Uptake of Lp (a) lipoprotein by cultured fibroblasts

Functional interrogation of cellular Lp(a) uptake by genome-scale CRISPR screening

An elevated level of lipoprotein(a), or Lp(a), in the bloodstream has been causally linked to the development of atherosclerotic cardiovascular disease and calcific aortic valve stenosis. Steady state levels of circulating lipoproteins are modulated by their rate of clearance, but the identity of the Lp(a) uptake receptor(s) has been controversial. In this study, we performed a genome-scale CRISPR screen to functionally interrogate all potential Lp(a) uptake regulators in HuH7 cells. Strikingly, the top positive and negative regulators of Lp(a) uptake in our screen were LDLR and MYLIP, encoding the LDL receptor and its ubiquitin ligase IDOL, respectively. We also found a significant correlation for other genes with established roles in LDLR regulation. No other gene products, including those previously proposed as Lp(a) receptors, exhibited a significant effect on Lp(a) uptake in our screen. We validated the functional influence of LDLR expression on HuH7 Lp(a) uptake, confirmed in vitro binding between the LDLR extracellular domain and purified Lp(a), and detected an association between loss-of-function LDLR variants and increased circulating Lp(a) levels in the UK Biobank cohort. Together, our findings support a central role for the LDL receptor in mediating Lp(a) uptake by hepatocytes.

Lipoprotein(a) catabolism: a case of multiple receptors

Lipoprotein(a) [Lp(a)] is an apolipoprotein B (apoB)-containing plasma lipoprotein similar in structure to low-density lipoprotein (LDL). Lp(a) is more complex than LDL due to the presence of apolipoprotein(a) [apo(a)], a large glycoprotein sharing extensive homology with plasminogen, which confers some unique properties onto Lp(a) particles. ApoB and apo(a) are essential for the assembly and catabolism of Lp(a); however, other proteins associated with the particle may modify its metabolism. Lp(a) specifically carries a cargo of oxidised phospholipids (OxPL) bound to apo(a) which stimulates many proinflammatory pathways in cells of the arterial wall, a key property underlying its pathogenicity and association with cardiovascular disease (CVD). While the liver and kidney are the major tissues implicated in Lp(a) clearance, the pathways for Lp(a) uptake appear to be complex and are still under investigation. Biochemical studies have revealed an exceptional array of receptors that associate with Lp(a) either via its apoB, apo(a), or OxPL components. These receptors fall into five main categories, namely 'classical' lipoprotein receptors, toll-like and scavenger receptors, lectins, and plasminogen receptors. The roles of these receptors have largely been dissected by genetic manipulation in cells or mice, although their relative physiological importance for removal of Lp(a) from the circulation remains unclear. The LPA gene encoding apo(a) has an overwhelming effect on Lp(a) levels which precludes any clear associations between potential Lp(a) receptor genes and Lp(a) levels in population studies. Targeted approaches and the selection of unique Lp(a) phenotypes within populations has nevertheless allowed for some associations to be made. Few of the proposed Lp(a) receptors can specifically be manipulated with current drugs and, as such, it is not currently clear whether any of these receptors could provide relevant targets for therapeutic manipulation of Lp(a) levels. This review summarises the current status of knowledge about receptor-mediated pathways for Lp(a) catabolism.

Roles of the low density lipoprotein receptor and related receptors in inhibition of lipoprotein(a) internalization by proprotein convertase subtilisin/kexin type 9

Elevated plasma concentrations of lipoprotein(a) (Lp(a)) are a causal risk factor for cardiovascular disease. The mechanisms underlying Lp(a) clearance from plasma remain unclear, which is an obvious barrier to the development of therapies to specifically lower levels of this lipoprotein. Recently, it has been documented that monoclonal antibody inhibitors of proprotein convertase subtilisin/kexin type 9 (PCSK9) can lower plasma Lp(a) levels by 30%. Since PCSK9 acts primarily through the low density lipoprotein receptor (LDLR), this result is in conflict with the prevailing view that the LDLR does not participate in Lp(a) clearance. To support our recent findings in HepG2 cells that the LDLR can act as a bona fide receptor for Lp(a) whose effects are sensitive to PCSK9, we undertook a series of Lp(a) internalization experiments using different hepatic cells, with different variants of PCSK9, and with different members of the LDLR family. We found that PCSK9 decreased Lp(a) and/or apo(a) internalization by Huh7 human hepatoma cells and by primary mouse and human hepatocytes. Overexpression of human LDLR appeared to enhance apo(a)/Lp(a) internalization in both types of primary cells. Importantly, internalization of Lp(a) by LDLR-deficient mouse hepatocytes was not affected by PCSK9, but the effect of PCSK9 was restored upon overexpression of human LDLR. In HepG2 cells, Lp(a) internalization was decreased by gain-of-function mutants of PCSK9 more than by wild-type PCSK9, and a loss-of function variant had a reduced ability to influence Lp(a) internalization. Apo(a) internalization by HepG2 cells was not affected by apo(a) isoform size. Finally, we showed that very low density lipoprotein receptor (VLDLR), LDR-related protein (LRP)-8, and LRP-1 do not play a role in Lp(a) internalization or the effect of PCSK9 on Lp(a) internalization. Our findings are consistent with the idea that PCSK9 inhibits Lp(a) clearance through the LDLR, but do not exclude other effects of PCSK9 such as on Lp(a) biosynthesis.

The complexity of lipoprotein (a) lowering by PCSK9 monoclonal antibodies

Since 2012, clinical trials dedicated to proprotein convertase subtilisin kexin type 9 (PCSK9) inhibition with monoclonal antibodies (mAbs) have unambiguously demonstrated robust reductions not only in low-density lipoprotein (LDL) cholesterol (LDL-C) but also in lipoprotein (a) [Lp(a)] levels. The scientific literature published prior to those studies did not provide any evidence for a link between PCSK9 and Lp(a) metabolism. More recent investigations, either in vitro or in vivo, have attempted to unravel the mechanism(s) by which PCSK9 mAbs reduce circulating Lp(a) levels, with some showing a specific implication of the LDL receptor (LDLR) in Lp(a) clearance whereas others found no significant role for the LDLR in that process. This elusive pathway appears clearly distinct from that of the widely prescribed statins that also enhance LDLR function but do not lower circulating Lp (a) levels in humans. So how does PCSK9 inhibition with mAbs reduce Lp(a)? This still remains to be established.

