Transvascular low-density lipoprotein transport in patients with diabetes mellitus (type 2): a noninvasive in vivo isotope technique

Low-density lipoprotein: a versatile nanoscale platform for targeted delivery

Low-density lipoprotein (LDL) is a small lipoprotein that plays a vital role in controlling lipid metabolism. LDL has a delicate nanostructure with unique physicochemical properties: superior payload capacity, long residence time in circulation, excellent biocompatibility, smaller size, and natural targeting. In recent decades, the superiority and feasibility of LDL particles as targeted delivery carriers have attracted much attention. In this review, we introduce the structure, composition, advantages, defects, and reconstruction of LDL delivery systems, summarize their research status and progress in targeted diagnosis and therapy, and finally look forward to the clinical application of LDL as an effective delivery vehicle.

Levels of atherogenic lipoproteins are unexpectedly reduced in interstitial fluid from type 2 diabetes patients

At a given level of serum cholesterol, patients with T2D have an increased risk of developing atherosclerosis compared with nondiabetic subjects. We hypothesized that T2D patients have an increased interstitial fluid (IF)-to-serum gradient ratio for LDL, due to leakage over the vascular wall. Therefore, lipoprotein profiles in serum and IF from 35 T2D patients and 35 healthy controls were assayed using fast performance liquid chromatography. The IF-to-serum gradients for VLDL and LDL cholesterol, as well as for apoB, were clearly reduced in T2D patients compared with healthy controls. No such differences were observed for HDL cholesterol. Contrary to our hypothesis, the atherogenic VLDL and LDL particles were not increased in IF from diabetic patients. Instead, they were relatively sparser than in healthy controls. The most probable explanation to our unexpected finding is that these lipoproteins are more susceptible to retainment in the extravascular space of these patients, reflecting a more active uptake by, or adhesion to, tissue cells, including macrophages in the vascular wall. Further studies are warranted to further characterize the mechanisms underlying these observations, which may be highly relevant for the understanding of why the propensity to develop atherosclerosis is increased in T2D.

The plasma concentration of HDL-associated apoM is influenced by LDL receptor-mediated clearance of apoB-containing particles

ApoM is mainly associated with HDL. Nevertheless, we have consistently observed positive correlations of apoM with plasma LDL cholesterol in humans. Moreover, LDL receptor deficiency is associated with increased plasma apoM in mice. Here, we tested the idea that plasma apoM concentrations are affected by the rate of LDL receptor-mediated clearance of apoB-containing particles. We measured apoM in humans each carrying one of three different LDL receptor mutations (n = 9) or the apoB3500 mutation (n = 12). These carriers had increased plasma apoM (1.34 ± 0.13 µM, P = 0.003, and 1.23 ± 0.10 µM, P = 0.02, respectively) as compared with noncarriers (0.93 ± 0.04 µM). When we injected human apoM-containing HDL into Wt (n = 6) or LDL receptor-deficient mice (n = 6), the removal of HDL-associated human apoM was delayed in the LDL receptor-deficient mice. After 2 h, 54 ± 5% versus 90 ± 8% (P < 0.005) of the initial amounts of human apoM remained in the plasma of Wt and LDL receptor-deficient mice, respectively. Finally, we compared the turnover of radio-iodinated LDL and plasma apoM concentrations in 45 normocholesterolemic humans. There was a negative correlation between plasma apoM and the fractional catabolic rate of LDL (r = -0.38, P = 0.009). These data suggest that the plasma clearance of apoM, despite apoM primarily being associated with HDL, is influenced by LDL receptor-mediated clearance of apoB-containing particles.

Increased LDL cholesterol and CRP in infants of mothers with type 1 diabetes

BACKGROUND

Proatherogenic stimuli during foetal life may predispose to development of atherosclerosis in adulthood. Elevated plasma low-density lipoprotein (LDL) cholesterol and C-reactive protein (CRP) expression is associated with increased risk of atherosclerosis.

