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

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

Role of an intramolecular contact on lipoprotein uptake by the LDL receptor.

The LDL receptor (LDLR) is an endocytic receptor that plays a major role in the clearance of atherogenic lipoproteins from the circulation. During the endocytic process, the LDLR first binds lipoprotein at the cell surface and then traffics to endosomes, where the receptor releases bound lipoprotein. Release is acid-dependent and correlates with the formation of an intramolecular contact within the receptor. Human mutations at residues that form the contact are associated with familial hypercholesterolemia (FH) and the goal of the present study was to determine the role of contact residues on LDLR function. We show that mutations at nine contact residues reduce the ability of the LDLR to support lipoprotein uptake. Unexpectedly, only four of the mutations (W515A, W541A, H562Y and H586Y) impaired acid-dependent lipoprotein release. The remaining mutations decreased the lipoprotein-binding capacity of the LDLR through either reduction in the number of surface receptors (H190Y, K560W, H562Y and K582W) or reduction in the fraction of surface receptors that were competent to bind lipoprotein (W144A and W193A). We also examined three residues, distal to the contact, which were predicted to be necessary for the LDLR to adopt the acidic conformation. Of the three mutations we tested (G293S, F362A and G375S), one mutation (F362A) reduced lipoprotein uptake. Together, these data suggest that the intramolecular interface plays multiple roles in LDLR function.

VLDL-TG kinetics: a dual isotope study for quantifying VLDL-TG pool size, production rates, and fractional oxidation in humans.

Very-low-density lipoproteins (VLDLs) are large, complex particles containing both surface proteins (e.g., ApoB100) and core lipids, e.g., cholesterol and triglycerides (TG). Whereas ApoB100 kinetics have been thoroughly studied, accurate measurement of VLDL-TG kinetics have proven difficult due to either complex mathematics or laborious procedures. The present study was therefore designed to measure VLDL-TG kinetics by dual isotope ex vivo labeled VLDL-TG tracers and well-established kinetics equations (bolus injection or the primed continuous infusion). Ten healthy Caucasian men [age, 23 +/- 3 yr old (mean +/- SD); body mass index, 24.7 +/- 1.3 kg/m(2)] were included in the study. VLDL-TG rate of appearance (Ra) was measured using a dual-tracer technique ([9,10-(3)H]-labeled VLDL-TG and [1-(14)C]-labeled VLDL-TG) to allow comparison of various bolus decay curve fits with the Ra obtained by the primed continuous infusion (PCI; considered the gold standard). In addition, VLDL-TG fatty acid oxidation was measured as (14)CO(2) in exhaled breath, using the hyamine trapping technique. Following a bolus injection, tracer decay was better described by a biexponential than a monoexponential fit (r(2) = 0.99 +/- 0.01 vs. 0.97 +/- 0.04, respectively, P = 0.01). VLDL-TG Ra calculated using the PCI correlated significantly with the biexponential fit (rho = 0.62, P < 0.05), whereas this was not the case for the monoexponential fit (rho = -0.18, P = not significant). VLDL-TG Ra using the best fit of the bolus injection method (biexponential) was less than values obtained by the constant infusion technique [biexponential, 34.3 (range, 27.1-69.6) vs. PCI, 44.4 (range, 33.0-72.7), P < 0.05]. Fractional oxidation of VLDL-TG was 37.2 +/- 8.8% at 240 min corresponding to 198.8 +/- 55.9 kcal/day or 10.6 +/- 3.3% of resting energy expenditure (REE). Our data demonstrate that VLDL-TG Ra measured by a biexponential fit to a bolus decay curve correlates well with VLDL-TG Ra measured by a primed continuous infusion, and therefore that a "second" peripheral VLDL-TG compartment with rapid exchange of TG exists. VLDL-TG volume of distribution is therefore greater than previously anticipated. Finally our data supports that VLDL-TG contributes quantitatively to REE.

Lipoprotein metabolism during acute inhibition of lipoprotein lipase in the cynomolgus monkey.

