The two major splice variants of scavenger receptor BI differ by their interactions with lipoproteins and cellular localization in endothelial cells

Abstract

The scavenger receptor SR-BI facilitates the transport of both HDL and LDL through endothelial cells. Its two splice variants, SR-BIvar1 and SR-BIvar2, differ in their carboxyterminal domains. Only the one of SR-BIvar1 contains the putative binding sites for the adapter proteins PDZK1 and DOCK4, which limit the cell surface abundance and internalization of the receptor.To investigate the cellular localization of the SR-BI variants and their interaction with lipoproteins in endothelial cells, EA.hy926 cells were stably transfected with vectors encoding untagged, GFP- or mCherry-tagged constructs of the two SR-BI variants. Additionally, the cells were transfected with shRNAs against PDZK1 or DOCK4.Microscopy investigation showed that SR-BIvar1 was predominantly localized on the cell surface together with clathrin whereas SR-BIvar2 was absent from the cell surface but retrieved in endosomes and lysosomes. Accordingly, only SR-BIvar1 increased lipoprotein binding to endothelial while HDL and LDL uptake were enhanced by both variants. Silencing of PDZK1 or DOCK3 only reduced HDL association in SR-BIvar2 overexpressing cells while LDL association was reduced both in wild type and SR-BIvar2 overexpressing cells.In conclusion, either SR-BI variant facilitates the uptake of HDL and LDL into endothelial cells, however by different mechanisms and trafficking routes. This dual role may explain why the loss of DOCK4 or PDZK1 differently affects the uptake of HDL and LDL in different endothelial cells.