Assembly of Very Low Density Lipoprotein: A Two-Step Process of Apolipoprotein B Core Lipidation

Rustaeus, Sabina; Lindberg, Karin; Stillemark, Pia; Claesson, Catharina; Asp, Lennart; Larsson, Thomas; Borén, Jan; Olofsson, Sven-Olof

doi:10.1093/jn/129.2.463s

Cited by 69 publications

(53 citation statements)

References 21 publications

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“…Recent data (summarized in Refs. 3,52) support the model that a small apoB-containing lipoprotein of HDL density is the "primordial" particle that results from the MTP-dependent phase of initial lipidation in the ER. These primordial particles can be secreted directly as lipoproteins in the HDL density range or they can be further lipidated within the cell to VLDL density and then secreted.…”

Section: Discussionsupporting

confidence: 70%

The Triple Threat to Nascent Apolipoprotein B

Fisher¹,

Pan²,

Chen³

et al. 2001

Journal of Biological Chemistry

176

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We previously showed that ⍀-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that ⍀-3-induced degradation preferentially reduces the secretion of large, assembled apoBlipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but ⍀-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that ⍀-3 fatty acids induced a striking loss of apoB 100 from the Golgi, while sparing apoB 100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the ⍀-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.Apolipoprotein B (apoB), 1 the major protein of atherogenic lipoproteins, is synthesized primarily by hepatic and intestinal cells. Most studies have focused on apoB metabolism in the liver, given the greater contribution to the plasma apoB pool made by that organ and the availability of relatively convenient primary and transformed hepatic cell models. The apoB message level and translational rate in hepatic cells are largely constitutive, and so secretory control is achieved primarily through co-and post-translational degradation of the protein (e.g. see Refs. 2-4 for recent reviews). Two specific mechanisms for the destruction of newly synthesized apoB in hepatic cells have been characterized. The first is endoplasmic reticulum-associated degradation (ERAD). Newly synthesized apoB in the endoplasmic reticulum (ER) is initially complexed with small amounts of lipid that are thought to be shuttled by the microsomal triglyceride transfer protein (MTP) (5). During severe lipid deprivation (6, 7) or MTP deficiency (8, 9), this initial lipidation fails, and the apoB becomes ubiquitinylated, which targets it for degradation by proteasomes (10 -14).The second mechanism for degradation of newly synthesized apoB is the re-uptake pathway. Re-uptake can occur after fully assembled apoB-containing particles have been exported across the plasma membrane but before they have diffused away from the vicinity of the cell by traversing the unstirred water layer that is adjacent...

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Section: Discussionsupporting

confidence: 70%

The Triple Threat to Nascent Apolipoprotein B

Fisher¹,

Pan²,

Chen³

et al. 2001

Journal of Biological Chemistry

176

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show abstract

“…Overexpression of DGAT1 and the ACATs in McA-RH7777 cells offered us the potential to speculate on the mechanism whereby the bulk addition of core lipids is added to nascent apoB-lipoproteins (52,53,62). Interestingly, we found that when cells transfected with DGAT were stimulated with OA, they secreted VLDL with high TG:CE ratios; when cells were transfected with either ACAT1 or ACAT2, they secreted VLDL with closer to equivalent amounts of TG and CE.…”

Section: Effect Of Oleic Acid On the Stimulation Of Apob Secretion Bymentioning

confidence: 77%

“…Several recent studies have supported a model in which the assembly of apoB-containing VLDL particles occurs in two discrete steps (52,53), with the merging of a lipid-poor nascent apoB-lipoprotein and a TG-rich droplet comprising the second step. Our present findings raised the question whether the lipid-rich VLDL secreted would be specifically TG-enriched, regardless of the enzyme that was overexpressed, or alternatively, would have a lipid core reflecting the enzyme that was specifically increased.…”

Section: Effect Of Oleic Acid On the Stimulation Of Apob Secretion Bymentioning

confidence: 95%

Overexpression of Human Diacylglycerol Acyltransferase 1, Acyl-CoA:Cholesterol Acyltransferase 1, or Acyl-CoA:Cholesterol Acyltransferase 2 Stimulates Secretion of Apolipoprotein B-containing Lipoproteins in McA-RH7777 Cells

