Fluorescence-Based Transport Assays Revisited in a Human Renal Proximal Tubule Cell Line

Apical transport is key in renal function, and the activity of efflux transporters and receptor-mediated endocytosis is pivotal in this process. The conditionally immortalized proximal tubule epithelial cell line (ciPTEC) endogenously expresses these systems. Here, we used ciPTEC to investigate the activity of three major efflux transporters, viz. breast cancer resistance protein (BCRP), multidrug resistance protein 4 (MRP4) and P-glycoprotein (P-gp), as well as protein uptake through receptor-mediated endocytosis using a fluorescence-based setup for transport assays. To this end, cells were exposed to Hoechst33342, chloromethylfluorescein-diacetate (CMFDA) and calcein-AM in presence or absence of model inhibitors for BCRP (KO143), P-gp (PSC833) or MRP's (MK571). Overexpression cell lines MDCKII-BCRP and MDCKII-P-gp were used as positive controls and membrane vesicles over-expressing one transporter were used to determine substrate and inhibitor specificities. Receptor-mediated endocytosis was investigated by determining the intracellular accumulation of fluorescently labeled receptor associated protein (RAP-GST). In ciPTEC, BCRP and P-gp showed similar expressions and activities while MRP4 was more abundantly expressed. Hoechst33342, GS-MF and calcein are retained in the presence of KO143, MK571 and PSC833, showing clearly redundancy between the transporters. Noteworthy is the fact that both KO143 and MK571 can block BCRP, P-gp and MRP's, while PSC833 appears a potent inhibitor for BCRP and P-gp, but not the MRP's. Furthermore, ciPTEC accumulate RAP-GST in intracellular vesicles in a dose and time dependent manner, which was reduced in megalin-deficient cells. In conclusion, fluorescent probe-based assays are fast and reproducible in determining apical transport mechanisms, in vitro. We demonstrate that typical substrates and inhibitors are not specific for the designated transporters, reflecting the complex interactions that can take place in vivo. The set of tools we describe are also compatible with innovative kidney culture models, and allows studying transport mechanisms that are central to drug absorption, disposition and detoxification.