Lipoprotein metabolism in hepatic lipase deficiency: studies on the turnover of apolipoprotein B and on the effect of hepatic lipase on high density lipoprotein

A study of associations between CUBN, HNF1A, and LIPC gene polymorphisms and coronary artery disease

The aim of this study was to identify novel genetic markers related to coronary artery disease (CAD) using a whole-exome sequencing (WES) approach and determine any associations between the selected gene polymorphisms and CAD prevalence. CUBN, HNF1A and LIPC gene polymorphisms related to CAD susceptibility were identified using WES screening. Possible associations between the five gene polymorphisms and CAD susceptibility were examined in 452 CAD patients and 421 control subjects. Multivariate logistic regression analyses indicated that the CUBN rs2291521GA and HNF1A rs55783344CT genotypes were associated with CAD (GG vs. GA; adjusted odds ratio [AOR] = 1.530; 95% confidence interval [CI] 1.113–2.103; P = 0.002 and CC vs. CT; AOR = 1.512; 95% CI 1.119–2.045; P = 0.007, respectively). The CUBN rs2291521GA and HNF1A rs55783344CT genotype combinations exhibited a stronger association with CAD risk (AOR = 2.622; 95% CI 1.518–4.526; P = 0.001). Gene-environment combinatorial analyses indicated that the CUBN rs2291521GA, HNF1A rs55783344CT, and LIPC rs17269397AA genotype combination and several clinical factors (fasting blood sugar (FBS), high-density lipoprotein (HDL), and low-density lipoprotein (LDL) levels) were associated with increased CAD risk. The CUBN rs2291521GA, HNF1A rs55783344CT, and LIPC rs17269397AA genotypes in conjunction with abnormally elevated cholesterol levels increase the risk of developing CAD. This exploratory study suggests that polymorphisms in the CUBN, HNF1A, and LIPC genes can be useful biomarkers for CAD diagnosis and treatment.

Causes and Consequences of Hypertriglyceridemia

Elevations in plasma triglyceride are the result of overproduction and impaired clearance of triglyceride-rich lipoproteins-very low-density lipoproteins (VLDL) and chylomicrons. Hypertriglyceridemia is characterized by an accumulation in the circulation of large VLDL-VLDL₁-and its lipolytic products, and throughout the VLDL-LDL delipidation cascade perturbations occur that give rise to increased concentrations of remnant lipoproteins and small, dense low-density lipoprotein (LDL). The elevated risk of atherosclerotic cardiovascular disease in hypertriglyceridemia is believed to result from the exposure of the artery wall to these aberrant lipoprotein species. Key regulators of the metabolism of triglyceride-rich lipoproteins have been identified and a number of these are targets for pharmacological intervention. However, a clear picture is yet to emerge as to how to relate triglyceride lowering to reduced risk of atherosclerosis.

FoxO transcription factors are required for hepatic HDL cholesterol clearance

Insulin resistance and type 2 diabetes are associated with low levels of high-density lipoprotein cholesterol (HDL-C). The insulin-repressible FoxO transcription factors are potential mediators of the effect of insulin on HDL-C. FoxOs mediate a substantial portion of insulin-regulated transcription, and poor FoxO repression is thought to contribute to the excessive glucose production in diabetes. In this work, we show that mice with liver-specific triple FoxO knockout (L-FoxO1,3,4), which are known to have reduced hepatic glucose production, also have increased HDL-C. This was associated with decreased expression of the HDL-C clearance factors scavenger receptor class B type I (SR-BI) and hepatic lipase and defective selective uptake of HDL cholesteryl ester by the liver. The phenotype could be rescued by re-expression of SR-BI. These findings demonstrate that hepatic FoxOs are required for cholesterol homeostasis and HDL-mediated reverse cholesterol transport to the liver.

The crucial roles of apolipoproteins E and C-III in apoB lipoprotein metabolism in normolipidemia and hypertriglyceridemia

PURPOSE OF REVIEW

To describe the roles of apolipoprotein C-III (apoC-III) and apoE in VLDL and LDL metabolism

RECENT FINDINGS

ApoC-III can block clearance from the circulation of apolipoprotein B (apoB) lipoproteins, whereas apoE mediates their clearance. Normolipidemia is sustained by hepatic secretion of VLDL and IDL subspecies that contain both apoE and apoC-III (VLDL E+C-III+). Most of this VLDL E+C-III+ is speedily lipolyzed, reduced in apoC-III content, and cleared from the circulation as apoE containing dense VLDL, IDL, and light LDL. In contrast, in hypertriglyceridemia, most VLDL is secreted with apoC-III but without apoE, and so it is not cleared until it loses apoC-III during lipolysis to dense LDL. In normolipidemia, the liver also secretes IDL and large and medium-size LDL, whereas in hypertriglyceridemia, the liver secretes more dense LDL with and without apoC-III. These pathways establish the hypertriglyceridemic phenotype and link it metabolically to dense LDL. Dietary carbohydrate compared with unsaturated fat suppresses metabolic pathways mediated by apoE that are qualitatively similar to those suppressed in hypertriglyceridemia.

SUMMARY

The opposing actions of apoC-III and apoE on subspecies of VLDL and LDL, and the direct secretion of LDL in several sizes, establish much of the basic structure of human apoB lipoprotein metabolism in normal and hypertriglyceridemic humans.

Genetic determinants of apolipoprotein B-100 kinetics

PURPOSE OF REVIEW

We review stable isotope tracer studies of apolipoprotein B-100 (apoB) kinetics concerning genetic polymorphisms and mutations that affect human lipoprotein metabolism.

RECENT FINDINGS

In obese men, the allelic combination of the apoB signal peptide, SP24, and cholesteryl ester transfer protein, CETP B1B1, is independently associated with lower VLDL apoB secretion. Microsomal triglyceride transfer protein -493G/T carriers have reduced IDL apoB and LDL apoB production as compared with controls. Mutations in cholesterol transporters (ATP-binding cassette transporter G8 and Niemann-Pick C1 Like 1) are associated with reduced VLDL apoB secretion and increased LDL apoB production and catabolism. The ATP-binding cassette transporter G8 400K variant is a significant, independent predictor of VLDL apoB secretion. Mutations in lipases (lipoprotein lipase and hepatic lipase) and transfer proteins (lecithin-cholesterol acyltransferase and cholesteryl ester transfer protein) alter their functional activity, which impact on VLDL and LDL kinetics.

SUMMARY

Mutations in genes that regulate intrahepatic apoB assembly and lipid substrate availability to the liver impact on VLDL apoB secretion. Lipoprotein tracer studies can provide functional insight into the potential impact of genetic polymorphisms in regulating apoB metabolism in humans.

