Diethylaminoethoxyhexestrol causes hypertriglyceridemia in guinea pigs

Treatment of rats, monkeys and man with diethylaminoethoxyhexestrol causes phospholipid storage in liver and other tissues. However, this drug has not been reported to alter plasma lipoprotein levels. When guinea pigs were treated with diethylaminoethoxyhexestrol, the fasting plasma triacylglycerol levels increased dramatically, from 43 to 1281 mg/dl, after only five doses of 12.5 mg/kg. Diethylaminoethoxyhexestrol-treated guinea pigs had reduced postheparin lipase activity. In addition, in vitro assays showed that this agent inhibited guinea pig postheparin lipoprotein lipase. It is hypothesized that diethylaminoethoxyhexestrol causes hypertriglyceridemia in guinea pigs because these animals are known to have low levels of serum activator for lipoprotein lipase and may be unusually susceptible to agents that inhibit lipoprotein lipase activity. The ability to produce hypertriglyceridemia in guinea pigs provides an animal model in which the metabolic consequences of hypertriglyceridemia can be studied.     

A selective deficiency of hepatic triacylglycerol lipase in guinea pigs

The properties of postheparin plasma triacylglycerol-hydrolyzing enzymes were investigated in guinea pig and rat. In rat, lipoprotein lipase and hepatic triacylglycerol lipase were separated on a heparin-Sepharose affinity chromatography. In postheparin plasma of guinea pig, however, hepatic triacylglycerol lipase was almost completely absent, while lipoprotein lipase was present. Hepatic triacylglycerol lipase was also deficient in the liver tissue extract of guinea pig. Plasma lipoprotein compositions of high-fat fed and control guinea pigs were analyzed. One of the outstanding changes found in high-fat fed animals was the presence of chylomicronemia. One guinea pig showed gross hyperlipemia with triacylglycerol concentrations of 2715 mg/100 ml. Plasma triacylglycerol concentrations of each lipoprotein fraction of very low density, intermediate density, low density and high density lipoproteins from high-fat fed animals were almost the same as those of the corresponding lipoprotein fractions from controls. Discussion was focused on the development of chylomicronemia in relation to the defects of triacylglycerol-hydrolyzing enzyme systems in this animal.     

Very low density lipoproteins and lipoprotein lipase in serum of rats deficient in essential fatty acids

Rats fed a diet deficient in essential fatty acids have a low level of serum very low density lipoproteins (VLDL). It was found that after intraperitoneal injection of heparin, deficient rats had a higher level of lipoprotein lipase activity in their plasma than did normal rats. VLDL isolated from serum of normal and deficient rats were compared as substrates for postheparin lipase of rat plasma. There was no significant difference in V(max) between the two preparations of lipoproteins, but the apparent K(m) for lipoproteins from deficient animals was significantly less than that for normal animals. These observations suggest that the low concentration of VLDL in deficient rats may be explained (a) by an increased activity of lipoprotein lipase in the tissues of these animals and (b) by the VLDL of deficient rats being more rapidly hydrolyzed at low concentrations by lipoprotein lipase than VLDL from normal rats.     

Nutritional regulation of lipoprotein lipase in guinea pig tissues

Glucose transport in guinea pig adipocytes has been shown to be markedly resistant to stimulation by insulin. Lipoprotein lipase is another transport catalyst in adipose tissue which is believed to be regulated by insulin. We have therefore studied how feeding-fasting affects lipoprotein lipase activity in guinea pig tissues. There was an even more marked decrease in adipose tissue lipoprotein lipase activity on fasting in guinea pigs (10-20 fold) than in rats or mice (4-5 fold). In adipocytes, the activity decreased only 2.5-4.5 fold; most of the change was in extracellular lipoprotein lipase. On glucose refeeding, the activity was rapidly restored. In the first 4 hours after glucose administration extracellular lipoprotein lipase activity increased to more than 10 times the amount present in adipocytes. After cycloheximide, lipoprotein lipase activity decreased with a half-life of 22 min. It is concluded that lipoprotein lipase is rapidly produced and turned over in guinea pig adipose tissue, and that the system is quite sensitive to feeding-fasting. In contrast to adipose tissue, there was no significant change in lipoprotein lipase activity in any other tissue on fasting. There was a strong correlation between the activities in heart and diaphragm muscle, but this correlation was independent of feeding-fasting.     

