Uptake of hypertriglyceridemic very low density lipoproteins and their remnants by HepG2 cells: the role of lipoprotein lipase, hepatic triglyceride lipase, and cell surface proteoglycans

In vitro hepatic steatosis model based on gut-liver-on-a-chip

Hepatic steatosis, also known as fatty liver disease, occurs due to abnormal lipid accumulation in the liver. It has been known that gut absorption also plays an important role in the mechanism underlying hepatic steatosis. Conventional in vitro cell culture models have limitations in recapitulating the mechanisms of hepatic steatosis because it does not include the gut absorption process. Previously, we reported development of a microfluidic gut-liver chip that can recapitulate the gut absorption of fatty acids and subsequent lipid accumulation in liver cells. In this study, we performed a series of experiments to verify that our gut-liver chip reproduces various aspects of hepatic steatosis. The absorption of fatty acids was evaluated under various culture conditions. The anti-steatotic effect of turofexorate isopropyl (XL-335) and metformin was tested, and both drugs showed different action mechanisms. In addition, the oxidative stress induced by lipid absorption was evaluated. Our results demonstrate the potential of the gut-liver chip for use as a novel, physiologically realistic in vitro model to study fatty liver disease.

Disialylated apolipoprotein C-III proteoform is associated with improved lipids in prediabetes and type 2 diabetes

The apoC-III proteoform containing two sialic acid residues (apoC-III2) has different in vitro effects on lipid metabolism compared with asialylated (apoC-III0) or the most abundant monosialylated (apoC-III1) proteoforms. Cross-sectional and longitudinal associations between plasma apoC-III proteoforms (by mass spectrometric immunoassay) and plasma lipids were tested in two randomized clinical trials: ACT NOW, a study of pioglitazone in subjects with impaired glucose tolerance (n = 531), and RACED (n = 296), a study of intensive glycemic control and atherosclerosis in type 2 diabetes patients. At baseline, higher relative apoC-III2 and apoC-III2/apoC-III1 ratios were associated with lower triglycerides and total cholesterol in both cohorts, and with lower small dense LDL in the RACED. Longitudinally, changes in apoC-III2/apoC-III1 were inversely associated with changes in triglycerides in both cohorts, and with total and small dense LDL in the RACED. apoC-III2/apoC-III1 was also higher in patients treated with PPAR-γ agonists and was associated with reduced cardiovascular events in the RACED control group. Ex vivo studies of apoC-III complexes with higher apoC-III2/apoC-III1 showed attenuated inhibition of VLDL uptake by HepG2 cells and LPL-mediated lipolysis, providing possible functional explanations for the inverse association between a higher apoC-III2/apoC-III1 and hypertriglyceridemia, proatherogenic plasma lipid profiles, and cardiovascular risk.

Myristic acid is associated to low plasma HDL cholesterol levels in a Mediterranean population and increases HDL catabolism by enhancing HDL particles trapping to cell surface proteoglycans in a liver hepatoma cell model

BACKGROUND

HDL-C plasma levels are modulated by dietary fatty acid (FA), but studies investigating dietary supplementation in FA gave contrasting results. Saturated FA increased HDL-C levels only in some studies. Mono-unsaturated FA exerted a slight effect while poly-unsaturated FA mostly increased plasma HDL-C.

AIMS

This study presents two aims: i) to investigate the relationship between HDL-C levels and plasma FA composition in a Sicilian population following a "Mediterranean diet", ii) to investigate if FA that resulted correlated with plasma HDL-C levels in the population study and/or very abundant in the plasma were able to affect HDL catabolism in an "in vitro" model of cultured hepatoma cells (HepG2).

RESULTS

plasma HDL-C levels in the population correlated negatively with myristic acid (C14:0, β = -0.24, p < 0.01), oleic acid (C18:1n9, β = -0.22, p < 0.01) and cis-11-Eicosenoic (C20:1n9, β = -0.19, p = 0.01) and positively with palmitoleic acid (C16:1, β = +0.19, p = 0.03). HepG2 cells were conditioned with FA before evaluating HDL binding kinetics, and only C14:0 increased HDL binding by a non-saturable pathway. After removal of heparan sulphate proteoglycans (HSPG) by heparinases HDL binding dropped by 29% only in C14:0 conditioned cells (p < 0.05). C14:0 showed also the highest internalization of HDL-derived cholesteryl esters (CE, +32% p = 0.01 vs. non-conditioned cells).

CONCLUSIONS

C14:0 was correlated with decreased plasma HDL-C levels in a Mediterranean population. C14:0 might reduce HDL-C levels by increasing HDL trapping to cell surface HSPG and CE stripping from bound HDL. Other mechanisms are to be investigated to explain the effects of other FA on HDL metabolism.

