Cideb regulates diet-induced obesity, liver steatosis, and insulin sensitivity by controlling lipogenesis and fatty acid oxidation

MicroRNA-7 mediates cross-talk between metabolic signaling pathways in the liver

MicroRNAs (miRNAs) have emerged as critical regulators of cellular metabolism. To characterise miRNAs crucial to the maintenance of hepatic lipid homeostasis, we examined the overlap between the miRNA signature associated with inhibition of peroxisome proliferator activated receptor-α (PPAR-α) signaling, a pathway regulating fatty acid metabolism, and the miRNA profile associated with 25-hydroxycholesterol treatment, an oxysterol regulator of sterol regulatory element binding protein (SREBP) and liver X receptor (LXR) signaling. Using this strategy, we identified microRNA-7 (miR-7) as a PPAR-α regulated miRNA, which activates SREBP signaling and promotes hepatocellular lipid accumulation. This is mediated, in part, by suppression of the negative regulator of SREBP signaling: ERLIN2. miR-7 also regulates genes associated with PPAR signaling and sterol metabolism, including liver X receptor β (LXR-β), a transcriptional regulator of sterol synthesis, efflux, and excretion. Collectively, our findings highlight miR-7 as a novel mediator of cross-talk between PPAR, SREBP, and LXR signaling pathways in the liver.

Triglyceride Metabolism in the Liver

Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid metabolism. Fatty acids accrue in liver by hepatocellular uptake from the plasma and by de novo biosynthesis. Fatty acids are eliminated by oxidation within the cell or by secretion into the plasma within triglyceride-rich very low-density lipoproteins. Notwithstanding high fluxes through these pathways, under normal circumstances the liver stores only small amounts of fatty acids as triglycerides. In the setting of overnutrition and obesity, hepatic fatty acid metabolism is altered, commonly leading to the accumulation of triglycerides within hepatocytes, and to a clinical condition known as nonalcoholic fatty liver disease (NAFLD). In this review, we describe the current understanding of fatty acid and triglyceride metabolism in the liver and its regulation in health and disease, identifying potential directions for future research. Advances in understanding the molecular mechanisms underlying the hepatic fat accumulation are critical to the development of targeted therapies for NAFLD. © 2018 American Physiological Society. Compr Physiol 8:1-22, 2018.

CIDE Family-Mediated Unique Lipid Droplet Morphology in White Adipose Tissue and Brown Adipose Tissue Determines the Adipocyte Energy Metabolism

White adipose tissue (WAT) stores energy as triacylglycerol in preparation for fasting state. In contrast, brown adipose tissue (BAT) consumes energy and produces heat in a cold environment. One of the major differences between these two adipose tissues is the morphology of the intracellular lipid droplet (LD), which is large and unilocular in WAT and small and multilocular in BAT. Although the fat-specific protein 27 alpha (FSP27α), belonging to the cell death-inducing DNA fragmentation factor A (DFFA)-like effector (Cide) family, was known to be indispensable for large unilocular LD formation in WAT, the mechanism that regulated small multilocular LD formation in BAT remained unknown. We recently uncovered that FSP27β, a novel isoform of FSP27 abundantly expressed in BAT, plays a crucial role in small multilocular LD formation by inhibiting the homodimerization of CideA in BAT. We speculate that unilocular LD formation is ideal for efficient lipid storage in WAT because lipolysis from the LD surface is restricted due to the minimum LD surface area. In addition, hydrolyzed free fatty acid (FFA) and glycerol can efficiently flow out into the circulation from the cell surface. In contrast, small multilocular LD formation is ideal for efficient intracellular lipolysis from the LD surface and the subsequent facilitation of FFA transport to mitochondria that are adjacent to LDs for β-oxidation in BAT. Thus, intracellular LD morphology is closely related to the functions and characteristics of adipose tissues. Given that the browning of adipose tissue leads to enhanced energy expenditure and the prevention of obesity, clarification of the mechanism with respect to intracellular LD formation is very meaningful.

Hepatitis B virus prevents excessive viral production via reduction of cell death-inducing DFF45-like effectors

The relationship between hepatitis B virus (HBV) infection and lipid accumulation remains largely unknown. In this study, we investigated the effect of HBV propagation on lipid droplet growth in HBV-infected cells and HBV-producing cell lines, HepG2.2.15 and HBV-inducible Hep38.7-Tet. The amount of intracellular triglycerides was significantly reduced in HBV-infected and HBV-producing cells compared with HBV-lacking control cells. Electron and immunofluorescent microscopic analyses showed that the average size of a single lipid droplet (LD) was significantly less in the HBV-infected and HBV-producing cells than in the HBV-lacking control cells. Cell death-inducing DFF45-like effectors (CIDEs) B and C (CIDEB and CIDEC), which are involved in LD expansion for the improvement of lipid storage, were expressed at a significantly lower level in HBV-infected or HBV-producing cells than in HBV-lacking control cells, while CIDEA was not detected in those cells regardless of HBV production. The activity of the CIDEB and CIDEC gene promoters was impaired in HBV-infected or HBV-producing cells compared to HBV-lacking control cells, while CIDEs potentiated HBV core promoter activity. The amount of HNF4α, that can promote the transcription of CIDEB was significantly lower in HBV-producing cells than in HBV-lacking control cells. Knockout of CIDEB or CIDEC significantly reduced the amount of supernatant HBV DNA, intracellular viral RNA and nucleocapsid-associated viral DNA, while the expression of CIDEB or CIDEC recovered HBV production in CIDEB- or CIDEC-knockout cells. These results suggest that HBV regulates its own viral replication via CIDEB and CIDEC.

New insight into inter-organ crosstalk contributing to the pathogenesis of non-alcoholic fatty liver disease (NAFLD)

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver dysfunction and a significant global health problem with substantial rise in prevalence over the last decades. It is becoming increasingly clear that NALFD is not only predominantly a hepatic manifestation of metabolic syndrome, but also involves extra-hepatic organs and regulatory pathways. Therapeutic options are limited for the treatment of NAFLD. Accordingly, a better understanding of the pathogenesis of NAFLD is critical for gaining new insight into the regulatory network of NAFLD and for identifying new targets for the prevention and treatment of NAFLD. In this review, we emphasize on the current understanding of the inter-organ crosstalk between the liver and peripheral organs that contributing to the pathogenesis of NAFLD.

CIDE gene expression in adipose tissue, liver, and skeletal muscle from obese and lean pigs

The expression of the cell death-inducing DNA fragmentation factor α-like effector (CIDE) family including Cidea, Cideb, and Cidec was significantly increased in mouse and human models of obesity. However, there was less information on these genes' expression in pigs. Here, we hypothesized that different fat accumulation between lean (Duroc×Landrace×Yorkshire gilts, DLY) and obese (Lantang) pigs was attributed to porcine CIDE-modulating lipid metabolism. Our data showed that Cidea and Cidec were expressed at a high level in adipose tissue, and at a relatively high level in skeletal muscle, whereas Cideb was mainly expressed in the liver in both breeds of pig. Lantang pigs had higher white adipose and skeletal muscle Cidea and Cidec mRNA abundance, and hepatic and muscle Cideb mRNA than DLY pigs. Lipid metabolism-related genes including sterol regulatory element binding protein 1c (SREBP-1c), hepatocyte nuclear factor-4α (HNF-4α), peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), fatty acid synthase (FASN), diacylglycerol O-acyltransferase 1 (DGAT1), and DGAT2 showed a higher expression level in adipose tissue from obese pigs than in that from lean pigs. Lantang pigs exhibited higher mRNA abundance for liver SREBP-1c, HNF-4α, and PGC-1α, and higher skeletal muscle SREBP-1c, HNF-4α, PGC-1α, and DGAT2 expression, as compared with DLY pigs. However, the perlipin2 mRNA levels in adipose tissues, liver, and skeletal muscle were significantly lower in obese pigs than in their lean counterparts. Furthermore, plasma non-esterified fatty acid (NEFA), glucose, and triacylglycerol (TAG) levels were greater in obese pigs than in lean pigs. Finally, data from correlation analysis further found that CIDE mRNA expression was positively correlated with back fat thickness (BFT), abdominal fat mass (AFM), and the levels of NEFA, TAG, and glucose in the two breeds. Collectively, these data revealed that the porcine CIDEs possibly modulated lipid metabolism and contributed to the development of fat deposition and obesity in Lantang pigs.

