A ligand-induced conformational change in apolipoprotein(a) enhances covalent Lp(a) formation

Lipoprotein(a) and cardiovascular disease

Elevated plasma levels of lipoprotein(a) (Lp(a)) are a prevalent, independent, and causal risk factor for atherosclerotic cardiovascular disease and calcific aortic valve disease. Lp(a) consists of a lipoprotein particle resembling low density lipoprotein and the covalently-attached glycoprotein apolipoprotein(a) (apo(a)). Novel therapeutics that specifically and potently lower Lp(a) levels are currently in advanced stages of clinical development, including in large, phase 3 cardiovascular outcomes trials. However, fundamental unanswered questions remain concerning some key aspects of Lp(a) biosynthesis and catabolism as well as the true pathogenic mechanisms of the particle. In this review, we describe the salient biochemical features of Lp(a) and apo(a) and how they underlie the disease-causing potential of Lp(a), the factors that determine plasma Lp(a) concentrations, and the mechanism of action of Lp(a)-lowering drugs.

Identification and analyses of inhibitors targeting apolipoprotein(a) kringle domains KIV-7, KIV-10, and KV provide insight into kringle domain function

Increased plasma concentrations of lipoprotein(a) (Lp(a)) are associated with an increased risk for cardiovascular disease. Lp(a) is composed of apolipoprotein(a) (apo(a)) covalently bound to apolipoprotein B of low-density lipoprotein (LDL). Many of apo(a)'s potential pathological properties, such as inhibition of plasmin generation, have been attributed to its main structural domains, the kringles, and have been proposed to be mediated by their lysine-binding sites. However, available small-molecule inhibitors, such as lysine analogs, bind unselectively to kringle domains and are therefore unsuitable for functional characterization of specific kringle domains. Here, we discovered small molecules that specifically bind to the apo(a) kringle domains KIV-7, KIV-10, and KV. Chemical synthesis yielded compound AZ-05, which bound to KIV-10 with a K_d of 0.8 μm and exhibited more than 100-fold selectivity for KIV-10, compared with the other kringle domains tested, including plasminogen kringle 1. To better understand and further improve ligand selectivity, we determined the crystal structures of KIV-7, KIV-10, and KV in complex with small-molecule ligands at 1.6-2.1 Å resolutions. Furthermore, we used these small molecules as chemical probes to characterize the roles of the different apo(a) kringle domains in in vitro assays. These assays revealed the assembly of Lp(a) from apo(a) and LDL, as well as potential pathophysiological mechanisms of Lp(a), including (i) binding to fibrin, (ii) stimulation of smooth-muscle cell proliferation, and (iii) stimulation of LDL uptake into differentiated monocytes. Our results indicate that a small-molecule inhibitor targeting the lysine-binding site of KIV-10 can combat the pathophysiological effects of Lp(a).

Small size apolipoprotein(a) isoforms enhance inflammatory and proteolytic potential of collagen-primed monocytes

Background

Atherosclerosis is an inflammatory process involving activation of monocytes recruited by various chemoattractant factors, among which lipoprotein(a) and its specific apolipoprotein apo(a). Lp(a) contains a specific apolipoprotein apo(a) which size is determined by a variable number of repeats of a specific structural domain, the kringle IV type 2 (IV-2). Lp(a) plasma concentration and apo(a) size is inversely correlated, and smaller apo(a) are major risk factors for coronary heart disease.

Design and methods

The aim of this study was to evaluate the effect of recombinant apo(a) isoforms (containing 10, 18 or 34 kringles) on monocytes interacting with type I collagen.

Results

Apo(a) isoforms stimulated reactive oxygen species (ROS) and matrix metalloproteinase-9 (MMP-9) production by monocytes, and not modified monocytes adhesion on type I collagen. This effect was specific of apo(a) since no effect was observed in the presence of plasminogen and was inversely related to apo(a) size. The lysine analogue 6-aminohexanoic acid which blocks the lysine binding sites (LBS), and carboxypeptidase B (CpB) which cleaves carboxy-terminal lysine residues, abolished apo(a)-induced ROS and MMP-9 production, highlighting an effect mediated by apo(a) lysing-binding sites.

Conclusions

These results indicate that activation of collagen-primed monocytes stimulated with apo(a) is a Kringle number-dependent effect and reinforce the hypothesis of a role for small size apo(a) isoforms in atherothrombosis.

