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Abstract |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Alveolar type II cells accumulate vitamin E preferentially from high-density lipoproteins (HDL) and express at least three receptors that are specific for HDL. The expression of these receptors increases in response to vitamin E deficiency. Beside receptors for specific lipid transfer from HDL, cubilin and megalin, several other receptors that mediate HDL-particle uptake were found in the lung. We hypothesize that alveolar type II cells also exhibit the HDL-particle uptake and that this process can be regulated by the vitamin E status. By confocal laser microscopy and flow cytometry we showed that type II cells accumulate protein-labeled HDL-particle. Vitamin E depletion in rats increased HDL-particle uptake in alveolar type II cells and the expression of megalin. The expression of cubilin did not change. Refeeding with vitamin E reversed HDL-particle uptake and megalin expression. Long-time incubation of type II cells with phorbol myristyl acetate (PMA) reduced HDL-holoparticle uptake and megalin expression. We assume that alveolar type II cells exhibit HDL-holoparticle uptake mediated by megalin and cubilin. Megalin represents the regulated element of the megalin/cubilin receptor-cooperation and can be modulated by protein kinase C.
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Introduction |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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It is generally accepted that the interaction of high-density
lipoprotein (HDL) and cell surface for the exchange of lipids is mediated by specific receptors. The uptake of lipids
from HDL
especially of vitamin E, cholesterol and cholesterol esterscan proceed in two ways: (i) via the selective uptake pathway, i.e., lipids are transported from the
receptor-bound HDL into the cells without internalization
of the lipoprotein particle (1, 2); and (ii) receptor-mediated, i.e., the uptake of the intact HDL-particle, a process
analogous with endocytosis of low-density lipoprotein (LDL)
by the LDL-receptor, usually via coated pits (3, 4).
The rodent scavenger receptor BI (SR-BI), its splicing variant SR-BII, and the human homolog CLA-1 are the only known receptors mediating selective lipid exchange with HDL (5). Numerous investigations have shown that SR-BI plays a major role in the metabolism of cholesterol, at least in the liver and in steroidogenic organs (8). Recently we showed that SR-BI may be also involved in the uptake of vitamin E in liver and alveolar type II cells (11).
Beside this uptake mechanism, unambiguous results on the endocytosis of HDL have shown that the uptake of lipids via transport of intact HDL particle seems to exist in some cells and tissues (12). Recent reports on the identification of receptors mediating the particulate uptake of HDL have supported this view. Convincing evidence has presented that the peripheral membrane protein cubilin is involved in the HDL-holoparticle uptake using cells and fluorescence-labeled HDL (13).
The candidate HDL-receptor cubilin was originally isolated as the intrinsic factor/vitamin B12 receptor from small intestine. The protein is expressed in a higher level in the kidney and especially in epithelial cells of the yolk sac (15). In 1997, the structural identity of cubilin and the kidney cell surface glycoprotein gp280 was demonstrated, but the functional role in the HDL-holoparticle uptake was demonstrated only recently (13). Cubilin is a membrane protein that does not contain typical transmembrane sequences and convincing evidence has been presented that cubilin and megalin interact in concert to mediate particulate uptake of HDL as well as albumin absorption. Megalin contains a short transmembrane sequence (18). Both cubilin and megalin are expressed in the lung (21, 22).
We have shown that alveolar type II cells can accumulate vitamin E preferentially from HDL and express at least three different HDL-specific receptors. The expression of these receptors in alveolar type II cells and the vitamin E uptake from HDL increases in response to vitamin E deficiency and decreases concomitantly after refeeding the vitamin (23, 24). Alveolar type II cells have a vital role in the assembly of lung surfactant. Vitamin E, a constituent of the alveolar surfactant, is supplemented during surfactant formation in type II cells (25). Vitamin E protects surfactant lipids against oxidation (26) and it is worth speculating that type II cells afford different uptake mechanism of vitamin E from HDL to maintain or to restore a sufficient vitamin E level in cells and surfactant.
In this paper we investigate if alveolar type II cells exhibit HDL-holoparticle uptake, whether they express the candidate receptors cubilin and megalin, and whether the vitamin E status affects the HDL-particle uptake and receptor expression.
