Expression of ABCG1, but not ABCA1, correlates with cholesterol release by cerebellar astroglia

Central nervous system lipoproteins mediate the exchange of cholesterol between cells and support synaptogenesis and neuronal growth. The primary source of lipoproteins in the brain is astroglia cells that synthesize and secrete apolipoprotein (apo) E in high density lipoprotein-like particles. Small quantities of apoA1, derived from the peripheral circulation, are also present in the brain. In addition to the direct secretion of apoE-containing lipoproteins from astroglia, glia-derived lipoproteins are thought to be formed by cholesterol efflux to extracellular apolipoproteins via ATP-binding cassette (ABC) transporters. We used cultured cerebellar murine astroglia to investigate the relationship among cholesterol availability, apoE secretion, expression of ABCA1 and ABCG1, and cholesterol efflux. In many cell types, cholesterol content, ABCA1 expression, and cholesterol efflux are closely correlated. In contrast, cholesterol enrichment of glia failed to increase ABCA1 expression, although ABCG1 expression and cholesterol efflux to apoA1 were increased. Moreover, the liver X receptor (LXR) agonist TO901317 up-regulated ABCA1 and ABCG1 expression in glia without stimulating cholesterol efflux. Larger lipoproteins were generated when glia were enriched with cholesterol, whereas treatment with the LXR agonist produced smaller particles that were eliminated when the glia were loaded with cholesterol. We also used glia from ApoE ^-/- mice to distinguish between direct lipoprotein secretion and the extracellular generation of lipoproteins. Our observations indicate that partially lipidated apoE, secreted directly by glia, is likely to be the major extracellular acceptor of cholesterol released from glia in a process mediated by ABCG1.