Characterization of the plasma lipoproteins of the genetically obese hyperlipoproteinemic Zucker fatty rat

The plasma lipoproteins of the Zucker fatty rat were characterized with respect to lipid and apoprotein composition, and results were compared with those obtained from lean controls. Information on apoproteins was obtained from gel filtration experiments and electrophoresis on polyacrylamide gels. Very low density lipoproteins (VLDL) were increased several-fold in fatties, and 78% of their mass was triglycerides compared with 60% in the controls. Low density (LDL) and high density (HDL) lipoproteins were increased by a factor of 2, although their compositions were similar to those of the controls. Levels of apoVLDL, apoLDL, and apoHDL were five, two and two times higher, respectively, in the fatties, and the two most rapidly moving subunit peptides on polyacrylamide gels were disproportionately elevated in the apoproteins. The slower of these two bands was present in relatively greater amounts than the faster one in fatties. If the slower peptide is an activator of lipoprotein lipase, analogous to the comparable subunit peptides of human apolipoproteins, plasmas of fatties could contain up to 10 times more lipase activator activity than control plasma. This finding, and the fact that adipose tissue lipoprotein lipase activity of fatties was about 150% of controls, suggests that fatties have increased capacities for VLDL catabolism. We have previously shown that hepatic VLDL secretory rates are higher than normal in these animals. The increased capacity for catabolism may be a response to the altered secretory rates.     

Hormone-sensitive lipase deficiency in mice changes the plasma lipid profile by affecting the tissue-specific expression pattern of lipoprotein lipase in adipose tissue and muscle.

Hormone-sensitive lipase (HSL) is believed to play an important role in the mobilization of fatty acids from triglycerides (TG), diglycerides, and cholesteryl esters in various tissues. Because HSL-mediated lipolysis of TG in adipose tissue (AT) directly feeds non-esterified fatty acids (NEFA) into the vascular system, the enzyme is expected to affect many metabolic processes including the metabolism of plasma lipids and lipoproteins. In the present study we examined these metabolic changes in induced mutant mouse lines that lack HSL expression (HSL-ko mice). During fasting, when HSL is normally strongly induced in AT, HSL-ko animals exhibited markedly decreased plasma concentrations of NEFA (-40%) and TG (-63%), whereas total cholesterol and HDL cholesterol levels were increased (+34%). Except for the increased HDL cholesterol concentrations, these differences were not observed in fed animals, in which HSL activity is generally low. Decreased plasma TG levels in fasted HSL-ko mice were mainly caused by decreased hepatic very low density lipid lipoprotein (VLDL) synthesis as a result of decreased NEFA transport from the periphery to the liver. Reduced NEFA transport was also indicated by a depletion of hepatic TG stores (-90%) and strongly decreased ketone body concentrations in plasma (-80%). Decreased plasma NEFA and TG levels in fasted HSL-ko mice were associated with increased fractional catabolic rates of VLDL-TG and an induction of the tissue-specific lipoprotein lipase (LPL) activity in cardiac muscle, skeletal muscle, and white AT. In brown AT, LPL activity was decreased. Both increased VLDL fractional catabolic rates and increased LPL activity in muscle were unable to provide the heart with sufficient NEFA, which led to decreased tissue TG levels in cardiac muscle. Our results demonstrate that HSL deficiency markedly affects the metabolism of TG-rich lipoproteins by the coordinate down-regulation of VLDL synthesis and up-regulation of LPL in muscle and white adipose tissue. These changes result in an "anti-atherogenic" lipoprotein profile.     

