Synthesis and secretion of apolipoprotein E by human placenta and choriocarcinoma cell lines

Abstract

The synthesis and secretion of apolipoproteins (apos) by cells from a human choriocarcinoma cell line, JAR, were examined by [ 35S]-methionine labeling followed by immunoprecipitation and SDS/PAGE. Apo E, but not aposA-I, A -IV, or B, was synthesized and secreted. Apo E was also synthesized by fragments of chorionic villi from human placenta and by another choriocarcinoma line, BeWo. Pulse-chase experiments with JAR cells revealed that apo E was initially synthesized as a 33 kDa protein followed by a 34 KDa protein, probably the result of glycosylation. The latter was secreted into the medium where it was detected coincident with a 21 22 kDa doublet, possibly proteolyticfragments ofapo E. Approximately 50 per cent of the apo E in the medium was complexed with lipid as indicated by ultracentrifugation at a density of 1.21 g/ml. The amount of apo E produced by JAR was not affected by preincubation with dibutyryl cAMP and theophylline, or by the cholesterol content of the cells. Following perfusion ofan isolated lobule ofhuman placenta with [ 14C]-amino acids, [ 14C]-apo E was detected by immunoprecipitation of the maternal and fetal perfusates with 88 per cent in the maternal perfusate. These studies suggest that apo E, which promotes receptor-mediated lipoprotein uptake, is secreted by the trophoblast to facilitate uptake of maternal lipoproteins.