Structure and metabolism of lipoprotein (a)

Lipoproteins, Cholesterol, and Atherosclerotic Cardiovascular Disease in East Asians and Europeans

One fifth of the world population live in East Asia comprising Japan, Korea, and China where ischemic heart disease, a major component of atherosclerotic cardiovascular disease (ASCVD), is the second most frequent cause of death. Each of low-density lipoproteins (LDL), remnant lipoproteins, and lipoprotein(a), summarized as non-high-density lipoproteins (non-HDL) or apolipoprotein B (apoB) containing lipoproteins, causes ASCVD. However, a significant proportion of the evidence on lipoproteins and lipoprotein cholesterol with risk of ASCVD came from White people mainly living in Europe and North America and not from people living in East Asia or of East Asian descent. With a unique biological, geohistorical, and social background in this world region, East Asians have distinctive characteristics that might have potential impact on the association of lipoproteins and lipoprotein cholesterol with risk of ASCVD. Considering the movement across national borders in the World, understanding of lipoprotein and lipoprotein cholesterol evidence on ASCVD in East Asia is important for both East Asian and non-East Asian populations wherever they live in the World.In this review, we introduce the biological features of lipoproteins and lipoprotein cholesterol and the evidence for their association with risk of ASCVD in East Asian and European populations. We also provide an overview of guideline recommendations for prevention of ASCVD in these two different world regions. Finally, specific preventive strategies and future perspectives are touched upon.

Assessment of Apolipoprotein(a) Isoform Size Using Phenotypic and Genotypic Methods

Apolipoprotein(a) (apo(a)) is the protein component that defines lipoprotein(a) (Lp(a)) particles and is encoded by the LPA gene. The apo(a) is extremely heterogeneous in size due to the copy number variations in the kringle-IV type 2 (KIV2) domains. In this review, we aim to discuss the role of genetics in establishing Lp(a) as a risk factor for coronary heart disease (CHD) by examining a series of molecular biology techniques aimed at identifying the best strategy for a possible application in clinical research and practice, according to the current gold standard.

Biohybrid Nanoparticles to Negotiate with Biological Barriers

Incapability of effective cross-talk with biological environments has partly impaired the in vivo functionality of nanoparticles (NPs). Homing, biodistribution, and function of NPs could be engineered through regulating their interactions with in vivo niches. Inspired by communications in biological systems, endowing a "biological identity" to synthetic NPs is one approach to control their biodistribution, and immunonegotiation profiles. This synthetic-biological combination is referred to as biohybrid NPs, which comprise both i) engineerable, readily producible, and trackable synthetic NPs as well as ii) biological moieties with the capability to cross-talk with immunological barriers. Here, the latest understanding on the in vivo interactions of NPs, biological barriers they face, and emerging methods for quantitative measurements of NPs' biodistribution are reviewed. Some key biomolecules that have emerged as negotiators with the immune system in the context of cancer and autoimmunity, and their inspirations on biohybrid NPs are introduced. Critical design considerations for efficient cross-talk between NPs and innate and adaptive immunity followed by hybridization methods are also discussed. Finally, clinical translation challenges and future perspectives regarding biohybrid NPs are discussed.

Recent advances in physiological lipoprotein metabolism

Research into lipoprotein metabolism has developed because understanding lipoprotein metabolism has important clinical indications. Lipoproteins are risk factors for cardiovascular disease. Recent advances include the identification of factors in the synthesis and secretion of triglyceride rich lipoproteins, chylomicrons (CM) and very low density lipoproteins (VLDL). These included the identification of microsomal transfer protein, the cotranslational targeting of apoproteinB (apoB) for degradation regulated by the availability of lipids, and the characterization of transport vesicles transporting primordial apoB containing particles to the Golgi. The lipase maturation factor 1, glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 and an angiopoietin-like protein play a role in lipoprotein lipase (LPL)-mediated hydrolysis of secreted CMs and VLDL so that the right amount of fatty acid is delivered to the right tissue at the right time. Expression of the low density lipoprotein (LDL) receptor is regulated at both transcriptional and post-transcriptional level. Proprotein convertase subtilisin/kexin type 9 (PCSK9) has a pivotal role in the degradation of LDL receptor. Plasma remnant lipoproteins bind to specific receptors in the liver, the LDL receptor, VLDL receptor and LDL receptor-like proteins prior to removal from the plasma. Reverse cholesterol transport occurs when lipid free apoAI recruits cholesterol and phospholipid to assemble high density lipoprotein (HDL) particles. The discovery of ABC transporters (ABCA1 and ABCG1) and scavenger receptor class B type I (SR-BI) provided further information on the biogenesis of HDL. In humans HDL-cholesterol can be returned to the liver either by direct uptake by SR-BI or through cholesteryl ester transfer protein exchange of cholesteryl ester for triglycerides in apoB lipoproteins, followed by hepatic uptake of apoB containing particles. Cholesterol content in cells is regulated by several transcription factors, including the liver X receptor and sterol regulatory element binding protein. This review summarizes recent advances in knowledge of the molecular mechanisms regulating lipoprotein metabolism.

Lipopeptide nanoparticles for potent and selective siRNA delivery in rodents and nonhuman primates

siRNA therapeutics have promise for the treatment of a wide range of genetic disorders. Motivated by lipoproteins, we report lipopeptide nanoparticles as potent and selective siRNA carriers with a wide therapeutic index. Lead material cKK-E12 showed potent silencing effects in mice (ED50 ∼ 0.002 mg/kg), rats (ED50 < 0.01 mg/kg), and nonhuman primates (over 95% silencing at 0.3 mg/kg). Apolipoprotein E plays a significant role in the potency of cKK-E12 both in vitro and in vivo. cKK-E12 was highly selective toward liver parenchymal cell in vivo, with orders of magnitude lower doses needed to silence in hepatocytes compared with endothelial cells and immune cells in different organs. Toxicity studies showed that cKK-E12 was well tolerated in rats at a dose of 1 mg/kg (over 100-fold higher than the ED50). To our knowledge, this is the most efficacious and selective nonviral siRNA delivery system for gene silencing in hepatocytes reported to date.

