FRINK Linked Data Fragments server

Matches in SemOpenAlex for { <https://semopenalex.org/work/W2170183132> ?p ?o ?g. }

• W2170183132 endingPage "2691" @default.
• W2170183132 startingPage "2681" @default.
• W2170183132 abstract "The cellular and molecular mechanisms responsible for lipoprotein [a] (Lp[a]) catabolism are unknown. We examined the plasma clearance of Lp[a] and LDL in mice using lipoproteins isolated from human plasma coupled to radiolabeled tyramine cellobiose. Lipoproteins were injected into wild-type, LDL receptor-deficient (Ldlr−/−), and apolipoprotein E-deficient (Apoe−/−) mice. The fractional catabolic rate of LDL was greatly slowed in Ldlr−/− mice and greatly accelerated in Apoe−/− mice compared with wild-type mice. In contrast, the plasma clearance of Lp[a] in Ldlr−/− mice was similar to that in wild-type mice and was only slightly accelerated in Apoe−/− mice. Hepatic uptake of Lp[a] in wild-type mice was 34.6% of the injected dose over a 24 h period. The kidney accounted for only a small fraction of tissue uptake (1.3%). To test whether apolipoprotein [a] (apo[a]) mediates the clearance of Lp[a] from plasma, we coinjected excess apo[a] with labeled Lp[a]. Apo[a] acted as a potent inhibitor of Lp[a] plasma clearance. Asialofetuin, a ligand of the asialoglycoprotein receptor, did not inhibit Lp[a] clearance.In summary, the liver is the major organ accounting for the clearance of Lp[a] in mice, with the LDL receptor and apolipoprotein E having no major roles. Our studies indicate that apo[a] is the primary ligand that mediates Lp[a] uptake and plasma clearance. The cellular and molecular mechanisms responsible for lipoprotein [a] (Lp[a]) catabolism are unknown. We examined the plasma clearance of Lp[a] and LDL in mice using lipoproteins isolated from human plasma coupled to radiolabeled tyramine cellobiose. Lipoproteins were injected into wild-type, LDL receptor-deficient (Ldlr−/−), and apolipoprotein E-deficient (Apoe−/−) mice. The fractional catabolic rate of LDL was greatly slowed in Ldlr−/− mice and greatly accelerated in Apoe−/− mice compared with wild-type mice. In contrast, the plasma clearance of Lp[a] in Ldlr−/− mice was similar to that in wild-type mice and was only slightly accelerated in Apoe−/− mice. Hepatic uptake of Lp[a] in wild-type mice was 34.6% of the injected dose over a 24 h period. The kidney accounted for only a small fraction of tissue uptake (1.3%). To test whether apolipoprotein [a] (apo[a]) mediates the clearance of Lp[a] from plasma, we coinjected excess apo[a] with labeled Lp[a]. Apo[a] acted as a potent inhibitor of Lp[a] plasma clearance. Asialofetuin, a ligand of the asialoglycoprotein receptor, did not inhibit Lp[a] clearance. In summary, the liver is the major organ accounting for the clearance of Lp[a] in mice, with the LDL receptor and apolipoprotein E having no major roles. Our studies indicate that apo[a] is the primary ligand that mediates Lp[a] uptake and plasma clearance. Lipoprotein [a] (Lp[a]) is a LDL-like lipoprotein that has been associated with increased risk of coronary heart disease, stroke, and restenosis (1Scanu A.M. The role of lipoprotein(a) in the pathogenesis of atherosclerotic cardiovascular disease and its utility as predictor of coronary heart disease events.Curr. Cardiol. Rep. 2001; 3: 385-390Google Scholar). Lp[a], which closely resembles LDL in lipid composition, contains a single apolipoprotein B-100 (apoB-100) molecule and an additional apolipoprotein, called apolipoprotein [a] (apo[a]), which is connected via a disulfide linkage to apoB-100 (1Scanu A.M. The role of lipoprotein(a) in the pathogenesis of atherosclerotic cardiovascular disease and its utility as predictor of coronary heart disease events.Curr. Cardiol. Rep. 2001; 3: 385-390Google Scholar). Apo[a] is a polymorphic glycoprotein that contains repeating domains of varying length that are homologous to kringle IV of plasminogen. More than 30 different isoforms of apo[a] have been described in humans, ranging in size from <300 kDa to >800 kDa (2Hobbs H.H. White A.L. Lipoprotein(a): intrigues and insights.Curr. Opin. Lipidol. 1999; 10: 225-236Google Scholar). Plasma levels of Lp[a] vary greatly between individuals, from <1 mg/dl to >100 mg/dl (2Hobbs H.H. White A.L. Lipoprotein(a): intrigues and insights.Curr. Opin. Lipidol. 