Novel Assay for Detection of LDL Transcytosis Across Coronary Endothelium Reveals an Unexpected Role for SR-B1

Introduction: Retention of LDL beneath the arterial endothelium initiates an inflammatory response culminating in atherosclerosis. How LDL crosses the endothelium to enter the arterial wall remains unknown. While LDL could conceivably pass between endothelial cells (paracellularly) or through them (transcytosis), electron microscopy studies in animals revealed LDL in intracellular vesicles and none at intercellular junctions. This, combined with the absence of endothelial injury or intercellular gaps in early atherosclerosis, suggests that transcytosis is the major route. However, technical challenges with studying transcytosis have made confirming and extending these findings difficult. We developed and validated a novel assay for measuring the transcytosis of native LDL across live human coronary artery endothelium in vitro. Using this assay, we propose to elucidate the regulation of LDL transcytosis and have identified a novel role for SR-B1. Methods and Results: Experiments were performed using primary human coronary artery endothelial monolayers. Transcytosis was quantified in single live cells in real time using total internal reflectance fluorescence microscopy. Transcytosis of LDL was saturable and inhibited by excess unlabeled LDL. By fluorescence microscopy we found that DiI-LDL colocalized significantly with scavenger receptor, class B, type 1 (SR-B1). Unexpectedly, overexpression of SR-BI resulted in increased LDL transcytosis, while knockdown of SR-BI by siRNA inhibited transcytosis. Excess HDL, the canonical SR-B1 ligand, also decreased LDL transcytosis. To confirm the occurrence of transcytosis in an intact vessel, we perfused murine aortas ex vivo with both LDL and dextran of a smaller molecular radius. We observed the accumulation of subendothelial LDL without dextran, indicating that transcytosis of LDL occurs in intact vessels. Conclusions: The accumulation of LDL in the subendothelial intima is the first step of atherosclerosis yet little is known about how it occurs. Our data suggests that transcytosis of LDL is an important contributor, particularly in the early stages of the disease. By identifying the mechanisms of transcytosis, our work could have important implications for its pathogenesis and therapy.