The effect of PPAR-alpha agonism on apolipoprotein metabolism in humans

Metabolic syndrome, diabetes and obesity are frequently associated with hypertriglyceridemia, hypercholesterolemia and low HDL levels, a phenotype known as atherogenic dyslipidemia. Atherogenic dyslipidemia and hypertriglyceridemia are frequently treated with fibric acid derivatives which activate the nuclear receptor PPAR-alpha leading to reduce plasma triglycerides and an increase in HDL cholesterol levels. The mechanism by which activation of PPAR-alpha with fibrates improves the plasma lipid profile in patients with atherogenic dyslipidemia and hypertriglyceridemia has been examined in several small studies measuring lipoprotein kinetics. The results of these studies indicate that the changes in lipoprotein metabolism observed in response to fibrate treatment vary according to lipoprotein phenotype. In general, fibrates act to reduce VLDL apoB-100 through enhanced fractional catabolism (clearance) of VLDL apoB-100 with additional effects on reducing VLDL apoB-100 production. LDL apoB-100 levels generally decrease in response to fibrates due to increased LDL fractional catabolism except in those patients with high to very high plasma triglyceride levels (>400mg/dL). Fibrates also increase HDL apoA-I and apoA-II levels by enhancing apoA-I and apoA-II production, although this is partially counteracted by increasing fractional catabolism of these apolipoproteins. The potent and specific PPAR-alpha agonist LY518674, reduced VLDL apoB-100 levels through enhanced fractional catabolism similar to what is seen with fibrates. In contrast to fibrates, LY518674 did not change HDL apoA-I levels in response to due to an increased turnover of apoA-I where an increased fractional catabolic rate entirely counteracted the increase in apoA-I production. The changes in apoB metabolism in response to PPAR-alpha activation with fibrates and specific PPAR-alpha agonists would be expected to reduce the risk of cardiovascular disease. However, the benefit of the enhanced turnover of HDL apoA-I in response to PPAR-alpha activation remains to be determined.

Improved cholesterol phenotype analysis by a model relating lipoprotein life cycle processes to particle size

Increased plasma cholesterol is a known risk factor for cardiovascular disease. Lipoprotein particles transport both cholesterol and triglycerides through the blood. It is thought that the size distribution of these particles codetermines cardiovascular disease risk. New types of measurements can determine the concentration of many lipoprotein size-classes but exactly how each small class relates to disease risk is difficult to clear up. Because relating physiological process status to disease risk seems promising, we propose investigating how lipoprotein production, lipolysis, and uptake processes depend on particle size. To do this, we introduced a novel model framework (Particle Profiler) and evaluated its feasibility. The framework was tested using existing stable isotope flux data. The model framework implementation we present here reproduced the flux data and derived lipoprotein size pattern changes that corresponded to measured changes. It also sensitively indicated changes in lipoprotein metabolism between patient groups that are biologically plausible. Finally, the model was able to reproduce the cholesterol and triglyceride phenotype of known genetic diseases like familial hypercholesterolemia and familial hyperchylomicronemia. In the future, Particle Profiler can be applied for analyzing detailed lipoprotein size profile data and deriving rates of various lipolysis and uptake processes if an independent production estimate is given.

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.

Lipoprotein alterations, hepatic lipase activity, and insulin sensitivity in subclinical hypothyroidism: response to L-T(4) treatment

Subclinical hypothyroidism (sH) has been associated with atherosclerotic cardiovascular disease even in the absence of hypercholesterolemia.

OBJECTIVE

Our study was designed to assess the hypothesis that other pro-atherogenic parameters, such as qualitative lipoprotein changes and insulin resistance, might be present in sH.

DESIGN AND METHODS

Twenty-one sH women were compared to 11 female controls matched for body mass index, menopausal status, and age. Before and after 6 months of levothyroxine (L-T(4)) treatment, we determined total cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides (TG), apoB levels, hepatic lipase (HL) activity in postheparin plasma samples, the chemical composition and copper-induced oxidation in isolated LDL and homeostasis model assessment (HOMA), quantitative insulin sensitivity check index, and insulinogenic index.

MAIN OUTCOME

Lipid profiles were similar between the two groups. No differences in LDL oxidability or the insulin sensitivity assessment parameters were found. HL activity was significantly lower in the sH patients: median (range), 13.1 (2.5-26.7) vs. 18.7 (7.9-28.1) micromol free fatty acids/mL, p < 0.04. The LDL-cholesterol/LDL-TG ratio was decreased in sH: 3.9 (1.8-5.5) vs. 4.7 (3.5-6.8), p < 0.02. HL negatively correlated with thyroid-stimulating hormone (TSH) levels (r = - 0.504, p < 0.01) and positively with LDL-cholesterol/LDL-TG (r = 0.46, p < 0.02). Posttreatment results for all these parameters did not differ significantly compared to baseline.

CONCLUSIONS

Increased levels of TSH are associated to a decrease in HL activity, explaining our findings of an LDL particle rich in TG. This qualitative lipoprotein alteration suggests a pro-atherogenic pattern in sH. Treatment with L-T(4), however, did not correct the basal lipid derangement.

Hepatic Lipase Deficiency Delays Atherosclerosis, Myocardial Infarction, and Cardiac Dysfunction and Extends Lifespan in SR-BI/Apolipoprotein E Double Knockout Mice

Objective— SR-BI/apolipoprotein (apo) E double knockout (dKO) mice exhibit many features of human coronary heart disease (CHD), including occlusive coronary atherosclerosis, cardiac hypertrophy, myocardial infarctions, and premature death. Here we determined the effects on this pathology of hepatic lipase (HL) deficiency, which has been shown to significantly modulate atherosclerosis.

Method and Results— The SR-BI/apoE/HL triple knockout (tKO) mice generated for this study lived significantly longer (37%) than corresponding dKO controls (average lifespans: 63.0±0.8 versus 46.0±0.3 days), despite their increased plasma cholesterol levels. At 6 weeks of age, compared with dKO mice, tKOs exhibited significantly less aortic root and coronary artery occlusive atherosclerosis, and improved cardiac structure and function. However, by 9 weeks of age the hearts of tKO mice exhibited lipid-rich coronary occlusions, myocardial infarctions, and cardiac dysfunction essentially identical to that of 6-week-old dKO mice.

Conclusions— HL-deficiency delays the onset and/or progression of atherosclerosis via a SR-BI–independent mechanism. Extent of occlusive coronary arterial lesions was more closely associated with cardiac dysfunction and lifespan than the amount of aortic root atherosclerosis, suggesting that these occlusions in dKO mice are responsible for ischemia, myocardial infarctions, and premature death.

