Elizabeth P. Newberry

Decreased Hepatic Triglyceride Accumulation and Altered Fatty Acid Uptake in Mice with Deletion of the Liver Fatty Acid-binding Protein Gene

Liver fatty acid-binding protein (L-Fabp) is an abundant cytosolic lipid-binding protein with broad substrate specificity, expressed in mammalian enterocytes and hepatocytes. We have generated mice with a targeted deletion of the endogenous L-Fabp gene and have characterized their response to alterations in hepatic fatty acid flux following prolonged fasting. Chow-fed L-Fabp–/– mice were indistinguishable from wild-type littermates with regard to growth, serum and tissue lipid profiles, and fatty acid distribution within hepatic complex lipid species. In response to 48-h fasting, however, wild-type mice demonstrated a ∼10-fold increase in hepatic triglyceride content while L-Fabp–/– mice demonstrated only a 2-fold increase. Hepatic VLDL secretion was decreased in L-Fabp–/– mice suggesting that the decreased accumulation of hepatic triglyceride was not the result of increased secretion. Fatty acid oxidation, as inferred from serum β-hydroxybutyrate levels, was increased in response to fasting, although the increase in L-Fabp–/– mice was significantly reduced in comparison to wild-type controls, despite comparable induction of PPARα target genes. Studies in primary hepatocytes revealed indistinguishable initial rates of oleate uptake, but longer intervals revealed reduced rates of uptake in fasted L-Fabp–/– mice. Oleate incorporation into cellular triglyceride and diacylglycerol was reduced in L-Fabp–/– mice although incorporation into phospholipid and cholesterol ester was no different than wild-type controls. These data point to an inducible defect in fatty acid utilization in fasted L-Fabp–/– mice that involves targeting of substrate for use in triglyceride metabolism.

Protection against Western diet–induced obesity and hepatic steatosis in liver fatty acid–binding protein knockout mice†

Liver fatty acid–binding protein (L‐Fabp) regulates murine hepatic fatty acid trafficking in response to fasting. In this study, we show that L‐Fabp−/− mice fed a high‐fat Western diet for up to 18 weeks are less obese and accumulate less hepatic triglyceride than C57BL/6J controls. Paradoxically, both control and L‐Fabp−/− mice manifested comparable glucose intolerance and insulin resistance when fed a Western diet. Protection against obesity in Western diet–fed L‐Fabp−/− mice was not due to discernable changes in food intake, fat malabsorption, or heat production, although intestinal lipid secretion kinetics were significantly slower in both chow‐fed and Western diet–fed L‐Fabp−/− mice. By contrast, there was a significant increase in the respiratory exchange ratio in L‐Fabp−/− mice, suggesting a shift in energy substrate use from fat to carbohydrate, findings supported by an approximately threefold increase in serum lactate. Microarray analysis revealed increased expression of genes involved in lipid synthesis (fatty acid synthase, squalene epoxidase, hydroxy‐methylglutaryl coenzyme A reductase), while genes involved in glycolysis (glucokinase and glycerol kinase) were decreased in L‐Fabp−/− mice. Fatty acid synthase expression was also increased in the skeletal muscle of L‐Fabp−/− mice. In conclusion, L‐Fabp may function as a metabolic sensor in regulating lipid homeostasis. We suggest that L‐Fabp−/− mice are protected against Western diet–induced obesity and hepatic steatosis through a series of adaptations in both hepatic and extrahepatic energy substrate use. (HEPATOLOGY 2006;44:1191–1205.)

Compensatory increase in hepatic lipogenesis in mice with conditional intestine-specific Mttp deficiency.

Microsomal TG transfer protein (MTTP) is required for the assembly and secretion of TG (TG)-rich lipoproteins from both enterocytes and hepatocytes. Liver-specific deletion of Mttp produced a dramatic reduction in plasma very low density lipoprotein-TG and virtually eliminated apolipoprotein B100 (apoB100) secretion yet caused only modest reductions in plasma apoB48 and apoB48 secretion from primary hepatocytes. These observations prompted us to examine the phenotype following intestine-specific Mttp deletion because murine, like human enterocytes, secrete virtually exclusively apoB48. We generated mice with conditional Mttp deletion in villus enterocytes (Mttp-IKO), using a tamoxifen-inducible, intestine-specific Cre transgene. Villus enterocytes from chow-fed Mttp-IKO mice contained large cytoplasmic TG droplets and no chylomicron-sized particles within the secretory pathway. Chow-fed, Mttp-IKO mice manifested steatorrhea, growth arrest, and decreased cholesterol absorption, features that collectively recapitulate the phenotype associated with abetalipoproteinemia. Chylomicron secretion was reduced dramatically in vivo, in conjunction with an ∼80% decrease in apoB48 secretion from primary enterocytes. Additionally, although plasma and hepatic cholesterol and TG content were decreased, Mttp-IKO mice demonstrated a paradoxical increase in both hepatic lipogenesis and very low density lipoprotein secretion. These findings establish distinctive features for MTTP involvement in intestinal chylomicron assembly and secretion and suggest that hepatic lipogenesis undergoes compensatory induction in the face of defective intestinal TG secretion.

