Lipoprotein lipase (LpL) affects low density lipoprotein (LDL) flux through vascular tissue: evidence that LpL increases LDL accumulation in vascular tissue

A new, powerful player in lipoprotein metabolism: brown adipose tissue

Important causes for modern epidemics such as obesity, diabetes, and cardiovascular disease are over- and malnutrition. Dietary as well as endogenous lipids are transported through the bloodstream in lipoproteins, and disturbances in lipoprotein metabolism are associated with atherosclerosis, heart disease, and diabetes. Recent findings reveal biological principles-how lipoproteins, in particular triglyceride-rich lipoproteins, are metabolized and what factors regulate their processing. The fate of triglycerides delivered by lipoproteins is quite simple: either they can be stored or they can be utilized for combustion or biosynthetic pathways. In the healthy state, fatty acids derived from triglycerides can be burned in the heart, muscle, and other organs for actual work load, or they can be stored in white adipose tissue. The combination of storage and combustion is realized in brown adipose tissue (BAT), a peripheral organ that was long thought to be only of relevance in small mammals: Recent data however prove that BAT plays an important role in human adults. Here, we will review recent insights on how BAT controls triglyceride clearance and the possible implications for the treatment of chronic diseases caused by lipid mishandling.

Triglyceride-rich lipoprotein lipolysis releases neutral and oxidized FFAs that induce endothelial cell inflammation

Triglyceride-rich lipoprotein (TGRL) lipolysis products provide a pro-inflammatory stimulus that can alter endothelial barrier function. To probe the mechanism of this lipolysis-induced event, we evaluated the pro-inflammatory potential of lipid classes derived from human postprandial TGRL by lipoprotein lipase (LpL). Incubation of TGRL with LpL for 30 min increased the saturated and unsaturated FFA content of the incubation solutions significantly. Furthermore, concentrations of the hydroxylated linoleates 9-hydroxy ocatadecadienoic acid (9-HODE) and 13-HODE were elevated by LpL lipolysis, more than other measured oxylipids. The FFA fractions elicited pro-inflammatory responses inducing TNFalpha and intracellular adhesion molecule expression and reactive oxygen species (ROS) production in human aortic endothelial cells (HAECs). The FFA-mediated increase in ROS was blocked by both the cytochrome P450 2C9 inhibitor sulfaphenazole and NADPH oxidase inhibitors. Compared with linoleate, 13-HODE was found to be a more potent inducer of ROS production in HAECs, an activity that was insensitive to both NADPH oxidase and cytochrome P450 inhibitors. Therefore, although the oxidative metabolism of FFA in endothelial cells can produce inflammatory responses, TGRL lipolysis can also release preformed mediators of oxidative stress (e.g., HODEs) that may influence endothelial cell function in vivo by stimulating intracellular ROS production.

Sialic acid as a protective barrier against neointima development

Arterial sialic acid (SA) has been shown to attenuate the binding of fibrinogen and low-density lipoproteins (LDL) to the vessel wall, presumably protecting against atherosclerosis. This study was aimed to assess the effect of changes in SA content in intimal thickening, an early step in the development of atherosclerosis. New Zealand white rabbits were subjected to bilateral carotid periarterial collaring, followed by in situ-perfusion with neuroaminidase (random artery) and with vehicle (contralateral control artery). The efficiency of SA removal was evaluated in perfusates and arterial homogenates, and arterial tissue samples were obtained 7 and 14 days after the intervention to assess morphological changes. Neuraminidase significantly reduced SA by 16.7%. Arterial desialylation was associated with a significantly increased neointimal formation. Proliferation of smooth muscle cells (SMCs), assessed by incorporation of bromo-2'-deoxyuridine into replicating DNA was also significantly increased in desialylated arteries. In addition, immunohistochemical studies showed a slightly stronger oxidized-LDL (ox-LDL) immunostaining in neointima of desialylated arteries than in control vessels. A mild reduction of SA increases intimal thickening, at least partly due to an enhanced proliferation of SMCs, and may facilitate the accretion of atherogenic lipoproteins, providing evidence for the potential role of SA in the protection against neointimal development.

