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Lipoprotein lipase and hepatic lipase

their relationship with HDL subspecies Lp(A-I) and Lp(A-I,A-II)
Open AccessPublished:June 01, 2003DOI:https://doi.org/10.1194/jlr.M300091-JLR200
      HDL subspecies Lp(A-I) and Lp(A-I,A-II) have different anti-atherogenic potentials. To determine the role of lipoprotein lipase (LPL) and hepatic lipase (HL) in regulating these particles, we measured these enzyme activities in 28 healthy subjects with well-controlled Type 1 diabetes, and studied their relationship with Lp(A-I) and Lp(A-I,A-II). LPL was positively correlated with the apolipoprotein A-I (apoA-I), cholesterol, and phospholipid mass in total Lp(A-I), and with the apoA-I in large Lp(A-I) (r ≥ 0.58, P ≥ 0.001). HL was negatively correlated with all the above Lp(A-I) parameters plus Lp(A-I) triglyceride (r ≥ −0.53, P ≤ 0.003). No correlation was detected between LPL and Lp(A-I,A-II). However, HL was inversely correlated with total Lp(A-I,A-II) phospholipid, and with large Lp(A-I,A-II) (r ≥ 0.50, P ≤ 0.006). Similar studies were performed with phospholipid transfer protein (PLTP). Only total Lp(A-I) triglyceride in women (not men) (r = 0.71, P = 0.009) was significantly correlated with PLTP activity. These observations indicate that LPL and HL play major roles in determining the level and composition of plasma Lp(A-I), particularly large Lp(A-I), but not with Lp(A-I,A-II) level.
      Furthermore, select correlations of LPL and/or HL with the apoA-I, cholesterol, and triglyceride of Lp(A-I) but not Lp(A-I,A-II) imply that the apoA-I and lipid of Lp(A-I) and Lp(A-I,A-II) are not fully equilibrated.
      Epidemiological studies performed in the past few decades have firmly established a negative association between human plasma HDL level and the incidence of coronary artery disease (CAD). Recent case-control and angiographic studies have found that this inverse relationship involves primarily species of certain sizes within HDL2 and HDL3 (
      • Johansson J.
      • Carlson L.A.
      • Landou C.
      • Hamsten A.
      High density lipoproteins and coronary atherosclerosis. A strong inverse relation with the largest particles is confined to normotriglyceridemic patients.
      ,
      • Cheung M.C.
      • Brown B.G.
      • Wolf A.C.
      • Albers J.J.
      Altered particle size distribution of apolipoprotein A-I-containing lipoproteins in subjects with coronary artery disease.
      ). Apolipoprotein A-I (apoA-I) and apoA-II are the major protein constituents of HDL. Two major populations of particles distinguishable by the presence or absence of apoA-II [Lp(A-I,A-II) and Lp(A-I)] exist in human HDL (
      • Cheung M.C.
      • Albers J.J.
      Characterization of lipoprotein particles isolated by immunoaffinity chromatography: particles containing A-I and A-II and particles containing A-I but no A-II.
      ). Both can be found in the HDL2 and HDL3 density subfractions (
      • James R.W.
      • Proudfoot A.
      • Pometta D.
      Immunoaffinity fractionation of high-density lipoprotein subclasses 2 and 3 using anti-apolipoprotein A-I and A-II immunosorbent gels.
      ,
      • Cheung M.C.
      • Segrest J.P.
      • Albers J.J.
      • Cone J.T.
      • Brouillette C.G.
      • Chung B.H.
      • Kashyap M.
      • Glasscock M.A.
      • Anantharamaiah G.M.
      Characterization of high density lipoprotein subspecies: structural studies by single vertical spin ultracentrifugation and immunoaffinity chromatography.
      ). They are catabolized at different rates (
      • Rader D.J.
      • Castro G.
      • Zech L.A.
      • Fruchart J.C.
      • Brewer H.B.
      In vivo metabolism of apolipoprotein A-I on high density lipoprotein particles LpA-I and LpA-I,A-II.
      ,
      • Tilly-Kiesi M.
      • Lichtenstein A.
      • Joven J.
      • Vilella E.
      • Cheung M.C.
      • Carrasco W.V.
      • Ordovas J.M.
      • Dolinikowski G.
      • Schaefer E.J.
      Impact of gender on the metabolism of apolipoprotein A-I in HDL subclasses LpAI and LpAI:AII in older subjects.
      ), and have been shown to respond differently to pharmacological (
      • Atmah R.F.
      • Shepherd J.
      • Packard C.J.
      Subpopulations of apolipoprotein A-I in human high density lipoproteins. Their metabolic properties and response to drug therapy.
      ,
      • Cheung M.C.
      • Zhao X.Q.
      • Davis J.W.
      • Albers J.J.
      • Chait A.
      • Brown B.G.
      Antioxidant supplements block the response of HDL to simvastatin-niacin therapy in patients with coronary artery disease and low HDL.
      ), dietary (
      • Delplanque B.
      • Richard J.L.
      • Jacotot B.
      Influence of diet on the plasma levels and distribution of apoA-I-containing lipoprotein particles.
      ,
      • Cheung M.C.
      • Lichtenstein A.H.
      • Schaefer E.J.
      Effects of a diet restricted in saturated fatty acids and cholesterol on the composition of apolipoprotein A-I-containing lipoprotein particles in the fasting and fed states.
      ), and hormonal (
      • Cheung M.C.
      • Walden C.E.
      • Knopp R.H.
      Comparison of the effects of triphasic oral contraceptives with desogestrel or levonorgestrel on apolipoprotein A-I-containing high-density lipoprotein particles.
      ,
      • Brinton E.A.
      Oral estrogen replacement therapy in postmenopausal women selectively raises levels and production rates of lipoprotein A-I and lowers hepatic lipase activity without lowering the fractional catabolic rate.
      ) perturbations, suggesting that they are distinct metabolic entities. Insofar as there is evidence that Lp(A-I) and Lp(A-I,A-II) and their components differ in their antiatherogenic potential and functional properties (
      • Puchois P.
      • Kandoussi A.
      • Fievet P.
      • Fourier J.L.
      • Bertrand M.
      • Koren E.
      • Fruchart J.C.
      Apolipoprotein A-I-containing lipoproteins in coronary artery disease.
      ,
      • Decossin C.
      • Castro G.
      • Derudas B.
      • Bertrand M.
      • Fruchart J-C.
      • Fievet C.
      Subclasses of LpA-I in coronary artery disease: distribution and cholesterol efflux ability.
      ,
      • Mowri H-O.
      • Patsch W.
      • Smith L.C.
      • Gotto A.M.
      • Patsch J.R.
      Different reactivities of high density lipoprotein 2 subfractions with hepatic lipase.
      ), and that CAD is more strongly associated with abnormalities in HDL particle size than with low HDL cholesterol level (
      • Johansson J.
      • Carlson L.A.
      • Landou C.
      • Hamsten A.
      High density lipoproteins and coronary atherosclerosis. A strong inverse relation with the largest particles is confined to normotriglyceridemic patients.
      ,
      • Cheung M.C.
      • Brown B.G.
      • Wolf A.C.
      • Albers J.J.
      Altered particle size distribution of apolipoprotein A-I-containing lipoproteins in subjects with coronary artery disease.
      ), it is important to know how their level, composition, and particle size profile are regulated.
      There are a number of proteins currently believed to play important roles in HDL metabolism. These include lipoprotein lipase (LPL), hepatic lipase (HL), plasma phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), and lecithin cholesterol acyltransferase (LCAT) [reviewed in refs. (
      • Eisenberg S.
      High density lipoprotein metabolism.
      ,
      • Huuskonen J.
      • Olkkonen V.M.
      • Jauhiainen M.
      • Ehnholm C.
      The impact of phospholipid transfer protein (PLTP) on HDL metabolism.
      )]. In this study, we focus on three of them: LPL, HL, and PLTP, and their relationship with HDL particles with and without apoA-II. LPL has been consistently shown to be positively, and HL inversely, associated with HDL2 (
      • Kuusi T.
      • Ehnholm C.
      • Viikari J.
      • Harkonen R.
      • Vartiainen E.
      • Puska P.
      • Taskinen M-R.
      Postheparin plasma lipoprotein and hepatic lipase are determinants of hypo- and hyperalphalipoproteinemia.
      ,
      • Nevin D.N.
      • Zambon A.
      • Furlong C.E.
      • Richter R.J.
      • Humbert R.
      • Hokanson J.E.
      • Brunzell J.D.
      Paraoxonase genotypes, lipoprotein lipase activity, and HDL.
      ,
      • Murdoch S.J.
      • Carr M.C.
      • Hokanson J.E.
      • Brunzell J.D.
      • Albers J.J.
      PLTP activity in premenopausal women. Relationship with lipoprotein lipase, HDL, LDL, body fat, and insulin resistance.
      ,
      • Kahri J.
      • Per-Henrik G.
      • Viberti G.
      • Elliott T.
      • Taskinen M-R.
      Regulation of apolipoprotein A-1-containing lipoproteins in IDDM.
      ,
      • Frenais R.
      • Nazih H.
      • Ouguerram K.
      • Maugeais C.
      • Zair Y.
      • Bard J.M.
      • Charbonnel B.
      • Magot T.
      • Krempf M.
      In vivo evidence for the role of lipoprotein lipase activity in the regulation of apolipoprotein A1 metabolism: a kinetic study in control subjects and patients with type 2 diabetes mellitus.
      ,
      • De Oliveira e Silva E.
      • Kong M.
      • Han Z.
      • Starr C.
      • Kass E.M.
      • Juo S.H.
      • Foster D.
      • Dansky H.M.
      • Merkel M.
      • Cundey K.
      • Brinton E.A.
      • Breslow J.L.
      • Smith J.D.
      Metabolic and genetic determinants of HDL metabolism and hepatic lipase activity in normolipidemic females.
      ). We hypothesized that both are major but opposing determinants of large Lp(A-I) particles. In vitro HL has been shown to convert the Lp(A-I) and Lp(A-I,A-II) in HDL2 to smaller particles (
      • Mowri H-O.
      • Patsch W.
      • Smith L.C.
      • Gotto A.M.
      • Patsch J.R.
      Different reactivities of high density lipoprotein 2 subfractions with hepatic lipase.
      ), and PLTP has been shown to promote the conversion of both Lp(A-I) and Lp(A-I,A-II) into larger and smaller particles (
      • Albers J.J.
      • Wolfbauer G.W.
      • Cheung M.C.
      • Day J.R.
      • Ching A.F.T.
      • Lok S.
      • Tu A-Y.
      Functional expression of human and mouse plasma phospholipid transfer protein: effect of recombinant and plasma PLTP on HDL subspecies.
      ). Thus, we also hypothesized that both HL and PLTP participate in regulating the particle size of these two populations of HDL. To test these hypotheses, we measured the postheparin plasma LPL and HL activities and plasma PLTP activity in 28 healthy subjects with well-controlled Type 1 diabetes, and determined the association of their activities with Lp(A-I) and Lp(A-I,A-II) level and composition. This report presents our findings, and discusses their metabolic implications on these apo-specific HDL particles.