Cell death-inducing DNA fragmentation factor A-like effector A and fat-specific protein 27β coordinately control lipid droplet size in brown adipocytes

Adipose tissue stores neutral lipids and is a major metabolic organ involved in regulating whole-body energy homeostasis. Triacylglycerol is stored as unilocular large lipid droplets (LDs) in white adipocytes and as multilocular small LDs in brown adipocytes. Proteins of the cell death-inducing DNA fragmentation factor A-like effector (Cide) family include CideA, CideB, and fat-specific protein of 27 (FSP27). Of these, FSP27 has been shown to play a crucial role in the formation of unilocular large LDs in white adipocytes. However, the mechanisms by which brown adipocytes store small and multilocular LDs remain unclear. An FSP27 isoform, FSP27β, was recently identified. We herein report that CideA and FSP27β are mainly expressed in brown adipose tissue and that FSP27β overexpression inhibits CideA-induced LD enlargements in a dose-dependent manner in COS cells. Furthermore, RNAi-mediated FSP27β depletion resulted in enlarged LDs in HB2 adipocytes, which possess the characteristics of brown adipocytes. Brown adipocytes in FSP27-knock-out mice that express CideA, but not FSP27β, had larger and fewer LDs. Moreover, we confirmed that FSP27β and CideA form a complex in brown adipose tissue. Our results suggest that FSP27β negatively regulates CideA-promoted enlargement of LD size in brown adipocytes. FSP27β appears to be responsible for the formation of small and multilocular LDs in brown adipose tissue, a morphology facilitating free fatty acid transport to mitochondria adjacent to LDs for oxidation in brown adipocytes.

The Complete Genome Sequence of a Second Distinct Betabaculovirus from the True Armyworm, Mythimna unipuncta

The betabaculovirus originally called Pseudaletia (Mythimna) sp. granulovirus #8 (MyspGV#8) was examined by electron microscopy, host barcoding PCR, and determination of the nucleotide sequence of its genome. Scanning and transmission electron microscopy revealed that the occlusion bodies of MyspGV#8 possessed the characteristic size range and morphology of betabaculovirus granules. Barcoding PCR using cytochrome oxidase I primers with DNA from the MyspGV#8 collection sample confirmed that it had been isolated from the true armyworm, Mythimna unipuncta (Lepidoptera: Noctuidae) and therefore was renamed MyunGV#8. The MyunGV#8 genome was found to be 144,673 bp in size with a nucleotide distribution of 49.9% G+C, which was significantly smaller and more GC-rich than the genome of Pseudaletia unipuncta granulovirus H (PsunGV-H), another M. unipuncta betabaculovirus. A phylogeny based on concatenated baculovirus core gene amino acid sequence alignments placed MyunGV#8 in clade a of genus Betabaculovirus. Kimura-2-parameter nucleotide distances suggested that MyunGV#8 represents a virus species different and distinct from other species of Betabaculovirus. Among the 153 ORFs annotated in the MyunGV#8 genome, four ORFs appeared to have been obtained from or donated to the alphabaculovirus lineage represented by Leucania separata nucleopolyhedrovirus AH1 (LeseNPV-AH1) during co-infection of Mythimna sp. larvae. A set of 33 ORFs was identified that appears only in other clade a betabaculovirus isolates. This clade a-specific set includes an ORF that encodes a polypeptide sequence containing a CIDE_N domain, which is found in caspase-activated DNAse/DNA fragmentation factor (CAD/DFF) proteins. CAD/DFF proteins are involved in digesting DNA during apoptosis.

The size matters: regulation of lipid storage by lipid droplet dynamics

Adequate energy storage is essential for sustaining healthy life. Lipid droplet (LD) is the subcellular organelle that stores energy in the form of neutral lipids and releases fatty acids under energy deficient conditions. Energy storage capacity of LDs is primarily dependent on the sizes of LDs. Enlargement and growth of LDs is controlled by two molecular pathways: neutral lipid synthesis and atypical LD fusion. Shrinkage of LDs is mediated by the degradation of neutral lipids under energy demanding conditions and is controlled by neutral cytosolic lipases and lysosomal acidic lipases. In this review, we summarize recent progress regarding the regulatory pathways and molecular mechanisms that control the sizes and the energy storage capacity of LDs.

Omic studies reveal the pathogenic lipid droplet proteins in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is an epidemic metabolic condition driven by an underlying lipid homeostasis disorder. The lipid droplet (LD), the main organelle involved in neutral lipid storage and hydrolysis, is a potential target for NAFLD therapeutic treatment. In this review, we summarize recent progress elucidating the connections between LD-associated proteins and NAFLD found by genome-wide association studies (GWAS), genomic and proteomic studies. Finally, we discuss a possible mechanism by which the protein 17β-hydroxysteroid dehydrogenase 13 (17β-HSD13) may promote the development of NAFLD.

The progress and challenges in metabolic research in China

Metabolism refers to a chain of chemical reactions converting food/fuel into energy to conduct cellular processes, including the synthesis of the building blocks of the body, such as proteins, lipids, nucleic acids, and carbohydrates, and the elimination of nitrogenous wastes. Metabolic chain reactions are catalyzed by various enzymes that are orchestrated in specific pathways. Metabolic pathways are important for organisms to grow and reproduce, maintain their structures, and respond to their environments. The coordinated regulation of metabolic pathways is important for maintaining metabolic homeostasis. The key steps and crucial enzymes in these pathways have been well investigated. However, the crucial regulatory factors and feedback (or feedforward) mechanisms of nutrients and intermediate metabolites of these biochemical processes remain to be fully elucidated. In addition, the roles of these enzymes and regulatory factors in controlling metabolism under physiological and pathological conditions are largely unknown. In particular, metabolic dysregulation is closely linked to the development of many diseases, including obesity, fatty liver, diabetes, cancer, cardiovascular, cerebrovascular, and neurodegenerative diseases. Therefore, metabolism, an old area of biochemistry, has attracted much attention in the last decade. With substantially increased government funding, the involvement of talented researchers, an improved infrastructure and scientific environment over the last ten years, the basic research in the field of metabolism in China has dramatically advanced. Here, we have summarized the major discoveries of scientists in China in the last decade in the area of metabolism. Due to the vast amount of information, we focused this review on specific aspects of metabolism, particularly metabolic regulation and lipid metabolism in vertebrates. © 2016 IUBMB Life, 68(11):847-853, 2016.