1999; 10: 225-236Google Scholar). The source of most of this variability in plasma concentrations is variability in the production rate of Lp[a] (3Rader D.J. Cain W. Ikewaki K. Talley G. Zech L.A. Usher D. B. Brewer Jr, H. The inverse association of plasma lipoprotein(a) concentrations with apolipoprotein(a) isoform size is not due to differences in Lp(a) catabolism but to differences in production rate.J. Clin. Invest. 1994; 93: 2758-2763Google Scholar, 4Rader D.J. Cain W. Zech L.A. Usher D. B. Brewer Jr, H. Variation in lipoprotein(a) concentrations among individuals with the same apolipoprotein (a) isoform is determined by the rate of lipoprotein(a) production.J. Clin. Invest. 1993; 91: 443-447Google Scholar), which, in turn, is controlled largely by the apo[a] gene locus (5Boerwinkle E. Leffert C.C. Lin J. Lackner C. Chiesa G. Hobbs H.H. Apolipoprotein(a) gene accounts for greater than 90% of the variation in plasma lipoprotein(a) concentrations.J. Clin. Invest. 1992; 90: 52-60Google Scholar). Apo[a] is synthesized by hepatocytes and rapidly associates with LDL after secretion to form Lp[a] in the sinusoids of the liver (2Hobbs H.H. White A.L. Lipoprotein(a): intrigues and insights.Curr. Opin. Lipidol. 1999; 10: 225-236Google Scholar, 6Koschinsky M.L. Marcovina S.M. Structure-function relationships in apolipoprotein(a): insights into lipoprotein(a) assembly and pathogenicity.Curr. Opin. Lipidol. 2004; 15: 167-174Google Scholar). Although the steps involved in Lp[a] production are known, the major mechanisms involved in Lp[a] clearance from plasma are not currently known. It has been suggested that the kidney might play a major role in Lp[a] clearance, after several clinical studies reported increased plasma Lp[a] levels in patients with renal failure (7Kostner K.M. Clodi M. Bodlaj G. Watschinger B. Horl W. Derfler K. Huber K. Decreased urinary apolipoprotein (a) excretion in patients with impaired renal function.Eur. J. Clin. Invest. 1998; 28: 447-452Google Scholar, 8Sechi L.A. Zingaro L. Carli S. De Sechi G. Catena C. Falleti E. Dell'Anna E. Bartoli E. Increased serum lipoprotein(a) levels in patients with early renal failure.Ann. Intern. Med. 1998; 129: 457-461Google Scholar). In addition, Kronenberg et al. (9Kronenberg F. Trenkwalder E. Lingenhel A. Friedrich G. Lhotta K. Schober M. Moes N. Konig P. Utermann G. Dieplinger H. Renovascular arteriovenous differences in Lp[a] plasma concentrations suggest removal of Lp[a] from the renal circulation.J. Lipid Res. 1997; 38: 1755-1763Google Scholar) used measurements of renovascular arteriovenous differences in Lp[a] plasma concentrations in patients without renal insufficiency to demonstrate that the human kidney may play an active role in Lp[a] catabolism. Several receptors that mediate the binding and uptake of lipoproteins containing apoB-100 have been proposed as receptors for Lp[a] catabolism. These include the LDL receptor (LDLR) (10Wiklund O. Angelin B. Olofsson S.O. Eriksson M. Fager G. Berglund L. Bondjers G. Apolipoprotein(a) and ischaemic heart disease in familial hypercholesterolaemia.Lancet. 1990; 335: 1360-1363Google Scholar, 11Floren C.H. Albers J.J. Bierman E.L. Uptake of Lp (a) lipoprotein by cultured fibroblasts.Biochem. Biophys. Res. Commun. 1981; 102: 636-639Google Scholar, 12Krempler F. Kostner G.M. Roscher A. Haslauer F. Bolzano K. Sandhofer F. Studies on the role of specific cell surface receptors in the removal of lipoprotein (a) in man.J. Clin. Invest. 1983; 71: 1431-1441Google Scholar, 13Havekes L. Vermeer B.J. Brugman T. Emeis J. Binding of Lp(a) to the low density lipoprotein receptor of human fibroblasts.FEBS Lett. 1981; 132: 169-173Google Scholar, 14Mbewu A.D. Bhatnagar D. Durrington P.N. Hunt L. Ishola M. Arrol S. Mackness M. Lockley P. Miller J.P. Serum lipoprotein(a) in patients heterozygous for familial hypercholesterolemia, their relatives, and unrelated control populations.Arterioscler. Thromb. 