Effect of lecithin:cholesterol acyltransferase on distribution of apolipoprotein A-IV among lipoproteins of human plasma

Apolipoprotein F concentration, activity, and the properties of LDL controlling ApoF activation in hyperlipidemic plasma

Apolipoprotein F (ApoF) modulates lipoprotein metabolism by selectively inhibiting cholesteryl ester transfer protein activity on LDL. This ApoF activity requires that it is bound to LDL. How hyperlipidemia alters total plasma ApoF and its binding to LDL are poorly understood. In this study, total plasma ApoF and LDL-bound ApoF were quantified by ELISA (n = 200). Plasma ApoF was increased 31% in hypercholesterolemic plasma but decreased 20% in hypertriglyceridemia. However, in donors with combined hypercholesterolemia and hypertriglyceridemia, the elevated triglyceride ameliorated the rise in ApoF caused by hypercholesterolemia alone. Compared with normolipidemic LDL, hypercholesterolemic LDL contained ∼2-fold more ApoF per LDL particle, whereas ApoF bound to LDL in hypertriglyceridemia plasma was <20% of control. To understand the basis for altered association of ApoF with hyperlipidemic LDL, the physiochemical properties of LDL were modified in vitro by cholesteryl ester transfer protein ± LCAT activities. The time-dependent change in LDL lipid composition, proteome, core and surface lipid packing, LDL surface charge, and LDL size caused by these factors were compared with the ApoF binding capacity of these LDLs. Only LDL particle size correlated with ApoF binding capacity. This positive association between LDL size and ApoF content was confirmed in hyperlipidemic plasmas. Similarly, when in vitro produced and enlarged LDLs with elevated ApoF binding capacity were incubated with LPL to reduce their size, ApoF binding was reduced by 90%. Thus, plasma ApoF levels and the activation status of this ApoF are differentially altered by hypercholesterolemia and hypertriglyceridemia. LDL size is a key determinate of ApoF binding and activation.

Mechanism and Physiologic Significance of the Suppression of Cholesterol Esterification in Human Interstitial Fluid

Cholesterol esterification in high density lipoproteins (HDLs) by lecithin:cholesterol acyltransferase (LCAT) promotes unesterified cholesterol (UC) transfer from red cell membranes to plasma in vitro. However, it does not explain the transfer of UC from most peripheral cells to interstitial fluid in vivo, as HDLs in afferent peripheral lymph are enriched in UC. Having already reported that the endogenous cholesterol esterification rate (ECER) in lymph is only 5% of that in plasma, we have now explored the underlying mechanism. In peripheral lymph from 20 healthy men, LCAT concentration, LCAT activity (assayed using an optimized substrate), and LCAT specific activity averaged, respectively, 11.8, 10.3, and 84.9% of plasma values. When recombinant human LCAT was added to lymph, the increments in enzyme activity were similar to those when LCAT was added to plasma. Addition of apolipoprotein AI (apo AI), fatty acid-free albumin, Intralipid, or the d < 1.006 g/ml plasma fraction had no effect on ECER. During incubation of lymph plus plasma, the ECER was similar to that observed with buffer plus plasma. When lymph was added to heat-inactivated plasma, the ECER was 11-fold greater than with lymph plus buffer. Addition of discoidal proteoliposomes of apo AI and phosphatidycholine (PC) to lymph increased ECER 10-fold, while addition of apo AI/PC/UC disks did so by only six-fold. We conclude that the low ECER in lymph is due to a property of the HDLs, seemingly substrate inhibition of LCAT by excess cell-derived UC. This is reversed when lymph enters plasma, consequent upon redistribution of UC from lymph HDLs to plasma lipoproteins.

Human serum preβ1-high density lipoprotein levels are independently and negatively associated with coronary artery diseases

BACKGROUND

Serum preβ1-high density lipoprotein (preβ1-HDL) was defined by two-dimensional non-denaturing linear gel electrophoresis and apolipoprotein A-I immuno-blotting. Serum preβ1-HDL seems to play an important role in reverse cholesterol transport, a well-known anti-atherosclerosis process. However, there are still debatable questions for its quantification and coronary artery disease (CAD) relevance.

METHODS

We isolated the preβ1-HDL using a new native polyacrylamide gel electrophoresis (PAGE) system and lipid pre-staining serum. We established a two-demensional gel electrophoresis system.

RESULTS

We measured the preβ1-HDL in Tangier disease patients and subjects with cholesterol ester transfer protein (CETP) mutation. The preβ1-HDL is clearly separated from lipid-free apoA-I monomer and cannot be converted into other HDL particles under lecithin-cholesterol acyltransferase (LCAT) inhibition. This preβ1-HDL is a spheroidal particle with the highest apoA-1/cholesterol ratio and highest density (≥1.21 g/ml), as compared with all other HDLs. Importantly, we found that serum from subjects with Tangier disease or with cholesterol ester transfer protein (CETP) mutation have no detectible preβ1-HDL particles. We recruited a total of 102 subjects underwent diagnostic coronary angiography and measured their preβ1-HDL levels. Among them, 56 had no stenosis of coronary artery and 46 were diagnosed as CAD, which was predefined as the presence of a luminal diameter stenosis ≥50 % in at least 1 major coronary artery territory. We found that preβ1-HDL is independently and negatively associated with the severity of the coronary artery stenosis (Gensini score).

