Chinese hamster ovary cells expressing a cell surface-anchored form of hepatic lipase. Characterization of low density lipoprotein and chylomicron remnant uptake and selective uptake of high density lipoprotein-cholesteryl ester

Localisation and regulation of cholesterol transporters in the human hair follicle: mapping changes across the hair cycle

Cholesterol has long been suspected of influencing hair biology, with dysregulated homeostasis implicated in several disorders of hair growth and cycling. Cholesterol transport proteins play a vital role in the control of cellular cholesterol levels and compartmentalisation. This research aimed to determine the cellular localisation, transport capability and regulatory control of cholesterol transport proteins across the hair cycle. Immunofluorescence microscopy in human hair follicle sections revealed differential expression of ATP-binding cassette (ABC) transporters across the hair cycle. Cholesterol transporter expression (ABCA1, ABCG1, ABCA5 and SCARB1) reduced as hair follicles transitioned from growth to regression. Staining for free cholesterol (filipin) revealed prominent cholesterol striations within the basement membrane of the hair bulb. Liver X receptor agonism demonstrated active regulation of ABCA1 and ABCG1, but not ABCA5 or SCARB1 in human hair follicles and primary keratinocytes. These results demonstrate the capacity of human hair follicles for cholesterol transport and trafficking. Future studies examining the role of cholesterol transport across the hair cycle may shed light on the role of lipid homeostasis in human hair disorders.

The cross-sectional study of hepatic lipase SNPs and plasma lipid levels

By the combination of meta‐analysis, the data of the 1,000 Genomes Project Phase 3, and the promoter sequence of hepatic lipase (LIPC), we performed the cross‐sectional study to explore the associations of four variants (rs1077835; rs1077834; rs1800588 [C‐514T], and rs2070895 [G‐250A]) in LIPC promoter with plasma lipid levels. Our results indicate that the first and the next three of the four SNPs are, respectively, reported to be associated with the decreased and increased HDL‐c level. Meta‐analysis of 87 studies with 101,988 participants indicates that HDL‐c level in rs1800588 (C‐514T) (pooled mean difference = 0.03, 95%CI (0.03, 0.04), p < .001) and rs2070895 (G‐250A) (pooled mean difference = 0.07, 95%CI (0.05, 0.09), p < .001) is higher in allele T or A carriers. Similarly, LDL‐c, TC, TG, and BMI levels are generally increased in T or A alleles carriers. We failed to conduct the meta‐analysis of rs1077835 and rs1077834 due to the limited previous reports. Data from the 1,000 Genomes indicate that the allele frequencies of the four SNPs in total or subpopulations are almost equal to each other. The paired value r² and D' of the four SNPs are larger than 0.8, which indicate the linkage disequilibrium of the four variants. The analysis of LIPC promoter indicate that C‐514T and G‐250A are, respectively, located in transcriptional factor binding sites of USF1and Pbx1b, which may partly explain the effect of the two SNPs on the decreased LIPC activity in the alleles carriers and the corresponding increased plasma lipids hydrolyzed by LIPC. These results may help us to better understand the different effects of the four SNPs on the plasma lipid levels among subpopulations and offer clues for future clinical treatment of dyslipidemia‐related diseases.

High-density lipoprotein metabolism and the human embryo

BACKGROUND

High-density lipoprotein (HDL) appears to be the dominant lipoprotein particle in human follicular fluid (FF). The reported anti-atherogenic properties of HDL have been attributed in part to reverse cholesterol transport. The discoveries of the scavenger receptor class B type I (SR-BI) and the ATP-binding cassette A1 lipid (ABCA1) transporter have generated studies aimed at unraveling the pathways of HDL biogenesis, remodeling and catabolism. The production of SR-BI and ABCA1 knockout mice as well as other lipoprotein metabolism-associated mutants has resulted in reduced or absent fertility, leading us to postulate the existence of a human hepatic-ovarian HDL-associated axis of fertility. Here, we review an evolving literature on the role of HDL metabolism on mammalian fertility and oocyte development.

METHODS

An extensive online search was conducted of published articles relevant to the section topics discussed. All relevant English language articles contained in Pubmed/Medline, with no specific time frame for publication, were considered for this narrative review. Cardiovascular literature was highly cited due to the wealth of relevant knowledge on HDL metabolism, and the dearth thereof in the reproductive field.

RESULTS

Various vertebrate models demonstrate a role for HDL in embryo development and fertility. In our clinical studies, FF levels of HDL cholesterol and apolipoprotein AI levels were negatively associated with embryo fragmentation, but not with embryo cell cleavage rate. However, the HDL component, paraoxonase 1 arylesterase activity, was positively associated with embryo cell cleavage rate.

CONCLUSIONS

HDL contributes to intra-follicular cholesterol homeostasis which appears to be important for successful oocyte and embryo development.

The -250G/A polymorphism in the hepatic lipase gene promoter influences the postprandial lipemic response in healthy men

BACKGROUND AND AIM

The -250G/A promoter polymorphism of the hepatic lipase gene has been associated with changes in the activity of the enzyme. We investigated whether this polymorphism modifies the postprandial response of triacylglycerol-rich lipoproteins (TRL) in young normolipemic males.

METHODS AND RESULTS

Fifty-one healthy apolipoprotein (apo) E3/E3 male volunteers (30 G/G and 21 carriers of the A allele) underwent a vitamin A fat-loading test and blood samples were drawn every hour until the 6th, and every 2h and 30 min until the 11th. Total plasma cholesterol and triacylglycerols (TG), as well as cholesterol, TG and retinyl palmitate (RP) in TRL, isolated by ultracentrifugation, were determined. Carriers of the A allele showed a higher response (P=0.008), a higher area under the curve (AUC; P=0.022) and a lower RP peak time (P=0.029) in small TRL during the postprandial response, as well as a lower peak time in total plasma TG levels (P=0.034) and large TRL-TG (P=0.033) than subjects who were homozygous for the G allele.

CONCLUSION

Our data indicate that the presence of the A allele in the -250G/A promoter polymorphism of the hepatic lipase gene is associated with a higher postprandial lipemic response.

Consequences of over-expression of rat Scavenger Receptor, SR-BI, in an adrenal cell model

Background

The plasma membrane scavenger receptor, SR-BI, mediates the 'selective uptake' process by which cholesteryl esters (CE) from exogenously supplied HDL are taken up by target cells. Recent work suggests that dimer and higher order oligomeric forms of the SR-BI protein are important to this process. SR-BI has been shown to be particularly associated with microvilli and microvillar channels found at the cell surface of steroidogenic cells, and a study with the hormone stimulated adrenal gland has shown impressive changes in the size and complexity of the microvillar compartment as the mass of CE uptake (and accompanying steroidogenesis) fluctuates. In the present study, we examine a cell line in which we overexpress the SR-BI protein to determine if morphological, biochemical and functional events associated with SR-BI in a controlled cell system are similar to those observed in the intact mammalian adrenal which is responsive to systemic factors.

