Can lipoprotein lipase be the culprit in cholesteryl ester accretion in smooth muscle cells in atheroma?

Bovine aortic smooth muscle cells and human skin fibroblasts were incubated with beta-very low density lipoprotein (beta VLDL) isolated from cholesterol-fed rabbits and labeled with [3H]cholesteryl oleate. Addition of lipoprotein lipase resulted in a 3.2-4.8-fold increase in cell associated radioactivity of which 45-61% was in free cholesterol, i.e., derived after intracellular hydrolysis. After exposure of smooth muscle cells to beta VLDL for up to 9 days and 60 min sodium heparin wash at 4 degrees C to remove extracellular surface bound lipoprotein, cellular cholesterol increase was 2 micrograms in controls and in the presence of lipoprotein lipase (LPL) it was tenfold higher. Addition of [3H]cholesteryl ester labeled beta VLDL during the last 48 h of incubation showed that 30-40% of total cellular label was in free cholesterol. This value represents the minimal cellular uptake of the added lipoprotein cholesteryl ester. Addition of recombinant apolipoprotein (apo) E to smooth muscle cells incubated with beta VLDL and [3H]oleate induced no further increase in [3H]cholesteryl oleate. We propose that following LPL-mediated binding of beta VLDL to heparan sulphate, this complex either undergoes endocytosis, or translocation of cholesteryl ester into the smooth muscle cells (SMC) occurs without endocytosis of the entire particle. The present results indicate that in the aortic wall macrophage-derived lipoprotein lipase could play a role in cholesteryl ester accretion in smooth muscle cells during atherogenesis.

Persistence of increased cholesteryl ester in human skin fibroblasts is caused by residual exogenous sphingomyelinase and is reversed by phospholipid liposomes

Human skin fibroblasts (HSF) were exposed to sphingomyelinase 50 or 5 mU/ml for 60 min, washed with 5 mM EDTA or 20% serum and with phosphate-buffered saline, and postincubated for 24 h in the presence of [14C]16:0 sphingomyelin (SP) liposomes. The recovery of up to 48% of label in the medium in ceramide provided evidence of persistence of sphingomyelinase activity. The rate of hydrolysis of [14C]16:0 SP remained the same irrespective of whether the liposomes were added immediately after the wash, or 3 or 6 h thereafter. In HSF labeled with [3H]cholesterol exposure to 50 mU/ml of sphingomyelinase for 60 min resulted in an increase in labeled cholesteryl ester (CE) at 6 and 24 h of postincubation. Addition of sphingomyelin liposomes reduced markedly the fraction of cellular labeled cholesteryl ester recovered after 24 h, while phosphatidylcholine liposomes were not effective. When the enzyme concentration had been reduced 5-20 fold the effect of sphingomyelin liposomes on cellular 3H-CE was evident already after 6 h of postincubation and some effect was seen also with phosphatidylcholine liposomes. Increase in the concentrations of SP liposomes to 150 micrograms/ml restored labeled cholesteryl ester to control values at 24 h. A significant reduction occurred also with 18:1 phosphatidylcholine liposomes but labeled cholesteryl ester remained 50-100% higher when compared with 18:1 or 18:2 SP. No correlation was seen between the rates of cholesteryl ester decrease and free cholesterol efflux into the medium. The inability to remove residual sphingomyelinase by regular washing procedures exaggerates and prolongs the recovery period of sphingomyelin during postincubation and delays the return of the cholesteryl ester pool to control levels. This can be counteracted by addition of phospholipid liposomes that can compete for the enzyme with the plasma membrane sphingomyelin and also substitute the hydrolyzed molecule in the plasma membrane to impede cholesterol flow to cell interior.

