Proteomics of apolipoproteins and associated proteins from plasma high-density lipoproteins

Association of apolipoprotein A1 and B with kidney function and chronic kidney disease in two multiethnic population samples

BACKGROUND

Circulating lipoproteins and their protein constituents, apolipoproteins, are risk factors for chronic kidney disease (CKD). The associations between apolipoprotein A1, apolipoprotein B and their ratio with glomerular filtration rate estimated from the new CKD Epidemiology Collaboration (CKD-EPI) equation (eGFR) are not well studied in the general population.

METHODS

Associations between apolipoprotein A1, B and their ratio with the outcomes of eGFR, CKD (eGFR<60 mL/min/1.73 m2) and albuminuria were examined in the Atherosclerosis Risk in Communities study (ARIC, n=10,292, 1996-98) and the Third National Health and Nutrition Examination Survey (NHANES III, n=7023, 1988-91). Cross-sectional multivariable-adjusted analyses were performed using linear and logistic regression. Prospective analyses related baseline apolipoprotein levels to subsequent CKD incidence over 10 years using the ARIC Carotid MRI follow-up cohort (n=1659).

RESULTS

Higher apolipoprotein A1 quartiles were associated with a lower prevalence of CKD [Q4 versus Q1: odds ratio (OR) 0.73, P-trend=0.02 in ARIC; Q4 versus Q1: OR 0.53, P-trend<0.01 in NHANES III] as well as with higher eGFR (P-trend<0.01 in ARIC and NHANES III). No consistent significant associations were found for apolipoprotein B in either study. The apolipoprotein B/A1 ratio was significantly associated with eGFR across quartiles in both studies (P-trend<0.01) and with CKD in ARIC (Q4 versus Q1: OR 1.23, P-trend=0.01). Prospectively, there were trends for the association of apolipoproteins with incident CKD [Q4 versus Q1: incidence rate ratio (IRR)=0.68 for apolipoprotein A1, P-trend=0.1; Q4 versus Q1: IRR=1.35 for apolipoprotein B, P-trend=0.2]. Associations were not systematically stronger when comparing traditional lipids (total cholesterol, low-density lipoprotein or high-density lipoprotein) to apolipoproteins.

CONCLUSIONS

Higher serum apolipoprotein A1 was associated with lower prevalence of CKD and higher eGFR estimated by the CKD-EPI equation in two large multiethnic population-based samples. While apolipoprotein B showed no consistent associations, a higher apolipoprotein B/A1 ratio was significantly associated with lower eGFR in both studies. The direction and magnitude of the longitudinal associations between apolipoproteins and CKD incidence were overall similar to those observed cross-sectionally. No consistent differences became apparent between traditional lipids and apolipoproteins.

Predictive value of different HDL particles for the protection against or risk of coronary heart disease

The inverse relationship between plasma HDL levels and the risk of developing coronary heart disease is well established. The underlying mechanisms of this relationship are poorly understood, largely because HDL consist of several functionally distinct subpopulations of particles that are continuously being interconverted from one to another. This review commences with an outline of what is known about the origins of individual HDL subpopulations, how their distribution is regulated, and describes strategies that are currently available for isolating them. We then summarise what is known about the functionality of specific HDL subpopulations, and how these findings might impact on cardiovascular risk. The final section highlights major gaps in existing knowledge of HDL functionality, and suggests how these deficiencies might be addressed. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Multidimensional profiling of plasma lipoproteins by size exclusion chromatography followed by reverse-phase protein arrays

