SYNTHESIS AND SECRETION OF APOLIPOPROTEIN-E OCCUR INDEPENDENTLY OF SYNTHESIS AND SECRETION OF APOLIPOPROTEIN-B-CONTAINING LIPOPROTEINS IN HEPG2 CELLS

The synthesis and secretion of apolipoprotein (apo) E, a major protein component of very low density lipoproteins, were examined in the huma n hepatoma cell line HepG2 under metabolic conditions known to stimula te lipogenesis and the production of apoB-containing lipoproteins. Whe n HepG2 cells were incubated in the presence of fetal bovine serum (5 or 10%) or canine chylomicron remnants (5 or 10-mu-g of protein), the secretion of triglycerides and cholesteryl esters of lipoproteins of d < 1.063 g/ml increased, as determined by the incorporation of [C-14]a cetate. Determination of the distribution of apoE among media lipoprot eins by agarose column chromatography showed that the majority of secr eted apoE was associated with large lipoproteins when cells were incub ated with fetal bovine serum. However, immunoblot analysis of total me dia apoE revealed that incubating cells with or without the lipogenic factors had no effect on the amount of apoE secreted. Pulse-chase and continuous labeling experiments demonstrated that the synthesis and se cretion of apoE did not vary under the different metabolic conditions, even though there was a 5-fold increase in apoB secretion in response to increased lipogenesis. Neither apoE nor apoB mRNA levels responded to the lipogenic stimuli. We conclude that the synthesis and secretio n of apoE are independent of the production of apoB-containing lipopro teins in HepG2 cells.