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.

Bile acids have the gall to function as hormones

In this issue of Cell Metabolism, Thomas et al. (2009) show that specific activation of the bile-acid-activated G protein-coupled receptor TGR5 improves pancreatic and hepatic function and impairs the development of obesity following administration of a high-fat diet.

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.

Chromosome translocations and fusion genes in breast cancer

Abstract #2021

Background: Little is known about chromosome translocations in the common epithelial cancers such as breast cancer, in spite of the central role played by translocations and consequent gene fusions in haematopoietic cancers.
 Methods: We present a comprehensive analysis by array painting of the chromosome translocations of four breast cancer cell lines, DU4475, HCC1806, HCC1187 and ZR-75-30. In array painting chromosomes are isolated by flow cytometry, amplified and hybridized to DNA microarrays. All breakpoints, totalling nearly 250, were mapped to at least 1Mb resolution and most balanced breakpoints were mapped to about 2kb resolution using custom oligonucleotide arrays. The remaining unbalanced breakpoints were mapped to around 20kb by identifying copy number steps in Affymetrix SNP6 array hybrizations obtained by the Sanger Institute's Cancer Genome Project. Breast tumours in parraffin section in tissue microarrays were screened by FISH to see whether selected breakpoints found in the cell lines are present in breast tumours.
 Results: We found at least 12 reciprocal translocations in the four cell lines, substantially more than expected, and many more rearrangements were balanced for at least one participating chromosome. Many of the breakpoints were within or adjacent to cancer-relevant genes, and three of the translocations have already been shown to form fusion transcripts, RIF1-PKD1L1, PUM1-TRERF1 and TAX1BP1-AHCY. For selected genes targetted by the translocations, about 100 breast tumours were screened for breaks. Breaks were found in two to six cases for several of the genes, confirming that some of them were broken in breast tumours. For example two cases of unbalanced breakage were identified in PKD1L1, and these were confirmed by array-CGH.
 Discussion: Our results suggest that breast cancers have fusion genes, and support the emerging view that chromosome rearrangements are likely to play a significant role in common epithelial cancers.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 2021.

The ABCs of sterol transport

Mammalian cells have developed various responses to minimize accumulation of unesterified cholesterol, as the latter can result in cell toxicity and death [reviewed in this edition by Björkhem (Björkhem, I. 2009. Are side-chain oxidized oxysterols regulators also in vivo? J. Lipid Res. In press)]. These responses include esterification to sequester excess sterol in intracellular lipid droplets, repression of both cholesterol synthesis and LDL receptor expression (thus reducing endocytosis of LDL), and induction of a panoply of genes that promote sterol efflux and affect lipid metabolism. The nuclear receptor liver-X-receptor (LXR) functions as a cellular "sterol sensor" and plays a critical role in these latter transcriptional changes [reviewed in this edition by Glass (Shibata, N., and Glass C, K. 2009. Regulation of macrophage function in inflammation and atherosclerosis. J. Lipid Res. In press)]. Activation of LXR by either endogenous oxysterols or synthetic agonists induces the expression of many genes, including those encoding ATP-binding cassette (ABC) transporters ABCA1, ABCG1, ABCG5, and ABCG8. As discussed below, these four proteins function to promote sterol efflux from cells.

Loss of ABCG1 results in chronic pulmonary inflammation

ABCG1, a member of the ATP-binding cassette transporter superfamily, is highly expressed in multiple cells of the lung. Loss of ABCG1 results in severe pulmonary lipidosis in mice, with massive deposition of cholesterol in both alveolar macrophages and type 2 cells and the accumulation of excessive surfactant phospholipids. These observations are consistent with ABCG1 controlling cellular sterol metabolism. Herein, we report on the progressive and chronic inflammatory process that accompanies the lipidosis in the lungs of Abcg1-/- mice. Compared with wild-type animals, the lungs of aged chow-fed mice deficient in ABCG1 show distinctive signs of inflammation that include macrophage accumulation, lymphocytic infiltration, hemorrhage, eosinophilic crystals, and elevated levels of numerous cytokines and cytokine receptors. Analysis of bronchoalveolar lavages obtained from Abcg1-/- mice revealed elevated numbers of foamy macrophages and leukocytes and the presence of multiple markers of inflammation including crystals of chitinase-3-like proteins. These data suggest that cholesterol and/or cholesterol metabolites that accumulate in Abcg1-/- lungs can trigger inflammatory signaling pathways. Consistent with this hypothesis, the expression of a number of cytokines was found to be significantly increased following increased cholesterol delivery to either primary peritoneal macrophages or Raw264.7 cells. Finally, cholesterol loading of primary mouse macrophages induced cytokine mRNAs to higher levels in Abcg1-/-, as compared with wild-type cells. These results demonstrate that ABCG1 plays critical roles in pulmonary homeostasis, balancing both lipid/cholesterol metabolism and inflammatory responses.

