Synchrotron-based X-ray fluorescence microscopy reveals accumulation of polymyxins in single human alveolar epithelial cells

Tobramycin Reduces Pulmonary Toxicity of Polymyxin B via Inhibiting the Megalin-Mediated Drug Uptake in the Human Lung Epithelial Cells

Accumulation of polymyxins in the lung epithelial cells can lead to increased mitochondrial oxidative stress and pulmonary toxicity. Aminoglycosides and polymyxins are used, via intravenous and pulmonary delivery, against multidrug-resistant Gram-negative pathogens. Our recent in vitro and animal studies demonstrated that the co-administration of polymyxins with aminoglycosides decreases polymyxin-induced pulmonary toxicity. The aim of this study was to investigate the in vitro transport and uptake of polymyxin B and tobramycin in human lung epithelial Calu-3 cells and the mechanism of reduced pulmonary toxicity resulting from this combination. Transport, intracellular localization, and accumulation of polymyxin B and tobramycin were investigated using doses of 30 mg/L polymyxin B, 70 mg/L tobramycin, and the combination of both. Adding tobramycin significantly (p < 0.05) decreased the polymyxin B-induced cytotoxicity in Calu-3 cells. The combination treatment significantly reduced the transport and uptake of polymyxin B and tobramycin in Calu-3 cells, compared to each drug alone, which supported the reduced pulmonary toxicity. We hypothesized that cellular uptake of polymyxin B and tobramycin shared a common transporter, megalin. We further investigated the megalin expression of Calu-3 cells using confocal microscopy and evaluated megalin activity using a megalin substrate, FITC-BSA, and a megalin inhibitor, sodium maleate. Both polymyxin B and tobramycin significantly inhibited FITC-BSA uptake by Calu-3 cells in a concentration-dependent manner. Sodium maleate substantially inhibited polymyxin B and tobramycin transport and cellular accumulation in the Calu-3 cell monolayer. Our study demonstrated that the significantly reduced uptake of polymyxin B and tobramycin in Calu-3 cells is attributed to the mechanism of action that determines that polymyxin B and tobramycin share a common transporter, megalin.

The use of synchrotron X-ray fluorescent imaging to study distribution and content of elements in chemically fixed single cells: a case study using mouse pancreatic beta-cells

Synchrotron X-ray fluorescence microscopy (SXRF) presents a valuable opportunity to study the metallome of single cells because it simultaneously provides high-resolution subcellular distribution and quantitative cellular content of multiple elements. Different sample preparation techniques have been used to preserve cells for observations with SXRF, with a goal to maintain fidelity of the cellular metallome. In this case study, mouse pancreatic beta-cells have been preserved with optimized chemical fixation. We show that cell-to-cell variability is normal in the metallome of beta-cells due to heterogeneity and should be considered when interpreting SXRF data. In addition, we determined the impact of several immunofluorescence (IF) protocols on metal distribution and quantification in chemically fixed beta-cells and found that the metallome of beta-cells was not well preserved for quantitative analysis. However, zinc and iron qualitative analysis could be performed after IF with certain limitations. To help minimize metal loss using samples that require IF, we describe a novel IF protocol that can be used with chemically fixed cells after the completion of SXRF.

Polymyxin Induces Significant Transcriptomic Perturbations of Cellular Signalling Networks in Human Lung Epithelial Cells

Inhaled polymyxins are increasingly used to treat pulmonary infections caused by multidrug-resistant Gram-negative pathogens. We have previously shown that apoptotic pathways, autophagy and oxidative stress are involved in polymyxin-induced toxicity in human lung epithelial cells. In the present study, we employed human lung epithelial cells A549 treated with polymyxin B as a model to elucidate the complex interplay of multiple signalling networks underpinning cellular responses to polymyxin toxicity. Polymyxin B induced toxicity (1.0 mM, 24 h) in A549 cells was assessed by flow cytometry and transcriptomics was performed using microarray. Polymyxin B induced cell death was 19.0 ± 4.2% at 24 h. Differentially expressed genes (DEGs) between the control and polymyxin B treated cells were identified with Student’s t-test. Pathway analysis was conducted with KEGG and Reactome and key hub genes related to polymyxin B induced toxicity were examined using the STRING database. In total we identified 899 DEGs (FDR < 0.01), KEGG and Reactome pathway analyses revealed significantly up-regulated genes related to cell cycle, DNA repair and DNA replication. NF-κB and nucleotide-binding oligomerization domain-like receptor (NOD) signalling pathways were identified as markedly down-regulated genes. Network analysis revealed the top 5 hub genes (i.e., degree) affected by polymyxin B treatment were PLK1(48), CDK20 (46), CCNA2 (42), BUB1 (40) and BUB1B (37). Overall, perturbations of cell cycle, DNA damage and pro-inflammatory NF-κB and NOD-like receptor signalling pathways play key roles in polymyxin-induced toxicity in human lung epithelial cells. Noting that NOD-like receptor signalling represents a group of key sensors for microorganisms and damage in the lung, understanding the mechanism of polymyxin-induced pulmonary toxicity will facilitate the optimisation of polymyxin inhalation therapy in patients.

Polymyxin-Induced Metabolic Perturbations in Human Lung Epithelial Cells

Inhaled polymyxins are associated with toxicity in human lung epithelial cells that involves multiple apoptotic pathways. However, the mechanism of polymyxin-induced pulmonary toxicity remains unclear. This study aims to investigate polymyxin-induced metabolomic perturbations in human lung epithelial A549 cells. A549 cells were treated with 0.5 or 1.0 mM polymyxin B or colistin for 1, 4, and 24 h. Cellular metabolites were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS), and significantly perturbed metabolites (log₂ fold change [log₂FC] ≥ 1; false-discovery rate [FDR] ≤ 0.2) and key pathways were identified relative to untreated control samples. At 1 and 4 h, very few significant changes in metabolites were observed relative to the untreated control cells. At 24 h, taurine (log₂FC = -1.34 ± 0.64) and hypotaurine (log₂FC = -1.20 ± 0.27) were significantly decreased by 1.0 mM polymyxin B. The reduced form of glutathione (GSH) was significantly depleted by 1.0 mM polymyxin B at 24 h (log₂FC = -1.80 ± 0.42). Conversely, oxidized glutathione (GSSG) was significantly increased by 1.0 mM both polymyxin B (log₂FC = 1.38 ± 0.13 at 4 h and 2.09 ± 0.20 at 24 h) and colistin (log₂FC = 1.33 ± 0.24 at 24 h). l-Carnitine was significantly decreased by 1.0 mM of both polymyxins at 24 h, as were several key metabolites involved in biosynthesis and degradation of choline and ethanolamine (log₂FC ≤ -1); several phosphatidylserines were also increased (log₂FC ≥ 1). Polymyxins perturbed key metabolic pathways that maintain cellular redox balance, mitochondrial β-oxidation, and membrane lipid biogenesis. These mechanistic findings may assist in developing new pharmacokinetic/pharmacodynamic strategies to attenuate the pulmonary toxicities of inhaled polymyxins and in the discovery of new-generation polymyxins.