A new sandwich enzyme immunoassay for measurement of plasma pre-beta1-HDL levels

Pre-beta1-HDL, a putative discoid-shaped high density lipoprotein (HDL) of approximately 67-kDa mass that migrates with pre-beta mobility in agarose gel electrophoresis, contains apolipoprotein A-I (apoA-I), phospholipids, and unesterified cholesterol. It participates in the retrieval of cholesterol from peripheral tissues. In this study we established a new sandwich enzyme immunoassay (EIA) for measuring plasma pre-beta1-HDL using mouse anti-human pre-beta1-HDL monoclonal antibody (MAb 55201) and goat anti-human apoA-I polyclonal antibody. MAb 55201 reacted with apoA-I in lipoprotein [A-I] with molecular mass less than 67 kDa, and with pre-beta1-HDL separated by nondenaturing two-dimensional electrophoresis, whereas it did not react with apoA-I in alpha-HDL. Pre-beta1-HDL levels measured by this method declined when incubated at 37 degrees C for 2 h, whereas this decrease was not observed in the presence of 2 mM lecithin:cholesterol acyltransferase inhibitor 5,5'-dithiobis (2-nitrobenzoic acid). To clarify the clinical significance of measuring pre-beta1-HDL by this method, 47 hyperlipidemic subjects [male/female 22/25; age 55 +/- 14 years; body mass index 25 +/- 4.5 kg/m(2); total cholesterol (TC) 245 +/- 64 mg/dl; triglyceride (TG) 232 +/- 280 mg/dl; HDL cholesterol (HDL-C) 51 +/- 23 mg/dl] and 25 volunteers (male/female 15/10; age 36 +/- 9.3 years; body mass index 23 +/- 3.5 kg/m(2); TC 183 +/- 28 mg/dl; TG 80 +/- 34 mg/dl; HDL-C 62 +/- 15 mg/dl) were involved. Plasma pre-beta1-HDL levels were significantly higher in hyperlipidemic subjects than in volunteers (39.3 +/- 10.1 vs. 22.5 +/- 7.5 mg/ml, P < 0.001) whereas plasma apoA-I levels did not differ (144.2 +/- 28.4 vs. 145.3 +/- 16.3 mg/dl).These results indicate that this sandwich EIA method specifically recognizes apoA-I associated with pre-beta1-HDL.     

Initial interaction of apoA-I with ABCA1 impacts in vivo metabolic fate of nascent HDL

We investigated the in vivo metabolic fate of pre-beta HDL particles in human apolipoprotein A-I transgenic (hA-I (Tg)) mice. Pre-beta HDL tracers were assembled by incubation of [(125)I]tyramine cellobiose-labeled apolipoprotein A-I (apoA-I) with HEK293 cells expressing ABCA1. Radiolabeled pre-beta HDLs of increasing size (pre-beta1, -2, -3, and -4 HDLs) were isolated by fast-protein liquid chromatography and injected into hA-I (Tg)-recipient mice, after which plasma decay, in vivo remodeling, and tissue uptake were monitored. Pre-beta2, -3, and -4 had similar plasma die-away rates, whereas pre-beta1 HDL was removed 7-fold more rapidly. Radiolabel recovered in liver and kidney 24 h after tracer injection suggested increased (P < 0.001) liver and decreased kidney catabolism as pre-beta HDL size increased. In plasma, pre-beta1 and -2 were rapidly (<5 min) remodeled into larger HDLs, whereas pre-beta3 and -4 were remodeled into smaller HDLs. Pre-beta HDLs were similarly remodeled in vitro with control or LCAT-immunodepleted plasma, but not when incubated with phospholipid transfer protein knockout plasma. Our results suggest that initial interaction of apoA-I with ABCA1 imparts a unique conformation that partially determines the in vivo metabolic fate of apoA-I, resulting in increased liver and decreased kidney catabolism as pre-beta HDL particle size increases.     

Apolipoprotein distribution in human lipoproteins separated by polyacrylamide gradient gel electrophoresis

