Monocarboxylate transporter (MCT) isoforms 1, 2 and 4 transport lactate in an electroneutral cotransport of 1:1 with H⁺. We could recently show that transport activity of MCT1/4 is enhanced by cytosolic carbonic anhydrase (CA) isoform II (1-4). Transport activity of MCT2 on the other hand, is not augmented by CAII, but instead by extracellular, membrane-bound CAIV (5).

The aim of the present study was to investigate possible interaction between the tumor-associated, extacellular CAIX and different MCTs. Therefore, MCT 1, 2 and 4 were heterologously expressed in Xenopus laevis oocytes, either alone or together with CAIX, and activity of the proteins was determined by measuring changes in intracellular or surface H⁺ with ion-selective microelectrodes during application of lactate and CO₂/HCO₃^-. In addition, the expressed proteins were visualised by antibody stainings and possible changes in protein levels were tested by Western blot analyses.

Expression of CAIX resulted in a robust protein level and catalytic activity as determined by Western blot analyses and mass spectrometry. H⁺ surface measurements confirmed membrane insertion of CAIX with its catalytic domain located extracellularly. Coexpression of CAIX enhanced transport activity of MCT1 and 4, but not of MCT2. The augmentation of MCT1/4 mediated H⁺/lactate cotransport was already detected in the nominal absence of CA's substrates and was persistent in the presence of the CA inhibitor ethoxyzolamide, indicating a non-catalytic facilitation of transport activity. In contrast, CAIX protein injected into the oocyte failed to enhance activity of MCT1/4.

The data show that CAIX can be successfully expressed and correctly inserted into the plasma membrane in Xenopus oocytes. Moreover, expressed, but not injected, CAIX augments transport activity of MCT1/4 in a non-catalytic manner, whereas MCT2 activity is not affected, demonstrating a MCT isoform-specific augmentation by CAIX. These findings could further help to understand the complex mechanisms that regulate lactate and H⁺ transport in cells and tissues, especially in tumors with high MCT4 and CAIX expression.

Supported by the DFG (DE 231/24-1, GRK 845/3).

References: 

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