Glycation of low-density lipoprotein results in the time-dependent accumulation of cholesteryl esters and apolipoprotein B-100 protein in primary human monocyte-derived macrophages

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.

RAGE signaling regulates the progression of diabetic complications

Diabetes, the ninth leading cause of death globally, is expected to affect 642 million people by 2040. With the advancement of an aging society, the number of patients with diabetes having multiple underlying diseases, such as hypertension, obesity, and chronic inflammation, is increasing. Thus, the concept of diabetic kidney disease (DKD) has been accepted worldwide, and comprehensive treatment of patients with diabetes is required. Receptor for advanced glycation endproducts (RAGE), a multiligand receptor, belonging to the immunoglobulin superfamily is extensively expressed throughout the body. Various types of ligands, including advanced glycation endproducts (AGEs), high mobility group box 1, S100/calgranulins, and nucleic acids, bind to RAGE, and then induces signal transduction to amplify the inflammatory response and promote migration, invasion, and proliferation of cells. Furthermore, the expression level of RAGE is upregulated in patients with diabetes, hypertension, obesity, and chronic inflammation, suggesting that activation of RAGE is a common denominator in the context of DKD. Considering that ligand–and RAGE–targeting compounds have been developed, RAGE and its ligands can be potent therapeutic targets for inhibiting the progression of DKD and its complications. Here, we aimed to review recent literature on various signaling pathways mediated by RAGE in the pathogenesis of diabetic complications. Our findings highlight the possibility of using RAGE–or ligand–targeted therapy for treating DKD and its complications.

The Role of Advanced Glycation End Products on Dyslipidemia

Disorders of lipoprotein metabolism and glucose homeostasis are common consequences of insulin resistance and usually co-segregate in patients with metabolic syndrome and type 2 diabetes mellitus (DM). Insulin-resistant subjects are characterized by atherogenic dyslipidemia, a specific lipid pattern which includes hypertriglyceridemia, reduced high-density lipoprotein cholesterol level, and increased proportion of small, dense low-density lipoprotein (LDL). Chronic hyperglycemia favors the processes of non-enzymatic glycation, leading to the increased production of advanced glycation end products (AGEs). Apart from direct harmful effects, AGEs are also potent inducers of oxidative stress and inflammation. In addition, increased AGEs' production may induce further qualitative modifications of small, dense LDL particles, converting them to glycated LDLs. These particles are even more atherogenic and may confer an increased cardiovascular risk. In this narrative review, we summarize the available evidence of the pathophysiological role and clinical importance of circulating AGEs and glycated LDLs in patients with dyslipidemia, particularly those with DM and related complications. In addition, we discuss recent advances and the issues that should be improved regarding laboratory assessment of AGEs and glycated LDLs, as well as the possibilities for their therapeutic modulation.

Advanced glycation endproducts in diabetes-related macrovascular complications: focus on methylglyoxal

Diabetes is associated with vascular injury and the onset of macrovascular complications. Advanced glycation endproducts (AGEs) and the AGE precursor methylglyoxal (MGO) have been identified as key players in establishing the relationship between diabetes and vascular injury. While most research has focused on the link between AGEs and vascular injury, less is known about the effects of MGO on vasculature. In this review, we focus on the mechanisms linking AGEs and MGO to the development of atherosclerosis. AGEs and MGO are involved in many stages of atherosclerosis progression. However, more research is needed to determine the exact mechanisms underlying these effects. Nevertheless, AGEs and MGO could represent valid therapeutic targets for the macrovascular complications of diabetes.

The Glyoxalase System Is a Novel Cargo of Amniotic Fluid Stem-Cell-Derived Extracellular Vesicles

The glyoxalase system is a ubiquitous cellular metabolic pathway whose main physiological role is the removal of methylglyoxal (MG). MG, a glycolysis byproduct formed by the spontaneous degradation of triosephosphates glyceraldehyde-3-phosphate (GA3P) and dihydroxyacetonephosphate (DHAP), is an arginine-directed glycating agent and precursor of the major advanced glycation end product arginine-derived, hydroimidazolone (MG-H1). Extracellular vesicles (EVs) are a heterogeneous family of lipid-bilayer-vesicular structures released by virtually all living cells, involved in cell-to-cell communication, specifically by transporting biomolecules to recipient cells, driving distinct biological responses. Emerging evidence suggests that included in the EVs cargo there are different metabolic enzymes. Specifically, recent research has pointed out that EVs derived from human amniotic fluid stem cell (HASC-EVs) contain glycolytic pay-off phase enzymes, such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Since GAPDH catalyzes the sixth step of glycolysis using as a substrate GA3P, from which MG spontaneously origins, we wanted to investigate whether MG-derived MG-H1, as well as glyoxalases, could be novel molecule cargo in these EVs. By using immunoassays and spectrophotometric methods, we found, for the first time ever, that HASC-EVs contain functional glyoxalases and MG-H1, pioneering research to novel and exciting roles of these eclectic proteins, bringing them to the limelight once more.