      METHODS

      Subjects and blood samples

      The subjects with well-controlled Type 1 diabetes (16 men and 12 women) were 28 of the 61 participants of the Diabetes Control and Complications Trials (DCCT) recruited between 1983 and 1989 who were enrolled in the Seattle cohort of the Epidemiology of Diabetes Intervention and Complication (EDIC) trial at the time of this study. EDIC is an ongoing observational follow-up study of the DCCT, a clinical trial conducted between 1982 and 1993 that involved 1,441 subjects with Type 1 diabetes for 1 to 15 years at study entry. The DCCT cohort was randomly assigned to intensive or conventional diabetes treatment and followed for 6.5 years on average (
      • The Diabetes Control and Complications Group
      The Diabetes Control and Complications Trial: design and methodologic considerations for the feasibility phase.
      ). Subjects were excluded from the DCCT at baseline if they had total cholesterol level greater than three standard deviations above the mean for sex and age as defined by the Lipid Research Clinics Population Studies (
      • United States Department of Health and Human Services
      Department of Health and Human Services: The Prevalence Study.
      ), a calculated LDL above 190 mg/dl, a cardiovascular abnormality (such as major electrocardiogram abnormalities, peripheral vascular disease symptoms, a clinical history of cardiovascular disease, or hypertension), or a body weight >30% above ideal body weight as defined by the 1983 Metropolitan Life Insurance norms (
      • Metropolitan Life Insurance Company
      Metropolitan height and weight tables.
      ). At the end of the DCCT in 1993, all surviving participants were eligible for EDIC. The study was approved by the Human Subject Review Committee of the University of Washington, and informed consent was obtained from all subjects before entering the study. Two blood samples were collected from each subject after a 12–14 h fast, one before and one 10 min after a heparin bolus of 60 U/kg was given. Preheparin and postheparin blood samples were collected into EDTA-containing and lithium heparin-containing tubes, respectively. The blood samples were promptly centrifuged at 4°C at 3,000 rpm for 15 min to obtain plasma. Fresh preheparin plasma was used for lipid and apolipoprotein determination, and for HDL particle isolation, or immediately frozen at −70°C for the measurement of PLTP activity. Postheparin plasma samples that were promptly frozen at −70°C were used for measuring lipase activities.

