Screening And Functional Study Of Cytoplasmic Domain-binding Proteins Of Type I/II Class A Scavenger Receptors

Class A scavenger receptor (SR-A) is a unique membrane glycoprotein and plays an important role in intracellular lipid accumulation in macrophages. However, the mechanism underlying the internalization of the receptor-ligand complexes into cells remains unclear. To elucidate the modulation manner of SR-A in foam cell formation, a pulldown assay was performed and glucose regulated protein 78 (GRP78) was identified to bind with the cytoplasmic domain of SR-A. Immunoprecipitation and indirect immunofluorescence assay demonstrated the specific binding and co-localization of GRP78 with SR-A in cells. Fluorescence resonance energy transfer experiments confirmed the direct interaction between GRP78 and SR-A in living cells. Overexpression of GRP78 significantly inhibited lipid accumulation in cells. Knockdown of GRP78 resulted in an up-regulation of acLDL uptake by SR-A, but did not alter cellular SR-A expression and binding ability. Loading of acetylated LDL (acLDL), a specific ligand of SR-A, to cells reduced binding of GRP78 to SR-A and subsequently prompted further cellular uptake of acLDL. The negatively regulatory effect of GRP78 on SR-A internalization might be partly attributed to the suppression of c-Jun-NH2-terminal kinase 2 signaling pathway. Our results suggest that GRP78 may act as an inhibitor of SR-A mediated internalization of modified LDL into macrophages.