Pleiotropic Roles of Bile Acids in Metabolism de Aguiar Vallim, Thomas Q.; Tarling, Elizabeth J.; Edwards, Peter A.
Cell metabolism,
05/2013, Letnik:
17, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Enzymatic oxidation of cholesterol generates numerous distinct bile acids that function both as detergents that facilitate digestion and absorption of dietary lipids, and as hormones that activate ...four distinct receptors. Activation of these receptors alters gene expression in multiple tissues, leading to changes not only in bile acid metabolism but also in glucose homeostasis, lipid and lipoprotein metabolism, energy expenditure, intestinal motility and bacterial growth, inflammation, liver regeneration, and hepatocarcinogenesis. This review covers the roles of specific bile acids, synthetic agonists, and their cognate receptors in controlling these diverse functions, as well as their current use in treating human diseases.
Four members of the mammalian ATP binding cassette (ABC) transporter G subfamily are thought to be involved in transmembrane (TM) transport of sterols. The residues responsible for this transport are ...unknown. The mechanism of action of ABCG1 is controversial and it has been proposed to act at the plasma membrane to facilitate the efflux of cellular sterols to exogenous high-density lipoprotein (HDL). Here we show that ABCG1 function is dependent on localization to intracellular endosomes. Importantly, localization to the endosome pathway distinguishes ABCG1 and/or ABCG4 from all other mammalian members of this superfamily, including other sterol transporters. We have identified critical residues within the TM domains of ABCG1 that are both essential for sterol transport and conserved in some other members of the ABCG subfamily and/or the insulin-induced gene 2 (INSIG-2). Our conclusions are based on studies in which (i) biotinylation of peritoneal macrophages showed that endogenous ABCG1 is intracellular and undetectable at the cell surface, (ii) a chimeric protein containing the TM of ABCG1 and the cytoplasmic domains of the nonsterol transporter ABCG2 is both targeted to endosomes and functional, and (iii) ABCG1 colocalizes with multiple proteins that mark late endosomes and recycling endosomes. Mutagenesis studies identify critical residues in the TM domains that are important for ABCG1 to alter sterol efflux, induce sterol regulatory element binding protein-2 (SREBP-2) processing, and selectively attenuate the oxysterol-mediated repression of SREBP-2 processing. Our data demonstrate that ABCG1 is an intracellular sterol transporter that localizes to endocytic vesicles to facilitate the redistribution of specific intracellular sterols away from the endoplasmic reticulum (ER).
Cellular lipid requirements are achieved through a combination of biosynthesis and import programs. Using isotope tracer analysis, we show that type I interferon (IFN) signaling shifts the balance of ...these programs by decreasing synthesis and increasing import of cholesterol and long chain fatty acids. Genetically enforcing this metabolic shift in macrophages is sufficient to render mice resistant to viral challenge, demonstrating the importance of reprogramming the balance of these two metabolic pathways in vivo. Unexpectedly, mechanistic studies reveal that limiting flux through the cholesterol biosynthetic pathway spontaneously engages a type I IFN response in a STING-dependent manner. The upregulation of type I IFNs was traced to a decrease in the pool size of synthesized cholesterol and could be inhibited by replenishing cells with free cholesterol. Taken together, these studies delineate a metabolic-inflammatory circuit that links perturbations in cholesterol biosynthesis with activation of innate immunity.
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•Identification of a cholesterol metabolism-type I interferon (IFN) inflammatory circuit•Type I interferon reprograms cholesterol homeostasis•Perturbing cholesterol synthesis engages type I IFN signaling•STING/TBK1 links cholesterol metabolism with type I interferon pathway
Cholesterol metabolism and type I interferon response are co-regulated in macrophages, creating an immuno-metabolic circuit that allows innate immune cells to coordinate metabolism changes with immune activation required for antiviral responses.
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.
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•Induction of Lpcat3 expression by LXRs promotes phospholipid remodeling•LXR-Lpcat3 activation drives unsaturated fatty acid incorporation into phospholipids•Lpcat3 activity in liver modulates lipid-induced ER stress and inflammation•Lpcat3 affects inflammation through regulation of membrane c-Src activity
FXR agonists are used to treat non-alcoholic fatty liver disease (NAFLD), in part because they reduce hepatic lipids. Here, we show that FXR activation with the FXR agonist GSK2324 controls hepatic ...lipids via reduced absorption and selective decreases in fatty acid synthesis. Using comprehensive lipidomic analyses, we show that FXR activation in mice or humans specifically reduces hepatic levels of mono- and polyunsaturated fatty acids (MUFA and PUFA). Decreases in MUFA are due to FXR-dependent repression of Scd1, Dgat2, and Lpin1 expression, which is independent of SHP and SREBP1c. FXR-dependent decreases in PUFAs are mediated by decreases in lipid absorption. Replenishing bile acids in the diet prevented decreased lipid absorption in GSK2324-treated mice, suggesting that FXR reduces absorption via decreased bile acids. We used tissue-specific FXR KO mice to show that hepatic FXR controls lipogenic genes, whereas intestinal FXR controls lipid absorption. Together, our studies establish two distinct pathways by which FXR regulates hepatic lipids.
