Identification and characterization of rat serum lipoprotein subclasses. Isolation by chromatography on agarose columns and sequential immunoprecipitation

Exchange of apolipoprotein A-I between lipid-associated and lipid-free states: a potential target for oxidative generation of dysfunctional high density lipoproteins

An important event in cholesterol metabolism is the efflux of cellular cholesterol by apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins (HDL). Lipid-free apoA-I is the preferred substrate for ATP-binding cassette A1, which promotes cholesterol efflux from macrophage foam cells in the arterial wall. However, the vast majority of apoA-I in plasma is associated with HDL, and the mechanisms for the generation of lipid-free apoA-I remain poorly understood. In the current study, we used fluorescently labeled apoA-I that exhibits a distinct fluorescence emission spectrum when in different states of lipid association to establish the kinetics of apoA-I transition between the lipid-associated and lipid-free states. This approach characterized the spontaneous and rapid exchange of apoA-I between the lipid-associated and lipid-free states. In contrast, the kinetics of apoA-I exchange were significantly reduced when apoA-I on HDL was cross-linked with a bi-functional reagent or oxidized by myeloperoxidase. Our observations support the hypothesis that oxidative damage to apoA-I by myeloperoxidase limits the ability of apoA-I to be liberated in a lipid-free form from HDL. This impairment of apoA-I exchange reaction may be a trait of dysfunctional HDL contributing to reduced ATP-binding cassette A1-mediated cholesterol efflux and atherosclerosis.

Diminished macrophage cholesterol removal rate by the altered HDL metabolism in the Nagase analbuminemic rat

Dyslipoproteinemia of the Nagase analbuminemic rat (NAR) is characterized by elevated concentrations of VLDL and LDL attributed to increased rates of liver lipoprotein synthesis. Increased lysophosphatidylcholine (LPC) in NAR HDL has been attributed to high plasma LCAT activity. We show here that, as compared with Sprague-Dawley rats (SDR), NAR plasma triacylglycerol (TAG), total cholesterol (TC), HDL TAG, protein, total phospholipids (PL), LPC, and PS are increased. These alterations rendered the NAR HDL particle more susceptible to the activity of the enzyme hepatic lipoprotein lipase (HL), which otherwise was unaltered in our study. Fractional catabolic rates in blood of the autologous 125I-apoHDL (median and lower quartile values), were, respectively, 0.231 and 1.645 (n = 10) in NAR as compared with 0.140 and 0.109 (n = 10) in SDR (P = 0.012), corresponding to synthesis rates of HDL protein of 89.8 +/- 33.7 mg/d in NAR and 17.4 +/- 6.5 mg/d in SDR (P = 0.0122). Furthermore, Swiss mouse macrophage free-cholesterol (FC) efflux rates, measured as the percent [14C]-cholesterol efflux/6 h, were 8.2 +/- 2.3 (n = 9) in NAR HDL and 11.2 +/- 3.2 (n = 10) in SDR HDL (P = 0.03). Therefore, in NAR the modification of the HDL composition slows down the cell FC efflux rate, and together with the increased rate of plasma HDL metabolism influences the reverse cholesterol transport system.

Effect of oncotic pressure on apolipoprotein A-I metabolism in the rat

The nephrotic syndrome is characterized by reduced plasma albumin and colloid osmotic pressure (pi), urinary protein loss and hyperlipidemia. High-density lipoprotein (HDL) and the level of apo A-I, the principal apolipoprotein in HDL, is increased in nephrotic rats and rats with hereditary analbuminemia (NAR)--animals with virtually no albumin in plasma and reduced plasma pi, but without proteinuria, suggesting that urinary protein loss is not responsible for increased plasma apo A-I levels. We conducted these studies to determine the mechanism responsible for increased plasma apo A-I levels in the nephrotic syndrome and NAR and to determine whether reduced plasma pi or albumin was responsible for increased apo A-I. We first measured the clearance of 125I apo A-I HDL in NAR and rats with passive Heymann nephritis (HN) compared with normal Sprague Dawley (SD) control. Both the clearance of apo A-I and fractional apo A-I turnover rate (FTR) were significantly reduced both in HN (7.40 +/- 2.18% plasma pool/hr) and NAR (5.63 +/- 1.12) compared with SD (9.87 +/- 0.75). Total apo A-I turnover rate, which in steady state equals apo A-I synthesis rate, was also significantly increased in both HN (487 +/- 127 micrograms/100 g body weight/hr) and NAR (253 +/- 16), compared with SD (216 +/- 19). Thus decreased apo A-I catabolism and increased synthesis both contributed to increased apo A-I levels in HN and NAR. We then infused either f3p4roncotic human albumin or ficoll into two additional groups of HN for days in quantities sufficient to maintain plasma pi within the normal range.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies with etofibrate in the rat. Part II: A comparison of the effects of prolonged and acute administration on plasma lipids, liver enzymes and adipose tissue lipolysis