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.

Lipoprotein [a] is cleared from the plasma primarily by the liver in a process mediated by apolipoprotein [a]

The cellular and molecular mechanisms responsible for lipoprotein [a] (Lp[a]) catabolism are unknown. We examined the plasma clearance of Lp[a] and LDL in mice using lipoproteins isolated from human plasma coupled to radiolabeled tyramine cellobiose. Lipoproteins were injected into wild-type, LDL receptor-deficient (Ldlr-/-), and apolipoprotein E-deficient (Apoe-/-) mice. The fractional catabolic rate of LDL was greatly slowed in Ldlr-/- mice and greatly accelerated in Apoe-/- mice compared with wild-type mice. In contrast, the plasma clearance of Lp[a] in Ldlr-/- mice was similar to that in wild-type mice and was only slightly accelerated in Apoe-/- mice. Hepatic uptake of Lp[a] in wild-type mice was 34.6% of the injected dose over a 24 h period. The kidney accounted for only a small fraction of tissue uptake (1.3%). To test whether apolipoprotein [a] (apo[a]) mediates the clearance of Lp[a] from plasma, we coinjected excess apo[a] with labeled Lp[a]. Apo[a] acted as a potent inhibitor of Lp[a] plasma clearance. Asialofetuin, a ligand of the asialoglycoprotein receptor, did not inhibit Lp[a] clearance. In summary, the liver is the major organ accounting for the clearance of Lp[a] in mice, with the LDL receptor and apolipoprotein E having no major roles. Our studies indicate that apo[a] is the primary ligand that mediates Lp[a] uptake and plasma clearance.

Lipoprotein(a) and apolipoprotein(a) isoforms and proteinuria in patients with moderate renal failure

BACKGROUND

Atherosclerotic diseases are a major cause of death in patients with renal failure. Increased serum concentrations of lipoprotein(a) [Lp(a)] have been established as a genetically controlled risk factor for these diseases and have been demonstrated in patients with moderate renal failure, suggesting that this lipoprotein contributes to the increased cardiovascular risk seen in these patients. Variable alleles at the apolipoprotein(a) [apo(a)] gene locus are the main determinants of the serum Lp(a) level in the general population. The purpose of this study was to investigate apo(a) isoforms in patients with moderate renal failure and mild proteinuria (less than 1.0 g/day).

METHODS

In 250 consecutive subjects recruited at a hypertension clinic, we assessed the renal function by 24-hour creatinine clearance, proteinuria, and microalbuminuria, as well as the prevalence of atherosclerotic disease, and we also measured apo(a) isoforms, serum albumin, and Lp(a) concentrations.

RESULTS

Moderate impairment of renal function (creatinine clearance, 30 to 89 ml/min per 1.73 m2 of body surface area) was found in 97 patients. Lp(a) levels were significantly greater in patients with moderate renal failure (21.7+/-23.9 mg/dl) as compared with patients with normal renal function (15.6+/-16.4 mg/dl, P<0.001), and an inverse correlation was observed between log Lp(a) and creatinine clearance (r = -0.181, P <0.01). However, no difference was found in the frequency of low molecular weight apo(a) isoforms between patients with normal (25.5%) and impaired (27.8%) renal function. Only patients with the smallest size apo(a) isoforms exhibited significantly elevated levels of Lp(a), whereas the large-size isoforms had similar concentrations in patients with normal and impaired renal function. No significant relationship was found between serum Lp(a) and proteinuria. Clinical and laboratory evidence of one or more events attributed to atherosclerosis was found in 9.8% of patients with normal renal function and 25.8% of patients with moderate renal failure (P<0.001).

CONCLUSIONS

These results indicate that renal failure per se or other genes beside the apo(a) gene locus are responsible for the elevation of serum Lp(a) levels in patients with moderate impairment of renal function. The elevation of Lp(a) levels occurs independently of the level of proteinuria and may contribute to the risk for atherosclerotic disease in these patients.

An individual with a healthy phenotype in spite of a pathogenic LDL receptor mutation (C240F)

Familial hypercholesterolemia (FH) is caused by a defect in the function of the low density lipoprotein (LDL) receptor and inherited in an autosomal, codominant way. In this study we present a 13-year-old girl, compound heterozygote for the LDL receptor mutations C240F and Y167X. Fibroblasts from the patient showed very low cholesterol esterification rate, LDL uptake, and degradation compared to normal fibroblasts (< 2%, 8%, and < 2%, respectively). The C240F mutant was expressed in LDL receptor deficient CHOMldlA7 cells. Analysis of cell extracts by immunoblotting demonstrated delayed processing of the mutated LDL receptor, which was accumulated as a precursor protein of normal size. A high molecular weight form of the receptor was also detectable in these cells, which probably reflects cross-linking through the unpaired cysteine residue in the binding domain. Cells expressing the C240F mutant protein were unable to mediate uptake and degradation of LDL. The two siblings of the index case also carried the C240F mutation, but surprisingly one of them (a 17-year-old brother) showed no signs of hypercholesterolemia. This observation is consistent with the view that there may be cholesterol lowering mechanisms that can be activated, perhaps by mutations in known or hitherto unknown genes.

