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Biomedical subjects

A Angel

Publications and source records attributed to A Angel.

At least 73 records · Page 4Linked to original sources

Selective uptake of HDL cholesterol ester by human fat cells.

In humans, high-density lipoprotein (HDL)-cholesterol ester turnover exceeds that of HDL apoproteins by severalfold or more, suggesting an independent catabolic fate of these constituents. The present study investigated the cellular uptake and dissociation of HDL labeled in its apoproteins with 125I and in its cholesterol ester with [3H]cholesteryl palmityl ether, a nonhydrolyzable cholesterol ester analogue. Approximately 50% of cell-associated 125I-HDL2 and 125I-HDL3 was released from prelabeled adipose cells by incubating the latter in the presence or absence of unlabeled lipoproteins for 2 h. The uptake of HDL-cholesterol ester by human fat cells as reflected by [3H]cholesteryl palmityl ether was 5-18 times greater than that predicted from the uptake of 125I-HDL2 and 125I-HDL3 and was irreversible. Analysis of dissociated 125I-HDL3 demonstrated changes to both higher and lower density fractions compared with the starting material. There was a high correlation between the cellular uptake of HDL3-cholesterol ester and HDL3-apoprotein uptakes (r = 0.90, P less than 0.01), suggesting that HDL-cholesterol ester uptake requires a specific apoprotein interaction or binding step. The selective uptake and retention of HDL-cholesterol ester by isolated adipocytes implies that human fat tissue may play a role in regulating the lipid composition of plasma HDL.

Adipose Tissue↗

Interactions of high density lipoprotein subclasses (HDL2 and HDLc) with dog adipocytes: selective effects of cholesterol and saturated fat feeding.

Adipose tissue is a cholesterol storage organ and derives its cholesterol primarily from circulating lipoproteins. The present study shows that adipocytes isolated from canine omental fat tissue interact specifically with high density lipoprotein subfractions lacking or enriched in apolipoprotein E, namely canine high density lipoprotein-2 (HDL2) and HDLc, respectively. While 125I-labeled HDL2 binding was inhibited similarly by both excess unlabeled HDLc and HDL2, 125I-labeled HDLc interaction was inhibited by its homologous ligand only. Paired studies showed that the amount of HDLc associated with adipocytes was significantly higher compared to HDL2. The effect of a short-term cholesterol and saturated fat feeding on adipocyte-HDL interaction was examined using fat cells obtained from dogs before and again 3 weeks after a diet supplemented with cholesterol (1% w/w) and saturated fat (30% lard, w/w). Significant increases in body weight and omental fat cell weight occurred after fat feeding. The amount of 125I-labeled HDL2 that could be bound to adipocytes increased after the diet, whether expressed on a per cell basis (P less than 0.005) or per unit cell surface (P less than 0.025). The amount of cell-associated 125I-labeled HDLc, however, was not significantly affected by the cholesterol-rich diet. The characteristics of HDLc and HDL2 dissociation were assessed by examining the release of labeled lipoproteins from adipocytes preincubated with 125I-labeled HDLc and 125I-labeled HDL2. HDL2 dissociation from adipocytes was significantly decreased (P less than 0.05) following the diet and may explain in part the apparent increase in cell-associated 125I-labeled HDL2.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗

The role of apolipoprotein A-I and apolipoprotein A-II in high-density lipoprotein binding to human adipocyte plasma membranes.

Adipocyte plasma membranes purified from omental fat tissue biopsies of massively obese subjects possess specific binding sites for high-density lipoprotein (HDL3). This binding was independent of apolipoprotein E as HDL3 isolated from plasma of an apolipoprotein E-deficient individual was bound to a level comparable to that of normal HDL3. To examine the importance of apolipoprotein A-I, the major HDL3 apolipoprotein, in the specific binding of HDL3 to human adipocytes, HDL3 modified to contain varying proportions of apolipoproteins A-I and A-II was prepared by incubating normal HDL3 particles with different amounts of purified apolipoprotein A-II. As the apolipoproteins A-I-to-A-II ratio in HDL3 decreased, the binding of these particles to adipocyte plasma membranes was reduced. Compared to control HDL3, a 92 +/- 3.1% reduction (mean +/- S.E., n = 3) in maximum binding capacity was observed along with an increased binding affinity for HDL3 particles in which almost all of the apolipoprotein A-I had been replaced by A-II. The uptake of HDL cholesteryl ester by intact adipocytes as monitored by [3H]cholesteryl ether labeled HDL3, was also significantly reduced (about 35% reduction, P less than 0.005) by substituting apolipoprotein A-II for A-I in HDL3. These data suggest that HDL binding to human adipocyte membranes is mediated primarily by apolipoprotein A-I and that optimal delivery of cholesteryl ester from HDL to human adipocytes is also dependent on apolipoprotein A-I.

