Delipidation of insect lipoprotein,lipophorin, affects its binding to the lipophorin receptor,LpR: Implications for the role of LpR-mediated endocytosis |
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| Authors: | Sigrid D Roosendaal Jan M Van Doorn Karine M Valentijn Dick J Van der Horst Kees W Rodenburg |
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| Institution: | 1. Division of Endocrinology and Metabolism, Department of Biology and Institute of Biomembranes, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands;2. Department of Molecular Cell Biology, Section Electron Microscopy, Leiden University Medical Center, Postal zone S-01-P, P.O. Box 9600, 2300 RC Leiden, The Netherlands;3. Division of Biological Chemistry and Drug Discovery, College of Life Sciences, University of Dundee, Dundee DD1 5EH, Scotland, United Kingdom;4. School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JR, Scotland, United Kingdom;2. Institute for Clinical Chemistry and Laboratory Medicine, University of Regensburg, 93042 Regensburg, Germany;4. Laboratorio de Morfología Celular, Unidad Mixta Centre D舗Investigació Príncep Felipe-Universitat de València Estudis Generals (CIPF-UVEG), Centro de Investigación Biomédica en Red, Enfermedades Neurodegenerativas (CIBERNED), 46013 Valencia, Spain;1. College of Food Science, Sichuan Agricultural University, Ya’an 625014, Sichuan, China;2. College of Literature, Sichuan Agricultural University, Ya’an 625014, Sichuan, China |
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| Abstract: | The insect lipophorin receptor (LpR), an LDL receptor (LDLR) homologue that is expressed during restricted periods of insect development, binds and endocytoses high-density lipophorin (HDLp). However, in contrast to LDL, HDLp is not lysosomally degraded, but recycled in a transferrin-like manner, leaving a function of receptor-mediated uptake of HDLp to be uncovered. Since a hallmark of circulatory HDLp is its ability to function as a reusable shuttle that selectively loads and unloads lipids at target tissues without being endocytosed or degraded, circulatory HDLp can exist in several forms with respect to lipid loading. To investigate whether lipid content of the lipoprotein affects binding and subsequent endocytosis by LpR, HDLp was partially delipidated in vitro by incubation with α-cyclodextrin, yielding a particle of buoyant density 1.17 g/mL (HDLp-1.17). Binding experiments demonstrated that LpR bound HDLp-1.17 with a substantially higher affinity than HDLp both in LpR-transfected Chinese hamster ovary (CHO) cells and isolated insect fat body tissue endogenously expressing LpR. Similar to HDLp, HDLp-1.17 was targeted to the endocytic recycling compartment after endocytosis in CHO(LpR) cells. The complex of HDLp-1.17 and LpR appeared to be resistant to endosomal pH, as was recently demonstrated for the LpR–HDLp complex, corroborating that HDLp-1.17 is recycled similar to HDLp. This conclusion was further supported by the observation of a significant decrease with time of HDLp-1.17-containing vesicles after endocytosis of HDLp-1.17 in LpR-expressing insect fat body tissue. Collectively, our results indicate that LpR favors the binding and subsequent endocytosis of HDLp-1.17 over HDLp, suggesting a physiological role for LpR in selective endocytosis of relatively lipid-unloaded HDLp particles, while lipid reloading during their intracellular itinerary might result in decreased affinity for LpR and thus allows recycling. |
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