A post-embedding immunoelectron-microscopic demonstration of apolipoprotein-B-containing lipoprotein particles in hepatocytes of fetal rats

Apolipoprotein B (apoB) is an essential component of chylomicrons, very low density lipoproteins, and low density lipoproteins. ApoB is a palmitoylated protein. To investigate the role of palmitoylation in lipoprotein function, a palmitoylation site was mapped to Cys-1085 and removed by mutagenesis. Secreted lipoprotein particles formed by nonpalmitoylated apoB were smaller and denser and failed to assemble a proper hydrophobic core. Indeed, the relative concentrations of nonpolar lipids were three to four times lower in lipoprotein particles containing mutant apoB compared with those containing wild-type apoB, whereas levels of polar lipids isolated from wild-type or mutant apoB lipoprotein particles appeared identical. Palmitoylation localized apoB to large vesicular structures corresponding to a subcompartment of the endoplasmic reticulum, where addition of neutral lipids was postulated to occur. In contrast, nonpalmitoylated apoB was concentrated in a dense perinuclear area corresponding to the Golgi compartment. The involvement of palmitoylation as a structural requirement for proper assembly of the hydrophobic core of the lipoprotein particle and its intracellular sorting represent novel roles for this posttranslational modification.

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Abnormal concentrations of CII, CIII, and E apolipoproteins among apolipoprotein B-containing, B-free, and A-I-containing lipoprotein particles in hemodialysis patients

Clinical Chemistry ◽

10.1093/clinchem/37.3.387 ◽

1991 ◽

Vol 37 (3) ◽

pp. 387-393 ◽

Cited By ~ 15

Author(s):

Naji Alsayed ◽

RegIs Rebourcet

Keyword(s):

Total Cholesterol ◽

Apolipoprotein B ◽

Marked Decrease ◽

Hemodialysis Patients ◽

Serum Concentrations ◽

Dialysis Patients ◽

Apo B ◽

Lipoprotein Particles ◽

Apo E ◽

Anephric Patients

Abstract Serum concentrations of total cholesterol, triglycerides, and apolipoproteins (apo) A-I, B, CII, CIII, and E in 36 hemodialysis patients and nine anephric patients were compared with the concentrations in 34 normolipidemic subjects. The dialysis patients displayed a moderate hypertriglyceridemia (1.94 +/- 0.12 vs 1.09 +/- 0.11 mmol/L in controls, mean +/- SEM; P less than 0.001), apo CIII concentrations were also increased (130.2 +/- 2.1 vs 108.4 +/- 0.7 mg/L; P less than 0.001), whereas apo CII (34.5 +/- 0.5 vs 36 +/- 0.5 mg/L; P less than 0.05), apo E (22.7 +/- 0.3 vs 27.9 +/- 0.2 mg/L; P less than 0.001), and apo A-I (1.18 +/- 0.05 vs 1.31 +/- 0.04 g/L; P less than 0.05) were decreased. Concentrations of serum apo B were normal (0.86 +/- 0.03 vs 0.97 +/- 0.07 g/L). In the hemodialysis patients, apo CIII concentrations were increased in apo B-containing lipoproteins (30.1 +/- 0.5 vs 25.0 +/- 0.1 mg/L; P less than 0.001), whereas CII and E were decreased below control values (14.4 +/- 0.2 vs 16.8 +/- 0.1, and 8.2 +/- 0.2 vs 11.4 +/- 0.1 mg/L, respectively; P less than 0.001 each). By calculation, non-B-containing lipoproteins in the hemodialysis group had increased concentrations of apo CIII (100.1 +/- 2.1 vs 83.3 +/- 0.7 mg/L; P less than 0.001) and decreased amounts of apo E (14.5 +/- 0.4 vs 16.4 +/- 0.3 mg/L; P less than 0.001); apo CII content was unchanged (20.1 +/- 0.5 vs 19.3 +/- 0.5 mg/L). Results for apo CII, CIII, and E among apo A-I-containing lipoproteins in both normolipidemic and hemodialysis groups were similar to those in non-B-containing lipoproteins. Finally, the sole significant (P less than 0.01) difference between the anephric and hemodialysis groups was the lower apo E concentrations in the former group. Accumulation of triglyceride-rich lipoproteins in hemodialysis patients may thus be related to the enrichment of apo CIII in apo B-containing lipoproteins and to a marked decrease in the apo CII and E contents.

