Cooperation of acylglycerol hydrolases in neuronal lipolysis

Genetic and biochemical evidence has established DDHD-domain containing 2 (DDHD2) as the principal triacylglycerol (TAG) hydrolase in neuronal lipolysis of cytosolic lipid droplets. In this issue of Journal of Lipid Research, Hofer et al. report that DDHD2 cooperates with adipose triglyceride lipase, the principal TAG hydrolase in adipose lipolysis, contributing to cytosolic hydrolysis of both TAG and diacylglycerols in murine neuroblastoma cells and primary cortical neurons via different configurations of the lipases. This finding highlights the complexity of cytosolic acylglycerol hydrolysis and raises many new questions in the field of lipid metabolism.

Hypoxia-Induced Adipose Lipolysis Requires Fibroblast Growth Factor 21

Fibroblast growth factor 21 (FGF21) is a recently discovered hepatokine that regulates lipid and glucose metabolism and is upregulated in response to numerous physiological and pathological stimuli. Herein, we demonstrate that both physical and chemical hypoxia increase the systemic and hepatic expression of FGF21 in mice; by contrast, hypoxia induces a reduction of FGF21 expression in hepatocytes, indicating that hypoxia-induced FGF21 expression is differentially regulated in intact animals and in hepatocytes. Furthermore, we demonstrate that hypoxia treatment increases hormone-sensitive lipase-mediated adipose tissue lipolysis in mice, which is reduced in Fgf21 knockout mice, thereby implying that FGF21 plays a critical role in hypoxia-related adipose lipolysis. Adipose tissue lipolysis causes an increase in the amount of circulating free fatty acids, which leads to the activation of peroxisome proliferators-activated receptor alpha and an increased expression of FGF21 in hepatocytes. We further show that hypoxia-induced elevation of reactive oxygen species, but not the hypoxia-inducible factor, is responsible for the lipolysis and FGF21 expression. In conclusion, our data clearly demonstrate that FGF21 plays a critical role in hypoxia-induced adipose lipolysis, which induces hepatic expression of FGF21. Clarification of hypoxia-regulated FGF21 regulation will enhance our understanding of the pathophysiology of hypoxia-related diseases, such as sleep disorders and metabolic diseases.

Trafficking of cholesterol from lipid droplets to mitochondria in bovine luteal cells: Acute control of progesterone synthesis

The corpus luteum is a transient endocrine gland that synthesizes and secretes the steroid hormone, progesterone, which is vital for establishment and maintenance of pregnancy. Luteinizing hormone (LH) via activation of protein kinase A (PKA) acutely stimulates luteal progesterone synthesis via a complex process, converting cholesterol via a series of enzymatic reactions, into progesterone. Lipid droplets in steroidogenic luteal cells store cholesterol in the form of cholesterol esters, which are postulated to provide substrate for steroidogenesis. Early enzymatic studies showed that hormone sensitive lipase (HSL) hydrolyzes luteal cholesterol esters. In this study, we tested whether HSL is a critical mediator of the acute actions of LH on luteal progesterone production. Using LH-responsive bovine small luteal cells our results reveal that LH, forskolin, and 8-Br cAMP-induced PKA-dependent phosphorylation of HSL at Ser563 and Ser660, events known to promote HSL activity. Small molecule inhibition of HSL activity and siRNA-mediated knock down of HSL abrogated LH-induced progesterone production. Moreover, western blotting and confocal microscopy revealed that LH stimulates phosphorylation and translocation of HSL to lipid droplets. Furthermore, LH increased trafficking of cholesterol from the lipid droplets to the mitochondria, which was dependent on both PKA and HSL activation. Taken together, these findings identify a PKA/HSL signaling pathway in luteal cells in response to LH and demonstrate the dynamic relationship between PKA, HSL, and lipid droplets in luteal progesterone synthesis.