1991; 11: 940-946Google Scholar, 15Utermann G. Hoppichler F. Dieplinger H. Seed M. Thompson G. Boerwinkle E. Defects in the low density lipoprotein receptor gene affect lipoprotein (a) levels: multiplicative interaction of two gene loci associated with premature atherosclerosis.Proc. Natl. Acad. Sci. USA. 1989; 86: 4171-4174Google Scholar), megalin/gp330 (16Niemeier A. Willnow T. Dieplinger H. Jacobsen C. Meyer N. Hilpert J. Beisiegel U. Identification of megalin/gp330 as a receptor for lipoprotein(a) in vitro.Arterioscler. Thromb. Vasc. Biol. 1999; 19: 552-561Google Scholar), the LDL receptor-related protein (LRP) (17Marz W. Beckmann A. Scharnagl H. Siekmeier R. Mondorf U. Held I. Schneider W. Preissner K.T. Curtiss L.K. Gross W. et al.Heterogeneous lipoprotein (a) size isoforms differ by their interaction with the low density lipoprotein receptor and the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor.FEBS Lett. 1993; 325: 271-275Google Scholar), and the VLDL receptor (18Argraves K.M. Kozarsky K.F. Fallon J.T. Harpel P.C. Strickland D.K. The atherogenic lipoprotein Lp(a) is internalized and degraded in a process mediated by the VLDL receptor.J. Clin. Invest. 1997; 100: 2170-2181Google Scholar). The latter two receptors can mediate binding to lipoproteins through apoE. According to the secretion-capture model, apoE that is secreted by the liver rapidly binds to remnants associated with heparan sulfate proteoglycans. These captured lipoproteins are then cleared by receptor-mediated endocytosis (19Mahley R.W. Ji Z.S. Remnant lipoprotein metabolism: key pathways involving cell-surface heparan sulfate proteoglycans and apolipoprotein E.J. Lipid Res. 1999; 40: 1-16Google Scholar). ApoE has been proposed to play a similar role in Lp[a] catabolism (20van Barlingen H.H. Kleinveld H.A. Erkelens D.W. de Bruin T.W. Lipoprotein lipase-enhanced binding of lipoprotein(a) [Lp(a)] to heparan sulfate is improved by apolipoprotein E (apoE) saturation: secretion-capture process of apoE is a possible route for the catabolism of Lp(a).Metabolism. 1997; 46: 650-655Google Scholar). The current studies were designed to test potential pathways responsible for Lp[a] uptake in mice through the use of a radioiodinated, intracellularly trapped ligand, tyramine cellobiose (TC), which was covalently attached to human Lp[a] and injected into mouse models in the C57BL/6 genetic background. Our results demonstrate that in this model, the liver is the major organ responsible for the plasma clearance of Lp[a] and that the kidney plays a very minor role under normal conditions. In addition, we examined the role of the LDLR, apoE, and the asialoglycoprotein receptor (ASGPR) in the clearance of Lp[a] in mice. These studies show that the LDLR, apoE, and the ASGPR do not have a major role in Lp[a] catabolism in mice. Lastly, we demonstrated that the administration of excess apo[a] can effectively block the plasma clearance of Lp[a], suggesting that it is the apo[a] moiety that mediates the plasma clearance of Lp[a]. Wild-type C57BL/6, LDL receptor-deficient (Ldlr−/−), and apoE-deficient (Apoe−/−) mice were obtained from Jackson Laboratories (Bar Harbor, ME). Mice were maintained on a chow diet under a 12 h light/dark cycle. Lp[a] was isolated from single-donor plasma as described previously (3Rader D.J. Cain W. Ikewaki K. Talley G. Zech L.A. Usher D. B. Brewer Jr, H. The inverse association of plasma lipoprotein(a) concentrations with apolipoprotein(a) isoform size is not due to differences in Lp(a) catabolism but to differences in production rate.J. Clin. Invest. 1994; 93: 2758-2763Google Scholar, 4Rader D.J. Cain W. Zech L.A. Usher D. B. Brewer Jr, H. Variation in lipoprotein(a) concentrations among individuals with the same apolipoprotein (a) isoform is determined by the rate of lipoprotein(a) production.J. Clin. Invest. 1993; 91: 443-447Google Scholar, 21Rader D.J. Mann W.A. Cain W. Kraft H.G. Usher D. Zech L.A. Hoeg J.M. Davignon J. Lupien P. Grossman M. et al.The low density lipoprotein receptor is not required for normal catabolism of Lp(a) in humans.J. Clin. Invest. 