Primary sequence mapping of human apolipoprotein B-100 epitopes. Comparisons of trypsin accessibility and immunoreactivity and implication for apoB conformation

Betaine Alleviates High-Fat Diet Induced Excessive Lipid Deposition in Gibel Carp Hepatopancreas and L8824 Cells by Enhancing VLDL Secretion through HNF4α/MTTP Pathway

Betaine, a methyl donor, plays a crucial role in lipid metabolism. Previous studies have shown that appropriate betaine supplementation in a high-fat diet reduces triglycerides (TG) of serum and hepatopancreas in fish. However, the underlying mechanism remains unclear. This study examined whether betaine can enhance the secretion of very low-density lipoprotein (VLDL) and sought to identify the specific mechanisms through which this enhancement occurs. A lipid accumulation model was established in gibel carp and L8824 cells using a high-fat diet and oleic acid, respectively. Different doses of betaine (1, 4, and 16 g/kg in the diet; 400 μmol in cell culture) were administered, and measurements were taken for lipid deposition, gene expression of HNF4α, MTTP, and ApoB, as well as the regulation of Mttp and Apob promoters by HNF4α. The results showed that betaine supplementation mitigated lipid droplet accumulation, TG levels, and VLDL production induced by the high-fat diet in gibel carp hepatopancreas and L8824 cells. Moreover, betaine not only increased VLDL content in the cell culture supernatant but also reversed the inhibitory effects of the high-fat diet on protein expression of MTTP, ApoB, and HNF4α in both gibel carp hepatopancreas and L8824 cells. Additionally, HNF4α exhibits transactivating activity on the promoter of Mttp in gibel carp. These findings suggest that betaine supplementation exerts its effects through the HNF4α/MTTP/ApoB pathway, promoting the assembly and secretion of VLDL and effectively reducing lipid accumulation in the hepatopancreas of farmed gibel carp fed a high-fat diet.

Hsp40s play distinct roles during the initial stages of apolipoprotein B biogenesis

Apolipoprotein B (ApoB) is the primary component of atherogenic lipoproteins, which transport serum fats and cholesterol. Therefore, elevated levels of circulating ApoB are a primary risk factor for cardiovascular disease. During ApoB biosynthesis in the liver and small intestine under nutrient-rich conditions, ApoB cotranslationally translocates into the endoplasmic reticulum (ER) and is lipidated and ultimately secreted. Under lipid-poor conditions, ApoB is targeted for ER Associated Degradation (ERAD). Although prior work identified select chaperones that regulate ApoB biogenesis, the contributions of cytoplasmic Hsp40s are undefined. To this end, we screened ApoB-expressing yeast and determined that a class A ER-associated Hsp40, Ydj1, associates with and facilitates the ERAD of ApoB. Consistent with these results, a homologous Hsp40, DNAJA1, functioned similarly in rat hepatoma cells. DNAJA1 deficient cells also secreted hyperlipidated lipoproteins, in accordance with attenuated ERAD. In contrast to the role of DNAJA1 during ERAD, DNAJB1-a class B Hsp40-helped stabilize ApoB. Depletion of DNAJA1 and DNAJB1 also led to opposing effects on ApoB ubiquitination. These data represent the first example in which different Hsp40s exhibit disparate effects during regulated protein biogenesis in the ER, and highlight distinct roles that chaperones can play on a single ERAD substrate.

Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity

Familial hypercholesterolaemia (FH) is an inherited autosomal dominant disorder resulting from defects in the low-density lipoprotein receptor (LDLR), in the apolipoprotein B (APOB) or in the proprotein convertase subtilisin/kexin type 9 (PCSK9) genes. In the majority of the cases FH is caused by mutations occurring within LDLR, while only few mutations in APOB and PCSK9 have been proved to cause disease. p.(Arg3527Gln) was the first mutation in APOB being identified and characterized. Recently two novel pathogenic APOB variants have been described: p.(Arg1164Thr) and p.(Gln4494del) showing impaired LDLR binding capacity, and diminished LDL uptake. The objective of this work was to analyse the structure of p.(Arg1164Thr) and p.(Gln4494del) variants to gain insight into their pathogenicity. Secondary structure of the human ApoB100 has been investigated by infrared spectroscopy (IR) and LDL particle size both by dynamic light scattering (DLS) and electron microscopy. The results show differences in secondary structure and/or in particle size of p.(Arg1164Thr) and p.(Gln4494del) variants compared with wild type. We conclude that these changes underlie the defective binding and uptake of p.(Arg1164Thr) and p.(Gln4494del) variants. Our study reveals that structural studies on pathogenic variants of APOB may provide very useful information to understand their role in FH disease.

Low-density lipoprotein modified by myeloperoxidase in inflammatory pathways and clinical studies

Oxidation of low-density lipoprotein (LDL) has a key role in atherogenesis. Among the different models of oxidation that have been studied, the one using myeloperoxidase (MPO) is thought to be more physiopathologically relevant. Apolipoprotein B-100 is the unique protein of LDL and is the major target of MPO. Furthermore, MPO rapidly adsorbs at the surface of LDL, promoting oxidation of amino acid residues and formation of oxidized lipoproteins that are commonly named Mox-LDL. The latter is not recognized by the LDL receptor and is accumulated by macrophages. In the context of atherogenesis, Mox-LDL accumulates in macrophages leading to foam cell formation. Furthermore, Mox-LDL seems to have specific effects and triggers inflammation. Indeed, those oxidized lipoproteins activate endothelial cells and monocytes/macrophages and induce proinflammatory molecules such as TNFα and IL-8. Mox-LDL may also inhibit fibrinolysis mediated via endothelial cells and consecutively increase the risk of thrombus formation. Finally, Mox-LDL has been involved in the physiopathology of several diseases linked to atherosclerosis such as kidney failure and consequent hemodialysis therapy, erectile dysfunction, and sleep restriction. All these issues show that the investigations of MPO-dependent LDL oxidation are of importance to better understand the inflammatory context of atherosclerosis.