Regulation of lipoprotein lipase by dibutyryl cAMP, cholera toxin, Hepes and heparin in F1 heart-cell cultures

Regulation of lipoprotein lipase was studied in mesenchymal rat heart-cell cultures. Treatment of the cultures with dibutyryl cyclic AMP or with cholera toxin resulted in an increase in LPL activity and a comparable increase in LPL mRNA. When the cells were exposed to 100 mM Hepes for 24 h, total enzyme activity rose 2-fold and LPL mRNA increased 2.4-fold. After 72 h, there was a 3-fold increase in LPL mRNA and a 4-fold rise in cellular LPL activity, while medium activity increased 20-fold. Exposure of the cultures to heparin for 24 h resulted in a 3.2-fold increase in total activity and a 36-fold increase in medium activity. This increase was not accompanied by any rise in LPL mRNA. Addition of actinomycin D to control dishes for 24 h resulted in a 33% reduction in LPL mRNA and a 43% reduction in enzyme activity. These values were 71% and 56%, respectively, in Hepes-treated cells, indicating that no stabilization of LPL mRNA occurred under these conditions. It can be concluded that in mesenchymal rat heart-cells in culture cAMP and cholera toxin upregulate lipoprotein lipase at the level of transcription. The increase in LPL activity after 24 h exposure to Hepes could be compatible with transcriptional regulation, while exposure to heparin is not accompanied by a change in LPL mRNA.

Effects of diets rich in monounsaturated fatty acids on plasma lipoproteins--the Jerusalem Nutrition Study. II. Monounsaturated fatty acids vs carbohydrates

Seventeen male Yeshiva students were randomly allocated to a crossover study with two 12-wk dietary periods of monounsaturated fatty acids (MUFAs) vs a carbohydrate (CHO)-rich diet while concentrations of saturated (SFAs) and polyunsaturated (PUFAs) fatty acids were kept similar. Total plasma cholesterol (TC) decreased significantly by approximately 7.7% and low-density-lipoprotein cholesterol (LDL-C) by 14.4% on the MUFA diet, whereas on the CHO diet no significant change in cholesterol concentrations occurred, in contrast to that predicted by the equations of Keys and Hegsted. Concentrations of high-density-lipoprotein cholesterol (HDL-C) did not change significantly on either diet. On the MUFA diet there was a significantly lower proneness to peroxidation of plasma and LDL lipids and less extensive metabolism of conditioned LDL by peritoneal macrophages. We conclude that dietary MUFAs lower TC and LDL-C concentrations, independently of other dietary fatty acids and in addition may reduce the susceptibility of LDL to oxidative stress.

Modulation of sphingomyelinase-induced cholesterol esterification in fibroblasts, CaCo2 cells, macrophages and smooth muscle cells

The present study has focused on three questions concerning the effect of sphingomyelinase on release of free cholesterol from the plasma membrane and its intracellular translocation: (i) Can one change the direction of the flow of cholesterol? (ii) Can one modulate the flow? (iii) May such a mechanism be relevant in atherogenesis? (i) The results obtained show that even in the presence of potent nonlipoprotein cholesterol acceptors in the medium, the intracellular flow of cholesterol is not reduced as measured by cholesterol esterification. Moreover, in sphingomyelinase-treated cells, cholesterol efflux in presence of nonlipoprotein acceptors was not enhanced even when intracellular esterification was inhibited. (ii) Modulation of the sphingomyelinase induced cholesterol flow can be obtained by 100 microM verapamil which reduces it. In human skin fibroblast, interference with the delivery of free cholesterol to its site of esterification was found in the presence of brefeldin A. (iii) Aortic smooth muscle cells in culture are sensitive to low concentrations of sphingomyelinase and the increase in esterified cholesterol is evident also after exposure to the enzyme for 24 h. The present results suggest that in the plasma membrane, free cholesterol bound to sphingomyelin may be in a compartment which renders it more available for transport to the cell interior than for efflux. In view of the sensitivity of aortic smooth muscle cells to sphingomyelinase, this mechanism for enhanced esterification of cholesterol could be relevant to the transformation of arterial smooth muscle cells into foam cells in the process of atherogenesis.

