Organic cation transporter function in different in vitro models of human lung epithelium

Organic cation transporters (OCTs/OCTNs) in human primary alveolar epithelial cells

Alveolar epithelium, besides exerting a key role in gas exchange and surfactant production, plays important functions in host defense and inflammation. Pathological conditions associated to alveolar dysfunction include Acute Respiratory Distress Syndrome (ARDS), asthma, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). The use of predictive in vitro models of human alveolar epithelium is nowadays required for the study of disease mechanisms, as well as of pharmacokinetic parameters of pulmonary drugs delivery. Here, we employed a novel 3D model of human alveoli, namely EpiAlveolar™, consisting of primary alveolar epithelial cells, pulmonary endothelial cells and fibroblasts, that reflects properly the in vivo-like conditions. In EpiAlveolar™ we performed a characterization of Organic Cation Transporters (OCTs and OCTNs) expression and activity and we found that OCTN2, OCT1 and OCT3 are expressed on the basolateral membrane; instead, ATB^0,+ transporter for cationic and neutral amino acids, which shares with OCTN2 the affinity for carnitine as substrate, is readily detectable and functional at the apical side. We also show that these transporters differentially interact with anticholinergic drugs. Overall, our findings reveal close similarities of EpiAlveolar™ with the tracheal/bronchial epithelium (EpiAirway™ model) and entrust this alveolar tissue as a potential tool for the screening of biopharmaceuticals molecules.

In vitro and ex vivo models in inhalation biopharmaceutical research - advances, challenges and future perspectives

Oral inhalation results in pulmonary drug targeting and thereby reduces systemic side effects, making it the preferred means of drug delivery for the treatment of respiratory disorders such as asthma, chronic obstructive pulmonary disease or cystic fibrosis. In addition, the high alveolar surface area, relatively low enzymatic activity and rich blood supply of the distal airspaces offer a promising pathway to the systemic circulation. This is particularly advantageous when a rapid onset of pharmacological action is desired or when the drug is suffering from stability issues or poor biopharmaceutical performance following oral administration. Several cell and tissue-based in vitro and ex vivo models have been developed over the years, with the intention to realistically mimic pulmonary biological barriers. It is the aim of this review to critically discuss the available models regarding their advantages and limitations and to elaborate further which biopharmaceutical questions can and cannot be answered using the existing models.

Organic Cation Transporters in the Lung-Current and Emerging (Patho)Physiological and Pharmacological Concepts

Organic cation transporters (OCT) 1, 2 and 3 and novel organic cation transporters (OCTN) 1 and 2 of the solute carrier 22 (SLC22) family are involved in the cellular transport of endogenous compounds such as neurotransmitters, l-carnitine and ergothioneine. OCT/Ns have also been implicated in the transport of xenobiotics across various biological barriers, for example biguanides and histamine receptor antagonists. In addition, several drugs used in the treatment of respiratory disorders are cations at physiological pH and potential substrates of OCT/Ns. OCT/Ns may also be associated with the development of chronic lung diseases such as allergic asthma and chronic obstructive pulmonary disease (COPD) and, thus, are possible new drug targets. As part of the Special Issue "Physiology, Biochemistry and Pharmacology of Transporters for Organic Cations", this review provides an overview of recent findings on the (patho)physiological and pharmacological functions of organic cation transporters in the lung.

