Phenotypic comparison of common mouse strains developing high-fat diet-induced hepatosteatosis

Seasonal light hours modulate peripheral clocks and energy metabolism in mice

Except for latitudes close to the equator, seasonal variation in light hours can change dramatically between summer and winter. Yet investigations into the interplay between energy metabolism and circadian rhythms typically use a 12 h light:12 h dark photoperiod corresponding to the light duration at the equator. We hypothesized that altering the seasonal photoperiod affects both the rhythmicity of peripheral tissue clocks and energy homeostasis. Mice were housed at photoperiods representing either light hours in summer, winter, or the equinox. Mice housed at a winter photoperiod exhibited an increase in the amplitude of rhythmic lipid metabolism and a modest reduction in fat mass and liver triglyceride content. Comparing melatonin-proficient and -deficient mice, the effect of seasonal light on energy metabolism was largely driven by differences in the rhythmicity of food intake and not melatonin. Together, these data indicate that seasonal light impacts energy metabolism by modulating the timing of eating.

Mammalian models of diabetes mellitus, with a focus on type 2 diabetes mellitus

Although no single animal model replicates all aspects of diabetes mellitus in humans, animal models are essential for the study of energy balance and metabolism control as well as to investigate the reasons for their imbalance that could eventually lead to overt metabolic diseases such as type 2 diabetes mellitus. The most frequently used animal models in diabetes mellitus research are small rodents that harbour spontaneous genetic mutations or that can be manipulated genetically or by other means to influence their nutrient metabolism and nutrient handling. Non-rodent species, including pigs, cats and dogs, are also useful models in diabetes mellitus research. This Review will outline the advantages and disadvantages of selected animal models of diabetes mellitus to build a basis for their most appropriate use in biomedical research.

Metabolic differences and differentially expressed genes between C57BL/6J and C57BL/6N mice substrains

C57BL/6J (B6J) and C57BL/6N (B6N) mice are the most frequently used substrains in C57BL/6 (B6) inbred mice, serving as physiological models for in vivo studies and as background strains to build transgenic mice. However, the differences in metabolic phenotypes between B6J and B6N mice are not coherent, and genotypic differences in metabolically important tissues have not been well studied. The phenotypic differences between B6J and B6N substrains have often been attributed to the role of the nicotinamide nucleotide transhydrogenase (Nnt) gene, whereby B6J has a spontaneous missense mutation of Nnt. Nevertheless, phenotypic differences between the two cannot be explained by Nnt mutations alone, especially in metabolic traits. Therefore, we aimed to investigate the genetic cause of the phenotypic differences between B6J and B6N mice. Determining consistent genetic differences across multiple tissues involved in metabolic traits such as subcutaneous and visceral white adipose tissues, brown adipose tissue, skeletal muscle, liver, hypothalamus, and hippocampus, may help explain phenotypic differences in metabolism between the two substrains. We report candidate genes along with comparative data on body weight, tissue weight, blood components involved in metabolism, and energy balance of B6J and B6N mice. Insulin degrading enzyme, adenylosuccinate synthase 2, and ectonucleotide triphosphate diphosphohydrolase 4 were highly expressed in B6J mice compared with those in B6N mice, and Nnt, WD repeat and FYVE domain containing 1, and dynein light chain Tctex-type 1 were less expressed in B6J mice compared with those in B6N mice in all seven tissues. Considering the extremely wide use of both substrains and their critical importance in generating transgenic and knock-out models, these findings guide future research across several interrelated fields.

Normal Pregnancy-Induced Islet Beta Cell Proliferation in Mouse Models That Are Deficient in Serotonin-Signaling

During mouse pregnancy placental lactogens stimulate prolactin receptors on pancreatic islet beta cells to induce expression of the tryptophan hydroxylase Tph1, resulting in the synthesis and secretion of serotonin. Presently, the functional relevance of this phenomenon is unclear. One hypothesis is that serotonin-induced activation of 5-HT_2B receptors on beta cells stimulates beta cell proliferation during pregnancy. We tested this hypothesis via three different mouse models: (i) total Tph1KO mice, (ii) 129P2/OlaHsd mice, which are incompetent to upregulate islet Tph1 during pregnancy, whereas Tph1 is normally expressed in the intestine, mammary glands, and placenta, and (iii) Htr2b-deficient mice. We observed normal pregnancy-induced levels of beta cell proliferation in total Tph1KO mice, 129P2/OlaHsd mice, and in Htr2b^-/- mice. The three studied mouse models indicate that islet serotonin production and its signaling via 5-HT_2B receptors are not required for the wave of beta cell proliferation that occurs during normal mouse pregnancy.

Hepatobiliary Thyroid Hormone Deficiency Impacts Bile Acid Hydrophilicity and Aquaporins in Cholestatic C57BL/6J Mice

Women are more prone to develop either hypothyroidism or cholesterol gallstones than men. However, a male predominance in cholesterol gallstones under hypothyroidism was reported. Recently, a novel pathogenic link between thyroid hormone (TH) deficiency and cholesterol gallstones has been described in male mice. Here, we investigate if TH deficiency impacts cholesterol gallstone formation in females by the same mechanism. Three-month-old C57BL/6J mice were randomly divided into a control, a TH deficient, a lithogenic, and a lithogenic + TH deficient group and diet-treated for two, four, and six weeks. Gallstone prevalence, liver function tests, bile composition, hepatic gene expression, and gallbladder aquaporin expression and localization were investigated. Cholesterol gallstones were observed in lithogenic + TH deficient but not lithogenic only female mice. Diminished hydrophilicity of primary bile acids due to decreased gene expression of hepatic detoxification phase II enzymes was observed. A sex-specific expression and localization of hepatobiliary aquaporins involved in transcellular water and glycerol permeability was observed under TH deficient and lithogenic conditions. TH deficiency promotes cholesterol gallstone formation in female C57BL/6J mice by the same mechanism as observed in males. However, cholesterol gallstone prevalence was lower in female than male C57BL/6J mice. Interestingly, the sex-specific expression and localization of hepatobiliary aquaporins could protect female C57BL/6J mice to cholestasis and could reduce biliary water transport in male C57BL/6J mice possibly contributing to the sex-dependent cholesterol gallstone prevalence under TH deficiency.

Thyroid hormones as a disease modifier and therapeutic target in nonalcoholic steatohepatitis

INTRODUCTION

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease worldwide and closely interconnected to the metabolic syndrome. Liver-specific and systemic signaling pathways orchestrating glucose and fatty acid metabolism contribute to intrahepatic accumulation of lipids and inflammatory processes eventually causing disease progression to nonalcoholic steatohepatitis (NASH), liver fibrosis, and cirrhosis. Since a high number of key regulatory genes regarding liver homeostasis are directly mediated via thyroid hormone (TH) signaling, targeting TH receptors (TRs) represent a promising therapeutic potential for the treatment of NAFLD.

AREAS COVERED

In this review, we elucidate the effects of TH on metabolic regulations in the liver via local availability and actions. We discuss recent advances and the potential impact of thyromimetics in basic research and clinical trials including liver-targeted and TRβ-specific agents for the treatment of NAFLD.

EXPERT OPINION

Unselective TR targeting can be accompanied by negative side effects due to high TRβ expression in other organs and TRα-mediated effects. Recent advances in drug development and the introduction of liver-targeted thyromimetics selectively activating TRβ such as Resmetirom (MGL-3196) and VK2809 bring new hope of translating the knowledge on local TH effects into effective hepatic lipid-clearing therapies against NASH.

Multi-Omics Approach Reveals Dysregulation of Protein Phosphorylation Correlated with Lipid Metabolism in Mouse Non-Alcoholic Fatty Liver

Obesity caused by overnutrition is a major risk factor for non-alcoholic fatty liver disease (NAFLD). Several lipid intermediates such as fatty acids, glycerophospholipids and sphingolipids are implicated in NAFLD, but detailed characterization of lipids and their functional links to proteome and phosphoproteome remain to be elucidated. To characterize this complex molecular relationship, we used a multi-omics approach by conducting comparative proteomic, phoshoproteomic and lipidomic analyses of high fat (HFD) and low fat (LFD) diet fed mice livers. We quantified 2447 proteins and 1339 phosphoproteins containing 1650 class I phosphosites, of which 669 phosphosites were significantly different between HFD and LFD mice livers. We detected alterations of proteins associated with cellular metabolic processes such as small molecule catabolic process, monocarboxylic acid, long- and medium-chain fatty acid, and ketone body metabolic processes, and peroxisome organization. We observed a significant downregulation of protein phosphorylation in HFD fed mice liver in general. Untargeted lipidomics identified upregulation of triacylglycerols, glycerolipids and ether glycerophosphocholines and downregulation of glycerophospholipids, such as lysoglycerophospholipids, as well as ceramides and acylcarnitines. Analysis of differentially regulated phosphosites revealed phosphorylation dependent deregulation of insulin signaling as well as lipogenic and lipolytic pathways during HFD induced obesity. Thus, this study reveals a molecular connection between decreased protein phosphorylation and lipolysis, as well as lipid-mediated signaling in diet-induced obesity.

