Catabolism of lipoprotein-X induced by infusion of 10% fat emulsion

Controversy over the atherogenicity of lipoprotein-X

PURPOSE OF REVIEW

Lipoprotein-X (Lp-X) is an abnormal lipoprotein containing abundant free cholesterol and phospholipids, as well as some apolipoprotein E (apoE). Serum Lp-X increases in patients with cholestasis and lecithin-cholesterol acyltransferase deficiency, as well as in those receiving intravenous lipid emulsion. Lp-X is often associated with skin xanthomas in cholestatic patients. However, earlier studies showed that Lp-X is not taken up by murine macrophages, but exerts antiatherogenic actions. In this review, we discuss the heterogeneity of Lp-X and its potential atherogenicity.

RECENT FINDINGS

Mass spectrometry revealed that Lp-X of cholestatic patients is similar in lipid composition to low-density lipoprotein (LDL) and high-density lipoprotein, but not to bile acids, suggesting that Lp-X is synthesized in the liver. Palmar xanthomas appear in patients with cholestasis, but regress over months after improvement of hypercholesterolemia. Lp-X isolated from cholestatic patients is rich in apoE, and causes more lipid accumulation than oxidized LDL and acetyl LDL in human monocyte-derived macrophages.

SUMMARY

Lp-X is heterogeneous in apoE content. Lp-X is taken up in cholestatic patients by apoE-recognizing lipoprotein receptors. Further research is warranted to fully understand the atherogenicity of Lp-X and the clinical significance of elevated Lp-X concentrations, particularly in cholestatic patients.

Review: The lipemia of sepsis: triglyceride-rich lipoproteins as agents of innate immunity

Bacterial endotoxin (LPS) elicits dramatic responses in the host including elevated plasma lipid levels due to the increased synthesis and secretion of triglyceride (TG)-rich lipoproteins by the liver, and the inhibition of lipoprotein lipase. This cytokine-induced hyperlipoproteinemia, clinically termed the `lipemia of sepsis', was customarily thought to represent the mobilization of lipid stores to fuel the host response to infection. However, since lipoproteins can also bind and neutralize LPS, we hypothesize that TG-rich lipoproteins (VLDL and chylomicrons) are also components of an innate, non-adaptive host immune response to infection. Herein we review data demonstrating the capacity of lipoproteins to bind LPS, protect against LPS-induced toxicity, and modulate the overall host response to this bacterial toxin. Lastly, we propose a pathway whereby lipoprotein-bound LPS may represent a novel, endogenous mechanism for regulating the hepatic acute phase response.

Triglyceride-rich lipoproteins as agents of innate immunity

Bacterial endotoxin (i.e., lipopolysaccharide [LPS]) elicits dramatic responses in the host, including elevated plasma lipid levels due to increased synthesis and secretion of triglyceride-rich lipoproteins by the liver and inhibition of lipoprotein lipase. This cytokine-induced hyperlipoproteinemia, clinically termed the "lipemia of sepsis," was customarily thought to involve the mobilization of lipid stores to fuel the host response to infection. However, because lipoproteins can also bind and neutralize LPS, we have long postulated that triglyceride-rich lipoproteins (very-low-density lipoproteins and chylomicrons) are also components of an innate, nonadaptive host immune response to infection. Recent research demonstrates the capacity of lipoproteins to bind LPS, protect against LPS-induced toxicity, and modulate the overall host response to this bacterial toxin.

Effects of a medium-chain triacylglycerol/long-chain triacylglycerol fat emulsion containing a reduced ratio of phospholipid to triacylglycerol in pediatric surgical patients

OBJECTIVE

Medium-chain triacylglycerol (MCT) has been shown to provide better nutritional support than long-chain triacylglycerol (LCT). We compared the efficacy of MCT/LCT fat emulsions containing a usual (0.12) or a decreased (0.06) ratio of phospholipid to triacylglycerol (PL:TG) in pediatric patients under surgical stress.

METHODS

Three patient groups (n=10 in each) received equivalent amounts of glucose (12 g.kg(-1).d(-1)) and amino acids (2 g.kg(-1).d(-1)), but group A received a 10% MCT fat emulsion (PL:TG 0.06), group B received a 20% MCT fat emulsion (PL:TG 0.06), and group C received a 10% MCT/LCT fat emulsion (PL:TG 0.12) in amounts of 1.5 g.kg(-1).d(-1) in a randomized study. Total parenteral nutrition was given for 7 d. Blood samples were collected before total parenteral nutrition administration and on days 4 and 7 for determination of various biochemical indexes.

