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.     

Infection induces a positive acute phase apolipoprotein E response from a negative acute phase gene: role of hepatic LDL receptors

Apolipoprotein E (apoE) plays important roles in lipid homeostasis, anti-inflammation, and host defense. Since tissue apoE mRNA levels have been reported to decrease during inflammatory responses, we were surprised to find that plasma apoE levels were significantly elevated during septic infections in both humans and mice. This apparent paradox was also observed during lipopolysaccharide-induced acute inflammation in mice: plasma levels of apoE increased up to 4-fold despite sharply decreased apoE gene expression in the liver, macrophages, and extrahepatic tissues. We hypothesized that apoE levels were augmented by decreased plasma clearance. Our analysis revealed that apoE associated principally with HDL in mice and that apoE was cleared from the circulation principally via LDL receptors. The acute inflammatory response decreased LDL receptor expression in the liver and significantly reduced the rate of apoE clearance. In contrast, the same inflammatory stimuli increased LDL receptor expression in macrophages. Our results define a novel acute phase mechanism that increases circulating apoE levels as apoE production decreases. Diminished hepatic LDL receptor expression may thus cooperate with elevated LDL receptor expression in macrophages to facilitate the forward transport of apoE and its associated lipids to these key defense cells.     

HDL and electronegative LDL exchange anti- and pro-inflammatory properties

Electronegative LDL [LDL(–)] is a minor modified LDL subfraction present in blood with inflammatory effects. One of the antiatherogenic properties of HDL is the inhibition of the deleterious effects of in vitro modified LDL. However, the effect of HDL on the inflammatory activity of LDL(–) isolated from plasma is unknown. We aimed to assess the putative protective role of HDL against the cytokine released induced in monocytes by LDL(–). Our results showed that LDL(–) cytokine release was inhibited when LDL(–) was coincubated with HDL and human monocytes and also when LDL(–) was preincubated with HDL and reisolated prior to cell incubation. The addition of apoliprotein (apo)AI instead of HDL reproduced the protective behavior of HDL. HDL preincubated with LDL(–) promoted greater cytokine release than native HDL. Incubation of LDL(–) with HDL decreased the electronegative charge, phospholipase C-like activity, susceptibility to aggregation and nonesterified fatty acid (NEFA) content of LDL(–), whereas these properties increased in HDL. NEFA content in LDL appeared to be related to cytokine production because NEFA-enriched LDL induced cytokine release. HDL, at least in part through apoAI, inhibits phospholipase-C activity and cytokine release in monocytes, thereby counteracting the inflammatory effect of LDL(–). In turn, HDL acquires these properties and becomes inflammatory.     

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.     

Interstitial fluid lipoproteins

While a wide variety of techniques has been used to collect samples of interstitial fluid, most of our detailed knowledge about the composition of interstitial fluid lipoproteins has come from lymph collection studies. The considerable variability of lymph data probably reflects the effect of variable metabolic modification and different capillary permeabilities on the lipoprotein composition of interstitial fluid. All density classes of plasma lipoproteins are present in lymph. In peripheral lymph, the lymph/plasma concentration ratios of lipoproteins vary from 0.03 for VLDL-sized particles to 0.2 for HDL. Lymph from more permeable vascular beds, such as lung and myocardium, contains proportionately more lipoproteins. Their lymph/plasma concentration ratios vary from 0.1 to 0.6. In general, lymph lipoproteins are more heterogeneous in size than their plasma counterparts. Lymph HDL and LDL contain larger and smaller particles than their plasma equivalents. Lymph lipoproteins have unusual shapes (square packing and discoidal), chemical compositions, and molecular charge, which suggest de novo formation and/or extensive peripheral modification. Lymph HDL and LDL are enriched in free cholesterol. Lymph HDL also has increased cholesterol/protein and phospholipid/protein (especially sphingomyelin) ratios (Sloop, C.H., L. Dory, and P.S. Roheim, unpublished observations). Lymph HDL apoprotein composition differs from that of plasma, with an increase in apoE and apoA-IV content relative to apoA-I. These discoidal HDL particles may be products of an initial stage of reverse cholesterol transport. We believe further study of their metabolic fate would give important information concerning the later stages of reverse cholesterol transport.     