The heterogeneity of serum lipoproteins (excluding very low density (VLDL) and intermediate density (IDL) lipoproteins) and that of lipoproteins secreted by HepG2 cells has been studied by immunoblot analysis of the apolipoprotein composition of the particles separated by polyacrylamide gradient gel electrophoresis (GGE) under nondenaturing conditions. The reactions of antibodies to apoA-I, apoA-II, apoE, apoB, apoD, and apoA-IV have revealed discrete bands of particles which differ widely in size and apolipoprotein composition. GGE of native serum lipoproteins demonstrated that apoA-II is present in lipoproteins of limited size heterogeneity (apparent molecular mass 345,000 to 305,000) and that apoB is present in low density lipoproteins (LDL) and absent from all smaller or denser lipoproteins. In contrast, serum apoA-I, E, D, and A-IV are present in very heterogeneous particles. Serum apoA-I is present mainly in particles of 305 to 130 kDa where it is associated with apoA-II, and in decreasing order of immunoreactivity in particles of 130-90 kDa, 56 kDa, 815-345 kDa, and finally within the size range of LDL, all regions where there is little detectable apoA-II. Serum apoE is present in three defined fractions, one within the size range of LDL, one containing heterogeneous particles between 640 and 345 kDa, and one defined fraction at 96 kDa. Serum apoD is also present in three defined fractions, one comigrating with LDL, one containing heterogeneous particles between 390 and 150 kDa, and one band on the migration front. Most of serum apoA-IV is contained in a band comigrating with albumin. GGE of centrifugally prepared LDL shows the presence of apoB, apoE, and apoD, but not that of apoA-I. However, the particles containing apoA-I, which, in serum, migrated within the LDL size range and as bands of 815 to 345 kDa, were recovered upon centrifugation in the d greater than 1.21 g/ml fraction. GGE of high density lipoproteins (HDL) indicated that most of apoA-I, A-II, and A-IV were present in lipoproteins of the same apparent molecular mass (390-152 kDa). ApoD tended to be associated with large HDL, and this was also significant for HDL apoE, which is present in lipoproteins ranging from 640 to 275 kDa. GGE of very high density lipoproteins (VHDL) presented some striking features, one of which was the occurrence of apolipoproteins in very discrete bands of different molecular mass. ApoA-II was bimodally distributed at 250-175 kDa and 175-136 kDa, the latter fraction also containing apoA-I.(ABSTRACT TRUNCATED AT 400 WORDS)     

Non-transferrin-bound iron species in the serum of hypotransferrinaemic mice.

Serum from homozygous hypotransferrinaemic mice (a mixed group of males and females, aged 6-8 wk) was found to contain low levels of iron (mean 0.9 +/- 0.5 microM (SEM, n = 4), as assayed by conventional serum iron assays. Similarly, low levels of non-transferrin-bound iron were determined with a nitrilotriacetate chelation assay (1.3 +/- 0.4 microM, n = 4) (Singh, S., Hider, R.C. and Porter, J.B. (1990) Analytical Biochemistry 186, 320-323). Mononuclear Fe (citrate) was undectable by electron paramagnetic resonance spectroscopy (EPR). Significantly larger quantities of iron (16 +/- 5 microM, n = 8) were detected by the bleomycin assay (Gutteridge, J.M.C., Rowley, D.A. and Halliwell, B. (1981) Biochemical Journal 199, 263-265), while non-haem iron assay or atomic absorption spectrophotometry revealed up to 96 microM iron. Haemoglobin iron was detectable at approximately 10 microM by spectrophotometry. Ferri-haem was undetectable by EPR spectroscopy. Serum ferritin levels of 641 +/- 128 micrograms/l (n = 14) in hypotransferrinaemic mice (wild-types 44 +/- 6 micrograms/l, n = 14) were observed and these cannot account for the non-transferrin-bound iron. Hypotransferrinaemic mouse serum therefore contains large quantities of non-transferrin-bound iron which is unreactive in some assays used to detect such iron in human iron overload. Fractionation by Sephadex G200 chromatography revealed three distinct species with apparent molecular weights of > or = 150 kDa, 40-80 kDa and 1-5 kDa. The iron may be distinguished from known extracellular iron proteins and haem-proteins by its availability to hot acid extractions.     

Effects of thyroid status and fasting on hepatic metabolism of apolipoprotein A-I

Metabolism of apolipoprotein (apo)A-I was studied in normal and chow-fed hyperthyroid rats, in 24-h fasted untreated male rats, and in rats after thyroparathyroidectomy (TXPTX). Rats were made hyperthyroid by administration of T3 (9.6 micrograms/day) or T4 (30 micrograms/day) with an Alzet osmotic minipump. Hyperthyroidism produced a similar two- to threefold elevation in plasma levels of apoA-I in male or female animals. During treatment with T3, plasma levels of T3 ranged from 200 to 400 ng/dl and did not correlate with plasma apoA-I levels. The net mass secretion and synthesis ([3H]leucine incorporation) of apoA-I by perfused livers from male hyperthyroid rats was elevated, while secretion of albumin was not different than that of euthyroid rats. Furthermore, the incorporation of [3H]leucine into total perfusate and hepatic protein was not altered by hyperthyroidism. The effect of thyroid hormone on apoA-I synthesis, therefore, does not appear to be a general effect on protein synthesis. After longer periods of treatment (28 days) with T3 (9.6 micrograms/day), hepatic apoA-I production decreased from that observed after 7 or 14 days of treatment, yet plasma apoA-I concentrations remained elevated. Plasma T3 decreased from 100 ng/dl to 40 ng/dl, in the hypothyroid rat resulting from TXPTX, but the plasma concentration of apoA-I did not change during the 2-week experimental period. The net secretion of apoA-I by livers from hypothyroid animals was depressed and albumin was uneffected compared to the euthyroid. Overnight fasting of euthyroid rats did not alter hepatic apoA-I secretion or plasma apoA-I levels, although under fasting conditions we had reported that hepatic output of apoB and E of VLDL is depressed. The addition of oleic acid to the perfusion medium, sufficient to stimulate VLDL production, did not affect net hepatic secretion of apoA-I by livers from euthyroid, hyperthyroid, or hypothyroid rats. In summary, hepatic synthesis of apoA-I appears to be controlled independently of other apo-lipoproteins and secretory proteins (albumin). Hepatic apoA-I synthesis is sensitive to thyroid status, increased in the hyperthyroid and decreased in the hypothyroid state. The specific stimulation of hepatic synthesis and secretion of apoA-I in the hyperthyroid state, however, tends to normalize over an extended period, perhaps from compensatory effects of a hormonal nature.     

