Transcription Factor MAFF (MAF Basic Leucine Zipper Transcription Factor F) Regulates an Atherosclerosis Relevant Network Connecting Inflammation and Cholesterol Metabolism.

BACKGROUND: Coronary artery disease (CAD) is a multifactorial condition with both genetic and exogenous causes. The contribution of tissue-specific functional networks to the development of atherosclerosis remains largely unclear. The aim of this study was to identify and characterize central regulators and networks leading to atherosclerosis. METHODS: Based on several hundred genes known to affect atherosclerosis risk in mouse (as demonstrated in knockout models) and human (as shown by genome-wide association studies), liver gene regulatory networks were modeled. The hierarchical order and regulatory directions of genes within the network were based on Bayesian prediction models, as well as experimental studies including chromatin immunoprecipitation DNA-sequencing, chromatin immunoprecipitation mass spectrometry, overexpression, small interfering RNA knockdown in mouse and human liver cells, and knockout mouse experiments. Bioinformatics and correlation analyses were used to clarify associations between central genes and CAD phenotypes in both human and mouse. RESULTS: The transcription factor MAFF (MAF basic leucine zipper transcription factor F) interacted as a key driver of a liver network with 3 human genes at CAD genome-wide association studies loci and 11 atherosclerotic murine genes. Most importantly, expression levels of the low-density lipoprotein receptor (LDLR) gene correlated with MAFF in 600 CAD patients undergoing bypass surgery (STARNET [Stockholm-Tartu Atherosclerosis Reverse Network Engineering Task]) and a hybrid mouse diversity panel involving 105 different inbred mouse strains. Molecular mechanisms of MAFF were tested in noninflammatory conditions and showed positive correlation between MAFF and LDLR in vitro and in vivo. Interestingly, after lipopolysaccharide stimulation (inflammatory conditions), an inverse correlation between MAFF and LDLR in vitro and in vivo was observed. Chromatin immunoprecipitation mass spectrometry revealed that the human CAD genome-wide association studies candidate BACH1 (BTB domain and CNC homolog 1) assists MAFF in the presence of lipopolysaccharide stimulation with respective heterodimers binding at the MAF recognition element of the LDLR promoter to transcriptionally downregulate LDLR expression. CONCLUSIONS: The transcription factor MAFF was identified as a novel central regulator of an atherosclerosis/CAD-relevant liver network. MAFF triggered context-specific expression of LDLR and other genes known to affect CAD risk. Our results suggest that MAFF is a missing link between inflammation, lipid and lipoprotein metabolism, and a possible treatment target.

Focal middle cerebral artery ischemia in rats via a transfemoral approach using a custom designed microwire.

OBJECTIVES: The aim of this study was to develop a reliable and repeatable method of inducing focal middle cerebral artery occlusion (MCAo) in rats without ligation of the external carotid artery (ECA), while reducing the risk of subarachnoid hemorrhage. METHODS: We prototyped microwires with different diameters (0.0120 inch, 0.0115 inch, 0.0110 inch), materials, and construction methods (coil-on-core, extruded polymer jacket-on-core). Under fluoroscopic guidance and using femoral artery access, the microwires were navigated into the internal carotid artery of male Wistar rats (n=50, weight 376±64 g) to induce MCAo for 1 or 2 h. We performed neurological assessments at baseline, and at 3, 24, 72, and 168 h after MCAo. MRI measurements were performed on a 9.4 T scanner at 1 and 7 days post-injury. RESULTS: The 0.0115 inch microwire with polymer jacket-on-core provided the most successful outcome. At 1 and 7 days post-injury, we observed similar infarction volumes for 1 and 2 h MCAo in the MRI study. Infarcted lesion volumes in both MCAo groups were significantly reduced at 7 days compared with 1 day post-injury. The trend in longitudinal changes for the scores of different neurological assessments was confirmed to be significant after the injury, but both groups showed a similar trend of neurological deficits over the course of the study. CONCLUSIONS: We have developed a reliable and repeatable MCAo method in rats, allowing for precise occlusion of the MCA under direct fluoroscopic visualization without alteration of the cerebral hemodynamics associated with ECA ligation. The custom designed microwire can also be sized for targeted focal ischemia in larger animals.