Shedding of syndecan-1 from human hepatocytes alters very low density lipoprotein clearance

We recently showed that the heparan sulfate proteoglycan syndecan-1 mediates hepatic clearance of triglyceride-rich lipoproteins in mice based on systemic deletion of syndecan-1 and hepatocyte-specific inactivation of sulfotransferases involved in heparan sulfate biosynthesis. Here, we show that syndecan-1 expressed on primary human hepatocytes and Hep3B human hepatoma cells can mediate binding and uptake of very low density lipoprotein (VLDL). Syndecan-1 also undergoes spontaneous shedding from primary human and murine hepatocytes and Hep3B cells. In human cells, phorbol myristic acid induces syndecan-1 shedding, resulting in accumulation of syndecan-1 ectodomains in the medium. Shedding occurs through a protein kinase C-dependent activation of ADAM17 (a disintegrin and metalloproteinase 17). Phorbol myristic acid stimulation significantly decreases DiD (1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate)-VLDL binding to cells, and shed syndecan-1 ectodomains bind to VLDL. Although mouse hepatocytes appear resistant to induced shedding in vitro, injection of lipopolysaccharide into mice results in loss of hepatic syndecan-1, accumulation of ectodomains in the plasma, impaired VLDL catabolism, and hypertriglyceridemia.

CONCLUSION

These findings suggest that syndecan-1 mediates hepatic VLDL turnover in humans as well as in mice and that shedding might contribute to hypertriglyceridemia in patients with sepsis.

Postprandial triglyceride-rich lipoproteins induce hepatic insulin resistance in HepG2 cells independently of their receptor-mediated cellular uptake

Non-alcoholic fatty liver disease (NAFLD) is associated with hepatic insulin resistance with the molecular basis of this association being not well understood. Here we studied the effect of hepatic triglyceride accumulation induced by postprandial triglyceride-rich lipoproteins (TGRL) on hepatic insulin sensitivity in HepG2 cells. Incubation of HepG2 cells with purified TGRL particles induced hepatocellular triglyceride accumulation paralleled by diminished insulin-stimulated glycogen content and glycogen synthase activity. Accordingly, insulin-induced inhibition of glycogen synthase phosphorylation as well as insulin-induced GSK-3 and AKT phosphorylation were reduced by TGRL. The effects of TGRL were dependent on the presence of apolipoproteins and more pronounced for denser TGRL. Moreover, TGRL effects required the presence of heparan sulfate-proteoglycans on the cell membrane and lipase activity but were independent of the cellular uptake of TGRL particles by receptors of the LDL receptor family. We suggest postprandial lipemia to be an important factor in the pathogenesis of NAFLD.

Streptococcal serum opacity factor increases the rate of hepatocyte uptake of human plasma high-density lipoprotein cholesterol

Serum opacity factor (SOF), a virulence determinant of Streptococcus pyogenes, converts plasma high-density lipoproteins (HDL) to three distinct species: lipid-free apolipoprotein (apo) A-I, neo HDL, a small discoidal HDL-like particle, and a large cholesteryl ester-rich microemulsion (CERM) that contains the cholesterol esters (CE) of up to ∼400000 HDL particles and apo E as its major protein. Similar SOF reaction products are obtained with HDL, total plasma lipoproteins, and whole plasma. We hypothesized that hepatic uptake of CERM-CE via multiple apo E-dependent receptors would be faster than that of HDL-CE. We tested our hypothesis using human hepatoma cells and lipoprotein receptor-specific Chinese hamster ovary (CHO) cells. The uptake of [(3)H]CE by HepG2 and Huh7 cells from HDL after SOF treatment, which transfers >90% of HDL-CE to CERM, was 2.4 and 4.5 times faster, respectively, than from control HDL. CERM-[(3)H]CE uptake was inhibited by LDL and HDL, suggestive of uptake by both the LDL receptor (LDL-R) and scavenger receptor class B type I (SR-BI). Studies in CHO cells specifically expressing LDL-R and SR-BI confirmed CERM-[(3)H]CE uptake by both receptors. RAP and heparin inhibit CERM-[(3)H]CE but not HDL-[(3)H]CE uptake, thereby implicating LRP-1 and cell surface proteoglycans in this process. These data demonstrate that SOF treatment of HDL increases the rate of CE uptake via multiple hepatic apo E receptors. In so doing, SOF might increase the level of hepatic disposal of plasma cholesterol in a way that is therapeutically useful.

Hepatic heparan sulfate proteoglycans and endocytic clearance of triglyceride-rich lipoproteins

Hypertriglyceridemia, characterized by the accumulation of triglyceride-rich lipoproteins in the blood, affects 10-20% of the population in western countries and increases the risk of atherosclerosis, coronary artery disease, and pancreatitis. The etiology of hypertriglyceridemia is complex, and much interest exists in identifying and characterizing the biological and environmental factors that affect the synthesis and turnover of plasma triglycerides. Genetic studies in mice have recently identified that heparan sulfate proteoglycans are a class of receptors that mediate the clearance of triglyceride-rich lipoproteins in the liver. Heparan sulfate proteoglycans are expressed by endothelial cells that line the hepatic sinusoids and the underlying hepatocytes, and are present in the perisinusoidal space (space of Disse). This chapter discusses the dependence of lipoprotein binding on heparan sulfate structure and the identification of hepatocyte syndecan-1 as the primary proteoglycan that mediates triglyceride-rich lipoprotein clearance.