1995; 95: 1403-1408Google Scholar). Briefly, plasma was immediately brought to 0.01% NaN3, 0.01% Na2EDTA, and 1 mM benzamidine and adjusted to a density of 1.21 g/ml, and total lipoproteins were isolated by ultracentrifugation. The total lipoprotein fraction was dialyzed against phosphate buffer (0.1 M sodium phosphate, 0.01% NaN3, 0.01% Na2EDTA, and 1 mM benzamidine, pH 7.4) and then applied to a lysine-Sepharose column (Amersham Pharmacia) equilibrated with phosphate buffer. The column was then washed with phosphate buffer, and the unbound fraction, which contained LDL, was collected. The bound fraction, which is composed mostly of Lp[a], was eluted with phosphate buffer containing 100 mM ε-aminocaproic acid (ACA). This fraction was brought to 7.5% (w/w) CsCl and centrifuged in a Beckman 60Ti rotor at 50,000 rpm and 15°C for 27 h. The self-forming density gradient was pumped from the centrifuge tubes. Those fractions containing purified Lp[a] were pooled and dialyzed against PBS (0.1 M NaCl, 0.01 M sodium phosphate, 0.01% NaN3, 0.01% Na2EDTA, and 1 mM benzamidine, pH 7.4). The unbound fraction from the lysine-Sepharose column was dialyzed against histidine buffer (0.2 M NaCl, 0.025 M histidine, and 0.01% Na2EDTA, pH 6.0) and applied to a column containing PBE94 (Bio-Rad) equilibrated with histidine buffer. The column was washed with histidine buffer, and the unbound fraction, which contained LDL, was collected and dialyzed against PBS. This fraction was concentrated using an Amicon stirred pressure cell, and the density was then adjusted to 1.35 g/ml with NaBr. Five milliliters of this solution was placed into 25 ml Beckman 60Ti centrifuge tubes and overlaid with a 0–25% NaBr gradient. The tubes were centrifuged for 90 min at 15°C and 50,000 rpm. The density gradient was pumped from the centrifuge tubes, and those fractions containing purified LDL were pooled and dialyzed against PBS. Apo[a] was prepared by a modification of the procedure described by Edelstein et al. (22Edelstein C. Mandala M. Pfaffinger D. Scanu A.M. Determinants of lipoprotein(a) assembly: a study of wild-type and mutant apolipoprotein(a) phenotypes isolated from human and rhesus monkey lipoprotein(a) under mild reductive conditions.Biochemistry. 1995; 34: 16483-16492Google Scholar). Lp[a] was brought to a final concentration of 100 mM ACA and 2 mM dithioerythritol and incubated at room temperature for 1 h. The reaction mixture was dialyzed against PBS for ∼1 h and adjusted to a density of 1.3 g/ml with NaBr. Approximately 5 ml of the density-adjusted reaction mix was placed into a Beckman 60Ti centrifuge tube and overlaid with a 10–25% NaBr gradient to a volume of 25 ml. This was centrifuged at 50,000 rpm and 15°C for 90 min to separate apo[a] from the LDL released by reduction and intact Lp[a]. The density gradient was pumped from the centrifuge tubes, and those fractions containing purified apo[a] were pooled, dialyzed against PBS, and concentrated using Vivaspin concentrators (Vivascience). The BCA protein assay (Pierce) was used to quantify all lipoprotein and apolipoprotein preparations. When Lp[a] was coinjected with excess apo[a], we estimated the molar excess of apo[a] as follows. We determined that the apparent molecular weight of apo[a] used in this study was the same as that of apoB-100 and made the simplifying assumption that an equimolar amount of Lp[a] would have twice the amount of protein as apo[a] when measured by the BCA assay. The fold molar excess was then calculated as (micrograms of apo[a] injected)/(micrograms of Lp[a] injected × 0.5). Lipoproteins, apo[a], and asialofetuin (from fetal calf serum; Sigma) were directly iodinated with either Na131I or Na125I (New England Nuclear) using a modification of the iodine monochloride procedure (4Rader D.J. Cain W. Zech L.A. Usher D. B. Brewer Jr, H. Variation in lipoprotein(a) concentrations among individuals with the same apolipoprotein (a) isoform is determined by the rate of lipoprotein(a) production.J. Clin. Invest. 1993; 91: 443-447Google Scholar); alternatively, TC was iodinated with either Na131I or Na125I and then chemically coupled to LDL or Lp[a] using the procedure described by Pittman and Taylor (23Pittman R.C. A. Taylor Jr, C. Methods for assessment of tissue sites of lipoprotein degradation.Methods Enzymol. 