Jorgensen E V, Gwynne J T, Handwerger S
Department of Pediatrics, Duke University Medical Center, Durham, North Carolina 27710.
Endocrinology. 1989 Dec;125(6):2915-21. doi: 10.1210/endo-125-6-2915.
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, -AII, 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 micrograms/ml (1.3 x 10(-7) M) and 9.8 +/- 3.2 x 10(5) binding sites/trophoblast cell, and 2) low affinity binding sites with a Kd of 172.8 +/- 64.8 micrograms/ml (2.3 x 10(-6) M) and an estimated 3.2 x 10(6) 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.
高密度脂蛋白（HDL3）能与多种类型的细胞高亲和力结合，但关于这种结合的性质和生物学意义仍存在争议。我们最近证实，HDL及载脂蛋白（apo）-AI、-AII和-CI可刺激人滋养层细胞中胎盘催乳素（hPL）释放出现特异性的剂量依赖性增加。为了研究HDL3结合与hPL释放刺激之间的可能关系，我们对[125I]HDL3与人滋养层细胞富集部分的结合进行了表征。结合研究是在通过对胶原酶/透明质酸酶分散的胎盘组织进行等密度离心分离得到的滋养层细胞以及无apo-E的HDL3（密度为1.125 - 1.215 g/ml）上进行的。在37℃下进行2小时的结合研究的Scatchard分析显示有两类结合位点：1）高亲和力结合位点，解离常数（Kd）为9.7±2.2微克/毫升（1.3×10−7 M），每个滋养层细胞有9.8±3.2×105个结合位点；2）低亲和力结合位点，Kd为172.8±64.8微克/毫升（2.3×10−6 M），估计每个细胞有3.2×106个位点。正如在肝细胞和其他细胞中所发现的，在较低的孵育温度下，每个滋养层细胞的HDL3结合位点数（而非结合亲和力）会减少。此外，HDL3与滋养层细胞的结合不需要钙，并且不受细胞预先用链霉蛋白酶或胰蛋白酶处理的影响。然而，滋养层细胞上的HDL3结合位点并非HDL3所特有的。不刺激hPL释放的低密度脂蛋白（密度为1.063 - 1.055 g/ml），在摩尔基础上与HDL3一样能有效结合到滋养层细胞的高亲和力和低亲和力结合位点上。此外，不竞争与滋养层细胞高亲和力结合的硝化HDL3可刺激hPL释放。尽管HDL3与滋养层细胞结合的特性与其他细胞相似，但这些结果强烈表明，HDL3与高亲和力结合位点的结合对于HDL介导的hPL释放并非必不可少。
