Large Apo Research Articles

Dalcetrapib and anacetrapib increase apolipoprotein E-containing HDL in rabbits and humans

The large HDL particles generated by administration of cholesteryl ester transfer protein inhibitors (CETPi) remain poorly characterized, despite their potential importance in the routing of cholesterol to the liver for excretion, which is the last step of the reverse cholesterol transport. Thus, the effects of the CETPi dalcetrapib and anacetrapib on HDL particle composition were studied in rabbits and humans. The association of rabbit HDL to the LDL receptor (LDLr) in vitro was also evaluated. New Zealand White rabbits receiving atorvastatin were treated with dalcetrapib or anacetrapib. A subset of patients from the dal-PLAQUE-2 study treated with dalcetrapib or placebo were also studied. In rabbits, dalcetrapib and anacetrapib increased HDL-C by more than 58% (P < 0.01) and in turn raised large apo E-containing HDL by 66% (P < 0.001) and 59% (P < 0.01), respectively. Additionally, HDL from CETPi-treated rabbits competed with human LDL for binding to the LDLr on HepG2 cells more than control HDL (P < 0.01). In humans, dalcetrapib increased concentrations of large HDL particles (+69%, P < 0.001) and apo B-depleted plasma apo E (+24%, P < 0.001), leading to the formation of apo E-containing HDL (+47%, P < 0.001) devoid of apo A-I. Overall, in rabbits and humans, CETPi increased large apo E-containing HDL particle concentration, which can interact with hepatic LDLr. The catabolism of these particles may depend on an adequate level of LDLr to contribute to reverse cholesterol transport.

LIPOPROTEIN (A) LEVELS PREDICT PULSE WAVE VELOCITY IN SUBJECTS IN PRIMARY PREVENTION FOR CARDIOVASCULAR DISEASE WITH LARGE APO(A) ISOFORMS: DATA FROM THE BRISIGHELLA HEART STUDY

Objective: In the last decades, high serum levels of lipoprotein(a) (Lp(a)) have been associated with increased cardiovascular disease (CVD) risk, in particular among individuals with smaller apolipoprotein(a) (apo(a)) isoforms than those with larger sizes. The aim of our analysis was to evaluate whether Lp(a) levels could predict early vascular aging, and whether smaller apo(a) isoforms have a predictive value of vascular aging different than larger apo(a) isoforms in a cohort of subjects free from CVD. Design and method: We considered the data of a subset of Brisighella Heart Study (BHS) participants free from CVD (462 men and 516 women) who were clinically evaluated during the 2012 BHS population survey. Predictors of arterial stiffness -measured as carotid-femoral pulse wave velocity (cf-PWV)- were estimated by the application of a step-wise linear regression model. Results: In our cohort, there were 511 subjects with small apo(a) size and 467 subjects with large apo(a) isoforms. Subjects with larger apo(a) isoform sizes had significantly lower serum levels of Lp(a). In the BHS subpopulation sample, cfPWV was predicted by age, systolic blood pressure (SBP), serum levels of high-density lipoprotein cholesterol (HDL-C), triglycerides (TG) and sex, being higher HDL-C serum levels and female sex associated to lower values of cfPWV. In subjects with smaller apo(a) isoform sizes, predictors of cf-PWV were age, SBP, sex and serum levels of HDL-C, being higher HDL-C serum levels and female sex associated to lower values of cfPWV. In subjects with larger apo(a) isoform sizes, cf-PWV was predicted by age, SBP, serum levels of Lp(a) and sex, being female sex associated to lower values of cfPWV. Conclusions: In our subpopulation sample, Lp(a) did not predict cfPWV. However, among subjects with larger apo(a) isoform sizes Lp(a) was a significant predictor of arterial stiffness.

Lipoprotein(a) Serum Levels Predict Pulse Wave Velocity in Subjects in Primary Prevention for Cardiovascular Disease with Large Apo(a) Isoforms: Data from the Brisighella Heart Study.