Sib-pair analysis detects elevated Lp(a) levels and large variation of Lp(a) concentration in subjects with familial defective ApoB

Whether or not Lp(a) plasma levels are affected by the apoB R3500Q mutation, which causes Familial Defective apoB (FDB), is still a matter of debate. We have analyzed 300 family members of 13 unrelated Dutch index patients for the apoB mutation and the apolipoprotein(a) [apo(a)] genotype. Total cholesterol, LDL-cholesterol, and lipoprotein(a) [Lp(a)] concentrations were determined in 85 FDB heterozygotes and 106 non-FDB relatives. Mean LDL levels were significantly elevated in FDB subjects compared to non-FDB relatives (P < 0.001). Median Lp(a) levels were not different between FDB subjects and their non-FDB relatives. In contrast, sib-pair analysis demonstrated a significant effect of the FDB status on Lp(a) levels. In sib pairs identical by descent for apo(a) alleles but discordant for the FDB mutation (n = 11) each sib with FDB had a higher Lp(a) level than the corresponding non-FDB sib. Further, all possible sib pairs (n = 105) were grouped into three categories according to the absence/presence of the apoB R3500Q mutation in one or both subjects of a sib pair. The variability of differences in Lp(a) levels within the sib pairs increased with the number (0, 1, and 2) of FDB subjects present in the sib pair. This suggests that the FDB status increases Lp(a) level and variability, and that apoB may be a variability gene for Lp(a) levels in plasma.

Interaction of native and oxidized lipoprotein(a) with human mesangial cells and matrix

The trapping of apolipoprotein(a) and apolipoprotein B-100 in glomeruli of patients with the nephrotic syndrome seems to be linked to a less favorable course of renal disease. To evaluate the potential role of lipoprotein(a) as a mediator of glomerular injury, we measured uptake of native lipoprotein(a) [Lp(a)] and oxidatively modified Lp(a) by cultured human mesangial cells and matrix and studied the effects of Lp(a) on mesangial cell DNA-synthesis and cellular proliferation. Uptake of Lp(a) by mesangial cells occurred at a significantly lower affinity (KD 16 micrograms/ml vs. 39 micrograms/ml) and a lower maximum degradative capacity (6.7-fold) than for LDL. Specificity of receptor mediated uptake was 50% for Lp(a) compared to 84% for LDL. Oxidative modification of both Lp(a) and LDL was accompanied by a significant decrease in uptake and degradative capacities. Due to the limited uptake, native and oxidatively modified Lp(a) had only marginal effects on intracellular cholesterol metabolism, which was measured as inhibition of sterol synthesis and stimulation of cholesterol esterification. However, binding of Lp(a), oxidized Lp(a) and oxidized LDL to extracellular mesangial matrix was enhanced compared to LDL. Furthermore, incubation of mesangial cells with Lp(a) and oxLp(a) in concentrations of 2.5 micrograms/ml and higher resulted in a decrease of DNA synthesis. Regardless of the oxidative status, a maximal suppression of DNA synthesis was observed at 20 micrograms/ml Lp(a). Native Lp(a) also blunted the stimulatory effects of PDGF on mesangial cell DNA-synthesis. Lp(a) did not alter basal TGF-beta transcription in human mesangial cells. The avid interaction of Lp(a) and modified lipoproteins with mesangial matrix provides a concept for the enhanced entrapment of these lipoproteins in the diseased glomerulum. Native Lp(a) is a poor ligand for the LDL receptor; oxidation of Lp(a) even lowers the affinity towards this receptor. Further studies must be carried out to clarify the pathophysiological significance of Lp(a) trapping in the mesangial matrix.

The role of lipoprotein(a) in atherogenesis and thrombosis

Lipoprotein(a) [Lp(a)] represents an important independent risk factor for atherosclerotic cardiovascular disease. Lp(a) constitutes a class of low-density lipoprotein-like particles that are structurally heterogeneous due to variability within the distinguishing apoprotein, apolipoprotein(a) [Apo(a)]. Apo(a) bears a high degree of homology to the fibrinolytic zymogen, plasminogen, the parent molecule of the serine protease plasmin. Apo(a) contains a variable number of tandemly repeated triple-loop units called kringles, which appear to mediate Lp(a)'s interactions with fibrin and cell surface receptors. Although the mechanism of its atherogenicity is unknown, Lp(a) has been implicated in the delivery of cholesterol to the injured blood vessel, in blockade of plasmin generation on fibrin and cell surfaces, and as a stimulus for smooth muscle cell proliferation. In addition, new members of the plasminogen/Apo(a) gene family have been defined, creating a potential link between Lp(a) and the control of angiogenesis in both health and disease. Pharmacologic therapy of elevated Lp(a) levels has been only modestly successful; apheresis remains the most effective therapeutic modality.

Effects of lipoprotein(a) and low density lipoprotein on growth of mitogen-stimulated human umbilical vein endothelial cells

We investigated the effects of lipoprotein(a) (Lp(a)) and low density lipoprotein (LDL) on proliferation of human umbilical vein endothelial cells (HUVECs). Both Lp(a) and LDL stimulated the growth of HUVECs synergistically with basic fibroblast growth factor and insulin in a dose-dependent manner. The potency of Lp(a) to promote the cell proliferation was 40% less than that of LDL. Addition of anti-transforming growth factor-beta 1 neutralizing antibody into the medium could not diminish the difference of HUVECs proliferation by Lp(a) and LDL. However, addition of anti-LDL receptor antibody suppressed HUVECs proliferation to the same level and sequestered the difference by the two lipoproteins. Moreover, cholesteryl ester content incubated with Lp(a) was 50% less than that with LDL. These results suggest that Lp(a) has less effect on HUVECs proliferation and cholesterol delivery to the cells than LDL. Therefore, Lp(a) may play a role as an atherogenic lipoprotein by delaying the repair of endothelium after injury.