Inhibition of Lipolysis With Acipimox Attenuates Postburn White Adipose Tissue Browning and Hepatic Fat Infiltration

Extensive burn injuries promote an increase in the lipolysis of white adipose tissue (WAT), a complication that enhances postburn hypermetabolism contributing to hyperlipidemia and hepatic steatosis. The systemic increase of free fatty acids (FFAs) due to burn-induced lipolysis and subsequent organ fatty infiltration may culminate in multiple organ dysfunction and, ultimately, death. Thus, reducing WAT lipolysis to diminish the mobilization of FFAs may render an effective means to improve outcomes postburn. Here, we investigated the metabolic effects of Acipimox, a clinically approved drug that suppresses lipolysis via inhibition of hormone-sensitive lipase (HSL). Using a murine model of thermal injury, we show that specific inhibition of HSL with Acipimox effectively suppresses burn-induced lipolysis in the inguinal WAT leading to lower levels of circulating FFAs at 7 days postburn (P < 0.05). The FFA substrate shortage indirectly repressed the thermogenic activation of adipose tissue after injury, reflected by the decrease in protein expression of key browning markers, UCP-1 (P < 0.001) and PGC-1α (P < 0.01). Importantly, reduction of FFA mobilization by Acipimox significantly decreased liver weight and intracellular fat accumulation (P < 0.05), suggesting that it may also improve organ function postburn. Our data validate the pharmacological inhibition of lipolysis as a potentially powerful therapeutic strategy to counteract the detrimental metabolic effects induced by burn.

Ghrelin stimulates fatty acid oxidation and inhibits lipolysis in isolated muscle from male rats

Ghrelin is classically known as a central appetite-stimulating hormone but has recently been recognized to have a significant role in peripheral tissue energy metabolism. However, the direct effects of ghrelin on skeletal muscle, a major site for glucose and lipid disposal, remain understudied. We found that the two major ghrelin isoforms, acylated and unacylated ghrelin, were able to significantly increase skeletal muscle fatty acid oxidation (~20%) while incorporation of fatty acids into major lipid pools remained unchanged. The increase in fatty acid oxidation was accompanied by increases in acetyl-CoA carboxylase phosphorylation, a downstream target of AMPK. Ghrelin isoforms had no independent effect on lipolysis under unstimulated conditions, but nearly completely abolished epinephrine-stimulated lipolysis. This effect was generally, but not consistently related to a blunting in the phosphorylation of HSL activation sites, Ser660 and 563. Taken together, these findings suggest that ghrelin isoforms have a direct, acute effect on fatty acid oxidation and lipolysis.

3,5-Diiodo-L-Thyronine Exerts Metabolically Favorable Effects on Visceral Adipose Tissue of Rats Receiving a High-Fat Diet

When administered to rats receiving a high-fat diet (HFD), 3,5-diiodo-L-thyronine (3,5-T2) [at a dose of 25 μg/100 g body weight (BW)] is known to increase energy expenditure and to prevent HFD-induced adiposity. Here, we investigated which cellular and molecular processes in visceral white adipose tissue (VAT) contributed to the beneficial effect of 3,5-T2 over time (between 1 day and 4 weeks following administration). 3,5-T2 programmed the adipocyte for lipolysis by rapidly inducing hormone sensitive lipase (HSL) phosphorylation at the protein kinase A-responsive site Ser⁵⁶³, accompanied with glycerol release at the 1-week time-point, contributing to the partial normalization of adipocyte volume with respect to control (N) animals. After two weeks, when the adipocyte volumes of HFD-3,5-T2 rats were completely normalized to those of the controls (N), 3,5-T2 consistently induced HSL phosphorylation at Ser⁵⁶³, indicative of a combined effect of 3,5-T2-induced adipose lipolysis and increasing non-adipose oxidative metabolism. VAT proteome analysis after 4 weeks of treatment revealed that 3,5-T2 significantly altered the proteomic profile of HFD rats and produced a marked pro-angiogenic action. This was associated with a reduced representation of proteins involved in lipid storage or related to response to oxidative stress, and a normalization of the levels of those involved in lipogenesis-associated mitochondrial function. In conclusion, the prevention of VAT mass-gain by 3,5-T2 occurred through different molecular pathways that, together with the previously reported stimulation of resting metabolism and liver fatty acid oxidation, are associated with an anti adipogenic/lipogenic potential and positively impact on tissue health.

