Effect of glycation on the properties of lipoprotein(a)

Protein glycation in diabetes mellitus

Diabetes mellitus is the ninth leading cause of mortality worldwide. It is a complex disease that manifests as chronic hyperglycemia. Glucose exposure causes biochemical changes at the proteome level as reflected in accumulation of glycated proteins. A prominent example is hemoglobin A1c (HbA1c), a glycated protein widely accepted as a diabetic indicator. Another emerging biomarker is glycated albumin which has demonstrated utility in situations where HbA1c cannot be used. Other proteins undergo glycation as well thus impacting cellular function, transport and immune response. Accordingly, these glycated counterparts may serve as predictors for diabetic complications and thus warrant further inquiry. Fortunately, modern proteomics has provided unique analytic capability to enable improved and more comprehensive exploration of glycating agents and glycated proteins. This review broadly covers topics from epidemiology of diabetes to modern analytical tools such as mass spectrometry to facilitate a better understanding of diabetes pathophysiology. This serves as an attempt to connect clinically relevant questions with findings of recent proteomic studies to suggest future avenues of diabetes research.

Lipoprotein(a) metabolism: potential sites for therapeutic targets

Lipoprotein(a) [Lp(a)] resembles low-density lipoprotein (LDL), with an LDL lipid core and apolipoprotein B (apoB), but contains a unique apolipoprotein, apo(a). Elevated Lp(a) is an independent risk factor for coronary and peripheral vascular diseases. The size and concentration of plasma Lp(a) are related to the synthetic rate, not the catabolic rate, and are highly variable with small isoforms associated with high concentrations and pathogenic risk. Apo(a) is synthesized in the liver, although assembly of apo(a) and LDL may occur in the hepatocytes or plasma. While the uptake and clearance site of Lp(a) is poorly delineated, the kidney is the site of apo(a) fragment excretion. The structure of apo(a) has high homology to plasminogen, the zymogen for plasmin and the primary clot lysis enzyme. Apo(a) interferes with plasminogen binding to C-terminal lysines of cell surface and extracellular matrix proteins. Lp(a) and apo(a) inhibit fibrinolysis and accumulate in the vascular wall in atherosclerotic lesions. The pathogenic role of Lp(a) is not known. Small isoforms and high concentrations of Lp(a) are found in healthy octogenarians that suggest Lp(a) may also have a physiological role. Studies of Lp(a) function have been limited since it is not found in commonly studied small mammals. An important aspect of Lp(a) metabolism is the modification of circulating Lp(a), which has the potential to alter the functions of Lp(a). There are no therapeutic drugs that selectively target elevated Lp(a), but a number of possible agents are being considered. Recently, new modifiers of apo(a) synthesis have been identified. This review reports the regulation of Lp(a) metabolism and potential sites for therapeutic targets.

In vivo oxidizability of LDL in type 2 diabetic patients in good and poor glycemic control

We aimed to determine if increased non-enzymatic glycosylation of the LDL was sufficient to increase the susceptibility to in vivo oxidation of the LDL particles. Twenty-two type 2 diabetic patients (11 males and 11 females) were included in this study. They were enrolled on the basis of good [glycated hemoglobin (HbA1c) < 7%] and poor glycemic control [(HbA1c) > 8%]. LDL were isolated by sequential ultracentrifugation and analyzed by capillary electrophoresis (CE) for diene conjugate content and for electronegativity. The glyc-LDL levels were increased in all diabetic type 2 patients, peaking in the diabetic subjects in poor diabetic control (17.3 +/- 8.07%). The LDL content of diene conjugates was similar between the two groups (6.65 +/- 0.77% for the patients with good glycemic control versus 6.88 +/- 0.74% for those with poor glycemic control; P = 0.49) as was the electrophoretic mobility ((-1.14544 +/- 0.089) x 10(-4) cm2/(V s) for the patients with good glycemic control and (-1.13666 +/- 0.073) x 10(-4) cm2/(V s) for those with poor glycemic control; P = 0.80). The susceptibility to in vivo oxidation of LDL from type 2 diabetic patients in poor glycemic control did not differ from that of well-controlled diabetic patients. LDL glycosylation was not able to increase the oxidizability of LDL in the diabetic patients with poor glycemic control.

