Presence of CLA-1 and HDL binding sites on syncytiotrophoblast brush border and basal plasma membranes of human placenta

It is now known that rapid placental and fetal development is associated with elevated levels of circulating high density lipoprotein (HDL) in pregnant women. The main structure implicated in the maternal-fetal exchange is the syncptiotrophoblast, composed of a brush border membrane (BBM), facing the mother, and a basal plasma membrane (BPM), facing the fetus. In order to understand the mechanisms controlling the placental and fetal supplies of cholesterol, we purified both BBM and BPM and verified the presence of HDL binding sites in these membranes. Binding studies using I-125-HDL3 show a single affinity binding site on BPM which has a dissociation constant (K-d) Of 3.45 +/- 0.43 mu g protein/ml and a maximal binding capacity (B-max) of 5.46 +/- 1.69 mu g protein/mg membrane proteins. In BBM, we observed two affinity binding sites, one with K-d of 0.62 +/- 0.83 mu g protein/ml and another one with a K-d of 6.57 +/- 0.87 mu g protein/ml. Their B-max values were 0.54 +/- 0.11 and 2.34 +/- 0.39 mu g of HDL3/mg membrane proteins, respectively. CLA-1, a putative HDL-receptor of 85 kDa, was detected on both BPM and BBM, together with two apo A-l binding sites of 110 and 96 kDa on BPM and BBM, respectively. These results provide further evidence that human placenta possesses specific sites for HDL binding, underlining the important role of maternal HDL in the transfer of cholesterol from the maternal circulation to the placenta and the fetus. (C) 1999 Harcourt Publishers Ltd.