      HDL particles

      The HDL particles containing both apoA-I and apoA-II [Lp(A-I, A-II)] and those containing apoA-I but not A-II [Lp(A-I)] were isolated from fresh plasma samples by established sequential dextran sulfate, anti-A-II, and anti-A-I chromatography (
      • Cheung M.C.
      • Albers J.J.
      Characterization of lipoprotein particles isolated by immunoaffinity chromatography: particles containing A-I and A-II and particles containing A-I but no A-II.
      ,
      • Cheung M.C.
      • Wolf A.C.
      In vitro transformation of apoA-I-containing lipoprotein subpopulations: role of lecithin:cholesterol acyltransferase and apoB-containing lipoproteins.
      ). The distribution of plasma apoA-I between Lp(A-I) and Lp(A-I,A-II) was determined by quantifying the apoA-I in these lipoproteins with proper adjustment for recovery (
      • Cheung M.C.
      • Brown B.G.
      • Wolf A.C.
      • Albers J.J.
      Altered particle size distribution of apolipoprotein A-I-containing lipoproteins in subjects with coronary artery disease.
      ,
      • Cheung M.C.
      • Albers J.J.
      Characterization of lipoprotein particles isolated by immunoaffinity chromatography: particles containing A-I and A-II and particles containing A-I but no A-II.
      ). The HDL size species were separated by nondenaturing gradient polyacrylamide gel electrophoresis using precast 4–30% gels (Alamo Gel, Inc., San Antonio, TX), and thyroglobulin, apoferritin, lactate dehydrogenase, and bovine albumin as calibration proteins. Gels were visualized for proteins with Coomassie Blue G-250, and scanned with a laser densitometer. The LKB 2400 GelScan XL® software was used to integrate and calculate the percent distribution of Lp(A-I) and Lp(A-I,A-II) in four size intervals: small, 7.0–8.2 nm; medium, 8.2–9.2 nm; large, 9.2–11.2 nm; and very large, 11.2–17.0 nm Stokes diameter as described (
      • Cheung M.C.
      • Brown B.G.
      • Wolf A.C.
      • Albers J.J.
      Altered particle size distribution of apolipoprotein A-I-containing lipoproteins in subjects with coronary artery disease.
      ). These size intervals were established based on the clustering of particles seen in healthy, normolipidemic subjects (
      • Cheung M.C.
      • Wolf A.C.
      • Brunzell J.D.
      Metabolic pathways of HDL subpopulations: physiological implications of in vitro observations.
      ). The concentration of apo(A-I+A-II) and apoA-I in HDL particles with and without apoA-II, respectively, in each size interval was calculated by multiplying the plasma concentration of apoA-I and apoA-II associated with these particles by the relative percent of particles in each size interval.

      Postheparin lipase activity

      Postheparin plasma samples that had been immediately frozen and stored at −70°C were used for the determination of total lipolytic activity as previously described (
      • Iverius P.H.
      • Brunzell J.D.
      Human adipose tissue lipoprotein lipase: changes with feeding and relation to postheparin plasma enzyme.
      ). Glycerol tri[1-14C]oleate (Amersham Pharmacia Biotech, Arlington Heights, IL) -labeled triglyceride, lecithin, and albumin were incubated with postheparin plasma for 60 min at 37°C, and the liberated 14C-labeled free fatty acids were then extracted and counted. LPL activity was calculated as the lipolytic activity removed from plasma by incubation with a specific monoclonal antibody against LPL (5D2), and HL activity was determined as the activity remaining after incubation with the LPL antibody. Enzyme activity is expressed as nanomoles of free fatty acid released per min/ml plasma at 37°C. For each assay, a bovine milk LPL standard was used to correct for interassay variation, and a human postheparin plasma standard was included to monitor interassay variation.

      PLTP activity

      The plasma phospholipid transfer activity mediated by PLTP was determined by measuring the transfer of [14C]phosphatidylcholine from phospholipid liposomes to HDL using an established radioassay (
      • Cheung M.C.
      • Wolfbauer G.
      • Albers J.J.
      Plasma phospholipid mass transfer rate: relationship to plasma phospholipid and cholesteryl ester transfer activities and lipid parameters.
      ). Three plasma samples, stored at −70°C until use, were included in each assay to control for interassay variation.

      Other analytical methods

      Standard enzymatic methods performed on a Hitachi 917 automatic analyzer (Roche Diagnostics, Indianapolis, IN) were used to determine fasting blood glucose and cholesterol, triglyceride, and phospholipid in plasma or fractionated lipoproteins (
      • Warnick G.R.
      Enzymatic methods for the quantification of lipoprotein lipids.
      ). Plasma HDL cholesterol concentration was measured from the supernatant fraction, following precipitation by dextran sulfate and magnesium chloride (
      • Warnick G.R.
      • Benderson J.
      • Albers J.J.
      Dextran sulfate-Mg2+ precipitation procedure for quantitation of high-density lipoprotein cholesterol.
      ). Cholesterol in VLDLs and LDLs was calculated using the Friedewald equation (
      • Friedewald W.T.
      • Levy R.I.
      • Fredrickson D.S.
      Estimation of the concentration of low density lipoprotein cholesterol in plasma without use of the ultracentrifuge.
      ). ApoA-I, apoA-II, and apoB were measured with a Behring nephelometer using Behring reagents (Behring Diagnostic, Inc., Somerville, NJ) and calibrated with the Northwest Lipid Research Laboratories calibrator. The concentration of plasma Lp(A-I) and Lp(A-I,A-II) was determined by measuring the apoA-I, apoA-II, and lipid in these particles isolated as described above. Glycosylated hemoglobin was measured as hemoglobin A1c (HbA1c) using ion-exchange HPLC (BioRad).

      Statistical analysis

      The Mann-Whitney U test was used to compare the various measured parameters between men and women. Spearman rank order correlation analyses were used to determine the relationship between LPL, HL, or PLTP activity and lipoproteins. All analyses were carried out using SPSS statistical software. In view of multiple comparison, only P values of ≤0.01 (two-tailed) are reported as significant unless otherwise indicated.

