Adipocyte triglyceride lipase expression in human obesity

Studies on the antiobesity effect of zinc-α2-glycoprotein in the ob/ob mouse

OBJECTIVE

To investigate the mechanism of the lipid depletion by zinc-α(2)-glycoprotein (ZAG).

DESIGN

Studies were conducted in the ob/ob mouse, or on isolated adipocytes from these animals or their lean counterparts.

RESULTS

Treatment of these animals for 15 days with ZAG (100  μg, intravenously, daily) resulted in a reduction of body weight of 6.55  g compared with phosphate-buffered saline-treated controls, without a change in food or water intake, but with a 0.4 °C rise in rectal temperature. ZAG-treated mice had a 30% reduction in carcass fat mass and a twofold increase in weight of brown adipose tissue. Epididymal adipocytes from ZAG-treated mice showed an increased expression of ZAG and hormone-sensitive lipase (HSL), and this was maintained for a further 3 days in the absence of ZAG. There was an increased lipolytic response to isoproterenol, which was retained for 3 days in vitro in the absence of ZAG. Expression of HSL was also increased in subcutaneous and visceral adipose tissue, as was also adipose triglyceride lipase (ATGL). There was a rapid loss of labelled lipid from epididymal adipose tissue of ZAG-treated mice, but not from the other depots, reflecting the difference in sensitivity to lipolytic stimuli. The increased expression of HSL and ATGL may involve the extracellular signal-regulated kinase (ERK) pathway, as the active (phospho) form was upregulated in all adipose depots after ZAG administration, whereas in vitro studies showed induction of HSL and ATGL by ZAG to be attenuated by PD98059, an inhibitor of the ERK pathway.

CONCLUSION

These results suggest that ZAG not only induces direct lipolysis, but also sensitizes adipose tissue to other lipolytic stimuli.

Triacylglycerol lipases and metabolic control: implications for health and disease

Fatty acids derived from the hydrolysis of adipose tissue and skeletal muscle triacylglycerol (TG) are an important energy substrate at rest and during physical activity. This review outlines the identification of the new TG lipase, adipose triglyceride lipase, the current understanding of how cellular TG lipases are regulated, and the implications for understanding the integrated control of TG lipolysis. Furthermore, this review outlines recent advances that propose a "revised" role for TG lipases in cellular function, metabolic homeostasis, and disease prevention.

Adipose tissue lipolysis

PURPOSE OF REVIEW

Adipose tissue lipolysis is a critical pathway for the maintenance of energy homeostasis through the degradation of triglycerides and the release of fatty acids into the circulation. The understanding of the cellular factors regulating triglyceride hydrolysis and the metabolic function of lipases has considerably expanded in the last few years, revealing an unexpected complexity. This review aims at describing recent discoveries related to the lipolytic pathway and its regulatory mechanisms.

RECENT FINDINGS

Considerable progress has been made in understanding the role and the mechanisms of activation of the lipolytic enzymes. Recent discoveries have dramatically altered the view of adipose tissue lipolysis and highlighted the importance of additional molecular actors in regulating this process. Catecholamines, natriuretic peptides, and insulin are considered to be the major regulators of lipolysis in humans. However, autocrine/paracrine factors such as metabolites and prostaglandins may also participate in its regulation.

SUMMARY

The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity. Unraveling the molecular events occurring during regulation of lipolysis may lead to novel therapeutic targets.

Contributions of adipocyte lipid metabolism to body fat content and implications for the treatment of obesity

Obesity is a chronic disease that increases susceptibility to various diseases, particularly cardiovascular dysfunction, type 2 diabetes, and some types of cancer. In this review, we highlighted recent evidence in mouse models that support a potential benefit of increasing adipose lipid utilization through stimulating lipolysis in adipose tissue and fatty acid oxidation. Brown adipocyte development within white adipose tissue of humans suggests that mouse models may be applicable to human obesity. Consequently, new therapies should target adipose tissue to specifically reduce fat mass through controlled triglyceride utilization.

Dissociation between fatty liver and insulin resistance in humans carrying a variant of the patatin-like phospholipase 3 gene

OBJECTIVE

In a genome-wide association scan, the rs738409 C>G single nucleotide polymorphism (SNP) in the patatin-like phospholipase 3 gene (PNPLA3) was strongly associated with increased liver fat but not with insulin resistance estimated from fasting values. We investigated whether the SNP determines liver fat independently of visceral adiposity and whether it may even play a role in protecting from insulin resistance.