To clarify the role of lipoprotein lipase (LPL) in the catabolism of nascent and circulating very low density lipoproteins (VLDL) and in the conversion of VLDL to low density lipoproteins (LDL), studies were performed in which LPL activity was inhibited in the cynomolgus monkey by intravenous infusion of inhibitory polyclonal or monoclonal antibodies. Inhibition of LPL activity resulted in a three- to fivefold increase in plasma triglyceride levels within 3 h. Analytical ultracentrifugation and gradient gel electrophoresis demonstrated an increase predominantly in more buoyant, larger VLDL (Sf 400-60). LDL and high density lipoprotein (HDL) cholesterol levels fell during this same time period, whereas triglyceride in LDL and HDL increased. Kinetic studies, utilizing radiolabeled human VLDL, demonstrated that LPL inhibition resulted in a marked decrease in the catabolism of large (Sf 400-100) VLDL apolipoprotein B (apoB). The catabolism of more dense VLDL (Sf 60-20) was also inhibited, although to a lesser extent. However, there was a complete block in the conversion of tracer in both Sf 400-100 and 60-20 VLDL apoB into LDL during LPL inhibition. Similarly, endogenous labeling of VLDL using [3H]leucine demonstrated that in the absence of LPL, no radiolabeled apoB appeared in LDL. We conclude that although catabolism of dense VLDL continues in the absence of LPL, this enzyme is required for the generation of LDL.

High density lipoproteins, but not other lipoproteins, provide a vehicle for sterol transport to bile.

Unesterified cholesterol (UC) that is taken up by the liver from lipoproteins is rapidly mixed by exchange with liver UC. Thus, it is not possible to quantitate the transport of UC from different lipoproteins into bile using radiolabeled UC. However, plant sterols do not exchange with UC and are secreted in bile with the same kinetics as UC. To compare the contribution to bile of sterols from different lipoproteins, we perfused isolated rat livers with VLDL, LDL, and HDL that were obtained from patients with hereditary phytosterolemia and were rich in plant sterols. After 30-min recirculating perfusions, hepatic concentrations of plant sterols were not different after different lipoproteins were perfused. However, biliary plant sterol secretion was markedly different: with the perfusion of either VLDL or LDL there was no increase in plant sterols in bile, but with perfusion of HDL, the secretion of plant sterols was increased two- to threefold (P = 0.0005). The increase in biliary plant sterols was detected 5-10 min after HDL was added to perfusates and was similarly large for each of three individual plant sterols that was tracked. Results show that when sterol transport from lipoproteins into bile can be determined, only HDL provides a vehicle for UC elimination in bile that is consistent with its putative function in reverse cholesterol transport.

[Effect of corticosterone and plasma lipoproteins on working capacity and ultrastructure of isolated rat heart].

Blood lipoproteins (very low density--VLDL, low density--LDL and high density--HDL) penetrate into the myocardium through capillary endothelial layer via receptor mediated endocytosis (all lipoproteins), nonreceptor uptake, or along interendothelial gaps (LDL). In the myocardium lipoproteins are captured by interstitial macrophages and are subjected to degradation in secondary liposomes. Their action on myocardium results in development of perivascular swelling and constriction of substantial portion of capillaries. However part of capillaries (about 20%) stay in a condition of vasodilation. Destructive changes revealed (myelin figures, mild lysis of myofibrils) are presumably caused by activation of lysosomal proteinases. Corticosterone lowered coronary flow velocity, while parameters of working capacity of the heart remained at control level. Combined use of corticosterone and VLDL suppressed myocardial functional activity, to a great extent because of diminishment of coronary flow velocity. Corticosterone and LDL exerted less pronounced negative effect. Corticosterone and HDL caused improvement of parameters of cardiac working capacity.

Postprandial VLDL-TG metabolism in type 2 diabetes.

