Advanced glycation end-products (AGEs) and functionality of reverse cholesterol transport in patients with type 2 diabetes and in mouse models.

AIMS/HYPOTHESIS: We investigated the contribution of AGEs to the impairment of reverse cholesterol transport (RCT) variables in diabetic individuals and in two animal models of diabetic obesity and of renal impairment. METHODS: The capacity of plasma and HDL from 26 individuals with moderately controlled type 2 diabetes to support cholesterol efflux was compared with 26 age- and sex-matched individuals without diabetes. We also compared the rates of RCT in vivo in two animal models: db/db mice and mice with chronic renal failure. RESULTS: Diabetic individuals had characteristic dyslipidaemia and higher levels of plasma AGEs. The capacity of whole plasma, ApoB-depleted plasma and isolated HDL to support cholesterol efflux was greater for diabetic patients compared with controls despite their lower HDL-cholesterol levels. The capacity of plasma to support cholesterol efflux correlated with plasma levels of cholesteryl ester transfer protein and levels of ApoB, but not with levels of AGE. RCT was severely impaired in db/db mice despite elevated HDL-cholesterol levels and no change in AGE concentration, whereas RCT in uraemic mice was unaffected despite elevated AGE levels. CONCLUSIONS/INTERPRETATION: AGEs are unlikely to contribute significantly to the impairment of RCT in type 2 diabetes.

Cholesterol efflux from macrophages is influenced differentially by plasmas from overweight insulin-sensitive and -resistant subjects.

OBJECTIVE: In vitro measurements of cholesterol efflux from macrophages have recently been shown to associate with cardiovascular risk. We investigated whether cholesterol efflux from macrophages incubated with plasmas from overweight/obese subjects with metabolic syndrome was influenced by the presence of insulin resistance. METHODS: Plasmas were obtained from 47 men and women with metabolic syndrome, of whom 25 were found to be insulin resistant (IR) and 22 insulin sensitive (IS) (Matsuda, De Fronzo equation based on oral glucose tolerance test). Activated human macrophage THP-1 cells in which cholesterol had been radiolabelled were incubated with the subjects' plasmas to allow calculation of % cholesterol efflux. RESULTS: Body mass index and waist measurements, as well as plasma lipid levels, did not differ between the two groups. Homeostatic model assessment-insulin resistance value as well as plasma insulin and leptin concentrations were higher in IR subjects. Cholesterol efflux was found to be significantly greater with plasmas from IR subjects (9.1%) than from IS subjects (6.7%) (P=0.005). Further, cholesterol efflux was significantly inversely associated with insulin sensitivity index (P<0.001), directly with arterial insulin concentration (P<0.001) and directly with cholesteryl ester transfer protein (CETP) mass (P=0.044). CONCLUSION: Plasmas from overweight subjects with insulin resistance induced greater in vitro cholesterol efflux compared with IS subjects. Efflux inversely correlated with insulin sensitivity suggesting an increase in reverse cholesterol transport in the IR state that may lead to greater transfer of cholesterol to apoB lipoproteins from high-density lipoproteins via CETP as a factor in the association between IR and atherosclerosis.

The effects of dietary weight loss with or without exercise training on liver enzymes in obese metabolic syndrome subjects.

AIM: Insulin resistance and visceral adiposity are predisposing factors for fatty liver disease. The main objectives of this study were (i) to compare the effects of caloric restriction (CR) alone or together with moderate-intensity aerobic exercise training (CR+EX) on liver enzymes, a surrogate marker of liver injury, in obese metabolic syndrome (MetS) subjects and (ii) to identify anthropometric, metabolic, cardiovascular and dietary predictors of changes in liver enzymes. METHODS: Sedentary men and women (n = 63), aged 55 ± 6 (s.d.) years with body mass index 32.7 ± 4.1 kg/m(2) and confirmed MetS, were randomized to 12-week CR, CR+EX or no treatment (Control). RESULTS: Weight loss averaged 7.6% in the CR and 9.1% in the CR+EX group (time effect, p < 0.001; group effect, p = 0.11); insulin sensitivity improved by 49 and 45%, respectively (both p < 0.001). Fitness (maximal oxygen consumption) increased by 19% in the CR+EX group only (p < 0.001). Alanine aminotransferase (ALT) levels decreased by 20% in the CR and 24% in the CR+EX group (time effect, both p < 0.001; group effect, p = 0.68); corresponding values for γ-glutamyltransferase (GGT) were -28 and -33%, respectively (time effect, both p < 0.001; group effect, p = 0.28). Reduction in abdominal fat mass (measured by DXA from L1 to L4) independently predicted ΔALT (r = 0.42, p = 0.005) and ΔGGT (r = 0.55, p < 0.001), whereas change in dietary saturated fat intake was independently associated with ΔALT (r = 0.35, p = 0.03). CONCLUSIONS: Reductions in central adiposity and saturated fat intake are key drivers of improvement in liver enzymes during lifestyle interventions. Exercise training did not confer significant incremental benefits in this study.