STAT5 activation induced by diabetic LDL depends on LDL glycation and occurs via src kinase activity

Advanced glycation end products (AGEs) have been implicated in the accelerated vascular injury occurring in diabetes. We recently reported that LDL prepared from type 2 diabetic patients (dm-LDL), but not normal LDL (n-LDL) triggered signal transducers and activators of transcription STAT5 activation and p21(waf) expression in endothelial cells (ECs). The aims of the present study were to investigate the role of LDL glycation in dm-LDL- mediated signals and to analyze the molecular mechanisms leading to STAT5 activation. We found that glycated LDL (gly-LDL) triggered STAT5 activation, the formation of a prolactin inducible element (PIE)-binding complex containing STAT5, and increased p21(waf) expression through the activation of the receptor for AGE (RAGE). We also demonstrated that dm-LDL and gly-LDL, but not n-LDL treatment induced the formation of a stable complex containing the activated STAT5 and RAGE. Moreover, gly-LDL triggered src but not JAK2 kinase activity. Pretreatment with the src kinase inhibitor PP1 abrogated both STAT5 activation and the expression of p21(waf) induced by gly-LDL. Consistently, gly-LDL failed to activate STAT5 in src(-/-) fibroblasts. Collectively, our results provide evidence for the role of glycation in dm-LDL-mediated effects and for a specific role of src kinase in STAT5-dependent p21(waf) expression.

Influence of lipoprotein(a) on restenosis after femoropopliteal percutaneous transluminal angioplasty in Type 2 diabetic patients

OBJECTIVE

The influence of vascular morphology and metabolic parameters including lipoprotein(a) (Lp(a)) on restenosis after peripheral angioplasty has been compared in Type 2 diabetes (DM) vs. non-diabetic patients (ND).

RESEARCH DESIGN AND METHODS

The clinical course and risk profile of 132 (54 DM vs. 78 ND) patients with peripheral arterial occlusive disease (PAD) were observed prospectively following femoropopliteal angioplasty (PTA). Clinical examination, oscillometry, ankle brachial blood pressure index (ABI) and the toe systolic blood pressure index (TSPI) were used during follow-up. Duplex sonography and reangiography were also used to verify suspected restenosis or reocclusion.

RESULTS

At the time of intervention patients with DM had a lower median Lp(a) of 9 vs. 15 mg/dl (P < 0.01) in patients without diabetes. Recurrence within 1 year after PTA occurred in 25 diabetic (= 46%, Lp(a) 12 mg/dl) and 30 non-diabetic (= 38%, Lp(a) 48 mg/dl) patients. DM patients with 1 year's patency had a median Lp(a) of 7 vs. 11 mg/dl in non-diabetic patients (P < 0.05). However, 12 months after angioplasty Lp(a) correlated negatively with the ABI (r = -0.44, P < 0.01) in diabetic and in non-diabetic patients (r = -0.20, P < 0.05). The probability of recurrence after PTA continuously increased with higher levels of Lp(a) in each subgroup of patients.

CONCLUSIONS

Our data indicate that Lp(a) is generally lower in those with peripheral arterial occlusive disease and Type 2 diabetes than in non-diabetic individuals. The increased risk for restenosis with rising levels of Lp(a) is set at a lower Lp(a) in diabetes and may be more harmful for diabetic patients.

