Lipolysis - a highly regulated multi-enzyme complex mediates the catabolism of cellular fat stores

Brown and beige adipose tissues: phenotype and metabolic potential in mice and men

With the recent rediscovery of brown fat in adult humans, our outlook on adipose tissue biology has undergone a paradigm shift. While we attempt to identify, recruit, and activate classic brown fat stores in humans, identification of beige fat has also raised the possibility of browning our white fat stores. Whether such transformation of human white fat depots can be achieved to enhance the whole body oxidative potential remains to be seen. Evidence to date, however, largely points toward a major oxidative role only for classic brown fat depots, at least in rodents. White fat stores seem to provide the main fuel for sustaining thermogenesis via lipolysis. Interestingly, molecular markers consistent with both classic brown and beige fat identity can be observed in human supraclavicular depot, thereby complicating the discussion on beige fat in humans. Here, we review the recent advances made in our understanding of brown and beige fat in humans and mice. We further provide an overview of their plausible physiological relevance to whole body energy metabolism.

Gastrointestinal factors regulating lipid droplet formation in the intestine

Cytoplasmic lipid droplets (CLD) are considered as neutral lipid reservoirs, which protect cells from lipotoxicity. It became clear that these fascinating dynamic organelles play a role not only in energy storage and metabolism, but also in cellular lipid and protein handling, inter-organelle communication, and signaling among diverse functions. Their dysregulation is associated with multiple disorders, including obesity, liver steatosis and cardiovascular diseases. The central aim of this review is to highlight the link between intra-enterocyte CLD dynamics and the formation of chylomicrons, the main intestinal dietary lipid vehicle, after overviewing the morphology, molecular composition, biogenesis and functions of CLD.

Macrophage activation marker sCD163 correlates with accelerated lipolysis following LPS exposure: a human-randomised clinical trial

BACKGROUND

Macrophage activation determined by levels of soluble sCD163 is associated with obesity, insulin resistance, diabetes mellitus type 2 (DM2) and non-alcoholic fatty liver disease (NAFLD). This suggests that macrophage activation is involved in the pathogenesis of conditions is characterised by adaptions in the lipid metabolism. Since sCD163 is shed to serum by inflammatory signals including lipopolysaccharides (LPS, endotoxin), we investigated sCD163 and correlations with lipid metabolism following LPS exposure.

METHODS

Eight healthy male subjects were investigated on two separate occasions: (i) following an LPS exposure and (ii) following saline exposure. Each study day consisted of a four-hour non-insulin-stimulated period followed by a two-hour hyperinsulinemic euglycemic clamp period. A ³H-palmitate tracer was used to calculate the rate of appearance (Ra_palmitate). Blood samples were consecutively obtained throughout each study day. Abdominal subcutaneous adipose tissue was obtained for western blotting.

RESULTS

We observed a significant two-fold increase in plasma sCD163 levels following LPS exposure (P < 0.001), and sCD163 concentrations correlated positively with the plasma concentration of free fatty acids, Ra_palmitate, lipid oxidation rates and phosphorylation of the hormone-sensitive lipase at serine 660 in adipose tissue (P < 0.05, all). Furthermore, sCD163 concentrations correlated positively with plasma concentrations of cortisol, glucagon, tumour necrosis factor (TNF)-α, interleukin (IL)-6 and IL-10 (P < 0.05, all).

CONCLUSION

We observed a strong correlation between sCD163 and stimulation of lipolysis and fat oxidation following LPS exposure. These findings support preexisting theory that inflammation and macrophage activation play a significant role in lipid metabolic adaptions under conditions such as obesity, DM2 and NAFLD.

Modulatory Effects of Breed, Feeding Status, and Diet on Adipogenic, Lipogenic, and Lipolytic Gene Expression in Growing Iberian and Duroc Pigs

Meat quality depends on tissue composition which is in turn influenced by different factors, such as diet, genotype, age, or sex. We evaluated the effects of breed, 24 h fasting, and dietary energy source (HO: oleic acid versus CH: carbohydrates) on the expression of candidate genes involved in adipogenesis, lipogenesis, and lipolysis in the adipose tissue from Iberian and Duroc growing pigs. The Iberian pigs showed greater feed intake, backfat thickness, and saturated fatty acids (SFA) content in the subcutaneous fat, whereas the Duroc pigs had greater ham weight and polyunsaturated fatty acids (PUFA) content. In both breeds, the diet induced changes in the fatty acid (FA) composition of subcutaneous fat samples. The HO group had higher monounsaturated fatty acids (MUFA) and oleic acid, and lower SFA than the CH group. Regarding gene expression, breed and feeding status (fasting versus postprandial) had significant effects on gene expression, with quantitative interactions between them, while diet showed negligible effects. In general, adipogenic and lipogenic genes were upregulated in the Iberian pigs and in postprandial samples. In contrast, the expression of lipolytic genes showed complex interaction effects. Our results agree with the phenotypic differences between the Iberian and Duroc breeds and with the inhibition of lipogenesis by fasting. Quantitative interactions between breed and feeding status effects were observed, which indicates a different response to fasting of the two breeds, with the obese Iberian breed showing a more stable expression of lipogenic genes. These results highlight the complexity of lipid metabolism regulation, especially in relation to lipolysis processes.

A Proximity Labeling Strategy Provides Insights into the Composition and Dynamics of Lipid Droplet Proteomes

Lipid droplet (LD) functions are regulated by a complement of integral and peripheral proteins that associate with the bounding LD phospholipid monolayer. Defining the composition of the LD proteome has remained a challenge due to the presence of contaminating proteins in LD-enriched buoyant fractions. To overcome this limitation, we developed a proximity labeling strategy that exploits LD-targeted APEX2 to biotinylate LD proteins in living cells. Application of this approach to two different cell types identified the vast majority of previously validated LD proteins, excluded common contaminating proteins, and revealed new LD proteins. Moreover, quantitative analysis of LD proteome dynamics uncovered a role for endoplasmic reticulum-associated degradation in controlling the composition of the LD proteome. These data provide an important resource for future LD studies and demonstrate the utility of proximity labeling to study the regulation of LD proteomes.

Fucoxanthin and Its Metabolite Fucoxanthinol Do Not Induce Browning in Human Adipocytes

Rodent studies suggest that the antiobesity effects of fucoxanthin relate to activation of brown fat and conversion of white adipocytes to the brown phenotype. To evaluate the browning effect in human adipocytes, we investigated the genes involved in browning and measured the oxygen consumption rate (OCR). Data were analyzed by one way ANOVA. Relative to control, fucoxanthinol (1 μM, 0.1 μM, 0.01 μM, 1 nM, 0.1 nM), the metabolite present in human plasma, stimulated lipolysis acutely (mean ± SEM: 4.2 ± 0.8, 3.1 ± 0.6, 4.1 ± 0.9, 3.8 ± 0.7, 3.8 ± 0.7, respectively, p < 0.01). There was no effect on OCR or the mRNA expression of UCP1, CPT-1β, and GLUT4, the genes associated with browning of adipose tissue, when human adipocytes were treated with fucoxanthin or fucoxanthinol. -mRNA expression of PGC-1α, PPARα, PPARγ, PDK4, FAS, and the lipolytic enzymes was not significantly altered by fucoxanthinol treatment (p > 0.05). Thus, in human adipocytes, fucoxanthin and its metabolite do not stimulate conversion of white adipocytes to the brown phenotype.

