High-Density Lipoprotein Biogenesis: Defining the Domains Involved in Human Apolipoprotein A-I Lipidation

The first step in removing cholesterol from a cell is the ATP-binding cassette transporter 1 (ABCA1)-driven transfer of cholesterol to lipid-free or lipid-poor apolipoprotein A-I (apoA-I), which yields cholesterol-rich nascent high-density lipoprotein (nHDL) that then matures in plasma to spherical, cholesteryl ester-rich HDL. However, lipid-free apoA-I has a three-dimensional (3D) conformation that is significantly different from that of lipidated apoA-I on nHDL. By comparing the lipid-free apoA-I 3D conformation of apoA-I to that of 9-14 nm diameter nHDL, we formulated the hypothetical helical domain transitions that might drive particle formation. To test the hypothesis, ten apoA-I mutants were prepared that contained two strategically placed cysteines several of which could form intramolecular disulfide bonds and others that could not form these bonds. Mass spectrometry was used to identify amino acid sequence and intramolecular disulfide bond formation. Recombinant HDL (rHDL) formation was assessed with this group of apoA-I mutants. ABCA1-driven nHDL formation was measured in four mutants and wild-type apoA-I. The mutants contained cysteine substitutions in one of three regions: the N-terminus, amino acids 34 and 55 (E34C to S55C), central domain amino acids 104 and 162 (F104C to H162C), and the C-terminus, amino acids 200 and 233 (L200C to L233C). Mutants were studied in the locked form, with an intramolecular disulfide bond present, or unlocked form, with the cysteine thiol blocked by alkylation. Only small amounts of rHDL or nHDL were formed upon locking the central domain. We conclude that both the N- and C-terminal ends assist in the initial steps in lipid acquisition, but that opening of the central domain was essential for particle formation.

What does procollagen C-endopeptidase enhancer protein 2 have to do with HDL-cholesteryl ester uptake? Or how I learned to stop worrying and love reverse cholesterol transport?

PURPOSE OF REVIEW

The purpose of this study is to provide an update on the role HDL apolipoprotein A-I plays in reducing the risk of cardiovascular disease (CVD) and how it relates to reverse cholesterol transport (RCT).

RECENT FINDINGS

Despite numerous studies showing that plasma HDL cholesterol concentrations are correlated with a reduced risk of CVD, pharmacologic elevation of HDL has not shown any beneficial effects to date. In contrast, studies correlating the measure of an individual's plasma cholesterol efflux capacity show greater promise as a tool for assessing CVD risk. Although ATP-binding cassette transporter 1-mediated lipidation of apoA-I is considered the principal source of plasma HDL, it represents only one side of the RCT pathway. Equally important is the second half of the RCT pathway in which the liver scavenger receptor class B1 selectively removes HDL cholesteryl esters for excretion. The combined action of the two enzyme systems is reflected in the overall steady-state concentration of plasma HDL cholesterol. For example, reduced ATP-binding cassette transporter 1-mediated production of nascent HDL lowers plasma HDL concentration, just as an increase in cholesteryl ester uptake by scavenger receptor class B1 reduces HDL levels. Thus, the complexity of intravascular HDL metabolism suggests that steady-state plasma HDL concentrations do not provide adequate information regarding an individual's HDL quality or function. Herein, we describe a new player, procollagen C-endopeptidase enhancer 2, which shows atheroprotective function and influences both sides of RCT by enhancing production and catabolism of HDL cholesteryl esters.

SUMMARY

The discovery of a new molecule, procollagen C-endopeptidase enhancer 2, implicated in the regulation of HDL cholesteryl ester concentrations suggests that the extracellular matrix and the proteins that regulate its function represent a new and as yet unexplored realm of HDL cholesterol metabolism.

