Epitopes close to the apolipoprotein B low density lipoprotein receptor-binding site are modified by advanced glycation end products

Modification of lipoprotein metabolism and function driving atherogenesis in diabetes

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disease, characterized by raised blood glucose levels and impaired lipid metabolism resulting from insulin resistance and relative insulin deficiency. In diabetes, the peculiar plasma lipoprotein phenotype, consisting in higher levels of apolipoprotein B-containing lipoproteins, hypertriglyceridemia, low levels of HDL cholesterol, elevated number of small, dense LDL, and increased non-HDL cholesterol, results from an increased synthesis and impaired clearance of triglyceride rich lipoproteins. This condition accelerates the development of the atherosclerotic cardiovascular disease (ASCVD), the most common cause of death in T2DM patients. Here, we review the alteration of structure, functions, and distribution of circulating lipoproteins and the pathophysiological mechanisms that induce these modifications in T2DM. The review analyzes the influence of diabetes-associated metabolic imbalances throughout the entire process of the atherosclerotic plaque formation, from lipoprotein synthesis to potential plaque destabilization. Addressing the different pathophysiological mechanisms, we suggest improved approaches for assessing the risk of adverse cardiovascular events and clinical strategies to reduce cardiovascular risk in T2DM and cardiometabolic diseases.

Role of APOC3 3238C/G, APOB 12669G/A and SCARB1 1050C/T polymorphisms, their expression in patients of HIV-associated lipodystrophy

Apolipoproteins and Scavenger Receptor Class B1 (SCARB1) proteins are involved in the etiology of HIV-associated lipodystrophy (HIVLD). APOC3 3238C/G, APOB 12669G/A and SCARB1 1050C/T polymorphisms were linked with increased level of APOB, TG, HDL-C and risk of cardiovascular diseases (CVDs). Hence, we evaluated the genetic variations of APOC3 3238C/G, APOB 12669G/A and SCARB1 1050C/T in 187 patients of HIV (64 with HIVLD, 123 without HIVLD) and 139 healthy controls using PCR-RFLP and expression by qPCR. The genotypes of SCARB1 1050 TT and APOB 12669AA showed a risk to severe HIVLD (P = 0.23, OR = 4.95; P = 0.16, OR = 2.02). The APOC3 3238 GG genotype was associated with a lesser risk of severe HIVLD (P = 0.07, OR = 0.22). The APOB 12669 GA genotype was associated with a greater risk of HIVLD severity in patients with impaired LDL, triglyceride (TG), and cholesterol levels (P = 0.34, OR = 4.13; P = 0.25, OR = 3.64; P = 0.26, OR = 5.47). Similarly, APOB 12669AA genotypes in the presence of impaired triglyceride levels displayed the susceptibility to severity of HIVLD (P = 0.77, OR = 2.91). APOB 12669 GA genotype along with impaired HDL and cholesterol levels indicated an increased risk for HIVLD acquisition among patients without HIVLD (P = 0.42, OR = 2.42; P = 0.26, OR = 2.27). In patients with and without HIVLD, APOC3 3238CG genotypes having impaired cholesterol and glucose levels had higher risk for severity and development of HIVLD (P = 0.13, OR = 2.84, P = 0.34, OR = 1.58; P = 0.71, OR = 1.86; P = 0.14, OR = 2.30). An increased expression of APOB and SCARB1 genes were observed in patients with HIVLD (+0.51 vs. -0.93; +4.78 vs. +3.29), and decreased expression of APOC3 gene was observed in patients with HIVLD (-0.35 vs. -1.65). In conclusion, the polymorphisms mentioned above were not associated with the modulation of HIVLD. However, in the presence of impaired triglyceride, HDL, cholesterol and glucose levels, APOB 12669AA and 12669 GA, APOC3 3238CG genotypes indicated a risk for the development and severity of HIVLD.