Apolipoprotein E and lipoprotein lipase increase triglyceride-rich particle binding but decrease particle penetration in arterial wall

OBJECTIVE

Liver-derived apolipoprotein E (apoE) decreases atherosclerosis without altering the circulating concentrations of plasma lipoproteins. We evaluated the effects of apoE and lipoprotein lipase (LpL) on the interactions of triglyceride-rich particles (TGRPs) in the arterial wall.

METHODS AND RESULTS

Quantitative fluorescence microscopy was used to study the interactions of TGRPs (25- to 35-nm diameter) in the arterial wall. Carotid arteries were harvested from rats, placed in a perfusion chamber, and perfused with fluorescently labeled TGRPs. In the absence of apoE or LpL, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine-TGRP (100 microg neutral lipid/mL) was poorly retained in the arterial wall. The addition of either apoE (10 microg/mL) or LpL (10 microg/mL) increased TGRP accumulation 220% and 100%, respectively. This effect was attenuated by heparin (10.0 IU/mL). Histological analyses of cross sections from these vessels demonstrate that in the absence of apoE or LpL, there is deep penetration of lipid into the arterial wall. With the addition of either apoE or LpL, arterial wall penetration of TGRP is blocked.

CONCLUSIONS

These results demonstrate that although apoE and LpL increase arterial wall accumulation of TGRPs, these proteins also reduce the penetration of TGRPs into the arterial wall. We postulate that this may represent a novel antiatherogenic property of apoE and LpL.

Endothelial lipase: a new lipase on the block

Endothelial lipase (EL) is a newly described member of the triglyceride lipase gene family. It has a considerable molecular homology with lipoprotein lipase (LPL) (44%) and hepatic lipase (HL) (41%). Unlike LPL and HL, this enzyme is synthesized by endothelial cells and functions at the site where it is synthesized. Furthermore, its tissue distribution is different from that of LPL and HL. As a lipase, EL has primarily phospholipase A1 activity. Animals that overexpress EL showed reduced HDL cholesterol levels. Conversely, animals that are deficient in EL showed a marked elevation in HDL cholesterol levels, suggesting that it plays a physiologic role in HDL metabolism. Unlike LPL and HL, EL is located in the vascular endothelial cells and its expression is highly regulated by cytokines and physical forces, suggesting that it may play a role in the development of atherosclerosis. However, there is only a limited amount of information available about this enzyme. Some of our unpublished data in addition to previously published data support the possibility that the enzyme plays a role in the formation of atherosclerotic lesion.

Activity and concentration of lipoprotein lipase in post-heparin plasma and the extent of coronary artery disease

Numerous studies have found polymorphisms in the lipoprotein lipase (LPL) gene to be associated with the risk of coronary artery disease (CAD), implicating LPL in the development of atherothrombotic disease. It remains controversial, however, whether LPL acts in a pro- or anti-atherogenic fashion. We quantitated activity and concentration of LPL in post-heparin plasma from 194 male patients undergoing coronary angiography. HDL cholesterol was significantly associated with LPL activity quartiles (1.09+/-0.26 the highest vs. 0.96+/-0.25 mmol/l the lowest quartile, P<0.01). There was also a trend towards higher total (5.61+/-1.33 vs. 5.16+/-1.44 mmol/l, P=0.059) and LDL cholesterol (3.92+/-1.39 vs. 3.46+/-1.06 mmol/l, P=0.09) with higher LPL activity. In contrast, measures of CAD extent showed no differences between LPL quartiles (P>0.30 for prior myocardial infarction, number of diseased vessels, Gensini and extent scores). Additionally, there was no difference in LPL activity (CAD: n=158, 168+/-70 nmol/ml/min, no CAD: n=36, 180+/-89 nmol/ml/min, P=0.47) or concentration (280+/-121 ng/ml and 288+/-111 ng/ml, P=0.72) between patients with and without CAD. Our data show that, in spite of an association with lipoprotein parameters, LPL in post-heparin plasma is unrelated to the presence or the extent of CAD. Therefore, lipoprotein lipase determination in plasma does not appear to be a useful marker in the assessment of CAD risk.