Lack of association of apolipoprotein E polymorphism with plasma Lp(a) levels in the Chinese

Apolipoprotein E (apoE) polymorphism and its influence on plasma lipids, lipoproteins, lipoprotein (a) [Lp(a)] and apolipoproteins was studied in 536 (270 males and 266 females) healthy Chinese in Singapore. From analysis of variance with age and BMI as covariates, apoE genotype was found to exert a significant influence on plasma total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) and apoB in females. Its effect in males was marginally significant only on LDL-C. In both sexes, plasma TC, LDL-C and apoB were lower in those who were E2-3 than in those who were E3-3. There was no significant difference in log-transformed Lp(a) level between the apoE genotypes after adjusting for the confounding effect of LDL-C in addition to age and BMI. The percentage variance (R2 x 100) of the lipid traits explained by apoE polymorphism in the females was 4.94% for plasma TC, 5.85% for LDL-C and 4.25% for apoB. We conclude that: 1) epsilon 2 allele had a lowering effect on plasma TC, LDL-C and apoB; 2) apoE polymorphism did not have any significant influence on Lp(a) concentration; and 3) the effect of apoE polymorphism on plasma TC, LDL-C and apoB was gender-specific, with a stronger influence in females than in males.

The low density lipoprotein receptor is not required for normal catabolism of Lp(a) in humans

Lipoprotein(a) [Lp(a)] is an atherogenic lipoprotein which is similar in structure to low density lipoproteins (LDL). The role of the LDL receptor in the catabolism of Lp(a) has been controversial. We therefore investigated the in vivo catabolism of Lp(a) and LDL in five unrelated patients with homozygous familial hypercholesterolemia (FH) who have little or no LDL receptor activity. Purified 125I-Lp(a) and 131I-LDL were simultaneously injected into the homozygous FH patients, their heterozygous FH parents when available, and control subjects. The disappearance of plasma radioactivity was followed over time. As expected, the fractional catabolic rates (FCR) of 131I-LDL were markedly decreased in the homozygous FH patients (mean LDL FCR 0.190 d-1) and somewhat decreased in the heterozygous FH parents (mean LDL FCR 0.294 d-1) compared with controls (mean LDL FCR 0.401 d-1). In contrast, the catabolism of 125I-Lp(a) was not significantly different in the homozygous FH patients (mean FCR 0.251 d-1), heterozygous FH parents (mean FCR 0.254 d-1), and control subjects (mean FCR 0.287 d-1). In summary, absence of a functional LDL receptor does not result in delayed catabolism of Lp(a), indicating that the LDL receptor is not a physiologically important route of Lp(a) catabolism in humans.

Cellular binding and degradation of lipoprotein (a)

Lp(a) is an important contributing factor to the development of atherosclerosis, and in structure is similar to LDL. Given the central role of the LDL receptor (LDL-R) in the metabolism of LDL, we felt that a study of the binding and degradation of Lp(a) facilitated by the LDL-R of human monocyte derived macrophages (HMDM) would be of value in understanding its pathological nature. In this study we compared equimolar amounts of Lp(a) and LDL and found that nearly equal amounts of Lp(a) and LDL bound to the LDL-R of HMDM at 4 degrees C, however the affinity of both lipoproteins was much lower than has been observed for the LDL-R of fibroblasts, being 0.80 muM for Lp(a) and 0.23 muM for LDL. The binding of Lp(a) to HMDM could be competed by 63% with a 50-fold excess of LDL. Degradation of Lp(a) at 37 degree C, unlike 4 degrees C binding, was mainly nonspecific (75% of total Lp(a) degradation) and when compared on an equimolar basis, nearly 6 times more LDL than Lp(a) was processed by the LDL-R pathway in 5 hr. Lower degradation of Lp(a) appears to be the result of lower binding at 37 degree C and a lower degradation rate when compared to LDL. It was not caused by increased intracellular accumulation or retroendocytosis. Degradation of both lipoproteins was only modestly affected by up and down regulation of the LDL-R. Because the binding of LDL at 4 degrees C and degradation at 37 degree C is mainly LDL-R specific, whereas only the 4 degree C binding of Lp(a) is so, suggests that the poor LDL-R dependent degradation of Lp(a) at 37 degree C is caused by a conformational change that is inducted in Lp(a) upon lowering the temperature to 4 degree C which allows better recognition of Lp(a) by the HMDM LDL-R.

Binding and degradation of lipoprotein(a) and LDL by primary cultures of human hepatocytes. Comparison with cultured human monocyte-macrophages and fibroblasts

Although lipoprotein(a) (Lp[a]) has structural similarities to low-density lipoprotein (LDL) that include the presence of apolipoprotein B100, there is some disagreement over the strength of its interaction with the LDL receptor and its cellular catabolism by the LDL receptor-mediated pathway. To clarify this subject we evaluated LDL receptor-mediated binding and degradation of Lp(a) and LDL in three human cell lines. The binding of 50 nmol/L Lp(a) at 37 degrees C to the LDL receptor of primary hepatocytes, macrophages, and fibroblasts was only 10%, 29%, and 29% of the respective value obtained with 50 nmol/L LDL. Analysis of 4 degrees C binding curves indicated that Lp(a) and LDL had equal affinities for the LDL receptor of fibroblasts, whereas maximal binding of Lp(a) was remarkably lower than that of LDL. LDL receptor-mediated degradation of 50 nmol/L Lp(a) in hepatocytes, macrophages, and fibroblasts was only 17%, 22%, and 26%, respectively, of the value obtained with 50 nmol/L LDL and varied greatly among the cells in that it was lowest in hepatocytes, an order of magnitude greater in macrophages, and two orders of magnitude greater in fibroblasts. In contrast, the nonspecific degradation rate of Lp(a) was similar to that of LDL in each of the three tested cell lines. However, the proportion of the degradation of Lp(a) that was nonspecific varied greatly, being 76%, 58%, and 33% in hepatocytes, macrophages, and fibroblasts, respectively. These studies indicate that not only is Lp(a) recognized by the LDL receptor but also that, in fibroblasts, Lp(a) and LDL have equal affinities for the LDL receptor, although Lp(a) has a much lower receptor occupancy than LDL. Additionally, they show that there are great cellular differences in the LDL receptor-mediated degradation of Lp(a). If these results can be extrapolated in vivo, where normal LDL levels are 40- to 50-fold higher than those of Lp(a), it would be unlikely that the hepatic LDL receptor is significantly involved in the degradation of Lp(a).