An apoA-I mimetic peptide containing a proline residue has greater in vivo HDL binding and anti-inflammatory ability than the 4F peptide

Modifying apolipoprotein (apo) A-I mimetic peptides to include a proline-punctuated α-helical repeat increases their anti-inflammatory properties as well as allows better mimicry of full-length apoA-I function. This study compares the following mimetics, either acetylated or biotinylated (b): 4F (18mer) and 4F-proline-4F (37mer, Pro). b4F interacts with both mouse HDL (moHDL) and LDL in vitro. b4F in vivo plasma clearance kinetics are not affected by mouse HDL level. Administration of biotinylated peptides to mice demonstrates that b4F does not associate with lipoproteins smaller than LDL in vivo, though it does associate with fractions containing free hemoglobin (Hb). In contrast, bPro specifically interacts with HDL. b4F and bPro show opposite binding responses to HDL by surface plasmon resonance. Administration of acetylated Pro to apoE^−/− mice significantly decreases plasma serum amyloid A levels, while acetylated 4F does not have this ability. In contrast to previous reports that inferred that 4F associates with HDL in vivo, we systematically examined this potential interaction and demonstrated that b4F does not interact with HDL in vivo but rather elutes with Hb-containing plasma fractions. bPro, however, specifically binds to moHDL in vivo. In addition, the number of amphipathic α-helices and their linker influences the anti-inflammatory effects of apoA-I mimetic peptides in vivo.     

Enhanced vascular permeability facilitates entry of plasma HDL and promotes macrophage-reverse cholesterol transport from skin in mice

Reverse cholesterol transport (RCT) pathway from macrophage foam cells initiates when HDL particles cross the endothelium, enter the interstitial fluid, and induce cholesterol efflux from these cells. We injected [³H]cholesterol-loaded J774 macrophages into the dorsal skin of mice and measured the transfer of macrophage-derived [³H]cholesterol to feces [macrophage-RCT (m-RCT)]. Injection of histamine to the macrophage injection site increased locally vascular permeability, enhanced influx of intravenously administered HDL, and stimulated m-RCT from the histamine-treated site. The stimulatory effect of histamine on m-RCT was abolished by prior administration of histamine H1 receptor (H1R) antagonist pyrilamine, indicating that the histamine effect was H1R-dependent. Subcutaneous administration of two other vasoactive mediators, serotonin or bradykinin, and activation of skin mast cells to secrete histamine and other vasoactive compounds also stimulated m-RCT. None of the studied vasoactive mediators affected serum HDL levels or the cholesterol-releasing ability of J774 macrophages in culture, indicating that acceleration of m-RCT was solely due to increased availability of cholesterol acceptors in skin. We conclude that disruption of the endothelial barrier by vasoactive compounds enhances the passage of HDL into interstitial fluid and increases the rate of RCT from peripheral macrophage foam cells, which reveals a novel tissue cholesterol-regulating function of these compounds.     

Composition, morphology and distribution of high-density lipoproteins in plasma and peripheral lymph: effect of feeding cholesterol and saturated fat

In euthyroid dogs fed a diet rich in cholesterol and saturated fat, the cholesterol concentration in both plasma and peripheral lymph increased progressively with the appearance of HDLc (d 1.006-1.063). This HDLc fraction was heterogeneous and could be separated into 'slow' and 'fast' migrating fractions by Pevikon block electrophoresis. On SDS-polyacrylamide gel electrophoresis, plasma 'slow' HDLc was appreciably enriched in apolipoprotein (apo) E, while plasma and lymph 'fast' HDLc were apo E-poor. In contrast, no apo E was visible in lymph 'slow' HDLc in either plasma or lymph HDL2 fractions (d 1.087-1.21). The interstitial HDL fractions containing apo A-IV ('fast' HDLc and HDL2) were also rich in free cholesterol, implying that apo A-IV-containing particles are involved in reverse cholesterol transport. Plasma and peripheral lymph HDL2 and 'fast' HDLc cholesterol/protein ratios were not different, whereas lymph 'slow' HDLc was 24% that of plasma, indicating that interstitial 'slow' HDLc was poor in cholesterol compared to plasma. This marked reduction in lymph 'slow' HDLc cholesterol suggests that this particle was either selectively retarded from egress by the endothelial barrier, or that interstitial 'slow' HDLc represents a depleted particle involved in the delivery of cholesterol to peripheral tissues. These findings taken together support the hypothesis that interstitial 'slow' HDLc may represent a particle involved in cholesterol ester delivery, in contrast with HDL2 and 'fast' HDLc, which could serve as an efflux acceptor of tissue free cholesterol. This study demonstrates significant heterogeneity of interstitial peripheral lymph lipoproteins compared to plasma lipoproteins, and indicates selective distribution of these particles in the extravascular space.     