Protein glycation - biomarkers of metabolic dysfunction and early-stage decline in health in the era of precision medicine

Protein glycation provides a biomarker in widespread clinical use, glycated hemoglobin HbA1c (A1C). It is a biomarker for diagnosis of diabetes and prediabetes and of medium-term glycemic control in patients with established diabetes. A1C is an early-stage glycation adduct of hemoglobin with glucose; a fructosamine derivative. Glucose is an amino group-directed glycating agent, modifying N-terminal and lysine sidechain amino groups. A similar fructosamine derivative of serum albumin, glycated albumin (GA), finds use as a biomarker of glycemic control, particularly where there is interference in use of A1C. Later stage adducts, advanced glycation endproducts (AGEs), are formed by the degradation of fructosamines and by the reaction of reactive dicarbonyl metabolites, such as methylglyoxal. Dicarbonyls are arginine-directed glycating agents forming mainly hydroimidazolone AGEs. Glucosepane and pentosidine, an intense fluorophore, are AGE covalent crosslinks. Cellular proteolysis of glycated proteins forms glycated amino acids, which are released into plasma and excreted in urine. Development of diagnostic algorithms by artificial intelligence machine learning is enhancing the applications of glycation biomarkers. Investigational glycation biomarkers are in development for: (i) healthy aging; (ii) risk prediction of vascular complications of diabetes; (iii) diagnosis of autism; and (iv) diagnosis and classification of early-stage arthritis. Protein glycation biomarkers are influenced by heritability, aging, decline in metabolic, vascular, renal and skeletal health, and other factors. They are applicable to populations of differing ethnicities, bridging the gap between genotype and phenotype. They are thereby likely to find continued and expanding clinical use, including in the current era of developing precision medicine, reporting on multiple pathogenic processes and supporting a precision medicine approach.

Synergistic effects of fructose and glucose on lipoprotein risk factors for cardiovascular disease in young adults

BACKGROUND

Fructose consumption increases risk factors for cardiometabolic disease. It is assumed that the effects of free sugars on risk factors are less potent because they contain less fructose. We compared the effects of consuming fructose, glucose or their combination, high fructose corn syrup (HFCS), on cardiometabolic risk factors.

METHODS

Adults (18-40 years; BMI 18-35 kg/m²) participated in a parallel, double-blinded dietary intervention during which beverages sweetened with aspartame, glucose (25% of energy requirements (ereq)), fructose or HFCS (25% and 17.5% ereq) were consumed for two weeks. Groups were matched for sex, baseline BMI and plasma lipid/lipoprotein concentrations. 24-h serial blood samples were collected at baseline and at the end of intervention. Primary outcomes were 24-h triglyceride AUC, LDL-cholesterol (C), and apolipoprotein (apo)B. Interactions between fructose and glucose were assessed post hoc.

FINDINGS

145 subjects (26.0 ± 5.8 years; body mass index 25.0 ± 3.7 kg/m²) completed the study. As expected, the increase of 24-h triglycerides compared with aspartame was highest during fructose consumption (25%: 6.66 mmol/Lx24h 95% CI [1.90 to 11.63], P = 0.0013 versus aspartame), intermediate during HFCS consumption (25%: 4.68 mmol/Lx24h 95% CI [-0.18 to 9.55], P = 0.066 versus aspartame) and lowest during glucose consumption. In contrast, the increase of LDL-C was highest during HFCS consumption (25%: 0.46 mmol/L 95% CI [0.16 to 0.77], P = 0.0002 versus aspartame) and intermediate during fructose consumption (25%: 0.33 mmol/L 95% CI [0.03 to 0.63], P = 0.023 versus aspartame), as was the increase of apoB (HFCS-25%: 0.108 g/L 95%CI [0.032 to 0.184], P = 0.001; fructose 25%: 0.072 g/L 95%CI [-0.004 to 0.148], P = 0.074 versus aspartame). The post hoc analyses showed significant interactive effects of fructose*glucose on LDL-C and apoB (both P < 0.01), but not on 24-h triglyceride (P = 0.340).

CONCLUSION

A significant interaction between fructose and glucose contributed to increases of lipoprotein risk factors when the two monosaccharides were co-ingested as HFCS. Thus, the effects of HFCS on lipoprotein risks factors are not solely mediated by the fructose content and it cannot be assumed that glucose is a benign component of HFCS. Our findings suggest that HFCS may be as harmful as isocaloric amounts of pure fructose and provide further support for the urgency to implement strategies to limit free sugar consumption.