METHODS AND RESULTS

In this study, we examined how maternal type 1 diabetes affects foetal plasma LDL cholesterol and CRP. In comparison with healthy mothers, the plasma LDL cholesterol was not increased in the mothers with diabetes, however, the umbilical-cord plasma LDL cholesterol was increased in their infants. CRP was increased in infants of mothers with diabetes and high haemoglobin A1c (HbA1c, > or = 6.2%). Human placenta expresses microsomal triglyceride transfer protein (MTP), which facilitates secretion of apolipoprotein B-containing lipoproteins. Microsomal triglyceride transfer activity was slightly higher (11%) in placentas from mothers with diabetes and HbA1c > or = 6.2% compared with the controls.

CONCLUSION

The results suggest that maternal type 1 diabetes increases the foetal plasma LDL cholesterol and CRP concentration and thus might predispose the offspring to development of atherosclerosis.

Polymorphisms in apolipoprotein B and risk of ischemic stroke

CONTEXT

Apolipoprotein B levels associate with risk of ischemic stroke. APOB polymorphisms may influence levels of apolipoprotein B and low-density lipoprotein (LDL), but whether they associate with risk of ischemic stroke is unknown.

OBJECTIVE

We tested the hypothesis that the APOB T71I, A591V, P2712L, R3611Q, E4154K, and N4311S polymorphisms associate with risk of ischemic stroke in the general population and performed in vivo human LDL turnover studies of E4154K heterozygotes vs. K4154K homozygotes.

DESIGN

This was a prospective study (the Copenhagen City Heart Study) with 23-yr, 100% complete follow-up.

SETTING

The study was conducted with a Danish general population.

PARTICIPANTS

A total of 9157 women and men aged 20-80+ yr participated in the study.

MAIN OUTCOME MEASURES

Risk of ischemic cerebrovascular disease and ischemic stroke, apolipoprotein B and LDL levels, and LDL fractional catabolic rate were measured. The hypothesis was formulated before genotyping.

RESULTS

APOB K4154K homozygotes had an age-adjusted hazard ratio of 0.4 (95% confidence interval 0.2-0.9) for ischemic cerebrovascular disease and 0.2 (0.1-0.7) for ischemic stroke relative to E4154E homozygotes. Corresponding multifactorially adjusted hazard ratios were 0.5 (0.2-1.0) and 0.2 (0.1-0.8). Furthermore, E4154K heterozygotes and K4154K homozygotes had lower levels of apolipoprotein B and LDL cholesterol, compared with E4154E homozygotes. Finally, E4154K heterozygotes had an increased fractional catabolic rate of LDL relative to E4154E homozygotes. None of the other polymorphisms studied influenced risk of ischemic stroke.

CONCLUSIONS

APOB K4154K homozygosity predicts a 3- to 5-fold reduction in risk of ischemic cerebrovascular disease and ischemic stroke. This may be explained by lower plasma levels of apolipoprotein B and LDL cholesterol caused by an increased catabolism of LDL particles, although another yet-unknown mechanism is also possible.

Lipoprotein profiles in plasma and interstitial fluid analyzed with an automated gel-filtration system

BACKGROUND

High-quality methods for lipoprotein characterization are warranted in studies on various metabolic diseases.

MATERIALS AND METHODS

An automated system for size-exclusion chromatography (SEC) of lipoproteins using commercially available components is described. Cholesterol or triglyceride content in separated lipoproteins from plasma and interstitial fluid (IF) was continuously determined on-line using microlitre sample volumes.

RESULTS

The lipoprotein assay showed a good concordance with the classic ultra-centrifugation/precipitation technique using fresh or frozen samples. Determination of lipoproteins in IF obtained from vacuum-induced skin blisters from 18 healthy subjects revealed that very low density lipoprotein (VLDL), low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol levels were 18%, 19% and 25%, respectively, of concomitant plasma concentrations. The size-exclusion chromatography (SEC) system also allows for triglyceride determination on-line and it could be shown that the system is advantageous for an accurate determination of triglycerides in conditions when there are high levels of glycerol, e.g. in mice and in patients with hyperglycerolaemia (pseudo-hypertriglyceridaemia).

