Glycation of very low density lipoprotein from rat plasma impairs its catabolism

The Impact of the Blood Lipids Levels on Arterial Stiffness

Arterial stiffness is a recognized predictor of cardiovascular morbidity and death. It is an early indicator of arteriosclerosis and is influenced by numerous risk factors and biological processes. The lipid metabolism is crucial and standard blood lipids, non-conventional lipid markers and lipid ratios are associated with arterial stiffness. The objective of this review was to determine which lipid metabolism marker has a greater correlation with vascular aging and arterial stiffness. Triglycerides (TG) are the standard blood lipids that have the strongest associations with arterial stiffness, and are often linked to the early stages of cardiovascular diseases, particularly in patients with low LDL-C levels. Studies often show that lipid ratios perform better overall than any of the individual variables used alone. The relation between arterial stiffness and TG/HDL-C has the strongest evidence. It is the lipid profile of atherogenic dyslipidemia that is found in several chronic cardio-metabolic disorders, and is considered one of the main causes of lipid-dependent residual risk, regardless of LDL-C concentration. Recently, the use of alternative lipid parameters has also been increasing. Both non-HDL and ApoB are very well correlated with arterial stiffness. Remnant cholesterol is also a promising alternative lipid parameter. The findings of this review suggest that the main focus should be on blood lipids and arterial stiffness, especially in individuals with cardio-metabolic disorders and residual cardiovascular risk.

Heparin binding triggers human VLDL remodeling by circulating lipoprotein lipase: Relevance to VLDL functionality in health and disease

Hydrolysis of VLDL triacylglycerol (TG) by lipoprotein lipase (LpL) is a major step in energy metabolism and VLDL-to-LDL maturation. Most functional LpL is anchored to the vascular endothelium, yet a small amount circulates on TG-rich lipoproteins. As circulating LpL has low catalytic activity, its role in VLDL remodeling is unclear. We use pre-heparin plasma and heparin-sepharose affinity chromatography to isolate VLDL fractions from normolipidemic, hypertriglyceridemic, or type-2 diabetic subjects. LpL is detected only in the heparin-bound fraction. Transient binding to heparin activates this VLDL-associated LpL, which hydrolyses TG, leading to gradual VLDL remodeling into IDL/LDL and HDL-size particles. The products and the timeframe of this remodeling closely resemble VLDL-to-LDL maturation in vivo. Importantly, the VLDL fraction that does not bind heparin is not remodeled. This relatively inert LpL-free VLDL is rich in TG and apoC-III, poor in apoE and apoC-II, shows impaired functionality as a substrate for the exogenous LpL or CETP, and likely has prolonged residence time in blood, which is expected to promote atherogenesis. This non-bound VLDL fraction increases in hypertriglyceridemia and in type-2 diabetes but decreases upon diabetes treatment that restores the glycemic control. In stark contrast, heparin binding by LDL increases in type-2 diabetes triggering pro-atherogenic LDL modifications. Therefore, the effects of heparin binding are associated negatively with atherogenesis for VLDL but positively for LDL. Collectively, the results reveal that binding to glycosaminoglycans initiates VLDL remodeling by circulating LpL, and suggest heparin binding as a marker of VLDL functionality and a readout for treatment of metabolic disorders.

Dyslipidemia in Pediatric Type 2 Diabetes Mellitus

PURPOSE OF REVIEW

Cardiovascular (CV) disease is a major cause of mortality in type 2 diabetes mellitus (T2D). Dyslipidemia is prevalent in children with T2D and is a known risk factor for CVD. In this review, we critically examine the epidemiology, pathophysiology, and recommendations for dyslipidemia management in pediatric T2D.

RECENT FINDINGS

Dyslipidemia is multifactorial and related to poor glycemic control, insulin resistance, inflammation, and genetic susceptibility. Current guidelines recommend lipid screening after achieving glycemic control and annually thereafter. The desired lipid goals are low-density lipoprotein cholesterol (LDL-C) < 100 mg/dL, high-density lipoprotein cholesterol (HDL-C) > 35 mg/dL, and triglycerides (TG) < 150 mg/dL. If LDL-C remains > 130 mg/dL after 6 months, statins are recommended with a treatment goal of < 100 mg/dL. If fasting TG are > 400 mg/dL or non-fasting TG are > 1000 mg/dL, fibrates are recommended. Although abnormal levels of atherogenic TG-rich lipoproteins, apolipoprotein B, and non-HDL-C are commonly present in pediatric T2D, their measurement is not currently considered in risk assessment or management.

Pathophysiology of diabetic dyslipidaemia: where are we?

