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The Role of Monocarboxylate Transporter 2 and 4 in the Transport of -Hydroxybutyric Acid in Mammalian Cells

Department of Pharmaceutical Sciences, School of Pharmacy and Pharmaceutical Sciences, University at Buffalo, State University of New York, Buffalo, New York

Monocarboxylate transporter 1 (MCT1) is an important determinant of the renal transport of the drug of abuse, -hydroxybutyric acid (GHB). The objective of this study was to investigate the role of MCT2 and MCT4, present in tissues including intestine, kidney, skeletal muscle, and brain, in the membrane transport of GHB and the MCT substrate L-lactate. mRNA and protein of MCT2 and MCT4 were expressed in MDA-MB231 cells, as detected by reverse transcription-polymerase chain reaction and Western blot analysis; MCT1 and MCT3 were not detected. The uptake of GHB or L-lactate by MDA-MB231 cells was pH-dependent but not sodium-dependent. The concentration-dependent uptake of GHB was best fitted to a single-transporter model with a diffusional clearance component (K_m of 17.6 ± 1.5 mM, V_max of 50.6 ± 9.0 nmol · mg^–1 min^–1 and diffusional clearance of 0.20 ± 0.07 µl · mg^–1 min^–1). On the other hand, the concentration-dependent uptake of L-lactate was best fitted to a two-transporter model (K_m of 21 ± 2.5 and 3.0 ± 1.5 mM, and V_max of 268 ± 72 and 62.9 ± 42.2 nmol · mg^–1min^–1, respectively). The uptake of GHB and L-lactate was inhibited by MCT inhibitors -cyano-4-hydroxycinnamate (CHC), phloretin, and p-chloromercuribenzoic acid; CHC inhibited GHB and L-lactate uptake with IC₅₀ values of 1.71 ± 0.39 and 0.71 ± 0.11 mM, respectively. Small interfering RNA treatment to silence MCT2 or MCT4 significantly decreased their protein expression and the uptake of L-lactate and GHB; however, the decrease in GHB uptake with MCT2 inhibition was smaller than that for MCT4. This investigation demonstrated that GHB is a substrate for both MCT2 and MCT4; these transporters may be important in the nonlinear disposition of GHB, as well as influencing its tissue distribution.