Homocysteine induces protein kinase C activation and stimulates c-Fos and lipoprotein lipase expression in macrophages

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease in human diabetes. Among the multiple factors that may account for the atherogenicity of homocysteine (Hcys) in patients with diabetes, macrophage (Mo) lipoprotein lipase (LPL) has unique features in that it is increased in human diabetes and acts as a proatherogenic factor in the arterial wall. In the present study, we determined the direct regulatory effect of Hcys on Mo LPL gene expression and secretion. Incubation of J774 Mo with Hcys increased, in a time- and dose-dependent manner, LPL mRNA expression and secretion. Induction of LPL gene expression was biphasic, peaking at 1 and 6 h. Whereas Hcys treatment increased protein kinase C (PKC) activity in Mo, pretreatment of Mo with PKC inhibitors totally suppressed Hcys-induced LPL mRNA expression. Hcys also increases the levels of c-fos mRNA in Mo and enhanced nuclear protein binding to the AP-1 sequence of the LPL gene promoter. Overall, these results demonstrate that Hcys stimulates Mo LPL at both the gene and protein levels and that Hcys-induced LPL mRNA expression requires PKC activation. They also suggest a possible role of c-fos in the stimulatory effect of Hcys on Mo LPL mRNA expression. These observations suggest a new mechanism by which Hcys may exert its proatherogenic effects in human diabetes.

Marked neointimal lipoprotein lipase increase in distinct models of proclivity to atherosclerosis: a feature independent of endothelial layer integrity

Lipoprotein lipase (LPL) in the arterial wall has been proposed to enhance the retention of apoB-containing lipoproteins, an early event in atherosclerosis. As the neointima is considered the primary site of lipid accumulation in atherogenesis, the arterial expression and location of LPL was investigated in distinct experimental models of neointimal formation in normolipidemic rabbits and rats. Neointima elicited by balloon aortic denudation or raised beneath an anatomically intact endothelial layer by placing a silastic collar around the common carotid artery, both showed a striking LPL immunostaining that mostly co-localized with neointimal smooth muscle cells. Besides, increased LPL protein and mRNA in deendothelialized aortas was demonstrated by Western and Northern blot analysis, respectively, suggesting an enhanced expression of LPL in injured arteries. It was concluded that LPL is increased in neointima developed in either denuded vessels or arteries with a preserved endothelium, a finding which suggests that LPL abundance may be an attribute of the neointima, whatever the stimulus that promotes its formation. On the basis of former evidence concerning the role of LPL in lipid retention, this study provides a possible explanation for the injury-induced vessel susceptibility to atherosclerosis, and the particular proneness of the neointimal layer to lipid accretion.

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.

Proliferative effect of lipoprotein lipase on human vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) proliferation is a key event in the development and progression of atherosclerotic lesions. Accumulating evidence suggests that lipoprotein lipase (LPL) produced in the vascular wall may exert proatherogenic effects. The aim of the present study was to examine the effect of LPL on VSMC proliferation. Incubation of growth-arrested human VSMCs with purified endotoxin-free bovine LPL for 48 and 72 hours, in the absence of any added exogenous lipoproteins, resulted in a dose-dependent increase in VSMC growth. Addition of VLDLs to the culture media did not further enhance the LPL effect. Treatment of growth-arrested VSMCs with purified human or murine LPL (1 microg/mL) led to a similar increase in cell proliferation. Neutralization of bovine LPL by the monoclonal 5D2 antibody, irreversible inhibition, or heat inactivation of the lipase suppressed the LPL stimulatory effect on VSMC growth. Moreover, preincubation of VSMCs with the specific protein kinase C inhibitors calphostin C and chelerythrine totally abolished LPL-induced VSMC proliferation. In LPL-treated VSMCs, a significant increase in protein kinase C activity was observed. Treatment of VSMCs with heparinase III (1 U/mL) totally inhibited LPL-induced human VSMC proliferation. Taken together, these data indicate that LPL stimulates VSMC proliferation. LPL enzymatic activity, protein kinase C activation, and LPL binding to heparan sulfate proteoglycans expressed on VSMC surfaces are required for this effect. The stimulatory effect of LPL on VSMC proliferation may represent an additional mechanism through which the enzyme contributes to the progression of atherosclerosis.