Syndecan-1 is the primary heparan sulfate proteoglycan mediating hepatic clearance of triglyceride-rich lipoproteins in mice

Elevated plasma triglyceride levels represent a risk factor for premature atherosclerosis. In mice, accumulation of triglyceride-rich lipoproteins can occur if sulfation of heparan sulfate in hepatocytes is diminished, as this alters hepatic lipoprotein clearance via heparan sulfate proteoglycans (HSPGs). However, the relevant HSPG has not been determined. In this study, we found by RT-PCR analysis that mouse hepatocytes expressed the membrane proteoglycans syndecan-1, -2, and -4 and glypican-1 and -4. Analysis of available proteoglycan-deficient mice showed that only syndecan-1 mutants (Sdc1-/- mice) accumulated plasma triglycerides. Sdc1-/- mice also exhibited prolonged circulation of injected human VLDL and intestinally derived chylomicrons. We found that mice lacking both syndecan-1 and hepatocyte heparan sulfate did not display accentuated triglyceride accumulation compared with single mutants, suggesting that syndecan-1 is the primary HSPG mediating hepatic triglyceride clearance. Immunoelectron microscopy showed that syndecan-1 was expressed specifically on the microvilli of hepatocyte basal membranes, facing the space of Disse, where lipoprotein uptake occurs. Abundant syndecan-1 on wild-type murine hepatocytes exhibited saturable binding of VLDL and inhibition by heparin and facilitated degradation of VLDL. Furthermore, adenovirus-encoded syndecan-1 restored binding, uptake, and degradation of VLDL in isolated Sdc1-/- hepatocytes and the lipoprotein clearance defect in Sdc1-/- mice. These findings provide the first in vivo genetic evidence that syndecan-1 is the primary hepatocyte HSPG receptor mediating the clearance of both hepatic and intestinally derived triglyceride-rich lipoproteins.

The hepatic uptake of VLDL in lrp-ldlr-/-vldlr-/- mice is regulated by LPL activity and involves proteoglycans and SR-BI

LPL activity plays an important role in preceding the VLDL remnant clearance via the three major apolipoprotein E (apoE)-recognizing receptors: the LDL receptor (LDLr), LDL receptor-related protein (LRP), and VLDL receptor (VLDLr). The aim of this study was to determine whether LPL activity is also important for VLDL remnant clearance irrespective of these receptors and to determine the mechanisms involved in the hepatic remnant uptake. Administration of an adenovirus expressing LPL (AdLPL) into lrp(-)ldlr(-/-)vldlr(-/-) mice reduced both VLDL-triglyceride (TG) and VLDL-total cholesterol (TC) levels. Conversely, inhibition of LPL by AdAPOC1 increased plasma VLDL-TG and VLDL-TC levels. Metabolic studies with radiolabeled VLDL-like emulsion particles showed that the clearance and hepatic association of their remnants positively correlated with LPL activity. This hepatic association was independent of the bridging function of LPL and HL, since heparin did not reduce the liver association. In vitro studies demonstrated that VLDL-like emulsion particles avidly bound to the cell surface of primary hepatocytes from lrp(-)ldlr(-/-)vldlr(-/-) mice, followed by slow internalization, and involved heparin-releaseable cell surface proteins as well as scavenger receptor class B type I (SR-BI). Collectively, we conclude that hepatic VLDL remnant uptake in the absence of the three classical apoE-recognizing receptors is regulated by LPL activity and involves heparan sulfate proteoglycans and SR-BI.

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.

Differential uptake of subfractions of triglyceride-rich lipoproteins by THP-1 macrophages

It is well known that raised plasma triglycerides (TG) are positively linked to the development of coronary heart disease. However, triglycerides circulate in a range of distinct lipoprotein subfractions and the relative atherogenicity of these subfractions is not clear. In this study, three fractions of triglyceride rich lipoprotein (TRL) were isolated from normolipidaemic males according to their differing Svedberg flotation (S(f)) rates: chylomicron (CM, S(f)>400), very low-density lipoprotein (VLDL)-1 (S(f) 60-400) and VLDL-2 (S(f) 20-60). These fractions were incubated with THP-1 monocyte-derived macrophages for determination of cholesterol and TG accumulation, in the presence and absence of the lipoprotein lipase (LPL) inhibitor orlistat. Expression of LDL receptor related protein (LRP) and apolipoprotein B48 receptor (apoB48R) was also examined in both differentiating monocytes, and monocyte-derived macrophages, incubated with TRL. VLDL-1 caused a significantly greater accumulation of TG within macrophages compared to VLDL-2. Binding studies also tended to show a greater preference for VLDL-1. No change in expression of LRP or apoB48R was observed in fully differentiated macrophages incubated with VLDL-1, VLDL-2 or CM, although a greater expression of LRP mRNA was observed in differentiating monocytes exposed to VLDL-1, compared to those incubated with CM or VLDL-2. TG loading in response to all three TRL fractions was blocked by orlistat, suggesting that it is likely that the major pathway for uptake of TG was hydrolysis by LPL. Calculations suggested that direct uptake of particles accounts for between 12 and 25% of total TAG uptake. In conclusion, THP monocyte-derived macrophages demonstrate a preference for VLDL-1, both through the LPL pathway and by direct uptake of whole particles.