Altered hepatic triglyceride content after partial hepatectomy without impaired liver regeneration in multiple murine genetic models

Liver regeneration is impaired following partial hepatectomy (PH) in mice with genetic obesity and hepatic steatosis and also in wild‐type mice fed a high‐fat diet. These findings contrast with other data showing that liver regeneration is impaired in mice in which hepatic lipid accumulation is suppressed by either pharmacologic leptin administration or by disrupted glucocorticoid signaling. These latter findings suggest that hepatic steatosis may actually be required for normal liver regeneration. We have reexamined this relationship using several murine models of altered hepatic lipid metabolism. Liver fatty acid (FA) binding protein knockout mice manifested reduced hepatic triglyceride (TG) content compared to controls, with no effect on liver regeneration or hepatocyte proliferation. Examination of early adipogenic messenger RNAs revealed comparable induction in liver from both genotypes despite reduced hepatic steatosis. Following PH, hepatic TG was reduced in intestine‐specific microsomal TG transfer protein deleter mice, which fail to absorb dietary fat, increased in peroxisome proliferator activated receptor alpha knockout mice, which exhibit defective FA oxidation, and unchanged (from wild‐type mice) in liver‐specific FA synthase knockout mice in which endogenous hepatic FA synthesis is impaired. Hepatic TG increased in the regenerating liver in all models, even in animals in which lipid accumulation is genetically constrained. However, in no model—and over a >90‐fold range of hepatic TG content—was liver regeneration significantly impaired following PH. Conclusion: Although hepatic TG content is widely variable and increases during liver regeneration, alterations in neither exogenous or endogenous lipid metabolic pathways, demonstrated to promote or diminish hepatic steatosis, influence hepatocyte proliferation. (HEPATOLOGY 2008.)

Trehalose inhibits solute carrier 2A (SLC2A) proteins to induce autophagy and prevent hepatic steatosis

The disaccharide trehalose blocks glucose uptake in hepatocytes and induces autophagy that prevents fatty liver disease. A sugary inhibitor of liver disease The accumulation of lipids in hepatocytes that occurs in nonalcoholic fatty liver disease (NAFLD) can result in liver failure or liver cancer. Trehalose is a ubiquitous sugar that is present in the food consumed by animals. DeBosch et al. determined that trehalose blocked glucose uptake into cells by inhibiting glucose transporters in the plasma membrane, which induced a “starvation”-like response that activated autophagy even in the presence of adequate nutrients and glucose. Furthermore, providing trehalose to mice that are a model of NAFLD prevented lipid accumulation in the liver. As noted by Mardones et al. in the associated Focus, trehalose, which has been previously under investigation to treat neurodegenerative diseases characterized by toxic protein aggregates, may be a “silver bullet” for treating diseases resulting from inadequate cellular degradative metabolism. Trehalose is a naturally occurring disaccharide that has gained attention for its ability to induce cellular autophagy and mitigate diseases related to pathological protein aggregation. Despite decades of ubiquitous use as a nutraceutical, preservative, and humectant, its mechanism of action remains elusive. We showed that trehalose inhibited members of the SLC2A (also known as GLUT) family of glucose transporters. Trehalose-mediated inhibition of glucose transport induced AMPK (adenosine 5′-monophosphate–activated protein kinase)–dependent autophagy and regression of hepatic steatosis in vivo and a reduction in the accumulation of lipid droplets in primary murine hepatocyte cultures. Our data indicated that trehalose triggers beneficial cellular autophagy by inhibiting glucose transport.