Evidence that phospholipids play a key role in pre-beta apoA-I formation and high-density lipoprotein remodeling

The initial plasma acceptor of unesterified cholesterol and phospholipids from peripheral cells has been identified as pre-beta migrating, lipid-free, or lipid-poor apolipoprotein (apo) A-I (pre-beta apoA-I). Pre-beta apoA-I is formed when plasma factors, such as cholesteryl ester transfer protein (CETP), remodel high-density lipoproteins (HDL). The aim of this study is to determine how phospholipids influence pre-beta apoA-I formation during the CETP-mediated remodeling of HDL. Reconstituted HDL (rHDL) containing either 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC), 1-palmitoyl-2-linoleoyl phosphatidylcholine (PLPC), 1-palmitoyl-2-arachidonyl phosphatidylcholine (PAPC), or 1-palmitoyl-2-docosahexanoyl phosphatidylcholine (PDPC) as the only phospholipid were prepared. The rHDL were comparable in size and core lipid/protein molar ratio and contained only cholesteryl esters in their core and apoA-I as the sole apolipoprotein. The (POPC)rHDL, (PLPC)rHDL, (PAPC)rHDL, and (PDPC)rHDL were respectively incubated for 0-24 h with CETP and microemulsions containing triolein and either POPC, PLPC, PAPC, or PDPC. The rate at which the rHDL were depleted of core lipids and remodeled to small particles varied widely with (POPC)rHDL < (PLPC)rHDL < (PDPC)rHDL approximately (PAPC)rHDL. Pre-beta apoA-I was not formed in the (POPC)rHDL incubations. Pre-beta apoA-I was apparent by 24 h in the (PLPC)rHDL incubations and by 12 h in the (PAPC)rHDL and (PDPC)rHDL incubations. The enhanced formation of pre-beta apoA-I in the (PAPC)rHDL and (PDPC)rHDL incubations reflected the increased core lipid depletion of the particles combined with the destabilization and progressive exclusion of apoA-I from the particle surface. In conclusion, these results show that phospholipids play a key role in the CETP-mediated remodeling of rHDL and pre-beta apoA-I formation.     

Molecular and cellular physiology of apolipoprotein A-I lipidation by the ATP-binding cassette transporter A1 (ABCA1)

The dynamics of ABCA1-mediated apoA-I lipidation were investigated in intact human fibroblasts induced with 22(R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Specific binding parameters of (125)I-apoA-I to ABCA1 at 37 degrees C were determined: K(d) = 0.65 microg/ml, B(max) = 0.10 ng/microg cell protein. Lipid-free apoA-I inhibited the binding of (125)I-apoA-I to ABCA1 more efficiently than pre-beta(1)-LpA-I, reconstituted HDL particles r(LpA-I), or HDL(3) (IC(50) = 0.35 +/- 1.14, apoA-I; 1.69 +/- 1.07, pre-beta(1)-LpA-I; 17.91 +/- 1.39, r(LpA-I); and 48.15 +/- 1.72 microg/ml, HDL(3)). Treatment of intact cells with either phosphatidylcholine-specific phospholipase C or sphingomyelinase affected neither (125)I-apoA-I binding nor (125)I-apoA-I/ABCA1 cross-linking. We next investigated the dynamics of apoA-I lipidation by monitoring the kinetic of apoA-I dissociation from ABCA1. The dissociation of (125)I-apoA-I from normal cells at 37 degrees C was rapid (t((1/2)) = 1.4 +/- 0.66 h; n = 3) but almost completely inhibited at either 15 or 4 degrees C. A time course analysis of apoA-I-containing particles released during the dissociation period showed nascent apoA-I-phospholipid complexes that exhibited alpha-electrophoretic mobility with a particle size ranging from 9 to 20 nm (designated alpha-LpA-I-like particles), whereas lipid-free apoA-I incubated with ABCA1 mutant (Q597R) cells was unable to form such particles. These results demonstrate that: 1) the physical interaction of apoA-I with ABCA1 does not depend on membrane phosphatidylcholine or sphingomyelin; 2) the association of apoA-I with lipids reduces its ability to interact with ABCA1; and 3) the lipid translocase activity of ABCA1 generates alpha-LpA-I-like particles. This process plays in vivo a key role in HDL biogenesis.     