Cardiovascular disease is a major cause of morbidity and mortality in patients with type 2 diabetes mellitus, with a two- to fourfold increase in cardiovascular disease risk compared with non-diabetic individuals. Abnormalities in lipid metabolism that are observed in the context of type 2 diabetes are among the major factors contributing to an increased cardiovascular risk. Diabetic dyslipidaemia includes not only quantitative lipoprotein abnormalities, but also qualitative and kinetic abnormalities that, together, result in a shift towards a more atherogenic lipid profile. The primary quantitative lipoprotein abnormalities are increased triacylglycerol (triglyceride) levels and decreased HDL-cholesterol levels. Qualitative lipoprotein abnormalities include an increase in large, very low-density lipoprotein subfraction 1 (VLDL1) and small, dense LDLs, as well as increased triacylglycerol content of LDL and HDL, glycation of apolipoproteins and increased susceptibility of LDL to oxidation. The main kinetic abnormalities are increased VLDL1 production, decreased VLDL catabolism and increased HDL catabolism. In addition, even though LDL-cholesterol levels are typically normal in patients with type 2 diabetes, LDL particles show reduced turnover, which is potentially atherogenic. Although the pathophysiology of diabetic dyslipidaemia is not fully understood, the insulin resistance and relative insulin deficiency observed in patients with type 2 diabetes are likely to contribute to these lipid changes, as insulin plays an important role in regulating lipid metabolism. In addition, some adipocytokines, such as adiponectin or retinol-binding protein 4, may also contribute to the development of dyslipidaemia in patients with type 2 diabetes.

Enhanced nonenzymatic glycation of eye lens proteins in experimental diabetes mellitus: an approach for the study of protein alterations as mediators of normal aging phenomena

The levels of advanced nonenzymatic glycation endproducts (ACE) were investigated by spectrofluorimetry in eye lens proteins obtained from rats with experimental diabetes of 3 and 6 months duration and from normal age-matched control rats. Diabetic animals showed higher AGE levels at both times studied. However the older control animals showed protein ACE levels comparable to those of the experimental 3 months diabetic group. These data suggest that a pathological phenomenon such as enhanced nonenzymatic glycation, associated to diabetic hyperglycemia, can be considered as a process leading to an accelerated aging of proteins. Thus experimental diabetes mellitus may be used as a model to investigate physiological protein senescence.

Hyperglycemia and glycation in diabetic complications

Diabetes mellitus is a multifactorial disease, classically influenced by genetic determinants of individual susceptibility and by environmental accelerating factors, such as lifestyle. It is considered a major health concern,as its incidence is increasing at an alarming rate, and the high invalidating effects of its long-term complications affect macro- and microvasculature, heart, kidney, eye, and nerves. Increasing evidence indicates that hyperglycemia is the initiating cause of the tissue damage occurring in diabetes, either through repeated acute changes in cellular glucose metabolism, or through the long-term accumulation of glycated biomolecules and advanced glycation end products (AGEs). AGEs represent a heterogeneous group of chemical products resulting from a nonenzymatic reaction between reducing sugars and proteins, lipids, nucleic acids, or a combination of these.The glycation process (glucose fixation) affects circulating proteins (serum albumin, lipoprotein, insulin, hemoglobin),whereas the formation of AGEs implicates reactive intermediates such as methylglyoxal. AGEs form cross-links on long-lived extracellular matrix proteins or react with their specific receptor RAGE, resulting inoxidative stress and proinflammatory signaling implicated in endothelium dysfunction, arterial stiffening, and microvascular complications. This review summarizes the mechanism of glycation and of AGEs formation and the role of hyperglycemia, AGEs, and oxidative stress in the pathophysiology of diabetic complications.

Effect of spermine on lipid profile and HDL functionality in the streptozotocin-induced diabetic rat model

This study aimed to investigate the effect of spermine (Spm) as a chemical chaperone and glycation inhibitor on the lipid profile and HDL functionality in the short- and long-term treatment of the STZ-induced diabetic rats. Male Wistar rats were divided into 4 groups (control, n=7; diabetic, n=9). Two groups (named 2 and 3) were injected intraperitoneally with streptozotocin. Control rats (named 1 and 4) were injected with vehicle alone. The treatment of diabetic and control animals (groups 3 and 4) with 60 micromol/l of Spm in drinking water was begun. The study continued up to the end of the fifth month. The serum glucose and insulin level, AGE formation, lipid profile, paraoxonase 1 (PON1), and lecithin: cholesterol acyl transferase (LCAT) activities were measured. Significantly lower plasma PON1, and LCAT activities and higher serum AGE, TG, TC and LDL-c, and lower HDL-c were seen in diabetic rats as compared to control groups (P<0.01). The increased AGE, TG, TC and LDL-c levels in diabetic groups decreased gradually after receiving Spm. In addition, due to Spm administration, an increase in the HDL-c level was observed after the first month of the experiment (P<0.01). The increase in the PON1 and LCAT activities in the diabetic group that received Spm was significant after the second and the forth month of the experiment, P<0.02 and P<0.05, respectively. In conclusion, spermine administration attenuated the changed parameters to near normal values in diabetic rats. Spermine, despite a lack of significant changes on glucose metabolism and insulin secretion, was found to improve diabetes complications.