1986; 129: 612-628Google Scholar). Typically, 0.1 μmol of TC was iodinated using Iodobeads (Pierce) and then chemically coupled to ∼5 mg of protein with 0.1 μmol of cyanuric chloride. The purity of the radiolabeled preparations was verified using SDS-PAGE and 1% agarose native gels (Ciba Corning). Radioiodinated lipoproteins in saline were injected via a tail vein. Blood was drawn from the orbital sinus into heparin-coated tubes, and plasma was separated by low-speed centrifugation. Radioactivity in 10 μl aliquots was quantified in a γ counter. The fractional catabolic rates (FCRs) were calculated using SAAM II (SAAM Institute, Seattle, WA). Tissue uptake of proteins labeled with TC was examined in mice housed in metabolic cages (four mice per cage). One day after injection of radiotracer, the mice were anesthetized and bled from the inferior vena cava immediately followed by perfusion with saline. Unless stated otherwise, organs were removed whole, weighed, and counted. Tissue samples were taken from visceral adipose tissue, muscle from the hind leg, and skin from the abdomen. These samples were weighed and counted. The accumulations of radiotracer in skin, muscle, and adipose tissue were calculated by assuming that these tissues accounted for 16.5, 38.4, and 6.4% (w/w), respectively, of the mass of a mouse (24Brown R.P. Delp M.D. Lindstedt S.L. Rhomberg L.R. Beliles R.P. Physiological parameter values for physiologically based pharmacokinetic models.Toxicol. Ind. Health. 1997; 13: 407-484Google Scholar). Feces and urine were collected and counted. The injected dose was calculated using the plasma counts at 5 min as follows: (counts per minute per milliliter of plasma at 5 min) × (mouse weight in grams) × (0.035 ml of plasma per gram of mouse body weight). The counts remaining in the plasma at 24 h were calculated as follows: (counts per minute per milliliter of plasma at 24 h) × (mouse weight in grams) × (0.035 ml of plasma per gram of mouse body weight). The tissue distribution of radiotracer is expressed as a percentage of the injected dose. All statistical analyses were performed using the unpaired two-tailed Student's t-test. Lipoproteins were either radiolabeled directly by the iodine monochloride method (4Rader D.J. Cain W. Zech L.A. Usher D. B. Brewer Jr, H. Variation in lipoprotein(a) concentrations among individuals with the same apolipoprotein (a) isoform is determined by the rate of lipoprotein(a) production.J. Clin. Invest. 1993; 91: 443-447Google Scholar) or, alternatively, radiolabeled TC was chemically cross-linked to lipoproteins by the method of Pittman and Taylor (23Pittman R.C. A. Taylor Jr, C. Methods for assessment of tissue sites of lipoprotein degradation.Methods Enzymol. 1986; 129: 612-628Google Scholar). For each study, lipoprotein preparations were analyzed by SDS-PAGE under reducing and nonreducing conditions. A representative gel is shown in Fig. 1A. The Lp[a] used in this study had a single apo[a] isoform with the same apparent molecular weight as apoB-100, and this was verified by immunoblotting (data not shown). Nondenaturing agarose gel electrophoresis was also performed (Fig. 1B), and for all preparations, a single band with preβ mobility was observed. Lipoproteins labeled by either of the two methods showed essentially identical migration patterns when examined using both electrophoretic methods. The 125I-TC-Lp[a] contained ∼10% of the label on apo[a] and the remainder on apoB-100 (data not shown). To assess which organs or tissues were responsible for Lp[a] uptake and catabolism, male C57BL/6 mice were injected simultaneously with both 125I-TC-Lp[a] and 131I-Lp[a]. For comparison, another set of male mice was injected with 125I-TC-LDL and 131I-LDL. The clearance of LDL was significantly faster than the clearance of Lp[a] for both 125I-TC-labeled lipoproteins (P = 0.02) and 131I-labeled lipoproteins (P = 0.0002) (Fig. 2). Furthermore, the plasma clearance of 125I-TC-labeled lipoproteins was similar to that of the 131I-labeled forms. The plasma clearance of 125I-TC-LDL (3.0 ± 0.2 d−1) and 131I-LDL (3.2 ± 0.3 d−1) were not significantly different (P = 0.3), whereas the clearance of 125I-TC-Lp[a] (2.0 ± 0.2 d−1) was slightly slower than that of 131I-Lp[a] (2.5 ± 0.3 d−1) (P = 0.03). After 24 h, the animals were euthanized and tissues counted. The results are shown in Fig. 3, and values are expressed as percentages of the injected dose retained by each tissue. The tissue distribution of TC-Lp[a] and TC-LDL were quite similar. The liver was the major organ that accumulated the TC-labeled lipoproteins, 21.3 ± 3.1% of the injected Lp[a] and 20.5 ± 2.5% of the injected LDL. The kidney contained only 1.3 ± 0.3% of the Lp[a] and <1% of the LDL. Intestinal tissue contained 2.1 ± 0.3% and 1.9 ± 0.5% of the Lp[a] and LDL, respectively. Combined values for the stomach, spleen, adrenals, testes, heart, lungs, and brain contained <2% of either lipoprotein. We used