Dissimilar effects of Brefeldin A on cholesteryl ester and triacylglycerol metabolism in CaCo2 and HepG2 cells as compared to peritoneal macrophages

The effect of Brefeldin A (BFA) on lipid metabolism was studied in two cell lines and in primary cultures of peritoneal macrophages. In both CaCo2 and HepG2 cells, which are models for human liver and intestine, addition of BFA resulted in a 2-10-fold increase in recovery of labeled cholesteryl ester when the cells had been prelabeled with free cholesterol or with [3H]oleic acid. This effect was linear for up to 6 h and could be elicited with doses of BFA as low as 0.03 micrograms/ml. The increase in cholesteryl ester induced by BFA was completely abolished by the ACAT inhibitor (Sandoz 58-035) and partly by forskolin. Intracellular hydrolysis of labeled cholesteryl ester was studied in the presence of the ACAT inhibitor and while in the controls 30-40% was hydrolyzed in 6 h, the values were 7-16% in the BFA treated cells. The slower rate of hydrolysis in the BFA treated cells could not account for the entire increase of cholesteryl ester and there was also no decrease in cholesteryl ester secretion. Even though activation of acyl-CoA:cholesterol acyltransferase by BFA was not demonstrated in cell homogenates, we hypothesize that in the intact cell the BFA induced increase in cholesteryl ester might have been related to the pronounced increase in modified endoplasmic reticulum which results from the dispersion of the Golgi apparatus. In the macrophages, BFA at doses of 0.25-1 micrograms/ml resulted in a 90% reduction in the incorporation of [3H]oleic acid into triacyglycerol. Incorporation of [3H]oleic acid into triacyglycerol in CaCo2 cells was not affected by BFA. In view of the ever-increasing use of BFA in cell biology, it seems important to emphasize that BFA may affect different pathways of lipid metabolism in various cells.

Fish oil ingestion in smokers and nonsmokers enhances peroxidation of plasma lipoproteins

The effect of fish oil ingestion (10 g MaxEPA/day) on the susceptibility of plasma lipoproteins to peroxidation was examined in 20 smokers (study A and B) and 22 nonsmokers (study C). The subjects were examined at the onset of each study (baseline values), divided into control and experimental groups and reexamined 4 weeks later. Smokers were examined 40 h after abstention from smoking (0 time) and 90 min after acute smoking (4-6 cigarettes). The parameters studied were TBARS, which provide an indication of peroxidative injury, and metabolism of conditioned LDL by macrophages as a biological indicator. These parameters were significantly higher (P less than 0.05-0.001) when the 90 min values of smokers were compared to time 0. After 4 weeks of fish oil ingestion, a significant rise above baseline values (33-50%) in plasma and LDL TBARS was found in smokers examined at time 0 and after acute smoking. Peroxidative modification of LDL isolated from smokers fed fish oil resulted in significantly higher TBARS (34-41%) and its metabolism by macrophages was higher (65-139%) compared to baseline values. In nonsmokers, the baseline values of the above parameters were lower than in smokers. Ingestion of fish oil resulted in a significant rise in TBARS in plasma (33%), LDL (137%), conditioned LDL (36-40%) and metabolism of conditioned LDL (70%) by macrophages. In 6 nonsmokers and 4 smokers, 400 mg of vitamin E/day were given with the fish oil. In the nonsmokers, vitamin E counteracted the effect of fish oil more effectively than in the smokers. In the light of the present results, indiscriminate recommendation of fish oil supplementation to the population at large should be cautioned.