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A role for palmitoylation in the quality control, assembly and secretion of apolipoprotein B

Biochemical Journal ◽

10.1042/bj20030951 ◽

2004 ◽

Vol 377 (1) ◽

pp. 121-130 ◽

Cited By ~ 7

Author(s):

Gonzalo L. VILAS ◽

Luc G. BERTHIAUME

Keyword(s):

Quality Control ◽

Apolipoprotein B ◽

Neutral Lipids ◽

Low Density Lipoprotein ◽

Density Lipoprotein ◽

Low Density ◽

Wild Type ◽

Er Quality Control ◽

Lipoprotein Particles ◽

Secretion Efficiency

ApoB (apolipoprotein B)-containing lipoprotein particles, such as chylomicrons, very-low-density and low-density lipoprotein particles, transport triacylglycerol and cholesteryl esters in the bloodstream. A palmitoylation site was previously mapped to Cys-1085 in a functional truncated apoB variant (apoB-29) and abolished by mutagenesis. This Cys-1085Ser mutation resulted in secretion of smaller and denser lipoprotein particles containing 80% less cholesteryl ester and triacylglycerol than wild-type controls. We show that palmitoylation of apoB-29 occurs in the ER (endoplasmic reticulum), stimulates the ER–Golgi transport rate of apoB-29 almost 2-fold, doubles the secretion efficiency of wild-type apoB-29 in comparison with (Cys-1085Ser)apoB-29 and reduces significantly the association of wild-type apoB-29 with calnexin in comparison with (Cys-1085Ser)apoB-29. While non-palmitoylated apoB-29 co-localized extensively with constitutively secreted transferrin, wild-type apoB-29 did so only partially and was enriched in ER extensions. Our results suggest that palmitoylation of apoB regulates the biogenesis of nascent apoB-containing lipoprotein particles by concentrating apoB in a specialized ER compartment and by stimulating dissociation from constituents of the ER quality-control machinery. This reduced interaction would lead to a faster ER–Golgi transit time and a higher secretion efficiency of wild-type apoB-29. Palmitoylation could regulate the amount of apoB available for secretion of neutral lipids.

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Lipoprotein particles in the jejunal mucosa of fetal rats

Developmental Biology ◽

10.1016/0012-1606(75)90142-6 ◽

1975 ◽

Vol 43 (1) ◽

pp. 204-211 ◽

Cited By ~ 9

Author(s):

Ki M. Mak ◽

Jerry S. Trier

Keyword(s):

Jejunal Mucosa ◽

Lipoprotein Particles ◽

Fetal Rats

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Assembly of Lipoprotein Particles Containing Apolipoprotein-B: Structural Model for the Nascent Lipoprotein Particle

Biophysical Journal ◽

10.1529/biophysj.104.046235 ◽

2005 ◽

Vol 88 (4) ◽

pp. 2789-2800 ◽

Cited By ~ 46

Author(s):

Paul E. Richardson ◽

Medha Manchekar ◽

Nassrin Dashti ◽

Martin K. Jones ◽

Anne Beigneux ◽

...

Keyword(s):

Apolipoprotein B ◽

Structural Model ◽

Lipoprotein Particles ◽

Lipoprotein Particle

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Tunicamycin induces ubiquitination and degradation of apolipoprotein B in HepG2 cells

Biochemical Journal ◽

10.1042/bj3530493 ◽

2001 ◽

Vol 353 (3) ◽

pp. 493-501 ◽

Cited By ~ 16

Author(s):

Wei LIAO ◽

Lawrence CHAN

Keyword(s):

Apolipoprotein B ◽

Hepg2 Cells ◽

Proteasome Inhibitor ◽

Plasma Lipoproteins ◽

Apo B ◽

Net Production ◽

Lipoprotein Particles ◽

Essential Component ◽

Intracellular Degradation

Apolipoprotein (apo) B-100 is an essential component of atherogenic plasma lipoproteins. Previous studies have demonstrated that the production of apoB-100 is regulated largely by intracellular degradation at both the co-translational and post-translational levels and that proteasome-mediated and non-proteasome-mediated pathways are involved in this process. ApoB-100 is a glycoprotein. The present study was undertaken to address the question of whether the inhibition of N-linked glycosylation with tunicamycin would interfere with apoB-100 production. We demonstrated that the treatment of HepG2 cells with tunicamycin decreased the net production of apoB-100 by enhancing co-translational degradation of the protein. This effect of tunicamycin was partly prevented by lactacystin, a specific proteasome inhibitor. Because lactacystin only partly reversed the effects of tunicamycin on apoB biogenesis, tunicamycin seemed also to induce apoB co-translational degradation in HepG2 cells by one or more non-proteasomal pathways. Furthermore, tunicamycin increased apoB ubiquitination approx. 4-fold. The proportion of the newly synthesized apoB-100 that was secreted and incorporated into the nascent lipoprotein particles was unaffected by tunicamycin. Thus the tunicamycin-mediated inhibition of N-linked glycosylation interferes with the production of apoB-100 that is mediated by both proteasomal and non-proteasomal pathways.

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