Loss of carnitine palmitoyltransferase 1a reduces docosahexaenoic acid-containing phospholipids and drives sexually dimorphic liver disease in mice

BACKGROUND AND AIMS

Genome and epigenome wide association studies identified variants in carnitine palmitoyltransferase 1a (CPT1a) that associate with lipid traits. The goal of this study was to determine the role of liver-specific CPT1a on hepatic lipid metabolism.

APPROACH AND RESULTS

Male and female liver-specific knockout (LKO) and littermate controls were placed on a low-fat or high-fat diet (60% kcal fat) for 15 weeks. Mice were necropsied after a 16 h fast, and tissues were collected for lipidomics, matrix-assisted laser desorption ionization mass spectrometry imaging, kinome analysis, RNA-sequencing, and protein expression by immunoblotting. Female LKO mice had increased serum alanine aminotransferase levels which were associated with greater deposition of hepatic lipids, while male mice were not affected by CPT1a deletion relative to male control mice. Mice with CPT1a deletion had reductions in DHA-containing phospholipids at the expense of monounsaturated fatty acids (MUFA)-containing phospholipids in whole liver and at the level of the lipid droplet (LD). Male and female LKO mice increased RNA levels of genes involved in LD lipolysis (Plin2, Cidec, G0S2) and in polyunsaturated fatty acid metabolism (Elovl5, Fads1, Elovl2), while only female LKO mice increased genes involved in inflammation (Ly6d, Mmp12, Cxcl2). Kinase profiling showed decreased protein kinase A activity, which coincided with increased PLIN2, PLIN5, and G0S2 protein levels and decreased triglyceride hydrolysis in LKO mice.

CONCLUSIONS

Liver-specific deletion of CPT1a promotes sexually dimorphic steatotic liver disease (SLD) in mice, and here we have identified new mechanisms by which females are protected from HFD-induced liver injury.

Loss of Carnitine Palmitoyltransferase 1a Reduces Docosahexaenoic Acid-Containing Phospholipids and Drives Sexually Dimorphic Liver Disease in Mice

Background and Aims

Genome and epigenome wide association studies identified variants in carnitine palmitoyltransferase 1a (CPT1a) that associate with lipid traits. The goal of this study was to determine the impact by which liver-specific CPT1a deletion impacts hepatic lipid metabolism.

Approach and Results

Six-to-eight-week old male and female liver-specific knockout (LKO) and littermate controls were placed on a low-fat or high-fat diet (HFD; 60% kcal fat) for 15 weeks. Mice were necropsied after a 16 hour fast, and tissues were collected for lipidomics, matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI), kinome analysis, RNA-sequencing, and protein expression by immunoblotting. Female LKO mice had increased serum alanine aminotransferase (ALT) levels which were associated with greater deposition of hepatic lipids, while male mice were not affected by CPT1a deletion relative to male control mice. Mice with CPT1a deletion had reductions in DHA-containing phospholipids at the expense of monounsaturated fatty acids (MUFA)-containing phospholipids in both whole liver and at the level of the lipid droplet (LD). Male and female LKO mice increased RNA levels of genes involved in LD lipolysis ( Plin2 , Cidec , G0S2 ) and in polyunsaturated fatty acid (PUFA) metabolism ( Elovl5, Fads1, Elovl2 ), while only female LKO mice increased genes involved in inflammation ( Ly6d, Mmp12, Cxcl2 ). Kinase profiling showed decreased protein kinase A (PKA) activity, which coincided with increased PLIN2, PLIN5, and G0S2 protein levels and decreased triglyceride hydrolysis in LKO mice.

Conclusions

Liver-specific deletion of CPT1a promotes sexually dimorphic steatotic liver disease (SLD) in mice, and here we have identified new mechanisms by which females are protected from HFD-induced liver injury.

Graphical Summary

Enhancement of High-Density Lipoprotein-Associated Protease Inhibitor Activity Prevents Atherosclerosis Progression

Background

Inflammatory cells within atherosclerotic lesions secrete various proteolytic enzymes that contribute to lesion progression and destabilization, increasing the risk for an acute cardiovascular event. The relative contributions of specific proteases to atherogenesis is not well understood. Elastase is a serine protease, secreted by macrophages and neutrophils, that may contribute to the development of unstable plaque. We have previously reported interaction of endogenous protease-inhibitor proteins with high-density lipoprotein (HDL), including alpha-1-antitrypsin, an inhibitor of elastase. These findings support a potential role for HDL as an endogenous modulator of protease activity. In this study, we test the hypothesis that enhancement of HDL-associated elastase inhibitor activity is protective against atherosclerotic lesion progression.

Methods

We designed an HDL-targeting protease inhibitor (HTPI) that binds to HDL and confers elastase inhibitor activity. Lipoprotein binding and the impact of HTPI on atherosclerosis was examined using mouse models.

Results

HTPI is a small (1.6 kDa) peptide with an elastase inhibitor domain, a soluble linker, and an HDL-targeting domain. When incubated with human plasma ex vivo , HTPI predominantly binds to HDL. Intravenous administration of HTPI to mice resulted in its binding to plasma HDL and increased elastase inhibitor activity on isolated HDL. Accumulation of HTPI within plaque was observed after systemic administration to Apoe -/- mice. To examine the effect of HTPI treatment on atherosclerosis, prevention and progression studies were performed using Ldlr -/- mice fed Western diet. In both study designs, HTPI-treated mice had reduced lipid deposition in plaque. Histology and immunofluorescence staining of aortic root sections were used to examine the impact of HTPI on lesion morphology and inflammatory features.

Conclusions

These data support the hypothesis that HDL-associated anti-elastase activity can improve the atheroprotective potential of HDL and highlight the potential utility of HDL enrichment with anti-protease activity as an approach for stabilization of atherosclerotic lesions.

Functional Exploration of Conserved Sequences in the Distal Face of Angiotensinogen-Brief Report

BACKGROUND

Angiotensinogen (AGT) is an essential component in the renin-angiotensin system. AGT has highly conserved sequences in the loop and β-sheet regions among species; however, their functions have not been studied.

METHODS

Adeno-associated viral vector (AAV) serotype 2/8 encoding mouse AGT with mutations of conserved sequences in the loop (AAV.loop-Mut), β-sheet (AAV.βsheet-Mut), or both regions (AAV.loop/βsheet-Mut) was injected into male hepatocyte-specific AGT-deficient (hepAGT-/-) mice in an LDL (low-density lipoprotein) receptor-deficient background. AAV containing mouse wild-type AGT (AAV.mAGT) or a null vector (AAV.null) were used as controls. Two weeks after AAV administration, all mice were fed a western diet for 12 weeks. To determine how AGT secretion is regulated in hepatocytes, AAVs containing the above mutations were transducted into HepG2 cells.

RESULTS

In hepAGT-/- mice infected with AAV.loop-Mut or βsheet-Mut, plasma AGT concentrations, systolic blood pressure, and atherosclerosis were comparable to those in AAV.mAGT-infected mice. Interestingly, plasma AGT concentrations, systolic blood pressure, and atherosclerotic lesion size in hepAGT-/- mice infected with AAV.loop/βsheet-Mut were not different from mice infected with AAV.null. In contrast, hepatic Agt mRNA abundance was elevated to a comparable magnitude as AAV.mAGT-infected mice. Immunostaining showed that AGT protein was accumulated in hepatocytes of mice infected with AAV.loop/βsheet-Mut or HepG2 cells transducted with AAV.loop/βsheet-Mut. Accumulated AGT was not located in the endoplasmic reticulum.

CONCLUSIONS

The conserved sequences in either the loop or β-sheet region individually have no effect on AGT regulation, but the conserved sequences in both regions synergistically contribute to the secretion of AGT from hepatocytes.

Abstract 437: Carnitine Palmitoyltransferase 1a Modulates Lipoprotein Metabolism

Background: Nonalcoholic fatty liver disease (NAFLD) affects almost 1 billion people worldwide and is associated with risk factors such as obesity and dyslipidemia. Studies show linked variants and methylation status of carnitine palmitoytransferase 1a (CPT1a) to disrupt very low-density lipoprotein (VLDL) cholesterol and triglyceride (TG) levels. We exhibit liver-specific deletion of CPT1a in mice lowers plasma cholesterol and TG while exacerbating NAFLD and inflammation.

Methods: Eight-week old Cpt1a floxed mice with the human apoB100 transgene (Cpt1a fl/fl /B100 Tg ) were administered control adenoassociated virus (AAV) or AAV encoding Cre-recombinase under control of a liver specific promoter (TBG-Cre). Control and Cpt1a liver-specific knock out (LKO) mice were placed on low-fat control or western-type diet (WTD; 42% kcal fat, 0.2% cholesterol) for 16 weeks. Body weights were recorded weekly and body composition by MRI was performed at the study midpoint and end. Tissues and plasma were collected and analyzed for lipid composition and gene and protein expression by QPCR and immunoblotting, respectively.

Results: CPT1a LKO mice had lower plasma cholesterol and TG irrespective of diet. The reduction in plasma cholesterol was limited to the LDL pool in FPLC-fractionated plasma. Hepatic TG was elevated in mice fed WTD and LKO Cpt1a mice. Loss of hepatic Cpt1a had no effect on hepatic cholesterol in male mice, but increased total and free cholesterol by 2- and 2.5-fold, respectively, in females. The rise in hepatic free cholesterol in female Cpt1a LKO mice associated with increases in Kupffer cell ( Clec4f ) and collagen ( Col1a1 ) gene expression. Free cholesterol levels were not related to differences in transcripts for enzymes involved in cholesterol synthesis or secretion ( Srebp2, Hmgcr, Hmgcs, Soat2, Mttp ), hepatic clearance ( Ldlr ), metabolism ( Cpy7a1, Cyp8b1 ), or biliary secretion ( Abcg5, Abcg8 ).

