Nonenzymatic glycosylation of low density lipoproteins in vitro. Effects on cell-interactive properties

LOX-1 in Cardiovascular Disease: A Comprehensive Molecular and Clinical Review

LOX-1, ORL-1, or lectin-like oxidized low-density lipoprotein receptor 1 is a transmembrane glycoprotein that binds and internalizes ox-LDL in foam cells. LOX-1 is the main receptor for oxidized low-density lipoproteins (ox-LDL). The LDL comes from food intake and circulates through the bloodstream. LOX-1 belongs to scavenger receptors (SR), which are associated with various cardiovascular diseases. The most important and severe of these is the formation of atherosclerotic plaques in the intimal layer of the endothelium. These plaques can evolve into complicated thrombi with the participation of fibroblasts, activated platelets, apoptotic muscle cells, and macrophages transformed into foam cells. This process causes changes in vascular endothelial homeostasis, leading to partial or total obstruction in the lumen of blood vessels. This obstruction can result in oxygen deprivation to the heart. Recently, LOX-1 has been involved in other pathologies, such as obesity and diabetes mellitus. However, the development of atherosclerosis has been the most relevant due to its relationship with cerebrovascular accidents and heart attacks. In this review, we will summarize findings related to the physiologic and pathophysiological processes of LOX-1 to support the detection, diagnosis, and prevention of those diseases.

Proteins in human body fluids contain in vivo antigen analog of the melibiose-derived glycation product: MAGE

Melibiose-derived AGE (MAGE) is an advanced glycation end-product formed in vitro in anhydrous conditions on proteins and protein-free amino acids during glycation with melibiose. Our previous studies revealed the presence of MAGE antigen in the human body and tissues of several other species, including muscles, fat, extracellular matrix, and blood. MAGE is also antigenic and induces generation of anti-MAGE antibody. The aim of this paper was to identify the proteins modified by MAGE present in human body fluids, such as serum, plasma, and peritoneal fluids. The protein-bound MAGE formed in vivo has been isolated from human blood using affinity chromatography on the resin with an immobilized anti-MAGE monoclonal antibody. Using mass spectrometry and immunochemistry it has been established that MAGE epitope is present on several human blood proteins including serum albumin, IgG, and IgA. In serum of diabetic patients, mainly the albumin and IgG were modified by MAGE, while in healthy subjects IgG and IgA carried this modification, suggesting the novel AGE can impact protein structure, contribute to auto-immunogenicity, and affect function of immunoglobulins. Some proteins in peritoneal fluid from cancer patients modified with MAGE were also observed and it indicates a potential role of MAGE in cancer.

The Effect of Pyridoxine Hydrochloride Supplementation on Leptin, Adiponectin, Glycemic Indices, and Anthropometric Indices in Obese and Overweight Women

Obesity has reached epidemic proportions globally. Among several methods for treating obesity, the use of dietary supplements is common recently. One supplement that can help in this regard might be vitamin B6 in high doses. The objective of this study was to evaluate the effect of pyridoxine hydrochloride supplementation on anthropometric indices, body composition, visceral adiposity index (VAI), and metabolic status in obese and overweight women. In this randomized controlled clinical trial, 44 obese and overweight women aged 18-50 years were selected and divided randomly into 2 groups: an intervention group (receiving 80 mg pyridoxine hydrochloride supplement for 8 weeks) and a control group (receiving placebo for 8 weeks). In the pyridoxine hydrochloride group, weight (p = 0.03), body mass index (p = 0.023), fat mass (p = 0.003), waist circumference (p = 0.005), VAI (p = 0.001), fasting insulin, insulin resistance (homeostasis model assessment of insulin resistance; HOMA-IR), total cholesterol, low-density lipoprotein, triglycerides (TG) and leptin (p < 0.001) decreased whereas adiponectin (p < 0.001) increased in comparison to the baseline values. There was a significant difference in fat mass, VAI, fasting insulin, HOMA-IR, and TG between pyridoxine hydrochloride and control groups following intervention in adjusted models (p < 0.05). The findings suggest that vitamin B6 supplementation may be effective in reducing BMI and improving body composition and biochemical factors associated with obesity.

Trial Registration

Iranian Registry of Clinical Trials Identifier: IRCT20181002041206N1.

Regulation of Macrophage Apoptosis and Atherosclerosis by Lipid-Induced PKCδ Isoform Activation

RATIONALE

Activation of monocytes/macrophages by hyperlipidemia associated with diabetes mellitus and obesity contributes to the development of atherosclerosis. PKCδ (protein kinase C δ) expression and activity in monocytes were increased by hyperlipidemia and diabetes mellitus with unknown consequences to atherosclerosis.

OBJECTIVE

To investigate the effect of PKCδ activation in macrophages on the severity of atherosclerosis.

METHODS AND RESULTS

PKCδ expression and activity were increased in Zucker diabetic rats. Mice with selective deletion of PKCδ in macrophages were generated by breeding PKCδ flox/flox mice with LyzM-Cre and ApoE^-/- mice (MPKCδKO/ApoE^-/- mice) and studied in atherogenic (AD) and high-fat diet (HFD). Mice fed AD and HFD exhibited hyperlipidemia, but only HFD-fed mice had insulin resistance and mild diabetes mellitus. Surprisingly, MPKCδKO/ApoE^-/- mice exhibited accelerated aortic atherosclerotic lesions by 2-fold versus ApoE^-/- mice on AD or HFD. Splenomegaly was observed in MPKCδKO/ApoE^-/- mice on AD and HFD but not on regular chow. Both the AD or HFD increased macrophage number in aortic plaques and spleen by 1.7- and 2-fold, respectively, in MPKCδKO/ApoE^-/- versus ApoE^-/- mice because of decreased apoptosis (62%) and increased proliferation (1.9-fold), and not because of uptake, with parallel increased expressions of inflammatory cytokines. Mechanisms for the increased macrophages in MPKCδKO/ApoE^-/- were associated with elevated phosphorylation levels of prosurvival cell-signaling proteins, Akt and FoxO3a, with reduction of proapoptotic protein Bim associated with PKCδ induced inhibition of P85/PI3K.

