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

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 (HDL3) binding and its eventual subsequent internalization in both these purified placental preparations. Isolated term placental microvilli were used for binding of [125I]HDL3 (devoid of apoprotein E). HDL3 were conjugated to colloidal gold for ultrastructural visualization of binding and internalization in syncytiotrophoblast in culture. Saturable binding of HDL3 was identified. Scatchard analysis revealed a Kd value of 24.2 +/- 8.0 micrograms HDL3 protein/ml and a maximum binding capacity at 4 degrees C of 128.2 +/- 54.5 micrograms HDL3 protein/mg of membrane protein. These sites have broad specificity: both LDL and acetyl-LDL were able to partially inhibit the HDL3 binding. Ultrastructural study confirms that gold-HDL3 bind specifically to syncytiotrophoblast membrane. However, after incubation at 37 degrees C, an internalization process similar to those described for gold-LDL and gold-acetyl-LDL was not observed for gold-HDL3. These results demonstrate specific HDL3 binding without internalization. The physiological significance of an HDL3 membranous interaction and the placental steroidogenesis remains to be established.