Statin Inhibition of Monocarboxylate Transporter (MCT)-Mediated Lactate Transport in HK-2 Cells

NS Bakar1, SE Jenkinson2, F Kamali0,1, CDA Brown0,2. 1Institute of Cellular Medicine, Medical School, Newcastle University, Newcastle Upon Tyne,NE2 4HH, UK, 2Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle Upon Tyne,NE2 4HH, UK

Introduction: 3-Hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (statins) successfully lower LDL-cholesterol and the risk of atherosclerosis. They are generally considered to be safe, although statin-induced myopathy is relatively common. Membrane transporters may play a crucial role in determining statin side effects. However, little is known regarding the interaction of monocarboxylate transporters (MCT) with statins.

Study aims: To investigate whether statins affect MCT function in human proximal tubular epithelial cells (HK-2).

Methods: The presence of MCT1 (transcribed by SLC16A1 gene) in HK-2 cells at mRNA level was confirmed by quantitative Polymerase Chain Reaction (qPCR) array.Initial rates of 3H labelled DL-lactate uptake (1µCi/mL) were used to measure MCT1 function. The inhibition of [3H]-DL-lactate uptake was assessed in the presence or absence of a range of statins and compared to that of the well defined MCT inhibitors, phloretin and α-cyano-4-hydroxycinnamate (CHC).

Results: In the functional assay, initial rate of uptake of [3H]-DL-lactate (100µM) into HK-2 cells was shown to be both proton- (769 ±144 pmol mg-1 min-1 at pH 5.5 vs 98 ± 23 pmol mg-1 min-1 at pH 7.4, p<0.001) and Na+-dependent with the Vmax (at pH 5.5) in the presence of Na+ (769 ± 144 pmol mg-1 min-1) significantly higher (p<0.05) compared to that seen in the absence of Na+ (203 ± 108 pmol mg-1 min-1). That [3H]-DL-lactate uptake was significantly higher in the presence of Na+ initially suggested the existence of the sodium-coupled MCT1 (SMCT1; SLC5A8) in the HK-2 cells. However, qPCR analysis demonstrated no SMCT1 expression at mRNA level in HK-2 cells and the effect of Na+ upon uptake relates to maintenance of proton gradient across cell membrane by Na+/H+ exchange. Consistent with expression of MCT1, lactate uptake was significantly inhibited in the presence of phloretin and CHC by 84.02 ± 2.4% and 41.87 ± 10.6 %, respectively (p<0.0001). Lactate uptake was also inhibited in the presence of simvastatin, atorvastatin or rosuvastatin with apparent Ki values of 0.43 ± 0.2 mM, 0.59 ± 0.3 mM and 3.06 ± 1.1 mM, respectively. In contrast, pravastatin had no significant inhibitory effect on MCT-mediated lactate uptake.

Conclusions: These findings suggest that lipophilic statins; atorvastatin and simvastatin may be potential inhibitors of MCT transporters and exhibit relatively higher affinity to MCT1 than the less lipophilic statins; rosuvastatin and pravastatin.