CONCLUSIONS

Accelerated development of atherosclerosis induced by insulin resistance and hyperlipidemia may be partially limited by PKCδ isoform activation in the monocytes, which decreased its number and inflammatory responses in the arterial wall.

Glycated apolipoprotein A-I assay by combination of affinity chromatography and latex immunoagglutination

The degree of glycation of plasma apolipoprotein A-I was measured by a combination of gel filtration, boronate affinity chromatography and latex immunoagglutination. The plasma concentrations of apolipoprotein A-I determined by this combination method (y) correlated well with those determined by turbidimetric immunoassay (x) (y=1.12x + 1.9, r=0.964). The inter- and intra-assay coefficients of variation in the glycated apolipoprotein A-I assay were 4.1-5.0% and 4.0-4.4%, respectively. Interference from plasma glucose at concentrations up to 55.1 mmol/L was eliminated by gel filtration. Labile glycated apolipoprotein A-I did not interfere with the measurement of glycated apolipoprotein A-I. Reference values for glycated apolipoprotein A-I were determined to be 2.4-4.0% (n=140), with no significant difference between men and women. The mean concentration of plasma glycated apolipoprotein A-I in patients with uncontrolled diabetes mellitus (5.11%) was significantly higher than in normal subjects (3.12%, P<0.001). The method is simple, rapid and highly sensitive for determination of the glycation level of plasma apolipoprotein A-I.

Glycated phosphatidylethanolamine promotes macrophage uptake of low density lipoprotein and accumulation of cholesteryl esters and triacylglycerols

Non-enzymatic glycation of low density lipoprotein (LDL) has been suggested to be responsible for the increase in susceptibility to atherogenesis of diabetic individuals. Although the association of lipid glycation with this process has been investigated, the effect of specific lipid glycation products on LDL metabolism has not been addressed. This study reports that glucosylated phosphatidylethanolamine (Glc-PtdEtn), the major LDL lipid glycation product, promotes LDL uptake and cholesteryl ester (CE) and triacylglycerol (TG) accumulation by THP-1 macrophages. Incubation of THP-1 macrophages at a concentration of 100 micrograms/ml protein LDL specifically enriched (10 nmol/mg LDL protein) with synthetically prepared Glc-PtdEtn resulted in a significant increase in CE and TG accumulation when compared with LDL enriched in non-glucosylated PtdEtn. After a 24-h incubation with LDL containing Glc-PtdEtn, the macrophages contained 2-fold higher CE (10.11 +/- 1.54 micrograms/mg cell protein) and TG (285.32 +/- 4.38 micrograms/mg cell protein) compared with LDL specifically enriched in non-glucosylated PtdEtn (CE, 3.97 +/- 0.95, p < 0.01 and TG, 185.57 +/- 3.58 micrograms/mg cell protein, p < 0.01). The corresponding values obtained with LDL containing glycated protein and lipid were similar to those of LDL containing Glc-PtdEtn (CE, 11.9 +/- 1.35 and TG, 280.78 +/- 3.98 micrograms/mg cell protein). The accumulation of both neutral lipids was further significantly increased by incubating the macrophages with Glc-PtdEtn LDL exposed to copper oxidation. By utilizing the fluorescent probe, 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI), a 1.6-fold increase was seen in Glc-PtdEtn + LDL uptake when compared with control LDL. Competition studies revealed that acetylated LDL is not a good competitor for DiI Glc-PtdEtn LDL (5-6% inhibition), whereas glycated LDL gave an 80% inhibition, and LDL + Glc-PtdEtn gave 93% inhibition of uptake by macrophages. These results indicate that glucosylation of PtdEtn in LDL accounts for the entire effect of LDL glycation on macrophage uptake and CE and TG accumulation and, therefore, the increased atherogenic potential of LDL in hyperglycemia.

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.

Modified lipoproteins, cytokines and macrovascular disease in non-insulin-dependent diabetes mellitus

The processes of glycation and oxidation play a significant role in the acceleration of atherosclerosis in diabetes mellitus. Glycation is thought not only to increase the susceptibility of low-density lipoprotein (LDL) to oxidation but also to enhance the propensity of vessel wall structural proteins to bind extravasated plasma proteins, including LDL, and thus to contribute to a more marked oxidative modification of LDL. Glycated and oxidized lipoproteins induce cholesteryl ester accumulation in human macrophages and may promote platelet and endothelial cell dysfunction. Furthermore, these modified lipoproteins have the ability to trigger an autoimmune response that leads to the formation of autoantibodies and subsequently to the formation of immune complexes containing LDL. Both the modified lipoproteins and the immune complexes formed with autoantibodies reactive with modified lipoproteins may be responsible for several alternative and not mutually exclusive pathways leading to foam cell formation, macrophage activation and endothelial cell damage and may thus be of potential significance in initiating and/or contributing to the acceleration of the development of atherosclerosis. In this review we discuss how modified LDL affects lipoprotein metabolism, how immune complexes containing LDL induce the transformation of macrophages into foam cells and promote macrophage activation leading to the release of cytokines and thus initiating a sequence of events leading to endothelial cell damage and to the recruitment and activation of leucocytes. We also summarize our work showing that macrophage activation by LDL containing immune complexes leads to a paradoxical increase in LDL-receptor expression thus further impairing cholesterol homeostasis and enhancing the development of atheromatous lesions.