Coordinate Transcriptional Repression of Liver Fatty Acid-binding Protein and Microsomal Triglyceride Transfer Protein Blocks Hepatic Very Low Density Lipoprotein Secretion without Hepatosteatosis

Unlike the livers of humans and mice, and most hepatoma cells, which accumulate triglycerides when treated with microsomal triglyceride transfer protein (MTP) inhibitors, L35 rat hepatoma cells do not express MTP and cannot secrete very low density lipoprotein (VLDL), yet they do not accumulate triglyceride. In these studies we show that transcriptional co-repression of the two lipid transfer proteins, liver fatty acid-binding protein (L-FABP) and MTP, which cooperatively shunt fatty acids into de novo synthesized glycerolipids and the transfer of lipids into VLDL, respectively, act together to maintain hepatic lipid homeostasis. FAO rat hepatoma cells express L-FABP and MTP and demonstrate the ability to assemble and secrete VLDL. In contrast, L35 cells, derived as a single cell clone from FAO cells, do not express L-FABP or MTP nor do they assemble and secrete VLDL. We used these hepatoma cells to elucidate how a conserved DR1 promoter element present in the promoters of L-FABP and MTP affects transcription, expression, and VLDL production. In FAO cells, the DR1 elements of both L-FABP and MTP promoters are occupied by peroxisome proliferator-activated receptor α-retinoid X receptor α (RXRα), with which PGC-1β activates transcription. In contrast, in L35 cells the DR1 elements of both L-FABP and MTP promoters are occupied by chicken ovalbumin upstream promoter transcription factor II, and transcription is diminished. The combined findings indicate that peroxisome proliferator-activated receptor α-RXRα and PGC-1β coordinately up-regulate L-FABP and MTP expression, by competing with chicken ovalbumin upstream promoter transcription factor II for the DR1 sites in the proximal promoters of each gene. Additional studies show that ablation of L-FABP prevents hepatic steatosis caused by treating mice with an MTP inhibitor. Our findings show that reducing both L-FABP and MTP is an effective means to reduce VLDL secretion without causing hepatic steatosis.

Stimulus-selective Inhibition of Rat Osteocalcin Promoter Induction and Protein-DNA Interactions by the Homeodomain Repressor Msx2

Osteocalcin (OC) is a matrix calcium-binding protein expressed in osteoblasts and odontoblasts undergoing mineralization. OC expression is up-regulated in part by signals initiated by basic fibroblast growth factor (FGF2), cyclic AMP or forskolin (FSK), and calcitriol via defined elements and DNA-protein interactions in the OC promoter. We identified the OC gene as a target for transcriptional suppression by Msx2, a homeodomain transcription factor that controls ossification in the developing skull. In this study, we examine the effects of Msx2 expression on OC promoter activation (luciferase reporter) by FGF2/FSK and calcitriol in MC3T3-E1 osteoblasts. Expression of Msx2 decreases basal activity of the 1-kilobase (−1050 to +32) rat OC promoter by 80%; however, the promoter is still inducible 3-fold by calcitriol. By contrast, OC promoter induction by FGF2/FSK is completely abrogated by Msx2. Because intrinsic Msx2 DNA binding activity is not required for the Msx2 suppressor function, we assessed whether Msx2 represses OC activation by regulating DNA-protein interactions at the FGF2 response element (OCFRE) and compared these interactions with those occurring at the calcitriol response element (VDRE). Treatment of MC3T3-E1 cells with FGF2/FSK or calcitriol up-regulates specific DNA-protein interactions at the OCFRE or VDRE, respectively, as detected by gel shift assay. Preincubation of crude nuclear extracts with recombinant glutathioneS-transferase (GST)-Msx2 dose-dependently inhibits OCFRE DNA binding activity, whereas GST has no effect. Msx2 itself does not bind the OCFRE. Residues 132–148 required for Msx2 core suppressor function in transfection assays are also required to inhibit OCFRE DNA binding activity. By contrast, GST-Msx2 has no effect on calcitriol-regulated DNA-protein interactions at the VDRE. Using gel shift as an assay, the OCFRE DNA-binding protein OCFREB was purified to about 50% homogeneity from MG63 osteosarcoma cells. Recombinant Msx2 inhibits purified OCFREB DNA binding activity, whereas the Msx2 variant lacking residues 132–148 is inactive. Thus, Msx2 abrogates up-regulation of the OC promoter by FGF2/FSK in part by inhibiting OCFREB binding to the OCFRE.

