Pathophysiology of lipoprotein transport

Genetic determinants of cardiometabolic risk: a proposed model for phenotype association and interaction

This review provides a translational and unifying summary of metabolic syndrome genetics and highlights evidence that genetic studies are starting to unravel and untangle origins of the complex and challenging cluster of disease phenotypes. The associated genes effectively express in the brain, liver, kidney, arterial endothelium, adipocytes, myocytes, and β cells. Progression of syndrome traits has been associated with ectopic lipid accumulation in the arterial wall, visceral adipocytes, myocytes, and liver. Thus, it follows that the genetics of dyslipidemia, obesity, and nonalcoholic fatty liver disease are central in triggering progression of the syndrome to overt expression of disease traits and have become a key focus of interest for early detection and for designing prevention and treatments. To support the "birds' eye view" approach, we provide a road-map depicting commonality and interrelationships between the traits and their genetic and environmental determinants based on known risk factors, metabolic pathways, pharmacologic targets, treatment responses, gene networks, pleiotropy, and association with circadian rhythm. Although only a small portion of the known heritability is accounted for and there is insufficient support for clinical application of gene-based prediction models, there is direction and encouraging progress in a rapidly moving field that is beginning to show clinical relevance.

Specific and scavenger low-density lipoprotein receptors involved in the disturbed lipid metabolism of patients with non-insulin-dependent diabetes mellitus are independent of obesity

Comparative studies were performed on monocyte-derived macrophages (MDMs), prepared by a 72-hour incubation of blood monocytes obtained from patients with non-insulin-dependent diabetes mellitus (NIDDM) and age-matched obese and non-obese controls. The MDMs, after a 72-hour culturing, expressed both specific and scavenger low-density lipoprotein (LDL) receptors on their surfaces. To study the binding capacity of both receptor types, [125I]LDL and [125I] acetylated LDL (acLDL) were applied to cells and the labeled ligands were then monitored to estimate the rate of intracellular degradations. The LDL-induced inhibition of endogenous cholesterol synthesis and the acLDL-triggered apolipoprotein (apo) E secretion were also studied, as the biological marker of receptor activation. The results indicate that the binding capacities of both specific and scavenger LDL receptors were not reduced in MDMs of diabetic patients. However, the intracellular degradation after LDL incorporation was decreased. The LDL-induced inhibition of cholesterol synthesis and the acLDL-transmitted apo E secretion were also found to be decreased in the MDMs of patients with NIDDM as compared with the obese and non-obese control groups. The NIDDM-induced impaired signal transduction of both specific and scavenger LDL receptors suggests an unclarified functional alteration of both receptor structures.

Cholesterol concentrations in diabetic patients in Nnewi, Nigeria

Twenty out-patient and twenty in-patient adult Nigerian type II diabetics and twenty healthy subjects matched for sex, age and weight, were studied at Nnamdi Azikiwe University Teaching Hospital, Nnewi, Nigeria. Mean total cholesterol and fasting blood glucose concentrations were higher in the diabetics. The out-patients had significantly higher total cholesterol and fasting blood glucose values than their in-patient counterparts. The possible cardiovascular risk of the raised cholesterol level in the diabetics was discussed. We recommend that for effective management of diabetic patients, periodic measurement of cholesterol level is necessary. An integrated enlightenment programme to acquaint the out-patients with the health benefits of dietary and drug compliance and occasional hospitalisation for better monitoring would be desirable.

Longitudinal study of cyclosporine and lipids in patients undergoing bone marrow transplantation

The metabolic effects of intravenous cyclosporine on lipids and lipoproteins were studied in 29 allogeneic bone marrow recipients compared with 13 autologous bone marrow patients not requiring cyclosporine therapy. Patients were monitored continuously from 5 days prior to 27 days following transplantation; cyclosporine treatment was initiated 4 days before transplantation. Fasting lipid and lipoprotein levels were measured in serial blood samples throughout the study period. Nutritional supplementation, conditioning regimens and concomitant medications were not significantly different between groups. Furthermore, no significant differences in age, weight, lipid, or lipoprotein levels were found at baseline between the patient groups. Cholesterol, triglyceride, low-density lipoprotein cholesterol, and very low-density lipoprotein cholesterol levels remained unchanged in autologous patients. As compared with baseline values, plasma total cholesterol increased by an average of 26 percent in allogeneic transplantation patients receiving cyclosporine. Similarly, the ratio of low-density lipoprotein to high-density lipoprotein cholesterol was fourfold greater in those patients treated with cyclosporine compared to the autologous group. We conclude that cyclosporine appears to elevate cholesterol levels. Neither acute graft vs host disease nor changes in hepatic function could explain the differences in plasma cholesterol levels between groups.