Methods

Y1-BS1 mouse adrenocortical cells were transiently transfected using rat SR-BI-pcDNA6-V5-His, rat SR-BI-pcDNA6-cMyc-His or control pcDNA6-V5-His vector construct using a CaPO₄ precipitation technique. Twenty four hours after transfection, cells were treated with, or without, Bt₂cAMP, and SR-BI expression, CE uptake, and steroidogenesis was measured. SR-BI dimerization and cell surface architectural changes were assessed using immunoelectron microscopic techniques.

Results

Overexpression of the scavenger receptor protein, SR-BI, in Y1-BS1 cells results in major alterations in cell surface architecture designed to increase uptake of HDL supplied-CEs. Changes include [1] the formation of crater-like erosions of the surface with multiple double membraned channel structures lining the craters, and [2] dimerized formations of SR-BI lining the newly formed craters and associated double membraned channels.

Conclusion

These data show that overexpression of the scavenger receptor protein, SR-BI (accompanied by suitable hormone treatment and lipoproteins) in susceptible mammalian cells – is associated with increased cholesterol uptake and SR-BI dimerization within a much enlarged and architecturally complex microvillar compartment. These changes duplicate the structural, biochemical and functional changes related to the uptake of HDL CEs normally signaled by the action of ACTH on intact adrenal tissue.

Lipolytic and ligand-binding functions of hepatic lipase protect against atherosclerosis in LDL receptor-deficient mice

To elucidate the separate contributions of the lipolytic versus ligand-binding functions of hepatic lipase (HL) to lipoprotein metabolism and atherosclerosis, and to investigate the role of the low density lipoprotein receptor (LDLr) in these processes, we compared mice expressing catalytically active HL (HL-WT) with mice expressing inactive HL (HL-S145G) in a background lacking endogenous HL and the LDLr (LDLr-KOxHL-KO). HL-WT and HL-S145G reduced (P < 0.05 for all) cholesterol (55% vs. 20%), non-HDL-cholesterol (63% vs. 22%), and apolipoprotein B (apoB; 34% vs. 16%) by enhancing the catabolism of autologous (125)I-apoB-intermediate density lipoprotein (IDL)/LDL (fractional catabolic rate in day(-1): 6.07 +/- 0.25, LDLr-KOxHL-WT; 4.76 +/- 0.30, LDLr-KOxHL-S145G; 3.70 +/- 0.13, LDLr-KOxHL-KO); HL-WT had a greater impact on the concentration, composition, particle size, and catabolism of apoB-containing lipoproteins (apoB-Lps) and HDL. Importantly, consistent with the changes in apoB-Lps, atherosclerosis in LDLr-KOxHL-KO mice fed a regular chow diet (RCD) was reduced by both HL-WT and HL-S145G (by 71% and 51% in cross-sectional analysis, and by 85% and 67% in en face analysis; P < 0.05 for all). These data identify physiologically relevant but distinct roles for the lipolytic versus ligand-binding functions of HL in apoB-Lp metabolism and atherosclerosis and demonstrate that their differential effects on these processes are mediated by changes in catabolism via non-LDLr pathways. These changes, evident even in the presence of apoE, establish an antiatherogenic role of the ligand-binding function of HL in LDLr-deficient mice.

Attenuated corticosterone response to chronic ACTH stimulation in hepatic lipase-deficient mice: evidence for a role for hepatic lipase in adrenal physiology

Hepatic lipase (HL), a liver-expressed lipolytic enzyme, hydrolyzes triglycerides and phospholipids in lipoproteins and promotes cholesterol delivery through receptor-mediated whole particle and selective cholesterol uptake. HL activity also occurs in the adrenal glands, which utilize lipoprotein cholesterol to synthesize glucocorticoids in response to pituitary ACTH. It is likely that the role of adrenal HL is to facilitate delivery of exogenous cholesterol for glucocorticoid synthesis. On this basis, we hypothesized that HL deficiency would blunt the glucocorticoid response to ACTH. Furthermore, because exogenous cholesterol also is derived from the LDL receptor (LDLR) pathway, we hypothesized that LDLR deficiency would blunt the response to ACTH. To test these hypotheses, we compared the corticosterone response to eight daily ACTH injections in HL-deficient (hl-/-), LDLR-deficient (Ldlr-/-), and HL- and LDLR-doubly deficient (Ldlr-/- hl-/-) mice with that in wild-type (WT) mice. Plasma corticosterone levels were measured on days 2, 5, and 8. Differences in plasma corticosterone levels between genotypes were analyzed by Kruskal-Wallis one-way ANOVA on ranks and pairwise multiple comparisons by Dunn's test. Our results demonstrate a trend toward reductions in plasma corticosterone levels on day 2 and significant reductions on day 5 and day 8 in the knockout models. Thus, on day 5, plasma corticosterone levels were reduced by 57, 70, and 73% (all P < 0.05) and on day 8 by 76, 59, and 63% (all P < 0.05) in hl-/-, Ldlr-/-, and Ldlr-/- hl-/- mice, respectively. These results demonstrate that HL deficiency, like LDLR deficiency, blunts the adrenal response to chronic ACTH stimulation and suggest a novel role for HL in adrenal physiology.

Removal of chylomicron remnants in transgenic mice overexpressing normal and membrane-anchored hepatic lipase

The LDL receptor and the LDL receptor-related protein (LRP) mediate the removal of chylomicron remnants. The LRP pathway involves sequestration of particles in the space of Disse. It has been proposed that either alone or in combination with other factors, such as apolipoprotein E and proteoglycans, hepatic lipase (HL) may contribute to the sequestration of chylomicron remnants. To test this hypothesis, we generated two lines of transgenic mice producing rat HL as a native or as a membrane-anchored form. These animals express HL at levels similar to normal rat. Chylomicron remnants were perfused in a single nonrecirculating pass into the livers of the rat HL transgenic, HL-deficient, and wild-type (WT) mice for 20 min, and the rate of chylomicron remnant removal was measured. Chylomicron remnants were removed at a rate of approximately 50% per pass in WT mice. It was slightly increased in both transgenic mice and reduced in HL-deficient mice compared with the WT mice. Confocal microscopy of liver sections showed that a modest amount of HL colocalized with chylomicron remnant clusters in the transgenic mice, suggesting that HL is a component of the LRP-proteoglycan clusters. These data suggest that HL helps to direct cholesterol to the tissues in which it is localized by a nonenzymatic mechanism.