Effects of reconstituted HDL on charge-based LDL subfractions as characterized by capillary isotachophoresis

Modified LDL in human plasma including small, dense LDL (sdLDL) and oxidized LDL carries a more negative charge than unmodified LDL and is atherogenic. We examined the effects of apolipoprotein A-I (apoA-I)/POPC discs on charge-based LDL subfractions as determined by capillary isotachophoresis (cITP). Three normal healthy subjects and seven patients with metabolic disorders were included in the study. LDL in human plasma was separated into two major subfractions, fast- and slow-migrating LDL (fLDL and sLDL), by cITP. Normal LDL was characterized by low fLDL, and mildly oxidized LDL in vitro and mildly modified LDL in human plasma were characterized by increased fLDL. Moderately oxidized LDL in vitro and moderately modified LDL in a patient with hypertriglyceridemia and HDL deficiency were characterized by both increased fLDL and a new LDL subfraction with a faster mobility than fLDL [very-fast-migrating LDL as determined by cITP (vfLDL)]. cITP LDL subfractions with faster electrophoretic mobility (fLDL vs. sLDL, vfLDL vs. fLDL) were associated with an increased content of sdLDL. Incubation of a plasma fraction with d>1.019 g/ml (depleted of triglyceride-rich lipoproteins) in the presence of apoA-I/POPC discs at 37 degrees C greatly decreased vfLDL and fLDL but increased sLDL. Incubation of whole plasma from patients with an altered distribution of cITP LDL subfractions in the presence of apoA-I/POPC discs also greatly decreased fLDL but increased sLDL. ApoA-I/POPC discs decreased the cITP fLDL level, the free cholesterol concentration, and platelet-activating factor acetylhydrolase activity in the sdLDL subclasses (d=1.040-1.063 g/ml) and increased the size of LDL. ApoA-I/POPC discs reduced charge-modified LDL in human plasma by remodeling cITP fLDL into sLDL subfractions.     

Effects of apolipoprotein B-100 on the metabolism of a lipid microemulsion model in rats

In previous studies, it was shown that lipid microemulsions resembling LDL (LDE) but not containing protein, acquire apolipoprotein E when injected into the bloodstream and bind to LDL receptors (LDLR) using this protein as ligand. Aiming to evaluate the effects of apolipoprotein (apo) B-100 on the catabolism of these microemulsions, LDE with incorporated apo B-100 (LDE-apoB) and native LDL, all labeled with radioactive lipids were studied after intraarterial injection into Wistar rats. Plasma decay curves of the labels were determined in samples collected over 10 h and tissue uptake was assayed from organs excised from the animals sacrificed 24 h after injection. LDE-apo B had a fractional clearance rate (FCR) similar to native LDL (0.40 and 0.33, respectively) but both had FCR pronouncedly smaller than LDE (0.56, P<0.01). Liver was the main uptake site for LDE, LDE-apoB, and native LDL, but LDE-apoB and native LDL had lower hepatic uptake rates than LDE. Pre-treatment of the rats with 17α-ethinylestradiol, known to upregulate LDLR, accelerated the removal from plasma of both LDE and LDE-apoB, but the effect was greater upon LDE than LDE-apoB. These differences in metabolic behavior documented in vivo can be interpreted by the lower affinity of LDLR for apo B-100 than for apo E, demonstrated in in vitro studies. Therefore, our study shows in vivo that, in comparison with apo E, apo B is a less efficient ligand to remove lipid particles such as microemulsions or lipoproteins from the intravascular compartment.     

Thematic review series: Patient-Oriented Research. What have we learned about HDL metabolism from kinetics studies in humans?

Plasma measurements of lipids, lipoproteins, and apolipoproteins provide information on the static levels of these fractions without providing key information on the dynamic fluxes of lipoproteins in the circulation. Kinetics studies, in contrast, provide additional information on the production and clearance rates of lipoproteins and the flow of lipids and apolipoproteins through lipoprotein fractions. This information is crucial in accurately delineating the metabolism of HDL in plasma, because plasma concentrations of HDL are the net result of the de novo production and catabolism of HDL as well as the recycling of HDL particles and the contribution to HDL from components of other lipoproteins. Studies aimed at measuring the metabolism of HDL particles have shown that HDL metabolism in vivo is complex and consists of multiple components. Kinetics studies provide a window into the metabolism of HDL, allowing us to better understand the mechanisms of HDL decrease in human conditions and the functionality of HDL particles. Here, we review the progress in our understanding of HDL metabolism derived from in vivo kinetics studies, focusing primarily on studies in humans but also reviewing key studies in animal models.     

