Progranulin (PGRN) is a secreted glycoprotein expressed in neurons and glia that is implicated in neuronal survival on the basis that mutations in the GRN gene causing haploinsufficiency result in a familial form of frontotemporal dementia (FTD). Recently, a direct interaction between PGRN and tumor necrosis factor receptors (TNFR I/II) was reported and proposed to be a mechanism by which PGRN exerts anti-inflammatory activity, raising the possibility that aberrant PGRN–TNFR interactions underlie the molecular basis for neuroinflammation in frontotemporal lobar degeneration pathogenesis. Here, we report that we find no evidence for a direct physical or functional interaction between PGRN and TNFRs. Using coimmunoprecipitation and surface plasmon resonance (SPR) we replicated the interaction between PGRN and sortilin and that between TNF and TNFRI/II, but not the interaction between PGRN and TNFRs. Recombinant PGRN or transfection of a cDNA encoding PGRN did not antagonize TNF-dependent NFκB, Akt, and Erk1/2 pathway activation; inflammatory gene expression; or secretion of inflammatory factors in BV2 microglia and bone marrow-derived macrophages (BMDMs). Moreover, PGRN did not antagonize TNF-induced cytotoxicity on dopaminergic neuroblastoma cells. Last, co-addition or pre-incubation with various N- or C-terminal-tagged recombinant PGRNs did not alter lipopolysaccharide-induced inflammatory gene expression or cytokine secretion in any cell type examined, including BMDMs from Grn+/− or Grn−/− mice. Therefore, the neuroinflammatory phenotype associated with PGRN deficiency in the CNS is not a direct consequence of the loss of TNF antagonism by PGRN, but may be a secondary response by glia to disrupted interactions between PGRN and Sortilin and/or other binding partners yet to be identified.
Excessive activity of hepatic atypical protein kinase (aPKC) is proposed to play a critical role in mediating lipid and carbohydrate abnormalities in obesity, the metabolic syndrome, and type 2 diabetes mellitus. In previous studies of rodent models of obesity and type 2 diabetes mellitus, adenoviral-mediated expression of kinase-inactive aPKC rapidly reversed or markedly improved most if not all metabolic abnormalities. Here, we examined effects of 2 newly developed small-molecule PKC-ι/λ inhibitors. We used the mouse model of heterozygous muscle-specific knockout of PKC-λ, in which partial deficiency of muscle PKC-λ impairs glucose transport in muscle and thereby causes glucose intolerance and hyperinsulinemia, which, via hepatic aPKC activation, leads to abdominal obesity, hepatosteatosis, hypertriglyceridemia, and hypercholesterolemia. One inhibitor, 1H-imidazole-4-carboxamide, 5-amino-1-[2,3-dihydroxy-4-[(phosphonooxy)methyl]cyclopentyl-[1R-(1a,2b,3b,4a)], binds to the substrate-binding site of PKC-λ/ι, but not other PKCs. The other inhibitor, aurothiomalate, binds to cysteine residues in the PBl-binding domains of aPKC-λ/ι/ζ and inhibits scaffolding. Treatment with either inhibitor for 7 days inhibited aPKC, but not Akt, in liver and concomitantly improved insulin signaling to Akt and aPKC in muscle and adipocytes. Moreover, both inhibitors diminished excessive expression of hepatic, aPKC-dependent lipogenic, proinflammatory, and gluconeogenic factors; and this was accompanied by reversal or marked improvements in hyperglycemia, hyperinsulinemia, abdominal obesity, hepatosteatosis, hypertriglyceridemia, and hypercholesterolemia. Our findings highlight the pathogenetic importance of insulin signaling to hepatic PKC-ι in obesity, the metabolic syndrome, and type 2 diabetes mellitus and suggest that 1H-imidazole-4-carboxamide, 5-amino-1-[2,3-dihydroxy-4-[(phosphonooxy)methyl]cyclopentyl-[1R-(1a,2b,3b,4a)] and aurothiomalate or similar agents that selectively inhibit hepatic aPKC may be useful treatments.
