Functional clarification of MCT1-mediated transport of monocarboxylic acids at the blood-brain barrier using in vitro cultured cells and in vivo BUI studies

Purpose, To prove the functional significance of monocarboxylic acid transp orter, MCT1 at the blood-brain barrier (BBB) for the passage of both endoge nous and exogenous monocarboxylic acids into the central nervous system. Methods. Monocarboxylic acid transport at the BBB was studied in rats by us ing a newly established immortalized brain capillary endothelial cell (BCEC ) line, RBEC1, and the results were compared with those obtained by using p rimary cultured BCECs, cells stably expressed with rat MCT1, and the in viv o brain uptake index (BUI) method. Results, The cell line, RBEC1 meets various morphological and enzymatic cri teria of BCECs and appears to be suitable for the study of BBB transport of monocarboxylic acids. The presence of MCT1-transcript in RBEC1 was confirm ed by the RT-PCR method, as previously observed in isolated brain capillari es. A typical substrate of MCT1, lactic acid, was taken up by RBEC1 in a st ereospecific and saturable manner. The value of the kinetic parameter Km sh owed good agreement with values previously obtained in studies using an in vivo BUI and in vitro MCT1-transfected cells. An organic weak acid, benzoic acid, which has been considered to cross biological membranes by passive d iffusion, exhibited carrier-mediated transport properties, such as saturati on, pH dependence, and stereospecific inhibition in RBEC1, similar to those we observed in primary cultured rat BCECs. The Km values in RBEC1, in prim ary cultured BCECs and in the in vivo BUI method were comparable and well a greed with that obtained in MCT1-transfected cells, suggesting that the tra nsport features of benzoic acid observed by in vitro methods well reflect t he in vivo transport activity. Furthermore, hybrid depletion of MCT1 in RBE C1 using an antisense oligonucleotide against rat MCT1 abolished the satura ble transport of benzoic acid. Conclusions. These observations show that MCT1 plays a significant role in the transport of monocarboxylic acids, including the exogenous organic weak acid benzoic acid, as well as native lactic acid.