RESEARCH DESIGN AND METHODS

Liver fat was measured by (1)H magnetic resonance spectroscopy and total and visceral fat by magnetic resonance tomography in 330 subjects. Insulin sensitivity was estimated during an oral glucose tolerance test and the euglycemic-hyperinsulinemic clamp (n = 222). PNPLA3 and tumor necrosis factor-alpha mRNA and triglyceride content were measured in liver biopsies from 16 subjects.

RESULTS

Liver fat correlated strongly with insulin sensitivity (P < 0.0001) independently of age, sex, total fat, and visceral fat. G allele carriers of the SNP rs738409 had higher liver fat (P < 0.0001) and an odds ratio of 2.38 (95% CI 1.37-4.20) for having fatty liver compared to C allele homozygotes. Interestingly, insulin sensitivity (oral glucose tolerance test: P = 0.99; clamp: P = 0.32), serum C-reactive protein levels, lipids, or liver enzymes (all P > 0.14) were not different among the genotypes. Additional adjustment for liver fat actually revealed increased insulin sensitivity in more obese carriers of the G allele (P = 0.01). In liver biopsies triglyceride content correlated positively with expression of the proinflammatory gene tumor necrosis factor-alpha in C allele homozygotes (n = 6, P = 0.027) but not in G allele carriers (n = 10, P = 0.149).

CONCLUSIONS

PNPLA3 may be an important key to understand the mechanisms discriminating fatty liver with and without metabolic consequences.

Modulation of myocellular fat stores: lipid droplet dynamics in health and disease

Storage of fatty acids as triacylglycerol (TAG) occurs in almost all mammalian tissues. Whereas adipose tissue is by far the largest storage site of fatty acids as TAG, subcellular TAG-containing structures—referred to as lipid droplets (LD)—are also present in other tissues. Until recently, LD were considered inert storage sites of energy dense fats. Nowadays, however, LD are increasingly considered dynamic functional organelles involved in many intracellular processes like lipid metabolism, vesicle trafficking, and cell signaling. Next to TAG, LD also contain other neutral lipids such as diacylglycerol. Furthermore, LD are coated by a monolayer of phospholipids decorated with a variety of proteins regulating the delicate balance between LD synthesis, growth, and degradation. Disturbances in LD-coating proteins may result in disequilibrium of TAG synthesis and degradation, giving rise to insulin-desensitizing lipid intermediates, especially in insulin-responsive tissues like skeletal muscle. For a proper and detailed understanding, more information on processes and players involved in LD synthesis and degradation is necessary. This, however, is hampered by the fact that research on LD dynamics in (human) muscle is still in its infancy. A rapidly expanding body of knowledge on LD dynamics originates from studies in other tissues and other species. Here, we aim to review the involvement of LD-coating proteins in LD formation and degradation (LD dynamics) and to extrapolate this knowledge to human skeletal muscle and to explore the role of LD dynamics in myocellular insulin sensitivity.

Neutral lipid storage disease: genetic disorders caused by mutations in adipose triglyceride lipase/PNPLA2 or CGI-58/ABHD5

Neutral lipid storage disease (NLSD) is a group of autosomal recessive disorders characterized by the excessive accumulation of neutral lipids in multiple tissues. Recently, two genes, adipose triglyceride lipase (ATGL/PNPLA2) and comparative gene identification-58 (CGI-58/ABHD5), have been shown to cause NLSD. ATGL specifically hydrolyzes the first fatty acid from triacylglycerols (TG) and CGI-58/ABHD5 stimulates ATGL activity by a currently unknown mechanism. Mutations in both the ATGL and the CGI-58 genes are associated with systemic TG accumulation, yet the resulting clinical manifestations are not identical. Patients with defective ATGL function suffer from more severe myopathy (NLSDM) than patients with defective CGI-58 function. On the other hand, CGI-58 mutations are always associated with ichthyosis (NLSDI), which was not observed in patients with defective ATGL function. These observations indicate an ATGL-independent function of CGI-58. This review summarizes recent findings with the goal of relating structural variants of ATGL and CGI-58 to functional consequences in lipid metabolism.