BACKGROUND: Type 2 diabetes is associated with excess postprandial lipemia due to accumulation of chylomicrons and VLDL particles. This is a risk factor for development of cardiovascular disease. However, whether the excess lipemia is associated with an impaired suppression of VLDL-TG secretion and/or reduced clearance into adipose tissue is unknown. OBJECTIVE: We measured the postprandial VLDL-TG secretion, clearance and adipose tissue storage to test the hypothesis that impaired postprandial suppression of VLDL-TG secretion, combined with impaired VLDL-TG storage in adipose tissue, is associated with excess postprandial lipemia. DESIGN: We studied 11 men with type 2 diabetes and 10 weight-matched non-diabetic men using ex-vivo labeled VLDL-TG tracers during an oral high-fat mixed-meal tolerance test to measure postprandial VLDL-TG secretion, clearance and storage. In addition, adipose tissue biopsies were analyzed for LPL activity and cellular storage factors. RESULTS: Men with type 2 diabetes had greater postprandial VLDL-TG concentration compared to non-diabetic men. However, postprandial VLDL-TG secretion rate was similar in the two groups with equal suppression of VLDL-TG secretion rate (≈50%) and clearance rate. In addition, postprandial VLDL-TG storage was similar in the two groups in both upper body and lower body subcutaneous adipose tissue. CONCLUSIONS: Despite greater postprandial VLDL-TG concentration, men with type 2 diabetes have similar postprandial suppression of VLDL-TG secretion and a similar ability to store VLDL-TG in adipose tissue compared to non-diabetic men. This may indicate that abnormalities in postprandial VLDL-TG metabolism are a consequence of obesity/insulin resistance more than a result of type 2 diabetes per se.

Interaction of lipoproteins with type II pneumocytes in vitro: morphological studies, uptake kinetics and secretion rate of cholesterol.

Apart from dipalmitoyl phosphatidylcholine, cholesterol is the most abundant surfactant lipid. About 90 to 99% of cholesterol of the alveolar surfactant is derived from serum lipoproteins. The aim of this study was to identify the lipoprotein which preferentially supplements type II pneumocytes with cholesterol destined for surfactant production. Ultrastructural investigations revealed that type II pneumocytes bind and take up HDL, LDL and VLDL. Binding and uptake of VLDL occurred even in the presence of excess LDL indicating that, besides LDL receptors, type II pneumocytes express additional binding sites for VLDL. Type II pneumocytes in primary culture are able to take up cholesterol added in the form of HDL, LDL and VLDL. Cholesterol uptake was lowest from HDL and highest from VLDL. The maximal velocity of cholesterol uptake from VLDL was more than three times that of cholesterol uptake from LDL. The half-maximal saturation of cholesterol uptake from VLDL was nearly half that of LDL. From these kinetic data and the distribution of free cholesterol among the serum lipoproteins, we calculated that the cholesterol uptake from VLDL is more than three times that of cholesterol uptake from LDL. In double-labeling experiments type II pneumocytes secreted palmitic acid-labeled phospholipids together with labeled free cholesterol taken up from lipoproteins. The secretion rates of both phospholipids and free cholesterol were stimulated to nearly the same extent by isoproterenol. From our results we conclude that type II pneumocytes interact specifically with HDL, LDL and VLDL. Cholesterol taken up in the form of the individual lipoproteins shows no difference in its availability for the formation of cholesterol ester and surfactant by type II pneumocytes in vitro. Based on the kinetic studies, it appears that VLDL is the major gateway through which cholesterol is provided to satisfy the cholesterol requirements of type II pneumocytes for the synthesis of surfactant.

Peroxynitrite-modified 99mTc-beta-VLDL: tissue distribution and plasma clearance rate.

Free radicals superoxide (O(2)(-)) and nitric oxide (*NO) are generated by blood vessels and can rapidly react to produce a peroxynitrite anion (ONOO(-)), a powerful oxidant that modifies lipoproteins making them more atherogenic. The aim of this study was to investigate the effect of peroxynitrite-induced modifications on beta-very-low-density lipoprotein (beta-VLDL) as to its biodistribution and plasma clearance rate, as well as the uptake of these particles by THP-1 cells. After being injected into New Zealand White rabbits, the peroxynitrite-modified beta-VLDL (99mTc-per-beta-VLDL) was cleared from circulation faster than the native beta-VLDL (99mTc-nat-beta-VLDL) in both normocholesterolemic rabbits (NC) and in hypercholesterolemic rabbits (HC). In HC rabbits, the fractional clearance of 99mTc-labeled beta-VLDL was significantly lower than in NC rabbits. The in vivo studies showed that accumulation of 99mTc-labeled beta-VLDL, expressed per gram of tissue, followed the decreasing order: kidney > liver > spleen > adrenal gland >or= lung > aortic arch > heart >or= abdominal aorta > thoracic aorta > psoas muscle. The high accumulation in the kidneys suggests the processing of 99mTc-labeled apolipoproteins by receptors present in kidney cells. The accumulation of 99mTc-nat-beta-VLDL in the whole organ was the following: liver > kidney > heart > spleen > adrenal gland > aorta in HC and NC rabbits. The uptake of 99mTc-per-beta-VLDL by the spleen was greater than the uptake by the heart in both groups. The in vitro studies showed that the uptake of 99mTc-per-beta-VLDL by THP-1 cells was higher than that of 99mTc-nat-beta-VLDL. These results show that peroxynitrite-modified beta-VLDL is rapidly removed from plasma and accumulates in several tissues, mainly in the liver and kidney. This may be particularly important in hypercholesterolemic situations that could favor the accumulation of native and peroxynitrite-modified beta-VLDL in several tissues.