In the last decades, high serum levels of lipoprotein(a) (Lp(a)) have been associated with increased cardiovascular disease (CVD) risk, in particular among individuals with smaller apolipoprotein(a) (apo(a)) isoforms than those with larger sizes. The aim of our analysis was to evaluate whether Lp(a) levels could predict early vascular aging, and whether smaller apo(a) isoforms had a predictive value for vascular aging different than larger apo(a) isoforms in a cohort of subjects free from CVD. We considered the data of a subset of Brisighella Heart Study (BHS) participants free from CVD (462 men and 516 women) who were clinically evaluated during the 2012 BHS population survey. Predictors of arterial stiffness, measured as carotid-femoral pulse wave velocity (cfPWV) were estimated by the application of a step-wise linear regression model. In our cohort, there were 511 subjects with small apo(a) size and 467 subjects with large apo(a) isoforms. Subjects with larger apo(a) isoform sizes had significantly lower serum levels of Lp(a). In the BHS subpopulation sample, cfPWV was predicted by age, systolic blood pressure (SBP), serum levels of high-density lipoprotein cholesterol (HDL-C), triglycerides (TG) and sex, higher HDL-C serum levels and female sex associated with lower values of cfPWV. In subjects with smaller apo(a) isoform sizes, predictors of cfPWV were age, SBP, sex and serum levels of HDL-C, being higher HDL-C serum levels and female sex associated to lower values of cfPWV. In subjects with larger apo(a) isoform sizes, cfPWV was predicted by age, SBP, serum levels of Lp(a) and sex, with female sex associated with lower values of cfPWV. In our subpopulation sample, Lp(a) did not predict cfPWV. However, in subjects with large apo(a) isoform sizes, Lp(a) was a significant predictor of arterial stiffness.

PCSK9 in African Americans and Caucasians in Relation to Lp(a) Level, Apo(a) Size and Heritability.

ContextInhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) reduces lipoprotein(a) [Lp(a)] levels, but the association of PCSK9 with Lp(a) level and its major determinant, apolipoprotein(a) [apo(a)] size, is not fully understood.ObjectiveTo assess the relationship between PCSK9, Lp(a) level, apo(a) size, age, and ethnicity/race.DesignCross-sectionalSettingGeneral populationParticipantsHealthy African Americans and Caucasians (n = 267); age range: 6 to 74 years.InterventionsNone.Main outcome measure(s)PCSK9 levels, apo(a) isoform and LPA allele sizes, and isoform-specific Lp(a) levels.ResultsPlasma PCSK9 levels were significantly higher in African Americans vs Caucasians, in females vs males, and in adults vs children. PCSK9 levels were not associated with total plasma Lp(a) levels either in all participants or in ethnicity-specific analyses. However, PCSK9 levels were significantly positively associated with isoform-specific Lp(a) levels carried by the larger apo(a) size in all participants (r = 0.139, P = 0.0361). In ethnicity/race analyses, a significant association was seen for African Americans (r = 0.268, P = 0.0199), but not for Caucasians. In contrast, there were no significant associations of PCSK9 with isoform-specific Lp(a) levels for the smaller apo(a) sizes in all participants nor in ethnic-specific analyses. Furthermore, heritability (h2) analyses revealed a significant heritability for PCSK9 level in both ethnic groups, with a higher estimate in Caucasians than in African Americans (47% vs 22%, respectively).ConclusionsAmong African Americans, but not Caucasians, PCSK9 levels were associated with isoform-specific Lp(a) levels carried on larger, but not smaller, apo(a) sizes. The findings illustrate a diverging relationship of PCSK9 with isoform-specific Lp(a) levels across ethnicity.