High-level lipoprotein [a] expression in transgenic mice: evidence for oxidized phospholipids in lipoprotein [a] but not in low density lipoproteins

Efforts to elucidate the role of lipoprotein [a] (Lp[a]) in atherogenesis have been hampered by the lack of an animal model with high plasma Lp[a] levels. We produced two lines of transgenic mice expressing apolipoprotein [a] (apo[a]) in the liver and crossed them with mice expressing human apolipoprotein B-100 (apoB-100), generating two lines of Lp[a] mice. One had Lp[a] levels of approximately 700 mg/dl, well above the 30 mg/dl threshold associated with increased risk of atherosclerosis in humans; the other had levels of approximately 35 mg/dl. Most of the LDL in mice with high-level apo[a] expression was covalently bound to apo[a], but most of the LDL in the low-expressing line was free. Using an enzyme-linked sandwich assay with monoclonal antibody EO6, we found high levels of oxidized phospholipids in Lp[a] from high-expressing mice but not in LDL from low-expressing mice or in LDL from human apoB-100 transgenic mice (P <0.00001), even though all mice had similar plasma levels of human apoB-100. The increase in oxidized lipids specific to Lp[a] in high-level apo[a]-expressing mice suggests a mechanism by which increased circulating levels of Lp[a] could contribute to atherogenesis.     

Oxidized lipoprotein lipids and atherosclerosis

Plasma lipoproteins contain variable amounts of lipid oxidation products (LOP), which are known to impair normal physiological functions and stimulate atherosclerotic processes. Recent evidence indicates that plasma lipoproteins are active carriers of LOP, low-density lipoprotein (LDL) directing transport toward peripheral tissues, and high-density lipoprotein (HDL) being active in the reverse transport. It has been proposed that the lipoprotein-specific transport of LOP could play a role in atherosclerosis-related effects of LDL and HDL. This article gives an overview of the present knowledge of lipoprotein LOP transport and its association with the risk of atherosclerosis and cardiovascular diseases (CVD). Evidence of the significance of lipoprotein LOP transport comes mainly from studies of physiological oxidative stress and is supported by studies of the functionality apolipoprotein A-1 mimetic peptides. A large body of data has accumulated indicating that lipoprotein LOP transport is connected to the risk of atherosclerosis. While high levels of LOP carried by LDL are indicative of elevated risk, high LOP level in HDL appears to associate with protection. If confirmed, the proposed lipoprotein LOP transport function would affect conception of the etiology of atherosclerosis, but would not conflict current views of the pathophysiological mechanisms. It could open new perspectives, such as the dietary origin of LOP, and the protective function of HDL in clearance of LOP. Focusing on LOP could give additional tools especially for prevention and diagnosis, but would not radically change the management of atherosclerosis and CVD.     

Visualization of the binding, endocytosis, and transcytosis of low- density lipoprotein in the arterial endothelium in situ

We investigated the interaction and transport of low-density lipoprotein (LDL) through the arterial endothelium in rat aorta and coronary artery, by perfusing in situ native, untagged human, and rat LDL. The latter was rendered electron-opaque after it interacted with the endothelial cell and was subsequently fixed within tissue. We achieved LDL electron-opacity by an improved fixation procedure using 3,3'-diaminobenzidine, and mordanting with tannic acid. The unequivocal identification of LDL was implemented by reacting immunocytochemically the perfused LDL with anti LDL-horseradish peroxidase conjugate. Results indicate that LDL is taken up and internalized through two parallel compartmented routes. (a) A relatively small amount of LDL is taken up by endocytosis via: (i) a receptor-mediated process (adsorptive endocytosis) that involved coated pits/vesicles, and endosomes, and, probably, (ii) a receptor-independent process (fluid endocytosis) carried out by a fraction of plasmalemmal vesicles. Both mechanisms bringing LDL to lysosomes supply cholesterol to the endothelial cell itself. (b) Most circulating LDL is transported across the endothelial cell by transcytosis via plasmalemmal vesicles which deliver LDL to the other cells of the vessel wall. Endocytosis is not enhanced by increasing LDL concentration, but the receptor-mediated internalization decreases at low temperature. Transcytosis is less modified by low temperature but is remarkably augmented at high concentration of LDL. While the endocytosis of homologous (rat) LDL is markedly more pronounced than that of heterologous (human) LDL, both types of LDL are similarly transported by transcytosis. These results indicate that the arterial endothelium possesses a dual mechanism for handling circulating LDL: by a high affinity process, endocytosis secures the endothelial cells' need for cholesterol; by a low-affinity nonsaturable uptake process, transcytosis supplies cholesterol to the other cells of the vascular wall, and can monitor an excessive accumulation of plasma LDL. Since in most of our experiments we used LDL concentrations above those found in normal rats, we presume that at low LDL concentrations saturable high-affinity uptake would be enhanced in relation to nonsaturable pathways.     