脂蛋白酯酶与动脉粥样硬化

脂蛋白酯酶(1ipopmtein lipase,LPL)是调节脂蛋白代谢的一种关键酶,如具有水解血浆脂蛋白中三酰甘油的作用等．体内LPL减少会导致血三酰甘油升高和高密度脂蛋白胆固醇降低,增加患动脉粥样硬化的危险．通过提高LPL的活性可以抑制动脉粥样硬化的发生发展．已有的研究说明NO-1886促进心肌和脂肪组织LPL mRNA表达,提高心肌、脂肪组织、骨骼肌和血液中LPL活性,因而改善脂蛋白代谢,抑制动脉粥样硬化．     

Insulin-mediated modifications of myocardial lipoprotein lipase and lipoprotein metabolism

Recirculating organ perfusion in vitro was conducted with hearts from control rats, animals given a single dose of streptozotocin (65 mg/kg) 48 h earlier, and streptozotocin-treated rats administered insulin (5 units), 2 h prior to organ perfusion. During 45-min perfusions, the lipolysis of very low density lipoprotein (VLDL) triglyceride was significantly less in hearts from diabetics than in controls (41.9 +/- 7.3% of control). This was associated with significant reductions in heparin-releasable (functional) lipoprotein lipase and tissue lipoprotein lipase of perfused hearts. The decreases in VLDL triglyceride metabolism and the levels of myocardial lipoprotein lipase were completely reversed by treatment of diabetic rats with insulin 2 h prior to study. Similar improvement of VLDL triglyceride metabolism and increases in myocardial lipoprotein lipase activity were observed in hearts from diabetic rats by direct addition of 100 milliunits/ml of insulin to the recirculating perfusion media. Under these conditions, the increase in both fractions of lipoprotein lipase in response to insulin was completely inhibited, and utilization of VLDL triglyceride was partially inhibited by pre-perfusion with cycloheximide for 10 min. The data derived from either VLDL triglyceride lipolysis in organ perfusion or direct measurement of myocardial lipoprotein lipase demonstrate a direct effect of insulin on myocardial lipoprotein lipase activity, and suggest that the response to insulin may be due in part to effects on protein synthesis.     

Changes in the concentration of plasma lipoproteins and apoproteins following the administration of Triton WR 1339 to rats.

Changes in whole plasma and lipoprotien apoprotein concentrations were determined after a single injection of Triton WR 1339 into rats. Concentrations of apoproteins A-I (an activator of lecithin:cholesterol acyl transferase), arginine-rich apoprotein (ARP), and B apoprotein were measured by electroimmunoassay. The content of C-II apoprotein (an activaor of lipoprotein lipase) was estimated by the ability of plasma and lipoprotein fractions to promote hydrolysis of triglyceride in the presence of cow's milk lipase and also by isoelectric focusing on polyacrylamide gels. Apoproteins C-II and A-I were rapidly removed from high density lipoprotein (HDL) after Triton treatment and were recovered in the d 1.21 g/ml infranate fraction. A-I was then totally cleared from the plasma within 10--20 hr after injection. Arginine-rich apoprotein was removed from HDL and also partially cleared from the plasma. The rise in very low density lipoprotein (vldl) apoprotein that followed the removal of apoproteins from HDL was mostly antributed to the B apoprotein, although corresponding smaller increases were observed in VLDL ARP and C apoproteins. The triglyceride:cholesterol, triglyceride:protein, and B:C apoprotein ratios of VLDL more closely resembled nascent rather than plasma VLDL 10 hr after Triton injection. These studies suggest that the detergent may achieve its hyperlipidemic effct by disrupting HDL and thus removing the A-I and C-II proteins from a normal activating environment compirsing VLDL, HDL, and the enzymes. The possible involvement of intact HDL in VLDL catabolism is discussed in relation to other recent reports which also suggest that abnormalities of the VLDL-LDL system may be due to the absence of normal HDL.     

Effects of cholestyramine on lipoprotein levels and metabolism in Syrian hamsters.

Oral administration of cholestyramine to adult male hamsters not only induced a marked decrease in plasma concentrations of cholesterol and LDL but had a similar lowering effect on plasma triacyglycerol and VLDL concentrations. The hypotriglyceridaemic effects of resin administration were not due to an increase in lipoprotein lipase, as post-heparin plasma lipoprotein lipase activities were unchanged, but rather to a 35% decrease in VLDL synthesis. Measurement of the disappearance rate of apolipoprotein B from VLDL after i.v. injection of 125I-labelled hamster or human VLDL into control and cholestyramine-fed recipient animals showed a 2-times lower T1/2 in the drug-treated animals. The fraction of VLDL apolipoprotein B, recovered at any time after injection in the LDL, was equal or higher in cholestyramine-fed animals as compared to controls. These data indicate that the lowering in plasma LDL by cholestyramine in male hamsters is due not only to LDL receptor up-regulation but also to a lower rate of VLDL synthesis. No indications were found for a decreased efficiency of VLDL to LDL conversion in cholestyramine-fed animals.     

