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High Density Lipoprotein3 Binding and Biological Action: High Affinity Binding Is not Necessary for Stimulation of Placental Lactogen Release from Trophoblast Cells^*

Departments of Pediatrics and Physiology Duke University Medical Center, Durham, North Carolina 27710
the Department of Medicine, University of North Carolina Chapel Hill, North Carolina 27514

Address all correspondence and requests for reprints to: Dr. E. Verena Jorgensen, Division of Pediatric Endocrinology, Box 690, All Children's Hospital, 801 Sixth Street South, St. Petersburg, Florida 33701.

Abstract

High density lipoprotein (HDL₃) binds with high affinity to many types of cells, but controversy exists concerning the nature and biological significance of the binding. We have recently demonstrated that HDL and apoproteins (apo)-AI, -All, and -CI stimulate a specific and dose-dependent increase in placental lactogen (hPL) release from human trophoblast cells. To examine the possible relationship between HDL₃ binding and stimulation of hPL release, we have characterized the binding of [¹²⁵I]HDL₃ to an enriched fraction of hPL-producing trophoblast cells. Binding studies were performed on trophoblast cells isolated by isopycnic centrifugation of collagenase/hyaluronidase- dispersed placental tissue and apo-E free-HDL3 (density, 1.125–1.215 g/ml). Scatchard analysis of binding studies performed at 37 C for 2 h revealed two classes of binding sites: 1) high affinity binding sites with a Kd of 9.7 ± 2.2 fµg/ml (1.3 x 10^–7 M) and 9.8 ± 3.2 x 105 binding sites/trophoblast cell, and 2) low affinity binding sites with a K_d of 172.8 ± 64.8 µg/ml (2.3 x 10^–6 M) and an estimated 3.2 x 106 sites/cell. As has been found in hepatocytes and other cells, the number of HDL3- binding sites per trophoblast cell (but not the binding affinity) decreased at lower incubation temperatures. In addition, HDL₃ binding to trophoblasts cells did not require calcium and was not affected by prior treatment of the cells with pronase or trypsin. HDL₃-binding sites on trophoblast cells, however, were not specific for HDL₃. Low density lipoprotein (density, 1.063–1.055 g/ml), which does not stimulate hPL release, was nearly as potent on a molar basis as HDL₃ in binding to the high and low affinity binding sites on trophoblast cells. Furthermore, nitrated HDL₃, which does not compete for high affinity binding to trophoblast cells, stimulated hPL release. Although the characteristics of HDL₃ binding to trophoblast cells are similar to those of other cells, these results strongly suggest that the binding of HDL₃ to high affinity binding sites is not essential for HDL-mediated hPL release.

Footnotes

^* Presented in part at the 71st Annual Meeting of The Endocrine Society, Seattle, WA, 1989. This work was supported by NIH Grant HD-07447.

Received August 7, 1989.

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