Organic Cation Transporters (OCTs) in EpiAirway™, A Cellular Model of Normal Human Bronchial Epithelium

Organic cation transporters (OCTs) and novel organic cation transporters (OCTNs) are responsible for drug delivery in the intestine and kidney; in the lung, OCTs mediate inhaled drugs’ transport, although their physiological role in airways remains poorly understood. The studies addressing OCTs/OCTNs in human airways were mostly performed in immortal or transformed cell lines; here, we studied OCTs in EpiAirway™, a recently developed in vitro model of normal bronchial epithelium. Calu-3 monolayers were used for comparison. The activity of OCTs was evaluated by measuring the uptake of 1-methyl-4-phenylpyridinium (MPP⁺) at the apical and basolateral side of monolayers and protein expression through Western Blot analysis. OCTs and OCTNs expression, along with that of Amino acid Transporter B^0,+ (ATB^0,+)transporter, was determined by measuring the number of mRNA molecules through quantitative Polymerase Chain Reaction (qPCR). The interaction of the transporters with bronchodilators was also assessed. Results highlight significant differences between Calu-3 cells and EpiAirway™, since, in the latter, OCTs are active only on the basolateral membrane where they interact with the bronchodilator ipratropium. No activity of OCTs is detectable at the apical side; there, the most abundant carrier is, instead, SLC6A14/ATB^0,+, that can thus be potentially listed among organic cation transporters responsible for drug delivery in the lung.

Comparison of Various Cell Lines and Three-Dimensional Mucociliary Tissue Model Systems to Estimate Drug Permeability Using an In Vitro Transport Study to Predict Nasal Drug Absorption in Rats

Recently, various types of cultured cells have been used to research the mechanisms of transport and metabolism of drugs. Although many studies using cultured cell systems have been published, a comparison of different cultured cell systems has never been reported. In this study, Caco-2, Calu-3, Madin–Darby canine kidney (MDCK), EpiAirway and MucilAir were used as popular in vitro cell culture systems, and the permeability of model compounds across these cell systems was evaluated to compare barrier characteristics and to clarify their usefulness as an estimation system for nasal drug absorption in rats. MDCK unexpectedly showed the best correlation (r = 0.949) with the fractional absorption (F_n) in rats. Secondly, a high correlation was observed in Calu-3 (r = 0.898). Also, Caco-2 (r = 0.787) and MucilAir (r = 0.750) showed a relatively good correlation with F_n. The correlation between F_n and permeability to EpiAirway was the poorest (r = 0.550). Because EpiAirway forms leakier tight junctions than other cell culture systems, the paracellular permeability was likely overestimated with this system. On the other hand, because MDCK formed such tight cellular junctions that compounds of paracellular model were less likely permeated, the paracellular permeability could be underestimated. Calu-3, Caco-2 and MucilAir form suitable cellular junctions and barriers, indicating that those cell systems enable the precise estimation of nasal drug absorption.

OCTN2-Mediated Acetyl-l-Carnitine Transport in Human Pulmonary Epithelial Cells In Vitro

The carnitine transporter OCTN2 is associated with asthma and other inflammatory diseases. The aims of this work were (i) to determine carnitine uptake into freshly isolated human alveolar type I (ATI)-like epithelial cells in primary culture, (ii) to compare the kinetics of carnitine uptake between respiratory epithelial in vitro cell models, and (iii) to establish whether any cell line was a suitable model for studies of carnitine transport at the air-blood barrier. Levels of time-dependent [³H]-acetyl-l-carnitine uptake were similar in ATI-like, NCl-H441, and Calu-3 epithelial cells, whereas uptake into A549 cells was ~5 times higher. Uptake inhibition was more pronounced by OCTN2 modulators, such as l-Carnitine and verapamil, in ATI-like primary epithelial cells compared to NCl-H441 and Calu-3 epithelial cells. Our findings suggest that OCTN2 is involved in the cellular uptake of acetyl-l-carnitine at the alveolar epithelium and that none of the tested cell lines are optimal surrogates for primary cells.

Reduced folate carrier-mediated methotrexate transport in human distal lung epithelial NCl-H441 cells

Objectives

We had previously found that reduced folate carrier (RFC; SLC19A1) is mainly involved in an influx of transport of methotrexate (MTX), a folate analogue, using alveolar epithelial A549 cells. Therefore, we examined MTX uptake in NCl-H441 (H441) cells, another in vitro alveolar epithelial model, focusing on the localization of RFC in the present study.