Nobiletin induces brown adipocyte-like phenotype and ameliorates stress in 3T3-L1 adipocytes

Browning of white adipocytes (beiging) is an attractive therapeutic strategy against obesity and its associated metabolic complications. Nobiletin (NOB) is a polymethoxylated flavone present in citrus fruits and has been reported to have anti-obesity effects. Here, we report that nobiletin exerts dual modulatory effects on adipocytes via induction of browning in 3T3-L1 white adipocytes and amelioration of stress in adipocytes. Nobiletin-induced beiging was investigated by determining expression levels of beige-specific genes and proteins by RT-PCR and immunoblot analysis, respectively. Nobiletin treatment rapidly elevated the expression levels of beige-specific genes such as Cd137, Cidea, Tbx1, and Tmem26. Further, nobiletin enhanced expression of the key transcription factors C/EBPβ, PPARδ, and PPARα, which are responsible for remodeling of white adipocytes. Nobiletin also strikingly activated HIB1B brown adipocytes and induced mitochondrial biogenesis in 3T3-L1 white adipocytes. In addition, nobiletin altered the expression of several lipid metabolism-related proteins such as ACOX1, CPT1, FAS, p-PLIN, SREBP and SIRT1. Moreover, nobiletin ameliorated stress in adipocytes by inhibiting expression levels of key stress molecules such as JNK and c-JUN. Nobiletin-induced browning could be mediated by tight regulation of kinases, as nobiletin induced PKA and p-AMPK at the protein expression level, and inhibition of PKA and p-AMPK by H-89 and dorsomorphin, respectively, abolished expression of the thermogenic markers PGC-1α and UCP1. Taken together, our findings suggest that nobiletin plays a modulatory role in adipocytes via induction of browning in 3T3-L1 white adipocytes and activation of HIB1B brown adipocytes combined with amelioration of stress in adipocytes, thereby exhibiting therapeutic potential against obesity.

Impact of training state on fasting-induced regulation of adipose tissue metabolism in humans

Recruitment of fatty acids from adipose tissue is increased during fasting. However, the molecular mechanisms behind fasting-induced metabolic regulation in human adipose tissue and the potential impact of training state in this are unknown. Therefore the aim of the present study was to investigate 1) fasting-induced regulation of lipolysis and glyceroneogenesis in human adipose tissue as well as 2) the impact of training state on basal oxidative capacity and fasting-induced metabolic regulation in human adipose tissue. Untrained [maximal oxygen uptake (V̇o_2max) < 45 ml·min^-1·kg^-1] and trained subjects (V̇o_2max > 55 ml·min^-1·kg^-1) fasted for 36 h, and abdominal subcutaneous adipose tissue biopsies were obtained 2, 12, 24, and 36 h after a standardized meal. Adipose tissue oxidative phosphorylation complexes, phosphoenolpyruvate carboxykinase, and pyruvate dehydrogenase (PDH)-E1α protein as well as PDH kinase (PDK) 2, PDK4, and PDH phosphatase 2 mRNA content were higher in trained subjects than in untrained subjects. In addition, trained subjects had higher adipose tissue hormone-sensitive lipase Ser660 phosphorylation and adipose triglyceride lipase protein content as well as higher plasma free fatty acid concentration than untrained subjects during fasting. Moreover, adipose tissue PDH phosphorylation increased with fasting only in trained subjects. Taken together, trained subjects seem to possess higher basal adipose tissue oxidative capacity as well as higher capacity for regulation of lipolysis and for providing substrate for glyceroneogenesis in adipose tissue during fasting than untrained subjects. NEW & NOTEWORTHY This study shows for the first time higher protein content of lipolytic enzymes and higher oxidative phosphorylation protein in adipose tissue from trained subjects than from untrained subjects during fasting. Furthermore, trained subjects had higher capacity for adipose tissue glyceroneogenesis than untrained subjects.

Ymr210wp leads to the accumulation of phospholipids and steryl esters in yeast

Ymr210w was identified as a MAG (Monoacylglycerol) lipase. The accumulation of the phospholipids in the ymr210wΔ was not clearly understood. It was expressed in S. cerevisiae using pYES2/CT vector and His-tag purified recombinant protein confirmed TAG lipase activity. To further evaluate the role of YMR210w, ester hydrolase activity was also confirmed with pNP-acetate, pNP-butyrate and pNP - palmitate. GC-MS lipid profiling of ymr210wΔ showed an increase in the 15:0 Pentadecanoic acid by 76% among the total lipids. Phospholipid, Erucic acid 22:1 (Δ13) showed 43% increase while steryl esters showed significant changes with 16:0 hexadecanoic acid augmentations by 80% and 18:0 Octadecanoic acid by 165% when compared to wild type (WT). Increase in the steryl ester and TAG content supports the accumulation of lipid bodies in ymr210wΔ strain when compared with WT cells.

Modulation of Lipid Droplet Metabolism-A Potential Target for Therapeutic Intervention in Flaviviridae Infections

Lipid droplets (LDs) are endoplasmic reticulum (ER)-related dynamic organelles that store and regulate fatty acids and neutral lipids. They play a central role in cellular energy storage, lipid metabolism and cellular homeostasis. It has become evident that viruses have co-evolved in order to exploit host lipid metabolic pathways. This is especially characteristic of the Flaviviridae family, including hepatitis C virus (HCV) and several flaviviruses. Devoid of an appropriate lipid biosynthetic machinery of their own, these single-strand positive-sense RNA viruses can induce dramatic changes in host metabolic pathways to establish a favorable environment for viral multiplication and acquire essential components to facilitate their assembly and traffic. Here we have reviewed the current knowledge on the intracellular life cycle of those from the Flaviviridae family, with particular emphasis on HCV and dengue virus (DENV), and their association with the biosynthesis and metabolism of LDs, with the aim to identify potential antiviral targets for development of novel therapeutic interventions.

Disturbed bovine mitochondrial lipid metabolism: a review

In mammals, excess energy is stored primarily as triglycerides, which are mobilized when energy demands arise and cannot be covered by feed intake. This review mainly focuses on the role of long chain fatty acids in disturbed energy metabolism of the bovine species. Long chain fatty acids regulate energy metabolism as ligands of peroxisome proliferator-activated receptors. Carnitine acts as a carrier of fatty acyl groups as long-chain acyl-CoA derivatives do not penetrate the mitochondrial inner membrane. There are two different types of disorders in lipid metabolism which can occur in cattle, namely the hypoglycaemic-hypoinsulinaemic and the hyperglycaemic-hyperinsulinaemic type with the latter not always associated with ketosis. There is general agreement that fatty acid β-oxidation capability is limited in the liver of (ketotic) cows. In accord, supplemental L-carnitine decreased liver lipid accumulation in periparturient Holstein cows. Of note, around parturition concurrent oxidation of fatty acids in skeletal muscle is highly activated. Also peroxisomal β-oxidation in liver of dairy cows may be part of the hepatic adaptations to a negative energy balance (NEB) to break down fatty acids. An elevated blood concentration of nonesterified fatty acids is one of the indicators of NEB in cattle among others like increased β-hydroxy butyrate concentration, and decreased concentrations of glucose, insulin, and insulin-like growth factor-I. Assuming that liver carnitine concentrations might limit hepatic fatty acid oxidation capacity in dairy cows, further study of the role of acyl-CoA dehydrogenases and/or riboflavin in bovine ketosis is warranted.