CONCLUSION

We established a novel and simple method for human serum preβ1-HDL quantification. We found that human lower preβ1-HDL is an independent predictor for severer coronary artery stenosis.

Conversion of lipid transfer inhibitor protein (apolipoprotein F) to its active form depends on LDL composition

Lipid transfer inhibitor protein (LTIP) exists in both active and inactive forms. Incubation (37°C) of plasma causes LTIP to transfer from a 470 kDa inactive complex to LDL where it is active. Here, we investigate the mechanisms underlying this movement. Inhibiting LCAT or cholesteryl ester transfer protein (CETP) reduced incubation-induced LTIP translocation by 40-50%. Blocking both LCAT and CETP completely prevented LTIP movement. Under appropriate conditions, either factor alone could drive maximum LTIP transfer to LDL. These data suggest that chemical modification of LDL, the 470 kDa complex, or both facilitate LTIP movement. To test this, LDL and the 470 kDa fraction were separately premodified by CETP and/or LCAT activity. Modification of the 470 kDa fraction had no effect on subsequent LTIP movement to native LDL. Premodification of LDL, however, induced spontaneous LTIP movement from the native 470 kDa particle to LDL. This transfer depended on the extent of LDL modification and correlated negatively with changes in the LDL phospholipid + cholesterol-to-cholesteryl ester + triglyceride ratio. We conclude that LTIP translocation is dependent on LDL lipid composition, not on its release from the inactive complex. Compositional changes that reduce the surface-to-core lipid ratio of LDL promote LTIP binding and activation.

Apolipoprotein A-IMilano/POPC complex attenuates post-ischemic ventricular dysfunction in the isolated rabbit heart

Irreversible myocardial injury is a potential consequence of coronary artery revascularization. Reperfusion leads to the production of oxidized products that can damage myocardium. High-density lipoproteins (HDL) are effective at removing oxidized lipids. We hypothesized that a synthetic HDL preparation, comprising recombinant apolipoprotein A-I(Milano) (apoA-I(M)) complexed with 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) (apoA-I(M)/POPC) would protect the heart from reperfusion injury. The ex vivo model consisted of rabbit hearts perfused by the Langendorff method. Hearts were equilibrated with Krebs-Henseleit buffer (10 min), pretreated with either apoA-I(M)/POPC (0.45 mg/mL) or vehicle (10 min), subjected to global ischemia (30 min) and reperfused for 60 min. ApoA-I(M)/POPC (n=7) prevented the left ventricular end-diastolic pressure elevation observed in the vehicle group (n=6) at the end of reperfusion (p<0.05). During reperfusion, coronary artery perfusion pressure increased in the controls (p<0.001), but not with apoA-I(M)/POPC. ApoA-I(M)/POPC reduced the release of creatine kinase at the end of the ischemic period (p<0.001). It also reduced cardiac left ventricle muscle lipid hydroperoxides by 46% (p<0.05). Direct comparison of the antioxidant potential indicated that recombinant apoA-I(M) was much more potent than apoA-I in attenuating low-density lipoprotein oxidation. Electron microscopy showed that apoA-I(M)/POPC prevented mitochondrial granulation, disorganization and sarcomere contraction band formation indicative of reperfusion injury. The apoA-I(M)/POPC complex thus appears to reduce reperfusion injury under global ischemic conditions, and may therefore have therapeutic application in the reduction of myocardial ischemia.

Immune-regulation of the apolipoprotein A-I/C-III/A-IV gene cluster in experimental inflammation

Apolipoprotein A-IV is a member of the apo A-I/C-III/A-IV gene cluster. In order to investigate its hypothetical coordinated regulation, an acute phase was induced in pigs by turpentine oil injection. The hepatic expression of the gene cluster as well as the plasma levels of apolipoproteins were monitored at different time periods. Furthermore, the involvement of the inflammatory mediators' interleukins 1 and 6 and tumor necrosis factor in the regulation of this gene cluster was tested in cultured pig hepatocytes, incubated with those mediators and apo A-I/C-III/A-IV gene cluster expression at the mRNA level was measured. In response to turpentine oil-induced inflammation, a decreased hepatic apo A-IV mRNA expression was observed (independent of apo A-I and apo C-III mRNA) not correlating with the plasma protein levels. The distribution of plasma apo A-IV experienced a shift from HDL to larger particles. In contrast, the changes in apo A-I and apo C-III mRNA were reflected in their corresponding plasma levels. Addition of cytokines to cultured pig hepatocytes also decreased apo A-IV and apo A-I mRNA levels. All these results show that the down-regulation of apolipoprotein A-I and A-IV messages in the liver may be mediated by interleukin 6 and TNF-alpha. The well-known HDL decrease found in many different acute-phase responses also appears in the pig due to the decreased expression of apolipoprotein A-I and the enlargement of the apolipoprotein A-IV-containing HDL.