Inactivation of the Glucose-Dependent Insulinotropic Polypeptide Receptor Improves Outcomes following Experimental Myocardial Infarction

Incretin hormones exert pleiotropic metabolic actions beyond the pancreas. Although the heart expresses both incretin receptors, the cardiac biology of GIP receptor (GIPR) action remains incompletely understood. Here we show that GIPR agonism did not impair the response to cardiac ischemia. In contrast, genetic elimination of the Gipr reduced myocardial infarction (MI)-induced ventricular injury and enhanced survival associated with reduced hormone sensitive lipase (HSL) phosphorylation; it also increased myocardial triacylglycerol (TAG) stores. Conversely, direct GIPR agonism in the isolated heart reduced myocardial TAG stores and increased fatty acid oxidation. The cardioprotective phenotype in Gipr^-/- mice was partially reversed by pharmacological activation or genetic overexpression of HSL. Selective Gipr inactivation in cardiomyocytes phenocopied Gipr^-/- mice, resulting in improved survival and reduced adverse remodeling following experimental MI. Hence, the cardiomyocyte GIPR regulates fatty acid metabolism and the adaptive response to ischemic cardiac injury. These findings have translational relevance for developing GIPR-based therapeutics.

Homozygous LIPE mutation in siblings with multiple symmetric lipomatosis, partial lipodystrophy, and myopathy

Despite considerable progress in identifying causal genes for lipodystrophy syndromes, the molecular basis of some peculiar adipose tissue disorders remains obscure. In an Israeli-Arab pedigree with a novel autosomal recessive, multiple symmetric lipomatosis (MSL), partial lipodystrophy and myopathy, we conducted exome sequencing of two affected siblings to identify the disease-causing mutation. The 41-year-old female proband and her 36-year-old brother reported marked accumulation of subcutaneous fat in the face, neck, axillae, and trunk but loss of subcutaneous fat from the lower extremities and progressive distal symmetric myopathy during adulthood. They had increased serum creatine kinase levels, hypertriglyceridemia and low levels of high-density lipoprotein cholesterol. Exome sequencing identified a novel homozygous NC_000019.9:g.42906092C>A variant on chromosome 19, leading to a NM_005357.3:c.3103G>T nucleotide change in coding DNA and corresponding p.(Glu1035*) protein change in hormone sensitive lipase (LIPE) gene as the disease-causing variant. Sanger sequencing further confirmed the segregation of the mutation in the family. Hormone sensitive lipase is the predominant regulator of lipolysis from adipocytes, releasing free fatty acids from stored triglycerides. The homozygous null LIPE mutation could result in marked inhibition of lipolysis from some adipose tissue depots and thus may induce an extremely rare phenotype of MSL and partial lipodystrophy in adulthood associated with complications of insulin resistance, such as diabetes, hypertriglyceridemia and hepatic steatosis. © 2016 Wiley Periodicals, Inc.

Theophylline-Based KMUP-1 Improves Steatohepatitis via MMP-9/IL-10 and Lipolysis via HSL/p-HSL in Obese Mice

KMUP-1 (7-[2-[4-(2-chlorobenzene)piperazinyl]ethyl]-1,3-dimethylxanthine) has been reported to cause hepatic fat loss. However, the action mechanisms of KMUP-1 in obesity-induced steatohepatitis remains unclear. This study elucidated the steatohepatitis via matrix metallopeptidase 9 (MMP-9) and tumor necrosis factor α (TNFα), and related lipolysis via hormone sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) by KMUP-1. KMUP-1 on steatohepatitis-associated HSL/p-HSL/ATGL/MMP-9/TNFα/interleukin-10 (IL-10) and infiltration of M1/M2 macrophages in obese mice were examined. KMUP-1 was administered by oral gavage from weeks 1–14 in high-fat diet (HFD)-supplemented C57BL/6J male mice (protection group) and from weeks 8–14, for 6 weeks, in HFD-induced obese mice (treatment group). Immunohistochemistry (IHC) and hematoxylin and eosin (H&E) staining of tissues, oil globules number and size, infiltration and switching of M1/M2 macrophages were measured to determine the effects on livers. IL-10 and MMP-9 proteins were explored to determine the effects of KMUP-1 on M1/M2 macrophage polarization in HFD-induced steatohepatitis. Long-term administration of KMUP-1 reversed HFD-fed mice increased in body weight, sGOT/sGPT, triglyceride (TG) and glucose. Additionally, KMUP-1 decreased MMP-9 and reactive oxygen species (ROS), and increased HSL/p-HSL and IL-10 in HFD mice livers. In conclusion, KMUP-1, a phosphodiesterase inhibitor (PDEI), was shown to reduce lipid accumulation in liver tissues, suggesting that it could be able to prevent or treat steatohepatitis induced by HFD.