Increased retention of LDL from type 1 diabetic patients in atherosclerosis-prone areas of the murine arterial wall

BACKGROUND AND AIMS

Type 1 diabetes accelerates the development of atherosclerotic cardiovascular diseases. Retention of low-density lipoprotein (LDL) in the arterial wall is a causal step in atherogenesis, but it is unknown whether diabetes alters the propensity of LDL for retention. The present study investigated whether LDL from type 1 diabetic and healthy non-diabetic subjects differed in their ability to bind to the arterial wall in a type 1 diabetic mouse model.

METHODS

Fluorescently-labeled LDL obtained from type 1 diabetic patients or healthy controls was injected into mice with type 1 diabetes. The amount of retained LDL in the atherosclerosis-prone inner curvature of the aortic arch was quantified by fluorescence microscopy. Healthy control LDL was in vitro glycated, analyzed for protein glycation by LC-MS/MS, and tested for retention propensity.

RESULTS

Retention of LDL from type 1 diabetic patients was 4.35-fold higher compared to LDL from nondiabetic subjects. Nuclear magnetic resonance (NMR) spectroscopy analysis of LDL revealed no differences in the concentration of the atherogenic small dense LDL between type 1 diabetic and non-diabetic subjects. In vitro glycation of LDL from a non-diabetic subject increased retention compared to non-glycated LDL. LC-MS/MS revealed four new glycated spots in the protein sequence of ApoB of in vitro glycated LDL.

CONCLUSIONS

LDL from type 1 diabetic patients showed increased retention at atherosclerosis-prone sites in the arterial wall of diabetic mice. Glycation of LDL is one modification that may increase retention, but other, yet unknown, mechanisms are also likely to contribute.

Susceptibility of LDL and its subfractions to glycation

PURPOSE OF REVIEW

To highlight the potential importance of glycation as an atherogenic modification of LDL, factors determining glycated apolipoprotein B in vivo and susceptibility of LDL to glycation in vitro. We also discuss the distribution of glycated apolipoprotein B across different LDL subfractions in healthy controls, patients with type 2 diabetes and metabolic syndrome.

RECENT FINDINGS

Small, dense LDL, which is known to be most closely associated with atherogenesis, is more preferentially glycated in vivo and more susceptible to glycation in vitro than more buoyant LDL. Glycation and oxidation of LDL appear to be intimately linked. In patients with type 2 diabetes, plasma glycated apolipoprotein B correlated with small, dense LDL apolipoprotein B, but not with HbA1c. Glycated apolipoprotein B is significantly lower in statin-treated type 2 diabetes compared with those not on statins.

SUMMARY

Glycation of LDL occurs chiefly because of the nonenzymatic reaction of glucose and its metabolites with the free amino groups of lysine of which apolipoprotein B is rich. Higher concentrations of glycated LDL are present in diabetes than in nondiabetic individuals and metabolic syndrome. Even in nondiabetic individuals, however, there is generally more circulating glycated LDL than oxidatively modified LDL. Probably, oxidation and glycation of LDL are partially interdependent and indisputably coexist, and both prevent LDL receptor-mediated uptake and promote macrophage scavenger receptor-mediated LDL uptake. The recognition that LDL glycation is at least as important as oxidation in atherogenesis may lead to improvements in our understanding of its mechanism and how to prevent it.

Immunochemical analysis of the electronegative LDL subfraction shows that abnormal N-terminal apolipoprotein B conformation is involved in increased binding to proteoglycans