Regulation of ATGL expression mediated by leptin in vitro in porcine adipocyte lipolysis

Adipose triglyceride lipase (ATGL), as an adipose-enriched protein, is able to hydrolyze triglycerides and plays an important part in triglyceride lipolysis of fat tissue. Leptin, an adipocyte cytokine, can increase the fat decomposition process. Many phenomena indicate that ATGL has a close relationship with leptin's promoting the hydrolysis of triglycerides. However, the regulatory mechanism of ATGL in leptin's promoting fat hydrolysis has not been directly and systematically studied yet. This study demonstrated that ATGL was expressed in vitro by leptin regulation. The amount of ATGL mRNA increased and the amount of ATGL protein decreased based on a dose-dependent manner when leptin concentrations ranged from 5 to 50 ng/ml were used to treat fully differentiated porcine adipocytes for 3 h. In addition, this study revealed that JAK-STAT and MAPK signaling pathways, as well as PPAR gamma all played important roles in the ATGL expression mediated by leptin.

Pigment epithelium-derived factor contributes to insulin resistance in obesity

Obesity is a major risk factor for insulin resistance; however, the factors linking these disorders are not well defined. Herein, we show that the noninhibitory serine protease inhibitor, pigment epithelium-derived factor (PEDF), plays a causal role in insulin resistance. Adipocyte PEDF expression and serum levels are elevated in several rodent models of obesity and reduced upon weight loss and insulin sensitization. Lean mice injected with recombinant PEDF exhibited reduced insulin sensitivity during hyperinsulinemic-euglycemic clamps. Acute PEDF administration activated the proinflammatory serine/threonine kinases c-Jun terminal kinase and extracellular regulated kinase in both muscle and liver, which corresponded with reduced insulin signal transduction. Prolonged PEDF administration stimulated adipose tissue lipolysis, resulted in ectopic lipid deposition, and reduced insulin sensitivity, while neutralizing PEDF in obese mice enhanced insulin sensitivity. Overall, these results identify a causal role for PEDF in obesity-induced insulin resistance.

Triglyceride lipases alter fuel metabolism and mitochondrial gene expressionThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

Fatty acids derived from the hydrolysis of adipose tissue and skeletal muscle triacylglycerol (TG) are an important energy substrate at rest and during prolonged moderate-intensity exercise. Hormone sensitive lipase (HSL) was long considered to be the rate-limiting enzyme for adipocyte and skeletal muscle TG lipolysis. However, the understanding of TG lipolysis regulation was recently challenged by the finding that adipose TG lipase (ATGL) is the predominant TG lipase in adipose tissue and an important regulator of TG degradation in skeletal muscle. Thus, it is now proposed that ATGL and HSL regulate lipolysis in a serial manner, with ATGL cleaving the first fatty acid and HSL the second fatty acid of TG. Further to this biochemical evaluation, the generation and metabolic characterization of ATGL−/− and HSL−/− mice have revealed distinct phenotypes. ATGL−/− mice are obese, exhibit impaired thermogenesis, oxidize more carbohydrate, and die prematurely due to cardiac dysfunction. Studies in HSL−/− mice report defective β-adrenergic stimulated lipolysis, protection against high-fat diet-induced obesity, and possible impairments in insulin secretion. This review outlines the current understanding of the cellular regulation of TG lipases, lipolytic regulation, and the functional implications of manipulating ATGL and HSL in vivo.

Genome-wide association scan meta-analysis identifies three Loci influencing adiposity and fat distribution

To identify genetic loci influencing central obesity and fat distribution, we performed a meta-analysis of 16 genome-wide association studies (GWAS, N = 38,580) informative for adult waist circumference (WC) and waist-hip ratio (WHR). We selected 26 SNPs for follow-up, for which the evidence of association with measures of central adiposity (WC and/or WHR) was strong and disproportionate to that for overall adiposity or height. Follow-up studies in a maximum of 70,689 individuals identified two loci strongly associated with measures of central adiposity; these map near TFAP2B (WC, P = 1.9x10(-11)) and MSRA (WC, P = 8.9x10(-9)). A third locus, near LYPLAL1, was associated with WHR in women only (P = 2.6x10(-8)). The variants near TFAP2B appear to influence central adiposity through an effect on overall obesity/fat-mass, whereas LYPLAL1 displays a strong female-only association with fat distribution. By focusing on anthropometric measures of central obesity and fat distribution, we have identified three loci implicated in the regulation of human adiposity.

