Ultrastructural visualization of the internalization of low density lipoprotein by human placental cells

Abstract

Low density lipoproteins (LDL) were conjugated to colloidal gold to visualize the route for internalization of LDL in the cultured cells of human term placenta. Cells were obtained from placental villi (caesarian section) by a standard trypsin-DNase dispersion method followed in some cases by a Percoll gradient centrifugation step. Employing electron microscopy it was observed that after 3 days of culture, cells obtained by trypsin-DNase dispersion were a mixture of macrophages, mononucleated cells and large multinucleated cells. When the cells were incubated for 3 days after the Percoll purification, essentially multinucleated cells identical to the syncytiotrophoblast were present. The number of LDL receptor was increased by preincubation in medium with lipoprotein depleted serum. In binding experiments cells incubated at 4 degrees C for 2 h with medium containing gold LDL conjugates showed gold LDL attached to the plasma membrane without characteristic localization. After incubation with gold LDL at 37 degrees C for various times, the three cellular types showed ligand internalization. Gold LDL endocytosis involved first coated pits but also uncoated plasmalemmal invaginations. Then gold LDL was further observed in coated and non coated vesicles, smooth walled endosomes, multivesicular bodies and tubular vesicles. Lastly free gold particles were observed in lysosome like dense bodies. These results prove the internalization of gold LDL conjugates by human cultured placental cells, particularly by syncytiotrophoblast like multinucleated cells. This accumulation of LDL (the major cholesterol carrying protein in humans) is recognised to be responsible for the exogenous cholesterol supply indispensable to the progesterone biosynthesis and cellular growth of the placenta.

Human placental Fcγ‐binding proteins in the maternofetal transfer of IgG

“…Its biological action is thought to be mediated through high affinity EGF receptors that are present both in apical and basal syncytiotrophoblast plasma membrane, reflecting both maternal and fetal influence on trophoblast production of hCG and placental lactogen (272,5). LDL, the main source of cholesterol required for plasma membrane structural components and placental steroid synthesis, is taken up by LDL receptors on syncytiotrophoblasts and degraded (362,217). Unlike IgG, LDL is not found in basal vesicles of syncytiotrophoblasts (316).…”

“…Several other molecules are taken up by syncytiotrophoblasts; insulin, EGF, LDL and transferrin all bind to specific receptors on syncytiotrophoblast surface and are endocytosed into coated vesicles or uncoated plasmalemmal invaginations (252,91,272,354,217,109,331). Maternal insulin has a growth promoting effect on the trophoblast particularly in the first trimester (78).…”

Intrauterine growth restriction is associated with alterations in placental lipoprotein receptors and maternal lipoprotein composition

“…In the human placenta, binding sites for LDL (16), VLDL, (41) and HDL (1) have been identified on the microvillous membrane of the syncytiotrophoblast. LDL-R-related protein (LRP-1) is a multiligand cell-surface receptor expressed in trophoblast during gestation (8) that binds apolipoprotein E-containing particles (4), such as VLDL.…”

Endocytic and Transcytotic Processes in Villous Syncytiotrophoblast: Role in Nutrient Transport to the Human Fetus

“…The number of LDLR (on the cell surface and intracellularly) is subjected to negative feed‐back regulation by free cholesterol as shown in nonsteroidogenic cells (68,74). This also holds true for placental STB (70). In contrast, estrogen stimulates LDLR expression in cultured STB and in placental minces, resulting in enhanced progesterone production (75,76).…”

“…In view of the vast amount of knowledge on the intracellular pathways of LDL and LDLR in fibroblasts and their regulation by cholesterol (68,69) it is surprising how little information is available on the localization, trafficking and even regulation of LDLR in human villous STB ( in situ ). LDLR are expressed mainly at the apical membrane of STB as shown by binding of gold‐labeled LDL in cultured STB (70) and by isolation of LDLR from purified placental brush border membranes (71). Following internalization of gold‐LDL in cultured STB at 37 °C, the ligand was demonstrated within clathrin‐coated pits, in small plasma membrane invaginations, in coated vesicles, endosomes, multivesicular bodies and lysosome‐like bodies where gold particles appeared free of LDL.…”