LXR signaling couples sterol metabolism to proliferation in the acquired immune response

Cholesterol is essential for membrane synthesis; however, the mechanisms that link cellular lipid metabolism to proliferation are incompletely understood. We demonstrate here that cellular cholesterol levels in dividing T cells are maintained in part through reciprocal regulation of the LXR and SREBP transcriptional programs. T cell activation triggers induction of the oxysterol-metabolizing enzyme SULT2B1, consequent suppression of the LXR pathway for cholesterol transport, and promotion of the SREBP pathway for cholesterol synthesis. Ligation of LXR during T cell activation inhibits mitogen-driven expansion, whereas loss of LXRbeta confers a proliferative advantage. Inactivation of the sterol transporter ABCG1 uncouples LXR signaling from proliferation, directly linking sterol homeostasis to the antiproliferative action of LXR. Mice lacking LXRbeta exhibit lymphoid hyperplasia and enhanced responses to antigenic challenge, indicating that proper regulation of LXR-dependent sterol metabolism is important for immune responses. These results implicate LXR signaling in a metabolic checkpoint that modulates cell proliferation and immunity.

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.

ABCG1 and ABCG4 are coexpressed in neurons and astrocytes of the CNS and regulate cholesterol homeostasis through SREBP-2

Here, we describe the initial characterization of Abcg4(-/-) mice and identify overlapping functions of ABCG4 and ABCG1 in the brain. Histological examination of tissues from Abcg4(+/-)/nlsLacZ and Abcg1(+/-)/nlsLacZ mice demonstrates that coexpression of Abcg4 and Abcg1 is restricted to neurons and astrocytes of the central nervous system (CNS). Interestingly, Abcg4 mRNA is undetectable outside the CNS, in contrast with the broad tissue and cellular expression of Abcg1. We also used primary astrocytes, microglia, neurons, and macrophages to demonstrate that the expression of Abcg1, but not Abcg4, is induced after the activation of liver X receptor. Cellular localization studies demonstrated that both proteins reside in RhoB-positive endocytic vesicle membranes. Furthermore, overexpression of either ABCG1 or ABCG4 increased the processing of sterol-regulatory element binding protein 2 (SREBP-2) to the transcriptionally active protein, thus accounting for the observed increase in the expression of SREBP-2 target genes and cholesterol synthesis. Consistent with these latter results, we show that the expression levels of the same SREBP-2 target genes are repressed in the brains of Abcg1(-/-) and, to a lesser extent, Abcg4(-/-) mice. Based on the results of the current study, we propose that ABCG1 and ABCG4 mediate the intracellular vesicular transport of cholesterol/sterols within both neurons and astrocytes to regulate cholesterol transport in the brain.

Regulation of the sodium/sulfate co-transporter by farnesoid X receptor alpha

Fxralpha is known to regulate a variety of metabolic processes, including bile acid, cholesterol, and carbohydrate metabolism. In this study, we show direct evidence that Fxralpha is a key player in maintaining sulfate homeostasis. We identified and characterized the sodium/sulfate co-transporter (NaS-1; Slc13a1) as an Fxralpha target gene expressed in the kidney and intestine. Electromobility shift assays, chromatin immunoprecipitation, and promoter reporter studies identified a single functional Fxralpha response element in the second intron of the mouse Slc13a1 gene. Treatment of wild-type mice with GW4064, a synthetic Fxralpha agonist, induced Slc13a1 mRNA in the intestine and kidney. Slc13a1 mRNA was also induced in the kidney and intestine of wild-type, but not Fxralpha-/- mice, after treatment with the hepatotoxin alpha-naphthylisothiocyanate, which is known to result in elevated blood bile acid levels. Finally, we observed a decrease in Slc13a1 mRNA in the kidney and intestine of Fxralpha-/- mice and a corresponding increase in urinary excretion of free sulfates as compared with wild-type mice. These results demonstrate that mouse Slc13a1 is a novel Fxralpha target gene expressed in the kidney and intestine and that in the absence of Fxralpha, mice waste sulfate into the urine. Thus, Fxralpha is necessary for normal sulfate homeostasis in vivo.