Adipose Tissue↗

HDL clearance and receptor-mediated catabolism of LDL are reduced in hypothyroid rats.

Hypercholesterolemia associated with hypothyroidism is due partly to increased plasma LDL and partly to increased HDL cholesterol concentrations. The increase in LDL cholesterol has been shown to be secondary to reduced plasma clearance of LDL. To determine which catabolic route was thyroid dependent, the present study examined the effects of hypothyroidism on the receptor-mediated pathway and the 'receptor-independent' pathway of LDL metabolism. Wistar rats (327 +/- 22 g; mean +/- SD) were made hypothyroid by feeding propylthiouracil (0.1%, w/w) and rat 131I-LDL (rLDL; d = 1.019-1.050) and 125I-methylated-LDL (rLDL-CH3) were simultaneously injected i.v. after which the rates of clearance of labelled LDL in plasma were determined over 0-54 h. Total LDL and 'receptor-independent' clearances were represented by clearance of 131I-rLDL and 125I-rLDL-CH3 respectively and the difference between the two represented high affinity receptor-mediated clearance. The data were analyzed using Matthews' model and the fractional catabolic rates (FCR) were calculated. The FCR of rLDL clearance via the receptor-mediated pathway was 0.1042 +/- 0.0112 pools/h (n = 6) in controls vs. 0.0613 +/- 0.0079 pools/h (n = 6) in hypothyroid animals (P less than 0.01). The FCR via the 'receptor-independent' pathway was 0.0642 +/- 0.0040 pools/h (n = 6) in controls vs. 0.0561 +/- 0.0036 pools/h (n = 6) in hypothyroid animals (not significant). The plasma HDL cholesterol concentration was also increased in hypothyroid rats (70.4 +/- 6.7 mg/dl) compared to control (53.3 +/- 3.1 mg/dl) (P less than 0.025).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

Regional variation in high-density lipoprotein binding to human adipocyte plasma membranes of massively obese subjects.

Obesity is associated with significant changes in cholesterol and lipoprotein metabolism. High density lipoprotein (HDL) cholesterol is often reduced and adipose tissue cholesterol stores are increased in obese individuals. This prompted a study on the binding of HLD fractions (HDL2 and HDL3) to adipocyte plasma membranes obtained from massively obese subjects (BMI greater than 37 kg m-2) undergoing gastroplasty. Regional variation in HDL binding to these adipocyte plasma membranes was demonstrated. Membranes derived from the abdominal subcutaneous depot exhibited similar binding affinity (Kd) but higher binding capacity (Bmax) for HDL2 and HDL3 than that from the omental depot. There was significant inter-individual variation in Bmax but the amount of HDL2 or HDL3 bound to the two depots of the same individual was positively correlated (HDL2, r = 0.66, P less than 0.05; HDL3, r = 0.88, P less than 0.01). While HDL2 binding showed a higher affinity (lower Kd) than HDL3, a significant positive correlation existed between HDL2 and HDL3 binding to the same adipocyte membranes (r = 0.89, P less than 0.01). A significant inverse correlation (P less than 0.05) was also observed between HDL2 and HDL3 binding to adipocyte membranes and plasma HDL-cholesterol concentration. These results suggest that adipose tissue is an important site of HDL metabolism and the subcutaneous fat depot may play a proportionally more significant role due to its higher HDL binding capacity. It is further suggested that increased HDL binding and metabolism by the expanded adipose tissue mass may contribute to reduced plasma HDL-cholesterol levels frequently associated with obesity.

Abdomen↗

Regional variation in HDL metabolism in human fat cells: effect of cell size.

