HDL Apolipoprotein Research Articles

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.

Lack of enteral nutrition during critical illness is associated with profound decrements in biliary lipid concentrationsrted by the Dutch Organization for Scientific Research, project no. 902-23-097.rted by the Dutch Organization for Scientific Research, project no. 902-23-097.rted by the Dutch Organization for Scientific Research, project no. 902-23-097.

Food in the intestine drives the enterohepatic circulation of bile components. We investigated whether parenteral or enteral delivery of nutrients alters serum and biliary lipids in critically ill patients. Eight intensive care unit (ICU) patients who had received >/= 5 d of total parenteral nutrition (TPN) were compared with 8 ICU patients who had fasted for >/=5 d. Both groups were studied before and after 5 d of enteral nutrition (EN). Each patient served as his or her own control. Duodenal bile was analyzed for biliary lipid content and serum lipids were determined simultaneously. Duodenal bile samples from 18 healthy persons served as controls. Bile salt concentrations in all ICU patients were 17% of control values before EN (P < 0.005) and 34% of control values after 5 d of EN (P < 0.005). Phospholipid concentrations were 12% of control before EN (P < 0. 0005) but increased almost 4-fold after EN (P < 0.0005). Biliary cholesterol concentrations were 20% of control values before EN (P < 0.001) and did not improve afterward. No difference in bile composition was observed between fasted ICU patients and those who received TPN. The inverse correlation between the severity of illness and biliary lipid concentrations observed before EN disappeared with enteric stimulation. The low serum concentrations of HDL cholesterol and apolipoprotein A-I increased significantly with EN in all ICU patients. Lack of EN during critical illness was associated with profound decrements in biliary lipid concentrations that normalized partially after 5 d of EN. We hypothesize that loss of enteric stimulation in ICU patients impairs hepatic lipid metabolism.

Beneficial Effects of Fibrates on Apolipoprotein A-I Metabolism Occur Independently of Any Peroxisome Proliferative Response

In humans, fibrates are frequently used normolipidemic drugs. Fibrates act by regulating genes involved in lipoprotein metabolism via activation of the peroxisome proliferator-activated receptor-alpha (PPARalpha) in liver. In rodents, however, fibrates induce a peroxisome proliferation, leading to hepatomegaly and possibly hepatocarcinogenesis. Although this peroxisome proliferative response appears not to occur in humans, it remains controversial whether the beneficial effects of fibrates on lipoprotein metabolism can occur dissociated from such undesirable peroxisomal response. Here, we assessed the influence of fenofibrate on lipoprotein metabolism and peroxisome proliferation in the rabbit, an animal that, contrary to rodents and similar to humans, is less sensitive to peroxisome proliferators. First, we demonstrate that in normal rabbits, fenofibrate given at a high dose for 2 weeks does not influence serum concentrations or intestinal mRNA levels of the HDL apolipoprotein apoA-I. Therefore, the study was continued with human apoA-I transgenic rabbits that overexpress the human apoA-I gene under control of its homologous promoter, including its PPAR-response elements. In these animals, fenofibrate increases serum human apoA-I concentrations via an increased expression of the human apoA-I gene in liver. Interestingly, liver weight or mRNA levels and activity of fatty acyl-CoA oxidase, a rate-limiting and marker enzyme of peroxisomal beta-oxidation, remain unchanged after fenofibrate. Expression of the human apoA-I transgene in rabbit liver suffices to confer fibrate-mediated induction of serum apoA-I. Furthermore, these data provide in vivo evidence that the beneficial effects of fibrates on lipoprotein metabolism occur mechanistically dissociated from any deleterious activity on peroxisome proliferation and possibly hepatocarcinogenesis.

Regulation of cellular lipid metatbolism in macrophages by endogenous apo E and exogenous HDL and lipid-free apolipoproteins

Regulation of cellular lipid metatbolism in macrophages by endogenous apo E and exogenous HDL and lipid-free apolipoproteins

Pentadecanoic acid in serum as a marker for intake of milk fat: relations between intake of milk fat and metabolic risk factors