Keratinocyte-specific Expression of Fatty Acid Transport Protein 4 Rescues the Wrinkle-free Phenotype in Slc27a4/Fatp4 Mutant Mice

FATP4 (fatty acid transport protein 4; also known as SLC27A4) is the most widely expressed member of a family of six long chain fatty acid transporters. FATP4 is highly expressed in enterocytes and has therefore been proposed to be a major importer of dietary fatty acids. Two independent mutations in Fatp4 cause mice to be born with thick, tight, shiny, “wrinkle-free” skin and a defective skin barrier; they die within hours of birth from dehydration and restricted movements. In contrast, induced keratinocyte-specific deficiency of FATP4 in adult mice causes only mild skin abnormalities. Therefore, whether the loss of FATP4 from skin or a systemic gestational metabolic defect causes the severe skin defects and neonatal lethality remain important unanswered questions. To investigate the basis for the phenotype, we first generated wild-type tetraploid/mutant diploid aggregates that should lead to rescue of any abnormalities caused by loss of FATP4 from the placenta. However, the skin phenotype was not ameliorated. We then generated transgenic mice expressing exogenous FATP4 either widely or specifically in suprabasal keratinocytes, and we bred the transgenes onto the Fatp4-/- background. Both modes of FATP4 expression led to rescue of the neonatally lethal skin defects, and the resulting mice were viable and fertile. Keratinocyte expression of an FATP4 variant with mutations in the acyl-CoA synthetase domain did not provide any degree of rescue. We conclude that expression of FATP4 with an intact acyl-CoA synthetase domain in suprabasal keratinocytes is necessary for normal skin development and that FATP4 functions in establishing the cornified envelope.

Liver fatty acid binding protein (L-Fabp) modulates murine stellate cell activation and diet-induced nonalcoholic fatty liver disease.

Activation of hepatic stellate cells (HSCs) is crucial to the development of fibrosis in nonalcoholic fatty liver disease. Quiescent HSCs contain lipid droplets (LDs), whose depletion upon activation induces a fibrogenic gene program. Here we show that liver fatty acid‐binding protein (L‐Fabp), an abundant cytosolic protein that modulates fatty acid (FA) metabolism in enterocytes and hepatocytes, also modulates HSC FA utilization and in turn regulates the fibrogenic program. L‐Fabp expression decreased 10‐fold following HSC activation, concomitant with depletion of LDs. Primary HSCs isolated from L‐FABP−/− mice contain fewer LDs than wild‐type (WT) HSCs, and exhibit up‐regulated expression of genes involved in HSC activation. Adenoviral L‐Fabp transduction inhibited activation of passaged WT HSCs and increased both the expression of prolipogenic genes and also augmented intracellular lipid accumulation, including triglyceride and FA, predominantly palmitate. Freshly isolated HSCs from L‐FABP−/− mice correspondingly exhibited decreased palmitate in the free FA pool. To investigate whether L‐FABP deletion promotes HSC activation in vivo, we fed L‐FABP−/− and WT mice a high‐fat diet supplemented with trans‐fatty acids and fructose (TFF). TFF‐fed L‐FABP−/− mice exhibited reduced hepatic steatosis along with decreased LD abundance and size compared to WT mice. In addition, TFF‐fed L‐FABP−/− mice exhibited decreased hepatic fibrosis, with reduced expression of fibrogenic genes, compared to WT mice. Conclusion: L‐FABP deletion attenuates both diet‐induced hepatic steatosis and fibrogenesis, despite the observation that L‐Fabp paradoxically promotes FA and LD accumulation and inhibits HSC activation in vitro. These findings highlight the importance of cell‐specific modulation of hepatic lipid metabolism in promoting fibrogenesis in nonalcoholic fatty liver disease. (Hepatology 2013)

Targeted Deletion of the Murine apobec-1 Complementation Factor (acf) Gene Results in Embryonic Lethality

ABSTRACT apobec-1 complementation factor (ACF) is an hnRNP family member which functions as the obligate RNA binding subunit of the core enzyme mediating C-to-U editing of the nuclear apolipoprotein B (apoB) transcript. ACF binds to both apoB RNA and apobec-1, the catalytic cytidine deaminase, which then results in site-specific posttranscriptional editing of apoB mRNA. Targeted deletion of apobec1 eliminates C-to-U editing of apoB mRNA but is otherwise well tolerated. However, the functions and potential targets of ACF beyond apoB mRNA editing are unknown. Here we report the results of generating acf knockout mice using homologous recombination. While heterozygous acf+/ − mice were apparently healthy and fertile, no viable acf − / − mice were identified. Mutant acf − / − embryos were detectable only until the blastocyst (embryonic day 3.5 [E3.5]) stage. No acf − / − blastocysts were detectable following implantation at E4.5, and isolated acf − / − blastocysts failed to proliferate in vitro. Small interfering RNA knockdown of ACF in either rat (apobec-1-expressing) or human (apobec-1-deficient) hepatoma cells decreased ACF protein expression and induced a commensurate increase in apoptosis. Taken together, these data suggest that ACF plays a crucial role, which is independent of apobec-1 expression, in cell survival, particularly during early embryonic development.