Conclusions: Liver-specific deletion of CPT1a reduces plasma LDL-cholesterol and increases cholesterol levels in female, but not male mice. The increase in hepatic free cholesterol did not alter expression of cholesterol-responsive genes, suggesting cholesterol accumulation outside of the regulatory pool.

Abstract 527: Loss Of Hepatic Insulin Signaling Promotes Apolipoprotein Ai Retention In Liver And Alters HDL In Plasma In Mice

Background: Insulin signaling regulates a variety of features of hepatic lipid and lipoprotein metabolism. Liver-specific deletion of insulin receptor (LIRKO) reduces plasma HDL, an effect attributed to a reduction in deiodinase 2 expression and apolipoprotein (Apo) AI mRNA expression. We report a concomitant accumulation of ApoAI protein in the liver of LIRKO mice in a yet to be fully characterized compartment within hepatocytes.

Methods: Mice harboring loxP sites within the insulin receptor gene (IR fl/fl ) were administered a control adenoassociated viral vector (AAV-Empty) or an AAV encoding Cre recombinase under the control of a liver-specific promoter (AAV_TBG-Cre) at 8 weeks of age to generate Control and LIRKO mice. Male and female mice were evaluated for the abundance of ApoAI in liver and total and FPLC-fractionated plasma. ApoAI secretion rates were evaluated in primary hepatocytes from both genotypes. Subcellular localization of ApoAI in liver of Control and LIRKO mice was evaluated by immunofluorescence microscopy and biochemical fractionation.

Results: The loss of hepatic insulin receptor and signaling was confirmed by rtPCR, immunoblot analysis and glucose tolerance test. Whereas ApoAI was unaffected in LIRKO mice compared to controls, protein levels were reduced in plasma and elevated in liver. In contrast, no changes were observed for ApoB or ApoE in either liver or plasma. FPLC fractionation of plasma indicates a shift in HDL size and an accumulation of ApoM in the HDL fractions. Secretion rates of newly synthesized ApoAI were modestly reduced in primary hepatocytes isolated from LIRKO mice. Immunolocalization of ApoAI demonstrated accumulation in large, membrane-bound organelles located peripherally in cells. These organelles stained positively for LampI and other markers of the endosomal-lysosomal system. However, markers of the endosomal-lysosomal compartment in perinuclear regions of the cells were negative for ApoAI.

Conclusions: Insulin signaling is a regulator of ApoAI secretion that alters the intracellular itinerary of ApoAI in the secretory and endocytic pathways and alters the plasma HDL proteome in mice.

Abstract 439: Abcg5 Abcg8 Are Essential To Maintain Sterol Balance When Challenged With Dietary Cholesterol In Mice

Background: ABCG5/ABCG8 (G5/G8) is the primary biliary cholesterol transporter and accounts for >70% of biliary cholesterol secretion. Mice and humans maintain fecal cholesterol excretion under a variety of conditions that disrupt biliary cholesterol secretion, indicating the presence of a non-biliary route for cholesterol elimination. Cholesterol can be actively secreted from the intestine, a pathway termed transintestinal cholesterol excretion (TICE). The present study investigates rates of intestinal cholesterol secretion in mice lacking G5/G8 to determine if adaptive responses within the gut facilitate cholesterol excretion in the absence of the primary biliary cholesterol transporter.

Methods: Wild-type (WT) and whole body Abcg5/Abcg8 deficient mice (G5/G8 KO) were maintained on a plant sterol free (PSF) diet to prevent the development of experimental sitosterolemia and potential confounding effects of biologically active phytosterols. Mice were anesthetized, the gall bladder cannulated, and the intestine perfused with model bile to simultaneously determine biliary and intestinal cholesterol secretion rates. Sterol balance studies were conducted in mice sequentially maintained on PSF diet, a PSF diet supplemented with 0.2% cholesterol, and standard rodent chow. Mice were allowed to acclimate to each diet for 14 days, then placed on wire-bottom cages for fecal collection. At termination of the experiment, basal bile, plasma, and tissues were collected. Fecal neutral sterols (FNS) were analyzed by GC-MS.

Results: Despite a substantial reduction in biliary cholesterol secretion in G5/G8 KO mice, differences in intestinal cholesterol secretion rates were only observed in males, with KO mice excreting less cholesterol into the intestinal lumen. FNS did not differ between genotypes when maintained on the PSF diet. On chow diet, FNS increased 2x in both genotypes, but did not differ across genotypes. When PSF diet was supplemented with 0.2% cholesterol, FNS increased by 3x in WT mice, but were unchanged in G5/G8 KO mice, relative to PSF diet levels.

Conclusions: Abcg5/Abcg8 is essential to maintain sterol balance when cholesterol is present in the diet. TICE is unable to compensate for Abcg5/Abcg8 deficiency in cholesterol fed mice.

Abstract 438: The CETP/ApoB100 Transgenic Mouse As A Model For Metabolic Syndrome

Background: Metabolic syndrome (MetS) is a cluster of risk factors that predisposes individuals to develop atherosclerotic cardiovascular disease, type 2 diabetes mellitus, and nonalcoholic fatty liver disease. Dyslipidemia is an essential element of MetS. Mice lack cholesterol ester transfer protein (CETP) and rapidly clear ApoB-containing lipoprotein and are resistant to the development of dyslipidemia and clinically relevant metabolic phenotypes when challenged with high fat, high cholesterol Western-type diet (WTD). We hypothesized that the presence of CETP and ApoB-containing lipoproteins would sensitize mice to the development of dyslipidemia and exacerbate metabolic phenotypes.

Approach and Results: Male and female (n=7-11) C57Bl6/J mice harboring a human CETP transgene, a human ApoB100 transgene, or both transgenes were placed on a WTD (42% kCal fat, 0.2% cholesterol). Adiposity, insulin sensitivity, glucose tolerance and plasma cholesterol were assessed at the midpoint and termination of the study. Mice were anesthetized, basal bile was collected (30 min), euthanized via cardiac puncture, and tissues dissected and snap frozen. One lobe of the liver was additionally fixed for histological analysis. In both females and males, CETP/ApoB100 and ApoB100 transgenic mice showed a robust increase in total plasma cholesterol and LDL-C. The presence of CETP further increased LDL-C in ApoB100 expressing mice and modestly reduced HDL-C. Differences in body weight were not observed across genotypes. However, ApoB100 expression reduced adiposity in females, but not males. Conversely, the presence of either ApoB100 or CETP impaired insulin sensitivity in male, but not female mice. Liver histology showed increased hepatocyte ballooning in ApoB100 and CETP/ApoB100 transgenic mice, but other measures of NAFLD were unaffected.

Conclusions: Mice expressing CETP/ApoB100 transgenes develop dyslipidemia more consistent with human lipoprotein profiles. The humanized lipoprotein profile reveals sexually dimorphic responses to WTD.

Abstract 452: Classification And Effect Of Correctors On ABCG8 Sitosterolemia Associated Cytosolic Mutants

Background: Sitosterolemia is a rare form of Familial Hypercholesterolemia (FH) which is unique from other forms of FH due to the accumulation of phytosterols in the plasma and tissues. Sitosterolemia is autosomal recessive and caused by mutations in either the ABCG5 or ABCG8 gene. ABCG5 and ABCG8 form a heterodimer (ABCG5/ABCG8) that functions at the apical surface of hepatocytes and enterocytes to promote cholesterol and phytosterol excretion. Small molecule modulators classified as either correctors or potentiators have therapeutic benefit when used to treat mutants of ABCB4 (PFIC3) or ABCC7 (Cystic Fibrous) that have impaired folding, stability, or activity. There are over 40 missense mutations in ABCG5/ABCG8 which have been clinically confirmed in patients with Sitosterolemia.

Methods: We developed a classification system for ABCG5 and ABCG8 mutations based on the underlying molecular defect for ABCG5/ABCG8 dysfunction (maturation, trafficking, activity, etc.). We used site directed mutagenesis to introduce ABCG8 missense mutations located in the N-terminal, cytosolic domain of human ABCG8. Normal and mutant ABCG5/ABCG8 constructs were transiently transfected into a human hepatocyte cell line, Huh-7, and analyzed for heterodimerization, trafficking beyond the ER, and localization at the cell surface.

Results: Of the ABCG8 cytosolic mutants studied, R184H, L196Q, L228P, and R263Q failed to traffic beyond the ER to form a stable heterodimer (class II) while P231T, T400K, N409D, N409I, and P415H were trafficking competent.

Conclusions: Forty four percent (4 of 9) of cytosolic, ABCG8 mutations that cause Sitosterolemia are due to failures in complex formation and trafficking beyond the ER. Fifty six (5 of 9) percent were trafficking competent, indicating a stable ABCG5/G8 complex, but loss of activity due to impaired ATP hydrolysis or substrate binding. ABCG8 mutants may be potentially restored by proteostatic regulators and/or potentiators & correctors that have shown to be effective in disease-causing mutations in other ABC transporters.