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Materials and Methods |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Feeding Regime
Wistar rats (body weight 80-90 g) from a local animal facility received the vitamin E-depleted diet (< 3 mg 
-tocopherol/kg; Altromin, Lage, Germany) ad libitum for 5 wk. Some of these rats received an -tocopherol-supplemented diet (400 mg RRR--
tocopherol/kg, Altromin) ad libitum for three days. The control
diet contained ~ 12 mg -tocopherol/kg; (Altromin).
Alveolar Type II Cell Preparation
The animals were anesthetized by intraperitoneal injection of 30 mg sodium pentobarbital in 1 ml heparin solution (5,000 IU). Alveolar type II cells were isolated by elastase digestion and "panning" cells on IgG-coated bacteriologic plastic dishes according to Dobbs and coworkers (27). Viability (90-95%) was judged by trypan blue staining and purity (83-90%) by Harris type hematoxylin staining of the isolated cells. Freshly isolated alveolar type II cells were suspended in Dubecco's Minimal Essential Medium (DMEM; Gibco BRL, Life Technologies, Paisley, Scotland) containing 10% (vol/vol) fetal calf serum, and aliquots were immediately used for the subsequent confocal laser microscopy and flow cytometric analysis (FACS). For the Western blot analysis, aliquots of the cell suspensions were shock-frozen in liquid N2 and stored at -85°C for maximum of 10 d.
Isolation and Protein Labeling of HDL
Ethylenediaminetetraacetic acid (EDTA) plasma from human males was isolated after phlebotomy. Lipoproteins were isolated by KBr/ NaCl-density gradient ultracentrifugation for 24 h in a Beckman SW 41 Ti rotor at 35,000 rpm as described by Chapman and colleagues (28), except that all gradient solutions contained 0.05% (wt/ vol) NaEDTA and 0.02% (wt/vol) NaN3. Fractions of 0.5 ml were removed from the meniscus downward by aspiration, and the content of apolipoproteins was analyzed by SDS-PAGE to avoid cross-contamination of HDL. ApoE-containing particles were removed from HDL fractions by affinity chromatography on heparin-Sepharose according to the method of Wilson and coworkers (29). The pooled density gradient fractions were desalted and transferred into TBS (50 mM Tris/HCl, pH 7.4, 150 mM NaCl) by passage through a column of Sephadex G-25 (PD-10; Amersham Pharmacia Biotech, Freiburg, Germany).
For the protein-labeling of HDL we used the Alexa Fluor Protein Labeling Kit from Molecular Probes (Leiden, The Netherlands) and followed the protocol of the manufacturer.
Alexa-labeling of HDL-proteins was controlled by SDS-PAGE in 14% polyacrylamide gels that were visualized using silver staining. Fluorescence signals of the fractionated proteins were directly detected using Kodak X-OMAT films (Eastman Kodak Company, Rochester, NY) (Figure 1).
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HDL-Holoparticle Uptake by Alveolar Type II Cells
Freshly isolated type II cells were incubated in the presence of 100 µg protein-labeled HDL (Alexa-HDL) per ml for 3 h at 37°C. Thereafter the fluorescence of Alexa-HDL bound to the surface of type II cells were quenched using the Phagotest Kit (OPREGEN Pharma, Heidelberg, Germany). Briefly, to 0.5 × 106 type cells suspended in 100 µl ice-cold Mg2+ and Ca2+ free Dulbecco's phosphate-buffered saline (PBS; Life Technologies, Karlsruhe, Germany) we added 100 µl ice-cold Quench-solution of the Phagotest Kit. After 2 to 3 s 1 ml ice-cold PBS was added and centrifuged for 5 min at 900 × g and 4°C. The cells were washed with PBS and thereafter resuspended in 50 µl PBS. A nonquenched and a quenched aliquot of the cell preparations were used for the flow cytometric analysis or for the confocal laser microscopy, respectively.
The effect of long-time incubation of type II cells with phorbol myristyl acetate (PMA) was measured using 106 freshly isolated type II cells that were incubated in the presence of 10
5 M
PMA for 20 h at 37°C. PMA was solubilized in dimethyl sulfoxide; the control cell were incubated in the presence of the same final dimethyl sulfoxide concentration (0.001% vol/vol). Thereafter, the adherent cells were detached with trypsin-EDTA (Gibco,
Life Technologies, Karlsruhe, Germany) 1:10 (vol/vol) diluted with PBS, washed, and used for the Alexa-HDL uptake experiments.