MAFG is a transcriptional repressor of bile acid synthesis and metabolism.

Specific bile acids are potent signaling molecules that modulate metabolic pathways affecting lipid, glucose and bile acid homeostasis, and the microbiota. Bile acids are synthesized from cholesterol in the liver, and the key enzymes involved in bile acid synthesis (Cyp7a1, Cyp8b1) are regulated transcriptionally by the nuclear receptor FXR. We have identified an FXR-regulated pathway upstream of a transcriptional repressor that controls multiple bile acid metabolism genes. We identify MafG as an FXR target gene and show that hepatic MAFG overexpression represses genes of the bile acid synthetic pathway and modifies the biliary bile acid composition. In contrast, loss-of-function studies using MafG(+/-) mice causes de-repression of the same genes with concordant changes in biliary bile acid levels. Finally, we identify functional MafG response elements in bile acid metabolism genes using ChIP-seq analysis. Our studies identify a molecular mechanism for the complex feedback regulation of bile acid synthesis controlled by FXR.

Upregulation of endogenous farnesyl diphosphate synthase overcomes the inhibitory effect of bisphosphonate on protein prenylation in Hela cells.

Nitrogen-containing bisphosphonates (N-BPs) such as zoledronic acid (ZOL) are the gold standard treatment for diseases of excessive bone resorption. N-BPs inactivate osteoclasts via inhibition of farnesyl diphosphate synthase (FPPS), thereby preventing the prenylation of essential small GTPases. Not all patients respond to N-BP therapy to the same extent, and some patients, for example with tumour-associated bone disease or Paget's disease, appear to develop resistance to N-BPs. The extent to which upregulation of FPPS might contribute to these phenomena is not clear. Using quantitative PCR and western blot analysis we show that levels of FPPS mRNA and protein can be upregulated in HeLa cells by culturing in lipoprotein deficient serum (LDS) or by over-expression of SREBP-1a. Upregulated, endogenous FPPS was predominantly localised to the cytosol and did not co-localise with peroxisomal or mitochondrial markers. Upregulation of endogenous FPPS conferred resistance to the inhibitory effect of low concentrations of ZOL on the prenylation of the small GTPase Rap1a. These observations suggest that an increase in the expression of endogenous FPPS could confer at least partial resistance to the pharmacological effect of N-BP drugs such as ZOL in vivo.

LXRs regulate ER stress and inflammation through dynamic modulation of membrane phospholipid composition.

The fatty acyl composition of phospholipids determines the biophysical character of membranes and impacts the function of membrane proteins. Here, we define a nuclear receptor pathway for the dynamic modulation of membrane composition in response to changes in cellular lipid metabolism. Ligand activation of liver X receptors (LXRs) preferentially drives the incorporation of polyunsaturated fatty acids into phospholipids through induction of the remodeling enzyme Lpcat3. Promotion of Lpcat3 activity ameliorates endoplasmic reticulum (ER) stress induced by saturated free fatty acids in vitro or by hepatic lipid accumulation in vivo. Conversely, Lpcat3 knockdown in liver exacerbates ER stress and inflammation. Mechanistically, Lpcat3 modulates inflammation both by regulating inflammatory kinase activation through changes in membrane composition and by affecting substrate availability for inflammatory mediator production. These results outline an endogenous mechanism for the preservation of membrane homeostasis during lipid stress and identify Lpcat3 as an important mediator of LXR effects on metabolism.

Impaired cholesterol efflux in senescent macrophages promotes age-related macular degeneration.

