Alterations of high density lipoproteins induced by thyroid hormones in man and rat.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
G Ponsin
Publications and source records attributed to G Ponsin.
Cholesterol-induced alteration of apolipoprotein A-I conformation in reassembled high density lipoprotein.
Recent reports have shown that apolipoprotein A-I (apo A-I), the major protein of high density lipoprotein (HDL) may exist in different conformational states. We studied the effects of apolipoprotein A-II and/or cholesterol on the conformation of apo A-I in reassembled HDL. Analysis of tryptophan fluorescence quenching in the presence of iodine suggested that cholesterol increased the number of apo A-I tryptophan residues accessible to the aqueous phase, but decreased their mean degree of hydration. These observations cannot be totally explained on the basis of the effect of cholesterol on phospholipid viscosity as determined by fluorescence anisotropy of diphenyl hexatriene. We did not observe any effect of apo A-II on the conformation of apo A-I.
Apolipoprotein A-I↗
Hepatic lipase activity during oral and parenteral 17 beta-estradiol replacement therapy: high-density lipoprotein increase may not be antiatherogenic.
OBJECTIVE: Hepatic lipase activity is involved in the removal of cholesterol and phospholipid from plasma high-density lipoprotein (HDL) to the liver. Changes in hepatic lipase are responsible for some lipoprotein modifications observed during estrogen administration (i.e., increased HDL levels). The aim of this study was to compare the effects of alternative routes of administration of estrogen on hepatic lipase activity and lipoprotein metabolism. DESIGN, PATIENTS: The effects of oral and parenteral estradiol (E2) replacement therapy on post-heparin hepatic lipase were studied in the short-term (6 weeks) in postmenopausal women. INTERVENTIONS: Ten patients received 2 mg/d or oral micronized E2 and 10 patients 3 mg/d of percutaneous E2. RESULTS: Both treatments significantly increased plasma E2 levels. Hepatic lipase decreased (-33%) significantly (P less than 0.05), and the phospholipids and free cholesterol content of HDL and HDL3 increased significantly (P less than 0.05) during oral but not during parenteral treatment. CONCLUSIONS: The short-term pharmacological effect of E2 on hepatic lipase depends on the route of administration. The inhibition of this enzyme may reduce the removal of HDL-cholesterol by the liver. The expected vascular benefits of such a pharmacological increase in HDL are questionable.
Administration, Cutaneous↗
[Modifications, due to nutrition, of methyl and methylene 1H NMR signals in human plasma and normal serum samples].
The line width test has been realised on twelve normal healthy persons (seven men and five women). Blood was collected at different time during one day. The results obtained when blood samples was taking after eating, showed an appreciable decrease in the average methylene and methyl line width. The same test realised on clear serum samples added up with increasing quantities of triglycerids rich lipoproteins leads to the same result.
Blood Chemical Analysis↗
Defect in the net transport of cholesterol from cultured fibroblasts to serum of hypothyroid patients.
Explore the source record for details and available documents.
Biological Transport↗
Non enzymatic glycation of apolipoprotein A-I. Effects on its self-association and lipid binding properties.
In diabetic patients, hyperglycaemia results in the non enzymatic glycation of many proteins including apolipoprotein A-I. We purified glycated apo A-I and compared its lipid binding properties to those of normal apo A-I. Analysis of tryptophan fluorescence spectra and of fluorescence quenching in the presence of iodine showed that glycation of apo A-I induces a decrease in the stability of the lipid-apoprotein interaction and in that of the apoprotein self-association. Repetitive ultracentrifugations of High Density Lipoprotein (HDL) samples containing radioiodinated apo A-I or glycated apo A-I revealed that glycation of the apoprotein facilitates its dissociation from HDL. These results suggest that the non enzymatic glycation of apo A-I may affect the structural cohesion of HDL particles.
Algorithms↗
Relationship between structure and metabolism of HDL apolipoproteins: study with synthetic peptides.
Explore the source record for details and available documents.
Animals↗
Characterization and metabolism of glycated high density lipoproteins in diabetic patients.
Explore the source record for details and available documents.
Adult↗
Direct evidence for the modulation of human platelet cytosolic free Ca2+ by intracellular cyclic AMP produced with a photoactivatable derivative.