Role of macrophage-derived lipoprotein lipase in lipoprotein metabolism and atherosclerosis

Lipoprotein lipase (LPL) synthesis by macrophages is upregulated in early atherogenesis, implicating the possible involvement of LPL in plaque formation. However, it is still unclear whether macrophage-derived LPL displays a proatherosclerotic or an antiatherosclerotic role in atherosclerotic lesion development. In this study, the role of macrophage-derived LPL on lipid metabolism and atherosclerosis was assessed in vivo by transplantation of LPL-deficient (LPL-/-) and wild-type (LPL+/+) bone marrow into C57BL/6 mice. Eight weeks after bone marrow transplantation (BMT), serum cholesterol levels in LPL-/--->C57BL/6 mice were reduced by 8% compared with those in LPL+/+-->C57BL/6 mice (P:<0.05, n=16), whereas triglycerides were increased by 33% (P:<0.05, n=16). Feeding the mice a high-cholesterol diet increased serum cholesterol levels in LPL-/--->C57BL/6 and LPL+/+-->C57BL/6 mice 5-fold and 9-fold, respectively, resulting in a difference of approximately 50% (P:<0. 01) after 3 months on the diet. No effects on triglyceride levels were observed under these conditions. Furthermore, serum apolipoprotein E levels were reduced by 50% in the LPL-/--->C57BL/6 mice compared with controls under both dietary conditions. After 3 months on a high-cholesterol diet, the atherosclerotic lesion area in LPL-/--->C57BL/6 mice was reduced by 52% compared with controls. It can be concluded that macrophage-derived LPL plays a significant role in the regulation of serum cholesterol, apolipoprotein E, and atherogenesis, suggesting that specific blockade of macrophage LPL production may be beneficial for decreasing atherosclerotic lesion development.

Localization of lipoprotein lipase in the diabetic heart: regulation by acute changes in insulin

Vascular endothelium-bound lipoprotein lipase (LPL) is rate limiting for free fatty acid (FFA) transport into tissues. In streptozotocin (STZ)-diabetic rats, we have previously demonstrated an increased heparin-releasable LPL activity from perfused hearts. Because heparin can traverse the endothelial barrier, conventional Langendorff retrograde perfusion of the heart with heparin could release LPL from both the capillary luminal and abluminal surfaces. To determine the precise location of the augmented LPL, a modified Langendorff retrograde perfusion was used to isolate the enzyme at the coronary lumen from that in the interstitial effluent. In response to heparin, a 4-fold increase in LPL activity and protein mass was observed in the coronary perfusate after 2 weeks of STZ diabetes. Release of LPL activity into the interstitial fluid of control hearts was slow but progressive, whereas in diabetic hearts, peak enzyme activity was observed within 1 to 2 minutes after heparin, followed by a gradual decline. Immunohistochemical studies of myocardial sections confirmed that the augmented LPL in diabetic hearts was mainly localized at the capillary endothelium. To study the acute effects of insulin on endothelial LPL activity, we examined rat hearts at various times after the onset of hyperglycemia. An increased heparin-releasable LPL activity in diabetic rats was demonstrated shortly (6 to 24 hours) after STZ injection or after withdrawal from exogenous insulin. Heparin-releasable coronary LPL activity was also increased after an overnight fast. These studies indicate that the intravascular heparin-releasable fraction of cardiac LPL activity is acutely regulated by short-term changes in insulin rather than glucose. Thus, during short periods (hours) of hypoinsulinemia, increased LPL activity at the capillary endothelium can increase the delivery of FFAs to the heart. The resultant metabolic changes could induce the subsequent cardiomyopathy that is observed in the chronic diabetic rat.

Modified LDL-mediated increases in endothelial layer permeability are attenuated with 17 beta-estradiol