Key hepatic signatures of human and mouse nonalcoholic steatohepatitis: A transcriptome-proteome data meta-analysis

BACKGROUND

Despite the global prevalence of nonalcoholic fatty liver disease (NAFLD), its pathophysiology remains unclear. In this study, we established highly confident nonalcoholic steatohepatitis (NASH) gene signatures and evaluated the pathological mechanisms underlying NASH through a systematic meta-analysis of transcriptome and proteome datasets obtained from NASH patients and mouse models.

METHODS

We analyzed NASH transcriptome datasets from 539 patients and 99 mice. A whole-liver tissue proteome dataset was used to confirm the protein level dysregulation of NASH signatures significant in both humans and mice.

RESULTS

In total, 254 human and 1,917 mouse NASH gene signatures were established. Up-regulated genes of 254 human signatures were associated with inflammation, steatosis, apoptosis, and extracellular matrix organization, whereas down-regulated genes were associated with response to metal ions and lipid and amino acid metabolism. When different mouse models were compared against humans, models with high fat and high fructose diet most closely resembled the genetic features of human NAFLD. Cross-species analysis revealed 66 genes that were concordantly dysregulated between human and mouse NASH. Among these, 14 genes were further validated to be dysregulated at the protein level. The resulting 14 genes included some of the well-established NASH associated genes and a promising NASH drug target. Functional enrichment analysis revealed that dysregulation of amino acid metabolism was the most significant hepatic perturbation in both human and mouse NASH.

CONCLUSIONS

We established the most comprehensive hepatic gene signatures for NASH in humans and mice to date. To the best of our knowledge, this is the first study to collectively analyze the common signatures between human and mouse NASH on a transcriptome-proteome scale.

Propensity to endoplasmic reticulum stress in deer mouse fibroblasts predicts skin inflammation and body weight gain

The unfolded protein response (UPR) is involved in the pathogenesis of metabolic disorders, yet whether variations in the UPR among individuals influence the propensity for metabolic disease remains unexplored. Using outbred deer mice as a model, we show that the intensity of UPR in fibroblasts isolated early in life predicts the extent of body weight gain after high-fat diet (HFD) administration. Contrary to those with intense UPR, animals with moderate UPR in fibroblasts and therefore displaying compromised stress resolution did not gain body weight but developed inflammation, especially in the skin, after HFD administration. Fibroblasts emerged as potent modifiers of this differential responsiveness to HFD, as indicated by the comparison of the UPR profiles of fibroblasts responding to fatty acids in vitro, by correlation analyses between UPR and proinflammatory cytokine-associated transcriptomes, and by BiP (also known as HSPA5) immunolocalization in skin lesions from animals receiving HFD. These results suggest that the UPR operates as a modifier of an individual's propensity for body weight gain in a manner that, at least in part, involves the regulation of an inflammatory response by skin fibroblasts. This article has an associated First Person interview with the first author of the paper.

Nicotinamide riboside has minimal impact on energy metabolism in mouse models of mild obesity

Supplementation with precursors of NAD has been shown to prevent and reverse insulin resistance, mitochondrial dysfunction, and liver damage in mouse models of diet-induced obesity. We asked whether the beneficial effects of supplementation with the NAD precursor nicotinamide riboside (NR) are dependent on mouse strain. We compared the effects of NR supplementation on whole-body energy metabolism and mitochondrial function in mildly obese C57BL/6N and C57BL/6J mice, two commonly used strains to investigate metabolism. Male C57BL/6N and C57BL/6J mice were fed a high-fat diet (HFD) or standard chow with or without NR supplementation for 8 weeks. Body and organ weights, glucose tolerance, and metabolic parameters as well as mitochondrial O2 flux in liver and muscle fibers were assessed. We found that NR supplementation had no influence on body or organ weight, glucose metabolism or hepatic lipid accumulation, energy expenditure, or metabolic flexibility but increased mitochondrial respiration in soleus muscle in both mouse strains. Strain-dependent differences were detected for body and fat depot weight, fasting blood glucose, hepatic lipid accumulation, and energy expenditure. We conclude that, in mild obesity, NR supplementation does not alter metabolic phenotype in two commonly used laboratory mouse strains.

Hypothyroidism Increases Cholesterol Gallstone Prevalence in Mice by Elevated Hydrophobicity of Primary Bile Acids

Background: Thyroid hormone (TH) deficiency has been associated with increased cholesterol gallstone prevalence. Hypothyroidism impacts hepatic lipid homeostasis, biliary secretion, gallbladder motility, and gallstone (LITH) gene expression, all potential factors contributing to cholesterol gallstone disease (CGD). However, how TH deficiency may lead to gallstone formation is still poorly understood. Therefore, we performed molecular studies in a CGD mouse model under lithogenic conditions and modulation of TH status. Methods: Male, three-month-old C57BL/6 mice were randomly divided into a control (euthyroid) group, a hypothyroid (hypo) group, a gallstone (litho) group, and a gallstone+hypothyroid (litho+hypo) group and were treated for 2, 4, and 6 weeks (n = 8/treatment period). Gallstone prevalence, biliary composition and cholesterol crystals, hepatic expression of genes participating in cholesterol, bile acid (BA), and phosphatidylcholine synthesis (Hmgcr, Cyp7a1, Pcyt1a), and canalicular transport (Abcg5, Bsep, Abcb4) were investigated. Results: Increased cholesterol gallstone prevalence was observed in hypothyroid mice under lithogenic diet after 4 and 6 weeks of treatment (4 weeks: 25% vs. 0%; 6 weeks: 75% vs. 37.5%). Interestingly, neither the composition of the three main biliary components, cholesterol, BAs, and phosphatidylcholine, nor the hepatic expression of genes involved in synthesis and transport could explain the differences in cholesterol gallstone formation in the mice. However, TH deficiency resulted in significantly increased hydrophobicity of primary BAs in bile. Furthermore, downregulation of hepatic sulfonation enzymes Papss2 and Sult2a8 as well as diminished biliary BA sulfate concentrations in mice were observed under hypothyroid conditions all contributing to a lithogenic biliary milieu as evidenced by microscopic cholesterol crystals and macroscopic gallstone formation. Conclusions: We describe a novel pathogenic link between TH deficiency and CGD and suggest that the increased hydrophobic character of biliary BAs due to the diminished expression of hepatic detoxification enzymes promotes cholesterol crystal precipitation and enhances cholesterol gallstone formation in the bile of hypothyroid mice.

Genetic Background but Not Intestinal Microbiota After Co-Housing Determines Hyperoxaluria-Related Nephrocalcinosis in Common Inbred Mouse Strains

Calcium oxalate (CaOx) crystal formation, aggregation and growth is a common cause of kidney stone disease and nephrocalcinosis-related chronic kidney disease (CKD). Genetically modified mouse strains are frequently used as an experimental tool in this context but observed phenotypes may also relate to the genetic background or intestinal microbiota. We hypothesized that the genetic background or intestinal microbiota of mice determine CaOx crystal deposition and thus the outcome of nephrocalcinosis. Indeed, Casp1^-/-, Cybb^-/- or Casp1^-/-/Cybb^-/- knockout mice on a 129/C57BL/6J (B6J) background that were fed an oxalate-rich diet for 14 days did neither encounter intrarenal CaOx crystal deposits nor nephrocalcinosis-related CKD. To test our assumption, we fed C57BL/6N (B6N), 129, B6J and Balb/c mice an oxalate-rich diet for 14 days. Only B6N mice displayed CaOx crystal deposits and developed CKD associated with tubular injury, inflammation and interstitial fibrosis. Intrarenal mRNA expression profiling of 64 known nephrocalcinosis-related genes revealed that healthy B6N mice had lower mRNA levels of uromodulin (Umod) compared to the other three strains. Feeding an oxalate-rich diet caused an increase in uromodulin protein expression and CaOx crystal deposition in the kidney as well as in urinary uromodulin excretion in B6N mice but not 129, B6J and Balb/c mice. However, backcrossing 129 mice on a B6N background resulted in a gradual increase in CaOx crystal deposits from F2 to F7, of which all B6N/129 mice from the 7^th generation developed CaOx-related nephropathy similar to B6N mice. Co-housing experiments tested for a putative role of the intestinal microbiota but B6N co-housed with 129 mice or B6N/129 (3^rd and 6^th generation) mice did not affect nephrocalcinosis. In summary, genetic background but not the intestinal microbiome account for strain-specific crystal formation and, the levels of uromodulin secretion may contribute to this phenomenon. Our results imply that only littermate controls of the identical genetic background strain are appropriate when performing knockout mouse studies in this context, while co-housing is optional.