Abstract 526: Molecular Characterization Of Sitosterolemia-associated Mutants Of ABCG5

Background: The proteostasis network is essential for the maintenance of cellular integrity by ensuring the correct folding of newly synthesized proteins. However, the high fidelity of this system contributes to diseases such as Cystic Fibrosis (CF, ABCC7), Progressive Familial Intrahepatic Cholestasis Type 3 (PFIC3, ABCB4), and Sitosterolemia (ABCG5/ABCG8). Interference with proteostasis and allowing sub-optimally folded ATP-Binding Cassette (ABC) transporters to transit to the cell surface partially restores function, with clinical benefit for CF and PFIC3. We sought to delineate the molecular underpinnings of Sitosterolemia, an autosomal recessive form of Familial Hypercholesterolemia (FH), characterized by accumulation of phytosterols in the plasma and tissues. ABCG5 ABCG8 form an obligate heterodimer at the surface of the liver and small intestine, and mediates sterol transport into bile and the intestinal lumen. Case studies reveal 57 loss of function and over 40 missense mutations of ABCG5 associated with Sitosterolemia. Recent pharmaceutics show promise with rescue of mutations in CF and PFIC3 with small molecule chaperones and potentiators. Through our analysis of ABCG5 missense mutations causing Sitosterolemia, we anticipate rescue by these compounds as well.

Methods: We established a classification system for missense mutations of ABCG5, similar to CF and PFIC3. Mutations were generated by site-directed mutagenesis, and confirmed through Sanger sequencing. Native and mutant ABCG5 were co-transfected with native ABCG8 into human hepatocytes and evaluated for protein abundance and trafficking beyond the endoplasmic reticulum (ER), by SDS-PAGE and immunoblot analysis.

Results: Nine mutants of ABCG5 have been generated. Co-expression of ABCG5 with ABCG8 i n vitro demonstrated I68N, A98G, E146Q, and R419P successfully traffic while R284S, T305R, R389H, R419H, and N437K were arrested within the proteostasis network.

Conclusions: Similar to CF and PFIC3, Sitosterolemia-associated mutations result in compromised heterodimer formation and trafficking to the apical surface. Small molecule chaperones and potentiators may partially rescue the mutants and provide clinical benefit for Sitosterolemia.

Abstract 528: Characterization Of Sitosterolemia-causing Mutations In The Extracellular Domain Of Abcg8

Background: ABCG5 and ABCG8 form a heterodimer that promotes biliary secretion and opposes intestinal absorption of cholesterol and phytosterols. Whole-genome and exome sequencing has revealed over 2000 variants of ABCG5 and ABCG8, many of which are predicted to be pathogenic or likely pathogenic. Missense mutations in clinically confirmed cases of Sitosterolemia are of particular interest for their potential to reveal structure-function relationships in ABCG5 ABCG8 and other ABC transporter family members. Mutations in ABCG5 ABCG8 with clinically confirmed Sitosterolemia generally cluster within the ATP binding cassette and transmembrane spanning segments that form the substrate-binding domain. However, four disease-causing mutations occur in the short extracellular loop in ABCG8, suggesting that these mutations may reveal novel elements of the ABCG5 ABCG8 function.

Methods: Mutants of ABCG8 were generated by site-directed mutagenesis. Plasmids encoding normal ABCG5 and normal or mutant ABCG8s were co-transfected into human Huh7 hepatocytes, lysates prepared, and analyzed by SDS-PAGE and immunoblot analysis. Maturation and trafficking of the ABCG5 ABCG8 heterodimer beyond the endoplasmic reticulum was assessed by the appearance of the higher molecular weight, mature form of each glycoprotein.

Results: Mutants G574R, G575D, and L596R failed to support trafficking of ABCG5 ABCG8 and were designated as Class II mutants causative in Sitosterolemia. Only G574E supported maturation of the ABCG5 ABCG8 transporter in cultured human hepatocytes and remains unclassified.

Conclusion: Formation and trafficking of the ABCG5 ABCG8 heterodimer is dependent on key residues within the third extracellular loop in ABCG8. Maturation of ABCG5 ABCG8 is tolerant of substitutions in glycine 574, including the addition of a larger and negatively charged residue. However, neither substitution forms a functional ABCG5 ABCG8 transporter.

Age and sex are associated with the plasma lipidome: findings from the GOLDN study

Background

Developing an understanding of the biochemistry of aging in both sexes is critical for managing disease throughout the lifespan. Lipidomic associations with age and sex have been reported, but prior studies are limited by measurements in serum rather than plasma or by participants taking lipid-lowering medications.

Methods

Our study included lipidomic data from 980 participants aged 18–87 years old from the Genetics of Lipid-Lowering Drugs and Diet Network (GOLDN). Participants were off lipid-lowering medications for at least 4 weeks, and signal intensities of 413 known lipid species were measured in plasma. We examined linear age and sex associations with signal intensity of (a) 413 lipid species; (b) 6 lipid classes (glycerolipids, glycerophospholipids, sphingolipids, sterol lipids, fatty acids, and acylcarnitines); and (c) 15 lipid subclasses; as well as with the particle sizes of three lipoproteins.

Results

Significant age associations were identified in 4 classes, 11 subclasses, 147 species, and particle size of one lipoprotein while significant sex differences were identified in 5 classes, 12 subclasses, 248 species, and particle sizes of two lipoproteins. For many lipid species (n = 97), age-related associations were significantly different between males and females. Age*sex interaction effects were most prevalent among phosphatidylcholines, sphingomyelins, and triglycerides.

Conclusion

We identified several lipid species, subclasses, and classes that differ by age and sex; these lipid phenotypes may serve as useful biomarkers for lipid changes and associated cardiovascular risk with aging in the future. Future studies of age-related changes throughout the adult lifespan of both sexes are warranted.

Trial registration

ClinicalTrials.gov NCT00083369; May 21, 2004.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-021-01456-2.

Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8

Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8. ABCG5 ABCG8 encodes a pair of ABC half transporters that form a heterodimer (G5G8), which then traffics to the surface of hepatocytes and enterocytes and promotes the secretion of cholesterol and xenosterols into the bile and the intestinal lumen. We review the literature from the initial description of the disease, the discovery of its genetic basis, current therapy, and what has been learned from animal, cellular, and molecular investigations of the transporter in the twenty years since its discovery. The genomic era has revealed that there are far more carriers of loss of function mutations and likely pathogenic variants of ABCG5 ABCG8 than previously thought. The impact of these variants on G5G8 structure and activity are largely unknown. We propose a classification system for ABCG5 ABCG8 mutants based on previously published systems for diseases caused by defects in ABC transporters. This system establishes a framework for the comprehensive analysis of disease-associated variants and their impact on G5G8 structure-function.

The prorenin receptor and its soluble form contribute to lipid homeostasis

Obesity is associated with alterations in hepatic lipid metabolism. We previously identified the prorenin receptor (PRR) as a potential contributor to liver steatosis. Therefore, we aimed to determine the relative contribution of PRR and its soluble form, sPRR, to lipid homeostasis. PRR-floxed male mice were treated with an adeno-associated virus with thyroxine-binding globulin promoter-driven Cre to delete PRR in the liver [liver PRR knockout (KO) mice]. Hepatic PRR deletion did not change the body weight but increased liver weights. The deletion of PRR in the liver decreased peroxisome proliferator-activated receptor gamma (PPARγ) and triglyceride levels, but liver PRR KO mice exhibited higher plasma cholesterol levels and lower hepatic low-density lipoprotein receptor (LDLR) and Sortilin 1 (SORT1) proteins than control (CTL) mice. Surprisingly, hepatic PRR deletion elevated hepatic cholesterol, and up-regulated hepatic sterol regulatory element-binding protein 2 (SREBP2) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA-R) genes. In addition, the plasma levels of sPRR were significantly higher in liver PRR KO mice than in controls. In vitro studies in HepG2 cells demonstrated that sPRR treatment upregulated SREBP2, suggesting that sPRR could contribute to hepatic cholesterol biosynthesis. Interestingly, PRR, total cleaved and noncleaved sPRR contents, furin, and Site-1 protease (S1P) were elevated in the adipose tissue of liver PRR KO mice, suggesting that adipose tissue could contribute to the circulating pool of sPRR. Overall, this work supports previous works and opens a new area of investigation concerning the function of sPRR in lipid metabolism and adipose tissue-liver cross talk.NEW & NOTEWORTHY Hepatic PRR and its soluble form, sPRR, contribute to triglyceride and cholesterol homeostasis and hepatic inflammation. Deletion of hepatic PRR decreased triglyceride levels through a PRR-PPARγ-dependent mechanism but increased hepatic cholesterol synthesis through sPRR-medicated upregulation of SREBP-2. Our study highlighted a new paradigm of cross talk between the liver and the adipose tissue involving cholesterol and sPRR.