Plasma metabolism of apoB-containing lipoproteins in patients with hepatic lipase deficiency

The metabolism of apoB-containing lipoproteins was investigated in the fasted state in three complete and three partial hepatic lipase (HL)-deficient subjects as well as in seven normotriglyceridemic (NTG) and two hypertriglyceridemic (HTG) controls using a 12 h primed-constant infusion of L-[5,5,5-D(3)]-leucine. Two males with complete HL deficiency had increased plasma pool sizes of VLDL and IDL apoB-100 due to substantial reductions in fractional catabolic rate (FCR) of VLDL and IDL apoB-100 compared with both NTG and HTG controls. Reductions in LDL apoB-100 production rate (PR) were also observed in these two patients compared with NTG and HTG controls. Complete HL deficiency in the female proband was associated with normal VLDL apoB-100 kinetics, while plasma IDL apoB-100 pool size was increased by 124% due to an 82% decrease in the FCR of IDL apoB-100. The FCR and PR of LDL apoB-100 were reduced by 64 and 51%, respectively, in the proband compared with sex-matched controls. Partial HL-deficient patients were characterized by apoB-containing lipoprotein metabolism similar to that of controls. These results indicate that complete HL deficiency is associated with a potentially atherogenic apoB-containing lipoprotein metabolism that can be modulated considerably by secondary factors such as gender and abdominal obesity.

Omega-3 fatty acids alter lipoprotein subfraction distributions and the in vitro conversion of very low density lipoproteins to low density lipoproteins

The purpose of this study was to determine the effects of a fish oil concentrate (FOC) on the in vitro conversion of very low density lipoproteins (VLDL) to intermediate (IDL) and low density lipoproteins (LDL). Six hypertriglyceridemic patients were randomly allocated to receive either placebo (olive oil) or FOC (1 g/14 kg body weight/day) for 4 weeks in a crossover study with a 4-week washout period. The FOC provided 3 g of eicosapentaenoic + docosahexaenoic acid per 70 kg of body weight, and it lowered plasma triglyceride and VLDL cholesterol levels by 35% and 42%, respectively. Decreases in the largest particles (VLDL(1)) were primarily responsible, with no effect noted in smaller VLDL particles (VLDL(2) and VLDL(3)). The FOC increased LDL cholesterol levels by 25% (P < 0.06) but did not affect LDL particle size. VLDL(1) and VLDL(3) were incubated in vitro with human postheparin lipases. Although triglycerides from both types of VLDL were hydrolyzed to the same extent with both treatments, particles isolated during the FOC phase were more readily converted into IDL and LDL than were control particles. These data suggest that the marine omega3 fatty acids may enhance the propensity of VLDL to be converted to LDL, partly explaining the decreased VLDL and increased LDL levels in FOC-treated patients.

Evidence that hepatic lipase deficiency in humans is not associated with proatherogenic changes in HDL composition and metabolism

The aim of the present study was to characterize the composition and metabolism of HDL in subjects with complete hepatic lipase (HL) deficiency. Analyses were carried out in three complete and three partial HL-deficient subjects as well as in eight normotriglyceridemic (NTG) and two hypertriglyceridemic controls. Complete HL deficiency was associated with hypertriglyceridemia and with a 3.5-fold increase in HDL-triglyceride (TG) levels. The in vivo kinetics of apolipoprotein A-I (apoA-I) and apoA-II (d < 1.25 g/l) were studied in the fasted state using a primed-constant infusion of l-(5,5,5-D3)leucine for 12 h. Complete HL deficiency was associated with a reduced fractional catabolic rate of apoA-I in the HL-deficient female proband (-47%) and in her two brothers (-21%) compared with gender- and TG-matched controls. Total plasma and HDL from complete HL-deficient patients were able to mediate normal cholesterol efflux from human skin fibroblasts labeled with [3H]cholesterol. Complete HL deficiency was also associated with normal levels of prebeta-migrating apoA-I-containing HDL separated by two-dimensional gel electrophoresis and with an accumulation of large HDL particles compared with NTG controls. These results suggest that HL activity is important for adequate HDL metabolism, although its presence may not be necessary for normal HDL-mediated reverse cholesterol transport.

Lipoprotein metabolism in subjects with hepatic lipase deficiency

A heritable deficiency of hepatic lipase (HL) provides insights into the physiologic function of HL in vivo. The metabolism of apolipoprotein B (apoB)-100 in very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) and of apoA-I and apoA-II in high-density lipoprotein (HDL) particles lipoprotein (Lp)(AI) and Lp(AI:AII) was assessed in 2 heterozygous males for compound mutations L334F/T383M or L334F/R186H, with 18% and 22% of HL activity, respectively, compared with 6 control males. Subjects were provided with a standard Western diet for a minimum of 3 weeks. At the end of the diet period, apo kinetics was assessed using a primed-constant infusion of [5,5,5-(2)H(3)] leucine. Mean plasma triglyceride (TG) and HDL cholesterol levels were 55% and 12% higher and LDL cholesterol levels 19% lower in the HL patients than control subjects. A higher proportion of apoB-100 was in the VLDL than IDL and LDL fractions of HL patients than control subjects due to a lower VLDL apoB-100 fractional catabolic rate (FCR) (4.63 v 9.38 pools/d, respectively) and higher hepatic production rate (PR) (33.24 v 10.87 mg/kg/d). Delayed FCR of IDL (2.78 and 6.31 pools/d) and LDL (0.128 and 0.205 pools/d) and lower PR of IDL (3.67 and 6.68 mg/kd/d) and LDL 4.57 and 13.07 mg/kg/d) was observed in HL patients relative to control subjects, respectively. ApoA-I FCR (0.09 and 0.13 pools/d) and PR (4.01 and 6.50 mg/kg/d) were slower in Lp(AI:AII) particles of HL patients relative to control subjects, respectively, accounting for the somewhat higher HDL cholesterol levels. HL deficiency may result in a lipoprotein pattern associated with low heart disease risk.