Interconversion between apolipoprotein A-I-containing lipoproteins of pre-beta and alpha electrophoretic mobilities.

Apolipoprotein (apo) A-I-containing lipoproteins can be separated into two subfractions, pre-beta HDL and alpha HDL (high density lipoproteins), based on differences in their electrophoretic mobility. In this report we present results indicating that these two subfractions are metabolically linked. When plasma was incubated for 2 h at 37 degrees C, apoA-I mass with pre-beta electrophoretic mobility disappeared. This shift in apoA-I mass to alpha electrophoretic mobility was blocked by the addition of either 1.4 mM DTNB or 10 mM menthol to the plasma prior to incubation, suggesting that lecithin:cholesterol acyltransferase (LCAT) activity was involved. There was no change in the electrophoretic mobility of either pre-beta HDL or alpha HDL when they were incubated with cholesterol-loaded fibroblasts. However, after exposure to the fibroblasts, the cholesterol content of the pre-beta HDL did increase approximately sixfold, suggesting that pre-beta HDL can associate with appreciable amounts of cellular cholesterol. Pre-beta HDL-like particles appear to be generated by the incubation of alpha HDL with cholesteryl ester transfer protein (CETP) and either very low density lipoproteins (VLDL) or low density lipoproteins (LDL). This generation of pre-beta HDL-like particles was documented both by immunoelectrophoresis and by molecular sieve chromatography. Based on these findings, we propose a cyclical model in which 1) apoA-I mass moves from pre-beta HDL to alpha HDL in connection with the action of LCAT and the generation of cholesteryl esters within the HDL, and 2) apoA-I moves from alpha HDL to pre-beta HDL in connection with the action of CETP and the movement of cholesteryl esters out of the HDL. Additionally, we propose that the relative plasma concentrations of pre-beta HDL and alpha HDL reflect the movement of cholesteryl esters through the HDL. Conditions that result in the accumulation of HDL cholesteryl esters will be associated with low concentrations of pre-beta HDL, whereas conditions that result in the depletion of HDL cholesteryl esters will be associated with elevated concentrations of pre-beta HDL. This postulate is consistent with published findings in patients with hypertriglyceridemia and LCAT deficiency.     

GPI-specific phospholipase D associates with an apoA-I- and apoA-IV-containing complex

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is abundant in serum and associates with high density lipoproteins (HDL). We have characterized the distribution of GPI-PLD among lipoproteins in human plasma. Apolipoprotein (apo)-specific lipoproteins containing apoB (Lp[B]), apoA-I and A-II (Lp[A-I, A-II]), or apoA-I only (Lp[A-I]) were isolated using dextran sulfate and immunoaffinity chromatography. In six human plasma samples with HDL cholesterol ranging from 39 to 129 mg/dl, 79 +/- 14% (mean +/- SD) of the total plasma GPI-PLD activity was associated with Lp[A-I], 9 +/- 12% with Lp[A-I, A-II], and 1 +/- 1% with Lp[B]; and 11 +/- 10% was present in plasma devoid of these lipoproteins. Further characterization of the GPI-PLD-containing lipoproteins by gel-filtration chromatography and nondenaturing polyacrylamide and agarose gel electrophoresis revealed that these apoA-I-containing particles/complexes were small (8 nm) and migrated with pre-beta particles on agarose electrophoresis. Immunoprecipitation of GPI-PLD with a monoclonal antibody to GPI-PLD co-precipitated apoA-I and apoA-IV but little or no apoA-II, apoC-II, apoC-III, apoD, or apoE. In vitro, apoA-I but not apoA-IV or bovine serum albumin interacted directly with GPI-PLD, but did not stimulate GPI-PLD-mediated cleavage of a cell surface GPI-anchored protein. Thus, the majority of plasma GPI-PLD appears to be specifically associated with a small, discrete, and minor fraction of lipoproteins containing apoA-I and apoA-IV. -- Deeg, M. A., E. L. Bierman, and M. C. Cheung. GPI-specific phospholipase D associates with an apoA-I- and apoA-IV-containing complex. J. Lipid Res. 2001. 42: 442--451.     

An erythropoietin fusion protein comprised of identical repeating domains exhibits enhanced biological properties.