Pierre CL, Macias‐Velasco JF, Hughes JW, Kunzmann M, Schmidt H, Wayhart JP, Lawson HA. Spontaneous restoration of functional β-cell mass in obese SM/J mice

Maintenance of functional β-cell mass is critical to preventing diabetes, but the physiological mechanisms that cause β-cell populations to thrive or fail in the context of obesity are unknown. High fat-fed SM/J mice spontaneously transition from hyperglycemic-obese to normoglycemic-obese with age, providing a unique opportunity to study β-cell adaptation. Here, we characterize insulin homeostasis, islet morphology, and β-cell function during SM/J's diabetic remission. As they resolve hyperglycemia, obese SM/J mice dramatically increase circulating and pancreatic insulin levels while improving insulin sensitivity. Immunostaining of pancreatic sections reveals that obese SM/J mice selectively increase β-cell mass but not α-cell mass. Obese SM/J mice do not show elevated β-cell mitotic index, but rather elevated α-cell mitotic index. Functional assessment of isolated islets reveals that obese SM/J mice increase glucose-stimulated insulin secretion, decrease basal insulin secretion, and increase islet insulin content. These results establish that β-cell mass expansion and improved β-cell function underlie the resolution of hyperglycemia, indicating that obese SM/J mice are a valuable tool for exploring how functional β-cell mass can be recovered in the context of obesity.

Susceptibility of male wild type mouse strains to antipsychotic-induced weight gain

While both men and women gain weight as a side effect of antipsychotic (AP) treatment, studies in mice have found only female mice are susceptible to weight gain. Therefore, to we set out to identify a strain of male mice that gain significant weight in response to APs which could better model AP-induced weight gain observed in humans. These studies determined that male Balb/c mice developed late onset olanzapine-induced weight gain. Patients often take APs for many years and thus understanding AP-mediated changes in food intake, energy expenditure and body weight regulation is particularly important.

Role of Sirt3 in Differential Sex-Related Responses to a High-Fat Diet in Mice

Metabolic homeostasis is differently regulated in males and females. Little is known about the mitochondrial Sirtuin 3 (Sirt3) protein in the context of sex-related differences in the development of metabolic dysregulation. To test our hypothesis that the role of Sirt3 in response to a high-fat diet (HFD) is sex-related, we measured metabolic, antioxidative, and mitochondrial parameters in the liver of Sirt3 wild-type (WT) and knockout (KO) mice of both sexes fed with a standard or HFD for ten weeks. We found that the combined effect of Sirt3 and an HFD was evident in more parameters in males (lipid content, glucose uptake, pparγ, cyp2e1, cyp4a14, Nrf2, MnSOD activity) than in females (protein damage and mitochondrial respiration), pointing towards a higher reliance of males on the effect of Sirt3 against HFD-induced metabolic dysregulation. The male-specific effects of an HFD also include reduced Sirt3 expression in WT and alleviated lipid accumulation and reduced glucose uptake in KO mice. In females, with a generally higher expression of genes involved in lipid homeostasis, either the HFD or Sirt3 depletion compromised mitochondrial respiration and increased protein oxidative damage. This work presents new insights into sex-related differences in the various physiological parameters with respect to nutritive excess and Sirt3.

Identification of hub genes and key pathways of dietary advanced glycation end products‑induced non‑alcoholic fatty liver disease by bioinformatics analysis and animal experiments

Non-alcoholic fatty liver disease (NAFLD) is a common chronic liver disease. Advanced glycation end products (AGEs) negatively affect the liver and accelerate NAFLD progression; however, the underlying mechanisms remain unclear. The present study aimed to examine the effect and mechanism of dietary AGEs on the mouse liver using bioinformatics and in vivo experimental approaches. Gene expression datasets associated with NAFLD were obtained from the Gene Expression Omnibus and differentially expressed genes (DEGs) were identified using GEO2R. Functional enrichment analyses were performed using the Database for Annotation, Visualization and Integrated Discovery and a protein-protein interaction network for the DEGs was constructed using the Search Tool for the Retrieval of Interacting Genes database. MCODE, a Cytoscape plugin, was subsequently used to identify the most significant module. The key genes involved were verified in a dietary AGE-induced non-alcoholic steatohepatitis (NASH) mouse model using reverse transcription-quantitative PCR (RT-qPCR). The 462 DEGs associated with NAFLD in the two datasets, of which 34 overlapping genes were found in two microarray datasets. Functional analysis demonstrated that the 34 DEGs were enriched in the ‘PPAR signaling pathway’, ‘central carbon metabolism in cancer’, and ‘cell adhesion molecules (CAMs)’. Moreover, four hub genes (cell death-inducing DFFA-like effector a, cell death-inducing DFFA-like effector c, fatty acid-binding protein 4 and perilipin 4) were identified from a protein-protein interaction network and were verified using RT-qPCR in a mouse model of NASH. The results suggested that AGEs and their receptor axis may be involved in NAFLD onset and/or progression. This integrative analysis identified candidate genes and pathways in NAFLD, as well as DEGs and hub genes related to NAFLD progression in silico and in vivo.

Effects of Propolis Extract and Propolis-Derived Compounds on Obesity and Diabetes: Knowledge from Cellular and Animal Models

Propolis is a natural product resulting from the mixing of bee secretions with botanical exudates. Since propolis is rich in flavonoids and cinnamic acid derivatives, the application of propolis extracts has been tried in therapies against cancer, inflammation, and metabolic diseases. As metabolic diseases develop relatively slowly in patients, the therapeutic effects of propolis in humans should be evaluated over long periods of time. Moreover, several factors such as medical history, genetic inheritance, and living environment should be taken into consideration in human studies. Animal models, especially mice and rats, have some advantages, as genetic and microbiological variables can be controlled. On the other hand, cellular models allow the investigation of detailed molecular events evoked by propolis and derivative compounds. Taking advantage of animal and cellular models, accumulating evidence suggests that propolis extracts have therapeutic effects on obesity by controlling adipogenesis, adipokine secretion, food intake, and energy expenditure. Studies in animal and cellular models have also indicated that propolis modulates oxidative stress, the accumulation of advanced glycation end products (AGEs), and adipose tissue inflammation, all of which contribute to insulin resistance or defects in insulin secretion. Consequently, propolis treatment may mitigate diabetic complications such as nephropathy, retinopathy, foot ulcers, and non-alcoholic fatty liver disease. This review describes the beneficial effects of propolis on metabolic disorders.

Diabetes Type 2 and Kisspeptin: Central and Peripheral Sex-Specific Actions

Kisspeptin (KP) plays a major role in the regulation of reproduction governed by the hypothalamic-pituitary-gonadal (HPG) axis. However, recent findings suggest that the KP system is present not only centrally (at the level of the hypothalamus), but also in the peripheral organs crucial for the control of metabolism. The KP system is sexually differentiated in the hypothalamus, and it is of particular interest to study whether sex-specific responses to type 2 diabetes (DM2) exist centrally and peripherally. As collection of data is limited in humans, animal models of DM2 are useful to understand crosstalk between metabolism and reproduction. Sex-specific variations in the KP system reported in animals suggest a need for the development of gender specific therapeutic strategies to treat DM2.

Vanin 1: Its Physiological Function and Role in Diseases

The enzyme vascular non-inflammatory molecule-1 (vanin 1) is highly expressed at gene and protein level in many organs, such as the liver, intestine, and kidney. Its major function is related to its pantetheinase activity; vanin 1 breaks down pantetheine in cysteamine and pantothenic acid, a precursor of coenzyme A. Indeed, its physiological role seems strictly related to coenzyme A metabolism, lipid metabolism, and energy production. In recent years, many studies have elucidated the role of vanin 1 under physiological conditions in relation to oxidative stress and inflammation. Vanin's enzymatic activity was found to be of key importance in certain diseases, either for its protective effect or as a sensitizer, depending on the diseased organ. In this review, we discuss the role of vanin 1 in the liver, kidney, intestine, and lung under physiological as well as pathophysiological conditions. Thus, we provide a more complete understanding and overview of its complex function and contribution to some specific pathologies.