Lipolysis and lipid mobilization in human adipose tissue

Triacylglycerol (TAG) stored in adipose tissue (AT) can be rapidly mobilized by the hydrolytic action of the three main lipases of the adipocyte. The non-esterified fatty acids (NEFA) released are used by other tissues during times of energy deprivation. Until recently hormone-sensitive lipase (HSL) was considered to be the key rate-limiting enzyme responsible for regulating TAG mobilization. A novel lipase named adipose triglyceride lipase/desnutrin (ATGL) has been identified as playing an important role in the control of fat cell lipolysis. Additionally perilipin and other proteins of the surface of the lipid droplets protecting or exposing the TAG core of the droplets to lipases are also potent regulators of lipolysis. Considerable progress has been made in understanding the mechanisms of activation of the various lipases. Lipolysis is under tight hormonal regulation. The best understood hormonal effects on AT lipolysis concern the opposing regulation by insulin and catecholamines. Heart-derived natriuretic peptides (i.e., stored in granules in the atrial and ventricle cardiomyocytes and exerting stimulating effects on diuresis and natriuresis) and numerous autocrine/paracrine factors originating from adipocytes and other cells of the stroma-vascular fraction may also participate in the regulation of lipolysis. Endocrine and autocrine/paracrine factors cooperate and lead to a fine regulation of lipolysis in adipocytes. Age, anatomical site, sex, genotype and species differences all play a part in the regulation of lipolysis. The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity and probably in several inborn errors of metabolism.

Regulation of carboxylesterase 1 (CES1) in human adipose tissue

Carboxylesterase 1 (CES1) has recently been suggested to play a role in lipolysis. Our aim was to study the regulation of CES1 expression in human adipose tissue. In the SOS Sib Pair Study, CES1 expression was higher in obese compared with lean sisters (n = 78 pairs, P = 8.7 × 10⁻¹⁸) and brothers (n = 12 pairs, P = 0.048). CES1 expression was higher in subcutaneous compared with omental adipose tissue in lean (P = 0.027) and obese subjects (P = 0.00036), and reduced during diet-induced weight loss (n = 24, weeks 8, 16, and 18 compared to baseline, P < 0.0001 for all time points). CES1 expression was higher in isolated adipocytes compared with intact adipose tissue (P = 0.0018) and higher in large compared with small adipocytes (P = 4.1 × 10⁻⁶). Basal and stimulated lipolysis was not different in individuals with high, intermediate, and low expression of CES1. Thus, CES1 expression was linked to body fat and adipocyte fat content but not to lipolytic activity.

Protein kinase A regulatory subunits in human adipose tissue: decreased R2B expression and activity in adipocytes from obese subjects

OBJECTIVE

In human adipocytes, the cAMP-dependent pathway mediates signals originating from beta-adrenergic activation, thus playing a key role in the regulation of important metabolic processes, i.e., lipolysis and thermogenesis. Cyclic AMP effects are mainly mediated by protein kinase A (PKA), whose R2B regulatory isoform is the most expressed in mouse adipose tissue, where it protects against diet-induced obesity and fatty liver development. The aim of the study was to investigate possible differences in R2B expression, PKA activity, and lipolysis in adipose tissues from obese and nonobese subjects.

RESEARCH DESIGN AND METHODS

The expression of the different PKA regulatory subunits was evaluated by immunohistochemistry, Western blot, and real-time PCR in subcutaneous and visceral adipose tissue samples from 20 nonobese and 67 obese patients. PKA activity and glycerol release were evaluated in total protein extract and adipocytes isolated from fresh tissue samples, respectively.

RESULTS

Expression techniques showed that R2B was the most abundant regulatory protein, both at mRNA and protein level. Interestingly, R2B mRNA levels were significantly lower in both subcutaneous and visceral adipose tissues from obese than nonobese patients and negatively correlated with BMI, waist circumference, insulin levels, and homeostasis model assessment of insulin resistance. Moreover, both basal and stimulated PKA activity and glycerol release were significantly lower in visceral adipose tissue from obese patients then nonobese subjects.

CONCLUSIONS

Our results first indicate that, in human adipose tissue, there are important BMI-related differences in R2B expression and PKA activation, which might be included among the multiple determinants involved in the different lipolytic response to beta-adrenergic activation in obesity.