Apolipoprotein H is not affected by in vitro glycosylation

Increased nonenzymatic glycosylation of all major classes of apolipoproteins has been demonstrated in diabetes. In this work we deal with the in vitro nonenzymatic glycosylation of apolipoprotein H, whose role in lipid metabolism is still poorly understood and whose levels increase in diabetes. Apolipoprotein H was isolated from human plasma and purified through a combination of affinity chromatography and continuous elution electrophoresis. The in vitro glycosylation was performed by incubating purified apolipoprotein H with high concentration of glucose. Our results indicate that the in vitro nonenzymatic glycosylation has no effect on the physical properties of apolipoprotein H, despite the fact that this apolipoprotein contains a high number of lysine residues. Since the in vitro concentration of glucose was far higher than the levels normally found in diabetic subjects, it is unlikely for apolipoprotein H to become glycosylated in diabetes.

Glyc-oxidized LDL impair endothelial function more potently than oxidized LDL: role of enhanced oxidative stress

Hypercholesterolemia is associated with impairment of endothelial function due to increased levels of LDL. In diabetic patients, however, attenuation of endothelial function occurs even under normocholesterolemic conditions. Here we assessed whether glycation of LDL potentiates their influence on endothelial function, with particular emphasis on the oxidizability of LDL and the role of O2-. Human LDL was glycated by dialyzation for 7 days against buffer containing 200 mmol/l glucose, or sham-treated without glucose, and oxidized by incubation with Cu2+. Glycation significantly enhanced the oxidizability of LDL, as detected by diene formation and by electrophoretic mobility (27.5 mm for oxidized LDL vs. 34 mm for oxidized glycated LDL at 20 h of oxidation). Isolated rings of rabbit aorta were superfused with physiological salt solution, and isometric tension was recorded. Incubation of the aortic rings with sham-treated or with glycated LDL, not oxidized, had no influence on acetylcholine-induced, endothelium-dependent relaxation. Exposure of the aortic rings to oxidized non-glycated LDL caused a significant inhibition (30% at 1 microM acetylcholine) of the endothelium-dependent relaxation only in the presence of diethyl-dithiocarbamate (DDC), an inhibitor of the endogenous superoxide dismutase (SOD). Incubation of aortic rings with oxidized glycated LDL attenuated endothelium-dependent relaxation even in the absence of DDC (by 31% at 1 microM acetylcholine). The presence of DDC potentiated the inhibition of relaxation (65% inhibition at 1 microM acetylcholine), and co-incubation with exogenous SOD and catalase prevented the inhibition of relaxation, indicating a mediator role of O2-. Endothelium-independent relaxation induced by forskolin was unaffected by any of the lipoproteins. Using a chemiluminescence assay, significantly increased O2- production of aortic rings pretreated with oxidized glycated LDL (4101 +/- 360 counts/s) in comparison to control rings (753 +/- 81 counts/s) or arteries pretreated with oxidized non-glycated LDL (2358 +/- 169 counts/s) could be detected, suggesting that enhanced NO-inactivation by O2- could be the underlying mechanism for the stronger impairment of endothelium-dependent dilations by oxidized glycated LDL. Glycation increases the oxidizability of LDL and potentiates its endothelium-damaging influence. The likely mechanism for attenuation of endothelium-dependent dilations is increased formation of O2-, resulting in inactivation of nitric oxide. This mechanism may play an important role in diabetic patients and may contribute to disturbed organ perfusion.

Influence of APOH protein polymorphism on apoH levels in normal and diabetic subjects

