Expression of scavenger receptor class B type 1 (SR-BI) promotes microvillar channel formation and selective cholesteryl ester transport in a heterologous reconstituted system.
In the "selective" cholesteryl ester (CE) uptake process, surface-associated lipoproteins [high density lipoprotein (HDL) and low density lipoprotein] are trapped in the space formed between closely apposed surface microvilli (microvillar channels) in hormone-stimulated steroidogenic cells. This is the same location where an HDL receptor (SR-BI) is found. In the current study, we sought to understand the relationship between SR-BI and selective CE uptake in a heterologous insect cell system. Sf9 (Spodoptera frugiperda) cells overexpressing recombinant SR-BI were examined for (i) SR-BI protein by Western blot analysis and light or electron immunomicroscopy, and (ii) selective lipoprotein CE uptake by the use of radiolabeled or fluorescent (BODIPY-CE)-labeled HDL. Noninfected or infected control Sf9 cells do not express SR-BI, show microvillar channels, or internalize CEs. An unexpected finding was the induction of a complex channel system in Sf9 cells expressing SR-BI. SR-BI-expressing cells showed many cell surface double-membraned channels, immunogold SR-BI, apolipoprotein (HDL) labeling of the channels, and high levels of selective HDL-CE uptake. Thus, double-membraned channels can be induced by expression of recombinant SR-BI in a heterologous system, and these specialized structures facilitate both the binding of HDL and selective HDL-CE uptake.
Reaven
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Geriatric Research, Education, and Clinical Center, Department of Veterans Affairs, Palo Alto Health Care System, Palo Alto, CA 94304, USA. ereaven@ispchannel.com
Leers-Sucheta
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S
Nomoto
A
A
Azhar
S
S
D000818:Animals; D001692:Biological Transport; D015153:Blotting, Western / Q000379:methods; D001896:Boron Compounds; D018955:CD36 Antigens / Q000096:biosynthesis* / Q000235:genetics; D002460:Cell Line; D002788:Cholesterol Esters / Q000378:metabolism*; D005456:Fluorescent Dyes; D007457:Iodine Radioisotopes; D007553:Isotope Labeling; D008075:Lipoproteins, HDL / Q000378:metabolism; D053440:Lipoproteins, HDL3; D008565:Membrane Proteins; D008854:Microscopy, Electron / Q000379:methods; D008856:Microscopy, Fluorescence / Q000379:methods; D008871:Microvilli / Q000378:metabolism; D011485:Protein Binding; D051381:Rats; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein / Q000096:biosynthesis* / Q000235:genetics; D051116:Receptors, Scavenger; D051122:Scavenger Receptors, Class B; D018411:Spodoptera; D014316:Tritium
C095489:4,4-difluoro-4-bora-3a,4a-diaza-s-indacene; D001896:Boron Compounds; D018955:CD36 Antigens; D002788:Cholesterol Esters; D005456:Fluorescent Dyes; D007457:Iodine Radioisotopes; D008075:Lipoproteins, HDL; D053440:Lipoproteins, HDL3; D008565:Membrane Proteins; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; C454739:Scarb1 protein, mouse; C495125:Scarb1 protein, rat; D051122:Scavenger Receptors, Class B; D014316:Tritium