The ligand-binding function of hepatic lipase modulates the development of atherosclerosis in transgenic mice

To investigate the separate contributions of the lipolytic versus ligand-binding function of hepatic lipase (HL) to plasma lipoprotein metabolism and atherosclerosis, we compared mice expressing catalytically active wild-type HL (HL-WT) and inactive HL (HL-S145G) with no endogenous expression of mouse apoE or HL (E-KO x HL-KO, where KO is knockout). HL-WT and HL-S145G reduced plasma cholesterol (by 40 and 57%, respectively), non-high density lipoprotein cholesterol (by 48 and 61%, respectively), and apoB (by 36 and 44%, respectively) (p < 0.01), but only HL-WT decreased high density lipoprotein cholesterol (by 67%) and apoA-I (by 54%). Compared with E-KO x HL-KO mice, both active and inactive HL lowered the pro-atherogenic lipoproteins by enhancing the catabolism of autologous (125)I-apoB very low density/intermediate density lipoprotein (VLDL/IDL) (fractional catabolic rates of 2.87 +/- 0.04/day for E-KO x HL-KO, 3.77 +/- 0.03/day for E-KO x HL-WT, and 3.63 +/- 0.09/day for E-KO x HL-S145G mice) and (125)I-apoB-48 low density lipoprotein (LDL) (fractional catabolic rates of 5.67 +/- 0.34/day for E-KO x HL-KO, 18.88 +/- 1.72/day for E-KO x HL-WT, and 9.01 +/- 0.14/day for E-KO x HL-S145G mice). In contrast, the catabolism of apoE-free, (131)I-apoB-100 LDL was not increased by either HL-WT or HL-S145G. Infusion of the receptor-associated protein (RAP), which blocks LDL receptor-related protein function, decreased plasma clearance and hepatic uptake of (131)I-apoB-48 LDL induced by HL-S145G. Despite their similar effects on lowering pro-atherogenic apoB-containing lipoproteins, HL-WT enhanced atherosclerosis by up to 50%, whereas HL-S145G markedly reduced aortic atherosclerosis by up to 96% (p < 0.02) in both male and female E-KO x HL-KO mice. These data identify a major receptor pathway (LDL receptor-related protein) by which the ligand-binding function of HL alters remnant lipoprotein uptake in vivo and delineate the separate contributions of the lipolytic versus ligand-binding function of HL to plasma lipoprotein size and metabolism, identifying an anti-atherogenic role of the ligand-binding function of HL in vivo.

Hepatic lipase, lipoprotein metabolism, and atherogenesis

The role of hepatic lipase as a multifunctional protein that modulates lipoprotein metabolism and atherosclerosis has been extensively documented over the last decade. Hepatic lipase functions as a lipolytic enzyme that hydrolyzes triglycerides and phospholipids present in circulating plasma lipoproteins. Hepatic lipase also serves as a ligand that facilitates lipoprotein uptake by cell surface receptors and proteoglycans, thereby directly affecting cellular lipid delivery. Recently, another process by which hepatic lipase modulates atherogenic risk has been identified. Bone marrow transplantation studies demonstrate that hepatic lipase present in aortic lesions markedly alters aortic lesion formation even in the absence of changes in plasma lipids. These multiple functions of hepatic lipase, which facilitate not only plasma lipid metabolism but also cellular lipid uptake, can be anticipated to have a major and complex impact on atherogenesis. Consistently, human and animal studies support proatherogenic and antiatherogenic roles for hepatic lipase. The concept of hepatic lipase as mainly a lipolytic enzyme that reduces atherogenic risk has evolved into that of a complex protein with multiple functions that, depending on genetic background and sites of expression, can have a variable effect on atherosclerosis.

Association of hepatic lipase with proteoglycans stimulates the production of proteoglycans in vivo and in vitro

HL is synthesized in hepatocytes and functions while bound to heparan sulfate proteoglycans (HSPGs) in sinusoidal endothelial cells. The HL-mediated uptake of lipoprotein requires cell-surface HSPG. The present study tested whether HL plays a role in the production of HSPG. The production of HSPG in Chinese hamster ovary (CHO) cells was determined by measuring the incorporation of (35)SO(4) into PGs. HL-producing HL-CHO cells showed approximately 30% more cellular PG than did wild-type (WT) cells. In contrast, PG production in cells producing a membrane-anchored HL-glycophosphatidylinositol (GPI) that was not bound to HSPG was virtually identical to that in WT cells. When purified HL was added to the WT- or HL-GPI cells, PG production increased significantly to a level similar to that of the HL-secreting cells, suggesting that the binding of HL to HSPG triggered the increased HSPG production. Heparin reduced PG production in HL-producing cells, confirming that PG production is stimulated only when HL is present as a ligand for HSPG. Real-time PCR and Northern blots demonstrated that PG production was significantly reduced in animals lacking HL. Together, these data suggest that the binding of HL to PG on the cell surface exerts a positive feedback on cellular PG production.

Endogenously produced endothelial lipase enhances binding and cellular processing of plasma lipoproteins via heparan sulfate proteoglycan-mediated pathway

Endothelial lipase (EL) is a new member of the triglyceride lipase gene family, which includes lipoprotein lipase (LpL) and hepatic lipase (HL). Enzymatic activity of EL has been studied before. Here we characterized the ability of EL to bridge lipoproteins to the cell surface. Expression of EL in wild-type Chinese hamster ovary (CHO)-K1 but not in heparan sulfate proteoglycan (HSPG)-deficient CHO-677 cells resulted in 3-4.4-fold increases of 125I-low density lipoprotein (LDL) and 125I-high density lipoprotein 3 binding (HDL3). Inhibition of proteoglycan sulfation by sodium chlorate or incubation of cells with labeled lipoproteins in the presence of heparin (100 microg/ml) abolished bridging effects of EL. An enzymatically inactive EL, EL-S149A, was equally effective in facilitating lipoprotein bridging as native EL. Processing of LDL and HDL differed notably after initial binding via EL to the cell surface. More than 90% of the surface-bound 125I-LDL was destined for internalization and degradation, whereas about 70% of the surface-bound 125I-HDL3 was released back into the medium. These differences were significantly attenuated after HDL clustering was promoted using antibody against apolipoprotein A-I. At equal protein concentration of added lipoproteins the ratio of HDL3 to VLDL bridging via EL was 0.092 compared with 0.174 via HL and 0.002 via LpL. In summary, EL mediates binding and uptake of plasma lipoproteins via a process that is independent of its enzymatic activity, requires cellular heparan sulfate proteoglycans, and is regulated by ligand clustering.

Biliary lipid secretion, bile acid metabolism, and gallstone formation are not impaired in hepatic lipase-deficient mice

Whereas hepatic lipase (HL) has been implicated in lipoprotein metabolism and atherosclerosis, its role in controlling biliary lipid physiology has not been reported. This work characterizes plasma lipoprotein cholesterol, hepatic cholesterol content, bile acid metabolism, biliary cholesterol secretion, and gallstone formation in HL-deficient mice and C57BL/6 controls fed standard chow, a cholesterol-supplemented diet, or a lithogenic diet. Compared with C57BL/6 controls, HL knockout mice exhibited increased basal plasma high-density lipoprotein (HDL) cholesterol as well as reduced cholesterol levels transported in large lipoproteins in response to cholesterol-enriched diets. Hepatic cholesterol content and biliary cholesterol secretion of chow-fed HL knockout and wild-type mice were not different and increased similarly in both strains after feeding dietary cholesterol or a lithogenic diet. There were no differences in biliary bile acid secretion, bile acid pool size and composition, or fecal bile acid excretion between HL-deficient and control mice. HL knockout mice had a similar prevalence of gallstone formation as compared with control mice when both strains were fed with a lithogenic diet. In conclusion, the deficiency of HL has no major impact on the availability of lipoprotein-derived hepatic cholesterol for biliary secretion; HL expression is not essential for diet-induced gallstone formation in mice.

