Development and polarization of cationic amino acid transporters and regulators in the human placenta

We have investigated L-arginine transport systems in the human placental syncytiotrophoblast across gestation using purified microvillous (MVM) and basal (BM) plasma membrane vesicles. In MVM from first-trimester and term placentas, L-arginine transport was by systems y+ and y+L. In BM (term placentas), however, there was evidence for system y+L only. The Michaelis constant of system y+L was significantly lower (P <0.05) in first-trimester compared with term MVM and lower in term MVM compared with BM (P <0.05). There was no functional evidence for system b0+ in term MVM or BM. Cationic amino acid transporter (CAT) 1, CAT 4, and 4F2hc were detected using RT-PCR in placentas throughout gestation, rBAT was not detected in term placentas. An ~85-kDa and an ~135-kDa protein was detected by Western blotting in MVM under reducing and nonreducing conditions, respectively, consistent with the 4F2hc monomer and the 4F2hc-light chain dimer, and their expression was significantly higher (P <0.05) in term compared with first-trimester MVM. These proteins were not detected in BM despite functional evidence for system y+L. These data suggest different roles for 4F2hc in the development and polarization of cationic amino acid transporters in the syncytiotrophoblast.