Regulation of hepatic Insig-2 by the farnesoid X receptor

Activation of the farnesoid X receptor (FXRalpha) affects genes controlling many pathways, including those involved in bile acid and glucose homeostasis. Here we report that a critical gene involved in cholesterol homeostasis, Insig-2, was induced when mice or cultured cells were treated with FXRalpha agonists or infected with constitutively active FXRalpha. No such induction was observed in agonist-treated FXRalpha-/- mice. Further analysis, which included EMSAs, reporter gene activation, and chromatin immunoprecipitation, identified two functional FXRalpha response elements within intron 2 of the mouse Insig-2 gene. In addition to increasing hepatic Insig-2 protein levels in wild-type mice, FXRalpha activation also reduced lanosterol 14alpha-demethylase mRNA levels and 3-hydroxy-3-methylglutaryl-coenzyme A reductase protein levels. Together, these changes likely account for the decrease in cholesterol synthesis observed after activation of FXR in primary hepatocytes. In conclusion, the current study links hepatic FXRalpha activation to regulation of genes involved in cholesterol synthesis.

FXR, a multipurpose nuclear receptor

The farnesoid X receptor (FXR) is a ligand-activated transcription factor and a member of the nuclear receptor superfamily. In the past six years, remarkable inroads have been made into determining the functional importance of FXR. This receptor has been shown to have crucial roles in controlling bile acid homeostasis, lipoprotein and glucose metabolism, hepatic regeneration, intestinal bacterial growth and the response to hepatotoxins. Thus, the development of FXR agonists might prove useful for the treatment of diabetes, cholesterol gallstones, and hepatic and intestinal toxicity.

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.

FXR deficiency causes reduced atherosclerosis in Ldlr-/- mice

OBJECTIVE

Based on the observation that Fxr-/- mice exhibit a proatherogenic lipoprotein profile, we investigated the role of FXR in the development of atherosclerosis.

METHODS AND RESULTS

Administration of a western diet to Fxr-/- mice or wild-type mice does not result in the development of significant atherosclerotic lesions. Consequently we generated Fxr-/- Ldlr-/- (DKO) mice and compared lesion development with Ldlr-/- mice. After 16 weeks on a Western diet, en face analysis of the aorta indicated that the male DKO mice had reduced atherosclerotic lesions as compared with Ldlr-/- mice. Plasma low-density lipoprotein cholesterol and high-density lipoprotein cholesterol levels were reduced by 40% to 50%, whereas triglyceride levels increased 4-fold in the male DKO mice. Finally, peritoneal macrophages freshly isolated from male DKO mice had reduced expression of CD36 mRNA and decreased neutral lipid accumulation, as compared with Ldlr-/- mice.

CONCLUSIONS

FXR deficiency in male, but not female, Ldlr-/- mice results in a reduction in the size of atherosclerotic lesions in the aorta. The reduction in atherosclerosis may result from a decrease in plasma low-density lipoprotein cholesterol, coupled with reduced expression of CD36 in macrophages of DKO mice.

ATP-binding cassette transporter G1 and lipid homeostasis

PURPOSE OF REVIEW

This review briefly discusses the ATP-binding cassette transporter G (ABCG) family members and emphasizes recent studies that identify ABCG1 as a key regulator of cellular lipid homeostasis.

RECENT FINDINGS

The in-vivo importance of ABCG1 has recently been demonstrated with both loss-of-function and gain-of-function studies in mice. Administration of a diet high in both fat and cholesterol to ABCG1 mice results in massive cholesterol accumulation in both the liver and lungs. In contrast, lipid accumulation is greatly attenuated in transgenic mice that express both the murine and human ABCG1 genes. Despite the observed tissue lipid accumulation, plasma lipid levels and lipoprotein cholesterol distribution are not significantly different between wild-type, ABCG1, and hABCG1 transgenic mice. Other studies show that ABCG1 expression is induced following activation of the nuclear receptor LXR and that over expression of ABCG1 results in increased efflux of cellular cholesterol to HDL or phospholipid vesicles.

SUMMARY

The ABCG1 transporter plays a key role in regulating cellular cholesterol and lipid homeostasis. Elucidation of the molecular mechanism by which ABCG1 controls sterol flux should provide critical information that may link ABCG1 to the reverse cholesterol transport pathway or diseases such as atherosclerosis.

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.