Electronegative LDL (LDL(-)) is a minor subfraction of modified LDL present in plasma. Among its atherogenic characteristics, low affinity to the LDL receptor and high binding to arterial proteoglycans (PGs) could be related to abnormalities in the conformation of its main protein, apolipoprotein B-100 (apoB-100). In the current study, we have performed an immunochemical analysis using monoclonal antibody (mAb) probes to analyze the conformation of apoB-100 in LDL(-). The study, performed with 28 anti-apoB-100 mAbs, showed that major differences of apoB-100 immunoreactivity between native LDL and LDL(-) concentrate in both terminal extremes. The mAbs Bsol 10, Bsol 14 (which recognize the amino-terminal region), Bsol 2, and Bsol 7 (carboxyl-terminal region) showed increased immunoreactivity in LDL(-), suggesting that both terminal extremes are more accessible in LDL(-) than in native LDL. The analysis of in vitro-modified LDLs, including LDL lipolyzed with sphingomyelinase (SMase-LDL) or phospholipase A(2) (PLA(2)-LDL) and oxidized LDL (oxLDL), suggested that increased amino-terminal immunoreactivity was related to altered conformation due to aggregation. This was confirmed when the aggregated subfractions of LDL(-) (agLDL(-)) and oxLDL (ag-oxLDL) were isolated and analyzed. Thus, Bsol 10 and Bsol 14 immunoreactivity was high in SMase-LDL, ag-oxLDL, and agLDL(-). The altered amino-terminal apoB-100 conformation was involved in the increased PG binding affinity of agLDL(-) because Bsol 10 and Bsol 14 blocked its high PG-binding. These observations suggest that an abnormal conformation of the amino-terminal region of apoB-100 is responsible for the increased PG binding affinity of agLDL(-).

Carbamylated LDL

Nonenzymatic modification of protein by cyanate, that is, carbamylation, has received new attention due to its apparent relevance in atherosclerosis. For example, carbamylation of low-density lipoprotein (LDL) is an important mechanism that potentially impacts high-risk atherosclerotic individuals with increased urea (renal insufficiency) or thiocyanate (tobacco smoking). Carbamylated LDL (cLDL) is increased in patients with end-stage kidney disease, especially those with atherosclerosis. In addition, cLDL exhibits distinct cytotoxic effects when tested in vitro on endothelial cells, induces the expression of adhesion molecules, and aggravates the monocyte adhesion to endothelial cells. It also facilitates the proliferation of vascular smooth-muscle cell (VSMC). Studies of potential pharmacological interruption of these processes in vivo may lead to discoveries of novel therapies for atherosclerosis.

Evaluation of serum biomarkers in nutritional disorders: glycated apolipoprotein B, fasting serum glucose, fructosamine, stable and labile glycated hemoglobin in diabetic and non-diabetic subjects

Glycated apolipoprotein B (ApoB-G), a non enzymatically glycated protein, has recently been associated with myocardial infarction. Our aim is to evaluate, in diabetic and non diabetic subjects, the relationship of ApoB-G with serum fasting glucose, fructosamine, stable and labile fractions of glycated hemoglobin ((S)HbA(1c), (L)HbA(1c), respectively) and insulin. The subjects were recruited from a previous study on ApoB-G and myocardial infarction: 141 of them were studied, 43 with and 98 without diabetes. ApoB-G was measured using a monoclonal antibody, and linear regression and correlation were used for statistical analysis of the data. ApoB-G was higher in diabetic than in non diabetic subjects. There was a statistically significant correlation of ApoB-G with triglycerides (r = 0.38, p = 0.01) in diabetic subjects, and with total proteins (r = 0.37, p = 0.0002), triglycerides (r = 0.34, p = 0.0007), and cholesterol (r = 0.23, p = 0.02) in non diabetic subjects. In the most parsimonious multiple linear regression model of ApoB-G on all the other serum variables, there was a statistically significant association of ApoB-G with triglycerides, in both diabetic and non diabetic subjects. The main results of this study suggest that serum ApoB-G is associated with serum triglycerides in both diabetic and non diabetic subjects.

Advanced glycation end-product of low density lipoprotein activates the toll-like 4 receptor pathway implications for diabetic atherosclerosis

OBJECTIVE

Diabetes is a major risk factor for coronary heart disease. Accumulation of advanced glycation end-products (AGEs) attributable to hyperglycemia in diabetics promotes the development of atherosclerosis. However, the underlying mechanisms remain unclear.