Two-week administration of engineered Escherichia coli establishes persistent resistance to diet-induced obesity even without antibiotic pre-treatment

Adverse alterations in the composition of the gut microbiota have been implicated in the development of obesity and a variety of chronic diseases. Re-engineering the gut microbiota to produce beneficial metabolites is a potential strategy for treating these chronic diseases. N-acyl-phosphatidylethanolamines (NAPEs) are a family of bioactive lipids with known anti-obesity properties. Previous studies showed that administration of Escherichia coli Nissle 1917 (EcN) engineered with Arabidopsis thaliana NAPE synthase to produce NAPEs imparted resistance to obesity induced by a high-fat diet that persisted after ending their administration. In prior studies, mice were pre-treated with ampicillin prior to administering engineered EcN for 8 weeks in drinking water. If use of antibiotics and long-term administration are required for beneficial effects, implementation of this strategy in humans might be problematic. Studies were therefore undertaken to determine if less onerous protocols could still impart persistent resistance and sustained NAPE biosynthesis. Administration of engineered EcN for only 2 weeks without pre-treatment with antibiotics sufficed to establish persistent resistance. Sustained NAPE biosynthesis by EcN was required as antibiotic treatment after administration of the engineered EcN markedly attenuated its effects. Finally, heterologous expression of human phospholipase A/acyltransferase-2 (PLAAT2) in EcN provided similar resistance to obesity as heterologous expression of A. thaliana NAPE synthase, confirming that NAPEs are the bioactive mediator of this resistance.

In Vivo Rodent Models of Type 2 Diabetes and Their Usefulness for Evaluating Flavonoid Bioactivity

About 40% of the world’s population is overweight or obese and exist at risk of developing type 2 diabetes mellitus (T2D). Obesity is a leading pathogenic factor for developing insulin resistance (IR). It is well established that IR and a progressive decline in functional β-cell mass are hallmarks of developing T2D. In order to mitigate the global prevalence of T2D, we must carefully select the appropriate animal models to explore the cellular and molecular mechanisms of T2D, and to optimize novel therapeutics for their safe use in humans. Flavonoids, a group of polyphenols, have drawn great interest for their various health benefits, and have been identified in naturally occurring anti-diabetic compounds. Results from many clinical and animal studies demonstrate that dietary intake of flavonoids might prove helpful in preventing T2D. In this review, we discuss the currently available rodent animal models of T2D and analyze the advantages, the limitations of each T2D model, and highlight the potential anti-diabetic effects of flavonoids as well as the mechanisms of their actions.

A comparative analysis of hepatic pathological phenotypes in C57BL/6J and C57BL/6N mouse strains in non-alcoholic steatohepatitis models

C57BL/6J (BL6J) and C57BL/6N (BL6N) inbred substrains are most widely used to understand the pathological roles of target molecules in a variety of diseases, including non-alcoholic steatohepatitis (NASH), based on transgenic mouse technologies. There are notable differences in the metabolic phenotypes, including glucose tolerance, between the BL6J and BL6N substrains, but the phenotypic differences in NASH are still unknown. We performed a comparative analysis of the two mouse substrains to identify the pathological phenotypic differences in NASH models. In the CCl₄-induced NASH model, the BL6J mice exhibited a more severe degree of oxidative stress and fibrosis in the liver than the BL6N mice. In contrast, in the high-fat diet-induced NASH model, more accumulation of hepatic triglycerides but less weight gain and liver injury were noted in the BL6J mice than in the BL6N mice. Our findings strongly suggest caution be exercised with the use of unmatched mixed genetic background C57BL6 mice for studies related to NASH, especially when generating conditional knockout C57BL6 mice.

Non-proteolytic ubiquitin modification of PPARγ by Smurf1 protects the liver from steatosis

Nonalcoholic fatty liver disease (NAFLD) is characterized by abnormal accumulation of triglycerides (TG) in the liver and other metabolic syndrome symptoms, but its molecular genetic causes are not completely understood. Here, we show that mice deficient for ubiquitin ligase (E3) Smad ubiquitin regulatory factor 1 (Smurf1) spontaneously develop hepatic steatosis as they age and exhibit the exacerbated phenotype under a high-fat diet (HFD). Our data indicate that loss of Smurf1 up-regulates the expression of peroxisome proliferator-activated receptor γ (PPARγ) and its target genes involved in lipid synthesis and fatty acid uptake. We further show that PPARγ is a direct substrate of Smurf1-mediated non-proteolytic lysine 63 (K63)-linked ubiquitin modification that suppresses its transcriptional activity, and treatment of Smurf1-deficient mice with a PPARγ antagonist, GW9662, completely reversed the lipid accumulation in the liver. Finally, we demonstrate an inverse correlation of low SMURF1 expression to high body mass index (BMI) values in human patients, thus revealing a new role of SMURF1 in NAFLD pathogenesis.

Reduced expression of thyroid hormone receptor β in human nonalcoholic steatohepatitis

Hepatic thyroid hormone signaling has an important role in the development and progression of nonalcoholic steatohepatitis (NASH). While the systemic levels of thyroid hormone might remain stable, there is evidence that the intracellular signaling machinery consisting of transporters, deiodinases and receptors could be altered in NASH. However, clinical material from human liver biopsies of individuals with NASH has not been studied to date. In a cross-sectional study, we analyzed 85 liver biopsies from patients with different stages of NASH that underwent bariatric surgery. Using qPCR, we analyzed gene expression of thyroid hormone transporters NTCP (SLC10A1), MCT8 (SLC16A2) and OATP1C1 (SLCO1C1), thyroid hormone receptor α and β (THRA and THRB) and deiodinase type I, II and III (DIO1, DIO2, DIO3). The expression was correlated with serum TSH, triglyceride, HbA1c and NASH score and corrected for age or gender if required. While DIO2, DIO3 and SLCO1C1 were not expressed in human liver, we observed a significant negative correlation of THRB and DIO1 with age, and SLC16A2 with gender. THRB expression was also negatively associated with serum triglyceride levels and HbA1c. More importantly, its expression was inversely correlated with NASH score and further declined with age. Our data provide unique insight into the mRNA expression of thyroid hormone transporters, deiodinases and receptors in the human liver. The findings allow important conclusions on the intrahepatic mechanisms governing thyroid hormone action, indicating a possible tissue resistance to the circulating hormone in NASH, which becomes more prominent in advanced age.

Diet-Induced Dysbiosis and Genetic Background Synergize With Cystic Fibrosis Transmembrane Conductance Regulator Deficiency to Promote Cholangiopathy in Mice

The most typical expression of cystic fibrosis (CF)-related liver disease is a cholangiopathy that can progress to cirrhosis. We aimed to determine the potential impact of environmental and genetic factors on the development of CF-related cholangiopathy in mice. Cystic fibrosis transmembrane conductance regulator (Cftr)^-/- mice and Cftr ^+/+ littermates in a congenic C57BL/6J background were fed a high medium-chain triglyceride (MCT) diet. Liver histopathology, fecal microbiota, intestinal inflammation and barrier function, bile acid homeostasis, and liver transcriptome were analyzed in 3-month-old males. Subsequently, MCT diet was changed for chow with polyethylene glycol (PEG) and the genetic background for a mixed C57BL/6J;129/Ola background (resulting from three backcrosses), to test their effect on phenotype. C57BL/6J Cftr ^-/- mice on an MCT diet developed cholangiopathy features that were associated with dysbiosis, primarily Escherichia coli enrichment, and low-grade intestinal inflammation. Compared with Cftr ^+/+ littermates, they displayed increased intestinal permeability and a lack of secondary bile acids together with a low expression of ileal bile acid transporters. Dietary-induced (chow with PEG) changes in gut microbiota composition largely prevented the development of cholangiopathy in Cftr ^-/- mice. Regardless of Cftr status, mice in a mixed C57BL/6J;129/Ola background developed fatty liver under an MCT diet. The Cftr ^-/- mice in the mixed background showed no cholangiopathy, which was not explained by a difference in gut microbiota or intestinal permeability, compared with congenic mice. Transcriptomic analysis of the liver revealed differential expression, notably of immune-related genes, in mice of the congenic versus mixed background. In conclusion, our findings suggest that CFTR deficiency causes abnormal intestinal permeability, which, combined with diet-induced dysbiosis and immune-related genetic susceptibility, promotes CF-related cholangiopathy.

