The role of amino acids in ApoB100 synthesis and catabolism in human HepG2 cells.
The object of this study was to examine the effects of extracellular amino acids on hepatic apoB100 metabolism using a transformed human liver cell line (HepG2) as a hepatocyte model. The data demonstrate that reduced extracellular amino acids resulted in marked increases in apoB100 concentrations in the medium (87 +/- 10% increase; p < 0.002). These data contrast with the effects of amino acids on secretion of total hepatic protein into the medium, which increased by 541 +/- 83% at 5.5 mg/ml amino acids compared to the low (0.5 mg/ml) amino acid medium (p < 0.01), pointing to a positive relationship between the concentration of amino acids in the medium and overall secretion of protein by the HepG2 cells. No significant effect of medium amino acid concentration on intracellular synthesis of cholesterol ester, cholesterol, triglyceride, or apoA1 secretion was evident. Amino acids also affected the activity of the low density lipoprotein pathway: at lower medium amino acid concentration, specific cell-associated radioactivity and cellular degradation were increased (117 +/- 24 and 233 +/- 13% at 0.5 versus 5.5 mg/ml amino acids, respectively). Pulse-chase analysis demonstrated that there was no difference in secretion efficiency of apoB (36 +/- 6 versus 46 +/- 8% in low versus high amino acid medium, respectively), but that the initial amount of [3H]apoB synthesized was greater in the low amino acid medium (10.5 +/- 3.8 versus 5.7 +/- 1.7 x 10(3) dpm [3H]apoB/mg of cell protein; p < 0.05). In contrast, the initial amount of [3H]albumin synthesized was much greater in the high amino acid medium (26.6 +/- 5.0 versus 54.6 +/- 19.0 x 10(3) dpm [3H]albumin/mg of cell protein in low versus high amino acid medium, respectively; p < 0.05). Slot blot analysis of apoB mRNA was 87 +/- 22% higher in lower amino acid medium as compared to the high amino acid medium (p < 0.01). These results demonstrate that amino acids have a profound negative regulatory effect on apoB synthesis and secretion and may shed light on the pathogenesis of some clinical dyslipidemias such as the increased plasma apoB levels in patients treated with continuous ambulatory peritoneal dialysis.
Zhang
Z
Z
McGill Unit for the Prevention of Cardiovascular Disease, Royal Victoria Hospital, Montreal, Quebec, Canada.
Sniderman
A D
AD
Kalant
D
D
Vu
H
H
Monge
J C
JC
Tao
Y
Y
Cianflone
K
K
D000418:Albumins / Q000096:biosynthesis / Q000378:metabolism; D000596:Amino Acids / Q000502:physiology*; D053299:Apolipoprotein B-100; D001055:Apolipoproteins B / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D003470:Culture Media; D006801:Humans; D012333:RNA, Messenger / Q000378:metabolism
D000418:Albumins; D000596:Amino Acids; D053299:Apolipoprotein B-100; D001055:Apolipoproteins B; D003470:Culture Media; D012333:RNA, Messenger