METHODS AND RESULTS

The advanced glycation end-product of low-density-lipoprotein (AGE-LDL) induced proinflammatory cytokine production in human coronary artery endothelial cells and human- and mouse-macrophages. AGE-LDL stimulated cytokine synthesis was markedly reduced in mouse macrophages with a TLR4 loss-of-function mutation. Coimmunoprecipitation experiments indicated AGE-LDL interacts with TLR4, RAGE, and CD36. Incubation of cultured macrophages with TLR4, RAGE, or CD36 antibodies inhibited AGE-LDL stimulation of tumor necrosis factor (TNF)alpha production. A competitive binding inhibitor of TLR4 blocked AGE-LDL binding to the receptor. After transfection of a HEK293 cell system with wild-type TLR4, AGE-LDL activated a signaling pathway including p38 alpha, JNK, and ERK1 kinases and AP1, Elk1, and NF-kappaB transcription factors; the net result being increased cytokine production. These effects were absent when cells were transfected with empty plasmid. Two common polymorphisms in TLR4, D299G and T399I, reduced the response of TLR4 to lipopolysaccharide (LPS) but had no effect on AGE-LDL signaling.

CONCLUSIONS

These results indicate that AGE-LDL activates a TLR4-mediated signaling pathway, thus inducing proinflammatory cytokine production. This mechanism may partly explain the increased risk of atherosclerosis observed in diabetics.

Glycation as an atherogenic modification of LDL

PURPOSE OF REVIEW

To highlight the potential importance of glycation as an atherogenic modification of LDL in both diabetic and nondiabetic people.

RECENT FINDINGS

Small dense LDL which is known to be most closely associated with atherogenesis is more susceptible to glycation than more buoyant LDL. Glycation and oxidation of LDL appear to be intimately associated.

SUMMARY

Glycation of LDL occurs chiefly due to the nonenzymatic reaction of glucose and its metabolites with the free amino groups of lysine in which LDL is rich. Higher concentrations of glycated LDL are present in diabetic than in nondiabetic individuals, but even in the latter, there is generally more circulating glycated LDL than oxidatively modified LDL. Probably, oxidation and glycation of LDL are at least partially interdependent, but both prevent LDL receptor-mediated uptake and promote macrophage scavenger receptor uptake. The recognition that LDL glycation is at least as important as oxidation in atherogenesis may lead to improvements in our understanding of its mechanism and how to prevent it.

Modified Low-Density Lipoproteins and High-Density Lipoproteins

It has long been known that the oxidative state of the various plasma lipoproteins modulates platelet aggregability, thereby contributing to atherogenesis. Low-density lipoprotein (LDL), occurring in vivo both in the native and oxidised forms, interacts directly with platelets, by binding to specific receptors. While the identity of the receptors for native LDL and some subfractions of high-density lipoproteins (HDL) remains disputed, apoE-containing HDL(2) binds to LRP8. The nature of these interactions as well as the distinction between candidate receptor proteins was elucidated using covalently modified apolipoproteins, which pointed to the participation of apolipoproteins in high affinity binding. However, the platelet effects initiated by binding of native lipoproteins remain controversial. Some of this ambiguity can be traced to the fact that native LDL inevitably undergoes substantial oxidisation upon modification, including by radiolabelling. The platelet-activating effects provoked by oxidised LDL are irrefutable, but many details remain unknown. The role of CD36 in platelet binding by oxidised LDL is well established, although additional receptors may exist. Much less is known about the interaction of oxidised HDL with platelets, since platelet activation was observed in some, but not all studies. Various frequently applied in vitro oxidation methods produce modified lipoprotein species that may not be relevant in vivo. Based on the reported modifications obtained by in vitro oxidation of LDL, early investigations focused mainly on the formation and the eventual effects of oxidised lipids. More recently, alterations to lipoproteins performed using hypochloric acid and myeloperoxidase redirected the attention to the role of modified apoproteins in triggering platelet responses.

The expression of apolipoprotein B epitopes is normal in LDL of diabetic and end-stage renal disease patients

AIMS/HYPOTHESIS

When LDLs are exposed to glucose in vitro, glycation of apolipoprotein B100 (apoB) leads to a loss in its affinity for the LDL receptor and reproducible alterations in the immunoreactivity of specific apoB epitopes, including several epitopes close to the LDL receptor binding site. The aim of this work was to determine if similar immunological changes are observed in vivo in LDLs of diabetic and end-stage renal disease (ESRD) patients.

