Functional analysis of the missense APOC3 mutation Ala23Thr associated with human hypotriglyceridemia.
We have shown that expression of apolipoprotein (apo) C-III promotes VLDL secretion from transfected McA-RH7777 cells under lipid-rich conditions. To determine structural elements within apoC-III that confer to this function, we contrasted wild-type apoC-III with a mutant Ala23Thr originally identified in hypotriglyceridemia subjects. Although synthesis of [(3)H]glycerol-labeled TAG was comparable between cells expressing wild-type apoC-III (C3wt cells) or Ala23Thr mutant (C3AT cells), secretion of [(3)H]TAG from C3AT cells was markedly decreased. The lowered [(3)H]TAG secretion was associated with an inability of C3AT cells to assemble VLDL(1). Moreover, [(3)H]TAG within the microsomal lumen in C3AT cells was 60% higher than that in C3wt cells, yet the activity of microsomal triglyceride-transfer protein in C3AT cells was not elevated. The accumulated [(3)H]TAG in C3AT microsomal lumen was mainly associated with lumenal IDL/LDL-like lipoproteins. Phenotypically, this [(3)H]TAG fractionation profiling resembled what was observed in cells treated with brefeldin A, which at low dose specifically blocked the second-step VLDL(1) maturation. Furthermore, lumenal [(35)S]Ala23Thr protein accumulated in IDL/LDL fractions and was absent in VLDL fractions in C3AT cells. These results suggest that the presence of Ala23Thr protein in lumenal IDL/LDL particles might prevent effective fusion between lipid droplets and VLDL precursors. Thus, the current study reveals an important structural element residing within the N-terminal region of apoC-III that governs the second step VLDL(1) maturation.
Sundaram
Meenakshi
M
Department of Biochemistry, Ottawa Institute of Systems Biology, University of Ottawa, Canada.
Zhong
Shumei
S
Bou Khalil
Maroun
M
Zhou
Hu
H
Jiang
Zhenghui G
ZG
Zhao
Yang
Y
Iqbal
Jahangir
J
Hussain
M Mahmood
MM
Figeys
Daniel
D
Wang
Yuwei
Y
Yao
Zemin
Z
D000409:Alanine / Q000235:genetics; D001056:Apolipoproteins C / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D020126:Brefeldin A / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D005786:Gene Expression Regulation; D006801:Humans; D052439:Lipid Metabolism Disorders / Q000235:genetics*; D053419:Lipoproteins, IDL / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000378:metabolism; D008079:Lipoproteins, VLDL / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism; D008958:Models, Molecular; D020125:Mutation, Missense; D011485:Protein Binding / Q000187:drug effects; D017433:Protein Structure, Secondary; D013912:Threonine / Q000235:genetics; D014280:Triglycerides / Q000378:metabolism*
D001056:Apolipoproteins C; D002352:Carrier Proteins; D053419:Lipoproteins, IDL; D008077:Lipoproteins, LDL; D008079:Lipoproteins, VLDL; D014280:Triglycerides; C106092:microsomal triglyceride transfer protein; D020126:Brefeldin A; D013912:Threonine; D000409:Alanine