Recent observations on the role of hemostatic determinants in the development of the atherothrombotic plaque

Recent evidence suggests that hemostatic determinants play a major role in the evolution of the atherothrombotic plaque. Platelets can serve as cholesterol donors for macrophages, thereby facilitating foam cell formation. Lipoprotein(a) inhibits fibrinolysis and may also contribute to atherogenesis by serving as a ligand for the scavenger receptor. By complexing with fibrin(ogen) in atheromatous lesions, lipoprotein(a) attenuates clearance of this protein, promoting atherogenesis and vascular dysfunction. These observations suggest that thrombotic determinants are critical for the development of the atheromatous plaque, and may guide the appropriate selection of potential therapeutic options in the future.

Changes in lipoprotein(a), LDL-cholesterol and apolipoprotein B in homozygous familial hypercholesterolaemic patients treated with dextran sulfate LDL-apheresis

We evaluated the effect of periodical treatment with LDL-apheresis by adsorption to dextran sulfate (Liposorber LA-15) on several aspects related to LDL and Lipoprotein(a) metabolisms, in three homozygous familial hypercholesterolaemic patients with LDL receptor deficiency. The dextran sulfate columns retained apolipoprotein B-containing particles with high affinity and capacity, in such a way that the treatment of a volume of plasma equivalent to three times the patient plasma volume resulted in an 85% decrease of circulating LDL-cholesterol and Lipoprotein(a). The continuous treatment with LDL-apheresis was highly beneficial for these patients since an average plasma concentration lower than 200 mg dl-1 for LDL-cholesterol, and lower than 25 mg dl-1 for Lipoprotein(a) could be achieved by treating the patients once a week. After each apheresis treatment, plasma concentrations of these metabolites progressively returned to the pretreatment, steady-state, levels. The analysis of the rates of return allowed us to estimate the fractional catabolic rates. FCRs of LDL-cholesterol were 0.052, 0.049 and 0.047 pools day-1, and those of apolipoprotein B, 0.065, 0.045 and 0.050 pools day-1 in the three subjects, respectively. These values are much lower than those in normolipidaemic individuals as observed by others, and are in accordance with the LDL-receptor deficiency condition of our patients. Two of them had highly elevated Lipoprotein(a) plasma concentrations, and their FCRs of Lipoprotein(a) were calculated to be 0.112 and 0.066 pools day-1.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo kinetics of lipoprotein(a) in homozygous Watanabe heritable hyperlipidaemic rabbits

In vivo kinetics of lipoprotein(a) [Lp(a)] were investigated in homozygous Watanabe heritable hyperlipidaemic (WHHL) rabbits (an animal model of familial hypercholesterolemia (FH)), and in normolipidemic Japanese White rabbits (controls). 125I-labelled Lp(a) and 131I-labelled LDL were simultaneously injected intravenously. Blood samples were then taken periodically. Kinetic parameters were calculated from the plasma radioactivity decay curves. The fractional catabolic rates (FCRs) of both Lp(a) and LDL (1.355 +/- 0.189 pools per day and 1.278 +/- 0.397 pools per day, respectively) in the WHHL rabbits were significantly (P < 0.005) smaller than those in the control rabbits (2.008 +/- 0.083 pools per day and 2.855 +/- 0.759 pools per day, respectively). In WHHL rabbits, the FCRs of Lp(a) and LDL were similar. However, in control rabbits, the FCR of Lp(a) was significantly (P < 0.01) smaller than that of LDL. In WHHL rabbit organs, the mean ratio of 125I-Lp(a): 131I-LDL, 48 h after injection, normalized to the corresponding isotope ratio in plasma, were 1.525, 1.020, 1.819 and 1.967, in liver, kidney, spleen and bile, respectively. These values were significantly higher than the corresponding values in control rabbits (0.590, 0.677, 0.862 and 0.766, respectively). Our data strongly suggest that Lp(a) clearance is not entirely dependent upon LDL receptors and may be mediated by some other mechanisms.

Heterogeneous lipoprotein (a) size isoforms differ by their interaction with the low density lipoprotein receptor and the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor

Lipoprotein (a) (Lp(a)) is a complex of low density lipoprotein (LDL) with apolipoprotein (apo) (a). To examine the size distribution of Lp(a), plasma was separated by fast flow gel filtration and Lp(a):B complexes were determined in the eluate by enzyme immunoassays, in which detection was performed with monoclonal antibodies specific for apoB. Lp(a):B particles displayed apparent molecular masses (M(r)) of 2 x 10(6) to at least 10 x 10(6). Lp(a) size isoforms differed by the expression of apoB epitopes and their interaction with cultured human skin fibroblasts. LDL was more effective in inhibiting binding, uptake, and degradation of low M(r) Lp(a) than of high M(r) Lp(a). In contrast, Glu-plasminogen, alpha 2-macroglobulin and tissue-type plasminogen activator were more effective in competing for the cellular degradation of high M(r) Lp(a) than of low M(r) Lp(a). Ligand blotting revealed that Lp(a) bound to the low density lipoprotein receptor, the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and to two other endosomal membrane proteins. We propose that the LDL receptor preferentially internalizes low M(r) Lp(a), whereas LRP may have a role in the clearance of high M(r) Lp(a).