tissue samples to estimate the amount of TC-labeled lipoproteins that accumulated in the skin, muscle, and adipose tissue (see Methods). Both the muscle and skin retained a considerable amount of lipoprotein. The muscle contained 11.7 ± 2.0% of the Lp[a] and 7.9 ± 4.4% of the LDL, whereas the skin retained 3.2 ± 1.1% of the Lp[a] and 4.0 ± 0.3% of the LDL. Adipose tissue retained only a small amount of label. As expected, the tissue distribution of 131I-LDL and 131I-Lp[a] that lacked the trapped TC ligand was considerably different from that of the TC-labeled lipoproteins (Fig. 3). The liver retained relatively little LDL and Lp[a] when these were not coupled to TC. Most of the labeled lipoprotein retained by tissues was found in the muscle and skin. The largest amount of radiolabel was recovered in the urine and accounted for 43% and 39% of the injected dose for LDL and Lp[a], respectively. This is an underestimation because there were some urine losses during some of the mouse bleeds. Indeed, in a similar study, when blood samples were not taken over the course of the urine collection, the radiolabel recovered in the urine accounted for almost all of the counts not recovered in the carcass when lipoproteins were labeled directly with iodine (data not shown). To determine whether the 125I-TC-Lp[a] injected into mice was still intact in the plasma after 24 h, plasma samples were adjusted to 7.5% (w/w) CsCl and ultracentrifuged to separate Lp[a] from LDL. Under these conditions, LDL floats near the top of the self-forming density gradient, whereas Lp[a] is found toward to the bottom. Figure 4clearly shows that almost all of the radiolabel in the plasma after 24 h was found as intact Lp[a]. Free apo[a], which would be found in the bottom of the tube, was not detected. Wild-type, Ldlr−/−, and Apoe−/− male mice were injected with both 125I-TC-Lp[a] and 131I-TC-LDL. The plasma clearance of LDL was considerably slower in Ldlr−/− mice than in wild-type controls (1.47 vs. 2.96 d−1; P = 0.0002), whereas the clearance was accelerated considerably in Apoe−/− mice (4.69 vs. 2.96 d−1; P < 0.0001) (Fig. 5A). In contrast, the plasma clearance rate of Lp[a] was not affected significantly by the absence of the LDLR and was modestly increased by 15% in the Apoe−/− mouse (Fig. 5B) compared with the wild-type mouse (2.95 vs. 2.43 d−1; P = 0.01). The distribution of the trapped TC ligand among the tissues of the three mouse strains is shown in Table 1. As shown previously for wild-type mice, the uptake of radiolabel by different tissues was very similar for TC-Lp[a] and TC-LDL when injected into wild-type mice, with the liver accounting for 23.2% and 21.4% of the injected dose for TC-Lp[a] and TC-LDL, respectively. LDLR deficiency resulted in a markedly lower accumulation of TC-LDL in the liver (8.0%) but only a small decrease in the accumulation of TC-Lp[a] in the liver (19.5%). Apoe−/− mice had a marked increase in the accumulation of TC-LDL in the liver (41.8%) but only a small increase in the accumulation of TC-Lp[a] in the liver (27.7%).TABLE 1Tissue distribution of 125I-TC-Lp[a] and 131I-TC-LDLTC-LDLTC-Lp[a]OrganApoe−/−Wild TypeLdlr−/−Apoe−/−Wild TypeLdlr−/−Liver41.76 ± 0.68aP < 0.0005.21.39 ± 3.118.00 ± 0.56aP < 0.0005.27.74 ± 1.40bP < 0.05.23.17 ± 2.3419.53 ± 1.72bP < 0.05.Intestine and contents6.10 ± 0.58bP < 0.05.9.26 ± 1.235.76 ± 1.12bP < 0.05.4.78 ± 0.58bP < 0.05.6.42 ± 1.094.08 ± 1.06bP < 0.05.Feces23.8513.606.299.487.216.2Stomach and contents0.36 ± 0.060.34 ± 0.170.81 ± 0.30bP < 0.05.0.40 ± 0.040.40 ± 0.220.88 ± 0.35bP < 0.05.Adrenals0.29 ± 0.12bP < 0.05.0.13 ± 0.050.04 ± 0.01bP < 0.05.0.07 ± 0.020.03 ± 0.010.02 ± 0.01Spleen0.98 ± 0.051.48 ± 0.620.81 ± 0.040.93 ± 0.030.91 ± 0.270.43 ± 0.03bP < 0.05.Gonads0.09 ± 0.00bP < 0.05.0.13 ± 0.020.11 ± 0.050.08 ± 0.010.10 ± 0.020.07 ± 0.03Heart0.07 ± 0.020.06 ± 0.010.07 ± 0.020.17 ± 0.050.13 ± 0.030.09 ± 0.02Lungs0.19 ± 0.030.23 ± 0.070.27 ± 0.090.35 ± 0.100.34 ± 0.130.20 ± 0.04Brain0.02 ± 0.000.03 ± 0.010.02 ± 0.01bP < 0.05.0.02 ± 0.000.03 ± 0.010.01 ± 0.01bP < 0.05.Skin3.13 ± 0.114.66 ± 3.405.56 ± 3.085.97 ± 1.475.75 ± 3.173.67 ± 1.96Muscle2.91 ± 0.358.61 ± 6.186.77 ± 1.784.58 ± 0.6311.39 ± 8.105.54 ± 1.53Adipose0.47 ± 0.201.39 ± 2.682.72 ± 4.120.71 ± 0.332.08 ± 4.202.48 ± 4.11Kidney1.40 ± 0.081.10 ± 0.430.61 ± 0.140.94 ± 0.051.06 ± 