Abdominal obesity is related to reduced plasma high-density lipoprotein (HDL) cholesterol, and both are associated with cardiovascular disease risk. We have observed that plasma membranes from abdominal subcutaneous adipocytes have a greater HDL binding capacity than omental fat cell plasma membranes. The present study examined whether these binding characteristics could be due to differences in fat cell size or cholesterol concentration between the two adipose depots. Abdominal subcutaneous and deep omental fat were obtained from massively obese patients at surgery. Subcutaneous abdominal fat cells were significantly larger and their cellular cholesterol content greater than omental adipocytes. The uptake of HDL by collagenase-isolated fat cells was studied by incubating the cells for 2 h at 37 degrees C with 10 micrograms/ml 125I-HDL2 or 125I-HDL3. In both depots, the cellular uptake of 125I-HDL2 and 125I-HDL3 was specifically inhibited by addition of 25-fold excess unlabeled HDL and a close correlation was observed between the cellular uptake of 125I-HDL2 and 125I-HDL3. In obese patients, the uptake of 125I-HDL was higher in subcutaneous cells than in omental cells [5.85 +/- 0.53 vs. 2.74 +/- 0.30 pmol X 2 h-1. (10(6) cells)-1]. The cellular 125I-HDL uptake was significantly correlated with adipocyte size and fat cell cholesterol content but not with adipocyte cholesterol concentration. These results suggest that the higher HDL uptake observed in subcutaneous cells compared with omental cells in obesity is the result of differences in adipocyte size rather than differences in the cholesterol concentration (cholesterol-to-triglyceride ratio).(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗

Effects of weight loss in massive obesity on insulin and C-peptide dynamics: sequential changes in insulin production, clearance, and sensitivity.

In massively obese patients hyperinsulinemia and insulin insensitivity usually improve with weight loss. To clarify the mechanism of these reversible abnormalities eight nondiabetic massively obese patients were studied before and at intervals (3 months and 1 yr) after weight loss following gastroplasty. Insulin dynamics were studied during the hyperglycemic clamp (change in glucose, 7 mmol L-1 for 2 h) by measuring the area under the insulin and C-peptide response curves, representing, respectively, systemic insulin response and insulin production. Compared to lean age-matched normal subjects the massively obese patients had the expected fasting hyperinsulinemia and an exaggerated insulin response (P less than 0.05). Within 3 months and after an approximately 20% weight loss, they had a marked reduction in the systemic insulin response but no change in the C-peptide response. Therefore, the reduction in insulin response was due to enhanced hepatic insulin clearance rather than reduced insulin production. Thus, the liver serves a gate-keeping role in regulating the systemic insulin response to a glucose challenge. With additional weight loss of 14% and then weight maintenance, insulin clearance was further increased, and a reduction in insulin production became evident, since the C-peptide response was reduced. Exogenous insulin clearance was measured using the euglycemic clamp technique before and after weight loss. Insulin clearance was initially lower in the massively obese patients compared to that in the normal subjects (P less than 0.05) and increased toward normal with weight loss (P less than 0.05). Similarly, insulin sensitivity, as measured by the ratio of glucose metabolised per U endogenous insulin, normalized with weight loss and weight maintenance. Thus, after significant weight loss followed by weight maintenance at a reduced, but not ideal, level, insulin clearance, production, and sensitivity all reverted to normal. These findings suggest that adipose mass per se may not be exclusively responsible for altered insulin and glucose dynamics in obesity and that additional factors associated with obesity, such as nutrient load, adipose distribution, fat cell size, or fatty acid flux, play a contributing role.

Adult↗

Interaction of high density lipoprotein with adipocytes in a new patient with Tangier disease.

A 56-year-old man, the offspring of a consanguineous first cousin marriage, presented with clinical, morphological, and biochemical features of familial deficiency of high-density lipoproteins (Tangier disease). Of 8 first- and second-degree relatives examined, 4 had either plasma apo A-I or HDL cholesterol concentrations 2 standard deviations below normal population mean on at least 1 occasion. The patient and a majority of his relatives also had high plasma apo B concentrations relative to their levels of cholesterol. Adipose tissue biopsy was undertaken to study HDL interaction with the patients' cells. Specific uptake of HDL3 was demonstrated in adipocytes of this patient, but was decreased relative to a control of similar fat cell size. However, no marked difference in fat cell cholesterol content was observed between the Tangier patient and the control. Thus it appears unlikely that adipocytes play a role in the etiology of Tangier disease.