The fatty acid composition of the diet is known to be partially reflected by the fatty acid composition of serum lipids. We examined whether pentadecanoic acid (15:0) in serum lipids can be used as a marker for intake of milk fat, the major dietary source of 15:0. We also investigated the relations between intake of milk fat and cardiovascular disease risk factors. Sixty-two 70-y-old men completed 7-d dietary records. The intake of milk products was studied in relation to the proportions of 15:0 in serum cholesterol esters and phospholipids, as well as to the clinical characteristics of these men, by using Spearman's rank correlation. The proportions of 15:0 in serum cholesterol esters were positively related to butter intake (r = 0.36. P = 0.004) and to the total amount of fat from milk products (r = 0.46, P < 0.0001): 15:0 in phospholipids was related to the amount of fat from milk and cream (r = 0.34, P = 0.008) and to the total amount of fat from milk products (r = 0.34, P = 0.008). Inverse associations were found between intake of milk products and body mass index, waist circumference, LDL-HDL ratio, HDL triacylglycerols, and fasting plasma glucose, whereas relations to HDL cholesterol and apolipoprotein A-I tended to be positive. The results suggest that 15:0 in serum can be used as a marker for intake of milk fat. The explanation for the inverse associations between the intake of milk products and certain cardiovascular risk factors is not known.

Laminin interactions with the apoproteins of acute-phase HDL: preliminary mapping of the laminin binding site on serum amyloid A

During AA amyloidosis, the major basement membrane components, collagen type-IV (C-IV), entactin, laminin and perlecan codeposit both spatially and temporally with AA fibrils. Our previous work demonstrated that laminin, and collagen type-IV, can associate with high affinity to a mixture of mouse acute-phase serum amyloid A isoforms (apoSAA1, apoSAA2 andapoSAA3). However, laminin also bound to residual HDL from which apoSAAs were extracted. To characterize further laminin binding specificity for acute-phase HDL apolipoproteins, we have systematically isolated the acute-phase apoSAAs, apoA-I, apoA-II and the apoCs (I, II and III) by reverse phase high pressure liquid chromatography (RP-HPLC), and tested their laminin binding activities individually by ELISA. All the apoSAAs tested bound laminin saturably and with high affinity (Kd 2 nM). In addition, apoA-I also showed laminin binding activity (Kd a 4.6 nM). Specific binding for apoA-II and the apo-Cs was not detected. To localize the laminin binding site on apoSAA, we generated defined CNBr fragments of apoSAA I and apoSAA2, purified them by RP-HPLC, and tested their laminin binding activity by ELISA. A 53 residue peptide corresponding to residues 24-76 ofapoSAA2 had the highest laminin binding activity, followed by an 80 residue peptide corresponding to residues 24-103 of apoSAA1, both of which contain a 30 residue sequence that has changed little during evolution. In addition, a 7 residue peptide (residues 17-23), which is common to both apoSAA1 and apoSAA2, also had laminin binding activity. We postulate that laminin facilitates AA amyloidogenesis by sequestering apoSAA and providing a “surface” on which heparan sulfate-dependent fibrillogenic nucleation events can take place.

Severe atherosclerosis and hypoalphalipoproteinemia in the staggerer mouse, a mutant of the nuclear receptor RORalpha.

Hypoalphalipoproteinemia is the most common lipoprotein abnormality in patients with coronary artery disease, yet its causes are unknown. We show that the homozygous staggerer (sg/sg) mutant mouse, which carries a deletion within the nuclear receptor RORalpha gene, develops severe atherosclerosis when maintained on an atherogenic diet. In addition, sg/sg mice display a profound hypoalphalipoproteinemia, which is associated with decreased plasma levels of the major HDL proteins, apolipoprotein (apo) A-I and apoA-II. This decrease in HDL levels in sg/sg mice is due to lowered apoA-I gene expression in the intestine but not in the liver. ApoA-II gene expression is unaffected. These results suggest that the RORalpha gene contributes to the plasma HDL level and susceptibility to atherosclerosis.

