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Journal of Lipid Research, Vol 37, 1198-1206, Copyright © 1996 by Lipid Research, Inc.
X Wu, A Shang, H Jiang and HN Ginsberg
The present study was aimed at defining the roles of intracellular
triglyceride pools in apolipoprotein B secretion from HepG2 cells. Oleic
acid (0.2 mmol/L) in the medium stimulated both triglyceride synthesis and
apolipoprotein B secretion. Stimulation of apolipoprotein B secretion was
lost about 30-40 min after oleic acid was removed from the medium, despite
the finding that most newly synthesized triglyceride was still present in
the cells. This suggested that only a small fraction of newly synthesized
triglyceride was transferred to a pool available for assembly of nascent
apoB into lipoproteins. Using cell fractionation, we analyzed two
triglyceride pools in HepG2 cells: a microsomal pool and a cytoplasmic
pool. Oleic acid-induced increases in the microsomal pool were small and
short-lived due to secretion; this pool, therefore, is a
"secretion-coupled" pool. The large majority of newly synthesized
triglyceride was in a cytosolic pool that was not associated with secretion
of apoB. Dibutyryl cAMP treatment was associated with a 3-fold increase in
the mobilization of the triglyceride droplets. Apolipoprotein B secretion,
however, was not increased, suggesting that the amount of triglyceride that
entered the "secretion-coupled" pool after hydrolysis and re-esterification
of cytoplasmic triglyceride was inadequate to stimulate apolipoprotein B
secretion. In summary, the majority of newly synthesized triglyceride,
whether derived from exogenous or endogenous fatty acids, is rapidly
shifted to a cytoplasmic pool that does not play a regulatory role in
apolipoprotein B secretion. The presence of a very small "secretion-
coupled" pool of triglyceride in HepG2 cells likely explains the high rates
of degradation of nascent apolipoprotein B, and the low rates of secretion
of lipid-poor lipoproteins.
ARTICLES
Low rates of apoB secretion from HepG2 cells result from reduced delivery of newly synthesized triglyceride to a "secretion-coupled" pool
Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA.
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