Probucol: pharmacology and clinical application

Increased levels of cholesterol, LDL-cholesterol, and VLDL-cholesterol are known risk factors for the development of atherosclerotic vascular disease. Probucol is an orally active agent that can reduce total cholesterol and LDL-cholesterol. The drug also reduces HDL-cholesterol. However, it may modify the lipoprotein in such a way that removal of cholesterol from peripheral tissues is still enhanced. Probucol also has antioxidant activity, which may inhibit the oxidative modification of LDL that contributes to lipid deposition in blood vessel walls. Probucol is a well tolerated agent. However, its ability to prolong life in human subjects still needs to be determined.

Lovastatin: an HMG-CoA reductase inhibitor for lowering cholesterol

Increased levels of cholesterol, LDL-cholesterol, and VLDL-cholesterol are known risk factors for the development of coronary artery disease. There are multiple drugs that can be used for lowering cholesterol, including lovastatin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme step in cholesterol synthesis in the body. The pharmacology of this novel agent is discussed in this article.

Lovastatin: a new cholesterol-lowering agent

Lovastatin is a potent new drug for lowering serum cholesterol through inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the rate-limiting enzyme for cholesterol biosynthesis. Metabolic studies with lovastatin in healthy volunteers and patients with hypercholesterolemia suggest reduced synthesis of low-density lipoprotein cholesterol (LDL-C) as well as enhanced catabolism LDL-C mediated through LDL receptors as the principal mechanisms for lipid-lowering effects. Total cholesterol and LDL-C are reduced by 30% or more on average when added to baseline therapy, with the effects being more pronounced in nonfamilial than in familial hypercholesterolemia. Optimal dosing appears to be 20 mg given twice a day. The most common adverse effects are gastrointestinal, while the most serious are elevated transaminase levels and the potential for lens opacities. Lovastatin is the first of a new class of lipid-lowering agents, and is effective when added to diet therapy or in combination with other drugs.

Prevalence and clinical correlates of beta-migrating very-low-density lipoprotein. Lipid Research Clinics Program Prevalence Study

The prevalence and clinical associations of beta-migrating very-low-density lipoprotein (beta-very-low-density lipoprotein), an atherogenic lipoprotein associated with type III hyperlipidemia, were compared in men and women using and not using estrogen-containing hormones. Beta-very-low-density lipoprotein was more common in men, 0.54 percent or one in 185, than in women not taking hormones, 0.30 percent or one in 333, and was least common in women taking hormones, 0.04 percent or one in 2,500. Beta-very-low-density lipoprotein-positive men and women were heavier and had higher triglyceride, cholesterol, and very-low-density lipoprotein cholesterol levels compared with beta-very-low-density lipoprotein-negative subjects, but 30 percent of the men and 21 percent of the nonuser women did not have elevated triglyceride or cholesterol levels. The very-low-density lipoprotein cholesterol/total triglyceride ratio exceeded 0.30 in 55 percent and 54 percent of beta-very-low-density lipoprotein-positive men and women compared with 3.3 percent and 3.9 percent of beta-very-low-density lipoprotein-negative men and women. Ninety-three percent of the beta-very-low-density lipoprotein-positive subjects had some lipoprotein lipid abnormality and 60 percent had an abnormal glucose, liver, or renal function test result or high alcohol or dietary cholesterol intake. In conclusion, beta-migrating very-low-density lipoprotein is more common in men than in women, consistent with a protective effect of estrogen. About half of the time, beta-very-low-density lipoprotein is associated with an elevated very-low-density lipoprotein cholesterol/total triglyceride ratio, indicating that beta-very-low-density lipoprotein can be present independent of other markers of type III hyperlipidemia. Nonetheless, beta-very-low-density lipoprotein signifies abnormal lipoprotein metabolism of some kind in almost every instance, is frequently associated with treatable medical conditions, and may on pathophysiologic grounds be considered important in its own right in predicting atherosclerosis risk. Beta-very-low-density lipoprotein-associated conditions should attract medical attention.