Apolipoprotein A1 Gene Polymorphisms and Risk of Early Coronary Disease

Genetic studies have identified polymorphisms at the apolipoprotein (Apo) A1 gene associated with HDL cholesterol and apolipoprotein levels, and a relationship between the severity of coronary artery disease and polymorphisms at the 5′-end of Apo A1 has been also reported. This study was designed to examine the relationship between polymorphism at the Apo A1 gene and the risk of early coronary artery disease. Furthermore, the association of the polymorphism with the classical risk factors was analyzed. A total of 176 male patients (mean age 43 ± 5 years) diagnosed as having unstable angina (53 cases) or myocardial infarction (123 cases) were prospectively evaluated. Data referring to hypertension, diabetes and tobacco consumption were recorded. The levels of total cholesterol, HDL cholesterol, Apo A1 and B and triglycerides were determined. DNA was obtained from the 176 patients and from 200 controls. In order to determine the Apo A1 genotypes at two polymorphic sites (G/A at –75 bp, and C/T at +83 bp), DNA was PCR amplified and digested with MspI. The frequency of carriers of the rare allele at the –75 bp site (M1–) was 0.34 in cases and 0.24 in controls (p < 0.05). The frequencies of the M1– allele among patients with angina and myocardial infarction were 0.43 (p = 0.009, angina vs. controls) and 0.30, respectively. No significant association between this polymorphism and other cardiovascular risk factors was found. No difference in the frequencies for carriers of the rare allele at the +83-bp polymorphism (M2) was observed among patients with angina (0.08 vs. 0.07) or myocardial infarction (0.04 vs. 0.07), and no association between this polymorphism and tobacco, hypertension and diabetes was noted. Patients carrying the rare M2– allele had a lower concentration of total cholesterol compared to those without this allele (183 ± 29 vs. 223 ± 54, p < 0.04) and HDL cholesterol was also lower among patients carrying the M2– (26 ± 4 vs. 33 ± 9, p < 0.02). In our community male patients with a diagnosis of coronary artery disease and age less than 50 years showed a higher frequency of the M1– allele at the –75-bp site of the Apo A1 gene. There was a significant increase in the frequency of the M1– allele in patients with unstable angina and no association with risk factors. In the +83-bp polymorphism there was no difference in the allelelic frequencies or the risk factors, except for the HDL cholesterol and total cholesterol where the patients with the allele M2– had lower levels than those homozygous for the M2+.

Variable lipemic response to dietary soy protein in healthy, normolipemic men

We found previously that dietary soy protein, compared with casein, reduced plasma LDL cholesterol and increased HDL cholesterol concentrations in healthy women and men. However, there was considerable variation among individuals. The aim of this study was to characterize the lipoprotein responsiveness of individuals to examine whether different response patterns could be identified. Nine normolipemic men consumed 2 liquid-formula diets of identical composition except that the protein component was either soy protein or casein. After 1 mo of consuming each diet, the subjects’ plasma HDL cholesterol (P < 0.01) and apolipoprotein (apo) A-I (P < 0.05) concentrations were increased by the soy-protein diet whereas the ratio of LDL cholesterol to HDL cholesterol was decreased (P < 0.01); total cholesterol, triacylglycerol, LDL cholesterol, apo B and apo A-II were insignificantly affected. In 5 individuals, however, soy protein reduced mean LDL cholesterol, LDL2 cholesterol, and LDL2 apo B concentrations by 26% and plasma apo B by 16%, whereas HDL cholesterol increased by 11%. In 3 other individuals, soy protein increased mean HDL cholesterol by 17% and plasma apo A-I by 12%, but did not lower LDL. In 1 subject, soy protein decreased LDL2 cholesterol by 11% and increased plasma triacylglycerol by 40%, but neither HDL cholesterol nor apo A-I increased. We identified 3 types of lipemic responses to dietary soy protein involving a reduction in atherogenic LDL and increase in antiatherogenic HDL. In most subjects, the effects on both LDL and HDL were favorable, although fewer experienced either an increase in HDL or a decrease in LDL2. Am J Clin Nutr 1998; 68(suppl):1380S–4S.

In vivo evidence for increased apolipoprotein A-I catabolism in subjects with impaired glucose tolerance.

