High density lipoprotein interaction with human placenta: Biochemical and ultrastructural characterization of binding to microvillous receptor and lack of internalization

Abstract

Specific receptor and internalization process for low density lipoprotein (LDL) and modified LDL (acetyl-LDL) have been well characterized in placental microvilli and in trophoblastic cells in culture. The aim of this study was to investigate high density lipoprotein (HDL 3) binding and its eventual subsequent internalization in both these purified placental preparations. Isolated term placental microvilli were used for binding of [ 125I]HDL 3 (devoid of apoprotein E). HDL 3 were conjugated to colloidal gold for ultrastructural visualization of binding and internalization in syncytiotrophoblast in culture. Saturable binding of HDL 3 was identified. Scatchard analysis revealed a K d value of 24.2 ± 8.0 μg HDL 3 protein/ml and a maximum binding capacity at 4°C of 128.2 ± 54.5 μg HDL 3 protein/mg of membrane protein. These sites have broad specificity: both LDL and acetyl-LDL were able to partially inhibit the HDL 3 binding. Ultrastructural study confirms that gold-HDL 3 bind specifically to syncytiotrophoblast membrane. However, after incubation at 37°C, an internalization process similar to those described for gold-LDL and gold-acetyl-LDL was not observed for gold-HDL 3 These results demonstrate specific HDL 3 binding without internalization. The physiological significance of an HDL 3 membranous interaction and the placental steroidogenesis remains to be established.