Stigmasterol stimulates transintestinal cholesterol excretion independent of liver X receptor activation in the small intestine

Despite advances in healthcare, cardiovascular disease (CVD) remains the leading cause of death in the United States. Elevated levels of plasma cholesterol are highly predictive of CVD and stroke and are the principal driver of atherosclerosis. Unfortunately, current cholesterol lowering agents, such as statins, are not known to reverse atherosclerotic disease once it has been established. In preclinical models, agonists of nuclear receptor, LXR, have been shown to reduce and reverse atherosclerosis. Phytosterols are bioactive non-cholesterol sterols that act as LXR agonists and regulate cholesterol metabolism and transport. We hypothesized that stigmasterol would act as an LXR agonist and alter intestinal cholesterol secretion to promote cholesterol elimination. Mice were fed a control diet, or a diet supplemented with stigmasterol (0.3% w/w) or T0901317 (0.015% w/w), a known LXR agonist. In this experiment we analyzed the sterol content of bile, intestinal perfusate, plasma, and feces. Additionally, the liver and small intestine were analyzed for relative levels of transcripts known to be regulated by LXR. We observed that T0901317 robustly promoted cholesterol elimination and acted as a strong LXR agonist. Stigmasterol promoted transintestinal cholesterol secretion through an LXR-independent pathway.

Simultaneous Determination of Biliary and Intestinal Cholesterol Secretion Reveals That CETP (Cholesteryl Ester Transfer Protein) Alters Elimination Route in Mice

OBJECTIVE

Determine the impact of CETP (cholesteryl ester transfer protein) on the route of cholesterol elimination in mice. Approach and Results: We adapted our protocol for biliary cholesterol secretion with published methods for measuring transintestinal cholesterol elimination. Bile was diverted and biliary lipid secretion maintained by infusion of bile acid. The proximal small bowel was perfused with bile acid micelles. In high-fat, high-cholesterol-fed mice, the presence of a CETP transgene increased biliary cholesterol secretion at the expense of transintestinal cholesterol elimination. The increase in biliary cholesterol secretion was not associated with increases in hepatic SR-BI (scavenger receptor BI) or ABCG5 (ATP-binding cassette G5) ABCG8. The decline in intestinal cholesterol secretion was associated with an increase in intestinal Niemann-Pick disease, type C1, gene-like 1 mRNA. Finally, we followed the delivery of HDL (high-density lipoprotein) or LDL (low-density lipoprotein) cholesteryl esters (CE) from plasma to bile and intestinal perfusates. HDL-CE favored the biliary pathway. Following high-fat feeding, the presence of CETP directed HDL-CE away from the bile and towards the intestine. The presence of CETP increased LDL-CE delivery to bile, whereas the appearance of LDL-CE in intestinal perfusate was near the lower limit of detection.

CONCLUSIONS

Biliary and intestinal cholesterol secretion can be simultaneously measured in mice and used as a model to examine factors that alter cholesterol elimination. Plasma factors, such as CETP, alter the route of cholesterol elimination from the body. Intestinal and biliary cholesterol secretion rates are independent of transhepatic or transintestinal delivery of HDL-CE, whereas LDL-CE was eliminated almost exclusively in the hepatobiliary pathway.

Bioinformatic analysis of endogenous and exogenous small RNAs on lipoproteins

To comprehensively study extracellular small RNAs (sRNA) by sequencing (sRNA-seq), we developed a novel pipeline to overcome current limitations in analysis entitled, “Tools for Integrative Genome analysis of Extracellular sRNAs (TIGER)”. To demonstrate the power of this tool, sRNA-seq was performed on mouse lipoproteins, bile, urine and livers. A key advance for the TIGER pipeline is the ability to analyse both host and non-host sRNAs at genomic, parent RNA and individual fragment levels. TIGER was able to identify approximately 60% of sRNAs on lipoproteins and >85% of sRNAs in liver, bile and urine, a significant advance compared to existing software. Moreover, TIGER facilitated the comparison of lipoprotein sRNA signatures to disparate sample types at each level using hierarchical clustering, correlations, beta-dispersions, principal coordinate analysis and permutational multivariate analysis of variance. TIGER analysis was also used to quantify distinct features of exRNAs, including 5ʹ miRNA variants, 3ʹ miRNA non-templated additions and parent RNA positional coverage. Results suggest that the majority of sRNAs on lipoproteins are non-host sRNAs derived from bacterial sources in the microbiome and environment, specifically rRNA-derived sRNAs from Proteobacteria. Collectively, TIGER facilitated novel discoveries of lipoprotein and biofluid sRNAs and has tremendous applicability for the field of extracellular RNA.

ABCG5 and ABCG8: more than a defense against xenosterols

The elucidation of the molecular basis of the rare disease, sitosterolemia, has revolutionized our mechanistic understanding of how dietary sterols are excreted and how cholesterol is eliminated from the body. Two proteins, ABCG5 and ABCG8, encoded by the sitosterolemia locus, work as obligate dimers to pump sterols out of hepatocytes and enterocytes. ABCG5/ABCG8 are key in regulating whole-body sterol trafficking, by eliminating sterols via the biliary tree as well as the intestinal tract. Importantly, these transporters keep xenosterols from accumulating in the body. The sitosterolemia locus has been genetically associated with lipid levels and downstream atherosclerotic disease, as well as formation of gallstones and the risk of gallbladder cancer. While polymorphic variants raise or lower the risks of these phenotypes, loss of function of this locus leads to more dramatic phenotypes, such as premature atherosclerosis, platelet dysfunction, and thrombocytopenia, and, perhaps, increased endocrine disruption and liver dysfunction. Whether small amounts of xenosterol exposure over a lifetime cause pathology in normal humans with polymorphic variants at the sitosterolemia locus remains largely unexplored. The purpose of this review will be to summarize the current state of knowledge, but also highlight key conceptual and mechanistic issues that remain to be explored.

Abstract 597: Disruptions in Hepatic Insulin Signaling Reveal an Enterohepatic Signaling Pathway that Regulates the ABCG5 ABCG8 Sterol Transporter and Biliary Cholesterol Secretion

Insulin resistance is associated with increased risk for cholesterol gallstones as well as the development of diabetic dyslipidemia. HDL is the primary cholesterol carrier in the reverse cholesterol transport (RCT) pathway, the process by which cholesterol is delivered from peripheral organs to the liver for elimination in bile. Therefore, we hypothesized that insulin signaling regulates hepatobiliary cholesterol transport in the RCT pathway. To test the role of insulin signaling, we utilized mice harboring insulin receptor flanked by loxP sites (IR fl/fl ) in combination with adenoassociated viral vectors containing no transgene (empty) or Cre recombinase to generate control and liver insulin receptor knock out (LIRKO) mice, respectively. As with previous LIRKO models, our mice showed markedly reduced insulin receptor mRNA and protein in liver, but not skeletal muscle or adipose tissue, and impaired glucose tolerance. LIRKO mice had increased biliary cholesterol secretion as well as increased expression of the ABCG5 ABCG8 sterol transporter, the primary mediator of biliary cholesterol secretion. Levels of SR-BI, the primary HDL receptor, were unchanged as were rates of HDL clearance from plasma and selective delivery of HDL cholesterol to the liver. We also observed increased ileal fibroblast growth factor (FGF)15 mRNA. Wild-type mice treated with FGF19 exhibited increased ABCG5 ABCG8 protein expression in the liver. Immunofluorescence microscopy was used to examine the subcellular localization of ABCG5 in the liver following FGF19 administration. Under control conditions, G5 appeared in puncta, diffusely distributed within hepatocytes. Following treatment with FGF19, G5 signal intensity was substantially increased and juxtaposed to zonula occcludin-1 (ZO-1), a tight junction protein the delineates the canalicular channels within the liver. In conclusion, depletion of hepatic insulin receptors increases G5G8 abundance and promotes its localization to the canalicular surface. This effect is associated with an increase in ileal FGF15 expression which promotes its localization to the apical surface and drives biliary cholesterol secretion.

Genetic Variants in HSD17B3, SMAD3, and IPO11 Impact Circulating Lipids in Response to Fenofibrate in Individuals With Type 2 Diabetes

Individuals with type 2 diabetes (T2D) and dyslipidemia are at an increased risk of cardiovascular disease. Fibrates are a class of drugs prescribed to treat dyslipidemia, but variation in response has been observed. To evaluate common and rare genetic variants that impact lipid responses to fenofibrate in statin-treated patients with T2D, we examined lipid changes in response to fenofibrate therapy using a genomewide association study (GWAS). Associations were followed-up using gene expression studies in mice. Common variants in SMAD3 and IPO11 were marginally associated with lipid changes in black subjects (P < 5 × 10-6 ). Rare variant and gene expression changes were assessed using a false discovery rate approach. AKR7A3 and HSD17B13 were associated with lipid changes in white subjects (q < 0.2). Mice fed fenofibrate displayed reductions in Hsd17b13 gene expression (q < 0.1). Associations of variants in SMAD3, IPO11, and HSD17B13, with gene expression changes in mice indicate that transforming growth factor-beta (TGF-β) and NRF2 signaling pathways may influence fenofibrate effects on dyslipidemia in patients with T2D.

Abstract 390: The Absence of Abcg5 Abcg8 Reveals a Sexually Dimorphic Adaption to Impaired Biliary Cholesterol Secretion

Objective: The ABCG5 ABCG8 (G5G8) sterol transporter is the primary mechanism for biliary cholesterol secretion, but mice maintain fecal sterol excretion in its absence. The mechanism by which mice maintain sterol excretion in the absence of this pathway is not known. Transintestinal cholesterol excretion (TICE) is an alternative pathway to hepatobiliary secretion. We investigated the impact of G5G8 deficiency on TICE in the absence of Sitosterolemia.