SUBJECTS, MATERIALS AND METHODS

The immunoreactivity of LDLs isolated from 14 diabetic patients with normal renal function and 13 patients with ESRD was studied with a panel of 25 well-characterised anti-apoB monoclonal antibodies.

RESULTS

Although diabetic and ESRD LDLs showed evidence of glycation modification, none of the changes in the apoB immunoreactivity induced by glucose in vitro was observed in vivo, including those for epitopes close to the LDL receptor binding domain.

CONCLUSIONS/INTERPRETATION

These results suggest that in vivo glycation of LDLs is a complex process that is not mimicked by in vitro exposure of LDLs to high concentrations of glucose. This questions the clinical significance of the in vitro glycation studies used to understand the pathophysiological consequences of LDL glycation in diabetes and ESRD.

Hyperglycemia, lipoprotein glycation, and vascular disease

Hyperlipidemia and its treatment are currently recognized as important modulators of cardio-vascular mortality in the presence of disordered glucose control. On the other hand, the effects of hyperglycemia and its treatment on hyperlipidemia are not widely appreciated. Hyperglycemia is commonly associated with an increase in intestinal lipoproteins and a reduction in high-density lipoprotein (HDL). This could be a consequence of hyperglycemia-induced glycation of lipoproteins, which reduces the uptake and catabolism of the lipoproteins via the classical low-density lipoprotein (LDL) receptor. A high dietary carbohydrate load increases the glycation of intestinal lipoproteins, prolongs their circulation, and increases their plasma concentration. Hyperglycemia also leads to inhibition of lipoprotein lipase, further aggravating hyperlipidemia. Circulating advanced glycation end-products (AGEs) also bind lipoproteins and delay their clearance, a mechanism that has particularly been implicated in the dyslipidemia of diabetic nephropathy. As uptake via scavenger receptors is not inhibited, glycation increases the proportion of lipoproteins that are taken up via inflammatory cells and decreases the proportion taken up by hepatocytes via classical LDL receptors. This promotes the formation of atheromatous plaques and stimulates inflammation. Hyperglycemia increases the formation of oxidized LDL and glycated LDL, which are important modulators of atherosclerosis and cardiovascular death. The risk of cardiovascular death is increased by even short-term derangement of blood sugar control, owing perhaps to the glycation of lipoproteins and other critical proteins. Glycated LDL could prove very useful in measuring the effect of hyperglycemia on cardiovascular disease, its risk factors, and its complications. Comparing different glucose-lowering and lipid-lowering drugs in respect to their influence on glycated LDL could increase knowledge of the mechanism by which they alter cardiovascular risk.

Glycation of low-density lipoproteins by methylglyoxal and glycolaldehyde gives rise to the in vitro formation of lipid-laden cells

AIMS/HYPOTHESIS

Previous studies have implicated the glycoxidative modification of low-density lipoprotein (LDL) by glucose and aldehydes (apparently comprising both glycation and oxidation), as a causative factor in the elevated levels of atherosclerosis observed in diabetic patients. Such LDL modification can result in unregulated cellular accumulation of lipids. In previous studies we have characterized the formation of glycated, but nonoxidized, LDL by glucose and aldehydes; in this study we examine whether glycation of LDL, in the absence of oxidation, gives rise to lipid accumulation in arterial wall cell types.

METHODS

Glycated LDLs were incubated with macrophage, smooth muscle, or endothelial cells. Lipid loading was assessed by HPLC analysis of cholesterol and individual esters. Oxidation was assessed by cholesterol ester loss and 7-ketocholesterol formation. Cell viability was assessed by lactate dehydrogenase release and cell protein levels.

RESULTS

Glycation of LDL by glycolaldehyde and methylglyoxal, but not glucose (in either the presence or absence of copper ions), resulted in cholesterol and cholesterol ester accumulation in macrophage cells, but not smooth muscle or endothelial cells. The extent of lipid accumulation depends on the degree of glycation, with increasing aldehyde concentration or incubation time, giving rise to greater extents of particle modification and lipid accumulation. Modification of lysine residues appears to be a key determinant of cellular uptake.