Apolipoprotein A-IV inhibits experimental colitis

The antiatherogenic properties of apoA-IV suggest that this protein may act as an anti-inflammatory agent. We examined this possibility in a mouse model of acute colitis. Mice consumed 3% dextran sulfate sodium (DSS) in their drinking water for 7 days, with or without daily intraperitoneal injections of recombinant human apoA-IV. apoA-IV significantly and specifically delayed the onset, and reduced the severity and extent of, DSS-induced inflammation, as assessed by clinical disease activity score, macroscopic appearance and histology of the colon, and tissue myeloperoxidase activity. Intravital fluorescence microscopy of colonic microvasculature revealed that apoA-IV significantly inhibited DSS-induced leukocyte and platelet adhesive interactions. Furthermore, apoA-IV dramatically reduced the upregulation of P-selectin on colonic endothelium during DSS-colitis. apoA-IV knockout mice exhibited a significantly greater inflammatory response to DSS than did their WT littermates; this greater susceptibility to DSS-induced inflammation was reversed upon exogenous administration of apoA-IV to knockout mice. These results provide the first direct support for the hypothesis that apoA-IV is an endogenous anti-inflammatory protein. This anti-inflammatory effect likely involves the inhibition of P-selectin-mediated leukocyte and platelet adhesive interactions.

Plasma distribution of apoA-IV in patients with coronary artery disease and healthy controls

Recent studies showed lower apolipoprotein A-IV (apoA-IV) plasma concentrations in patients with coronary artery disease (CAD). The actual distribution of the antiatherogenic apoA-IV in human plasma, however, is discussed controversially and it was never investigated in CAD patients. We therefore developed a gentle technique to separate the various apoA-IV-containing plasma fractions. Using a combination of precipitation of all lipoproteins with 40% phosphotungstic acid and 4 M MgCl2, as well as immunoprecipitation of all apoA-I-containing particles with an anti-apoA-I antibody, we obtained three fractions of apoA-IV: lipid-free apoA-IV (about 4% of total apoA-IV), apoA-IV associated with apoA-I (LpA-I:A-IV, 12%), and apoA-I-unbound but lipoprotein-containing apoA-IV (LpA-IV, 84%). We compared these three apoA-IV fractions between 52 patients with a history of CAD and 52 age- and sex-matched healthy controls. Patients had significantly lower apoA-IV levels when compared to controls (10.28 +/- 3.67 mg/dl vs. 11.85 +/- 2.82 mg/dl, P = 0.029), but no major differences for the three plasma apoA-IV fractions. We conclude that our gentle separation method reveals a different distribution of apoA-IV than in many earlier studies. No major differences exist in the apoA-IV plasma distribution pattern between CAD patients and controls. Therefore, the antiatherogenic effect of apoA-IV has to be explained by other functional properties of apoA-IV (e.g., the antioxidative characteristics).

Interfacial exclusion pressure determines the ability of apolipoprotein A-IV truncation mutants to activate cholesterol ester transfer protein

We used a panel of recombinant human apolipoprotein (apo) A-IV truncation mutants, in which pairs of 22-mer alpha-helices were sequentially deleted along the primary sequence, to examine the impact of protein structure and interfacial activity on the ability of apoA-IV to activate cholesterol ester transfer protein. Circular dichroism and fluorescence spectroscopy revealed that the secondary structure, conformation, and molecular stability of recombinant human apoA-IV were identical to the native protein. However, deletion of any of the alpha-helical domains in apoA-IV disrupted its tertiary structure and impaired its molecular stability. Surprisingly, determination of the water/phospholipid interfacial exclusion pressure of the apoA-IV truncation mutants revealed that, for most, deletion of amphipathic alpha-helical domains increased their affinity for phospholipid monolayers. All of the truncation mutants activated the transfer of fluorescent-labeled cholesterol esters between high and low density lipoproteins at a rate higher than native apoA-IV. There was a strong positive correlation (r = 0.790, p = 0.002) between the rate constant for cholesterol ester transfer and interfacial exclusion pressure. We conclude that molecular interfacial exclusion pressure, rather than specific helical domains, determines the degree to which apoA-IV, and likely other apolipoproteins, facilitate cholesterol ester transfer protein-mediated lipid exchange.

Apolipoprotein A-IV polymorphisms and diet-gene interactions

Apolipoprotein A-IV is a 46kDa glycoprotein that is synthesized by intestinal enterocytes and is incorporated into the surface of nascent chylomicrons. Considerable evidence suggests that apolipoprotein A-IV plays a role in intestinal lipid absorption and chylomicron assembly. We have proposed that polymorphisms that alter the interfacial behavior of apolipoprotein A-IV may modulate the physical properties and metabolic fate of plasma chylomicrons. Of the reported genetic polymorphisms of apolipoprotein A-IV, two, Q360H and T347S, are known to occur at high frequencies among the world populations. Biophysical studies have established that the Q360H isoprotein displays higher lipid affinity; conversely the T347S isoprotein is predicted to be less lipid avid. Recent studies have shown that the Q360H polymorphism is associated with increased postprandial hypertriglyceridemia, a reduced low-density lipoprotein response to dietary cholesterol in the setting of a moderate fat intake, an increased high-density lipoprotein response to changes in total dietary fat content, and lower body mass and adiposity; the T347S polymorphism appears to confer the opposite effects. Studies on the diet-gene interactions of other apolipoprotein A-IV alleles are needed, as are studies on the interactions between apolipoprotein A-IV alleles and other apolipoprotein polymorphisms.

