Modulation of lipoprotein lipase activity in mouse peritoneal macrophages by recombinant human tumor necrosis factor

Physiological regulation of lipoprotein lipase

The enzyme lipoprotein lipase (LPL), originally identified as the clearing factor lipase, hydrolyzes triglycerides present in the triglyceride-rich lipoproteins VLDL and chylomicrons. LPL is primarily expressed in tissues that oxidize or store fatty acids in large quantities such as the heart, skeletal muscle, brown adipose tissue and white adipose tissue. Upon production by the underlying parenchymal cells, LPL is transported and attached to the capillary endothelium by the protein GPIHBP1. Because LPL is rate limiting for plasma triglyceride clearance and tissue uptake of fatty acids, the activity of LPL is carefully controlled to adjust fatty acid uptake to the requirements of the underlying tissue via multiple mechanisms at the transcriptional and post-translational level. Although various stimuli influence LPL gene transcription, it is now evident that most of the physiological variation in LPL activity, such as during fasting and exercise, appears to be driven via post-translational mechanisms by extracellular proteins. These proteins can be divided into two main groups: the liver-derived apolipoproteins APOC1, APOC2, APOC3, APOA5, and APOE, and the angiopoietin-like proteins ANGPTL3, ANGPTL4 and ANGPTL8, which have a broader expression profile. This review will summarize the available literature on the regulation of LPL activity in various tissues, with an emphasis on the response to diverse physiological stimuli.

Calorie restriction in mice does not affect LDL reverse cholesterol transport in vivo

Calorie restriction (CR) prolongs life in animals, but may reduce plasma HDL, important in reverse cholesterol transport (RCT). The effect of CR, 60% of an ad libitum (AL) diet, on cholesterol removal from rectus femoris muscle injected with cationized LDL, was studied in C57BL male mice. RCT in vivo, on CR and AL diet, and cholesterol efflux from macrophages exposed to CR or AL sera, was similar, despite a 22% reduction in plasma HDL-cholesterol (HDL-C). In CR fed mice total cholesterol (TC) and phospholipid (T-PL) decreased by 32% and 38%, while HDL-C and HDL-PL decreased by 22% and 16% only, resulting in increased HDL-PL/T-PL ratio, which enhanced RCT. Partial re-feeding (CR-RF, 70% of AL) induced normalization of plasma lipids (excluding triglycerides), while HDL-PL/T-PL remained elevated. Thus, as CR did not interfere with RCT in vivo, it could possibly be beneficial to patients at risk for coronary heart disease.

In CCR2-/- mice monocyte recruitment and egress of LDL cholesterol in vivo is impaired

Recruitment of macrophages plays an important role in initiation of atheroma, but their involvement in cholesterol clearance during regression is unknown. We developed a mouse model to quantitate cholesterol clearance from a depot of cationized LDL injected into a leg muscle, which evokes a sterile inflammatory reaction. In the CCR2(-/-) mice, cholesterol clearance was significantly slower than in C57BL controls because of decrease in cholesteryl ester (CE) hydrolysis, which is mandatory prior to cholesterol efflux. In CCR2(-/-) mice, macrophage recruitment to the injected site, identified by immunohistochemistry, was markedly delayed. CE hydrolysis was also significantly reduced in thioglycollate elicited peritoneal exudate cells of CCR2(-/-) mice, related to paucity of macrophages in the cell differential. The present study provides definite evidence that recruitment of macrophages is required for LDL cholesterol clearance, which plays a prominent role in regression of an atheroma.

