Functional Clarification of MCT1-Mediated Transport of Monocarboxylic Acids at the Blood-Brain Barrier Using In Vitro Cultured Cells and In Vivo BUI Studies
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Purpose. To prove the functional significance of monocarboxylic acidtransporter, MCT1 at the blood-brain barrier (BBB) for the passage ofboth endogenous and exogenous monocarboxylic acids into the centralnervous system.
Methods. Monocarboxylic acid transport at the BBB was studied inrats by using a newly established immortalized brain capillaryendothelial cell (BCEC) line, RBEC1, and the results were compared withthose obtained by using primary cultured BCECs, cells stably expressedwith rat MCT1, and the in vivo brain uptake index (BUI) method.
Results. The cell line, RBEC1 meets various morphological andenzymatic criteria of BCECs and appears to be suitable for the studyof BBB transport of monocarboxylic acids. The presence ofMCT1-transcript in RBEC1 was confirmed by the RT-PCR method, aspreviously observed in isolated brain capillaries. A typical substrate ofMCT1, lactic acid, was taken up by RBEC1 in a stereospecific andsaturable manner. The value of the kinetic parameter Km showed goodagreement with values previously obtained in studies using an in vivoBUI and in vitro MCT1-transfected cells. An organic weak acid,benzoic acid, which has been considered to cross biological membranesby passive diffusion, exhibited carrier-mediated transport properties,such as saturation, pH dependence, and stereospecific inhibition inRBEC1, similar to those we observed in primary cultured rat BCECs.The Km values in RBEC1, in primary cultured BCECs and in thein vivo BUI method were comparable and well agreed with that obtainedin MCT1-transfected cells, suggesting that the transport features ofbenzoic acid observed by in vitro methods well reflect thein vivo transport activity. Furthermore, hybrid depletion of MCT1 in RBEC1using an antisense oligonucleotide against rat MCT1 abolished thesaturable transport of benzoic acid.
Conclusions. These observations show that MCT1 plays a significantrole in the transport of monocarboxylic acids, including the exogenousorganic weak acid benzoic acid, as well as native lactic acid.
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