The role of apolipoprotein A-IV in the regulation of food intake

Apolipoprotein A-IV (apo A-IV) is a glycoprotein synthesized by the human intestine. In rodents, both the small intestine and liver secrete apo A-IV, but the small intestine is the major organ responsible for the circulating apo A-IV. Intestinal apo A-IV synthesis is markedly stimulated by fat absorption and appears not to be mediated by the uptake or reesterification of fatty acids to form triglycerides. Rather, the formation of chylomicrons acts as a signal for the induction of intestinal apo A-IV synthesis. Intestinal apo A-IV synthesis is also enhanced by a factor from the ileum, probably peptide tyrosine-tyrosine. The inhibition of food intake by apo A-IV is mediated centrally. The stimulation of intestinal synthesis and the secretion of apo A-IV by lipid absorption are rapid; thus, apo A-IV likely plays a role in the short-term regulation of food intake. Other evidence suggests that apo A-IV may also be involved in the long-term regulation of food intake and body weight. Chronic ingestion of a high-fat diet blunts the intestinal apo A-IV response to lipid feeding and may explain why the chronic ingestion of a high-fat diet predisposes both animals and humans to obesity.

Human apolipoprotein A-IV metabolism within triglyceride-rich lipoproteins and plasma

In order to investigate the metabolism of apo A-IV within TRL and plasma, we assessed TRL and plasma apo A-IV kinetics in 19 and 4 subjects, respectively, consuming an average US diet for a 6-week period. At the end of this diet study, each subject received a primed-constant infusion of deuterated leucine over a 15 h time period with hourly feeding, and blood samples were drawn at 10 time points. TRL was separated by ultracentrifugation. Apo A-IV was isolated by immunoprecipitation and/or SDS-PAGE. Apo A-IV concentrations were determined by immunoelectrophoresis. Stable isotope tracer/tracee ratios were measured by gas chromatography/mass spectrometry, and the data were analyzed by multicompartmental modeling. The mean concentrations of plasma and TRL apo A-IV during the isotope infusion period were 21.0+/-3.2 and 0.66+/-0.25 mg/dl, respectively, and these values were 11.5 and 30.5% higher than those of fasting samples. The mean TRL and plasma apo A-IV residence times (RT) were 1.97+/-0.57 and 2.71+/-0.65 days, and transport rates (TR) were 0.17+/-0.19 and 3.90+/-1.24 mg/kg per day, respectively. There were significant correlations between TRL apo A-IV concentrations and TR (r(2)=0.79, P<0.001), and between TRL apo A-IV pool size and TRL cholesterol levels (r(2)=0.29, P=0.02). Our data indicated that; (1) TRL apo A-IV has a RT of 1.97 days which is similar to that earlier reported for HDL apo A-IV; (2) Apo A-IV recirculates between TRL and other slowly turning over pools; (3) the primary determinant of TRL apo A-IV levels is its TR; and (4) there is no correlation between TRL apo A-IV and apo B48 fractional catabolism in TRL.

Effects of age, gender, and lifestyle factors on plasma apolipoprotein A-IV concentrations

Apolipoprotein (apo) A-IV is a protein component of triglyceride (TG)-rich lipoproteins and high density lipoproteins (HDL). Plasma apo A-IV levels were measured by immunoelectrophoresis and these values were related to other biological variables in 723 middle aged and elderly men and women (more than 90% of them were Caucasian) prior to participation in a lifestyle modification program. Apo A-IV may play an important function in regulating lipid absorption, reverse cholesterol transport, and food intake. The data are consistent with the following concepts: (1) apo A-IV levels are significantly and positively correlated with age (r = 0.159, P < 0.05) in all subjects, with plasma apo A-I levels in both men (r = 0.194, P < 0.001) and women (r = 0.213, P < 0.001), and with apo E (r=0.111, P<0.05) and TG levels (r =0.120, P <0.05) in men; (2) apo A-IV levels are inversely correlated with body mass index (r = 0.170, P <0.05) in women; (3) female subjects on hormone replacement therapy have significantly lower plasma apo A-IV levels (by 4.1%, P < 0.05) than normal controls; (4) diabetic subjects have significantly higher apo A-IV levels (by 21%, P < 0.01) than normal subjects; (5) there is no significant effect of smoking, alcohol intake, and apo A-IV-1/2 genotype on apo A-IV levels. The data indicate that plasma apo A-IV levels are significantly affected by age, diabetes, and hormone replacement therapy.

Common apolipoprotein A-IV variants are associated with differences in body mass index levels and percentage body fat

OBJECTIVE

To determine the relationship between two common apoA-IV variants (Thr347-->Ser; Gln360-->His), and body mass index (BMI) and percentage body fat.

DESIGN

Cross-sectional study.

SUBJECTS

Eight-hundred and forty-eight subjects screened for participation in ongoing clinical studies.

MEASUREMENTS

ApoA-IV genotype, body mass index, waist-to-hip ratio and percentage body fat by bioelectric impedance.

RESULTS

Participants had an average age of 41+/-12 y and an average BMI of 28.2+/-5.5 kg/m2. Individuals homozygous for the Ser347 allele had higher BMI (32.3+/-6.6 vs 28.6+/-5.3 kg/m2; P<0.01) and percentage body fat (36.9+/-7.8 vs 31.0+/-9.6%; P<0.05) compared with individuals homozygous for Thr347. In contrast, the presence of at least one copy of the His360 allele was associated with lower BMI (27.2+/-5.0 vs 28.4+/-5.6 kg/m2; P<0.05) and percentage body fat (28.6+/-8.2 vs 30.7+/-9.1%; P<0.05). The genotype effects persisted after normalization of the data for the potential confounding effects of gender, age and race. When grouped by BMI percentile, the frequency of the Ser347/Ser347 genotype increased while the frequency of the His360 allele decreased with increasing BMI.