Diallyl disulphide inhibits apolipoprotein(a) expression in HepG2 cells through the MEK1-ERK1/2-ELK-1 pathway

Background

Lipoprotein(a) [LP(a)] is implicated as a common and independent risk factor for cardiovascular diseases. The therapeutic options currently available for reducing plasma LP(a) concentrations are limited. Diallyl disulphide (DADS), the main component of garlic, regulates lipid metabolism in hepatocytes and adipocytes through ERK1/2 signalling. This study aimed to assess the effect of DADS on apolipoprotein(a) [apo(a)] in HepG2 cells. We also determined the effects of DADS on apo(a) expression and secretion in HepG2 cells as well as the underlying mechanisms.

Methods

We examined the role of DADS on apo(a) expression in HepG2 cells by treating cell with different concentrations of DADS (10, 20, 40 and 80 μg/mL) for 24 h or treating cells with 40 μg/mL DADS for 0, 6, 12, 24 and 48 h. Then we used quantitative real-time PCR to analysis apo(a) mRNA levels, used Western blot to analysis apo(a) protein levels and used enzyme-linked immunosorbent assay to test apo(a) secreted levels. To farther determined the role of DADS, we applied Transfection of small interfering RNA to knockdown ELK-1levels and applied PD98059, a specific inhibitor of ERK1/2, to block ERK1/2 signal.

Results

The results show DADS inhibited apo(a) at both the mRNA and protein levels in HepG2 cells in a dose-dependent manner. DADS-mediated inhibition of apoa(a) expression in HepG2 cells was attenuated when the cells were cultured in medium containing PD98059 (ERK1/2 inhibitor) or were transfected with siRNAs against MEK1 or ELK-1. Overexpression of apo(a) yielded similar results.

Conclusions

This study reveals that DADS can downregulate apo(a) expression in a dose-dependent manner via the MEK-ERK12-ELK-1 pathway.

Lipoprotein(a) catabolism is regulated by proprotein convertase subtilisin/kexin type 9 through the low density lipoprotein receptor

Elevated levels of lipoprotein(a) (Lp(a)) have been identified as an independent risk factor for coronary heart disease. Plasma Lp(a) levels are reduced by monoclonal antibodies targeting proprotein convertase subtilisin/kexin type 9 (PCSK9). However, the mechanism of Lp(a) catabolism in vivo and the role of PCSK9 in this process are unknown. We report that Lp(a) internalization by hepatic HepG2 cells and primary human fibroblasts was effectively reduced by PCSK9. Overexpression of the low density lipoprotein (LDL) receptor (LDLR) in HepG2 cells dramatically increased the internalization of Lp(a). Internalization of Lp(a) was markedly reduced following treatment of HepG2 cells with a function-blocking monoclonal antibody against the LDLR or the use of primary human fibroblasts from an individual with familial hypercholesterolemia; in both cases, Lp(a) internalization was not affected by PCSK9. Optimal Lp(a) internalization in both hepatic and primary human fibroblasts was dependent on the LDL rather than the apolipoprotein(a) component of Lp(a). Lp(a) internalization was also dependent on clathrin-coated pits, and Lp(a) was targeted for lysosomal and not proteasomal degradation. Our data provide strong evidence that the LDLR plays a role in Lp(a) catabolism and that this process can be modulated by PCSK9. These results provide a direct mechanism underlying the therapeutic potential of PCSK9 in effectively lowering Lp(a) levels.

Apolipoprotein(a) stimulates vascular endothelial cell growth and migration and signals through integrin alphaVbeta3

Elevated plasma concentrations of Lp(a) [lipoprotein(a)] are an emerging risk factor for atherothrombotic disease. Apo(a) [apolipoprotein(a)], the unique glycoprotein component of Lp(a), contains tandem repeats of a plasminogen kringle (K) IV-like domain. In the light of recent studies suggesting that apo(a)/Lp(a) affects endothelial function, we evaluated the effects of apo(a)/Lp(a) on growth and migration of cultured HUVECs (human umbilical-vein endothelial cells). Two full-length r-apo(a) [recombinant apo(a)] variants (12K and 17K), as well as Lp(a), were able to stimulate HUVEC growth and migration to a comparable extent; 17K r-apo(a) also decreased the levels of total and active transforming growth factor-beta secreted by these cells. Using additional r-apo(a) variants corresponding to deletions and/or site-directed mutants of various kringle domains in the molecule, we were able to determine that the observed effects of full-length r-apo(a) on HUVECs were dependent on the presence of a functional lysine-binding site(s) in the apo(a) molecule. With respect to signalling events elicited by apo(a) in HUVECs, we found that 17K treatment of the cells increased the phosphorylation level of FAK (focal adhesion kinase) and MAPKs (mitogen-activated protein kinases), including ERK (extracellular-signal-regulated kinase), p38 and JNK (c-Jun N-terminal kinase). In addition, we showed that LM609, the function-blocking antibody to integrin alphaVbeta3, abrogated the effects of 17K r-apo(a) and Lp(a) on HUVECs. Taken together, the results of the present study suggest that the apo(a) component of Lp(a) signals through integrin alphaVbeta3 to activate endothelial cells.