DGAT1-Dependent Lipid Droplet Biogenesis Protects Mitochondrial Function during Starvation-Induced Autophagy

Lipid droplets (LDs) provide an "on-demand" source of fatty acids (FAs) that can be mobilized in response to fluctuations in nutrient abundance. Surprisingly, the amount of LDs increases during prolonged periods of nutrient deprivation. Why cells store FAs in LDs during an energy crisis is unknown. Our data demonstrate that mTORC1-regulated autophagy is necessary and sufficient for starvation-induced LD biogenesis. The ER-resident diacylglycerol acyltransferase 1 (DGAT1) selectively channels autophagy-liberated FAs into new, clustered LDs that are in close proximity to mitochondria and are lipolytically degraded. However, LDs are not required for FA delivery to mitochondria but instead function to prevent acylcarnitine accumulation and lipotoxic dysregulation of mitochondria. Our data support a model in which LDs provide a lipid buffering system that sequesters FAs released during the autophagic degradation of membranous organelles, reducing lipotoxicity. These findings reveal an unrecognized aspect of the cellular adaptive response to starvation, mediated by LDs.

Lipases and lipid droplet-associated protein expression in subcutaneous white adipose tissue of cachectic patients with cancer

Background

Cancer cachexia is a multifactorial metabolic syndrome characterized by marked loss of adipose tissue and skeletal muscle. Fat loss from adipose tissue in cancer cachexia is partly the result of increased lipolysis. Despite the growing amount of studies focused on elucidating the mechanisms through which lipolysis-related proteins regulate the lipolytic process, there are scarce data concerning that profile in the adipose tissue of cancer cachectic patients. Considering its fundamental importance, it was our main purpose to characterize the expression of the lipolysis-related proteins in the white adipose tissue of cachectic cancer patients.

Methods

Patients from the University Hospital were divided into three groups: control, cancer cachexia (CC), and weight-stable cancer patients (WSC). To gain greater insight into adipose tissue wasting during cancer cachexia progression, we have also analyzed an experimental model of cachexia (Walker 256 carcinosarcoma). Animals were divided into: control, intermediate cachexia (IC) and terminal cachexia (TC). Subcutaneous white adipose tissue of patients and epidydimal white adipose tissue of animals were investigated regarding molecular aspects by determining the protein content and gene expression of hormone-sensitive lipase (HSL), adipose triglyceride lipase (ATGL), comparative gene identification-58 (CGI-58), perilipin 1, leptin, adiponectin, visfatin, and tumour necrosis factor alpha (TNF-alpha).

Results

We found augmented lipolysis in CC associated with increased HSL expression, as well as upregulation of ATGL expression and reduction in perilipin 1 content. In IC, there was an imbalance in the secretion of pro- and anti-inflammatory factors. The alterations at the end-stage of cachexia were even more profound, and there was a reduction in the expression of almost all proteins analyzed in the animals.

Conclusions

Our findings show that cachexia induces important morphological, molecular, and humoral alterations in the white adipose tissue, which are specific to the stage of the syndrome.

Pioglitazone Enhances Cytosolic Lipolysis, β-oxidation and Autophagy to Ameliorate Hepatic Steatosis

Non-alcoholic fatty liver disease closely contributes to the development of obesity and insulin resistance. Even though pioglitazone has been reported to effectively lessen hepatic steatosis in human studies, its molecular mechanism remains unclear. This study is designed to investigate the regulation of cytosolic lipolysis, β-oxidation and autophagy by pioglitazone in a mice model of high fat diet (HFD) and cell model incubated with palmitic acid. Our results revealed hepatic steatosis was apparently induced by HFD and it was significantly reversed by pioglitazone. The serum insulin and hepatic triglyceride content was significantly decreased by co-administered pioglitazone with HFD. Hepatic expression of cytosolic-lipolysis related proteins (ATGL, HSL), β-oxidation (CPT-1A) and autophagy-related proteins (ATG7, LC3, LAL) was significantly enhanced by pioglitazone. Knockdown PPARα/PPARγ in AML12 cells significantly and proportionally reduced the expressions of ATGL, CPT-1A and LC3II, which was induced by pioglitazone. Furthermore, facilitation of the autophagic flux by pioglitazone was obviously blocked by lysosomal inhibitor, leupeptin, to demonstrate accumulation of the LC3II and intracellular lipid in AML12 cells. Our results demonstrated that pioglitazone attenuating the hepatic steatosis may be mediated by enhancing cytosolic lipolysis, β-oxidation and autophagy in a PPARα and PPARγ dependent manner.

Minimal alteration in muscle lipid genes following stabilized weight loss

Variations in skeletal muscle peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), carntine palmitoyltransferase-1 (CPT-1), perilipin protein 2 (PLIN2), and adipose tissue triglyceride lipase (ATGL), and comparative gene identification-58 (CGI-58) have been described as playing important roles in the metabolic regulation of lipid oxidation, and may influence intramyocellular lipid (IMCL) and muscle lipid droplet size (LDS). While acute changes in caloric balance and/or aerobic capacity may affect lipid metabolism, the influence of sustained weight loss derived from caloric restriction with weight loss (CWL) compared with exercise training with weight loss (EWL) on the abovementioned parameters has not been fully elucidated. Using a combination of metabolic feeding and/or supervised exercise training, we evaluated the influence of stabilized weight loss elicited by CWL compared with EWL without the confounding influence of acute alterations in caloric balance on molecular markers of mitochondrial metabolism and lipid droplet size in middle-aged overweight individuals with impaired glucose tolerance. There were no significant changes in PGC-1α, CPT-1, PLIN2, ATGL and, CGI-58 messenger RNA (mRNA) in CWL and EWL. While there were no changes in ATGL mRNA in CWL, there was a strong trend (P = 0.05) for the ΔATGL mRNA in EWL with stabilized weight loss. There were no significant changes in IMCL or LDS within skeletal muscle in CWL or EWL, respectively. In conclusion, under the conditions of chronic caloric balance following dietary or exercise-based interventions, mediators of mitochondrial function, IMCL and LDS, were largely unaffected. Future studies should focus on intervention-based changes in protein expression and/or phosphorylation and the relationship to physiological endpoints.

The perilipin family of lipid droplet proteins: Gatekeepers of intracellular lipolysis

Lipid droplets in chordates are decorated by two or more members of the perilipin family of lipid droplet surface proteins. The perilipins sequester lipids by protecting lipid droplets from lipase action. Their relative expression and protective nature is adapted to the balance of lipid storage and utilization in specific cells. Most cells of the body have tiny lipid droplets with perilipins 2 and 3 at the surfaces, whereas specialized fat-storing cells with larger lipid droplets also express perilipins 1, 4, and/or 5. Perilipins 1, 2, and 5 modulate lipolysis by controlling the access of lipases and co-factors of lipases to substrate lipids stored within lipid droplets. Although perilipin 2 is relatively permissive to lipolysis, perilipins 1 and 5 have distinct control mechanisms that are altered by phosphorylation. Here we evaluate recent progress toward understanding functions of the perilipins with a focus on their role in regulating lipolysis and autophagy. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Aldehyde dehydrogenase 1 a1 regulates energy metabolism in adipocytes from different species

BACKGROUND

Survival and longevity of xenotransplants depend on immune function and ability to integrate energy metabolism between cells from different species. However, mechanisms for interspecies cross talk in energy metabolism are not well understood. White adipose tissue stores energy and is capable of mobilization and dissipation of energy as heat (thermogenesis) by adipocytes expressing uncoupling protein 1 (Ucp1). Both pathways are under the control of vitamin A metabolizing enzymes. Deficient retinoic acid production in aldehyde dehydrogenase 1 A1 (Aldh1a1) knockout adipocytes (KO) inhibits adipogenesis and increases thermogenesis. Here we test the role Aldh1a1 in regulation of lipid metabolism in xenocultures.