Glycation of lipoproteins and accelerated atherosclerosis in non-insulin-dependent diabetes mellitus

We used a new and remarkably simple method to examine the extent of in vivo lipoprotein glycation in type II diabetic patients with atherosclerosis and diabetic patients with no complications. Serum glycated lipoprotein levels were determined by agarose gel film electrophoresis in 48 non-diabetic control subjects and 39 diabetic patients, of whom 26 had no complications and 13 had atherosclerotic heart disease. Fasting serum glucose, glycohemoglobin and serum fructosamine concentrations (indicators of glycemia) and total cholesterol, triglyceride, low-density lipoprotein-, very low-density lipoprotein- and high-density lipoprotein-cholesterol concentrations and the low-density lipoprotein/high-density lipoprotein ratio (serum lipid profile) were also determined in the control and diabetic subjects. Glycated low-density lipoprotein and very low-density lipoprotein concentrations were significantly increased in diabetic patients compared with controls; but only glycated very low-density lipoprotein was significantly increased in atherosclerotic patients compared with diabetics without complications. The lipid profile parameters were not significantly increased in patients compared with controls. In diabetics, especially those with poorly controlled hyperglycemia and atherosclerosis, glycation of lipoprotein fractions might be more important than serum lipid and lipoprotein abnormalities. The significant correlation between atherosclerosis and glycated very low-density lipoprotein, suggests that very low-density lipoprotein glycation could be responsible for the development of atherosclerosis in diabetes.

Modified low density lipoprotein and its constituents augment cytokine-activated vascular cell adhesion molecule-1 gene expression in human vascular endothelial cells

Early features in the pathogenesis of atherosclerosis include accumulation of oxidized LDL (oxLDL) and endothelial expression of the vascular adhesion molecule VCAM-1. Because antioxidants inhibit endothelial VCAM-1 expression, we tested the hypothesis that oxLDL functions as a prooxidant signal in atherogenesis to augment VCAM-1 activation by inflammatory signals. Cultured human aortic endothelial cells (HAECs) or human umbilical vein endothelial cells (HUVECs) were incubated with unmodified LDL, oxLDL, or glycated LDL for 48 h. No change in VCAM-1, intercellular cell adhesion molecule-1 (ICAM-1), or E-selectin expression from control was observed by ELISA. However, dose-response and time course studies demonstrated that oxLDL enhanced VCAM-1 expression induced by the cytokin tumor necrosis factor alpha (TNF alpha) 63% in HAECs and 45% in HUVECs over unmodified LDL or control. Using flow cytometry analysis, oxLDL augmented TNF alpha-induced VCAM-1 expression in a uniform HAEC population. oxLDL had no effect on E-selection induction. oxLDL augmented TNF alpha-induced ICAM-1 expression 44% in HAECs but not in HUVECs. Glycated LDL augmented TNF alpha-induced VCAM-1 expression 35% in HAECs but not HUVECs. Similar results were obtained with 13-HPODE or lysophosphatidylcholine, significant components of oxLDL. 13-HPODE augmented TNF alpha-induced mRNA accumulation and transcriptional activation of VCAM-1 in HAECs. These results suggest that as long-term regulatory signals, specific oxidized fatty acid and phospholipid components of oxLDL augment the ability of vascular endothelial cells to express cytokine-mediated VCAM-1. These studies link oxidant signals conferred by oxLDL to oxidation-sensitive regulatory mechanisms controlling the expression of endothelial cell adhesion molecules involved in early atherosclerosis.

Effect of glycation on the properties of lipoprotein(a)

Lipoprotein(a) [Lp(a)] was glycated by incubation in vitro with glucose (0 to 200 mmol/L), and its properties were compared with native Lp(a) and native and glycated LDL. Glucose was incorporated into Lp(a) in proportions that mirrored the distribution of lysines between apolipoprotein (apo) B-100 and apo(a). Because the kringle IV domains of apo(a) are lysine poor, only 10% of glucose bound to apo(a), whereas 90% was attached to the apoB-100 of Lp(a). Approximately 3% of the lysines of both Lp(a) and LDL were modified, which is a level comparable with that observed in LDL isolated from diabetic individuals. Glucose uptake by Lp(a) and LDL was almost identical and was linear as a function of concentration and time. Glycation increased the negative charge of Lp(a) and LDL as monitored by electrophoresis and ion-exchange chromatography and also reduced the affinity of Lp(a) and LDL for heparin-Sepharose. Glycation did not affect the lysine-binding property of Lp(a) or generate measurable malondialdehyde oxidation adducts. The catabolism of glycated Lp(a) by human monocyte-derived macrophages (HMDMs), like that of native Lp(a), was largely LDL receptor independent. Both glycated Lp(a) and LDL were degraded at a comparatively faster rate and stimulated greater cholesteryl ester formation than their unmodified counterparts. However, the degradation rate of glycated Lp(a) was approximately four- to fivefold slower and its stimulation of cholesteryl ester formation was ninefold lower than that of either form of LDL. These results show that Lp(a) can be glycated nonenzymatically in vitro, that the incorporation of glucose is dependent on the distribution of lysines between apo(a) and apoB-100, and that glycation does not affect the lysine-binding properties of Lp(a). Furthermore, glycation produced modest increases in the degradation rate of Lp(a) and associated cholesteryl ester synthesis by HMDMs. Based on these data, glycation does not appear to significantly enhanced the atherogenic potential of unmodified Lp(a).