CONCLUSIONS

These data suggest a role for apoA-IV in fat storage or mobilization and that genetic variations in the apoA-IV gene may play a role in the development of obesity.

Ileal lipid infusion stimulates jejunal synthesis of apolipoprotein A-IV without affecting mRNA levels

We examined the effect of ileal infusions of lipid emulsion on mRNA levels and biosynthesis of apolipoprotein A-IV (apo A-IV) in jejunal Thiry-Vella fistulas in rats. The rats were surgically prepared with jejunal Thiry-Vella fistulas; after recovery they were deprived of food, equipped with ileal infusion cannulas, then given 8 hr ileal infusions of fatty acid/monoglyceride emulsions. Mucosal synthesis and transcript levels of apo A-IV in the Thiry-Vella loop were then measured. Lipid infusion produced a two-fold stimulation in incorporation of 3H-leucine into apo A-IV-specific protein, but had no significant effect on apo A-IV mRNA levels. These results support the hypothesis that a lipid-elicited, distal gut-derived, systemic signal stimulates the production of apo A-IV by a post-transcriptional mechanism.

Intestinal synthesis and lymphatic secretion of apolipoprotein A-IV after cessation of duodenal fat infusion: mediation by bile

We tested whether secretion of apolipoprotein (apo) A-IV depends upon intestinal triglyceride (TG) transport by comparing output kinetics of TG and apo A-IV during and after duodenal lipid infusion in lymph-fistula rats. Lipid infusion (triolein, 40 mumol/h, 8 h) produced increases in lymphatic TG and apo A-IV output. After 8 h, triolein infusate was replaced with glucose-saline; TG output returned to basal levels 4-5 h later. However, apo A-IV output continued at significantly elevated levels until 20 h after the start of the experiment. Bile diversion blocked this continued output of A-IV during the post-lipid period, and resulted in basal TG output that was 75% lower than in bile-intact rats. Return of bile or low-dose triolein infusion (5 mumol/h) into the intestine reversed these effects. There were no differences in hepatic synthesis or filtration of plasma A-IV into lymph between bile-intact and bile-diverted groups. Intestinal A-IV synthesis was elevated in both groups even during the post-lipid period. The results support the hypothesis that intestinal triglyceride transport drives apo A-IV secretion, and suggest the existence of a bile-dependent, post-translational mechanism for the control of lymphatic apo A-IV output.

Regulation of intestinal apolipoprotein A-IV synthesis

Apolipoprotein (apo) A-IV is a protein synthesized, in humans, only by the small intestine. It has a molecular weight of 46 000 Da. This paper summarizes the evidence supporting its role as a satiety factor following the ingestion of fat. This function of apo A-IV is unique and not shared by other apolipoproteins, including apo A-I. The satiety effect of apo A-IV is centrally mediated. The mechanism of how apo A-IV inhibits food intake is not clear but it probably acts by inhibiting both gastric acid secretion as well as gastric motility. Lipid absorption stimulates apo A-IV synthesis and secretion by the jejunum. In addition to lipid feeding, there is evidence that a factor which is released as a result of lipid absorption in the distal small intestine also stimulates the synthesis and release of apo A-IV by the jejunum. This factor is probably PYY.

PYY stimulates synthesis and secretion of intestinal apolipoprotein AIV without affecting mRNA expression

We tested whether exogenous peptide YY (PYY) can stimulate synthesis and lymphatic secretion of intestinal apolipoprotein AIV (apo AIV). Rats with mesenteric lymph fistulas and right atrial cannulas were given continuous intravenous infusions of control vehicle or PYY at 25, 50, 75, 100, or 200 pmol . kg-1 . h-1. PYY (75-200 pmol . kg-1 . h-1) stimulated lymphatic apo AIV output from 1.5- to 3.5-fold higher than basal output. In separate experiments, PYY (100 pmol . kg-1 . h-1) produced a 60% increase in jejunal mucosal apo AIV synthesis but had no effect on mucosal apo AIV mRNA levels at doses up to 200 pmol . kg-1 . h-1. Finally, exogenous PYY infusion (100 pmol . kg-1 . h-1) produced a plasma PYY increment of 30 pM compared with an increment of 18.7 pM in response to ileal infusion of lipid. These results support the hypothesis that PYY may be an endocrine mediator of the effects of distal gut lipid on production and release of intestinal apo AIV, likely via a posttranscriptional mechanism of action.

Antioxidant protection of LDL by physiological concentrations of 17 beta-estradiol. Requirement for estradiol modification

BACKGROUND

Exposure to estrogens reduces the risk for coronary artery disease and associated clinical events; however, the mechanisms responsible for these observations are not clear. Supraphysiological levels of estrogens act as antioxidants in vitro, limiting oxidation of low-density lipoprotein (LDL), an event implicated in atherogenesis. We investigated the conditions under which physiological concentrations of 17 beta-estradiol (E2) inhibit oxidative modification of LDL.

METHODS AND RESULTS

Plasma incubated with E2 (0.1 to 100 nmol/L) for 4 hours yielded LDL that demonstrated a dose-related increase in resistance to oxidation by Cu2+ as measured by conjugated diene formation. This effect was dependent on plasma, because incubation of isolated LDL with E2 at these concentrations in buffered saline produced no effect on Cu(2+)-mediated oxidation. Incubation of plasma with E2 had no effect on LDL alpha-tocopherol content or cholesteryl ester hydroperoxide formation during the 4-hour incubation. Plasma incubation with [3H]E2 was associated with dose-dependent association of 3H with LDL. High-performance liquid chromatographic analysis of LDL derived from plasma incubated with [3H]E2 indicated that the majority of the associated species were not detectable as authentic E2 but as nonpolar forms of E2 that were susceptible to base hydrolysis consistent with fatty acid esterification of E2. Plasma-mediated association of E2 and subsequent antioxidant protection was inhibited by 5,5'-dithiobis(2-nitrobenzoic acid), an inhibitor of plasma acyltransferase activity.