Effect of an anti-lipoprotein lipase serum on plasma triglyceride removal.

Anti-lipoprotein lipase sera injected intravenously in roosters blocked quantitatively the catabolism of very low density lipoprotein (VLDL) triglyceride. Antibodies were produced in rabbits immunized with highly purified lipoprotein lipase (LPL, glycerol ester hydrolase, E C 3.1.1.3) prepared from chicken adipose tissue. Following anti-LPL serum injection there was a linear increase in plasma triglyceride concentration. The rate of entry of triglyceride in plasma was estimated from the rate of triglyceride accumulation in the plasma of animals injected with anti-LPL serum, or from the disappearance curve of biologically labelled VLDL. In instances where both measurements were conducted in the same animals there was very close agreement between the two procedures. Inhibition of VLDL triglyceride catabolism of anti-LPL serum provided a way to characterize newly secreted VLDL that exhibited a broad spectrum of particle sizes with a median of 625 A degrees. They contained 76.2 +/- 1.2% triglyceride and had a high ratio of free to ester cholesterol (2.46 +/- 0.45). In control VLDL samples there was 46.1% triglyceride, and the ratio of free to ester cholesterol was 1.19. The complete inhibition of triglyceride removal by an antiserum prepared against adipose tissue LPL demonstrates that the NaCl-inhibited, serum-activated lipase prepared by affinity chromatography on heparin-Sepharose and concanavalin A-Sepharose columns is the enzyme responsible in vivo for the catabolism of VLDL triglyceride. Further, the kinetics of triglyceride accumulation in the plasma provide evidence that the site of degradation of VLDL triglyceride is within the plasma compartment.     

Effects of threonine-poor apolipoproteins on post-heparin plasma hepatic triacylglycerol lipase and lipoprotein lipase

Whole-irradiated rabbit pre-heparin plasma had an important inhibitory effect on hepatic triacylglycerol lipase and lipoprotein lipase activities, whereas control rabbit pre-heparin plasma slightly inhibited hepatic triacylglycerol lipase activity at a high concentration and enhanced lipoprotein lipase activity. As some apolipoproteins were known to modulate these two lipolytic enzymes, the inhibitory effects of irradiated rabbit plasma were investigated in apolipoproteins. Three apolipoproteins, with isoelectric points of about 6.58, 6.44 and 6.12, characterized by their low content in threonine (threonine-poor apolipoproteins) were produced in high concentrations in rabbit VLDL and HDL after irradiation. The effects of these apolipoproteins on control rabbit post-heparin plasma hepatic triacylglycerol lipase and extrahepatic lipoprotein lipase were studied. Threonine-poor apolipoproteins substantially inhibited the hepatic triacylglycerol lipase activity and enhanced the apolipoprotein C-II-stimulated activity of lipoprotein lipase. The amounts of these apolipoproteins in triacylglycerol-rich lipoprotein particles may determine the lipolytic activity of lipoprotein lipase and hepatic triacylglycerol lipase in triacylglycerol hydrolysis. The existence of another inhibitor of lipoprotein lipase remains to be determined.     