Identification of copy number variations in three Chinese horse breeds using 70K single nucleotide polymorphism BeadChip array

Copy number variation (CNV), an essential form of genetic variation, has been increasingly recognized as one promising genetic marker in the analysis of animal genomes. Here, we used the Equine 70K single nucleotide polymorphism genotyping array for the genome-wide detection of CNVs in 96 horses from three diverse Chinese breeds: Debao pony (DB), Mongolian horse (MG) and Yili horse (YL). A total of 287 CNVs were determined and merged into 122 CNV regions (CNVRs) ranging from 199 bp to 2344 kb in size and distributed in a heterogeneous manner on chromosomes. These CNVRs were integrated with seven existing reports to generate a composite genome-wide dataset of 1558 equine CNVRs, revealing 69 (56.6%) novel CNVRs. The majority (69.7%) of the 122 CNVRs overlapped with 438 genes, whereas 30.3% were located in intergenic regions. Most of these genes were associated with common CNVRs, which were shared by divergent horse breeds. As many as 60, 42 and 91 genes overlapping with the breed-specific ss were identified in DB, MG and YL respectively. Among these genes, FGF11, SPEM1, PPARG, CIDEB, HIVEP1 and GALR may have potential relevance to breed-specific traits. These findings provide valuable information for understanding the equine genome and facilitating association studies of economically important traits with equine CNVRs in the future.

Cell-death-inducing DFFA-like Effector B Contributes to the Assembly of Hepatitis C Virus (HCV) Particles and Interacts with HCV NS5A

Hepatitis C virus (HCV) uses components of the very-low-density lipoprotein (VLDL) pathway for assembly/release. We previously reported that hepatocyte nuclear factor 4α (HNF4α) participates in HCV assembly/release through downstream factors those participate in VLDL assembly/secretion. Cell-death-inducing DFFA-like effector B (CIDEB) is an important regulator of the VLDL pathway. CIDEB is required for entry of HCV particles from cell culture (HCVcc), but the effects of CIDEB on the post-entry steps of the HCV lifecycle are unclear. In the present study, we determined that CIDEB is required for HCV assembly in addition to HCVcc entry. Furthermore, CIDEB interacts with the HCV NS5A protein, and the N terminus of CIDEB and the domain I of NS5A are involved in this interaction. Moreover, CIDEB silencing impairs the association of apolipoprotein E (ApoE) with HCV particles. Interestingly, CIDEB is also required for the post-entry stages of the dengue virus (DENV) life cycle. Collectively, these results indicate that CIDEB is a new host factor that is involved in HCV assembly, presumably by interacting with viral protein, providing new insight into the exploitation of the VLDL regulator CIDEB by HCV.

Lipid droplet-associated proteins in atherosclerosis (Review)

Accumulation of atherosclerotic plaques in arterial walls leads to major cardiovascular diseases and stroke. Macrophages/foam cells are central components of atherosclerotic plaques, which populate the arterial wall in order to remove harmful modified low‑density lipoprotein (LDL) particles, resulting in the accumulation of lipids, mostly LDL‑derived cholesterol ester, in cytosolic lipid droplets (LDs). At present, LDs are recognized as dynamic organelles that govern cellular metabolic processes. LDs consist of an inner core of neutral lipids surrounded by a monolayer of phospholipids and free cholesterol, and contain LD‑associated proteins (LDAPs) that regulate LD functions. Foam cells are characterized by an aberrant accumulation of cytosolic LDs, and are considered a hallmark of atherosclerotic lesions through all stages of development. Previous studies have investigated the mechanisms underlying foam cell formation, aiming to discover therapeutic strategies that target foam cells and intervene against atherosclerosis. It is well established that LDAPs have a major role in the pathogenesis of metabolic diseases caused by dysfunction of lipid metabolism, and several studies have linked LDAPs to the development of atherosclerosis. In this review, several foam cell‑targeting pathways have been described, with an emphasis on the role of LDAPs in cholesterol mobilization from macrophages. In addition, the potential of LDAPs as therapeutic targets to prevent the progression and/or facilitate the regression of the disease has been discussed.

Nuclear receptor-mediated regulation of lipid droplet-associated protein gene expression in adipose tissue

In adipose tissues, nuclear receptors (NRs) have important metabolic actions on cellular lipid-storing capacity through targeted gene regulation. Lipid droplets (LDs) are the organelles for intracellular triacylglycerol (TAG) storage and are present in all eukaryotic cells. They are small in most cells, but in white adipocytes, they can occupy 90% of the cytoplasm. LDs consist of a TAG core surrounded by a phospholipid monolayer and an array of associated proteins that determine size, stability, inter-droplet interaction, and lipid storage capacity. The genes that encode these proteins are more highly expressed in brown compared with white fat, correlating with the greater LD surface area in multilocular brown adipocytes. Gene expression profiling reveals that most NRs are present in adipose tissues, with some showing greater expression in brown compared with white fat, including peroxisome proliferator-activated receptor (PPAR) α, estrogen-related receptor α, and NURR1. NR signaling is important for the regulated expression of most genes that encode LD-associated proteins. For example, estradiol signals via estrogen receptor α to regulate the levels of PLIN1 and the lipase ATGL controlling LD size and total lipid accumulation. PPARγ is essential for adipocyte differentiation and function, and analysis of data obtained through chromatin immunoprecipitation followed by high-throughput DNA sequencing shows that it binds to the promoters of many genes encoding LD proteins in adipocytes. Of these genes, the greatest PPARγ binding was to regulatory regions for Plin1, Cidec, and G0s2. NRs represent an important target for controlling LD dynamics in diseases affected by altered fat storage encompassing obesity and lipodystrophy, which are an increasing health problem.

Differential Roles of Cell Death-inducing DNA Fragmentation Factor-α-like Effector (CIDE) Proteins in Promoting Lipid Droplet Fusion and Growth in Subpopulations of Hepatocytes

Lipid droplets (LDs) are dynamic subcellular organelles whose growth is closely linked to obesity and hepatic steatosis. Cell death-inducing DNA fragmentation factor-α-like effector (CIDE) proteins, including Cidea, Cideb, and Cidec (also called Fsp27), play important roles in lipid metabolism. Cidea and Cidec are LD-associated proteins that promote atypical LD fusion in adipocytes. Here, we find that CIDE proteins are all localized to LD-LD contact sites (LDCSs) and promote lipid transfer, LD fusion, and growth in hepatocytes. We have identified two types of hepatocytes, one with small LDs (small LD-containing hepatocytes, SLHs) and one with large LDs (large LD-containing hepatocytes, LLHs) in the liver. Cideb is localized to LDCSs and promotes lipid exchange and LD fusion in both SLHs and LLHs, whereas Cidea and Cidec are specifically localized to the LDCSs and promote lipid exchange and LD fusion in LLHs. Cideb-deficient SLHs have reduced LD sizes and lower lipid exchange activities. Fasting dramatically induces the expression of Cidea/Cidec and increases the percentage of LLHs in the liver. The majority of the hepatocytes from the liver of obese mice are Cidea/Cidec-positive LLHs. Knocking down Cidea or Cidec significantly reduced lipid storage in the livers of obese animals. Our data reveal that CIDE proteins play differential roles in promoting LD fusion and lipid storage; Cideb promotes lipid storage under normal diet conditions, whereas Cidea and Cidec are responsible for liver steatosis under fasting and obese conditions.