Procollagen C-endopeptidase Enhancer Protein 2 (PCPE2) Reduces Atherosclerosis in Mice by Enhancing Scavenger Receptor Class B1 (SR-BI)-mediated High-density Lipoprotein (HDL)-Cholesteryl Ester Uptake

Studies in human populations have shown a significant correlation between procollagen C-endopeptidase enhancer protein 2 (PCPE2) single nucleotide polymorphisms and plasma HDL cholesterol concentrations. PCPE2, a 52-kDa glycoprotein located in the extracellular matrix, enhances the cleavage of C-terminal procollagen by bone morphogenetic protein 1 (BMP1). Our studies here focused on investigating the basis for the elevated concentration of enlarged plasma HDL in PCPE2-deficient mice to determine whether they protected against diet-induced atherosclerosis. PCPE2-deficient mice were crossed with LDL receptor-deficient mice to obtain LDLr(-/-), PCPE2(-/-) mice, which had elevated HDL levels compared with LDLr(-/-) mice with similar LDL concentrations. We found that LDLr(-/-), PCPE2(-/-) mice had significantly more neutral lipid and CD68+ infiltration in the aortic root than LDLr(-/-) mice. Surprisingly, in light of their elevated HDL levels, the extent of aortic lipid deposition in LDLr(-/-), PCPE2(-/-) mice was similar to that reported for LDLr(-/-), apoA-I(-/-) mice, which lack any apoA-I/HDL. Furthermore, LDLr(-/-), PCPE2(-/-) mice had reduced HDL apoA-I fractional clearance and macrophage to fecal reverse cholesterol transport rates compared with LDLr(-/-) mice, despite a 2-fold increase in liver SR-BI expression. PCPE2 was shown to enhance SR-BI function by increasing the rate of HDL-associated cholesteryl ester uptake, possibly by optimizing SR-BI localization and/or conformation. We conclude that PCPE2 is atheroprotective and an important component of the reverse cholesterol transport HDL system.

Abstract 116: Mechanism of ApoA-I Attenuation of Inflammation Associated with Atherosclerosis

Cytokines/chemokines and their receptors have been an important consideration when investigating the role of inflammation in atherosclerosis. In humans and animal models, monocyte recruitment and accumulation within the artery wall occurs in response to elevated plasma cholesterol levels. To mimic this process in a mouse model of atherosclerosis, our lab has utilized the LDLr-/-, ApoA-I-/- double knockout mouse. Our studies have shown that LDLr-/-, ApoA-I-/- mice have increased immune cell mobilization driving the progression of atherosclerosis when compared to LDLr-/- mice, and that treating mice with frequent, low doses (200 μg) of subcutaneous administered lipid-free apoA-I or rHDL reverses the atherogenic process. Based on these studies we wanted to investigate which inflammatory pathways were most affected by apoA-I treatment. To do this, we carried out RT-PCR to obtain the relative mRNA abundance on 40 different markers of inflammation in two genotypes of atherogenic diet fed mice. Each genotype of mice was divided into two subsets. One subset was fed an atherogenic diet for 12 weeks while the other subset received 12 weeks of diet and subcutaneous injections of 200 μg of apoA-I for the last 6 weeks of the study. Spleen was isolated from all animals and their inflammatory gene mRNA expression examined. Most significantly both genotypes showed a large decrease in IL-3 mRNA expression but also showed a significant increase in CD163 mRNA expression with apoA-I treatment. Decrease in splenic IL-3 expression is a significant mediator of inflammation regression because of the role IL-3 plays in monocyte production inside the spleen. Other investigators have shown that myeloid cells reserved in the spleen give rise to monocytes in an IL-3 rich environment. IL-3 develops and maintains circulating monocytes, which may accumulate in the arteries during atherosclerosis. This suggests apoA-I treatment affects IL-3 levels in the spleen, and thus the amount of monocytes in circulation. An increase in CD163 mRNA expression in the spleen implies higher accumulation of monocytes inside the spleen, and out of circulation. This suggests that apoA-1 treatment can decrease the amount of monocytes in the arteries, which can be a result of reverse in the atherogenic process.