The bridging function of hepatic lipase clears plasma cholesterol in LDL receptor-deficient “apoB-48-only” and “apoB-100-only” mice

Hepatic lipase clears plasma cholesterol by lipolytic and nonlipolytic processing of lipoproteins. We hypothesized that the nonlipolytic processing (known as the bridging function) clears cholesterol by removing apoB-48- and apoB-100-containing lipoproteins by whole particle uptake. To test our hypotheses, we expressed catalytically inactive human HL (ciHL) in LDL receptor deficient "apoB-48-only" and "apoB-100-only" mice. Expression of ciHL in "apoB-48-only" mice reduced cholesterol by reducing LDL-C (by 54%, 46 +/- 6 vs. 19 +/- 8 mg/dl, P < 0.001). ApoB-48 was similarly reduced (by 60%). The similar reductions in LDL-C and apoB-48 indicate cholesterol removal by whole particle uptake. Expression of ciHL in "apoB-100-only" mice reduced cholesterol by reducing IDL-C (by 37%, 61 +/- 19 vs. 38 +/- 12 mg/dl, P < 0.003). Apo-B100 was also reduced (by 27%). The contribution of nutritional influences was examined with a high-fat diet challenge in the "apoB-100-only" background. On the high fat diet, ciHL reduced IDL-C (by 30%, 355 +/- 72 vs. 257 +/- 64 mg/dl, P < 0.04) but did not reduce apoB-100. The reduction in IDL-C in excess of apoB-100 suggests removal either by selective cholesteryl ester uptake, or by selective removal of larger, cholesteryl ester-enriched particles. Our results demonstrate that the bridging function removes apoB-48- and apoB-100-containing lipoproteins by whole particle uptake and other mechanisms.

Effects on Lipoprotein Subclasses of Combined Expression of Human Hepatic Lipase and Human apoB in Transgenic Rabbits

Objective— The effects of combined expression of human hepatic lipase (HL) and human apolipoprotein B (apoB) on low-density lipoprotein (LDL) subclasses were examined in rabbits, a species naturally deficient in HL activity.

Methods and Results— In apoB-transgenic rabbit plasma, >80% of the protein was found in the 1.006- to 1.050-g/mL fraction. Gradient gel electrophoresis (GGE) of this fraction revealed two distinct species, designated large and small LDL. A denser fraction (d=1.050 to 1.063 g/mL) contained small LDL as well as another discrete LDL subspecies, designated very small LDL. Expression of HL resulted in reductions in protein concentrations in the 1.006- to 1.050-g/mL density-gradient subfractions containing large (6.5±4.1 versus 32.6±12.0 mg/dL, P <0.005) and small LDL (59.6±17.4 versus 204.3±50.3 mg/dL, P <0.002). A concomitant small but not significant increase in protein concentration in the denser LDL fraction (48.0±28.2 versus 44.6±18.2 mg/dL) was due primarily to an increase in very small LDL (25.9±3.1 versus 9.6±5.4% of total LDL GGE densitometric area, P <0.002).

Conclusion— These findings support a direct role for HL in regulating total plasma LDL concentrations as well as in the production of smaller, denser LDL from larger, more buoyant precursors.

Endocytosis of hepatic lipase and lipoprotein lipase into rat liver hepatocytes in vivo is mediated by the low density lipoprotein receptor-related protein

In isolated cell studies, the internalization and degradation of hepatic lipase (HL) has been linked to its binding to the low density lipoprotein receptor-related protein (LRP). We have utilized the receptor-associated protein (RAP), a universal inhibitor of high affinity ligand binding to LRP, to evaluate the participation of LRP in the endocytosis of HL and lipoprotein lipase (LPL). We isolated a total endosome fraction from rat livers after a 30-min infusion of recombinant RAP, administered as a glutathione S-transferase conjugate (GST-RAP). GST-RAP infusion had no effect on the concentration of HL in liver homogenates, but its concentration in blood plasma increased progressively by 20%, and enrichment over homogenate of HL in endosomes was reduced by 50% as compared with infusion of GST alone. The concentrations of LPL in liver and plasma were 1.4 and 0.5%, respectively, those of HL, but endosomal enrichment of the two enzymes was similar ( approximately 10-fold). GST-RAP infusion had no effect on the concentration of LPL in liver but increased its concentration in blood plasma by 250% and reduced its endosomal enrichment by 95% or greater. GST-RAP infusion also reduced endosomal enrichment of LRP by 40%, but enrichment of several other endocytic receptors was unaffected. Endosomal enrichment of several membrane trafficking proteins associated with the endocytic pathway in hepatocytes was unaffected by GST-RAP with the exception of early endosome endosome antigen 1, which was reduced by 85%. We conclude that HL is partially and LPL almost exclusively taken up into rat hepatocytes after binding to the endocytic receptor LRP.

Plasma lipoprotein lipase, hepatic lipase activities, VLDL, LDL compositions at different times of hemodialysis

The effects of hemodialysis duration (HD) on lipoprotein lipase (LPL) and hepatic lipase (HL) activities and very low density lipoprotein (VLDL), low density lipoproteins (LDL) amounts and compositions were investigated in 58 patients, divided according to HD: GI: under 1 year, GII: 1-5 years, GIII: 5-13 years. HL and LPL activities were reduced in GIII versus GI (P<0.01) and 47% of GIII patients had negligible HL activity. LPL and HL activities were correlated with HD (r=-0.80, P<0.001). Apo C-III concentrations were correlated with HD (r=0.58, P<0.05). Compared with controls, triacylglycerols (TG) were increased in GI, GII (P<0.01) and GIII (P<0.001), and were correlated with HD (r=0.75, P<0.05). VLDL amounts and VLDL-cholesteryl esters (CE) were enhanced in GIII versus GI and GII (P<0.05). VLDL-TG and VLDL-phospholipids (PL) were correlated with HD (r=0.60, P<0.05). LDL-apolipoproteins and unesterified cholesterol (UC) were increased in GII versus GI (P<0.05) and in GIII versus GII and GI (P<0.01). LDL-PLs were decreased in GIII versus GI (P<0.05). Compared with controls, LDL-TGs were higher in GI and GII (P<0.01) and in GIII (P<0.05). Long-term treatment with acetate hemodialysis using cuprophane membrane does not improve lipolytic activity decrease and lipoprotein alterations generated by chronic renal failure (CRF).

Characterization of a novel mutation causing hepatic lipase deficiency among French Canadians

Individuals with hepatic lipase (HL) deficiency are often characterized by elevated levels of triglycerides (TGs) and cholesterol. The aim of the present study was to characterize the molecular defect leading to severe HL deficiency in a Québec-based kindred. In the proband and two of her brothers, the very low to undetectable HL activity resulted from compound heterozygosity for two rare HL gene mutations, a previously unknown missense mutation in exon 5 designated A174T and the previously reported T383M mutation in exon 8 of the HL gene. The mutation at codon 174 resulted in the substitution of alanine for threonine, a polar amino acid, in a highly conserved nonpolar region of the protein involved in the catalytic activity of the enzyme. The severe HL deficiency among the three related compound heterozygotes was associated with a marked TG enrichment of LDL and HDL particles. The two men with severe HL deficiency also presented with abdominal obesity, which appeared to amplify the impact of HL deficiency on plasma TG-rich lipoprotein levels. Our results demonstrated that HL deficiency in this Québec kindred is associated with an abnormal lipoprotein-lipid profile, which may vary considerably in the presence of secondary factors such as abdominal obesity.