Metabolic syndrome and extensive adipose tissue inflammation in morbidly obese Göttingen minipigs

OBJECTIVE

The worldwide prevalence of obesity has increased to 10% in men and 15% in women and is associated with severe comorbidities such as diabetes, cancer, and cardiovascular disease. Animal models of obesity are central to experimental studies of disease mechanisms and therapeutic strategies. Diet-induced obesity (DIO) models in rodents have provided important insights into the pathophysiology of obesity and, in most instances, are the first in line for exploratory pharmacology studies. To deepen the relevance towards translation to human patients, we established a corresponding DIO model in Göttingen minipigs (GM).

METHODS

Young adult female ovariectomized GM were fed a high-fat/high-energy diet for a period of 70 weeks. The ration was calculated to meet the requirements and maintain body weight (BW) of lean adult minipigs (L-GM group) or increased stepwise to achieve an obese state (DIO-GM group). Body composition, blood parameters and intravenous glucose tolerance were determined at regular intervals. A pilot chronic treatment trial with a GLP1 receptor agonist was conducted in DIO-GM. At the end of the study, the animals were necropsied and a biobank of selected tissues was established.

RESULTS

DIO-GM developed severe subcutaneous and visceral adiposity (body fat >50% of body mass vs. 22% in L-GM), increased plasma cholesterol, triglyceride, and free fatty acid levels, insulin resistance (HOMA-IR >5 vs. 2 in L-GM), impaired glucose tolerance and increased heart rate when resting and active. However, fasting glucose concentrations stayed within normal range throughout the study. Treatment with a long-acting GLP1 receptor agonist revealed substantial reduction of food intake and body weight within four weeks, with increased drug sensitivity relative to observations in other DIO animal models. Extensive adipose tissue inflammation and adipocyte necrosis was observed in visceral, but not subcutaneous, adipose tissue of DIO-GM.

CONCLUSIONS

The Munich DIO-GM model resembles hallmarks of the human metabolic syndrome with extensive adipose tissue inflammation and adipocyte necrosis reported for the first time. DIO-GM may be used for evaluating novel treatments of obesity and associated comorbidities. They may help to identify triggers and mechanisms of fat tissue inflammation and mechanisms preventing complete metabolic decompensation despite morbid obesity.

Short-Term Fish Oil Treatment Changes the Composition of Phospholipids While Not Affecting the Expression of Mfsd2a Omega-3 Transporter in the Brain and Liver of the 5xFAD Mouse Model of Alzheimer's Disease

Long-term fish oil (FO) supplementation is able to improve Alzheimer’s disease (AD) pathology. We aimed to determine the impact of short-term fish oil (FO) intake on phospholipids composition and plaque pathology in 5xFAD mice, a widely used animal model of AD. A 3-week-long FO supplementation administered at 3 months of age decreased the number of dense core plaques in the 5xFAD cortex and changed phospholipids in the livers and brains of wild-type (Wt) and 5xFAD mice. Livers of both genotypes responded by increase of n-3 and reciprocal decrease of n-6 fatty acids. In Wt brains, FO supplementation induced elevation of n-3 fatty acids and subsequent enhancement of n-6/n-3 ratio. However, in 5xFAD brains the improved n-6/n-3 ratio was mainly due to FO-induced decrease in arachidonic and adrenic n-6 fatty acids. Also, brain and liver abundance of n-3 fatty acids were strongly correlated in Wts, oppositely to 5xFADs where significant brain-liver correlation exists only for n-6 fatty acids. Expression of omega-3 transporter Mfs2a remained unchanged after FO supplementation. We have demonstrated that even a short-term FO intake improves the phospholipid composition and has a significant effect on plaque burden in 5xFAD brains when applied in early stages of AD pathology.

Animal models of obesity and diabetes mellitus

More than one-third of the worldwide population is overweight or obese and therefore at risk of developing type 2 diabetes mellitus. In order to mitigate this pandemic, safer and more potent therapeutics are urgently required. This necessitates the continued use of animal models to discover, validate and optimize novel therapeutics for their safe use in humans. In order to improve the transition from bench to bedside, researchers must not only carefully select the appropriate model but also draw the right conclusions. In this Review, we consolidate the key information on the currently available animal models of obesity and diabetes and highlight the advantages, limitations and important caveats of each of these models.

Regulation of fatty acid trafficking in liver by thioesterase superfamily member 1

Thioesterase superfamily member 1 (Them1) is an acyl-CoA thioesterase that is highly expressed in brown adipose tissue, where it functions to suppress energy expenditure. Lower Them1 expression levels in the liver are upregulated in response to high-fat feeding. Them1-/- mice are resistant to diet-induced obesity, hepatic steatosis, and glucose intolerance, but the contribution of Them1 in liver is unclear. To examine its liver-specific functions, we created conditional transgenic mice, which, when bred to Them1-/- mice and activated, expressed Them1 exclusively in the liver. Mice with liver-specific Them1 expression exhibited no changes in energy expenditure. Rates of fatty acid oxidation were increased, whereas hepatic VLDL triglyceride secretion rates were decreased by hepatic Them1 expression. When fed a high-fat diet, Them1 expression in liver promoted excess steatosis in the setting of reduced rates of fatty acid oxidation and preserved glycerolipid synthesis. Liver-specific Them1 expression did not influence glucose tolerance or insulin sensitivity, but did promote hepatic gluconeogenesis in high-fat-fed animals. This was attributable to the generation of excess fatty acids, which activated PPARα and promoted expression of gluconeogenic genes. These findings reveal a regulatory role for Them1 in hepatocellular fatty acid trafficking.

Sulfonolipids as novel metabolite markers of Alistipes and Odoribacter affected by high-fat diets

The gut microbiota generates a huge pool of unknown metabolites, and their identification and characterization is a key challenge in metabolomics. However, there are still gaps on the studies of gut microbiota and their chemical structures. In this investigation, an unusual class of bacterial sulfonolipids (SLs) is detected in mouse cecum, which was originally found in environmental microbes. We have performed a detailed molecular level characterization of this class of lipids by combining high-resolution mass spectrometry and liquid chromatography analysis. Eighteen SLs that differ in their capnoid and fatty acid chain compositions were identified. The SL called “sulfobacin B” was isolated, characterized, and was significantly increased in mice fed with high-fat diets. To reveal bacterial producers of SLs, metagenome analysis was acquired and only two bacterial genera, i.e., Alistipes and Odoribacter, were revealed to be responsible for their production. This knowledge enables explaining a part of the molecular complexity introduced by microbes to the mammalian gastrointestinal tract and can be used as chemotaxonomic evidence in gut microbiota.

Brain Control of Plasma Cholesterol Involves Polysialic Acid Molecules in the Hypothalamus

The polysialic acid (PSA) is a large glycan that is added to cell-surface proteins during their post-translational maturation. In the brain, PSA modulates distances between cells and controls the plasticity of the nervous system. In the hypothalamus, PSA is involved in many aspects of energy balance including food intake, osmoregulation, circadian rhythm, and sleep. In this work, we investigated the role of hypothalamic PSA in the regulation of plasma cholesterol levels and distribution. We report that HFD consumption in mice rapidly increased plasma cholesterol, including VLDL, LDL, and HDL-cholesterol. Although plasma VLDL-cholesterol was normalized within the first week, LDL and HDL were still elevated after 2 weeks upon HFD. Importantly, we found that hypothalamic PSA removal aggravated LDL elevation and reduced HDL levels upon HFD. These results indicate that hypothalamic PSA controls plasma lipoprotein profile by circumventing the rise of LDL-to-HDL cholesterol ratio in plasma during overfeeding. Although mechanisms by which hypothalamic PSA controls plasma cholesterol homeostasis remains to be elucidated, these findings also suggest that low level of hypothalamic PSA might be a risk factor for dyslipidemia and cardiovascular diseases.