Frontotemporal dementia (FTD) is a neurodegenerative disease with hallmark deficits in social and emotional function. Heterozygous loss-of-function mutations in GRN, the progranulin gene, are a common genetic cause of the disorder, but the mechanisms by which progranulin haploinsufficiency causes neuronal dysfunction in FTD are unclear. Homozygous progranulin knockout (Grn−/−) mice have been studied as a model of this disorder and show behavioral deficits and a neuroinflammatory phenotype with robust microglial activation. However, homozygous GRN mutations causing complete progranulin deficiency were recently shown to cause a different neurological disorder, neuronal ceroid lipofuscinosis, suggesting that the total absence of progranulin may have effects distinct from those of haploinsufficiency. Here, we studied progranulin heterozygous (Grn+/−) mice, which model progranulin haploinsufficiency. We found that Grn+/− mice developed age-dependent social and emotional deficits potentially relevant to FTD. However, unlike Grn−/− mice, behavioral deficits in Grn+/− mice occurred in the absence of gliosis or increased expression of tumor necrosis factor–α. Instead, we found neuronal abnormalities in the amygdala, an area of selective vulnerability in FTD, in Grn+/− mice. Our findings indicate that FTD-related deficits due to progranulin haploinsufficiency can develop in the absence of detectable gliosis and neuroinflammation, thereby dissociating microglial activation from functional deficits and suggesting an important effect of progranulin deficiency on neurons.
Among organelles, lipid droplets (LDs) uniquely constitute a hydrophobic phase in the aqueous environment of the cytosol. Their hydrophobic core of neutral lipids stores metabolic energy and membrane components, making LDs hubs for lipid metabolism. In addition, LDs are implicated in a number of other cellular functions, ranging from protein storage and degradation to viral replication. These processes are functionally linked to many physiological and pathological conditions, including obesity and related metabolic diseases. Despite their important functions and nearly ubiquitous presence in cells, many aspects of LD biology are unknown. In the past few years, the pace of LD investigation has increased, providing new insights. Here, we review the current knowledge of LD cell biology and its translation to physiology.
organelle; energy metabolism; triacylglycerol; membranes; obesity; oil; fat
Excessive deposition of fat in the liver (hepatic steatosis) is frequently accompanied by hepatic insulin resistance. Whether this correlation is due to a causal relationship between the conditions has been the subject of considerable debate, and the literature abounds with conflicting data and theories. Here we provide a perspective by defining the problem and its challenges, analyzing the possible causative relationships, and drawing some conclusions.
triglycerides; liver; diabetes
Review aberrations of insulin signaling to atypical protein kinase C (aPKC) in muscle and liver that generate cardiovascular risk factors, including, obesity, hypertriglyceridemia, hypercholesterolemia, insulin resistance and glucose intolerance in type 2 diabetes mellitus (T2DM), and obesity-associated metabolic syndrome (MetSyn).
aPKC and/or Akt mediate insulin effects on glucose transport in muscle, and synthesis of lipids, cytokines and glucose in liver. In T2DM, whereas Akt and aPKC activation are diminished in muscle, and hepatic Akt activation is diminished, hepatic aPKC activation is conserved. Imbalance between muscle and hepatic aPKC activation (and expression of PKC-ι in humans) by insulin results from differential downregulation of insulin receptor substrates that control phosphatidylinositol 3-kinase. Conserved activation of hepatic aPKC in hyperinsulinemic states of T2DM, obesity and MetSyn is problematic as excessive activation of aPKC-dependent lipogenic, gluconeogenic and proinflammatory pathways increases cardiovascular risk factors. Indeed, selective inhibition of hepatic aPKC by adenoviral-mediated expression of kinase-inactive aPKC, or newly-developed small-molecule biochemicals, dramatically improves abdominal obesity, hepatosteatosis, hypertriglyceridemia, hypercholesterolemia, insulin resistance and glucose intolerance in murine models of obesity and T2DM.
Hepatic aPKC is a unifying target for treating multiple clinical abnormalities that increase cardiovascular risk in insulin-resistant states of obesity, MetSyn and T2DM.
Atypical Protein Kinase C; Obesity; Metabolic Syndrome; Type 2 Diabetes Mellitus; Insulin Signaling in Liver and Muscle
Lipid droplets (LDs) are the major fat storage organelles in eukaryotic cells, but how their size is regulated is unknown. Using genetic screens in C. elegans for LD morphology defects in intestinal cells, we found that mutations in atlastin, a GTPase required for homotypic fusion of endoplasmic reticulum (ER) membranes, cause not only ER morphology defects, but also a reduction in LD size. Similar results were obtained after depletion of atlastin or expression of a dominant-negative mutant, whereas overexpression of atlastin had the opposite effect. Atlastin depletion in Drosophila fat bodies also reduced LD size and decreased triglycerides in whole animals, sensitizing them to starvation. In mammalian cells, co-overexpression of atlastin-1 and REEP1, a paralog of the ER tubule-shaping protein DP1/REEP5, generates large LDs. The effect of atlastin-1 on LD size correlates with its activity to promote membrane fusion in vitro. Our results indicate that atlastin-mediated fusion of ER membranes is important for LD size regulation.