Pathologic angiogenesis mediated by abnormally polarized macrophages plays a central role in common age-associated diseases such as atherosclerosis, cancer, and macular degeneration. Here we demonstrate that abnormal polarization in older macrophages is caused by programmatic changes that lead to reduced expression of ATP binding cassette transporter ABCA1. Downregulation of ABCA1 by microRNA-33 impairs the ability of macrophages to effectively efflux intracellular cholesterol, which in turn leads to higher levels of free cholesterol within senescent macrophages. Elevated intracellular lipid polarizes older macrophages to an abnormal, alternatively activated phenotype that promotes pathologic vascular proliferation. Mice deficient for Abca1, but not Abcg1, demonstrate an accelerated aging phenotype, whereas restoration of cholesterol efflux using LXR agonists or miR-33 inhibitors reverses it. Monocytes from older humans with age-related macular degeneration showed similar changes. These findings provide an avenue for therapeutic modulation of macrophage function in common age-related diseases.

MicroRNA-144 regulates hepatic ATP binding cassette transporter A1 and plasma high-density lipoprotein after activation of the nuclear receptor farnesoid X receptor.

RATIONALE: The bile acid receptor farnesoid X receptor (FXR) regulates many aspects of lipid metabolism by variouscomplex and incompletely understood molecular mechanisms. We set out to investigate the molecular mechanisms for FXR-dependent regulation of lipid and lipoprotein metabolism. OBJECTIVE: To identify FXR-regulated microRNAs that were subsequently involved in regulating lipid metabolism. METHODS AND RESULTS: ATP binding cassette transporter A1 (ABCA1) is a major determinant of plasma high-density lipoprotein (HDL)-cholesterol levels. Here, we show that activation of the nuclear receptor FXR in vivo increases hepatic levels of miR-144, which in turn lowers hepatic ABCA1 and plasma HDL levels. We identified 2 complementary sequences to miR-144 in the 3' untranslated region of ABCA1 mRNA that are necessary for miR-144-dependent regulation. Overexpression of miR-144 in vitro decreased both cellular ABCA1 protein and cholesterol efflux to lipid-poor apolipoprotein A-I protein, whereas overexpression in vivo reduced hepatic ABCA1 protein and plasma HDL-cholesterol. Conversely, silencing miR-144 in mice increased hepatic ABCA1 protein and HDL-cholesterol. In addition, we used tissue-specific FXR-deficient mice to show that induction of miR-144 and FXR-dependent hypolipidemia requires hepatic, but not intestinal, FXR. Finally, we identified functional FXR response elements upstream of the miR-144 locus, consistent with direct FXR regulation. CONCLUSIONS: We have identified a novel pathway involving FXR, miR-144, and ABCA1 that together regulate plasma HDL-cholesterol.

Loss of FXR protects against diet-induced obesity and accelerates liver carcinogenesis in ob/ob mice.

Farnesoid X receptor (FXR) is known to play important regulatory roles in bile acid, lipid, and carbohydrate metabolism. Aged (>12 months old) Fxr(-/-) mice also develop spontaneous liver carcinomas. In this report, we used three mouse models to investigate the role of FXR deficiency in obesity. As compared with low-density lipoprotein receptor (Ldlr) knockout (Ldlr(-/-)) mice, the Ldlr(-/-)Fxr(-/-) double-knockout mice were highly resistant to diet-induced obesity, which was associated with increased expression of genes involved in energy metabolism in the skeletal muscle and brown adipose tissue. Such a striking effect of FXR deficiency on obesity on an Ldlr(-/-) background led us to investigate whether FXR deficiency alone is sufficient to affect obesity. As compared with wild-type mice, Fxr(-/-) mice showed resistance to diet-induced weight gain. Interestingly, only female Fxr(-/-) mice showed significant resistance to diet-induced obesity, which was accompanied by increased energy expenditure in these mice. Finally, we determined the effect of FXR deficiency on obesity in a genetically obese and diabetic mouse model. We generated ob(-/-)Fxr(-/-) mice that were deficient in both Leptin and Fxr. On a chow diet, ob(-/-)Fxr(-/-) mice gained less body weight and had reduced body fat mass as compared with ob/ob mice. In addition, we observed liver carcinomas in 43% of young (<11 months old) Ob(-/-)Fxr(-/-) mice. Together these data indicate that loss of FXR prevents diet-induced or genetic obesity and accelerates liver carcinogenesis under diabetic conditions.