Triglyceride-rich lipoproteins are associated with hypertension in preeclampsia

Disorders of the lipoprotein metabolism are a major cause of endothelial dysfunction that may result in hypertension and proteinuria, clinical hallmarks of preeclampsia (PE). Lipoproteins and low-density lipoprotein (LDL) subfractions were investigated in 15 women with severe PE and compared with 23 women with a normal course of pregnancy. Compared with normal pregnancy, in PE apolipoprotein (apo)B in very low-density lipoprotein was increased by 76% (P = 0.008), and the triglyceride content of intermediate dense lipoproteins (IDL) was increased by 51% (P < 0.001); cholesterol and apoB in LDL were decreased by 26% (P = 0.005) and 23% (P = 0.016), respectively. Although not significant, the LDL profile was dominated by the most buoyant LDL-1. ApoB in the most dense LDL (dLDL), namely LDL-5 and LDL-6, was significantly decreased by 49% (P < 0.001) and 55% (P < 0.001), respectively. Diastolic blood pressure was positively correlated with the triglyceride content of IDL (r = 6.31; P < 0.001 and r = 0.352; P = 0.033 by partial correlation controlling for the presence or absence of PE) and negatively correlated with the concentration of apoB in dLDL (r = -0.500; P = 0.002). In addition, IDL triglycerides correlated negatively with infant birth weight percentile (r = -0.373; P = 0.027) and positively with proteinuria (r = 0.430; P = 0.014). Low birth weight was associated with high IDL triglycerides and low rather than high concentrations of dLDL. Triglyceride-rich remnants are known to cause endothelial dysfunction. Because the triglyceride content of IDL was positively correlated with elevated blood pressure and proteinuria, triglyceride-rich remnant lipoproteins might contribute to the pathophysiology of PE.

Metabolic origins and clinical significance of LDL heterogeneity

LDLs in humans comprise multiple distinct subspecies that differ in their metabolic behavior and pathologic roles. Metabolic turnover studies suggest that this heterogeneity results from multiple pathways, including catabolism of different VLDL and IDL precursors, metabolic remodeling, and direct production. A common lipoprotein profile designated atherogenic lipoprotein phenotype is characterized by a predominance of small dense LDL particles. Multiple features of this phenotype, including increased levels of triglyceride rich lipoprotein remnants and IDLs, reduced levels of HDL and an association with insulin resistance, contribute to increased risk for coronary heart disease compared with individuals with a predominance of larger LDL. Increased atherogenic potential of small dense LDL is suggested by greater propensity for transport into the subendothelial space, increased binding to arterial proteoglycans, and susceptibility to oxidative modification. Large LDL particles also can be associated with increased coronary disease risk, particularly in the setting of normal or low triglyceride levels. Like small LDL, large LDL exhibits reduced LDL receptor affinity compared with intermediate sized LDL. Future delineation of the determinants of heterogeneity of LDL and other apoB-containing lipoproteins may contribute to improved identification and management of patients at high risk for atherosclerotic disease.

Hepatic lipase: a pro- or anti-atherogenic protein?

Hepatic lipase (HL) plays a role in the metabolism of pro- and anti-atherogenic lipoproteins affecting their plasma level and composition. However, there is controversy regarding whether HL accelerates or retards atherosclerosis. Its effects on different lipoprotein classes show that, potentially, HL may promote as well as decrease atherogenesis. Studies in animals with genetically modulated HL expression show that it depends on the model used whether HL acts pro- or anti-atherogenic. In humans, HL activity seems to correlate inversely with atherosclerosis in (familial) hypercholesterolemia, and positively in hypertriglyceridemia. In normolipidemia, HL activity is weakly associated with coronary artery disease (CAD). Genetically low or absent HL activity is usually associated with increased CAD risk, especially if plasma lipid transport is impaired due to other factors. Since HL promotes the uptake of lipoproteins and lipoprotein-associated lipids, HL may affect intracellular lipid content. We hypothesize that the prime role of HL is to maintain, in concert with other factors (e.g., lipoprotein receptors), intracellular lipid homeostasis. This, and the uncertainties about its impact on human atherosclerosis, makes it difficult to predict whether HL is a suitable target for intervention to lower CAD risk. First, the physiological meaning of changes in HL activity under different conditions should be clarified.

Thyroid disease and lipids

The composition and the transport of lipoproteins are seriously disturbed in thyroid diseases. Overt hypothyroidism is characterized by hypercholesterolaemia and a marked increase in low-density lipoproteins (LDL) and apolipoprotein B (apo A) because of a decreased fractional clearance of LDL by a reduced number of LDL receptors in the liver. The high-density lipoprotein (HDL) levels are normal or even elevated in severe hypothyroidism because of decreased activity of cholesteryl-ester transfer protein (CETP) and hepatic lipase (HL), which are enzymes regulated by thyroid hormones. The low activity of CETP, and more specifically of HL, results in reduced transport of cholesteryl esters from HDL(2) to very low-density lipoproteins (VLDL) and intermediate low-density lipoprotein (IDL), and reduced transport of HDL(2) to HDL(3). Moreover, hypothyroidism increases the oxidation of plasma cholesterol mainly because of an altered pattern of binding and to the increased levels of cholesterol, which presents a substrate for the oxidative stress. Cardiac oxygen consumption is reduced in hypothyroidism. This reduction is associated with increased peripheral resistance and reduced contractility. Hypothyroidism is often accompanied by diastolic hypertension that, in conjunction with the dyslipidemia, may promote atherosclerosis. However, thyroxine therapy, in a thyrotropin (TSH)-suppressive dose, usually leads to a considerable improvement of the lipid profile. The changes in lipoproteins are correlated with changes in free thyroxine (FT(4)) levels. Hyperthyroidism exhibits an enhanced excretion of cholesterol and an increased turnover of LDL resulting in a decrease of total and LDL cholesterol, whereas HDL are decreased or not affected. The action of thyroid hormone on Lp(a) lipoprotein is still debated, because both decrease or no changes have been reported. The discrepancies are mostly because of genetic polymorphism of apo(a) and to the differences between the various study groups. Subclinical hypothyroidism (SH) is associated with lipid disorders that are characterized by normal or slightly elevated total cholesterol levels, increased LDL, and lower HDL. Moreover, SH has been associated with endothelium dysfunction, aortic atherosclerosis, and myocardial infarction. Lipid disorders exhibit great individual variability. Nevertheless, they might be a link, although it has not been proved, between SH and atherosclerosis.