Calcium Reduces Liver Injury in Mice on a High-Fat Diet: Alterations in Microbial and Bile Acid Profiles

A high-fat “Western-style” diet (HFWD) promotes obesity-related conditions including non-alcoholic steatohepatitis (NASH), the histologic manifestation of non-alcoholic fatty liver disease (NAFLD). In addition to high saturated fat and processed carbohydrates, the typical HFWD is deficient in calcium. Calcium-deficiency is an independent risk factor for many conditions associated with the Western-style diet. However, calcium has not been widely evaluated in the context of NAFLD. The goal of the present study was to determine if dietary calcium supplementation could protect mice fed a HFWD from NAFLD, specifically by decreasing non-alcoholic steatohepatitis (NASH) and its down-stream consequences. Male C57BL/6NCrl mice were maintained for 18-months on a HFWD containing dietary calcium at either 0.41 gm/kg feed (unsupplemented) or 5.25 gm/kg feed (supplemented). Although there was no difference in body weight or steatosis, calcium-supplemented mice were protected against downstream consequences of hepatic steatosis, manifested by lower inflammation, less fibrosis, and by lower overall histologic NAFLD activity scores (NAS). Calcium supplementation correlated with distinctly segregating gut fecal and cecal microbial communities as defined by 16S rRNA gene sequence. Further, calcium supplementation also correlated with decreased hepatic concentration of the major conjugated murine primary bile acid, tauro-β-muricholic acid (as well as a decrease in the parent unconjugated bile acid). Thus, calcium was protective against progression of diet-induced hepatic steatosis to NASH and end-stage liver disease, suggesting that calcium supplementation may effectively protect against adverse hepatic consequences of HFWD in cases where overall diet modification cannot be sustained. This protective effect occurred in concert with calcium-mediated gut microbial community shifts and alterations of the hepatic bile acid pool.

Differential Insulin Secretion of High-Fat Diet-Fed C57BL/6NN and C57BL/6NJ Mice: Implications of Mixed Genetic Background in Metabolic Studies

Many metabolic studies employ tissue-specific gene knockout mice, which requires breeding of floxed gene mice, available mostly on C57BL/6N (NN) genetic background, with cre or Flp recombinase-expressing mice, available on C57BL/6J (JJ) background, resulting in the generation of mixed C57BL/6NJ (NJ) genetic background mice. Recent awareness of many genetic differences between NN and JJ strains including the deletion of nicotinamide nucleotide transhydrogenase (nnt), necessitates examination of the consequence of mixed NJ background on glucose tolerance, beta cell function and other metabolic parameters. Male mice with NN and NJ genetic background were fed with normal or high fat diets (HFD) for 12 weeks and glucose and insulin homeostasis were studied. Genotype had no effect on body weight and food intake in mice fed normal or high fat diets. Insulinemia in the fed and fasted states and after a glucose challenge was lower in HFD-fed NJ mice, even though their glycemia and insulin sensitivity were similar to NN mice. NJ mice showed mild glucose intolerance. Moreover, glucose- but not KCl-stimulated insulin secretion in isolated islets was decreased in HFD-fed NJ vs NN mice without changes in insulin content and beta cell mass. Under normal diet, besides reduced fed insulinemia, NN and NJ mice presented similar metabolic parameters. However, HFD-fed NJ mice displayed lower fed and fasted insulinemia and glucose-induced insulin secretion in vivo and ex vivo, as compared to NN mice. These results strongly caution against using unmatched mixed genetic background C57BL/6 mice for comparisons, particularly under HFD conditions.

Susceptibility of Different Mouse Wild Type Strains to Develop Diet-Induced NAFLD/AFLD-Associated Liver Disease

Although non-alcoholic and alcoholic fatty liver disease have been intensively studied, concerning pathophysiological mechanisms are still incompletely understood. This may be due to the use of different animal models and resulting model-associated variation. Therefore, this study aimed to compare three frequently used wild type mouse strains in their susceptibility to develop diet-induced features of non-alcoholic/alcoholic fatty liver disease. Fatty liver disease associated clinical, biochemical, and histological features in C57BL/6, CD-1, and 129Sv WT mice were induced by (i) high-fat diet feeding, (ii) ethanol feeding only, and (iii) the combination of high-fat diet and ethanol feeding. Hepatic and subcutaneous adipose lipid profiles were compared in CD-1 and 129Sv mice. Additionally hepatic fatty acid composition was determined in 129Sv mice. In C57BL/6 mice dietary regimens resulted in heterogeneous hepatic responses, ranging from pronounced steatosis and inflammation to a lack of any features of fatty liver disease. Liver-related serum biochemistry showed high deviations within the regimen groups. CD-1 mice did not exhibit significant changes in metabolic and liver markers and developed no significant steatosis or inflammation as a response to dietary regimens. Although 129Sv mice showed no weight gain, this strain achieved most consistent features of fatty liver disease, apparent from concentration alterations of liver-related serum biochemistry as well as moderate steatosis and inflammation as a result of all dietary regimens. Furthermore, the hepatic lipid profile as well as the fatty acid composition of 129Sv mice were considerably altered, upon feeding the different dietary regimens. Accordingly, diet-induced non-alcoholic/alcoholic fatty liver disease is most consistently promoted in 129Sv mice compared to C57BL/6 and CD-1 mice. As a conclusion, this study demonstrates the importance of genetic background of used mouse strains for modeling diet-induced non-alcoholic/alcoholic fatty liver disease.

Genetic and pharmacological inhibition of vanin-1 activity in animal models of type 2 diabetes

Vanins are enzymes that convert pantetheine to pantothenic acid (vitamin B5). Insights into the function of vanins have evolved lately, indicating vanin-1 to play a role in inflammation, oxidative stress and cell migration. Moreover, vanin-1 has recently gained attention as a novel modulator of hepatic glucose and lipid metabolism. In the present study, we investigated the role of vanin-1 in the development of hepatic steatosis and insulin resistance in animal models of obesity and diabetes. In addition, we evaluated the potency of RR6, a novel pharmacological vanin-1 inhibitor, as an anti-diabetic drug. Increased vanin activity was observed in plasma and liver of high fat diet (HFD)-induced obese mice, as well as ZDF-diabetic rats. Ablation of vanin-1 (Vnn1^−/− mice) mildly improved glucose tolerance and insulin sensitivity in HFD-fed mice, but had no effects on body weight, hepatic steatosis or circulating lipid levels. Oral administration of RR6 for 8 days completely inhibited plasma vanin activity, but did not affect hepatic glucose production, insulin sensitivity or hepatic steatosis in ZDF-diabetes rats. In conclusion, absence of vanin-1 activity improves insulin sensitivity in HFD-fed animals, yet short-term inhibition of vanin activity may have limited value as an anti-diabetic strategy.

Effects of high fat diets on rodent liver bioenergetics and oxidative imbalance

Human metabolic diseases can be mimicked in rodents by using dietary interventions such as high fat diets (HFD). Nonalcoholic fatty liver disease (NAFLD) develops as a result of HFD and the disease may progress in a manner involving increased production of oxidants. The main intracellular source of these oxidants are mitochondria, which are also responsible for lipid metabolism and thus widely recognized as important players in the pathology and progression of steatosis. Here, we review publications that study redox and bioenergetic effects of HFD in the liver. We find that dietary composition and protocol implementations vary widely, as do the results of these dietary interventions. Overall, all HFD promote steatosis, changes in β-oxidation, generation and consequences of oxidants, while effects on body weight, insulin signaling and other bioenergetic parameters are more variable with the experimental models adopted. Our review provides a broad analysis of the bioenergetic and redox changes promoted by HFD as well as suggestions for changes and specifications in methodologies that may help explain apparent disparities in the current literature.

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High fat diets (HFDs) induce steatosis, even with no weight changes .

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HFDs activate β-oxidation.

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HFDs promote oxidative imbalance.

An Overview of Murine High Fat Diet as a Model for Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a worldwide epidemic, which by all predictions will only increase. To help in combating the devastating array of phenotypes associated with T2DM a highly reproducible and human disease-similar mouse model is required for researchers. The current options are genetic manipulations to cause T2DM symptoms or diet induced obesity and T2DM symptoms. These methods to model human T2DM have their benefits and their detractions. As far as modeling the majority of T2DM cases, HFD establishes the proper etiological, pathological, and treatment options. A limitation of HFD is that it requires months of feeding to achieve the full spectrum of T2DM symptoms and no standard protocol has been established. This paper will attempt to rectify the last limitation and argue for a standard group of HFD protocols and standard analysis procedures.

The genetic architecture of NAFLD among inbred strains of mice

To identify genetic and environmental factors contributing to the pathogenesis of non-alcoholic fatty liver disease, we examined liver steatosis and related clinical and molecular traits in more than 100 unique inbred mouse strains, which were fed a diet rich in fat and carbohydrates. A >30-fold variation in hepatic TG accumulation was observed among the strains. Genome-wide association studies revealed three loci associated with hepatic TG accumulation. Utilizing transcriptomic data from the liver and adipose tissue, we identified several high-confidence candidate genes for hepatic steatosis, including Gde1, a glycerophosphodiester phosphodiesterase not previously implicated in triglyceride metabolism. We confirmed the role of Gde1 by in vivo hepatic over-expression and shRNA knockdown studies. We hypothesize that Gde1 expression increases TG production by contributing to the production of glycerol-3-phosphate. Our multi-level data, including transcript levels, metabolite levels, and gut microbiota composition, provide a framework for understanding genetic and environmental interactions underlying hepatic steatosis.