Leptin, but not adiponectin, is a predictor of recurrent cardiovascular events in men: results from the LIPID study.

OBJECTIVE: To investigate the relationships between plasma leptin and adiponectin levels and recurrent cardiovascular events (cardiovascular death, nonfatal myocardial infarction and stroke) in men with earlier acute coronary syndromes. DESIGN, SUBJECTS AND MEASUREMENTS: A nested case-control study examined circulating leptin and adiponectin levels in plasma obtained 4-6 years after entry into the Long-Term Intervention with Pravastatin in Ischaemic Disease (LIPID) trial. Plasma was assayed from 184 men who suffered recurrent events within 4.4 years after blood collection and 184 matched controls who remained free of further events. The association between cardiovascular events and the explanatory variables was examined by conditional logistic regression analysis. RESULTS: Relative risk (RR) increased across increasing leptin quartiles; the highest quartile compared with the lowest quartile was related to the highest risk (P for trend=0.002); the increased risk remained after adjustment for risk factors (P=0.018) or for obesity (P=0.038), but in the final model (adjusted for randomized treatment, other drugs, LIPID risk score, age and body mass index), the risk was attenuated (RR=1.61, 95% CI: 0.72-3.57, P for trend=0.34). Adiponectin did not predict cardiovascular events. Subjects randomly allocated to pravastatin had 6% lower leptin levels (P=0.04) than those allocated to placebo. CONCLUSION: Plasma leptin was a significant and independent predictor of recurrent cardiovascular events (cardiovascular death, nonfatal myocardial infarction and stroke) in men with earlier acute coronary syndromes.

Turnover of plasma esterified cholesterol in normocholesterolemic and hypercholesterolemic subjects and its relation to body build.

The turnover rate of plasma total esterified cholesterol was measured after the intravenous injection of tritiated mevalonic acid in 16 men with coronary heart disease. Four subjects were normocholesterolemic and 12 were hypercholesterolemic; among the latter, 3 suffered from familial hypercholesterolemia and 6 were overweight. The turnover rate of plasma esterified cholesterol was highest among the overweight hypercholesterolemic subjects and least among the subjects with familial hypercholesterolemia. Hypercholesterolemic subjects of normal weight had turnover rates similar to those found in normocholesterolemic men. Significant correlations were found between body surface or body weight and the turnover rate of plasma esterified cholesterol. We conclude that the factors which determine the development of hypercholesterolemia are not identical in all hypercholesterolemic subjects and that when hypercholesterolemia is associated with overweight there is an increased formation of at least plasma esterified cholesterol.

Precursor-product relationship between pools of very low density lipoprotein triglyceride.

The process of removal of triglyceride from the plasma may involve a sequential conversion of larger to smaller glyceride-rich lipoproteins. This has been studied within the species of lipoproteins comprising the very low density lipoproteins (VLDL) which transport the bulk of endogenously formed triglyceride. Palmitic acid-(14)C which was used to label the plasma glycerides was administered either as a prolonged constant infusion or as a pulse label. The specific activity-time curves of triglyceride fatty acids (TGFA) were analyzed both in total VLDL and in two subfractions of VLDL. The nature of the curves for total VLDL that were observed during the constant infusions were consistent with slow isotopic equilibration of precursors of VLDL-TGFA or with the presence of a precursor-product relationship between different components of VLDL-TGFA. The curves did not indicate any detectable differences in (fractional) turnover rates of independently metabolized pools of VLDL-TGFA. Differences in the specific activity-time curves of TGFA in two subfractions of VLDL (Sf > 100 and Sf 20-100) were consistent with a precursor-product relationship between TGFA in the two subfractions; again there was no indication of significant differences in (fractional) turnover rates. The specific activity-time curves of TGFA in the two subfractions of VLDL that were obtained with single injections of radio-palmitate showed a consistent difference in the rates at which TGFA became labeled in the two subfractions, being slower in the Sf 20-100 fraction. The findings from all experiments when considered together, were compatible with a precursor-product relationship that suggested that larger VLDL were converted to progressively smaller species as triglyceride was being removed.