Depletion of Myostatin b Promotes Somatic Growth and Lipid Metabolism in Zebrafish

Myostatin (MSTN) is a negative regulator of myogenesis in vertebrates. Depletion of mstn resulted in elevated muscle growth in several animal species. However, the report on the complete ablation of mstn in teleost fish has not yet become available. In this study, two independent mstnb-deficient mutant lines in zebrafish were generated with the TALENs technique. In the mstnb-deficient zebrafish, enhanced muscle growth with muscle fiber hyperplasia was achieved. Beginning at the adult stage (80 days postfertilization), the mstnb-deficient zebrafish exhibited increased circumferences and body weights compared with the wild-type sibling control fish. Although the overall total lipid/body weight ratios remained similar between the mstnb-deficient zebrafish and the control fish, the distribution of lipids was altered. The size of the visceral adipose tissues became smaller while more lipids accumulated in skeletal muscle in the mstnb-deficient zebrafish than in the wild-type control fish. Based on the transcriptional expression profiles, our results revealed that lipid metabolism, including lipolysis and lipogenesis processes, was highly activated in the mstnb-deficient zebrafish, which indicated the transition of energy metabolism from protein-dependent to lipid-dependent in mstnb-deficient zebrafish. Our mstnb-deficient model could be valuable in understanding not only the growth trait regulation in teleosts but also the mechanisms of teleost energy metabolism.

Lipid droplets and steroidogenic cells

Lipid droplets (LDs) in steroidogenic tissues have a cholesteryl ester (CE) core surrounded by a phospholipid monolayer that is coated with associated proteins. Compared with other tissues, they tend to be smaller in size and more numerous in numbers. These LDs are enriched with PLIN1c, PLIN2 and PLIN3. Both CIDE A and B are found in mouse ovary. Free cholesterol (FC) released upon hormone stimulation from LDs is the preferred source of cholesterol substrate for steroidogenesis, and HSL is the major neutral cholesterol esterase mediating the conversion of CEs to FC. Through the interaction of HSL with vimentin and StAR, FC is translocated to mitochondria for steroid hormone production. Proteomic analyses of LDs isolated from loaded primary ovarian granulosa cells, mouse MLTC-1 Leydig tumor cells and mouse testes revealed LD associated proteins that are actively involved in modulating lipid homeostasis along with a number of steroidogenic enzymes. Microscopy analysis confirmed the localization of many of these proteins to LDs. These studies broaden the role of LDs to include being a platform for functional steroidogenic enzyme activity or as a port for transferring steroidogenic enzymes and/or steroid intermediates, in addition to being a storage depot for CEs.

Lipid droplets and associated proteins in the skin: basic research and clinical perspectives

Lipid droplets (LDs), the major organelles handling fat storage, comprise a hydrophobic neutral lipid core surrounded by a phospholipid monolayer embedded with a protein miscellany. Although lipids of the stratum corneum are essential for the skin barrier, and progressive lipid accumulation culminating in cell disruption is the hallmark of sebaceous differentiation, only a few studies touched on skin LD and associated proteins so far. Here, after briefly introducing the basic facts about LD and associated proteins, we discuss how forthcoming studies may unveil novel players in skin lipid metabolism and candidate target proteins for treating skin diseases.

Cidec promotes the differentiation of human adipocytes by degradation of AMPKα through ubiquitin-proteasome pathway

BACKGROUND

We previously showed that Cidec was localized on the surface of lipid droplets and could promote the differentiation of human adipocytes, but the molecular mechanism was still unknown.

METHODS & RESULTS

In this study, we first sought to identify proteins that interact with Cidec using yeast two-hybrid system. The results revealed that Cidec could directly interact with AMPKα1 subunit. We further showed that AMPKα levels decreased while Cidec increased during the adipogenic differentiation of human adipocytes. Meanwhile, we observed that the increased Cidec could reduce AMPKα level in adipocytes, and the downregulation of AMPKα could help to promote the differentiation of adipocytes. The results of co-immunoprecipitation and immunofluorescent proved that Cidec biochemically interacted and co-localized with AMPKα1, which meant Cidec was a regulator for AMPKα stability through an ubiquitin-proteasome pathway.

CONCLUSION

Our data suggested that Cidec could interact with and down-regulate AMPKα through an ubiquitin-proteasome degradation pathway, which provided a possible mechanism of Cidec in promoting human adipocytes differentiation.

GENERAL SIGNIFICANCE

Our work proposed a new possible mechanism for human adipogenesis, and also provided a potential role of AMPKα as a target in treating obesity or obesity-related diseases.

The Lipid Droplet Protein Hypoxia-inducible Gene 2 Promotes Hepatic Triglyceride Deposition by Inhibiting Lipolysis

The liver is a major site of glucose, fatty acid, and triglyceride (TG) synthesis and serves as a major regulator of whole body nutrient homeostasis. Chronic exposure of humans or rodents to high-calorie diets promotes non-alcoholic fatty liver disease, characterized by neutral lipid accumulation in lipid droplets (LD) of hepatocytes. Here we show that the LD protein hypoxia-inducible gene 2 (Hig2/Hilpda) functions to enhance lipid accumulation in hepatocytes by attenuating TG hydrolysis. Hig2 expression increased in livers of mice on a high-fat diet and during fasting, two states associated with enhanced hepatic TG content. Hig2 expressed in primary mouse hepatocytes localized to LDs and promoted LD TG deposition in the presence of oleate. Conversely, tamoxifen-inducible Hig2 deletion reduced both TG content and LD size in primary hepatocytes from mice harboring floxed alleles of Hig2 and a cre/ERT2 transgene controlled by the ubiquitin C promoter. Hepatic TG was also decreased by liver-specific deletion of Hig2 in mice with floxed Hig2 expressing cre controlled by the albumin promoter. Importantly, we demonstrate that Hig2-deficient hepatocytes exhibit increased TG lipolysis, TG turnover, and fatty acid oxidation as compared with controls. Interestingly, mice with liver-specific Hig2 deletion also display improved glucose tolerance. Taken together, these data indicate that Hig2 plays a major role in promoting lipid sequestration within LDs in mouse hepatocytes through a mechanism that impairs TG degradation.

Thyroid dysfunction associated with follicular cell steatosis in obese male mice and humans

Adult thyroid dysfunction is a common endocrine disorder associated with an increased risk of cardiovascular disease and mortality. A recent epidemiologic study revealed a link between obesity and increased prevalence of hypothyroidism. It is conceivable that excessive adiposity in obesity might lead to expansion of the interfollicular adipose (IFA) depot or steatosis in thyroid follicular cells (thyroid steatosis, TS). In this study, we investigated the morphological and functional changes in thyroid glands of obese humans and animal models, diet-induced obese (DIO), ob/ob, and db/db mice. Expanded IFA depot and TS were observed in obese patients. Furthermore, DIO mice showed increased expression of lipogenesis-regulation genes, such as sterol regulatory element binding protein 1 (SREBP-1), peroxisome proliferator-activated receptor γ (PPARγ), acetyl coenzyme A carboxylase (ACC), and fatty acid synthetase (FASN) in the thyroid gland. Steatosis and ultrastructural changes, including distension of the endoplasmic reticulum (ER) and mitochondrial distortion in thyroid follicular cells, were uniformly observed in DIO mice and genetically obese mouse models, ob/ob and db/db mice. Obese mice displayed a variable degree of primary thyroid hypofunction, which was not corrected by PPARγ agonist administration. We propose that systemically increased adiposity is associated with characteristic IFA depots and TS and may cause or influence the development of primary thyroid failure.