The Epoxyeicosatrienoic Acid Pathway Enhances Hepatic Insulin Signaling and is Repressed in Insulin-Resistant Mouse Liver

Although it is widely accepted that ectopic lipid accumulation in the liver is associated with hepatic insulin resistance, the underlying molecular mechanisms have not been well characterized.

Here we employed time resolved quantitative proteomic profiling of mice fed a high fat diet to determine which pathways were affected during the transition of the liver to an insulin-resistant state. We identified several metabolic pathways underlying altered protein expression. In order to test the functional impact of a critical subset of these alterations, we focused on the epoxyeicosatrienoic acid (EET) eicosanoid pathway, whose deregulation coincided with the onset of hepatic insulin resistance. These results suggested that EETs may be positive modulators of hepatic insulin signaling. Analyzing EET activity in primary hepatocytes, we found that EETs enhance insulin signaling on the level of Akt. In contrast, EETs did not influence insulin receptor or insulin receptor substrate-1 phosphorylation. This effect was mediated through the eicosanoids, as overexpression of the deregulated enzymes in absence of arachidonic acid had no impact on insulin signaling. The stimulation of insulin signaling by EETs and depression of the pathway in insulin resistant liver suggest a likely role in hepatic insulin resistance. Our findings support therapeutic potential for inhibiting EET degradation.

Variation in Type 2 Diabetes-Related Phenotypes among Apolipoprotein E-Deficient Mouse Strains

We recently have found that apolipoprotein E-deficient (Apoe(-/-)) mice with the C57BL/6 background develop type 2 diabetes when fed a Western diet for 12 weeks. In the present study we constructed multiple Apoe(-/-) mouse strains to find diabetes-related phenotyptic variations that might be linked to atherosclerosis development. Evaluation of both early and advanced lesion formation in aortic root revealed that C57BL/6, SWR/J, and SM/J Apoe(-/-) mice were susceptible to atherosclerosis and that C3H/HeJ and BALB/cJ Apoe(-/-) mice were relatively resistant. On a chow diet, fasting plasma glucose varied among strains with C3H/HeJ having the highest (171.1 ± 9.7 mg/dl) and BALB/cJ the lowest level (104.0 ± 6.6 mg/dl). On a Western diet, fasting plasma glucose rose significantly in all strains, with C57BL/6, C3H/HeJ and SWR/J exceeding 250 mg/dl. BALB/cJ and C3H/HeJ were more tolerant to glucose loading than the other 3 strains. C57BL/6 was sensitive to insulin while other strains were not. Non-fasting blood glucose was significantly lower in C3H/HeJ and BALB/cJ than C57BL/6, SM/J, and SWR/J. Glucose loading induced the 1st and the 2nd phase of insulin secretion in BALB/cJ, but the 2nd phase was not observed in other strains. Morphological analysis showed that BALB/cJ had the largest islet area (1,421,493 ± 61,244 μm(2)) and C57BL/6 had the smallest one (747,635 ± 41,798 μm(2)). This study has demonstrated strain-specific variations in the metabolic and atherosclerotic phenotypes, thus laying the basis for future genetic characterization.

Genetic manipulation of cardiac Hsp72 levels does not alter substrate metabolism but reveals insights into high-fat feeding-induced cardiac insulin resistance

Heat shock protein 72 (Hsp72) protects cells against a variety of stressors, and multiple studies have suggested that Hsp72 plays a cardioprotective role. As skeletal muscle Hsp72 overexpression can protect against high-fat diet (HFD)-induced insulin resistance, alterations in substrate metabolism may be a mechanism by which Hsp72 is cardioprotective. We investigated the impact of transgenically overexpressing (Hsp72 Tg) or deleting Hsp72 (Hsp72 KO) on various aspects of cardiac metabolism. Mice were fed a normal chow (NC) or HFD for 12 weeks from 8 weeks of age to examine the impact of diet-induced obesity on metabolic parameters in the heart. The HFD resulted in an increase in cardiac fatty acid oxidation and a decrease in cardiac glucose oxidation and insulin-stimulated cardiac glucose clearance; however, there was no difference in Hsp72 Tg or Hsp72 KO mice in these rates compared with their respective wild-type control mice. Although HFD-induced cardiac insulin resistance was not rescued in the Hsp72 Tg mice, it was preserved in the skeletal muscle, suggesting tissue-specific effects of Hsp72 overexpression on substrate metabolism. Comparison of two different strains of mice (BALB/c vs. C57BL/6J) also identified strain-specific differences in regard to HFD-induced cardiac lipid accumulation and insulin resistance. These strain differences suggest that cardiac lipid accumulation can be dissociated from cardiac insulin resistance. Our study finds that genetic manipulation of Hsp72 does not lead to alterations in metabolic processes in cardiac tissue under resting conditions, but identifies mouse strain-specific differences in cardiac lipid accumulation and insulin-stimulated glucose clearance.

Dietary effects on liver tumor burden in mice treated with the hepatocellular carcinogen diethylnitrosamine

BACKGROUND & AIMS

Mice exposed to the hepatocellular carcinogen diethylnitrosamine at 2 weeks of age have a high risk of developing primary liver tumors later in life. Previous studies have demonstrated that diethylnitrosamine-treated mice have increased tumor burden when fed an obesigenic "Western" diet rich in lard fat and sugar. However, the role of dietary fats vs. sugars in the promotion of liver cancer is poorly understood. The aim of this study was to determine how altering dietary fats vs. sugars affects tumor burden in the diethylnitrosamine model.

METHODS

C57BL/6N mice were treated with diethylnitrosamine at 2 weeks of age and, from 6 to 32 weeks of age, fed one of five diets that differed in fat and sugar content, including normal chow, ketogenic, and Western diets.

RESULTS

Mice fed sugar-rich diets had the greatest tumor burden irrespective of dietary fat content. In contrast, mice fed a high-fat low-sugar diet had the least tumor burden despite obesity and glucose intolerance. When evaluated as independent variables, tumor burden was positively correlated with hepatic fat accumulation, postprandial insulin, and liver IL-6, and inversely correlated with serum adiponectin. In contrast, tumor burden did not correlate with adiposity, fasting insulin, or glucose intolerance. Furthermore, mice fed high sugar diets had lower liver expression of p21 and cleaved caspase-3 compared to mice fed low sugar diets.

CONCLUSIONS

These data indicate that dietary sugar intake contributes to liver tumor burden independent of excess adiposity or insulin resistance in mice treated with diethylnitrosamine.

Combining metabolomic non-targeted GC×GC-ToF-MS analysis and chemometric ASCA-based study of variances to assess dietary influence on type 2 diabetes development in a mouse model

Insulin resistance (IR) lies at the origin of type 2 diabetes. It induces initial compensatory insulin secretion until insulin exhaustion and subsequent excessive levels of glucose (hyperglycemia). A high-calorie diet is a major risk factor contributing to the development of this metabolic disease. For this study, a time-course experiment was designed that consisted of two groups of mice. The aim of this design was to reproduce the dietary conditions that parallel the progress of IR over time. The first group was fed with a high-fatty-acid diet for several weeks and followed by 1 week of a low-fatty-acid intake, while the second group was fed with a low-fatty-acid diet during the entire experiment. The metabolomic fingerprint of C3HeB/FeJ mice liver tissue extracts was determined by means of two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-ToF-MS). This article addresses the application of ANOVA-simultaneous component analysis (ASCA) to the found metabolomic profile. By performing hyphenated high-throughput analytical techniques together with multivariate chemometric methodology on metabolomic analysis, it enables us to investigate the sources of variability in the data related to each experimental factor of the study design (defined as time, diet and individual). The contribution of the diet factor in the dissimilarities between the samples appeared to be predominant over the time factor contribution. Nevertheless, there is a significant contribution of the time-diet interaction factor. Thus, evaluating the influences of the factors separately, as it is done in classical statistical methods, may lead to inaccurate interpretation of the data, preventing achievement of consistent biological conclusions.

High fat diet-induced modifications in membrane lipid and mitochondrial-membrane protein signatures precede the development of hepatic insulin resistance in mice

Objective

Excess lipid intake has been implicated in the pathophysiology of hepatosteatosis and hepatic insulin resistance. Lipids constitute approximately 50% of the cell membrane mass, define membrane properties, and create microenvironments for membrane-proteins. In this study we aimed to resolve temporal alterations in membrane metabolite and protein signatures during high-fat diet (HF)-mediated development of hepatic insulin resistance.