FXR regulates organic solute transporters alpha and beta in the adrenal gland, kidney, and intestine

Expression of the farnesoid X receptor (FXR; NR1H4) is limited to the liver, intestine, kidney, and adrenal gland. However, the role of FXR in the latter two organs is unknown. In the current study, we performed microarray analysis using RNA from H295R cells infected with constitutively active FXR. Several putative FXR target genes were identified, including the organic solute transporters alpha and beta (OSTalpha and OSTbeta). Electromobility shift assays and promoter-reporter studies identified functional farnesoid X receptor response elements (FXREs) in the promoters of both human genes. These FXREs are conserved in both mouse genes. Treatment of wild-type mice with 3-(2,6-dichlorophenyl)-4-(3'-carboxy-2-chloro-stilben-4-yl)-oxymethyl-5-isopropyl-isoxazole (GW4064), a synthetic FXR agonist, induced OSTalpha and OSTbeta mRNAs in the intestine and kidney. Both mRNAs were also induced when wild-type, but not FXR-deficient (FXR-/-), adrenals were cultured in the presence of GW4064. OSTalpha and OSTbeta mRNA levels were also induced in the adrenals and kidneys of wild-type, but not FXR-/-, mice after the increase of plasma bile acids in response to the hepatotoxin alpha-naphthylisothiocyanate. Finally, overexpression of human OSTalpha and OSTbeta facilitated the uptake of conjugated chenodeoxycholate and the activation of FXR target genes. These results demonstrate that OSTalpha and OSTbeta are novel FXR target genes that are expressed in the adrenal gland, kidney, and intestine.

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.

Alpha-crystallin is a target gene of the farnesoid X-activated receptor in human livers

Alpha-crystallins comprise 35% of soluble proteins in the ocular lens and possess chaperone-like functions. Furthermore, the alphaA subunit (alphaA-crystallin) of alpha crystallin is thought to be "lens-specific" as only very low levels of expression were detected in a few non-lenticular tissues. Here we report that human alphaA-crystallin is expressed in human livers and is regulated by farnesoid X-activated receptor (FXR) in response to FXR agonists. AlphaA-crystallin was identified in a microarray screen as one of the most highly induced genes after treatment of HepG2 cells with the synthetic FXR ligand GW4064. Northern blot and quantitative real-time PCR analyses confirmed that alphaA-crystallin expression was induced in HepG2-derived cell lines and human primary hepatocytes and hepatic stellate cells in response to either natural or synthetic FXR ligands. Transient transfection studies and electrophoretic mobility shift assays revealed a functional FXR response element located in intron 1 of the human alphaA-crystallin gene. Importantly, immunohistochemical staining of human liver sections showed increased alphaA-crystallin expression in cholangiocytes and hepatocytes. As a member of the small heat shock protein family possessing chaperone-like activity, alphaA-crystallin may be involved in protection of hepatocytes from the toxic effects of high concentrations of bile acids, as would occur in disease states such as cholestasis.

A role for the apoptosis inhibitory factor AIM/Spalpha/Api6 in atherosclerosis development

Macrophages play a central role in the development of atherosclerosis through the accumulation of oxidized LDL (oxLDL). AIM (Spalpha/Api6) has previously been shown to promote macrophage survival; however, its function in atherogenesis is unknown. Here we identify AIM as a critical factor that protects macrophages from the apoptotic effects of oxidized lipids. AIM protein is induced in response to oxLDL loading and is highly expressed in foam cells within atherosclerotic lesions. Interestingly, both expression of AIM in lesions and its induction by oxidized lipids require the action of LXR/RXR heterodimers. AIM-/- macrophages are highly susceptible to oxLDL-induced apoptosis in vitro and undergo accelerated apoptosis in atherosclerotic lesions in vivo. Moreover, early atherosclerotic lesions in AIM-/-LDLR-/- double knockout mice are dramatically reduced when compared to AIM+/+LDLR-/- controls. We conclude that AIM production facilitates macrophage survival within atherosclerotic lesions and that loss of AIM decreases early lesion development by increasing macrophage apoptosis.

Activation of the nuclear receptor FXR induces fibrinogen expression: a new role for bile acid signaling

Three genes, fibrinogen-alpha (FBGalpha), -beta, and -gamma, encode proteins that make up the mature FBG protein complex. This complex is secreted from the liver and plays a key role in coagulation in response to vascular disruption. We identified all three FBG genes in a screen designed to isolate genes that are regulated by the farnesoid X receptor (FXR; NR1H4). Treatment of human hepatoma cells with either naturally occurring or synthetic [3-(2,6-dichlorophenyl)-4-(3'-carboxy-2-chloro-stilben-4-yl)-oxymethyl-5-isopropyl-isoxazole] FXR ligands resulted in the induction of transcripts for all three genes. The induction of FBGbeta mRNA in response to activated FXR appears to be a primary transcriptional response, as it is blocked by actinomycin D but not by cycloheximide. Four FXR isoforms were recently identified that differ either at their N termini and/or by the presence of four amino acids in the hinge region. Interestingly, the activities of the human FBGbeta promoter-reporter constructs were highly induced by FXR isoforms that lack the four amino acid insert. The observation that all three FBG subunits are induced by specific FXR isoforms, in response to FXR ligands, suggests that bile acids and FXR modulate fibrinolytic activity.

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.