      RESULTS

      Subject characteristics

      The characteristics of the 28 subjects are summarized in Table 1. They were between 33 and 54 years old, with a mean age of 44.6 years, mean HbA1c of 7.6%, fasting glucose of 154 mg/dl, and body mass index (BMI) of 26.4 kg/m2. No gender differences were observed in these parameters.
      TABLE 1Subject characteristics
      All

      (n = 28)
      Women

      (n = 12)
      Men

      (n = 16)
      Age, years44.6 ± 5.644.3 ± 5.444.9 ± 5.9
      Body mass index, kg/m226.4 ± 3.425.9 ± 3.226.7 ± 3.6
      Hemoglobin A1c, %7.6.±1.07.7 ± 1.07.6 ± 1.1
      Fasting glucose, mg/dl154 ± 78136 ± 48166 ± 94
      Cholesterol, mg/dl172 ± 32177 ± 26169 ± 37
      Triglyceride, mg/dl82.4 ± 67.168.8 ± 38.292.6 ± 82.3
      ApoB, mg/dl80.3 ± 20.477.4 ± 18.782.5 ± 22.0
      LDL cholesterol, mg/dl98.5 ± 25.496.3 ± 20.6100.1 ± 29.0
      ApoA-I, mg/dl143 ± 28161 ± 25129 ± 21
      P ≤ 0.001.
      HDL2 cholesterol, mg/dl12.9 ± 6.816.7 ± 5.910.1 ± 6.2
      P ≤ 0.01.
      HDL3 cholesterol, mg/dl44.0 ± 10.250.2 ± 8.339.4 ± 9.1
      P ≤ 0.005.
      HDL triglyceride, mg/dl13.3 ± 10.816.0 ± 9.811.2 ± 11.3
      LPL activity, nmol/ml/min380 ± 120417 ± 114353 ± 121
      HL activity, nmol/ml/min324 ± 158245 ± 54384 ± 184
      P ≤ 0.01.
      PLTP activity, μmol/ml/h14.6 ± 1.714.6 ± 2.014.6 ± 1.5
      HL, hepatic lipase; LPL, lipoprotein lipase; PLTP, phospholipid transfer protein. Numbers are mean ± SD. Values denote significant difference between men and women by Mann-Whitney U test.
      a P ≤ 0.001.
      b P ≤ 0.01.
      c P ≤ 0.005.

      Plasma lipid and HDL particles

      All subjects had normal plasma cholesterol, with the highest being 218 mg/dl. All but two men had plasma triglyceride <180 mg/dl. Men and women had comparable total cholesterol, triglyceride, LDL, and apoB. However, men had significantly lower HDL2 and HDL3 cholesterol (Table 1). They also had lower plasma apoA-I (Table 1) and fewer Lp(A-I) particles (Table 2), particularly those in the medium-(8.2–9.2 nm) and large-(9.2–11.2 nm) sized range.
      TABLE 2Particle size profiles of Lp(A-I) and Lp(A-I,A-II)
      All

      (n = 28)
      Women

      (n = 12)
      Men

      (n = 16)
      mean ± SD
      Lp(A-I) particles
       ApoA-I (mg/dl)54.3 ± 21.269.6 ± 15.242.9 ± 17.6
      P ≤ 0.001.
       Size species (mg/dl)
      Calculated as apoA-I and apo(A-I+A-II) in Lp(A-I) and Lp(A-I,A-II), respectively, in each size interval.
      Small (7.0–8.2 nm)10.6 ± 3.511.6 ± 4.89.8 ± 1.9
      Medium (8.2–9.2 nm)14.4 ± 6.620.0 ± 5.410.2 ± 3.7
      P ≤ 0.001.
      Large (9.2–11.2 nm)23.2 ± 13.830.9 ± 10.217.4 ± 13.7
      P ≥ 0.005.
      Very large (11.2–17.0 nm)6.2 ± 2.17.1 ± 1.65.5 ± 2.2
      Lp(A-I,A-II) particles
       ApoA-I (mg/dl)87.2 ± 13.690.0 ± 16.085.1 ± 11.5
       ApoA-II (mg/dl)32.5 ± 5.033.7 ± 6.331.5 ± 3.6
       Size species (mg/dl)
      Calculated as apoA-I and apo(A-I+A-II) in Lp(A-I) and Lp(A-I,A-II), respectively, in each size interval.
      Small (7.0–8.2 nm)37.1 ± 14.430.9 ± 11.541.7 ± 15.0
      Medium (8.2–9.2 nm)53.4 ± 12.359.2 ± 14.449.0 ± 8.5
      Large (9.2–11.2 nm)22.1 ± 8.426.1 ± 6.019.1 ± 8.9
      P ≤ 0.01.
      Very large (11.2–17.0 nm)7.1 ± 2.47.4 ± 2.56.9 ± 2.4
      Values denote significant difference between men and women by Mann-Whitney U test.
      a P ≤ 0.001.
      b P ≥ 0.005.
      c P ≤ 0.01.
      d Calculated as apoA-I and apo(A-I+A-II) in Lp(A-I) and Lp(A-I,A-II), respectively, in each size interval.
      Although plasma Lp(A-I,A-II) levels were similar in men and women, differences in the size profile of these particles were observed. Men had significantly fewer large (9.2–11.2 nm) particles (P = 0.007), and tended to have more small particles with Stokes diameter <8.2 nm (P = 0.03) (Table 2).

