Circulating Lipoprotein Concentrations Research Articles

Bilirubin Decreases Macrophage Cholesterol Efflux and ATP-Binding Cassette Transporter A1 Protein Expression.

BackgroundMild but chronically elevated circulating unconjugated bilirubin is associated with reduced total and low‐density lipoprotein cholesterol concentration, which is associated with reduced cardiovascular disease risk. We aimed to investigate whether unconjugated bilirubin influences macrophage cholesterol efflux, as a potential mechanism for the altered circulating lipoprotein concentrations observed in hyperbilirubinemic individuals.Methods and ResultsCholesterol efflux from THP‐1 macrophages was assessed using plasma obtained from normo‐ and hyperbilirubinemic (Gilbert syndrome) humans (n=60 per group) or (heterozygote/homozygote Gunn) rats (n=20 per group) as an acceptor. Hyperbilirubinemic plasma from patients with Gilbert syndrome and Gunn rats induced significantly reduced cholesterol efflux compared with normobilirubinemic plasma. Unconjugated bilirubin (3–17.1 μmol/L) exogenously added to plasma‐ or apolipoprotein A1–supplemented media also decreased macrophage cholesterol efflux in a concentration‐ and time‐dependent manner. We also showed reduced protein expression of the ATP‐binding cassette transporter A1 (ABCA1), a transmembrane cholesterol transporter involved in apolipoprotein A1–mediated cholesterol efflux, in THP‐1 macrophages treated with unconjugated bilirubin and in peripheral blood mononuclear cells obtained from hyperbilirubinemic individuals. Furthermore, we demonstrated that bilirubin accelerates the degradation rate of the ABCA1 protein in THP‐1 macrophages.ConclusionsCholesterol efflux from THP‐1 macrophages is decreased in the presence of plasma obtained from humans and rats with mild hyperbilirubinemia. A direct effect of unconjugated bilirubin on cholesterol efflux was demonstrated and is associated with decreased ABCA1 protein expression. These data improve our knowledge concerning bilirubin's impact on cholesterol transport and represent an important advancement in our understanding of bilirubin's role in cardiovascular disease.

Circulating Lipoprotein Concentrations Correlate with Total but Not Free Heme in Different Sickle Cell Disease Genotypes

In chronic hemolytic states, hemoglobin is released in the bloodstream and free heme is formed upon hemoglobin oxidation. Free heme has been shown to trigger vaso-occlusion in animal models of sickle cell disease (SCD) by activating endothelial cells through toll-like receptor 4 (TLR4). Serum levels of glycoproteins specialized in clearing hemoglobin and heme, such as haptoglobin and hemopexin, are typically depleted in SCD plasma. High-density (HDL) and low-density lipoproteins (LDL) can also bind heme and work as the first line of defense against heme toxicity. Heme can react with LDL and generate oxidized LDL (ox-LDL), another TLR4 ligand.We aimed to investigate the importance of lipoproteins as heme scavengers by comparing SCD patients with homozygous sickle cell anemia (SS) patients and double heterozygous hemoglobin SC disease (SC) patients, who present with higher and lower hemolytic rates, respectively.Eighty-three patients (aged 18-68 years old, 44 female) participated in this study: 43 SS (12 receiving hydroxyurea - HU) and 40 SC. Exclusion criteria were pregnancy, blood transfusion, pain crisis, or use of lipid lowering drugs in the past 3 months. Peripheral venous blood samples were collected in tubes with EDTA for complete blood counts and without antiocoagulant for serum separation after centrifugation at 500g for 20 min. Serum lactate dehydrogenase, total cholesterol and HDL were measured by conventional enzymatic-colorimetric methods on a Roche Hitachi Modular automation system (Roche, Germany). LDL was calculated by Friedewald equation. Serum oxidized LDL (ox-LDL), total and free heme were determined with commercially available ELISA assays (Mercodia Oxidized LDL ELISA, Heme QuantiChrom Assay, Bioassay Systems, USA). Statistical analysis was performed with SPSS v.22.0.LDH and reticulocyte count were significantly higher in SS patients than SC, and hemoglobin was significantly lower (P =0.001). Total heme was significantly higher in SS patients taking HU (SSHU) (90 ± 59 mM) compared with SC patients (66 ± 18 mM), P =0.01. Total cholesterol was higher in SC than in SS patients, 141 ± 28 vs. 107 ± 20 mg/dL, respectively, P =0.01). We found no significant differences between groups regarding LDL (P =0.08), HDL (P =0.10), ox-LDL (P =0.52). Total heme correlated negatively with total cholesterol (r=-0.33, P =0.037) and HDL (r=-0.49, P =0.001) in the whole population, regardless of genotype or treatment with HU. Surprisingly, free heme did not differ between SS, SSHU, or SC groups of patients (P=0.19), and did not correlate with cholesterol levels.The negative correlation between total heme and total cholesterol supports that lipoproteins contribute to defend the body against heme toxicity. Despite HDL, LDL, and ox-LDL not being significantly different among the groups, the stronger negative correlation between total heme and HDL suggests that this cholesterol fraction may be more important in heme scavenging. We hypothesize that more intense hemolysis may lead to the production of heme capable of oxidizing lipoproteins, which can then be rapidly cleared by macrophages, explaining hypocholesterolemia in SS patients. Similar concentrations of free heme and ox-LDL in all groups suggests that, in steady state, their concentrations are kept relatively constant regardless of hemolytic rate. Since circulating concentrations of these TLR4 ligands did not differ between SC and SS patients in our study, this might indicate that the contribution of hemolysis to endothelial dysfunction and to differences in severity and in the variety of complications between these genotypes may be better explained by TLR4 activation by bound heme rather than by activation by free heme or ox-LDL.Financial support: FAPESP/CAPES/CNPq DisclosuresNo relevant conflicts of interest to declare.