Adipose Tissue↗

Modifications and degradation of high density lipoproteins.

It is evident that lipoprotein modifications, degradation and clearance from plasma and interstitial compartments involves both cellular and extracellular processing. Cellular uptake of the intact particle as a whole and/or selective removal of constituent apoproteins and lipids by various parenchymal cells goes on continuously. Regulation of these processes undoubtedly varies tissue to tissue and much remains to be clarified in human tissues in vivo. The metabolic effects of chemical, proteolytic, and lipolytic modification of lipoproteins secondary to transient cellular encounters (e.g. during transit through endothelial barriers, or reversible binding to cells) on apolipoprotein clearance remains to be defined. It is likely that multiple post-secretory modifications occur and together represent subtle regulatory events that modulate lipid shuttle functions and cellular targetting properties of HDL particles.

Adipose Tissue↗

Effects of massive obesity on insulin sensitivity and insulin clearance and the metabolic response to insulin as assessed by the euglycemic clamp technique.

Insulin sensitivity was studied in nine nondiabetic massively obese patients (one male and eight females ages 39.0 +/- 2.7 years, body mass index 47.1 +/- 1) by the euglycemic clamp technique (40 microU/m2/min) and compared to seven lean control subjects (three males and three females, ages 34.8 +/- 2.5 years, body mass index 23 +/- 1.1). Fasting plasma glucose, immunoreactive insulin, and C-peptide concentrations were higher in the massively obese patients than in the controls (P less than 0.025). Following exogenous insulin infusion, immunoreactive glucagon and C-peptide concentrations decreased similarly in the massively obese patients and controls, indicating normal sensitivity of the alpha and beta cell to insulin. Glucose uptake (M) expressed either as mg X min-1 of fat free mass was significantly reduced in the massively obese patients compared to the controls (P less than 0.001). Similarly, the M/I ratio (glucose uptake per unit of insulin) was significantly reduced in the massively obese patients (P less than 0.001). Free fatty acids and glycerol concentrations measured in the fasting state were significantly elevated in the massively obese patients (free fatty acids 678 +/- 51 v 467 +/- 55 mumol/L, P less than 0.05; glycerol 97 +/- 9 v 59 +/- 11 mumol/L, P less than 0.02). The effects of insulin on antilipolysis was assessed by measuring the reductions in free fatty acids and glycerol concentration during the glucose clamp study. Although the absolute levels remained higher in the massively obese patients, inhibition of lipolysis was similar in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Hydroxybutyric Acid↗

Enhanced binding of low and high density lipoproteins to human adipocyte plasma membranes: effects of temperature and proteases.

The specific binding of 125I-labelled low density lipoprotein ([125I]LDL to human adipocyte plasma membranes was higher at 37 than at 0 degree C. Prior treatment of membranes with pronase had no effect on LDL binding measured at 0 degree C but consistently stimulated binding at 37 degrees C. Plasmin was similar to pronase in enhancing LDL-specific binding, but thrombin was not as effective. 125I-labelled high density lipoprotein ([125I]HDL2) specific binding to human adipocyte plasma membranes was similarly sensitive to temperature and pronase treatment. Addition of the protease inhibitor aprotinin in the adipocyte membrane binding assay significantly reduced [125I]LDL binding at 37 degrees C (p less than 0.05), suggesting the involvement of a protease activity intrinsic to the lipoproteins and (or) membranes. These data demonstrate that both LDL and HDL binding in human adipocyte plasma membranes can be "up-regulated" by specific proteolytic perturbations in a temperature-dependent manner.

Adipose Tissue↗

Animal models of myoclonus using 1,2-dihydroxybenzene (catechol) and chloralose.