Detection of coronary atherosclerotic plaques with superficial proteoglycans and foam cells using real-time intrinsic fluorescence spectroscopy

OBJECTIVES

The protein components of low-density lipoprotein (LDL), oxidized LDL and proteoglycans such as versican contain tryptophan, an amino acid with characteristic fluorescence features at 308 nm excitation wavelength. We hypothesize that intrinsic fluorescence spectroscopy at 308 nm excitation wavelength IFS308, a method suitable for clinical use, can identify coronary artery lesions with superficial foam cells (SFCs) and/or proteoglycans.

METHODS

We subjected 119 human coronary artery specimens to in vitro fluorescence and reflectance spectroscopy. We used 5 basis spectra to model IFS308, and extracted their contributions to each individual IFS308 spectrum. A diagnostic algorithm using the contributions of Total Tryptophan and fibrous cap to IFS308 was built to identify specimens with SFCs and/or proteoglycans in their top 50 μm.

RESULTS

We detected SFCs and/or proteoglycans, such as versican or the glycosaminoglycan hyaluronan, in 24 fibrous cap atheromas or pathologic intimal thickening (PIT) lesions. An algorithm using the contributions of Total Tryptophan and fibrous cap to IFS308 was able to identify these segments with 92% sensitivity and 80% specificity.

CONCLUSION

We were able to establish a set of characteristic LDL, oxidized LDL, versican and hyaluronan fluorescence spectra, ready to be used for real-time diagnosis. The IFS(308) technique detects SFCs and/or proteoglycans in fibrous cap atheromas and PIT lesions. SFCs and proteoglycans are histological markers of vulnerable plaques, and this study is a step further in developing an invasive clinical tool to detect the vulnerable atherosclerotic plaque.

The Effects of Extended Release Niacin in Combination with Omega 3 Fatty Acid Supplements in the Treatment of Elevated Lipoprotein (a)

Objective. To assess the effectiveness of niacin/fish oil combination therapy in reducing Lipoprotein (a) [Lp(a)] levels after twelve weeks of therapy. Background. Lipoprotein (a) accumulates in atherosclerotic lesions and promotes smooth muscle cell growth and is both atherogenic and thrombogenic. A clinical trials of combination therapy for the reduction of Lp(a) has not been previously reported. Methods. The study was an observational study following subjects with an elevated Lp(a) (>70 nmol/L) to assess impact of 12 weeks of combination Omega 3FA, niacin, and the Mediterranean diet on Lp(a). Results. Twenty three patients were enrolled with 7 patients lost to follow up and 2 patients stopped due to adverse events. The average Lp(a) reduction in the remaining 14 subjects after 12 weeks of combination therapy was 23%  ± 17% [P = .003] with a significant association of the reduction of Lp(a) with increasing baseline levels of Lp(a) [R² = 0.633, P = .001]. Conclusions. There was a significant reduction in Lp(a) levels with combination therapy. A more pronounced effect was noted in patients with higher baseline levels of Lp(a).

Lipoprotein (a) and its immune complexes in dyslipidemic subjects

OBJECTIVES

To investigate plasma levels of lipoprotein (a) [Lp(a)] and low-density lipoprotein (LDL)-circulating immune complexes (IC) in subjects with various dyslipidemias.

METHODS

Plasma Lp(a), Lp(a)-IC, and LDL-IC levels were determined by enzyme-linked immunosorbent assays (ELISAs) in 198 subjects with various dyslipidemias and 34 control subjects.

RESULTS

Hypertriglyceridemic subjects exhibited the lowest plasma Lp(a) levels, while hypercholesterolemic subjects exhibited the highest levels. Subjects with mixed hyperlipidemia had intermediate plasma Lp(a) concentrations, which were significantly lower than those of subjects with normal lipid levels. Interestingly, we also found that hypertriglyceridemic subjects had the lowest plasma Lp(a)-IC and LDL-IC levels, while hypercholesterolemic subjects exhibited the highest levels. Triglyceride (TG) levels were negatively correlated with Lp(a) (r = -0.15, P < 0.05), Lp(a)-IC (r = -0.20, P < 0.01), and LDL-IC (r = -0.214, P < 0.01) concentrations. Furthermore, significantly positive relationships were found between Lp(a)-IC and Lp(a) levels (r = 0.65, P < 0.001) and between LDL-IC and LDL-C levels (r = 0.43, P < 0.001).

CONCLUSIONS

The results argue for a regulatory role of TG on plasma Lp(a) and its circulating immune complexes in subjects with various dyslipidemias. The circulating levels of these immune complexes levels are likely to change with different concentrations of Lp(a) and LDL.

Lipoprotein(a) changes during natural menstrual cycle and ovarian stimulation with recombinant and highly purified urinary FSH