We have previously reported that intraplatelet "cyclic AMP jumps" produced with newly synthesized photoactivatable cyclic AMP analogue, inhibited washed rat platelet aggregation and serotonin release as induced by thrombin. Using the same approach on human platelets, thrombin-induced platelet aggregation was dose-dependently inhibited only when a flash was delivered. The mechanism of action of intraplatelet cyclic AMP as resulting from photolysis could be by controlling the level of cytosolic Ca2+. In order to test this hypothesis, the same protocol was used on human platelets preloaded with the internal Ca2+ fluorescent indicator, Quin 2, we found that the extent and the rate of the rise of the cytosolic Ca2+ induced by thrombin were dramatically decreased, in the presence of the photoactivatable cyclic AMP, only following photoirradiation. In addition, the flashes were produced, in the presence of photoactivatable cyclic AMP, after the thrombin-induced rise of internal Ca2+ had reached its peak. In these conditions, photoirradiation caused a rapid fall in fluorescence. These experiments provide the first direct evidence that intracellular cyclic AMP is involved in the control of platelet cytosolic Ca2+ by inhibition of its mobilization and by stimulation of its sequestration.
Aminoquinolines↗
High density lipoprotein alterations induced by bezafibrate in healthy male volunteers.
The alterations of HDL structure and metabolism induced by bezafibrate administration were studied in healthy male volunteers. As usually observed in hyperlipaemic patients, bezafibrate induced a decrease of the plasma concentrations of apo B and LDL-cholesterol and an increase of that of HDL-cholesterol. Analysis of HDL by gradient polyacrylamide gel electrophoresis revealed that bezafibrate administration resulted in a change of the particle size distribution likely suggesting a drop of the HDL2/HDL3 ratio. This was accompanied by a 30% enhancement of the plasma concentration of apoprotein A-II, while that of apoprotein A-I remained unchanged. These data suggest an increase of the HDL concentration, preferentially in the HDL3 subfraction. In spite of these HDL alterations, there was no evidence of change in the three stages of the reverse pathway of cholesterol, since bezafibrate did not induce any significant alteration in the in vitro properties of plasma with respect to (a) cholesterol transport from cultured cells, (b) cholesterol esterification, and (c) transfer of cholesteryl esters from HDL to VLDL-LDL.
Adult↗
Time-course of alterations of high density lipoproteins (HDL) during thyroxine administration to hypothyroid women.
Serum HDL cholesterol, apolipoproteins AI and AII and post heparin lipolytic activities (PHLA) have been measured in a group of fourteen hypothyroid women without ovarian oestrogen secretion before and during a 2-month thyroxine treatment. The more rapid and consistent observed event was a decrease in apo AI levels (164 +/- 5 vs. 149 +/- 5 mg dl-1, mean +/- SEM, P less than 0.05) correlated (r = 0.79, P less than 0.05) to a slight increment of PHLA. A slight decrease in apo AII concentration was seen only after 5 days (25 +/- 2 vs. 22 +/- 2 mg dl-1, P less than 0.05) and in HDL cholesterol only after 60 days (1.3 +/- 0.6 vs. 1.1 +/- 0.5 mmol 1-1, P less than 0.05). Apo AI, HDL2 and HDL3 cholesterol were measured in another group of seven hypothyroid postmenopausal women before and after a 2-month thyroxine treatment. We observed a decrease in HDL2 cholesterol (1.69 +/- 0.20 vs. 1.17 +/- 0.09 mmol 1-1, P less than 0.02) with no changes in HDL3 cholesterol (0.88 +/- 0.09 vs. 0.99 +/- 0.06 mmol 1-1, NS). The decrease in HDL2 cholesterol correlated (r = 0.72, P = 0.05) with that for apo AI. The differential influence of thyroxine (T4) administration on the major HDL components might reflect changes in HDL composition due to the multiple effects of thyroid hormones on lipid metabolism. It can be hypothesized that the decrease in apo AI and HDL2 cholesterol concentrations are due, at least in part, to the increase in hepatic lipase activity.
Adolescent↗
Lipid-peptide association and activation of lecithin:cholesterol acyltransferase. Effect of alpha-helicity.