The composition of lipoproteins and the association of proteins with various particles are of much interest in the context of cardiovascular disease. Here, we describe a technique for the multidimensional analysis of lipoproteins and their associated apolipoproteins. Plasma is separated by size exclusion chromatography (SEC), and fractions are analyzed by reverse-phase arrays. SEC fractions are spotted on nitrocellulose slides and incubated with different antibodies against individual apolipoproteins or antibodies against various apolipoproteins. In this way, tens of analytes can be measured simultaneously in 100 μl of plasma from a single SEC separation. This methodology is particularly suited to simultaneous analysis of multiple proteins that may change their distribution to lipoproteins or alter their conformation, depending on factors that influence circulating lipoprotein size or composition. We observed changes in the distribution of exchangeable apolipoproteins following addition of recombinant apolipoproteins or interaction with exogenous compounds. While the cholesteryl ester transfer protein (CETP)-dependent formation of pre-β-HDL was inhibited by the CETP inhibitors torcetrapib and anacetrapib, it was not reduced by the CETP modulator dalcetrapib. This finding was elucidated using this technique.

Extracellular modifications of HDL in vivo and the emerging concept of proteolytic inactivation of preβ-HDL

PURPOSE OF REVIEW

Both quantity and quality of the circulating HDL particle matter for the optimal antiatherogenic potential of HDL. This review summarizes various mechanisms capable of inducing extracellular modifications of HDL and reducing the function of HDL subclasses as cholesterol acceptors. Special emphasis is laid on the proteolytic inactivation of lipid-poor preβ-migrating HDL (preβ-HDL).

RECENT FINDINGS

HDL particles can undergo functional inactivation in vivo. During atherogenesis, different cell types in the arterial intima release enzymes into the intimal fluid, potentially capable of causing structural and chemical modifications of the various components present in the lipid core or in the polar surface of the HDL particles. Enzymatic oxidation, lipolysis and proteolysis, and nonenzymatic glycosylation are among the HDL modifications that adversely affect HDL functionality. Proteolysis of preβ-HDL by various proteases present in the arterial intima has emerged as a potential mechanism that impairs the efficiency of HDL to promote cholesterol efflux from macrophage foam cells, the mast cell-derived neutral protease chymase being a prime example of such impairment. A paradigm of proteolytic inactivation of preβ-HDL in vivo is emerging.

SUMMARY

Several extracellular enzymes present in the arterial intima may compromise various cardioprotective functions of HDL. Observations on proteolysis of specific lipid-poor HDL subpopulations in vivo constitute the basis for future studies evaluating the actual impact of proteolytic microenvironments on the initiation and progression of atherosclerotic lesions.

Proteomics of Lipoprotein(a) identifies a protein complement associated with response to wounding

Lipoprotein(a) [Lp(a)] is a major independent risk factor for cardiovascular disease. Twenty percent of the general population exhibit levels above the risk threshold highlighting the importance for clinical and basic research. Comprehensive proteomics of human Lp(a) will provide significant insights into Lp(a) physiology and pathogenicity. Using liquid chromatography-coupled mass spectrometry, we established a high confidence Lp(a) proteome of 35 proteins from highly purified particles. Protein interaction network analysis and functional clustering revealed proteins assigned to the two major biological processes of lipid metabolism and response to wounding. The latter includes the processes of coagulation, complement activation and inflammatory response. Furthermore, absolute protein quantification of apoB-100, apo(a), apoA1, complement C3 and PON1 gave insights into the compositional stoichiometry of associated proteins per particle. Our proteomics study has identified Lp(a)-associated proteins that support a suggested role of Lp(a) in response to wounding which points to mechanisms of Lp(a) pathogenicity at sites of vascular injury and atherosclerotic lesions. This study has identified a high confidence Lp(a) proteome and provides an important basis for further comparative and quantitative analyses of Lp(a) isolated from greater numbers of plasma samples to investigate the significance of associated proteins and their dynamics for Lp(a) pathogenicity.