New insight into the pathophysiology of lipid abnormalities in type 2 diabetes

Lipid abnormalities in patients with type 2 diabetes are likely to play an important role in the development of atherogenesis. These lipid disorders include not only quantitative but also qualitative abnormalities of lipoproteins which are potentially atherogenic. The main quantitative abnormalities are increased triglyceride levels, related to an augmented hepatic production of VLDL and a reduction of both VLDL and IDL catabolism, and decreased HDL-Cholesterol levels due to an accelerated HDL catabolism. The main qualitative abnormalities include large VLDL particles (VLDL1), relatively rich in triglycerides, small dense LDL particles, increase in triglyceride content of LDL and HDL, glycation of apolipoproteins and increased susceptibility of LDL to oxidation. Moreover, although plasma LDL-cholesterol level is usually normal in type 2 diabetic patients, LDL particles show significant kinetic abnormalities, such as reduced turn-over, which is potentially harmful. The pathophysiology of lipid abnormalities in type 2 diabetes is not yet totally explained. However, insulin resistance and the "relative" insulin deficiency, observed in patients with type 2 diabetes, are likely to play a crucial role since insulin has an important function in the regulation of lipid metabolism. In addition, it is not excluded that adipocytokines, such as adiponectin, could play a role in the pathophysiology of lipid abnormalities in type 2 diabetes.

The effect of high-dose simvastatin on triglyceride-rich lipoprotein metabolism in patients with type 2 diabetes mellitus

Statins decrease triglycerides (TGs) in addition to decreasing low density lipoprotein-cholesterol. Although the mechanism for the latter effect is well understood, it is still unclear how TG decrease is achieved with statin therapy. Because hypertriglyceridemia is common in obese patients with type 2 diabetes mellitus, we studied triglyceride-rich lipoprotein triglyceride (TRL-TG) turnover in 12 such subjects using stable isotopically labeled glycerol. The diabetic subjects were studied after 12 weeks of placebo and after a similar course of therapy with simvastatin (80 mg daily) in a single-blind design. The results were compared with those from six nonobese nondiabetic control subjects. Simvastatin therapy reduced serum TGs by 35% in the diabetic subjects. Compared with the control subjects, TRL-TG secretion was almost 2-fold higher in the diabetic subjects (45.4 +/- 4.9 vs. 24.4 +/- 1.9 micromol/min; P < 0.002) and was unaffected by simvastatin therapy. However, TRL-TG clearance was significantly increased in the diabetic subjects during simvastatin treatment compared with placebo (0.25 +/- 0.03 vs. 0.16 +/- 0.02 pools/h; P < 0.002). This change was accompanied by a 49% increase in preheparin plasma lipase activity (P < 0.03) and a 21% increase in postheparin LPL activity (P < 0.01). Together, these findings provide strong evidence that the effect of statins on serum TGs is related to an increase in LPL activity, resulting in accelerated delipidation of TRL particles. The effect of high-dose simvastatin on triglyceride-rich lipoprotein metabolism in patients with type 2 diabetes mellitus.

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.

Fatty acid metabolism is enhanced in type 2 diabetic hearts

The metabolic phenotype of hearts has been investigated using rodent models of type 2 diabetes which exhibit obesity and insulin resistance: db/db and ob/ob mice, and Zucker fatty and ZDF rats. In general, cardiac fatty acid (FA) utilization is enhanced in type 2 diabetic hearts, with increased rates of FA oxidation (db/db, ob/ob and ZDF models) and increased FA esterification into cellular triacylglycerols (db/db hearts). Hearts from db/db and ob/ob mice and ZDF rat hearts all have elevated levels of myocardial triacylglycerols, consistent with enhanced FA utilization. A number of mechanisms may be responsible for enhanced FA utilization in type 2 diabetic hearts: (i) increased FA uptake into cardiac myocytes and into mitochondria; (ii) altered mitochondrial function, with up-regulation of uncoupling proteins; and (iii) stimulation of peroxisome proliferator-activated receptor-alpha. Enhanced cardiac FA utilization in rodent type 2 diabetic models is associated with reduced cardiac contractile function, perhaps as a consequence of lipotoxicity and/or reduced cardiac efficiency. Similar results have been obtained with human type 2 diabetic hearts, suggesting that pharmacological interventions that can reduce cardiac FA utilization may have beneficial effects on contractile function.

