Uptake of lipoprotein-associated α-tocopherol by primary porcine brain capillary endothelial cells

From the severe neurological syndromes resulting from Vitamin E deficiency, it is evident that an adequate supply of the brain with alpha-tocopherol (alpha TocH), the biologically most active member of the vitamin E family, is of utmost importance. However, uptake mechanisms of alpha TocH in cells constituting the blood-brain barrier are obscure. Therefore, we studied the interaction of low (LDL) and high (HDL) density lipoproteins (the major carriers of alpha TocH in the circulation) with monolayers of primary porcine brain capillary endothelial cells (pBCECs) and compared the ability of these two lipoprotein classes to transfer lipoprotein-associated alpha TocH to pBCECs. With regard to potential binding proteins, we could identify the presence of the LDL receptor and a putative HDL, binding protein with an apparent molecular mass of 100 kDa. At 4 degrees C, pBCECs bound LDL with high affinity (K-D = 6 nM) and apolipoprotein E-free HDL3 with low affinity (98 nM). The binding capacity was 20,000 (LDL) and 200,000 (HDL3) lipoprotein particles per cell. alpha TocH uptake was approximately threefold higher from HDL3 than from LDL when [C-14]alpha TocH-labeled lipoprotein preparations were used. The majority of HDS-associated alpha TocH was taken up in a lipoprotein particle-independent manner, exceeding HDL3 holoparticle uptake 8- to 20-fold. This uptake route is less important for LDL-associated alpha TocH (alpha TocH uptake similar to 1.5-fold higher than holoparticle uptake). In line with tracer experiments, mass transfer studies with unlabeled lipoproteins revealed that alpha TocH uptake from HDL, was almost fivefold more efficient than from LDL, Biodiscrimination studies indicated that uptake efficacy for the eight different stereoisomers of synthetic alpha TocH is nearly identical. Our findings indicate that HDL could play a major role in supplying the central nervous system with alpha TocH in vivo.