Modulation of lipid synthesis, apolipoprotein biogenesis, and lipoprotein assembly by butyrate.

Short-chain fatty acids (SCFAs) are potent modulators of the growth, function, and differentiation of intestinal epithelia. In addition, high-fiber diets may protect against the development of atherosclerosis because of their cholesterol-lowering effects due, in large part, to SCFA production, liver sterol metabolism, and bile acid excretion. Although the small gut plays a major role in dietary fat transport and contributes substantially to plasma cholesterol and lipoprotein homeostasis, the impact of SCFAs on intestinal lipid handling remains unknown. In the present study, the modulation of lipid synthesis, apolipoprotein biogenesis, and lipoprotein secretion by butyrate was investigated in Caco-2 cells plated on permeable polycarbonate filters, which permit separate access to the upper and lower compartments of the monolayers. Highly differentiated and polarized cells (20 days of culture) were incubated for 20 h with 20 mM butyrate in the apical medium. In the presence of [14C]oleic acid, butyrate led to a significant reduction of secreted, labeled triglycerides (27%; P < 0.01) and phospholipids (25%; P < 0.05). Similarly, butyrate significantly decreased the incorporation of [14C]acetate into exported cholesteryl ester (49%; P < 0.005). As expected from these results, with [14C]oleic acid as a precursor, butyrate significantly (P < 0.05) diminished the delivery of radiolabeled chylomicrons and very low-density lipoproteins. In parallel, [35S]methionine pulse labeling of Caco-2 cells revealed the concomitant inhibitory effect of butyrate on the synthesis of apolipoproteins B-48 (28%; P < 0.05) and A-I (32%; P < 0.01). Collectively, our data indicate that butyrate may influence lipid metabolism in Caco-2 cells, thus suggesting a potential regulation of intestinal fat absorption and circulating lipoprotein concentrations.

Effects of Treatment of Secondary Hyperparathyroidism on the Lipid Profile in Patients on Hemodialysis

Abnormalities in circulating lipoprotein concentrations are a characteristic finding in patients undergoing dialytic therapy. A substantial number of these patients display type IV hyperlipoproteinemia. Certain data suggest that secondary hyperparathyroidism may induce disturbances in lipid metabolism. To evaluate the effects of pulse calcitriol therapy on the lipid profile in these patients, we undertook a prospective study in 12 patients on stable bicarbonate hemodialysis. Lipid parameters comprising cholesterol and the low- as well as the high-density lipoprotein subfractions, triglycerides, apolipoproteins A and B, serum parathyroid hormones (iPTH), alkaline phosphatase, calcium, phosphorus, hematocrit, and blood urea were obtained prior to commencement of pulse calcitriol therapy and again 8–10 weeks later. Calcitriol therapy was associated with a decrease in serum iPTH levels (701 ± 103.9 vs. 220.3 ± 50.5 pmol/l; p < 0.001). Significant increases in high-density lipoprotein cholesterol (32.8 ± 2.7 vs. 38.8 ± 2.3 mmol/l; p < 0.05) and apolipoprotein A-I (107.8 ± 6.1 vs. 121.8 ± 5.8 g/l; p < 0.05) were noted during the course of the study. Moreover, serum iPTH correlated inversely with high-density lipoprotein cholesterol and apolipoprotein A-I. There were no changes in other lipid parameters except for low-density lipoprotein cholesterol which showed a tendency to increase. We conclude that in short-term study, pulse oral calcitriol therapy is associated with an improvement in the lipid profile in patients with secondary hyperparathyroidism. However, it remains to be established whether ameliorating the uremic dyslipidemia results in any long-term clinical benefits.

Lipoprotein Metabolism in Normal and Uremic Patients

Abnormalities in circulating lipoprotein concentrations are a characteristic finding both in patients with uremia and in patients undergoing dialysis. These patients tend to have elevated triglyceride (TG) concentrations and low concentrations of high-density lipoprotein (HDL) cholesterol. Elevations of low-density lipoprotein (LDL) are not usually observed unless the patients have undergone renal transplantation and are receiving therapy with immune suppressive medications. Hypertriglyceridemia and low HDL may be the consequence of decreased actions of lipoprotein lipase (LPL), the endothelial cell-bound enzyme that degrades circulating lipoprotein triglyceride. A poorly characterized circulating inhibitor to this enzyme is found in uremic plasma. Preliminary data suggest that high-flux dialysis with polysulfone (PS) membranes improves the lipoprotein abnormalities and decreases circulating LPL inhibitors. Whether such therapy will alter the incidence of coronary morbidity and mortality in patients with end-stage renal failure remains to be tested.

The effect of portacaval shunt on hepatic lipoprotein metabolism in familial hypercholesterolemia

The hyperlipidemia observed in familial hypercholesterolemia can be reduced by portacaval anastomosis. We report the effects of a portacaval shunt on hepatic morphology and biosynthetic pathways crucial to hepatic cholesterol homeostasis in homozygous receptor-negative familial hypercholesterolemia. Portacaval anastomosis was associated with a dramatic change in hepatocyte morphology, 28% reduction in plasma low-density lipoprotein concentration, and a decrease in hepatic total and free cholesterol content by 27 and 75%, respectively. Furthermore, the rate-limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase was decreased by 56%. Finally, the reduced binding of low-density lipoproteins to hepatic membranes preoperatively was increased following the portacaval shunt. These combined results indicate that the changes in circulating lipoprotein concentrations observed after portacaval shunt are due to alterations in the metabolic consequences of the defective recognition of lowdensity lipoproteins by the liver of familial hypercholesterolemic subjects.