Concentrations of acetyl–coenzyme A and nicotinamide adenine dinucleotide (NAD+) affect histone acetylation and thereby couple cellular metabolic status and transcriptional regulation. We report that the ketone body d-β-hydroxybutyrate (βOHB) is an endogenous and specific inhibitor of class I histone deacetylases (HDACs). Administration of exogenous βOHB, or fasting or calorie restriction, two conditions associated with increased βOHB abundance, all increased global histone acetylation in mouse tissues. Inhibition of HDAC by βOHB was correlated with global changes in transcription, including that of the genes encoding oxidative stress resistance factors FOXO3A and MT2. Treatment of cells with βOHB increased histone acetylation at the Foxo3a and Mt2 promoters, and both genes were activated by selective depletion of HDAC1 and HDAC2. Consistent with increased FOXO3A and MT2 activity, treatment of mice with βOHB conferred substantial protection against oxidative stress.
Lipid droplets (LDs) are cellular storage organelles for neutral lipids that vary in size and abundance according to cellular needs. Physiological conditions that promote lipid storage rapidly and markedly increase LD volume and surface. How the need for surface phospholipids is sensed and balanced during this process is unknown. Here, we show that phosphatidylcholine (PC) acts as a surfactant to prevent LD coalescence, which otherwise yields large, lipolysis resistant LDs and triglyceride (TG) accumulation. The need for additional PC to coat the enlarging surface during LD expansion is provided by the Kennedy pathway, which is activated by reversible targeting of the rate-limiting enzyme, CTP:phospho-cholin cytidylyltransferase (CCT), to growing LD surfaces. The requirement, targeting, and activation of CCT to growing LDs were similar in cells of Drosophila and mice. Our results reveal a mechanism to maintain PC homeostasis at the expanding LD monolayer through targeted activation of a key PC synthesis enzyme.
Lipid droplets (LDs) store metabolic energy and membrane lipid precursors. With excess metabolic energy, cells synthesize triacylglycerol (TG) and form LDs that grow dramatically. It is unclear how TG synthesis relates to LD formation and growth. Here, we identify two LD subpopulations: smaller LDs of relatively constant size, and LDs that grow larger. The latter population contains isoenzymes for each step of TG synthesis. Glycerol-3-phosphate acyltransferase 4 (GPAT4), which catalyzes the first and rate-limiting step, relocalizes from the endoplasmic reticulum (ER) to a subset of forming LDs, where it becomes stably associated. ER-to-LD targeting of GPAT4 and other LD-localized TG synthesis isozymes is required for LD growth. Key features of GPAT4 ER-to-LD targeting and function in LD growth are conserved between Drosophila and mammalian cells. Our results explain how TG synthesis is coupled with LD growth and identify two distinct LD subpopulations based on their capacity for localized TG synthesis.
Lipid droplets (LDs) are dynamic, cytosolic lipid-storage organelles found in nearly all cell types. Too many or too few LDs during excess or deficient fat storage lead to many different human diseases. Recent insights into LD biology and LD protein functions shed new light on mechanisms underlying those metabolic pathologies. These findings will likely provide opportunities for treatment of diseases associated with too much or too little fat.
atherosclerosis; lipid droplet; lipodystrophy; metabolic syndrome; triglyceride storage
Dissecting the role of insulin in the complex regulation of triglyceride metabolism is necessary for understanding dyslipidemia and steatosis. Liver Insulin Receptor Knockout (LIRKO) mice show that in the physiological context of feeding, hepatic insulin signaling is not required for the induction of mTORC1, an upstream activator of the lipogenic regulator, SREBP-1c. Feeding induces SREBP-1c mRNA in LIRKO livers, though not to the extent observed in controls. A high fructose diet also partially induces SREBP-1c and lipogenic gene expression in LIRKO livers. Insulin signaling becomes more important in the pathological context of obesity, as knockdown of the insulin receptor in ob/ob mice, a model of Type 2 diabetes, using anti-sense oligonucleotides, abolishes the induction of SREBP-1c and its targets by obesity and ameliorates steatosis. Thus, insulin-independent signaling pathways can partially compensate for insulin in the induction of SREBP-1c by feeding but the further induction by obesity/Type 2 diabetes is entirely dependent upon insulin.