Human triglyceride-rich lipoproteins impair glucose metabolism and insulin signalling in L6 skeletal muscle cells independently of non-esterified fatty acid levels

AIMS/HYPOTHESIS

Elevated fasting and postprandial plasma levels of triglyceride-rich lipoproteins (TGRLs), i.e. VLDL/remnants and chylomicrons/remnants, are a characteristic feature of insulin resistance and are considered a consequence of this state. The aim of this study was to investigate whether intact TGRL particles are capable of inducing insulin resistance.

METHODS

We studied the effect of highly purified TGRLs on glycogen synthesis, glycogen synthase activity, glucose uptake, insulin signalling and intramyocellular lipid (IMCL) content using fully differentiated L6 skeletal muscle cells.

RESULTS

Incubation with TGRLs diminished insulin-stimulated glycogen synthesis, glycogen synthase activity, glucose uptake and insulin-stimulated phosphorylation of Akt and glycogen synthase kinase 3. Insulin-stimulated tyrosine phosphorylation of IRS-1, and IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase (PI3K) activity were not impaired by TGRLs, suggesting that these steps were not involved in the lipoprotein-induced effects on glucose metabolism. The overall observed effects were time- and dose-dependent and paralleled IMCL accumulation. NEFA concentration in the incubation media did not increase in the presence of TGRLs indicating that the effects observed were solely due to intact lipoprotein particles. Moreover, co-incubation of TGRLs with orlistat, a potent active-site inhibitor of various lipases, did not alter TGRL-induced effects, whereas co-incubation with receptor-associated protein (RAP), which inhibits interaction of TGRL particles with members of the LDL receptor family, reversed the TGRL-induced effects on glycogen synthesis and insulin signalling.

CONCLUSIONS/INTERPRETATION

Our data suggest that the accumulation of TGRLs in the blood stream of insulin-resistant patients may not only be a consequence of insulin resistance but could also be a cause for it.

Remnant Lipoprotein-Induced Smooth Muscle Cell Proliferation Involves Epidermal Growth Factor Receptor Transactivation

Background— Remnant lipoproteins (RLPs) have been shown to play a causative role during atherosclerosis. Furthermore, it is known that vascular smooth muscle cell (SMC) proliferation is crucial for the development of atherosclerosis and restenosis after percutaneous coronary intervention. We examined the direct effect of RLPs on the proliferation and signal transduction of SMCs.

Methods and Results— Incubation in the presence of RLPs (20 mg cholesterol per dL) for 48 hours induced rat aortic SMC proliferation (2.3-fold over medium alone). RLPs also induced the phosphorylation of epidermal growth factor (EGF) receptor in SMCs, which was followed by the activation of mitogen-activated protein kinases. Moreover, the activation of protein kinase C (PKC) as well as the shedding of membrane-bound soluble heparin-binding EGF-like growth factor (HB-EGF) was observed after RLP treatment of SMCs, whereas PKC inhibitors and metalloprotease inhibitors inhibited RLP-induced EGF receptor transactivation and HB-EGF shedding in SMCs. Furthermore, anti-HB-EGF neutralizing antibody inhibited RLP-induced EGF receptor transactivation. Phosphorylation of EGF receptor and HB-EGF shedding were also observed in the aortas of apolipoprotein E–knockout mice but not in those of C57BL6 mice.

Conclusions— These results suggest that RLPs transactivate EGF receptor via PKC and HB-EGF shedding from SMCs, resulting in SMC proliferation.

Hepatic lipase and dyslipidemia: interactions among genetic variants, obesity, gender, and diet

Hepatic lipase (HL) plays a central role in LDL and HDL remodeling. High HL activity is associated with small, dense LDL particles and with reduced HDL2 cholesterol levels. HL activity is determined by an HL gene promoter polymorphism, by gender (lower in premenopausal women), and by visceral obesity with insulin resistance. The activity is affected by dietary fat intake and selected medications. There is evidence for an interaction of the HL promoter polymorphism with visceral obesity, dietary fat intake, and with lipid-lowering medications in determining the level of HL activity. The dyslipidemia with high HL activity is a potentially proatherogenic lipoprotein profile in the metabolic syndrome, in Type 2 diabetes, and in familial combined hyperlipidemia.