Transport pathways of beta-VLDL by aortic endothelium of normal and hypercholesterolemic rabbits.

The uptake and transport of beta-VLDL by the aortic endothelium was investigated in normal and hyperlipidemic rabbits fed a cholesterol-enriched diet for 1 week to 5 months. Weekly (in the first month) or every other week afterwards, animals were given one of the following probes: (a) [125I]-beta-VLDL injected in vivo and after 24 h the whole aorta or its intima and media were separately collected and examined by spectrometry and autoradiography; (b) [125I]-beta-VLDL coupled to the fluorescent probe 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate perfused in situ for 1-2 h and aorta examined by radioassay and fluorescence microscopy; (c) beta-VLDL-gold complex perfused in situ for 10-15 min and aortic fragments examined by electron microscopy. In addition, cryosections of aortic wall were processed for the immunocytochemical detection of apolipoprotein B and apolipoprotein E. The results showed that both in normal and hyperlipidemic rabbits, the aortic endothelium transports plasma beta-VLDL by a dual pathway: (i) endocytosis involving coated pits and vesicles, endosomes, multivesicular bodies and lysosomes, and (ii) transcytosis, the predominant process, carried out by plasmalemmal vesicles. Both processes, and especially transcytosis, are markedly increased in hyperlipidemia leading to progressive accumulation of beta-VLDL or/and its components in the subendothelial extracellular matrix. In prelesional stages of atherogenesis, beta-VLDL-gold complexes or deposits of apo B and apo E were detected in close association with extracellular liposomes. With the appearance of intimal macrophage-derived foam cells, the immunoperoxidase reaction product, revealing the presence of the two apolipoproteins, could also be seen in intracellular lipid inclusions.

Accumulation of lipoprotein fractions and subfractions in the arterial wall, determined in an in vitro perfusion system.

A large proportion of a dense subfraction of LDL in plasma is coupled with an increased risk of coronary artery disease, CAD. This may reflect an increased inflow of such LDL subfractions into the intima, since the inflow of lipoproteins is supposed to be inversely related to the size of the particles. In order to evaluate this possibility we used an in vitro perfusion system for aortic intima-media from rabbits with experimental atherosclerosis. The uptake of human VLDL, LDL, HDL and subfractions of LDL (LDL1, 1.019-1.035 and LDL2, 1.035-1.063 g/ml) in lesions and non-involved areas was studied. Our results indicate that particle size is an important factor for the clearance of lipoproteins into the arterial tissue, both for plaques (VLDL 7.6, LDL 25, HDL 58 nl/mg wet wt./h) and in other areas (VLDL 3.8, LDL 4.1, HDL 12 nl/mg wet wt./h). Interestingly, the uptake of LDL2 was as much as 1.5-1.9 times higher than LDL1. This supports the view that an increased lipid load in the arterial wall may be one mechanism behind the association between denser LDL and CAD. Our data also suggest that the difference between LDL uptake in plaque (576 nl/mg wet wt.) and other areas (48 nl/mg wet wt.) not only reflects a rapid clearance but a large distribution volume of the intima (plaque > 60%, non-involved areas 5.7%).

The metabolism of the xenobiotic triacylglycerols, rac-1- and sn-2- (3-phenoxybenzoyl)-dipalmitoylglycerol, following intravenous administration to the rat.