Autoantibodies Against Methylglyoxal-Modified Apolipoprotein B100 and ApoB100 Peptide Are Associated With Less Coronary Artery Atherosclerosis and Retinopathy in Long-Term Type 1 Diabetes

OBJECTIVE

Methylglyoxal (MGO), a reactive aldehyde forming advanced glycation end products (AGEs), is increased in diabetes and recognized by the immune system, resulting in anti-AGE-specific autoantibodies. The association of these immune responses with macro- and microvascular complications in type 1 diabetes remains unclarified. We investigated associations between MGO-modified apolipoprotein B100 (apoB100) and apoB100 peptide 5 (MGO-p5) autoantibodies and coronary atherosclerosis and retinopathy in type 1 diabetes.

RESEARCH DESIGN AND METHODS

IgM and IgG against MGO-apoB100 and MGO-p5 were measured by ELISA in plasma from 103 subjects with type 1 diabetes and 63 control subjects (Dialong study) and in a replication cohort of 27 subjects with type 1 diabetes (Oslo study). Coronary atherosclerosis was assessed by computed tomography coronary angiography or intravascular ultrasound. Retinopathy was classified by retinal photos.

RESULTS

MGO-apoB100 IgM and MGO-p5 IgM levels were higher in subjects with diabetes with no coronary artery stenosis compared with subjects with significant stenosis (median [interquartile range]: 96.2 arbitrary units [AU] [71-126.8] vs. 54 AU [36.1-85.4], P = 0.003 for MGO-apoB100; and 77.4 AU [58-106] vs. 36.9 AU [28.9-57.4], P = 0.005 for MGO-p5). MGO-apoB100 IgM and MGO-p5 IgM were associated with less severe coronary stenosis after adjusting for confounders (odds ratio 0.2 [95% CI 0.05-0.6], P = 0.01; and 0.22 [0.06-0.75], P = 0.02). The inverse association of MGO-p5 IgM and coronary stenosis was confirmed in the replication cohort. Subjects with proliferative retinopathy had significantly lower MGO-apoB100 IgM and MGO-p5 IgM than those with background retinopathy.

CONCLUSIONS

Autoantibodies against AGE-modified apoB100 are inversely associated with coronary atherosclerosis and proliferative retinopathy, suggesting vascular protective effects of these autoantibodies in type 1 diabetes.

Glycosylation of human plasma lipoproteins reveals a high level of diversity, which directly impacts their functional properties

AIMS

Human plasma lipoproteins are known to contain various glycan structures whose composition and functional importance are starting to be recognized. We assessed N-glycosylation of human plasma HDL and LDL and the role of their glycomes in cellular cholesterol metabolism.

METHODS

N-glycomic profiles of native and neuraminidase-treated HDL and LDL were obtained using HILIC-UHPLC-FLD. Relative abundance of the individual chromatographic peaks was quantitatively expressed as a percentage of total integrated area and N-glycan structures present in each peak were elucidated by MALDI-TOF MS. The capacity of HDL to mediate cellular efflux of cholesterol and the capacity of LDL to induce cellular accumulation of cholesteryl esters were evaluated in THP-1 cells.

RESULTS

HILIC-UHPLC-FLD analysis of HDL and LDL N-glycans released by PNGase F resulted in 22 and 18 distinct chromatographic peaks, respectively. The majority of N-glycans present in HDL (~70%) and LDL (~60%) were sialylated with one or two sialic acid residues. The most abundant N-glycan structure in both HDL and LDL was a complex type biantennary N-glycan with one sialic acid (A2G2S1). Relative abundances of several N-glycan structures were dramatically altered by the neuraminidase treatment, which selectively removed sialic acid residues. Native HDL displayed significantly greater efficacy in removing cellular cholesterol from THP-1 cells as compared to desialylated HDL (p < 0.05). Cellular accumulation of cholesteryl esters in THP-1 cells was significantly higher after incubations with desialylated LDL particles as compared to native LDL (p < 0.05).