RESULTS

Serum phospholipid concentrations were significantly higher in group C than in group A or B on days 4 and 7 (P<0.05). Serum triacylglycerol and cholesterol concentrations and the very-low-density lipoprotein percentage were also significantly higher in group C than in group A or B on days 4 and 7 (P<0.05). The high-density lipoprotein percentage was significantly higher in group B on days 4 and 7 (P<0.05).

CONCLUSIONS

In pediatric patients under surgical stress, a total parenteral nutrition regimen containing an MCT/LCT fat emulsion with a decreased PL:TG ratio (0.06) is likely to result in partly better lipid and lipoprotein metabolism than an emulsion containing the usual ratio (0.12).

Severe Hypercholesterolemia Associated with Decreased Hepatic Triglyceride Lipase Activity and Pseudohyponatremia in Patients after Allogeneic Stem Cell Transplantation

A 55-year-old woman with Ph-negative acute lymphoblastic leukemia in primary induction failure received allogeneic peripheral blood stem cell transplantation from her HLA-compatible sister. Pseudohyponatremia developed due to extreme hypercholesterolemia of 4091 mg/dL accompanied by lipoprotein X and lipoprotein Y. The hypercholesterolemia was caused by cholestasis due to chronic GVHD and ischemic cholangiopathy. In addition, we found that hepatic triglyceride lipase (HTGL) activity was severely decreased, which could be another novel factor causing extreme hypercholesterolemia after allogeneic transplantation. The total cholesterol has been gradually decreasing followed by the improvement of cholestasis with bezafibrate, ursodeoxycholic acid and prednisone treatments, and by a slight increase in HTGL-protein. To our knowledge, this is the first report to describe the association of decreased HTGL with extreme hypercholesterolemia after allogeneic transplantation.

Endogenous lipoproteins impact the response to endotoxin in humans

OBJECTIVE

To determine whether endogenous lipoproteins can abrogate the host response to lipopolysaccharide (LPS) in vivo.

DESIGN

Randomized, placebo-controlled study.

SETTING

Urban public hospital with academic affiliation.

SUBJECTS

Eighteen healthy, normolipidemic, normal weight volunteers, 21-35 yrs of age.

INTERVENTIONS

Fasting and postprandial (hypertriglyceridemic) subjects were injected with endotoxin (LPS, Lot EC-5, 4 ng/kg = 20 endotoxin units/kg) as either a bolus or following preincubation of the LPS with autologous whole blood vs. saline. In addition, LPS-induced cytokine production was determined ex vivo to examine the capacity of fasting vs. hypertriglyceridemic whole blood to attenuate the effect of large, potentially lethal concentrations of LPS in humans.

MEASUREMENTS

Vital signs were recorded and serial blood samples analyzed for changes in white blood cell count, cytokine, and stress hormone levels over 24 hrs. The distribution of lipoproteins in fasting and postprandial blood after preincubation was determined using 125I-LPS.

MAIN RESULTS

Endogenous lipoproteins abrogated the host response to LPS in vivo, but only after preincubation with the LPS. Peak oral temperature (p < .05) and white blood cell count (p < .05), and plasma tumor necrosis factor (TNF)-alpha (p < .01) and adrenocorticotropic hormone levels (p < .03) were significantly reduced in volunteers injected with LPS preincubated with whole blood vs. LPS preincubated with saline. Approximately 80% of the LPS was bound to lipoproteins after preincubation with either fasting or hypertriglyceridemic blood. Thus, protection was associated with lipoprotein binding. In addition, hypertriglyceridemic but not fasting blood inhibited the ex vivo TNF-alpha response to large, highly toxic doses of LPS (p < .05). Without the preincubation of lipoproteins with LPS, there was a trend for an exaggerated clinical and TNF-alpha response in the hypertriglyceridemic subjects.

CONCLUSION

Preincubation of LPS with whole blood promotes lipoprotein-LPS binding and is associated with an attenuated response to this toxic macromolecule. Although the clinical relevance of these data requires further elucidation, our results continue to support a lipid-based therapeutic strategy to combat gram-negative sepsis.

APA hamster model for diabetic atherosclerosis. 2. Analysis of lipids and lipoproteins

