Characterization of monocarboxylate transport in human kidney HK-2 cells

The objectives of this study were to characterize the expression and function of monocarboxylate transporters (MCTs) in human kidney HK-2 cells and to compare the expression of MCTs in HK-2 cells to that found in human kidney. mRNA and protein expression of MCTs were determined by RT-PCR and Western analyses, respectively, while immunofluorescence staining was used to determine the membrane localization of MCT1. The driving force, transport kinetics, and inhibition of two MCT substrates, D-lactate and butyrate, were characterized in HK-2 cells. mRNA of MCT1, -2, -3, -4 isoforms were present in HK-2 cells and in human kidney cortex. MCT1 was present predominantly on the basal membranes of HK-2 cells. The cellular uptake of D-lactate and butyrate exhibited pH- and concentration-dependence (D-lactate, K_m of 26.5 ± 2.2 mM and V_max of 72.0 ± 14.5 nmol mg^-1 min^-1; butyrate, K_m of 0.8 ± 0.3 mM, V_max of 29.3 ± 2.5 nmol mg^-1 min^-1, and a diffusional clearance of 2.1 μL mg^-1 min^-1). The uptake of D-lactate and butyrate by HK-2 cells was inhibited by MCT analogues and the classical MCT inhibitors α-cyano-4-hydroxycinnamate, pCMB, and phloretin. The uptake of D-lactate and butyrate by HK-2 cells significantly decreased after transfection with small-interference RNA for MCT1. In summary, MCTs were present in both HK-2 cells and human kidney cortex, and HK-2 cells exhibited polarized MCT expression and pH-dependent transport of D-lactate and butyrate. Our results also support the usefulness of HK-2 cells as an in vitro model for studying monocarboxylate transport in renal proximal tubule cells.