CONCLUSIONS

N-glycome of human plasma lipoproteins reveals a high level of diversity, which directly impacts functional properties of the lipoproteins.

A comparison of the effects of low- and high-dose atorvastatin on lipoprotein metabolism and inflammatory cytokines in type 2 diabetes: Results from the Protection Against Nephropathy in Diabetes with Atorvastatin (PANDA) randomized trial

BACKGROUND

Statin therapy is recommended in type 2 diabetes (T2DM) although views on treatment intensity and therapeutic targets remain divided.

OBJECTIVES

Our objectives were to compare the effects of high-intensity and moderate-intensity atorvastatin treatment on lipoprotein metabolism and inflammatory markers and how frequently treatment goals are met in high-risk T2DM patients.

METHODS

Patients with T2DM and albuminuria (urinary albumin:creatinine ratio >5 mg/mmol, total cholesterol <7 mmol/L, proteinuria <2 g/d, creatinine <200 μmol/L) were randomized to receive atorvastatin 10 mg (n = 59) or 80 mg (n = 60) daily. Baseline and 1-year follow-up data are reported.

RESULTS

Patients were at high cardiovascular disease risk (observed combined mortality and nonfatal cardiovascular disease annual event rate 4.8%). The non-high-density lipoprotein cholesterol (HDL-C) goal of <2.6 mmol/L was achieved in 72% of participants receiving high-dose atorvastatin, but only in 40% on low-dose atorvastatin (P < .005). The proportion achieving apolipoprotein B (apoB) <0.8 g/L on high-dose and low-dose atorvastatin was 82% and 70%, respectively (NS). Total cholesterol, triglycerides, low-density lipoprotein (LDL) cholesterol, non-HDL-C, oxidized LDL, apoB, glyc-apoB, apolipoprotein E, and lipoprotein-associated phospholipase A2 decreased significantly, more so in participants on high-dose atorvastatin. Adiponectin increased and serum amyloid A decreased without dose dependency. Neither dose produced significant changes in HDL-C, cholesterol efflux, high-sensitivity C-reactive protein, glycated hemoglobin, serum paraoxonase-1, lecithin:cholesterol acyltransferase, or cholesteryl ester transfer protein.

CONCLUSIONS

High-dose atorvastatin is more effective in achieving non-HDL-C therapeutic goals and in modifying LDL-related parameters. Recommended apoB treatment targets may require revision. Despite the increase in adiponectin and the decrease in serum amyloid A, HDL showed no change in functionality.

The myeloperoxidase-derived oxidant hypothiocyanous acid inhibits protein tyrosine phosphatases via oxidation of key cysteine residues

Phosphorylation of protein tyrosine residues is critical to cellular processes, and is regulated by kinases and phosphatases (PTPs). PTPs contain a redox-sensitive active site Cys residue, which is readily oxidized. Myeloperoxidase, released from activated leukocytes, catalyzes thiocyanate ion (SCN(-)) oxidation by H2O2 to form hypothiocyanous acid (HOSCN), an oxidant that targets Cys residues. Dysregulated phosphorylation and elevated MPO levels have been associated with chronic inflammatory diseases where HOSCN can be generated. Previous studies have shown that HOSCN inhibits isolated PTP1B and induces cellular dysfunction in cultured macrophage-like cells. The present study extends this previous work and shows that physiologically-relevant concentrations of HOSCN alter the activity and structure of other members of the wider PTP family (including leukocyte antigen-related PTP, PTP-LAR; T-cell PTP, TC-PTP; CD45 and Src homology phosphatase-1, Shp-1) by targeting Cys residues. Isolated PTP activity, and activity in lysates of human monocyte-derived macrophages (HMDM) was inhibited by 0-100 µM HOSCN with this being accompanied by reversible oxidation of Cys residues, formation of sulfenic acids or sulfenyl-thiocyanates (detected by Western blotting, and LC-MS as dimedone adducts), and structural changes. LC-MS/MS peptide mass-mapping has provided data on the modified Cys residues in PTP-LAR. This study indicates that inflammation-induced oxidants, and particularly myeloperoxidase-derived species, can modulate the activity of multiple members of the PTP superfamily via oxidation of Cys residues to sulfenic acids. This alteration of the balance of PTP/kinase activity may perturb protein phosphorylation and disrupt cell signaling with subsequent induction of apoptosis at sites of inflammation.