Glycation and oxidation of human low density lipoproteins reduces heparin binding and modifies charge

The effects of glycation and oxidation of human low density lipoproteins (LDL) on heparin binding were studied and compared with modifications in the charge of the particles. Glycation of LDL at a molar ratio of 4 mol glucose mol-1 apoB, decreases affinity for heparin, as shown by heparin-agarose affinity chromatography since salt molarity needed for elution decreases from 550 mmol l-1 for control LDL (c-LDL) to 350 mmol l-1 for glycated LDL (glc-LDL). Oxidized LDL (oxi-LDL) shows marked heterogeneity, most of the fractions having decreased affinity. Heparin-agarose affinity chromatography of LDL preparations shows the presence of a small (5-7%), low-affinity fraction in euglycaemic human plasma LDL (c-LDL). Its elution volume coincides with both glc-LDL and a fraction of oxi-LDL, suggesting it may contain glycated and oxidized molecules present in plasma. DEAE-Trisacryl anion exchange chromatography elution profiles of c-LDL preparations shows the presence of a more electronegative fraction accounting for about 10% of total protein. This fraction elutes with 260 mmol l-1 NaCl instead of 130 mmol l-1 for the main fraction, it roughly coincides with elution volumes of main peaks of glc-LDL and oxi-LDL. Results indicate that glycated particles may be present in this fraction. Our data demonstrate then that glycation, and to a lesser degree, oxidation of LDL reduce affinity to heparin. From an analytical approach, modified LDL can be separated from the bulk of native LDL both by DEAE and heparin-agarose chromatographies.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycation and oxidation: a role in the pathogenesis of atherosclerosis

Reactions involving glycation and oxidation of proteins and lipids are believed to contribute to atherogenesis. Glycation, the nonenzymatic binding of glucose to protein molecules, can increase the atherogenic potential of certain plasma constituents, including low-density lipoprotein (LDL). Glycation of LDL is significantly increased in diabetic patients compared with normal subjects, even in the presence of good glycemic control. Metabolic abnormalities associated with glycation of LDL include diminished recognition of LDL by the classic LDL receptor; increased covalent binding of LDL in vessel walls; enhanced uptake of LDL by macrophages, thus stimulating foam cell formation; increased platelet aggregation; formation of LDL-immune complexes; and generation of oxygen free radicals, resulting in oxidative damage to both the lipid and protein components of LDL and to any nearby macromolecules. Oxidized lipoproteins are characterized by cytotoxicity, potent stimulation of foam cell formation by macrophages, and procoagulant effects. Combined glycation and oxidation, "glycoxidation," occurs when oxidative reactions affect the initial products of glycation, and results in irreversible structural alterations of proteins. Glycoxidation is of greatest significance in long-lived proteins such as collagen. In these proteins, glycoxidation products, believed to be atherogenic, accumulate with advancing age: in diabetes, their rate of accumulation is accelerated. Inhibition of glycation, oxidation, and glycoxidation may form the basis of future antiatherogenic strategies in both diabetic and nondiabetic individuals.

Interactions of LDL and modified LDL with mesangial cells and matrix

Hyperlipidemia may play a role in the progression of diabetic and other renal diseases. Low density lipoprotein (LDL) and other proteins including extracellular matrix components undergo nonenzymatic glycation in vivo. We examined the effects of glycation of LDL as occurs in diabetes (4 to 8%) on binding and uptake by mesangial cells and their proliferation. The glycation of LDL (g-LDL) significantly decreased its binding and uptake by mesangial cells by 15 to 20%, indicating that glycated LDL binds to the LDL receptor, but with lower affinity than LDL. Both LDL and g-LDL modestly stimulated [3H] thymidine incorporation into mesangial cells at 5 to 10 micrograms/ml. Native, oxidized (Ox-LDL) and glycated LDL all bound to the extracellular matrix generated by rat mesangial cells in culture. The binding of LDL, Ox-LDL and g-LDL to mesangial matrix was two to four times higher than to mesangial cells. Binding of LDL and g-LDL was significantly higher to glycolaldehyde modified matrix, which serves as an in vitro model for nonenzymatic glycation end-product cross-linking of matrix which occurs in long-standing diabetes. Based on these findings, we propose that glycation of LDL decreases its binding and uptake by the LDL receptor of mesangial cells and may slow its catabolism. Furthermore, LDL bound to extracellular mesangial matrix can undergo oxidation and generate cytotoxic LDL components. This process may be further enhanced by advanced glycation of the mesangial matrix in diabetes, contributing to glomerular pathology.

Increased permeability across the blood-nerve barrier of albumin glycated in vitro and in vivo from patients with diabetic polyneuropathy