Caloric restriction and intermittent fasting alter hepatic lipid droplet proteome and diacylglycerol species and prevent diabetes in NZO mice

Caloric restriction and intermittent fasting are known to improve glucose homeostasis and insulin resistance in several species including humans. The aim of this study was to unravel potential mechanisms by which these interventions improve insulin sensitivity and protect from type 2 diabetes. Diabetes-susceptible New Zealand Obese mice were either 10% calorie restricted (CR) or fasted every other day (IF), and compared to ad libitum (AL) fed control mice. AL mice showed a diabetes prevalence of 43%, whereas mice under CR and IF were completely protected against hyperglycemia. Proteomic analysis of hepatic lipid droplets revealed significantly higher levels of PSMD9 (co-activator Bridge-1), MIF (macrophage migration inhibitor factor), TCEB2 (transcription elongation factor B (SIII), polypeptide 2), ACY1 (aminoacylase 1) and FABP5 (fatty acid binding protein 5), and a marked reduction of GSTA3 (glutathione S-transferase alpha 3) in samples of CR and IF mice. In addition, accumulation of diacylglycerols (DAGs) was significantly reduced in livers of IF mice (P=0.045) while CR mice showed a similar tendency (P=0.062). In particular, 9 DAG species were significantly reduced in response to IF, of which DAG-40:4 and DAG-40:7 also showed significant effects after CR. This was associated with a decreased PKCε activation and might explain the improved insulin sensitivity. In conclusion, our data indicate that protection against diabetes upon caloric restriction and intermittent fasting associates with a modulation of lipid droplet protein composition and reduction of intracellular DAG species.

Metabolic syndrome in pediatrics

The ever growing prevalence of childhood obesity is being accompanied by an increase in the pediatric population of diseases once believed to be exclusive of the adulthood such as the metabolic syndrome (MS). The MS has been defined as the link between insulin resistance, hypertension, dyslipidemia, impaired glucose tolerance, and other metabolic abnormalities associated with an increased risk of atherosclerotic cardiovascular diseases in adults. In this review, we will discuss the peculiar aspects of the pediatric MS and the role of novel molecules and biomarkers in its pathogenesis.

Intestinal triacylglycerol synthesis in fat absorption and systemic energy metabolism

The intestine plays a prominent role in the biosynthesis of triacylglycerol (triglyceride; TAG). Digested dietary TAG is repackaged in the intestine to form the hydrophobic core of chylomicrons, which deliver metabolic fuels, essential fatty acids, and other lipid-soluble nutrients to the peripheral tissues. By controlling the flux of dietary fat into the circulation, intestinal TAG synthesis can greatly impact systemic metabolism. Genes encoding many of the enzymes involved in TAG synthesis have been identified. Among TAG synthesis enzymes, acyl-CoA:monoacylglycerol acyltransferase 2 and acyl-CoA:diacylglycerol acyltransferase (DGAT)1 are highly expressed in the intestine. Their physiological functions have been examined in the context of whole organisms using genetically engineered mice and, in the case of DGAT1, specific inhibitors. An emerging theme from recent findings is that limiting the rate of TAG synthesis in the intestine can modulate gut hormone secretion, lipid metabolism, and systemic energy balance. The underlying mechanisms and their implications for humans are yet to be explored. Pharmacological inhibition of TAG hydrolysis in the intestinal lumen has been employed to combat obesity and associated disorders with modest efficacy and unwanted side effects. The therapeutic potential of inhibiting specific enzymes involved in intestinal TAG synthesis warrants further investigation.

Inhibiting monoacylglycerol acyltransferase 1 ameliorates hepatic metabolic abnormalities but not inflammation and injury in mice

Abnormalities in hepatic lipid metabolism and insulin action are believed to play a critical role in the etiology of nonalcoholic steatohepatitis. Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol, which is the penultimate step in one pathway for triacylglycerol synthesis. Hepatic expression of Mogat1, which encodes an MGAT enzyme, is increased in the livers of mice with hepatic steatosis, and knocking down Mogat1 improves glucose metabolism and hepatic insulin signaling, but whether increased MGAT activity plays a role in the etiology of nonalcoholic steatohepatitis is unclear. To examine this issue, mice were placed on a diet containing high levels of trans fatty acids, fructose, and cholesterol (HTF-C diet) or a low fat control diet for 4 weeks. Mice were injected with antisense oligonucleotides (ASOs) to knockdown Mogat1 or a scrambled ASO control for 12 weeks while remaining on diet. The HTF-C diet caused glucose intolerance, hepatic steatosis, and induced hepatic gene expression markers of inflammation, macrophage infiltration, and stellate cell activation. Mogat1 ASO treatment, which suppressed Mogat1 expression in liver and adipose tissue, attenuated weight gain, improved glucose tolerance, improved hepatic insulin signaling, and decreased hepatic triacylglycerol content compared with control ASO-treated mice on HTF-C chow. However, Mogat1 ASO treatment did not reduce hepatic diacylglycerol, cholesterol, or free fatty acid content; improve histologic measures of liver injury; or reduce expression of markers of stellate cell activation, liver inflammation, and injury. In conclusion, inhibition of hepatic Mogat1 in HTF-C diet-fed mice improves hepatic metabolic abnormalities without attenuating liver inflammation and injury.

Investigating the antiviral role of cell death-inducing DFF45-like effector B in HCV replication

Cell-death-inducing DFF45-like effector B (CIDEB) is an apoptotic host factor, which was recently found to also regulate hepatic lipid homeostasis. Herein we delineate the relevance of these dual roles of CIDEB in apoptosis and lipid metabolism in the context of hepatitis C virus (HCV) replication. We demonstrate that HCV upregulates CIDEB expression in human serum differentiated hepatoma cells. CIDEB overexpression inhibits HCV replication in HCV replicon expressing Huh7.5 cells, while small interfering RNA knockdown of CIDEB expression in human serum differentiated hepatoma cells promotes HCV replication and secretion of viral proteins. Furthermore, we characterize a CIDEB mutant, KRRA, which is deficient in lipid droplet clustering and fusion and demonstrate that CIDEB-mediated inhibition of HCV is independent of the protein's lipid droplet fusogenic role. Our results suggest that higher levels of CIDEB expression, which favour an apoptotic role for the host factor, inhibit HCV. Collectively, our data demonstrate that CIDEB can act as an anti-HCV host factor and contribute to altered triglyceride homeostasis.

Dissecting adipose tissue lipolysis: molecular regulation and implications for metabolic disease

Lipolysis is the process by which triglycerides (TGs) are hydrolyzed to free fatty acids (FFAs) and glycerol. In adipocytes, this is achieved by sequential action of adipose TG lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase. The activity in the lipolytic pathway is tightly regulated by hormonal and nutritional factors. Under conditions of negative energy balance such as fasting and exercise, stimulation of lipolysis results in a profound increase in FFA release from adipose tissue (AT). This response is crucial in order to provide the organism with a sufficient supply of substrate for oxidative metabolism. However, failure to efficiently suppress lipolysis when FFA demands are low can have serious metabolic consequences and is believed to be a key mechanism in the development of type 2 diabetes in obesity. As the discovery of ATGL in 2004, substantial progress has been made in the delineation of the remarkable complexity of the regulatory network controlling adipocyte lipolysis. Notably, regulatory mechanisms have been identified on multiple levels of the lipolytic pathway, including gene transcription and translation, post-translational modifications, intracellular localization, protein-protein interactions, and protein stability/degradation. Here, we provide an overview of the recent advances in the field of AT lipolysis with particular focus on the molecular regulation of the two main lipases, ATGL and HSL, and the intracellular and extracellular signals affecting their activity.

