Metabolic Effects of Blocking Lactate Transport in Brain Cortical Tissue Slices Using an Inhibitor Specific to MCT1 and MCT2

Abstract

A novel inhibitor of lactate transport, AR-C122982, was used to study the effect of inhibiting the monocarboxylate transporters MCT1 and MCT2 on cortical brain slice metabolism. We studied metabolism of L-[3-13C]lactate, and D-[1-13C]glucose under a range of conditions. Experiments using L-[3-13C]lactate showed that the inhibitor AR-C122982 altered exchange of lactate. Under depolarizing conditions, net flux of label from D-[1-13C]glucose was barely altered by 10 or 100 nM AR-C122982. In the presence of AMPA or glutamate there were increases in net flux of label and metabolic pool sizes. These data suggest lactate may supply compartments in the brain not usually directly accessed by glucose. In general, it would appear that movement of lactate between cell types is not essential for metabolic activity, with the heavy metabolic workloads imposed being unaffected by inhibition of MCT1 and MCT2. Further experiments investigating the mechanism of operation of AR-C122982 are necessary to corroborate this finding.

The inhibition of monocarboxylate transporter 2 (MCT2) by AR-C155858 is modulated by the associated ancillary protein

“…In mammalian cells, embigin is the preferred endogenous binding partner of MCT2 [22] and thus MCT2-mediated lactate transport is likely to be considerably less sensitive to inhibition by AR-C155858 than that mediated by MCT1. This may justify the cautious use of AR-C155858 to dissect out the different roles of MCT1 (very sensitive to AR-C155858), MCT2 (less sensitive to AR-C155858) and MCT4 (insensitive to AR-C155858) in the metabolism of tissues such as the brain, as has been reported by Bröer and colleagues [41]. However, the development of totally isoform-specific inhibitors of MCTs that are not influenced by the associated ancillary protein is clearly desirable.…”

AR-C155858 is a potent inhibitor of monocarboxylate transporters MCT1 and MCT2 that binds to an intracellular site involving transmembrane helices 7–10

“…In this context it may be noted that inhibition of MCT1 and MCT2 by the organomercurial reagent p-chloromercuribenzene sulfonate also depends on the associated ancillary protein [38]. Our results would also imply that interpretation of metabolic studies in which AR-C122982 has been used to discriminate between MCT1- and MCT2-mediated lactate transport [49] may not be straightforward.…”

Pyruvate Carboxylation in Different Model Systems Studied by 13C MRS

“…In experiments with brain tissue slices metabolizing [1- 13 C]glucose it was possible to show evidence of pyruvate carboxylation in aspartate [50]. Increased carboxylation was detected when the slices were incubated with AMPA or exogenous glutamate [50] suggesting that increased neuronal work increases carboxylation. These results contrast with the above mentioned results using K + depolarization where no increased carboxylation was detected and where the workload is greater in astrocytes.…”

“…These results contrast with the above mentioned results using K + depolarization where no increased carboxylation was detected and where the workload is greater in astrocytes. Interestingly, inhibition of the monocarboxylate transporters MCT1 and MCT2 using AR-C122982 at 10 nM increased carboxylation significantly but increasing the inhibition further by using 100 nM AR-C122982 reduced carboxylation [50]. …”