This prospective, randomized, controlled study compared the effects of recombinant human FSH (r-hFSH) and highly purified urinary FSH (u-hFSH HP) on lipoprotein(a) [Lp(a)] concentrations in women undergoing ovarian stimulation. Fifty infertile women were randomly allocated into two equally sized treatment groups (n = 25 per group). Thirty normal ovulation women were recruited as controls. The infertile women received u-hFSH or r-hFSH 150 IU/day starting on cycle day 2. From cycle day 6 the dose was adjusted according to ovarian response. Human chorionic gonadotrophin 10,000 IU was administered once there was at least one follicle > or =18 mm in diameter. The luteal phase was supported with progesterone 50 mg/day for at least 15 days. Repeated measurements of Lp(a) concentrations were performed during both stimulated and natural cycles. A significant increase in luteal phase Lp(a) concentrations was detected in the stimulated cycles, whereas no significant changes in serum Lp(a) concentrations were observed during natural cycles. There were no significant differences between the urinary and recombinant FSH effects on serum Lp(a). The luteal Lp(a) increase was transitory because after 1 month Lp(a) concentrations returned to baseline values if pregnancy failed to occur; in pregnant women persistent increased Lp(a) concentrations were found at the 8th week. The percentage changes in serum Lp(a) were positively correlated with the luteal progesterone increase (r = 0.40, P < 0.05), but not with follicular or luteal oestradiol increase. The women with low baseline Lp(a) (< or =5 mg/dl) had a greater increase of the Lp(a) concentrations at midluteal phase than women with baseline Lp(a) >5 mg/dl. In conclusion, the recombinant or urinary hFSH administration does not directly influence Lp(a) concentrations. The luteal Lp(a) increase in stimulated cycles is not related to gonadotrophin treatment per se, but appears to be related to the high luteal progesterone concentrations, physiologically or pharmacologically determined. Our results also suggest that the sensitivity to the progesterone changes could be related to apolipoprotein(a) phenotype.

Impact of apolipoprotein(a) on in vitro angiogenesis

Angiostatin, which consists of the kringle I-IV domains of plasminogen and which is secreted into urine, is an efficient inhibitor of angiogenesis and tumor growth. Because N-terminal apolipoprotein(a) [apo(a)] fragments, which also contain several types of kringle IV domains, are found in urine as well, we evaluated the potential angiostatic properties of these urinary apo(a) fragments and of a recombinant form of apo(a) [r-apo(a)]. We used human microvascular endothelial cell (hMVEC)-based in vitro assays of tube formation in 3-dimensional fibrin matrixes. Purified urinary apo(a) fragments or r-apo(a) inhibited the basic fibroblast growth factor/tumor necrosis factor-alpha-induced formation of capillary-like structures. At concentrations varying from 0.2 to 10 microgram/mL, urinary apo(a) fragments inhibited tube formation by as much as 70%, whereas there was complete inhibition by r-apo(a). The highest concentrations of both inhibitors also reduced urokinase plasminogen activator production of basic fibroblast growth factor-induced hMVEC proliferation. The inhibitors had no effect on plasminogen activator inhibitor-1 expression. If our in vitro model for angiogenesis is valid for the in vivo situation as well, our data point toward the possibility that apo(a) may also be physiologically operative in modulating angiogenesis, as the concentration of free apo(a) found in humans exceeds that tested herein.

Prospective association of lipoprotein(a) concentrations and apo(a) size with coronary heart disease among men in the Multiple Risk Factor Intervention Trial

In this nested case-control study, lipoprotein (a) [Lp(a)] concentrations and apo(a) isoform size were measured in serum samples obtained from men participating in the prospective Multiple Risk Factor Intervention Trial (MRFIT). Serum from men aged 35 to 57 years and stored for up to 20 years were analyzed for Lp(a) levels (n=736) and isoform size (n=487), respectively. Cases involved nonfatal myocardial infarctions (MI; n=98), documented during the active phase of the study that ended on February 28, 1982 and coronary heart disease (CHD) deaths (n=148) monitored through 1990. Median Lp(a) levels did not differ between cases and controls and mean apo(a) size did not vary between cases and controls in the entire study population. When adjusted for age and Lp(a) concentration, logistic regression analysis indicated that small apo(a) isoforms were associated with CHD deaths among smokers (OR 3.31; 95% CI 1.07-10.28).

Evaluation of a new apolipoprotein(a) isoform-independent assay for serum Lipoprotein(a)

The risk factor, Lipoprotein(a), [(Lp(a)], has been measured in numerous clinical studies by a variety of immunochemical assay methods. It is becoming apparent that for many of these assays antibody specificity towards the apolipoprotein(a) [apo(a)] repetitive component [the kringle 4-type 2 repeats] and apo(a) size heterogeneity can significantly affect the accuracy of serum Lp(a) measurements. To address this issue, we investigated whether our current in house Lp(a) [Mercodia] assay showed such bias compared to a recently available assay [Apo-Tek], claiming to possess superior capability for isoform-independent measurement of Lp(a). Levels of Lipoprotein(a) by both Apo-Tek and Mercodia assays correlated inversely with apo(a) isoform sizes. No significant differences were observed between assays in ranges of Lp(a) concentration within each isoform group. The Mercodia assay exhibited similar isoform-independent behaviour to that of Apo-Tek for the quantitation of serum Lipoprotein(a). Essentially identical results were obtained by the two methods, suggesting that Mercodia assay's capture monoclonal antibody also (as is the case for Apo-Tek) does not recognize the kringle 4-type 2 repetitive domain of apo(a). Correlation of Lp(a) concentrations in patient specimens between Apo-Tek and Mercodia assays showed good agreement, although an overall higher degree of imprecision and non-linearity was noted for the Apo-Tek procedure. A change-over to the Apo-Tek assay would therefore not improve on our current assessment of risk contribution from Lp(a) for atherosclerotic vascular disease in individuals with measurable levels of circulating Lipoprotein(a).

[Phenotypic expression of Lp(a) in Spanish children and adolescents]

BACKGROUND

To know the distribution of phenotypes Lp(a) in an young population.

METHODS

Lipoprotein levels, lipoprotein(a), apolipoproteins and the Lp(a) phenotypes were determined in 105 children, selected according to their cholesterol concentrations.

RESULTS

The Lp(a) concentrations were significantly higher in group with low molecular weight respect to group with high molecular weight. The most frequent isoform was S3.

CONCLUSIONS

The Lp(a) concentrations correlate inversely with the molecular weight of Apo(a) isoforms.