The hematopoietic growth factor erythropoietin (Epo) initiates its intracellular signaling cascade by binding to and inducing the homodimerization of two identical receptor molecules. We have now constructed and expressed in COS cells a cDNA encoding a fusion protein consisting of two complete human Epo domains linked in tandem by a 17-amino acid flexible peptide. On SDS-polyacrylamide gel electrophoresis, the Epo-Epo fusion protein migrated as a broad band with an average apparent molecular mass of 76 kDa, slightly more than twice the average apparent molecular mass of Epo, 37 kDa. Enzymatic N-deglycosylation resulted in an Epo-Epo species that migrated on SDS-polyacrylamide gel electrophoresis as a narrow band with an average apparent molecular mass of 39 kDa. The specific activity of the Epo-Epo fusion protein in vitro (1,007 IU/microgram; 76 IU/pmol) was significantly greater than that of Epo (352 IU/microgram; 13 IU/pmol). Moreover, secretion of Epo-Epo by COS cells was 8-fold greater than that of Epo. Subcutaneous administration of a single dose of Epo-Epo to mice resulted in a significant increase in red blood cell production within 7 days. In contrast, administration of an equivalent dose of conventional recombinant Epo was without effect. The pharmacokinetic behavior of Epo-Epo differed significantly from that of Epo. The results suggest that Epo-Epo may have important biological and therapeutic advantages.     

Monocyte-derived macrophage and alveolar macrophage fibronectin production and cathepsin D activity

Alveolar macrophages are thought to play an important role in ongoing tissue breakdown and repair processes in the normal lung. The secretion and regulation of cathepsin D (important for the final breakdown of collagen) and fibronectin (involved in the healing process) in human peripheral blood monocytes (PBM) and pulmonary alveolar macrophages (PAM) were investigated. Cathepsin D enzyme activity was measured by quantitating the TCA-soluble fragments of [3H]hemoglobin. Freshly isolated PBM contained less cell-associated cathepsin D activity than did freshly isolated PAM (314 +/- 35 micrograms/10(6) cells vs 381 +/- 35 micrograms/10(6) cells, respectively). After 7-10 days in culture, cell-associated enzyme levels in both PBM and PAM were significantly increased (P less than 0.001 for PBM; P less than 0.0001 for PAM). In addition, freshly isolated PAM secreted more cathepsin D than did freshly isolated PBM (5.8 +/- 3.2 micrograms/10(6) cells vs 0.83 +/- 0.83 micrograms/10(6) cells, P less than 0.02). In the presence of LPS (10 micrograms/ml), cell-associated cathepsin D was inhibited in both PBM and PAM. With the addition of gamma-IFN (500 U/ml), both cell-associated and secreted enzyme were increased in freshly isolated and 10-day-cultured PBM and PAM. In parallel studies, fibronectin secretion (by ELISA assay) in both PBM and PAM increased over time in culture. LPS had no effect on PBM or PAM secretion of human fibronectin while gamma-IFN increased PBM and PAM fibronectin levels. Thus, both macrophage cathepsin D activity and fibronectin secretion are increased by gamma-interferon while macrophage cathepsin D activity, but not fibronectin secretion, is decreased by LPS. These studies demonstrate that human macrophage cathepsin D activity is actively modulated by inflammatory mediators and that macrophage mediators of tissue breakdown and repair are not modulated synchronously.     

Insulin-like growth factor binding protein secretion by muscle cells: effect of cellular differentiation and proliferation

Several cell types have been shown to secrete insulin-like growth factor binding proteins (IGF-BP) in vitro. Since IGF-BP influences cell responsiveness to IGF, three muscle cell types were investigated to determine if they produced IGF-BP and to identify factors that regulate IGF-BP secretion. Porcine smooth muscle cells (pSMC), rat L6 skeletal muscle cells, and mouse BC3H-1 myocytes were used. IGF-BP activity in serum-free conditioned media was quantitated with a polyethylene glycol precipitation method. All three cell types secreted IGF-BP activity into the medium. Insulin was a potent stimulant of IGF-BP secretion for each cell type. Specifically, 1 microgram/ml insulin increased the IGF-BP concentration in conditioned media from 10.5 +/- 1.3 to 15.0 +/- 1.5 ng/ml in confluent L6 myotubes, from 42.5 +/- 11.1 to 90.5 +/- 9.8 ng/ml in confluent BC3H-1 cells, and from 2.1 +/- 0.1 to 3.8 +/- 0.1 ng/ml in confluent pSMC. L6 myotubes required more insulin (8 micrograms/ml) to achieve a half-maximal stimulation of IGF-BP secretion than confluent pSMC, differentiation deficient L6.DD cells or BC3H-1 cells, where half-maximal stimulation occurred between 125 and 300 ng/ml. L6 myoblasts were 40-fold more sensitive to insulin stimulation of IGF-BP secretion than L6 myotubes. IGF-I, although it interferes with the assay and thereby lowers the amount of detectable IGF-BP, stimulated the secretion of IGF-BP from all three cell types. Dexamethasone, (10(-7) M) decreased IGF-BP secretion into the media by approximately 50% for all three cell types. Affinity cross-linking and ligand blotting of 125I-IGF-I to conditioned media from each cell type showed (IGF-BP)-(IGF-I) complexes with molecular weights ranging 32-40 kDa (24-32 kDa for IGF-BP and 7.5 kDa for IGF-I). Insulin stimulated cell proliferation for both L6 myoblasts and BC3H-1 myocytes. This cell proliferative response was associated with an increase in IGF-BP secretion/cell in response to insulin. In contrast dexamethasone decreased L6 myoblast proliferation and decreased IGF-BP secretion/cell. We conclude that IGF-BP is secreted by each muscle cell type and that the state of cellular differentiation or quiescence influences its basal and insulin-stimulated secretion. Insulin and IGF-I are stimulators of IGF-BP secretion, whereas dexamethasone inhibits IGF-BP secretion. Because these hormones control muscle cell growth and differentiation, the IGF-BP may play an important regulatory role in these processes.     