Simplified assays of lipolysis enzymes for drug discovery and specificity assessment of known inhibitors

Lipids are used as cellular building blocks and condensed energy stores and also act as signaling molecules. The glycerolipid/ fatty acid cycle, encompassing lipolysis and lipogenesis, generates many lipid signals. Reliable procedures are not available for measuring activities of several lipolytic enzymes for the purposes of drug screening, and this resulted in questionable selectivity of various known lipase inhibitors. We now describe simple assays for lipolytic enzymes, including adipose triglyceride lipase (ATGL), hormone sensitive lipase (HSL), sn-1-diacylglycerol lipase (DAGL), monoacylglycerol lipase, α/β-hydrolase domain 6, and carboxylesterase 1 (CES1) using recombinant human and mouse enzymes either in cell extracts or using purified enzymes. We observed that many of the reported inhibitors lack specificity. Thus, Cay10499 (HSL inhibitor) and RHC20867 (DAGL inhibitor) also inhibit other lipases. Marked differences in the inhibitor sensitivities of human ATGL and HSL compared with the corresponding mouse enzymes was noticed. Thus, ATGListatin inhibited mouse ATGL but not human ATGL, and the HSL inhibitors WWL11 and Compound 13f were effective against mouse enzyme but much less potent against human enzyme. Many of these lipase inhibitors also inhibited human CES1. Results describe reliable assays for measuring lipase activities that are amenable for drug screening and also caution about the specificity of the many earlier described lipase inhibitors.

Sodium acetate decreases phosphorylation of hormone sensitive lipase in isoproterenol-stimulated 3T3-L1 mature adipocytes

Lipolysis, the process of hydrolysis of stored triacylglycerol into glycerol and non-esterified fatty acids (NEFA), is reported to be reduced by short chain fatty acids (SCFA) but the mechanism of this inhibition is poorly understood. The aim of this study was to measure the phosphorylation at serine residue 563 of hormone sensitive lipase with and without exposure to sodium acetate. Using the 3T3-L1 cell line, we identified that stimulating the cells with isoproterenol increased phosphorylated hormone sensitive lipase (pHSL) expression by 60% compared with the basal state. In the presence of the SCFA acetate in stimulated cells, pHSL decreased by 15% compared with stimulated cells alone. These results were mirrored by the NEFA release from stimulated cells that had significantly decreased in the presence of sodium acetate after 60 min (from 0.53 µmol mg(-1) protein to 0.41 µmol mg(-1) protein, respectively, P = 0.004); and 180 min (1.73 µmol mg(-1) protein to 1.13 µmol mg(-1) protein, P = 0.020); however, treatment had no effect on glycerol release (P = 0.109). In conclusion, exposure to 4 mM acetate reduced the level of phosphorylation of HSL(SER563) in mature 3T3-L1 adipocytes and led to a significant reduction in NEFA release, although glycerol release was not affected.

Regulation of energy metabolism by long-chain fatty acids

In mammals, excess energy is stored primarily as triglycerides, which are mobilized when energy demands arise. This review mainly focuses on the role of long chain fatty acids (LCFAs) in regulating energy metabolism as ligands of peroxisome proliferator-activated receptors (PPARs). PPAR-alpha expressed primarily in liver is essential for metabolic adaptation to starvation by inducing genes for beta-oxidation and ketogenesis and by downregulating energy expenditure through fibroblast growth factor 21. PPAR-delta is highly expressed in skeletal muscle and induces genes for LCFA oxidation during fasting and endurance exercise. PPAR-delta also regulates glucose metabolism and mitochondrial biogenesis by inducing FOXO1 and PGC1-alpha. Genes targeted by PPAR-gamma in adipocytes suggest that PPAR-gamma senses incoming non-esterified LCFAs and induces the pathways to store LCFAs as triglycerides. Adiponectin, another important target of PPAR-gamma may act as a spacer between adipocytes to maintain their metabolic activity and insulin sensitivity. Another topic of this review is effects of skin LCFAs on energy metabolism. Specific LCFAs are required for the synthesis of skin lipids, which are essential for water barrier and thermal insulation functions of the skin. Disturbance of skin lipid metabolism often causes apparent resistance to developing obesity at the expense of normal skin function.