Regulation of adipocyte lipolysis

In adipocytes the hydrolysis of TAG to produce fatty acids and glycerol under fasting conditions or times of elevated energy demands is tightly regulated by neuroendocrine signals, resulting in the activation of lipolytic enzymes. Among the classic regulators of lipolysis, adrenergic stimulation and the insulin-mediated control of lipid mobilisation are the best known. Initially, hormone-sensitive lipase (HSL) was thought to be the rate-limiting enzyme of the first lipolytic step, while we now know that adipocyte TAG lipase is the key enzyme for lipolysis initiation. Pivotal, previously unsuspected components have also been identified at the protective interface of the lipid droplet surface and in the signalling pathways that control lipolysis. Perilipin, comparative gene identification-58 (CGI-58) and other proteins of the lipid droplet surface are currently known to be key regulators of the lipolytic machinery, protecting or exposing the TAG core of the droplet to lipases. The neuroendocrine control of lipolysis is prototypically exerted by catecholaminergic stimulation and insulin-induced suppression, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and perilipin. Interestingly, in recent decades adipose tissue has been shown to secrete a large number of adipokines, which exert direct effects on lipolysis, while adipocytes reportedly express a wide range of receptors for signals involved in lipid mobilisation. Recently recognised mediators of lipolysis include some adipokines, structural membrane proteins, atrial natriuretic peptides, AMP-activated protein kinase and mitogen-activated protein kinase. Lipolysis needs to be reanalysed from the broader perspective of its specific physiological or pathological context since basal or stimulated lipolytic rates occur under diverse conditions and by different mechanisms.

Cideb facilitates the lipidation of chylomicrons in the small intestine

Cell death-inducing DFF45-like effector b (Cideb), an endoplasmic reticulum (ER)- and lipid droplet (LD)-associated protein, has been shown to play a critical role in maintaining hepatic lipid homeostasis by promoting the lipidation and maturation of VLDL particles. Here, we observed that Cideb is expressed in the jejunum and ileum sections of the small intestine, and its expression was induced by high-fat diet. Intragastric gavage with lipids resulted in the retention of lipids in the intestine in Cideb-deficient mice. In addition, we observed that mice with Cideb deficiency exhibited reduced intestinal chylomicron-TG secretion and increased lipid accumulation in the enterocytes. The sizes of chylomicrons secreted from the small intestine of Cideb-deficient mice were also smaller than those from wild-type mice. Furthermore, the overexpression of Cideb increased TG secretion and reduced lipid accumulation in the enterocyte-like Caco-2 cells. In addition, we proved that Cideb was localized to the ER and LDs and could interact with ApoB48 in Caco-2 cells. Overall, these data revealed that Cideb plays an important role in controlling intestinal chylomicron lipidation.

Cell death-inducing DFFA-like effector b is required for hepatitis C virus entry into hepatocytes

The molecular mechanism of the hepatic tropism of hepatitis C virus (HCV) remains incompletely defined. In vitro hepatic differentiation of pluripotent stem cells produces hepatocyte-like cells (HLCs) permissive for HCV infection, providing an opportunity for studying liver development and host determinants of HCV susceptibility. We previously identified the transition stage of HCV permissiveness and now investigate whether a host protein whose expression is induced during this transition stage is important for HCV infection. We suppressed the expression of a liver-specific protein, cell death-inducing DFFA-like effector b (CIDEB), and performed hepatocyte function and HCV infection assays. We also used a variety of cell-based assays to dissect the specific step of the HCV life cycle that potentially requires CIDEB function. We found CIDEB to be an essential cofactor for HCV entry into hepatocytes. Genetic interference with CIDEB in stem cells followed by hepatic differentiation leads to HLCs that are refractory to HCV infection, and infection time course experiments revealed that CIDEB functions in a late step of HCV entry, possibly to facilitate membrane fusion. The role of CIDEB in mediating HCV entry is distinct from those of the well-established receptors, as it is not required for HCV pseudoparticle entry. Finally, HCV infection effectively downregulates CIDEB protein through a posttranscriptional mechanism.

IMPORTANCE

This study identifies a hepatitis C virus (HCV) entry cofactor that is required for HCV infection of hepatocytes and potentially facilitates membrane fusion between viral and host membranes. CIDEB and its interaction with HCV may open up new avenues of investigation of lipid droplets and viral entry.

Targeting Hepatic Glycerolipid Synthesis and Turnover to Treat Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of metabolic abnormalities ranging from simple hepatic steatosis (accumulation of neutral lipid) to development of steatotic lesions, steatohepatitis, and cirrhosis. NAFLD is extremely prevalent in obese individuals and with the epidemic of obesity; nonalcoholic steatohepatitis (NASH) has become the most common cause of liver disease in the developed world. NASH is rapidly emerging as a prominent cause of liver failure and transplantation. Moreover, hepatic steatosis is tightly linked to risk of developing insulin resistance, diabetes, and cardiovascular disease. Abnormalities in hepatic lipid metabolism are part and parcel of the development of NAFLD and human genetic studies and work conducted in experimentally tractable systems have identified a number of enzymes involved in fat synthesis and degradation that are linked to NAFLD susceptibility as well as progression to NASH. The goal of this review is to summarize the current state of our knowledge on these pathways and focus on how they contribute to etiology of NAFLD and related metabolic diseases.

Steatosis in the liver

Accumulation of triacylglycerols within the cytoplasm of hepatocytes to the degree that lipid droplets are visible microscopically is called liver steatosis. Most commonly, it occurs when there is an imbalance between the delivery or synthesis of fatty acids in the liver and their disposal through oxidative pathways or secretion into the blood as a component of triacylglycerols in very low density lipoprotein. This disorder is called nonalcoholic fatty liver disease (NAFLD) in the absence of alcoholic abuse and viral hepatitis, and it is often associated with insulin resistance, obesity and type 2 diabetes. Also, liver steatosis can be induced by many other causes including excessive alcohol consumption, infection with genotype 3 hepatitis C virus and certain medications. Whereas hepatic triacylglycerol accumulation was once considered the ultimate effector of hepatic lipotoxicity, triacylglycerols per se are quite inert and do not induce insulin resistance or cellular injury. Rather, lipotoxic injury in the liver appears to be mediated by the global ongoing fatty acid enrichment in the liver, paralleling the development of insulin resistance. A considerable number of fatty acid metabolites may be responsible for hepatic lipotoxicity and liver injury. Additional key contributors include hepatic cytosolic lipases and the "lipophagy" of lipid droplets, as sources of hepatic fatty acids. The specific origin of the lipids, mainly triacylglycerols, accumulating in liver has been unraveled by recent kinetic studies, and identifying the origin of the accumulated triacylglycerols in the liver of patients with NAFLD may direct the prevention and treatment of this condition.