The effect of chelerythrine on PKC and megalin expression and
on the HDL-holoparticle uptake was measured by preincubation
of 106 type II cells in presence of 2 µM chelerythrine chloride
(Sigma, Deisenhofen, Germany) for 18 h at 37°C. Thereafter the
cells were washed with PBS and used for the Alexa-HDL uptake
experiment and for Western blot analysis.
Confocal Laser Microscopy
2.5 × 106 freshly isolated type II cells in suspension were incubated with 100 µg HDL labeled with Alexa 488 (Molecular Probes) in 1 ml of DMEM/0.1% BSA for 3 h at 37°C. Thereafter they were washed and then fixed in 1% paraformaldehyde in 250 M Hepes for 10 min at 3°C. To visualize the cell boundaries after the uptake period, the cells were washed and then incubated for 20 min at 3°C with tetramethylrhodamine isothiocyanate (TRITC)- labeled Maclura pomifera lectin (MPA) from Sigma and fixed. The cells were analyzed by confocal laser scanning microscope (CLSM), equipped with an argon/krypton laser (Leica Lasertechnik, Heidelberg, Germany). Images were captured using a 40× NA 1.3 oil objective to fluorescent exitation and emission spectra for Alexa 488 (exitation 490 nm, emission 520 nm) and for TRITC (exitation 541 nm, emission 572 nm). With the dual-channel system of the confocal microscope, dual-emission (535/590 nm) images were simultaneously recorded with a scanning speed at 16 s/frame (512 lines). Sections were made through the center of the cells. Representative samples are shown.
Flow Cytometric Analysis
To quantify protein-labeled HDL uptake, lung cells were resuspended in FACS Buffer consisting of PBS supplemented with 2% FBS (Biochrom KG, Berlin, Germany) and 0.2% sodium acid (Boehringer Ingelheim Bioproducts, Heidelberg, Germany) at 0.4 × 107cells/ml. HDL-particle uptake was measured by incubation of type II cells in the presence of Alexa-HDL followed by quenching the fluorescence of Alexa-HDL bound to the surface of type II cells as outlined above.
All expression data are given as or calculated based on the difference between the mean fluorescence intensity (MFI) of the target cell population.
Determination of the Megalin mRNA Expression
Real-time quantitative polymerase chain reaction primers.
For the amplification of glycoprotein 330 (GP 330) two primers, namely 5'-GCGAATACAGGTGCGACCAT-3' (forward) and 5'-TTA TAGCAGGCTCCATTGGCA-3' (reverse) were used. This primer set defines a 100-bp fragment in the highly conserved pol region and corresponds to position 4,073-4,137 of the glycoprotein 330 (GenBank accession number NM-030827). The primers were obtained from Genset SA (Paris, France). and were designed using the Primer Express software (PE Applied Biosystems, Foster City, CA) following guidlines suggested in the Primer Express applications-based primer design manual.
Real-time quantitative polymerase chain reaction conditions.
Gene expression analysis was performed using real-time polymerase chain reaction (PCR) (GeneAmp 5,700 Sequence Detection System; PE Applied Biosystems). First-strand cDNA was synthesized using SuperScript II (Invitrogen, Carlsbad, CA) with 1 µg of total RNA as template. The reactions contained 5 µM random hexamer oligonucleotide primers, 10 mM dithioerythritol, 0.5 mM dNTP mix, 1× first-strand buffer, and 20 units of SuperScript II in a total volume of 20 µl. RNA was denatured at 65°C for 5 min, chilled on ice, and incubated at room temperature for 10 min before adding reverse transcriptase. The reaction was allowed to proceed for 40 min at 42°C and was stopped by heating to 70°C for 15 min.