Apolipoprotein (apo)H (also known as beta 2 glycoprotein-I) is a glycoprotein synthesized by liver cells and it is present in the blood associated with plasma lipoproteins. APOH displays a genetically determined structural polymorphism: three alleles (APOH*1, APOH*2, APOH*3) at a single locus on chromosome 17 code for different isoforms, and population studies have shown that APOH*2 is the most frequent allele. This paper assesses the relation between APOH phenotypes and plasma apoH levels in a population composed of 278 healthy subjects (243 H2/2, 32 H3/2, 2 H3/3, 1 H2/1; allele frequencies APOH*1 0.002, APOH*2 0.934, APOH*3 0.064) and 245 diabetics (212 H2/2, 30 H3/2, 3 H3/3; allele frequencies APOH*2 0.927 and APOH*3 0.073). Determination of apoH levels by competitive ELISA gave a mean value of 26.3 +/- 9.8 mg/dl for all subjects, 22.6 +/- 7.7 in normals vs 30.6 +/- 10.3 in diabetics (p = 0.0001), and 23.0 +/- 7.9, 19.3 +/- 5.4 and 18.5 +/- 3.5 mg/dl for H2/2, H3/2 and H3/3 in normals and 31.1 +/- 10.1, 28.2 +/- 10.8 and 15.7 +/- 9.0 mg/dl in diabetics, respectively. ANCOVA of the adjusted data revealed a significant difference in apoH levels for the three phenotypes in both the normal subjects (p = 0.01) and the diabetics (p = 0.02). ANCOVA of the whole samples of subjects, controlling for diabetes as well as age, sex and total cholesterol, indicated a substantial effect of phenotype, independent of the other variables (p = 0.0007).

Apolipoprotein H levels in diabetic subjects: correlation with cholesterol levels

To assess the relationship between apolipoprotein H (apo H) plasma levels and lipid metabolism in diabetes mellitus, we have examined the correlation between apo H plasma concentration and the main plasma lipid levels in 127 non-insulin-dependent (NIDDM) and 118 insulin-dependent (IDDM) diabetes mellitus patients. The data are compared with those in 286 nondiabetics. Our data show a significant increase in plasma apo H in diabetic as opposed to nondiabetic subjects (NIDDM, 29.9 +/- 10.8 mg/dL; IDDM, 31.3 +/- 9.9; controls, 22.5 +/- 7.7; F = 53.3, P = .0001). The relation between plasma lipids and apo H was simultaneously evaluated in the three groups with inclusion of diabetes, sex, body mass index (BMI), and age as covariates in the model. This analysis showed a strong positive correlation (P = .0009) between apo H and total cholesterol, and a weaker positive correlation with triglycerides ([TGs] P = .016). The correlation between apo H and hemoglobin A1c (HbA1c) levels in diabetics (P = .03) highlights the importance of glycemic control for plasma levels of this apoprotein, which is highly glycated. Although the role of apo H in lipid metabolism is still uncertain, recent investigations on the possible relation between plasma apo H levels and increased plasma lipids and thrombotic risk could explain the increased atherosclerotic risk in diabetic patients.

Evaluation of serum glycated lipoprotein(a) levels in noninsulin-dependent diabetic patients

OBJECTIVE

Serum Lp(a) levels are generally considered unaffected by non-insulin-dependent diabetes mellitus (NIDDM). However, high Lp(a) concentrations as well as an increased rate of nonenzymatic glycation of proteins may be involved in degenerative diabetic complications.

DESIGN AND METHODS

We measured serum glycated Lp(a) levels in 17 NIDDM patients, as compared to 14 normoglycaemic controls. Glycated proteins were separated from nonglycated ones by boronate affinity chromatography, and specific proteins assayed by immunonephelometric methods in both fractions.

RESULTS

The percentage of glycated Lp(a) was 1.5 +/- 0.4% (mean +/- SD) in the control group, and was significantly higher in NIDDM patients: 4.3 +/- 1.5% (p < 0.01). The basal level of Lp(a) glycation was lower than that of other proteins, particularly apo B (4.0 +/- 0.7%). By contrast, the variations of glycated Lp(a) levels were of greater amplitude (+ 187%) than those of glycated apo B (+ 67%). Glycated Lp(a) values were significantly elevated in patients with micro and macrovascular complications in comparison with uncomplicated patients.

CONCLUSIONS

These results suggest that glycated Lp(a) may be considered a potentially interesting parameter in the pathophysiology of diabetic vascular complications.

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).