CONCLUSIONS

The described system should be of value in studies where detailed lipoprotein analysis is warranted and particularly when significant sample series with small volumes are available. Our data also suggest that there is a 4-5.5-fold concentration gradient between plasma and IF for the three major plasma lipoproteins.

Increased transvascular lipoprotein transport in diabetes: association with albuminuria and systolic hypertension

CONTEXT

Diabetes is associated with a highly increased risk of atherosclerosis, especially if hypertension or albuminuria is present.

OBJECTIVE

We hypothesized that the increased transvascular lipoprotein transport in diabetes may be further accelerated if hypertension or albuminuria is present, possibly explaining increased intimal lipoprotein accumulation and thus atherosclerosis.

DESIGN

The study was cross-sectional and was performed in 1999-2002.

SETTING

The study took place in the referral center.

PATIENTS

The patients included 60 with diabetes mellitus (27 with type 1 diabetes and 33 with type 2 diabetes) and 42 healthy controls. All were randomly recruited.

MAIN OUTCOME MEASURE

We used an in vivo method for measurement of transvascular transport of low-density lipoprotein (LDL). Autologous 131I-LDL was reinjected iv, and the 1-h fractional escape rate was taken as an index of transvascular transport.

RESULTS

Transvascular LDL transport was 1.8 (1.6-2.0), 2.3 (2.0-2.6), and 2.6 (1.3-4.0)%/[h x (liter/m2)] in healthy controls, diabetic controls, and diabetes patients with systolic hypertension or albuminuria, respectively (P = 0.013; F = 4.5; df =2; ANOVA). These differences most likely were not caused by altered hepatic LDL receptor expression, glycosylation of LDL, small LDL size, or medicine use.

CONCLUSIONS

Transvascular LDL transport is increased in patients with diabetes mellitus, especially if systolic hypertension or albuminuria is present. Accordingly, lipoprotein flux into the arterial wall could be increased in these patients, possibly explaining accelerated development of atherosclerosis.

Pathogenesis and management of diabetic dyslipidemia

Patients with diabetes mellitus have a 2- to 4-fold increased risk of atherosclerotic cardiovascular, peripheral vascular, and cerebrovascular disease, which are the leading causes of morbidity and mortality in this population. Several epidemiological studies have shown an association between diabetic dyslipidemia, which is characterized by hypertriglyceridemia, low levels of high density lipoprotein-cholesterol, postprandial lipemia and small, dense low density lipoprotein-cholesterol (LDL-C) particles, and the occurrence of cardiovascular disease. Other studies have established the beneficial effects of lipid lowering on the reduction of major coronary events in diabetic patients. The recent National Cholesterol Education Program (NCEP) Adult Treatment Panel III (ATP III) guidelines emphasize diabetes as a coronary heart disease risk equivalent. The NCEP ATP III states that elevated LDL-C is a major risk factor for coronary heart disease, and the primary goal of risk-reduction therapy is the reduction of LDL-C levels to 100 mg/dL. This article defines and describes diabetic dyslipidemia and its etiology and pathogenesis, as well as reviewing guidelines and recommendations for treatment of this disorder. Treatment of diabetic dyslipidemia includes 1) lifestyle modifications: physical activity and a diet low in saturated fats and cholesterol and high in complex carbohydrates and fiber; and 2) pharmacological treatment with (i) oral antihyperglycemic agents: metformin and thiazolidinediones; (ii) weight reduction drugs: orlistat and sibutramine and; (iii) lipid-lowering drugs: HMG-CoA reductase inhibitors, fibric acid derivatives, nicotinic acid, and bile acid sequestrants.