Methods and Results: We compared both hepatobiliary and transintestinal cholesterol excretion rates in wild-type (WT) and G5G8 deficient mice of both sexes. WT and G5G8 were maintained on a plant-sterol free diet from the time of weaning to prevent the development of secondary phenotypes associated with Sitosterolemia. Biliary and intestinal cholesterol secretion rates were determined by biliary diversion with simultaneous perfusion of the proximal 10 cm of the small bowel. Among WT mice, biliary cholesterol secretion was greater in female mice compared to males. Conversely, male mice exhibited greater rates of TICE than females. As expected, WT mice had higher biliary cholesterol secretion rates than their G5G8 deficient littermates. However, the decline in biliary cholesterol secretion was far less in male mice compared to females in the absence of G5G8. In female mice, the absence of G5G8 resulted in a two-fold increase in TICE, whereas males were unaffected.

Conclusion: Female mice are more dependent upon the biliary pathway for cholesterol excretion, whereas males are more dependent upon TICE. G5G8 independent pathways are present for both biliary and intestinal cholesterol secretion. Female and male mice differ in their adaptation to G5G8 deficiency in order to maintain fecal sterol excretion.

Abstract 181: Insulin Receptor Signaling Regulates ApoA-I Secretion from Hepatocytes

The aim of the study was to assess the effect of insulin receptor (IR) signaling on hepatic apoA-I metabolism. The experimental approach was to compare hepatic apoA-I expression, secretion and cellular localization in IRfl/fl mice and LIRKO mice in which hepatic insulin receptors were specifically deleted by AAV delivered Cre-recombinase. Results showed that IR mRNA and protein levels were markedly reduced in the livers of LIRKO mice compared to control IRfl/fl mice. As expected, LIRKO mice exhibited decreased glucose tolerance and reduced hepatic insulin signaling. Knockdown of hepatic IR decreased plasma HDL cholesterol and apoA-I levels. Whereas apoA-I mRNA levels were similar in LIRKO and control hepatocytes, apoA-I protein levels were increased in both the liver and primary hepatocytes isolated from LIRKO mice. In contrast to apoA-I, apoE and apoB protein levels in the liver and in cultured hepatocytes, as well as in the plasma, were similar in LIRKO and control mice. ApoA-I accumulation in LIRKO hepatocytes was associated with a decreased rate of apoA-I secretion. Immunofluorescence staining demonstrated that apoA-I accumulated in LIRKO hepatocytes in membrane bound inclusions. These inclusions shared markers characteristic of early, late and recycling endosomes, and of lysosomes. We conclude that IR-mediated insulin signaling plays an important role in hepatic apoA-I secretion and consequent nascent HDL formation. Reduced apoA-I secretion from liver into the circulation may contribute to the lower HDL levels typically associated with insulin resistance.

Effect of peripheral circadian dysfunction on metabolic disease in response to a diabetogenic diet

BMAL1 is a core component of the transcription/translation machinery that regulates central and peripheral circadian rhythms that coordinate behavior and metabolism, respectively. Our objective was to determine the impact of BMAL1 in adipose alone or in combination with liver on metabolic phenotypes. Control, adipose-Bmal1 knockout (ABKO), and liver- and adipose-Bmal1 knockout (LABKO) female mice were placed in TSE System metabolic chambers for metabolic phenotyping. A second cohort of male mice was fed a control or diabetogenic diet, and body weight and composition, glucose tolerance, insulin sensitivity, and serum and hepatic lipids were measured. Both female ABKO and LABKO mice exhibited increased food consumption compared with control mice. ABKO mice also exhibited increased overall activity predominantly during the light phase compared with both control and LABKO mice and were protected from increased weight gain. When the male cohort was challenged with a diabetogenic diet, LABKO mice had increased body weight due to increased fat mass compared with control and ABKO mice. However, these mice did not present further impairments in glycemic control, adipose inflammation, or liver injury. LABKO mice had increased hepatic cholesterol and elevated expression of cholesterol synthesis and uptake genes. Our data indicate that deletion of this allele in adipose or in combination with liver alters feeding behavior and locomotor activity. However, obesity is exacerbated only with the combination of liver and adipose deletion.

GRP78 rescues the ABCG5 ABCG8 sterol transporter in db/db mice

OBJECTIVE

Mice lacking leptin (ob/ob) or its receptor (db/db) are obese, insulin resistant, and have reduced levels of biliary cholesterol due, in part, to reduced levels of hepatic G5G8. Chronic leptin replacement restores G5G8 abundance and increases biliary cholesterol concentrations, but the molecular mechanisms responsible for G5G8 regulation remain unclear. In the current study, we used a series of mouse models to address potential mechanisms for leptin-mediated regulation of G5G8.

METHODS AND RESULTS

We acutely replaced leptin in ob/ob mice and deleted hepatic leptin receptors in lean mice. Neither manipulation altered G5G8 abundance or biliary cholesterol. Similarly, hepatic vagotomy had no effect on G5G8. Alternatively, G5G8 may be decreased in ob/ob and db/db mice due to ER dysfunction, the site of G5G8 complex assembly. Overexpression of the ER chaperone GRP78 using an adenoviral vector restores ER function and reduces steatosis in ob/ob mice. Therefore, we determined if AdGRP78 could rescue G5G8 in db/db mice. As in ob/ob mice, AdGRP78 reduced expression of lipogenic genes and plasma triglycerides in the db/db strain. Both G5 and G8 protein levels increased as did total biliary cholesterol, but in the absence of changes in G5 or G8 mRNAs. The increase in G5G8 was associated with increases in a number of proteins, including the ER lectin chaperone, calnexin, a key regulator of G5G8 complex assembly.

CONCLUSIONS

Leptin signaling does not directly regulate G5G8 abundance. The loss of G5G8 in mice harboring defects in the leptin axis is likely associated with compromised ER function.

The combination of ezetimibe and ursodiol promotes fecal sterol excretion and reveals a G5G8-independent pathway for cholesterol elimination

Previous studies suggest an interdependent relationship between liver and intestine for cholesterol elimination from the body. We hypothesized that a combination of ursodiol (Urso) and ezetimibe (EZ) could increase biliary secretion and reduce cholesterol reabsorption, respectively, to promote cholesterol excretion. Treatment with Urso increased hepatic ABCG5 ABCG8 (G5G8) protein and both biliary and fecal sterols in a dose-dependent manner. To determine whether the drug combination (Urso-EZ) further increased cholesterol excretion, mice were treated with Urso alone or in combination with two doses of EZ. EZ produced an additive and dose-dependent increase in fecal neutral sterol (FNS) elimination in the presence of Urso. Finally, we sequentially treated wide-type and G5G8-deficient mice with Urso and Urso-EZ to determine the extent to which these effects were G5G8 dependent. Although biliary and FNS were invariably lower in G5G8 KO mice, the relative increase in FNS following treatment with Urso alone or the Urso-EZ combination was not affected by genotype. In conclusion, Urso increases G5G8, biliary cholesterol secretion, and FNS and acts additively with EZ to promote fecal sterol excretion. However, the stimulatory effect of these agents was not G5G8 dependent.

ABCD2 identifies a subclass of peroxisomes in mouse adipose tissue

ATP-binding cassette transporter D2 (D2) is an ABC half transporter that is thought to promote the transport of very long-chain fatty acyl-CoAs into peroxisomes. Both D2 and peroxisomes increase during adipogenesis. Although peroxisomes are essential to both catabolic and anabolic lipid metabolism, their function, and that of D2, in adipose tissues remain largely unknown. Here, we investigated the D2 localization and the proteome of D2-containing organelles, in adipose tissue. Centrifugation of mouse adipose homogenates generated a fraction enriched with D2, but deficient in peroxisome markers including catalase, PEX19, and ABCD3 (D3). Electron microscopic imaging of this fraction confirmed the presence of D2 protein on an organelle with a dense matrix and a diameter of ∼ 200 nm, the typical structure and size of a microperoxisome. D2 and PEX19 antibodies recognized distinct structures in mouse adipose. Immunoisolation of the D2-containing compartment confirmed the scarcity of PEX19 and proteomic profiling revealed the presence of proteins associated with peroxisome, endoplasmic reticulum (ER), and mitochondria. D2 is localized to a distinct class of peroxisomes that lack many peroxisome proteins, and may associate physically with mitochondria and the ER.

ABCD2 alters peroxisome proliferator-activated receptor α signaling in vitro, but does not impair responses to fenofibrate therapy in a mouse model of diet-induced obesity

Fenofibrate is a peroxisome proliferator-activated receptor (PPAR) α ligand that has been widely used as a lipid-lowering agent in the treatment of hypertriglyceridemia. ABCD2 (D2) is a peroxisomal long-chain acyl-CoA transporter that is highly induced by fenofibrate in the livers of mice. To determine whether D2 is a modifier of fibrate responses, wild-type and D2-deficient mice were treated with fenofibrate for 14 days. The absence of D2 altered expression of gene clusters associated with lipid metabolism, including PPARα signaling. Using 3T3-L1 adipocytes, which express high levels of D2, we confirmed that knockdown of D2 modified genomic responses to fibrate treatment. We next evaluated the impact of D2 on effects of fibrates in a mouse model of diet-induced obesity. Fenofibrate treatment opposed the development of obesity, hypertriglyceridemia, and insulin resistance. However, these effects were unaffected by D2 genotype. We concluded that D2 can modulate genomic responses to fibrates, but that these effects are not sufficiently robust to alter the effects of fibrates on diet-induced obesity phenotypes.