      Postheparin lipolytic activities and lipoproteins

      Postheparin plasma LPL and HL activities of the subjects are shown in Table 1. Men and women had comparable LPL activity, but men had significantly higher HL activity. Neither activity had any significant correlation with total or non-HDL cholesterol and triglyceride, and neither had any significant association with HbA1c, a measure of glycemic control (data not shown). As expected, LPL activity was positively associated (r = 0.59, P = 0.001) and HL activity was negatively associated (r = −0.50, P = 0.007) with HDL cholesterol. The correlations between these lipolytic activities and HDL particles are shown in Table 3. Both activities were strongly associated with the amounts of apoA-I, cholesterol, and phospholipid in Lp(A-I), and with the level of these particles with Stokes diameter >9.2 nm, but only HL activity demonstrated a significant association with Lp(A-I) triglyceride. All correlations were positive with LPL (r between 0.51 and 0.62, P ≤ 0.006) and negative with HL (r between −0.48 and −0.56, P ≤ 0.01). A significant inverse relationship was also detected between HL activity and Lp(A-I,A-II) phospholipid, as well as the amounts of these particles in the 9.2–11.2 nm size interval. In contrast, no correlation was found between LPL activity and the lipid and size profile of Lp(A-I,A-II). Furthermore, no relationship was detected between the two major protein components of Lp(A-I,A-II) and the lipases.
      TABLE 3Correlations between post heparin plasma lipolytic activities and HDL particle composition and size
      LPL ActivityHL Activity
      Lp(A-I)Lp(A-I,A-II)Lp(A-I)Lp(A-I,A-II)
      (n = 28)rPrPrPrP
      Particle protein and lipid (mg/dl)
       ApoA-I0.600.0010.180.368−0.560.002−0.160.424
       ApoA-II0.030.880−0.190.339
       Cholesterol0.62<0.0010.410.030−0.550.003−0.380.048
       Phospholipid0.580.0010.220.265−0.550.002−0.500.006
       Triglyceride0.200.299−0.370.053−0.530.003−0.160.405
      Size species (mg/dl)
      Correlation was performed with the mass of apoA-I and apo(A-I + A-II) in Lp(A-I) and Lp(A-I,A-II), respectively.
       Small (7.0–8.2 nm)0.070.707−0.080.692−0.210.2930.300.119
       Medium (8.2–9.2 nm)0.410.0310.170.388−0.440.019−0.220.267
       Large (9.2–11.2 nm)0.600.0010.360.062−0.560.002−0.69<0.001
       Very large (11.2–17.0 nm)0.510.0060.220.252−0.480.010−0.280.146
      Numbers in bold type denote significant correlation at P ≤ 0.01.
      a Correlation was performed with the mass of apoA-I and apo(A-I + A-II) in Lp(A-I) and Lp(A-I,A-II), respectively.

      PLTP activity and lipoproteins

      Recently, we found a clear gender difference in the association of PLTP activity with lipoproteins and metabolic parameters in a population of 134 subjects (
      • Cheung M.C.
      • Knopp R.H.
      • Retzlaff B.
      • Kennedy H.
      • Wolfbauer G.
      • Albers J.J.
      Association of plasma phospholipid transfer protein activity with IDL and buoyant LDL: impact of gender and adiposity.
      ). In view of this, correlation analysis between PLTP and lipoproteins was performed with all 28 subjects as well as with the gender subgroups. In the group of all subjects, PLTP was significantly and positively correlated with total and LDL cholesterol (r = 0.52, P = 0.004 and r = 0.48, P = 0.01, respectively) (Table 4). Gender subgroup analysis revealed that these correlations were statistically significant only in women. Furthermore, in the women, PLTP activity was also positively associated with BMI (r = 0.76, P = 0.005), apoB (r = 0.72, P = 0.008), Lp(A-I) triglyceride (r = 0.71, P = 0.009), and the amounts of small (<8.2 nm) Lp(A-I) (r = 0.68, P = 0.015) and small Lp(A-I,A-II) particles (r = 0.66, P = 0.02). No correlation was observed between PLTP and age, glycosylated hemoglobin, total A-I, A-II, HDL, HDL2, and HDL3 cholesterol in either men or women or the entire group.
      TABLE 4Gender-specific correlations between plasma PLTP activity and metabolic or lipoprotein parameters
      All

      (n = 28)
      Women

      (n = 12)
      Men

      (n = 16)
      rPrPrP
      Body mass index0.340.0790.760.005 0.000.996
      Fasting glucose0.220.2780.610.047 0.010.965
      Cholesterol (CH)0.520.0040.680.016 0.270.305
      Triglyceride (TG)0.210.2890.470.124 0.000.996
      ApoB0.350.0640.720.008−0.0290.914
      LDL CH0.480.0100.720.008 0.290.282
      TG in Lp(A-I)0.330.0880.710.009 0.000.991
      TG in Lp(A-I,A-II)0.210.2800.440.152 0.010.978
      Small Lp(A-I)

        (7.0–8.2 nm)
      0.400.0350.680.015 0.120.668
      Small Lp(A-I,A-II)

        (7.0–8.2 nm)
      0.040.8580.660.020−0.390.140
      Units of parameters are listed in Table 1. P ≤ 0.02 are in bold font.