Mutation in apolipoprotein B associated with hypobetalipoproteinemia despite decreased binding to the low density lipoprotein receptor

Mutations in apolipoprotein B (APOB) may reduce binding of low density lipoprotein (LDL) to the LDL receptor and cause hypercholesterolemia. We showed that heterozygotes for a new mutation in APOB have hypobetalipoproteinemia, despite a reduced binding of LDL to the LDL receptor. APOB R3480P heterozygotes were identified among 9,255 individuals from the general population and had reduced levels of apoB-containing lipoproteins. Most surprisingly, R3480P LDL bound with lower affinity to the LDL receptor than non-carrier LDL in vitro, and these results were confirmed by turnover studies of LDL in vivo. In very low density lipoprotein (VLDL) turnover studies, the amount of VLDL converted to LDL in R3480P heterozygotes was substantially reduced, suggesting that this was the explanation for the hypobetalipoproteinemia observed in these individuals. Our findings emphasized the importance of combining in vitro studies with both human in vivo and population-based studies, as in vitro studies often have focused on very limited aspects of complex mechanisms taken out of their natural context.

Statin therapy improves brachial artery vasodilator function in patients with Type 1 diabetes and microalbuminuria

AIMS

Type 1 diabetes mellitus patients with microalbuminuria have endothelial dysfunction associated with the degree of albuminuria but not with LDL-cholesterol levels. Lipid-lowering agents such as statins may still be of benefit as they can correct endothelial dysfunction by both lipid and non-lipid mechanisms. We therefore examined the effects of atorvastatin on brachial artery endothelial dysfunction in these patients.

METHODS

In a double-blind, randomized crossover study, 16 Type 1 diabetes mellitus patients with microalbuminuria received 6 weeks of atorvastatin 40 mg/day or placebo, separated by a 4-week washout. Brachial artery, endothelium-dependent, flow-mediated dilatation (FMD) and endothelium-independent, glyceryl trinitrate-mediated dilatation (GTNMD) were measured.

RESULTS

Compared with placebo, atorvastatin produced a significant decrease in apolipoprotein B (34.2%), LDL-cholesterol (44.1%) (all P < 0.001), and oxidized-LDL (35.7%, P = 0.03). There was a non-significant increase in plasma cGMP (P = 0.13) on atorvastatin. FMD and GTNMD increased significantly on atorvastatin (FMD: atorvastatin +1.8 +/- 0.4%; placebo +0.2 +/- 0.4%, P = 0.007); (GTNMD: atorvastatin +1.3 +/- 0.9%; placebo -1.2 +/- 0.6%, P = 0.04). An increase in cGMP was independently correlated with an increase in FMD on atorvastatin (adjusted (R2) 0.41, P = 0.02).

CONCLUSION

Atorvastatin improves endothelium-dependent and independent vasodilator function of the brachial artery in Type 1 diabetes mellitus patients with microalbuminuria. This may relate to pleiotropic effects of statins, in particular reduced oxidative stress and increased availability of nitric oxide.

Insulin resistance and endothelial dysfunction in type 2 diabetes patients with or without microalbuminuria

OBJECTIVE

To evaluate the relationship between insulin resistance and endothelial function in type 2 diabetes patients with or without microalbuminuria and to explore the pathophysiological mechanisms of the increased macrovascular risk in type 2 diabetes mellitus.

METHODS

Twelve type 2 diabetes patients with microalbuminuria (urinary albumin 30-300 g/mg creatinine, DM-MA) and 12 type 2 diabetes patients without microalbuminuria (urinary albumin <30 g/mg creatinine, DM-NA) were recruited, matched for their sex, age, body mass index (BMI), diabetes duration, antidiabetic therapy. Their hemoglobin (HbA1C) was less than 7.5% and the blood pressure was lower than 140/90 mmHg. None had evidence of macrovascular disease and serum cholesterol levels were normal. Twelve healthy volunteers (C) were enrolled as controls. All subjects received a hyperinsulinemic euglycemic clamp study (insulin infusion rate, 120 mU/m2/min) to assess the peripheral glucose disposal rate (GDR) in the steady state and had a high-resolution ultrasonography to measure vasodilation in the brachial artery diameter in response to reactive hyperemia (endothelium-dependent) and administration of glyceryl trinitrate (endothelium-independent). Plasma free fatty acids (FFAs), plasminogen activator inhibitor type I (PAI-1), and von Willebrand factor (vWF) were also measured.