Macrophage-specific expression of human lipoprotein lipase accelerates atherosclerosis in transgenic apolipoprotein e knockout mice but not in C57BL/6 mice

Transgenic mice with macrophage-specific expression of human (hu) lipoprotein lipase (LPL) were generated to determine the contribution of macrophage LPL to atherogenesis. Macrophage specificity was accomplished with the scavenger receptor A promoter. Complete characterization demonstrated that macrophages from these mice expressed huLPL mRNA and secreted enzymatically active huLPL protein. Expression of huLPL was macrophage specific, because total RNA isolated from heart, thymus, lung, liver, muscle, and adipose tissues was devoid of huLPL mRNA. Macrophage-specific expression of huLPL did not exacerbate lesions in aortas of C57BL/6 mice even after 32 weeks on an atherosclerotic diet. However, when expressed in apolipoprotein E knockout background, the extent of occlusion in the aortic sinus region of male huLPL+ mice increased 51% (n=9 to 11, P<0.002) compared with huLPL- mice after they had been fed a Western diet for 8 weeks. The proatherogenic effect of macrophage LPL was confirmed in serial sections of the aorta obtained after mice had been fed a Western diet for 3 weeks. By immunohistochemical analysis, huLPL protein was detected in the lesions of huLPL+ mice but not in huLPL- mice. Our results establish that macrophage LPL accelerates atherosclerosis in male apolipoprotein E knockout mice.

Sustained elevated plasma aprotinin concentration in mice following intraperitoneal injections of w/o emulsions incorporating aprotinin

This study was initiated to test the feasibility of w/o emulsions as a sustained release system for aprotinin following intraperitoneal injection in mice. The emulsion was well tolerated in mice and sustained release was observed over a period of 96 h. The time for maximum plasma concentration of aprotinin was 10 min and 12 h after injection of a control solution and the emulsion dosage form, respectively. Furthermore, the hemolytic activity of the emulsion constituents was low indicating a low acute toxicological potential of the emulsion. The present study also showed that the lipolytic activity in peritoneal exudate from mice is important for the clearance of oily vehicles from the peritoneal cavity with lipolytic rate constants ranging from 50 to 130 nmol free fatty acid released/min/mg exudate protein at 37 degrees C, pH 8.5. It was concluded that the w/o emulsion was well suited to provide sustained elevated plasma aprotinin concentrations in mice.

Diabetic state induces lipid loading and altered expression and secretion of lipoprotein lipase in human monocyte-derived macrophages

Non-insulin-dependent diabetes mellitus (NIDDM) is frequently associated with macroangiopathies and coronary heart diseases. Lipoprotein lipase (LPL), an enzyme known to undergo significant functional alterations in diabetic state, is also a potential atherogenic protein. Since, to the best of our knowledge, there are no data concerning LPL secreted by macrophages of NIDDM patients we conducted a study to assess the expression and activity of LPL secreted by monocyte-derived macrophages from NIDDM patients with cardiovascular complications versus cardiovascular patients without diabetes (controls). Isolated cells from NIDDM patients, after 7 days in culture in the presence of 20% autologous serum, readily exhibit a foam cell phenotype, in contrast to the cells from controls. Macrophages were mainly loaded with triglycerides, whose cellular amount was well correlated to triglyceridemia of NIDDM subjects. Concomitantly, macrophages from NIDDM patients displayed a approximately six-fold decrease of mRNA expression and a approximately two-fold reduction of the activity of secreted LPL, as compared to control cells. These data suggest that in complicated diabetic state, macrophage loading leading to foam cell formation is accelerated, at least in part, due to a diminished expression and activity of LPL. These observations add and extend the data that may explain the occurrence of accelerated atherogenesis and of the atherosclerotic complications associated with diabetes.

Lipoprotein lipase, a key role in atherosclerosis?

Lipoprotein lipase (LPL) plays a central role in lipid metabolism and transport by catalysing the hydrolysis of triacylglycerol-rich lipoproteins. The importance of LPL expressed by the adipose tissue and muscles in the provision of non-esterified fatty acids and 2-monoacylglycerol for tissue utilisation is well established. However, recent studies on LPL expressed by cells of the vascular wall, particularly macrophages, have identified additional actions of the enzyme that contribute to the promotion of foam cell formation and atherosclerosis. This review deals with the role of LPL in atherosclerosis, and its regulation by mediators that are known to be present in the lesion.