CONCLUSIONS

Exposure of LDL to physiological levels of E2 in a plasma milieu is associated with enhanced resistance to Cu(2+)-mediated oxidation and incorporation of E2 derivatives into LDL. This antioxidant capacity may be another means by which E2 limits coronary artery disease in women.

Control of synthesis and secretion of intestinal apolipoprotein A-IV by lipid

Apolipoprotein (apo) A-IV, a component of intestinally secreted, triacylglycerol-rich lipoproteins, has recently been proposed as a physiological controller of gastric function and food intake. Thus, it is important to understand the mechanisms involved in the control of expression, synthesis and secretion of apo A-IV. Apo A-IV is a member of a closely linked, multigene cluster which includes apolipoproteins A-I and C-III. Expression and synthesis of apo A-IV display marked variability with regard to species, tissue, stage of development and response to hormones, but intestinal apo A-IV is consistently stimulated by dietary lipid. The precise molecular mechanisms underlying the response of apo A-IV to lipid have not been clearly defined. Most evidence supports the hypothesis that some aspect of lipid transport is necessary for the apo A-IV response, but only part of this response may be due to a direct effect of intestinal lipid: recent findings suggest a connection between intestinal production of apo A-IV and hormonal and/or neural factors associated with operation of the "ileal brake." Thus, apo A-IV may play an integrative role in the modulation of both upper gastrointestinal function and ingestive behavior.

Activation of human plasma cholesteryl ester transfer protein by human apolipoprotein A-IV

Function of apolipoprotein (apo) A-IV was studied for its role in cholesteryl ester transfer protein (CETP; lipid transfer protein, LTP) reaction between lipid microemulsions having the diameter of low density lipoprotein, being compared to apoA-I. CETP hardly catalyzed lipid transfer without apolipoproteins. ApoA-IV bound to the surface of the microemulsion in equilibrium with a similar affinity to that of other helical apolipoproteins, and activated the transfer reaction by CETP of cholesteryl ester, triacylglycerol and phosphatidylcholine between the emulsions. The rate of the transfer reaction of cholesteryl ester and triacylglycerol was directly proportional to the amount of the bound apoA-IV to the surface of the emulsion. For phosphatidylcholine, activation was less effective until 40% of total binding capacity of lipid emulsion was occupied by the apolipoprotein. Cholesteryl ester was highly preferred by CETP over triacylglycerol when equal amount of these lipids was present in the core of the apoA-IV-activated emulsion, resulting in almost no triacylglycerol transfer. However, when the emulsion has the core exclusively of triacylglycerol, triacylglycerol was transferred by CETP with the rate in the same order as that of cholesteryl ester transfer. These findings were all comparable to the results with apoA-I, and also consistent with our previous observation for other amphiphilic helical apolipoproteins such as apoA-II, E and C-III.

Lipoproteins containing apolipoprotein A-IV but not apolipoprotein A-I take up and esterify cell-derived cholesterol in plasma

Two-dimensional nondenaturing polyacrylamide gradient gel electrophoresis (2D-PAGGE) identifies distinct apoA-I-or apoE-containing subclasses of high-density lipoproteins (HDLs), each of which plays a different role in reverse cholesterol transport. In this study we used 2D-PAGGE to investigate the role of apoA-IV-containing lipoproteins in reverse cholesterol transport in native plasma. Incubation of 2D electrophoretograms with anti-apoA-IV antibodies identified up to three subclasses of particles. The smaller particle subclasses, LpA-IV-1 and LpA-IV-2, were found in every plasma sample. The largest particle subclass, LpA-IV-3, was observed in fewer than 10% of the plasmas analyzed. 2D-PAGGE of apoA-I-deficient plasma and apoA-I-depleted plasma and anti-apoA-I immunosubtracting 2D-PAGGE of normal plasma revealed that LpA-IV-1 and LpA-IV-2 do not contain apoA-I. The importance of LpA-IV-1 and LpA-IV-2 for uptake and esterification of cell-derived cholesterol was investigated using pulse-chase incubations of plasma with [3H]cholesterol-labeled fibroblasts followed by anti-apoA-I immunosubtracting 2D-PAGGE. During 1-minute pulse incubation with cells, [3H]cholesterol was taken up by gamma-LpE > LpA-IV-1 > pre-beta 1-LpA-I > LpA-IV-2 (">" denotes "more than"). During subsequent chase incubation without cells, proportionately less radioactivity disappeared from LpA-IV-1 and LpA-IV-2 than from pre-beta 1-LpA-I and gamma-LpE. During 5-minute pulse incubations, radioactive cholesteryl esters were formed in pre-beta 3-LpA-I > alpha-LpA-I > LpA-IV-1 > LpA-IV-2. The fractional estertification rate was highest in pre-beta 2-LpA-I and lowest in alpha-LpA-I.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex-specific effects of the glutamine/histidine polymorphism in apo A-IV on HDL metabolism

