Severe Hypertriglyceridemia, large lipoproteins and protection against atherosclerosis

Remnant cholesterol and risk of myocardial infarction in patients with coronary artery disease undergoing revascularization

BACKGROUND

Despite substantial reduction in low-density lipoprotein cholesterol (LDL-C), patients develop recurrent cardiovascular events. Remnant cholesterol (RC), the cholesterol content of triglyceride-rich lipoproteins, is a potential contributor to this residual risk.

OBJECTIVES

To investigate the association between RC and risk for myocardial infarction (MI) in patients with coronary artery disease, and examine whether the predictive value of RC is retained beyond non-high-density lipoprotein cholesterol (non-HDL-C).

METHODS

Data on 9451 patients undergoing coronary revascularization in a single center. RC was calculated as total cholesterol minus high-density lipoprotein cholesterol minus LDL-C (estimated using Martin-Hopkins equation). Cox-regression models were used to estimate the association between RC and risk for MI. Discordance analyses were performed to examine the correlation between RC and non-HDL-C (or LDL-C) in relation to MI risk.

RESULTS

Mean age was 65±11 years; 67% presented with acute coronary syndrome. During median follow-up of 9.6 years, 1690 patients developed MI. After multivariable adjustment including lipid-lowering therapies and non-HDL-C, RC was associated with higher MI risk: hazard ratio (95% confidence interval): 1.36 (1.20-1.56) and 1.58 (1.35-1.85) in those with RC levels ≥75th (32.6 mg/dL) and ≥90th (41.8 mg/dL) percentile, compared to RC <50th percentile (25.5 mg/dL). When RC and non-HDL-C (or LDL-C) levels were discordant, the level of RC better reflected the risk for MI.

CONCLUSIONS

Elevated RC is a risk factor for MI independent of lipid-lowering therapies and non-HDL-C, providing further support that RC may serve as a residual cardiovascular risk marker and potential treatment target in patients with coronary artery disease.

Clinical features and outcomes of severe, very severe, and extreme hypertriglyceridemia in a regional health service

BACKGROUND

Comprehensive data on severe hypertriglyceridemia (HTG) in the general population setting are limited and of importance due to the increase in metabolic risk factors and novel therapies under development.

OBJECTIVE

To investigate contributing causes and outcomes of severe to extreme HTG.

METHODS

Regional database retrospectively analyzed for subjects with severe HTG. Adverse outcomes were investigated in correlation to HTG severity, with follow-up initiating at first documentation of HTG > 1000 mg/dL.

RESULTS

A total of 3091 subjects with severe (peak triglycerides 1000-1999 mg/dL; n = 2590), very severe (2000-2999 mg/dL; n = 369), and extreme (≥3000 mg/dL; n = 132) HTG were identified. Mean age was 48 ± 12 years; 73% males. Obesity (48%) and diabetes (62%) were main contributing factors. During follow-up (median 101 months), 4.7% subjects had pancreatitis, 4.7% myocardial infarction, and 6% stroke. Compared with severe HTG, the multivariate-adjusted hazard ratio for pancreatitis was 3.22 (95% confidence interval 2.21-4.70) for individuals with very severe HTG and 5.55 (3.53-8.71) for those with extreme HTG, P < .0001. In contrast, the extent of HTG severity at these levels was not associated with worse cardiovascular outcomes or death. Most subjects (81%) achieved triglyceride levels <500 mg/dL, associated with lower risk for developing pancreatitis but not myocardial infarction or stroke.

CONCLUSIONS

Severity of HTG is closely related to cardiometabolic conditions, with a stepwise increase in the risk for pancreatitis, particularly if not attaining reduced triglyceride levels during the follow-up. In contrast, whereas mild-to-moderate HTG is a known established cardiovascular risk factor, very severe and extreme HTG may not further increase the risk for myocardial infarction, stroke, or mortality.

Triglycerides and heart disease: still a hypothesis?

The purpose of this article is to review the basic and clinical science relating plasma triglycerides and cardiovascular disease. Although many aspects of the basic physiology of triglyceride production, its plasma transport, and its tissue uptake have been known for several decades, the relationship of plasma triglyceride levels to vascular disease is uncertain. Are triglyceride-rich lipoproteins, their influence on high-density lipoprotein and low-density lipoprotein, or the underlying diseases that lead to defects in triglyceride metabolism the culprit? Animal models have failed to confirm that anything other than early fatty lesions can be produced by triglyceride-rich lipoproteins. Metabolic products of triglyceride metabolism can be toxic to arterial cells; however, these studies are primarily in vitro. Correlative studies of fasting and postprandial triglycerides and genetic diseases implicate very-low-density lipoprotein and their remnants and chylomicron remnants in atherosclerosis development, but the concomitant alterations in other lipoproteins and other risk factors obscure any conclusions about direct relationships between disease and triglycerides. Genes that regulate triglyceride levels also correlate with vascular disease. Human intervention trials, however, have lacked an appropriately defined population and have produced outcomes without definitive conclusions. The time is more than ripe for new and creative approaches to understanding the relationship of triglycerides and heart disease.