Syrian hamsters of the APA strain (APA hamsters) have recently been shown to have atheromatous lesions in the aortic arches under diabetic condition induced by a single injection of streptozotocin (SZ). In that model, fatty streaks, which are the initial lesions of atherogenesis, develop by 6 weeks after the injection (WAI). In this study, we evaluated plasma lipid concentrations and lipoprotein profiles in diabetic APA hamsters at 6 WAI to reveal the early stage of atherogenesis clinicopathologically. As a result, by biochemical analysis, hyperglycemic APA hamsters showed signs of hypercholesterolemia and hypertriglyceridemia. Low-density lipoprotein (LDL) cholesterol significantly increased, but high-density lipoprotein (HDL) cholesterol significantly decreased. Agarose gel electrophoresis showed an obvious increase in the fractions of chylomicron, LDL and abnormal lipoprotein. Plasma LDL in diabetic animals was in a state more susceptible to oxidization. In addition, a significant increase in glycated LDL was also found in the diabetic animals by enzyme linked immunosorbent assay (ELISA). Moreover, lipid peroxidation product (4-hydroxynonenal (4 HNE))-adducted proteins and advanced glycation end-products (AGE) were immunohistochemically detected in the foam cells of the fatty streaks. These results revealed that diabetic APA hamsters had hyperlipidemia characterized by increases in chylomicron, LDL and abnormal lipoprotein, and suggested that oxidized LDL and/or glycated LDL might be actively uptaken by macrophages and play an important role in the initial stage of atherogenesis.

Effect of intravenous lipid emulsions on hepatic cholesterol metabolism

BACKGROUND

Total parenteral nutrition offers the chance of survival to children who have had extensive gut resections or gut failure. However, in infants it is often associated with serious complications including cholestatic liver disease. The causes of these complications remain unclear, although it has been suggested that the lipid emulsions used in total parenteral nutrition may be responsible.

METHODS

An in vitro system was developed to study the effect of lipid emulsions on hepatic cholesterol metabolism using cultured hepatocytes.

RESULTS

Incubations of Hep G2 cells with medium containing Intralipid (Pharmacia and Upjohn, Milton Keynes, UK) demonstrated that the fat emulsion mediated a powerful dose-dependent but reversible inhibition of cholesterol uptake. In addition Intralipid was shown to stimulate the efflux of cholesterol from Hep G2 cells. The component or components of the Intralipid responsible for these effects and the mechanism by which they act remain to be established.

CONCLUSIONS

Intravenous lipid emulsions may interfere with hepatic cholesterol metabolism in vivo. This may have implications for the development of total parenteral nutrition-associated cholestasis in neonates.

Nutrition in the intensive care unit

Nutritional support has become a routine part of the care of the critically ill patient. It is an adjunctive therapy, the main goal of which is to attenuate the development of malnutrition, yet the effectiveness of nutritional support is often thwarted by an underlying hostile metabolic milieu. This requires that these metabolic changes be taken into consideration when designing nutritional regimens for such patients. There is also a need to conduct large, multi-center studies to acquire more knowledge of the cost-benefit and cost effectiveness of nutritional support in the critically ill.

Structure and metabolic fate of triacylglycerol- and phospholipid-rich particles of commercial parenteral fat emulsions

The lipid emulsions used in parenteral nutrition are constituted of particles rich in triacylglycerols (TAG) called artificial chylomicrons (200-500 nm in diameter; monolayer of phospholipids [PL] enveloping a TAG core) and PL-rich particles called liposomes (diameter inferior to 80 nm; bilayer of PL around an aqueous phase), which represent the excess emulsifier. Introduced into the circulation, the two populations of particles come into contact with circulating lipoproteins and cell membranes and experience the same overall fate: exchanges and transfers of lipids and apolipoproteins, enzymatic hydrolysis of TAG and PL, and internalization by different tissues. The relative importance of these different metabolic processes varies depending on the type of particle. The artificial chylomicrons undergo a hydrolysis of their TAG by lipoprotein lipase, with a release of fatty acids and formation of smaller particles of remnants, which are rapidly removed by the liver. In delivering fatty acids to the tissue, artificial chylomicrons fulfill an energy transport function similar to the natural chylomicrons. The liposomes hold little energy interest, and they also have deleterious effects when infused in excess. They inhibit the lipolysis of artificial chylomicrons and, by actively capturing endogenous cholesterol, they stimulate tissue cholesterogenesis and accumulate in the blood as lipoprotein-X, a long-lived abnormal lipoprotein. To limit as much as possible the metabolic perturbations due to the intravenous administration of exogenous PL, the emulsion has to be infused at a low rate, and should contain the minimal amount of excess PL.

Clearance of artificial triacylglycerol particles

In this review we discuss the metabolism of parenteral emulsions in relation to their natural counterpart, the chylomicrons. A major reaction is lipoprotein lipase-mediated hydrolysis of triglycerides at the vascular endothelium in extrahepatic tissues. The lipase is retained at the cell surface by interactions with heparan sulfate proteoglycans but can move along the surface. Lipoproteins and emulsion particles are initially steered to the endothelium by electrostatic forces. These weak interactions are reinforced by recruitment of lipase molecules. Small particles, whether injected as such or formed as remnants of larger particles, are catabolized mainly through receptor-mediated endocytosis in the liver. In contrast, many of the larger particles are removed by other, less well defined, mechanisms.