Changes in lipoprotein subfraction concentration and composition in healthy individuals treated with the CETP inhibitor anacetrapib

We investigated the effects of the cholesteryl ester (CE) transfer protein inhibitor anacetrapib (ANA) on plasma lipids, lipoprotein subfraction concentrations, and lipoprotein composition in 30 healthy individuals. Participants (n = 30) were randomized to ANA 20 mg/day, 150 mg/day, or placebo for 2 weeks. Changes in concentration of lipoprotein subfractions were assessed using ion mobility, and compositional analyses were performed on fractions separated by density gradient ultracentrifugation. ANA 150 mg/day versus placebo resulted in significant decreases in LDL-cholesterol (26%) and apo B (29%) and increases in HDL-cholesterol (82%). Concentrations of medium and small VLDL, large intermediate density lipoprotein (IDL), and medium and small LDL (LDL2a, 2b, and 3a) decreased whereas levels of very small and dense LDL4b were increased. There was enrichment of triglycerides and reduction of CE in VLDL, IDL, and the densest LDL fraction. Levels of large buoyant HDL particles were substantially increased, and there was enrichment of CE, apo AI, and apoCIII, but not apoAII or apoE, in the mid-HDL density range. Changes in lipoprotein subfraction concentrations and composition with ANA 20 mg/day were similar to those for ANA 150 mg/day but were generally smaller in magnitude. The impact of these changes on cardiovascular risk remains to be determined.     

Dalcetrapib and anacetrapib increase apolipoprotein E-containing HDL in rabbits and humans

The large HDL particles generated by administration of cholesteryl ester transfer protein inhibitors (CETPi) remain poorly characterized, despite their potential importance in the routing of cholesterol to the liver for excretion, which is the last step of the reverse cholesterol transport. Thus, the effects of the CETPi dalcetrapib and anacetrapib on HDL particle composition were studied in rabbits and humans. The association of rabbit HDL to the LDL receptor (LDLr) in vitro was also evaluated. New Zealand White rabbits receiving atorvastatin were treated with dalcetrapib or anacetrapib. A subset of patients from the dal-PLAQUE-2 study treated with dalcetrapib or placebo were also studied. In rabbits, dalcetrapib and anacetrapib increased HDL-C by more than 58% (P < 0.01) and in turn raised large apo E-containing HDL by 66% (P < 0.001) and 59% (P < 0.01), respectively. Additionally, HDL from CETPi-treated rabbits competed with human LDL for binding to the LDLr on HepG2 cells more than control HDL (P < 0.01). In humans, dalcetrapib increased concentrations of large HDL particles (+69%, P < 0.001) and apo B-depleted plasma apo E (+24%, P < 0.001), leading to the formation of apo E-containing HDL (+47%, P < 0.001) devoid of apo A-I. Overall, in rabbits and humans, CETPi increased large apo E-containing HDL particle concentration, which can interact with hepatic LDLr. The catabolism of these particles may depend on an adequate level of LDLr to contribute to reverse cholesterol transport.     

Impact of LDL apheresis on atheroprotective reverse cholesterol transport pathway in familial hypercholesterolemia

In familial hypercholesterolemia (FH), low HDL cholesterol (HDL-C) levels are associated with functional alterations of HDL particles that reduce their capacity to mediate the reverse cholesterol transport (RCT) pathway. The objective of this study was to evaluate the consequences of LDL apheresis on the efficacy of the RCT pathway in FH patients. LDL apheresis markedly reduced abnormal accelerated cholesteryl ester transfer protein (CETP)-mediated cholesteryl ester (CE) transfer from HDL to LDL, thus reducing their CE content. Equally, we observed a major decrease (-53%; P < 0.0001) in pre-β1-HDL levels. The capacity of whole plasma to mediate free cholesterol efflux from human macrophages was reduced (-15%; P < 0.02) following LDL apheresis. Such reduction resulted from a marked decrease in the ABCA1-dependent efflux (-71%; P < 0.0001) in the scavenger receptor class B type I-dependent efflux (-21%; P < 0.0001) and in the ABCG1-dependent pathway (-15%; P < 0.04). However, HDL particles isolated from FH patients before and after LDL apheresis displayed a similar capacity to mediate cellular free cholesterol efflux or to deliver CE to hepatic cells. We demonstrate that rapid removal of circulating lipoprotein particles by LDL apheresis transitorily reduces RCT. However, LDL apheresis is without impact on the intrinsic ability of HDL particles to promote either cellular free cholesterol efflux from macrophages or to deliver CE to hepatic cells.     

