The lipolytic processing of triglyceride-rich lipoproteins (e.g., chylomicrons, very low density lipoproteins) by lipo- protein lipase (LPL) is the central event in plasma triglyc- eride metabolism and plays a crucial role in the delivery of lipid nutrients to parenchymal cells

This article is available online at http://www.jlr.org The lipolytic processing of triglyceride-rich lipoproteins (e.g., chylomicrons, very low density lipoproteins) by lipoprotein lipase (LPL) is the central event in plasma triglyceride metabolism and plays a crucial role in the delivery of lipid nutrients to parenchymal cells (e.g., adipocytes, myocytes) ( 1–4 ). LPL is synthesized by parenchymal cells and secreted into the interstitial spaces, but it needs to reach the capillary lumen in order to hydrolyze the triglycerides in plasma lipoproteins. Recent studies showed that GPIHBP1, a glycosylphosphatidylinositol-anchored protein of capillary endothelial cells, binds LPL avidly ( 5 ) and is required for the transport of LPL to the capillary lumen ( 6 ). In the absence of GPIHBP1, LPL accumulates in the interstitial spaces around parenchymal cells and is unable to process triglyceride-rich lipoproteins in the bloodstream, resulting in markedly elevated plasma triglyceride levels and interfering with the delivery of lipid nutrients to parenchymal cells ( 5–7 ). Although GPIHBP1 is essential for the delivery of LPL to the luminal face of capillaries, the cellular mechanisms for moving GPIHBP1 and LPL across endothelial cells are poorly defi ned. It is unclear whether GPIHBP1 and LPL move unidirectionally from the basolateral face of endothelial cells to LPL’s site of action along the capillary lumen or whether “backwards traffi cking” also occurs (i.e., movement of GPIHBP1 and LPL from the lumen to the basolateral face of cells). A second issue is whether GPIHBP1 Abstract Lipoprotein lipase (LPL) is secreted into the interstitial spaces by adipocytes and myocytes but then must be transported to the capillary lumen by GPIHBP1, a glycosylphosphatidylinositol-anchored protein of capillary endothelial cells. The mechanism by which GPIHBP1 and LPL move across endothelial cells remains unclear. We asked whether the transport of GPIHBP1 and LPL across endothelial cells was unior bidirectional. We also asked whether GPIHBP1 and LPL are transported across cells in vesicles and whether this transport process requires caveolin-1. The movement of GPIHBP1 and LPL across cultured endothelial cells was bidirectional. Also, GPIHBP1 moved bidirectionally across capillary endothelial cells in live mice. The transport of LPL across endothelial cells was inhibited by dynasore and genistein, consistent with a vesicular transport process. Also, transmission electron microscopy (EM) and dual-axis EM tomography revealed GPIHBP1 and LPL in invaginations of the plasma membrane and in vesicles. The movement of GPIHBP1 across capillary endothelial cells was effi cient in the absence of caveolin-1, and there was no defect in the internalization of LPL by caveolin-1-defi cient endothelial cells in culture. Our studies show that GPIHBP1 and LPL move bidirectionally across endothelial cells in vesicles and that transport is effi cient even when caveolin-1 is absent. —Davies, B. S. J., C. N. Goulbourne, R. H. Barnes II, K. A. Turlo, P. Gin, S. Vaughan, D. J. Vaux, A. Bensadoun, A. P. Beigneux, L. G. Fong, and S. G. Young. Assessing mechanisms of GPIHBP1 and lipoprotein lipase movement across endothelial cells. J. Lipid Res. 2012. 53: 2690–2697.