The in vivo kinetics of the HDL apolipoproteins (apo) A-I and A-II were studied in six subjects with impaired glucose tolerance (IGT) and six control subjects with normal glucose tolerance (NGT), using a stable isotope approach. During a 12-h primed constant infusion of L-[ring-13C6]-phenylalanine, tracer enrichment was determined in apoA-I and apoA-II from ultracentrifugally isolated HDL. The rates of HDL apoA-I and apoA-II production and catabolism were estimated using a one-compartment model-based analysis. Triglycerides were higher in IGT subjects (1.33 +/- 0.21 vs. 0.84 +/- 0.27 mmol/l, P < 0.05), but were within the normal range. HDL cholesterol and apoA-I levels were significantly lower in subjects with IGT (1.07 +/- 0.15 vs. 1.36 +/- 0.14 mmol/l, P < 0.05; 0.94 +/- 0.10 vs. 1.34 +/- 0.07 g/l, P < 0.01). In IGT subjects, HDL composition was significantly altered, characterized by an increase in HDL triglycerides (4.9 +/- 1.9 vs. 3.2 +/- 1.0%, P < 0.05) and HDL phospholipids (34.7 +/- 2.6 vs. 27.5 +/- 5.8%, P < 0.05) and a decrease in HDL cholesteryl esters (10.1 +/- 2.0 vs. 12.7 +/- 2.9%, P < 0.05) and HDL apoA-I (31.5 +/- 4.4 vs. 43.2 +/- 2.4%, P < 0.05). The mean fractional catabolic rate (FCR) of HDL apoA-I was significantly higher in IGT subjects (0.34 +/- 0.05 vs. 0.26 +/- 0.03 day(-1), P < 0.01), while the HDL apoA-I production rate (PR), as well as the PR and FCR of HDL apoA-II, showed no differences between the two groups. There were significant correlations between HDL apoA-I FCR and the following parameters: HDL apoA-I (r = -0.902, P < 0.001), HDL cholesterol (r = -0.797, P = 0.001), plasma triglycerides (r = 0.743, P < 0.01), HDL triglycerides (r = 0.696, P < 0.01), and cholesterol ester transfer protein activity (r = 0.646, P < 0.01). We observed a strong positive association between increased apoA-I catabolism and insulin (r = 0.765, P < 0.01) and proinsulin (r = 0.797, P < 0.01) concentrations. These data support the hypothesis that the decrease in HDL cholesterol and apoA-I levels in IGT is principally the result of an enhanced apoA-I catabolism. The latter seems to be an early metabolic finding in IGT even when other lipid parameters, especially plasma triglycerides, still appear to be not or only weakly affected.

Mechanism of action of fibrates on lipid and lipoprotein metabolism.

Treatment with fibrates, a widely used class of lipid-modifying agents, results in a substantial decrease in plasma triglycerides and is usually associated with a moderate decrease in LDL cholesterol and an increase in HDL cholesterol concentrations. Recent investigations indicate that the effects of fibrates are mediated, at least in part, through alterations in transcription of genes encoding for proteins that control lipoprotein metabolism. Fibrates activate specific transcription factors belonging to the nuclear hormone receptor superfamily, termed peroxisome proliferator-activated receptors (PPARs). The PPAR-alpha form mediates fibrate action on HDL cholesterol levels via transcriptional induction of synthesis of the major HDL apolipoproteins, apoA-I and apoA-II. Fibrates lower hepatic apoC-III production and increase lipoprotein lipase--mediated lipolysis via PPAR. Fibrates stimulate cellular fatty acid uptake, conversion to acyl-CoA derivatives, and catabolism by the beta-oxidation pathways, which, combined with a reduction in fatty acid and triglyceride synthesis, results in a decrease in VLDL production. In summary, both enhanced catabolism of triglyceride-rich particles and reduced secretion of VLDL underlie the hypotriglyceridemic effect of fibrates, whereas their effect on HDL metabolism is associated with changes in HDL apolipoprotein expression.

Dietary Cholesterol Affects Serum Lipids, Lipoproteins and LDL Metabolism in Cynomolgus Monkeys in a Dose-Dependent Manner

To examine the mechanism(s) underlying the cholesterolemic response to dietary cholesterol and saturated fatty acids, low density lipoprotein (LDL) metabolism was studied in two groups of cynomolgus monkeys fed diets containing 30 or 36% of total energy as fat. At each dietary fat level, the same group of monkeys was sequentially fed three dietary cholesterol concentrations as egg yolk in the following sequence: low (0.01 mg/kJ), medium (0.03 mg/kJ) and high (0.05 mg/kJ) for 30, 32 and 24 wk, respectively. Dietary polyunsaturated and monounsaturated fatty acids were the same in the two groups; the 6% difference in fat was due to the saturated fatty acids, 12:0 and 14:0. Serum total cholesterol, LDL cholesterol and LDL apolipoprotein B concentrations increased (P < 0.05) with dietary cholesterol in a dose-dependent manner in both fat groups. These elevations were the result of generally increasing LDL apolipoprotein B production rates, concomitant with reduced LDL apolipoprotein B fractional clearance at the high cholesterol intake. Serum HDL cholesterol and HDL apolipoprotein A-I concentrations were not affected in a consistent manner. These results demonstrate that cynomolgus monkeys are hyperresponsive to dietary cholesterol compared with humans, suggesting that this model may be useful in identifying metabolic and genetic predictors for hyperresponsiveness to dietary cholesterol in humans as well as assessing the metabolic heterogeneity of responses to dietary cholesterol.