METHODS

Murine wide-type (WT) and KO pre-adipocytes were encapsulated into a poly-L-lysine polymer that allows exchange of humoral factors <32kD via nanopores. Encapsulated murine adipocytes were co-incubated with primary differentiated canine adipocytes. Then, expression of adipogenic and thermogenic genes in differentiated canine adipocytes was detected by real-time polymerase chain reaction (PCR). The regulatory factors in WT and KO cells were identified by comparison of secretome using proteomics and in transcriptome by gene microarray.

RESULTS

Co-culture of encapsulated mouse KO vs WT adipocytes increased expression of peroxisome proliferator-activated receptor gamma (Pparg), but reduced expression of its target genes fatty acid binding protein 4 (Fabp4), and adipose triglyceride lipase (Atgl) in canine adipocytes, suggesting inhibition of PPARγ activation. Co-culture with KO adipocytes also induced expression of Ucp1 in canine adipocytes compared to expression in WT adipocytes. Cumulatively, murine KO compared to WT adipocytes decreased lipid accumulation in canine adipocytes. Comparative proteomics revealed significantly higher levels of vitamin A carriers, retinol binding protein 4 (RBP4), and lipokalin 2 (LCN2) in KO vs WT adipocytes.

CONCLUSIONS

Our data demonstrate the functional exchange of regulatory factors between adipocytes from different species for regulation of energy balance. RBP4 and LCN2 appear to be involved in the transport of retinoids for regulation of lipid accumulation and thermogenesis in xenocultures. While the rarity of thermogenic adipocytes in humans and dogs precludes their use for autologous transplantation, our study demonstrates that xenotransplantation of engineered cells could be a potential solution for the reduction in obesity in dogs and a strategy for translation to patients.

Omega-3 polyunsaturated fatty acids as a treatment strategy for nonalcoholic fatty liver disease

Obese and type 2 diabetic (T2DM) patients have a high prevalence of nonalcoholic fatty liver disease (NAFLD). NAFLD is a continuum of chronic liver diseases ranging from benign hepatosteatosis to nonalcoholic steatohepatitis (NASH), cirrhosis and primary hepatocellular cancer (HCC). Because of its strong association with the obesity epidemic, NAFLD is rapidly becoming a major public health concern worldwide. Surprisingly, there are no FDA approved NAFLD therapies; and current therapies focus on the co-morbidities associated with NAFLD, namely, obesity, hyperglycemia, dyslipidemia, and hypertension. The goal of this review is to provide background on the disease process, discuss human studies and preclinical models that have examined treatment options. We also provide an in-depth rationale for the use of dietary ω3 polyunsaturated fatty acid (ω3 PUFA) supplements as a treatment option for NAFLD. This focus is based on recent studies indicating that NASH patients and preclinical mouse models of NASH have low levels of hepatic C_20-22 ω3 PUFA. This decline in hepatic PUFA may account for the major phenotypic features associated with NASH, including steatosis, inflammation and fibrosis. Finally, our discussion will address the strengths and limitations of ω3 PUFA supplements use in NAFLD therapy.

Intracellular lipid droplets support osteoblast function

Bone formation is an osteoblast-specific process characterized by high energy demands due to the secretion of matrix proteins and mineralization vesicles. While glucose has been reported as the principle fuel source for osteoblasts, recent evidence supports the tenet that osteoblasts can utilize fatty acids as well. Although the ability to accumulate lipid droplets has been demonstrated in many cell types, there has been little evidence that osteoblasts possess this characteristic. The current study provides evidence that osteoblastogenesis is associated with lipid droplet accumulation capable of supplying energy substrates (fatty acids) required for the differentiation process. Understanding the role of fatty acids in metabolic programming of the osteoblast may lead to novel approaches to increase bone formation and ultimately bone mass.

Lipid droplet growth and adipocyte development: mechanistically distinct processes connected by phospholipids

The differentiation of preadipocytes into mature adipocytes is accompanied by the growth and formation of a giant, unilocular lipid droplet (LD). Mechanistically however, LD growth and adipogenesis are two different processes. Recent studies have uncovered a number of proteins that are able to regulate both LD dynamics and adipogenesis, such as SEIPIN, LIPIN and CDP-Diacylglycerol Synthases. It appears that phospholipids, phosphatidic acid in particular, play a critical role in both LD budding/growth and adipocyte development. This review summarizes recent advances, and aims to provide a better understanding of LD growth as well as adipogenesis, two critical aspects in mammalian fat storage. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Mild mitochondrial uncoupling induces HSL/ATGL-independent lipolysis relying on a form of autophagy in 3T3-L1 adipocytes

Obesity is characterized by an excessive triacylglycerol accumulation in white adipocytes. Various mechanisms allowing the tight regulation of triacylglycerol storage and mobilization by lipid droplet-associated proteins as well as lipolytic enzymes have been identified. Increasing energy expenditure by inducing a mild uncoupling of mitochondria in adipocytes might represent a putative interesting anti-obesity strategy as it reduces the adipose tissue triacylglycerol content (limiting alterations caused by cell hypertrophy) by stimulating lipolysis through yet unknown mechanisms, limiting the adverse effects of adipocyte hypertrophy. Herein, the molecular mechanisms involved in lipolysis induced by a mild uncoupling of mitochondria in white 3T3-L1 adipocytes were characterized. Mitochondrial uncoupling-induced lipolysis was found to be independent from canonical pathways that involve lipolytic enzymes such as HSL and ATGL. Finally, enhanced lipolysis in response to mitochondrial uncoupling relies on a form of autophagy as lipid droplets are captured by endolysosomal vesicles. This new mechanism of triacylglycerol breakdown in adipocytes exposed to mild uncoupling provides new insights on the biology of adipocytes dealing with mitochondria forced to dissipate energy.

Rosmarinic acid suppresses adipogenesis, lipolysis in 3T3-L1 adipocytes, lipopolysaccharide-stimulated tumor necrosis factor-α secretion in macrophages, and inflammatory mediators in 3T3-L1 adipocytes

Background: Rosmarinic acid (RA) is a natural phenol carboxylic acid with many promising biological effects. It may be a suitable candidate for improving obesity-related adipose tissue dysfunction.

Objective: We aimed to investigate the therapeutic use of RA as an anti-obesity agent by measuring its effects on adipogenesis, lipolysis, and messenger RNA (mRNA) expression of major adipokines in 3T3-L1 adipocytes; and its effects on lipopolysaccharide (LPS)-induced tumor necrosis factor-α (TNF-α) secretion in macrophages and inflammatory mediators in 3T3-L1 adipocytes incubated with macrophage-conditioned medium (MCM).

Methods: 3T3-L1 preadipocytes were used to explore how RA affects adipogenesis, as well as the involvement of phosphorylated extracellular signal-regulated kinase-1/2 (p-ERK1/2) and mothers against decapentaplegic homolog 3 (p-Smad3). 3T3-L1 preadipocytes were also differentiated into mature adipocytes to explore how RA affects basal and isoproterenol- and forskolin-stimulated lipolysis; and how RA affects key adipokines’ mRNA expression. RAW 264.7 macrophages were stimulated with LPS in the absence or presence of RA to explore RA’s effects on TNF-α secretion. MCM was collected and 3T3-L1 adipocytes were incubated with MCM to explore RA’s effects on interleukin-6 (IL-6), IL-1β, monocyte chemoattractant protein-1 (MCP-1), and RANTES mRNA expression.