Lipid peroxidation generates biologically active phospholipids including oxidatively N-modified phospholipids

Peroxidation of membranes and lipoproteins converts "inert" phospholipids into a plethora of oxidatively modified phospholipids (oxPL) that can act as signaling molecules. In this review, we will discuss four major classes of oxPL: mildly oxygenated phospholipids, phospholipids with oxidatively truncated acyl chains, phospholipids with cyclized acyl chains, and phospholipids that have been oxidatively N-modified on their headgroups by reactive lipid species. For each class of oxPL we will review the chemical mechanisms of their formation, the evidence for their formation in biological samples, the biological activities and signaling pathways associated with them, and the catabolic pathways for their elimination. We will end by briefly highlighting some of the critical questions that remain about the role of oxPL in physiology and disease.

Platelets and smooth muscle cells affecting the differentiation of monocytes

Background

Atherosclerosis is characterised by the formation of plaques. Monocytes play a pivotal role in plaque development as they differentiate into foam cells, a component of the lipid core whilst smooth muscle cells (SMC) are the principal cell identified in the cap. Recently, the ability of monocytes to differentiate into a myriad of other cell types has been reported. In lieu of these findings the ability of monocytes to differentiate into SMCs/smooth muscle (SM)-like cells was investigated.

Method and Results

Human monocytes were co-cultured with platelets or human coronary aortic SMCs and then analysed to assess their differentiation into SMCs/SM-like cells. The differentiated cells expressed a number of SMC markers and genes as determined by immunofluorescence staining and quantitative polymerase chain reaction (qPCR). CD array analysis identified marker expression profiles that discriminated them from monocytes, macrophages and foam cells as well as the expression of markers which overlapped with fibroblast and mesenchymal cells. Electron microscopy studies identified microfilaments and increased amounts of rough endoplasmic reticulum indicative of the SM- like cells, fibroblasts.

Conclusions

In the appropriate environmental conditions, monocytes can differentiate into SM-like cells potentially contributing to cap formation and plaque stability. Thus, monocytes may play a dual role in the development of plaque formation and ultimately atherosclerosis.

Apolipoprotein A-I glycation by glucose and reactive aldehydes alters phospholipid affinity but not cholesterol export from lipid-laden macrophages

Increased protein glycation in people with diabetes may promote atherosclerosis. This study examined the effects of non-enzymatic glycation on the association of lipid-free apolipoproteinA-I (apoA-I) with phospholipid, and cholesterol efflux from lipid-loaded macrophages to lipid-free and lipid-associated apoA-I. Glycation of lipid-free apoA-I by methylglyoxal and glycolaldehyde resulted in Arg, Lys and Trp loss, advanced glycation end-product formation and protein cross-linking. The association of apoA-I glycated by glucose, methylglyoxal or glycolaldehyde with phospholipid multilamellar vesicles was impaired in a glycating agent dose-dependent manner, with exposure of apoA-I to both 30 mM glucose (42% decrease in kslow) and 3 mM glycolaldehyde (50% decrease in kfast, 60% decrease in kslow) resulting is significantly reduced affinity. Cholesterol efflux to control or glycated lipid-free apoA-I, or discoidal reconstituted HDL containing glycated apoA-I (drHDL), was examined using cholesterol-loaded murine (J774A.1) macrophages treated to increase expression of ATP binding cassette transporters A1 (ABCA1) or G1 (ABCG1). Cholesterol efflux from J774A.1 macrophages to glycated lipid-free apoA-I via ABCA1 or glycated drHDL via an ABCG1-dependent mechanism was unaltered, as was efflux to minimally modified apoA-I from people with Type 1 diabetes, or controls. Changes to protein structure and function were prevented by the reactive carbonyl scavenger aminoguanidine. Overall these studies demonstrate that glycation of lipid-free apoA-I, particularly late glycation, modifies its structure, its capacity to bind phospholipids and but not ABCA1- or ABCG1-dependent cholesterol efflux from macrophages.

