The limitations of renal epithelial cell line HK-2 as a model of drug transporter expression and function in the proximal tubule

Acquiring a mechanistic understanding of the processes underlying the renal clearance of drug molecules in man has been hampered by a lack of robust in vitro models of human proximal tubules. Several human renal epithelial cell lines derived from the renal cortex are available, but few have been characterised in detail in terms of transporter expression. This includes the HK-2 proximal tubule cell line, which has been used extensively as a model of nephrotoxicity. The aim of this study was to investigate the expression and function of drug transporters in HK-2 cells and their suitability as an in vitro model of the human proximal tubule. qPCR showed no mRNA expression of the SLC22 transporter family (OAT1, OAT3, OCT2) in HK-2 cells compared to renal cortex samples. In contrast, SLC16A1 (MCT1), which is important in the uptake of monocarboxylates, and SLCO4C1 (OATP4C1) were expressed in HK-2 cells. The functional expression of these transporters was confirmed by uptake studies using radiolabelled prototypic substrates dl-lactate and digoxin, respectively. The mRNA expression of apical membrane efflux transporters ABCB1 (MDR1) and several members of the ABCC family (multidrug resistance proteins, MRPs) was shown by qPCR. ABCG1 (BCRP) was not detected. The efflux of Hoechst 33342, a substrate for MDR1, was blocked by MDR1 inhibitor cyclosporin A, suggesting the functional expression of this transporter. Similarly, the efflux of the MRP-specific fluorescent dye glutathione methylfluorescein was inhibited by the MRP inhibitor MK571. Taken together, the results of this study suggest that HK-2 cells are of limited value as an in vitro model of drug transporter expression in the human proximal tubule.