High-density lipoprotein proteome dynamics in human endotoxemia

Background

A large variety of proteins involved in inflammation, coagulation, lipid-oxidation and lipid metabolism have been associated with high-density lipoprotein (HDL) and it is anticipated that changes in the HDL proteome have implications for the multiple functions of HDL. Here, SELDI-TOF mass spectrometry (MS) was used to study the dynamic changes of HDL protein composition in a human experimental low-dose endotoxemia model. Ten healthy men with low HDL cholesterol (0.7+/-0.1 mmol/L) and 10 men with high HDL cholesterol levels (1.9+/-0.4 mmol/L) were challenged with endotoxin (LPS) intravenously (1 ng/kg bodyweight). We previously showed that subjects with low HDL cholesterol are more susceptible to an inflammatory challenge. The current study tested the hypothesis that this discrepancy may be related to differences in the HDL proteome.

Results

Plasma drawn at 7 time-points over a 24 hour time period after LPS challenge was used for direct capture of HDL using antibodies against apolipoprotein A-I followed by subsequent SELDI-TOF MS profiling. Upon LPS administration, profound changes in 21 markers (adjusted p-value < 0.05) were observed in the proteome in both study groups. These changes were observed 1 hour after LPS infusion and sustained up to 24 hours, but unexpectedly were not different between the 2 study groups. Hierarchical clustering of the protein spectra at all time points of all individuals revealed 3 distinct clusters, which were largely independent of baseline HDL cholesterol levels but correlated with paraoxonase 1 activity. The acute phase protein serum amyloid A-1/2 (SAA-1/2) was clearly upregulated after LPS infusion in both groups and comprised both native and N-terminal truncated variants that were identified by two-dimensional gel electrophoresis and mass spectrometry. Individuals of one of the clusters were distinguished by a lower SAA-1/2 response after LPS challenge and a delayed time-response of the truncated variants.

Conclusions

This study shows that the semi-quantitative differences in the HDL proteome as assessed by SELDI-TOF MS cannot explain why subjects with low HDL cholesterol are more susceptible to a challenge with LPS than those with high HDL cholesterol. Instead the results indicate that hierarchical clustering could be useful to predict HDL functionality in acute phase responses towards LPS.

The relationship between high density lipoprotein subclass profile and apolipoprotein concentrations

The HDL fraction in human plasma is heterogeneous in terms of size, shape, composition, and surface charge. The HDL subclasses contents were quantified by 2-dimensional non-denaturing gel electrophoresis, immunoblotting, and image analysis. This research review systematically analyzed the relationship between the contents of HDL subclasses and the concentrations and ratios of the 5 major plasma apolipoproteins (apo). As the concentration of apoA-I increases, the contents of all HDL subclasses increase significantly. The most significant association was observed between large-sized HDL2b contents and apoA-I. ApoA-II played a dual function in the contents of HDL subclasses, and both small-sized HDL3b and HDL3a and large-sized HDL2b tended to increase with apoA-II concentration. An increase in the concentrations of apoC-II, C-III, and B-100 resulted in higher levels of small-sized HDL particles and lower levels of large-sized HDL particles. Plasma apoB- 100, apoC-II, and apoC-III appear to play a coordinated role in assembly of HDL particles and the determination of their contents. Higher concentrations of apoA-I could inhibit the reduction in content of large-sized HDL2b effected by apoB-100, C-II, and C-III. The preβ1-HDL contents increased significantly and those of HDL2b declined progressively with an increased apoB-100/apoA-I or a decreased apoC-III/apoC-II ratio. In summary, each apo has distinct but interrelated roles in HDL particle generation and metabolism. ApoA-I and apoC-II concentrations are independent determinants of HDL subtypes in circulation and apoA-I levels might be a more powerful factor to influence HDL subclasses distribution. Moreover, apoB- 100/apoA-I ratio could reliably and sensitively reflect the HDL subclass profile.

The versatility of HDL: a crucial anti-inflammatory regulator

BACKGROUND

Low levels of plasma high-density lipoprotein (HDL) represent a major cardiovascular risk factor and therefore raising HDL has been proposed to positively affect patients with atherosclerotic heart disease. However, the current evidence that raising HDL per se will reduce atherosclerosis and thereby cardiovascular events still remains controversial.