The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues

Because cholesterol is a precursor for the synthesis of steroid hormones, steroidogenic tissues have evolved multiple pathways to ensure adequate supplies of cholesterol. These include synthesis, storage as cholesteryl esters, and import from lipoproteins. In addition to endocytosis via members of the low-density lipoprotein receptor superfamily, steroidogenic cells acquire cholesterol from lipoproteins by selective lipid uptake. This pathway, which does not involve lysosomal degradation of the lipoprotein, is mediated by the scavenger receptor class B type I (SR-BI). SR-BI is highly expressed in steroidogenic cells, where its expression is regulated by various trophic hormones, as well as in the liver. Studies of genetically manipulated strains of mice have established that SR-BI plays a key role in regulating lipoprotein metabolism and cholesterol transport to steroidogenic tissues and to the liver for biliary secretion. In addition, analysis of SR-BI-deficient mice has shown that SR-BI expression is important for alpha-tocopherol and nitric oxide metabolism, as well as normal red blood cell maturation and female fertility. These mouse models have also revealed that SR-BI can protect against atherosclerosis. If SR-BI plays similar physiological and pathophysiological roles in humans, it may be an attractive target for therapeutic intervention in cardiovascular and reproductive diseases.

HDL regulates the displacement of hepatic lipase from cell surface proteoglycans and the hydrolysis of VLDL triacylglycerol

We have previously shown that hepatic lipase (HL) is inactive when bound to purified heparan sulfate proteoglycans and can be liberated by HDL and apolipoprotein A-I (apoA-I), but not by LDL or VLDL. In this study, we show that HDL is also able to displace HL directly from the surface of the hepatoma cell line, HepG2, and Chinese hamster ovary cells stably overexpressing human HL. ApoA-I is more efficient at displacing cell surface HL than is HDL, and different HDL classes vary in their ability to displace HL from the cell surface. HDL2s have a greater capacity to remove HL from the cell surface and intracellular compartments, as compared with the smaller HDL particles. The different HDL subclasses also uniquely affect the activity of the enzyme. HDL2 stimulates HL-mediated hydrolysis of VLDL-triacylglycerol, while HDL3 is inhibitory. Inhibition of VLDL hydrolysis appears to result from a decreased interlipoprotein shuttling of HL between VLDL and the smaller, more dense HDL particles. This study suggests that high HDL2 levels are positively related to efficient triacylglycerol hydrolysis by their ability to enhance the liberation of HL into the plasma compartment and by a direct stimulation of VLDL-triacylglycerol hydrolysis.

Lipoprotein lipase (LpL) on the surface of cardiomyocytes increases lipid uptake and produces a cardiomyopathy

Lipoprotein lipase is the principal enzyme that hydrolyzes circulating triglycerides and liberates free fatty acids that can be used as energy by cardiac muscle. Although lipoprotein lipase is expressed by and is found on the surface of cardiomyocytes, its transfer to the luminal surface of endothelial cells is thought to be required for lipoprotein lipase actions. To study whether nontransferable lipoprotein lipase has physiological actions, we placed an alpha-myosin heavy-chain promoter upstream of a human lipoprotein lipase minigene construct with a glycosylphosphatidylinositol anchoring sequence on the carboxyl terminal region. Hearts of transgenic mice expressed the altered lipoprotein lipase, and the protein localized to the surface of cardiomyocytes. Hearts, but not postheparin plasma, of these mice contained human lipoprotein lipase activity. More lipid accumulated in hearts expressing the transgene; the myocytes were enlarged and exhibited abnormal architecture. Hearts of transgenic mice were dilated, and left ventricular systolic function was impaired. Thus, lipoprotein lipase expressed on the surface of cardiomyocytes can increase lipid uptake and produce cardiomyopathy.

Endothelial cell-derived lipase mediates uptake and binding of high-density lipoprotein (HDL) particles and the selective uptake of HDL-associated cholesterol esters independent of its enzymic activity

Endothelial cell-derived lipase (EDL) is a new member of the lipase gene family with high sequence homology with lipoprotein lipase (LPL). EDL is a phospholipase with very little triacylglycerol lipase activity. To investigate the effects of EDL on binding and uptake of high-density lipoprotein (HDL), as well as on the selective uptake of HDL-derived cholesterol esters (CEs), HepG2 cells were infected with adenovirus coding for EDL. For comparison, cells were also infected with LPL and with lacZ as a control. Both HDL binding and particle uptake were increased 1.5-fold and selective HDL-CE uptake was increased 1.8-fold in EDL-infected HepG2 cells compared with controls. The effect of LPL was less pronounced, resulting in 1.1-fold increase in particle uptake and 1.3-fold increase in selective uptake. Inhibition of the enzymic activity with tetrahydrolipstatin (THL) significantly enhanced the effect of EDL, as reflected by a 5.2-fold increase in binding, a 2.6-fold increase in particle uptake and a 1.1-fold increase in CE selective uptake compared with incubations without THL. To elucidate the mechanism responsible for the effects of THL, we analysed the abundance of heparin-releasable EDL protein from infected HepG2 cells upon incubations with THL, HDL and free (non-esterified) fatty acids (FFAs). In the presence of THL, vastly more EDL protein remained bound to the cell surface. Additionally, HDL and FFAs reduced the amount of cell-surface-bound EDL, suggesting that fatty acids that are liberated from phospholipids in HDL release EDL from the cell surface. This was substantiated further by the finding that, in contrast with EDL, the amount of cell-surface-bound enzymically inactive mutant EDL (MUT-EDL) was not reduced in the presence of HDL and foetal calf serum. The increased amount of cell-surface-bound MUT-EDL in the presence of THL suggested that the enzymic inactivity of MUT-EDL, as well as an augmenting effect of THL that is independent of its ability to inactivate the enzyme, are responsible for the increased amount of cell-surface-bound EDL in the presence of THL. Furthermore, in cells expressing MUT-EDL, binding and holoparticle uptake were markedly higher compared with cells expressing the active EDL, and could be increased further in the presence of THL. Despite 1.7-fold higher binding and 1.8-fold higher holoparticle uptake, the selective CE uptake by MUT-EDL-expressing cells was comparable with EDL-expressing cells and was even decreased 1.3-fold with THL. Experiments in CLA-1 (CD-36 and LIMPII analogous 1, the human homologue of scavenger receptor class B type I)-deficient HEK-293 cells demonstrated that EDL alone has the ability to stimulate HDL-CE selective uptake independently of CLA-1. Thus our results demonstrate that EDL mediates both HDL binding and uptake, and the selective uptake of HDL-CE, independently of lipolysis and CLA-1.

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.

Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis

The steroidogenic tissues have a special requirement for cholesterol, which is used as a substrate for steroid hormone biosynthesis. In many species this cholesterol is obtained from plasma lipoproteins by a unique pathway in which circulating lipoproteins bind to the surface of the steroidogenic cells and contribute their cholesteryl esters to the cells by a 'selective' process in which the whole lipoprotein particle does not enter the cell. This review describes the lipoprotein selective cholesteryl ester uptake process and its specific partnership with the HDL receptor, scavenger receptor class BI (SR-BI). It describes the characteristics of the selective pathway, and the molecular properties, localization, regulation, anchoring sites and potential mechanisms of action of SR-BI in facilitating cholesteryl ester uptake by steroidogenic cells.

Apolipoprotein A-I regulates lipid hydrolysis by hepatic lipase

Association of hepatic lipase (HL) with pure heparan sulfate proteoglycans (HSPG) has little effect on hydrolysis of high density lipoprotein (HDL) particles, but significantly inhibits (>80%) the hydrolysis of low (LDL) and very low density lipoproteins (VLDL). Lipolytic inhibition is associated with a differential ability of the lipoproteins to remove HL from the HSPG. LDL and VLDL are unable to displace HL, whereas HDL readily displaces HL from the HSPG. These data show that HSPG-bound HL is inactive. Purified apolipoprotein (apo) A-I is more efficient than HDL at liberating HL from HSPG, and HL displacement is associated with the direct binding of apoA-I to HSPG. However, displacement of HL by apoA-I does not enhance hydrolysis of VLDL particles. This appears due to the direct inhibition of HL by apoA-I. Both apoA-I and HDL are able to inhibit VLDL lipid hydrolysis by up to 60%. Inhibition of VLDL hydrolysis is associated with the binding of apoA-I to the surface of the VLDL particle and a concomitant decreased affinity for HL. These data show that apoA-I can regulate lipid hydrolysis by HL by liberating/activating the enzyme from cell surface proteoglycans and by directly modulating lipoprotein binding and hydrolysis.

Charting the fate of the "good cholesterol": identification and characterization of the high-density lipoprotein receptor SR-BI

Risk for cardiovascular disease due to atherosclerosis increases with increasing concentrations of low-density lipoprotein (LDL) cholesterol and is inversely proportional to the levels of high-density lipoprotein (HDL) cholesterol. The receptor-mediated control of plasma LDL levels has been well understood for over two decades and has been a focus for the pharmacologic treatment of hypercholesterolemia. In contrast, the first identification and characterization of a receptor that mediates cellular metabolism of HDL was only recently reported. This receptor, called scavenger receptor class B type I (SR-BI), is a fatty acylated glycoprotein that can cluster in caveolae-like domains on the surfaces of cultured cells. SR-BI mediates selective lipid uptake from HDL to cells. The mechanism of selective lipid uptake is fundamentally different from that of classic receptor-mediated endocytic uptake via coated pits and vesicles (e.g. the LDL receptor pathway) in that it involves efficient receptor-mediated transfer of the lipids, but not the outer shell proteins, from HDL to cells. In mice, SR-BI plays a key role in determining the levels of plasma HDL cholesterol and in mediating the regulated, selective delivery of HDL-cholesterol to steroidogenic tissues and the liver. Significant alterations in SR-BI expression can result in cardiovascular and reproductive disorders. SR-BI may play a similar role in humans; thus, modulation of its activity may provide the basis of future approaches to the treatment and prevention of atherosclerotic disease.

The role of the high-density lipoprotein receptor SR-BI in cholesterol metabolism

The HDL receptor scavenger receptor class B type I (SR-BI), which mediates selective HDL cholesterol uptake, plays a role in murine HDL metabolism, reverse cholesterol transport and whole-body cholesterol homeostasis. SR-BI is found in the liver, where its expression is regulated by estrogen, dietary cholesterol and fat, and controls murine plasma HDL cholesterol levels and bile cholesterol secretion. SR-BI is also highly expressed in rodent steroidogenic cells, where it facilitates cholesterol uptake for storage or steroid hormone synthesis and where its expression is regulated by trophic hormones. The detailed mechanism(s) underlying SR-BI-mediated selective cholesterol uptake have not yet been elucidated. Further analysis of the molecular and cellular bases of SR-BI regulation and function should provide new insights into the physiology and pathophysiology of cholesterol metabolism.

In vivo evidence for both lipolytic and nonlipolytic function of hepatic lipase in the metabolism of HDL

To investigate the in vivo role that hepatic lipase (HL) plays in HDL metabolism independently of its lipolytic function, recombinant adenovirus (rAdV) expressing native HL, catalytically inactive HL (HL-145G), and luciferase control was injected in HL-deficient mice. At day 4 after infusion of 2 x 10(8) plaque-forming units of rHL-AdV and rHL-145G-AdV, similar plasma concentrations were detected in postheparin plasma (HL=8.4+/-0.8 microg/mL and HL-145G=8.3+/-0.8 microg/mL). Mice expressing HL had significant reductions of cholesterol (-76%), phospholipids (PL; -68%), HDL cholesterol (-79%), apolipoprotein (apo) A-I (-45%), and apoA-II (-59%; P<0.05 for all), whereas mice expressing HL-145G decreased their cholesterol (-49%), PL (-40%), HDL cholesterol (-42%), and apoA-II (-89%; P<0.005 for all) but had no changes in apoA-I. The plasma kinetics of (125)I-labeled apoA-I HDL, (131)I-labeled apoA-II HDL, and [(3)H]cholesteryl ester (CE) HDL revealed that compared with mice expressing luciferase control (fractional catabolic rate [FCR] in d(-1): apoA-I HDL=1.3+/-0.1; apoA-II HDL=2.1+/-0; CE HDL=4.1+/-0.7), both HL and HL-145G enhanced the plasma clearance of CEs and apoA-II present in HDL (apoA-II HDL=5.6+/-0.5 and 4.4+/-0.2; CE HDL=9.3+/-0. 0 and 8.3+/-1.1, respectively), whereas the clearance of apoA-I HDL was enhanced in mice expressing HL (FCR=4.6+/-0.3) but not HL-145G (FCR=1.4+/-0.4). These combined findings demonstrate that both lipolytic and nonlipolytic functions of HL are important for HDL metabolism in vivo. Our study provides, for the first time, in vivo evidence for a role of HL in HDL metabolism independent of lipolysis and provides new insights into the role of HL in facilitating distinct metabolic pathways involved in the catabolism of apoA-I- versus apoA-II-containing HDL.

Influence of the HDL receptor SR-BI on atherosclerosis

The scavenger receptor class B, type I (SR-BI) is an HDL receptor that mediates selective cholesterol uptake from HDL to cells. In rodents, SR-BI has a critical influence on plasma HDL-cholesterol concentration and structure, the delivery of cholesterol to steroidogenic tissues, female fertility, and biliary cholesterol concentration. SR-BI can also serve as a receptor for non-HDL lipoproteins and appears to play an important role in reverse cholesterol transport. Recent studies involving the manipulation of SR-BI expression in mice, either using adenovirus-mediated or transgenic hepatic overexpression or using homologous recombination for complete functional ablation, indicate that the expression of SR-BI protects against atherosclerosis. If SR-BI has a similar activity in humans, it may become an attractive target for therapeutic intervention.

