Expression of scavenger receptor class B type 1 (SR-BI) promotes microvillar channel formation and selective cholesteryl ester transport in a heterologous reconstituted system

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Abstract
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. Sf 9 ( 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 Sf 9 cells do not express SR-BI, show microvillar channels, or internalize CEs. An unexpected finding was the induction of a complex channel system in Sf 9 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.