SUN-580 PCSK9 and Lp(a): Association Between PCSK9 Level and Larger Apo(a) Isoform Size in African-Americans and Caucasians

Introduction: An elevated level of lipoprotein(a) [Lp(a)] is an independent causal risk factor for cardiovascular disease. Non-genetic factors do not appreciably influence Lp(a) levels due to a strong genetic control. However, inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) has been shown to reduce Lp(a) levels. The association of PCSK9 with Lp(a) level and its major genetic determinant—apolipoprotein(a) [apo(a)] size—is not fully understood. In this study, we assessed the relationship between PCSK9, Lp(a) level, apo(a) size, age, and race/ethnicity. Methods: Healthy Caucasian and African-American families were recruited from the general population (age range: 6–74 years, N=267). PCSK9 and Lp(a) levels were assayed enzymatically; apo(a) isoform and LPA allele sizes and isoform-specific Lp(a) levels were determined. Results: In all participants, PCSK9 levels differed significantly by race/ethnicity, age, and sex. Thus, the mean PCSK9 levels were significantly higher in African-Americans vs. Caucasians (104 ± 29 vs. 95 ± 30 ng/mL, respectively, p=0.020), in adults vs. children (102 ± 29 vs. 92 ± 31 ng/mL, respectively, p=0.001) and in females vs. males (103 ± 30 vs. 94 ± 29 ng/mL, respectively, p=0.007). PCSK9 levels were not associated with total plasma Lp(a) levels neither in all participants nor in ethnicity-specific analyses. However, PCSK9 levels were significantly and positively associated with isoform-specific Lp(a) levels carried by the larger apo(a) size in all subjects (r=0.139, p=0.0361). In race/ethnicity analyses, a significant association was seen for African-Americans (r=0.268, p=0.0199), but not for Caucasians. In contrast, there were no significant associations of PCSK9 with isoform-specific Lp(a) levels for the smaller apo(a) sizes in all participants nor in ethnic-specific analyses. Of note, PCSK9 levels were significantly negatively associated with the larger apo(a) isoform sizes in all participants (r=-0.139, p=0.0366). Although significant in both groups, heritability of PCSK9 level was higher in Caucasians than in African-Americans (47% vs. 22%, respectively). Conclusions: Among African-Americans, but not Caucasians, PCSK9 levels were associated with isoform-specific Lp(a) levels carried on larger, but not smaller, apo(a) sizes. The findings illustrate a diverging relationship of PCSK9 with isoform-specific Lp(a) levels.

The role of lipoprotein (a) in chronic kidney disease: Thematic Review Series: Lipoprotein (a): Coming of Age at Last

Lipoprotein (a) [Lp(a)] and its measurement, structure and function, the impact of ethnicity and environmental factors, epidemiological and genetic associations with vascular disease, and new prospects in drug development have been extensively examined throughout this Thematic Review Series on Lp(a). Studies suggest that the kidney has a role in Lp(a) catabolism, and that Lp(a) levels are increased in association with kidney disease only for people with large apo(a) isoforms. By contrast, in those patients with large protein losses, as in the nephrotic syndrome and continuous ambulatory peritoneal dialysis, Lp(a) is increased irrespective of apo(a) isoform size. Such acquired abnormalities can be reversed by kidney transplantation or remission of nephrosis. In this Thematic Review, we focus on the relationship between Lp(a), chronic kidney disease, and risk of cardiovascular events.

Genetic determination of lipoprotein(a) and its association with cardiovascular disease: convenient does not always mean better

Genetic determination of lipoprotein(a) and its association with cardiovascular disease: convenient does not always mean better

Apolipoprotein (a) Isoforms and the Risk of Vascular Disease: A Systematic Review of 40 Studies Involving 58,000 Participants

