Carrier-mediated Transport of Folic Acid in BeWo Cell Monolayers as a Model of the Human Trophoblast

Abstract

Using cultured BeWo cells as a model of human trophoblast, we investigated whether carrier-mediated transport of folic acid occurs. BeWo cells, which were derived from human choriocarcinoma, were cultured on a tissue culture plate or in a permeation chamber. When the cells reached confluence, drug uptake or transport experiments were performed. The uptake of [3H]folic acid by BeWo cells occurred at a much lower rate at 4°C than at 37°C. The uptake of [3H]folic acid was saturable at higher concentrations and inhibited by typical metabolic inhibitors, sodium azide and 2,4-dinitrophenol. The uptake of [3H]folic acid was significantly increased with decreasing pH of the incubation buffer and markedly inhibited by 4,4′-diidothiocyanostilbene-2,2′-disulfonic acid (DIDS). Analogs of folic acid, methotrexate and 5-methyltetrahydrofolate, inhibited the uptake of [3H]folic acid by BeWo cells. Kinetic analysis using Lineweaver-Burk plots revealed that methotrexate competitively inhibited the uptake of [3H]folic acid and folic acid competitively inhibited the uptake of [3H]methotrexate. In transport experiments, the permeation of [3H]folic acid from the apical-to-basal side was greater than that from the basal-to-apical side, and the transport of [3H]folic acid from the apical-to-basal side was inhibited by an excess of folic acid. The findings obtained in the present study confirm the existence of an asymmetric, carrier-mediated transport system for folic acid and its analog, methotrexate, across BeWo cells, a representative of the human trophoblast.