The metabolism of 3-phenoxybenzoic acid (3PBA) in the form of triacylglycerol conjugates was compared with that of non-esterified 3PBA. Three radiolabeled triacylglycerols (rac-1-(3-phenoxy-[ring-14C]-benzoyl)-2,3-dipalmitoylglycerol (1(3PBA)DPG), sn-2-(3-phenoxy-[ring-14C]benzoyl)-1,3-dipalmitoylglycerol (2(3PBA)DPG) and the "natural" tri-[1-14C]oleoylglycerol) were incorporated into rat VLDL. Nonesterified 3PBA was prepared in rat serum albumin solution. Each preparation was administered i.v. to rats and serial blood samples were taken during the subsequent 6 hr. Urine and faeces were collected and tissue residues determined at 6 hr and 48 hr after administration. Biphasic elimination of 3PBA was observed with half-lives of 18 min and 2 hr. The triacylglycerols showed a rapid first phase and a longer second phase half-life: trioleoylglycerol 26 hr, 1(3PBA)DPG 7.6 hr and 2(3PBA)DPG 17.3 hr. The majority (63-76%) of 3PBA (whether esterified or not) was eliminated within 24 hr in urine, which contained similar profiles of metabolites. The triacylglycerols gave rise to higher tissue residues than did non-esterified 3PBA, particularly in adipose tissue which alone was not significantly depleted of radioactivity between 6 and 48 hr. The results accord with the rapid association of the VLDL-(3PBA)DPG complexes with lipoprotein lipase of the capillary epithelium, followed by hydrolysis to 3PBA, metabolism and elimination but with a proportion being redistributed into adipose tissue, re-esterified and then eliminated relatively slowly.

MDL 29311, an analog of probucol, decreases triglycerides in rats by increasing hepatic clearance of very-low-density lipoprotein.

MDL 29311 is an analog of probucol that shares probucol's antioxidant and antiatherogenic properties. When fed to rats as a 1% dietary admixture, MDL 29311 decreased triglyceride levels by 65% without affecting total or high-density lipoprotein (HDL) cholesterol levels. Under the same conditions, probucol decreased triglyceride levels by 23% and total cholesterol levels by 29% (with a corresponding decrease in HDL cholesterol level). MDL 29311 treatment did not affect the rate of triglyceride entry into the plasma. However, MDL 29311-treated rats cleared in vivo-labeled very-low-density lipoprotein (VLDL)-associated [3H]-triglyceride ([3H]-VLDL) over threefold faster than control rats. This increase in clearance led to increased levels of [3H]-lipid in liver and decreased [3H]-lipid in fat, muscle, diaphragm, and kidney of MDL 29311-treated rats 1.5 to 2.0 minutes after injection of [3H]-VLDL. MDL 29311 treatment had no effect on lipoprotein lipase (LPL) or hepatic triglyceride lipase (H-TGL) activities, or on plasma apolipoprotein (apo) C-II-dependent LPL activation. Intravenously injected [3H]-VLDL was allowed to circulate in MDL 29311-treated or control rats for 1 minute, and the undiluted plasma was then perfused through rat livers in a recirculating system. The [3H] in MDL 29311 plasma was cleared threefold faster (t1/2, 1.3 v 3.8 minutes) than the [3H] in control plasma by control livers. Conversely, the [3H] in control plasma was cleared slowly (t1/2 = 3.5 minutes) by the livers of MDL 29311-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of L-carnitine treatment on very low density lipoprotein kinetics in the hyperlipidemic rabbit.

This study examined the hypolipidemic effect of 4 weeks of L-carnitine treatment (170 mg/kg b.w./day) in New Zealand White rabbits fed a high fat diet (5% corn oil/0.5% cholesterol). Specifically, [3H] glycerol and [125I] very low density lipoprotein (VLDL) turnover studies were conducted to examine the effect of treatment on VLDL kinetics. The masses of plasma VLDL-triglycerides (VLDL-TG) and VLDL-apoprotein B (VLDL-apoB) were significantly increased by the high-fat diet. Four weeks of treatment with L-carnitine significantly reduced these masses. Kinetic analysis indicated that fat feeding reduced the fractional catabolic rates (FCRs) of VLDL-TG and VLDL-apoB relative to chow-fed controls. The transport of these VLDL components was not altered by the diet. L-carnitine treatment had no effect on the FCRs of VLDL-TG and VLDL-apoB or on the transport of VLDL-apoB. Yet, treatment significantly lowered the transport of VLDL-TG. These data indicate that the lipid-lowering effect of L-carnitine in this animal model was due, in part, to a decrease in the transport and not due to an alteration in the fractional catabolic rate of VLDL-TG.