Characterization of dog prenodal peripheral lymph lipoproteins. Evidence for the peripheral formation of lipoprotein-unassociated apoA-I with slow pre-beta electrophoretic mobility

Dog plasma and prenodal peripheral lymph apoA-I distribution was examined by nondenaturing gradient gel electrophoresis-immunoblot analysis. In control dogs, plasma apoA-I could be localized to two distinct populations of particles with modal diameters of 8.4 nm and 10.4 nm. The smaller sized population accounted for over 50% of plasma apoA-I. Peripheral lymph apoA-I distribution was significantly different. The percentage of apoA-I localized to the 10.4 nm population was reduced by 40% and the modal diameter of the smaller HDL apoA-I population was significantly decreased by 0.1 nm. Additionally, peripheral lymph apoA-I could be localized to particles smaller than albumin (lipoprotein-unassociated apoA-I). The presence of lipoprotein-unassociated apoA-I particles was confirmed by gel filtration chromatography. Immunoblots of column fractions subjected to agarose electrophoresis revealed that these particles had slow pre-beta electrophoretic mobility. In dogs fed an atherogenic diet, lipoprotein-unassociated apoA-I particles with slow pre-beta electrophoretic mobility could be found in both plasma and peripheral lymph. With increasing degree of hypercholesterolemia, the relative amount of plasma lipoprotein-unassociated apoA-I tended to increase. In peripheral lymph, an increasing degree of hypercholesterolemia was associated with a decrease in the relative amount of lipoprotein-unassociated apoA-I. Instead, a population of large apoA-I particles (11-25 nm) became increasingly prominent.     

Synthesis and secretion of plasminogen activator inhibitor 1 by human endothelial cells in vitro. Effect of active site mutagenized tissue-type plasminogen activator

The effects of recombinant tissue-type plasminogen activator (rt-PA) and of an inactive mutant of rt-PA, obtained by mutagenesis of the active site Ser478 to Ala (rt-PA-Ala478), on the synthesis and secretion of plasminogen activator inhibitor-1 (PAI-1) by human umbilical vein endothelial cells (HUVEC) in culture were studied. Under base-line conditions, PAI-1 antigen secretion was 4.3 +/- 1.0 micrograms (mean +/- S.D., n = 8) per 10(6) cells in 24 h. This PAI-1 had a low specific activity (6,000 +/- 1,600 units/mg) and Mr of 50,000, which was not altered by addition of rt-PA. In HUVEC cultured with 2 micrograms/ml rt-PA-Ala478, PAI-1 antigen secretion was 2.1 +/- 0.8 micrograms (n = 5) per 10(6) cells in 24 h with a specific activity of 120,000 +/- 42,000 units/mg and Mr of 50,000. Addition of rt-PA to this conditioned medium resulted in generation of three main components: 16% migrated as an Mr 106,000 rt-PA.PAI-1 complex, 16% as an Mr 81,000 degraded rt-PA.PAI-1 complex and the remainder as an Mr 45,000 degradation product of PAI-1. HUVEC cultured with 2 micrograms/ml rt-PA secreted 3.9 +/- 0.6 micrograms (n = 8) PAI-1 antigen per 10(6) cells within 24 h, of which 20-50% occurred as intact or degraded complexes with t-PA (Mr 106,000 and 81,000) and the rest as an inactive Mr 45,000 degradation product of PAI-1. PAI-1 mRNA levels, determined by Northern blot analysis and expressed relative to beta-actin mRNA levels, were very similar for HUVEC cultured in the absence or the presence of rt-PA or rt-PA-Ala478. It is concluded that PAI-1 is secreted by HUVEC in culture in fully active form which spontaneously inactivates. PAI-1 can be stabilized by addition of rt-PA-Ala478 to the culture medium, resulting in a 20-fold increase in specific activity. Interaction of rt-PA with active PAI-1 produces both t-PA.PAI-1 complex and an inactive degradation product of PAI-1.     