-Current research suggests that estrogen may have primary effects on the artery wall. To investigate the mechanisms of female sex hormone actions in the artery wall, we used an isolated, perfused, rat carotid artery model to examine the effects of estradiol on the rates of accumulation of normal (N-LDL) and minimally modified (MM-LDL) low density lipoprotein in ovariectomized rats. N-LDL, MM-LDL, and oxidized LDL (OX-LDL) were fluorescently labeled and perfused into individual arteries. The rate of LDL accumulation was measured by quantitative fluorescence microscopy before and after treatment with estradiol (1 nmol/L, 272 pg/mL). Estradiol had no effect on the rate of N-LDL accumulation (45+/-12 versus 48+/-15 ng cholesterol per cm2 per h). However, estradiol significantly decreased the rate of MM-LDL (240+/-48 versus 160+/-48 ng cholesterol per cm2 per h; P<0.05) and OX-LDL (191+/-53 versus 112+/-36 ng cholesterol per cm2 per h; P<0.05) accumulation. Further experiments showed that perfusion of unlabeled MM-LDL (100 microgram/mL) increased endothelial layer permeability when the rate of accumulation of a water-soluble, fluorescently labeled, reference molecule (64 000-molecular weight dextran) was determined before and after perfusion of MM-LDL (319+/-96 versus 510+/-191 ng per cm2 per h, n=6 arteries; P<0.05). Estradiol prevented the expected increase in the rate of dextran accumulation when perfused with MM-LDL (control, 415+/-49 ng per cm2 per h and MM-LDL+estradiol, 415+/-160 ng per cm2 per h). Our studies show that estradiol prevents compromise of the endothelial barrier mediated by MM-LDL and attenuates accumulation of MM-LDL in the artery wall.

Inhibition of LPL expression in human monocyte-derived macrophages is dependent on LDL oxidation state: a key role for lysophosphatidylcholine

The regulation of macrophage lipoprotein lipase (LPL) secretion and mRNA expression by atherogenic lipoproteins is of critical relevance to foam cell formation. LPL is present in arterial lesions and constitutes a bridging ligand between lipoproteins, proteoglycans, and cell receptors, thus favoring macrophage lipoprotein uptake and lipid accumulation. We investigated the effects of native and of oxidized lipoproteins on the expression of LPL in an in vitro human monocyte-macrophage system. Exposure of mature macrophages (day 12) to highly copper-oxidized human low density lipoprotein (LDL) (100 microg protein per milliliter) led to marked reduction in the expression of LPL activity (-62%, P<0.01) and mRNA level (-47%, P<0.05); native LDL, acetylated LDL, and LDL oxidized for <6 hours were without effect. The reduction in LPL activity became significant at a threshold of 6 hours of LDL oxidation (-31%, P<0.05). Among the biologically active sterols formed during LDL oxidation, only 7beta-hydroxycholesterol (5 microg/mL) induced a minor reduction in macrophage LPL activity, whereas 25-hydroxycholesterol was without effect. By contrast, lysophosphatidylcholine, whose LDL content increased in parallel with the degree of oxidation, induced significant reductions in LPL activity and mRNA levels at concentrations of 2 to 20 micromol/L (-34% to -53%, P<0.01). Our results demonstrate that highly oxidized LDL (>6-hour oxidation) exerts negative feedback on LPL secretion in human monocytes-macrophages via a reduction in mRNA levels. By contrast, native LDL and mildly oxidized LDL (<6-hour oxidation) did not exert a feedback effect on LPL expression. We speculate that the content of lysophosphatidylcholine and, to a lesser degree, of 7beta-hydroxycholesterol in oxidized LDLs is responsible for the downregulation of LPL activity and mRNA abundance in human monocyte-derived macrophages and may therefore modulate LPL-mediated pathways of lipoprotein uptake during conversion of macrophages to foam cells.

Streptozotocin-induced diabetes enhances cardiac heparin-releasable lipoprotein lipase activity in spontaneously hypertensive rats

Vascular endothelial-bound lipoprotein lipase (LPL), also known as heparin-releasable LPL, catalyzes the breakdown of the triglyceride component of lipoproteins and is rate-limiting for free fatty acid transport to tissues. We previously demonstrated that heparin-releasable LPL activity increases in diabetic Wistar rat hearts, whereas with the development of hypertension in spontaneously hypertensive rats (SHR), there is a concomitant and progressive reduction in LPL activity. The objective of the present study was to examine the regulation of cardiac LPL activity in SHR-diabetic rats. Heparin perfusion of the isolated Langendorff heart induced the release of LPL activity. SHR hearts demonstrated a reduction in peak heparin-releasable LPL activity, relative to Wistar controls. However, induction of streptozotocin-induced diabetes in SHR, as in Wistar rats, increased peak heparin-releasable LPL activity in perfused hearts. The elevated heparin-releasable LPL peak could not be accounted for by enhanced LPL synthesis in that both cellular and surface-bound LPL activities in myocytes from SHR-diabetic rats were low relative to control. Chronic (12-day) insulin treatment of SHR-diabetic rats reduced the augmented heparin-releasable LPL activity and increased cell-associated LPL activity. Moreover, acute (90-minute) treatment of SHR-diabetic rats with rapid-acting insulin also reduced the heparin-releasable LPL activity to normal, although it had no effect on the low cellular LPL activity. These results demonstrate that the diabetes-induced augmentation of cardiac LPL counteracts the reduction in enzyme activity associated with hypertension. This may serve to increase the delivery of free fatty acid to the heart, and the resultant metabolic changes may lead to the severe cardiomyopathy observed in the hypertensive-diabetic rat heart.