Methods

Transport function of RFC in H441 cells was studied using [3H]MTX.

Key findings

The uptake of MTX was increased remarkably after pretreatment of the cell monolayer with ethylenediaminetetraacetic acid (EDTA) in H441 cells but not in A549 cells, indicating the contribution of the basolaterally located transporter. In addition, folic acid and thiamine monophosphate, RFC inhibitors, inhibited the uptake of MTX from the basolateral side of the H441 cells. In order to compare the function of RFC on the apical and basolateral sides of the cells, the uptake of MTX from each side was examined using a Transwell chamber. Intracellular MTX amounts from the basolateral side were found to be significantly higher than those from the apical side.

Conclusions

These findings suggest that the distribution of MTX in the lung alveolar epithelial cells may be mediated by basolaterally located RFC in alveolar epithelial cells.

Expression analysis of human solute carrier (SLC) family transporters in nasal mucosa and RPMI 2650 cells

With nearly 400 members, the solute-linked carrier (SLC) superfamily is one of the most important gene classes concerning the disposition of drugs and the transport of physiological substrates in the human body. The mapping of related transport proteins is already well advanced for the intestines, kidneys and liver, but it has recently been brought into focus for various respiratory epithelia. The aim of this study was to evaluate the expression of several SLC transporters in differently cultured RPMI 2650 cells, as well as in specimens of the human nasal mucosa. The expression profiles of PEPT2, OATP1A2, OATP4C1, OCT2, OCTN1 and OCTN2 were investigated at the gene and protein levels by performing RT-PCR, western blot analysis and immunohistological staining. Uptake assays using appropriate substrates and inhibitory substances were performed to compare the activity of peptide, organic anion and organic cation transporters, respectively, among the three models. Expression of the six SLC transporters under investigation was confirmed at the mRNA and protein levels in human nasal mucosa ex vivo as well as in RPMI 2650 cells grown under different culture conditions. The functionality was almost equal among all of the models for the PEPT and OCT(N) transporters, while the functional activity of the OATP transporters was more pronounced for both in vitro models than for excised nasal tissue. Despite negligible variations in transporter capacities, the RPMI 2650 cell cultures and freshly isolated human nasal epithelium showed nearly comparable expression patterns for the examined SLC proteins. Therefore, in vitro models based on the RPMI 2650 cell line could provide helpful data during the preclinical investigation of intranasally administered drug formulations and in the development of strategies to target nasal drug transporters for either local or systemic drug delivery.

Novel therapeutic roles for surfactant-inositols and -phosphatidylglycerols in a neonatal piglet ARDS model: a translational study

The biological and immune-protective properties of surfactant-derived phospholipids and phospholipid subfractions in the context of neonatal inflammatory lung disease are widely unknown. Using a porcine neonatal triple-hit acute respiratory distress syndrome (ARDS) model (repeated airway lavage, overventilation, and LPS instillation into airways), we assessed whether the supplementation of surfactant (S; poractant alfa) with inositol derivatives [inositol 1,2,6-trisphosphate (IP3) or phosphatidylinositol 3,5-bisphosphate (PIP2)] or phosphatidylglycerol subfractions [16:0/18:1-palmitoyloleoyl-phosphatidylglycerol (POPG) or 18:1/18:1-dioleoyl-phosphatidylglycerol (DOPG)] would result in improved clinical parameters and sought to characterize changes in key inflammatory pathways behind these improvements. Within 72 h of mechanical ventilation, the oxygenation index (S+IP3, S+PIP2, and S+POPG), the ventilation efficiency index (S+IP3 and S+POPG), the compliance (S+IP3 and S+POPG) and resistance (S+POPG) of the respiratory system, and the extravascular lung water index (S+IP3 and S+POPG) significantly improved compared with S treatment alone. The inositol derivatives (mainly S+IP3) exerted their actions by suppressing acid sphingomyelinase activity and dependent ceramide production, linked with the suppression of the inflammasome nucleotide-binding domain, leucine-rich repeat-containing protein-3 (NLRP3)-apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)-caspase-1 complex, and the profibrotic response represented by the cytokines transforming growth factor-β1 and IFN-γ, matrix metalloproteinase (MMP)-1/8, and elastin. In addition, IκB kinase activity was significantly reduced. S+POPG and S+DOPG treatment inhibited polymorphonuclear leukocyte activity (MMP-8 and myeloperoxidase) and the production of interleukin-6, maintained alveolar-capillary barrier functions, and reduced alveolar epithelial cell apoptosis, all of which resulted in reduced pulmonary edema. S+DOPG also limited the profibrotic response. We conclude that highly concentrated inositol derivatives and phosphatidylglycerol subfractions in surfactant preparations mitigate key inflammatory pathways in inflammatory lung disease and that their clinical application may be of interest for future treatment of the acute exudative phase of neonatal ARDS.