Each sample was amplified in quadruplicate using real-time quantitative PCR. The thermocycling parameters were: 50°C for 2 min, 95°C for 10 min, then 40 cycles of 95°C for 15 s and 60°C for 60 s. PCR reactions were performed in 25-µl volumes containing TaqMan Universal PCR Master Mix (2X; PE Applied Biosystems), 300 nM of each primer, and 5 µl of each cDNA mixture. A housekeeping gene, GAPDH, was used as an endogenous control to account for variability in the conversion efficiency of the reverse transcription reaction.
Western Blot Analysis
The concentration of cubilin and megalin was estimated by Western blot analysis. The lysis of alveolar type II cells, membrane preparation, and SDS-PAGE was performed as outlined in previous communications (23, 24).
Briefly, stored alveolar type II cells were resuspended in homogenization buffer (20 mM Tris/HCl, pH 7.5, 1 mM EDTA, 1 mM EGTA, 250 mM saccharose, 1 mM DTT, 1 mM PMSF, 10 µg/ml
aprotinin, 10 µg/ml leupeptin, and a Boehringer protease inhibitor
tablet/10 ml) and the cells were sonicated (2 × 20 s; Sonoplus HD60,
Bandelin Electronics, Berlin, Germany). The particulate fraction
was collected by centrifugation at 100,000 × g for 1 h and solubilized
in homogenization buffer comprising 1% (wt/vol) Triton X-100
for 2 h on ice. Insoluble material was removed by centrifugation at
10,000 × g. All steps were performed between 0 and 4°C. The supernatants were subjected to electrophoresis and immunoblotting.
The procedures for separating proteins by standard SDS-PAGE on
6% gels, for electroblotting to nitrocellulose and for visualizing the
protein bands using rabbit anti-cubilin antiserum (a generous gift of
Mats Paulsson, Institute of Biochemistry, University of Cologne,
Germany) and anti-megalin (Rabbit anti-Pleo-lipoprotein receptor;
RDI, Flanders, NY) have all been published previously (18, 19).
Peroxidase-conjugated second antibodies were purchased from RDI
and rabbit anti-PKC antibodies that recognizes the -, 
I-, II-, and
-isoform of PKC (Calbiochem, La Jolla, CA). The peroxidase/
chemiluminescence bands on Kodak X-OMAT films were quantified by scanning using a densitometer with automatic calibration
(Image Master DTS; Pharmacia, Uppsala, Sweden) and GS-710 Imaging Densitometer (Bio-Rad, Hercules, CA).
Other Methods
Vitamin E and its oxidation product, vitamin E-quinone, in plasma and alveolar type II cells were determined by high performance liquid chromatography (HPLC) of the hexane extracts according to Catignani and Bieri (30). For the determination of total protein we used Bradford's reagent (Sigma).
Statistical Analysis
The number of independent experiments (n) given in tables and the legend of figures designates the number of animals of the different feeding groups. For the calculation of the significance of differences between mean values we used Student's t test.
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Results |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Vitamin E Status of Alveolar Type II Cells
As shown in previous experiments, feeding a vitamin E- depleted diet caused a decrease of the vitamin E content in type II cells and refeeding with a vitamin E-enriched diet increased the vitamin E content of type II cells (23, 33). In the present experiment (n = 3 animals per feeding group) the vitamin E content of type II cells decreased from 45 ± 7 µg/ 106 cells in control to 12 ± 1 µg/106 cells in the vitamin E- depleted group. Refeeding a vitamin E-enriched diet caused an increase of vitamin E in type II cells to 43 ± 3 µg/106 cells.
Characterization of Alexa-Labeled HDL
Alelxa-labeled HDL analyzed by SDS-PAGE shows that the HDL-apolipoproteins were labeled and that the Alexa-labeled HDL preparation did not contain apolipoprotein E (Figure 1).
Confocal Laser Microscopy
The confocal laser microscopic investigations of equatorial levels of alveolar type II cells preincubated with Alexa- labeled HDL showed fluorescence-labeled particles inside of the cells surrounding the nucleus (Figures 2A-2D). Staining the surface membrane of type II cells (red) showed that protein-labeled HDL (green) was indeed within the cells and not attached to the outer leaflet of the surface membrane.