Interaction of Lp(a) and of apo(a) with liver cells

Lipoprotein(a) (Lp[a]) is a lipoprotein of high atherogenicity with unknown function. Although it binds in vitro to the low-density lipoprotein (LDL) receptor, it is not clear whether this mechanism also operates in vivo. We studied the interaction of Lp(a) and of apoprotein(a) (apo[a]) with hepatoma cells (HepG2 and Hep3B) with the following results. (1) HepG2 cells exhibited saturable high-affinity binding of LDLs, whereas the majority of Lp(a) binding was of low affinity and nonsaturable. Preincubation of HepG2 cells with LDL markedly reduced cholesterol biosynthesis, but Lp(a) had a much lower effect. (2) When HepG2 cells were preincubated for 48 to 72 hours with Lp(a) or apo(a), 125I-LDL binding was increased by a factor of > 2. During this time, up to approximately 1 microgram of apo(a) per 1 milligram cell protein was found to be cell associated in an undegraded form. Monoclonal antibodies against the LDL receptor did not prevent the increase in LDL binding stimulated by apo(a). (3) Coincubation with LDL caused a significant increase of Lp(a) degradation by HepG2 cells that was probably caused by an increase of Lp(a) uptake in a "hitchhiking"-like process.

Relation of lipoprotein(a) to coronary heart disease and duplexsonographic findings of the carotid arteries in heterozygous familial hypercholesterolemia

In familial hypercholesterolemia (FH) elevated Lp(a) concentrations are more frequent than in the general Caucasian population, but the clinical relevance of Lp(a) as a risk-factor in this group of patients is controversial. In 91 adult patients with heterozygous FH due to LDL-receptor defect we analyzed the correlation between Lp(a) concentrations, presence of coronary heart disease (CHD) and degree of atherosclerosis of the carotid arteries assessed by duplex scan. Coronary heart disease was present in 32 patients (24 males, 8 females). In the group without CHD the median of the Lp(a) distribution was 23 mg/dl, in the group with CHD 43 mg/dl (P < 0.05). The median of Lp(a) was 8 mg/dl in patients without pathological changes in the duplex scan of the carotids, 13 mg/dl in the group with intimal thickening, 25 mg/dl in patients with non-obstructing plaques, and 45 mg/dl in presence of > 30% luminal obstruction (P < 0.01). The role of Lp(a) as an independent risk factor was analyzed by stepwise logistic regression together with age, sex, LDL-, HDL-cholesterol, serum triglycerides, smoking status and presence of hypertension. For the prediction of CHD only age, HDL cholesterol and gender reached statistical significance. Lp(a) was, however, the lipoprotein parameter with the highest discriminative strength for the presence of a pathological duplex scan (P = 0.016), followed by LDL- (P = 0.03), and HDL-cholesterol (P = 0.03). These results provide direct evidence for a close correlation between Lp(a) and the rate of progression of atherosclerosis in FH, already at early, asymtomatic stages.

A PvuII polymorphism of the low density lipoprotein receptor gene is not associated with plasma concentrations of low density lipoproteins including LP(a)

Lipoprotein(a) [Lp(a)] is a low density lipoprotein (LDL), in which apolipoprotein B-100 (apo B-100) is attached to apolipoprotein(a) [apo(a)], a glycoprotein of variable size. Lp(a) may be as atherogenic as LDL. In normal populations, Lp(a) concentrations in plasma are largely determined by the apo(a) gene locus on chromosome 6, but regulation of synthesis and catabolism of Lp(a) is poorly understood. In some studies, a PvuII restriction fragment length polymorphism (RFLP) in the LDL receptor gene seems to affect concentrations of LDL in plasma, and other studies have indicated that Lp(a) catabolism could be mediated by the LDL receptor. We therefore expected that the PvuII polymorphism in the LDL receptor gene might be associated with Lp(a) levels in 170 Caucasian men aged 40 years, selected to have a high representation of low molecular weight apo(a) phenotypes. However, plasma concentrations of cholesterol, LDL-cholesterol, HDL-cholesterol, triglycerides and Lp(a) were all unrelated to the LDL receptor gene PvuII polymorphism both in the group as a whole and when it was subgrouped by apo(a) phenotype. Therefore our data do not support the concept that this particular LDL receptor gene polymorphism is associated with LDL receptor function, and our data therefore neither support nor rule out a role for the LDL receptor in Lp(a) catabolism.

Growth hormone treatment of growth hormone-deficient adults results in a marked increase in Lp(a) and HDL cholesterol concentrations

The effects of growth hormone treatment of adults with adult-onset pituitary insufficiency on lipoproteins and apolipoproteins were investigated. Nine patients, one women and eight men (age range, 34-58 years), who had been treated for pituitary tumors were studied. They had complete pituitary insufficiency with a duration of at least 1 year. All patients received replacement therapy with thyroid hormones, glucocorticoids, and gonadal steroids. The study had a double-blind, placebo-controlled, crossover design for active treatment with recombinant human growth hormone (0.25-0.5 units/kg per week s.c. given each evening) for 6 months. Fasting serum levels of cholesterol; triglycerides; high density lipoprotein and low density lipoprotein cholesterol; apolipoproteins A-I, B, and E; and lipoprotein (a) were measured before and after 6 and 26 weeks of treatment. Lipoprotein (a) concentrations increased markedly during treatment and were about twice as high compared with pretreatment levels. Serum cholesterol and low density lipoprotein cholesterol concentrations were decreased after 6 weeks of treatment, but levels had returned to pretreatment levels after 26 weeks. High density lipoprotein cholesterol concentrations increased during treatment and were significantly higher than pretreatment levels after 26 weeks of treatment. Serum triglyceride concentrations did not change significantly, but in two patients with marked hypertriglyceridemia, growth hormone treatment resulted in a marked decrease. Serum concentrations of apolipoproteins A-I, B, and E did not change significantly, but changes in apolipoprotein A-I and B concentrations were in parallel to those observed for high density lipoprotein cholesterol and low density lipoprotein cholesterol, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamine/histidine polymorphism in apo A-IV affects plasma concentrations of lipoprotein(a) and fibrin split products in coronary heart disease patients