Chylomicron remnant metabolism studied with a new breath test in postmenopausal women with and without type 2 diabetes mellitus

OBJECTIVES

The kinetic basis for the effect of type 2 diabetes mellitus (DM) on postprandial lipoproteins has not been fully established. We investigated chylomicron remnant metabolism using a stable isotope breath test and fasting measurements of plasma apolipoprotein (apo) B-48 and apoC-III concentrations in postmenopausal women with and without type 2 DM.

PATIENTS

Twenty-four postmenopausal women without DM and 14 postmenopausal women with diet-controlled DM of similar age and body mass index (BMI) were studied in the postabsorptive state.

METHODS

The fractional catabolic rate (FCR) of an intravenously injected chylomicron remnant-like emulsion was determined from the appearance of 13CO2 in the breath using isotope-ratio mass spectrometry and multicompartmental modelling. apoB-48, a marker of particle number of intestinal lipoproteins, was determined immunoelectrophoretically. apoC-III was measured by immunoturbidimetric assay.

RESULTS

Compared with the nondiabetic women, the women with DM had significantly higher plasma apoB-48 concentration (16.40 +/- 1.18 mg/l vs. 13.0 +/- 0.9 mg/l; mean +/- standard error mean; P = 0.021), higher plasma apoC-III concentration (204.24 +/- 15.18 mg/l vs. 170.74 +/- 10.75 mg/l; P = 0.042) and lower FCR of the chylomicron remnant-like emulsion (0.06 +/- 0.05 pools/h vs. 0.12 +/- 0.02 pools/h; P < 0.001). In the diabetic patients, the FCR of the emulsion was correlated significantly with plasma apoB-48 levels (r = -0.641, P = 0.007) but not with apoC-III levels.

CONCLUSIONS

In postmenopausal women, diabetes mellitus appears to decrease the catabolism of chylomicron remnants and result in an accumulation of these particles in plasma. This may chiefly be due to decreased clearance by hepatic receptors related to an effect of insulin resistance. Impairment in the catabolism of chylomicron remnants may contribute to increased risk of atherosclerosis in postmenopausal women with type 2 diabetes mellitus.

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.

Diminished rate of mouse peritoneal macrophage cholesterol efflux is not related to the degree of HDL glycation in diabetes mellitus

The efflux of (14)C-cholesterol from mouse peritoneal macrophages mediated by in vivo and in vitro glycation of intact HDL(3) and by HDL(3) apolipoproteins was investigated. Cholesterol-laden cells were incubated a long time with HDL(3) from control subjects (C), poorly controlled diabetes mellitus patients (D) and with HDL C submitted to in vitro glycation (G), as well as with all their respectively isolated apolipoproteins. A diminished cholesterol efflux rate occurred in incubations with intact HDL(3) D but not with intact HDL(3)G or with apoHDL(3)C, G or D. The specific binding of (125)I-HDL(3)G to the cell receptor, obtained upon incubation in the absence and in the presence of excess unlabelled HDL(3), was lower than the control. The role of apoE secretion by cholesterol-laden macrophages on cholesterol efflux was analyzed by incubating apoE knockout and control mice macrophages with HDL C or HDL G: a lower cholesterol efflux was observed from apoE knockout macrophages but glycation of HDL(3) did not influence this process either. The diminished capacity to remove cholesterol by the HDL drawn from diabetic subjects must be attributed to other modifications of the lipoproteins, except for non enzymatic glycation. Thus, events that impair the cell cholesterol removal in diabetes mellitus are multifaceted.

Postprandial metabolism of apolipoprotein B-48- and B-100-containing particles in type 2 diabetes mellitus: relations to angiographically verified severity of coronary artery disease

The aim of the present cross-sectional angiographic study was to examine if there is a relationship between the severity of CAD and postprandial lipemia in patients with type 2 diabetes mellitus. Special emphasis was directed to determining the contribution of apolipoprotein B-48 (apoB-48)-containing and B-100 (apoB-100)-containing triglyceride-rich particles to the magnitude of postprandial lipemia and degree of CAD. The role of apolipoprotein E (apoE) phenotype as a modulator of postprandial lipemia was also evaluated. The severity of CAD was determined by a quantitative coronary angiography and the subjects were classified into two groups based on the presence (severe CAD) or absence (mild CAD) of at least 50% stenosis in a major coronary vessel. The study population consisted of 43 subjects (31 men and 12 women) with fair glycemic control and comparable fasting lipids and body mass index. Postprandial responses of TG, apoB-48 and apoB-100 in lipoprotein subfractions (chylomicrons, VLDL1, VLDL2 and IDL) were determined after a fat load. Type 2 diabetic patients exhibited the classical dyslipidemia of the insulin resistance syndrome and delayed clearance of both hepatic and intestinal particles. Fasting or postprandial lipid or lipoprotein measurements, including apoB-48 and apoB-100 concentrations, did not differ between the groups. The presence or absence of apoE-4 allele did not significantly influence postprandial lipemia. The severity of the most significant coronary stenosis in angiography correlated with plasma and with chylomicron area under curve (AUC) for TG (n=27) and chylomicron AUC for apoB-48 (n=20). The strongest correlate of maximal stenosis was area under incremental curve (AUIC) for apoB-100 in IDL fraction (r=0.548, P=0. 012, n=20). In conclusion, postprandial apoB-48 and apoB-100 metabolism in triglyceride rich lipoproteins is distorted in type 2 diabetic patients, even in those with only mild CAD. The data suggest that postprandial change in small remnant particle numbers may contribute to the severity of CAD in type 2 diabetes.