Hepatic hepatocyte nuclear factor 4α is essential for maintaining triglyceride and cholesterol homeostasis.

OBJECTIVE: Loss-of-function mutations in human hepatocyte nuclear factor 4α (HNF4α) are associated with maturity-onset diabetes of the young and lipid disorders. However, the mechanisms underlying the lipid disorders are poorly understood. In this study, we determined the effect of acute loss or augmentation of hepatic HNF4α function on lipid homeostasis. METHODS AND RESULTS: We generated an adenovirus expressing LacZ (Ad-shLacZ) or short hairpin RNA of Hnf4α (Ad-shHnf4α). Tail vain injection of C57BL/6J mice with Ad-shHnf4α reduced hepatic Hnf4α expression and resulted in striking phenotypes, including the development of fatty liver and a >80% decrease in plasma levels of triglycerides, total cholesterol, and high-density lipoprotein cholesterol. These latter changes were associated with reduced hepatic lipogenesis and impaired very-low-density lipoprotein secretion. Deficiency in hepatic Hnf4α did not affect intestinal cholesterol absorption despite decreased expression of genes involved in bile acid synthesis. Consistent with the loss-of-function data, overexpression of Hnf4α induced numerous genes involved in lipid metabolism in isolated primary hepatocytes. Interestingly, many of these HNF4α-regulated genes were not induced in wild-type mice that overexpressed hepatic Hnf4α. Because of selective gene regulation, mice overexpressing hepatic Hnf4α had unchanged plasma triglyceride levels and decreased plasma cholesterol levels. CONCLUSIONS: Loss of hepatic HNF4α results in severe lipid disorder as a result of dysregulation of multiple genes involved in lipid metabolism. In contrast, augmentation of hepatic HNF4α activity lowers plasma cholesterol levels but has no effect on plasma triglyceride levels because of selective gene regulation. Our data indicate that hepatic HNF4α is essential for controlling the basal expression of numerous genes involved in lipid metabolism and is indispensable for maintaining normal lipid homeostasis.

Genome-wide interrogation of hepatic FXR reveals an asymmetric IR-1 motif and synergy with LRH-1.

We used mouse hepatic chromatin enriched with an FXR antibody and chromatin immunoprecipitation-sequencing (ChIP-seq) to evaluate FXR binding on a genome-wide scale. This identified 1656 FXR-binding sites and 10% were located within 2 kb of a transcription start site which is much higher than predicted by random occurrence. A motif search uncovered a canonical nuclear receptor IR-1 site, consistent with in vitro DNA-binding studies reported previously. A separate nuclear receptor half-site for monomeric receptors such as LRH-1 was co-enriched and FXR activation of four newly identified promoters was significantly augmented by an LRH-1 expression vector in a co-transfection assay. There were 1038 genes located within 20 kb of a peak and a gene set enrichment analysis showed that genes identified by our ChIP-seq analysis are highly correlated with genes activated by an FXR-VP16 adenovirus in primary mouse hepatocytes providing functional relevance to the genome-wide binding study. Gene Ontology analysis showed FXR-binding sites close to many genes in lipid, fatty acid and steroid metabolism. Other broad gene clusters related to metabolism, transport, signaling and glycolysis were also significantly enriched. Thus, FXR may have a much wider role in cellular metabolism than previously appreciated.

Impaired development of atherosclerosis in Abcg1-/- Apoe-/- mice: identification of specific oxysterols that both accumulate in Abcg1-/- Apoe-/- tissues and induce apoptosis.