Diverse effect of ethnicity on plasma lipoprotein[a] levels in heterozygote patients with familial hypercholesterolemia

Plasma lipids, lipoproteins, and lipoprotein[a] (Lp[a]) levels were determined in 216 members of 14 families with familial hypercholesterolemia (FH). Ninety-nine subjects harbored a mutant low density lipoprotein (LDL) receptor allele as confirmed by molecular genetic analysis. Four different mutant alleles were identified, each in a defined genetic group, Druze, Christian-Arabs, and Ashkenazi and Sephardic Jews. The findings in FH subjects (cases) were compared with their nonaffected family members (controls). Plasma Lp[a] levels increased with age in the controls but not in cases and were different among the four genetic groups. Mean plasma Lp[a] levels were significantly higher in cases (33 mg/dl) than in controls (22 mg/dl). Plasma LDL cholesterol levels were raised in cases of the four genetic groups to a similar extent, in contrast to the mean plasma Lp[a] that varied. The Lp[a] level was higher by 30-33% in cases from the Druze, Christian-Arabs, and Jewish-Ashkenazi groups but by 110% in the Jewish-Sephardic group. Apo[a] isoform distribution was similar in cases and controls within each genetic group. Lp[a] levels were highest in subjects with LpS1 isoform, in particular in cases from the Jewish-Sephardic group. These data indicate that the higher Lp[a] levels in FH heterozygotes cannot be attributed solely to lack of functional LDL receptor molecules but possibly reflect multiple gene interactions.

Expression of lipoprotein lipase mRNA in rat heart is localized mainly to mesenchymal cells as studied by in situ hybridization

The expression of lipoprotein lipase mRNA (LPL mRNA) was studied in rat hearts by use of a sulfur-35-labeled antisense mRNA probe. Rats were studied under three conditions: fed, fasted, and injected with cholera toxin (an irreversible agonist of adenylate cyclase) and then fasted. The highest LPL activity was found in the hearts of cholera toxin-injected, fasted rats. After injection of cholera toxin, LPL mRNA levels were 3.5-fold higher than those from fed rats. Using in situ hybridization, we studied the site of expression of LPL mRNA under the same three experimental conditions. In sections of hearts from cholera toxin-injected, fasted rats, concentrations of autoradiographic grains, representing the site of LPL mRNA, were seen over interstitial elements, which comprise capillary and perivascular cells. A more diffuse and sparse reaction was seen over cardiac myocytes and was not always distinguishable from background. A similar but much less definitive localization was seen in sections of hearts from fasted rats. The present results indicate that in the rat heart, the main site of LPL synthesis and processing, especially after stimulation with an irreversible agonist of adenylate cyclase, is localized to interstitial elements rather than to adult cardiac myocytes.

Metabolism of 3-[3H]sphingosine sphingomyelin labeled with [14C]palmitic or [14C]linoleic acid by Hep G2 cells and rat liver in vivo

The metabolism of sphingomyelin labeled with 3-[3H]sphingosine and [14C]16:0 or [14C]18:2 fatty acid was studied in cultured Hep G2 cells or macrophages and after injection into rats. In pulse-chase experiments, the loss of 3H and 14C-label was more rapid when the cells had been pulsed with 18:2 than with 16:0 sphingomyelin. At the end of 24 h chase, the labeled ceramide contained more [14C]18:2 fatty acid than [14C]16:0. In addition, the 3H-label derived from 3-[3H]sphingomyelin was recovered also as free sphingosine. After injection in vivo, more [3H]sphingosine-labeled sphingomyelin was present in the liver 3 and 24 h after injection of 16:0 than after injection of 18:2 sphingomyelin. The ratio of [3H]ceramide derived from 16:0 sphingomyelin to that derived from 18:2 sphingomyelin as percent of injected dose was 1.84 3 h after injection and 1.31 after 24 h. The ratio of 3H/14C in liver ceramide was 6.4 3 h after injection of 18:2 sphingomyelin and 3.4 after 16:0 sphingomyelin. The present results show that 3-[3H]sphingomyelin is metabolized quite extensively and that the fate of the sphingosine moiety is related to the type of fatty acid present in the phospholipid. These findings indicate that there is little or no reutilization of 18:2 ceramide for sphingomyelin formation and suggest that sphingosine derived from 18:2 sphingomyelin is channeled primarily for catabolism.