Exercise training has little effect on HDL levels and metabolism in men with initially low HDL cholesterol

Low concentrations of high-density lipoprotein cholesterol (HDL-C) are a recognized risk factor for atherosclerotic cardiovascular disease. Exercise is often recommended to increase HDL-C, but the effect of exercise training on HDL levels and metabolism in subjects with low HDL concentrations is not well defined. The present study compared the HDL response to 12 months of supervised endurance exercise training without weight loss in 17 men aged 26–49 years with initially low (<40 mg/dl, N=7) or normal (>44 mg/dl, N=10) HDL-C levels. HDL-C levels and HDL apolipoprotein metabolism were assessed while the subjects consumed controlled diets before and after the year of training. Increases in total (5.1±2.8 versus 1.9±4.2 mg/dl, P=0.08) and HDL2 (3.8±2.9 versus 0.4±1.1 mg/dl, P=0.01) cholesterol were greater in men with normal initial HDL-C levels. Catabolic rates for HDL apolipoproteins decreased 7–14% and biological half-lives increased 10–15% after exercise training in subjects with normal HDL, but were unchanged in the low HDL-C group. HDL apolipoprotein synthetic rates were not consistently affected by exercise training in either group. Postheparin lipoprotein lipase activity increased 27%, the clearance rate of intravenous triglycerides increased 14%, and apolipoprotein B levels decreased 16% with training in subjects with normal HDL-C but were unchanged in the low HDL-C group. We conclude that the ability to increase HDL-C levels through endurance exercise training is limited in subjects with low initial HDL-C, possibly because exercise training in such subjects fails to alter triglyceride metabolism.

Effects of testosterone replacement on HDL subfractions and apolipoprotein A-I containing lipoproteins

Effects of testosterone replacement on HDL subfractions and apolipoprotein A-I containing lipoproteins

Plasma and lipoprotein lipid composition and hepatic antioxidant status in spontaneously hypertensive (SHR) and normotensive (WKY) rats

Plasma and lipoprotein lipid composition and endogenous hepatic antioxidant status were investigated in hypertensive, 14-week-old spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats fed a standard commercial rat chow. Total plasma calcium and magnesium concentrations were similar between both rat strains; however, systolic blood pressure in SHR was greater than in WKY at 13 weeks of age (197 +/- 12 vs. 132 +/- 14 mmHg; p < or = 0.05), confirming hypertension in SHR. Total plasma cholesterol and triacylglycerol concentrations were lower (p < or = 0.05) in SHR compared with WKY. A lower (p < 0.05) HDL cholesterol level in SHR plasma resulted in a higher LDL to HDL cholesterol ratio compared with WKY counterparts. No significant differences in the relative proportion of HDL apolipoprotein A-I fraction were observed between SHR and WKY. Both SHR VLDL and HDL triacylglycerol fractions were lower (p < 0.05) in SHR than WKY. Analysis of liver antioxidant enzyme activities showed no differences in rat liver superoxide dismutase (SOD), but lower (p < 0.05) liver glutathione peroxidase (GSH-Px) activity in SHR. However, liver glutathione (GSH) levels were similar in SHR and WKY counterparts. A possible compensatory effect to the oxidative status of SHR was suggested by the significant (p < 0.05) increase in both liver catalase (CAT) and glutathione reductase (GSSG-Red) activities. Despite these results, in vitro oxidative challenge studies with H2O2 demonstrated a greater susceptibility of liver to GSH depletion in the SHR, although no parallel change in thiobarbituric acid reactive substances (TBARS) production was observed. The comparatively lower plasma cholesterol observed in hypertensive SHR paralleled specific differences in liver catalase and glutathione redox antioxidant enzyme activities.

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 triglyc-erides 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 Thr3 83 to Met that results in impaired secretion and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipi-demia 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. All of the current evidence consistently suggests that HL deficiency is in a state of increased susceptibility to heart disease that requires the coexistence of an independent cause of hyperlipidemia for the development of premature CAD. Any detrimental effects of high levels of HL are speculative at this time.