In Caucasians, a histidine for glutamine substitution (Gln-->His) at residue 360 in apolipoprotein (apo) A-IV leads to an electrophoretically detectable polymorphism whose contribution to lipid metabolism regulation is controversial. In this study of 426 male and 188 female coronary heart disease patients, we analyzed the impact of this polymorphism on lipid metabolism, particularly high-density lipoprotein (HDL). The frequency of the rarer apo A-IV (360:His) allele was .069. This polymorphism exerted opposite effects in men and women in terms of serum concentrations of total cholesterol; triglycerides; HDL cholesterol; LDL cholesterol; lipoprotein (Lp) A-I; and apo A-I, A-II, and B. Only the difference in Lp A-I levels between male apo A-IV (360:Gln/Gln) homozygotes and apo A-IV (360:Gln/His) heterozygotes was significant (P < .05). In randomly selected subgroups of 38 male and 15 female apo A-IV (360:Gln/His) heterozygotes and 104 male and 15 female apo A-IV (360:Gln) homozygotes, heterozygosity for apo A-IV (360:Gln/His) in both sexes was associated with lower plasma cholesteryl ester transfer protein (CETP) activity (P < .05) and higher serum apo A-IV concentrations (P < .01 in men). Moreover, only men had significantly higher mean plasma activity levels of lecithin:cholesterol acyltransferase (LCAT) (P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Compensatory increase in intestinal apolipoprotein A-IV mRNA levels in the experimental nephrotic rat

Using experimental nephrotic rats, we investigated the potential feedback regulation of apolipoproteins (apos) at their hepatic and intestinal synthetic sites. Nephrotic syndrome (NS) was induced in rats by puromycin aminonucleoside (PAN) with a single intraperitoneal injection (120 mg/kg). In nephrotic rats, we observed a 60% reduction in serum apo A-IV levels despite a 3.4-fold increase in jejunum and a 1.5-fold increase in ileum apo A-IV mRNA levels, although hepatic apo A-IV levels were unchanged compared with those in pair-fed control rats. A strikingly positive correlation was observed between daily urinary excretion of apo A-IV and its mRNA levels in jejunum (r = .856, P < .01; n = 10) and ileum (r = .710, P < .05; n = 10). On the other hand, nephrotic rats had an 8.2-fold increase in serum apo A-I level associated with a 4.6-fold increase in hepatic and a small but significant increase in jejunum apo A-I mRNA levels. Compared with the fractional catabolic loss of albumin or apo A-IV, that of apo A-I was small and suggests a diminished level of glomerular filtration, leading to a further elevation in serum apo A-I level. Barring nonspecific effects of PAN, these data suggest that reduction of serum apo A-IV level due to urinary loss may directly upregulate mRNA levels in the small intestine. Alternatively, it may be the result of an effective filtration of a serum component unassociated with lipoproteins that normally and site-specifically reduces apo A-I and apo A-IV mRNA transcription.

Structure of the apolipoprotein A-IV/lipid discoidal complexes: an attenuated total reflection polarized Fourier transform infrared spectroscopy study

Discoidal lipid particles were prepared from a reaction mixture containing apo A-IV and dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) in the molar ratio of 185:1 (lipid/protein). The complexes were isolated by gel filtration and characterized in terms of composition and size. Infrared attenuated total reflection spectroscopy was used to estimate the secondary structure of apolipoprotein A-IV and the orientation of its amphipathic alpha-helices with respect to the lipid hydrocarbon chains. In addition, infrared spectra were analyzed in terms of the conformation and organization of different regions of the lipid molecules in the particles. This approach has been applied successfully to reconstituted HDL particles prepared from a reaction mixture containing DPPC and apo A-I in the molar ratio of 150:1 (Wald, J.H., Goormaghtigh, E., De Meutter, J., Ruysschaert, J.M. and Jonas, A. (1990) J. Biol. Chem. 265, 20044-20050). Apo A-IV helicity increased for the protein bound to DMPC or DPPC but the increase was more pronounced for the apo A-IV/DMPC particles. In both complexes, the alpha helical amphipathic segments of the protein were parallel to the lipid acyl chains and no significant modification of the overall organization of the lipid molecules in the lipid bilayer was observed. The presence of apo A-IV seems only to affect the conformation of the lipid hydrocarbon chains in close contact with the protein in the discoidal particles.

Characterization of apoA-IV-containing lipoprotein particles isolated from human plasma and interstitial fluid

Apolipoprotein (apo) A-IV has been proposed to play a role in reverse cholesterol transport. ApoA-IV-containing lipoprotein particles (A-IVLp) were isolated from human plasma and interstitial fluid and characterized by immunoaffinity chromatography. Two major A-IVLp subpopulations, lipoprotein particles containing apoA-IV with apoA-I (LpA-I:A-IV) and lipoprotein particles containing apoA-IV without apoA-I (LpA-IV), were identified. The larger subpopulation of A-IVLp is the LpA-IV that represents 70% (protein mass) of the initial particles. Only 5.8% of apoA-IV was recovered in the retained fraction after affinity chromatography with an anti-apoA-I immunosorbent. ApoA-I, apoA-II, apoA-IV, apoB, apoC-III, apoD, apoE, apoH, lecithin: cholesterol acyltransferase (LCAT), cholesteryl ester transfer (CET) protein, proline-rich protein, and a protein of Mr 59,000 were detected in the A-IVLp. These particles contain more than 20% triglycerides (lipid mass). ApoA-IV-containing particles that were isolated from plasma are heterogeneous in size, consisting of two major populations with Stokes' diameters of 10.3 nm and 9.3 nm. Both subpopulations of A-IVLp contain LCAT and CET activities and promote cholesterol efflux from cholesterol-preloaded adipose cells. These data support the hypothesis that A-IVLp particles may be involved in reverse cholesterol transport.