0.160.82 ± 0.09bP < 0.05.Urine17.166.622.5911.697.163.97Plasma7.47 ± 0.23bP < 0.05.11.02 ± 1.4826.27 ± 3.00aP < 0.0005.11.90 ± 1.13bP < 0.05.16.33 ± 1.6116.32 ± 2.04Apoe−/− , apolipoprotein E-deficient; Ldlr−/− , LDL receptor-deficient; Lp[a], lipoprotein [a]; TC, tyramine cellobiose. Mice were injected with both 125I-TC-Lp[a] and 131I-TC-LDL. Wild-type (n = 7), Ldlr−/− (n = 4), and Apoe−/− (n = 3) mice were housed in separate metabolic cages. Animals were euthanized after 24 h, and the organs and tissue samples were collected. Plasma samples were also taken, and the results are shown in Fig. 4. The results are expressed as percentages of the injected dose. Data for feces and urine are from pooled samples. t-tests were used to compare each tissue in the knockout lines with wild-type tissues.a P < 0.0005.b P < 0.05. Open table in a new tab Apoe−/− , apolipoprotein E-deficient; Ldlr−/− , LDL receptor-deficient; Lp[a], lipoprotein [a]; TC, tyramine cellobiose. Mice were injected with both 125I-TC-Lp[a] and 131I-TC-LDL. Wild-type (n = 7), Ldlr−/− (n = 4), and Apoe−/− (n = 3) mice were housed in separate metabolic cages. Animals were euthanized after 24 h, and the organs and tissue samples were collected. Plasma samples were also taken, and the results are shown in Fig. 4. The results are expressed as percentages of the injected dose. Data for feces and urine are from pooled samples. t-tests were used to compare each tissue in the knockout lines with wild-type tissues. Pittman and Taylor (23Pittman R.C. A. Taylor Jr, C. Methods for assessment of tissue sites of lipoprotein degradation.Methods Enzymol. 1986; 129: 612-628Google Scholar) showed in rat and in rabbit that trapped radiotracer from TC-labeled plasma protein did not redistribute between tissues after initial uptake, within 24 h after injection. However, they did detect significant leakage of radiolabel from the liver, which appeared in the gut contents and the feces. Because the liver is a major site for Lp[a] and LDL uptake in mice, we believe that radiotracer appearing in the gut contents and feces is probably hepatic in origin. Therefore, a better estimate for the hepatic uptake and catabolism of LDL and Lp[a] in mice would be obtained by adding the amounts recovered in the liver, gut contents, and feces. The net hepatic contribution to plasma clearance was calculated and expressed as a percentage of the injected dose (Fig. 6). In these studies, the intestinal tissue and contents were not separated. We estimated the net hepatic clearance as the sum of the label found in the liver, intestine with contents, and feces. Because intestinal tissue contained only ∼2% of the injected dose for both TC-LDL and TC-Lp[a] in wild-type mice, the error caused by including the intestinal tissue in the net hepatic uptake should be relatively small. In wild-type mice, the net hepatic uptake of TC-labeled LDL after 24 h was 41.7 ± 2.3% of the injected dose. Hepatic uptake of TC-LDL was decreased by >2-fold in Ldlr−/− mice to 18.7 ± 0.7% (P < 0.0001), whereas hepatic uptake in Apoe−/− mice was increased nearly 2-fold to 70.25 ± 4.0% (P < 0.0001). In contrast, hepatic uptake of TC-Lp[a] showed a small decrease in Ldlr−/− mice to 27.9 ± 1.3% compared with 34.6 ± 3.1% in wild-type mice (P = 0.003), and it increased in Apoe−/− mice to 39.33 ± 2.1% (P = 0.04). Apo[a] that was purified from Lp[a] had the same apparent molecular weight as apoB-100 when analyzed by SDS-PAGE under reducing conditions and, as expected, migrated considerably faster under nonreducing conditions (Fig. 1C). The plasma clearance of 125I-TC-apo[a] was monitored in four male mice, and after 24 h, tissues were harvested and counted. Plasma clearance of apo[a] was faster than that of Lp[a], with a FCR of 7.38 ± 0.53 d−1. The tissue distribution of 125I-TC-apo[a] was similar to that for Lp[a] (Fig. 3C). The liver was the predominant site of apo[a] catabolism and retained 34.7 ± 1.9% of the injected dose, whereas the kidney retained only 0.9 ± 0.2%. The hepatic contribution to plasma clearance (" @default.
• W2170183132 created "2016-06-24" @default.
• W2170183132 creator A5010842897 @default.
• W2170183132 creator A5018054391 @default.
• W2170183132 creator A5052727162 @default.
• W2170183132 creator A5055785240 @default.
• W2170183132 creator A5066826351 @default.
