Uptake and transport of high‐density lipoprotein (HDL) and HDL‐associated α‐tocopherol by an in vitro blood–brain barrier model

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Abstract
The present study aimed to investigate pathways that contribute to uptake and transcytosis of high-density lipoproteins (HDLs) and HDL-associated α-tocopherol (αTocH) across an in vitro model of the blood–brain barrier (BBB). In primary porcine brain capillary endothelial cells HDL-associated αTocH was taken up in 10-fold excess of HDL holoparticles, indicating efficient selective uptake, a pathway mediated by scavenger receptor class B, type I (SR-BI). SR-BI was present in caveolae of brain capillary endothelial cells and expressed almost exclusively at the apical membrane. Disruption of caveolae with methyl-β-cyclodextrin (CDX) resulted in (mis)sorting of SR-BI to the basolateral membrane. Immunohistochemistry of porcine brain cryosections revealed SR-BI expression on brain capillary endothelial cells and presumably astrocytic endfeet. HDL-associated [14C]αTocH taken up by brain capillary endothelial cells was recovered in sucrose gradient fractions containing the majority of cellular caveolin-1, the major caveolae-associated protein. During mass transfer studies using αTocH-enriched HDL, approximately 50% of cellular αTocH was recovered with the bulk of cellular caveolin-1 and SR-BI. Efflux experiments revealed that a substantial amount of cell-associated [14C]αTocH could be mobilized into the culture medium. In addition, apical-to-basolateral transport of HDL holoparticles and HDL-associated αTocH was saturable. Results from the present study suggest that part of cerebral apolipoprotein A-I and αTocH originates from plasma HDL transcytosed across the BBB and that caveolae-located SR-BI facilitates selective uptake of HDL-associated αTocH at the BBB.