Lipoprotein (a) in android obesity and NIDDM: a new member in 'the metabolic syndrome'

The 'metabolic syndrome' is a special clinical entity characterized by upper body segment obesity (android obesity), together with one or more of a constellation of metabolic disorders that includes glucose intolerance, which may amount to frank diabetes mellitus, hypertension, cardiovascular lesions, hyperuricemia, and dyslipidemias (hypercholesterolemia, hypertriglyceridemia and reduced serum HDL). Recently, lipoprotein (Lp) (a) proved to be a new member in this syndrome. Lp(a) has the distinctive feature of containing apolipoprotein (a), which is a glycoprotein linked to apo B100, and has a similarity to plasminogen; it is also structurally related to LDL. Lp(a) is a macromolecular complex which is genetically determined, and has been identified as an independent risk factor for premature coronary artery disease (CAD). It is elevated in diabetic and non-diabetic android obese subjects, and aggravates the atherogenic effect of diabetes mellitus. Lp(a) is poorly influenced either by dietary measures or by hypolipidemic drugs. Unfortunately, few pharmacologic agents, such as niacin, nicotinic acid, sex hormones (estrogen and testosterone), alcohol and neomycin, affect Lp(a).

Effects of exercise on lipoprotein(a)

Lipoprotein(a) [Lp(a)] is a unique lipoprotein complex in the blood. At high levels (> 30 mg/dl), Lp(a) is considered an independent risk factor for cardiovascular diseases. Serum Lp(a) levels are largely genetically determined, remain relatively constant within a given individual, and do not appear to be altered by factors known to influence other lipoproteins (e.g. lipid-lowering drugs, dietary modification and change in body mass). Since regular exercise is associated with favourable changes in lipoproteins in the blood, recent attention has focused on whether serum Lp(a) levels are also influenced by physical activity. Population and cross-sectional studies consistently show a lack of association between serum Lp(a) levels and regular moderate physical activity. Moreover, exercise intervention studies extending from 12 weeks to 4 years indicate that serum Lp(a) levels do not change in response to moderate exercise training, despite improvements in fitness level and other lipoprotein levels in the blood. However, recent studies suggest the possibility that serum Lp(a) levels may increase in response to intense load-bearing exercise training, such as distance running or weight lifting, over several months to years. Cross-sectional studies have reported abnormally high serum Lp(a) levels in experienced distance runners and body builders who train for 2 to 3 hours each day. However, the possible confounding influence of racial or ethnic factors in these studies cannot be discounted. Recent intervention studies also suggest that 9 to 12 months of intense exercise training may elevate serum Lp(a) levels. However, these changes are generally modest (10 to 15%) and, in most individuals, serum Lp(a) levels remain within the recommended range. It is unclear whether increased serum Lp(a) levels after intense exercise training are of clinical relevance, and whether certain Lp(a) isoforms are more sensitive to the effects of exercise training. Since elevation of both low density lipoprotein cholesterol (LDL-C) and Lp(a) levels in the blood exerts a synergistic effect on cardiovascular disease risk, attention should focus on changing lifestyle factors to decrease LDL-C (e.g. dietary intervention) and increase high density lipoprotein cholesterol (e.g. exercise) levels in the blood.

Lipoprotein(a) as a risk factor for ischemic heart disease: metaanalysis of prospective studies

Although in vitro studies support a pathophysiologic role for lipoprotein(a) [Lp(a)] in the development of atherosclerosis, and retrospective studies consistently report that there is a relationship between Lp(a) and ischemic heart disease (IHD), the conclusions drawn from prospective studies about this relationship have been inconsistent. To address this issue, we have performed a metaanalysis of data available from prospective studies. Lp(a) concentrations expressed as mass units vary markedly between studies, reflecting the need for assay standardization. In 12 of 14 prospective studies, Lp(a) concentrations are higher in subjects who later develop IHD (cases) than in those who do not (controls), although there is variation in the size of the effect. Sample storage temperature may contribute to this variability. When the studies are analyzed collectively, Lp(a) concentrations are significantly higher in cases than in controls, and the extent of the effect is similar in men and women. These findings provide evidence in support of a causal role for Lp(a) in the development of atherosclerosis. Measurement of Lp(a) may be useful to guide management of individuals with a family history of IHD or with existing disease. The separation in values between cases and controls is not, however, sufficient to allow the use of Lp(a) as a screening test in the general population.

Therapeutic efficiency of lipoprotein(a) reduction by low-density lipoprotein immunoapheresis

This study was performed to investigate the effect of low-density lipoprotein (LDL) immunoapheresis on lipoprotein(a) [Lp(a)] reduction in patients with heterozygous and homozygous familial hyperlipidemia (N=16) and insufficient response to lipid-lowering agents. By desorption of approximately 5,700+/-500 mL of plasma, a mean reduction in total cholesterol of 62% (P < .001) and in LDL-cholesterol of 70% (P < .001) was achieved. Lp(a), which was elevated at study entry in seven of these patients (82.1+/-34.3 mg/dL; range, 48 to 148 mg/dL), was reduced during the initial LDL-apheresis procedure by 74.8%+/-14.1% (P < .001). Long-term apheresis treatment performed at weekly intervals resulted in an mean reduction in Lp(a) pretreatment values to 39.1+/-28.5 mg/dL (-54%; P < .001). Desorbed Lp(a) was measured at the waste of the columns for 31 apheresis treatments. Lp(a) concentration of the column waste was higher in patients with elevated serum Lp(a) pretreatment values as compared with those with Lp(a) serum values within the normal range (elevated Lp(a), 1,420+/-380 mg; without elevated Lp(a), 235+/-190 mg; P < .001). The rate of return of Lp(a) following apheresis treatment scheduled at weekly intervals was comparable to that of LDL-cholesterol.

