PHARMACOGENETICS OF MONOCARBOXYLATE TRANSPORTERS

Abstract

In this study, we studied the effect of genetic polymorphism and drug/food interaction on members of SLC superfamily, SLC16A family of proton-linked membrane transport proteins known as monocarboxylate transporters (MCTs). The genetic polymorphism in SLC16A1 and SLC16A3 genes that may be present in the ethnic Chinese (n = 95) and Indian (n = 96) groups of Singapore population have been examined. The promoter, coding region and exon-intron junctions of the SLC16A1 and SLC16A3 genes encoding the MCT1 and MCT4 transporters were screened for genetic variations by DNA sequencing. A total of 21 genetic variations, including 14 novel ones, were found in SLC16A1 gene. Of the 3 nonsynonymous variants, the 303T>G (Ile101Met) was predicted by PolyPhen and SIFT as having a potentially detrimental effect on MCT1 activity. In contrast, the 1282 G>A (Val428Ile) and 1470 T>A (Asp490) were speculated to be benign. A total of 46 genetic variants were detected in SLC16A3 gene, of which 33 are novel. Among the 5 nonsynonymous variants, only 44 C>T (Ala15Val) was predicted by both bioinformatics algorithms as having damaging effect on MCT4 protein function, whereas 55 G>A (Gly19Ser), 574G>A (Val192Met) and 916G>A (Gly306Ser) had conflicting results between the SIFT and PolyPhen programs. Lastly, the 641C>T (Ser214Phe) was predicted to be a benign variant.

A common MCT1 polymorphism, 1470 T>A (Asp490Glu), has been identified in both ethnic Chinese and Indian groups. Given that MCT1 is the only MCT isoform found in the human erythrocyte and contributes up to 90% of lactate influx, we chose to study human erythrocytes to investigate the effect of this polymorphism in the MCT1-mediated lactate influx. The 10-assay grouped data suggests that human MCT1 system exhibited a low affinity but high capacity transport mechanism for the transport of lactate into red blood cells. Besides, low interindividual variations were observed in the total- and MCT1-mediated lactate influx into human erythrocytes in the local Chinese population. In addition, the data shows that T allele of 1470 T>A (Asp490Glu) may have a defective effect in transport function and appear to have a lower transport activity. However, additional studies involving larger samples are required to verify the impact of 1470 T>A (Asp490Glu) polymorphism in MCT1 transporter activity.

The diet-interaction via MCT1 inhibition may affect lactate shuttling and drug disposition. Therefore, the effects of various dietary and pharmacological agents on MCT1 were examined on human erythrocyte lactate influx. Of these, diflunisal, diclofenac, mefenamic acid, meclofenamic acid, tolfenamic acid, luteolin, etodalac, phloretin and morin exhibited strong inhibition on MCT1-mediated L-lactate influx. The inhibitory concentrations of some of these tested NSAIDs are in the range of physiologically relevant concentrations that are achievable from the oral intake. Therefore, a clinical trial was designed to investigate the effect of one of the tested NSIADs, diflunisal, on lactate distribution between plasma and erythrocytes. In this study, the subjects were pre-treated with 5 oral doses of placebo or diflunisal (500mg) followed by a single dose infusion of sodium lactate at the infusion rate of 5.6mg/kg/min for 30 min. We observed small differences between placebo- and diflunisal-treated groups. These differences, however, were not statistically significant. The data suggests that given dose of diflunisal was insufficient toinhibit lactate transport in the blood for this study.