Recombinant human interleukin-1 suppresses lipoprotein lipase activity, but not expression of lipoprotein lipase mRNA in mesenchymal rat heart cell cultures

The effect of human recombinant interleukin-1 (IL-1) on the regulation of lipoprotein lipase (LPL) was studied in rat heart mesenchymal cell cultures. A time-dependent reduction in enzyme activity occurred with a 30% fall after 1 h. The suppression of enzyme activity was accompanied by a commensurate reduction in enzyme mass. The reduction in LPL activity was most prominent in the heparin releasable pool; IL-1 treatment resulted in a 7.2-8.3-fold decrease in the functional compartment and a 2.5-2.8-fold decrease in residual cellular activity. The effect of IL-1 could be prevented by the addition of the IL-1 inhibitor. However, in contradistinction to the effect of tumor necrosis factor (TNF), there was no change in LPL mRNA in cultures treated with IL-1. The present results show that the regulation of LPL in mesenchymal heart cell cultures by IL-1 occurs posttranscriptionally, as has been shown in 3T3 cells. The more pronounced effect on LPL activity in the functional pool suggests that IL-1 treatment might have influenced also the processing and/or transport of the enzyme to the cell surface.

Effects of diets rich in monounsaturated fatty acids on plasma lipoproteins--the Jerusalem Nutrition Study: high MUFAs vs high PUFAs

Twenty-six Yeshiva students were randomly assigned to a 24-wk crossover study of monounsaturated fatty acid (MUFA) vs polyunsaturated fatty acid (PUFA) diets (50% carbohydrate, 32% fat, 18% protein) fed alternately during two 12-wk periods. Total plasma cholesterol (TC) decreased significantly by approximately 10% and approximately 16% on the MUFA and PUFA diets, respectively. Plasma triglyceride response was variable. Low-density-lipoprotein cholesterol (LDL-C) decreased in both groups with an additional significant effect between periods. Concentrations of high-density-lipoprotein cholesterol did not change significantly. LDL-receptor status in fresh monocytes, affinity of LDL towards the LDL receptor in cultured fibroblasts, zonal-centrifugation profiles, and lipoprotein composition were not significantly different between the diets. There was a significantly higher tendency toward lipid peroxidation on the PUFA diet, as ascertained by more thiobarbituric acid-reactive-substances formation on that diet. Dietary PUFA results in somewhat lower TC and LDL-C concentrations whereas with MUFA the susceptibility of LDL to oxidative stress is lower.

The expression of lipoprotein lipase activity and mRNA in mesenchymal rat heart cell cultures is modulated by bFGF

The effect of basic fibroblast growth factor (FGF) on lipoprotein lipase (LPL) production was studied in mesenchymal rat heart cell cultures. Addition of FGF to culture medium containing 20% serum resulted in a 3-fold increase in LPL activity. The minimal effective dose of FGF was 10 ng/ml and the increase occurred after exposure for 48 h. Addition of FGF was effective during the first week in culture, when enzyme activity was increasing, but not after 11 days when the cultures were superconfluent and the enzyme activity was high. Addition of FGF to serum-poor medium was able to replace serum required to sustain LPL activity. In FGF-treated cultures, more LPL activity was present in the functional pool, but not in the medium, than in the controls. The increase in enzymic activity was accompanied by an increase in enzyme mass and in LPL mRNA.