A hepatic lipase gene promoter polymorphism attenuates the increase in hepatic lipase activity with increasing intra-abdominal fat in women

High hepatic lipase (HL) activity is associated with an atherogenic lipoprotein profile of small, dense LDL particles and lower HDL(2)-C. Intra-abdominal fat (IAF) is positively associated with HL activity. A hepatic lipase gene (LIPC) promoter variant (G-->A(-250)) is associated with lower HL activity, higher HDL(2)-C, and less dense LDL particles. To determine whether the LIPC promoter polymorphism acts independently of IAF to regulate HL, 57 healthy, premenopausal women were studied. The LIPC promoter A allele was associated with significantly lower HL activity (GA/AA=104+/-34 versus GG=145+/-57 nmoles x mL(-1) x min(-1), P=0.009). IAF was positively correlated with HL activity (r=0.431, P<0.001). Multivariate analysis revealed a strong relationship between both the LIPC promoter genotype (P=0. 001) and IAF (P<0.001) with HL activity. The relationship between IAF and HL activity for carriers and noncarriers of the A allele was curvilinear with the carriers having a lower apparent maximum level of plasma HL activity compared with noncarriers (138 versus 218 nmoles x mL(-1) x min(-1), P<0.001). In addition, the LIPC A allele was associated with a significantly higher HDL(2)-C (GA/AA=16+/-7 versus GG=11+/-5 mg/dL, P=0.003). We conclude that the LIPC promoter A allele attenuates the increase in HL activity due to IAF in premenopausal women.

Low density lipoprotein uptake: holoparticle and cholesteryl ester selective uptake

Low density lipoproteins (LDL) contain apolipoprotein B-100 and are cholesteryl ester-rich, triglyceride-poor macromolecules, arising from the lipolysis of very low density lipoproteins. This review will describe the receptors responsible for uptake of whole LDL particles (holoparticle uptake), and the selective uptake of LDL cholesteryl ester. The LDL-receptor mediates the internalization of whole LDL through an endosomal-lysosomal pathway, leading to complete degradation of LDL. Increasing LDL-receptor expression by pharmacological intervention efficiently reduces blood LDL concentrations. The lipolysis stimulated receptor and LDL-receptor related protein may also lead to complete degradation of LDL in presence of free fatty acids and apolipoprotein E- or lipase-LDL complexes, respectively. Selective uptake of LDL cholesteryl ester has been demonstrated in the liver, especially in rodents and humans. This activity brings five times more LDL cholesteryl ester than the LDL-receptor to human hepatoma cells, suggesting that it is a physiologically significant pathway. The lipoprotein binding site of HepG2 cells mediates this process and recognizes all lipoprotein classes. Scavenger receptor class B type I and CD36, which mediate the selective uptake of high density lipoprotein cholesteryl ester, are potentially involved in LDL cholesteryl ester selective uptake, since they both bind LDL with high affinity. It is not known whether they are identical to the uncloned lipoprotein binding site and if the selective uptake of LDL cholesteryl ester produces a less atherogenic particle. If this is verified, pharmacological up-regulation of LDL cholesteryl ester selective uptake may become another therapeutic approach for reducing blood LDL-cholesterol levels and the risk of atherosclerosis.

Somatic gene therapy for dyslipidemias

Somatic gene transfer is a valuable tool for the in vivo evaluation of lipoprotein metabolism. It has been used to dissect metabolic pathways, to establish structure-function relationships of various gene products, and to evaluate conventional lipid-lowering and novel therapeutic genes for the treatment of lipoprotein disorders. In this article we review some general aspects of somatic gene therapy and the different vehicles used for the delivery of therapeutic genes. We highlight some recent advances in adenoviral vector development that make this vector an attractive system for clinical trials.

Targeting HDL-mediated cellular cholesterol efflux for the treatment and prevention of atherosclerosis

The hallmark of the atherosclerotic lesion is the overaccumulation of cholesterol in arterial wall cells. As no pathway exists for the degradation of cholesterol in peripheral cells, a mechanism is necessary to prevent its accumulation to toxic levels in these cells and to allow its delivery to the liver for excretion in bile. Promoting this reverse cholesterol transport pathway is believed to be the main cardioprotective action of high density lipoprotein (HDL). The rate-limiting step in this pathway is likely the initial removal of cholesterol from peripheral cells by HDL. The pathway HDL utilizes for inducing efflux of excess cellular cholesterol represents an important and as-yet untapped mechanism to employ for the treatment and prevention of atherosclerotic vascular disease. This review summarizes the potential cardioprotective actions of HDL, the mechanisms of HDL-mediated cellular cholesterol efflux, and evidence that the specific pathway of cholesterol removal by HDL may be enhanced and used as a novel target in the therapy of atherosclerosis.

Atheroprotective mechanisms of HDL

The aim of this review was to bring together results obtained from studies on different aspects of HDL as related to CHD and atherosclerosis. As atherosclerosis is a multistep process, the various components of HDL can intervene at different stages, such as induction of monocyte adhesion molecules, prevention of LDL modification and removal of excess cholesterol by reverse cholesterol transport. Transgenic technology has provided a model for atherosclerosis, and permitted evaluation of the contributions of different HDL components towards the global effect. The availability of apo AIV transgenic mice amplified the results obtained from apo AI overexpressors with respect to prevention of atherosclerosis. Prevention of atherosclerosis in apo E deficient mice by relatively small amounts of macrophage derived apo E may open new possibilities for therapeutic intervention. Contrary to early notions, increased plasma levels of CETP, even in the presence of low but functionally normal HDL, were atheroprotective. The extent to which paraoxonase and apo J participate in prevention of human atherosclerosis needs further evaluation. The findings that LCAT overexpression in rabbits was atheroprotective in contrast to increase in atherosclerosis in h LCAT tg mice, which was only partially corrected by CETP expression, call for some caution in the extrapolation of results from transgenic animals to humans. The important discovery of SR-BI as the receptor for selective uptake of CE from HDL revived interest in the clearance of CE from plasma. This pathway supplies also the vital precursor for steroidogenesis in adrenals and gonads and was shown to be dependent on apo AI.

The selective pathway and a high-density lipoprotein receptor (SR-BI) in ovarian granulosa cells of the mouse

We recently reported that rat luteinized ovary tissue and primary cultures of rat ovarian granulosa cells reveal a remarkably tight functional correlation between expressed selective uptake of lipoprotein cholesteryl esters and the expression of an HDL receptor protein, scavenger receptor, class B, type I (SR-BI). In the current study, we examine these same processes in C57 mouse granulosa cells and report a different correlation. Unlike the rat cells, non-hormone stimulated mouse granulosa cells are able to effectively carry out their selective pathway functions and secrete HDL-derived progestins despite low levels of SR-BI and barely detectable levels of SR-BII (an isoform of SR-BI). Once stimulated with trophic hormones or Bt2cAMP, small (30-40%) increases are observed in selective pathway functions, but major (approximately 20-fold) increases are seen in SR-BI and SR-BII expression: thus, relatively little is gained in selective cholesteryl ester uptake by mouse granulosa cells even though SR-BI and SR-BII levels are greatly increased. The importance of the HDL receptor proteins to the selective pathway remains clear, however, since a significant portion of the selective process in both basal and stimulated granulosa cells is inhibitable by the use of blocking antibody. Another surface protein, caveolin, previously reported to co-localize with SR-BI in mouse cells shows no change in expression during periods when SR-BI/BII levels are undergoing major shifts.