Human triglyceride-rich lipoprotein apo E kinetics and its relationship to LDL apo B-100 metabolism

Apolipoprotein (apo) E is a multifunctional protein that can act as a ligand for lipoprotein receptors. The receptor-mediated clearance of the triglyceride-rich lipoproteins (TRL) chylomicrons and VLDL from plasma is, in part, dependent on apo E. Enrichment of VLDL with apo E is thought to enhance receptor-mediated clearance of VLDL resulting in a low rate of conversion of VLDL to LDL. However, the kinetic mechanism controlling the concentration of apo E in VLDL is not known. We conducted kinetic studies on apo E in the TRL fraction (d < 1.006 g/ml) and apo B-100 in the TRL and LDL (d = 1.019-1.063 g/ml) fractions to assess the kinetic determinants of apo E concentration in TRL and to determine the effects that TRL apo E production and clearance rates have on the production rate of LDL apo B-100. Nineteen males between the ages of 24 and 73 underwent a primed-constant infusion with deuterated leucine tracer in the constantly-fed state. Apo B-100 from TRL and LDL, and apo E from TRL were isolated and their tracer incorporation measured by gas chromatography/mass spectrometry. The residence time and production rates of each protein were determined from the kinetic data using the SAAM II modeling program. The residence time and production rate of TRL apo E were about one-half that of TRL apo B-100 (1.8 +/- 1.0 vs. 2.9 +/- 2.1 h and 14.5 +/- 11.0 vs. 27.6 +/- 17.3 mg/kg per day, respectively). The production rate of TRL apo E was weakly correlated with the production rate of TRL apo B-100 (r = 0.424, P = 0.07). Multiple regression analysis showed that the residence time of TRL apo B-100 and the relative TRL apo E production rate (relative to the TRL apo B100 production rate) were negatively associated with LDL apo B-100 production rate, accounting for 68% of its variability. We conclude that (1) the concentration of apo E in TRL is highly correlated to its production rate, suggesting that production rate regulates the TRL apo E concentration, and (2) individuals with a relatively short TRL apo B-100 residence time and those producing TRL with a relatively low apo E content have the highest LDL apo B-100 production rates.

Alterations of apolipoprotein B metabolism in HIV-infected patients with antiretroviral combination therapy

BACKGROUND

Dyslipidemia (predominantly hypertriglyceridemia) is frequently seen in patients receiving antiretroviral combination therapy (ART). However, the underlying mechanisms and long-term risks (e.g., cardiovascular events) are still unclear.

OBJECTIVES/METHODS

In 5 patients with ART-associated dyslipidemia, stable isotope labeled amino acid tracer (d3-Leu) kinetic analysis over 12 days was used to investigate the metabolism of apolipoprotein B-containing lipoproteins (very low density lipoproteins [VLDL]1, VLDL2, intermediate density lipoproteins [IDL] and low density lipoproteins [LDL]). Data were compared with those in 6 healthy normolipidemic controls.

RESULTS

The patients under ART showed significantly increased fasting triglycerides (359 vs. 77 mg/dl) and VLDL (54 vs. 15 mg/dl), compared with controls. They had significantly higher total cholesterol (213 vs. 157 mg/dl) and there was a nonsignificant trend toward higher LDL (136 vs. 93 mg/dl), and toward lower HDL (26 vs. 46 mg/dl). The ratio of large, buoyant LDL1 over small, dense LDL2 was markedly reduced in patients under ART (0.80 vs. 2.00). Total apo B synthesis was significantly increased (25.5 vs. 14.5 mg/kg/d) and shifted toward triglyceride rich VLDL1 (18.5 vs. 8.7 mg/kg/d) in patients receiving ART. There was also a significantly reduced rate of apo B lipoprotein transfer from VLDL1 to VLDL2 (3.7 vs. 20.7 pools/d). In addition, all patients revealed insulin resistance.

CONCLUSIONS

These data indicate that increased triglycerides in HIV-infected patients with ART are primary due to reduced rates of VLDL transfer into denser lipoproteins implying a lower rate of lipoprotein lipase-mediated delipidation. In addition, total apo B synthesis was increased and shifted toward triglyceride-rich VLDL1. Overall, this lipoprotein profile in patients with ART-associated dyslipidemia implies an increased risk for cardiovascular events.

The V73M mutation in the hepatic lipase gene is associated with elevated cholesterol levels in four Dutch pedigrees with familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is a heritable lipid disorder characterized by multiple lipoprotein phenotypes within a single family. Previously, we have shown an increased incidence of mutations in the LPL gene which was associated with elevated levels of very low density lipoprotein (VLDL) and decreased levels of high density lipoprotein among the families studied. Now, we report the results of our study on the hepatic lipase gene. We found the HL V73M variant to be present in four FCHL families. By means of a pedigree-based maximum log-likelihood method we analyzed the effect of this variant on the lipid levels in these families. Carriers of the HL V73M variant revealed significantly higher levels of total cholesterol (P < 0.01) and apoB (P <0.01). These findings show that the HL V73M mutant explains another part of the variability in the phenotype observed among FCHL family members, compared with mutations in the LPL gene. Family analysis shows that in these FCHL families, carriers of mutations in the LPL or HL genes have an increased risk for FCHL compared with their non-carrier relatives.

Hepatic lipase and HDL metabolism

Hepatic lipase is a lipolytic enzyme that has been suggested to have a role in HDL metabolism. Evidence suggests that HDL-cholesterol level is at least partly regulated by hepatic lipase level. Recent studies have shown that hepatic lipase not only hydrolyzes triglyceride and phospholipid in HDL, but also stimulates HDL cholesterol ester uptake by hepatocytes. Therefore, hepatic lipase, together with lipid transfer proteins, determines both HDL-cholesterol level and its function in reverse cholesterol transport. These conclusions are based on observations from in-vitro model substrate studies, cell culture studies, transgenic animal studies, and clinical studies. At present time, it is not known whether hepatic lipase action increases or decreases risk of developing atherosclerosis.