A description of the physiological effects and pharmacological actions of catechol (1,2-dihydroxybenzene) and chloralose (monoglucochloralose) on muscular activity in animals has been presented. Catechol produces a stimulus-sensitive state wherein a tactile stimulus to a limb evokes a jerk in the muscles of that limb. This jerk can consist of three components: a polysynaptic spinal reflex, a periphero-corticospinal reflex, and a brainstem reflex. Chloralose also produces a stimulus-sensitive state with two components: a polysynaptic spinal reflex in the stimulated limb and a presumed spino-bulbo-spinal reflex activation of all body flexor muscles. The relevance of these agents as animal models for the mechanism of generation of human myoclonic activity has been discussed.

Acetylcholine↗

Arrest of neuropathy and myopathy in abetalipoproteinemia with high-dose vitamin E therapy.

A 16-year-old girl, one of dizygotic twins, presented in 1976 complaining of a 1-year history of a lack of coordination and an inability to run. The results of biochemical tests confirmed the diagnosis of classic abetalipoproteinemia. In addition to the recognized neurologic features of this disorder, she had a reduced evoked motor unit potential and markedly elevated serum levels of muscle enzymes, which suggested myositis. The serum vitamin E level was markedly decreased. Oral therapy with vitamin E, 800 mg daily, was begun, and in 1981 the dosage was increased to 3200 mg daily. Over the 7 years of follow-up she improved clinically, there was an increase in the evoked motor unit potential, the serum levels of some of the muscle enzymes decreased to normal, and the serum and tissue vitamin E levels increased significantly. It was concluded that treatment with high doses of vitamin E was responsible for the arrest of the usually progressive neuropathy and myopathy.

Abetalipoproteinemia↗

Cellular localization of metallothionein in the lung following repeated cadmium inhalation.

Cadmium and metallothionein (MT) levels were measured in lungs derived from Lewis rats exposed 0, 5, 10, 15, 20, 24, or 30 times to a Cd acetate aerosol (1.6 mg/m3). Cd burden rose from 2.21 micrograms in control lungs to 42.55 micrograms in lungs of animals exposed 30 times to the Cd aerosol. Pulmonary MT, expressed as nmoles Cd-thionein, paralleled Cd retention. Levels increased from 7.5 nmol in control animals to a value of 340.5 nmol in animals exposed 30 times to Cd. Energy dispersive X-ray microanalysis and the laser micro-probe analyzer were used to localize Cd within the lung. Lung tissue was prepared by a variety of chemical and physical fixation procedures. Cd localization proved unsuccessful by these techniques, presumably because the cellular Cd levels were below the limits of detectability. Using a immunohistochemical procedure which employed an antibody to MT, pulmonary MT was found in alveolar epithelial cells, fibroblasts, and lymphocytes. The MT was restricted to the nuclei of these cells.

Aerosols↗

Relationship of diet to the fatty acid composition of human adipose tissue structural and stored lipids.

The habitual intake of 20 healthy free-living subjects was determined by two 7-day food records. Documented fatty acid intakes were utilized to examine the influence of fatty acid intake on fatty acid composition of stored and structural lipids in subcutaneous adipose tissue. Subjects with higher intakes of saturated fatty acids exhibited increased levels of total saturated fatty acids and decreased polyunsaturated fatty acids in adipose tissue triglycerides (p less than 0.01). The dietary P/S ratio was significantly related to the saturated and polyunsaturated content of stored lipids. In the phospholipid fraction, relationships were found between dietary C18:2(6) and the P/S ratio of phosphatidylcholine (p less than 0.05). The essential fatty acid content of the two phospholipids studied was related to the dietary fats consumed. Relationships were identified between major fatty acids in the triglyceride and phospholipid fraction. Although diet was found to relate to fatty acid composition, the structural lipids in human adipose tissue appear more resistant to compositional change than stored triglycerides.

Adipose Tissue↗

Characterization of high density lipoprotein binding to human adipocyte plasma membranes.