Conclusion: Smaller apolipoprotein (a) (apo[a]) isoforms confer an approximately twofold higher risk of ischemic stroke or coronary heart disease than larger isoforms of apo(a). Summary: Lipoprotein(a) (Lp[a]) is composed of a glycoprotein molecule, apo(a), and a low-density lipoprotein (LDL) particle. Apo(a) is responsible for the properties of Lp(a). (Marcovina SM et al [Am J Cardiol 1998:82:57U-66U]; McLean JW et al [Nature 1987;330:132-7]). Increased circulating Lp(a) concentration is associated with increased risk of coronary heart disease (CHD) and stroke and is independent of other conventional risk factors for vascular disease, including total cholesterol level. The overall additive risk of abnormalities of Lp(a) is only about one-quarter that seen with LDL cholesterol level (JAMA 2009;302:412-23). However, specific Lp(a) subtypes may confer higher cardiovascular risk. If that is the case, analysis for Lp(a) subtypes may be useful in the stratification of vascular risk. The authors postulated that Lp(a) particles associated with smaller rather than larger apo(a) isoforms may result in higher cardiovascular risk. They analyzed information from 40 studies published between January 1970 and June 2009 that reported an association between apo(a) isoforms and the risk of ischemic stroke or CHD. This involved 11,396 patients and 46,938 controls in 36 studies that used comparable phenotyping and analytic methods to assess apo(a) isoform size. These studies yielded a combined relative risk for CHD of 2.08 (95% confidence interval [CI], 1.67-2.58) for subjects with smaller vs larger apo(a) isoforms. There was substantial heterogeneity among the studies (I2 = 85%; 95% CI, 80%-89%). Heterogeneity was mainly explained by differences in analytical approaches and laboratory methods. Six studies of ischemic stroke used comparable phenotypic methods with a combined relative risk of 2.14 (95% CI, 1.85-2.97). Comment: Apo(a) size heterogeneity is a function of a copy number variation of one protein domain, kringle IV type 2, the gene for which exists in 5 to 50 identically repeated copies. Copy number variation of the gene confers marked heterogeneity in the molecular mass of the apo(a) isoform (Boffa MB [Clin Biochem 2004;37:333-43]). Apo(a) subtyping has been clinically limited because it adds a relatively modest incremental risk compared with other biomarkers for cardiovascular disease. However, this study indicates there are subtypes of apo(a) that may be worth looking for. It will need to be determined whether smaller apo(a) isoforms have sufficient relevance in determining vascular risk independent from Lp(a) concentration when compared to other more conventional risk factors for atherosclerosis.

Lipoprotein(a) levels, apo(a) isoform size, and coronary heart disease risk in the Framingham Offspring Study

The aim of this study was to assess the independent contributions of plasma levels of lipoprotein(a) (Lp(a)), Lp(a) cholesterol, and of apo(a) isoform size to prospective coronary heart disease (CHD) risk. Plasma Lp(a) and Lp(a) cholesterol levels, and apo(a) isoform size were measured at examination cycle 5 in subjects participating in the Framingham Offspring Study who were free of CHD. After a mean follow-up of 12.3 years, 98 men and 47 women developed new CHD events. In multivariate analysis, the hazard ratio of CHD was approximately two-fold greater in men in the upper tertile of plasma Lp(a) levels, relative to those in the bottom tertile (P < 0.002). The apo(a) isoform size contributed only modestly to the association between Lp(a) and CHD and was not an independent predictor of CHD. In multivariate analysis, Lp(a) cholesterol was not significantly associated with CHD risk in men. In women, no association between Lp(a) and CHD risk was observed. Elevated plasma Lp(a) levels are a significant and independent predictor of CHD risk in men. The assessment of apo(a) isoform size in this cohort does not add significant information about CHD risk. In addition, the cholesterol content in Lp(a) is not a significant predictor of CHD risk.

Association of Apo(a)isoform size with dyslipoproteinemia in male Venous Thrombosis patients

Background Lp(a) is a proatherogenic lipoprotein that may also be prothrombotic. Apo(a) size isoforms have differential effects on fibrinolysis. Whereas Lp(a) concentrations have been linked to venous thromboembolic disease (VTE) risk, apo(a) polymorphisms in VTE have not been studied. Methods We used a standardized high resolution agarose gel electrophoresis technique to determine apo(a) isoform size, and a Lp(a) immunoassay insensitive to apo(a) size to measure Lp(a) concentration in 46 men with VTE and 46 age-matched healthy controls. Results Apo(a) isoform distribution in VTE cases and controls was bimodal and VTE patients tended to have more medium-sized isoforms K 4-(19-27) (54.3% vs. 34.8%, p = 0.06). Cases and controls had the same median predominant apo(a) size isoform (23.5 K 4 repeats) and comparable Lp(a) concentrations. However, subgroup analysis based on apo(a) isoform size (K 4 ≤ 23 or K 4 ≥ 24) revealed that cases in the K 4 ≥ 24 subgroup had higher Lp(a) concentrations than the controls in this isofrom subgroup (14.5 mmol vs. 6.6 mmol, p = 0.029). Also, dyslipoproteinemia (smaller LDL and HDL particles, higher LDL and lower HDL parameters) was strongly associated with VTE only in this larger apo(a) isoform group. Conclusions These observations provide the first evidence that determination of apo(a) isoforms may provide useful novel insights into VTE risk.