Fractional esterification rate of HDL particles in patients with type 2 diabetes. Relation to coronary heart disease risk factors.

To study the fractional esterification rate of cholesterol on HDL particles (FERHDL) in adults with type 2 diabetes and assess its correlation with serum lipids and other coronary heart disease (CHD) risk factors. FERHDL was measured in 90 adult (57 men, 33 women) patients by an isotopic assay method involving several steps, including preparation of VLDL- and LDL-depleted plasma, labeling of the sample with a trace amount of tritiated cholesterol, separation of free and esterified cholesterol fractions by chromatography post incubation, and subsequent counting of radioactivity in the individual fractions. Male patients have higher FERHDL values than their female counterparts. When HDL cholesterol was controlled for in a multivariate regression analysis, the sex factor was not significant. There was a significant positive correlation between FERHDL and plasma total cholesterol (r = 0.32), triglycerides (r = 0.82), apolipoprotein B (apo B; r = 0.48), insulin (r = 0.46), BMI (r = 0.31), and waist-to-hip ratio (WHR; r = 0.50). There was a negative correlation between FERHDL and HDL cholesterol (r = -0.76) and apolipoprotein AI (r = -0.60). When both HDL cholesterol and triglycerides were controlled for, the only significant correlation was between FERHDL and BMI. Non-insulin-requiring type 2 diabetic patients have FERHDL, which correlated positively with triglycerides and negatively with HDL cholesterol. The positive correlation of FERHDL with serum insulin, WHR, total cholesterol, and apo B, but not that with BMI, loses its significance when HDL cholesterol and triglycerides are controlled. The sex difference between men and women in FERHDL also loses its significance when HDL cholesterol is controlled.

Phosphoproteins regulated by the interaction of high-density lipoprotein with human skin fibroblasts.

Interaction of HDL with cells activates protein kinase C (PKC), a process that may be important in stimulating efflux of excess cellular cholesterol. Here we report that HDL treatment of cholesterol-loaded fibroblasts increases 32P labeling of three acidic phosphoproteins. These phosphoproteins, called pp80, pp27, and pp18 based on apparent M(r) in kD, were also phosphorylated by acute treatment of cells with phorbol myristate acetate, suggesting that they are regulated in response to PKC activation. The HDL-stimulated phosphorylation of pp80 and pp18 was significant after only 30 seconds and was sustained for at least 30 and 120 minutes, respectively, while increased phosphorylation of pp27 was transient, reaching a maximum at 10 minutes. Both pp27 and pp18 were phosphorylated on serine/threonine residues, whereas pp80 was phosphorylated on serine/threonine and tyrosine residues. Immunoprecipitation studies suggested that pp80 is the myristoylated alanine-rich C kinase substrate protein, but the identities of pp27 and pp18 are unknown. HDL and trypsin-digested HDL stimulated phosphorylation of pp80 and pp27, while purified apoA-I, apoA-II, or apoE had no stimulatory effects, indicating that the active component in HDL was trypsin resistant and unlikely to be an apolipoprotein. Conversely, HDL, apoA-I, apoA-II, and apoE all stimulated pp18 phosphorylation, while trypsin-digested HDL had less effect, consistent with pp18's being responsive to HDL apolipoproteins. Treatment of cholesterol-depleted cells with apoA-I also stimulated phosphorylation of pp18, but only transiently. These results suggest that HDL interaction with cells activates diverse PKC-mediated pathways that target different phosphoproteins. Of these three phosphoproteins, only pp18 has a phosphorylation response consistent with its being involved in apolipoprotein-mediated lipid transport.

1.P.280 A common substitution lle405Val in the cholesteryl ester transfer protein (CETP) is associated with increased HDL cholesterol and apolipoprotein Al. Studies in a general population sample of 8,989 women and men

1.P.280 A common substitution lle405Val in the cholesteryl ester transfer protein (CETP) is associated with increased HDL cholesterol and apolipoprotein Al. Studies in a general population sample of 8,989 women and men

4.P.282 Effects of testosterone replacement on HDL subfractions and apolipoprotein A-I containing lipoproteins in hypogonadal men

4.P.282 Effects of testosterone replacement on HDL subfractions and apolipoprotein A-I containing lipoproteins in hypogonadal men