A series of apolipopeptides, in which single proline substitutions were made at various sites in the 20-residue sequence, have been synthesized and tested. These peptides have nearly the same hydrophobic content, but very different helical contents, in a structure-making solvent. The affinity of these peptides for phospholipids was evaluated on the basis of their intrinsic tryptophan fluorescence and equilibrium dialysis against model high density lipoproteins. Proline substitutions at one end of the peptide had little or no effect on the fluorescence, circular dichroism, affinity for model high density lipoproteins, or activation of human plasma lecithin:cholesterol acyltransferase. By contrast, there was a dramatic change in all of these variables as the site of substitution was moved progressively closer to the middle of the peptide. All of these data suggested that a helix breaker that is substituted at the midpoint of a helical surface-associating peptide will greatly reduce its affinity for phospholipid surfaces. These results demonstrate that helicity and hydrophobicity are independent determinants of the affinity of an apolipopeptide for a phospholipid surface.
Amino Acid Sequence↗
In vivo interaction of synthetic acylated apopeptides with high density lipoproteins in rat.
The metabolism of synthetic peptide analogues of high density lipoprotein (HDL) apoproteins has been studied in the rat. These compounds are 15-amino acid lipid associating peptides (LAPs) bearing acyl chains of various lengths (0-16 carbon units). After injection of each 125I-LAP, the serum decay curves suggested a two-compartment process with a clearance rate decreasing when the acyl chain lengths increased. The similarity between the apparent half-life of C16-LAP and that of apoprotein A-I as well as the chromatographic analysis of rat serum were consistent with a partitioning of the LAPs between HDL and the aqueous phase. This was strongly dependent upon the acyl chain length of the LAPs. The distribution volumes of the 125I-LAPs in organs were measured 10 min after injection. The results were analyzed using a model explicitly predicting the organ distribution volumes of HDL and the equilibrium constant (Keq) of the binding of each LAP to HDL. HDL distributed significantly in the adrenals (250 microliters/g), liver (80 microliters/g), and ovaries (55 microliters/g), but not in the kidneys. This suggests that the binding of HDL apoproteins to kidneys, reported by others, was due to the uptake of free apoproteins. The Keqs exhibited a log-linear relationship with respect to the acyl chain length of the LAPs. Each carbon unit added to the acyl chain decreased the free energy of association by a constant value (0.3 kcal mol-1). This clearly showed a strict hydrophobic effect similar to that previously observed in vitro.
Animals↗
Abnormal interaction of the human apolipoprotein A-I variant [Lys107----0] with high density lipoproteins.
Several isoforms of apoprotein A-I [apoA-I], the major apoprotein of high density lipoproteins [HDL], have been described. We compared the in vivo and in vitro properties of normal human apoA-I with those of apoA-I [Lys107----0]. Fluorescence and circular dichroic spectra showed that deletion of Lys107 decreases apoprotein self-association. In vivo metabolic studies in the rat indicated that the interaction of apoA-I [Lys107----0] with HDL was lower than normal. We conclude that deletion of Lys107 results in a reorganization of the apoprotein structure that decreases its potential to form hydrophobic associations.
Animals↗
Equilibrium of apoproteins between high density lipoprotein and the aqueous phase: modelling of in vivo metabolism.
There is a growing body of evidence that apoproteins of the high density lipoproteins (HDL) exchange between lipoprotein particles through the aqueous phase and that there is always a finite but physiologically important quantity of lipid-free apoprotein in plasma. We have studied the theoretical consequences of this concept on the catabolism of apoproteins and developed a testable model for studying the in vivo metabolism of HDL apoproteins. This model describes the putative modification of the apparent tissue distribution spaces of the apoprotein that would result from a change in the partitioning of this apoprotein between HDL and the aqueous phase. The main parameters predicted by the model include the tissue distribution spaces of free monomeric apoprotein, those of HDL-bound apoprotein and the partition coefficient of the apoprotein between HDL and the aqueous phase in blood. We have designed several ways of testing this model in vivo including the use of model synthetic apoproteins. This model can be generalized to a number of other binding systems.
Animals↗
In vitro binding of synthetic acylated lipid-associating peptides to high-density lipoproteins: effect of hydrophobicity.