Oral nicotine induces an atherogenic lipoprotein profile

Male squirrel monkeys were used to evaluate the effect of chronic oral nicotine intake on lipoprotein composition and metabolism. Eighteen yearling monkeys were divided into two groups: 1) Controls fed isocaloric liquid diet; and 2) Nicotine primates given liquid diet supplemented with nicotine at 6 mg/kg body wt/day. Animals were weighed biweekly, plasma lipid, glucose, and lipoprotein parameters were measured monthly, and detailed lipoprotein composition, along with postheparin plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activity, was assessed after 24 months of treatment. Although nicotine had no effect on plasma triglyceride or high density lipoproteins (HDL), the alkaloid caused a significant increase in plasma glucose, cholesterol, and low density lipoprotein (LDL) cholesterol plus protein while simultaneously reducing the HDL cholesterol/plasma cholesterol ratio and animal body weight. Levels of LDL precursors, very low density (VLDL) and intermediate density (IDL) lipoproteins, were also lower in nicotine-treated primates while total postheparin lipase (LPL + HTGL) activity was significantly elevated. Our data indicate that long-term consumption of oral nicotine induces an atherogenic lipoprotein profile (increases LDL, decreases HDL/total cholesterol ratio) by enhancing lipolytic conversion of VLDL to LDL. These results have important health implications for humans who use smokeless tobacco products or chew nicotine gum for prolonged periods.     

Resistance of a very low density lipoprotein subpopulation from familial dysbetalipoproteinemia to in vitro lipolytic conversion to the low density lipoprotein density fraction

In vitro lipolysis of very low density lipoprotein (VLDL) from normolipidemic and familial dysbetalipoproteinemic plasma by purified bovine milk lipoprotein lipase was studied using the combined single vertical spin and vertical autoprofile method of lipoprotein analysis. Lipolysis of normolipidemic plasma supplemented with autologous VLDL resulted in the progressive transformation of VLDL to low density lipoprotein (LDL) via intermediate density lipoprotein (IDL) with the transfer of the excess cholesterol to high density lipoprotein (HDL). At the end of 60 min lipolysis, 92-96% of VLDL triglyceride was hydrolyzed, and, with this process, greater than 95% of the VLDL cholesterol and 125-I-labeled VLDL protein was transferred from the VLDL to the LDL and HDL density region. When VLDL from the plasma of an individual with familial dysbetalipoproteinemia was substituted for VLDL from normolipidemic plasma, less than 50% of the VLDL cholesterol and 65% of 125I-labeled protein was removed from the VLDL density region, although 84-86% of VLDL triglyceride was lipolyzed. Analysis of familial dysbetalipoproteinemic VLDL fractions from pre- and post-lipolyzed plasma showed that the VLDL remaining in the postlipolyzed plasma (lipoprotein lipase-resistant VLDL) was richer in cholesteryl ester and tetramethylurea-insoluble proteins than that from prelipolysis plasma; the major apolipoproteins in the lipoprotein lipase-resistant VLDL were apoB and apoE. During lipolysis of normolipidemic VLDL containing trace amounts of 125I-labeled familial dysbetalipoproteinemic VLDL, removal of VLDL cholesterol was nearly complete from the VLDL density region, while removal of 125I-labeled protein was only partial. A competition study for lipoprotein lipase, comparing normolipidemic and familial dysbetalipoproteinemic VLDL to an artificial substrate ([3H]triolein), revealed that normolipidemic VLDL is clearly better than familial dysbetalipoproteinemic VLDL in competing for the release of 3H-labeled free fatty acids. The results of this study suggest that, in familial dysbetalipoproteinemic individuals, a subpopulation of VLDL rich in cholesteryl ester, apoB, and apoE is resistant to in vitro conversion by lipoprotein lipase to particles having LDL-like density. The presence of this lipoprotein lipase-resistant VLDL in familial dysbetalipoproteinemic subjects likely contributes to the increased level of cholesteryl ester-rich VLDL and IDL in the plasma of these subjects.     

Acute inhibition of hepatic lipase and increase in plasma lipoproteins after alcohol intake

Chronic alcohol intake is associated with an increase in fasting plasma high density lipoproteins (HDL). To study alcohol's acute effects on plasma lipoproteins, we measured plasma lipoprotein concentrations and activities of postheparin plasma lipases in nine normolipemic males after ingestion of 40 g of ethanol (as whiskey). After alcohol there was no change in lipoprotein lipase activity but hepatic lipase was decreased to 67% of baseline at 6 hr. There were associated increases in HDL phospholipids (12 mg/dl) and cholesterol (10 mg/dl) resulting in prominence of larger, lipid-enriched HDL particles. Changes were most pronounced in the HDL3 and HDL2a subclasses. Very low density lipoprotein (VLDL) phospholipids and cholesterol were also increased by 13 and 9 mg/dl, respectively, with no significant change in triglycerides. Changes in lipoproteins and lipase were largely reversed 10 hr after alcohol intake. The transient increases in VLDL and HDL lipids after alcohol may result in part from acute inhibition of hepatic lipase activity. The results suggest a role of hepatic lipase in the catabolism of phospholipids of VLDL and possibly HDL.     