Leptin is associated with the size of the apolipoprotein(a) particle in African tribal populations living on fish or vegetarian diet

ObjectiveApolipoprotein(a) [or apo(a)] isoform size, which is strongly genetically determined, showed significant association with the cardiovascular risk. Subjects on a fish diet have lower lipoprotein(a) levels, larger apo(a) isoform sizes and lower leptin levels than their vegetarian diet counterparts. We hypothesized that leptin may contribute to a potential association between the type of diet and the size of apo(a) isoforms. MethodsAnthropometric data, dietary nutrients, lipoprotein profile, plasma leptin levels, and apo(a) isoforms were evaluated in two related homogenous African tribal populations of Tanzania, one on a primarily freshwater fish diet (n=278), and the other on a vegetarian diet (n=326). ResultsWe observed a strong negative association between leptin levels and size of each of the apo(a) isoforms in both fish and vegetable diet groups, and in both genders. However, leptin was not associated with levels of lipoprotein(a). In multivariate analysis, a strong and independent association between leptin and size of apo(a) isoforms was observed. The size of apo(a) isoforms was strongly associated with high and low leptin states. Subjects with low leptins had 30% larger sizes of apo(a) isoforms than their high leptin counterparts. ConclusionsHigh leptin subjects have smaller, potentially more atherogenic, apo(a) isoform sizes than low leptin ones. We suggest that omega-3 rich diet can influence the levels of apo(a) and/or Lp(a) even though they are mainly genetically determined. These findings may have implications for understanding the interaction between leptin and cardiovascular risk.

Genome-wide association study of plasma lipoprotein(a) levels identifies multiple genes on chromosome 6q

Plasma lipoprotein(a) (Lp[a]) level is an independent risk factor of cardiovascular disease that is under strong genetic control. We conducted a genome-wide association study of plasma Lp(a) in 386 members of a founder population that adheres to a communal lifestyle, proscribes cigarette smoking, and prepares and eats meals communally. We identified associations with 77 single nucleotide polymorphisms (SNPs) spanning 12.5 Mb on chromosome 6q26-q27 that met criteria for genome-wide significance (P <or= 1.3 x 10(-7)) and were within or flanking nine genes, including LPA. We show that variation in at least six genes in addition to LPA are significantly associated with Lp(a) levels independent of each other and of the kringle IV repeat polymorphism in the LPA gene. One novel SNP in intron 37 of the LPA gene was also associated with Lp(a) levels and carotid artery disease number in unrelated Caucasians (P = 7.3 x 10(-12) and 0.024, respectively), also independent of kringle IV number. This study suggests a complex genetic architecture of Lp(a) levels that may involve multiple loci on chromosome 6q26-q27.

The apolipoprotein(a) gene: Linkage disequilibria at three loci differs in African Americans and Caucasians