Effect of HELP-LDL-apheresis on outcomes in patients with advanced coronary atherosclerosis and severe hypercholesterolemia

In the secondary prevention of coronary artery disease (CAD) the beneficial effect of lipid lowering is no longer controversial. LDL-apheresis is a feasible therapy for effective lipid lowering in patients refractory to diet and cholesterol lowering drugs. To assess the impact of the HELP-therapy (heparin-induced, extracorporeal LDL precipitation) on patients' clinical outcome and coronary angiography, we set up a prospective trial, in which patients with advanced coronary atherosclerosis and severe hypercholesterolemia resistant to diet and drug therapy were treated with LDL-apheresis. A total of 44 patients were treated with adjunctive weekly HELP-therapy for 15.5 +/- 9.5 months. The mean levels of total cholesterol (Chol), LDL-cholesterol (LDL-C), Lp(a), and fibrinogen at baseline were 308.0 +/- 69.7, 231.8 +/- 72.7, 82.2 +/- 54.1, and 356.1 +/- 94.1 mg/dl, respectively. LDL-apheresis caused a mean per treatment reduction of 44.8 +/- 8.7, 55.5 +/- 8.6, 60.8 +/- 10.2, and 53.8 +/- 6.5% of Chol, LDL-C, Lp(a), and fibrinogen, resulting in mean treatment interval values of 190.4 +/- 33.7, 116.3 +/- 28.9, 51.9 +/- 33.1, and 213.7 +/- 148.9 mg/dl, respectively. Improvement of the clinical status (exercise tolerance, anti-anginal drug use, angina pectoris) was found in 73%, no change in 11%, and deterioration in 16% of the cases. Four patients died cardiac death. The maximal bicycle exercise work load of the patients increased significantly from 101 +/- 41 to 119 +/- 46 W (P < 0.001). Ten (40%) out of 25 patients who underwent follow-up angiography revealed CAD progression, whereas two (8%) patients had CAD regression. Despite angiographic deterioration eight out of ten progressors (80%) improved clinically. In patients with advanced coronary atherosclerosis and severe hypercholesterolemia HELP-therapy can safely and effectively lower LDL-C, Lp(a), and fibrinogen. The chronic weekly HELP-treatment results in clinical improvement in the majority of patients, even in those patients with angiographically shown CAD progression.

Alcohol withdrawal-induced change in lipoprotein(a): association with the growth hormone/insulin-like growth factor-I (IGF-I)/IGF-binding protein-1 (IGFBP-1) axis

Lipoprotein(a) [Lp(a)] is an important risk factor for cardiovascular disease. Alcohol is one of the few nongenetic factors that lower Lp(a) levels, but the metabolic mechanisms of this action are unknown. Alcohol inhibits the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. Alcohol might also affect IGF-binding protein-1 (IGFBP-1), which is an acute inhibitor of IGF-I. We studied how alcohol withdrawal affects Lp(a) levels and the GH/IGF-I/IGFBP-1 axis. Male alcohol abusers (n=27; 20 to 64 years old) were monitored immediately after alcohol withdrawal for 4 days. Twenty-six healthy men, mainly moderate drinkers, served as control subjects. Fasting blood samples were drawn to determine Lp(a), IGF-I, and IGFBP-1 (by ELISA, RIA, and immunoenzymometric assay, respectively). Nocturnal (12 hours) urine collection was performed in 9 alcoholics and 11 control subjects for GH analyses (RIA). The groups were similar in age and body mass index. Lp(a), GH, and IGF-I tended to be lower and IGFBP-1 higher in the alcoholics immediately after alcohol withdrawal than in the control subjects. During the 4-day observation in alcoholics, Lp(a) levels increased by 64% and IGF-I levels by 41%, whereas IGFBP-1 levels decreased by 59% (P<.001 after ANOVA for all comparisons). Urinary GH levels tended to decline. The increase in Lp(a) correlated inversely with the changes in IGFBP-1 (r= -.63, P<.001, n=27) and GH (r=-.70, P<.05, n=9), but not with IGF-I. In multiple regression analysis, the main predictors for the increase in Lp(a) were IGFBP-1 and urinary GH. In conclusion, alcohol withdrawal induces interrelated and potentially atherogenic changes in Lp(a) and IGFBP-1 levels.

Tracking of serum lipoprotein (a) concentration and its contribution to serum cholesterol values in children from 7 to 36 months of age in the STRIP Baby Study. Special Turku Coronary Risk Factor Intervention Project for Babies

We investigated the tracking phenomenon of serum lipoprotein (a) concentrations and assessed the impact of serum concentration of lipoprotein (a) cholesterol on total cholesterol concentrations in children from 7 to 36 months of age. Serum samples for lipoprotein (a) and cholesterol determinations at 7, 13, 24 and 36 months were prospectively obtained from 430 children. Serum lipoprotein (a) was determined using immunoradiometric assay. A strong correlation was observed between lipoprotein (a) concentrations at 7 and 36 months of age (r = 0.88, P < 0.001). Seventy-eight per cent to 86% of the children in the lowest and highest lipoprotein (a) quintiles at 13 months remained in the respective quintiles at 36 months. The average contribution of lipoprotein (a) cholesterol to total cholesterol varied from 0.5% to 3.2% (individual variation 0.13-32.39%) depending on the type of milk received and the age of the children. At 7 months the contribution was 0.44% in breast-fed and 0.93% in formula-fed infants (P < 0.0001). The tracking phenomenon of serum lipoprotein (a) concentrations is strong already in early childhood. The contribution of lipoprotein (a) cholesterol to serum total cholesterol concentration should be taken into account when the changes in serum cholesterol levels are interpreted in the first year of life.