Glycation of apolipoprotein E impairs its binding to heparin: identification of the major glycation site

The increased glycation of plasma apolipoproteins represents a possible major factor for lipid disturbances and accelerated atherogenesis in diabetic patients. The glycation of apolipoprotein E (apoE), a key lipid-transport protein in plasma, was studied both in vivo and in vitro. ApoE was shown to be glycated in plasma very low density lipoproteins of both normal subjects and hyperglycemic, diabetic patients. However, diabetic patients with hyperglycemia showed a 2-3-fold increased level of apoE glycation. ApoE from diabetic plasma showed decreased binding to heparin compared to normal plasma apoE. The rate of Amadori product formation in apoE in vitro was similar to that for albumin and apolipoproteins A-I and A-II. The glycation of apoE in vitro significantly decreased its ability to bind to heparin, a critical process in the sequestration and uptake of apoE-containing lipoproteins by cells. Diethylenetriaminepentaacetic acid, a transition metal chelator, had no effect on the loss of apoE heparin-binding activity, suggesting that glycation rather than glycoxidation is responsible for this effect. In contrast, glycation had no effect on the interaction of apoE with amyloid beta-peptide. ApoE glycation was demonstrated to be isoform-specific. ApoE(2) showed a higher glycation rate and the following order was observed: apoE(2)>apoE(4)>apoE(3). The major glycated site of apoE was found to be Lys-75. These findings suggest that apoE is glycated in an isoform-specific manner and that the glycation, in turn, significantly decreases apoE heparin-binding activity. We propose that apoE glycation impairs lipoprotein-cell interactions, which are mediated via heparan sulfate proteoglycans and may result in the enhancement of lipid abnormalities in hyperglycemic, diabetic patients.

Lipase activities in post-heparin plasma and tissues, and susceptibilities of lipoproteins in experimental diabetic rats

Heparin administration to diabetic rats caused no change in VLDL, an increase in IDL and a decrease in LDL on electrophoretic analysis of plasma lipoproteins, while the administration to control rats markedly decreased VLDL and increased IDL and LDL. Both hepatic triglyceride lipase (HTGL) and lipoprotein lipase (LPL) activities in the postheparin plasma were lower in the diabetic rats than in the controls, and the reduction of HTGL activity was greater than that of LPL activity in the diabetic rats. The LPL activity in the adipose tissue was lower in the diabetic rats than in the controls, but the activities in the cardiac and skeletal muscles were similar in the two rats. The HTGL-catalyzed fatty acid (FA) releases from the diabetic VLDL and IDL were lower than those from the normal rat VLDL and IDL, while the LPL-catalyzed FA release in the diabetic rats was not different from those in the controls. The decreases in LPL and HTGL activities and the markedly impaired susceptibility of IDL to HTGL coincide well with the postheparin changes in plasma lipoproteins in diabetic rats, an increase in IDL and a decrease in LDL.

Serum glycated lipoproteins in type II diabetic patients with and without complications

The extent of lipoprotein glycation was assessed in 82 patients diagnosed as diabetes mellitus type II. Glycated lipoprotein levels in serum were determined by agarose gel film electrophoresis in 48 non-diabetic control subjects, 56 diabetics with complications and 26 diabetics with no complications. All glycated lipoproteins were higher in the patients when the diabetics were compared to the control group, but glycated very low-density lipoprotein (VLDL) was the only glycated lipoprotein fraction that was observed to be higher in the diabetic patients with complications compared to the patients without complications (P < 0.01). Triglycerides and VLDL-cholesterol were also increased in the diabetics, but there was no significant difference in these variables when the patients without complications were compared to the control subjects. There was no difference in total cholesterol and low-density lipoprotein (LDL)-cholesterol levels of the diabetic patients compared to the controls.

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.