Results: During the preadipocyte differentiation process, RA suppressed peroxisome proliferator-activated receptor-γ and CCAAT/enhancer binding protein-α, and activated p-ERK1/2 and p-Smad3; inhibition of adipogenesis by RA was partially restored following treatment with p-ERK1/2 and p-Smad3 inhibitors. In mature adipocytes, RA inhibited basal lipolysis; phosphodiesterase-3 inhibitor reversed this. RA also inhibited isoproterenol- and forskolin-stimulated glycerol and free fatty acid release, and the phosphorylation of hormone-sensitive lipase and perilipin. RA had no effects on leptin, adiponectin, resistin, or visfatin mRNA expression. RA suppressed TNF-α mRNA expression and secretion in LPS-stimulated RAW 264.7 macrophages; and reduced LPS-MCM-induced IL-6, IL-1β, MCP-1, and RANTES mRNA expression in 3T3-L1 adipocytes.

Conclusions: RA exerts inhibitory effects on adipogenesis, lipolysis, and inflammation. RA could be a promising natural product for improving adipose mobilization in obesity.

Serum levels of fatty acid binding protein 4 and fat metabolic markers in relation to catecholamines following exercise

BACKGROUND

Lipolysis is stimulated by activation of adrenergic inputs to adipose tissues. Our recent study showed that serum concentrations of fatty acid binding protein 4 (FABP4) are robustly elevated in patients with acute myocardial infarction and ventricular tachyarrhythmia, that display a marked activation of the sympathetic nervous system (SNS). However, it remains unknown whether circulating FABP4 concentrations are associated with exercise-induced SNS activation.

METHODS

Thirty one healthy volunteers underwent cardiopulmonary exercise testing on a cycle ergometer up to the workload levels below and above anaerobic threshold, low- and high-intensity exercise, respectively. Serial blood samplings were performed before and after exercise.

RESULTS

High-intensity exercise significantly increased serum concentrations of FABP4 and catecholamines, and their concentrations declined fast thereafter in a similar fashion. These changes were accompanied by little, if any, changes in other metabolic markers. Regardless of adiposity, percent change from baseline to peak FABP4 levels (%FABP4) was comparable in all subjects. Stepwise regression analysis revealed that %FABP4 was highly correlated with that in norepinephrine.

CONCLUSIONS

Our study reveals the significant correlation between circulating FABP4 and norepinephrine levels during exercise testing. Together with the fact that FABP4 is secreted from adipocytes via β-adrenergic-mediated lipolytic mechanisms, this study suggests FABP4 as a potential biomarker for adrenergic overdrive.

Lipid Droplet-Associated Hydrolase Promotes Lipid Droplet Fusion and Enhances ATGL Degradation and Triglyceride Accumulation

Lipid droplet (LD)-associated hydrolase (LDAH) is a newly identified LD protein abundantly expressed in tissues that predominantly store triacylglycerol (TAG). However, how LDAH regulates TAG metabolism remains unknown. We found that upon oleic acid loading LDAH translocalizes from the ER to newly formed LDs, and induces LD coalescence in a tubulin-dependent manner. LDAH overexpression and downregulation in HEK293 cells increase and decrease, respectively, TAG levels. Pulse and chase experiments show that LDAH enhances TAG biogenesis, but also decreases TAG turnover and fatty acid release from cells. Mutations in predicted catalytic and acyltransferase motifs do not influence TAG levels, suggesting that the effect is independent of LDAH’s enzymatic activity. However, a LDAH alternative-splicing variant missing 90 amino acids at C-terminus does not promote LD fusion or TAG accumulation, while it still localizes to LDs. Interestingly, LDAH enhances polyubiquitination and proteasomal degradation of adipose triglyceride lipase (ATGL), a rate limiting enzyme of TAG hydrolysis. Co-expression of ATGL reverses the changes in LD phenotype induced by LDAH, and both proteins counterbalance their effects on TAG stores. Together, these studies support that under conditions of TAG storage in LDs LDAH plays a primarily lipogenic role, inducing LD growth and enhancing degradation of ATGL.

Role of cyclooxygenase-mediated metabolites in lipid metabolism and expression of some immune-related genes in juvenile grass carp (Ctenopharyngodon idellus) fed arachidonic acid

Cyclooxygenase (COX) catalyzes the conversion of arachidonic acid (ARA) to prostaglandins, and COX-mediated metabolites play important roles in the regulation of lipid metabolism and immunity in mammals. However, such roles of COX in fish remain largely unknown. In this study, we designed three semi-purified diets, namely ARA-free (control), ARA, and ARA + acetylsalicylic acid (ASA; a COX inhibitor), and used them to feed grass carp (27.65 ± 3.05 g) for 8 weeks. The results showed that dietary ARA significantly increased the amount of ARA in the hepatopancreas, muscle, and kidney (P < 0.05), whereas this increase was reduced by dietary ASA. The hepatopancreatic prostaglandin E₂ content increased in the ARA group, and this increase was inhibited by ASA (P < 0.05). ARA decreased the lipid content in the hepatopancreas, whereas ASA recovered lipid content to a significant level (P < 0.05). ARA significantly decreased the messenger RNA (mRNA) expression levels of fatty acid synthase and stearoyl-CoA desaturase in the hepatopancreas (P < 0.05). However, ASA did not rescue the mRNA expression of these genes (P > 0.05). Interestingly, ARA significantly enhanced the level of peroxisome proliferator-activated receptor α gene expression, and this increase was attenuated by ASA (P < 0.05). Finally, ARA significantly enhanced the mRNA expression of myeloid differentiation factor 88 (MyD88) in the kidney, and ASA attenuated the expression of toll-like receptor 22 and MyD88 (P < 0.05). In conclusion, our findings suggest that COX metabolites play important roles in the inhibition of lipid accumulation in the hepatopancreas of grass carp fed with ARA and that regulation of gene expression promotes lipid catabolism rather than lipogenic activities. Additionally, these eicosanoids might participate in the upregulation of immunity-related genes in the kidney.

Silencing of AQP3 induces apoptosis of gastric cancer cells via downregulation of glycerol intake and downstream inhibition of lipogenesis and autophagy

Gastric cancer (GC) has a poor prognosis and is a leading cause of cancer-related death. Optimal therapeutic targets have not been identified. AQP3 is capable of transporting glycerol across the cytomembrane. Previous studies have shown that AQP3 is involved in proliferation, invasion and migration by regulating glycerol and lipid metabolism in diverse cancer cell types. However, the potential roles of glycerol and lipid metabolism in AQP3-related cell apoptosis in GC remain unclear. In this study, we observed that AQP3 expression was upregulated in tumor tissues, and positively correlated with tumor size, lymph node metastasis and glycerol concentration in human GC samples. Silencing of AQP3 resulted in decreased glycerol intake and impaired lipid synthesis, which contributed to increased cell apoptosis. Furthermore, inhibition of autophagy induced by AQP3 knockdown promoted cell apoptosis. Administration of either glycerol or rapamycin restored cell viability, and overexpression of AQP3 increased cell viability by upregulating cellular glycerol metabolism and autophagy. Our study demonstrates that the increase in cell apoptosis of AQP3-deficient GC cells is a consequence of reduced glycerol uptake and lipogenesis and is associated with autophagy inhibition induced by AQP3 deficiency.