      DISCUSSION

      LPL is the key enzyme that hydrolyzes core triglyceride in circulating chylomicrons and VLDLs [reviewed in refs. (
      • Brunzell J.D.
      • Deeb S.S.
      Familial lipoprotein lipase deficiency, apo CII deficiency and hepatic lipase deficiency.
      ,
      • Goldberg I.J.
      Lipoprotein lipase and lipolysis: central roles in lipoprotein metabolism and atherogenesis.
      )]. Although the transfer of surface protein and lipid from chylomicron and VLDL remnant particles to HDL induced by LPL hydrolysis of triglyceride is the accepted mechanism behind the positive association between LPL and HDL, the extent to which these lipoprotein surface remnants affect the level and composition of Lp(A-I) and Lp(A-I,A-II) in vivo is not known. Our observation that LPL activity is significantly and positively associated with the apoA-I, cholesterol, and phospholipid of Lp(A-I) but not of Lp(A-I,A-II) suggests that HDL particles without apoA-II may be the preferential acceptors of surface remnants of triglyceride-rich lipoproteins. Consistent with this, LPL activity is positively and significantly associated with the amounts of large (>9.2 nm) Lp(A-I) particles but not with large Lp(A-I,A-II) particles. Thus these data provide evidence for the first time that the efficiency of triglyceride hydrolysis by LPL results in greater impact (direct or indirect) on the level, composition, and particle size of Lp(A-I) than on those of Lp(A-I,A-II).
      HL can hydrolyze both triglyceride and phospholipids. Its physiological substrates were originally believed to be the core remnants of triglyceride-rich lipoproteins and LDL [reviewed in refs. (
      • Brunzell J.D.
      • Deeb S.S.
      Familial lipoprotein lipase deficiency, apo CII deficiency and hepatic lipase deficiency.
      ,
      • Taskinen M-R.
      • Kuusi T.
      Enzymes involved in triglyceride hydrolysis.
      )]. However, the increase in HDL level and size in HL deficiency (
      • Brunzell J.D.
      • Deeb S.S.
      Familial lipoprotein lipase deficiency, apo CII deficiency and hepatic lipase deficiency.
      ,
      • Taskinen M-R.
      • Kuusi T.
      Enzymes involved in triglyceride hydrolysis.
      ,
      • Zambon A.
      • Deeb S.S.
      • Hokanson J.E.
      • Brown B.G.
      • Brunzell J.D.
      Common variants in the promoter of the hepatic lipase gene are associated with lower levels of hepatic lipase activity, buoyant LDL, and higher HDL2 cholesterol.
      ) and the inverse association between HL and HDL2 consistently observed in normolipidemic and dyslipidemic individuals (
      • Kuusi T.
      • Ehnholm C.
      • Viikari J.
      • Harkonen R.
      • Vartiainen E.
      • Puska P.
      • Taskinen M-R.
      Postheparin plasma lipoprotein and hepatic lipase are determinants of hypo- and hyperalphalipoproteinemia.
      ,
      • Murdoch S.J.
      • Carr M.C.
      • Hokanson J.E.
      • Brunzell J.D.
      • Albers J.J.
      PLTP activity in premenopausal women. Relationship with lipoprotein lipase, HDL, LDL, body fat, and insulin resistance.
      ,
      • Kahri J.
      • Per-Henrik G.
      • Viberti G.
      • Elliott T.
      • Taskinen M-R.
      Regulation of apolipoprotein A-1-containing lipoproteins in IDDM.
      ,
      • De Oliveira e Silva E.
      • Kong M.
      • Han Z.
      • Starr C.
      • Kass E.M.
      • Juo S.H.
      • Foster D.
      • Dansky H.M.
      • Merkel M.
      • Cundey K.
      • Brinton E.A.
      • Breslow J.L.
      • Smith J.D.
      Metabolic and genetic determinants of HDL metabolism and hepatic lipase activity in normolipidemic females.
      ,
      • Zambon A.
      • Deeb S.S.
      • Hokanson J.E.
      • Brown B.G.
      • Brunzell J.D.
      Common variants in the promoter of the hepatic lipase gene are associated with lower levels of hepatic lipase activity, buoyant LDL, and higher HDL2 cholesterol.
      ) indicate that HL is a key enzyme in processing the triglyceride and phospholipid in these lipoproteins, especially in large HDLs. What is not known, however, is whether HL hydrolyzes the phospholpid and triglyceride in Lp(A-I) and Lp(A-I,A-II) to the same extent, and what the ultimate impact of HL on these apo-specific HDL particles is in vivo.
      Mowri et al. reported that the lipolysis of triglycerides and phospholipid by HL was higher in HDL2 particles with apoA-II than in those without apoA-II in vitro. Also, the different lipolytic rates in HDL2 particles with and without apoA-II correlated with the size reduction of these HDL particles (
      • Mowri H-O.
      • Patsch W.
      • Smith L.C.
      • Gotto A.M.
      • Patsch J.R.
      Different reactivities of high density lipoprotein 2 subfractions with hepatic lipase.
      ). On the basis of these observations, we had expected that HL would show a stronger correlation with the phospholipid, triglyceride, and size profile of Lp(A-I,A-II) than with those of Lp(A-I). Our data that HL activity was significantly correlated with the triglyceride of Lp(A-I) but not with that of Lp(A-I,A-II) are inconsistent with that report. This discrepancy could perhaps be explained by the studies of Hime, Barter, and Rye (
      • Hime N.J.
      • Barter P.J.
      • Rye K-A.
      The influence of apolipoproteins on the hepatic lipase-mediated hydrolysis of high density lipoprotein phospholipid and triacylglycerol.
      ) and their prediction that at low substrate concentrations (such as in the in vitro experiments of Mowri et al.), the amount of HL interacting with HDL and hydrolyzing HDL lipids might be much greater in particles with apoA-II than in those without apoA-II. At high substrate concentrations (such as in the vascular compartment), HL would hydrolyze more lipids in HDL particles without apoA-II than in those with apoA-II.
      In contrast, HL activity was significantly correlated with the phospholipid in both Lp(A-I) and Lp(A-I,A-II). These data imply that the rate of HL-mediated phospholipid hydrolysis is comparable between Lp(A-I) and Lp(A-I,A-II). Alternatively, this may mean that phospholipids are transferred more readily than triglycerides between these two populations of HDL particles. As HL depletes the lipid components of HDL, it also reduces HDL size. We had expected that HL would therefore be negatively correlated with large HDL particles and positively correlated with small HDLs. Here we find that it correlated more with large Lp(A-I) and Lp(A-I,A-II) than with small particles. This could be explained by our observation that PLTP also plays some roles in regulating the level of small HDL particles (Table 4), and the current concept that the conversion of HDL from large to small HDL upon HL-mediated hydrolysis requires further processing by other factors, such as CETP (
      • Eisenberg S.
      High density lipoprotein metabolism.
      ,
      • Kahri J.
      • Per-Henrik G.
      • Elliott T.
      • Viberti G.C.
      • Taskinen M-R.
      Plasma cholesteryl ester transfer protein and its relationship to plasma lipoproteins and apolipoprotein A-1-containing lipoproteins in IDDM patients with microalbuminuria and clinical nephropathy.