Dimyristoylphosphotidylcholine induces conformational changes in apoB that lowers lipoprotein(a)

Lipoprotein(a) [Lp(a)] is assembled by the binding of apolipoprotein B (apoB) lysine residues on LDL to lysine binding sites in apolipoprotein(a) [apo(a)] and the subsequent formation of a disulphide bond between apoB and apo(a). In this study, we induced changes in apoB conformation by adding phospholipids to LDL and tested the effect of the altered apoB conformation on Lp(a) assembly. The addition of dimyristoylphosphatidylcholine (DMPC) to isolated LDL induced a decrease in the alpha-helical content of apoB and increased the immunoreactivity of the apoB C terminus toward monoclonal antibodies in the region. These conformational changes were associated with a reduction in the ability of the DMPC-modified LDL to form Lp(a) in in vitro assays. Furthermore, administration of DMPC to Lp(a) transgenic mice lead to a significant but transient decrease in Lp(a) levels (18.6% decrease at 2 h, P < 0.001) which coincided with the association of DMPC with LDL in plasma. Our study shows that changes in apoB conformation in the C-terminal region alter the exposure of sequences required for Lp(a) assembly and reduce the formation of Lp(a) both in vitro and in vivo. We conclude that manipulation of LDL surface phospholipids alters Lp(a) levels.

Trehalose and 6-aminohexanoic acid stabilize and renature glucose-6-phosphate dehydrogenase inactivated by glycation and by guanidinium hydrochloride

A number of naturally occurring small organic molecules, primarily involved in maintaining osmotic pressure in the cell, display chaperone-like activity, stabilizing the native conformation of proteins and protecting them from various kinds of stress. Most of them are sugars, polyols, amino acids or methylamines. In addition to their intrinsic protein-stabilizing activity, these small organic stress molecules regulate the activity of some molecular chaperones, and may stabilize the folded state of proteins involved in unfolding or in misfolding diseases, such as Alzheimer's and Parkinson's diseases, or alpha1-antitrypsin deficiency and cystic fibrosis, respectively. Similar to molecular chaperones, most of these compounds have no substrate specificity, but some specifically stabilize certain proteins, e.g., 6-aminohexanoic acid (AHA) stabilizes apolipoprotein A. In the present work, the capacity of 6-aminohexanoic acid to stabilize non-specifically other proteins is demonstrated. Both trehalose and AHA significantly protect glucose-6-phosphate dehydrogenase (G6PD) against glycation-induced inactivation, and renatured enzyme already inactivated by glycation and by guanidinium hydrochloride (GuHCl). To the best of our knowledge, there are no data on the effect of these compounds on protein glycation. The correlation between the recovery of enzyme activity and structural changes indicated by fluorescence spectroscopy and Western blotting contribute to better understanding of the protein stabilization mechanism.

Structure-function relationships in apolipoprotein(a): insights into lipoprotein(a) assembly and pathogenicity

PURPOSE OF REVIEW

Lipoprotein(a) is a structurally and functionally unique lipoprotein consisting of the glycoprotein apolipoprotein(a) covalently linked to LDL. Lipoprotein(a) is assembled extracellularly by a two-step mechanism, still incompletely understood, in which initial non-covalent interactions between apolipoprotein(a) and apolipoprotein B precede specific disulfide bond formation. Elevated concentrations of plasma lipoprotein(a) are a risk factor for a variety of vascular diseases, including coronary heart disease, ischaemic stroke and venous thrombosis. Whereas many pathogenic mechanisms have been proposed for lipoprotein(a), it remains to be conclusively demonstrated which mechanisms are relevant to human disease.