Evidence for kinetic heterogeneity among human low density lipoproteins

The kinetics of low density lipoprotein apolipoprotein B (LDL apo B) metabolism are usually determined using turnover techniques in which radioiodinated LDL apo B is injected as a bolus into plasma, and serial plasma and urinary radioactivity samples are taken. The metabolic parameter of interest usually estimated from such data is the fractional catabolic rate (FCR). Two methods are normally employed to obtain an estimate of the FCR. One, the so-called Matthews' analysis, assumes plasma LDL apo B metabolism can be described by a single plasma pool while the other is determined by calculating the ratio of urinary radioactivity excreted to mean plasma radioactivity per day. Both of these methods assume LDL apo B is kinetically homogeneous, thus ignoring the evidence that LDL is biochemically heterogeneous in some individuals. If this biochemical heterogeneity manifests itself as kinetic heterogeneity, then the use of these data to estimate the FCR will not permit the resolution of the finer details of potential metabolic defects. This paper addresses the question of kinetic homogeneity and heterogeneity of LDL apo B within the context of several integrated kinetic models of increasing complexity. Each model fits reasonably the turnover data and hence cannot be rejected on the basis of failure to be compatible with the data. However, the models have strikingly different physiologic interpretations while providing essentially the same estimate for the FCR. Thus LDL apo B metabolism appears to be more complex than originally believed, and the models provide a framework within which to design new experiments to distinguish among them.

The use of heparin-sepharose affinity chromatography to separate two distinct lipoproteins from the low density lipoprotein fraction of rat plasma

Heparin-Sepharose affinity chromatography resolved rat lipoproteins isolated in the density range 1.019-1.063 g/ml into two distinct fractions. The first, which eluted with 150 mM NaCl, was shown by immunodiffusion and by polyacrylamide gel electrophoresis to contain predominantly apo E. On this basis the major lipoprotein present is identified as high density lipoprotein1 (HDL1). The second, eluted with 325 mM NaCl, contains predominantly apoprotein B by the same techniques. The main lipoprotein present is therefore identified as low density lipoprotein (LDL). The lipid composition of the two lipoproteins is similar. Both are rich in cholesterol ester, with a free to ester cholesterol ratio of about 1 to 2.5. From lipid and protein analyses on the two fractions, the plasma concentrations of HDL1 and LDL can be calculated to be about 14 and 12 mg/100 ml, respectively. Their mean diameters are 13 +/- 2 nm and 18 +/- 5 nm, respectively.

Lipoprotein lipase and hepatic lipase activity after heparin administration in abetalipoproteinemia and hypobetalipoproteinemia

The purpose of this study was to examine whether an absence of triglyceride-rich lipoproteins (chylomicrons and very-low-density lipoproteins) in plasma is associated with any changes in the enzyme activity of lipoprotein lipase or hepatic lipase after heparin administration. To study this, the activities of hepatic lipase and lipoprotein lipase were determined in control subjects, in two patients with heterozygous hypobetalipoproteinemia, and in three patients with phenotypic abetalipoproteinemia after administration of heparin. Both enzymes showed normal activity in the patients with hypobetalipoproteinemia, but showed consistently reduced activity in the patients with abetalipoproteinemia. Hepatic lipase activity in plasma samples from these three patients obtained 15 minutes after intravenous injection of heparin was 55%, 87%, and 46% of that of the controls, whereas corresponding values in plasma samples obtained 30 minutes after heparin were 47%, 70%, and 57%, respectively. Lipoprotein lipase activity in the three patients with abetalipoproteinemia was 46%, 29%, and 34% of that of the controls in the samples obtained 15 minutes after heparin injection, whereas the values obtained after 30 minutes were 53%, 64%, and 47% of that of the controls. We conclude that an inherent absence of triglyceride-rich lipoproteins, as occurs in abetalipoproteinemia, is associated with reduced enzyme activity of both hepatic lipase and lipoprotein lipase in plasma after heparin administration.

Nutritional care of the critically ill patient

Nutritional care is a supportive measure. Just as ventilatory support does not "cure" underlying pulmonary disease, nutritional support may not in itself resolve the disease that causes inadequate intake of nutrients. In the setting of multimodality care in the ICU, the provision of calories and protein to maintain the body cell mass is the initial goal. Such an approach will minimize feeding complications yet prevent nutritional debility. An acceptably low rate of complications is possible but can only be achieved by strict attention to detail. When the acute phase of the disease resolves, more vigorous nutritional support is possible and will restore body composition to normal.

