ASC system activity is altered by development of cell polarity in trophoblast from human placenta

Pathways of neutral amino acid uptake were investigated in vitro during differentiation of primary cultures of trophoblast isolated from full-term human placentas and a clone (b30) of the BeWo cell line. Inhibition of initial alanine (0.1 microM) uptake by 2-(methylamino)isobutyric acid and unlabeled alanine revealed two Na(+)-dependent systems and one Na(+)-independent transporter. Characterization of these transporters, by selective inhibition, suggested system A, ASC, and L-like transporters. Concomitant with formation of microvillous membrane and syncytium, system ASC activity decreased from 16.1 +/- 2.8 pmol.mg DNA-1.min-1 at 24 h to 2.4 +/- 1.1 pmol.mg DNA-1.min-1 at 72 h. Na(+)-independent alanine uptake increased from 6.0 +/- 2.0 to 12.9 +/- 0.9 pmol.mg DNA-1.min-1 at 24 and 72 h, respectively. Similarly, alpha-(methylamino)isobutyric acid-insensitive, Na(+)-dependent activity in b30 cells (100 microM alanine) decreased from 6.5 +/- 1.6 to 1.2 +/- 1.2 nmol.mg DNA-1.min-1 for control and forskolin-treated cells, respectively. We conclude that membrane specialization accompanying fusion and differentiation of the cytotrophoblast to form syncytiotrophoblast results in a polarization of neutral amino acid transport systems.