      ). Enhanced uptake and degradation of small HDL particles mediated by CETP and the lipases could also contribute to the weaker correlation between HL and small HDL (
      • Collet X.
      • Tall A.R.
      • Serajuddin H.
      • Guendouzi K.
      • Royer L.
      • Oliveira H.
      • Barbaras R.
      • Jiang X-C.
      • Francone O.L.
      Remodeling of HDL by CETP in vivo and by CETP and hepatic lipase in vitro results in enhanced uptake of HDL CE by cells expressing scavenger receptor B-1.
      ).
      Plasma PLTP has been shown to be instrumental in the net mass transfer of phospholipid and free cholesterol from the lower density lipoproteins to HDL in vitro (
      • Cheung M.C.
      • Wolfbauer G.
      • Albers J.J.
      Plasma phospholipid mass transfer rate: relationship to plasma phospholipid and cholesteryl ester transfer activities and lipid parameters.
      ,
      • Tall A.R.
      • Krumholz S.
      • Olivercrona T.
      Plasma phospholipid transfer protein enhances transfer and exchange of phospholipids between very low density lipoproteins and high density lipoproteins during lipolysis.
      ,
      • Tollefson J.H.
      • Ravnik S.
      • Albers J.J.
      Isolation and characterization of a phospholipid transfer protein (LTP-II) from human plasma.
      ,
      • Murdoch S.J.
      • Breckenridge W.C.
      Effect of lipid transfer proteins on lipoprotein lipase induced transformation of VLDL and HDL.
      ) and in PLTP-deficient mice (
      • Jiang X.C.
      • Bruce C.
      • Mar J.
      • Lin M.
      • Ji Y.
      • Francone O.L.
      • Tall A.R.
      Targeted mutation of plasma phospholipid transfer protein gene markedly reduced high-density lipoprotein levels.
      ). It has also been shown to induce the fusion of HDL to form particles larger and smaller than the parent molecules in vitro (
      • Albers J.J.
      • Wolfbauer G.W.
      • Cheung M.C.
      • Day J.R.
      • Ching A.F.T.
      • Lok S.
      • Tu A-Y.
      Functional expression of human and mouse plasma phospholipid transfer protein: effect of recombinant and plasma PLTP on HDL subspecies.
      ,
      • Tu A.Y.
      • Nishida H.I.
      • Nishida T.
      High density lipoprotein conversion mediated by human plasma phospholipid transfer protein.
      ,
      • Von Eckardstein A.
      • Jauhianinen M.
      • Huang Y.
      • Metso J.
      • Langer C.
      • Pusinen P.
      • Wu S.
      • Ehnholm C.
      • Assmann G.
      Phospholipid transfer protein mediated conversion of high density lipoproteins generates pre-β1-HDL.
      ). These observations suggest that PLTP may be involved in the metabolism of HDL as well as VLDL and LDL. However, its physiological significance in humans has been difficult to confirm in the absence of any known case of PLTP deficiency, and results of studies performed to determine its relationship to the different classes of lipoproteins have been inconsistent (
      • Murdoch S.J.
      • Carr M.C.
      • Hokanson J.E.
      • Brunzell J.D.
      • Albers J.J.
      PLTP activity in premenopausal women. Relationship with lipoprotein lipase, HDL, LDL, body fat, and insulin resistance.
      ,
      • Cheung M.C.
      • Knopp R.H.
      • Retzlaff B.
      • Kennedy H.
      • Wolfbauer G.
      • Albers J.J.
      Association of plasma phospholipid transfer protein activity with IDL and buoyant LDL: impact of gender and adiposity.
      ,
      • Dullaart R.P.
      • Sluiter W.J.
      • Dikkeschei L.D.
      • Hoogenberg K.
      • Van Tol A.
      Effect of adiposity on plasma lipid transfer protein activities: a possible link between insulin resistance and high density lipoprotein metabolism.
      ,
      • Cheung M.C.
      • Wolfbauer G.
      • Brown B.G.
      • Albers J.J.
      Relationship between plasma phospholipid transfer protein activity and HDL subclasses among patients with low HDL and cardiovascular disease.
      ). Because we have shown that PLTP can cause size changes in both Lp(A-I) and Lp(A-I,A-II) (
      • Albers J.J.
      • Wolfbauer G.W.
      • Cheung M.C.
      • Day J.R.
      • Ching A.F.T.
      • Lok S.
      • Tu A-Y.
      Functional expression of human and mouse plasma phospholipid transfer protein: effect of recombinant and plasma PLTP on HDL subspecies.
      ), we had expected to see a significant association between plasma PLTP activity and the size profile of these particles. However, among the four size species in Lp(A-I) and Lp(A-I,A-II), PLTP only revealed a trend of correlation with Lp(A-I) particles smaller than 8.2 nm in the 28 subjects. Because we have recently found that gender-related factors can modify the relationship between PLTP and lipoproteins in 134 nondiabetic subjects (
      • Cheung M.C.
      • Knopp R.H.
      • Retzlaff B.
      • Kennedy H.
      • Wolfbauer G.
      • Albers J.J.
      Association of plasma phospholipid transfer protein activity with IDL and buoyant LDL: impact of gender and adiposity.
      ), correlation between PLTP activity and the various metabolic and lipid parameters was analyzed further with the two gender subgroups. This additional analysis revealed that in women but not in men, ∼45% of the variation in Lp(A-I) and Lp(A-I,A-II) particles smaller than 8.2 nm could be attributed to plasma PLTP activity. Association between PLTP activity and the three size species of Lp(A-I) and Lp(A-I,A-II) with Stokes diameter >8.2 nm was less evident. Taken together, our data suggest that among HL, LPL, and PLTP, the lipases appeared to be stronger determinants of the amounts of large HDL particles while PLTP appeared to have a greater impact on small HDL particles. Furthermore, as previously reported for nondiabetic subjects, the impact of PLTP on HDL in subjects with well-controlled Type 1 diabetes is also gender dependent.
      In addition to particle size, there was also a significant positive correlation between PLTP activity and Lp(A-I) triglyceride. We do not have any explanation for this relationship at present, but it may be related to an earlier observation that PLTP can promote CETP-mediated cholesteryl ester and triglyceride exchange between VLDL and HDL in vitro (
      • Tall A.R.
      • Krumholz S.
      • Olivercrona T.
      Plasma phospholipid transfer protein enhances transfer and exchange of phospholipids between very low density lipoproteins and high density lipoproteins during lipolysis.
      ). If this is the case in vivo, then the significant correlation between PLTP and Lp(A-I) triglyceride would further suggest that Lp(A-I) is the preferential HDL acceptor of triglyceride transferred from the lower density lipoproteins. In women but not in men, PLTP activity was also significantly and positively associated with BMI, apoB, and LDL cholesterol. These associations are entirely consistent with those observed in 134 nondiabetic subjects (
      • Cheung M.C.
      • Knopp R.H.
      • Retzlaff B.
      • Kennedy H.
      • Wolfbauer G.
      • Albers J.J.
      Association of plasma phospholipid transfer protein activity with IDL and buoyant LDL: impact of gender and adiposity.
      ). The strong association between PLTP and apoB or LDL cholesterol supports the recent suggestion that PLTP may play a role in regulating the secretion of apoB-containing lipoproteins (