Lipoprotein composition as a component in the lipoprotein clearance defect in experimental diabetes

The hypertriglyceridemia associated with streptozotocin-induced diabetes in rats is largely reflected in the plasma lipoproteins of density less than 1.006 g/ml. Analysis of the plasma apolipoproteins of these rats indicated marked alterations in both the total levels and in the lipoprotein distribution of the major apolipoproteins. In whole plasma, diabetes was associated with significant increases in apolipoprotein (apo)-AIV, apo-AI, and apo-B (mainly in the intestinally derived apo-B240) and a marked decrease in apo-E. In the d less than 1.006 g/ml lipoprotein fraction (very-low-density lipoproteins (VLDL], there were significant increases in apo-B240, apo-AI, and apo-AIV and decreased levels of apo-E and the C apolipoproteins. The decrease in apo-C was primarily due to lower levels of apo-CII, and the ratio of the lipoprotein lipase inhibitor, apo-CIII, to the lipoprotein lipase activator, apo CII, was significantly increased over that in controls. The comparative clearance of triglycerides of VLDL particles from control and diabetic rat plasma was tested in recirculating heart perfusion in vitro. During 45-min perfusions of hearts from control donor rats, lipolysis of triglycerides of VLDL from diabetic rats was only 63-64% of that using plasma VLDL from control rats. Perfusion of hearts from diabetic rats with VLDL from control rats gave lipolysis values of only 53% of that obtained with normal hearts. Where both the VLDL and hearts were obtained from diabetic rats, lipolysis was 23% of that observed when both the lipoprotein and the organ were from control rats. The data suggest that in addition to depressed lipoprotein lipase activity in the tissue from diabetic rats, there are also major compositional changes in circulating lipoproteins which may contribute to defective triglyceride clearance from the circulation.     

Lipoprotein lipase-catalyzed hydrolysis of phosphatidylcholine of guinea pig very low density lipoproteins and discoidal complexes of phospholipid and apolipoprotein: effect of apolipoprotein C-II on the catalytic mechanism

To elucidate the mechanism by which apolipoprotein C-II (apoC-II) enhances the activity of lipoprotein lipase (LpL), discoidal phospholipid complexes were prepared with apoC-III and di[(14)C]palmitoyl phosphatidylcholine (DPPC) and containing various amounts of apoC-II. The rate of DPPC hydrolysis catalyzed by purified bovine milk LpL was determined on the isolated complexes. The rate of hydrolysis was optimal at pH 8.0. Analysis of enzyme kinetic data over a range of phospholipid concentrations revealed that the major effect of apoC-II was to increase the maximal velocity (V(max)) some 50-fold with a limited effect on the Michaelis constant (K(m)). V(max) of the apoC-III complex containing no apoC-II was 9.2 nmol/min per mg LpL vs. 482 nmol/min per mg LpL for the complex containing only apoC-II. The effect of apoC-II on enzyme kinetic parameters for LpL-catalyzed hydrolysis of DPPC complexes was compared to that on the parameters for hydrolysis of DPPC and trioleoylglycerol incorporated into guinea pig very low density lipoproteins (VLDL(p)) which lack the equivalent of human apoC-II. Tri[(3)H]oleoylglycerol-labeled VLDL(p) were obtained by perfusion of guinea pig liver with [(3)H]oleic acid. Di[(14)C]palmitoyl phosphatidylcholine was incorporated into the VLDL(p) by incubation of VLDL(p) with sonicated vesicles of di[(14)C]palmitoyl phosphatidylcholine and purified bovine liver phosphatidylcholine exchange protein. The rates of LpL-catalyzed hydrolysis of trioleoylglycerol and DPPC were determined at pH 7.4 and 8.5 in the presence and absence of apoC-II. In the presence of apoC-II, the V(max) for DPPC hydrolysis in guinea pig VLDL(p) increased at both pH 7.4 and pH 8.5 (2.4- and 3.2-fold, respectively); the value of K(m) did not change at either pH (0.23 mm). On the other hand, the kinetic value of K(m) for triacylglycerol hydrolysis in the presence of apoC-II decreased at both pH 7.4 (3.05 vs. 0.54 mm) and pH 8.5 (2.73 vs. 0.62 mm). These kinetic studies suggest that apoC-II enhances phospholipid hydrolysis by LpL in apoC-III-DPPC discoidal complexes and VLDL(p) mainly by increasing the V(max) of the enzyme for the substrates, whereas the activator protein primarily causes a decrease in the apparent K(m) for triacylglycerol hydrolysis.-Shirai, K., T. J. Fitzharris, M. Shinomiya, H. G. Muntz, J. A. K. Harmony, R. L. Jackson and D. M. Quinn. Lipoprotein lipase-catalyzed hydrolysis of phosphatidylcholine of guinea pig very low density lipoproteins and discoidal complexes of phospholipid and apolipoprotein: effect of apolipoprotein C-II on the catalytic mechanism.     

