Effect of overexpression of human SR-AI on oxLDL uptake and apoptosis in 293T cells.

Type I class A macrophage scavenger receptor (SR)-AI plays an important role in foam cell formation and in apoptosis in atherosclerosis, however the mechanism remains unclear. Therefore, we generated a pEGFP-C1-SR-AI plasmid construct for transient transfection of 293T human embryonic kidney cells and observed if SR-AI expression led: (i) to foam cell formation or apoptosis; and (ii) to expression of apoptosis-related genes Bcl-2 and Bak-1 in cells treated with oxidized low-density lipoprotein (oxLDL). The pEGFP-C1 (empty vector) transfected cell line was used as a control. Transfection efficiency of each group was >90% and transfected cells expressed functional SR-AI protein. Binding and uptake of 3,3'-dioctadecylindocarbocyanine-labeled oxLDL (DiI-oxLDL) were verified by flow cytometry; increases in the rate of oxLDL binding and uptake were observed in pEGFP-C1-SR-AI transfected 293T cells and incubation with oxLDL also led to increased apoptosis (≈50%) compared with controls. A decrease in Bcl-2 and an increase in Bak-1 mRNA and protein expression were observed in pEGFP-C1-SR-AI transfected cells compared with controls. We conclude that transient over-expression of SR-AI leads to an increase in oxLDL uptake and binding in a non-macrophage cell line. In addition, over-expression of SR-AI induced non-macrophage cell apoptosis via downregulation of Bcl-2 and upregulation of Bak-1 expression. We conclude that the 293T cell expression described here is a model for foam cell formation. These results may form the basis of further research into SR-AI structure and function (including lipoprotein uptake, apoptosis modulation and adhesion), which may give an insight into the progression of atherosclerosis in vivo.