Molecular basis for homo-dimerization of the CIDE domain revealed by the crystal structure of the CIDE-N domain of FSP27

FSP27 (CIDE-3 in humans) plays critical roles in lipid metabolism and apoptosis and is known to be involved in regulation of lipid droplet (LD) size and lipid storage and apoptotic DNA fragmentation. Given that CIDE-containing proteins including FSP27 are associated with many human diseases including cancer, aging, diabetes, and obesity, studies of FSP27 and other CIDE-containing proteins are of great biological importance. As a first step toward elucidating the molecular mechanisms of FSP27-mediated lipid droplet growth and apoptosis, we report the crystal structure of the CIDE-N domain of FSP27 at a resolution of 2.0 Å. The structure revealed a possible biologically important homo-dimeric interface similar to that formed by the hetero-dimeric complex, CAD/ICAD. Comparison with other structural homologues revealed that the PB1 domain of BEM1P, ubiquitin-like domain of BAG6 and ubiquitin are structurally similar proteins. Our homo-dimeric structure of the CIDE-N domain of FSP27 will provide important information that will enable better understanding of the function of FSP27.

Dietary Lipid During Late-Pregnancy and Early-Lactation to Manipulate Metabolic and Inflammatory Gene Network Expression in Dairy Cattle Liver with a Focus on PPARs

Polyunsaturated (PUFA) long-chain fatty acids (LCFAs) are more potent in eliciting molecular and tissue functional changes in monogastrics than saturated LCFA. From −21 through 10 days relative to parturition dairy cows were fed no supplemental LCFA (control), saturated LCFA (SFAT; mainly 16:0 and 18:0), or fish oil (FISH; high-PUFA). Twenty-seven genes were measured via quantitative RT-PCR in liver tissue on day −14 and day 10. Expression of nuclear receptor co-activators (CARM1, MED1), LCFA metabolism (ACSL1, SCD, ACOX1), and inflammation (IL6, TBK1, IKBKE) genes was lower with SFAT than control on day −14. Expression of SCD, however, was markedly lower with FISH than control or SFAT on both −14 and 10 days. FISH led to further decreases in expression on day 10 of LCFA metabolism (CD36, PLIN2, ACSL1, ACOX1), intracellular energy (UCP2, STK11, PRKAA1), de novo cholesterol synthesis (SREBF2), inflammation (IL6, TBK1, IKBKE), and nuclear receptor signaling genes (PPARD, MED1, NRIP1). No change in expression was observed for PPARA and RXRA. The increase of DGAT2, PLIN2, ACSL1, and ACOX1 on day 10 versus −14 in cows fed SFAT suggested upregulation of both beta-oxidation and lipid droplet (LD) formation. However, liver triacylglycerol concentration was similar among treatments. The hepatokine FGF21 and the gluconeogenic genes PC and PCK1 increased markedly on day 10 versus −14 only in controls. At the levels supplemented, the change in the profile of metabolic genes after parturition in cows fed saturated fat suggested a greater capacity for uptake of fatty acids and intracellular handling without excessive storage of LD.

Dynamic changes in lipid droplet-associated proteins in the "browning" of white adipose tissues

The morphological and functional differences between lipid droplets (LDs) in brown (BAT) and white (WAT) adipose tissues will largely be determined by their associated proteins. Analysing mRNA expression in mice fat depots we have found that most LD protein genes are expressed at higher levels in BAT, with the greatest differences observed for Cidea and Plin5. Prolonged cold exposure, which induces the appearance of brown-like adipocytes in mice WAT depots, was accompanied with the potentiation of the lipolytic machinery, with changes in ATGL, CGI-58 and G0S2 gene expression. However the major change detected in WAT was the enhancement of Cidea mRNA. Together with the increase in Cidec, it indicates that LD enlargement through LD-LD transference of fat is an important process during WAT browning. To study the dynamics of this phenotypic change, we have applied 4D confocal microscopy in differentiated 3T3-L1 cells under sustained β-adrenergic stimulation. Under these conditions the cells experienced a LD remodelling cycle, with progressive reduction on the LD size by lipolysis, followed by the formation of new LDs, which were subjected to an enlargement process, likely to be CIDE-triggered, until the cell returned to the basal state. This transformation would be triggered by the activation of a thermogenic futile cycle of lipolysis/lipogenesis and could facilitate the molecular mechanism for the unilocular to multilocular transformation during WAT browning. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.

Time-dependent network analysis reveals molecular targets underlying the development of diet-induced obesity and non-alcoholic steatohepatitis

Prolonged high-fat diet leads to the development of obesity and multiple comorbidities including non-alcoholic steatohepatitis (NASH), but the underlying molecular basis is not fully understood. We combine molecular networks and time course gene expression profiles to reveal the dynamic changes in molecular networks underlying diet-induced obesity and NASH. We also identify hub genes associated with the development of NASH. Core diet-induced obesity networks were constructed using Ingenuity pathway analysis (IPA) based on 332 high-fat diet responsive genes identified in liver by time course microarray analysis (8 time points over 24 weeks) of high-fat diet-fed mice compared to normal diet-fed mice. IPA identified five core diet-induced obesity networks with time-dependent gene expression changes in liver. These networks were associated with cell-to-cell signaling and interaction (Network 1), lipid metabolism (Network 2), hepatic system disease (Network 3 and 5), and inflammatory response (Network 4). When we merged these core diet-induced obesity networks, Tlr2, Cd14, and Ccnd1 emerged as hub genes associated with both liver steatosis and inflammation and were altered in a time-dependent manner. Further, protein-protein interaction network analysis revealed Tlr2, Cd14, and Ccnd1 were interrelated through the ErbB/insulin signaling pathway. Dynamic changes occur in molecular networks underlying diet-induced obesity. Tlr2, Cd14, and Ccnd1 appear to be hub genes integrating molecular interactions associated with the development of NASH. Therapeutics targeting hub genes and core diet-induced obesity networks may help ameliorate diet-induced obesity and NASH.

Parasitic nematode-induced modulation of body weight and associated metabolic dysfunction in mouse models of obesity

Obesity is associated with a chronic low-grade inflammation characterized by increased levels of proinflammatory cytokines that are implicated in disrupted metabolic homeostasis. Parasitic nematode infection induces a polarized Th2 cytokine response and has been explored to treat autoimmune diseases. We investigated the effects of nematode infection against obesity and the associated metabolic dysfunction. Infection of RIP2-Opa1KO mice or C57BL/6 mice fed a high-fat diet (HFD) with Nippostrongylus brasiliensis decreased weight gain and was associated with improved glucose metabolism. Infection of obese mice fed the HFD reduced body weight and adipose tissue mass, ameliorated hepatic steatosis associated with a decreased expression of key lipogenic enzymes/mediators, and improved glucose metabolism, accompanied by changes in the profile of metabolic hormones. The infection resulted in a phenotypic change in adipose tissue macrophages that was characterized by upregulation of alternative activation markers. Interleukin-13 (IL-13) activation of the STAT6 signaling pathway was required for the infection-induced attenuation of steatosis but not for improved glucose metabolism, whereas weight loss was attributed to both IL-13/STAT6-dependent and -independent mechanisms. Parasitic nematode infection has both preventive and therapeutic effects against the development of obesity and associated features of metabolic dysfunction in mice.