Lipoprotein (a) concentrations in patients with various dyslipidaemias

Although the genetic background is the most important determinant of lipoprotein (a) (Lp(a)) concentration other factors, such as coexistent dyslipidaemia, could modify its levels. We undertook the present study to examine the serum Lp(a) concentration in various dyslipidaemias and to reveal any correlation of serum Lp(a) concentration with the other lipid parameters in a large group of dyslipidaemic Greek patients. A total of 242 patients followed as outpatients in our lipid clinic were studied. The patients were stratified into four main groups. Patients with cholesterol levels greater than 5.17 mmol/L but normal triglycerides were regarded as hypercholesterolaemic (n=85), patients with triglycerides greater than 2.25 mmol/L but normal cholesterol levels as hypertriglyceridaemic (n=51), patients with both increased cholesterol and triglyceride levels as having mixed hyperlipidaemia (n=62), and finally patients with decreased (<0.90 mmol/L) high-density lipoprotein (HDL) cholesterol but normal cholesterol and triglyceride levels as having primary hypoalphalipoproteinaemia (n=44). Hypercholesterolaemic patients exhibited the highest serum Lp(a) levels, while hypertriglyceridaemic patients exhibited the lowest. Patients with mixed hyperlipidaemia had intermediate serum Lp(a) concentration, which was significantly higher than that of hypertriglyceridaemic patients but significantly lower than that of hypercholesterolaemic patients. Interestingly, patients with low serum HDL-cholesterol levels presented with low serum Lp(a) concentration similar to that of hypertriglyceridaemic patients. In hypercholesterolaemic patients no correlation was found between serum total and low-density lipoprotein (LDL) cholesterol nor apolipoprotein B (apoB) levels and Lp(a) concentration. On the contrary, in hypertriglyceridaemic patients an inverse correlation was observed between serum triglycerides and Lp(a) concentration. After dividing the hypertriglyceridaemic patients into one group with elevated (>1.3 g/L) serum apoB levels (n=32) and another group with normal apoB levels (n=19), we found that the median serum Lp(a) concentration was three times higher in hyperapoB patients compared to patients with normal apoB levels. We conclude that serum Lp(a) levels are different in various types of primary hyperlipidaemia and are modulated according to the type of lipid elevation.

Association of Lp(a) rather than integrally-bound apo(a) with triglyceride-rich lipoproteins of human subjects

The majority of apolipoprotein (a) [apo(a)] in plasma is characteristically associated with Lipoprotein (a) [Lp(a)], having a buoyant density (1.05-1.08 g/ml) intermediate between low density lipoproteins (LDL) and high density lipoproteins (HDL). In the fed (postprandial) state or in the presence of fasting (endogenous) hypertriglyceridemia, a small proportion of plasma apo(a) is found in the density < 1.006 g/ml fraction of plasma, associated with larger and less dense triglyceride-rich lipoproteins (TRL). In order to further characterize the presence of apo(a) in ultracentrifugally-separated TRL (UTC-TRL), this lipoprotein fraction was isolated from plasma obtained in the fed state (three hours after an oral fat load) from healthy normolipidemic subjects (Lp(a): 38 +/- 8 mg/dl (mean +/- S.E.), n = 4) and also from plasma obtained after an overnight fast from hypertriglyceridemic patients (plasma TG: 8.16 +/- 2.00 mmol/l, Lp(a): 41 +/- 3 mg/dl, n = 18). Apo(a) in 3 h-postprandial UTC-TRL (5 +/- 2% of total plasma apo(a)) and in hypertriglyceridemic UTC-TRL (8 +/- 2% total apo(a)) was separable by electrophoresis and/or gel chromatography (FPLC) from the majority of UTC-TRL lipid. Apo(a) in UTC-TRL fractions had slow pre-beta electrophoretic mobility and was isolated in a lipoprotein size-range smaller than VLDL and larger than LDL, consistent with it being Lp(a). Recentrifugation of UTC-TRL resulted in the majority of apo(a) being recovered in the density > 1.006 g/ml fraction. Addition of proline to plasma samples before ultracentrifugation (final concentration: 0.1 M) substantially reduced the amount of Lp(a) in UTC-TRL. TRL separated from plasma by FPLC contained less apo(a) (2-5% of total plasma apo(a)), but this apo(a) was also readily dissociable from TRL lipid, had slow pre-beta electrophoretic mobility, and was associated with a lipoprotein with the size of Lp(a). Our data suggest that apo(a) in the TRL fraction of subjects with postprandial triglyceridemia or endogenous hypertriglyceridemia is not an integral component of plasma VLDL or chylomicrons, but represents the presence of non-covalently bound Lp(a).

Lipoprotein (a) distribution in a Nigerian population

OBJECTIVES

To determine the distribution and determinants of lipoprotein (a) (Lp(a)) concentration among Nigerians.

METHODS

Subjects were recruited from civil servants living in Benin City, Nigeria. The height and weight of the individuals were measured and their use of alcohol and tobacco estimated by questionnaire. Laboratory analyses of blood samples involved Lp(a), total cholesterol (TC), high-density lipoprotein (HDLc), HDL2c, HDL3c, triglyceride (TG) and insulin.

RESULTS

The analyses indicate that the Lp(a) concentrations are elevated among Nigerian populations and more skewed towards high levels than is observed for caucasian and oriental groups. The median levels for Lp(a) were 24.0 mg dl-1 and 19.0 mg dl-1 for women and men, respectively. This difference was significant (P < 0.05) but after stratifying by age, only the 45-54 year-old group of women (30.1 mg dl-1) had significantly higher (p < 0.001) median concentrations of Lp(a) than men (18.4 mg dl-1). Age, 20-64, had no influence on Lp(a) levels in men but in women Lp(a) concentrations increased significantly with age (p < 0.05). Among males alcohol consumption, smoking and body mass index (BMI) were not related to Lp(a) concentrations but a significant effect (p < 0.05) was noted for waist-hip ratio (WHR). Among females no relationship was observed between Lp(a) levels and alcohol consumption, BMI and WHR. All serum lipids measured (TC, HDLc, HDL2c, HDL3c, low-density lipoprotein (LDLc), and TG) were correlated with Lp(a) concentrations among men. A significant association with TC and LDLc remained after correcting for Lp(a) cholesterol. Among women, the Lp(a) levels were associated with TC, HDLc, HDL3c, and LDLc but not with HDL2c, and TG. The correlations with TC and LDLc were not significant after correcting for Lp(a) cholesterol. Insulin did not correlate with Lp(a) levels in either men or women.