ALS is a devastating neurodegenerative disease primarily affecting motor neurons. Mutations in TDP-43 cause some forms of the disease, and cytoplasmic TDP-43 aggregates accumulate in degenerating neurons of most ALS patients. Thus, strategies aimed at targeting the toxicity of cytoplasmic TDP-43 aggregates may be effective. Here we report results from two genome-wide loss-of-function TDP-43 toxicity suppressor screens in yeast. The strongest suppressor of TDP-43 toxicity was deletion of Dbr1, which encodes RNA lariat debranching enzyme. We show that in the absence of Dbr1 enzymatic activity intronic lariats accumulate in the cytoplasm and likely act as decoys to sequester TDP-43 away from interfering with essential cellular RNAs and RNA-binding proteins. Knockdown of Dbr1 in a human neuronal cell line or in primary rodent neurons is also sufficient to rescue TDP-43 toxicity. Our findings provide insight into TDP-43 cytotoxicity and suggest decreasing Dbr1 activity could be a potential therapeutic approach for ALS.
We present a consolidated view of the complexity and challenges of designing studies for measurement of energy metabolism in mouse models, including a practical guide to the assessment of energy expenditure, energy intake and body composition and statistical analysis thereof. We hope this guide will facilitate comparisons across studies and minimize spurious interpretations of data. We recommend that division of energy expenditure data by either body weight or lean body weight and that presentation of group effects as histograms should be replaced by plotting individual data and analyzing both group and body-composition effects using analysis of covariance (ANCOVA).
Progranulin (GRN) mutations cause frontotemporal dementia (FTD), but GRN’s function in the CNS remains largely unknown. To identify the pathways downstream of GRN, we used weighted gene co-expression network analysis (WGCNA) to develop a systems-level view of transcriptional alterations in a human neural progenitor model of GRN-deficiency. This highlighted key pathways such as apoptosis and ubiquitination in GRN deficient human neurons, while revealing an unexpected major role for the Wnt signaling pathway, which was confirmed by analysis of gene expression data from postmortem FTD brain. Furthermore, we observed that the Wnt receptor Fzd2 was one of only a few genes up-regulated at 6 weeks in a GRN knockout mouse, and that FZD2 reduction caused increased apoptosis, while its upregulation promoted neuronal survival in vitro. Together, these in vitro and in vivo data point to an adaptive role for altered Wnt signaling in GRN deficiency-mediated FTD, representing a potential therapeutic target.
Progranulin; Frontotemporal Dementia; Wnt; Fzd2; WGCNA
A complex between the ER resident protein FATP1 and the lipid droplet–localized DGAT2 protein facilitates lipid droplet expansion in C. elegans and mammalian cells.
At the subcellular level, fat storage is confined to the evolutionarily conserved compartments termed lipid droplets (LDs), which are closely associated with the endoplasmic reticulum (ER). However, the molecular mechanisms that enable ER–LD interaction and facilitate neutral lipid loading into LDs are poorly understood. In this paper, we present evidence that FATP1/acyl-CoA synthetase and DGAT2/diacylglycerol acyltransferase are components of a triglyceride synthesis complex that facilitates LD expansion. A loss of FATP1 or DGAT2 function blocked LD expansion in Caenorhabditis elegans. FATP1 preferentially associated with DGAT2, and they acted synergistically to promote LD expansion in mammalian cells. Live imaging indicated that FATP1 and DGAT2 are ER and LD resident proteins, respectively, and electron microscopy revealed FATP1 and DGAT2 foci close to the LD surface. Furthermore, DGAT2 that was retained in the ER failed to support LD expansion. We propose that the evolutionarily conserved FATP1–DGAT2 complex acts at the ER–LD interface and couples the synthesis and deposition of triglycerides into LDs both physically and functionally.