Effect of TNF on triacylglycerol in cultured vascular smooth muscle cells

Smooth muscle cells (SMC) isolated from bovine aorta or human saphenous vein were cultured and used to study the putative effect of recombinant human tumor necrosis factor (TNF) on lipid metabolism in vascular cells. Addition of TNF to the culture medium for 24-48 h resulted in an increase of [3H]oleic acid uptake and esterification into lipids. The effect could be seen already with 0.3 ng/ml and was maximal with 30 ng/ml. The effect of TNF was mainly on the incorporation of [3H]oleic acid into triacylglycerol which increased by 140% in the bovine cells. There was also a significant increase in [3H]cholesteryl ester. In the human SMC there was a 40% increase in [3H]oleic acid into total lipids, while the rise in [3H]triacylglycerol ranged between 60-90%. TNF did not modulate cellular triacyglycerol synthesis in cultured mouse peritoneal macrophages. Since TNF was shown to be synthesized and secreted not only by macrophages but also by smooth muscle cells, it could play an autocrine role in lipid metabolism during development of atherosclerotic lesions. The cellular population of the lesions, i.e., predominance of macrophages or smooth muscle cells, could determine the relative proportion of triacylglycerol accumulation.

Removal of cholesteryl ester from hepatic reticuloendothelial cells in vivo is not enhanced by plasma cholesteryl ester transfer protein

The putative role of cholesteryl ester transfer protein (CETP) in the removal of cholesteryl ester from hepatic reticuloendothelial cells in vivo was studied in hamsters. The parameter tested was retention of [3H]cholesteryl linoleyl ether ([3H]CLE), a nonhydrolysable analog of cholesteryl ester, in the liver after injection of [3H]CLE labeled acetylated LDL, which is targetted to nonparenchymatous littoral cells. In hamsters fed laboratory chow, plasma cholesteryl ester transfer activity (CETA) was 10.6 +/- 0.9 units and the retention of [3H]CLE in the liver 28 days after injection was 86% of the 4 h value. It was about 55% in rats fed the same diet, in which CETA was not detectable. When the diet was supplemented with 2% cholesterol and 15% margarine, CETA activity in hamsters increased 2-fold, yet no change in retention of [3H]CLE in liver was seen after 28 days. In rats, the retention of [3H]CLE in the liver was also not changed by the dietary fat supplementation. These results do not support the role of CETP in vivo in removal of cholesteryl ester from intact reticuloendothelial cells.

Effect of vitamin C and E supplementation on susceptibility of plasma lipoproteins to peroxidation induced by acute smoking

The effect of acute smoking on plasma lipoproteins was studied in seventeen smokers. In study 1, 7 subjects were examined prior to and 2 weeks after supplementation with vitamin C. In study 2, the effect of acute smoking was first determined in 10 additional subjects and subsequently they were divided into 3 groups, 3 and 4 subjects were supplemented with vitamin C or E, respectively, for 4 weeks, and 3 remained untreated. Plasma and LDL TBARS were examined at time zero (i.e., 40-48 h after total abstention from smoking) and at 90 min after acute smoking (5-7 cigarettes). In all 17 subjects examined prior to vitamin supplementation, significantly higher TBARS values were found in plasma, native LDL and LDL conditioned with smooth muscle cells (SMC) when the 90 min values were compared to 0 time. The LDL isolated after 90 min and conditioned with SMC was metabolized more extensively by mouse peritoneal macrophages than its zero time counterpart. The differences between the 0 time and 90 min values were not seen after the subjects had been supplemented with vitamin C for 2 or 4 weeks or with vitamin E for 4 weeks. The present results indicate that acute smoking exerts an oxidative stress on plasma lipoproteins and that higher plasma levels of natural antioxidants, such as vitamins C and E have a protective role.