A glutamine/histidine polymorphism at residue 360 in apolipoprotein (apo) A-IV that generates two electrophoretically detectable isoforms, apo A-IV-1 and apo A-IV-2, affects the plasma concentration of lipoprotein(a) (Lp[a]) in a healthy population. To verify this unexpected association we analyzed the effect of the apo A-IV polymorphism on Lp(a) serum concentrations in 275 male coronary heart disease patients. Allele frequencies of apo A-IV-1 and apo A-IV-2 were 0.917 and 0.083, respectively. In addition, apo A-IV-1/2 heterozygotes showed a 30% lower geometric mean concentration of Lp(a) than apo A-IV-1/1 homozygotes in this study. The relative frequency of Lp(a) concentrations > 20 mg/dl was significantly increased by a factor of 2.25 in apo A-IV-1/1 homozygotes. Other lipid parameters were not significantly affected by this apo A-IV polymorphism. Because of the relations between Lp(a) and the fibrinolytic system, we also analyzed the effect of the apo A-IV polymorphism on hemostatic variables. Apo A-IV-1/2 heterozygosity was associated with a 70% higher geometric mean plasma concentration of D-dimer, i.e., proteolytic fragments of cross-linked fibrin. Plasma concentrations of prothrombin fragments F1 + F2, fibrinogen, plasminogen, and plasminogen activator inhibitor-1 were unaffected. In conclusion, our results indicate a hitherto unappreciated role of the apo A-IV gene or a closely linked locus for the regulation of Lp(a) metabolism and hemostasis and also possibly for atherosclerosis and thrombosis.

Changes in Lp(a) lipoprotein levels during the treatment of hypercholesterolaemia with simvastatin

Thirty-six patients with total serum cholesterol levels above 6.5 mmol/l and Lipoprotein(a) levels above 100 mg.l-1 were evaluated in a 24 week double-blind, placebo controlled, cross-over study to assess the possible changes in Lp(a) during treatment with the HMG CoA reductase inhibitor simvastatin. The median plasma Lp(a) increased from 359 to 464 mg.l-1 during simvastin treatment as compared to placebo (not significant). Individual changes in Lp(a) varied. In a multivariate linear regression analysis the individual change in Lp(a) was correlated with the baseline Lp(a) (r = 0.64), the change in serum triglycerides (r = 0.48) and the baseline apolipoprotein B (r = 0.36). Differences between the Lp(a) phenotypes may explain some of the varied Lp(a) responses. It appears that the effect of simvastatin on the Lp(a) level in individuals is usually insignificant, but in patients with a high Lp(a) simvastatin may further increase it.

Isolation and characterization of two sub-species of Lp(a), one containing apo E and one free of apo E

Lipoprotein Lp(a) was isolated by immunoaffinity chromatography using anti apolipoprotein B and anti apolipoprotein (a) immunosorbents. Besides apolipoproteins (a) and B, this fraction was shown to contain apolipoproteins C and E. Therefore, it was decided to further purify this crude Lp(a) into particles containing apolipoprotein E and particles free of apo E, using chromatography with an anti apolipoprotein E immunosorbent. Lp(a), free of apolipoprotein E was cholesterol ester rich and triacylglycerol poor and was found mainly in the LDL size range. In contrast, Lp(a) containing apolipoprotein E was triacylglycerol rich and was distributed mainly in the VLDL and IDL size range. Binding of these two fractions, one containing apo E and one free of it, to the apo B/E receptor of HeLa cells was studied. Both fractions bound to the receptor but the one containing apo E had a better affinity than the one free of apo E. Further studies are needed to identify the clinical importance of these two different entities.

Acute reduction of lipoprotein(a) by tissue-type plasminogen activator

BACKGROUND

Lipoprotein(a) [Lp(a)] is a low density lipoprotein-like particle whose apolipoprotein B (apo B) moiety is disulfide-linked to apo(a), a plasminogen-like inhibitor of fibrinolysis in vitro. We hypothesized that plasma concentrations of Lp(a) are acutely affected by intravenous tissue-type plasminogen activator (t-PA).

METHODS AND RESULTS

Patients with unstable angina were randomized to receive either intravenous t-PA (n = 15) or placebo (n = 11). Two-way ANOVA using repeated measures revealed a significant effect of t-PA on concentrations of Lp(a) (p = 0.026). There was a 48% fall in Lp(a) from baseline concentrations in the t-PA group at 12 hours (p = 0.031) but not at 72 hours. Lp(a) in the placebo group was unchanged.

CONCLUSIONS

We conclude that t-PA produces a sharp and substantial but reversible reduction in plasma Lp(a). These data suggest that Lp(a) concentration is not as static in vivo as had been believed and might be acutely modifiable through some mechanism that induces its removal from the freely circulating state.

Hormonal regulation of serum Lp (a) levels. Opposite effects after estrogen treatment and orchidectomy in males with prostatic carcinoma

Serum concentrations of lipoprotein (a) [Lp (a)] were determined in two groups of elderly males suffering from prostatic carcinoma, who were randomized to treatment with estrogen (n = 15) or orchidectomy (n = 16). Estrogen was given as oral ethinylestradiol, 150 micrograms daily, combined with intramuscular polyestradiol phosphate, 80 mg/mo. The baseline levels were similar in both groups, but 6 mo after initiation of therapy, serum Lp (a) levels were decreased approximately 50% in the estrogen-treated group (P less than 0.001) in contrast to a 20% increase (P less than 0.01) in the orchidectomized group. Concomitantly, LDL cholesterol decreased by 30% and HDL cholesterol increased by almost 60% in the estrogen-treated patients. There was no relationship between the change in LDL cholesterol and Lp (a) reduction. In conclusion, Lp (a) levels in males were found to drastically decrease upon estrogen treatment and to increase after orchidectomy, suggesting that sex hormones, and particularly estrogens, exert a regulatory role on the serum Lp (a) level in man.