Lipid Droplets: Formation to Breakdown

One of the most exciting areas of cell biology during the last decade has been the study of lipid droplets. Lipid droplets allow cells to store non-polar molecules such as neutral lipids in specific compartments where they are sequestered from the aqueous environment of the cell yet can be accessed through regulated mechanisms. These structures are highly conserved, appearing in organisms throughout the phylogenetic tree. Until somewhat recently, lipid droplets were widely regarded as inert, however progress in the field has continued to demonstrate their vast roles in a number of cellular processes in both mitotic and post-mitotic cells. No doubt the increase in the attention given to lipid droplet research is due to their central role in current pressing human diseases such as obesity, type-2 diabetes, and atherosclerosis. This review provides a mechanistic timeline from neutral lipid synthesis through lipid droplet formation and size augmentation to droplet breakdown.

Piceatannol Reduces Fat Accumulation in Caenorhabditis elegans

Excess fat accumulation and abnormal metabolism are involved in numerous diseases and thus the research on identification of compounds that can regulate energy homeostasis could significantly facilitate the current effort to prevent and/or treat metabolic disorders. Piceatannol, one of the natural stilbenes, was previously found to decrease lipid accumulation of 3T3-L1 adipocytes. However, its role in fat metabolism in vivo is not known. Thus, Caenorhabditis elegans as an animal model was used in the current study to determine the effect of piceatannol on fat accumulation and its underlying mechanisms. The results showed that 50 and 100 μM piceatannol significantly reduced fat accumulation of wild-type worms grown in normal and high-glucose conditions without altering the growth rate, worm length, pumping rate, or moving speed. The current study further indicated that piceatannol decreased the expression of sbp-1 (encodes an ortholog of mammalian sterol regulatory element-binding protein) and its target gene fasn-1 (encodes an ortholog of fatty acid synthase) as well as increased the expression of hosl-1 (encodes an ortholog of hormone-sensitive lipase) in glucose-treated worms. These data suggested that piceatannol reduced fat accumulation in C. elegans by suppression of genes involved in lipid synthesis and possibly through stimulation of lipolysis. Given that piceatannol exerts similar effects in both C. elegans and 3T3-L1 cells, our finding could provide a mechanistic insight into the role of piceatannol in lipid metabolism in mammals.

Adipose tissue lipolysis and remodeling during the transition period of dairy cows

Elevated concentrations of plasma fatty acids in transition dairy cows are significantly associated with increased disease susceptibility and poor lactation performance. The main source of plasma fatty acids throughout the transition period is lipolysis from adipose tissue depots. During this time, plasma fatty acids serve as a source of calories mitigating the negative energy balance prompted by copious milk synthesis and limited dry matter intake. Past research has demonstrated that lipolysis in the adipose organ is a complex process that includes not only the activation of lipolytic pathways in response to neural, hormonal, or paracrine stimuli, but also important changes in the structure and cellular distribution of the tissue in a process known as adipose tissue remodeling. This process involves an inflammatory response with immune cell migration, proliferation of the cellular components of the stromal vascular fraction, and changes in the extracellular matrix. This review summarizes current knowledge on lipolysis in dairy cattle, expands on the new field of adipose tissue remodeling, and discusses how these biological processes affect transition cow health and productivity.

Insulin resistance and β-cell function in Colombian mestizo and Embera-Chamí populations and their relation with adiposity degree

BACKGROUND AND OBJECTIVE

Insulin resistance (IR) is a condition favored by metabolic and endocrine changes experienced by adipose tissue in the context of obesity. The prevalence and the presentation of both IR and obesity vary among the populations, and may be affected by ancestral genetic composition among other factors. The aim of this study was to compare the presence of IR and obesity in Amerindians of the Embera-Chamí ethnicity and Colombian mestizo population.

PATIENTS AND METHODS

A sample of 630 individuals, 471 mestizos and 159 Amerindians of the Embera-Chamí ethnicity, from the general population of Colombia were studied. For each participant, anthropometric and biochemical measurements, as well as blood pressure and the Homeostatic Model Assessment (HOMA) of IR and β-cell function (%B) were recorded. These values were compared between the two populations.

RESULTS

While prevalence of central obesity was similar in both populations (48.7% and 42.6% in the mestizo and Embera groups respectively; p=0.148), body mass index (BMI) values suggested a higher prevalence of overweight in the Embera than in mestizo population (43.4% Embera, 31.8% mestizo; p=0.027). Despite the similarities in the prevalence of HOMA-IR and HOMA-%B status between both populations, the Embera population had a significantly greater pancreatic β-cell function, higher insulin levels, and better glucose control, across BMI and central obesity categories, than the mestizo population.

CONCLUSION

There are differences in aspects related to energy metabolism between the samples of the mestizo and Amerindian populations analyzed.

Adipocyte-Tumor Cell Metabolic Crosstalk in Breast Cancer

The tumor stroma is a heterogeneous ecosystem comprising matrix, fibroblasts, and immune cells and has an important role in cancer progression. Adipocytes constitute a major component of breast stroma, and significant emerging evidence demonstrates a reciprocal metabolic adaptation between stromal adipocytes and breast cancer (BC) cells. Recent observations promote a model where adipocytes respond to cancer cell-derived endocrine and paracrine signaling to provide metabolic substrates, which in turn drive enhanced cancer cell proliferation, invasion, and treatment resistance. Further defining the mechanisms that underpin this dynamic interaction between stromal adipocytes and BC cells, especially in the context of obesity, may identify novel therapeutic strategies. These will become increasingly important in addressing the clinical challenges presented by obesity and metabolic syndromes.

Intramyocellular fat storage in metabolic diseases

Over the past decades, obesity and its metabolic co-morbidities such as type 2 diabetes (T2D) developed to reach an endemic scale. However, the mechanisms leading to the development of T2D are still poorly understood. One main predictor for T2D seems to be lipid accumulation in "non-adipose" tissues, best known as ectopic lipid storage. A growing body of data suggests that these lipids may play a role in impairing insulin action in metabolic tissues, such as liver and skeletal muscle. This review aims to discuss recent literature linking ectopic lipid storage and insulin resistance, with emphasis on lipid deposition in skeletal muscle. The link between skeletal muscle lipid content and insulin sensitivity, as well as the mechanisms of lipid-induced insulin resistance and potential therapeutic strategies to alleviate lipotoxic lipid pressure in skeletal muscle will be discussed.

Adipocyte STAT5 deficiency promotes adiposity and impairs lipid mobilisation in mice

AIMS/HYPOTHESIS

Dysfunction of lipid metabolism in white adipose tissue can substantially interfere with health and quality of life, for example in obesity and associated metabolic diseases. Therefore, it is important to characterise pathways that regulate lipid handling in adipocytes and determine how they affect metabolic homeostasis. Components of the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway are involved in adipocyte physiology and pathophysiology. However, the exact physiological importance of the STAT family member STAT5 in white adipose tissue is yet to be determined. Here, we aimed to delineate adipocyte STAT5 functions in the context of lipid metabolism in white adipose tissue.

METHODS

We generated an adipocyte specific knockout of Stat5 in mice using the Adipoq-Cre recombinase transgene followed by in vivo and in vitro biochemical and molecular studies.

RESULTS

Adipocyte-specific deletion of Stat5 resulted in increased adiposity, while insulin resistance and gluconeogenic capacity was decreased, indicating that glucose metabolism can be improved by interfering with adipose STAT5 function. Basal lipolysis and fasting-induced lipid mobilisation were diminished upon STAT5 deficiency, which coincided with reduced levels of the rate-limiting lipase of triacylglycerol hydrolysis, adipose triglyceride lipase (ATGL, encoded by Pnpla2) and its coactivator comparative gene identification 58 (CGI-58). In a mechanistic analysis, we identified a functional STAT5 response element within the Pnpla2 promoter, indicating that Pnpla2 is transcriptionally regulated by STAT5.