On the anti-atherogenic effect of the antioxidant BHT in cholesterol-fed rabbits: inverse relation between serum triglycerides and atheromatous lesions

We have shown that inclusion of the antioxidant butylated hydroxytoluene (BHT) in the diet protects against development of atherosclerotic lesions in cholesterol-fed rabbits. In parallel, BHT treatment results in increased plasma triglyceride levels. The present study explores the relationship between the triglyceride-inducing and protective effects of BHT in two different studies. The combined material contains 22 rabbits fed cholesterol and 18 rabbits fed cholesterol in combination with 1% BHT. In the BHT group there was an inverse relationship between triglyceride exposure/cholesterol exposure and extent of lesions with r=0.74 (P=0.0005). Our results show that increased triglyceride exposure parallels the anti-atherogenic effect of BHT. There was no significant correlation between atheromatosis and serum BHT levels. beta-very low density lipoprotein (beta-VLDL) from cholesterol and BHT animals was triglyceride-enriched and smaller compared to beta-VLDL from cholesterol-fed animals, but there was no significant association between the anti-atherogenic effect of BHT and particle size or apolipoprotein pattern of LDL or beta-VLDL. LDL isolated from rabbits treated with cholesterol and BHT was less sensitive to oxidative modification than LDL isolated from rabbits treated with cholesterol only. In conclusion, our results demonstrate that the degree of triglyceride exposure may be an important modulator of the anti-atherogenic effect of an antioxidant.

Potential downregulation of HMG-CoA reductase after prolonged administration of P-407 in C57BL/6 mice

This study investigated the potential alteration in the amount of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase messenger RNA (mRNA) and lipoprotein lipase (LPL) mRNA in the livers of C57BL/6 mice after long-term (200 days) treatment with the nonionic surfactant called poloxamer 407 (P-407). Previously, P-407 has been used to produce a dose-controlled hyperlipidemic state in C57BL/6 mice with subsequent formation of atherosclerotic lesions. Five groups of mice were studied; controls (C); mice fed a standard chow diet enriched with only cholic acid (CH); mice fed the high-cholesterol, high-fat Paigen diet (HF); mice treated with 0.5 g/kg P-407 every third day (P); and mice administered 0.5 g/kg P-407 every third day while consuming a diet identical to that of mice in group CH (PC). Neither a significant (p < 0.05) weight loss nor alteration in liver enzymes (AST and ALT) were observed for any group throughout the study when compared with the control mice. Total plasma cholesterol (CHOL) was significantly elevated compared with controls for mice in groups HF, P, and PC, whereas total plasma triglycerides (TG) were significantly increased for mice in only groups P and PC. Long-term ingestion of a high-fat diet or a diet enriched in cholic acid resulted in a significant (p < 0.05) reduction in HDL-CHOL when compared with controls. Plasma samples assayed at 200 days for mice in groups HF and P showed a shift in the lipoprotein fraction distribution primarily to VLDL-CHOL as compared with mice in group C in which, as expected, most of the CHOL was contained in the HDL fraction. The biologic activity of HMG-CoA reductase assayed in hepatic microsomal homogenates was significantly reduced for mice in groups CH (p < 0.01), HF (p < 0.01), and PC (p < 0.05), but not for mice in group P, when compared with control. A statistical analysis of the data demonstrated significant (p < 0.05) reductions in the HMG-CoA reductase mRNA levels in hepatic tissue for all treatment groups relative to mRNA levels determined for mice in group C. In contrast, no treatment group demonstrated a significant difference in hepatic LPL mRNA levels when compared with mRNA levels determined for control animals. These data demonstrate that P-407 administration to C57BL/6 mice significantly decreased the amount of HMG-CoA reductase mRNA detected in liver.