RESULTS

The GDR was lower in type 2 diabetes patients with microalbuminuria (7.90 +/- 1.79 mg/kg/min) than in patients without microalbuminuria (9.46 +/- 1.59 mg/kg/min, P<0.05), and the GDR in both diabetes groups was decreased, compared with the findings from the healthy control group (13.06 +/- 1.98 mg/kg/min, P<0.01). Plasma FFAs concentration was different among the three groups (DM-MA-1008 +/- 229 mol/l, DM-NA-675 +/- 201 mol/l, P<0.01; C-364 +/- 169 mol/l, P<0.01). Endothelium-dependent vasodilation was impaired in the microalbuminuric patients (8.0 +/- 3.8%) compared with the normoalbuminuria patients (9.7 +/- 4.3%, P>0.05) and the healthy controls (14.2 +/- 5.0, P<0.05). Plasma PAI-1 and vWF levels increased in the microalbuminuric patients (61.9 +/- 18.2 ng/ml, 126.8 (76.5-212.7) %) compared with the levels in the normoalbuminuric patients (45.4 +/- 16.3 ng/ml, 87.9 (84.2-114) %) (PAI-1, P<0.05; vWF, P>0.05) and in the healthy controls (33.6 +/- 10.6 ng/ml, 67.2 (61.5-75.4) %, both P<0.01). In addition, the partial correlation analysis revealed a significant positive correlation between GDR and endothelium-dependent vasodilation (r=0.465, P<0.01, n=36), and a negative correlation between GDR and plasma FFA levels (P<0.05).

CONCLUSIONS

Compared with type 2 diabetes patients with normoalbuminuria, patients with microalbuminuria had more severe insulin resistance, more prominent endothelial dysfunction, and higher plasma FFA, PAI-1, and vWF levels. Therefore we speculate that insulin resistance and endothelial dysfunction may act within the metabolic syndrome to increase the cardiovasular risk in this subset of patients, and improving insulin resistance and endothelial dysfunction may reduce the morbidity and mortality from macrovascular complications in type 2 diabetes patients.

Transvascular lipoprotein transport in patients with chronic renal disease

BACKGROUND

While increased plasma cholesterol is a well-established cardiovascular risk factor in the general population, this is not so among patients with chronic renal disease. We hypothesized that the transvascular lipoprotein transport, in addition to the lipoprotein concentration in plasma, determines the degree of atherosclerosis among patients with chronic renal disease.

METHODS

We used an in vivo method for measurement of transvascular transport of low-density lipoprotein (LDL) in 21 patients with chronic renal disease and in 42 healthy control patients. Autologous 131-iodinated LDL was reinjected intravenously, and the 1-hour fractional escape rate was taken as index of transvascular transport.

RESULTS

Transvascular LDL transport tended to be lower in patients with chronic renal disease than in healthy control patients [3.3 (95% CI 2.4-4.2) vs. 4.2 (3.7-4.2)%/hour; NS]. However, this tendency disappeared when transvascular LDL transport was corrected for distribution volume of LDL [1.7 (1.2-2.2) vs. 1.8 (1.6-2.0) %/(hour x (L/m(2))); NS]. There was significant variation in transvascular LDL transport between diabetic patients with chronic renal disease, nondiabetic patients with chronic renal disease, and healthy control patients [5.0 (3.2-7.8) vs. 3.0 (2.2-3.8) vs. 4.2 (3.6-4.8) %/hour; P < 0.01 after adjustment for distribution volume of LDL]. This variation was unlikely caused by altered hepatic LDL receptor expression or glycosylation of LDL in diabetes patients.

CONCLUSION

Transvascular LDL transport may be increased in diabetic patients with chronic renal disease, suggesting that lipoprotein flux into the arterial wall is increased. A similar mechanism does not operate in nondiabetic patients with chronic renal disease.