α-Defensins Induce a Post-translational Modification of Low Density Lipoprotein (LDL) That Promotes Atherosclerosis at Normal Levels of Plasma Cholesterol

Approximately one-half of the patients who develop clinical atherosclerosis have normal or only modest elevations in plasma lipids, indicating that additional mechanisms contribute to pathogenesis. In view of increasing evidence that inflammation contributes to atherogenesis, we studied the effect of human neutrophil α-defensins on low density lipoprotein (LDL) trafficking, metabolism, vascular deposition, and atherogenesis using transgenic mice expressing human α-defensins in their polymorphonuclear leukocytes (Def^+/+). Accelerated Def^+/+ mice developed α-defensin·LDL complexes that accelerate the clearance of LDL from the circulation accompanied by enhanced vascular deposition and retention of LDL, induction of endothelial cathepsins, increased endothelial permeability to LDL, and the development of lipid streaks in the aortic roots when fed a regular diet and at normal plasma levels of LDL. Transplantation of bone marrow from Def^+/+ to WT mice increased LDL clearance, increased vascular permeability, and increased vascular deposition of LDL, whereas transplantation of WT bone marrow to Def^+/+ mice prevented these outcomes. The same outcome was obtained by treating Def^+/+ mice with colchicine to inhibit the release of α-defensins. These studies identify a potential new link between inflammation and the development of atherosclerosis.     

Remodeling of human plasma lipoproteins by detergent perturbation

Detergent perturbation, the treatment of total human plasma lipoproteins (TLP) with sodium cholate and its subsequent removal, has been used to study lipoprotein dynamics and stability. At physiological TLP concentrations, detergent perturbation converts low-density lipoproteins (LDL) and high-density lipoproteins (HDL) to higher-particle weight species with the concomitant release of apo A-I but not apo A-II as a lipid-poor species. Detergent perturbation of isolated HDL also releases lipid-poor apo A-I and forms larger HDL species, whereas detergent perturbation of an isolated LDL has no effect on its size. A model is presented in which detergent perturbation induces transfer of PC from metastable HDL and LDL to mixed micelles with sodium cholate. The remaining LDL and HDL are unstable because of the loss of their surface components, phospholipid and/or apo A-I, and fuse to give larger LDL and HDL particles. These effects on HDL, i.e., PC transfer, apo A-I dissociation, and particle fusion, emulate the activity of human plasma phospholipid transfer protein. Thus, detergent perturbation is a new and potentially powerful method for determining lipoprotein stability, studying the mechanisms for remodeling of plasma lipoproteins, and preparing new forms of HDL and LDL with unique interactions with lipoprotein transporters and receptors.     

Responses of plasma apolipoproteins to gonadectomy and androgen substitution in male rats

To obtain information on testosterone effects on plasma apolipoproteins, the amount and composition of apo-proteins in lipoproteins of 5 density classes (VLDL, LDL, HDL2b, HDL2a, HDL3) was estimated in 3 groups of adult male rats: normal control rats, castrated rats, and rats injected daily with testosterone propionate (200 micrograms/day) for one week after castration. Apoproteins were separated by sodium dodecylsulfate polyacrylamide gel electrophoresis after ultracentrifugation of plasma, and determined colorimetrically. Total amount of apoprotein carried in LDL (d = 1.006-1.063 g/ml) and HDL2b (d = 1.063-1.100 g/ml) was higher in castrated than in control rats, but was not significantly different from controls in testosterone substituted rats. LDL apo B and HDL2b apo E were higher in castrated than in normal rats; control levels were observed in androgen substituted rats. Except for a greatly increased relative amount of HDL2b apo E, and a decreased percentage of HDL2b apo A-I in castrated rats, there were no significant alterations by castration of apoprotein composition of the lipoproteins. The results raise the question whether the androgenic state might affect processes related to the effects of plasma LDL apo B and HDL apo E.