HepG2 cell LDL receptor activity and the accumulation of apolipoprotein B and E in response to docosahexaenoic acid and cholesterol.

Abstract

In the present study, the accumulation of apolipoproteins (apo) A-I, B, and E in culture medium was measured after 0, 3, 6, 12, and 24 h of incubation with 150 microM docosahexaenoic acid complexed to 75 microM bovine serum albumin (BSA-22:6), either in the presence or absence of 50 micrograms/ml cholesterol and 4 micrograms/ml 25-hydroxycholesterol (C/25-OH). HepG2 cells incubated with BSA + C/25-OH for 24 h accumulated approximately 2.0-fold greater apoE and B as compared to BSA-treated cells. Moreover, HepG2 cell apoB accumulation after 24 h of BSA-22:6 treatment was approximately 2.0-fold greater than apoB accumulation from cells treated with BSA alone. When BSA-22:6 and C/25-OH were both included in the incubation, apoB accumulation was approximately 5.0-fold greater than BSA-treated cells. Comparative studies using BSA-18:1 were carried out for 24 h and showed similar levels of apoA-I, B, and E accumulation in culture medium as compared to BSA-22:6-treated cells. In addition, apoA-I, B, and E mRNA abundance were found to be unaffected by type of fatty acid treatment or length of incubation, averaging 48.2 +/- 7.5, 222 +/- 33.6, and 17.1 +/- 0.7 pg mRNA/micrograms RNA (mean +/- SEM), respectively. As the accumulation of apoB and apoE in culture medium may be modified by HepG2 cell LDL receptor expression, LDL receptor mRNA abundance and LDL receptor activity were quantified at various times over the course of the study. By 6 h of BSA + C/25-OH treatment, LDL receptor mRNA was reduced approximately 2.3-fold, while receptor activity was reduced approximately 1.5-fold, as compared to BSA controls. In an experiment designed to determine uptake of HepG2 cell lipoproteins, 3H-labeled apoB-containing lipoproteins derived from HepG2 cells were prepared. The 3H-labeled lipoproteins were 1.25-fold more likely to be removed from the media of HepG2 cells treated with BSA than from cells treated with BSA + C/25-OH. From these results, we postulate that HepG2 cell LDL receptor activity mediates the removal of apoB, E-containing lipoproteins from culture medium and contributes to the lower accumulation of apoB and E observed in culture medium from cells treated with BSA as compared to cells treated with C/25-OH.