Abstract 320: Procollagen C-Endopeptidase Enhancer Protein 2 Enhances SR-BI Mediated HDL Cholesterol Uptake and Reduces Atherosclerosis in Mice

Epidemiological studies have shown an inverse correlation between plasma high density lipoprotein (HDL) concentrations and cardiovascular disease risk. At variance with these observations, clinical trials that significantly raised plasma HDL-C levels did not have improved clinical outcomes, emphasizing the importance of understanding HDL function. Recently, a significant correlation has been reported between human procollagen c-endopeptidase enhancer protein 2 (PCPE2) single nucleotide polymorphisms and HDL. PCPE2, a 52 kDa glycoprotein found in the extracellular matrix enhances cleavage of C-terminal procollagen by bone morphogenetic protein 1. Mice lacking PCPE2 have elevated concentrations of enlarged plasma HDL, a phenomenon associated with defective cholesterol efflux. HDL synthesis depends on ABCA1-mediated lipid efflux to lipid-poor apoA-I balanced by cholesterol uptake through hepatic scavenger receptor class B type I (SR-BI). Our studies focused on investigating if the elevated concentration of enlarged plasma HDL in PCPE2 deficient mice was atheroprotective. PCPE2 deficient mice were crossed with LDLr -/- mice (SKO) giving LDLr -/- , PCPE2 -/- (DKO) mice that had elevated HDL levels compared to SKO mice. Despite elevated HDL levels, we found that DKO mice had significantly more lipid and CD68+ infiltration into the aortic root, similar to that reported for LDLr -/- , apoA-I -/- mice that lack plasma apoA-I/HDL. Furthermore, DKO mice showed reduced HDL apoA-I fractional clearance and reverse cholesterol transport rates compared to SKO mice suggesting PCPE2 plays a significant role in HDL remodeling and/or cholesterol uptake by the liver. To test the effect of PCPE2 on SR-BI function we incubated 3 H-cholesteryl ether ( 3 H-CE) enriched HDL from SKO and DKO mice with CHO cells overexpressing PCPE2. Compared to CHO control cells, overexpression of PCPE2 increased 3 H-CE HDL uptake that was independent of HDL particle origin. Western Blot analysis showed no difference in SR-BI expression between control and PCPE2 transfected cells, suggesting that PCPE2 enhanced SR-BI function and promoted HDL cholesterol ester uptake. We conclude that PCPE2 is atheroprotective and an essential component of the reverse cholesterol transport system.

Abstract 224: Formation of Cholesterol-Enriched Nascent HDL Requires Helices 5 and 7

Heart disease claims nearly 400,000 individuals in the US per year. Large population studies have repeatedly demonstrated an inverse relationship between HDL concentration and risk. Despite the strong correlation, pharmacological trials that raise plasma HDL have shown little efficacy, suggesting that HDL function should be monitored. Studies were initiated into the mechanism of cholesterol efflux leading to nascent HDL (nHDL) formation. The most productive cholesterol removal occurs when lipid-poor apoA-I, the main structural protein in nHDL, forms particles after interacting with ABCA1. ApoA-I acquires cholesterol and phospholipid at the cell surface forming 10-12 nm diameter nHDL whose composition resembles lipid rafts. Because lipidated apoA-I has a significantly different 3-D conformation compared to its lipid-poor counterpart, the structural modifications leading to nHDL formation were investigated. First, we completed a 3-D solution structure of lipid-poor apoA-I using lysine specific chemical cross-linking in conjunction with specifically engineered double cysteine-containing apoA-I mutants. Strategically placed cysteines form disulfide bonds (locked) when residues are within ~0.5 nm. If no disulfide bond formed (unlocked) then the two cysteines were separated by more than ~0.5 nm. Both methods rely on mass spectrometry to identify amino acid sequence and distance constraints within each 10 repeating helices comprising 80% of full-length apoA-I. By comparing lipid-poor apoA-I conformation to our previously described 3-D structure of apoA-I on 10-12 nm nHDL, we formulated a series of hypothetical helical domain transitions that might drive protein-lipid interaction and particle formation. To test, 10 different double cysteine mutants were employed in either their “locked” (oxidized) or “unlocked” (reduced) state by evaluating their ability to form recombinant HDL using synthetic phospholipid or to form nHDL using HEK cells. After determining HDL particle yield, size and composition from all locked and unlocked double cysteine apoA-I mutants we conclude that both the N- and C-terminal ends of apoA-I are essential for the first steps in lipid acquisition, but that the central helices 5, 6 and 7 are essential for nHDL formation.