Methods

We induced hepatosteatosis by feeding C3HeB/FeJ male mice an HF enriched with long-chain polyunsaturated C18:2n6 fatty acids for 7, 14, or 21 days. Longitudinal changes in hepatic insulin sensitivity were assessed via the euglycemic-hyperinsulinemic clamp, in membrane lipids via t-metabolomics- and membrane proteins via quantitative proteomics-analyses, and in hepatocyte morphology via electron microscopy. Data were compared to those of age- and litter-matched controls maintained on a low-fat diet.

Results

Excess long-chain polyunsaturated C18:2n6 intake for 7 days did not compromise hepatic insulin sensitivity, however, induced hepatosteatosis and modified major membrane lipid constituent signatures in liver, e.g. increased total unsaturated, long-chain fatty acid-containing acyl-carnitine or membrane-associated diacylglycerol moieties and decreased total short-chain acyl-carnitines, glycerophosphocholines, lysophosphatidylcholines, or sphingolipids. Hepatic insulin sensitivity tended to decrease within 14 days HF-exposure. Overt hepatic insulin resistance developed until day 21 of HF-intervention and was accompanied by morphological mitochondrial abnormalities and indications for oxidative stress in liver. HF-feeding progressively decreased the abundance of protein-components of all mitochondrial respiratory chain complexes, inner and outer mitochondrial membrane substrate transporters independent from the hepatocellular mitochondrial volume in liver.

Conclusions

We assume HF-induced modifications in membrane lipid- and protein-signatures prior to and during changes in hepatic insulin action in liver alter membrane properties – in particular those of mitochondria which are highly abundant in hepatocytes. In turn, a progressive decrease in the abundance of mitochondrial membrane proteins throughout HF-exposure likely impacts on mitochondrial energy metabolism, substrate exchange across mitochondrial membranes, contributes to oxidative stress, mitochondrial damage, and the development of insulin resistance in liver.

Thermoneutral housing is a critical factor for immune function and diet-induced obesity in C57BL/6 nude mice

OBJECTIVES

Obesity-related cancers represent public health burdens of the first order. Nevertheless, suitable mouse models to unravel molecular mechanisms linking obesity to human cancer are still not available. One translational model is the immunocompromised Foxn1 (winged-helix/forkead transcription factor) nude mouse transplanted with human tumor xenografts. However, most xenograft studies are conducted in nude mice on an in-bred BALB/c background that entails protection from diet-induced obesity. To overcome such resistance to obesity and its sequelae, we here propose the dual strategy of utilizing Foxn1 nude mice on a C57BL/6 background and housing them at their thermoneutral zone.

METHODS

C57BL/6 nude and corresponding wild-type mice, housed at 23 or 33 °C, were subjected to either low-fat diet or high-fat diet (HFD). Energy expenditure, locomotor activity, body core temperature, respiratory quotient as well as food and water intake were analyzed using indirect calorimetry. Immune function at different housing temperatures was assessed by using an in vivo cytokine capture assay.

RESULTS

Our data clearly demonstrate that conventional housing protects C57BL/6 nude mice from HFD-induced obesity, potentially via increased energy expenditure. In contrast, HFD-fed C57BL/6 nude mice housed at thermoneutral conditions develop adiposity, increased hepatic triglyceride accumulation, adipose tissue inflammation and glucose intolerance. Moreover, increased circulating levels of lipopolysaccharide-driven cytokines suggest a greatly enhanced immune response in C57BL/6 nude mice housed at thermoneutrality.

CONCLUSION

Our data reveals mild cold stress as a major modulator for energy and body weight homeostasis as well as immune function in C57BL/6 nude mice. Adjusting housing temperatures to the thermoneutral zone may ultimately be key to successfully study growth and progression of human tumors in a diet-induced obese environment.

Defective insulin secretory response to intravenous glucose in C57Bl/6J compared to C57Bl/6N mice

Objective

The C57Bl/6J (Bl/6J) mouse is the most widely used strain in metabolic research. This strain carries a mutation in nicotinamide nucleotide transhydrogenase (Nnt), a mitochondrial enzyme involved in NADPH production, which has been suggested to lead to glucose intolerance and beta-cell dysfunction. However, recent reports comparing Bl/6J to Bl/6N (carrying the wild-type Nnt allele) under normal diet have led to conflicting results using glucose tolerance tests. Thus, we assessed glucose-stimulated insulin secretion (GSIS), insulin sensitivity, clearance and central glucose-induced insulin secretion in Bl/6J and N mice using gold-standard methodologies.

Methods

GSIS was measured using complementary tests (oral and intravenous glucose tolerance tests) and hyperglycemic clamps. Whole-body insulin sensitivity was assessed using euglycemic-hyperinsulinemic clamps. Neurally-mediated insulin secretion was measured during central hyperglycemia.

Results

Bl/6J mice have impaired GSIS compared to Bl/6N when glucose is administered intravenously during both a tolerance test and hyperglycemic clamp, but not in response to oral glucose. First and second phases of GSIS are altered without changes in whole body insulin sensitivity, insulin clearance, beta-cell mass or central response to glucose, thereby demonstrating defective beta-cell function in Bl/6J mice.

Conclusions

The Bl/6J mouse strain displays impaired insulin secretion. These results have important implications for choosing the appropriate test to assess beta-cell function and background strain in genetically modified mouse models.

Enhanced glucose tolerance in pancreatic-derived factor (PANDER) knockout C57BL/6 mice

Pancreatic-derived factor (PANDER; also known as FAM3B) is a uniquely structured protein strongly expressed within and secreted from the endocrine pancreas. PANDER has been hypothesized to regulate fasting and fed glucose homeostasis, hepatic lipogenesis and insulin signaling, and to serve a potential role in the onset or progression of type 2 diabetes (T2D). Despite having potentially pivotal pleiotropic roles in glycemic regulation and T2D, there has been limited generation of stable animal models for the investigation of PANDER function, and there are no models on well-established genetic murine backgrounds for T2D. Our aim was to generate an enhanced murine model to further elucidate the biological function of PANDER. Therefore, a pure-bred PANDER knockout C57BL/6 (PANKO-C57) model was created and phenotypically characterized with respect to glycemic regulation and hepatic insulin signaling. The PANKO-C57 model exhibited an enhanced metabolic phenotype, particularly with regard to enhanced glucose tolerance. Male PANKO-C57 mice displayed decreased fasting plasma insulin and C-peptide levels, whereas leptin levels were increased as compared with matched C57BL/6J wild-type mice. Despite similar peripheral insulin sensitivity between both groups, hepatic insulin signaling was significantly increased during fasting conditions, as demonstrated by increased phosphorylation of hepatic PKB/Akt and AMPK, along with mature SREBP-1 expression. Insulin stimulation of PANKO-C57 mice resulted in increased hepatic triglyceride and glycogen content as compared with wild-type C57BL/6 mice. In summary, the PANKO-C57 mouse represents a suitable model for the investigation of PANDER in multiple metabolic states and provides an additional tool to elucidate the biological function and potential role in T2D.

PPAR-alpha dependent regulation of vanin-1 mediates hepatic lipid metabolism

BACKGROUND & AIMS

Peroxisome proliferator-activated receptor alpha (PPARα) is a key regulator of hepatic fat oxidation that serves as an energy source during starvation. Vanin-1 has been described as a putative PPARα target gene in liver, but its function in hepatic lipid metabolism is unknown.

METHODS

We investigated the regulation of vanin-1, and total vanin activity, by PPARα in mice and humans. Furthermore, the function of vanin-1 in the development of hepatic steatosis in response to starvation was examined in Vnn1 deficient mice, and in rats treated with an inhibitor of vanin activity.

RESULTS

Liver microarray analyses reveals that Vnn1 is the most prominently regulated gene after modulation of PPARα activity. In addition, activation of mouse PPARα regulates hepatic- and plasma vanin activity. In humans, consistent with regulation by PPARα, plasma vanin activity increases in all subjects after prolonged fasting, as well as after treatment with the PPARα agonist fenofibrate. In mice, absence of vanin-1 exacerbates the fasting-induced increase in hepatic triglyceride levels. Similarly, inhibition of vanin activity in rats induces accumulation of hepatic triglycerides upon fasting. Microarray analysis reveal that the absence of vanin-1 associates with gene sets involved in liver steatosis, and reduces pathways involved in oxidative stress and inflammation.

CONCLUSIONS

We show that hepatic vanin-1 is under extremely sensitive regulation by PPARα and that plasma vanin activity could serve as a readout of changes in PPARα activity in human subjects. In addition, our data propose a role for vanin-1 in regulation of hepatic TG levels during fasting.