A decreased expression of angiopoietin-like 3 is protective against atherosclerosis in apoE-deficient mice

KK/Snk mice (previously KK/San) possessing a recessive mutation (hypl) of the angiopoietin-like 3 (Angptl3) gene homozygously exhibit a marked reduction of VLDL due to the decreased Angptl3 expression. Recently, we proposed that Angptl3 is a new class of lipid metabolism modulator regulating VLDL triglyceride (TG) levels through the inhibition of lipoprotein lipase (LPL) activity. In this study, to elucidate the role of Angptl3 in atherogenesis, we investigated the effects of hypl mutation against hyperlipidemia and atherosclerosis in apolipoprotein E knockout (apoEKO) mice. ApoEKO mice with hypl mutation (apoEKO-hypl) exhibited a significant reduction of VLDL TG, VLDL cholesterol, and plasma apoB levels compared with apoEKO mice. Hepatic VLDL TG secretion was comparable between both apoE-deficient mice. Turnover studies revealed that the clearance of both [3H]TG-labeled and 125I-labeled VLDL was significantly enhanced in apoEKO-hypl mice. Postprandial plasma TG levels also decreased in apoEKO-hypl mice. Both LPL and hepatic lipase activities in the postheparin plasma increased significantly in apoEKO-hypl mice, explaining the enhanced lipid metabolism. Furthermore, apoEKO-hypl mice developed 3-fold smaller atherogenic lesions in the aortic sinus compared with apoEKO mice. Taken together, the reduction of Angptl3 expression is protective against hyperlipidemia and atherosclerosis, even in the absence of apoE, owing to the enhanced catabolism and clearance of TG-rich lipoproteins.

The low density lipoprotein receptor-related protein complexes with cell surface heparan sulfate proteoglycans to regulate proteoglycan-mediated lipoprotein catabolism

It has been proposed that clearance of cholesterol-enriched very low density lipoprotein (VLDL) particles occurs through a multistep process beginning with their initial binding to cell-surface heparan sulfate proteoglycans (HSPG), followed by their uptake into cells by a receptor-mediated process that utilizes members of the low density lipoprotein receptor (LDLR) family, including the low density lipoprotein receptor-related protein (LRP). We have further explored the relationship between HSPG binding of VLDL and its subsequent internalization by focusing on the LRP pathway using a cell line deficient in LDLR. In this study, we show that LRP and HSPG are part of a co-immunoprecipitable complex at the cell surface demonstrating a novel association for these two cell surface receptors. Cell surface binding assays show that this complex can be disrupted by an LRP-specific ligand binding antagonist, which in turn leads to increased VLDL binding and degradation. The increase in VLDL binding results from an increase in the availability of HSPG sites as treatment with heparinase or competitors of glycosaminoglycan chain addition eliminated the augmented binding. From these results we propose a model whereby LRP regulates the availability of VLDL binding sites at the cell surface by complexing with HSPG. Once HSPG dissociates from LRP, it is then able to bind and internalize VLDL independent of LRP endocytic activity. We conclude that HSPG and LRP together participate in VLDL clearance by means of a synergistic relationship.

Atorvastatin attenuates remnant lipoprotein-induced monocyte adhesion to vascular endothelium under flow conditions

Remnant lipoproteins have been reported to play a causative role in atherogenesis. We investigated the effect of remnant-like lipoprotein particles (RLPs) on monocyte-endothelial interaction and their potential regulation by atorvastatin. Monocytic U937 cells were incubated with RLPs isolated from hypertriglyceridemia subjects and their adhesion to human umbilical vein endothelial cells (HUVECs) was examined under flow conditions. Incubation of U937 cells with 15 micro g protein/mL RLPs increased their adhesion to HUVECs activated with IL-1beta (untreated: 6.8+/-1.6 cells/HPF versus RLPs: 16.2+/-3.3 cells/HPF, P<0.05). Flow cytometric analysis revealed that incubation with RLPs increased expression levels of CD11a, CD18, and CD49d in U937 cells. Moreover, RLP-induced RhoA activation as well as FAK activation was seen in U937 cells, and RLP-induced RhoA activation seemed to be involved with PKC-dependent signaling. To explore the effect of atorvastatin on RLP-induced U937 cell adhesion to HUVECs, U937 cells were incubated with RLPs in the presence of atorvastatin. Pretreatment of U937 cells with 10 micro mol/L atorvastatin significantly decreased RLP-induced U937 cell adhesion to activated HUVECs (RLP 15.2+/-1.5 cells/HPF versus atorvastatin+RLP 10.2+/-1.0 cells/HPF; P<0.05) and decreased the enhanced integrin expression in RLP-treated U937 cells. Atorvastatin also inhibited RLP-induced RhoA activation and FAK activation in U937 cells. In summary, RLPs induced monocyte adhesion to vascular endothelium by sequential activation of PKC, RhoA, FAK, and integrins, indicating a role of remnant lipoproteins in vascular inflammation during atherogenesis. Atorvastatin attenuated this enhanced monocyte adhesion to HUVECs, suggesting an antiinflammatory role for this compound.