Effects of platelet-derived growth factor on the synthesis of lipoprotein lipase in human monocyte-derived macrophages

Lipoprotein lipase (LPL), which is secreted by the two predominant cell types in atherosclerotic plaque, macrophages and smooth muscle cells, may be involved in atherosclerosis by generating atherogenic remnant lipoproteins. We investigated the effects of platelet-derived growth factor (PDGF)-BB on the synthesis of LPL by human monocyte-derived macrophages. These cells were cultured in the presence of PDGF-BB for 8 days, after which the enzyme activity, mass, and mRNA levels of LPL were determined. The effect of PDGF-BB was time-dependent and dose-dependent at concentrations of 1 to 10 ng/mL. At 10 ng/mL PDGF-BB enhanced twofold to 2.3-fold the secretion of LPL, and a pulse-labeling study with [35S]methionine revealed that 10 ng/mL PDGF-BB significantly increased the synthesis of LPL. Northern blotting analysis showed that the LPL mRNA level increased dose dependently in macrophages treated with PDGF-BB, and 10 ng/mL PDGF-BB enhanced twofold the expression of LPL mRNA. The protein kinase C inhibitor staurosporine suppressed the effect of PDGF-BB on LPL activity. These results indicate that PDGF-BB stimulated transcription of the LPL gene in human monocyte-derived macrophages through protein kinase C activation and resulted in an increased synthesis of LPL. Therefore, we hypothesize that the augmented synthesis of LPL by PDGF-BB modulates atherosclerosis by influencing lipoprotein metabolism in the vascular wall.

Effect of transforming growth factor-beta on lipoprotein lipase in rat mesenchymal heart cell cultures

The effect of recombinant transforming growth factor-beta 2 (rTGF-beta 2) on lipoprotein lipase (LPL) synthesis was studied in mesenchymal rat heart cell cultures. Addition of rTGF-beta 2 to culture medium containing 20% serum resulted in a time-dependent decrease in LPL activity. With 10 ng/ml a 30% fall occurred after 12 h and only 20% of enzyme activity remained after 24 h with 5 or 10 ng/ml. The minimal effective dose of rTGF-beta 2 was 0.1 ng/ml and a 20% decrease occurred after exposure for 24 h. Antibodies specific to TGF-beta 2 blocked this effect. The decrease in enzymic activity was accompanied by a decrease in enzyme mass and LPL mRNA. Addition of rTGF-beta 2 was effective only during the first week in culture, when enzyme activity was increasing but not after 12 days when the cultures were overconfluent, and the enzyme activity was high.

Expression of lipoprotein lipase mRNA and secretion in macrophages isolated from human atherosclerotic aorta

The expression of lipoprotein lipase (LPL) mRNA and the LPL activity were studied in macrophages (CD14 positive) from human atherosclerotic tissue. Macrophages were isolated after collagenase digestion by immunomagnetic isolation. About 90% of the cells were foam cells with oil red O positive lipid droplets. To analyze the mRNA expression, PCR with specific primers for LPL was used. Arterial macrophages were analyzed directly after isolation and the data showed low expression of LPL mRNA when compared with monocyte-derived macrophages. To induce the expression of LPL mRNA in macrophages, PMA was used. When incubating arterial macrophages with PMA for 24 h we could not detect any increase in LPL mRNA levels. Similarly, the cells secreted very small amounts of LPL even after PMA stimulation. In conclusion, these studies show a very low expression of LPL mRNA in the CD14-positive macrophage-derived foam cells isolated from human atherosclerotic tissue. These data suggest that the CD14-positive cells are a subpopulation of foam cells that express low levels of lipoprotein lipase, and the lipid content could be a major factor for downregulation of LPL. However, the cells were isolated from advanced atherosclerotic lesions, and these findings may not reflect the situation in early fatty streaks.