Liang¹,

Oelkers

Cui-ying³

et al. 2004

Journal of Biological Chemistry

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“…Many of the synthesized lipids are exported from the liver in the form of apolipoprotein-containing particles known as very low density lipoproteins (VLDLs), precursors of low-density lipoproteins. The VLDLs are composed of a nonpolar core of triglycerides and cholesteryl esters surrounded by a monolayer amphipathic coating of protein, phospholipids, and cholesterol (14,15). VLDLs are formed in the luminal space within the endoplasmic reticulum (ER), whereas the lipids that are required for their formation are synthesized on the cytosolic side of the ER membrane (16).…”

Section: Hepatitis C Virus (Hcv)mentioning

confidence: 99%

Hepatitis C Virus Nonstructural Proteins Inhibit Apolipoprotein B100 Secretion

Domitrovich

Felmlee

Siddiqui

2005

Journal of Biological Chemistry

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Host genes involved in lipid metabolism are differentially regulated during the early stages of hepatitis C virus (HCV) infection. The majority of lipids synthesized in the liver are exported to other tissues in the form of lipoproteins. The formation of these lipoproteins is dependent upon the association of triglycerides with apolipoprotein B100. Using the HCV subgenomic replicon expression system, we show that secretion of apoB100 is significantly reduced. Inhibition of apoB100 degradation by ALLN did not improve secretion. Triglyceride levels as well as microsomal triglyceride transfer protein mRNA and activity levels were reduced in replicon-expressing cells, indicating potential reasons for the observed decrease. Further evidence is presented for the interaction between the HCV nonstructural protein 5A and apoB100. These results provide further insight into the alteration of lipid metabolism by HCV. Hepatitis C virus (HCV)3 infection is a major health problem worldwide. HCV infection causes chronic hepatitis in up to 60 -80% of infected adults (1) and is associated with steatosis, cirrhosis, and hepatocellular carcinoma (2). The HCV viral genome consists of a positive sense single-stranded RNA 9.6 kb in length and encodes a polyprotein precursor of ϳ3000 amino acids (3). The resulting polyprotein is cleaved into at least three structural proteins (the core protein and the envelope proteins E1 and E2) and a variety of nonstructural proteins (p7 through NS5A/B) (3, 4). Many of the nonstructural proteins are essential for productive viral replication (5).The development of selectable subgenomic HCV RNA replicons has led to important advances in the field of HCV. Recently, several groups have reported the full-length infectious tissue culture system (6 -8), which is likely to further our understanding of the infectious process. The subgenomic replicons are bicistronic constructs composed of the HCV internal ribosomal entry site (IRES), the neomycin phosphotransferase (neo) gene, and the encephalomyocarditis IRES, which mediates the translation of the HCV nonstructural proteins NS3 through NS5, followed by the 3Ј-noncoding region (9).Chronic HCV is characterized by several histological features of the liver such as bile duct damage, lymphoid follicles, low serum cholesterol levels, and, in ϳ50 -60% of the cases, steatosis (10, 11). Recently, genomic analysis conducted on the livers of HCV-infected chimpanzees revealed that the transcript levels of genes involved in lipid metabolism and homeostasis are altered (12, 13).The liver is a major site of lipid synthesis. Many of the synthesized lipids are exported from the liver in the form of apolipoprotein-containing particles known as very low density lipoproteins (VLDLs), precursors of low-density lipoproteins. The VLDLs are composed of a nonpolar core of triglycerides and cholesteryl esters surrounded by a monolayer amphipathic coating of protein, phospholipids, and cholesterol (14, 15). VLDLs are formed in the luminal space within the endoplasmic reticulum (ER), ...

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Assembly of Very Low Density Lipoprotein: A Two-Step Process of Apolipoprotein B Core Lipidation

Cited by 69 publications

References 21 publications

The Triple Threat to Nascent Apolipoprotein B

The Triple Threat to Nascent Apolipoprotein B

Overexpression of Human Diacylglycerol Acyltransferase 1, Acyl-CoA:Cholesterol Acyltransferase 1, or Acyl-CoA:Cholesterol Acyltransferase 2 Stimulates Secretion of Apolipoprotein B-containing Lipoproteins in McA-RH7777 Cells

Hepatitis C Virus Nonstructural Proteins Inhibit Apolipoprotein B100 Secretion

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