Acceleration of biliary cholesterol secretion restores glycemic control and alleviates hypertriglyceridemia in obese db/db mice

OBJECTIVE

Recent studies support a role for cholesterol in the development of obesity and nonalcoholic fatty liver disease. Mice lacking the ABCG5 ABCG8 (G5G8) sterol transporter have reduced biliary cholesterol secretion and are more susceptible to steatosis, hepatic insulin resistance, and loss of glycemic control when challenged with a high-fat diet. We hypothesized that accelerating G5G8-mediated biliary cholesterol secretion would correct these phenotypes in obese mice.

APPROACH AND RESULTS

Obese (db/db) male and their lean littermates were administered a cocktail of control adenovirus or adenoviral vectors encoding ABCG5 and ABCG8 (AdG5G8). Three days after viral administration, measures of lipid and glucose homeostasis were determined, and tissues were collected for biochemical analyses. AdG5G8 increased biliary cholesterol and fecal sterol elimination. Fasting glucose and triglycerides declined, and glucose tolerance improved in obese mice expressing G5G8 compared with mice receiving control adenovirus. These changes were associated with a reduction in phosphorylated eukaryotic initiation factor 2α and c-Jun N-terminal kinase in liver, suggesting alleviation of endoplasmic reticulum stress. Phosphorylated insulin receptor and protein kinase B were increased, indicating restored hepatic insulin signaling. However, there was no reduction in hepatic triglycerides after the 3-day treatment period.

CONCLUSIONS

Accelerating biliary cholesterol secretion restores glycemic control and reduces plasma triglycerides in obese db/db mice.

New developments in selective cholesteryl ester uptake

PURPOSE OF REVIEW

Selective lipid uptake (SLU) is known to be a major pathway of lipoprotein cholesterol metabolism in experimental animals and humans, but remains poorly understood. This review provides a brief overview of SLU mediated by the HDL receptor scavenger receptor B-type I (SR-BI), and highlights several surprising new findings related to the impact of SLU pathways in cholesterol homeostasis.

RECENT FINDINGS

Under certain conditions, SR-BI-mediated SLU contributes to reverse cholesterol transport (RCT) independently of ABCG5/G8-mediated biliary cholesterol secretion, implying a novel trafficking mechanism. Hepatic SR-BI expression and RCT are decreased in diabetic mice. Farnesoid X receptor (FXR) and the microRNAs miR-185, miR-96 and miR-223 are emerging therapeutic targets for increasing SR-BI expression. SR-BI-independent selective cholesteryl ester uptake is a newly characterized pathway in macrophage foam cells.

SUMMARY

New findings underscore the importance of SR-BI-mediated SLU in hepatic SLU and RCT, while indicating that further investigation is needed to define SLU pathways, including SR-BI-independent macrophage selective cholesteryl ester uptake. The intracellular trafficking of cholesterol in these pathways appears to be critical to their normal function and is a major subject of ongoing studies.

Mechanism of rapid elimination of lysophosphatidic acid and related lipids from the circulation of mice

Lysophosphatidic acid (LPA) is a bioactive lipid mediator. Concentrations of the major LPA species in mouse plasma decreased uniformly following administration of a potent selective inhibitor of the LPA-generating lysophospholipase D autotaxin, identifying an active mechanism for removal of LPA from the circulation. LPA, akylglycerol phosphate (AGP), sphingosine 1-phosphate (S1P), and a variety of structural mimetics of these lipids, including phosphatase-resistant phosphonate analogs of LPA, were rapidly eliminated (t1/2 < 30 s) from the circulation of mice following intravenous administration of a single bolus dose without significant metabolism in situ in the blood. These lipids accumulated in the liver. Elimination of intravenously administered LPA was blunted by ligation of the hepatic circulation, and ∼90% of LPA administered through the portal vein was accumulated by the isolated perfused mouse liver at first pass. At early times following intravenous administration, more LPA was associated with a nonparenchymal liver cell fraction than with hepatocytes. Primary cultures of nonparenchymal liver cells rapidly assimilated exogenously provided LPA. Our results identify hepatic uptake as an important determinant of the bioavailability of LPA and bioactive lysophospholipid mimetics and suggest a mechanism to explain changes in circulating LPA levels that have been associated with liver dysfunction in humans.

The ABCG5 ABCG8 sterol transporter opposes the development of fatty liver disease and loss of glycemic control independently of phytosterol accumulation

ABCG5 and ABCG8 form a complex (G5G8) that opposes the absorption of plant sterols but is also expressed in liver where it promotes the excretion of cholesterol into bile. Hepatic G5G8 is transcriptionally regulated by a number of factors implicated in the development of insulin resistance and nonalcoholic fatty liver disease. Therefore, we hypothesized that G5G8 may influence the development of diet-induced obesity phenotypes independently of its role in opposing phytosterol absorption. G5G8 knock-out (KO) mice and their wild type (WT) littermates were challenged with a plant sterol-free low fat or high fat (HF) diet. Weight gain and the rise in fasting glucose were accelerated in G5G8 KO mice following HF feeding. HF-fed G5G8 KO mice had increased liver weight, hepatic lipids, and plasma alanine aminotransferase compared with WT controls. Consistent with the development of nonalcoholic fatty liver disease, macrophage infiltration, the number of TUNEL-positive cells, and the expression of proinflammatory cytokines were also increased in G5G8 KO mice. Hepatic lipid accumulation was associated with increased peroxisome proliferator activated receptor γ, CD36, and fatty acid uptake. Phosphorylation of eukaryotic translation initiation factor 2α (eiF2α) and expression of activating transcription factor 4 and tribbles 3 were elevated in HF-fed G5G8 KO mice, a pathway that links the unfolded protein response to the development of insulin resistance through inhibition of protein kinase B (Akt) phosphorylation. Phosphorylation of Akt and insulin receptor was reduced, whereas serine phosphorylation of insulin receptor substrate 1 was elevated.

The absence of ABCD2 sensitizes mice to disruptions in lipid metabolism by dietary erucic acid

ABCD2 (D2) is a peroxisomal transporter that is highly abundant in adipose tissue and promotes the oxidation of long-chain MUFA. Erucic acid (EA, 22:1ω9) reduces very long chain saturated fatty acids in patients with X-linked adrenoleukodystrophy but promotes dyslipidemia and dilated cardiomyopathy in rats. To determine the role of D2 in the metabolism of EA, we challenged wild-type and D2 deficient mice (D2 KO) with an enriched EA diet. In D2 KO mice, dietary EA resulted in the rapid expansion of adipose tissue, adipocyte hypertrophy, hepatic steatosis, and the loss of glycemic control. However, D2 had no impact on the development of obesity phenotypes in two models of diet-induced obesity. Although there was a significant increase in EA in liver of D2 KO mice, it constituted less than 2% of all fatty acids. Metabolites of EA (20:1, 18:1, and 16:1) were elevated, particularly 18:1, which accounted for 50% of all fatty acids. These data indicate that the failure to metabolize EA in adipose results in hepatic metabolism of EA, disruption of the fatty acid profile, and the development of obesity and reveal an essential role for D2 in the protection from dietary EA.

Abstract 60: The ABCG5 ABCG8 Sterol Transporter Opposes Insulin Resistance and Fatty Liver Disease Independent of Phytosterol Accumulation

ABCG5 and ABCG8 form a complex (G5G8) that opposes the absorption of plant sterols, but is also expressed in liver where it promotes the excretion of cholesterol into bile. Hepatic G5G8 is transcriptionally regulated by a number of factors implicated in the development of insulin resistance and nonalcoholic fatty liver disease (NAFLD). Therefore, we hypothesized that G5G8 may influence the development of diet-induced obesity phenotypes independently of its role in opposing phytosterol absorption. G5G8 knockout (KO) mice and their wild type (WT) littermates were challenged with plant sterol free low fat (LF) or high fat (HF) diets. Weight gain and the rise in fasting glucose were accelerated in G5G8 KO mice following HF feeding. HF-fed G5G8 KO mice had increased liver weight, hepatic lipids and plasma ALT compared to WT controls. Consistent with the development of NAFLD, macrophage infiltration, the number of TUNEL positive cells and the expression of proinflammatory cytokines were also increased in G5G8 KO mice, but there was no evidence of fibrosis. Hepatic lipid accumulation was associated with increased mRNA levels for PPARγ and PPARγ target genes, but not genes involved in lipogenesis. Phosphorylation of eukaryotic translation initiation factor 2α (eiF2α) and expression activating transcription factor 4 (ATF4) and tribbles 3 (Trb3) were elevated in HF-fed G5G8 KO mice, a pathway that links the unfolded protein response (UPR) to the development if insulin resistance through inhibition of protein kinase B (Akt) phosphorylation. However, the remaining components of the UPR were unchanged.

Conclusion: G5G8 plays a previously unappreciated role in the development of insulin resistance and fatty liver disease by opposing hepatic accumulation of cholesterol and phosphorylation of eiF2α.