A common promoter polymorphism in the hepatic lipase gene (LIPC-480C>T) is associated with an increase in coronary calcification in type 1 diabetes

Type 1 diabetes is associated with coronary heart disease (CHD) and coronary artery calcification (CAC), a measure of subclinical CHD. The hepatic lipase gene promoter polymorphism (LIPC-480C>T) is a common variant affecting lipid metabolism. This study examined the relation between the LIPC-480C>T and CAC in type 1 diabetes. In the type 1 diabetic patients studied, 56% had CAC >0 Agatston units (AU). These subjects had a longer duration of diabetes (26.2 +/- 1.3 vs. 17.8 +/- 1.4 years; P < 0.001), lower HDL cholesterol levels (55.7 +/- 2.4 vs. 61.0 +/- 2.5 mg/dl; P = 0.05), higher triglyceride levels (101 +/- 17.3 vs. 66 +/- 7.6 mg/dl; P < 0.05), and higher diastolic blood pressure (79.7 +/- 1.0 vs. 76.0 +/- 1.4 mmHg; P < 0.05). The LIPC-480 T allele was more common in subjects with CAC (frequency = 0.31 +/- 0.05 vs. 0.14 +/- 0.04; P = 0.006). The proportion with CAC was 44% in LIPC-480CC subjects, 71% in heterozygotes, and 83% in LIPC-480TT subjects (P < 0.01). LIPC-480 T allele frequency increased as the amount of CAC increased (P = 0.007). LIPC-480 genotype was independently associated with the CAC (odds ratio = 2.90, 95% CI 1.22-6.92, P < 0.05) after adjusting for duration of diabetes, age, sex, diastolic blood pressure, HDL cholesterol, and triglyceride levels. In conclusion, the LIPC-480C>T polymorphism was associated with subclinical CHD in type 1 diabetes. This genetic variant may identify subjects in which early intervention to prevent CHD may be appropriate.

Clinical significance of preheparin serum lipoprotein lipase mass in coronary vasospasm

The present study investigated the clinical significance of preheparin serum lipoprotein lipase (LPL) mass in coronary vasospasm by examining its relationship with the acetylcholine-induced coronary artery response in patients without angiographically demonstrable atherosclerotic coronary artery disease (CAD). The subjects were 39 men who had suspected CAD and who underwent coronary angiography. Coronary vasospasm was defined as a marked luminal narrowing or total occlusion provoked by the intracoronary administration of acetylcholine. Preheparin LPL mass was lower (p<0.05) in 25 subjects in whom vasospasm was induced by the acetylcholine provocation test than in the 14 subjects with a negative response. As regards preheparin LPL mass, the subjects with multiple vessel spasm had significantly low concentrations (p<0.05) compared with single vessel spasm, although serum lipid levels were not significantly different. Multiple regression analysis revealed only preheparin LPL mass had a significant absolute t-value (2.016) among the coronary risk factors. Low preheparin LPL mass is interpreted as reflecting an impaired acetylcholine-induced coronary relaxation in coronary vasospasm and preheparin LPL mass may be useful as a marker of early stage coronary atherosclerosis that is not detectable by angiography.

The metabolic availability of dietary triacylglycerols from two high oleic oils during the postprandial period does not depend on the amount of oleic acid ingested by healthy men

Monounsaturated oils, virgin olive oil (VOO) and high oleic sunflower oil (HOSO) are suggested to have selective physiologic effects on humans in the fasting state. The aim of the study was to evaluate whether two oils with equal amounts of oleic acid but with different compositions of minor fatty acids and triacylglycerol molecular species (TAG) could produce different triacylglycerol-rich lipoprotein (TRL)-TAG responses in the postprandial state. Eight normolipidemic men consumed the following three meals in random order on separate occasions with 2 wk between meals: control meal, control meal plus VOO and control meal plus HOSO. Plasma total TAG and TRL-TAG were measured hourly for 7 h after ingestion. TAG and sn-2 positional fatty acids within TAG were analyzed in the TRL fraction. Plasma total TAG concentrations in response to the dietary oils did not differ. However, TRL triglyceridemia was significantly lower after VOO intake (P < 0.05). The molecular species in the TRL fraction returned toward basal levels more quickly (P < 0.05) after VOO than HOSO intake. 2-Positional fatty acid analysis demonstrated higher proportions of stearic and palmitic acids and a lower proportion of oleic acid (P < 0.05) in TRL-TAG derived from HOSO. This study shows that VOO intake results in attenuated postprandial TAG concentration and faster TRL-TAG disappearance from blood compared with HOSO, suggesting that the oleic acid content may not be the main factor affecting TAG metabolism. Minor fatty acids such as linoleic acid and the 2-positional distribution of saturated stearic and palmitic acids into the TAG molecule may be important determinants of postprandial lipemia in normolipidemic men.