Lipoprotein(a) (Lp(a)) is an independent, genetically regulated cardiovascular risk factor. Lp(a) plasma levels are largely determined by the apolipoprotein(a) (apo(a)) component, and differ across ethnicity. Although a number of polymorphisms in the apo(a) gene have been identified, apo(a) genetic regulation is not fully understood. To study the relation between apo(a) gene variants, we constructed haplotypes and assessed linkage equilibrium in African Americans and Caucasians for three widely studied apo(a) gene polymorphisms (apo(a) size, +93 C/T and pentanucleotide repeat region (PNR)). Apo(a) size allele frequency distributions were different across ethnicity ( p < 0.01). For African Americans, PNR frequencies were similar across apo(a) sizes, suggesting linkage equilibrium. For Caucasians, the PNR and the PNR–C/T haplotype frequencies differed for large and small apo(a), with the T and PNR 9 alleles associated with large apo(a) size ( p < 0.0002); also, the PNR 9 allele was more common on a T allele, while PNR 8 was more common on a C allele. On a C allele background, small PNR alleles were more common and the PNR 10 allele less common among African Americans than Caucasians ( p < 0.001). The ethnic difference in apo(a) size distribution remained controlling for C/T and PNR alleles ( p = 0.023). In conclusion, allele and haplotype frequencies and the nature of the linkage disequilibrium differed between African Americans and Caucasians at three apo(a) gene polymorphisms.

ApoE genotype affects allele-specific apo[a] levels for large apo[a] sizes in African Americans: the Harlem-Basset Study

The genetic variability of apolipoprotein E (apoE) influences plasma lipoprotein levels, and allele frequencies differ between African Americans and Caucasians. As African Americans have higher lipoprotein [a] (Lp[a]) levels than Caucasians, we investigated the effects of the apoE gene on allele-specific apolipoprotein [a] (apo[a]) levels across ethnicity. We determined apo[a] sizes, allele-specific apo[a] levels (i.e., levels associated with alleles defined by size), and the apoE gene polymorphism in 231 African Americans and 336 Caucasians. African Americans, but not Caucasians, with the apo E2 genotype had lower levels of Lp[a] compared with those with the apo E4 genotype (9.6 vs. 11.2 nmol/l; P = 0.034, expressed as square root levels). Distribution of apo[a] alleles across apoE genotypes were similar between African Americans and Caucasians. Among African Americans with large apo[a], the allele-specific apo[a] level was significantly lower among epsilon2 carriers compared with epsilon3 or epsilon4 carriers (5.4 vs. 6.6 and 7.4 nmol/l, respectively; P < 0.005, expressed as square root levels). In contrast, there was no significant difference in allele-specific apo[a] levels across apoE genotypes among Caucasians. For large apo[a] sizes, apoE genotype contributed to the observed African American-Caucasian differences in allele-specific apo[a] levels.

Lipoprotein(a) in the Cerebrospinal Fluid of Neurological Patients with Blood–Cerebrospinal Fluid Barrier Dysfunction

Lipoprotein(a) [Lp(a)] is a recognized pathogenic particle in human plasma, but its presence in the cerebrospinal fluid and its possible role in the central nervous system have not been documented. We tested the hypothesis that apolipoprotein(a) [apo(a)], free or as a component of the Lp(a) particle, can cross the blood-cerebrospinal fluid barrier and be found in the cerebrospinal fluid of patients affected by neurologic pathologies. We studied paired cerebrospinal fluid/serum samples from 77 patients with inflammatory (n=20) or noninflammatory (n=34) blood-cerebrospinal fluid barrier dysfunction and without blood-cerebrospinal fluid barrier dysfunction (n=23). We used ELISA to measure Lp(a) concentrations and Western blot and immunodetection to analyze apo(a) isoforms in native and reducing conditions. Entire Lp(a) with either small or large apo(a) isoforms was present in the cerebrospinal fluid of patients with blood-cerebrospinal fluid barrier dysfunction, regardless of its pathogenesis. Multiple linear regression analysis showed that both serum Lp(a) concentration (P=0.003) and cerebrospinal fluid/serum albumin ratio (P<0.001) were predictors of the Lp(a) concentration in cerebrospinal fluid. Our results demonstrate that Lp(a) can cross a dysfunctional blood-cerebrospinal fluid barrier. The unusual presence of Lp(a) in the cerebrospinal fluid could extend some of its known pathogenic effects to the central nervous system.