Phytosterols differentially influence ABC transporter expression, cholesterol efflux and inflammatory cytokine secretion in macrophage foam cells

Phytosterol supplements lower low-density lipoprotein (LDL) cholesterol, but accumulate in vascular lesions of patients and limit the anti-atherosclerotic effects of LDL lowering in apolipoprotein E (Apo E)-deficient mice, suggesting that the cholesterol-lowering benefit of phytosterol supplementation may not be fully realized. Individual phytosterols have cell-type specific effects that may be either beneficial or deleterious with respect to atherosclerosis, but little is known concerning their effects on macrophage function. The effects of phytosterols on ABCA1 and ABCG1 abundance, cholesterol efflux and inflammatory cytokine secretion were determined in cultured macrophage foam cells. Among the commonly consumed phytosterols, stigmasterol increased expression of ABCA1 and ABCG1 and increased efflux of cholesterol to apolipoprotein (Apo) AI and high-density lipoprotein (HDL). Campesterol and sitosterol had no effect on ABCA1 or ABCG1 levels. Sitosterol had no effect on cholesterol efflux to Apo AI or HDL, whereas campesterol had a modest but significant reduction in cholesterol efflux to HDL in THP-1 macrophages. Whereas stigmasterol blunted aggregated LDL (agLDL) induced increases in tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β secretion, sitosterol exacerbated these effects. The presence of campesterol had no effect on agLDL-induced inflammatory cytokine secretion from THP-1 macrophages. In conclusion, the presence of stigmasterol in modified lipoproteins promoted cholesterol efflux and suppressed inflammatory cytokine secretion in response to lipid loading in macrophage foam cells. While campesterol was largely inert, the presence of sitosterol increased the proinflammatory cytokine secretion.

Cyclooxygenase-2 deficiency attenuates adipose tissue differentiation and inflammation in mice

Obesity is associated with a variety of disorders and is a significant health problem in developed countries. One factor controlling the level of adiposity is the differentiation of cells into adipocytes. Adipocyte differentiation requires expression of peroxisome proliferator-activated receptor γ (PPARγ), which is activated by ligands to regulate expression of genes involved in adipocyte differentiation. Although 15-deoxy-Δ(12,14)-prostaglandin (PG) J(2) (15d-PGJ(2)) has long been known to be a potent activator of PPARγ, the importance of its synthesis in adipose tissue in vivo is not clear. The current study utilized mice deficient in cyclooxygenase-2 (COX-2) to examine the role of COX-2-derived PGs as in vivo modulators of adiposity. As compared with strain- and age-matched wild-type controls, the genetic deficiency of COX-2 resulted in a significant reduction in total body weight and percent body fat. Although there were no significant differences in food consumption between groups, COX-2-deficient mice showed increased metabolic activity. Epididymal adipose tissue from wild-type mice produced a significantly greater level of 15d-PGJ(2), as compared with adipose tissue isolated from mice deficient in COX-2. Furthermore, production of the precursor required for 15d-PGJ(2) formation, PGD(2), was also significantly reduced in COX-2-deficient adipose tissue. The expression of markers for differentiated adipocytes was significantly reduced in adipose tissue from COX-2-deficient mice, whereas preadipocyte marker expression was increased. Macrophage-dependent inflammation was also significantly reduced in adipose tissue of COX-2-deficient mice. These findings suggest that reduced adiposity in COX-2-deficient mice results from attenuated PPARγ ligand production and adipocyte differentiation.

ABCD2 is abundant in adipose tissue and opposes the accumulation of dietary erucic acid (C22:1) in fat

The ATP binding cassette transporter, ABCD2 (D2), is a peroxisomal protein whose mRNA has been detected in the adrenal, brain, liver, and fat. Although the role of this transporter in neural tissues has been studied, its function in adipose tissue remains unexplored. The level of immunoreactive D2 in epididymal fat is >50-fold of that found in brain or adrenal. D2 is highly enriched in adipocytes and is upregulated during adipogenesis but is not essential for adipocyte differentiation or lipid accumulation in day 13.5 mouse embryonic fibroblasts isolated from D2-deficient (D2(-/-)) mice. Although no differences were appreciated in differentiation percentage, total lipid accumulation was greater in D2(-/-) adipocytes compared with the wild type. These results were consistent with in vivo observations in which no significant differences in adiposity or adipocyte diameter between wild-type and D2(-/-) mice were observed. D2(-/-) adipose tissue showed an increase in the abundance of 20:1 and 22:1 fatty acids. When mice were challenged with a diet enriched in erucic acid (22:1), this lipid accumulated in the adipose tissue in a gene-dosage-dependent manner. In conclusion, D2 is a sterol regulatory element binding protein target gene that is highly abundant in fat and opposes the accumulation of dietary lipids generally absent from the triglyceride storage pool within adipose tissue.

The ABCG5 ABCG8 sterol transporter and phytosterols: implications for cardiometabolic disease

PURPOSE OF REVIEW

This review summarizes recent developments in the activity, regulation, and physiology of the ABCG5 ABCG8 (G5G8) transporter and the use of its xenobiotic substrates, phytosterols, as cholesterol lowering agents in the treatment of cardiovascular disease. Recent progress has significant implications for the role of G5G8 and its substrates in complications associated with features of the metabolic syndrome.

RECENT FINDINGS

Recent reports expand the clinical presentation of sitosterolemia to include platelet and adrenal dysfunction. The G5G8 sterol transporter is critical to hepatobiliary excretion of cholesterol under nonpathological conditions and has been linked to the cholesterol gallstone susceptibility. Finally, the cardiovascular benefits of cholesterol lowering through the use of phytosterol supplements were offset by vascular dysfunction, suggesting that alternative strategies to reduced cholesterol absorption offer greater benefit.

SUMMARY

Insulin resistance elevates G5G8 and increases susceptibility to cholesterol gallstones. However, this transporter is critical for the exclusion of phytosterols from the absorptive pathways in the intestine. Challenging the limits of this protective mechanism through phytosterol supplementation diminishes the cardioprotective benefits of cholesterol lowering in mouse models of cardiovascular disease.

Transport of maternal cholesterol to the fetus is affected by maternal plasma cholesterol concentrations in the golden Syrian hamster

The fetus has a high requirement for cholesterol and synthesizes cholesterol at elevated rates. Recent studies suggest that fetal cholesterol also can be obtained from exogenous sources. The purpose of the current study was to examine the transport of maternal cholesterol to the fetus and determine the mechanism responsible for any cholesterol-driven changes in transport. Studies were completed in pregnant hamsters with normal and elevated plasma cholesterol concentrations. Cholesterol feeding resulted in a 3.1-fold increase in the amount of LDL-cholesterol taken up by the fetus and a 2.4-fold increase in the amount of HDL-cholesterol taken up. LDL-cholesterol was transported to the fetus primarily by the placenta, and HDL-cholesterol was transported by the yolk sac and placenta. Several proteins associated with sterol transport and efflux, including those induced by activated liver X receptor, were expressed in hamster and human placentas: NPC1, NPC1L1, ABCA2, SCP-x, and ABCG1, but not ABCG8. NPC1L1 was the only protein increased in hypercholesterolemic placentas. Thus, increasing maternal lipoprotein-cholesterol concentrations can enhance transport of maternal cholesterol to the fetus, leading to 1) increased movement of cholesterol down a concentration gradient in the placenta, 2) increased lipoprotein secretion from the yolk sac (shown previously), and possibly 3) increased placental NPC1L1 expression.

Diabetic HDL-associated myristic acid inhibits acetylcholine-induced nitric oxide generation by preventing the association of endothelial nitric oxide synthase with calmodulin

In the current study, we examined whether diabetes affected the ability of HDL to stimulate nitric oxide (NO) production. Using HDL isolated from both diabetic humans and diabetic mouse models, we found that female HDL no longer induced NO synthesis, despite containing equivalent amounts of estrogen as nondiabetic controls. Furthermore, HDL isolated from diabetic females and males prevented acetylcholine-induced stimulation of NO generation. Analyses of both the human and mouse diabetic HDL particles showed that the HDLs contained increased levels of myristic acid. To determine whether myristic acid associated with HDL particles was responsible for the decrease in NO generation, myristic acid was added to HDL isolated from nondiabetic humans and mice. Myristic acid-associated HDL inhibited the generation of NO in a dose-dependent manner. Importantly, diabetic HDL did not alter the levels of endothelial NO synthase or acetylcholine receptors associated with the cells. Surprisingly, diabetic HDL inhibited ionomycin-induced stimulation of NO production without affecting ionomycin-induced increases in intracellular calcium. Further analysis indicated that diabetic HDL prevented calmodulin from interacting with endothelial NO synthase (eNOS) but did not affect the activation of calmodulin kinase or calcium-independent mechanisms for stimulating eNOS. These studies are the first to show that a specific fatty acid associated with HDL inhibits the stimulation of NO generation. These findings have important implications regarding cardiovascular disease in diabetic patients.

Defects in the Leptin Axis Reduce Abundance of the ABCG5-ABCG8 Sterol Transporter in Liver

ABGG5 (G5) and ABCG8 (G8) are ABC half-transporters that dimerize within the endoplasmic reticulum, traffic to the cell surface, and mediate cholesterol excretion into bile. Mice harboring defects in the leptin axis (db/db and ob/ob) have reduced biliary cholesterol concentrations. Rapid weight loss brought about by administration of leptin or dietary restriction increases biliary cholesterol excretion. We hypothesized that the reduction in biliary cholesterol in mice harboring defects in the leptin axis is associated with a reduction in G5G8 transporters and that levels of the transporter would increase with leptin administration and dietary restriction. We examined mRNA and protein levels for G5 and G8 in db/db and ob/ob mice. In both models G5 and G8 protein levels were reduced. In ob/ob mice, both leptin administration and dietary restriction increased G5 and G8 protein and biliary cholesterol concentrations. Finally, we examined the effects of tauroursodeoxycholate, which has been shown to increase biliary cholesterol excretion and function as a molecular chaperone. Tauroursodeoxycholate increased G5 and G8 protein and biliary cholesterol concentrations in both wild-type and db/db mice. Our results indicate that the mechanism for reduced biliary cholesterol excretion in db/db and ob/ob mice involves reductions in G5 and G8 protein levels and that this may occur at the level of G5G8 heterodimer assembly within the endoplasmic reticulum.