Structural peculiarities of the binding of very low density lipoproteins and low density lipoproteins to the LDL receptor in hypertriglyceridemia: role of apolipoprotein E

Very low (VLDL) and low density lipoproteins (LDL) were isolated from plasma of patients with the E3/3 phenotype which were divided into three groups based on their plasma triglyceride content: low (TG<200 mg/dl, TG(l)), intermediate (200<300 mg/dl, TG(i)300 mg/dl, TG(h)). The protein density (PD) on the VLDL and LDL surface was calculated from lipoprotein composition and protein location was studied by tryptophan fluorescence quenching by I(-) anions at 25 degrees C and 40 degrees C. A comparison of the TG(h) with the TG(l) group revealed a significant (<0.05) increase of the PD parameter as much as 21% for VLDL, but not for LDL where this parameter did not change for any group; generally, PD(LDL) values were 3.2-3.8-fold lower than PD(VLDL). In accordance with this difference, the tryptophan accessibility f in VLDL vs. LDL was lower at both temperatures. There were temperature-induced changes of the f parameter in opposite directions for these lipoproteins. The difference in f value gradually decreased for VLDL in the direction TG(l)TG(i)TG(h) while for LDL there was a U-shaped dependence for these groups. The Stern-Volmer quenching constant K(S-V) which is sensitive to both temperature and viscosity, did not change for VLDL, but K(S-V)(LDL) was 2-3-fold higher for the TG(i) group compared to the other two. The efficiencies of VLDL and LDL binding to the LDL receptor (LDLr) in vitro were compared by solid-phase assay free of steric hindrance observed in cell binding. The maximal number of binding sites did not change for either type of particles and between groups. The association constant K(a) and apolipoprotein (apo) E/apoB mole ratio values all increased significantly for VLDL, but not for LDL, in comparison of the TG(i+h) with the TG(l) group. Based on VLDL and LDL concentrations in serum and on the affinity constant values obtained in an in vitro assay, VLDL concentrations corresponding to 50% inhibition of LDL binding (IC(50)) were calculated in an assumption of the competition of both ligands for LDLr in vivo; the mean values of IC(50) decreased 2-fold when plasma TG exceeded 200 mg/dl. The functional dependences of K(a)(VLDL), IC(50) and apoE content in VLDL (both fractional and absolute) and in serum on TG content in the whole concentration range studied were fitted to a saturation model. For all five parameters, the mean half-maximum values TG(1/2) were in the range 52-103 mg/dl. The efficiency of protein-protein interactions is suggested to differ in normolipidemic vs. HTG-VLDL and apoE content and/or protein density on VLDL surface may be the primary determinant(s) of the increased binding of HTG-VLDL to the LDL receptor. ApoCs may compete with apoE for the binding to the VLDL lipid surface as plasma triglyceride content increases. The possible competition of VLDL with LDL for the catabolism site(s) in vivo, when plasma TG increases, could explain the atherogenic action of TG-rich lipoproteins. Moreover, the 'dual action' hypothesis on anti-atherogenic action of apoE-containing high density lipoproteins (HDL) in vivo is suggested: besides the well-known effect of HDL as cholesteryl ester catabolic outway, the formation of a transient complex of apoE-containing discs appearing at the site of VLDL TG hydrolysis by lipoprotein lipase with VLDL particles proposed in our preceding paper promotes the efficient uptake of TG-rich particles; in hypertriglyceridemia due to the diminished HDL content this uptake seems to be impaired which results in the increased accumulation of the remnants of TG-rich particles. This explains the observed increase in cholesterol and triglyceride content in VLDL and LDL, respectively, due to the CETP-mediated exchange of cholesteryl ester and triglyceride molecules between these particles.

Functional variants in the lipoprotein lipase gene and risk cardiovascular disease

The current report is a quantitative review of the relationship between lipoprotein lipase gene variants and cardiovascular disease based on published population-based studies. Sixteen studies, representing 17,630 individuals, report allelic distribution for lipoprotein lipase gene variants among patients and control individuals. Patient outcomes included clinical cardiovascular disease events, documented coronary disease based on angiography, or intimal media thickening by B-mode ultrasonography. Mantel-Haenszel stratified analysis was used to compute a summary odds ratio and 95% confidence intervals for the association between rare allele in the lipoprotein lipase gene and disease status. Because of potential differing effects associated with different lipoprotein lipase variants, each lipoprotein lipase mutant allele was considered separately. The lipoprotein lipase D9N/-93G to T allele has a summary odds ratio of 2.03 (95% confidence interval 1.30-3.18), indicating a twofold increase in risk of coronary disease for carriers with this allelic variant. The summary odds ratio for the relationship of the rare lipoprotein lipase G188E variant with cardiovascular disease is 5.25 (95% confidence interval 1.54-24.29). The lipoprotein lipase N291S allele is associated with a marginal increase in cardiovascular disease (summary odds ratio 1.25, 95% confidence interval 0.99-1.60, P = 0.07). However, there is stronger evidence for a positive association in certain populations. The summary odds ratio for lipoprotein lipase S447X allele is 0.81 (95% confidence interval 0.65-1.0), which indicates a cardioprotective effect of this lipoprotein lipase gene variant. Thus, lipoprotein lipase gene variants are associated with differential susceptibility to cardiovascular disease.

Hepatic lipase: new insights from genetic and metabolic studies

Hepatic lipase catalyses the hydrolysis of triglycerides and phospholipids in all major classes of lipoproteins. Genetic deficiency of this enzyme is associated with a unique plasma lipoprotein profile, characterized by hypertriglyceridemia and elevated concentrations of intermediate density lipoproteins and HDL. Recent studies have identified common polymorphisms in the hepatic lipase gene that are associated with low hepatic lipase activity and increased concentrations of large HDL. Association studies using these polymorphisms are elucidating the effects of variation in hepatic lipase activity on plasma lipoprotein concentrations and susceptibility to coronary atherosclerosis.

Lamellar lipoproteins uniquely contribute to hyperlipidemia in mice doubly deficient in apolipoprotein E and hepatic lipase

Remnants of triglyceride-rich lipoproteins containing apolipoprotein (apo) B-48 accumulate in apo E-deficient mice, causing pronounced hypercholesterolemia. Mice doubly deficient in apo E and hepatic lipase have more pronounced hypercholesterolemia, even though remnants do not accumulate appreciably in mice deficient in hepatic lipase alone. Here we show that the doubly deficient mice manifest a unique lamellar hyperlipoproteinemia, characterized by vesicular particles 600 A-1,300 A in diameter. As seen by negative-staining electron microscopy, these lipoproteins also contain an electron-lucent region adjacent to the vesicle wall, similar to the core of typical lipoproteins. Correlative chemical analysis indicates that the vesicle wall is composed of a 1:1 molar mixture of cholesterol and phospholipids, whereas the electron-lucent region appears to be composed of cholesteryl esters (about 12% of the particle mass). Like the spherical lipoproteins of doubly deficient mice, the vesicular particles contain apo B-48, but they are particularly rich in apo A-IV. We propose that cholesteryl esters are removed from spherical lipoproteins of these mice by scavenger receptor B1, leaving behind polar lipid-rich particles that fuse to form vesicular lipoproteins. Hepatic lipase may prevent such vesicular lipoproteins from accumulating in apo E-deficient mice by hydrolyzing phosphatidyl choline as scavenger receptor B1 removes the cholesteryl esters and by gradual endocytosis of lipoproteins bound to hepatic lipase on the surface of hepatocytes.