RECENT FINDINGS

Structural and functional studies have verified that apolipoprotein(a) kringle 4 types 6-8 contain lysine binding sites of a weaker affinity for lysine analogues than kringle 4 type 10. Recent evidence has conclusively shown a role for kringle 4 types 7 and 8 in lipoprotein(a) assembly. Moreover, apolipoprotein(a) has been shown to undergo a conformational change, from a closed to an open form, which accelerates the rate of covalent lipoprotein(a) assembly. Functional studies in vitro have identified the domains in apolipoprotein(a) that mediate its inhibitory effects on fibrin clot lysis, binding to fibrin and other biological substrates, and pro-inflammatory and anti-angiogenic properties.

SUMMARY

Extensive structure-function studies of apolipoprotein(a) have begun to yield important insights into the domains in apolipoprotein(a) that mediate lipoprotein(a) assembly and the pathogenic effects of this lipoprotein. Continued investigations of these relationships will contribute critically to unravelling the many outstanding questions about lipoprotein(a) metabolism and pathophysiology.

Quantitative Evaluation of the Contribution of Weak Lysine-binding Sites Present within Apolipoprotein(a) Kringle IV Types 6–8 to Lipoprotein(a) Assembly

During lipoprotein(a) (Lp(a)) assembly, non-covalent interactions between apolipoprotein(a) (apo(a)) and low density lipoprotein precede specific disulfide bond formation. Studies have shown that the non-covalent step involves an interaction between the weak lysine-binding sites (WLBS) present within each of apo(a) kringle IV types 6, 7, and 8 (KIV(6-8)), and two lysine residues (Lys(680) and Lys(690)) within the NH(2) terminus of the apolipoprotein B-100 (apoB) component of low density lipoprotein. In the present study, we introduced single point mutations (E56G) into each of the WLBS present in apo(a) KIV(6-8) and expressed these mutations in the context of a 17-kringle (17K) recombinant apo(a) variant. Single mutations that disrupt the WLBS in KIV(6), KIV(7), and KIV(8), as well as mutants that disrupt the WLBS in both KIV(6) and KIV(7), or both KIV(7) and KIV(8), were assessed for their ability to form non-covalent and covalent Lp(a) complexes. Our results demonstrate that both apo(a) KIV(7) and KIV(8), but not KIV(6), are required for maximally efficient non-covalent and covalent Lp(a) assembly. Single mutations in the WLBS of KIV(7) or KIV(8) resulted in a 3-fold decrease in the affinity of 17K recombinant apo(a) for apoB, and a 20% reduction in the rate of covalent Lp(a) formation. Tandem mutations in the WLBS in both KIV(7) and KIV(8) resulted in a 13-fold reduction in the binding affinity between apo(a) and apoB, and a 75% reduction in the rate of the covalent step of Lp(a) formation. We also showed that KIV(7) and KIV(8) specifically bind with high affinity to apoB-derived peptides containing Lys(690) or Lys(680), respectively. Taken together, our data demonstrate that specific interactions between apo(a) KIV(7) and KIV(8) and Lys(680) and Lys(690) in apoB mediate a high affinity non-covalent interaction between apo(a) and low density lipoprotein, which dictates the efficiency of covalent Lp(a) formation.

Report of the National Heart, Lung, and Blood Institute Workshop on Lipoprotein(a) and Cardiovascular Disease: Recent Advances and Future Directions

It has been estimated that ∼37% of the US population judged to be at high risk for developing coronary artery disease (CAD), based on the National Cholesterol Education Program guidelines, have increased plasma lipoprotein(a) [Lp(a)], whereas Lp(a) is increased in only 14% of those judged to be at low risk. Therefore, the importance of establishing a better understanding of the relative contribution of Lp(a) to the risk burden for CAD and other forms of vascular disease, as well as the underlying mechanisms, is clearly evident. However, the structural complexity and size heterogeneity of Lp(a) have hindered the development of immunoassays to accurately measure Lp(a) concentrations in plasma. The large intermethod variation in Lp(a) values has made it difficult to compare data from different clinical studies and to achieve a uniform interpretation of clinical data. A workshop was recently convened by the National Heart, Lung, and Blood Institute (NHLBI) to evaluate our current understanding of Lp(a) as a risk factor for atherosclerotic disorders; to determine how future studies could be designed to more clearly define the extent to which, and mechanisms by which, Lp(a) participates in these processes; and to present the results of the NHLBI-supported program for the evaluation and standardization of Lp(a) immunoassays. This report includes the most recent data presented by the workshop participants and the resulting practical and research recommendations.