Scavenger receptor class B, type I is expressed in porcine brain capillary endothelial cells and contributes to selective uptake of HDL-associated vitamin E

It is clearly established that an efficient supply to the brain of alpha -tocopherol (alpha TocH), the most biologically active member of the Vitamin E family, is of the utmost importance for proper neurological functioning. Although the mechanism of uptake of alpha TocH into cells constituting the brood-brain barrier (BBB) is obscure, we previously demonstrated that high-density lipoprotein (HDL) plays a major role in the supply of alpha TocH to porcine brain capillary endothelial cells (pBCECs). Here we studied whether a porcine analogue of human and rodent scavenger receptor class B, type I mediates selective (without concomitant lipoprotein particle internalization) uptake of HDL-associated alpha TocH in a similar manner to that described for HDL-associated cholesteryl esters (CEs). In agreement with this hypothesis we observed that a major proportion of alpha TocH uptake by pBCECs occurred by Selective uptake, exceeding HDL3 holoparticle uptake by up to 13-fold. The observation that selective uptake of MDL-associated CE exceeded HDL3 holoparticle up to fourfold Suggested that a porcine analogue of SR-BI (pSR-BI) may be involved in lipid uptake at the BBB. In line with the observation of selective lipid uptake, RT-PCR and northern and western blot analyses revealed the presence of pSR-BI in cells constituting the BBB. Adenovirus-mediated overexpression of the human analogue of SR-BI (hSR-BI) in pBCECs resulted in a fourfold increase in selective HDL-associated alpha TocH uptake. In accordance with the proposed function of SR-BI, selective HDL-CE uptake was increased sixfold in Chinese hamster ovary cells stably transfected with murine SR-BI (mSR-BI). Most importantly stable mSR-BI overexpression mediated a twofold increase in HDL-associated [C-14]alpha TocH selective uptake in comparison with control cells. In line with tracer experiments, mass transfer studies with unlabelled lipoproteins revealed that mSR-BI overexpression resulted in a twofold increase in endogenous HDL3-associated alpha TocH uptake. The results of this Study indicate that SR-BI promotes the uptake of HDL-associated alpha TocH into cells constituting the BBB and plays an important role during the supply of the CNS with this indispensable micronutrient.