Abstract
Receptor mediated endocytosis of LDL is known to be very important in regulating plasma cholesterol levels. While the principal details of LDL uptake by cells seem to be elucidated, there are still conflicting reports concerning the manner of display of B,E receptors in the plasma membrane before they are trapped in coated pits. To clarify whether B,E receptors are inserted into the plasma membrane uniformly, individually and at widely dispersed sites, or as groups of loosely associated receptors (plaques or clusters), we performed experiments to visualize B,E receptors at the cell surface. Our experiments included the use of LDL-gold complexes, the application of anti-apo B-100 antibodies to detect receptor-bound LDL molecules as well as the application of monoclonal anti-receptor antibodies. In every set of experiments electron microscopy of platinum-carbon replicas revealed a loose cluster arrangement of B,E receptors in fixed and in unfixed fibroblasts and hepatocytes. With the label-fracture technique we were able to demonstrate that the clusters of B,E receptors are mostly located nearby but not in coated pits. Using different ligands for detection of B,E receptors in hepatocytes we got evidence that, in addition to B,E receptors, additional lipoprotein receptors, that putatively recognize apo E, must be present in the plasma membrane of this cell type. This receptor differs from the B,E receptor in that it is displayed in a more dispersed form. From our results we conclude that 1) it is possible to discriminate different labeling patterns of receptors with the aid of morphological techniques and that 2) B,E receptors are initially inserted and displayed as aggregates and remain in this form until they reach coated pits.
Zusammenfassung
Die rezeptorvermittelte Endozytose von LDL (Low density lipoprotein) ist ein wesentlicher Vorgang bei der Regulation des Plasmacholesterinspiegels. Während die grundlegenden Vorgänge der LDL-Aufnahme durch Zellen bekannt sind, ist die Verteilung der B,E-Rezeptoren in der Plasmamembran strittig. Um zu klären, ob B,E-Rezeptoren in die Plasmamembran als einzelne Moleküle, weit voneinander entfernt, oder in Form von Gruppen (Clustern) eingebaut werden, haben wir B,E-Rezeptoren mit Hilfe verschiedener experimenteller Ansätze auf der Zelloberfläche von Fibroblasten und Hepatozyten nachgewiesen. Unsere Experimente umfassen die Anwendung von LDL-Goldkomplexen, den Einsatz von anti-Apo-B-100-Antikörpern, um rezeptorgebundene LDLMoleküle nachzuweisen, und den Einsatz von anti-B,E-Rezeptor-Antikörpern. Die elektronenmikroskopische Auswertung von Platin-Kohlenstoffrepliken zeigt, daß B,E-Rezeptoren auf fixierten und unfixierten Fibroblasten und Hepatozyten ausschließlich in Clustern angeordnet sind, unabhängig von der eingesetzten Detektionsmethode. Mit der Label-Fracture-Technik läßt sich zeigen, daß diese Cluster meistens in der Nähe von Coated pits lokalisiert sind. Unsere Ergebnisse lassen weiterhin Rückschlüsse darauf zu, daß in der Plasmamembran der Hepatozyten weitere Lipoproteinrezeptoren, die vermutlich APO (Apolipoprotein) E erkennen, existieren. Diese Rezeptoren unterscheiden sich von B,ERezeptoren in ihrem Verteilungsmuster in der Zellmembran. Unsere Ergebnisse zeigen damit erstens, daß unterschiedliche Verteilungsmuster von Rezeptoren mit morphologischen Techniken gezeigt werden können und zweitens, daß B,ERezeptoren im Gegensatz zu anderen Zelloberflächenrezeptoren ausschließlich in Form von Clustern in der Plasmamembran angeordnet sind.
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© 1992 Springer Fachmedien Wiesbaden
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Harrach, B., Robenek, H. (1992). Distribution of B,E receptors in the plasma membrane of fibroblasts and hepatocytes. In: Heinle, H., Schulte, H., Schaefer, H.E. (eds) Arteriosklerotische Gefäßerkrankungen. Vieweg+Teubner Verlag, Wiesbaden. https://doi.org/10.1007/978-3-663-19646-4_29
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DOI: https://doi.org/10.1007/978-3-663-19646-4_29
Publisher Name: Vieweg+Teubner Verlag, Wiesbaden
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