Identification of PLTP as an LXR target gene and apoE as an FXR target gene reveals overlapping targets for the two nuclear receptors

Abstract

Affymetrix microarray data and Northern blot assays demonstrated that phospholipid transfer protein (PLTP) was induced 6-fold when either murine or human macrophages were incubated in the presence of ligands for the liver X receptor (LXR) and the retinoid X receptor. Two functional LXR response elements (LXREs) were identified and characterized in the proximal promoter of the human PLTP gene. One LXRE corresponds to a traditional direct repeat separated by 4 bp. However, the second LXRE is novel in that it corresponds to an inverted repeat separated by 1 bp, and is identical to the farnesoid X receptor response element. These studies demonstrate that PLTP is a direct target for activated LXR and farnesoid X receptor (FXR). In addition, apolipoprotein E (apoE), a known LXR target gene in macrophages, was shown to be activated in liver cells by FXR ligands.Taken together, the current data suggest that a small number of genes that currently include PLTP, apoE, and apoC-II, are induced in macrophages by activated LXR and in liver by activated FXR. Phospholipid transfer protein (PLTP) is a member of the lipid transfer/lipopolysaccharide binding protein gene family that includes CETP, the lipopolysaccharide binding protein, and the bactericidal/permeability increasing protein (1). PLTP has broad substrate specificity and functions to transfer phospholipids from triglyceride rich particles (VLDL and chylomicrons) to HDL during lipoprotein lipolysis (2). PLTP mRNA is expressed in a number of tissues, including liver, ovary, thymus, and placenta (3). Recent studies have demonstrated that PLTP is a target gene for activated farnesoid X receptor (FXR) (4, 5). This conclusion was based on the findings that i ) FXR ligands induced PLTP mRNA levels in cultured human hepatocytes (4) and in the livers of wild type but not FXR null mice (6), and ii ) the proximal promoter of the human gene contains a functional FXR response element (FXRE) (4, 5).In the current report, we utilized murine and human macrophages to demonstrate that PLTP is also an liver X receptor (LXR) target gene. Since macrophages do not express FXR, the results demonstrate that PLTP can be regulated independently by either receptor. Apolipoprotein E (apoE) is a known LXR target gene in macrophages (7,8). We now demonstrate that the hepatic expression of apoE is induced by FXR ligands. Taken together, these studies provide evidence that a subset of genes, which currently includes PLTP, apoE, and apoC-II (4-8), are induced following activation of LXR or FXR and control lipid metabolism. EXPERIMENTAL PROCEDURES Reagents, RNA isolation, Northern blot analysis, transient transfections, and reporter gene assaysExpression plasmids, sources of all reagents, and the indicated procedures have been described (6, 7, 9, 10). T0901317, chenodeoxycholate (CDCA), LG100153 (LG), and GW4064 (GW) were obtained from Cayman Chemical (Ann Arbor, MI), Sigma (St. Louis, MO), Dr. R. Heyman (Ligand Pharmaceuticals, La Jolla, CA), and Dr. T. Willson (GlaxoSmithKline), respectivel...

Cholesterol-lowering effect of bezafibrate is independent of peroxisome proliferator-activated receptor activation in mice

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Protein kinase C α modulates liver X receptor α transactivation

“…Since their discovery, multiple LXR target genes that are involved in cholesterol and lipid transport and homeostasis have been identified. These include the ATPbinding cassette (ABC) transporters, ABCA1, ABCG1, ABCG5 and ABCG8, lipoprotein and lipoprotein-remodeling enzymes such as apolipoprotein E, lipoprotein lipase, cholesterol ester transfer protein as well as lipogenic proteins such as sterol response element-binding protein (SREBP)-1c, and fatty acid synthase (Costet et al 2000, Mak et al 2002a,b, Ulven et al 2004. Despite increasing knowledge of the physiological function and mechanisms of action of LXR, little is known about the mechanisms by which LXR itself is regulated.…”

Enlargement of High Density Lipoprotein in Mice via Liver X Receptor Activation Requires Apolipoprotein E and Is Abolished by Cholesteryl Ester Transfer Protein Expression

“…The resulting phenotype was largely from a failure to up-regulate cholesterol 7␣-hydroxylase (Cyp7a), the rate-limiting step in the conversion of cholesterol to bile acids. Since then, multiple LXR target genes, including ABCA1 (10), apoE (11), CETP (12), and PLTP (13,14), have been identified, indicating the critical role of LXR in regulating HDL metabolism. It has also been reported recently that LXRs also play a central role in regulating fatty acid biosynthesis (15,16), glucose metabolism (17), and inflammation process (18,19).…”