AIMS

In this review, we discuss the diverse anti-atherogenic and anti-inflammatory properties of HDL in the light of recent findings indicating that the quality rather than the mere quantity of HDL determines its beneficial effects against atherosclerosis. More specifically, we will focus on the conspicuous anti-inflammatory properties of HDL as this might contribute to the overall beneficial effects of HDL in diseased patients such as modulation of costimulatory/adhesion molecule expression, cytokine production and inhibition of the prototypical proinflammatory transcription factor NF-κB.

RESULTS

A range of clinical disorders share permanent inflammation as a characteristic hallmark including coronary artery disease, chronic kidney disease, diabetes mellitus or rheumatoid arthritis and also display distinct qualitative changes in the HDL compartment. Loss of anti-inflammatory functions of HDL is emerging as an important risk factor for disease progression and survival in these clinical entities.

CONCLUSIONS

It will be important to define the anti-inflammatory effects of HDL at the molecular level and to dissect the manifold functional implications to develop both novel functional assays that enable meaningful outcome studies and foster new therapeutic concepts in patients with altered HDL function.

Proteomic characterization of human plasma high density lipoprotein fractionated by gel filtration chromatography

Plasma levels of high density lipoprotein cholesterol (HDL-C) are inversely proportional to the incidence of cardiovascular disease. Recent applications of modern proteomic technologies have identified upward of 50 distinct proteins associated with HDL particles with many of these newly discovered proteins implicating HDL in nonlipid transport processes including complement activation, acute phase response and innate immunity. However, almost all MS-based proteomic studies on HDL to date have utilized density gradient ultracentrifugation techniques for HDL isolation prior to analysis. These involve high shear forces and salt concentrations that can disrupt HDL protein interactions and alter particle function. Here, we used high-resolution size exclusion chromatography to fractionate normal human plasma to 17 phospholipid-containing subfractions. Then, using a phospholipid binding resin, we identified proteins that associate with lipoproteins of various sizes by electrospray ionization mass spectrometry. We identified 14 new phospholipid-associated proteins that migrate with traditionally defined HDL, several of which further support roles for HDL in complement regulation and protease inhibition. The increased fractionation inherent to this method allowed us to visualize HDL protein distribution across particle size with unprecedented resolution. The observed heterogeneity across subfractions suggests the presence of HDL particle subpopulations each with distinct protein components that may prove to impart distinct physiological functions.

Lipidomics as a tool for the study of lipoprotein metabolism

Although technologies for lipidomic and proteomic investigations have developed very recently, lipidomic and proteomic studies of plasma lipoproteins have already provided several impressive examples of detailed characterization of distinct metabolic pathways potentially involved in lipoprotein metabolism in both health and disease states (obesity, insulin resistance, fatty liver disease) as well as under lifestyle and dietary modification (fish consumption, carbohydrates, probiotics) and lipid-modifying treatments (statins, low-density lipoprotein apheresis). Available lipidomic methodologies have facilitated detailed characterization of lipid classes and molecular species present in plasma as well as in lipoprotein fractions. Together with emerging proteomic techniques, lipidomics of plasma lipoproteins will soon provide molecular details of lipoprotein composition, which will ultimately be translated into integrated knowledge of the structure, metabolism, and function of lipoproteins in health and disease.

Low clusterin levels in high-density lipoprotein associate with insulin resistance, obesity, and dyslipoproteinemia

OBJECTIVE

To determine whether obesity and insulin resistance associate with changes in the protein content of high-density lipoprotein (HDL) in 2 different groups of men by using targeted proteomics.