Studies on the turnover of triglyceride and esterified cholesterol in subjects with the nephrotic syndrome.

Factors involved in the hyperlipidemia of nephrosis have been studied in seven patients. The turnover of triglyceride was measured in plasma very low density lipoproteins after the injection of glycerol-(14)C. The turnover of esterified cholesterol was measured in whole plasma and in very low density lipoproteins after the injection of mevalonic acid-2-(3)H. Urine protein loss was found to be significantly correlated with the plasma concentrations of triglyceride and free cholesterol, suggesting that increasing loss of protein is associated with the formation of larger lipoproteins. Lactescent plasmas were found in the subjects with the greatest protein loss. The turnover rate of triglyceride tended to be higher among subjects with higher than with lower triglyceride concentrations and was on the average higher than among six normotriglyceridemic subjects. However, there was also evidence for decreased clearance of glyceride from plasma. The hypertriglyceridemia of nephrosis appeared to reflect both increased formation of glyceride and decreased removal of glyceride from plasma. The turnover of esterified cholesterol was significantly higher in whole plasma of nephrotic subjects than in normocholesterolemic nonnephrotic patients. Esterified cholesterol turnover in very low density lipoproteins was raised in the two subjects in whom a major part of total esterified cholesterol was carried in this lipoprotein fraction. These studies were repeated in one subject after remission was induced. The cessation of urinary loss of protein was associated with reductions in the concentrations and turnover of triglyceride and esterified cholesterol. The increased turnover of plasma lipids in nephrosis may reflect the general increase in the formation of protein.

Studies on the in vivo and in vitro distribution of apolipoprotein A-IV in human plasma and lymph.

To investigate the unique distribution in plasma of apolipoprotein A-IV (apo A-IV) we have determined, in a series of in vitro and in vivo studies, the redistribution among lipoproteins of 125I-apo A-IV. Free 125I-apo A-IV associated predominantly with high density lipoprotein (HDL) (72 +/- 3.5%) in incubations with plasma, and with triglyceride-rich lipoproteins (TRL) (65 +/- 3.0%) in incubations with lymph, rather than with the lipoprotein-deficient fraction (LDF) where greater than 90% of apo A-IV resides. Incubations with 125I-apo A-IV (incorporated within HDL or TRL) also resulted in similar redistributions of label. Specific radioactivities of apo A-IV in HDL and in TRL were of a similar order and 15-fold higher than those in LDF. However, when 125I-apo A-IV in LDF was incubated with plasma, 57 +/- 2.6% of label remained in the LDF, though the specific radioactivity of apo A-IV in HDL was 15-fold higher than in LDF. Thus, apo A-IV apparently exchanges freely between TRL, HDL, and a part of apo A-IV in LDF, but most of apo A-IV in LDF is refractive to free exchange or transfer. In vivo experiments carried out in five subjects, in which 125I-apo A-IV was injected within TRL, HDL, or LDF, were consistent with the in vitro data in showing rapid exchange of label among plasma apo A-IV containing fractions with much higher specific radioactivities in HDL than in LDF (10-30-fold). However, the small fraction of apo A-IV in LDF that did become labeled was removed from plasma in a biexponential fashion and at the same rate as from HDL. Thus, only a small fraction of the bulk of apo A-IV in plasma LDF exchanges freely with apo A-IV in TRL and HDL, suggesting that apo A-IV in LDF exists in at least two pools. This is consistent with our previous findings that apo A-IV in plasma is present in two distinct complexes with lipids and other peptides.

Cholesterol metabolism in human obesity.