Effect of induced hypercholesterolemia in rabbits on functional responses of isolated large proximal and small distal coronary arteries

We studied the effects of hypercholesterolemia on the vascular responses of proximal and distal parts of the rabbit coronary circulation in two consecutive studies. For 12 weeks, New Zealand White rabbits were fed a control diet or a diet with 1% cholesterol dissolved in either 3% coconut oil (study A) or ether (study B). Isolated proximal epicardial and distal intramyocardial coronary arteries from control and hypercholesterolemic rabbits were mounted for isometric tension recording in a double myograph. In study A for hypercholesterolemic rabbits (n = 12), the maximal relaxation and sensitivity to acetylcholine (ACh) were significantly decreased in proximal coronary segments contracted with 30 mmol/l potassium solution compared with segments from control rabbits (n = 13). The only change observed in distal coronary segments was a slight decrease in relaxation in response to low ACh concentrations (10(-8) and 3 x 10(-8) mol/l). However, in study B for proximal coronary and distal coronary segments from hypercholesterolemic rabbits (n = 13), the area under the ACh relaxation curve was increased compared with that of control rabbits (n = 12). Other parameters that were similarly affected in studies A and B include the following: 1) proximal coronary segments from hypercholesterolemic rabbits were more sensitive to sodium nitroprusside (SNP) than were those from control rabbits, but this was not true for distal coronary segments; 2) endothelial removal from arterial segments of control rabbits induced a significant increase in sensitivity and maximal relaxation to SNP of proximal coronary and distal coronary arteries; 3) in segments from hypercholesterolemic rabbits, the absence of endothelium did not alter the response of proximal coronary segments to SNP but did augment the relaxation of distal coronary segments to SNP; 4) the maximal response to 5-hydroxytryptamine in proximal coronary arteries from hypercholesterolemic rabbits was increased compared with those from control rabbits, whereas such changes were not observed in distal coronary arteries; and 5) histological examination showed the presence of atheromatous plaques in proximal coronary but not in distal coronary segments from treated animals. In conclusion, the present investigation demonstrates that induced hypercholesterolemia alters both the structure and function of proximal parts of the coronary circulation. In distal coronary arteries of hypercholesterolemic rabbits, the only change observed was an impaired endothelium-dependent cholinergic relaxation, but even this change appeared to be dependent on the manner in which cholesterol was added to the diet, although parallel studies are required to confirm this.

The antioxidant butylated hydroxytoluene protects against atherosclerosis

Rabbits fed a 1% cholesterol diet with or without the antioxidant butylated hydroxytoluene (BHT) developed typical atherosclerotic lesions. The addition of BHT gave higher levels of total cholesterol (+40%), triglycerides (+250%), low density lipoprotein (LDL), and very low density lipoprotein (VLDL) in plasma. Despite the lower plasma lipid levels, the degree of atherosclerosis of the aortic surface was considerably higher in rabbits fed cholesterol than in the group treated with cholesterol and BHT. The mean atherosclerotic involvement was 18.6 +/- 4.4% in the former group and 5.9 +/- 1.7% in the latter group (p = 0.02). In all animals, there was a high correlation between the area of the arterial lesion and cholesterol content (r = 0.96). Serum levels of cholesterol autooxidation products (7-ketocholesterol and cholesterol 5 alpha,6 alpha-epoxide) were lower in the group of rabbits treated with BHT (p less than 0.005). Serum levels of vitamin E were slightly higher in the BHT group. There was no significant difference in the clearance of beta-VLDL between the two treatment groups after using either beta-VLDL from cholesterol-fed animals or beta-VLDL from BHT-fed animals. The results are in accord with the contention that oxidative modification of lipoproteins is important for the development of atherosclerosis and that antioxidants may have a protective effect. At present, however, other explanations cannot be completely excluded, for example, effects of antioxidants on immunologic factors or monocyte adhesion.

Dehydroepiandrosterone feeding prevents aortic fatty streak formation and cholesterol accumulation in cholesterol-fed rabbit

The concentration of dehydroepiandrosterone sulfate (DHEA-S) in human plasma is higher than any other steroid. Recent evidence has suggested an inverse relationship between plasma DHEA levels and the development of coronary atherosclerosis in humans. We used the cholesterol-fed rabbit model to investigate whether DHEA feeding would diminish aortic fatty streak formation in this model. Fifteen New Zealand White rabbits were fed rabbit chow supplemented with 0.5% cholesterol (wt/wt). Seven animals were, in addition, fed DHEA, 0.5% of diet (wt/wt). Animals were sacrificed after 2 months, and the aortic involvement with fatty streaks was evaluated by computerized planimetry of Sudan IV-stained aortas and by chemical analysis of aortic wall lipids. Compared to controls, DHEA-fed animals had similar plasma levels of total, very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) cholesterol, corticoids, and estrogens. DHEA-fed animals had higher plasma levels of total, VLDL, and LDL triglycerides and lower HDL triglycerides than did controls. DHEA feeding resulted in 30% and 40%, respectively, inhibition of fatty streak formation by chemical analysis and planimetry. We conclude that DHEA feeding inhibits the development of aortic fatty streaks in cholesterol-fed rabbits, independent of changes in plasma total and LDL cholesterol levels of DHEA conversion to estrogens or corticoids.