High-density lipoprotein impedes glycation of low-density lipoprotein

Glycation of low-density lipoprotein (LDL) increases its atherogenicity, but whether high-density lipoprotein (HDL) can protect LDL against glycation is not known. LDL and HDL were isolated from 32 volunteers with serum HDL cholesterol concentrations ranging from 0.76 to 2.01 (mean = 1.36) mmol/L. Glycation of LDL was induced by incubation with 0-80 mmol/L glucose for 7 days at 37°C under nitrogen in the presence of and absence of human HDL. Glycation of LDL apolipoprotein B (apoB) doubled at glucose 50 and 80 mmol/L (both p < 0.001), and this increase was ameliorated by HDL. In the absence of glucose, 0.11 (0.01) [mean (standard error, SE)] mg apoB/mg LDL protein was glycated increasing to 0.22 (0.02) mg/mg at glucose 80 mmol/L in the absence of HDL, but remaining at 0.13 (0.01) mg/mg when autologous HDL was present. Heterologous HDL from a further study of 12 healthy participants was similarly effective in impeding LDL apoB glycation. HDL impeded not only glycation but also the lipid peroxidation, free amino group consumption and increased electrophoretic mobility of LDL which accompanied glycation. HDL from participants with higher serum paraoxonase1 (PON1) was more effective in impeding glycation and the related processes. In conclusion, HDL can impede the glucose-induced glycoxidation of LDL. PON1 may be important for this function of HDL.

Small dense LDL is more susceptible to glycation than more buoyant LDL in Type 2 diabetes

Glycation of apoB (apolipoprotein B) of LDL (low-density lipoprotein) increases its atherogenicity. Concentrations of both serum glyc-apoB (glycated apoB) and SD-LDL (small dense LDL) (syn LDL3; D=1.044–1.063 g/ml) are increased in diabetes and are closely correlated. We studied whether SD-LDL is more susceptible to glycation in vitro than more buoyant LDL in statin- and non-statin-treated Type 2 diabetes mellitus. Serum SD-LDL apoB and glyc-apoB on statins was 20±2 (means±S.D.) and 3.6±0.41 compared with 47±3 and 5.89±0.68 mg/dl in those not receiving statins (P<0.001 and <0.01, respectively). There was a dose-dependent increase in glycation on incubation of LDL subfractions with glucose, which was accompanied by an increase in LPO (lipid peroxide) and electrophoretic mobility and a decrease in free amino groups. SD-LDL was more susceptible to these changes than more buoyant LDL. Both SD-LDL and more buoyant LDL from statin-treated patients were less susceptible to glycation. There were fewer free amino groups on LDL subfractions from statin-treated patients, which may contribute to this resistance. In conclusion, greater susceptibility of SD-LDL to glycation is likely to contribute to the raised levels of circulating glyc-apoB in diabetes. Statins are associated with lower levels of both SD-LDL and glyc-apoB.

Inhibition of lysosomal function in macrophages incubated with elevated glucose concentrations: a potential contributory factor in diabetes-associated atherosclerosis

OBJECTIVE

People with diabetes have an elevated risk of atherosclerosis. The accumulation of lipid within macrophage cells in the artery wall is believed to arise via the uptake and subsequent processing of modified low-density lipoproteins (LDL) via the endo-lysosomal system. In this study the effects of prolonged exposure to elevated glucose upon macrophage lysosomal function was examined to determine whether this contributes to modulated protein catabolism.

METHODS

Human monocytes were isolated from white-cell concentrates and differentiated, in vitro, into monocyte-derived macrophages over 11 days in medium containing 5-30 mmol/L glucose. Murine macrophage-like J774A.1 cells were incubated similarly. Lysosomal cathepsin (B, D, L and S) and acid lipase activities were assessed using fluorogenic substrates; cathepsin protein levels were examined by Western blotting. Lysosomal numbers were examined using the lysomotropic fluorescent dye LysoTracker DND-99, measurement of aryl sulfatase activity, and quantification of lysosome-associated membrane glycoprotein-1 (LAMP-1) by Western blotting.

RESULTS

Exposure to elevated glucose, but not mannitol, resulted in a concentration-dependent decrease in the activity, and to a lesser extent protein levels, of four lysosomal cathepsins. Acid lipase activity was also significantly reduced. Arysulfatase activity, LAMP-1 levels and lysosomal numbers were also decreased at the highest glucose concentrations, though to a lesser extent.

CONCLUSION

Long term exposure of human and murine macrophage cells to elevated glucose levels result in a depression of lysosomal proteolytic and lipase activities. This may result in decreased clearance and cellular accumulation of (lipo)proteins and contribute to the accumulation of modified proteins and lipids in diabetes-associated atherosclerosis.

