In this study, we developed a method to analyze liposomal binding to a cell membrane receptor using fluorescence-labeled liposomes and demonstrated that scavenger class B type 1 (SR-B1) plays a crucial role in binding of liposomes containing phosphatidylcholine (PC) to HEK293T cell membrane and phosphatidic acid (PA) can modulate it. Site-directed mutagenesis of SR-B1 revealed that S112F and T175A mutations in its ectodomain abrogated binding and endocytosis of PC liposomes in HEK293T cells. K151A and K156A mutations attenuated their binding and endocytosis too. Although the effects of mutations on binding and endocytosis were similar between PC liposomes and PC/PA and PA liposomes, SR-B1 dependency appeared to be PC > PC/PA > PA liposomes. Our data indicate that (i) nanoparticles including high-density lipoprotein (HDL), silica, and liposomes bind to a common or close site of SR-B1, and (ii) PC/PA and PA liposomes bind not only to SR-B1 but also other receptor(s) in HEK293T cells. In addition, PC/PA liposomes induced lipid droplet (LD) formation in HEK293T cells more than PC liposomes. Treatment of HEK293T cells with SR-B1 siRNA suppressed PC/PA liposome-induced LD formation. Taken together, our results demonstrate that SR-B1 plays an essential role in binding PC-containing liposomes and the subsequent induction of cellular responses, while PA can modulate them.
Keywords: Lipid droplets; Liposomes; Phosphatidic acid; Phosphatidylcholine; Scavenger receptor class B type 1.
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