Stable isotope tracer dilution for quantifying very low-density lipoprotein-triacylglycerol kinetics in man.

BACKGROUND & AIM: A number of approaches have been employed in the past to measure very low-density lipoprotein (VLDL) triacylglycerol (TG) kinetics in humans in vivo, varying in the selection of tracer and mode of administration. All, however, make use of labeled TG precursors and more or less complicated mathematical models to derive the kinetic parameters of interest. The aim of the present study was to develop a conceptually straightforward method, based on the traditional tracer infusion technique, for quantifying VLDL-TG production rates in man using stable isotopes. METHOD: Our approach involves ingestion of [U-13C3]glycerol to endogenously label the glycerol in VLDL-TG, plasmapheresis, isolation of the newly 13C-labeled VLDL from plasma, and administration within the next 2-3 days via a primed constant autologous reinfusion. This procedure produces enough tracer for a priming dose plus 2-3 h of infusion. In the physiological conditions examined (basal and hyperglycemic states, fat- and carbohydrate-rich diets), with almost 3-fold ranging VLDL-TG pool sizes, a steady state in plasma VLDL-TG glycerol tracer-to-tracee ratio was readily achieved within 2 h. Consequently, calculations are made according to the isotope dilution principle, thus avoiding assumptions implicit in more complicated models. CONCLUSION: The stable isotope VLDL-TG tracer dilution method offers an alternative and reliable tool for the determination of endogenous VLDL-TG kinetics in man under a variety of metabolic states.

Plasma clearance and biodistribution of oxidatively modified 99mTc-beta-VLDL in rabbits.

The biodistribution and removal from plasma (measured as fractional clearance rate, FCR, per hour) of native and oxidatively modified 99mtechnetium-labeled beta-very low density lipoprotein (99mTc-beta-VLDL) were investigated in hypercholesterolemic (HC) and control (C) three-month old New Zealand rabbits. The intracellular accumulation of beta-VLDL labeled with 99mTc was studied in vitro in THP-1 cells and monocyte-derived macrophages isolated from rabbits. After intravenous injection into C rabbits, copper-oxidized beta-VLDL (99mTc-ox-beta-VLDL) was cleared from the circulation faster (0.362 +/- 0.070/h) than native beta-VLDL (99mTc-nat-beta-VLDL, 0.241 +/- 0.070/h). In contrast, the FCR of 99mTc-ox-beta-VLDL in HC rabbits was lower (0.100 +/- 0.048/h) than that of 99mTc-nat-beta-VLDL (0.163 +/- 0.043/h). The hepatic uptake of radiolabeled lipoproteins was lower in HC rabbits (0.114 +/- 0.071% injected dose/g tissue for 99mTc-nat-beta-VLDL and 0.116 +/- 0.057% injected dose/g tissue for 99mTc-ox-beta-VLDL) than in C rabbits (0.301 +/- 0.113% injected dose/g tissue for 99mTc-nat-beta-VLDL and 0.305 +/- 0.149% injected dose/g tissue for 99mTc-ox-beta-VLDL). The uptake of 99mTc-nat-beta-VLDL and 99mTc-ox-beta-VLDL by atherosclerotic aorta lesions isolated from HC rabbits (99mTc-nat-beta-VLDL: 0.033 +/- 0.012% injected dose/g tissue and 99mTc-ox-beta-VLDL: 0.039 +/- 0.017% injected dose/g tissue) was higher in comparison to that of non-atherosclerotic aortas from C rabbits (99mTc-nat-beta-VLDL: 0.023 +/- 0.010% injected dose/g tissue and 99mTc-ox-beta-VLDL: 0.019 +/- 0.010% injected dose/g tissue). However, 99mTc-nat-beta-VLDL and 99mTc-ox-beta-VLDL were taken up by atherosclerotic lesions at similar rates. In vitro studies showed that both monocyte-derived macrophages isolated from rabbits and THP-1 macrophages significantly internalized more 99mTc-ox-beta-VLDL than 99mTc-nat-beta-VLDL. These results indicate that in cholesterol-fed rabbits 99mTc-ox-beta-VLDL is slowly cleared from plasma and accumulates in atherosclerotic lesions. However, although the extent of in vitro uptake of 99mTc-ox-beta-VLDL by macrophages was high, the in vivo accumulation of this radiolabeled lipoprotein by atherosclerotic lesions did not differ from that of 99mTc-nat-beta-VLDL.

