Identification of three cationic amino acid transporters in placental trophoblast: Cloning, expression, and characterization of hCAT-1

The concentrative transfer of amino acids from maternal to fetal blood is e ssential to fetal growth and metabolism. Cationic amino acids are transport ed across the placental microvillous and basal membranes by multiple pathwa ys which act to mediate maternal/fetal transport. To identify the cationic amino acid transporters of human placenta, total RNA was harvested from cul tured trophoblast and from the BeWo choriocarcinoma cell line, b30 clone, a nd used for reverse transcription (RT) and polymerase chain reaction (PCR). Primers based on published sequences identified expression of mRNAs for hC ATs-1, -2B, and -4. RT-PCR yielded a 2.1 kb hCAT-1 cDNA which was cloned. h CAT-1 cRNA injection into Xenopus laevis oocytes stimulated saturable lysin e uptake (K-m similar to 100 mu M). In the presence of Na+, uptake was inhi bited by leucine, homoserine, and alanine but not by valine and glutamate. These transport characteristics are comparable to those of system y(+) in p lacental basal membrane, but differ from those of the same system in microv illous membrane. The identification, cloning, and characterization of multi ple human placental cationic amino acid transporters has the potential to f acilitate molecular investigation of transport by the maternal- and fetal-f acing membranes of placental trophoblast and increase understanding of the mechanism of transplacental amino acid transfer.