Uptake and Degradation of Lipoproteins by Human Trophoblastic Cells in Primary Culture*

Abstract

It has been shown previously that human placental trophoblastic cells use principally lipoprotein-cholesterol for progesterone synthesis. In the present investigation, human placental trophoblastic cells in primary culture were employed to study the kinetics of uptake and degradation of lipoproteins by trophoblastic tissue. Maximal uptake of [125I]iodo-low density lipoprotein ([125I]LDL) by these cells was achieved by 5-6 h, while degradation of [125I]iodo-LDL was linear from 30 min to 32 h. Uptake and degradation of [125I]iodo-LDL as a function of substrate concentration was studied. The apparent dissociation constants (KD) for both uptake and degradation processes were calculated from these data and were found to be similar (1.03 X 10(-8) and 6.8 X 10(-9) M, respectively), a finding which is suggestive that these processes are causally related. Chloroquine inhibited [125I]iodo-LDL degradation by these cells, a finding that is indicative that LDL degradation is dependent on lysosomal proteolytic activity. When nonradiolabeled high density lipoprotein (HDL) was added to the culture medium, there was little effect on either uptake or degradation of [125I]iodo-LDL; however, when nonradiolabeled LDL was added to the culture medium the net uptake and degradation of [125I]iodo-LDL were diminished progressively as the concentration of nonradiolabeled LDL was increased, findings which are indicative of specificity of the LDL receptor. Uptake and degradation of [125I]iodo-HDL by these cells as a function of substrate concentration were examined. Uptake of [125I]iodo-HDL increased in a linear fashion as the concentration of [125I]iodo-HDL was increased up to 1000 microgram protein/ml; however, degradation of [125I]iodo-HDL was negligible. It is concluded that utilization of lipoproteins by human placental trophoblastic cells is mediated by high affinity, low capacity receptors specific for LDL. These cells do not degrade the protein component of HDL.