• W2170183132 creator A5067149045 @default.
• W2170183132 creator A5081502914 @default.
• W2170183132 date "2005-12-01" @default.
• W2170183132 modified "2023-10-06" @default.
• W2170183132 title "Lipoprotein [a] is cleared from the plasma primarily by the liver in a process mediated by apolipoprotein [a]" @default.
• W2170183132 cites W1516430535 @default.
• W2170183132 cites W1531104076 @default.
• W2170183132 cites W1575753161 @default.
• W2170183132 cites W1894209555 @default.
• W2170183132 cites W1964777313 @default.
• W2170183132 cites W1967523354 @default.
• W2170183132 cites W1967662295 @default.
• W2170183132 cites W1968921241 @default.
• W2170183132 cites W1975168532 @default.
• W2170183132 cites W1975403309 @default.
• W2170183132 cites W1979264461 @default.
• W2170183132 cites W1981135463 @default.
• W2170183132 cites W1985025463 @default.
• W2170183132 cites W1985099812 @default.
• W2170183132 cites W1986353574 @default.
• W2170183132 cites W1991387414 @default.
• W2170183132 cites W1993729162 @default.
• W2170183132 cites W1994273046 @default.
• W2170183132 cites W1996358023 @default.
• W2170183132 cites W1998023491 @default.
• W2170183132 cites W2002146594 @default.
• W2170183132 cites W2006030777 @default.
• W2170183132 cites W2010758833 @default.
• W2170183132 cites W2013493542 @default.
• W2170183132 cites W2013525819 @default.
• W2170183132 cites W2019345011 @default.
• W2170183132 cites W2023996177 @default.
• W2170183132 cites W2025043970 @default.
• W2170183132 cites W2031110027 @default.
• W2170183132 cites W2032838825 @default.
• W2170183132 cites W2039988386 @default.
• W2170183132 cites W2053607106 @default.
• W2170183132 cites W2058795920 @default.
• W2170183132 cites W2061688860 @default.
• W2170183132 cites W2062899313 @default.
• W2170183132 cites W2068733672 @default.
• W2170183132 cites W2071679006 @default.
• W2170183132 cites W2077688960 @default.
• W2170183132 cites W2079790612 @default.
• W2170183132 cites W2081157663 @default.
• W2170183132 cites W2083800045 @default.
• W2170183132 cites W2089361884 @default.
• W2170183132 cites W2089940523 @default.
• W2170183132 cites W2093535758 @default.
• W2170183132 cites W2104937328 @default.
• W2170183132 cites W2105042387 @default.
• W2170183132 cites W2113106232 @default.
• W2170183132 cites W2117988339 @default.
• W2170183132 cites W2147029096 @default.
• W2170183132 cites W2150436995 @default.
• W2170183132 cites W2160728277 @default.
• W2170183132 cites W2161230420 @default.
• W2170183132 cites W2166747423 @default.
• W2170183132 cites W2182476310 @default.
• W2170183132 cites W2186577366 @default.
• W2170183132 cites W2399722862 @default.
• W2170183132 cites W947931977 @default.
• W2170183132 doi "https://doi.org/10.1194/jlr.m500249-jlr200" @default.
• W2170183132 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/16150825" @default.
• W2170183132 hasPublicationYear "2005" @default.
• W2170183132 type Work @default.
• W2170183132 sameAs 2170183132 @default.
• W2170183132 citedByCount "98" @default.
• W2170183132 countsByYear W21701831322012 @default.
• W2170183132 countsByYear W21701831322013 @default.
• W2170183132 countsByYear W21701831322014 @default.
• W2170183132 countsByYear W21701831322015 @default.
• W2170183132 countsByYear W21701831322016 @default.
• W2170183132 countsByYear W21701831322017 @default.
• W2170183132 countsByYear W21701831322018 @default.
• W2170183132 countsByYear W21701831322019 @default.
• W2170183132 countsByYear W21701831322020 @default.
• W2170183132 countsByYear W21701831322021 @default.
• W2170183132 countsByYear W21701831322022 @default.
• W2170183132 countsByYear W21701831322023 @default.
• W2170183132 crossrefType "journal-article" @default.
• W2170183132 hasAuthorship W2170183132A5010842897 @default.
• W2170183132 hasAuthorship W2170183132A5018054391 @default.
• W2170183132 hasAuthorship W2170183132A5052727162 @default.
• W2170183132 hasAuthorship W2170183132A5055785240 @default.
• W2170183132 hasAuthorship W2170183132A5066826351 @default.
• W2170183132 hasAuthorship W2170183132A5067149045 @default.
• W2170183132 hasAuthorship W2170183132A5081502914 @default.
• W2170183132 hasBestOaLocation W21701831321 @default.
• W2170183132 hasConcept C126322002 @default.
• W2170183132 hasConcept C126894567 @default.

• next