Morphological fatty changes and function of the liver, serum free fatty acids, and triglycerides during parenteral nutrition

Morphological fatty changes and function of the liver and serum free fatty acids and triglycerides were investigated in 37 catabolic patients (22 men, 15 women) given different parenteral nutrition regimens. In the glucose group energy was supplied as carbohydrate alone, in the lipid group as carbohydrates and fats, and in addition both groups received amino acids. In the amino acid group amino acids were given in excess and less energy was supplied as carbohydrates. Each patient served as his own control. During parenteral nutrition liver steatosis rose from 5% to 35% (p less than 0.001) in the glucose group and from 7% to 23% (p less than 0.01) in the amino acid group, but no increase occurred in the lipid group. Liver fat accumulation was associated with the rises in serum aminotransferase activities and with the lack of or a poor rise in serum prothrombin and proconvertin. The conjugation function of the liver was not disturbed. No cholestatis was found. During lipid infusion serum free fatty acids increased to 4.41 mmol/l (p less than 0.01) and serum triglycerides to 3.06 mmol/l (p less than 0.01), but they decreased to normal range 12 h after lipid infusion was stopped. In the glucose and amino acid groups serum free fatty acid levels fell, as expected, below the normal range. Serum triglycerides rose 1.4-fold (p less than 0.05) in the amino acid group. On the basis of liver tests and histological examination steatosis in the liver caused only a minor disturbance in hepatocellular integrity. The very high levels of serum free fatty acids and triglycerides during lipid infusion may be harmful in certain pathological states.

Severe hypertriglyceridaemia responding to insulin and nicotinic acid therapy

A patient with unusually severe hypertriglyceridaemia (serum concentration initially 258 mmol/l or 22600 mg/dl) and hypercholesterolaemia is reported and discussed. The triglyceride elevation was found to reside within the very low density lipoprotein fraction and was probably attributable to the combination of diabetes mellitus and familial hypertriglyceridaemia. Treatment with insulin and restriction of dietary carbohydrate led to a 50% reduction in the triglyceride concentration, and the addition of nicotinic acid in modest doses led ultimately to a complete normalization of the patient's lipid values. A close correlation was noted between the falling triglyceride concentration and the rising serum sodium concentration during the course of successful therapy. Overall, it is felt likely that this patient's severe and reversible hypertriglyceridaemia was on the basis of excessively rapid lipolysis leading to high concentrations of very low density lipoprotein production. Combined therapy with insulin and nicotinic acid is recommended for other patients of this nature.

Hormonal and substrate responses to a standard meal in normal and hypertriglyceridemic subjects

Postprandial plasma insulin, glucose, growth hormone, cortisol and free fatty acid responses to a standard mixed meal were determined and compared to fasting plasma triglyceride levels in normal and hypertriglyceridemic subjects. The hypertriglyceridemic subjects had significantly higher postprandial plasma insulin levels than did normals, but no significant differences were seen between the two groups to any of the other variables. Postprandial plasma insulin levels were highly correlated to fasting plasma triglyceride levels, but there was no significant correlation between any of the other variables and fasting plasma triglyceride levels.

Depression of glucose utilization by Intralipid in the post-traumatic period: an experimental study

We investigated the accelerated clearance of Intralipid (IL) in the immediate post-traumatic period and the influence of the concomitant rise in fatty acid ((FFA) metabolism on carbohydrate tolerance. As a result we postulate that intermediary products of fatty acid oxidation inhibit key enzymes in the glycolytic pathway. The fatty acidemia and its metabolic sequlae can be avoided by intermittent Intralipid supplementation (at low rates) during TPN. It will assure (1) a larger carbohydrate-to-fat caloric ratio during infusion and (2) cyclical regeneration of the enzyme systems involved in lipid metabolism.

Why very low density lipoprotein levels are normal in familial hyperchylomicronaemia

The anomalous finding that very low density lipoprotein levels are relatively normal in patients with familial hyperchylomicronaemia has never been satisfactorily explained, particularly in view of the marked reduction or absence of peripheral lipoprotein lipase activity characteristic of this condition. I propose that the discrepancy between the plasma levels of the two triglyceride-rich lipoprotein fractions in these patients is due to the secretion by the liver of triglyceride in the form of chylomicron-like particles, rather than as very low density lipoprotein. The proposed "switch" in the spectrum of lipoproteins secreted by the liver is probably contingent upon the activity of the hepatic lipase present on the liver cell plasma membrane.