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 wi th elevated levels of circulating high density lipoprotein (HDL) in pregnan t women. The main structure implicated in the maternal-fetal exchange is th e syncptiotrophoblast, composed of a brush border membrane (BBM), facing th e mother, and a basal plasma membrane (BPM), facing the fetus. In order to understand the mechanisms controlling the placental and fetal supplies of c holesterol, we purified both BBM and BPM and verified the presence of HDL b inding sites in these membranes. Binding studies using I-125-HDL3 show a si ngle affinity binding site on BPM which has a dissociation constant (K-d) O f 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 aff inity binding sites, one with K-d of 0.62 +/- 0.83 mu g protein/ml and anot her one with a K-d of 6.57 +/- 0.87 mu g protein/ml. Their B-max values wer e 0.54 +/- 0.11 and 2.34 +/- 0.39 mu g of HDL3/mg membrane proteins, respec tively. 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 a nd BBM, respectively. These results provide further evidence that human pla centa possesses specific sites for HDL binding, underlining the important r ole of maternal HDL in the transfer of cholesterol from the maternal circul ation to the placenta and the fetus. (C) 1999 Harcourt Publishers Ltd.