CONCLUSIONS/INTERPRETATION

Our findings reveal an essential role for STAT5 in maintaining lipid homeostasis in white adipose tissue and provide a rationale for future studies into the potential of STAT5 manipulation to improve outcomes in metabolic diseases.

The effects of detraining and training on adipose tissue lipid droplet in obese mice after chronic high-fat diet

Background

It is well known that exercise promotes lipolysis by stimulating the lipid droplet (LD) signaling pathway. However, few studies have been conducted to examine the effect of detraining with high fat diet (HFD) and training effects after long-term HFD. Here, we investigated the effect of detraining and training on adipose tissue LD pathway in diet-induced obese mice after continuous HFD.

Methods

Seventy male C57BL/6 mice were randomly assigned into a Normal diet + Sedentary group (ND, n = 10) or a High-fat diet + Sedentary group (HF, n = 50); in the HF group, obesity was induced by a 45% fat chow for six weeks. For the subsequent eight weeks, the HF group was randomly subdivided into an HF (n = 30) or an HF + training group (HFT, n = 20), and the HFT group was subjected to treadmill training while on an HFD. Following this eight-week period, the HFT group stopped exercising (HFT-DT group, n = 10), and the mice in the HF group were randomly subdivided into an HF (n = 10) or an HF + training group (HF-T, n = 10). After training and detraining, abdominal visceral fat was obtained and analyzed by histological staining and western blot.

Results

Treadmill exercise decreased body weight and fat mass (P <0.05), and increased the levels of PKA, perilipin1, CGI-58, ATGL, and HSL (P <0.05) after eight weeks of training. Following eight weeks of detraining, the levels of PKA and HSL were decreased (P <0.05); however, exercise after chronic HFD increased the levels of PKA, perilipin1, CGI-58, ATGL, and HSL (P <0.05), and decreased body weight and fat mass (P <0.05).

Conclusions

Regardless of dietary restrictions, exercise is an effective treatment for obesity, owing to the regulation of LD signaling proteins. Moreover, the effects of regular exercise after chronic HFD were similar to those of exercise in the absence of HFD. Therefore, although obesity is induced by chronic HFD, exercise without dietary change is sufficiently effective for obesity treatment regardless of the preceding HFD period.

The why, when and how of lipid droplet diversity

Lipid droplets are the universal cellular organelles for the transient or long-term storage of lipids. The number, size and composition of lipid droplets vary greatly within cells in a homogenous population as well as in different cell types. The variability of intracellular lipid-storage organelles reflects the diversification of lipid droplet composition and function. Lipid droplet diversification results, for example, in two cellular lipid droplet populations that are prone to diminish and grow, respectively. The aberrant accumulation or depletion of lipids are hallmarks or causes of various human pathologies. Thus, a better understanding of the origins of lipid droplet diversification is not only a fascinating cell biology question but also potentially serves to improve comprehension of pathologies that entail the accumulation of lipids. This Commentary covers the lipid droplet life cycle and highlights the early steps during lipid droplet biogenesis, which we propose to be the potential driving forces of lipid droplet diversification.

Lipoquality control by phospholipase A2 enzymes

The phospholipase A₂ (PLA₂) family comprises a group of lipolytic enzymes that typically hydrolyze the sn-2 position of glycerophospholipids to give rise to fatty acids and lysophospholipids. The mammalian genome encodes more than 50 PLA₂s or related enzymes, which are classified into several subfamilies on the basis of their structures and functions. From a general viewpoint, the PLA₂ family has mainly been implicated in signal transduction, producing bioactive lipid mediators derived from fatty acids and lysophospholipids. Recent evidence indicates that PLA₂s also contribute to phospholipid remodeling for membrane homeostasis or energy production for fatty acid β-oxidation. Accordingly, PLA₂ enzymes can be regarded as one of the key regulators of the quality of lipids, which I herein refer to as lipoquality. Disturbance of PLA₂-regulated lipoquality hampers tissue and cellular homeostasis and can be linked to various diseases. Here I overview the current state of understanding of the classification, enzymatic properties, and physiological functions of the PLA₂ family.

Beyond thermoregulation: metabolic function of cetacean blubber in migrating bowhead and beluga whales

The processes of lipid deposition and utilization, via the gene leptin (Lep), are poorly understood in taxa with varying degrees of adipose storage. This study examines how these systems may have adapted in marine aquatic environments inhabited by cetaceans. Bowhead (Balaena mysticetus) and beluga whales (Delphinapterus leucas) are ideal study animals-they possess large subcutaneous adipose stores (blubber) and undergo bi-annual migrations concurrent with variations in food availability. To answer long-standing questions regarding how (or if) energy and lipid utilization adapted to aquatic stressors, we quantified variations in gene transcripts critical to lipid metabolism related to season, age, and blubber depth. We predicted leptin tertiary structure conservation and assessed inter-specific variations in Lep transcript numbers between bowheads and other mammals. Our study is the first to identify seasonal and age-related variations in Lep and lipolysis in these cetaceans. While Lep transcripts and protein oscillate with season in adult bowheads reminiscent of hibernating mammals, transcript levels reach up to 10 times higher in bowheads than any other mammal. Data from immature bowheads are consistent with the hypothesis that short baleen inhibits efficient feeding. Lipolysis transcripts also indicate young Fall bowheads and those sampled during Spring months limit energy utilization. These novel data from rarely examined species expand the existing knowledge and offer unique insight into how the regulation of Lep and lipolysis has adapted to permit seasonal deposition and maintain vital blubber stores.

Hepatic Retinyl Ester Hydrolases and the Mobilization of Retinyl Ester Stores

For mammals, vitamin A (retinol and metabolites) is an essential micronutrient that is required for the maintenance of life. Mammals cannot synthesize vitamin A but have to obtain it from their diet. Resorbed dietary vitamin A is stored in large quantities in the form of retinyl esters (REs) in cytosolic lipid droplets of cells to ensure a constant supply of the body. The largest quantities of REs are stored in the liver, comprising around 80% of the body’s total vitamin A content. These hepatic vitamin A stores are known to be mobilized under times of insufficient dietary vitamin A intake but also under pathological conditions such as chronic alcohol consumption and different forms of liver diseases. The mobilization of REs requires the activity of RE hydrolases. It is astounding that despite their physiological significance little is known about their identities as well as about factors or stimuli which lead to their activation and consequently to the mobilization of hepatic RE stores. In this review, we focus on the recent advances for the understanding of hepatic RE hydrolases and discuss pathological conditions which lead to the mobilization of hepatic RE stores.

Novel Browning Agents, Mechanisms, and Therapeutic Potentials of Brown Adipose Tissue

Nonshivering thermogenesis is the process of biological heat production in mammals and is primarily mediated by brown adipose tissue (BAT). Through ubiquitous expression of uncoupling protein 1 (Ucp1) on the mitochondrial inner membrane, BAT displays uncoupling of fuel combustion and ATP production in order to dissipate energy as heat. Because of its crucial role in regulating energy homeostasis, ongoing exploration of BAT has emphasized its therapeutic potential in addressing the global epidemics of obesity and diabetes. The recent appreciation that adult humans possess functional BAT strengthens this prospect. Furthermore, it has been identified that there are both classical brown adipocytes residing in dedicated BAT depots and "beige" adipocytes residing in white adipose tissue depots that can acquire BAT-like characteristics in response to environmental cues. This review aims to provide a brief overview of BAT research and summarize recent findings concerning the physiological, cellular, and developmental characteristics of brown adipocytes. In addition, some key genetic, molecular, and pharmacologic targets of BAT/Beige cells that have been reported to have therapeutic potential to combat obesity will be discussed.

