Uptake of lipoprotein-associated alpha-tocopherol by primary porcine braincapillary 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, i s of utmost importance. However, uptake mechanisms of alpha TocH in cells c onstituting the blood-brain barrier are obscure. Therefore, we studied the interaction of low (LDL) and high (HDL) density lipoproteins (the major car riers of alpha TocH in the circulation) with monolayers of primary porcine brain capillary endothelial cells (pBCECs) and compared the ability of thes e two lipoprotein classes to transfer lipoprotein-associated alpha TocH to pBCECs. With regard to potential binding proteins, we could identify the pr esence of the LDL receptor and a putative HDL, binding protein with an appa rent 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 p articles per cell. alpha TocH uptake was approximately threefold higher fro m 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 lip oprotein particle-independent manner, exceeding HDL3 holoparticle uptake 8- to 20-fold. This uptake route is less important for LDL-associated alpha T ocH (alpha TocH uptake similar to 1.5-fold higher than holoparticle uptake) . In line with tracer experiments, mass transfer studies with unlabeled lip oproteins revealed that alpha TocH uptake from HDL, was almost fivefold mor e efficient than from LDL, Biodiscrimination studies indicated that uptake efficacy for the eight different stereoisomers of synthetic alpha TocH is n early identical. Our findings indicate that HDL could play a major role in supplying the central nervous system with alpha TocH in vivo.