Document Detail

Inhibition of N-linked glycosylation results in retention of intracellular apo[a] in hepatoma cells, although nonglycosylated and immature forms of apolipoprotein[a] are competent to associate with apolipoprotein B-100 in vitro.
MedLine Citation:
PMID:  9717723     Owner:  NLM     Status:  MEDLINE    
Apolipoprotein[a] (apo[a]) is a highly polymorphic glycoprotein that forms a covalent complex with apolipoprotein B-100 (apoB-100), producing a lipoprotein species referred to as lipoprotein[a] (Lp[a]). We have studied the effects of alterations in glycosylation of apo[a] on its intracellular processing and secretion as well as its ability to associate with low density lipoprotein (LDL) apoB-100. HepG2 cells transfected with a 6 kringle IV (6 K-IV) apo[a] minigene were treated with tunicamycin, an inhibitor of N-linked glycosylation, which eliminated apo[a]-B-100 complexes from the media. Tunicamycin treatment also reduced secretion of the 6 K-IV apo[a] protein from transfected McA-RH7777 cells by approximately 50%, but completely eliminated secretion of apo[a] species containing 9 and 17 K-IV repeats. Mixing experiments, performed with radiolabeled media (+/-tunicamycin) from transfected McA-RH7777 cells, demonstrated no alteration in the extent of association of apo[a] with human LDL. Similar mixing experiments using culture media from glycosylation-defective mutant chinese hamster ovary (CHO) cells transfected with the same apo[a] minigene showed identical results. Apo[a] secretion was demonstrated in all mutant cell lines in the absence of either N- or O-linked (or both) glycosylation. The mechanisms underlying the reduced secretion of apo[a] from transfected hepatoma cells were examined by pulse-chase radiolabeling and apo[a] immunoprecipitation. Tunicamycin treatment altered the efficiency of precursor apo[a] processing from the ER by increasing its ER retention time. The increased accumulation of precursor apo[a] in the ER was associated with alterations in the kinetics of association with two resident endoplasmic reticulum (ER) chaperone proteins, calnexin and BiP. These findings suggest that the glycosylation state and size of apo[a] appear to play a role in regulating its efficient exit from the endoplasmic reticulum. However, neither N- nor O-linked glycosylation of apo[a] exerts a major regulatory role in its covalent association with apoB-100.
D K Bonen; F Nassir; A M Hausman; N O Davidson
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Publication Detail:
Type:  In Vitro; Journal Article; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Journal of lipid research     Volume:  39     ISSN:  0022-2275     ISO Abbreviation:  J. Lipid Res.     Publication Date:  1998 Aug 
Date Detail:
Created Date:  1998-11-16     Completed Date:  1998-11-16     Revised Date:  2007-11-14    
Medline Journal Info:
Nlm Unique ID:  0376606     Medline TA:  J Lipid Res     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  1629-40     Citation Subset:  IM    
Department of Medicine, University of Chicago, IL 60637, USA.
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MeSH Terms
Apolipoprotein B-100
Apolipoproteins A / chemistry*,  genetics,  metabolism*
Apolipoproteins B / metabolism*
CHO Cells
Calcium-Binding Proteins / metabolism
Carcinoma, Hepatocellular / metabolism
Carrier Proteins / metabolism
Endoplasmic Reticulum / metabolism
Heat-Shock Proteins*
Lipoproteins, LDL / metabolism
Molecular Chaperones / metabolism
Protein Precursors / chemistry,  metabolism
Protein Processing, Post-Translational / drug effects
Tumor Cells, Cultured
Tunicamycin / pharmacology
Grant Support
Reg. No./Substance:
0/Apolipoprotein B-100; 0/Apolipoproteins A; 0/Apolipoproteins B; 0/Calcium-Binding Proteins; 0/Carrier Proteins; 0/Heat-Shock Proteins; 0/Lipoproteins, LDL; 0/Molecular Chaperones; 0/Protein Precursors; 0/molecular chaperone GRP78; 11089-65-9/Tunicamycin; 139873-08-8/Calnexin

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