The blood-nerve transfer of human plasma albumin glycated with D-glucose was investigated by measuring the permeability coefficient-surface area product (PS) of the blood-nerve barrier to radioiodinated albumin in normal adult rat sciatic nerve. Human albumin (ALB) from normal individuals, freshly isolated by CM-Affi-Gel Blue affinity chromatography, was glycated in vitro for 1, 3, 10, 19, and 30 weeks. Glycated ALB (gALB) was separated from the nonglycated form by boronate-affinity chromatography. The efficiency of this separation was assessed by chromatography of ALB glycated with [14C]glucose and by rechromatography of isolated ALB and gALB after radioiodination. The gALB was also shown to have a higher molecular weight and be completely separated from ALB after SDS/pore gradient electrophoresis in a Tris borate/EDTA buffer. After 1 week of glycation, the gALB PS was 2.2-fold greater than the ALB PS (0.724 +/- 0.063 x 10(-6) vs. 0.328 +/- 0.053 x 10(-6) ml.g-1.s-1; mean +/- SD; P less than 0.0001) and it increased with the time of glycation reaching a maximum value of 16.2-fold greater at 30 weeks (4.656 +/- 1.117 x 10(-6) vs. 0.288 +/- 0.042 x 10(-6) ml.g-1.s-1; mean +/- SD; P less than 0.0001). No change was observed in the residual endoneurial plasma volume. In addition, the PS of gALB isolated from patients with diabetic polyneuropathy was significantly increased (P less than 0.0001) compared to the PS for ALB isolated from the same patients. It is hypothesized that the increased permeability of gALB and presumably other glycated serum components across the blood-nerve barrier, as well as the observed quantitative increase in ALB, IgG, and IgM in sural nerve biopsies from patients with diabetic polyneuropathy contribute to the development of diabetic polyneuropathy over a prolonged period of time by mechanisms that might involve osmotic changes in the nerve microenvironment, direct toxic effects of glycated macromolecules on cells within the endoneurium, or nerve damage by classical immunological mechanisms due to trapping of glycated immunoglobulins within nerve.

Glycated low density lipoprotein catabolism is increased in rabbits with alloxan-induced diabetes mellitus

Hyperglycaemia in diabetes mellitus is responsible for the process of non-enzymatic glycosylation of different proteins. Since we did not find elevated glycated apolipoprotein B levels in diabetic patients, an altered glycated apolipoprotein B metabolism was suspected in diabetic patients. Experiments in normal rabbits showed that non-reductive (in vitro) glycated low density lipoprotein (gly-LDL) was cleared at a slower rate than control LDL and thus stayed longer in the circulation (vascular mean residence time: 10 vs 8 h, p less than 0.001). The body mean residence time for gly-LDL was 22 h vs 17 h for control LDL. In diabetic animals the catabolic parameters of both LDL preparations changed towards a faster clearance, the effect being greatest for gly-LDL (total mean residence times of gly-LDL pre-diabetic: 19 h, diabetic: 16 h; control LDL pre-diabetic and diabetic: 14 h). The difference in clearance between glycated and control LDL was thus strongly reduced. Virtually no antibody complexed to gly-LDL could be measured. The results suggest an increased activity of the non-receptor mediated pathway in diabetes mellitus, possibly co-responsible for an increased atherosclerotic risk.

Doxazosin therapy in the treatment of diabetic hypertension

The incidence of cardiovascular disease in non-insulin-dependent diabetes mellitus (NIDDM) has not been reduced by the control of hyperglycemia alone. Hypertension and dyslipidemia may be the major determinants of macrovascular disease in these patients. With the high prevalence of hypertension in NIDDM, antihypertensive drugs are likely to be important determinants of an atherogenic lipid profile. To date, there is no completed major randomized controlled trial of antihypertensive treatment outcome in a diabetic population, and as such, drug choice for the treatment of diabetic hypertension is often based on evidence extrapolated from studies in nondiabetic groups. However, two short-term studies have assessed the effects of doxazosin antihypertensive therapy in subjects with NIDDM. Both studies showed that the significant reduction in blood pressure with doxazosin treatment was associated with favorable effects on the serum lipid profile. In one study, contrasting adverse effects of atenolol treatment on glycemic control, lipids, and lipoproteins were observed. Doxazosin therapy was associated with a trend toward correcting the disturbances of lipoprotein metabolism characteristic of NIDDM. These metabolic effects, combined with effective lowering of blood pressure by doxazosin, may be important determinants of cardiovascular disease in the long term.

Pyridoxine reduces cholesterol and low-density lipoprotein and increases antithrombin III activity in 80-year-old men with low plasma pyridoxal 5-phosphate

We have previously observed that pyridoxine treatment reduced plasma total cholesterol (TC) and low-density lipoprotein (LDL) cholesterol concentrations and increased antithrombin III (AT III) activity in atherosclerotic patients with subnormal plasma pyridoxal 5-phosphate (PLP) levels. In order to confirm these results, we selected 17 males with low plasma PLP levels from a group of 122 80-year-old males in whom PLP has been determined. After supplementation with 120 mg of pyridoxine per day for 8 weeks their mean plasma TC and LDL cholesterol concentrations were decreased by 10% (p less than 0.01) and 17% (p less than 0.001), respectively. There was no effect on high-density lipoprotein cholesterol and triglycerides but plasma AT III activity was increased by 6% (p less than 0.05). The mechanism by which pyridoxine acts is unclear but it is hypothesized that pyridoxine-derived PLP may enhance the catabolism of LDL and the activity of AT III by inhibiting their glycosylation.

Glycation of the human erythrocyte glucose transporter in vitro and its functional consequences

Glycation of human erythrocyte membrane proteins was induced by incubation in vitro with high concentrations (80 mM or 200 mM) of D-glucose for 3 or 6 days. The extent of glycation was quantified from the covalent incorporation of 3H by reduction of the glucose glycation products with NaB3H4. For membranes incubated for 3 days with 80 mM-D-glucose, glycation in vitro of Band 4.5 (containing the glucose transporter) was equivalent to 0.11 mol of glucose/mol of glucose transporter, compared with 3H labelling in 3-day-incubated control membranes of 0.055 mol of glucose/mol of glucose transporter. In membranes incubated for 6 days with 200 mM-D-glucose, glycation increased to 0.21 mol of glucose/mol of glucose transporter, whereas the controls without glucose had 0.11 mol of glucose/mol of glucose transporter. Glycation in vitro was accompanied by a fall in the Bmax of binding of [3H]cytochalasin B (a competitive inhibitor of glucose transport), without any change in the binding affinity. The data suggest that glycated glucose transporters have decreased ability to bind cytochalasin B. It is proposed that glycation can alter glucose transporter activity.