Regulation of RAGE for attenuating progression of diabetic vascular complications

Diabetic angiopathy including micro- and macroangiopathy is concerned with high rate of morbidity and mortality in patients with long-standing diabetes. Receptor for advanced glycation end products (RAGE) and its ligands have been considered as important pathogenic triggers for the progression of the vascular injuries in diabetes. The deleterious link between RAGE and diabetic angiopathy has been demonstrated in animal studies. Preventive and therapeutic strategies focusing on RAGE and its ligand axis may be of great importance in relieving diabetic vascular complications and reducing the burden of disease.

Effect of exposure of human monocyte-derived macrophages to high, versus normal, glucose on subsequent lipid accumulation from glycated and acetylated low-density lipoproteins

During atherosclerosis monocyte-derived macrophages accumulate cholesteryl esters from low-density lipoproteins (LDLs) via lectin-like oxidised LDL receptor-1 (LOX-1) and class AI and AII (SR-AI, SR-AII) and class B (SR-BI, CD36) scavenger receptors. Here we examined the hypothesis that hyperglycaemia may modulate receptor expression and hence lipid accumulation in macrophages. Human monocytes were matured into macrophages in 30 versus 5 mM glucose and receptor expression and lipid accumulation quantified. High glucose elevated LOX1 mRNA, but decreased SR-AI, SR-BI, LDLR, and CD36 mRNA. SR-BI and CD36 protein levels were decreased. Normo- and hyperglycaemic cells accumulated cholesteryl esters from modified LDL to a greater extent than control LDL, but total and individual cholesteryl ester accumulation was not affected by glucose levels. It is concluded that, whilst macrophage scavenger receptor mRNA and protein levels can be modulated by high glucose, these are not key factors in lipid accumulation by human macrophages under the conditions examined.

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.

Cytokines, macrophage lipid metabolism and foam cells: implications for cardiovascular disease therapy

Cardiovascular disease is the biggest killer globally and the principal contributing factor to the pathology is atherosclerosis; a chronic, inflammatory disorder characterized by lipid and cholesterol accumulation and the development of fibrotic plaques within the walls of large and medium arteries. Macrophages are fundamental to the immune response directed to the site of inflammation and their normal, protective function is harnessed, detrimentally, in atherosclerosis. Macrophages contribute to plaque development by internalizing native and modified lipoproteins to convert them into cholesterol-rich foam cells. Foam cells not only help to bridge the innate and adaptive immune response to atherosclerosis but also accumulate to create fatty streaks, which help shape the architecture of advanced plaques. Foam cell formation involves the disruption of normal macrophage cholesterol metabolism, which is governed by a homeostatic mechanism that controls the uptake, intracellular metabolism, and efflux of cholesterol. It has emerged over the last 20 years that an array of cytokines, including interferon-γ, transforming growth factor-β1, interleukin-1β, and interleukin-10, are able to manipulate these processes. Foam cell targeting, anti-inflammatory therapies, such as agonists of nuclear receptors and statins, are known to regulate the actions of pro- and anti-atherogenic cytokines indirectly of their primary pharmacological function. A clear understanding of macrophage foam cell biology will hopefully enable novel foam cell targeting therapies to be developed for use in the clinical intervention of atherosclerosis.

Small-dense LDL and LDL glycation in metabolic syndrome and in statin-treated and non-statin-treated type 2 diabetes

Small-dense LDL (SD-LDL) has been particularly implicated in atherosclerosis. It has previously been reported that in non-diabetic people SD-LDL is preferentially glycated. The distribution of glycated apolipoprotein B (glyc-apoB) in lipoproteins in metabolic syndrome (MS) and in type 2 diabetes has not previously been studied. Plasma apoB and glyc-apoB were determined in different apoB-containing lipoproteins including buoyant and SD-LDL in MS (n=18) and type 2 diabetes (DM) [n=48; 12 statin-untreated (DM-S) and 36 statin-treated (DM+S)]. Plasma glyc-apoB was 5.6 ± 0.9, 3.5 ± 0.5 and 4.0 ± 0.2 mg/dl in DM-S, DM+S and MS, respectively. The glycated proportion of SD-LDL-apoB was greater than buoyant LDL in all groups. SD-LDL contributed most to plasma glyc-apoB in DM-S, because SD-LDL-apoB was higher in DM-S than in MS and DM+S (p < 0.001). Plasma glyc-apoB correlated with SD-LDL-apoB (r=0.74, p < 0.0001 in diabetes and r=0.53, p < 0.001 in MS), but not with HbA(1c). SD-LDL is preferentially glycated in type 2 diabetes and MS. Its concentration is a stronger determinant of plasma glycapoB than glycaemia. Statin-induced changes in its level may be important in decreasing apoB glycation in diabetes. These findings may explain the small effect of improving glycaemia relative to statin treatment in reducing atherosclerosis risk in type 2 diabetes and the increased risk in MS even before the onset of type 2 diabetes.