Protective effect of apolipoprotein E2 on coronary artery disease in African Americans is mediated through lipoprotein cholesterol

We studied the relationship of apolipoprotein E (apoE) isoforms and coronary artery disease (CAD) in 224 African Americans and 326 Caucasians undergoing diagnostic coronary angiography. The presence of CAD was defined as >50% stenosis in at least one artery. ApoE allele frequencies were 0.12, 0.62, and 0.26 for epsilon 2, epsilon 3, and epsilon 4, respectively, in African Americans and 0.08, 0.78, and 0.14 for epsilon 2, epsilon 3, and epsilon 4, respectively, in Caucasians. Among African Americans, CAD was present in 9 of 34 epsilon 2 carriers (26%), significantly smaller (P < 0.05) in proportion compared with 39 of 82 epsilon 3 carriers and 43 of 92 epsilon 4 carriers (48% and 47%, respectively), suggesting a protective effect of the epsilon 2 allele. No such difference was seen in Caucasians. In African Americans but not Caucasians, LDL cholesterol was lower in epsilon 2 carriers than in epsilon 3 and epsilon 4 carriers (106 vs. 127 and 134 mg/dl, respectively; P < 0.005). After adjusting for lipid levels, the association between apoE2 and CAD was no longer significant. Thus, the protective effect of apoE2 seen in African Americans could be explained by a favorable lipid profile in epsilon 2 carriers, whereas in Caucasians, the absence of such a protective effect could be attributable to the lack of effect of apoE2 on the lipid profile.

Apo[a] size and PNR explain African American-Caucasian differences in allele-specific apo[a] levels for small but not large apo[a

Apolipoprotein [a] (apo[a]) gene size is a major predictor of lipoprotein [a] level. To determine genetic predictors of allele-specific apo[a] levels beyond gene size, we evaluated the upstream C/T and pentanucleotide repeat (PNR) polymorphisms. We determined apo[a] sizes, allele-specific apo[a] levels, and C/T and PNR in 215 Caucasians and 139 African Americans. For Caucasians, apo[a] size affected allele-specific levels substantially greater in subjects with apo[a] < 24 K4; for African Americans, the size effect was smaller than in Caucasians, <24 K4, but did not decrease at higher repeats. In both groups, the level decreased with increasing size of the other allele. Controlling for apo[a] sizes, PNR decreased allele-specific apo[a] levels in Caucasians with increasing PNR > 8. In a multiple regression model, apo[a] allele size and size and expression of the other apo[a] allele (and PNR > 8 for Caucasians) significantly predicted allele-specific apo[a] levels. For a common PNR 8 allele, predicted values were similar in the two ethnicities for small size apo[a]. Allele-specific apo[a] levels were influenced by the other allele size and expression. Observed differences between Caucasians and African Americans in allele-specific apo[a] levels were explained for small apo[a] sizes by the other allele size and PNR; the ethnicity differences remain unexplained for larger sizes.

Mechanism for the homocysteine-enhanced antifibrinolytic potential of lipoprotein(a) in human plasma

Lipoprotein(a) and total plasma homocysteine levels are now established as independent atherothrombogenic risk factors. A distinctive pathophysiological feature of lipoprotein(a) is its antifibrinolytic activity, an effect dependent on plasma concentration and high affinity for fibrin of its small size apo(a) component. A stimulating effect of homocysteine on purified lipoprotein(a) has been proposed. However, little is known about their specific interactions in human plasma. We demonstrate by immunochemical, ligand-binding and plasminogen activation studies, that homocysteine modifies the structure and function of lipoprotein(a) in human plasma; it reduces the apo(a)/apoB disulfide bond causing the appearance of free apo(a) with high affinity for fibrin that inhibits plasminogen binding and plasmin formation (r= -0.995, p =0.002). These effects were evident particularly in plasma samples containing lipoprotein(a) with low affinity for fibrin and more than 22 kringles apo(a) isoforms. In contrast, for plasmas containing high fibrin affinity lipoprotein(a) (less than 22 kringles apo[a] isoforms) no significant change neither in fibrin binding nor in plasmin formation was observed. Furthermore, isolated apo(a) recombinants (10 to 34 kringles) that have been shown to display size-independent high affinity for fibrin were not affected by homocysteine, thus confirming lipoprotein(a) as its main target. These results suggest that the pro-atherogenic role already conferred to lipoprotein(a) by small apo(a) isoforms may be extended to large apo(a) isoforms if released in plasma by homocysteine, as this mechanism reveals their high fibrin affinity. Lipoprotein(a) and homocysteine may therefore constitute, if acting in concert, a new risk factor for athero-thrombotic vascular disease.