Hepatic lipase mediates an increase in selective uptake of high-density lipoprotein-associated cholesteryl esters by human Hep 3B hepatoma cells in culture

Selective uptake of high-density lipoprotein- (HDL-) associated cholesteryl esters (CE), i.e. lipid uptake independent from particle uptake, delivers CE to the liver and steroidogenic tissues in vivo. In vitro, besides hepatocytes and steroidogenic cells many other cell types selectively take up HDL CE. Hepatic lipase (HL) stimulates the internalisation of apoprotein (apo) B-containing lipoproteins by hepatocytes independent from lipolysis. In this study the role of HL in the hepatic metabolism of apo A-I-containing lipoproteins, i.e. HDL, was investigated. HDL3 (d = 1.125-1.21 g/ml) was radiolabeled in its protein (125I) and in its CE moiety ([3H]cholesteryl oleyl ether, ([3H]CEt)). HL originated from tissue culture media of hepatoma cells and from post-heparin plasma. Human Hep 3B hepatoma cells incubated in medium containing radiolabeled HDL3. In the absence of HL, the rate of apparent HDL3 particle uptake according to the lipid tracer ([3H]CEt) was in most cases in approximately 10-fold excess on that due to the protein label (125I), indicating selective CE uptake from HDL3. Addition of HL to these incubations increased the cellular uptake of [3H]CEt and of 125I from HDL3 and quantitatively the most prominent effect was an up to approximately 2.5-fold stimulation of apparent selective CE uptake ([3H]CEt-125I). This increase in selective CE uptake was observed in the presence of tetrahydrolipstatin, an inhibitor of the catalytically active site of HL, suggesting that this HL effect is independent from lipolysis. HL binds to cell surface heparan sulfate proteoglycans. To explore the role of these molecules for the HL effect on selective CE uptake, hepatoma cells were depleted of proteoglycans or Chinese hamster ovary (CHO) cells deficient in proteoglycan synthesis were used. Proteoglycan-deficiency reduced the HL-mediated increase in selective uptake by more than 80%. To investigate if low-density lipoprotein (LDL) receptors or the LDL receptor-related protein (LRP) are involved in the HL effect on selective CE uptake, murine embryonic fibroblasts (MEF) were used which are deficient in these receptors; alternatively, monensin, an inhibitor of endocytosis was present in the medium of Hep 3B cells during the uptake assay for labeled HDL3. These experiments yielded no evidence for a role of LDL receptors or LRP in the HL-mediated increase in selective CE uptake. In summary, HL mediates an increase in HDL3 selective CE uptake by human Hep 3B hepatoma cells. This HL effect is independent from lipolysis and independent from LRP and LDL receptors. However this HL effect is susceptible to cell surface proteoglycan deficiency. The potential physiologic implication is that HL modifies HDL selective CE uptake by the liver in vivo and such an effect could play a role in reverse cholesterol transport.

The efficient cellular uptake of high density lipoprotein lipids via scavenger receptor class B type I requires not only receptor-mediated surface binding but also receptor-specific lipid transfer mediated by its extracellular domain

The class B type I scavenger receptor, (SR-BI), is a member of the CD36 superfamily of proteins and is a physiologically relevant, high affinity cell surface high density lipoprotein (HDL) receptor that mediates selective lipid uptake. The mechanism of selective lipid uptake is fundamentally different from that of classic receptor-mediated uptake via coated pits and vesicles (e.g. the low density lipoprotein receptor pathway) in that it involves efficient transfer of the lipids, but not the outer shell proteins, from HDL to cells. The abilities of SR-BI and CD36, both of which are class B scavenger receptors, to bind HDL and mediate cellular uptake of HDL-associated lipid when transiently expressed in COS cells were examined. For these experiments, the binding of HDL to cells was assessed using either 125I- or Alexa (a fluorescent dye)-HDL in which the apolipoproteins on the surface of the HDL particles were covalently modified. Lipid transfer was measured using HDL noncovalently labeled by the fluorescent lipid 1,1'-dioctadecyl-3,3, 3',3'-tetramethylindocarbocyanine perchlorate. Although both mSR-BI and human CD36 (hCD36) could mediate the binding of HDL in a punctate pattern across the surfaces of cells, only mSR-BI efficiently mediated the transfer of lipid to the cells. Analysis of point mutants established that the major sites of fatty acylation of mSR-BI are Cys462 and Cys470 and that fatty acylation is not required for receptor clustering, HDL binding, or efficient lipid transfer. Generation of mSR-BI/hCD36 domain swap chimeras showed that the differences in lipid uptake activities between mSR-BI and hCD36 were not due to differences between their two sets of transmembrane and cytoplasmic domains but rather result from differences in their large extracellular loop domains. These results show that high affinity binding to a cell surface receptor is not sufficient to ensure efficient cellular lipid uptake from HDL. Thus, SR-BI-mediated binding combined with SR-BI-dependent facilitated transfer of lipid from the HDL particle to the cell appears to be the most likely mechanism for the bulk of the selective uptake of cholesteryl esters from HDL to the liver and steroidogenic tissues.

Interaction between ApoB and hepatic lipase mediates the uptake of ApoB-containing lipoproteins

Hepatic lipase (HL) on the surface of hepatocytes and endothelial cells lining hepatic sinusoids, the adrenal glands, and the ovary hydrolyzes triglycerides and phospholipids of circulating lipoproteins. Its expression significantly enhances low density lipoprotein (LDL) uptake via the LDL receptor pathway. A specific interaction between LPL, a homologous molecule to HL, and apoB has been described (Choi, S. Y., Sivaram, P., Walker, D. E., Curtiss, L. K., Gretch, D. G., Sturley, S. L., Attie, A. D., Deckelbaum, R. J., and Goldberg, I. J. (1995) J. Biol. Chem. 270, 8081-8086). The present studies tested the hypothesis that HL enhances the uptake of lipoproteins by a specific interaction of HL with apoB. On a ligand blot, HL bound to apoB26, 48, and 100 but not to apoE or apoAI. HL binding to LDL in a plate assay with LDL-coated plates was significantly greater than to bovine serum albumin-coated plates. Neither heat denatured HL nor bacterial fusion protein of HL bound to LDL in the plate assays. 125I-LDL bound to HL-saturated heparin-agarose gel with a Kd of 52 nM, and somewhat surprisingly, this binding was not inhibited by excess LPL. In cell culture experiments HL enhanced the uptake of 125I-LDL at both 4 and 37 degreesC. The enhanced binding and uptake of LDL was significantly inhibited by monoclonal anti-apoB antibodies. In contrast to LPL, both amino- and carboxyl-terminal antibodies blocked the apoB interaction with HL to the same extent. Thus, we conclude that there is a unique interaction between HL and apoB that facilitates the uptake of apoB-containing lipoproteins by cells where HL is present.