[Technetium 99m Tc labeled lipoproteins. I. Preparation and evaluation of quality]. / Lipoproteiny znacené techneciem 99m Tc. I. Príprava a hodnocení jakosti.

Technetium 99m Tc labelled lipoproteins are novel diagnostic agents suitable for the study of the lipoprotein metabolism and prospectively for picturing the specific receptors. The paper reports the first results of the preparation and quality evaluation of very low density lipoproteins (VLDL) labelled with technetium 99m Tc. This radionuclide is, due to its advantageous properties, preferentially employed in nuclear medicine. The present paper resulted in a successful attempt to bind technetium 99m Tc to a lipoprotein carrier with selective transport and targeted organ-specific biodistribution.

Measuring very low density lipoprotein-triglyceride kinetics in man in vivo: how different the various methods really are.

PURPOSE OF REVIEW: The purpose of this article is to briefly outline the methods that are currently available for the determination of very low density lipoprotein-triglyceride (VLDL-TG) kinetics in man in vivo. RECENT FINDINGS: A number of novel methodologies have been developed over the years for quantifying VLDL-TG production, clearance, and turnover rates. Besides the splanchnic arteriovenous balance technique, tracer methods with radioactive and, more recently, stable isotopes have been widely used. Most of the latter approaches utilize an isotopically labelled substrate, such as glycerol, fatty acid or acetate, which is eventually incorporated into a VLDL-TG moiety, and monitor the time course of change in specific activity or enrichment. A procedure of in vivo labelling of VLDL-TG with stable isotopes and use of the labelled VLDL-TG as a tracer has also been described in man. There is, however, considerable variability in estimates of VLDL-TG kinetics obtained by the various techniques, which cannot be readily attributed to normal physiological variation. Still, a large part of this discrepancy may be related to differences in VLDL-TG pool size within the normal range, which seem to account for approximately 40-50% of the total variance in VLDL-TG kinetics in both men and women. SUMMARY: Several methods are available for quantifying VLDL-TG kinetics in man in vivo, varying in the selection of tracer, mode of administration and sampling, and data analysis. These inherent features, along with different pool sizes, result in multifold variable estimations of VLDL-TG kinetic parameters.

Metabolism of canine beta-very low density lipoproteins in normal and cholesterol-fed dogs.

Cholesteryl ester-rich beta-very low density lipoproteins (beta-VLDL) are beta-migrating lipoproteins that accumulate in the plasma of cholesterol-fed animals and of patients with type III hyperlipoproteinemia. There are two distinct fractions: fraction I beta-VLDL are chylomicron remnants of intestinal origin, and fraction II beta-VLDL are cholesterol-rich VLDL of hepatic origin. The liver rapidly clears fraction I beta-VLDL from the plasma of both normal and cholesterol-fed dogs. The liver also clears fraction II beta-VLDL rapidly and efficiently from the plasma of normal dogs by receptor-mediated uptake. In cholesterol-fed dogs the clearance is biphasic: an initial rapid die-away of about 30% to 40% of the injected dose within 5 minutes, followed by a slow clearance of plasma radioactivity (a half-life of more than 20 hours). The rapid, initial phase of fraction II beta-VLDL clearance appears to be related to sequestration of the lipoproteins presumably on endothelial cells and is apparently associated with lipolytic processing. Treatment of the fraction II beta-VLDL with lipoprotein lipase abolishes this rapid phase. In the cholesterol-fed dog, the slow, late phase of clearance corresponds to the conversion of fraction II beta-VLDL to the smaller, denser intermediate and low density lipoproteins (IDL and LDL), which are slowly cleared from the plasma. It is concluded that fraction II beta-VLDL are catabolized in the normal dog by rapid uptake mediated at least in part by the apo B,E(LDL) receptor of hepatic parenchymal cells. In cholesterol-fed dogs, in which these receptors are markedly down-regulated, fraction II beta-VLDL are apparently initially bound to endothelial cells and converted to IDL and LDL by lipolytic processing.