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Flow Cytometric Determination of the HDL-Particle Uptake Kinetic
For the flow cytometric analysis, we used the same protocol of the HDL-holoparticle uptake by type II cells as for the confocal laser microscopy. Type II cells were incubated with protein-labeled HDL and the uptake was differentiated from binding of protein-labeled HDL using the quenching method. To show that the fluorescence determined after quenching represents the uptake of protein-labeled HDL, we measured the inhibitory effect of nonlabeled HDL. Figure 3 shows that a 40-fold excess of nonlabeled HDL caused a significant reduction of the Alexa-HDL uptake. The flow cytometric histograms showed a shift to lower fluorescence (Figure 3A), and the Alexa-HDL uptake decreased to ~ 25% of control (Figure 3B).
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Figure 4 shows that HDL-holoparticle uptake increased with the concentration of Alexa-HDL. From these data we calculated the uptake constant to be 38 µg-labeled HDL protein/ml.
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Effect of the Vitamin E Status on the Expression of Megalin on the mRNA Level
Table 1 shows that the values of the quantitative PCR did not significantly change between the feeding groups. The result indicates that the expression of megalin mRNA in alveolar type II cells did not change in response to the alimentary vitamin E status of rats.
Effect of the Vitamin E Status on the Expression of Megalin and Cubilin
Vitamin E depletion caused an increase of the megalin expression (Figures 5A and 5C) in parallel to the HDL-particle uptake (Figure 5D), whereas the expression of cubilin did not change (Figure 5B). Refeeding the vitamin again reduced the expression of megalin.
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Effect of the Vitamin E Status on the HDL-Particle Uptake
Figure 5D shows the HDL-particle uptake by type II cells in relation to the alimentary vitamin E status of rats. Vitamin E deficiency of rats increased the HDL-particle uptake of freshly isolated alveolar type II cells and refeeding a vitamin E-enriched diet reduced this uptake to values obtained for the control collective.
Effect of Long-time Incubation of Type II Cells with
PMA on PKC and Megalin Expression and
HDL-Particle Uptake
In alveolar type II cells, vitamin E modulates the activation
of protein kinase C (PKC). We measured the effect of
PKC inhibition by long-time incubation of type II cells
with PMA on HDL-particle uptake and megalin expression. Figure 6 shows that PMA incubation for 20 h reduced
the expression of PKC and megalin (Figures 6A, 6B, and
6C) as well as the HDL-particle uptake (Figure 6D), whereas
the cubilin expression did not change (results not shown).
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Effect of Preincubation of Type II Cells with Chelerythrine
on PKC, Cubilin and Megalin Expression, and on the
HDL-Particle Uptake
To verify the PKC effect on HDL-holoparticle uptake
and megalin expression, we preincubated type II cells in
the presence of chelerythrine, a specific inhibitor of PKC
(Figure 7). The mechanism of the PKC inhibition differs
among chelerythrine preincubation and long-time incubation of type II cells with PMA, whereas the effect on
HDL-holoparticle uptake and megalin expression was the same. The densitometric analysis of the Western blots
showed that chelerythrine caused in relation to control a
decrease of PKC to 86% and of megalin to 65%, whereas
cubilin did not change (100%). The HDL-holoparticle uptake decreased to 73% of control.
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Discussion |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Based on the present results, we conclude that alveolar
type II cell exhibit HDL-holoparticle uptake. The determination of the kinetic data of HDL-particle uptake (Kd: 38 µg HDL-protein/ml) shows that the uptake is of the same
magnitude as the affinity of HDL to SR-BI (Kd: 30 µg
apoA1/ml; Ref. 5) and to MDP (Kd: 25 µg apoA1/ml; Ref.
24). Assuming that the Kd values of HDL-binding to SR-BI and MDP reflect the magnitude of the specific lipid
transfer of vitamin E from HDL, both mechanismsHDL-particle uptake and specific lipid transfermight have the
same physiologic meaning for supplementation of type II
cells with vitamin E. This idea is underlined by the results
that vitamin E depletion increased not only the expression
of SR-BI, HB2, and MDP (23, 24), but also HDL-particle
uptake, and that refeeding the vitamin largely reversed
these effects (Figure 4D and Refs. 23, 24).