A simultaneous study of the metabolism of apolipoprotein B and albumin in nephrotic patients

BACKGROUND

The nephrotic syndrome is characterized by proteinuria, hypoalbuminemia and hyperlipidemia. Despite intensive research it is not clear at present what the causal links are between these pathological findings.

METHODS

Stable isotope labeled amino acid tracer kinetic analysis was used to simultaneously investigate the metabolism of four apolipoprotein B-containing lipoproteins (VLDL1, VLDL2, IDL and LDL) and albumin in seven patients with nephrotic syndrome and marked hypercholesterolemia, in two additional nephrotic patients with concomitant renal failure and mixed hyperlipidemia, and in a matched group of normolipidemic controls.

RESULTS

Increased concentrations of VLDL2, IDL and LDL were due to (a) impaired VLDL2 and IDL delipidation, (b) reduced LDL catabolism, and (c) a trend towards an increased rate of total apolipoprotein B production. The rate of fractional albumin elimination was three times higher in patients than in controls and the rate of albumin synthesis was increased by 45%. No correlations were detectable between rates of apolipoprotein B production and the rate of albumin synthesis.

CONCLUSIONS

The results of this study suggest that hyperlipidemia in nephrotic syndrome is predominantly the result of delayed lipoprotein delipidation and catabolism. There is no evidence that it is driven by a general increase of the rate of hepatic protein synthesis.

Hepatic lipase deficiency

Hepatic lipase (HL) is an enzyme that is made primarily by hepatocytes (and also found in adrenal gland and ovary) and hydrolyzes phospholipids and triglycerides of plasma lipoproteins. It is secreted and bound to the hepatocyte surface and readily released by heparin. It is a member of the lipase superfamily and is homologous to 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 postabsorptive triglyceride-poor HDL. HL plays a secondary role in the clearance of chylomicron remnants by the liver. Human post-heparin HL activity is inversely correlated with intermediate density lipoprotein cholesterol concentration only in subjects with a hyperlipidemia involving VLDL. This is consistent with intermediate-density lipoproteins being a substrate for HL. HDL cholesterol has been reported to be inversely correlated to HL activity, and on this basis it has been suggested that lowering HL would increase HDL cholesterol. However, the correlation could also be due to a common hormonal factor such as estrogen, which has been shown to up-regulate apoAI and HDL cholesterol and lower HL. A striking feature of severe deficiency of HL is the increase in HDL cholesterol and apolipoprotein AI and an approximately 10-fold increase in HDL triglyceride. Hyper-alpha-triglyceridemia is not a feature of antiatherogenic HDL. HL binds not only to heparan, but also to the LDL receptor-related protein. 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 Ser267 to Phe that results in an inactive enzyme and a mutation of Thr383 to Met that results in impaired secretion and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease. Recently, it has been reported that mutations affecting the structure of HL (e.g., T383M) are relatively frequent in the Finnish population. A C-to-T polymorphism in the promotor region of the HL gene is associated with lowered HL activity and less strongly with increased HDL cholesterol. In summary, there is a good understanding of what HL does in lipoprotein metabolism; however, there is little understanding of its physiological importance, that is, why HL does what it does. (ABSTRACT TRUNCATED)

Hepatic lipase affects both HDL and ApoB-containing lipoprotein levels in the mouse

Transgenic mice were created overproducing a range of human HL (hHL) activities (4-23-fold increase) to further examine the role of hepatic lipase (HL) in lipoprotein metabolism. A 5-fold increase in heparin releasable HL activity was accompanied by moderate (approx. 20%) decreases in plasma total and high density lipoprotein (HDL) cholesterol and phospholipid (PL) but no significant change in triglyceride (TG). A 23-fold increase in HL activity caused a more significant decrease in plasma total and HDL cholesterol, PL and TG (77%, 64%, 60%, and 24% respectively), and a substantial decrease in lipoprotein lipids amongst IDL, LDL and HDL fractions. High levels of HL activity diminished the plasma concentration of apoA-I, A-II and apoE (76%, 48% and 75%, respectively). In contrast, the levels of apoA-IV-containing lipoproteins appear relatively resistant to increased titers of hHL activity. Increased hHL activity was associated with a progressive decrease in the levels and an increase in the density of LpAI and LpB48 particles. The increased rate of disappearance of 125I-labeled human HDL from the plasma of hHL transgenic mice suggests increased clearance of HDL apoproteins in the transgenic mice. The effect of increased HL activity on apoB100-containing lipoproteins was more complex. HL-deficient mice have substantially decreased apoB100-containing low density lipoproteins (LDL) compared to controls. Increased HL activity is associated with a transformation of the lipoprotein density profile from predominantly buoyant (VLDL/IDL) lipoproteins to more dense (LDL) fractions. Increased HL activity from moderate (4-fold) to higher (5-fold) levels decreased the levels of apoB100-containing particles. Thus, at normal to moderately high levels in the mouse, HL promotes the metabolism of both HDL and apoB-containing lipoproteins and thereby acts as a key determinant of plasma levels of both HDL and LDL.

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.

Lipoprotein heterogeneity and apolipoprotein B metabolism

The apolipoprotein B containing lipoproteins VLDL, IDL, and LDL exhibit variation in their structure, function, and metabolism. These major lipoprotein classes can be fractionated into apparently discrete components by density gradient centrifugation or affinity chromatography. Examination of the behavior of subfractions in vivo reveals the presence of metabolic channels within the VLDL-LDL delipidation cascade so that the pedigree of a lipoprotein in part determines its metabolic fate. Evidence from VLDL and LDL apoB turnovers together with epidemiological data allows the construction of a quantitative model for the generation of small, dense LDL. This lipoprotein subspecies is one component of the dyslipidemic syndrome known as the atherogenic lipoprotein phenotype, a common disorder in those at risk for coronary heart disease. Understanding lipoprotein heterogeneity is an essential step in the further discovery of the pathogenesis of atherosclerosis and in the tailoring of pharmacologic treatment for subjects at risk.