Prebeta-migrating high density lipoprotein: quantitation in normal and hyperlipidemic plasma by solid phase radioimmunoassay following electrophoretic transfer

A quantitative solid phase immunoassay has been developed for the determination of the mass of electrophoretically separated prebeta apolipoprotein A-I (apoA-I) in human plasma. Conditions have been identified for the quantitative transfer and immunoblotting of the apolipoprotein in the absence of organic solvents or detergents. In normolipidemic plasma, the prebeta-migrating fraction of apoA-I represented 4.2 +/- 1.8% of total apoA-I (61 +/- 26 micrograms of apoA-I per ml of plasma). Significantly higher levels were found in hypercholesterolemia of genetic origin, in primary and secondary hypertriglyceridemia, and in congenital lecithin:cholesterol acyltransferase deficiency. In all cases prebeta-migrating apoA-I consisted in large part of low molecular weight lipoprotein species, compared to the size of the major, alpha-migrating apoA-I fraction.     

Purification and characterization of photosystem I and photosystem II core complexes from wild-type and phycocyanin-deficient strains of the cyanobacterium Synechocystis PCC 6803

Highly photoactive Photosystem I (PS I) and Photosystem II (PS II) core complexes have been isolated from the cyanobacterium Synechocystis Pasteur Culture Collection (PCC) 6803 and a phycocyanin-deficient mutant, enriched in PS II. Cell breakage using glass beads was followed by sucrose density gradient centrifugation and two high-performance liquid chromatography steps involving anion-exchange and hydroxyapatite. The PS I core complex has an apparent molecular mass of 300 +/- 20 kDa (including a detergent shell of about 50 kDa) and contains subunits of approximately 60, approximately 60, 18.5, 18.5, 16, 15, 10.5, 9.5, and 6.5 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblots; its antenna size is 75 +/- 5 chlorophyll/P-700. The PS II core complex has an apparent molecular mass of 310 +/- 20 kDa (including the detergent shell); subunits of 43, 37, 33, 29, and 10-11 kDa were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. The antenna size of the average PS II complex is 45 +/- 5 chlorophyll/primary quinone electron acceptor (QA). This preparation procedure also yields, as a byproduct, a highly purified cytochrome b6f complex. This complex contains four subunits of 38, 24, 19, and 15 kDa and b- and c-type cytochromes in a ratio of 2:1. Its apparent molecular mass of 180 +/- 20 kDa (including the detergent shell) is consistent with a monomeric complex.     

Phospholipid transfer protein (PLTP) causes proteolytic cleavage of apolipoprotein A-I

Plasma phospholipid transfer protein (PLTP) is a factor that plays an important role in HDL metabolism. In this study we present data suggesting that PLTP has an inherent protease activity. After incubation of HDL3 in the presence of purified plasma PLTP, the d < 1.25 g/ml particles (fusion particles) contained intact 28.2 kDa apoA-I while the d > 1.25 g/ml fraction (apoA-I-PL complexes) contained, in addition to intact apoA-I, a cleaved 23 kDa form of apoA-I. Purified apoA-I was also cleaved by PLTP and produced a similar 23 kDa apoA-I fragment. The cleavage of apoA-I increased as a function of incubation time and the amount of PLTP added. The process displayed typically an 8-10 h lag or induction period, after which the cleavage proceeded in a time-dependent manner. This lag-phase was necessary for the development of the cleavage activity during incubation at 37 degrees C. The specific apoA-I cleavage activity of different PLTP preparations varied between 0.4-0.8 microg apoA-I degraded/h per 1000 nmol per h of PLTP activity. The 23 kDa apoA-I fragment reacted with monoclonal antibodies specific for the N-terminal part of apoA-I, indicating that the apoA-I cleavage occurred in the C-terminal portion. The apoA-I cleavage products were further characterized by mass spectrometry. The 23 kDa fragment yielded a mass of 22.924 kDa, demonstrating that the cleavage occurs in the C-terminal portion of apoA-I between amino acid residues 196 (alanine) and 197 (threonine). The intact apoA-I and the 23 kDa fragment revealed identical N-terminal amino acid sequences. The cleavage of apoA-I could be inhibited with APMSF and chymostatin, suggesting that it is due to a serine esterase-type of proteolytic activity. Recombinant PLTP produced in CHO cells or using the baculovirus-insect cell system caused an apoA-I cleavage pattern identical to that obtained with plasma PLTP. The present results raise the question of whether PLTP-mediated proteolytic cleavage of apoA-I might affect plasma HDL metabolism by generating a novel kinetic compartment of apoA-I with an increased turnover rate.     

Biochemical characterization and substrate specificity of rat prostate kallikrein (S3): comparison with tissue kallikrein, tonin and T-kininogenase.