Lipoprotein lipase increases lipoprotein binding to the artery wall and increases endothelial layer permeability by formation of lipolysis products

Mechanisms responsible for the accumulation of low-density lipoprotein (LDL) were investigated in a new model, the perfused hamster aorta. To do this, we developed a method to study LDL flux in real time in individually perfused arteries; each artery served as its own control. Using quantitative fluorescence microscopy, the rates of LDL accumulation and efflux were separately determined. Perfusion of arteries with buffer plus lipoprotein lipase (LpL) increased LDL accumulation 5-fold (0.1 +/- 0.03 mV/min [control] versus 0.5 +/- 0.05 mV/min [LpL]) by increasing LDL retention in the artery wall. This effect was blocked by heparin and monoclonal antibodies directed against the amino-terminal region of apolipoprotein B (apo B). This suggests that specific regions of apo B are involved in LDL accumulation within arteries. Also, the effect of hydrolysis of triglyceride-rich lipoproteins on endothelial barrier function was studied. We compared endothelial layer permeability using a water-soluble reference molecule, fluorescently labeled dextran. When LpL was added to hypertriglyceridemic plasma, dextran accumulation within the artery wall increased > 4-fold (0.024 +/- 0.01 mV/min [control] versus 0.098 +/- 0.05 mV/min [LpL]). Under the same conditions, LpL increased LDL accumulation approximately 3-fold (0.016 +/- 0.003 mV/min [control] versus 0.047 +/- 0.013 mV/min [LpL]). Rapid efflux of LDL from the artery wall indicated that increased endothelial layer permeability was the primary mechanism during periods of increased lipolysis. Our data demonstrate two LpL-mediated effects that may increase the amount of LDL in the artery wall. These findings may pertain to the observed relationship between increased postprandial lipemia and atherosclerosis.

Lipoprotein lipase correlates positively and hepatic lipase inversely with calcific atherosclerosis in homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (FH) is a rare genetic disorder that leads to premature atherosclerosis due to a defective LDL receptor. There is, however, a large degree of phenotypic heterogeneity at the level of atherosclerosis even in patients with identical mutations of the LDL receptor protein. Lipoprotein lipase (LPL) and hepatic lipase (HL) are crucial enzymes in lipoprotein metabolism, and both have been proposed as having proatherogenic as well as antiatherogenic effects. To evaluate a potential role for these enzymes in the severity of atherosclerosis, we correlated postheparin LPL mass and activity as well as HL activity with the volume of total calcific atherosclerosis (heart and thoracic aorta), coronary artery calcific atherosclerosis, and Achilles tendon width as measured by computed tomography in 15 FH homozygotes. LPL dimer and total mass were positively correlated with all three parameters (r = .65 to .87, P < .01) as was LPL activity (r = .52 to .63, P < .05). HL activity was negatively correlated with total and coronary artery calcified lesion volume (r = -.55 to .57, P < .05). In a multiple regression model of the coronary artery lesion volume, LPL dimer mass and HL activity together accounted for 84% of the variability (r = .92, P < .0001). In a multiple regression model of the total calcified lesion volume, HL activity, total cholesterol, age, and LPL dimer mass together accounted for 85% of the variability (r = .92, P = .0005). These data demonstrate a significant correlation of LPL mass and activity with the extent of calcific atherosclerosis in homozygous FH. It is not clear whether LPL is the cause or consequence of the observed correlation, but if the association between LPL and coronary artery lesions is also present in patients with other genetic dyslipoproteinemias, LPL could constitute a new risk factor for cardiovascular disease.