CONCLUSIONS/INTERPRETATION

These results are consistent with LDL glycation, in the absence of oxidation, being sufficient for rapid lipid accumulation by macrophage cells. Aldehyde-mediated "carbonyl-stress" may therefore facilitate the formation of lipid-laden (foam) cells in the artery wall.

Retina expresses microsomal triglyceride transfer protein: implications for age-related maculopathy

The principal extracellular lesions of age-related maculopathy (ARM), the leading cause of vision loss in the elderly, involve Bruch's membrane (BrM), a thin vascular intima between the retinal pigment epithelium (RPE) and its blood supply. With age, 80-100 nm solid particles containing esterified cholesterol (EC) accumulate in normal BrM, and apolipoprotein B (apoB) immunoreactivity is detectable in BrM- and ARM-associated lesions. Yet little evidence indicates that increased plasma cholesterol is a risk factor for ARM. To determine if RPE is capable of assembling its own apoB-containing lipoprotein, we examined RPE for the expression of microsomal triglyceride transfer protein (MTP), which is required for this process. Consistent with previous evidence for apoB expression, MTP is expressed in RPE, the ARPE-19 cell line, and, unexpectedly, retinal ganglion cells, which are neurons of the central nervous system. De novo synthesis and secretion of neutral lipid by ARPE-19 was supported by high levels of radiolabeled EC and triglyceride in medium after supplementation with oleate. Lipoprotein assembly and secretion is implicated as a constitutive retinal function and a plausible candidate mechanism involved in forming extracellular cholesterol-containing lesions in ARM. The pigmentary retinopathy and neuropathy of abetalipoproteinemia (Mendelian Inheritance of Man 200100; Bassen-Kornzwieg disease), which is caused by mutations in the MTP gene, may involve loss of function at the retina.

Impact of in vivo glycation of LDL on platelet aggregation and monocyte chemotaxis in diabetic Psammomys obesus

Psammomys obesus (sand rat) is an appropriate model to highlight the development of hyperinsulinemia, insulin resistance, obesity, and diabetes. This animal species, with genetically predetermined diabetes, acquires non-insulin dependent diabetes mellitus when exposed to energy-rich diets. In the present study, we explored the possibility that glycation of LDL may occur in diabetes-prone P. obesus and affect platelet and macrophage functions. The glycation of LDL, isolated from diabetic animals, was significantly (P < 0.05) higher (40%) than that of control animals. The incubation of platelets with glycated LDL enhanced the reactivity of platelets by 32-44% depending on the aggregating agents (thrombin, collagen, ADP). Furthermore, LDL derived from diabetic rats were chemotactic for normal monocytes and stimulated the incorporation of [14C]oleate into cellular cholesteryl esters. The enhancement of platelet aggregation and cholesterol esterification in monocytes may contribute toward the accelerated development of atherosclerotic cardiovascular disease in diabetic P. obesus animals. This study also illustrates the relevance of studying atherosclerosis in the P. obesus animal model, as it shows an increased tendency to develop diet-induced diabetes, which is associated with cardiovascular disorders.

Early-glycation of apolipoprotein E: effect on its binding to LDL receptor, scavenger receptor A and heparan sulfates

Glycation is responsible for disruption of lipoprotein functions leading to the development of atherosclerosis in diabetes. The effects of apolipoprotein E (apoE) glycation were investigated with respect to its interaction with receptors. The interaction of apoE with the low density lipoprotein receptor (LDL-R) and scavenger receptor A (SR-A) was measured by competition experiments performed using, respectively, on a human fibroblast cell line 125I-LDL, and on a murine macrophage cell line (J774) 125I-acetylated LDL, and unlabeled apoE/phospholipid complexes. Glycated apoE binding to heparin and heparan sulfates (HS) was assessed by surface plasmon resonance (SPR) technology. Site-directed mutagenesis was then performed on Lys-75, the major glycation site of the protein. The prepared mutant protein proved to be useful as a tool to study the role of Lys-75 in apoE glycation. The findings showed that, although glycation has no effect on apoE binding either to the LDL-R or to SR-A, it impairs its binding to immobilized heparin and HS. The glycation of Lys-75 was found to be proceed rapidly and contributed significantly to total protein glycation. We propose that, in the case of diabetes, glycation may lead to the atherogenicity of apoE-containing lipoproteins disturbing their uptake via the HS proteoglycan pathway.