Increased transvascular low density lipoprotein transport in insulin dependent diabetes: a mechanistic model for development of atherosclerosis

BACKGROUND

The increased risk of atherosclerosis associated with diabetes cannot be explained by conventional cardiovascular risk factors alone. We hypothesized that transvascular lipoprotein transport may be increased in patients with diabetes, possibly explaining increased intimal lipoprotein accumulation and thus atherosclerosis.

METHODS

We used an in vivo method for measurement of transvascular transport of low density lipoprotein (LDL) and applied it in 24 patients with insulin dependent diabetes mellitus (type 1) and in 30 healthy controls. LDL was individually sampled and autologous 131-iodinated LDL was reinjected intravenously in addition to 125-iodinated albumin, and the 1-h fractional escape rates were taken as indices of transvascular transport.

RESULTS

Transvascular LDL transport was 5.7+/-2.2 and 4.0+/-1.8%/h in patients with diabetes and controls (P<0.005); equivalent values for albumin were 6.8+/-2.5 and 5.4+/-2.0%/h (P<0.05). This difference most likely was not caused by altered hepatic LDL receptor expression, glycosylation of LDL, small LDL size, nephropathy, statin use, or different plasma insulin levels in diabetes patients.

CONCLUSION

Transvascular LDL transport may be increased in patients with type 1 diabetes. This suggests that lipoprotein flux into the arterial wall is increased in people with type 1 diabetes, possibly explaining accelerated development of atherosclerosis.

Dyslipidaemia

The lowering of serum cholesterol is increasingly recognised as essential in the prevention of coronary heart disease and other atherosclerotic disease. The success of statin trials and the need to deploy these drugs effectively in the population has led increasingly to the identification of many people whose serum cholesterol, triglycerides, and HDL-cholesterol require clinical assessment, and frequently treatment. Lipid disorders are mainly straightforward, but some are complex or resistant to simple treatment strategies. I have reviewed the clinical manifestations of disordered lipid metabolism (dyslipidaemia) and its management.

Diabetic dyslipidaemia: from basic research to clinical practice

The recognition that the increase of plasma triglyceride rich lipoproteins (TRLs) is associated with multiple alterations of other lipoproteins species that are potentially atherogenic has expanded the picture of diabetic dyslipidaemia. The discovery of heterogeneity within major lipoprotein classes VLDL, LDL and HDL opened new avenues to reveal the specific pertubations of diabetic dyslipidaemia. The increase of large VLDL 1 particles in Type 2 diabetes initiates a sequence of events that generates atherogenic remnants, small dense LDL and small dense HDL particles. Together these components comprise the atherogenic lipid triad. Notably the malignant nature of diabetic dyslipidaemia is not completely shown by the lipid measures used in clinical practice. The key question is what are the mechanisms behind the increase of VLDL 1 particles in diabetic dyslipidaemia? Despite the advances of recent years, our understanding of VLDL assembly and secretion is still surprisingly incomplete. To date it is still unclear how the liver is able to regulate the amount of triglycerides incorporated into VLDL particles to produce either VLDL 1 or VLDL 2 particles. The current evidence suggests that the machinery driving VLDL assembly in the liver includes (i) low insulin signalling via PI-3 kinase pathway that enhances lipid accumulation into "nascent " VLDL particles (ii) up-regulation of SREBP-1C that stimulates de novo lipogenesis and (iii) excess availability of "polar molecules" in hepatocytes that stabilizes apo B 100. Recent data suggest that all these steps could be fundamentally altered in Type 2 diabetes explaining the overproduction of VLDL apo B as well as the ability of insulin to suppress VLDL 1 apo B production in Type 2 diabetes. Recent discoveries have established the transcription factors including PPARs, SREBP-1 and LXRs as the key regulators of lipid assembly in the liver. These observations suggest these factors as a new target to tailor more efficient drugs to treat diabetic dyslipidaemia.