Contribution of FAT/CD36 to the regulation of skeletal muscle fatty acid oxidation: an overview

Long chain fatty acids (LCFAs) are an important substrate for ATP production within the skeletal muscle. The process of LCFA delivery from adipose tissue to muscle mitochondria involves many regulatory steps. Recently, it has been recognized that LCFA oxidation is not only dependent on LCFA delivery to the muscle, but also on regulatory steps within the muscle. Increasing selected fatty acid binding proteins/transporters on the plasma membrane facilitates a very rapid LCFA increase into the muscle, independent of any changes in LCFA delivery to the muscle. Such a mechanism of LCFA transporter translocation is activated by muscle contraction. Intramuscular triacylglycerols may also be hydrolysed to provide fatty acids for mitochondrial oxidation, particularly during exercise, when hormone-sensitive lipase and other enzymes are activated. Mitochondrial LCFA entry is also highly regulated. This however does not involve only the malonyl CoA carnitine palmitoyltransferase-I (CPTI) axis. Exercise-induced fatty acid entry into mitochondria is also regulated by at least one of the proteins (FAT/CD36) that also regulates plasma membrane fatty acid transport. Among individuals, differences in mitochondrial fatty acid oxidation appear to be correlated with the content of mitochondrial CPTI and FAT/CD36. This paper provides a brief overview of mechanisms that regulate LCFA uptake and oxidation in skeletal muscle during exercise and in obesity. We focus largely on our own work on FAT/CD36, which contributes to regulating, in a coordinated fashion, LCFA uptake across the plasma membrane and the mitochondrial membrane. Very little is known about the roles of FATP1-6 on fatty acid transport in skeletal muscle.

Fate of fat: the role of adipose triglyceride lipase in lipolysis

Lipolysis, the coordinated catabolism of triacylglycerol (TG) stored in cellular lipid droplets, provides fatty acids, di-, and monoglycerides. These products are important energy substrates, precursors for other lipids, or lipid signaling molecules. Following their discovery by Hollenberg, C.H., Raben, M.S., and Astwood, E.B.(1961) and Vaughan, M., Berger, J.E., and Steinberg, D. (1964), hormone-sensitive lipase (HSL) and monoacylglycerol lipase stayed in the focus of research for three decades. Within the last decade, however, it became evident that the lipolytic pathway is incompletely understood. Studies on the regulation of lipolysis and the characterization of HSL-deficient mice indicated that additional previously unrecognized factors that contribute to fat catabolism must exist. This led to the discovery of the perilipin, adipophilin, Tip47 (PAT) family of lipid droplet binding proteins and the identification of a novel TG hydrolase named adipose triglyceride lipase (ATGL). This review focuses on the importance of ATGL as TG lipase within the "lipolytic machinery" and the current knowledge of molecular mechanisms that regulate ATGL activity.

Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores

Fatty acids (FAs) are essential components of all lipid classes and pivotal substrates for energy production in all vertebrates. Additionally, they act directly or indirectly as signaling molecules and, when bonded to amino acid side chains of peptides, anchor proteins in biological membranes. In vertebrates, FAs are predominantly stored in the form of triacylglycerol (TG) within lipid droplets of white adipose tissue. Lipid droplet-associated TGs are also found in most nonadipose tissues, including liver, cardiac muscle, and skeletal muscle. The mobilization of FAs from all fat depots depends on the activity of TG hydrolases. Currently, three enzymes are known to hydrolyze TG, the well-studied hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL), discovered more than 40 years ago, as well as the relatively recently identified adipose triglyceride lipase (ATGL). The phenotype of HSL- and ATGL-deficient mice, as well as the disease pattern of patients with defective ATGL activity (due to mutation in ATGL or in the enzyme's activator, CGI-58), suggest that the consecutive action of ATGL, HSL, and MGL is responsible for the complete hydrolysis of a TG molecule. The complex regulation of these enzymes by numerous, partially uncharacterized effectors creates the "lipolysome," a complex metabolic network that contributes to the control of lipid and energy homeostasis. This review focuses on the structure, function, and regulation of lipolytic enzymes with a special emphasis on ATGL.