OBJECTIVE: To generate Abcg1(-/-) Apoe(-/-) mice to understand the mechanism and cell types involved in changes in atherosclerosis after loss of ABCG1. METHODS AND RESULTS: ABCG1 is highly expressed in macrophages and endothelial cells, 2 cell types that play important roles in the development of atherosclerosis. Abcg1(-/-) Apoe(-/-) and Apoe(-/-) mice and recipient Apoe(-/-) mice that had undergone transplantation with bone marrow from Apoe(-/-) or Abcg1(-/-) Apoe(-/-) mice were fed a Western diet for 12 or 16 weeks before quantification of atherosclerotic lesions. These studies demonstrated that loss of ABCG1 from all tissues, or from only hematopoietic cells, was associated with significantly smaller lesions that contained increased numbers of TUNEL- and cleaved caspase 3-positive apoptotic Abcg1(-/-) macrophages. We also identified specific oxysterols that accumulate in the brains and macrophages of the Abcg1(-/-) Apoe(-/-) mice. These oxysterols promoted apoptosis and altered the expression of proapoptotic genes when added to macrophages in vitro. CONCLUSIONS: Loss of ABCG1 from all tissues or from only hematopoietic cells results in smaller atherosclerotic lesions populated with increased apoptotic macrophages, by processes independent of ApoE. Specific oxysterols identified in tissues of Abcg1(-/-) Apoe(-/-) mice may be critical because they induce macrophage apoptosis and the expression of proapoptotic genes.

Differential expression and function of ABCG1 and ABCG4 during development and aging.

ABCG1 and ABCG4 are highly homologous members of the ATP binding cassette (ABC) transporter family that regulate cellular cholesterol homeostasis. In adult mice, ABCG1 is known to be expressed in numerous cell types and tissues, whereas ABCG4 expression is limited to the central nervous system (CNS). Here, we show significant differences in expression of these two transporters during development. Examination of beta-galactosidase-stained tissue sections from Abcg1(-/-)LacZ and Abcg4(-/-)LacZ knockin mice shows that ABCG4 is highly but transiently expressed both in hematopoietic cells and in enterocytes during development. In contrast, ABCG1 is expressed in macrophages and in endothelial cells of both embryonic and adult liver. We also show that ABCG1 and ABCG4 are both expressed as early as E12.5 in the embryonic eye and developing CNS. Loss of both ABCG1 and ABCG4 results in accumulation in the retina and/or brain of oxysterols, in altered expression of liver X receptor and sterol-regulatory element binding protein-2 target genes, and in a stress response gene. Finally, behavioral tests show that Abcg4(-/-) mice have a general deficit in associative fear memory. Together, these data indicate that loss of ABCG1 and/or ABCG4 from the CNS results in changes in metabolic pathways and in behavior.

Identification of novel pathways that control farnesoid X receptor-mediated hypocholesterolemia.

Farnesoid X receptor (FXR) plays important regulatory roles in bile acid, lipoprotein, and glucose homeostasis. Here, we have utilized Fxr(-/-) mice and mice deficient in scavenger receptor class B type I (SR-BI), together with an FXR-specific agonist and adenovirus expressing hepatocyte nuclear factor 4alpha or constitutively active FXR, to identify the mechanisms by which activation of FXR results in hypocholesterolemia. We identify a novel pathway linking FXR to changes in hepatic p-JNK, hepatocyte nuclear factor 4alpha, and finally SR-BI. Importantly, we demonstrate that the FXR-dependent increase in SR-BI results in both hypocholesterolemia and an increase in reverse cholesterol transport, a process involving the transport of cholesterol from peripheral macrophages to the liver for excretion into the feces. In addition, we demonstrate that FXR activation also induces an SR-BI-independent increase in reverse cholesterol transport and reduces intestinal cholesterol absorption. Together, these data indicate that FXR is a promising therapeutic target for treatment of hypercholesterolemia and coronary heart disease.