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•Non-steroidal agonists of FXR significantly decrease intestinal lipid absorption•FXR decreases hepatic triglycerides independently of SHP and SREBP1C•FXR activation reduces expression of three key lipogenic genes, Scd1, Lpin1, and Dgat2•Intestinal and hepatic FXR are both required to decrease hepatic triglycerides
The nuclear receptor FXR lowers hepatic triglycerides to protect against the onset of NAFLD. Clifford et al. demonstrate that activation of FXR decreases hepatic triglycerides through two distinct mechanisms. First, via bile-acid-dependent decreases in intestinal lipid absorption and second, through selective changes in lipogenesis.
Highlights ► ATP binding cassette (ABC) transporters utilize the energy derived from ATP hydrolysis to transport substrates across membrane bilayers. ► There are 48 human ABC transporters and ...approximately half are thought to be involved in the transport of lipids and lipid-related compounds. ► ABC transporters are found in almost every type of intracellular organelle, and individual transporters have multiple functions depending on their cellular context.
The liver X receptors (LXRs) are transcriptional regulators of lipid homeostasis that also have potent anti-inflammatory effects. The molecular basis for their anti-inflammatory effects is ...incompletely understood, but has been proposed to involve the indirect tethering of LXRs to inflammatory gene promoters. Here we demonstrate that the ability of LXRs to repress inflammatory gene expression in cells and mice derives primarily from their ability to regulate lipid metabolism through transcriptional activation and can occur in the absence of SUMOylation. Moreover, we identify the putative lipid transporter Abca1 as a critical mediator of LXR's anti-inflammatory effects. Activation of LXR inhibits signaling from TLRs 2, 4 and 9 to their downstream NF-κB and MAPK effectors through Abca1-dependent changes in membrane lipid organization that disrupt the recruitment of MyD88 and TRAF6. These data suggest that a common mechanism-direct transcriptional activation-underlies the dual biological functions of LXRs in metabolism and inflammation.
Endothelial cells transduce mechanical forces from blood flow into intracellular signals required for vascular homeostasis. Here we show that endothelial NOTCH1 is responsive to shear stress, and is ...necessary for the maintenance of junctional integrity, cell elongation, and suppression of proliferation, phenotypes induced by laminar shear stress. NOTCH1 receptor localizes downstream of flow and canonical NOTCH signaling scales with the magnitude of fluid shear stress. Reduction of NOTCH1 destabilizes cellular junctions and triggers endothelial proliferation. NOTCH1 suppression results in changes in expression of genes involved in the regulation of intracellular calcium and proliferation, and preventing the increase of calcium signaling rescues the cell-cell junctional defects. Furthermore, loss of Notch1 in adult endothelium increases hypercholesterolemia-induced atherosclerosis in the descending aorta. We propose that NOTCH1 is atheroprotective and acts as a mechanosensor in adult arteries, where it integrates responses to laminar shear stress and regulates junctional integrity through modulation of calcium signaling.
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.
Autoantibodies to multiple cytokines have been identified and some, including antibodies against granulocyte-macrophage colony-stimulating factor (GM-CSF), have been associated with increased ...susceptibility to infection. High levels of GM-CSF autoantibodies that neutralize signaling cause autoimmune pulmonary alveolar proteinosis (aPAP), an ultrarare autoimmune disease characterized by accumulation of excess surfactant in the alveoli, leading to pulmonary insufficiency. Defective GM-CSF signaling leads to functional deficits in multiple cell types, including macrophages and neutrophils, with impaired phagocytosis and host immune responses against pulmonary and systemic infections. In this article, we review the role of GM-CSF in aPAP pathogenesis and pulmonary homeostasis along with the increased incidence of infections (particularly opportunistic infections). Therefore, recombinant human GM-CSF products may have potential for treatment of aPAP and possibly other infectious and pulmonary diseases due to its pleotropic immunomodulatory actions.
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