Mapping the ABC transporter landscape in the human lung: A comprehensive characterisation of P-gp, MRP1 and BCRP expression and functional activity in human lung epithelial cells

Active efflux transporters can play a substantial role in the drug disposition of their substrates in the human body. Efflux transporters like P-glycoprotein (P-gp), multidrug resistance associated protein 1 (MRP1) and breast cancer resistance protein (BCRP) were described to be expressed in the human lung. However, there is conflicting data reported regarding their expression and functional activity in the human lung epithelium in vitro. In this study, the expression and functional efflux of P-gp, MRP1 and BCRP was investigated in cell lines and primary cells derived from human upper (16HBE14o-, Calu-3, NHBE) and lower airways (NCI-H441, A549, hAELVi, Arlo and hAEpC). Additionally, the impact of culture condition, air liquid interface (ALI) vs liquid covered condition (LCC), on transporters' expression levels and efflux was evaluated. Gene and protein expression analysis revealed a ubiquitous expression of MRP1, while BCRP was present in most cells, except Calu-3, Arlo and hAEpC and P-gp was absent in all cells apart from Calu-3. Culturing the cells at ALI vs. LCC condition had only a significant impact on P-gp and MRP1 protein expression levels in Calu-3, which were both reduced at LCC. The functional activity of the expressed efflux transporters was demonstrated by the intracellular accumulation of fluorophore substrates in the absence and presence of transporter specific inhibitors. However, bidirectional transport studies using drug substrates did not result in any observed efflux mediated by MRP1 and BCRP across any of the lung epithelial cells. Only P-gp, expressed in Calu-3 at ALI, showed functional activity in this experimental setting. In addition, it was observed that not all tested cells are suitable for studying pulmonary drug disposition processes in vitro due to their inability to form a tight cell barrier. Overall, the results of this study indicate differences in drug transporters' expression levels across human lung epithelial cells. Moreover, the efflux of model fluorophores for P-gp, MRP1 and BCRP was determined, whereas no such efflux was observed for MRP1 and BCRP when using drugs as substrates.