Relation of the size and intracellular sorting of apoB to the formation of VLDL 1 and VLDL 2

Abstract

In this study, we tested the hypothesis that two separate pathways, the two-step process and an apolipoprotein B (apoB) size-dependent lipidation process, give rise to different lipoproteins. Expression of apoB-100 and C-terminally truncated forms of apoB-100 in McA-RH7777 cells demonstrated that VLDL particles can be assembled by apoB size-dependent linear lipidation, resulting in particles whose density is inversely related to the size of apoB. This lipidation results in a LDL-VLDL 2 particle containing apoB-100. VLDL 1 is assembled by the two-step process by apoB-48 and larger forms of apoB but not to any significant amount by apoB-41. The major amount of intracellular apoB-80 and apoB-100 banded with a mean density of 1.10 g/ml. Its formation was dependent on the sequence between apoB-72 and apoB-90. This dense particle, which is retained in the cell, possibly by chaperones or association with the microsomal membrane, is a precursor of secreted VLDL 1. The intracellular LDL-VLDL 2 particles formed during size-dependent lipidation appear to be the precursors of intracellular VLDL 1. We propose that the dense apoB-100 intracellular particle is converted to LDL-VLDL 2 by size-dependent lipidation. LDL-VLDL 2 is secreted or converted to VLDL 1 by the uptake of the major amount of triglycerides. -Stillemark-Billton, P., C. Beck, J. Immunoelectron microscopy (1) and kinetic studies (2, 3) indicate that VLDLs are assembled in two major steps (4, 5). The first step occurs during the translation of apolipoprotein B (apoB) and gives rise to a premature particle (2, 6) we refer to as a primordial lipoprotein. Major amounts of lipid are added in the second step, resulting in bona fide VLDL (2, 7, 8); a second precursor of VLDL, an apoB-free "lipid droplet" in the smooth endoplasmic reticulum (1, 9) whose assembly requires microsomal triglyceride transfer protein (MTP), may also be involved (10). VLDLs are secreted in two forms: large, triglyceride-rich VLDL 1 and smaller, triglyceride-poor VLDL 2. Overproduction of VLDL 1 is linked to conditions such as insulin resistance and type II diabetes (11).The lengths of C-terminally truncated forms of apoB-100 are inversely related to the amount of lipid in the lipoproteins they assemble; assembly with apoB-100 results in VLDL (12). This size-dependent lipidation of apoB does not fit the two-step model; in particular, it cannot explain why apoB-48 has the ability to assemble VLDL but apoB-40 lacks this ability (8).In this study, we tested the hypothesis that two separate pathways, the two-step process and an apoB size-dependent lipidation process, give rise to different lipoproteins. Our results indicate that although apoB can assemble VLDL 1 once it reaches the size of apoB-48, the size-dependent process gives rise to LDL-VLDL 2 particles first when apoB-100 is reached. In contrast to apoB-48 (2), the major intracellular form of apoB-100 is much denser than expected from the size/density relation, and it is retained in the secretory pathway. The sequence between apoB-7...

Role of ADP Ribosylation Factor 1 in the Assembly and Secretion of ApoB-100–Containing Lipoproteins

“…The migration in the gradient of VLDL species (VLDL 1 and VLDL 2) with different assembly pathways has recently been reported. 20 When the cells were cultured in the absence of oleic acid, the majority of the apoB-100 formed bands in the VLDL 1 and VLDL 2 density ranges; a small amount banded in the low-density lipoprotein (LDL) density range and was included in the VLDL 2 fraction ( Figure 1A). In the presence of oleic acid, the majority of apoB-100 was present in the VLDL 1 density region ( Figure 1B).…”

ApoA-V

“…Pre-VLDL is retained in the cell, partly by its interaction with chaperones (BiP and PDI in particular). 18 The pre-VLDL particle cannot be secreted unless more neutral lipids (triglycerides and cholesterol esters) are added. This addition of lipids occurs in two ways.…”

“…First, a size-dependent lipidation converts pre-VLDL to VLDL 2 (ie, the triglyceride-poor VLDL). 18 Second, VLDL 2 can receive a bulk load of lipids, forming VLDL 1 (the triglyceride-rich VLDL). 18 The prerequisite for this bulk lipidation is that apoB should have a size of at least apoB-48 18 and that the protein is translocated out of the rough ER.…”

“…18 Second, VLDL 2 can receive a bulk load of lipids, forming VLDL 1 (the triglyceride-rich VLDL). 18 The prerequisite for this bulk lipidation is that apoB should have a size of at least apoB-48 18 and that the protein is translocated out of the rough ER. This transfer involves an ARF1 (ADP ribosylation factor 1)/COP I (coatomere I)-dependent sorting process from the ERGIC (ER-Golgi intermediate compartment) to the cis-Golgi.…”

Cardiovascular and hematopoietic responses to volatile benzene exposure.

“…Assembly of VLDL begins with lipidation of apolipoprotein B100 (apoB100) by microsomal triglyceride transfer protein (MTP) in the rough endoplasmic reticulum of the liver and this leads to generation of triglyceride-poor VLDL particles (VLDL2) (186) and additionally lipidated to form mature VLDL1 (187). VLDL is secreted from the liver and converted to IDL by lipoprotein lipase in the periphery.…”