An experiment was undertaken to test whether in severe obesity cholesterol production rates obtained by isotope kinetic analysis (two-pool compartmental analysis) are comparable to those measured by chemical sterol balance techniques. Eight severely obese but normocholesterolemic patients were studied by the balance method, and five of these eight were studied by compartmental analysis. Cholesterol turnover was 10% higher by compartmental analysis. In the entire group of eight patients cholesterol turnover was greater than twice that found previously in nonobese patients studied under similar conditions with bile acids and neutral sterols both participating in the increase. This increment was directly related to excess body fat and to adipose cellularity, with correlation co-efficients of 0.66 and 0.72, respectively. The amount of cholesterol in the slowly turning over pool B was related to degree of adiposity, but that in plasma and in pool A did not differ from values in nonobese patients.

Diurnal fluctuations in triglyceride, free fatty acids, and insulin during sucrose consumption and insulin infusion in man.

Serial changes in circulating triglyceride, free fatty acids (FFA), insulin, and glucose have been measured in human subjects fed sucrose as the sole source of calories for 2- or 3-day periods. The sucrose was given either during the day with overnight fasting (19 subjects) or as continual 3-hour meals during the day and night (seven subjects). Insulin was infused overnight in five additional subjects on the day-feeding regimen to determine the effect on triglyceride concentration. The concentration of triglyceride increased during the study in all subjects, but there was a clear diurnal pattern in the response which was present even in the continual feeding studies. The rise in triglyceride occurred mainly overnight, and during the day there was frequently a fall in the concentration. The overnight increase was significantly less when insulin was infused. There were also diurnal fluctuations in FFA and insulin in both daytime and continual feeding regimens. The plasma FFA, like triglyceride, rose during the night and fell during the day while the insulin rose during the day and fell overnight. Separate statistical analysis of the daytime and overnight changes revealed that the changes in triglyceride were significantly but negatively correlated with changes in insulin during both periods. The changes in triglyceride and FFA were positively correlated during the day but not significantly related during the night. The data show that when sucrose is eaten for 2 or 3 days, there is a general increase in triglyceride concentration upon which are superimposed major diurnal fluctuations in the concentrations of triglyceride, insulin, and FFA. It is suggested that the highly significant inverse relationship between changes in triglyceride and insulin may be mediated through an effect of insulin on triglyceride removal.

Distribution and turnover of cholesterol in humans.

The relationships between some parameters of cholesterol metabolism and body weight were studied in 22 subjects. Cholesterol-4-(14)C, complexed with plasma lipoprotein, was injected intravenously and from the resultant specific activity-time curves a number of indexes of cholesterol turnover were calculated. These were based on the two-pool model previously described by Goodman and Noble and included estimates of the sizes of the two pools, the production rate of cholesterol in the system, the rate constants for cholesterol removal from the two pools and transfer between the pools, and the metabolic clearance of cholesterol. Single and multiple regression analysis was used to define the relationships between the turnover and distribution of cholesterol and the total weight and fat content of the body. The amount of cholesterol in the more rapidly turning over pool A, which probably includes cholesterol in liver, plasma, erythrocytes, and part of the viscera such as intestine, varied from 14.9 to 32.7 g. The mean value for the extraplasma part of pool A was 17.9 g. Making certain assumptions it was possible to derive estimates of the probable lower and upper values for size of pool B (exchangeable cholesterol in tissues other than in pool A), which were, on average, 35 and 60 g. The daily production rate of cholesterol (assumed to be equivalent to total turnover rate) varied between 0.73 and 1.68 g/day. The production rate of cholesterol and the size of pool B were significantly related to total body, and particularly to excess body, weight. When the plasma content was excluded, the amount of cholesterol in pool A was not related to weight. For a body of ideal weight the production rate was 1.10 g/day and the size of pool B between 32 and 53 g. For each kilogram of excess weight the expected increments were 0.0220 g/day and 0.90 g, respectively. The plasma cholesterol concentration was not related to the production rate or to the amount of cholesterol in the two pools. It was, however, inversely related to the fractional rate of removal from pool A and to the metabolic clearance rate of cholesterol which suggests that inadequate excretion could be of importance in the development of hypercholesterolemia.