High lipoprotein(a) levels and small apolipoprotein(a) sizes are associated with endothelial dysfunction in a multiethnic cohort

ObjectivesThis study sought to determine the effect of lipoprotein(a), or Lp(a), levels and apolipoprotein(a), or apo(a), sizes on endothelial function and to explore ethnic differences in their effects. BackgroundAlthough high levels of Lp(a) have been shown to confer increased cardiovascular risk in Caucasians, its significance in non-Caucasian populations is uncertain. The pathogenic role of the apo(a) component of Lp(a) is also unclear. MethodsThe relationship of Lp(a) levels and apo(a) sizes to endothelial function was examined in a multiethnic cohort of 89 healthy subjects (age 42 ± 9 years; 50 men, 39 women) free of other cardiac risk factors. Endothelium-dependent, flow-mediated dilation (FMD) and endothelium-independent, nitrate-induced dilation (NTG) were assessed by ultrasound imaging of the brachial artery. ResultsPlasma Lp(a) levels were lowest in Caucasians (18.3 ± 21.1 mg/dl, n = 40); intermediate in Hispanics (30.2 ± 30.5 mg/dl, n = 21); and highest in African Americans (68.8 ± 46.0 mg/dl, n = 28). Lipoprotein(a) levels were found to correlate inversely to FMD (r = −0.33, p < 0.005) but not to NTG (r = 0.06, p = 0.60). This association remained significant after adjusting for gender (p = 0.002). In addition, subjects with small apo(a) size of ≤22 kringle 4 repeats had significantly lower FMD than those with large apo(a) (2.23 ± 2.37% vs. 6.26 ± 4.29%, p < 0.0001), irrespective of Lp(a) levels. ConclusionsThese findings support an independent role of Lp(a) in atherogenesis, an effect that is particularly evident in African Americans. The proatherogenic property of Lp(a) can be attributed in part to its apo(a) component.

Lipoprotein size and atherosclerosis susceptibility in Apoe(-/-) and Ldlr(-/-) mice.

Two hypercholesterolemic mouse models, the apo-E-deficient mouse (Apoe(-/-)) and the LDL receptor-deficient mouse (Ldlr(-/-)), have been used extensively as animal models of atherogenesis. Total plasma cholesterol levels in chow-fed Apoe(-/-) mice are much higher than in Ldlr(-/-) mice. In a recent study, we managed to even-up the cholesterol levels in Apoe(-/-) mice and Ldlr(-/-) mice by making both models homozygous for the Apob(100) (apo B-100-only) allele. On a chow diet, apo-E-deficient apo B-100-only mice (Apoe(-/-)Apob(100/100)) and LDL receptor-deficient apo B-100-only mice (Ldlr(-/-)Apob(100/100)) had similar total plasma cholesterol levels ( approximately 300 mg/dL). The plasma of Ldlr(-/-)Apob(100/100) mice contained large numbers of small lipoproteins, whereas the plasma of Apoe(-/-)Apob(100/100) mice contained much lower levels of much larger lipoproteins. Interestingly, the Ldlr(-/-)Apob(100/100) mice developed far more extensive atherosclerotic lesions than the Apoe(-/-)Apob(100/100) mice. The finding of substantially more atherosclerosis in Ldlr(-/-)Apob(100/100) mice than in Apoe(-/-)Apob(100/100) mice, despite nearly identical cholesterol levels, suggests that large numbers of small apo B-100-containing lipoproteins are far more atherogenic than lower numbers of large apo B-100-containing lipoproteins.