Functional asymmetry of nucleotide-binding domains in ABCG5 and ABCG8

The ATP-binding cassette half-transporters ABCG5 (G5) and ABCG8 (G8) promote secretion of neutral sterols into bile, a major pathway for elimination of sterols. Mutations in either ABCG5 or ABCG8 cause sitosterolemia, a recessive disorder characterized by impaired biliary and intestinal sterol secretion, sterol accumulation, and premature atherosclerosis. The mechanism by which the G5G8 heterodimer couples ATP hydrolysis to sterol transport is not known. Here we examined the roles of the Walker A, Walker B, and signature motifs in the nucleotide-binding domains (NBD) of G5 and G8 using recombinant adenoviruses to reconstitute biliary sterol transport in G5G8-deficient mice. Mutant forms of each half-transporter were co-expressed with their wild-type partners. Mutations at crucial residues in the Walker A and Walker B domains of G5 prevented biliary sterol secretion, whereas mutations of the corresponding residues in G8 did not. The opposite result was obtained when mutations were introduced into the signature motif; mutations in the signature domain of G8 prevented sterol transport, but substitution of the corresponding residues in G5 did not. Taken together, these findings indicate that the NBDs of G5 and G8 are not functionally equivalent. The integrity of the canonical NBD formed by the Walker A and Walker B motifs of G5 and the signature motif of G8 is essential for G5G8-mediated sterol transport. In contrast, mutations in key residues of the NBD formed by the Walker A and B motifs of G8 and the signature sequence of G5 did not affect sterol secretion.

Missense Mutations in ABCG5 and ABCG8 Disrupt Heterodimerization and Trafficking

Mutations in ABCG5 (G5) or ABCG8 (G8) cause sitosterolemia, an autosomal recessive disease characterized by sterol accumulation and premature atherosclerosis. G5 and G8 are ATP-binding cassette (ABC) half-transporters that must heterodimerize to move to the apical surface of cells. We examined the role of N-linked glycans in the formation of the G5/G8 heterodimer to gain insight into the determinants of folding and trafficking of these proteins. Site-directed mutagenesis revealed that two asparagine residues (Asn(585) and Asn(592)) are glycosylated in G5 and that G8 has a single N-linked glycan attached to Asn(619). N-Linked glycosylation of G8 was required for efficient trafficking of the G5/G8 heterodimer, but mutations that abolished glycosylation of G5 did not prevent trafficking of the heterodimer. Both G5 and G8 are bound by the lectin chaperone, calnexin, suggesting that the calnexin cycle may facilitate folding of the G5/G8 heterodimer. To determine the effects of 13 disease-causing missense mutations in G5 and G8 on formation and trafficking of the G5/G8 heterodimer, mutant forms of the half-transporters were expressed in CHO-K1 cells. All 13 mutations reduced trafficking of the G5/G8 heterodimer from the endoplasmic reticulum to the Golgi complex, and most prevented the formation of stable heterodimers between G5 and G8. We conclude that the majority of the molecular defects in G5 and G8 that cause sitosterolemia impair transport of the sterol transporter to the cell surface.

ABCG5 and ABCG8 are obligate heterodimers for protein trafficking and biliary cholesterol excretion

ABCG5 (G5) and ABCG8 (G8) are ATP-binding cassette (ABC) transporters that limit intestinal absorption and promote biliary excretion of neutral sterols. Mutations in either ABCG5 or ABCG8 result in an identical clinical phenotype, suggesting that these two half-transporters function as heterodimers. Expression of both G5 and G8 is required for either protein to be transported to the plasma membrane of cultured cells. In this paper we used immunofluorescence microscopy to confirm, in vivo, that G5 is localized to the apical membranes of mouse enterocytes and hepatocytes. Other ABC half-transporters function as homodimers or as heterodimers with other subfamily members. To determine whether G5 or G8 complex with other ABCG half-transporters, we co-expressed G1, G2, and G4 with either G5 or G8 in cultured cells. G1, G2, and G4 co-immunoprecipitated with G5, and G4 co-immunoprecipitated with G8, but the putative dimers were retained in the endoplasmic reticulum (ER). Adenovirus-mediated expression of either G5 or G8 in the liver of G5G8 null mice resulted in ER retention of the expressed proteins and no increase in biliary cholesterol. In contrast, co-expression of G5 and G8 resulted in transit of the proteins out of the ER and a 10-fold increase in biliary cholesterol concentration. Finally, adenoviral expression of G2 in the presence or absence of G5 or G8 failed to promote sterol excretion into bile. These experiments indicate that G5 and G8 function as obligate heterodimers to promote sterol excretion into bile.

ABCG5 and ABCG8 are obligate heterodimers for protein trafficking and biliary cholesterol excretion

ABCG5 (G5) and ABCG8 (G8) are ATP-binding cassette (ABC) transporters that limit intestinal absorption and promote biliary excretion of neutral sterols. Mutations in either ABCG5 or ABCG8 result in an identical clinical phenotype, suggesting that these two half-transporters function as heterodimers. Expression of both G5 and G8 is required for either protein to be transported to the plasma membrane of cultured cells. In this paper we used immunofluorescence microscopy to confirm, in vivo, that G5 is localized to the apical membranes of mouse enterocytes and hepatocytes. Other ABC half-transporters function as homodimers or as heterodimers with other subfamily members. To determine whether G5 or G8 complex with other ABCG half-transporters, we co-expressed G1, G2, and G4 with either G5 or G8 in cultured cells. G1, G2, and G4 co-immunoprecipitated with G5, and G4 co-immunoprecipitated with G8, but the putative dimers were retained in the endoplasmic reticulum (ER). Adenovirus-mediated expression of either G5 or G8 in the liver of G5G8 null mice resulted in ER retention of the expressed proteins and no increase in biliary cholesterol. In contrast, co-expression of G5 and G8 resulted in transit of the proteins out of the ER and a 10-fold increase in biliary cholesterol concentration. Finally, adenoviral expression of G2 in the presence or absence of G5 or G8 failed to promote sterol excretion into bile. These experiments indicate that G5 and G8 function as obligate heterodimers to promote sterol excretion into bile.

Effect of estrogen on scavenger receptor BI expression in the rat

High-dose 17alpha-ethinyl estradiol treatment is associated with increased adrenal and decreased hepatic levels of scavenger receptor class B type 1 (SR-BI) in rats. In this paper we explored the mechanisms responsible for the differential regulation of SR-BI by estrogen in these two tIssues. Previously it was shown that estrogen-treated rats are profoundly hypolipidemic due to increased hepatic low density lipoprotein receptor (LDLR) activity, and that this effect is not maintained with hypophysectomy. To determine if the reduction in hepatic SR-BI was a direct or indirect effect of estrogen, we treated hypophysectomized rats with high-dose estrogen; the levels of SR-BI expression did not change in the livers or adrenals of these animals. To determine if the absence of response to estrogen in the adrenals of hypophysectomized animals was due to the absence of adrenocorticotropic hormone (ACTH), we examined the effect of estrogen treatment on SR-BI expression in animals treated with dexamethasone, which inhibits endogenous ACTH production. The administration of dexamethasone completely inhibited the increase in SR-BI expression in the adrenals of estrogen-treated rats. From these studies we conclude that estrogen does not have a direct effect on SR-BI expression in either the liver or the adrenals. In the liver, the decrease in SR-BI is dependent on the estrogen-induced increase in LDLR activity, and in the adrenal glands, ACTH is required for the estrogen-associated increase in expression of SR-BI.

Coexpression of ATP-binding cassette proteins ABCG5 and ABCG8 permits their transport to the apical surface

Mutations in either ATP-binding cassette (ABC) G5 or ABCG8 cause sitosterolemia, an autosomal recessive disorder of sterol trafficking. To determine the site of action of ABCG5 and ABCG8, we expressed recombinant, epitope-tagged mouse ABCG5 and ABCG8 in cultured cells. Both ABCG5 and ABCG8 underwent N-linked glycosylation. When either protein was expressed individually in cells, the N-linked sugars remained sensitive to Endoglycosidase H (Endo H). When ABCG5 and ABCG8 were coexpressed, the attached sugars were Endo H-resistant and neuraminidase-sensitive, indicating that the proteins were transported to the trans-Golgi complex. The mature, glycosylated forms of ABCG5 and ABCG8 coimmunoprecipitated, consistent with heterodimerization of these two proteins. The Endo H-sensitive forms of ABCG5 and ABCG8 were confined to the endoplasmic reticulum (ER), whereas the mature forms were present in non-ER fractions in cultured hepatocytes. Immunoelectron microscopy revealed ABCG5 and ABCG8 on the plasma membrane of these cells. In polarized WIF-B cells, recombinant ABCG5 localized to the apical (canalicular) membrane when coexpressed with ABCG8, but not when expressed alone. To our knowledge this is the first direct demonstration that trafficking of an ABC half-transporter to the cell surface requires the presence of its dimerization partner.