Apolipoprotein B is synthesized in selected human non-hepatic cell lines but not processed into mature lipoprotein

ApoB48 as an Efficient Regulator of Intestinal Lipid Transport

Fatty meals induce intestinal secretion of chylomicrons (CMs) containing apolipoprotein (Apo) B48. These CMs travel via the lymphatic system before entering the circulation. ApoB48 is produced after post-transcriptional RNA modification by Apobec-1 editing enzyme, exclusively in the small intestine of humans and most other mammals. In contrast, in the liver where Apobec-1 editing enzyme is not expressed (except in rats and mice), the unedited transcript encodes a larger protein, ApoB100, which is used in the formation of very low-density lipoproteins (VLDL) to transport liver-synthesized fat to peripheral tissues. Apobec-1 knockout (KO) mice lack the ability to perform ApoB RNA editing, and thus, express ApoB100 in the intestine. These mice, maintained on either a chow diet or high fat diet, have body weight gain and food intake comparable to their wildtype (WT) counterparts on the respective diet; however, they secrete larger triglyceride (TG)-rich lipoprotein particles and at a slower rate than the WT mice. Using a lymph fistula model, we demonstrated that Apobec-1 KO mice also produced fewer CMs and exhibited reduced lymphatic transport of TG in response to duodenal infusion of TG at a moderate dose; in contrast, the Apobec-1 KO and WT mice had similar lymphatic transport of TG when they received a high dose of TG. Thus, the smaller, energy-saving ApoB48 appears to play a superior role in comparison with ApoB100 in the control of intestinal lipid transport in response to dietary lipid intake, at least at low to moderate lipid levels.

Charged amino acid residues 997-1000 of human apolipoprotein B100 are critical for the initiation of lipoprotein assembly and the formation of a stable lipidated primordial particle in McA-RH7777 cells

We previously demonstrated that a portion, or perhaps all, of the residues between 931 and 1000 of apolipoprotein (apo) B100 are required for the initiation of apoB-containing particle assembly. Based on our structural model of the first 1000 residues of apoB (designated as apoB:1000), we hypothesized that this domain folds into a three-sided lipovitellin-like "lipid pocket" via a hairpin-bridge mechanism. We proposed that salt bridges are formed between four tandem charged residues 717-720 in the turn of the hairpin bridge and four tandem complementary residues 997-1000 located at the C-terminal end of the model. To identify the specific motif within residues 931 and 1000 that is critical for apoB particle assembly, apoB:956 and apoB:986 were produced. To test the hairpin-bridge hypothesis, the following mutations were made: 1) residues 997-1000 deletion (apoB:996), 2) residues 717-720 deletion (apoB:1000Delta717-720), and 3) substitution of charged residues 997-1000 with alanines (apoB:996 + 4Ala). Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. 1) ApoB:956 did not form stable particles and was secreted as large lipid-rich aggregates. 2) ApoB:986 formed both a lipidated particle that was denser than HDL(3) and large lipid-rich aggregates. 3) Compared with wild-type apoB:1000, apoB:1000Delta717-720 displayed the following: (i) significantly diminished capacity to form intact lipidated particles and (ii) increased propensity to form large lipid-rich aggregates. 4) In striking contrast to wild-type apoB:1000, (i) apoB:996 and apoB:996 + 4Ala were highly susceptible to intracellular degradation, (ii) only a small proportion of the secreted proteins formed stable HDL(3)-like lipoproteins, and (iii) a majority of the secreted proteins formed large lipid-rich aggregates. We conclude that the first 1000 amino acid residues of human apoB100 are required for the initiation of nascent apoB-containing lipoprotein assembly, and residues 717-720 and 997-1000 play key roles in this process, perhaps via a hairpin-bridge mechanism.

Lipid droplets are arrested in the ER membrane by tight binding of lipidated apolipoprotein B-100

Apolipoprotein B-100 (ApoB) is a major component of very-low-density lipoproteins, and is deposited in a region around lipid droplets (LDs) called the ;ApoB-crescent'. The ApoB-crescent is thought to be related to ApoB degradation because it drastically increases when proteasome or autophagy is inhibited. In the present study, we found that ApoB-crescents were significantly reduced when ApoB lipidation was suppressed by either the inhibition or knockdown of the microsomal triglyceride-transfer protein. By contrast, ApoB-crescents increased under conditions that are presumed to cause lipidated ApoB abnormalities in secretory compartments. By electron microscopic analyses, we identified the ApoB-crescent as a thin cholesterol-rich ER cistern fused to an LD, and - topologically - this structure is equivalent to a lipid-ester globule between the two leaflets of the ER membrane. ApoB localized in the thin cisternal lumen, and its binding to LDs was resistant to alkaline treatment. Overexpression of ADRP or TIP47 suppressed the increase in the number of ApoB-crescents, whereas knockdown of these proteins had the opposite effect. From these results, we inferred that the ApoB-crescent is formed by an LD that is arrested in the ER membrane by tight binding of lipidated ApoB to its luminal surface. We suggest that ApoB processing and LD formation are closely linked.

Microsomal triglyceride transfer protein activity is not required for the initiation of apolipoprotein B-containing lipoprotein assembly in McA-RH7777 cells

We previously demonstrated that the N-terminal 1000 amino acid residues of human apolipoprotein (apo) B (designated apoB:1000) are competent to fold into a three-sided lipovitellin-like lipid binding cavity to form the apoB "lipid pocket" without a structural requirement for microsomal triglyceride transfer protein (MTP). Our results established that this primordial apoB-containing particle is phospholipid-rich (Manchekar, M., Richardson, P. E., Forte, T. M., Datta, G., Segrest, J. P., and Dashti, N. (2004) J. Biol. Chem. 279, 39757-39766). In this study we have investigated the putative functional role of MTP in the initial lipidation of apoB:1000 in stable transformants of McA-RH7777 cells. Inhibition of MTP lipid transfer activity by 0.1 microm BMS-197636 and 5, 10, and 20 microm of BMS-200150 had no detectable effect on the synthesis, lipidation, and secretion of apoB:1000-containing particles. Under identical experimental conditions, the synthesis, lipidation, and secretion of endogenous apoB100-containing particles in HepG2 and parental untransfected McA-RH7777 cells were inhibited by 86-94%. BMS-200150 at 40 microm nearly abolished the secretion of endogenous apoB100-containing particles in HepG2 and parental McA-RH cells but caused only 15-20% inhibition in the secretion of apoB: 1000-containing particles. This modest decrease was attributable to the nonspecific effect of a high concentration of this compound on hepatic protein synthesis, as reflected in a similar (20-25%) reduction in albumin secretion. Suppression of MTP gene expression in stable transformants of McA-RH7777 cells by micro-interfering RNA led to 60-70% decrease in MTP mRNA and protein levels, but it had no detectable effect on the secretion of apoB:1000. Our results provide a compelling argument that the initial addition of phospholipids to apoB:1000 and initiation of apoB-containing lipoprotein assembly occur independently of MTP lipid transfer activity.