Freshly isolated human adipocytes showed specific uptake of 125I-labeled human high density lipoprotein (HDL2 and HDL3), a portion of which could be released by subsequent incubation with excess unlabeled ligand. To study the mechanism of HDL binding, sucrose gradient-purified adipocyte plasma membranes were incubated with radioiodinated lipoprotein particles under equilibrium conditions in the absence (total binding) or presence (nonspecific binding) of 100-fold excess unlabeled ligand. Specific binding of HDL2 and HDL3, calculated by subtracting nonspecific from total binding, was Ca++ independent, unaffected by EDTA, and not abolished by pronase treatment of the membranes. Modification of HDL3 by reductive methylation or cyclohexanedione treatment also failed to affect its binding to adipocyte plasma membranes. High salt concentration (200 mM NaCl) inhibited specific binding of HDL2 and HDL3 but had no effect on LDL binding. A significant portion of 125I-HDL2 or 125I-HDL3 binding was consistently inhibited by adding excess unlabeled LDL, but this inhibition was incomplete as compared with a similar molar excess of unlabeled HDL2 or HDL3. The role of apoproteins (apo) in HDL binding to adipocyte membranes was examined by comparing binding of HDL2 and HDL3 isolated from normal, abetalipoproteinemic (abeta) and apo E-deficient (apo E0) plasma. Specific binding was observed with all normal and mutant HDL particles. Furthermore, a significant portion (61-78%) of abeta-HDL2, apo E0-HDL2, and apo E0-HDL3 binding was inhibited by adding 100-fold excess of unlabeled low density lipoproteins (LDL). The cross-competition of LDL and HDL binding was confirmed by the ability of normal, abeta, and apo E0-HDL2 to completely inhibit 125I-LDL binding. These data suggest that HDL binding is independent of apo E and that the responsible apoprotein(s) of HDL complete with LDL-apo B for binding to the same or closely related site in the adipocyte plasma membrane. Normal and apo E0-HDL3 binding was also completely inhibited by normal HDL2, which suggested that HDL2 and HDL3 probably bind to the same site. Scatchard analysis of normal HDL2, normal HDL3, and apo E0-HDL3 binding data best fitted a one-component binding profile with similar equilibrium dissociation constants (40-96 nM). HDL3 binding was found to be effectively inhibited by anti-human apo AI or anti-human apo AII, but not by anti-human apo B antisera. This binding was also unaffected by monoclonal anti-human apo B or E antibodies known to inhibit binding of apo B or apo E containing lipoprotein to the LDL receptor of cultured fibroblasts. These findings, taken together, suggest that human fat cells possess HDL binding sites with apo AI and /or apo AII specificity. The significant but partial inhibition of HDL2 and HDL3 binding by LDL along with the complete inhibition of LDL binding by HDL2 and HDL3 tends to exclude a single binding site that interacts both lipoproteins and favors the interpretation that LDL and HDL particles bind to multiple recognition sites or to different conformation of the same lipoprotein binding domain on the human fat cell.

Adipose Tissue↗

Characterization of low density lipoprotein binding to human adipocytes and adipocyte membranes.

125I-labeled low density lipoprotein (LDL) binding to purified plasma membranes prepared from freshly isolated human adipocytes was saturable, specific, and displaceable by unlabeled ligand. The maximum specific binding capacity measured at saturating concentrations of 125I-LDL was 1.95 +/- 1.17 micrograms of LDL bound/mg of membrane protein (mean +/- S.D., n = 16). In contrast to cultured fibroblasts, specific binding of LDL to adipocyte membranes was calcium-independent, was not affected by EDTA or NaCl, and was not destroyed by pronase. Plasma membranes purified directly from homogenized adipose tissue also showed calcium-independent LDL specific binding (0.58 +/- 0.33 micrograms of LDL bound/mg of membrane protein, mean +/- S.D. n = 11). Specific binding, internalization, and degradation of 125I-methylated LDL was demonstrated in isolated adipocytes and competition experiments showed that native and methylated LDL interacted with adipocytes through some common recognition mechanism(s). Compared to native LDL, specific binding of methylated LDL to adipocyte membranes was significantly reduced (43%), indicating that interaction of LDL with adipocyte was dependent in part on the lysine residues of apolipoprotein B. LDL binding to adipocyte plasma membranes was also competitively inhibited by human high density lipoprotein subfractions HDL2 and HDL3. Thus, LDL metabolism in mature adipocytes appears to be regulated by mechanisms distinctly different from a variety of cultured mesenchymal cells. In addition, the ability of adipocytes to bind, internalize, and degrade significant amounts of methylated LDL supports the view that adipose tissue is involved in the metabolism of modified lipoproteins in vivo.

Adipose Tissue↗