Interleukin-4 regulates lipid metabolism by inhibiting adipogenesis and promoting lipolysis

Long-term cytokine-mediated inflammation is a risk factor for obesity and type 2 diabetes mellitus (T2DM). Our previous studies reveal significant associations between promoter single nucleotide polymorphisms (SNPs) of interleukin (IL)-4 and T2DM, as well as between SNPs in genes encoding IL-4/IL-4 receptor and high density lipoproteins. Our animal study reveals that IL-4 regulates glucose/lipid metabolism by promoting glucose tolerance and inhibiting lipid deposits. The above results strongly suggest the involvement of IL-4 in energy homeostasis. In the present study, we focus on examining the regulatory mechanism of IL-4 to lipid metabolism. Our results show that IL-4 inhibits adipogenesis by downregulating the expression of peroxisome proliferator-activated receptor-γ and CCAAT/enhancer-binding protein-α. Additionally, IL-4 promotes lipolysis by enhancing the activity and translocation of hormone sensitive lipase (HSL) in mature adipocytes, which suggests that IL-4 plays a pro-lipolytic role in lipid metabolism by boosting HSL activity. Our results demonstrate that IL-4 harbors pro-lipolysis capacity by inhibiting adipocyte differentiation and lipid accumulation as well as by promoting lipolysis in mature adipocytes to decrease lipid deposits. The above findings uncover the novel roles of IL-4 in lipid metabolism and provide new insights into the interactions among cytokine/immune responses, insulin sensitivity, and metabolism.

Lipolytic actions of secretin in mouse adipocytes

Secretin (Sct), a classical gut hormone, is now known to play pleiotropic functions in the body including osmoregulation, digestion, and feeding control. As Sct has long been implicated to regulate metabolism, in this report, we have investigated a potential lipolytic action of Sct. In our preliminary studies, both Sct levels in circulation and Sct receptor (SctR) transcripts in adipose tissue were upregulated during fasting, suggesting a potential physiological relevance of Sct in regulating lipolysis. Using SctR knockout and Sct knockout mice as controls, we show that Sct is able to stimulate lipolysis in vitro in isolated adipocytes dose- and time-dependently, as well as acute lipolysis in vivo. H-89, a protein kinase A (PKA) inhibitor, was found to attenuate lipolytic effects of 1 μM Sct in vitro, while a significant increase in PKA activity upon Sct injection was observed in the adipose tissue in vivo. Sct was also found to stimulate phosphorylation at 660^ser of hormone sensitive lipase (HSL) and to bring about the translocation of HSL from cytosol to the lipid droplet. In summary, our data demonstrate for the first time the in vivo and in vitro lipolytic effects of Sct, and that this function is mediated by PKA and HSL.

Alteration of cardiac glucose metabolism in association to low birth weight: experimental evidence in lambs with left ventricular hypertrophy

OBJECTIVE

Intrauterine growth restriction that results in low birth weight (LBW) has been linked to the onset of pathological cardiac hypertrophy. An altered transition from a fetal to an adult energy metabolism phenotype, with increased reliance on glucose rather than fatty acids for energy production, could help explain this connection. We have therefore investigated cardiac metabolism in relation to left ventricular hypertrophy in LBW lambs, at 21days after birth.

MATERIALS/METHODS

The expression of regulatory molecules involved in cardiac glucose and fatty acid metabolism was measured using real-time PCR and Western blotting. A section of the left ventricle was fixed for Periodic Acid Schiff staining to determine tissue glycogen content.