To contribute to the understanding of the hypolipidemic action of etofibrate, which is the 1,2-ethandiol ester of clofibric acid and nicotinic acid, 300 mg of this drug/kg body weight or of the medium were administered daily by a stomach tube to normolipidemic rats. Some animals were decapitated at the 10th day of daily treatment (prolonged treatment), whereas others were studied at different times after one single administration (acute treatment). In animals on prolonged treatment etofibrate decreased plasma levels of cholesterol, triacylglycerols, free fatty acids (FFA) and glycerol, as well as the total and unesterified cholesterol concentrations, in liver microsomes. In these rats, etofibrate increased the activity of liver cytosolic glycerol-3-P dehydrogenase, whereas it decreased the activity of both microsomal HMG-CoA reductase and cholesterol 7 alpha-hydroxylase and did not affect acyl-CoA: cholesterol acyltransferase (ACAT). At 3, 5 and 7 h after acute treatment, etofibrate decreased plasma levels of triacylglycerols, glycerol and FFA, and this effect disappeared at 24 h, whereas plasma cholesterol did not change 3 h after etofibrate but decreased at 5 and 7 h and remained low after 24 h, and a similar change was found in the liver microsomes free cholesterol concentration. However, with the exception of a significant reduction in cytosolic glycerol-3-P dehydrogenase at 7 h and in ACAT at 5 h, acute etofibrate treatment did not affect the activity of the liver enzymes studied. At low concentrations (10(-5) M) in the incubation medium, etofibrate decreased the release of both FFA and glycerol by epididymal fat pad pieces incubated in vitro. These findings together with those previously reported by us in rats using a similar etofibrate treatment protocol [6] indicate that etofibrate decreases the availability of lipolytic products in the liver by acting on their release from adipose tissue and on their intrinsic hepatic metabolism. Consequently, this drug would decrease liver VLDL triacylglycerol synthesis and secretion, which together with facilitating the clearance of circulating triacylglycerols causes its hypotriglyceridemic effect. The hypocholesterolemic effect of etofibrate after acute treatment may be a secondary consequence of the reduced liver VLDL production caused by decreased adipose tissue lipolysis, but after prolonged treatment, this effect also seems to be influenced by the inhibition of HMG-CoA reductase activity which would reduce cholesterol synthesis.

Regulation of hepatic and non-hepatic apolipoprotein A-I and E gene expression during liver regeneration

In this study, we have determined what tissues other than liver express apolipoprotein (apo) A-I and apo E genes during liver regeneration at the level of the specific mRNAs, and have compared these findings with the serum values of high-density lipoprotein (HDL), low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL). Our results show that liver and intestine express most of the apo A-I mRNA during liver regeneration. Although apo E mRNA is expressed principally by the liver, its expression is reduced in liver during regeneration but is increased markedly in non-hepatic tissues, such as in intestine, kidney, lung and brain. These results suggest that humoral or circulating factors released during liver regeneration influence apolipoprotein E gene expression, not only in hepatic but also in non-hepatic tissue.

Triiodothyronine increases rat apolipoprotein A-I synthesis and alters high-density lipoprotein composition in vivo

The effects of altered serum 3,3',5-triiodothyronine levels on rat lipoprotein metabolism were examined. Daily injections of the hormone (50 micrograms/100 g body mass) over a period of six days led to an increase of 6.4-fold in the hepatic mRNA level for apolipoprotein(apo)A-I, and a 21% increase in serum apoA-I levels. 12h after a single injection of 3,3',5-triiodothyronine the rate of [14C]leucine incorporation into apoA-I increased 2.1 fold. Conversely, in hypothyroid rats there was a decrease in hepatic mRNA levels for apoA-I and a decreased rate of [14C]leucine incorporation into apoA-I. The increase in hepatic apoA-I mRNA levels following 3,3',5-triiodothyronine treatment occurred prior to significant changes in serum triacylglycerol levels. High-density lipoprotein (HDL) particles isolated from the serum of hyperthyroid rats were smaller and enriched in apoA-I compared to apoA-IV and apoE. Similar changes in HDL composition were observed following in vitro incubations of normal rat serum with purified rat apoA-I. The results suggest that during altered thyroid status, changes in serum HDL size and composition occur in association with significant changes in apoA-I gene expression.

Effect of lipid transfer protein on plasma lipids, apolipoproteins and metabolism of high-density lipoprotein cholesteryl ester in the rat

The role of human plasma lipid transfer protein (LTP) in lipoprotein metabolism was studied in the rat, a species without endogenous cholesteryl ester and triacylglycerol transfer activity. Partially purified human LTP was injected intravenously into rats. The plasma activity was between 1.5- and 4-fold that of human plasma during the experiments. 6 h after the injection of LTP, a significant increase in serum apoB, and no significant changes in serum total cholesterol, free cholesterol, triacylglycerols, apoA-I, apoE, or apoA-IV were noted. Cholesterol was increased in very-low density and low-density lipoproteins (VLDL and LDL) and decreased in large-sized apoE-rich HDL. ApoA-I-containing particles with a size smaller than in normal rats were present in serum of LTP-treated rats. The mean diameter of HDL particles decreased and apoE, normally present on large-sized HDL, was present on smaller sized particles. The metabolic fate of cholesteryl ester, originally associated with HDL, was studied by injection of [3H]cholesteryl linoleyl ether-labelled apoA-I-rich HDL in the absence and in the presence of LTP. The disappearance of [3H]cholesteryl linoleyl ether, injected as part of apoA-I-rich HDL, from serum was increased in the LTP-treated rats; the t1/2 changed from 3.9 to 2.2 h, resulting in an increased accumulation of [3H]cholesteryl linoleyl ether in the liver. This can be explained by the redistribution of HDL [3H]cholesteryl linoleyl ether to VLDL and LDL in the presence of LTP, leading to the combined contribution of VLDL, LDL and HDL to the hepatic uptake. The present findings show profound effects of LTP on the chemical composition of HDL subspecies, the size of HDL and on the plasma turnover and hepatic uptake of cholesteryl esters originally present in apo A-I-rich HDL.