Emerging new paradigms for ABCG transporters.

Every cell is separated from its external environment by a lipid membrane. Survival depends on the regulated and selective transport of nutrients, waste products and regulatory molecules across these membranes, a process that is often mediated by integral membrane proteins. The largest and most diverse of these membrane transport systems is the ATP binding cassette (ABC) family of membrane transport proteins. The ABC family is a large evolutionary conserved family of transmembrane proteins (>250 members) present in all phyla, from bacteria to Homo sapiens, which require energy in the form of ATP hydrolysis to transport substrates against concentration gradients. In prokaryotes the majority of ABC transporters are involved in the transport of nutrients and other macromolecules into the cell. In eukaryotes, with the exception of the cystic fibrosis transmembrane conductance regulator (CFTR/ABCC7), ABC transporters mobilize substrates from the cytoplasm out of the cell or into specific intracellular organelles. This review focuses on the members of the ABCG subfamily of transporters, which are conserved through evolution in multiple taxa. As discussed below, these proteins participate in multiple cellular homeostatic processes, and functional mutations in some of them have clinical relevance in humans.

FXR signaling in metabolic disease.

Farnesoid X receptor (FXR), a member of the nuclear receptor superfamily, has been shown to be important in controlling numerous metabolic pathways; these include roles in maintaining bile acid, lipid and glucose homeostasis, in preventing intestinal bacterial infection and gallstone formation and in modulating liver regeneration and tumorigenesis. The accumulating data suggest that FXR may be a pharmaceutical target for the treatment of certain metabolic diseases.

Deletion of the transmembrane transporter ABCG1 results in progressive pulmonary lipidosis.

We show that mice lacking the ATP-binding cassette transmembrane transporter ABCG1 show progressive and age-dependent severe pulmonary lipidosis that recapitulates the phenotypes of different respiratory syndromes in both humans and mice. The lungs of chow-fed Abcg1(-/-) mice, >6-months old, exhibit extensive subpleural cellular accumulation, macrophage, and pneumocyte type 2 hypertrophy, massive lipid deposition in both macrophages and pneumocytes and increased levels of surfactant. No such abnormalities are observed at 3 months of age. However, gene expression profiling reveals significant changes in the levels of mRNAs encoding key genes involved in lipid metabolism in both 3- and 8-month-old Abcg1(-/-) mice. These data suggest that the lungs of young Abcg1(-/-) mice maintain normal lipid levels by repressing lipid biosynthetic pathways and that such compensation is inadequate as the mice mature. Studies with A-549 cells, a model for pneumocytes type 2, demonstrate that overexpression of ABCG1 specifically stimulates the efflux of cellular cholesterol by a process that is dependent upon phospholipid secretion. In addition, we demonstrate that Abcg1(-/-), but not wild-type macrophages, accumulate cholesterol ester droplets when incubated with surfactant. Together, these data provide a mechanism to explain the lipid accumulation in the lungs of Abcg1(-/-)mice. In summary, our results demonstrate that ABCG1 plays essential roles in pulmonary lipid homeostasis.

Impaired development of atherosclerosis in hyperlipidemic Ldlr-/- and ApoE-/- mice transplanted with Abcg1-/- bone marrow.