Catabolism of very low density lipoprotein B apoprotein in man.

The turnover and the catabolic fate of the B apoprotein of very low density lipoprotein (VLDL-B) was studied in 15 normal and hyperlipidemic subjects using reinjected autologous VLDL labeled with radioiodine. The specific radioactivity-time curve of the B apoprotein in total VLDL (S(f)20-400) was multiexponential but conformed to a two-pool model during the first 48 h of catabolism. The flux was highest in several hypertriglyceridemic subjects. The mass of pool A exceeded the intravascular content of VLDL-B by 30% on average, indicating extravascular metabolism of VLDL. The two-pool model might reflect the input of several populations of particles or heterogeneity of catabolic processes or pools. The flux of B apoprotein was also measured in several subclasses of VLDL, in smaller intermediate density lipoproteins, and in low density lipoproteins (LDL). In three subjects the flux was similar in S(f) 60-400 and in S(f) 12-60 lipoproteins, suggesting that VLDL was catabolized at least to a particle in the density range S(f) 12-60. Subsequent catabolism appeared to proceed by two pathways: in normotriglyceridemic subjects, B apoprotein flux in the S(f) 20-400 and in S(f) 12-20 lipoproteins was similar, whereas in hypertriglyceridemic subjects flux through S(f) 12-20 accounted for only part of the VLDL-B flux. The flux of low density lipoprotein B apoprotein (LDL-B), which is believed to be derived from VLDL catabolism, was calculated from the area between the specific activity time curves of VLDL-B and LDL-B. In subjects with normal plasma triglyceride concentration, LDL-B flux was from 91% to 113% of that of VLDL-B; but in three hypertriglyceridemic subjects showing high rates of VLDL-B transport, LDL-B flux was only one-third that of VLDL-B. This suggests that when VLDL-B flux is high, VLDL is substantially catabolized by a route other than through LDL and possibly leaves the circulation as a particle in the S(f) 20-60 density range.

Suppression by diets rich in fish oil of very low density lipoprotein production in man.

The highly polyunsaturated fatty acids in fish oils lower the plasma triglyceride concentration. We have studied the effect of a diet rich in fish oil on the rate of production of the triglyceride-transporting very low density lipoprotein (VLDL). Seven subjects, five normal and two with hypertriglyceridemia received up to 30% of daily energy needs from a fish oil preparation that was rich in eicosapentaenoic acid and docosahexaenoic acid, omega-3 fatty acids with five and six double bonds, respectively. Compared with a diet similarly enriched with safflower oil (in which the predominant fatty acid is the omega-6 linoleic acid, with two double bonds), the fish oil diet lowered VLDL lipids and B apoprotein concentrations profoundly. High density lipoprotein lipids and A1 apoprotein were also lowered, but the effect on low density lipoprotein (LDL) concentration was inconsistent. The daily production or flux of VLDL apoprotein B, calculated from reinjected autologous 125I-labeled lipoprotein, was substantially less in six subjects studied after 3 wk of fish oil, compared with after safflower oil. This effect on flux was more consistent than that on the irreversible fractional removal rate, which was increased in the four normolipidemic but inconsistent in the hypertriglyceridemic subjects. This suggests that fish oil reduced primarily the production of VLDL. The daily production of VLDL triglyceride, calculated from the kinetics of the triglyceride specific radioactivity-time curves after [3H]glycerol was injected, also showed very substantial reductions in five subjects studied. The marked suppression in VLDL apoprotein B and VLDL triglyceride formation was found not to be due to diminished plasma total free fatty acid or plasma eicosapentaenoic flux, calculated during constant infusions of [14C]eicosapentaenoic acid and [3H]oleic acid in four subjects. In two subjects there was presumptive evidence for substantial independent influx of LDL during the fish oil diet, based on the precursor-product relationship between the intermediate density lipoprotein and LDL apoprotein B specific radioactivity-time curves.

Dietary supplementation with chickpeas for at least 5 weeks results in small but significant reductions in serum total and low-density lipoprotein cholesterols in adult women and men.