Reversal of multidrug resistance in human lung cancer cells by delivery of 3-octadecylcarbamoylacrylic acid-cisplatin-based liposomes

Liposome-based drug delivery system would be an innovative and promising candidate to circumvent multidrug resistance (MDR) of cisplatin (CDDP). However, the reversal efficacy of liposomal CDDP was severely impaired by weak cellular uptake and insufficient intracellular drug release. In this study, 3-octadecylcarbamoylacrylic acid–CDDP nanocomplex (OMI–CDDP–N)-based liposomes (OCP-L) with high cellular uptake and sufficient intracellular drug release were designed to circumvent MDR of lung cancer. OMI–CDDP–N was synthesized through a pH-sensitive monocarboxylato and an O→Pt coordinate bond, which is more efficient than CDDP. Also, OCP-L incorporated with OMI–CDDP–N could induce effective cellular uptake, enhanced nuclear distribution, and optimal cellular uptake kinetics. In particular, OCP-L presented superior effects on enhancing cell apoptosis and in vitro cytotoxicity in CDDP-resistant human lung cancer (A549/CDDP) cells. The mechanisms of MDR reversal in A549/CDDP cells by OCP-L could attribute to organic cation transporter 2 restoration, ATPase copper-transporting beta polypeptide suppression, hypoxia-inducible factor 1 α-subunit depletion, and phosphatidylinositol 3-kinase/Akt pathway inhibition. These results demonstrated that OCP-L may provide an effective delivery of CDDP to resistant cells to circumvent MDR and enhance the therapeutic index of the chemotherapy.

Autologous Co-culture of Primary Human Alveolar Macrophages and Epithelial Cells for Investigating Aerosol Medicines. Part I: Model Characterisation

The development of new formulations for pulmonary drug delivery is a challenge on its own. New in vitro models which address the lung are aimed at predicting and optimising the quality, efficacy and safety of inhaled drugs, to facilitate the more rapid translation of such products into the clinic. Reducing the complexity of the in vivo situation requires that such models reproducibly reflect essential physiological factors in vitro. The choice of cell types, culture conditions and the experimental set-up, can affect the outcome and the relevance of a study. In the alveolar space of the lung, epithelial cells and alveolar macrophages are the most important cell types, forming an efficient cellular barrier to aerosols. Our aim was to mimic this barrier with primary human alveolar cells. Cell densities of alveolar macrophages and epithelial cells, isolated from the same human donor, were optimised, with a focus on barrier properties. The combination of 300,000 epithelial cells/cm2 together with 100,000 macrophages/cm2 showed a functional barrier (transepithelial electrical resistance > 500ω.cm2). This cell model was combined with the Pharmaceutical Aerosol Deposition Device on Cell Cultures. The functionality of the in vitro system was investigated with spray-dried fluorescently labelled poly(lactic-co-glycolic) acid particles loaded with ovalbumin as a model drug.