Identification of Three Cationic Amino Acid Transporters in Placental Trophoblast: Cloning, Expression, and Characterization of hCAT-1

Abstract.

The concentrative transfer of amino acids from maternal to fetal blood is essential to fetal growth and metabolism. Cationic amino acids are transported across the placental microvillous and basal membranes by multiple pathways which act to mediate maternal/fetal transport. To identify the cationic amino acid transporters of human placenta, total RNA was harvested from cultured trophoblast and from the BeWo choriocarcinoma cell line, b30 clone, and used for reverse transcription (RT) and polymerase chain reaction (PCR). Primers based on published sequences identified expression of mRNAs for hCATs-1, -2B, and -4. RT-PCR yielded a 2.1 kb hCAT-1 cDNA which was cloned. hCAT-1 cRNA injection into Xenopus laevis oocytes stimulated saturable lysine uptake (K _m ∼100 μm). In the presence of Na⁺, uptake was inhibited by leucine, homoserine, and alanine but not by valine and glutamate. These transport characteristics are comparable to those of system y⁺ in placental basal membrane, but differ from those of the same system in microvillous membrane. The identification, cloning, and characterization of multiple human placental cationic amino acid transporters has the potential to facilitate molecular investigation of transport by the maternal- and fetal-facing membranes of placental trophoblast and increase understanding of the mechanism of transplacental amino acid transfer.