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.The human brain contains 15-20% of total body cholesterol but represents only ϳ5% of total body weight (1). Cholesterol is a key component of all membranes, including myelin, in the central nervous system (CNS), 4 and cholesterol is synthesized continuously, albeit at a low rate, in the adult brain (1). All cholesterol in the CNS is synthesized within the CNS rather than being imported from the periphery (2). As a mechanism for maintaining cholesterol homeostasis in the CNS, cholesterol can be converted into the more polar 24-hydroxycholesterol, which is released into the circulation by a subset of neurons (3, 4). Because cholesterol metabolism and distribution are not uniform across all cell types of the brain, an efficient system is necessary for the transport of cholesterol, and probably other lipids, among cells of the CNS. In the CNS, as in the periphery, cholesterol exchange between cells is mediated by lipoproteins. Lipoproteins in the CNS are derived from glia (astrocytes and microglia), which synthesize and secrete apolipoprotein (apo) E and apoJ (5-7) as well as apoD (8). Unlike the plasma, in which apoA1 is the most abundant apolipoprotein, apoE is the major apolipoprotein in the CNS (9, 10). Glia-derived lipoproteins play important roles in the brain by enhancing synaptogenesi...