Diabetic dyslipidaemia

Dyslipidaemia is an important component of the metabolic syndrome observed in patients with type 2 diabetes, and is characterized by moderate hypertriglyceridaemia and low levels of high-density lipoprotein (HDL) cholesterol concentrations. Dyslipidaemia contributes to increased vascular risk and is therefore a good target for therapeutic intervention in the form of glycaemic control, lifestyle measures and hypolipidaemic drugs. It is proposed that lipid abnormalities in type 2 diabetes are secondary consequences of insulin resistance. Any approach that lowers insulin resistance would be anticipated to have a beneficial effect on dyslipidaemia, but in many cases patients with type 2 diabetes fail to achieve normal lipidaemia through diet, exercise and glycaemic control. Subgroup analyses of major clinical trials suggests that lipid-lowering therapy reduces CHD risk in patients with diabetes, but trials performed specifically in populations of patients with diabetes are ongoing. Until then, patients with type 2 diabetes who have established CHD or high individual risk already warrant aggressive lipid-lowering pharmacotherapy. In the author's view, when ongoing studies are complete it is likely that most patients with type 2 diabetes will be prescribed lipid-lowering drugs.

Nonoxidative modifications of lipoproteins in atherogenesis

The key initiating event in atherosclerosis is the retention of plasma lipoproteins in the subendothelial matrix. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. There is considerable evidence that many of these biological responses, notably macrophage cholesteryl ester loading (foam cell formation), require subendothelial modification of the retained lipoproteins. Oxidation of lipoproteins is one such modification that likely occurs in vivo and promotes certain atherogenic events, but oxidation cannot explain all aspects of atherogenesis, including certain elements of macrophage foam cell formation. For this reason, there has been renewed interest in other modifications of lipoproteins that may be important in atherogenesis. This review addresses five such lipoprotein modifications, namely aggregation, glycation, immune complex formation, proteoglycan complex formation, and conversion to cholesterol-rich liposomes. The focus is on the evidence that these modifications occur in atherosclerotic lesions and on the potential role of these modified lipoproteins in atherogenesis, with an emphasis on macrophage foam cell formation.

Characterization of low-density lipoprotein uptake by murine macrophages exposed to Chlamydia pneumoniae

Exposure to Chlamydia pneumoniae is correlated with atherosclerosis in a variety of clinical and epidemiological studies, but how the organism may initiate and promote the disease is poorly understood. One pathogenic mechanism could involve modulation of macrophage function by C. pneumoniae. We recently demonstrated that C. pneumoniae induces macrophages to accumulate excess cholesterol and develop into foam cells, the hallmark of early atherosclerotic lesions. To determine if C. pneumoniae-induced foam cell formation involved increased uptake of low-density lipoprotein (LDL), the current study examined macrophage association of a fluorescent carbocyanine (DiI)-labeled LDL following infection. C. pneumoniae enhanced the association of DiI-LDL with macrophages in a dose-dependent manner with respect to both C. pneumoniae and DiI-LDL. Interestingly, increased association was inhibited by native LDL and occurred in the absence of oxidation byproducts and in the presence of antioxidants. However, enhanced DiI-LDL association occurred without the participation of the classical Apo B/E native LDL receptor, since C. pneumoniae increased DiI-LDL association and induced foam cell formation in macrophages isolated from LDL-receptor-deficient mice. Surprisingly, DiI-LDL association was inhibited not only by unlabeled native LDL but also by high-density lipoprotein, very low density lipoprotein, and oxidized LDL. These data indicate that exposure of macrophages to C. pneumoniae increases the uptake of LDL and foam cell formation by an LDL-receptor-independent mechanism.