The pathogenic mechanisms of frontotemporal dementia (FTD) remain poorly understood. Here we generated multiple induced pluripotent stem cell (iPSC) lines from a control subject, a patient with sporadic FTD, and an FTD patient with a novel GRN mutation (PGRN S116X). In neurons and microglia differentiated from PGRN S116X iPSCs, the levels of intracellular and secreted progranulin were reduced, establishing patient-specific cellular models of progranulin haploinsufficiency. Through a systematic screen of inducers of cellular stress, we found that PGRN S116X neurons, but not sporadic FTD neurons, exhibited increased sensitivity to staurosporine and other kinase inhibitors. Moreover, the serine/threonine kinase S6K2, a component of the PI3K and MAPK pathways, was specifically downregulated in PGRN S116X neurons. Both increased sensitivity to kinase inhibitors and reduced S6K2 were rescued by progranulin expression. Our findings identify cell-autonomous, reversible defects in patient neurons with progranulin deficiency and provide a new model for studying progranulin-dependent pathogenic mechanisms and testing potential therapies.
frontotemporal dementia; haploinsufficiency; iPSCs; kinases; microglia; neurons; progranulin; stress; S6K2
Congenital diarrheal disorders (CDDs) are a collection of rare, heterogeneous enteropathies with early onset and often severe outcomes. Here, we report a family of Ashkenazi Jewish descent, with 2 out of 3 children affected by CDD. Both affected children presented 3 days after birth with severe, intractable diarrhea. One child died from complications at age 17 months. The second child showed marked improvement, with resolution of most symptoms at 10 to 12 months of age. Using exome sequencing, we identified a rare splice site mutation in the DGAT1 gene and found that both affected children were homozygous carriers. Molecular analysis of the mutant allele indicated a total loss of function, with no detectable DGAT1 protein or activity produced. The precise cause of diarrhea is unknown, but we speculate that it relates to abnormal fat absorption and buildup of DGAT substrates in the intestinal mucosa. Our results identify DGAT1 loss-of-function mutations as a rare cause of CDDs. These findings prompt concern for DGAT1 inhibition in humans, which is being assessed for treating metabolic and other diseases.
Progranulin (PGRN) is a widely expressed secreted protein that is linked to inflammation. In humans, PGRN haploinsufficiency is a major inherited cause of frontotemporal dementia (FTD), but how PGRN deficiency causes neurodegeneration is unknown. Here we show that loss of PGRN results in increased neuron loss in response to injury in the CNS. When exposed acutely to 1-methyl-4-(2′-methylphenyl)-1,2,3,6-tetrahydrophine (MPTP), mice lacking PGRN (Grn–/–) showed more neuron loss and increased microgliosis compared with wild-type mice. The exacerbated neuron loss was due not to selective vulnerability of Grn–/– neurons to MPTP, but rather to an increased microglial inflammatory response. Consistent with this, conditional mutants lacking PGRN in microglia exhibited MPTP-induced phenotypes similar to Grn–/– mice. Selective depletion of PGRN from microglia in mixed cortical cultures resulted in increased death of wild-type neurons in the absence of injury. Furthermore, Grn–/– microglia treated with LPS/IFN-γ exhibited an amplified inflammatory response, and conditioned media from these microglia promoted death of cultured neurons. Our results indicate that PGRN deficiency leads to dysregulated microglial activation and thereby contributes to increased neuron loss with injury. These findings suggest that PGRN deficiency may cause increased neuron loss in other forms of CNS injury accompanied by neuroinflammation.
Hepatitis C virus (HCV) infection is closely tied to the lipid metabolism of liver cells. Here, we identify the triglyceride-synthesizing enzyme DGAT1 as an important host factor for HCV infection; DGAT1 interacts with the viral nucleocapsid core and is required for the trafficking of core to lipid droplets. Inhibition of DGAT1 activity or RNAi-mediated knockdown severely impairs infectious virion production, implicating DGAT1 as a new target for antiviral therapy.
Mice lacking acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1), an enzyme that catalyzes the terminal step in triacylglycerol synthesis, have enhanced insulin sensitivity and are protected from obesity, a result of increased energy expenditure. In these mice, factors derived from white adipose tissue (WAT) contribute to the systemic changes in metabolism. One such factor, adiponectin, increases fatty acid oxidation and enhances insulin sensitivity. To test the hypothesis that adiponectin is required for the altered energy and glucose metabolism in DGAT1-deficient mice, we generated adiponectin-deficient mice and introduced adiponectin deficiency into DGAT1-deficient mice by genetic crosses. Although adiponectin-deficient mice fed a high-fat diet were heavier, exhibited worse glucose tolerance, and had more hepatic triacylglycerol accumulation than wild-type controls, mice lacking both DGAT1 and adiponectin, like DGAT1-deficient mice, were protected from diet-induced obesity, glucose intolerance, and hepatic steatosis. These findings indicate that adiponectin is required for normal energy, glucose, and lipid metabolism but that the metabolic changes induced by DGAT1-deficient WAT are independent of adiponectin and are likely due to other WAT-derived factors. Our findings also suggest that the pharmacological inhibition of DGAT1 may be useful for treating human obesity and insulin resistance associated with low circulating adiponectin levels.