High macrophage lipoprotein lipase expression and secretion are associated in inbred murine strains with susceptibility to atherosclerosis

To test the possibility that variations in macrophage lipoprotein lipase (LPL) secretion may constitute one of the hereditary components of atherosclerosis, we evaluated LPL gene expression and secretion in macrophages harvested from inbred mouse strains differing in their susceptibility to the diet-induced development of atherosclerosis. Inflammatory peritoneal macrophages harvested from atherosclerosis-susceptible C57BL/6J mice showed twofold to threefold higher basal LPL mass, activity, and mRNA levels than those isolated from atherosclerosis-resistant C3H/HeN mice. We determined LPL secretion and gene expression in the susceptible C57BL/6J (B), resistant A/J (A), and A x B/B x A recombinant inbred strains of mice typed as atherosclerosis resistant (A-like) or atherosclerosis susceptible (B-like). Macrophage LPL secretion and mRNA expression were twofold higher in the susceptible C57BL/6J (B) mice than in the resistant A/J (A) mice. Significantly higher LPL secretion, activity, and gene expression were found in recombinant inbred mouse strains that typed B-like than in those typed A-like. These results indicate that susceptibility to atherosclerosis is associated in inbred mouse strains with high LPL secretion and mRNA levels, whereas lower LPL secretion and mRNA expression are observed in atherosclerosis-resistant mice. These observations suggest a contributive role for LPL in the development of atherosclerosis.

Macrophages and smooth muscle cells express lipoprotein lipase in human and rabbit atherosclerotic lesions

Lipoprotein lipase (LPL; EC 3.1.1.34) may promote atherogenesis by producing remnant lipoproteins on the endothelial surface and by acting on lipoproteins in the artery wall. In vitro, smooth muscle cells and macrophages synthesize LPL, but in human carotid lesions only a few smooth muscle cells were reported to contain LPL protein. Endothelial cells do not synthesize LPL in vitro, but in normal arteries intense immunostaining for LPL is present on the endothelium. We used Northern blot analysis, in situ hybridization, and immunocytochemistry of human and rabbit arteries to determine cellular distribution and the site of the synthesis of LPL in atherosclerotic lesions. Northern blot analysis showed that LPL mRNA was detectable in macrophage-derived foam cells isolated from arterial lesions of "ballooned" cholesterol-fed rabbits. In situ hybridization studies of atherosclerotic lesions with an antisense riboprobe showed a strong hybridization signal for LPL mRNA in some, but not all, lesion macrophages, which were mostly located in the subendothelial and edge areas of the lesions. Also, some smooth muscle cells in lesion areas also expressed LPL mRNA. Immunocytochemistry of frozen sections of rabbit lesions with a monoclonal antibody to human milk LPL showed intense staining for LPL protein in macrophage-rich intimal lesions. The results suggest that lesion macrophages and macrophage-derived foam cells express LPL mRNA and protein. Some smooth muscle cells in the lesion areas also synthesize LPL. These data are consistent with an important role for LPL in atherogenesis.

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.

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.

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.

Interaction of lipoprotein lipase with subendothelial extracellular matrix

We have analyzed the binding of lipoprotein lipase (LPL) to the subendothelial extracellular matrix produced by cultured endothelial cells. Binding was linear up to a concentration of 0.5 microgram/ml (10 nM) enzyme used in this study, and equilibrium was achieved after 2 h of incubation with bovine 125I-LPL at 4 degrees C. Heparin and heparan sulfate effectively inhibited the binding of LPL to extracellular-matrix-coated plates; chondroitin sulfate had no effect, while high concentrations of dermatan sulfate or keratan sulfate inhibited binding of LPL to extracellular matrix by only 40%. Basic fibroblast growth factor (bFGF) did not affect LPL binding, while antithrombin-III (AT-III) caused up to a 50% inhibition of enzyme binding to extracellular matrix. alpha-Thrombin. 5.10(-6) M, and its esterolytically inactive derivative, DIP-alpha-thrombin, effectively inhibited binding of LPL to extracellular-matrix-coated plates. alpha-Thrombin was also able to release the extracellular-matrix-bound LPL in an active form. Extracellular-matrix-bound LPL detached into medium containing triolein emulsion and/or serum, and was catalytically active after being released. Extracellular-matrix-bound LPL lost 30% of its activity following incubation at 37 degrees C for 4 h. in contrast to soluble LPL which lost 75% of its activity. It is plausible to conclude from these data that in vivo the subendothelial basement membrane, similarly to extracellular matrix, sequesters and stabilizers LPL secreted into the subendothelial space by non-endothelial cells, and thus may play an important role in determining the route of LPL from its site of synthesis to its site of action.

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.