Deleterious effects of reactive aldehydes and glycated proteins on macrophage proteasomal function: possible links between diabetes and atherosclerosis

People with diabetes experience chronic hyperglycemia and are at a high risk of developing atherosclerosis and microvascular disease. Reactions of glucose, or aldehydes derived from glucose (e.g. methylglyoxal, glyoxal, or glycolaldehyde), with proteins result in glycation that ultimately yield advanced glycation end products (AGE). AGE are present at elevated levels in plasma and atherosclerotic lesions from people with diabetes, and previous in vitro studies have postulated that the presence of these materials is deleterious to cell function. This accumulation of AGE and glycated proteins within cells may arise from either increased formation and/or ineffective removal by cellular proteolytic systems, such as the proteasomes, the major multi-enzyme complex that removes proteins within cells. In this study it is shown that whilst high glucose concentrations fail to modify proteasome enzyme activities in J774A.1 macrophage-like cell extracts, reactive aldehydes enhanced proteasomal enzyme activities. In contrast BSA, pre-treated with high glucose for 8 weeks, inhibited both the chymotrypsin-like and caspase-like activities. BSA glycated using methylglyoxal or glycolaldehyde, also inhibited proteasomal activity though to differing extents. This suppression of proteasome activity by glycated proteins may result in further intracellular accumulation of glycated proteins with subsequent deleterious effects on cellular function.

Increased serum levels of methylglyoxal-derived hydroimidazolone-AGE are associated with increased cardiovascular disease mortality in nondiabetic women

OBJECTIVE

To investigate the association of the levels of methylglyoxal-derived hydroimidazolone AGE modified proteins (MG-H1-AGE) with cardiovascular disease (CVD) mortality in an 18-year follow-up study in Finnish nondiabetic and diabetic subjects.

METHODS

The study design was a nested case-control study. Serum MG-H1-AGE levels in samples drawn at baseline were measured with a DELFIA type immunoassay in 220 diabetic subjects and 61 nondiabetic subjects who died from CVD during the follow-up, and age- and gender-matched 157 diabetic subjects and 159 nondiabetic subjects who did not die from CVD.

RESULTS

In type 2 diabetic subjects serum MG-H1-AGE levels were similar in subjects who died from CVD and in subjects who did not, 32.6 (24.6-42.1) (median (interquartile range)) vs. 31.3 (22.5-40.7)U/mL (p=0.281). In nondiabetic subjects serum MG-H1 levels were significantly higher in subjects who died from CVD than in subjects who were alive, 35.4 (28.1-44.7) vs. 31.3 (24.2-38.6)U/mL (p=0.025). Corresponding MG-H1 levels were 41.2 (35.6-58.7) vs. 31.1 (26.7-35.7)U/mL, p=0.003, in women, and 34.4 (26.3-41.2) vs. 32.0 (22.8-40.3)U/mL, p=0.270, in men. Multivariate logistic regression analysis showed a significant association of serum levels of MG-H1-AGE with CVD mortality in nondiabetic women (adjusted p=0.021), but not in nondiabetic men.

CONCLUSIONS

Our 18-year follow-up study shows that high baseline serum levels of MG-H1 type of AGE modified proteins were associated with CVD mortality in nondiabetic women, but not in nondiabetic men or in diabetic subjects.

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.

Derangements of intravascular remodeling of lipoproteins in type 2 diabetes mellitus: consequences for atherosclerosis development

In type 2 diabetes mellitus, elevated fasting and postprandial plasma triglycerides, small dense low-density lipoprotein particles, low high-density lipoprotein (HDL) cholesterol levels, and increased action of lipid transfer proteins may enhance peripheral lipid accumulation and increase cardiovascular risk. Despite low HDL cholesterol, plasma's ability to stimulate cellular cholesterol efflux, reflecting an early step in the reverse cholesterol transport pathway, appears to be maintained, perhaps implicating a compensatory mechanism.