ABCA1 and Scavenger Receptor Class B, Type I, Are Modulators of Reverse Sterol Transport at an in Vitro Blood-Brain Barrier Constituted of Porcine Brain Capillary Endothelial Cells

Ute Panzenboeck,Zoltan Balazs,Andrea Sovic,Andelko Hrzenjak,Sanja Levak-Frank,Andrea Wintersperger,Ernst Malle,Wolfgang Sattler

doi:10.1074/jbc.m207601200

Abstract

The objective of the present study was to investigate the involvement of key players in reverse cholesterol/24(S)OH-cholesterol transport in primary porcine brain capillary endothelial cells (pBCEC) that constitute the BBB. We identified that, in addition to scavenger receptor class B, type I (SR-BI), pBCEC express ABCA1 and apolipoprotein A-I (apoA-I) mRNA and protein. Studies on the regulation of ABCA1 by the liver X receptor agonist 24(S)OH-cholesterol revealed increased ABCA1 expression and apoA-I-dependent [3H]cholesterol efflux from pBCEC. In unpolarized pBCEC, high density lipoprotein, subclass 3 (HDL3)-dependent [3H]cholesterol efflux, was unaffected by 24(S)OH-cholesterol treatment but was enhanced 5-fold in SR-BI overexpressing pBCEC. Efflux of cellular 24(S)-[3H]OH-cholesterol was highly efficient, independent of ABCA1, and correlated with SR-BI expression. Polarized pBCEC were cultured on porous membrane filters that allow separate access to the apical and the basolateral compartment. Addition of cholesterol acceptors to the apical compartment resulted in preferential [3H]cholesterol efflux to the basolateral compartment. HDL3 was a better promoter of basolateral [3H]cholesterol efflux than lipid-free apoA-I. Basolateral pretreatment with 24(S)OH-cholesterol enhanced apoA-I-dependent basolateral cholesterol efflux up to 2-fold along with the induction of ABCA1 at the basolateral membrane. Secretion of apoA-I also occurred preferentially to the basolateral compartment, where the majority of apoA-I was recovered in an HDL-like density range. In contrast, 24(S)-[3H]OH-cholesterol was mobilized efficiently to the apical compartment of the in vitro BBB by HDL3, low density lipoprotein, and serum. These results suggest the existence of an autoregulatory mechanism for removal of potentially neurotoxic 24(S)OH-cholesterol. In conclusion, the apoA-I/ABCA1- and HDL/SR-BI-dependent pathways modulate polarized sterol mobilization at the BBB.

Highlights

In the past few years substantial evidence has accumulated for some neurodegenerative disorders being tightly coupled to lipid and/or lipoprotein metabolism in the peripheral circulation
Expression of ABCA1 and ApoA-I by Primary porcine brain capillary endothelial cells (pBCEC)—Because ABCA1 and apolipoprotein A-I (apoA-I) represent key players in reverse cholesterol transport (RCT), we initially analyzed their expression by pBCEC
Among the various endothelial cell types investigated by RT-PCR, pBCEC and HUVEC exhibited a prominent signal for ABCA1 mRNA, followed by EAHY hybridoma cells, whereas no mRNA was detected in RBE4 cells (Fig. 1A)

Summary

Introduction

In the past few years substantial evidence has accumulated for some neurodegenerative disorders being tightly coupled to lipid and/or lipoprotein metabolism in the peripheral circulation. Decreased serum HDL cholesterol and apolipoprotein A-I (apoA-I) concentrations correlate with the severity of Alzheimer’s disease [5]. The integrity of the BBB itself strongly depends on cellular cholesterol homeostasis Another major difference is a unique strategy of the brain to secrete cholesterol. Cholesterol and 24(S)OH-Cholesterol Transport at the BBB conversion to 24(S)OH-cholesterol being the major pathway for the maintenance of brain cholesterol homeostasis, 24(S)OHcholesterol levels are elevated in cerebrospinal fluid of Alzheimer’s patients [12] and in plasma correlate with the severity of dementia [13], probably as a consequence of increased cholesterol turnover. ABCA1 has been identified as the primary gatekeeper for eliminating tissue cholesterol, because it mediates the apolipoprotein-dependent transfer of intracellular cholesterol and phospholipid to lipid-free apoA-I (14 –16). Porcine brain endothelial cells have been reported to synthesize apoA-I [23], and apoA-I abundantly present in the central nervous system is obviously transported across the BBB [8]

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Conclusion