RESULTS

There was increased abundance of insulin signalling pathway proteins (phospho-insulin receptor, insulin receptor and phospho-Akt) and the glucose transporter (GLUT)-1, but no change in GLUT-4 or glycogen content in the heart of LBW compared to ABW lambs. There was, however, increased abundance of cardiac pyruvate dehydrogenase kinase 4 (PDK-4) in LBW compared to ABW lambs. There were no significant changes in the mRNA expression of components of the peroxisome proliferator activated receptor regulatory complex or proteins involved in fatty acid metabolism.

CONCLUSION

We concluded that LBW induced left ventricular hypertrophy was associated with increased GLUT-1 and PDK-4, suggesting increased glucose uptake, but decreased efficacy for the conversion of glucose to ATP. A reduced capacity for energy conversion could have significant implications for vulnerability to cardiovascular disease in adults who are born LBW.

Effects of short-term refeeding on the expression of genes involved in lipid metabolism in chicks (Gallus gallus)

The aim of this study was to analyze the expression patterns of key genes involved in lipid metabolism in response to feeding in chicks. A total of 18 thirteen day-old male chicks were fasted for 12h. The mRNA levels of the genes in the liver and white adipose tissue were analyzed after 0, 2, and 4h of refeeding. The mRNA levels of sterol regulatory element-binding protein (SREBP) 1, liver X receptor α, peroxisome proliferator-activated receptor (PPAR) γ, acetyl-CoA carboxylase α and fatty acid synthase were significantly increased after 2h of refeeding. In contrast, the mRNA levels of PPARα and carnitine palmitoyltransferase 1a were significantly decreased after 2h of refeeding. The mRNA level of acyl-CoA oxidase was significantly decreased after 4h of refeeding. The mRNA levels of cholesterol metabolism-related genes such as SREBP2 and 3-hydroxy-3-methylglutaryl-CoA reductase were significantly increased after 2h of refeeding. In the white adipose tissue, the mRNA level of PPARγ was significantly increased after 2h of refeeding, whereas the mRNA level of adipose triglyceride lipase was significantly decreased after 4h of refeeding. These results demonstrated that expression of lipid metabolism-related genes is regulated by short-term refeeding in chicks.

Role of intramyocelluar lipids in human health

Intramyocellular lipid (IMCL) is predominantly stored as intramuscular triglyceride (IMTG) in lipid droplets and is utilized as metabolic fuel during physical exercise. IMTG is also implicated in muscle insulin resistance (IR) in type 2 diabetes. However, it has become apparent that lipid moieties such as ceramide and diacylglycerol are the likely culprits of IR. This article reviews current knowledge of IMCL-mediated IR and important areas of investigation, including myocellular lipid transport and lipid droplet proteins. Several crucial questions remain unanswered, such as the identity of specific ceramide and diacylglycerol species that mediate IR in human muscle and their subcellular location. Quantitative lipidomics and proteomics of targeted subcellular organelles will help to better define the mechanisms underlying pathological IMCL accumulation and IR.

Fucoidan from Marine Brown Algae Inhibits Lipid Accumulation

In this study, we elucidated the inhibitory effect of fucoidan from marine brown algae on the lipid accumulation in differentiated 3T3-L1 adipocytes and its mechanism. The treatment of fucoidan in a dose-dependent manner was examined on lipid inhibition in 3T3-L1 cells by using Oil Red O staining. Fucoidan showed high lipid inhibition activity at 200 μg/mL concentration (P < 0.001). Lipolytic activity in adipocytes is highly dependent on hormone sensitive lipase (HSL), which is one of the most important targets of lipolytic regulation. Here, we examined the biological response of fucoidan on the protein level of lipolysis pathway. The expressed protein levels of total hormone sensitive lipase (HSL) and its activated form, phosphorylated-HSL were significantly increased at concentration of 200 μg/mL fucoidan. Furthermore, insulin-induced 2-deoxy-D-[³H] glucose uptake was decreased up to 51% in fucoidan-treated cells as compared to control. Since increase of HSL and p-HSL expression and decrease of glucose uptake into adipocytes are known to lead to stimulation of lipolysis, our results suggest that fucoidan reduces lipid accumulation by stimulating lipolysis. Therefore, these results suggest that fucoidan can be useful for the prevention or treatment of obesity due to its stimulatory lipolysis.