To identify genes that are transcriptionally activated when human macrophages accumulate excess lipids, we employed the mRNA differential display technique using RNA isolated from human monocyte-macrophages incubated in the absence or presence of acetylated low density lipoprotein and sterols (cholesterol and 25-hydroxycholesterol). These studies identified a mRNA whose levels were highly induced in lipid-loaded macrophages. The mRNA encoded the human White protein, a member of the ATP-binding cassette (ABC) transporter superfamily of proteins. The mRNA levels of ABC8, the murine homolog of the human white gene, were also induced when a murine macrophage cell line, RAW264.7, was incubated with acetylated low density lipoprotein and sterols. Additional studies demonstrated that white/ ABC8 mRNA levels were induced by specific oxysterols that included 25-, 20(S)-, and 22(R)-hydroxycholesterol, and by a retinoid X receptor-specific ligand. Furthermore, the oxysterol-mediated induction of ABC8 expression in mouse peritoneal macrophages was dependent on the presence of the nuclear oxysterol receptors, liver X receptors (LXRs). Macrophages derived from mice lacking both LXR␣ and LXR␤ failed to up-regulate the expression of ABC8 following incubation with 22(R)-hydroxycholesterol. Oxysterol-dependent induction of white/ABC8 mRNA was blocked by actinomycin D but not by cycloheximide treatment of cells. We conclude that the white and ABC8 genes are primary response genes that are transcriptionally activated by specific oxysterols and that this induction is mediated by the LXR subfamily of nuclear hormone receptors. These data strongly support the hypothesis that white/ABC8 has a role in cellular sterol homeostasis.An early event in the development of the fatty streak in the artery wall involves the entry of circulating monocytes into the subendothelial space and their subsequent differentiation into macrophages (1, 2). In the presence of oxidized or modified forms of LDL, 1 these macrophages accumulate cytoplasmic lipid droplets that contain excess cholesteryl esters (1-3). The latter cells are termed macrophage "foam" cells, because the lipid droplets give them a characteristic foamy appearance when viewed under the microscope (1). The importance of monocyte/macrophages in the development of fatty streaks and the more advanced atherosclerotic lesions can be gauged from the effect of deletions or mutations of genes that are involved in either monocyte recruitment into the artery wall or their subsequent differentiation into macrophages. For example, mice defective in monocyte chemoattractant protein-1 (4), the monocyte chemoattractant protein-1 receptor (5), or macrophagecolony-stimulating factor (6, 7) exhibit reduced levels of atherosclerotic lesions when compared with normal mice.Relatively little is known about the alterations in gene expression that occur when macrophages accumulate excess lipids to become macrophage foam cells. In previous attempts to identify such genes, macrophages were incubated with Ac-LDL to pr...

The synthesis of 7␣-hydroxylated bile acids from oxysterols requires an oxysterol 7␣-hydroxylase encoded by the Cyp7b1 locus. As expected, mice deficient in this enzyme have elevated plasma and tissue levels of 25-and 27-hydroxycholesterol; however, levels of another major oxysterol, 24-hydroxycholesterol, are not increased in these mice, suggesting the presence of another oxysterol 7␣-hydroxylase. Here, we describe the cloning and characterization of murine and human cDNAs and genes that encode a second oxysterol 7␣-hydroxylase. The genes contain 12 exons and are located on chromosome 6 in the human (CYP39A1 locus) and in a syntenic position on chromosome 17 in the mouse (Cyp39a1 locus). CYP39A1 is a microsomal cytochrome P450 enzyme that has preference for 24-hydroxycholesterol and is expressed in the liver. The levels of hepatic CYP39A1 mRNA do not change in response to dietary cholesterol, bile acids, or a bile acid-binding resin, unlike those encoding other sterol 7␣-hydroxylases. Hepatic CYP39A1 expression is sexually dimorphic (female > male), which is opposite that of CYP7B1 (male > female). We conclude that oxysterol 7␣-hydroxylases with different substrate specificities exist in mice and humans and that sexually dimorphic expression patterns of these enzymes in the mouse may underlie differences in bile acid metabolism between the sexes.Two metabolic pathways that differ in their initial steps produce 7␣-hydroxylated bile acids (1). In one pathway, cholesterol (5-cholesten-3␤-ol) is first converted into 7␣-hydroxycholesterol (cholest-5-ene-3␤,7␣-diol) by the enzyme cholesterol 7␣-hydroxylase, which is encoded by the Cyp7a1 gene in mice. In the other pathway, cholesterol is first converted into one of several oxysterols prior to being 7␣-hydroxylated by oxysterol 7␣-hydroxylase, which is encoded by the Cyp7b1 gene. The 7␣-hydroxylated intermediates produced by these different initiating steps are subsequently converted into primary bile acids by a series of shared enzymes in the liver (2).Cholesterol 7␣-hydroxylase shows a marked preference for cholesterol as a substrate and is only weakly active against other sterols (3), whereas oxysterol 7␣-hydroxylase prefers 25-hydroxycholesterol (cholest-5-ene-3␤,25-diol) and 27-hydroxycholesterol (cholest-5-ene-3␤,27-diol) (4, 5). In agreement with the latter preference, Cyp7b1 knockout mice accumulate these two oxysterols in their plasma and tissues (6). The levels of the other major oxysterol, 24-hydroxycholesterol (cholest-5-ene-3␤,24-diol), are near normal in these animals (6). In contrast, a human with a complete absence of oxysterol 7␣-hydroxylase activity accumulated 24-hydroxycholesterol as well as 25-and 27-hydroxycholesterol in his plasma (7). These observations suggest that the human enzyme, unlike the mouse enzyme, may act on all three oxysterol substrates.The observed differences in the oxysterol accumulation patterns in mice and humans that express no oxysterol 7␣-hydroxylase are not due to differences in the biosynthesis of oxysterols, as cholester...