To measure the effect of hydrophobicity on the binding of model apoproteins to lipoproteins, we synthesized a 15 amino acid lipid-associating peptide (LAP) with acyl chains of various lengths (0-18 carbons) bound to the N-terminal amino acid through a peptide bond. The acylated LAPs preferentially bound to high-density lipoprotein (HDL) and were activators of lecithin:cholesterol acyltransferase. Circular dichroic spectra indicated that the LAP association with phospholipid was accompanied by increased alpha-helical structure. The LAPs self-associated in solution as judged from tryptophan fluorescence analysis. These characteristics, which are comparable to those of apolipoprotein A-I, were strongly dependent upon the acyl chain length of the LAPs. The equilibrium constants (Keq) for the association of LAPs to reassembled HDL were measured by equilibrium dialysis at several temperatures. At 37 degrees C, Keq increased by 3 orders of magnitude as the number of carbon units was increased from 0 to 16; there was a log-linear relationship between Keq and the acyl chain length. The free energy of association (delta Ga) decreased by a constant value for each methylene unit added to the acyl chain (0.35 kcal mol-1), clearly demonstrating a strict hydrophobic effect. This change of delta Ga was enthalpy rather than entropy driven. Our data show that, with all other parameters including putative alpha-helicity, sequence, and molecular weight being constant, the binding of a lipid-associating peptide to lipoprotein is governed by its hydrophobicity.
Apolipoprotein A-I↗
Control of thyrotropin glycosylation in normal rat pituitary cells in culture: effect of thyrotropin-releasing hormone.
Regulation of glycosylation of TSH was studied in primary cultures of normal rat pituitary cells. [3H]Glucosamine or [3H]proline incorporation into immunoprecipitable TSH and trichloroacetic acid-precipitable proteins was measured after incubation periods ranging from 4-72 h. TSH release was assessed by RIA of TSH in the medium. TRH (30 nM) specifically increased the glycosylation of TSH despite the fact that it did not stimulate [3H]proline incorporation into the hormone even after 72 h of continuous labeling. The TRH-stimulated [3H]glucosamine-labeled TSH was completely recovered in the incubation medium. Effective concentrations of TRH were in the same range as those necessary for stimulation of TSH release (10(-10) - 10(-6) M). Somatostatin (50 nM) and T3 (10 microM) antagonized TRH effects on both TSH release and glycosylation. Stages of TSH glycosylation were discriminated by the addition to the culture medium of tunicamycin (10 micrograms/ml) or monensin (25 microM), which are known to inhibit core and terminal glycosylation of proteins, respectively. Medium [3H]glucosamine-labeled TSH was fully glycosylated, whereas a large part of the intracellular hormone was only core glycosylated. This suggests that terminal glycosylation of TSH could be related to hormone secretion. TRH stimulated essentially only terminal glycosylation of TSH. No alteration of core glycosylation of the hormone was observed after TRH treatment. The stimulating effect of TRH on terminal glycosylation of TSH is probably related to its ability to stimulate hormone release.
Animals↗
Evidence for a coupling of prolactin secretion and synthesis by rat pituitary cells in culture.
The relationship between the release and the synthesis of prolactin by rat pituitary cells in culture was studied using a microtubule-disrupting drug, vinblastine. (1) Prolactin secretion was inhibited by vinblastine in short-term incubations. Vinblastine did not act via the dopamine pathway, since a potent anti-dopaminergic drug, fluphenazine, was unable to reverse the inhibiting action of the antimicrotubular agent. (2) Continuous treatment by vinblastine induced a progressive decrease of the rate of incorporation of [3H]leucine in prolactin. The half-inhibition time was about 2 days. This inhibition of prolactin synthesis was selective, since total protein synthesis remained unaffected. (3) Measurements of radioimmunoassayable prolactin showed that the inhibition of hormone release by vinblastine led to a transient increase of the intracellular content of prolactin. The phase of over-accumulation was followed by a progressive reduction of the total (cell + medium) prolactin. This result is in agreement with the observed inhibition of de novo synthesis of prolactin and indicates that a degradation process takes place in pituitary cells in culture. In conclusion, the use of vinblastine allows us to demonstrate that the rate of prolactin synthesis is dependent upon the secretory status of the cell.
Animals↗