The candidate receptor cubilin, most probably a peripheral membrane protein (31), needs the cooperation with megalin for HDL-particle uptake (18). Megalin is a protein with a single transmembrane domain and a short cytoplasmic domain (31, 32). The expression of megalin and cubilin (21, 22) indicates that this receptor cooperation may be also responsible for the HDL-holoparticle uptake by alveolar type II cells. It is interesting to note that the expression of megalin increased in parallel to the HDL-particle uptake in response to vitamin E depletion, whereas the cubilin expression did not change (Figure 4). It might be hypothesized that the transmembrane receptor protein megalin represents the regulatory element of the receptor cooperation for HDL-particle uptake with cubulin that is extracellularly localized. Refeeding a vitamin E-enriched diet to rats reduced HDL-particle uptake and to a lesser extent the megalin expression in type II cell membranes. The latter result may be due to the relatively long half-life of megalin (30.) However, it might also be hypothesized that not only the concentration of the megalin protein but also a loss of the megalin-cubilin association could be responsible for the stronger refeeding-induced reduction of the HDL-particle uptake. The alimentary vitamin E status of rats did not change the expression of megalin on the mRNA level (Table 1). Therefore, the results indicate that the cellular vitamin E status induces an intracellular redistribution of megalin rather then an increase of the protein concentration. However, an increased protein expression on the translation level cannot be excluded.
Megalin contains three sites that can be phosphorylated
by protein kinase C (PKC) (31, 32) and vitamin E depletion caused a 20-fold increase of PKC activity (33). Therefore, it might be hypothesized that the PKC-mediated
modulation of the phosphorylation status of megalin might
affect its activity in HDL-particle uptake. If that is so,
long-time incubation of type II cells with PMA, or incubation in presence of chelerythrine that reduces PKC activity in type II cells, should have the opposite effect on HDL-particle uptake and megalin expression as vitamin E depletion. However, in comparison with the modulation of
the PKC activity by vitamin E depletion, the effect of
PMA-long-time incubation and chelerythrine preincubation on the PKC activity exhibited a significantly smaller effect (11, 33) and correspondingly a smaller effect on particulate HDL-uptake and megalin expression. Nevertheless, the significant inhibition of the HDL-particle uptake,
and the reduced megalin expression as result of long-time
incubation of type II cells in the presence of PMA and
chelerythrine, indicates that PKC-mediated modulation of
the phosphorylation status of megalin might be regulative for the particulate HDL-uptake. Cubilin expression did
not show any changes in response to these metabolic situations that change HDL-holoparticle uptake in parallel with
megalin expression. This may be caused by the localization
of cubilin. Cubilin has no classic membrane-spanning segment and seems to be attached to the outside of the membrane via the amino-terminal region and possibly by palmitoylation (31). Therefore, cubilin needs the cooperation with the transmembrane receptor megalin to realize and
regulate HDL particle uptake. The cytoplasmic segment
of megalin contains the PKC phosphorylation sides and
may easily be available for modifications that affect its activity in the particulate uptake of HDL, whereas its extracellular segment or the extracellularly attached cubilin can
hardly be modified.
From our results we conclude that type II cells possess different mechanisms for HDL-lipid uptake, namely specific lipid transfer and HDL-holoparticle uptake. Because the vitamin E status regulates the expression of receptor types responsible for both mechanisms, probably via the modulation of the PKC activity, it may be assumed that maintenance of a sufficient vitamin E level in type II cells depends first and foremost on its interaction with HDL. Therefore, these two different mechanisms guarantee the alveolar type II cell interaction with HDL.
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Footnotes |
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Address correspondence to: Bernd Rüstow, Ph.D., Department of Neonatology, University Hospital Charité, Humboldt University of Berlin, Schumannstr. 20/21, 10098 Berlin, Germany. E-mail: bernd.ruestow{at}charite.de
(Received in original form November 16, 2002 and in revised form April 4, 2002).
Abbreviations: Dulbecco's Minimal Essential Medium, DMEM; flow cytometric analysis, FACS; high-density lipoprotein, HDL; high-performance liquid chromatography, HPLC; mean fluorescence intensity, MFI; low-density lipoprotein, LDL; Dulbecco's phosphate-buffered saline, PBS; protein kinase C, PKC; phorbol myristyl acetate, PMA; scavenger receptor type BI, SR-BI.
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References |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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