Regulation and function of triacylglycerol lipases in cellular metabolism

The ability to store energy in the form of energy-dense TAG (triacylglycerol) and to mobilize these stores rapidly during times of low carbohydrate availability (fasting or famine) or during heightened metabolic demand (exercise or cold-stress) is a highly conserved process essential for survival. Today, in the presence of nutrient excess and sedentary lifestyles, the regulation of this pathway is viewed as an important therapeutic target for disease prevention, as elevated circulating fatty acids in obesity contribute to many aspects of the metabolic syndrome including hepatic steatosis, atherosclerosis and insulin resistance. In the present review, we discuss the metabolic regulation and function of TAG lipases with a focus on HSL (hormone-sensitive lipase), ATGL (adipose triacylglycerol lipase) and newly identified members of the lipolytic proteome.

Storing up trouble: does accumulation of intramyocellular triglyceride protect skeletal muscle from insulin resistance?

1. Insulin resistance occurs when normal amounts of insulin are inadequate to produce a normal insulin response from cells. This is important in the context of whole-body glucose homeostasis because skeletal muscle is the main tissue for insulin-stimulated glucose disposal. 2. In obesity, lipid deposition in peripheral tissues, such as skeletal muscle, is linked to the activation of stress kinases and the development of insulin resistance. Accumulation of intramyocellular triglyceride (IMTG) is positively associated with insulin resistance; however, it is unknown whether IMTG causes insulin resistance or protects cells from insulin resistance by preventing the accrual of bioactive lipid metabolites. 3. The role of IMTG in the development of insulin resistance is not resolved. Stable overexpression of the triglyceride lipase adipose triglyceride lipase (ATGL) reduced IMTG content in myotubes, but resulted in a concomitant increase in diacylglycerol (DAG) and ceramide and caused insulin resistance. Increasing TG content by muscle-specific overexpression of diacylglycerol acyltransferase (DGAT) 1 protected mice from insulin resistance. Conversely, overexpression of DGAT2 in glycolytic muscle resulted in accumulation of TG and ceramide and insulin resistance in these tissues. This was sufficient to induce whole-body insulin and glucose insensitivity. 4. It is unlikely that IMTG causes cause insulin resistance directly. Instead, it appears as though TG accumulates in skeletal muscle to sequester fatty acids and to protect from the deleterious actions of lipids, such as ceramide and DAG. Whether lipase inhibitors are viable therapeutics to prevent obesity induced insulin resistance is unknown, but future studies examining tissue-specific ATGL/hormone-sensitive lipase knockouts will hopefully resolve this question.

The patatin-like lipase family in Gallus gallus

Background

In oviparous species, genes encoding proteins with functions in lipid remodeling, such as specialized lipases, may have evolved to facilitate the assembly and utilization of yolk lipids by the embryo. The mammalian gene family of patatin-like phospholipases (PNPLAs) has received significant attention, but studies in other vertebrates are lacking; thus, we have begun investigations of PNPLA genes in the chicken (Gallus gallus).

Results

We scanned the draft chicken genome using human PNPLA sequences, and performed PCR to amplify and sequence orthologous cDNAs. Full-length cDNA sequences of galline PNPLA2/ATGL, PNPLA4, -7, -8, -9, and the activator protein CGI-58, as well as partial cDNA sequences of avian PNPLA1, -3, and -6 were obtained. The high degree of sequence identities (~50 to 80%) between the avian and human orthologs suggests conservation of important enzymatic functions. Quantitation by qPCR of the transcript levels of PNPLAs and CGI-58 in 21 tissues indicates that expression patterns and levels diverge greatly between species. A particularly interesting tissue in which certain PNPLAs may contribute to physiological specialization is the extraembryonic yolk sac.

Conclusion

Knowledge about the exact in-vivo functions of PNPLAs in any system is still sparse. Thus, studies about the temporal expression patterns and functions of the enzymes identified here, and of other already known extracellular lipases and co-factors, in the yolk sac and embryonic tissues during embryogenesis are called for. Based on the information obtained, further studies are anticipated to provide important insights of the roles of PNPLAs in the yolk sac and embryo development.

Impairment of nutritional regulation of adipose triglyceride lipase expression with age

OBJECTIVE

Fasting-induced lipolysis becomes less effective with age. We have studied whether nutritional regulation of adipose triglyceride lipase (ATGL)--with an important role in lipolysis in low energy states--is affected by age.