Identification of the proteoglycan binding site in apolipoprotein B48

An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. Previously we identified Site B (residues 3359-3369) in apolipoprotein (apo) B100 as the proteoglycan binding sequence in low density lipoproteins (LDLs) and showed that the atherogenicity of apoB-containing lipoproteins is linked to their affinity for artery wall proteoglycans. However, both apoB100- and apoB48-containing lipoproteins are equally atherogenic even though Site B lies in the carboxyl-terminal half of apoB100 and is absent in apoB48. If binding to proteoglycans is a key step in atherogenesis, apoB48-containing lipoproteins must bind to proteoglycans via other proteoglycan binding sites in the amino-terminal 48% of apoB. In vitro studies have identified five clusters of basic amino acids in delipidated apoB48 that bind negatively charged glycosaminoglycans. To determine which of these sites is functional on LDL particles, we analyzed the proteoglycan binding activity of recombinant human LDLs from transgenic mice or rat hepatoma cells. Substitution of neutral amino acids for the basic amino acids in Site B-Ib (residues 84-94) abolished the proteoglycan binding activity of recombinant apoB53. Carboxyl-truncated apoB80 bound biglycan with higher affinity than apoB100 and apoB48. ApoB80 in which Site B was mutated had the same affinity for proteoglycans as apoB48. These data support the hypothesis that the carboxyl terminus of apoB100 "masks" Site B-Ib, the amino-terminal proteoglycan binding site, and that this site is exposed in carboxyl-truncated forms of apoB. The presence of a proteoglycan binding site in the amino-terminal region of apoB may explain why apoB48- and apoB100-containing lipoproteins are equally atherogenic.

Proteasome-mediated degradation of apolipoprotein B targets both nascent peptides cotranslationally before translocation and full-length apolipoprotein B after translocation into the endoplasmic reticulum

A major portion of newly synthesized apolipoprotein B (apoB) is degraded intracellularly. This degradation has been demonstrated to be mediated largely by the ubiquitin-proteasome pathway. We examined whether nascent apoB polypeptides or full-length apoB is selectively retrotranslocated from the endoplasmic reticulum into the cytosol for degradation. Herein, we found that full-length apoB as well as partial-length apoB peptides are ubiquitinated in HepG2 cells, and ubiquitination is an exclusively cytosolic process. Calnexin, which binds specifically to glycoproteins, has been postulated to promote apoB folding and complete translocation; we found that ubiquitinated apoB is bound to calnexin, suggesting that ubiquitinated apoB is glycosylated. In addition to calnexin binding, we have other pieces of evidence that the full-length intracellular ubiquitinated apoB is glycosylated, because (i) it binds to concanavalin A, and (ii) glycan can be demonstrated in the full-length ubiquitinated apoB by a chemical detection method involving oxidation of adjacent hydroxyl groups in the glycan moiety. Because glycosylation occurs inside the endoplasmic reticulum, the full-length glycosylated apoB must have been retrotranslocated into the cytosol for ubiquitination and proteasome-mediated degradation. Next we synchronized translation in HepG2 cells by puromycin treatment. A pulse-chase experiment using [35S]methionine labeling of intracellular apoB in these synchronized cells demonstrated that nascent partial-length apoB peptides are also ubiquitinated cotranslationally. We conclude that the ubiquitin proteasome-mediated degradation of apoB targets both nascent peptides cotranslationally before translocation as well as full-length apoB after its translocation into the endoplasmic reticulum.

Structure of apolipoprotein B-100 of human low density lipoproteins

We have analyzed low density lipoproteins (LDL) apolipoprotein (apop) B structure by direct sequence analysis of LDL apo B-100 tryptic peptides. Native LDL were digested with trypsin, and the products were fractionated on a Sephadex G-50 column. The partially digested apo B-100 still associated with lipids was recovered in the void volume (designated trypsin-nonreleasable, TN, peptides). The released peptides (designated trypsin-releasable, TR, peptides) in subsequent peaks were repurified on two successive high-performance liquid chromatography (HPLC) columns. The TN peak was delipidated and redigested with trypsin, and the resulting peptides were purified on two successive HPLC columns. Using this approach, we sequenced over 88% of LDL apo B-100, extending and refining our previous study (Nature 1986;323:738-742) which covered 52% of the protein. TN peptides made up 31%, and the TR peptides, 34% of the apo B-100 sequence; 23.7% were found under both TN and TR categories. Based on its differential trypsin releasability, apo B-100 can be divided into five domains: 1) residues 1----1000, largely TR; 2) residues 1001----1700, alternating TR and TN; 3) residues 1701----3070, largely TN; 4) residues 3071----4100, mainly TR and mixed; and 5) residues 4101----4536, almost exclusively TN. Domain 1 contained 14 of the 25 Cys residues in apo B. Domain 4 encompassed seven N-glycosylation sites, and contained the putative receptor binding domains. All 19 potential N-glycosylation sites were directly sequenced: 16 were found to be glycosylated and three were not. Three pairs of disulfide bridges were also mapped. Finally, a combination of cDNA sequencing, direct mRNA sequencing, and comparison of published apo B-100 sequences allowed us to identify specific amino acid residues within apo B-100 that seem to represent bona fide allelic variations. Our study provides information on LDL apo B-100 structure that will be important to our understanding of its conformation and metabolism.