AIM: To compare the effects of a chickpea-supplemented diet and those of a wheat-supplemented diet on human serum lipids and lipoproteins. METHODS: Forty-seven free-living adults participated in a randomized crossover weight maintenance dietary intervention involving two dietary periods, chickpea-supplemented and wheat-supplemented diets, each of at least 5 weeks duration. RESULTS: The serum total cholesterol and low-density lipoprotein cholesterol levels were significantly lower (both p < 0.01) by 3.9 and 4.6%, respectively, after the chickpea-supplemented diet as compared with the wheat-supplemented diet. Protein (0.9% of energy, p = 0.01) and monounsaturated fat (3.3% of total fat, p < 0.001) intakes were slightly but significantly lower and the carbohydrate intake significantly higher (1.7% of energy, p < 0.001) on the chickpea-supplemented diet as compared with the wheat-supplemented diet. Multivariate analyses suggested that the differences in serum lipids were mainly due to small differences in polyunsaturated fatty acid and dietary fibre contents between the two intervention diets. CONCLUSIONS: Inclusion of chickpeas in an intervention diet results in lower serum total and low-density lipoprotein cholesterol levels as compared with a wheat-supplemented diet.

HDL3 degradation by proteases in isolated rat intestinal mucosal cells.

Human 125I-labelled HDL3 is degraded by isolated rat intestinal mucosal cells. In our experimental conditions, lipoprotein degradation occurred by two different mechanisms. In one, lipoprotein was degraded within the cell, following binding and internalisation. In the other, degradation occurred in the medium, which seemed to contain protease activity released from cells during incubation. Though lipoprotein-deficient serum apparently interfered with degradation in the medium, bovine serum albumin had no such effect. The lysosomal inhibitor, chloroquine, reduced degradation by 60% without inhibiting HDL binding. Intestinal cell extracts contained at least two different proteases, with pH optima of 4.5 and 8.0, respectively. Comparing HDL and LDL degradation on a molar basis, more HDL particles were degraded by the cell-free extracts at pH 4.5. This degradation was activated by dithiothreitol and was inhibited by iodoacetic acid. From these observations we conclude that HDL3 is taken up by the rat intestinal mucosal cell through a specific binding site and subsequently degraded by a thiol-dependent protease in the lysosome.

In vivo transfer of cholesteryl esters from high density lipoproteins to very low density lipoproteins in man.

The fate of cholesteryl esters in high density lipoprotein (HDL) was studied to determine whether the transfer of esterified cholesterol from HDL to other plasma lipoproteins occurred to a significant extent in man. HDL cholesteryl ester, labelled in vitro with [3H] cholesterol, was injected into human subjects. Labelling of cholesteryl esters in very low density (VLDL) occurred rapidly and by 3 h, the esterified cholesterol in VLDL reached peak specific radioactivity. The removal rate of cholesteryl esters from HDL appeared to be exponential and of the order of 0.2/h; calculation of the apparent flux was about 150 mg/h which approximates reported values for total cholesterol esterification in human plasma in vivo. The rapid rate of labelling of VLDL from HDL suggests that the transfer of HDL cholesteryl esters to VLDL may represent a significant pathway for the disposal of HDL cholesterol.

Metabolism of human plasma triacylglycerol-rich lipoproteins in rodent macrophages: capacity for interaction at beta-VLDL receptor.

The capacity of human plasma triacylglycerol-rich lipoproteins to be metabolized by rat macrophages was studied with plasma triacylglycerol-rich lipoproteins obtained from subjects with fasting chylomicronemia or from normal subjects after a fat meal. Triacylglycerol-rich lipoproteins were separated by chromatography into two fractions designated TRL1 and TRL2; from their composition and changing concentration during alimentary lipemia, TRL1 contained a higher proportion of chylomicron remnants than TRL2. Degradation of 125I-labeled TRL1 was greater than that of 125I-labeled TRL2. In competition studies with 125I-labeled beta-VLDL from cholesterol-fed rabbits, unlabeled TRL1 displaced beta-VLDL as completely as did unlabeled beta-VLDL, being slightly more potent than TRL2, which contained less apolipoprotein E than TRL1. This reflected common interaction at receptors that probably included both beta-VLDL and B/E receptors, since: (1) in fresh macrophages, VLDL from hypertriglyceridemic subjects partially displaced beta-VLDL; (2) in B/E receptor-repressed macrophages, TRL1 maintained capacity to totally displace beta-VLDL. This was confirmed in experiments with J774 murine macrophages in which triacylglycerol-rich lipoproteins and beta-VLDL displaced each other equally, whereas LDL was ineffective in displacing beta-VLDL. Furthermore, monoclonal antibodies raised against apolipoprotein B48 and reacting strongly with LDL, failed to inhibit the binding of triacylglycerol-rich lipoprotein to the macrophages. This indicates an interaction through apolipoprotein E which is present in high concentration in triacylglycerol-rich lipoprotein as well as in beta-VLDL. It applies to triacylglycerol-rich particles derived from either the intestine (chylomicron remnants) or the liver (VLDL remnants from hypertriglyceridemic subjects).