DESIGN

Wistar rats of different ages (from 1 to 13 months) were distributed in control and fasted groups (14 h-food deprivation). ATGL mRNA expression was measured in different adipose depots at different ages and in only one depot at 13 months by reverse transcription (RT)-PCR. ATGL protein levels were determined at 3 and 7 months (not at 13 months) by western blot. Nonesterified fatty acid (NEFA), insulin and leptin levels were assessed in serum by enzymatic assays.

RESULTS

ATGL expression was dependent on regional fat distribution, with higher levels in brown than in white adipose tissue depots; and was affected by age: ATGL mRNA was increased with age in the brown adipose tissue and was decreased in two of the studied white depots, the inguinal and retroperitoneal, not being affected in the epididymal and mesenteric. Age also affected ATGL nutritional regulation: fasting increased ATGL gene expression and protein levels in the different white adipose depots of the youngest rats (up to the age of 5 months), whereas there was no change in the oldest rats studied (7 and 13-months old). This was in agreement with the pattern of NEFA levels, which did not increase in serum of fasted rats in the oldest animals, whereas other homeostatic parameters, such as insulin and leptin, responded to fasting independently of age. ATGL expressed by brown adipose tissue was not affected by feeding conditions at any age.

CONCLUSION

Nutritional regulation of ATGL expression in white adipose tissue is impaired with age, which could contribute to the increased difficulty for mobilizing lipids when animals are exposed to nutritional stress such as fasting.

Fat-specific protein 27 regulates storage of triacylglycerol

FSP27 (fat-specific protein 27) is a member of the cell death-inducing DNA fragmentation factor-alpha-like effector (CIDE) family. Although Cidea and Cideb were initially characterized as activators of apoptosis, recent studies have demonstrated important metabolic roles for these proteins. In this study, we investigated the function of another member of this family, FSP27 (Cidec), in apoptosis and adipocyte metabolism. Although overexpression of FSP27 is sufficient to increase apoptosis of 293T and 3T3-L1 cells, more physiological levels of expression stimulate spontaneous lipid accumulation in several cell types without induction of adipocyte genes. Increased triacylglycerol is likely due to decreased beta-oxidation of nonesterified fatty acids. Altered flux of fatty acids into triacylglycerol may be a direct effect of FSP27 function, which is localized to lipid droplets in 293T cells and 3T3-L1 adipocytes. Stable knockdown of FSP27 during adipogenesis of 3T3-L1 cells substantially decreases lipid droplet size, increases mitochondrial and lipid droplet number, and modestly increases glucose uptake and lipolysis. Expression of FSP27 in subcutaneous adipose tissue of a human diabetes cohort decreases with total fat mass but is not associated with measures of insulin resistance (e.g. homeostasis model assessment). Together, these data indicate that FSP27 binds to lipid droplets and regulates their enlargement.

Adipose triglyceride lipase regulation of skeletal muscle lipid metabolism and insulin responsiveness

Adipose triglyceride lipase (ATGL) is important for triglyceride (TG) metabolism in adipose tissue, and ATGL-null mice show increased adiposity. Given the apparent importance of ATGL in TG metabolism and the association of lipid deposition with insulin resistance, we examined the role of ATGL in regulating skeletal muscle lipid metabolism and insulin-stimulated glucose disposal. ATGL expression in myotubes was reduced by small interfering RNA and increased with a retrovirus encoding GFP-HA-ATGL. ATGL was also overexpressed in rats by in vivo electrotransfer. ATGL was down-regulated in skeletal muscle of obese, insulin-resistant mice and negatively correlated with intramyocellular TG levels. ATGL small interfering RNA in myotubes reduced TG hydrolase activity and increased TG content, whereas ATGL overexpression induced the reciprocal response, indicating that ATGL is an essential TG lipase in skeletal muscle. ATGL overexpression in myotubes increased the oxidation of fatty acid liberated from TG and diglyceride and ceramide contents. These responses in cells were largely recapitulated in rats overexpressing ATGL. When ATGL protein expression and TG hydrolase activity in obese, insulin-resistant rats were restored to levels observed in lean rats, TG content was reduced; however, the insulin resistance induced by the high-fat diet persisted. In conclusion, ATGL TG hydrolysis in skeletal muscle is a critical determinant of lipid metabolism and storage. Although ATGL content and TG hydrolase activity are decreased in obese, insulin-resistant phenotypes, overexpression does not rescue the condition, indicating reduced ATGL is unlikely to be a primary cause of obesity-associated insulin resistance.