The conformation of lipid-free human apolipoprotein A-I in solution

Apolipoprotein AI (apoA-I) is the principal acceptor of lipids from ATP-binding cassette transporter A1, a process that yields nascent high density lipoproteins. Analysis of lipidated apoA-I conformation yields a belt or twisted belt in which two strands of apoA-I lie antiparallel to one another. In contrast, biophysical studies have suggested that a part of lipid-free apoA-I was arranged in a four-helix bundle. To understand how lipid-free apoA-I opens from a bundle to a belt while accepting lipid it was necessary to have a more refined model for the conformation of lipid-free apoA-I. This study reports the conformation of lipid-free human apoA-I using lysine-to-lysine chemical cross-linking in conjunction with disulfide cross-linking achieved using selective cysteine mutations. After proteolysis, cross-linked peptides were verified by sequencing using tandem mass spectrometry. The resulting structure is compact with roughly four helical regions, amino acids 44-186, bundled together. C- and N-terminal ends, amino acids 1-43 and 187-243, respectively, are folded such that they lie close to one another. An unusual feature of the molecule is the high degree of connectivity of lysine40 with six other lysines, lysines that are close, for example, lysine59, to distant lysines, for example, lysine239, that are at the opposite end of the primary sequence. These results are compared and contrasted with other reported conformations for lipid-free human apoA-I and an NMR study of mouse apoA-I.

Abstract 430: PCPE2 a Novel Determinant in HDL Biogenesis and Metabolism

Recent attempts to reduce cardiovascular events by pharmacologically raising plasma HDL concentration have not proven efficacious. This has lead to investigations on how to increase functional versus dysfunctional plasma HDL. Our work shows that a functional subfraction from ABCA1 mediated biogenesis of nascent HDL (nHDL) consists of 3 apoA-I, 105 phosphotidylcholine, 27 sphingomyelin and 108 free cholesterol molecules, compositionally reminiscent of lipid rafts. Our studies are now focused on procollagen C-proteinase enhancer 2 (PCPE2), first described by Francone, OL, et al. (2011), to play a significant role in the ABCA1 mediated efflux of cholesterol to apoA-I. Based on previous studies, we hypothesize that PCPE2 binds lipid-free apoA-I thereby aligning specific helices among 3 molecules of apoA-I for lipidation. To test this, we created loss-of-function apoA-I mutant proteins to identify specific helical repeats that participated in nHDL formation. The various mutations span the central helices as well as the N- and C- termini of apoA-I. Opposing cysteines were placed within 3-5 å using a composite 3-D model of lipid-free apoA-I conformation. The mutant apoA-I disulfide “locked” (oxidized) and “unlocked” (reduced) status was monitored using mass spectrometric sequencing. Using both locked and unlocked versions of each of the mutant proteins, secondary structure as well as the extent of lipidation was investigated. In addition, studies, utilizing LDL receptor, PCPE2 double knockout mice (LDLr -/- , PCPE2 -/- ) fed an atherogenic diet have been carried out. Despite having a higher HDL concentration LDLr -/- , PCPE2 -/- mice had significantly more aortic lipid deposition compared to LDLr -/- mice. The absence of PCPE2 substantially reduced the ability to efflux cholesterol suggesting that the HDL was dysfunctional. These data strongly suggest that PCPE2 participates in lipidation by helping apoA-I to either open or target a site on ABCA1 that promotes apoA-I opening.