Preheparin serum lipoprotein lipase mass is negatively related to coronary atherosclerosis

In preheparin serum, there exists lipoprotein lipase (LPL) mass with little activity. The clinical significance of this preheparin serum LPL mass (preheparin LPL mass) is unclear. We studied the levels of preheparin LPL mass in patients with coronary atherosclerosis, comparing the results with those in healthy men. We also evaluated the correlation between preheparin LPL mass and the severity of coronary atherosclerosis by comparing with other risk factors such as age, smoking, family history, hypertension, hyperuricemia, diabetes mellitus, total cholesterol, triglyceride, high density lipoprotein-cholesterol and body mass index. The subjects, 70 men presenting with symptoms of coronary artery disease, underwent coronary angiographic examination. Significant narrowness was defined as > or = 75%. Control group comprised 77 men who had annual health checks and showed no abnormal findings. Preheparin LPL mass in the stenosis group was lower than normal coronary group and also than the control group. Multivariate analysis showed that preheparin LPL mass had the highest t-value (-2.53) for the number of lesions among the risk factors listed above. These results suggest that low preheparin LPL mass may be deeply involved in the progression of coronary atherosclerosis.

Enhancement of preheparin serum lipoprotein lipase mass by bezafibrate administration

To clarify the clinical implication of preheparin serum lipoprotein lipase mass (preheparin LpL mass), we studied the relationships between preheparin LpL mass and serum lipids, including midband lipoproteins, which migrate between very low density lipoproteins and low density lipoproteins on polyacrylamide gel disc electrophoresis, in hyperlipidemias. And we also studied the changes of preheparin LpL mass in hypertriglyceridemic patients during bezafibrate administration, which is known to enhance LpL activity in postheparin plasma. Preheparin LpL mass correlated positively with high-density lipoprotein-cholesterol (HDL-C) (r=0.418, P<0.01) and negatively with triglyceride (TG) (r=-0.256, P<0.01), but did not correlate with total cholesterol (TC) in 64 hyperlipidemic (type IIa, IIb and IV) patients. The midband lipoproteins were observed in 80% of hypertriglyceridemic patients (32/40). Preheparin LpL mass in midband lipoprotein-positive subjects was lower significantly than that in midband-negative subjects. When bezafibrate (400 mg/day) was administrated to 40 hypertriglyceridemic patients for 4 months, TG level significantly decreased (-49+/-7%, P<0.01), TC levels decreased (-11+/-4%, not significant), and HDL-C levels increased (+27+/-4%, P<0.01). The midband lipoproteins disappeared in 95% of patients. Preheparin LpL mass significantly increased (+25+/-6%, P<0. 0005). In nine patients who stopped bezafibrate, TG levels significantly increased (+49+/-7%, P<0.01) and HDL-C levels decreased (-27+/-4%, P<0.01). Preheparin LPL mass significantly decreased (-25+/-6%, P<0.0005). These results suggested that bezafibrate administration enhanced preheparin LpL mass. And it might be implicated that enhanced LpL production by bezafibrate could reflect an increase of preheparin LpL mass.

Complete atherosclerosis regression after human ApoE gene transfer in ApoE-deficient/nude mice

The apolipoprotein E (apoE)-deficient mouse is a relevant animal model of human atherosclerosis. Although the prevention of atherosclerosis development has been documented after somatic gene transfer into animal models, regression of lesions remains to be demonstrated. Thus, we used this genetically defined mouse model nn the nude background to show atherosclerosis regression. ApoE-deficient nude mice were infected with 5 x 10(8) or 10(9) plaque-forming units of a first-generation adenovirus encoding human apoE cDNA. The secretion of human apoE resulted in a rapid decrease of total cholesterol, which normalized the hypercholesterolemic phenotype within 14 days (from 600+/-100 to <100 microg/mL). Transgene expression was observed during a period of >4 months, with a normalization of cholesterol and triglyceride levels during 5 months. At that time, we successfully reinjected the recombinant adenovirus and observed the appearance of the human protein as well as the correction of lipoprotein phenotype. In mice killed 6 months-after the first infection, we observed a dose-dependent regression of fatty streak lesions in the aorta. We showed sustained expression of a transgene with a first-generation adenoviral vector and a correction of dyslipoproteinemia phenotype leading to lesion regression. These data demonstrate that somatic gene transfer can induce plaque regression.