A tissue kallikrein-like enzyme encoded by S3 mRNA was purified to homogeneity from rat prostate gland. The apparent molecular mass of the prostate enzyme is 32 kDa as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). The intact 32 kDa enzyme is split into two bands of lower molecular mass, 18 and 14 kDa, under reducing conditions on SDS-PAGE. NH2-terminal amino acid sequence analyses of the intact enzyme and heavy and light chains revealed the identity to the translated sequence of a prostate kallikrein cDNA (S3). Isoelectric focusing indicated that the prostate enzyme is a basic protein with pI of 7.30-7.45. Specific activities of the prostate kallikrein toward angiotensin I, angiotensinogen and rat low M(r) kininogen as well as tripeptide chromogenic substrates were compared with those of tissue kallikrein, tonin and T-kininogenase. The kinin-releasing activity is inhibited by leupeptin, antipain, benzamidine and soybean trypsin inhibitor. A sensitive and specific radioimmunoassay for the rat prostate kallikrein shows that the immunoreactive kallikrein levels in prostate and submandibular gland were 23.78 +/- 2.62 micrograms/mg protein (n = 5) and 12.29 +/- 2.25 micrograms/mg protein (n = 5), respectively. The results indicate that the prostate kallikrein S3 is expressed at high levels in both prostate and submandibular glands.     

Quantitative changes in steroidogenic organelles in the corpus luteum of the pregnant rat in relation to progestin secretion on day 16 and in the morning and afternoon of day 22

The present study was carried out to determine whether the major steroidogenic organelles of luteal cells quantitatively reflect variations in ovarian steroid secretion rates during pregnancy in the rat. Assessments were made on day 16 and in the morning (AM) and afternoon (PM) of day 22 (term is day 23). Ovarian steroidogenesis differs both quantitatively and qualitatively between these stages of pregnancy, so together they provide an ideal physiological model to study structural-functional relationships in the ovary. Corpora lutea of five rats were examined at each stage after progesterone and 20 alpha-hydroxypregn-4-en-3-one (20 alpha-OHP) secretion rates had been determined by a venous outflow technique. Total progestin secretion (progesterone plus 20 alpha-OHP) fell from 32.5 +/- 5.2 to 9.8 +/- 1.2 micrograms/hr per ovary (mean +/- SEM) between day 16 and day 22 AM but then increased to 22.6 +/- 1.4 micrograms/hr per ovary by day 22 PM. The total volume of luteal cell cytoplasm was slightly greater at day 22 AM and PM than at day 16. Similarly, the volumes of smooth endoplasmic reticulum (SER), lipid droplets, and membrane-bound granules all increased, but the volume of mitochondria decreased slightly. In contrast, the surface areas of SER and the inner and outer mitochondrial membranes did not change between day 16 and day 22 AM but then increased substantially by day 22 PM. Therefore, steroid secretion rates per unit area of steroidogenic membrane showed no consistent pattern over the stages examined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Efflux of lipid from fibroblasts to apolipoproteins: dependence on elevated levels of cellular unesterified cholesterol.

Earlier work from this laboratory showed that enrichment of cells with free cholesterol enhanced the efflux of phospholipid to lipoprotein acceptors, suggesting that cellular phospholipid may contribute to high density lipoprotein (HDL) structure and the removal of sterol from cells. To test this hypothesis, we examined the efflux of [3H]cholesterol (FC) and [32P]phospholipid (PL) from control and cholesterol-enriched fibroblasts to delipidated apolipoproteins. The percentages of [3H]cholesterol and [32P]phospholipid released from control cells to human apolipoprotein A-I were 2.2 +/- 0.5%/24 h and 0.8 +/- 0.1%/24 h, respectively. When the cellular cholesterol content was doubled, efflux of both lipids increased substantially ([3H]FC efflux = 14.6 +/- 3.6%/24 h and [32P]PL efflux = 4.1 +/- 0.3%/24 h). Phosphatidylcholine accounted for 70% of the radiolabeled phospholipid released from cholesterol-enriched cells. The cholesterol to phospholipid molar ratio of the lipid released from cholesterol-enriched cells was approximately 1. This ratio remained constant throughout an incubation time of 3 to 48 h, suggesting that there was a coordinate release of both lipids. The concentrations of apoA-I, A-II, A-IV, E, and Cs that promoted half-maximal efflux of phospholipid from cholesterol-enriched fibroblasts were 53, 30, 68, 137, and 594 nM, respectively. With apoA-I and A-IV, these values for half-maximal efflux of phospholipid were identical to the concentrations that resulted in half-maximal efflux of cholesterol. Agarose gel electrophoresis of medium containing apoA-I that had been incubated with cholesterol-enriched fibroblasts revealed a particle with alpha to pre-beta mobility. We conclude that the cholesterol content of cellular membranes is an important determinant in the ability of apolipoproteins to promote lipid removal from cells. We speculate that apolipoproteins access cholesterol-phosphatidylcholine domains within the plasma membrane of cholesterol-enriched cells, whereupon HDL is generated in the extracellular compartment. The release of cellular lipid to apolipoproteins may serve as a protective mechanism against the potentially damaging effects of excess membrane cholesterol.