Role of advanced glycation end-products (AGE) in late diabetic complications

To evaluate accumulation of advanced glycation end-products (AGE) in diabetes and its possible correlation with late diabetic complications, AGE levels were measured by spectrofluorimetry in eye lens and sciatic nerve proteins and isolated tail tendon collagen of rats with experimental diabetes of 3- and 6-month duration. The values obtained were compared to those from age-matched control rats and correlated with cataract presence and somatosensory evoked potential (SEP) alterations. Diabetic animals had increased AGE levels in all tissues at both times; cataract developed in 29% of diabetic rats at 3 months and in 57% at 6 months; SEP conduction velocity was reduced in diabetic animals both at 3 (54.5 +/- 1.8 S.E.M. m/s vs. 73.9 +/- 1.0, P < 0.0001) and 6 months (59.5 +/- 1.4 vs. 71.5 +/- 1.6, P < 0.0001) from diabetes induction. No eye lens AGE level differences were observed when cataract presence was considered. Interestingly, in diabetic rats, increased sciatic nerve AGE levels were associated with reduced SEP. These data show that: (1) AGE levels are increased as early as 3 months from development of hyperglycemia; (2) other factors, in addition to an enhanced rate of fluorescent AGE formation, might play important roles in the pathogenesis of diabetic cataract; (3) increased peripheral nerve AGE levels are associated with SEP alterations.

Fluorescence properties of oxidised human plasma low-density lipoproteins

Appearance of fluorescence emission between 380-550 nm (lambda exc 350-400 nm) in freshly prepared low-density lipoprotein from asymptomatic normolipemic human plasma revealed the presence of in vivo oxidative modification of its protein moiety. Low-density lipoprotein elicited seven fluorophores in three dimensional fluorescence spectrogram. Assignment of fluorescent chemical structures originating from oxidative modification of the protein moiety of low-density lipoprotein has been made with the help of second derivative fluorescence spectroscopy.

Evidence for a short-term stimulatory effect of insulin on cholesterol synthesis in newly insulin-treated diabetic patients

To gain further insight into the effects of insulin on cholesterol synthesis in humans, 19 newly insulin-treated diabetic patients were studied before any insulin treatment (study day 1) and after a few days of optimized glycemic control with a continuous intravenous insulin infusion (study day 2). The patients were divided into two groups according to their clinical characteristics and laboratory disorders. Groups I and II consisted, respectively, of 10 newly diagnosed type I diabetic patients and nine type II diabetic patients with secondary failure to oral antidiabetic drugs. Cholesterol synthesis was estimated from the determination of serum lathosterol, a metabolic precursor in the cholesterol pathway, and from the serum lathosterol to cholesterol ratio. Serum cholesterol (millimolar, mean +/- SEM) remained unchanged in both groups. After insulin therapy (study day 2), serum lathosterol (micromolar) and the serum lathosterol to cholesterol ratio (molar ratio x 10(3)) were significantly increased as compared with baseline (study day 1). Serum lathosterol levels were as follows: 9.9 +/- 2.0 versus 4.1 +/- 0.4 (P < .02) in group I, and 9.9 +/- 0.8 versus 5.7 +/- 0.7 (P < .005) in group II; serum lathosterol to cholesterol ratios were 2.10 +/- 0.39 versus 0.86 +/- 0.11 (P < .005) in group I, and 1.92 +/- 0.12 versus 0.98 +/- 0.10 (P < .001) in group II. The data indicate that in newly insulin-treated diabetic patients, short-term intensive insulin therapy has a stimulatory effect on cholesterol synthesis and even results in cholesterol overproduction.

The dyslipoproteinemias of diabetes

Atherosclerosis is the most frequent complication of diabetes. There are many potentially atherogenic factors in diabetes that may underlie this problem. Of these, one is the group of dyslipoproteinemias. In diabetes there are both qualitative and quantitative changes in the plasma lipoproteins. Based on pathophysiological and epidemiological data, these may be among the many factors that can result in early macrovascular disease. Furthermore, at least one of the dyslipoproteinemias--hypertriglyceridemia--is associated with insulin resistance and therefore could aggravate glucose intolerance. Thus, on theoretical grounds it is reasonable to postulate that treating the dyslipoproteinemias of diabetes would reduce atherosclerotic disease. However, to date there have been no intervention studies specifically designed to test this postulate in the diabetic population. One such study, the Diabetes Atherosclerosis Intervention Study, is currently in progress.