Expression of CIDE proteins in clear cell renal cell carcinoma and their prognostic significance

Clear cell renal cell carcinoma (ccRCC) is the major and aggressive subtype of renal cell carcinoma. It is known to derive its histologic appearance from accumulation of abundant lipids and glycogens. The cell death-inducing DFF45-like effector (CIDE) family has been characterized as the lipid droplet proteins involved in the metabolism of lipid storage droplets. The purpose of this study was to evaluate the expression of CIDE proteins in ccRCC cells and to investigate their prognostic significance. We examined consecutive patients with sporadic ccRCC, who underwent nephrectomy, to measure their mRNA and protein expression of CIDE proteins. We found that Cidec and ADRP expression were significantly up-regulated in ccRCC, compared with normal kidney tissues. Cideb was down-regulated. We also found that Cideb was expressed more in low-grade ccRCC than in high-grade tumors. To further clarify the relationship between Cideb expression and patient prognosis, we evaluated 57 ccRCC patients followed up for 120 months. Reduced ccRCC Cideb expression was associated with a higher Fuhrman nuclear grade. Patients with high Cideb expression had better overall survival rate than those with low expression (p < 0.05). Cideb expression was an independent predictor of survival (p = 0.001). Although the biologic function of Cideb in ccRCC remains unknown, the expression level of Cideb might be a novel predictor of prognosis in ccRCC.

Chronic overexpression of PNPLA3I148M in mouse liver causes hepatic steatosis

A genetic variant in PNPLA3 (PNPLA3(I148M)), a triacylglycerol (TAG) hydrolase, is a major risk factor for nonalcoholic fatty liver disease (NAFLD); however, the mechanism underlying this association is not known. To develop an animal model of PNPLA3-induced fatty liver disease, we generated transgenic mice that overexpress similar amounts of wild-type PNPLA3 (PNPLA3(WT)) or mutant PNPLA3 (PNPLA3(I148M)) either in liver or adipose tissue. Overexpression of the transgenes in adipose tissue did not affect liver fat content. Expression of PNPLA3(I148M), but not PNPLA3(WT), in liver recapitulated the fatty liver phenotype as well as other metabolic features associated with this allele in humans. Metabolic studies provided evidence for 3 distinct alterations in hepatic TAG metabolism in PNPLA3(I148M) transgenic mice: increased formation of fatty acids and TAG, impaired hydrolysis of TAG, and relative depletion of TAG long-chain polyunsaturated fatty acids. These findings suggest that PNPLA3 plays a role in remodeling TAG in lipid droplets, as they accumulate in response to food intake, and that the increase in hepatic TAG levels associated with the I148M substitution results from multiple changes in hepatic TAG metabolism. The development of an animal model that recapitulates the metabolic phenotype of the allele in humans provides a new platform in which to elucidate the role of PNLPA3(I148M) in NAFLD.

Purification, crystallization and preliminary X-ray crystallographic analysis of the CIDE-N domain of Fsp27

Fsp27, a member of the CIDE protein family which is selectively expressed in adipocytes, has emerged as a novel regulator for unilocular lipid droplet (LD) formation, lipid metabolism, differentiation of adipocytes and insulin sensitivity. An LD is a subcellular compartment that is used by adipocytes for the efficient storage of fats. The CIDE-N domain of Fsp27 functions as a recruitment platform that induces the correct configuration of the Fsp27 CIDE-C domain to facilitate LD fusion. This study reports the high-yield expression of the mouse Fsp27 CIDE-N domain in Escherichia coli; a two-step purification protocol with high efficiency was established and crystallographic analysis was performed. The purity of the recombinant Fsp27 was >95% as assessed by SDS-PAGE. Crystals were obtained at 291 K using 28% polyethylene glycol 4000 as a precipitant. Diffraction data were collected to 1.92 Å resolution and the crystal belonged to space group P6(5), with unit-cell parameters a=b=63.3, c=37.4 Å, α=β=90, γ=120°. The components of the crystal were identified by ion-trap LC/MS/MS spectrometric analysis. The structure has been solved by molecular replacement and refinement is in progress.

CIDE proteins and lipid metabolism

Lipid homeostasis is maintained through the coordination of lipid metabolism in various tissues, including adipose tissue and the liver. The disruption of lipid homeostasis often results in the development of metabolic disorders such as obesity, diabetes mellitus, liver steatosis, and cardiovascular diseases. Cell death-inducing DNA fragmentation factor 45-like effector family proteins, including Cidea, Cideb, and Fsp27 (Cidec), are emerging as important regulators of various lipid metabolic pathways and play pivotal roles in the development of metabolic disorders. This review summarizes the latest cell death-inducing DNA fragmentation factor 45-like effector protein discoveries related to the control of lipid metabolism, with emphasis on the role of these proteins in lipid droplet growth in adipocytes and in the regulation of very low-density lipoprotein lipidation and maturation in hepatocytes.

Cidea promotes hepatic steatosis by sensing dietary fatty acids

High levels of dietary saturated fat have been closely associated with the development of hepatic steatosis, but the factors that mediate this process remain elusive. Here, we observed that the level of cell death-inducing DNA fragmentation factor-alpha-like effector a (Cidea) expression was highly correlated with the severity of hepatic steatosis in humans. Overexpression of Cidea in mouse liver resulted in increased hepatic lipid accumulation and the formation of large lipid droplets (LDs). In contrast, mice with a Cidea deficiency had decreased lipid accumulation and alleviated hepatic steatosis when they received a high-fat-diet feeding or in ob/ob mice. Furthermore, the knockdown of Cidea in livers of ob/ob mice resulted in significantly reduced hepatic lipid accumulation and smaller LDs. Importantly, we observed that Cidea expression in hepatocytes was specifically induced by saturated fatty acids (FAs), and such induction was reduced when sterol response element-binding protein (SREBP)1c was knocked down. In contrast, the overexpression of SREBP1c restored the saturated FA-induced expression of Cidea. In addition, we observed that the stability of Cidea protein in hepatocytes increased significantly in response to treatment with FAs.

CONCLUSION

Cidea plays critical roles in promoting hepatic lipid accumulation and in the development of hepatic steatosis by acting as a sensor that responds to diets that contain FAs.

Opposing roles of cell death-inducing DFF45-like effector B and perilipin 2 in controlling hepatic VLDL lipidation

Regulation of hepatic very low density lipoprotein (VLDL) assembly and maturation is crucial in controlling lipid homeostasis and in the development of metabolic disorders, including obesity, hepatic steatosis, and insulin resistance. Cideb, a member of cell death-inducing DFF45-like effector (CIDE) protein family, has been previously shown to promote VLDL lipidation and maturation. However, the precise subcellular location of Cideb-mediated VLDL lipidation and the factors modulating its activity remain elusive. In addition to its localization to endoplasmic reticulum (ER) and lipid droplets (LD), we observed that Cideb was also localized to the Golgi apparatus. Mature and lipid-rich VLDL particles did not accumulate in the Golgi apparatus in Cideb(-/-) livers. Interestingly, we observed that hepatic perilipin 2/adipose differentiation-related protein (ADRP) levels were markedly increased in Cideb(-/-) mice. Liver-specific knockdown of perilipin 2 in Cideb(-/-) mice resulted in the reduced accumulation of hepatic triglycerides (TAG), increased VLDL-TAG secretion, and the accumulation of mature TAG-rich VLDL in the Golgi apparatus. These data reveal that Cideb and perilipin 2 play opposing roles in controlling VLDL lipidation and hepatic lipid homeostasis.