Plasma apolipoprotein A-IV metabolism in patients with chronic renal disease

The plasma concentration and distribution of apolipoprotein A-IV were investigated in normotriglyceridaemic patients with end-stage renal disease and compared with those in a sex- and age-matched control group with normal renal function. A three-fold elevated plasma mean concentration of apolipoprotein A-IV was found in patients with end-stage renal disease treated by haemo- or peritoneal dialysis (58.5 +/- 18.9 mg dl-1 or 50.5 +/- 12.2 mg dl-1, respectively) compared with the controls (18.3 +/- 6.4 mg dl-1). The plasma distribution of apolipoprotein A-IV was studied in patients treated by haemodialysis and in controls by gel permeation chromatography. In the haemodialysis group, 40.3% of the apolipoprotein A-IV was found to be associated with the fraction of high density lipoproteins, whereas the rest (59.7%) was not associated with lipoproteins. This distribution was significantly different from that in the control group (24.8% vs. 75.2%, 0.01 less than P less than 0.05). The elevated plasma concentrations of apolipoprotein A-IV in the patients are not related to triglyceride levels and therefore are unlikely to result from an impaired catabolism of triglyceride-rich lipoproteins. The accumulation of apolipoprotein A-IV in high density lipoproteins from patients with end-stage renal disease might reflect the impaired reversed cholesterol transport mechanisms which are believed to be a major cause of the high prevalence of atherosclerotic diseases in these patients.

Hepatic lipase promotes a loss of apolipoprotein A-I from triglyceride-enriched human high density lipoproteins during incubation in vitro

Studies have been performed to investigate a possible mechanism to account for the low concentrations of apolipoproteins A-I (apo A-I) in subjects with hypertriglyceridemia. Incubation of human plasma in vitro with canine hepatic lipase resulted in the hydrolysis of approximately half the triglyceride in the high density lipoproteins (HDLs), but little change in the concentrations of other HDL constituents. However, when the plasma was supplemented with cholesteryl ester transfer protein and very low density lipoproteins to enrich the HDL with triglyceride, hepatic lipase promoted not only a significant reduction in HDL triglyceride acquired by the lipid transfer process but also an enhanced transfer of cholesteryl esters out of the particles. These changes were accompanied by a marked loss of apo A-I from HDL, which was demonstrated independently by ultracentrifugation, size-exclusion chromatography, and gradient gel-immunoblot analysis. The apo A-I lost from HDL was recovered in the "lipoprotein-free" fraction of plasma. The results of these studies indicate that primary reductions in the concentration of HDL core lipids in vitro are accompanied by a secondary loss of apo A-I from HDL. While recognizing the need for caution in any extrapolation from observations made in vitro to what may occur in vivo, these studies are nevertheless consistent with a proposition that the low concentration of apo A-I in subjects with hypertriglyceridemia is secondary to the reduced concentration of HDL core lipids in such subjects.

Evidence in vitro that hepatic lipase reduces the concentration of apolipoprotein A-I in rabbit high-density lipoproteins

Incubation of rabbit plasma in vitro with hepatic lipase resulted in the hydrolysis of triacylglycerol in high-density lipoproteins (HDL) and a reduction in HDL particle size. These changes were accompanied by a decrease in the concentration of apolipoprotein A-I (apo A-I) in the HDL. The loss of apo A-I was demonstrated independently by ultracentrifugation, size exclusion chromatography and gradient gel-immunoblot analysis. It was unrelated to hydrolysis of HDL phospholipids but did correlate with the reduction in HDL particle size. These studies suggest that the concentration of apo A-I in HDL may be influenced by factors which regulate the metabolism of HDL core lipid constituents.

Preferential redistribution of lipoprotein-unassociated apoA-IV to an HDL subpopulation with a high degree of LCAT modification

The relationship between LCAT mediated HDL modification and the redistribution of lipoprotein-unassociated apoA-IV to HDL was investigated in vitro. Immunoaffinity-isolated rat lipoprotein-unassociated apoA-IV was added to apoB-, apoE-, apoA-IV depleted, [3H]cholesterol labelled rat plasma and incubated at 37 degrees C. The addition of lipoprotein-unassociated apoA-IV resulted in a modest (10%) but significant reduction in the rate of cholesterol esterification. Incubations conducted in the presence of active LCAT led to a time-dependent increase in the amount of the 3H label retained by an anti-apoA-IV immunoaffinity column. Lipoproteins retained by the anti-apoA-IV immunoaffinity column had experienced a greater conversion of [3H]cholesterol to [3H]cholesteryl esters (48% esterification at 30 min) than the unretained lipoproteins (19% esterification at 30 min). These data suggest that during the course of LCAT-induced cholesterol esterification, lipoprotein-unassociated apoA-IV transfers to a subpopulation of HDL which has been modified by LCAT to a greater extent than the remaining HDL. Further analysis of the data demonstrates that 48% cholesterol esterification is sufficient to allow apoA-IV to be accommodated on the surface of an HDL particle.