METHODS AND RESULTS

Insulin resistance and obesity are hallmarks of type 2 diabetes mellitus and the metabolic syndrome, which confer an increased risk of cardiovascular disease. Recent studies suggest that the protein cargo of HDL makes important contributions to the lipoprotein's cardioprotective effects. In a discovery study, we used isotope dilution mass spectrometry to quantify the relative concentrations of 5 proteins previously implicated in HDL's cardioprotective effects in 3 groups of healthy subjects: lean insulin-sensitive, lean insulin-resistant, and obese insulin-resistant individuals. We validated our findings in a different group of subjects. The clusterin concentration in HDL strongly and negatively associated with insulin resistance and body mass index in both populations. HDL clusterin levels were lower in subjects with low HDL and high triglycerides, key components of the metabolic syndrome. There was an inverse correlation between clusterin levels in HDL and very-low-density lipoprotein/low-density lipoprotein.

CONCLUSIONS

Clusterin levels in HDL are lower in men with reduced insulin sensitivity, higher body mass index, and an unfavorable lipid profile. Our observations raise the possibility that clusterin depletion contributes to the loss of HDL's cardioprotective properties.

Exploring genetic determinants of plasma total cholesterol levels and their predictive value in a longitudinal study

BACKGROUND

Plasma total cholesterol (TC) levels are highly genetically determined. Although ample evidence of genetic determination of separate lipoprotein cholesterol levels has been reported, using TC level directly as a phenotype in a relatively large broad-gene based association study has not been reported to date.

METHODS AND RESULTS

We genotyped 361 single nucleotide polymorphisms (SNPs) across 243 genes based on pathways potentially relevant to cholesterol metabolism in 3575 subjects that were examined thrice over 11 years. Twenty-three SNPs were associated with TC levels after adjustment for multiple testing. We used 12 of them (rs7412 and rs429358 in APOE, rs646776 in CELSR2, rs1367117 in APOB, rs6756629 in ABCG5, rs662799 in APOA5, rs688 in LDLR, rs10889353 in DOCK7, rs2304130 in NCAN, rs3846662 in HMGCR, rs2275543 in ABCA1, rs7275 in SMARCA4) that were confirmed in previous candidate association or genome-wide-association studies to define a gene risk score (GRS). Average TC levels increased from 5.23 ± 0.82 mmol/L for those with 11 or less cholesterol raising alleles to 6.03 ± 1.11 mmol/L for those with 18 or more (P for trend<0.0001). The association with TC levels was slightly stronger when the weighted GRS that weighted the magnitude of allelic effects was used.

CONCLUSION

A panel of common genetic variants in the genes pivotal in cholesterol metabolism could possibly help identify those people who are at risk of high cholesterol levels.

Clinical implications of the BARI 2D and COURAGE trials of coronary artery disease

The results of the Bypass Angioplasty Revascularization Investigation 2 Diabetes (BARI 2D) and Clinical Outcomes Utilizing Revascularization and Aggressive Drug Evaluation (COURAGE) clinical trials raise important questions about the prevention and treatment of coronary heart disease among type 2 diabetics and nondiabetics. The BARI 2D and COURAGE trials showed that pharmacological therapy is as effective as surgery or angioplasty, because of the incidence of total mortality and cardiovascular disease events. The results are consistent with the clinical trials of lipid-lowering, antihypertensive therapy. The efficacy of lowering glycohemoglobin below 7% or benefits of specific glucose-lowering drugs is still unresolved. The BARI 2D trial focused on more advanced atherosclerotic disease. An important question is whether newly incident diabetics should be screened for subclinical atherosclerosis and treated aggressively with pharmacological therapy as in the BARI 2D trial. We are still uncertain whether raising high-density lipoprotein cholesterol will provide further benefit in reducing coronary heart disease. Better measurement of plaque morphology, determinants of hypercoagulable status, and drugs to reduce thrombosis and plaque are of high priority. Longer follow-up of the BARI 2D and COURAGE trials will provide important information about the risks of cardiovascular disease events and disability. It will be important but difficult to translate the results of the trials to community practice under the current health care system.