The role of hepatic lipase in lipoprotein metabolism and atherosclerosis

In addition to its traditional role in the hydrolysis of lipoprotein triglycerides and phospholipids, recent studies have implicated hepatic lipase in other aspects of cellular lipid and/or lipoprotein metabolism and atherosclerosis. Hepatic lipase may serve as a ligand that mediates the interaction of lipoproteins to cell surface receptors and/or proteoglycans as well as modulating aortic lesion development in different animal models. Over the past several years significant advances have been made in our understanding of new, alternative mechanisms by which hepatic lipase may modulate lipoprotein metabolism and the development of atherosclerosis in vivo.

Targeted mutation reveals a central role for SR-BI in hepatic selective uptake of high density lipoprotein cholesterol

Scavenger receptor BI (SR-BI) is a cell surface receptor that binds high density lipoproteins (HDL) and mediates selective uptake of HDL cholesteryl esters (CE) in transfected cells. To address the physiological role of SR-BI in HDL cholesterol homeostasis, mice were generated bearing an SR-BI promoter mutation that resulted in decreased expression of the receptor in homozygous mutant (designated SR-BI att) mice. Hepatic expression of the receptor was reduced by 53% with a corresponding increase in total plasma cholesterol levels of 50-70% in SR-BI att mice, attributable almost exclusively to elevated plasma HDL. In addition to increased HDL-CE, HDL phospholipids and apo A-1 levels were elevated, and there was an increase in HDL particle size in mutant mice. Metabolic studies using HDL bearing nondegradable radiolabels in both the protein and lipid components demonstrated that reducing hepatic SR-BI expression by half was associated with a decrease of 47% in selective uptake of CE by the liver, and a corresponding reduction of 53% in selective removal of HDL-CE from plasma. Taken together, these findings strongly support a pivotal role for hepatic SR-BI expression in regulating plasma HDL levels and indicate that SR-BI is the major molecule mediating selective CE uptake by the liver. The inverse correlation between plasma HDL levels and atherosclerosis further suggests that SR-BI may influence the development of coronary artery disease.

Overexpression of hepatic lipase in transgenic mice decreases apolipoprotein B-containing and high density lipoproteins. Evidence that hepatic lipase acts as a ligand for lipoprotein uptake

To determine the mechanisms by which human hepatic lipase (HL) contributes to the metabolism of apolipoprotein (apo) B-containing lipoproteins and high density lipoproteins (HDL) in vivo, we developed and characterized HL transgenic mice. HL was localized by immunohistochemistry to the liver and to the adrenal cortex. In hemizygous (hHLTg+/0) and homozygous (hHLTg+/+) mice, postheparin plasma HL activity increased by 25- and 50-fold and plasma cholesterol levels decreased by 80% and 85%, respectively. In mice fed a high fat, high cholesterol diet to increase endogenous apoB-containing lipoproteins, plasma cholesterol decreased 33% (hHLTg+/0) and 75% (hHLTg+/+). Both apoB-containing remnant lipoproteins and HDL were reduced. To extend this observation, the HL transgene was expressed in human apoB transgenic (huBTg) and apoE-deficient (apoE-/-) mice, both of which have high plasma levels of apoB-containing lipoproteins. (Note that the huBTg mice that were used in these studies were all hemizygous for the human apoB gene.) In both the huBTg,hHLTg+/0 mice and the apoE-/-,hHLTg+/0 mice, plasma cholesterol decreased by 50%. This decrease was reflected in both the apoB-containing and the HDL fractions. To determine if HL catalytic activity is required for these decreases, we expressed catalytically inactive HL (HL-CAT) in apoE-/- mice. The postheparin plasma HL activities were similar in the apoE-/- and the apoE-/-,HL-CAT+/0 mice, reflecting the activity of the endogenous mouse HL and confirming that the HL-CAT was catalytically inactive. However, the postheparin plasma HL activity was 20-fold higher in the apoE-/-,hHLTg+/0 mice, indicating expression of the active human HL. Immunoblotting demonstrated high levels of human HL in postheparin plasma of both apoE-/-,hHLTg+/0 and apoE-/-,HL-CAT+/0 mice. Plasma cholesterol and apoB-containing lipoprotein levels were approximately 60% lower in apoE-/-,HL-CAT+/0 mice than in apoE-/- mice. However, the HDL were only minimally reduced. Thus, the catalytic activity of HL is critical for its effects on HDL but not for its effects on apoB-containing lipoproteins. These results provide evidence that HL can act as a ligand to remove apoB-containing lipoproteins from plasma.

Heparan sulfate proteoglycans participate in hepatic lipaseand apolipoprotein E-mediated binding and uptake of plasma lipoproteins, including high density lipoproteins

High density lipoprotein (HDL) particles and HDL cholesteryl esters are taken up by both receptor-mediated and non-receptor-mediated pathways. Here we show that cell surface heparan sulfate proteoglycans (HSPG) participate in hepatic lipase (HL)- and apolipoprotein (apo) E-mediated binding and uptake of mouse and human HDL by cultured hepatocytes. The HL secreted by HL-transfected McA-RH7777 cells enhanced both HDL binding at 4 degrees C (approximately 2-4-fold) and HDL uptake at 37 degrees C (approximately 2-5-fold). The enhanced binding and uptake of HDL were partially inhibited by the 39-kDa protein, an inhibitor of low density lipoprotein receptor-related protein (LRP), but were almost totally blocked by heparinase, which removes the sulfated glycosaminoglycan chains from HSPG. Therefore, HL may mediate the uptake of HDL by two pathways: an HSPG-dependent LRP pathway and an HSPG-dependent but LRP-independent pathway. The HL-mediated binding and uptake of HDL were only minimally reduced when catalytically inactive HL or LRP binding-defective HL was substituted for wild-type HL, indicating that much of the HDL uptake required neither HL binding to the LRP nor lipolytic processing. To study the role of HL in facilitating the selective uptake of cholesteryl esters, we used HDL into which radiolabeled cholesteryl ether had been incorporated. HL increased the selective uptake of HDL cholesteryl ether; this enhanced uptake was reduced by more than 80% by heparinase but was unaffected by the 39-kDa protein. Like HL, apoE enhanced the binding and uptake of HDL (approximately 2-fold) but had little effect on the selective uptake of HDL cholesteryl ether. In the presence of HL, apoE did not further increase the uptake of HDL, and at a high concentration apoE impaired or decreased the HL-mediated uptake of HDL. Therefore, HL and apoE may utilize similar (but not identical) binding sites to mediate HDL uptake. Although the relative importance of cell surface HSPG in the overall metabolism of HDL in vivo remains to be determined, cultured hepatocytes clearly displayed an HSPG-dependent pathway that mediates the binding and uptake of HDL. This study also demonstrates the importance of HL in enhancing the binding and uptake of remnant and low density lipoproteins via an HSPG-dependent pathway.