The phospholipid transfer protein gene is a liver X receptor target expressed by macrophages in atherosclerotic lesions

The liver X receptors (LXRs) are members of the nuclear receptor superfamily that are activated by oxysterols. In response to ligand binding, LXRs regulate a variety of genes involved in the catabolism, transport, and uptake of cholesterol and its metabolites. Here we demonstrate that LXRs also regulate plasma lipoprotein metabolism through control of the phospholipid transfer protein (PLTP) gene. LXR ligands induce the expression of PLTP in cultured HepG2 cells and mouse liver in vivo in a coordinate manner with known LXR target genes. Moreover, plasma phospholipid transfer activity is increased in mice treated with the synthetic LXR ligand GW3965. Unexpectedly, PLTP expression was also highly inducible by LXR in macrophages, a cell type not previously recognized to express this enzyme. The ability of synthetic and oxysterol ligands to regulate PLTP mRNA in macrophages and liver is lost in animals lacking both LXRalpha and LXRbeta, confirming the critical role of these receptors. We further demonstrate that the PLTP promoter contains a high-affinity LXR response element that is bound by LXR/RXR heterodimers in vitro and is activated by LXR/RXR in transient-transfection studies. Finally, immunohistochemistry studies reveal that PLTP is highly expressed by macrophages within human atherosclerotic lesions, suggesting a potential role for this enzyme in lipid-loaded macrophages. These studies outline a novel pathway whereby LXR and its ligands may modulate lipoprotein metabolism.     

Identification of human low-density lipoprotein receptor as a novel target gene regulated by liver X receptor alpha

Liver X receptor alpha (LXRalpha) is a member of the nuclear receptor superfamily that is activated by oxysterols, and plays a pivotal role in regulating the metabolism, transport and uptake of cholesterol. Here, we demonstrate that LXRalpha also regulates the low-density lipoprotein receptor (LDLR) gene, which mediates the endocytic uptake of LDL cholesterol in the liver. An LXR agonist induced the expression of LDLR in cultured hepatoblastoma cells. Moreover, the LDLR promoter contained an LXR response element that was recognized by LXRalpha/RXRalpha (retinoid X receptor alpha) heterodimers in hepatoblastoma cells. These results suggest a novel pathway whereby LXRalpha might modulate cholesterol metabolism.     

LXR/RXR activation enhances basolateral efflux of cholesterol in CaCo-2 cells

Regulation of gene expression of ATP-binding cassette transporter (ABC)A1 and ABCG1 by liver X receptor/retinoid X receptor (LXR/RXR) ligands was investigated in the human intestinal cell line CaCo-2. Neither the RXR ligand, 9-cis retinoic acid, nor the natural LXR ligand 22-hydroxycholesterol alone altered ABCA1 mRNA levels. When added together, ABCA1 and ABCG1 mRNA levels were increased 3- and 7-fold, respectively. T0901317, a synthetic non-sterol LXR agonist, increased ABCA1 and ABCG1 gene expression 11- and 6-fold, respectively. ABCA1 mass was increased by LXR/RXR activation. T0901317 or 9-cis retinoic acid and 22-hydroxycholesterol increased cholesterol efflux from basolateral but not apical membranes. Cholesterol efflux was increased by the LXR/RXR ligands to apolipoprotein (apo)A-I or HDL but not to taurocholate/phosphatidylcholine micelles. Actinomycin D prevented the increase in ABCA1 and ABCG1 mRNA levels and the increase in cholesterol efflux induced by the ligands. Glyburide, an inhibitor of ABCA1 activity, attenuated the increase in basolateral cholesterol efflux induced by T0901317. LXR/RXR activation decreased the esterification and secretion of cholesterol esters derived from plasma membranes. Thus, in CaCo-2 cells, LXR/RXR activation increases gene expression of ABCA1 and ABCG1 and the basolateral efflux of cholesterol, suggesting that ABCA1 plays an important role in intestinal HDL production and cholesterol absorption.     

Cholesterol secretion and homeostasis in chondrocytes: a liver X receptor and retinoid X receptor heterodimer mediates apolipoprotein A1 expression.

Cholesterol is required for chondrocyte differentiation and bone formation. Apolipoprotein A1 (apoA-1) plays a major role in lipoprotein clearance and cholesterol redistribution. We report here that apoA-1 is expressed during chondrocyte differentiation in vitro and in vivo. In differentiating chondrocytes, the expression of the liver X receptor (LXR) is modulated and its expression correlates to the expression of apoA-1. The expression of other LXR target genes related to cholesterol homeostasis such as ABCA1 cholesterol transporter and sterol regulatory element-binding protein 1 (SREBP1) is similarly regulated. Small molecule ligands activating either LXR or retinoid X receptor (RXR) lead to a dramatic increase in apoA-1 mRNA and protein expression in cultured chondrocytes. These ligands strongly induce ABCA1 cholesterol transporter expression and effectively mediate cholesterol efflux from hypertrophic chondrocytes. In addition, we report that, in the same cells, the ligands down modulate Serum Amyloid A expression induced by bacterial lipopolysaccharide. Our studies provide evidence that LXR/RXR mediate a fine regulation of cholesterol homeostasis in differentiating chondrocytes.