Comparison of binding and degradation of high density lipoprotein by intestinal mucosal cells, fibroblasts and adrenal cortical cells in culture.

In this study we have compared the binding and degradation of human high density lipoprotein (HDL3), devoid of apolipoprotein E, by rat intestinal (mucosal) and adrenal cells and by human fibroblasts in culture. Binding of HDL3 to adrenal and intestinal cells was characterised by saturable, specific processes whereas skin fibroblasts from normal humans did not convincingly demonstrate saturability and had a lower affinity and capacity compared with adrenal and intestinal cells. Post-receptor events also appeared to differ. Cells from the adrenal cortex and gut showed similar binding affinities for HDL3 but the capacity for binding and for degrading HDL3 was much higher with intestinal cells. The large amounts of HDL degraded by intestinal cells suggest a specific role for the gut in HDL catabolism, and that, in the rat, intestinal cholesterol may be derived from circulating HDL. Finally, it is suggested that rat adrenal cortical and intestinal mucosal cells possess surface receptors for HDL3 which differ from the LDL receptor.

Slower removal of intestinal apolipoprotein B-48 than of apolipoprotein B-100 in severely hypertriglyceridemic subjects.

Apolipoprotein B, the major structural protein of triacylglycerol-rich lipoprotein, occurs in two immunologically distinct forms, termed apolipoproteins B-100 and B-48. In man, the former is associated with triacylglycerol-rich lipoproteins of hepatic origin and the latter with intestinal triacylglycerol-rich lipoproteins. We have studied the rates of removal of the two proteins when 125I-labelled triacylglycerol-rich lipoproteins were reinjected into six severely hypertriglyceridemic subjects showing hyperchylomicronemia, and into two normal subjects. The specific radioactivities of apolipoproteins B-100 and B-48 were determined over periods of up to 30 h. In all six hyperlipemic subjects the removal of apolipoprotein B-100 was either significantly faster than that of apolipoprotein B-48 (in four) or similar in the two who cleared triacylglycerol-rich lipoproteins very slowly. When triacylglycerol-rich lipoproteins from two hyperlipemic subjects (in whom apolipoprotein B-48 was cleared more slowly) were injected into two normal subjects both B apolipoproteins were cleared at similar rates. Since apolipoprotein B-48 appears to be a marker for remnants of intestinal particles, its slower removal than that of apolipoprotein B-100 in severe hypertriglyceridemia suggests that one metabolic defect associated with the hyperchylomicronemia is defective removal of chylomicron remnants.

The hormonal stimulation of high-density lipoprotein binding, internalization and degradation by cultured rat adrenal cortical cells.

Several studies have shown that high-density lipoprotein stimulates steroidogenesis in rat tissues which have been treated with pituitary hormones. To determine whether these hormones can directly affect receptors for high-density lipoprotein, we have incubated cultured rat adrenal cortical cells with 125I-labeled human HDL3 and studied the effect of corticotrophin on the binding, internalization and degradation of this lipoprotein. ACTH stimulated all these parameters of HDL metabolism in a dose-dependent manner with maximal stimulation occurring between 10 and 20 mU/ml. The effect was temperature-dependent and showed target cell specificity. Although the hormone stimulated binding and internalization 5-6-fold, degradation of HDL3 was substantially less (2-fold) than anticipated. This suggests that the lipoprotein was taken up by vesicles resembling receptosomes which escape fusion with lysosomes; thus degradation of entrapped particles does not occur, while transfer of cholesterol for steroidogenesis is unaffected.