Metabolism of very low- and low-density lipoproteins isolated from normolipidaemic type 2 (non-insulin-dependent) diabetic patients by human monocyte-derived macrophages

The very low- and low-density lipoprotein fractions were isolated from 16 normolipidaemic Type 2 (non-insulin-dependent) diabetic patients in good to fair glycaemic control and from corresponding age-, sex-, and race-matched, non-diabetic control subjects. Rates of cholesteryl ester synthesis averaged 268 +/- 31 vs 289 +/- 40 pmol 14C-cholesteryl oleate.mg cell protein-1.20 h-1 for very low- and 506 +/- 34 vs 556 +/- 51 pmol 14C-cholesteryl oleate.mg cell protein-1.20 h-1 for low-density lipoproteins isolated from the Type 2 diabetic patients and control subjects, respectively, when they were incubated with human macrophages. A group of approximately one-third of the patients was selected for separate analyses because very low-density lipoproteins isolated from these patients did stimulate more cholesteryl ester synthesis when incubated with macrophages. There were no significant differences in the lipid composition of the lipoproteins isolated from the three groups of subjects. The relative proportion of apoprotein C to apoprotein E was significantly decreased (p less than 0.002) in the very low-density lipoproteins from diabetic patients and was further decreased in samples from these selected diabetic patients. The apoprotein C-I content of very low-density lipoproteins isolated from diabetic patients was increased compared to control subjects and was further increased in samples from the selected diabetic patients (p less than 0.02). There were no significant differences in the proportions of apoproteins C-III-0, C-III-1, or C-III-2 among the three groups. These studies suggest that in normolipidaemic Type 2 diabetic patients, the apoprotein composition of VLDL is abnormal and this may alter VLDL macrophage interactions and thus contribute to the increased prevalence of atherosclerosis in diabetic patients.

Comparison of arterial intimal clearances of LDL from diabetic and nondiabetic cholesterol-fed rabbits. Differences in intimal clearance explained by size differences

Arterial intimal clearances of low density lipoproteins (LDL) from diabetic cholesterol-fed rabbits (D-LDL) and LDL from nondiabetic cholesterol-fed rabbits (N-LDL) were compared. In six experiments, D-LDL and N-LDL were isolated from a diabetic and a nondiabetic rabbit, were iodinated with 125I and 131I, respectively, were mixed, and were reinjected into the same two rabbits as well as into a normal rabbit. Fractional catabolic rates for D-LDL and N-LDL in normal rabbits were 0.065 and 0.074 h-1 (p less than 0.05), respectively. For five of the six pairs of LDL, the D-LDL was smaller than N-LDL as determined by gel filtration. The arterial permeability to N-LDL, when normalized for differences in arterial cholesterol content, did not appear to differ between diabetic and nondiabetic rabbits. The relative arterial intimal clearance (D-LDL/N-LDL) in arteries from diabetic and nondiabetic rabbits was inversely related to the relative molecular weight (D-LDL/N-LDL). For example, when the molecular weight of D-LDL was as low as 60% of that of N-LDL (i.e., the diameter of D-LDL was reduced 16%), the intimal clearance of D-LDL was 40% larger than that of N-LDL. When, on the other hand, molecular weights and diameters of the two LDL were similar, the intimal clearance was also quite similar. These results suggest that arterial intimal clearance of LDL from diabetic and nondiabetic cholesterol-fed rabbits is comparable unless the two types of LDL have a different size.

Binding and transcytosis of glycoalbumin by the microvascular endothelium of the murine myocardium: evidence that glycoalbumin behaves as a bifunctional ligand

The binding and transport of glycoalbumin (gA) by the endothelium of murine myocardial microvessels were studied by perfusing in situ 125I-gA or gA-gold complexes (gA-Au) and examining the specimens by radioassays and EM, respectively. After a 3-min perfusion, the uptake of radioiodinated gA is 2.2-fold higher than that of native albumin; it is partially (approximately 55%) competed by either albumin or D-glucose, and almost completely abolished by the concomitant administration of both competitors or by gA. D-mannose and D-galactose are not effective competitors. Unlike albumin-gold complexes that bind restrictively to plasmalemmal vesicles, gA-Au labels the plasma-lemma proper, plasmalemmal vesicles open on the lumen, and most coated pits. Competing albumin prevents gA-Au binding to the membrane of plasmalemmal vesicles, while glucose significantly reduces the ligand binding to plasmalemma proper. Competition with albumin and glucose gives additive effects. Transcytosis of gA-Au, already detected at 3 min, becomes substantial by 30 min. No tracer exit via intercellular junctions was detected. gA-Au progressively accumulates in multivesicular bodies. The results of the binding and competition experiments indicate that the gA behaves as a bifunctional ligand which is recognized by two distinct binding sites: one, located on the plasma membrane, binds as a lectin the glucose residues of gA; whereas the other, confined to plasmalemmal vesicles, recognizes presumably specific domains of the albumin molecule.