CONCLUSIONS

Lp(a) concentrations are high in Nigerians, particularly among women, and the association between the Lp(a) concentrations and other lipoproteins is stronger than in white populations.

A prospective case-control study of lipoprotein(a) levels and apo(a) size and risk of coronary heart disease in Stanford Five-City Project participants

Lipoprotein(a) [Lp(a)] is formed by the assembly of LDL particles and a carbohydrate-rich protein, apolipoprotein(a) [apo(a)], which has a high degree of structural homology with plasminogen. While the majority of retrospective studies have found an association between Lp(a) level and cardiovascular disease (CVD), the few prospective studies to date have reported contradictory results. We conducted a nested case-control study using the participants in the Stanford Five-City Project, a long-term CVD prevention trial. Participants with an incident possible or definite myocardial infarction or coronary death were matched to a single control subject for age, sex, ethnicity, residence in a treatment or control city, and time of survey. This process yielded 134 case-control pairs, 90 male and 44 female, for whom plasma was available for analysis of Lp(a). Lp(a) values in nanomoles per liter were determined by an enzyme-linked immunoassay that measures Lp(a) independently of apo(a) size polymorphism. Apo(a) size isoforms were determined by SDS-agarose gel electrophoresis. Median Lp(a) level in male cases was almost double that in control subjects (41.8 versus 21.2 nmol/L; P < .01); in female cases, median Lp(a) was 34% higher than in control subjects (32.5 versus 21.2 nmol/L), but this difference was not statistically significant. Among the male cases, there was an increased frequency of small apo(a) isoforms, while no significant difference was found in apo(a) size between female cases and control subjects. The association between Lp(a) level and case-control status in men was independent of total, HDL, and non-HDL cholesterol levels, as well as apo(a) size isoform, cigarette smoking, blood pressure, and obesity. In men, the most efficient threshold value of Lp(a) concentration for separating cases and control subjects was 35 nmol/L; the odds ratio for being a case above this level compared with below was 2.84 (95% confidence interval: 1.53-5.27, P < .001). This study provides strong evidence that Lp(a) level is a prospective, independent risk factor for developing coronary artery disease in men and indicates that the size of apo(a) may also play a role. The lack of a significant association in women deserves further evaluation in larger studies.

Lipoprotein(a): structural implications for pathophysiology

The assembly between a low-density lipoprotein particle and apolipoprotein(a), a highly carbohydrate-rich protein, gives origin to a peculiar class of lipoproteins, only found in the hedgehog, primates, and humans, termed lipoprotein(a). Apolipoprotein(a), which shares a high degree of sequence homology with the fibrinolytic proenzyme plasminogen, is linked to the apolipoprotein B-100 component of low-density lipoprotein via a disulfide bond and confers distinct biochemical and metabolic properties to lipoprotein(a). Because of its peculiar structural features and the observed correlation between high lipoprotein(a) levels and the development of a variety of atherosclerotic disorders, this lipoprotein has become the focus of an intense research effort. Although accumulation of lipoprotein(a) in the vessel wall at sites of vascular injury has been clearly evidenced, the mechanism(s) by which lipoprotein(a) exerts its pathogenic effect in this milieu remain largely unknown. It has been hypothesized that the pathological effect of lipoprotein(a) is related either to its similarity to low-density lipoprotein (i.e., a pro-atherogenic effect) or to the apolipoprotein(a) similarity to plasminogen (i.e., a pro-thrombotic/anti-fibrinolytic effect). However, it is probable that both components contribute to the pathogenicity of lipoprotein(a). The fact that lipoprotein(a) levels are largely genetically determined, varying widely among individuals and racial groups, adds additional elements to the scientific interest that surrounds this lipoprotein. Both clinical and biochemical studies of lipoprotein(a) have been complicated by the high degree of structural heterogeneity of apolipoprotein(a), which is considered the most polymorphic protein in human plasma. Our aim in this paper is to provide an overview of the most salient structural features of lipoprotein(a) and their possible pathophysiological implications.

Hormone replacement therapy lowers plasma Lp(a) concentrations. Comparison of cyclic transdermal and continuous estrogen-progestin regimens

To study the responses of serum lipoproteins, apoproteins (apo's), and lipoprotein(a) (Lp[a]) to two frequently used hormone replacement therapies (HRTs), 120 postmenopausal women were randomly allocated to receive either transdermal therapy consisting of 28-day cycles with patches that delivered 17 beta-estradiol (50 micrograms/d) combined with cyclic oral medroxyprogesterone acetate (10 mg/d for 12 days per cycle) or continuous oral 17 beta-estradiol (2 mg/d) together with norethisterone acetate (1 mg/d) for 12 months. Blood samples were taken before and at 6 and 12 months of HRT. Concentrations of serum total, low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol decreased by 14% (P < .001), 17% (P < .001), and 9% (P < .001) in the oral HRT group. Respective changes were 5.7% (P < .001), 4.8% (P < .05), and 4.7% (NS) in the transdermal group. Serum triglycerides remained unchanged in the oral group but decreased by 15.7% (P < .001) in the transdermal group. We observed only trivial changes in serum apo B levels. The changes in apo A-I levels paralleled those of HDL cholesterol in the oral HRT group. The concentration of serum Lp(a) decreased by 31% (P < .001) and 16% (P < .001) in the two groups. The combination of progestin and transdermal estrogen was not associated with any further change of Lp(a). The decrement in Lp(a) during therapy was positively associated with baseline Lp(a) levels in both groups (r = .96, P < .001 and r = .88, P < .001). Thus, both HRT regimens were highly effective in lowering elevated Lp(a) levels in postmenopausal women. The divergent responses of LDL and HDL cholesterol in the two HRT groups may influence the potential cardioprotective effects of the two HRT regimens. Prospective trials are needed to define the long-term effects with respect to coronary heart disease risk.