Smoking depresses adipose lipoprotein lipase response to oral glucose

Adipose tissue lipoprotein lipase was studied in smokers (n = 17) aged 18-47 years and compared with enzyme activity in non-smokers of comparable age (n = 8) and a second time in some of the subjects 5-9 weeks after cessation of smoking (n = 7). Serum cotinine levels served to validate the smoking status of the subjects. Fasting enzyme activity was similar in smokers and non-smokers, when expressed per 10(6) cells, but was significantly increased when normalized for cell size. When lipoprotein lipase was determined in the same individual 4 h after an oral glucose load, a significant decrease (P less than 0.002) occurred in the smokers, while enzyme activity rose in the nonsmokers (P less than 0.02). A tendency for enzyme activity to rise after oral glucose was seen in ex-smokers, which did not reach statistical significance. Even though the mean serum insulin and glucose levels did not differ in the three groups of subjects, the per cent decrease in lipoprotein lipase after oral glucose in smokers was negatively correlated with insulin release into serum in the same subject, i.e., the greater the insulin release, the less the decrease in lipoprotein lipase activity. We would like to propose that the lower body weight in smokers is related to the paradoxical response of adipose tissue lipoprotein lipase to carbohydrate and that the reversal of this behaviour contributes to the weight gain often observed after cessation of smoking.

Recent developments in atherogenesis

This review attempts to highlight some new developments in the study of atherosclerosis with respect to lipoprotein and cellular elements of the aortic wall. The studies reviewed have shown that in addition to LDL (low density lipoprotein), atherogenic lipoproteins comprise also beta-VLDL (very low density lipoprotein), Lp (a) (lipoprotein (a)), peroxidatively modified LDL and remnant lipoproteins. The latter two were shown to interact preferentially with macrophages, the main cellular elements of fatty streaks. New information has also accrued with regard to improved identification of macrophage and smooth muscle derived foam cells and the presence of T-lymphocytes in the atheroma has been firmly established. The interaction of the various lipoproteins with the cellular elements and the role of lymphokines and growth factors in the formation of the atheroma are reviewed.

Cholesteryl ester transfer activity in hamster plasma: increase by fat and cholesterol rich diets

We investigated the presence of cholesteryl ester transfer activity (CETA) in plasma of hamsters kept on various dietary regimens. In hamsters kept on a regular diet, CETA activity was about 5 units/4 mg protein of d greater than 1.21 g/ml fraction of plasma, as compared to about 35 units present in human d greater than 1.21 g/ml fraction. Addition of 15% margarine or butter alone or together with 2% cholesterol resulted in a 2-3-fold increase in plasma CETA. The increase in plasma CETA was correlated with plasma cholesterol levels (r = 0.78; P less than 0.001) and plasma triacylglycerol levels (r = 0.56, P less than 0.001). Hamsters consuming the cholesterol + butter-supplemented diets had the highest plasma CETA, cholesterol and triacylglycerol levels, while CETA in plasma of rats and mice remained nondetectable even after 4 weeks on the diet. The causal relation between hypercholesterolemia, hypertriglyceridemia and evaluation in CETA in hamsters remains to be elucidated.

Phosphatidylinositol-specific phospholipase C releases lipoprotein lipase from the heparin releasable pool in rat heart cell cultures

The effect of phosphatidylinositol-specific phospholipase C (PI-PLC) on the release of lipoprotein lipase was studied in F1 heart cell cultures. Exposure of the cultures for 10 min to PI-PLC resulted in a 2-fold increase in the release of lipoprotein lipase (LPL) into the culture medium. PI-PLC released LPL from the heparin-releasable pool and PI-PLC was not effective in cultures pretreated with heparin. Insulin had no influence on the release of LPL from the heart cell cultures, even though it enhanced the uptake of 2-deoxy[3H]glucose by these cells. In cultures labeled with 35S, treatment with PI-PLC resulted in an increase in the release of 35S-labeled proteoglycan. PI-PLC was also effective in enhancing the release of bovine LPL exogenously bound to cultured aortic smooth muscle cells. The findings that PI-PLC was not effective after heparin, that it did release exogenously added LPL to cell cultures and that it released 35S-labeled proteoglycan, were interpreted to indicate that PI-PLC apparently acts on the release of LPL in an indirect manner, releasing heparan sulphate to which LPL is bound. As there is a previously described correlation between circulating LPL and the heparin-releasable LPL, we hypothesize that the activity of PI-PLC in the endothelial cell membrane or plasma phosphatidyl-specific phospholipase D regulates the plasma LPL levels.