triglyceride; adipose tissue; gene knockout; acyl-coenzyme A:diacyl-glycerol acyltransferase 1
Acyl CoA:diacylglycerol acyltransferase (DGAT) is a ubiquitously expressed enzyme that catalyzes the final reaction in the major pathways of triglyceride synthesis. Mice lacking DGAT1 (Dgat–/–) demonstrate significant changes in lipid metabolism in several tissues, including the skin. Here we report the effects of DGAT1 deficiency on fur and sebaceous glands. Adult Dgat–/– mice had dry fur and hair loss, which were associated with atrophic sebaceous glands and fur lipid abnormalities. As a result, Dgat–/– mice had impaired water repulsion and defective thermoregulation after water immersion. These phenotypes were mostly absent in Dgat–/– mice with leptin deficiency, indicating an unexpected role for leptin in modulating the skin phenotype. Our findings indicate that DGAT1 plays an important role in normal fur and sebaceous gland physiology and provide evidence that leptin modulates these processes in the skin.
Calorie restriction results in leanness, which is linked to metabolic conditions that favor longevity. We show here that deficiency of the triglyceride synthesis enzyme acyl CoA:diacylglycerol acyltransferase 1 (DGAT1), which promotes leanness, also extends longevity without limiting food intake. Female DGAT1-deficient mice were protected from age-related increases in body fat, tissue triglycerides, and inflammation in white adipose tissue. This protection was accompanied by increased mean and maximal life spans of ~25% and ~10%, respectively. Middle-aged Dgat1−/− mice exhibited several features associated with longevity, including decreased levels of circulating insulin growth factor 1 (IGF1) and reduced fecundity. Thus, deletion of DGAT1 in mice provides a model of leanness and extended lifespan that is independent of calorie restriction.
DGAT1; adipose tissue; longevity; triglycerides; calorie restriction
The class IA phosphatidylinsositol 3-kinases (PI3Ks) form a critical node in the insulin metabolic pathway; however, the precise roles of the different isoforms of this enzyme remain elusive. Using tissue-specific gene inactivation, we demonstrate that p110α catalytic subunit of PI3K is a key mediator of insulin metabolic actions in the liver. Thus, deletion of p110α in liver results in markedly blunted insulin signaling with decreased generation of PIP3 and loss of insulin activation of Akt, defects that could not be rescued by overexpression of p110β. As a result, mice with hepatic knockout of p110α display reduced insulin sensitivity, impaired glucose tolerance, and increased gluconeogenesis, hypolipidemia, and hyperleptinemia. The diabetic syndrome induced by loss of p110α in liver did not respond to metformin treatment. Together, these data indicate that the p110α isoform of PI3K plays a fundamental role in insulin signaling and control of hepatic glucose and lipid metabolism.
Nonalcoholic fatty liver disease, characterized by accumulation of triacylglycerols (TG) and other lipids in the liver, often accompanies obesity and is a risk factor for nonalcoholic steatohepatitis and fibrosis. To treat or prevent fatty liver, a thorough understanding of hepatic fatty acid and TG metabolism is crucial. To investigate the role of acyl CoA:diacylglycerol acyltransferase 1 (DGAT1), a key enzyme of TG synthesis, in fatty liver development, we studied mice with global and liver-specific knockout of Dgat1. DGAT1 was required for hepatic steatosis induced by high-fat diet and prolonged fasting, which are both characterized by delivery of exogenous fatty acids to the liver. Studies in primary hepatocytes showed that DGAT1 deficiency protected against hepatic steatosis by reducing synthesis and increasing the oxidation of fatty acids. In contrast, lipodystrophy (aP2-SREBP-1c436) and liver X receptor activation (T0901317), which increase de novo fatty acid synthesis in liver, caused steatosis independently of DGAT1. Pharmacologic inhibition of Dgat1 with antisense oligonucleotides protected against fatty liver induced by a high-fat diet. In conclusion, our findings identify a specific role for hepatic DGAT1 in esterification of exogenous fatty acids and indicate that DGAT1 contributes to hepatic steatosis induced by this mechanism.
Triglyceride synthesis; obesity; diabetes; DGAT; fatty liver