The amplified detection of free and bound methoxatin (PQQ) with nitroblue tetrazolium redox reactions: insights into the PQQ-locus

Porcine kidney diamine oxidase, a PQQ-enzyme, can be directly measured by formazan production with putrescine and nitroblue tetrazolium. This cyclic reaction in air is unaffected by superoxide dimutase, suggesting a two electron transfer between substrate-reduced PQQ-locus and nitroblue tetrazolium, without intermediate formation of superoxide. With albumin-bound PQQ and detergent-exposed PQQ-loci, glycine can be oxidized by PQQ and electrons repetitively transferred through PQQ-sites to nitroblue tetrazolium, the rate of formazan production detecting picomoles of exposed PQQ-locus. Exposed PQQ-loci are also reducible with NaCNBH3. Nitroblue tetrazolium, reoxidizes the reduced PQQ-locus with formazan production. These experiments suggest that the PQQ-locus of quinoproteins contains a [ketone-ketoimine in equilibrium with ketoamine] redox center.

Stimulation of cholesteryl ester synthesis in human monocyte-derived macrophages by low-density lipoproteins from type 1 (insulin-dependent) diabetic patients: the influence of non-enzymatic glycosylation of low-density lipoproteins

Diabetes mellitus is an independent risk factor in the development of atherosclerosis. In this study we aimed to demonstrate whether there is an abnormal interaction between low-density lipoproteins from diabetic patients and human macrophages. We measured cholesteryl ester synthesis and cholesteryl ester accumulation in human monocyte-derived macrophages (obtained from non-diabetic donors) incubated with low density lipoproteins from Type 1 (insulin-dependent) diabetic patients in good or fair glycaemic control. Low density lipoproteins from the diabetic patients stimulated more cholesteryl ester synthesis than low density lipoproteins from non-diabetic control subjects (7.19 +/- 1.19 vs 6.11 +/- 0.94 nmol/mg cell protein/20 h, mean +/- SEM, p less than 0.05). The stimulation of cholesteryl ester synthesis by low density lipoproteins isolated from diabetic patients was paralleled by a significant increase in intracellular cholesteryl ester accumulation (p less than 0.02). There were no significant differences in the lipid composition of low density lipoproteins between the diabetic and control groups. Non-enzymatic glycosylation of low density lipoproteins was higher in the diabetic group (p less than 0.01) and correlated significantly with cholesteryl ester synthesis (r = 0.58). Similarly, low-density lipoproteins obtained from non-diabetic subjects and glycosylated in vitro stimulated more cholesteryl ester synthesis in macrophages than control low density lipoproteins. The increase in cholesteryl ester synthesis and accumulation by cells exposed to low density lipoproteins from diabetic patients seems to be mediated by an increased uptake of these lipoproteins by macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)

Estimation and characterization of glycosylated carbonic anhydrase I in erythrocytes from patients with diabetes mellitus

Glycosylated form of carbonic anhydrase isozyme I was found in human erythrocytes. The percent of glycosylated enzyme of the total erythrocyte carbonic anhydrase I of patients with diabetes mellitus was significantly higher than that from normal controls. Characterization of the glycosylated carbonic anhydrase I was studied using an enzyme purified from diabetic patients. The glycosylated enzyme showed a slightly acidic isoelectric point in comparison with that of a nonglycosylated enzyme. The specific activity of the glycosylated enzyme was approximately 40% of that of the normal enzyme, and the immunological activity decreased to 52% of that of the normal enzyme. Estimation of carbohydrates which may form a ketoamine linkage with the enzyme was studied using [3H]-labelled glycosylated enzyme synthesized by incubation of the enzyme with [3H]-D-glucose in vitro, and it was found that one mol of glucose binds to one mol of enzyme. Exposure of red cells to a higher concentration of glucose in diabetics brought about glycosylation of carbonic anhydrase, which is associated with its low activity enzymatically and immunologically.

Glycosylation of low density lipoprotein in patients with type 1 (insulin-dependent) diabetes: correlations with other parameters of glycaemic control

Glycosylation of low density lipoproteins obtained from 16 patients with Type 1 (insulin-dependent) diabetes and from 16 age-, sex-, and race-matched controls, was determined. The diabetic patients were normolipaemic and were in good or fair glycaemic control. Eleven patients performed home blood glucose monitoring. Glycosylation of low density lipoproteins in the diabetic patients was significantly higher (p less than 0.001) than in the control subjects, and was significantly correlated with haemoglobin A1c, (p less than 0.01), glycosylation of plasma proteins, (p less than 0.001), and mean home blood glucose, (p less than 0.01). This study confirms that, in diabetic patients, increased glycosylation of low density lipoprotein occurs to an extent which correlates closely with other commonly used indices of glycaemic control.

Limited nonenzymatic glucosylation of low-density lipoprotein does not alter its catabolism in tissue culture

This study examines the effects of various degrees of chemical modification of low-density lipoprotein (LDL) on its catabolism by various cell types. Moderate glucosylation of LDL does not alter its interaction with the high-affinity receptor present on human fibroblasts at concentration of 5-2000 micrograms LDL-cholesterol/ml. Only heavily glucosylated LDL (more than 12 lysine residues glucosylated per apolipoprotein B) or LDL glucosylated in the presence of Na(CN)BH3, i.e., conditions not expected to occur in diabetes, inhibit receptor-mediated internalisation and degradation. Moderately glucosylated LDL is also readily recognized by cultured rat hepatocytes and porcine endothelial cells. Human monocyte-derived macrophages accumulate cholesteryl ester when incubated with acetylated LDL for 12 days but no enhanced cholesteryl ester formation was found when native or glucosylated LDL (3.3 lysines glucosylated per apolipoprotein B) were used.