A Luciferase-fragment Complementation Assay to Detect Lipid Droplet-associated Protein-Protein Interactions

A critical challenge for all organisms is to carefully control the amount of lipids they store. An important node for this regulation is the protein coat present at the surface of lipid droplets (LDs), the intracellular organelles dedicated to lipid storage. Only limited aspects of this regulation are understood so far. For the probably best characterized case, the regulation of lipolysis in mammals, some of the major protein players have been identified, and it has been established that this process crucially depends on an orchestrated set of protein-protein interactions. Proteomic analysis has revealed that LDs are associated with dozens, if not hundreds, of different proteins, most of them poorly characterized, with even fewer data regarding which of them might physically interact. To comprehensively understand the mechanism of lipid storage regulation, it will likely be essential to define the interactome of LD-associated proteins.Previous studies of such interactions were hampered by technical limitations. Therefore, we have developed a split-luciferase based protein-protein interaction assay and test for interactions among 47 proteins from Drosophila and from mouse. We confirmed previously described interactions and identified many new ones. In 1561 complementation tests, we assayed for interactions among 487 protein pairs of which 92 (19%) resulted in a successful luciferase complementation. These results suggest that a prominent fraction of the LD-associated proteome participates in protein-protein interactions.In targeted experiments, we analyzed the two proteins Jabba and CG9186 in greater detail. Jabba mediates the sequestration of histones to LDs. We successfully applied our split luciferase complementation assay to learn more about this function as we were e.g. able to map the interaction between Jabba and histones. For CG9186, expression levels affect the positioning of LDs. Here, we reveal the ubiquitination of CG9186, and link this posttranslational modification to LD cluster induction.

Divergent functions of endotrophin on different cell populations in adipose tissue

Endotrophin is a cleavage product of collagen 6 (Col6) in adipose tissue (AT). Previously, we demonstrated that endotrophin serves as a costimulator to trigger fibrosis and inflammation within the unhealthy AT milieu. However, how endotrophin affects lipid storage and breakdown in AT and how different cell types in AT respond to endotrophin stimulation remain unknown. In the current study, by using a doxycycline-inducible mouse model, we observed significant upregulation of adipogenic genes in the white AT (WAT) of endotrophin transgenic mice. We further showed that the mice exhibited inhibited lipolysis and accelerated hypertrophy and hyperplasia in WAT. To investigate the effects of endotrophin in vitro, we incubated different cell types from AT with conditioned medium from endotrophin-overexpressing 293T cells. We found that endotrophin activated multiple pathological pathways in different cell types. Particularly in 3T3-L1 adipocytes, endotrophin triggered a fibrotic program by upregulating collagen genes and promoted abnormal lipid accumulation by downregulating hormone-sensitive lipolysis gene and decreasing HSL phosphorylation levels. In macrophages isolated from WAT, endotrophin stimulated higher expression of the collagen-linking enzyme lysyl oxidase and M1 proinflammatory marker genes. In the stromal vascular fraction isolated from WAT, endotrophin induced upregulation of both profibrotic and proinflammatory genes. In conclusion, our study provides a new perspective on the effect of endotrophin in abnormal lipid accumulation and a mechanistic insight into the roles played by adipocytes and a variety of other cell types in AT in shaping the unhealthy microenvironment upon endotrophin treatment.

The Role of Cytochrome P450 Epoxygenases, Soluble Epoxide Hydrolase, and Epoxyeicosatrienoic Acids in Metabolic Diseases

Metabolic diseases are associated with an increased risk of developing cardiovascular disease. The features comprising metabolic diseases include obesity, insulin resistance, hyperglycemia, hyperlipidemia, and hypertension. Recent evidence has emerged showcasing a role for cytochrome P450 epoxygenases, soluble epoxide hydrolase, and epoxyeicosatrienoic acids (EETs) in the development and progression of metabolic diseases. This review discusses the current knowledge related to the modulation of cytochrome P450 epoxygenases and soluble epoxide hydrolase to alter concentrations of biologically active EETs, resulting in effects on insulin resistance, lipid metabolism, obesity, and diabetes. Future areas of research to address current deficiencies in the understanding of these enzymes and their eicosanoid metabolites in various aspects of metabolic diseases are also discussed.

Global Changes in Lipid Profiles of Mouse Cortex, Hippocampus, and Hypothalamus Upon p53 Knockout

Comprehensive lipidomic profiling in three different brain tissues (cortex, hippocampus, and hypothalamus) of mouse with p53 deficiency was performed by nanoflow liquid chromatography-tandem mass spectrometry (nLC-MS/MS) and the profile was compared with that of the wild type. p53 gene is a well-known tumour suppressor that prevents genome mutations that can cause cancers. More than 300 lipids (among 455 identified species), including phospholipids (PLs), sphingolipids, ceramides (Cers), and triacylglycerols (TAGs) were quantitatively analysed by selective reaction monitoring (SRM) of nanoflow ultrahigh performance liquid chromatography-electrospray ionization-tandem mass spectrometry (nUPLC-ESI-MS/MS). Among the three different neural tissues, hypothalamus demonstrated the most evident lipid profile changes upon p53 knockout. Alterations of PLs containing acyl chains of docosahexaenoic acid and arachidonic acid (highly enriched polyunsaturated fatty acids in the nervous system) were examined in relation to cell apoptosis upon p53 knockout. Comparison between sphingomyelins (SMs) and Cers showed that the conversion of SM to Cer did not effectively progress in the hypothalamus, resulting in the accumulation of SMs, possibly due to the inhibition of apoptosis caused by the lack of p53. Furthermore, TAGs were considerably decreased only in the hypothalamus, indicative of lipolysis that led to substantial weight loss of adipose tissue and muscles.

Adipocyte-specific overexpression of retinol-binding protein 4 causes hepatic steatosis in mice

There is considerable evidence that both retinoids and retinol-binding protein 4 (RBP4) contribute to the development of liver disease. To understand the basis for this, we generated and studied transgenic mice that express human RBP4 (hRBP4) specifically in adipocytes. When fed a chow diet, these mice show an elevation in adipose total RBP4 (mouse RBP4 + hRBP4) protein levels. However, no significant differences in plasma RBP4 or retinol levels or in hepatic or adipose retinoid (retinol, retinyl ester, and all-trans-retinoic acid) levels were observed. Strikingly, male adipocyte-specific hRBP4 mice fed a standard chow diet display significantly elevated hepatic triglyceride levels at 3-4 months of age compared to matched littermate controls. When mice were fed a high-fat diet, this hepatic phenotype, as well as other metabolic phenotypes (obesity and glucose intolerance), worsened. Because adipocyte-specific hRBP4 mice have increased tumor necrosis factor-α and leptin expression and crown-like structures in adipose tissue, our data are consistent with the notion that adipose tissue is experiencing RBP4-induced inflammation that stimulates increased lipolysis within adipocytes. Our data further establish that elevated hepatic triglyceride levels result from increased hepatic uptake of adipose-derived circulating free fatty acids. We obtained no evidence that elevated hepatic triglyceride levels arise from increased hepatic de novo lipogenesis, decreased hepatic free fatty acid oxidation, or decreased very-low-density lipoprotein secretion.

CONCLUSION

Our investigations establish that RBP4 expressed in adipocytes induces hepatic steatosis arising from primary effects occurring in adipose tissue. (Hepatology 2016;64:1534-1546).