OBJECTIVE: The lungs of Abcg1-/- mice accumulate macrophage foam cells that contain high levels of unesterified and esterified cholesterol, consistent with a role for ABCG1 in facilitating the efflux of cholesterol from macrophages to high-density lipoprotein (HDL) and other exogenous sterol acceptors. Based on these observations, we investigated whether loss of ABCG1 affects foam cell deposition in the artery wall and the development of atherosclerosis. METHODS AND RESULTS: Bone marrow from wild-type or Abcg1-/- mice was transplanted into Ldlr-/- or ApoE-/- mice. After administration of a high-fat/high-cholesterol diet, plasma and tissue lipid levels and atherosclerotic lesion size were quantified and compared. Surprisingly, transplantation of Abcg1-/- bone marrow cells resulted in a significant reduction in lesion size in both mouse models, despite the fact that lipid levels increased in the lung, spleen, and kidney. Lesions of Ldlr-/- mice transplanted with Abcg1-/- cells contained increased numbers of apoptotic cells. Consistent with this observation, in vitro studies demonstrated that Abcg1-/- macrophages were more susceptible to oxidized low-density lipoprotein (ox-LDL)-dependent apoptosis than Abcg1+/+ cells. CONCLUSIONS: Diet-induced atherosclerosis is impaired when atherosclerotic-susceptible mice are transplanted with Abcg1-/- bone marrow. The demonstration that Abcg1-/- macrophages undergo accelerated apoptosis provides a mechanism to explain the decrease in the atherosclerotic lesions.

Activation of the nuclear receptor FXR improves hyperglycemia and hyperlipidemia in diabetic mice.

Farnesoid X receptor (FXR) plays an important role in maintaining bile acid and cholesterol homeostasis. Here we demonstrate that FXR also regulates glucose metabolism. Activation of FXR by the synthetic agonist GW4064 or hepatic overexpression of constitutively active FXR by adenovirus-mediated gene transfer significantly lowered blood glucose levels in both diabetic db/db and wild-type mice. Consistent with these data, FXR null mice exhibited glucose intolerance and insulin insensitivity. We further demonstrate that activation of FXR in db/db mice repressed hepatic gluconeogenic genes and increased hepatic glycogen synthesis and glycogen content by a mechanism that involves enhanced insulin sensitivity. In view of its central roles in coordinating regulation of both glucose and lipid metabolism, we propose that FXR agonists are promising therapeutic agents for treatment of diabetes mellitus.

A role for FXR and human FGF-19 in the repression of paraoxonase-1 gene expression by bile acids.

Paraoxonase-1 (PON1), an enzyme that metabolizes organophosphate insecticides, is secreted by the liver and transported in the blood complexed to HDL. In humans and mice, low plasma levels of PON1 have also been linked to the development of atherosclerosis. We previously reported that hepatic Pon1 expression was decreased when C57BL/6J mice were fed a high-fat, high-cholesterol diet supplemented with cholic acid (CA). In the current study, we used wild-type and farnesoid X receptor (FXR) null mice to demonstrate that this repression is dependent upon CA and FXR. PON1 mRNA levels were also repressed when HepG2 cells, derived from a human hepatoma, were incubated with natural or highly specific synthetic FXR agonists. In contrast, fibroblast growth factor-19 (FGF-19) mRNA levels were greatly induced by these same FXR agonists. Furthermore, treatment of HepG2 cells with recombinant human FGF-19 significantly decreased PON1 mRNA levels. Finally, deletion studies revealed that the proximal -230 to -96 bp region of the PON1 promoter contains regulatory element(s) necessary for promoter activity and bile acid repression. These data demonstrate that human PON1 expression is repressed by bile acids through the actions of FXR and FGF-19.

ABCG1 has a critical role in mediating cholesterol efflux to HDL and preventing cellular lipid accumulation.

Here we demonstrate that the ABC transporter ABCG1 plays a critical role in lipid homeostasis by controlling both tissue lipid levels and the efflux of cellular cholesterol to HDL. Targeted disruption of Abcg1 in mice has no effect on plasma lipids but results in massive accumulation of both neutral lipids and phospholipids in hepatocytes and in macrophages within multiple tissues following administration of a high-fat and -cholesterol diet. In contrast, overexpression of human ABCG1 protects murine tissues from dietary fat-induced lipid accumulation. Finally, we show that cholesterol efflux to HDL specifically requires ABCG1, whereas efflux to apoA1 requires ABCA1. These studies identify Abcg1 as a key gene involved in both cholesterol efflux to HDL and in tissue lipid homeostasis.