Testosterone-induced suppression of lipoprotein(a) in normal men; relation to basal lipoprotein(a) level

The concentration of lipoprotein(a) [Lp(a)] in human plasma is largely genetically determined and is inversely correlated to the size of apolipoprotein(a) [apo(a)]. Additionally, Lp(a) values are relatively stable within individuals and are only marginally susceptible to therapeutic treatment. The aim of our study was to evaluate the effect of exogenous testosterone on plasma Lp(a) concentration. The study was carried out on 19 healthy men who were receiving weekly intramuscular injections of 200 mg testosterone enanthate. Lp(a) values were determined at multiple time-points by a double monoclonal antibody-based enzyme immunoassay. This method is not sensitive to variation in Lp(a) size and the values are expressed in nmol/l. Apo(a) size isoforms were determined by agarose gel electrophoresis followed by immunoblotting. No correlation was found between the baseline Lp(a) values and the baseline values of testosterone or estradiol. The Lp(a) response to testosterone treatment varied widely among subjects and was dependent upon the pretreatment Lp(a) concentration. For 10 subjects with low Lp(a) values (< 25 nmol/l), no significant decrease in Lp(a) was observed while, for the nine individuals with Lp(a) values > 25 nmol/l, there was a significant and consistent reduction in Lp(a) ranging from 25 to 59%. Lp(a) levels returned to baseline values following cessation of testosterone administration. Apo(a) size polymorphism did not appear to play a role in the determination of Lp(a) response to testosterone.

Lipoprotein(a) in health and disease

Lipoprotein(a) [Lp(a)] represents an LDL-like particle to which the Lp(a)-specific apolipoprotein(a) is linked via a disulfide bridge. It has gained considerable interest as a genetically determined risk factor for atherosclerotic vascular disease. Several studies have described a correlation between elevated Lp(a) plasma levels and coronary heart disease, stroke, and peripheral atherosclerosis. In healthy individuals, Lp(a) plasma concentrations are almost exclusively controlled by the apo(a) gene locus on chromosome 6q2.6-q2.7. More than 30 alleles at this highly polymorphic gene locus determine a size polymorphism of apo(a). There exists an inverse correlation between the size (molecular weight) of apo(a) isoforms and Lp(a) plasma concentrations. The standardization of Lp(a) quantification is still an unresolved task due to the large particle size of Lp(a), the presence of two different apoproteins [apoB and apo(a)], and the large size polymorphism of apo(a) and its homology with plasminogen. A working group sponsored by the IFCC is currently establishing a stable reference standard for Lp(a) as well as a reference method for quantitative analysis. Aside from genetic reasons, abnormal Lp(a) plasma concentrations are observed as secondary to various diseases. Lp(a) plasma levels are elevated over controls in patients with nephrotic syndrome and patients with end-stage renal disease. Following renal transplantation, Lp(a) concentrations decrease to values observed in controls matched for apo(a) type. Controversial data on Lp(a) in diabetes mellitus result mainly from insufficient sample sizes of numerous studies. Large studies and those including apo(a) phenotype analysis came to the conclusion that Lp(a) levels are not or only moderately elevated in insulin-dependent patients. In noninsulin-dependent diabetics, Lp(a) is not elevated. Conflicting data also exist from studies in patients with familial hypercholesterolemia. Several case-control studies reported elevated Lp(a) levels in those patients, suggesting a role of the LDL-receptor pathway for degradation of Lp(a). However, recent turnover studies rejected that concept. Moreover, family studies also revealed data arguing against an influence of the LDL receptor for Lp(a) concentrations. Several rare diseases or disorders, such as LCAT- and LPL-deficiency as well as liver diseases, are associated with low plasma levels or lack of Lp(a).

Lipoprotein(a) immunoassays: comparison of a semi-quantitative latex method and two monoclonal enzyme immunoassays

Lipoprotein(a) is considered an independent risk factor for atherosclerosis. A variety of analytical methods have been proposed for lipoprotein(a) measurement, the majority of which require dedicated instruments and are costly to perform, particularly when the aim is to screen for high lipoprotein(a) concentrations in large populations. We evaluated the sensitivity and specificity of a newly developed semi-quantitative latex method to assess its suitability for identifying subjects with high lipoprotein(a) levels. Based on clinical data, a sensitivity limit of 20 mg/dl of total lipoprotein(a) particle was selected for the latex method. Results obtained by the latex method on 204 subjects were compared with two enzyme immunoassays using two anti-apo(a) monoclonal antibodies with different specificities. In one assay, the detecting monoclonal antibody (MAb a-5) is directed against an epitope present in a variable number depending on the apo(a) size isoforms in lipoprotein(a), while the other assay the detecting monoclonal antibody (MAb a-40) is directed against an epitope present only once in lipoprotein(a) particles, irrespective of their apo(a) size. Both the latex method and the MAb a-5 assay demonstrated a 100% sensitivity, in that no false-negative results were found using the MAb a-40 assay as the gold standard. Eleven subjects (5.4%) were misclassified as false positive by MAb a-5 assay and 23 (11.3%) were misclassified by the latex method. Based on its 100% sensitivity and 89% specificity, we conclude that the lipoprotein(a) latex method is a cost-effective rapid approach for screening large populations.