Enhanced uptake and impaired intracellular metabolism of low density lipoprotein complexed with anti-low density lipoprotein antibodies

We have previously shown that incubation of human macrophages with antigen-antibody complexes prepared with native human low density lipoprotein (LDL) and rabbit anti-LDL antibodies (LDL-ICs) results in an increased intracellular accumulation of cholesteryl esters (CEs) and induces a marked increase in the number of LDL receptors. To determine whether the increased CE accumulation in these cells occurred during incubation of the cells with LDL-ICs or whether it was secondary to the uptake of LDL by overexpressed LDL receptors, we incubated human macrophages with LDL-ICs for 22 hours, followed by incubation with native LDL for another 20 hours. We found that about 90% of the accumulated CEs could be accounted for by the first incubation with LDL-ICs. We then proceeded to show that the CEs accumulated during incubation of cells with LDL-ICs was secondary to enhanced uptake and impaired degradation of the LDL complexed with immunoglobulin G (IgG) (LDL-IC), which led to a marked intracellular accumulation of undergraded LDL (levels 199-fold higher than those obtained when the cells were incubated with the same concentration of native LDL not complexed with IgG). We have also shown that not all CEs accumulated in these cells were derived from accumulation of undegraded LDL and that some of them were derived from the reesterification of free cholesterol released during hydrolysis of LDL. LDL-ICs promoted increased CE accumulation and foam cell formation at concentrations as low as 25 micrograms/ml. To determine which receptors were involved in the uptake of LDL-ICs, we performed experiments in which the uptake of LDL-ICs was competitively inhibited with heat-aggregated gamma globulin, native LDL, beta-very low density lipoprotein, or acetylated LDL. Our results demonstrated that LDL-IC uptake was most effectively inhibited by heat-aggregated gamma globulin, partially inhibited by native LDL or by a monoclonal antibody to the LDL receptor, and not inhibited by acetylated LDL or beta-very low density lipoprotein. Thus, we conclude that the majority of LDL-ICs are taken up through Fc gamma receptors. Finally, we investigated whether the increase in LDL receptor expression was dependent on the receptor pathway used by the LDL-ICs, and we were able to demonstrate that when macrophages were incubated with LDL-ICs prepared with F(ab')2 fragments of the anti-LDL antibody, LDL receptor expression was not enhanced. Therefore, we postulate that the uptake of LDL-ICs through Fc gamma receptors results in an uncoupling of the normal regulation of the LDL receptor expression.

Morphological detection and quantification of lipoprotein(a) deposition in atheromatous lesions of human aorta and coronary arteries

Lipoprotein(a), as an atherogenic particle, represents an independent risk factor for coronary heart disease. In the present study the morphological distribution of apoprotein (a) and apoprotein B within the arterial wall is described. Apoprotein B, a constituent of very low-density lipoprotein, low-density lipoprotein and lipoprotein(a) has previously been demonstrated in atheromatous lesions. Lipoprotein(a) possesses an additional protein, designated apoprotein (a). Autopsy material (n = 74) from the left coronary artery and from the thoracic aorta has been examined by means of immunohistochemistry and both apoprotein (a) and apoprotein B were detected, primarily associated with the extracellular matrix and accumulating in lesions in the arterial wall. The staining pattern for both antigens was almost always found to be congruent, suggesting that the detection of (a)-antigen has to be attributed at least in part to the presence of lipoprotein(a). It is concluded that both low-density lipoprotein and lipoprotein(a) have an important role in the pathogenesis of atherosclerosis.

Overexpression of human low density lipoprotein receptors leads to accelerated catabolism of Lp(a) lipoprotein in transgenic mice

Lp(a) lipoprotein purified from human plasma bound with high affinity to isolated bovine LDL receptors on nitrocellulose blots and in a solid-phase assay. Lp(a) also competed with 125I-LDL for binding to human LDL receptors in intact fibroblasts. Binding led to cellular uptake of Lp(a) with subsequent stimulation of cholesterol esterification. After intravenous injection, human Lp(a) was cleared slowly from the plasma of normal mice. The clearance was markedly accelerated in transgenic mice that expressed large amounts of LDL receptors. We conclude that the covalent attachment of apo(a) to apo B-100 in Lp(a) does not interfere markedly with the ability of apo B-100 to bind to the LDL receptor and that this receptor has the potential to play a major role in clearance of Lp(a) from the circulation of intact humans.

Effect of simvastatin on plasma lipids, apolipoproteins and lipoprotein particles in patients with primary hypercholesterolaemia

The effect of treatment with simvastatin, a new HMG-CoA reductase inhibitor, has been investigated in 27 patients with primary hypercholesterolaemia. It produced a significant decrease of cholesterol and phospholipids in plasma, LDL and apolipoprotein B-containing lipoproteins. Plasma apolipoproteins B, C-III and E were also significantly lowered. The concentration of lipoprotein particles recognized by monoclonal antibodies (BL3, BL5 and BL7), associated with atherosclerotic disease, was also lowered by the treatment. Lipoproteins LpA-II:A-I were not changed, while LpA-I, which has been suggested to be the protective fraction of the apo A-I-containing lipoproteins, was slightly and inconsistently increased.

Effect of HELP-LDL-apheresis on serum concentrations of human lipoprotein(a): kinetic analysis of the post-treatment return to baseline levels

In addition to LDL, Lp(a) can be quantitatively eliminated from plasma in an extracorporeal LDL-apheresis procedure based on precipitation with heparin at pH 5.12. The rates of return of post-apheresis Lp(a) and LDL-cholesterol concentrations to baseline levels were investigated in six individuals with familial hypercholesterolaemia (one homozygote, five heterozygote) and one normolipaemic individual. The first-order disappearance constants (kappa) were derived for LDL and Lp(a) according to Apstein et al. The kappa values for LDL in the homozygous FH and the normolipaemic individual were 0.082 and 0.43 respectively while the heterozygous FH patients had kappa values intermediate between the two (median 0.231; range 0.116-0.261). The first-order disappearance constants of Lp(a) did not correlate with those of LDL. The homozygous FH and normolipaemic individuals had Lp(a) kappa values of 0.158 and 0.199 respectively; the corresponding values in the heterozygous FH patients were: median 0.142; range 0.045-0.179.