Effect of environmental tobacco smoke on LDL accumulation in the artery wall

BACKGROUND

Previous research has shown that exposure to environmental tobacco smoke (ETS) increases the risk of atherosclerosis. To test the hypothesis that exposure to ETS increases LDL accumulation in the artery wall, we developed a model to measure the rate of LDL accumulation in individually perfused rat carotid arteries after the artery had been perfused with plasma taken from rats exposed to ETS (ETS-plasma).

METHODS AND RESULTS

Rats were exposed to ETS in a chamber in which steady-state sidestream smoke was continuously circulating. After exposure, blood from the animals was collected. Carotid arteries from unexposed rats were perfused first with normal plasma containing fluorescently labeled LDL. Then the same arteries (10 arteries from five rats) were perfused with ETS-plasma plus fluorescently labeled LDL. Photometric measurements were made during perfusion of the arteries with fluorescently labeled LDL, and rate of LDL accumulation (mV/min) and lumen volume (mV) (volume of fluorescently labeled LDL solution) were determined. Perfusion with ETS-plasma increased the rate of LDL accumulation (mean +/- SEM, 6.9 +/- 1.8 mV/min) compared with control (1.6 +/- 0.40 mV/min, P < or = .02). LDL accumulation was primarily dependent on LDL interaction with ETS-plasma rather than the interaction of ETS-plasma with the artery wall. Also, ETS-plasma significantly increased lumen volume (43.3 +/- 5.1 mV) compared with control (35.1 +/- 4.4 mV, P < or = .005).

CONCLUSIONS

Exposure to ETS acutely increased LDL accumulation in perfused arteries. Repeated exposure to ETS may represent important early events in atherogenesis.

Accelerated atherosclerosis in mice lacking tumor necrosis factor receptor p55

TNF-alpha (TNF) is produced primarily from macrophages and promotes numerous inflammatory reactions associated with atherosclerosis including the induction of vascular adhesion molecules and the recruitment and proliferation of monocyte/macrophages. There are two receptors known to elicit TNF responses, termed p55 and p75. Since p55 is thought to play the primary role in inflammatory processes, we postulated that the absence of p55 in mice would protect against atherosclerosis. In contrast, C57BL/6 mice lacking p55 had aortic sinus lesion sizes 2.3-fold larger than C57BL/6 wild type mice when fed an atherogenic diet (37,123 +/- 3485 microm2 versus 16, 688 +/- 2887 microm2, respectively, p < 0.0004). Plasma lipid levels were not different between strains. A 3-fold increase in the uptake and degradation of acetylated low density lipoprotein for p55-null as compared with wild type mice was demonstrated in cultured peritoneal macrophages. Immunohistochemical staining for scavenger receptor protein in the aortic sinus was more intense in lesions from the p55-null mice as compared with wild type controls. Our results support the concept that increased scavenger receptor activity contributes to excessive fatty streak formation. We conclude that TNF p55 receptors protect against atherosclerotic lesion development in the mouse.

Injury produces early rise in lipoprotein lipase activity in rabbit aorta

The mechanisms following intimal injury predisposing towards atherosclerotic changes have not been fully elucidated. We speculated that a local increase in the enzyme lipoprotein lipase (LPL) might explain a higher susceptibility of the damaged intima to lipid accretion, and so we investigated the effect of balloon endothelial denudation on LPL activity and cholesterol content (LPLa and Cholc, respectively), in aortas from normolipidemic male New Zealand white rabbits. Arteries were obtained from injured and control animals after 2, 6, 8 and 10 weeks to evaluate the shortest period after de-endothelialization necessary to detect LPLa changes. Injury resulted in a 4-fold LPLa rise (P < 0.01), as early as 2 weeks, and the enzymatic activity remained increased throughout the study period. A mild but significant 22% Cholc increase (P < 0.03) was found after 2 weeks of injury, even in this normolipidemic rabbit model. We conclude that physical damage to the intima markedly and soon increases LPLa. This finding might account for the higher lipid accumulation by injured vessels, providing additional support to the hypothesis of LPL as an atherogenic mediator.