Contribution of the receptor pathway to low density lipoprotein catabolism in humans. New methods for quantitation

Receptor-mediated catabolism of low density lipoprotein (LDL) by cultured cells depends on the presence of functionally significant arginine and lysine residues on the lipoprotein apoprotein. When these are blocked, the recognition process is abolished, and catabolism of the modified lipoprotein is restricted to other mechanisms. Accurate discrimination between the activities of the receptor and nonreceptor pathways in vivo depends critically on the metabolic properties of this chemically modified lipoprotein. Here we report our experiences with two lysine-modified LDL tracers, glucosylated LDL (GLC-LDL) and 2-hydroxyacetaldehyde-treated LDL (HOET-LDL). The fractional clearance rate of GLC-LDL (0.25 +/- 0.05 pools/day, n = 5) was 50% of that of control material (0.51 +/- 0.09 pools/day) injected simultaneously into normal subjects. The HOET-LDL was also retarded in its clearance. Here, however, the fractional clearances of the control (0.37 +/- 0.06 pools/day, n = 6) and modified lipoprotein (0.19 +/- 0.03 pools/day) were lower than those obtained by the glucosylation procedure. We suspect that the prolonged incubation required for glucosylation of LDL artifactually accelerated its catabolism. The HOET-LDL does not suffer from this defect and seems to be a better tracer of the receptor-independent pathway. In a group of 10 subjects, HOET-LDL was metabolically indistinguishable from 1,2 cyclohexanedione-treated, arginine-modified LDL.

New concepts about the pathogenesis of atherosclerosis in diabetes mellitus

New concepts about the pathogenesis of atherosclerosis in diabetes mellitus are presented. Emphasis is given to alterations of endothelial function, as indicated by von Willebrand factor activity, prostacyclin release, and fibrinolytic activity in diabetes mellitus. Previous work on platelet aggregation and arachidonic acid metabolism is updated and recent findings are emphasized. The atherogenic mix of elevated low-density lipoprotein cholesterol and low high-density lipoprotein cholesterol levels in uncontrolled diabetes mellitus is noted. The lipid hypothesis is extended by consideration of very low-density lipoprotein and intermediate-density lipoprotein metabolism in diabetes. Lipoprotein-cell interactions that may contribute to atherosclerosis are reviewed and suggestions are made for future research in order to clarify the pathogenesis of atherosclerosis in diabetes mellitus.

A novel method for generating region-specific monoclonal antibodies to modified proteins. Application to the identification of human glucosylated low density lipoproteins

Modifications of plasma lipoprotein structure and function resulting from in vivo post-translational nonenzymatic glycosylation may play a role in the premature atherosclerosis of patients with diabetes mellitus. This report describes the generation and characterization of six unique murine monoclonal antibodies that bind glucosylated human plasma lipoproteins, but do not react with normal plasma lipoproteins. This was accomplished by immunizing mice with homologous glucosylated low density lipoprotein. In competitive inhibition radioimmunoassays, the dominant epitope recognized by these antibodies on glucosylated low density lipoprotein was identified as glucitollysine, the reduced hexose alcohol form of glucose conjugated to the epsilon amino group of lysine. Each of these antibodies was capable of identifying glucitollysine epitopes on all reduced glucosylated proteins studied, including high density lipoprotein, albumin, hemoglobin, and transferrin. These antibodies were also capable of identifying and quantitating glucitollysine residues on the total plasma proteins and isolated lipoproteins of normal and diabetic individuals after reduction of the proteins with NaBH4. Preliminary data suggest that diabetic total plasma proteins and isolated lipoproteins contain at least threefold more immunochemically detectable glucitollysine residues than nondiabetic plasma proteins and lipoproteins. The technique described in this report should allow production of region-specific antibodies to any immunogenic modification of a protein.

Altered glycosylation and sialylation of serum proteins and lipid bound sialic acids in chronic renal failure

Abnormalities in the metabolism of carbohydrates, lipids and proteins are well documented in chronic renal failure (CRF). We have studied the extent of glycosylation and sialylation of serum proteins and also the levels of lipid-bound sialic acids in nondiabetic patients with CRF maintained on haemodialysis. The total neutral sugar content (TNS) and the non-enzymatic glycosylation (NEG) of serum proteins, the protein-bound sialic acids (PSA) and the lipid-bound sialic acids (LSA) were found to be significantly elevated in the patients as compared to a group of healthy controls. A single dialysis therapy produced some alterations in these measurements. The TNS correlated significantly with the NEG and also with the PSA. No correlation existed between total lipids. None of these measurements, however, correlated with the serum creatinine values. The established as well as putative pathophysiological consequences of these findings are discussed.

Measurement of receptor-independent metabolism of low-density lipoprotein. An application of glycosylated low-density lipoprotein

Incubation of human low-density lipoprotein (LDL) with glucose results in a nonenzymatic formation of a Schiff base between the monosaccharide and lysyl residues of apolipoprotein B. Increasing the percentage of lysyl residues of apolipoprotein B modified by glycosylation decreases the fractional catabolic rate of the glycosylated LDL, and decreases the metabolism of the glycosylated LDL by human skin fibroblasts. The glycosylated LDL, containing 20-40% of total lysyl residues of apoprotein B modified, was metabolized at a slow rate by both human skin fibroblasts and mouse peritoneal macrophages. These results led to the suggestion that glycosylated LDL is primarily catabolized via a receptor-independent process. Assuming LDL catabolism occurs via receptor-dependent and receptor-independent processes, the ratio of (fractional catabolic rate of glycosylated LDL)/(fractional catabolic rate of native LDL) should be an estimate of the percentage of LDL catabolism via the receptor-independent process. From the fractional catabolic rates of glucose-LDL (20-40% of lysyl residues modified) and galactose-LDL (30-60% of lysyl residues modified) 41% and 30% respectively, of LDL catabolism occurred by a receptor-independent process.