MDL 29311, an analog of probucol, decreases triglycerides in rats by increasing hepatic clearance of very-low-density lipoprotein

MDL 29311 is an analog of probucol that shares probucol's antioxidant and antiatherogenic properties. When fed to rats as a 1% dietary admixture, MDL 29311 decreased triglyceride levels by 65% without affecting total or high-density lipoprotein (HDL) cholesterol levels. Under the same conditions, probucol decreased triglyceride levels by 23% and total cholesterol levels by 29% (with a corresponding decrease in HDL cholesterol level). MDL 29311 treatment did not affect the rate of triglyceride entry into the plasma. However, MDL 29311-treated rats cleared in vivo-labeled very-low-density lipoprotein (VLDL)-associated [3H]-triglyceride ([3H]-VLDL) over threefold faster than control rats. This increase in clearance led to increased levels of [3H]-lipid in liver and decreased [3H]-lipid in fat, muscle, diaphragm, and kidney of MDL 29311-treated rats 1.5 to 2.0 minutes after injection of [3H]-VLDL. MDL 29311 treatment had no effect on lipoprotein lipase (LPL) or hepatic triglyceride lipase (H-TGL) activities, or on plasma apolipoprotein (apo) C-II-dependent LPL activation. Intravenously injected [3H]-VLDL was allowed to circulate in MDL 29311-treated or control rats for 1 minute, and the undiluted plasma was then perfused through rat livers in a recirculating system. The [3H] in MDL 29311 plasma was cleared threefold faster (t1/2, 1.3 v 3.8 minutes) than the [3H] in control plasma by control livers. Conversely, the [3H] in control plasma was cleared slowly (t1/2 = 3.5 minutes) by the livers of MDL 29311-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased susceptibility of glycated very low density lipoproteins to lipoprotein lipase in vitro and prevention by glutathione

In vitro glycation of very low density lipoprotein (VLDL) reduced the susceptibility to lipoprotein lipase (LPL) as the level of glycation increased. Addition of reduced glutathione to an incubation medium of serum and glucose interfered with glycation of serum proteins when the concentration of reduced glutathione was higher than 10 mM. At concentrations higher than 25 mM, it also significantly prevented the glycation induced reduction of fatty acid releases from VLDL by LPL. There were no such effects on glycation of serum protein and the fatty acid release from the addition of aminoguanidine. By contrast, addition of D-lysine enhanced glycation of serum proteins by glucose and further decreased fatty acid release from VLDL by LPL. From these results, it is suggested that glycation of VLDL decreases the susceptibility of VLDL to LPL. Delayed catabolism of VLDL in diabetic patients is considered partly caused by glycation of apoproteins, which renders VLDL less sensitive to LPL, in addition to the decreased LPL activity in diabetes mellitus.

In vitro degradation of very low density lipoprotein from diabetic patients by lipoprotein lipase

Fatty acid release by incubation with lipoprotein lipase (LPL) in vitro from very low density lipoproteins (VLDL) obtained from diabetic patients was low compared with that from healthy subjects, though the compositions were similar in both VLDL. Percentages of the large size VLDL decreased and those of the small size VLDL increased after the incubation with LPL. At the same time, on polyacrylamide gel disk electrophoresis, the smaller catabolic products from these VLDL appeared at a similar position to that of low density lipoproteins (LDL) and at the running front where high density lipoproteins (HDL) had migrated. The amount of the small size VLDL and the LDL-like lipoproteins produced from diabetic VLDL were less than those from normal VLDL and inversely correlated with the percent decrease of the large original size VLDL. This fact suggests that VLDL from diabetic patients are poor substrates for LPL compared with normal VLDL.

Hypertriglyceridemia is exacerbated by slow lipolysis of triacylglycerol-rich lipoproteins in fed but not fasted streptozotocin diabetic rats

Hydrolysis by endothelial lipases of triacylglycerol-rich lipoproteins of diabetic origin were compared to lipoproteins of non-diabetic origin. The plasma lipoprotein fraction of density < 1.006 g/ml, including chylomicrons and VLDL, were incubated in vitro with post-heparin plasma (PHP) lipases. The lipoproteins of diabetic origin were hydrolysed at a significantly slower rate than lipoproteins from normal rats by the lipoprotein lipase component of PHP. However, if rats were fasted for 16 h prior to lipoprotein recovery, no differences in rates of VLDL hydrolysis were observed. Slower hydrolysis of lipoproteins of diabetic origin reflected a decrease in the apolipoprotein CII/CIII ratio and other changes in the apolipoprotein profile. To assess whether diabetic rats were less able to clear triacylglycerol independent of changes in the nature of the lipoproteins, we monitored the clearance of chylomicron-like lipid emulsions in hepatectomized rats. In vivo, emulsion triacylglycerol hydrolysis was not slowed due to diabetes. However, control and diabetic rats, which had been fasted for 16 h, cleared triacylglycerol at about twice the rate of fed rats. Triacylglycerol secretion rates in diabetic and control rats were similar, whether fed or fasted. We conclude that in streptozocin diabetic rats, hypertriglyceridemia was not due to overproduction of chylomicron- or VLDL-triacylglycerol, nor to decreased endothelial lipase activities. Rather, in fed diabetic rats, the triacylglycerol-rich lipoproteins are poorer substrates for lipoprotein lipase. This may lead to slower formation of remnants which would exacerbate slow remnant removal. VLDL of diabetic origin were hydrolysed as efficiently as VLDL from control donors, suggesting that in the fed state the lipolytic defect may be specific for chylomicrons.