The role of hepatic lipase in lipoprotein metabolism

Hepatic lipase (HL) is one of two major lipases released from the vascular bed by intravenous injection of heparin. HL hydrolyzes phospholipids and triglycerides of plasma lipoproteins and is a member of a lipase superfamily that includes lipoprotein lipase and pancreatic lipase. The enzyme can be divided into an NH2-terminal domain containing the catalytic site joined by a short spanning region to a smaller COOH-terminal domain. The NH2-terminal portion contains an active site serine in a pentapeptide consensus sequence, Gly-Xaa-Ser-Xaa-Gly, as part of a classic Ser-Asp-His catalytic triad, and a putative hinged loop structure covering the active site. The COOH-terminal domain contains a putative lipoprotein-binding site. The heparin-binding sites may be distributed throughout the molecule, with the characteristic elution pattern from heparin-sepharose determined by the COOH-terminal domain. Of the three N-linked glycosylation sites, Asn-56 is required for efficient secretion and enzymatic activity. HL is hypothesized to directly couple HDL lipid metabolism to tissue/cellular lipid metabolism. The potential significance of the HL pathway is that it provides the hepatocyte with a mechanism for the uptake of a subset of phospholipids enriched in unsaturated fatty acids and may allow the uptake of cholesteryl ester, free cholesterol and phospholipid without catabolism of HDL apolipoproteins. HL can hydrolyze triglyceride and phospholipid in all lipoproteins, but is predominant in the conversion of intermediate density lipoproteins to LDL and the conversion of post-prandial triglyceride-rich HDL into the post-absorptive triglyceride-poor HDL. It has been suggested that enzymatically inactive HL can play a role in hepatic lipoprotein uptake forming a 'bridge' by binding to the lipoprotein and to the cell surface. This raises the interesting possibility that production and secretion of mutant inactive HL could promote clearance of VLDL remnants. We have described a rare family with HL deficiency. Affected patients are compound heterozygotes for a mutation of Ser267Phe that causes an inactive enzyme and a mutation of Thr383Met that results in impaired secretion of HL and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease.

Apolipoprotein E and atherosclerosis: insight from animal and human studies

Major advances have been made in our understanding of the role of apolipoprotein E (apoE) in the onset and development of atherosclerosis. Increasing evidence from both animal and human studies suggests that apoE is able to protect against atherosclerosis by: a) promoting efficient uptake of triglyceride-rich lipoproteins from the circulation; b) maintaining normal macrophage lipid homeostasis; c) playing a role in cellular cholesterol efflux and reverse cholesterol transport; d) acting as an antioxidant; e) inhibiting platelet aggregation; and f) modulating immune function. In humans, apoE is polymorphic, and this genetic variation has a strong effect on its antiatherogenic characteristics. Thus, compared to the epsilon3 allele, the epsilon4 allele promotes atherosclerosis, whereas the epsilon2 allele is either pro- or anti-atherogenic, depending on the influence of both environmental and genetic factors. ApoE and its gene are prime targets for therapeutic intervention aimed at preventing or treating atherosclerotic vascular disease.

Receptor and non-receptor mediated uptake of chylomicron remnants by the liver

The uptake of chylomicron remnants by rodent liver is mediated by proteins residing on the microvillous surface of hepatocytes and occurs in two steps. First, initial removal of the remnants from the blood occurs through binding to the low density lipoprotein (LDL) receptor via apo E and to hepatic lipase via polar lipids and proteins on the remnant surface. Second, chylomicron remnants are taken up into the cell mainly by the LDL receptor and follow the classical receptor-mediated pathway of endocytosis. The LDL receptor-related protein (LRP), which binds weakly to chylomicron remnants via apo E, does not appear to have a significant role in the initial removal process. The remnant particles can, however, be enriched with proteoglycan-bound apo E present on hepatocytic microvilli, which increases their affinity for LRP to the extent that they are subject to endocytosis by this receptor, particularly when the LDL receptor is deficient or down-regulated. Hepatic lipase can also mediate binding of remnants to LRP, for which it has high affinity. Lipolysis of remnant lipids by hepatic lipase may promote but is not required for interaction of remnants with the endocytic receptors. Proteoglycan-bound hepatic lipase may also mediate endocytosis of chylomicron remnants independent of apo E, so that hepatic catabolism of these particles is not completely dependent upon this apoprotein. Available data from experiments in vivo thus indicate redundancy of both steps of hepatic uptake of chylomicron remnants, consistent with the centrality of this process in nutrient delivery.

Kinetics of triglyceride rich lipoproteins: chylomicrons and very low density lipoproteins

Lipoprotein dynamics are complex during the postprandial state. A significant rise in chylomicron concentration is associated with increased competition for LPL with VLDL particles. This results in an increased concentration of large VLDL. The concentration of small VLDL is reduced as a result of diminished conversion of large to small VLDL. Such changes, induced in the postprandial state, complicate the application and development of models that describe lipoprotein particle kinetics. The development of models that integrate chylomicron and VLDL particle information, rather than surrogate markers, together with data including other variables will provide insight into the complexity of lipoprotein metabolism in the postprandial state.