Mechanisms of enhancement of cholesteryl ester transfer protein activity by lipolysis

Lipoprotein lipase enhances the cholesteryl ester transfer protein (CETP)-mediated transfer of cholesteryl esters from plasma high density lipoproteins (HDL) to very low density lipoproteins (VLDL). In time course studies the stimulation of cholesteryl ester transfer by bovine milk lipase was correlated with accumulation of fatty acids in VLDL remnants. As the amount of fatty acid-poor albumin in the incubations was increased, there was decreased accumulation of fatty acids in VLDL remnants and a parallel decrease in the stimulation of cholesteryl ester transfer by lipolysis. Addition of sodium oleate to VLDL and albumin resulted in stimulation of the CETP-mediated transfer of cholesteryl esters from HDL to VLDL. The stimulation of transfer of cholesteryl esters into previously lipolyzed VLDL was abolished by lowering the pH from 7.5 to 6.0, consistent with a role of lipoprotein ionized fatty acids. CETP-mediated cholesteryl ester transfer from HDL to VLDL was also augmented by phosholipase A2 and by a bacterial lipase which lacked phospholipase activity. When VLDL and HDL were re-isolated after a lipolysis experiment, both lipoproteins stimulated CETP activity. Postlipolysis VLDL and HDL bound much more CETP than native VLDL or HDL. Lipolysis of apoprotein-free phospholipid/triglyceride emulsions also resulted in enhanced binding of CETP to the emulsion particles. Incubation conditions which abolished the enhanced cholesteryl ester transfer into VLDL remnants reduced binding of CETP to remnants, emulsions, and HDL. In conclusion, the enhanced CETP-mediated transfer of cholesteryl esters from HDL to VLDL during lipolysis is related to the accumulation of products of lipolysis, especially fatty acids, in the lipoproteins. Lipids accumulating in VLDL remnants and HDL as a result of lipolysis may augment binding of CETP to these lipoproteins, leading to more efficient transfer of cholesteryl esters from HDL to VLDL.     

Lipid metabolism in endotoxic rats: decrease in hepatic triglyceride lipase activity

Studies were conducted to investigate the effect of E. coli endotoxin administration on hepatic triglyceride lipase (H-TGL) activity in rats, since H-TGL activity is known to behave differently from lipoprotein lipase (LPL) activity in various situations. Plasma triglyceride and free fatty acid concentrations were markedly elevated in animals after injection of endotoxin. Cholesterol and phospholipids were also increased significantly. Lipoprotein analysis by ultracentrifugation showed that the most pronounced increase of lipoproteins was in the VLDL and IDL fractions. Triglyceride lipase activities in post-heparin plasma were markedly decreased. A selective assay for H-TGL activity using a specific antibody revealed that this enzyme as well as LPL is significantly decreased (26% of control) in endotoxic animals. Thus, the increase of VLDL and IDL appears to result from the decrease of both of LPL and H-TGL.