinpoint the exact antifungal target of iturin A, we used homologous recombination to knock out and overexpress erg3, a key gene in ergosterol synthesis. Saccharomyces cerevisiae and Aspergillus carbonarius were transformed using the LiAc/SS-DNNPEG and Agrobacterium-mediated transformation (AMT), respectively. Surprisingly, increasing ergosterol content did not augment antifungal activity. Furthermore, iturin A's antifungal activity against S. cerevisiae was reduced while it pre-incubation with voltage-gated potassium (Kv) channel inhibitor, indicating that Kv activation was responsible for cell death. Iturin A was found to activate the Kv protein, stimulating K+ efflux from cell. In vitro tests confirmed interaction between iturin A and Kv protein. This study highlights Kv as one of the precise targets of iturin A in its antifungal activity, offering a novel target for the development of antifungal medications.

Overexpression of PDR16 Confers Amphotericin B Resistance in a PMP3-Dependent Manner in Yeast Saccharomyces cerevisiae

Invasive fungal infections in humans with compromised immune systems are the primary cause of morbidity and mortality, which is becoming more widely acknowledged. Amphotericin B (AmB) is one of the antifungal drugs used to treat such infections. AmB binds with plasma membrane ergosterol, inducing cellular ions to leak and causing cell death. Reduction in ergosterol content and modification of cell walls have been described as AmB resistance mechanisms. In addition, when the sphingolipid level is decreased, the cell becomes more susceptible to AmB. Previously, PDR16, a gene that encodes phosphatidylinositol transfer protein in Saccharomyces cerevisiae, was shown to enhance AmB resistance upon overexpression. However, the mechanism of PDR16-mediated AmB resistance is not clear. Here, in this study, it was discovered that a plasma membrane proteolipid 3 protein encoded by PMP3 is essential for PDR16-mediated AmB resistance. PDR16-mediated AmB resistance does not depend on ergosterol, but a functional sphingolipid biosynthetic pathway is required. Additionally, PMP3-mediated alteration in membrane integrity abolishes PDR16 mediated AmB resistance, confirming the importance of PMP3 in the PDR16 mediated AmB resistance.

Antifungal drug resistance in Candida: a special emphasis on amphotericin B

Invasive fungal infections in humans caused by several Candida species, increased considerably in immunocompromised or critically ill patients, resulting in substantial morbidity and mortality. Candida albicans is the most prevalent species, although the frequency of these organisms varies greatly according to geographic region. Infections with C. albicans and non-albicans Candida species have become more common, especially in the past 20 years, as a result of aging, immunosuppressive medication use, endocrine disorders, malnourishment, extended use of medical equipment, and an increase in immunogenic diseases. Despite C. albicans being the species most frequently associated with human infections, C. glabrata, C. parapsilosis, C. tropicalis, and C. krusei also have been identified. Several antifungal drugs with different modes of action are approved for use in clinical settings to treat fungal infections. However, due to the common eukaryotic structure of humans and fungi, only a limited number of antifungal drugs are available for therapeutic use. Furthermore, drug resistance in Candida species has emerged as a result of the growing use of currently available antifungal drugs against fungal infections. Amphotericin B (AmB), a polyene class of antifungal drugs, is mainly used for the treatment of serious systemic fungal infections. AmB interacts with fungal plasma membrane ergosterol, triggering cellular ion leakage via pore formation, or extracting the ergosterol from the plasma membrane inducing cellular death. AmB resistance is primarily caused by changes in the content or structure of ergosterol. This review summarizes the antifungal drug resistance exhibited by Candida species, with a special focus on AmB.