Lipoprotein lipase binds to low density lipoprotein receptors and induces receptor-mediated catabolism of very low density lipoproteins in vitro

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of plasma triglycerides, promotes binding and catabolism of triglyceride-rich lipoproteins by various cultured cells. Recent studies demonstrate that LPL binds to three members of the low density lipoprotein (LDL) receptor family, including the LDL receptor-related protein (LRP), GP330/LRP-2, and very low density lipoprotein (VLDL) receptors and induces receptor-mediated lipoprotein catabolism. We show here that LDL receptors also bind LPL and mediate LPL-dependent catabolism of large VLDL with Sf 100-400. Up-regulation of LDL receptors by lovastatin treatment of normal human foreskin fibroblasts (FSF cells) resulted in an increase in LPL-induced VLDL binding and catabolism to a level that was 10-15-fold greater than in LDL receptor-negative fibroblasts, despite similar LRP activity in both cell lines. This indicates that the contribution of LRP to LPL-dependent degradation of VLDL is small when LDL receptors are maximally up-regulated. Furthermore studies in LRP-deficient murine embryonic fibroblasts showed that the level of LPL-dependent degradation of VLDL was similar to that in normal murine embryonic fibroblasts. LPL also promoted the internalization of protein-free triglyceride emulsions; lovastatin-treatment resulted in 2-fold higher uptake in FSF cells, indicating that LPL itself could bind to LDL receptors. However, the lower induction of emulsion catabolism as compared with native VLDL suggests that LPL-induced catabolism via LDL receptors is only partially dependent on receptor binding by LPL and instead is primarily due to activation of apolipoproteins such as apoE. A fusion protein between glutathione S-transferase and the catalytically inactive carboxyl-terminal domain of LPL (GST-LPLC) also induced binding and catabolism of VLDL. However GST-LPLC was not as active as native LPL, indicating that lipolysis is required for a maximal LPL effect. Mutations of critical tryptophan residues in GST-LPLC that abolished binding to VLDL converted the protein to an inhibitor of lipoprotein binding to LDL receptors. In solid-phase assays using immobilized receptors, LDL receptors bound to LPL in a dose-dependent manner. Both LPL and GST-LPLC promoted binding of VLDL to LDL receptor-coated wells. These results indicate that LPL binds to LDL receptors and suggest that the carboxyl-terminal domain of LPL contributes to this interaction.

COOH-terminal disruption of lipoprotein lipase in mice is lethal in homozygotes, but heterozygotes have elevated triglycerides and impaired enzyme activity

The role of the enzyme lipoprotein lipase (LPL) in atherosclerosis is uncertain. To generate an animal model of LPL deficiency, we targeted the LPL gene in embryonic stem cells with a vector designed to disrupt the COOH terminus of the protein and used these cells to generate LPL-deficient mice. Germ line transmission of the disrupted LPL allele was achieved with two chimeric males, and offspring from each of these animals were phenotypically identical. Pups homozygous (-/-) for LPL deficiency died within 48 h of birth with extreme elevations of serum triglycerides (13,327 mg/dl) associated with essentially absent LPL enzyme activity in heart and carcass. Newborn heterozygous (+/-) LPL-deficient pups had lower LPL enzyme activity and higher triglycerides (370 versus 121 mg/dl) than wild type (+/+) littermates. Adult heterozygotes had higher triglycerides than wild type mice with ad libitum feeding (236 mg/dl for +/- versus 88 mg/dl for +/+) and after fasting for 4 h (98 mg/dl for +/- versus 51 for +/+) or 12 h (109 mg/dl for +/- versus 56 mg/dl for +/+). Triglycerides were present as very low density lipoprotein particles and chylomicrons, but high density lipoprotein cholesterol levels were not decreased in +/- animals. Plasma heparin-releasable LPL activity was 43% lower in +/- versus +/+ adult animals. LPL activity, mRNA, and protein were lower in the tissues of +/- versus +/+ mice. Homozygous LPL deficiency caused by disruption of the COOH terminus of the enzyme is lethal in mice. Heterozygous LPL deficiency caused by this mutation is associated with mild to moderate hypertriglyceridemia without affecting static HDL cholesterol levels. Heterozygous LPL-deficient mice could be useful for determining if hypertriglyceridemia, independently or in combination with other discrete defects, influences atherosclerosis.