      • Jiang X.C.
      • Qin S.
      • Qiao C.
      • Kawano K.
      • Lin M.
      • Skold A.
      • Xiao X.
      • Tall A.R.
      Apolipoprotein B secretion and atherosclerosis are decreased in mice with phospholipid-transfer protein deficiency.
      ).
      The relationships among HDL particles are highly complex and have yet to be fully elucidated. The original concept of reversible interconversion of HDL2- and HDL3-density subclasses (
      • Eisenberg S.
      High density lipoprotein metabolism.
      ) was recently challenged when kinetic studies showed that small HDL particles were converted in a unidirectional manner to medium or large HDLs (
      • Colvin P.L.
      • Moriguchi E.
      • Barrett H.
      • Parks J.
      • Rudel L.
      Production rate determines plasma concentration of large high density lipoprotein in non-human primates.
      ,
      • Colvin P.L.
      • Moriguchi E.
      • Barrett P.H.R.
      • Parks J.S.
      • Rudel L.L.
      Small HDL particles containing two apoA-I molecules are precursors in vivo to medium and large HDL particles containing three and four apoA-I molecules in nonhuman primates.
      ). This may be relevant to the differential correlation between LPL, HL, or PLTP and small or large particles observed in this study. Although apoA-I transfers among HDL particles, there has been evidence of the existence of exchangeable and nonexchangeable pools of apoA-I in HDL (
      • Atmah R.F.
      • Shepherd J.
      • Packard C.J.
      Subpopulations of apolipoprotein A-I in human high density lipoproteins. Their metabolic properties and response to drug therapy.
      ,
      • Cheung M.C.
      • Wolf A.C.
      • Knopp R.H.
      • Foster D.M.
      Protein transfer between A-I-containing lipoprotein subpopulations: evidence of non-transferable A-I in particles with A-II.
      ). Little is known regarding the movements of lipid among HDL particles. The present study provides some insight into this area. Specifically, significant correlations of LPL and HL with the cholesterol and/or triglyceride of Lp(A-I) but not of Lp(A-I,A-II) suggest that the lipids in these two populations of particles are not fully equilibrated.
      Although we have based the focus of the discussion on the lipolytic activity of LPL and HL and the lipid transfer ability of PLTP, both LPL and HL can also enhance the binding of lipoproteins to cells and promote cellular lipoprotein uptake and degradation (
      • Rinninger F.
      • Brundert M.
      • Brosch I.
      • Donarski N.
      • Budzinski R.M.
      • Greten H.
      Lipoprotein lipase mediates an increase in selective uptake of HDL-associated cholesteryl esters by cells in culture independent of scavenger receptor B1.
      ,
      • Medh J.D.
      • Fry G.L.
      • Bowen S.L.
      • Ruben S.
      • Wong H.
      • Chappell D.A.
      Lipoprotein lipase and hepatic lipase promoted very low density lipoprotein degradation proceeds via an apolipoprotein E-dependent mechanism.
      ,
      • Lambert G.
      • Chase M.B.
      • Dugi K.
      • Bensadoun A.
      • Brewer Jr., H.B.
      • Santamarina-Fojo S.
      Hepatic lipase promotes the selective uptake of high density lipoprotein-cholesteryl esters via the scavenger receptor B1.
      ), and there is evidence that PLTP plays a role in cell cholesterol efflux (
      • Wolfbauer G.
      • Albers J.J.
      • Oram J.F.
      Phospholipid transfer protein enhances removal of cellular cholesterol and phospholipids by high-density lipoprotein apolipoproteins.
      ). Thus, it is possible that the observed correlation between LPL, HL, or PLTP and HDL particles also reflects, in part, the roles of these proteins in lipoprotein-cell interaction, uptake, degradation, and efflux.
      While all the subjects in this study have Type 1 diabetes, their HbA1c level was not significantly associated with HL, LPL, or Lp(A-I) level (data not shown). Therefore, the correlations between the lipases and Lp(A-I) observed in this study are independent of glycemic control. Insofar as neither LPL, HL, nor PLTP appeared to be major determinants of plasma Lp(A-I,A-II) level, the regulation of this population of HDL particles in the vascular compartment remains to be investigated. We observed over a decade ago that both LCAT and the apoB-containing lipoproteins are required to induce significant changes in Lp(A-I,A-II) in vitro (
      • Cheung M.C.
      • Wolf A.C.
      In vitro transformation of apoA-I-containing lipoprotein subpopulations: role of lecithin:cholesterol acyltransferase and apoB-containing lipoproteins.
      ). Recently, Clay et al. found that LCAT can promote the formation of Lp(A-I,A-II) in the presence of LDL through a fusion process (
      • Clay M.A.
      • Pyle D.
      • Rye K-A.
      • Barter P.J.
      Formation of spherical, reconstituted high density lipoproteins containing both apolipoproteins A-I and A-II is mediated by lecithin:cholesterol acyltransferase.
      ). On the basis of these studies, we speculate that apoB-containing lipoproteins and LCAT may be important regulators of plasma Lp(A-I, A-II).
      In summary, we have studied the postheparin plasma LPL and HL activities, plasma PLTP activity, and HDL particles in 28 subjects with well-controlled Type 1 diabetes, and determined the relationship of these lipolytic and lipid transfer activities with the characteristics of Lp(A-I) and Lp(A-I,A-II) to gain further insights into the role of these proteins in the metabolism of apo-specific HDL particles. Our observations suggest that Lp(A-I)s may be the preferred acceptors of excess surface remnants of triglyceride-rich lipoproteins, and that both LPL and HL play important but opposing regulatory roles in plasma Lp(A-I) concentration and lipid composition, and the amounts of large Lp(A-I). In contrast, PLTP appears to exert a greater impact on the amounts of small HDL rather than of large HDL. The lack of correlation between LPL, HL, or PLTP and the apoA-I and A-II in Lp(A-I,A-II) suggests that none of these three proteins are major determinants of total amounts of plasma Lp(A-I,A-II), even though HL is inversely correlated with Lp(A-I,A-II) phospholipid and large Lp(A-I,A-II). Select correlation of LPL and HL with the apoA-I, cholesterol and/or triglyceride of Lp(A-I) but not of Lp(A-I,A-II) implies that the apoA-I and lipid of Lp(A-I), and Lp(A-I,A-II) are not fully equilibrated. The present data also confirm our observation in an earlier study (
      • Goldberg I.J.
      Lipoprotein lipase and lipolysis: central roles in lipoprotein metabolism and atherogenesis.
      ) that PLTP may play a role in the metabolism of apoB-containing lipoproteins, and that gender-related factors such as differences in HL activity seen in this study can modulate the impact of PLTP on lipoprotein.

      Acknowledgments

      This work was supported by National Institutes of Health Grants DK-02456, N01-DK-6-2203, and HL-30086, and by the University of Washington Clinical Nutrition Research Unit (DK-35816) and General Research Center (RR37). The authors thank Alegria Aquino-Albers and Steve Hashimoto for their technical expertise.

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