Non-enzymatic glycation of apolipoprotein B in the sera of diabetic and non-diabetic subjects

Utilising a combination of m-aminophenyl-borate affinity chromatography and an immunoradiometric assay for apolipoprotein B (apo B), we have developed a specific and highly sensitive (6 ng/ml) procedure for the assay of glycated apo B. We studied 52 diabetic patients, 50 non-diabetic control subjects and 12 patients heterozygous for familial hypercholesterolaemia (FH). Both insulin-dependent and non-insulin dependent diabetics were included in our study. Total apo B in the diabetics (108 +/- 5 mg/dl; mean +/- S.E.M) was increased (controls: 95 +/- 4 mg/dl; P less than 0.05). In the FH group the serum apo B concentration (216 +/- 24 mg/dl) was significantly higher (P less than 0.001) than both the other groups studied. Both the serum glycated apo B concentration (9.3 +/- 0.8 mg/dl versus 4.8 +/- 0.7 mg/dl) and the percentage glycated apo B (7.9 +/- 0.4% compared to 3.9 +/- 0.2%) were significantly higher in the diabetics than in non-diabetic controls (P less than 0.001). A positive correlation was found between the percentage of glycated apo B and glycated haemoglobin (r = 0.65; P less than 0.001) and fasting glucose concentration (r = 0.52; P less than 0.001) in diabetics. The percentage of glycated apo B in FH patients was not significantly different from controls, but the serum concentration of glycated apo B, because of the greatly increased total level of apo B was raised (8.2 +/- 1.4 mg/dl) to a similar extent to that of the diabetics.

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.

Effect of long-term insulin deficiency and insulin treatment on the composition of triglyceride-rich lipoproteins and triglyceride turnover in rats

The effect of long-term (4 months) insulin deficiency on triglyceride turnover was examined using Triton WR1339 in rats. Triglyceride secretion rate was estimated in rats 2 weeks and 4 months after induction of diabetes with 40 mg/kg of streptozotocin. By the second week diabetic rats showed prominent hyperglycemia and the plasma insulin level was very low. In spite of a lower triglyceride secretion rate compared to non-diabetic control rats, diabetic rats showed normotriglyceridemia. Thus, the estimated fractional catabolic rate for plasma triglyceride was decreased in the diabetic rats of 2 weeks duration. By the fourth month diabetic rats still showed a suppressed triglyceride secretion rate but plasma triglyceride was markedly higher than in the non-diabetic control rats. Therefore, their estimated fractional catabolic rate for plasma triglyceride was severely suppressed. They also showed hyperglycemia and hypercholesterolemia. The triglyceride-rich lipoprotein particles obtained after Triton injection in long-term diabetic rats were significantly cholesterol-enriched and triglyceride-depleted compared to control rats. These changes were already present in 2-week diabetic rats but the magnitude was significantly smaller that those in long-term diabetic rats. All of these abnormalities (including triglyceride turnover and the particle composition) were almost normalized by 2 weeks of insulin treatment (6 units/day). Thus, it was concluded from the present data that duration of insulin deficiency is an important determinant of triglyceride removal rate from the circulation in rats. Further modification of lipid composition of triglyceride-rich lipoprotein particles by long-term insulin-deficiency could be one of the reasons for this removal defect.(ABSTRACT TRUNCATED AT 250 WORDS)

[Anomalies of lipid metabolism in diabetes mellitus]

Atheroma is by far the most common cause of mortality in diabetic patients (66 to 75% of deaths). Several physiopathological mechanisms are suspected to account for the greater frequency and severity of atheroma in diabetes. Among these, lipid abnormalities hold first rank and include not only quantitative but also qualitative abnormalities of lipoproteins altering their kinetics and bindings to membrane receptors. The main quantitative abnormalities are an increase of triglycerides and very low density lipoproteins (VLDL) and a fall in high density lipoproteins (HDL) and their HDL2 subfraction. Qualitative abnormalities include non-enzymatic glucosylation of apoproteins, changes in lipoprotein size and increase in their triglyceride content, and excessive oxidation of low density lipoproteins (LDL). Both quantitative and qualitative abnormalities of lipoproteins are present in non-insulin-dependent diabetes, whereas only qualitative abnormalities are observed, as a rule, in treated insulin-dependent diabetes. The physiopathology of lipid metabolism disorders is complex, possibly multifactorial and still imperfectly known. However, such factors as modification of insulin status, hyperglycaemia and obesity frequently associated with diabetes, are thought to be involved.